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.     

In2O3 nanocubes/Ti3C2Tx MXene composites for enhanced methanol gas sensing properties at room temperature

Highly active two-dimensional (2D) nanocomposites, integrating the unique merits of individual components and synergistic effects of composites, have been recently receiving attention for gas sensing. In this work, In₂O₃ nanocubes/Ti₃C₂T_x MXene nanocomposites were synthesized using In₂O₃ nanocubes and layered Ti₃C₂T_x MXene via a facile hydrothermal self-assembly method. Characterization results indicated that the In₂O₃ nanocubes with sizes approximately 20–130 nm in width were well dispersed on the surface of layered Ti₃C₂T_x MXene to form numerous heterostructure interfaces. Based on the synergistic effects of electronic properties and gas-adsorption capabilities, In₂O₃ nanocubes/Ti₃C₂Tx MXene nanocomposites exhibited high response (29.6%–5 ppm) and prominent selectivity to methanol at room temperature. Meanwhile, the low detection concentration could be reduced to ppm-level, the response/recovery times are shortened to 6.5/3.5 s, excellent linearity and outstanding repeatability. The strategy of compositing layered MXene with metal oxide semiconductor provides a novel pathway for the future development of room temperature gas sensors.     

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.     

In_2O_3纳米陶瓷纤维无纺布的静电纺丝制备及气敏特性研究

采用溶胶-凝胶法结合静电纺丝技术制备了直径20~60 nm的超细氧化铟(In2O3)纳米陶瓷纤维及纳米陶瓷纤维无纺布。采用XRD,IR,SEM,HR-TEM,TGA等分析方法对纳米纤维的形貌和显微结构进行了表征,并研究了其气敏特性。由700℃下煅烧的该超细In2O3纳米纤维所制备的气敏元件具有较好的反应和选择性,对甲醛气体表现出较快的响应和恢复速度。     

Synthesis of few-layered Ti3C2Tx/ WO3 nanorods foam composite material for NO2 gas sensing at low temperature

The few-layered Ti₃C₂T_x/WO₃ nanorods foam composite material was synthesized by electrostatic self-assembly and bidirectional freeze-drying technologies. The phase structure and microstructure of synthesized samples was characterized by XRD, FESEM, TEM and their gas sensing properties estimated via a self-designed equipment with four test channels. The results demonstrate WO₃ nanorods were successfully anchored on the surface and between layers of few-layered Ti₃C₂T_x MXene by electrostatic self-assembly strategy and the composite material simultaneously has a low-density foam morphology by means of bidirectional freeze-drying processes. There exists a typical heterostructure at the interfaces owing to the inseparable contact between the few-layered Ti₃C₂T_x MXene and WO₃ nanorods. Compared with the original WO₃ nanorods, the few-layered Ti₃C₂T_x/WO₃ nanorods foam composite material displays excellent gas sensing properties for NO₂ detection at low temperature, in particular the optimal value of gas sensing response (R_g/R_a) reaches to 89.46 toward 20 ppm NO₂ at 200 °C. The gas sensing mechanism was also discussed. The increase of gas sensitivity is attributed to a fact that during the reaction process of gas sensing, the excellent conductivity of the few-layered Ti₃C₂T_x MXene provided faster transport channels of free carriers, and the heterojunctions formed by few-layered Ti₃C₂T_x MXene and WO₃ nanorods enhanced the carriers separation efficiency. Meanwhile, the low-density layered structure of few-layered Ti₃C₂T_x/WO₃ nanorods foam composite material provides convenient diffusion paths for gas molecules to the surface of WO₃ nanorods.     

In2O3 nanosheets array directly grown on Al2O3 ceramic tube and their gas sensing performance

Arrayed In₂O₃ nanosheets were synthesized directly via a two-step solution approach on an Al₂O₃ ceramic tube. Their morphology and structure were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV–Vis absorption spectroscopy, and scanning electron microscopy (SEM). The results reveal that the length of each nanosheet is about 1 µm, the width of the bottom of nanosheet is about 200 nm. Importantly, the In₂O₃ nanosheets with large specific surface area possess highly sensing performance for ethanol detection. The response value to 100 ppm ethanol is about 45 at an operating temperature of 280 °C, and the response and recovery time are extremely short. It is expected that the directly grown In₂O₃ nanosheets with large specific surface area and excellent sensing properties will become a promising functional material in monitoring and detecting ethanol.     

Effect of dispersant on chain formation capability of TiO2 nanoparticles under low frequency electric fields for NO2 gas sensing applications

In this study, the effect of Dolapix CE64 as dispersant on the deposition mechanism and chain formation of TiO₂ nanoparticles in the different frequencies under non-uniform AC electric field was investigated. The optical microscope (OM) images of the deposition patterns obtained at the frequency of 1 Hz and 100 Hz in non-aqueous media one containing dispersant and the other being dispersant free revealed that the addition of charge inducing agents (dispersant) has drastically changed deposition mechanism enabling particles to enter the gap leaving the electrodes surfaces uncoated. At 10 kHz, it was observed that by the introduction of Dolapix CE64 the TiO₂ nanoparticles were able to form chain-like patterns along the electric field lines bridging the interelectrode gap. The obtained pearl chain formation (PCF) was employed to fabricate an NO₂ gas sensor which showed a good response to the target gas at 450, 500 and 550 ˚C.     

NO2 gas sensing properties of Au-functionalized porous ZnO nanosheets enhanced by UV irradiation

Porous ZnO nanosheets were synthesized by thermal evaporation. The morphology, crystal structure, and sensing properties of the ZnO nanosheets to NO₂ gas at room temperature under UV illumination were examined. Au nanoparticles with diameters of a few tens of nanometers were distributed over the ZnO nanosheets. The responses of the multiple networked nanosheet gas sensors were improved 1.8–3.3 fold by Au functionalization at NO₂ concentrations ranging from 1 to 5 ppm. Furthermore, the Au-functionalized ZnO nanosheet gas sensors showed a considerably enhanced response at room temperature under ultraviolet (UV) illumination. In addition, the mechanisms through which the gas sensing properties of ZnO nanosheets are enhanced by Au functionalization and UV irradiation are discussed.     

Ethanol sensing properties of networked In2O3 nanorods decorated with Cr2O3-nanoparticles

In₂O₃ nanorods decorated with Cr₂O₃ nanoparticles were synthesized by thermal evaporation of In₂S₃ powder in an oxidizing atmosphere followed by solvothermal deposition of Cr₂O₃ and their ethanol gas sensing properties were examined. The pristine and Cr₂O₃-decorated In₂O₃ nanorods exhibited responses of ~524% and ~1053%, respectively, to 500-ppm ethanol at 200 °C. The Cr₂O₃-decorated In₂O₃ nanorod sensor showed stronger electrical response to ethanol gas at 200 °C than the pristine In₂O₃ nanorod counterpart. The former also showed faster response and recovery than the latter. The pristine and Cr₂O₃-decorated In₂O₃ nanorod sensors showed the strongest response to ethanol gas at 250 and 200 °C, respectively. The Cr₂O₃-decorated In₂O₃ nanorod sensor showed selectivity for ethanol gas over other reducing gases. The underlying mechanism for the enhanced response, sensing speed and selectivity of the Cr₂O₃-decorated In₂O₃ nanorod sensor for ethanol gas is discussed.     

Fabrication of Ti3C2Tx/In2O3 nanocomposites for enhanced ammonia sensing at room temperature

Ti₃C₂T_x, as a novel two-dimensional material, displays promising prospects in NH₃ detection at room temperature. However, the NH₃ detection limit of pristine Ti₃C₂T_x is still a major research concern. Therefore, it is important to explore new Ti₃C₂T_x-based nanocomposites for better NH₃-sensing performance. In the present experiment, Ti₃C₂T_x/In₂O₃ nanocomposites were successfully synthesized by ultrasonication and characterized by XRD, FESEM, TEM, XPS, and BET. The optimal Ti₃C₂T_x/In₂O₃-based sensor had a high response of 63.8% (30.4 times higher than that of pristine Ti₃C₂T_x) to 30 ppm NH₃ at room temperature. In addition, the optimal Ti₃C₂T_x/In₂O₃-based sensor had stable repeatability, excellent selectivity, and long-term stability, while exhibiting excellent potential for NH₃ detection at room temperature.     

Fabrication and NO2 gas sensing performance of TeO2-core/CuO-shell heterostructure nanorod sensors

TeO₂-nanostructured sensors are seldom reported compared to other metal oxide semiconductor materials such as ZnO, In₂O₃, TiO₂, Ga₂O₃, etc. TeO₂/CuO core-shell nanorods were fabricated by thermal evaporation of Te powder followed by sputter deposition of CuO. Scanning electron microscopy and X-ray diffraction showed that each nanorod consisted of a single crystal TeO₂ core and a polycrystalline CuO shell with a thickness of approximately 7 nm. The TeO₂/CuO core-shell one-dimensional (1D) nanostructures exhibited a bamboo leaf-like morphology. The core-shell nanorods were 100 to 300 nm in diameter and up to 30 μm in length. The multiple networked TeO₂/CuO core-shell nanorod sensor showed responses of 142% to 425% to 0.5- to 10-ppm NO₂ at 150°C. These responses were stronger than or comparable to those of many other metal oxide nanostructures, suggesting that TeO₂ is also a promising sensor material. The responses of the core-shell nanorods were 1.2 to 2.1 times higher than those of pristine TeO₂ nanorods over the same NO₂ concentration range. The underlying mechanism for the enhanced NO₂ sensing properties of the core-shell nanorod sensor can be explained by the potential barrier-controlled carrier transport mechanism.

PACS

61.46. + w; 07.07.Df; 73.22.-f     

Ultrasensitive NO2 gas sensing performance of two dimensional ZnO nanomaterials: Nanosheets and nanoplates

Highly sensitive NO₂ gas sensors with low detection limit are vital for practical application in air pollution monitoring. Here, the NO₂ gas sensing performance of porous ZnO nanosheets and nanoplates were investigated, with different shape and thickness. It was found that ultra-thin ZnO nanoplates had a higher sensitivity than coral-like ZnO nanosheets. The results were attributed to the high specific surface and very small thickness of the ultrathin nanoplates. The nanoplates have indeed a thickness of 15 nm compared to that of the nanosheets which is 100 nm, and a BET surface area of 75 m²/g, while that of the nanosheets is 6 m²/g. The chemosensor based on ultra-thin ZnO nanoplates shows a response (calculated as the ratio between the resistance of the sensor in the presence of the gas and in its absence) of 76 to 0.5 ppm of NO₂ at 200 °C, with a theoretical detection limit of 3 parts per trillion and a selectivity higher than 760 towards acetone, ethanol, isopropyl alcohol, triethylamine, SO₂ and CO. The specific surface and the small thickness of the ultra-thin nanoplates contribute to its highly improved sensing performance, making it ideal for NO₂ gas sensing.     

Hydrothermally grown ZnO nanorods arrays for selective NO2 gas sensing: Effect of anion generating agents

Vertically aligned ZnO nanorods (ZNRs) arrays with various aspect ratios were deposited by using a simple and inexpensive hydrothermal route at relatively low temperature of 90 °C. The influence of hydroxide anion generating agents in the solution on the growth of ZNRs arrays was studied. Hexamethylenetetramine (HMTA) and ammonia were used as hydroxide anion generating agents while polyethyleneimine (PEI) as structure directing agent. The combined effect of these three agents plays a crucial role in the growth of ZNRs arrays with respect to their rod length and diameter, which controls the aspect ratio. The deposited ZNRs exhibited hexagonal wurtize crystal structure with preferred orientation along (002) plane. The highly crystalline nature and pure phase formation of ZNRs was confirmed from FT-Raman studies. The maximum gas response (R_g/R_a) of 67.5 was observed for high aspect ratio ZNRs, deposited with combination of HMTA, ammonia as well as PEI. The enhancement in gas response can be attributed to high surface area (45 cm²/g) and desirable surface accessibility in high aspect ratio ZNRs. Fast response–recovery characteristics, especially a much quicker gas response time of 32 s and recovery time of 530 s were observed at 100 ppm NO₂ gas concentration.     

Surface tuning of halloysite nanotubes with Fe3O4 and 3-D MnO2 nanoflakes for highly selective and sensitive acetone gas sensing

Nanosized M-HNTs-MnO₂ (Magnetic halloysite nanotubes-manganese dioxide) nanocomposite was synthesized by the reduction-precipitation method followed by the hydrothermal process. The existence of MnO₂ nanoflakes on M-HNTs represents 3-D nanostructures without stacking of nanotubes and agglomeration. The sensor-based on M-HNTs-MnO₂ nanocomposites exhibits higher sensor response (R_air/R_gas = 35.6) to 100 ppm of acetone gas at operating temperature (150 °C), with a short response-recovery time (3 s/7 s). The M-HNTs-MnO₂ nanocomposite sensor shows excellent potential to act as a low cost, low-temperature sensor for acetone gas, with high acetone selectivity under high humidity conditions and with the interference of other gases. The high surface to volume ratio, three-dimensional nanostructure, and strong interactions between M-HNTs and MnO₂ nanoflakes are accountable for the improvement of acetone sensing performance. Based on the high acetone selectivity, high stability and fast dynamic response, the M-HNTs and MnO₂ sensor is an extremely appropriate candidate for a low-cost acetone sensor, and the projected approach offers a way to develop gas sensors that can be function at low temperatures for a wide variety of applications.     

Realization of low-temperature and selective NO2 sensing of SnO2 nanowires via synergistic effects of Pt decoration and Bi2O3 branching

Metal oxide semiconductors with branched structures, such as branched nanowires (b-NWs), have promising properties for being used in gas sensors. In this work, we synthesized Pt-decorated Bi₂O₃-branched SnO₂ nanowires (NWs). NO₂ sensing studies revealed the superior capacity of a Pt-decorated Bi₂O₃-branched SnO₂ NWs gas sensor relative to pristine and branched SnO₂ gas sensors, and it worked at near room temperature (50 °C). The increased sensing capacity was related to the synergistic effects of Pt decoration and Bi₂O₃ branching, particularly the morphology of the gas sensor with branched structures, the promising effects of Pt as a noble metal with good catalytic activity, and the generation of homo- and heterojunctions in the Pt-decorated Bi₂O₃-branched SnO₂ NWs gas sensor. The results obtained in this work are useful for design and development of NO₂ gas sensors using a simple strategy, which can be easily extended to various metal oxides.     

Improvement of gas sensing property for two-dimensional Ti3C2Tx treated with oxygen plasma by microwave energy excitation

Two-dimensional layered Ti₃C₂T_x MXene was prepared through hydrothermal etching method with LiF and hydrochloric (HCl) acid. Ti₃C₂T_x was further treated with oxygen plasma activated by microwave energy to obtain the activated Ti₃C₂T_x at different temperatures ranging from 350 °C to 550 °C. The gas-sensing properties of raw Ti₃C₂T_x and Ti₃C₂T_x activated with oxygen microwave plasma were tested toward different volatile organic compounds gases. The results indicated that Ti₃C₂T_x activated at 500 °C exhibited excellent gas-sensing properties at room temperature (25 °C) to 100 ppm ethanol with a value of 22.47, which is attributed to the enhancement of the amount of oxygen functional groups and defects on the MXene Ti₃C₂T_x film, and in turn to lead to more oxygen molecules adsorption and desorption reaction in the active defect sites. The enhancement of ethanol-sensing performance demonstrated that the activated Ti₃C₂T_x possess great potential in gas sensing.     

ZnO-enhanced In2O3-based sensors for n-butanol gas

A series of high-response and fast-response/recovery n-butanol gas sensors was fabricated by adding ZnO to In₂O₃ in varying molar ratios to form ZnO-In₂O₃ nanocomposites via a facile co-precipitation hydrothermal method. Morphological characterizations revealed that the shape of pure In₂O₃ was changed from irregular cubes into irregular nanoparticles, 30–50?nm in size, with the addition of ZnO. Compared with the pure In₂O₃ gas sensor, the ZnO-In₂O₃ gas sensor exhibits superior n-butanol sensing performance. With the introduction of ZnO, the response of the sensor to n-butanol was improved from 17 to 99.5 at 180?°C for a [Zn]:[In] molar ratio of 1:1. In addition, the ZnO-In₂O₃ gas sensors show a reduced optimal working temperature, excellent selectivity to n-butanol, and good repeatability. The response of the ZnO-enhanced In₂O₃-based sensors showed a strong linear relationship with the n-butanol gas concentration, allowing for the quantitative detection of n-butanol gas.     

Cu2O quantum dots modified by RGO nanosheets for ultrasensitive and selective NO2 gas detection

Real-time monitoring of trace NO₂ emission has been an emerging challenge in environment and health sectors lately. Aiming to overcome this challenge, NO₂ gas sensors based on cuprous oxide quantum dots (Cu₂O QDs) anchored onto reduced graphene oxide (RGO) nanosheets serving as a sensitive layer were prepared in this report. Apart from a series of purposive measurements, various characterization techniques such as XRD, Raman, XPS and TEM were employed as well to assist the exploration of sensors performance to NO₂ gas. The experimental results revealed a 580% response enhancement for prepared RGO/Cu₂O sensors compared with pure RGO counterparts, as well as an excellent selectivity. In a specific experiment, the sensing response attained 4.8% and 29.3% toward 20 ppb and 100 ppb NO₂ respectively at 60 °C, which was larger than most Cu₂O based resistive gas sensors. Moreover, further subtle modulation of this RGO/Cu₂O nanocomposites led to a preferable room-temperature response of 37.8% toward 100 ppb NO₂, which also offered a favorable stability of 98.1% response retention after four exposures within ten days. The obtained results imply that the prepared RGO/Cu₂O QDs sensors possess a competitive capability of trace NO₂ detection.     

Nanostructured SnO2 thin films for NO2 gas sensing applications

We report the synthesis of nanostructured SnO₂ by a simple inexpensive sol–gel spin coating method using m-cresol as a solvent. This method facilitates rapid synthesis at comparatively lower temperature enabling formation of nanostructures suitable for gas-sensing applications. Various physicochemical techniques have been used for the characterization of SnO₂ thin films. X-ray diffraction analysis confirmed the single-phase formation of tetragonal SnO₂ having crystallite size 5–10 nm. SnO₂ showed highest response (19%) with 77.90% stability toward 100 ppm nitrogen dioxide (NO₂) at 200 °C. The response time of 7 s and recovery time of 20 min were also observed with the same operating parameters. The probable mechanism is proposed to explain the selective response toward nitrogen dioxide. Impedance spectroscopy studies showed that the response to nitrogen dioxide is mainly contributed by grain boundaries. The reproducibility and stability study of SnO₂ sensor confirmed its candidature for detection of NO₂ gas at low concentration (10–100 ppm) and lower operating temperature.