# Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction
## 2 Quantum Dots-Based Photocatalyst for Hydrogen Evolution
### Carbon Quantum Dots Based Hybrid Photocatalyst
[109] employed the open hollow structure and doping strategy to prepare CQDs-doped Fe/Co/Ni phosphides with a unique open hollow structure, which enhanced the spillover rate of the gas phase, increased the number of active centers, and achieved a significant enhancement in seawater electrolysis performance. [PERSON] et al. [28] reported on CQDs/g-C\({}_{3}\)N\({}_{4}\) nanocomposites prepared by the in situ synthesis of CQDs on the surface of g-C\({}_{3}\)N\({}_{4}\) using urea and glucose precursors, which were investigated for use in photocatalytic H\({}_{2}\) generation. The results show that the as-prepared CQDs/g-C\({}_{3}\)N\({}_{4}\) hybrids exhibited an excellent H\({}_{2}\) evolution rate of 2.34 umol g\({}^{-1}\cdot\)h\({}^{-1}\), which was 4 times greater than that of pristine g-C\({}_{3}\)N\({}_{4}\). The enhancement in photocatalytic activity is primarily due to the creation of a built-in electric field at the interface between the carbon quantum dots (CQDs) and graphitic carbon nitride (g-C\({}_{3}\)N\({}_{4}\)). This built-in electric field significantly improves the separation of photoinduced electron-hole pairs. Furthermore, the close integration of CQDs with g-C\({}_{3}\)N\({}_{4}\) forms an effective electron transfer (ET) channel, which enhances the photocatalytic efficiency for hydrogen (H\({}_{2}\)) production. The up-conversion photoluminescence (UCPL) property of CQDs plays a significant role in photocatalysis by enabling the conversion of low-energy photons to higher energy states. This process expands the spectrum of light that is usable for photocatalytic reactions, as it allows near-infrared (NIR) or even infrared (IR) light, which carry lower energy than visible light, to be upconverted to a higher energy level that can be utilized by the photocatalyst. When CQDs were combined with a semiconductor (such as g-C\({}_{3}\)N\({}_{4}\)), the CQDs not only extended the light response range of g-C\({}_{3}\)N\({}_{4}\), but also accelerated the transfer of photoinduced charges by promoting electron excitation to higher energy levels, where they can participate in redox reactions more effectively [58]. Moreover, CQDs could be used as an electron reservoir to effectively improve the photogenerated charge carriers' separation efficiency, even in the NIR region. [PERSON] et al. [28] have reported that the integration of CQDs into g-C\({}_{3}\)N\({}_{4}\) affords CQDs-implanted g-C\({}_{3}\)N\({}_{4}\) nanotubes (CCTs). Compared with those of g-C\({}_{3}\)N\({}_{4}\) and CQDs/g-C\({}_{3}\)N\({}_{4}\), the PL emission intensity of CCTs is dramatically decreased, indicating that the CQDs implanted with sp\({}^{2}\)-hybridized nitrogen as an electron buffer can considerably facilitate excited electron transfer; this, in turn, further inhibits the recombination of photogenerated electron-hole pairs. Thus, the as-prepared CCTs exhibited increased visible light absorption and decreased the energy barrier of the H\({}_{2}\) generation reaction, showing significantly high photocatalytic performance with a H\({}_{2}\) generation rate of 3538.3 umol g\({}^{-1}\cdot\)h\({}^{-1}\). [PERSON] et al. [110] constructed a system of indium phosphide quantum dots and carbon quantum dots (InP QDs/CQDs), and the average rate of photocatalytic decomposition of hydrogen sulfide for hydrogen production in the InP QDs/CQDs system was increased by a factor of 2.1 compared with that of InP QDs alone, suggesting that the introduction of the CQDs could effectively improve the separation efficiency of the photogenerated carriers. GQDs are special CQDs that comprise one or a few layers of graphene of a small size (<10 nm), and exhibit various unique characteristics, including an abundance of surface functional groups and fluorescence properties, and they can serve as occatalysts to facilitate the migration of photogenerated charge carriers, leading to the improvement of photocatalytic performance [57; 111]. Moreover, GQDs can extend the light absorption range as a result of their up-conversion luminescence property.