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.     

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.     

Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases

The hydrogen-based economy is one of the possible approaches toward to eliminate the problem of global warming, which are increases because of the gathering of greenhouse gases. Palladium (Pd) is well-known material having a strong affinity to the hydrogen absorbing property and thus appropriate material to embed in the membrane for the improvement of selective permeation of hydrogen gas. In present work, we have functionalized polycarbonate (PC) membranes with the help of UV irradiation to embed the Pd nanoparticles in pores as well as on the surface of the PC membrane. Use of Pd Nanoparticles is helpful to enhance the H₂ selectivity over other gases (CO₂, N_2, etc.). Also, the UV based modification of membrane increases the attachment of Pd Nanoparticles. Further to enhance the Pd nanoparticles attachment, we used PVP binder with Pd nanoparticles solution. Gas permeability measurements of functionalized PC membranes have been carried out, and better selectivity of hydrogen has been found in the functionalized and Pd nanoparticle binded membrane. PC membrane with 48 h UV irradiated and Pd NPs with PVP have been found to have maximum selectivity and permeability for H₂ gas. All the samples being characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and UV–Vis spectroscopy for their morphological and structural investigation.     

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.     

Carbon-supported PdSn–SnO2 catalyst for ethanol electro-oxidation in alkaline media

Carbon-supported PdSn–SnO₂ with high electrical catalytic activity for ethanol oxidation in alkaline solution was synthesized using an impregnation reduction method. XRD analysis of the as-prepared PdSn–SnO₂/C revealed that the Pd diffraction peaks shifted to lower 2θ values with respect to the corresponding peaks of the Pd/C catalyst, indicating that Sn doping could shrink the Pd crystalline lattice. XPS measurements confirmed the existence of Sn and SnO₂ in the PdSn–SnO₂/C catalysts. The prepared PdSn–SnO₂/C catalysts presented a remarkably higher electrocatalytic activity than that of the Pd–Sn/C and Pd/C catalysts. This was mainly because the easy adsorption-dissociation of OH_ads over the SnO₂ surface changed the electronic effect and accelerated the adsorption of ethanol on the surface of Pd, thus enhancing the overall ethanol oxidation kinetics and contributing to a significant improvement in the catalytic activity.     

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.     

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.     

The performance of Pt nanoparticles supported on Sb2O5.SnO2, on carbon and on physical mixtures of Sb2O5.SnO2 and carbon for ethanol electro-oxidation

Pt nanoparticles were supported on Sb₂O₅.SnO₂ (ATO), on carbon and on physical mixtures of ATO and carbon by an alcohol-reduction process using ethylene glycol as reducing agent. The obtained materials were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Their performance for ethanol oxidation was investigated at room temperature by chronoamperometry and in a direct ethanol fuel cell (DEFC) at 100 °C. Pt nanoparticles supported on a physical mixture of ATO and carbon showed a significant increase of performance for ethanol oxidation compared to Pt nanoparticles supported on ATO or on carbon.     

Ultrasonic assisted synthesis of a nano-sized Co2SnO4/graphene: A potential material for electrochemical hydrogen storage application

A facile and rapid sonochemical method has been developed for the synthesis of Co₂SnO₄ nanostructures in presence of glucose as a green capping agent, for the first time. The effect of different parameters such as ultrasonic irradiation, time irradiation, basic agent and solvent were studied to reach optimum size and morphology conditions. The optimized Co₂SnO₄ nanostructures anchored onto graphene sheets and Co₂SnO₄/Graphene nanocomposite synthesized through pre-graphenization, successfully. In this paper, hydrogen storage capacity of optimized Co₂SnO₄ nanostructures and Co₂SnO₄/Graphene nanocomposite compared for the first time. Co₂SnO₄/Graphene nanocomposite show more excellent electrochemical performance than pure Co₂SnO₄ nanoparticles. It was found that the synergistic effect between Co₂SnO₄ nanoparticles and graphene sheets can improve the electrochemical performance of this hybrid composites electrode. After 25 cycles, the discharging capacities of the Co₂SnO₄ nanostructure and Co₂SnO₄/Graphene nanocomposite electrode reach 1190 and 2700 mAh/g, respectively.     

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.     

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.     

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.     

Investigation of WO3/ZnO thin-film heterojunction-based Schottky diodes for H2 gas sensing

A comparative study of Schottky diode hydrogen gas sensors based on Pd/WO₃/Si and Pd/WO₃/ZnO/Si structure is presented in this work. Atomic force microscopy and X-ray photoelectron spectroscopy reveal that the WO₃ sensing layer grown on ZnO has a rougher surface and better stoichiometric composition than the one grown on the Si substrate. Analysis of the I–V characteristics and dynamic response of the two sensors when exposed to different hydrogen concentrations and various temperatures indicate that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10 times higher sensitivity, and shorter response and recovery times (105 s and 25 s, respectively) towards 10,000-ppm H₂/air at 423 K. Study on the energy band diagram of the diode suggests that the barrier height is modulated by the WO₃/ZnO heterojunction, which could be verified by the symmetrical sensing properties of the Pd/WO₃/ZnO/Si gas sensor with respect to applied voltage.     

DFT study of H2 adsorption on Pd-decorated single walled carbon nanotubes with C-vacancies

Carbon nanotubes are considered important materials for hydrogen storage. Although the C–H interaction is very weak at room temperature, the incorporation of highly dispersed Pd nanoparticles increases the H₂ adsorption on carbon surfaces. In this work we performed density functional theory studies of H₂ adsorption on single walled carbon nanotubes (SWCNTs) with C-vacancies and a Pd decoration. We used the VASP and SIESTA codes. Our calculations show that Pd adsorption is favored on the C-vacancies of the (5,5) SWCNT, while H₂ adsorption also occurs preferentially on C-defective sites.     

Design and preparation of CNT@SnO2 core-shell composites with thin shell and its application for ethanol oxidation

A novel electrocatalyst structure of carbon nanotubes (CNT) coated with thin SnO₂ and Pt (Pt/(CNT@SnO₂)) is reported. The CNT@SnO₂ composites with a thin shell (about 2 nm) are prepared by a simple chemical-solution method. The Pt/(CNT@SnO₂) catalyst is prepared by first microwave heating H₂PtCl₆ in NaOH ethylene glycol solution and then depositing Pt nanoparticles onto CNT@SnO₂ composites. High-resolution transmission electron microscopy and X-ray diffraction show that crystalline SnO₂ of about 2 nm thickness is coated uniformly on the surface of the CNTs. Pt nanoparticles of about 3.2 nm in diameter are homogenously dispersed on the SnO₂ surface. Electrochemical studies were carried out using cyclic voltammetry and chronoamperometry. The results showed that Pt/(CNT@SnO₂) catalysts have much higher catalytic activity and CO-tolerance for ethanol electro-oxidation than that of Pt/CNT.     

Photocatalytic H2 production from acetic acid solution over CuO/SnO2 nanocomposites under UV irradiation

The CuO/SnO₂ composites have been prepared by the simple co-precipitation method and further characterized by the XRD, FESEM and Raman spectroscopy. The photocatalytic H₂ production from acetic acid (HAc) solution over CuO/SnO₂ photocatalyst has been investigated at room temperature under UV irradiation. Effects of CuO loading, photocatalyst concentration, acetic acid concentration and pH on H₂ production have been systematically studied. Compared with pure SnO₂, the 33.3 mol%CuO/SnO₂ composite exhibited approximately twentyfold enhancement of H₂ production. The H₂ yield is about 0.66 mol-H₂/mol-HAc obtained under irradiation for prolonged time. The Langmuir-type model is applied to study the dependence of hydrogen production rate on HAc concentration. A possible mechanism for photocatalytic degradation of acetic acid over CuO/SnO₂ photocatalyst is proposed as well. Our results provide a method for pollutants removal with simultaneous hydrogen generation. Due to simple preparation, high H₂ production activity and low cost, the CuO/SnO₂ photocatalyst will find wide application in the coming future of hydrogen economy.     

Platinum nanoparticles embedded in layer-by-layer films from SnO2/polyallylamine for ethanol electrooxidation

Self-assembled films from SnO₂ and polyallylamine (PAH) were deposited on gold via ionic attraction by the layer-by-layer (LbL) method. The modified electrodes were immersed into a H₂PtCl₆ solution, a current of 100 μA was applied, and different electrodeposition times were used. The SnO₂/PAH layers served as templates to yield metallic platinum with different particle sizes. The scanning tunnel microscopy images show that the particle size increases as a function of electrodeposition time. The potentiodynamic profile of the electrodes changes as a function of the electrodeposition time in 0.5 mol L⁻¹ H₂SO₄, at a sweeping rate of 50 mV s⁻¹. Oxygen-like species are formed at less positive potentials for the Pt–SnO₂/PAH film in the case of the smallest platinum particles. Electrochemical impedance spectroscopy measurements in acid medium at 0.7 V show that the charge transfer resistance normalized by the exposed platinum area is 750 times greater for platinum electrode (300 kΩ cm²) compared with the Pt–SnO₂/PAH film with 1 min of electrodeposition (0.4 kΩ cm²). According to the Langmuir–Hinshelwood bifunctional mechanism, the high degree of coverage with oxygen-like species on the platinum nanoparticles is responsible for the electrocatalytic activity of the Pt–SnO₂/PAH concerning ethanol electrooxidation. With these features, this Pt–SnO₂/PAH film may be grown on a proton exchange membrane (PEM) in direct ethanol fuel cells (DEFC).     

Ethanol oxidation on carbon supported (PtSn)alloy/SnO2 and (PtSnPd)alloy/SnO2 catalysts with a fixed Pt/SnO2 atomic ratio: Effect of the alloy phase characteristics

To evaluate the effect of the alloy phase characteristics on the ethanol oxidation activity, carbon supported (PtSnPd)_alloy/SnO₂ catalysts were prepared and their electrocatalytic activity compared with that of carbon supported (PtSn)_alloy/SnO₂. Pt-Sn-Pd/C samples in the atomic ratio (1:1:0.3) and (1:1:1) were characterized by energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). XRD analysis shows the presence of fcc Pt reflexions, shifted to lower angles, and SnO₂ reflexions. By comparison with the XRD patterns of carbon supported Pt-Sn (1:1) and Pt-Pd (3:1) samples, prepared by the same method, the formation of ternary PtSnPd alloys is postulated. The crystallite size of the ternary catalysts is smaller than that of both binary Pt-Sn/C (1:1) and Pt-Pd/C (3:1) catalysts. Chronoamperometry experiments and tests in direct ethanol fuel cells of the as-prepared catalysts shows that the activity for ethanol oxidation of (PtSn)_alloy/SnO₂ is higher than that of (PtSnPd)_alloy/SnO₂. This result, obtained with the same Pt/SnO₂ atomic ratio in all the samples, indicates the critical role of the alloy phase characteristics of these catalysts on their activity for ethanol oxidation.     

Palladium nanoparticle deposition onto the WO3 surface through hydrogen reduction of PdCl2: Characterization and gasochromic properties

Nowadays, gasochromic Pd/WO₃ coatings as optically switchable materials have become more applicable for hydrogen sensors and smart windows. In this study, WO₃ films were prepared by Pulsed Laser Deposition (PLD) and spin-coating sol-gel techniques. For deposition of Pd, first a layer of PdCl₂ was obtained via a simple drop-drying process by dropping PdCl₂ solution onto WO₃ substrates and drying them at room temperature. Then Pd nanoparticles were synthesized via hydrogen gas exposure that causes reduction of the PdCl₂ layer. According to Scanning Electron Microscope (SEM) observations before hydrogen reduction, many individual nanoparticles or fractal-like constructions of palladium were formed in the PdCl₂ layer in which the fractal branches were distorted after hydrogen treatment. Surface chemistry of the observed Pd nanoparticles was studied using X-ray Photoelectron Spectroscopy (XPS) at different stages of the reduction process. The results showed that after hydrogen treatment, the chlorine atoms were desorbed from the PdCl₂ layer and a metallic Pd layer remained on the surface of WO₃. Gasochromic properties in the presence of H₂ or O₂ gases for different PdCl₂ amounts revealed that the rate and saturated level of coloring depends on the PdCl₂ amounts as well as on the preparation method of the WO₃ substrates due to different porosities.