Electrochemical adsorbate-induced substrate restructuring: gold(110) in aqueous bromide electrolytes

The electrode potential-dependent atomic structure of ordered Au(110) in bromide electrolytes is examined by in situ scanning tunneling microscopy, focusing on the ordered adlayer arrangements formed on the (1×1) substrate terraces, and especially the adsorbate-induced nanoscale restructuring which is observed to occur at higher potentials. Comparison is made with the previously examined behavior of Au(110) in iodide electrolytes. Several ordered bromine structures were identified, all featuring ‘close-packed’ Br rows along the (001) direction, i.e. perpendicular to the (110) ‘rails’. At lower coverages, θBr<0.75, rectangular packing was commonly observed, featuring also Br rows parallel to the (110) direction. The Br binding sites were determined unambiguously from ‘potentiodynamic’ STM data, featuring juxtaposed substrate and adlayer domains created by potential-step perturbations during image acquisition. The rectangular unit cells contain Br atoms lying within the (110) troughs, and exhibit periodic variations from ‘long twofold’ to fourfold-hollow binding sites. These include (3×1) and (4×1) unit cells, corresponding to θBr=0.66 and 0.75, respectively. At similar potentials to the latter, an alternative (4×1) structure can be formed having the same coverage, yet with quasi-hexagonal Br packing. Increasing the potential further yields erosion of terrace edges, especially along the (001) direction, and the appearance of arrays of single atomic-layer gold ‘nanorods’, 1.8 nm wide, also oriented along (001). These strings eventually self-assemble into ordered arrays, with rod separations of 0.9–1.2 nm. The restructuring can be reversed rapidly by returning the potential to lower values. The ordered restructuring is markedly different from that observed previously in the Au(110)–I system, which features ‘miniterrace stripes’ largely paralleling the (11̄0) direction. These structural differences can be understood in terms of the known dissimilarities in the close-packed Br and I adlattices; the strict (001) orientation of the nanorods in the Au(110)–Br system is apparently triggered by the similarly oriented Br strings. The widths of the restructured gold miniterraces in both electrolytes are surmised to be determined by adsorbate-induced dipole repulsion forces.