# Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction
## 2 Quantum Dots-Based Photocatalyst for Hydrogen Evolution
### Carbon Quantum Dots Based Hybrid Photocatalyst  
Carbon quantum dots (CQDs) are 0D carbon nanomaterials with a conjugated \(\pi\) structure; they comprise sp\({}^{2}\)/sp\({}^{3}\) hybridized carbon atoms, and exhibit unique optical properties and electrical properties [107]. These CQDs exhibit special characteristics, including broad optical absorption and the up-conversion of fluorescence emissions [27; 29; 108]. In this regard, CQDs can be used as the cocatalyst for the photosystem, and the development of CQDs-based photocatalysts for H\({}_{2}\) production has attracted immense attention in recent years. The most fundamental issues in photocatalysis include the low light absorption, fast recombination of photogenerated charge carriers, and slow surface reaction rate within the photocatalysts [8; 59]. Here, during photocatalytic H\({}_{2}\) generation with CQDs-based photocatalysts, CQDs can act as a photosensitizer to improve the solar light absorption of the photocatalysts and/or an electron reservoir to inhibit the recombination of charge carriers, resulting in enhanced photocatalytic performance. Hence, various semiconductor photocatalysts have been combined with CQDs to enhance the visible light absorption and improve the migration of photogenerated electron-hole pairs, thus enhancing  
Figure 7: The photocatalytic mechanism of g-C\({}_{3}\)N\({}_{4}\)@Ti\({}_{3}\)C\({}_{2}\) QD composites. Reprinted with permission from Ref. [53]. photocatalytic efficiency [22; 24; 25; 27]. Moreover, CQDs can even act as single photocatalysts for photocatalysis [28]. For example, CQDs serve as a cocatalyst in the photocatalytic H\({}_{2}\) generation reaction. They can reserve electrons from the photocatalyst to improve efficiency via the rapid migration of photogenerated electron-hole pairs. Typically, CQDs serve as a reservoir for electrons to improve efficiency via the separation of electron-hole pairs in TiO\({}_{2}\). [PERSON] et al. [108] prepared GQDs via the alkali-mediated hydrothermal method, and CQDs/(001)-faceted anatase TiO\({}_{2}\) composites were prepared by the further hydrothermal deposition of GQDs on (001) TiO\({}_{2}\) nanosheets for effective photocatalytic H\({}_{2}\) evolution. The results show that CQDs implanted into (001)TiO\({}_{2}\) sheets act as electron traps to accelerate the migration of photogenerated electron-hole pairs, further leading to the improvement of photocatalytic performance. The CQDs/(001)TiO\({}_{2}\) hybrid photocatalyst with 5 wt. % of CQDs exhibits a significantly higher photocatalytic performance, with the H\({}_{2}\) generation rate of 79.3 umol g\({}^{-1}\cdot\)h\({}^{-1}\), which value is 8 times that of bare (001)TiO\({}_{2}\). Moreover, the CQDS/(001)TiO\({}_{2}\) hybrids exhibit good stability over 30 h. Similarly, [PERSON] et al. [22] reported that CQDS/TiO\({}_{2}\) nanosheets with (001) facets of TiO\({}_{2}\)-001 exhibit an excellent photocatalytic performance with a significantly higher H\({}_{2}\) generation rate and long durability. They CQDs were synthesized by the electrochemical method and further combined with TiO\({}_{2}\) nanosheets under heating conditions to produce CQDs/TiO\({}_{2}\)-001 photocatalysts. The enhancement in photocatalytic efficiency is attributed to a synergistic effect between the highly active (001) facet of TiO\({}_{2}\)-001 and CQDs; the more active facet (001) can promote electron transfer, and the CQDs serve as an electron trap to suppress the recombination of photogenerated charge carriers. Additionally, the integration of CQDs enhances the absorption of visible light, as they function as photosensizers. The formation of a Ti-O-C bond between CQDs and TiO\({}_{2}\)-001 further leads to a yet higher photocatalytic efficiency for CQDs/TiO\({}_{2}\)-001. Figure 8 shows the mechanism of photocatalytic H\({}_{2}\) production by CQDs/TiO\({}_{2}\)-001 hybrids. [PERSON] et al.