Hydrogen behavior in gasochromic tungsten oxide films investigated by elastic recoil detection analysis

Changes in the composition and crystalline structure of gasochromic tungsten oxide films resulting from the incorporation of hydrogen were investigated; the oxide films were prepared by reactive RF magnetron sputtering on SiO₂ and glassy carbon substrates simultaneously. X-ray diffraction analysis of the deposited films at 600 °C showed a uniaxial oriented structure in the (0 1 0) plane of monoclinic WO₃ for both substrates. The elastic recoil detection analysis (ERDA) and Rutherford backscattering spectroscopy (RBS) for the films on glassy carbon revealed that the hydrogen impurity was uniformly distributed up to a concentration of 0.24 H/W. The Pd-coated films on SiO₂ turned blue when they were exposed to a mixture of Ar and 5% H₂ gases. When the sample became colored, the hydrogen concentration in the film increased to 0.47 H/W and the crystalline structure of the film changed from monoclinic to tetragonal. These results indicated that the gasochromic coloration of the tungsten oxide films coincided with incorporation of hydrogen atoms into the crystalline lattice, corresponding to the formation of hydrogen tungsten bronze (H_xWO₃).