Unlocking the effects of Cu doping in heavy-metal-free AgIn5S8 quantum dots for highly efficient photoelectrochemical solar energy conversion

Colloidal quantum dots (QDs) containing toxic lead and cadmium compounds, i.e., Pb/Cd(S,Se,Te), have been widely studied for hydrogen generation because of their promising optoelectronic properties and efficient photocatalytic activities. However, it is still desirable to construct heavy-metal-free QDs with high photoconversion efficiencies through careful design and doping with more environmentally benign alternatives. Here, we design and synthesize Cu-doped AgIn5S8 (Cu–AIS) QDs that contain Cu T2 state sites between the Ag 4d and S 3p orbitals, producing adjustable bandgaps and slowing photogenerated charge recombination compared to pristine AIS QDs. We demonstrate that this Cu-doping configuration not only lowers the activation energy barrier with a more negative potential, but it also leads to the efficient separation and transportation of photogenerated electrons and holes. As proof-of-concept, we fabricate a photoelectrochemical (PEC) cell based on a Cu–AIS QD-sensitized TiO2 photoanode and achieve a maximum saturated photocurrent density of ∼9.8 mA cm−2 at 0.75 V versus a reversible hydrogen electrode (RHE). Furthermore, photostability experiments demonstrate that the operational long-term stability of the Cu–AIS QDs is much better than that of an AIS QD-based PEC cell. Overall, heavy-metal-free Cu–AIS QDs pave the way to the design and synthesis of environmentally friendly, high-efficiency, and cost-effective PEC systems for solar-driven hydrogen generation, and other optoelectronic devices, such as photovoltaics and photodetectors.