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.     

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.     

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.     

Effective monitoring and classification of hydrogen and ammonia gases with a bilayer Pt/SnO2 thin film sensor

The monitoring and classification of different gases, such as H₂ and NH₃ using a low-cost resistive semiconductor sensor is preferred in practical applications in hydrogen energy, breath analysis, air pollution monitoring, industrial control, and etc. Herein, porous bi-layer Pt/SnO₂ thin film sensors were fabricated to enhance H₂ and NH₃ sensing performance for effective monitoring and classification. Different Pt film thicknesses of 2, 5, 10, and 20 nm were deposited on 150 nm SnO₂ film-based sensors by sputtering method to optimize the response to H₂ and NH₃ gases. Gas sensing results showed that the fabricated Pt/SnO₂ films significantly improved the sensor response to NH₃ and H₂ compared to pure SnO₂ thin film. The sensors based on 5 and 10 nm Pt catalyst layers presented the highest responses to H₂ and NH₃, respectively. The optimal working temperature for NH₃ was in the range from 250 °C to 350 °C, and that for H₂ gas is less than 200 °C. The response of Pt/SnO₂ sensors to CH₄, CO, H₂S, and liquefied petroleum gas was much lower than that to NH₃ and H₂ supporting the high selectivity. On the basis of sensing results at different working temperatures or Pt thicknesses, we applied a radar plot and linear discriminant analysis methods to distinguish NH₃ and H₂. The results showed that H₂ and NH₃ could be classified without any confusion with different Pt layer thicknesses at a working temperature of 250 °C.     

Experimental investigation of natural polysaccharide-based mixed matrix membrane modified with graphene oxide and Pd-nanoparticles for enhanced gas separation performance

In this work, we proposed a mixed matrix membrane prepared by using a glycerol modified guar gum (GGP) polymer matrix incorporated with graphene oxide (GO). The influence of varying GO concentration on the gas separation performance was investigated and 2 wt% was found to be the optimum concentration for high performance. The 2 wt% GO mixed matrix membranes were further modified with Pd nanoparticles. When GO, and Pd nanoparticles were mixed, CO₂ permeability increased by 49.94%, while the permeability of H₂ gas molecules decreased by 98.11%, respectively, compared to the pristine GGP membrane. The selectivity of CO₂/H₂ was obtained as 18.27. The glass transition temperature of the membrane increased from 85 to 95.2 °C, tensile strength and elongation of the break were significantly improved by 29.09% and 84.37% through the addition of Pd and GO into the membrane. The scanning electron microscopy revealed a dense top surface after GO nanosheets incorporation. Further, the thermogravimetric analysis proposes that the modified membrane is thermally stable than GGP. Henceforth, the study suggests GO incorporation and Pd nanoparticles modification of guar gum membrane is a promising gas separation membrane with potentially high selectivity for CO₂ gas.     

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%.     

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.     

Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies

2D SnO₂ disks with excellent purity and crystallinity were synthesized through a low cost, facile hydrothermal process and were characterized in terms of their morphological, structural, optical and electrochemical properties. The 2D disk-like morphology of synthesized SnO₂ presented the average thickness of ∼1 μm and possessed the typical rutile tetragonal phase for the SnO₂ with preferred growth along (100) plane. As-synthesized SnO₂ disks were used for the fabrication of gas sensors for reducing gases like H₂, CO, and C₃H₈. With the optimized temperature at 400 °C, the as-synthesized SnO₂ electrode expressed the gas responses of 14.7, 9.3 and 8.1 for H₂, CO, and C₃H₈, respectively. Contrary, the reasonable response times of 4 s, 3 s, and 8 s and the recovery times of 331 s, 201 s, and 252 s were recorded for H₂, CO, and C₃H₈ gases, respectively. The DFT studies conducted herein suggest that the adsorbed oxygenated species act as a primary redox mediator for gas sensing reaction between reductive gases like H₂, CO and C₃H_8, and SnO₂ sensor. From DFT analysis, a very low heat of adsorption (≤0.2 eV) estimated which suggested the physisorption of the H₂ molecules on the surface of the sensing material (i.e. SnO₂). In contrast, the deposited oxygen atom forms strong chemical bonds with O_2c and O_3c sites. The oxygen atom bonded to O_2c site control the conductivity of the sensor better than the O_3c sites.     

Effect of hydroxyl groups on the oxygen vacancy and hydrogen-sensitive properties of CeO2 in different morphologies

The “hydroxyl-oxygen vacancy model” was of great importance for the catalytic and physicochemical properties of metal oxides. Here, we proposed a simple method to construct a hydroxyl-oxygen vacancy model for one-step reduction of palladium chloride through ascorbic acid rich in hydroxyl, loaded on rod, spherical, nanoparticles and cubic CeO₂, to achieve the Ce⁴⁺/Ce³⁺ transition and the generation of Pd NPs. The surface-loading Pd ions activated the lattice oxygen of CeO₂, and promoted the overflow of reverse oxygen at the Pd–O–Ce interface. The increasing in Ce³⁺ occupancy altered the reduction performance, oxygen vacancy number and active Pd valence state of the catalyst, and greatly contributed to the response and recovery times. Among the obtained sensors, the 0.50 wt % Pd/CeO₂–C/R sensor show a response amplitude of 190/196% to 1000 ppm H₂ at 120 °C with a response/recovery time of only 1/3 s. These excellent results are mainly attributed to the chemisorbed oxygen and Ce³⁺ after ascorbic acid reducing Pd are found to be 4.84 and 1.57 times higher than pure CeO₂. The hydroxyl-oxygen vacancy model may open up a new avenue for detecting hydrogen sensing.     

The effect of SnO2 on enhancing electrocatalytic property of palladium toward formic acid oxidation

Although palladium (Pd) based materials are considered the best catalyst for formic acid oxidation reaction (FAOR), they are still confronted with a lot of barriers, such as the growth/sintering of Pd nanoparticles (NPs) and the accumulation of adsorbed poisoning intermediates. Herein, tin dioxide (SnO₂) decorated carbon black was utilized as the catalyst carrier to synthesize Pd/SnO₂/C for FAOR. The introduction of SnO₂ significantly reduced the particle size of Pd NPs and forming the Pd–O–Sn structure. Compared with Pd/C, Pd/SnO₂/C owned higher concentration of O_ads and less adsorption amount of poisoning intermediates. The oxygen atoms adsorbed on Pd surface were rapidly transferred to SnO₂ due to the spillover effect. The FAOR reaction kinetic results showed that the introduction of SnO₂ accelerated the diffusion rate of formic acid on the electrode surface. Pd/SnO₂/C exhibited high specific activity (5.97 mA cm⁻²), excellent durability, and high anti-CO poisoning ability toward FAOR due to the introduction of SnO₂.     

Two operating modes of palladium film hydrogen sensor based on suspended micro hotplate

Palladium film hydrogen sensor based on suspended micro hotplate has been fabricated to operate at elevated temperature with low power consumption. Below 150 °C, the response of the sensor to H₂ is represented by an increase in resistance. At higher temperature, the phenomenon of resistance reduction appears when it comes into contact with H₂. We have researched the reasons for this phenomenon and proposed that the sensitive mechanism is the redox reaction of Pd film on the suspended structure. The suspended substrate can affect the temperature at which redox of the Pd film occurs, and be sensitive to the changes of the surrounding gas stream. When the working temperature is 400 °C, the magnitude of response (S) changes to −0.4% within 2 s for 200 ppm H₂, and S changes to −3% within 10 s for 4000 ppm H₂. This micro hotplate based hydrogen sensor can control the range of operating temperature according to the performance requirements.     

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.     

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.     

SnO2 nanoparticles dispersed carboxylated Poly(arylene ether sulfones) nanocomposites for proton exchange membrane fuel cell (PEMFC) applications

The new poly (arylene ether sulfone) (CPAEs) polymer, and carboxylated through simple Thiol Ene reaction, is characterized by FTIR, ¹H NMR. The SnO₂ nanoparticles are synthesized via alkaline and template free, one-pot hydrothermal method and characterized using HRTEM analysis. SnO₂ nanoparticles in dispersed CPAEs polymer is synthesized and examined by PXRD, SEM and TGA analyses. Further, the typical properties of bare CPAEs and 1%, 2% and 3% SnO₂ NPs of dispersed CPAEs nanocomposite membranes such as water uptake, swelling ratio, ion exchange capacity, proton conductivity and oxidative stability are evaluated. The PXRD pattern suggests the successful formation of amorphous natured CPAEs polymer and tetragonal rutile structured in SnO₂ NPs. It is observed that the SEM images indicate SnO₂ NPs, bare CPAEs polymers as spherical and form wavelike morphology. It is also noted that the HR-TEM image has identified SnO₂ NPs as non-uniform in size with an average particle size of 4 nm. 3% SnO₂ NPs loaded with CPAEs nanocomposite membrane exhibits an IEC value at 0.78 mmol/g-1 and a proton conductivity value of around 1.49 × 10^?3 S/cm^?1 at 100 °C. It shows excellent oxidative stability with a value of 12.3% degradation after being exposed to Fenton reagent at 68 °C for 8 h.     

Stabilized zirconia-based sensor utilizing SnO2-based sensing electrode with an integrated Cr2O3 catalyst layer for sensitive and selective detection of hydrogen

A highly selective hydrogen (H₂) sensor has been successfully developed by using an yttria-stabilized zirconia (YSZ)-based mixed-potential-type sensor utilizing SnO₂ (+30 wt.% YSZ) sensing electrode (SE) with an intermediate Al₂O₃ barrier layer which was coated with a catalyst layer of Cr₂O₃. The sensor utilizing SnO₂ (+30 wt.% YSZ)-SE was found to be capable of detecting H₂ and propene (C₃H₆) sensitively at 550 °C. In order to enhance the selectivity towards H₂, a selective C₃H₆ oxidation catalyst was employed to minimize unwanted responses caused by interfering gases. Among the examined metal oxides, Cr₂O₃ facilitated the selective oxidation of C₃H₆. However, the addition or lamination of Cr₂O₃ to SnO₂ (+30 wt.% YSZ)-SE was found to diminish the sensing responses to all examined gases. Therefore, an intermediate layer of Al₂O₃ was sandwiched between the SE layer and the catalyst layer to prevent the penetration of Cr₂O₃ particles into the SE layer. The sensor using SnO₂ (+30 wt.% YSZ)-SE coated with a catalyst layer of Cr₂O₃ as well as an intermediate layer of Al₂O₃ exhibited a sensitive response toward H₂, with only minor responses toward other examined gases at 550 °C under humid conditions (21 vol.% O₂ and 1.35 vol.% H₂O in N₂ balance). A linear relationship was observed between sensitivity and H₂ concentration in the range of 20–800 ppm on a logarithmic scale. The results of sensing performance evaluation and polarization curve measurements indicate that the sensing mechanism is based on the mixed-potential model.     

Hydrogen sensing properties of ZnO nanorods: Effects of annealing,temperature and electrode structure

In this study, the hydrogen (H₂) sensing properties of vertically aligned zinc oxide (ZnO) nanorods were investigated depending on annealing, Pd coating, temperature and electrode structure. ZnO nanorods were fabricated by using hydrothermal method on a glass substrate and an indium tin oxide (ITO) coated glass substrate. In order to determine the effects of annealing on the H₂ sensor performance, the nanorods were heated at 500 °C in dry air. H₂ sensing measurements were done in the temperature range of 25–200 °C. It was found that, the sensor response of Pd coated ZnO nanorods were much higher than the un-coated nanorods due to the catalytic effect of Pd thin film. Moreover, the un-annealed samples showed better sensor response than the annealed samples due to the number of oxygen deficiency. In addition, the lateral electrode structure showed higher sensor response than the sandwich electrode structure.     

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.     

Improved hydrogen storage performance of defected carbon nanotubes with Pd spillover catalysts dispersed using supercritical CO2 fluid

Hydrogen storage properties of carbon nanotubes (CNTs) modified by oxidative etching and decoration of Pd spillover catalysts are investigated. A mixed H₂SO₄/H₂O₂ solution containing ferrous ions (Fe²⁺) is useful to open the caps, to shorten the length, and to generate defects on CNTs. The Pd catalysts are deposited on the CNTs with the aid of supercritical carbon dioxide (scCO₂); as a result, a highly dispersed Pd nanoparticles and an intimate connection between Pd and carbon surface can be obtained. Combination of the two approaches can optimize a hydrogen spillover reaction on CNTs, resulting in a superior hydrogen storage capacity of 1.54 wt% (at 25 °C and 6.89 MPa), which corresponds to an enhancement factor of ∼4.5 as compared to that of pristine CNTs.     

Ultra-sensitive hydrogen gas sensors based on Pd-decorated tin dioxide nanostructures: Room temperature operating sensors

We have investigated the fabrication of hydrogen gas sensors based on networks of Pd nanoparticles (NPs) deposited tin dioxide nanowires (NWs). SnO₂ NWs with tin NPs attached on the surface were obtained by a simple thermal evaporation of SnO crystalline powders. The tin dioxide NWs were decorated with Pd NPs by the reduction process in Pd ion solution. The sensors showed ultra-high sensitivity (∼1.2 × 10⁵%) and fast response time (∼2 s) upon exposure to 10,000 ppm H₂ at room temperature. These sensors were also found to enable a significant electrical conductance modulation upon exposure to extremely low concentrations (40 ppm) of H₂ in the air. Our fabrication method of sensors combining with Pd NPs, Sn NPs and n-type semiconducting SnO₂ NWs allows optimized catalytic and depletion effect and results the production of highly-sensitive H₂ sensors that exhibit a broad dynamic detection range, fast response times, and an ultra-low detection limit.     

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.