Pd Layer Research Articles

DFT Study on Electronic Interactions of Pt, Pd and Au Atoms with γ-Al2O3

The metal-support electronic interaction of dispersed Pt, Pd and Au layers on γ-Al2O3 is studied by density functional theory (DFT) calculations. The results indicate that electrons transfer significantly between the contacting layers of Au, Pt or Pd and the γ-Al2O3. Fukui function calculation results exhibit the electrophilicity of Al and O atoms on the Pt-supported surface is the greatest, while their nucleophilicity is the weakest. DOS calculation results demonstrate that the metal d orbital and O 2p orbital participate in the interactions, and the interaction between Pt 5d orbital and O 2p orbital is the strongest.DOI: http://dx.doi.org/10.5755/j01.ms.24.3.17855

Engineering of Hollow PdPt Nanocrystals via Reduction Kinetic Control for Their Superior Electrocatalytic Performances.

Synthesis of hollow metal nanocrystals (NCs) is greatly attractive for their high active surface areas, which gives rise to excellent catalytic activity. Taking PdPt alloy nanostructure as an example, we designed a synthetic tactic for the preparation of hollow metal nanostructures by delicate control over the difference in the reduction kinetic of metal precursors. At a high reduction rate difference, the Pd layer forms from H2PdCl4 and is subsequently etched, leading to the formation of a hollow space. A solid PdPt structure is achieved when the reduction rate of Pd and Pt precursor is comparable. Obviously, the hollow space and composition are tunable as well by adjusting the reduction rate difference. More importantly, the prepared hollow PdPt nanostructures exhibit a branched outer, porous wall, and rough hollow interior. The branched outer and rough hollow interior provide the higher density of unsaturated atoms, whereas the porous wall serves as channels connecting the inner, outer, and reactive agents. Moreover, the periodic self-consistent density function theory suggests that the d-band theory density of state of the PdPt nanoalloys is upshifted in comparison to the monometallic component, which will beneficial for improvement in their catalytic performances. Electrocatalytic tests reveal that the PdPt bimetallic NCs, especially for Pt32Pd68 nanostructures, show excellent catalytic activity and stability toward methanol oxidation reaction owing to their special structures as well as compositions.

Spin transfer torque switching of Co/Pd multilayers and Gilbert damping of Co-based multilayers

The critical current density (Jc0) and thermal stability factor Δ for the spin transfer torque switching of [Pt/Co]6/Cu/[Co/Pd]3 nanopillars with various Co/Pd layer compositions were investigated. Moreover, the Gilbert damping constants α of Co/Pd and Co/Pt multilayers (MLs) with various layer compositions were also studied to understand the variations in the Jc0 and Δ of the nanopillars. The Jc0 and Δ of Co/Pd MLs were found to be almost independent of the pillar diameters, and the effective reversal size contributing to Δ was found to be much smaller than the physical pillar diameter. From the spin transfer torque (STT) switching of Co/Pd MLs with various layer compositions, Jc0 was found to gradually increase with increasing thickness ratio of Pd and Co layers, tPd/tCo, up to tPd/tCo = 2, and further increase in tPd/tCo resulted in the decrease in Jc0. On the other hand, Δ was roughly independent of the thickness ratio. The Gilbert damping constants α of Co/Pd and Co/Pt MLs increased with increasing thickness ratio of noble metal and Co layers, tNM/tCo. The large damping constant of the Co/Pt ML compared with that of the Co/Pd ML means that Co/Pt and Co/Pd MLs are appropriate for use as reference and memory layers, respectively. The increase in α with tPd/tCo is considered to be responsible for the increase in Jc0 up to tPd/tCo = 2.

Independent Band Modulation in 2D van der Waals Heterostructures via a Novel Device Architecture.

Benefiting from the technique of vertically stacking 2D layered materials (2DLMs), an advanced novel device architecture based on a top‐gated MoS2/WSe2 van der Waals (vdWs) heterostructure is designed. By adopting a self‐aligned metal screening layer (Pd) to the WSe2 channel, a fixed p‐doped state of the WSe2 as well as an independent doping control of the MoS2 channel can be achieved, thus guaranteeing an effective energy‐band offset modulation and large through current. In such a device, under specific top‐gate voltages, a sharp PN junction forms at the edge of the Pd layer and can be effectively manipulated. By varying top‐gate voltages, the device can be operated under both quasi‐Esaki diode and unipolar‐Zener diode modes with tunable current modulations. A maximum gate‐coupling efficiency as high as ≈90% and a subthreshold swing smaller than 60 mV dec−1 can be achieved under the band‐to‐band tunneling regime. The superiority of the proposed device architecture is also confirmed by comparison with a traditional heterostructure device. This work demonstrates the feasibility of a new device structure based on vdWs heterostructures and its potential in future low‐power electronic and optoelectronic device applications.

Probing Interfacial Electronic and Catalytic Properties on Well-Defined Surfaces by Using In Situ Raman Spectroscopy.

Heterogeneous metal interfaces play a key role in determining the mechanism and performance of catalysts. However, in situ characterization of such interfaces at the molecular level is challenging. Herein, two model interfaces, Pd and Pt overlayers on Au single crystals, were constructed. The electronic structures of these interfaces as well as effects of crystallographic orientation on them were analyzed by shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) using phenyl isocyanide (PIC) as a probe molecule. A clear red shift in the frequency of the C≡N stretch (νNC ) was observed, which is consistent with X-ray photoelectron spectroscopy (XPS) data and indicates that the ultrathin Pt and Pd layers donate their free electrons to the Au substrates. Furthermore, in situ electrochemical SHINERS studies showed that the electronic effects weaken Pt-C/Pd-C bonds, leading to improved surface activity towards CO electrooxidation.

Probing Interfacial Electronic and Catalytic Properties on Well‐Defined Surfaces by Using In Situ Raman Spectroscopy

AbstractHeterogeneous metal interfaces play a key role in determining the mechanism and performance of catalysts. However, in situ characterization of such interfaces at the molecular level is challenging. Herein, two model interfaces, Pd and Pt overlayers on Au single crystals, were constructed. The electronic structures of these interfaces as well as effects of crystallographic orientation on them were analyzed by shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS) using phenyl isocyanide (PIC) as a probe molecule. A clear red shift in the frequency of the C≡N stretch (νNC) was observed, which is consistent with X‐ray photoelectron spectroscopy (XPS) data and indicates that the ultrathin Pt and Pd layers donate their free electrons to the Au substrates. Furthermore, in situ electrochemical SHINERS studies showed that the electronic effects weaken Pt−C/Pd−C bonds, leading to improved surface activity towards CO electrooxidation.

Superior Reliability of SAC105 Solder on BGA Package Pad with NiPdAu Coating

The reliability of Sn-Ag-Cu (SAC) solder welded with Cu-organic solderability preservative (OSP) and NiAu- or NiPdAu-coated ball grid array (BGA) package pads has been investigated after isothermal aging. Moreover, the interfacial reaction and diffusion reaction are discussed. The intermetallic compound (IMC) layer was found to be too thick to withstand large stresses, leading to poor reliability of the solder joints on Cu-OSP pads. Additionally, for NiAu-coated pads, not only irregular Au-rich phases but also more diffused Ni without the barrier effect of Pd promoted formation of IMC in the solder balls, resulting in a potential mismatch with the initial solder. Compared with the NiAu-coated pad, the IMC layer that formed between the NiPdAu-coated pads and SAC105 solder was thinner, indicating an effective diffusion barrier effect of the Pd layer in the NiPdAu coating. Hence, SAC105 solder showed superior reliability on BGA package pads coated with NiPdAu.

Regulating Surface Facets of Metallic Aerogel Electrocatalysts by Size-Dependent Localized Ostwald Ripening.

It is well known that the activity and stability of electrocatalysts are largely dependent on their surface facets. In this work, we have successfully regulated surface facets of three-dimensional (3D) metallic Au m- n aerogels by salt-induced assembly of citrate-stabilized gold nanoparticles (Au NPs) of two different sizes and further size-dependent localized Ostwald ripening at controlled particle number ratios, where m and n represent the size of Au NPs. In addition, 3D Au m- n-Pd aerogels were further synthesized on the basis of Au m- n aerogels and also bear controlled surface facets because of the formation of ultrathin Pd layers on Au m- n aerogels. Taking the electrooxidation of small organic molecules (such as methanol and ethanol) by the resulting Au m- n and Au m- n-Pd aerogels as examples, it is found that surface facets of metallic aerogels with excellent performance can be regulated to realize preferential surface facets for methanol oxidation and ethanol oxidation, respectively. Moreover, they also indeed simultaneously bear high activity and excellent stability. Furthermore, their activities and stability are also highly dependent on the area ratio of active facets and inactive facets on their surfaces, respectively, and these ratios are varied via the mismatch of sizes of adjacent NPs. Thus, this work not only demonstrates the realization of the regulation of the surface facets of metallic aerogels by size-dependent localized Ostwald ripening but also will open up a new way to improve electrocatalytic performance of 3D metallic aerogels by surface regulation.

Duplex Pd/ceramic/Pd composite membrane for sweep gas-enhanced CO2 capture

Supported, thin-layered Pd membranes are promising tools for pre-combustion carbon capture because they can deliver simultaneously high-pressure CO2 and H2 streams as needed for sequestration and power generation, respectively. Moreover, the diluent required for the gas turbine fuel can be advantageously added as sweep gas in the membrane separator to boost H2 permeation. A composite membrane has been prepared with Pd layers on both sides of a porous ceramic tube to eliminate the mass transfer resistance resulting from sweep gas diffusion into the support of conventional Pd/ceramic membranes. The H2 permeability of the Pd/ceramic/Pd membrane (1.01 ×10−8 mol m−1 s−1 Pa−0.5 at 773 K) is similar to that of a supported single Pd layer membrane but its H2/N2 selectivity (18033) is enhanced by an order of magnitude at 773 K and ΔP = 500 kPa. Intriguingly, the selectivity of the double Pd layer membrane improves with increasing pressure difference ΔP while that of Pd/ceramic membranes declines. This unusual pressure dependence originates from the elevated pressure within the ceramic of the double Pd layer membrane. The sweep gas-induced mass transfer resistance grows rapidly with increasing permeate pressure and sweep gas amount attenuating the single gas H2 flux of the Pd/ceramic membrane by 67% at 1 MPa permeate pressure. This resistance remains significant under CO2 capture conditions albeit concentration polarization on the feed side has an even stronger permeation-retarding effect in that situation. The CO2 capture rate and purity benefit greatly from the enhanced high-pressure H2 selectivity of Pd/ceramic/Pd membranes which renders them attractive tools for any kind of H2 separation task.

Hydrogen storage in Ti-based metal hydrides investigated by elastic recoil detection analysis (ERDA)

Hydrogen storage capacity of CP-Ti, Ti-6Al-4V alloy, and Ti-6Al-4V alloy coated with a thin Pd layer was investigated. All samples were hydrogenated at 550 °C for 3 h in 15% hydrogen and 85% argon gas mixture. The hydrogen content and its depth profile were investigated using elastic recoil detection analysis (ERDA) while the X-ray diffraction was used for phase analysis. The highest average content of hydrogen of (41 ± 60) at.% was absorbed in CP-Ti while (31 ± 4) at.% H was determined in Ti-6Al-4V alloy. The presence of a thin Pd catalytic layer improved the hydrogen absorption of alloy to (36 ± 3) at.% H. With the beam energy of 3 MeV, H was profiled up to (4520 ± 410) × 1015 atoms/cm2, (3250 ± 300) × 1015 atoms/cm2 and (3950 ± 360) × 1015 atoms/cm2 in CP-Ti, Ti-6Al-4V alloy and in the coated system respectively.The TiH2 hydrides were found in all three systems in addition to Pd3Ti phase in the Pd/Ti-6Al-4V coated system.

Self-Rectifying and Forming-Free Resistive-Switching Device for Embedded Memory Application

The self-rectifying resistive-switching (RS) device is one of the most promising solutions to overcome the sneaking current issue in a 3D vertical crossbar array. In this letter, we report a CMOS-compatible, forming-free, self-rectifying resistive random access memory device with high uniformity and low operation voltage ( 109) characteristics. After introducing a 3-nm HfO2 thin film between Pd and WOx layer, Schottky contact of Pd/HfO2 was formed, which resulted in rectifying property. The HfO2 layer also served as an oxygen reservoir for RS in WOx layer. This novel memory device with excellent performance is a promising candidate for future high density embedded memory application.

Temperature dependence of photoinduced hydrogen production and simultaneous separation in TiO2 nanotubes/palladium bilayer membrane

Temperature dependence of photoinduced hydrogen production and simultaneous separation was examined in a bilayer membrane comprised by an anodized TiO2 nanotube array (TNA) and a palladium layer. This membrane was fabricated by transferring a TNA embedded in a titanium sheet onto an electroless-plated palladium film. Sacrificial water splitting with methanol was photocatalytically performed under ultraviolet (UV) irradiation and only generated hydrogen gas was concurrently separated with the Pd layer. The H2 production rate (rH2) with the membrane at various temperatures was evaluated by using a home-made characterization system. The measured rH2 showed larger values at higher temperature of the membrane and increased abruptly after several hours of UV irradiation. This inflection behavior appeared earlier at the higher temperature of the membrane, which can be related to the permeation and adsorption characteristics of hydrogen in the Pd layer.

(Invited) Use of ALD by SLRR for the Growth of Layered Bi-Metallic Structures and Alloy Thin Films with Specific Functionality

The ingenious idea of surface limited deposition is among the brightest achievements of Radoslav Adzic. The proposed by his group in the beginning of this century approach of surface limited redox replacement (SLRR) for realization of monolayer thick coatings with catalytically active metals and alloys was used as foundation from others for the development of epitaxial thin film growth methods. Central stage in this direction takes the use of SLRR approach for coating nanoparticles and continuous nanoporous metal layers with ultrathin films of metal, layered bi-metallic structures, and/or alloys with specific functionality. Successful outcome of this coating renders the final product remarkably active and durable catalyst. In this talk, the direct use of SLRR based protocols for the growth of layered bi-metallic films and alloys with targeted functionality. Specific consideration will be given to co- deposition modes in the systems Ag-Cu and Au-Cu. Factors controlling the quality of the deposited films in terms of morphology, orientation and composition will be comparatively considered. Emphasis will be placed on the thermodynamic miscibility of the metals participating in the deposition process as a factor controlling the final growth outcome in both cases; layered structures with sharp interface boundary in the Ag-Cu system and a single phase alloy in the Au-Cu system upon application of identical growth protocols. Results of electrochemical and SEM / STM characterization of the morphology, structure and/or orientation of the deposited films will be presented and critically discussed. Also, the parting behavior of both components upon anodic polarization scans will be considered in detail and studied with emphasis of post-anodization morphological characteristics of the remaining structures. The discussion on the catalyst processing component based on coating of above described bi-metallic structures with ultra thin Pt and/or Pd layers will be the conclusion of this talk. It will be garnered by the demonstration of results of activity and durability testing of accordingly developed catalysts for organic fuel oxidation and/or nitrogen electrochemistry. Finally, a glimpse into the future of this rapidly developing field catalyst design at the atomic scale will be also provided.

Dielectric properties of thin Cr2O3 films grown on elemental and oxide metallic substrates

In an attempt to optimize leakage characteristics of \ensuremath{\alpha}-$\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ thin films, its dielectric properties were investigated at local and macroscopic scale. The films were grown on Pd(111), Pt(111), and ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ (0001), supported on $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ substrate. The local conductivity was measured by conductive atomic force microscopy mapping of $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ surfaces, which revealed the nature of defects that formed conducting paths with the bottom Pd or Pt layer. A strong correlation was found between these electrical defects and the grain boundaries revealed in the corresponding topographic scans. In comparison, the $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ film on ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ exhibited no leakage paths at similar tip bias value. Electrical resistance measurements through e-beam patterned top electrodes confirmed the resistivity mismatch between the films grown on different electrodes. The x-ray analysis attributes this difference to the twin free $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ growth on ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ seeding.

Highly conductive PdCoO2 ultrathin films for transparent electrodes

We report on the successful synthesis of highly conductive PdCoO2 ultrathin films on Al2O3 (0001) by pulsed laser deposition. The thin films grow along the c-axis of the layered delafossite structure of PdCoO2, corresponding to the alternating stacking of conductive Pd layers and CoO2 octahedra. The thickness-dependent transport measurement reveals that each Pd layer has a homogeneous sheet conductance as high as 5.5 mS in the samples thicker than the critical thickness of 2.1 nm. Even at the critical thickness, high conductivity exceeding 104 S cm−1 is achieved. Optical transmittance spectra exhibit high optical transparency of PdCoO2 thin films particularly in the near-infrared region. The concomitant high values of electrical conductivity and optical transmittance make PdCoO2 ultrathin films promising transparent electrodes for triangular-lattice-based materials.

Sequential interfacial reactions of SAC305 solder joints with thin ENEPIG surface finishes

Sequential interfacial reactions of Sn‐3.0Ag‐0.5Cu solder on a new multilayer metallization, ENEPIG (electroless nickel‐electroless palladium‐immersion gold) with a 0.1‐μm‐thin Ni(P) layer (thin‐ENEPIG), during reflowing were evaluated in this study. After 20‐second reflow process, layer‐shaped (Au,Cu)Sn4 intermetallic compounds formed at the interface, and thin Ni and Pd layers underneath the Au layer remained unreacted on the polycrystalline Cu substrate. On the other hand, Au, Pd, and Ni tri‐layers in the thin ENEPIG layer were exhausted and (Cu,Ni)6Sn5 intermetallic compounds formed at the interface after reflowing for 30 seconds.

Pd supported on NC@SiC as an efficient and stable catalyst for 4-carboxybenzaldehyde hydrogenation

NC@SiC prepared by a facile pyrolysis method was used as the support of Pd catalyst for the hydrogenation of 4-carboxybenzaldehyde. The structure of Pd/NC@SiC was characterized by N2 adsorption-desorption, ICP-OES, SEM, HRTEM and XPS. The Pd/NC@SiC catalyst showed superior activity compared with Pd/SiC due to its hierarchically mesoporous and microporous structure and the synergistic effect between Pd and N-doped carbon layer. In addition, Pd/NC@SiC also exhibited a good durability under high temperature and high pressure H2, which was attributed to its highly dispersed Pd nanoparticles stabilized by the N-doped carbon structure.

Current Density Contribution to Plasmonic Enhancement Effects in Metal–Semiconductor–Metal Photodetectors

This work studies how the local current density in a metal–semiconductor–metal photodetector (MSM PD) corresponds to the plasmonic enhancement and therefore affects the overall enhancement of the device. For this type of semiconductor photodetector, the enhancement of incident light due to plasmonic structures is most critical inside the substrate, where the photocurrent is generated. This work develops a relationship between the total device optical enhancement and the current density by considering the average optical enhancement, weighted by the current density in the GaAs layer of a simulated MSM PD. This corresponds to an increased overall current in the device. Effects of the wire and nanoslit widths on the total weighted optical enhancement were studied. The results showed that both widths have a significant impact on the total weighted optical enhancement, improving it by two orders of magnitude when using the smallest possible wire widths and nanoslits.

Synergistically Enhanced Oxygen Reduction Electrocatalysis by Subsurface Atoms in Ternary PdCuNi Alloy Catalysts

AbstractFor Pd‐based alloy catalysts, the selection of metallic alloying elements and the construction of composition‐gradient surface and subsurface layers are critical in achieving superior electrocatalytic activities in, e.g., the oxygen reduction reaction (ORR). Based on the Pd‐containing alloy, highly monodispersed PdCuNi ternary alloy nanocrystals are prepared through a wet‐chemical approach, and a solution‐based oxidative surface treatment protocol is utilized to activate the surface of the nanocrystals. A drastically enhanced ORR activity can be achieved by removing the surface Ni and Cu atoms through the surface treatment protocol. The treated catalyst demonstrates a mass activity of 0.45 A mgPd−1 in alkaline medium, 5 and 2.4 times those of commercial Pt/C and Pd/C, respectively. The first‐principle calculation result suggests the critical roles of the coexistence of Ni and Cu atoms and their synergistic interaction beneath the outmost pure Pd layer in optimizing the oxygen binding energy for ORR. The calculation also suggests that the optimal binding energy of oxygen requires an appropriate Ni/Cu ratio in the subsurface layer. This work demonstrates a class of high‐performance Pt‐free ternary alloy ORR catalysts and may provide a general guideline for the structural design of Pd‐based ternary alloy catalysts.

Amorphous Pd-assisted H2 detection of ZnO nanorod gas sensor with enhanced sensitivity and stability

For monitoring H2 concentrations in air, diverse resistive gas sensors have been demonstrated. In particular, Pd-decorated metal oxides have shown remarkable selectivity and sensing response for H2 detection. In this work, H2 sensing behavior of amorphous Pd layer covering ZnO nanorods (am-Pd/ZnO NRs) is investigated. This is the first report on the enhanced gas sensing performance attained by using an amorphous metal layer. The amorphous Pd layer is generated by reduction reaction with a strong reducing agent (NaBH4), and it covers the ZnO nanorods completely with a thickness of 2–5 nm. For comparison, crystalline Pd nanoparticles-decorated ZnO nanorods (c-Pd/ZnO NRs) are produced using a milder reducing agent like hydrazine. Comparing the c-Pd/ZnO NRs sensor and other previously reported hydrogen sensors based on the crystalline Pd and metal oxides, the am-Pd/ZnO NRs sensor exhibits a remarkable sensing response (12,400% at 2% H2). The enhancement is attributed to complete cover of the amorphous Pd layer on the ZnO NRs, inducing larger interfaces between the Pd and ZnO. In addition, the amorphous Pd layer prevents surface contamination of the ZnO NRs. Therefore, the am-Pd/ZnO NRs sensor maintains initial sensing performance even after 5 months.