Charge-dependent atomic-scale structures of high-index and ( 110) gold electrode surfaces as revealed by scanning tunneling microscopy

Abstract

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 the terrace-step morphology and superstructures as a function of the crystallographic orientation. Six faces, Au(221), (331), (533), (311), (210), and (410), two each lying in the three main zones of the unit projected stereographic triangle, were selected to investigate the role of the step orientation and terrace width for non-vicinal faces. Data for the low-index surface Au(llO) are included for comparison with Au(331) and (221), since all three feature formally a n(111)−(111) terrace-step structure. Measurements of the 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 (Epzc) for each surface and to check the potential-dependent surface stability. The two surfaces in the (111)−(100) zone, Au(533) and (311), both display essentially (1 × 1) (i.e. bulk-termination) atomic structures at positive electrode charges (i.e. for E >E ), yet exhibit significant surface relaxation at negative charges involving edge-atom depression and row buckling. For Au(221) and (331), lying in the (111)−(110) zone, however, such surface relaxation is seen even at positive electrode charges. This behavioral distinction can be understood on the basis of the differing step structures. Moreover, Au(331) undergoes a reversible (1 × 2) reconstruction at negative charges, involving row pairing. This reconstruction is compared with that observed for Au(110), which also involves “row pairing” but for which several distinct local microstructures can be distinguished. The faces in the (100)−(110) zone, Au(210) and (410), exhibit locally ordered atomic arrangements indicative of an essentially bulk-termination structure. The longer-range superstructures for the high-index faces, specifically involving domain-edge propagation across monoatomic steps, exhibit systematic trends consistent with an effectively attractive step-step interaction at distances within ca. 5 Å.