Gasochromic behavior of sol-gel derived Pd doped peroxopolytungstic acid (W-PTA) nano-composite films

The behavior of sol-gel prepared thin films exhibiting a gasochromic effect ; i.e., a reversibly change in colour from transparent when in air to blue when in H-2, has been studied. The films were prepared from a Pd (PdCl2) do ped peroxopolytungstic acid sol using a dip-coating technique. Transmission electron microscopy together with selected area electron diffractrometry r evealed that the films consist of monoclinic and hexagonal nanocrystalline grains (similar to 2-5 nm) embedded in an amorphous phase. This amorphous h ydrated phase was established using Infrared (IR) and Raman spectroscopy. C haracteristic vibrations observed in the in-situ IR spectra of the coloured and bleached states revealed the presence and the importance of terminal W =O and W-OH2 groups in the colouring of the films. Colouring/bleaching chan ges of Pd doped W-PTA films observed using in-situ UV-Vis spectroscopy are described in terms of Pd concentration, and the number of reducing/oxidisin g cycles. The rate of colouring/bleaching is greater for films containing a higher concentration of Pd but the change in the optical density does not increase, i.e., shows saturation.