Crack‐Free Periodic Porous Thin Films Assisted by Plasma Irradiation at Low Temperature and Their Enhanced Gas‐Sensing Performance

Homogenous thin films are preferable for high‐performance gas sensors because of their remarkable reproducibility and long‐term stability. In this work, a low‐temperature fabrication route is presented to prepare crack‐free and homogenous metal oxide periodic porous thin films by oxygen plasma irradiation instead of high temperature annealing by using a sacrificial colloidal template. Rutile SnO₂ is taken as an example to demonstrate the validity of this route. The crack‐free and homogenous porous thin films are successfully synthesized on the substrates in situ with electrodes. The SnO₂ porous thin film obtained by plasma irradiation is rich in surface OH groups and hence superhydrophilic. It exhibits a more homogenous structure and lower resistance than porous films generated by annealing. More importantly, such thin films display higher sensitivity, a lower detection threshold (100 ppb to acetone) and better durability than those that have been directly annealed, resulting in enhanced gas‐sensing performance. The presented method could be applied to synthesize other metal oxide homogenous thin films and to fabricate gas‐sensing devices with high performances.     

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。     

Reduced graphene oxide-porous In2O3 nanocubes hybrid nanocomposites for room-temperature NH3 sensing

Metal oxide semiconductors (MOS)-reduced graphene oxide (rGO) nanocomposites have attracted great attention for room-temperature gas sensing applications. The development of novel sensing materials is the key issue for the effective detection of ammoniagas at room temperature. In the present work, the novel reduced graphene oxide (rGO)-In₂O₃ nanocubes hybrid materials have been prepared via a simple electrostatic self-assembly strategy. Characterization results exhibit that the intimate interfacial contact between In₂O₃ nanocubes and the rGO sheets are achieved. Particularly, the as-prepared rGO/In₂O₃ nanocomposites displayed high sensitivity, fast response and excellent selectivity towards ammonia (NH₃) at room-temperature, which clearly uncovers the merit of structural design and rational integration with rGO sheets. The superior gas sensing performance of the rGO/In₂O₃ nanocomposites can be attributed to the synergetic effects of rGO sheets and porous In₂O₃ nanocubes. The reported synthesis offers a general approach to rGO/MOS-based semiconductor composites for room-temperature gas sensing applications.     

Amphiphilic block copolymers directed synthesis of mesoporous nickel-based oxides with bimodal mesopores and nanocrystal-assembled walls

Mesoporous late-transition metal oxides have great potential in applications of energy,catalysis and chemical sensing due to their unique physical and chemical properties.However,their synthesis via the flexible and scalable soft-template method remain a great challenge,due to the weak organic-inorganic interaction between the frequently used surfactants(e.g.,Pluronic-type block copolymers) and metal oxide precursors,and the low crystallization temperature of metal oxides.In this study,ordered mesoporous NiO with dual mesopores,high surface area and well-interconnected crystalline porous frameworks have been successfully synthesized via the facile solvent evaporation-induced co-assembly(EICA) method,by using lab-made amphiphilic diblock copolymer polystyrene-b-poly(4-vinylpyridine)(PS-b-P4 VP) as both the structure-directing agent(the soft template) and macromolecular chelating agents for nickel species,THF as the solvent,and nickel acetylacetonate(Ni(acac)2) as inorganic precursor.Similarly,by using Ni(acac)2 and Fe(acac)3 as the binary precursors,ordered mesoporous Fedoped NiO materials can be obtained,which have bimodal mesopores of large mesopores(32.5 nm) and secondary mesopores(4.0-11.5 nm) in the nanocrystal-assembled walls,high specific surface areas(～74.8 m~2/g) and large pore value(～0.167 cm~3/g).The obtained mesoporous Fe-doped NiO based gas sensor showed superior ethanol sensing performances with good sensitivity,high selectivity and fast response-recovery dynamics.     

Amphiphilic block copolymers directed synthesis of mesoporous nickel-based oxides with bimodal mesopores and nanocrystal-assembled walls

Ordered mesoporous Fe-doped NiO with dual mesopores, high surface area and well-interconnected crystalline porous frameworks have been synthesized via solvent evaporation-induced co-assembly (EICA) method, by using PS-b-P4VP as structure-directing agent, Ni(acac)₂ and Fe (acac)₃ as binary inorganic precursor, and showed superior ethanol sensing performances with good sensitivity, high selectivity and fast response-recovery dynamics.     

Metal-organic frameworks-derived hierarchical ZnO structures as efficient sensing materials for formaldehyde detection

Semiconducting metal oxides have been considered as effective approach for designing high-performance chemical sensing materials. In this paper, a kind of metal-organic frameworks ZIF-8 was used as sacrificed template to prepare porous ZnO hollow nanocubes for the application in gas sensing. It is found that changing calcination temperature and solvent can greatly influence the morphology of the material, which finally affects the gas sensing performance. Acetylene-sensing properties of the sensors were investigated in detail. It can be clearly seen that the material used methanol as reaction solvent with the decomposition at 350 °C for 2 h (ZnO-350-M) showed the optimal formaldehyde-sensing behaviors compared with other materials prepared in this experiment. The dynamic transients of the ZnO-350-M-based sensors demonstrated a high response value (about 10), fast response and recovery rate (4 s and 4 s, respectively) and good selectivity towards 100 ppm (part per million) formaldehyde as well as a low detectable limit (1 ppm). As exemplified for the sensing investigation towards formaldehyde, the porous ZnO hollow nanocubes showed a significantly improved chemical sensitivity due to the highly synergistic effects from the well exposed surfaces, defect states and the robust ZnO.     

金属氧化物异质结气体传感器气敏增强机理

金属氧化物异质结由于费米能级效应、不同组分之间的协同作用,常被用来提高电阻型金属氧化物半导体气体传感器的气敏特性。本文简述了近年来国内外金属氧化物异质结材料的类别,主要分为混合氧化物结构、层状结构、第二相粒子修饰结构、一维纳米结构和核-壳结构;重点综述了金属氧化物异质结的气敏增强机理,包括异质结效应、协同效应、催化溢流效应、响应反型、载流子分离及微结构调控六大机理;分析了当前异质结气体传感器面临的瓶颈。最后对纳米异质结气体传感器的发展进行了展望,今后金属氧化物异质结气体传感器可以从明确异质结界面机理展开,这将为自下而上地设计出符合实际需要的气体传感器提供一定参考。     

Meso- to Macro-Porous Oxides as Semiconductor Gas Sensors

The present article reviews our latest studies directed to improving gas sensing properties of semiconductor metal oxides, SnO₂ and TiO₂, by controlling their porous structure in the range of nano- to micro-meter size. Thermally stable mesoporous oxide powders and submicron-size porous thick films were fabricated by a templating method employing supramolecular surfactants or polymethylmethacrylate microspheres and also by a sol–gel process in a polyethylene glycol matrix. Advantages of these methods in improving the sensing properties are overviewed and then remaining problems are discussed.     

Enhanced CO sensing properties of Pd modified ZnO porous nanosheets

Noble metal is usually used to improve the gas sensing performance of metal oxide semiconductor (MOS) due to its better catalytic properties. In this work, we reported a synthesis of Pd/ZnO nanocomposite by an in situ reduction with ascorbic acid (AA). It was found that Pd/ZnO sensor has excellent selectivity to CO and the response of the Pd/ZnO sensor towards 100 ppm CO was as high as 15 (R_a/R_g), obviously higher than that of the pristine ZnO sensor (1.4) when the working temperature is 220 °C. Moreover, the pure ZnO sensor almost has no selectivity to CO, but the Pd/ZnO sensor has excellent selectivity to CO, which may be ascribed to the electronic sensitization of Pd. Our present results demonstrate that the Pd can significantly improve the gas-sensing performance of metal oxide semiconductor and the obtained sensor has great potential in monitoring coal mine gas.     

Rational construction and triethylamine sensing performance of foam shaped α-MoO3@SnS2 nanosheets

Owing to their high surface area, stable structure and easy fabrication, composite nanomaterials with encapsulation structures have attracted considerable research interest as sensing materials to detect volatile organic compounds. Herein, a hydrothermal route is designed to prepare foam shaped α-MoO₃@SnS₂ nanosheets that exhibit excellent sensing performance for triethylamine（TEA）. The developed sensor,based on α-MoO₃@SnS₂ nanosheets, displays a high ...     

Semiconductor Gas Sensors: Dry Synthesis and Application

Since the development of the first chemoresistive metal oxide based gas sensors, transducers with innovative properties have been prepared by a variety of wet‐ and dry‐deposition methods. Among these, direct assembly of nanostructured films from the gas phase promises simple fabrication and control and with the appropriate synthesis and deposition methods nm to μm thick films, can be prepared. Dense structures are achieved by tuning chemical or vapor deposition methods whereas particulate films are obtained by deposition of airborne, mono‐ or polydisperse, aggregated or agglomerated nanoparticles. Innovative materials in non‐equilibrium or sub‐stoichiometric states are captured by rapid cooling during their synthesis. This Review presents some of the most common chemical and vapor‐deposition methods for the synthesis of semiconductor metal oxide based detectors for chemical gas sensors. In addition, the synthesis of highly porous films by novel aerosol methods is discussed. A direct comparison of structural and chemical properties with sensing performance is given.     

以MOFs为前驱体的多孔金属氧化物催化剂研究进展

多孔金属氧化物具有高比表面积、大孔径、特殊的形貌和结构特性,广泛应用于催化、锂离子电池、太阳能电池、气敏传感器等领域。金属有机骨架材料(MOFs)是一类具有周期性网络结构的新型多孔晶体材料,在气体存储、气体分离、催化等领域具有重要的应用价值。近年来,以MOFs为前驱体制备多孔碳和多孔金属氧化物成为MOFs应用领域一个新的研究热点。本文主要综述了以MOFs为前驱体制备的多孔金属氧化物和多孔金属氧化物/碳复合物在CO氧化、催化产氢、异丁烷脱氢、环已烯氧化、醇直接氧化为酯、醛氧化酰胺化反应、光催化降解有机物和氧还原反应等方面的应用。     

多孔氧化物块体材料的制备及其对重金属离子吸附研究进展

目前去除重金属离子的方法较多,其中吸附法因操作简单、经济性好而被广泛使用。多孔氧化物块体材料作为新兴的吸附材料具有比表面积高、机械强度高及可回收性好等特点,但吸附选择性差以及孔结构单一的缺陷限制了其在水体重金属处理中的应用。本文详细叙述了多孔氧化物块体材料的多种制备方法,并分别讨论了各方法的优势及其不足。介绍了近几年研究较多的多孔氧化物块体材料及其制备方法与特性。最后,对多孔氧化物块体材料对Pb、Cd及Cr等重金属离子吸附性能的影响因素及改善措施方面进行了总结分析,并指出多种材料的复合、材料表面的接枝改性及分级多孔结构的构建会增强对重金属离子的吸附性能。期望本文为多孔氧化物块体材料的制备及其在重金属离子吸附方面的应用提供参考。     

Gas-Sensing Activity of Amorphous Copper Oxide Porous Nanosheets

In this paper, the gas-sensing properties of copper oxide porous nanosheets in amorphous and highly crystalline states were comparatively investigated on the premise of almost the same specific surface area, morphology and size. Unexpectedly, the results show that amorphous copper oxide porous nanosheets have much better gas sensing properties than highly crystalline copper oxide to a serious of volatile organic compounds, and the lowest detection limit (LOD) of the amorphous copper oxide porous nanosheets to methanal is even up to 10 ppb. By contrast, the LOD of the highly crystalline copper oxide porous nanosheets to methanal is 95 ppb. Experiments prove that the oxygen vacancies contained in the amorphous copper oxide porous nanosheets play a key role in improving gas sensitivity, which greatly improve the chemical activity of the materials, especially for the adsorption of molecules containing oxygen-groups such as methanal and oxygen.     

Plasmonic Au-Metal Oxide Nanocomposites for High-Temperature and Harsh Environment Sensing Applications

Noble metal nanoparticles like Au have long been admired for their brilliant colour, significantly influenced by plasmon resonance. When embedded in metal oxides, they exhibit unique properties which make them an excellent choice for sensing in high-temperature and harsh environment atmospheres. In this review, the various morphologies of Au nanoparticles (AuNPs) used in combination with metal oxides for sensing gases at temperatures greater than 300 °C are discussed. Theoretical discussions on the plasmon resonance properties of AuNPs as well as computational techniques like finite difference time domain (FDTD), are often used for understanding and correlating their extinction spectra and are briefed initially. The sensing properties of AuNPs embedded on a metal oxide matrix (such as TiO₂, SiO₂, NiO etc) for quantifying multiple analytes are then elucidated. The effect of high temperature as well as gas environments including corrosive atmospheres on such nanocomposites, and the different approaches to comprehend them are presented. Finally, techniques and methods to improve on the challenges associated with the realization and integration such Au-metal oxide plasmonic nanostructures for applications such as combustion monitoring, fuel cells, and other applications are discussed.     

Elucidating the Mechanism of Working SnO2 Gas Sensors Using Combined Operando UV/Vis,Raman, and IR Spectroscopy

SnO₂ is the most widely used metal oxide gas‐sensing material but a detailed understanding of its functioning is still lacking despite its relevance for applications. To gain new mechanistic insight into SnO₂ gas sensors under working conditions, we have developed an operando approach based on combined UV/Vis, Raman, and FTIR spectroscopy, allowing us for the first time to relate the sensor response to the concentration of oxygen vacancies in the metal oxide, the nature of the adsorbates, and the gas‐phase composition. We demonstrate with the example of ethanol gas sensing that the sensor resistance is directly correlated with the number of surface oxygen vacancies and the presence of surface species, in particular, acetate and hydroxy groups. Our operando results enable an assessment of mechanistic models proposed in the literature to explain gas sensor operation. Owing to their fundamental nature, our findings are of direct relevance also for other metal oxide gas sensors.     

Porous tin oxide nanostructured microspheres for sensor applications

We have sought to enhance the sensitivity of conductometric gas microsensors through the design and fabrication of porous, three-dimensional tin oxide nanoparticle structures. Electrostatically controlled layer-by-layer processing in aqueous solutions was used to decorate sacrificial latex microspheres with Sb:SnO2 nanoparticles. To evaluate their sensing performance, these structures were then deposited as films, via micropipetting, on MEMS micro-hot-plate platforms with interdigitated electrodes. Prior to gas testing, rapid heating of the micro-hot-plates was used to remove the sacrificial latex templates, thereby revealing a 3-D structure composed of interconnected spherical tin oxide nanoparticle shells with porous ultrathin walls. Changes in film conductance, caused by exposure to test gases (methanol, carbon monoxide, benzene, water) in a dry air background, were measured at different temperatures. Hollow nanoparticle microsphere films exhibited partial selectivity for these different gases, good dynamic range at different temperatures and gas concentrations, and good repeatability and stability over long runs. These films also yielded approximately 3-fold and 5-fold increases in sensitivity to methanol when compared to SnO2 polycrystalline chemical vapor deposition films and Sb:SnO2 microporous nanoparticle films, respectively. Gains in sensitivity are attributed to the multiscale porous architecture of the hollow microsphere films. This architecture promotes gas diffusion and increases the active surface area.     

Hierarchical NiCo2O4 microspheres assembled by nanorods with p-type response for detection of triethylamine

The morphological and structural design provides an efficient protocol to optimize the performance of gas sensing materials. In this work, a gas sensor with high sensitivity for triethylamine (TEA) detection is developed based on p-type NiCo₂O₄ hierarchical microspheres. The NiCo₂O₄ microspheres, synthesized by a hydrothermal route, have a three-dimensional (3D) urchin-like structure assembled by nanorod building blocks. The structure-property correlation has been investigated by powder X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscope, scanning electron microscope, N₂ adsorption-desorption tests and comprehensive gas sensing experiments. The influence of calcination temperature on the morphological structure and sensing performances has been investigated. Results reveal that the material annealed at 300 °C has a very large specific surface area of 125.27 m²/g, thereby demonstrating the best TEA sensing properties including high response and low limit of detection (145 ppb), good selectivity and stability. The further increase of the calcination temperature leads to the collapse of the 3D hierarchical structure with significantly decreased surface area, which is found to decline the sensing performances. This work indicates the promise of ternary p-type metal oxide nanostructures for application in highly sensitive gas sensors.     

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.