Understanding the band-edge electronic structure and charge-transfer dynamics in size-confined nanostructures is critical for developing advanced materials used in energy conversion applications, such as green hydrogen production, organic pollutant decomposition and solar cells. [1] In this study, we present a series of high-surface-area mesoporous materials comprising continuous networks of interconnected zinc indium sulfide (ZnIn₂S₄) nanocrystals with tunable diameters (ranging from ~4 to ~12 nm). [2] This development enables a detailed investigation of size-dependent effects on charge-transfer dynamics and photochemical performance within these nanostructures. Using a combination of spectroscopic and (photo)electrochemical techniques as well as theoretical calculations, we elucidated the influence of nanocrystal size on the electronic structure, including band-edge positions, charge density profiles and charge-transfer kinetics. The results show that reducing the size of ZnIn₂S₄ nanocrystals enhances interfacial charge-transfer kinetics and charge separation rates, improving the ability of photogenerated carriers to drive water-splitting reactions. [3] Consequently, the photocatalytic H₂ evolution activity of these materials is among the highest reported for single-component sulfide photocatalysts. However, for ultrasmall nanocrystals, charge transfer and separation kinetics reveal that surface sulfur vacancies, which generate mid-gap states at the interface, are significant contributors to reduced photocurrent and photocatalytic performance. 

These findings offer valuable insights for the rational design of semiconductor nanostructures through synthetic engineering, aiming at the development of high-performance catalysts for zero-carbon energy applications.