| As a common semiconductor gas sensing material,with its high sensitivity,fast response,low energy consumption and cost,simple operation,etc.,transition metal oxides have been widely used in industrial applications and academic research.CoO is selected for the first time as a gas sensitive material in this study.By controlling the type of solvent,hydrothermal time,temperature and heat treatment system,the CoO with different morphologies and structures was synthesized.By doping transition metal ion thermal stability and gas sensing properties of CoO was optimized,and the influence of different morphology,phase structure and ion doping on gas sensing properties was investigated.In the synthesis of CoO nanomaterials with different morphologies,the nano-CoO with different dimensions,such as zero dimension,one dimension and two dimensions were prepared by changing the conditions of cobalt source,structure directing agent,hydrothermal temperature and time.The phase structure of CoO nanorods was controlled by the change of heat treatment temperature and holding time,and the cubic phase,mixed phase and tetragonal phase of CoO were successfully synthesized.Different morphologies makes a big difference in the sensitivity to ethanol gas of nano-CoO materials.Such as in zero dimensional CoO materials,the nanoparticles with smaller grain size and the lager porosity is more responsive to ethanol;and there is a big gap on gas sensing performance between nanosheets and nanoparticles,which is attributed to the specific surface area of nanoparticles is greater than that of nanosheets.The larger the specific surface area is,the more adsorption sites to the gas molecules of the surface the nano-CoO material can provide,and then the sensitive properties of the material are enhanced.Therefore,the improvement of grain size,pore structure and specific surface area of nano-CoO materials has important effect on improving the sensitive response of ethanol gas.The different phase structures of nano-CoO materials have important influence on the gas sensing properties.The working temperature of tetragonal phase CoO is 110°C,which is lower than the cubic and mixed phase of 130°C.In response to various gases,the tetragonal CoO has obvious advantages,especially for 100 ppm ethanol,which reaches 30,while the others are 19.56 and 11.14,respectively.The tetragonal CoO is able to cause the increase of the amount of gas response,due to its relatively low symmetry and relatively larger electric dipole moment,which leads to the enhancement of the adsorption force between the polar gas molecules and the material.And the sensitization on the surface of material reacts more thoroughly,so the tetragonal CoO exhibits a more dominant performance on the sensitive response to the polar gas.The doping of transition metal Fe2+ has a certain effect on the phase composition,microstructure,thermal stability and gas sensing properties of CoO nanorods.With Fe2+ doping,the CoO nanorods tetragonal phase gradually disappeared and transformed into cubic phase and the morphology also changed.The nanorods self-assembled effect is more obvious,the diameter reduces 50% compared with no-doping samples,and the length to diameter ratio is as high as 50 to 80,so that the surface of the product is highly improved.After doping,the oxygen vacancies on the surface of the CoO nanorods graded structure are increased,and the doping of Fe2+ also increases the oxidation phase transition temperature of CoO.Based on the improvement of surface area and the increase of surface oxygen defects,the gas sensing performance is greatly improved and optimized. |
