Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

This article extensively reviews the recent development of semiconductor metal oxide gas sensors for environmentally hazardous gases including NO2, NO, N2O, H2S, CO, NH3, CH4, SO2 and CO2. The gas sensing properties of differently-prepared metal oxides and loaded metal oxides towards nine environmentally hazardous gases have been individually compared and digested. Promising materials for sensitive and selective detection of each hazardous gas have been identified. For instance, unloaded WO3 nanostructures are the most promising candidates for NO2 sensing while metal catalyst loaded WO3 and gold-loaded SnO2 sensors are among the most effective for NO and N2O sensing, respectively. Moreover, related gas-sensing mechanisms are comprehensively discussed.

Semiconducting metal oxides as sensors for environmentally hazardous gases

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

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Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review

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

Graphene, a single, one-atom-thick sheet of carbon atoms arranged in a honeycomb lattice and the two-dimensional building block for carbon materials, has attracted great interest for a wide range of applications. Due to its superior properties such as thermo-electric conduction, surface area and mechanical strength, graphene materials have inspired huge interest in sensing of various chemical species. In this timely review, we discuss the recent advancement in the field of graphene based gas sensors with emphasis on the use of modified graphene materials. Further, insights of theoretical and experimental aspects associated with such systems are also discussed with significance on the sensitivity and selectivity of graphene towards various gas molecules. The first section introduces graphene, its synthesis methods and its physico-chemical properties. The second part focuses on the theoretical approaches that discuss the structural improvisations of graphene for its effective use as gas sensing materials. The third section discusses the applications of pristine and modified graphene materials in gas sensing applications. Various graphene modification methods are discussed including using dopants and defects, decoration with metal/metal oxide nanoparticles, and functionalization with polymers. Finally, a discussion on the future challenges and perspectives of this enticing field of graphene sensors for gas detection is provided.

Recent advances in graphene based gas sensors

Graphene-based nanocomposites have proven to be very promising materials for gas sensing applications. In this paper, we present a general approach for the preparation of graphene‐WO3 nanocomposites. Graphene‐WO3 nanocomposite thin-layer sensors were prepared by drop coating the dispersed solution onto the alumina substrate. These nanocomposites were used for the detection of NO2 for the first time. TEM micrographs revealed that WO3 nanoparticles were well distributed on graphene nanosheets. Three different compositions (0.2, 0.5 and 0.1 wt%) of graphene with WO3 were used for the gas sensing measurements. It was observed that the sensor response to NO2 increased nearly three times in the case of graphene‐WO3 nanocomposite layer as compared to a pure WO3 layer at room temperature. The best response of the graphene‐WO3 nanocomposite was obtained at 250 C. (Some figures may appear in colour only in the online journal)

https://iopscience.iop.org/article/10.1088/0957-4484/23/20/205501/pdf

Faster response of NO 2 sensing in graphene-WO 3 nanocomposites

Ni- and/or Al-doped and undoped SnO2 thick film gas sensors were prepared using screen printing technique and tested for their LPG gas sensitivity. Tin oxide powder was prepared using a chemical precipitation technique. The sensitivity, optimum working temperature and response time were investigated in relation to dopants as well as preparation route. The results show that the gas sensitivity is affected not only by the additive but the way it is added into the sensor material. The results indicated a reduction in grain size on Al and Ni doping. The results on resistance, response and recovery time were explained in terms of n–p junction formation between SnO2 and NiO, which increases the depletion barrier height.

Effect of Ni doping on thick film SnO2 gas sensor

Ammonia gas sensing properties of a single layer WO 3 thick film and a double layer sensor structure having a supported catalyst on it have been investigated. The single layer sensor exhibited low response to NH 3 . An enhancement in gas response was achieved by doping WO 3 with Pt, Pd or Au. Ammonia sensing properties of the WO 3 thick film was improved markedly by overcoating a platinum catalyzed silica-niobia layer onto the WO 3 film surface. Such a double layer structure not only increased the gas response, but decreased the response time also.

Kiran Jain

Papers

Faster response of NO 2 sensing in graphene-WO 3 nanocomposites

Effect of Ni doping on thick film SnO2 gas sensor

Highly sensitive NH3 sensor using Pt catalyzed silica coating over WO3 thick films

Study on ZnO-doped tin oxide thick film gas sensors

At room temperature graphene/SnO2 is better than MWCNT/SnO2 as NO2 gas sensor