AVS 53rd International Symposium
    Surface Science Thursday Sessions
       Session SS1-ThM

Paper SS1-ThM9
Water Adsorption from Submonolayer to Multilayer Coverage on TiO2(110) and Fe2O3(0001) by In Situ Spectroscopy

Thursday, November 16, 2006, 10:40 am, Room 2002

Session: Reactivity of Oxide Surfaces I
Presenter: S. Yamamoto, Stanford Synchrotron Radiation Laboratories
Authors: G. Ketteler, Chalmers University of Technology, Sweden
S. Yamamoto, Stanford Synchrotron Radiation Laboratories
H. Bluhm, LBNL
T. Kendelewicz, Stanford University
K. Andersson, Stanford Synchrotron Radiation Laboratory
D.E. Starr, LBNL
G.E. Brown Jr., Stanford University
A. Nilsson, Stanford Synchrotron Radiation Laboratory
M. Salmeron, LBNL
Correspondent: Click to Email
Despite their importance in environmental, chemical and biological sciences, the bonding structure of water at interfaces in equilibrium with vapor remains poorly understood. We have studied the adsorption of water on a rutile TiO@sub 2@(110) and hematite Fe@sub 2@O@sub 3@(0001) single crystal from submonolayer to several multilayer coverage by in situ X-ray photoemission at temperatures above 270 K in the presence of vapor pressures of up to 1.5 Torr. We find that water adsorption occurs in distinct steps that are determined by the availability of surface sites for hydrogen bonding. On TiO@sub 2@(110), very acidic sites such as O-vacancy defects present in high vacuum conditions disappear rapidly to form ~0.25 ML of OH species on bridging positions when the vapor pressure becomes larger than 10@super -4@ Torr. A similar coverage of molecular water binds strongly to these acidic sites by charge transfer from the oxygen atom of water to the hydrogen atoms of the OH group (hydrogen bonding). This acts as a nucleation site for further water adsorption on both sides along the Ti troughs. We find no indication of a hydroxylation of regular terrace sites. The enthalpy and entropy of condensation over a wide coverage range were determined by analysis of equilibrium isobars. On Fe@sub 2@O@sub 3@(0001), defects as well as regular terrace sites are hydroxylated prior to water adsorption. Water adsorption occurred for higher relative humidity than on TiO@sub 2@(110) (5x10@super -3@ vs. 10@super -6@ % RH), and most likely it is bound to terrace OH groups. Based on these findings and NEXAFS we were able to propose a structure model for the H-bonded structures that determine water adsorption on rutile(110) and hematite(0001) from submonolaye to multilayer coverage. We will correlate the affinity of oxide surfaces to react with and bind water with the acido-basic properties of different surface functionalities.