| In the current, hydrogen gas is regarded as one of the best clean and efficient energy have been expanding into various fields such as fuel cell vehicles, rocket engines, chemical production, and so on. However, the small molecular size, the flammable and explosive properties of hydrogen gas make its detection an important issue. Therefore, there is increasing demand for hydrogen sensor during its transport, storage and use. A robust hydrogen sensor should have low detection limits, fast response and recovery time as well as low interference with various environmental conditions.The tetragonalcrystal TiO2is an important n-type semiconducting material, which have a wide range of applications in areas such as photocatalyst, dye-sensitized solar cells, biosensing, gas sensing and so on. With the advent of nanotechnology, TiO2nanotube arrays have been developed for gas sensing instead of thin films due to their high-aspect-radio as well as high sensing abilities. In addition, the low cost, miniaturization and mechanical stability should also be taken into account for practical applications. However, there are still many aspects of this approach that require work, including detection limits at room temperature, time response upon switching away from the hydrogen ambient, in particular the sensing mechanism.In this paper, hydrogen sensors have been fabricated from highly ordered TiO2nanotube arrays by anodization in an ethylene glycol solution containing NH4F. The effects of the anodization voltage and time of the anodization nanotube arrays formed on the surface of pure titanium were investigated. To research the sensing mechanism, TiO2nanoholes and thin films were be fabricated by anodiztion and rf magnetic sputtering, respectively. And three types of sensors were then annealed at500for3h in oxygen environment. The crystal phases and the microstructure were observed with X-ray diffraction and scanning electron microscope, respectively. Morphology of the individual nanotube was observed by a transmission electron microscope. For the sensor measurements, the separation of the Pt electrodes was coated by DC sputtering onto the three types of samples. The result from the sensor measurements showed that the vertically oriented TiO2nanotube arrays with Pt electrodes exhibits a tremendous variation in resistance of7orders of magnitude and short response time of13s at room temperature, on exposure to2000ppm hydrogen. Even at the low hydrogen concentration, the sensitivities can shill reach to2or3orders of magnitude. The experimental results appear to indicate that the sensing mechanism of TiO2-based hydrogen sensors included both the effect of chemisorbed oxygen and that of a Schottky barrier. The modulation of the Schottky barrier effects the resistance as follow:The H atoms dissociates into H+ions which diffuse into Pt and settle on the Pt/TiO2interface results in reduction of the barrier height. The low response of the sensors with Pt/Ti electrodes compared to that with Pt electrodes confirms that the Schottky barrier height modulation at the Pt/TiO2interface is one of important factors affecting hydrogen response. |
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