Methanol adsorption and dissociation on GaP(110) studied by ambient pressure X-ray photoelectron spectroscopy

Abstract

Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to investigate methanol (CH₃OH) adsorption and reaction on the GaP(110) surface. Exposure of CH₃OH to GaP(110) at room temperature led to the formation of at least four different surface species as indicated by analysis of C 1s and O 1s XPS features. By combining AP-XPS data with density functional theory calculations, the surface species were identified as methoxy (CH₃O*), formaldehyde (CH₂O*), and paired methanol (p-CH₃O*H) and methoxy (p-CH₃O*) species, where “paired” means that they belong to a hydrogen-bonded methoxy-methanol complex. Asterisk * here indicates an adsite. The formation of CH₂O* via the dehydrogenation of CH₃O* was shown to be limited by the availability of vacant phosphorus (P) sites on GaP(110). With an increase in CH₃OH pressure, the fractional coverage of CH₃O* species reached 0.55, and the surface P sites were completely saturated with hydrogen. Under a constant CH₃OH pressure of 0.5 Torr, the surface concentration of the paired species and of CH₂O* remained constant until 400 K. At higher temperatures, thermally driven reactions led to a significant increase in the concentration of surface CH_x* species, which suggests that C-O bond cleavage of the CH₃O group is the dominant decomposition mechanism on GaP(110). Based on the reactivity of GaP(110) toward CH₃OH dehydrogenation, elevated temperatures and CH₃OH pressures may be used to functionalize this surface.