# Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction
## 1 Introduction  
With the increase in energy demands and environmental pollution, it is imperative to develop new approaches to acquiring environmentally friendly energy. Hydrogen has attracted remarkable interest as one of the most green and renewable energy sources to substitute non-renewable energy, such as fossil fuels. At the same time, solar energy is also a clean, sustainable and renewable natural energy. The conversion of solar energy into hydrogen fuels has been regarded as one of the most eco-friendly strategies to substitute fossil fuels, and it will greatly alleviate the global energy crisis and environmental pollution [1; 2; 3; 4; 5]. Therefore, hydrogen production from water splitting has attracted more and more attention from researchers [6; 7; 8; 9; 10]. However, the technology is still far from industrial applicability due to its low photocatalytic efficiency. The development of efficient photocatalysts for H\({}_{2}\) evolution is crucial for the realization of a sustainable energy future. Ongoing research in this field focuses on improving the efficiency, stability, and cost-effectiveness of photocatalytic systems to make them commercially viable and scalable. In general, three major steps are involved in photocatalytic H\({}_{2}\) production: (1) light absorption by the semiconductor to generate electrons and holes; (2) photogeneratedcharge separation and migration; and (3) the photogenerated electrons are reacted with water to produce hydrogen (Figure 1) [8; 10]. These three processes determine the overall photocatalytic efficiency of water splitting. The key challenge in photocatalysis is to design semiconductor photocatalysts that can efficiently absorb light, allow photogenerated charge carriers' separation and transfer, and drive the chemical reactions needed for H2 production. An effective method to reduce recombination and facilitate the surface reaction required for H2 production involves the cocatalyst loading strategy, undertaken to improve the photocatalytic efficiency of H2 evolution reactions. A cocatalyst attached to a semiconductor photocatalyst is well known to play important roles in high-efficiency photocatalytic water splitting for H2 production:  
The cocatalyst can improve light absorption;  
The cocatalyst can reduce the activation energy or overpotential of the H2 generation reaction, leading to a fast surface reaction;  
The cocatalyst can boost the photogenerated charge carriers' separation and transfer;  
The cocatalyst can suppress photocorrosion and increase the durability of the photocatalyst. Typical H2 evolution cocatalysts include noble metals such as Pt, Au, and Ag, and transition metals such as Co, Ni, and Cu, as well as transition metal sulfides or phosphides (e.g., MoS2, WS2, NiS, Ni2P, CoP, and FeP and so on), which can trap photogenerated electrons and provide the additional active sites for hydrogen generation. Particularly, noble metals such as Pt nanoparticles, when used as cocatalysts, also have an important promotional effect on the optical properties of the photocatalyst, affecting the surface plasmon resonance (SPR), which could significantly enhance the photocatalytic H2 generation efficiency. Notably, the band gap and CB and VB levels are the most important factors determining the performance of a photocatalyst. Generally, the photocatalytic H2 generation reaction is a thermodynamically feasible process when the CB level of a semiconductor photocatalyst is more negative than the reduction potential of H+/H2 [11]. Therefore, to achieve efficient H2 evolution efficiency, a semiconductor photocatalyst must fulfill several requirements: (i) high visible light utilization; (ii) low recombination of charge carriers; (iii) the CB level should be greater than the hydrogen reduction potential (H+/H2) level; (iv) it should be stable under extreme environments, and (v) cost-effectiveness. Much effort has been devoted to the search for a photocatalyst that can satisfy these requirements [12; 13; 14].