2.2.2. Reduced Graphene Oxide and Graphene Oxide

Since the discovery of graphene (G) and/or graphene oxide (GO) [50], they have been widely studied [51–53]. GO consists of graphene nanosheets with epoxy or hydroxyl group-modified basal planes and carbonyl/carboxylic acid-modified edges [54–56]. In contrast to graphite, GO can be exfoliated easily and dispersed in aqueous solution because of these hydrophilic groups on the surface [57–59]. Hence, a few studies have used graphene oxide as the supporting material. For instance, in a recent study done by Peng et al., GO-cadmium sulfide was immobilized and well dispersed on a graphene oxide sheet. Increasing the GO-loading up to 5 wt % promoted the hydrogen generation rate to a maximum of 314 μmol h−<sup>1</sup> [60]. Hou et al. found that the introduction of GO can extend the lifetime of the photoexcited carriers because they can act as both electron transporter and electron acceptor [61].

A number of authors have also driven the further development of reduced graphene oxide. For instance, Zhang et al. enhanced solar photocatalytic hydrogen production by introducing reduced graphene oxide nanosheets and ZnxCd1−xS. That research provided a green method for using reduced graphene oxide (RGO) as a support material to enhance the performance of ZnxCd1−xS photocatalyst, for the hydrogen production of RGO-Zn0.8Cd0.2S was 450% higher than that of pristine Zn0.8Cd0.2S [62]. Similar effects after the incorporation of reduced graphene oxide have been evidenced in other works, such as studies of ZnO-CdS/RGO [63] and ternary NiS/ZnxCd1−xS/RGO nanocomposites [64]. Table 6 presents the photocatalytic H2 production properties of graphene oxide loaded with composite photocatalysts.


**Table 6.** Photocatalytic H2 production for graphene oxide loaded with composite photocatalysts.

#### 2.2.3. Conductive Polymer

Many papers have reported on the improvement of photocatalytic activity due to the loading of conductive polymers, such as polythiophene, polypyrrole, PEDOT, and PSS. Conductive polymers are capable of inducing charge separation in the composite photocatalysts [65]. A recent study showed that polyaniline (PANI)/ZnS synthesized by solvothermal method increases the hydrogen evolution rate up to 6750 μmol h−1g−<sup>1</sup> because PANI has unique electron and hole transporting properties [66]. PANI-ZnS composite photocatalysts exhibit improved dispersibility, light harvesting, and photocurrent response. Wang et al. [67] also reported on conducting polymers (such as polypyrrole, poly-3,4-ethylenedioxythiophene (PEDOT), and PANI) on the surface of CdS nanorods. It was found that the rate of hydrogen production of polyaniline@CdS was almost 5 times that of PEDOT@CdS and 3 times that of polypyrrole@CdS. That study showed that polyaniline is an efficient conducting material for modifying CdS nanomaterials, for it enables better light penetration than does a polypyrrole shell. Zieli ´nska et al. [68] reported that the hydrogen production of PANI/NaTaO3 photocatalyst under UV light irradiation was about twice that of pristine NaTaO3 photocatalyst. PANI/NaTaO3 exhibits a lower PL spectrum than that of NaTaO3, indicating a slower recombination of photoexcited charge carriers. The photoluminescence (PL) spectrum confirmed that the enhancement resulted from the efficient charge separation process. Sasikala et al. studied the photocatalytic hydrogen production performance of MoS2-PANI-CdS photocatalysts [69]. The incorporation of MoS2 and PANI improved the visible light absorption ability and improved the lifetime of photoexcited electron–hole pairs of the composite photocatalysts. To achieve high photocatalytic activity, 4% MoS2 and 5% PANI are the optimum amounts for the composite photocatalysts. The enhanced light absorption and lifetime of the photoexcited charge carriers result in improved activity of the photocatalysts. Figure 4 presents a possible mechanism for the electron transfer of MoS2-PANI-CdS photocatalysts. Incorporating polyaniline helps to separate photoinduced charges across the ZnS-polyaniline interfaces.

**Figure 4.** A proposed mechanism for the electron transfer of MoS2-PANI-CdS photocatalysts when illuminated. Figure adapted from [69].
