The Structure And The Oxygen Sensitivity Of TiO₂ Oxygen Sensitive Thin Films With Fe Ion Doping

Now days, a highly sensitive and fast response oxygen gas sensor is in demand to control oxygen concentration in food preservation, combustion processes in automobile engines and industries such as steel, chemical etc. Various semiconductor oxides have been proposed and have shown sensitivity towards oxygen at different operating temperature. All these materials show change in their resistivity on exposure to oxygen gas. Among these materials, rutile TiO₂ is a potential cansidate because of its low cost, stable phase even at higher temperature, higher oxygen sensitivity etc.The Fe³⁺ doped TiO₂ oxygen sensitive thin films were prepared by sol-gel method, and deposited on quartz glass substrate and single crystal silicon substrate respectively. The phase structure of the thin films were measured by X—ray diffraction (XRD). The morphologies were observed by scanning electron microscope (SEM). The oxygen sensitivity of the thin films were denoted by S=R / R₀, where R was the resistivity of the rutile TiO₂ thin films exposed to oxygen gas at 300℃, and R₀ was the resisitivity of the rutile TiO₂ thin films in vacuum.The study has demonstrated that the anatase phase appeared with a small grain size dispersed homogeneously in the thin film. The rutile phase appeared with a morphology of special clustered grains and large size in the thin film.It is exhibited evidently that the formation of the rutile phase in the TiO₂ thin film was dependent on doping Fe³⁺ in the system. Without doping Fe³⁺, the formatiom of the rutile in the thin films began with about 9000℃, and the thin films must be post calcined at 900℃ with 1h. With the Fe³⁺ doping, the formation of the rutile in the thin films were 800℃ and without post calcined. It shown that the formation temperature wasabout 100℃ lower with Fe³⁺ doped than that without doping Fe³⁺ . In addition, the content of the rutile phase was changed with the varying of Fe³⁺ doping. When Fe³⁺ doping was lower, with substituting a small size of Fe ions for a large size of Ti ions, the Fe can go into the TiO₂ lattice as substitutional metal dopant. The trivalent Fe dopant increased theconcentration of oxygen vacancies in TiO2 lattice, which in turn accelerated the transformation from the anatase phase to the rutile phase by reducing the strain energy in the lattice. The the content of the rutile phase increased with the increasing of Fe3+ doping. When Fe3+ doping was over, the doping of Fe ions increased the distortion of the TiO2 lattice, and the congtent of the rutile phase decreased with the increasing of Fe3+ doping. The largest content of the rutile phase appeared in the TiC>2 thin film while doping Fe3+ reached a proper quantities.The crystallization of the TiC>2 thin films have the same regularity no matter the thin films were deposited on quartz glass substrate or on single crystal silicon substrate. But compared with these substrates, the thin films deposited on single crystal silicon substrate have a better construction the the thin films deposited on quartz glass substrate.In this work, we have prepared the porous TiC>2 thin films with the addition of polyethylene glycol(PEG). It was shown that with the addition of PEG and with the prolong of the calcined time, the porosity of the rutile TiO2 thin films increased. Besides affecting the porosity of the rutile TiC>2 thin films, the addition of PEG affect the hydrolysis and the polycondensation reaction of TiC>2, and affect the crystallization of the rutile TiO2 thin films too.The oxygen sensitivity of Fe doped TiC>2 thin films was controled by the concentration of the oxygen vacancies and the content of the rutile phase in TiO2 thin films. The increasing of the concentration of the oxygen vacancies increased the adsorption of the oxygen, the oxygen sensitivity of rutile TiC>2 thin films was increased. The migration-rate of the oxygen vacancy in rutile TiC>2 is higher than the anatase TiO2, the oxygen sensitivity of TiC>2 thin films would increased with the increasing of the concent of the rutile in TiO2 thin films. The Fe3+ doping increased the oxygen vacancy of the rutile T1O2 thin films and it increased the content of the rutile TiC>2 in the films, which in turn increased the oxygen sensitivity of the rutile TiO2 thin films.In addition, the porosity of the thin films increased the specific surface area of the thin films and the porosity of the thin films also increased the diffusion of the materials. The oxygen sensitivity of the thinfilms increased with the increasing of the porosities. The addition of PEG not only increased the porosities in the rutile TiO2 thin films, but also increased the content of the rutile in TiO2 thin films, which in turn increased the oxygen sensitivity of the rutile TiO2 thin films.In conclusion, the crystalline ability of the rutile TiO2 thin films was improved by the doption of Fe. The porous TiO2 thin films were successfully prepared by addition of PEG, and the porosity of the rutile TiO2 thin films were increased with the addition of the PEG. The oxygen sensitivity of the TiO2 thin films was increased when Fe doping reached a proper quantities, the oxygen sensitivity of the TiO2 thin films also increased with the addition of PEG...