Pd Films Research Articles

Alloyed Au/Pd nanoparticles formed by laser ablation of thin films in liquid

The formation of alloyed Au/Pd nanoparticles (NPs) from thin magnetron-deposited Au–Pd films by nanosecond laser ablation in water is investigated. The films weight loss dependence is studied by thickness, intensity and laser pulses per point. Prepared NPs are transferred onto the silica wafer and characterized by scanning and transmission electron microscopy as well as optical and energy-dispersive x-ray spectroscopies. NPs size distribution was obtained using the dynamic light scattering (DLS) method.

Optimisation and Stability of Rh Particles on Noble Metal Films Immobilised on H+ Conducting Solid Polymer Electrolyte in Attaining Efficient Nitrate Removal.

During the electrocatalytic reduction of nitrate, nitrite is often evolved as a product along with ammonia due to the sluggish nitrite-to-ammonia conversion process compared to the nitrate-to-nitrite conversion step. Rhodium metal has been proven to enhance nitrite-to-ammonia conversion rates, yielding ammonia as the only final product. In the present article, we have shown how effectively Rh nanoparticles immobilized on Pt and Pd films deposited on H+ conducting Nafion-117 membranes eliminate intermediate nitrite ions during the progress of the nitrate reduction reaction in a flow type reactor. In this research, we also demonstrated the optimization of Rh nanoparticles on the cathode surface to attain effective nitrate reduction along with a reproducibility check. The dissolution of loosely held Rh nanoparticles on the cathodic surface was observed, which tends to be redeposited during cathodic electrolysis, causing stable performance. Finally, Tafel analysis was performed to show the relative kinetic feasibility of the Rh-modified Pt and Pd electrodes in attaining nitrate reduction reactions in neutral medium.

Hydrogen sensing by plasmon decoupling effect in nanostructured Pd/Au films

Plasmon coupling effect occurs in plasmonic nanostructures when interparticle distances are in the order of particle size leading to spectral shifts in the plasmonic band. This effect has been recently highlighted for measurement of fluctuations in the interparticle distance at nanoscale level. In this study, nanostructured thin Au films were deposited on quartz substrates by pulsed laser deposition (PLD) for sensing of hydrogen gas. A blue shift from 730 to 560 nm in LSPR of Au films was observed when substrate temperatures rises from 25 to 600 °C due to variation in morphology of films from a continuous surface composed of tiny agglomerates to granular surface composed of bigger particles with increased interparticle spacing. For plasmon coupling sensing of hydrogen, a thin Pd film was deposited on top of nanostructured Au films. Upon hydrogen exposure, up to12 nm blue shift within few seconds was observed depending on hydrogen concentration. Based on field emission scanning electron microscope (FESEM) images and finite-difference time-domain (FDTD) simulations, this plasmon sensing is explained by hydrogen-induced decoupling due to the formation of surface stresses in Pd, which can affect the LSPR via an increase in interparticle spacing of Au nanoislands. • Nanostructured Au/quartz films were deposited prepared by pulsed laser deposition at different substrate temperatures. • The interparticle distance of Au films increases with temperature, leading to blue shift in the gold plasmonic absorption. • A thin Pd layer as hydrogen catalyst was deposited on Au/quartz samples by DC magnetron sputtering. • Hydrogen interaction led to a further blue shift, which was greater for films with smaller interparticle distance. • Using finite difference time domain (FDTD) simulation, this effect was attributed to a plasmon decoupling effect.

Low pressure CVD growth of 2D PdSe2 thin film and its application in PdSe2-MoSe2 vertical heterostructure

Palladium diselenide (PdSe2) is a novel member of the transition metal dichalcogenide family with layer dependent bandgap in the infrared regime with potential applications in many electronic and optoelectronic devices. Low pressure chemical vapor deposition (LPCVD) could be an effective way to synthesize large area 2D PdSe2 materials at low growth temperatures creating new opportunities for the widescale applications of PdSe2. Here, we report LPCVD growth of PdSe2 for the first time at a growth temperature down to 250 °C, which is significantly lower than what was previously reported. The 2 nm Pd films became 8 nm PdSe2 after selenization in the temperature range of 250 °C–375 °C and no thickness variation with growth temperature was observed in our atomic force microscopy study. Raman study showed narrowing of PdSe2 related peaks with increasing growth temperature suggesting improved structural quality of the films. X-ray photoelectron spectroscopy study confirmed complete selenization of the thin films to the lowest growth temperature of 250 °C. Electrical transport properties study showed resistance of the devices decrease with increasing growth temperature possibly due to the improvement of crystallinity. We also found that the devices show p-type behavior with mobilities up to 1 cm2 V−1 s−1. The good electrical quality of the film was further confirmed by demonstrating its application in fabricating PdSe2/MoSe2 vertical heterojunction which showed rectification behavior with a rectification ratio of up to 232. Kelvin probe force microscopy confirmed that the rectification behavior was originated from the work function difference of 0.76 eV between MoSe2 and PdSe2.

Dislocations, texture and stress development in hydrogen-cycled Pd thin films: An in-situ X-ray diffraction study

For Pd thin films, microstructural changes involved during hydrogen cycling provide the information needed to predict and optimize the film's mechanical strength. In this paper, a systematic study of the morphology, microstructure, texture, and stress has been performed on Pd thin films during hydrogen loading and deloading cycles at room temperature. Pd thin films of similar morphology were prepared by magnetron sputtering on substrates of different compliances, i.e., Si-oxide, Titanium (Ti) and Polyimide (PI). The evolution of the morphology, grain-orientation distribution (texture), state of stress, and dislocation densities are analyzed for each of the film substrate types for 20 hydrogen loading/deloading cycles. The lattice expansion and contraction caused by the transition from Pd to Pd-hydride and back result in a strong stress increase. This stress increase stabilizes after a few cycles by grain boundary motion that leads to a gradual enhancement of the (111) texture and changes in the dislocation density for Pd films that are strongly clamped on to an oxidized Si(100) wafer substrate with an intermediate layer (Ti or PI). For Pd on PI, the stress is also partly released by a crack-based (crack widening/growth/propagation) pathway. Pd films on Ti and PI do not buckle or blister after 20 hydrogen cycles. By providing a sufficiently compliant substrate the traditional problems of buckle-delamination of a film on a stiff substrate are mitigated. • Pd thin films: microstructure and mechanical strength during hydrogen cycling studied. • Cyclic deformation resulted in irreversible texture changes − sharpening of (111)-fiber texture. • An open morphology restricts internal stress from amplifying during hydrogen uptake. • All Pd films contain similar kinds of defects, that differ only by their concentration. • The flexible PI inter-layer provides enhanced adhesive support, preventing strain localization and film delamination.

Effect of Co, Pd and Pt ultra-thin films on the Ni-silicide formation: investigating the sandwich configuration

The effect of Co, Pd and Pt ultrathin films on the kinetics of the formation of Ni-silicide by reactive diffusion is investigated. 50 nm Ni/1 nm X/ 50 nm Ni (X = Co, Pd, Pt) deposited on Si(100) substrates are studied using in-situ and ex-situ measurements by X-ray diffraction (XRD). The presence of Co, Pd or Pt thin films in between the Ni layers delays the formation of the metal rich phase compared to the pure Ni/Si system and thus these films act as diffusion barriers. A simultaneous silicide formation (δ-Ni2Si and NiSi phases) different from the classic sequential formation is found during the consumption of the top Ni layer for which Ni has to diffuse through the barrier. A model for the simultaneous growth in the presence of a barrier is developed, and simulation of the kinetics measured by XRD is used to determine the permeability of the different barriers. Atom probe tomography (APT) of the Ni/Pd/Ni system shows that the Pd layer is located between the Ni top layer and δ-Ni2Si during the silicide growth, in accordance with a silicide formation controlled by Ni diffusion through the Pd layer. The effect of the barrier on the silicide formation and properties is discussed.

Scalable production of single 2D van der Waals layers through atomic layer deposition: bilayer silica on metal foils and films

The self-limiting nature of atomic layer deposition (ALD) makes it an appealing option for growing single layers of two-dimensional van der Waals (2D-VDW) materials. In this paper it is demonstrated that a single layer of a 2D-VDW form of SiO2 can be grown by ALD on Au and Pd polycrystalline foils and epitaxial films. The silica was deposited by two cycles of bis(diethylamino) silane and oxygen plasma exposure at 525 K. Initial deposition produced a three-dimensionally disordered silica layer; however, subsequent annealing above 950 K drove a structural rearrangement resulting in 2D-VDW. The annealing could be performed at ambient pressure. Surface spectra recorded after annealing indicated that the two ALD cycles yielded close to the silica coverage obtained for 2D-VDW silica prepared by precision SiO deposition in ultra-high vacuum (UHV). Analysis of ALD-grown 2D-VDW silica on a Pd(111) film revealed the co-existence of amorphous and incommensurate crystalline 2D phases. In contrast, ALD growth on Au(111) films produced predominantly the amorphous phase while SiO deposition in UHV led to only the crystalline phase, suggesting that the choice of Si source can enable phase control.

Growth and morphology of thin epitaxial Pd films on Au(001) film grown on Fe-buffered MgO(001) substrate

We have investigated the growth of thin Pd films on a Au(001) surface, which was a Au(001) film grown on a Fe-buffered MgO(001) substrate, at 290 K up to 4 monolayer (ML) thickness and the effect of post-annealing (PA) at 470 K. The surface morphology and structure are determined using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). The LEED patterns of the as-grown and PA films indicate that the Pd(001) films grow epitaxially on the Au(001) surface with noticeable in-pain lattice expansion of 4.5% relative to the bulk Pd, indicating a tensely strained tetragonal deformation. The Pd film does not grow in the layer-by-layer growth mode at 290 K. Instead, numerous Pd islands grow anisotropically along the 〈110〉 direction. The surface morphology of the films improves significantly after the PA at 470 K due to the coalescence of the elongated Pd islands. Consequently, the room temperature growth of the Pd layers followed by PA at 470 K enables us to fabricate the high-quality epitaxial Pd(001) film, which has large and atomically flat terraces without Au segregation. Additional Pd growth up to 8 ML thickness onto such 4 ML PA films kept at 470 K is also studied by STM. This specific three-step method facilitates the quasi-layer-by-layer growth and opens a chance for practical use in the future.

Improved hydrogen sorption properties of Pd in protic and aprotic ionic liquids effected by superacid addition

The mixtures of selected protic and aprotic ionic liquids with their parent superacids were used as electrolytes in the process of hydrogen electrosorption in thin (ca. 0.5 µm) Pd films. The properties of ionic liquids and their mixtures with superacids were characterized with the use of electrochemical and physicochemical methods to show the perspectives of their potential utilization as electrolytes in hydride/protonic systems. Based on the electrochemical studies it was possible to estimate the minimum concentrations of the superacids in protic and aprotic ionic liquids which give maximum hydrogen absorption capacities in Pd (H/Pd). Unusual, previously unnoticed ‘two-step’ α→β phase transition was observed on the hydrogen electrosorption isotherm of Pd for the process of hydrogen absorption from the protic ionic liquids for some superacid concentrations. 1H NMR spectroscopy of the protic ionic liquids enables to discuss the relation between proton mobility and hydrogen electrosorption properties.

Surface plasmon resonance sensing with thin films of palladium and platinum - quantitative and real-time analysis.

Surface plasmon resonance (SPR) is a highly useful technique in biology and is gradually becoming useful also for materials science. However, measurements to date have been performed almost exclusively on gold, which limits the possibility to probe chemical modifications of other metals. In this work we show that 20 nm Pd and Pt films work "fairly well" for quantitative SPR sensing of organic films despite the high light absorption. In the interval between total reflection and the SPR angle, high intensity changes occur when a film is formed on the surface. Fresnel models accurately describe the full angular spectra and our data analysis provides good resolution of surface coverage in air (a few ng cm-2). Overall, the Pd sensors behave quite similarly to 50 nm gold in terms of sensitivity and field extension, although the noise level in real-time measurements is ∼5 times higher. The Pt sensors exhibit a longer extension of the evanescent field and ∼10 times higher noise compared to gold. Yet, formation of organic layers a few nm in thickness can still be monitored in real-time. As a model system, we use thiolated poly(ethylene glycol) to make Pd and Pt protein repelling. Our findings show how SPR can be used for studying chemical modifications of two metals that are important in several contexts, for instance within heterogeneous catalysis. We emphasize the advantages of simple sample preparation and accurate quantitative analysis in the planar geometry by Fresnel models.

1Pa3-3 Effects of Au atoms deposition on Pd film for hydrogen gas sensor using wireless and electrodeless quartz crystal resonator

1Pa3-3 Effects of Au atoms deposition on Pd film for hydrogen gas sensor using wireless and electrodeless quartz crystal resonator

Multilayer crystal-amorphous Pd-based nanosheets on Si/SiO2 with interface-controlled ion transport for efficient hydrogen storage

This contribution shows an unusually high hydrogen storage of multilayer amorphous (A)-crystalline (C) Pd–Si based nanosheets when stacked in the right order. Samples with A/C/A/C/A stacking sequence exhibit 40 and 12 times larger hydrogen sorption than monolithic crystalline and amorphous samples, respectively. The maximum capacitance calculated from the fitting of electrochemical impedance measurements of the same sample is twice larger than that of the conventional polycrystalline Pd films of similar thickness. Five times higher diffusion coefficient calculated from modified Cottrell equation is obtained compared to specimens with C/A/C/A/C stacking. For the A/C/A/C/A multilayers, nanobubbles with diameters of 1–2 nm are homogeneously distributed at Si/SiO2 interface, and PdHx crystal formation in these regions confirms hydrogen-metal interactions. Furthermore, corrosion-resistant amorphous top layer permits larger amounts of hydrogen ion transfer to inner layers. Thus, hydrogen storage and production can be enhanced by smart design of multilayers targeted for proton exchange membrane electrolysis or fuel cells.

Suspended Palladium/Polymer Bilayer for High-Contrast and Fast Hydrogen Sensors.

Hydrogen sensing is extremely essential for hydrogen-related applications due to the explosibility of hydrogen gas (H2). Here, we first present a high-contrast and fast optical hydrogen sensor, which is a partially suspended Pd/PMMA bilayer on a PDMS substrate with a microgroove array on the surface. The suspended structure reduces constraints from the substrate on the Pd film, leading to a large wrinkling amplitude and fast response rate during hydrogenation. The PMMA film can protect the Pd film from the poisonous impurities in the air and improve the flexibility of the bilayer. When exposed to 4% H2 mixed with air, the reflectance of the sensor drops down from 43 to 4% at 600 nm wavelength, in which the corresponding reflectance contrast, defined as the ratio of the reflectances before and after exposure to hydrogen, is 10.75. Such a high reflectance variation results from the light scattering induced by the wrinkling of the suspended Pd/PMMA bilayer during hydrogenation. Meanwhile, the sensor has a fast response that the reflectance can decrease from 43 to 33% within 0.6 s. Moreover, the sensor shows good recyclability and hydrogen selectivity. These excellent performances suggest that our suspended Pd/PMMA bilayer has great potential for practical hydrogen detection.

Effect of increasing interaction interface on perpendicular magnetic anisotropy of Pd/Co2MnSi/Pd multilayers

Interfacial interaction induced perpendicular magnetic anisotropy (PMA) is one of the research hotspots of magnetic recording thin films. The PMA of Pd/Co2MnSi(CMS)/Pd multilayers can be improved by increasing the interfaces of heavy metal/ferromagnetic (HM/FM). In order to study the effect of adding HM/FM interface on the PMA of multilayers, a Pd layer was inserted into the CMS of Pd/CMS/Pd film to form Pd/CMS(t/2)/Pd/CMS(t/2)/Pd multilayer structure with quadruple Pd/CMS interface, and the [Pd/CMS(t/2)]N was inserted into the Pd/CMS(t/2)/Pd film to form Pd/CMS(t/2)[Pd/CMS(t/2)]N/Pd multilayer structure. The results showed that the PMA and MS of Pd/CMS(t/2)/Pd/CMS(t/2)/Pd multilayers were enhanced. The maximum Keff was 52.3% higher than that of the Pd/CMS(t)/Pd structure. The increase of MS can effectively reduce the switching current density of the magnetic tunnel junction, and then reduce the energy consumption of magnetic record writing. For the Pd/CMS(3.5)/[Pd(0.8)/CMS(3.5)]N/Pd multilayers, the PMA peaked at N = 1. With the increase of N, the PMA decreases and gradually changes from PMA to IMA. This indicates that the increase of more Pd/CMS interfaces does not increase the PMA further. In addition, when N = 3 and 4, the expression of Keff values is inconsistent with the corresponding hysteresis loops, and the exchange bias effect of in-plane hysteresis loops appears at N = 4 and N = 5.

Coherent-incoherent crossover of the intrinsic spin Hall effect in Pd

We report the coherent-incoherent crossover of the spin Hall effect in Pd. By measuring spin-orbit torques generated by oxygen-incorporated Pd films, we find two distinct regimes of the spin Hall effect; one in which the spin Hall conductivity is insensitive to the electric conductivity, and the other in which the spin Hall conductivity decreases with decreasing electric conductivity. The crossover between the two regimes is consistent with the prediction of the coherent-incoherent crossover of the spin Hall effect, which is analogous to that of the anomalous Hall effect. Unlike the anomalous Hall effect, for which the crossover has been observed for a wide class of ferromagnets, the crossover of the spin Hall effect has been confirmed only for Pt and Pt-based alloys. The observation of the crossover of the spin Hall effect for Pd supports the generality of this phenomenon, illustrating an important correspondence between the spin Hall effect and the anomalous Hall effect.

Microenvironment Effects on Electrocatalytic Oxygen Reduction: The Role of Acid Electrolyte Anions

The surface-electrolyte microenvironment plays an important role in the performance of electrochemical systems. Better understanding the role that electrolyte ions play at or near the surface is an avenue to optimizing the local electrocatalyst reaction microenvironment in renewable electrochemical energy technologies such as fuel cells, electrolyzers, and batteries. Focusing on the oxygen reduction reaction (ORR), as one of the key half reactions often involved in these electrochemical technologies, better understanding acid electrolyte anion effects may help facilitate the design of non-Pt catalyst material alternatives for proton exchange membrane fuel cells (PEMFCs) and/or engineer better electrolyte-electrode interfaces with tuned local microenvironments. Previous work has focused on studying anion effects in acidic media using Pt electrocatalysts, and changes in performance as a function of anion species have been generally attributed to competitive adsorption. In addition to acid electrolyte anion effects not being widely studied outside of on Pt materials, possible anion effects outside competitive adsorption on the ORR have, in general, not been investigated in detail. Therefore, to better understand acid electrolyte anion effects on the ORR activity and selectivity of both strong and weak oxygen-binding materials, smooth Pd and Ag thin films specifically, we employed cyclic voltammetry with a rotating (ring) disk electrode to investigate the ORR performance response of these metals in various electrolytes (HClO4, HNO3, H2SO4, H3PO4, HCl, and HBr) at pH 1. Moreover, we employ extensive physical characterization methods to measure exposed electrocatalyst surface area (atomic force microscopy, AFM), composition & electronic structure (x-ray photoelectron spectroscopy, XPS), and mass/thickness (inductively coupled plasma optical emission spectrometry, ICP-OES) of the thin films before and after ORR testing in each electrolyte. In this work, we demonstrate anion identity plays a significant role in modifying ORR activity and selectivity on both smooth thin film Ag and Pd polycrystalline electrocatalysts; notably including decreased hydrogen peroxide selectivity in nitric acid compared to in the other investigated electrolytes. Moreover, we combine physics-based modeling and data science to gain insight into the fundamental nature of anion interactions in the electrochemical double layer microenvironment that result in the observed performance changes as a function of anion species. These insights provide guidance on opportunities to improve performance for ORR catalysts in PEMFCs and related applications.

Hydrogen sensing by RGB investigation of gasochromic coloration of MoO3/Pd-coated polyester fabrics

Recently, colorimetric sensors capable of detecting explosive and toxic molecules have attracted much attention for applications in health and environmental monitoring. Here, we demonstrate the detection of hydrogen gas using gasochromic fabrics and evaluate their performance with a commercial RGB color sensor module. For this purpose, first a thin Pd film was sputter deposited onto the polyester fabric and then amorphous MoO3 films (several microns) were deposited by RF reactive magnetrons in various O2/Ar ratios (from 2.5 to 12.5 O2). Field emission scanning electron microscope (FESEM) identified the morphological changes of the surface at each coating step and showed that the fibrous structure of the fabrics was preserved after coating of the films, which is suitable for better gas adsorption. Exposure to hydrogen caused the obtained MoO3/Pd/polyester samples to turn blue-gray at room temperature and the UV–Vis spectra showed an increase in optical absorption. Their sensitivity was evaluated in the presence of different H2 concentrations from 250 to 10,000 ppm by recording the time variation of color difference (RGB Euclidean distance). A significant correlation was found between absorption spectrum and RGB analysis with respect to oxygen content. The color variation showed a linear behavior with respect to hydrogen concentration for all samples. Response and recovery times, reversibility and reproducibility in the presence of hydrogen gas were also evaluated using the RGB readout data. Overall, the RGB module optimally depicts the sensing characteristics of the gasochromic fabrics in terms of reversibility, sensitivity and reproducibility, which may contribute to their commercial development.

Effect of Pd/ZnO Morphology on Surface Acoustic Wave Sensor Response.

Laser deposition was used to obtain Pd/ZnO bilayers, which were used as sensing layers in surface acoustic wave (SAW) sensors. The effect of laser deposition parameters such as deposition pressure, laser energy per pulse, laser wavelength or pulse duration on the porosity of the Pd and ZnO films used in the sensors was studied. The effect of the morphology of the Pd and ZnO components on the sensor response to hydrogen was assessed. Deposition conditions producing more porous films lead to a larger sensor response. The morphology of the ZnO component of the bilayer is decisive and has an influence on the sensor properties in the same order of magnitude as the use of a bilayer instead of a single Pd or ZnO layer. The effect of the Pd film morphology is considerably smaller than that of ZnO, probably due to its smaller thickness. This has implications in other bilayer material combinations used in such sensors and for other types of analytes.

Exploiting the Combination of Displacement and Chemical Plating for a Tailored Electroless Deposition of Palladium Films on Copper

Various formulations for electroless deposition, to obtain continuous nanometre-sized and micrometre-sized films of palladium on copper, were compared. We deposited ultrathin films using displacement plating formulations. We obtained continuous films with an equivalent thickness between 6 and 22 nm, measured by exploiting the K-ratio method with SEM-EDS of Pd layers. The Pd films obtained in this step of the work represent a cost-effective catalytic substrate. As a second step, we selected chemical plating as the procedure to obtain palladium films with a thickness in the micrometre range. An ammonia-based Pd chemical plating bath represent one of the most effective chemical plating formulations. To prevent copper substrates from being damaged by ammonia, displacement plating with palladium was also applied as a pre-treatment to make the use of these plating baths a viable way to obtain thicker palladium coatings. Palladium films showing good adherence, compact morphology, and a thickness over 1.5 μm were obtained, proving that the combination of two different electroless techniques was the key to develop a sustainable procedure for micrometre-sized palladium coatings, which could substitute electroplating of Pd in galvanic industry for decorative applications.

Water structure at the multilayers of palladium deposited at nanostructured Au electrodes

In situ ATR-SEIRAS experiments were carried out to describe water structure at the surface of multilayers of Pd modified nanopatterned gold film electrodes in contact with aqueous 0.1 M HClO 4 solution. The results show that the surface water becomes more ordered when the thickness of the Pd film increases. The “ice-like water” band becomes stronger and for the electrode with 5 ML Pd@Au, the ratio of “ice-like water” to “liquid-like water” becomes larger than that in bulk water. Concomitantly, the population of “monomers” or small water clusters diminishes. All surface water bands increase when hydrogen adsorption takes place. The bending band corresponding to “monomers” or “multimers” water is observed chiefly at the potentials of hydrogen adsorption.