Light-activated room-temperature gas sensors based on metal oxide nanostructures: A review on recent advances

Many recent efforts are directed toward developing high-performance gas sensors based on metal oxide nanostructures operating at room temperature, as it lowers the power consumption, simplifies the device fabrication as well as improves the safety and stability of the sensors. The light-activated gas sensing technology was intensively studied because of its high effectiveness in improving the gas sensing performance of metal oxide nanostrctures at room temperature. This review is covers comprehensive advances in the emerging and feasible approaches for improving nanostructured metal oxide-based gas sensors by light activation, especially the progresses made in the last five years. We first summarize the effects of light-activation on gas sensing behavior of metal oxide nanostructures with some new insights into the related mechanisms. For enhancing the light-activated gas-sensing performance some possible strategies are then introduced, which include the modification of the size, dimension, nanoarchitecture, porous or hierarchical structure and doping or defect engineering, as well as the construction of nanocomposite sensing materials. Finally, some recent developments in light source and device structure design towards low power gas sensor systems are discussed. We hope that this review would provide some useful information to the design of light-activated metal oxide gas sensors operating at room temperature.     

Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review

Since the sensing capability of semiconducting metal oxides was demonstrated in the 1960s, solid state gas sensors based on these materials have attracted considerable attention from both scientific and practical point of view. Because of the promising characteristics for detecting toxic gases and volatile organic compounds (VOCs) compared to conventional techniques, these devices are expected to play a key role in environmental monitoring, chemical process control, personal safety and so on in the near future. Therefore, in recent years, intensive studies have been conducted to improve their sensing performances, particularly to increase the sensitivity and detection limit of such devices. This can be accomplished by using metal oxide nanostructures with various shapes such as nanoparticles, nanowires, nanorods and nanotubes having sizes in the nanometer range. Owing to the high surface-to-volume ratios and consequently large number of surface sites exposed to target gas, nanostructured metal oxides enable a larger gas-sensing layer interaction and hence a higher sensitivity in comparison with conventional materials.This article extensively reviews recent developments in this field, focusing the attention on the detection of some common VOCs, including acetone (C₃H₆O), acetylene (C₂H₂), benzene (C₆H₆), cyclohexene (C₆H₁₀), ethanol (C₂H₅OH), formaldehyde (HCHO), n-butanol (C₄H₉OH), methanol (CH₃OH) toluene (C₇H₈), and 2-propanol (C₃H₈O), by means of conductometric solid state sensors based on nanostructured semiconducting metal oxides.     

A review of recent developments in tin dioxide nanostructured materials for gas sensors

Gas sensors based on SnO₂ nanostructures have been extensively investigated in recent years. Many recent investigations have focused on synthesizing 0D, 1D, 2D and 3D SnO₂ nanostructures with high sensing capacity. This work presents a review of the recent developments in pure, doped and metal oxide functionalized SnO₂ nanostructured gas sensors, emphasizing the main SnO₂ preparation methods and the working principle of SnO₂ gas sensors. Most studies have shown that doping, coupled with a high surface area, can significantly improve SnO₂ sensing properties. Sensing response, response/recovery times, and operating temperature can be modulated by the synergistic effect between these two factors. In general, fine nanoparticles, mesoporous materials, hollow and 3D nanostructures combined with additives such as Pt, Pd, Cu, Ni, Ag and Al have shown the best improvements in gas sensing.     

基于金属氧化物的乙醇检测气敏材料的研究进展

金属氧化物型半导体气体传感器是目前常用的乙醇检测手段,深入研究和改进金属氧化物型半导体材料是提升传感器性能的重要方式。本文首先论述了气敏检测的机理和影响因素,并综述了近年来发展的主要金属氧化物型半导体气敏材料,重点介绍了不同微观结构的Co₃O₄、ZnO、SnO₂及掺杂金属氧化物材料、氧化物异质结等的研究和发展情况,对它们的合成方法、结构特点以及结构与乙醇气敏性能之间的关系进行了探讨。分析表明,减小材料颗粒尺寸、构建大比表面积多孔结构、掺杂和复合改性,是提升金属氧化物材料气敏性能的有效措施。此外,基于传感器微小化的趋势,以微机电系统（MEMS）工艺为基础的微型传感器成为气体传感器的发展趋势。然而,目前针对金属氧化物气敏材料的制备依然缺乏一定的理论指导,气体检测缺乏相应的机理研究,亟需物理、化学、材料等多学科的相互结合,促进乙醇等半导体气体传感器的进一步发展。     

沸石及其复合物气体传感器的研究与应用

半导体金属氧化物是一种常见的气敏材料,以该类型材料作为敏感材料可以设计出具有不同传感原理的气体传感器,但选择性和灵敏度不佳却一直是该类气体传感器的不足。为了解决该问题,常将气敏材料与沸石进行复合,制备金属氧化物/沸石气体传感器,利用沸石独特的物理、化学特性来改善金属氧化物的气敏特性。近年来,许多研究者对金属氧化物/沸石气体传感器进行了研究,使该类传感器对目标气体的选择性与灵敏度均有了提升。为了更好地总结已有的研究内容,以气体传感器的检测原理为主线,对金属氧化物/沸石气体传感器进行了总结,结合沸石对气敏特性的改善进行归纳梳理,从传感器的制备方法、气敏特性和敏感机理等多方面进行了详细的整理和分析,为后续此类工作的开展提供基础。     

Development of Agile Titania Sensors Via High-Temperature Reductive Etching Process (HiTREP©): I. Structural Reorganization

High selectivity, enhanced sensitivity, short response times and long shelf-life are much sought-after features in solid-state chemical sensors for the detection and metering of gas(es) of interest. Because the sensing mechanism of semiconducting oxides is invariably surface dominated, benign microscopic features are desirable to realize a useful sensing material. In principle, such morphological features could be incorporated in a number of semiconducting oxides by employing a technique based on thermodynamic consideration of the metal/metal oxide coexistence. By dynamically modulating the equilibrium oxygen partial pressure across the metal/metal oxide proximity line, renewed formulation and growth of an oxide surface on an atomic/submolecular level with exotic morphological features under conditions of oxygen "deprivation" or "enrichment" has been achieved practically in a number of potential ceramic sensor systems. In the case of oxides that are not amenable to such classical oxygen partial pressure modulation, a novel high-temperature reductive etching process (HiTREP^©) could be exploited to recreate the smart nanofeatures to impart the desired accentuation effect. This surface modification method was applied to a new commercially available aqueous plasma electro-deposited (PED) titania thick film, and the microscopic results of this strategy are presented.     

Effect of sintering temperature on the electrical and gas sensing properties of tin oxide powders

Tin oxide is an n-type semiconducting material having superior properties that can be utilized in several applications. The warning and detection of several dangerous gases in the environment are possible by utilizing gas sensors. The comprehensive functionality of these sensors could help to reduce the risk of severe health hazards and unexpected explosion risks. Tin oxide-based gas sensors exhibit reliable gas sensing performances along with respectful sensitivity and selectivity. Tin oxides in micro-and nano-particle forms provide an extremely high surface-to-volume ratio, which is favorable for gas sensors. Processing and synthesis of tin oxide particles accompany high-temperature processes, and this paper focuses on studying the effect of sintering temperatures on the structural and grain size of the commercially available tin oxide particles. The surface morphology of the tin oxide samples sintered at three different temperatures of 1100, 1200, 1300 ^°C shows a clear difference in the grain size and further affecting the dielectric properties of the materials. The gas sensing performances of three tin oxide samples are investigated by fabricating a pellet-type gas sensor. The sensor with the sintering temperature of 1200 ^°C exhibits the best gas-sensing performance with high response and low limit of detection (LOD). Our results suggest that the sintering temperature plays a vital role in deciding the dielectric properties and grain sizes, which are important parameters that affect the gas sensing behavior of tin oxide micro-and nano-particles.     

Hierarchical Assembly of Multifunctional Oxide-based Composite Nanostructures for Energy and Environmental Applications

Composite nanoarchitectures represent a class of nanostructured entities that integrates various dissimilar nanoscale building blocks including nanoparticles, nanowires, and nanofilms toward realizing multifunctional characteristics. A broad array of composite nanoarchitectures can be designed and fabricated, involving generic materials such as metal, ceramics, and polymers in nanoscale form. In this review, we will highlight the latest progress on composite nanostructures in our research group, particularly on various metal oxides including binary semiconductors, ABO(3)-type perovskites, A(2)BO(4) spinels and quaternary dielectric hydroxyl metal oxides (AB(OH)(6)) with diverse application potential. Through a generic template strategy in conjunction with various synthetic approaches- such as hydrothermal decomposition, colloidal deposition, physical sputtering, thermal decomposition and thermal oxidation, semiconductor oxide alloy nanowires, metal oxide/perovskite (spinel) composite nanowires, stannate based nanocompostes, as well as semiconductor heterojunction-arrays and networks have been self-assembled in large scale and are being developed as promising classes of composite nanoarchitectures, which may open a new array of advanced nanotechnologies in solid state lighting, solar absorption, photocatalysis and battery, auto-emission control, and chemical sensing.     

Nanostructured Tin Oxide as a Surface‐Enhanced Raman Scattering Substrate for the Detection of Nitroaromatic Compounds

Sensing of low concentrations of two nitroaromatic compounds, 1,2‐dinitrotoluene and 2‐nitrophenol, is presented. The sensing mechanism is based on surface‐enhanced Raman scattering (SERS) using nanostructured tin oxide as the SERS‐active substrate. The SnO_x nanostructures are synthesized by a simple solgel method and doped with Ag and Au. The Raman signal of a low concentration of the analyte, otherwise extremely weak, becomes significant when the analytes are attached to these substrates. Doping of SnO_x nanopowders with Ag and Au leads to a further increase in the Raman intensities. This study demonstrates the scope of ceramic–metal nanocomposites as convenient solid‐state SERS sensors for low‐level detection.     

Review of recent progress on graphene-based composite gas sensors

Development of gas sensors for detecting toxic, harmful, flammable, and explosive gases has always been very popular research direction. Graphene is considered potential chemi-resistive gas sensing material owing to its high specific surface area and good conductivity. Recent studies have shown that graphene-based gas sensors doped with metals, polymers, and metal oxides have good sensitivity, selectivity, and repeatability. Moreover, they are superior to traditional gas sensors. In this review, sensing mechanism of such composite sensors is introduced. In addition, research status on various sensors is discussed, and their advantages and disadvantages are summarized. Possible improvement methods are proposed as well. Finally, several common problems characteristic of graphene-based gas sensors are described, together with some critical ideas for improving their performance.     

Effect of the morphology of solution‐grown ZnO nanostructures on gas‐sensing properties

Despite the great potential of zinc oxide (ZnO) nanostructures as a sensing material for high‐performance gas sensors, the correlation between the morphology of ZnO nanostructure and its gas‐sensing performance has not been systematically investigated yet. In this work, ZnO nanostructures with controlled morphologies were synthesized by low‐temperature solution route and chemical bath deposition method. Thin film gas sensors were fabricated from the nanostructures and the sensor performance such as the response, recovery time, and stability was examined for several gases. It is demonstrated that the gas‐sensing performance of a ZnO nanostructure sensor is strongly influenced by its morphology. One dimensional ZnO nanocones are highly promising for practical application to gas sensors, due to their large surface area per unit mass and unique conical structure.     

多孔分级结构金属氧化物气体传感器研究概述

多孔和分级结构因其大的表面积和有助于气体运输的特点而拥有理想的气敏性能。本文分别综述了应用于气体传感的多孔和分级结构金属氧化物的主要制备方法,并介绍了一种新的基于生物模板合成多孔分级结构金属氧化物的方法。由于其形貌中团聚较少,多孔分级结构的气敏性能较传统纳米结构的更为优秀。     

Adsorption,catalysis, and reactions on the surfaces of metal nano-oxides

Nanomaterials based on metal oxides are considered. Special attention is given to adsorption, because this step determines physicochemical properties of nanostructured materials. The main processes are considered that occur on the surface of metal nano-oxides in the course of adsorption and the nature of chemoresistance. A model is presented that explains the increasing sensitivity of semiconductor sensor materials with a decrease in the grain size. The potential of the use of metal and metal oxide nanoparticles in catalysis and photocatalysis is discussed. Examples are given for the selective synthesis of α-mercaptopyridine on the surface of TiO₂ with supported silver nanoparticles with a diameter of <1 nm. Possible problems that might appear when nanoparticles are used in large-scale manufactures are discussed. Promising examples of the use of magnesium and calcium oxide nanoparticles for the destruction of toxic substances, specifically 3,3-dimethyl-2-butylmethylphosphoxofluoride and dichloroethyl sulfide at room temperature are analyzed. The method of cryoformation is considered that makes it possible to create new nanomaterials for use in catalysis, in gas sensors, and for modifying pharmaceuticals to reach a higher biological activity.     

导电聚合物气敏传感器研究进展

气敏传感器是利用材料的气敏特性实现目标气体浓度检测的电子元器件,在生产安全、环境监测、临床医学等领域均有广泛应用。气敏材料主要分为金属氧化物半导体材料、导电聚合物(CP)材料、金属有机框架材料。导电聚合物因其成本低、易于合成,在室温下对氨气等有害气体表现出良好的响应的特点而受到广泛关注。近年来导电聚合物复合物的研究也极大地提高了导电聚合物的气敏性能。分析了导电聚合物电阻调控机理,重点介绍了近年来对氨气、二氧化氮、硫化氢等气体的导电聚合物及其复合物的气敏传感器的研究进展,简要介绍了导电高分子在甲醇、三乙胺、一氧化碳等气体检测中的研究情况,最后展望了导电聚合物在气体传感领域的应用前景。     

Zinc ferrite based gas sensors: A review

Flammable, explosive and toxic gases, such as hydrogen, hydrogen sulfide and volatile organic compounds vapor, are major threats to the ecological environment safety and human health. Among the available technologies, gas sensing is a vital component, and has been widely studied in literature for early detection and warning. As a metal oxide semiconductor, zinc ferrite (ZnFe₂O₄) represents a kind of promising gas sensing material with a spinel structure, which also shows a fine gas sensing performance to reducing gases. Due to its great potentials and widespread applications, this article is intended to provide a review on the latest development in zinc ferrite based gas sensors. We first discuss the general gas sensing mechanism of ZnFe₂O₄ sensor. This is followed by a review of the recent progress about zinc ferrite based gas sensors from several aspects: different micro-morphology, element doping and heterostructure materials. In the end, we propose that combining ZnFe₂O₄ which provides unique microstructure (such as the multi-layer porous shells hollow structure), with the semiconductors such as graphene, which provide excellent physical properties. It is expected that the mentioned composites contribute to improving selectivity, long-term stability, and other sensing performance of sensors at room or low temperature.     

Surface bound nanostructures of ternary r-GO / Mn3O4/V2O5 system for room temperature selectivity of hydrogen gas

Room temperature detection of highly sensitive Hydrogen (H₂) gas sensing material preparation was taken as a major objective in this present work. Herein, a novel one pot hydrothermal method is proposed for the synthesis of ternary r-GO decorated Manganese oxide (Mn₃O₄) and Vanadium pentoxide (V₂O₅) nanocomposite. The significant electrical conductivity of r-GO plays an important role here to enhance the sensing property. Tunable band bending features of metal oxides over the r-GO surface makes the composite works at room temperature with high selectivity of H₂. The optical, structural and morphological characteristics were analyzed by UV–Visible spectroscopy (UV–Vis), X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray Photoelectron spectroscopy (XPS), Scanning electron microscope (SEM), and High Resolution-Transmission Electron Microscope (HR-TEM). The sensing results reveals that the present nanocomposite is selectively sensitive towards H₂ with sensitivity value of (175%) at room temperature with response time (82 s) and recovery time (92 s). To investigate the low detection limit gas concentration was varied in the range from 20 ppm to 50 ppm. The synergetic sensing performance and stability of this nanocomposite could be due to the formation of metal oxides with perspicuous nanostructures as heterojunction decorated over the r-GO stack layer.     

Detection of low ppm carbon monoxide with charge ordered LuFe2O4 gas sensor – A novel sensing mechanism

The field of solid state gas sensing is dominated by metal oxide semiconductors, whose modality of detecting volatiles is primarily governed by the chemisorption kinetics of the surface dwelling oxygen adatoms. The performance of these sensors is thus related to the quality of the sensor surface and is influenced by constraints like surface deterioration, foreign contamination and humidity absorption. In order to mitigate these limitations, charge ordered spin frustrated lutetium ferrite has been investigated as a sensing material for detection of low concentrations of carbon monoxide. The lutetium ferrite based sensors have a unique sensing mechanism where its already collapsed charge order is partially restored in presence of carbon monoxide, allowing it to mitigate the constraints pertinent to the quality of the sensor surface. The partial restoration of charge order is recognized by a significant nonlinear increase in the sensor impedance, when the toxic gas is dispensed in its ambience. Normally, the phenomenon of charge order collapsing and restoration in multiferroics is produced by external impetus in form of optical, thermal or magnetic stimuli. The scope of producing a similar effect through modulation of external ambience has immense possibility not only in the field of multiferroics but also in gas sensing where the dilemmas associated with surface detection might be mitigated through this approach.     

Preparation of Nanostructured ZnO Thin Films Using Magnetron Sputtering for the Gas Sensors Applications

ZnO is one of the most promising transparent conducting oxide materials, which widely used in thin film gas sensors. In this research, the dependence of the thermal oxidation time on structural, morphological and gas sensing properties of ZnO thin films is investigated. ZnO nanostructures are synthesized by using DC magnetron sputtering for deposition of pure zinc layers on glass substrates and then thermal oxidation of deposited zinc layers to produce zinc oxide (ZnO) thin films. Obtained results from X-ray diffraction revealed that the degree of crystallinity and the average grain size of the ZnO deposited thin films enhance with increasing the thermal oxidation time. Surface topography and growth behavior of ZnO thin films have important role in optimization of gas sensing properties of these films. In this study, scanning electron microscopy and atomic force microscopy have been used to investigate the effective parameters related to the surface topography of the films. Obtained results from these analyzes revealed that the surface topography of ZnO deposited samples strongly depend on thermal oxidation time. Also the effect of thermal oxidation time on the performance of ZnO gas sensors is investigated. The results indicated that the ethanol gas sensing properties of ZnO samples improve with decreasing the size of grains.     

Characterization and gas sensing properties of bead-like ZnO using multi-walled carbon nanotube templates

We report bead-like ZnO nanostructures for gas sensing applications, synthesized using multi-walled carbon nanotube (MWCNT) templates. The ZnO nanostructures are grown following a two-step process: in the first, ZnO nanoparticles are synthesized on MWCNTs by thermal evaporation of a Zn powder; and in the second, the hybrid nanostructures are heat-treated at 800 °C. Scanning and transmission electron microscopy images indicate that the bead-like ZnO nanostructures have surface protuberances with nanoparticle sizes ranging from 20 to 60 nm, and a well-crystallized hexagonal structure. Gas sensors based on multiple-networked bead-like ZnO showed considerably enhanced electrical responses and better stability to both oxidizing (NO₂) and reducing (CO) gases compared with previously reported nanostructured gas sensors, even if the response to CO gas was slow to increase. Both the NO₂ and CO gas sensing properties increased dramatically when the working temperature was increased up to 300 °C. The response sensitivities measured were 2953%, 5079%, 9641%, 3568%, and 3777% to 20 ppm NO₂ at 200, 250, 300, 350 and 400 °C, respectively. For CO gas on the other hand, the response sensitivities were 107%, 110%, 114%, 118%, and 122% at 5, 10, 20, 50, and 100 ppm concentrations, respectively. For concentrations between 5 and 20 ppm, the recovery time of the oxidizing gas was much shorter than the response time. The origin of the NO₂/CO gas sensing mechanism of the bead-like ZnO nanostructures is discussed.     

金属氧化物半导体气敏传感器的研究和开发进展

综述了近期国内外金属氧化物半导体气敏传感器的研究和开发进展；阐述了半导体气敏材料的气敏作用及机理；展望了今后半导体气敏传感器技术的发展趋向。