# Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction
## 2 Quantum Dots-Based Photocatalyst for Hydrogen Evolution  
By summarizing the application of QDs in photocatalytic hydrogen production in recent years, it has been found that the application of QDs in photocatalytic hydrogen evolution can be divided into three categories, which are the use of single semiconductor QDs as photocatalysts; semiconductor-quantum-dot-based hybrid photocatalysts; metal QDs and MXene QDs used as cocatalysts; carbon QDs-based hybrid photocatalysts. Table 1 provides a detailed summary of the reported examples of the QDs utilized for photocatalytic H\({}_{2}\) generation. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Photocatalyst** & **Cocatalyst** & **Reaction** & **Light** & **H\({}_{2}\) Generation** & **Referenced** & **QDs** \\  & **Medium** & **Source** & **(mmol h\({}^{-1}\) g\({}^{-1}\))** & **Sample/Enhancement Factor** \({}^{\ast}\) & **Ref. ** & **Size** \\ \hline ZnCdS QDs & Na\({}_{2}\)SO\({}_{3}\) and Na\({}_{2}\)S & 300 W Xe lamp (\(\lambda\geq\) 420 nm) & 3.70 & bulk ZnCdS/5.2 & [33] & 4.5-7.8 \\ \hline CuInfS QDs & ascorbic acid & LED light source (470 nm) & 116 & Cu\({}_{2}\)S/18 & [34] & 3 \\ \hline Cd\({}_{0.6}\)/Mo\({}_{0.3}\)Se & Na\({}_{2}\)SO\({}_{3}\) and Na\({}_{2}\)S & 300 W Xe lamp (\(\lambda\geq\) 420 nm) & 0.911 & MoSe\({}_{2}\) & [35] & 3 \\ \hline CdS QDs & Na\({}_{2}\)SO\({}_{3}\) and Na\({}_{2}\)S & 300 W Xe lamp & 0.410 & Bulk CdS/5 & [36] & 6 \\ \hline CdSe/CdS/ZnS QDs & Na\({}_{2}\)SO\({}_{3}\) and Na\({}_{2}\)S & 300 W Xe lamp & 0.195 & CdSe QDs/6.32 & [37] & 7.2 \\ \hline CdSe QDs/g-C\({}_{3}\)N\({}_{4}\) & L-ascorbic acid & 500 W Hg lamp (\(\lambda\geq\) 420 nm) & 0.615 & g-C\({}_{3}\)N\({}_{4}\)/76 & [38] & 2-3 \\ \hline CdCO\({}_{3}\)/CdS QDs & Na\({}_{2}\)SO\({}_{3}\) and Na\({}_{2}\)S & 300 W Xe lamp (\(\lambda\geq\) 420 nm) & 1.93 & & [39] & 5 \\ \hline CdS QDs/Pt/In\({}_{2}\)O\({}_{3}\) & \multirow{2}{*}{Pt/In\({}_{2}\)O\({}_{3}\)} & \multirow{2}{*}{Lactic acid} & 300 W Xe lamp (\(\lambda\geq\) 420 nm) & 1.03 & & [40] & \\ \cline{3-3} CuInS\({}_{2}\) QDs/CN & & Triethanolamine (TEOA) & 300 W Xe lamp (\(\lambda\geq\) 420 nm) & 1.92 & CN/2.6 & [41] & 5 \\ \hline ZnAgInS QDs/2D & \multirow{2}{*}{Ascorbic acid} & 300 W Xe lamp (\(\lambda\geq\) 400 nm) & 40.1 & & [42] & 3-5 \\ \cline{3-3} CdSe nanosheet & & & & & & [43] & 7-8 \\ \hline CdSe QDs/WS\({}_{2}\) & \multirow{2}{*}{Lactic acid} & 300 W Xe lamp (\(\lambda\geq\) 420 nm) & 14 & & [43] & 7-8 \\ \cline{3-3} \cline{5-3}  
A semiconductor particle smaller than its Bohr exciton diameter can exhibit quantum confinement effects, which distinguish QDs from their bulk counterparts. In this case, the charge carriers become spatially confined, such that the energies of photogenerated electrons and holes are increased. As shown in Figure 3a, bulk CdSe has a moderate bandgap (Eg = 1.74 eV), while 2.0 nm CdSe particles have a notably large bandgap (2.88 eV). The conduction band energy (E\({}_{\text{CB}}\)) is increased (electrons in the conduction band become more reduced), and the valence band energy (E\({}_{\text{VB}}\)) is decreased. Smaller QDs are thus expected to more easily engage in interfacial electron transfer with H\({}_{2}\) evolution occatalysts than larger QDs. Indeed, according to [PERSON] theory, quantum confined systems can exhibit high rates of interfacial charge transfer owing to the increased energies of confined electron-hole pairs, which is of benefit to applications in solar-to-fuel conversions, such as artificial photosynthetic H\({}_{2}\) evolution. While bulk CdSe and 6.0 nm QDs are not excellent photocatalysts, the smaller particles are, with their normalized rate of H\({}_{2}\) evolution increasing when smaller QDs are used (Figure 3b). Therefore, it is clear that optimizing QD size distribution is an effective and facile pathway to increasing the rate of solar H\({}_{2}\) evolution from QD-based photosystems [15]. Semiconductor QDs typically comprise group II-VI, III-V, or IV-VI elements.