Analysis of the physical structure of nanometric WOx/ZrO2 using electrical measurements

The structure of WOx/ZrO2 was studied by X-ray diffraction, laser Raman spectroscopy and measurement of electrical properties using impedance spectroscopy. Results from classical analysis were consistent with a structure comprising nanometric ZrO2 particles covered by a WOx surface layer. Based on this information we modelled the impedance spectra as the superposition of two contributions. The values of the electrical properties estimated from our model indicated the presence of a dielectric and a semiconductor. The first phase had electrical properties closely matching the reported values for ZrO2, whereas the semiconductor phase was assigned to a non-stoichiometric WOx phase. The tungsten-bearing species had temperature-dependent properties and play an important role in the ac response of the studied system and also in oxidation-reduction processes. The activation energy is 1.3 eV for ZrO2, whereas WOx has two slightly different energy values (2.4 and 2.1 eV) in different temperature ranges. Use of impedance spectroscopy provides valuable information about the surface structure as well as the contribution of the bulk, which may be important in catalysis. (orig.)