Fabrication And Gas Sensing Property Of Cuo-ZnO Heterojunction Materials

As oxide semiconductor sensing materials, ZnO is an n-type semiconductor with awide-band gap of3.37eV, CuO is a p-type semiconductor with a narrow band gap of1.2eV,extensive research has been conducted due to their high chemical stability, low cost and goodflexibility in fabrication. However, due to a single semiconductor oxide as gas sensor materialhas defects such as low response and poor selectivity, so it might be a good solution tocombine n-type ZnO with p-type CuO in improving the gas-sensing properties.A facile approach for synthesis of flower-like p-CuO/n-ZnO heterojunction nanorodswere established in our experiments. CuO/ZnO nanorods were prepared by co-precipitation ofCuO nanoparticles on the hydrothermally grown ZnO nanorods using CTAB as surfactant.The ethanol gas-sensing properties of CuO/ZnO nanorods were evaluated with differentethanol vapor concentrations at the working temperature of300oC. The response of0.25:1CuO/ZnO nanorod sensors to100ppm ethanol was98.8,2.5times which of ZnO only sample,with a response and recovery time of7s,9s, respectively. The response of low concentrationas1ppm ethanol can reach the value of9.68. The enhanced ethanol response is mainlyattributed to a wider depletion layer on the CuO/ZnO surface resulted from the formation ofp-n heterojunctions between p-CuO nanoparticles and n-type ZnO nanorods. Resisitance ofthe sensing device might change greatly as the thickness of the depletion layer may changegreatly when reductant atmosphere approches the surface of the sensing device.In this dissertation, CuO lamellar microsphere was synthesized through a hydrothermalmethod. The average diameter of microspheres size is about3-4μm, and the thickness of thelamella becomes thinner as the reaction time increases. When the reaction time in synthesis is9h, the thickness of the lamella is27nm. The results of gas-sensing mesurement show thatthe response of the sample to10and500ppm ethanol are4.5and20.8, respectively, with ashort response and recovery time in20s and good ethanol selectivity at260oC.In order to study the influence of heterojunction on the gas sensing properties further,ZnO/CuO microsphere was synthesized through a hydrothermal method. When the reactiontime is30min, the sample morphology is mainly CuO microspheres with a size of3μm. Asthe reaction time increases, ZnO nanorods emerge on the CuO microsphere surface. When thetime is9h, ZnO nanorods are basically wrapped on CuO microspheres. The optimumworking temperature of these sensing devices is300oC. Ethanol gas-sensing performance ofCuO/ZnO microspheres show stong relation with the reaction time in the synthesis, when thereaction time is9h, the response of the obtained sample to10ppm ethanol is6.2, and that is32to400ppm ethanol, with a response and recovery time of10s and13s, respectively.