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.     

Surface and interface properties of monolayer graphene on hydrophobic and hydrophilic ultrananocrystalline diamond structures for hydrogen sensing applications

Here, for the first time, we synthesize hybrid hydrophilic and hydrophobic nanocarbon materials with reliable and stable gas sensing performance. The hybrid monolayer graphene (Gr)–nitrogen and argon (N₂ and Ar) gas incorporated ultra-nanocrystalline diamond (Gr/N₂@UNCD and Gr/Ar@UNCD) structures were synthesized using a microwave plasma enhanced chemical vapor deposition (MPECVD) method. The presented nanohybrid combinations have a unique morphology with diamond defects (sp³) covered by a graphene sheet (sp²). Sample sensors with metal electrodes were fabricated to study the H₂ gas sensing properties of the material. Thus, the as-fabricated Gr/N₂@UNCD exhibited higher sensor response (14.6%) than those of the as-fabricated Gr, N-UNCD and Gr/Ar@UNCD (3.6, 1.07 and 11.2%) based devices. The Gr/N₂@UNCD nanohybrid based sensor showed outstanding repeatability, selectivity and stability over ～56 days. The substantial improvement in the H₂ sensing performance of the as-fabricated Gr/N₂@UNCD nanohybrid based sensor was attributed to the modifications in surface morphology and resistance. The partial-hydrophobic surface of Gr/N₂@UNCD alters the beneficial resistivity and improved absorption, which assists in the efficient transport of electrons and H₂ gas molecules. The hybrid nanostructure of Gr-N₂@UNCD exhibits several unique properties that paves the way to future opportunities for advanced gas sensor fabrication.     

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.     

Boron nitride nanotubes (BNNTs) decorated Pd-ternary alloy (Pd63·2Ni34·3Co2.5) for H2 sensing

The micro-electro-mechanical system (MEMS)-based field effect transistor (FET) sensor for hydrogen detection was fabricated by modifying the gate electrode with boron nitride nanotubes (BNNTs) decorated Pd-ternary alloy (Pd_63·2Ni_34·3Co_2.5) as a hydrogen sensing layer Electro-thermal properties of the micro-heater embedded under sensor membrane were analyzed by a finite element method (FEM) simulation. The structural and morphological properties of the gate electrode were studied by Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FESEM). A variation in gate potential is observed due to the H₂ atmosphere that leads to the variation in the depletion region, therefore, changing the current in the channel (BNNTs decorated Pd-ternary alloy). The BNNTs-decorated Pd ternary alloy displayed high sensing response, fast response and recovery time for H₂ gas, low power consumption, long-term stability, and wide detection range from 1 to 5000 ppm H₂. The drain current of the H₂ FET sensor varied significantly at hydrogen gas exposure and increased with H₂ concentration. As proposed H₂ FET sensor can be utilized to the H₂ leak detection system for safe applications.     

Dual gas sensing properties of graphene-Pd/SnO2 composites for H2 and ethanol: Role of nanoparticles-graphene interface

In the present work, role of palladium (Pd) and tin oxide (SnO₂) nanoparticles (NPs) deposited on graphene has been investigated in terms of dual gas sensing characteristics of ethanol and H₂ between two temperatures. The incorporation of nanoparticles into graphene has been observed which results a large change in the sensing response towards these gases. It is investigated that, incorporation of isolated Pd NPs on the graphene facilitates the room temperature sensing of H₂ gas with fast response and recovery time whereas, isolated SnO₂ NPs on graphene enables the detection of ethanol at 200 °C. However, combination of isolated Pd and SnO₂ NPs on graphene shows improved sensitivity and good selectivity towards H₂ and ethanol, usually not observed in chemiresistive gas sensors. Catalytic PdH interaction and corresponding change in work function of nanoparticles on hydrogenation resulting in modifications in electronic exchange between Pd, SnO₂ and graphene are responsible for the observed behavior. These results are important for developing a new class of chemiresistive type gas sensor based on change in the electronic properties of the graphene and NPs interfaces.     

Characteristics of resistivity-type hydrogen sensing based on palladium-graphene nanocomposites

We describe the characteristics of resistivity-type hydrogen (H₂) sensors made of palladium (Pd)-graphene nanocomposites. The Pd-graphene composite was synthesized by a simple chemical route capable of large production. Synthesis of Pd nanoparticles (PdNPs) of various sizes decorated on graphene flakes were easily controlled by varying the concentration of Pd precursors. Resistivity H₂ sensors were fabricated from these Pd-graphene composites and evaluated with various concentrations of H₂ and interfering gases at different temperatures. Characteristics for sensitivity, selectivity, response time and operating life were studied. The results from testing the Pd-graphene indicated a potential for hydrogen sensing materials at low temperature with good sensitivity and selectivity. Specifically H₂ was measurable with concentrations ranging from 1 to 1000 ppm in laboratory air, with a very low detection limit of 0.2 ppm. The response of the sensors is almost linear. The resistivity of sensors changed approximately 7% in its resistance with 1000 ppm H₂ even at room temperature. The robust mechanical properties of graphene, which supported these PdNPs, exhibit structural stability and durability in H₂ sensors for at least six months. Moreover, the advantages in this work are experimental reproducibility in synthesis Pd-graphene composite and sensor fabrication process.     

Room temperature response and enhanced hydrogen sensing in size selected Pd-C core-shell nanoparticles: Role of carbon shell and Pd-C interface

In the present work, the effect of carbon shell around size selected palladium (Pd) nanoparticles on hydrogen (H₂) sensing has been studied by investigating the sensing response of Pd-C core-shell nanoparticles having a fixed core size and different shell thickness. The H₂ sensing response of sensors based on Pd and Pd-C nanoparticles deposited on SiO₂ and graphene substrate has been measured over a temperature range of 25 °C–150 °C. It is observed that Pd-C nanoparticle sensor shows higher sensitivity with increase in shell thickness and faster response/recovery in comparison to that of Pd nanoparticle samples. Pd-C nanoparticles show room temperature H₂ sensitivity in contrast to Pd nanoparticles which respond only at higher temperatures. Role of carbon shell is also understood by investigating H₂ sensing properties of Pd and Pd-C nanoparticles on graphene substrates. These results show that higher catalytic activity and electronic interaction at Pd-C interface, a complete coverage and protection of Pd surface by carbon and presence of structural defects in nanoparticle core are important for room temperature and higher sensing response.     

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.     

Hydrogen sensor using the Pd film supported on anodic aluminum oxide

Highly sensitive hydrogen gas sensors were fabricated using a microelectromechanical system (MEMS) and anodic aluminum oxides (AAOs) process. MEMS based gas sensor platform was designed with the multi-layer type for Pd film morphology manipulations. The operating temperature of the micro heater was positively correlated with the heater. Hydrogen sensing response of the sensor showed a good positive linearity as the gas sensitivity increased with increasing hydrogen concentration. The hydrogen sensitivity (defined as ratio of sensor resistances in air and after the hydrogen gas injection) was 0.638% at hydrogen concentration of 2000 ppm. The H₂ sensitivity was very dependent on the thickness and morphology of Pd-nanosized film. The gas sensitivity and response properties showed different behaviors when palladium film was deposited on the anodic aluminum oxide (AAO) layer. The hydrogen sensitivity for the Pd on AAO layer was about 0.783% at the hydrogen concentration of 2000 ppm. The sensitivity of the Pd-AAO layer improved with respect to the pure Pd thin film due to nanoporous nature of AAO.     

Nano-bitter gourd like structured CuO for enhanced hydrogen gas sensor application

Hydrogen gas sensing were investigated using nano-bitter gourd like structured CuO material synthesized via a chemical route. Morphology of CuO was revealed using FE-SEM and TEM image analysis. CuO phase confirmation and molecular structural fingerprint were verified by XRD and Raman analysis respectively. Elemental composition and atomic states of elements were investigated by EDS and XPS techniques respectively. A remarkable high gas response of 175% was recorded by CuO sensor towards 100 ppm hydrogen (H₂) at the operating temperature 200 °C with response time 150 s. The lowest detection of H₂ was observed at 2 ppm concentration with the gas response of 5%. The gas response was studied as functions of different operating temperatures and concentrations. Transient gas response and stability were also confirmed for CuO sensor. Hydrogen sensing mechanism of CuO sensor was elucidated.     

Fast response of hydrogen sensor using palladium nanocube-TiO2 nanofiber composites

In this work, we investigated the properties of resistivity type hydrogen (H₂) sensor for monitoring in H₂ gas. The H₂ sensor was made of Pd nanocube (NCs) and TiO₂ nanofiber (NFs) composites. The Pd NCs was synthesized by seed-mediated growth and TiO₂ nanofiber was synthesized via electrospinning method. The two nanomaterials are then converted into nanocomposites by ultrasonication process. Pd NCs-TiO₂ NFs composite was characterized by scanning electron microscope (SEM) and high resolution transmission electron microscope (HRTEM). The H₂ sensing properties including the response/recovery time, the response value and linearity of the synthesized samples were investigated toward to various H₂ concentrations (0.6, 0.8 and 1%). The response of H₂ sensor is S = 40.8% and the response/recovery time are 25/1 s with 0.6% at working temperature of 150 °C. Moreover, the H₂ sensor has excellent cross-selectivity for H₂ compared to ethanol, nitrogen dioxide and isopropyl alcohol.     

Understanding on the hydrogen detection of plasma sprayed tin oxide/tungsten oxide (SnO2/WO3) sensor

The plasma spray technique was well proven in producing metal oxide based gas sensors in the last two decades using different powder feedstocks. However, limited research was made to fabricate hydrogen gas sensor from tin oxide layer coated over tungsten oxide layer. This paper attempts to interpret the hydrogen gas sensing performances of plasma sprayed coating derived by depositing tin oxide layer over tungsten oxide (SnO₂/WO₃) layer. Plasma sprayed SnO₂/WO₃ sensor showed maximum response of 90% at 150 °C in contrast to stand-alone WO₃ (89% at 350 °C) and stand-alone SnO₂ (89% at 250 °C). The lower operating temperature of SnO₂/WO₃ sensor without compromising gas response was attributed to the WO₃–SnO₂ hetero-junction. SnO₂/WO₃ sensor showed selective sensing towards hydrogen with respect to carbon monoxide and methane gases. This sensor also possessed repeatable characteristics after 39 days from the initial measurement. In a nut-shell, plasma spayed SnO₂/WO₃ sensor showed stability of base resistance, repeatability after successive response and recovery cycles, selective sensing towards 500 ppm H₂ with significant magnitude of gas response of 90%, response time of 35 s and recovery time of 269 s at a temperature of 150 °C.     

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 polyaniline nanofiber-based carbon monoxide sensor for hydrogen fuel cell application

The sensing of carbon monoxide (CO) impurity contained in hydrogen fuel is a challenging work in the field of low temperature proton exchange membrane fuel cell (PEMFC). In the present work a chemiresistive gas sensor based on polyaniline (PANI) nanofibers was developed to detect CO in hydrogen. The sensor was fabricated by a template-free electrochemical polymerization of aniline on an interdigitated electrode. The most distinctive feature of the fabricated sensor was the formation of a horizontally oriented, monolayered PANI nanofiber network on the insulating gap area. The gas sensing character of the PANI nanosensor was evaluated by measuring the change in electrical resistance when gas atmosphere was changed from pure hydrogen to mixtures of CO in hydrogen. The results demonstrated that the PANI nanosensor had an excellent responding ability on CO in hydrogen with a concentration as low as 1 ppm. The influences of parameters, such as nanostructure, doping level, dopants, and CO concentrations, on the sensing characters of the nanosensor were discussed. The responding mechanism was attributed to the different binding sites of CO and H₂ with PANI: H₂ with the protonated amine nitrogen atoms and CO with the unprotonated amine nitrogen atoms. In view of its novel sensing mechanism and high sensing performance, the fabricated sensor is very promising to be applied as a new type of CO sensor to prevent the catalysis poisoning of PEMFC.     

Ultra-fast response and highly selectivity hydrogen gas sensor based on Pd/SnO2 nanoparticles

It is still a challenging task to achieve the rapid detection of hydrogen (H₂) with the rapid development of hydrogen energy sector. In this work, the H₂ sensing capabilities of pristine and Pd-modified SnO₂ nanoparticles with the size of ～7 nm were systematically evaluated. The SnO₂ nanoparticles were synthesized via hydrothermal method and Pd modification was performed using impregnation route. Pd modification remarkably upgraded the H₂ sensing performances compared with the pristine SnO₂ gas sensor. The working temperature of SnO₂ decreased from 300 °C to 125 °C after Pd loading. Among the prepared Pd/SnO₂ gas sensors, 0.50 at.% Pd/SnO₂ sensor exhibited the highest response magnitude of 254 toward 500 ppm H₂ and rapid response/recovery time of 1/22 s at 125 °C. The enhanced H₂ sensing capabilities by Pd modification may be related to the catalytic effect and the resistance modulation.     

Synthesis and characterizations of highly responsive H2S sensor using p-type Co3O4 nanoparticles/nanorods mixed nanostructures

High-quality p-type semiconducting Co₃O₄ with mixed morphology of nanoparticles/nanorods are synthesized using a hydrothermal route for high response and selective hydrogen sulphide (H₂S) sensor application. XRD and Raman studies revealed the crystal structure and molecular bonding for obtained Co₃O_4, respectively. The nanoparticles/nanorods-like structures were confirmed for Co₃O₄ using FESEM and TEM analysis. The EDS and XPS spectra analysis were carried out for elemental composition and chemical atomic states of Co₃O₄. The Co₃O₄ sensor is investigated for gas sensing properties in dynamic conditions. The sensor exhibited the highest selectivity towards H₂S among various hydrogen-contained gases at 225 °C. The sensor revealed a high response of 357% and 44% for 100 and 10 ppm H₂S gas concentrations, respectively. The Co₃O₄ sensor exhibited a systematic dynamic resistance response for 100–10 ppm range H₂S gas. The excellent dynamic resistance repeatability of the sensor was shown towards 25 ppm H₂S gas. The response of Co₃O₄ sensor was investigated at different operating temperatures and H₂S concentrations. The sensor stability and H₂S sensing mechanism for the Co₃O₄ sensor have been reported. Highly uniform and mixed nanostructures of Co₃O₄ can be the potential sensor material for real-time high-performance H₂S sensor application.     

Synthesis and characterization of TiO2 doped polyaniline composites for hydrogen gas sensing

The Polyaniline (PANI) and Titanium dioxide (TiO₂)/PANI composite thin film based chemiresistor type gas sensors for hydrogen (H₂) gas sensing application are presented in this paper. Pure PANI and TiO₂/PANI composites with different wt% of TiO₂ were synthesized by chemical oxidative polymerization of aniline using ammonium persulfate in acidic medium at 0-5 °C. Thin films of PANI and TiO₂/PANI composites were deposited on copper (Cu) interdigited electrodes (IDE) by spin coating method to prepare the chemiresistor sensor. Finally, the response of these chemiresistor sensors for H₂ gas was evaluated by monitoring the change in electrical resistance at room temperature. It was observed that the TiO₂/PANI composite thin film based chemiresistor sensors show a higher response as compared to pure PANI sensor. The structural and optical properties of these composite films have been characterized by X-ray diffraction (XRD) and UV-Visible (UV-Vis) spectroscopy respectively. Morphological and structural properties of these composites have also been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) respectively.     

Selective and tunable H2 adsorption/sensing performance of W-doped graphene under external electric fields: A DFT study

W-doped graphene and its selective gas adsorption/sensing performance are studied through first-principles density functional theory (DFT) calculations. A single W atom is stably anchored into the graphene plane with a high binding energy of ?9.325 eV. The W-doped graphene interacts more strongly with H₂ compared to NH₃, CH₄, CO, SO₂ or H₂S. The H₂ adsorption system also has a higher adsorption energy of ?1.035 eV. Furthermore, the W-doped graphene exhibits the highest sensor response to H₂ with the largest number of transferred charges and the biggest change in the band gap. A negative electric field improves the interaction between the H₂ and the W-doped graphene by increasing the adsorption energy and promoting charge transfer. However, the adsorption of the H₂ is significantly weakened upon the application of a positive electric field; the adsorbed H₂ is easily desorbed from the W-doped graphene with a modulated recovery time as short as ～4.099 s at room temperature (300 K) upon a +0.4 V Å^?1 increase in the electric field. These results reveal that the W-doped graphene has promising selective and tunable H₂ adsorption/sensing performance upon the application of external electric fields.     

Efficient room temperature hydrogen gas sensing based on graphene oxide and decorated porous silicon

In this study, a triple system for a hydrogen gas sensor was fabricated using graphene oxide, palladium nanoparticles, and porous silicon as a substrate. The fabricated sample was investigated by energy dispersive X-ray spectroscopy, field emission scanning electron microscopy, and Raman spectroscopy. Field emission scanning electron microscopy images displayed a relatively uniform distribution of Palladium nanoparticles over porous silicon. In addition, it was observed that the graphene oxide nanosheets accumulated over the Palladium nanoparticles. Hydrogen-sensing measurements demonstrated that the fabricated system can even detect hydrogen at 200 ppm and 15 °C. The formation of palladium hydride was the main mechanism for detection. In fact, this structure caused a change in resistance through the creation of new electron pathways. Furthermore, the H₂ concentration showed a linear function to the reciprocal of the response time; this suggests that the sensing kinetics of the sample depends on the atomization of hydrogen molecules, which occurs via Pd nanoparticles. Moreover, the fabricated sample displayed significant selectivity for hydrogen gas compared to other examined gases.     

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.