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

The interaction of water with the (001) surface of alpha-Cr2O3 was examined with temperature programmed desorption (TPD), high resolution electron ene rgy-loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). Two alpha-Cr2O3(001) surfaces were examined, both of which were grown on al pha-Al2O3(001) substrates using oxygen plasma-assisted molecular beam epita xy (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 i n-plane lattice spacing of the alpha-Cr2O3(001) surface on alpha-Fe2O3/alph a-Al2O3(001) was 2% expansively strained relative to the unstrained alpha-C r2O3(001) surface grown directly on alpha-Al2O3(001). Both the strained and unstrained surfaces exhibited similar water TPD behavior, with the possibl e exception that the desorption states of water on the strained surface wer e shifted slightly to lower temperatures relative to those on the unstraine d surface. Water adsorbs on alpha-Cr2O3(001) in both molecular and dissocia tive states, with the former desorbing in TPD at 295 K and the latter at 34 5 K. TPD uptake measurements and XPS data suggest that each surface Cr3+ at om has the capacity to bind two water molecules, one in a molecular state a nd one in a dissociative state. Water in the dissociative state is comprise d of two distinct OH groups based on HREELS, one of which is a terminal gro up 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 26 45 and 2120 cm(-1) with D2O adsorption. The low loss energy for the bridgin g OH/OD group indicates its involvement in a very strong hydrogen-bonded in teraction with another species, presumably the oxygen atom of the terminal OH group. This pairing behavior is likely responsible for the first-order d esorption kinetics observed for the recombinative desorption state at 345 K . The hydrogen-bonding interaction is unusually strong, as exemplified by t he very low nu(OH) frequency for the bridging OH group. Studies on the oxyg en pre-exposed surface indicate that oxygen atoms, formed by O-2 dissociati on, block the H2O dissociative channel but do not block the molecular adsor ption channel. (C) 2000 Elsevier Science B.V. All rights reserved.