Electrochemical surface nanostructuring of copper using fluoride and chloride

Copper is an active electrocatalyst for different energy conversion reactions which performance depends on the structure of the surface active sites. Here we propose a simple strategy to generate (n10) or (100)x(110) motifs on copper with tailored roughness. We electrochemically oxidize and reduce a copper electrode in solutions containing a mixture of sodium fluoride and sodium chloride with total concentration of 0.1 M: (0.1 -x) M NaF + x M NaCl. Chloride acts as a ligand agent inducing surface re-faceting and the growth of (n10) sites. Fluoride, which weakly adsorbs on copper, mainly causes an increase of the electroactive area and surface rugosity. With fluoride and chloride mixtures, we form (n10) motifs that appear covered by nanometric clusters, progressively increasing the density of defects. The nanoscale clusters almost coalesce with very high fluoride/chloride molar ratio solutions. To address the change in surface geometry on copper, we recorded the voltametric fingerprints of the modified surfaces with lead underpotential deposition (UPD) and compared them with the lead UPD-response of model copper single facets. Our work provides a direct link between an electrochemical response and ex-situ structural characterization describing, in detail, the effect of halides on the surface nanostructuring of copper.