气体传感器的发展现状、半导体金属氧化物（SMO）气体传感器的优势与挑战、ZnO作为气敏材料的特点及其研究进展

摘要: 以三氧化钨为基材,通过掺杂制作了半导体型氯化氢气体传感器,研究元件的气敏特性。以钨酸钠和浓盐酸为反应物,采用溶胶-凝胶法,制备出具有特殊结构的三氧化钨。通过XRD、SEM、TEM等微观分析手段,发现该材料具有层片状结构,结构松散,平均粒径约17纳米。研究发现,掺杂适量的氧化铝可以大大提高三氧化钨对氯化氢气体的灵敏度,但元件稳定性以及选择性比较差,通过掺杂少量的氧化锌,虽然降低了元件的灵敏度,却可以大大提高元件的稳定性,适当提高加热电压,可抑制元件对NO2等气体的灵敏度,提高选择性。本文对三氧化钨的气敏机理及特性进行分析。

摘要: 科技的发展和生产力水平的不断提高, 给人们生活带来极大便利的同时, 由于能源消耗及其附加污染物的生成, 环境面临巨大的挑战。其中大气污染是我们关注的主要方面之一。NO 2 作为一种主要的大气污染物, 对水体、土壤和大气都会造成极大威胁, 如地表水、土壤酸化以及光化学烟雾等。同时对人体健康也有很大危害, 如引发呼吸道疾病和心脑血管疾病。因此, 制备适用于环境监测且工作温度和检测限较低且灵敏度高的便携式气体传感器来确定环境中NO 2 浓度的大小十分重要。本文概述几种典型的N型半导体金属氧化物NO 2 气体传感器的研究进展, 对NO 2 传感器在环境监测方面的应用和今后的研究方向进行了展望。

摘要: 由氮氧化物引起的环境污染一直是一个非常严峻的问题, 我们需要利用气体传感器对氮氧化物进行检测以便预防和解决污染问题。金属氧化物半导体式气体传感器受到科研学者的广泛关注。大量实验数据表明, 制备具有介孔结构的半导体材料, 将会提升传感器在工作温度、气体检测限、稳定性和选择性等方面的性能。本文将从响应机理、性能参数和影响因素方面对金属氧化物半导体式气体传感器进行简要介绍并具体综述了具有介孔结构的钨基、锌基和锡基NOx气体传感器的研究现状。

摘要: 随着市场对有机挥发性气体检测需求的提高，半导体气体传感器因其工作温度低、循环稳定性良好、响应恢复时间短等特性受到了广泛关注。三乙胺是一种重要的化工原料，然而，其挥发性特性及刺激性气味和低毒性对人类的生命安全造成了威胁。近年来国内外有不少三乙胺气体检测的研究论文和产品介绍，但全面性的综述文章极少。本文以前期学者对三乙胺气体检测的工作为抓手，首先讲述了三乙胺传感器的响应机理和提高气敏响应的内在因素，总结了以不同金属氧化物为基质的复合型半导体传感器对三乙胺气体的检测情况，并详细介绍了不同材料形貌与气敏性能之间的关系。另外，对部分材料的气敏增强机理作了简要概述。最终，对三乙胺气体传感领域做出展望，并为后续开展相关领域工作提供了理论基础。

制备了一种负载NiO纳米粒子的ZnO纳米花杂化材料，并对其进行了XRD分析、形貌分析及气体传感性能测试。结果表明，表面负载的NiO纳米粒子调节了ZnO纳米花的导电通道；与纯ZnO材料相比，NiO负载量优化后的异质结气敏材料对二甲苯的响应值提高了3.3倍；传感器的选择性得到提高，响应时间缩短。从更广泛的角度提出了通过引入表面异质结构进行功能化修饰高性能半导体金属氧化物气体传感器的方法。

摘要: 随着微机电系统(MEMS)的发展，运用该技术的半导体传感器也跟着迅速发展，逐渐走向微型化、集成化和智能化。基于MEMS的微加热板(MHP)的金属氧化物甲烷传感器具有功耗小、响应快等优点，广泛应用于甲烷检测。其中，氧化锌(ZnO)甲烷敏感材料因其灵敏度高、中毒效应小、工作温度低等优点，广受关注。但是，该敏感材料制备的传感器响应性能依然受加热温度及热量分布的强烈影响。使用有限元分析(FEA)软件COMSOL中的Multiphysics模块对物理场中的温度进行仿真分析与比较，揭示了在相同工作条件下加热电极结构对温度分布的影响，优选的微加热板达到300 ℃时需要75 mW左右的功率。在商用微加热板的叉指电极上采用无遮挡全表面溅射氧化锌敏感材料构建ZnO薄膜甲烷传感器，并使用合肥微纳公司HIS9010测试了气体传感器的响应。采用静态测量的方法向1 L的气体腔内注射甲烷气体，经过测试，与现在不同形貌的ZnO相比，本课题组使用的磁控溅射制备的氧化锌薄膜气体传感器，在(1000～10000)×10 −6 甲烷浓度区间内响应线性度比较好，对浓度为10000×10 −6 的甲烷响应值达到了30。与国内外商用甲烷传感器的甲烷响应性能进行了对比，结果表明本课题组制作传感器响应更高，更具有应用优势。

摘要: 通过自牺牲模板法制备Y掺杂ZnO的半导体材料（Zn-MOF-Y），并对合成材料进行表征。Zn-MOF-Y的微观特征表现为由纳米颗粒组成的纳米球为ZnO和YF 3 的复合材料，含有Zn、O、Y、F这4种元素，其比表面积为15.7844 m 2 /g，平均孔径为15 nm。对以Zn-MOF-Y为传感材料制备的半导体气体传感器进行气敏测试，其对100 mg/L 的NO 2 的响应值为20，传感器的响应与浓度呈现线性关系；对100 mg/L的NO 2 的响应/恢复时间为82/64 s，并且该传感器具有优良的稳定性。由于该传感器具有以上优点，这为其以后的实际应用提供了可能。

摘要: 气体传感器在现代科学技术领域发挥着重要作用, 它们被广泛用于公共安全、污染监测、呼吸分析、智能家居等。由于其低成本、高灵敏度、易制造的优势, 促使众多研究人员对气体传感器展开广泛研究。尤其是金属氧化物半导体(MOS)型气体传感器, 此类传感器可检测150多种有毒有害气体, 对具有易挥发、易燃易爆性质的乙醇(C 2 H 5 OH)有机挥发性气体(VOCs)具有良好的气敏特性。本文回顾了近一年来最先进的MOS电阻型乙醇传感器, 详细阐述了传感器对乙醇气体的传感机制; 重点介绍了不同MOS型传感器对乙醇的敏感特性, 分析了诸如金属掺杂、异质结复合材料以及MOS分等级结构对乙醇传感器性能的影响。通过分析可得, SnO 2 、ZnO基传感器对乙醇的传感性能最佳, 特别是经金属掺杂和由不同导电类型半导体构建的异质结MOS传感器。

测试环境中的水汽会干扰金属氧化物半导体气敏材料的气敏反应，导致基线电阻漂移、响应灵敏度降低等问题，从而显著影响传感器对目标气体的检测。对水汽干扰气体传感器气敏反应的机制进行了简要阐述，重点对当前抗湿检测的思路进行了归纳总结。当前较为广泛开发的抗湿检测思路包括筛滤作用、捕水作用、传感器表面调控3大类，不同抗湿思路的组合可实现金属氧化物半导体气敏材料抗湿维稳和响应性能的同步提升。

随着物联网的快速发展,各领域对气体监测的需求越来越大,基于先进微机电系统(Micro-electro-mechanical systems,MEMS)技术的金属氧化物半导体(Metal oxide semiconductor,MOS)气体传感器在过去几十年里取得了很大发展。MEMS微热板的多样化设计、MOSs纳米结构的多样化以及机器学习算法的出现为MEMS的传感性能以及智能传感系统的构建提供了很大助力。本文从MEMS气体传感器的分类、制备和应用以及传感器阵列的构建等方面综述了金属氧化物半导体MEMS气体传感器的最新研究进展,并对MEMS基气体传感器的发展前景进行了总结和展望。

摘要: 气体传感器的核心介质为气敏薄膜,而薄膜本身的特性有关键的影响,主要体现在薄膜微观结构如晶粒尺寸、膜厚、空隙率和有效表面积等方面。溶胶-凝胶法由于有方法简单及成膜温度低等优点,得到了广泛研究与应用。本文综述了溶胶-凝胶法制备的金属氧化物薄膜微纳结构与气敏机理进来的研究进展,结果表明最佳的晶粒尺寸约为10nm,最佳膜厚约为110 nm。在晶粒尺寸控制方面,通过控制煅烧温度与时间及在溶胶-凝胶过程中加入不同的添加剂,可有效优化晶粒尺寸提高灵敏度。最后,从能带结构角度总结了气敏传感器的电学特性及荷电传输机理,讨论了热电子发射理论和电子隧穿理论起主导作用时的薄膜微纳结构。

摘要: 肺癌的高发生率和高致死率使得其早期筛查与早诊早治的重要性日益突出。相比传统的侵入性检查方法，基于气体传感器的呼气分析技术因其低成本、无创性以及快速诊断的优势，成为一种极具潜力的肺癌早期筛查手段。其中，易集成、操作简便的金属氧化物半导体气体传感器备受关注。本文综述了半导体气体传感器在肺癌早期筛查中的最新研究进展。首先介绍了低剂量计算机断层扫描（LDCT）技术、肺癌呼气生物标志物以及传感器的工作原理；其次总结了新型金属氧化物半导体气体传感器对肺癌标志物气体的检测性能及其增强机理；最后讨论了电子鼻技术及其智能算法在肺癌早期筛查中的应用现状与挑战，并展望了基于半导体气体传感器的肺癌早筛技术的发展前景。

金属氧化物半导体具有较好的气敏性，基于金属氧化物半导体H 2 S气体传感特性得到了广泛研究. 然而，随着气体检测精细程度的增加，需要设计具有更优性能的纳米材料，来实现气体传感器检测下限和灵敏度的提高. 同其他维度纳米材料相比，一维结构纳米材料由于具有良好的结晶度、较大的比表面积和独特的电子输运特性，在H 2 S气敏性能提升上有明显优势. 因此，本文主要以H 2 S气体为主体，综述了基于金属氧化物半导体不同一维结构纳米材料的特点和一维结构纳米材料H 2 S气体传感器的研究进展. 讨论了金属氧化物半导体基一维结构纳米材料对H 2 S气体传感的影响和气敏机理. 最后，对金属氧化物半导体基一维结构纳米材料H 2 S气体传感器的性能改进和未来应用前景进行了展望.

原子层沉积（ALD）是一种可以在原子尺度上沉积金属和金属氧化物传感材料的通用技术。本文综述了用于气体传感器的金属和金属氧化物半导体(MOS)异质结构，重点讨论了利用ALD技术制备三种MOS基异质结构，包括在MOS上沉积金属催化剂、在MOS上沉积金属氧化物、和基于MOS的核壳异质结构，并对这些异质结构的气敏性能进行了详细的分析和讨论。同时，对ALD技术合成MOS气敏材料的未来发展和面临的挑战进行了讨论。

摘要: 丙酮是一种有毒有害的有机气体, 广泛存在于工业生产及家居生活中, 同时也是糖尿病的诊断物之一, 因此准确实时的丙酮检测手段是安全生产和人体健康所迫切需要的。金属氧化物半导体气体传感器具有灵敏度高、成本低、可靠性高等优点, 在有毒有害气体检测领域应用广泛。本文综述了近年来丙酮气敏传感材料的研究进展, 对气敏材料的结构与形貌、气敏机理与改性手段等方面进行了介绍, 分析了其研究现状以及未来的发展趋势。

… , since the synergistic effects contribute to amplify the sensor signal. In … ZnO-based gas sensors, focusing on how the morphology and structure of these materials influence on the sensor …

Abstract The release of toxic and harmful gases seriously endangers human health and social and environmental safety. Sensitive determination of these gases is urgently needed to meet the demands of people and society. ZnO-based gas sensors have been widely used due to fast response, low detection limit, high selectivity, reliable performance and low manufacturing cost. In this review, we highlight the possible sensing mechanism of n-type ZnO materials, and discuss various strategies for enhancing sensitivity and selectivity of ZnO-based gas sensing materials, including regulation of the material morphology, composite of materials, photoexcitation, etc. Then, the current challenges of ZnO-based gas sensing materials are outlined, followed with the future research interests and directions of these materials.

The most extensively explored gas sensors are metal oxide semiconductor materials owing to their high selectivity, stability and cost-effective, simple synthesis technique. Despite these characteristics, they can operate at low...

… in metal oxide semiconductor (MOS) gas sensors. However, a clear … a ZnO single nanowire sensor as a platform to investigate the sensitization of Pt catalyst. Pt has been loaded on ZnO …

… Ag nanoparticles the ZnO sensor shows superior sensitivity 479 % towards 300 ppm hydrogen gas concentration, whereas 101 % response was noted for the pure ZnO sensor at 250 C …

Abstract Monitoring and detection of triethylamine (TEA) gas are indispensable to environmental and human safety. However, TEA gas sensor suffers from low response. Herein, unique nanosized-Pt-decorated hierarchical ZnO microspheres were prepared. Compared with pristine ZnO microspheres and Pt-c-ZnO (Pt nanoparticles deposited on commercial ZnO), the obtained Pt-ZnO exhibited the best TEA gas sensing behaviors, in terms of lower working temperature (200 °C), better selectivity and long-term stability. Notably, the response value of Pt-ZnO sensor towards 100 ppm TEA was up to 242, which was 50 and 16-fold higher than those of pristine ZnO and Pt-c-ZnO sensor, respectively. The superior properties were attributed to structural advantages as well as the synergistic effect between Pt and ZnO. The sensing mechanism was thoroughly clarified by the electron-sinker effect of Pt. It was further corroborated by Kelvin probe and density function theory (DFT) simulation. Moreover, higher surface adsorption oxygen activity and lower TEA adsorption energy of this optimal sample were demonstrated by temperature programmed desorption (O2-TPD) and DFT simulation. Based on the aforementioned advantages, Pt-ZnO becomes a satisfactory candidate for TEA gas sensors. Significantly, this work opens a new avenue for the understanding the mechanism for performance enhancement under.

… Dynamic measurement method of gas sensors can bring a … gases with the same functional group. In this paper, four alcohol homologue gases were detected by a ZnO gas sensor. The …

… surface area, ZnO/ZnO homojunctions, and structural defects. We developed a highly sensitive NO 2 gas sensor with improved reliability using morphologically engineered ZnO, which …

… [4], [5] including gas sensors [6], [7], [8], [9]. Among metal oxide semiconductors, ZnO can be used effectively as gas sensor due to exceptional properties like large number of active sites…

The aim of the current research is to utilize pure ZnO, and Pt nanoparticles loaded ZnO pencil-like microstructures to develop a highly selective and stable H 2 gas sensor. The …

… The 2% Au/ZnO possessed a higher response and lower limit of detection, demonstrating … /ZnO sensor proposed in this work has a potential application for an ethanolamine gas sensor. …

This study uses ultrasonic grinding to grind ZnO powder to 10–20-nanometer nanoparticles (NPs), and these are integrated with a MEMS structure to form a ZnO-NPs/MEMS gas sensor. Measuring 1 ppm NH3 gas and operating at room temperature, the sensor response for the ZnO-NPs/MEMS gas sensor is around 39.7%, but the origin-ZnO powder/MEMS gas sensor is fairly unresponsive. For seven consecutive cycles, the ZnO-NPs/MEMS gas sensor has an average sensor response of about 40% and an inaccuracy of <±2%. In the selectivity of the gas, the ZnO-NPs/MEMS gas sensor has a higher response to NH3 than to CO, CO2, H2, or SO2 gases because ZnO nanoparticles have a greater surface area and more surface defects, so they adsorb more oxygen molecules and water molecules. These react with NH3 gas to increase the sensor response.

In this paper, the ZnO target was synthesized by the solid-state reaction method and a nanostructured thin film was deposited by the RF (radio frequency) magnetron sputtering method on a Multi-Sensor-Platform. The obtained ZnO nanostructured film was investigated as the sensing material. Energy-Dispersive X-ray (EDX) analysis indicated the existence of La in the synthesized ZnO material. Scanning Electron Microscope (SEM) images of the film showed the grain sizes in the range of 20–40 nm. Sensor performance characteristics such as a dynamic response, response and recovery times, and ethanol detection range were investigated at 50–300 °C. A sensitivity was observed at extremely low concentrations of ethanol (0.7 ppm). The minimum response and recovery times of the sensor corresponding to 675 ppm ethanol vapor concentration at 250 °C were found to be 14 s and 61 s, respectively. The sensor showed a high response, good selectivity, fast response/recovery behavior, excellent repeatability toward ethanol vapor, and low sensitivity toward humidity. These characteristics enable the use of a ZnO based sensor for ethanol detecting applications.

Developments of advanced sensors for NO 2 gas with high sensitivity and selectivity are essential but challenging. In this work, a Ce-doped ZnO nanoarray (termed as CZO) was …

Although they are not as favorable as other influential gas sensors, metal-oxide semiconductor-based chemiresistors ensure minimal surface reactivity, restricting their gas selectivity, gas response, and reaction kinetics, particularly when functioning at room temperature (RT). A hybrid design, which includes metal-oxide/carbon nanostructures and passivation with specific gas filtration layers, can address the concerns of surface reactivity. We present a novel hierarchical nanostructured zinc oxide (ZnO), decorated with graphitic carbon (GC) and synthesized via a wet-chemical strategy, which is then followed by the self-assembly of a zeolitic imidazolate framework (ZIF-8). Because of its large surface area, high porosity, and efficient inspection of other analyte (interfering) gases, the ZnO@GC can provide intensified surface reactivity at RT. In the present study, such a hybrid sensor confirmed extraordinary gas sensing properties, which was characterized by excellent H2 selectivity, fast response, rapid recovery kinetics, and high gas response (ΔR/R0 ∼ 124.6%@10 ppm), particularly in extremely humid environments. The results reveal that adsorption sites provided by the ZIF-8 template-based ZnO@GC frameworks facilitate the adsorption and desorption of H2.

Abstract In this work, LaCoO3 (LCO) nanoparticles were synthesized by sol-gel method and modified on the surface of ZnO. The LaCoO3-modified ZnO (LCO/ZnO) nanometer flake materials were successfully prepared, the microstructure, surface properties and internal composition of which were analyzed by various characterization tools. Compared with the traditional ZnO sensor, LCO/ZnO sensor has been greatly improved in terms of gas-sensitive response, response time and recovery time. At the optimal operating temperature of 320 °C, the maximum response of LCO/ZnO sensor to 100 ppm ethanol gas can reach 55, which is 6 times higher than that of pure ZnO sensor. Meanwhile, the response time and recovery time of LCO/ZnO sensor were reduced to 2.8 and 9.7 s, respectively. All the results demonstrate that LCO is an excellent catalyst for improving the gas-sensitive performance of metal oxide semiconductor sensors. The first principle was used to analyze the surface properties, and study the sensitization mechanism of LCO in detail from the adsorption process of surface oxygen, heterojunction action and LCO catalytic oxidation process for ethanol sensing. The improvement of the sensing performance of LCO/ZnO sensor was attributed to the increase of surface adsorbed oxygen content and the strong catalytic oxidation activity of LCO.

… This study aimed to develop a gas sensor based on a ZnO-SnO 2 nanocomposite fabricated … gas. The response, recovery, and selectivity of a series of sensors employing ZnO-SnO 2 …

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… Then we dip LSFO on ZnO nanofiber and grow it on the interdigital gold electrode substrate for gas sensors. The results show that the ZnO/LSFO heterostructure gas sensor has a good …

Semiconductor metal oxides (SMOxs) are widely used in gas sensors due to their excellent sensing properties, abundance, and ease of manufacture. The best examples of these sensing materials are SnO2 and TiO2 that have wide band gap and offer unique set of functional properties; the most important of which are electrical conductivity and high surface reactivity. There has been a constant development of SMOx sensor materials in the literature that has been accompanied by the improvement of their gas-sensitive properties for the gas detection. This review is dedicated to compiling of these efforts in order to mark the achievements in this area. The main material-specific aspects that strongly affect the gas sensing properties and can be controlled by the synthesis method are morphology/nanostructuring and dopants to vary crystallographic structure of MOx sensing material.

Identifying disease biomarkers and detecting hazardous, explosive, flammable, and polluting gases and chemicals with extremely sensitive and selective sensor devices remains a challenging and time-consuming research challenge. Due to their exceptional characteristics, semiconducting metal oxides (SMOxs) have received a lot of attention in terms of the development of various types of sensors in recent years. The key performance indicators of SMOx-based sensors are their sensitivity, selectivity, recovery time, and steady response over time. SMOx-based sensors are discussed in this review based on their different properties. Surface properties of the functional material, such as its (nano)structure, morphology, and crystallinity, greatly influence sensor performance. A few examples of the complicated and poorly understood processes involved in SMOx sensing systems are adsorption and chemisorption, charge transfers, and oxygen migration. The future prospects of SMOx-based gas sensors, chemical sensors, and biological sensors are also discussed.

The usage of the gas sensor has been increasing very rapidly in the industry and in daily life for various potential applications. In the recent years, metal oxide semiconductors (MOS) become the primary choice for designing highly sensitive, stable, and low‐cost real‐life applications‐based gas sensors due to their inherent physical and chemical properties. Researchers have proposed numerous sensing mechanisms to explain the functionality of MOS‐based gas sensors. In this review, we have comprehensively covered different sensing mechanisms used for MOS. We have also discussed different parameters affecting the sensitivity and selectivity of the gas sensors. Moreover, the different techniques used to enhance the gas sensing response of MOS‐based sensors are also extensively covered. And finally, we give our prospective on recent opportunities and challenges on the future applications of MOS‐based gas sensors.

With the emerging of the Internet of Things, chemiresistive gas sensors have been extensively applied in industrial production, food safety, medical diagnosis, and environment detection, etc. Considerable efforts have been devoted to improving the gas‐sensing performance through tailoring the structure, functions, defects and electrical conductivity of sensitive materials. Among the numerous sensitive materials, mesoporous semiconductor metal oxides possess unparalleled properties, including tunable pore size, high specific surface area, abundant metal–oxygen bonds, and rapid mass transfer/diffusion behavior (Knudsen diffusion), which have been regarded as the most potential sensitive materials. Herein, the synthesis strategies for mesoporous metal oxides are overviewed, the classical functionalization techniques of sensitive materials are also systemically summarized as a highlight, including construction of mesoporous structure, regulation of micro‐nano structure (i.e., heterojunctions), noble metal sensitization (e.g., Au, Pt, Ag, Pd) and heteroatomic doping (e.g., C, N, Si, S). In addition, the structure–function relationship of sensitive materials has been discussed at molecular‐atomic level, especially for the chemical sensitization effect, elucidating the interface adsorption/catalytic mechanism. Moreover, the challenges and perspectives are proposed, which will open a new door for the development of intelligent gas sensor in various applications.

While numerous types of gas sensors have been developed for various industries and applications such as the automotive industry, environmental monitoring, and personal safety, nanoscale chemiresistive gas sensors have gained significant research interest due to several advantages such as high sensitivity, low power consumption, and portability. An essential component of these gas sensors is the sensing material where metal oxide semiconductor (MOS) materials are the most prevalent sensing material. Since the adoption of nanoscale synthesis methods for sensing materials development, such as electrospinning and hydrothermal synthesis, many novel 1D MOS‐based nanostructured sensing materials have been demonstrated to enhance gas sensing performance. Overall, nanoengineering approaches and mechanisms for enhancement of gas sensing performance of 1D metal oxide‐based sensing materials are systematically discussed and categorized into several overarching strategies, such as tuning of materials dimension, morphology, and composition. Furthermore, integration of 1D sensing nanomaterials into sensor devices are discussed from the perspective of different chemiresistive sensor architectures and device fabrication methods. Finally, this review also discusses use of 1D MOS materials for emerging and novel electronic nose applications.

Volatile compounds not only contribute to the distinct flavors and aromas found in foods and beverages, but can also serve as indicators for spoilage, contamination, or the presence of potentially harmful substances. As the odor of food raw materials and products carries valuable information about their state, gas sensors play a pivotal role in ensuring food safety and quality at various stages of its production and distribution. Among gas detection devices that are widely used in the food industry, metal oxide semiconductor (MOS) gas sensors are of the greatest importance. Ongoing research and development efforts have led to significant improvements in their performance, rendering them immensely useful tools for monitoring and ensuring food product quality; however, aspects related to their limited selectivity still remain a challenge. This review explores various strategies and technologies that have been employed to enhance the selectivity of MOS gas sensors, encompassing the innovative sensor designs, integration of advanced materials, and improvement of measurement methodology and pattern recognize algorithms. The discussed advances in MOS gas sensors, such as reducing cross-sensitivity to interfering gases, improving detection limits, and providing more accurate assessment of volatile organic compounds (VOCs) could lead to further expansion of their applications in a variety of areas, including food processing and storage, ultimately benefiting both industry and consumers.

Latest progress on flexible room temperature (FRT) gas sensor based on metal oxide semiconductors (MOS) is comprehensively reviewed. FRT gas sensor based on pristine MOS and MOS modified with noble metal nanoparticles, organic polymers, carbon based materials and transition metal dichalcogenide materials are meticulously reviewed. The gas sensing mechanism of MOS chemiresistive gas sensors are introduced and the applications, future perspectives, and challenges of FRT gas sensors are also proposed. With the rapid development of the Internet of Things, there is a great demand for portable gas sensors. Metal oxide semiconductors (MOS) are one of the most traditional and well-studied gas sensing materials and have been widely used to prepare various commercial gas sensors. However, it is limited by high operating temperature. The current research works are directed towards fabricating high-performance flexible room-temperature (FRT) gas sensors, which are effective in simplifying the structure of MOS-based sensors, reducing power consumption, and expanding the application of portable devices. This article presents the recent research progress of MOS-based FRT gas sensors in terms of sensing mechanism, performance, flexibility characteristics, and applications. This review comprehensively summarizes and discusses five types of MOS-based FRT gas sensors, including pristine MOS, noble metal nanoparticles modified MOS, organic polymers modified MOS, carbon-based materials (carbon nanotubes and graphene derivatives) modified MOS, and two-dimensional transition metal dichalcogenides materials modified MOS. The effect of light-illuminated to improve gas sensing performance is further discussed. Furthermore, the applications and future perspectives of FRT gas sensors are also discussed.

There is an increasing need for the development of low-cost and highly sensitive gas sensors for environmental, commercial, and industrial applications in various areas, such as hazardous gas monitoring, safety, and emission control in combustion processes. Considering this, resistive-based gas sensors using metal oxide semiconductors (MOSs) have gained special attention owing to their high sensing performance, high stability, and low cost of synthesis and fabrication. The relatively low final costs of these gas sensors allow their commercialization; consequently, they are widely used and available at low prices. This review focuses on the important MOSs with different morphologies, including quantum dots, nanowires, nanofibers, nanotubes, hierarchical nanostructures, and other structures for the fabrication of resistive gas sensors.

… The sensing mechanism of gas sensor based on metal oxides semiconductors is mainly … target gases. For example, for n-type semiconductors, the gas response (S) of the sensor is …

… explores metal oxide semiconductor (MOS) gas sensors with prime … of a gas sensor. The paper discusses various aspects for a sensor and encompasses with several other gas sensors …

… adsorption/desorption of gases. In metal oxide semiconductor gas sensors, the sensitivity and … discussed the MXene heterojunction with metal oxides for the detection of various gases. …

With the development of internet of things and artificial intelligence electronics, metal oxide semiconductor (MOS)‐based sensing materials have attracted increasing attention from both fundamental research and practical applications. MOS materials possess intrinsic physicochemical properties, tunable compositions, and electronic structure, and are particularly suitable for integration and miniaturization in developing chemiresistive gas sensors. During sensing processes, the dynamic gas–solid interface interactions play crucial roles in improving sensors’ performance, and most studies emphasize the gas–MOS chemical reactions. Herein, from a new view angle focusing more on physical gas–solid interactions during gas sensing, basic theory overview and latest progress for the dynamic process of gas molecules including adsorption, desorption, and diffusion, are systematically summarized and elucidated. The unique electronic sensing mechanisms are also discussed from various aspects including molecular interaction models, gas diffusion mechanism, and interfacial reaction behaviors, where structure–activity relationship and diffusion behavior are overviewed in detail. Especially, the surface adsorption–desorption dynamics are discussed and evaluated, and their potential effects on sensing performance are elucidated from the gas–solid interfacial regulation perspective. Finally, the prospect for further research directions in improving gas dynamic processes in MOS gas sensors is discussed, aiming to supplement the approaches for the development of high‐performance MOS gas sensors.

In this review, the different important parameters having significant effects on the sensing behavior of semiconductor metal oxide (SMO) based chemiresistive gas sensors have been …

To meet requirements in air quality monitoring, sensors are required that can measure the concentration of gaseous pollutants at concentrations down to the ppb and ppt levels, while at the same time they exhibiting high sensitivity, selectivity, and short response/recovery times. Among the different sensor types, those employing metal oxide semiconductors (MOSs) offer great promises as they can be manufactured in easy/inexpensive ways, and designed to measure the concentration of a wide range of target gases. MOS sensors rely on the adsorption of target gas molecules on the surface of the sensing material and the consequent capturing of electrons from the conduction band that in turn affects their conductivity. Despite their simplicity and ease of manufacturing, MOS gas sensors are restricted by high limits of detection (LOD; which are typically in the ppm range) as well as poor sensitivity and selectivity. LOD and sensitivity can in principle be addressed by nanostructuring the MOSs, thereby increasing their porosity and surface-to-volume ratio, whereas selectivity can be tailored through their chemical composition. In this paper we provide a critical review of the available techniques for nanostructuring MOSs using chemiresistive materials, and discuss how these can be used to attribute desired properties to the end gas sensors. We start by describing the operating principles of chemiresistive sensors, and key material properties that define their performance. The main part of the paper focuses on the available methods for synthesizing nanostructured MOSs for use in gas sensors. We close by addressing the current needs and provide perspectives for improving sensor performance in ways that can fulfill requirements for air quality monitoring.

Semiconductor metal oxide (SMO) gas sensors have attracted considerable attention for detecting environmental pollution, as well as the accidental leakage of flammable, explosive, and toxic gases. SMOs are known to exhibit high sensitivity, fast response time, and excellent selectivity towards various types of gases. Many new strategies have been implemented to improve these characteristics. Among the materials produced by these methods, nanomaterials (NMs) synthesized by electrospinning have unprecedented advantages, including catalyst introduction, morphological control, thermodynamic stability, unique physicochemical properties, composition adjustment, and rapid adsorption-desorption rates of the NMs, and are appealing for the designing highly sensitive and selective gas sensors. This review highlights the latest findings on the design and fabrication of electrospun gas sensors for detecting various gases including hydrogen (H2), methane (CH4), nitrogen monoxide (NO), hydrogen sulfide (H2S), ammonia (NH3), ethanol (C2H5OH), acetone (CH3COCH3), formaldehyde (HCHO) and toluene (C6H5CH3). Studies have indicated that NMs with different shapes (e.g., nanotubes, nanowires, nanoflowers, nanosheets, nanorods, nanofilms, and nanofibers) and compositions (single-phase SMOs, modified SMOs, nanocomposites of SMOs, and SMOs combined with carbon nanomaterials) display high response values, long-term stability, low humidity dependence, fast response/recovery times, and low detection limits for gases. Finally, conclusions and future perspectives for gas sensors based on the electrospinning technique are discussed.

… , metal oxide semiconductors (MOSs) gas sensors based on the advanced microelectromechanical systems (MEMS) … of MEMS-based MOSs gas sensors in terms of the …

This paper describes the design and development of low power Micro Electro Mechanical Systems (MEMS) microheater and metal oxide semiconductor CO 2 sensor. To achieve low …

… These challenges result in variable sensor responses and reliability concerns, highlighting the need for innovative solutions to unlock the full potential of MEMS gas sensors in the …

… Metal oxide semiconductor gas sensors have attracted particular attention due to their merits of high sensitivity and easy integration. However, their insufficient selectivity severely …

In this study, a simple and cost-effective metal oxide semiconductor (MOS) gas sensor, which can be fabricated utilizing only two photolithography steps, was designed and developed through the planar microelectromechanical systems (MEMS) technique. Ball-milled porous tin dioxide nanoparticle clusters were precisely drop-coated onto the integrated microheater region and subsequently characterized using a helium ion microscope (HIM). The spatial suspension of the silicon nitride platform over the silicon substrate provides superior thermal isolation and thus dramatically reduces the power consumption of the microheater. The well-designed microheater exhibits excellent thermal uniformity, which was verified both computationally and experimentally. The as-fabricated sensors were tested for ethanol gas sensing at various operating temperatures with different concentrations. At the optimal work temperature of ∼400 °C, our gas sensors demonstrated a respectable sensitivity to 1 ppm ethanol, which is the lower detection limit to most commercial products. Moreover, stable performance over repetitive testing was observed. The innovative sensor developed here is a promising candidate for portable gas sensing devices and various other commercial applications.

The MEMS gas sensor is one of the most promising gas sensors nowadays due to its advantage of small size, low power consumption, and easy integration. It has been widely applied in energy components, portable devices, smart living, etc. The performance of the gas sensor is largely determined by the sensing materials, as well as the fabrication methods. In this review, recent research progress on H2, CO, NO2, H2S, and NH3 MEMS sensors is surveyed, and sensing materials such as metal oxide semiconductors, organic materials, and carbon materials, modification methods like construction of heterostructures, doping, and surface modification of noble metals, and fabrication methods including chemical vapor deposition (CVD), sputtering deposition (SD), etc., are summarized. The effect of materials and technology on the performance of the MEMS gas sensors are compared.

… of room-temperature detection of perovskite and metal oxide … -mechanical system (MEMS) gas sensing arrays and electronic … future prospects of metal oxide gas sensors are presented. …

Highly sensitive gas sensors with remarkably low detection limits are attractive for diverse practical application fields including real-time environmental monitoring, exhaled breath diagnosis, and food freshness analysis. Among various chemiresistive sensing materials, noble metal-decorated semiconducting metal oxides (SMOs) have currently aroused extensive attention by virtue of the unique electronic and catalytic properties of noble metals. This review highlights the research progress on the designs and applications of different noble metal-decorated SMOs with diverse nanostructures (e.g., nanoparticles, nanowires, nanorods, nanosheets, nanoflowers, and microspheres) for high-performance gas sensors with higher response, faster response/recovery speed, lower operating temperature, and ultra-low detection limits. The key topics include Pt, Pd, Au, other noble metals (e.g., Ag, Ru, and Rh . ), and bimetals-decorated SMOs containing ZnO, SnO_2, WO_3, other SMOs (e.g., In_2O_3, Fe_2O_3, and CuO), and heterostructured SMOs. In addition to conventional devices, the innovative applications like photo-assisted room temperature gas sensors and mechanically flexible smart wearable devices are also discussed. Moreover, the relevant mechanisms for the sensing performance improvement caused by noble metal decoration, including the electronic sensitization effect and the chemical sensitization effect, have also been summarized in detail. Finally, major challenges and future perspectives towards noble metal-decorated SMOs-based chemiresistive gas sensors are proposed. Recent progress in noble metal-decorated (NM-D) semiconducting metal oxides (SMOs) gas sensors are summarized. Gas sensing mechanisms related to noble metal decoration are carefully discussed. Crucial challenges facing the development of NM-D SMOs gas sensors are analyzed.

… microelectromechanical systems (MEMS) technology to fabricate a gallium oxide (GO) film/MEMS gas sensor. … In terms of gas measurements, GO film that is annealed at 236.82 C with a …

Abstract This study produces Co3O4 nanoparticles (NPs) using ultrasonic wave grinding technology. The experimental results show that the average diameter of Co3O4-NPs is about 6 nm. Au nanoparticles are adsorbed onto the surface of Co3O4-NPs and integrated with a MEMS structure to form a Au/Co3O4-NPs/MEMS NO2 gas sensor. The diameter of Au nanoparticle is approximately 1 nm. For a 100-ppb NO2 gas concentration, an Au/Co3O4-NPs/MEMS gas sensor has the greatest sensor response and has a higher sensitivity to NO2 than other gases (SO2, NH3, CO, CO2) when NO2 gas is introduced at 10 ppm and at the optimal operating temperature of 136 ℃.

This article reports for the first time a quadrilateral microelectromechanical sensor (MEMS) metal oxide (MOx) gas sensor cell with four different sensitive materials based on complete semiconductor process. In order to improve the strength, symmetry of temperature distribution, and thermal stability of the structure, the design path of the Pt heater extends to the substrate along the four supporting beams, so that the joule heat generated and heat conduction in the core area are symmetrical. Four pairs of detection electrodes and microheater are designed on the same layer and arranged in parallel; thus, the process steps are simplified and the overlapping and stacking of metals can be avoided. Each pair of detection electrodes corresponds to a type of patterned sensing material film. The sensor cell was fabricated using typical 4 in MEMS process, and four kinds of sensing materials (SnO2/Au, SnO2, In2O3/Au, and ZnO/Au) were completed by four times of sputtering process and patterned through UV lithography and lift-off, realizing the semiconductor process of the entire sensor cell. Five gases (ethanol, ethyl acetate, trimethylamine, ammonia, and hydrogen sulfide) were tested to verify the performance of the sensor cell, and the results show that the sensor cell has differential responses to the test gases. Furthermore, the k-nearest neighbor (KNN) pattern algorithm is used to distinguish the five gases with an accuracy of 99.33%. The sensor cell is promising in gas identification applications.

A low-power multichannel metal oxide semiconductor (MOS) gas sensor device is presented, including a gas sensor with four gas detecting channels. The device adopts a T-shaped structure having a bridge-type single strip microheater along its horizontal axis, which also houses the four gas detecting channels. The device was designed and fabricated using MEMS technology with SnO2 deposited by sputtering as the sensing layer. Based on the temperature gradient of the microheater, at the same heating voltage, the device has a different temperature at each channel. Gas tests for four environmental air pollutants were carried out, and all four channels responded to H2, CO, NO2, and NH3 gases with a total power consumption of 21.92 mW. The responses characteristics of two sets of symmetrical channels are different under the same input heating voltage. The response change trend of each channel is also different with different target gas as the input heating voltage changes. [2021-0243]

With the growing demand for chemical information collection from the environment, miniaturized size and high sensitivity labeled microelectromechanical systems (MEMS) gas molecular sensing devices have emerged as a promising element in the development of machine olfactory. However, prolonged exposure to gas analytes often induces slow chemical transformations on the sensing film surface, leading to reduced chemical activity, performance degradation, and mechanical failures such as membrane cracking or delamination. In the real market, especially under harsh working environments, long‐term stability is a critical quality metric in gas sensor development. Therefore, the pursuit of MEMS gas sensors that offer both high sensitivity and extended lifespan becomes indispensable yet challenging. Thus, this review provides a comprehensive overview of recent studies and achievements in MEMS gas sensors, highlighting efforts aimed at enhancing their working stability. Key areas of focus include advancements in the chemical and physical characteristics of sensing films, as well as improvements in device structures. Furthermore, current limitations, perspectives, and future possibilities for designing and fabricating MEMS gas sensors with long‐term stability are discussed.

… Severe health issues occur when human beings are exposed to NO 2 gas, which is formed … 2 gas sensors are highly needed at low concentration gas detections. ZnO gas sensor with …

Many types of MEMS gas sensors based on metal-oxide semiconductors (MOS) are reported in recent years. For the fabrication of the gas sensors, it is still a big challenge to achieve wafer-level patterning and good compatibility for the MOS thin film architecture. In this paper, we report a wafer-level patterning method using photolithography to increase the patterning efficiency. When compared with devices using dip-coating, our devices show comparable response and excellent reproducibility. Besides, we prove the feasibility of this method for patterning different materials onto various electrodes structure. This method provides a potential for mass production of MEMS gas sensors where high efficiency and throughout are crucial.

Air pollution is a severe concern globally as it disturbs the health conditions of living beings and the environment because of the discharge of acetone molecules. The metal-oxide-semiconductor (MOS) nanomaterials are crucial for developing efficient sensors because their outstanding chemical and physical properties empowering the inclusive developments in gas sensor productivity. This review presents the ZnO nanostructures' state-of-the-art and notable growth and their structural, morphological, electronic, optical, and acetone-sensing properties. The key parameters such as response, gas detection limit, sensitivity, reproducibility, response and recovery time, selectivity, and stability of the acetone sensor have been discussed. Further, the gas-sensing mechanism models of the metal-oxide-semiconductors for acetone sensing are reported and discussed. Finally, future possibilities and challenges of the metal-oxide-semiconductor (ZnO) based gas sensor for acetone detection have also been explored.

Research focus on control growth of nanostructures, understanding of sensing mechanism through band model, LUMO energy, defect state density. Further, role of electrode for sensing and substrate for devices reliability has been discussed.

In this work, we report on the synthesis of four morphologies of ZnO, namely, nanoparticles, nanorods, nanosheets, and nanoflowers, from a single precursor Zn(CH3COO)2·2H2O under different reaction conditions. The synthesised nanostructured materials were characterised using powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy, UV–Vis, XPS analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and nitrogen sorption at 77 K. The XRD, FTIR, and Raman analyses did not reveal any significant differences among the nanostructures, but differences in the electronic properties were noted among the different morphologies. The TEM and SEM analyses confirmed the four different morphologies of the ZnO nanostructures. The textural characteristics revealed that the specific surface areas were different, being 1.3, 6.7, 12.7, and 26.8 m2/g for the nanoflowers, nanoparticles, nanorods, and nanosheets, respectively. The ZnO nanostructures were then mixed with carbon nanoparticles (CNPs) and cellulose acetate (CA) to make nanocomposites that were then used as sensing materials in solid-state sensors to detect methanol, ethanol, and isopropanol vapour at room temperature. The sensors’ responses were recorded in relative resistance. When detecting methanol, 6 out of 12 sensors were responsive, and the most sensitive sensor was the composite with a mass ratio of 1:1:1 of ZnO nanorods:CNPs:CA with a sensitivity of 0.7740 Ω ppm−1. Regarding the detection of ethanol vapour, 9 of the 12 sensors were responsive, and the 3:1:1 mass ratio with ZnO nanoparticles was the most sensitive at 4.3204 Ω ppm−1. Meanwhile, with isopropanol, 5 out of the 12 sensors were active and, with a sensitivity of 3.4539 Ω ppm−1, the ZnO nanoparticles in a 3:1:1 mass ratio were the most sensitive. Overall, the response of the sensors depended on the morphology of the nanostructured ZnO materials, the mass ratio of the sensing materials in the composites, and the type of analyte. The sensing mechanism was governed by the surface reaction on the sensing materials rather than pores hindering the analyte molecules from reaching the active site, since the pore size is larger than the kinetic diameter of the analyte molecules. Generally, the sensors responded well to the ethanol analyte, rather than methanol and isopropanol. This is due to ethanol molecules displaying a more enhanced electron-donating ability.

In this comprehensive study, Ce-doped ZnO nanostructures were hydrothermally synthesized with varying Ce concentrations (0.5%, 1.0%, 1.5%, and 2.0%) to explore their gas-sensing capabilities, particularly towards NO2. Structural characterization revealed that as Ce doping increased, crystal size exhibited a slight increment while band gap energies decreased. Notably, the 0.5% Ce-doped ZnO nanostructure demonstrated the highest NO2 gas response of 8.6, underscoring the significance of a delicate balance between crystal size and band gap energy for optimal sensing performance. The selectivity of the 0.5% Ce-doped ZnO nanostructures to NO2 over other gases like H2, acetone, NH3, and CO at a concentration of 100 ppm and an optimized temperature of 250 °C was exceptional, highlighting its discriminatory prowess even in the presence of potential interfering gases. Furthermore, the sensor displayed reliability and reversibility during five consecutive tests, showcasing consistent performance. Long-term stability testing over 30 days revealed that the gas response remained almost constant, indicating the sensor's remarkable durability. In addition to its robustness against humidity variations, maintaining effectiveness even at 41% humidity, the sensor exhibited impressive response and recovery times. While the response time was swift at 11.8 s, the recovery time was slightly prolonged at 56.3 s due to the strong adsorption of NO2 molecules onto the sensing material hindering the desorption process. The study revealed the intricate connection between Ce-doping levels, structure, and gas-sensing. It highlighted the 0.5% Ce-doped ZnO nanostructure as a highly selective, reliable, and durable NO2 gas sensor, with implications for future environmental monitoring and safety.

… in the domain of Zno based gas sensors. Srinivasan et al. … ZnO nanostructures, and the techniques used to create them, after which we have provided an overview of ZnO nano-sensors …

… The ZnO nanostructures were made by combining the successive ionic layer adsorption and … of the gas sensors can be improved by control of the morphology of ZnO nanostructures. …

Abstract Three different ZnO nanostructures (nanorods/flowers/spheres) were prepared through facile hydrothermal method or water bath, whose sensing properties to NO2 were investigated in detail at room temperature activated by UV light (365 nm LED). The sensing results showed that ZnO nanospheres exhibit the highest response of 29.4 to 5 ppm NO2, which can be attributed to their biggest specific surface area and the most amount of adsorbed oxygen species on the surface. In contrast, ZnO nanorods show the fastest response and recovery speed (9 and 18 s to 5 ppm NO2, respectively) due to their highest crystallinity, least surface defects and unidirectional electron transfer path, which is much faster than most reported literatures. With regard to the ZnO nanoflowers, both the gas-sensing response and the response and recovery speed are between ZnO nanorods and ZnO nanospheres. Furthermore, the stability, selectivity, and the moisture resistance of three different ZnO nanostructures were also researched.

… gas sensors, As a result, ZnO nanoparticles are commonly utilized to make effective gas sensors … on nitrogen dioxide (NO 2 ) gas sensor advances based on ZnO nanoparticles, X-ray …

… ZnO nanostructured thin films. The prepared films were characterized structurally, optically, and morphologically. The sensor … the acetone gas sensor operating at lower temperatures. …

… gas response was achieved by optimising doping concentration. When ZnO and Sn doped ZnO nanostructures … C provided the best gas response. Using several saturated solutions, the …

… of flexible gas sensors utilizing ZnO/Kevlar™, entailing the modification of ZnO morphology. … be a feasible and interesting substrate for nanostructured ZnO-based NH3 gas sensors. …

In this work, we have synthesized the ZnO Hexagonal pyramid structures (ZnO HPs) for low detection limit of NO 2 gas sensors by a facile hydrothermal method. The structural, …

Development of sensor materials based on metal oxide semiconductors (MOS) for selective gas sensors is challenging for the tasks of air quality monitoring, early fire detection, gas leaks search, breath analysis, etc. An extensive range of sensor materials has been elaborated, but no consistent guidelines can be found for choosing a material composition targeting the selective detection of specific gases. Fundamental relations between material composition and sensing behavior have not been unambiguously established. In the present review, we summarize our recent works on the research of active sites and gas sensing behavior of n-type semiconductor metal oxides with different composition (simple oxides ZnO, In2O3, SnO2, WO3; mixed-metal oxides BaSnO3, Bi2WO6), and functionalized by catalytic noble metals (Ru, Pd, Au). The materials were variously characterized. The composition, metal-oxygen bonding, microstructure, active sites, sensing behavior, and interaction routes with gases (CO, NH3, SO2, VOC, NO2) were examined. The key role of active sites in determining the selectivity of sensor materials is substantiated. It was shown that the metal-oxygen bond energy of the MOS correlates with the surface acidity and the concentration of surface oxygen species and oxygen vacancies, which control the adsorption and redox conversion of analyte gas molecules. The effects of cations in mixed-metal oxides on the sensitivity and selectivity of BaSnO3 and Bi2WO6 to SO2 and VOCs, respectively, are rationalized. The determining role of catalytic noble metals in oxidation of reducing analyte gases and the impact of acid sites of MOS to gas adsorption are demonstrated.

… Gas selectivity is necessary to differentiate between multiple gas species that may be present … In this work, studies on gas selectivity in semiconducting metal oxide sensors are reviewed …

The demand for highly functional chemical gas sensors has surged due to the increasing awareness of human health to monitor metabolic disorders or noncommunicable diseases, safety measures against harmful greenhouse and/or explosive gases, and determination of food freshness. Over the years of dedicated research, several types of chemiresistive gas sensors have been realized with appreciable sensitivities toward various gases. However, critical issues such as poor selectivity and sluggish response/recovery speeds continue to impede their widespread commercialization. Specifically, the mechanisms behind the selective response of some chemiresistive materials toward specific gas analytes remain unclear. In this review, we discuss state-of-the-art strategies employed to attain gas-selective chemiresistive materials, with particular emphasis on materials design, surface modification or functionalization with catalysts, defect engineering, material structure control, and integration with physical/chemical gas filtration media. The nature of material surface–gas interactions and the supporting mechanisms are elucidated, opening opportunities for optimizing the materials design, fine-tuning the gas sensing performance, and guiding the selection of the most appropriate materials for the accurate detection of specific gases. This review concludes with recommendations for future research directions and potential opportunities for further selectivity improvements.

Metal oxide nanoparticles have been widely utilized for the fabrication of functional gas sensors to determine various flammable, explosive, toxic, and harmful gases due to their advantages of low cost, fast response, and high sensitivity. However, metal oxide-based gas sensors reveal the shortcomings of high operating temperature, high power requirement, and low selectivity, which limited their rapid development in the fabrication of high-performance gas sensors. The combination of metal oxides with two-dimensional (2D) nanomaterials to construct a heterostructure can hybridize the advantages of each other and overcome their respective shortcomings, thereby improving the sensing performance of the fabricated gas sensors. In this review, we present recent advances in the fabrication of metal oxide-, 2D nanomaterials-, as well as 2D material/metal oxide composite-based gas sensors with highly sensitive and selective functions. To achieve this aim, we firstly introduce the working principles of various gas sensors, and then discuss the factors that could affect the sensitivity of gas sensors. After that, a lot of cases on the fabrication of gas sensors by using metal oxides, 2D materials, and 2D material/metal oxide composites are demonstrated. Finally, we summarize the current development and discuss potential research directions in this promising topic. We believe in this work is helpful for the readers in multidiscipline research fields like materials science, nanotechnology, chemical engineering, environmental science, and other related aspects.

… ZnO nanoneedles-based gas sensor was studied for the detection of isopropanol at low concentrations (below 10 ppm). The pure and Fe-doped ZnO … The morphologies of the sensing …

… gas sensors, the review includes a thorough study of gas sensors and the factors relating to sensing mechanisms. … ], gas adsorption and desorption on the surface of ZnO semiconductor …

Abstract In this paper, Pd/ZnO–SnO2 hollow nanofibers prepared by electrospinning and magnetron sputtering. The sensing materials exhibited extraordinary hydrogen (H2) sensing behavior – the response reached 171 at 270 °C, and besides, the response time and recovery time were as low as 19 s and less than 1 s at 240 °C, with steady high selectivity and good repeatability. Such an enhanced sensing performance could be attributed to the Pd/ZnO/SnO2 ternary heterojunctions formed in the hollow nanofibers. Moreover, the materials were studied by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and especially the morphology formation process was further explained. The existence of Pd was confirmed to increase the resistance modulation range of heterojunction. In addition, the gas sensing mechanism based on the structure was proposed.

… sensing of ZnO (0001) surface. Adsorption configurations and electronic properties of ZnO and Pt-doped ZnO … enough to judge the sensitivity of gas sensors, and one has to consider all …

… homologous gases in combination with oxygen vacancy modulation of ZnO gas sensors. … n-butyl alcohol were measured by ZnO gas sensors with different oxygen vacancies content …

Abstract The chemiresistor-type sensors based on Ag-ZnO nanostructures have shown excellent sensing properties to NO2 under visible light irradiation, however, they cannot escape from the destiny of the performances degraded by long-term operation. So, aiming at improving the long-term stability, the revelation of the essence of performance degradation is crucial and also a great challenge. Here, we comparatively investigated the change rules of NO2 sensing properties after running the as-fabricated Ag-ZnO nanoparticles-based sensor multiple times during 60 days under UV and visible light irradiation. It is demonstrated that the room temperature light-activated (especially UV light) NO2 sensing performances (response and sensitivity) of the sensor degrade after running multiple times. The degraded performances can be attributed to the deteriorated crystal quality and increased oxygen vacancy defects of the sensing materials, and oxidization of more Ag0 into Ag+ on the ZnO surfaces, which greatly increase the free charge carrier concentration in the dark and decrease photogenerated charge carrier concentration under light irradiation. Correspondingly, the photoconductivity of the device declines and its output resistance signal weakens. This work offers not only a fundamental understanding of the performance degradation of the light-activated room temperature NO2 gas sensor, but also a strategy for the study on the performance degradation mechanism of other devices.

… The ZnO has the potential to form a number of nanostructures with various morphologies … gas sensing properties. Herein, hydrogen sulphide (H 2 S) gas sensing properties of the ZnO …

… The results showed that ZnO/Ti 3 C 2 T x composites with few-layer MXene had a higher … ZnO and gas-sensing properties. Finally, the gas sensing mechanism of few-layer MXene/ZnO …

Abstract The design of hetero-nanojunctions can greatly amplify the gas-sensing performance of conventional metal oxide gas sensors. In this study, the composite nanofibers (NFs) of ZnO–SnO2 internal heterojunctions (HJs) and the mixed NFs of ZnO/SnO2 external HJs were realized by using the electrospinning technique. Then, the gas-sensing characteristics of the as-synthesized NFs toward H2S and NO2 gases were systematically investigated and compared. The effects of the internal and external nanojunctions of the hetero-NF-based gas sensors were also explored. Results showed that the ZnO–SnO2 composite NFs demonstrate a higher response than those of the ZnO/SnO2 mixed NFs, as well as bare ZnO and SnO2 NFs. Also, the cross responses of the hetero-NF sensors toward 200 ppm CO, 250 ppm H2, and 250 ppm NH3 were examined. The gas-sensing mechanism of the internal and external HJs of composite ZnO/SnO2 and mixed ZnO/SnO2 NFs was analyzed. The remarkable enhanced gas-sensing performance of hetero-NFs was mainly attributed to the formation of internal HJs.

Abstract Catalytic and sensing performance of ZnO/Ag nanocomposites greatly depends on the synthesis method used for its production, presenting both challenges and opportunities for optimization. A polymer-network gel process is one of the promising techniques that enables cheap and efficient synthesis of photoactive ZnO/Ag nanocomposites. However, the mechanisms responsible for the formation of ZnO/Ag interfaces are not completely understood, and the photocatalytic efficiency of the ZnO/Ag nanocomposites produced by a simple gel process is not sufficient. In this paper we describe a comparative study on the one- and two-step polymer network gel synthesis techniques, and investigate the mechanisms that account for the significant differences in the observed performance of ZnO/Ag produced by these two methods. The ZnO/Ag nanocomposites synthesized by the two-step method exhibit remarkably enhanced photocatalytic efficiency for Methylene blue, Methyl orange, Rhodamine B, Malachite Green and Formaldehyde (HCHO) under UV light irradiation and simulated sunlight. As the gas sensing material, the ZnO/Ag nanocomposites synthesized by the two-step method show the higher sensitivity and better stability and selectivity. The microstructures and optical characteristics of the samples were revealed by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, ultraviolet visible (UV–vis) spectroscopy, room temperature photoluminescence measurements, and energy dispersive X-ray (EDS) analysis. Samples synthesized by the two-step process possess a more uniform dispersion of Ag nanoparticles, a smaller Ag crystallite size, and a narrower bandgap. Chemically, more oxygen vacancies and hydroxyl radicals are present in the samples synthesized by the two-step process. Jointly, these factors favor enhanced photocatalytic degradation of organic pollutants, and sensitivities of the sensors.

… @ZnO composites was reduced from 200 C to 160 C. Among them, the gas sensor fabricated by the sample SnO 2 @ZnO 0.33 … The enhanced gas sensing mechanism of SnO 2 @ZnO …

Fast detection of hydrogen gas leakage or its release in different environments, especially in large electric vehicle batteries, is a major challenge for sensing applications. In this study, the morphological, structural, chemical, optical, and electronic characterizations of ZnO:Eu nanowire arrays are reported and discussed in detail. In particular, the influence of different Eu concentrations during electrochemical deposition was investigated together with the sensing properties and mechanism. Surprisingly, by using only 10 μM Eu ions during deposition, the value of the gas response increased by a factor of nearly 130 compared to an undoped ZnO nanowire and we found an H2 gas response of ∼7860 for a single ZnO:Eu nanowire device. Further, the synthesized nanowire sensors were tested with ultraviolet (UV) light and a range of test gases, showing a UV responsiveness of ∼12.8 and a good selectivity to 100 ppm H2 gas. A dual-mode nanosensor is shown to detect UV/H2 gas simultaneously for selective detection of H2 during UV irradiation and its effect on the sensing mechanism. The nanowire sensing approach here demonstrates the feasibility of using such small devices to detect hydrogen leaks in harsh, small-scale environments, for example, stacked battery packs in mobile applications. In addition, the results obtained are supported through density functional theory-based simulations, which highlight the importance of rare earth nanoparticles on the oxide surface for improved sensitivity and selectivity of gas sensors, even at room temperature, thereby allowing, for instance, lower power consumption and denser deployment.

This paper reports synthesis, properties and gas sensing applications of ZnO nanoflowers and CdO–ZnO nanorices prepared by hydrothermal process. The morphological …

In this study, we report on the synthesis of the NiO/ZnO heterostructured nanowires by a facile two-step liquid phase route and their gas sensing characteristics employing Au …

… rGO/ZnO NFs exhibited the highest gas … sensor exhibits an ultralow detection limit of 4.9 ppt and excellent stability towards H 2 S. A novel sulfuration-desulfuration sensing mechanism is …