Origin of photoelectrochemical CO2 reduction on bare Cu(In,Ga)S2 (CIGS) thin films in aqueous media without co-catalysts†

Photoelectrochemical (PEC) CO₂ reduction (CO₂R) on semiconductors provides a promising route to convert CO₂ to fuels and chemicals. However, most semiconductors are not stable under CO₂R conditions in aqueous media and require additional protection layers for long-term durability. To identify materials that would be stable and yield CO₂R products in aqueous conditions, we investigated bare Cu(In,Ga)S₂ (CIGS) thin films. We synthesized CIGS thin films by sulfurizing a sputtered Cu–In–Ga metal stack. The as-synthesized CIGS thin films are Cu-deficient and have a high enough bandgap (1.7 eV) suitable to perform CO₂R. The bare CIGS photocathodes had faradaic yields of 14% for HCOO⁻ and 30% for CO in 0.1 M KHCO₃ electrolyte without the use of any co-catalysts under 1 sun illumination at an applied bias of −0.4 V vs. RHE and operated stably for 80 min. Operando Raman spectroscopy under CO₂R conditions showed that the dominant A₁ mode of CIGS was unaffected during operation. Post-mortem X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analysis suggests that the CO₂R stability could be related to self-protection caused by the in situ formation of oxides/hydroxides of Ga and In during operation. Density functional theory (DFT) calculations also reveal that Ga and In are the preferential sites for the adsorption of CO₂R products, particularly HCOO⁻. These results show that CIGS is a promising semiconductor material for performing direct semiconductor/electrolyte reactions in aqueous media for the PEC CO₂R.