Band-edge alignment in ultra-narrow InAs1-xSbx photocathodes enabling selective solar-driven CO2-to-liquid conversion

Abstract

Selective photoelectrochemical CO2 reduction technologies typically rely on wide-bandgap semiconductors, which provide sufficient photovoltage but low solar utilization. In this study, we demonstrate that the absolute band-edge alignment, rather than the bandgap size, governs CO2 reduction selectivity in the ultra-narrow-bandgap (<0.5 eV) InAs1-xSbx photocathode. Single-phase zinc-blende alloys with tunable conduction and valence band positions, while maintaining high carrier mobility, are obtained via molecular beam epitaxy. Under 1-sun irradiation in CO2-saturated bicarbonate, intermediate compositions (x ≈ 0.5–0.7) achieve a Faradaic efficiency of ∼70 % for C1–C2 oxygenates at −0.7 V vs. RHE, while suppressing H₂ evolution by a factor of more than 10 compared to InAs. UV photoelectron spectroscopy and DFT calculations reveal that antimony incorporation shifts both the conduction and valence band edges toward vacuum, thereby weakening H* adsorption and modulating interfacial energetics to favor multi-electron CO2 reduction. This study presents a practical design framework that demonstrates how ultra-narrow-bandgap III–V alloys can enable efficient and selective CO2-to-liquid conversion when their band alignment and surface energetics are precisely engineered.