Structural kinetics studies on phase transitions of the Bi UPD layer between the (2 x 2) and (p x square root[3]) structures using surface X-ray diffraction.

The kinetics of the phase transition between the (2 x 2) and (p x square root[3])-Bi structures on Au(111) was investigated using electrochemical methods and time-resolved surface X-ray diffraction. The temporal changes in the current value and the diffracted X-ray intensity that originated from the (2 x 2)-Bi overlayer were monitored during the phase transitions at various over-potentials. The phase transition models and kinetics parameters were deduced from each of the current and X-ray intensity transient curves. We also carried out comparative studies of the phase transition from the structural and electrochemical points of view. For the (p x square root[3]) --> (2 x 2) phase transition, the phase transition models determined by the X-ray and electrochemical measurements were a surface-diffusion controlled instantaneous nucleation-growth process and a Langmuir process, respectively. For the reverse transition, the phase transition models determined by X-ray and electrochemical measurements were a Langmuir adsorption process and a surface diffusion controlled nucleation-growth process, respectively. Our results revealed that the current transient curve does not always reflect the phase transition model in both cases and suggest that a structural analysis is fundamental in the phase transition studies. The disagreements between the phase transition models and their mechanisms are discussed.