**1. Introduction**

Water and wastewater treatment brings new challenges due to the occurrence of new and refractory contaminants produced by anthropogenic activities [1, 2]. In the past few years, water depuration aimed to remove particles, the bulk of organic matter and inactivate microorganisms. However, nowadays, the degradation not only of organic pollutants at trace levels but its precursors and by-products is especially pursued in drinking water treatment systems. For instance, the biological and chemical degradation of fluoroquinolone antibiotics results in the generation of some by-products displaying antibiotic residual activity. Biological and abiotic degradation of personal care products, such as triclosan and triclocarban, leads to the emergence of polychlorinated biphenyls and dioxins. Some bacterial residues in surface water bodies, such as microcystin and geosmin, may impact in the organoleptic properties of drinking water as well as express toxicity, while some iodinated pharmaceuticals may be precursors of trihalomethanes. Under this scenario, the most advanced water treatment systems should aim to completely mineralize the organic pollutants, in order to take the risks of water contamination to the minimum. The complete mineralization of organic pollutants can be warranted by few processes, such as photocatalysis [3–7]. This process is based on the generation of highly reactive •OH radicals, which are able to fully oxidize organic molecules. Heterogeneous photocatalysis process has shown to be efficient in the degradation of organic pollutants in water, avoiding the transport and use of potentially hazardous materials, such as acids or H<sup>2</sup> O2 , while the catalyst can be recovered and reused in several cycles [8, 9].

**2. Fundamentals on the formation and photoexcitation of TiO2**

**Figure 1.** Photoactivation and charge carriers transfer in a semiconductor/conductor heterounion.

A heterostructure or junction is defined as the interfacial union of two or more components. In photocatalysis, heterounions are formed to improve the efficiency of a semiconductor either by redshifting the light absorption or through the decrease of the recombination of the hole-electron pairs [12, 17, 18]. Heterostructures are commonly built up by combining a specific semiconductor with one or more materials, such as metals, semiconductors or organic

TiO2-Low Band Gap Semiconductor Heterostructures for Water Treatment Using Sunlight…

Semiconductor/metal junctions are heterostructures based on the deposition of metallic nanoparticles on crystalline semiconductors. Such union is able to hinder the electron/hole recombination through the sequestration of the photoelectrons from the conduction band of the semiconductor to the surface of the metallic nanoparticles [12] (**Figure 1**). The differences in the Fermi level reached in the semiconductor/metal junction triggers the sequestration of photoelectrons; although other factors, such as the higher work function and electronegativity of the metal, favor the transference of the charge carriers [16, 19, 20]. The defects created in the semiconductor/conductor junction are proposed as the main route of the electron transfer from one material to the other. The Schottky barrier created in the junction impedes the return of the charge carriers to the conduction band of the semiconductor. Typically, low loadings of metallic nanoparticles tend to be the optimal for achieving the highest potential of the electron trap process. However, when this optimal loading is surpassed, the electron traps are converted into charge carriers recombination sites, leading to the dramatic drop of the photocatalytic performance [21–23]. Even when the electron trap effect is unable to significantly redshift the light absorption of the heterostructure, some photocatalytic activity can be achieved by the surface plasmon resonance (SPR) effect, which is expressed by the oscillation of the electrons in the conduction band of the metallic nanoparticles when electromagnetic excitation at a determined wavelength is provided [24–26]. Some extent of photocatalytic activity has been attributed to SPR effect, although this cannot be considered as determinant in the photodegradation of pollutants in water when compared with that achieved under UV light irradiation.

**heterostructures**

molecules for example, azo dyes or polymers.

**-based** 

307

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TiO<sup>2</sup> is a widely used semiconductor in heterogeneous photocatalysis due to its high activity and stability [10–12]. This harmless material is currently used not only for water treatment but in food preparation and disinfection of surgical equipment. Anatase is the most photoactive phase of TiO<sup>2</sup> , followed by rutile; however, the wide band gap value of both phases (3.0–3.2 eV) results in the activation of these materials under UV-A light irradiation [13]. In order to take the photocatalysis process toward sustainability, it is necessary to develop materials with high photo-response under sunlight irradiation. Given that sunlight comprises only 4–5% of UV-A light, the need to find nanostructures capable to absorb visible light—which composes 50% of the sunlight spectrum [14]—becomes imperative. A growing number of modifications of TiO<sup>2</sup> at nanometric scale have been performed in order to achieve the complete photoactivation of this semiconductor under visible light irradiation. Doping with nonmetal atoms has given partially positive results, since the redshift of the absorption edge of TiO<sup>2</sup> increases in turn the recombination rate of the hole-electron pair. The development of heterostructures based on TiO<sup>2</sup> coupled to low band gap semiconductors could be an efficient approach to improve the photocatalytic conversion of contaminants in water [12, 15, 16]. This chapter explores the fundamentals of the synthesis and photoactivation of the TiO<sup>2</sup> -low band gap semiconductor heterostructures, and presents some of the reported data on the photocatalytic activity of these materials, in order to bring light on their potential use for water purification at a higher scale.

#### **2. Fundamentals on the formation and photoexcitation of TiO2 -based heterostructures**

**1. Introduction**

306 Titanium Dioxide - Material for a Sustainable Environment

TiO<sup>2</sup>

higher scale.

Water and wastewater treatment brings new challenges due to the occurrence of new and refractory contaminants produced by anthropogenic activities [1, 2]. In the past few years, water depuration aimed to remove particles, the bulk of organic matter and inactivate microorganisms. However, nowadays, the degradation not only of organic pollutants at trace levels but its precursors and by-products is especially pursued in drinking water treatment systems. For instance, the biological and chemical degradation of fluoroquinolone antibiotics results in the generation of some by-products displaying antibiotic residual activity. Biological and abiotic degradation of personal care products, such as triclosan and triclocarban, leads to the emergence of polychlorinated biphenyls and dioxins. Some bacterial residues in surface water bodies, such as microcystin and geosmin, may impact in the organoleptic properties of drinking water as well as express toxicity, while some iodinated pharmaceuticals may be precursors of trihalomethanes. Under this scenario, the most advanced water treatment systems should aim to completely mineralize the organic pollutants, in order to take the risks of water contamination to the minimum. The complete mineralization of organic pollutants can be warranted by few processes, such as photocatalysis [3–7]. This process is based on the generation of highly reactive •OH radicals, which are able to fully oxidize organic molecules. Heterogeneous photocatalysis process has shown to be efficient in the degradation of organic pollutants in water, avoiding the transport and

 is a widely used semiconductor in heterogeneous photocatalysis due to its high activity and stability [10–12]. This harmless material is currently used not only for water treatment but in food preparation and disinfection of surgical equipment. Anatase is the

of both phases (3.0–3.2 eV) results in the activation of these materials under UV-A light irradiation [13]. In order to take the photocatalysis process toward sustainability, it is necessary to develop materials with high photo-response under sunlight irradiation. Given that sunlight comprises only 4–5% of UV-A light, the need to find nanostructures capable to absorb visible light—which composes 50% of the sunlight spectrum [14]—becomes

performed in order to achieve the complete photoactivation of this semiconductor under visible light irradiation. Doping with nonmetal atoms has given partially positive results,

low band gap semiconductors could be an efficient approach to improve the photocatalytic conversion of contaminants in water [12, 15, 16]. This chapter explores the funda-

heterostructures, and presents some of the reported data on the photocatalytic activity of these materials, in order to bring light on their potential use for water purification at a

of the hole-electron pair. The development of heterostructures based on TiO<sup>2</sup>

O2

, followed by rutile; however, the wide band gap value

, while the catalyst can be

at nanometric scale have been


coupled to

increases in turn the recombination rate

use of potentially hazardous materials, such as acids or H<sup>2</sup>

imperative. A growing number of modifications of TiO<sup>2</sup>

mentals of the synthesis and photoactivation of the TiO<sup>2</sup>

since the redshift of the absorption edge of TiO<sup>2</sup>

recovered and reused in several cycles [8, 9].

most photoactive phase of TiO<sup>2</sup>

A heterostructure or junction is defined as the interfacial union of two or more components. In photocatalysis, heterounions are formed to improve the efficiency of a semiconductor either by redshifting the light absorption or through the decrease of the recombination of the hole-electron pairs [12, 17, 18]. Heterostructures are commonly built up by combining a specific semiconductor with one or more materials, such as metals, semiconductors or organic molecules for example, azo dyes or polymers.

Semiconductor/metal junctions are heterostructures based on the deposition of metallic nanoparticles on crystalline semiconductors. Such union is able to hinder the electron/hole recombination through the sequestration of the photoelectrons from the conduction band of the semiconductor to the surface of the metallic nanoparticles [12] (**Figure 1**). The differences in the Fermi level reached in the semiconductor/metal junction triggers the sequestration of photoelectrons; although other factors, such as the higher work function and electronegativity of the metal, favor the transference of the charge carriers [16, 19, 20]. The defects created in the semiconductor/conductor junction are proposed as the main route of the electron transfer from one material to the other. The Schottky barrier created in the junction impedes the return of the charge carriers to the conduction band of the semiconductor. Typically, low loadings of metallic nanoparticles tend to be the optimal for achieving the highest potential of the electron trap process. However, when this optimal loading is surpassed, the electron traps are converted into charge carriers recombination sites, leading to the dramatic drop of the photocatalytic performance [21–23]. Even when the electron trap effect is unable to significantly redshift the light absorption of the heterostructure, some photocatalytic activity can be achieved by the surface plasmon resonance (SPR) effect, which is expressed by the oscillation of the electrons in the conduction band of the metallic nanoparticles when electromagnetic excitation at a determined wavelength is provided [24–26]. Some extent of photocatalytic activity has been attributed to SPR effect, although this cannot be considered as determinant in the photodegradation of pollutants in water when compared with that achieved under UV light irradiation.

**Figure 1.** Photoactivation and charge carriers transfer in a semiconductor/conductor heterounion.

The synthesis of semiconductor/semiconductor nanocomposites provides an efficient way to redshift the photoactivation of TiO<sup>2</sup> -based materials. When these materials are exposed to visible light, the low band gap semiconductor is activated, producing the hole–electron pair. Then, the photoelectrons migrate from the surface of the low band gap semiconductor to the conduction band of TiO<sup>2</sup> , as an effect of the difference in the redox potential between both semiconductors (**Figure 2**). The process leads to the drop of the hole-electron pair recombination rate, resulting in the oxidation process in the low band gap semiconductor, and the reduction process on the TiO<sup>2</sup> surface [17, 27].

the HOMO and LUMO positions of the semiconductors in the heterostructure, leading to the

TiO2-Low Band Gap Semiconductor Heterostructures for Water Treatment Using Sunlight…

http://dx.doi.org/10.5772/intechopen.76501

309

In some advanced approaches, metallic nanoparticles are settled in the heterounion of the two semiconductors. Noble metal nanoparticles can act as electron mediators, transporting the charge carriers from one semiconductor to another, increasing with this the electron trap effect. Heterostructures based on p-n semiconductors have shown a notable increase in the charge separation when noble metal nanoparticles are added in the semiconductor heterounion. These materials, known as all solid-state Z schemes, provide highly reductive photo-

Photoelectrons are transported from semiconductor II to semiconductor I via the metallic nanoparticles. At the same time, photoelectrons are formed in semiconductor I and photoholes in the HOMO of semiconductor I are recombined with photoelectrons coming from the LUMO of semiconductor II. This kind of schemes impedes the recombination of the charge carriers with the highest oxidative and reductive potential, increasing not only the photocatalytic performance of the components, but bringing chances to photodegrade more recalcitrant

displays higher photocatalytic activity due to the transfer of the charge carriers from the semiconductor to the polymer, as shown in **Figure 5b**. Even when the lifetime of the charge carriers is increased, visible light-driven activity is not improved; however, graphene materials can be used as an excellent electron mediator in all solid Z schemes. On the other hand, organic molecules, which are able generate the triplet state under visible light irradiation, can act as sensitizers when

Semiconductor/semiconductor junctions can be classified depending on either the type of semiconductors that are being coupled or by the band structure they present. Considering the type of semiconductors, the semiconductor/semiconductor heterostructures can be classified

nanoparticles are deposited on graphene sheets, the semiconductor/polymer junction

surface (**Figure 5a**). In this case, electrons are injected from the sensitized

based all solid Z scheme when photoactivation when UV

electrons and highly oxidative photo-holes by the process shown in **Figure 4** [28].

pollutants because of the increment of the oxidative potential of the charge carriers.

molecule to the LUMO of the semiconductor, triggering the photocatalytic process [29].

**2.1. Classification of the semiconductors/semiconductor heterostructures**

**Figure 4.** Photoactivation and charge carriers transfer in a TiO<sup>2</sup>

decrement in the photocatalytic performance [15, 17, 18].

When TiO<sup>2</sup>

deposited on the TiO<sup>2</sup>

light activation occurs.

When the composite is photoactivated under UV light irradiation, the hole-electron pairs are produced in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of each semiconductor. Charge carriers are then transported and accumulated in the HOMO and LUMO of one of the semiconductors, as a function of gradient in the potential of the bands (**Figure 3**). For this kind of schemes, p-n heterostructures have shown the best results in the separation of the photo-formed charge carriers. However, in some cases, decreasing of the redox potential of charge carriers can occur, depending of

**Figure 2.** Photoactivation and charge carriers transfer in a TiO<sup>2</sup> -low band gap semiconductor heterounion when photoactivation with visible light occurs.

**Figure 3.** Photoactivation and charge carriers transfer in a TiO<sup>2</sup> -low band gap semiconductor heterounion when photoactivation with UV light occurs.

the HOMO and LUMO positions of the semiconductors in the heterostructure, leading to the decrement in the photocatalytic performance [15, 17, 18].

In some advanced approaches, metallic nanoparticles are settled in the heterounion of the two semiconductors. Noble metal nanoparticles can act as electron mediators, transporting the charge carriers from one semiconductor to another, increasing with this the electron trap effect. Heterostructures based on p-n semiconductors have shown a notable increase in the charge separation when noble metal nanoparticles are added in the semiconductor heterounion. These materials, known as all solid-state Z schemes, provide highly reductive photoelectrons and highly oxidative photo-holes by the process shown in **Figure 4** [28].

Photoelectrons are transported from semiconductor II to semiconductor I via the metallic nanoparticles. At the same time, photoelectrons are formed in semiconductor I and photoholes in the HOMO of semiconductor I are recombined with photoelectrons coming from the LUMO of semiconductor II. This kind of schemes impedes the recombination of the charge carriers with the highest oxidative and reductive potential, increasing not only the photocatalytic performance of the components, but bringing chances to photodegrade more recalcitrant pollutants because of the increment of the oxidative potential of the charge carriers.

When TiO<sup>2</sup> nanoparticles are deposited on graphene sheets, the semiconductor/polymer junction displays higher photocatalytic activity due to the transfer of the charge carriers from the semiconductor to the polymer, as shown in **Figure 5b**. Even when the lifetime of the charge carriers is increased, visible light-driven activity is not improved; however, graphene materials can be used as an excellent electron mediator in all solid Z schemes. On the other hand, organic molecules, which are able generate the triplet state under visible light irradiation, can act as sensitizers when deposited on the TiO<sup>2</sup> surface (**Figure 5a**). In this case, electrons are injected from the sensitized molecule to the LUMO of the semiconductor, triggering the photocatalytic process [29].
