**Meet the editor**

Dr. Yunjin Yao's research focuses on synthesis, characterization, and application of functional nanomaterials and pollution control principles and applications of various environmental technologies, particularly various advanced oxidation processes in water and air. He has published more than 50 original journal publications (including top-ranking journals, such as *Applied Catalysis B:* 

*Environmental*, *Journal of Hazardous Materials*, *Water Research*, and *Chemical Engineering Journal*) and 13 patents and delivered over 20 plenary/keynote/ invited presentations. His current projects include chemical and photocatalytic degradation kinetics and mechanisms of organic pollutants. Apart from his personal research, he also serves as an editorial board member of *American Chemical Science Journal, Journal of Applied Chemical Science International,* and *Journal of Analytical, Bioanalytical and Separation Techniques*.

Contents

**Preface VII**

Hideto Miyabe

**Fabrication 19**

**Visible Light Irradiation 1**

**Inactivation of Bacteria 29**

Shan Cong and Zhigang Zhao

**Photocatalysts 47**

**Photocatalyst 67**

Saidu

Chapter 2 **Rice Husk for Photocatalytic Composite Material**

Masahide Yasuda and Jin Matsumoto

Chapter 1 **Organic Reactions Promoted by Metal-Free Organic Dyes Under**

Diana Rakhmawaty Eddy, Atiek Rostika Noviyanti, Solihudin Solihudin, Safri Ishmayana and Roekmi-ati Tjokronegoro

Chapter 3 **Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light**

Chapter 4 **Carbon Quantum Dots: A Component of Efficient Visible Light**

Chapter 5 **Graphene-Semiconductor Composites as Visible Light-Induced**

Chapter 6 **Role of Graphene in Photocatalytic Solar Fuel Generation 87**

Chapter 7 **CQD-Based Composites as Visible-Light Active Photocatalysts**

Abdullahi Baba Makama, Muneer Umar and Shettima Abdulkadir

Ozlem Altintas Yildirim and Teoman Ozturk

Babak Adeli and Fariborz Taghipour

**for Purification of Water 111**

## Contents

#### **Preface XI**


#### Chapter 2 **Rice Husk for Photocatalytic Composite Material Fabrication 19** Diana Rakhmawaty Eddy, Atiek Rostika Noviyanti, Solihudin Solihudin, Safri Ishmayana and Roekmi-ati Tjokronegoro


## Chapter 8 **Visible Light–Driven Hydrogen Production by Carbon based Polymeric Materials 137**

Preface

are presented.

efficient, and facile in fabrication.

hance the photocatalytic activity of carbon materials.

The increasing worldwide energy shortage and environmental pollution issues are becom‐ ing serious threats to the development of human society. Over the past two decades, sun‐ light has been served as the most ideal power to drive the chemical reactions, resolve the energy shortage, and remove pollution. Particularly, semiconductor-based photocatalysis has been considered as an economic, safe, renewable, and clean technology, for various ap‐ plications, such as photodegradation of organic compounds, water splitting, and selective organic transformations. Nevertheless, the wide band gap and the low solar-energy utiliza‐ tion efficiency still remain the "bottleneck" to satisfy the requirements of practical applica‐ tions. Thus, a key challenge is to develop novel photocatalysts that are stable, abundant,

Recently, highly graphitic carbons and nanostructured *sp*<sup>2</sup> carbons (e.g., nanotubes, graphe‐ nes, fibers, graphite powder, and graphitic carbon nitride (g-C3N4)) with excellent properties have been widely used in optical and electronic devices, chemical sensors, energy genera‐ tion and storage, as well as environmental remediation. As is known, the photocatalytic process involves the generation of charge carriers, that is, electrons and holes induced by light. However, the pristine carbon material is usually restricted by unsatisfactory photoca‐ talytic efficiency and practical applications due to the insufficient solar light absorption, the low-surface area, and the fast recombination of photogenerated electron-hole pairs. General‐ ly, each photocatalytic reaction would involve three processes: photon absorption, electronhole charge generation and charge separation, and catalytic surface reactions. Therefore, various modification strategies, such as elemental and molecular doping, preparation of mesoporous carbon materials, and combination of conductive materials, are adopted to en‐

In this book, we intend to describe the great potential of efficient and low-cost carbon-based materials in various realms, such as the decomposition of pollutants, the water-gas shift re‐ action, and the photocatalytic evolution of H2. Finally, the current challenges and the crucial issues of carbon-based photocatalysts that need to be addressed for the future applications

**Yunjin Yao**

Hefei University of Technology, China

Palas Baran Pati, SU-IL In and Haining Tian

## Preface

Chapter 8 **Visible Light–Driven Hydrogen Production by Carbon based**

Palas Baran Pati, SU-IL In and Haining Tian

**Polymeric Materials 137**

**VI** Contents

The increasing worldwide energy shortage and environmental pollution issues are becom‐ ing serious threats to the development of human society. Over the past two decades, sun‐ light has been served as the most ideal power to drive the chemical reactions, resolve the energy shortage, and remove pollution. Particularly, semiconductor-based photocatalysis has been considered as an economic, safe, renewable, and clean technology, for various ap‐ plications, such as photodegradation of organic compounds, water splitting, and selective organic transformations. Nevertheless, the wide band gap and the low solar-energy utiliza‐ tion efficiency still remain the "bottleneck" to satisfy the requirements of practical applica‐ tions. Thus, a key challenge is to develop novel photocatalysts that are stable, abundant, efficient, and facile in fabrication.

Recently, highly graphitic carbons and nanostructured *sp*<sup>2</sup> carbons (e.g., nanotubes, graphe‐ nes, fibers, graphite powder, and graphitic carbon nitride (g-C3N4)) with excellent properties have been widely used in optical and electronic devices, chemical sensors, energy genera‐ tion and storage, as well as environmental remediation. As is known, the photocatalytic process involves the generation of charge carriers, that is, electrons and holes induced by light. However, the pristine carbon material is usually restricted by unsatisfactory photoca‐ talytic efficiency and practical applications due to the insufficient solar light absorption, the low-surface area, and the fast recombination of photogenerated electron-hole pairs. General‐ ly, each photocatalytic reaction would involve three processes: photon absorption, electronhole charge generation and charge separation, and catalytic surface reactions. Therefore, various modification strategies, such as elemental and molecular doping, preparation of mesoporous carbon materials, and combination of conductive materials, are adopted to en‐ hance the photocatalytic activity of carbon materials.

In this book, we intend to describe the great potential of efficient and low-cost carbon-based materials in various realms, such as the decomposition of pollutants, the water-gas shift re‐ action, and the photocatalytic evolution of H2. Finally, the current challenges and the crucial issues of carbon-based photocatalysts that need to be addressed for the future applications are presented.

> **Yunjin Yao** Hefei University of Technology, China

**Chapter 1**

**Provisional chapter**

**Organic Reactions Promoted by Metal-Free Organic**

Although dyes have received much attention as the visible light-activated photocatalysts, the use of metal-free organic dyes in synthetic organic chemistry is still limited. This chapter summarizes the recent progress in the visible light photocatalysis promoted by metalfree organic dyes. Eosin Y is the typical organic dyes to induce the photoredox catalysis. Recently, other organic dyes such as Rose Bengal, fluorescein, and methylene blue have been studied as photocatalysts to promote the single-electron transfer processes.

The use of abundant sunlight as a clean source of energy is an important aim of green chemistry. In recent years, dyes have attracted a great deal of attention as the visible light-activated photocatalysts in synthetic organic chemistry. However, these studies have mainly concentrated on the redox transformations using transition metal dyes such as ruthenium or iridium photocatalysts [1–9]. In contrast, the use of metal-free dyes still remains rather underdeveloped, although organic dyes are more environmentally friendly and cheaper. Eosin Y is the typical organic dyes to induce the photoredox catalysis [10]. Recently, Rose Bengal, fluorescein, methylene blue, and other organic dyes have been studied as photocatalysts to promote the single-electron transfer processes [11–13]. Additionally, 3-cyano-1-methylquinolinium, 9-mesityl-10-methylacridinium ion, and acridinium salts were developed as organic photo-

**Keywords:** photocatalyst, organocatalyst, dye, catalysis, visible light, radical

**Organic Reactions Promoted by Metal-Free Organic** 

DOI: 10.5772/intechopen.70507

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

The photoredox cycle is initiated by the visible light irradiation of dye in the ground state to produce the high-energy excited state of dye (Dye\*) (**Figure 1**). Two distinctive pathways

**Dyes Under Visible Light Irradiation**

**Dyes Under Visible Light Irradiation**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70507

Hideto Miyabe

**Abstract**

**1. Introduction**

catalysts [12, 13].

Hideto Miyabe

**Provisional chapter**

### **Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation Dyes Under Visible Light Irradiation**

**Organic Reactions Promoted by Metal-Free Organic** 

DOI: 10.5772/intechopen.70507

#### Hideto Miyabe Hideto Miyabe Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70507

#### **Abstract**

Although dyes have received much attention as the visible light-activated photocatalysts, the use of metal-free organic dyes in synthetic organic chemistry is still limited. This chapter summarizes the recent progress in the visible light photocatalysis promoted by metalfree organic dyes. Eosin Y is the typical organic dyes to induce the photoredox catalysis. Recently, other organic dyes such as Rose Bengal, fluorescein, and methylene blue have been studied as photocatalysts to promote the single-electron transfer processes.

**Keywords:** photocatalyst, organocatalyst, dye, catalysis, visible light, radical

### **1. Introduction**

The use of abundant sunlight as a clean source of energy is an important aim of green chemistry. In recent years, dyes have attracted a great deal of attention as the visible light-activated photocatalysts in synthetic organic chemistry. However, these studies have mainly concentrated on the redox transformations using transition metal dyes such as ruthenium or iridium photocatalysts [1–9]. In contrast, the use of metal-free dyes still remains rather underdeveloped, although organic dyes are more environmentally friendly and cheaper. Eosin Y is the typical organic dyes to induce the photoredox catalysis [10]. Recently, Rose Bengal, fluorescein, methylene blue, and other organic dyes have been studied as photocatalysts to promote the single-electron transfer processes [11–13]. Additionally, 3-cyano-1-methylquinolinium, 9-mesityl-10-methylacridinium ion, and acridinium salts were developed as organic photocatalysts [12, 13].

The photoredox cycle is initiated by the visible light irradiation of dye in the ground state to produce the high-energy excited state of dye (Dye\*) (**Figure 1**). Two distinctive pathways

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

**Figure 1.** Dye-catalyzed photoredox cycle.

from dye in the excited state (Dye\*) are described for the mechanism of visible light photoredox catalysis. The reductive property of Dye\* can be used in the presence of a sacrificial electron acceptor. In other words, Dye\* serves as an electron donor leading the radical cation species of Dye. In contrast, Dye\* also acts as an electron acceptor in the presence of a sacrificial electron donor.

### **2. Eosin Y and eosin B**

Eosin Y (EY) is the typical organic dye to induce the synthetically useful photoredox transformations [14–30]. EY that absorbed visible light populates in the lowest excited singlet state. The subsequent spin-forbidden singlet-triplet intersystem crossing affords EY in the excited triplet state. A variety of photoredox transformations are induced by the single-electron transfer from EY in the excited triplet state. In 2014, König provides the review article concerning the utility of EY as a photocatalyst in synthetic organic chemistry [10]. Therefore, this section highlights the recent remarkable progress in the EY-catalyzed photoredox transformations.

The EY-catalyzed generation of aryl radicals from aryl diazonium salts was studied by König's group [14]. The direct C−H bond arylation of heteroarenes with aryl diazonium salts was achieved by employing only 1 mol% of EY (**Figure 2**). The arylation of furan with diazonium salt **1** in dimethyl sulfoxide (DMSO) proceeded smoothly under the visible light irradiation to give the desired coupling product **2** in 85% yield. This transformation proceeds through the radical mechanism. Initially, aryl radical is produced by the single-electron transfer from the excited EY (EY\*) to aryl diazonium salt **1**. The addition of aryl radical to furan leads to the formation of the radical intermediate **A**, which is further oxidized to cation intermediate **B**. Final deprotonation gives the coupling product **2**. Next, the EY-catalyzed arylation of simple arenes with fluorinated aryl bromides was developed [15]. In the presence of EY (5 mol%) and triethylamine as an electron donor, the direct arylation using 1-bromo-2,3,4,5,6-pentafluorobenzene **3** and benzene gave the coupling product **4** in 85% yield. The mechanistic investigations reveal that the photooxidation of

triethylamine by the excited EY (EY\*), and the subsequent single-electron transfer from the radical anion species of EY to **3** leads to the formation of the polyfluorinated aryl radical. The mild visible light-mediated generation of aryl radicals from diazonium salts was also investigated by Wangelin's group [16–18]. The coupling reaction catalyzed by EY was investigated with no use of

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation

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3

The vinyl sulfones were synthesized by the EY-catalyzed reaction of alkenes with sodium aryl sulfinates [21]. The reaction of 1,2-dihydronaphthalene **6** with sodium benzenesulfinate **5** was performed in the presence of EY (10 mol%) and nitrobenzene as a terminal oxidant (**Figure 3**).

any sacrificial oxidants [19, 20].

**Figure 2.** EY-catalyzed generation of aryl radicals and coupling reactions.

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation http://dx.doi.org/10.5772/intechopen.70507 3

**Figure 2.** EY-catalyzed generation of aryl radicals and coupling reactions.

from dye in the excited state (Dye\*) are described for the mechanism of visible light photoredox catalysis. The reductive property of Dye\* can be used in the presence of a sacrificial electron acceptor. In other words, Dye\* serves as an electron donor leading the radical cation species of Dye. In contrast, Dye\* also acts as an electron acceptor in the presence of a sacrificial

Eosin Y (EY) is the typical organic dye to induce the synthetically useful photoredox transformations [14–30]. EY that absorbed visible light populates in the lowest excited singlet state. The subsequent spin-forbidden singlet-triplet intersystem crossing affords EY in the excited triplet state. A variety of photoredox transformations are induced by the single-electron transfer from EY in the excited triplet state. In 2014, König provides the review article concerning the utility of EY as a photocatalyst in synthetic organic chemistry [10]. Therefore, this section highlights the recent remarkable progress in the EY-catalyzed photoredox transformations.

The EY-catalyzed generation of aryl radicals from aryl diazonium salts was studied by König's group [14]. The direct C−H bond arylation of heteroarenes with aryl diazonium salts was achieved by employing only 1 mol% of EY (**Figure 2**). The arylation of furan with diazonium salt **1** in dimethyl sulfoxide (DMSO) proceeded smoothly under the visible light irradiation to give the desired coupling product **2** in 85% yield. This transformation proceeds through the radical mechanism. Initially, aryl radical is produced by the single-electron transfer from the excited EY (EY\*) to aryl diazonium salt **1**. The addition of aryl radical to furan leads to the formation of the radical intermediate **A**, which is further oxidized to cation intermediate **B**. Final deprotonation gives the coupling product **2**. Next, the EY-catalyzed arylation of simple arenes with fluorinated aryl bromides was developed [15]. In the presence of EY (5 mol%) and triethylamine as an electron donor, the direct arylation using 1-bromo-2,3,4,5,6-pentafluorobenzene **3** and benzene gave the coupling product **4** in 85% yield. The mechanistic investigations reveal that the photooxidation of

electron donor.

**2. Eosin Y and eosin B**

**Figure 1.** Dye-catalyzed photoredox cycle.

2 Visible-Light Photocatalysis of Carbon-Based Materials

triethylamine by the excited EY (EY\*), and the subsequent single-electron transfer from the radical anion species of EY to **3** leads to the formation of the polyfluorinated aryl radical. The mild visible light-mediated generation of aryl radicals from diazonium salts was also investigated by Wangelin's group [16–18]. The coupling reaction catalyzed by EY was investigated with no use of any sacrificial oxidants [19, 20].

The vinyl sulfones were synthesized by the EY-catalyzed reaction of alkenes with sodium aryl sulfinates [21]. The reaction of 1,2-dihydronaphthalene **6** with sodium benzenesulfinate **5** was performed in the presence of EY (10 mol%) and nitrobenzene as a terminal oxidant (**Figure 3**).

visible light irradiation. The desired coumarin **10** was obtained in 78% isolated yield. Initially, *tert*-butoxyl radical is produced by the single-electron transfer from the excited EY (EY\*) to TBHP. The cyclization reaction is promoted by the addition of sulfonyl radical, generated from sulfinic acid **9** and *tert*-butoxyl radical, to alkyne moiety of **8**. The coumarin **10** is formed *via* the oxidation of the cyclized radical intermediate **E** by the radical cation species of EY.

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation

The EY-catalyzed cyclization of 2-isocyanobiphenyls with arylsulfonyl chlorides took place

base, the EY-catalyzed reaction of benzenesulfonyl chloride **11** and 2-isocyanobiphenyl **12** proceeded smoothly to give the 6-phenyl-substituted phenanthridine **13** in 79% yield (**Figure 5**). Initially, the single-electron transfer from the excited EY (EY\*) to sulfonyl chloride **11** gives the phenyl radical, which adds to isocyanide **12** to form the imidoyl radical intermediate **F**. The subsequent cyclization gives the cyclized radical intermediate **G**, which is oxidized by the

EY could be used as the photocatalyst for the 5-*exo*-trig cyclization of iminyl radicals generated from *O*-aryl oximes [24]. Among several aryl oximes evaluated, 2,4-dinitro-substituted aryl oxime **14** has the excellent reactivity due to its low reduction potential (**Figure 6**). In the presence of cyclohexadiene (CHD) as a H-donor, EY-catalyzed photoreaction of **14** gave the cyclized product **15** in 78% yield. In this transformation, the iminyl radical **I** is generated *via* the reduction of **14** by the excited EY (EY\*) followed by the fragmentation of radical anion **H**. The cyclization of **I** gives the cyclized *C*-centered radical **J**, which abstracts H-atom from CHD to give the desired product **15**. Furthermore, the formation of product **15** was observed even

HPO<sup>4</sup>

http://dx.doi.org/10.5772/intechopen.70507

N under the visible

as a

5

under the oxidant-free visible light irradiation conditions [23]. In the presence of K<sup>2</sup>

radical cation species of EY. Finally, the deprotonation leads to **13**.

in the absence of EY, when the MeCN solution of **14** was treated with Et<sup>3</sup>

**Figure 5.** EY-catalyzed cyclization of 2-isocyanobiphenyl.

**Figure 3.** EY-catalyzed reaction using sodium benzenesulfinate.

The desired vinyl sulfone **7** was obtained in 99% yield. In this reaction, sodium sulfinate **5** is oxidized by the excited EY (EY\*) to give the sulfonyl radical, which attacks the double bond of **6** to form the radical intermediate **C**. Nitrobenzene oxidizes the radical cation species of EY to give EY in the ground state and the radical anion species **D** of nitrobenzene, which reacts with radical intermediate **C** to give the vinyl sulfone **7**.

The oxidative cyclization reaction between 3-phenylpropiolate **8** and 4-methylbenzenesulfinic acid **9** was studied (**Figure 4**) [22]. In the presence of EY (1 mol%) and *tert*-butyl hydroperoxide (TBHP), the reaction between **8** and **9** was performed in MeCN–H<sup>2</sup> O (1:1, v/v) under the

**Figure 4.** EY-catalyzed reaction using 4-methylbenzenesulfinic acid.

visible light irradiation. The desired coumarin **10** was obtained in 78% isolated yield. Initially, *tert*-butoxyl radical is produced by the single-electron transfer from the excited EY (EY\*) to TBHP. The cyclization reaction is promoted by the addition of sulfonyl radical, generated from sulfinic acid **9** and *tert*-butoxyl radical, to alkyne moiety of **8**. The coumarin **10** is formed *via* the oxidation of the cyclized radical intermediate **E** by the radical cation species of EY.

The EY-catalyzed cyclization of 2-isocyanobiphenyls with arylsulfonyl chlorides took place under the oxidant-free visible light irradiation conditions [23]. In the presence of K<sup>2</sup> HPO<sup>4</sup> as a base, the EY-catalyzed reaction of benzenesulfonyl chloride **11** and 2-isocyanobiphenyl **12** proceeded smoothly to give the 6-phenyl-substituted phenanthridine **13** in 79% yield (**Figure 5**). Initially, the single-electron transfer from the excited EY (EY\*) to sulfonyl chloride **11** gives the phenyl radical, which adds to isocyanide **12** to form the imidoyl radical intermediate **F**. The subsequent cyclization gives the cyclized radical intermediate **G**, which is oxidized by the radical cation species of EY. Finally, the deprotonation leads to **13**.

EY could be used as the photocatalyst for the 5-*exo*-trig cyclization of iminyl radicals generated from *O*-aryl oximes [24]. Among several aryl oximes evaluated, 2,4-dinitro-substituted aryl oxime **14** has the excellent reactivity due to its low reduction potential (**Figure 6**). In the presence of cyclohexadiene (CHD) as a H-donor, EY-catalyzed photoreaction of **14** gave the cyclized product **15** in 78% yield. In this transformation, the iminyl radical **I** is generated *via* the reduction of **14** by the excited EY (EY\*) followed by the fragmentation of radical anion **H**. The cyclization of **I** gives the cyclized *C*-centered radical **J**, which abstracts H-atom from CHD to give the desired product **15**. Furthermore, the formation of product **15** was observed even in the absence of EY, when the MeCN solution of **14** was treated with Et<sup>3</sup> N under the visible

**Figure 5.** EY-catalyzed cyclization of 2-isocyanobiphenyl.

The desired vinyl sulfone **7** was obtained in 99% yield. In this reaction, sodium sulfinate **5** is oxidized by the excited EY (EY\*) to give the sulfonyl radical, which attacks the double bond of **6** to form the radical intermediate **C**. Nitrobenzene oxidizes the radical cation species of EY to give EY in the ground state and the radical anion species **D** of nitrobenzene, which reacts

The oxidative cyclization reaction between 3-phenylpropiolate **8** and 4-methylbenzenesulfinic acid **9** was studied (**Figure 4**) [22]. In the presence of EY (1 mol%) and *tert*-butyl hydroperox-

O (1:1, v/v) under the

with radical intermediate **C** to give the vinyl sulfone **7**.

**Figure 4.** EY-catalyzed reaction using 4-methylbenzenesulfinic acid.

**Figure 3.** EY-catalyzed reaction using sodium benzenesulfinate.

4 Visible-Light Photocatalysis of Carbon-Based Materials

ide (TBHP), the reaction between **8** and **9** was performed in MeCN–H<sup>2</sup>

**3. Rose Bengal**

Rose Bengal (RB) was widely used as a visible light-activated photocatalyst [32–44]. Tan's group studied the photoredox catalysis using RB [32–36]. RB was a good catalyst for the dehydrogenative coupling reaction between tetrahydroisoquinolines and nitroalkanes (**Figure 9**) [33]. In the presence of RB (5 mol%), the reaction of *N*-phenyl-tetrahydroisoquinoline **20** with

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation

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7

obtained relative to the reaction performed in open air; thus, air is important for this reaction as an oxidant. Additionally, this reaction was expanded to the dehydrogenative Mannich

, a much lower yield was

nitromethane gave the adduct **21** in 92% yield. In the absence of O<sup>2</sup>

**Figure 8.** Eosin B-catalyzed phosphorylation of benzothiazole.

**Figure 9.** RB-catalyzed reaction of tetrahydroisoquinoline.

reaction using enamine nucleophiles generated from ketones and pyrrolidine.

**Figure 6.** EY-catalyzed cyclization of 2,4-dinitro-substituted aryl oxime.

light irradiation. In this case, the visible light-mediated electron transfer would be induced by the formation of donor-acceptor complex between Et<sup>3</sup> N and 2,4-dinitrophenyl group of **14**.

The EY-induced photocatalysis was applied to the radical cascade cyclization of polyenes [25]. The photocatalytic cascade cyclization of polyene **16** proceeded by employing EY (**Figure 7**). Hexafluoro-2-propanol (HFIP) was identified as the optimal solvent. The cyclized product **17** was obtained in 93% yield with the excellent diastereoselectivity *via* the radical cation intermediate **K** generated by the single-electron transfer from **16** to the excited EY (EY\*). In this process, the OH moiety of **16** would act as a terminator.

Eosin B is also the active catalyst under the visible light irradiation [31]. The C−H functionalization of thiazole derivatives with diarylphosphine oxides was achieved by the eosin B-catalyzed photoredox process. When eosin B was employed as a photocatalyst, the phosphorylation of benzothiazole **18** with diphenylphosphine oxide proceeded effectively to give the phosphorylation product **19** in 87% yield (**Figure 8**). In this transformation, hydrogen (H<sup>2</sup> ) is the only by-product.

**Figure 7.** EY-catalyzed cascade cyclization of polyene.

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation http://dx.doi.org/10.5772/intechopen.70507 7

**Figure 8.** Eosin B-catalyzed phosphorylation of benzothiazole.

#### **3. Rose Bengal**

light irradiation. In this case, the visible light-mediated electron transfer would be induced

The EY-induced photocatalysis was applied to the radical cascade cyclization of polyenes [25]. The photocatalytic cascade cyclization of polyene **16** proceeded by employing EY (**Figure 7**). Hexafluoro-2-propanol (HFIP) was identified as the optimal solvent. The cyclized product **17** was obtained in 93% yield with the excellent diastereoselectivity *via* the radical cation intermediate **K** generated by the single-electron transfer from **16** to the excited EY (EY\*). In this

Eosin B is also the active catalyst under the visible light irradiation [31]. The C−H functionalization of thiazole derivatives with diarylphosphine oxides was achieved by the eosin B-catalyzed photoredox process. When eosin B was employed as a photocatalyst, the phosphorylation of benzothiazole **18** with diphenylphosphine oxide proceeded effectively to give the phosphorylation

N and 2,4-dinitrophenyl group of **14**.

) is the only by-product.

by the formation of donor-acceptor complex between Et<sup>3</sup>

**Figure 6.** EY-catalyzed cyclization of 2,4-dinitro-substituted aryl oxime.

6 Visible-Light Photocatalysis of Carbon-Based Materials

process, the OH moiety of **16** would act as a terminator.

**Figure 7.** EY-catalyzed cascade cyclization of polyene.

product **19** in 87% yield (**Figure 8**). In this transformation, hydrogen (H<sup>2</sup>

Rose Bengal (RB) was widely used as a visible light-activated photocatalyst [32–44]. Tan's group studied the photoredox catalysis using RB [32–36]. RB was a good catalyst for the dehydrogenative coupling reaction between tetrahydroisoquinolines and nitroalkanes (**Figure 9**) [33]. In the presence of RB (5 mol%), the reaction of *N*-phenyl-tetrahydroisoquinoline **20** with nitromethane gave the adduct **21** in 92% yield. In the absence of O<sup>2</sup> , a much lower yield was obtained relative to the reaction performed in open air; thus, air is important for this reaction as an oxidant. Additionally, this reaction was expanded to the dehydrogenative Mannich reaction using enamine nucleophiles generated from ketones and pyrrolidine.

**Figure 9.** RB-catalyzed reaction of tetrahydroisoquinoline.

Next, the combination of graphene oxide and RB was studied in the reaction between tetrahydroisoquinolines with TMSCN or TMSCF<sup>3</sup> [34]. In the presence of RB (5 mol%) and graphene oxide (50 wt%), the reaction of **20** with TMSCN proceeded effectively to give the adduct **22** in 99% yield, while the yield of **22** decreased to 45% in the absence of graphene oxide. The use of graphene oxide as a cocatalyst improves the reaction rates and yields.

New method for the synthesis of Meyers's bicyclic lactams was developed by using RB photocatalysis [37–39]. This cascade transformation is the one-pot reaction which begins from furan substrates (**Figure 10**) [37]. Despite the extraordinary complexity of reaction cascade, the reaction between 2-methylfuran **23** and L-serine ethyl ester **24** led to the formation of bicyclic lactam **25** in 68% yield. At first, RB promotes the photooxidation of methylfuran **23** with singlet oxygen in MeOH. The intermediate **L** is formed by the in situ reduction of hydroperoxy with Me<sup>2</sup> S. The next reaction of **L** with L-serine ethyl ester **24** gives the intermediate **M**, which is converted to 2-pyrrolidinone **N** *via* imino enal. Actually, 2-pyrrolidinone **N** could be isolated by the flash column chromatography using silica gel neutralized by trimethylamine. Finally, treatment of **N** with TFA gives the bicyclic lactam **25** as the final product of one-pot reaction cascade.

The aerobic visible light-promoted indole C3 formylation reaction was achieved by using RB as a photocatalyst and *N*,*N*,*N*′,*N*′-tetramethylenediamine (TMEDA) as a one-carbon source (**Figure 11**) [40]. Upon the irradiation of visible light, the reaction of *N*-methylindole **26** with TMEDA in the presence of RB (5 mol%) and KI as an additive under air afforded 3-formyl-*N*-methylindole **27** in 70% yield. This transformation proceeds *via* the addition of *N*-methylindole **26** to iminium ion **O** generated by the oxidation of TMEDA. Next, the C3 thiocyanation reaction of indoles was developed by using ammonium thiocyanate (NH<sup>4</sup> SCN) as a thiocyanate radical source [41]. In the presence of RB (1 mol%), the reaction of indole **28** with NH<sup>4</sup> SCN gave the adduct **29** in 98% yield. In this reaction, thiocyanate radical is generated by the single-electron transfer between thiocyanate anion and the excited RB (RB\*). The thiocyanate radical adds to indole **28**. The subsequent oxidation leads to **29**.

**Figure 11.** RB-catalyzed functionalization of C3 in indoles.

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation

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9

**Figure 12.** RB-catalyzed oxidative cyclization of benzimidamide.

**Figure 10.** RB-catalyzed synthesis of bicyclic lactam.

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation http://dx.doi.org/10.5772/intechopen.70507 9

**Figure 11.** RB-catalyzed functionalization of C3 in indoles.

Next, the combination of graphene oxide and RB was studied in the reaction between tetrahy-

oxide (50 wt%), the reaction of **20** with TMSCN proceeded effectively to give the adduct **22** in 99% yield, while the yield of **22** decreased to 45% in the absence of graphene oxide. The use of

New method for the synthesis of Meyers's bicyclic lactams was developed by using RB photocatalysis [37–39]. This cascade transformation is the one-pot reaction which begins from furan substrates (**Figure 10**) [37]. Despite the extraordinary complexity of reaction cascade, the reaction between 2-methylfuran **23** and L-serine ethyl ester **24** led to the formation of bicyclic lactam **25** in 68% yield. At first, RB promotes the photooxidation of methylfuran **23** with singlet oxygen in MeOH. The intermediate **L** is formed by the in situ reduction of hydroper-

which is converted to 2-pyrrolidinone **N** *via* imino enal. Actually, 2-pyrrolidinone **N** could be isolated by the flash column chromatography using silica gel neutralized by trimethylamine. Finally, treatment of **N** with TFA gives the bicyclic lactam **25** as the final product of one-pot

The aerobic visible light-promoted indole C3 formylation reaction was achieved by using RB as a photocatalyst and *N*,*N*,*N*′,*N*′-tetramethylenediamine (TMEDA) as a one-carbon source (**Figure 11**) [40]. Upon the irradiation of visible light, the reaction of *N*-methylindole **26** with TMEDA in the presence of RB (5 mol%) and KI as an additive under air afforded 3-formyl-*N*-methylindole **27** in 70% yield. This transformation proceeds *via* the addition of *N*-methylindole **26** to iminium ion **O** generated by the oxidation of TMEDA. Next, the C3 thiocyanation reaction of indoles was developed by using ammonium thiocyanate (NH<sup>4</sup>

as a thiocyanate radical source [41]. In the presence of RB (1 mol%), the reaction of indole **28**

ated by the single-electron transfer between thiocyanate anion and the excited RB (RB\*). The

thiocyanate radical adds to indole **28**. The subsequent oxidation leads to **29**.

SCN gave the adduct **29** in 98% yield. In this reaction, thiocyanate radical is gener-

S. The next reaction of **L** with L-serine ethyl ester **24** gives the intermediate **M**,

graphene oxide as a cocatalyst improves the reaction rates and yields.

[34]. In the presence of RB (5 mol%) and graphene

SCN)

droisoquinolines with TMSCN or TMSCF<sup>3</sup>

8 Visible-Light Photocatalysis of Carbon-Based Materials

oxy with Me<sup>2</sup>

reaction cascade.

with NH<sup>4</sup>

**Figure 10.** RB-catalyzed synthesis of bicyclic lactam.

**Figure 12.** RB-catalyzed oxidative cyclization of benzimidamide.

The new method for the synthesis of quinazolines was developed by using RB as a photocatalyst (**Figure 12**) [42]. In the presence of RB (1 mol%), CBr<sup>4</sup> as an oxidant, and K<sup>2</sup> CO<sup>3</sup> as a base, the oxidative carbon-carbon bond-forming cyclization of *N*-benzyl-*N*′-phenyl benzimidamide **30** proceeded smoothly under the visible light irradiation. The desired quinazoline **31** was obtained in 88% yield. In this transformation, CBr<sup>3</sup> radical is generated by the reaction between CBr<sup>4</sup> and the radical anion species of RB. Next, the iminium ion intermediate **Q** is formed from the radical cation intermediate **P** by the association of CBr<sup>3</sup> radical. Finally, the intramolecular Friedel-Craft reaction of iminium ion **Q** leads to quinazoline **31**.

The utility of rhodamine B as a water-soluble photocatalyst was demonstrated in the aqueous-

ceeded smoothly to give the product **35** in 90% yield (**Figure 14**). In this transformation, the

was proposed. This electron transfer process was supported by the fluorescence quenching

 into I<sup>2</sup> . NEt as a

O pro-

NEt. In

F7 I 11

F7 I in H<sup>2</sup>

http://dx.doi.org/10.5772/intechopen.70507

) of rhodamine B to *i*-C<sup>3</sup>

) to the radical cation spe-

, because the oxidation potential of

I. Additionally, the aqueous-medium radical addition-

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation

medium carbon-carbon bond-forming radical reactions [47]. In the presence of (*i*-Pr)<sup>2</sup>

cyclization-trapping reaction of **36** proceeded effectively even in the absence of (*i*-Pr)<sup>2</sup>

reductive quencher, the rhodamine B-catalyzed reaction of alkene **34** with *i*-C<sup>3</sup>

F7

photo-induced electron transfer from the excited singlet state (*S*<sup>1</sup>

this transformation, the single-electron transfer from iodine ion (I−

cies of rhodamine B in an ion pair would proceed to give I<sup>2</sup>

**Figure 14.** Rhodamine B-catalyzed aqueous-medium radical reactions.

of rhodamine B with addition of *i*-C<sup>3</sup>

rhodamine B is positive enough to oxidize I−

### **4. Fluorescein and rhodamine B**

The utility of fluorescein was demonstrated in the alkoxycarboxylation of aryldiazonium salts using CO gas [45]. In the presence of 0.5 mol% of fluorescein as a photocatalyst, treatment of diazonium tetrafluoroborate **32** with CO (80 atm pressure) in methanol under the irradiation of visible light gave methyl ester **33** in 80% yield (**Figure 13**). Initially, the phenyl radical is generated from diazonium **32** by the single-electron transfer from the excited state of fluorescein (Dye\*). Next, benzoyl radical **R** is formed *via* trapping of CO molecule by phenyl radical. The methyl ester **33** is obtained *via* the oxidation of benzoyl radical **R** by the reactive radical cation species of dye followed by trapping of the resulting benzylidyneoxonium **S** with methanol. Furthermore, the dual catalytic system using photocatalyst and gold catalyst was studied by Glorius's group [46].

**Figure 13.** Fluorescein-catalyzed alkoxycarboxylation of aryldiazonium.

The utility of rhodamine B as a water-soluble photocatalyst was demonstrated in the aqueousmedium carbon-carbon bond-forming radical reactions [47]. In the presence of (*i*-Pr)<sup>2</sup> NEt as a reductive quencher, the rhodamine B-catalyzed reaction of alkene **34** with *i*-C<sup>3</sup> F7 I in H<sup>2</sup> O proceeded smoothly to give the product **35** in 90% yield (**Figure 14**). In this transformation, the photo-induced electron transfer from the excited singlet state (*S*<sup>1</sup> ) of rhodamine B to *i*-C<sup>3</sup> F7 I was proposed. This electron transfer process was supported by the fluorescence quenching of rhodamine B with addition of *i*-C<sup>3</sup> F7 I. Additionally, the aqueous-medium radical additioncyclization-trapping reaction of **36** proceeded effectively even in the absence of (*i*-Pr)<sup>2</sup> NEt. In this transformation, the single-electron transfer from iodine ion (I− ) to the radical cation species of rhodamine B in an ion pair would proceed to give I<sup>2</sup> , because the oxidation potential of rhodamine B is positive enough to oxidize I− into I<sup>2</sup> .

The new method for the synthesis of quinazolines was developed by using RB as a pho-

as a base, the oxidative carbon-carbon bond-forming cyclization of *N*-benzyl-*N*′-phenyl benzimidamide **30** proceeded smoothly under the visible light irradiation. The desired

ion intermediate **Q** is formed from the radical cation intermediate **P** by the association of

The utility of fluorescein was demonstrated in the alkoxycarboxylation of aryldiazonium salts using CO gas [45]. In the presence of 0.5 mol% of fluorescein as a photocatalyst, treatment of diazonium tetrafluoroborate **32** with CO (80 atm pressure) in methanol under the irradiation of visible light gave methyl ester **33** in 80% yield (**Figure 13**). Initially, the phenyl radical is generated from diazonium **32** by the single-electron transfer from the excited state of fluorescein (Dye\*). Next, benzoyl radical **R** is formed *via* trapping of CO molecule by phenyl radical. The methyl ester **33** is obtained *via* the oxidation of benzoyl radical **R** by the reactive radical cation species of dye followed by trapping of the resulting benzylidyneoxonium **S** with methanol. Furthermore, the dual catalytic system using photocatalyst and gold catalyst was

radical. Finally, the intramolecular Friedel-Craft reaction of iminium ion **Q** leads to

as an oxidant, and K<sup>2</sup>

and the radical anion species of RB. Next, the iminium

radical is gener-

CO<sup>3</sup>

tocatalyst (**Figure 12**) [42]. In the presence of RB (1 mol%), CBr<sup>4</sup>

ated by the reaction between CBr<sup>4</sup>

10 Visible-Light Photocatalysis of Carbon-Based Materials

**4. Fluorescein and rhodamine B**

studied by Glorius's group [46].

**Figure 13.** Fluorescein-catalyzed alkoxycarboxylation of aryldiazonium.

CBr<sup>3</sup>

quinazoline **31**.

quinazoline **31** was obtained in 88% yield. In this transformation, CBr<sup>3</sup>

**Figure 14.** Rhodamine B-catalyzed aqueous-medium radical reactions.

### **5. Methylene blue and acridine red**

Methylene blue (MB) is a member of the thiazine dye family. Scaiano's group used MB as a photocatalyst under the visible light irradiation [48]. The radical trifluoromethylation of electron-rich heterocycles was studied by the use of Togni's reagent **39** as a CF<sup>3</sup> radical source (**Figure 15**). The trifluoromethylation of 3-methylindole **38** proceeded with good yield at low catalyst concentration, when *N*,*N*,*N′*,*N′*-tetramethylenediamine (TMEDA) was used as an electron donor. In the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as an electron donor, the reaction of terminal alkene **41** with Togni's reagent **39** also gave the hydrotrifluoromethylation product **42** in 67% yield as a major product, because the fully reduced form of MB, leuco-MB, acts as a hydrogen source [49]. The possible mechanism for the catalytic formation of CF<sup>3</sup> radical is shown. The visible light-excited MB (MB\*) is readily quenched by aliphatic amines such as TMEDA or DBU to form the semi-reduced MB as a radical anion and an α-amino radical. CF<sup>3</sup> radical is generated *via* the reduction of Togni's reagent **39** with semireduced MB and/or an α-amino radical.

Me<sup>2</sup>

catalyst [51].

ously cyclized to form **46**.

**6. Riboflavin tetraacetate**

mol%) and [Fe(TPA)(MeCN)<sup>2</sup>

](ClO<sup>4</sup> )2

**Figure 16.** MB-catalyzed reactions of furan with amines.

**Figure 17.** Acridine red-catalyzed thiolation of tetrahydrofuran.

](ClO<sup>4</sup> )2

[Fe(TPA)(MeCN)<sup>2</sup>

S (**Figure 16**). The use of 2-(3,4-dimethoxyphenyl)ethanamine **45** instead of benzylamine led to the formation of tricycle **46** *via* Pictet-Spengler cyclization process. For Pictet-Spengler cyclization, HCOOH was added as an acid leading to *N*-acyliminium ion, which spontane-

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation

http://dx.doi.org/10.5772/intechopen.70507

13

New phenothiazine-based organic dye was also developed as a visible light-activated photo-

Acridine red was used as a photocatalyst for the visible light-induced direct thiolation of ethers (**Figure 17**) [52]. The thiolation of tetrahydrofuran (THF) using diphenyl disulfide **47** was carried out in the presence of acridine red (2 mol%) and *tert*-butyl hydroperoxide (TBHP) as an oxidant. The reaction occurred at ambient conditions to give α-arylthioether **48** in 82% yield. This transformation proceeds *via* the generation of radical intermediate from THF,

Riboflavin tetraacetate (RFT) is an effective photocatalyst for the visible light-driven organic reactions. The aerobic oxidation of alkyl benzenes to ketones and carboxylic acids was investigated through a dual catalysis using RFT and the tris(2-pyridylmethyl)amine-iron complex

(TPA=tris(2-pyridylmethyl)amine) [53]. When a mixture of RFT (10

(2 mol%) was employed, the oxidation of 4-ethylanisole

which reacts with diphenyl disulfide **47** to afford 2-(phenylthio)-tetrahydrofuran **48**.

The one-pot transformation of furans into 5-hydroxy-1*H*-pyrrol-2(5*H*)-ones was investigated by using MB as a photocatalyst [50]. In the presence of MB (2 mol%) and oxygen, the reaction of furan **43** with benzylamine gave lactam **44** in 72% yield *via* the reduction of the intermediate by

**Figure 15.** MB-catalyzed trifluoromethylation using Togni's reagent.

Me<sup>2</sup> S (**Figure 16**). The use of 2-(3,4-dimethoxyphenyl)ethanamine **45** instead of benzylamine led to the formation of tricycle **46** *via* Pictet-Spengler cyclization process. For Pictet-Spengler cyclization, HCOOH was added as an acid leading to *N*-acyliminium ion, which spontaneously cyclized to form **46**.

New phenothiazine-based organic dye was also developed as a visible light-activated photocatalyst [51].

Acridine red was used as a photocatalyst for the visible light-induced direct thiolation of ethers (**Figure 17**) [52]. The thiolation of tetrahydrofuran (THF) using diphenyl disulfide **47** was carried out in the presence of acridine red (2 mol%) and *tert*-butyl hydroperoxide (TBHP) as an oxidant. The reaction occurred at ambient conditions to give α-arylthioether **48** in 82% yield. This transformation proceeds *via* the generation of radical intermediate from THF, which reacts with diphenyl disulfide **47** to afford 2-(phenylthio)-tetrahydrofuran **48**.

**Figure 16.** MB-catalyzed reactions of furan with amines.

**5. Methylene blue and acridine red**

12 Visible-Light Photocatalysis of Carbon-Based Materials

formation of CF<sup>3</sup>

an α-amino radical. CF<sup>3</sup>

reduced MB and/or an α-amino radical.

**Figure 15.** MB-catalyzed trifluoromethylation using Togni's reagent.

Methylene blue (MB) is a member of the thiazine dye family. Scaiano's group used MB as a photocatalyst under the visible light irradiation [48]. The radical trifluoromethylation of

(**Figure 15**). The trifluoromethylation of 3-methylindole **38** proceeded with good yield at low catalyst concentration, when *N*,*N*,*N′*,*N′*-tetramethylenediamine (TMEDA) was used as an electron donor. In the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as an electron donor, the reaction of terminal alkene **41** with Togni's reagent **39** also gave the hydrotrifluoromethylation product **42** in 67% yield as a major product, because the fully reduced form of MB, leuco-MB, acts as a hydrogen source [49]. The possible mechanism for the catalytic

aliphatic amines such as TMEDA or DBU to form the semi-reduced MB as a radical anion and

The one-pot transformation of furans into 5-hydroxy-1*H*-pyrrol-2(5*H*)-ones was investigated by using MB as a photocatalyst [50]. In the presence of MB (2 mol%) and oxygen, the reaction of furan **43** with benzylamine gave lactam **44** in 72% yield *via* the reduction of the intermediate by

radical is shown. The visible light-excited MB (MB\*) is readily quenched by

radical is generated *via* the reduction of Togni's reagent **39** with semi-

radical source

electron-rich heterocycles was studied by the use of Togni's reagent **39** as a CF<sup>3</sup>

**Figure 17.** Acridine red-catalyzed thiolation of tetrahydrofuran.

### **6. Riboflavin tetraacetate**

Riboflavin tetraacetate (RFT) is an effective photocatalyst for the visible light-driven organic reactions. The aerobic oxidation of alkyl benzenes to ketones and carboxylic acids was investigated through a dual catalysis using RFT and the tris(2-pyridylmethyl)amine-iron complex [Fe(TPA)(MeCN)<sup>2</sup> ](ClO<sup>4</sup> )2 (TPA=tris(2-pyridylmethyl)amine) [53]. When a mixture of RFT (10 mol%) and [Fe(TPA)(MeCN)<sup>2</sup> ](ClO<sup>4</sup> )2 (2 mol%) was employed, the oxidation of 4-ethylanisole **49** proceeded effectively under the visible light irradiation to give 4-acetylanisole **50** in 80% yield (**Figure 18**). In this oxidation, the iron complex acts as a catalyst for not only oxidation of **49** but also disproportionation of hydrogen peroxide H<sup>2</sup> O2 which is obtained in the RFTcatalyzed oxidation of **49**.

**Author details**

Address all correspondence to: miyabe@huhs.ac.jp

Edition. 2009;**48**:9785-9786

2015;**13**:9152-9167

2015;**51**:3290-3301

2015;**54**:15632-15641

2016;**14**:3849-3862

Reviews. 2013;**42**:97-113

Communications. 2014;**50**:6688-6699

in organic synthesis. ACS Catalysis. 2014;**4**:355-360

School of Pharmacy, Hyogo University of Health Sciences, Minatojima, Chuo-ku, Japan

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RFT also catalyzed the aerobic oxidation of sulfides to sulfoxides without overoxidation to sulfones [54]. In the presence of RFT (2 mol%), sulfide **51** was transformed chemoselectively to the corresponding sulfoxide **52** in 91% yield (**Figure 19**).

**Figure 18.** RFT-catalyzed oxidation of 4-ethylanisole.

**Figure 19.** RFT-catalyzed oxidation of sulfide.

### **7. Concluding remarks**

Organic dyes that absorbed visible light induce the synthetically valuable photochemical transformations. The metal-free photocatalysis using organic dyes rapidly progresses in the last few years. In addition to the organic dyes shown in this chapter, Fukuzumi' group has developed 3-cyano-1-methylquinolinium and 9-mesityl-10-methylacridinium ions as photocatalysts [13]. More recently, Nicewicz' group has studied the photocatalysis using acridinium salts [12]. These visible light-induced catalysis disclosed a broader aspect of the utility of organic photocatalysts for synthetic organic chemistry. This chapter will inspire creative new contributions to organic chemists.

## **Author details**

Hideto Miyabe

Address all correspondence to: miyabe@huhs.ac.jp

School of Pharmacy, Hyogo University of Health Sciences, Minatojima, Chuo-ku, Japan

### **References**

**Figure 19.** RFT-catalyzed oxidation of sulfide.

**Figure 18.** RFT-catalyzed oxidation of 4-ethylanisole.

**7. Concluding remarks**

contributions to organic chemists.

**49** proceeded effectively under the visible light irradiation to give 4-acetylanisole **50** in 80% yield (**Figure 18**). In this oxidation, the iron complex acts as a catalyst for not only oxidation

RFT also catalyzed the aerobic oxidation of sulfides to sulfoxides without overoxidation to sulfones [54]. In the presence of RFT (2 mol%), sulfide **51** was transformed chemoselectively

Organic dyes that absorbed visible light induce the synthetically valuable photochemical transformations. The metal-free photocatalysis using organic dyes rapidly progresses in the last few years. In addition to the organic dyes shown in this chapter, Fukuzumi' group has developed 3-cyano-1-methylquinolinium and 9-mesityl-10-methylacridinium ions as photocatalysts [13]. More recently, Nicewicz' group has studied the photocatalysis using acridinium salts [12]. These visible light-induced catalysis disclosed a broader aspect of the utility of organic photocatalysts for synthetic organic chemistry. This chapter will inspire creative new

O2

which is obtained in the RFT-

of **49** but also disproportionation of hydrogen peroxide H<sup>2</sup>

to the corresponding sulfoxide **52** in 91% yield (**Figure 19**).

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**Chapter 2**

)

, while

**Provisional chapter**

**Rice Husk for Photocatalytic Composite Material**

**Rice Husk for Photocatalytic Composite Material** 

DOI: 10.5772/intechopen.72704

As a semiconductor, zinc oxide (ZnO) has better UV absorbing properties compared to other semiconductor materials, and therefore, it has better dye degrading abilities. However, ZnO tends to agglomerate, which lead to poor degradation compared to the other semiconductors. In this study, to overcome the agglomeration of ZnO, silica (SiO<sup>2</sup>

was combined with ZnO. The composite was tested for its photocatalytic activity. The

photocatalyst was fabricated on a glass plate. In order to investigate the addition

was supported on ZnO with 8% composition. The opti-

95/5, as indicated by high degradation activity,

 on ZnO, X-ray diffraction (XRD) and scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS) was used. The result of the XRD analysis demonstrates similar peak results with ZnO XRD data from ICSD 157132 with a hexagonal structure. The results indicate that the ZnO structure did not change after the addition of SiO<sup>2</sup>

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

The textile industry is developing at a rapid pace, and this has a positive impact on garments development. However, it also increases the negative impact through their industrial waste, especially textile dye. One of the means to degrade dye is by the use of a semiconductor material that has photocatalytic activity [1]. Utilization of semiconductors in photocatalysis is an interesting topic, attributable to its ability to degrade compounds with ultraviolet light facilitation [2].

Diana Rakhmawaty Eddy, Atiek Rostika Noviyanti,

Diana Rakhmawaty Eddy, Atiek Rostika Noviyanti,

Solihudin Solihudin, Safri Ishmayana and

Solihudin Solihudin, Safri Ishmayana and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72704

SEM-EDS results showed that SiO<sup>2</sup>

mal composition was found to be ZnO/SiO<sup>2</sup>

which can degrade up to 89% methylene blue.

**Keywords:** rice husk, silica, photocatalysis, ZnO, methylene blue

**Fabrication**

**Fabrication**

Roekmi-ati Tjokronegoro

**Abstract**

ZnO/SiO<sup>2</sup>

of SiO<sup>2</sup>

**1. Introduction**

Roekmi-ati Tjokronegoro


**Provisional chapter**

### **Rice Husk for Photocatalytic Composite Material Fabrication Fabrication**

**Rice Husk for Photocatalytic Composite Material** 

DOI: 10.5772/intechopen.72704

Diana Rakhmawaty Eddy, Atiek Rostika Noviyanti, Solihudin Solihudin, Safri Ishmayana and Roekmi-ati Tjokronegoro Solihudin Solihudin, Safri Ishmayana and Roekmi-ati Tjokronegoro Additional information is available at the end of the chapter

Diana Rakhmawaty Eddy, Atiek Rostika Noviyanti,

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72704

#### **Abstract**

[43] Fidaly K, Ceballos C, Falguières A, Veitia MSI, Guy A, Ferroud C. Visible light photoredox organocatalysis: A fully transition metal-free direct asymmetric α-alkylation of

[44] Barata-Vallejo S, Yerien DE, Postigo A. Benign perfluoroalkylation of aniline derivatives through photoredox organocatalysis under visible-light irradiation. European Journal of

[45] Guo W, Lu LQ, Wang Y, Wang YN, Chen JR, Xiao WJ. Metal-free, room-temperature, radical alkoxycarbonylation of aryldiazonium salts through visible-light photoredox

[46] Hopkinson MN, Sahoo B, Glorius F. Dual photoredox and gold catalysis: Intermolecular multicomponent oxyarylation of alkenes. Advanced Synthesis & Catalysis. 2014;**356**:

[47] Yoshioka E, Kohtani S, Jichu T, Fukazawa T, Nagai T, Kawashima A, Takemoto Y, Miyabe H. Aqueous-medium carbon−carbon bond-forming radical reactions catalyzed by excited rhodamine B as a metal-free organic dye under visible light irradiation. Journal

[48] Pitre SP, McTiernan CD, Ismaili H, Scaiano JC. Metal-free photocatalytic radical trifluoromethylation utilizing methylene blue and visible light irradiation. ACS Catalysis.

[49] Yamamoto S, Fujiyama Y, Shiozaki M, Sueishi Y, Nishimura N. Hydride transfer reactions of leuco methylene blue and leuco thionine with some *p*-benzoquinones. Journal of

[50] Kalaitzakis D, Kouridaki A, Noutsias D, Montagnon T, Vassilikogiannakis G. Methylene blue as a photosensitizer and redox agent: Synthesis of 5-hydroxy-1*H*-pyrrol-2(5*H*)-ones

[51] Park JH, Ko KC, Kim E, Park N, Ko JH, Ryu DH, Ahn TK, Lee JY, Son SU. Photocatalysis by phenothiazine dyes: Visible-light-driven oxidative coupling of primary amines at

[52] Zhu X, Xie X, Li P, Guo J, Wang L. Visible-light-induced direct thiolation at α-C(sp<sup>3</sup>

[53] Mihldorf B, Wolf R. C-H photooxygenation of alkyl benzenes catalyzed by riboflavin tetraacetate and a non-heme iron catalyst. Angewandte Chemie International Edition.

[54] Neveselý T, Svobodová E, Chudoba J, Sikorski M, Cibulka R. Efficient metal-free aerobic

photooxidation of sulfides to sulfoxides mediated by a vitamin B<sup>2</sup>

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Organic Chemistry. 2015;**2015**:7869-7875

18 Visible-Light Photocatalysis of Carbon-Based Materials

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ambient temperature. Organic Letters. 2012;**14**:5502-5505

2794-2800

2014;**4**:2530-2535

2016;**18**:1546-1549

2016;**55**:427-430

As a semiconductor, zinc oxide (ZnO) has better UV absorbing properties compared to other semiconductor materials, and therefore, it has better dye degrading abilities. However, ZnO tends to agglomerate, which lead to poor degradation compared to the other semiconductors. In this study, to overcome the agglomeration of ZnO, silica (SiO<sup>2</sup> ) was combined with ZnO. The composite was tested for its photocatalytic activity. The ZnO/SiO<sup>2</sup> photocatalyst was fabricated on a glass plate. In order to investigate the addition of SiO<sup>2</sup> on ZnO, X-ray diffraction (XRD) and scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS) was used. The result of the XRD analysis demonstrates similar peak results with ZnO XRD data from ICSD 157132 with a hexagonal structure. The results indicate that the ZnO structure did not change after the addition of SiO<sup>2</sup> , while SEM-EDS results showed that SiO<sup>2</sup> was supported on ZnO with 8% composition. The optimal composition was found to be ZnO/SiO<sup>2</sup> 95/5, as indicated by high degradation activity, which can degrade up to 89% methylene blue.

**Keywords:** rice husk, silica, photocatalysis, ZnO, methylene blue

#### **1. Introduction**

)-H

derivative and visible

The textile industry is developing at a rapid pace, and this has a positive impact on garments development. However, it also increases the negative impact through their industrial waste, especially textile dye. One of the means to degrade dye is by the use of a semiconductor material that has photocatalytic activity [1]. Utilization of semiconductors in photocatalysis is an interesting topic, attributable to its ability to degrade compounds with ultraviolet light facilitation [2].

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

TiO<sup>2</sup> is usually used as a photocatalyst because it is stable compared to the other photocatalytic agents. However, TiO<sup>2</sup> absorbs less UV light compared to ZnO. Therefore, ZnO can degrade more dye. However, in reality, ZnO degrades less dye compared to TiO<sup>2</sup> , since ZnO tends to agglomerate [3]. To overcome this problem, SiO<sup>2</sup> can be added to ZnO. Pure ZnO degrades 40% dye in 60 minutes. However, when SiO<sup>2</sup> was added, the degradation increased and showed better results compared to pure ZnO. The addition of SiO<sup>2</sup> to ZnO achieved optimum photocatalytic activity at ZnO/SiO<sup>2</sup> 90/10 weight ratio with 99% dye degradation efficiency [4].

methylene blue solutions with concentration values of 0.2, 0.4, 0.6 and 0.8 ppm. The absor-

methylene blue in a test tube. It was followed by irradiation using a mercury lamp for 4 hours. Every 2 hours, 2 mL of sample was collected, and its methylene blue content was determined using a visible spectrophotometer at 660 nm. The assay was performed for all the fabricated

weight ratio, as described in previous studies [4, 7]. The fabrication was conducted in distilled water, while assay was performed in two different solvents, i.e., distilled water and methanol. According to the assay results, both solvents yielded the same ZnO attachment level on the glass slide. Therefore, both water and methanol are effective as solvents, as previously pro-

with a magnetic stirrer at 500 rpm for 2 hours, as recommended by prior work [4].

remove impurities that can interfere with the attachment of ZnO/SiO<sup>2</sup>

attaches strongly onto the glass slide (**Figure 1**).

**Figure 2**. It can be observed on the 500× magnification that the ZnO/SiO<sup>2</sup>

hydroxyl radical generation by ZnO to degrade the dye.

uniformly, and have a higher porosity compared to the ZnO/SiO<sup>2</sup>

Sonication with ~30 kHz for 90 minutes was conducted to homogenize the ZnO/SiO<sup>2</sup>

no agglomeration occurs. Prior to the coating, the glass slide was cleaned using acetone to

photocatalyst was fabricated to give final mass of 3 g with 95/5, 90/10, 85/15

to completely disperse in water, the suspension was stirred

onto the glass slide, it was heated to 450°C. The sur-

95/5 weight ratio. The result of SEM is illustrated in

addition. It appears that the particles do not attach

to ZnO is significantly effective to facilitate a uniform spread

was dried in a 40°C oven for 12 hours to remove excess water so that

is attached should be a flat surface, so that when the surface is washed

to be washed [4].

SEM-EDS and XRD was performed to investigate the ZnO coating onto glass slides. Complete

formly with low porosity. A porous structure started to appear at 1000× magnification. The particles attachment affects the efficiency of methylene blue degradation, since it determines

so that

. The glass slide that was

layer attached uni-

layer, as shown in **Figure 2**. It

layer was inserted into 50 mL of 1 ppm

Rice Husk for Photocatalytic Composite Material Fabrication

http://dx.doi.org/10.5772/intechopen.72704

21

bance was plotted against concentration, and used as a standard curve.

For photocatalytic assay, a glass side with ZnO/SiO<sup>2</sup>

**2.4. Photocatalytic assay**

**3. Results and discussions**

posed [7]. In order for ZnO and SiO<sup>2</sup>

To increase the attachment of ZnO/SiO<sup>2</sup>

it is easier for the agglomerated ZnO/SiO<sup>2</sup>

analysis was performed for ZnO/SiO<sup>2</sup>

**Figure 3** shows the ZnO layer without SiO<sup>2</sup>

appears that the addition of SiO<sup>2</sup>

coated with ZnO/SiO<sup>2</sup>

face where ZnO/SiO<sup>2</sup>

**3.2. Characterization results**

the ZnO/SiO<sup>2</sup>

 **photocatalyst**

slides.

**3.1. ZnO/SiO2**

The ZnO/SiO<sup>2</sup>

Rice husk is composed of about 20% paddy grain [5]. It is composed of mainly cellulose (~32%), silica (~22%) and lignin (~16%) [6]. After milling, rice husk becomes a major waste product, and is not utilized optimally. In 2015, Indonesia produced 75 million tonnes of rice (Central Agency on Statistics, bps.go.id), and therefore, rice husk becomes abundant and a cheap source for silica.

The efficiency of degradation and decolourization is affected by the stability of ZnO layer and ZnO layer morphology. An experiment was conducted by fabricating ZnO/SiO<sup>2</sup> composite with 95/5, 90/10, and 85/15 weight ratio. The results indicated good results when the composite was used to degrade methylene blue with synthetic SiO<sup>2</sup> made from rice husk as the supporting material. Therefore, the present study aims to fabricate ZnO/SiO<sup>2</sup> with less agglomeration [4].

### **2. Experimental section**

## **2.1. Isolation of SiO2**

Rice husk was carbonated at 400°C for 6 hours, followed by increasing the heat to 700°C in an argon atmosphere for 4 hours. The carbon was then ground and seized using a 100 mesh sieve. The carbon was suspended in with a mole ratio 1/3/150 = silica:potassium carbonate: water and refluxed for 150 minutes. The mixture was filtered and the filtrate was allowed to cool. The SiO<sup>2</sup> was precipitated and collected after filtering the solution.

#### **2.2. Fabrication of ZnO/SiO2 nanocomposite**

Three weight ratio of ZnO/SiO<sup>2</sup> , namely, 100/0, 95/5, 90/10 and 85/15, were prepared to give 3 g of total mass. The solid mixture was then suspended in 100 mL of distilled water. The suspension was stirred using a magnetic stirrer (500 rpm) for 2 hours, followed by sonication for 90 minutes (Elma ultrasonic LC 30H). The ZnO/SiO<sup>2</sup> suspension was dropped onto a glass slide (1 × 3 cm) with a pipette until the entire glass surface was covered. The slide was dried at 40°C for 12 hours, followed by calcinations at 450°C for 1 hour. The slide was washed using distilled water, and the layer was characterized using X-ray diffraction (XRD) and scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS).

#### **2.3. Determination of maximum wavelength and standard curve**

The absorbance of 1 ppm methylene blue solution was recorded at 500–700 nm in wavelength. The wavelength at which the highest absorbance was detected was used to measure the concentration of methylene blue. The standard curve was made by measuring the absorbance of methylene blue solutions with concentration values of 0.2, 0.4, 0.6 and 0.8 ppm. The absorbance was plotted against concentration, and used as a standard curve.

#### **2.4. Photocatalytic assay**

TiO<sup>2</sup>

agents. However, TiO<sup>2</sup>

is usually used as a photocatalyst because it is stable compared to the other photocatalytic

Rice husk is composed of about 20% paddy grain [5]. It is composed of mainly cellulose (~32%), silica (~22%) and lignin (~16%) [6]. After milling, rice husk becomes a major waste product, and is not utilized optimally. In 2015, Indonesia produced 75 million tonnes of rice (Central Agency on Statistics, bps.go.id), and therefore, rice husk becomes abundant and a cheap source for silica. The efficiency of degradation and decolourization is affected by the stability of ZnO layer and

with 95/5, 90/10, and 85/15 weight ratio. The results indicated good results when the composite

Rice husk was carbonated at 400°C for 6 hours, followed by increasing the heat to 700°C in an argon atmosphere for 4 hours. The carbon was then ground and seized using a 100 mesh sieve. The carbon was suspended in with a mole ratio 1/3/150 = silica:potassium carbonate: water and refluxed for 150 minutes. The mixture was filtered and the filtrate was allowed to

3 g of total mass. The solid mixture was then suspended in 100 mL of distilled water. The suspension was stirred using a magnetic stirrer (500 rpm) for 2 hours, followed by sonication

slide (1 × 3 cm) with a pipette until the entire glass surface was covered. The slide was dried at 40°C for 12 hours, followed by calcinations at 450°C for 1 hour. The slide was washed using distilled water, and the layer was characterized using X-ray diffraction (XRD) and scanning

The absorbance of 1 ppm methylene blue solution was recorded at 500–700 nm in wavelength. The wavelength at which the highest absorbance was detected was used to measure the concentration of methylene blue. The standard curve was made by measuring the absorbance of

was precipitated and collected after filtering the solution.

 **nanocomposite**

electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS).

**2.3. Determination of maximum wavelength and standard curve**

ZnO layer morphology. An experiment was conducted by fabricating ZnO/SiO<sup>2</sup>

more dye. However, in reality, ZnO degrades less dye compared to TiO<sup>2</sup>

showed better results compared to pure ZnO. The addition of SiO<sup>2</sup>

agglomerate [3]. To overcome this problem, SiO<sup>2</sup>

was used to degrade methylene blue with synthetic SiO<sup>2</sup>

for 90 minutes (Elma ultrasonic LC 30H). The ZnO/SiO<sup>2</sup>

material. Therefore, the present study aims to fabricate ZnO/SiO<sup>2</sup>

40% dye in 60 minutes. However, when SiO<sup>2</sup>

photocatalytic activity at ZnO/SiO<sup>2</sup>

20 Visible-Light Photocatalysis of Carbon-Based Materials

**2. Experimental section**

**2.2. Fabrication of ZnO/SiO2**

Three weight ratio of ZnO/SiO<sup>2</sup>

**2.1. Isolation of SiO2**

cool. The SiO<sup>2</sup>

absorbs less UV light compared to ZnO. Therefore, ZnO can degrade

, since ZnO tends to

composite

to ZnO achieved optimum

can be added to ZnO. Pure ZnO degrades

made from rice husk as the supporting

suspension was dropped onto a glass

with less agglomeration [4].

was added, the degradation increased and

90/10 weight ratio with 99% dye degradation efficiency [4].

, namely, 100/0, 95/5, 90/10 and 85/15, were prepared to give

For photocatalytic assay, a glass side with ZnO/SiO<sup>2</sup> layer was inserted into 50 mL of 1 ppm methylene blue in a test tube. It was followed by irradiation using a mercury lamp for 4 hours. Every 2 hours, 2 mL of sample was collected, and its methylene blue content was determined using a visible spectrophotometer at 660 nm. The assay was performed for all the fabricated slides.

## **3. Results and discussions**

#### **3.1. ZnO/SiO2 photocatalyst**

The ZnO/SiO<sup>2</sup> photocatalyst was fabricated to give final mass of 3 g with 95/5, 90/10, 85/15 weight ratio, as described in previous studies [4, 7]. The fabrication was conducted in distilled water, while assay was performed in two different solvents, i.e., distilled water and methanol. According to the assay results, both solvents yielded the same ZnO attachment level on the glass slide. Therefore, both water and methanol are effective as solvents, as previously proposed [7]. In order for ZnO and SiO<sup>2</sup> to completely disperse in water, the suspension was stirred with a magnetic stirrer at 500 rpm for 2 hours, as recommended by prior work [4].

Sonication with ~30 kHz for 90 minutes was conducted to homogenize the ZnO/SiO<sup>2</sup> so that no agglomeration occurs. Prior to the coating, the glass slide was cleaned using acetone to remove impurities that can interfere with the attachment of ZnO/SiO<sup>2</sup> . The glass slide that was coated with ZnO/SiO<sup>2</sup> was dried in a 40°C oven for 12 hours to remove excess water so that the ZnO/SiO<sup>2</sup> attaches strongly onto the glass slide (**Figure 1**).

To increase the attachment of ZnO/SiO<sup>2</sup> onto the glass slide, it was heated to 450°C. The surface where ZnO/SiO<sup>2</sup> is attached should be a flat surface, so that when the surface is washed it is easier for the agglomerated ZnO/SiO<sup>2</sup> to be washed [4].

#### **3.2. Characterization results**

SEM-EDS and XRD was performed to investigate the ZnO coating onto glass slides. Complete analysis was performed for ZnO/SiO<sup>2</sup> 95/5 weight ratio. The result of SEM is illustrated in **Figure 2**. It can be observed on the 500× magnification that the ZnO/SiO<sup>2</sup> layer attached uniformly with low porosity. A porous structure started to appear at 1000× magnification. The particles attachment affects the efficiency of methylene blue degradation, since it determines hydroxyl radical generation by ZnO to degrade the dye.

**Figure 3** shows the ZnO layer without SiO<sup>2</sup> addition. It appears that the particles do not attach uniformly, and have a higher porosity compared to the ZnO/SiO<sup>2</sup> layer, as shown in **Figure 2**. It appears that the addition of SiO<sup>2</sup> to ZnO is significantly effective to facilitate a uniform spread

of ZnO when attached onto the glass slide surface, which can then reduce the porosity of the

95/5 weight ratio, it is clear that the composite can produce more hydroxyl radical, which

The EDS results are presented in **Figure 4**. EDS is used to investigate the composition of ZnO

8.38% weight respectively. However, according to the calculation results, the composition of Zn and Si should be 76.23 and 2.30% weight respectively. A higher Si composition and a lower Zn composition compared to the theoretical value is due to the ZnO being carried

as a hydroxyl radical generator. Based on the comparison results of ZnO to ZnO/SiO<sup>2</sup>

layer. A lower porosity leads to a higher ZnO attached to the glass slide, which acts

95/5 weight ratio and (B) ZnO with 500× magnification.

mixture. With 95/5 weight ratio, Zn and Si composition was 61.64 and

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with

23

ZnO/SiO<sup>2</sup>

and SiO<sup>2</sup>

leads to a more effective dye degradation.

in ZnO/SiO<sup>2</sup>

**Figure 3.** Micrograph of (A) ZnO/SiO<sup>2</sup>

**Figure 4.** EDS results of ZnO/SiO<sup>2</sup>

95/5 weight ratio.

**Figure 1.** ZnO/SiO<sup>2</sup> and ZnO.

**Figure 2.** Micrograph of ZnO/SiO<sup>2</sup> at 95/5 weight ratio at (A) 500-, (B) 1000-, (C) 2500-, and (D) 5000-fold magnification.

of ZnO when attached onto the glass slide surface, which can then reduce the porosity of the ZnO/SiO<sup>2</sup> layer. A lower porosity leads to a higher ZnO attached to the glass slide, which acts as a hydroxyl radical generator. Based on the comparison results of ZnO to ZnO/SiO<sup>2</sup> with 95/5 weight ratio, it is clear that the composite can produce more hydroxyl radical, which leads to a more effective dye degradation.

The EDS results are presented in **Figure 4**. EDS is used to investigate the composition of ZnO and SiO<sup>2</sup> in ZnO/SiO<sup>2</sup> mixture. With 95/5 weight ratio, Zn and Si composition was 61.64 and 8.38% weight respectively. However, according to the calculation results, the composition of Zn and Si should be 76.23 and 2.30% weight respectively. A higher Si composition and a lower Zn composition compared to the theoretical value is due to the ZnO being carried

**Figure 3.** Micrograph of (A) ZnO/SiO<sup>2</sup> 95/5 weight ratio and (B) ZnO with 500× magnification.

**Figure 1.** ZnO/SiO<sup>2</sup>

and ZnO.

22 Visible-Light Photocatalysis of Carbon-Based Materials

**Figure 2.** Micrograph of ZnO/SiO<sup>2</sup>

at 95/5 weight ratio at (A) 500-, (B) 1000-, (C) 2500-, and (D) 5000-fold magnification.

away during the washing process, especially the agglomerated ZnO. The agglomeration may occur when sonication is extensively long. When more ZnO is washed away, a more porous structure occurs on the layer (**Figure 3**). A high porosity on the layer reduces the ability of ZnO to form hydroxyl radical, which then reduces the rate of degradation. From the EDS results (**Figure 3**), it was proven that 8.38% of SiO<sup>2</sup> is supported on the ZnO particles. It was also found that around 1.22% weight was not detected by the EDS method, which indicates the presence of impurities in the layer.

This result confirmed that SiO<sup>2</sup>

amorphous property (**Figure 6**).

correlation.

**3.4. Photocatalysis assay**

above 80%. When SiO<sup>2</sup>

**Figure 6.** XRD of SiO<sup>2</sup>

.

used to fabricate ZnO/SiO<sup>2</sup>

The wavelength at which maximum absorbance of methylene blue was detected is 660 nm. The maximum wavelength has the highest sensitivity, and therefore the measurement of methylene blue concentration was conducted at this wavelength to the minimize error rate. The correlation of the standard curve was 0.9909, which shows that the method has good

In the photocatalysis assay, samples were collected four times every hour, and the concentration of methylene blue was determined by measuring the absorbance of the samples and converted to a concentration using a standard curve. The percentage of the efficiency of methylene blue degradation was obtained by comparing the concentration of methylene blue

effective compared to the other samples, as it can degrade up to 52.5% of methylene blue.

ratio of 95/5. 90/10 and 85/15 showed 89.95, 82.57 and 37.40% degradation percentage respectively. The photocatalysts with 95/5 and 90/10 weight ratio is more effective in degrading methylene blue dye compared to 85/15 weight ratio, as indicated by percentage degradation

before and after radiation. The efficiency of degradation is presented in **Figure 7**.

As shown in **Figure 7**, after 1 hour of radiation, ZnO/SiO<sup>2</sup>

leads to a lower hydroxyl radical formation. ZnO/SiO<sup>2</sup>

pared to the other samples. After 4 hours of irradiation, ZnO/SiO<sup>2</sup>

After 3 hours of irradiation, ZnO/SiO<sup>2</sup>

lower. This is likely caused by SiO<sup>2</sup>

**3.3. Maximum wavelength and standard curve of methylene blue**

is SiO<sup>2</sup>

Rice Husk for Photocatalytic Composite Material Fabrication

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from rice husk with an

25

with 95/5 weight ratio was more

photocatalysts with 85/15 weight ratio

photocatalysts with weight

with 90/10 weight ratio degrades the dye better com-

covering ZnO that should be act as photocatalysts, which

was used by more than 10%, the ability of the photocatalysts became

To investigate the effect of SiO<sup>2</sup> on the ZnO structure, XRD was used. The result of XRD is presented in **Figure 5**. It was found that peaks at 32.33°, 34.67° and 36.85° of ZnO/SiO<sup>2</sup> was shifted compared to standard ZnO. This shift occurred as a result of the SiO<sup>2</sup> attachment on ZnO. The intensity of the peaks indicates that there is no difference between peaks of ZnO/ SiO<sup>2</sup> compared to standard ZnO (ICSD 157132 standard), which has a hexagonal crystal structure. This result is in agreement with previously published results, which describe that layers of ZnO have a hexagonal structure.

The results indicate that the addition of SiO<sup>2</sup> does not change the ZnO crystal structure. However, the addition of SiO<sup>2</sup> can increase the distribution of ZnO on the glass slide, thus resulting in a more homogen layer, which leads to no agglomeration. The XRD of ZnO/SiO<sup>2</sup> shows that the SiO<sup>2</sup> peak appears with very low intensity at 2θ = 20–25° (**Figure 5**). The low intensity peak appears because the SiO<sup>2</sup> amount in ZnO/SiO<sup>2</sup> was low, i.e. 5%. The peak at 2θ = 23.5° with low intensity belongs to SiO<sup>2</sup> from rice husk with an amorphous property.

**Figure 5.** XRD of ZnO/SiO<sup>2</sup> and ZnO standard (ICSD 157132).

This result confirmed that SiO<sup>2</sup> used to fabricate ZnO/SiO<sup>2</sup> is SiO<sup>2</sup> from rice husk with an amorphous property (**Figure 6**).

#### **3.3. Maximum wavelength and standard curve of methylene blue**

The wavelength at which maximum absorbance of methylene blue was detected is 660 nm. The maximum wavelength has the highest sensitivity, and therefore the measurement of methylene blue concentration was conducted at this wavelength to the minimize error rate. The correlation of the standard curve was 0.9909, which shows that the method has good correlation.

#### **3.4. Photocatalysis assay**

away during the washing process, especially the agglomerated ZnO. The agglomeration may occur when sonication is extensively long. When more ZnO is washed away, a more porous structure occurs on the layer (**Figure 3**). A high porosity on the layer reduces the ability of ZnO to form hydroxyl radical, which then reduces the rate of degradation. From the EDS

also found that around 1.22% weight was not detected by the EDS method, which indicates

presented in **Figure 5**. It was found that peaks at 32.33°, 34.67° and 36.85° of ZnO/SiO<sup>2</sup>

ZnO. The intensity of the peaks indicates that there is no difference between peaks of ZnO/

resulting in a more homogen layer, which leads to no agglomeration. The XRD of ZnO/SiO<sup>2</sup>

 compared to standard ZnO (ICSD 157132 standard), which has a hexagonal crystal structure. This result is in agreement with previously published results, which describe that layers

shifted compared to standard ZnO. This shift occurred as a result of the SiO<sup>2</sup>

is supported on the ZnO particles. It was

does not change the ZnO crystal structure.

from rice husk with an amorphous property.

was low, i.e. 5%. The peak at

was

attachment on

on the ZnO structure, XRD was used. The result of XRD is

can increase the distribution of ZnO on the glass slide, thus

peak appears with very low intensity at 2θ = 20–25° (**Figure 5**). The low

amount in ZnO/SiO<sup>2</sup>

results (**Figure 3**), it was proven that 8.38% of SiO<sup>2</sup>

the presence of impurities in the layer.

24 Visible-Light Photocatalysis of Carbon-Based Materials

To investigate the effect of SiO<sup>2</sup>

of ZnO have a hexagonal structure.

However, the addition of SiO<sup>2</sup>

shows that the SiO<sup>2</sup>

**Figure 5.** XRD of ZnO/SiO<sup>2</sup>

and ZnO standard (ICSD 157132).

The results indicate that the addition of SiO<sup>2</sup>

intensity peak appears because the SiO<sup>2</sup>

2θ = 23.5° with low intensity belongs to SiO<sup>2</sup>

SiO<sup>2</sup>

In the photocatalysis assay, samples were collected four times every hour, and the concentration of methylene blue was determined by measuring the absorbance of the samples and converted to a concentration using a standard curve. The percentage of the efficiency of methylene blue degradation was obtained by comparing the concentration of methylene blue before and after radiation. The efficiency of degradation is presented in **Figure 7**.

As shown in **Figure 7**, after 1 hour of radiation, ZnO/SiO<sup>2</sup> with 95/5 weight ratio was more effective compared to the other samples, as it can degrade up to 52.5% of methylene blue. After 3 hours of irradiation, ZnO/SiO<sup>2</sup> with 90/10 weight ratio degrades the dye better compared to the other samples. After 4 hours of irradiation, ZnO/SiO<sup>2</sup> photocatalysts with weight ratio of 95/5. 90/10 and 85/15 showed 89.95, 82.57 and 37.40% degradation percentage respectively. The photocatalysts with 95/5 and 90/10 weight ratio is more effective in degrading methylene blue dye compared to 85/15 weight ratio, as indicated by percentage degradation above 80%. When SiO<sup>2</sup> was used by more than 10%, the ability of the photocatalysts became lower. This is likely caused by SiO<sup>2</sup> covering ZnO that should be act as photocatalysts, which leads to a lower hydroxyl radical formation. ZnO/SiO<sup>2</sup> photocatalysts with 85/15 weight ratio

**Figure 6.** XRD of SiO<sup>2</sup> .

less efficient because SiO<sup>2</sup>

that the addition of SiO<sup>2</sup>

**4. Conclusion**

**Author details**

**References**

and Roekmi-ati Tjokronegoro

SiO<sup>2</sup>

ratio showed the highest dye degradation.

**Figure 8** shows the comparison between ZnO/SiO<sup>2</sup>

methylene blue degradation of up to 89.95%, while ZnO without SiO<sup>2</sup>

of the ZnO. The optimal composition was found to be ZnO/SiO<sup>2</sup>

\*Address all correspondence to: diana.rahmawati@unpad.ac.id

Chemical Engineering Science. 2001;**56**:547-555

nanoparticles on glass plate. Desalination. 2009;**249**:1371-1376

Padjadjaran, Jatinangor, Sumedang, Indonesia

degradation activity, which can degrade up to 89% methylene blue.

is not a semiconductor material, so it cannot produce free electrons

increases the effectiveness of the phototocatalyst, as indicated by higher

and ZnO photocatalytic activity. It is clear

Rice Husk for Photocatalytic Composite Material Fabrication

http://dx.doi.org/10.5772/intechopen.72704

can only degrade 37.40%.

95/5, as indicated by a high

was

27

and hydroxyl radicals. According to Behnajady et al. [8], less hydroxyl radicals will lead to a lower dye reduction efficiency. **Figure 7** confirms this, where photocatalysts with 95/5 weight

According to Soltani et al. (2015), this is caused because the ZnO attachment without SiO<sup>2</sup>

not uniform on the glass slide surface that tends to form a porous structure. The addition of

The addition of silica from rice husk to ZnO photocatalysts improves the spread of the particles uniformly on the glass slide. The addition of the silica does not alter the crystal structure

Diana Rakhmawaty Eddy\*, Atiek Rostika Noviyanti, Solihudin Solihudin, Safri Ishmayana

[1] Selvam K, Swaminathan K, Chae K-S. Decolourization of azo dyes and a dye industry effluent by a white rot fungus *Thelephora* sp. Bioresource Technology. 2003;**88**:115-119 [2] Dijkstra MF, Buwalda H, de Jong AWF, Michorius A, Winkelman JGM, Beenackers AACM. Experimental comparison of three reactor designs for photocatalytic water purification.

[3] Behnajady MA, Moghaddam SG, Modirshahla N, Shokri M. Investigation of the effect of heat attachment method parameters at photocatalytic activity of immobilized ZnO

Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas

to ZnO assisted the attachment to form a uniform layer, and this leads to lower porosity.

**Figure 7.** Efficiency of methylene blue degradation using ZnO/SiO<sup>2</sup> photocatalysis with various ZnO/SiO<sup>2</sup> weight ratio. Irradiation time was 4 hours.

**Figure 8.** Comparison of methylene blue degradation efficiency using ZnO/SiO<sup>2</sup> compared to ZnO photocatalyst.

less efficient because SiO<sup>2</sup> is not a semiconductor material, so it cannot produce free electrons and hydroxyl radicals. According to Behnajady et al. [8], less hydroxyl radicals will lead to a lower dye reduction efficiency. **Figure 7** confirms this, where photocatalysts with 95/5 weight ratio showed the highest dye degradation.

**Figure 8** shows the comparison between ZnO/SiO<sup>2</sup> and ZnO photocatalytic activity. It is clear that the addition of SiO<sup>2</sup> increases the effectiveness of the phototocatalyst, as indicated by higher methylene blue degradation of up to 89.95%, while ZnO without SiO<sup>2</sup> can only degrade 37.40%. According to Soltani et al. (2015), this is caused because the ZnO attachment without SiO<sup>2</sup> was not uniform on the glass slide surface that tends to form a porous structure. The addition of SiO<sup>2</sup> to ZnO assisted the attachment to form a uniform layer, and this leads to lower porosity.

### **4. Conclusion**

The addition of silica from rice husk to ZnO photocatalysts improves the spread of the particles uniformly on the glass slide. The addition of the silica does not alter the crystal structure of the ZnO. The optimal composition was found to be ZnO/SiO<sup>2</sup> 95/5, as indicated by a high degradation activity, which can degrade up to 89% methylene blue.

### **Author details**

Diana Rakhmawaty Eddy\*, Atiek Rostika Noviyanti, Solihudin Solihudin, Safri Ishmayana and Roekmi-ati Tjokronegoro

\*Address all correspondence to: diana.rahmawati@unpad.ac.id

Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Padjadjaran, Jatinangor, Sumedang, Indonesia

### **References**

**Figure 8.** Comparison of methylene blue degradation efficiency using ZnO/SiO<sup>2</sup>

**Figure 7.** Efficiency of methylene blue degradation using ZnO/SiO<sup>2</sup>

26 Visible-Light Photocatalysis of Carbon-Based Materials

Irradiation time was 4 hours.

compared to ZnO photocatalyst.

photocatalysis with various ZnO/SiO<sup>2</sup>

weight ratio.


[4] Soltani RD, Shams Khoramabadi G, Godini H, Noorimotlagh Z. The application of ZnO/ SiO<sup>2</sup> nanocomposite for the photocatalytic degradation of a textile dye in aqueous solutions in comparison with pure ZnO nanoparticles. Desalination and Water Treatment. 2015;**56**:2551-2558

**Chapter 3**

**Provisional chapter**

**Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for**

This chapter describes the photocatalysis action of (dihydroxo)tetraphenyl¬porphyrinato complexes of high valent P (V), Ge (IV), and Sb (V) (P(tpp), Ge(tpp), and Sb(tpp)). These chromophores were fixed onto silica gel (SiO2) through Coulombic forces and hydrogen bonding between axial hydroxo ligands and silanol groups to produce M(tpp)/SiO2 (M = P, Ge, and Sb) composites. M(tpp)/SiO2 were applied to the photo-inactivation of *Escherichia coli* and *Legionella pneumophila*. Moreover, M(tpp)/SiO2 was subjected to practical experiments for the photoinactivation of *L. pneumophila* naturally occurring in a cooling tower and a public fountain. It is noteworthy that 80 g of Sb(tpp)/SiO2 catalyst, containing 40 mg of Sb(tpp) maintained a concentration of Legionella species below 100 CFU/100 mL for 120 days in 13 m3 of water in a fountain under sunlight exposure. The photoinactivation proceeded through the liberation of M(tpp) from SiO2, adsorption of M(tpp) inside bacteria, and generation of reactive oxygen species, such as singlet oxygen, under visible light irradiation, thus resulting in bacteria apoptosis. Based on these results, we developed water-soluble porphyrins by modification of P and Sb porphyrin axial ligands to alkyloxo, alkylethylenedioxy, and alkylpyridinium groups. These water-soluble porphyrins were applied to the photodynamic inactivation of E. coli and

**Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for** 

DOI: 10.5772/intechopen.70004

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

generates a hydroxyl radical, which works as a strong oxidizing reagent

and reproduction in any medium, provided the original work is properly cited.

Photocatalysis has received much attention as an environmentally friendly process for degrading organic compounds and bacteria in contaminated water. It is well known that UV-light

**Keywords:** high valent metal, P(V)-porphyrin, Ge(IV)-porphyrin, Sb(V)-porphyrin,

*Escherichia coli*, *Legionella pneumophila*, *Saccharomyces cerevisiae*

**Visible Light Inactivation of Bacteria**

**Visible Light Inactivation of Bacteria**

Masahide Yasuda and Jin Matsumoto

Masahide Yasuda and Jin Matsumoto

http://dx.doi.org/10.5772/intechopen.70004

*Saccharomyces cerevisiae*.

**1. Introduction**

irradiation of TiO2

**Abstract**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter


**Provisional chapter**

### **Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria Visible Light Inactivation of Bacteria**

**Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for** 

DOI: 10.5772/intechopen.70004

Masahide Yasuda and Jin Matsumoto Masahide Yasuda and Jin Matsumoto Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70004

#### **Abstract**

[4] Soltani RD, Shams Khoramabadi G, Godini H, Noorimotlagh Z. The application of ZnO/

[5] Madrid R, Nogueira CA, Margarindo F. Production and characterization of amorphous silica from rice husk waste. In: The 4th International Conference on Engineering for

[6] Prasad R, Pandey M. Rice husk ash as a renewable source for the production of value added silica gel and its application: An overview. Bulletin of Chemical Reaction

[7] Behnajady MA, Modirshahla N. Photooxidative degradation of Malachite Green (MG)

[8] Behnajady MA, Modirshahla N, Daneshvar N, Rabbani M. Photocatalytic degradation of C.I. Acid Red 27 by immobilized ZnO on glass plates in continuous-mode. Journal of

: Influence of operational parameters and kinetic modeling. Dyes and

Waste and Biomass Valorisation; 10-13 September 2012; Porto, Portugal; 2012

 nanocomposite for the photocatalytic degradation of a textile dye in aqueous solutions in comparison with pure ZnO nanoparticles. Desalination and Water Treatment.

SiO<sup>2</sup>

2015;**56**:2551-2558

28 Visible-Light Photocatalysis of Carbon-Based Materials

by UV/H<sup>2</sup>

O2

Pigments. 2006;**70**:54-59

Engineering & Catalysis. 2012;**7**(1):1-25

Hazardous Materials. 2007;**140**:257-263

This chapter describes the photocatalysis action of (dihydroxo)tetraphenyl¬porphyrinato complexes of high valent P (V), Ge (IV), and Sb (V) (P(tpp), Ge(tpp), and Sb(tpp)). These chromophores were fixed onto silica gel (SiO2) through Coulombic forces and hydrogen bonding between axial hydroxo ligands and silanol groups to produce M(tpp)/SiO2 (M = P, Ge, and Sb) composites. M(tpp)/SiO2 were applied to the photo-inactivation of *Escherichia coli* and *Legionella pneumophila*. Moreover, M(tpp)/SiO2 was subjected to practical experiments for the photoinactivation of *L. pneumophila* naturally occurring in a cooling tower and a public fountain. It is noteworthy that 80 g of Sb(tpp)/SiO2 catalyst, containing 40 mg of Sb(tpp) maintained a concentration of Legionella species below 100 CFU/100 mL for 120 days in 13 m3 of water in a fountain under sunlight exposure. The photoinactivation proceeded through the liberation of M(tpp) from SiO2, adsorption of M(tpp) inside bacteria, and generation of reactive oxygen species, such as singlet oxygen, under visible light irradiation, thus resulting in bacteria apoptosis. Based on these results, we developed water-soluble porphyrins by modification of P and Sb porphyrin axial ligands to alkyloxo, alkylethylenedioxy, and alkylpyridinium groups. These water-soluble porphyrins were applied to the photodynamic inactivation of E. coli and *Saccharomyces cerevisiae*.

**Keywords:** high valent metal, P(V)-porphyrin, Ge(IV)-porphyrin, Sb(V)-porphyrin, *Escherichia coli*, *Legionella pneumophila*, *Saccharomyces cerevisiae*

### **1. Introduction**

Photocatalysis has received much attention as an environmentally friendly process for degrading organic compounds and bacteria in contaminated water. It is well known that UV-light irradiation of TiO2 generates a hydroxyl radical, which works as a strong oxidizing reagent

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

for various microorganisms in aqueous solutions [1]. In 1985, Matsunaga et al. reported that Pt-loaded TiO2 , under irradiation of >380 nm light, was capable of photoinactivation of various microorganisms, such as *Lactobacillus acidophilus*, *Saccharomyces cerevisiae*, *Escherichia coli*, and *Chlorella vulgaris* [2]. However, the UV light used for activation of TiO2 is harmful to humans and only low amounts of the correct wavelengths are found in sunlight. Therefore, a photocatalyst that operates under irradiation from visible light emitted from the sun or a fluorescent lamp has been earnestly desired. Many researchers have focused much attention on porphyrins and metalloporphyrins, which can absorb visible light with high-absorption coefficients. Among the various metalloporphyrin chromophores, we selected axially dihydroxo-substituted tetraphenylporphyrin complexes (M(tpp)) of high valent metals, such as P(V), Ge(IV), and Sb(V) (**Figure 1**). To support M(tpp) on a carrier, we used silica gel (SiO2 ), which has a high transparency to visible light and a strong binding force to porphyrins. Thus, M(tpp)/SiO2 composites were prepared by adsorption of M(tpp) on SiO2 and the composites operated as photocatalysts under visible light irradiation. Moreover, the porphyrins were subjected to water solubilization for medical field applications.

In this chapter, we will cover M(tpp)/SiO2 and water-soluble porphyrins that were applied to the photosensitized inactivation of *E. coli* and *S*. *cerevisiae* in a reaction vessel and *Legionella pneumophila* in naturally occurring environments such as cooling tower and public fountains.

high. High efficiency of triplet state formation is advantageous for energy transfer to 3

compared with the divalent metal tetraphenylporphyrin complexes of Zn(II) (−1.31 V), Ni(II) (−1.18 V), and Pb(II) (−1.10 V) [8]. This fact shows that M(tpp)s (M = P, Ge, Sb) have powerful oxidation abilities under light irradiation. Moreover, the toxicity of M(tpp) is low. The lethal dose 50 (LD50) of Sb(tpp) [9] and P(tpp) [10] was more than 2000 mg/kg. Therefore, M(tpp) can

) under excitation at the Q-band and fluorescence lifetimes (*τ*<sup>F</sup>

To perform the photoreaction in an aqueous solution, less water-soluble M(tpp) was fixed

ing between axial hydroxo ligands and silanol groups. The general procedure for M(tpp)/

a MeOH−toluene solution (1:4 v/v, 500 mL) containing Sb(tpp) (60 mg) and the mixture was allowed to stand for 18 h. MeOH was evaporated from the solution at 40°C under reduced pressure (**Figure 2**). The treated silica gel was isolated by filtration and then dried under

(M = Ge, Sb) have high oxidation abilities, M(tpp)/SiO2

oxidation of cycloalkenes [12] and acetone [4] and the dechlorination of 4-chlorophenol [13]. The photocatalytic reactions were performed using the setup depicted in **Figure 3**, where the reactant was supplied by a continuous flow system. In a spiral-type apparatus (**Figure 3A**), the reactant solution is fed continuously from a holder to a spiral glass tube packed with the

red values of M(tpp)s were −0.74 to −0.93 V vs. Ag/AgNO3

Fluorescence maxima. The values in parenthesis are excitation energy in eV.

**red/Vb λmax/nmc**

.

.

<sup>d</sup>Molar absorption coefficients (*ε*) of M(tpp) in MeCN. The *ε* of P(tpp) was measured in MeOH.

 **(***ε* **/104**

P(tpp) 1.20<sup>g</sup> −0.93<sup>g</sup> 424 (31.2) 554 (1.82) 607 (2.04) 0.0416 (5.4) [3] Ge(tpp) 0.95 −0.83 420 (77.6) 554 (2.19) 596 (2.08) 0.1500 (4.7) [4, 5] Sb(tpp) 1.17 −0.74 419 (41.6) 552 (3.09) 596 (2.08) 0.0518 (1.6) [6, 7]

**Soret band Q-band λmax/(***E***0–0/eV)e** *Φ***<sup>F</sup>**

Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria

be safely used in living environmental fields.

Silysia) were used. M(tpp) was fixed on SiO2

reduced pressure at 40°C to give Sb(tpp)/SiO2

**2.2. Preparation of porphyrins/silica gel composites**

to form M(tpp)/SiO2

preparation is described for the case of Sb(tpp)/SiO2

Fuji Silysia, Japan) and silica gel beads (0.85–1.70 mm*φ*, 306 m2

**2.3. Photocatalytic oxidation of organic compounds using M(tpp)/SiO2**

*E*1/2

a

b

c

e

f

g

**M(tpp)** *E***1/2**

**ox/Va** *E***1/2**

Half peak of oxidation potential *vs* Ag/AgNO3

Half peak of reduction potential *vs* Ag/AgNO3

Absorption maxima of M(tpp).

Fluorescence quantum yield (*Φ*<sup>F</sup>

**Table 1.** Properties of M(tpp).

Unpublished results.

SiO2

onto porous SiO2

Since M(tpp)/SiO2

O2 . The 31

, which were relatively positive

).

g−1, CARIACT Q–10, Fuji

 **(M = Ge, Sb)**

(70 g) was added to

was applied to the

[11]. Silica gel powder (300 mesh, 40 μm*φ*, BW300

 **M−1 cm−1)d Fluorescence Ref.**

 **(***τ***<sup>F</sup> /ns)f**

http://dx.doi.org/10.5772/intechopen.70004

as follows. SiO2

. The content of Sb(tpp) was 0.087 wt%.

through Coulombic forces and hydrogen bond-

**Figure 1.** P(tpp), Ge(tpp), and Sb(tpp).

#### **2. Preparation of the photocatalysts**

#### **2.1. Porphyrin chromophores**

The porphyrin chromophores studied were (dihydroxo) tetraphenylporphyrinatophosphorus chloride (P(tpp)), (dihydroxo)tetraphenylporphyrinatogermanium (Ge(tpp)), and (dihydroxo)tetraphenylporphyrinatoantimony bromide (Sb(tpp)). Axial hydroxo ligands were bonded to the metals through stable covalent bonds. P(tpp) and Sb(tpp) are cationic complexes, whereas Ge(tpp) is a neutral complex. **Table 1** lists the physicochemical properties of the M(tpp)s, such as oxidation potentials (*E*1/2 ox vs. Ag/AgNO3 ), reduction potentials (*E*1/2 red), absorption maxima (*λ*max), and molar absorption coefficients (*ε*). Since M(tpp) has absorptions at the Soret band around 420 nm and Q-bands at 550 nm with high *ε*, they can capture even weak light emitted from a fluorescent lamp and the sun. The fluorescence maxima (λF ), quantum yields (*Φ*<sup>F</sup> ), and lifetimes (τF ) were obtained from fluorescence spectra. Since *Φ*<sup>F</sup> values were low, it was suggested that the efficiency of triplet state formation was relatively


a Half peak of oxidation potential *vs* Ag/AgNO3 .

b Half peak of reduction potential *vs* Ag/AgNO3 .

c Absorption maxima of M(tpp).

<sup>d</sup>Molar absorption coefficients (*ε*) of M(tpp) in MeCN. The *ε* of P(tpp) was measured in MeOH.

e Fluorescence maxima. The values in parenthesis are excitation energy in eV.

f Fluorescence quantum yield (*Φ*<sup>F</sup> ) under excitation at the Q-band and fluorescence lifetimes (*τ*<sup>F</sup> ).

g Unpublished results.

for various microorganisms in aqueous solutions [1]. In 1985, Matsunaga et al. reported that

ous microorganisms, such as *Lactobacillus acidophilus*, *Saccharomyces cerevisiae*, *Escherichia coli*,

humans and only low amounts of the correct wavelengths are found in sunlight. Therefore, a photocatalyst that operates under irradiation from visible light emitted from the sun or a fluorescent lamp has been earnestly desired. Many researchers have focused much attention on porphyrins and metalloporphyrins, which can absorb visible light with high-absorption coefficients. Among the various metalloporphyrin chromophores, we selected axially dihydroxo-substituted tetraphenylporphyrin complexes (M(tpp)) of high valent metals, such as P(V), Ge(IV), and Sb(V) (**Figure 1**). To support M(tpp) on a carrier, we used silica gel (SiO2

which has a high transparency to visible light and a strong binding force to porphyrins. Thus,

operated as photocatalysts under visible light irradiation. Moreover, the porphyrins were

the photosensitized inactivation of *E. coli* and *S*. *cerevisiae* in a reaction vessel and *Legionella pneumophila* in naturally occurring environments such as cooling tower and public fountains.

The porphyrin chromophores studied were (dihydroxo) tetraphenylporphyrinatophosphorus chloride (P(tpp)), (dihydroxo)tetraphenylporphyrinatogermanium (Ge(tpp)), and (dihydroxo)tetraphenylporphyrinatoantimony bromide (Sb(tpp)). Axial hydroxo ligands were bonded to the metals through stable covalent bonds. P(tpp) and Sb(tpp) are cationic complexes, whereas Ge(tpp) is a neutral complex. **Table 1** lists the physicochemical properties of

absorption maxima (*λ*max), and molar absorption coefficients (*ε*). Since M(tpp) has absorptions at the Soret band around 420 nm and Q-bands at 550 nm with high *ε*, they can capture even weak light emitted from a fluorescent lamp and the sun. The fluorescence maxima (λF

values were low, it was suggested that the efficiency of triplet state formation was relatively

ox vs. Ag/AgNO3

), reduction potentials (*E*1/2

) were obtained from fluorescence spectra. Since *Φ*<sup>F</sup>

red),

),

composites were prepared by adsorption of M(tpp) on SiO2

subjected to water solubilization for medical field applications.

In this chapter, we will cover M(tpp)/SiO2

30 Visible-Light Photocatalysis of Carbon-Based Materials

**2. Preparation of the photocatalysts**

the M(tpp)s, such as oxidation potentials (*E*1/2

), and lifetimes (τF

**2.1. Porphyrin chromophores**

**Figure 1.** P(tpp), Ge(tpp), and Sb(tpp).

quantum yields (*Φ*<sup>F</sup>

and *Chlorella vulgaris* [2]. However, the UV light used for activation of TiO2

, under irradiation of >380 nm light, was capable of photoinactivation of vari-

is harmful to

and the composites

and water-soluble porphyrins that were applied to

),

Pt-loaded TiO2

M(tpp)/SiO2

**Table 1.** Properties of M(tpp).

high. High efficiency of triplet state formation is advantageous for energy transfer to 3 O2 . The *E*1/2 red values of M(tpp)s were −0.74 to −0.93 V vs. Ag/AgNO3 , which were relatively positive compared with the divalent metal tetraphenylporphyrin complexes of Zn(II) (−1.31 V), Ni(II) (−1.18 V), and Pb(II) (−1.10 V) [8]. This fact shows that M(tpp)s (M = P, Ge, Sb) have powerful oxidation abilities under light irradiation. Moreover, the toxicity of M(tpp) is low. The lethal dose 50 (LD50) of Sb(tpp) [9] and P(tpp) [10] was more than 2000 mg/kg. Therefore, M(tpp) can be safely used in living environmental fields.

#### **2.2. Preparation of porphyrins/silica gel composites**

To perform the photoreaction in an aqueous solution, less water-soluble M(tpp) was fixed onto porous SiO2 to form M(tpp)/SiO2 [11]. Silica gel powder (300 mesh, 40 μm*φ*, BW300 Fuji Silysia, Japan) and silica gel beads (0.85–1.70 mm*φ*, 306 m2 g−1, CARIACT Q–10, Fuji Silysia) were used. M(tpp) was fixed on SiO2 through Coulombic forces and hydrogen bonding between axial hydroxo ligands and silanol groups. The general procedure for M(tpp)/ SiO2 preparation is described for the case of Sb(tpp)/SiO2 as follows. SiO2 (70 g) was added to a MeOH−toluene solution (1:4 v/v, 500 mL) containing Sb(tpp) (60 mg) and the mixture was allowed to stand for 18 h. MeOH was evaporated from the solution at 40°C under reduced pressure (**Figure 2**). The treated silica gel was isolated by filtration and then dried under reduced pressure at 40°C to give Sb(tpp)/SiO2 . The content of Sb(tpp) was 0.087 wt%.

#### **2.3. Photocatalytic oxidation of organic compounds using M(tpp)/SiO2 (M = Ge, Sb)**

Since M(tpp)/SiO2 (M = Ge, Sb) have high oxidation abilities, M(tpp)/SiO2 was applied to the oxidation of cycloalkenes [12] and acetone [4] and the dechlorination of 4-chlorophenol [13]. The photocatalytic reactions were performed using the setup depicted in **Figure 3**, where the reactant was supplied by a continuous flow system. In a spiral-type apparatus (**Figure 3A**), the reactant solution is fed continuously from a holder to a spiral glass tube packed with the

**Figure 2.** Silica gel-supported M(tpp) catalyst (M(tpp)/SiO2 ).

photocatalyst. Sample irradiation was performed using a fluorescent lamp (22 W). The oval mirror-type apparatus (**Figure 3B**) consisted of a fluorescent lamp (18 W), an oval mirror, and a reactor (20 mm*φ* × 500 mm; 150 mL) packed with the photocatalyst. The fluorescent lamp was set on one focus of the oval mirror, and the reactor was set at another focus. The visible light emitted from the fluorescent lamp was concentrated onto the reactor. The reactant solution was fed continuously from reservoirs into the reactors.

Photo-oxidation of MeOH to HCHO was performed by Ge(tpp)/SiO2

on a spiral glass tube (4 mm*φ* × 2.5 m) packed with Ge(tpp)/SiO2

**Figure 4.** Photo-oxidation of cycloalkenes using Sb(tpp)/SiO2

as an O-radical generator. The photo-oxidation by Ge(tpp)/SiO2

Dechlorination of 4-chlorophenol (4-CP) using Sb(tpp)/SiO2

mirror-type apparatus (**Figure 3B**) [13]. Here, Fe(NO3

**Figure 5.** Photo-oxidation of methanol using Ge(tpp)/SiO2

CD3

high. Sb(tpp)/SiO2

ΔG = *E*1/2

ox − *E*1/2

the adsorption of 4-CP on the SiO2

state of Sb(tpp) (*E*0−0 = 1.63 eV) to Fe3+ ion (*E*1/2

red − *E*0–0, where *E*1/2

using a spiral type reactor (**Figure 3A**). Irradiation with a fluorescent light was performed

TMS.

Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria

continuously into the spiral glass tube from the reservoir that contained an aerated aqueous solution of MeOH (50 mM in 200 mL) [4]. HCHO (30.6 μM) was formed after 180 h with a turnover number (TON) of 3.0 (**Figure 5**). The isotope effect for the photo-oxidation of metha-

OH. Therefore, it was suggested that the oxidation occurs through hydrogen abstraction. The generation of a Ge-O• species was generated by excitation of Ge(tpp). Here, Ge(tpp) acts

)3

(0.087 wt% Sb(tpp)) was loaded into the oval mirror reactor. Before irradia-

ethylbenzene to produce alcohols and aldehydes/ketones. In the cases of cumene, methylcy-

acceptor for the dechlorination of 4-CP, since the oxidation potential of 4-CP was relatively

.

tion, the aqueous solution of 4-CP (initial concentration was 493 μM) was fed for 3 h under dark conditions, and the initial concentration of 4-CP decreased to 400 mM, probably due to

and 1,4-benzoquinone (205 μM). Fe2+ (811 μM) was produced as a consequence of the reduc-

was responsible for the photodechlorination initiation, since Rehm-Weller equation calculated that the free energy change (ΔG) for the electron transfer from the excited triplet

ox (Sb(tpp)) = 1.17 V vs Ag/AgNO3

concentration of 4-CP decreased from 400 to 6 μM along with the formation of Cl−

tion of Fe3+ (**Figure 6**). Electron transfer from the excited triplet state of Sb(tpp)/SiO2

nol was found to be 2.1 from the ratios of slopes in the time-conversion plots for CH3

clopentane, and methylcyclohexane, the corresponding alcohols were produced.

at room temperature

OH and

33

(12 g). The reactant was fed

http://dx.doi.org/10.5772/intechopen.70004

was applied to toluene and

was performed using the oval

as the electron

(233 μM)

to Fe3+

) would be −0.15 V:

. The resulting reactive

was used instead of O2

. Upon irradiation with the fluorescent lamp for 72 h, the

red = −0.31 V vs Ag/AgNO3

The photocatalytic oxidations of cycloalkenes using oxygen were performed in a spiral type reactor (**Figure 3A**). Irradiation was directed onto a spiral glass tube (4 mm*Φ* × 2.5 m) containing photocatalyst (6.0 g) to which CH2 Cl2 solutions (150 mL) of cycloalkenes (0.1 M) were fed continuously from the reservoir [12]. To reduce silanol group effects, Sb(tpp)/SiO2 was modified by capping with (Me3 Si)<sup>2</sup> NH to give Sb(tpp)/SiO2 TMS. The photocatalytic oxidation of cyclohexene on Sb(tpp)/SiO2 TMS in CH2 Cl2 produced *cis*-1,2-epoxycyclohexane (28%), 1,2-cyclohexanediol (27%), and 2-cyclohexen-1-ol (38%) along with a small amount of 2-cyclohexene-1-one (7%) at 99.8% conversion after irradiation for 56 h (**Figure 4**). Similarly, the photocatalytic oxidation was applied to cyclooctene and 1-methyl-cyclohexene, which mainly resulted in diol formation.

**Figure 3.** Setups for photocatalytic reactions: spiral type (A) and oval mirror type (B).

Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria http://dx.doi.org/10.5772/intechopen.70004 33

**Figure 4.** Photo-oxidation of cycloalkenes using Sb(tpp)/SiO2 TMS.

photocatalyst. Sample irradiation was performed using a fluorescent lamp (22 W). The oval mirror-type apparatus (**Figure 3B**) consisted of a fluorescent lamp (18 W), an oval mirror, and a reactor (20 mm*φ* × 500 mm; 150 mL) packed with the photocatalyst. The fluorescent lamp was set on one focus of the oval mirror, and the reactor was set at another focus. The visible light emitted from the fluorescent lamp was concentrated onto the reactor. The reactant solu-

).

The photocatalytic oxidations of cycloalkenes using oxygen were performed in a spiral type reactor (**Figure 3A**). Irradiation was directed onto a spiral glass tube (4 mm*Φ* × 2.5 m) con-

NH to give Sb(tpp)/SiO2

Cl2

1,2-cyclohexanediol (27%), and 2-cyclohexen-1-ol (38%) along with a small amount of 2-cyclohexene-1-one (7%) at 99.8% conversion after irradiation for 56 h (**Figure 4**). Similarly, the photocatalytic oxidation was applied to cyclooctene and 1-methyl-cyclohexene, which mainly

solutions (150 mL) of cycloalkenes (0.1 M) were

produced *cis*-1,2-epoxycyclohexane (28%),

TMS. The photocatalytic oxida-

was

Cl2

fed continuously from the reservoir [12]. To reduce silanol group effects, Sb(tpp)/SiO2

TMS in CH2

tion was fed continuously from reservoirs into the reactors.

Si)<sup>2</sup>

**Figure 3.** Setups for photocatalytic reactions: spiral type (A) and oval mirror type (B).

taining photocatalyst (6.0 g) to which CH2

**Figure 2.** Silica gel-supported M(tpp) catalyst (M(tpp)/SiO2

32 Visible-Light Photocatalysis of Carbon-Based Materials

modified by capping with (Me3

resulted in diol formation.

tion of cyclohexene on Sb(tpp)/SiO2

Photo-oxidation of MeOH to HCHO was performed by Ge(tpp)/SiO2 at room temperature using a spiral type reactor (**Figure 3A**). Irradiation with a fluorescent light was performed on a spiral glass tube (4 mm*φ* × 2.5 m) packed with Ge(tpp)/SiO2 (12 g). The reactant was fed continuously into the spiral glass tube from the reservoir that contained an aerated aqueous solution of MeOH (50 mM in 200 mL) [4]. HCHO (30.6 μM) was formed after 180 h with a turnover number (TON) of 3.0 (**Figure 5**). The isotope effect for the photo-oxidation of methanol was found to be 2.1 from the ratios of slopes in the time-conversion plots for CH3 OH and CD3 OH. Therefore, it was suggested that the oxidation occurs through hydrogen abstraction. The generation of a Ge-O• species was generated by excitation of Ge(tpp). Here, Ge(tpp) acts as an O-radical generator. The photo-oxidation by Ge(tpp)/SiO2 was applied to toluene and ethylbenzene to produce alcohols and aldehydes/ketones. In the cases of cumene, methylcyclopentane, and methylcyclohexane, the corresponding alcohols were produced.

$$\begin{array}{cccc} \mathsf{CH}^{3}\mathsf{OH} & \xrightarrow{h\nu,\,\mathrm{O}\_{2}} & \mathsf{H}\mathsf{CH}\mathsf{CH} \\ & \mathsf{H}\_{2}\mathsf{O} & \\ & & \mathsf{H}\_{2}\mathsf{O} & \\ \end{array}$$

**Figure 5.** Photo-oxidation of methanol using Ge(tpp)/SiO2 .

Dechlorination of 4-chlorophenol (4-CP) using Sb(tpp)/SiO2 was performed using the oval mirror-type apparatus (**Figure 3B**) [13]. Here, Fe(NO3 )3 was used instead of O2 as the electron acceptor for the dechlorination of 4-CP, since the oxidation potential of 4-CP was relatively high. Sb(tpp)/SiO2 (0.087 wt% Sb(tpp)) was loaded into the oval mirror reactor. Before irradiation, the aqueous solution of 4-CP (initial concentration was 493 μM) was fed for 3 h under dark conditions, and the initial concentration of 4-CP decreased to 400 mM, probably due to the adsorption of 4-CP on the SiO2 . Upon irradiation with the fluorescent lamp for 72 h, the concentration of 4-CP decreased from 400 to 6 μM along with the formation of Cl− (233 μM) and 1,4-benzoquinone (205 μM). Fe2+ (811 μM) was produced as a consequence of the reduction of Fe3+ (**Figure 6**). Electron transfer from the excited triplet state of Sb(tpp)/SiO2 to Fe3+ was responsible for the photodechlorination initiation, since Rehm-Weller equation calculated that the free energy change (ΔG) for the electron transfer from the excited triplet state of Sb(tpp) (*E*0−0 = 1.63 eV) to Fe3+ ion (*E*1/2 red = −0.31 V vs Ag/AgNO3 ) would be −0.15 V: ΔG = *E*1/2 ox − *E*1/2 red − *E*0–0, where *E*1/2 ox (Sb(tpp)) = 1.17 V vs Ag/AgNO3 . The resulting reactive

**Figure 7**. The glass tube was shaken at 160 rpm and irradiated for 2 h using a fluorescent lamp (Panasonic FL-15ECW, Japan, λ = 400–723 nm, maximum intensity at 545 nm, 10.5 W cm−2) set above a reciprocal shaker. Aliquots (0.1 mL) were taken from the reaction mixture at 20 min intervals and plated on agar plates in triplicate. The colonies that appeared after incubation for 24 h at 30°C were counted. The amount of living cells (*B* in cell mL−1) was defined as the

amount of *E. coli*. Upon irradiation for 60 min, the concentration of *E. coli* decreased from the

experiments, irradiation in the absence of the photocatalyst and photocatalyst without irra-

lived to contact directly with the bacteria. Therefore, it was suggested that the excited triplet

We postulated that photoinactivation obeyed a Michaelis-Menten type mechanism, which

(A) and P(tpp)/SiO2

in phosphate buffer solution (10 mL) for *E. coli* and water (10 mL) for *L. pneumophila* under irradiation of fluorescent

cell mL–1 for *E. coli* and 6.4 × 103

(○), and dark in the absence of M(tpp)/SiO2

Similar photoinactivation studies of *E. coli* were performed using P(tpp)/SiO2

**Figure 8B**. Under a nitrogen atmosphere, photoinactivation using P(tpp)/SiO2

) of *E. coli*, where *B*<sup>0</sup>

http://dx.doi.org/10.5772/intechopen.70004

cells mL−1) to about 40 cells mL−1. Its survival ratio was 0.6%.

Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria

demonstrated bactericidal activity under visible light irradiation. Control

(triplet energy = 1.63 eV) underwent energy transfer to molecular oxygen

O2

in aqueous solutions to sterilize *E. coli* cells using visible light irradiation.

has bactericidal activity only with light activation. Incident light was absorbed

. However, the excited singlet state of Sb(tpp)/SiO2

is the initial

35

was too short-

(10 mg) in

−*B*

(C) under

(▲), visible

(△). Reaction conditions: initial

did not occur.

of *E. coli*, showing that the

). Thus, we found that Sb(tpp)/SiO2

in the ground state. Plots of *B*<sup>0</sup>

cells) [10]. The survival plots are shown in

(B) and *L. pneumophila* by Sb(tpp)/SiO2

cells mL−1 for *L. pneumophila,* M(tpp)/SiO2 = 10 mg

( ), dark conditions in the presence of M(tpp)/SiO2

average number of the colonies of *E. coli* in the triplicate plates.

diation, were also performed. Each control run maintained the *B*<sup>0</sup>

(triplet energy = 0.98 eV) to generate singlet oxygen (<sup>1</sup>

involves an interaction between bacteria and P(tpp)/SiO2

phosphate buffer (10 mL) containing *E. coli* (10<sup>4</sup>

**Figure 8.** Photoinactivation of *E. coli* by Sb(tpp)/SiO2

light irradiation in the absence of M(tpp)/SiO2

concentration of bacteria = 1.0 × 10<sup>4</sup>

lamps (light intensity = 21 W cm–2).

visible light irradiation in the presence of M(tpp)/SiO2

**Figure 8A** shows the time courses of survival ratio (100 *B*/*B*<sup>0</sup>

initial concentration (6.4 × 103

exclusively by Sb(tpp)/SiO2

O2

Thus, Sb(tpp)/SiO2

state of Sb(tpp)/SiO2

could generate <sup>1</sup>

Sb(tpp)/SiO2

**Figure 6.** Dechlorination of 4-chlorophenol (4-CP) using Sb(tpp)/SiO2 under irradiation from a fluorescent lamp.

dicationic Sb(tpp)/SiO2 undergoes the hole transfer to 4-CP adsorbed on SiO2 and solved in the aqueous solution. The resulting 4-CP cation radical allows for the nucleophilic addition of H<sup>2</sup> O. The substitution of Cl with OH via the hydroxy adduct followed by the oxidation by Fe3+ gives 1,4-benzoquinone.

#### **3. Photoinactivation of** *E. coli* **by M(tpp)/SiO2 (M = P, Sb)**

Our first experiment for photoinactivation using Sb(tpp)/SiO2 was reported in 2003 against *E. coli* [10]. Photoinactivation of *E. coli* was enacted as follows: *E. coli* k-12 (IFO3335) was cultured aerobically at 30°C for 8 h in a basal medium (pH 6.5) consisting of 1% bactotriptone, 0.5% yeast extract, and 1% NaCl. After centrifugation of the cultured broth at 8500 g for 10 min, the harvested cells were washed with physiological saline and then resuspended in the saline. Phosphate buffer (9.0 mL, 100 mM, pH 7.0), the cell suspension of *E. coli* (1.0 mL, ca. 10<sup>4</sup> cells mL−1), and Sb(tpp)/SiO2 (10 mg) were introduced into an L-type glass tube (length 180 mm, diameter 15 mm) that was set on a reciprocal shaker in the apparatus shown in

**Figure 7.** Apparatus used for photoinactivation of bacteria experiments.

**Figure 7**. The glass tube was shaken at 160 rpm and irradiated for 2 h using a fluorescent lamp (Panasonic FL-15ECW, Japan, λ = 400–723 nm, maximum intensity at 545 nm, 10.5 W cm−2) set above a reciprocal shaker. Aliquots (0.1 mL) were taken from the reaction mixture at 20 min intervals and plated on agar plates in triplicate. The colonies that appeared after incubation for 24 h at 30°C were counted. The amount of living cells (*B* in cell mL−1) was defined as the average number of the colonies of *E. coli* in the triplicate plates.

**Figure 8A** shows the time courses of survival ratio (100 *B*/*B*<sup>0</sup> ) of *E. coli*, where *B*<sup>0</sup> is the initial amount of *E. coli*. Upon irradiation for 60 min, the concentration of *E. coli* decreased from the initial concentration (6.4 × 103 cells mL−1) to about 40 cells mL−1. Its survival ratio was 0.6%. Thus, Sb(tpp)/SiO2 demonstrated bactericidal activity under visible light irradiation. Control experiments, irradiation in the absence of the photocatalyst and photocatalyst without irradiation, were also performed. Each control run maintained the *B*<sup>0</sup> of *E. coli*, showing that the Sb(tpp)/SiO2 has bactericidal activity only with light activation. Incident light was absorbed exclusively by Sb(tpp)/SiO2 . However, the excited singlet state of Sb(tpp)/SiO2 was too shortlived to contact directly with the bacteria. Therefore, it was suggested that the excited triplet state of Sb(tpp)/SiO2 (triplet energy = 1.63 eV) underwent energy transfer to molecular oxygen (triplet energy = 0.98 eV) to generate singlet oxygen (<sup>1</sup> O2 ). Thus, we found that Sb(tpp)/SiO2 could generate <sup>1</sup> O2 in aqueous solutions to sterilize *E. coli* cells using visible light irradiation.

dicationic Sb(tpp)/SiO2

Fe3+ gives 1,4-benzoquinone.

cells mL−1), and Sb(tpp)/SiO2

**3. Photoinactivation of** *E. coli* **by M(tpp)/SiO2**

**Figure 6.** Dechlorination of 4-chlorophenol (4-CP) using Sb(tpp)/SiO2

34 Visible-Light Photocatalysis of Carbon-Based Materials

**Figure 7.** Apparatus used for photoinactivation of bacteria experiments.

Our first experiment for photoinactivation using Sb(tpp)/SiO2

of H<sup>2</sup>

10<sup>4</sup>

undergoes the hole transfer to 4-CP adsorbed on SiO2

 **(M = P, Sb)**

under irradiation from a fluorescent lamp.

(10 mg) were introduced into an L-type glass tube (length

O. The substitution of Cl with OH via the hydroxy adduct followed by the oxidation by

the aqueous solution. The resulting 4-CP cation radical allows for the nucleophilic addition

*E. coli* [10]. Photoinactivation of *E. coli* was enacted as follows: *E. coli* k-12 (IFO3335) was cultured aerobically at 30°C for 8 h in a basal medium (pH 6.5) consisting of 1% bactotriptone, 0.5% yeast extract, and 1% NaCl. After centrifugation of the cultured broth at 8500 g for 10 min, the harvested cells were washed with physiological saline and then resuspended in the saline. Phosphate buffer (9.0 mL, 100 mM, pH 7.0), the cell suspension of *E. coli* (1.0 mL, ca.

180 mm, diameter 15 mm) that was set on a reciprocal shaker in the apparatus shown in

and solved in

was reported in 2003 against

Similar photoinactivation studies of *E. coli* were performed using P(tpp)/SiO2 (10 mg) in phosphate buffer (10 mL) containing *E. coli* (10<sup>4</sup> cells) [10]. The survival plots are shown in **Figure 8B**. Under a nitrogen atmosphere, photoinactivation using P(tpp)/SiO2 did not occur. We postulated that photoinactivation obeyed a Michaelis-Menten type mechanism, which involves an interaction between bacteria and P(tpp)/SiO2 in the ground state. Plots of *B*<sup>0</sup> −*B*

**Figure 8.** Photoinactivation of *E. coli* by Sb(tpp)/SiO2 (A) and P(tpp)/SiO2 (B) and *L. pneumophila* by Sb(tpp)/SiO2 (C) under visible light irradiation in the presence of M(tpp)/SiO2 ( ), dark conditions in the presence of M(tpp)/SiO2 (▲), visible light irradiation in the absence of M(tpp)/SiO2 (○), and dark in the absence of M(tpp)/SiO2 (△). Reaction conditions: initial concentration of bacteria = 1.0 × 10<sup>4</sup> cell mL–1 for *E. coli* and 6.4 × 103 cells mL−1 for *L. pneumophila,* M(tpp)/SiO2 = 10 mg in phosphate buffer solution (10 mL) for *E. coli* and water (10 mL) for *L. pneumophila* under irradiation of fluorescent lamps (light intensity = 21 W cm–2).

values against irradiation time gave a linear correlation. From the slopes of the plots, the rate for photoinactivation was determined to be proportional to the amounts of P(tpp)/SiO2 and light intensity. Thus, adsorption of *E. coli* on P(tpp)/SiO2 was important for efficient photoinactivation using P(tpp)/SiO2 under visible light irradiation.

### **4. Photoinactivation of** *Legionella* **species in naturally occurring environments**

*L. pneumophila* has become one of the most aggressive pathogens since the first outbreak of Legionnaires' disease in Philadelphia in 1976 [14]. The natural habitats for *L. pneumophila* include a wide range of aquatic bodies, such as lakes, streams, and artificially constructed aquatic reservoirs (hot springs, fountains, and cooling towers). Large outbreaks of Legionnaires' disease have been reported in Portugal, the Netherlands, and Spain [15–17]. On the other hand, photocatalytic treatments have received much attention as an environmentally friendly process to inactivate bacteria in contaminated water. In 2002-2003, our group carried out the photoinactivation of *Legionella* species occurring in bacteria's natural habitats, such as in cooling towers and public fountains using a use of Sb(tpp)/SiO2 [18].

that consisted of seven fluorescent lamps (18 W, 43 mm*φ* × 50 cm) and the Sb(tpp)/SiO2

**Figure 9.** Practical experiment in a cooling tower: the cylindrical apparatus (B) setup in the cooling tower (A).

vessel, the Sb(tpp)/SiO2

experiment.

was filled with 13 m3

determined by the colony counting method.

Thus, the bactericidal effects of Sb(tpp)/SiO2

**4.3. Practical experiments in water fountain**

lyst (4.0 kg, 0.05 wt% Sb(tpp)) was used. Water in the holder (800 L) of the cooling tower was pumped into a cylindrical vessel at a rate of 28 L min−1, and then, the treated water was returned to the holder. The average retention time was calculated to be 26 s. In the cylindrical

lamps at ambient temperature. Sampling of the water was carried out at outlet at 3–7 day intervals. At the same time, the atmospheric temperatures were recorded as the average value of the highest temperature of Miyazaki city during the sampling day and 2 days prior. The sample water (1.0 L) was filtrated through a membrane filter (0.45 μm, HA, Millipore) under reduced pressure into the vessel (100 mL) containing the microbes adhering to the membrane filter, an aqueous solution (5 mL) was added, and the vessel was shaken vigorously. Saturated aqueous KCl (5 mL, pH 2.2) containing 0.2 M HCl was added, and the vessel was shaken vigorously. After standing for exactly 20 min at room temperature, the prepared solution was ready for plating as described in Section 4.1. The amounts of *Legionella* species were

Under the conditions without any bactericidal treatments, *Legionella* species occurred in a range from 20 to 139 CFU in the holder of the cooling tower, as shown in **Figure 10**. After the bactericidal apparatus was active, the concentrations of *Legionella* species decreased to levels below the detection limit. This was maintained until the irradiation treatment ceased. Seven days after the irradiation ceased, detectable amounts of *Legionella* species reappeared.

Practical experiments were performed in a public fountain of Miyazaki city (**Figure 11A**) that

of water [18]. Photoinactivation of the fountain was examined using a

catalyst was irradiated by visible light emitted from the fluorescent

Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria

http://dx.doi.org/10.5772/intechopen.70004

were practically confirmed in this cooling tower

cata-

37

#### **4.1. Photoinactivation of** *L. pneumophila*

Initially, the photoinactivation of *L. pneumophila* was examined with Sb(tpp)/SiO2 using L-type glass tubes in the apparatus shown in **Figure 7** in a similar manner to the method described for *E. coli* [10]. A phosphate buffer (0.1 M, pH 7.0, 10 mL) containing a cell suspension of *L. pneumophila* (6.4 × 105 CFU/100 mL), and Sb(tpp)/SiO2 (10 mg) was introduced into the L-type glass tube (length 18 cm, diameter 1.5 cm) and irradiated with a fluorescent lamp [18]. Aliquots (0.1 mL) of the reaction mixture were directly plated on a selective medium for *Legionella* species, i.e., WYOα agar medium (Eiken Chemicals Co., Ltd, Japan) consisting of glycine (3 g), vancomycin (5 mg), polymixin B (106 IU), and amphotericin B (80 mg). The colonies of *L. pneumophila* appeared after incubation for 7 days at 36°C. Wet, smooth, and bluishwhite colonies were counted on triplicate plates. The cell concentration of *L. pneumophila* was represented in colony formation units in 100 mL of the aqueous solution (CFU/100 mL).

**Figure 8C** shows the time courses for *Legionella* species survival. Upon irradiation for 60 min in the presence of the Sb(tpp)/SiO2 , the concentration of *Legionella* species apparently decreased from the initial concentration (6.4 × 105 CFU) to 4 × 103 CFU. Its survival ratio was 0.6%. In the control experiments, irradiation in the absence of Sb(tpp)/SiO2 , in the presence of Sb(tpp)/SiO2 without irradiation, and in the absence of Sb(tpp)/SiO2 and irradiation, each runs maintained the initial concentration of *Legionella* species. Thus, Sb(tpp)/SiO2 was confirmed to have the photocatalytic activity that could sterilize *Legionella* species.

#### **4.2. Practical experiments in a cooling tower**

The bactericidal experiment was performed in a cooling tower (**Figure 9A**) that was located in a building in Miyazaki city [18]. A cylindrical apparatus (200 mm*φ* × 500 mm, **Figure 9B**)

**Figure 9.** Practical experiment in a cooling tower: the cylindrical apparatus (B) setup in the cooling tower (A).

that consisted of seven fluorescent lamps (18 W, 43 mm*φ* × 50 cm) and the Sb(tpp)/SiO2 catalyst (4.0 kg, 0.05 wt% Sb(tpp)) was used. Water in the holder (800 L) of the cooling tower was pumped into a cylindrical vessel at a rate of 28 L min−1, and then, the treated water was returned to the holder. The average retention time was calculated to be 26 s. In the cylindrical vessel, the Sb(tpp)/SiO2 catalyst was irradiated by visible light emitted from the fluorescent lamps at ambient temperature. Sampling of the water was carried out at outlet at 3–7 day intervals. At the same time, the atmospheric temperatures were recorded as the average value of the highest temperature of Miyazaki city during the sampling day and 2 days prior. The sample water (1.0 L) was filtrated through a membrane filter (0.45 μm, HA, Millipore) under reduced pressure into the vessel (100 mL) containing the microbes adhering to the membrane filter, an aqueous solution (5 mL) was added, and the vessel was shaken vigorously. Saturated aqueous KCl (5 mL, pH 2.2) containing 0.2 M HCl was added, and the vessel was shaken vigorously. After standing for exactly 20 min at room temperature, the prepared solution was ready for plating as described in Section 4.1. The amounts of *Legionella* species were determined by the colony counting method.

Under the conditions without any bactericidal treatments, *Legionella* species occurred in a range from 20 to 139 CFU in the holder of the cooling tower, as shown in **Figure 10**. After the bactericidal apparatus was active, the concentrations of *Legionella* species decreased to levels below the detection limit. This was maintained until the irradiation treatment ceased. Seven days after the irradiation ceased, detectable amounts of *Legionella* species reappeared. Thus, the bactericidal effects of Sb(tpp)/SiO2 were practically confirmed in this cooling tower experiment.

#### **4.3. Practical experiments in water fountain**

values against irradiation time gave a linear correlation. From the slopes of the plots, the rate for photoinactivation was determined to be proportional to the amounts of P(tpp)/SiO2

*L. pneumophila* has become one of the most aggressive pathogens since the first outbreak of Legionnaires' disease in Philadelphia in 1976 [14]. The natural habitats for *L. pneumophila* include a wide range of aquatic bodies, such as lakes, streams, and artificially constructed aquatic reservoirs (hot springs, fountains, and cooling towers). Large outbreaks of Legionnaires' disease have been reported in Portugal, the Netherlands, and Spain [15–17]. On the other hand, photocatalytic treatments have received much attention as an environmentally friendly process to inactivate bacteria in contaminated water. In 2002-2003, our group carried out the photoinactivation of *Legionella* species occurring in bacteria's natural habitats,

under visible light irradiation.

**4. Photoinactivation of** *Legionella* **species in naturally occurring** 

such as in cooling towers and public fountains using a use of Sb(tpp)/SiO2

Initially, the photoinactivation of *L. pneumophila* was examined with Sb(tpp)/SiO2

L-type glass tubes in the apparatus shown in **Figure 7** in a similar manner to the method described for *E. coli* [10]. A phosphate buffer (0.1 M, pH 7.0, 10 mL) containing a cell suspen-

CFU/100 mL), and Sb(tpp)/SiO2

the L-type glass tube (length 18 cm, diameter 1.5 cm) and irradiated with a fluorescent lamp [18]. Aliquots (0.1 mL) of the reaction mixture were directly plated on a selective medium for *Legionella* species, i.e., WYOα agar medium (Eiken Chemicals Co., Ltd, Japan) consisting of

nies of *L. pneumophila* appeared after incubation for 7 days at 36°C. Wet, smooth, and bluishwhite colonies were counted on triplicate plates. The cell concentration of *L. pneumophila* was represented in colony formation units in 100 mL of the aqueous solution (CFU/100 mL).

**Figure 8C** shows the time courses for *Legionella* species survival. Upon irradiation for 60

The bactericidal experiment was performed in a cooling tower (**Figure 9A**) that was located in a building in Miyazaki city [18]. A cylindrical apparatus (200 mm*φ* × 500 mm, **Figure 9B**)

0.6%. In the control experiments, irradiation in the absence of Sb(tpp)/SiO2

maintained the initial concentration of *Legionella* species. Thus, Sb(tpp)/SiO2

have the photocatalytic activity that could sterilize *Legionella* species.

without irradiation, and in the absence of Sb(tpp)/SiO2

light intensity. Thus, adsorption of *E. coli* on P(tpp)/SiO2

activation using P(tpp)/SiO2

36 Visible-Light Photocatalysis of Carbon-Based Materials

**4.1. Photoinactivation of** *L. pneumophila*

glycine (3 g), vancomycin (5 mg), polymixin B (106

decreased from the initial concentration (6.4 × 105

**4.2. Practical experiments in a cooling tower**

min in the presence of the Sb(tpp)/SiO2

Sb(tpp)/SiO2

sion of *L. pneumophila* (6.4 × 105

**environments**

and

using

was important for efficient photoin-

[18].

(10 mg) was introduced into

CFU. Its survival ratio was

and irradiation, each runs

, in the presence of

was confirmed to

IU), and amphotericin B (80 mg). The colo-

, the concentration of *Legionella* species apparently

CFU) to 4 × 103

Practical experiments were performed in a public fountain of Miyazaki city (**Figure 11A**) that was filled with 13 m3 of water [18]. Photoinactivation of the fountain was examined using a

**Figure 10.** Time-course plots of the amounts of *Legionella* species ( ) in the cooling tower along with the atmospheric temperature (○). The cylindrical photo-bactericidal apparatus was operated from October 1 to October 21, 2002. Conditions: catalyst, Sb(tpp)/SiO2 (4 kg); water, ca. 800 L; flow rate, 28 L min−1; and average retention time, 26 s.

leaf-type of photoinactivation apparatus (200 mm*φ* × 50 mm, **Figure 11B**) containing the Sb(tpp)/SiO2 catalyst (80 g, 0.05 wt% Sb(tpp)), which operated under sunlight irradiation. The determination of viable cell numbers of *Legionella* species was carried out in the manner described in Section 4.2. After the leaf-type apparatus had been installed into the fountain, the concentrations of *Legionella* species in the fountain were continuously kept below 30 CFU under sunlight irradiation (**Figure 12**). After the leaf-type apparatus was removed, the concentrations of *Legionella* species gradually increased to reach 100 CFU, which is the environmental quality standard, within 42 days after the removal.

As a result of the practical experiments, it is noteworthy that 80 g of Sb(tpp)/SiO2

experiment was performed using a leaf-type photo-bactericidal apparatus containing Sb(tpp)/SiO2

the average of the highest temperature of Miyazaki city for three days before the sampling day.

of water below 100 CFU for 120 days.

ICP. Before use, the Sb content in Sb(tpp)/SiO2

with the original spectra of Sb(tpp)/SiO2

**4.4. Mechanism for photoinactivation using M(tpp)/SiO2**

13 m3

that contained 13 m3

Moreover, Sb(tpp)/SiO2

which contained 40 mg of Sb(tpp), could maintain the concentration of *Legionella* species in

**Figure 12.** Time-conversion plots of the amount of *Legionella* species ( ) in public fountain photoinactivation. The

on May 28, 2003 and was removed from the fountain on August 22, 2003. Atmospheric temperature (○) was recorded as

of water between April 9 to October 29, 2003. The leaf-type apparatus was installed into the fountain

Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria

http://dx.doi.org/10.5772/intechopen.70004

39

Elemental analyses of the catalysts before and after use in the fountain were performed with

good agreement with the Sb content (72 ppm) calculated for the 0.05 wt% of Sb(tpp) content in the catalyst. After 3 months of use in the fountain, the Sb content decreased from 80 to 17 ppm. On the other hand, Na, Mg, Al, and Ca largely increased, resulting in ion-absorption on SiO2

ning microscopy (CLSM). It was found that the fluorescence coming from the surface of the catalyst keep the similar shapes to the original catalyst, but the intensity was weaker compared

of the catalyst maintained the original intensity. Therefore, it is suggested that Sb(tpp) was eliminated from the surface of the catalyst. Irradiation of fluorescent light on the Sb(tpp)/SiO2 catalyst in deionized water did not sufficiently account for the spectral change and decrease in total Sb. Therefore, that the cationic Sb(tpp) chromophore was exchanged with alkali metal ions in the bulk water on the surface of the catalyst under irradiation is strongly suggested.

catalyst used in the fountain was analyzed by a confocal laser scan-

catalyst,

(80 g) in a fountain

.

was measured to be 80 ppm, which was in

. On the other hand, the fluorescence from the inside

**Figure 11.** Practical experiments in a public fountain: The leaf-type apparatus (B) was set in the fountain (A). (C) Picture indicates the leaf-type apparatus set in the pool of the fountain.

Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria http://dx.doi.org/10.5772/intechopen.70004 39

**Figure 12.** Time-conversion plots of the amount of *Legionella* species ( ) in public fountain photoinactivation. The experiment was performed using a leaf-type photo-bactericidal apparatus containing Sb(tpp)/SiO2 (80 g) in a fountain that contained 13 m3 of water between April 9 to October 29, 2003. The leaf-type apparatus was installed into the fountain on May 28, 2003 and was removed from the fountain on August 22, 2003. Atmospheric temperature (○) was recorded as the average of the highest temperature of Miyazaki city for three days before the sampling day.

As a result of the practical experiments, it is noteworthy that 80 g of Sb(tpp)/SiO2 catalyst, which contained 40 mg of Sb(tpp), could maintain the concentration of *Legionella* species in 13 m3 of water below 100 CFU for 120 days.

### **4.4. Mechanism for photoinactivation using M(tpp)/SiO2**

leaf-type of photoinactivation apparatus (200 mm*φ* × 50 mm, **Figure 11B**) containing the

**Figure 10.** Time-course plots of the amounts of *Legionella* species ( ) in the cooling tower along with the atmospheric temperature (○). The cylindrical photo-bactericidal apparatus was operated from October 1 to October 21, 2002.

The determination of viable cell numbers of *Legionella* species was carried out in the manner described in Section 4.2. After the leaf-type apparatus had been installed into the fountain, the concentrations of *Legionella* species in the fountain were continuously kept below 30 CFU under sunlight irradiation (**Figure 12**). After the leaf-type apparatus was removed, the concentrations of *Legionella* species gradually increased to reach 100 CFU, which is the environmental quality

**Figure 11.** Practical experiments in a public fountain: The leaf-type apparatus (B) was set in the fountain (A). (C) Picture

catalyst (80 g, 0.05 wt% Sb(tpp)), which operated under sunlight irradiation.

(4 kg); water, ca. 800 L; flow rate, 28 L min−1; and average retention time, 26 s.

Sb(tpp)/SiO2

Conditions: catalyst, Sb(tpp)/SiO2

38 Visible-Light Photocatalysis of Carbon-Based Materials

standard, within 42 days after the removal.

indicates the leaf-type apparatus set in the pool of the fountain.

Elemental analyses of the catalysts before and after use in the fountain were performed with ICP. Before use, the Sb content in Sb(tpp)/SiO2 was measured to be 80 ppm, which was in good agreement with the Sb content (72 ppm) calculated for the 0.05 wt% of Sb(tpp) content in the catalyst. After 3 months of use in the fountain, the Sb content decreased from 80 to 17 ppm. On the other hand, Na, Mg, Al, and Ca largely increased, resulting in ion-absorption on SiO2 . Moreover, Sb(tpp)/SiO2 catalyst used in the fountain was analyzed by a confocal laser scanning microscopy (CLSM). It was found that the fluorescence coming from the surface of the catalyst keep the similar shapes to the original catalyst, but the intensity was weaker compared with the original spectra of Sb(tpp)/SiO2 . On the other hand, the fluorescence from the inside of the catalyst maintained the original intensity. Therefore, it is suggested that Sb(tpp) was eliminated from the surface of the catalyst. Irradiation of fluorescent light on the Sb(tpp)/SiO2 catalyst in deionized water did not sufficiently account for the spectral change and decrease in total Sb. Therefore, that the cationic Sb(tpp) chromophore was exchanged with alkali metal ions in the bulk water on the surface of the catalyst under irradiation is strongly suggested. A similar phenomena were observed in the case of P(tpp)/SiO2 [10]. Moreover, when Sb(tpp) was tightly fixed on SiO2 through covalent bonds, no photoinactivation occurred [19].

Therefore, the liberation of the Sb(tpp) chromophore from SiO2 is necessary for photoinactivation, as shown in **Figure 13**. Sb(tpp) can dissolve slightly in water (the water solubility (*C*W) of Sb(tpp) is low (0.08 mM)). The liberated Sb(tpp) might be adsorbed by the bacteria and induce apoptosis under visible light irradiation. After the photoinactivation, Sb(tpp) might separate from the SiO2 and move to the pool of the fountain. This may explain the loss of Sb(tpp) from Sb(tpp)/SiO2 .

(ca. 1 × 10<sup>4</sup>

dark conditions at all.

**Figure 14.** Water-soluble porphyrins (**1**–**3**).

cell mL−1) and compounds **1**–**3** (5–2000 nM). The solution was irradiated using

Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria

http://dx.doi.org/10.5772/intechopen.70004

41

a fluorescent lamp (λmax = 545 nm) under aerobic conditions on a reciprocal shaker. The amounts of bacteria (cell mL−1) at every irradiation time were determined by the colony counting method. Under dark conditions, the bacterial counts were maintained at the initial levels. Thus, it was confirmed that all porphyrins (**1**–**3**) have no bactericidal activity under

From the plots of the survival ratios against irradiation time, the bactericidal activity of porphyrins was evaluated by the half life (*T*1/2 in min), which was the time required for the initial bacteria concentration to be halved. Moreover, the minimum concentrations ([*P*]) of porphyrins were adjusted until *T*1/2 was found in the range of 0–120 min. The [*P*] values and *T*1/2 are shown in **Table 2**. As [*P*] values and *T*1/2 are smaller, the bactericidal activity of porphyrins is higher. The most active sensitizer for *S. cerevisiae* was **2e** and **2f**, whose [*P*] value was 5.0 nM. However, **2e** and **2f** were ineffective for *E. coli*. Tricationic complexes **3a**−**3c** were effective for *E. coli.* The bioaffinity of the porphyrins can be related to the structure of the bacterial cell wall. In the case of *S. cerevisiae*, whose cell wall consisted of hydrophobic peptidoglycan, water-soluble porphyrins (**2e**–**2f**) having hydrophobic character had the highest bioaffinity. On the other hand, polycationic porphyrins (**3**) had the highest bioaffinity toward gramnegative *E. coli*, whose cell wall consisted of phospholipids, lipopolysaccharides, lipoteichoic acids, and lipoproteins [30]. Moreover, it was found that **3** interacted strongly with human serum albumin (HSA) [31], and the addition of HSA was effective for inducing photoinactivation of *S. cerevisiae* using **3** [29]. Recently, reviews on photoinactivation by water-soluble

porphyrins have been published by Almeida at al. [20] and our group [32].

**Figure 13.** Possible mechanism for photoinactivation of bacteria by M(tpp)/SiO2 .

### **5. Photosensitized inactivation using water-soluble porphyrins**

As mentioned in the previous section, it was found that Sb(tpp) dissolved in water was responsible for the photoinactivation of bacteria. Therefore, in our next study, we intended to inactivate bacteria using water-soluble porphyrins. Water-soluble porphyrins have received much attention in connection with photoinactivation [20] and photodynamic therapy (PDT) [21–24] ever since the first report on photoinactivation of *E. coli* by water-soluble *meso*-substituted cationic porphyrins appeared in 1996 [25]. For the biological application of porphyrins, water solubility is an important characteristic in the handling of porphyrins in an aqueous solution. We modified the axial ligands of P and Sb porphyrins by installing alkyloxo (**1**) [26], alkylethylenedioxy (**2**) [27, 28], and alkylpyridinium groups (**3**) [20, 29] (**Figure 14**). The water solubility (*C*W in mM) is shown in **Table 2**. The quantum yields for the formation of <sup>1</sup> O2 were determined to be 0.65 for **1b**, 0.53 for **1d** [26], 0.62 for **2b**, 0.69 for **2c**, 0.73 for **2d** [27], 0.88 for **3g**, and 0.87 for **3c** [29].

We show the results of photoinactivation of *S. cerevisiae* and *E. coli* using water-soluble P and Sb porphyrins (**1**–**3**). Photoinactivation was enacted using the apparatus shown in **Figure 7**. The porphyrin solution, a bacteria suspension, and buffer solution (or water) were introduced into L-type glass tubes, resulting in a solution (10 mL) containing bacteria cells

Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria http://dx.doi.org/10.5772/intechopen.70004 41

**Figure 14.** Water-soluble porphyrins (**1**–**3**).

A similar phenomena were observed in the case of P(tpp)/SiO2

Therefore, the liberation of the Sb(tpp) chromophore from SiO2

was tightly fixed on SiO2

40 Visible-Light Photocatalysis of Carbon-Based Materials

**3g**, and 0.87 for **3c** [29].

[10]. Moreover, when Sb(tpp)

is necessary for photoinactivation,

.

O2 were

through covalent bonds, no photoinactivation occurred [19].

as shown in **Figure 13**. Sb(tpp) can dissolve slightly in water (the water solubility (*C*W) of Sb(tpp) is low (0.08 mM)). The liberated Sb(tpp) might be adsorbed by the bacteria and induce apoptosis under visible light irradiation. After the photoinactivation, Sb(tpp) might separate from the SiO2 and move to the pool of the fountain. This may explain the loss of Sb(tpp) from Sb(tpp)/SiO2

**5. Photosensitized inactivation using water-soluble porphyrins**

**Figure 13.** Possible mechanism for photoinactivation of bacteria by M(tpp)/SiO2

As mentioned in the previous section, it was found that Sb(tpp) dissolved in water was responsible for the photoinactivation of bacteria. Therefore, in our next study, we intended to inactivate bacteria using water-soluble porphyrins. Water-soluble porphyrins have received much attention in connection with photoinactivation [20] and photodynamic therapy (PDT) [21–24] ever since the first report on photoinactivation of *E. coli* by water-soluble *meso*-substituted cationic porphyrins appeared in 1996 [25]. For the biological application of porphyrins, water solubility is an important characteristic in the handling of porphyrins in an aqueous solution. We modified the axial ligands of P and Sb porphyrins by installing alkyloxo (**1**) [26], alkylethylenedioxy (**2**) [27, 28], and alkylpyridinium groups (**3**) [20, 29] (**Figure 14**). The water

.

solubility (*C*W in mM) is shown in **Table 2**. The quantum yields for the formation of <sup>1</sup>

determined to be 0.65 for **1b**, 0.53 for **1d** [26], 0.62 for **2b**, 0.69 for **2c**, 0.73 for **2d** [27], 0.88 for

We show the results of photoinactivation of *S. cerevisiae* and *E. coli* using water-soluble P and Sb porphyrins (**1**–**3**). Photoinactivation was enacted using the apparatus shown in **Figure 7**. The porphyrin solution, a bacteria suspension, and buffer solution (or water) were introduced into L-type glass tubes, resulting in a solution (10 mL) containing bacteria cells (ca. 1 × 10<sup>4</sup> cell mL−1) and compounds **1**–**3** (5–2000 nM). The solution was irradiated using a fluorescent lamp (λmax = 545 nm) under aerobic conditions on a reciprocal shaker. The amounts of bacteria (cell mL−1) at every irradiation time were determined by the colony counting method. Under dark conditions, the bacterial counts were maintained at the initial levels. Thus, it was confirmed that all porphyrins (**1**–**3**) have no bactericidal activity under dark conditions at all.

From the plots of the survival ratios against irradiation time, the bactericidal activity of porphyrins was evaluated by the half life (*T*1/2 in min), which was the time required for the initial bacteria concentration to be halved. Moreover, the minimum concentrations ([*P*]) of porphyrins were adjusted until *T*1/2 was found in the range of 0–120 min. The [*P*] values and *T*1/2 are shown in **Table 2**. As [*P*] values and *T*1/2 are smaller, the bactericidal activity of porphyrins is higher. The most active sensitizer for *S. cerevisiae* was **2e** and **2f**, whose [*P*] value was 5.0 nM. However, **2e** and **2f** were ineffective for *E. coli*. Tricationic complexes **3a**−**3c** were effective for *E. coli.* The bioaffinity of the porphyrins can be related to the structure of the bacterial cell wall. In the case of *S. cerevisiae*, whose cell wall consisted of hydrophobic peptidoglycan, water-soluble porphyrins (**2e**–**2f**) having hydrophobic character had the highest bioaffinity. On the other hand, polycationic porphyrins (**3**) had the highest bioaffinity toward gramnegative *E. coli*, whose cell wall consisted of phospholipids, lipopolysaccharides, lipoteichoic acids, and lipoproteins [30]. Moreover, it was found that **3** interacted strongly with human serum albumin (HSA) [31], and the addition of HSA was effective for inducing photoinactivation of *S. cerevisiae* using **3** [29]. Recently, reviews on photoinactivation by water-soluble porphyrins have been published by Almeida at al. [20] and our group [32].


species, such as <sup>1</sup>

our laboratories.

**Acknowledgements**

large contributions.

**Author details**

Japan

**References**

Masahide Yasuda\* and Jin Matsumoto

Letters. 2010;**39**:874-875

2014;**118**:3926-3933

\*Address all correspondence to: yasuda@cc.miyazaki-u.ac.jp

O2

the cell walls and inside the cells.

, is generated by energy transfer from the porphyrins to O2

Silica Gel–Supported P-, Ge-, and Sb-Porphyrins for Visible Light Inactivation of Bacteria

Thus, porphyrins are useful chromophores for catalysis and sensitization in a biological application. The application of water-soluble M-porphyrins to PDT is currently underway in

This work was partially supported by a Grant-in-Aid for Scientific Research (C) (16K05847) from Japan Society for the Promotion of Science (JSPS). Also, the authors would like to express grateful acknowledgment to many students belonged in our laboratory during 2003 to present when visible light inactivation has been investigated in our laboratory for their

Department of Applied Chemistry, Faculty of Engineering, University of Miyazaki, Miyazaki,

[1] Fujishima A, Rao TN, Tryk DA. Titanium dioxide photocatalysis. Journal of Photo-

[2] Matsunaga T, Tomoda R, Nakajima T, Wake H. Photoelectrochemical sterilization of microbial cells by semiconductor powders. FEMS Microbiology Letters. 1985;**29**:211-214

[3] Fujitsuka M, Cho DW, Tojo S, Inoue A, Shiragami T, Yasuda M, Majima T. Electron transfer from axial ligand to S1- and S2-excited phosphorus tetraphenylporphyrin. The

[4] Shiragami T, Shiraki R, Makise R, Matsumoto J, Yasuda M. Di(hydroxo)porphyrin GeIV complex/silica gel composite as visible light-assisted radical generator. Chemistry

[5] Fujitsuka M, Shiragami T, Cho DW, Tojo S, Yasuda M, Majima T. Solvent dynamics regulated electron transfer in S2-excited Sb and Ge tetraphenylporphyrins with an electron donor substituent at the *meso*-position. The Journal of Physical Chemistry A.

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molecules on

43

http://dx.doi.org/10.5772/intechopen.70004

a Water solubility of porphyrins in mM.

b Experiment with \* was performed in the presence of human serum albumin (HSA, 400 nM).

c Unpublished results.

**Table 2.** Photoinactivation of *S. cerevisiae* and *E. coli* by water-soluble porphyrins (**1**, **2**, and **3**).

### **6. Conclusion and perspectives**

Since aqueous solutions are more transparent for visible light than ultraviolet, visible light photocatalysts work best for the photocatalytic reactions in aqueous solution. Moreover, visible light photocatalysts take advantage of the photocatalytic reactions under sunlight irradiation, since sunlight consists of 52% visible, 42% infrared, 6% UV-A, and 0.5% UV-B light. We showed two methods to photoinactivate bacteria: one method is the dispersion of M(tpp)/ SiO2 (M = P, Ge, and Sb) in water, which is applicable to open system in naturally occurring environments; the other method is the water solubilization of M-porphyrins (M = P and Sb), which can be used in a closed system. M(tpp) and M-porphyrins can interact with bacteria through adsorption onto cell walls and absorption into the cells. Under irradiation, reactive species, such as <sup>1</sup> O2 , is generated by energy transfer from the porphyrins to O2 molecules on the cell walls and inside the cells.

Thus, porphyrins are useful chromophores for catalysis and sensitization in a biological application. The application of water-soluble M-porphyrins to PDT is currently underway in our laboratories.

### **Acknowledgements**

This work was partially supported by a Grant-in-Aid for Scientific Research (C) (16K05847) from Japan Society for the Promotion of Science (JSPS). Also, the authors would like to express grateful acknowledgment to many students belonged in our laboratory during 2003 to present when visible light inactivation has been investigated in our laboratory for their large contributions.

### **Author details**

Masahide Yasuda\* and Jin Matsumoto

\*Address all correspondence to: yasuda@cc.miyazaki-u.ac.jp

 Department of Applied Chemistry, Faculty of Engineering, University of Miyazaki, Miyazaki, Japan

### **References**

**6. Conclusion and perspectives**

Water solubility of porphyrins in mM.

Unpublished results.

SiO2

a

b

c

Since aqueous solutions are more transparent for visible light than ultraviolet, visible light photocatalysts work best for the photocatalytic reactions in aqueous solution. Moreover, visible light photocatalysts take advantage of the photocatalytic reactions under sunlight irradiation, since sunlight consists of 52% visible, 42% infrared, 6% UV-A, and 0.5% UV-B light. We showed two methods to photoinactivate bacteria: one method is the dispersion of M(tpp)/

Experiment with \* was performed in the presence of human serum albumin (HSA, 400 nM).

**Table 2.** Photoinactivation of *S. cerevisiae* and *E. coli* by water-soluble porphyrins (**1**, **2**, and **3**).

**Entry** *n m C***W/mM** *S. cerevisiae***<sup>b</sup>** *E. coli*

42 Visible-Light Photocatalysis of Carbon-Based Materials

**1a** 0 0.10 50 380 – – **1b** 1 0.13 50 192 – – **1c** 6 1.09 50 14 – – **1d** 10 2.10 40 22 – – **1e** 12 2.21 50 17 – – **1f** 14 2.40 50 21 – – **2a** 1 3 17.4 500 23 – – **2b** 1 2 13.9 300 81 – – **2c** 2 2 13.0 200 31 – – **2d** 4 2 5.38 50 55 – – **2e** 6 2 2.07 5 64 – – **2f** 6 1 1.11 5 69 – – **3a** 2 >120 \*50 \*32 250<sup>c</sup> 32<sup>c</sup> **3b** 4 112 \*50 \*36 250<sup>c</sup> 53<sup>c</sup> **3c** 6 63.6 50 44 250<sup>c</sup> 120<sup>c</sup> **3d** 1 3.35 50 20 2000 66 **3e** 4 6.10 \*30 \*85 2000 27 **3f** 5 3.80 – – 500 29 **3g** 6 5.84 \*20 \*72 500 31 **3h** 7 6.00 – – 400 24 **3i** 8 3.80 – – 500 63

**[***P***]/nM** *T***1/2/min [***P***]/nM** *T***1/2/min**

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[6] Tsunami SI, Tanaka K, Matsumoto J, Shiragami T, Yasuda M. Effects of an axial amino ligand on the spectroscopic and electrochemical properties of amino(methoxo)- (tetraphenylporphyrinato)antimony(V) complexes. Bulletin of the Chemical Society of

[7] Andou Y, Shiragami T, Shima K, Yasuda M. Synthesis and fluorescence study of a (2-naphthoxy)polyoxalkoxyantimony(V) tetraphenylporphyrin complex. Journal of

[8] Felton RH, Linschitz H. Polarographic reduction of porphyrins and electron spin resonance of porphyrin anions. Journal of the American Chemical Society. 1966;**88**:1113-1116

[9] Shiragami T, Matsumoto J, Inoue H, Yasuda M. Antimony porphyrin complexes as visible-light driven photocatalyst. Journal of Photochemistry and Photobiology C:

[10] Fueda Y, Suzuki H, Komiya Y, Asakura Y, Shiragami T, Matsumoto J, Yokoi H, Yasuda M. Bactericidal effect of silica gel-supported porphyrinatophosphorus(V) catalysts on *Escherichia coli* under visible-light irradiation. Bulletin of the Chemical Society of Japan.

[11] Yokoi H, Shiragami T, Hirose J, Kawauchi T, Hinoue K, Fueda Y, Nobuhara K, Akazaki I, Yasuda M. Bactericidal effect of a silica gel-supported porphyrinatoantimony(V) complex under visible light irradiation. World Journal of Microbiology and Biotechnology.

[12] Shiragami T, Makise RI, Inokuchi Y, Matsumoto J, Inoue H, Yasuda M. Efficient photocatalytic oxidation of cycloalkenes by dihydroxo(tetraphenylporphyrunato) antimony supported on silica gel under visible light irradiation. Chemistry Letters. 2004;**33**:736-737

[13] Shiragami T, Shimizu Y, Hinoue K, Fueta Y, Nobuhara K, Akazaki I, Yasuda M. Silica gelsupported porphyrinatoantimony(V) complex acting as visible-light driven photocatalyst for dechlorination of chlorophenols in aqueous solution. Journal of Photochemistry

[14] Chang FY, Yu VL. *Legionella* infection. In: Fishman AP, Elias JA, Fishman JA, Grippi MA, Kaiser LR, and Senior R, editors. Fishman's Pulmonary Diseases and Disorders. Vol. 2,

[15] Fernández JA, López P, Orozco D, Merino J. Clinical study of an outbreak of Legionnaire's disease in Alcoy, southeastern Spain. European Journal of Clinical Microbiology &

[16] Lettinga KD, Verbon A, Weverling GJ, Schellekens JFP, Den Boer JW, Yzerman EPF, Prins J, Boersma WG, Van Ketel RJ, Prins JM, Speelman P. Legionnaires' disease at a Dutch flower show: Prognostic factors and impact of therapy. Emerging Infectious Diseases

[17] Correia AM, Gonçalves G, Reis J, Cruz JM, Castro e Freitas JA. An outbreak of legionnaires' disease in a municipality in northern Portugal. Euro Surveillance. 2001;**6**:121-124

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[31] Matsumoto J, Kubo T, Shinbara T, Matsuda N, Shiragami T, Fujitsuka M, Majima T, Yasuda M. Spectroscopic analysis of the interaction of human serum albumin with tricationic phosphorus porphyrins bearing axial pyridinio groups. Bulletin of the Chemical Society of Japan. 2013;**86**:1240-1247

**Chapter 4**

**Provisional chapter**

**Carbon Quantum Dots: A Component of Efficient**

**Carbon Quantum Dots: A Component of Efficient** 

10.5772/intechopen.70801

Carbon quantum dots (CQDs) have been developed as a new member of nanocarbons, characterized by the relatively easy preparation from a wide spectrum of carbonaceous precursors through either bottom-up or top-down routes. Attractive optoelectronic properties have been observed with CQDs, including efficient light absorption, variable photoluminescence (PL), unique up-conversion PL and prominent electron transport ability, which make CQDs an important component with great potential in the design of efficient visible light-driven photocatalysts. In this chapter, detailed contribution of CQDs to the enhanced visible light-driven photocatalysis will be included, in the classification of the role as electron mediator, photosensitizer, spectral converter and sole photocatalyst.

**Keywords:** carbon quantum dots, photocatalysis, visible light, charge carrier, photoluminescence

Carbon dots have emerged as a new class of quantum dot-like nanocarbons, which are typically constituted by discrete, quasi-spherical nanoparticles with sizes below 10 nm. The first discovery of quantum dot-like nanocarbons can be dated back to 2004, when carbon nanoparticles with sizes at about 1 nm were isolated by Xu et al. [1] as a byproduct of preparation of single-walled carbon nanotubes, separated from carbon soot produced by arc discharge. However, it was until 2 years later, these new kind of nanocarbons have been documented as "carbon quantum dots" (CQDs) and boosted widespread interests, since the synthesis of

fluorescent carbon nanoparticles with diameter less than 10 nm from Sun's group [2].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Distinct from other well developed nanocarbons, such as fullerenes, carbon nanotubes and graphene, CQDs show their unique advantages, including isotropic shapes, ultrafine dimensions,

**Visible Light Photocatalysts**

**Visible Light Photocatalysts**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Shan Cong and Zhigang Zhao

Shan Cong and Zhigang Zhao

http://dx.doi.org/10.5772/intechopen.70801

**Abstract**

**1. Introduction**

[32] Matsumoto J, Shiragami T, Hirakawa K, Yasuda M. Water-solubilization of P(V) and Sb(V) porphyrins and their photobiological application. International Journal of Photoenergy. 2015;**2015**:Article ID 148964

**Provisional chapter**

### **Carbon Quantum Dots: A Component of Efficient Visible Light Photocatalysts Visible Light Photocatalysts**

**Carbon Quantum Dots: A Component of Efficient** 

10.5772/intechopen.70801

Shan Cong and Zhigang Zhao Shan Cong and Zhigang Zhao Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70801

#### **Abstract**

[31] Matsumoto J, Kubo T, Shinbara T, Matsuda N, Shiragami T, Fujitsuka M, Majima T, Yasuda M. Spectroscopic analysis of the interaction of human serum albumin with tricationic phosphorus porphyrins bearing axial pyridinio groups. Bulletin of the Chemical

[32] Matsumoto J, Shiragami T, Hirakawa K, Yasuda M. Water-solubilization of P(V) and Sb(V) porphyrins and their photobiological application. International Journal of Photo-

Society of Japan. 2013;**86**:1240-1247

46 Visible-Light Photocatalysis of Carbon-Based Materials

energy. 2015;**2015**:Article ID 148964

Carbon quantum dots (CQDs) have been developed as a new member of nanocarbons, characterized by the relatively easy preparation from a wide spectrum of carbonaceous precursors through either bottom-up or top-down routes. Attractive optoelectronic properties have been observed with CQDs, including efficient light absorption, variable photoluminescence (PL), unique up-conversion PL and prominent electron transport ability, which make CQDs an important component with great potential in the design of efficient visible light-driven photocatalysts. In this chapter, detailed contribution of CQDs to the enhanced visible light-driven photocatalysis will be included, in the classification of the role as electron mediator, photosensitizer, spectral converter and sole photocatalyst.

**Keywords:** carbon quantum dots, photocatalysis, visible light, charge carrier, photoluminescence

### **1. Introduction**

Carbon dots have emerged as a new class of quantum dot-like nanocarbons, which are typically constituted by discrete, quasi-spherical nanoparticles with sizes below 10 nm. The first discovery of quantum dot-like nanocarbons can be dated back to 2004, when carbon nanoparticles with sizes at about 1 nm were isolated by Xu et al. [1] as a byproduct of preparation of single-walled carbon nanotubes, separated from carbon soot produced by arc discharge. However, it was until 2 years later, these new kind of nanocarbons have been documented as "carbon quantum dots" (CQDs) and boosted widespread interests, since the synthesis of fluorescent carbon nanoparticles with diameter less than 10 nm from Sun's group [2].

Distinct from other well developed nanocarbons, such as fullerenes, carbon nanotubes and graphene, CQDs show their unique advantages, including isotropic shapes, ultrafine dimensions,

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

tunable surface functionalities as well as the simple, fast and cheap preparations, which are attractive for a host of applications. Meanwhile, as a new member of quantum dot materials, CQDs also show potential as replacement for traditional toxic metal-based quantum dots currently in use, benefiting from their highly hydrophilic surface, high resistance to photobleaching, easy functionalization, chemical inertness, low toxicity and good biocompatibility. Until now, the applications of CQDs have been widely demonstrated in several fields, ranging from sensing, bioimaging to catalysis and energy conversion/storage.

oxidation, followed by gel electrophoresis separation to extract CQDs [1]. Subsequently, laser ablation of graphite using Ar as a carrier gas in the presence of water vapor was proved to be effective for the preparation of CQDs by means of the top-down route [2]. To date, CQDs have already been produced from graphite [10], graphene [11], carbon nanotubes (CNTs) [12] and other materials with sp2 carbon structure as a product of dimension reduction, meanwhile, through various physicochemical processes, including arc discharge [1], laser ablation [2], electrochemical exfoliation [13, 14] and chemical oxidation [15, 16]. However, to break down the chemical inert carbon structure of sp2 covalence bonds, the top-down process is invariably complicated with harsh conditions, some may be environmentally harmful, and thus not suitable for a large-scale production. In addition, effective control over the particle size of

Carbon Quantum Dots: A Component of Efficient Visible Light Photocatalysts

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49

On the contrary, the bottom-up methods offer opportunities to control the growth of CQDs by using organic molecular as carbon precursors. Carbonization reaction can be applied to various kinds of molecules, followed by aggregation in solution to produce CQDs. Pulsed laser irradiation of toluene [17], hydrothermal treatment of citric acid [18], electrochemical carbonization of low-molecular-weight alcohols [19] and microwave-assisted pyrolysis of citric acid formamide solution [20] have already been utilized for the preparation of CQDs. Recently, biomass molecules, such as sucrose [21], glucose [22], cellulose [23] and amino acid [24], have attracted great attentions as suitable precursors for the preparation of CQDs via dehydrate and further carbonize. Moreover, raw biomass is also suitable precursor for the preparation of CQDs, as a strategy potential for large-scale production. For instance, Sahu et al. [25] prepared 0.4 g of CQDs from a mixture of orange juice with ethanol by hydrothermal treatment at 120°C for 2.5 h (**Figure 2A**). While using chicken eggs as raw material, Chen and coworkers [26] prepared CQDs via plasma-induced pyrolysis with a yield up to 10 g, suitable for applications as fluorescent inks (**Figure 2B**). In comparison with the liquid biomass as precursor, solid biomass may provide a more practical solution to scale up the synthesis of CQDs due to their much longer shelf life. For example, hair can be used as carbon source for the synthesis of CQDs, in utilization of its main component of keratin. A one-step thermal treatment of hair at 200°C for about 24 h was reported by Guo et al. to obtain CQDs, with the yield estimated to

**Figure 2.** (A) Illustration of the formation of CQDs from orange juice by hydrothermal treatment. (B) Fabrication of CQDs from chicken egg and their application as fluorescent carbon inks. Reprinted with permission [25, 26]. Copyright

2012, Royal Society of Chemistry; Copyright 2012, Wiley-VCH.

CQDs product is often difficult to realize in such top-down processes.

**2.2. Bottom-up route**

This chapter will start with a brief overview about the preparation routes of CQDs, followed by a quick snapshot of their crystalline and electronic structure as well as the structureinduced optical properties. Then, the role of CQDs as an active component in modern visible light photocatalysis will be discussed in detail, based on a short review about works published in recent years.

### **2. Preparation methods**

The preparation of CQDs can be generally classified into "top-down" and "bottom-up" routes (**Figure 1**). The former involves breaking down of carbonaceous materials via chemical, electrochemical or physical approaches. On the contrary, the latter is realized by carbonization of small organic molecules followed by chemical fusion. A detailed review about the preparation of CQDs can be found in several review articles [3–9], and only a brief overview of recent progresses about the CQDs preparation will be given here.

### **2.1. Top-down route**

The first reported CQDs that isolated by chance from arc-discharged soot were produced by the top-down method, in which the carbonaceous soot was cleaved through nitric acid

**Figure 1.** Schematic illustration of CQD preparation via "top-down" and"bottom-up" approaches.

oxidation, followed by gel electrophoresis separation to extract CQDs [1]. Subsequently, laser ablation of graphite using Ar as a carrier gas in the presence of water vapor was proved to be effective for the preparation of CQDs by means of the top-down route [2]. To date, CQDs have already been produced from graphite [10], graphene [11], carbon nanotubes (CNTs) [12] and other materials with sp2 carbon structure as a product of dimension reduction, meanwhile, through various physicochemical processes, including arc discharge [1], laser ablation [2], electrochemical exfoliation [13, 14] and chemical oxidation [15, 16]. However, to break down the chemical inert carbon structure of sp2 covalence bonds, the top-down process is invariably complicated with harsh conditions, some may be environmentally harmful, and thus not suitable for a large-scale production. In addition, effective control over the particle size of CQDs product is often difficult to realize in such top-down processes.

#### **2.2. Bottom-up route**

tunable surface functionalities as well as the simple, fast and cheap preparations, which are attractive for a host of applications. Meanwhile, as a new member of quantum dot materials, CQDs also show potential as replacement for traditional toxic metal-based quantum dots currently in use, benefiting from their highly hydrophilic surface, high resistance to photobleaching, easy functionalization, chemical inertness, low toxicity and good biocompatibility. Until now, the applications of CQDs have been widely demonstrated in several fields, ranging from

This chapter will start with a brief overview about the preparation routes of CQDs, followed by a quick snapshot of their crystalline and electronic structure as well as the structureinduced optical properties. Then, the role of CQDs as an active component in modern visible light photocatalysis will be discussed in detail, based on a short review about works pub-

The preparation of CQDs can be generally classified into "top-down" and "bottom-up" routes (**Figure 1**). The former involves breaking down of carbonaceous materials via chemical, electrochemical or physical approaches. On the contrary, the latter is realized by carbonization of small organic molecules followed by chemical fusion. A detailed review about the preparation of CQDs can be found in several review articles [3–9], and only a brief overview of recent

The first reported CQDs that isolated by chance from arc-discharged soot were produced by the top-down method, in which the carbonaceous soot was cleaved through nitric acid

**Figure 1.** Schematic illustration of CQD preparation via "top-down" and"bottom-up" approaches.

sensing, bioimaging to catalysis and energy conversion/storage.

progresses about the CQDs preparation will be given here.

lished in recent years.

**2.1. Top-down route**

**2. Preparation methods**

48 Visible-Light Photocatalysis of Carbon-Based Materials

On the contrary, the bottom-up methods offer opportunities to control the growth of CQDs by using organic molecular as carbon precursors. Carbonization reaction can be applied to various kinds of molecules, followed by aggregation in solution to produce CQDs. Pulsed laser irradiation of toluene [17], hydrothermal treatment of citric acid [18], electrochemical carbonization of low-molecular-weight alcohols [19] and microwave-assisted pyrolysis of citric acid formamide solution [20] have already been utilized for the preparation of CQDs. Recently, biomass molecules, such as sucrose [21], glucose [22], cellulose [23] and amino acid [24], have attracted great attentions as suitable precursors for the preparation of CQDs via dehydrate and further carbonize. Moreover, raw biomass is also suitable precursor for the preparation of CQDs, as a strategy potential for large-scale production. For instance, Sahu et al. [25] prepared 0.4 g of CQDs from a mixture of orange juice with ethanol by hydrothermal treatment at 120°C for 2.5 h (**Figure 2A**). While using chicken eggs as raw material, Chen and coworkers [26] prepared CQDs via plasma-induced pyrolysis with a yield up to 10 g, suitable for applications as fluorescent inks (**Figure 2B**). In comparison with the liquid biomass as precursor, solid biomass may provide a more practical solution to scale up the synthesis of CQDs due to their much longer shelf life. For example, hair can be used as carbon source for the synthesis of CQDs, in utilization of its main component of keratin. A one-step thermal treatment of hair at 200°C for about 24 h was reported by Guo et al. to obtain CQDs, with the yield estimated to

**Figure 2.** (A) Illustration of the formation of CQDs from orange juice by hydrothermal treatment. (B) Fabrication of CQDs from chicken egg and their application as fluorescent carbon inks. Reprinted with permission [25, 26]. Copyright 2012, Royal Society of Chemistry; Copyright 2012, Wiley-VCH.

be about 95% [27]. Thanks to the diversity and plenty of biomass, the large-scale synthesis of CQDs from biomass is of great potential especially in view of practical applications.

## **3. Structures and properties of CQDs**

#### **3.1. Structures**

Generally, CQDs are quasi-spherical nanoparticles with diameter less than 10 nm, which can be amorphous or crystalline nanoparticles built up with sp2 carbon clusters. A typical fringe spacing of 0.34 nm can readily be distinguished from the high resolution transmission electron microscopy (HR-TEM) images of well crystalized CQDs, corresponding to the (002) interlayer spacing of graphite (**Figure 3A**) [28]. While as a special example of CQDs, graphene quantum dots (G-CQDs) usually possess a good crystallinity and layered structure (composed by less than 4, mostly a single graphene layer), with a crystal lattice spacing of 0.24 nm, corresponding to the (100) in-plane lattice parameter of graphene (**Figure 3B**) [29].

The modulated electronic structure of CQDs is unique among carbonaceous materials, which is dramatically size dependent. According to the quantum confinement effect (QCE), a reduced band gap of CQDs can be expected when the dot size increases. Li et al. [30] performed theoretical calculations to investigate the relationship between luminescence and cluster size of CQDs, who confirmed that the band gap reduced gradually as the size of CQDs increased (**Figure 3C**). Meanwhile, variable functional groups (such as hydroxyl and carboxyl) anchored on the surface of CQDs are indispensable for their excellent aqueous solubility, which can also greatly affect the electronic structure of CQDs. It has been demonstrated that different functional groups (either electron withdrawing or electron donating) simultaneously modulate the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels of CQDs, however, with relative small change in the band gap [31]. For instance, an electron donating group could raise both the HOMO and LUMO levels to higher energy, while an electron withdrawing group would cause a prominent down-shift of the total energy levels (**Figure 3D**). In addition, doping with heteroatoms such as nitrogen, sulfur were found to alter the electronic structures of CQDs significantly. The incorporation of ∼1 at% of S into CQDs could effectively modify their electronic structure by introducing S-related energy levels between π and π\* orbitals of CQDs, which would diversify the electron transition pathways and facilitate inter band crossing of electrons (**Figure 3E**) [32].

efficient multiphoton up-conversion, all of which can be effectively tailored by the artificial control of the size, shape, surface and doping, based on the remarkable quantum confinement

**Figure 3.** (A) TEM images of CQDs. (B) HR-TEM image of sp2 domains G-CQDs. (C) LUMO-HOMO gap dependence on the size of CQDs. (D) Energy diagrams of CQDs functionalized with different groups. (E) The proposed energy level diagram of S-doped CQDs. Reprinted with permission [28–32]. Copyright 2011, Wiley-VCH; copyright 2015, Royal Society of Chemistry; copyright 2010, Wiley-VCH; copyright 2015, American Chemical Society; copyright 2014, Royal

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CQDs are effective photon-harvesting agents especially in short-wavelength region, due to the π-π\* transition of C═C bonds. Typically, CQDs show strong optical absorption in the UV region (260–320 nm) with a tail extending into the visible range, in which a maximum peak around 230 nm is assigned to the π-π\* transition of aromatic C═C bonds and a shoulder at 300 nm is assigned to the n-π\* transition of C═O bonds or other surface groups [33]. In addition, the surface functional groups may also greatly contribute to the absorption at the UV-visible region, since a gradual red-shifted absorption can be observed after modification with some passivating agents. For instance, the absorbance of CQDs was found to increase to longer wavelength in the range of 350–550 nm after surface passivation with 4,7,10-trioxa-1,13-tridecanediamine (TTDDA) [34]. Moreover, doping with heteroatom is also effective to

effect (QCE), surface effect and edge effect.

*3.2.1. Absorbance*

Society of Chemistry.

Overall, the diversity of modulations for CQDs, from size control to surface functional groups exchange and heteroatom doping, makes it possible in the delicate control over their electronic structures and optical properties, showing potential as active components in light harvesting and utilizations.

#### **3.2. Optical properties**

Most notable, unique optical properties are discovered with CQDs, including wide-spectrum light harvesting from ultraviolet to near-infrared (NIR), tunable photoluminescence (PL) and Carbon Quantum Dots: A Component of Efficient Visible Light Photocatalysts http://dx.doi.org/10.5772/intechopen.70801 51

**Figure 3.** (A) TEM images of CQDs. (B) HR-TEM image of sp2 domains G-CQDs. (C) LUMO-HOMO gap dependence on the size of CQDs. (D) Energy diagrams of CQDs functionalized with different groups. (E) The proposed energy level diagram of S-doped CQDs. Reprinted with permission [28–32]. Copyright 2011, Wiley-VCH; copyright 2015, Royal Society of Chemistry; copyright 2010, Wiley-VCH; copyright 2015, American Chemical Society; copyright 2014, Royal Society of Chemistry.

efficient multiphoton up-conversion, all of which can be effectively tailored by the artificial control of the size, shape, surface and doping, based on the remarkable quantum confinement effect (QCE), surface effect and edge effect.

#### *3.2.1. Absorbance*

be about 95% [27]. Thanks to the diversity and plenty of biomass, the large-scale synthesis of

Generally, CQDs are quasi-spherical nanoparticles with diameter less than 10 nm, which can be amorphous or crystalline nanoparticles built up with sp2 carbon clusters. A typical fringe spacing of 0.34 nm can readily be distinguished from the high resolution transmission electron microscopy (HR-TEM) images of well crystalized CQDs, corresponding to the (002) interlayer spacing of graphite (**Figure 3A**) [28]. While as a special example of CQDs, graphene quantum dots (G-CQDs) usually possess a good crystallinity and layered structure (composed by less than 4, mostly a single graphene layer), with a crystal lattice spacing of 0.24 nm, correspond-

The modulated electronic structure of CQDs is unique among carbonaceous materials, which is dramatically size dependent. According to the quantum confinement effect (QCE), a reduced band gap of CQDs can be expected when the dot size increases. Li et al. [30] performed theoretical calculations to investigate the relationship between luminescence and cluster size of CQDs, who confirmed that the band gap reduced gradually as the size of CQDs increased (**Figure 3C**). Meanwhile, variable functional groups (such as hydroxyl and carboxyl) anchored on the surface of CQDs are indispensable for their excellent aqueous solubility, which can also greatly affect the electronic structure of CQDs. It has been demonstrated that different functional groups (either electron withdrawing or electron donating) simultaneously modulate the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels of CQDs, however, with relative small change in the band gap [31]. For instance, an electron donating group could raise both the HOMO and LUMO levels to higher energy, while an electron withdrawing group would cause a prominent down-shift of the total energy levels (**Figure 3D**). In addition, doping with heteroatoms such as nitrogen, sulfur were found to alter the electronic structures of CQDs significantly. The incorporation of ∼1 at% of S into CQDs could effectively modify their electronic structure by introducing S-related energy levels between π and π\* orbitals of CQDs, which would diversify the electron transition pathways and facilitate inter band crossing of electrons (**Figure 3E**) [32].

Overall, the diversity of modulations for CQDs, from size control to surface functional groups exchange and heteroatom doping, makes it possible in the delicate control over their electronic structures and optical properties, showing potential as active components in light har-

Most notable, unique optical properties are discovered with CQDs, including wide-spectrum light harvesting from ultraviolet to near-infrared (NIR), tunable photoluminescence (PL) and

CQDs from biomass is of great potential especially in view of practical applications.

ing to the (100) in-plane lattice parameter of graphene (**Figure 3B**) [29].

**3. Structures and properties of CQDs**

50 Visible-Light Photocatalysis of Carbon-Based Materials

**3.1. Structures**

vesting and utilizations.

**3.2. Optical properties**

CQDs are effective photon-harvesting agents especially in short-wavelength region, due to the π-π\* transition of C═C bonds. Typically, CQDs show strong optical absorption in the UV region (260–320 nm) with a tail extending into the visible range, in which a maximum peak around 230 nm is assigned to the π-π\* transition of aromatic C═C bonds and a shoulder at 300 nm is assigned to the n-π\* transition of C═O bonds or other surface groups [33]. In addition, the surface functional groups may also greatly contribute to the absorption at the UV-visible region, since a gradual red-shifted absorption can be observed after modification with some passivating agents. For instance, the absorbance of CQDs was found to increase to longer wavelength in the range of 350–550 nm after surface passivation with 4,7,10-trioxa-1,13-tridecanediamine (TTDDA) [34]. Moreover, doping with heteroatom is also effective to tailor the absorption of CQDs. As reported by Qu et al. [35], the absorption band of S,N-codoped CQDs moved into the visible region of 550–595 nm, showing potential as visible lightharvesting agent.

#### *3.2.2. Photoluminescence (PL)*

Tunable photoluminescence, with an emission peak ranging from deep ultraviolet to visible or even NIR region, is one of the most fascinating features of CQDs. Usually, these observed tunable PL properties are in close relationship with the QCE or surface effects of CQDs.

It is wildly accepted that the PL of CQDs originates from the emissive surface energy traps upon stabilization, since surface passivation is often indispensable for photoluminescent CQDs with high quantum yield (**Figure 4A**) [36]. Meanwhile, some researchers suggested that the PL in CQDs may be a result of radiative recombination of surface-confined electrons and holes, based on the observation that PL of CQDs can be effectively quenched by both electron donors and acceptors [37]. Significantly, this notion indicates that photo-excited CQDs are both good electron acceptors and electron donors. While in some cases, quantum size effect is the main reason for the observed size-dependent optical properties of CQDs. As reported by Li et al. [30], CQDs emitted blue, green, yellow, and orange, respectively, along with increased dot size under identical λex (**Figure 4B**). Since no appreciable change in the PL spectra after further H<sup>2</sup> plasma treatment to remove surface oxygen species, the luminescence emissions of CQDs was attributed to the quantum-sized graphite structure rather than the carbon-oxygen surface. However, contradict results also existed, as Bao et al. [28] observed a red-shifted PL emission peak when the sizes of CQDs decreased at the same λex. Based on the assumption that smaller CQDs were easily oxidized to take on more oxygen-related surface states, they suggested the red-shifted PL of CQDs was a result of variations in surface states rather than a size effect.

Therefore, the origin of the displayed optical properties of CQDs is still in debate, which may be probably in relationship with the competition between the emissive sites and non-radiative trap sites on the surface, as well as the quantum confinement effects determined by particle sizes.

#### *3.2.3. Up-conversion photoluminescence (UCPL)*

In contrary to the conventional down-conversion fluorescence emissions (Stokes PL), certain CQDs are also found to display up-conversion fluorescence emissions (anti-Stokes PL), with the emission wavelength shorter than the excitation wavelength, arising from the simultaneous absorption of two or multiple longer wavelength photons. UCPL is an attractive optical property of CQDs, which enables many promising applications, especially for photocatalysis in the visible to NIR spectrum.

at 450 nm was emitted from CQDs under an excitation of 800 nm femtosecond pulsed laser, and the UCPL intensity was found to relate to the excitation power density (**Figure 4C**).

**Figure 4.** (A) UV/vis absorption (Abs) and PL emission spectra (recorded for progressively longer excitation wavelengths from 320 to 520 nm) of PEG1500N passivated CQDs. (B) Optical images of CQDs illuminated under while (above, daylight lamp) and UV light (bottom, 365 nm). (C) UCPL spectra of CQDs under the excitation of a femtosecond pulsed laser at 800 nm. (D) Schematic illustration of various typical electronic transition processes of CQDs. Normal PL of small size (a) and large size (b); UCPL of large size (c) and large size (d). Reprinted with permission [30, 36, 39]. Copyright 2009,

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However, it was still under debate whether the multiphoton-active process was sufficient to explain the UCPL phenomenon of CQDs. In view of the practically constant energy difference of 1.1 eV between the excitation and emission, Shen et al. [40] proposed an anti-stokes transition mechanism using the energy level structural model, who postulated that low energy photons could excite π electrons (in intermediate levels) to LUMO followed by relaxation into σ orbital (HOMO) for the emission of a shorter wavelength photon (**Figure 4D**). Therefore, the constant energy difference here between excitation and emission light is ascribed to the

energy gap between π and σ orbitals.

Wiley-VCH; copyright 2010, Wiley-VCH; copyright 2013, Wiley-VCH.

The visible emission of ∼5 nm CQDs under the excitation of 800 nm femtosecond pulsed laser was first recorded by Cao et al. [38], who proposed a two-photon mechanism for the up-conversion emission. Later, several other groups observed similar UCPL from CQDs prepared via different synthetic routes. For instance, Gan et al. [39] demonstrated that blue light centered

Carbon Quantum Dots: A Component of Efficient Visible Light Photocatalysts http://dx.doi.org/10.5772/intechopen.70801 53

tailor the absorption of CQDs. As reported by Qu et al. [35], the absorption band of S,N-codoped CQDs moved into the visible region of 550–595 nm, showing potential as visible light-

Tunable photoluminescence, with an emission peak ranging from deep ultraviolet to visible or even NIR region, is one of the most fascinating features of CQDs. Usually, these observed tunable PL properties are in close relationship with the QCE or surface effects of CQDs.

It is wildly accepted that the PL of CQDs originates from the emissive surface energy traps upon stabilization, since surface passivation is often indispensable for photoluminescent CQDs with high quantum yield (**Figure 4A**) [36]. Meanwhile, some researchers suggested that the PL in CQDs may be a result of radiative recombination of surface-confined electrons and holes, based on the observation that PL of CQDs can be effectively quenched by both electron donors and acceptors [37]. Significantly, this notion indicates that photo-excited CQDs are both good electron acceptors and electron donors. While in some cases, quantum size effect is the main reason for the observed size-dependent optical properties of CQDs. As reported by Li et al. [30], CQDs emitted blue, green, yellow, and orange, respectively, along with increased dot size under identical λex (**Figure 4B**). Since no appreciable change in the PL

emissions of CQDs was attributed to the quantum-sized graphite structure rather than the carbon-oxygen surface. However, contradict results also existed, as Bao et al. [28] observed a red-shifted PL emission peak when the sizes of CQDs decreased at the same λex. Based on the assumption that smaller CQDs were easily oxidized to take on more oxygen-related surface states, they suggested the red-shifted PL of CQDs was a result of variations in surface states

Therefore, the origin of the displayed optical properties of CQDs is still in debate, which may be probably in relationship with the competition between the emissive sites and non-radiative trap sites on the surface, as well as the quantum confinement effects determined by particle

In contrary to the conventional down-conversion fluorescence emissions (Stokes PL), certain CQDs are also found to display up-conversion fluorescence emissions (anti-Stokes PL), with the emission wavelength shorter than the excitation wavelength, arising from the simultaneous absorption of two or multiple longer wavelength photons. UCPL is an attractive optical property of CQDs, which enables many promising applications, especially for photocatalysis

The visible emission of ∼5 nm CQDs under the excitation of 800 nm femtosecond pulsed laser was first recorded by Cao et al. [38], who proposed a two-photon mechanism for the up-conversion emission. Later, several other groups observed similar UCPL from CQDs prepared via different synthetic routes. For instance, Gan et al. [39] demonstrated that blue light centered

plasma treatment to remove surface oxygen species, the luminescence

harvesting agent.

*3.2.2. Photoluminescence (PL)*

52 Visible-Light Photocatalysis of Carbon-Based Materials

spectra after further H<sup>2</sup>

rather than a size effect.

*3.2.3. Up-conversion photoluminescence (UCPL)*

in the visible to NIR spectrum.

sizes.

**Figure 4.** (A) UV/vis absorption (Abs) and PL emission spectra (recorded for progressively longer excitation wavelengths from 320 to 520 nm) of PEG1500N passivated CQDs. (B) Optical images of CQDs illuminated under while (above, daylight lamp) and UV light (bottom, 365 nm). (C) UCPL spectra of CQDs under the excitation of a femtosecond pulsed laser at 800 nm. (D) Schematic illustration of various typical electronic transition processes of CQDs. Normal PL of small size (a) and large size (b); UCPL of large size (c) and large size (d). Reprinted with permission [30, 36, 39]. Copyright 2009, Wiley-VCH; copyright 2010, Wiley-VCH; copyright 2013, Wiley-VCH.

at 450 nm was emitted from CQDs under an excitation of 800 nm femtosecond pulsed laser, and the UCPL intensity was found to relate to the excitation power density (**Figure 4C**).

However, it was still under debate whether the multiphoton-active process was sufficient to explain the UCPL phenomenon of CQDs. In view of the practically constant energy difference of 1.1 eV between the excitation and emission, Shen et al. [40] proposed an anti-stokes transition mechanism using the energy level structural model, who postulated that low energy photons could excite π electrons (in intermediate levels) to LUMO followed by relaxation into σ orbital (HOMO) for the emission of a shorter wavelength photon (**Figure 4D**). Therefore, the constant energy difference here between excitation and emission light is ascribed to the energy gap between π and σ orbitals.

Moreover, Wen et al. [41] recently argued that the observed multiphoton excitation properties in some CQDs might be artificial and could originate from the normal fluorescence, which was excited by the second diffraction leaking in the monochromator of the fluorescence spectrophotometer. Therefore, great cautions should be paid to the observed UCPL of CQDs, and it was suggested that the exact up-conversion from CQDs would be excited under a high enough power density from a femtosecond pulsed laser.

electron mediator in placement of noble metals for new design of photocatalysts, CQDs show excellent electron conductivity as well as large electron-storage capacity, benefiting from the

On the one hand, CQDs can act as conductors for efficient charge carrier transport between adjacent semiconductors, due to the electron conductivity from the Ohmic contact at the semiconductor-CQDs interfaces. Miao et al. [42] fabricated a ternary visible light-driven photocata-

lytic efficiency for RhB degradation under visible irradiation (>420 nm), which was about 4.0,

thermodynamically favorable electron transfer upon band alignment, CQDs was believed to

On the other hand, CQDs can act as electron sink to accept photogenerated electrons from adjacent semiconductors, and thus to suppress the electron-hole recombination, benefiting from the large electron-storage capacity of carbon nanostructures. Huang et al. [43] obtained CQDs/

diation (λ > 420 nm). In a possible photocatalytic mechanism, the authors inferred that CQDs

both the separation and transfer efficiency of photogenerated charge carriers (**Figure 5B**). Wu et al. [44] demonstrated a case of a bi-quantum dot (QD) photocatalyst for overall water

photocatalysts via a simple hydrothermal route, which showed eight times larger

N4

. Reprinted with permission [42, 43]. Copyright 2017, Elsevier; copyright 2017, American Chemical Society.

ration of electron-hole pairs, responsible for the improved photocatalysis (**Figure 5A**).

N4

and g-C<sup>3</sup>

/CQDs/AgBr, by the in-situ growth of AgBr nanoparticles

Carbon Quantum Dots: A Component of Efficient Visible Light Photocatalysts

O4

QDs were combined together. Without any co-catalysts

QDs were believed to be reduction and oxidation reaction

O4

evolution of 0.92 μmol h−1 was

QDs. In such a two-electron path-

and AgBr. (B) CQDs as electron sink at the surface

/AgBr, respectively. Due to the

http://dx.doi.org/10.5772/intechopen.70801

55

to AgBr for an improved sepa-

alone under visible irra-

hybrids, to improve

nanosheets. The nanocomposite exhibited improved photocata-

N4

N4

N4

inherent sp2 graphitic carbon compositions.

N4

act as mediators to promote the electron transfer from g-C<sup>3</sup>

transient photocurrent response in comparison with that of ZnFe2

or sacrificial reagents but under solar light irradiation, H2

achieved with such hybrids, about four times that of BiVO4

**Figure 5.** (A) CQDs as electron conductor at the interface of C<sup>3</sup>

could function as an electron reservoir and transporter over CQDs/ZnFe2

5.3 and 2.3 times higher than that of AgBr, g-C<sup>3</sup>

lyst with the construction of g-C<sup>3</sup>

splitting, in which CQDs and BiVO4

way reaction, CQDs and BiVO4

on CQDs modified g-C<sup>3</sup>

ZnFe2 O4

of ZnFe2 O4

### **4. Applications in photocatalysis**

Based on the generation of charge carriers such as electrons and holes induced by photon absorption, photocatalysis has found great potentials in the application of both environmental remediation and energy conversion, ranging from pollutant degradation to H<sup>2</sup> generation and CO2 reduction. To generate carriers with sufficient redox potentials, wide band gap semiconductors (such as TiO2 ) are often used as photocatalysts, but at the cost of limited utilization of only high energy UV irradiation (~5% of sunlight). At the same time, recombination of charge carriers pervasively exists during their migration through the bulk to the surface in semiconductor particles, where to initiate photocatalytic reactions. Therefore, evolutions of photocatalytic system to overcome the low-usage of sunlight as well as high recombination rate of the photo-induced charge carriers is still an urgent need to improve the overall quantum efficiency for practical applications. Interestingly, all of the optical and structural properties of the CQDs mentioned above meet with the requirement of photocatalytic reactions (such as utilization toward solar spectrum, fast migration of charge carriers and efficient surface redox reactions), indicating the promising application of CQDs in photocatalysis. As a matter of fact, CQDs have shown their importance as a versatile component especially in visible light-driven photocatalysis since their discovery, ascribing to the tunable optical properties (such as absorbance, PL and UCPL) for excellent solar spectrum utilizations, also due to the electron accepting and transporting features of carbon nanomaterials that can direct the flow of photogenerated charge carriers. Therefore, CQDs can act not only as an efficient photocatalyst alone, but also as a multifunctional constituent in the design of photocatalyst for broadened light response and promoted electron-hole separation. In this section, we will summarize the fundamentally multifaceted roles of CQDs as an important component in visible light-driven photocatalysis.

#### **4.1. Electron mediators**

Photon-induced charge carriers with a sufficient amount that can migrate to the surface of semiconductors are at the core of photocatalytic reactions. Since charge carriers can easily be traps by various random defects, to collect or mediate the flow of charge carriers for efficient separation is necessary for an effective photocatalyst design. Noble metals with a large work function are commonly deposited onto the surface of semiconductors to act as an electron sink and help in an efficient separation of charge carriers. However, the use of noble metals shows a main drawback of poor economic viability due to their high expense. As a potential electron mediator in placement of noble metals for new design of photocatalysts, CQDs show excellent electron conductivity as well as large electron-storage capacity, benefiting from the inherent sp2 graphitic carbon compositions.

Moreover, Wen et al. [41] recently argued that the observed multiphoton excitation properties in some CQDs might be artificial and could originate from the normal fluorescence, which was excited by the second diffraction leaking in the monochromator of the fluorescence spectrophotometer. Therefore, great cautions should be paid to the observed UCPL of CQDs, and it was suggested that the exact up-conversion from CQDs would be excited under a high

Based on the generation of charge carriers such as electrons and holes induced by photon absorption, photocatalysis has found great potentials in the application of both environmen-

tion of only high energy UV irradiation (~5% of sunlight). At the same time, recombination of charge carriers pervasively exists during their migration through the bulk to the surface in semiconductor particles, where to initiate photocatalytic reactions. Therefore, evolutions of photocatalytic system to overcome the low-usage of sunlight as well as high recombination rate of the photo-induced charge carriers is still an urgent need to improve the overall quantum efficiency for practical applications. Interestingly, all of the optical and structural properties of the CQDs mentioned above meet with the requirement of photocatalytic reactions (such as utilization toward solar spectrum, fast migration of charge carriers and efficient surface redox reactions), indicating the promising application of CQDs in photocatalysis. As a matter of fact, CQDs have shown their importance as a versatile component especially in visible light-driven photocatalysis since their discovery, ascribing to the tunable optical properties (such as absorbance, PL and UCPL) for excellent solar spectrum utilizations, also due to the electron accepting and transporting features of carbon nanomaterials that can direct the flow of photogenerated charge carriers. Therefore, CQDs can act not only as an efficient photocatalyst alone, but also as a multifunctional constituent in the design of photocatalyst for broadened light response and promoted electron-hole separation. In this section, we will summarize the fundamentally multifaceted roles of CQDs as an important component in vis-

Photon-induced charge carriers with a sufficient amount that can migrate to the surface of semiconductors are at the core of photocatalytic reactions. Since charge carriers can easily be traps by various random defects, to collect or mediate the flow of charge carriers for efficient separation is necessary for an effective photocatalyst design. Noble metals with a large work function are commonly deposited onto the surface of semiconductors to act as an electron sink and help in an efficient separation of charge carriers. However, the use of noble metals shows a main drawback of poor economic viability due to their high expense. As a potential

reduction. To generate carriers with sufficient redox potentials, wide band gap semi-

) are often used as photocatalysts, but at the cost of limited utiliza-

generation

tal remediation and energy conversion, ranging from pollutant degradation to H<sup>2</sup>

enough power density from a femtosecond pulsed laser.

**4. Applications in photocatalysis**

54 Visible-Light Photocatalysis of Carbon-Based Materials

and CO2

conductors (such as TiO2

ible light-driven photocatalysis.

**4.1. Electron mediators**

On the one hand, CQDs can act as conductors for efficient charge carrier transport between adjacent semiconductors, due to the electron conductivity from the Ohmic contact at the semiconductor-CQDs interfaces. Miao et al. [42] fabricated a ternary visible light-driven photocatalyst with the construction of g-C<sup>3</sup> N4 /CQDs/AgBr, by the in-situ growth of AgBr nanoparticles on CQDs modified g-C<sup>3</sup> N4 nanosheets. The nanocomposite exhibited improved photocatalytic efficiency for RhB degradation under visible irradiation (>420 nm), which was about 4.0, 5.3 and 2.3 times higher than that of AgBr, g-C<sup>3</sup> N4 and g-C<sup>3</sup> N4 /AgBr, respectively. Due to the thermodynamically favorable electron transfer upon band alignment, CQDs was believed to act as mediators to promote the electron transfer from g-C<sup>3</sup> N4 to AgBr for an improved separation of electron-hole pairs, responsible for the improved photocatalysis (**Figure 5A**).

On the other hand, CQDs can act as electron sink to accept photogenerated electrons from adjacent semiconductors, and thus to suppress the electron-hole recombination, benefiting from the large electron-storage capacity of carbon nanostructures. Huang et al. [43] obtained CQDs/ ZnFe2 O4 photocatalysts via a simple hydrothermal route, which showed eight times larger transient photocurrent response in comparison with that of ZnFe2 O4 alone under visible irradiation (λ > 420 nm). In a possible photocatalytic mechanism, the authors inferred that CQDs could function as an electron reservoir and transporter over CQDs/ZnFe2 O4 hybrids, to improve both the separation and transfer efficiency of photogenerated charge carriers (**Figure 5B**). Wu et al. [44] demonstrated a case of a bi-quantum dot (QD) photocatalyst for overall water splitting, in which CQDs and BiVO4 QDs were combined together. Without any co-catalysts or sacrificial reagents but under solar light irradiation, H2 evolution of 0.92 μmol h−1 was achieved with such hybrids, about four times that of BiVO4 QDs. In such a two-electron pathway reaction, CQDs and BiVO4 QDs were believed to be reduction and oxidation reaction

**Figure 5.** (A) CQDs as electron conductor at the interface of C<sup>3</sup> N4 and AgBr. (B) CQDs as electron sink at the surface of ZnFe2 O4 . Reprinted with permission [42, 43]. Copyright 2017, Elsevier; copyright 2017, American Chemical Society.

center, respectively. As an effective electron acceptor, CQDs received electrons from VB of BiVO4 before transferring to surface-adsorbed water for hydrogen production.

Therefore, as electron mediators, CQDs show noble metal comparable ability as both conductor and sink for an efficient charge separation in association with semiconductor photocatalysts. At the same time, the relative low cost and easy preparation make CQDs an available alternative to the expensive noble metal as electron mediators in new design of photocatalysts.

#### **4.2. Photosensitizers**

Extending light harvesting, especially by increasing the absorbance of visible photons, has been a popular strategy to achieve improved efficiency for modern photocatalysis. While the extended absorption spectrum of semiconductors usually means narrowed bandgap and thus decreased redox ability, which often obstructs the design of visible light-driven photocatalysts. Using visible light photosensitizers that can transfer photogenerated electrons to the CB of semiconductor in contact has been widely employed to address this conflict. Quantum dots of CdS, CdSe, PbS, are well known photosensitizers with a relative narrow bandgap to achieve good light harvesting efficiency, however, still suffer from some inherent drawbacks, including high cost, instability against photocorrosion and dissolution of toxic heavy-metal ions. Recently discovered CQDs, as an intriguing member of the nanocarbon family, have shown great potentials as photosensitizers due to their broad absorption spectra and large absorption coefficients.

Zhang et al. [45] prepared CQDs with tunable size of 2–5 nm from graphitic polymeric micelles, which showed high water solubility due to the surface decorated carboxyl groups. The obtained CQDs were then loaded onto the surface of P25 TiO2 through a facial dispersionevaporation route, followed by thermal treatment at 300°C in air to remove organic passivation layer for a close contact at hetero interfaces. The cooperation of CQDs and TiO2 was believed to be responsible for the visible light photocatalytic degradation of MB, since either CQDs or TiO2 alone showed negligible activities. CQDs probably acted as a photosensitizer in the CQDs/TiO2 hybrid, which injected visible light excited electrons into the CB of TiO2 , responsible for the remarkably enhanced visible light photocatalytic activity (**Figure 6A**). Wu et al. [46] suggested that the surface groups of CQDs were not necessarily removed before using as photosensitizers. CQDs with an average size of 30 nm were prepared through hydrothermal carbonization of pentosan, followed by the preparation of CQDs/TiO2 composite through directly drying from the suspension of TiO2 in a CQDs aqueous solution. The CQDs with abundant oxygen-containing groups could act as a photosensitizer to absorb visible light, with excited electrons transferred to the CB of TiO2 . These sensitized electrons then reacted with O2 adsorbed on TiO2 surface to produce active oxygen radicals (O2 − ), which were responsible for the observed MB degradation. In another case, Yu et al. [47] reported the efficient photocatalytic H<sup>2</sup> evolution activity of CQDs/P25 composite under both UV-vis and visible light (λ > 450 nm) irradiation without the aid of any noble metal cocatalyst. Different from pure P25, CQDs/P25 exhibited visible light-driven photocatalytic activity for H2 production, ascribing to the photosensitizing ability of CQDs. Just like organic dyes, CQDs sensitized P25 into a visible light responsive "dyade" structure through the newly formed Ti─O─C bond, in

which photo-induced electrons transferred from the excited CQDs to CB states of TiO2

/CQDs hybrids and MB concentration (C/C0

production mechanism on the CQDs/TiO2

N4

2015, Wiley-VCH; copyright 2014, Royal Society of Chemistry; copyright 2016, Elsevier.

The characterized sp2 carbon composition of CQDs also contributes to the charge transfer ability as photosensitizers. In a recent report of Liu et al. [48], CQDs synthesized from bee pol-

thermal treatment. The composite has overcome the limited visible light absorption of C<sup>3</sup>

evolution of 88.1 μmol/h, 3.02 folds of pristine C<sup>3</sup>

N4

actions were suggested to effectively expand the visible light absorption via photosensitiza-

Overall, due to the excellent light harvesting ability over a broad spectrum, as well as the effective chemical binding toward semiconductor surface, carbon dots have shown their potential as alternatives to the scarce and toxic heavy-metal compounds that are currently

The UCPL of CQDs is of particular interests in this context, promising an emission of shorter wavelength photons upon the simultaneous absorption of two or multiple longer wavelength

tion and hence suppress the recombination of photo-induced carriers (**Figure 6C**).

production (**Figure 6B**).

/CQDs composites, and the phototcatalytic performance of C3

evolution rate under visible (λ > 420 nm) irradiation. Reprinted with permission [45, 47, 48]. Copyright

nanosheets for a 2D/0D type photocatalysts via hydro-

with varied CQDs contents under visible (λ > 450 nm) irradiation. (C)

Carbon Quantum Dots: A Component of Efficient Visible Light Photocatalysts

http://dx.doi.org/10.5772/intechopen.70801

57

N4

) versus photocatalytic reaction time. (B)

under UV and visible light (λ > 450 nm), and

N4

nanosheets network via π-π stacking inter-

lowed by reactions with protons for H<sup>2</sup>

evolution rates on CQDs/TiO2

N4

**Figure 6.** (A) TEM image of the TiO2

Schematic illustration for the H2

Schematic illustration of the C<sup>3</sup>

photocatalytic H<sup>2</sup>

for the H<sup>2</sup>

C3 N4

lens were used to couple with g-C<sup>3</sup>

dispersed CQDs uniformly anchored to the C3

used as photosensitizer for visible light harvesting.

showing an optimal H<sup>2</sup>

**4.3. Spectral converters**

, fol-

N4 ,

nanosheets. Well

and 0.2 wt%-CQDs/

Carbon Quantum Dots: A Component of Efficient Visible Light Photocatalysts http://dx.doi.org/10.5772/intechopen.70801 57

**Figure 6.** (A) TEM image of the TiO2 /CQDs hybrids and MB concentration (C/C0 ) versus photocatalytic reaction time. (B) Schematic illustration for the H2 production mechanism on the CQDs/TiO2 under UV and visible light (λ > 450 nm), and photocatalytic H<sup>2</sup> evolution rates on CQDs/TiO2 with varied CQDs contents under visible (λ > 450 nm) irradiation. (C) Schematic illustration of the C<sup>3</sup> N4 /CQDs composites, and the phototcatalytic performance of C3 N4 and 0.2 wt%-CQDs/ C3 N4 for the H<sup>2</sup> evolution rate under visible (λ > 420 nm) irradiation. Reprinted with permission [45, 47, 48]. Copyright 2015, Wiley-VCH; copyright 2014, Royal Society of Chemistry; copyright 2016, Elsevier.

which photo-induced electrons transferred from the excited CQDs to CB states of TiO2 , followed by reactions with protons for H<sup>2</sup> production (**Figure 6B**).

The characterized sp2 carbon composition of CQDs also contributes to the charge transfer ability as photosensitizers. In a recent report of Liu et al. [48], CQDs synthesized from bee pollens were used to couple with g-C<sup>3</sup> N4 nanosheets for a 2D/0D type photocatalysts via hydrothermal treatment. The composite has overcome the limited visible light absorption of C<sup>3</sup> N4 , showing an optimal H<sup>2</sup> evolution of 88.1 μmol/h, 3.02 folds of pristine C<sup>3</sup> N4 nanosheets. Well dispersed CQDs uniformly anchored to the C3 N4 nanosheets network via π-π stacking interactions were suggested to effectively expand the visible light absorption via photosensitization and hence suppress the recombination of photo-induced carriers (**Figure 6C**).

Overall, due to the excellent light harvesting ability over a broad spectrum, as well as the effective chemical binding toward semiconductor surface, carbon dots have shown their potential as alternatives to the scarce and toxic heavy-metal compounds that are currently used as photosensitizer for visible light harvesting.

#### **4.3. Spectral converters**

center, respectively. As an effective electron acceptor, CQDs received electrons from VB of

Therefore, as electron mediators, CQDs show noble metal comparable ability as both conductor and sink for an efficient charge separation in association with semiconductor photocatalysts. At the same time, the relative low cost and easy preparation make CQDs an available alternative to the expensive noble metal as electron mediators in new design of photocatalysts.

Extending light harvesting, especially by increasing the absorbance of visible photons, has been a popular strategy to achieve improved efficiency for modern photocatalysis. While the extended absorption spectrum of semiconductors usually means narrowed bandgap and thus decreased redox ability, which often obstructs the design of visible light-driven photocatalysts. Using visible light photosensitizers that can transfer photogenerated electrons to the CB of semiconductor in contact has been widely employed to address this conflict. Quantum dots of CdS, CdSe, PbS, are well known photosensitizers with a relative narrow bandgap to achieve good light harvesting efficiency, however, still suffer from some inherent drawbacks, including high cost, instability against photocorrosion and dissolution of toxic heavy-metal ions. Recently discovered CQDs, as an intriguing member of the nanocarbon family, have shown great potentials as photosensitizers due to their broad absorption spectra and large

Zhang et al. [45] prepared CQDs with tunable size of 2–5 nm from graphitic polymeric micelles, which showed high water solubility due to the surface decorated carboxyl groups.

evaporation route, followed by thermal treatment at 300°C in air to remove organic passivation layer for a close contact at hetero interfaces. The cooperation of CQDs and TiO2

believed to be responsible for the visible light photocatalytic degradation of MB, since either

responsible for the remarkably enhanced visible light photocatalytic activity (**Figure 6A**). Wu et al. [46] suggested that the surface groups of CQDs were not necessarily removed before using as photosensitizers. CQDs with an average size of 30 nm were prepared through hydrothermal carbonization of pentosan, followed by the preparation of CQDs/TiO2

CQDs with abundant oxygen-containing groups could act as a photosensitizer to absorb vis-

responsible for the observed MB degradation. In another case, Yu et al. [47] reported the effi-

ible light (λ > 450 nm) irradiation without the aid of any noble metal cocatalyst. Different from

ascribing to the photosensitizing ability of CQDs. Just like organic dyes, CQDs sensitized P25 into a visible light responsive "dyade" structure through the newly formed Ti─O─C bond, in

pure P25, CQDs/P25 exhibited visible light-driven photocatalytic activity for H2

alone showed negligible activities. CQDs probably acted as a photosensitizer

hybrid, which injected visible light excited electrons into the CB of TiO2

surface to produce active oxygen radicals (O2

evolution activity of CQDs/P25 composite under both UV-vis and vis-

through a facial dispersion-

in a CQDs aqueous solution. The

. These sensitized electrons then

−

was

com-

), which were

production,

,

The obtained CQDs were then loaded onto the surface of P25 TiO2

posite through directly drying from the suspension of TiO2

ible light, with excited electrons transferred to the CB of TiO2

adsorbed on TiO2

before transferring to surface-adsorbed water for hydrogen production.

BiVO4

**4.2. Photosensitizers**

56 Visible-Light Photocatalysis of Carbon-Based Materials

absorption coefficients.

CQDs or TiO2

reacted with O2

cient photocatalytic H<sup>2</sup>

in the CQDs/TiO2

The UCPL of CQDs is of particular interests in this context, promising an emission of shorter wavelength photons upon the simultaneous absorption of two or multiple longer wavelength photons. Such a spectral converting ability of CQDs provides an attractive strategy for the design of visible light- or even NIR-responsive photocatalysts. A large number of recent work concerning about the photocatalytic applications of CQDs have suggested the contribution of their UCPL properties. Ke et al. [49] used TiO2 photocatalysts modified with CQDs to obtain a 3.6 times higher rate of MB degradation under visible light irradiation (λ > 420 nm) when compared with pure TiO2 , and the enhanced photocatalysis was mainly ascribed to the increased amount of oxidative radicals in hypothesis of the up-conversion process from CQDs (**Figure 7A**). Similar results have also been observed with CQDs/g-C<sup>3</sup> N4 hybrids, as shown by Zhang et al. [50]. After impregnation of CQDs into g-C<sup>3</sup> N4 , a 3.7-fold enhancement in phenol degradation rate constant can be obtained compared with that of the pristine g-C<sup>3</sup> N4 under visible light irradiation (λ > 400 nm). As suggested by the authors, light of λ > 550 nm could be converted to λ < 460 nm photons by CQDs, and subsequently excited g-C<sup>3</sup> N4 to generate electron-hole pairs (**Figure 7B**). UCPL character of CQDs that enhanced the production of excitons by extending the visible light-absorption region was supposed to be one important contribution to the observed enhanced photocatalysis, along with the efficient charge separation arising from the band structure alignment.

was observed under UV irradiation, but only with negligible efficiency under visible light. Subsequently, the authors proposed that the up-converted PL may not play the crucial role for the improved activity in this system, instead, molecular oxygen activated by the oxygen defects on the surface of N-CQDs may be responsible for the organic removal under UV

Carbon Quantum Dots: A Component of Efficient Visible Light Photocatalysts

In fact, no direct evidence has been provided to confirm the contribution of UCPL to the improved photocatalytic activities postulated in many CQDs based photocatalytic systems. The generally poor efficiency of up-conversion process in most cases, if such an effect exists, may lead to its small influence compared to other optoelectronic contributions. Yu et al. [52] magnified the UCPL of CQDs by means of fluorescence resonance energy transfer (FRET) assisted up-conversion process, in which rhodamine B (RhB) was combined with naked CQDs of 3.4 nm to make a FRET pair. The FRET effect was used to magnify the up-conversion

generating 18 times higher photocurrent at λ > 500 nm than those without RhB molecules

The origin of light absorption in CQDs stems from π-π\* (C═C) and n-π\* (C═O) transitions in the core and on the surface of the particles, respectively, which means that the optoelectronic properties of CQDs will be readily tuned by both size modulation and chemical modification. The size effect largely arises from the confinement, causing a separation of π and π\* orbitals prominently when the domain size is less than 10 nm. At the same time, CQDs rich in surface oxygen groups are usually p-doped due to the larger electronegativity of oxygen atoms relative to carbon atoms. Replacing oxygen functional groups on the CQDs surface with nitrogencontaining groups can readily transform CQDs into an n-type semiconductor. In addition to surface modification, direct substitution with heteroatoms in the graphene lattice also greatly affects the optical and electronic properties of CQDs. These excellent properties have gained CDs with an adjustable band structure together with broadband light-absorption ability,

Although CQDs alone as photocatalysts show great potential, relatively few examples of CQDs solely with satisfying photocatalytic activity have been recorded to date, partially due to the relative poor electron transfer inside the CQDs. Doping with heteroatoms has been proved to be an effective way to promote electron transfer and thus the performance of the CQDs as photocatalyst. Cu–N co-doping of CQDs was achieved by Wu et al. [53] through

of the Cu─N doping, the electron accepting and donating abilities of CQDs could be both enhanced, together with the increased electric conductivity. These merits ultimately facilitated the entire electron-transfer process in CQDs and further improved the photocatalytic oxidation of 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate (1,4-DHP). In another case provided by Martindale et al. [54], N doped CQDs were synthesized by pyrolysis of aspartic acid (Asp) in air at 320°C. CQDs with N doping showed increased specific activity of

PO4

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59

[Cu(EDTA)] as precursor (**Figure 8A**). As a result

evolution system, with an

particles and thus

of CQDs, with the up-converted photons subsequently sensitized Ag<sup>3</sup>

which inspire their applications as efficient photocatalyst.

7950 μmolH2 (gCD)−1 h−1 in the solar (AM1.5G) photocatalytic H2

an one-step pyrolytic synthesis with Na2

irradiation.

(**Figure 7C**).

**4.4. Sole photocatalyst**

However, some researchers clarified that UCPL of CQDs might not be the main reason for the improved photocatalytic activity after CQDs introduction. In a recent work of Di et al. [51], improved photocatalytic degradation of ciprofloxacin on N-CQDs/BiPO4 composite

**Figure 7.** (A) UV-vis absorption spectra of TiO2 and CQDs/TiO2 hybrid, and the EPR spectra of hydroxyl radicals over the CQDs/TiO2 hybrid under visible irradiation (λ > 420 nm). (B) Apparent rate constants of phenol degradation over CQDs modified C<sup>3</sup> N4 photocatalysts with varied loading amount, and the schematic illustration of the possible photocatalytic process on the C3 N4 /CQDs hybrid under visible light (λ > 400 nm). (C) Transient photocurrent responses at an applied potential of 0 V under visible irradiation (λ > 500 nm) for photocatalysts with different composition, and the schematic energy band diagram of the RhB/CQDs/Ag<sup>3</sup> PO4 nanocomposite. Reprinted with permission [49, 50, 52]. Copyright 2017, Elsevier; copyright 2016, Elsevier; copyright 2016, Elsevier.

was observed under UV irradiation, but only with negligible efficiency under visible light. Subsequently, the authors proposed that the up-converted PL may not play the crucial role for the improved activity in this system, instead, molecular oxygen activated by the oxygen defects on the surface of N-CQDs may be responsible for the organic removal under UV irradiation.

In fact, no direct evidence has been provided to confirm the contribution of UCPL to the improved photocatalytic activities postulated in many CQDs based photocatalytic systems. The generally poor efficiency of up-conversion process in most cases, if such an effect exists, may lead to its small influence compared to other optoelectronic contributions. Yu et al. [52] magnified the UCPL of CQDs by means of fluorescence resonance energy transfer (FRET) assisted up-conversion process, in which rhodamine B (RhB) was combined with naked CQDs of 3.4 nm to make a FRET pair. The FRET effect was used to magnify the up-conversion of CQDs, with the up-converted photons subsequently sensitized Ag<sup>3</sup> PO4 particles and thus generating 18 times higher photocurrent at λ > 500 nm than those without RhB molecules (**Figure 7C**).

#### **4.4. Sole photocatalyst**

photons. Such a spectral converting ability of CQDs provides an attractive strategy for the design of visible light- or even NIR-responsive photocatalysts. A large number of recent work concerning about the photocatalytic applications of CQDs have suggested the contribution

obtain a 3.6 times higher rate of MB degradation under visible light irradiation (λ > 420 nm)

the increased amount of oxidative radicals in hypothesis of the up-conversion process from

in phenol degradation rate constant can be obtained compared with that of the pristine g-C<sup>3</sup>

under visible light irradiation (λ > 400 nm). As suggested by the authors, light of λ > 550 nm

erate electron-hole pairs (**Figure 7B**). UCPL character of CQDs that enhanced the production of excitons by extending the visible light-absorption region was supposed to be one important contribution to the observed enhanced photocatalysis, along with the efficient charge separa-

However, some researchers clarified that UCPL of CQDs might not be the main reason for the improved photocatalytic activity after CQDs introduction. In a recent work of Di et al. [51], improved photocatalytic degradation of ciprofloxacin on N-CQDs/BiPO4 composite

CQDs (**Figure 7A**). Similar results have also been observed with CQDs/g-C<sup>3</sup>

could be converted to λ < 460 nm photons by CQDs, and subsequently excited g-C<sup>3</sup>

and CQDs/TiO2

PO4

hybrid under visible irradiation (λ > 420 nm). (B) Apparent rate constants of phenol degradation over CQDs

potential of 0 V under visible irradiation (λ > 500 nm) for photocatalysts with different composition, and the schematic

photocatalysts with varied loading amount, and the schematic illustration of the possible photocatalytic

/CQDs hybrid under visible light (λ > 400 nm). (C) Transient photocurrent responses at an applied

shown by Zhang et al. [50]. After impregnation of CQDs into g-C<sup>3</sup>

photocatalysts modified with CQDs to

N4

, a 3.7-fold enhancement

N4

hybrids, as

N4

to gen-

, and the enhanced photocatalysis was mainly ascribed to

N4

hybrid, and the EPR spectra of hydroxyl radicals over the

nanocomposite. Reprinted with permission [49, 50, 52]. Copyright 2017,

of their UCPL properties. Ke et al. [49] used TiO2

tion arising from the band structure alignment.

**Figure 7.** (A) UV-vis absorption spectra of TiO2

energy band diagram of the RhB/CQDs/Ag<sup>3</sup>

Elsevier; copyright 2016, Elsevier; copyright 2016, Elsevier.

CQDs/TiO2

modified C<sup>3</sup>

process on the C3

N4

N4

when compared with pure TiO2

58 Visible-Light Photocatalysis of Carbon-Based Materials

The origin of light absorption in CQDs stems from π-π\* (C═C) and n-π\* (C═O) transitions in the core and on the surface of the particles, respectively, which means that the optoelectronic properties of CQDs will be readily tuned by both size modulation and chemical modification. The size effect largely arises from the confinement, causing a separation of π and π\* orbitals prominently when the domain size is less than 10 nm. At the same time, CQDs rich in surface oxygen groups are usually p-doped due to the larger electronegativity of oxygen atoms relative to carbon atoms. Replacing oxygen functional groups on the CQDs surface with nitrogencontaining groups can readily transform CQDs into an n-type semiconductor. In addition to surface modification, direct substitution with heteroatoms in the graphene lattice also greatly affects the optical and electronic properties of CQDs. These excellent properties have gained CDs with an adjustable band structure together with broadband light-absorption ability, which inspire their applications as efficient photocatalyst.

Although CQDs alone as photocatalysts show great potential, relatively few examples of CQDs solely with satisfying photocatalytic activity have been recorded to date, partially due to the relative poor electron transfer inside the CQDs. Doping with heteroatoms has been proved to be an effective way to promote electron transfer and thus the performance of the CQDs as photocatalyst. Cu–N co-doping of CQDs was achieved by Wu et al. [53] through an one-step pyrolytic synthesis with Na2 [Cu(EDTA)] as precursor (**Figure 8A**). As a result of the Cu─N doping, the electron accepting and donating abilities of CQDs could be both enhanced, together with the increased electric conductivity. These merits ultimately facilitated the entire electron-transfer process in CQDs and further improved the photocatalytic oxidation of 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate (1,4-DHP). In another case provided by Martindale et al. [54], N doped CQDs were synthesized by pyrolysis of aspartic acid (Asp) in air at 320°C. CQDs with N doping showed increased specific activity of 7950 μmolH2 (gCD)−1 h−1 in the solar (AM1.5G) photocatalytic H2 evolution system, with an

**5. Summary and outlook**

**Acknowledgements**

**Author details**

**References**

Shan Cong\* and Zhigang Zhao

scale production especially for technological purpose.

\*Address all correspondence to: Scong2012@sinano.ac.cn

Chinese Academy of Sciences (CAS), SuzhouPR, China

After a decade of extensive investigation, CQDs have already been developed to be one of the most important members of modern nanocarbon family. Unexpected optoelectronic properties have been observed with this initially emergent nanocarbon material, including UV-visible absorbance, tunable PL and unique UCPL, together with electronic conductivity, showing the opportunity of further adjustment through surface functionalization and heteroatom doping. Specially, the excellent light harvesting capability and efficient photo-induced electron transfer ability make CQDs an exceptional candidate in visible light-driven photocatalytic applications, which play multifaceted roles as electron mediator, photosensitizer, spectral converter, or photocatalyst. Characterized by low cost, easy preparation, water soluble and nontoxic, CQDs have served as a common replacement for toxic heavy metal-based QDs and high-expense noble metals as co-catalysts. In addition, benefiting from the generally simple and economic synthesis of CQDs out of a wide spectrum of biomass carbon sources, the application of CQDs as photocatalytic component is of great potential in view of large-

Carbon Quantum Dots: A Component of Efficient Visible Light Photocatalysts

http://dx.doi.org/10.5772/intechopen.70801

61

This work was supported by the National Natural Science Foundation of China (51372266, 51572286, and 21503266) and the Outstanding Youth Fund of Jiangsu Province (BK20160011).

Key Lab of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics,

[1] Xu X, Ray R, Gu Y, Ploehn HJ, Gearheart L, Raker K, Scrivens WA. Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments. Journal of the American Chemical Society. 2004;**126**(40):12736-12737. DOI: 10.1021/ja040082h [2] Zhou J, Booker C, Li R, Zhou X, Sham TK, Sun X, Ding Z. An electrochemical avenue to blue luminescent nanocrystals from multiwalled carbon nanotubes (MWCNTs). Journal

of the American Chemical Society. 2007;**129**(4):744-745. DOI: 10.1021/ja0669070

**Figure 8.** (A) (a) The synthetic procedure, (b) TEM images and (c) UV/vis absorption spectra of doped CQDs. (B) (a) UV/vis absorption spectra of N doped CQDs (g-N-CD), graphite CQDs (g-CD) and amorphous CQDs (a-CD), (b) photocatalytic H<sup>2</sup> generation with various amounts of CQDs (0.1–1 mg) under simulated solar irradiation (AM 1.5G, 100 mW cm−2). Reprinted with permission [53–55]. Copyright 2015, Wiley-VCH; copyright 2017, Wiley-VCH; copyright 2014, Wiley-VCH.

order of magnitude improvement realized compared to undoped CQDs (**Figure 8B**). The authors suggested that nitrogen doping in CQDs increased the efficiency of hole-scavenging through electron donor effect and thereby significantly extended the lifetime of the photogenerated electrons.

Moreover, the electronic structure of CQDs can also be dramatically changed after heteroatomic doping. By treating GO in NH<sup>3</sup> at 500°C followed by oxidation with a modified Hummers' method, Yeh et al. [55] obtained N doped CQDs and applied as visible light (λ > 420 nm) driven photocatalyst for the overall water-splitting. Based on the electrochemical Mott-Schottky analysis, N doped CDs showed both p- and n-type conductivities, with the n-conductivity caused by embedding nitrogen atoms in the graphene frame and the p-conductivity by grafting oxygen functionalities on the graphene surface. The inherent sp2 clusters of CQDs served as the junction between the p- and n-domains to form buitin p-n diodes, which were responsible for an internal Z-scheme charge transfer at the QD interface, similar to that of biological photosynthesis (**Figure 8C**).

Therefore, by suitably adjusting the size, defect densities, atom doping and surface functional groups, it is possible to tune the electronic structure of CQDs and thus the optical response over a wide spectrum, allowing the potential application as effective photocatalysts with full utilization of the solar light.

### **5. Summary and outlook**

After a decade of extensive investigation, CQDs have already been developed to be one of the most important members of modern nanocarbon family. Unexpected optoelectronic properties have been observed with this initially emergent nanocarbon material, including UV-visible absorbance, tunable PL and unique UCPL, together with electronic conductivity, showing the opportunity of further adjustment through surface functionalization and heteroatom doping. Specially, the excellent light harvesting capability and efficient photo-induced electron transfer ability make CQDs an exceptional candidate in visible light-driven photocatalytic applications, which play multifaceted roles as electron mediator, photosensitizer, spectral converter, or photocatalyst. Characterized by low cost, easy preparation, water soluble and nontoxic, CQDs have served as a common replacement for toxic heavy metal-based QDs and high-expense noble metals as co-catalysts. In addition, benefiting from the generally simple and economic synthesis of CQDs out of a wide spectrum of biomass carbon sources, the application of CQDs as photocatalytic component is of great potential in view of largescale production especially for technological purpose.

### **Acknowledgements**

This work was supported by the National Natural Science Foundation of China (51372266, 51572286, and 21503266) and the Outstanding Youth Fund of Jiangsu Province (BK20160011).

### **Author details**

order of magnitude improvement realized compared to undoped CQDs (**Figure 8B**). The authors suggested that nitrogen doping in CQDs increased the efficiency of hole-scavenging through electron donor effect and thereby significantly extended the lifetime of the photogen-

**Figure 8.** (A) (a) The synthetic procedure, (b) TEM images and (c) UV/vis absorption spectra of doped CQDs. (B) (a) UV/vis absorption spectra of N doped CQDs (g-N-CD), graphite CQDs (g-CD) and amorphous CQDs (a-CD), (b)

100 mW cm−2). Reprinted with permission [53–55]. Copyright 2015, Wiley-VCH; copyright 2017, Wiley-VCH; copyright

generation with various amounts of CQDs (0.1–1 mg) under simulated solar irradiation (AM 1.5G,

Moreover, the electronic structure of CQDs can also be dramatically changed after het-

Hummers' method, Yeh et al. [55] obtained N doped CQDs and applied as visible light (λ > 420 nm) driven photocatalyst for the overall water-splitting. Based on the electrochemical Mott-Schottky analysis, N doped CDs showed both p- and n-type conductivities, with the n-conductivity caused by embedding nitrogen atoms in the graphene frame and the p-conductivity by grafting oxygen functionalities on the graphene surface. The inherent sp2 clusters of CQDs served as the junction between the p- and n-domains to form buitin p-n diodes, which were responsible for an internal Z-scheme charge transfer at the QD interface, similar

Therefore, by suitably adjusting the size, defect densities, atom doping and surface functional groups, it is possible to tune the electronic structure of CQDs and thus the optical response over a wide spectrum, allowing the potential application as effective photocatalysts with full

at 500°C followed by oxidation with a modified

erated electrons.

photocatalytic H<sup>2</sup>

2014, Wiley-VCH.

eroatomic doping. By treating GO in NH<sup>3</sup>

60 Visible-Light Photocatalysis of Carbon-Based Materials

to that of biological photosynthesis (**Figure 8C**).

utilization of the solar light.

Shan Cong\* and Zhigang Zhao

\*Address all correspondence to: Scong2012@sinano.ac.cn

Key Lab of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS), SuzhouPR, China

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**Chapter 5**

**Provisional chapter**

**Graphene-Semiconductor Composites as Visible Light-**

Graphene-based composites produced by the incorporation of graphene into suitable semiconductors doped with various metals enable to induce the unique properties of the graphene, such as extended light absorption range, charge separation, and so high dye adsorption capacity. Therefore, graphene-based composites can provide to enhance the overall photocatalytic performance of the base semiconductor under the visible-light, and to open up new pathways to high-performance photocatalyst for the future applications. This chapter of the book focuses on the structural and optical properties of the graphene-semiconductor-based composite structure. Furthermore, final photocatalytic properties of the graphene-based composites can be controlled by applying different synthesis routes. Common synthesis methods of the graphene-semiconductor composites such as sol-gel, solution mixing, *in situ* growth, hydrothermal growth, and solvothermal method are discussed on the resultant visible-light photocatalytic property of the these composites. At the same time, doping of the graphene-semiconductor material with metal ions also allows an improvement of the visible light-induced photocatalytic activity. Therefore, studies related with the effect of the dopant agent on the visible light

**Keywords:** graphene-semiconductor composites, photocatalyst, visible light degradation,

Environmental water contaminations originated from the organic dye compounds used in textile and paint industries and household chemicals are a continuous threat to human health. Generally, water treatment methods include primary and secondary treatment steps. While primary treatment of water is designed to physically remove organic solids by sedimentation and flotation, secondary treatment is aimed to biologically remove the residual organics from

**Graphene-Semiconductor Composites as Visible** 

DOI: 10.5772/intechopen.70709

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Induced Photocatalyst**

**Light-Induced Photocatalyst**

http://dx.doi.org/10.5772/intechopen.70709

**Abstract**

**1. Introduction**

Ozlem Altintas Yildirim and Teoman Ozturk

Ozlem Altintas Yildirim and Teoman Ozturk

Additional information is available at the end of the chapter

photocatalytic activity are also reviewed in this chapter.

structural-electronic properties, synthesis methods

Additional information is available at the end of the chapter


**Provisional chapter**

### **Graphene-Semiconductor Composites as Visible Light-Induced Photocatalyst Light-Induced Photocatalyst**

**Graphene-Semiconductor Composites as Visible** 

DOI: 10.5772/intechopen.70709

Ozlem Altintas Yildirim and Teoman Ozturk Ozlem Altintas Yildirim and Teoman Ozturk

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70709

#### **Abstract**

[52] Yu S, Lee SY, Umh HN, Yi J. Energy conversion of sub-band-gap light using naked carbon nanodots and rhodamine B. Nano Energy. 2016;**26**:479-487. DOI: 10.1016/j.

[53] Wu W, Zhan L, Fan W, Song J, Li X, Li Z, Wang R, Zhang J, Zheng J, Wu M, Zeng H. Cu–N dopants boost electron transfer and photooxidation reactions of carbon dots. Angewandte Chemie, International Edition. 2015;**54**(22):6540-6544. DOI: 10.1002/

[54] Martindale BCM, Hutton GAM, Caputo CA, Prantl S, Godin R, Durrant JR, Reisner E. Enhancing light absorption and charge transfer efficiency in carbon dots through graphitization and core nitrogen doping. Angewandte Chemie, International Edition.

[55] Yeh TF, Teng CY, Chen SJ, Teng H. Nitrogen-doped graphene oxide quantum dots as photocatalysts for overall water-splitting under visible light illumination. Advanced

2017;**56**(23):6459-6463. DOI: 10.1002/anie.201700949

Materials. 2014;**26**(20):3297-3303. DOI: 10.1002/adma.201305299

nanoen.2016.06.008

66 Visible-Light Photocatalysis of Carbon-Based Materials

anie.201501912

Graphene-based composites produced by the incorporation of graphene into suitable semiconductors doped with various metals enable to induce the unique properties of the graphene, such as extended light absorption range, charge separation, and so high dye adsorption capacity. Therefore, graphene-based composites can provide to enhance the overall photocatalytic performance of the base semiconductor under the visible-light, and to open up new pathways to high-performance photocatalyst for the future applications. This chapter of the book focuses on the structural and optical properties of the graphene-semiconductor-based composite structure. Furthermore, final photocatalytic properties of the graphene-based composites can be controlled by applying different synthesis routes. Common synthesis methods of the graphene-semiconductor composites such as sol-gel, solution mixing, *in situ* growth, hydrothermal growth, and solvothermal method are discussed on the resultant visible-light photocatalytic property of the these composites. At the same time, doping of the graphene-semiconductor material with metal ions also allows an improvement of the visible light-induced photocatalytic activity. Therefore, studies related with the effect of the dopant agent on the visible light photocatalytic activity are also reviewed in this chapter.

**Keywords:** graphene-semiconductor composites, photocatalyst, visible light degradation, structural-electronic properties, synthesis methods

#### **1. Introduction**

Environmental water contaminations originated from the organic dye compounds used in textile and paint industries and household chemicals are a continuous threat to human health. Generally, water treatment methods include primary and secondary treatment steps. While primary treatment of water is designed to physically remove organic solids by sedimentation and flotation, secondary treatment is aimed to biologically remove the residual organics from

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

wastewater by utilizing microorganisms in a controlled environment [1]. As an advanced treatment step, desalination technologies such as reverse osmosis, electrodialysis, ion exchange, and freeze desalination were applied. However, the above-listed traditional biological treatment techniques can be ineffective in decolorizing dye molecules. Due to the specific needs during the degradation of organic pollutants, there has been a strong interest in the development of efficient treatment method.

[15], sensors [16], nano electronics [17], energy conversion [18], and water treatment [4]. In the past few decades, there is an ever-increasing interest in the photocatalytic applications of

Graphene-Semiconductor Composites as Visible Light-Induced Photocatalyst

To improve the photocatalytic efficiency of the semiconductor-based metal oxides and graphenebased photocatalysts under the visible-light, integration of semiconductor metal oxides with graphene has been one the most popular approach due to theoretically large specific-surface area and high intrinsic electron mobility of graphene at room temperature [3]. Graphene behaves as an electron acceptor in this type composite photocatalysts. Generally, degradation of the organic dye molecules takes place with anchoring onto the surface of semiconducting particles by means of covalent bonding and ion-pair type association. In the literature, different types of graphenebased composites (graphene-semiconductor, graphene oxide (GO)-semiconductor, and reduced

can be found. Graphene-semiconductor composites have been fabricated through simple mixing and/or sonication, sol-gel process, liquid-phase, hydrothermal, and solvothermal methods. Furthermore, as a strategy to improve visible-light photocatalytic efficiency of graphene-based composites, doping semiconductor with metal ions is an effective way. In this chapter of the book, structural and optical properties of the graphene-semiconductor composites, general synthesis methods, and photocatalytic properties of these composites under the visible light are discussed. Finally, the effect of the doping on the resultant visible light-induced photocatalytic

**2. The graphene-semiconductor composites; structural and optical** 

materials draw attention because of offering high photocatalytic activity [6].

Over the last decades, spreading of diseases due to organic dye compounds caused by noxious chemicals has become a global public health issue. Thus, scientists have focused on the studies of the wastewater treatment and antibacterial applications required to use a photocatalyst material. In the studies to prevent the spreading of the dye molecules, carbonaceous

Carbon is one of the most abundant elements in the world and is necessary for life in the nature. Carbon has different forms or allotropes which include buckminsterfullerene (or just simply fullerene), diamond, graphite, etc. The discovery of buckminsterfullerene (C60) [20] which is a cluster of 60 carbon atoms brought the Nobel Prize in Chemistry in 1996 to Kroto, Curl, and Smalley. On the other hand, diamond is considered as the hardest material, whereas graphite is extremely soft. Graphite is a naturally occurring mineral and has parallel carbon atom layers which are named as graphene, so that graphene can be defined as a single and flat

(2D) honeycomb lattice. Graphene was first isolated as a single-layer from graphite in 2004 by Novoselov et al. using the mechanical exfoliation technique [7]. This technique is based on the repeated peeling of thin flakes from a piece of graphite with a scotch tape. This success


, ZnO, CdS, Zn<sup>2</sup>

http://dx.doi.org/10.5772/intechopen.70709

SnO<sup>4</sup>

, etc.)

69

graphene oxide-semiconductor; as a semiconductor material TiO2

activity of graphene-semiconductor composites is reported.

layer of graphite which consists of sp2

graphene [19].

**properties**

Recently, in order to degrade organic dye compounds, many researchers have studied on the photocatalyst material which is usually a wide band gap semiconductor in the presence of solar energy. The photocatalytic reaction mechanism is based on the ability to create electron-hole pairs under photo illumination. For photocatalytic devices, sunlight, which is abundantly available as natural irradiation energy, and UV irradiation sources can be conveniently exploited for the irradiation of semiconducting materials. For practical applications, development of an efficient, reproducible, and cost-effective visible light-induced photocatalyst is desirable for the large-scale production of catalyst. Recently, many research papers are dedicated to the semiconductor-based [zinc oxide (ZnO), cadmium sulfide (CdS), cadmium selenide (CdSe), iron oxide (Fe<sup>2</sup> O3 ), tin oxide (SnO<sup>2</sup> ), zinc sulfide (ZnS), etc.] [2], carbonaceous-based (nanotubes, fullerenes, graphene, etc.) [3], and semiconductor-carbonaceous composite photocatalysts [4]. Over the recent decades, semiconductor-based photocatalysts have attracted great attention in water purification because of their high photosensitivity, environment-friendly non-toxic nature, and low cost [5].

Although semiconductor-based photocatalysts, especially TiO2 , have been recognized as the most effective catalysts and are widely used in the removal of toxic or hazardous organic pollutants, several fundamental issues must be addressed before the photocatalysts are efficiently used for the industrial applications. For example, large bandgap of the TiO2 photocatalyst provides only absorbance of UV light with a wavelength of less than 385 nm, which accounts for less than 10% of solar irradiation. In addition, fast recombination of electron-hole pairs limits the applicability of TiO2 as photocatalyst. These problems are the cause of low photosensitivity and high cost, and makes these types of photocatalyst unfavorable for large scale production.

In the family of carbon nanomaterials, graphene-based photocatalyst activated under the visible light seems to be the preferred candidates because of their outstanding advantages such as low cost, innocuous nature, ease of availability/processing, and suitability of their structure to create composites with semiconductors [6]. Graphene is a two-dimensional material having sp2 -bonded carbon atoms tightly packed into a two-dimensional honeycomb structure. After discovering by Novoselov et al. in 2004 [7], graphene has been attracting more and more attention because of their unique properties such as excellent mobility of charge carriers at room temperature (100,000 cm2 V−1 s−1) [8], theoretically large surface area (2630 m2 g−1) [9], optical transparency [10], excellent mechanical strength (2.4 ± 0.4 TPa) [11], high thermal conductivity (~5000 W m−1 K−1) [12], exhibiting half-integer room-temperature quantum Hall effect [13], capacity of sustaining large electrical current density (108 A cm−2) [14], etc. It has attracted a great attention for their potential applications in variety fields such as catalyst [15], sensors [16], nano electronics [17], energy conversion [18], and water treatment [4]. In the past few decades, there is an ever-increasing interest in the photocatalytic applications of graphene [19].

wastewater by utilizing microorganisms in a controlled environment [1]. As an advanced treatment step, desalination technologies such as reverse osmosis, electrodialysis, ion exchange, and freeze desalination were applied. However, the above-listed traditional biological treatment techniques can be ineffective in decolorizing dye molecules. Due to the specific needs during the degradation of organic pollutants, there has been a strong interest in the develop-

Recently, in order to degrade organic dye compounds, many researchers have studied on the photocatalyst material which is usually a wide band gap semiconductor in the presence of solar energy. The photocatalytic reaction mechanism is based on the ability to create electron-hole pairs under photo illumination. For photocatalytic devices, sunlight, which is abundantly available as natural irradiation energy, and UV irradiation sources can be conveniently exploited for the irradiation of semiconducting materials. For practical applications, development of an efficient, reproducible, and cost-effective visible light-induced photocatalyst is desirable for the large-scale production of catalyst. Recently, many research papers are dedicated to the semiconductor-based [zinc oxide (ZnO), cadmium sulfide (CdS), cadmium

), tin oxide (SnO<sup>2</sup>

ceous-based (nanotubes, fullerenes, graphene, etc.) [3], and semiconductor-carbonaceous composite photocatalysts [4]. Over the recent decades, semiconductor-based photocatalysts have attracted great attention in water purification because of their high photosensitivity,

most effective catalysts and are widely used in the removal of toxic or hazardous organic pollutants, several fundamental issues must be addressed before the photocatalysts are effi-

catalyst provides only absorbance of UV light with a wavelength of less than 385 nm, which accounts for less than 10% of solar irradiation. In addition, fast recombination of electron-hole

photosensitivity and high cost, and makes these types of photocatalyst unfavorable for large

In the family of carbon nanomaterials, graphene-based photocatalyst activated under the visible light seems to be the preferred candidates because of their outstanding advantages such as low cost, innocuous nature, ease of availability/processing, and suitability of their structure to create composites with semiconductors [6]. Graphene is a two-dimensional material hav-

attracted a great attention for their potential applications in variety fields such as catalyst


ciently used for the industrial applications. For example, large bandgap of the TiO2

), zinc sulfide (ZnS), etc.] [2], carbona-

as photocatalyst. These problems are the cause of low

, have been recognized as the

A cm−2) [14], etc. It has

photo-

ment of efficient treatment method.

68 Visible-Light Photocatalysis of Carbon-Based Materials

selenide (CdSe), iron oxide (Fe<sup>2</sup>

pairs limits the applicability of TiO2

scale production.

ing sp2

O3

environment-friendly non-toxic nature, and low cost [5].

Although semiconductor-based photocatalysts, especially TiO2

effect [13], capacity of sustaining large electrical current density (108

To improve the photocatalytic efficiency of the semiconductor-based metal oxides and graphenebased photocatalysts under the visible-light, integration of semiconductor metal oxides with graphene has been one the most popular approach due to theoretically large specific-surface area and high intrinsic electron mobility of graphene at room temperature [3]. Graphene behaves as an electron acceptor in this type composite photocatalysts. Generally, degradation of the organic dye molecules takes place with anchoring onto the surface of semiconducting particles by means of covalent bonding and ion-pair type association. In the literature, different types of graphenebased composites (graphene-semiconductor, graphene oxide (GO)-semiconductor, and reduced graphene oxide-semiconductor; as a semiconductor material TiO2 , ZnO, CdS, Zn<sup>2</sup> SnO<sup>4</sup> , etc.) can be found. Graphene-semiconductor composites have been fabricated through simple mixing and/or sonication, sol-gel process, liquid-phase, hydrothermal, and solvothermal methods. Furthermore, as a strategy to improve visible-light photocatalytic efficiency of graphene-based composites, doping semiconductor with metal ions is an effective way. In this chapter of the book, structural and optical properties of the graphene-semiconductor composites, general synthesis methods, and photocatalytic properties of these composites under the visible light are discussed. Finally, the effect of the doping on the resultant visible light-induced photocatalytic activity of graphene-semiconductor composites is reported.

## **2. The graphene-semiconductor composites; structural and optical properties**

Over the last decades, spreading of diseases due to organic dye compounds caused by noxious chemicals has become a global public health issue. Thus, scientists have focused on the studies of the wastewater treatment and antibacterial applications required to use a photocatalyst material. In the studies to prevent the spreading of the dye molecules, carbonaceous materials draw attention because of offering high photocatalytic activity [6].

Carbon is one of the most abundant elements in the world and is necessary for life in the nature. Carbon has different forms or allotropes which include buckminsterfullerene (or just simply fullerene), diamond, graphite, etc. The discovery of buckminsterfullerene (C60) [20] which is a cluster of 60 carbon atoms brought the Nobel Prize in Chemistry in 1996 to Kroto, Curl, and Smalley. On the other hand, diamond is considered as the hardest material, whereas graphite is extremely soft. Graphite is a naturally occurring mineral and has parallel carbon atom layers which are named as graphene, so that graphene can be defined as a single and flat layer of graphite which consists of sp2 -bonded carbon atoms packed into a two-dimensional (2D) honeycomb lattice. Graphene was first isolated as a single-layer from graphite in 2004 by Novoselov et al. using the mechanical exfoliation technique [7]. This technique is based on the repeated peeling of thin flakes from a piece of graphite with a scotch tape. This success has led to an incredible increase in theoretical and experimental studies about graphene in the recent years. Geim and Novoselov were awarded the Nobel Prize in Physics 2010 because of the producing, isolating, identifying and characterizing graphene.

Exhibiting of the photocatalytic properties of the graphene-based materials depends on the creating free radicals in the photocatalytic reaction. However, fast recombination of the electron-hole pair, which is formed under the light source, is the major disadvantage of the using of the graphene as a photocatalyst material. In order to overcome this problem, chemical modification of the graphene is done with bioactive nanoparticles or light sensitive materials. Due to the expensive cost of gold and silver, which are commonly used as nanoparticles [40, 41], the applications of the photocatalyst composite formed with these elements and graphene is limited in large scale applications. The chemical modification of the graphene with light sensi-

sensitive material used most commonly up till now. However, an alternative material which will form a composite structure with graphene is still being sought due to small portion solar

and Honda in 1972 [42], there is a growing interest in the photocatalysis of semiconductors. Many types of semiconductor photocatalysts have been reported up to now including TiO2

low cost, nontoxicity, chemical stability, and its highly oxidizing. However, the applications

and its large band gap (3.2 eV) that requires the UV light irradiation. **Figure 1** shows a schematic illustration of the photocatalytic reaction mechanism. With the absorption of the photon energy, which has greater than that of the band gap of the semiconductor material, electron in the valance band becomes excited and jumps to the conduction band. Hence, a positively charged hole forms in the valence band. As a result of the recombination with the electron, this the positively charged hole can oxidize water molecules and provide to form hyper-reactive hydroxyl free radicals (•OH). These radicals induce the photodegradation of the dye

has been widely used in the light-induced photochemical reactions because of its

are restricted due to fast recombination rate of photogenerated electron-hole pairs

is the light

,

71

with photoelectrolysis by Fujishima

http://dx.doi.org/10.5772/intechopen.70709

, ZnS, etc. Among the many different semiconductor materi-

Graphene-Semiconductor Composites as Visible Light-Induced Photocatalyst

tive materials is another way to improve photocatalytic activity of graphene. TiO2

light usage of TiO2

ZnO, CdS, CdSe, Fe<sup>2</sup>

als, TiO2

of TiO2

molecules.

.

O3 , SnO<sup>2</sup>

Since the seminal discovery of water splitting on TiO<sup>2</sup>

**Figure 1.** A schematic illustration of the photocatalytic reaction mechanism.

Apart from the exfoliation method, alternative ways for the producing of graphene have been developed: epitaxial growth on silicon carbide is based on the evaporating silicon from SiC with heating and leaving a thin carbon layer [21]. Chemical vapor deposition on metal surfaces is one of the fabricate methods of graphene. A transition metal (nickel) film is exposed to a hydrocarbon gas at high temperature under ambient pressure. Cooling the sample, solubility of carbon in the transition metal film decreases and an ultrathin graphene layer is formed over the metal surface [22, 23].

Besides these methods, the chemical reduction of graphene oxide (GO) derived from chemical exfoliation of graphite is one of efficient and low cost ways in order to obtain large scale graphene which was first reported by Ruoff's group [24–27]. GO is a strongly oxidized form of graphene which can be obtained using oxidizing agents from graphite oxide. Graphite oxide is generally obtained with Hummers' method [28] which involves the oxidation of graphite powder in the mixture of concentrated sulfuric acid (H2 SO<sup>4</sup> ), sodium nitrate (NaNO3 ), and potassium permanganate (KMnO4 ). After oxidation, the layers of graphite oxide, which consists of GO sheets exhibit hydrophilicity, allows water to intercalate between the sheets and disperse them. So, the exfoliation of graphite oxide is facilitated and a layer graphene or GO is produced via ultrasonic treatment in aqueous media. GO contains various types of oxygen containing functional groups such as hydroxyl, epoxide, and carbonyl groups caused by heavily oxidation. Due to the disruption of the conjugated structure by functionalization, covalent bonds of carbon and oxygen atoms become more intense and the resistance of the layer increases, so the conductivity of GO decreases. For this reason, a more efficient chemical reduction method is required in order to obtain GO with high conductivity. In other words, GO needs to be reduced by removing the oxygen-bearing group. The term of the reduced GO (rGO) is used to refer to as chemically modified graphene, chemically derived graphene, or reduced graphene [29].

There are various GO reduction methods which can be achieved through chemical, thermal, or electrochemical ways [30, 31]. In order to obtain rGO with chemical way, reducing agents are used such as hydrazine (N2 H4 ) [27], hydroquinone (HQ) [32] sodium borohydride (NaBH4 ) [33], and hydriodic acid (HI) [34]. Hydrazine, which is one of the most widely used reducing agents, has been used to reduce GO in order to remove the functional groups. However, hydrazine is highly toxic and incorporates C─N groups which increase the resistance of GO sheets due to scattering electrons. During the thermal reduction, GO is exposed to a heating process with the evolved gases. Decomposing of the epoxy and hydroxyl groups with temperature generates high pressures that separate the graphene sheets [35–37]. However, thermal reduction damages the structure of graphene sheets and causes mass loss [31, 35].

Another method is electrochemical reduction, where the electron behaves as the reducing agent. In the electrochemical approach, a thin GO film is deposited onto the surface of the substrates such as flexible plastic, glass, indium tin oxide, and glassy carbon [38, 39]. Reference and auxiliary electrodes are placed and the electrochemical reduction process is performed on the GO with cyclic voltammetry in a buffer electrolyte. Oxygen-containing functional groups in GO can be reduced by the applying various voltages.

Exhibiting of the photocatalytic properties of the graphene-based materials depends on the creating free radicals in the photocatalytic reaction. However, fast recombination of the electron-hole pair, which is formed under the light source, is the major disadvantage of the using of the graphene as a photocatalyst material. In order to overcome this problem, chemical modification of the graphene is done with bioactive nanoparticles or light sensitive materials. Due to the expensive cost of gold and silver, which are commonly used as nanoparticles [40, 41], the applications of the photocatalyst composite formed with these elements and graphene is limited in large scale applications. The chemical modification of the graphene with light sensitive materials is another way to improve photocatalytic activity of graphene. TiO2 is the light sensitive material used most commonly up till now. However, an alternative material which will form a composite structure with graphene is still being sought due to small portion solar light usage of TiO2 .

has led to an incredible increase in theoretical and experimental studies about graphene in the recent years. Geim and Novoselov were awarded the Nobel Prize in Physics 2010 because of

Apart from the exfoliation method, alternative ways for the producing of graphene have been developed: epitaxial growth on silicon carbide is based on the evaporating silicon from SiC with heating and leaving a thin carbon layer [21]. Chemical vapor deposition on metal surfaces is one of the fabricate methods of graphene. A transition metal (nickel) film is exposed to a hydrocarbon gas at high temperature under ambient pressure. Cooling the sample, solubility of carbon in the transition metal film decreases and an ultrathin graphene layer is formed

Besides these methods, the chemical reduction of graphene oxide (GO) derived from chemical exfoliation of graphite is one of efficient and low cost ways in order to obtain large scale graphene which was first reported by Ruoff's group [24–27]. GO is a strongly oxidized form of graphene which can be obtained using oxidizing agents from graphite oxide. Graphite oxide is generally obtained with Hummers' method [28] which involves the oxidation of graphite pow-

GO sheets exhibit hydrophilicity, allows water to intercalate between the sheets and disperse them. So, the exfoliation of graphite oxide is facilitated and a layer graphene or GO is produced via ultrasonic treatment in aqueous media. GO contains various types of oxygen containing functional groups such as hydroxyl, epoxide, and carbonyl groups caused by heavily oxidation. Due to the disruption of the conjugated structure by functionalization, covalent bonds of carbon and oxygen atoms become more intense and the resistance of the layer increases, so the conductivity of GO decreases. For this reason, a more efficient chemical reduction method is required in order to obtain GO with high conductivity. In other words, GO needs to be reduced by removing the oxygen-bearing group. The term of the reduced GO (rGO) is used to refer to as chemically modified graphene, chemically derived graphene, or reduced graphene [29]. There are various GO reduction methods which can be achieved through chemical, thermal, or electrochemical ways [30, 31]. In order to obtain rGO with chemical way, reducing agents are

[33], and hydriodic acid (HI) [34]. Hydrazine, which is one of the most widely used reducing agents, has been used to reduce GO in order to remove the functional groups. However, hydrazine is highly toxic and incorporates C─N groups which increase the resistance of GO sheets due to scattering electrons. During the thermal reduction, GO is exposed to a heating process with the evolved gases. Decomposing of the epoxy and hydroxyl groups with temperature generates high pressures that separate the graphene sheets [35–37]. However, thermal reduction damages the structure of graphene sheets and causes mass loss [31, 35].

Another method is electrochemical reduction, where the electron behaves as the reducing agent. In the electrochemical approach, a thin GO film is deposited onto the surface of the substrates such as flexible plastic, glass, indium tin oxide, and glassy carbon [38, 39]. Reference and auxiliary electrodes are placed and the electrochemical reduction process is performed on the GO with cyclic voltammetry in a buffer electrolyte. Oxygen-containing functional groups

SO<sup>4</sup>

), sodium nitrate (NaNO3

). After oxidation, the layers of graphite oxide, which consists of

) [27], hydroquinone (HQ) [32] sodium borohydride (NaBH4

), and potas-

)

the producing, isolating, identifying and characterizing graphene.

over the metal surface [22, 23].

70 Visible-Light Photocatalysis of Carbon-Based Materials

sium permanganate (KMnO4

used such as hydrazine (N2

der in the mixture of concentrated sulfuric acid (H2

H4

in GO can be reduced by the applying various voltages.

Since the seminal discovery of water splitting on TiO<sup>2</sup> with photoelectrolysis by Fujishima and Honda in 1972 [42], there is a growing interest in the photocatalysis of semiconductors. Many types of semiconductor photocatalysts have been reported up to now including TiO2 , ZnO, CdS, CdSe, Fe<sup>2</sup> O3 , SnO<sup>2</sup> , ZnS, etc. Among the many different semiconductor materials, TiO2 has been widely used in the light-induced photochemical reactions because of its low cost, nontoxicity, chemical stability, and its highly oxidizing. However, the applications of TiO2 are restricted due to fast recombination rate of photogenerated electron-hole pairs and its large band gap (3.2 eV) that requires the UV light irradiation. **Figure 1** shows a schematic illustration of the photocatalytic reaction mechanism. With the absorption of the photon energy, which has greater than that of the band gap of the semiconductor material, electron in the valance band becomes excited and jumps to the conduction band. Hence, a positively charged hole forms in the valence band. As a result of the recombination with the electron, this the positively charged hole can oxidize water molecules and provide to form hyper-reactive hydroxyl free radicals (•OH). These radicals induce the photodegradation of the dye molecules.

**Figure 1.** A schematic illustration of the photocatalytic reaction mechanism.

There has been considerable interest on the enhancement of photocatalytic activity by the combination of carbonaceous materials with semiconductors because of its extraordinary structural and electronic properties. The pioneering work of Williams et al. about UV-induced photocatalytic reduction of GO [43] has increased the interest in graphene-based semiconductor photocatalysts. They exposed a mixture of graphene oxide and TiO2 to UV light in ethanol. When the holes that occur as a result of excitation are consumed to produce ethoxy radicals, the electrons accumulate within TiO2 particles and reduce the oxygen containing functional groups on GO surface. Zhang et al. observed a significant enhancement in the photocatalytic activity of the photodegradation of methylene blue (MB) with chemically bonded graphene TiO2 P25 (with particle size around 25 nm) nanocomposite photocatalyst [44]. These nanocomposite photocatalysts were obtained with the reduction of GO and loading of P25 using hydrothermal method.

thermodynamically stable phase of ZnO [53]. ZnO has similar characteristics to TiO2

due to UV irradiation as a photocatalyst and degrade the pollutants [55].

**3. Common synthesis methods of the graphene-semiconductor** 

semiconductors. In recent years, semiconductors such as SnO<sup>2</sup>

which will be mentioned in section 4.

of these syntheses are given below.

gated the photocatalytic activity of the graphene-TiO2

**composites**

**3.1. Sol-gel method**

of graphene-TiO2

**3.2. Solution mixing method**

TiO2

its large band gap (3.37 eV) and UV light sensitivity. ZnO has been used in the photocatalytic reduction of GO in ethanol by Williams and Kamat [54]. Similar to mechanism of graphene-

Although there have been a lot progress in the photocatalysis of graphene-based semiconductor composites, very small portion of the solar spectrum is utilized due to UV light sensitive

CdSe [59] have been used as photocatalysts in graphene-semiconductor composites under visible light. These semiconductors generally have been chosen due to their proportional narrow band gaps. For example, the energy band gap of CdSe is 1.67 eV, so it can be used under visible light in order to use solar spectrum efficiently. Also, use of graphene enhances the photoactivity of graphene-semiconductor composites due to separation of charges, suppression of the charge recombination, and facilitation of charge transfer. There is a great challenge to improve the efficient conversion of solar spectrum in the visible light region photocatalysis

At the present time, many different methods have been developed to synthesize graphenesemiconductor composites. Most commonly used synthesis methods are sol-gel, solution mixing, *in situ* growth, hydrothermal, and solvothermal methods. More detailed explanations

Sol-gel method is the process of the suspending of particles in a liquid and placing of these

ites employing a sol-gel method using tetrabutyl titanate (TBOT) and GO as starting materials. At first, they dispersed the graphene sheets into ethanol solvent, and then dropped TBOT into the suspension. The obtained sol was dried to form precursor. Finally, graphene/TiO2 composites were synthesized by annealing the precursor for the crystallization. They investi-

from water photo-splitting under UV-Vis illumination. Liu et al. [61] reported a preparation

Solution mixing method is one of the most common and easiest methods for the preparation of graphene/semiconductor composites which are prepared using the suspensions of graphene

composites using titanium precursors and rGO by a sol-gel method and

particles onto a surface from a solution. Zhang et al. [60] synthesized graphene-TiO2

observed an enhancement in the photocatalytic activity under visible light.

composites, electron-hole pairs generated by UV excited ZnO create electron-hole pairs

[56], BiVO4

Graphene-Semiconductor Composites as Visible Light-Induced Photocatalyst

http://dx.doi.org/10.5772/intechopen.70709

due to

73

[57], CdS [58], and

compos-

composites using hydrogen evolution

Photocatalytic enhancement in the graphene-semiconductor composites can be explained with three mechanisms due to unique properties of graphene: (i) extension the wavelength of the absorbed light due to the chemical bonds which narrow the band gap of semiconductor and transparency of graphene; (ii) suppression of the recombination of photogenerated electron hole pairs. The excited electrons transfer from the conduction band of semiconductor to graphene which is an acceptor due to its long π conjugation structure [45]. With fast charge transportation and effective separation of electron-hole pairs, the charge recombination is suppressed; (iii) increasing absorptivity of pollutants. Model dyes or contaminant molecules usually have an aromatic structure and they could be conjugated to the aromatic regions of the graphene. This absorptivity increases the amount of contaminant molecules on the catalyst and improves a significant increase of photocatalytic degradation [44–46]. The role of adsorption of graphene in photocatalysis was studied by Liu et al. [47]. They have shown that rGO captures dye molecules from water through adsorption and photoinduced electrons from TiO2 through surface during photocatalytic process. The photocatalytic mechanism in graphene-TiO2 composites can be briefly summarized as follows:

TiO2 + hν → TiO2 (h+ + e<sup>−</sup> ) TiO2 (e<sup>−</sup> ) + O2 + graphene → TiO2 + O2 <sup>−</sup> + graphene (e<sup>−</sup> TiO2 (h+ ) + OH<sup>−</sup> → TiO2 + •OH

In order to increase the photocatalytic activity of TiO2 , it is required that the slowing of the charge recombination rate and the reduction of the energy band gap. These processes have been done with increasing the porosity and surface area [48], doping with metals [49] or nonmetals [50], and surface coupling with metals [51] or semiconductors [52]. While there is a great challenge and effort to explain the mechanism of photocatalytic enhancement observed in graphene-semiconductor composites, it is still unclear.

)

Apart from TiO2 , there are some semiconductors which have been studied in the photocatalysis of graphene-semiconductors composites as photocatalysts. One of these semiconductors, ZnO, is a promising candidate for environmental contaminations due to its strong oxidizing power, good photocatalytic activity, and low cost. The crystal structures of ZnO exist in rocksalt, zinc blende, and wurtzite (hexagonal) forms. Wurtzite is the most commonly used and thermodynamically stable phase of ZnO [53]. ZnO has similar characteristics to TiO2 due to its large band gap (3.37 eV) and UV light sensitivity. ZnO has been used in the photocatalytic reduction of GO in ethanol by Williams and Kamat [54]. Similar to mechanism of graphene-TiO2 composites, electron-hole pairs generated by UV excited ZnO create electron-hole pairs due to UV irradiation as a photocatalyst and degrade the pollutants [55].

Although there have been a lot progress in the photocatalysis of graphene-based semiconductor composites, very small portion of the solar spectrum is utilized due to UV light sensitive semiconductors. In recent years, semiconductors such as SnO<sup>2</sup> [56], BiVO4 [57], CdS [58], and CdSe [59] have been used as photocatalysts in graphene-semiconductor composites under visible light. These semiconductors generally have been chosen due to their proportional narrow band gaps. For example, the energy band gap of CdSe is 1.67 eV, so it can be used under visible light in order to use solar spectrum efficiently. Also, use of graphene enhances the photoactivity of graphene-semiconductor composites due to separation of charges, suppression of the charge recombination, and facilitation of charge transfer. There is a great challenge to improve the efficient conversion of solar spectrum in the visible light region photocatalysis which will be mentioned in section 4.

### **3. Common synthesis methods of the graphene-semiconductor composites**

At the present time, many different methods have been developed to synthesize graphenesemiconductor composites. Most commonly used synthesis methods are sol-gel, solution mixing, *in situ* growth, hydrothermal, and solvothermal methods. More detailed explanations of these syntheses are given below.

#### **3.1. Sol-gel method**

There has been considerable interest on the enhancement of photocatalytic activity by the combination of carbonaceous materials with semiconductors because of its extraordinary structural and electronic properties. The pioneering work of Williams et al. about UV-induced photocatalytic reduction of GO [43] has increased the interest in graphene-based semiconduc-

When the holes that occur as a result of excitation are consumed to produce ethoxy radicals,

groups on GO surface. Zhang et al. observed a significant enhancement in the photocatalytic activity of the photodegradation of methylene blue (MB) with chemically bonded graphene

Photocatalytic enhancement in the graphene-semiconductor composites can be explained with three mechanisms due to unique properties of graphene: (i) extension the wavelength of the absorbed light due to the chemical bonds which narrow the band gap of semiconductor and transparency of graphene; (ii) suppression of the recombination of photogenerated electron hole pairs. The excited electrons transfer from the conduction band of semiconductor to graphene which is an acceptor due to its long π conjugation structure [45]. With fast charge transportation and effective separation of electron-hole pairs, the charge recombination is suppressed; (iii) increasing absorptivity of pollutants. Model dyes or contaminant molecules usually have an aromatic structure and they could be conjugated to the aromatic regions of the graphene. This absorptivity increases the amount of contaminant molecules on the catalyst and improves a significant increase of photocatalytic degradation [44–46]. The role of adsorption of graphene in photocatalysis was studied by Liu et al. [47]. They have shown that rGO captures dye molecules from water through adsorption and photoinduced electrons

through surface during photocatalytic process. The photocatalytic mechanism in

)

, there are some semiconductors which have been studied in the photocataly-

, it is required that the slowing of the

composites can be briefly summarized as follows:

<sup>−</sup> + graphene (e<sup>−</sup>

charge recombination rate and the reduction of the energy band gap. These processes have been done with increasing the porosity and surface area [48], doping with metals [49] or nonmetals [50], and surface coupling with metals [51] or semiconductors [52]. While there is a great challenge and effort to explain the mechanism of photocatalytic enhancement observed

sis of graphene-semiconductors composites as photocatalysts. One of these semiconductors, ZnO, is a promising candidate for environmental contaminations due to its strong oxidizing power, good photocatalytic activity, and low cost. The crystal structures of ZnO exist in rocksalt, zinc blende, and wurtzite (hexagonal) forms. Wurtzite is the most commonly used and

 P25 (with particle size around 25 nm) nanocomposite photocatalyst [44]. These nanocomposite photocatalysts were obtained with the reduction of GO and loading of P25 using

to UV light in ethanol.

particles and reduce the oxygen containing functional

tor photocatalysts. They exposed a mixture of graphene oxide and TiO2

the electrons accumulate within TiO2

72 Visible-Light Photocatalysis of Carbon-Based Materials

TiO2

from TiO2

TiO2 (e<sup>−</sup>

TiO2 (h+

graphene-TiO2

TiO2 + hν → TiO2

Apart from TiO2

 (h+ + e<sup>−</sup> )

) + OH<sup>−</sup> → TiO2 + •OH

) + O2 + graphene → TiO2 + O2

In order to increase the photocatalytic activity of TiO2

in graphene-semiconductor composites, it is still unclear.

hydrothermal method.

Sol-gel method is the process of the suspending of particles in a liquid and placing of these particles onto a surface from a solution. Zhang et al. [60] synthesized graphene-TiO2 composites employing a sol-gel method using tetrabutyl titanate (TBOT) and GO as starting materials. At first, they dispersed the graphene sheets into ethanol solvent, and then dropped TBOT into the suspension. The obtained sol was dried to form precursor. Finally, graphene/TiO2 composites were synthesized by annealing the precursor for the crystallization. They investigated the photocatalytic activity of the graphene-TiO2 composites using hydrogen evolution from water photo-splitting under UV-Vis illumination. Liu et al. [61] reported a preparation of graphene-TiO2 composites using titanium precursors and rGO by a sol-gel method and observed an enhancement in the photocatalytic activity under visible light.

#### **3.2. Solution mixing method**

Solution mixing method is one of the most common and easiest methods for the preparation of graphene/semiconductor composites which are prepared using the suspensions of graphene and semiconductor nanoparticles. Paek et al. [62] prepared graphene-SnO<sup>2</sup> composites using solution mixing method. They first prepared graphene sheets via the chemical reduction of graphite oxide, and then obtained SnO<sup>2</sup> nanoparticles through the controlled hydrolysis of SnCl<sup>4</sup> with NaOH. Reduced graphene sheets were dispersed in the ethylene glycol, and reassembled in the presence of SnO<sup>2</sup> nanoparticles to form the composites. Bell et al. [63] prepared rGO-TiO2 composites by mixing suspensions of TiO2 powder with the GO suspension ultrasonically and exposing the suspension to UV light. Sonication has been used to dissolve GO in non-polar solvents. Using a similar technique, Akhavan and Ghaderi [64] prepared the graphene-TiO2 thin films and studied the antibacterial activity of the graphene-TiO<sup>2</sup> composite thin films under solar light irradiation.

nanotube nanocomposites prepared with hydrothermal method and used these nanocomposites as photocatalysts for the hydrogen evolution and the degradation of methyl orange under

**4. Visible light photocatalytic activity of the graphene-semiconductor** 

In recent decades, graphene-semiconductor-assisted photocatalysts have attracted considerable attention in not only wastewater treatment but also antibacterial applications due to their high photosensitivity, low cost, and environment-friendly non-toxic nature [70]. Widely used

cal needs as a photocatalytic material, since activation of these catalysts require high-energy UV light, resulting in low efficiency in the visible and near infrared regions. The low separation efficiency of electron-hole pairs and photo-corrosion are other principle problems that inhibit the photocatalytic activity of these semiconductors. In order to solve these limiting factors and enhance the visible light photocatalytic activity of the semiconductors, considerable efforts have been spent on the improvement of the photocatalytic activity by means of noble metal deposition [71], transition-metal doping [2], and semiconductor combination [72]. According to purely used semiconductor, better photocatalytic activities are obtained when the above listed methods are used. However, the recombination of electron-hole species

Semiconductor-matrix system is another method to improve photocatalytic property due to impede the recombination of electron-hole species in the semiconductors by efficient electron transport matrices, such as carbon nanostructures or conductive polymer films, or by molecular electron delay semiconductor structures. Among these techniques, carbonaceous nanomaterials seems to be the most promising matrix system due to enhancement of the photocatalytic properties of semiconductors due to their unique structures that can bring attractive characteristic to photocatalyst materials. Photocatalyst and the delocalized conjugated carbonaceous materials have similar energy level providing an interface hybrid effect between these materials. This effect results in rapid charge separation and slow charge recom-

photocatalyst due to its excellent photocatalytic performance, high stability, low cost, and relatively low toxicity, in comparison to other semiconductor metal oxides [73]. Recently,

composite material such as chemical stability and inertness in acid and basic media, unusual tunable structural, electronic and chemical properties [74, 75], the lack of reproducibility, and the weakening of the light intensity at catalyst surface have still remained as challenges


in the semiconductors is still a problem for the photocatalytic applications.

, ZnO, CdS, CdSe, etc., do not fulfill all the practi-

Graphene-Semiconductor Composites as Visible Light-Induced Photocatalyst

http://dx.doi.org/10.5772/intechopen.70709

75

has provoked great interest as

, TiO2

. Although unique advantages of carbon-based materials as

fixed activated

visible-light irradiation.

photocatalyst semiconductors such as TiO2

bination in the electron-transfer process.

carbon, and carbon-coated TiO2

TiO2

Among the various types of semiconductor materials, TiO2

composites can be divided into three main groups; carbon-doped TiO2

**composites**

#### **3.3.** *In situ* **growth method**

*In situ* is derived from Latin meaning in the reaction mixture in chemistry. *In situ* growth is based on the direct reaction of graphene and semiconductor nanoparticles in solution, and widely used to prepare graphene-semiconductor composites. Lambert et al. [65] synthesized GO-TiO2 composites via the hydrolysis of TiF4 in the presence of aqueous dispersions of GO then obtained rGO-TiO2 adding hydrazine after sonication. Zhang et al. [66] prepared rGO-SnO<sup>2</sup> and rGO-TiO2 composites using *in situ* method. They reduced GO dispersion to rGO with adding SnCl<sup>2</sup> or TiCl3 . Sn2+ and Ti3+ ions were converted to SnO<sup>2</sup> or TiO2 nanoparticles and rGO-semiconductor composites were studied for the photocatalytic properties for degrading rhodamine B (RhB) under visible light. Li and Cao [67] obtained graphene-ZnO composites via a chemical deposition route. GO suspension was mixed with Zn2+ ions containing solution then NaOH was added to the mixture. The powder obtained from the separated and dried solution was dispersed in NaBH4 solution. After hydrothermal treatment, graphene-ZnO composites were obtained and used in the degradation of RhB under UV or visible light irradiation.

#### **3.4. Hydrothermal growth method**

Hydrothermal method can be defined as the process of growing crystals in the presence of water as a solvent at high temperature and pressure in an autoclave which is a closed reaction vessel. This method is one of the conventional methods in order to obtain graphene-semiconductor composites. Zhang et al. [44] obtained a chemically bonded graphene-TiO2 (P25) nanocomposite with graphene oxide and P25, using a facile one-step hydrothermal method. In their work, GO was dissolved in a solution of distilled water and P25 was added to the solution. Then suspension was placed in an autoclave so GO was reduced and P25 nanoparticles were deposited on the graphene simultaneously. They measured an enhancement in the reaction rate of the photodegradation of MB using graphene-P25 nanocomposites under UV and visible light. Liu et al. [68] demonstrated a one-pot hydrothermal approach to prepare rGO-ZnO composites and applied these composites as photocatalyst in order to decompose MB under UV and visible light. GO was dispersed in aqueous solution with ultrasonication and then Zn powders were mixed with GO suspension. Zn powder was used as a reducing agent and a precursor of ZnO. Ye et al. [69] prepared CdS-graphene and CdS-carbon nanotube nanocomposites prepared with hydrothermal method and used these nanocomposites as photocatalysts for the hydrogen evolution and the degradation of methyl orange under visible-light irradiation.

and semiconductor nanoparticles. Paek et al. [62] prepared graphene-SnO<sup>2</sup>

composites by mixing suspensions of TiO2

graphite oxide, and then obtained SnO<sup>2</sup>

74 Visible-Light Photocatalysis of Carbon-Based Materials

ite thin films under solar light irradiation.

composites via the hydrolysis of TiF4

or TiCl3

rated and dried solution was dispersed in NaBH4

sembled in the presence of SnO<sup>2</sup>

**3.3.** *In situ* **growth method**

GO then obtained rGO-TiO2

rGO with adding SnCl<sup>2</sup>

visible light irradiation.

and rGO-TiO2

**3.4. Hydrothermal growth method**

SnCl<sup>4</sup>

pared rGO-TiO2

graphene-TiO2

GO-TiO2

rGO-SnO<sup>2</sup>

solution mixing method. They first prepared graphene sheets via the chemical reduction of

ultrasonically and exposing the suspension to UV light. Sonication has been used to dissolve GO in non-polar solvents. Using a similar technique, Akhavan and Ghaderi [64] prepared the

*In situ* is derived from Latin meaning in the reaction mixture in chemistry. *In situ* growth is based on the direct reaction of graphene and semiconductor nanoparticles in solution, and widely used to prepare graphene-semiconductor composites. Lambert et al. [65] synthesized

ticles and rGO-semiconductor composites were studied for the photocatalytic properties for degrading rhodamine B (RhB) under visible light. Li and Cao [67] obtained graphene-ZnO composites via a chemical deposition route. GO suspension was mixed with Zn2+ ions containing solution then NaOH was added to the mixture. The powder obtained from the sepa-

graphene-ZnO composites were obtained and used in the degradation of RhB under UV or

Hydrothermal method can be defined as the process of growing crystals in the presence of water as a solvent at high temperature and pressure in an autoclave which is a closed reaction vessel. This method is one of the conventional methods in order to obtain graphene-semiconductor composites. Zhang et al. [44] obtained a chemically bonded graphene-TiO2

nanocomposite with graphene oxide and P25, using a facile one-step hydrothermal method. In their work, GO was dissolved in a solution of distilled water and P25 was added to the solution. Then suspension was placed in an autoclave so GO was reduced and P25 nanoparticles were deposited on the graphene simultaneously. They measured an enhancement in the reaction rate of the photodegradation of MB using graphene-P25 nanocomposites under UV and visible light. Liu et al. [68] demonstrated a one-pot hydrothermal approach to prepare rGO-ZnO composites and applied these composites as photocatalyst in order to decompose MB under UV and visible light. GO was dispersed in aqueous solution with ultrasonication and then Zn powders were mixed with GO suspension. Zn powder was used as a reducing agent and a precursor of ZnO. Ye et al. [69] prepared CdS-graphene and CdS-carbon

with NaOH. Reduced graphene sheets were dispersed in the ethylene glycol, and reas-

thin films and studied the antibacterial activity of the graphene-TiO<sup>2</sup>

composites using

compos-

nanopar-

(P25)

powder with the GO suspension

in the presence of aqueous dispersions of

solution. After hydrothermal treatment,

or TiO2

nanoparticles through the controlled hydrolysis of

nanoparticles to form the composites. Bell et al. [63] pre-

adding hydrazine after sonication. Zhang et al. [66] prepared

composites using *in situ* method. They reduced GO dispersion to

. Sn2+ and Ti3+ ions were converted to SnO<sup>2</sup>

## **4. Visible light photocatalytic activity of the graphene-semiconductor composites**

In recent decades, graphene-semiconductor-assisted photocatalysts have attracted considerable attention in not only wastewater treatment but also antibacterial applications due to their high photosensitivity, low cost, and environment-friendly non-toxic nature [70]. Widely used photocatalyst semiconductors such as TiO2 , ZnO, CdS, CdSe, etc., do not fulfill all the practical needs as a photocatalytic material, since activation of these catalysts require high-energy UV light, resulting in low efficiency in the visible and near infrared regions. The low separation efficiency of electron-hole pairs and photo-corrosion are other principle problems that inhibit the photocatalytic activity of these semiconductors. In order to solve these limiting factors and enhance the visible light photocatalytic activity of the semiconductors, considerable efforts have been spent on the improvement of the photocatalytic activity by means of noble metal deposition [71], transition-metal doping [2], and semiconductor combination [72]. According to purely used semiconductor, better photocatalytic activities are obtained when the above listed methods are used. However, the recombination of electron-hole species in the semiconductors is still a problem for the photocatalytic applications.

Semiconductor-matrix system is another method to improve photocatalytic property due to impede the recombination of electron-hole species in the semiconductors by efficient electron transport matrices, such as carbon nanostructures or conductive polymer films, or by molecular electron delay semiconductor structures. Among these techniques, carbonaceous nanomaterials seems to be the most promising matrix system due to enhancement of the photocatalytic properties of semiconductors due to their unique structures that can bring attractive characteristic to photocatalyst materials. Photocatalyst and the delocalized conjugated carbonaceous materials have similar energy level providing an interface hybrid effect between these materials. This effect results in rapid charge separation and slow charge recombination in the electron-transfer process.

Among the various types of semiconductor materials, TiO2 has provoked great interest as photocatalyst due to its excellent photocatalytic performance, high stability, low cost, and relatively low toxicity, in comparison to other semiconductor metal oxides [73]. Recently, TiO2 -carbonaceus-based composites attract great attention for their potential applications in air and water purification due to their outstanding photocatalytic activity. These types of composites can be divided into three main groups; carbon-doped TiO2 , TiO2 fixed activated carbon, and carbon-coated TiO2 . Although unique advantages of carbon-based materials as composite material such as chemical stability and inertness in acid and basic media, unusual tunable structural, electronic and chemical properties [74, 75], the lack of reproducibility, and the weakening of the light intensity at catalyst surface have still remained as challenges [76, 77]. Great photocatalytic activity under the visible light is another limiting factor that impedes the photocatalytic marketing.

proposed that the effective separation of photo-induced charges induced by TiO<sup>2</sup>

**5. Effects of doping on the visible light photocatalytic property of the** 

Generally, in order to improve visible light-induced photocatalytic activity of the semiconductor-based, carbonaceous-based, and semiconductor-carbonaceous composite photocatalysts, the band gap energy of photocatalyst material should be reduced or split into several sub-gaps. Doping semiconductor-based photocatalyst material with various noble metals such as gold, palladium, and silver and transition metals such as cobalt, chrome, copper, iron, molybdenum, etc. become an exciting area in research [5, 82, 83]. Yildirim and Durucan reported that electronic band structure of the semiconductor ZnO nanoparticles can be tunable with substitutional incorporation of Cu ions into ZnO crystal lattice providing to the absorbing more photons [84]. Therefore, transition metal doping enables semiconductor photocatalyst to expand its photo response from the UV light region into the visible range and to

Graphene-Semiconductor Composites as Visible Light-Induced Photocatalyst

http://dx.doi.org/10.5772/intechopen.70709

For the graphene-semiconductor-based composites, in order to improve the visible lightinduced photocatalytic activity, the manipulation and optimization of charge carrier transfer across the interface between semiconductors and graphene is important [58]. Up to now, researchers have expressed an opinion that an intimate interfacial contact is a key aspect for the sufficient utilization of electron conductivity of graphene so enhancement of the photocatalytic activity of the graphene-semiconductor-based composites [85, 86]. However, tunable interfacial atomic charge carrier transfer pathways are also important for the photocatalytic activity of the composites. Atomic charge carrier transfer pathways can be controlled by means of the rational synergy interaction between incorporated individual components in

Zhang et al. reported a detailed study to investigate the effect of the various types of metal ion (Ca2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) addition into CdS which is a well-known II–VI semiconductor with suitable band gap (2.4 eV) matching well with the spectrum of sunlight [87]. Incorporation of the metal ions was performed by using the simple solvothermal method. They proposed a mechanism for the formation of the metal interfacial layer matrix between semiconductor CdS and graphene and found that incorporation of the small amount of metal ions into the interfacial layer matrix between the semiconductor and graphene considerably enhance the visible light-induced photocatalytic activity of graphene-semiconductor composites. Interfacial layer is maintained by the electrostatic attractive interaction between negatively charged oxygen atoms of the functional groups on the GO and positively charged metal ions. Furthermore, binding interaction between defect sites on the GO sheet and metal ions could also promote to more effectively attaching of metal ions onto the GO sheet. Zhang et al. also found that for the highest photocatalytic activity under the visible light irradiation, the optimal weight addition ratio of graphene is 5 wt.%. It is quite interesting to observe the best

with exposed {001} facets and graphene plays an effective role.

**graphene-semiconductor composites**

decompose organic pollutants more effectively.

graphene-semiconductor composites [3].

nanosheets

77

Zhang et al. synthesized TiO2 (P25)-graphene composite via a one-step hydrothermal method and investigated photocatalytic activity of the composites under the both UV and visible light [44]. They found that under both the light sources, the P25-graphene composite structure exhibited better photocatalytic degradation of the dye and enhanced charge separation and transportation properties than that of the pure P25 and P25-carbon nanoparticle structure. They also found that absorption spectra of pure P25 and P25-graphene composite structure shows a red shift to higher wavelength for composite structure exhibiting a photo-responding range at approximately 430–440 nm which corresponds to the violet-blue region in electromagnetic spectrum. Therefore, semiconductor-graphene composite structure offers a more efficient utilization of the solar spectrum under visible light irradiation and expected to assist its use in practical environmental protection issues. The improvement in the photocatalytic property of the P25-graphene composite structure can be attributed to its extended photo responding range by means of chemical bonds and selective adsorption of the aromatic dye on the catalyst. This noncovalent adsorption is driven by the π-π stacking between aromatic regions of the graphene and dye molecule. Similar adsorption has also been reported for the conjugation between aromatic molecules and carbon nanotubes [78].

Li et al. reported that graphene-CdS visible light driven photocatalysts composites act as the clean and renewable hydrogen production path through water splitting [58]. They found that under the visible light, pure CdS exhibited little photocatalytic activity due to the rapid recombination of CB-electrons and VB-holes. The photocatalytic activity of CdS semiconductor was improved by the graphene addition since larger surface area of graphene provides more active adsorption sites acting as photocatalytic reaction centers. Graphene also provides a reduction in the recombination probability of the photo-excited electron-hole pairs and an increasing the number of the charge carriers to form reactive species. However, there is saturation graphene amount in the visible light-induced photocatalytic activity due to "shielding effect." The high weight addition of graphene into composite structure reduces the light irradiation depth due to its opacity, thus and reaction medium restricts the efficiency of graphene in promoting the photocatalytic activity of graphene-semiconductor composites [44, 79, 80]. Due to intrinsic "shielding effect" leading to achieve an optimal photocatalytic activity enhancement with graphene addition into semiconductor material, generally 5 wt.% graphene is incorporated into composite structure.

For the investigation of visible light photocatalytic mechanism, Wang et al. reported synthesis of high-quality nanosized anatase ultra-thin TiO2 nanosheets grown on graphene nanosheets with {001} facets via a simple one-pot solvothermal synthetic route [81]. They proposed that TiO2 /graphene nanocomposites show better visible light photocatalytic activity than pure TiO2 and P25 due to prefer transferring of the photo-induced electrons via Ti─O─C between TiO2 and C interaction. This electron transection delays the recombination of photoinduced charge carriers and extends the carrier lifetime, therefore provides to the enhancement of photocatalytic performance of the composite structure under the visible light. For this enhancement of the visible light photocatalytic activity of the composite structure, Wang et al. proposed that the effective separation of photo-induced charges induced by TiO<sup>2</sup> nanosheets with exposed {001} facets and graphene plays an effective role.

## **5. Effects of doping on the visible light photocatalytic property of the graphene-semiconductor composites**

[76, 77]. Great photocatalytic activity under the visible light is another limiting factor that

and investigated photocatalytic activity of the composites under the both UV and visible light [44]. They found that under both the light sources, the P25-graphene composite structure exhibited better photocatalytic degradation of the dye and enhanced charge separation and transportation properties than that of the pure P25 and P25-carbon nanoparticle structure. They also found that absorption spectra of pure P25 and P25-graphene composite structure shows a red shift to higher wavelength for composite structure exhibiting a photo-responding range at approximately 430–440 nm which corresponds to the violet-blue region in electromagnetic spectrum. Therefore, semiconductor-graphene composite structure offers a more efficient utilization of the solar spectrum under visible light irradiation and expected to assist its use in practical environmental protection issues. The improvement in the photocatalytic property of the P25-graphene composite structure can be attributed to its extended photo responding range by means of chemical bonds and selective adsorption of the aromatic dye on the catalyst. This noncovalent adsorption is driven by the π-π stacking between aromatic regions of the graphene and dye molecule. Similar adsorption has also been reported for the

Li et al. reported that graphene-CdS visible light driven photocatalysts composites act as the clean and renewable hydrogen production path through water splitting [58]. They found that under the visible light, pure CdS exhibited little photocatalytic activity due to the rapid recombination of CB-electrons and VB-holes. The photocatalytic activity of CdS semiconductor was improved by the graphene addition since larger surface area of graphene provides more active adsorption sites acting as photocatalytic reaction centers. Graphene also provides a reduction in the recombination probability of the photo-excited electron-hole pairs and an increasing the number of the charge carriers to form reactive species. However, there is saturation graphene amount in the visible light-induced photocatalytic activity due to "shielding effect." The high weight addition of graphene into composite structure reduces the light irradiation depth due to its opacity, thus and reaction medium restricts the efficiency of graphene in promoting the photocatalytic activity of graphene-semiconductor composites [44, 79, 80]. Due to intrinsic "shielding effect" leading to achieve an optimal photocatalytic activity enhancement with graphene addition into semiconductor material, generally 5 wt.% gra-

For the investigation of visible light photocatalytic mechanism, Wang et al. reported syn-

nanosheets with {001} facets via a simple one-pot solvothermal synthetic route [81]. They

induced charge carriers and extends the carrier lifetime, therefore provides to the enhancement of photocatalytic performance of the composite structure under the visible light. For this enhancement of the visible light photocatalytic activity of the composite structure, Wang et al.

/graphene nanocomposites show better visible light photocatalytic activity

and P25 due to prefer transferring of the photo-induced electrons via Ti─O─C

and C interaction. This electron transection delays the recombination of photo-

nanosheets grown on graphene

conjugation between aromatic molecules and carbon nanotubes [78].

phene is incorporated into composite structure.

proposed that TiO2

than pure TiO2

between TiO2

thesis of high-quality nanosized anatase ultra-thin TiO2

(P25)-graphene composite via a one-step hydrothermal method

impedes the photocatalytic marketing.

76 Visible-Light Photocatalysis of Carbon-Based Materials

Zhang et al. synthesized TiO2

Generally, in order to improve visible light-induced photocatalytic activity of the semiconductor-based, carbonaceous-based, and semiconductor-carbonaceous composite photocatalysts, the band gap energy of photocatalyst material should be reduced or split into several sub-gaps. Doping semiconductor-based photocatalyst material with various noble metals such as gold, palladium, and silver and transition metals such as cobalt, chrome, copper, iron, molybdenum, etc. become an exciting area in research [5, 82, 83]. Yildirim and Durucan reported that electronic band structure of the semiconductor ZnO nanoparticles can be tunable with substitutional incorporation of Cu ions into ZnO crystal lattice providing to the absorbing more photons [84]. Therefore, transition metal doping enables semiconductor photocatalyst to expand its photo response from the UV light region into the visible range and to decompose organic pollutants more effectively.

For the graphene-semiconductor-based composites, in order to improve the visible lightinduced photocatalytic activity, the manipulation and optimization of charge carrier transfer across the interface between semiconductors and graphene is important [58]. Up to now, researchers have expressed an opinion that an intimate interfacial contact is a key aspect for the sufficient utilization of electron conductivity of graphene so enhancement of the photocatalytic activity of the graphene-semiconductor-based composites [85, 86]. However, tunable interfacial atomic charge carrier transfer pathways are also important for the photocatalytic activity of the composites. Atomic charge carrier transfer pathways can be controlled by means of the rational synergy interaction between incorporated individual components in graphene-semiconductor composites [3].

Zhang et al. reported a detailed study to investigate the effect of the various types of metal ion (Ca2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) addition into CdS which is a well-known II–VI semiconductor with suitable band gap (2.4 eV) matching well with the spectrum of sunlight [87]. Incorporation of the metal ions was performed by using the simple solvothermal method. They proposed a mechanism for the formation of the metal interfacial layer matrix between semiconductor CdS and graphene and found that incorporation of the small amount of metal ions into the interfacial layer matrix between the semiconductor and graphene considerably enhance the visible light-induced photocatalytic activity of graphene-semiconductor composites. Interfacial layer is maintained by the electrostatic attractive interaction between negatively charged oxygen atoms of the functional groups on the GO and positively charged metal ions. Furthermore, binding interaction between defect sites on the GO sheet and metal ions could also promote to more effectively attaching of metal ions onto the GO sheet. Zhang et al. also found that for the highest photocatalytic activity under the visible light irradiation, the optimal weight addition ratio of graphene is 5 wt.%. It is quite interesting to observe the best catalytic activity from 5 wt.% graphene addition since generally less than 5 wt.% graphene containing graphene-semiconductor composites exhibit a proper synergy interaction between graphene and the semiconductor due to the shielding effect of graphene. Graphene addition maintains the intimate interfacial contact between the semiconductor and graphene transfer charge carriers and expands the lifetime more effectively under visible light irradiation. Therefore, with the introducing small amount of metal ions into composite structure, the photocatalytic activity of the composite is considerably improved. Under visible light irradiation, this composite structure can be used anaerobic reduction of nitro compound and the aerobic selective oxidation of alcohol.

The second photocatalytic route includes the excitation of dye under visible-light irradiation.

graphene oxide, and to lastly Ag nanoparticle surface as shown in **Figure 2**. Therefore, dye molecules provide also to an improvement of the photocatalytic ability by means of acting as a visible light sensitizer and offering a self-degradation. For the both routes, while reaction of the excited electrons and dissolved oxygen molecules produces oxygen peroxide radicals, reaction between the positive charged hole and the hydroxide ion derived from water forms hydroxyl radicals. As a result of these reactions, an enhancement of the photocatalytic activity

pared with the sol-gel method and contained different amount of graphene (0–2 wt.%). These

ties, which favored the catalytic reaction. Due to different oxide states of Mn (MnO, MnO<sup>2</sup>

In recent decades, photocatalysis is attracting more and more attention due to their potential for solving environmental problems originated from organic dye compounds which cause to serious detrimental effects for living ecosystems as a result of water pollution. Up to now, many research papers and review articles are dedicated to development of the photocatalytic materi-

materials widely used in the removal of toxic or hazardous organic pollutants. However, due to their wide band gap, semiconductor-based photocatalysts can only absorb UV light comprising for less than 10% of solar irradiation. Therefore, expanding the absorption spectral range to exploit the more abundant source of light energy and even poor illumination of interior lighting coming from the sun is critical in development of the photocatalyst. According to literature, the photocatalytic activity of semiconductor materials can be improved by the graphene addition since graphene with larger surface area provides more active adsorption sites acting as photocatalytic reaction centers. Graphene also provides a reduction in the recombination probability of the photo-excited electron-hole pairs and an increasing the number of the charge carriers to form reactive species. For this purpose, graphene-semiconductor-based composite structure is one of the most popular approach due to their outstanding/combining advantages such as theoretically large specific surface area and high intrinsic electron mobility of graphene at room temperature, low cost, innocuous nature, ease of availability/processing,

In literature, a wide variety of the growth methods have been reported such as sol-gel, solution mixing, *in situ* growth, hydrothermal growth, and solvothermal method. Among these methods, solution mixing technique is one of the most common and easiest the method for the fabrication of graphene-semiconductor composite structures which are synthesized by using

, and nonstoichiometric MnOX/Mn) Mn incorporation provides multiple valence states which supports electron transfer and so redox reaction on the surface of the composite catalyst.


Graphene-Semiconductor Composites as Visible Light-Induced Photocatalyst

, then to

79

,


http://dx.doi.org/10.5772/intechopen.70709


and ZnO have been used as two most preferred

In this way, photogenerated electrons excited from dye could transfer to first TiO<sup>2</sup>


als. Semiconductor metal oxide especially TiO<sup>2</sup>

and suitability of their structure to create composites with each other.

Lu et al. reported that manganese oxides supported on TiO2

lysts [90]. MnOX active component was incorporated into TiO2

of GO-TiO2

Mn2 O3

MnOx incorporated TiO2

**6. Conclusions**

In order to improve the visible light-induced photocatalytic activity, doping was found to be helpful due to tailoring the electronic properties of the composites. Mechanism of the organic dye degradation by the semiconductor catalyst is based on two photocatalytic routes for the excitation under visible-light irradiation. A schematic representation of the photocatalytic reaction mechanism for the Ag-doped TiO2 -rGO composite structure is shown in **Figure 2**. The first photocatalytic route is based on the excitation of the semiconductor TiO<sup>2</sup> . Light irradiation causes to electron excitation forming photogenerated electron in the conduction band (CB)/hole in the valence band (VB). Photoinduced electron transfer could take place from CB of TiO2 (−4.2 eV) to work function of the graphene oxide (−4.42 eV) [88]. Furthermore, the work function of the Ag nanoparticles (−4.74 eV) is located at a lower energy level than that of GO [89]. Therefore, photoinduced electrons could easily move from GO to Ag nanoparticle surface and chemical reactions with the dye molecules occur as photogenerated charges move to the particle surface. The recombination of photoinduced electrons and holes is minimized, thus reactive oxygen radicals (•O2 − ) is formed as a reaction of these electrons with adsorbed oxidants (usually O2 ). At the same time, the holes in the VB could react with H2 O/OH<sup>−</sup> to generate hydroxyl radicals (•OH) which contribute to dye degradation.

**Figure 2.** A proposed mechanism for the photodegradation of dyes over TiO2 -Ag-rGO composite under visible light irradiation.

The second photocatalytic route includes the excitation of dye under visible-light irradiation. In this way, photogenerated electrons excited from dye could transfer to first TiO<sup>2</sup> , then to graphene oxide, and to lastly Ag nanoparticle surface as shown in **Figure 2**. Therefore, dye molecules provide also to an improvement of the photocatalytic ability by means of acting as a visible light sensitizer and offering a self-degradation. For the both routes, while reaction of the excited electrons and dissolved oxygen molecules produces oxygen peroxide radicals, reaction between the positive charged hole and the hydroxide ion derived from water forms hydroxyl radicals. As a result of these reactions, an enhancement of the photocatalytic activity of GO-TiO2 -Ag nanocomposites is expected.

Lu et al. reported that manganese oxides supported on TiO2 -graphene nanocomposite catalysts [90]. MnOX active component was incorporated into TiO2 -graphene nanocomposites prepared with the sol-gel method and contained different amount of graphene (0–2 wt.%). These MnOx incorporated TiO2 -graphene catalysts exhibit excellent structure and electrical properties, which favored the catalytic reaction. Due to different oxide states of Mn (MnO, MnO<sup>2</sup> , Mn2 O3 , and nonstoichiometric MnOX/Mn) Mn incorporation provides multiple valence states which supports electron transfer and so redox reaction on the surface of the composite catalyst.

### **6. Conclusions**

catalytic activity from 5 wt.% graphene addition since generally less than 5 wt.% graphene containing graphene-semiconductor composites exhibit a proper synergy interaction between graphene and the semiconductor due to the shielding effect of graphene. Graphene addition maintains the intimate interfacial contact between the semiconductor and graphene transfer charge carriers and expands the lifetime more effectively under visible light irradiation. Therefore, with the introducing small amount of metal ions into composite structure, the photocatalytic activity of the composite is considerably improved. Under visible light irradiation, this composite structure can be used anaerobic reduction of nitro compound and the aerobic

In order to improve the visible light-induced photocatalytic activity, doping was found to be helpful due to tailoring the electronic properties of the composites. Mechanism of the organic dye degradation by the semiconductor catalyst is based on two photocatalytic routes for the excitation under visible-light irradiation. A schematic representation of the photocatalytic

diation causes to electron excitation forming photogenerated electron in the conduction band (CB)/hole in the valence band (VB). Photoinduced electron transfer could take place from CB

work function of the Ag nanoparticles (−4.74 eV) is located at a lower energy level than that of GO [89]. Therefore, photoinduced electrons could easily move from GO to Ag nanoparticle surface and chemical reactions with the dye molecules occur as photogenerated charges move to the particle surface. The recombination of photoinduced electrons and holes is minimized,

(−4.2 eV) to work function of the graphene oxide (−4.42 eV) [88]. Furthermore, the

). At the same time, the holes in the VB could react with H2

The first photocatalytic route is based on the excitation of the semiconductor TiO<sup>2</sup>

−

erate hydroxyl radicals (•OH) which contribute to dye degradation.

**Figure 2.** A proposed mechanism for the photodegradation of dyes over TiO2


) is formed as a reaction of these electrons with adsorbed

. Light irra-

O/OH<sup>−</sup>


to gen-

selective oxidation of alcohol.

78 Visible-Light Photocatalysis of Carbon-Based Materials

of TiO2

irradiation.

reaction mechanism for the Ag-doped TiO2

thus reactive oxygen radicals (•O2

oxidants (usually O2

In recent decades, photocatalysis is attracting more and more attention due to their potential for solving environmental problems originated from organic dye compounds which cause to serious detrimental effects for living ecosystems as a result of water pollution. Up to now, many research papers and review articles are dedicated to development of the photocatalytic materials. Semiconductor metal oxide especially TiO<sup>2</sup> and ZnO have been used as two most preferred materials widely used in the removal of toxic or hazardous organic pollutants. However, due to their wide band gap, semiconductor-based photocatalysts can only absorb UV light comprising for less than 10% of solar irradiation. Therefore, expanding the absorption spectral range to exploit the more abundant source of light energy and even poor illumination of interior lighting coming from the sun is critical in development of the photocatalyst. According to literature, the photocatalytic activity of semiconductor materials can be improved by the graphene addition since graphene with larger surface area provides more active adsorption sites acting as photocatalytic reaction centers. Graphene also provides a reduction in the recombination probability of the photo-excited electron-hole pairs and an increasing the number of the charge carriers to form reactive species. For this purpose, graphene-semiconductor-based composite structure is one of the most popular approach due to their outstanding/combining advantages such as theoretically large specific surface area and high intrinsic electron mobility of graphene at room temperature, low cost, innocuous nature, ease of availability/processing, and suitability of their structure to create composites with each other.

In literature, a wide variety of the growth methods have been reported such as sol-gel, solution mixing, *in situ* growth, hydrothermal growth, and solvothermal method. Among these methods, solution mixing technique is one of the most common and easiest the method for the fabrication of graphene-semiconductor composite structures which are synthesized by using the suspensions of semiconductor nanoparticles and graphene. For the graphene-semiconductor-based composites, to enhance the visible light-induced photocatalytic activity, doping was found to be helpful due to tailoring the electronic properties of the composites providing the manipulation and optimization of charge carrier transfer across the interface between semiconductors and graphene and a reduction in the recombination probability of the photoexcited electron-hole pairs. This chapter of the book is a review of the studies for the development of highly photocatalytically active graphene-semiconductor composite under visible light irradiation.

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### **Author details**

Ozlem Altintas Yildirim1 \* and Teoman Ozturk2


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1 Department of Metallurgical and Materials Engineering, Selcuk University, Konya, Turkey

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) to light

**Provisional chapter**

**Role of Graphene in Photocatalytic Solar Fuel**

**Role of Graphene in Photocatalytic Solar Fuel** 

DOI: 10.5772/intechopen.72623

One of the most promising methods for conversion and storage of solar energy is in the form of the chemical bonds of an energy carrier, such as hydrogen or light hydrocarbons. However, the traditional methods to harness and store solar energy are simply too expensive to be implemented on a large scale. It has been documented that the recombination of photo-induced charge carriers is the greatest source of inefficiency in photocatalytic systems. In the last decade, graphene derivatives and their functionalized nanostructures were extensively utilized for various roles to improve the efficiency of photocatalytic solar fuel generation. These include photocatalyst/redox active sites via band gap and defect density engineering, charge acceptor due to their excellent carrier mobility, a solid-state charge mediator by electronic band alignment, and light absorber by taking advantage of their photoluminescence characteristics at the nanoscale. This chapter aims to provide an authoritative and in-depth review on the properties and application of graphene derivatives, as well as the recent advances in the design of graphene-based photocatalytic systems. The knowledge extracted from the presented materials can be applied to other applications dealing with surface chemistry, interfacial science, and optoelectronic device fabrication.

**Keywords:** solar fuel generation, graphene, photocatalyst, water splitting,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

places the photocatalytic splitting of water and the conversion of carbon dioxide (CO<sup>2</sup>

Direct production of fuels from sunlight is an attractive route to address the energy crisis facing humanity in the twenty-first century, because it inherently provides a method for extracting energy during the night and for cost-effectively dispatching and distributing energy in the existing infrastructure for use in the residential, industrial, and transportation sectors [1]. That

**Generation**

**Abstract**

CO2

**1. Introduction**

reduction

**Generation**

Babak Adeli and Fariborz Taghipour

Babak Adeli and Fariborz Taghipour

http://dx.doi.org/10.5772/intechopen.72623

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter


**Provisional chapter**

### **Role of Graphene in Photocatalytic Solar Fuel Generation Generation**

**Role of Graphene in Photocatalytic Solar Fuel** 

DOI: 10.5772/intechopen.72623

Babak Adeli and Fariborz Taghipour

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[79] Huang X, Qi X, Boey F, Zhang H. Graphene-based composites. Chemical Society Reviews.

[80] Zhang N, Zhang Y, Yang M-Q, Tang Z-R, Xu Y-J. A critical and benchmark comparison on graphene-, carbon nanotube-, and fullerene-semiconductor nanocomposites as visible light photocatalysts for selective oxidation. Journal of Catalysis. 2013;**299**:210-221

[81] Wang W-S, Wang D-H, Qu W-G, Lu L-Q, Xu A-W. Large ultrathin anatase TiO<sup>2</sup> nanosheets with exposed {001} facets on graphene for enhanced visible light photocata-

[82] Ahmad M, Yingying S, Nisar A, Sun H, Shen W, Wei M, Zhu J. Synthesis of hierarchical flower-like ZnO nanostructures and their functionalization by Au nanoparticles for improved photocatalytic and high performance Li-ion battery anodes. Journal of

[83] Georgekutty R, Seery MK, Pillai SC. A highly efficient Ag-ZnO photocatalyst: Synthesis, properties, and mechanism. The Journal of Physical Chemistry C. 2008;**112**:13563-13570

[84] Altıntaş Yıldırım Ö, Durucan C. Room temperature synthesis of Cu incorporated ZnO nanoparticles with room temperature ferromagnetic activity: Structural, optical and

as efficient photocatalysts for hydrogen evolution. The Journal of Physical Chemistry C.

[86] Xiang Q, Yu J, Jaroniec M. Preparation and enhanced visible-light photocatalytic H2

[87] Zhang N, Yang M-Q, Tang Z-R, Xu Y-J. Toward improving the graphene–semiconductor composite photoactivity via the addition of metal ions as generic interfacial mediator.

for high performance water disinfection and decontamination under solar irradiation.

[89] Yoo D-H, Cuong TV, Luan VH, Khoa NT, Kim EJ, Hur SH, Hahn SH. Photocatalytic performance of a Ag/ZnO/CCG multidimensional heterostructure prepared by a solution-

[90] Lu X, Song C, Chang C-C, Teng Y, Tong Z, Tang X. Manganese oxides supported on TiO<sup>2</sup>

graphene nanocomposite catalysts for selective catalytic reduction of NOX with NH3

low temperature. Industrial and Engineering Chemistry Research. 2014;**53**:11601-11610

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N4

[88] Liu L, Bai H, Liu J, Sun DD. Multifunctional graphene oxide-TiO<sup>2</sup>

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at

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magnetic characterization. Ceramics International. 2016;**42**:3229-3238

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Materials Chemistry. 2011;**21**:7723-7729

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ACS Nano. 2014;**8**:623-633

production activity of graphene/C3

Babak Adeli and Fariborz Taghipour Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72623

#### **Abstract**

One of the most promising methods for conversion and storage of solar energy is in the form of the chemical bonds of an energy carrier, such as hydrogen or light hydrocarbons. However, the traditional methods to harness and store solar energy are simply too expensive to be implemented on a large scale. It has been documented that the recombination of photo-induced charge carriers is the greatest source of inefficiency in photocatalytic systems. In the last decade, graphene derivatives and their functionalized nanostructures were extensively utilized for various roles to improve the efficiency of photocatalytic solar fuel generation. These include photocatalyst/redox active sites via band gap and defect density engineering, charge acceptor due to their excellent carrier mobility, a solid-state charge mediator by electronic band alignment, and light absorber by taking advantage of their photoluminescence characteristics at the nanoscale. This chapter aims to provide an authoritative and in-depth review on the properties and application of graphene derivatives, as well as the recent advances in the design of graphene-based photocatalytic systems. The knowledge extracted from the presented materials can be applied to other applications dealing with surface chemistry, interfacial science, and optoelectronic device fabrication.

**Keywords:** solar fuel generation, graphene, photocatalyst, water splitting, CO2 reduction

### **1. Introduction**

Direct production of fuels from sunlight is an attractive route to address the energy crisis facing humanity in the twenty-first century, because it inherently provides a method for extracting energy during the night and for cost-effectively dispatching and distributing energy in the existing infrastructure for use in the residential, industrial, and transportation sectors [1]. That places the photocatalytic splitting of water and the conversion of carbon dioxide (CO<sup>2</sup> ) to light

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

hydrocarbons, driven solely by sunlight, among the most promising approaches. These were studied extensively in the last decade [2]. However, visible-light water splitting and CO<sup>2</sup> reduction are inherently associated with inefficiencies and complicated processes. For instance, the possible products of these processes may include H2 , HCOOH, HCHO, CH3 OH, CO, and CH4 , which are selective and dependent on many competing factors (e.g., reaction kinetics, redox potentials of photo-induced charge carriers, morphology, crystalline structure, exposed crystalline facets and the surface properties of the utilized photocatalyst, and redox active sites) [3]. In addition, the energy levels of the photo-induced charges are relative to the electronic band structure of the employed semiconductor; thus, the desired photocatalyst must possess a matching molecular orbital structure corresponding to the redox potentials of the reaction. Electrons contain the energy of the lowest unoccupied molecular orbital (LUMO) and holes pose the potential energy of the highest occupied molecular orbital (HOMO). These energy levels are also known as the bottom of the semiconductor's conduction band (CB) and the top of the semiconductor's valence band (VB) and are shown in **Figure 1**. For a photocatalytic reaction to proceed, the photo-induced charges must pose a suitable energy level corresponding to redox potential of the reaction. Taking the solar water-splitting reaction as an example, the photo-excited electrons and the holes must contain more negative and more positive energy compared to the water reduction (0 eV vs. NHE) and the water oxidation potential (1.23 eV vs. NHE), respectively. Upon photo-excitation and the generation of photo-induced electrons and holes, the charge carriers can reach to the electrolyte and participate in redox reactions at CB and VB, as demonstrated in **Figure 1**.

remained too low for them to be feasibly commercialized. Such inefficiencies are primarily attributed in the recombination of photo-induced charges, displayed by red dashed-line in **Figure 1**, which occurs mainly at the grain boundaries and the crystalline defects within the bulk of photocatalyst, where the diffusion path of charge carriers is considerable, and/ or the density redox active sites are not sufficient [4]. In the last few years, various strategies—emphasizing the nanoscale morphology and exposed crystallographic facets, as well as increasing the specific surface area and the number of redox active sites—have led to outstanding improvements. However, these are not enough to put the commercialization of solar

als and fabrication routes of advance structures has hindered further consideration of some

In the last decade, carbon-based materials, such as carbon nanotubes (CNTs), graphene, graphene oxide, carbon quantum dots, carbon fibers, activated carbon, and carbon black, have been the focus of intense research, owing to their peculiar characteristics, such as tunable electronic band structure, ultra-high specific surface area, tailored crystalline structure, and reactive crystallographic facets. Among them, graphene derivatives have grasped researchers' attention, due to their effectiveness as redox active sites, tunable defect-density active sites, short carrier's diffusion paths, and high electron mobility, as well as efficient light harvesting

Over 18,000 articles related to graphene were published from 2004 to 2014. In the last decade, due to the advances in materials sciences and nanotechnology, tailoring the optical, structural, and electrochemical characteristics of graphene-based photocatalysts at the nanoscale toward quantum efficiency (QE) improvement has been extensively studied. This chapter aims to present the recent advances in the application of graphene-based materials in solar fuel generation

review articles previously published on topics related to the subject of this chapter [3, 5–9].

Dating back to October 2004, a revolution in science and technology was triggered when Novoselov et al. [10] had prepared stable 2D sheets of carbon atoms at ambient conditions, the so-called graphene nanosheets. Graphene, as an allotrope of carbon, is an isolated monolayer

bond with a π-electron cloud and is considered as one of the most important materials in the current century. Graphene soon became one of the attractive components in photonic device fabrication, fuel conversion, fuel storage, environment, sensing, and catalysis, owing to its outstanding mechanical, thermal, optical, and electrical properties. Photocatalytic applications, highly conductive graphene nanosheets (2D) and quantum dots (zero-dimensional) with a massive surface area and ultra-active catalytic facets, particularly on the dangling crystallography edges, are excellent materials for hybridization with prominent photocatalysts to enhance the separation of photo-excited charges and the active surface area for the redox reactions.

reduction in sight. Moreover, the abundance of photocatalyst materi-

Role of Graphene in Photocatalytic Solar Fuel Generation

http://dx.doi.org/10.5772/intechopen.72623

89

conversion. Readers are encouraged to reach out to comprehensive

honeycomb lattice hybridized C─C

water splitting and CO<sup>2</sup>

via water splitting and CO<sup>2</sup>

**2. Graphene derivative materials**

sheet containing atoms that are tightly packed into an sp2

within their two-dimensional (2D) crystallography.

candidate materials.

Despite the efforts in utilizing metal oxide, sulfide, and nitride photocatalysts, as well as their binary and ternary solid solutions, the efficiencies of the solar fuel generation processes have

**Figure 1.** Band gap energy (E<sup>g</sup> ), valence band (VB), and conduction band (CB) potential of semiconductors. The figure also shows processes (photo-excitation, charge transportation, and charge recombination) that occur upon striking the surface of a semiconductor by a light photon with energy greater than the semiconductor's band gap. Once photoinduced charges reach the surface of the semiconductor, several possible products can proceed (i.e., water splitting and CO2 conversion). The potential of the redox reactions is denoted by Eo .

remained too low for them to be feasibly commercialized. Such inefficiencies are primarily attributed in the recombination of photo-induced charges, displayed by red dashed-line in **Figure 1**, which occurs mainly at the grain boundaries and the crystalline defects within the bulk of photocatalyst, where the diffusion path of charge carriers is considerable, and/ or the density redox active sites are not sufficient [4]. In the last few years, various strategies—emphasizing the nanoscale morphology and exposed crystallographic facets, as well as increasing the specific surface area and the number of redox active sites—have led to outstanding improvements. However, these are not enough to put the commercialization of solar water splitting and CO<sup>2</sup> reduction in sight. Moreover, the abundance of photocatalyst materials and fabrication routes of advance structures has hindered further consideration of some candidate materials.

In the last decade, carbon-based materials, such as carbon nanotubes (CNTs), graphene, graphene oxide, carbon quantum dots, carbon fibers, activated carbon, and carbon black, have been the focus of intense research, owing to their peculiar characteristics, such as tunable electronic band structure, ultra-high specific surface area, tailored crystalline structure, and reactive crystallographic facets. Among them, graphene derivatives have grasped researchers' attention, due to their effectiveness as redox active sites, tunable defect-density active sites, short carrier's diffusion paths, and high electron mobility, as well as efficient light harvesting within their two-dimensional (2D) crystallography.

Over 18,000 articles related to graphene were published from 2004 to 2014. In the last decade, due to the advances in materials sciences and nanotechnology, tailoring the optical, structural, and electrochemical characteristics of graphene-based photocatalysts at the nanoscale toward quantum efficiency (QE) improvement has been extensively studied. This chapter aims to present the recent advances in the application of graphene-based materials in solar fuel generation via water splitting and CO<sup>2</sup> conversion. Readers are encouraged to reach out to comprehensive review articles previously published on topics related to the subject of this chapter [3, 5–9].

### **2. Graphene derivative materials**

hydrocarbons, driven solely by sunlight, among the most promising approaches. These were

tion are inherently associated with inefficiencies and complicated processes. For instance, the

which are selective and dependent on many competing factors (e.g., reaction kinetics, redox potentials of photo-induced charge carriers, morphology, crystalline structure, exposed crystalline facets and the surface properties of the utilized photocatalyst, and redox active sites) [3]. In addition, the energy levels of the photo-induced charges are relative to the electronic band structure of the employed semiconductor; thus, the desired photocatalyst must possess a matching molecular orbital structure corresponding to the redox potentials of the reaction. Electrons contain the energy of the lowest unoccupied molecular orbital (LUMO) and holes pose the potential energy of the highest occupied molecular orbital (HOMO). These energy levels are also known as the bottom of the semiconductor's conduction band (CB) and the top of the semiconductor's valence band (VB) and are shown in **Figure 1**. For a photocatalytic reaction to proceed, the photo-induced charges must pose a suitable energy level corresponding to redox potential of the reaction. Taking the solar water-splitting reaction as an example, the photo-excited electrons and the holes must contain more negative and more positive energy compared to the water reduction (0 eV vs. NHE) and the water oxidation potential (1.23 eV vs. NHE), respectively. Upon photo-excitation and the generation of photo-induced electrons and holes, the charge carriers can reach to the electrolyte and participate in redox reactions at CB and

Despite the efforts in utilizing metal oxide, sulfide, and nitride photocatalysts, as well as their binary and ternary solid solutions, the efficiencies of the solar fuel generation processes have

also shows processes (photo-excitation, charge transportation, and charge recombination) that occur upon striking the surface of a semiconductor by a light photon with energy greater than the semiconductor's band gap. Once photoinduced charges reach the surface of the semiconductor, several possible products can proceed (i.e., water splitting and

), valence band (VB), and conduction band (CB) potential of semiconductors. The figure

.

, HCOOH, HCHO, CH3

reduc-

,

OH, CO, and CH4

studied extensively in the last decade [2]. However, visible-light water splitting and CO<sup>2</sup>

possible products of these processes may include H2

88 Visible-Light Photocatalysis of Carbon-Based Materials

VB, as demonstrated in **Figure 1**.

**Figure 1.** Band gap energy (E<sup>g</sup>

conversion). The potential of the redox reactions is denoted by Eo

CO2

Dating back to October 2004, a revolution in science and technology was triggered when Novoselov et al. [10] had prepared stable 2D sheets of carbon atoms at ambient conditions, the so-called graphene nanosheets. Graphene, as an allotrope of carbon, is an isolated monolayer sheet containing atoms that are tightly packed into an sp2 honeycomb lattice hybridized C─C bond with a π-electron cloud and is considered as one of the most important materials in the current century. Graphene soon became one of the attractive components in photonic device fabrication, fuel conversion, fuel storage, environment, sensing, and catalysis, owing to its outstanding mechanical, thermal, optical, and electrical properties. Photocatalytic applications, highly conductive graphene nanosheets (2D) and quantum dots (zero-dimensional) with a massive surface area and ultra-active catalytic facets, particularly on the dangling crystallography edges, are excellent materials for hybridization with prominent photocatalysts to enhance the separation of photo-excited charges and the active surface area for the redox reactions.

Early studies showed that monolayer graphene can be successfully isolated and studied; while a 2D crystalline graphene nanosheet is known to be thermodynamically unstable, its properties are not yet well known [11]. Graphene is a zero band gap semimetal with a small overlap between its HOMO and LUMO [12–14]. In fact, the electrical, mechanical, optical, and thermal properties of graphene are very similar to those of single-walled carbon nanotubes (SWCNTs), while it can be prepared at significantly lower cost [11]. In particular, graphene's massive theoretical specific surface area (~2600 m<sup>2</sup> g−1 [15]), high mobility of charge carriers (~10,000 cm<sup>2</sup> V−1 s−1 at room temperature, approaching to 200,000 cm<sup>2</sup> V−1 s−1 for lower carrier densities and temperatures [16]), plus its excellent thermal conductivity (3000–5000 W m−1 K−1 [17]), 97.7% optical transmittance, and <0.1% reflectance [18] (monolayer graphene nanosheet) make it an excellent choice for light harvesting and the fabrication of energy conversion devices [15, 16, 19].

**3. Role of graphene in solar fuel generation**

is discussed, and examples from literature are presented.

catalysts that meet all the required conditions.

atoms, creating delocalized π (fill band) and π\*

hybridized structure is formed from s, px

as graphenes VB and CB [35, 36].

2pz

The molecular energy state of carbon is unique and contains 1s2

**3.1. Graphene as photocatalyst**

The improvements in the efficiencies of graphene-based photocatalyst are likely the outcome of these various functionalities and cannot be attributed to one individual phenomenon. In the following sections, the role of graphene derivatives in visible light-driven fuel generation

There are a handful of materials that can carry out visible light solar fuel generation reaction, exhibiting properties such as short band gap to harvest visible light, and suitable band-edge potential corresponding to redox potentials must be formed within the photocatalyst crystallography. Even then, the photo-generated charges must diffuse through a highly crystalline structure and interact with molecules and ions at highly reactive surface sites. Favorably, graphene as an abundant and cost-effective compound, is among the few nonmetallic photo-

contribute accordingly in various crystalline structures. As displayed in **Figure 2a**, the sp2

the surrounding atoms through three strong covalent σ bonds. In this structure, the remaining

The Fermi level in pristine graphene is located at the points connecting the valence and conduction bands in momentum space, as shown in **Figure 2b**, which is also known as the Dirac point [36]. Graphene exhibits an intrinsic n-type character [38]. Because of a small overlap between its valence and conduction bands, graphene has been characterized as a semimetal and/or a

**Figure 2.** (a) The 3D band structure of graphene. (b) The linear dispersion and the band structure at the Dirac point in

graphene [37]. Reprinted with permission from [37]. Copyright 2010 American Chemical Society.

orbital, perpendicular to the graphene plane, overlapped with the one in the neighboring

, and py

, 2s2

, and 2p2

orbitals on each carbon atom connected to

Role of Graphene in Photocatalytic Solar Fuel Generation

http://dx.doi.org/10.5772/intechopen.72623

91

(empty band) orbitals, which are also known

orbitals, which

Within a few years, graphene-derivative materials, such as graphene oxide (GO), reduced graphene oxide (rGO), nanoribbons, quantum dots (QDs), and their functionalized nanostructures, demonstrated even more exciting characteristics. GO is a 2D carbon nanomaterial with many merits, such as low manufacturing cost, facile mass production, fascinating chemistry, and remarkable semiconducting behavior [5, 20–25]. The oxygen-containing functional groups on the surface of GO make it readily dispersed in aqueous solutions and effectively interact with other organic and inorganic compounds, as well as ionic species [26]. The surface of GO nanosheets is mainly decorated with epoxide (═O) and hydroxyl (─OH) groups, while small composition of carbonyl (─C═O) and carboxyl (─COOH) groups are linked to the nanosheet's edges [27]. These oxygen functionalities allow GO and rGO to interact with a wide range of precursors and structures through noncovalent, covalent, and/or ionic interactions [15, 18]. In addition, various densities of sp3 hybridization in the GO structure create a wide range of interesting characteristics. Unlike sp<sup>2</sup> hybridization, which is attributed to the bonding of carbon atoms to the neighboring carbon atoms (which are not connected with hydroxy or epoxy groups) or oxygen in the form of carbonyl or carboxyl groups, sp3 hybridization forms when carbon atoms are bonded to epoxy or hydroxyl groups [28–30]. Density functional theory (DFT) studies, in agreement with experimental observations, confirm the role of oxygen functional groups on the optical properties of graphene over a wide range [29–31]. An intense blue shift in the electron energy loss spectrum (EELS) of GO was observed when the concentration of epoxy and hydroxyl functional groups increased [29, 30]. Interestingly, an increase in the density of carbonyl groups affects the GO EELS; as such, at O/C ~37.5% a red shift about 1.0 eV compared to the pristine graphene is observed [29]. Therefore, the density of surface functionalities can be precisely tuned to control the optical and electrochemical properties of graphene derivatives over a wide range.

QDs, a multilayer zero-dimensional structure, contain sp<sup>2</sup> hybridized honeycomb carbon, have attracted enormous attention, and have been extensively studied for a wide range of applications—energy conversion in particular [32, 33]. Within their ultra-small sizes (typically less than 100 nm), their optical and electronic properties can be tailored, and a well-defined band gap can be formed [34]. Therefore, graphene QDs are like highly crystalline inorganic semiconductor QDs with superior physically and chemically reactive facets [32].

### **3. Role of graphene in solar fuel generation**

The improvements in the efficiencies of graphene-based photocatalyst are likely the outcome of these various functionalities and cannot be attributed to one individual phenomenon. In the following sections, the role of graphene derivatives in visible light-driven fuel generation is discussed, and examples from literature are presented.

### **3.1. Graphene as photocatalyst**

Early studies showed that monolayer graphene can be successfully isolated and studied; while a 2D crystalline graphene nanosheet is known to be thermodynamically unstable, its properties are not yet well known [11]. Graphene is a zero band gap semimetal with a small overlap between its HOMO and LUMO [12–14]. In fact, the electrical, mechanical, optical, and thermal properties of graphene are very similar to those of single-walled carbon nanotubes (SWCNTs), while it can be prepared at significantly lower cost [11]. In particular, graphene's massive theoretical specific surface area (~2600 m<sup>2</sup> g−1 [15]), high mobility of charge carriers (~10,000 cm<sup>2</sup> V−1 s−1 at room temperature, approaching to 200,000 cm<sup>2</sup> V−1 s−1 for lower carrier densities and temperatures [16]), plus its excellent thermal conductivity (3000–5000 W m−1 K−1 [17]), 97.7% optical transmittance, and <0.1% reflectance [18] (monolayer graphene nanosheet) make it an excellent choice for light harvesting and the fabrication of energy conversion

Within a few years, graphene-derivative materials, such as graphene oxide (GO), reduced graphene oxide (rGO), nanoribbons, quantum dots (QDs), and their functionalized nanostructures, demonstrated even more exciting characteristics. GO is a 2D carbon nanomaterial with many merits, such as low manufacturing cost, facile mass production, fascinating chemistry, and remarkable semiconducting behavior [5, 20–25]. The oxygen-containing functional groups on the surface of GO make it readily dispersed in aqueous solutions and effectively interact with other organic and inorganic compounds, as well as ionic species [26]. The surface of GO nanosheets is mainly decorated with epoxide (═O) and hydroxyl (─OH) groups, while small composition of carbonyl (─C═O) and carboxyl (─COOH) groups are linked to the nanosheet's edges [27]. These oxygen functionalities allow GO and rGO to interact with a wide range of precursors and structures through noncovalent, covalent,

which is attributed to the bonding of carbon atoms to the neighboring carbon atoms (which are not connected with hydroxy or epoxy groups) or oxygen in the form of carbonyl or car-

groups [28–30]. Density functional theory (DFT) studies, in agreement with experimental observations, confirm the role of oxygen functional groups on the optical properties of graphene over a wide range [29–31]. An intense blue shift in the electron energy loss spectrum (EELS) of GO was observed when the concentration of epoxy and hydroxyl functional groups increased [29, 30]. Interestingly, an increase in the density of carbonyl groups affects the GO EELS; as such, at O/C ~37.5% a red shift about 1.0 eV compared to the pristine graphene is observed [29]. Therefore, the density of surface functionalities can be precisely tuned to control the optical and electrochemical properties of graphene derivatives over a

have attracted enormous attention, and have been extensively studied for a wide range of applications—energy conversion in particular [32, 33]. Within their ultra-small sizes (typically less than 100 nm), their optical and electronic properties can be tailored, and a well-defined band gap can be formed [34]. Therefore, graphene QDs are like highly crystalline inorganic

semiconductor QDs with superior physically and chemically reactive facets [32].

hybridization forms when carbon atoms are bonded to epoxy or hydroxyl

hybridization in the

hybridized honeycomb carbon,

hybridization,

and/or ionic interactions [15, 18]. In addition, various densities of sp3

QDs, a multilayer zero-dimensional structure, contain sp<sup>2</sup>

GO structure create a wide range of interesting characteristics. Unlike sp<sup>2</sup>

devices [15, 16, 19].

90 Visible-Light Photocatalysis of Carbon-Based Materials

boxyl groups, sp3

wide range.

There are a handful of materials that can carry out visible light solar fuel generation reaction, exhibiting properties such as short band gap to harvest visible light, and suitable band-edge potential corresponding to redox potentials must be formed within the photocatalyst crystallography. Even then, the photo-generated charges must diffuse through a highly crystalline structure and interact with molecules and ions at highly reactive surface sites. Favorably, graphene as an abundant and cost-effective compound, is among the few nonmetallic photocatalysts that meet all the required conditions.

The molecular energy state of carbon is unique and contains 1s2 , 2s2 , and 2p2 orbitals, which contribute accordingly in various crystalline structures. As displayed in **Figure 2a**, the sp2 hybridized structure is formed from s, px , and py orbitals on each carbon atom connected to the surrounding atoms through three strong covalent σ bonds. In this structure, the remaining 2pz orbital, perpendicular to the graphene plane, overlapped with the one in the neighboring atoms, creating delocalized π (fill band) and π\* (empty band) orbitals, which are also known as graphenes VB and CB [35, 36].

The Fermi level in pristine graphene is located at the points connecting the valence and conduction bands in momentum space, as shown in **Figure 2b**, which is also known as the Dirac point [36]. Graphene exhibits an intrinsic n-type character [38]. Because of a small overlap between its valence and conduction bands, graphene has been characterized as a semimetal and/or a

**Figure 2.** (a) The 3D band structure of graphene. (b) The linear dispersion and the band structure at the Dirac point in graphene [37]. Reprinted with permission from [37]. Copyright 2010 American Chemical Society.

semiconductor with zero band gap energy [13, 39, 40]. This orbital structure creates high conductivity greater than that for silver, which is the least resistive metallic material [41].

By negative and positive doping of graphene's lattice, the state of the Fermi level around the Dirac point can be tuned; therefore, the electronic characteristics of graphene can be tailored. This unique characteristic is attributed to the increase in the concentration of charge carriers by two orders of magnitude, to obtain n- or p-type graphene [17, 35]. For instance, the Fermi level of graphene can be shifted below the Dirac point so p-type characteristics can be formed between graphene and the rutile TiO2 (110) surface, whereas the holes are accumulated on graphene and electrons are localized in the CB energy state of TiO<sup>2</sup> [38]. Therefore, the graphene electronic structure can be considered a photocatalyst design enabler, as the nanoscale p-n junction can be formed between localized p- and n-doped islands within the 2D crystallography.

From defect-free monolayer graphene to 3D aggregates of multilayer graphene, graphene nanoribbons, rGO, and finally GO, the band gap can be tuned within a wide range through engineering the morphology of the synthesized sample or surface modifications with functional groups. For instance, graphene nanoribbons are 1–100 nm wide, long strips of graphene, where a band gap is formed in the band structure of nanoribbon graphene when the width of the strips is reduced to less than 20 nm [42]. Due to such strange characteristics, graphene nanoribbons, nanomesh, and quantum dots exhibit semiconducting behavior with a band gap less than 0.5 eV [36]. Another strategy to engineer the band gap of graphene derivatives is intermolecular hybridization. An increase in the conductivity of graphene as a result of incorporation with another carbon compound was reported. For example, graphene derivatives–carbon nanotube composites, which are promising materials for transparent conductive materials, exhibited reduced surface resistance from their original 660 Ω sq.−1 and 890 Ω sq.−1 for rGO, and multiwalled CNTs, respectively, to ~100 Ω sq.−1 for the hybrid, although hybridization reduces the optical transmittance of the composite [43]. Similar observations were also reported for CNTs grown on graphene utilized for optoelectronic applications [44–46].

consideration, the thermal energy was shown to be very effective in the reduction of GO and the preparation of conductive film at the early stages (T < 700°C); even at excessive tem-

**Figure 3.** (a) Gradual chemical reduction of GO: the band gap energy and schematic structure of rGO at various stages of

American Chemical Society. (b) Percentage of different carbon bonds as a function of GO thermal reduction obtained via

of thermal reduction: Lattice at (c) room temperature; (d) ~100°C; (e) ~220°C; and (f) ~500°C, where the dark gray area

carbon and the corresponding oxygen concentration [49]. (c–f) Structural model of GO at various stages

the C–C bonds cannot be realized. From this result, Mattevi and coworkers [49] proposed a

in the course of thermal reduction, the connection between the original conductive graphitic clusters (shown as dark gray in **Figure 3c**) is formed, which leads to development of the sp2 network (**Figure 3d**). Electrical conductivity of polycrystalline graphite (1.25 × 10<sup>3</sup> S cm−1) can

genated groups. Thus, the interaction between molecular orbitals of carbon atoms and oxygen functionalities forms a forbidden electronic band as high as 2.7 eV, while maintaining sufficient carrier mobility to transfer the photo-induced charges. Crystalline defects create preferential bonding sites for the adsorption and deposition of atoms and molecules, which can be employed for the fabrication of interaction with active species in electrolyte. However, more

network (**Figure 3e**) and further enhanced up to the point that approaches the one for

network must be extensively interrupted by functionalization of various oxy-

fraction of ~0.87 when a great portion of the sp<sup>2</sup>

network cannot be eliminated and complete healing of

bonds to oxygen-containing groups [49]. Reprinted with

Role of Graphene in Photocatalytic Solar Fuel Generation

http://dx.doi.org/10.5772/intechopen.72623

93

changes from 3.5 to 1.0 eV [48]. Reprinted with permission from [48]. Copyright 2012

fraction of 0.95 (**Figure 3f**).

network. As shown in **Figure 3c**–**f**,

islands are connected via

reduction reactions, the

peratures (T~1000°C), the entire sp<sup>3</sup>

the chemical reduction showing Eg

XPS. Inset: sp<sup>2</sup>

indicates sp2

single-layer graphene (6 × 10<sup>3</sup> S cm−1) at an sp2

be obtained at an sp2

an sp2

graphene sp2

mechanistic model to describe the evolving conductive sp2

clusters and the light gray area represents sp3

permission from [49]. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

To accomplish a suitable band gap for solar water splitting and CO<sup>2</sup>

Owing to its large band gap, GO is characterized as an insulating material. Optical measurements of the carriers' lifetime in various wavelengths show an ns-scale decay, indicating semiconducting behavior of GO [47]. Chemical, electrochemical, and photo-induced reduction of GO to rGO are facile and functional routes to tune the band gap of semiconducting nanosheet-like compounds over a wide range, with conductivity up to 30,000 Sm−1 [30, 47]. Mathkar et al. [48] studied the controlled reduction of GO using hydrazine vapor while monitoring the optical and compositional properties of the obtained rGO. They successfully controlled the band gap of rGO from 3.5 eV (sp<sup>3</sup> rich) to 1.0 eV (sp<sup>2</sup> rich), as demonstrated in **Figure 3a**.

A similar transition pattern was reported by Mattevi et al. [49] who investigated the surface composition and structure of GO at various stages of thermal reduction. As presented in **Figure 3b**, functionality was developed between the density of sp2 bonds and the reductionderiving force (reduction temperature) that exhibited the transformation of an individual GO nanosheet to rGO with a variety of electronic properties. Taking this transition pattern into

semiconductor with zero band gap energy [13, 39, 40]. This orbital structure creates high con-

By negative and positive doping of graphene's lattice, the state of the Fermi level around the Dirac point can be tuned; therefore, the electronic characteristics of graphene can be tailored. This unique characteristic is attributed to the increase in the concentration of charge carriers by two orders of magnitude, to obtain n- or p-type graphene [17, 35]. For instance, the Fermi level of graphene can be shifted below the Dirac point so p-type characteristics can be

the graphene electronic structure can be considered a photocatalyst design enabler, as the nanoscale p-n junction can be formed between localized p- and n-doped islands within the

From defect-free monolayer graphene to 3D aggregates of multilayer graphene, graphene nanoribbons, rGO, and finally GO, the band gap can be tuned within a wide range through engineering the morphology of the synthesized sample or surface modifications with functional groups. For instance, graphene nanoribbons are 1–100 nm wide, long strips of graphene, where a band gap is formed in the band structure of nanoribbon graphene when the width of the strips is reduced to less than 20 nm [42]. Due to such strange characteristics, graphene nanoribbons, nanomesh, and quantum dots exhibit semiconducting behavior with a band gap less than 0.5 eV [36]. Another strategy to engineer the band gap of graphene derivatives is intermolecular hybridization. An increase in the conductivity of graphene as a result of incorporation with another carbon compound was reported. For example, graphene derivatives–carbon nanotube composites, which are promising materials for transparent conductive materials, exhibited reduced surface resistance from their original 660 Ω sq.−1 and 890 Ω sq.−1 for rGO, and multiwalled CNTs, respectively, to ~100 Ω sq.−1 for the hybrid, although hybridization reduces the optical transmittance of the composite [43]. Similar observations were also reported for CNTs grown on graphene utilized for optoelectronic

Owing to its large band gap, GO is characterized as an insulating material. Optical measurements of the carriers' lifetime in various wavelengths show an ns-scale decay, indicating semiconducting behavior of GO [47]. Chemical, electrochemical, and photo-induced reduction of GO to rGO are facile and functional routes to tune the band gap of semiconducting nanosheet-like compounds over a wide range, with conductivity up to 30,000 Sm−1 [30, 47]. Mathkar et al. [48] studied the controlled reduction of GO using hydrazine vapor while monitoring the optical and compositional properties of the obtained rGO. They successfully

A similar transition pattern was reported by Mattevi et al. [49] who investigated the surface composition and structure of GO at various stages of thermal reduction. As presented in

deriving force (reduction temperature) that exhibited the transformation of an individual GO nanosheet to rGO with a variety of electronic properties. Taking this transition pattern into

rich) to 1.0 eV (sp<sup>2</sup>

(110) surface, whereas the holes are accumu-

[38]. Therefore,

rich), as demonstrated

bonds and the reduction-

ductivity greater than that for silver, which is the least resistive metallic material [41].

lated on graphene and electrons are localized in the CB energy state of TiO<sup>2</sup>

formed between graphene and the rutile TiO2

92 Visible-Light Photocatalysis of Carbon-Based Materials

controlled the band gap of rGO from 3.5 eV (sp<sup>3</sup>

**Figure 3b**, functionality was developed between the density of sp2

2D crystallography.

applications [44–46].

in **Figure 3a**.

**Figure 3.** (a) Gradual chemical reduction of GO: the band gap energy and schematic structure of rGO at various stages of the chemical reduction showing Eg changes from 3.5 to 1.0 eV [48]. Reprinted with permission from [48]. Copyright 2012 American Chemical Society. (b) Percentage of different carbon bonds as a function of GO thermal reduction obtained via XPS. Inset: sp<sup>2</sup> carbon and the corresponding oxygen concentration [49]. (c–f) Structural model of GO at various stages of thermal reduction: Lattice at (c) room temperature; (d) ~100°C; (e) ~220°C; and (f) ~500°C, where the dark gray area indicates sp2 clusters and the light gray area represents sp3 bonds to oxygen-containing groups [49]. Reprinted with permission from [49]. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

consideration, the thermal energy was shown to be very effective in the reduction of GO and the preparation of conductive film at the early stages (T < 700°C); even at excessive temperatures (T~1000°C), the entire sp<sup>3</sup> network cannot be eliminated and complete healing of the C–C bonds cannot be realized. From this result, Mattevi and coworkers [49] proposed a mechanistic model to describe the evolving conductive sp2 network. As shown in **Figure 3c**–**f**, in the course of thermal reduction, the connection between the original conductive graphitic clusters (shown as dark gray in **Figure 3c**) is formed, which leads to development of the sp2 network (**Figure 3d**). Electrical conductivity of polycrystalline graphite (1.25 × 10<sup>3</sup> S cm−1) can be obtained at an sp2 fraction of ~0.87 when a great portion of the sp<sup>2</sup> islands are connected via an sp2 network (**Figure 3e**) and further enhanced up to the point that approaches the one for single-layer graphene (6 × 10<sup>3</sup> S cm−1) at an sp2 fraction of 0.95 (**Figure 3f**).

To accomplish a suitable band gap for solar water splitting and CO<sup>2</sup> reduction reactions, the graphene sp2 network must be extensively interrupted by functionalization of various oxygenated groups. Thus, the interaction between molecular orbitals of carbon atoms and oxygen functionalities forms a forbidden electronic band as high as 2.7 eV, while maintaining sufficient carrier mobility to transfer the photo-induced charges. Crystalline defects create preferential bonding sites for the adsorption and deposition of atoms and molecules, which can be employed for the fabrication of interaction with active species in electrolyte. However, more importantly, defects in the structure of graphene significantly enhance the density of dangling C─O bonds at the edges, which is very reactive for various redox reactions. The density of crystallographic defects must be optimized, as in contrast to aforementioned advantages; high density of structural defects increases the resistivity of graphene.

the N-doped graphene QDs divided by the original sp<sup>2</sup>

the photo-induced electrons and holes recombined at this contact, providing charges at

**Figure 4.** Hydrogen evolution from 20% MeOH solution and pure water using GO and 5 wt% Pt loaded GO photocatalysts (a) under UV–Vis irradiation and (b) under Vis irradiation. (c–f) Color variation of GO photocatalyst in a 20% MeOH solution indicating the self-reduction of GO (c) before irradiation; (d) at the start of irradiation; (e) after 30 min; and (f) after 2 h [50]. Reprinted with permission from [50]. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

**Figure 5.** Band structure of GO samples (a) before and (b) after UV irradiation. GO1, GO2, and GO3 denoted for GO samples processed at oxidized through Hummers method for 4, 12, and 24 h [51]. Reprinted with permission from [51].

Copyright 2011 American Chemical Society.

suitable energy states for overall water splitting, is demonstrated in **Figure 6**.

islands (ohmic contacts). Thus,

95

http://dx.doi.org/10.5772/intechopen.72623

Role of Graphene in Photocatalytic Solar Fuel Generation

GO has exhibited photo-induced activity for reduction reactions by promoting electrons to their conduction band upon photon absorption. For example, GO was used for the UV-assisted reduction of biological samples (reduction of resazurin to resorufin), showing no sign of degradation at 350 nm [24]. Yeh et al. [50] investigated the functionality of GO for UV–Vis-induced sacrificial hydrogen evolution from 20% MeOH aqueous solution. Over the course of the reaction, in addition to the proton reduction at the active sites, evidently a portion of the photoinduced electrons interacted with the surface oxygen functional groups; thus, the band gap of the spent photocatalyst was reduced to 2.4 eV from its original 4.3 eV. Interestingly, the bare GO sample generated 17,000 μmol within 6 h under a 400 W high pressure mercury lamp, which was considered higher than the one loaded with 5 wt% Pt and far above the amount of hydrogen obtained from pure water (280 μmol) for the same period (**Figure 4a**). The low activity of the Pt-loaded sample is likely attributed to surface coverage of GO with opaque Pt nanoparticles.

GO has also exhibited visible-light (λ > 400 nm) activity in MeOH solution, as shown in **Figure 4b**, reached QE = 0.01%, which is significantly lower than that observed under a mercury lamp (QE = 2.70%). The stable evolution of hydrogen even at low band gap energy not only suggests that the CB of semiconducting GO is laid down at a suitable energy state corresponding to H+ /H2 potential but also indicates that the transitional decay in the GO band gap is attributed to the upward shift of the VB, which is responsible for the lack of O<sup>2</sup> evolution, even in the presence of a scavenging Ag+ ion [50]. This hypothesis can be extracted from the spectroscopic measurements, indicating that the removal of oxygen-containing groups on the surface leads to the reduction in the band gap through shifting the VB maximum upward, while the CB potential remained nearly unchanged at −0.75 to −0.71 eV versus Ag/AgCl [51, 52]. Further oxidization of GO increases the band gap and provides sufficient overpotential at the GO molecular orbital for an O2 evolution reaction. However, the activity of the photocatalyst is expected to decline via photo-reduction, as shown for GO before and after photo reduction in **Figure 5a** and **b**, respectively [51].

The valence band maximum (VBM) and conduction band minimum (CBM) potential of GO can be tuned through doping, so within the localized sp2 islands, both p- and n-type conductivities can be formed [30]. Therefore, individual islands may be activated for one photocatalytic half-reaction corresponding to their electrical characteristics. GO is intrinsically p-type due to electron-withdrawing oxygen functional groups on its surface [17]. Replacing these oxygen-containing groups with nitrogen, via noninvasive routes such as high temperature ammonolysis [53], results in n-type GO [30]. N-doped graphene oxide quantum dots (NGO–QDs) containing 6% carbon-bonding composition with N1 s/ C1s = 2.9% posed a 2.2 eV band gap and exhibited overall water splitting under visiblelight irradiation, comparable to that of the Rh2-yCryO3 loaded GaN:ZnO solid solution photocatalyst, without the use of any precious metals [53]. The origin of this activity can be explained through the co-existence of p- and n-type conductivity within the lattice of the N-doped graphene QDs divided by the original sp<sup>2</sup> islands (ohmic contacts). Thus, the photo-induced electrons and holes recombined at this contact, providing charges at suitable energy states for overall water splitting, is demonstrated in **Figure 6**.

importantly, defects in the structure of graphene significantly enhance the density of dangling C─O bonds at the edges, which is very reactive for various redox reactions. The density of crystallographic defects must be optimized, as in contrast to aforementioned advantages; high

GO has exhibited photo-induced activity for reduction reactions by promoting electrons to their conduction band upon photon absorption. For example, GO was used for the UV-assisted reduction of biological samples (reduction of resazurin to resorufin), showing no sign of degradation at 350 nm [24]. Yeh et al. [50] investigated the functionality of GO for UV–Vis-induced sacrificial hydrogen evolution from 20% MeOH aqueous solution. Over the course of the reaction, in addition to the proton reduction at the active sites, evidently a portion of the photoinduced electrons interacted with the surface oxygen functional groups; thus, the band gap of the spent photocatalyst was reduced to 2.4 eV from its original 4.3 eV. Interestingly, the bare GO sample generated 17,000 μmol within 6 h under a 400 W high pressure mercury lamp, which was considered higher than the one loaded with 5 wt% Pt and far above the amount of hydrogen obtained from pure water (280 μmol) for the same period (**Figure 4a**). The low activity of the Pt-loaded sample is likely attributed to surface coverage of GO with opaque Pt

GO has also exhibited visible-light (λ > 400 nm) activity in MeOH solution, as shown in **Figure 4b**, reached QE = 0.01%, which is significantly lower than that observed under a mercury lamp (QE = 2.70%). The stable evolution of hydrogen even at low band gap energy not only suggests that the CB of semiconducting GO is laid down at a suitable energy state corre-

troscopic measurements, indicating that the removal of oxygen-containing groups on the surface leads to the reduction in the band gap through shifting the VB maximum upward, while the CB potential remained nearly unchanged at −0.75 to −0.71 eV versus Ag/AgCl [51, 52]. Further oxidization of GO increases the band gap and provides sufficient overpotential at the

is expected to decline via photo-reduction, as shown for GO before and after photo reduction

The valence band maximum (VBM) and conduction band minimum (CBM) potential of

conductivities can be formed [30]. Therefore, individual islands may be activated for one photocatalytic half-reaction corresponding to their electrical characteristics. GO is intrinsically p-type due to electron-withdrawing oxygen functional groups on its surface [17]. Replacing these oxygen-containing groups with nitrogen, via noninvasive routes such as high temperature ammonolysis [53], results in n-type GO [30]. N-doped graphene oxide quantum dots (NGO–QDs) containing 6% carbon-bonding composition with N1 s/ C1s = 2.9% posed a 2.2 eV band gap and exhibited overall water splitting under visible-

photocatalyst, without the use of any precious metals [53]. The origin of this activity can be explained through the co-existence of p- and n-type conductivity within the lattice of

attributed to the upward shift of the VB, which is responsible for the lack of O<sup>2</sup>

GO can be tuned through doping, so within the localized sp2

light irradiation, comparable to that of the Rh2-yCryO3

potential but also indicates that the transitional decay in the GO band gap is

ion [50]. This hypothesis can be extracted from the spec-

evolution reaction. However, the activity of the photocatalyst

evolution, even

islands, both p- and n-type

loaded GaN:ZnO solid solution

density of structural defects increases the resistivity of graphene.

94 Visible-Light Photocatalysis of Carbon-Based Materials

nanoparticles.

sponding to H+

/H2

GO molecular orbital for an O2

in **Figure 5a** and **b**, respectively [51].

in the presence of a scavenging Ag+

**Figure 4.** Hydrogen evolution from 20% MeOH solution and pure water using GO and 5 wt% Pt loaded GO photocatalysts (a) under UV–Vis irradiation and (b) under Vis irradiation. (c–f) Color variation of GO photocatalyst in a 20% MeOH solution indicating the self-reduction of GO (c) before irradiation; (d) at the start of irradiation; (e) after 30 min; and (f) after 2 h [50]. Reprinted with permission from [50]. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

**Figure 5.** Band structure of GO samples (a) before and (b) after UV irradiation. GO1, GO2, and GO3 denoted for GO samples processed at oxidized through Hummers method for 4, 12, and 24 h [51]. Reprinted with permission from [51]. Copyright 2011 American Chemical Society.

of dye molecules through the redox reaction. Pt-loaded rGO nanosheets as electron mediators for Eosin Y (EY) dye molecules has exhibited 9.3% QE under visible light, which is significantly greater than those recorded for EY-Pt and EY-rGO composites (**Figure 7a**). Due to the position of the energy bands, the excited electrons are transferred from the CB of the dye molecule to rGO

Further tailoring the band structure is shown to be highly effective, as the 5 wt% Pt-loaded GO nanosheets cosensitized by 1:1 M EY: Rose Bengal (RB) dye molecules reached quantum yield as high as 37.3% in 15 V% TEOA solution under two 450 W Xe lamps adjusted for 520

The process of transportation of an excited electron-hole pair from an emitter or "donor" to an absorbing medium, or "acceptor" is called nonradiative energy transfer (NRET). According to a study by Raja and coworkers, the rate of NRET in layered materials such as graphene and MoS<sup>2</sup>

function of number of layers by comparing the decay rates of quantum dot fluorescence when the

the population of charge carriers decays relatively slowly (a luminescence lifetime of 5 ns) in the

to high NRET rate, the photoluminescence (PL) lifetime decay decreased by an order of magnitude. Interestingly, their study indicates that the PL lifetime (inversely proportional to NRET

(**Figure 8b**). This finding confirms the strong electron acceptor role of graphene and provides hints for fabrication of highly efficient energy conversion/storage and optoelectronic devices.

In the first demonstration of graphene-based electron acceptors, Liu et al. [64] reported a solution-processable functionalized graphene electron-accepter with poly(3-octylthiophene) and poly(3-hexylthiophene) donor materials, reaching power conversion efficiency of 1.4% at 100 mW cm−2AM 1.5G. In 2008, Williams et al. [65] discovered a room-temperature technique for the in situ reduction of GO via the transfer of electrons from semiconductor bulk to nanosheets.

generated electron transfer from EY to rGO, and eventually to Pt, due to the composite energy level diagram [61].

Reprinted with permission from [61]. Copyright 2011 American Chemical Society.

evolution over EY-Pt, EY-RGO, and EY-RGO-Pt photocatalysts and (b) photo-

rate) drops and increases by increasing the number of layers in graphene and MoS<sup>2</sup>

is a

97

, respectively

[63]. As illustrated with a gray line in **Figure 8a**,

Role of Graphene in Photocatalytic Solar Fuel Generation

http://dx.doi.org/10.5772/intechopen.72623

). After planting QDs on 2D materials, due

and eventually to Pt active sires, as demonstrated in **Figure 7b**.

and 550 nm irradiation [62].

**Figure 7.** (a) The time courses of H<sup>2</sup>

**3.2. Graphene as electron acceptor**

chromophores are placed on graphene and MoS<sup>2</sup>

absence of the acceptor parties (graphene and MoS<sup>2</sup>

**Figure 6.** Mechanism of overall water splitting on p-type and n-type conductivity in GO lattice interconnected with sp2 ohmic contact [53]. Reprinted with permission from [53]. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Tan et al. [54], for the first time, reported the activity of isolated GO nanosheets for the conversion of CO2 to methane (0.0628 μmol g−1 h−1 for 2 h) under visible light (15 W energy-saving daylight bulb source), although in situ photo-induced removal of oxygen functional groups resulted in a gradual decrease in methanol generation. CO2 conversion to MeOH was also reported over modified graphene oxides with various band gap energies [55]. The phosphatemodified graphene with band gap ~3.2–4.4 eV produced methanol from CO<sup>2</sup> at 0.172 μmol gr−1 h−1, nearly six times higher than the one measured for commercial TiO2 under a 300 W halogen lamp. Since the photoexcitation in GO attributed to the surface oxygenated bonds, in this system, surface modification facilitated the electron/hole pair generation, which resulted in oxygen evolution and CO2 reduction at the VB and CB, respectively [53].

Due to the high redox activity of crystalline defects in 2D carbon structures, graphene hybridized light absorber antenna have been the subject of photo-electro-chemical research. Dye sensitization has emerged as an effective technique to harvest a visible portion of the solar spectrum, and an alternative to the utilization of costly inorganic semiconductors [56–60]. A positive charge on the surface of dye molecules promotes their association and interfacial contact with negatively charged GO through electrostatic attraction. Latorre-Sánchez and coworkers [52] have functionalized the surface of GO with various degrees of oxidation (~10% carbon-oxygen content), hybridized it with a series of dye molecules (up to 39 wt%), and tested the fabricated composite for H<sup>2</sup> evolution in 20 V% methanol solution under 532 nm monochromatic light. In their experiment, the composite of cationic and anodic Ru dye complexes were anchored to the interlayer space of multilayer GO. The cationic [Ru(bipy)<sup>3</sup> ] 2+ exhibited the highest total corrected hydrogen evolution (with regard to dye absorption spectra), while the anodic Ru polypyridyl complex (N719) demonstrated the highest hydrogen evolution rate after nearly 100 min induction period. The induction period for N719 dye is likely attributed to the activation period, and/or the relocation of dye molecules through the redox reaction. Pt-loaded rGO nanosheets as electron mediators for Eosin Y (EY) dye molecules has exhibited 9.3% QE under visible light, which is significantly greater than those recorded for EY-Pt and EY-rGO composites (**Figure 7a**). Due to the position of the energy bands, the excited electrons are transferred from the CB of the dye molecule to rGO and eventually to Pt active sires, as demonstrated in **Figure 7b**.

Further tailoring the band structure is shown to be highly effective, as the 5 wt% Pt-loaded GO nanosheets cosensitized by 1:1 M EY: Rose Bengal (RB) dye molecules reached quantum yield as high as 37.3% in 15 V% TEOA solution under two 450 W Xe lamps adjusted for 520 and 550 nm irradiation [62].

#### **3.2. Graphene as electron acceptor**

Tan et al. [54], for the first time, reported the activity of isolated GO nanosheets for the conver-

**Figure 6.** Mechanism of overall water splitting on p-type and n-type conductivity in GO lattice interconnected with

ohmic contact [53]. Reprinted with permission from [53]. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA,

daylight bulb source), although in situ photo-induced removal of oxygen functional groups

reported over modified graphene oxides with various band gap energies [55]. The phosphate-

halogen lamp. Since the photoexcitation in GO attributed to the surface oxygenated bonds, in this system, surface modification facilitated the electron/hole pair generation, which resulted

Due to the high redox activity of crystalline defects in 2D carbon structures, graphene hybridized light absorber antenna have been the subject of photo-electro-chemical research. Dye sensitization has emerged as an effective technique to harvest a visible portion of the solar spectrum, and an alternative to the utilization of costly inorganic semiconductors [56–60]. A positive charge on the surface of dye molecules promotes their association and interfacial contact with negatively charged GO through electrostatic attraction. Latorre-Sánchez and coworkers [52] have functionalized the surface of GO with various degrees of oxidation (~10% carbon-oxygen content), hybridized it with a series of dye molecules (up to 39 wt%), and tested the fabricated composite for H<sup>2</sup> evolution in 20 V% methanol solution under 532 nm monochromatic light. In their experiment, the composite of cationic and anodic Ru dye complexes were anchored to the interlayer space of

tion (with regard to dye absorption spectra), while the anodic Ru polypyridyl complex (N719) demonstrated the highest hydrogen evolution rate after nearly 100 min induction period. The induction period for N719 dye is likely attributed to the activation period, and/or the relocation

modified graphene with band gap ~3.2–4.4 eV produced methanol from CO<sup>2</sup>

gr−1 h−1, nearly six times higher than the one measured for commercial TiO2

]

resulted in a gradual decrease in methanol generation. CO2

to methane (0.0628 μmol g−1 h−1 for 2 h) under visible light (15 W energy-saving

reduction at the VB and CB, respectively [53].

2+ exhibited the highest total corrected hydrogen evolu-

conversion to MeOH was also

at 0.172 μmol

under a 300 W

sion of CO2

sp2

Weinheim.

in oxygen evolution and CO2

96 Visible-Light Photocatalysis of Carbon-Based Materials

multilayer GO. The cationic [Ru(bipy)<sup>3</sup>

The process of transportation of an excited electron-hole pair from an emitter or "donor" to an absorbing medium, or "acceptor" is called nonradiative energy transfer (NRET). According to a study by Raja and coworkers, the rate of NRET in layered materials such as graphene and MoS<sup>2</sup> is a function of number of layers by comparing the decay rates of quantum dot fluorescence when the chromophores are placed on graphene and MoS<sup>2</sup> [63]. As illustrated with a gray line in **Figure 8a**, the population of charge carriers decays relatively slowly (a luminescence lifetime of 5 ns) in the absence of the acceptor parties (graphene and MoS<sup>2</sup> ). After planting QDs on 2D materials, due to high NRET rate, the photoluminescence (PL) lifetime decay decreased by an order of magnitude. Interestingly, their study indicates that the PL lifetime (inversely proportional to NRET rate) drops and increases by increasing the number of layers in graphene and MoS<sup>2</sup> , respectively (**Figure 8b**). This finding confirms the strong electron acceptor role of graphene and provides hints for fabrication of highly efficient energy conversion/storage and optoelectronic devices.

In the first demonstration of graphene-based electron acceptors, Liu et al. [64] reported a solution-processable functionalized graphene electron-accepter with poly(3-octylthiophene) and poly(3-hexylthiophene) donor materials, reaching power conversion efficiency of 1.4% at 100 mW cm−2AM 1.5G. In 2008, Williams et al. [65] discovered a room-temperature technique for the in situ reduction of GO via the transfer of electrons from semiconductor bulk to nanosheets.

**Figure 7.** (a) The time courses of H<sup>2</sup> evolution over EY-Pt, EY-RGO, and EY-RGO-Pt photocatalysts and (b) photogenerated electron transfer from EY to rGO, and eventually to Pt, due to the composite energy level diagram [61]. Reprinted with permission from [61]. Copyright 2011 American Chemical Society.

this composite, rGO serves as an active site for H2

quantum efficiency at 420 nm (Na<sup>2</sup>

composite with n-type rGO-TiO2

consists of the BiVO<sup>4</sup>

CB of BiVO<sup>4</sup>

SrTiO3

rGO and over 140 times of the bare TaON.

as an electron mediator for efficient charge transfer within composite components. Due to the higher VB level of CdS, the photo-excited holes are promptly transferred to CdS and eventually to electrolyte and reduce the rate of recombination. By adding rGO and the Pt cocatalyst, the photoinduced electrons sink into rGO and subsequently into Pt, driven by their working function differentiation, which further reduces the recombination losses. The 1 wt% CdS-content ternary rGO hybridized core-shell composite decorated with 0.4% Pt reached 31% sacrificial apparent

Z-scheme photocatalytic systems have demonstrated tremendous potential for efficient

through the Z-scheme mechanism was published in 1997 by Sayama et al. [86]. Since then, extensive studies have placed emphasis on the structure and catalytic behavior of individual Z-schematic components and their electronic interaction. Z-scheme water splitting is particularly of interest because the wide range of reduction and oxidation light-absorbing components provides design flexibilities for solar fuel generation. The conventional Z-scheme

the electron mediators must efficiently transfer electrons by adsorbing and desorbing onto and from the surfaces of photocatalysts, unstable semiconductors, such as metal sulfides, are not considered good candidates for Z-scheme water splitting [89]. Besides its role as an electron acceptor in semiconductor composites, graphene derivatives were used as a solidstate electron mediator for Z-scheme photocatalytic systems. Graphene-based compounds are excellent conductive platforms as solid-state electron mediators and provide enormous

Recently, Iwashina et al. [89] synthesized a series of p-type metal sulfides (HER) in ternary

0.1 wt% Pt cocatalyst exhibited the highest activity for water splitting, yielding QE = 1.3% under 380 nm monochromatic irradiation. In this system, no appreciable gas evolution was

The effectiveness of rGO as a solid-state electron mediator for Z-schematic water splitting

evolving photocatalyst was investigated, as demonstrated in **Figure 9a**, which suggests enormous potential for electrolyte-independent overall water splitting [90, 91]. The electrons in the

water reduction and oxidization, respectively. Therefore, as depicted in **Figure 9b**, upon visible

evolution, respectively. Due to this effective electron mediator characteristic of rGO, as shown

:Rh and BiVO<sup>4</sup>

:Rh are transferred to the rGO mediator and recombined at the conductive support. The


(OER). Among the various composites, CuGaS<sup>2</sup>

possibilities for the commercialization of Z-scheme solar-fuel-generation technology.

Z-schematic mechanism and the efficient contribution of rGO as an electron mediator.

Ru-complex cocatalyst [Ru(2,2′-bipyridine)(4,4′-diphosphonate-2,2′-bipyridine)(CO)<sup>2</sup>

solar energy conversion. The first report on the stoichiometric water splitting into H<sup>2</sup>

S-Na<sup>2</sup> SO3

design is adapted through electron transfer from an O2

evolving photocatalyst via ionic electron mediators, such as IO3−/I<sup>−</sup>

observed in the absence of rGO, and/or either Pt-loaded CuGaS2

as O2

light excitation, the electrons in the CB of BiVO<sup>4</sup>

free electrons and holes on the SrTiO3

and the holes in the VB of SrTiO<sup>3</sup>

evolution, as discussed in Section 3.1, as well

Role of Graphene in Photocatalytic Solar Fuel Generation

http://dx.doi.org/10.5772/intechopen.72623

aqueous solution), over twice that of the one without


and Fe3+/Fe<sup>2</sup>

or TiO2

:Rh cannot meet the energy requirements for

and the holes in the electron-donor level of

surface freely participated in the H2

and O2

99


[87, 88]. Since

loaded with

, indicating the

decorated with

and O2

] 2+ as H2 -

**Figure 8.** PL lifetime decay of QDs deposited on 2D graphene and MoS<sup>2</sup> (a) and the impact of graphene and MoS<sup>2</sup> thickness on the PL lifetime of charge carriers (b) [63]. Reprinted with permission from [63]. Copyright 2016 American Chemical Society.

This discovery facilitated the utilization of graphene derivatives as electron acceptors for solar fuel generation. The interesting work of Ng and coworkers [66] is one of the early studies of such efforts through the fabrication of BiVO<sup>4</sup> hybridized GO followed by in situ photo deposition.

In the last decade, various combinations of semiconductors and graphene-derivative electron acceptors and active sites were studied for effective solar fuel generation. Thus far, the combination of graphene derivatives and promising semiconducting photocatalyst via facile techniques was reported, including metal oxides [67–70], nitrides [71, 72], oxynitrides [26, 73], sulfides, and oxysulfides [74–77], as well as ternary composites [78] and those based on abundant Earth materials [79–81]. To realize the effectiveness of such hybridization, a recent study by Tang and coworkers [82] can be discussed. An advanced ternary photocatalytic system of TiO2 nanotubedecorated CdS nanoparticles in composite with rGO nanosheets were synthesized and tested for photocatalytic hydrogen evolution where (1) the light absorption had been extended to the visible region; (2) the photo-excited charge separation significantly boosted and the electron path was configured to CdS(CB)TiO2(CB)rGO; and (3) CdS photo-corrosion was suppressed because of the rGO nanosheet protection (60 h stable H<sup>2</sup> evolution), via the so-called sheltering effect.

Readers are encouraged to read interesting review articles published recently on related topics, such as the one published by Xiang et al. [3] and Low et al. [6].

#### **3.3. Graphene as electron mediator**

Ternary nanocomposites of semiconductors and carbon-based materials can be designed and fabricated to tailor the absorption spectrum and to enhance the catalytic performance for solar fuel generation. The visible-light-responsive semiconductors were incorporated into the existing composites to further absorb the incident photons [82–85]. Effective electronic interaction between two photo-responsive components forms a recombination contact between the two and thus suppresses the unwanted recombination lost within one intrinsic component.

Hou et al. [78] reported an outstanding visible-light hydrogen evolution over the CdS-core TaONshell rGO ternary composite prepared via the hydrothermal-assisted ion-exchange technique. In this composite, rGO serves as an active site for H2 evolution, as discussed in Section 3.1, as well as an electron mediator for efficient charge transfer within composite components. Due to the higher VB level of CdS, the photo-excited holes are promptly transferred to CdS and eventually to electrolyte and reduce the rate of recombination. By adding rGO and the Pt cocatalyst, the photoinduced electrons sink into rGO and subsequently into Pt, driven by their working function differentiation, which further reduces the recombination losses. The 1 wt% CdS-content ternary rGO hybridized core-shell composite decorated with 0.4% Pt reached 31% sacrificial apparent quantum efficiency at 420 nm (Na<sup>2</sup> S-Na<sup>2</sup> SO3 aqueous solution), over twice that of the one without rGO and over 140 times of the bare TaON.

Z-scheme photocatalytic systems have demonstrated tremendous potential for efficient solar energy conversion. The first report on the stoichiometric water splitting into H<sup>2</sup> and O2 through the Z-scheme mechanism was published in 1997 by Sayama et al. [86]. Since then, extensive studies have placed emphasis on the structure and catalytic behavior of individual Z-schematic components and their electronic interaction. Z-scheme water splitting is particularly of interest because the wide range of reduction and oxidation light-absorbing components provides design flexibilities for solar fuel generation. The conventional Z-scheme design is adapted through electron transfer from an O2 -evolving photocatalyst to an H2 evolving photocatalyst via ionic electron mediators, such as IO3−/I<sup>−</sup> and Fe3+/Fe<sup>2</sup> [87, 88]. Since the electron mediators must efficiently transfer electrons by adsorbing and desorbing onto and from the surfaces of photocatalysts, unstable semiconductors, such as metal sulfides, are not considered good candidates for Z-scheme water splitting [89]. Besides its role as an electron acceptor in semiconductor composites, graphene derivatives were used as a solidstate electron mediator for Z-scheme photocatalytic systems. Graphene-based compounds are excellent conductive platforms as solid-state electron mediators and provide enormous possibilities for the commercialization of Z-scheme solar-fuel-generation technology.

This discovery facilitated the utilization of graphene derivatives as electron acceptors for solar fuel generation. The interesting work of Ng and coworkers [66] is one of the early studies of such

thickness on the PL lifetime of charge carriers (b) [63]. Reprinted with permission from [63]. Copyright 2016 American

In the last decade, various combinations of semiconductors and graphene-derivative electron acceptors and active sites were studied for effective solar fuel generation. Thus far, the combination of graphene derivatives and promising semiconducting photocatalyst via facile techniques was reported, including metal oxides [67–70], nitrides [71, 72], oxynitrides [26, 73], sulfides, and oxysulfides [74–77], as well as ternary composites [78] and those based on abundant Earth materials [79–81]. To realize the effectiveness of such hybridization, a recent study by Tang and

decorated CdS nanoparticles in composite with rGO nanosheets were synthesized and tested for photocatalytic hydrogen evolution where (1) the light absorption had been extended to the visible region; (2) the photo-excited charge separation significantly boosted and the electron path was configured to CdS(CB)TiO2(CB)rGO; and (3) CdS photo-corrosion was suppressed because

Readers are encouraged to read interesting review articles published recently on related top-

Ternary nanocomposites of semiconductors and carbon-based materials can be designed and fabricated to tailor the absorption spectrum and to enhance the catalytic performance for solar fuel generation. The visible-light-responsive semiconductors were incorporated into the existing composites to further absorb the incident photons [82–85]. Effective electronic interaction between two photo-responsive components forms a recombination contact between the two and thus suppresses the unwanted recombination lost within one intrinsic component.

Hou et al. [78] reported an outstanding visible-light hydrogen evolution over the CdS-core TaONshell rGO ternary composite prepared via the hydrothermal-assisted ion-exchange technique. In

coworkers [82] can be discussed. An advanced ternary photocatalytic system of TiO2

hybridized GO followed by in situ photo deposition.

(a) and the impact of graphene and MoS<sup>2</sup>

evolution), via the so-called sheltering effect.

nanotube-

efforts through the fabrication of BiVO<sup>4</sup>

98 Visible-Light Photocatalysis of Carbon-Based Materials

Chemical Society.

**Figure 8.** PL lifetime decay of QDs deposited on 2D graphene and MoS<sup>2</sup>

of the rGO nanosheet protection (60 h stable H<sup>2</sup>

**3.3. Graphene as electron mediator**

ics, such as the one published by Xiang et al. [3] and Low et al. [6].

Recently, Iwashina et al. [89] synthesized a series of p-type metal sulfides (HER) in ternary composite with n-type rGO-TiO2 (OER). Among the various composites, CuGaS<sup>2</sup> loaded with 0.1 wt% Pt cocatalyst exhibited the highest activity for water splitting, yielding QE = 1.3% under 380 nm monochromatic irradiation. In this system, no appreciable gas evolution was observed in the absence of rGO, and/or either Pt-loaded CuGaS2 or TiO2 , indicating the Z-schematic mechanism and the efficient contribution of rGO as an electron mediator.

The effectiveness of rGO as a solid-state electron mediator for Z-schematic water splitting consists of the BiVO<sup>4</sup> as O2 -evolution photocatalyst and Rh-doped SrTiO3 decorated with Ru-complex cocatalyst [Ru(2,2′-bipyridine)(4,4′-diphosphonate-2,2′-bipyridine)(CO)<sup>2</sup> ] 2+ as H2 evolving photocatalyst was investigated, as demonstrated in **Figure 9a**, which suggests enormous potential for electrolyte-independent overall water splitting [90, 91]. The electrons in the CB of BiVO<sup>4</sup> and the holes in the VB of SrTiO<sup>3</sup> :Rh cannot meet the energy requirements for water reduction and oxidization, respectively. Therefore, as depicted in **Figure 9b**, upon visible light excitation, the electrons in the CB of BiVO<sup>4</sup> and the holes in the electron-donor level of SrTiO3 :Rh are transferred to the rGO mediator and recombined at the conductive support. The free electrons and holes on the SrTiO3 :Rh and BiVO<sup>4</sup> surface freely participated in the H2 and O2 evolution, respectively. Due to this effective electron mediator characteristic of rGO, as shown in **Figure 9c**, the stoichiometric and stable visible-light driven Z-scheme overall water splitting for the BiVO<sup>4</sup> -rGO-SrTiO3 :Rh decorated with Ru-based HER cocatalyst system was observed.

The complex catalysts based on ruthenium have been extensively studied due to their potential in mimicking the plant photosynthesis process [92]. In particular, Ru complexes demonstrated stable and promising performances for CO2 conversion with high selectivity toward HCOOH [93]. The ruthenium complex reduction catalyst containing 2-thiophenyl benzimidazole ligands (schematic **Figure 10**) was studied for visible-light CO2 reduction while it was covalently anchored to GO through the epoxide groups on the GO surface [94]. The treatment of GO with chloroacetic acid leads to conversion of the ─OH and epoxide groups to ─COOH groups, which was further treated by thionyl chloride to transform GO─COOH to ─COCl functionalized GO. This 2D platform is ideal for interaction with the Ru-complex, as schematically illustrated in **Figure 10**. The Ru-complex-GO composite produced over 2000 μmol g−1 of formic acid in 20 h, without use of a sacrificial reagent.

Kuai and coworkers [95] fabricated TiO2 –CdS encapsulated rGO composite where graphene was used as a solid-sate electron mediator for the Z-schematic conversion of CO<sup>2</sup> in the presence of water vapor. Hydrothermally prepared CdS nanospheres, as shown in **Figure 11a** and **b**, were positively charged and wrapped inside rGO nanosheets via self-assembly, which was induced by electrostatic forces (**Figure 11c** and **d**), followed by kinetic-controlled deposition of TiO2 nanoparticles, as illustrated in **Figure 11e**. Such nanostructure is highly beneficial for Z-scheme photocatalysis, since rGO is positioned between two redox antenna/active sites, as demonstrated in **Figure 11f**, where ideal interfacial contact between the composite components can be maintained. Thus, the photo-induced electrons in TiO2 CB are transferred to rGO and subsequently recombine with the holes generated at the CdS nanosphere's VB, resulting in enhanced density of photo-generated electrons and holes on the CdS nanosphere and TiO2 nanoparticles, respectively. The photocatalytic efficiency of the CdS-rGO-TiO<sup>2</sup> Z-scheme system under 300 W Xe lamp irradiation was remarkably enhanced relative to pristine CdS, CdS-TiO2 , and CdS-rGO composites.

**3.4. Graphene as thermally-induced medium**

Copyright 2015 Royal Society of Chemistry.

Until now, the majority of studies on graphene-based photocatalysts have placed their focus on the role of graphene derivatives as transparent/flexible electrodes, electron acceptors, and electron mediators. Recently, the functionality of graphene-based compounds was investigated as a light absorber. In the recent years, it was concluded that graphene could absorb the entire solar spectrum because of its black color and zero band gap. Although such capability does not lead to active electron/hole pair generation for photocatalytic reaction, it does result in local high-temperature zones inside and on the surface of the photocatalyst, due to the photo-thermal effect, and enhances photocatalytic activity. According to a study by Gan and coworkers [96], such a photo-thermal effect contributes up to 38% in the photo-degradation of organic pollutants in the P25-rGO composite.

**Figure 10.** Surface functionalization of GO with a Ru catalyst complex [94]. Reprinted with permission from [94].

Role of Graphene in Photocatalytic Solar Fuel Generation

http://dx.doi.org/10.5772/intechopen.72623

101

Colloidal QDs exhibit high quantum efficiencies through band gap engineering methods and are known to be excellent absorbers and emitters at various wavelengths [63]. Graphene QDs can be prepared via green and facile techniques [97], and have demonstrated strong quantum confinement at sizes below 10 nm [98]. The challenge of employing the interesting PL properties of graphene QDs for visible-light photocatalytic applications is their excitation-dependent PL properties, which means that at various excitation wavelengths, different energies are induced in the orbital structure of QDs. Zhuo et al. [98] prepared graphene QDs through the facile ultrasound route and exhibited an excitation-independent PL peak at 407 nm. Due to this constant PL emission, rutile

**Figure 9.** (a) Schematic illustration of rGO-mediated Z-scheme water splitting in Ru-SrTiO3:Rh (HER) and BiVO<sup>4</sup> (OER) system. (b) Photo-induced charge transfer driven by the energy band structure of the composite. (c) Time-course of visible light Z-scheme overall water splitting of Ru-SrTiO<sup>3</sup> :Rh-rGO-BiVO<sup>4</sup> under a 300 W Xe lamp [90]. Reprinted with permission from [90]. Copyright 2011 American Chemical Society.

**Figure 10.** Surface functionalization of GO with a Ru catalyst complex [94]. Reprinted with permission from [94]. Copyright 2015 Royal Society of Chemistry.

#### **3.4. Graphene as thermally-induced medium**

in **Figure 9c**, the stoichiometric and stable visible-light driven Z-scheme overall water splitting

The complex catalysts based on ruthenium have been extensively studied due to their potential in mimicking the plant photosynthesis process [92]. In particular, Ru complexes demon-

HCOOH [93]. The ruthenium complex reduction catalyst containing 2-thiophenyl benzimid-

covalently anchored to GO through the epoxide groups on the GO surface [94]. The treatment of GO with chloroacetic acid leads to conversion of the ─OH and epoxide groups to ─COOH groups, which was further treated by thionyl chloride to transform GO─COOH to ─COCl functionalized GO. This 2D platform is ideal for interaction with the Ru-complex, as schematically illustrated in **Figure 10**. The Ru-complex-GO composite produced over 2000 μmol g−1

water vapor. Hydrothermally prepared CdS nanospheres, as shown in **Figure 11a** and **b**, were positively charged and wrapped inside rGO nanosheets via self-assembly, which was induced by electrostatic forces (**Figure 11c** and **d**), followed by kinetic-controlled deposition of TiO2 nanoparticles, as illustrated in **Figure 11e**. Such nanostructure is highly beneficial for Z-scheme photocatalysis, since rGO is positioned between two redox antenna/active sites, as demonstrated in **Figure 11f**, where ideal interfacial contact between the composite components can be main-

recombine with the holes generated at the CdS nanosphere's VB, resulting in enhanced density of

**Figure 9.** (a) Schematic illustration of rGO-mediated Z-scheme water splitting in Ru-SrTiO3:Rh (HER) and BiVO<sup>4</sup>

system. (b) Photo-induced charge transfer driven by the energy band structure of the composite. (c) Time-course of

:Rh-rGO-BiVO<sup>4</sup>

:Rh decorated with Ru-based HER cocatalyst system was observed.

conversion with high selectivity toward

–CdS encapsulated rGO composite where graphene was

CB are transferred to rGO and subsequently

Z-scheme system under 300 W Xe lamp irra-

reduction while it was

in the presence of

nanoparticles, respectively.

, and CdS-rGO composites.

under a 300 W Xe lamp [90]. Reprinted with

(OER)

for the BiVO<sup>4</sup>


100 Visible-Light Photocatalysis of Carbon-Based Materials

Kuai and coworkers [95] fabricated TiO2

tained. Thus, the photo-induced electrons in TiO2

The photocatalytic efficiency of the CdS-rGO-TiO<sup>2</sup>

visible light Z-scheme overall water splitting of Ru-SrTiO<sup>3</sup>

permission from [90]. Copyright 2011 American Chemical Society.

strated stable and promising performances for CO2

of formic acid in 20 h, without use of a sacrificial reagent.

azole ligands (schematic **Figure 10**) was studied for visible-light CO2

used as a solid-sate electron mediator for the Z-schematic conversion of CO<sup>2</sup>

photo-generated electrons and holes on the CdS nanosphere and TiO2

diation was remarkably enhanced relative to pristine CdS, CdS-TiO2

Until now, the majority of studies on graphene-based photocatalysts have placed their focus on the role of graphene derivatives as transparent/flexible electrodes, electron acceptors, and electron mediators. Recently, the functionality of graphene-based compounds was investigated as a light absorber. In the recent years, it was concluded that graphene could absorb the entire solar spectrum because of its black color and zero band gap. Although such capability does not lead to active electron/hole pair generation for photocatalytic reaction, it does result in local high-temperature zones inside and on the surface of the photocatalyst, due to the photo-thermal effect, and enhances photocatalytic activity. According to a study by Gan and coworkers [96], such a photo-thermal effect contributes up to 38% in the photo-degradation of organic pollutants in the P25-rGO composite.

Colloidal QDs exhibit high quantum efficiencies through band gap engineering methods and are known to be excellent absorbers and emitters at various wavelengths [63]. Graphene QDs can be prepared via green and facile techniques [97], and have demonstrated strong quantum confinement at sizes below 10 nm [98]. The challenge of employing the interesting PL properties of graphene QDs for visible-light photocatalytic applications is their excitation-dependent PL properties, which means that at various excitation wavelengths, different energies are induced in the orbital structure of QDs. Zhuo et al. [98] prepared graphene QDs through the facile ultrasound route and exhibited an excitation-independent PL peak at 407 nm. Due to this constant PL emission, rutile

**4. Conclusion and perspective**

and atmospheric CO2

**Author details**

and Fariborz Taghipour1,2\*

\*Address all correspondence to: fariborz.taghipour@ubc.ca

University of British Columbia, Vancouver, BC, Canada

Technology. 2013;**2**:Q118-Q126

Babak Adeli<sup>1</sup>

Canada

**References**

Graphene-derivatives, such as graphene oxide, reduced graphene oxide, and their functionalized materials are attractive components in optoelectronic device fabrication, owing to their peculiar optical, thermal, mechanical, and electrochemical properties. In particular, the 2D carbon nanostructure has proven to be an excellent candidate for the extraction of solar energy and

Graphene derivatives' ultra-high specific surface area, which promotes the rate of redox reactions; its tunable band gap, which controls their absorption spectrum; its controllable defects density, which promotes their reactivity; and its carrier mobility, which promotes charge separation, all offer design flexibility at the nanoscale. However, the performance of graphene-based photocatalytic systems is vastly unpredictable, and understanding the role of graphene in photocatalytic fuel generation is still under debate. Over 18,000 articles related to graphene were published from 2004 to 2014. This rapid progress seeks continuous publication of critical reviews to genuinely add to the existing literature and to discuss the state-of-the-art development. Looking at the fast-paced advances in nanotechnology and materials sciences, it is apparent that the use of solar energy is an indispensable reality. The sunlight-induced splitting of waste water

well as bacterial disinfection and pollutant decomposition. Therefore, the search for a highly active photocatalyst that can be produced through low-cost and scalable routes, exhibits a stable performance, and does not pose any threat to the environment is an extremely important task. Graphene derivatives are among the few candidates that meet all the aforementioned conditions.

1 Photoreaction Engineering Laboratory, Chemical and Biological Engineering Department,

2 Clean Energy Research Centre (CERC), University of British Columbia, Vancouver, BC,

[1] Adeli B, Taghipour FA. Review of synthesis techniques for gallium-zinc oxynitride solar-activated photocatalyst for water splitting. ECS Journal of Solid State Science and

[3] Xiang Q, Cheng B, Yu J. Graphene-based photocatalysts for solar-fuel generation.

[2] Crabtree GW, Lewis NS. Solar energy conversion. Physics Today. 2007;**60**:37-42

Angewandte Chemie International Edition. 2015;**54**:11350-11366

reduction offers the onsite production of clean and renewable energy, as

conversion.

103

Role of Graphene in Photocatalytic Solar Fuel Generation

http://dx.doi.org/10.5772/intechopen.72623

its chemical transformation to fuel through photocatalytic water splitting and CO<sup>2</sup>

**Figure 11.** SEM images of CdS NSs (a and b), CdS NSs/GO (c and d), CdS NSs/rGO/TiO<sup>2</sup> (e) and the HRTEM image of CdS NSs/rGO/TiO<sup>2</sup> (f) [95]. Reprinted with permission from [95]. Copyright 2015 Royal Society of Chemistry.

TiO2 -QDs showed nine-fold visible-light catalytic activity (since it has a narrower band gap than 407 nm), compared to that of anatase TiO<sup>2</sup> (a band gap larger than 407 nm).

Recently, free-standing vertical graphene exhibited high potential as a light absorber. Zhao and coworkers [99] proposed a detailed mechanism for plasma-enhanced chemical vapor deposition of free-standing vertical graphene nanosheets, which is validated by transmission electron microscopy observations at the nucleation and growth stages. Davami et al. [100] reported that such morphology absorbs up to 97% of incident photons, which is much higher than previous carbon-based absorber materials, such as forest CNTs. Although free-standing graphene absorbers are yet to be studied for the solar-fuel-generation application, preliminary research data suggest their great potential for future-generation photocatalytic applications.

### **4. Conclusion and perspective**

Graphene-derivatives, such as graphene oxide, reduced graphene oxide, and their functionalized materials are attractive components in optoelectronic device fabrication, owing to their peculiar optical, thermal, mechanical, and electrochemical properties. In particular, the 2D carbon nanostructure has proven to be an excellent candidate for the extraction of solar energy and its chemical transformation to fuel through photocatalytic water splitting and CO<sup>2</sup> conversion. Graphene derivatives' ultra-high specific surface area, which promotes the rate of redox reactions; its tunable band gap, which controls their absorption spectrum; its controllable defects density, which promotes their reactivity; and its carrier mobility, which promotes charge separation, all offer design flexibility at the nanoscale. However, the performance of graphene-based photocatalytic systems is vastly unpredictable, and understanding the role of graphene in photocatalytic fuel generation is still under debate. Over 18,000 articles related to graphene were published from 2004 to 2014. This rapid progress seeks continuous publication of critical reviews to genuinely add to the existing literature and to discuss the state-of-the-art development. Looking at the fast-paced advances in nanotechnology and materials sciences, it is apparent that the use of solar energy is an indispensable reality. The sunlight-induced splitting of waste water and atmospheric CO2 reduction offers the onsite production of clean and renewable energy, as well as bacterial disinfection and pollutant decomposition. Therefore, the search for a highly active photocatalyst that can be produced through low-cost and scalable routes, exhibits a stable performance, and does not pose any threat to the environment is an extremely important task. Graphene derivatives are among the few candidates that meet all the aforementioned conditions.

### **Author details**

Babak Adeli<sup>1</sup> and Fariborz Taghipour1,2\*

\*Address all correspondence to: fariborz.taghipour@ubc.ca

1 Photoreaction Engineering Laboratory, Chemical and Biological Engineering Department, University of British Columbia, Vancouver, BC, Canada

2 Clean Energy Research Centre (CERC), University of British Columbia, Vancouver, BC, Canada

### **References**

TiO2

CdS NSs/rGO/TiO<sup>2</sup>

407 nm), compared to that of anatase TiO<sup>2</sup>

102 Visible-Light Photocatalysis of Carbon-Based Materials


(f) [95]. Reprinted with permission from [95]. Copyright 2015 Royal Society of Chemistry.

Recently, free-standing vertical graphene exhibited high potential as a light absorber. Zhao and coworkers [99] proposed a detailed mechanism for plasma-enhanced chemical vapor deposition of free-standing vertical graphene nanosheets, which is validated by transmission electron microscopy observations at the nucleation and growth stages. Davami et al. [100] reported that such morphology absorbs up to 97% of incident photons, which is much higher than previous carbon-based absorber materials, such as forest CNTs. Although free-standing graphene absorbers are yet to be studied for the solar-fuel-generation application, preliminary research data suggest their great potential for future-generation photocatalytic applications.

**Figure 11.** SEM images of CdS NSs (a and b), CdS NSs/GO (c and d), CdS NSs/rGO/TiO<sup>2</sup>

(a band gap larger than 407 nm).

(e) and the HRTEM image of


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**Chapter 7**

Provisional chapter

**CQD-Based Composites as Visible-Light Active**

DOI: 10.5772/intechopen.74245

The unique physicochemical properties of carbon quantum dot-(CQD)-based photocatalysts, notably their exceptionally good light absorption in the UV and near-visible region, tunable photoluminescence, extraordinary upconversion photoluminescence, outstanding electron affinity, and photoinduced electron transfer, and electron mobility, have attracted considerable attention in different photocatalytic applications. In this review, we summarized the fundamental mechanism and thermodynamics of heterogeneous photocatalysis of aqueous pollutants and the fundamental multifaceted roles of CQDs in photoredox process. Furthermore, we discussed the recent developments in the use of CQD-based materials as visiblelight active photocatalysts in water purification. Finally, the challenges and future direction of CQD-based materials as photocatalytic materials for environmental decontamination were

Carbon quantum dots (CQDs) are a recently discovered class of carbon-based nanomaterials that are typically discrete, quasi-spherical, and less than 10 nm in size [1] (although CQDs of sizes >10 nm have been reported [2–5]). They are composed of sp2=sp3-hybridized carbon atoms, have various surface functional groups, and possess composition-dependent fluorescence. This relatively new material has attracted huge interest in many fields including, but not limited to, electrocatalysis [6], biosensing [7–10], bioimaging [11–14], chemical sensing [15],

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Keywords: carbon, CQDs, photocatalyst, upconversion, visible-light

CQD-Based Composites as Visible-Light Active

**Photocatalysts for Purification of Water**

Photocatalysts for Purification of Water

Abdullahi Baba Makama, Muneer Umar and

Abdullahi Baba Makama, Muneer Umar and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Shettima Abdulkadir Saidu

Shettima Abdulkadir Saidu

Abstract

highlighted.

1. Introduction

http://dx.doi.org/10.5772/intechopen.74245

#### **CQD-Based Composites as Visible-Light Active Photocatalysts for Purification of Water** CQD-Based Composites as Visible-Light Active Photocatalysts for Purification of Water

DOI: 10.5772/intechopen.74245

Abdullahi Baba Makama, Muneer Umar and Shettima Abdulkadir Saidu Abdullahi Baba Makama, Muneer Umar and Shettima Abdulkadir Saidu

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.74245

#### Abstract

The unique physicochemical properties of carbon quantum dot-(CQD)-based photocatalysts, notably their exceptionally good light absorption in the UV and near-visible region, tunable photoluminescence, extraordinary upconversion photoluminescence, outstanding electron affinity, and photoinduced electron transfer, and electron mobility, have attracted considerable attention in different photocatalytic applications. In this review, we summarized the fundamental mechanism and thermodynamics of heterogeneous photocatalysis of aqueous pollutants and the fundamental multifaceted roles of CQDs in photoredox process. Furthermore, we discussed the recent developments in the use of CQD-based materials as visiblelight active photocatalysts in water purification. Finally, the challenges and future direction of CQD-based materials as photocatalytic materials for environmental decontamination were highlighted.

Keywords: carbon, CQDs, photocatalyst, upconversion, visible-light

#### 1. Introduction

Carbon quantum dots (CQDs) are a recently discovered class of carbon-based nanomaterials that are typically discrete, quasi-spherical, and less than 10 nm in size [1] (although CQDs of sizes >10 nm have been reported [2–5]). They are composed of sp2=sp3-hybridized carbon atoms, have various surface functional groups, and possess composition-dependent fluorescence. This relatively new material has attracted huge interest in many fields including, but not limited to, electrocatalysis [6], biosensing [7–10], bioimaging [11–14], chemical sensing [15],

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and nanomedicine [16], due to their unique tunable photoluminescence (PL) properties, chemical inertness, high water solubility, ease and low cost of fabrication and, more importantly, low toxicity. Additionally, CQDs have also attracted considerable interest in various photocatalytic applications—environmental remediation [17–21], water splitting to produce H2 production [22–27], CO2 conversion [28–30], and synthesis of chemicals [28–33]—because when coupled with a semiconductor photocatalyst, CQDs can provide with several advantages, including improved light harvesting ability, efficient usage of the full spectrum of sunlight, efficient charge carrier separation, stability, and hinder charge recombination. Figure 1 illustrates the various applications of CQDs in photocatalysis (left) and their competitive optical and structural properties during each photocatalytic procedure (right).

Figure 1. Illustration showing the different areas of applications of CQDs (reproduced from [34]).

### 2. Principles of heterogeneous photocatalysis

The fundamentals of heterogeneous photocatalysis underlying the degradation of a pollutant employing a semiconductor catalyst have been intensively reported in many literatures [35–41].

First, the spatially separated charge carriers e�

may be summarized in a simplified way according to Eqs. (1)–(5).

Charge generation : SC þ hη ≥ E<sup>g</sup>

Charge recombination : SC e�

Charge separation : SC e�=h<sup>þ</sup> ! SC e�

vb; h<sup>þ</sup>

levels of the adsorbate species [40–42].

(reproduced from [43]).

cb=h<sup>þ</sup>

face where they can induce a redox reaction. At the surface, the electrons can be transferred to reduce an electron acceptor (A) (pathway 1). On the other hand, a donor species (D) can donate an electron to the surface hole and gets oxidized in the process (pathway 2). The probability and rate of the charge transfer processes for electrons and holes depends upon the respective positions of the band edges for the conduction and valence bands and the redox potential

Figure 2. Schematic illustration of the principles and fundamental kinetic requirements of heterogeneous photocatalysis

In competition with charge transfer to adsorbed species is electron and hole recombination. Recombination of the separated electron and hole can occur on the surface (pathway 3) or in the volume of the semiconductor particle (pathway 4). The electron-hole recombination can occur nonradioactively or radioactively to dissipate the input energy either as heat (E) or as a photon (hη) [44, 45], respectively. The widely postulated steps of heterogeneous photocatalysts

vb can migrate to the semiconductor sur-

CQD-Based Composites as Visible-Light Active Photocatalysts for Purification of Water

http://dx.doi.org/10.5772/intechopen.74245

113

! SC e�=h<sup>þ</sup> (1)

cb ! SC <sup>þ</sup> E or <sup>h</sup><sup>η</sup> (3)

vb (2)

cb; h<sup>þ</sup>

A semiconductor (SC) is a material that has a valence band (VB) and a nearly empty conduction band (CB) that are separated from one another by a band gap E<sup>g</sup> . In the ground state (i.e., at T ¼ 0 K), all the electrons are found in the VB of the SC. When illuminated by light of energy greater than or equal to the band gap (hv ≥ Eg) of a semiconductor, electrons in the VB are excited to the CB, leaving behind equal number of voids (or holes) in the valence band. The generation of electron-hole pairs e� cb=h<sup>þ</sup> vb is the first step in the heterogeneous photocatalysis of organic and inorganic compounds by semiconductors. This step is illustrated by the enlarged section of Figure 2. Once generated, the fate of the electron and hole can follow several de-excitation pathways as shown in Figure 2.

CQD-Based Composites as Visible-Light Active Photocatalysts for Purification of Water http://dx.doi.org/10.5772/intechopen.74245 113

and nanomedicine [16], due to their unique tunable photoluminescence (PL) properties, chemical inertness, high water solubility, ease and low cost of fabrication and, more importantly, low toxicity. Additionally, CQDs have also attracted considerable interest in various photocatalytic applications—environmental remediation [17–21], water splitting to produce H2 production [22–27], CO2 conversion [28–30], and synthesis of chemicals [28–33]—because when coupled with a semiconductor photocatalyst, CQDs can provide with several advantages, including improved light harvesting ability, efficient usage of the full spectrum of sunlight, efficient charge carrier separation, stability, and hinder charge recombination. Figure 1 illustrates the various applications of CQDs in photocatalysis (left) and their competitive optical

and structural properties during each photocatalytic procedure (right).

112 Visible-Light Photocatalysis of Carbon-Based Materials

2. Principles of heterogeneous photocatalysis

several de-excitation pathways as shown in Figure 2.

generation of electron-hole pairs e�

tion band (CB) that are separated from one another by a band gap E<sup>g</sup>

Figure 1. Illustration showing the different areas of applications of CQDs (reproduced from [34]).

cb=h<sup>þ</sup> vb

The fundamentals of heterogeneous photocatalysis underlying the degradation of a pollutant employing a semiconductor catalyst have been intensively reported in many literatures [35–41]. A semiconductor (SC) is a material that has a valence band (VB) and a nearly empty conduc-

(i.e., at T ¼ 0 K), all the electrons are found in the VB of the SC. When illuminated by light of energy greater than or equal to the band gap (hv ≥ Eg) of a semiconductor, electrons in the VB are excited to the CB, leaving behind equal number of voids (or holes) in the valence band. The

of organic and inorganic compounds by semiconductors. This step is illustrated by the enlarged section of Figure 2. Once generated, the fate of the electron and hole can follow

is the first step in the heterogeneous photocatalysis

. In the ground state

Figure 2. Schematic illustration of the principles and fundamental kinetic requirements of heterogeneous photocatalysis (reproduced from [43]).

First, the spatially separated charge carriers e� cb=h<sup>þ</sup> vb can migrate to the semiconductor surface where they can induce a redox reaction. At the surface, the electrons can be transferred to reduce an electron acceptor (A) (pathway 1). On the other hand, a donor species (D) can donate an electron to the surface hole and gets oxidized in the process (pathway 2). The probability and rate of the charge transfer processes for electrons and holes depends upon the respective positions of the band edges for the conduction and valence bands and the redox potential levels of the adsorbate species [40–42].

In competition with charge transfer to adsorbed species is electron and hole recombination. Recombination of the separated electron and hole can occur on the surface (pathway 3) or in the volume of the semiconductor particle (pathway 4). The electron-hole recombination can occur nonradioactively or radioactively to dissipate the input energy either as heat (E) or as a photon (hη) [44, 45], respectively. The widely postulated steps of heterogeneous photocatalysts may be summarized in a simplified way according to Eqs. (1)–(5).

$$\text{Charge generation}: \quad \mathbf{SC} + h\eta \left( \geq \mathbf{E}\_{\mathfrak{g}} \right) \to \mathbf{SC} \left( \mathbf{e}^{-}/\mathbf{h}^{+} \right) \tag{1}$$

Charge separation : SC e�=h<sup>þ</sup> ! SC e� cb; h<sup>þ</sup> vb (2)

$$\begin{array}{c} \text{Charge recombination}: \quad \mathbf{SC}(\mathbf{e}\_{\text{vb}}^{-}, \mathbf{h}\_{\text{cb}}^{+}) \to \mathbf{SC} + \mathbf{E} \text{ or } h\eta \end{array} \tag{3}$$

$$\text{Charge trapping} : \quad \text{SC}(\mathbf{e}\_{\text{vb}}^{-}, \mathbf{h}\_{\text{cb}}^{+}) + 2\text{Tr} \to \text{SC}(\mathbf{e}\_{\text{Tr}}^{-}, \mathbf{h}\_{\text{Tr}}^{+}) \tag{4}$$

$$\text{Reduction reaction} \quad \text{A} + \text{e}\_{\text{Tr}}^{-} \to \text{A}^{\bullet -} \tag{5}$$

$$\text{Oxidation reaction} \quad \mathbf{D} + \mathbf{h}\_{\text{Tr}}^{+} \to \mathbf{D}^{\bullet+} \tag{6}$$

While choosing a photocatalyst for application in environmental remediation, it is also essential to take into account the reduction potentials (E) of the substrate, as well as those of the intermediates that are formed during the photocatalytic reaction(s). However, the reduction potentials for different organic or inorganic compounds are usually dependent on the reaction conditions such as pH and the employed electrolyte. The reduction potential of the couples

Figure 4. Bandgap energy structure of several common semiconductors on a potential scale (V) vs. NHE/vacuum

These reactions refer to one-electron reduction reactions—the potentials of which are given versus the standard hydrogen electrode (SHE). Table 1 includes the reduction potentials of the

For example, over a TiO2 photocatalyst in pH 7 solution, reaction Eq. (9) is feasible because the

most important inorganic species that may be present in photocatalytic systems.

oxidation potential (E<sup>0</sup> H2O=OH• ð Þ¼þ2:27 V vs. SHE) of the hydroxyl radical.

H2O ionization : H<sup>þ</sup>

O2 isonosorption : O2 þ e�

oxidant þ e� ! Reductant (7)

oxidant þ e� þ nH<sup>þ</sup> ! Reductant (8)

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cb ¼ �0:52 V vs. SHE is sufficiently more negative than the reduction

vb ¼ þ2:53 V vs. SHE is sufficiently more positive than the

cb ! O2

ads þ h<sup>þ</sup>

•� (9)

vb ! OH• <sup>þ</sup> <sup>H</sup><sup>þ</sup> (10)

<sup>¼</sup> -0:28 V vs. SHE) of the superoxide radicals. Reaction (10) occurs

ads þ OH�

M=M•� refers to the reactions described by Eqs (7) and (8):

redox potential of the e�

2

because the redox potential of the h<sup>þ</sup>

potential (E<sup>0</sup> O2=O•�

(reproduced from [51]).

A thermodynamic requirement of spontaneous reaction is a negative change in Gibb's energy, (i.e., ΔG < 0) [46]. In a photocatalytic reaction, this condition is met when the quasi-Fermi (usually taken as the flat band potentials) energy levels of the photogenerated electrons and holes of the photocatalyst straddle the reduction/oxidation potentials of the substrate. In other words, the bottom of the photocatalyst's conduction bands (Emin CB ) must be located at a more negative potential than the reduction potential (E<sup>0</sup> red) of the substrate, while the top of its valence bands (Emax VB ) must be positioned more positively than the oxidation potential (E<sup>0</sup> ox) of the substrate [43, 47] as illustrated in Figure 3. Therefore, the spontaneity or otherwise the charge carrier-induced redox reactions, generically depicted by Eq. (5), is determined by the potential of the band edges of the semiconductor photocatalyst [48, 49].

As shown in Figure 4, the redox potential of the VB and the CB for different semiconductors varies between +4.0 and �1.5 V vs. normal hydrogen electrode (NHE), respectively. Many organic compounds have more negative oxidation potentials than the valence potential of most semiconductor photocatalysts. Accordingly, it is thermodynamically possible for the organic pollutants to be oxidized by photocatalysts. In contrast, fewer organic compounds can be reduced by photocatalyst since only a smaller number of them have a potential below that of the conduction band of most photocatalysts [50].

Figure 3. Thermodynamic constraints on the transfer of charge carriers to adsorbed molecules, and ΔE represents the kinetic overpotential of the reduction process. In this scheme, electrons and holes gain stability when moving down and up, respectively (reproduced from [43]).

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Charge trapping : SC e�

words, the bottom of the photocatalyst's conduction bands (Emin

potential of the band edges of the semiconductor photocatalyst [48, 49].

negative potential than the reduction potential (E<sup>0</sup>

114 Visible-Light Photocatalysis of Carbon-Based Materials

that of the conduction band of most photocatalysts [50].

valence bands (Emax

up, respectively (reproduced from [43]).

vb; h<sup>þ</sup> cb

A thermodynamic requirement of spontaneous reaction is a negative change in Gibb's energy, (i.e., ΔG < 0) [46]. In a photocatalytic reaction, this condition is met when the quasi-Fermi (usually taken as the flat band potentials) energy levels of the photogenerated electrons and holes of the photocatalyst straddle the reduction/oxidation potentials of the substrate. In other

the substrate [43, 47] as illustrated in Figure 3. Therefore, the spontaneity or otherwise the charge carrier-induced redox reactions, generically depicted by Eq. (5), is determined by the

As shown in Figure 4, the redox potential of the VB and the CB for different semiconductors varies between +4.0 and �1.5 V vs. normal hydrogen electrode (NHE), respectively. Many organic compounds have more negative oxidation potentials than the valence potential of most semiconductor photocatalysts. Accordingly, it is thermodynamically possible for the organic pollutants to be oxidized by photocatalysts. In contrast, fewer organic compounds can be reduced by photocatalyst since only a smaller number of them have a potential below

Figure 3. Thermodynamic constraints on the transfer of charge carriers to adsorbed molecules, and ΔE represents the kinetic overpotential of the reduction process. In this scheme, electrons and holes gain stability when moving down and

Reduction reaction A þ e�

Oxidation reaction D þ h<sup>þ</sup>

<sup>þ</sup> 2Tr ! SC e�

VB ) must be positioned more positively than the oxidation potential (E<sup>0</sup>

Tr; h<sup>þ</sup> Tr

(4)

CB ) must be located at a more

ox) of

red) of the substrate, while the top of its

Tr ! <sup>A</sup>•� (5)

Tr ! <sup>D</sup>•<sup>þ</sup> (6)

Figure 4. Bandgap energy structure of several common semiconductors on a potential scale (V) vs. NHE/vacuum (reproduced from [51]).

While choosing a photocatalyst for application in environmental remediation, it is also essential to take into account the reduction potentials (E) of the substrate, as well as those of the intermediates that are formed during the photocatalytic reaction(s). However, the reduction potentials for different organic or inorganic compounds are usually dependent on the reaction conditions such as pH and the employed electrolyte. The reduction potential of the couples M=M•� refers to the reactions described by Eqs (7) and (8):

$$\mathbf{\color{red}{\cdot \times}}\text{ oxidation} + \mathbf{\color{red}{\cdot \cdot}}\text{ -- }\text{Reductant} \tag{7}$$

$$\text{oxidation} + \text{e}^- + \text{nH}^+ \rightarrow \text{Reductant} \tag{8}$$

These reactions refer to one-electron reduction reactions—the potentials of which are given versus the standard hydrogen electrode (SHE). Table 1 includes the reduction potentials of the most important inorganic species that may be present in photocatalytic systems.

For example, over a TiO2 photocatalyst in pH 7 solution, reaction Eq. (9) is feasible because the redox potential of the e� cb ¼ �0:52 V vs. SHE is sufficiently more negative than the reduction potential (E<sup>0</sup> O2=O•� 2 <sup>¼</sup> -0:28 V vs. SHE) of the superoxide radicals. Reaction (10) occurs because the redox potential of the h<sup>þ</sup> vb ¼ þ2:53 V vs. SHE is sufficiently more positive than the oxidation potential (E<sup>0</sup> H2O=OH• ð Þ¼þ2:27 V vs. SHE) of the hydroxyl radical.

$$\mathbf{O}\_2 \text{ isonosorption}: \qquad \qquad \mathbf{O}\_2 + \mathbf{e}\_{\mathbf{cb}}^- \to \mathbf{O}\_2 \text{\textquotedbl{}}\tag{9}$$

$$\text{H}\_2\text{O ionization}: \text{H}^+ + \text{OH}^-\_{\text{ads}} + \text{h}^+\_{\text{vb}} \to \text{OH}^\bullet + \text{H}^+ \tag{10}$$


Moreover, it is in fact critical to provide an overpotential for each process, to initiate and subsequently drive the electron transfer process. Without an overpotential, even a good

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The absorption of large portion of incident solar radiation by a photocatalyst is one of the key factors for achieving useful efficiencies in photocatalysis [52]. For wide bandgap semiconductors such as TiO2, a common strategy for achieving this is through the use of photosensitizers. A photosensitizer is excited by low-energy photons to generate electrons that are subsequently injected into the conduction band of the wide bandgap photocatalyst. The common photosensitizers are usually characterized as expensive [53–55], toxic [56], unstable [57], and polluting [58]. These drawbacks limit the practical and large-scale application of conventional photosensitizers. Accordingly, it is essential to find an alternative photosensitizer material(s) that is free from these drawbacks and one that can harvest larger portion of the solar radiation. Carbon quantum dot appears to be such a material. It is easy to produce, cheap, nontoxic, green, stable, and abundant [1, 58, 59]. Additionally, it displays strong blue photoluminescence and good optical absorption in the UV and near-visible region [60]. Futhermore, CQDs exhibit upconversion photoluminescence properties [19, 61–63] and exhibit promising electron transfer properties [64, 65]. These optical characteristics of CQDs render them promising candidates as photosensitizers for photocatalytic applications. Three possible photsensitization mechanisms are proposed for CQDs in the literature. Firstly, CQD can be excited by a low-energy

vb) and electrons (e�

results in the extended light absorption range as shown in Figure 5(c) [67].

photocatalyst (such as TiO2) to initiate a reaction as shown in Figure 5(a) [19]. Secondly, the CQDs with upconversion PL properties can convert longer wavelength light to the short wavelength light, which in turn can excite a wide bandgap photocatalyst (ZnO) to form

Recently, several reports have been published that show CQDs to dramatically extend the optical absorption range of wide and narrow bandgap photocatalysts to the entire visiblelight range and beyond [18–21, 58, 60, 62, 68–71]. For example, in a recent publication, Ye et al. [58] reported using CQDs to extend the light absorption range of BiVO4 to the entire visible range. The resulting CQD/BiVO4 composite photocatalyst achieved an absolute photocurrent density Jabs of 9:2 mA cm�<sup>2</sup> as shown in Figure 6(a). A photoanode fashioned from the composite (NiOOH/FeOOH/CQDs/BiVO4) achieved a photocurrent density of 5:99 mA cm�<sup>2</sup> at 1.23 V vs. RHE under AM 1.5G in KH2PO4 aqueous solution without a hole scavenger (pH 7) and record a high applied bias photon-to-current efficiency of 2.29% at 0.6 V vs. RHE. Ren et al. [18]

vb pairs (Figure 5(b) [66]). Finally, addition of CQDs may lead to bandgap narrowing of the semiconductor owing to the chemical bonding between semiconductor and CQD, which

cb). Under favorable thermodynamic condi-

cb is injected into the conduction band of a wide bandgap

photocatalyst cannot ensure sufficiently a high rate of reaction.

3.1. Extends the optical absorption range of photocatalysts

radiation to generate holes (h<sup>þ</sup>

tion, the photogenerated e�

e� cb=h<sup>þ</sup>

3. Beneficial roles of CQD materials in photocatalysis

a Reduction potential referring to one-electron reduction vs. SHE in the case of proton involvement, and E values may be used to estimate the standard potentials Eo as defined by the Nernst equation.

Table 1. Reduction potentials of some species that may be involved in photocatalytic systems (adapted from [51]).

Moreover, it is in fact critical to provide an overpotential for each process, to initiate and subsequently drive the electron transfer process. Without an overpotential, even a good photocatalyst cannot ensure sufficiently a high rate of reaction.

### 3. Beneficial roles of CQD materials in photocatalysis

#### 3.1. Extends the optical absorption range of photocatalysts

Reductiom reaction E<sup>a</sup>

116 Visible-Light Photocatalysis of Carbon-Based Materials

Fe<sup>3</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! Fe<sup>2</sup><sup>þ</sup> 0.570

Ag<sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! Ag 0.800

HS• <sup>þ</sup> <sup>e</sup>� ! HS� 1.150

CN� þ e� ! CN� 1.900 Br� þ e� ! Br� 2.000 Cl• <sup>þ</sup> <sup>e</sup>� ! Cl� 2.200–2.600

<sup>2</sup> þ e� ! 2Cl� 2.300

<sup>F</sup>• <sup>þ</sup> <sup>e</sup>� ! <sup>F</sup>� 3.600

aq �2.870

<sup>2</sup> �1.900

<sup>2</sup> 0.420–0.600

<sup>2</sup> 0.420–0.600

<sup>2</sup> 0.590

<sup>2</sup> 0.870–1.040

<sup>2</sup> 1.000

� 1.270–1.420

<sup>3</sup> 1.500

<sup>3</sup> 2.300–2.600

<sup>4</sup> 2.430

used to estimate the standard potentials Eo as defined by the Nernst equation.

H2O2 <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! H2O <sup>þ</sup> HO• 0.800 pH 7

<sup>2</sup> þ 2H<sup>þ</sup> þ e� ! H2O2 0.940 pH 7

<sup>1</sup> þ H<sup>þ</sup> þ e� ! H2O2 1.420 pH 9

OH• <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! H2O 1.800–2.180 pH 7

OH• <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! H2O 2.590–2.850 pH 0

<sup>2</sup> �0.330

<sup>3</sup> <sup>þ</sup> 2OH� �2.470

<sup>2</sup> �0.037 pH 0

<sup>3</sup> 0.630 pH >8

<sup>3</sup> 1.070 pH 7

<sup>3</sup> 1.190–1.600 pH >11

<sup>3</sup> 1.800 pH 7

Reduction potential referring to one-electron reduction vs. SHE in the case of proton involvement, and E values may be

Table 1. Reduction potentials of some species that may be involved in photocatalytic systems (adapted from [51]).

aq þ e� ! e�

CO2 <sup>þ</sup> <sup>e</sup>� ! CO•�

O2 <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! HO•�

•�

<sup>3</sup> <sup>þ</sup> <sup>e</sup>� ! SO<sup>2</sup>�

<sup>2</sup> þ e� ! HO�

<sup>2</sup> þ e� ! NO�

<sup>2</sup> þ H<sup>þ</sup> þ e� ! HO�

<sup>3</sup> þ H<sup>þ</sup> þ e� ! HCO�

O2 <sup>þ</sup> <sup>e</sup>� ! <sup>O</sup>•�

Cl2 <sup>þ</sup> <sup>e</sup>� ! Cl•�

I2 þ e� ! I

SO•�

HO•

NO•

O•�

O•�

CO•�

I • <sup>þ</sup> <sup>e</sup>� ! <sup>I</sup>

HO•

CO•�

Cl•�

NO•

SO•�

a

O3 <sup>þ</sup> <sup>e</sup>� ! <sup>O</sup>•�

<sup>3</sup> <sup>þ</sup> <sup>e</sup>� ! CO2�

O3 <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! HO•

<sup>3</sup> þ e� ! NO�

<sup>4</sup> <sup>þ</sup> <sup>e</sup>� ! SO<sup>2</sup>�

<sup>4</sup> <sup>þ</sup> H2O <sup>þ</sup> <sup>e</sup>� ! SO•�

SO2�

, (V) Remark

The absorption of large portion of incident solar radiation by a photocatalyst is one of the key factors for achieving useful efficiencies in photocatalysis [52]. For wide bandgap semiconductors such as TiO2, a common strategy for achieving this is through the use of photosensitizers. A photosensitizer is excited by low-energy photons to generate electrons that are subsequently injected into the conduction band of the wide bandgap photocatalyst. The common photosensitizers are usually characterized as expensive [53–55], toxic [56], unstable [57], and polluting [58]. These drawbacks limit the practical and large-scale application of conventional photosensitizers. Accordingly, it is essential to find an alternative photosensitizer material(s) that is free from these drawbacks and one that can harvest larger portion of the solar radiation. Carbon quantum dot appears to be such a material. It is easy to produce, cheap, nontoxic, green, stable, and abundant [1, 58, 59]. Additionally, it displays strong blue photoluminescence and good optical absorption in the UV and near-visible region [60]. Futhermore, CQDs exhibit upconversion photoluminescence properties [19, 61–63] and exhibit promising electron transfer properties [64, 65]. These optical characteristics of CQDs render them promising candidates as photosensitizers for photocatalytic applications. Three possible photsensitization mechanisms are proposed for CQDs in the literature. Firstly, CQD can be excited by a low-energy radiation to generate holes (h<sup>þ</sup> vb) and electrons (e� cb). Under favorable thermodynamic condition, the photogenerated e� cb is injected into the conduction band of a wide bandgap photocatalyst (such as TiO2) to initiate a reaction as shown in Figure 5(a) [19]. Secondly, the CQDs with upconversion PL properties can convert longer wavelength light to the short wavelength light, which in turn can excite a wide bandgap photocatalyst (ZnO) to form e� cb=h<sup>þ</sup> vb pairs (Figure 5(b) [66]). Finally, addition of CQDs may lead to bandgap narrowing of the semiconductor owing to the chemical bonding between semiconductor and CQD, which results in the extended light absorption range as shown in Figure 5(c) [67].

Recently, several reports have been published that show CQDs to dramatically extend the optical absorption range of wide and narrow bandgap photocatalysts to the entire visiblelight range and beyond [18–21, 58, 60, 62, 68–71]. For example, in a recent publication, Ye et al. [58] reported using CQDs to extend the light absorption range of BiVO4 to the entire visible range. The resulting CQD/BiVO4 composite photocatalyst achieved an absolute photocurrent density Jabs of 9:2 mA cm�<sup>2</sup> as shown in Figure 6(a). A photoanode fashioned from the composite (NiOOH/FeOOH/CQDs/BiVO4) achieved a photocurrent density of 5:99 mA cm�<sup>2</sup> at 1.23 V vs. RHE under AM 1.5G in KH2PO4 aqueous solution without a hole scavenger (pH 7) and record a high applied bias photon-to-current efficiency of 2.29% at 0.6 V vs. RHE. Ren et al. [18]

Figure 5. (a) Schematic illustration of the sensitization mechanism of CQDs based on (a) photoexcitation of CQDs [19], (b) upconversion photoluminescence [66], and (c) narrowing of band gap [67].

reported that coupling of CQDs with MoSe2 resulted in a composite, CQD/MoSe2 photocatalyst that is active over the entire range of the solar spectrum (Figure 6(b)).

Carbon quantum dots have also been applied as visible-light photosensitizer for non-noble metal H2-evolution catalyst [24, 60, 68]. Martindale et al. used CQDs as a photosensitizer to energize a nickel-based molecular catalyst, Ni-bis-(diphosphine) (NiP) to produce H2 under visible-light irradiation [24, 60]. A proposed scheme of H2 production in the homogeneous CQD-NiP system is presented in Figure 6(c). Irradiation of photoluminescent CQDs with visible-light results in the direct transfer of photoexcited electrons to the catalyst NiP with subsequent reduction of aqueous protons to H2. The electron donor EDTA [60] or TCEP/EDTA [24] quenches the photogenerated holes in the CQDs. In a similar work, McCormick and coworkers [68] used a PVP-coated CQD as a photosensitizer for a nickel nanoparticle (NiNP) catalyst (Figure 6(d)) to produce H2 at a much higher quantum yield of 6%. In this report, it was observed that the fluorescence quantum yield of CQDs increases with increasing PVP coating. However, H2 production decreased when the PVP coating of CQD is greater than 20%. A total of 330 H2/g CQD was collected from a 20% PVP-coated CDQ/Ni nanoparticle system at a pH of 4.51 after five irradiation with a 470-LED light source.

Upconversion photoluminescence property (UCPL) of CQDs was used to improve the visiblelight and NIR response of narrow bandgap composite photocatalysts. For example, Liu and coworkers [72] used the upconversion property of nitrogen-doped CQDs to improve the

Figure 6. (a) Spectrum of the solar irradiance of AM 1.5G (ASTM G173-03) and the spectra showing the efficiencies of light absorption (LHE) of the BiVO4 and CQDs/BiVO4 photoanodes at 300–800 nm [58]. (b) Diffuse absorption spectra of MoSe2 and the CQD/MoSe2 (denoted as MC- in the plot) composites with different CQD contents [18]. Photosensitization of metal catalyst: (c) molecular NiP catalyst [60] and (d) nickel nanoparticle (NiNP) catalyst [68] under visible-light irradiation. (e) Sensitization of Bi20TiO32 by upconversion photoluminescence property of CQDs under NIR radiation [62].

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Figure 6. (a) Spectrum of the solar irradiance of AM 1.5G (ASTM G173-03) and the spectra showing the efficiencies of light absorption (LHE) of the BiVO4 and CQDs/BiVO4 photoanodes at 300–800 nm [58]. (b) Diffuse absorption spectra of MoSe2 and the CQD/MoSe2 (denoted as MC- in the plot) composites with different CQD contents [18]. Photosensitization of metal catalyst: (c) molecular NiP catalyst [60] and (d) nickel nanoparticle (NiNP) catalyst [68] under visible-light irradiation. (e) Sensitization of Bi20TiO32 by upconversion photoluminescence property of CQDs under NIR radiation [62].

reported that coupling of CQDs with MoSe2 resulted in a composite, CQD/MoSe2 photoca-

Figure 5. (a) Schematic illustration of the sensitization mechanism of CQDs based on (a) photoexcitation of CQDs [19],

Carbon quantum dots have also been applied as visible-light photosensitizer for non-noble metal H2-evolution catalyst [24, 60, 68]. Martindale et al. used CQDs as a photosensitizer to energize a nickel-based molecular catalyst, Ni-bis-(diphosphine) (NiP) to produce H2 under visible-light irradiation [24, 60]. A proposed scheme of H2 production in the homogeneous CQD-NiP system is presented in Figure 6(c). Irradiation of photoluminescent CQDs with visible-light results in the direct transfer of photoexcited electrons to the catalyst NiP with subsequent reduction of aqueous protons to H2. The electron donor EDTA [60] or TCEP/EDTA [24] quenches the photogenerated holes in the CQDs. In a similar work, McCormick and coworkers [68] used a PVP-coated CQD as a photosensitizer for a nickel nanoparticle (NiNP) catalyst (Figure 6(d)) to produce H2 at a much higher quantum yield of 6%. In this report, it was observed that the fluorescence quantum yield of CQDs increases with increasing PVP coating. However, H2 production decreased when the PVP coating of CQD is greater than 20%. A total of 330 H2/g CQD was collected from a 20% PVP-coated CDQ/Ni nanoparticle

talyst that is active over the entire range of the solar spectrum (Figure 6(b)).

(b) upconversion photoluminescence [66], and (c) narrowing of band gap [67].

118 Visible-Light Photocatalysis of Carbon-Based Materials

system at a pH of 4.51 after five irradiation with a 470-LED light source.

Upconversion photoluminescence property (UCPL) of CQDs was used to improve the visiblelight and NIR response of narrow bandgap composite photocatalysts. For example, Liu and coworkers [72] used the upconversion property of nitrogen-doped CQDs to improve the visible-light and near infra-red response of graphitic carbon nitride (g-C3N4). Similarly, Mao et al. [62] improved the visible-light and NIR response of Bi20TiO32 by coupling it with CQDs. The resulting composite photocatalyst exhibited improved photoactivity under NIR irradiation. Figure 6(e) shows the proposed CQD sensitization mechanism.

#### 3.2. Enhances charge separation and electron transfer

One of the main processes that limit the quantum efficiency of a photocatalytic system is the fast recombination of photogenerated charge carriers [40]. Therefore, to enhance the performance of a photocatalyst, it is essential to improve charge carrier separation and minimize the rate of their recombination. Several approaches have been devised to achieve this goal. These included strategies such as surface modification of the semiconductor particles with noble metals [73, 74], coupling of two semiconductor particles with different electronic levels [75–77], and using sacrificial reagents to scavenge for photogenerated electrons or holes [78–80]. Another strategy is to couple carbon-based materials with a photocatalyst because of their high charge storage capacity and electrical conductivity [81–83]. In particular, many reports have been recently published that show the beneficial effect of coupling of CQDs with photocatalysts [18–21, 63, 71, 84–86]. The intrinsic band gap and strong electron affinity [59, 65, 87] of CQDs give them the ability to readily accept photogenerated electrons from an electron donor such as a semiconductor with a more negative conduction band minimum. The transferred electrons are then shuttled freely along the conducting paths of the CQDs allowing for effective charge separation, stabilization, and prevention of charge recombination. The longer-lived charge carriers have greater probability to induce transformations, thus accounting for the much improved quantum efficiency of the CQD-based photocatalysts. Figure 7 depicts a proposed photoinduced electron transfer mechanism on a layered composite of CQD/CdS photocatalyst [88].

Results of time-resolved (TR) PL and photocurrent measurements corroborated the slow recombination rates and enhanced the separation of electron-hole pairs observed in CQDbased photocatalysts. Figure 7(b) and (d) showed the time-resolved photoluminescence (TRPL) measurements on pure Bi20TiO32 and CdSe and their CQD-based counterparts with different amounts of CQD. Because the TRPL plots of the CQD composite photocatalysts exhibited lower PL intensities, it implies that the CQDs' modification effectively inhibits the recombination of photogenerated electron-hole-electron pairs in the hybrid photocatalysts. Figure 7(c) and (e) showed the corresponding photocurrent responses. The higher photocurrent signals recorded for the CQD-based photocatalysts compared to the pure photocatalysts demonstrated that much more photogenerated charge carriers were produced and the electron-hole pairs could separate more efficiently [62, 89].

#### 3.3. Provides additional surface for adsorption and reaction

Another key role of CQD, which promotes photocatalytic activity, is its capacity to provide additional surface for adsorption and reaction of substrates. As a nanomaterial, CQDs possess a larger surface area to volume ratio, and thus, its composite with other nanomaterials potentially acquires much enhanced surface area, thereby increasing the adsorption capacity. Like other carbon materials, adsorption capacity of CQD originates from the flexible sp2-bonded carbon structure and the large surface area. Because the surface of CQDs contains a plethora of oxygenated functional groups [59], its adsorption capability depends on its interaction with the adsorbate. The aromatic regions of CQDs can form a π-π stacking interaction with organic adsorbates

and (d) and (e) pure for CdSe and CQD-based photocatalysts from Ref. [89].

Figure 7. (a) Illustration of the mechanism of photoinduced electron transfer on a layered CQD/CdS nanocomposite photocatalyst from Ref. [88]. Plots (b)–(e) show the time-resolved photoluminescence and transient photocurrent profiles of pure photocatalysts and their CQD-based counterparts. Charts (b) and (c) for Bi20TiO32 and CQD/Bi20TiO32 from [62],

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visible-light and near infra-red response of graphitic carbon nitride (g-C3N4). Similarly, Mao et al. [62] improved the visible-light and NIR response of Bi20TiO32 by coupling it with CQDs. The resulting composite photocatalyst exhibited improved photoactivity under NIR irradia-

One of the main processes that limit the quantum efficiency of a photocatalytic system is the fast recombination of photogenerated charge carriers [40]. Therefore, to enhance the performance of a photocatalyst, it is essential to improve charge carrier separation and minimize the rate of their recombination. Several approaches have been devised to achieve this goal. These included strategies such as surface modification of the semiconductor particles with noble metals [73, 74], coupling of two semiconductor particles with different electronic levels [75–77], and using sacrificial reagents to scavenge for photogenerated electrons or holes [78–80]. Another strategy is to couple carbon-based materials with a photocatalyst because of their high charge storage capacity and electrical conductivity [81–83]. In particular, many reports have been recently published that show the beneficial effect of coupling of CQDs with photocatalysts [18–21, 63, 71, 84–86]. The intrinsic band gap and strong electron affinity [59, 65, 87] of CQDs give them the ability to readily accept photogenerated electrons from an electron donor such as a semiconductor with a more negative conduction band minimum. The transferred electrons are then shuttled freely along the conducting paths of the CQDs allowing for effective charge separation, stabilization, and prevention of charge recombination. The longer-lived charge carriers have greater probability to induce transformations, thus accounting for the much improved quantum efficiency of the CQD-based photocatalysts. Figure 7 depicts a proposed photoinduced electron

tion. Figure 6(e) shows the proposed CQD sensitization mechanism.

transfer mechanism on a layered composite of CQD/CdS photocatalyst [88].

electron-hole pairs could separate more efficiently [62, 89].

3.3. Provides additional surface for adsorption and reaction

Results of time-resolved (TR) PL and photocurrent measurements corroborated the slow recombination rates and enhanced the separation of electron-hole pairs observed in CQDbased photocatalysts. Figure 7(b) and (d) showed the time-resolved photoluminescence (TRPL) measurements on pure Bi20TiO32 and CdSe and their CQD-based counterparts with different amounts of CQD. Because the TRPL plots of the CQD composite photocatalysts exhibited lower PL intensities, it implies that the CQDs' modification effectively inhibits the recombination of photogenerated electron-hole-electron pairs in the hybrid photocatalysts. Figure 7(c) and (e) showed the corresponding photocurrent responses. The higher photocurrent signals recorded for the CQD-based photocatalysts compared to the pure photocatalysts demonstrated that much more photogenerated charge carriers were produced and the

Another key role of CQD, which promotes photocatalytic activity, is its capacity to provide additional surface for adsorption and reaction of substrates. As a nanomaterial, CQDs possess a larger surface area to volume ratio, and thus, its composite with other nanomaterials potentially

3.2. Enhances charge separation and electron transfer

120 Visible-Light Photocatalysis of Carbon-Based Materials

Figure 7. (a) Illustration of the mechanism of photoinduced electron transfer on a layered CQD/CdS nanocomposite photocatalyst from Ref. [88]. Plots (b)–(e) show the time-resolved photoluminescence and transient photocurrent profiles of pure photocatalysts and their CQD-based counterparts. Charts (b) and (c) for Bi20TiO32 and CQD/Bi20TiO32 from [62], and (d) and (e) pure for CdSe and CQD-based photocatalysts from Ref. [89].

acquires much enhanced surface area, thereby increasing the adsorption capacity. Like other carbon materials, adsorption capacity of CQD originates from the flexible sp2-bonded carbon structure and the large surface area. Because the surface of CQDs contains a plethora of oxygenated functional groups [59], its adsorption capability depends on its interaction with the adsorbate. The aromatic regions of CQDs can form a π-π stacking interaction with organic adsorbates containing aromatic structures to enhance their adsorption. On the other hand, the presence of functional groups such as carbonyl, epoxy, hydroxyl, and amino groups on the surface of CQDs promotes the adsorption of a wide variety of molecules and metal ions. The increased adsorption of target reactants on the surface of photocatalyst increases their chance of reacting with photogenerated reactive oxidative species, thus enhancing photocatalytic activity of the composite CQDs-based photocatalyst [90–92].

CQD composite Targeted

Bi2MoO6-CDQ Ciprofloxacin (CIP)

Bi2MoO6-CDQ Bisphenol A

Bi2MoO6 � CDQ Tetracycline

CQDs/BiOI Tetracycline

Bi2WO6 � CDQ Tetracycline

Ag3PO4 � CQD Methyl orange (MO)

CQD � TiO2 Rhodamine B

irradiation.

pollutant

CDQ=Bi2WO6 Ciprofloxacin 300 W Xe lamp,

CQD/BiOBr Ciprofloxacin 300 W Xe lamp,

CQDs/BiOI Bisphenol A 300 W Xe lamp,

Bi2WO6-CDQ Bisphenol A 300 W Xe lamp,

CQD/BiOBr Bisphenol A 300 W Xe lamp,

chloride (TC)

chloride

chloride

(RhB)

CQDs/BiOI Rhodamine B 300 W Xe lamp,

CQD/BiOBr Rhodamine B 300 W Xe lamp,

CQDs=Ag=Ag2O Rhodamine B 150 W NIR lamp,

Bi2WO6-CDQ Rhodamine B 300 W Xe lamp,

CQDs/BiOI Rhodamine B 300 W Xe lamp,

CQDs/CdS Nitro-benzene 3 W LED light,

(BPA)

N-CQD Methylene blue > 420 nm light 10 μL cat., MB (3 mL,

300 W Xe lamp with 400-nm filter

≤ 400-nm filter

≤ 400 nm filter

300 W Xe lamp, 400-nm filter

≤ 420-nm filter

≤ 400-nm filter

≤ 400-nm filter

300 W Xe lamp, 400-nm filter

300 W Xe lamp, ≤ 420-nm filter

300 W Xe lamp, ≤ 400-nm filter

150 W Xe lamp, ≤ 420-nm filter

500 W Xe lamp, ≤ 420-nm filter

≤ 420-nm filter

≤ 400-nm filter

≤ 700-nm filter

≤ 400-nm filter

≤ 400-nm filter

≤ 420-nm filter

Visible-light source Experiment conditions η, % Reference

CQD-Based Composites as Visible-Light Active Photocatalysts for Purification of Water

100 mg cat., CIP (100 mL, 10 mg/L),

50 mg cat., CIP (100 mL, 10 mg/L),

100 mg cat., BPA (100 mL, 10 mg/L),

30 mg, 50 mg, 70 mg Cat., BPA (100 mL, 10 mg/L), 120 min

50 mg cat., BPA (100 mL, 10 mg/L),

100 mg cat., TC (100 mL, 20 mg/L),

30 mg, 50 mg, 70 mg Cat., TC (100 mL, 20 mg/L), 120 min

50 mg cat., TC (100 mL, 20 mg/L),

200 mg cat., MO (100 mL, 66 ppm),

100 mg cat., MB (100 mL, 10 ppm),

20 mg cat., RhB (100 mL, 10 mg/L),

50 mg cat., RhB (100 mL, 10 mg/L),

50 mg cat., RhB (100 mL, 10 mg/L),

30 mg cat., RhB (100 mL, 10 mg/L),

30 mg cat., RhB (15 mL, 10 mg/L),

120 min, þHCOONH4

30 mg, 50 mg, 70 mg Cat., RhB (100 mL, 10 mg/L), 30 min

),þNaBH4, 6.5 min

30 mg cat., CIP (100 mL, 10 mg/L), 4 h 44.3 [95]

50 mg cat., BPA (100 mL, 10 mg/L), 66 [95]

≈ 100 [93]

123

http://dx.doi.org/10.5772/intechopen.74245

88 [20]

87 [21]

54 [20]

99 [84]

— [21]

— [20]

68 [84]

— [21]

≈ 100 [70]

95.4 [71]

61 [84]

≈ 100 [95]

48 [94]

— [21]

94.9 [98]

— [88]

<sup>1</sup> � <sup>10</sup>�4mol dm3

C, 120 min

C, 120 min

30�

30�

120 min

30, 120 min

120 min

25 min

60 min

30 min

150 min

120 min

120 min

Table 2. Different CQD-based composites used as photocatalysts used for water purification under visible-light

MoSe2 � CDQ Cr^6+ > 420 nm light 180 min 99 [18]

120 min

### 4. CQD-based semiconductor composites as visible-light active photocatalysts for water treatment

Recently, a large number of studies [17–21, 85, 86, 93–95] have reported on the application of CQD/semiconductor nanocomposites as photocatalysts for water purification owing to the exceptional properties exhibited by the composites. Carbon quantum dots have been coupled with variety of metal-oxide semiconductors such as TiO2, Ag3PO4, SiO2, MoSe2, Bi2MoO6, and others and have found to be an excellent photocatalyst for the degradation of pollutants. Table 2 summarizes some of the CQD-based composites that have been investigated as photocatalysts for the degradation of aqueous pollutants.

Chen et al. [17] reported the fabrication of nitrogen doped (i.e., NCQD/Ag3PO4) complex photocatalyst with improved activity under visible-light irradiation. As shown in Figure 8(a), the activities of the hybrid NCQD/Ag3PO4 materials are significantly enhanced for the degradation of methyl orange (MO) compared with the pure Ag3PO4 under visible light (λ > 420 nm). A composite of NCQD/Ag3PO4 prepared from a 3 mL solution of 0.1 g CQD/L of distilled water denoted 3-NCQD/Ag3PO4 exhibited the highest activity. The 3-NCQD/Ag3PO4 sample eliminated 98% of the MO present in 18 min. In another report, Ren et al. [18] reported



containing aromatic structures to enhance their adsorption. On the other hand, the presence of functional groups such as carbonyl, epoxy, hydroxyl, and amino groups on the surface of CQDs promotes the adsorption of a wide variety of molecules and metal ions. The increased adsorption of target reactants on the surface of photocatalyst increases their chance of reacting with photogenerated reactive oxidative species, thus enhancing photocatalytic activity of the compos-

Recently, a large number of studies [17–21, 85, 86, 93–95] have reported on the application of CQD/semiconductor nanocomposites as photocatalysts for water purification owing to the exceptional properties exhibited by the composites. Carbon quantum dots have been coupled with variety of metal-oxide semiconductors such as TiO2, Ag3PO4, SiO2, MoSe2, Bi2MoO6, and others and have found to be an excellent photocatalyst for the degradation of pollutants. Table 2 summarizes some of the CQD-based composites that have been investigated as

Chen et al. [17] reported the fabrication of nitrogen doped (i.e., NCQD/Ag3PO4) complex photocatalyst with improved activity under visible-light irradiation. As shown in Figure 8(a), the activities of the hybrid NCQD/Ag3PO4 materials are significantly enhanced for the degradation of methyl orange (MO) compared with the pure Ag3PO4 under visible light (λ > 420 nm). A composite of NCQD/Ag3PO4 prepared from a 3 mL solution of 0.1 g CQD/L of distilled water denoted 3-NCQD/Ag3PO4 exhibited the highest activity. The 3-NCQD/Ag3PO4 sample eliminated 98% of the MO present in 18 min. In another report, Ren et al. [18] reported

Visible-light source Experiment conditions η, % Reference

100 mg cat., MB (100 mL, 10 mg/L),

50 mg cat., MB (100 mL, 10 mg/L),

50 mg cat., MB (100 mL, 20 mg/L),

30 mg cat., MB (100 mL, 50 mg/L),

+35% H2O2, 90 min

120 min

MB (15 mL, 5 mg/L), 100 min 35 [85]

— [20]

≈ 100 [94]

97.3 [96]

88 [97]

30 C, 120 min

150 min

CQD-TiO2 Methylene blue > 420 nm light MB (50 mg/L), 25 min ≈ 100 [86] CQD-TiO2 Methylene blue > 420 nm light 120 min 90 [19] CQD-SiO2 Methylene blue > 420 nm light MB (50 mg/L), 15 min ≈ 100 [86]

4. CQD-based semiconductor composites as visible-light active

ite CQDs-based photocatalyst [90–92].

122 Visible-Light Photocatalysis of Carbon-Based Materials

CQD composite Targeted

CQD=Bi2MoO6 Methylene blue (MB)

pollutant

CQDs=Ag=Ag2O Methylene blue 250 W Xe lamp,

CQD-TiO2 Methylene blue 500 W Xe lamp,

Fe2O3-CQD Methylene blue 400 W halogen,

Cu2O-CQD Methylene blue 400 W halogen,

photocatalysts for water treatment

photocatalysts for the degradation of aqueous pollutants.

300 W Xe lamp, ≤ 400-nm filter

≤ 420-nm filter

≤ 420-nm filter

≤ 400-nm filter

≤ 400-nm filter

Table 2. Different CQD-based composites used as photocatalysts used for water purification under visible-light irradiation.

improved activity for the reduction of Cr(VI) ion over CQD/MoSe2 composite photocatalyst compared to the activity over pure MoSe2 (Figure 8(b)). Similarly, Xie et al. also reported on the beneficial effects of complexing CQDs with Bi20TiO32 photocatalysts. Visible light active to improve the rate of charge transfer and reducing the rate of h<sup>þ</sup> vb-e� cb recombination in Bi20TiO32 photocatalysts. The resulting CQD/Bi20TiO32 composites exhibited improved photocatalytic activities as shown in Figure 8(c).

Li and co-workers attributed the enhanced photocatalytic activities of the nanocomposites under visible light to upconversion and efficient charge carrier separation in the nanocomposite systems. The former allows CQDs to emit short-wavelength photons after absorbing visible light. The emitted short-wavelength UV light then induces photoexcitation of the TiO2 and SiO2 to

position of the conduction band edge of the CQDs allows electron transfer from the surface of the semiconductors, thereby enhancing charge carrier separation and inhibiting recombination

Yu et al. [96] reported the synthesis of CQDs/Fe2O3 complex photocatalysts via a facile solventthermal process in an aqueous solution (Figure 10a). The photocatalytic activity of photocatalyst was evaluated by the degradation of MB as a model contaminant under visible-light irradiation. The results (Figure 10b) showed that coupling of CQDs with Fe2O3 greatly enhances the

Figure 9. (a) A representative SEM image of the SiO2/CQDs nanocomposite photocatalyst. Inset shows the corresponding HRTEM image. (b) Excitation-emission profile showing the upconversion PL property of the as-synthesized CQDs. (c) Degradation profile of MB in suspensions of different CQDs-based photocatalysts and control samples. (d) Scheme showing the postulated explanation of the mechanism of photocatalysis over CQDs-based photocatalyst. TiO2/CQDs are

cb and h<sup>þ</sup>

vb) redox pairs that go to initiate the process of MB degradation

vb available for photoreactions.

CQD-Based Composites as Visible-Light Active Photocatalysts for Purification of Water

•� and OH• and others. The appropriate

http://dx.doi.org/10.5772/intechopen.74245

125

generate the electron/hole (e�

resulting in the increased number of e�

used as a representative (reproduced from [86]).

cb=h<sup>þ</sup>

via the generation reactive oxygen species such as O2

Li et al. [86] first reported the exploitation of the unique optical properties of CQDs in photocatalysis. The team synthesized TiO2/CQDs and SiO2/CQDs nanocomposite systems (Figure 9a) from CQDs with upconversion photoluminescence (PL) properties (Figure 9b) and used them to investigate the degradation of aqueous methylene blue (MB) under visible light. The as-synthesized CQDs-based photocatalysts exhibited enhanced ability to promote the degradation of the dye compared to their precursor components (CDQs, TiO2, or SiO2) as shown in Figure 9c. Under the given experimental conditions (50 mgL�<sup>1</sup> MB solution, suspended TiO2/CQDs or SiO2/CQDs, 300 W halogen lamp irradiation) used, the MB was almost completely degraded in 15 and 25 min over SiO2/CQDs and TiO2/CQDs, respectively.

Figure 8. Photocatalytic performance of (a) MO over NCQD/Ag3PO4. (b) Cr(VI) over CQD/MoSe2. (c) Isoproturon over CQD/Bi20TiO32.

Li and co-workers attributed the enhanced photocatalytic activities of the nanocomposites under visible light to upconversion and efficient charge carrier separation in the nanocomposite systems. The former allows CQDs to emit short-wavelength photons after absorbing visible light. The emitted short-wavelength UV light then induces photoexcitation of the TiO2 and SiO2 to generate the electron/hole (e� cb=h<sup>þ</sup> vb) redox pairs that go to initiate the process of MB degradation via the generation reactive oxygen species such as O2 •� and OH• and others. The appropriate position of the conduction band edge of the CQDs allows electron transfer from the surface of the semiconductors, thereby enhancing charge carrier separation and inhibiting recombination resulting in the increased number of e� cb and h<sup>þ</sup> vb available for photoreactions.

improved activity for the reduction of Cr(VI) ion over CQD/MoSe2 composite photocatalyst compared to the activity over pure MoSe2 (Figure 8(b)). Similarly, Xie et al. also reported on the beneficial effects of complexing CQDs with Bi20TiO32 photocatalysts. Visible light active to

photocatalysts. The resulting CQD/Bi20TiO32 composites exhibited improved photocatalytic

Li et al. [86] first reported the exploitation of the unique optical properties of CQDs in photocatalysis. The team synthesized TiO2/CQDs and SiO2/CQDs nanocomposite systems (Figure 9a) from CQDs with upconversion photoluminescence (PL) properties (Figure 9b) and used them to investigate the degradation of aqueous methylene blue (MB) under visible light. The as-synthesized CQDs-based photocatalysts exhibited enhanced ability to promote the degradation of the dye compared to their precursor components (CDQs, TiO2, or SiO2) as shown in Figure 9c. Under the given experimental conditions (50 mgL�<sup>1</sup> MB solution, suspended TiO2/CQDs or SiO2/CQDs, 300 W halogen lamp irradiation) used, the MB was almost completely degraded in 15 and 25 min over SiO2/CQDs and TiO2/CQDs, respectively.

Figure 8. Photocatalytic performance of (a) MO over NCQD/Ag3PO4. (b) Cr(VI) over CQD/MoSe2. (c) Isoproturon over

vb-e�

cb recombination in Bi20TiO32

improve the rate of charge transfer and reducing the rate of h<sup>þ</sup>

activities as shown in Figure 8(c).

124 Visible-Light Photocatalysis of Carbon-Based Materials

CQD/Bi20TiO32.

Yu et al. [96] reported the synthesis of CQDs/Fe2O3 complex photocatalysts via a facile solventthermal process in an aqueous solution (Figure 10a). The photocatalytic activity of photocatalyst was evaluated by the degradation of MB as a model contaminant under visible-light irradiation. The results (Figure 10b) showed that coupling of CQDs with Fe2O3 greatly enhances the

Figure 9. (a) A representative SEM image of the SiO2/CQDs nanocomposite photocatalyst. Inset shows the corresponding HRTEM image. (b) Excitation-emission profile showing the upconversion PL property of the as-synthesized CQDs. (c) Degradation profile of MB in suspensions of different CQDs-based photocatalysts and control samples. (d) Scheme showing the postulated explanation of the mechanism of photocatalysis over CQDs-based photocatalyst. TiO2/CQDs are used as a representative (reproduced from [86]).

decomposition of MB under visible-light irradiation. The authors attributed the enhanced activity of the photocatalyst to the mesoporous architecture and large surface area of the α-Fe2O3, which provides more abundant active sites for the adsorption and degradation of MB to occur and also for the effective electron transfer from the conduction band of the Fe2O3 particle to the conducting networks of the loaded CQDs as shown in Figure 10c.

further functionalities to the composites such as high adsorption capacity, extended light absorption range, and improved charge separation properties along with high stability. Despite the confirmation of the excellent activity of CQDs-based photocatalystsic aforementioned reports, the applicability of the heterogeneous photocatalytic technology in large-scale water treatment is constrained by several key technical issues that have to be addressed.

CQD-Based Composites as Visible-Light Active Photocatalysts for Purification of Water

http://dx.doi.org/10.5772/intechopen.74245

127

The main constraints associated with the field scale application of CQD-based photocatalysts essentially lies in the design of (i) a catalyst that can utilize large portion of the solar spectra, (ii) catalyst immobilization strategy to provide a cost-effective solid-liquid separation, (iii) improvement in the photocatalytic operation for wider pH range and to minimize the addition of oxidant additives, (iv) new integrated or coupling system for enhanced photomineralization or photodisinfection kinetics, and (v) effective design of photocatalytic reactor system or parabolic solar collector for higher utilization of solar energy to reduce the electricity costs.

The authors gratefully acknowledge the financial support of the Tertiary Education Trust Fund

1 Department of Chemical Engineering Technology, The Federal Polytechnic, Nasarawa,

2 Department of Science Laboratory Technology, The Federal Polytechnic, Nasarawa, Nigeria

[1] Baker SN, Baker GA. Luminescent carbon nanodots: emergent nanolights. Angewandte Chemie (International Ed. in English). 2010;49(38):6726-6744. DOI: 10.1002/anie.200906623.

[2] Yang P, Zhao J, Wang J, Cui H, Li L, Zhu Z. Multifunctional nitrogen-doped carbon nanodots for photoluminescence, sensor, and visible- light-induced H2 production. ChemPhysChem. 2015;16(14):3058-3063. DOI: 10.1002/cphc.201500447. ISSN: 1439-7641

[3] Yang P, Zhao J, Wang J, Cao B, Li L, Zhu Z. Construction of Z-scheme carbon nanodots/ WO3 with highly enhanced photocatalytic hydrogen production. Journal of Materials

\*, Muneer Umar<sup>1</sup> and Shettima Abdulkadir Saidu<sup>2</sup>

Acknowledgements

Author details

Nigeria

References

Abdullahi Baba Makama1

of the Nigeria Ministry of Education.

\*Address all correspondence to: abmakama@hotmail.com

ISSN: 1521-3773 (Electronic); 1433-7851 (Linking)

Chemistry A. 2015;3(16):8256-8259. DOI: 10.1039/C5TA00657K

(Electronic); 1439-4235 (Linking)

Figure 10. (a) HRTEM image of the CQDs/Fe2O3 photocatalysts. (b) Comparison of MB degradation profile in the presence of different catalysts: (■) CQD/Fe2O3 without light, (Δ)Fe2O3, (▲)Fe2O3 <sup>þ</sup> H2O2, (○) CQD/Fe2O3, and (•) CQD/Fe2O3 þ H2O2 (reproduced from [96]).

#### 5. Conclusions and outlook

In summary, various types of carbon quantum dots/semiconductor composites have been investigated for the design of novel materials for water remediation via heterogenous photocatalysis. The coupling of CQDs with semiconductors synergistically enhances the activity of the photocatalysts for the degradation of various waterborne pollutants. The unique properties of CQDs, in conjunction with size-dependent properties of nanomaterials, induce further functionalities to the composites such as high adsorption capacity, extended light absorption range, and improved charge separation properties along with high stability. Despite the confirmation of the excellent activity of CQDs-based photocatalystsic aforementioned reports, the applicability of the heterogeneous photocatalytic technology in large-scale water treatment is constrained by several key technical issues that have to be addressed.

The main constraints associated with the field scale application of CQD-based photocatalysts essentially lies in the design of (i) a catalyst that can utilize large portion of the solar spectra, (ii) catalyst immobilization strategy to provide a cost-effective solid-liquid separation, (iii) improvement in the photocatalytic operation for wider pH range and to minimize the addition of oxidant additives, (iv) new integrated or coupling system for enhanced photomineralization or photodisinfection kinetics, and (v) effective design of photocatalytic reactor system or parabolic solar collector for higher utilization of solar energy to reduce the electricity costs.

### Acknowledgements

decomposition of MB under visible-light irradiation. The authors attributed the enhanced activity of the photocatalyst to the mesoporous architecture and large surface area of the α-Fe2O3, which provides more abundant active sites for the adsorption and degradation of MB to occur and also for the effective electron transfer from the conduction band of the Fe2O3 particle to the

In summary, various types of carbon quantum dots/semiconductor composites have been investigated for the design of novel materials for water remediation via heterogenous photocatalysis. The coupling of CQDs with semiconductors synergistically enhances the activity of the photocatalysts for the degradation of various waterborne pollutants. The unique properties of CQDs, in conjunction with size-dependent properties of nanomaterials, induce

Figure 10. (a) HRTEM image of the CQDs/Fe2O3 photocatalysts. (b) Comparison of MB degradation profile in the presence of different catalysts: (■) CQD/Fe2O3 without light, (Δ)Fe2O3, (▲)Fe2O3 <sup>þ</sup> H2O2, (○) CQD/Fe2O3, and (•)

conducting networks of the loaded CQDs as shown in Figure 10c.

126 Visible-Light Photocatalysis of Carbon-Based Materials

5. Conclusions and outlook

CQD/Fe2O3 þ H2O2 (reproduced from [96]).

The authors gratefully acknowledge the financial support of the Tertiary Education Trust Fund of the Nigeria Ministry of Education.

### Author details

Abdullahi Baba Makama1 \*, Muneer Umar<sup>1</sup> and Shettima Abdulkadir Saidu<sup>2</sup>

\*Address all correspondence to: abmakama@hotmail.com

1 Department of Chemical Engineering Technology, The Federal Polytechnic, Nasarawa, Nigeria

2 Department of Science Laboratory Technology, The Federal Polytechnic, Nasarawa, Nigeria

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**Chapter 8**

**Provisional chapter**

from earth abundant source—

evolution from water. After two decades of con-

**Visible Light–Driven Hydrogen Production by Carbon**

water—is a nice approach to find the solution of energy crisis and environmental protection. There are two half reactions; first, water oxidation into oxygen and proton and

tinuous attempts, there have been several efficient water oxidation photocatalysts introduced, whereas the proton reduction photocatalyst were relatively less explored. Major portion of reported photocatalysts for proton reduction are mainly derived from either noble metals or precious metals. Carbon-based organic photocatalysts have become attractive recently. These organic materials have several advantages like light weight, cheap, well-defined structure-property relationship and the most attractive one is better batch to batch reproducibility. Here, the reported organic photocatalysts and their performance are summarized which in fact help others to get an idea about ongoing progress in this area of research and to understand the basic designing principle for efficient

**Keywords:** solar fuels, photochemical hydrogen generation, proton reduction, photocatalysts,

Due to the limited storage of fossil fuels and the environmental hazards from burning of fossil fuels, there is urgent need for renewable energy sources. Harvesting solar energy is an ideal alternative solution. Silicon solar cell is already proven as an efficient technology for converting solar energy into electrical energy. In past two decades, enormous attempts have been done to replace silicon solar cells by organic carbon-based compound such as dyes and

carbon nitride, conjugated polymers, polymer dots, porous materials

**Visible Light–Driven Hydrogen Production by Carbon** 

DOI: 10.5772/intechopen.70444

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**based Polymeric Materials**

**based Polymeric Materials**

http://dx.doi.org/10.5772/intechopen.70444

**Abstract**

**1. Introduction**

Palas Baran Pati, SU-IL In and Haining Tian

Palas Baran Pati, SU-IL In and Haining Tian

Additional information is available at the end of the chapter

followed by proton reduction led to H2

photocatalysts for fuel production.

Converting solar energy into storable solar fuels such as H2

Additional information is available at the end of the chapter


**Provisional chapter**

### **Visible Light–Driven Hydrogen Production by Carbon based Polymeric Materials based Polymeric Materials**

**Visible Light–Driven Hydrogen Production by Carbon** 

DOI: 10.5772/intechopen.70444

Palas Baran Pati, SU-IL In and Haining Tian Palas Baran Pati, SU-IL In and Haining Tian Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70444

#### **Abstract**

[93] Arul V, Edison TNJI, Lee YR, Sethuraman MG. Biological and catalytic applications of green synthesized fluorescent N-doped carbon dots using Hylocereus undatus. Journal of Photochemistry and Photobiology B. 2017;168:142-148. DOI: 10.1016/j.jphotobiol.2017.

[94] Chen J, Che H, Huang K, Liu C, Shi W. Fabrication of a ternary plasmonic photocatalyst CQDs/Ag/Ag2O to harness charge flow for photocatalytic elimination of pollutants. Applied Catalysis B. 2016;192:134-144. DOI: 10.1016/J.APCATB.2016.03.056. URL: http://

[95] Xia J, Di J, Li H, Xu H, Li H, Guo S. Ionic liquid-induced strategy for carbon quantum dots/BiOX (X = Br, Cl) hybrid nanosheets with superior visible light-driven photocatalysis. Applied Catalysis. B, Environmental. 2016;181:260-269. DOI: 10.1016/J. APCATB.2015.07.035. URL: http://www.sciencedirect.com/science/article/pii/S0926337315

[96] Byong Yong Yu And Seung-Yeop Kwak. Carbon quantum dots embedded with mesoporous hematite nanospheres as efficient visible light-active photocatalysts. Journal of

[97] Li H, Liu R, Yang L, Huang H, Yu H, Ming H, Lian S, Lee S-T, Kang Z. Carbon quantum dots/Cu2O composites with protruding nanostructures and their highly efficient (near) infrared photocatalytic behavior. Journal of Materials Chemistry. 2012;22(34):17470-17475

[98] Di J, Xia J, Ji M, Xu L, Yin S, Zhang Q, Chen Z, Li H. Carbon quantum dots in situ coupling to bismuth oxyiodide via reactable ionic liquid with enhanced photocatalytic molecular oxygen activation performance. Carbon. 2016;98(Supplement C):613-623. DOI: 10.1016/j.carbon.2015.11.015. URL: http://www.sciencedirect.com/science/article/pii/S000

www.sciencedirect.com/science/article/pii/S0926337316302387

Materials Chemistry 2012;22(17):8345-8353

02.007

136 Visible-Light Photocatalysis of Carbon-Based Materials

300540

8622315304243

Converting solar energy into storable solar fuels such as H2 from earth abundant source water—is a nice approach to find the solution of energy crisis and environmental protection. There are two half reactions; first, water oxidation into oxygen and proton and followed by proton reduction led to H2 evolution from water. After two decades of continuous attempts, there have been several efficient water oxidation photocatalysts introduced, whereas the proton reduction photocatalyst were relatively less explored. Major portion of reported photocatalysts for proton reduction are mainly derived from either noble metals or precious metals. Carbon-based organic photocatalysts have become attractive recently. These organic materials have several advantages like light weight, cheap, well-defined structure-property relationship and the most attractive one is better batch to batch reproducibility. Here, the reported organic photocatalysts and their performance are summarized which in fact help others to get an idea about ongoing progress in this area of research and to understand the basic designing principle for efficient photocatalysts for fuel production.

**Keywords:** solar fuels, photochemical hydrogen generation, proton reduction, photocatalysts, carbon nitride, conjugated polymers, polymer dots, porous materials

#### **1. Introduction**

Due to the limited storage of fossil fuels and the environmental hazards from burning of fossil fuels, there is urgent need for renewable energy sources. Harvesting solar energy is an ideal alternative solution. Silicon solar cell is already proven as an efficient technology for converting solar energy into electrical energy. In past two decades, enormous attempts have been done to replace silicon solar cells by organic carbon-based compound such as dyes and

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

polymers for developing efficient solar energy harvesting cells [1, 2]. But all these solar cells can convert solar energy into electrical energy. From the total consumption of energy in our society, we can see that only around 20% of the energy consumed is electrical energy and the rest is used as fuel form. So, storing solar energy into fuels is another important solution to energy crisis.

have been successfully applied in the photocatalytic hydrogen production. These organic materials work as heterogeneous catalyst which associated with poor dispersibility in aqueous medium. The difficult dispersibility results in the low efficiency and stability of these materials. So, there is chance and need for improvement and betterment in this field to get

Visible Light–Driven Hydrogen Production by Carbon based Polymeric Materials

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139

Herein, in this chapter, we summarized and discussed the facile synthesis and latest developments of carbon-based photo-active organic materials usable for photochemical hydrogen generation. We will discuss the working principle and the thermodynamics behind the process for showing proton reduction ability to generate hydrogen. The discussion of available organic materials will be subdivided into following types: (a) covalent organic frameworks (COFs), (b) covalent triazine frameworks (CTFs), (c) carbon nitride-based polymers, (d) conjugated microporous polymers (CMP), (e) conjugated linear polymers and very recently introduced (f) polymer dots (Pdots). Our discussion particularly focuses on the modification of synthesis, structure and properties, aiming at better understanding of the intimate structureperformance relationship and further enhancement of photocatalytic activities. We believe that this chapter will be helpful for the readers to understand the working principle and structure-property relationship to devise a high-efficient photocatalyst. It is very timely on the

**2. Thermodynamics and driving force for proton reduction hydrogen** 

**Figure 1** shows the thermodynamics of water splitting (**Figure 1(a)**, at pH = 0) and photochemical hydrogen generation steps from water. Both the half reaction water oxidation and proton reduction are pH dependent. For the efficient water oxidation, the band gap of the conjugated material should be higher than 1.23 eV with well-stabilized oxidized state of the material

**Figure 1.** (a) Energy diagram of water splitting reaction. (b) Photochemical hydrogen generation energy diagram in the

highly efficient organic photocatalysts for photochemical hydrogenation.

aspect of renewable energy source finding.

**generation process**

presence of a sacrificial electron donor.

The light-induced water splitting into chemical fuels, oxygen and hydrogen, by sunlight represents an unlimited source of energy from naturally abundant resources. To carry out the process, it requires a photocatalyst and the key half reactions of water splitting are the oxidation of water to form oxygen and protons and the reduction of the protons to form hydrogen. The standard reduction potentials of these processes (pH = 0) are 1.23 V and 0 V vs. Normal Hydrogen Electrode (NHE), respectively, and the catalyst has to utilise photons of sufficient energy to generate at least this potential in order to split water, in practise a significant over potential usually results in a potential of at least 2V being required [3]. Groundbreaking work by Fujishima and Honda in 1972 showed that hydrogen generation *via* water splitting is possible using photocatalysts based on oxide semiconductors capable of adsorbing ultraviolet light energy [4]. For efficient hydrogen generation, development of efficient proton reduction catalyst is very essential. Enormous attempts have been done during the last few years to develop efficient proton reduction catalyst. Molecular catalyst-photosensitizer assemblies and inorganic material photocatalysts have been intensively developed [5]. On the other hand, organic photocatalysts have also attracted intense interests from scientists due to their facile structure modification, less environmental footprints, tunable light absorption and promising photocatalytic reactivity.

Conjugated materials can absorb visible light because of their delocalized π-system. These properties have led to applications in organic electronics and organic photonics [2]. In early 1985, Yanagida et al. have used linear poly(p-phenylene) for hydrogen generation which is known as the first report on carbon-based material for photochemical hydrogen generation [6]. This material only showed very low activity (apparent quantum yield (AQY) = 0.006%) and worked under ultraviolet (UV)-light (λ >366 nm). Another disadvantage was the insolubility, which delimits the further advancement on these materials. After two decades, carbon nitride has established itself as an efficient proton reduction catalyst [7]. Carbon nitride (g-C<sup>3</sup> N4 ) shows good efficiency on proton reduction in the presence of some co-catalysts such as Ni and Ni-P [8]. In the presence of co-catalyst, carbon nitride even can oxidize water [9]. But in the absence of co-catalysts, carbon nitride showed unsatisfactory performance for proton reduction. Due to the fixed structure of carbon nitride, there are few chances to tune the properties mainly light harvesting ability, which delimits the efficiency of the materials. Due to the high activity of carbon nitride and the restriction towards further improvement, scientists started to look and use conjugated materials for solar light-driven hydrogen production. The major advantage of the carbon-based organic semiconductor is that easily synthetic and purifying process and better batch to batch reproducibility. Well-established structure-property relationship helps to design and modify the structure to improve the performance of the materials. Very recently, various pure organic semiconducting materials such as poly(azomethine)s [10], covalent organic framework [11], phenyl-triazine oligomers [12], microporous organic nanorods [13], heptazine networks [14] and polybenzothiadiazoles [15] have been successfully applied in the photocatalytic hydrogen production. These organic materials work as heterogeneous catalyst which associated with poor dispersibility in aqueous medium. The difficult dispersibility results in the low efficiency and stability of these materials. So, there is chance and need for improvement and betterment in this field to get highly efficient organic photocatalysts for photochemical hydrogenation.

polymers for developing efficient solar energy harvesting cells [1, 2]. But all these solar cells can convert solar energy into electrical energy. From the total consumption of energy in our society, we can see that only around 20% of the energy consumed is electrical energy and the rest is used as fuel form. So, storing solar energy into fuels is another important solution to

The light-induced water splitting into chemical fuels, oxygen and hydrogen, by sunlight represents an unlimited source of energy from naturally abundant resources. To carry out the process, it requires a photocatalyst and the key half reactions of water splitting are the oxidation of water to form oxygen and protons and the reduction of the protons to form hydrogen. The standard reduction potentials of these processes (pH = 0) are 1.23 V and 0 V vs. Normal Hydrogen Electrode (NHE), respectively, and the catalyst has to utilise photons of sufficient energy to generate at least this potential in order to split water, in practise a significant over potential usually results in a potential of at least 2V being required [3]. Groundbreaking work by Fujishima and Honda in 1972 showed that hydrogen generation *via* water splitting is possible using photocatalysts based on oxide semiconductors capable of adsorbing ultraviolet light energy [4]. For efficient hydrogen generation, development of efficient proton reduction catalyst is very essential. Enormous attempts have been done during the last few years to develop efficient proton reduction catalyst. Molecular catalyst-photosensitizer assemblies and inorganic material photocatalysts have been intensively developed [5]. On the other hand, organic photocatalysts have also attracted intense interests from scientists due to their facile structure modification, less environmental footprints, tunable light absorption and promising

Conjugated materials can absorb visible light because of their delocalized π-system. These properties have led to applications in organic electronics and organic photonics [2]. In early 1985, Yanagida et al. have used linear poly(p-phenylene) for hydrogen generation which is known as the first report on carbon-based material for photochemical hydrogen generation [6]. This material only showed very low activity (apparent quantum yield (AQY) = 0.006%) and worked under ultraviolet (UV)-light (λ >366 nm). Another disadvantage was the insolubility, which delimits the further advancement on these materials. After two decades, carbon nitride has established itself as an efficient proton reduction catalyst [7]. Carbon nitride

) shows good efficiency on proton reduction in the presence of some co-catalysts such

as Ni and Ni-P [8]. In the presence of co-catalyst, carbon nitride even can oxidize water [9]. But in the absence of co-catalysts, carbon nitride showed unsatisfactory performance for proton reduction. Due to the fixed structure of carbon nitride, there are few chances to tune the properties mainly light harvesting ability, which delimits the efficiency of the materials. Due to the high activity of carbon nitride and the restriction towards further improvement, scientists started to look and use conjugated materials for solar light-driven hydrogen production. The major advantage of the carbon-based organic semiconductor is that easily synthetic and purifying process and better batch to batch reproducibility. Well-established structure-property relationship helps to design and modify the structure to improve the performance of the materials. Very recently, various pure organic semiconducting materials such as poly(azomethine)s [10], covalent organic framework [11], phenyl-triazine oligomers [12], microporous organic nanorods [13], heptazine networks [14] and polybenzothiadiazoles [15]

energy crisis.

138 Visible-Light Photocatalysis of Carbon-Based Materials

photocatalytic reactivity.

(g-C<sup>3</sup> N4 Herein, in this chapter, we summarized and discussed the facile synthesis and latest developments of carbon-based photo-active organic materials usable for photochemical hydrogen generation. We will discuss the working principle and the thermodynamics behind the process for showing proton reduction ability to generate hydrogen. The discussion of available organic materials will be subdivided into following types: (a) covalent organic frameworks (COFs), (b) covalent triazine frameworks (CTFs), (c) carbon nitride-based polymers, (d) conjugated microporous polymers (CMP), (e) conjugated linear polymers and very recently introduced (f) polymer dots (Pdots). Our discussion particularly focuses on the modification of synthesis, structure and properties, aiming at better understanding of the intimate structureperformance relationship and further enhancement of photocatalytic activities. We believe that this chapter will be helpful for the readers to understand the working principle and structure-property relationship to devise a high-efficient photocatalyst. It is very timely on the aspect of renewable energy source finding.

### **2. Thermodynamics and driving force for proton reduction hydrogen generation process**

**Figure 1** shows the thermodynamics of water splitting (**Figure 1(a)**, at pH = 0) and photochemical hydrogen generation steps from water. Both the half reaction water oxidation and proton reduction are pH dependent. For the efficient water oxidation, the band gap of the conjugated material should be higher than 1.23 eV with well-stabilized oxidized state of the material

**Figure 1.** (a) Energy diagram of water splitting reaction. (b) Photochemical hydrogen generation energy diagram in the presence of a sacrificial electron donor.

to donate photogenerated hole for water oxidation to oxygen and proton. The excited state should be higher in energy to donate photoexcited electron to produce hydrogen upon reduction of proton. In case of photochemical hydrogen production (**Figure 1(b)**), the photoexcited material gets reduced by oxidizing sacrificial electron donor and the reduced material carries out the reduction of proton to produce hydrogen. There are many sacrificial donors used for the reaction; the most popular ones are triethanolamine (TEOA) and sodium ascorbate.

### **3. Materials**

#### **3.1. Covalent organic framework**

Covalent organic frameworks (COFs) are highly ordered two- or three-dimensional crystalline polymers [16, 17]. These materials specially attract the interest in the area of photochemical hydrogen generation due to high surface area and tuneable pore size, which are essential criteria for heterogeneous catalysis [18]. By varying the molecular synthons, the optical and electrical properties of the resulted COFs can be nicely tuned. Stegbauer et al. enriched the library on this area by synthesizing and characterizing a series of COFs and further they successfully applied these materials for photochemical hydrogen generation in the presence of co-catalyst and a sacrificial electron donor [11]. Acid-catalysed Schiff base reaction resulted a two-dimensional (2D) mesoporous hydrazine-based COFs (TFTP-COF) with honeycombtype network (**Figure 2**). It showed a high surface area of 1603 m<sup>2</sup> g−1 with a pore size of 3.8 nm. These polymers with a band gap of 2.8 eV show light absorption ability in the visible range. It shows high hydrogen evolution performance (230 μmol h−1 g−1) when Pt nanoparticle and sodium ascorbate were used as co-catalyst and sacrificial electron donor, respectively, under visible light (>420 nm) irradiation. Indeed, it showed higher hydrogen evolution rate (1970 μmol h−1 g−1) with a AQY of 2.2% when triethanolamine (TEOA) was used instead of sodium ascorbate as sacrificial electron donor.

Again Vyas et al. further synthesize a series of 2D azine-linked COFs with varying amount of nitrogen by solvothermal method from hydrazine and triphenylarylaldehydes [19]. The high BET-surface area (1537 m<sup>2</sup> g−1) suggests that the COFs are highly porous. All COFs absorb in the UV region and the blue part of the visible region with an absorption edge around 465–475 nm which led to an optical band gap of 2.6–2.7 eV (**Figure 3**). The **N3 -COF** showed hydrogen evolution rate of 1703 μmol h−1 g−1 when TEOA used as sacrificial electron donor and hexachloroplatinic acid for in situ generation of Pt-nanoparticle.

**3.2. Covalent triazine-based frameworks (CTFs)**

the presence of ZnCl2

It is believed that due to incorporation of electronegative nitrogen centre the H2

carbon, blue: nitrogen, red: oxygen). Reproduced with permission from RSC; see Ref. [11].

can be strongly enhanced. On this aspect, triazine or heptazine framework in the organic polymeric system can be beneficial for photochemical hydrogen production. CTFs are typically prepared by ionothermal trimerization of aromatic nitriles such as 1,4-dicyanobenzene (DCB), 1,3,5-tris(4-cyanophenyl)benzene (TCPB) and 2,6-dicyanopyridine (DCP) process in

**Figure 2.** Acetic acid-catalysed hydrazone formation furnishes a mesoporous 2D network with a honeycomb type in plane structure. (a) Scheme showing the condensation of the two monomers to form the TFPT-COF. (b) TFPT-COF with a cofacial orientation of the aromatic building blocks, constituting a close-to eclipsed primitive hexagonal lattice (grey:

microporous structure with high surface area. Typically, CTFs are black in colour with very low band gap (~1 eV) which in fact is insufficient for both photochemical water splitting and generated proton reduction. Schwinghammer et al. synthesized yellow phenyl-triazine oligomers by reducing carbonization effect through lowering temperature (300 °C) and prolonging reaction time (150 h) [12]. These synthesized CTFs showed the band gap in the range of 3.2–3.3 eV. In the presence of Pt as co-catalyst and TEOA as sacrificial donor, the most active CTF sample showed a hydrogen evolution rate of 1076 (±278) μmol h−1 g−1. Although the hydrogen

which acts both as catalyst and solvent [21]. These systems are typically

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evolution

The above study suggests that with the increase of N containing in the central core of COFs the photocatalytic activity is increased so further they tried to introduced N atom in the peripheral unit of COF also [20]. They introduced pyridine containing azine-linked COFs by solvothermal process between **PTP-CHO** and hydrazine hydrate (**Figure 4**). Photocatalytic hydrogen evolution rate was tested in the presence of TEOA as electron donor and hexachloroplatinic acid as in situ formation for Pt-nanoparticle used as co-catalyst. The hydrogen evolution rate is lower for **PTP-COF** (83.83 μmol h−1 g−1) than **N3 -COF** (1703 μmol h−1 g−1).

Visible Light–Driven Hydrogen Production by Carbon based Polymeric Materials http://dx.doi.org/10.5772/intechopen.70444 141

**Figure 2.** Acetic acid-catalysed hydrazone formation furnishes a mesoporous 2D network with a honeycomb type in plane structure. (a) Scheme showing the condensation of the two monomers to form the TFPT-COF. (b) TFPT-COF with a cofacial orientation of the aromatic building blocks, constituting a close-to eclipsed primitive hexagonal lattice (grey: carbon, blue: nitrogen, red: oxygen). Reproduced with permission from RSC; see Ref. [11].

#### **3.2. Covalent triazine-based frameworks (CTFs)**

to donate photogenerated hole for water oxidation to oxygen and proton. The excited state should be higher in energy to donate photoexcited electron to produce hydrogen upon reduction of proton. In case of photochemical hydrogen production (**Figure 1(b)**), the photoexcited material gets reduced by oxidizing sacrificial electron donor and the reduced material carries out the reduction of proton to produce hydrogen. There are many sacrificial donors used for the reaction; the most popular ones are triethanolamine (TEOA) and sodium ascorbate.

Covalent organic frameworks (COFs) are highly ordered two- or three-dimensional crystalline polymers [16, 17]. These materials specially attract the interest in the area of photochemical hydrogen generation due to high surface area and tuneable pore size, which are essential criteria for heterogeneous catalysis [18]. By varying the molecular synthons, the optical and electrical properties of the resulted COFs can be nicely tuned. Stegbauer et al. enriched the library on this area by synthesizing and characterizing a series of COFs and further they successfully applied these materials for photochemical hydrogen generation in the presence of co-catalyst and a sacrificial electron donor [11]. Acid-catalysed Schiff base reaction resulted a two-dimensional (2D) mesoporous hydrazine-based COFs (TFTP-COF) with honeycomb-

3.8 nm. These polymers with a band gap of 2.8 eV show light absorption ability in the visible range. It shows high hydrogen evolution performance (230 μmol h−1 g−1) when Pt nanoparticle and sodium ascorbate were used as co-catalyst and sacrificial electron donor, respectively, under visible light (>420 nm) irradiation. Indeed, it showed higher hydrogen evolution rate (1970 μmol h−1 g−1) with a AQY of 2.2% when triethanolamine (TEOA) was used instead of

Again Vyas et al. further synthesize a series of 2D azine-linked COFs with varying amount of nitrogen by solvothermal method from hydrazine and triphenylarylaldehydes [19]. The high

in the UV region and the blue part of the visible region with an absorption edge around

hydrogen evolution rate of 1703 μmol h−1 g−1 when TEOA used as sacrificial electron donor

The above study suggests that with the increase of N containing in the central core of COFs the photocatalytic activity is increased so further they tried to introduced N atom in the peripheral unit of COF also [20]. They introduced pyridine containing azine-linked COFs by solvothermal process between **PTP-CHO** and hydrazine hydrate (**Figure 4**). Photocatalytic hydrogen evolution rate was tested in the presence of TEOA as electron donor and hexachloroplatinic acid as in situ formation for Pt-nanoparticle used as co-catalyst. The hydrogen evolution rate is lower for **PTP-COF** (83.83 μmol h−1 g−1) than **N3**

465–475 nm which led to an optical band gap of 2.6–2.7 eV (**Figure 3**). The **N3**

and hexachloroplatinic acid for in situ generation of Pt-nanoparticle.

g−1) suggests that the COFs are highly porous. All COFs absorb

g−1 with a pore size of

**-COF** showed

**-COF**

type network (**Figure 2**). It showed a high surface area of 1603 m<sup>2</sup>

sodium ascorbate as sacrificial electron donor.

BET-surface area (1537 m<sup>2</sup>

(1703 μmol h−1 g−1).

**3. Materials**

**3.1. Covalent organic framework**

140 Visible-Light Photocatalysis of Carbon-Based Materials

It is believed that due to incorporation of electronegative nitrogen centre the H2 evolution can be strongly enhanced. On this aspect, triazine or heptazine framework in the organic polymeric system can be beneficial for photochemical hydrogen production. CTFs are typically prepared by ionothermal trimerization of aromatic nitriles such as 1,4-dicyanobenzene (DCB), 1,3,5-tris(4-cyanophenyl)benzene (TCPB) and 2,6-dicyanopyridine (DCP) process in the presence of ZnCl2 which acts both as catalyst and solvent [21]. These systems are typically microporous structure with high surface area. Typically, CTFs are black in colour with very low band gap (~1 eV) which in fact is insufficient for both photochemical water splitting and generated proton reduction. Schwinghammer et al. synthesized yellow phenyl-triazine oligomers by reducing carbonization effect through lowering temperature (300 °C) and prolonging reaction time (150 h) [12]. These synthesized CTFs showed the band gap in the range of 3.2–3.3 eV. In the presence of Pt as co-catalyst and TEOA as sacrificial donor, the most active CTF sample showed a hydrogen evolution rate of 1076 (±278) μmol h−1 g−1. Although the hydrogen

**Figure 3.** Design (a) and synthesis (b) of the **Nx -COFs**. Absorption spectra of precursor aldehydes **Nx -Alds** in dichloromethane at 22°C (c). Hydrogen production monitored over 8 h using **Nx -COFs** as photocatalyst in the presence of triethanolamine as sacrificial electron donor (d). Reproduced with permission from Nature Publication Group; see Ref. [19].

and tri-*s*-triazine (heptazine), are co-existed to separately construct the ideal polymeric carbon nitride units [22–25]. Being energetically higher s-triazine is thermodynamically unfavourable to synthesize carbon nitride with pure s-triazine unit. In fact, poly(triazine imide) (PTI) and melon, which belong to typical s-triazine and tri-*s*-triazine structure, respectively, are always experimentally synthesized to obtain the binary carbon nitride polymers with 2D electronic structures [22–29]. PTI always features high level of crystallinity, whereas most of the melons are amorphous or semi-crystalline in structure. Besides, quite different optical properties were also generated due to the different degrees of conjugation. In this case, these two counterparts showed

**Figure 5.** (a) Possible ring- and chain-like oligomers, (b) average hydrogen evolution curve and (c) quantum yield with

**Figure 4.** Synthesis of precursor **PTP-CHO** and **PTP-COF**. The in-plane unit cell is indicated on the right. Reproduced

evolution. Ham et al. first reported

evolution rate under visible light irradiation in the presence

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143

different photocatalytic performances for photocatalytic H<sup>2</sup>

a function of wavelength. Reproduced with permission from RSC; see Ref. [12].

exhibited a steady H<sup>2</sup>

that PTI/Li<sup>+</sup>

Cl<sup>−</sup>

with permission from RSC; see Ref. [20].

evolution rate (121 μmol h−1 g−1) is low but most active CTF is active in photochemical hydrogen production in the absence of co-catalyst. The apparent quantum yield (AQY) of the highest active CTF is determined to be 5.5% at 400 (±20) nm. On an intension of efficient CTFs by manipulating structural and optical properties, Wu et al. introduced the DCB-based **CTF-T1** by incorporating methyl group in monomer through trifluoromethanesulfonic acid-catalysed reaction. The optical band gap of devised CTF was determined to be 2.9 eV. The CTF showed water splitting to produce H<sup>2</sup> in the presence of sacrificial electron donor. The AQY of hydrogen evolving reaction was calculated to 2.4% in 400–440 nm **(Figure 5**).

#### **3.3. Carbon nitride-based polymers**

Since 2009, after successful demonstration in water splitting reaction, carbon nitride analogues become matter of interest. Commonly, it is believed that two different analogues, namely *s*-triazine Visible Light–Driven Hydrogen Production by Carbon based Polymeric Materials http://dx.doi.org/10.5772/intechopen.70444 143

**Figure 4.** Synthesis of precursor **PTP-CHO** and **PTP-COF**. The in-plane unit cell is indicated on the right. Reproduced with permission from RSC; see Ref. [20].

and tri-*s*-triazine (heptazine), are co-existed to separately construct the ideal polymeric carbon nitride units [22–25]. Being energetically higher s-triazine is thermodynamically unfavourable to synthesize carbon nitride with pure s-triazine unit. In fact, poly(triazine imide) (PTI) and melon, which belong to typical s-triazine and tri-*s*-triazine structure, respectively, are always experimentally synthesized to obtain the binary carbon nitride polymers with 2D electronic structures [22–29]. PTI always features high level of crystallinity, whereas most of the melons are amorphous or semi-crystalline in structure. Besides, quite different optical properties were also generated due to the different degrees of conjugation. In this case, these two counterparts showed different photocatalytic performances for photocatalytic H<sup>2</sup> evolution. Ham et al. first reported that PTI/Li<sup>+</sup> Cl<sup>−</sup> exhibited a steady H<sup>2</sup> evolution rate under visible light irradiation in the presence

evolution rate (121 μmol h−1 g−1) is low but most active CTF is active in photochemical hydrogen production in the absence of co-catalyst. The apparent quantum yield (AQY) of the highest active CTF is determined to be 5.5% at 400 (±20) nm. On an intension of efficient CTFs by manipulating structural and optical properties, Wu et al. introduced the DCB-based **CTF-T1** by incorporating methyl group in monomer through trifluoromethanesulfonic acid-catalysed reaction. The optical band gap of devised CTF was determined to be 2.9 eV. The CTF showed

of triethanolamine as sacrificial electron donor (d). Reproduced with permission from Nature Publication Group; see

Since 2009, after successful demonstration in water splitting reaction, carbon nitride analogues become matter of interest. Commonly, it is believed that two different analogues, namely *s*-triazine

gen evolving reaction was calculated to 2.4% in 400–440 nm **(Figure 5**).

dichloromethane at 22°C (c). Hydrogen production monitored over 8 h using **Nx**

in the presence of sacrificial electron donor. The AQY of hydro-

**-COFs**. Absorption spectra of precursor aldehydes **Nx**

**-Alds** in

**-COFs** as photocatalyst in the presence

water splitting to produce H<sup>2</sup>

Ref. [19].

**3.3. Carbon nitride-based polymers**

**Figure 3.** Design (a) and synthesis (b) of the **Nx**

142 Visible-Light Photocatalysis of Carbon-Based Materials

**Figure 5.** (a) Possible ring- and chain-like oligomers, (b) average hydrogen evolution curve and (c) quantum yield with a function of wavelength. Reproduced with permission from RSC; see Ref. [12].

of sacrificial agents and co-catalyst (Pt, 5 wt.%), whereas it showed low activity in comparison to that of Pt/g-C3 N4 which may be assigned to the weak optical absorption in the visible light region [30]. Schwinghammer et al. modify the PTI/Li<sup>+</sup> Cl<sup>−</sup> by using 4-amino-2, 6-dihydroxypyrimidine (**4AP**) as the dopant [31]. This modified PTI showed red-shifted absorption spectrum compared to the crystalline PTI and more similar to the melon structure, which improves the visible light absorption. By using this doped PTI, AQY of hydrogen evolution can be improved five times (3.4%), which is more than that of raw melon (0.6%). By supramolecular aggregation allowed by ionic melt polycondensation (IMP) of melamine and 2,4,6-triaminopyrimidine as dopants, Bhunia et al. introduced a new crystalline carbon nitride which exhibited an AQY of 15% for photocatalytic H<sup>2</sup> production at 400 nm [32]. Thus, molecular doping proved itself as very efficient technique for efficient H<sup>2</sup> production. Tri-*s*-triazine-based g-C<sup>3</sup> N4 is commonly prepared from the thermal polymerization of carbon and nitrogen containing organic and inorganic materials such as melamine, cyanamide, urea, thiourea, ammonium thiocyanate and guanidine sulphate [33–41]. These materials are known as efficient photocatalysts for splitting of water. The g-C<sup>3</sup> N4 polymers prepared from different starting materials usually show different properties such as surface morphology, optical absorption, charge carrier ability and electronic band structure. So, particularly by this way, the performance for photochemical reaction can be varied. To create porosity in the structure, strategies, such as hard template method, self-assembly (SA) and exfoliation, are conducted to improve the photocatalytic performances. Maeda et al. successfully introduced the spine nanosphere carbon nitride *in situ* deposited with 3 wt% Pt as co-catalysts as efficient photocatalyst for hydrogen generation with a hydrogen evolution rate (HER) of 574 μmol h−1 [42]. The AQY for H<sup>2</sup> evolution at 420 nm can reach a value of 9.6%.

by changing the monomers. Optical properties are also tuned by different amount of monomers which reflect in the range of optical band gap between 1.94 and 2.95 eV with an absorption band edge 445–588 nm. When in a water/methanol mix solvent in the presence of TEOA, photocatalytic reaction to generate hydrogen of these materials was tested. **CP-CMP10** shows the highest activity with a hydrogen evolution rate of 17.4 μmol h−1. These polymers even showed hydrogen evolution performance without any co-catalyst such as Pt. It showed the optimal AQY for visible

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production is 2.62% in the presence of 80 vol.% diethylamine as the sacrificial agent.

Subsequently, Li et al. introduced donor-acceptor type microporous polymer by co-polymerization method [44]. Two types of monomeric units, electron-rich units (**M2**-**M4**) and electron-deficient (**M1**) units, were copolymerized with different ration of weak donor biphenyl and weak acceptor bipyridine to produce a series of four polymers (**Figure 7**). The calculated band gaps of all polymers were in the range of 2.0–2.8 eV. The light absorbing ability of these polymers is significantly improved. Charge separation is facilitated due to the intra-molecular charge transfer, which is in fact very essential to generate excited holes and electrons for water oxidation and proton reduction, respectively. **PCP4e** showed the best performance for photocatalytic hydrogen production with a HER of 33.0 μmol h−1. Bipyridyl-substituted polymers showed better performance than the biphenyl-containing polymer which can be attributed due to the presence of more electronegative N-atom, which may facilitate hydrogen adsorp-

On the other hand, Yang et al. reported benzothiadiazole-based donor-acceptor type conjugated microporous polymers as photocatalysts for hydrogen generation [15]. Benzothiadiazole unit is highly explored in organic conjugated materials for electronic devices due to its unique electron withdrawing ability. By co-polymerization technique starting with benzene and benzothiadiazole as basic monomeric unit and changing the substitution in benzene ring, they are able to synthesize a series of conjugated polymer from 1D to 3D in nature (**Figure 8**). **B-BT-1,4**

**Figure 7.** Structures of monomers and synthesis of PCP photocatalysts by Suzuki polycondensation and photographs of

the prepared **PCPa** and **PCPe** photocatalysts. Reproduced with permission from ACS; see Ref. [44].

tion through hydrogen bonding to enhance the proton reduction process.

light H2

#### **3.4. Conjugated microporous polymers (CMPs)**

Conjugated polymers with extended π-conjugation have been intensively explored in organic electronics and semiconductor device due to electronic and physical properties. Copolymerization of various monomeric units (building block) led to conjugated polymers, which allow to design various kind polymers with different electronic properties by varying the structural modification in monomeric units. Very recently, this kind of conjugated microporous polymers (CMPs) were successfully applied for photochemical hydrogen generation. Sprick et al. reported a series of CMP by varying the ratio of monomeric unit (**Figure 6**) [43]. The surface area is also tuneable

**Figure 6.** Schematic representation of reactions for the synthesis of CP-CMPs. Optical properties of the conjugated copolymer photocatalysts. Reproduced with permission from ACS; see Ref. [43].

by changing the monomers. Optical properties are also tuned by different amount of monomers which reflect in the range of optical band gap between 1.94 and 2.95 eV with an absorption band edge 445–588 nm. When in a water/methanol mix solvent in the presence of TEOA, photocatalytic reaction to generate hydrogen of these materials was tested. **CP-CMP10** shows the highest activity with a hydrogen evolution rate of 17.4 μmol h−1. These polymers even showed hydrogen evolution performance without any co-catalyst such as Pt. It showed the optimal AQY for visible light H2 production is 2.62% in the presence of 80 vol.% diethylamine as the sacrificial agent.

of sacrificial agents and co-catalyst (Pt, 5 wt.%), whereas it showed low activity in comparison

rimidine (**4AP**) as the dopant [31]. This modified PTI showed red-shifted absorption spectrum compared to the crystalline PTI and more similar to the melon structure, which improves the visible light absorption. By using this doped PTI, AQY of hydrogen evolution can be improved five times (3.4%), which is more than that of raw melon (0.6%). By supramolecular aggregation allowed by ionic melt polycondensation (IMP) of melamine and 2,4,6-triaminopyrimidine as dopants, Bhunia et al. introduced a new crystalline carbon nitride which exhibited an AQY

pared from the thermal polymerization of carbon and nitrogen containing organic and inorganic materials such as melamine, cyanamide, urea, thiourea, ammonium thiocyanate and guanidine sulphate [33–41]. These materials are known as efficient photocatalysts for splitting of water. The

 polymers prepared from different starting materials usually show different properties such as surface morphology, optical absorption, charge carrier ability and electronic band structure. So, particularly by this way, the performance for photochemical reaction can be varied. To create porosity in the structure, strategies, such as hard template method, self-assembly (SA) and exfoliation, are conducted to improve the photocatalytic performances. Maeda et al. successfully introduced the spine nanosphere carbon nitride *in situ* deposited with 3 wt% Pt as co-catalysts as efficient photocatalyst for hydrogen generation with a hydrogen evolution rate (HER) of 574

evolution at 420 nm can reach a value of 9.6%.

Conjugated polymers with extended π-conjugation have been intensively explored in organic electronics and semiconductor device due to electronic and physical properties. Copolymerization of various monomeric units (building block) led to conjugated polymers, which allow to design various kind polymers with different electronic properties by varying the structural modification in monomeric units. Very recently, this kind of conjugated microporous polymers (CMPs) were successfully applied for photochemical hydrogen generation. Sprick et al. reported a series of CMP by varying the ratio of monomeric unit (**Figure 6**) [43]. The surface area is also tuneable

**Figure 6.** Schematic representation of reactions for the synthesis of CP-CMPs. Optical properties of the conjugated

copolymer photocatalysts. Reproduced with permission from ACS; see Ref. [43].

which may be assigned to the weak optical absorption in the visible light

production at 400 nm [32]. Thus, molecular doping proved itself as

by using 4-amino-2, 6-dihydroxypy-

N4

is commonly pre-

Cl<sup>−</sup>

production. Tri-*s*-triazine-based g-C<sup>3</sup>

to that of Pt/g-C3

g-C<sup>3</sup> N4 N4

144 Visible-Light Photocatalysis of Carbon-Based Materials

of 15% for photocatalytic H<sup>2</sup>

μmol h−1 [42]. The AQY for H<sup>2</sup>

**3.4. Conjugated microporous polymers (CMPs)**

very efficient technique for efficient H<sup>2</sup>

region [30]. Schwinghammer et al. modify the PTI/Li<sup>+</sup>

Subsequently, Li et al. introduced donor-acceptor type microporous polymer by co-polymerization method [44]. Two types of monomeric units, electron-rich units (**M2**-**M4**) and electron-deficient (**M1**) units, were copolymerized with different ration of weak donor biphenyl and weak acceptor bipyridine to produce a series of four polymers (**Figure 7**). The calculated band gaps of all polymers were in the range of 2.0–2.8 eV. The light absorbing ability of these polymers is significantly improved. Charge separation is facilitated due to the intra-molecular charge transfer, which is in fact very essential to generate excited holes and electrons for water oxidation and proton reduction, respectively. **PCP4e** showed the best performance for photocatalytic hydrogen production with a HER of 33.0 μmol h−1. Bipyridyl-substituted polymers showed better performance than the biphenyl-containing polymer which can be attributed due to the presence of more electronegative N-atom, which may facilitate hydrogen adsorption through hydrogen bonding to enhance the proton reduction process.

On the other hand, Yang et al. reported benzothiadiazole-based donor-acceptor type conjugated microporous polymers as photocatalysts for hydrogen generation [15]. Benzothiadiazole unit is highly explored in organic conjugated materials for electronic devices due to its unique electron withdrawing ability. By co-polymerization technique starting with benzene and benzothiadiazole as basic monomeric unit and changing the substitution in benzene ring, they are able to synthesize a series of conjugated polymer from 1D to 3D in nature (**Figure 8**). **B-BT-1,4**

**Figure 7.** Structures of monomers and synthesis of PCP photocatalysts by Suzuki polycondensation and photographs of the prepared **PCPa** and **PCPe** photocatalysts. Reproduced with permission from ACS; see Ref. [44].


**Figure 8.** Structures of two series of polybenzothiadiazoles with different molecular designs and porosity data and electrochemical properties of the polymers. Reproduced with permission from Wiley-VCH; see Ref. [15].

itself showed photocatalytic activity with a HER of 12.0 μmol h−1. Upon addition of 3 wt% Pt as cocatalyst, the HER was enhanced to 116.0 μmol h−1. The AQY of **B-BT-1,4** for hydrogen evolution was found to be as high as 4.01%.

Chu et al. reported crystalline polyimide (PI) via high temperature dehydration reaction between melamine (MA) and pyromellitic dianhydride (PMDA) (**Figure 9**) [45]. A series of PIs with different optical band gaps (2.56–3.39 eV) were synthesized and the degree of polymerization is controlled by tuning the heating temperature. These polymers showed photocatalytic activity to produce hydrogen from water in the presence of Pt and TEOA as cocatalyst and sacrificial donor, respectively. Out of all PIs, **PI-325** showed the best activity with a HER of 150 μmol h−1 and 15.2 μmol h−1 under UV and visible light irradiation, respectively. **PI-325** showed AQY of 0.2% at 420 nm light irradiation.

**Figure 9.** Reaction scheme for the synthesis of PI, UV-vis absorption spectra of PI samples processed at different

under visible light irradiation (λ>420 nm). Reproduced with permission from Elsevier; see Ref. [45].

**Figure 10.** Molecular structures of the building units for the **ANW** materials and time course of H2

**ANW** networks from water containing 10vol. % triethanolamine as electron donor (λ>300 nm). Reproduced with

evolution from a 10 vol% aqueous methanol solution by Pt-deposited PI samples

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production through

temperatures and time course of H2

permission from RSC; see Ref. [10].

In early 2010, Schwab et al. successfully introduced a series of imine-based conjugate poly (azomethine) materials [10]. The Schiff-base condensation reaction were adopted between the 1,3,5-tris(4- aminophenyl)benzene (**1**) as amine residue with different aldehydes such as 1,4-phthalaldehyde (**2**), naphthalene-2,6-dicarbaldehyde (**3**), anthracene-2,6-dicarbaldehyde (**4**) and anthracene-9,10-dicarbaldehyde (**5**) to synthesize the polymers. The optical band gaps of the synthesized polymers were located in the range of 1.96–2.38 eV with an optical band edge of 532–632 nm. These polymers showed photocatalytic activity under UV-light irradiation with a HER of 7 μmol h−1 (**ANW-2**), whereas under visible light these polymers did not show any photocatalytic activity (**Figure 10**).

#### **3.5. Conjugated linear and planarized polymers**

As we mentioned before, Yanagida et al. in early 1985 reported poly(p-phenylene) as active photochemical hydrogen production catalyst under UV-light illumination [6]. Due to the solubility and low activity, there is no further advancement in this area. Recently, Sprick et al. synthesized and characterized a series of planar polymers by comprising phenylene and fluorine, carbazole, or dibenzo [b,d] thiophene [46]. From the study, they concluded that planar polymers showed higher photocatalytic activity to produce hydrogen. The pure copolymers Visible Light–Driven Hydrogen Production by Carbon based Polymeric Materials http://dx.doi.org/10.5772/intechopen.70444 147

itself showed photocatalytic activity with a HER of 12.0 μmol h−1. Upon addition of 3 wt% Pt as cocatalyst, the HER was enhanced to 116.0 μmol h−1. The AQY of **B-BT-1,4** for hydrogen

**Figure 8.** Structures of two series of polybenzothiadiazoles with different molecular designs and porosity data and

electrochemical properties of the polymers. Reproduced with permission from Wiley-VCH; see Ref. [15].

Chu et al. reported crystalline polyimide (PI) via high temperature dehydration reaction between melamine (MA) and pyromellitic dianhydride (PMDA) (**Figure 9**) [45]. A series of PIs with different optical band gaps (2.56–3.39 eV) were synthesized and the degree of polymerization is controlled by tuning the heating temperature. These polymers showed photocatalytic activity to produce hydrogen from water in the presence of Pt and TEOA as cocatalyst and sacrificial donor, respectively. Out of all PIs, **PI-325** showed the best activity with a HER of 150 μmol h−1 and 15.2 μmol h−1 under UV and visible light irradiation, respectively. **PI-325**

In early 2010, Schwab et al. successfully introduced a series of imine-based conjugate poly (azomethine) materials [10]. The Schiff-base condensation reaction were adopted between the 1,3,5-tris(4- aminophenyl)benzene (**1**) as amine residue with different aldehydes such as 1,4-phthalaldehyde (**2**), naphthalene-2,6-dicarbaldehyde (**3**), anthracene-2,6-dicarbaldehyde (**4**) and anthracene-9,10-dicarbaldehyde (**5**) to synthesize the polymers. The optical band gaps of the synthesized polymers were located in the range of 1.96–2.38 eV with an optical band edge of 532–632 nm. These polymers showed photocatalytic activity under UV-light irradiation with a HER of 7 μmol h−1 (**ANW-2**), whereas under visible light these polymers did not

As we mentioned before, Yanagida et al. in early 1985 reported poly(p-phenylene) as active photochemical hydrogen production catalyst under UV-light illumination [6]. Due to the solubility and low activity, there is no further advancement in this area. Recently, Sprick et al. synthesized and characterized a series of planar polymers by comprising phenylene and fluorine, carbazole, or dibenzo [b,d] thiophene [46]. From the study, they concluded that planar polymers showed higher photocatalytic activity to produce hydrogen. The pure copolymers

evolution was found to be as high as 4.01%.

146 Visible-Light Photocatalysis of Carbon-Based Materials

showed AQY of 0.2% at 420 nm light irradiation.

show any photocatalytic activity (**Figure 10**).

**3.5. Conjugated linear and planarized polymers**

**Figure 9.** Reaction scheme for the synthesis of PI, UV-vis absorption spectra of PI samples processed at different temperatures and time course of H2 evolution from a 10 vol% aqueous methanol solution by Pt-deposited PI samples under visible light irradiation (λ>420 nm). Reproduced with permission from Elsevier; see Ref. [45].

**Figure 10.** Molecular structures of the building units for the **ANW** materials and time course of H2 production through **ANW** networks from water containing 10vol. % triethanolamine as electron donor (λ>300 nm). Reproduced with permission from RSC; see Ref. [10].

(**P1**-**P7**) are also exhibited excellent visible light-induced H<sup>2</sup> evolution activities even without depositing any co-catalysts, whereas methanol and triethylamine mixture were used as the sacrificial electron donors. Out of all planar co-polymers, **P7** showed the highest photocatalytic activity with HER of 92 μmol h−1, while the rate was further found to be increased up to 116 μmol h−1 in the presence of Pt-nanoparticles as a co-catalyst. The AQY of hydrogen generation for **P7** was measured as 2.3% at 420 nm (**Figure 11**).

#### **3.6. Polymer dots (Pdots)**

Till discussed all materials behaved as heterogeneous catalyst because these all have poor dispersibility in water, to run the photocatalytic experiment in absolutely aqueous condition and the organic/water mixed solvent were commonly used to make good dispersibility. Another drawback is that majority materials were worked under UV light and also in the presence of a co-catalyst such as Pt. In the absence of co-catalyst and visible light, all show very unsatisfactory performances. Tian et al. used the conjugated polymer dots (Pdots), which is nicely dispersible in aqueous solution for photocatalytic proton reduction [47]. The Pdots of hydrophobic conjugated polymer (**PFBT**) were prepared through nanoprecipitation method by mixing with an amphiphilic co-polymers (**PS-PEG-COOH**). Typically, **PFBT** was dissolved in THF together with the co-polymer **PS-PEG-COOH** and then was poured into water. THF was removed later by continuously sonicating and Ar purging. The well-dispersed nanoparticles in water were obtained. Particle size of the nanoparticles was around 30–50 nm measured by dynamic light scattering (DLS). The optical band gap of PFBT Pdots was 2.38 eV. Without any co-catalyst in the presence of ascorbic acid as sacrificial donor, Pdots showed excellent photocatalytic activity with a HER of 8.3±0.2 mmol h−1 g−1. Pdots configuration is very important because the pristine polymer itself did not show activity under the same test condition. The AQY of hydrogen evolution was 0.5% at 445 nm. However, after 1.5 h, the Pdots deactivated and precipitated out from the aqueous medium by forming bigger particle (**Figure 12**).

respect to **Pdots 1**, **Pdots 2** shows even more stability and works for more than 4 hour. The

three injections of the headspace gas in the reaction flask. Reproduced with permission from Wiley-VCH; see Ref. [47].

**Figure 12.** Preparation of PFBT Pdots and the diagram for light-driven hydrogen generation; visible light-driven hydrogen generation from water at room temperature. Conditions: **PFBT** Pdots, ca. 16.8 mg/mL; ascorbic acid: 0.2 M; pH 4.0 (adjusted with 1 M NaOH); LED lamp: white light (l>420 nm); the kinetic curve is monitored by a hydrogen sensor

of **Pdots 2**, suggests that the evolving hydrogen is mainly coming from the water. Since this kind of conjugated polymers is prepared by Pd-catalysed copolymerization method, where the residual Pd is known to be a good catalyst for proton reduction and it could behave as co-catalyst for this photocatalytic reaction. However, from the Pd poisoning experiments with carbon monoxide (CO) and ethylenediaminetetraacetic acid (EDTA), it suggests that the Pdots

**Figure 13.** Molecular structures of three organic D–A polymers (**1**, **2** and **3**) and a co-polymer PS-PEG-COOH; lightdriven hydrogen generation of Pdots **1** (17 μg/ml), **2** (13 μg/ml) and **3** (19 μg/ml) in water at ambient temperature. Conditions: ascorbic acid: 0.2 M; pH 4.0 (adjusted with 1 M NaOH); white LED light (420 nm, 17 W, 5000 K). Inset: H<sup>2</sup>

bubbles observed during photocatalysis. Reproduced with permission from RSC; see Ref. [48].

O and NaOD instead of H<sup>2</sup>

generation of **Pdots 2** was measured as 0.6% at 550 nm. The isotopic labelling

O and NaOH in a photocatalytic experiment

generation confirmed by GC-MS, the three peaks are signals from

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149

AQY for H<sup>2</sup>

experiment, using D<sup>2</sup>

(see details in the Supporting Information). Inset: H<sup>2</sup>

In order to further improve the system and study the reactivity sites of photocatalysis, Tian et al. subsequently investigated structures insight for highly efficient Pdots by both experimental and theoretical prospect. Pdots prepared from **1** and **2** (**Figure 13**) with benzothiadiazole (BT) units showed photocatalytic activity with a HER of 8.3±0.2 mmol h−1 g−1 and 50±0.5 mmol h−1 g−1, whereas Pdots of **3** without such a unit did not show any photocatalytic activity. It suggests that the BT unit has crucial role in proton reduction reaction [48]. The theoretical study suggests that proton adsorption is facilitated due to the presence of N sites in the electron withdrawing BT unit which in fact enhances the photocatalytic activity. With

**Figure 11.** Schematic representation of reactions for the synthesis of planarized conjugated polymers. Reproduced with permission from Wiley-VCH; see Ref. [46].

Visible Light–Driven Hydrogen Production by Carbon based Polymeric Materials http://dx.doi.org/10.5772/intechopen.70444 149

(**P1**-**P7**) are also exhibited excellent visible light-induced H<sup>2</sup>

eration for **P7** was measured as 2.3% at 420 nm (**Figure 11**).

**3.6. Polymer dots (Pdots)**

148 Visible-Light Photocatalysis of Carbon-Based Materials

permission from Wiley-VCH; see Ref. [46].

depositing any co-catalysts, whereas methanol and triethylamine mixture were used as the sacrificial electron donors. Out of all planar co-polymers, **P7** showed the highest photocatalytic activity with HER of 92 μmol h−1, while the rate was further found to be increased up to 116 μmol h−1 in the presence of Pt-nanoparticles as a co-catalyst. The AQY of hydrogen gen-

Till discussed all materials behaved as heterogeneous catalyst because these all have poor dispersibility in water, to run the photocatalytic experiment in absolutely aqueous condition and the organic/water mixed solvent were commonly used to make good dispersibility. Another drawback is that majority materials were worked under UV light and also in the presence of a co-catalyst such as Pt. In the absence of co-catalyst and visible light, all show very unsatisfactory performances. Tian et al. used the conjugated polymer dots (Pdots), which is nicely dispersible in aqueous solution for photocatalytic proton reduction [47]. The Pdots of hydrophobic conjugated polymer (**PFBT**) were prepared through nanoprecipitation method by mixing with an amphiphilic co-polymers (**PS-PEG-COOH**). Typically, **PFBT** was dissolved in THF together with the co-polymer **PS-PEG-COOH** and then was poured into water. THF was removed later by continuously sonicating and Ar purging. The well-dispersed nanoparticles in water were obtained. Particle size of the nanoparticles was around 30–50 nm measured by dynamic light scattering (DLS). The optical band gap of PFBT Pdots was 2.38 eV. Without any co-catalyst in the presence of ascorbic acid as sacrificial donor, Pdots showed excellent photocatalytic activity with a HER of 8.3±0.2 mmol h−1 g−1. Pdots configuration is very important because the pristine polymer itself did not show activity under the same test condition. The AQY of hydrogen evolution was 0.5% at 445 nm. However, after 1.5 h, the Pdots deactivated and precipitated out from the aqueous medium by forming bigger particle (**Figure 12**).

In order to further improve the system and study the reactivity sites of photocatalysis, Tian et al. subsequently investigated structures insight for highly efficient Pdots by both experimental and theoretical prospect. Pdots prepared from **1** and **2** (**Figure 13**) with benzothiadiazole (BT) units showed photocatalytic activity with a HER of 8.3±0.2 mmol h−1 g−1 and 50±0.5 mmol h−1 g−1, whereas Pdots of **3** without such a unit did not show any photocatalytic activity. It suggests that the BT unit has crucial role in proton reduction reaction [48]. The theoretical study suggests that proton adsorption is facilitated due to the presence of N sites in the electron withdrawing BT unit which in fact enhances the photocatalytic activity. With

**Figure 11.** Schematic representation of reactions for the synthesis of planarized conjugated polymers. Reproduced with

evolution activities even without

**Figure 12.** Preparation of PFBT Pdots and the diagram for light-driven hydrogen generation; visible light-driven hydrogen generation from water at room temperature. Conditions: **PFBT** Pdots, ca. 16.8 mg/mL; ascorbic acid: 0.2 M; pH 4.0 (adjusted with 1 M NaOH); LED lamp: white light (l>420 nm); the kinetic curve is monitored by a hydrogen sensor (see details in the Supporting Information). Inset: H<sup>2</sup> generation confirmed by GC-MS, the three peaks are signals from three injections of the headspace gas in the reaction flask. Reproduced with permission from Wiley-VCH; see Ref. [47].

respect to **Pdots 1**, **Pdots 2** shows even more stability and works for more than 4 hour. The AQY for H<sup>2</sup> generation of **Pdots 2** was measured as 0.6% at 550 nm. The isotopic labelling experiment, using D<sup>2</sup> O and NaOD instead of H<sup>2</sup> O and NaOH in a photocatalytic experiment of **Pdots 2**, suggests that the evolving hydrogen is mainly coming from the water. Since this kind of conjugated polymers is prepared by Pd-catalysed copolymerization method, where the residual Pd is known to be a good catalyst for proton reduction and it could behave as co-catalyst for this photocatalytic reaction. However, from the Pd poisoning experiments with carbon monoxide (CO) and ethylenediaminetetraacetic acid (EDTA), it suggests that the Pdots

**Figure 13.** Molecular structures of three organic D–A polymers (**1**, **2** and **3**) and a co-polymer PS-PEG-COOH; lightdriven hydrogen generation of Pdots **1** (17 μg/ml), **2** (13 μg/ml) and **3** (19 μg/ml) in water at ambient temperature. Conditions: ascorbic acid: 0.2 M; pH 4.0 (adjusted with 1 M NaOH); white LED light (420 nm, 17 W, 5000 K). Inset: H<sup>2</sup> bubbles observed during photocatalysis. Reproduced with permission from RSC; see Ref. [48].

are the real photocatalysts. The DFT computation also suggests the unique structure of the Pdots from aggregated polymers benefits for lowering the energy barrier of proton reduction process via N atoms in BT units.

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### **4. Perspective**

Library of carbon-based organic photocatalysts are very few in comparison to the inorganic photocatalysts. The performance of these materials are still unsatisfactory but these organic materials have several advantages such as light-weight, easily processable and better reproducibility upon batch-to-batch variation. The organic photocatalysts are of great interest because there is a wide opportunity to introduce several new materials with different optical and electrochemical properties by small structural modification. By proper structural modification, the performance of these materials can be tuned and would be comparable with the inorganic materials. The major drawback of all of these organic carbon-based material is water dispersibility because of very poor solubility in water. The polymer dot encounters the solubility issue by some extent whereas rather need optimization to solve the associated problem such as unwanted aggregation along with light illumination, which causes deactivation and so on.

### **Acknowledgements**

The authors thanks the financial support from KA Wallenberg Foundation, Swedish Energy Agency, Sweden VR-Korea NRF joint climate change project, Stiftelsen Olle Engkvist Byggmästare and Göran Gustafssons Stiftelse.

### **Author details**

Palas Baran Pati<sup>1</sup> , SU-IL In<sup>2</sup> and Haining Tian1 \*

\*Address all correspondence to: haining.tian@kemi.uu.se

1 Physical Chemistry, Department of Chemistry-Ångström Laboratory, Uppsala University, Uppsala, Sweden

2 Department of Energy Systems Engineering, DGIST, Daegu, Republic of Korea

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are the real photocatalysts. The DFT computation also suggests the unique structure of the Pdots from aggregated polymers benefits for lowering the energy barrier of proton reduction

Library of carbon-based organic photocatalysts are very few in comparison to the inorganic photocatalysts. The performance of these materials are still unsatisfactory but these organic materials have several advantages such as light-weight, easily processable and better reproducibility upon batch-to-batch variation. The organic photocatalysts are of great interest because there is a wide opportunity to introduce several new materials with different optical and electrochemical properties by small structural modification. By proper structural modification, the performance of these materials can be tuned and would be comparable with the inorganic materials. The major drawback of all of these organic carbon-based material is water dispersibility because of very poor solubility in water. The polymer dot encounters the solubility issue by some extent whereas rather need optimization to solve the associated problem such as unwanted aggregation along with light illumination, which causes deactiva-

The authors thanks the financial support from KA Wallenberg Foundation, Swedish Energy Agency, Sweden VR-Korea NRF joint climate change project, Stiftelsen Olle Engkvist

\*

1 Physical Chemistry, Department of Chemistry-Ångström Laboratory, Uppsala University,

[1] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Chemical Reviews. Dye-sensitized

process via N atoms in BT units.

150 Visible-Light Photocatalysis of Carbon-Based Materials

**4. Perspective**

tion and so on.

**Author details**

Palas Baran Pati<sup>1</sup>

Uppsala, Sweden

**References**

**Acknowledgements**

Byggmästare and Göran Gustafssons Stiftelse.

, SU-IL In<sup>2</sup>

solar cells. 2016;**110**:6595

\*Address all correspondence to: haining.tian@kemi.uu.se

and Haining Tian1

2 Department of Energy Systems Engineering, DGIST, Daegu, Republic of Korea


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## *Edited by Yunjin Yao*

Carbon-based photocatalysis has been considered as an economic, safe, renewable, and clean technology, for various applications. However, the pristine carbon material is usually restricted by unsatisfactory photocatalytic efficiency and practical applications due to the insufficient solar light absorption, the low-surface area, and the fast recombination of photogenerated electron-hole pairs. Various modification strategies, such as elemental and molecular doping, preparation of mesoporous carbon materials, and combination of conductive materials, are adopted to enhance the photocatalytic activity of carbon materials. In this book, we intend to describe the great potential of efficient and low-cost carbon-based materials in various realms, such as photodegradation of organic compounds, water splitting, and selective organic transformations.

Published in London, UK © 2018 IntechOpen © Arsgera / iStock

Visible-Light Photocatalysis of Carbon-Based Materials

Visible-Light Photocatalysis

of Carbon-Based Materials

*Edited by Yunjin Yao*