Semiconducting Metal Oxide Based Sensors for Selective Gas Pollutant Detection

A review of some papers published in the last fifty years that focus on the semiconducting metal oxide (SMO) based sensors for the selective and sensitive detection of various environmental pollutants is presented

Review—Non-Invasive Monitoring of Human Health by Exhaled Breath Analysis: A Comprehensive Review

Exhaled human breath analysis is a very promisingfield of research work having great potential for diagnosis of diseases in non-invasive way. Breath analysis has attracted huge attention in thefield of medical diagnosis and disease monitoring in the last twodecades. VOCs/gases (Volatile Organic Compounds) in exhaled breath bear thefinger-prints of metabolic and biophysicalprocesses going on in human body. It’s a non-invasive, fast, non-hazardous, cost effective, and point of care process for diseasestate monitoring and environmental exposure assessment in human beings. Some VOCs/gases in exhaled breath are bio-markers ofdifferent diseases and their presence in excess amount is indicative of un-healthiness. Breath analysis has the potential for earlydetection of diseases. However, it is still underused and commercial device is yet not available owing to multiferrious challenges.This review is intended to provide an overview of major biomarkers (VOCs/gases) present in exhaled breath, importance of theiranalysis towards disease monitoring, analytical techniques involved, promising materials for breath analysis etc. Finally, relatedchallenges and limitations along with future scope will be touched upon.will be touched upon

Environmental Gas Sensors Based on Nanostructured Thin Films

Since the discovery of electron microscopes, nanomaterials and nanotechnology have been influencing academic and industrial research greatly for bringing out newer and better products with improved materials’ properties. In the field of environmental gas sensors too, the demonstration of nanomaterials for sensing various gases has become a common practice. Environmental gas pollution has turned out to be a huge concern in the society due to the progress of civilization. The awareness of health hazard for different toxic/polluting gases and rectification measure by imposing stricter norms has prompted extensive research to develop efficient gas sensors to detect trace level of pollution from various sources. Thin film, ultrathin film, and nanostructure materials of metal oxide semiconductor, polymer, metal, carbon nanotube, graphene, etc. with or without sensitizers have been investigated for sensing various toxic gases. New device structures have been fabricated to achieve high sensitivity, selectivity, fast response, etc. The microstructure and thickness of film are found to influence the performance greatly. Various methods of preparations and mechanism of sensing are being explored. All these aspects and the challenges were discussed in this chapter

Effects of Calcination Temperature and Acid-Base Properties on Mixed Potential Ammonia Sensors Modified by Metal Oxides

Mixed potential sensors were fabriated using yttria-stabilized zirconia (YSZ) as a solid electrolyte and a mixture of Au and various metal oxides as a sensing electrode. The effects of calcination temperature ranging from 600 to 1,000 °C and acid-base properties of the metal oxides on the sensing properties were examined. The selective sensing of ammonia was achieved by modification of the sensing electrode using MoO3, Bi2O3 and V2O5, while the use of WO3, Nb2O5 and MgO was not effective. The melting points of the former group were below 820 °C, while those of the latter group were higher than 1,000 °C. Among the former group, the selective sensing of ammonia was strongly dependent on the calcination temperature, which was optimum around melting point of the corresponding metal oxides. The good spreading of the metal oxides on the electrode is suggested to be one of the important factors. In the former group, the relative response of ammonia to propene was in the order of MoO3 > Bi2O3 > V2O5, which agreed well with the acidity of the metal oxides. The importance of the acidic properties of metal oxides for ammonia sensing was clarified

Detection of explosive markers using zeolite modified gas sensors

Detection of hidden explosive devices is a key priority for security and defence personnel around the globe. Electronic noses, based on metal oxide semiconductors (MOS), are a promising technology for creating inexpensive, portable and sensitive devices for such a purpose. An array of seven MOS gas sensors was fabricated by screen printing, based on WO3 and In2O3 inks. The sensors were tested against six gases, including four explosive markers: nitromethane, DMNB (2,3-dimetheyl-2,3-dinitrobutane), 2-ethylhexanol and ammonia. The gases were successfully detected with good sensitivity and selectivity from the array. Sensitivity was improved by overlaying or admixing the oxides with two zeolites, H-ZSM-5 and TS-1, and each showed improved responses to –NO2 and –OH moieties respectively. Admixtures in particular showed promise, with excellent sensitivity and good stability to humidity. Machine learning techniques were applied to a subset of the data and could accurately classify the gases detected, even when confounding factors were introduced

Exploitation of Unique Properties of Zeolites in the Development of Gas Sensors

The unique properties of microporous zeolites, including ion-exchange properties, adsorption, molecular sieving, catalysis, conductivity have been exploited in improving the performance of gas sensors. Zeolites have been employed as physical and chemical filters to improve the sensitivity and selectivity of gas sensors. In addition, direct interaction of gas molecules with the extraframework cations in the nanoconfined space of zeolites has been explored as a basis for developing new impedance-type gas/vapor sensors. In this review, we summarize how these properties of zeolites have been used to develop new sensing paradigms. There is a considerable breadth of transduction processes that have been used for zeolite incorporated sensors, including frequency measurements, optical and the entire gamut of electrochemical measurements. It is clear from the published literature that zeolites provide a route to enhance sensor performance, and it is expected that commercial manifestation of some of the approaches discussed here will take place. The future of zeolite-based sensors will continue to exploit its unique properties and use of other microporous frameworks, including metal organic frameworks. Zeolite composites with electronic materials, including metals will lead to new paradigms in sensing. Use of nano-sized zeolite crystals and zeolite membranes will enhance sensor properties and make possible new routes of miniaturized sensors

Nanotechnology Applications for Chemical and Biological Sensors

Recent discoveries indicate that when the materials are brought down to sizes in the range 1–100 nm, theseexhibit unique electrical, optical, magnetic, chemical, and mechanical properties. Methods have now beenestablished to obtain the monodisperse nanocrystals of various metallic and semiconducting materials, single-walled and multi-walled nanotubes of carbon and other metallic and non-metallic materials together withorganic nanomaterials such as supra-molecular nanostructures, dendrimers, hybrid composites with tailoredfunctionalities. The high surface-to-volume ratio with an added element of porosity makes these highly potentialcandidates for chemical and biological sensor applications with higher degree of sensitivity and selectivity ascompared to their bulk counterparts. The paper reviews the recent developments and applications of chemicaland biological sensors based on nanomaterials of various structural forms.Defence Science Journal, 2008, 58(5), pp.636-649, DOI:http://dx.doi.org/10.14429/dsj.58.168

NANOSTRUCTURED ARRAYS FOR SENSING AND ENERGY STORAGE APPLICATIONS

Vertically aligned multi walled carbon nanotube (MWCNT) arrays fabricated by xylene pyrolysis in anodized aluminum oxide (AAO) templates without the use of a catalyst, were integrated into a resistive sensor design. The steady state sensitivities as high as 5% and 10% for 100 ppm of NH3 and NO2 respectively at a flow rate of 750 sccm were observed. A study was undertaken to elucidate (i) the dependence of sensitivity on the thickness of amorphous carbon layers, (ii) the effect of UV light on gas desorption characteristics and (iii) the dependence of room temperature sensitivity on different NH3 and NO2 flow rates. An equivalent circuit model was developed to understand the operation and propose design changes for increased sensitivity. Multi Walled Carbon NanoTubes (MWCNTs) – Polymer composite based hybrid sensors were fabricated and integrated into a resistive sensor design for gas sensing applications. Thin films of MWCNTs were grown onto Si/SiO2 substrates via xylene pyrolysis using chemical vapor deposition technique. Polymers like PEDOT:PSS and Polyaniline (PANI) mixed with various solvents like DMSO, DMF, 2-Propanol and Ethylene Glycol were used to synthesize the composite films. These sensors exhibited excellent response and selectivity at room temperature when exposed to low concentrations (100ppm) of gases like NH3 and NO2. Effect of various solvents on the sensor response imparting selectivity to CNT – Polymer nanocomposites was investigated extensively. Sensitivities as high as 28% was observed for a MWCNT – PEDOT:PSS composite sensor when exposed to 100ppm of NH3 and -29.8% sensitivity for a MWCNT-PANI composite sensor to 100ppm of NO2. A novel nanostructured electrode design for Li based batteries and electrochemical capacitor applications was developed and tested. High density and highly aligned metal oxide nanowire arrays were fabricated via template assisted electrochemical deposition. Nickel and Molybdenum nanowires fabricated via cathodic deposition process were converted into respective oxides via thermal treatments and were evaluated as electrodes for batteries and capacitor applications via Cyclic Voltammetery (CV). Several chemical baths were formulated for the deposition of pristine molybdenum nanowires. Superior electrochemical performance of metal (Ni and Mo) oxide nanowires was observed in comparison to the previously reported nano-particle based electrodes

Room Temperature Gas Sensing Using Pure and Modified Metal Oxide Nanowires

Recently, various quasi 1D metal oxide semiconductor nanostructures (nanorods, nanowires, nanotubes, nanobelts) of various binary oxides have been found to be excellent materials for gas sensing. However, some of the sensitive gas sensors can work only at elevated temperatures. The sensing performance can be further improved when these oxides are doped with noble metal nanoparticles and form hetero-junction with other oxides, especially different types of metal oxide. These modifications can substantially change the surface properties as well as electronic properties due to their enhancement of the depletion layer at the metal nanoparticle-metal oxide nanowire and homo/hetero-interfaces. The objective of this dissertation study is to investigate the sensing performance of WO3, ZnO, NiO and TiO2 nanowires towards various air pollutant gases such as NH3, NO2, H2S and CO at room temperature. The sensing performance of pure metal oxide nanowires are further improved by doping these nanowires with noble metal nanoparticles and through the formation of n-p hetero-junction of two dissimilar oxides. Based on this study, it was found that pure ZnO and NiO nanowires show a high sensitivity and the best selectivity performance towards the ppm level NO2 (1 ppm) with respect to other interfering gases. On the other hand, both WO3/Ag and WO3-NiO gas sensors show enhanced sensing and highly selective performance towards H2S (~10ppm) at room temperature. Additionally, sensor response and recovery become faster with WO3/Ag than pure WO3 nanowires. The plausible reasons for such improvements with these surface modifications are discussed. This study provides a scientific foundation to engineer practical room-temperature gas sensors with enhanced performance