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 aqu eous perchloric acid electrolyte as revealed by in-situ scanning tunne ling microscopy (STM) under electrode potential control are reported w ith the objective of ascertaining the terrace-step morphology and supe rstructures as a function of the crystallographic orientation. Six fac es, Au(221), (331), (533), (311), (210), and (410), two each lying in the three main zones of the unit projected stereographic triangle, wer e selected to investigate the role of the step orientation and terrace width for non-vicinal faces. Data for the low-index surface Au(110) a re included for comparison with Au(331) and (221), since all three fea ture formally a n(111)-(111) terrace-step structure. Measurements of t he double-layer capacitance as a function of the electrode potential, E, in dilute perchloric acid were also undertaken in order to evaluate the potential of zero charge (E(pzc)) for each surface and to check t he potential-dependent surface stability. The two surfaces in the (111 )-(100) zone, Ae(533) and (311), both display essentially (1 X 1) (i.e . bulk-termination) atomic structures at positive electrode charges (i .e. for E > E(pzc)), yet exhibit significant surface relaxation at neg ative charges involving edge-atom depression and row buckling. For Au( 221) and (331), lying in the (111)-(110) zone, however, such surface r elaxation is seen even at positive electrode charges. This behavioral distinction can be understood on the basis of the differing step struc tures. Moreover, Au(331) undergoes a reversible (1 X 2) reconstruction at negative charges, involving row pairing. This reconstruction is co mpared with that observed for Au(110), which also involves ''row pairi ng'' but for which several distinct local microstructures can be disti nguished. The faces in the (100)-(110) zone, Au(210) and (410), exhibi t locally ordered atomic arrangements indicative of an essentially bul k-termination structure. The longer-range superstructures for the high -index faces, specifically involving domain-edge propagation across mo noatomic steps, exhibit systematic trends consistent with an effective ly attractive step-step interaction at distances within ca. 5 Angstrom .