High-performance and stable NH3 production using a TiO2-protected Si photocathode and patterned Au loading†

Crystalline silicon (c-Si) is a promising material for photoelectrochemical (PEC) ammonia (NH₃) production from nitrate (NO₃⁻) reduction owing to its appropriate band gap and optimal charge-transport properties. However, c-Si is not stable in aqueous solutions, causing the detachment of catalysts from the c-Si photoelectrode and resulting in a dramatic decrease in the performance. Furthermore, electrocatalysts on c-Si block light, therby reducing the PEC NH₃-production efficiency. Herein, we stabilized and increased the efficiency of the c-Si photocathode by TiO₂ deposition and loaded an optimized amount of Au using an e-beam patterning, respectively. We found that TiO₂ not only protects the c-Si photoelectrode from the electrolyte but also promotes strong bonding between Au and the c-Si photoelectrode. Notably, TiO₂ showed a synergistic effect with the Au electrocatalyst in increasing the faradaic efficiency (FE) of NO₃⁻ reduction for NH₃ production, which was further confirmed by density functional theory calculations. Overall, the Au-loaded TiO₂-protected c-Si photoelectrode showed a stable and record-high NH₃-production rate of 1590 ± 40 μg_NH3 cm⁻² h⁻¹ with an FE of 83.4% ± 5.6% at −0.35 V vs. the reversible hydrogen electrode.