Ammonia gas sensors based on chemically reduced graphene oxide sheets self-assembled on Au electrodes

We present a useful ammonia gas sensor based on chemically reduced graphene oxide (rGO) sheets by self-assembly technique to create conductive networks between parallel Au electrodes. Negative graphene oxide (GO) sheets with large sizes (>10 μm) can be easily electrostatically attracted onto positive Au electrodes modified with cysteamine hydrochloride in aqueous solution. The assembled GO sheets on Au electrodes can be directly reduced into rGO sheets by hydrazine or pyrrole vapor and consequently provide the sensing devices based on self-assembled rGO sheets. Preliminary results, which have been presented on the detection of ammonia (NH₃) gas using this facile and scalable fabrication method for practical devices, suggest that pyrrole-vapor-reduced rGO exhibits much better (more than 2.7 times with the concentration of NH₃ at 50 ppm) response to NH₃ than that of rGO reduced from hydrazine vapor. Furthermore, this novel gas sensor based on rGO reduced from pyrrole shows excellent responsive repeatability to NH₃. Overall, the facile electrostatic self-assembly technique in aqueous solution facilitates device fabrication, the resultant self-assembled rGO-based sensing devices, with miniature, low-cost portable characteristics and outstanding sensing performances, which can ensure potential application in gas sensing fields.     

Recent progress of Ga2O3-based gas sensors

In recent years, gas sensors fabricated from gallium oxide (Ga₂O₃) materials have aroused intense research interest due to the superior material properties of large dielectric constant, good thermal and chemical stability, excellent electrical properties, and good gas sensing. Over the past decades, Ga₂O₃-based gas sensors experienced rapid development. The long-term stable Ga₂O₃-based gas sensors for detecting oxygen and carbon monoxide have been commercialized and renowned with extremely good gas sensing characteristics. Recent pioneering studies also exhibit that the Ga₂O₃-based gas sensors possess great potentials in applications of detecting nitrogen oxides, hydrogen, volatile organic compounds and ammonia gases. This article presents recent advances in gas sensing mechanism, device performance parameters, influence factors, and applications of Ga₂O₃-based gas sensors. The impacts of influence factors, doping, material structure and device structure on the performance of gas sensors are discussed in detail. Finally, a brief overview of challenges and opportunities for the Ga₂O₃-based gas sensors is presented.     

Enhanced NO2 gas sensor device based on supramolecularly assembled polyaniline/silver oxide/graphene oxide composites

Herein, we report a successful synthesis of supramolecularly assembled polyaniline/silver oxide/graphene oxide composite (PANI/Ag₂O/GO) for enhanced NO₂ gas sensing application. The PANI/Ag₂O/GO composite was synthesized by facile stirring followed by an ultrasonication process. The prepared material was characterized by different techniques such as x-ray diffraction, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy and Raman-scattering spectroscopy. The detailed analysis revealed that the average crystallite sizes of PANI/Ag₂O and PANI/Ag₂O/GO composites were found to be 37.37 nm and 41.55 nm, respectively. FESEM and TEM analysis showed coral-like rough-surfaced and extensively agglomerated morphology for PANI and ultrathin flexible sheet-like morphology for GO. Ag₂O nanoparticles with diameters 20–30 nm were well incorporated in the GO sheets and PANI matrix in the case of PANI/Ag₂O/GO composites. The synthesized materials were used to make resistive sensor devices that had a high response to NO₂ gas. The fabricated sensors were examined at various temperatures to obtain the optimal sensing temperature. The fabricated NO₂ gas sensor device based on PANI/Ag₂O/GO composite exhibited a highest sensitivity of 5.85 for 25 ppm at an optimized temperature (100 °C) as compared to the pure PANI (2.5) and PANI/Ag₂O composite (3.25). Further, the fabricated sensor device based on PANI/Ag₂O/GO composite was also examined at different NO₂ gas concentrations.     

Room temperature NH3 gas sensor based on PMMA/RGO/ZnO nanocomposite films fabricated by in-situ solution polymerization

Effective detection of ammonia gas is of great importance due to its detrimental effects on human health, environment, and ecosystem. High-performance composite gas sensors are vital in accomplishing this goal. Herein, we investigate the performance of an ammonia (NH₃) gas sensor fabricated via dip-coating the silver interdigitated electrode for PMMA/RGO/ZnO (PRZ) nanocomposite solution with acetone as a solvent. The PRZ ternary nanocomposite was synthesized using the in-situ solution polymerization method and the resistive properties of the films assembled on the interdigitated electrode were analyzed, with respect to the fixed and varying ammonia gas concentrations, using LCR meter. When the sensor is operated in the controlled chamber containing ammonia gas at room temperature, the sensor responds rapidly to ammonia with a fast recovery of 13.02 s at a gas concentration of 350 ppm. The PRZ sensor exhibits high sensing percentage response (527%), excellent repeatability (four times), high sensitivity at low concentrations (less than 10 ppm), swift response and recovery times (1.94 s/13.02 s), and long-term stability (up to 90 days) with fluctuation of 3.2%, which signifies PRZ composite as a potential material for ammonia gas sensor. Aspects such as simplicity of the synthesis process and fabrication, excellent sensing performance, as well as fast response-recovery time at a particular gas concentration are noteworthy in this study. These features can be utilized for the detection of ammonia gas in chemical and biological fields.     

A review of recent developments in tin dioxide nanostructured materials for gas sensors

Gas sensors based on SnO₂ nanostructures have been extensively investigated in recent years. Many recent investigations have focused on synthesizing 0D, 1D, 2D and 3D SnO₂ nanostructures with high sensing capacity. This work presents a review of the recent developments in pure, doped and metal oxide functionalized SnO₂ nanostructured gas sensors, emphasizing the main SnO₂ preparation methods and the working principle of SnO₂ gas sensors. Most studies have shown that doping, coupled with a high surface area, can significantly improve SnO₂ sensing properties. Sensing response, response/recovery times, and operating temperature can be modulated by the synergistic effect between these two factors. In general, fine nanoparticles, mesoporous materials, hollow and 3D nanostructures combined with additives such as Pt, Pd, Cu, Ni, Ag and Al have shown the best improvements in gas sensing.     

Low concentration ethanol sensor based on graphene/ZnO nanowires

Metal-oxide based gas sensors are widely used as the gas sensing elements in industrial and residential areas. Many efforts have been made to increase sensitivity and reduce the working temperature of metal-oxide based gas sensors. In this paper, ZnO nanowires (NWs) were successfully grown on graphene (Gr) nanosheets by the hydrothermal method. The synthesized Gr/ZnO NWs nanocomposite were investigated as the sensing material. Not only is the sensor response much higher, it also works in a lower working temperature toward a low concentration of ethanol in comparison with pure ZnO NWs. The optimum working temperature is reduced from 200 °C in pure ZnO NWs to 125 in Gr/ZnO NWs sensor. The maximum response of the Gr/ZnO NWs sensor is 26, which is approximately enhanced twice as much as the pure ZnO NWs sensor. The lower limit of detection (LLOD) of the proposed sensor is as low as 1 ppm ethanol vapor. The sensor was shown a high response, good selectivity, fast response toward ethanol vapor, excellent repeatability, and low sensitivity toward a high relative humidity, as well as remarkable long-term stability.     

Light-activated room-temperature gas sensors based on metal oxide nanostructures: A review on recent advances

Many recent efforts are directed toward developing high-performance gas sensors based on metal oxide nanostructures operating at room temperature, as it lowers the power consumption, simplifies the device fabrication as well as improves the safety and stability of the sensors. The light-activated gas sensing technology was intensively studied because of its high effectiveness in improving the gas sensing performance of metal oxide nanostrctures at room temperature. This review is covers comprehensive advances in the emerging and feasible approaches for improving nanostructured metal oxide-based gas sensors by light activation, especially the progresses made in the last five years. We first summarize the effects of light-activation on gas sensing behavior of metal oxide nanostructures with some new insights into the related mechanisms. For enhancing the light-activated gas-sensing performance some possible strategies are then introduced, which include the modification of the size, dimension, nanoarchitecture, porous or hierarchical structure and doping or defect engineering, as well as the construction of nanocomposite sensing materials. Finally, some recent developments in light source and device structure design towards low power gas sensor systems are discussed. We hope that this review would provide some useful information to the design of light-activated metal oxide gas sensors operating at room temperature.     

NO2 gas sensing responses of In2O3 nanoparticles decorated on GO nanosheets

Nanomaterials offer a wide range of applications in environmental nanotechnology. Hazardous pollutants in the environment are needed to be detected and controlled effectively to avoid human health risks. In this paper, we described the fine-controlled growth of In₂O₃ nanoparticles embedded on GO nanosheets by a facile precipitation method. The In₂O₃@GO nanocomposites exhibited outstanding gas sensing performance as compared with pure In₂O₃ nanoparticles towards NO₂. At 225 °C, the sensor displayed high selectivity, best response (78) to 40 ppm NO₂, quick response, and recovery times of 106s/42s. The improved sensing performances of the nanocomposite were attributed to large surface area, high gas adsorption-desorption capability, and the formation of p-n heterojunctions between In₂O₃ nanoparticles and GO nanosheets. The excellent gas detecting activities validate In₂O₃@GO nanocomposites as a promising candidate in the NO₂ gas sensor industry.     

Polydopamine-assisted formation of Co3O4-nanocube-anchored reduced graphene oxide composite for high-performance supercapacitors

Tailoring transition-metal oxide nanoparticles with two-dimensional carbon has become a favorite way to improve their electrochemical performance. In this study, a composite of reduced graphene oxide was anchored by Co₃O₄ nanocubes and easily prepared with the assistance of polydopamine (PDA), using a combination of hydrothermal reaction and pyrolysis (Co₃O₄@PDA-rGO). Polydopamine, which possesses abundant catechol and amine groups, could be easily grafted onto graphene oxide to reduce the aggregation of graphene particles. Furthermore, PDA provided active sites, i.e., catechol and amine groups, which coordinated with Co²⁺, enabling enrichment of metal ions on the surface of graphene. After the pyrolysis of Co²⁺-containing PDA-grafted graphene at 400 °C, the Co²⁺ ions were converted into Co₃O₄ nanocubes, while the PDA carbonized to form N-doped porous carbon on the surface of graphene. The resulting product, Co₃O₄@PDA-rGO, demonstrated extraordinary supercapacitive behavior with good cycling stability owing to its unique porous structure as well as the intimate contact between Co₃O₄ and the carbon matrix.     

ZnO-capped nanorod gas sensors

This paper reports on the synthesis of pristine α-Fe₂O₃ nanorods and Fe₂O₃–ZnO core–shell nanorods using a combination of thermal oxidation and atomic layer deposition (ALD) techniques; the completed nanorods were then used for ethanol sensing studies. The crystal structure and morphology of the synthesized nanostructures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing properties of the pristine and core–shell nanorods for gas-phase ethanol were examined using different concentrations of ethanol (5–200 ppm) at different temperatures (150–250 °C). The XRD and SEM revealed the excellent crystallinity of the Fe₂O₃–ZnO core–shell nanorods, as well as their uniformity in terms of shape and size. The Fe₂O₃–ZnO core–shell nanorod sensor showed a stronger response to ethanol than the pristine Fe₂O₃ nanorod sensor. The response (i.e., the relative change in electrical resistance R_a/R_g) of the core–shell nanorod sensor was 22.75 for 100 ppm ethanol at 200 °C whereas that of the pristine nanorod sensor was only 3.85 under the same conditions. Furthermore, under these conditions, the response time of the Fe₂O₃–ZnO core–shell nanorods was 15.96 s, which was shorter than that of the pristine nanorod sensor (22.73 s). The core–shell nanorod sensor showed excellent selectivity to ethanol over other VOC gases. The improved sensing response characteristics of the Fe₂O₃–ZnO core–shell nanorod sensor were attributed to modulation of the conduction channel width and the potential barrier height at the Fe₂O₃–ZnO interface accompanying the adsorption and desorption of ethanol gas as well as to preferential adsorption and diffusion of oxygen and ethanol molecules at the Fe₂O₃–ZnO interface.     

Porous Si/SnO2 nanowires heterostructures for H2S gas sensing

In this work, a low-temperature (100 °C) gas sensor based on heterojunctions of porous Si and SnO₂ nanowires (NWs) is presented. Porous Si was obtained from p-Si wafers by electrochemical etching, and SnO₂ NWs were fabricated by a vapor-liquid-solid route. Different characterization techniques were used to verify the formation of porous Si/SnO₂ NW heterojunctions. H₂S gas sensing results showed enhanced gas sensing performance of the porous Si/SnO₂ NW sensor in comparison with that of a porous Si sensor. The reasons for such enhancement are discussed in detail. This study demonstrates the promising effects of SnO₂ NWs in combination with porous Si to realize low-temperature H₂S gas sensors that are highly compatible with existing Si processing technology.     

Preparation and gas-sensing performance of GO/SnO2/NiO gas-sensitive composite materials

In this study, a SnO₂/NiO composite material was prepared via a co-precipitation method. After calcination at 400 °C for 2 h, a binary composite material (SnO₂/NiO) with good crystallization was obtained. Then, a graphene oxide (GO)/SnO₂/NiO ternary composite material was prepared using a hydrothermal method, in which SnO₂/NiO performed secondary growth on the GO surface. The XRD results showed that SnO₂/NiO exhibited good crystallinity and proved the existence of a chemical bond, Sn–O–C, which was due to the formation of a chemical bond between GO and SnO₂/NiO. Lastly, GO/SnO₂/NiO was successfully prepared and coated on the surface of a gold electrode for gas sensitivity test. A good response to acetone gas in the concentration range of 10–500 ppm at 350 °C was determined. Compared with SnO₂/NiO, GO/SnO₂/NiO showed remarkable improvements in response time, recovery time, and sensitivity. At 350 °C, the sensitivity of acetone with a concentration of 50 ppm was 21.11, the response time was only 5 s, and the recovery time was 150 s. GO/SnO₂/NiO comprised two structures, chemical bond and p-n junction, which exerted a synergistic effect. GO/SnO₂/NiO indicated an excellent application prospect in acetone gas detection.     

金属氧化物半导体基三乙胺传感器研究进展

三乙胺是一种应用广泛但对人体有毒副作用的挥发性有机物,需要长期有效的监测,开发一种性能稳定、安全可靠的三乙胺气敏传感器,实现对环境中三乙胺气体浓度实时检测,对于三乙胺的安全储存、运输和使用等环节是至关重要的。金属氧化物半导体基气敏传感器具有制备简单、价格低廉、响应值高等优点,在三乙胺气体的检测中具有不可替代的作用。重点介绍了基于金属氧化物半导体的三乙胺传感器最新研究进展。综述了近年来包括掺杂、异质结、有机金属骨架和氧化还原石墨烯在内的关于金属氧化物半导体基三乙胺气敏材料的制备和性能等方面的研究成果。论述了金属氧化物半导体基复合材料对三乙胺气敏性能的机理。展望了金属氧化物基三乙胺气敏材料的未来研究方向。     

Highly sensitive gas sensors on low-cost nanostructured polymer substrates

SnO₂ thin-film gas sensors have been successfully fabricated on nanospiked polyurethane polymer surfaces, which are replicated by a low-cost soft nanolithography method from silicon nanospike structures formed with femtosecond laser irradiations. Measurements revealed significant response to carbon monoxide (CO) gas at room temperature, which is considerably different from the sensors of SnO₂ thin films coated on smooth surfaces that show no response to CO gas at room temperature. The high area/volume ratio and sharp structures of the nanospikes enhance the sensitivity of SnO₂ at room temperature. This will greatly decrease the electrical power consumption of the gas sensor and the cost for calibrations, and has great potential for application in other sensing systems.     

Synthesis and characterization of graphene and carbon nanotubes: A review on the past and recent developments

Carbon nanotubes (CNTs) and graphene have built broad interest in most areas of science and engineering because of their extraordinary physical, mechanical, thermal and optical properties. Graphene is a two-dimensional one-atom-thick planar sheet of sp²-bonded carbon atoms while CNTs are a cylindrical nanostructure which composed entirely of sp²-bonded carbon atoms as well. This review presents and discusses the past and current advancement of synthesis and characterization of graphene and CNTs. The review also concludes with a brief summary and an outlook on the challenges and future prospects in the growth of graphene and CNTs.     

Synthesis of porous CeO2-SnO2 nanosheets gas sensors with enhanced sensitivity

Two-dimension (2D) CeO₂-SnO₂ nanosheets with uniform size and small rhombus nanopores were synthesized by the hydrothermal method. The structure of CeO₂-SnO₂ nanosheets was confirmed by X-ray diffraction (XRD), energy dispersive spectrometer (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM). The gas sensing behaviors of the fabricated sensors were systematically investigated. Under optimum operating temperature (340 °C), the response to 100 ppm ethanol of the CeO₂-SnO₂ sensor was 44, which was 2 times larger than that of the SnO₂ sensor (about 19). The response and recovery time of the CeO₂-SnO₂ sensor were 25 s and 6 s, while that of the SnO₂ sensor were 29 s and 7 s, respectively. The results revealed that porous CeO₂-SnO₂ nanosheets enhanced the gas sensing properties and shortened the response/recovery time, which were attributed to the porous structure and the effect of the CeO₂-doping. In addition, the ethanol sensing mechanism was carefully discussed.     

Low temperature operated highly sensitive,selective and stable NO2 gas sensors using N-doped SnO2-rGO nanohybrids

In this study, we report synthesis of SnO₂-rGO and N-doped SnO₂-rGO nanohybrids by facile hydrothermal method. The XRD analysis of the synthesized nanohybrids revealed a tetragonal rutile structure of SnO₂ lattice.Further structural, chemical, morphological and optical properties of SnO₂-rGO and SnO₂-NrGO nanohybrids were investigated by Raman, X-Ray Photoelectron spectroscopy, Transmission Electron Microscopy, UV–Vis spectroscopy and Photoluminescence spectroscopy. Furthermore, the gas sensing properties of the synthesized nanohybrids were studied in detail. It was observed that SR2 and SRN2 nanohybrids exhibited superior NO₂ sensing response (55.2 and 84.5% respectively) at low operating temperature (120 °C) and low gas concentration (0.5 ppm). Moreover, SR2 and SRN2 also exhibited excellent selectively towards NO₂ along with remarkable stability upto 90% over 30 days. This improved performance can be attributed to the synergetic effect of small particle size, high defect concentration and high surface area due to incorporation of SnO₂ along with N doping in rGO. Therefore, SR2 and SRN2 can be utilized as effective NO₂ gas sensors.     

几种固体电解质型气体传感器的研究进展

固体电解质型气体传感器的应用范围越来越广,尤其是在检测大气中的有害成分COx,SOx,NOx等气体方面。本文简述了固体电解质气体传感器的发展及分类,并重点介绍了分别以硫酸盐、NASICON,β—Al2O3,LaF3为固体电解质的几种气体传感器。最后,对固体电解质型气体传感器未来的研究方向进行了展望。     

The role of functional groups on graphene oxide in epoxy nanocomposites

The role of functional groups on the surface of graphene oxide (GO) upon its ability to reinforce an epoxy resin has been investigated. It is known that a base-washing process removes oxidative debris from as-prepared GO and reduces the number of functional groups in the material. Both as-prepared (aGO) and base-washed graphene oxide (bwGO) fillers were incorporated into an epoxy resin matrix and the mechanical properties of the different nanocomposites were investigated. The best levels of reinforcement were found with the addition of low loadings of aGO while the bwGO gave inferior levels of reinforcement at the same loading level. Raman spectroscopy was used to both assess the dispersion of the fillers and efficiency of stress transfer to the GO in the nanocomposites during deformation. It was found that for a given filler loading the aGO materials had the most uniform dispersion of filler and the largest Raman band shifts per unit strain, indicating the importance of the presence of functional groups in both dispersing the GO and giving good interfacial stress transfer in the nanocomposites.     

Analytical modelling of monolayer graphene-based ion-sensitive FET to pH changes

Graphene has attracted great interest because of unique properties such as high sensitivity, high mobility, and biocompatibility. It is also known as a superior candidate for pH sensing. Graphene-based ion-sensitive field-effect transistor (ISFET) is currently getting much attention as a novel material with organic nature and ionic liquid gate that is intrinsically sensitive to pH changes. pH is an important factor in enzyme stabilities which can affect the enzymatic reaction and broaden the number of enzyme applications. More accurate and consistent results of enzymes must be optimized to realize their full potential as catalysts accordingly. In this paper, a monolayer graphene-based ISFET pH sensor is studied by simulating its electrical measurement of buffer solutions for different pH values. Electrical detection model of each pH value is suggested by conductance modelling of monolayer graphene. Hydrogen ion (H⁺) concentration as a function of carrier concentration is proposed, and the control parameter (Ƥ) is defined based on the electro-active ions absorbed by the surface of the graphene with different pH values. Finally, the proposed new analytical model is compared with experimental data and shows good overall agreement.