Band engineering in iron and silver co-doped double perovskite nanocrystals for selective photocatalytic CO2 reduction

Double metal cation halide perovskites are promising alternatives to lead halide perovskites due to their exceptional compositional flexibility and stability. However, their utilization in solar-light harvesting applications has been hindered by their large band gap and the complexity of producing doped or alloyed materials with desirable optoelectronic properties. In this study, we report the colloidal synthesis of iron-doped Cs2NaInCl6 double perovskite nanocrystals (NCs), leading to a significant extension of the absorption edge from 330 nm to 505 nm. We also demonstrate that simultaneous doping with Fe3+ and Ag+ ions allows significant reduction of the optical band gap and precise tuning of electronic band structures of the resulting NCs. The enhanced absorption in the visible region is attributed to the substitution of In-5s by the Fe-3d state, while the introduction of the Ag 4d state upshifts the valence band maximum, inducing a transformative change in the band structure, as confirmed by density functional theory (DFT) calculations. Remarkably, by precisely controlling the band positions of the Fe3+-doped Cs2Ag0.5Na0.5InCl6 NCs, we accomplished the selective photocatalytic reduction of CO2 into CH4, making them readily available for solar-energy conversion technologies.