Title : Size effect of nanoparticles on the sensing behaviour of semiconducting metal oxides
Abstract:
The interaction of semi conducting metal oxides with various toxic and combustible gases is very important for the operation of energy devices such as gas sensors. The electronic transport properties of such metal oxides strongly depend on the defect concentrations on the surface and in the bulk which becomes significant particularly when the material is comprised of nanoparticles [1]. In the present study, ZnO thin films and Indium-doped zinc oxide thin films were synthesized using a cost-effective chemical solution deposition technique. The films were deposited onto quartz substrate. X-Ray energy dispersive spectroscopy EDS were used to determine film composition [2]. For detailed information about the crystallographic structure of the films X-ray diffraction spectra was analysed. From the XRD results lattice parameters, size and micro strains were determined. Indium contents of 3 wt. % causes decreasing lattice parameters. Then with further indium insertion an increase of these parameters is observed. Preferential growth orientation was dependent on the indium contents. Only (002) planes have a marked tendency to grow normal to the substrate, consequently, for optimal dopant concentration the films are strongly c-axis oriented. The film microstructure was observed by FESEM techniques. Then surface morphology and microstructure were correlated with the results of the X-ray diffraction analysis. Since the gas sensing mechanism in these thin films involve surface oxidation, electron exchange and desorption of the reaction product, the measured resistance transients during response and recovery depends on a variety of factors interrelated to each other. For example, the response is related to the type (n or p) and concentration of intrinsic carriers, and their mobility. Also, the ratio of depletion layer width and grain size influences the response characteristics of the sensing elements. The processing conditions (deposition/post annealing temperature/time); influence the type and concentration of point defects, whereas film thickness influences the size and morphology of the grains. The intrinsic defect concentration may also be modified by aliovalent doping. The doping contents in turn could control the phase purity and defect concentration of the modified films. The depletion layer width and the grain size could be modified by aliovalent doping which in turn control the response characteristics of the deposited films. In view to this, the effect of processing routes with allied processing conditions, film thickness, and effect of aliovalent doping controlling the intrinsic defect, phase formation behaviour and film microstructure was studied systematically to identify their influence on the gas sensing characteristics by varying the test gas (viz. H2, CO, CH4, C4H10 etc) concentration and operating temperatures. To address the selectivity issue, feature extraction method of the conductance transients in conjunction with pattern recognition algorithms was considered as a viable tool to investigate the cross selectivity of the sensor elements towards various combustible gases.
