Nanostructured CuWO4/WO3-x films prepared by reactive magnetron sputtering for hydrogen sensing

High-purity films consisting of copper tungstate (CuWO4) and sub-stoichiometric tungsten oxide (WO3-x) were prepared by reactive sputter deposition. An original two-step deposition process was applied for their synthesis. First, a tungsten oxide thin film was deposited by dc magnetron sputtering from a W target in an Ar + O-2 gas mixture, afterward, rf sputtering of a Cu target in an Ar + O-2 gas mixture was employed to form a discontinuous CuWO4 layer at the top. This results in a formation of nanostructured branched islands of the tungstate. Bilayers with various layer thicknesses were investigated for the sensitivity to hydrogen gas as a conductometric sensor. The sensitivity changes remarkably with the thicknesses of individual layers. The maximum sensitivity was observed for the films with a layer thickness ratio of 5 nm/20 nm. The response was enhanced more than eight times compared to a 20 nm-thick tungsten oxide alone film. An explanation based on the formation of nano-sized n-n junctions is provided. In addition, a microscopy study of the bilayer growth is presented in detail. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.