Structure and Desorption Kinetics of Acetonitrile Thin Films on Pt(111) and on Graphene on Pt(111)

Acetonitrile thin films were prepared on Pt(111) and on graphene on Pt(111) using molecular beam techniques. Temperature programed desorption (TPD) experiments of acetonitrile on Pt(111) displayed first-order kinetics for monolayer desorption and zero-order kinetics for multilayer desorption. We used reflection adsorption infrared spectroscopy (RAIRS) to characterize the orientation of the monolayer on Pt(111). Acetonitrile molecules have been shown to adopt a bridging structure where the C≡N bond breaks to form two single bonds to underlying platinum atoms. When Tdₑₚₒₛᵢₜᵢₒₙ ≥ 120 K, our RAIRS spectra were consistent with this bridging configuration. However, when Tdₑₚₒₛᵢₜᵢₒₙ was lowered to 60 K, molecules retained the C≡N bond and adopted a vertical orientation with the nitrogen closest to the platinum. On graphene, the TPD experiments show that both the monolayer and multilayer desorb with zero-order kinetics. Our RAIRS spectra suggest that the monolayer consists of acetonitrile molecules oriented with their molecular axes parallel to the graphene substrate. This orientation did not change when Tdₑₚₒₛᵢₜᵢₒₙ was varied from 60 to 120 K. Our results provide a more complete picture of how acetonitrile molecules interact with Pt(111) and with graphene surfaces.