Research On Gas Sensors Based On Metal Oxide And Metal Sulfide Nanostructures

In the past 50 years, semiconductor gas sensor has been a hot topic that attracted the attention of the researchers all over the world. With the advantages of low cost, easy to prepare, portable and fit for mass production, gas sensors based on semiconductor enjoys a good potential application. In recent years, the marked progress of the studies about nanomaterials has made it possible to fabricate semiconductor gas sensor with good performances. Some of the sensors has reached the commercial level for qualitative and quantitative measurement. However, most of the semiconductor gas sensors are still faced with the kinds of problem, such as low response, slow response and recovery speed, poor selectivity. We still have a long way to go to obtain semiconductor gas sensors with high performances. The study about the semiconductor gas sensors will mainly focus on the following two respects:I. The first one is to develop the gas sensing properties of the traditional metal oxide.（1） Though there are various metal oxide semiconductor materials used in the gas sensing field, Fe₂O₃ and TiO₂ are two important metal oxide for gas sensing which can not be replaced because of their advantages of easy to prepare, low cost and good sensing stability. In order to improve the sensing property, it is a good choice to conduct the study in two ways: improving the availability of the materials and the ability of the materials to react with the gas. In particular, fabricating nanostructures with special morphology（such as hierarchical structure） and doping the sensing materials with transition metal are used as two useful methods to improve the sensing properties of the semiconductor materials.The electrospinning method is used to fabricate solid ?-/α-Fe₂O₃ nanobelts and hierarchical ?-/α-Fe₂O₃ nanobelts. The sensor based on the latter material shows better sensing performance to acetone gas. The reason is that the hierarchical ?-/α-Fe₂O₃ nanobelts are assembled from 0 dimensional nanoparticles which offer the nanobelts two merits: the 1 dimensional morphology is maintained while making use of the high specific surface area of the 0 dimensional nanoparticles. In this way, the aggregation of nanoparticles is avoided. At the same time, nanobelts have loose and porous structure which is different from the solid nanobelts. The BET analysis demonstrated that there are large amount of pore spaces with pore size under 7 nm formed in the materials. The pore spaces are beneficial for the diffusion and absorption of the gas molecules in the materials, getting higher response to acetone gas.（2） Doping transitional element into the semiconductor to change the physical and chemical properties is a good way to improve the gas sensing performances of the sensing materials. In this dissertation, the electrospinning method is used to fabricate Co doped TiO₂ and Ni doped TiO₂ bowl-like nanostructures.Firstly, the electrospinning method is used to fabricate Ni doped Ti O₂ bowl-like nanostructures with different Ni doping content（0 mol%, 2 mol%, 4 mol% and 6 mol%）. The sensor based on 2 mol% Ni doped TiO₂ bowl-like nanostructures shows the highest response among the sensors with different Ni doping content. At the optimum working temperature of 302 ?C, the response of the 2 mol% Ni doped TiO₂ bowl-like nanostructures to 100 ppm xylene is 4.4, which is more than 2 fold of the response of the pure TiO₂. There are several main reasons considered to be responsible for the high performane of the Ni doped TiO₂:more holes are formed when the Ni replaced Ti in the crystal lattice, the impurity energy level and the surface defects were induced by NiTiO3.Secondly, the electrospinning method is used to fabricate Co doped Ti O₂ bowl-like nanostructures with different Co doping content（0 mol%, 2 mol% and 4 mol%）. At the optimum working temperature of 344 ?C, the response of the 2 mol% Co doped TiO₂ bowl-like nanostructures to 100 ppm xylene is 6.3, which is more than 2 times larger than response of the pure Ti O₂.There are several reasons considered to be responsible for the high performane of the Co doped TiO₂: more holes are formed when Co was doped into TiO₂, the impurity energy level and the surface defects were induced by CoTiO3.It is worth mentioning that template is needed to fabricate metal oxide with hollow structure in general case. However, bowl-like nanostructures is prepared directly by the electrospay method witout using template. This method offers an easier way to prepare nanostructure with hollow morphology, which is very useful for the fabrication of sensors with low cost and large-scale production.II. The second expect is to explore unconventional semiconductor gas sensing materials.As the important II–VI group metal sulfide,CdS and ZnS have the advantages of cheap, chemical stable and easy to be prepared. They are used widely in the fields of photodetecting, optical waveguide, field effect transistor, solar cell and photocatalysis. CdS and ZnS has the similar crystal structure, physical and chemical properties with ZnO, which is a king of traditional gas sensing semiconductor material. However, there are few reports about the gas sensor based on II–VI group metal sulfide. As II–VI group metal sulfide have the similar physical and chemical properties with ZnO, it is meaningful to try to use II–VI group metal sulfide nanostructure as the gas sensing material, offering more possibilities for the realization of the gas sensor with high performance.（1） Solvothermal method is used to prepare single crystal CdS nanowire. The CdS nanowire has uniform morphology and smooth surface. The diameters of the nanowires are in the rage of 30-100 nm and the lengthes of the nanowires are about 10 μm. The CdS nanowire is used as the sensing material for the semiconductor gas sensor for the first time. At the optimum working temperatureof 206 °C, CdS nanowire gas senor shows a high response（S=14.9） to 100 ppm ethanol. What’s important, the sensor shows ultrafast response and recovery speed to 100 ppm ethanol gas. The reponse and recovery time is 0.4 s and 0.2 s, respectively, which is quicker than most of the reported ethanol senors. The reason for the super performance of the CdS nanowire can be attributed to the single-crystal structure and the weaker electronegativity of sulfide atom than that of oxygen atom.（2）One-step solvothermal method is used to prepare Zn1-xCdxS（x=0¹ï¼‰ nanorods, which are used to prepare semiconductor gas sensor. When x=0.4, the response of the Zn1-xCdxS sensor to 20 ppm ethanol reaches the the highest（S=12.8）. The corresponding response and recovery time is 2 s and 1 s,respectively. The dissertation discussed the influence on the sensing property of the stoichiometric ratio of cations in Zn1-xCdxS. It is demonstrated that it is a good way to develop the performance of the semiconductor based gas sensor by adjusting the stoichiometric ratio of the ternary compound.