Surface chemistry of glycine on Pt{111} in different aqueous environments

Abstract

Adsorption of glycine on Pt{111} under UHV conditions and in different aqueous environments was studied by XPS (UHV and ambient pressure) and NEXAFS. Under UHV conditions, glycine adsorbs in its neutral molecular state up to about 0.15 ML. Further deposition leads to the formation of an additional zwitterionic species, which is in direct contact with the substrate surface, followed by the growth of multilayers, which also consist of zwitterions. The neutral surface species is most stable and decomposes at 360K through a multi-step process which includes the formation of methylamine and carbon monoxide.When glycine and water are co-adsorbed in UHV at low temperatures (<170K) inter-layer diffusion is inhibited and the surface composition depends on the adsorption sequence. Water adsorbed on top of a glycine layer does not lead to significant changes in its chemical state. When glycine is adsorbed on top of a pre-adsorbed chemisorbed water layer or thick ice layer, however, it is found in its zwitterionic state, even at low coverage. No difference is seen in the chemical state of glycine when the layers are exposed to ambient water vapor pressure up to 0.2Torr at temperatures above 300K. Also the decomposition temperature stays the same, 360K, irrespective of the water vapor pressure. Only the reaction path of the decomposition products is affected by ambient water vapor.