High-Speed Surface X-Ray Diffraction for Monitoring Structural Changes of Electrode Surfaces: An Application to Methanol Oxidation on Pt(111)

Abstract

To deeply understand how electrochemical reactions proceed at the electrode/electrolyte interfaces, it is necessary to reveal the interface structures at atomic resolution and in real time. Scanning probe microscopies, synchrotron X-ray scattering and absorption spectroscopy, and surface-sensitive optical methods are often used to observe the interface structures in-situ. Among them, X-ray crystal truncation rod (CTR) scattering, which is one kind of surface X-ray scattering, is a powerful technique to determine the atomic structures across the interface. CTR scattering is widely used to determine static structures, but its application to time-resolved measurements is limited. In the previous time-resolved measurements [e.g. Tamura et al., J. Phys. Chem. B 108, 1992 (2004)], they monitored a specific point in reciprocal space during the electrochemical reaction. Such fixed-single-point measurements can monitor structural changes qualitatively, such as roughening/flattening of the electrode surface and a formation/destruction of a superstructure formed by the lateral periodic array of the adsorbates. However, these measurements can not reveal small positional changes (0.1~0.01 Å) of the interface atoms quantitatively, which often occur with a change of surface adsorbate species or a change of the oxidation state of the adsorbates and/or of the electrode surface. In order to determine the structural change quantitatively, simultaneous measurements of a CTR profile, whose range is wide enough to allow the structural analysis at atomic scale, is needed. In this study, we used an energy-dispersive X-ray CTR scattering technique [T. Matsushita et al., J. Appl. Phys. 110, 102209 (2011)], which enables the simultaneous measurement of a wide range of a CTR profile, to study the electrochemical oxidation of methanol on a Pt(111) electrode. In the methanol oxidation, it is widely accepted that CO molecules are strongly adsorbed on the Pt surface to cause the overpotential. The time-resolved measurement revealed that the height of the topmost atomic layer of Pt was changed by up to about 0.1 Å in the methanol oxidation. The behavior is closely related to the CO adsorption density, referring to the previous infrared spectroscopy measurements [Xia et al., Electrochem. Acta 41, 711 (1996)]. The CO-induced structural change shows a hysteresis with respect to the potential loop (0-1.0V vs. SHE), strongly supporting that the CO-adsorbed surface causes the overpotential. The CTR profile measurements during the potential step chronoamperometry indicates that the formation of a CO-desorbed domain takes several seconds to occur after a positive potential step (0 to 0.8 V), while the CO-rich domain forms within a time resolution of 1 s after a negative potential step (0.95 to 0.6 V).