Metal Oxide (MO)semiconductors are one of the most widely used materials in commercial gas sensor devices. The basic principle of chemoresistive gas sensor operation stems on the high sensitivity of electrical resistance to ambient gaseous conditions. Depending on whether the oxide is "p type" or "n type", the resistance increases (or decrease), when placed in atmosphere containing reducing (or oxidizing) gases. The study of conductometric metal oxide semiconductor gas sensors has dual importance in view of their technological device applications and understanding fundamental MO-gas interactions. Metal oxides based sensors offer high thermal, mechanical and chemical stability. A large number of MOs show good sensitivities to various gases like CO, NOX, SOX, NH3, alcohols and other Volatile Organic Compounds (VOCs). VOCs are very common hazardous pollutants in the environment. Gas sensors are in great demand for their various applications such as food quality control, fermentation industries, road safety, defence, environmental monitoring and other chemical industries. The aim of the study is to explore the possibility of advancements in semiconducting MO based gas sensor devices through tuning microstructural parameters along with chemical dopants or additives. And further to investigate the underlying mechanism of conductometric MO gas sensors. The novel synthesis method employed is based on the solution combustion method coupled with ultrasonically nebulized spray pyrolysis technique. The well studied SnO2 and relatively unexplored Cr2O3 oxide systems are selected for the study. The non-equilibrium processing conditions result in unique microstructure and defect chemistry. In addition, using this technique MO - Reduced Graphene Oxide (RGO) nanocomposite films has also been fabricated and its application to room temperature gas sensor devices is demonstrated. The thesis comprises of seven chapters. the following section describe the summery of individual chapters. The Chapter 1 describes the introduction and background literature of this technology. A brief review of developments in gas sensor technology so far has been enlisted. This chapter also gives a glimpse of applications of MO semiconductors based sensors. The underlying mechanism involved in the sensing reaction and the primary factors influencing the response of a gas sensor device are enlisted. Further in the later part of the chapter focused the material selection criteria, effect of additives/dopants and future prospects of the technology. The end of this chapter highlights the objective and scope of the work in this dissertation. In the Chapter 2 the the materials selection, characterization techniques and particularly the experimental setups used are elaborated. This includes the deposition method used, which is developed in our group and the the in house built gas sensing system including its working principles and various issues have been addressed. The Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) is a novel deposition method devised, which is a combination of conventional spray pyrolysis and solution combustion technique. Spray pyrolysis is versatile, economic and simple technique, which can be used for large area deposition of porous films. The intention is to exploit the exothermicity of combustion reaction in order to have high crystallinity, smaller crystallite size with high surface area, which are extremely important in gas sensor design and its efficiency. Further the gas sensing system and its operation are discussed in detail including the advantages of vertical sensing chamber geometry, wider analyte concentration range (ppm to percentage) obtained through vapor pressure data and simultaneous multi sensor characterization allowing better comparison. Here in this work, Chromium oxide (Cr2O3) and Tin oxide (SnO2) are selected as gas sensing materials for this work as a p-type and n-type metal oxide semiconductors respectively. Nevertheless Cr2O3 is a less explored gas sensing material as compared to SnO2, which is also being used in many commercially available gas sensor devices. Thus, studying and comparing gas sensing properties of a relatively novel and a well established material would justify the potential of the novel deposition technique developed. In Chapter 3, the effect of exothermic reaction between oxidizer and fuel, on the morphology, surface stoichiometry and observed gas sensing properties of Cr2O3 thin films deposited by UNSPACM, is studied. An elaborative study on the structural, morphological and surface stoichiometry of chromium oxide films is undertaken. Various deposition parameters have been optimized. An extensive and systematic gas sensing study is carried out on Cr2O3 films deposited, to achieve unique microstructure. The crystallinity and microstructure are investigated by varying the deposition conditions. Further, the effect of annealing in oxygen gas atmospheres on the films was also investigated. The gas sensing properties are studied for various VOCs, in temperature range 200 - 375 oC. The possible sensing mechanism and surface chemical processes involved in ethanol sensing, based on empirical results, are discussed. In chapter 4, the effect of 1% Pt doping on gas sensing properties of Cr2O3 thin films prepared by UNSPACM, is investigated. The chemical analysis is done using x-ray photoelectron spectroscopy to find the chemical state of Pt and quantification is done. The gas sensing is done towards gases like NO2, Methane and Ethanol. The enhancement in sensitivity and remarkable reduction in response as well as recovery times have been modeled with kinetic response analysis to study the variation with temperature as well as concentration. Further the analysis of observations and model fittings is discussed. The Chapter 5 deals with the defects induced ferromagnetism and gas sensing studies SnO2 nanoparticles prepared by solution combustion method. The structural, chemical analysis of as-synthesized and annealed SnO2 nanoparticles reveal gradual reduction in defect concentration of as-prepared SnO2. The findings of various characterization techniques along with optical absorption and magnetic studies to investigate the defect structure of the material are presented. As defects play crucial role in gas sensing properties of the metal oxide material, the defect induced room temperature ferromagnetism in undoped SnO2 has been used as a potential tool to probe the evidence of the defects. Finally a correlation is established between observed room temperature ferromagnetism and gas sensing studies and primary role of defects in gas sensing mechanism over microstructure is realized . The Chapter 6 presents the deposition of SnO2 thin films by UNSPACM method on glass substrates for gas sensing application. The readiness of UNSPACM in making sensor materials with unform dopant distribution is demonstrated in order to improve the sensor performance in terms of response and selectivity. The chemical composition, film morphology and gas sensing studies are reported. The SnO2 is doped with Cr and Pt to enhance the sensing properties of the material. The doped Oxide films are found to show enhancement in sensitivity and improve the selectivity of the films towards specific gases like NO2 and CO. Further in Chapter 7 an effort has been made to overcome the problem of high operating temperature of metal oxide gas sensors through use of Reduced Graphene Oxide (RGO) and metal oxide nanocomposite films. Although RGO shows room temperature response towards many toxic and hazardous gases but it exhibits poor sensor signal recovery. This has been successfully solved by making nanohybrids of RGO and SnO2. It not only improves the sensor signal kinetics but it enhances the sensitivity also. Thus this chapter endeavors towards low power consumption gas sensing devices. The key findings and future aspects are summarized in the Chapter 8.

Recently, there has been an increasing interest in the electronics world for those aspects related to semiconducting gas sensor (SGS) materials. In view of the increasingly strict legal limits for pollutant gas emissions, there is a great interest in developing high performance gas sensors for applications such as controlling air pollution and exhaust gases in automotive industry. In this way, semiconductor gas sensors offer good advantages with respect to other gas sensor devices, due to their simple implementation, low cost and good stability and sensitivity. The first part of the thesis is dedicated to the synthesis, film structural and sensitivity study of the Tin Oxide film deposited by RF sputtering, doped with noble metal Palladium (Pd). Effects on the Gas Sensitivity due to the deposition parameters like thickness of the film, Substrate temperature, Ar /O2 ratio of the sputtering environment, annealing temperature and duration and doping metal weight % into the Tin Oxide films are studied and the results are shown in detail. The sensitivity and selectivity of the gas sensing film is decided by the operating temperature i.e. the temperature of the gas sensing film while it is in the target gas ambience, Microheaters happen to be the very important component in the gas sensor especially with wide band gap semiconducting metal oxides films such as tin oxide, gallium oxide or indium oxides. Other than gas sensing microheater also finds applications in many areas like thermal dip pen nanolithography, polymerase chain reaction (PCR), fluid pumping with bubbles, in vitro fertilization etc. So in this report due importance was given for the design and fabrication of the microheater. Microheaters are the most power consuming element of the integrated Gas sensors. This is also an important reason for the extensive microheater work in this research. Six different heater patterns were simulated by considering low power and temperature uniformity as an important goals. Among them the best three patterns named Double spiral, “Fan” Shape and “S” shape were chosen for fabrication and both thermal and electrical characterization results of them were presented in detail in the Microheater section of the thesis. It is believed that the intelligent design and integration of the electronic circuitry (for drive, signal conditioning/compensation, and read-out) with the gas sensing element can mitigate some of the significant issues inherent in solid-state gas sensors, such as strong temperature and humidity dependence, signal drift, aging, poisoning, and weak selectivity. The sensitivity of the gas sensors which has been indicated as the dynamic change of resistance in wide range should be read out properly. Towards this aim a low cast high efficient readout circuit is designed and implemented. Temperature monitoring and controlling is a key phenomenon in the metal Oxide based gas sensors since the selectivity mainly depends on the operating temperature of the sensing film. So focus was also shown on the design and implementation of the temperature monitoring and control unit, which been presented in the last part of this thesis.