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

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.