Spectroscopic and Spectroelectrochemical Studies on Redox Properties of Zinc Sulfide and Their Consequences on Photocatalytic Activity

Understanding the relationships between the electronic structure (and therefore redox properties) of semiconductors and their photoactivity is crucial for a rational design of photocatalysts. In this work, we undertake such analysis with the help of emission and absorption UV-vis spectroscopy, as well as diffuse reflectance spectroelectrochemistry (SE-DRS). Herein, we focus on zinc sulfide, which is considered a photocatalyst with strong reducing properties but is also known for its efficient visible light photoluminescence. A series of ZnS materials with a different Zn:S ratio was synthesized, in an anaerobic and oxygen atmosphere. The synthesized samples were thoroughly studied in terms of morphological, photophysical, and photocatalytic properties. Two groups of materials (sulfur-rich and sulfur-deficient) could be distinguished, with significantly different properties, such as the crystal size, oxygen concentration, bandgap energy, distribution of electronic states, and photoelectrocatalytic and photocatalytic activities. We confirmed the presence of deep electronic states, which significantly decrease the reduction abilities of ZnS, some of which are surface states. Models of the electronic structures associated with possible photophysical processes were proposed for both sulfur-rich and sulfur-deficient materials. Although in proposed models the energy of photogenerated electrons is significantly lower than that usually considered based on the level of the conduction band edge, the photocatalytic tests revealed their sufficient potential to perform hydrogen evolution and CO2 reduction processes.