Comprehensive investigation on planar type of Pd–GaN hydrogen sensors

This paper reviews both static and dynamic characteristics of a planar-type Pd–GaN metal–semiconductor–metal (MSM) hydrogen sensor. The sensing mechanism of a metal–semiconductor (MS) hydrogen sensor was firstly reviewed to realize the sensing mechanism of the proposed sensor. Symmetrically bi-directional current–voltage characteristics associated with our sensor were indicative of easily integrating with other electrical/optical devices. In addition to the sensing current, the sensing voltage was also used as detecting signals in this work. With regard to sensing currents (sensing voltages), the proposed sensor was biased at a constant voltage (current) in a wide range of hydrogen concentration from 2.13 to 10,100 ppm H₂/N₂. Experimental results reveal that the proposed sensor exhibits effective barrier height variations (sensing responses) of 134 (173) and 20 mV (1) at 10,100 and 2.13 ppm H₂/N₂, respectively. A sensing voltage variation as large as 18 V was obtained at 10,100 ppm H₂/N₂, which is the highest value ever reported. If an accepted sensing voltage variation is larger than 3 (5) V, the detecting limit is 49.1 (98.9) ppm. Moreover, voltage transient response and current transient response to various hydrogen-containing gases were experimentally studied. The new finding is that the former response time is shorter than the latter one. Other dynamic measurements by switching voltage polarity and/or continuously changing hydrogen concentration were addressed, showing the proposed sensor is a good candidate for commonly used MS sensors.     

Hydrogen sensing characteristics of a Pd/Nickel oxide/GaN-based Schottky diode

Hydrogen sensing characteristics of a novel metal-oxide-semiconductor (MOS) Schottky diode are thoroughly investigated. The MOS structure consists of a gallium nitride (GaN)-based semiconductor system, a nickel oxide (NiO) layer, and palladium (Pd) catalytic materials. A well-prepared Pd/NiO/GaN-based diode shows several advantages in relation to hydrogen sensing, including a simple structure, high sensing speed, wide flexibility for operation under both forward and reverse applied voltages, and a good sensing response of 8.1 × 10³ under an applied forward voltage of 0.25 V, at 300 K in a 1% H₂/air ambience. Furthermore, under an applied reverse voltage of −2 V and at a high temperature of 573 K, this MOS diode shows a response as high as 1.8 × 10⁴ towards 1% H₂/air mixture gas. The Schottky diode sensor with a novel Pd/NiO/GaN structure demonstrated in this study is a promising candidate for high-performance hydrogen sensing applications.     

Sensing properties of resistive-type hydrogen sensors with a Pd–SiO2 thin-film mixture

Zigzag-shaped pure-Pd thin film and Pd–SiO₂ thin-film mixture as resistive-type hydrogen sensors were deposited on cover-glass substrates through a multiple-boat thermal evaporator. Temperature dependence of the resistance of the pure-Pd resistive-type sensor showed a relative sensitivity of 3.2% at 80 °C with a temperature coefficient of the resistance (TCR) of 0.058%/°C. Sensing properties of the Pd–SiO₂ resistive-type sensor responding to the presence of 1% H₂/N₂ are much better than those of the pure-Pd one, including a higher relative sensitivity (9%–7.7%), a faster response time (10 s–30 s), and a lower detection concentration limit (50 ppm–100 ppm). A higher dissociation rate and a faster diffusion rate due to porous-like properties and more hydrogen atoms caught due to oxygen associated with the Pd–SiO₂ thin-film mixture explain why the Pd–SiO₂ resistive-type sensor has a higher relative sensitivity with a shorter response time.     

High optical response NiO,Pd/NiO and Pd/WO3 hydrogen sensors

In this study, NiO and WO₃ oxide semiconductors were fabricated on glass substrates by RF Magnetron Sputtering technique. Structural and optical characterizations of the semiconductors were performed using XRD, SEM, and optical absorption measurements. NiO and WO₃ thin films were occasionally coated with palladium. In order to investigate the optical response of these semiconductors under hydrogen gas exposure, an optical gas sensor test system was installed and programmed. In both of the coated and uncoated cases, optical absorption changes due to hydrogen gas exposure on the surface were investigated. It was observed that these changes occur between 450 and 850 nm wave lengths range. The absorption in the NiO semiconductor was reduced between these wave lengths, while the absorption was increased in the WO₃ semiconductor. In the uncoated state, only NiO gave an optical response to hydrogen gas. While the palladium coated NiO (Pd/NiO) sensor had the best response and recovery times of respectively 70 s and 206 s for 2% fraction of H₂ gas at 300 °C constant temperature, the Pd/WO₃ sensor gave the best response time of 340 s. Palladium coating resulted in approximately 150% increase in the responses of the NiO sensors at higher H₂ concentration. The lower limit of H₂ sensing of the Pd/NiO sensors at 300 °C was at the H₂ fraction of 0.05%, while for Pd/WO₃ sensors this value was 0.025%.     

Hydrogen gas sensing based on polyaniline/anatase titania nanocomposite

Polyaniline (emeraldine)/anatase TiO₂ nanocomposite (PA-NC) was prepared by a chemical oxidative polymerization. The thin films of PA-NC for hydrogen gas sensing application were deposited on Cu-interdigited electrodes by spin coating technique. A study on characteristics of PA-NC thin films was demonstrated by a porous cylindrical morphology. The response and response/recovery time of sensors for hydrogen gas were evaluated by the change of TiO₂ wt% at environmental conditions. Resistance-sensing measurement was exhibited a high sensitivity about 1.63, a good Long-term response, low response time and recovery time about 83 s and 130 s, respectively, at 0.8 vol% hydrogen gas for PA-NC including 25% wt of anatase nanoparticles. The current–voltage characteristics of PA-NC gas sensors before and after hydrogen gas injection showed a nonlinear ohmic current. Moreover, we studied the formation of PA-NCs and their hydrogen gas sensing mechanism based on contact regions in PA-NC.     

An excellent room-temperature hydrogen sensor based on titania nanotube-arrays

Hydrogen sensors have been fabricated from highly ordered TiO₂ nanotube arrays through anodization of a Ti substrate in an ethylene glycol solution containing NH₄F. The vertically oriented TiO₂ nanotube arrays containing Pt electrodes exhibit an ability to detect a wide-range of hydrogen concentrations at room temperature. On exposure to 2000 ppm (parts per million) hydrogen, the sensors exhibit seven orders of magnitude change in resistance with a response time of 13 s at room temperature. The TiO₂ nanotube arrays sensor equipped with Pt electrodes exhibited a diode-type current–voltage (I–V) characteristic in air, but nearly ohmic behavior in hydrogen balanced with argon. A significant response to hydrogen was observed without the presence of oxygen in the base atmosphere. The response of two kinds of sensors with either Pt or Pt/Ti electrodes to 500 ppm hydrogen was measured and the results suggested that the excellent hydrogen sensing properties in air resulted primarily from the variation of the Schottky barrier height at the Pt/TiO₂ interface.     

MEMS-microhotplate-based hydrogen gas sensor utilizing the nanostructured porous-anodic-alumina-supported WO3 active layer

Practical microsensors for fast, highly sensitive hydrogen gas detection were fabricated by combining silicon integral technology for MEMS microhotplate platform with newly developed technological, electrical, and electrolytic conditions for forming nanostructured porous-anodic-alumina-templated WO₃ layer as the sensing material. The morphology–structure–property relationship for the nanostructured sensing layer was determined by scanning electron microscopy, X-ray diffraction, and through systematically investigating the sensor performance at various H₂ concentrations (5–1000 ppm) and operating temperatures (20–350 °C). The sensors showed superior sensitivity to hydrogen gas, with the lowest detection limit ever reported for WO₃ semiconductors (5 ppm), the fast response and recovery times (2–3 min), and the best sensitivity at 150 °C, which was 100 times higher than that of a reference sensor having a smooth WO₃ active film. The technology developed enables high-volume, low-cost, and low-power sensor-on-a-chip solution for a hydrogen-based energy economy where the use of highly sensitive and low-power-consuming devices is encouraged.     

MOS hydrogen sensor with very fast response based on ultra-thin thermal SiO2 film

An MOS capacitor-type hydrogen gas sensor was fabricated with the structure of Ni/SiO₂/Si by using conventional silicon wafer technologies. Grown by dry oxidation at 900°C, the thickness of the SiO₂ film was only 24 Å. At 150°C, comparing to another MOS capacitor with 148 Å-thick oxide and otherwise identical configurations, this sensor showed much faster response speed (the time interval to reach half of the magnitude of the steady-state signal, or t_50%, was only 4 s in response to 1% H₂ without deduction of the delay from the gas delivery system), as well as enhanced signal magnitude (about two times of the former for 1% H₂). Based on the hydrogen-binding to the traps in the bulk SiO₂, a mechanism was proposed to explain the very short response time on the device with the ultra-thin SiO₂. The gate leakage in the device is also discussed. The presented sensor demonstrates a promising step in designing low-cost H₂ detectors with very fast responses.     

Investigation of WO3/ZnO thin-film heterojunction-based Schottky diodes for H2 gas sensing

A comparative study of Schottky diode hydrogen gas sensors based on Pd/WO₃/Si and Pd/WO₃/ZnO/Si structure is presented in this work. Atomic force microscopy and X-ray photoelectron spectroscopy reveal that the WO₃ sensing layer grown on ZnO has a rougher surface and better stoichiometric composition than the one grown on the Si substrate. Analysis of the I–V characteristics and dynamic response of the two sensors when exposed to different hydrogen concentrations and various temperatures indicate that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10 times higher sensitivity, and shorter response and recovery times (105 s and 25 s, respectively) towards 10,000-ppm H₂/air at 423 K. Study on the energy band diagram of the diode suggests that the barrier height is modulated by the WO₃/ZnO heterojunction, which could be verified by the symmetrical sensing properties of the Pd/WO₃/ZnO/Si gas sensor with respect to applied voltage.     

Hydrogen sensing characteristics of Pd/SiO2-nanoparticles (NPs)/AlGaN metal-oxide-semiconductor (MOS) diodes

A Pd/SiO₂-nanoparticles (NPs)/AlGaN metal-oxide-semiconductor (MOS) structure is used to fabricate interesting Schottky diode-type hydrogen sensors. The employment of SiO₂-NPs could effectively increase the specific surface area of Pd catalytic metal and the Schottky barrier height. Good hydrogen sensing performance is obtained. Experimentally, as compared to a conventional Pd/AlGaN MS diode, a significant 34.5-fold improvement on hydrogen sensing response is obtained under an introduced 1% H₂/air gas at 300 K when a 10 wt% concentration of SiO₂-NPs is employed in the studied device. Yet, the increase in SiO₂-NP concentration relatively deteriorates the ability to detect very low hydrogen concentration levels (≦1 ppm H₂/air). In addition, the increase in SiO₂-NP concentration creates a decrease and increase on response and recovery time constants of transient behaviors, respectively.     

Study of sensitivity and response kinetics changes for SnO2 thin-film hydrogen sensors

We present results of investigations devoted to searching of ways for sol–gel derived SnO₂ thin-film hydrogen sensors performance improvement. To that end we studied the as-prepared sensors parameters changes in real time during short- and long-term operation in gas–air mixture and known operating temperature conditions. It was established that in comparison with as-prepared sensor parameters such initial operation mode leads to the rise of the sensors sensitivity on more than two orders of magnitude, improvement of the sensor parameters stability in time. Obtained experimental data are in a good accordance with known theoretical results.     

Flexible hydrogen gas sensor based on a capacitor-like Pt/TiO2/Pt structure on polyimide foil

Metal oxide semiconductor gas sensors of hydrogen with a typical capacitor-like Pt/TiO₂/Pt electrode arrangement exhibit excellent sensitivity to hydrogen even at room temperature. At the same time, very similar Pt/TiO₂/Pt cells can also be used as memristive elements exhibiting resistive switching between two resistive states, which has been recently exploited to create a gas sensor with built-in memory. Merging of these two functionalities within a single device also opens new possibilities for smart gas sensor arrays. However, so far such sensors have been prepared only on rigid substrates. In this work, a flexible hydrogen gas sensor with such capacitor-like Pt/TiO₂/Pt electrode arrangement fabricated on polyimide foil is presented and characterized in terms of hydrogen gas sensing properties and bending endurance. The sensor exhibits high response (R_air/R_H2) of more than 10⁵ to 10 000 ppm H₂ at 150 °C with minor decline at elevated humidity and is capable of room temperature operation. The lowest detected concentration was 3 ppm at 150 °C and 300 ppm at room temperature in dry conditions. Bending the sensor 10⁵ times over diameter of 10 mm led to slight improvement of the sensing performance.     

Hydrogen gas sensor based on mesoporous In2O3 with fast response/recovery and ppb level detection limit

Hydrogen gas sensors were fabricated using mesoporous In₂O₃ synthesized using hydrothermal reaction and calcination processes. Their best performance for the hydrogen detection was found at a working temperature of 260 °C with a high response of 18.0 toward 500 ppm hydrogen, fast response/recovery times (e.g. 1.7 s/1.5 s for 500 ppm hydrogen), and a low detection limit down to 10 ppb. Using air as the carrier gas, the mesoporous In₂O₃ sensors exhibited good reversibility and repeatability towards hydrogen gas. They also showed a good selectivity for hydrogen compared to other commonly investigated gases including NH₃, CO, ethyl alcohol, ethyl acetate, styrene, CH₂Cl₂ and formaldehyde. In addition, the sensors showed good long-term stability. The good sensing performance of these hydrogen sensors is attributed to the formation of mesoporous structures, large specific surface areas and numerous chemisorbed oxygen ions on the surfaces of the mesoporous In₂O₃.     

A novel hydrogen-sensitive sensor based on Pd nanorings/TNTs composite structure

Hydrogen sensors with a novel composite structure comprised of Pd nanorings distributed on TiO₂ nanotube arrays were developed and tested. Effect of the TiO₂ nanotube diameter size, Pd nanorings thickness on the sensors' hydrogen response characteristics were investigated. Time dependence of resistance of the Pd nanorings/TNTs composite structure on various hydrogen concentrations was also carried out and demonstrated good room temperature hydrogen sensitive characteristics. Optimized experiments demonstrated that the hydrogen sensor composed of 25 nm-thickness Pd nanorings distributed on the 77 nm-diameter size TiO₂ nanotube showed a fast response time (3.8 s) and high sensitivity (92.05%) at 0.8 vol% H₂. A hydrogen sensitive characteristics model is proposed and the Pd nanorings' important role in the hydrogen sensitive mechanisms is described. The hydrogen sensor's excellent hydrogen sensitive characteristics is ascribed to the Pd nanorings' quick and continual formation and breakage of multiple passages due to absorption and desorption of hydrogen atoms.     

Pd–WO3/reduced graphene oxide hierarchical nanostructures as efficient hydrogen gas sensors

Pd–WO₃ nanostructures were incorporated on graphene oxide (GO) and partially reduced graphene oxide (PRGO) sheets using a controlled hydrothermal process to fabricate effective hydrogen gas sensors. Pd–WO₃ nanostructures showed ribbon-like morphologies and Pd–WO₃/GO presented an irregular nanostructured form, while Pd–WO₃/PRGO exhibited a hierarchical nanostructure with a high surface area. Gas sensing properties of thin films of these materials were studied for different hydrogen concentrations (from 20 to 10,000 ppm) at various temperatures (from room temperature to 250 °C). Although adding GO in the Pd–WO₃, after hydrothermal process could increase the film conductivity, gas sensitivity was reduced to half, due to lower surface area of the irregular morphology in comparison with the ribbon-like morphology. The Pd–WO₃/PRGO films showed an optimum sensitivity (∼10 folds better than the sensitivity of Pd–WO₃/GO), and a fast response and recovery time (<1 min) at low temperature of 100 °C. Moreover, the Pd–WO₃/PRGO-based gas sensor was sensitive to 20 ppm concentration of hydrogen gas at room temperature. The results confirmed the effect of residual oxygen-containing functional groups of PRGO on the growth and morphology of Pd–WO₃ as well as gas sensing properties of metal oxide/graphene based hybrid nanostructures.     

Pd/Ag alloy as an application for hydrogen sensing

A highly sensitive H₂ gas sensor was fabricated using a Micro Electromechanical Systems (MEMS) procedure having an embedded micro-heater. The palladium-silver (Pd/Ag having stoichiometric ratios 77:23) thin film was deposited by the RF/DC magnetron sputtering and used as the hydrogen sensing layer designed as a zig-zag pattern. Morphological and structural properties of the Pd/Ag thin film was studied by Field emission scanning electron microscope (FESEM), Atomic force microscopy (AFM) and Energy Dispersive Analysis of X-rays respectively. The working temperature of the micro heater showed a linear relation with variations of the heater voltage. The electro thermal properties of the H₂ sensor were studied by finite element method (FEM). The sensing properties of the fabricated H₂ sensor as the change of electrical resistance were studied with respect to hydrogen concentration and temperature. Experimental results showed high sensor response and response time after application of the heater voltage. The sensing properties of the alloyed Pd/Ag thin film were more improved than those of pure palladium. The maximum sensor response (R_s) of the fabricated H₂ sensor was 14.26% for 1000 ppm H₂. The sensor response of the fabricated H₂ sensor showed linear behavior with the heater voltage (operating temperature) and positively corresponded with the hydrogen concentration.     

Facial fabrication of an inorganic/organic thermoelectric nanocomposite based gas sensor for hydrogen detection with wide range and reliability

A novel method for fabrication of a thermochemical hydrogen (TCH) gas sensor composed of platinum (Pt)-decorated graphene sheets and a thermoelectric (TE) polymer nanocomposite was investigated. The hydrogen sensing characterization for the device included gas response, response time (T₉₀), recovery time (D₁₀), and reliability testing, which were systematically conducted at room temperature with a relative humidity of 55%. Here, the Pt-decorated graphene sheets act as both an effective hydrogen oxidation surface and a heat-transfer TE polymer nanocomposite having low thermal conductivity. This property plays an important role in generating output voltage signal with a temperature difference between the top and bottom surfaces of the nanocomposite. As a result, our TCH gas sensor can detect the range of hydrogen from 100 ppm to percentage level with good linearity. The best response and recovery time revealed for the optimized TCH gas sensor were 23 s and 17 s under 1000 ppm H₂/air, respectively. This type of sensor can provide an important component for fabricating thermoelectric-based gas sensors with favorable gas sensing performance.     

High performance hydrogen sensor based on Pd/TiO2 composite film

Hydrogen sensors with fast response and recovery rate based on nanoporous palladium (Pd) and titanium dioxide (TiO₂) composite films supported by anodic aluminum oxide (AAO) template have been demonstrated. Nanoporous TiO₂ film was sprayed on the porous AAO templates, followed by Pd film deposited on TiO₂ layer by DC magnetron sputtering. We have researched the detection performance of the hydrogen sensors depending on different thickness of TiO₂ layer from 6 to 30 nm with keeping the thickness of Pd as 30 nm. The results have demonstrated the sensors with 10 nm thickness of TiO₂ achieve the best performance with a response/recovery time as short as 4/8s at 0.8% and 0.4% hydrogen concentration, respectively. The sensors exhibited very good performance under hydrogen concentrations from 0.4% to 1.8%.     

Aerogel sheet of carbon nanotubes decorated with palladium nanoparticles for hydrogen gas sensing

In the development of hydrogen sensors, it is required to meet the demands of both high sensor performance as well as the ease of fabrication for mass production. For this purpose we proposed a chemiresistive hydrogen sensors based on an aerogel sheet of carbon nanotubes decorated with palladium nanoparticles (CNT/Pd sheet). The fabrication process is straightforward that a dry-spun CNT aerogel sheet is suspended between concentric electrodes followed by depositing Pd nanoparticles on CNT sheets by thermal evaporation. The present CNT/Pd sheet sensors can detect hydrogen at concentrations as low as 2 ppm at room temperature with a detection range from 2 to 1000 ppm. The aerogel nature of CNT/Pd sheet contributes to low detection limit and broad detection range of the CNT/Pd sensor. Relations between hydrogen concentration and sensor response and response time, and the effects of temperature on sensor performance were investigated.     

Hydrogen sensors fabricated with sprayed NiO,NiO:Li and NiO:Li,Pt thin films

Hydrogen sensors have been prepared using NiO, NiO:Li and NiO:Li,Pt thin films deposited on alumina substrates by chemical spray deposition. X-ray diffraction indicates grain sizes of the order of 60 nm, which are in agreement with HRSEM images. The electrical response to hydrogen was studied at different work temperature (250 °C–450 °C) and at different hydrogen concentrations (3000–30,000 ppm). The NiO and NiO:Li sensors present maximum sensibilities in the range of work temperatures used, while the NiO:Li,Pt sensor has it at lower temperatures, promising to be a good hydrogen sensor at temperatures close to ambient. When tested at different concentrations the pure nickel oxide sensor and the sensor with platinum present a linear behavior, while the lithium-doped sensor presents a potential relation; in all cases, lithium-doped and with platinum on surface, sensitivity proved to be higher than that of pure nickel oxide. The use of platinum on surface sensors improves the response time from 6.6 min to 1.5 min.