6.1 Summary. Summary and Scope for Further Study

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1 Summary and scope for further study Summary and Scope for Further Study 6.1 Summary Detecting gases and determining their composition has constantly been on the increase in recent years. The demand for better environmental control and safety has increased research activities of solid-state gas sensors. Gas sensors using metal oxide semiconductors have been the subject of extensive investigations for more than three decades, primarily focusing on SnO 2. In more recent research, the interests shifted to some other promising metal oxides, with interesting properties as gas sensing materials. Using metal oxides has several advantages, features such as simplicity in device structure, low cost for fabrication, robustness in practical applications and adaptability to a wide variety of reducing or oxidising gases. Gas sensors based on nano metal oxide materials are the focus of current research activity of sensor designers. One of the characteristics of nanometric materials is their high surface area/volume ratio. Many studies, then, have focused on reducing the size of the metal oxides in the form of nanoparticles and nanowires. Also reactions at grain boundaries and complete depletion of carriers in the grains can strongly modify the material transport properties in nanostructured oxides In recent years the efforts of gas sensor designers were aimed at: (a) increasing of gas sensitivity and improvement of their selectivity (b) reduction of electric power consumption by resistive heater of gas sensors, and (c) decreasing of response and recovery times. One of the main 263

2 Chapter - 6 challenges to the developers of metal-oxide gas sensors is high selectivity without affecting the sensitivity of the sensor. In this present work we have made an attempt to address these problems. The aim of the present work is development of pure and doped semiconducting metal oxide for the detection of pollutant gases like NO 2 and H 2 S gases. The first part of this work is dedicated to the design and fabrication of a gas sensing measurement chamber for the purpose of sensing studies. The chamber was fabricated incorporating the facilities for both static and flow through method. In all the gas sensing measurements carried out we have used the static measurement method. To study the temperature dependent gas sensing properties of the semiconducting metal oxide gas sensors a microcontroller based PID temperature controller was designed. The temperature controller has an accuracy of C. A RTD was used as the temperature sensor. The temperature controller is able to vary temperature in the range of 50 to C. The resistance variation of sensor in presence of target gas was measured using a keithely 195A digital multimeter. A GPIB interface was used to interface the multimeter to the system. The second part of this work is devoted to the preparation of active layers for the sensing application and their electrical and structural characterization. Nanocrystalline thick and thin films based on semiconducting metal oxides are prepared as active layers for the purpose of gas sensing application. Semiconducting metal oxides like tungsten oxide, zinc oxide and indium oxide are selected for the active layer materials. The effect of doping on the gas sensing properties of pure metal oxides is studied by adding dopants like copper and indium. The prepared sensors are characterized using different analytical characterisation tools like XRD, SEM, EDS and Raman spectroscopy. All sensor films were annealed at C overnight prior to gas 264

3 Summary and scope for further study sensing studies to reduce the posterior material instabilities. Concentration dependent studies for all the sensors were obtained at the optimum operating temperature. Pure and copper doped nanocrystalline tungsten oxide powders were prepared and thick films of the sensor are obtained on glass substrate. Pure and doped thick film sensors are investigated for NO 2 and H 2 S gas sensing properties. The temperature dependent sensitivity studies are performed to obtain the optimum operating temperature. The addition of copper is found to enhance the NO 2 gas sensing property of the sensor. The 3wt% copper doped tungsten oxide thick film has sensitivity 4 times higher than the undoped for 7 ppm concentration. The incorporation of copper may lead to the formation of Cu-O-W bonds that creates adsorption sites of copper and tungsten due to their change in oxidation states. Hence the target gas molecules have more adsoption sites to react with the sensor thereby increasing the sensitivity of the sensor. With copper addition it is found that optimum operating temperature decreases from 200 to C. The H 2 S gas sensing investigation reveal that with copper addition the sensitivity of the tungsten oxide was decreased. Hence the copper doped studies performed show that the copper doped tungsten oxide sensors is more selective to NO 2 gas. Pure and indium doped ZnO thin film sensor prepared by spray pyrolysis is also studied for gas sensing application. The temperature dependent sensitivity studies are performed and optimum operating temperature for pure and doped sensors is found to be C. The doped film sensing studies proved that the pure ZnO itself is a promising candidate for gas sensing application for both the test gases studied. Contrary to enhancement effect in sensitivity by the presence of additives, it was found 265

4 Chapter - 6 that incorporation of indium decreases gas sensing ability of the sensor. This decrease in sensitivity is attributed to the increase in free electron concentration and a consequent decrease in resistivity occurs due to replacement of Zn 2+ cation by the In 3+ cation, which acts as a donor. No variation in optimum temperature is obtained due to the indium doping. NO 2 gas sensing studies performed on pure and copper doped indium oxide thick film sensors proved that with copper doping the sensitivity of pure films can be enhanced. Maximum sensitivity for pure and doped sensors is achieved at a temperature of C. Pure sensor showed a maximum sensitivity of 2.82 to 7 ppm concentration of NO 2 at C while 3wt% copper doped sensor showed a sensitivity of 8.83 for the same concentration and temperature. Two possible mechanisms are suggested for enhanced gas sensing. One is the increase in In 2+ centers that occur due to copper addition. Another mechanism is due to cuprous ions Cu +, which coexists with cupric ions Cu 2+ that can enhance the gas sensing property of doped sensors. With introduction of copper the response and recover time was also found to improve for NO 2 gas sensing. The improved response as well as recovery time performance of doped In 2 O 3 over pure In 2 O 3 is due to the catalytic interaction of copper ion with the gas species to be detected. H 2 S gas sensing investigation on pure and copper doped indium oxide thick film sensor also show an improved sensitivity for copper doping. It is attributed to p-n heterojunction formed by CuO-In 2 O 3. In presence of H 2 S gas CuO gets converted to metallic CuS which is very conducting and hence enhance the sensor performance. The room temperature studies performed presents that copper doped indium oxide sensors have a very response and recovery time. The 3wt% copper doped sensor has a recovery time of

5 Summary and scope for further study minutes for ppm of H 2 S gas concentration studied while pure sensor has a recovery time of minutes. The concentration dependent studies at room temperature show that as copper doping amount increased the sensitivity of the sensor also increased. The pure sensor has a sensitivity of 7.26 to ppm at room temperature while 3wt% copper doped sensor has a sensitivity of Scope of Further Study The scope of extending the current work for further research is suggested below. Reduction in particle size can enhance the gas sensing property considerably. To improve the gas sensor response it would be interesting to reduce the grain size under 20nm and to examine how the sensitivity and selectivity of sensors changes as size of these nanoparticles approach to Debye length. The stability of the obtained sensors should be confirmed by long term tests under real working conditions. Longer annealing temperature should also be explored in order to achieve high stability. Gas response studies depending on different annealing temperature can also be performed. The adsorption-desorption kinetics of the gas can drastically vary depending on the nature of the surface. A deliberate creation of defects, nonstoichiometry or incorporation of various materials on the surface can enhance the sensitivity and selectivity of the sensors. Addition of other metal additives can also be performed. Variation in surface morphology related studies can be further done for the purpose of gas sensing. Nanostructured materials like nanotubes, 267

6 Chapter - 6 nanowires, nanorods and nanofiber exhibit a greater response to gas sensing due to the larger surface area to volume ratio. These surface morphologies can enhance the detection levels to ppb range. An integrated micro machined gas sensor array, associated with pattern recognition (PARC) techniques, such as artificial neural networks (ANNs), is also proposed to overcome problems associated with poor selectivity encountered during operation of individual gas sensor