# Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction
## 2 Quantum Dots-Based Photocatalyst for Hydrogen Evolution
### Semiconductor-Quantum-Dot-Based Hybrid Photocatalyst
#### 2.2.3 Metal-Phosphide-Quantum-Dot-Based Hybrid Photocatalyst  
Metal phosphides (MxPy) have attracted much attention in the context of photocatalytic H\({}_{2}\) evolution from water splitting because of their low hydrogen evolution overpotential, tunable electronic structure, high electrical conductivity, and low price. They can not only act as photocatalysts--GaP and InP are well-known semiconductors--but they can also act as cocatalysts, with NiP and Fe\({}_{2}\)P exhibiting metallic behavior. Metal phosphide QDs are new cocatalysts that are attractive for their use in photocatalytic H\({}_{2}\) generation. For example, [PERSON] et al. [51] reported on a Ni\({}_{2}\)PQ QD/red P nanosheet photocatalyst with a p-n heterojunction that exhibited an excellent photocatalytic hydrogen generation rate. The results show that the internal electric field of the p-n heterojunction and the small size of uniform Ni\({}_{2}\)P QDs lead to the improved photocatalytic activity of  
Figure 5: (**a**) Schematic diagram of the energy band structure; (**b**) The mechanism by which Co\({}_{3}\)O\({}_{4}\) QDs/TiO\({}_{2}\) achieves photocatalytic overall water splitting. Reprinted with permission from Ref. [49].  
the Ni\({}_{2}\)P QD/red P nanosheets composite. During photocatalysis, the electrons in the CB of Ni\({}_{2}\)P QDs are rapidly transferred to red P nanosheets, after which H\({}^{+}\) is reduced to H\({}_{2}\). The photogenerated holes in the VB of red P are transferred to the Ni\({}_{2}\)P QDs, and then oxidize the sacrificial agents (Figure 6). In addition, Ni\({}_{2}\)P QDs are synthesized and highly dispersed on the red P nanosheet's surface, affording abundant photocatalytically active sites. [PERSON] et al. [95] achieved efficient photocatalytic H\({}_{2}\) precipitation via the in situ loading of Ni single-atom sites onto RP QDs; the presence of nickel atoms significantly lowered the energy barrier for electron transfer in the photocatalytic process. The Ni-P sites acted as electron antennae to attract light carriers to the solid-liquid interface, which activated the protons to initiate the effective H\({}_{2}\) production process, resulting in a high rate of H\({}_{2}\) production--224-fold higher than that of the original RP QDs.  
Researchers have extensively explored InP-based photocatalysts for use in photocatalysis due to their adjustable band gaps, high absorption coefficients, narrow emission line widths, and flexible composition [94,96]. Furthermore, InP exhibits a more significant degree of covalent bonding than cadmium selenide (CdSe), resulting in decreased electron-hole attraction and reduced photon coupling. Recently, InP quantum dots (QDs) have been utilized for photocatalytic hydrogen (H2) production. [PERSON] and [PERSON] et al. [97] have investigated InP/ZnS QDs for use in photocatalytic H\({}_{2}\) generation. By covering the surface of the InP QD with ZnS, the hydrogen evolution rate was enhanced compared with blank InP QDs. Under optimal conditions, an internal quantum yield of 31% at 525 nm can be achieved.