| The development of industry has brought much more treasures to people. However, more and more poisonous gases produced in the development process are not only non-friendly to the environment but also dangerous to people. With the fast development of the society, people need higher efficient methods to detect the poisonous gases. Gas sensors based on metal oxide semiconductors have the advantages of high-sensitivity, nice-selectivity as well as easy miniaturization and automation. They can be applied to a wide range of analytical tasks, such as industrial control and environmental monitoring. On the one hand, nano-crystalline particles, exhibiting a large surface area with diameter less than 100 nm, might be favorable for improving the sensitivity of gas-sensing material. Carrying out researches on the preparation methods of nano-materials is one of the new research directions of current gas sensing materials. On the other hand, surface modification by proper choice of additives or dopants is often used to improve the response of the gas sensing materials for particular applications. According to the above-mentioned directions, a series of investigations including preparation of nano-materials, structural analysis and actual application in gas sensing field have been performed in this dissertation. In part one, a series of nano-sized mixed oxides has been obtained by solid-solid chemical reaction at room temperature which is a new direction in preparing inorganic nano-materials. This method doesn't use solvent and avoids the effect of solvent. There are four steps in a typical proceeding of solid-state reaction: diffusion, reaction, nucleation and growth. The growth of the final particles is inhibited by the inorganic salts added in the reaction system. We have obtained Mg2Fe2O4, CdSnO3 and NiFe2O4 using cheap inorganic salts as precursors. The process used here is a high-yielding, low-cost procedure and environment friendly for the synthesis of the nano-materials above-mentioned. Furthermore, the noble metals, well known as active catalysts, have been confirmed to possess the promoting effects on many semiconductor gas sensors. So Pt, Pd and Au are used to better the gas sensing properties of the final materials obtained in this thesis by the impregnation technique. In chapter 2, n-type semi-conductive nanometer material MgFe2O4 was synthesized by solid state reaction of inorganic reagents MgSO4, Fe(NO3)3?9H2O, and NaOH. Conductance responses of the nanocrystalline MgFe2O4 thick film were measured by exposing the film to reducing gases like methane (CH4), hydrogen sulfide (H2S), liquefied petroleum gas (LPG) and ethanol gas (C2H5OH). It was found that the sensor exhibited various sensing responses to these gases at different operating temperature and the gas sensor can realize the detection of low concentration of H2S and ethanol at 160 oC and 335 oC, respectively. Furthermore, the sensor exhibited a fast response and a good recovery. Successive on and off responses could be repeated without observing major changes in the response signal. In chapter 3, CdSnO3, a semiconducting oxide with perovskite structure, was prepared by solid state reaction of inorganic reagents 3CdSO4?8H2O, SnCl4?5H2O and NaOH. Noble metal additive Pt of different concentrations from 0.1 at.% to 2 at.% was incorporated into CdSnO3 by impregnation technique and the effect of Pt on the gas sensing properties of nano-crystalline CdSnO3 was studied. Conductance responses of the nano-crystalline CdSnO3 thick films were measured by exposing the films to C2H5OH, CO, CH4, C4H10, gasoline and LPG at different operating temperatures. It was found that sensors doped with Pt exhibited good sensitivity and selectivity to the vapor of C2H5OH and the optimum sensitivity is 68.2 obtained with the sensor doped with 1.5 at.% Pt. In chapter 4, ultrafine NiFe2O4 powders were prepared by solid state reaction of inorganic reagents, Ni(Ac)2, Fe(NO3)3, and NaOH. Noble metals such as Au, Pd and Pt with different concentration were incorporated into NiFe2O4 by impregnation technique. The gas sensing characteristic of NiFe2O4 nanopowder with and without different noble metal dopants were investigated. The electrical resistance response to H2S gas of the sensors based on the materials was investigated at different operating temperature and different gas concentrations. The results show that NiFe2O4 is a p-type semiconductor and the gas-sensing response of the doped NiFe2O4 sensors was superior to that of the undoped ones. The sensor response increased linearly with the H2S gas concentration up to 100 ppm. The sensor with the 1.5 at.% Au doped NiFe2O4 showed excellent electrical resistance response towards 5ppm H2S gas and the sensor response was up to 35.8 at 300 oC. The 1.5 at.% Pt doped one was less sensitive to H2S but worked at lower temperature, 240 oC. The gas-sensing behavior of these materials with respect to various reducing gases like LPG, CH4, CO, C4H10 and H2 shows that the H2S gas sensor developed possesses an excellent selectivity. The interaction mechanism and correlations between the electricalresistance response and noble metal dopants were discussed. One-dimensional materials such as nanorods and nanowires have attracted much attention due to their specific physical properties and interesting application in nano-devices. The study of one dimensional (1D) materials has become a potential frontier in nanoscience and nanotechnology in the last few years. In part two, nano-sized flower-like ZnO and ZnO nano-rods were synthesized by a simple hydrothermal method and a solvothermal route, respectively (Chapter 5). The two methods are convenient, environment friendly, inexpensive and efficient process. The gas sensors based on the ZnO obtained by these two methods showed excellent responses and selectivity to ethanol gas and the responses and recoveries were both fast. In part three, ZnO nanorods were synthesized by a modified polyol process using inorganic reagents as the precursors (Chapter 6). The synthesis process without requirement of delicate equipments was a convenient, environment friendly, inexpensive and efficient method for preparing ZnO nanorods. Conductance responses of the ZnO nanorod thick film were measured by exposing the film to reducing gases like CH4, LPG, C4H10, H2 and C2H5OH. The results indicate that the device showed nice responses and selectivity to ethanol gas. Furthermore, the sensor exhibited a fast response and a good recovery property. In part four, a compound material of MWNTs coated with SnO2 was synthesized at ambient conditions, and the gas sensing properties of the material were studied (Chapter 7). Conductance responses of the compound materials were measured by exposing to reducing gases like CH4, CO, C4H10, LPG and ethanol gas. It was found that the device exhibited nearly non-sensing responses to CH4, CO and C4H10. While it showed good sensing responses to LPG and C2H5OH. Furthermore, the sensor exhibited a fast response and recovery within seconds and the gas-sensing responses increased linearly with the increment of the gas concentrations of LPG and ethanol. Many preparation methods described and nano-materials obtained in this dissertation aimed on the gas sensing materials. Their potential application area is not, however, limited to this kind of materials. In part five, nano-sized flower-like ZnO synthesized by a simple hydrothermal method was dispersed in the chitosan solution to form a ZnO/chitosan composite matrix for the fabrication of H2O2 biosensor in the complementary part of the thesis (Chapter 8). This composite combined the advantages of inorganic species (ZnO) and organic polymer (chitosan). The parameters affecting the fabrication and experimental conditions of biosensorswere optimized. Using hydroquinone as the mediator, the biosensor showed a fast response of less than 5 s with the linear range of 1.0×10-5 to 1.8×10-3 mol L-1H2O2 with a correlation coefficient of 0.995 (n =20). The detection limit of the sensor was found to be 2.0 μmol L-1, based on a signal-to-noise ratio of 3. The biosensor exhibited satisfactory reproducibility and stability and retained about 78% of its original response after 40 day storage in a phosphate buffer at 4 °C.
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