2.1. Use of ZnO/graphene photocatalysts

Photocatalysts are based on semiconductor materials, which are activated by radiation with higher energy than the bandgap of the used semiconductor, in order to create hole-electron pairs, once the electron passes from valence to conduction band. The promoted electron, toward conduction band, and the hole remained in valence band react in the photocatalyst interface with adsorbed substances in order to create reactive entities (free radicals and/or radical anions), which interact with contaminants to degrade them. All these processes imply the sorption on the photocatalyst's interface, its activation by radiation, creation of reactive species, and the recombination of electron to hole. The last could occur very fast that the creation of reactive

species does not take place at great extent, giving ineffective photodegradation process.

groups also consider the modifications with electron-rich species like graphene.

bismuth oxychloride have gained attention for photocatalytic process.

phenol is presented and discussed.

58 Photocatalysts - Applications and Attributes

2. Zinc oxide

parameters implied in the photocatalysis.

In this context, the research regarding photocatalysis, in recent years, is focused to decrease the bandgap of photocatalyst in order to use solar light rather than UV light, which implies additional cost during the treatment process. In addition, minimization of the recombination process, in order to improve the photocatalytic performance, is quite important too. Some approaches to face these issues consider the doping with metals; meanwhile other research

TiO2 has been the photocatalyst by excellence and has been widely studied in its pristine form and/or doped with metals like gold and other elements. Nevertheless, other metal oxides, such as ZnO, have shown better photocatalytic performance, especially when visible light is used. ZnO has also been doped with metals like gold or silver. More recently, other metal oxides like

In this chapter, discussion about the modification of ZnO with graphene is presented and discussed in terms of the implied mechanism (hybridization), in addition to the obtained results when such hybrid photocatalyst was used for the photodegradation of triclosan under visible light. Additionally, the modification method of ZnO with silver nanoparticles and its effect on photocatalytic performance for bisphenol A, Rhodamine B (RhB), and Triclosan is presented.

Finally, the use of an attractive photocatalyst, bismuth oxychloride (BiOCl), and its respective modification with silver and graphene oxide for RhB photodegradation along with the result and mechanism for the photocatalyst based on TiO2-BiOCl used for photodegradation of

The three different photocatalysts show how effective photocatalyst can be obtained and modified. The presented and discussed results contribute to understand some of the key

Zinc oxide (ZnO) is a II-VI group semiconductor with wurtzite structure with lattice parameters a = 0.3296 nm and c = 0.52065 nm. Zn2+ atoms are tetrahedrally coordinated with O2 atoms stacked alternately along the c axis so that d-electrons of zinc are hybridized with 2p-electrons of oxygen [2]. ZnO has the potential to become an important material for photocatalysis because it

is nontoxic (it is often included in pharmaceuticals), photostable, and low cost.

Nowadays, solar energy, radiant light, and heat from the sun are the most abundant available sources of clean energy. Thus, research studies and development of materials that can efficiently harvest solar irradiation and used for green environmental pollution management are essential. Photocatalysis, which could use renewable solar energy to activate the chemical reactions via oxidation and reduction, such as that occurs in advance oxidation processes (AOPs), is a sustainable technology to provide solution for environmental issue. This photocatalysis system has attracted great interest from science community as the most promising way to solve the environmental problems, especially getting rid of residual pollutants from wastewater stream.

In the field of photocatalysis, ZnO has emerged as the leading candidate for green environmental management systems because of its unique characteristics, such as wide bandgap (3.37 eV) in the near-UV spectral region, a large electron exciton binding energy of 60 eV at room temperature, strong oxidation ability, and good photocatalytic property [4]. It is a wellknown fact that ZnO occurs as white hexagonal crystal or white powder known as white zinc. ZnO crystallizes in the wurtzite structure and is available as large bulk single crystals [5]. As an important semiconductor material, ZnO has been applied in catalysis, rubber and paint industries, ceramic bodies, varistors, fertilizers, and cosmetics [6].

Recently, the development of ZnO with precisely controllable features has gained significant scientific interest. The electrical, optical, and magnetic properties of ZnO can be altered or improved by the use of ZnO in nanoscale and efforts have been developed to improve the properties of ZnO photocatalyst [7]. Moreover, ZnO is an environmental friendly material as it is compatible with living organisms [8]. Since ZnO has almost the same bandgap energy as TiO2 (3.2 eV), its photocatalytic capability is anticipated to be similar to that of TiO2. However, ZnO is relatively cheaper compared to TiO2, whereby the usage of titanium dioxide is uneconomic for large-scale water treatment operations [9]. The greatest advantage of ZnO is the ability to absorb a wide range of solar spectrum and more light quanta than some semiconducting metal oxides including the capacity to absorb visible light energy, which is due to its wide band energy. This results in fast recombination of photogenerated charges and thus caused low photocatalytic efficiency.

The recombination of photogenerated hole (hVB<sup>+</sup> ) and electron (eCB) is one of the major disadvantages in semiconductor photocatalysis. This recombination step lowers the quantum yield and causes energy wasting. Therefore, the e/h<sup>+</sup> recombination process should be inhibited to ensure efficient photocatalysis. Metal doping could counter the recombination problem with efficient charge separation between electrons and holes in ZnO photocatalyst. In addition, the dopants may trap electrons, reducing the chances of e/h<sup>+</sup> recombination that deactivate the photocatalytic system [10]. Furthermore, the generation of hydroxyl radicals and active oxygen species will greatly increase resulting from the enhancement in charge separation efficiency [11]. Semiconductors as graphene have been proven as a couple semiconductor that can improve the visible-light photocatalytic efficiency of ZnO due to its remarkable chemical, physical, and mechanical properties, such as large surface area (2600 m<sup>2</sup> /g), excellent electrical and thermal conductivity, high mechanical strength, flexibility, and efficient wide range of light adsorption. Due to the properties of graphene-based materials, several ranges of environmental applications have been developed such as absorption, transformation, and detection [12]. So far, numerous methods have been used to design and synthesize ZnO/ graphene hybrid photocatalysts with various morphologies. However, most of these methods rely on chemical and/or high-energy consumption resulting in a costly, environmentally hazardous, and especially inefficient photocatalyst for complete degradation of organic pollutants as triclosan, which has been classified as potential endocrine disrupting compound (EDC). Triclosan was ranked as the most abundant compound among all investigated pharmaceuticals and personal care products with its mean concentration of 12.6 3.8 mg/Kg in 110 biosolids samples collected from 94 US wastewater treatment plants across 32 states and the District of Columbia using EPA Method 1694 [13]. In addition, the highest initial concentrations of triclosan detected in municipal biosolids were 2715 and 1265 μg/Kg [14].

graphene sheets as dopant in ZnO nanocatalyst. It is important to mention that wurtzite has been the structure used in this study with a bandgap of 3.21 eV obtained by UV-vis spectrophotometer, while a value of 3.15 eV for ZnO/graphene composite was obtained. This reduction in the bandgap value is associated to a structure based on good interaction between ZnO nanocatalyst and graphene sheets, which creates intermediate energy levels between both materials, a property that allows to transfer the electrons promoted from the valence band to the conductive band of the ZnO semiconductor to graphene sheets that capture and retain the transferred electrons, improving the photocatalytic performance of ZnO as it is illustrated in Figure 2. This important first approximation has been the result of the evaluation of the effect of the loaded graphene amount in zinc oxide nano-photocatalyst. Graphene concentrations of 0.25 and 0.5 wt% were evaluated for this purpose and the obtained results showed enhancement of the ZnO photocatalytic activity under visible light radiation even using minimum contents of graphene sheets. Thus, 34 and 36% of the initial concentration of triclosan (8 ppm) was degraded using ZnO/graphene composites loaded with 0.25 and 0.5 wt% of graphene, respectively, and the bandgap values were 3.19 and 3.18 eV, for such ZnO/graphene composites. In contrast with 25% of initial triclosan degraded using pristine ZnO (bandgap: 3.21 eV). These results can be compared with those reported in previous investigations related with the photodegradation of triclosan using dopants such as rare-earth elements as Ce (47%) [17], metals such as Au (10% after 5 h) [18], Ag [19], and Cu [20]; as well as oxide compounds as

Figure 1. Schematic illustration of the interaction between ZnO photocatalyst doped with graphene sheets and its

Modified Metallic Oxides for Efficient Photocatalysis http://dx.doi.org/10.5772/intechopen.80834 61

interaction with triclosan.

MgO, WO3, TiO2, ZnO, or graphene oxide [21] using dopant contents up to 10%.

The adsorption properties of the as-prepare ZnO/graphene hybrid photocatalysts doped with different amounts of graphene sheets were one of the most important characteristics to improve the degradation efficiency of the ZnO. The specific area of ZnO and ZnO/graphene

Hydrothermal and chemical reductions have been the main methods studied to obtain ZnO/ graphene hybrid photocatalysts. However, photodeposition method [15] has been reported as an efficient method to generate hybrid photocatalyst to degrade pollutants. This study proposes the synthesis of ZnO/graphene semiconductors by developing a facile, cheap, environmentally and high reproducibility approach to obtain an efficient material to degrade organic pollutants, as triclosan (TCS), under visible light. For instance, it has been demonstrated that inductive irradiation method is possible to synthesize ZnO composites due to its polarity [16]. In particular, graphene has the ability to accept electrons efficiently due to the absence of oxygen chemical groups on its surface preventing the recombination and providing a favorable π-π conjugation between TCS and aromatic region of graphene (Figure 1). The trapped electrons on graphene react with the dissolved oxygen and water to form reactive superoxide and hydroxyl radicals, which further oxidizes triclosan.

The modified photodeposition method has resulted in a successful process to prepare ZnO/ graphene hybrid photocatalysts with enhanced photocatalytic activity under visible light radiation. The obtained results show degradation of 1% of triclosan (8 ppm) in absence of catalyst (photolysis) while this degradation percentage increases up to 42% using 0.5 wt% of

wide band energy. This results in fast recombination of photogenerated charges and thus

disadvantages in semiconductor photocatalysis. This recombination step lowers the quantum yield and causes energy wasting. Therefore, the e/h<sup>+</sup> recombination process should be inhibited to ensure efficient photocatalysis. Metal doping could counter the recombination problem with efficient charge separation between electrons and holes in ZnO photocatalyst. In addition, the dopants may trap electrons, reducing the chances of e/h<sup>+</sup> recombination that deactivate the photocatalytic system [10]. Furthermore, the generation of hydroxyl radicals and active oxygen species will greatly increase resulting from the enhancement in charge separation efficiency [11]. Semiconductors as graphene have been proven as a couple semiconductor that can improve the visible-light photocatalytic efficiency of ZnO due to its remarkable

electrical and thermal conductivity, high mechanical strength, flexibility, and efficient wide range of light adsorption. Due to the properties of graphene-based materials, several ranges of environmental applications have been developed such as absorption, transformation, and detection [12]. So far, numerous methods have been used to design and synthesize ZnO/ graphene hybrid photocatalysts with various morphologies. However, most of these methods rely on chemical and/or high-energy consumption resulting in a costly, environmentally hazardous, and especially inefficient photocatalyst for complete degradation of organic pollutants as triclosan, which has been classified as potential endocrine disrupting compound (EDC). Triclosan was ranked as the most abundant compound among all investigated pharmaceuticals and personal care products with its mean concentration of 12.6 3.8 mg/Kg in 110 biosolids samples collected from 94 US wastewater treatment plants across 32 states and the District of Columbia using EPA Method 1694 [13]. In addition, the highest initial concentra-

chemical, physical, and mechanical properties, such as large surface area (2600 m<sup>2</sup>

tions of triclosan detected in municipal biosolids were 2715 and 1265 μg/Kg [14].

and hydroxyl radicals, which further oxidizes triclosan.

Hydrothermal and chemical reductions have been the main methods studied to obtain ZnO/ graphene hybrid photocatalysts. However, photodeposition method [15] has been reported as an efficient method to generate hybrid photocatalyst to degrade pollutants. This study proposes the synthesis of ZnO/graphene semiconductors by developing a facile, cheap, environmentally and high reproducibility approach to obtain an efficient material to degrade organic pollutants, as triclosan (TCS), under visible light. For instance, it has been demonstrated that inductive irradiation method is possible to synthesize ZnO composites due to its polarity [16]. In particular, graphene has the ability to accept electrons efficiently due to the absence of oxygen chemical groups on its surface preventing the recombination and providing a favorable π-π conjugation between TCS and aromatic region of graphene (Figure 1). The trapped electrons on graphene react with the dissolved oxygen and water to form reactive superoxide

The modified photodeposition method has resulted in a successful process to prepare ZnO/ graphene hybrid photocatalysts with enhanced photocatalytic activity under visible light radiation. The obtained results show degradation of 1% of triclosan (8 ppm) in absence of catalyst (photolysis) while this degradation percentage increases up to 42% using 0.5 wt% of

) and electron (eCB) is one of the major

/g), excellent

caused low photocatalytic efficiency.

60 Photocatalysts - Applications and Attributes

The recombination of photogenerated hole (hVB<sup>+</sup>

Figure 1. Schematic illustration of the interaction between ZnO photocatalyst doped with graphene sheets and its interaction with triclosan.

graphene sheets as dopant in ZnO nanocatalyst. It is important to mention that wurtzite has been the structure used in this study with a bandgap of 3.21 eV obtained by UV-vis spectrophotometer, while a value of 3.15 eV for ZnO/graphene composite was obtained. This reduction in the bandgap value is associated to a structure based on good interaction between ZnO nanocatalyst and graphene sheets, which creates intermediate energy levels between both materials, a property that allows to transfer the electrons promoted from the valence band to the conductive band of the ZnO semiconductor to graphene sheets that capture and retain the transferred electrons, improving the photocatalytic performance of ZnO as it is illustrated in Figure 2. This important first approximation has been the result of the evaluation of the effect of the loaded graphene amount in zinc oxide nano-photocatalyst. Graphene concentrations of 0.25 and 0.5 wt% were evaluated for this purpose and the obtained results showed enhancement of the ZnO photocatalytic activity under visible light radiation even using minimum contents of graphene sheets. Thus, 34 and 36% of the initial concentration of triclosan (8 ppm) was degraded using ZnO/graphene composites loaded with 0.25 and 0.5 wt% of graphene, respectively, and the bandgap values were 3.19 and 3.18 eV, for such ZnO/graphene composites. In contrast with 25% of initial triclosan degraded using pristine ZnO (bandgap: 3.21 eV). These results can be compared with those reported in previous investigations related with the photodegradation of triclosan using dopants such as rare-earth elements as Ce (47%) [17], metals such as Au (10% after 5 h) [18], Ag [19], and Cu [20]; as well as oxide compounds as MgO, WO3, TiO2, ZnO, or graphene oxide [21] using dopant contents up to 10%.

The adsorption properties of the as-prepare ZnO/graphene hybrid photocatalysts doped with different amounts of graphene sheets were one of the most important characteristics to improve the degradation efficiency of the ZnO. The specific area of ZnO and ZnO/graphene hybrid photocatalysts was determined via N2 adsorption isotherms using the Brunauer Emmett Teller (BET) method. The results revealed that the graphene monolayers (44.2 g/m<sup>2</sup> ) showed the highest surface area for the analyzed pristine material, while ZnO had a specific area of 10.8 g/m<sup>2</sup> . Among the three investigated composites, the photocatalyst loaded with 0.5 wt% of graphene had the highest surface area (18.3 g/m2 ), followed by ZnO/graphene 0.25 wt% (14 g/m<sup>2</sup> ) and ZnO/graphene 0.1 wt% (13.3 g/m2 ) catalysts. Thus, the addition of graphene sheets increases the surface area of the hybrid catalysts up to 69% of the initial area of zinc oxide and improve their adsorption capacities, resulting in the first property of the asprepare hybrid composites to increase the degradation of triclosan, as it has been reported previously [22].

confirms the interaction between ZnO and GO given by sp3 defects. The results given hereinabove are consistent with the results in FTIR characterization, revealing the reestablishment of the conjugated graphene oxide network. In contrast, the ratio ID/IG for the composite prepared with graphene sheets was calculated and corresponds to the value of 1.04 (higher than ID/IG of pristine graphene, 0.36) indicating the presence of more defects in the graphene oxide lattice, which implies a decrease in the size of the in-plane sp<sup>2</sup> domains and formation of the defects and disorders in the graphene sheets, revealing the reestablishment of the conjugated graphene network (sp<sup>2</sup> carbon) [23] in the ZnO/GO photocatalysts due to the hybridization of graphene via photoirradiation method but resulting in a weak interaction due to the absence of chemical groups on the surface of graphene sheets as are presented on GO surface. The interaction between graphene oxide sheets was improved by the interaction of ZnO-polarized structure and the chemical surface of graphene oxide, which contains carbonyl, carboxyl,

The degradation curves of triclosan for ZnO/GO hybrid photocatalyst are presented in Figure 3. It is noticed that the composites obtained by the photodeposition method are the materials with the best performance to degrade triclosan under visible light radiation compared with photolysis

In addition to the photodegradation curves presented in Figure 3, the rate constants are presented

The results show the highest apparent degradation rate constant for the hybrid catalyst syn-

Figure 3. Triclosan photodegradation curves with ZnO/graphene and ZnO/GO hybrid photocatalysts under visible light.

semiconductor, corroborating the importance of interaction between ZnO and GO.

), almost four times higher than the pristine ZnO

Modified Metallic Oxides for Efficient Photocatalysis http://dx.doi.org/10.5772/intechopen.80834 63

experiment and with the degradation using the bare ZnO photocatalyst.

epoxy, and hydroxyl groups.

thesized with graphene oxide (0.003 min<sup>1</sup>

in Table 1.

Conditions: 23C, pH = 7.

In order to study the effect of the method of synthesis in the ZnO/graphene hybrids, composites were prepared by impregnation method with graphene contents of 0.5 wt% using continuous stirring in the absence of UV radiation. The resultant photocatalysts were tested to degrade triclosan under visible light radiation, and it was obtained that 15% of the initial concentration of triclosan was degraded, 53% less than the composites synthesized by photodeposition method as it can be observed in Figure 2.

The hybridization of ZnO with graphene has been studied and confirmed by Raman spectroscopy integrating the intensity ratio of the D to G bands characteristic for carbon materials. The G peak arises from the stretching of C-C bond of graphite materials and is highly sensitive to strain effects in sp<sup>2</sup> system, while D peak is caused by the disordered structure of graphene material. Regarding the sp<sup>3</sup> and sp<sup>2</sup> hybridizations ID/IG value, in the case of pristine graphene oxide was 0.93, indicating that the intensity of the G band is higher than D band, which results in a lower amount of sp<sup>3</sup> defects and less structural disorder. Compared with graphene oxide (0.93), the reduction of ID/IG ratio for ZnO/GO 0.5 wt% hybrid photocatalyst (ID/IG = 0.91) is observed, implying a reduction of sp<sup>3</sup> defects compared with pure graphene oxide. This fact

Figure 2. Degradation curves of triclosan under visible light radiation conditions: 23C, pH = 7.

confirms the interaction between ZnO and GO given by sp3 defects. The results given hereinabove are consistent with the results in FTIR characterization, revealing the reestablishment of the conjugated graphene oxide network. In contrast, the ratio ID/IG for the composite prepared with graphene sheets was calculated and corresponds to the value of 1.04 (higher than ID/IG of pristine graphene, 0.36) indicating the presence of more defects in the graphene oxide lattice, which implies a decrease in the size of the in-plane sp<sup>2</sup> domains and formation of the defects and disorders in the graphene sheets, revealing the reestablishment of the conjugated graphene network (sp<sup>2</sup> carbon) [23] in the ZnO/GO photocatalysts due to the hybridization of graphene via photoirradiation method but resulting in a weak interaction due to the absence of chemical groups on the surface of graphene sheets as are presented on GO surface. The interaction between graphene oxide sheets was improved by the interaction of ZnO-polarized structure and the chemical surface of graphene oxide, which contains carbonyl, carboxyl, epoxy, and hydroxyl groups.

hybrid photocatalysts was determined via N2 adsorption isotherms using the Brunauer Emmett Teller (BET) method. The results revealed that the graphene monolayers (44.2 g/m<sup>2</sup>

showed the highest surface area for the analyzed pristine material, while ZnO had a specific

graphene sheets increases the surface area of the hybrid catalysts up to 69% of the initial area of zinc oxide and improve their adsorption capacities, resulting in the first property of the asprepare hybrid composites to increase the degradation of triclosan, as it has been reported

In order to study the effect of the method of synthesis in the ZnO/graphene hybrids, composites were prepared by impregnation method with graphene contents of 0.5 wt% using continuous stirring in the absence of UV radiation. The resultant photocatalysts were tested to degrade triclosan under visible light radiation, and it was obtained that 15% of the initial concentration of triclosan was degraded, 53% less than the composites synthesized by

The hybridization of ZnO with graphene has been studied and confirmed by Raman spectroscopy integrating the intensity ratio of the D to G bands characteristic for carbon materials. The G peak arises from the stretching of C-C bond of graphite materials and is highly sensitive to strain effects in sp<sup>2</sup> system, while D peak is caused by the disordered structure of graphene material. Regarding the sp<sup>3</sup> and sp<sup>2</sup> hybridizations ID/IG value, in the case of pristine graphene oxide was 0.93, indicating that the intensity of the G band is higher than D band, which results in a lower amount of sp<sup>3</sup> defects and less structural disorder. Compared with graphene oxide (0.93), the reduction of ID/IG ratio for ZnO/GO 0.5 wt% hybrid photocatalyst (ID/IG = 0.91) is observed, implying a reduction of sp<sup>3</sup> defects compared with pure graphene oxide. This fact

0.5 wt% of graphene had the highest surface area (18.3 g/m2

photodeposition method as it can be observed in Figure 2.

Figure 2. Degradation curves of triclosan under visible light radiation conditions: 23C, pH = 7.

) and ZnO/graphene 0.1 wt% (13.3 g/m2

. Among the three investigated composites, the photocatalyst loaded with

area of 10.8 g/m<sup>2</sup>

62 Photocatalysts - Applications and Attributes

0.25 wt% (14 g/m<sup>2</sup>

previously [22].

)

), followed by ZnO/graphene

) catalysts. Thus, the addition of

The degradation curves of triclosan for ZnO/GO hybrid photocatalyst are presented in Figure 3. It is noticed that the composites obtained by the photodeposition method are the materials with the best performance to degrade triclosan under visible light radiation compared with photolysis experiment and with the degradation using the bare ZnO photocatalyst.

In addition to the photodegradation curves presented in Figure 3, the rate constants are presented in Table 1.

The results show the highest apparent degradation rate constant for the hybrid catalyst synthesized with graphene oxide (0.003 min<sup>1</sup> ), almost four times higher than the pristine ZnO semiconductor, corroborating the importance of interaction between ZnO and GO.

Figure 3. Triclosan photodegradation curves with ZnO/graphene and ZnO/GO hybrid photocatalysts under visible light. Conditions: 23C, pH = 7.


stronger than those for O2

trons [24] as shown in Figure 5.

development of ZnO/graphene hybrid photocatalysts.

2.2. ZnO modification by inorganic molecules: silver

•, which suggests the predominance of oxidative reactions whose

Modified Metallic Oxides for Efficient Photocatalysis http://dx.doi.org/10.5772/intechopen.80834 65

holes are responsible for the degradation of triclosan. Furthermore, ZnO photocatalyst presents visible light photocatalytic activity but generates stronger visible light after graphene hybridization, showing that graphene oxide is responsible for the visible light performance, which is induced by the injection of an excited electron from the lowest unoccupied molecular orbit (LUMO) of graphene to the conduction band (CB) of ZnO. The introduction of the graphene semiconductors can possibly cause the rapid separation of electron-hole pairs during irradiation [11] prolonging the electron-hole pair lifetime and accelerating the transfer rate of elec-

In conclusion, the adsorption properties, good interaction between ZnO- and graphene-based materials, chemical structure of graphene, method of synthesis, and concentration of the dopant used to hybridize ZnO catalyst are the most important properties that affect the

Silver nanoparticles are linked to the ZnO surface through the alkanethiol surfactant (Figure 6a). The stabilizer ligands COOH-(CH2)n-S-Ag keep nanoparticles as small as 7 nm in the solution and 15–26 nm in the Ag/ZnO photocatalyst, corroborated by XRD; although TEM analysis shows Ag nanoparticles of spherical shape and defined boundaries smallest as 3 nm over ZnO surface (Figure 6b-d). We studied the effect of the pH and time on the functionalization of ZnO nanoagglomerates by two methods denominated as photodeposition (PD) and impregnation (IMP) [15]. We propose to replace the ambiguous terms "doping" with functionalization when spoke of superficial ZnO modification. For instance, the sample 1%Ag/ZnO-PD11,1 synthesized by the PD method using 1 wt.% Ag at pH 11 and 1 h under UV light, a functionalization yield of 100% was corroborated by elemental analysis by inductively coupled plasma spectrometry ICP-OES and SEM-EDX. The UV irradiation produces free radicals (●OH, ●O) that degrade the ligand and release the silver nanoparticles onto the ZnO surface (Figure 6c). Byproducts of surfactant decomposition accumulate in ZnO are observed by IR. This functionalization of the

Figure 5. Schematic illustration of the photocatalytic process in the ZnO/GO hybrid semiconductor.

Table 1. Apparent rate constants for triclosan photodegradation under visible light.

Figure 4. EPR spectra of ZnO/GO hybrid photocatalyst irradiated under visible light in (a) water and (b) ethanol solvents. DMPO was used as a radical trapper.

The mechanism of the best as-prepare ZnO/GO hybrid composite was proposed. Thus, the ESR spin-trap technique (with DMPO) was used to monitor the reactive oxygen species generated during the irradiation of the hybrid photocatalyst and the results are shown in Figure 4. Both signals of DMPO-• OH and DMPO-O2 • are clearly observed when pristine ZnO and ZnO/GO were exposed to visible radiation. Therefore, a dual mechanism involving both hydroxyl radicals and superoxide radicals is expected in the photocatalytic process. However, the signals for hybrid photocatalysts are stronger than the signals in pristine ZnO, thus accounting for the higher and stable photocatalytic performance of hybrid composites than bare ZnO toward the degradation of triclosan. The hydroxyl radicals trapped by DMPO (DMPO-• OH) and superoxide radicals (DMPO-O2 •) for ZnO and hybrid photocatalysts were characterized by detecting four characteristic signals in water, and six signals in ethanol for DMPO-• OH and DMPO-O2 •, respectively. In both cases, it is noticed that the signals for hybrid material are more pronounced than those for the ZnO pristine sample, thus accounting for the better photocatalytic performance.

Thus, the enhanced photocatalytic activity of ZnO/GO photocatalyst is due to the introduction of carbon material, which promotes an increase in charge separation to effective utilization of electrons to produce more • OH and O2 • radicals. In this case, the signals of • OH radicals are stronger than those for O2 •, which suggests the predominance of oxidative reactions whose holes are responsible for the degradation of triclosan. Furthermore, ZnO photocatalyst presents visible light photocatalytic activity but generates stronger visible light after graphene hybridization, showing that graphene oxide is responsible for the visible light performance, which is induced by the injection of an excited electron from the lowest unoccupied molecular orbit (LUMO) of graphene to the conduction band (CB) of ZnO. The introduction of the graphene semiconductors can possibly cause the rapid separation of electron-hole pairs during irradiation [11] prolonging the electron-hole pair lifetime and accelerating the transfer rate of electrons [24] as shown in Figure 5.

In conclusion, the adsorption properties, good interaction between ZnO- and graphene-based materials, chemical structure of graphene, method of synthesis, and concentration of the dopant used to hybridize ZnO catalyst are the most important properties that affect the development of ZnO/graphene hybrid photocatalysts.

### 2.2. ZnO modification by inorganic molecules: silver

The mechanism of the best as-prepare ZnO/GO hybrid composite was proposed. Thus, the ESR spin-trap technique (with DMPO) was used to monitor the reactive oxygen species generated during the irradiation of the hybrid photocatalyst and the results are shown in

Figure 4. EPR spectra of ZnO/GO hybrid photocatalyst irradiated under visible light in (a) water and (b) ethanol solvents.

• are clearly observed when pristine

•) for ZnO and hybrid photocatalysts were

OH radicals are

•, respectively. In both cases, it is noticed that the signals for

• radicals. In this case, the signals of •

OH and DMPO-O2

ZnO and ZnO/GO were exposed to visible radiation. Therefore, a dual mechanism involving both hydroxyl radicals and superoxide radicals is expected in the photocatalytic process. However, the signals for hybrid photocatalysts are stronger than the signals in pristine ZnO, thus accounting for the higher and stable photocatalytic performance of hybrid composites than bare ZnO toward the degradation of triclosan. The hydroxyl radicals trapped by DMPO

characterized by detecting four characteristic signals in water, and six signals in ethanol for

hybrid material are more pronounced than those for the ZnO pristine sample, thus accounting

Thus, the enhanced photocatalytic activity of ZnO/GO photocatalyst is due to the introduction of carbon material, which promotes an increase in charge separation to effective utilization of

Figure 4. Both signals of DMPO-•

DMPO was used as a radical trapper.

64 Photocatalysts - Applications and Attributes

OH and DMPO-O2

electrons to produce more •

for the better photocatalytic performance.

OH) and superoxide radicals (DMPO-O2

Table 1. Apparent rate constants for triclosan photodegradation under visible light.

OH and O2

(DMPO-•

DMPO-•

Silver nanoparticles are linked to the ZnO surface through the alkanethiol surfactant (Figure 6a). The stabilizer ligands COOH-(CH2)n-S-Ag keep nanoparticles as small as 7 nm in the solution and 15–26 nm in the Ag/ZnO photocatalyst, corroborated by XRD; although TEM analysis shows Ag nanoparticles of spherical shape and defined boundaries smallest as 3 nm over ZnO surface (Figure 6b-d). We studied the effect of the pH and time on the functionalization of ZnO nanoagglomerates by two methods denominated as photodeposition (PD) and impregnation (IMP) [15]. We propose to replace the ambiguous terms "doping" with functionalization when spoke of superficial ZnO modification. For instance, the sample 1%Ag/ZnO-PD11,1 synthesized by the PD method using 1 wt.% Ag at pH 11 and 1 h under UV light, a functionalization yield of 100% was corroborated by elemental analysis by inductively coupled plasma spectrometry ICP-OES and SEM-EDX. The UV irradiation produces free radicals (●OH, ●O) that degrade the ligand and release the silver nanoparticles onto the ZnO surface (Figure 6c). Byproducts of surfactant decomposition accumulate in ZnO are observed by IR. This functionalization of the

Figure 5. Schematic illustration of the photocatalytic process in the ZnO/GO hybrid semiconductor.

stirring results in heterogeneously distributed silver nanoparticles of average size of 15 nm determined by Scherrer equation. The surfactant decomposes almost completely during the heating (at 300C) that releases AgNPs onto ZnO surface. As before, residuals S-H and C-H functionalizing the ZnO surface was observed by IR (Figure 6a). Similarly, the as-synthesized 1%Ag/ZnO-IMP samples absorb in visible region of the spectrum and silver nanoparticles

It is challenging to control the metallic particle size because the nanoparticles have the trend to form agglomerates. The deposition of silver nanoparticles over metallic oxides from preformed nanoparticles using, for example, chemical vapor deposition [27] has been used, but IMP and PD methods demonstrate to be a successful bulk functionalization of ZnO at room temperature and atmospheric pressure. The increase of photocatalytic efficiency by silvermodified ZnO is demonstrated on the degradation of endocrine disruptors (i.e., bisphenol-A), an emergent contaminant (i.e., triclosan), and a dye (i.e., RhB) under visible light, which represents an important achievement in the use of solar-driven Ag/ZnO photocatalysts.

Figure 6e-g shows the progress of the contaminant photodegradation under visible light

show that 25% of bisphenol-A (10 mg/L) was destroyed after 3 h using Ag/ZnO-PD, and it represents an improvement of 100% compared with ZnO. A total of 35% of initial triclosan (20 mg/L) was destroyed within 3 h using Ag/ZnO-IMP11, represents an improvement of 45% compared with ZnO. Finally, 90% of dye discoloration is obtained with Ag/ZnO-PD11, being

Similarly, the photocatalysts were tested under UV light (302 nm). The use of BET surface area normalization of the apparent rate constant (kapp/BET ssa) in photocatalysis was proposed to clearly demonstrate that ZnO nanoagglomerates is 400% faster than Pi-25, a TiO2 well-known and extensively used photocatalyst from Degussa. The bandgap energy of ZnO and titanium dioxide (TiO2) is basically the same (3.2 eV). However, the valence and conduction bands exhibit differences in electric potential values; whose reported values are in the range of 0.45 to 2.75 eV vs. NHE and 0.1 to 3.1 eV vs. NHE, for ZnO and TiO2, respectively [28, 29]. Thus, the photogenerated holes in ZnO have strong enough oxidizing power to decompose most organic compounds. Furthermore, 1%Ag/ZnO-PD11,1 photocatalytic activity is 370% faster than ZnO attributed to the photoexcited electron trapping in the metallic primary (e/h<sup>+</sup> pair)

effect of pH, photocatalyst dosage, and bisphenol-A concentration on the kinetic rate constant

The utilization of powder photocatalyst may end unpractical for industrial scales because of the technical challenges like efficient dispersion and finally difficult separation of photocatalyst after the reaction that may entail important energetic costs and sometimes even producing a secondary pollution. Besides, photocorrosion is an important drawback in photocatalysis, and the anchoring of silver has been proved to control its progress. The

, 8 W 3UV-38 UVP Inc. lamp, lab-made reactor) at 20C. The results

OH) for the oxidation of bisphenol-A. The

Modified Metallic Oxides for Efficient Photocatalysis http://dx.doi.org/10.5772/intechopen.80834 67

induce a surface-localized plasmon resonance.

(365 nm, 0.97 mW/cm2

only 20% better than ZnO.

and secondary active species (free radicals, i.e., •

2.3. ZnO immobilization by organic molecules: poly (acrylic acid)

were also studied extensively and reported [3].

Figure 6. FTIR (a) and TEM image (b) of the as-synthesized photocatalyst functionalized ZnO by silver nanoparticles. Schematic illustration of the functionalization by PD method. (c). Histogram of silver particle size (d). Photocatalytic degradation of bisphenol A (e), triclosan (f), and RhB (g).

photocatalyst surface with S-H, C-O, and hydroxyl groups (OH) creates defect sites advantageous in photocatalysis. Silver-experienced redox processes during PD functionalization, for instance, oxidation (Ag+ /Ag0 , +0.799 eV vs. SHE) by photogenerated holes (+2.75 eV, vs. SHE) and free radicals. They reduce again into the zero-valence form (Ag0 ) by photoexcited electrons. This method favors insertion of ionic silver Ag+ into ZnO crystalline structure perceived as an expansion of lattice parameters measured by XRD. The new attached silver possibly anchors on the surface defect sites of ZnO [25, 26]. The role of the metallic modifier in Ag/ZnO is to promote the pair electron-hole (e/h+ ) separation and to increase the photocatalyst sensitivity toward visible light, in our case, evidenced as an absorption in the visible region by UV-Vis spectroscopy.

The IMP functionalization mechanism is different in the sense that silver nanoparticles and ZnO interaction is a function of the reaction time for an optimum of 2 h. For instance, the sample 1%Ag/ZnO-IMP11,2 synthesized using 1 wt.% Ag at pH 11 and 2 h under vigorous stirring results in heterogeneously distributed silver nanoparticles of average size of 15 nm determined by Scherrer equation. The surfactant decomposes almost completely during the heating (at 300C) that releases AgNPs onto ZnO surface. As before, residuals S-H and C-H functionalizing the ZnO surface was observed by IR (Figure 6a). Similarly, the as-synthesized 1%Ag/ZnO-IMP samples absorb in visible region of the spectrum and silver nanoparticles induce a surface-localized plasmon resonance.

It is challenging to control the metallic particle size because the nanoparticles have the trend to form agglomerates. The deposition of silver nanoparticles over metallic oxides from preformed nanoparticles using, for example, chemical vapor deposition [27] has been used, but IMP and PD methods demonstrate to be a successful bulk functionalization of ZnO at room temperature and atmospheric pressure. The increase of photocatalytic efficiency by silvermodified ZnO is demonstrated on the degradation of endocrine disruptors (i.e., bisphenol-A), an emergent contaminant (i.e., triclosan), and a dye (i.e., RhB) under visible light, which represents an important achievement in the use of solar-driven Ag/ZnO photocatalysts.

Figure 6e-g shows the progress of the contaminant photodegradation under visible light (365 nm, 0.97 mW/cm2 , 8 W 3UV-38 UVP Inc. lamp, lab-made reactor) at 20C. The results show that 25% of bisphenol-A (10 mg/L) was destroyed after 3 h using Ag/ZnO-PD, and it represents an improvement of 100% compared with ZnO. A total of 35% of initial triclosan (20 mg/L) was destroyed within 3 h using Ag/ZnO-IMP11, represents an improvement of 45% compared with ZnO. Finally, 90% of dye discoloration is obtained with Ag/ZnO-PD11, being only 20% better than ZnO.

Similarly, the photocatalysts were tested under UV light (302 nm). The use of BET surface area normalization of the apparent rate constant (kapp/BET ssa) in photocatalysis was proposed to clearly demonstrate that ZnO nanoagglomerates is 400% faster than Pi-25, a TiO2 well-known and extensively used photocatalyst from Degussa. The bandgap energy of ZnO and titanium dioxide (TiO2) is basically the same (3.2 eV). However, the valence and conduction bands exhibit differences in electric potential values; whose reported values are in the range of 0.45 to 2.75 eV vs. NHE and 0.1 to 3.1 eV vs. NHE, for ZnO and TiO2, respectively [28, 29]. Thus, the photogenerated holes in ZnO have strong enough oxidizing power to decompose most organic compounds. Furthermore, 1%Ag/ZnO-PD11,1 photocatalytic activity is 370% faster than ZnO attributed to the photoexcited electron trapping in the metallic primary (e/h<sup>+</sup> pair) and secondary active species (free radicals, i.e., • OH) for the oxidation of bisphenol-A. The effect of pH, photocatalyst dosage, and bisphenol-A concentration on the kinetic rate constant were also studied extensively and reported [3].
