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.     

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.     

Palladium nanoparticle binding in functionalized track etched PET membrane for hydrogen gas separation

In this work, track-etched poly (ethylene terephthalate) (PET) membranes having different pore sizes were functionalized by the carboxylic groups and the amino groups. Palladium (Pd) nanoparticles of average diameter 5 nm were synthesized chemically and deposited onto pore walls as well as on the surface of these pristine and functionalized membranes. Effect of Pd nanoparticles binding on these membranes were explored and aminated membrane were found to bind more Pd nanoparticles due to its affinity. The morphology of these composite membranes is characterized by Scanning Electron Microscope (SEM) for confirmation of Pd nanoparticle deposition on pore wall as well as on the surface. Gas permeability of functionalized and non-functionalized membranes for hydrogen and carbon dioxide has been examined. From the gas permeability data of hydrogen (H₂) and carbon dioxide (CO₂) gases, it was observed that these membranes have higher permeability for H₂ as compared with CO₂. Due to absorption of hydrogen by Pd nanoparticles selectivity of H₂ over CO₂ was found higher as compared to without Pd embedded membranes. Such type of membranes can be used to develop hydrogen selective nanofilters for purification/separation technology.     

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.     

Reduced graphene oxide-supported Pd@Au bimetallic nano electrocatalyst for enhanced oxygen reduction reaction in alkaline media

Rational design and synthesis of core-shell bimetallic nanoparticles with tailored structural and functional properties is highly sought to realize clean and energy-efficient fuel cell systems. Herein, PdAu bimetallic nanoparticles (NPs) with core-shell morphology (Pd_Core–Au_Shell) were fabricated on the surface of reduced graphene oxide (RGO) support by a facile two-step protocol. In the first step, Pd_Core–Ag_Shell bimetallic NPs were synthesized on RGO support by reducing Pd²⁺and Ag⁺ ions with methyl ammonia borane (MeAB). Later, Pd_Core–Au_Shell bimetallic NPs were conveniently fabricated on RGO support via a galvanic replacement strategy involving sacrificial oxidation of metallic silver and reduction of gold ions. The resulting core/shell bimetallic NPs were characterized by X-ray diffraction (XRD), High-resolution transmission electron microscopy (HR-TEM), Energy dispersive X-ray spectroscopy (EDS), Fourier-Transform Infrared Spectroscopy (FT-IR) and cyclic voltrammetry (CV). The electrocatalytic performance of core/shell nanostructures for the room temperature oxygen reduction reaction (ORR) in alkaline media were systematically performed by CV. The electrode-area-normalized ORR activity of RGO-supported Pd_Core–Au_Shell NPs was higher than the corresponding commercially available carbon-supported Pt nanoparticles (Pt/C) at ?0.8 V vs Ag/AgCl (satd. KCl) (6.24 vs 5.34 mA cm^?2, respectively). Further, methanol-tolerant ORR activities of as-synthesized catalysts were also studied. The Au-on-Pd/RGO bimetallic NPs presented enhanced ORR activity both in presence and in the absence of methanol in comparison with a commercial Pt/C catalyst and as-synthesized Pd/RGO and Au/RGO catalysts. The enhanced catalytic activities of core/shell structures might be resulted owing to the optimized core/shell structure comprising of a small Pd core and a thin Au shell and synergistic effects offered by Pd and Au. The present synthesis protocol demonstrated for two-layer structure can be extended to multi-layered structures with desired functions and activities.     

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.     

Pd/NiO core/shell nanoparticles on La0.02Na0.98TaO3 catalyst for hydrogen evolution from water and aqueous methanol solution

Novel Pd/NiO core/shell nanoparticles (NPs) have been synthesized by a simple impregnation method with low temperature processing, in a ‘green’, scalable process using nontoxic chemicals. The cocatalyst consisting of a Pd core and a NiO shell formed simultaneously on the surface of La-doped NaTaO₃ photocatalyst. The Pd core both induces migration of photogenerated electrons from the bulk of the La_0.02Na_0.98TaO₃ and transfers electrons to the NiO shell. Without the NiO shell, Pd NPs show negligible H₂ production from water splitting, due to the rapid reaction between hydrogen and oxygen on the surface. On the other hand, the NiO shell allows the permeation of hydrogen and enables hydrogen reduction on Pd. The incorporation of NiO shell onto Pd remarkably enhances the photocatalytic performance of La-doped NaTaO₃ for hydrogen production from pure water. In addition, the core/shell structure can significantly enhance the stability of Pd during the photocatalytic reaction. Similar concepts could be extended to other applications, where the catalytic activity and stability are of concerns. The formation mechanism of the core/shell photocatalyst is proposed based on the high resolution transmission electron microscopy (HRTEM) images and X-ray absorption near-edge structure (XANES) analyses.     

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.     

On-chip growth of patterned ZnO nanorod sensors with PdO decoration for enhancement of hydrogen-sensing performance

In this study, we used a low-temperature hydrothermal technique to fabricate arrays of sensors with ZnO nanorods grown on-chip. The sensors on the glass substrate then were sputter decorated with Pd at thicknesses of 2, 4, and 8 nm and annealed at 650 °C in air for an hour. Scanning electron microscopy, high resolution transmission microscopy, X-ray diffraction, and surface analysis by X-ray photoelectron spectroscopy characterization demonstrated that decoration of homogenous PdO nanoparticles on the surface of ZnO nanorods had been achieved. The sensors were tested against three reducing gases, namely hydrogen, ethanol, and ammonia, at 350, 400, and 450 °C. The ZnO nanorods decorated with PdO particles from the 2 and 4 nm layers showed the highest responses to H₂ at 450 and 350 °C, respectively. These samples also generally exhibited better selectivity for hydrogen than for ethanol and ammonia at the same concentrations and at all tested temperatures. However, the ZnO nanorods decorated with PdO particles from the 8 nm layer showed a reverse sensing behaviour compared with the first two. The sensing mechanism behind these phenomena is discussed in the light of the spillover effect of hydrogen in contact with the PdO particles as well as the negative competition of the PdO thin film formed between the sensor electrodes during sputter decoration, Pd–Zn heterojunction that forms at high temperature and thus influences the conductivity of the ZnO nanorods.     

Selective deposition of Pd nanoparticles in porous PET membrane for hydrogen separation

Hydrogen purification based on Pd deposition in porous polymeric membranes show promising results for hydrogen permeability and selectivity. It is due to high absorption property of Pd nanoparticles. In this work, gas permeability of carboxylic group functionalized Polyethylene terephthalate (PET) membranes with different time of functionalization have been examined. It has been found that PET membrane having more –COOH group shows higher selectivity for Hydrogen (H₂). Further to improve the selectivity, these carboxylated PET membranes dipped in Pd nanoparticles solution for 6 h and found more selective for H₂ in comparison to Carbon dioxide (CO₂) and Nitrogen (N₂). As the carboxylation increases selectivity of H₂ improves drastically in the beginning and nearly get saturated after 24 h. Similar trend has been observed for these membranes after Pd nanoparticles deposition. Fourier transform infrared spectroscopy (FTIR) spectra of these membranes revealed that intensity of peaks related to –COOH group at 2968 cm^?1 & 1716 cm^?1 increases with functionalization time. Field Emission Scanning Electron Microscopy (FESEM) was used to study the surface morphology of membranes.     

2-D WO3 decorated with Pd for rapid gasochromic and electrical hydrogen sensing

The ultrathin two-dimensional (2D) nanomaterials display unique properties owing to their ultrahigh specific surface area and strong quantum confinement of electrons in two dimensions. In this work, we fabricated a rapid gasochromic and electrical hydrogen sensing system containing 2D WO₃ and Pd nanoparticles. 2D WO₃ nano-plates (NP) are synthesized using sol–gel method and Pd nanoparticles are coated on WO₃ by green photochemical deposition method. The sensor is fabricated by dispersing the 2D WO₃/Pd composite on filter paper. In presence of hydrogen gas, 2D WO₃/Pd composite produces visible change in color from brown to dark blue. With the fabricated sensor, as low as 0.1% H₂ gas in air at room temperature can be easily detected using electrical sensing scheme whereas for higher concentration from 1 to 100%, eye readable gasochromic scheme can be utilized. The use of 2D WO₃ decreased the response time in great deal compared to WO₃ nanoparticles indicating the advantage of 2D structure in fabricating rapid response H₂ sensors. The proposed method is simple and can be easily employed to large scale fabrication system for commercial applications.     

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.     

Hydrogen storage in platinum loaded single-walled carbon nanotubes

To study the hydrogen storage capacity, platinum (Pt) nanoparticles were deposited on single-walled carbon nanotubes (SWNT) using hexachloroplatinic acid (H₂PtCl₆·6H₂O) as a precursor. To verify Pt deposition on the surface of the SWNT, a Transmission Electron Microscope (TEM) was used to obtain surface morphology. The TEM images show that Pt nanoparticles were homogeneously distributed on the surface of SWNT. Commercial SWNT were also used to compare the results. Thermal Gravimetric Analysis at heating rate of 5 °C/min is measured for pure SWNT and Pt loaded SWNT. Before hydrogen storage measurements these samples were reduced in 10% of H₂ in Ar, flowing at 900 °C in a tubular furnace for 1 hour. Hydrogen storage capacity of these SWNT was investigated under 25 bar pressure and room temperature as well as liquid nitrogen temperature.     

Enhancement of hydrogen storage capacity of multi-walled carbon nanotubes with palladium doping prepared through supercritical CO2 deposition method

Pd doped Multi-Walled Carbon Nanotubes were prepared via supercritical carbon dioxide deposition method in order to enhance the hydrogen uptake capacity of carbon nanotubes at ambient conditions. A new bipyridyl precursor that enables reduction at moderate conditions was used during preparation of the sample. Both XRD analyses and TEM images confirmed that average Pd nanoparticle size distribution was around 10 nm. Hydrogen adsorption and desorption experiments at room temperature with very low pressures (0–0.133 bar) were conducted together with temperature programmed desorption (TPD) and reduction (TPR) experiments on undoped and doped materials to understand the complete hydrogen uptake profile of the materials. TPD experiments showed that Pd nanoparticles increased the hydrogen desorption activity at moderate temperatures around at 38 °C while for undoped materials it was determined around at 600 °C. Moreover, a drastic enhancement of hydrogen storage was recorded from 44 μmol/g sample for undoped material to 737 μmol/g sample for doped material through adsorption/desorption isotherms at room temperature. This enhancement, also verified by TPR, was attributed to spillover effect.     

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

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

Enhanced photo-fermentative hydrogen production of Rhodopseudomonas sp. nov. strain A7 by the addition of TiO2, ZnO and SiC nanoparticles

In order to strengthen photo-fermentative hydrogen production by different photocatalytic nanoparticles, hydrogen production by photo-fermentative bacteria with addition of TiO₂, ZnO and SiC nanoparticles in batch culture were investigated in this study. The results indicated that three nanoparticles could improve hydrogen production performance of Rhodopseudomonas sp. nov. strain A7 under respective optimal conditions. The hydrogen yield of 2.81 mol-H₂/mol-acetate was obtained when TiO₂ nanoparticles with a concentration of 300 mg/L and size of 25 nm. The concentration of ZnO nanoparticles was at 100 mg/L, hydrogen yield reached 2.64 mol-H₂/mol-acetate. Compared with TiO₂ and ZnO nanoparticles, SiC nanoparticles exhibited greatest potential for enhancing photo-hydrogen production. By addition of nano-SiC with concentration of 200 mg/L which was prepared at temperature of 1500 °C, the maximum hydrogen volume, average hydrogen content and hydrogen yield of strain A7 were achieved at 2272 mL-H₂/L-culture, 85.2% and 2.99 mol-H₂/mol-acetate, respectively. And hydrogen production was 18.6% higher than that of alone strain A7 without the addition of nanoparticles. Therefore, the addition of SiC nanoparticles is a promising strategy to improve photo-fermentative hydrogen production from wastewater.     

Pd/NH2-KIE-6 catalysts with exceptional catalytic activity for additive-free formic acid dehydrogenation at room temperature: Controlling Pd nanoparticle size by stirring time and types of Pd precursors

Pd nanoparticle size is one of important factors to determine the catalytic activity of formic acid dehydrogenation catalysts. Thus various approaches to minimization of Pd nanoparticles have been attempted. In this study, we first tried to decrease Pd nanoparticles size and increase Pd dispersion of Pd/NH₂-mesoporous silica (Pd/NH₂-KIE-6) catalysts by controlling only stirring time and types of Pd precursors. It was demonstrated that the stirring time and types of Pd precursors significantly affect the performance of the catalysts. As a result, the Pd/NH₂-KIE-6 exhibited the highest catalytic activity (TOF: 8185 mol H₂ mol catalyst^?1 h^?1) ever reported for additive-free formic acid dehydrogenation at room temperature. In addition, the Pd/NH₂-KIE-6 provided higher TOF even than the case with additives such as sodium formate. Considering that the catalytic activity of Pd-based catalysts for formic acid dehydrogenation was previously controlled by promoter, support type and surface chemistry of supports, controlling the stirring time and types of Pd precursors is novel and very intriguing solutions to go beyond the current kinetic limitation for formic acid dehydrogenation.     

Enhanced performance of direct peroxide/peroxide fuel cell by using ultrafine Nickel Ferric Ferrocyanide nanoparticles as the cathode catalyst

Direct peroxide-peroxide fuel cell (DPPFC) employing with H₂O₂ both as the fuel and oxidant is an attractive fuel cell due to its no intermediates, easy handling, low toxicity and expense. However, the major gap of DPPFC is the cathode performance as a result of the slow reaction kinetics of H₂O₂ electro-reduction and thus the target issue is to design cathode catalysts with high performance and low cost. Herein, different with using noble metal of state-of-the-art, we have successfully synthesized ultra-fine NiFe ferrocyanide (NiFeHCF) nanoparticles (the mean particles size is 2.5 nm) through a co-precipitation method, which is used as the cathode catalyst towards H₂O₂ reduction in acidic medium. The current density of H₂O₂ reduction on the resultant NiFeHCF electrode after the 1800 s test period at ?0.1, 0 and 0.1 V are 121, 93 and 76 mA cm², respectively. Meanwhile, a single two-compartment DPPFC cell with NiFeHCF nanoparticles as the cathode and Ni/Ni foam as the anode is assembled and displayed a stable OCP of 1.09 V and a peak power density of 36 mW cm^?2 at 20 °C, which is much higher than that of a DPPFC employed with Pd nano-catalyst as cathode.     

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.