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 WO3 thin film formed on a PET film by the sol-gel method using photo irradiation—based on gasochromism of WO3 was developed. By irradiating a thin film, which was prepared by using WO3 precursor solution synthesized by the sol-gel method, with ultraviolet rays, a high-purity WO3 film could be prepared on PET at low temperature. The sensor was structured as a polystyrene (PS) film containing palladium (Pd) laminated on a WO3 film. The WO3 layer was porous, so the PS containing Pd atoms solution penetrated the WO3 layer. WO3 reacted with hydrogen gas and instantly turned blue as the transmittance of the WO3 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 WO3 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
Highlights
In recent years, owing to the problems of global warming due to mass consumption of fossil fuel and the depletion of energy resources, development of alternative energy sources as a replacement for fossil energy has become urgent
By irradiating a thin film, which was prepared by using WO3 precursor solution synthesized by the sol-gel method, with ultraviolet rays, a high-purity WO3 film could be prepared on PET at low temperature
The WO3 thin film was used as an element for detecting hydrogen, and the “gasochromism” property of the thin film was used
Summary
In recent years, owing to the problems of global warming due to mass consumption of fossil fuel and the depletion of energy resources, development of alternative energy sources as a replacement for fossil energy has become urgent. Fardindoost et al reported that a WO3 thin film doped with Pd can be formed by the sol-gel method, and the resistance of the film changes significantly on contact with hydrogen gas [17] In both cases, heat treatment at high temperature was used to produce the WO3 films, and both films were formed on a substrate with high heat resistance, such as glass or alumina. In this paper, regarding our aim of manufacturing a high-sensitivity hydrogen sensor with flexibility at low cost, 1) the development of low-temperature technology for depositing a WO3 film by the sol-gel method with photo-irradiation and 2) forming the WO3 film on a PET film to form a flexible hydrogen sensor are reported
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