Low-Temperature Formation of a WO₃ Thin Film by the Sol-Gel Method Using Photo-Irradiation and Fabrication of a Flexible Hydrogen Sensor

Abstract

Hydrogen has been recently attracted much attention with respect to high energy-conversion efficiency and low environmental burden. However, hydrogen gas is dangerous due to an explosive gas and a fast combustion rate. Therefore, the development of hydrogen sensor with high accuracy and reliability that can detect hydrogen easily is required. Especially, a flexible hydrogen sensor is useful because it has a high degree of freedom with respect to the shape of location in which the sensor is to be located. A flexible hydrogen sensor—namely, a WO 3 thin film formed on a PET film by the sol-gel method using photo irradiation—based on gasochromism of WO 3 was developed. By irradiating a thin film, which was prepared by using WO 3 precursor solution synthesized by the sol-gel method, with ultraviolet rays, a high-purity WO 3 film could be prepared on PET at low temperature. The sensor was structured as a polystyrene (PS) film containing palladium (Pd) laminated on a WO 3 film. The WO 3 layer was porous, so the PS containing Pd atoms solution penetrated the WO 3 layer. WO 3 reacted with hydrogen gas and instantly turned blue as the transmittance of the WO 3 layer changed. The sensor showed high reactivity even for hydrogen concentration below 4% (1%, 0.5%, 0.25%, and 0.1%), which was the lower limit of hydrogen ignition, and a linear relationship between hydrogen concentration and change in transmittance was found. Moreover, the resistance of the WO 3 film significantly and instantaneously changed due to hydrogen-gas exposure, and the hydrogen concentration and resistance change showed a linear relationship. It is therefore possible to quantitatively detect low concentrations of hydrogen by using changes in transmittance and resistance as indices. Since these changes occur selectively under hydrogen at room temperature and normal pressure, they form the basis of a highly sensitive hydrogen sensor. Since the developed sensor is flexible, it has a high degree of freedom with respect to the shape of location in which the sensor is to be installed