Recent progress in applications of graphene oxide for gas sensing: A review

This paper is a review of the recent progress on gas sensors using graphene oxide (GO). GO is not a new material but its unique features have recently been of interest for gas sensing applications, and not just as an intermediate for reduced graphene oxide (RGO). Graphene and RGO have been well known gas-sensing materials, but GO is also an attractive sensing material that has been well studied these last few years. The functional groups on GO nanosheets play important roles in adsorbing gas molecules, and the electric or optical properties of GO materials change with exposure to certain gases. Addition of metal nanoparticles and metal oxide nanocomposites is an effective way to make GO materials selective and sensitive to analyte gases. In this paper, several applications of GO based sensors are summarized for detection of water vapor, NO₂, H₂, NH₃, H₂S, and organic vapors. Also binding energies of gas molecules onto graphene and the oxygenous functional groups are summarized, and problems and possible solutions are discussed for the GO-based gas sensors.     

Interplay between O2 and SnO2: oxygen ionosorption and spectroscopic evidence for adsorbed oxygen.

Tin dioxide is the most commonly used material in commercial gas sensors based on semiconducting metal oxides. Despite intensive efforts, the mechanism responsible for gas-sensing effects on SnO(2) is not fully understood. The key step is the understanding of the electronic response of SnO(2) in the presence of background oxygen. For a long time, oxygen interaction with SnO(2) has been treated within the framework of the "ionosorption theory". The adsorbed oxygen species have been regarded as free oxygen ions electrostatically stabilized on the surface (with no local chemical bond formation). A contradiction, however, arises when connecting this scenario to spectroscopic findings. Despite trying for a long time, there has not been any convincing spectroscopic evidence for "ionosorbed" oxygen species. Neither superoxide ions O(2)(-), nor charged atomic oxygen O,(-) nor peroxide ions O(2)(2-) have been observed on SnO(2) under the real working conditions of sensors. Moreover, several findings show that the superoxide ion does not undergo transformations into charged atomic oxygen at the surface, and represents a dead-end form of low-temperature oxygen adsorption on reduced metal oxide.     

金属有机框架及其衍生的金属氧化物在电阻式气体传感器中的应用

高灵敏和选择性的气体传感器对于实时监测大气中有毒有害气体和早期的疾病诊断具有重要的意义。目前,传统的气敏材料仍然存在着许多问题亟待解决,例如：选择性差、检测极限不够低、使用寿命短等。作为一种多孔的配位聚合物,金属有机框架材料(MOFs)由于其超高的比表面积和较大的孔隙率在气体传感器领域已经得到广泛的应用。利用MOFs自身或者由它们衍生的不同纳米结构的金属氧化物可以提升气体传感器的灵敏度和选择性,为制备新型高性能的气体传感器提供了新的思路和方向。本文结合金属氧化物半导体(MOS)的气敏机理,综述了不同结构的MOFs及其衍生的金属氧化物在电阻式气体传感器领域的研究进展,并对其应用前景和发展方向作出了展望。     

Porous metal oxides as gas sensors

Semiconducting metal oxides are frequently used as gas-sensing materials. Apart from large surface-to-volume ratios, well-defined and uniform pore structures are particularly desired for improved sensing performance. This article addresses the role of some key structural aspects in porous gas sensors, such as grain size and agglomeration, pore size or crack-free film morphology. New synthesis concepts, for example, the utilisation of rigid matrices for structure replication, allow to control these parameters independently, providing the opportunity to create self-diagnostic sensors with enhanced sensitivity and reproducible selectivity.     

HFIP-Functionalized Co3O4 Micro-Nano-Octahedra/rGO as a Double-Layer Sensing Material for Chemical Warfare Agents

Semiconductor metal oxides (SMO)-based gas-sensing materials suffer from insufficient detection of a specific target gas. Reliable selectivity, high sensitivity, and rapid response–recovery times under various working conditions are the main requirements for optimal gas sensors. Chemical warfare agents (CWA) such as sarin are fatal inhibitors of acetylcholinesterase in the nerve system. So, sensing materials with high sensitivity and selectivity toward CWA are urgently needed. Herein, micro-nano octahedral Co₃O₄ functionalized with hexafluoroisopropanol (HFIP) were deposited on a layer of reduced graphene oxide (rGO) as a double-layer sensing materials. The Co₃O₄ micro-nano octahedra were synthesized by direct growth from electrospun fiber templates calcined in ambient air. The double-layer rGO/Co₃O₄-HFIP sensing materials presented high selectivity toward DMMP (sarin agent simulant, dimethyl methyl phosphonate) versus rGO/Co₃O₄ and Co₃O₄ sensors after the exposure to various gases owing to hydrogen bonding between the DMMP molecules and Co₃O₄-HFIP. The rGO/Co₃O₄-HFIP sensors showed high stability with a response signal around 11.8 toward 0.5 ppm DMMP at 125 °C, and more than 75 % of the initial response was maintained under a saturated humid environment (85 % relative humidity). These results prove that these double-layer inorganic–organic composite sensing materials are excellent candidates to serve as optimal gas-sensing materials.     

Recent developments on application of nanometal-oxide based gas sensor arrays

It is shown that the sensor arrays can acquire more information on a given sample than an individual sensor. The main classes of gas-sensing materials include metal-oxide semiconductors. Gas sensors based on chemiresistive semiconducting metal-oxides have many potential benefits including, their very low cost, fast response, recovery time, simple electronic interface, ease of use, and ability to detect a large analytes. Recent advances in gas sensor arrays have shown the capability to incorporate nanomaterial based cross-reactive array. In this way it is possible to increase the surface/volume ratio of the sensing layer. Therefore the surface scattering is better influenced by adsorbed species and change in sensor conductivity is higher. Also the sensitivity of semiconductor oxide materials can be improved by using of nanoparticles. Gas sensor arrays based on nanotechnology can rapidly, sensitively, and selectively detect target molecules.     

Growth of Indium Oxide Nanowalls on Patterned Conducting Substrates: Towards Direct Fabrication of Gas Sensors

Nanowall materials are ideal two-dimensional structures with high surface-to-volume ratios and open edge geometries. We first report on the growth and characterization of indium oxide nanowalls on transparent and conducting indium tin oxide substrates. The nanosheets that compose the nanowalls are single-crystalline and are approximately 8 nm in thickness. The density and the lateral dimensions of the nanosheets on the substrate can be controlled by the growth time. Adopting a bridgework-like strategy, we directly construct indium oxide nanowall gas sensors on the patterned indium tin oxide substrates. The pattern lines on the substrates are etched using transparent plastic adhesive tape as shadow mask, which is both simple and cheap in comparison with the conventional photolithography technique. The sensors exhibit fast response/recovery behavior and good reproducibility to NO₂ gas under mild testing conditions, such as room temperature, ambient pressure, dry air background, and 1.5 V dc bias, and can achieve a detection limit as low as 50 ppb. We propose an assumption that the gas adsorption is composed of deep adsorption and probe adsorption to explain the interesting gas-sensing behavior of the indium oxide nanowalls. We suggest that the work reported herein, including the facile growth of indium oxide nanowalls, the bridgework-like strategy to directly construct electronic devices, and the high gas-sensing performance of the indium oxide nanowalls sensors, is a significant step towards the real applications of novel semiconductor nanostructures.     

MgFe2O4纳米颗粒及其气敏性研究

利用Sol-gel燃烧法合成氧化物MgFe2O4对氯气有很高的灵敏度,并利用XRD对产品的物相进行了分析;用TEM对粉体晶粒和形貌进行检测,将产物粉体制成气敏元件,采用静态配气法在气敏测试仪上进行了灵敏度和响应-恢复曲线的测试,研究证明镁铁复合氧化物对氯气有很好的气敏特性。     

Surface engineering of one-dimensional tin oxide nanostructures for chemical sensors

Nanostructured materials are promising candidates for chemical sensors due to their fascinating physicochemical properties. Among various candidates, tin oxide (SnO₂) has been widely explored in gas sensing elements due to its excellent chemical stability, low cost, ease of fabrication and remarkable reproducibility. We are presenting an overview on recent investigations on 1-dimensional (1D) SnO₂ nanostructures for chemical sensing. In particular, we focus on the performance of devices based on surface engineered SnO₂ nanostructures, and on aspects of morphology, size, and functionality. The synthesis and sensing mechanism of highly selective, sensitive and stable 1D nanostructures for use in chemical sensing are discussed first. This is followed by a discussion of the relationship between the surface properties of the SnO₂ layer and the sensor performance from a thermodynamic point of view. Then, the opportunities and recent progress of chemical sensors fabricated from 1D SnO₂ heterogeneous nanostructures are discussed. Finally, we summarize current challenges in terms of improving the performance of chemical (gas) sensors using such nanostructures and suggest potential applications. Contains 101 references.

Effect of defects in TiO2 nanoplates with exposed {001} facets on the gas sensing properties

The gas-sensing properties and mechanism of anatase TiO₂ with complete and defective {001} facets were explored and compared, which provides a more direct evidence for the high sensitivity of TiO₂ with the complete {001} crystal facets.     

Fast response speed of mechanically exfoliated MoS2 modified by PbS in detecting NO2

MoS₂, acting as a promising gas sensing material, has shown huge potential in monitoring of toxic and harmful gases at room temperature. However, MoS₂-based gas sensors still suffer from poor gas sensing performance such as poor sensitivity, long response time. Constructing the heterostructure is an effective approach to improve gas-sensing performance of MoS₂. Herein, PbS@MoS₂ composites synthesized by mechanical exfoliation combining with wet-chemical precipitation are used to investigate its performance in detecting NO₂ at room temperature. The response value of PbS@MoS₂ gas sensor against NO₂ is significantly improved compared with the pure MoS₂ gas sensor. At the same time, the modification with PbS also accelerates the response speed of MoS₂, and the response time is almost reduced by two orders of magnitude, from hundreds of seconds to less than ten seconds. The enhanced response value and fast response time are mainly benefited from the modulation effect of NO₂ to PbS@MoS₂ heterostructure and the mechanically exfoliated MoS₂ surface with few defects. This work can be expected to provide useful guidance for designing composite materials with excellent gas sensing properties.     

The bilayer lipid membrane as a basis for a selective sensor for ammonia

The development of an electrochemical method for the selective sensing of ammonia gas, based on a modified bilayer lipid membrane, is described. Membrane selectivity for ammonium ion is achieved through incorporation of the antibiotic nonactin as ion-carrier. The detection limits compare favourably with those for conventional ammonia gas-sensing electrodes, but the selectivity is much superior. Theoretical evaluation of the potential sensitivity of the new gas-sensor with respect to design parameters is described.     

Highly sensitive,humidity-tolerant and flexible NO2 sensors based on nanoplate Bi2Se3 film

Recently, two-dimension (2D) materials have fueled considerable interest in the field of gas sensing to cope urgent demands at specific scenarios. Unfortunately, the susceptibility to ambient humidity, and/or fragile operation stability always frustrate their further practicability. To overcome these drawbacks, we proposed one novel flexible gas sensor based on bismuth selenide (Bi₂Se₃) nanoplates for sensitive NO₂ detection at room temperature. The as-prepared Bi₂Se₃ sensor exhibited favorable sensing performance, including remarkable NO₂ selectivity, high response of 120% and fast response time of 81 s toward 5 ppm NO₂, an ultralow detection limit of 100 ppb, and nice stability. Besides, the excellent humidity tolerance and mechanical flexibility endowed Bi₂Se₃ sensors with admirable reliability under harsh working conditions. The first-principles calculation further revealed the insights of extraordinary NO₂ selectivity and the underlying gas-sensing mechanism.     

Sensitivity of Semiconductor Gas Sensors to Hydrogen and Oxygen in an Inert Gas Atmosphere

Semiconductor gas sensors with nine types of gas-sensing films were prepared and their sensitivity for hydrogen and oxygen in binary and ternary gas mixtures containing nitrogen in concentrations of 0–4 and 0–8 vol %, respectively was studied. The sensor temperature was varied from 200 to 500°C. Sensors based on an In₂O₃+ Al₂O₃(30 : 70) composite with platinum contact areas exhibited the best metrological and performance characteristics. The resistance of sensors heated to an optimum temperature of 400°C was measured as a function of the test gas concentration. In principle, the concentrations of the components in nitrogen can be determined to within 5 rel % with the use of the above functions.     

Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report

Gas sensors based on semiconducting materials have become of great interest to both sensor users and researchers. In this context, a huge number of publications have appeared in the literature which deal with metal oxide gas sensors, in general, and with the prototype material SnO₂, in particular. The amount of data published grows continuously and has led to a situation in which even experts in this field tend to lose an overview. The present review describes the reasons for this complexity and outlines unifying concepts to understand the huge amount of published, mostly empirical data. This leads to a comprehension of gas-sensing phenomena in both the application and research domains.     

Resistive gas sensors based on the composites of nanostructured carbonized polyaniline and Nafion

Due to constant necessity to have reliable and sensitive gas sensors in many contemporary technologies, there is a permanent need for development of new sensing platforms with good sensing properties. Here, we demonstrate a novel type of resistive gas sensors based on carbonized polyaniline/Nafion composites. The sensing mechanism of such sensors is based on the sorption of gases by the composites which induce Nafion swelling and decreasing of conductivity. Chemosensitive properties can be tuned by the (i) choice of carbon materials with different conductivities, (ii) Nafion content in the composite, and (iii) thickness of the composite layer. We have shown that the sensors respond to water, acetone, ethanol, and methanol vapors. For the last two cases, we have achieved high sensitivity, fast response, wide concentration range, and good recovery. The use of simultaneous two- and four-point techniques for these sensors provides an internal control of the sensor integrity.     

Designing State-of-the-Art Gas Sensors: From Fundamentals to Applications

Gas sensors are crucial in environmental monitoring, industrial safety, and medical diagnostics. Due to the rising demand for precise and reliable gas detection, there is a rising demand for cutting-edge gas sensors that possess exceptional sensitivity, selectivity, and stability. Due to their tunable electrical properties, high-density surface-active sites, and significant surface-to-volume ratio, nanomaterials have been extensively investigated in this regard. The traditional gas sensors utilize homogeneous material for sensing where the adsorbed surface oxygen species play a vital role in their sensing activity. However, their performance for selective gas sensing is still unsatisfactory because the employed high temperature leads to the poor stability. The heterostructures nanomaterials can easily tune sensing performance and their different energy band structures, work functions, charge carrier concentration and polarity, and interfacial band alignments can be precisely designed for high-performance selective gas sensing at low temperature. In this review article, we discuss in detail the fundamentals of semiconductor gas sensing along with their mechanisms. Further, we highlight the existed challenges in semiconductor gas sensing. In addition, we review the recent advancements in semiconductor gas sensor design for applications from different perspective. Finally, the conclusion and future perspectives for improvement of the gas sensing performance are discussed.     

Low-temperature H2 sensing in self-assembled organotin thin films

Self-assembled nanoporous tin-based hybrid thin films prepared by the sol-gel method from organically-bridged ditin hexaalkynides detect hydrogen gas from 50 to 200 °C at the 200-10,000 ppm level. This finding opens a fully new class of gas-sensing materials as well as a new opportunity to integrate organic functionality in gas sensing metal oxides.     

An Au clusters related spill-over sensitization mechanism in SnO(2)-based gas sensors identified by operando HERFD-XAS, work function changes, DC resistance and catalytic conversion studies

The role of Au additives in SnO(2)-based thick film gas sensors was investigated by a combination of operando investigation techniques, namely spectroscopic high energy resolved fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) and simultaneous DC resistance and work function change measurements. The results have shown that the Au is present in the form of small metallic particles at the surface of the host metal oxide without changing its bulk or surface electronic properties. The sensitization effect of Au can therefore be attributed to the "spill-over effect", meaning that the Au particles enrich the surface of the active metal oxide with oxygen species which consequently react with reducing gases such as CO and H(2). This is in contrast to the effect of Pd and Pt promoters which were found to be distributed at an atomic level on the surface and in the bulk of the supporting sensing material and therefore have a tremendous effect on its bulk and surface electronic properties.