Charge-Dependent Atomic-Scale Structures of High-Index and (110) Gold Electrode Surfaces as Revealed by Scanning Tunneling Microscopy

The atomic and nanoscale structures of high-index gold surfaces in aqueous perchloric acid electrolyte as revealed by in-situ scanning tunneling microscopy STM under electrode potential control are reported with the objective of ascertaining systematically the terrace-step morphology and superstructures as a function of the crystallographic orientation. Six faces, Au221, 331, 533, 311, 210, and 410, two each lying in the three fcc crystallographic zones, were selected to investigate the role of the step orientation and terrace width for non-vicinal faces. Data for the low-index surface Au110 are also included for comparison with Au331 and 221, since all three feature formally a n111-111 terrace-step structure. Measurements of the double-layer capacitance as a function of the electrode potential, E, in dilute 10 mM perchloric acid were also undertaken in order to evaluate the potential of zero charge E sub pzc for each surface and to check the potential-dependent surface stability. The two surfaces in the 111-100 zone, Au533 and 311, both display essentially 1 x 1 i.e bulk-termination atomic structures at positive electrode charges i.e. for E E sub pzc, yet exhibit significant surface relaxation at negative charges involving edge-atom depression and row buckling. For the surfaces in the 111-110 zone, Au221 and 331, however, such surface relaxation is seen even at positive electrode charges. This behavioral difference can be understood on the basis of the differing step structures present in the two zones. Moreover, Au331 undergoes a reversible 1 x 2 reconstruction at negative charges, involving row pairing.