Proton transfer in water–hydroxyl mixed overlayers on Pt(1 1 1): Combined approach of laser desorption and spatially-resolved X-ray photoelectron spectroscopy

Abstract

Proton transfer in water–hydroxyl mixed overlayers on a Pt(1 1 1) surface was studied by a combination of laser induced thermal desorption (LITD) method and spatially-resolved X-ray photoelectron spectroscopy (micro-XPS). The modulated pattern OH + H 2O/H 2O/OH + H 2O was initially prepared by the LITD method; vacant area with a 400 μm width was first formed in the mixed OH + H 2O overlayer by irradiation of focused laser pulses, and followed by refilling the vacant area with pure H 2O. Spatial distribution changes of OH and H 2O were measured as a function of time with the micro-XPS technique, which indicated that H 2O molecules in the central region flow into the OH + H 2O region. From quantitative analyses using a diffusion equation, we found that the proton transfer in the mixed overlayer consists of at least two pathways: direct proton transfer from H 2O to OH in the nearest site and the proton transfer to the next-nearest site via H 3O + formation. The time scale of first and second path was estimated to be 5.2 ± 0.9 ns and 48 ± 12 ns at 140 K, respectively. In the presence of water capping layer, however, the rate of proton transfer is reduced by an order of magnitude, which would be explained by peripatetic behavior of proton into H 2O capping layer.