Wet-Chemistry: A Promising Route to Realize Highly Efficient Semiconductor-Based Photocatalysis

Photocatalysis represents a promising approach to alleviate the environmental crisis and resources shortage. In principle, the semiconductors can be excited by appropriate photoenergy and generate the electron-hole pairs for the redox reactions on the surface. The highly efficient semiconductor-based photocatalysts are conventionally synthesized by solid-state methods. Or the photocatalysts were synthesized through the reactions with high temperature or pressure and dried into powder, which has to be redispersed well in water before carrying out the photocatalytic measurement. However, such synthetic process always meets the difficulty of uniform dispersion and the loss of surface reactivity. Our latest findings found the low-dimensional semiconductor photocatalysts prepared without drying process make it more efficient and facilely controllable in photocatalytic reactions. Furthermore, such method is flexible in modulating the surface chemistry and constructing robust platform for photocatalytic reactions. Here two examples will be presented:<br/><br/>With sufficiently high conduction band, ZnS is one of the typical photocatalysts for hydrogen evolution and CO₂ reduction. In our work, a series of ZnS-based nanocrystals with an average size of 3 nm were synthesized by mixing the precursor ZnSO₄ and Na₂S in water at room temperature. The as-synthesized nanocrystals were only rinsed with water and directly used in the subsequent photocatalytic experiments without any drying process. Such all-inorganic wet system showed an outstanding performance in the CO₂ conversion to formate with a high efficiency and selectivity. By simply changing the precursor composition and concentration, the ZnS systems could also be introduced dopants, cocatalysts and vacancies.<br/><br/>Molecularly nagatively-charged titanate monolayers are stably dispersed in aqueous solution but showed limited photocatalytic applications up to date. One of the drawbacks is the easy accumulation of metallic cocatalyst reaching to tens of nanometers by photodeposition that results in a fast recombination of charges. By exchanging the water with nucleophile solvents and reserving the outmost layer ligands, it is successful to realize the uniform dispersion of the ultrasmall metallic cocatalysts on the surface of the titanate monolayers. After rinsing the monolayers with pure water without drying process, a boosted photocatalytic activity can be achieved.<br/><br/>The presentation highlights the wet chemistry as a facile synthetic approach at ambient conditions, which greatly reduces the experimental time and efforts. The experiments disclose the importance of the wet surface and the possible loss of active sites and reactivity during drying process. The insights into the surface chemistry (large surface energy, rich active sites, tunable functional groups, etc.) may lay a significant groundwork for future investigations aiming at efficient photocatalytic systems.