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

Selective photoelectrochemical CO 2 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 CO 2 reduction selectivity in the ultra-narrow-bandgap (<0.5 eV) InAs 1-x Sb x 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 CO 2 -saturated bicarbonate, intermediate compositions (x ≈ 0.5–0.7) achieve a Faradaic efficiency of ∼70 % for C 1 –C 2 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 CO 2 reduction. This study presents a practical design framework that demonstrates how ultra-narrow-bandgap III–V alloys can enable efficient and selective CO 2 -to-liquid conversion when their band alignment and surface energetics are precisely engineered. • Ultra-narrow-gap InAs 1-X Sb X photocathodes achieve selective PEC CO 2 RR. • ∼70 % Faradaic efficiency for C 1 –C 2 oxygenates at −0.7 V vs RHE under 1-sun. • Sb-rich surfaces suppress HER via weaker H binding confirmed by EIS and ΔG(H). • UPS/XPS band alignment explains activity and selectivity on a vacuum scale. • Stable operation at −0.7 V; severe corrosion observed beyond −1.4 V.