HREELS, TPD and XPS study of the interaction of water with the α-Cr 2O 3(001) surface

Abstract

The interaction of water with the (001) surface of α-Cr 2O 3 was examined with temperature programmed desorption (TPD), high resolution electron energy-loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). Two α-Cr 2O 3(001) surfaces were examined, both of which were grown on α-Al 2O 3(001) substrates using oxygen plasma-assisted molecular beam epitaxy (MBE). The two surfaces differed in that one was grown with α-Fe 2O 3 interlayers whereas the other was grown directly on α-Al 2O 3(001). The in-plane lattice spacing of the α-Cr 2O 3(001) surface on α-Fe 2O 3/α-Al 2O 3(001) was 2% expansively strained relative to the unstrained α-Cr 2O 3(001) surface grown directly on α-Al 2O 3(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 α-Cr 2O 3(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 Cr 3+ 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 ν(OH) mode at 3600 cm −1 and the other of which is presumably a bridging group with a ν(OH) mode at 2885 cm −1. These losses shift to 2645 and 2120 cm −1 with D 2O 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 ν(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 H 2O dissociative channel but do not block the molecular adsorption channel.