Structural, vibrational, and gasochromic properties of porous WO3 films templated with a sol-gel organic-inorganic hybrid

The structure and the gasochromic properties of sol-gel-derived WO3 films with a monoclinic structure (m-WO3) were studied by focusing attention on the size of the monoclinic grains. The size of the m-WO3 grains is modified by the addition of an organic-inorganic hybrid to the initial peroxopolytungstic acid (W-PTA) sols which are based on chemically bonded poly-(propylene glycol) to triethoxysilane end-capping groups (ICS-PPG). The results obtained with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the heat treatment (500 oC) of WO3/ICS- IPG (0.5, 1, 2, 5, and 10 mol%) composite films results in a change of their morphology, and nanodimensional pores are formed between the grains. High-resolution TEM (HRTEM) analysis revealed the presence of an amorphous phase on the outside of the m-WO3 grains, whereas energy-dispersive x-ray spectra (EDXS) showed that this amorphous phase contained W and Si. Impregnation of the WO3/ICS-PPG film with H2PtCl6/i-propanol solution followed by heat treatment at 380 oC gave the films their gasochromic properties. Infrared and Raman spectroscopic studies of the WO3/ICS-PPG film confirmed the results of the corresponding HRTEM and EDXS analysis. In situ UV/Vis and in situ IR, spectra of the films were measured in hydrogen and in air, and coloring/bleaching changes and the corresponding kinetics were assessed. The IR spectra of gasochromically color d films showed that the mesoporous WO3/ICS-PPG (1 mol%) film transforms to tetragonal HxWO3 bronze. The IR spectra of the HxWO3 bronze are discussed with the aim to establish the existence of the metal-OH vibrations of gasochromically formed oxyhydroxide tungsten bronze. (author)