# 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  
Various IIB-VI semiconductor QDs are frequently used to form composites with other semiconductor photocatalysts to enhance the photocatalytic activity in the visible spectrum due to their special optical-electrical and size-tunability properties, corresponding to quantum confinement effects, which can solve issues of low solar energy utilization and high carrier recombination, in turn achieving the band structure engineering of semiconductors and improving the photocatalytic efficiency [72; 82; 83; 84; 85]. For example, CdS QDs have been combined with g-C\({}_{3}\)N\({}_{4}\) to improve photocatalytic hydrogen evolution efficiency. [PERSON] et al. [85] reported a strategy of growing CdS QDs in situ on a g-C\({}_{3}\)N\({}_{4}\) nanosheet surface, leading to the formation of a contact interface between g-C\({}_{3}\)N\({}_{4}\) and CdS QDs,in turn leading to the effective charge carriers' separation on both parts. On the other hand, the hybrid photocatalyst showed an increased number of photocatalytic reactive sites. Consequently, the resultant CdS QD/g-C\({}_{3}\)N\({}_{4}\) demonstrated significantly enhanced photocatalytic performances compared with pure g-C\({}_{3}\)N\({}_{4}\). [PERSON] et al. [86] synthesized a novel MoS\({}_{2}\) quantum dot-modified g-C\({}_{3}\)N\({}_{4}\) nanosheets/n-doped carbon dot heterojunction photocatalyst via the thermal polymerization and subsequent solvothermal method. The photocatalytic hydrogen precipitation rate of the composite was 212.41 umol g\({}^{-1}\) h\({}^{-1}\), which is 53 times higher than that of g-C3N4/MoS\({}_{2}\)-3%. Similarly, [PERSON] et al. [13] reported that g-C\({}_{3}\)N\({}_{4}\) decorated with CdS QDs exhibits effective photocatalytic performance with a H\({}_{2}\) evolution rate of 17.27 umol-h\({}^{-1}\) under visible light irradiation. This value is 9 times that of pure g-C\({}_{3}\)N\({}_{4}\), corresponding to the efficient separation and transportation of the photogenerated charge carriers. Moreover, the CdS QD/g-C\({}_{3}\)N\({}_{4}\) composite exhibits red-shift and strong absorption in the visible light region due to the introduction of CdS QDs. [PERSON] et al. [87] deposited CdS QDs on TiO\({}_{2}\) nanotube arrays (TNTAs) for visible-light-driven hydrogen production. The optimal CdS-TNTA sample exhibited the highest activity for H\({}_{2}\) generation, which can be explained by the fact that the introduction of CdS QDs could effectively enhance the visible light absorption and charge separation. However, excess CdS QDs easily lead to the formation of CdS particles. Similarly, with CdSe QDs, [PERSON] et al. [88] synthesized and carefully characterized [001]-TiO\({}_{2}\)/CdSe QD and [101]-TiO\({}_{2}\)/CdSe QD composites using mercaptopropionic acid as the linker. The photocatalytic activity of [001]-TiO\({}_{2}\)/CdSe QDs is greater than that of [101]-TiO\({}_{2}\)/CdSe QDs in relation to photocatalytic H\({}_{2}\) generation because of the fast migration of photogenerated electrons from the QDs to the [001]-TiO\({}_{2}\). [PERSON] et al. [40] reported a series of CdS QD/Pt/oxides for use in photocatalytic H\({}_{2}\) evolution, which exhibit superior photocatalytic performance to Pt/CdS, Pt/Ga\({}_{2}\)O\({}_{3}\), and Pt/In\({}_{2}\)O\({}_{3}\). Among these, CdS/Pt/In\({}_{2}\)O\({}_{3}\)-U demonstrated the highest photocatalytic activity and excellent stability due to the fast separation and transfer of the photogeneraed charge carriers with the aid of CdS QDs. In addition, ternary metal sulfide is a conventional photocatalytic hydrogen production catalyst; therefore, several researchers have investigated the combination of CdS QDs with ternary metal sulfide to improve the hydrogen evolution activity. For example, [PERSON] et al.