Water Adsorption on Pt(211): Insights from Heterodyne-Detected Vibrational Sum-Frequency Generation Spectroscopy

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In this paper, water adsorption on Pt(211) under ultrahigh vacuum conditions is studied by heterodyne-detected vibrational sum-frequency generation spectroscopy. The imaginary parts of the surface nonlinear susceptibility (Imχ (2) ) of intact water are successfully obtained in the OH stretching region for submonolayer water coverage, showing a negative band at 3000–3450 cm –1, which indicates an H-down configuration with the proton facing the Pt substrate. Although it is generally believed that water adsorption begins at the step, Imχ (2) signals do not show such a behavior and reveal no difference between the water at the step and the terrace. In addition, the bandwidth of the negative band is much larger than that observed for Pt(533) with a lower step density. These experimental results suggest that wetting on Pt(211) occurs simultaneously on both the step and terrace and that the water configuration is more inhomogeneously broadened than on the surfaces with the lower step density. DFT calculations predict possible adsorption structures on Pt(211) and suggest that the negative Imχ (2) signal is due to H-down water molecules at the terrace accepting hydrogen bonds from water molecules directly decorating the step edge.

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