HREELS, TPD and XPS study of the interaction of water with the α-Cr2O3(001) surface

The interaction of water with the (001) surface of alpha-Cr2O3 was examined with temperature programmed desorption (TPD), high resolution electron energy-loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). Two alpha-Cr2O3(001) surfaces were examined, both of which were grown on alpha-Al2O3(001) substrates using oxygen plasma-assisted molecular beam epitaxy (MBE). The two surfaces differed in that one was grown with alpha-Fe2O3 interlayers whereas the other was grown directly on alpha-Al2O3(001). The in-plane lattice spacing of the alpha-Cr2O3(001) surface on alpha-Fe2O3/alpha-Al2O3(001) was 2% expansively strained relative to the unstrained alpha-Cr2O3(001) surface grown directly on alpha-Al2O3(001). Both the strained and unstrained surfaces exhibited similar water TPD behavior, with the possible exception that the desorption states of water on the strained surface were shifted slightly to lower temperatures relative to those on the unstrained surface. Water adsorbs on alpha-Cr2O3(001) in both molecular and dissociative states, with the former desorbing in TPD at 295 K and the latter at 345 K. TPD uptake measurements and XPS data suggest that each surface Cr3+ atom has the capacity to bind two water molecules, one in a molecular state and one in a dissociative state. Water in the dissociative state is comprised of two distinct OH groups based on HREELS, one of which is a terminal group with a nu(OH) mode at 3600 cm(-1) and the other of which is presumably a bridging group with a nu(OH) mode at 2885 cm(-1). These losses shift to 2645 and 2120 cm(-1) with D2O adsorption. The low loss energy for the bridging OH/OD group indicates its involvement in a very strong hydrogen-bonded interaction with another species, presumably the oxygen atom of the terminal OH group. This pairing behavior is likely responsible for the first-order desorption kinetics observed for the recombinative desorption state at 345 K. The hydrogen-bonding interaction is unusually strong, as exemplified by the very low nu(OH) frequency for the bridging OH group. Studies on the oxygen pre-exposed surface indicate that oxygen atoms, formed by O-2 dissociation, block the H2O dissociative channel but do not block the molecular adsorption channel. (C) 2000 Elsevier Science B.V. All rights reserved.