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

Considering metal oxide nanoparticles as important technological materials, authors provide a comprehensive review of researches on metal oxide nanoparticles, their synthetic strategies, and techniques, nanoscale physicochemical properties, defining specific industrial applications in the various fields of applied nanotechnology. This work expansively reviews the recent developments of semiconducting metal oxide gas sensors for environmental gases including CO2, O2, O3, and NH3; highly toxic gases including CO, H2S, and NO2; combustible gases such as CH4, H2, and liquefied petroleum gas; and volatile organic compounds gases. The gas sensing properties of different metal oxides nanoparticles towards specific target gases have been individually discussed. Promising metal oxide nanoparticles for sensitive and selective detection of each gas have been identified. This review also categorizes metal oxides sensors by analyte gas and also summarizes the major techniques and synthesis strategies used in nanotechnology. Additionally, strategies, sensing mechanisms and related applications of semiconducting metal oxide materials are also discussed in detail. Related applications are innumerable trace to ultratrace-level gas sensors, batteries, magnetic storage media, various types of solar cells, metal oxide nanoparticles applications in catalysis, energy conversion, and antennas (including microstrip and patch-type optically transparent antennas), rectifiers, optoelectronic, and electronics.

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Metal oxide nanoparticles and their applications in nanotechnology

https://www.econstor.eu/bitstream/10419/244006/1/1693663112.pdf

Enhanced sensing performance of ZnO nanostructures-based gas sensors: A review

Chemiresistive gas sensors with low power consumption, fast response, and reliable fabrication process for a specific target gas have been now created for many applications. They require both sensitive nanomaterials and an efficient substrate chip for heating and electrical addressing. Herein, a near room working temperature and fast response triethylamine (TEA) gas sensor has been fabricated successfully by designing gold (Au)-loaded ZnO/SnO2 core-shell nanorods. ZnO nanorods grew directly on Al2O3 flat electrodes with a cost-effective hydrothermal process. By employing pulsed laser deposition (PLD) and DC-sputtering methods, the construction of Au nanoparticle-loaded ZnO/SnO2 core/shell nanorod heterostructure is highly controllable and reproducible. In comparison with pristine ZnO, SnO2, and Au-loaded ZnO, SnO2 sensors, Au-ZnO/SnO2 nanorod sensors exhibit a remarkably high and fast response to TEA gas at working temperatures as low as 40 °C. The enhanced sensing property of the Au-ZnO/SnO2 sensor is also discussed with the semiconductor depletion layer model introduced by Au-SnO2 Schottky contact and ZnO/SnO2 N-N heterojunction.

Near Room Temperature, Fast-Response, and Highly Sensitive Triethylamine Sensor Assembled with Au-Loaded ZnO/SnO2 Core–Shell Nanorods on Flat Alumina Substrates

This paper demonstrates a hydrogen gas sensor based on palladium-tin oxide- molybdenum disulfide (Pd-SnO 2 /MoS 2 ) ternary hybrid via hydrothermal route. The morphologies, microstructures and compositional characteristics of the Pd-SnO 2 /MoS 2 nanocomposite were sufficiently examined by X-ray diffraction (XRD), Raman spectroscopy (RS), nitrogen sorption analysis, energy dispersive spectrometer (EDS), scanning electron microscopy (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The gas-sensing performances of the Pd-SnO 2 /MoS 2 sensor were investigated by exposed to different concentrations of hydrogen gas from 30 ppm to 5000 ppm at room temperature. The experimental results showed that the hydrogen gas sensor has a quite sensitive response, swift response-recovery time, good repeatability and selectivity toward hydrogen gas. Furthermore, the effect of Pd loading in the hybrid on the hydrogen gas sensing was investigated. The sensing mechanism of the Pd-SnO 2 /MoS 2 sensor was attributed to the synergistic effect of the ternary nanostructures and the modulation of potential barrier with electron transfer. This work indicates that the as-prepared Pd-SnO 2 /MoS 2 composite is a candidate for detecting hydrogen gas in various applications at room temperature.

Room temperature hydrogen gas sensor based on palladium decorated tin oxide/molybdenum disulfide ternary hybrid via hydrothermal route

This work reports the synthesis and detailed investigation on ZnO–SnO 2 composite type hydrogen sensor prototype. The sensor material was structurally and morphologically characterized by X-ray diffraction technique and scanning electron microscopy, respectively. The gas sensing behaviour of the fabricated sensor prototype was investigated for varied concentration of test gases at different temperature. The cross-response of this sensor to other gases, viz. methane and carbon mono-oxide was also investigated, which showed good selectivity, excellent response and reproducibility to hydrogen at 150 °C.

ZnO–SnO2 based composite type gas sensor for selective hydrogen sensing

Humidity sensing by porous silicon (PS) layer is commonly reported either by capacitive sensing or by conductive sensing. A critical analysis of both capacitive and conductive sensing by microporous PS layer is presented here. The influences of parasitic capacitances and resistances unavoidably associated with the active porous layer on the measured changes in capacitance and resistance of the humidity sensor with variation of humidity are analysed. The role of contact geometry, signal frequency and porosity of PS layer are also discussed. It is shown that capacitive sensing is more sensitive in low frequency range owing to the relative contributions of parasitic components.

Role of parasitics in humidity sensing by porous silicon

A novel hygrometer is presented, comprising a capacitive humidity sensor with a porous silicon (PS) dielectric and electronics. The adsorption of water vapor by the PS layer leading to change of its effective dielectric constant is modeled with an effective medium approximation (EMA). A simple, but precise, phase-sensitive electronic circuit has been developed. This detects any change of phase of a sinusoidal signal transmitted through the PS dielectric and correlates to ambient humidity. It is outlined how the nonlinear response of the sensor is compensated through piecewise linearization. The sensor is tested in combination with the phase detection circuitry. Excellent linearity over the entire range of relative humidity is achieved. Experimental results show a resolution better than 0.1% and an accuracy of 2% (near the transition region) and better than 0.1% (otherwise). The response time is less than 10 s with good stability.

A hygrometer comprising a porous silicon humidity sensor with phase-detection electronics

Stability in photoluminescence of porous silicon

Thermally oxidized p-type Si [resistivity (5–10) Ω cm and 〈1 0 0〉 orientation] with oxide thickness of 245 nm was used as the substrate for the deposition of graphene by CVD. The formation of multilayer graphene (MLG) was confirmed by Raman spectroscopy. A heterojunction was fabricated by sol–gel coating of TiO2 on CVD grown graphene layer. The formation of titanium dioxide was verified by electron dispersion spectroscopy (EDS). The surfaces of TiO2, graphene as well as p-TiO2/n-graphene interface were characterized by scanning electron microscopy (SEM). Detail sensor study of the p-TiO2/n-graphene heterojunction was performed by taking two lateral catalytic metal (Pd) contacts deposited by e-beam evaporation. The response time of 16 s and the corresponding recovery time of 61 s were obtained for 0.5% H2 in air at 125 °C. The p-TiO2/n-graphene junction showed selectivity for H2 compared to methane. The stability study was performed and it showed almost the steady results over a period of 3 days as tested by the discrete measurements. The sensing mechanism was formulated using a simplified energy band diagram.

Jayoti Das

Papers

ZnO–SnO2 based composite type gas sensor for selective hydrogen sensing

Role of parasitics in humidity sensing by porous silicon

A hygrometer comprising a porous silicon humidity sensor with phase-detection electronics

Stability in photoluminescence of porous silicon

Studies on p-TiO2/n-graphene heterojunction for hydrogen detection