Study of Bi UPD structures on Au(1 0 0) using in situ surface X-ray scattering

The potential-dependent structure of the Bi UPD adlayer on Au(1 0 0) in 0.1 M HClO4 was examined by in situ surface X-ray scattering (SXS) measurements, revealing a complex sequence of different phases. Bi initially adsorbs in form of a disordered adlayer. With decreasing potential three ordered adlayer phases are formed: a (3 × 3) structure consisting of square Bi quadrumers, a c(√2 × 3√2) R45° structure, where the Bi adatoms are arranged in zigzag chains, and a quasi-hexagonal close-packed c(p × 2) adlayer, which is uniaxially incommensurate and compresses by ≈2% over the 30 mV stability range. According to surface crystallographic studies the local structural arrangement in the first two phases resembles that in the bulk Bi crystal. Furthermore, X-ray scattering voltammetry indicates kinetically hindered phase transitions between the ordered phases and allows to confirm that only the c(√2 × 3√2) R45° structure is active towards electrocatalytic H2O2 reduction.