Sensitive Hydrogen Sensor Research Articles

Highly sensitive hydrogen sensor based on graphite-InP or graphite-GaN Schottky barrier with electrophoretically deposited Pd nanoparticles

Depositions on surfaces of semiconductor wafers of InP and GaN were performed from isooctane colloid solutions of palladium (Pd) nanoparticles (NPs) in AOT reverse micelles. Pd NPs in evaporated colloid and in layers deposited electrophoretically were monitored by SEM. Diodes were prepared by making Schottky contacts with colloidal graphite on semiconductor surfaces previously deposited with Pd NPs and ohmic contacts on blank surfaces. Forward and reverse current-voltage characteristics of the diodes showed high rectification ratio and high Schottky barrier heights, giving evidence of very small Fermi level pinning. A large increase of current was observed after exposing diodes to flow of gas blend hydrogen in nitrogen. Current change ratio about 700,000 with 0.1% hydrogen blend was achieved, which is more than two orders-of-magnitude improvement over the best result reported previously. Hydrogen detection limit of the diodes was estimated at 1 ppm H2/N2. The diodes, besides this extremely high sensitivity, have been temporally stable and of inexpensive production. Relatively more expensive GaN diodes have potential for functionality at high temperatures.

Optimizing Hydrogen Sensing Behavior by Controlling the Coverage in Pd Nanoparticle Films

The response of quantum-conductance-based hydrogen sensors fabricated by controllable deposition of closely spaced Pd nanoparticle films between interdigital electrodes was investigated. Three typical response regions with different conductance–hydrogen pressure correlations were observed. The response characteristics of the devices were found to depend strongly on the nanoparticle coverage. In the low H2 pressure region, higher coverage gives higher sensitivity. In the high H2 pressure region, quantitative sensing can only be realized with low nanoparticle coverage. Optimizing the coverage allows the attainment of highly sensitive hydrogen sensors with a very wide quantitative working range, extending far beyond the hydrogen pressure region associated with the α-to-β phase transition of Pd.

Ultra Sensitive Fiber-Optic Hydrogen Sensor Based on High Order Cladding Mode

We demonstrate a simple but sensitive hydrogen gas sensor composed of a palladium-coated long-period fiber grating (LPG). By writing an LPG in a low core index fiber, high-order cladding modes are excited. As the palladium thin layer absorbs hydrogen, the effective refractive indexes of the cladding modes are affected, thus the resonant wavelengths of the LPG are changed with a high sensitivity. With 70-nm-thick coating, 7.5 nm of the hydrogen-induced spectral shift was achieved. The spectral response of the proposed sensor to hydrogen gas and its recovery with nitrogen gas are presented.

Highly sensitive hydrogen sensors based on co-sputtered platinum-activated tungsten oxide films

Platinum-activated tungsten oxide (Pt–WO 3) films for highly sensitive hydrogen (H 2) sensors were co-deposited using radio frequency (r.f.) sputtering with subsequent two-step heat treatments. The Pt–WO x films were annealed at 700 °C for 1 h and then thermally treated at 450 °C for 24 h or 48 h. The microstructure, crystalline phase and composition of the as-sputtered, annealed and thermally treated films were characterized with scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The sensors were tested for 30–200 ppm H 2 in air at different temperatures from 95 to 220 °C and with a relative humidity from 0 to 75%. The influences of Pt concentration and thermal treatment on the microstructure and the sensing performances of the Pt–WO 3 films are presented in this work.

Hydrogen gas sensing properties of PdO thin films with nano-sized cracks

We report on a novel method for the fabrication of highly sensitive hydrogen gas sensorsbased on palladium oxide thin films and have investigated their hydrogen sensing propertiesand nanostructures. To our knowledge, this is the first report on the use of palladium oxideand reduced palladium thin films as hydrogen sensors. The palladium oxide thin films weredeposited on thermally oxidized Si substrates using a reactive direct current (DC)magnetron sputtering system. Considerable changes in the resistance of the palladiumoxide thin films were observed when they were initially exposed to hydrogen gas, asa result of the reduction process. After the initial exposure to hydrogen gas ofPdO30%, its sensitivityincreased up to ∼ 4.5 × 103%. The morphology of the PdO surface was analyzed using a scanning electron microscope(SEM), in order to investigate the interactions between palladium oxide and hydrogen. TheSEM images showed a large number of nano-sized cracks on the surface of the palladiumoxide during the reduction process, which acted to increase the effective surface-to-volumeratio. The response behaviors of the reduced Pd films to hydrogen gas were reversible andhad an enhanced sensing property when compared with those of the pure Pd films. Inaddition, their sensitivities and response times were improved due to the nano-sizedcracks on the surfaces. The results demonstrate that palladium oxide and reducedpalladium thin films can be applied for use in highly sensitive hydrogen sensors.

Properties of gas sensors based on photochemically deposited nano‐ crystalline SnO2 films

AbstractWe fabricated highly sensitive room temperature hydrogen sensors based on SnO2 films. The films were deposited by the photochemical deposition. A solution containing SnSO4 with its pH adjusted to about 1.4 by HNO3 were dropped onto the glass substrate and irradiated with the UV light. After 5min irradiation, the substrate was washed with water and dried before dropping the new solution. The films were fabricated by repeating this process. The sample annealed at 200 °C showed current increase by a factor >104 within 1 min for 5000 ppm hydrogen at room temperature. The removal of hydrogen from the ambient caused only a small decrease in the current, but the current decreased by more than three order of magnitude almost instantly upon subsequent exposure to air. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation of highly selective, sensitive and stable hydrogen sensors based on Pd-doped tungsten trioxide

In this paper, highly selective, sensitive and stable hydrogen sensors based on Pd-doped tungsten trioxide were successfully prepared by promoting way. Two different morphologies of WO3 were obtained by chemical route synthesis (WO3 nano-particles and WO3 micro-lamellae), their morphologies were observed by SEM and TEM microscopy and their crystalline structures were determined by X-ray diffraction (XRD). To prepare Pd-doped WO3 films, palladium chloride salt/WO3 was highly dispersed in organic solution using sonication, the resultant Pd–WO3 dispersion was drop coated on the alumina substrate fitted with gold electrodes and annealed at 500∘C. The gas sensing properties of Pd-doped WO3 films to reducing gases (NH3, CO, H2) were studied at different operating temperatures. The sensors have shown high sensitivity and selectivity to hydrogen at working temperature of 200∘C. The sensors with low Pd loading (0.4 wt%) has shown the best stability and reproducibility. In these last conditions, the sensors have shown no obvious response to ammonia (≤1 ppm) and insignificant response to carbon monoxide (≤200 ppm).

Pd-Sensitized Single Vanadium Oxide Nanowires: Highly Responsive Hydrogen Sensing Based on the Metal−Insulator Transition

Exceptionally sensitive hydrogen sensors were produced using Pd-nanoparticle-decorated, single vanadium dioxide nanowires. The high-sensitivity arises from the large downward shift in the insulator to metal transition temperature following the adsorption on and incorporation of atomic hydrogen, produced by dissociative chemisorption on Pd, in the VO(2), producing approximately 1000-fold current increases. During a rapid initial process, the insulator to metal transition temperature is decreased by >10 degrees C even when exposed to trace amounts of hydrogen gas. Subsequently, hydrogen continues to diffuse into the VO(2) for several hours before saturation is achieved with only a modest change in the insulator to metal transition temperature but with a significant increase in the conductivity. The two time scales over which H-related processes occur in VO(2) likely signal the involvement of two distinct mechanisms influencing the electronic structure of the material one of which involves electron-phonon coupling pursuant to the modification of the vibrational normal modes of the solid by the introduction of H as an impurity.

Application of PdNi-coated Magnetic Ribbon to a Wireless Hydrogen Sensor

This paper describes a wireless hydrogen sensor that employs the resonance frequency change of PdNi-coated magnetic ribbons. A Pd0.9522Ni0.0478-coated magnetic ribbon exhibits a monotonous increase in resonance frequency with increases in hydrogen concentration. Because Pd absorbs hydrogen selectively and expands, the magnetic anisotropy in the ribbon changes from transversal single axis to isotropic in-plane. This weakens the ΔE effect and thus increases the resonance frequency. Consequently, PdNi-coated magnetic ribbon is confirmed as a sensitive wireless hydrogen sensor device.

Palladium–polyelectrolyte hybrid nanoparticles for hydrogen sensor in fuel cells

We prepared palladium–polyelectrolyte hybrid nanoparticles by using a metallization of polyelectrolyte. In this study, we selected polyacrylic acid (PAA) as a polyelectrolyte and reduced the palladium ions on the PAA by using ascorbic acids in order to form a unique spherically shaped mosslike hybrid nanoparticle. Palladium (Pd) can absorb hydrogen to become PdHx, and the storage of hydrogen increases the electrical resistance and volume of Pd materials. The use of this material is attracting growing interest as a reliable, cheap, ultracompact, and safe hydrogen sensor for use in fuel cells. We showed the utilization of the Pd–PAA hybrid nanoparticles as a highly sensitive hydrogen sensor that exhibited a switch response depending on volume expansion in a cyclic atmosphere exchange.

Highly sensitive hydrogen sensor based on suspended, functionalized single tungsten nanowire bridge

A highly sensitive hydrogen sensor based on suspended and functionalized single tungsten nanowire is presented. The sensing tungsten nanowire is synthesized by focused ion beam-chemical vapor deposition (FIB-CVD) using W(CO) 6 precursor gas on batch-fabricated microelectrodes. The nanowire measures 10 μm in length and 150 nm in diameter, and is suspended 50 μm above the wafer substrate connecting two microelectrodes to enhance the exposure of nanowire surface to target gas molecules for improved chemisorptions. The nanowire is then functionalized by palladium–platinum (Pd–Pt) sputtering process to be sensitive to the hydrogen molecules. The significant increase of nanowire resistance is observed whenever it is exposed to hydrogen molecules, and the resistance is recovered to the original value after hydrogen molecules are purged out. The experimentally characterized hydrogen sensor shows 11.52% increase of resistance to the exposure to 1% (10,000 ppm) hydrogen and 99% nitrogen gas mixture at room temperature and atmospheric pressure. The lowest hydrogen concentration measured is 0.001% (10 ppm) with 0.58% corresponding resistance increase.

Improving hydrogen detecting performance of a Pd/n-LTPS/glass thin film Schottky diode with a TiO2 interface layer

The hydrogen detecting performance of a Pd/ n -LTPS/glass thin film Schottky diode is significantly improved with a TiO 2 interface layer. The n -LTPS ( n -type low temperature polysilicon) thin film is an amorphous silicon (a-Si) film on a glass substrate with both excimer laser annealing and PH 3 gas plasma treating. Under the conditions of room temperature and −2 V bias, the addition of the TiO 2 interface layer promotes the relative sensitivity ratio from 290.4% to 1539.6% to 50 ppm H 2 . The response time is also reduced from 61 s to 40 s. Compared to the reported H 2 sensors with bulk Si or III–V compounds, the developed Pd/TiO 2 / n -LTPS/glass Schottky diode shows a high potential in development and realization of a low cost and highly sensitive hydrogen sensor.

Effects of Catalyst on Gasochromic Properties in Tungsten Oxide Films

Effects of catalyst on gasochromic properties in tungsten oxide films were investigated. Amorphous WO3 films coated with Pd and Pt catalysts were prepared on SiO2 substrates by employing reactive RF magnetron sputtering. To quantitatively estimate the thickness of the catalytic layer, the Rutherford backscattering spectroscopy (RBS) was employed. The coloration behavior was examined by measuring light transmittance of the samples exposed to 1% hydrogen diluted nitrogen gas. The field emission scanning electron microscope (FE-SEM) showed that particles with a dozes nm diameter of Pd and with a few nm of Pt grew on the surface of silicon substrates at beginning of deposition. The continuous catalytic layers were observed with increasing the amounts of more than 2 nm for Pd and 0.2 nm for Pt catalysts, respectively. When the continuous layers formed on WO3 surface, the excellent gasochromic properties were obtained. The results indicated that Pt catalysts were suitable for high sensitive hydrogen sensors consisting of gasochromic WO3 films.

Tin oxide nanocluster hydrogen and ammonia sensors

We have prepared sensitive hydrogen and ammonia sensors from thin films oftin nanoclusters with diameters between 3 and 10 nm. By baking the samples at200 °C in ambient air the clusters were oxidized, resulting in very stable films of tin oxide clusterswith similar diameters to the original Sn clusters. By monitoring the electrical resistance, itis shown that the cluster films are highly responsive to hydrogen and ammonia at relativelylow temperatures, thereby making them attractive for commercial applications in whichlow power consumption is required. Doping of the films by depositing Pd on top of theclusters resulted in much improved sensor response and response times. It is shown thatoptimal sensor properties are achieved for very thin cluster films (a few monolayers ofclusters).

Highly Sensitive Hydrogen Sensor Using Pt Nanoparticles Coated ZnO Single- and Multi- Nanowires

not Available.

Comprehensive study of pseudomorphic high electron mobility transistor (pHEMT)-based hydrogen sensor

Pseudomorphic high electron mobility transistors (pHEMTs) were applied to cover an extremely wide frequency range. The pHEMT structure combined with the hydrogen sensor concept was fabricated and simulated. The remarkable saturation current density explained that the sensor was suitable for operation at room temperature. In addition, the Schottky barrier height lowered was demonstrated. From the experimental and simulated data, it was found that not only the Schottky barrier height lowering but also the change of the conduction-band discontinuity (Δ E C) in a hydrogen-containing ambience influenced the characteristics of the hydrogen sensor. The related threshold voltage change of the hydrogen sensor was also manifested. Furthermore, two important parameters of pHEMT, i.e., on-state output drain current and off-state breakdown characteristics, under exposing to a hydrogen ambience were also discussed. The studied device was shown to be a candidate for a high-power and highly sensitive hydrogen sensor and pHEMT integrated circuit applications.

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.

Development of a highly sensitive porous Si-based hydrogen sensor using Pd nano-structures

A novel, resistance-based porous silicon sensor with Pd nano-structures as hydrogen sensing layer is presented. The sensor operates at room temperature. p-Type Si substrate is subjected to porous Si etching. The substrate is then coated with a thin layer of Pd and annealed at 900 °C. This results in some Pd getting oxidized on porous Si and a thin PdO layer forms on the surface of the substrate. The sensor was tested in the range of 0–1.5% hydrogen. The sensor responded in real time. Unlike conventional thin film-based resistive hydrogen sensors this sensor showed an inverse relationship between increased hydrogen concentration versus resistance. The mechanism driving the changed output is discussed.

In-line highly sensitive hydrogen sensor based on palladium-coated single-mode tapered fibers

We report a fiber-optic sensor for detecting low concentrations of hydrogen. The sensor is based on the absorption change of the evanescent fields in a Pd-coated, single-mode tapered fiber. We fabricated a collection of polarization-sensitive and polarization-insensitive devices and we studied the effect of the different parameters on the sensitivity of the device. We found that the sensor's response is wavelength dependent and it can be adjusted with the taper diameter. Dramatic transmission changes were observed when the sensors were exposed to hydrogen concentrations below 4%. We complement the experimental results with theoretical calculations of the attenuation constant of the fundamental fiber-mode.

Extreme Changes in the Electrical Resistance of Titania Nanotubes with Hydrogen Exposure

Extremely sensitive, drift‐free, and robust hydrogen sensors based on titania nanotube arrays (see Figure) are reported. Very large changes in the electrical conductivity of the nanotubes upon exposure to hydrogen gas have been measured, over one order of magnitude greater than the previous record. Dependence of the electrical conductivity on both temperature and nanotube pore size is found.