# Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction
## 3 Summary and Outlook  
In summary, quantum dots (QDs) have attracted considerable research attention from the scientific community. Compared to bulk materials, QDs exhibit various special properties, including band-gap tunability, an improved visible light absorption ability, and abundant surface active sites. Therefore, a great number of studies have reported on the application of QDs as efficient photocatalysts and cocatalysts for photocatalytic hydrogen production. This review provides a comprehensive description of the basic features of QDs, as well as their types and applications, in relation to photocatalytic hydrogen generation. Furthermore, semiconductor QDs serving as a single photocatalyst or in combination with other semiconductors for photocatalytic hydrogen production have been reviewed. Next, metal QDs and carbon QDs combined with other semiconductors for photocatalytic hydrogen production have also been discussed in detail. When optimized in terms of size distribution, shell materials and thickness, and surface ligands, QDs have excellent light absorption, exciton generation and charge separation properties. As such, QDs have been shown to be ideal components of photocatalysts. Although QDs-based hybrids exhibit an improved photocatalytic performance, the reported optimum photocatalytic efficiency is still not sufficient for large-scale industrial applications; thus, the new strategies set out to develop highly efficient QDs-based composites deserve more attention. Furthermore, the stability of QDs used in photocatalytic reaction processes is very important. QDs are often plagued by stability in use, which stems from the fact that there are always surface excitons. This leads to difficulties in reaching the ideal state during the cyclic testing of QDs. To overcome this drawback, the controlled assembly of two or more components has been investigated to enhance the stability of QDs' properties and structures, and to overcome some of the disadvantages of using single materials. As reported by [PERSON] et al. [113], CuO QDs were encapsulated in the pores of MIL-125(Ti) MOF and further combined with g-C3N4 to form a composite photocatalyst. Benefiting from the protection of the MIL-125(Ti) framework, the stability of the composite photocatalyst in the reaction system was significantly improved. In addition, the quantum confinement effects of QDs make them attractive for use in various applications, such as photocatalytic hydrogen generation, CO2 photoreduction, optoelectronic applications, lithium-ion batteries, and solar cells. It can be observed that band energies can be tuned by controlling the particle sizes of QDs according to the quantum confinement effects of QDs, opening up new avenues to enhance the photocatalytic efficiency. However, the design and study of QDs-based photocatalysts are still challenging as a result of several aspects, e.g., the up-conversion fluorescence effect of QDs, the structure-activity relationship and the structural engineering of QDs-based photocatalysts still need to be studied in depth. Future investigations may focus on the exploitation of novel QDs for efficient photocatalytic H2 generation. In addition, the mechanisms determining the role of QDs in QDs-based hybrid photocatalysts are not very clear; in order to discern how they operate during photocatalytic reactions, more efforts must be made. Notably,QDs-based hybrids are mainly investigated by use of a sacrificial donor; in short, the hydrogen evolution's half-reaction is under study. This is one of the more attractive and challenging topics that should be approached in future, in order to extend the application of QDs-based photocatalysts in efficient overall water splitting reactions to produce both hydrogen and oxygen. Conceptualization, X.G. and M.C. ; validation, Q.W. and Y.L. ; investigation, Q.W. ; resources and writing--original draft preparation, X.G. ; writing--review and editing, M.C.