Palladium Overlayers Research Articles

The spin polarization of palladium on magneto-electric Cr2O3

While induced spin polarization of a palladium (Pd) overlayer on antiferromagnetic and magneto-electric Cr2O3(0001) is possible because of the boundary polarization at the Cr2O3(0001), in the single domain state, the Pd thin film appears to be ferromagnetic on its own, likely as a result of strain. In the conduction band, we find the experimental evidence of ferromagnetic spin polarized in Pd thin films on a Cr2O3(0001) single crystal, especially in the thin limit, Pd thickness of around 1–4 nm. Indeed there is significant spin polarization in 10 Å thick Pd films on Cr2O3(0001) at 310 K, i.e. above the Néel temperature of bulk Cr2O3. While Cr2O3(0001) has surface moments that tend to align along the surface normal, for Pd on Cr2O3, the spin polarization contains an in-plane component. Strain in the Pd adlayer on Cr2O3(0001) appears correlated to the spin polarization measured in spin polarized inverse photoemission spectroscopy. Further evidence for magnetization of Pd on Cr2O3 is provided by measurement of the exchange bias fields in Cr2O3/Pd(buffer)/[Co/Pd] n exchange bias systems. The magnitude of the exchange bias field is, over a wide temperature range, virtually unaffected by the Pd thickness variation between 1 and 2 nm.

A novel Nb-based hydrogen purification membrane without catalytic palladium overlayer

Recently, group 5 metals and related alloys are considered as promising hydrogen permeation membranes to replace commercial palladium and its alloys. However, almost all these alloy membranes need to be coated with a noble Pd catalytic layer, which is complicated and costly. Therefore, one of the important R&D aspects is to explore "non-Pd" membranes. To address this challenge, we innovatively used vanadium and its oxide layer to replace Pd in the "core" materials of these membranes, and then developed a new type of Nb-based hydrogen purification membrane without a catalytic Pd overlayer. The base-metal Nb15Ti55Fe30 layer, produced by the directional solidification method, not only exhibits excellent hydrogen permeation performance, but also has good hydrogen embrittlement resistance. After testing, the novel composite membrane showed a high hydrogen permeability of 6.07 × 10−8 mol H2 m−1 s−1 Pa−1/2 at 773 K, which is ca. 3.8 times that of pure Pd. More importantly, the price of this novel membrane is at least two orders of magnitude cheaper than that of a Pd membrane. This work opens up a new way for large-scale practical application of Nb-based membranes, and the developed "non-Pd" composite membrane has wide application prospects.

X-ray photoelectron and resistivity studies of the Pd-covered Ce thin films

We have fabricated Ce/Pd bilayers grown on the SiO2 substrate by magnetron sputtering under ultrahigh vacuum. Usage of palladium layer on top of the Ce film appears to prevent effectively the sample oxidation. The thickness of the cerium films is between 10-200 nm and 10 nm-thick palladium overlayers are always used. We have performed in-situ X-ray photoelectron spectroscopy on the as-deposited films and ex-situ electrical resistivity measurements and XRD characterization. XPS confirms a very good quality of the samples. The analysis of the Ce 3d spectrum suggests that the f states of Ce are on the border between the fluctuating valence and localization. The resistivity measurements reveal a competition of the Kondo scattering, semiconducting and metallic behaviors as well as the influence of the dimensional effect.

Substantial effect of palladium overlayer deposition on the H2 sensing performance of TiO2/ITO nanocomposite

The fabrication of hybrid Pd overlayerd TiO2/ITO nanocomposite films by a novel two step deposition process was demonstrated. The influence of Pd overlayer deposition on the structural, morphological and optical properties of the composite were investigated and optimized for use as H2 sensor. The sample prepared with 4 min Pd deposition time revealed nanospherical grains with coconut grated morphology favorable for H2 adsorption. The reduced band gap of optimized samples was attributed to the large crystallite size exhibited by the Pd: TiO2 particles. The response of the TiO2/ITO sensor with Pd catalytic overlayer on the sensitivity was observed. The fast recovery time of 15 s recorded for our sensor was attributed to the desorption activity of the nano porous Pd/TiO2 sensing layer achieved by multilayer deposition.

(Invited) Carbon Dioxide Electroreduction at Ultrathin Palladium Layers Deposited on Copper Electrode

Electrochemical conversion of carbon dioxide into eligible fuels has become the subject of particular fundamental interest. Recently, we have reported electrochemical studies on CO2 electroreduction at multilayered catalyst composed of the monatomic layer of copper covering palladium overlayers (0.8-10 monolayers) deposited on the well-defined Au(111) surface [1]. In this study Carbon dioxide electroreduction has been studied at ultrathin palladium layers deposited on copper electrode. The Pd deposits have been prepared by using chemical methods. The spontaneous galvanic displacement and resulting uncontrolled deposition of the palladium on copper has been entirely eliminated by the use of palladium(II). This method allowed us for controlled and precise deposition of palladium layers by electrochemical methods. The deposits coverage was changed within the range 0.5 to 5 atomic monolayers. The effectiveness of the method and the durability of the obtained ultrathin palladium layers have been confirmed by X-ray photoelectron spectroscopy (XPS). XPS has been also used for detailed analysis of the electronic properties of the Pd/Cu systems. Significant chemical interactions of both elements resulted in a partial shift of the electron density from palladium to copper and in the changes in the symmetry of 3d palladium signals characteristic for two-dimensional palladium structures (Pd monolayers). A detailed analysis of the valence band structure of Pd/Cu was correlated to the catalytic activity of the studied systems. The unique properties of the valence band structure cannot be attributed to the simple superposition of pure metal bands (Pd and Cu). It has been shown that the increase in the electron density near the Fermi level results in a significant decrease in the overpotential of the carbon dioxide electroreduction. Analysis of the gaseous products obtained in the CO2 electroreduction was performed by gas chromatography. Adsorption of carbon dioxide on Pd/Cu layers was demonstrated by XPS measurements in the presence of 1 mbar CO2 in XPS measurements under near ambient pressure XPS (NAP-XPS). [1] A. Januszewska, R. Jurczakowski, P. Kulesza, Langmuir 30(2014)14314-14321

Platinum and Palladium Overlayers Dramatically Enhance the Activity of Ruthenium Nanotubes for Alkaline Hydrogen Oxidation

Templated vapor synthesis and thermal annealing were used to synthesize unsupported metallic Ru nanotubes with Pt or Pd overlayers. By controlling the elemental composition and thickness of these overlayers, we obtain nanostructures with very high alkaline hydrogen oxidation activity. Nanotubes with a nominal atomic composition of Ru0.90Pt0.10 display a surface-specific activity (2.4 mA/cm2) that is 35 times greater than that of pure Ru nanotubes at a 50 mV overpotential and ∼2.5 times greater than that of pure Pt nanotubes (0.98 mA/cm2). The surface-segregated structure also confers dramatically increased Pt utilization efficiency. We find a platinum-mass-specific activity of 1240 A/gPt for the optimized nanotube versus 280 A/gPt for carbon-supported Pt nanoparticles and 109 A/gPt for monometallic Pt nanotubes. We attribute the enhancement of both area- and platinum-mass-specific activity to the atomic-scale homeomorphism of the nanotube form factor with adlayer-modified polycrystals. In this case, subsu...

Characterization of grating-coupled long range surface plasmon polariton membrane waveguides.

The first demonstration of grating-coupled long range surface plasmon polaritons in cladded free-standing membrane waveguides is presented. Two different waveguide structures are explored: the first is a gold (Au) stripe embedded in a thin Cytop free-standing membrane, the other being the same structure but with a thin palladium (Pd) over-layer. The waveguides are excited with integrated grating couplers designed for a working wavelength of 1550 nm. The waveguides are characterized by applying a cutback technique with the Au waveguide loss measured as 3.4 dB/mm and the Pd/Au waveguide loss as 57 dB/mm. The wavelength dependency of the weakly reflecting optical cavity is also observed with a free spectral range of ~3.6 nm and a finesse of 2.1.

Modeling of long range surface plasmon polariton cladded membrane waveguides with integrated grating couplers as hydrogen sensors

The design of a long range surface plasmon polariton cladded membrane waveguide with grating couplers is proposed. The device consists of a gold stripe embedded in a thin Cytop membrane with a palladium over-layer and can be used as a hydrogen sensor. Input and output light coupling is achieved through integrated gold grating couplers directly on the waveguides. The design is approached through finite element method modeling. Waveguide and sensor designs are compared and discussed via a 2D modal analysis. The design and optimization of input and output grating couplers are also presented.

CO₂ electroreduction at bare and Cu-decorated Pd pseudomorphic layers: catalyst tuning by controlled and indirect supporting onto Au(111).

We report here the results of electrochemical studies on CO2 electroreduction at multilayered catalyst composed of the monatomic layer of copper covering palladium overlayers (0.8-10 monolayers) deposited on the well-defined Au(111) surface. These multilayered systems were obtained by successive underpotential deposition steps: Pd on Au(111) as well as Cu on Pd/Au(111). Low index orientation of Au substrate was chosen to compare Pd overlayers with bulk Pd(111), which is known to reduce CO2 to CO adsorbates in acidic solutions. The process of CO2 electroreduction was studied by using classical transient electrochemical methods. Catalytic activity of bare Pd layers was investigated in acidic and neutral solutions. In the latter case, much higher activity of Pd overlayers was observed. The results showed that the palladium layer thickness significantly changed the catalytic activities of both bare Pd overlayers and the one Cu monolayer covered electrodes toward CO2 electroreduction. Results show that catalytic activity can be finely tuned by using the multilayered near-surface-alloy approach.

DFT study of platinum and palladium overlayers on tungsten carbide: Structure and electrocatalytic activity toward hydrogen oxidation/evolution reaction

Platinum and palladium overlayers on W- and C-terminated WC(0001) surface, at coverage ranging from 0.25 ML to 2 ML, have been studied using DFT approach. Strong adhesion of metal monolayers to the WC support, accompanied by a modification of electronic structure, was evidenced. Calculated values of hydrogen binding energy on studied metal overlayers were correlated to available experimentally determined values of exchange current densities of hydrogen oxidation/evolution reaction (HOR/HER), resulting with the volcano curve with both explanatory and predictive power. None of the investigated metal/WC surfaces were found to exceed the Pt(111) surface in terms of catalytic activity toward HER/HOR. The obtained results indicate that WC may efficiently replace the precious metal support in the Pt (Pd) core–shell electrocatalyst for HER/HOR, but no synergism arising due to electronic effects of WC support was evidenced.

Phonon Conduction in Periodically Porous Silicon Nanobridges

Thermal conduction in periodically porous nanostructures is strongly influenced by phonon boundary scattering, although the precise magnitude of this effect remains open to investigation. This work attempts to clarify the impact of phonon-boundary scattering at room temperature using electrothermal measurements and modeling. Silicon nanobeams, prepared using electron beam lithography, were coated with a thin palladium overlayer, which serves as both a heater and thermometer for the measurement. The thermal conductivity along the length of the silicon nanobeams was measured using a steady-state Joule heating technique. The thermal conductivities of the porous nanobeams were reduced to as low as 3% of the value for bulk silicon. A Callaway-Holland model for the thermal conductivity was adapted to investigate the relative impact of boundary scattering, pore scattering, and phonon bandgap effects. Both the experimental data and the modeling showed a reduction in thermal conductivity with increasing pore diameter, although the experimentally measured value was up to an order of magnitude lower than that predicted by the model.

Alkene hydrogenation on metal surfaces: Why and when are Pd overlayers more efficient catalysts than bulk Pd?

Previous works have shown that palladium overlayers (Pd/Ni, Pd/Cu) are more active than pure Pd surfaces for alkene hydrogenation. These results have been ascribed to the specific nanostructure of the alloy surfaces. Here, we compare Pd(100), Pd(110) and Pd 8Ni 92(110) single-crystal surfaces toward 1,3-butadiene hydrogenation and hydrogen absorption, using a gas-phase static reactor. We show that the lower rate of butene formation on clean Pd surfaces can in fact be explained by the initial fast diffusion of hydrogen into the Pd crystal (conversely, hydrogen dissolution in Pd–Ni is negligible). However, the activity of Pd becomes higher at steady state, i.e. after several reaction cycles, due to the increase of the near-surface H concentration. Unlike the butane formation rate, the partial hydrogenation rate appears poorly affected by the Pd surface structure. These results suggest that, when hydrogen supply is rate-determining, hydrogen absorption effects can be more critical than structural effects for Pd-catalyzed hydrogenations.

Hydrogen evolution and oxidation—a prototype for an electrocatalytic reaction

Due to progress in the theory of electrocatalysis and in quantum chemistry, it has become possible to investigate the hydrogen reaction and perform quantitative calculations for the reaction rate. First, we demonstrate this with model calculations for the adsorption of hydrogen on Pt(111). In accordance with experimental data, we find hydrogen adsorption at a potential above the equilibrium potential and with an almost vanishing energy of activation. As a second example, we explain trends in the catalytic activity of palladium overlayers and clusters on Au(111) electrodes.

Effect of Current Density on Thermodynamic Properties of Nanocrystalline Palladium Capped Samarium Hydride Thin Film Switchable Mirrors

A 55 nm samarium film capped with a 10 nm palladium overlayer switched from a metallic reflecting to a semiconducting, transparent in visible state during ex-situ hydrogen loading via electrochemical means in 1 M KOH electrolytic aqueous solution at room temperature. The switching between metal to semiconductor was accompanied by measurement of transmittance during hydrogen loading/unloading. The effect of current density on switching and thermodynamic properties was studied between dihydride state (FCC phase) and trihydride state (hexagonal phase). From the plateau of partial pressure of hydrogen atx=2.6, enthalpy of formation was calculated at different current densities. The diffusion coefficients and switching kinetics are shown to depend on applied current density.

Effect of Palladium Overlayer on Samarium Hydride Thin Film Switchable Mirror: Electrochemical and Topographical Study

Samarium films with a typical thickness of 55 nm deposited by vacuum evaporation and covered with Pd overlayers with thicknesses of 5, 8, 11, and 15 nm were loaded with hydrogen and deloaded in a 1 M KOH solution galvanostatically at room temperature. Our study showed that palladium overlayer thickness plays the most crucial role in observing the fast transition between an as-deposited metallic state and a semiconducting nearly samarium trihydride state as well as in obtaining a very high optical contrast. The desorption of hydrogen from palladium-capped SmH3-δ films was studied by in situ transmission measurement during the electrochemical loading/unloading of hydrogen. It was shown that palladium cap layer thickness affects the deloading of hydrogen from the hydrided films. The effect of a palladium overlayer on the surface morphology of metallic and hydrogen-desorbed samarium films showed the sequence of nucleation and growth.

Metal–support interactions in systems palladium deposited on oxidized tungsten surfaces

The morphology of the palladium (Pd) overlayers on oxidized tungsten (W) tips has been studied by Field Emission Microscopy (FEM). The effect of thermal treatment on the interaction of Pd with the support and chemisorption of CO on variously treated Pd-containing samples has been investigated. The results are discussed in relation to complementary macroscopic experiments by synchrotron radiation excited photoelectron spectroscopy (SRPES) and thermally programmed desorption (TPD) of carbon monoxide (CO) on a polycrystalline W foil. A distinct influence of support pre-oxidation on the Pd layer growth has been demonstrated. Two types of oxidized supports have been used: tungsten with oxygen pre-adsorbed at room temperature (RT) and then heated to 700 K (WO x /W (RT) system) and tungsten oxidized at 1300 K (WO x /W (1300 K) system) in situ. The surface of WO x /W (1300 K) sample is fully oxidized in contrast to WO x /W (RT), where the presence of un-oxidized patches has been demonstrated by SRPES measurements. A Pd layer grows on the WO x /W (RT) surface mostly on the densely populated planes (1 1 0) and (2 1 1) of the W tip. Heating of this system up to 700 K results in disaggregation of the original Pd layer. Pd clusters on the tungsten tip oxidized at 1300 K are localized on the atomically rough (1 1 1) plane. The observed differences in CO adsorption on the aforementioned types of investigated samples can be attributed to differences in the chemical nature of their surfaces.

Electrochemical study of Pd capped samarium hydride thin film switchable mirror

A 55-nm samarium film capped with a 15-nm palladium overlayer switched from a metallic reflecting to a semiconducting, transparent in visible state during ex-situ hydrogen loading via electrochemical means in 1 M KOH electrolytic aqueous solution at room temperature. The optical transmittance exhibits a hysteresis effects on loading and unloading of hydrogen in SmH x thin film between x = 2 and 3. The hysteresis is discussed in terms of strain (and consequently stress) at the interface between the dihydride and trihydride. The diffusion coefficients and switching kinetics are shown to depend on applied current density. The changes of anodic overpotential with time during applied constant current discharge have been used to determine the electrochemical parameters the exchange current density i 0 and the symmetry factor β.

Electrochemically induced switching in SmHx thin films

Discovery of optical and electrical switching in yttrium and lanthanum hydride thin films in 1996, has triggered large scale investigations of rare earth hydride thin films. Both light and heavy rare earths have been shown to exhibit this so called ‘switchable mirror’ effect. Besides gas phase loading, solid as well as liquid electrolytes have been used as means of hydrogen loading. In this paper, we report on the ability of samarium films to switch between the reflecting and transparent states electrochemically by altering the hydrogen content. Samarium films of typical thickness 55nm deposited by vacuum evaporation and covered with Pd overlayer of thicknesses 5, 8 and 11nm were loaded with hydrogen and deloaded in a 1M KOH solution galvanostatically at room temperature. Our studies have shown that thickness of palladium overlayer plays the most crucial role in observing fast transition between the as deposited metallic and the semiconducting nearly samarium trihydride states as well as to obtain a very high optical contrast. The large reversible change in transparency between di and trihydride states suggests possibilities of use of samarium films as an optical shutter as well as a hydrogen sensor material.

Electrochemical loading of hydrogen in palladium capped samarium thin film: structural, electrical, and optical properties

A 50 nm samarium film capped with a 7 nm palladium overlayer switched from a metallic to semiconducting state during ex-situ hydrogen loading via electrochemical means at room temperature. The transition is accompanied by a change in transmittance measured during hydrogen loading and the associated optical appearance. The monitoring of working electrode (WE) potential, the transmittance and chi potential difference (Δ χ) has been used to identify the phases present during hydrogen loading. Deloading of hydrogen has been studied in open circuit potential condition. Glancing angle X-ray diffraction (GAXRD) studies show that the rhombohedral structure of metallic samarium film ( a 0=8.989 Å) changes to hexagonal structure of the SmH 3− δ film with average lattice parameters of a=3.775 Å and c=6.743 Å. A direct optical band gap of 2.9 eV has been obtained for SmH 3− δ film and 2.0 eV for SmH 2 ± ε film from reflectance and transmittance data. Removal of hydrogen from SmH 3− δ leads to the formation of localized states within the band whose signature is clearly seen in transmittance and Tauc’s plot curves of SmH 2 ± ε film. The Hall coefficient R H measured as a function of hydrogen concentration, changes from a metal-like value −14.23×10 -10 m 3/C to −1001.1×10 −10 m 3/C for SmH 3− δ films. On unloading hydrogen, the value of R H changes to −3.56×10 −10 m 3/C at the dihydride composition.

Desorption of Hydrogen from SmH3-δ Films: Effect of Palladium Overlayer Thickness

Desorption of hydrogen from palladium-capped SmH3-δ films of 55 nm thickness has been studied using optical measurements. Result show that the thickness of the palladium cap layer affects the deloading of hydrogen from the hydrided films. The direct band gap of the SmHx films, calculated from the (αhν)2 vs hν plots, has been found to decrease with increasing thickness of the palladium overlayer, a fact attributed to the enhanced deloading of hydrogen. The removal of hydrogen from these films also leads to the formation of localized states, whose signature is clearly visible in these plots.