AgxCu100−x Decorated Si Micropillars as Photocathodes for the Reduction of CO2

Abstract

This work reports the fabrication of p-type Si micropillar (MP) substrates decorated with Ag_xCu_100−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 Ag_xCu_100−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.