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 H₂, has been studied. The films were prepared from a Pd (PdCl₂) doped peroxopolytungstic acid sol using a dip-coating technique. Transmission electron microscopy together with selected area electron diffractrometry revealed that the films consist of monoclinic and hexagonal nanocrystalline grains (approximately 2-5 nm) embedded in an amorphous phase. This amorphous hydrated phase was established using Infrared (IR) and Raman spectroscopy. Colouring/bleaching changes 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/oxidizing cycles. The rate of colouring/bleaching is greater for films confining a higher concentration of Pd but the change in the optical density does not increase, i.e., shows saturation.