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This work reports the fabrication of p‐type Si micropillar (MP) substrates decorated with AgxCu100−x bimetallic nanoparticles and their application as photocathodes for CO2 photoelectrochemical reduction. Metal deposition by metal‐assisted chemical etching is chosen as the nanoparticle synthesis method, to explore for the first time its capabilities for 3D structures. It is found to be applicable, allowing a good control of the composition, with nanoparticles distributed along the entire MP, but with a coverage gradient from top to bottom. The AgxCu100−x decorated Si MPs photocathodes show enhanced light trapping compared to flat Si, with 45 % lower reflectance values in the visible and significantly higher catalytic activity, in terms of photocurrent density, overpotential and power savings (4.7 % for Ag50Cu50/Si MPs vs. 3 % for Ag50Cu50/flat‐Si). Si MPs coated with Ag50Cu50 and Ag20Cu80 provide the highest gain in potential (440 and 600 mV vs. bare Si MPs) and an increased selectivity towards high energy density products (i. e., CH4) compared to monometallic photocathodes. These are promising features for efficient light‐driven CO2 conversion. However, a significant metal loss is observed during photoelectrolysis, especially for Cu‐rich compositions. Suggestions to improve the photocathode performance in terms of metal coating homogeneity and catalyst stability are presented. [ABSTRACT FROM AUTHOR]

Abstract This work reports the fabrication of p‐type Si micropillar (MP) substrates decorated with AgxCu100−x bimetallic nanoparticles and their application as photocathodes for CO2 photoelectrochemical reduction. Metal deposition by metal‐assisted chemical etching is chosen as the nanoparticle synthesis method, to explore for the first time its capabilities for 3D structures. It is found to be applicable, allowing a good control of the composition, with nanoparticles distributed along the entire MP, but with a coverage gradient from top to bottom. The AgxCu100−x decorated Si MPs photocathodes show enhanced light trapping compared to flat Si, with 45 % lower reflectance values in the visible and significantly higher catalytic activity, in terms of photocurrent density, overpotential and power savings (4.7 % for Ag50Cu50/Si MPs vs. 3 % for Ag50Cu50/flat‐Si). Si MPs coated with Ag50Cu50 and Ag20Cu80 provide the highest gain in potential (440 and 600 mV vs. bare Si MPs) and an increased selectivity towards high energy density products (i. e., CH4) compared to monometallic photocathodes. These are promising features for efficient light‐driven CO2 conversion. However, a significant metal loss is observed during photoelectrolysis, especially for Cu‐rich compositions. Suggestions to improve the photocathode performance in terms of metal coating homogeneity and catalyst stability are presented.