# Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction
## 2 Quantum Dots-Based Photocatalyst for Hydrogen Evolution
### Semiconductor-Quantum-Dot-Based Hybrid Photocatalyst
#### 2.2.2 Metal-Oxide-QD-Based Hybrid Photocatalyst  
Except for metal chalcogenide QDs, a variety of metal oxide QDs including CuO, NiO, Co\({}_{3}\)O\({}_{4}\), SnO\({}_{2}\), WO\({}_{3}\), and Bi\({}_{2}\)O\({}_{3}\) have been explored for use as cocatalysts in the photocatalytic H\({}_{2}\) production reaction [49; 50; 91; 92]. In general, metal oxide quantum dots have higher photostability than sulfide/selenide quantum dots. Metal oxide quantum dots with low toxicity and good chemical stability have emerged as potential host catalysts for photocatalytic HER [93]. [PERSON] et al. [46] reported that a heterostructure created by the in situ seed-directed growth of MoS\({}_{2}\) quantum dots within ZnIn\({}_{2}\)S\({}_{4}\) not only establishes heterojunctions between MoS\({}_{2}\) and ZnIn\({}_{2}\)S\({}_{4}\) to spatially separate photogenerated electron-hole pairs, but also provides active sites for electron trapping to promote hydrogen evolution. Consequently, the MoS\({}_{2}\)-QD/ZnIn\({}_{2}\)S\({}_{4}\) composite achieves a high photocatalytic H\({}_{2}\) evolution rate of 7152 \(\upmu\)mol h\({}^{-1}\)\(\cdot\)g\({}^{-1}\), which is \(\sim\)9 times greater than that of pristine  
Figure 4: Schematic and photocatalytic activity of CdSe/CdS heterostructure QDs. Reprinted with permission from Ref. [33]. ZnIn\({}_{2}\)S\({}_{4}\). MoS\({}_{2}\)-QDs serve as a hydrogen evolution reaction cocatalyst and an electron transfer bridge. Similarly, [PERSON] et al. [48] have designed and synthesized high-dispersion transition metal sulfide (Co\({}_{3}\)S\({}_{4}\)) QDs instead of noble metal cocatalysts to anchor onto g-C\({}_{3}\)N\({}_{4}\) nanosheets. As-synthesized Co\({}_{3}\)S\({}_{4}\)/g-C\({}_{3}\)N\({}_{4}\) nanosheet (CNNS) nanohybrids exhibit abundant surface active sites and the fast migration of photogenerated electrons because of the highly dispersed cocatalyst Co\({}_{3}\)S\({}_{4}\) QDs; therefore, the Co\({}_{3}\)S\({}_{4}\)/CNNSs exhibit a remarkably high photocatalytic efficiency, with a photocatalytic H\({}_{2}\) generation rate of 20.5 mmol/g/h, which value is 555 times higher than that of pure CNNS. [PERSON] et al. [50] developed a facile sacrificial coating method to fabricate a NiO QD/TiO\({}_{2}\) heterojunction. The as-prepared NiO QD/TiO\({}_{2}\) exhibited an excellent H\({}_{2}\) evolution rate of 1.35 mmol h\({}^{-1}\).g\({}^{-1}\), which is 37 times greater than that of NiO/TiO\({}_{2}\), 56 times greater than that of pure TiO\({}_{2}\), and even approaches that of Pt/TiO\({}_{2}\). This excellent photocatalytic performance results from the small sizes of NiO QDs, which lead to a shorter transfer distance for photogenerated carriers and more surface active sites in the NiO QD/TiO\({}_{2}\) composite than that of NiO/TiO\({}_{2}\). [PERSON] et al. [49] reported that the Co\({}_{3}\)O\({}_{4}\) QD/TiO\({}_{2}\) heterojunction exhibits a good photocatalytic performance. The Co\({}_{3}\)O\({}_{4}\) QDs act as cocatalyst, play dual roles in photocatalysis to improve the visible light utilization, and tune the work function of TiO\({}_{2}\) to accelerate the H\({}_{2}\) production rate on the Co\({}_{3}\)O\({}_{4}\) QDs' surface. In addition, the holes in TiO\({}_{2}\) lead to the generation of oxygen. The mechanism of the overall water splitting by the Co\({}_{3}\)O\({}_{4}\) QDs/TiO\({}_{2}\) composite is displayed in Figure 5.  
[PERSON] et al. [92] fabricated tunable Z-scheme semiconductors comprising TiO\({}_{2}\)-supported WO\({}_{3-\chi}\) QDs by in situ solvothermal and hydrogen reduction methods. The synthesized hybrid photocatalyst exhibited improved photocatalytic performance. This improvement could be related to the formation of W\({}^{5+}\)/VO defects at the interface, which are crucial for the fabrication of Z-scheme photocatalysts. [PERSON] et al. [94] reported that Bi\({}_{2}\)O\({}_{3}\) clusters@TiO\({}_{2}\) (BT) nanostructures facilitate the enhancement of the photocatalytic H\({}_{2}\) evolution rate.