# Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction
## 1 Introduction
Recent advances have revealed that quantum dots (QDs) demonstrate promise for use in the photocatalytic H2 evolution reaction because of their unique optical-physical properties, including a good light absorption coefficient, quantum confinement effect, and optical stability. Due to the quantum-confined effect, QDs have demonstrated promise in efficient photocatalytic reactions. Compared to bulk semiconductor materials, semiconductor QDs show band-gap tunability, good visible light absorption, and more active sites. QDs are  
Figure 1: Schematic of water splitting over semiconductor photocatalysts. (1) the light absorption and the formation of the photogenerated electron–hole pairs; (2) the photogenerated carriers migrate to the surface of semiconstor; (3) the photogenerated carriers participate the surface reactions. defined as a class of quasi-zero-dimensional (0D) nanoparticles. The exciton Bohr radius is a critical length scale for QDs, as it defines the size range over which QDs exhibit quantum confinement effects. For a given material, QDs must be smaller than this radius so as to exhibit molecular-like behavior [15]. The photophysical properties of QDs, such as absorption, emission wavelength, and quantum yield, can be tuned by controlling their size. As the size of the QDs decreases, the bandgap increases due to quantum confinement, leading to a blue shift in absorption and emission spectra. As the size of the QDs increases, their energy levels begin to merge and form bands similar to those in bulk semiconductors. This leads to properties that are more typical of traditional semiconductor materials, such as a broad emission spectrum and less pronounced quantum confinement effects. As mentioned, Figure 2 illustrates the transition of electronic energy states from discrete molecules to a bulk material, with QDs lying in between these two extremes [16]. In semiconductor QDs, the electron and hole energy states are discrete and regular. Owing to quantization, the band gap of semiconductor QDs is enhanced in comparison with that of bulk semiconductors, with the CB and VB energy levels moving up and down, respectively. Then, the redox potentials of H\({}_{2}\) generation within QDs are easily adjusted by particle size regulation, and quantum confinement endows semiconductor QDs with tunable driving forces to make a majority of the redox reactions thermodynamically feasible. Besides this, compared to bulk semiconductors, QDs exhibit a short electron-hole diffusion distance due to their small size, facilitating the efficient migration of the charge carrier and weakening the adverse effects of interior recombination centers [14; 17; 18]. Moreover, the abundant surface active sites in QDs speed up the surface reaction, and then enhance the phototocatalytic performance. At the same time, the electron transfer (ET) route of semiconductor QDs is flexible, and can be used for the spatially restricted close interactions between electron donor-acceptor sites [19]. The quantum confinement effect in QDs has given rise to several fascinating optoelectronic features, leading to good visible light absorption, the rapid migration of charge carriers, and low reduction potentials, which are extremely important for photocatalytic H\({}_{2}\) production [20]. Except for semiconductor QDs, other photoactive metal or metal-oxide QDs and carbon QDs (CQDs) are also potential candidates, as they rely on sunlight for photocatalytic water splitting. In particular, CQDs are promising candidates for use in the development of photocatalysts [21; 22; 23; 24], which include delocalized conjugated \(\pi\) structures that can induce the rapid migration of charge carrier, leading to promoted photocatalytic efficiency [25; 26; 27]. Meanwhile, CQDs show the special optical property of up-conversion fluorescene emission due to their quantum confinement [27; 28; 29; 30; 31; 32], which expands the light-utilization range and improves the photocatalytic efficiency [23; 25].