Hydroxylation of ultra-thin films of α-Cr 2O 3(0 0 0 1) formed on Cr(1 1 0)

Abstract

X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) data on five to six monolayer thick anhydrous films of α-Cr 2O 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 surface hydroxylation of the thin films resulting from water dissociation. The XP O1s region of the hydroxylated film is characterized by the growth of a higher binding energy peak (531.4 eV) assigned to hydroxide groups in addition 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 corresponding 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 third peak assigned to Cr(III) cations bonded to hydroxide groups (577.2 to 577.4 eV). The STM data show that the terraces and defects of the anhydrous oxide film are equally modified by hydroxylation indicating that the reaction is not defect specific and that water is dissociated on the regular sites of the α-Cr 2O 3(0 0 0 1) terraces. A disordered and corrugated surface is produced by hydroxylation which suggests significant OH-induced surface 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⩽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 XPS intensity attenuation can be used to estimate the surface concentration of the hydroxylated surface. With increasing water doses, Cr(III) enrichment is observed at the oxide surface indicating OH-induced oxidation of the metallic substrate. A plateau is reached corresponding to a fully hydroxylated oxide film. The ratio of hydroxide groups to hydroxylated cations is found to be about 1 from the XPS data. Upon annealing, OH groups recombine and water desorbs with a relatively large profile consistent with the disordered structure of the hydroxylated surface.