Tungsten oxide is an n-type semiconductor with wide band gap energy. This metal oxide has many applications such as electrochromic, photocatalyst and gas sensors. Among the various properties, chemochromic properties in the presence of hydrogen gas have much attention. Hydrogen is a colorless, odorless and tasteless gas. As a result, more attention is needed to be paid to hydrogen as a clean fuel in household and traortations applications. It is a highly flammable fuel with a wide combustion range of 4–75%. Therefore, detection and leakage control of this gas is a challenging subject. In the presence of hydrogen gas changes the optical properties of tungsten oxide thin films and the colure of layer convert from a traarent to an absorbing blue state. This property makes a good option the tungsten oxide films as optical hydrogen sensor. In this study, at first the peroxopolytungstic acid (P-PTA) sol was prepared according to the method of Kudo et al. Then, the traarent orange sol has been spin- coated on glass substrates. In order to study the effect of annealing temperature on different properties of samples, the as-prepared films were annealed at different temperatures 100, 200, 300 and 400 ?C. In order to deposit palladium nanoparticles on tungsten oxide surface, drops of a 0.2 g/l PdCl 2 solution were transferred to the surface of layers by hydrogen reduction. By this method, the samples were strongly and uniformly colored. Among the samples of different annealing temperature, the sample of 400 ?C was better considerably in gasochromic switching rate as well as coloring depth. Structural, chemical, surface morphology, optical and hydrophilic properties of layers is examined. Amorphous WO 3 structure is observed in X-ray Diffraction (XRD) patterns of dried gel and un-annealed films, However heat treatment at 400 ?C, 500 ?C results in formation of monoclinic WO 3 structure, respectively. By the Fourier transform infrared (FTIR) spectroscopy were determined the chemical bonds of the samples. In order to study the morphology of the films surfaces, AFM was used. The result of this analysis showed that with increasing of annealing temperature increases the degree of crystallization. In order to study on the hydrophilicity property of layers, contact angle of water droplet with surface is measured by the contact angle measurement. The optical properties of layers were measured with a spectrophotometer (UV-Vis) at the spectral range 190–1100 nm. Keywords: Tungsten oxide, Sol-gel, Chemochromic properties, Palladium catalyst, Annealing temperature, Hydrogen reduction.
