One-Step Synthesis of Ag–Cu Bimetallic Nanoparticles on p-Si Photoelectrodes for Solar-Driven CO₂ Reduction

In the present work, we report the one-step synthesis of Ag x Cu 100– x bimetallic nanoparticles (NPs) on p-type silicon (p-Si) supports and their application as photocathodes for the photoelectrochemical CO 2 reduction reaction (PEC-CO 2 RR). Based on the metal-assisted chemical etching (MACE) of Si in HF medium, the proposed method allows the formation of NPs homogeneously distributed on the Si surface, with an excellent control of their bimetallic composition, surface coverage, and morphology by adjusting the experimental conditions (deposition time, precursor concentrations). Microstructural studies reveal a phase-separated polycrystalline structure for the bimetallic with a high dispersion of Ag and Cu nanocrystallites, remarkable for Ag/Cu nanostructures synthesized by soft chemistry. The analysis of the PEC-CO 2 RR shows a better performance of Ag 50 Cu 50 /p-Si than Ag/p-Si and Cu/p-Si, with an onset potential shift of 840 mV and a ratiometric power saved of ∼3% compared to an analogous metal electrode in CO 2 saturated 0.5 M NaHCO 3 . Photoelectrolysis tests under 1 sun illumination coupled with gas chromatography demonstrate the capacity of the bimetallic photocathode to guide the selectivity with the applied potential. Thus, CO 2 conversion to CO and CH 4 is achieved on Ag 50 Cu 50 /p-Si, with a maximum Faradaic efficiency (FE) of 26% and 18.2% at −0.72 and −0.87 V RHE, respectively. These values are consistent with recent research on Ag–Cu systems on Si for the PEC-CO 2 RR but are obtained at significantly lower overpotentials. These are the first results ever reported for the PEC-CO 2 RR on p-Si photocathodes decorated with AgCu NPs. The proposed deposition method is inexpensive, easily scalable, and can be extended to any bimetallic system whose elements are compatible with MACE of Si.