Thin Pd Layer Research Articles

RVC as new carbon material for batteries

Results of the last 10 years of our investigations on the use of reticulated vitreous carbon (RVC®) in various types of cells show that this kind of porous conductive glassy carbon can be successfully used as a matrix for active mass and as a current collector in many primary and secondary batteries. This paper focuses mainly on the use of RVC® modified with metals like Pt, Rh, Pd, Ni, Pb and their oxides in construction of the following types of cells: (1) zinc-carbon primary and secondary batteries, (2) rechargeable cell systems with NiOOH cathode, (3) electrochemical capacitors with Pd and Pd–Rh alloys, and (4) lead acid battery. The most important improvement in performance has been achieved with lead-acid and zinc-carbon batteries. Especially for the modified zinc-carbon battery, which has been constructed in the popular AA format, manufactured on a small (laboratory) scale, and tested according to the IEC norms as a battery for commercial use, the capacity has been higher by ca. 20% compared with the batteries available on the market. Also, results obtained for thin layers of Pd and their alloys deposited on RVC® are promising in terms of the application of these kinds of electrodes in electrochemical capacitors. The pseudocapacitance of a Pd–Rh/RVC® composition may reach 540 F g−1.

Evidence for large hydrogen capacity in single-walled carbon nanotubes encapsulated by thin Pd layers onto a Pd substrate

We report a study of hydrogen storage and its mechanism in a novel material, representing single-walled carbon nanotubes (SWCNTs) encapsulated by thin Pd layers onto a Pd substrate. A synergetic effect resulting in combination of the Pd and the SWCNT properties with regard to hydrogen has been achieved. We showed that adding SWCNTs increases the H 2-capacity of the Pd–SWCNT composite under electrochemical loading only by up to 25% relative to the Pd metal alone. At the same time, with regard to the added SWCNTs, such synergetic approach (providing high H 2 pressure from highly H-loaded massive Pd substrate into a small fraction of deposited SWCNT) allowed us to achieve a net capacity of 8–12 wt.%. H 2, thus, bringing a unique chance to study hydrogen storage mechanism in highly H-loaded SWCNT. Using ESR technique it was established that the Pd–C x π-complexes forming at the openings of SWCNTs could be considered as hydrogen adsorption sites, providing both high gravimetric capacity (H/C > 1) and low hydrogen binding energy in the Pd encapsulated SWCNT.

Growth and Characterization of GaN Nanowires for Hydrogen Sensors

We report on the growth and characterization of high-quality GaN nanowires for hydrogen sensors. We grew the GaN nanowires by catalytic chemical vapor deposition (CVD) using gold thin films as a catalyst on a Si wafer with an insulating SiO2 layer. Structural characterization of the as-grown nanowires by several methods shows that the nanowires are single-crystal wurtzite GaN.␣Photoluminescence measurements under 325 nm excitation show a near-band-edge emission peak around ∼3.4 eV. The hydrogen sensors are fabricated by contacting the as-grown GaN nanowires by source and drain electrodes and coating them with a thin layer of Pd. Hydrogen sensing experiments using the fabricated devices show high sensitivity response (ppm detection limit at room temperature) and excellent recovery. This work opens up the possibility of using high-quality GaN nanowire networks for hydrogen sensing applications.

Surface alloying and mixed valence in thin layers of Ce and Pd on Ru(0 0 0 1)

The Ce/Pd/Ru(0001) system has been studied by photoelectron spectroscopy and low energy electron diffraction. The Pd overlayer thicknesses were in the range from one to two monolayers. The effective Ce overlayer thicknesses were in the range from 0.5 to 1.5 monolayers. The interfaces were studied for annealing temperatures from room temperature up to 1030°C. A tendency of intermixing of Pd and Ce was observed already at room temperature. The estimated Ce valence and 4f–4d hybridization strength were found to be largest for the most Pd rich surfaces. The onset of desorption of Ce and Pd takes place at a temperature of 700°C, which is considerably lower than the temperature of onset of desorption of Ce from the Ce–Pd system. This is argued to be due to a weakening of the substrate bonds when stronger Ce–Pd bonds form. Intermixing between the Ru substrate and Pd and Ce was not observed. The low value of the sample work function that was recorded throughout these studies shows that Ce was always present at the outermost surface layer.

Electrical conductivity dependence of thin metallic films of Au and Pd as a top electrode in capacitor applications

Electrical conductivity dependence of thin metallic films of Au and Pd over the different perovskites was investigated. It is found from electrical properties that crystallographic growth orientation of Au and Pd thin layers attained from X-ray diffraction results indicate the slop of current (I)–voltage (V) plots. Besides, surface morphology and topography was considered using Field Emission Scanning Electron Microscopy and Atomic Force Microscopy, respectively. Obtained results showed the Stranski–Krastanov growth of the Pd and Au. Indeed, diminishing of the root-mean-square roughness of Pd/BiMnO3/SrTiO3 following by Au deposition should be concerned due to growth of Au onto the crack-like parts of the substrate. These crack-like parts appeared due to parasitic phases of the Bi–Mn–O system mainly Mn3O4 (l0l) and Mn3O4 (004l).The different response in the electrical properties of heterostructures suggests that electrical conductance of the Au and Pd thin metallic films have the crystallographic orientation dependence. Furthermore, polycrystallinity of the thin metallic films are desired in electrode applications due to increase the conductivity of the metallic layers.

Gasochromic Properties of Mg–Ni Switchable Mirror Thin Films on Flexible Sheets

In this work, we produced a switchable mirror thin film based on a Mg4Ni thin film with a Pd top layer deposited on transparent polymer sheets. The Mg4Ni thin film was prepared on poly(ethylene naphthalate) and poly(ethylene terephthalate) substrates by the DC magnetron cosputtering of Mg and Ni targets followed by in situ covering with a Pd thin layer. The gasochromic switching properties, durability, and response (hydrogenation and dehydrogenation kinetics) were investigated by loading 4% hydrogen in Ar. We found that Pd/Mg4Ni thin film mirrors on a poly(ethylene naphthalate) sheet have superior switching properties. The hydrogenation time (transition from the metallic state to the transparent state) of the sample is almost the same as the hydrogenation time of the same sample prepared on a glass substrate, whereas the dehydrogenation time (transition from the transparent state to the metallic state) is shorter than that of the glass substrate sample. In addition, the switching durability of the sample on the flexible sheet is extended to 200 switching cycles. These results show that the switchable mirror deposited on flexible sheets has a similar performance to the sample on glass substrates.

Preparation of Pdshell–Aucore/SiO2 catalyst and catalytic activity for acetylene hydrogenation

Various Pd shell thicknesses (0.12–1.5 nm) were synthesized on preformed Au particles of 5 nm size by seeded growth technique (15–80 at% Pd) using sodium citrate and tannic acid. The sols were characterized by UV–vis spectroscopy, TEM and high-resolution TEM (HRTEM) measurements. HRTEM confirmed the pseudomorphic growth of the Pd shell on the Au core. The Au/Pd core/shell particles were fixed onto SiO2 (Aerosil 200) support by PDDA polycation. The catalytic activity in acetylene hydrogenation and selectivity of competition between acetylene and propene were tested after O2 and H2 pretreatment. The samples even in “as prepared” state hydrogenated acetylene. The thin Pd layer (15–30 at% Pd) on Au provided higher hydrogenation activity than the thick Pd shell. However, thermal treatment of the samples in H2 stream causing Au/Pd intermixing shifted the activity maximum to higher Pd concentration (68–80 at% Pd). Comparison of the TOF (1/s) and selectivity values allowed us to conclude that the homogenized particle with 68–80 at% Pd shows better hydrogenation activity and selectivity than the thin Pd shell (15–30 at% Pd) on the Au core.

Hydrogen in thin Pd-based layers deposited on reticulated vitreous carbon—A new system for electrochemical capacitors

Reticulated vitreous carbon (RVC) has been used as a matrix for electrodeposition of thin layers of Pd and Pd-rich Pd–Rh alloys. It was found that RVC substrate does not affect qualitatively hydrogen absorption behavior of Pd-based deposits. Similarly to thin Pd or Pd alloy layers deposited on Au wires, the α–β phase transition controls the overall rate of hydrogen absorption and desorption into/from Pd-based/RVC electrodes. The possibility of the application of these materials as phase charging–discharging systems was investigated. The values of specific pseudocapacitance, specific power and specific energy were comparable with those for supercapacitors utilizing various redox reactions.

Nano-structured Pd-long period fiber gratings integrated optical sensor for hydrogen detection

A fiber-optic sensor for detecting hydrogen at various temperatures (30–200 °C) is reported in this paper. The sensor is composed of a long period fiber gratings coated with a nano-structured Pd thin layer as the sensing material. The sensing principle of the designed sensor is based on the change of the refractive index of the Pd thin layer upon exposure to hydrogen, which causes the resonance wavelength shift of the long period fiber gratings. The Pd layer prepared by sputter deposition has nanograins of 30–40 nm in size. The resonance wavelength of the long period fiber gratings coated with Pd decreases as hydrogen concentration increases from 0% to 16% at all temperatures. There is a significant wavelength shift of the sensor when the Pd film goes through the phase transition between α and β phases. The sensor has a short response time with a relatively slower recovery rate. The sensor maintains its functionality after several cycles of hydrogen detection and recovery.

Hydrogen generation and purification in a composite Pd hollow fiber membrane reactor: Experiments and modeling

“Pd nanopore” composite membranes are a novel class of H 2 permselective membranes in which a thin layer of Pd is grown within the pores of a supported nanoporous layer. In this work, Pd nanopore membranes and conventional Pd top-layer membranes were used in the generation of high-purity H 2 from the catalytic decomposition of anhydrous NH 3. An effective 4 μm thick Pd nanopore membrane and 13 μm thick Pd top-layer membrane were synthesized on 2 mm O.D. α-Al 2O 3 hollow fibers. The permeation features of the membranes were determined and the membranes were then employed in a single fiber packed-bed membrane reactor in which Ni-catalyzed NH 3 decomposition served as the test reaction, with conditions spanning a range of conditions (500–600 °C; 3–5 bar total retentate pressure; 60–1200 scc/h g cat space velocity). The NH 3 conversions in both the PBMRs were approximately 10% higher than in a packed-bed reactor (PBR) under similar conditions. The increase in conversion with the PBMR was attributed to the removal of H 2, which has an inhibitory effect on the forward kinetics of the reaction as per the Temkin-Pyzhev type rate mechanism. Reactor productivities in the range of 2 mol/s m 3 (at 85% H 2 utilization) to 7 mol/s m 3 (at 50% H 2 utilization) were obtained. The permeate stream purity exceeded 99.2% H 2. A two-dimensional pseudo-homogeneous model was successfully used to simulate the experimental results and to interpret the findings. Permeation and kinetic parameters were estimated in permeation and PBR experiments, respectively. Without any data fitting the PBMR model predictions demonstrated very good agreement with experimental trends. Together with an analysis of the characteristic times, the model determined that transverse transport of hydrogen in the catalyst bed limited PBMR performance. The model was used to determine the rate limiting step and to suggest ways in which the reactor productivities could be further improved.

Effect of deposition conditions on the response and durability of an Mg 4Ni film switchable mirror

We are developing switchable mirror thin films based on an Mg–Ni alloy which can be switched between mirror and transparent states. The Mg 4Ni alloy thin film was prepared on a glass substrate by DC magnetron co-sputtering of Mg and Ni targets and was subsequently covered in situ with a thin layer of Pd. The interface structure between Pd and Mg–Ni layers was found to have a crucial effect on the switching response and durability. By controlling the sputtering sequence and tailoring the interface structure, the switching response can be improved by 50–80% for the dehydrogenation process. The switching durability of the improved thin film is twice as long as that of a typical sample. We have also found that the switching response is dependent on the substrate temperature during sputtering. The dehydrogenation rate can be increased when the substrate is retained below room temperature.

First-Principles Calculations of the Atomic and Electronic Structures in Au-Pd Slab Interfaces

We performed first-principles calculations using the projector augmented-wave (PAW) method for Au/Pd slab interface models. The calculations of relaxed configurations and energies for the thin Pd layers (3 layers) stacking on Au (111) and Au (100) slabs with an epitaxial relationship represent that Pd overlayers have a lateral expansion in both cases. This trend is in good agreement with experimental results for Pd/Au slabs and Au-Pd core-shell nanoparticles, obtained by electron microscopy, X-ray diffraction, and positron annihilation. In addition, an intermixing configuration near the Au-Pd interface was shown to be more stable than the binary separated one.

Microbeam analysis of lateral inhomogeneity in depth penetration of Pd in porous silicon

Porous silicon (PS) is widely utilized in gas sensors. Palladium is a good choice to sensitize the surface of PS to hydrogen. Ideally for highest sensitivity of the sensor, all the pores of PS should be completely covered with palladium. Rutherford backscattering spectroscopy (RBS) technique is routinely used to determine depth profile of elements in thin layers. By using microbeam analysis as a complementary technique, we clearly observed the lateral image of penetrated Pd in depth. In this work, we used electrochemically anodized-silicon wafer doped with Pd by the electroless process. In our analysis, we intentionally characterized both the area of PS where Electric Field is applied during silicon etching (EF area) and the Field Free region (FF area) of the sample. Microbeam analysis show that EF area of the sample is covered with thicker palladium layer, but the pores are rarely filled in this area, while in the FF area of the sample, there is a thin palladium film on the silicon surface, but the pores are partially filled. We used a computational simulation method based on columnar pore structure and linear variation of Pd depth profile in pores and determined the lateral variation of Pd penetration in pores in both parts of the sample. These simulations showed that there are 180±80nm thick Pd layer with 300±200nm penetration in pores in EF region and 80±40nm thin Pd layer with 800±200nm penetrate in pores in FF region.

Evidence for large hydrogen storage capacity in single-walled carbon nanotubes encapsulated by electroplating Pd onto a Pd substrate

We report a study of hydrogen storage in an alternative material, representing single-walled carbon nanotubes (SWCNTs) encapsulated by thin Pd layers onto a Pd substrate. A synergetic effect resulting in combination of the Pd and the SWCNTs properties with regards to hydrogen has been achieved. Adding SWCNTs increases the ${\mathrm{H}}_{2}$ capacity of the Pd-SWCNT composite by up to 25% relative to Pd metal alone under electrochemical loading. This results in a storage capacity of $8--12\phantom{\rule{0.3em}{0ex}}\mathrm{wt}\phantom{\rule{0.2em}{0ex}}%$. with regard to the added SWCNTs.

Effect of thickness and porous structure of SiC layers on the spectral response of the Pd/SiC–pSi Schottky photodiodes

In this work, we study the effect of the thickness and porous structure of silicon carbide (PSC) layers on the electrical properties of Schottky photodiodes by using a palladium (Pd) layer deposited on non-porous silicon carbide (SiC) and porous-SiC (PSC) layers. The non-porous and porous-SiC layers were realized on a p-type silicon (Si(100)) substrate by pulsed laser deposition using a KrF laser (248nm) and thermal deposition of a thin Pd layer. The porous structure of the SiC layer deposited was developed by an electrochemical (anodization) method. The electrical measurements were made at room temperature (295K) in an air ambience. The effect of the porous surface structure and the thickness of the SiC layer were investigated by evaluating electrical parameters such as the ideality factor (n) and barrier height (ϕBp). The thickness of the porous layer significantly affects the electrical properties of the Schottky photodiodes. Analysis of current–voltage (I–V) characteristics showed that the forward current might be described by a classical thermal emission theory. The ideality factor determined by the I–V characteristics was found to be dependent on the SiC thickness a value For a thin SiC layer (0.16μm) n was around 1.325 with a barrier height 0.798eV, while for a thick layer (1.6μm), n and ϕBp were 1.026 and 0.890eV, respectively for Pd/SiC–pSi. These results indicate Schottky photodiodes with high performance are obtained for thicker SiC layer and for thin layer of PSC. This effect showed the uniformity of the SiC layer. In the same case the ideality factor (n) decreases for Pd/PSC–pSi(100) for low SiC thickness by report of Pd/PSC–pSi(100) Schottky photodiodes, but for Pd/PSC–pSi(100) n increase for large SiC thickness layer. We notice that the barrier height (ϕBp) was reversely depend by report of ideality factor. A spectral response value of (SR) of 34mA/W at λ=400nm was measured for Pd/0.16μm SiC–pSi Schottky photodiode with low SiC thickness. On the other hand, a value of SR=0.14mA/W at λ=900nm was obtained when we used PSC layer (Pd/PSC–pSi(100)). A reverse behaviour occurs for thicker SiC layer. Finally, it was found that the thickness and surface porous structure have strong effect on sensitivity.

Preparation of Thin Palladium Composite Membranes and Application to Hydrogen/Nitrogen Separation

Thin palladium composite membranes were prepared by modified electroless plating method on α-alumina supports and a dense Pd/α-Al 2O 3 composite membrane with high hydrogen flux, good selectivity for hydrogen was obtained. It was tested in a single gas permeation system for hydrogen permeance and hydrogen selectivity over nitrogen. The hydrogen permeance of the corresponding membrane was as high as 2.45 × 10 −6mol · m −2 · s −1 · Pa −1 and H 2/N 2 selectivity over 700 at 623K and a pressure difference of 0.1MPa. The main resistance of the composite membrane to H 2 permeation lies in the aluminum ceramic support rather than the thin Pd layer.

Structure and Performance of Pd-Nafion® Composite Membranes for DMFC

The Nafion membrane was modified using plasma activation and Palladium-sputtering in an effort to reduce methanol crossover in DMFC. The structure, morphology and chemical composition of Pd thin layer were characterizied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectrometer (EDX). It was shown that pure Pd metal adhered to the substract very well. The preliminary results in simulative DMFC test revealed that the Nafion112/Pd/Nafion112 composite membrane exhibited better performance in methanol inhibition than the Nafion112/Nafion112 composite membrane.

Influence of thickness and porous structure of SiC layers on the electrical properties of Pt/SiC-pSi and Pd/SiC-pSi Schottky diodes for gas sensing purposes

Abstract The aim of this work was to study the influence of the thickness and porous structure of silicon carbide (PSC) layers on the electrical properties of Schottky diodes for gas sensing purposes by using a platinum (Pt) or a palladium (Pd) layer deposited on non-porous silicon carbide (SiC) and porous SiC (PSC) layers. The Schottky diodes were used for the first time for H2 and hydrocarbon (C2H6) gas sensing. The non-porous and porous SiC layers were realized on a p-type silicon (Si(1 0 0)) substrate by pulsed laser deposition using a KrF laser (248 nm) and thermal deposition of a thin Pt and Pd layer. The porous structure of the SiC layer deposited was developed by an electrochemical (anodization) method. The properties of the porous SiC layers formed by this method were investigated by scanning electron microscopy (SEM). The electrical measurements were made at room temperature (295 K) in an air ambience using a cryostat chamber. The effect of the porous surface structure and the thickness of the SiC layer was investigated by evaluating electrical parameters such as the ideality factor (n), barrier height (ϕBp) and series resistance (Rs). The thickness of the porous layer significantly affects the electrical properties of the Schottky diodes. Analysis of current–voltage (I–V) characteristics showed that the forward current might be described by a classical thermal emission theory. The ideality factor determined by the I–V characteristics was found to be dependent on the SiC thickness. For a thin SiC layer (0.16 μm) n was around 1.293 with a barrier height 0.847 eV, while for a thick layer (1.6 μm), n and ϕBp were 1.110 and 0.812 eV, respectively, for both Pt/SiC-pSi and Pd/SiC-pSi. The low value of series resistance obtained using Cheung's method clearly indicated the high performance of the Schottky diode for large SiC layer thickness. This effect showed the uniformity of the SiC layer. C–V analysis revealed a decrease in the capacitance with increasing voltage. This decrease was less for a thin SiC layer (0.16 μm) than a thick SiC layer (1.6 μm), clearly indicating the role of the SiC thickness in the determining sensor capacitance values. Finally, high sensitivity (ΔI/I = 90%) and selectivity of the sensors were reached at low voltages below 1 V, by using the PSC layer with catalytic metals of Pt and Pd.

Pd/native nitride/n-GaAs structures as hydrogen sensors

In this study, a novel Pd/native nitride/ n -GaAs structure was developed and investigated as hydrogen sensor. Thin layers of native nitride on n -GaAs (1 0 0) epitaxial crystals was grown by anodic nitridation, and Pd thin layer as gate electrode was evaporated on the native nitride film in high vacuum. Electrical and hydrogen sensing properties of the structures were obtained from the current–voltage ( I – V ), capacitance–voltage ( C – V ) and capacitance–time ( C – t ) characteristics measured in a computer controlled flow set-up at temperatures of 50, 90 and 130 °C, respectively. The samples show a behavior like a Schottky diode. The thicker sample has smaller leakage current and greater breakdown voltage. The sensor responses are totally reversible with response and recovery times, 35 and 40 s, respectively. The device with the thickest native nitride film showed better sensitivity, and the detection limit was lower than 20 ppm for H 2 .

Semiconductor behavior of hydrided Dy thin films as a function of increasing hydrogen pressure

Polycrystalline Dy thin films, coated with a very thin Pd layer, have been deposited on glass substrates by electron beam evaporation and subsequently hydrided. For hydrogen partial pressures larger than 0.7 kPa, a semiconductor character is displayed in the optical properties of the DyH 2 + x thin films. A parametric model is applied to follow the evolution with hydrogen pressure of various parameters characterizing the absorption by the semiconductor DyH 2 + x thin films. The contribution of the free electrons to the absorption of light is also considered within a Drude model incorporating the presence of the hydrogen atoms as ionized impurity scattering centers.