# Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction
## 2 Quantum Dots-Based Photocatalyst for Hydrogen Evolution
### Single-Semiconductor QDs as Photocatalyst  
Just like the traditional semiconductor photocatalyst, semiconductor QDs alone can directly act as a photocatalyst in the H\({}_{2}\) evolution reaction; at the same time, they can also combine with other semiconductors or photocatalysts for use as materials to prepare hybrid photocatalysts for H\({}_{2}\) evolution reactions, which we will discuss in the next part. Transition metal sulfide quantum dot nanocrystals with large absorption coefficients, abundant active sites and quantum-limited domain effects are emerging as solar energy utilizers [64]. As well-known IIB-VI semiconductors, CdS photocatalyst systems have been extensively investigated for use in photocatalytic H\({}_{2}\) generation from water because of their special advantages, including their controllable morphology and suitable band gap. Hydrothermal and solvothermal methods are commonly used for the preparation of CdS [65; 66; 67; 68]. However, one of the more challenging aspects is the fast recombination of photogenerated charge carriers on the bulk CdS photocatalyst, which can easily lead to low photocatalytic activity [69; 70]. In this regard, CdS QDs with small particle sizes shows unique advantages, including the rapid migration of photogenerated electrons to the surfaces of CdS, the abundance of surface active sites, and the quantum confinement effect, which could effectively promote the separation efficiency of photogenerated charge carriers and further enhance the photocatalytic performance; therefore, the application of semiconductor QDs is extensive in photocatalytic H\({}_{2}\) production [71; 72]. For example, [PERSON] et al. [36] reported that mono-metal sulfide (MS) (M = Cd, Zn) QDs prepared via a modified gel crystal growth method exhibit higher H\({}_{2}\) evolution rates than those of bulk MS, corresponding to the synergistic effects of the QDs and the hydrogel. MS QDs demonstrate unique light absorption due to their extremely small size, compared with bulk MS photocatalysts. The unique up-conversion fluorescence effects of MS QDs extend the light absorption range and maximize the light absorption efficiency. Similarly, [PERSON] et al. [73] employed a simple effective method to enhance the photocatalytic H\({}_{2}\) evolution of CdSe QDs; after the surface modification with partial coverage of ZnS, the CdSe QDs exhibited high photocatalytic activity with a H\({}_{2}\) generation rate of 306.3 \(\upmu\)mol mg\({}^{-1}\cdot\)h\({}^{-1}\). [PERSON] et al. [74] reported that the fluorescence efficiency of molybdenum disulfide quantum dots (MoS\({}_{2}\) QDs) could be improved by monoatomic metal (Au, Ag, Pt, Cu) modification, and the fluorescence emission of MoS\({}_{2}\) QDs was enhanced by a factor of 4 under monoatomic Au modification. Figure 3: Quantum confinement effect in controlled photocatalytic H\({}_{2}\) evolution. (**a**) The potentials of the conduction and valence bands of CdSe change as the sizes of the particles vary from nanoscale to bulk. The upper circles illustrate the corresponding size variation. (**b**) The relative photocatalytic H\({}_{2}\) evolution rates are normalized with regard to photocatalyst sizes. Reprinted with permission from Ref. [15]. Compared to mono-metal chalcogenide QDs, multi-metal chalcogenide QDs can efficiently adjust the band gap through composition control, improving visible light utilization. Thus far, various ternary QDs, such as CdMoSe, CdTeSe, and CdZnSe, have been thoroughly investigated [35; 75; 76; 77]. The light absorption and band gap of Cd\({}_{1-\chi}\)Zn\({}_{\mathrm{x}}\)S as a representative can be tuned by the alteration of cationic mole ratios [78]. Similarly, [PERSON] et al. [35] prepared aqueous ternary Cd\({}_{\mathrm{x}}\)Mo\({}_{1-\chi}\)Se QDs via the modulation of the metal content, and revealed that the doping of Mo promotes the migration of photogenerated electrons of CdSe QDs, and leads to high photocatalytic activity. Moreover, the Cd\({}_{0.67}\)Mo\({}_{0.33}\)Se QDs exhibited good stability in recycling tests.