Tungsten oxide plays an important role in a variety of electrochromic devices, catalysts and chemical sensors. Doping of the surface of these devices by any active metal can be used to control their sensitivity and selectivity. Fundamental studies of the dependency of physical and chemical properties on the device structure are often performed on well-defined epitaxial (model) systems.
The epitaxial tungsten oxide thin films were prepared by radio-frequency plasma oxidation of W(110) surface followed by re-crystallisation at high temperature. Reflection High-Energy Electron Diffraction (RHEED) analysis showed that the films are composed of relatively large crystal grains having pseudo-cubic structure and (111) epitaxial plane parallel to the sample surface. The re-crystallisation process led to the partial reduction of tungsten oxide film. The lack of oxygen atoms in the structure is compensated by formation of crystallographic shear planes along <120> and <001> crystallographic directions.
In further step of experiment, the epitaxial tungsten oxide / W(110) system was used as a substrate for deposition of a small amount of Pd. The two population of three-dimensional Pd clusters having (111) epitaxial plane and exhibiting "double positioning" were observed. The number of clusters in each population reflected mutual symmetry and orientation of the tungsten oxide surface lattice and Pd epitaxial plane.
The electronic structure of the Pd / tungsten oxide / W(110) system before and after the Pd deposition was investigated by means of X-ray Photoelectron Spectroscopy (XPS) and Synchrotron Radiation Photoelectron Spectroscopy (SRPES) methods. The epitaxial tungsten oxide thin film exhibited well-defined oxidation states indicated by narrow components in the W4f spectrum which were not observed in the case of amorphous phase of tungsten oxide. The Pd deposition led to significant changes in the valence band structure but the detail analysis of W4f and Pd3d lines does not show direct interaction of Pd and W species. The epitaxial Pd / tungsten oxide / W(110) system is suitable for further model studies of heterogeneous catalysis and gas sensing properties of tungsten oxide based catalysts and gas sensors.