**2. Metal-sulfide photocatalysts**

In general terms, a good photocatalyst should have the following characteristics: effective charge-carrier separation, fast charge transfer, strong optical absorption, photochemical stability, low-cost production, and nontoxicity [52]. Among the several types of photocatalysts available, inorganic semiconductors have been intensely investigated in water remediation processes because they might fulfill, at least in selected cases, the above requirements. Inorganic materials considered as semiconductors exhibit bandgap energies in the range of 0.3–3.8 eV. In particular, TiO2 and TiO2-based heterogeneous photocatalysts have been the most explored semiconductor materials for photocatalytic applications because of the high free energy of photogenerated charge carriers, low-cost, and high chemical stability [53, 54]. However, both TiO2 polymorphs (anatase/rutile) show a wide bandgap (anatase 3.2 eV; rutile 3.0 eV), which limits photocatalytic applications of pure TiO2 to UV irradiated systems. Other semiconductor photocatalysts exhibiting narrower bandgaps have been investigated, which can replace TiO2 in certain conditions or that might act as a complementary phase in extending light absorption to the visible composite systems. Among these semiconductors, this chapter focus on the use of binary metal-sulfide compounds, with emphasis on their nanocrystalline forms. **Table 1** shows examples of metal sulfides investigated as photocatalysts and selected properties for the pure phases.

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


#### **Table 1.**

*Characteristics of macrocrystalline metal sulfides as photocatalysts in an aqueous medium [44].*

#### **Figure 2.**

*The scheme illustrates the widening of the bandgap energy of a certain semiconductor material, as particle size decreases from its macrocrystalline form (left) to the nanocluster regime (right). Quantum dots are nanocrystalline semiconductors (middle) that show quantum-size effects, corresponding to the intermediate situation between macrocrystalline materials and nanoclusters.*

A macrocrystalline metal sulfide (MS) semiconductor comprises a threedimensional network of ordered atoms (metal and S atoms) characterized by a band gap energy at a certain temperature. As particle size decreases, and below a certain threshold, the electronic band structure of the semiconductor changes with the widening of the bandgap energy [55, 56]. For semiconductor nanoclusters, that is molecular-like nanocrystals depicted on the right in **Figure 2**, an analogous interpretation applies, although the energy gap is usually understood as the energy separation between the frontier molecular orbitals HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital). Thus, as in the case of conventional photosemiconductors, the incidence of a photon with energy greater than this energetic separation originates in semiconductor nanocrystals (and nanoclusters) the formation of an electron–hole pair, often called exciton, which in the macrocrystalline material is dimensionally characterized by the Bohr exciton radius of that

semiconductor. The charge carriers in nanosized semiconductors migrate fast and participate in several photoprocesses, which include trapping and recombination [57, 58].

Metal sulfides can be explored in the macrocrystalline form as photocatalysts, for example, in aqueous suspensions, membranes, and thin films [59–61]. However, in the past decades, there has been intense research on their use as nanocrystalline materials, namely due to the possibility to explore quantum-size effects, as mentioned above. MS semiconductor nanocrystals (quantum dots) are small crystalline particles that exhibit quantum size-dependent optical and electronic properties [62, 63]. With typical dimensions in the range of 1–100 nm, these nanocrystals bridge the gap between those of molecules and micrometric crystals, displaying distinct optical behavior in relation to their bulk counterparts [64]. If the size of nanocrystals is smaller than the bulk exciton Bohr radius, the charge carriers become spatially confined, showing size-dependent absorption and fluorescence spectra with discrete electronic transitions at room temperature (**Figure 2**).

For instance, the optical spectra of colloids of nanocrystalline semiconductors show blue shifts in their absorption edges (or excitonic peaks) with decreasing particle diameters. Metal-sulfide nanocrystals that exhibit quantum size effects, that is, quantum dots, can be used as size-tuned light-absorption photosensitizers, namely in visible photocatalytic applications [44, 65–67]. Quantum size effects occurring in MS nanocrystals dispersed in aqueous suspensions, also affect the CB and VB redox levels, thus influencing redox reactions that involve the migration of photogenerated charge carriers to the particles'surfaces. Nanosized semiconductors have dimensions considerably superior to conventional molecular photosensitizers, which in comparison to the latter, present a broader absorption wavelength range, large density of states, and high optical extinction coefficients [62], hence favoring photon harvesting in photocatalytic applications.

Colloidal synthesis offers a wide range of chemical methods to obtain MS nanocrystals with controlled particle size distributions and particle shapes, thus with tailored bandgaps for diverse semiconductors and their solid solutions [68–71]. Furthermore, such colloids can be selected as nanodispersed systems showing strong visible-light absorption and size-tuned bandgap. However, these systems also show limitations, which deserve further research aiming their application as more efficient photocatalysts. Although certain MS is used as visible-light photocatalysts, the photogenerated electron–hole pairs are also susceptible to recombination. The occurrence of charge-carrier recombination limits their mobility from the bulk lattice to the particles'surface, thus decreasing the efficiency of the photocatalyst. Moreover, surface-sulfide anions (S2) in aqueous MS colloids are prone to oxidation, a process that gains more relevance due to the oxidative role of photogenerated holes at the surface [72, 73]. In fact, under light irradiation, sulfide anions can oxidize forming sulfate (SO4 <sup>2</sup>) or elemental sulfur (S0 ), causing the deactivation of the photocatalyst.

The inhibition of metal-sulfide photocorrosion is an important requirement for photocatalytic reactions, namely because the long lifetime of photogenerated electron–hole pairs and the chemical stability are essential for producing efficient photocatalysts. Several strategies have been reported that tackle this problem, such as modifying the crystal structure, size, and morphology of semiconductors [74, 75], combining with transition metal ions or cocatalysts [76, 77], producing heterojunctions, [78–80] and by adjusting the reaction parameters [81–83]. For instance, Bo *et al*. have reported that the interfacial interaction between both semiconductors in the MoS2/CdS heterostructures restrains the photocorrosion of MoS2.

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

The authors have shown that electrons photogenerated on the CB of CdS are transferred to the CB of MoS2 to participate in the H2 evolution reaction, while the holes on the VB of MoS2 migrate to the VB of CdS [79]. Huang *et al*. have shown that the growth of a larger bandgap semiconductor, such as ZnO, on a core with a smaller band gap as CdS improves the stability of the hybrid nanostructure and inhibits the photocorrosion of CdS particles [84]. In turn, Yi and Wang have found that the photocorrosion of CdS is significantly inhibited when cobalt ions or molybdate are injected into the CdS-lactic acid system. The photogenerated holes in the CdS are fastly captured by the transition metal ions, reducing the oxidation of S2 on the CdS surface [85, 86]. The coupling of metal-sulfide semiconductor photocatalysts with inorganic substrates might bring other advantages and several approaches have been reported [87, 88]. In this context, carbon nanomaterials have also been investigated as functional platforms that bring new potential to the application of these materials, including photocorrosion inhibition of the supported metal sulfides.
