Cu2NiSnS4/g-C3N4 S-scheme photocatalysts: interfacial surface trap states vs. hydrogen production†

Graphitic carbon nitride (g-C₃N₄), a two-dimensional semiconducting material, shows promise in energy conversion but faces challenges such as rapid charge carrier recombination and poor visible-light absorption. To address these issues, we integrated Cu₂NiSnS₄ (CNTS) with g-C₃N₄ using an ultrasonication-assisted microwave irradiation method and observed that incorporating g-C₃N₄ with 5 wt% CNTS produced 4.6 μmol of sacrificial hydrogen under direct sunlight irradiation over 4 h. This presents a significant 38-fold increase in photocatalytic hydrogen production compared to that of bare g-C₃N₄. However, increasing the CNTS loading beyond 5 wt% gradually decreased hydrogen production. Higher CNTS loading also caused gradual quenching of photoluminescence spectra, which contradicts the hydrogen evolution results. On the other hand, time-resolved photoluminescence measurements indicated a shorter charge carrier lifetime in the composite, suggesting higher non-radiative recombination and/or a faster charge carrier separation rate. The discrepancies between PL spectra, TRPL measurements, and hydrogen production suggest the presence of a higher density of surface trap states at the CNTS/g-C₃N₄ interface. These trap states likely facilitate faster charge separation at lower CNTS loadings but lead to increased non-radiative recombination at higher loadings, thereby reducing hydrogen production. The CNTS/g-C₃N₄ photocatalysts showed outstanding stability over a period of ten cycles under a xenon lamp. This work provides new insights into interfacial charge transfer dynamics in heterojunction photocatalysts.