Hydroxylation of ultra-thin films of alpha-Cr2O3(0001) formed on Cr(110)

X-ray photoelectron spectroscopy (XPS). thermal desorption spectroscopy (TD S), low energy electron diffraction (LEED) and scanning tunneling microscop y (STM) data on five to six monolayer thick anhydrous films of rCr(2)O(3)(0 0 0 1) epitaxially grown on Cr(1 1 0) single-crystal surfaces and exposed to water vapor at 300 K are reported. The results evidence reversible surfa ce hydroxylation of the thin films resulting from water dissociation. The X P Ols region of the hydroxylated film is characterized by the growth of a h igher binding energy peak (531.4 eV) assigned to hydroxide groups in additi on to the main peak corresponding to the O-2 anions of the oxide film (530. 7 to 530.9 eV). The Cr 2p(3,2) region is characterized by the two peaks cor responding to the substrate Cr(0) atoms (574.4 eV) and the Cr(III) cations (576.5 to 576.7 eV) initially present in the anhydrous oxide film plus a th ird peak assigned to Cr(III) cations bonded to hydroxide groups (577.2 to 5 77.4 eV). The STM data show that the terraces and defects of the anhydrous oxide film are equally modified by hydroxylation indicating that the reacti on is not defect specific and that water is dissociated on the regular site s of the alpha -Cr2O3(0 0 0 1) terraces. A disordered and corrugated surfac e is produced by hydroxylation which suggests significant OH-induced surfac e diffusion and rearrangement of the surface cation and anions planes. The LEED data show that the atomic lattice of the bulk of the oxide film is not modified in the conditions of hydroxylation tested (water doses less than or equal to 112.5 L) in agreement with the unmodified nanostructure of the oxide film observed by STM. An atomic layer by atomic layer model of the XP S intensity attenuation can be used to estimate the surface concentration o f the hydroxylated surface. With increasing water doses, Cr(III) enrichment is observed at the oxide surface indicating OH-induced oxidation of the me tallic substrate. A plateau is reached corresponding to a fully hydroxylate d oxide film. The ratio of hydroxide groups to hydroxylated cations is foun d to be about 1 from the XPS data. Upon annealing, OH groups recombine and water desorbs with a relatively large profile consistent with the disordere d structure of the hydroxylated surface. (C) 2001 Elsevier Science B.V. All rights reserved.