Metal Oxide Surfaces; Beyond UHV

Authors: Mahmoud Ahmed (University of Manchester)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: December 2014
Diamond Proposal Number(s): 6695 , 7622 , 7842 , 7985

Abstract: In this thesis, three experimental studies of metal oxide surfaces beyond UHV conditions are presented, in an attempt to bridge the so-called ‘pressure’ gap. In the first of these studies, surface X-ray diffraction has been employed to elucidate the surface structure of #-Cr2O3(0001) as a function of water partial pressure at room temperature. In ultra high vacuum, subsequent to exposure to ~ 2000 Langmuir of H2O, the surface is determined to be terminated by a partially occupied double layer of chromium atoms. No evidence of adsorbed OH/H2O is found under this regime. At a water partial pressure of ~ 30 mbar, a surface termination involving a single OH/H2O species bound atop to each surface Cr atom is obtained. Surface X-ray diffraction has also been employed to elucidate the geometry of the TiO2(011)/H2O interface at room temperature. In ultra high vacuum, a surface structure in quantitative agreement with previously published studies is found. Most notably at a water partial pressure of ~ 30 mbar, the interface geometry is determined to be consistent with the predicted structure emerging from ab initio calculations in which the surface undergoes transformation from a (2×1) reconstruction to a (1×1) unit cell. In the final investigation a procedure for non-UHV wet-chemical preparation of TiO2 single crystal substrates is detailed. The potential of this recipe is demonstrated through application to rutile-TiO2(110) and rutile-TiO2(011) substrates. Characterisation with scanning probe microscopy, low energy electron diffraction and auger electron spectroscopy, indicate that flat, well-ordered, carbon-free surfaces can be generated. Notably, in contrast to the (2×1) surface unit cell found for TiO2(011) prepared in ultra high vacuum, wet-chemical preparation results in a (4×1) termination; wet-chemically prepared TiO2(110) displays an unreconstructed (1x1) surface.

Subject Areas: Chemistry, Materials

Instruments: I07-Surface & interface diffraction

Other Facilities: XMaS-ESRF

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