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.     

Ultralow-Power Alcohol Vapor Sensors Using Chemically Functionalized Multiwalled Carbon Nanotubes

Alcohol sensors, batch fabricated by forming bundles of chemically functionalized multiwalled carbon nanotubes (f-CNTs) across Au electrodes on SiO₂/Si substrates using an AC electrophoretic technique, were developed for alcohol vapor detection using an ultralow input power of ~ 0.01 - 1 muW, which is lower than the power required for most commercially available alcohol sensors by more than four orders of magnitude. The multiwalled carbon nanotubes (MWCNTs) have been chemically functionalized with the COOH groups by oxidation. We found that the sensors are selective with respect to flow from air, water vapor, and alcohol vapor. The sensor response is linear for alcohol vapor concentrations from 1 to 21 ppm with a detection limit of 0.9 ppm. The transient response of these sensors is experimentally shown to be ~1 s and the variation of the responses at each concentration is within 10% for all of the tested sensors. The sensors could also easily be reset to their initial states by annealing the f-CNTs sensing elements at a current of 100-200 muA within ~ 100-200 s. We demonstrated that the response of the sensors can be increased by one order of magnitude after adding the functional group COOH onto the nanotubes, i.e., from ~0.9% of a bare MWCNTs sensor to ~9.6% of an f-CNTs sensor with a dose of 21 ppm alcohol vapor.     

Room temperature Cl(2) sensing using thick nanoporous films of Sb-doped SnO(2)

Thick film resistive Cl(2) sensors were fabricated using SnO(2) doped with Sb. The nanocrystalline powders of Sb-doped SnO(2) synthesized by a sol-gel method were compressed into an 800?μm thick pellet. The fabricated sensors were tested against gases like Cl(2), Br(2), HCl, NO, NO(2), CHCl(3), NH(3) and H(2). The highest response to Cl(2) was achieved in 0.1% Sb doping where an exposure to 3?ppm of Cl(2) gas led to a 500-fold increase in device resistance. The high sensitivity to Cl(2) is accompanied by minor interference due to other gases at room temperature. It was found that the SnO(2) doped with 0.1% Sb exhibited high response, selectivity (>100 in comparison to the gases described above) and short response time (～60?s) to Cl(2) at 3?ppm level at room temperature.     

Microwave-assisted synthesis of ZnO/MWCNT hybrid nanocomposites and their alcohol-sensing properties

Zinc oxide and multi-walled carbon nanotube (ZnO/MWCNT) hybrid nanocomposites were synthesised by microwave-assisted method using the mixed solution of zinc acetate dehydrate (Zn(CH₃COO)₂·2H₂O) and treated MWCNTs. The syntheses were carried out at various microwave irradiation powers. The characterisation of the as-synthesised nanocomposites was conducted by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD results revealed that the composites were composed of two phases of MWCNTs and hexagonal wurzite ZnO. The SEM results showed that the ZnO nanoparticles were well decorated on the surface of MWCNTs. The amount of ZnO nanoparticles and their size increased with increasing irradiation power. Thick-film sensors were fabricated onto interdigitated conducting electrodes using as-synthesised hybrid composites as sensing materials. The alcohol-sensing behaviour of the hybrid composite films was investigated. The results indicated that the irradiation power had significant influence on the sensing response of the sensors toward alcohol. The sensor fabricated from the composite synthesised at higher irradiation power exhibited an enhanced alcohol-sensing performance.     

Gas sensors based on MWCNTs-PVP composite films for 1,2-dichloroethane vapor detection

Multi-walled carbon nanotubes (MWCNTs) - polyvinylpyrrolidone (PVP) composite materials were applied as sensitive films of resistive gas sensors for the first time to detect 1,2-dichloroethane vapor. Four sensors containing different quantities of PVP were prepared. The results revealed that composite films got a larger sensing response compared with the MWCNTs single-layer film. In addition, sensitivity for 1,2-dichloroethane vapor was improved due to the addition of PVP. The repeatability and long-term stability of the composite film sensors were studied as well. Moreover, MWCNTs-PVP composite films had a good selectivity for 1,2-dichloroethane vapor.     

Effects of electrode configuration on polymer carbon-black composite chemical vapor sensor performance

The performance of polymer carbon-black composite chemical vapor sensors as a function of underlying electrode size and geometry has been studied. The sensor performance parameters investigated were sensor response magnitude to a toluene analyte (100, 500, and 1000 ppm), fundamental sensor noise in the presence of air, and two concentrations of toluene (100 and 500 ppm), and signal-to-noise ratio (100 and 500 ppm). An array of sensors with 42 different circular electrode configurations were designed, fabricated, and tested where electrode gap was varied from 10 to 500 /spl mu/m and the diameter of the sensors was varied from 30 to 2000 /spl mu/m. Each array of electrodes was coated with an approximately 1 /spl mu/m-thick layer of conducting polymer carbon-black composite with an insulating poly(alkylacrylate) polymer. The response magnitude, fundamental noise, and signal-to-noise ratio of each sensor was measured and compared to electrode geometry, such as electrode gap, aspect ratio, and overall size. No significant dependence of sensor response magnitude and noise to electrode configuration has been observed to be larger than the variation from sensor to sensor. However, the signal-to-noise ratio tended to decrease for sensors with the smallest scales.     

聚吡咯/二氧化钛复合薄膜的制备及气敏性研究

运用静电力自组装和原位化学氧化聚合相结合的方法制备了聚吡咯/纳米二氧化钛(PPy/TiO₂)复合薄膜, 并进行了紫外－可见光谱分析和原子力显微镜分析. 采用平面叉指电极制备了PPy/TiO₂复合薄膜气体传感器, 研究了其在常温下对有毒气体NH₃和CO的敏感性. 最后测试了该传感器的温度湿度特性. 结果表明, 该传感器对NH₃具有较高的灵敏度, 对CO几乎没有响应. 同时讨论了复合薄膜沉积时间对气敏特性的影响, 实验表明当沉积时间为20min时, 该传感器的NH₃敏感特性最好.     

Ultrasensitive chemiresistors based on electrospun TiO2 nanofibers

Nanostructured semiconducting metal oxides and particularly single nanowire devices offer exceptional gas sensitivity but at the expense of statistical variations and excessive noise levels. In this study TiO2/poly(vinyl acetate) composite nanofiber mats were directly electrospun onto interdigitated Pt electrode arrays, hot pressed at 120 degrees C, and calcined at 450 degrees C. This resulted in a novel multiple nanowire network composed of sheaths of 200-500 nm diameter cores filled with readily gas accessible approximately 10 nm thick single-crystal anatase fibrils. TiO2 nanofiber sensors tested for NO2, in dry air, exhibited exceptional sensitivity showing with, for example, a 833% increase in sensor resistance when exposed to 500 ppb NO2 at 300 degrees C, consistent with a detection limit estimated to be well below 1 ppb. Unusual response patterns were observed at high NO2 concentrations (> 12.5 ppm), consistent with n to p inversion of the surface-trap limited conduction facilitated by the high surface-to-volume ratio of this material.     

Iridium oxide as low temperature NO/sub 2/-sensitive material for work function-based gas sensors

This paper presents the results on work function-based NO/sub 2/-sensing properties of iridium-oxide thin films at 130/spl deg/C. Films of 20-nm and 100-nm thickness were deposited on silicon substrates using dc sputtering followed by annealing in oxygen ambient. Sensitivity of these films to different concentrations of NO/sub 2/, H/sub 2/, CO, Cl/sub 2/, and NH/sub 3/ in synthetic air was measured using a Kelvin probe. It was observed that work function of 20-nm-thick iridium-oxide film changed by /spl sim/100 mV on exposure to 5-ppm NO/sub 2/ (German safety limit). Cross sensitivity to other gases (except NH/sub 3/) and interference of humidity was found to be negligibly small. The film was incorporated as a gate electrode in a hybrid suspended gate field effect transistor (HSGFET) structure to examine its suitability in FET-type sensors. The films were characterized using Rutherford backscattering spectroscopy, X-ray diffraction analysis, and scanning electron microscopy to determine their composition, phase, and surface morphology. The results suggest that iridium-oxide film is a promising material for the realization of a FET-based NO/sub 2/ sensor.     

Preparation of modified MWCNTs-doped PANI nanorods by oxygen plasma and their ammonia-sensing properties

Multi-wall carbon nanotubes (MWCNTs)-doped polyaniline (PANI) nanopowders were prepared by chemical oxidation polymerization. Then, the MWCNTs-doped PANI nanopowders were modified by a radio frequency (RF) oxygen plasma source. The morphology and structure of modified MWCNTs-doped PANI nanorods were analyzed by SEM and FI-IR. Gas sensors were fabricated based on plasma modified MWCNTs-doped PANI nanorods to detect ammonia at room temperature. The response amplitude of the gas sensor based on modified MWCNTs-doped PANI nanorods was much higher than those of MWCNTs-doped PANI nanopowders and pure PANI nanopowders sensors, respectively, in ammonia concentration range of 10–150 ppm. Cross responses of modified MWCNTs-doped PANI nanorods sensor to ammonia, ethanol, formaldehyde, and toluene were tested. The sensor showed good selectivity and stability. The sensing mechanism of modified MWCNTs-doped PANI nanorods gas sensor was analyzed.     

High-sensitivity NO2 detection with carbon nanotube-gold nanoparticle composite films

Composite films of single-walled carbon nanotube mesh doped with alkanethiol monolayer protected gold clusters (MPCs) have been investigated for ultrahigh sensitivity detection of nitrogen dioxide. The response to NO2 (measured as increased conductance) of the composite materials increased with MPC loading until a threshold MPC loading level was achieved, after which no further enhancement of sensor response is observed. The total of about ten droplets of MPC solution had been cast atop the SWNT mesh. The detection limit for NO2 has been improved 9.6-fold, to 4.6 ppb, compared with that obtained with pure SWNT sensors. Ultraviolet illumination helps to speed up the sensor recovery. All tests were done under ambient conditions.     

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.     

Periodically porous top electrodes on vertical nanowire arrays for highly sensitive gas detection

Nanowires of various materials and configurations have been shown to be highly effective in the detection of chemical and biological species. In this paper, we report a novel, nanosphere-enabled approach to fabricating highly sensitive gas sensors based on ordered arrays of vertically aligned silicon nanowires topped with a periodically porous top electrode. The vertical array configuration helps to greatly increase the sensitivity of the sensor while the pores in the top electrode layer significantly improve sensing response times by allowing analyte gases to pass through freely. Herein, we show highly sensitive detection to both nitrogen dioxide (NO(2)) and ammonia (NH(3)) in humidified air. NO(2) detection down to 10 parts per billion (ppb) is demonstrated and an order-of-magnitude improvement in sensor response time is shown in the detection of NH(3).     

YSZ薄膜的制备及应用

研究了在ＮＡＳＩＣＯＮ基板上使用溅射法制备的ＹＳＺ薄膜的结构,晶相和导电性,实验表明,刚制备的ＹＳＺ薄膜７５０℃常规退火处理后,薄膜表面致密、均匀、无裂纹,具备良好的导电性能,ＹＳＺ／ＮＡＳＩＣＯＮ组合再加上Ｂａ（ＮＯ３）２辅助电极构成的ＮＯｘ气体传感器在４５０℃下响应迅速（响应时间约为３ｍｉｎ）、稳定、能检测到１０ｐｐｍ量级的ＮＯ２气体。     

Design of low cost gas sensor based on SrTiO3 and BaTiO3 films

We have prepared SrTiO3/BaTiO3 multilayer film on alumina substrates by a sol-gel technique and investigated their response for sensing ethanol vapor. The surface morphology of the films were characterized by atomic force microscope (AFM) showing that the grain size of the films increase up to 40 nm as the annealing temperature increased to 1000 degrees C. The ethanol sensors based on SrTiO3/BaTiO3 thin films were fabricated by applying interdigitated gold electrodes by sputtering technique. The ethanol sensing characteristics of SrTiO3/BaTiO3 thin films were quantified by the change in resistance of the sensors when they were exposed to ethanol. The optimum operating tempearature of these sensors was found to be 350 degrees C. In addition, the film annealed at 1000 degrees C exhibited p-type gas sensing behavior with the best sensitivity of 30-100 for low ethanol concentration in the range of 100-1000 ppm.     

Gas-sensing properties of catalytically modified WO/sub 3/ with copper and vanadium for NH/sub 3/ detection

Ammonia gas detection by pure and catalytically modified WO/sub 3/-based gas sensors was analyzed. Sensor response of pure tungsten oxide to NH/sub 3/ was unsatisfactory, probably due to the unselective oxidation of ammonia into NO/sub x/. Copper and vanadium were introduced in different concentrations and the resulting material was annealed at different temperatures in order to improve the sensing properties for NH/sub 3/ detection. The introduction of Cu and V as catalytic additives improved the sensor response to NH/sub 3/. Possible reaction mechanisms of NH/sub 3/ over these materials are discussed. Sensor responses to other gases like NO/sub 2/ or CO and interference of humidity on ammonia detection were also analyzed so as to choose the best sensing element.     

Improving the thromboresistivity of chemical sensors via nitric oxide release: fabrication and in vivo evaluation of NO-releasing oxygen-sensing catheters

The development and in vivo analytical performance of a nitric oxide (NO)-releasing amperometric oxygen sensor with greatly enhanced thromboresistivity are reported. Gas permeable coatings formulated with cross-linked silicone rubber (SR) containing NO-generating compounds (diazeniumdiolates) are shown to release NO for extended periods of time (> 20 h) while reducing platelet adhesion and activation. Oxygen-sensing catheters prepared by dip-coating the NO-releasing films over the outer SR tubes of the implantable devices display similar analytical response properties in vitro (sensitivity, selectivity, response times) when compared to analogous sensors prepared without the NO release coatings. Superior analytical accuracy (relative to blood PO2 values measured in vitro) and greatly reduced thrombus formation on the outer surface of the sensors are observed in vivo (in canine model) with the NO release PO2 sensors compared to control sensors (without NO release) implanted simultaneously within the same animals. Based on these preliminary studies, the use of NO release polymers to fabricate catheter-style chemical sensors may be a potential solution to lingering biocompatibility and concomitant performance problems encountered when attempting to employ such devices for continuous intravascular measurements of blood gases and electrolytes.     

N型Ag2Te@PEDOT:PSS复合材料薄膜的制备与热电性能

利用一步热还原法制备了聚3, 4-乙烯二氧噻吩:聚苯乙烯磺酸包覆的Te纳米线(Te@PEDOT:PSS)复合膜，然后将其浸入不同浓度的AgNO₃溶液中，通过Ag+与Te的反应使Te向Ag₂Te转变，从而使热电材料由P型的Te@PEDOT:PSS复合膜转化为N型的Ag₂Te@PEDOT:PSS复合膜。通过FESEM、TEM、XPS、XRD等表征手段揭示了掺杂过程中AgNO₃与Te@PEDOT:PSS复合膜的作用机制，探究了掺杂浓度对Ag₂Te@PEDOT:PSS复合膜热电性能的影响。反应中Ag₂Te@PEDOT:PSS复合膜的电导率随着AgNO₃溶液浓度的增加呈先增大后减小的趋势，主要是由于AgNO₃浓度较大时生成TeO₂造成的，Seebeck系数随着AgNO₃浓度增大而迅速减小，主要是由于反应生成的Ag₂Te为N型传导，当Ag₂Te所提供的电子数量超过Te提供的空穴数量时，材料的传导机制由P型变为N型，即Seebeck系数由正变负，随着AgNO₃浓度的增大，Seebeck系数的绝对值变大，当AgNO₃溶液浓度为10 mmol时，Seebeck系数为(?55.9±3.3) μV/K。当AgNO₃溶液浓度为20 mmol时，N型Ag₂Te@PEDOT:PSS复合膜的功率因子达最大值，为(8.4±0.7) μW/(m·K2)。      

Hybrid Films of Graphene and Carbon Nanotubes for High Performance Chemical and Temperature Sensing Applications

A hybrid composite material of graphene and carbon nanotube (CNT) for high performance chemical and temperature sensors is reported. Integration of 1D and 2D carbon materials into hybrid carbon composites is achieved by coupling graphene and CNT through poly(ionic liquid) (PIL) mediated‐hybridization. The resulting CNT/PIL/graphene hybrid materials are explored as active materials in chemical and temperature sensors. For chemical sensing application, the hybrid composite is integrated into a chemo‐resistive sensor to detect a general class of volatile organic compounds. Compared with the graphene‐only devices, the hybrid film device showed an improved performance with high sensitivity at ppm level, low detection limit, and fast signal response/recovery. To further demonstrate the potential of the hybrid films, a temperature sensor is fabricated. The CNT/PIL/graphene hybrid materials are highly responsive to small temperature gradient with fast response, high sensitivity, and stability, which may offer a new platform for the thermoelectric temperature sensors.     

原位聚合 ( PSS/ PANI) n复合膜的静电层2层自组装

基于静电相互作用 , 从苯胺单体出发原位聚合、现场掺杂、层2层自组装制备多层聚苯乙烯磺酸钠( PSS) /聚苯胺 ( PANI) 复合膜。为了明确控制 PSS/ PANI复合膜纳米结构的因素 , 以紫外2可见 (UV2Vis) 光谱跟踪 PSS/ PANI复合膜的成膜过程 , 系统地研究了基片性质、 氧化剂用量及沉积时间等溶液因素对单个双层复合膜纳米结构的影响 , 得到了制备单个双层复合膜的较优条件。在此基础上变换苯胺单体浓度 , 制备了不同纳米结构的多层复合膜 ( PSS/ PANI) n, 并用原子力显微镜 (AFM) 、 扫描电镜 (SEM) 及椭偏仪等对该多层复合薄膜的形貌结构进行了表征。结果表明 , 通过控制沉积条件 , 每双层复合膜的厚度可控制在 40～100 nm , 电导率可达- 12. 675 mS·cm , 并且可制备增长均匀的 8 个双层的 ( PSS/ PANI) 8复合膜。热失重分析 ( TGA) 表明 , ( PSS/ PANI) n多层复合膜的热稳定性优于普通 PSS/ PANI复合膜。