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.     

The effect of carbon nanotube dispersion on CO gas sensing characteristics of polyaniline gas sensor

Polyaniline is one of the most promising conducting polymers for gas sensing applications due to its relatively high stability and n or p type doping capability. However, the conventionally doped polyaniline still exhibits relatively high resistivity, which causes difficulty in gas sensing measurement. In this work, the effect of carbon nanotube (CNT) dispersion on CO gas sensing characteristics of polyaniline gas sensor is studied. The carbon nanotube was synthesized by Chemical Vapor Deposition (CVD) using acetylene and argon gases at 600 degrees C. The Maleic acid doped Emeradine based polyaniline was synthesized by chemical polymerization of aniline. CNT was then added and dispersed in the solution by ultrasonication and deposited on to interdigitated AI electrode by solvent casting. The sensors were tested for CO sensing at room temperature with CO concentrations in the range of 100-1000 ppm. It was found that the gas sensing characteristics of polyaniline based gas sensor were considerably improved with the inclusion of CNT in polyaniline. The sensitivity was increased and response/recovery times were reduced by more than the factor of 2. The results, therefore, suggest that the inclusion of CNT in MA-doped polyaniline is a promising method for achieving a conductive polymer gas sensor with good sensitivity, fast response, low-concentration detection and room-operating-temperature capability.     

Enhanced photocleavage of water using titania nanotube arrays

In this study highly ordered titania nanotube arrays of variable wall thickness are used to photocleave water under ultraviolet irradiation. We demonstrate that the wall thickness and length of the nanotubes can be controlled via anodization bath temperature. We find that the nanotube wall thickness is a key parameter influencing the magnitude of the photoanodic response and the overall efficiency of the water-splitting reaction. For 22 nm inner pore diameter nanotube arrays, those fabricated in a 5 degrees C anodization bath, 224 nm length and 34 nm wall thickness produced a photoanodic response that was thrice that of a nanotube array fabricated in a 50 degrees C anodization bath, 120 nm length and 9 nm wall-thickness. At high anodic polarization, above 1 V, the quantum efficiency under 337 nm illumination was greater than 90%. For the 5 degrees C anodization bath samples (22 nm pore-diameter, 34 nm wall thickness), upon 320-400 nm illumination at an intensity of 100 mW/cm(2), hydrogen gas was generated at the power-time normalized rate of 960 micromol/h W (24 mL/h W) at an overall conversion efficiency of 6.8%. To the best of our knowledge, this hydrogen generation rate is the highest reported for a titania-based photoelectrochemical cell.     

Fabrication of hydrogen sensors with transparent titanium oxide nanotube-array thin films as sensing elements

Transparent thin films comprised of highly ordered titania nanotube-arrays were grown from titanium thin films using an anodization technique, from which highly sensitive and selective hydrogen sensors that can operate at room temperature were fabricated. Titanium films sputter deposited on glass at 500 °C were anodized in a fluorine-containing electrolyte to obtain nanotube-array films. Precise monitoring of current during the anodization enabled removal of the samples from the anodization bath at a point where the remaining metal layer became discontinuous, without destroying the nanotube architecture. The samples were then annealed in oxygen at 420 °C to crystallize the nanotube-arrays as well as oxidize any un-anodized metallic regions, yielding transparent films comprised of titanium oxide nanotube-arrays. Herein, we discuss the morphology, structure and optical characterization of these films. When coated with a 10-nm discontinuous palladium layer, the optically transparent nanotube-array films serve as excellent hydrogen sensors, exhibiting a four-order magnitude drop in resistance with exposure to 1000 ppm hydrogen at room temperature.     

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.     

Temperature effects on resistance of aligned multiwalled carbon nanotube films

Electrical transport in vertically aligned films of multiwalled carbon nanotubes has been investigated in the -150 degrees C to 300 degrees C temperature range (all the tests were conducted in air at atmospheric pressure). In all the cases, the nanotube film exhibited a semi-conducting behavior, with the film resistance decreasing with increasing temperature. Removal of amorphous carbon contamination (via plasma etching) significantly improved the nanotube film's sensitivity to temperature changes (particularly in the 20 degrees C to 200 degrees C temperature range). All the of films tested in this study showed a consistent, repeatable behavior that was independent of the nanotube film length. The temperature sensitivity of the nanotube films was also found to be independent of the heating/cooling rates and without hysteresis. Because of the excellent repeatability and stability of the results, it is conceived that miniaturized temperature sensors could be designed using such aligned multiwalled nanotube films.     

Tuning Array Morphology for High‐Strength Carbon‐Nanotube Fibers

Vertically aligned carbon‐nanotube arrays are synthesized by chemical vapor deposition. Carbon‐nanotube fibers are directly spun from the obtained nanotube arrays and then tested mechanically. A strong correlation between the array morphologies and the mechanical properties of the fibers is observed: well‐aligned arrays yield fibers with much higher performance, while wavy and entangled arrays give poor fiber properties. More importantly, such array morphologies could be controlled by introducing hydrogen or oxygen during the nanotube synthesis. By simply switching the growth condition from 150 ppm oxygen addition to 2% hydrogen addition, the nanotube array changes from the wavy morphology to the well‐aligned morphology, and correspondingly the tensile strength of the resultant fibers could be increased by 4.5 times, from 0.29 GPa for the fibers spun from the oxygen‐assistance‐grown nanotube arrays to 1.3 GPa for the fibers spun from the hydrogen‐assistance‐grown nanotube arrays. The detailed effects of hydrogen and oxygen on the nanotube growth, especially on the growth rate and the array spinnability, are extensively studied. The formation mechanism of the different morphologies of the nanotube arrays and the failure mechanism of the nanotube fibers are also discussed in detail.     

Palladium mesowire arrays for fast hydrogen sensors and hydrogen-actuated switches

Arrays of mesoscopic palladium wires prepared by electrodeposition form the basis for hydrogen sensors and hydrogen-actuated switches that exhibit a response time ranging from 20 ms to 5 s, depending on the hydrogen concentration. These devices were constructed by electrodepositing palladium mesowires on a highly oriented pyrolytic graphite surface and then transferring these mesowires to a cyanoacrylate film supported on a glass slide. The application of silver contacts to the ends of 10-100 mesowires, arrayed electrically in parallel, produced sensors and switches that exhibited a high conductivity state in the presence of hydrogen and a low conductivity state in the absence of hydrogen. After an initial exposure to hydrogen, 15-50 nanoscopic gaps are formed in each mesowire. These nanoscopic gaps or "break junctions" close in the presence of hydrogen gas and reopen in its absence as hydrogen is reversibly occluded by the palladium grains in each wire, and the palladium lattice expands and contracts by several percent. The change in resistance for sensors and switches was related to the hydrogen concentration over a range from 1 to 10%.     

Enhanced gas sensing performance of SnO2/α-MoO3 heterostructure nanobelts

Extremely high sensitivity and low working temperature of gas sensors are realized from SnO(2)/α-MoO(3) heterostructure nanobelts. Their sensitivity against 500 ppm ethanol is up to 67.76 at the working temperature of 300?°C, which is higher than that of bare α-MoO(3) and SnO(2) nanostructures. Also the working temperature can be lowered down to 120?°C. Such behaviors are attributed to the variation of the junction barrier at the SnO(2)/α-MoO(3) interface. The present results imply that heterostructured 1D nanomaterials may yield gas sensors with improved characteristics, and can be applied to a wide range of gas sensors.     

Oxide Nanobelts as Conductometric Gas Sensors

Semiconducting oxide nanobelts have been obtained by vapor phase deposition. In this work we present the results obtained using tin oxide nanobelts as conductometric gas sensors. Electrical characterization showed that the nanobelts were sensitive to oxygen and environmental polluting species such as CO as well as ethanol for breath analyzers and food control applications. The sensor response, defined as the relative variation in conductance or resistance due to the introduction of the gas, is 200% for 30 ppm of CO at 623 K and 2500% for 10 ppm of ethanol at 623 K. We have studied the variation of the response as a function of the density of the nanobelts. The results demonstrate the potential of fabricating nanosize sensors using the integrity of a single nanobelt with sensitivity at the level of a few ppb and the necessity to control nanobelts density to optimize the sensing performances.     

Ethanol and hydrogen sensors based on ZnO nanoparticles and nanowires

Hydrogen sensing characteristics of thick films of nanoparticles of ZnO and 3% Mn or Co doped ZnO with and without impregnation of 1% Pt have been investigated over a wide concentration range. The sensor characteristics of the Pt impregnated ZnO and 3% Mn or Co-doped ZnO nanoparticles are found to be excellent with the sensitivity reaching values in the 1000-1500 range for 1000 ppm of H2 at 125 degrees C or lower. The sensitivity is quite high even for 10 ppm of H2. The response and recovery times are generally less than 10 s in the region of high sensitivity. These films exhibit very good sensing characteristics for ethanol, the sensitivity being well above 2000 at 50 degrees C for 1000 ppm of vapour. Nanowires of Pt-impregnated ZnO also exhibit satisfactory sensing characteristics, but the values of the sensitivity are somewhat lower. Humidity has negligible effect on the performance of these sensors.     

Synthesis of mesoporous tin oxide and its application as a LNG sensor

The nanostructured SnO2 gas sensor with Au electrodes and Pt heater has been fabricated as one unit via screen printing process. The gas sensor was tested for CH4 sensing behavior at 350 degrees C in the concentration range of 500-10,000 ppm. Those mesoporous SnO2 sensors exhibited the similar sensoring properties in CH4 and CO detection. The fast speed of response and high sensitivity were obtained for mesoporous tin oxide sensor as compared to non-porous one.     

Wide-range hydrogen sensing with Nb-doped TiO2 nanotubes

Anatase-type titania nanotubes doped with Nb element were fabricated through an anodization of Ti35Nb alloy substrate and further annealing at 450 °C. Hydrogen sensitivity of the Nb-doped TiO(2) nanotubes at room temperature was investigated through exposure of the nanotube samples to different hydrogen atmospheres. At room temperature, the Nb-doped nanotubes demonstrated a good sensitivity for wide-range detection of both dilute and high-concentration hydrogen atmospheres ranging from 50 ppm to 2% H(2). The Nb-doped nanotubes also presented remarkable reversibility and repeatability as well as a quick response to the hydrogen atmosphere. The Nb-doped titania nanotubes have great advantages as robust and wide-range hydrogen sensors operating at room temperature.     

Amorphous TiO(2) nanotube arrays for low-temperature oxygen sensors

Titania nanotube arrays (TNTA) were synthesized on a titanium substrate using anodic oxidation in an electrolyte containing ammonium fluoride and evaluated for low-temperature oxygen sensing. Their sensing properties were tested at different temperatures (50, 100, 150, 200, 250 and 300?°C) when exposed to various oxygen concentrations. The as-prepared TNTA are amorphous and exhibit much higher carrier concentration than that of annealed TNTA. Such amorphous TNTA show much higher sensitivity than that of annealed TNTA, SrTiO(3) and Ga(2)O(3) sensors. This sample demonstrates the lowest detectable oxygen concentration of 200 ppm, excellent recovery and good linear correlation at 100?°C. These results indicate that TNTA are indeed very attractive oxygen-sensing materials.     

Carbon nanotube-TiO(2) hybrid films for detecting traces of O(2)

Hybrid titania films have been prepared using an adapted sol-gel method for obtaining well-dispersed hydrogen plasma-treated multiwall carbon nanotubes in either pure titania or Nb-doped titania. The drop-coating method has been used to fabricate resistive oxygen sensors based on titania or on titania and carbon nanotube hybrids. Morphology and composition studies have revealed that the dispersion of low amounts of carbon nanotubes within the titania matrix does not significantly alter its crystallization behaviour. The gas sensitivity studies performed on the different samples have shown that the hybrid layers based on titania and carbon nanotubes possess an unprecedented responsiveness towards oxygen (i.e.?more than four times higher than that shown by optimized Nb-doped TiO(2) films). Furthermore, hybrid sensors containing carbon nanotubes respond at significantly lower operating temperatures than their non-hybrid counterparts. These new hybrid sensors show a strong potential for monitoring traces of oxygen (i.e.?≤10?ppm) in a flow of CO(2), which is of interest for the beverage industry.     

Surfactant assisted solid-state synthesis and gas sensor application of a SWCNT/SnO2 nanocomposite material

Although tin oxide has been the most widely investigated metal oxide material for gas detection, it suffers from the large resistance and high operating temperature. This could be overcome by hybridization with nanostructured carbon. In this work, tin oxide nanoparticles with ultrasmall sizes of 1-3 nm have been uniformly coated onto bundles of single-walled carbon nanotubes by a surfactant assisted solid state synthesis approach for the first time. Gas sensor properties of the as-synthesized nanocomposite material toward NO2 (from 5 to 60 ppm) are measured at 150 degrees C. Compared to the pure carbon tubes gas sensors, the nanocomposite gas sensor responds to NO2 in low concentrations with good linearity, high sensitivity, and fast recovery, while working at a relatively low temperature.     

双通TiO2纳米管阵列膜制备及应用研究

双通TiO_2纳米管阵列膜的成功制备可有效提高TiO_2纳米管阵列膜的光催化活性,同时还可扩展其在太阳能电池、气体敏感器等方面的应用,具有很强的实际意义。在大量文献调研的基础上,综述了双通TiO_2纳米管阵列膜的阳极氧化制备方法及其在催化与制氢、氢敏特性、分子过滤等方面的应用进展,并侧重讨论了双通TiO_2纳米管阵列膜的分离机制。最后,对双通TiO_2纳米管阵列膜的后续发展方向和应用前景进行了展望。     

Comparison of gas sensing properties based on hollow and porous α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanotubes

Hollow and porous α-Fe₂O₃ nanotubes were successfully synthesized by single nozzle electrospinning method followed by annealing treatment. The crystal structures and morphologies of the as-prepared materials were characterized by X-ray diffraction and scanning electron microscopy, respectively. The as-prepared materials were applied to construct gas sensor devices which gas sensing properties were further investigated. The obtained results revealed that porous α-Fe₂O₃ nanotube gas sensors exhibit a markedly enhanced gas sensing performance compared with hollow α-Fe₂O₃ nanotube gas sensors, which was about three times higher to 100 ppm acetone at 240 °C. Interestingly, hollow and porous α-Fe₂O₃ nanotube gas sensors both showed fast response–recovery time and good selectivity, but the porous ones possessed the shorter recovery time. The improved properties could be attributed to the unique morphology of porous nanotubes. Thus, further improvement of performance in metal-oxide-semiconductors materials could be realized by preparation the unique porous structures of nanotubes. Moreover, it is expected that porous metal-oxide-semiconductors nanotubes could be further design as promising candidates for gas sensing materials.     

Electrochemical fabrication of titania nanotube arrays with tunning nature of dimethyl sulfoxide and its application for hydrogen sensing

In the present work, tuning effects of dimethyl sulphoxide (DMSO) on the length, wall thickness, dimension and morphology of titania nanotube arrays fabricated by anodization was investigated. DMSO presented in both ethylene glycol and glycerol electrolytes provided excellent tunability in length, wall thickness and diameter of the produced TiO2 nanotube arrays by systematically varying the DMSO concentration. At the DMSO concentration of 12 wt% in ethylene glycol, the nanotube length could be up to 13.0 microm at 20 V for 8 h anodization. The TiO2 nanotube arrays produced in DMSO containing ethylene glycol or glycerol electrolytes displayed a high sensitivity to hydrogen at room temperature compared to the absence of DMSO. The further enhancement of resistance response could be achieved by coating a Pt and Pd layer on the surface of TiO2 nanotube-arrays. The presented work provided a simple way to control formation of TiO2 nanotube arrays through the tuning effects of DMSO in ethylene glycol or glycerol electrolytes.     

Trace detection of dissolved hydrogen gas in oil using a palladium nanowire array

The electrical resistance, R, of an array of 30 palladium nanowires is used to detect the concentration of dissolved hydrogen gas (H(2)) in transformer oil over the temperature range from 21 to 70 °C. The palladium nanowire array (PdNWA), consisting of Pd nanowires ～100 nm (width), ～20 nm (height), and 100 μm (length), was prepared using the lithographically patterned nanowire electrodeposition (LPNE) method. The R of the PdNWA increased by up to 8% upon exposure to dissolved H(2) at concentrations above 1.0 ppm and up to 2940 ppm at 21 °C. The measured limit-of-detection for dissolved H(2) was 1.0 ppm at 21 °C and 1.6 ppm at 70 °C. The increase in resistance induced by exposure to H(2) was linear with [H(2)](oil)(1/2) across this concentration range. A PdNWA sensor operating in flowing transformer oil has functioned continuously for 150 days.