Research On The Gas Sensing And Device Physics Of Epitaxial Heterojunction Of TiO₂Based Nanotubes

Titania nanotubes array has a broad prospect in the fields of hydrogen storage,photo catalytic degradation, dye sensitized solar cell and so on, due to its excellentphysical and chemical properties. At the same time, it has become novel gas sensingmaterials. The improvement of gas sensitivity and the acknowledgement of devicephysical properties would become the only way for the achievement from materials todevices.In this paper, electrochemical impedance spectroscopy （EIS） was used toresearch the electrochemical oxidation mechanism of titania nanotubes array, and Pdwas decorated on the surface by microemulsion electrodeposition. The research of gassensing and device properties of the Schottky barrier was conducted for the furtherinvestigation about the sensing mechanism towards reduced gases.Titania nanotubes array was fabricated via electrochemical oxidation method,and changes of current in different conditions were recorded. The preliminary studyon the mechanism was conducted by combining the SEM and EIS. Results indicatedthat the negative impedance appeared in low frequency may be related to diffusionactions of F^-、Ti⁴⁺å’ŒO^2-.Pd was decorated on the surface of titania nanotubes array by electrodeposition.Results showed that Pd particles become conglobation in60C, and they becomeevenly distributed plate-shaped in25C.Titania nanotubes array was fabricated on the wires, and Pd was epitaxially grewalong the tube walls by direct current deposition. Point contact Schottky barrier wasfabricated by touching one Ti wire to the other, and the gas sensing properties andmechanisms were investigated. Results indicated that pure nanotubes had no responseto H₂and CO. However, they had obvious response to the two kind of gases after thedecoration of Pd. The sensitivity of the point contact sensor to2000ppm H₂was about8500in70C, and the response and recovery time were less than60s. But the detection of CO need higher temperature, and sensitivity was about1.25to100ppmCO in200C. The mechanism of electron transfer in gas sensing process wasobtained by the EIS analysis and the current–voltage （I-V） characteristic measurement.And the device properties parameters, including Schottky barrier height and idealityfactor, were calculated based on the thermal electron emission model.