Metal oxide gas sensors are predominant solid-state gas detecting devices for domestic, commercial and industrial applications, which have many advantages such as low cost, easy production, and compact size However, the performance of such sensors is significantly influenced by the morphology and structure of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films to achieve highly-sensitive properties Lots of metal oxide nanostructures have been developed to improve the gas sensing properties such as sensitivity, selectivity, response speed, and so on Here, we provide a brief overview of metal oxide nanostructures and their gas sensing properties from the aspects of particle size, morphology and doping When the particle size of metal oxide is close to or less than double thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably, which would be called "small size effect", yet small size of metal oxide nanoparticles will be compactly sintered together during the film coating process which is disadvantage for gas diffusion in them In view of those reasons, nanostructures with many kinds of shapes such as porous nanotubes, porous nanospheres and so on have been investigated, that not only possessed large surface area and relatively mass reactive sites, but also formed relatively loose film structures which is an advantage for gas diffusion Besides, doping is also an effective method to decrease particle size and improve gas sensing properties Therefore, the gas sensing properties of metal oxide nanostructures assembled by nanoparticles are reviewed in this article The effect of doping is also summarized and finally the perspectives of metal oxide gas sensor are given

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Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review

Xin Zhou,†,‡ Songyi Lee,† Zhaochao Xu,* and Juyoung Yoon*,† †Department of Chemistry and Nanoscience, Ewha Womans University, Seoul 120-750, Republic of Korea ‡Research Center for Chemical Biology, Department of Chemistry, Yanbian University, Yanjii 133002, People’s Republic of China Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Shahekou, Dalian, Liaoning, People’s Republic of China

Recent Progress on the Development of Chemosensors for Gases.

Considering metal oxide nanoparticles as important technological materials, authors provide a comprehensive review of researches on metal oxide nanoparticles, their synthetic strategies, and techniques, nanoscale physicochemical properties, defining specific industrial applications in the various fields of applied nanotechnology. This work expansively reviews the recent developments of semiconducting metal oxide gas sensors for environmental gases including CO2, O2, O3, and NH3; highly toxic gases including CO, H2S, and NO2; combustible gases such as CH4, H2, and liquefied petroleum gas; and volatile organic compounds gases. The gas sensing properties of different metal oxides nanoparticles towards specific target gases have been individually discussed. Promising metal oxide nanoparticles for sensitive and selective detection of each gas have been identified. This review also categorizes metal oxides sensors by analyte gas and also summarizes the major techniques and synthesis strategies used in nanotechnology. Additionally, strategies, sensing mechanisms and related applications of semiconducting metal oxide materials are also discussed in detail. Related applications are innumerable trace to ultratrace-level gas sensors, batteries, magnetic storage media, various types of solar cells, metal oxide nanoparticles applications in catalysis, energy conversion, and antennas (including microstrip and patch-type optically transparent antennas), rectifiers, optoelectronic, and electronics.

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Metal oxide nanoparticles and their applications in nanotechnology

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.

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Semiconducting Metal Oxide Based Sensors for Selective Gas Pollutant Detection

Metal-oxide-semiconductor (MOS) based gas sensors have been considered a promising candidate for gas detection over the past few years. However, the sensing properties of MOS-based gas sensors also need to be further enhanced to satisfy the higher requirements for specific applications, such as medical diagnosis based on human breath, gas detection in harsh environments, etc. In these fields, excellent selectivity, low power consumption, a fast response/recovery rate, low humidity dependence and a low limit of detection concentration should be fulfilled simultaneously, which pose great challenges to the MOS-based gas sensors. Recently, in order to improve the sensing performances of MOS-based gas sensors, more and more researchers have carried out extensive research from theory to practice. For a similar purpose, on the basis of the whole fabrication process of gas sensors, this review gives a presentation of the important role of screening and the recent developments in high throughput screening. Subsequently, together with the sensing mechanism, the factors influencing the sensing properties of MOSs involved in material preparation processes were also discussed in detail. It was concluded that the sensing properties of MOSs not only depend on the morphological structure (particle size, morphology, pore size, etc.), but also rely on the defect structure and heterointerface structure (grain boundaries, heterointerfaces, defect concentrations, etc.). Therefore, the material-sensor integration was also introduced to maintain the structural stability in the sensor fabrication process, ensuring the sensing stability of MOS-based gas sensors. Finally, the perspectives of the MOS-based gas sensors in the aspects of fundamental research and the improvements in the sensing properties are pointed out.

Metal-oxide-semiconductor based gas sensors: screening, preparation, and integration

Thin films of pure nano-crystalline SnO 2 and WO 3 -doped SnO 2 in different concentrations (3 wt.% and 5 wt.%) are deposited using sol–gel spin coating technique on glass substrates. The structural and morphological properties of these films are investigated using XRD, TEM and AFM. The sensitivity and selectivity of these films are tested to different reducing and oxidizing gases such as SO 2 , NH 3 , NO 2 and ethanol. A SnO 2 thin film with 5 wt.% WO 3 shows very high responses to NO 2 (four orders of magnitude in the value of ( R g  −  R a )/ R a ; R g and R a are the resistance in NO 2 and in air, respectively) and excellent selectivity for NO 2 gas at a low operating temperature of 150 °C. The effect of WO 3 on the sensing characteristics of these films towards NO 2 is discussed.

Highly sensitive SnO2 thin film NO2 gas sensor operating at low temperature

Undoped and indium-doped SnO2 thin films with different doping concentrations have been prepared by sol–gel spin coating process on float glass substrates. XRD, TEM and AFM studies are carried out to investigate the effect of indium in SnO2 thin films. TEM studies reveal the dispersed nanoparticles of around 3 nm size (∼Debye length). In addition we have observed that indium doping improves the sensor response and selectivity towards NO2 gas at a low operating temperature and also resolves the problem of agglomeration of particles that is responsible for the lowering of sensor response and stability in the above mentioned particle size range. A very high sensor response of around 7200% has been observed in 10 wt% indium-doped SnO2 films to 500 ppm of NO2 gas at an operating temperature of 150 °C.

Effect of indium-doped SnO2 nanoparticles on NO2 gas sensing properties

This paper presents the gas sensing properties of MoO 3 (1, 5, 10 and 20 wt%) added SnO 2 thin films prepared by a sol–gel spin coating process on float glass substrates. The sensing responses of all these deposited films towards various gases (NO 2 , SO 2 , C 2 H 5 OH and NH 3 ) are examined. Microstructural studies have been carried out using XRD, atomic force microscopy (AFM) and transmission electron microscopy (TEM). The peculiarities of FT–IR spectra of the surface species arising from the addition of different concentrations of MoO 3 are analyzed. It has been observed that with an increase in doping concentration the reflectance decreases because of an increase in roughness and a decrease in the number of free electrons. The doped films show high selectivity towards NO 2 gas along with a sensor response of 360% at 170 °C to 500 ppm of gas with a 10 wt% MoO 3 added SnO 2 thin film.

Jaswinder Kaur

Papers

Highly sensitive SnO2 thin film NO2 gas sensor operating at low temperature

Effect of indium-doped SnO2 nanoparticles on NO2 gas sensing properties

Effect of MoO3 addition on the NO2 sensing properties of SnO2 thin films

Role of surface properties of MoO3-doped SnO2 thin films on NO2 gas sensing