| In2O3 is a n-type semiconductor oxide with direct band gap of 3.55-3.75 eV. having two kinds of crystal types: cubic crystal and hexagonal crystal.In2O3 has high conductivity and a large number of intrinsic donor such as oxygen hole defects, so, it has been widely used in light catalysis, sensors,light-emitting diodes and other fields. In recent years, it has been found that increasing the specific surface area of semiconductor materials and precious metals doping can improve the sensitivity of semiconductor gas sensor, which currently become the research direction of In2O3 nano-materials.In the paper, pure and Pd-doped flower-like In2O3 microstructures were synthesized via hydrothermal growth for the enhanced gas sensing properties.The morphology, crystal structure and chemical composition of the materials are characterized using SEM, TEM, XRD, EDS and XPS. Using the static volumetric method, gas sensing of the sensors is tested by CGS-1TP intelligent gas sensitive analysis system. Analysis conclusion is as follows:1. Pure and different content Pd-doped flower-like In2O3 microstructures have been successfully synthesized with hydrothermal method, corresponding to Pd:In molar ratios of 0, 0.5, 1, 2 and 4 mol%. As-prepared pure andPd-doped flower-like In2O3 microstructures are the body-centred cubic structure, which have diameters of about 5-7 μm. The influence of the doping process on the surface morphology of the Pd-doped In2O3 microflowers is little.Partial Pd is doped into the In2O3 lattice and the rest loaded on the surface.2. Oxidizing gas(NO2) sensing test results of pure and Pd-doped flower-like In2O3 microstructures show that the optimum woking temperature of pure In2O3 microflowers is 135 ℃, while, that of the 1 mol% Pd-doped flower-like In2O3 microstructures is 110 ℃. After Pd doping the best working temperature of flower-like In2O3 microstructure has been reduced. The 1 mol%Pd-doped In2O3 sensor exhibites the best gas-sensing properties. The response of 1 mol% Pd-doped flower-like In2O3 microstructures is 4048 with an almost3 fold enhancement in sensitivity compared with the pure one(1310) and has a response/recovery time of 180/90 s to 50 ppm NO2 at 110 ℃. The NO2 detection limit of the sensors is down to 0.5 ppm.3. Reducing gas(H2) sensing test results of pure and Pd-doped In2O3flower-like In2O3 microstructures show that the optimum operating temperature is determined to be about 210 oC for all the fabricated In2O3 sensors, indicating the best working temperature of flower-like In2O3 microstructure has not been changed by Pd doping. 1 mol% Pd-doped In2O3 microstructure reaches the maximum response value of 3.6, which is almost two times higher than the response(1.64) of the pure In2O3 microstructure to100 ppm H2 at 210 oC. The measured response/recovery times of 1 mol%Pd-doped In2O3 sensor is only 4/7 s, which are shorter than the response/recovery time(11/12 s) of pure In2O3, suggesting l mol% Pd-doped In2O3 sensor has the best sensing properties of H2.4. Through the analysis of the gas-sensing mechanism of pure and Pd-doped In2O3 flower-like In2O3 microstructures to oxidizing gas and reducing gas, we know that doping process of precious metals can accelerate the decomposition of O2 molecules on the surface of the materials and increase the surface adsorbed oxygen. Free electrons can transfer between the heterojunction interface, which make the width of the space charge layer changed. Optimizing the amount of the doped precious metals in the semiconductor materials can improve gas sensitive properties to the greatest extent. |
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