Mechanistic Insight into Donor–Acceptor Pair-Driven Photocatalysis: How S ^2– Dipole and ZnS Shell Regulate Sub-Bandgaps of Ag–In–S QDs

Donor–acceptor pairs (D–A pairs) are a unique structure of I–III–VI group quantum dots (QDs), which can effectively store charge carriers and suppress electron–hole recombination. However, the uneven energy level distribution of the donor–acceptor pair can affect the accuracy of the carrier relaxation behavior. In this work, the influence of D–A pairs with different densities on the carrier dynamics of Ag–In–S QDs was studied and the effective separation and storage of electrons and holes were achieved. Subsequently, the dipole moment effect of inorganic S2– was utilized to bring the sub-bandgap edges closer to the valence band (VB) and conduction band (CB), respectively, thereby optimizing the distribution of D–A pairs and suppressing the carrier recombination caused by energy levels near the center of the band gap. Density functional theory calculations and Fourier transform infrared spectroscopy (FTIR) characterization have demonstrated the feasibility of substituting MPA with inorganic S2– in theory and experiment. Through the energy band coupling effect of partially coated ZnS shell layers, a pathway for promoting electron transfer was provided, and the rate of electron diffusion to the surface was enhanced. Through the above strategies, the carrier dynamics in Ag–In–S QDs have been effectively optimized. Without constructing heterojunctions, the hydrogen production rate increased from the initial 0.023 mmol·g–1·h–1 to a maximum of 29.056 mmol·g–1·h–1, achieving a nearly 1200-fold growth. This research provides an approach to enhance the hydrogen production efficiency of green and pollution-free I–III–VI group QDs through photocatalysis.