Controlling Valence Band for Cu2Sn0.38Ge0.62S3 Particles via Li Substitution at Cu Sites

Cu2(Sn,Ge)S3 (CTGS) is a promising near-infrared-responsive photocathode material whose conduction band minimum can be tuned by varying the Sn/Ge ratio, whereas direct control of the valence band maximum (VBM) remains challenging because of the large Cu 3d contribution to the VBM. This study proposes a strategy for adjusting the VBM position for CTGS photocatalytic particles based on a high degree of substitution of monovalent alkali metal cations at Cu+ sites. Specifically, Li+, Na+, and K+ are introduced during solid-state synthesis under H2S flow while reducing the Cu precursor amount. The results show that Na+ and K+ were mainly enriched near the surfaces of CTGS particles because of their larger ionic radii, whereas Li+ could be incorporated into the bulk of the photocatalyst crystals. Substituting Li+ ions at Cu+ sites reduced the contribution of Cu 3d orbitals to VBM formation, thereby positively shifting the VBM position. Notably, Li+-substituted CTGS was still able to absorb near-infrared light up to 900 nm, despite having a wider bandgap than pristine CTGS. Consequently, a photocathode fabricated from Li+-substituted CTGS particles exhibited an increased cathodic photocurrent of –4.5 mA cm–2 at 0 V vs a reversible hydrogen electrode (RHE) and a positively shifted onset potential of 0.38 VRHE during photoelectrochemical hydrogen evolution under simulated sunlight. Device simulations suggested that the positive VBM shift improves the CdS/CTGS band alignment, increases band bending, and suppresses interfacial recombination. It is evident that doping with Li+ represents a promising approach to significantly adjusting the band structures of Cu-based multielement semiconductors.