Graphene decorated Pd-Ag nanoparticles for H2 sensing

Hydrogen sensor based on graphene nano-composite with Pd-Ag nanoparticles was fabricated by MEMS process. Structural and morphological properties of the sensing film were studied by an energy dispersive spectroscopy (EDS) and field emission scanning electron microscopy (FESEM), respectively. The H₂ sensing properties of as-formed sensor were investigated by measuring the resistance changes at different H₂ concentrations. The maximum gas response was 16.2% at 1000 ppm of H₂ gas. The gas sensitivity of the as-formed H₂ sensor showed linear behavior with the hydrogen concentration. Experimental results showed that the coupling of graphene with Pd/Ag alloy enhanced significantly hydrogen sensing performance.     

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.     

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.     

Room temperature hydrogen gas sensors of functionalized carbon nanotubes based hybrid nanostructure: Role of Pt sputtered nanoparticles

Fast detection of H₂ gas at room temperature has constantly remained a challenge. The metal-oxide based gas sensors have shown excellent sensing properties for gases like H₂, NO, CO and NH₃. In the present work, the H₂ gas sensing characteristics of multiwalled carbon nanotubes based hybrid sensor (F-MWCNTs/TiO₂/Pt) has been reported. The fabricated sensor shows 3.9% sensitivity for low concentration i.e. 0.05% of H₂ with good repeatability and stability at room temperature. The sensing response of F-MWCNTs/TiO₂/Pt is interrelated to change in their resistance on the introduction of H₂ gas and this phenomenon is required for deep understanding the effect of H₂ adsorption on their electronic conduction. The improvement in sensitivity of F-MWCNTs/TiO₂/Pt as compared to MWCNTs/TiO₂ towards H₂ is because of the catalytic role of dispersed Pt nanoparticles deposited by sputtering.     

Triphenylamine-functionalized graphene decorated with Pt nanoparticles and its application in photocatalytic hydrogen production

A new type of graphene-based nanohybrid was prepared from graphene nanosheets and 4-(diphenylamino)benzaldehyde (TPA-CHO) through 1,3-dipolar cycloaddition. The nanohybrid was modified by platinum nanoparticles via photodeposition. The nanohybrid and its Pt modified nanocomposite were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Ultraviolet–visible absorption (UV–vis), Fourier transform infrared (FTIR), and Raman spectra confirmed that triphenylamine moiety grafted on the graphene surface. The results of fluorescence quenching and photocurrent enhancement of the triphenylamine-functionalized graphene revealed that photoinduced electron transfer from triphenylamine moiety to the graphene sheet. The investigation of using the Pt modified graphene-based nanocomposite as a photocatalyst for H₂ evolution showed that under UV–vis light irradiation, the average H₂ evolution rate and the quantum efficiency is 2.3 μmol h⁻¹ and 0.45% mol E⁻¹, respectively. This work demonstrated a potential application of an organic sensitizer covalently functionalized graphene as a novel photocatalyst in the field of solar energy conversion.     

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.     

The structural and electronic properties of Li-doped fluorinated graphene and its application to hydrogen storage

By using first-principles density functional theory, a theoretical investigation of Li-doped fluorinated graphene and its application as a hydrogen storage media is performed. It is found that a mixture between sp³ and a higher degree of sp² of the carbon orbitals after doping with Li would restore the distorted fluorinated graphene, and a fluorinated graphene layer with Li adsorbed on single or double-sides could store hydrogen up to 9 or 16.2 wt%. Regarding the H₂ adsorption mechanism, it has been demonstrated that the enhanced electrostatic field around the Li atom originates from the increased charge transfer from Li to graphene and F atoms with more electronegativity. Hybridization interaction between Li and graphene is also responsible for the adsorption of H₂ molecules.     

Efficient hydrogen storage in defective graphene and its mechanical stability: A combined density functional theory and molecular dynamics simulation study

A combined density functional theory and molecular dynamics approach is employed to study modifications of graphene at atomistic level for better H₂ storage. The study reveals H₂ desorption from hydrogenated defective graphene structure, V₂₂₂, to be exothermic. H₂ adsorption and desorption processes are found to be more reversible for V₂₂₂ as compared to pristine graphene. Our study shows that V₂₂₂ undergoes brittle fracture under tensile loading similar to the case of pristine graphene. The tensile strength of V₂₂₂ shows slight reduction with respect to their pristine counterpart, which is attributed to the transition of sp² to sp³-like hybridization. The study also shows that the V₂₂₂ structure is mechanically more stable than the defective graphene structure without chemically adsorbed hydrogen atoms. The current fundamental study, thus, reveals the efficient recovery mechanism of adsorbed hydrogen from V₂₂₂ and paves the way for the engineering of structural defects in graphene for H₂ storage.     

Hybrid amorphous MoSx-graphene protected Cu2O photocathode for better performance in H2 evolution

This paper reports on a new strategy using a combination of graphene protective layer and amorphous molybdenum sulfide (MoS_x) H₂-evolving catalyst layer to form a hybrid coating layer that enhances both photocatalytic activity and stability of the p-Cu₂O photocathode for the solar H₂ generation in water. Graphene sheet was transferred from a copper substrate onto the p-Cu₂O electrode surface by a simple solution process, resulting in the creation of p-Cu₂O/Gr. A layer of MoS_x was deposited further to generate a hybrid p-Cu₂O/Gr/MoS_x photocathode. In a pH 7 phosphate buffer electrolyte and under 1 Sun light illumination, at the applied potential of 0 V vs. reversible hydrogen electrode (RHE), the resultant p-Cu₂O/Gr and p-Cu₂O/Gr/MoS_x show 7-fold superior catalytic activity and significant enhancement of the stability compared to a pristine p-Cu₂O photocathode. The obtained results demonstrate that the p-Cu₂O/Gr and p-Cu₂O/Gr/MoS_x photocathodes are very promising for photoelectrochemical cell water splitting.     

Sensors for anaerobic hydrogen measurement: A comparative study between a resistive PdAu based sensor and a commercial thermal conductivity sensor

Hydrogen sensors able to perform measurements in real time in anaerobic environment such as natural gas (NG) will greatly help the development of power to gas technology. For now, thermal conductivity (TC) gas sensors and Pd thin film based sensors have demonstrated their capability to measure H₂ in air and N₂ but there is still lack of testing in natural gas environment. In this study, the sensing performances (response, hysteresis, response time and selectivity) of two sensors were assessed in three anaerobic environments: N₂, CH₄, and NG. The first one is a homemade resistive sensor based on a PdAu thin film and the second one is a commercial thermal conductivity sensor. While most performances are equivalent for both technologies, only the PdAu sensor is able to detect selectively H₂, without any interfering effect with NG components. Thus, Pd based thin film sensors are promising for H₂ detection in anaerobic environments.     

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.     

Temperature dependence of sensing properties of hydrogen-sensitive extended-base heterojunction bipolar transistors

A planar-type metal-semiconductor-metal (MSM) hydrogen sensor forming on the collector layer was employed as an extended base (EB) of the InGaP-GaAs heterojunction bipolar transistors (HBTs). Then, hydrogen sensing transistors integrated were proposed and studied. After introducing sensing properties of the EB-hydrogen sensor, various sensing current gains defined were addressed for our hydrogen sensing transistor. Instead of the base current, N₂ and/or hydrogen-containing gases were used as a parameter while measuring common-emitter characteristics of the hydrogen sensing transistor at various temperatures. Experimental results show that maximum sensing base current gains in 1% H₂/N₂ is 330 at 25 °C while it is enhanced to 1800 at 50 °C, then to 2300 at 80 °C, and finally to 2800 at 110 °C. In contrast, a peak sensing collector current gain is as high as 1.2 × 10⁵ (4.3 × 10⁴) in 1% (0.01%) at 110 °C. In addition, response times obtained from the sensing diode (base) and collector currents in 0.01% H₂/N₂ are 485 (490) and 745 s at 25 °C. Together with important features including one power supply and low-power consumption, the proposed hydrogen sensing transistor is very promising for applications in detecting hydrogen.     

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/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.     

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.     

Planar solid-state amperometric hydrogen gas sensor based on Nafion®/Pt/nano-structured polyaniline/Au/Al2O3 electrode

The effects of Nafion^® film thickness and charges passed for the preparation of Pt and nano-structured polyaniline (nsPANi) on the sensing properties of a planar solid-state amperometric hydrogen gas sensor are investigated. The surface morphology, Pt loading and electroactive surface area (ESA) are analyzed by FESEM, inductively coupled plasma (ICP) and cyclic voltammetry (CV), respectively. The specific sensitivity of the hydrogen gas sensor can be effectively promoted by decreasing the thickness of the Nafion^® film and the charge passed for the electrodeposition of Pt and PANi (from 30 to 10 mC), respectively. The very low Pt loading of the sensing composite electrode is due to the use of the nanofibrous PANi as support, which remarkably promotes Pt utilization. The specific sensitivity and the response time of the hydrogen gas sensor based on the Nafion^® (5.7 μm)/Pt/nsPANi/Au/Al₂O₃ electrode with a Pt loading of 1.87 μg are found to be 338.50 μA ppm^?1 g^?1 and 100–250 s, respectively, for measuring 10–10,000 ppm H₂.     

Influence of the reduction of graphene oxide (rGO) on the structure and photoactivity of CdS-rGO hybrid systems

Solvothermal and chemical reduction of graphene oxide with N₂H₄ or HI affect the surface composition, rupture and delamination degree of reduced graphene oxide (rGO). Higher reduction and stacking of rGO was achieved by chemical reduction with HI, while solvothermal reduction and, especially, the chemical reduction with N₂H₄ lead to higher delamination of rGO. The incorporation of the different rGO to CdS implies changes in the characteristics and photoactivity of the CdS-rGO hybrids. A promoter effect was observed in all CdS-rGO hybrids respect to the photoactivity of bare CdS, observing the better photoactivity on the hybrid in which the graphene oxide was reduced with HI (CdS-rGO/HI). The variations in the photoactivity of CdS-rGO hybrids are analyzed in terms of changes in the structure, surface and light absorption ability of CdS and also analyzing the contact of CdS with rGO. The greater concentration of small CdS nanostructures with strong quantum confinement is in the origin of the enhancement in photoactivity observed in the CdS-rGO/HI hybrid.     

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.     

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.     

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.