Tungsten oxide thin films were prepared by an inorganic-sol-gel dip-coating process, where the sol was obtained by adding citric acid, as chelating agent, to the ammonia solution of tungstic acid. The resultant thin films were a mixture of monoclinic and tetragonal phases of WO3 and, after being pretreated at 600°C and sintered at 650°C, the average grain size of the polycrystalline films was about 500 nm. The gas-sensing properties of WO3 thin films were tested at temperatures ranging from 500° to 600°C and in nitrogen gas containing 5vol% O2 or 5vol% H2. The WO3 sensors exhibited a good sensitivity and response speed at the temperature of 550°C.
Sol–gel derived mesoporous Pt and Cr-doped WO3 thin films: the role played by mesoporosity and metal doping in enhancing the gas sensing properties