How do gold-nanocrystal surface facets affect their electrocatalytic activities and the benzocaine-oxidation mechanism?

Abstract

Here we unveil the effect of gold (Au) nanocrystals with different surface facets on electrocatalytic sensing activities using benzocaine as an analyte. To this end, we performed cyclic voltammetry, differential pulse voltammetry, and in-situ Raman spectroelectrochemical experiments by employing Au nanocrystals with different exposed surface facets as electrocatalytic sensors for benzocaine. The experiments were complemented by density-functional calculations, where the rhombic dodecahedra nanocrystals, enclosed by {110} facets, showed stronger interaction energy with benzocaine, followed by cubes ({100} facets) and octahedra ({111} facets). Results from in-situ Raman spectroelectrochemical measurements have also demonstrated that the benzocaine adsorption displayed a facet-dependent behavior, which supports that the analyte can be strongly adsorbed on the Au{110} facets in comparison to {100} and {111} counterparts. In this case, we propose an oxidative mechanism of the benzocaine reaction, in which benzocaine may be electrochemically adsorbed at Au nanocrystals through the –NH2 groups. After that, this –NH(Au) group is oxidized to –NO and, further to –NO2, in which different profiles were observed as a function of the Au nanocrystal facets. The effect of these facets was also observed in the oxygen reduction reaction; the rhombic dodecahedra presented a potential reduction of 200 mV, and a current increased by more than twenty times relative to the values of a commercial Au electrode (bulk). Indeed, our findings provide a deeper understanding of facet-dependent electrooxidation activity, mechanisms, and electrocatalytic detection of organic compounds.