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.     

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.     

On a transistor-type hydrogen gas sensor prepared by an electrophoretic deposition (EPD) approach

A Pd/GaN/AlGaN heterostructure field-effect transistor (HFET)-type hydrogen gas sensor, based on an electrophoretic deposition (EPD) approach, is fabricated and studied. Due to the formation of good Schottky gate contact by an EPD approach, the studied HFET shows improved DC performance including the suppressed gate current and better thermal stabilities on current–voltage (I–V) characteristics. This is mainly attributed to the reduction of interface trap density and improved Pd morphology. The EPD-based Pd morphologies are examined by X-ray diffraction, energy dispersive spectroscopy, Auger electron spectroscopy, scanning electron microscopy, and atomic force microscopy. For the used gate-dimension of 1 μm × 100 μm, an EPD-based HFET shows low gate current of 2.9 nA, maximum drain saturation current of 490 mA/mm, and maximum extrinsic transconductance of 78.9 mS/mm at room temperature. Also, solid thermal stabilities on maximum drain saturation current (−0.46 mA/mm K) and maximum extrinsic transconductance (−0.08 mS/mm K) are found as the temperature is increased from 300 to 600 K. For hydrogen gas sensing application, at 370 K, the maximum hydrogen sensitivity of 600.1 μA/mm ppm H₂/air under a 5 ppm H₂/air ambiance and fast response time (30 s) and recovery time (47 s) under a 10,000 ppm H₂/air ambiance are obtained. The EPD approach also demonstrates advantages of low cost, simple apparatus, easy process, little restriction on the shaped substrate, composited deposition, and adjustable alloy grain size. Therefore, the proposed EPD approach gives the promise for fabricating high-performance HFET devices and hydrogen gas sensors.     

Comparative study of Schottky diode type hydrogen sensors based on a honeycomb GaN nanonetwork and on a planar GaN film

We demonstrate Schottky diode type hydrogen (H₂) sensors both on a planar GaN film grown by Metal Organic Chemical Vapor Deposition and on a honeycomb GaN nanonetwork grown by Molecular Beam Epitaxy. The metal-semiconductor Pt/planar GaN film Schottky diode was fabricated and used as a H₂ sensor element with response time τ   of 80 s (10,000 ppm) and 2000 ppm limit of detection for hydrogen gas (LODH₂)$\left({\text{LOD}}_{{\text{H}}_2}\right)$ at 373 K. A significant improvement in H₂ detection is observed for the honeycomb GaN nanonetwork. The characteristics of the H₂ sensor on the honeycomb GaN nanonetwork are quantitatively studied in comparison with that on the planar GaN film. The response time τ is shortened by a factor of 27 (3 s versus   80 s) and the LODH₂${\text{LOD}}_{{\text{H}}_2}$ is lowered by two orders of magnitude, from 2000 to 50 ppm. Moreover, the operating temperature could be reduced to room temperature. Through analyzing the transient-state, we observed a reduction of activation energy E_a from 6.22 to 2.4 kcal/mol. The reduced activation energyE_a is regarded as the reason that leads to a superior H₂ detection of the honeycomb GaN nanonetwork in terms of response time τ and operating temperature.     

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.     

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.     

Unidirectional sensing characteristics of structured Au–GaN–Pt diodes for differential-pair hydrogen sensors

A new metal-semiconductor-metal (MSM) hydrogen sensor was proposed to avoid (or to reduce) false alarms due to temperature drift when it is used in differential-pair hydrogen-sensing systems. A GaN semiconductor layer together with Pt as catalytic metal and Au as Schottky metal was employed to structure an Au–GaN–Pt MSM sensor. In particular, the structured Au–GaN–Pt MSM sensor can function as an active sensor and a reference sensor, depending on the polarity of applied voltage, in a differential-pair sensing circuit. Possible sensing mechanisms associated with the Au–GaN–Pt MSM sensor were described first to include band diagrams and graphical analysis. Experimental results reveal that an active sensor by forward-biasing the Au–GaN–Pt MSM sensor responses well to hydrogen-containing gases (50, 500, and 5000 ppm H₂/N₂) at various temperatures (25 °C, 50 °C, 70 °C, and 90 °C). High sensing current gains over 10⁴ were obtained. Further, the Au–GaN–Pt MSM sensor can also be reverse-biased to act as a reference sensor which shows negligible responses to hydrogen-containing gases. The differential-pair sensing circuit with the proposed Au–GaN–Pt MSM sensor reduces false alarms due to ambient temperature variation while it provides a short detection time.     

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.     

Multifunctional dumbbell-shaped ZnO based temperature-dependent UV photodetection and selective H2 gas detection

In this article, a single step, chemically synthesized dumbbell-shaped ZnO microcrystal has been utilized to fabricate UV photodetectors and highly selective H₂ gas sensors. The structural, morphological, chemical states and optical properties were investigated via using various characterization tools. The dumbbell-shaped morphology acquires 1.2–1.4 μm length and diameters of 300–400 nm respectively. The temperature-dependent photodetection and gas sensing characteristics has been studied in the temperature range 298.15–433.15 K respectively. The maximum relative change in photoresponse and gas sensor response has been found at 358.15 K. The rise time and fall time have been reduced from ~2.4 sec to ~1.0 sec and ~3.1 sec–~2.2 sec respectively in the applied temperature range. Such behavior is attributed to the increasing desorption rate with a higher operating temperature. The maximum sensor response towards H₂ was found to be ~20% for 100 ppm concentration at an operating temperature of 358.15 K.     

A resonant photoacoustic cell for hydrogen gas detection

Hydrogen gas (H₂) detection plays an important role in many fields. With the continuous demand and development of clean energy, it is urgent to study new hydrogen gas sensors for stable and accurate H₂ detection. The purpose of this research is to develop a new H₂ sensor based on the resonant photoacoustic (PA) cell as the sensing element. The sensitivity of the resonant PA cell to the resonant frequency is sufficiently utilized. The optimization of its resonance frequency was investigated minutely for the H₂ sensor. Detection utilizes resonance frequency differences between H₂ and air as a sensing mechanism. The resonance frequency tracking is adopted and implemented by the field-programmable gate array (FPGA) device. The minimum detection limit of about 74 ppm for H₂ has been demonstrated by preliminary experiments. The response time of the sensor is about 5 s. This sensor detects concentrations ranging from 74 ppm to 100% in 1 atm. The preliminary test result shows that the H₂ sensor based on this structure has a larger application perspective.     

SnO2 functionalized AlGaN/GaN high electron mobility transistor for hydrogen sensing applications

In this study, we report on a demonstration of hydrogen sensing at low temperature using SnO₂ functionalized AlGaN/GaN high electron mobility transistors (HEMT). The SnO₂ dispersion was synthesized via a hydrothermal method and selectively deposited on the gate region of a HEMT device through a photolithography process. The high electron sheet carrier concentration of nitride HEMTs provides an increased sensitivity relative to simple Schottky diodes fabricated on GaN layers. The morphology and crystalline properties of the SnO₂-gate, together with the texture of the multilayer films on the device were investigated by SEM, HRTEM, EDS and XRD. The effects of annealing treatment on the crystalline properties of the SnO₂-gate, and gas sensing properties of SnO₂-gated HEMT sensors were studied. The SnO₂-gated HEMT sensor showed fast and reversible hydrogen gas sensing response at low temperature.     

Development of a hydrogen dual sensor for fuel cell applications

The development and application of a hydrogen dual sensor (HDS) for the application in the fuel cell (FC) field, is reported. The dual sensing device is based on a ceramic platform head with a semiconducting metal oxide layer (MOx) printed on Pt interdigitated contacts on one side and a Pt serpentine resistance on the back side. MOx layer acts as a conductometric (resistive) gas sensor, allowing to detect low H₂ concentrations in air with high sensitivity and fast response, making it suitable as a leak hydrogen sensor. The proposed Co-doped SnO₂ layer shows high sensitivity to hydrogen (R₀/R = 90, for 2000 ppm of H₂) at 250 °C in air, and with fast response (<3 s). Pt resistance serves as a thermal conductivity sensor, and can used to monitor the whole range of hydrogen concentration (0–100%) in the fuel cell feed line with short response-recovery times, lower than 13 s and 14 s, respectively. The effect of the main functional parameters on the sensor response have been evaluated by bench tests. The results demonstrate that the dual sensor, in spite of its simplicity and cheapness, is promising for application in safety and efficiency control systems for FC power source.     

Robust hydrogen detection system with a thermoelectric hydrogen sensor for hydrogen station application

A prototype hydrogen detection system using the micro-thermoelectric hydrogen sensor (micro-THS) was developed for the safety of hydrogen infrastructure systems, such as hydrogen stations. We have designed a detection part with a pressure proof enclosure adoptable for the international standard of Exd II CT3, and carried out an explosion strength test, explosion and fire hazard tests, and an impact test. The hydrogen sensing performance of the detection part of this prototype system showed a good linear relationship between the sensing signal and hydrogen concentrations in air, for a wide range of hydrogen concentrations from 10 ppm to 40,000 ppm (4 vol.%). This prototype detection system was installed in the outdoor field of the hydrogen station and the response for H₂ gas in air of 100 ppm, 1000 ppm, and 10000 ppm was tested monthly for 1 year.     

Enhancement of the room-temperature hydrogen sensing performance of MoO3 nanoribbons annealed in a reducing gas

Uniform-sized orthorhombic MoO₃ nanoribbons were synthesized by a simple hydrothermal method at 240 °C. The nanoribbons grew along the [001] orientation, with average length, width and thickness of approximately 20 μm, 270 nm and 90 nm, respectively. The obtained nanoribbons were further annealed in a hydrogen atmosphere at different temperatures to modify their surface states. The treatment of the nanoribbons at 300 °C significantly elevated the concentration of non-stoichiometric Mo⁵⁺ to 24.7%, much larger than the original concentration (∼14.8%). A positive relationship was found between the non-stoichiometric Mo⁵⁺, chemisorbed oxygen ion and sensor response. The sensor based on the MoO₃ nanoribbons treated at 300 °C exhibited a faster response time of approximately 10.9 s, and a higher sensor response of 17.3 towards 1000 ppm H₂, compared with the results of original tests (∼21 s and ∼5.7, respectively), indicating the significantly improved gas sensing performance of the treated MoO₃. Meanwhile, the sensor also exhibited excellent repeatability and selectivity toward hydrogen gas. The enhancement of the hydrogen gas sensing performance of treated MoO₃ nanoribbons was attributed to the more effective adjustment of the width of the depletion region on the nanoribbon surface and the height of the potential barrier at the junctions, induced by the interaction between hydrogen molecules and higher-concentration oxygen ions. Our research implied that the gas sensing performance of nanostructured metal oxides could be successfully enhanced through annealing in the reducing gas.     

High performance room temperature GaN-nanowires hydrogen gas sensor fabricated by chemical vapor deposition (CVD) technique

Large-scale synthesis of GaN nanowires was grown on c-sapphire substrate by chemical vapor deposition technique. X-ray diffraction, field emission scanning electron microscopy, μ-Raman and μ-photoluminescence analyses reveal that GaN nanowires crystallize within a hexagonal wurtzite-type structure with a considerably high yield, high aspect ratio of GaN NWs having an average diameter and length of 80 nm and up to several microns, respectively. A metal–semiconductor–metal (MSM) gas sensor using GaN nanowires was fabricated and hydrogen (H₂)-sensing performances were examined over broad range of concentrations (7–1000 ppm) and at various operating temperatures (25,100, 150 °C). The NWs demonstrated high sensitivity up to 255% upon exposure to 1000 ppm of H₂ gas at room temperature at a low power consumption of 60 μW. Additionally, at room temperature, the sensor exhibited a significant sensitivity of 83% when exposed to a very low H₂ gas concentration of 34 ppm then becomes 15% at ultra low level of 7 ppm. The sensing measurements of NWs based sensor for H₂ gas in the temperatures range of 25–150 °C were repeatable and reversible over a period of time of 50 min. The sensor exhibited free hysteresis phenomena after exposed to various H₂ concentrations at various temperatures. The high performance of the fabricated sensor was attributed mainly to the large surface-to-volume ratio as well as the high crystallinity of the synthesized GaN NWs.     

Room temperature H2 sensing using functionalized GaN nanotubes with ultra low activation energy

A single-step synthesis route of square shaped wurtzite GaN nanotubes is reported by a quasi-vapor–solid process with detailed growth kinetics involving surface energies and Ga mobility along different crystalline facets. A wet chemical route is used for the functionalization of GaN nanotubes with Pt nanoclusters of average diameter ∼1.6 (0.4) nm in order to instigate the formation of localized Schottky barrier, responsible for carrier transport in the sensing process. Catalytically enhanced dissociation of molecular H₂ down to the lowest detection limit of 25 ppm at room temperature, as compared to those of reported GaN systems has been shown. We report, for the first time, a very low activation energy value of 29.4 meV which will be useful in practical sensing of H₂ at room temperature without any application of bias.     

Reversible gasochromic hydrogen sensing of mixed-phase MoO3 with multi-layered Pt/Ni/Pt catalyst

Visual detection of hydrogen is important for hydrogen-powered vehicles and hydrogen fuel stations. However, there are few studies on such the sensing approach, particularly solving challenges about the endurance or reusability of the sensors. Here, our development of superior reversible gasochromic hydrogen sensors based on a novel combination of a multi-layered Pt–Ni catalyst and a mixed-phase MoO₃ active layer is introduced. The mixed α and β phases in the MoO₃ layer can provide more high-energy sites for gasochromic reactions. The Pt–Ni catalyst, where Ni serves as a modifier of surface atoms’ diffusivity, successfully converts the mostly irreversible gasochromic sensing mode of MoO₃ to the reversible gasochromic mode. Our hydrogen sensor shows coloration from grey to navy blue within 30 s, and its recovery occurs within 50 s. Furthermore, it can detect H₂ gas (5–1000 ppm) in both optical and electrical modes. Notably, our sensor is all constructed through dry processes, raising its potential for large-scale manufacture and integration.     

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.     

The decoration of TiO2/reduced graphene oxide by Pd and Pt nanoparticles for hydrogen gas sensing

Reduced graphene oxide (RGO) was used to improve the hydrogen sensing properties of Pd and Pt-decorated TiO₂ nanoparticles by facile production routes. The TiO₂ nanoparticles were synthesized by sol–gel method and coupled on GO sheets via a photoreduction process. The Pd or Pt nanoparticles were decorated on the TiO₂/RGO hybrid structures by chemical reduction. X-ray photoelectron spectroscopy demonstrated that GO reduction is done by the TiO₂ nanoparticles and Ti–C bonds are formed between the TiO₂ and the RGO sheets as well. Gas sensing was studied with different concentrations of hydrogen ranging from 100 to 10,000 ppm at various temperatures. High sensitivity (92%) and fast response time (less than 20 s) at 500 ppm of hydrogen were observed for the sample with low concentration of Pd (2 wt.%) decorated on the TiO₂/RGO sample at a relatively low temperature (180 °C). The RGO sheets, by playing scaffold role in these hybrid structures, provide new pathways for gas diffusion and preferential channels for electrical current. Based on the proposed mechanisms, Pd/TiO₂/RGO sample indicated better sensing performance compared to the Pt/TiO₂/RGO. Greater rate of spill-over effect and dissociation of hydrogen molecules on Pd are considered as possible causes of the enhanced sensitivity in Pd/TiO₂/RGO.     

Sensing mechanism of hydrogen gas sensor based on RF-sputtered ZnO thin films

The mechanism of hydrogen (H₂) gas sensing in the range of 200–1000 ppm of RF-sputtered ZnO films was studied. The I–V characteristics as a function of operating temperature proved the ohmic behaviour of the contacts to the sensor. The complex impedance spectrum (IS) of the ZnO films showed a single semicircle with shrinkage in the diameter as the temperature increased. The best fitting of these data proved that the device structure can be modelled as a single resistance-capacitance equivalent circuit. It was suggested that the conductivity mechanism in the ZnO sensor is controlled by surface reaction. The impedance spectrum also exhibited a decreased in semicircle radius as the hydrogen concentration was increased in the range from 200 ppm to 1000 ppm.