# Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction
## 2 Quantum Dots-Based Photocatalyst for Hydrogen Evolution
### Semiconductor-Quantum-Dot-Based Hybrid Photocatalyst
#### 2.2.1 Metal-Chalcogenide-QD-Based Hybrid Photocatalyst
[72] reported that CdS QD/graphene/ZnIn\({}_{2}\)S\({}_{4}\) composites exhibit excellent photocatalytic performance, with a H\({}_{2}\) production rate of 2.7 mmol-h\({}^{-1}\), due to the improved light absorption ability and the facilitated migration of photogenerated electrons resulting from the combination of CdS QDs and ZnIn\({}_{2}\)S\({}_{4}\). At the same time, the as-synthesized CdS QD/graphene/ZnIn\({}_{2}\)S\({}_{4}\) composite displays good stability, resulting from the high hydrothermal stability and the strong interaction between CdS QDs and ZnIn\({}_{2}\)S\({}_{4}\). [PERSON] et al. [89] prepared a CdS QD-sensitized Zn\({}_{1-x}\)Cd\({}_{x}\)S composite via a simple ion exchange route, and reported the especially high visible light photocatalytic H\({}_{2}\) production rate of 2128 umol g\({}^{-1}\).h\({}^{-1}\); this value is 53 times higher than that for CdS. It is also considerably greater than that for Pt/ZnS under both ultraviolet and visible light irradiation. The blue-shift in the absorption spectrum confirms the quantum size effect exhibited by CdS QDs; the introduction of CdS QDs modifies the energy levels of the conduction band (CB) and the valence band (VB) within the combined semiconductor structure, which facilitates electron transfer (ET) and amplifies the photocatalytic efficiency. [PERSON] et al. [90] proposed a CdSe/CdS QDs core/shell heterostructure via the growth of a layer of CdS on the surface of the CdSe QDs, and reported significantly improved hydrogen evolution efficiency. Transient absorption spectroscopy revealed the presence of several deep trap states on the pristine CdSe QD surface (Figure 4). These defect states could not produce H\({}_{2}\) because of the lower CB level than is necessary for H\({}_{2}\)O reduction. Through the growth of a CdS shell on the surface of the CdSe QDs, the deep trap states can be effectively passivated. Therefore, an increased number of electrons can be maintained at the CB and react with H+ to produce hydrogen. Thus far, Cd-based IIB-VI QDs have been frequently applied in photocatalytic H\({}_{2}\) generation. However, the toxicity of Cd may limit their further application. Therefore, researchers have gradually developed Cd-free QDs to replace Cd-based QDs, and some of these QDs have provided impressive results that are comparable to those of Cd-based QDs. For instance, [PERSON] et al. [71] reported a NiS\({}_{2}\) QDs/g-C\({}_{3}\)N\({}_{4}\) composite with the NiS\({}_{2}\) QDs anchored on g-C\({}_{3}\)N\({}_{4}\) nanosheets, exhibiting a significantly high photocatalytic efficiency with the hydrogen generation rate of 4.841 \(\upmu\)mol-h\({}^{-1}\), which value is nearly two times greater than that of the Pt/g-C\({}_{3}\)N\({}_{4}\) photocatalyst. Moreover, the composite exhibits high stability. From the results, we can see that NiS\({}_{2}\) QDs act as a cocatalyst to improve the photogenerated charge carriers' separation efficiency, and to increase the surface's active sites, leading to accelerated H\({}_{2}\) generation. Complex I-III-VI QDs, such as CuInS\({}_{2}\) and ZnS-AgInS\({}_{2}\) QDs, have been widely studied for use in photocatalytic H\({}_{2}\) evolution. The traditional method of preparation for ternary and multinary I-III-VI QDs is the hot injection method. An example of the hot injection method of Zn\({}_{x}\)Cu\({}_{y}\)-InS\({}_{1.5\times 0.5\times 0.5\)y QDs is the reaction of metal acetate and sulfur in octadecene with oleic acid and dodecanediol used as capping reagents. Aqueous synthesis methods are receiving more and more attention because of the promising biological and catalytic applications of I-III-VI QDs. With mercaptoacetic acid, mercaptopropionic acid and other thiol-containing amines/alcohols as ligands, different types of I-III-VI QDs can be synthesized. [PERSON] et al. [41] synthesized CuInS\({}_{2}\)/CN heterojunctions for photocatalytic water splitting.