En aquesta tesi es detalla la metodologia per obtindre sensors de gasos basats en òxid de tungstè nanoestructurat sobre suports micromecanitzats de silici. Aquesta nanoestructuració s’ha fet mitjançant una capa d’alúmina porosa como a motlle, pel que s’ha desenvolupat una metodologia per a compatibilitzar l'anodització de l’alumini, i altres metalls com el tungstè, amb els processos estàndards del silici. S’han desenvolupat dos tipus de capes nanoestructurades, nanotubs i nanopunts de WO3. Els nanotubs s’han obtingut depositant mitjançant polvorització catòdica reactiva la capa sensible sobre alúmina porosa recobrint les parets dels pors. Els nanopunts s’han obtingut anoditzant una bicapa d’alumini i tungstè, on la primera anodització crea la alúmina porosa i la segona fa créixer els nanopunts d’òxid de tungstè en la base dels pors. S’ha analitzat la composició, morfologia i funcionament com a sensors de gasos d’ambdós materials nanoestructurats i s’han comparat els resultats amb sensors basats en materials sense nanoestructuració. / En esta tesis se detalla la metodologia para obtener sensores de gases basados en óxido de tungsteno nanoestructurado sobre soportes micromecanizados de silicio. Dicha nanoestructuración se ha obtenido empleando una capa de alúmina porosa como molde, por lo que se desarrolla una metodología para compatibilizar la anodización del aluminio, y otros metales como el tungsteno, con los procesos estándares del silicio. Se han desarrollado dos tipos de capas nanoestructuradas, nanotubos y nanopuntos de WO3. Los nanotubos se han obtenido depositando por pulverización catódica reactiva la capa sensible sobre alúmina porosa recubriendo las paredes de sus poros. Los nanopuntos se han obtenido anodizando una bicapa de aluminio y tungsteno, donde la primera anodización crea la alúmina porosa y la segunda hace crecer los nanopuntos de óxido de tungsteno en la base de los poros. Se ha analizado la composición, morfología y funcionamiento como sensores de gases en ambos casos y se han comparado los resultados con los de sensores sin nanoestructuración. / This thesis shows the methodology to obtain nanostructured tungsten oxide layer as sensing material on silicon micromachined gas sensor devices. A porous anodised alumina layer was used as pattern to obtain it, so a technique has been developed to make compatible the anodising of aluminium and other metals like tungsten with the standard silicon processes. Two different nanostructuring approaches were developed, nanotube and nanodot based tungsten oxide layers. The WO3 nanotube layer has been obtained by the tungsten oxide deposition using reactive sputtering on the porous alumina layer. As a result a continuous sensing layer coats the pores without clogging them. WO3 nanodot layers were obtained by the anodising of an aluminium and tungsten bilayer, where the first anodising process grows the porous alumina layer and the second one generates the tungsten oxide nanodots in the end of the pores. Compositional and morphological studies and the study of their behaviour as gas sensors where conducted for the two nanomaterials. The results have been compared with the flat tungsten oxide layers on micromachined gas sensors.

abstract: There are increasing demands for gas sensors in air quality and human health monitoring applications. The qualifying sensor technology must be highly sensitive towards ppb level gases of interest, such as acetylene (C2H2), hydrogen sulfide (H2S), and volatile organic compounds. Among the commercially available sensor technologies, conductometric gas sensors with nanoparticles of oxide semiconductors as sensing materials hold significant advantages in cost, size, and cross-compatibility. However, semiconductor gas sensors must overcome some major challenges in thermal stability, sensitivity, humidity interference, and selectivity before potential widespread adoption in air quality and human health monitoring applications. The focus of this dissertation is to tackle these issues by optimizing the composition and the morphology of the nanoparticles, and by innovating the structure of the sensing film assembled with the nanoparticles. From the nanoparticles perspective, the thermal stability of tin oxide nanoparticles with different Al dopant concentrations was studied, and the results indicate that within certain range of doping concentration, the dopants segregated at the grain surface can improve the thermal stability by stabilizing the grain boundaries. From the sensing film perspective, a novel self-assembly approach was developed for copper oxide nanosheets and the sensor response towards H2S gas was revealed to decrease monotonically by more than 60% as the number of layers increase from 1 to 300 (thickness: 0.03-10 μm). Moreover, a sensing mechanism study on the humidity influence on H2S detection was performed to gain more understandings of the role of the hydroxyl group in the surface reaction, and humidity independent response was observed in the monolayer film at 325 ℃. With a more precise deposition tool (Langmuir-Blodgett trough), monolayer film of zinc oxide nanowires sensitized with gold catalyst was prepared, and highly sensitive and specific response to C2H2 in the ppb range was observed. Furthermore, the effect of surface topography of the monolayer film on stabilizing noble metal catalyst, and the sensitization mechanism of gold were investigated. Lastly, a semiconductor sensor array was developed to analyze the composition of gases dissolved in transformer oil to demonstrate the industrial application of this sensor technology. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

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Today there is a great deal of interest in the development of gas sensors for applications like air pollution monitoring, indoor environment control, detection of harmful gases in mines etc. Based on different sensing principles, a large variety of sensors such as semiconductor gas sensors, thermoelectric gas sensors, optical sensors and thermal conductivity sensors have been developed. The present thesis reports a detailed account of a novel method followed for the design and development of a thermoelectric gas sensor for sensing of Volatile Organic Compounds. Thermoelectric effect is one of the highly reliable and important working principles that is widely being put into practical applications. The thermoelectric property of semiconducting tin oxide film has been utilized in the sensor that has been developed. The thermoelectric property of semiconducting tin oxide film has been utilized in the sensor. The deposition parameters for sputtering of tin oxide film have been optimized to obtain a high seebeck coefficient. A test set-up to characterize the deposited films for their thermoelectric property has been designed and developed. A novel method of increasing the seebeck coefficient of tin oxide films has been successfully implemented. Thin films of chromium, copper and silver were used for this purpose. Deposition of the semiconducting oxide on strips of metal films has led to a noticeable increase in the seebeck coefficient of the oxide film without significantly affecting its thermal conductivity. The next part of our work involved development of a gas sensor using this thermoelectric film. These sensors were further tested for their response to volatile organic compounds. The sensor showed significant sensitivity to the test gases at relatively low temperatures. In addition to this, the developed sensor is also selective to acetone gas.

The atmosphere we live in contains various kinds of chemical species, natural and artificial, some of which are vital to our life, while many others are more or less harmful. The vital gases like oxygen, humidity have to be kept at adequate levels in the living atmosphere, whereas the hazardous and toxic gases like hydrocarbons, H2, volatile organic compounds, CO2, CO, NOx, SO2, NH3, O3 etc should be controlled to be under the designated levels. The measurement technology necessary for monitoring these gases has emerged, particularly as organic fuels and other chemicals have become essential in domestic and industrial life. In addition to other applications, environmental pollution monitoring and control has become a fundamental need in the recent years. Therefore, there has been an extensive effort to develop high-performance chemical sensors of small size, rugged construction, light weight, true portability, and with better sensing characteristics such as high sensitivity, fast response and recovery times, low drift, and high degree of specificity. Among the various types of gas sensors studied, solid state gas sensors based on semiconducting metal oxides are well established, due to their advantages over the other types, and hence cover a wide range of applications. However, the widespread application of these sensors has been hindered by limited sensitivity and selectivity. Various strategies have been employed in order to improved the performance parameters of these sensors. This thesis work has two major investigations, which form two parts of the thesis. The first part of this thesis describes the efforts to improve the sensing behaviour of one of the extensively studied metal oxide gas sensors, namely, ZnO, through a novel, ultrasonic-nebulised spray pyrolyis synthesis method, employing an aqueous combustion mixture (NSPACM). The second part of the thesis deals with the ideal of gas detection by optical means through the reversible phase transformation between V2O5 and V6O13 deposited by metalorganic chemical vapor deposition(MOCVD). The introductory chapter I deals with basics of chemical sensors and the characteristic sensing parameters. Different types of gas sensors based on the phenomena employed for sensing are discussed, with an emphasis on semiconducting metal oxide gas sensors. The importance of material selection for solid state gas sensors, depending on the purpose, location, and conditions of operation are discussed, supporting the assertion that semiconducting metal oxides are better suited to fulfill all the requirements of modern gas sensors. Some of the effective methods to improve performance parameters including the influence of grain size, microstructure, and surface doping are described., followed by the motivation of the present thesis. The part I of the thesis is based on the resistive semiconducting metal oxide, where the system investigated was ZnO. Part one comprises Chapters 2, 3 and 4. In Chapter 2, a brief introduction to the material properties of ZnO, followed by various synthesis techniques are discussed. An overview of spray pyrolysis and combustion synthesis is followed by the details of the method employed in the present study, namely NSPACM, which is based on the above two methods, for the formation of ZnO films. A detailed description of the film deposition system built in house is presented, followed by the deposition procedure and the parameters used. Thermal study of the combustion mixture and non-combustion precursor shows the importance of the fuel, along with oxidizer, in forming the film. The films formed using combustion mixture are found to be polycrystalline, whereas films formed without combustion were found to have preferred crystallographic orientation even on an amorphous substrate, which is explained on the basis of minimization of surface energy. The observed unique microstructure with fine crystallite size and porous morphology is attributed to the combustion method employed, which is interesting from the point of view of gas sensing. Chapter 3 concerns the gas sensing study of these ZnO films. The design of the home made gas sensing system is explained in detail. The study of electrode characteristics is followed by the important steps in gas sensing measurements. ZnO gas sensors were mainly studied for their selectivity between aliphatic and aromatic hydrocarbons. The results show two regions of temperature where the sensitivity peaks for aliphatic hydrocarbons, whereas aromatic hydrocarbons show a single sensitive region. This observation can pave the way for imparting selectivity. Possible reasons for the observed behavior are mentioned. Chapter 4 describes the chemical and physical modifications done to ZnO thin films by doping with catalysts, and through the use of x-y translational stage for large-area deposition.. Homogenous distribution of catalysts achieved by the NSPACM synthesis procedure, determined by the x-ray elemental mapping, is discussed. The addition of catalysts improved the sensing both because of catalytic effects and by promoting preferred crystallographic orientation, with Ni addition showing the better effects. The use of the x-y stage in producing the films with high orientation, which improved the gas sensing behavior, is explained. Part II of the thesis comprises Chapters 5,6 and 7, and describes a detailed study of V2O5 and V6O13 thin films deposited by MOCVD for optical sensing of chemical species. In Chapter 5, a brief introduction to chemical vapor deposition is given, followed by the importance of the characteristics of CVD precursors – in particular, the importance of their thermal behavior in film formation. This is followed by the importance of vapor pressure and partial pressure studies in the MOCVD of oxides of a multivalent metal such as vanadium. Various techniques of measuring vapor pressure are listed, followed by the details of the method used in the present study employing rising temperature thermogravimetry, based on the Langmuir equation. Thermogravimetric analysis performed, both at atmospheric as well as at low pressure, using commercial and home made apparatus, respectively is discussed. A detailed description of the home made setup is also presented. Chapter 6 describes the application of the vapor pressure and partial pressure studies to the deposition of films using MOCVD. Here, a detailed description of the vanadium oxide phase diagram and the stability of various phases is presented, which points the importance of precise parameter control during the deposition to obtain pure phases. The details of the CVD setup, followed by the procedure and parameters of deposition, are presented. The films deposited at various deposition temperatures, analyzed using XRD and SEM, are discussed. The effect of temperature on the growth is explained. The effect of vapor pressure is studied by varying the precursor vaporizer temperature, with a growth temperature maintained invariant. The influence of the amount of precursor on film growth, with a particular crystalline orientation and phase content, is explained followed by the description of the deposition of pure phases of V2O5 and V6O13 through the optimization of CVD parameters. Chapter 7 deals with the optical study of the films deposited by the above method. Here, the importance of two phases of vanadium oxide, V2O5 and V6O13, to the proposed gas sensing action, is presented. Their structural similarity in terms of polyhedral arrangement in the ab plane can be the basis of a reversible phase change. The difference in the optical transmittance in two phases forms the basis for the optical method for chemical sensing. The details of the laser-based optical sensing setup, its, design and the detection method, are explained. Studies on hydrocarbon sensing with vanadium, pentoxide films are also presented. The novelty in using reversible chemical transformation of a material system for detection of reducing and oxidizing gases in the ambient gases is discussed. Chapter 8 provides a summary of the present thesis, together with the main conclusions. The work reported in this thesis has been carried out by the candidate as part of the Ph.d training programme. She hopes that this would constitute a worthwhile contribution towards the understanding and subsequent application of ZnO and oxides of vanadium(V2O5 and V6O13) as novel gas sensors which will be useful for environmental protection, as well as for safety in industrial an domestic sectors.

The basic necessities to sustain life are – air, water and food. Although the harmful effects due to contaminated food or water are dangerous to life, these can be reduced/avoided by controlling the intake. Whereas, in case of air, the same amount of control cannot be exercised as there is very little, one can do in case of inhalation. Maximum damage to life is due to air contamination which can be detected and prevented by using gas sensors. The proper use of these sensors not only save lives, but also minimizes social and financial loss. The objective of this thesis work is to study and explore the use of p-type semiconducting material such as CuO, as a promising gas sensing material for organic compounds (VOCs), compatible with existing silicon fabrication technology. The Thesis consist of 7 chapters: Chapter 1 covers the general introduction about gas sensors, sensor parameters, criteria for the selection of sensing material, suitability of CuO as sensing material and a brief literature survey. The second chapter includes the selection of substrate, cleaning procedures and suitable deposition method. The deposition method used in the present thesis work is DC/RF magnetron sputtering. The reactive magnetron sputtering is employed during the deposition of CuO sensing films. It also includes basic introduction about some of the common material characterization techniques. This is followed by Chapter 3 which includes the optimization of sputtering process parameters such as applied power, working pressure, Ar-O2 ratio and substrate temperature for CuO sensing film and the effect of these on surface morphology. Information on the optimized sputtering parameters for electrode film (silver and gold) deposition has also been included in this chapter. In order to study the sensing behavior of the sensor, suitable testing set-up is necessary. This leads us to Chapter 4 that discusses the development of an in-house built sensor testing setup and its automization using MATLAB. The automated testing set-up facilitates off-time data plotting as well as real-time data plotting during the sensing process. To demonstrate the working of the set-up, some initial results obtained are also included in this chapter. After ascertaining the functioning of the automated gas sensor testing set-up, detailed study on the sensing behavior of nanostructured CuO films was performed. This information along with the necessary details is included in Chapter 5. The sensing response of nanostructured CuO films has been studied for different VOCs such as alcohol, toluene and benzene. The study carried out on the effect of different surface additives like multi-walled carbon nanotubes (MWNTs), gold or platinum on ethanol sensing has also been included in this chapter. During the use of MWNTs as surface additives, different concentrations of MWNTs – 0.01 mg, 0.05 mg and 0.1 mg have been dispersed on the CuO sensing film. The sample with lowest concentration of MWNTs exhibited highest sensitivity and lower response time. It is due to the fact that, higher concentrations of MWNTs do not result into uniform dispersion over the CuO films and cover the sensing film almost completely. Operating temperature is the most important factor affecting the performance of a gas sensor. In order to maintain the operating temperature for the portable sensor, the sensor is usually integrated with a heater. The chapter 6 deals with heater optimization including design, simulation and fabrication. In this chapter, microheater as well as macro-heaters were simulated and fabricated. The fabricated macro-heater is bonded with the sensor by eutectic bonding. One of the bonded samples was studied for its sensing response. The final chapter of the thesis deals with the conclusion of present research work and the possible further work on CuO gas sensor.

In this work the influence of solid electrolyte additives on the gas sensing properties of tin oxide layers was investigated systematically for the first time. NASICON (NAtrium, Super Ionic CONductor, Na(1+x)Zr2SixP(3-x)O12; 0 <= x <= 3) was used as a model for solid electrolyte additives. The structure of that material is ideally suitable for studies of the correlation between material parameters and the gas sensitivity of the layers. In the NASICON structure the content of mobile Na+-ions can be varied by a factor of four resulting in a simultaneous change of the ionic conductivity sigma(Na+) by approximately three orders of magnitude without considerable structural alterations. Powders of SnO2 and NASICON (x = 0; 2.2; 3) were prepared separately by means of sol-gel routes and mixed in a volume ratio of 80/20. Pastes were prepared from these powders with different compositions and screen printed on alumina substrates with a fourfold structure of thin film gold electrode combs. Four different compositions were characterised simultaneously at elevated temperatures in various gas atmospheres. The conductivity of the layers, when measured in air, decreases considerably with increasing Na+-content in the NASICON additive. This is correlated with enhanced activation energy of the electronic conductivity. The sensitivity of the layers to polar organic molecules like R-OH (alcohols), R-HO (aldehydes) and ROOH (carboxylic acids) is highly enhanced by the NASICON additive. This is observed especially on the admixtures with NASICON of high Na+-content (x = 2.2 and x = 3). On the other hand, the sensitivity to substances with mid-standing functional groups like 2-propanol or propanone can not be enhanced by NASICON additives. Furthermore the sensitivity of these composite layers to CO, H2, NH3, methane, propane, propene and toluene (all exposed as admixtures with air) is lower than the sensitivity of pure SnO2-layers. These observations are well correlated with the results of gas consumption measurements on SnO2/NASICON powders by means of FTIR spectroscopy. In spite of the lack of surface analytical data, a model of surface chemical gas reactions based on a triple phase boundary (SnO2/NASICON/gas atmosphere) was developed, which explains the experimental observations qualitatively. It is assumed that the decrease of the electronic conductivity as observed in the presence of NASICON additives with increasing Na+-content is due to an enhanced electron depletion layer. This is formed in the SnO2 grains by Na+/e- interactions across the SnO2/NASICON-interface. The enormous enhancement of the sensitivity to polar organic molecules may be due to specific nucleophilic interactions with the Na+-ions and coupled Na+/e--interactions at the triple phase reaction sites.

In this work the influence of solid electrolyte additives on the gas sensing properties of tin oxide layers was investigated systematically for the first time. NASICON (NAtrium, Super Ionic CONductor, Na(1+x)Zr2SixP(3-x)O12; 0 <= x <= 3) was used as a model for solid electrolyte additives. The structure of that material is ideally suitable for studies of the correlation between material parameters and the gas sensitivity of the layers. In the NASICON structure the content of mobile Na+-ions can be varied by a factor of four resulting in a simultaneous change of the ionic conductivity sigma(Na+) by approximately three orders of magnitude without considerable structural alterations. Powders of SnO2 and NASICON (x = 0; 2.2; 3) were prepared separately by means of sol-gel routes and mixed in a volume ratio of 80/20. Pastes were prepared from these powders with different compositions and screen printed on alumina substrates with a fourfold structure of thin film gold electrode combs. Four different compositions were characterised simultaneously at elevated temperatures in various gas atmospheres. The conductivity of the layers, when measured in air, decreases considerably with increasing Na+-content in the NASICON additive. This is correlated with enhanced activation energy of the electronic conductivity. The sensitivity of the layers to polar organic molecules like R-OH (alcohols), R-HO (aldehydes) and ROOH (carboxylic acids) is highly enhanced by the NASICON additive. This is observed especially on the admixtures with NASICON of high Na+-content (x = 2.2 and x = 3). On the other hand, the sensitivity to substances with mid-standing functional groups like 2-propanol or propanone can not be enhanced by NASICON additives. Furthermore the sensitivity of these composite layers to CO, H2, NH3, methane, propane, propene and toluene (all exposed as admixtures with air) is lower than the sensitivity of pure SnO2-layers. These observations are well correlated with the results of gas consumption measurements on SnO2/NASICON powders by means of FTIR spectroscopy. In spite of the lack of surface analytical data, a model of surface chemical gas reactions based on a triple phase boundary (SnO2/NASICON/gas atmosphere) was developed, which explains the experimental observations qualitatively. It is assumed that the decrease of the electronic conductivity as observed in the presence of NASICON additives with increasing Na+-content is due to an enhanced electron depletion layer. This is formed in the SnO2 grains by Na+/e- interactions across the SnO2/NASICON-interface. The enormous enhancement of the sensitivity to polar organic molecules may be due to specific nucleophilic interactions with the Na+-ions and coupled Na+/e--interactions at the triple phase reaction sites.