Nanosized titania derived from a novel sol-gel process for ammonia gas sensor applications

Highly redispersible anatase nanoparticles were prepared by a novel sol-gel based hydrothermal process for gas sensing applications. Thin titania films composed of nanoparticles were deposited on Au interdigital electrodes by dip-coating, annealed and tested in a gas test bench at 350 °C. The anatase films showed a very high sensitivity towards ammonia and no cross interference by CO₂, O₂ and C₃H₈. To classify the sensor as an ammonia gas sensor, a comparison with other sensor designs from literature has been performed.     

Acetylene sensor based on Pt/ZnO thick films as prepared by flame spray pyrolysis

ZnO nanoparticles loaded with 0.2-2.0 at.% Pt have been successfully produced in a single step by flame spray pyrolysis (FSP) technique using zinc naphthenate and platinum(II) acetylacetonate, as precursors dissolved in xylene and their acetylene sensing characteristics have been investigated. The particle properties were analyzed by XRD, BET, TEM, SEM and EDS. Under the 5/5 (precursor/oxygen) flame condition, ZnO nanoparticles and nanorods were observed. The crystallite sizes of ZnO spherical and hexagonal particles were found to be ranging from 5 to 20 nm while ZnO nanorods were seen to be 5-20 nm in width and 20-40 nm in length. In addition, very fine Pt nanoparticles with diameter of ∼1 nm were uniformly deposited on the surface of ZnO particles. From gas-sensing characterization, acetylene sensing characteristics of ZnO nanoparticles is significantly improved as Pt content increased from 0 to 2  at.%. The 2 at.% Pt loaded ZnO sensing film showed an optimum C₂H₂ response of ∼836 at 1% acetylene concentration and 300 °C operating temperature. A low detection limit of 50 ppm was obtained at 300 °C operating temperature. In addition, Pt loaded ZnO sensing films exhibited good selectivity towards hydrogen, methane and carbon monoxide.     

Influence of pH on particle size and sensing response of chemically synthesized chromium oxide nanoparticles to alcohols

Nanoparticles of chromium oxide have been synthesized by following a co-precipitation route at various pH values of the precursor solution. Structural and morphological analysis were carried out by using XRD and TEM techniques which revealed that the size of nanoparticles synthesized at pH 9 was smaller as compared to those synthesized at other pH values. The thick films of synthesized samples were deposited on alumina substrate and their sensing response to methanol, ethanol and isopropanol was investigated at different operating temperatures. It was observed that all the sensors gave optimum response at 250 °C. It has been observed that sample prepared at pH 9, being a collection of smallest particles as compared to other samples, exhibited high sensing response to alcohol vapour. Sensor response of all the samples tested was significantly higher towards isopropanol vapour than towards methanol or ethanol.In the present study the effect of particle size on intergranular activation energy has been studied as well. It was found that smaller particles possess high activation energy and exhibit higher sensing response as compared to that of larger particles. This type of study may help in the selection of particle suitable for gas sensing.     

Electrostatic sprayed SnO2 and Cu-doped SnO2 films for H2S detection

This paper presents the ability of electrostatic sprayed tin oxide (SnO₂) and tin oxide doped with copper oxide (1, 2, and 4 at.% Cu) films to detect different pollutant gases, i.e., H₂S, SO₂, and NO₂. The influence of a copper oxide dopant on the SnO₂ morphology is studied using scanning electron microscopy (SEM) technique, which reveals a small decrease in the porosity and particle size when the amount of dopant is increased. The sensing properties of the SnO₂ films are greatly improved by doping, i.e., the Cu-doped SnO₂ films have large response to low concentration (10 ppm) of H₂S at low operating temperature (100 °C). Furthermore, no cross-sensitivity to 1 ppm NO₂ and 20 ppm SO₂ is observed. Among the studied films, the 1 at.% Cu-doped SnO₂ layer is the most sensitive in the detection of all the studied gases.     

Novel NO2 gas sensor based on cuprous oxide thin films

In this paper we present the results concerning the characterization of cuprous oxide thin films fabricated by chemical deposition and rapid photothermal processing (RPP) method. The growth kinetic effects and influence of the RPP temperature on the chemical deposited cuprous oxide thin films microstructures were investigated by scanning electron microscopy and energy dispersive X-ray spectrometry. The effect of the electrical resistivity change of Cu₂O thin film layer in the presence of NO₂ is used for gas sensing measurements. Cuprous oxide layers are used as NO₂ gas sensitive material in a novel gas sensor element. It can be shown from experimental results that chemical bath deposition and rapid photothermal processing not only allows green materials preparation but also improves the performance and reliability over conventional methods of the production of sensors for continuous environmental monitoring.     

Sensing properties of organised films based on a bithiophene derivative

Highly reproducible optic and electrochemical sensors have been developed using organised films from a polar bithiophene derivative, the 5-(dimethylamino)-5′-nitro-2,2′-bithiophene (Me₂N–T₂–NO₂). The strength of the molecular dipole moment of this push–pull end-capped bithiophene has permitted to obtain highly ordered, homogeneous and reproducible films by using both the Langmuir–Blodgett and the casting techniques. The organisation of the molecules in LB films and cast films has been established by means of UV–vis, infrared and Raman spectroscopy and by AFM.Me₂N–T₂–NO₂ thin films possess appealing optical and electrochemical sensing capabilities. UV–vis spectra can be modified in the presence of a variety of volatile organic compounds and the sensitivity is related to the polarity of the gas analysed. Films can also be used as electrochemical sensors because the characteristics of the current/potential curves are sensitive to the nature of the electrolytic solution. The spectral changes accompanying the applied voltage could be used to produce ionochromic sensor electrodes.The structure of the films has an important impact in the sensing properties of the films and in their stability. The optical and electrochemical sensing properties of Langmuir–Blodgett films are more reproducible than those observed in cast films. This makes films prepared using the LB technique to be preferred as sensing devices. However the casting technique provides a fast method to obtain cheap and highly ordered sensors.     

α-MoO3/TiO2 core/shell nanorods: Controlled-synthesis and low-temperature gas sensing properties

Crystalline α-MoO₃/TiO₂ core/shell nanorods are fabricated by a hydrothermal method and subsequent annealing processes under H₂/Ar flow and in the ambient atmosphere. The shell layer is composed of crystalline TiO₂ particles with a diameter of 2-6 nm, and its thickness can be easily controlled in the range of 15-45 nm. The core/shell nanorods show enhanced sensing properties to ethanol vapor compared to bare α-MoO₃ nanorods. The sensing mechanism is different from that of other one-dimensional metal oxide core/shell nanostructures due to very weak response of TiO₂ nanoparticles to ethanol. The enhanced sensing properties can be explained by the change of type II heterojunction barrier formed at the interface between α-MoO₃ and TiO₂ in the different gas atmosphere. The present results demonstrate a novel sensing mechanism available for gas sensors with high performance.     

Ethanol sensing using CuO/MWNT thin film

Copper (II) oxide (CuO)/multiwall carbon nanotube (MWNT) thin film based ethanol-sensors were fabricated by dispersing CVD-prepared MWNTs in varying concentration over DC magnetron sputtered-CuO films. The responses of these sensors as a function of MWNT concentrations and temperatures were measured, and compared. The sensing response was the maximum at an operating temperature near 400 °C for all the samples irrespective of the MWNTs dispersed over them. At optimum operating temperature (T_opt) of 407 ± 1 °C, the response is linear for 100-700 ppm range and tends to saturate at higher concentrations. In comparison with bare CuO sample, the response of CuO/MWNT sensing films increased up to 50% in the linear range. The response improvement for 2500 ppm of ethanol was up to 90% compared to bare CuO sample. In addition, the sensing response time also reduced to around 23% for lowest ethanol concentration at T_opt. However, a decrease in the sensor response was observed on films with very high concentrations of MWNTs.     

Alterations in the complex refractive index of copper oxide thin films as sensing effect for hydrogen sulfide monitoring

Throughout the last three decades cuprous (Cu₂O) and cupric oxide (CuO) have been subject of extensive investigations of their material properties. This research was mainly driven by potential applicability as a photovoltaic or doping material. However, CuO/Cu₂O layers show a specific reaction towards hydrogen sulfide (H₂S), making it a good candidate as highly selective gas sensor material. On this account thin film samples of CuO and Cu₂O have been investigated with regard to their specific surface interactions with H₂S gas. Changes in morphology, chemical composition, and alterations in the complex refractive index have been thoroughly examined in order to understand possible sensing effects. Raman spectroscopy was used for verifying the films composition after heat treatment. Transmission and reflection characteristics in the extended UV/Vis regime (350–1,100 nm) of initially prepared samples and after exposure to well-defined doses of H₂S were recorded. A distinct increase in transmissivity was observed for Cu₂O films in the wavelength region λ = 550–900 nm. An initial conditioning effect was observed from consecutive measurements. Absorptivity characteristics and optical band gaps were derived, showing an absorptivity shift of CuO thin films after exposure towards H₂S. A specific optical read-out based on total internal reflection was set-up, offering a transient monitoring of the materials surface interactions with the gas phase. Changes in the response, in terms of intensity variations, were reproducibly shown for low concentrations of 5 ppm of H₂S.     

Selectivity of flame-spray-made Nb/ZnO thick films towards NO2 gas

Unloaded ZnO and Nb/ZnO nanoparticles containing 0.25, 0.5 and 1 mol.% Nb were produced in a single step by flame-spray pyrolysis (FSP) technique. The nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The BET surface area (SSA_BET) of the nanoparticles was measured by nitrogen adsorption. FSP yielded small Nb particles attached to the surface of the supporting ZnO nanoparticles, indicating a high SSA_BET. The morphology and accurate size of the primary particles were further investigated by TEM. Nb/ZnO nanoparticles paste composed of ethyl cellulose and terpineol as binder and solvent respectively was coated on Al₂O₃ substrate interdigitated with gold electrodes to form thick films by spin coating technique. After the sensing tests, the morphology and the cross-section of sensing film were analyzed by SEM and EDS analyses. The influence on a low dynamic range of Nb concentration on NO₂ response (0.1-4 ppm) of thick film sensor elements was studied at the operating temperatures ranging from 250 to 350 °C in the presence of dry air. The optimum Nb concentration was found be 0.5 mol.% and 0.5 mol.% Nb exhibited an optimum NO₂ response of ∼1640 and a short response time (27 s) for NO₂ concentration of 4 ppm at 300 °C.     

Temperature dependent H2S and Cl2 sensing selectivity of Cr2O3 thin films

The conductometric gas sensing characteristics of Cr₂O₃ thin films - prepared by electron-beam deposition of Cr films on quartz substrate followed by oxygen annealing - have been investigated for a host of gases (CH₄, CO, NO₂, Cl₂, NH₃ and H₂S) as a function of operating temperature (between 30 and 300 °C) and gas concentration (1-30 ppm). We demonstrate that these films are highly selective to H₂S at an operating temperature of 100 °C, while at 220 °C the films become selective to Cl₂. This result has been explained on the basis of depletion of chemisorbed oxygen from the surface of films due to temperature and/or interaction with Cl₂/H₂S, which is supported experimentally by carrying out the work function measurements using Kelvin probe method. The temperature dependent selectivity of Cr₂O₃ thin films provides a flexibility to use same film for the sensing of Cl₂ as well as H₂S.     

Photon stimulated sensor based on indium oxide nanoparticles I: Wide-concentration-range ozone monitoring in air

Ozone sensors are of great demand for monitoring high-concentration ozone for industrial applications and low-concentration ozone for protecting people's health, although commercial ozone sensors are limited in their detection range. In this work, it is demonstrated that compact energy-saving photostimulated ozone sensors based on indium oxide nanoparticles can detect ozone with a dynamical range over four orders of magnitude at room temperature. The photostimulated ozone sensor shows a very low cross response to NO₂, CO, and CO₂. Furthermore, the sensing signal is very reproducible, and no hysteresis effects were found in repeated measurements.     

Gas sensor with excellent selectivity to hydrogen gas

Gas sensors exhibiting high performance for various types of gases and excellent selectivity to hydrogen gas were prepared using sol–gel-derived proton-conducting glass films with well-designed Pt nanoparticles and manganese oxide as electrodes. Hydrogen and alcohols were oxidized at the Pt anode, followed by reduction at the MnO₂ cathode, resulting in an electromotive force. In contrast, when Pt nanoparticles coated with a polymer film were used as the anode, the alcohol species were not oxidized, but only the oxidization of hydrogen gas was detected. This sensor exhibited high selectivity to hydrogen gas and presents great potential for practical applications.     

精密压阻弹性体及力敏触觉传感器阵列

空气热氧化用多元醇溶剂热方法得到的金属钌纳米颗粒,可以得到纳米氧化钌颗粒,将其与金属钌纳米粉体混合,使其具有适当的聚集特性,将混合粉体与硅橡胶复合,呈现具有良好的压阻重复性和压阻敏感性,测量不同尺寸电极间的压阻行为,表明此种导电硅橡胶在毫米尺寸以上呈现一定的标度不变性,表明材料在一定尺寸范围内满足微型化要求,适用于集成的压阻弹性体应力测量阵列,我们制备了测量面载荷分布的阵列电极,设计了用于动态触觉传感测量的电路和支持软件,结果表明,针对压阻材料电阻/载荷敏感特性,通过每个压阻单元微秒级的信号获取和数据处理,可以实现毫米尺度的静态或动态载荷三维成像,该传感阵列的可重复和定量特性使其基本满足实用的人体工学器件要求。     

On the temperature dependence of the resistive and surface ionisation response of SnO2 gas sensing layers

Gas sensing experiments have been performed on SnO₂ thin films using a wide range of different analyte gases. In these experiments, the SnO₂ layers were specifically configured to observe the familiar resistive (RES) gas response alongside with the novel surface ionisation (SI) response. It is shown that the RES and SI responses, in general, occur in dissimilar temperature ranges and that both follow very different selectivity criteria. Microscopically, both kinds of response proceed through analyte-specific sequences of adsorption, surface reaction, charge transfer and desorption steps. A generalisation of the Ahlers model [1], originally developed to account for the bell-shaped temperature variation of the RES response of thin film metal oxide layers, is shown to quantitatively account for the whole range of SI data. Fits to experimental SI response vs. temperature curves allow the total energy input into the surface ionisation process to be determined and insights into the ionisation mechanism to be gained.     

Examination of Au/SnO2 core-shell architecture nanoparticle for low temperature gas sensing applications

Au/SnO₂ core-shell structure nanoparticles (NPs) were synthesized using two methods, microwave and conventional precipitation. In both cases, the size of the Au core was 12-18 nm and the thickness of the SnO₂ shell was 8-12 nm. The particle size of SnO₂ synthesized using the microwave and precipitation method was 3-5 nm and 1-2 nm, respectively. Upon heating to 400-600 °C, both particles maintained their core-shell morphology but the smaller SnO₂ particles prepared using the precipitation method were more sintered. The resistance changes in films of these particles were measured as a function of CO concentration. The Au/SnO₂ particles prepared using the microwave method showed higher sensor response than those prepared using the precipitation method, even providing a significant signal at testing temperatures approaching ambient conditions, thereby affording a new class of material for gas sensing. Both sets of core-shell particles showed higher sensor response than the SnO₂ nanoparticles. The role of the Au core as a catalyst in improving the adsorption and oxidation of CO gas is important for improving the low temperature response. In addition, the maintenance of the size of SnO₂ in the microwave method during sintering contributed to the higher response towards CO sensing.     

Sprayed CdIn2O4 thin films for liquefied petroleum gas (LPG) detection

Nanocrystalline cadmium indium oxide (CdIn₂O₄) thin films of different thicknesses were deposited by chemical spray pyrolysis technique and utilized as a liquefied petroleum gas (LPG) sensors. These CdIn₂O₄ films were characterized for their structural and morphological properties by means of X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The dependence of the LPG response on the operating temperature, LPG concentration and CdIn₂O₄ film thickness were investigated. The results showed that the phase structure and the LPG sensing properties changes with the different thicknesses. The maximum LPG response of 46% at the operation temperature of 673 K was achieved for the CdIn₂O₄ film of thickness of 695 nm. The CdIn₂O₄ thin films exhibited good response and rapid response/recovery characteristics to LPG.     

Semiconducting metal oxides as sensors for environmentally hazardous gases

This article extensively reviews the recent development of semiconductor metal oxide gas sensors for environmentally hazardous gases including NO₂, NO, N₂O, H₂S, CO, NH₃, CH₄, SO₂ and CO₂. The gas sensing properties of differently-prepared metal oxides and loaded metal oxides towards nine environmentally hazardous gases have been individually compared and digested. Promising materials for sensitive and selective detection of each hazardous gas have been identified. For instance, unloaded WO₃ nanostructures are the most promising candidates for NO₂ sensing while metal catalyst loaded WO₃ and gold-loaded SnO₂ sensors are among the most effective for NO and N₂O sensing, respectively. Moreover, related gas-sensing mechanisms are comprehensively discussed.     

Ammonia sensing properties of polypyrrole thin films at room temperature

Polypyrrole thin films were synthesized in situ by chemical polymerization. Fourier transform infrared spectroscopy revealed formation of polypyrrole. The morphological studies by scanning electron microscopy showed formation of uniform granular structure with average grain size of 0.6 μm. The film composition was characterized by X-ray photoelectron spectroscopy for chemical composition in polypyrrole film. These films were investigated for their sensing behaviour towards NH₃ and NO at room temperature. It has been observed that these films are selective for NH₃ and the sensitivity exhibited a linear response in range of 4-80 ppm.     

Micro-machined WO3-based sensors with improved characteristics

Characteristics of WO₃-based micro-machined sensors prepared using modified technologies of sensing layer deposition have been studied. The sensing films were deposited using two sputtering regimes. The first one included three interruptions of the deposition process. The second one comprised a deposition by using a floating regime that included three interruptions as well. In the first two interruptions the sputtering power was 100 W and in the last one the sputtering power was set to 280 W. Additionally to the operations of film deposition, annealing and lift-off processes were optimized. The micro-sensors showed high sensitivity and selectivity to oxidizing gases. The stability of the micro-sensors has been investigated as well. An explanation for the high sensitivity and selectivity of these new micro-sensors is presented in this study.