IR Spectroscopic investigations of gasochromic and electrochromic sol-gel-derived peroxotungstic acid/ormosil composite and crystalline WO3 films

We studied the gasochromic effect of amorphous peroxopolytungstic acid (W-PTA), W-PTA (ormosil) and crystalline WO3 films. These latter films were prepared after a heat treatment of W-PTA/ormosil films at 450°C. The ormosil served as a template, providing the monoclinic WO3 films with adequate porosity. The spill-over effect was attained by impregnating the porous WO3 crystalline films with H2PtCl6 followed by a heat treatment at 380°C. The amorphous films became gasochromic with the addition of PdCl2 to the corresponding W-PTA and W-PTA/ormosil sols.Structural features of all the films were studied with the help of infrared (IR) spectroscopy and transmission electron microscopy (TEM). In situ IR spectra of the films, performed in the presence of reducing (H2/Ar) and oxidising (O2/Ar) gases, revealed a reversible transformation of the monoclinic to the tetragonal HxWO3 phase. At the same time the coloration (reduction) of the amorphous films was accompanied by the formation of coordinated water molecules and increased numbers of W=O bonds. Gasochromic colouring/bleaching changes and the corresponding kinetics were assessed from in situ UV-visible transmission measurements on the films.