Research On Gas Sensors Based On Oxide Semiconductors With Hierarchical Structure

Here, we systematically investigated the design and synthesis of oxidesemiconductors with hierarchical structure, the relationship betweenmorphology/microstructure and sensing performance, the modification of sensingperformance using doping and composition, the application in gas sensors. Variousmorphologies of SnO2and-Fe2O3with hierarchical structure had been successfullyprepared by hydrothermal method. In order to further enhance the sensingperformances of the as-prepared hierarchical oxide semiconductors, the modificationswere carried out using the doping and composition. Several kind of high performanceof sensing devices had been developed. Main contents included:1、In order to alleviate the degradation of sensing performances of gas sensorsbased on oxide semiconductor nanoparticles, which derived from the aggregation ofnanoparticles, various morphologies of hierarchical oxide semiconductors withdifferent structural units were prepared.(1) We designed and synthesized hierarchical Fe2O3composed of primarynanoparticles and investigated their ethanol sensing properties. First, the C2H5OH andCH3COCH3sensors based on porous-Fe2O3hierarchical ellipsoids, which wereobtained by a facile one-step template-free hydrothermal method, were fabricated.The as-prepared porous-Fe2O3ellipsoids exhibited good selectivity to ethanol at250℃, the response of the sensor to100ppm ethanol was25.6. The high responsewas attributed to their porous structures. Secondly, amorphous Fe2O3hollownanospheres with double-shell structure were synthesized via a template-freehydrothermal method, which was based on the hydrolytic process of potassiumferricyanide. The obtained samples were tested for response to ethanol. The responseof sensor to100ppm C2H5OH was8.9and the time of response was1s. It was found that the sensor based on double shell hollow spheres exhibited better sensingperformance compared to that based on single shell structure. The enhancement insensitivity was contributed from their larger specific surface area and pore volume.(2) The gas sensors based on hierarchical nanostructures-Fe2O3composed ofnanorods were fabricated. First, urchin-like-Fe2O3hierarchical nanostructurescomposed of single-crystal nanorods had been obtained by a facile chemical solutionroute combined with subsequent calcination process. The results of sensing propertiesshowed that the optimum operating temperatures of the sensor to ethanol and acetonewere260and275℃. The responses of sensor to60、80、100and200ppm ethanolwere6.3、8.4、12and16.2, respectively and the time of response and recovery wasabout2and40s to100ppm ethanol. Secondly, the ethanol gas sensor based onurchin-like-Fe2O3hollow spheres was fabricated. The hollow spheres showedhigher sensitivity than the solid spheres under the same of operating temperature andconcentration. For example, the responses of hollow and solid spheres to100ppmethanol were12.6and9.2, respectively. Moreover, the sensor based on hollow spheresdisplayed fast response and the time of response was2s. The high sensitivity and fastresponse were attributed to hollow structure and good permeability, which wouldbenefit the diffusion and reaction of gas and enhanced the utilization rate of sensingmaterial.(3) Porous SnO2hierarchical architectures composed of nanosheets had beensynthesized by calcining the precipitates prepared through a facile one-stephydrothermal synthesis method, and their sensing properties were investigated. Gassensing tests showed that the sensor using porous SnO2hierarchical nanosheets afterannealing at600C for2h exhibited the highest sensitivity. The sensor showed a4-fold increase in gas response to100ppm C2H5OH in comparison with nanoparticlesat275C. The excellent gas sensing properties of flower-like SnO2hierarchicalstructures were attributed to its unique structures.2、We systematically investigated the modification of sensing material using thedoping and composition of semicomductor in order to enhance the sensingperformance of sensors. (1) First, to solve the bad selectivity of the sensor based on SnO2nanostructures,the hierarchical undoped and Cd-doped SnO2nanostructures had been synthesized viaa low-cost and environmentally friendly hydrothermal route, and their gas sensingproperties were investigated. It was found that the sensor based on pure SnO2nanorods exhibited an obvious response to ethanol, while the doped nanorods showedgood selectivity to H2S. The sensor based on the3.0wt%Cd-doped SnO2displayedexcellent selectivity toward H2S at the operating temperature275oC, giving a responseof about31to10ppm, which was about22times higher than that based on pure SnO2.Cd-doping could improve the formation of surface oxygen vacancy, which wasprobably responsible for the enhanced performance. Secondly, monodisperse anduniform pure and Cu-doped-Fe2O3cubes with a hierarchical architecture piled upnanoparticles as secondary units were obtained via a low-cost and environmentallyfriendly hydrothermal route and their gas sensing properties were investigated. Acomparative gas sensing study between the Cu-doped-Fe2O3and pure-Fe2O3cubes was performed to demonstrate the superior gas sensing properties of the dopedsamples. It was found that the sensor based on Cu-doped-Fe2O3(3.0wt%) had aresponse of19-100ppm C2H5OH, which was about3times higher than that based onpure-Fe2O3nanostructures at the same operating temperature (225oC).(2) Besides doping method, we also used semiconductor composition technologyfor modification of sensing material to realize the enhancement of sensingperformances. Double-shell SnO2/-Fe2O3hollow composites were synthesized forthe first time, and their ethanol sensing were tested. It was found that suchdouble-shell composites exhibited an enhanced ethanol sensing properties comparedwith the single-component SnO2hollow spheres. For example, at an ethanolconcentration of100ppm, the response of the SnO2/-Fe2O3composites was about16,which was about2times higher than that of the primary SnO2nanostructures. Theenhanced performance might be attributed to the novel structure as well as the changeof heterojuction barrier at the different gas atmosphere.