Fabrication And Gas Sensing Properties Of Nanostructure TiO₂Thin Films

In this thesis, the author focuses on the fabrication and gas sensing properties ofseveral TiO₂thin films with different morphologies such as porous thin films,nano-scale thin films, net-like films and nanotubes using chemical methods includingmicro-arc oxidation （MAO） process, liquid phase deposition （LPD） method andanodic oxidation. The gas sensing mechanism of TiO₂has been investigated based onthe I-V, Electrochemical impedance spectroscopy （EIS） and Mott-Schottky analysisof the as prepared samples.Firstly, in-situ anatase TiO₂thin films with a porous morphology were fabricatedon titanium substrate by micro-arc oxidation （MAO） process without the subsequentheat treatment. The obtained films exhibited the maximum H2gas response at250℃but showed no response at the temperature above300℃. EIS analysis reveals that theequivalent electron transfer circuit of grain and grain boundary could be representedelectrochemically by a resistor and a constant phase element （CPE） in parallel,respectively, by which a possible surface-controlled gas sensing mechanism isproposed. It is found that temperature dependent grain boundary is a crucial factorgoverning the gas sensing properties. The W6+and Fe3+ions doped TiO₂films werefabricated conveniently through adjusting the electrolyte composition andconcentration. It was found that the gas sensing response of the doped TiO₂films wasenhanced while the measuring temperature did not decrease generally. Newmorphology of net-like TiO₂thin films was successfully fabricated by micro-arcMAO process of Ti plates and a subsequent chemical treatment of the as-preparedMAO-TiO₂thin films. The modified TiO₂thin films have the characteristics of a lowresponse temperature （1502℃.）w hen exposed to HThe liquid phase deposition method （LPD） is convenient for the fabrication ofthin films on large area. In this thesis, crystalline anatase TiO₂thin films wereobtained on glass substrates at60℃,75℃and90℃, respectively, by lowtemperature liquid phase deposition （LPD） method without the subsequent heattreatment using （NH4）2TiF6and H3BO3as predecessors. The results show that the gas sensors signal decreases with the increasing deposition temperature. The TiO₂thinfilms obtained at deposition temperature of60℃exhibited the maximum H2gasresponse at350℃.TiO₂nanotube arrays were grown on titanium substrate using anodic oxidation inan aqueous ethylene glycol electrolyte containing0.5wt%NH4F in this thesis. Theas-fabricated TiO₂nanotube arrays show excellent hydrogen sensing response even ata low temperature below100℃. It was found that the working electrode andmeasuring bias act as crucial factors in detecting gas. The high hydrogen sensingresponse could be obtained by the Pd electrode at room temperature. The signals ofAu-TiO₂nanotube Schottky sensors increased almost linearly up to a local-maximumby increasing the applied reverse bias voltage, while little improvement on gassensitivity was observed with further increase of the applied reverse bias voltage.Based on the analysis of impedance and device characteristic, we supposed that thereverse bias voltage dependent hydrogen sensing was mainly attributed to the changeof the Schottky barrier height which was intrinsically determined by the Au-TiO₂configuration. The analysis of bias voltage influence on the hydrogen sensingproperties of the gas sensors might be helpful to provide some insights for thepractical design of metal oxides gas sensors.