Investigation On Performance Of Titanium Dioxide Thin Films Hydrogen Sensor

With the wide application of hydrogen as clean and efficient energy source,the security of hydrogen using has attracted broad attention due to possible explosion accidents when the content of hydrogen reaches 4%in the air.On one hand,sensors with excellent performance are urgently demanded for hydrogen detection in industry.On the other hand,the gas-sensing mechanism has not yet been fully understood.In this work,magnetron sputtering and hydrothermal method were combined to prepare TiO₂ film hydrogen sensor with excellent performance at room temperature?25±5??.The gas-sensing mechanism has been investigated,the results are as follows:1.Ti₂O₃ was used as the target material in the process of magnetron sputtering deposition.The sputtering process lasted for 15 min,and thin films were formed on the FTO substrates as the seed layer.The seed layer was then submitted to post annealing treatment.Rutile TiO₂ thin film with grains size ranging from 20nm to 200nm was obtained.2.Hydrothermal method was adopted to prepare TiO₂ films on the FTO substrate,with the reaction temperature of 150?,and reaction time of 8 hours.Function of the seed layer was investigated.The resutls have shown that,in addition to improving the films' crystallization and density,the TiO₂ seed layer had a positive impact on the growth along the[002]orientation of the thin films.Leakage current through FTO conductive substrate could be greatly reduced due to the existance of the seed layer,which separated FTO and TiO₂ film.The TiO₂ hydrogen sensor with seed layer has demonstrated excellent performance under different hydrogen concentration.The detecting limit of the hydrogen sensor with seed layer could be as low as 4ppm.3.The TiO₂ films with seed layer were further annealed at 400? for 20min.Compared to unannealed films sensor,the Resistance-Time curve of the sensor with the annealed TiO₂ film was smoother,the signal was more stable,the sensitivity increased,and the minimum detection limit was further decreased to 1 ppm.4.Working temperature has shown great influence on the performance of the hydrogen sensors.By increasing the test temperature to 100?,the performance of the sensor had been significantly improved.The resistance decreased to nearly 42.6%of the original resistance even when hydrogen concentration is 1ppm.The response and recovery time were also remarkablely reduced.5.Gas-sensing mechanism of the thin films was also discussed.It was found out that the sensing process related to both TiO₂ surface and Pt/TiO₂ schottky barrier.H2 preferred to be adsorbed onto the?002?surface and to stay above Ti4+ sites to form a stable state called H2-Ti?4?.Subsequently,partial H2 molecules were dissociated due to exsitance of the O2-nearby,and the formed atomic H diffused along the[002]direction to the sublayer,thus the resistance of the TiO₂ thin film decreased.At low hydrogen concentration,the charge carriers transported predominantly within the TiO₂ surface layer.With the increase of hydrogen concentration,the above processes also occurred on the surface of TiO₂ under Pt electrode,the resistance change of the sensor was then originated from the Pt/TiO₂ Schottky barrier change.The recovery processes of the hydrogen sensor refered to desorption of H2 from the surface of the oriented TiO₂ nanorods as well as desorption from the defect sites.