**4. Application of carbon-based semiconductor nanostructures in photocatalysis**

Metal sulfides, such as the binary compounds CdS, Ag2S, Bi2S3, and CuS, have been referenced in photocatalysis literature as efficient photon harvesters of visible-light radiation [116]. When supported on graphitic materials, these semiconductors improve the conductivity for electron capture and transport [51, 117–123]. There are

several methods of synthesis of metal sulfides coupled to rGO and GO substrates, which comprise solid-state, sonochemical, microwave irradiation, solvothermal, and hydrothermal methods [113, 120, 121, 124–138]. Our research group has developed a single-source method to prepare GO-based composites having supported metal sulfides. The type of metal sulfide generated *in situ* is determined by the metal dialkyldithiocarbamate complex employed as a single-molecule precursor, thus GObased nanocomposites of Ag2S, CuS, Bi2S3, and ZnS, are examples of such materials (**Figure 3**) [131]. In fact, this method is an extension of the sonochemical method first developed by Estrada *et al*. for decorating MWCNT, GO, and graphite with CdS obtained from the precursor cadmium(II) diethyldithiocarbamate [113].

Although CdS presents serious drawbacks for practical applications due to its wellknown toxicity, research on CdS-based nanomaterials provide helpful insights concerning the visible-light response and underlying mechanisms in semiconductor photocatalysis [139]. There are a number of studies reporting visible-light active heterostructures of CdS/rGO and CdS/GO, which were investigated as photocatalysts for the degradation of organic dyes [124, 126, 128]. These heterostructures showed higher photocatalytic efficiency than bare CdS and could be used for up to four cycles, without loss of activity. For instance, Zhang *et al*. developed visible-light irradiated CdS/graphene nanophotocatalysts for the photooxidation of alcohols and reduction of Cr(VI) ions in water [140]. Multicomponent photocatalysts of TiO2/CdS/rGO have shown higher photocatalytic activity than TiO2/rGO, for the photodegradation of RhB, MB, and *p*-chlorophenol, under visible-light irradiation [141, 142]. Wang *et al*. showed that nanocomposites based on heterojunctions of CdS and TiO2 nanoparticles were efficiently supported on rGO [141]. Such heterostructures prevented CdS photocorrosion due to the synergy that results from supporting such coupled semiconductor nanostructures on rGO (**Figure 4**). Similarly, the coupling of semiconducting phases, such as TiO2 and CdS or Ag2S, improves photon harvesting and charge separation and prevents the oxidation of the metal sulfides [142, 143].

**Figure 4.**

*The scheme illustrates visible-light photogeneration of oxygen radicals in a hybrid heterostructure composed of CdS (red)/TiO2(gray) supported on rGO sheets dispersed in an aqueous medium.*

The semiconductor Bi2S3 absorbs in the visible and NIR spectral range and does not pose serious toxicity concerns associated with CdS. Wang and coworkers showed that Bi2S3 immobilized on carbon dots have higher photocatalytic efficiency than their individual components, by investigating the degradation of MB and tetracycline under UV-, visible-, and NIR-light irradiation [144]. Khalid *et al*. synthesized nanorods of Bi2S3, which showed 87% efficiency in the degradation of Congo red dye, under UV-light irradiation over 90 minutes [145]. Chen *et al*. have reported improved photodegradation of 2,4-dichlorophenol irradiated with visible light in the presence of Bi2S3/rGO nanocomposites [137]. The authors also found that there is an optimal loading of Bi2S3 phases on carbon substrates, concluding that for higher contents of rGO less efficient photocatalytic systems are obtained. Similarly, for Ag2S/graphene, it was found that the performance of the photocatalyst depended on the relative amounts of semiconductor and graphene in the nanostructure. The authors have investigated samples with distinct graphene content (wt%: 2, 4, and 6), showing that in those conditions, the photodegradation of RhB, occurred most efficiently under visible-light irradiation in the presence of the sample 4 wt% in graphene [146].

Copper sulfide is a *p*-type semiconductor with phase-dependent properties; thus, the band gap energy range between 1.2 and 2.2 eV, depending on the crystalline form present [147–151]. This is an interesting aspect for photocatalytic applications because several crystalline phases have been reported for copper sulfide, showing the metal in distinct oxidation states, such as in chalcocite (Cu2S) and covellite (CuS). Additionaly, several nonstoichiometric phases (Cu2-xS) have been reported showing compositions that can be easily varied depending on the experimental conditions [152]. Hybrid nanostructures composed of copper sulfide and graphene (or graphene derivatives) show high potential in photocatalysis. For instance, it has been reported that hybrid nanostructures of CuS/rGO show superior photocatalytic activity as compared to the single-phase system composed of CuS nanoparticles, for the photodegradation of organic dyes under visible-light irradiation [121, 153–155] and UV-light irradiation [156]. El-Hout *et al*. have reported that CuS/rGO photocatalysts lead to the complete mineralization of malachite green after 90 minutes, under sunlight irradiation [157]. As previously mentioned, Shi *et al*. also stated that there is an optimal loading of CuS on rGO, showing that samples with 20% of rGO have better photocatalytic activity than samples containing 30% of rGO [120]. This has been explained by the effect on the stacking of graphene sheets and metal-sulfide particle aggregation, which results from the presence of a high amount of carbon nanomaterials (rGO or graphene) employed in the composite structure [155, 158]. Wang *et al*. have found that the

synergistic interaction occurring between graphene and metal-sulfide phases in CuS/graphene, with an impact on the electronic conductivity of graphene and CuS/graphene morphology, accounts for the observed stability and photoactivity of such heterostructures [159]. Matos *et al*. have synthesized hybrid composites comprising S-doped graphene decorated with CuS and Fe3O4 semiconductor phases, which showed higher photocatalytic ability than their individual components in the photodegradation of 4-nitrophenol, in addition, these photocatalysts could be recovered and reused in subsequent cycles [158].

Although ZnS is a non-absorbing material in the visible range due to its wide band gap energy (3.66 eV for blende structure and 3.77 eV for wurtzite structure), it has been found that ZnS coupled to carbon nanomaterials result in hybrid heterostructures with photocatalytic activity under visible-light irradiation [127, 133, 135]. Hence, Ming *et al*. have reported the degradation of ciprofloxacin, MB, and RhB under visible-light irradiation in the presence of ZnS/carbon nanostructures [160]. Also, Chen and Chakraborty have shown that under UV-light irradiation, the photodegradation of RhB and MO occurs more efficiently in the presence of the ZnS/graphene and ZnS/rGO photocatalysts, respectively [136, 161].
