**1. Introduction**

Photocatalyst is a term that combines two words—*photo*, which is related to light, and *catalyst*, which is a compound that does not change the thermodynamics of the reaction but changes its kinetics, by establishing new reaction routes with lower activation energy, without being consumed during the process. Hence, semiconducting photocatalysis involves chemical reactions that occur at the surfaces of certain semiconductor compounds when irradiated with light of a selected wavelength range. Typically, these reactions occur in a liquid medium using the photocatalyst in the solid state, thus the chemical process is generally termed heterogeneous photocatalysis. In

this work, the semiconductor photocatalyst is considered as part of a colloid or suspension, though this has not been always the case. For instance, thin films have been also applied namely for air purification. Examples of heterogeneous photocatalytic processes using semiconductor particles include photooxidation reactions, which have been exploited for the degradation of organic pollutants present in water [1–5]. Compared to more conventional water treatment methods, such as those based on adsorption and flocculation, which might require a subsequent step for the chemical degradation of the pollutant, in photocatalysis the pollutant is eliminated by aerobic photooxidation. Ideally, this oxidative process should generate carbon dioxide and water as the final products, that is, the complete mineralization of the organic pollutant, though this has been rarely achieved. As such, different remediation technologies can coexist in the same water treatment plant and, in several situations, their complementary role brings more efficient approaches. For example, adsorption and photocatalytic technologies can be implemented in different stages in the same water treatment plant. Even though, advanced oxidation processes based on the use of efficient photocatalysts have been regarded as a way to minimize the impact of CEC in water sources, which even in trace levels are harmful and for which conventional water treatments are ineffective.

Two main optical processes have been proposed considering the role of the semiconductor during a photocatalytic reaction, as illustrated in **Figure 1** for TiO2 photocatalysts. In direct photocatalysis, a photon with energy higher than the band gap energy of the semiconductor (h*ν1*) is absorbed and an electron (e�) is excited to the conduction band (CB), leaving a hole (h<sup>+</sup> ) in the valence band (VB). The band gap energy of the semiconductor is defined as the difference between the CB (bottom energy level) and the VB (top energy level). The photogenerated electron–hole pair (e�/�h<sup>+</sup> ) is responsible for reduction and oxidation reactions that take place at the surface of the photocatalyst particle in contact with the aqueous medium. The electron in the CB migrates to the surface of the semiconductor and participates in reduction reactions, and the hole in the VB diffuses to the photocatalyst surface and is involved in oxidation reactions. In addition, the dissolved O2 can accept photogenerated electrons to yield superoxide radicals (*O*�� <sup>2</sup> ) and photogenerated holes can oxidize H2O to form strong oxidant hydroxyl radicals (*HO*• ) (Eqs. (1) and (2)) [6, 7].

$$\text{e}\_{\text{CB}}^{-} + \text{O}\_{2} \rightarrow \text{O}\_{2}^{\bullet -} \tag{1}$$

$$h\_{\text{VB}}^{+} + H\_{2}O \to HO^{\bullet} + H^{+} \tag{2}$$

#### **Figure 1.**

*The schematic representation of the direct (hυ*1*) and indirect (hυ*2*) photochemical processes occurring in lightirradiated TiO2 nanoparticles, commonly used as photocatalysts in the form of aqueous colloids. Adapted from [6].*

*Nanomaterials of Carbon and Metal Sulfides in Photocatalysis DOI: http://dx.doi.org/10.5772/intechopen.109658*

On the other hand, in indirect photocatalysis, also known as photosensitized photocatalysis, the mechanism involves the photoexcitation (h*ν*2) of a second species (P) to an excited state from which an electron is injected into the CB of the semiconductor. This process has been observed in the degradation of contaminant organic dyes, which can also act as photosensitizers for cases in which the reduction potential of the excited state is negative enough for electron injection into the CB of the semiconductor [7]. In indirect photocatalysis, there is no generation of a VB hole and the semiconductor functions as an electron relay, thereby preventing undesired back reactions [7]. Nevertheless, this process is usually less efficient than direct photocatalysis due to the lower efficiency of the electron injection. Both direct and indirect photocatalysis convert the initially generated superoxide radicals into other reactive oxygen species with high oxidative power (Eqs. (3)-(7)), for example, with reduction potentials of 0.94 V (*O*•� <sup>2</sup> *=H*2*O*2), 1.29 V (*H*2*O*2*=H*2O) and 1.90 V (*HO*• *=HO*�) [6, 8]. Although such radicals are nonselective, they are effective in oxidizing organic contaminants, such as dye molecules [9–15], antibiotics [16–20], or pesticides [21–25], as well as for other sanitation applications, such as the elimination of pathogens [26–32].

$$\text{O}\_2^{\bullet-} + \text{H}^+ \to \text{HO}\_2^\bullet \tag{3}$$

$$\text{HO}\_2^\bullet + \text{HO}\_2^\bullet \to \text{H}\_2\text{O}\_2 + \text{O}\_2 \tag{4}$$

$$O\_2^{\bullet-} + HO\_2^{\bullet} \to O\_2 + HO\_2^{-} \tag{5}$$

$$\rm{H}\rm{O}\_{2}^{-} + \rm{H}^{+} \rightarrow \rm{H}\_{2}\rm{O}\_{2} \tag{6}$$

$$H\_2O\_2 + O\_2^{\bullet-} \rightarrow HO^\bullet + HO^- + O\_2 \tag{7}$$

In semiconductor photocatalysis, several fundamental aspects should be considered to develop the photocatalyst based on functional and operational criteria. Hence, light absorption (absorption coefficient and wavelength range), photoinduced charge separation, charge trapping, and charge transfer are among the key parameters for designing efficient photocatalytic systems [33, 34]. For instance, photogenerated electrons are unstable species in an excited state, which tend naturally to return to the ground state either via adsorbed hydroxyl radicals or by recombination with unreacted holes or structural traps on semiconductors [35–38]. Since these species are determinants in the efficiency of a photocatalyst, several research groups have explored strategies to increase the photoinduced charge separation to avoid charge recombination and consequently increase the lifetime of photogenerated electron/hole pairs. These strategies include (i) coupling of semiconductor photocatalysts with metal nanoparticles [39–41]; (ii) sensitization of the photocatalyst surface through physical or chemical adsorption of molecules that absorb visible light and are excited either to the singlet or triplet excited state [42] and; (iii) coupling of at least two semiconductor photocatalysts with different bandgap values [43, 44]. The presence of charge trapping sites in a semiconductor photocatalyst allows also the extension of the lifetime of the charge carriers from microseconds to milliseconds since in these sites there is greater charge-carrier stability. Although such trap sites are mostly located at the surface of a semiconductor photocatalyst, they may be present also on grain boundaries or in the bulk lattice, or even present as electron scavengers, such as O2. On the other hand, deeply stabilized trapped charges lose redox potential and increase the potential barrier for charge transfer at the semiconductor or water interface [45].

Thus, electron transfer reaction depends largely on structural parameters ascribed to the semiconductor photocatalyst, such as crystal facet structure, lattice surface, size, and morphology. Trapping mechanisms might be favorable if they allow photon activity to generate charge carriers, and permit charge carriers to reach the electron transfer regions. Otherwise, it could be disadvantageous for the overall photocatalytic process.

Several strategies have been proposed to adjust the physical and chemical properties of semiconductor photocatalysts to improve light absorption and charge transfer efficiency, reduce the recombination rate of photogenerated charge carriers, and accelerate surface reactions [46]. Examples of such strategies include metal-ion doping of the semiconductor [39–41], combination with distinct semiconductors that result in heterostructures [43, 44], and surface chemical functionalization using selected photosensitizers [42]. Noteworthy, the combination of inorganic semiconductors with carbonaceous materials, such as graphene and their structural derivatives, has also received great attention in the design of a new class of nanocomposite photocatalysts [47, 48]. The use of carbon nanostructures for supported semiconductor photocatalysts offers great advantages. Hence, depending on the carbon material, high electrically conductive nanostructures can act as scavengers of photogenerated electrons. Also, watercompatible nanomaterials promote the aqueous dispersion of the photocatalyst, which by achieving a high specific surface area enhances the adsorption capacity of the system [48]. Furthermore, surface functionalization of the carbon lattice confers functional chemical groups that might favor the subsequent attachment of semiconductor nanophases. A paradigmatic example of this situation is the application of graphene oxide as a nanoplatform for semiconductor photocatalysts, and notwithstanding limitations that can also arise such as photoreduction of the carbon substrate or the limited absorption by the photocatalyst [49–51].
