Two-dimensional (2D) nanostructures are highly attractive for fabricating nanodevices due to their high surface-to-volume ratio and good compatibility with device design. In recent years 2D nanostructures of various materials including metal oxides, graphene, metal dichalcogenides, phosphorene, BN and MXenes, have demonstrated significant potential for gas sensors. This review aims to provide the most recent advancements in utilization of various 2D nanomaterials for gas sensing. The common methods for the preparation of 2D nanostructures are briefly summarized first. The focus is then placed on the sensing performances provided by devices integrating 2D nanostructures. Strategies for optimizing the sensing features are also discussed. By combining both the experimental results and the theoretical studies available, structure-properties correlations are discussed. The conclusion gives some perspectives on the open challenges and future prospects for engineering advanced 2D nanostructures for high-performance gas sensors devices.

Two-Dimensional Nanostructured Materials for Gas Sensing

High-precision gas sensors operated at room temperature are attractive for various real-time gas monitoring applications, with advantages including low energy consumption, cost effectiveness and device miniaturization/flexibility. Studies on sensing materials, which play a key role in good gas sensing performance, are currently focused extensively on semiconducting metal oxide nanostructures (SMONs) used in the conventional resistance type gas sensors. This topical review highlights the designs and mechanisms of different SMONs with various patterns (e.g. nanoparticles, nanowires, nanosheets, nanorods, nanotubes, nanofilms, etc.) for gas sensors to detect various hazardous gases at room temperature. The key topics include (1) single phase SMONs including both n-type and p-type ones; (2) noble metal nanoparticle and metal ion modified SMONs; (3) composite oxides of SMONs; (4) composites of SMONs with carbon nanomaterials. Enhancement of the sensing performance of SMONs at room temperature can also be realized using a photo-activation effect such as ultraviolet light. SMON based mechanically flexible and wearable room temperature gas sensors are also discussed. Various mechanisms have been discussed for the enhanced sensing performance, which include redox reactions, heterojunction generation, formation of metal sulfides and the spillover effect. Finally, major challenges and prospects for the SMON based room temperature gas sensors are highlighted.

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Advances in designs and mechanisms of semiconducting metal oxide nanostructures for high-precision gas sensors operated at room temperature

Metal-oxide-semiconductor (MOS) based gas sensors have been considered a promising candidate for gas detection over the past few years. However, the sensing properties of MOS-based gas sensors also need to be further enhanced to satisfy the higher requirements for specific applications, such as medical diagnosis based on human breath, gas detection in harsh environments, etc. In these fields, excellent selectivity, low power consumption, a fast response/recovery rate, low humidity dependence and a low limit of detection concentration should be fulfilled simultaneously, which pose great challenges to the MOS-based gas sensors. Recently, in order to improve the sensing performances of MOS-based gas sensors, more and more researchers have carried out extensive research from theory to practice. For a similar purpose, on the basis of the whole fabrication process of gas sensors, this review gives a presentation of the important role of screening and the recent developments in high throughput screening. Subsequently, together with the sensing mechanism, the factors influencing the sensing properties of MOSs involved in material preparation processes were also discussed in detail. It was concluded that the sensing properties of MOSs not only depend on the morphological structure (particle size, morphology, pore size, etc.), but also rely on the defect structure and heterointerface structure (grain boundaries, heterointerfaces, defect concentrations, etc.). Therefore, the material-sensor integration was also introduced to maintain the structural stability in the sensor fabrication process, ensuring the sensing stability of MOS-based gas sensors. Finally, the perspectives of the MOS-based gas sensors in the aspects of fundamental research and the improvements in the sensing properties are pointed out.

Metal-oxide-semiconductor based gas sensors: screening, preparation, and integration

21st century has already seen huge progress in science and technology of small, highly sensitive gas sensors, which can selectively detect environmental toxins like NO x – the oxides of nitrogen – a byproduct of fossil fuel combustion. Into this bargain, public became more health-aware and environmental bodies grew stricter, stimulating analytical and material scientists to find new strategies from material synthesis to fabrication of NO x sensors in order to produce fast and reliable gas detectors. To the scientists, semiconducting metal oxides, owing to their low cost, easy processing, high gas response, good electrical properties and above all tunable structure at the nanoscale, always presented a first-hand choice for sensor fabrication. This article presents an overview of the most recent developments in semiconducting NO x gas sensors based on these metal oxide nanostructures and their applications in vehicle exhaust and environmental monitoring. A strong emphasis is presented on chemiresistor and field effect transistor devices using semiconducting metal oxides as active layers. The performance levels of these NO x sensors are compared to those of other devices as well as other semiconductor materials. Furthermore, keeping in mind the ultimate user demands, limitations of the current sensor technologies and future strategies are discussed.

NOx sensors based on semiconducting metal oxide nanostructures: Progress and perspectives

A gas sensor based on PbS colloidal quantum dots (CQDs) is constructed on a paper substrate, yielding flexible, rapid-response NO₂ gas sensors, fabricated from the solution phase. The devices are highly sensitive and fully recoverable at room temperature, which is attributed to the excellent access of gas molecules to the CQD surface, realized by surface ligand removal, combined with the desirable binding energy of NO₂ with the PbS CQDs.

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Physically Flexible, Rapid‐Response Gas Sensor Based on Colloidal Quantum Dot Solids

In this paper, a novel porous WO3 sensor was prepared by anodic oxidation of DC magnetron sputtered metallic tungsten (W) film deposited on alumina substrate. The structural and morphological properties of these films are investigated using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). Coral-like porous crystalline WO3 film with a grain size of about 9.3 nm was obtained after annealing of the anodized W film. The porous WO3 sensor achieved its maximum response value to NO2 at a low operating temperature of 150 °C. In comparison to sputtered WO3 sensor, the porous WO3 sensor showed markedly higher responses, much better response–recovery characteristics and lower optimal operating temperature to different concentration of NO2 gas due to its high specific surface area and small grain size.

NO2-sensing properties of porous WO3 gas sensor based on anodized sputtered tungsten thin film

Gas sensors based on V2O5 hierarchical structure networks were formed through a direct seed-induced hydrothermal growth of nanostructured V2O5 on the substrate attached a pair of patterned Pt electrodes. Porous flower- or honeycomb-like V2O5 hierarchical structures could be controlledly synthesized under different precursor (NH4VO3) concentrations. The as-prepared hierarchical structures of V2O5, especially the flower-like network with radially oriented ultrathin nanoneedles and nanoribbons and the overlaped nanowires as constituents, show favorable microstructure features for gas-sensing application. The ethanol gas sensing properties of V2O5 network-structured sensors were investigated at room temperature (20 °C) up to 300 °C over ethanol concentration ranging from 5 to 1000 ppm. The sensor based on V2O5 hierarchical structure network showed temperature-dependent p- to n-type response characteristic reversal, resulting in dual working temperature characteristic with the dual response extremes reached at room temperature (20 °C) and 250 °C respectively. The flower-like V2O5 network sensor exhibits perfect reversibility, high response value and fast response–recovery characteristic to ethanol gas at the dual working temperatures, due to the good interface performance and gas adsorption–desorption properties of the directly assembled porous network. At 250 °C, the flower-like V2O5 network sensor is much sensitive to both ethanol gas and NH3, while at room temperature, the sensor presents very good selectivity to ethanol gas.

Vanadium pentoxide hierarchical structure networks for high performance ethanol gas sensor with dual working temperature characteristic

Nanowires and nanosheets of tungsten oxide were synthesized by solvothermal method with different tungsten hexachloride (WCl 6 ) concentrations in 1-propanol solvent. The morphology and crystal structure of the tungsten oxide nanostructures were investigated by means of field emission scanning electron microscope, X-ray diffraction and transmission electron microscope. The specific surface area and pore size distribution were characterized by Brunauer–Emmett–Teller gas-sorption measurements. One-dimensional W 18 O 49 nanowire bundles were synthesized at the WCl 6 concentration of 0.01 M. With the concentration increasing to 0.02 M, the structure of the pure two-dimensional WO 3 nanosheets was formed. The NO 2 gas sensing properties of W 18 O 49 nanowires and WO 3 nanosheets were investigated at 100 °C up to 250 °C over NO 2 concentration ranging from 1 to 20 ppm. Both nanowires and nanosheets exhibit reversible response to NO 2 gas at different concentrations. In comparison to WO 3 nanosheets, W 18 O 49 nanowire bundles showed a much higher response value and faster response–recovery characteristics to NO 2 gas, especially a much quicker response characteristic with response time of 19 s at the concentration of 5 ppm.

Microstructure characterization and NO2-sensing properties of tungsten oxide nanostructures

The gas response of oxide-based sensors strongly depends on its surface properties. To explore the potential sensing ability of one-dimensional (1D) WO3 nanowire under existence of oxygen vacancy, the adsorption of NO2 molecule on the oxygen vacancy defected surface of WO3 nanowire was studied using density functional theory (DFT) calculations. Three kinds of stable oxygen vacancies were considered, and the three corresponding most energetically favorable adsorption configurations were constructed based on the calculation adsorption energy. The electronic properties including density of states, band structure and atomic Mulliken population were further studied. It is found that the existence of oxygen vacancies benefits NO2 adsorption on nanowire surface, and makes the interaction between molecule and vacancy-defected surface strengthened markedly. Consequently, the electronic structures and electronic properties of the defective WO3 nanowires were tuned obviously, causing a distinct change of the density of state at Fermi level and much more electrons extracted from the defective surface. The transferred electrons from the vacancy-defected WO3 nanowire was four to six times more, depending on different oxygen vacancies, than that from the stoichiometric nanowire, indicating a positive effect of oxygen vacancy on NO2-sensing response. The results highlight the possibility to develop high sensitive NO2 gas sensors through introducing oxygen vacancy defects in WO3 nanowire.

DFT study on interaction of NO2 with the vacancy-defected WO3 nanowires for gas-sensing

Quasi-orienting W 18 O 49 nanowire bundles were synthesized by solvothermal method. The microstructures and the NO 2 -sensing properties of the as-synthesized nanowires annealed at different temperatures were studied. To characterize the morphology and crystalline structure of the annealed tungsten oxide, field emission scanning electron microscope, X-ray diffraction and transmission electron microscope were employed. It was found that with annealing temperature increasing, the nanowire bundles became straighter and slightly thicker, and eventually a nonobelt-like structure was formed via the nanowires coalescence at 450 °C. Meanwhile, the monoclinic W 18 O 49 was transformed to monoclinic WO 3 when annealing temperature rising from 350 to 450 °C. In comparison to the W 18 O 49 nanowire bundles-based sensor, the sensor based on the 450 °C annealing-induced WO 3 nanobelt-like structure exhibited markedly higher response value and gas selectivity, as well as much better response-recovery characteristics to NO 2 gas due to its favorable microstructure feature to gas-sensing.

Yuxiang Qin

Papers

NO2-sensing properties of porous WO3 gas sensor based on anodized sputtered tungsten thin film

Vanadium pentoxide hierarchical structure networks for high performance ethanol gas sensor with dual working temperature characteristic

Microstructure characterization and NO2-sensing properties of tungsten oxide nanostructures

DFT study on interaction of NO2 with the vacancy-defected WO3 nanowires for gas-sensing

Effect of annealing on microstructure and NO2-sensing properties of tungsten oxide nanowires synthesized by solvothermal method