Ethanol sensing evaluation of sol–gel barium calcium ferrite

Hexaferrites are very attractive materials for high-frequency circuits and operating devices, nevertheless their use for sensing application is very rare. In the present work, BaCa₂Fe₁₆O₂₇ hexaferrite was synthesized by a simple sol–gel technique. Structural and microstructural information have been obtained by X-ray powder diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The gas sensing performance of BaCa₂Fe₁₆O₂₇ hexaferrite-based sensors was investigated for the detection of some common volatile organic compounds and gases like ethanol, acetone, methane and carbon monoxide. The results showed the sensors to exhibit high sensitivity, quick response/recovery and good reproducibility and stability towards 100 ppm ethanol. Moreover the sensors proved to be highly selective from measurements of cross sensitivity.     

Hierarchical assembly of SnO2 nanorod on spindle-like α-Fe2O3 for enhanced acetone gas-sensing performance

Preparing a heterojunction structure in different metal oxides is an efficacious method to improve the gas-sensing properties. In this article, a novelty SnO₂ nanorod/spindle-like Fe₂O₃ heterostructure was successfully fabricated through a simple two-step hydrothermal route. The morphological characterization revealed that the spindle-shaped Fe₂O₃ with length and diameter of 400 and 100 nm were firstly fabricated by a hydrothermal process, and then a large number of SnO₂ nanorods (lengths of 30 nm and diameterd of 8 nm) covered the spindle-shaped Fe₂O₃ uniformly. In order to facilitate better practical applications, the gas sensing performance of sensors based on SnO₂/Fe₂O₃ nanostructures and pure Fe₂O₃ nanospindles on volatile organic compounds were systematically studied. Gas sensing tests indicated that such hierarchical SnO₂/Fe₂O₃ heterostructures revealed improved acetone sensing performance compared to pure spindle-like Fe₂O₃, and the enhanced gas-sensitivity performance possibly be attributed to the synergistic effect and heterojunction of the interface between spindle-like Fe₂O₃ and SnO₂ nanorod. Additionally, this research on as-obtained SnO₂/Fe₂O₃ hierarchical assembly may provide a new insight and a rational strategy to upgrade the sensing performance of certain semiconductor metal oxide materials by rationally designing various novel layered nanostructures in the future.     

Facile fabrication and superior gas sensing properties of spongelike Co-doped ZnO microspheres for ethanol sensors

In this work, a novel strategy has been adopted for the synthesis of hybrid Co-doped ZnO (Co/ZnO) microspheres using the solvothermal method with a synergistic effect of ultrasonic and microwave radiation. The Co/ZnO microspheres were characterized by XRD, FE-SEM, XPS and BET techniques. Sensing tests revealed that the Co/ZnO microspheres exhibited highly better ethanol sensing properties than pure ZnO nanoparticles did, including lower limit of detection (less than 10?ppm), higher response (ca. 120–100?ppm ethanol), lower operating temperature (ca. 220?°C), faster response (10?s) and recovery time (5?s) and better selectivity. The superior gas sensing properties were mainly attributed to the incorporation of Co, which increased the amount of oxygen vacancies and adsorbed oxygen. The sensing mechanism has been explained by oxygen chemisorption on the ZnO surfaces and subsequent reactions of surface adsorbed oxygen species with the ethanol molecules.     

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.     

Synthesis of single-crystal SnO2 nanowires for NOx gas sensors application

Single-crystal SnO₂ nanowires (NWs) were successfully synthesized and characterized as sensing materials for long-term NO_x stability detection in environmental monitoring. Reproducible and selective growths of the SnO₂ NWs on a patterned, 5 nm-thick gold catalyst coated on a SiO₂/Si wafer as substrate were conducted by evaporating SnO powder source at 960 °C in a mixture of argon/oxygen ambient gas (Ar: 50 sccm/O₂: 0.5 sccm). The as-obtained products were characterized by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman scattering, and photoluminescence (PL). The SEM and HRTEM images revealed that the products are single-crystal SnO₂ NWs with diameter and length ranges of 70 nm–150 nm and 10 μm–100 μm, respectively. The three observed Raman peaks at 476, 633, and 774 cm⁻¹ indicated the typical rutile phase, which is in agreement with the XRD results. The NWs showed stable PL with an emission peak centered at around 620 nm at room-temperature, indicating the existence of oxygen vacancies in the NW samples. The electrical properties of synthesized SnO₂ NWs sensor were also investigated and it exhibited a negative temperature coefficient of resistance in the measured range (300–525 K). The calculated activation energy E_c of SnO₂ NWs was 0.186 eV. Moreover, the SnO₂ NW sensors exhibited good response to NO_x gas. The response of the sensors to 5 ppm NO_x reached 105% at an operating temperature of 200 °C.     

Effect of low H2 concentrations on the current-voltage characteristic of ZnO gas sensor

Abstract

Abstract

A self-heated ZnO gas sensor was prepared through thermal oxidation of Zn metal for 30 and 60?min in oxygen atmosphere. The XRD results confirmed the conversion of Zn metal to ZnO. The current-voltage measurements showed changes in forward current in the presence of hydrogen gas, with the concentration ranging from 40 to 160?ppm. A better resolution of current response due to H₂ concentrations was obtained for the samples with 30?min oxidation, although both samples showed enhanced response with the applied voltage. The barrier height of both samples was found to decrease with H₂ concentrations, thus aiding in the increase in current response.     

Synthesis and enhanced gas sensing properties of flower-like ZnO/α-Fe2O3 core-shell nanorods

This paper reports a facile two-step synthesis route for the preparation of flower-like ZnO/α-Fe₂O₃ nanorods (NRs). Flower-like ZnO NRs with the average diameter about 810 nm and length about 4.5 µm were firstly synthesized via a chemical solution method, and then ZnO NRs was coated with a continuous α-Fe₂O₃ layer to form ZnO/α-Fe₂O₃ core-shell structure through an ionic-layer adsorption and reaction method. The gas-sensing results show that the ZnO/α-Fe₂O₃ NRs exhibit excellent sensitivity, selectivity, and response-recovery capacity to ethanol vapor at a low optimum temperature of 240 °C. In particular, compared with pure ZnO NRs and α-Fe₂O₃ nanoparticles (NPs), the ZnO/α-Fe₂O₃ NRs show an obvious improvement in gas sensing properties. The substantial improvement of sensing properties may be attributed to the unique microstructure and heterojunction formed between ZnO and α-Fe₂O₃.     

气体传感器在监测化工空气污染中的应用

随着工业生产、环境监测等领域对气体传感器的精度、性能、稳定性方面的要求的提高,对气体传感器的研究和开发也越来越重要。文章提出了气体传感器在监测化工空气污染中的应用,主要介绍了气体传感器以及工业空气污染监测中常用的气体传感器———半导体式气体传感器。     

Low temperature TiO2 based gas sensors for CO2

Monitoring the level of CO₂, especially in closed spaces, is more and more required in technological applications, or in human activities. Since most of the literature data reveal CO₂ detection materials with high sensitivities over 300 °C, here we have concentrated on the gas sensing abilities of Cr doped TiO₂ thin films in front of CO₂, close to the room temperature and at atmospheric pressure. The films were obtained by RF reactive sputtering. The undoped films contain a mixture of anatase and rutile phases. With the increase of Cr content, the crystallites size decreases, and the films become pure rutile for a 4 at% Cr concentration. We found out that these material based sensors are more sensitive to CO₂ for higher Cr concentration, the optimum operating temperature approaching to the room temperature, determining in fact low energy consumption. The explanation is related to the observed increase of oxygen vacancies number (which we have evidenced and clarified), and also to the presence of the rutile phase, whose higher dielectric constant (compared to anatase), and its finer crystallites, determine a better gas sensing. More, the surface active area in front of CO₂ increases, as the films become rougher for higher Cr contents. The increase of Cr³⁺ percentage enhance the power of interaction with the adsorbed species (O₂ and/or CO₂). A grain boundary model was proposed for the thermal activation of the electrical conductivity. The energy barrier height at the grain boundary, the impurities concentration (characteristic parameters of this model) were calculated and found to agree well with the data in the literature.     

Optical Probes for Multiphase Flow Characterization: Some Recent Improvements

Concentration, velocity, size and interfacial area density are key parameters in the design of multiphase processes involving bubbles or droplets. Optical probes provide a relatively simple and cheap mean to measure these quantities. In this paper, recent advances in probe design and signal processing are summarized, and the performances of these systems are discussed. Finally, various applications are presented and known limitations of the technique are underlined.     

In situ formation of one-dimensional CoMoO4/MoO3 heterojunction as an effective trimethylamine gas sensor

In situ diffusion growth of CoMoO₄ nanoparticles on the surface of MoO₃ nanobelts was achieved through a facile method. The microstructures, morphologies, and chemical composition of the nanocomposites were investigated based on X-ray diffraction, field-emission electron scanning microscopy, transmission electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectra. The characterization results confirmed CoMoO₄ nanoparticles uniformly distributed on the surface of the MoO₃ nanobelts. The trimethylamine sensing properties of pristine MoO₃ and CoMoO₄/MoO₃ nanocomposites were investigated using a static system. The experimental results revealed that heterojunction structure of CoMoO₄/MoO₃ nanocomposites displayed low working temperature and enhanced sensing performance. The response of CoMoO₄/MoO₃ nanocomposites to 100 ppm trimethylamine was ～104.8 at 220 °C, which was four times higher than that of pure MoO₃ at 280 °C. The reason for the enhanced sensing properties of CoMoO₄/MoO₃ nanocomposites is attributed mainly to the formation of the p–n junction between the p-type CoMoO₄ nanoparticles and the n-type MoO₃ nanobelts.     

Effect of sintering temperature on the electrical and gas sensing properties of tin oxide powders

Tin oxide is an n-type semiconducting material having superior properties that can be utilized in several applications. The warning and detection of several dangerous gases in the environment are possible by utilizing gas sensors. The comprehensive functionality of these sensors could help to reduce the risk of severe health hazards and unexpected explosion risks. Tin oxide-based gas sensors exhibit reliable gas sensing performances along with respectful sensitivity and selectivity. Tin oxides in micro-and nano-particle forms provide an extremely high surface-to-volume ratio, which is favorable for gas sensors. Processing and synthesis of tin oxide particles accompany high-temperature processes, and this paper focuses on studying the effect of sintering temperatures on the structural and grain size of the commercially available tin oxide particles. The surface morphology of the tin oxide samples sintered at three different temperatures of 1100, 1200, 1300 ^°C shows a clear difference in the grain size and further affecting the dielectric properties of the materials. The gas sensing performances of three tin oxide samples are investigated by fabricating a pellet-type gas sensor. The sensor with the sintering temperature of 1200 ^°C exhibits the best gas-sensing performance with high response and low limit of detection (LOD). Our results suggest that the sintering temperature plays a vital role in deciding the dielectric properties and grain sizes, which are important parameters that affect the gas sensing behavior of tin oxide micro-and nano-particles.     

Fabrication and characterization of Ag-Decorated indium–tin-oxide nanoparticle based ethanol sensors using an enhanced electrophoretic method

In this paper different Indium Tin Oxide (ITO)-based ethanol vapor sensors (fabricated as thin films and nanoparticles) are presented, and their structural and sensing properties are investigated. An Electrophoretic Deposition (EPD)-Enhanced method is proposed for the fabrication of pure, and Ag-decorated indium tin oxide sensors. The proposed sensors are then compared to conventional indium tin oxide sensors fabricated by sputtering (thin film), and drop-casting method in terms of response, and working temperature. It is shown that the electrophoretic deposition method has decreased the final particle size of the indium tin oxide nanoparticles by limiting the agglomeration of nanoparticles, and increased the sparsity of the particles forming the sensing material. Results suggest that compared to the conventional sensors, the sensors fabricated by the proposed electrophoretic deposition method (i.e., the pure-indium tin oxide (EPD-TF), and the Ag-decorated indium tin oxide (Ag-EPD) sensors), has considerably better performance for the detection of the ethanol vapors, showing reduced working temperature (110 and 130 °C, respectively), higher response, and better selectivity over CO, methane, methanol, and acetone. Moreover, the response and recovery time of the proposed sensors were found to be lower than most of the previously reported indium tin oxide-based ethanol sensors, approving the positive effect of the electrophoretic method as a simple, controllable method for sensor fabrication.