CO.'S . 3 O . 4 Nanocomposite Controlled Preparation And NO X Gas Sensitive Properties

Facing on the issue that conventional semiconductor materials have commonly low response at room temperature,we have dedicated to ultra-sensitive NO_x sensors capable of operating at room temperature and to optimization their synthetic methods in the thesis.The p-type metal oxide semiconductors Co₃O₄ and CuO were chosen as starting materials to develop Co₃O₄/CuO and Co₃O₄/CuO-EG nanocomposite materials.The morphology and microstructure of new-synthesized composites were characterized by XRD,FT-IR,SEM,TEM and XPS as well as other analytic methods.The self-assembled gas-sensing test system was used to study the NO_x gas sensing performance of composites at room temperature,and the composite formation and gas-sensing mechanisms were explored.Firstly,porous hierarchical Co₃O₄/CuO nanosheets with different Cu molar ratios were synthesized by a simple hydrothermal-calcination method.We analyzed the effects of different Cu molar ratio on the morphology and gas sensitivity of Co₃O₄/CuO nanocomposite.Compared to pure Co₃O₄,the binary composite Co₃O₄/CuO shows significantly increased carrier density and enhanced gas sensing performance,which is evidently caused by CuO doping.Representatively,the CC2-1 sample,having a molar ratio of 2:1 for Co3O4 and CuO,features a mesoporous structure with highest porosity;and its nanocrystalline particles are cross-linked to form a two-dimensional porous layered nanosheet with an average thickness of about 5 nm.The sample has an excellent NO_x detection performance at room temperature（RT=21°C）,response to 1 000 ppm NO_x up to 14.16,response time as short as 2 s,and detection limit as low as 0.01 ppm;more importantly,it shows a good reversibility and selectivity.The excellent gas sensing performance of the CC series composites is attributed to the addition of a proper proportion of Cu and pore-forming agent NaHCO₃,which cooperatively form a unique porous hierarchical heterojunction structure.The formation of heterojunction structure allows the composite to show higher porosity and more defects,which eventually leads to a significant increase in the chemical adsorption capacity.Secondly,a three-dimensional composite material Co₃O₄/CuO-EG（labeled as CCG）was prepared by vacuum with aforementioned porous hierarchical Co3O4/CuO nanocomposite and carbon nanomaterial expanded graphite（EG）.The morphology structure and NO_x gas sensing performance at room temperature were subsequently studied.For example,the CCG sample with Co₃O₄ and CuO of 2:1 in a molar ratio exhibits excellent gas sensing performance at room temperature,including response to NO_x up to 16.1 and detection limit as low as 0.03 ppm,as well as displays good reversibility and selectivity.Unlike Co3O4/CuO series materials,three-phase CCG series materials have a 3D structure,in which EG acts as a template in the synthetic process and induces thinner and smaller nanosheets in size;and Co₃O₄/CuO cubic nanoparticles gradually grow in this process.The excellent gas sensing performance of CCG materials is assigned to the presence of a large number of heterostructures and defect structures in the 3D structure that are formed by nanoparticles-loaded nanosheets.In the conjunction with EG’s electron transport capability,CCGs’merits of abundant surface active centers and excellent chemical adsorption capacities greatly improve the gas sensing performance.In this paper,the facile doping modification method enables to effectively improve gas sensing performance of p-type metal oxide semiconductor nanocomposite,and also provides a fundamental understanding for materials that have desirable applications as NO_x gas sensor at room temperature.