Tailoring Cu d Orbital Electron Density in Nanocrystalline Alloy Au _<i>x</i> Cu _<i>y</i> -Decorated Si Nanowires for Photoelectrochemical Highly Selective Urea Synthesis

The photoelectrochemical (PEC) coreduction of CO₂ and nitrate provides a promising approach for urea green synthesis, yet it suffered sluggish photogenerated carrier dynamics and low selectivity. To address these, a nanocrystalline alloy Cu_<i>x</i>Au_<i>y</i>-modified Si nanowire (NW) photocathode was prepared. Under -0.1 V vs reversible hydrogen electrode, the optimized Cu_1.25Au-Si NWs achieves a maximum urea 87.15% Faradaic efficiency of urea synthesis and yields 28.61 μg cm^-2 h^-1 in a 100 h measurement under light illumination, among the highest reported PEC values. Spectra analysis and density functional theory calculations confirm that the tunability of the Cu d orbital electron density reduces the energy barrier of the key rate-determining step (*COOHNH₂ → *CONH₂) in CO₂ activation to 0.23 eV and enhances electron transport kinetics by strengthening the charge polarization and prolonged carrier lifetime at the heterojunction interface. This study provides new mechanistic insights and strategies for designing high-performance alloy-semiconductor photocathodes for urea synthesis with good durability.