Alcohol sensing of tin oxide thin film prepared by sol-gel process

The present paper describes the alcohol sensing characteristics of spin coated SnO₂ thin film deposited by using sol-gel process. The sensitivity of the film was measured at different temperatures and different concentrations of alcohol at ppm level. Alcohol detection result shows peak sensitivity at 623 K. The variation of sensitivity and ethanol concentration has shown a linear relationship up to 1150 ppm and after that it saturates. The response time measurement of the sensor was also observed and it was found that the response time is 30 sec. The results obtained favour the sol-gel process as a low cost method for the preparation of thin films with a high sensing characteristic.     

Sol-gel-based, planar waveguide sensor for water vapor

A water vapor sensor based on a combination of sol-gel processing and planar optical waveguide technologies has been developed. The indicator erythrosin B was entrapped in a thin sol-gel film (thickness ～100 nm) prepared from methyltriethoxysilane, dimethyldiethoxysilane, and tetraethoxysilane. This dye exhibits an increase in absorbance in the presence of liquid or gaseous water. The sol-gel layer containing the dye was deposited onto a sol-gel-derived, single-mode planar waveguide. Outcoupled light intensity measurements (at 514.5 nm) over a range of water vapor concentrations (in a nitrogen gas stream) yielded a response over a wide range of relative humidity (<1-～70%) at room temperature. Response and reversal times were less than 1 min, which may make this sensor attractive for real-time monitoring applications.     

Nafion film/K(+)-exchanged glass optical waveguide sensor for BTX detection

An optical waveguide (OWG) sensor for the detection of BTX gases is reported. The highly sensitive element of this sensor was made by coating the copper Nafion film over a single-mode potassium ion exchanged glass OWG. We used the OWG sensor to detect toluene gas as a typical example BTX gas. The sensor exhibits a linear response to toluene in the range of 0.25-4250 ppm with response and recovery times less than 25 s. The sensor has a short response time, high sensitivity, and good reversibility.     

H2S sensors based on SnO2 films: RGTO verses RF sputtering

Gas sensing characteristics of SnO₂ thin films prepared by RF sputtering have been investigated and compared to that of RGTO (Rheotaxially Grown and Thermally Oxidized) films. Both the sensor films exhibited a highly selective response towards H₂S with RF sputtered film showing better response characteristics. RF sputtered and RGTO films exhibited a maximum response of 54 and 15 towards 10 ppm of H₂S at an optimum operating temperature of 150 and 250 °C, respectively. Sputtered films exhibited a linear response in the wide concentration range from 500 ppb to 500 ppm while RGTO films were found to saturate for concentrations above 100 ppm. XPS investigations revealed that the RGTO films are more sub–stoichiometric or oxygen deficient than the sputtered films. Raman studies further indicates that the surface of sputtered and RGTO films are characterized by the presence of oxygen deficiency attributed to the “bridging-type” and deeper “in-plane/sub-bridging” oxygen vacancies, respectively. The improved response kinetics of the RF sputtered films is attributed to the presence of bridging type oxygen vacancies that facilitates the charge transfer between the sensor surface and H₂S molecules.     

A comparative study on the NO2 gas sensing properties of ZnO thin films, nanowires and nanorods

ZnO thin films were fabricated using the spin coating method, ZnO nanowires by cathodically induced sol-gel deposition by the means of an anodic aluminum oxide (AAO) template, and ZnO nanorods with the hydrothermal technique. For thin film preparation, a clear, homogeneous and stable ZnO solution was prepared by the sol-gel method using zinc acetate (ZnAc) precursor which was then coated on a glass substrate with a spin coater. Vertically aligned ZnO nanowires which were approximately 65 nm in diameter and 10 μm in length were grown in an AAO template by applying a cathodic voltage in aqueous zinc nitrate solution at room temperature. For fabrication of the ZnO nanorods, the sol-gel ZnO solution was coated on glass substrate by spin coating as a seed layer. Then ZnO nanorods were grown in zinc nitrate and hexamthylenetetramine aqueous solution. The ZnO nanorods are approximately 30 nm in diameter and 500 nm in length. The ZnO thin film, ZnO nanowires and nanorods were characterized by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). The NO₂ gas sensing properties of ZnO thin films, nanowires and nanorods were investigated in a dark chamber at 200 °C in the concentration range of 100 ppb-10 ppm. It was found that the response times of both ZnO thin films and ZnO nanorods were approximately 30 s, and the sensor response was depended on shape and size of ZnO nanostructures and electrode configurations.     

TiO(2)/LiCl-based nanostructured thin film for humidity sensor applications

A simple and straightforward method of depositing nanostructured thin films, based on LiCl-doped TiO(2), on glass and LiNbO(3) sensor substrates is demonstrated. A spin-coating technique is employed to transfer a polymer-assisted precursor solution onto substrate surfaces, followed by annealing at 520°C to remove organic components and drive nanostructure formation. The sensor material obtained consists of coin-shaped nanoparticles several hundred nanometers in diameter and less than 50 nm thick. The average thickness of the film was estimated by atomic force microscopy (AFM) to be 140 nm. Humidity sensing properties of the nanostructured material and sensor response times were studied using conductometric and surface acoustic wave (SAW) sensor techniques, revealing reversible signals with good reproducibility and fast response times of about 0.75 s. The applicability of this nanostructured film for construction of rapid humidity sensors was demonstrated. Compared with known complex and expensive methods of synthesizing sophisticated nanostructures for sensor applications, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), this work presents a relatively simple and inexpensive technique to produce SAW humidity sensor devices with competitive performance characteristics.     

Thin film chemical sensors based on p-CuO/n-ZnO heterocontacts

Previous work on bulk ceramic heterocontacts (n-ZnO/p-CuO) has indicated significant sensitivity to the presence of specific adsorbed chemical species. Here, these results are extended to thin film heterostructures fabricated via chemical solution methods. It is expected that thin film sensor architectures will possess significant advantages over their bulk counterparts. In this study, the desired properties of porosity and crystallinity have been optimized with respect to pyrolysis temperature for each ZnO and CuO sol-gel process. The results of microscopy and X-ray diffraction (XRD) indicated that an optimal balance of these two properties is achieved at a pyrolysis temperature of 250 °C. The CuO films were seen to possess a level of porosity significantly higher than that seen in the ZnO films, making them an ideal candidate for the top layer in a planar thin film heterostructure. Results of current-voltage measurements conducted in 4000 ppm hydrogen have confirmed that the inherent porosity of the CuO films led to an enhanced sensor response in CuO on ZnO heterostructures. Lastly, the fabrication and structural characterization of a mixed solution type heterostructure has been detailed. Atomic force microscopy and XRD data indicated the presence of ZnO pillars dispersed among a matrix of CuO.     

Device optimization of CO2 gas sensor using planar technology

A planar type Li+ ion based potentiometric CO2 micro gas sensor of size 2 x 3 mm has been fabricated on alumina substrate by combining thin and thick film technology. The heater, electrodes and electrolyte were deposited by thin film deposition technique and the sensing and reference electrodes were printed by silk screen printing technology. The optimal thickness and sintering temperature of electrolyte are 1.2 microm and 775 degrees C. The sensor with Li2CO3 and 20 mol% BaCO3 not only exhibits a good Nernstian behavior but also consistent results over a long time at 450 degrees C in dry as well as 70% RH humidity condition between 160-5000 ppm CO2 concentrations. The spreading effect of the sensing and reference materials was controlled by the addition of Al2O3:B2O3 (1:2 mol%) glass.     

Fabrication of a room temperature hydrogen sensor based on thin film of single-walled carbon nanotubes doped with palladium nanoparticles

A new sensor for the detection of hydrogen at parts per million (ppm) levels was fabricated by coating a thin film of palladium-doped activated single-walled carbon nanotube on the inner wall of a glass tube. The response of the sensor was based on the changes in the impedance of the sensor upon the adsorption of hydrogen molecules. The linear dynamic range of the sensor was from 1 to 50?ppm. The relative standard deviation of six replicate analyses of 5?ppm of H₂ was 2.1% and the limit of detection was 0.73?ppm for H₂ species. Humidity, methane and hydrogen sulphide did not have any serious effect on hydrogen recognition. Also, no interfering effect was observed when 20-fold excess (mass/mass) of carbon dioxide or carbon monoxide was present with hydrogen.     

WO3薄膜气敏光学传感特性研究

WO3薄膜是良好的光学气敏传感器材料.采用溶胶凝胶法制备了WO3掺杂薄膜,对样品在不同浓度氢气气氛中的气敏光学性质、敏感度及响应时间进行了测试、分析和计算,并结合光传输理论给出了气敏薄膜的光学变化机制,理论分析与实验结论吻合.     

Detection of acetone vapor using graphene on polymer optical fiber

Recently, many studies have been focused on the development of fiber optic sensor systems for various gases and vapors. In the present study, an intrinsic polymer optical fiber (POF) sensor using graphene is described for the purpose of acetone vapor sensing for the first time. Observations on the continuous measurement of acetone vapor in dehydrated air are presented. The principle of operation of sensor transduction relies on the dependence of the reflectance on the optical and geometric properties of the sensitive over layered when the vapor molecules are adsorbed on the graphene film. For the same purpose the CVD synthesized graphene film was transferred on the POF end. The synthesized graphene film thickness was evaluated using atomic force microscopy (AFM), Raman spectroscopy and transmission electron microscopy (TEM). For the preliminary evaluation using volatile organic compounds, we evaluated the sensor performance for acetone. Upon the interaction of the sensor with acetone vapor, the variation in the reflected light was monitored as a function of the acetone concentration. The sensor response shows a significant change in sensitivity as compared with the POF probe without a graphene coating. The present sensor shows a satisfactory response upon exposure to various concentrations of acetone vapor from 44 ppm to 352 ppm. To the best of our knowledge, the use of graphene film along with POF for the sensing of volatile organic compounds has not previously been reported.     

基于SnO2薄膜气体传感器的设计与制作（英文）

设计了一种带硅岛结构的基于SnO2薄膜材料的共面式气体传感器.利用有限元工具对传感器进行了稳态热分析,分析结果表明这种传感器在33.84 mW的功耗下最高温度达到400℃,气敏薄膜上温度分布均匀.详细阐述了传感器的制作过程,过程中总共使用4块掩模版用于光刻工艺.采用溶胶-凝胶法制备了SnO2纳米薄膜作为传感器的气敏元件.对传感器进行了气敏测试,实验结果表明该传感器拥有良好的气敏性能,在300℃下对50×10-6到2 000×10-6氢气的灵敏度逐渐递增,反应时间可控制在10 s以内.     

Indium oxide thin film based ammonia gas and ethanol vapour sensor

A sensor for ammonia gas and ethanol vapour has been fabricated using indium oxide thin film as sensing layer and indium tin oxide thin film encapsulated in poly(methyl methacrylate) (PMMA) as a miniature heater. For the fabrication of miniature heater indium tin oxide thin film was grown on special high temperature corning glass substrate by flash evaporation method. Gold was deposited on the film using thermal evaporation technique under high vacuum. The film was then annealed at 700 K for an hour. The thermocouple attached on sensing surface measures the appropriate operating temperature. The thin film gas sensor for ammonia was operated at different concentrations in the temperature range 323–493 K. At 473 K the sensitivity of the sensor was found to be saturate. The detrimental effect of humidity on ammonia sensing is removed by intermittent periodic heating of the sensor at the two temperatures 323K and 448 K, respectively. The indium oxide ethanol vapour sensor operated at fixed concentration of 400 ppm in the temperature range 293–393 K. Above 373 K, the sensor conductance was found to be saturate. With various thicknesses from 150–300 nm of indium oxide sensor there was no variation in the sensitivity measurements of ethanol vapour. The block diagram of circuits for detecting the ammonia gas and ethanol vapour has been included in this paper.     

Optical properties and sensing applications of lithium iron phosphate thin films

A composite optical waveguide sensor, consisting of lithium iron phosphate (LiFePO₄, LFP) as the sensing material, was constructed and utilized for the detection of volatile organic compound gases. Nano-LFP powder was prepared via the hydrothermal method and was subsequently utilized in a dip-coating procedure for the fabrication of LFP thin films. The effect of heat treating temperature on the refractive index of the thin films was studied. A glass optical waveguide gas sensor was fabricated by coating an LFP thin film on the surface of single-mode tin-diffused glass optical waveguide. The sensor was found to exhibit a linear response to xylene in the range of 50-1000 ppm, with response times of less than 5 s.     

Fluorescence behaviors of 5-dimethylamino-1-naphthalene-sulfonyl-functionalized self-assembled monolayer on glass wafer surface and its sensing properties for nitrobenzene

A fluorescent film was prepared by chemically immobilizing 5-dimethylamino-1-naphthalenesulfonyl (dansyl) onto an epoxide-terminated self-assembled monolayer on glass. Steady-state fluorescence emission measurements demonstrated that the sensing molecules were homogenously and stably immobilized on the substrate surface. Fluorescence quenching results showed that the film is sensitive to the presence of nitrobenzene and has a great selectivity to it as compared with other commonly used quenchers like nitromethane, sodium nitrite, potassium iodide and acrylamide. An assumption that a less polar intermediate phase, which was composed of the long flexible spacers, the dansyl moieties and the solvent within the layer, was formed on the substrate surface was proposed to rationalize the different response of the film to those common quenchers. The explanation was supported by the results from fluorescence anisotropy studies and fluorescence quenching experiments performed in different solvents. Fluorescence lifetime studies showed that the quenching of nitrobenzene to the emission of the film is static in nature, which has been attributed to the formation of a charge transfer complex between the quencher and the fluorophore. Furthermore, the response of the film to nitrobenzene is fast and reversible, suggesting that the film might be used as a sensing film for the compound and its analogs.     

Synthesis of indium oxide cubic crystals by modified hydrothermal route for application in room temperature flexible ethanol sensors

Indium oxide cubic crystals were prepared by using hexamethylenetetramine and indium chloride without the addition of any structure directing agents. The chemical route followed in the present work was a modified hydrothermal synthesis. The average crystallite size of the prepared cubes was found to be 40 nm. A blue emission at 418 nm was observed at room temperature when the sample was excited with a 380 nm Xenon lamp. This emission due to oxygen vacancies made the material suitable for gas sensing applications. The synthesized material was made as a composite film with polyvinyl alcohol which was more flexible than the films prepared on glass substrates. This flexible film was used as a sensing element and tested with ethanol vapours at room temperature. The film showed fast response as well as recovery to ethanol vapours with a sensor response of about 1.4 for 100 ppm of the gas.     

Gas sensor using single-wall carbon nanohorns

We fabricated a gas sensor using single-walled carbon nanohorns (SWNHs) produced by the gas-injected arc-in-water method. This gas sensor consisted of agglomerated SWNHs as a coating film between Al electrodes on a glass substrate and the shift of the electric resistance of this coating film caused by gas adsorption was monitored. Its sensing property was examined for the detection of NH₃ and O₃ at room temperature. It was confirmed that the electrical resistance of the SWNHs film increases with adsorption of NH₃, whereas the adsorption of O₃ induced the decrease of the resistance. A model to correlate the gas concentration and the sensing property was proposed focusing on the detection of NH₃ based on mono-layer adsorption and a second-order interaction of adsorbed gas molecules for charge transfer.     

Indium Tin Oxide Thin-Film Sensor for Detection of Volatile Organic Compounds (VOCs)

Indium tin oxide (ITO) (In₂O₃ + 17% SnO₂) thin films were grown on glass substrate by direct evaporation method. Two thick gold pads were deposited to take out contacts. The response of these films at different operating temperatures, when exposed to various volatile organic compounds (VOCs) such as methanol, ethanol, butanol, and acetone in the concentration range 200-2500 ppm was evaluated. Additionally, the effect of film thickness on the response charateristics of methanol and acetone was studied. The linearity and sensitivity of the sensors were measured. The ITO thin-film sensors showed a sensitivity of 0.256 ohms/ppm to acetone vapors, which was almost linear in the range 200-2500 ppm. In order to improve sensitivity and selectivity, a thin layer of various metal and metal oxides such as Cu and PbO was deposited on the sensor surface to work as catalytic layer and the effect on the performance of the sensor was studied. The response and recovery times of the sensor were determined for acetone vapors and were found to be 155 sec and 110 sec, respectively.     

Sol-gel thin-film immobilized soybean peroxidase biosensor for the amperometric determination of hydrogen peroxide in acid medium.

An acid-stable soybean-peroxidase biosensor was developed by immobilizing the enzyme in a sol-gel thin film. Methylene blue was used as a mediator because of its high electron-transfer efficiency. The sol-gel thin film and enzyme membrane were characterized by FT-IR, and the effects of pH, operating potential, and temperature were explored for optimum analytical performance by using the amperometric method. The H2O2 sensor exhibited a fast response (5 s), high sensitivity (27.5 microA/mM), as well as good thermostability and long-term stability. In addition, the performance of the biosensor was investigated using flow-injection analysis (FIA).     

Chemical gas sensor application of open-pore mesoporous thin films based on integrated optical polarimetric interferometry

Chemical gas sensors that employ integrated optical polarimetric interferometry were fabricated by the sol-gel synthesis of transparent mesoporous thin films of TiO2-P2O5 nanocomposite on tapered layers of TiO2 sputtered on tin-diffused glass waveguides. Atomic force microscopy images of the mesoporous thin film clearly show the open pore mouths on the film surface that favor rapid diffusion and adsorption of gas-phase analytes within the entire film. Adsorption of gas and vapor induces changes (Deltan) in the refractive index of the mesoporous thin film that lead to shifts in the phase difference between the fundamental transverse electric and magnetic modes simultaneously excited in the glass waveguide via single-beam incidence. Upon exposure to NH3 gas at concentrations as low as 100 ppb in dry air at room temperature, the sensor exhibits a reversible change in the phase difference with the response and recovery times of less than 60 and 90 s, respectively. It is unexpected that the sensor is unresponsive to either NO2 or C6H6 vapor, leading to a somewhat selective sensitivity to NH3. Determination of Deltan was carried out with a combination of the experimental results and the theoretical calculations. The sensor design represents a novel, effective, and easily accessible approach to mesoporous thin-film-based integrated optical chemical sensors.