Effect of the morphology on the mechanisms of the magnetization reversal of multilayered thin Co/Pd films

A comparative study of the magnetization curves of continuous and porous multilayered Pd_10nm/[Co_0.3nm]/Pd_0.55nm]15/Pd_2nm films deposited on an anodized TiO₂ template was performed by magnetometry. Based on the comparison of the dependences of coercive field H_C on angle θ between the easy-magnetization axis and the direction of external magnetic field H with theoretical dependences H_C(θ) for the magnetization reversal by domain walls motion (according to the Kondorski model) and the rotation of magnetic moments (by the Stoner–Wohlfarth model), the differences in the mechanisms of magnetization reversal for two mentioned types of the films were revealed. The correlation between the difference in the morphologies of the continuous and porous films and revealed change in the mechanisms of the magnetization reversal, as well as the changes in values of H_C and calculated constants of the magnetic anisotropy, is discussed.     

Hydrogen adsorption and desorption on the Pt and Pd subnano clusters — a review

In this review, we present our recent first principles studies on the sequential H₂ dissociative chemisorption and H desorption on the Pt _n and Pd _n clusters (n=2–9, 13). Upon full saturation by H atoms, the calculated H2 dissociative chemisorption energy and H desorption energy on Pt₁₃ and Pd₁₃ clusters are similar to the corresponding values on smaller close-packed clusters. Indeed, the catalytic performances of these subnano clusters do not vary significantly with the particle sizes or shapes. Instead, they are dependent on the surface metal atoms which can be accessed by H atoms. In addition to the coverage dependency of the H₂ dissociative chemisorption and H sequential desorption energies, the phase transition of both Pt₁₃ and Pd₁₃ from the icosahedral to fcc-like structures at certain H coverage was also investigated.      

Pd2+ reduction and gasochromic properties of colloidal tungsten oxide nanoparticles synthesized by pulsed laser ablation

Tungsten oxide nanoparticles were fabricated by a pulsed laser ablation method in deionized water using the first harmonic of a Nd:YAG laser (λ=1064 nm) at three different laser pulse energies (E1 =160, E2 =370 and E3 =500 mJ/pulse), respectively. The aim is to investigate the effect of laser pulse energy on the size distribution and gasochromic property of colloidal nanoparticles. The products were characterized by dynamic light scattering (DLS), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and UV-Vis spectroscopy. The results indicated that WO₃ nanoparticles were formed. After ablation, a 0.2 g/l PdCl₂ solution was added to activate the solution against hydrogen gas. In this process Pd²⁺ ions were reduced to deposit fine metallic Pd particles on the surface of tungsten oxide nanoparticles. The gasochromic response was measured by H₂ and O₂ gases bubbling into the produced colloidal Pd–WO₃. The results indicate that the number of unreduced ions (Pd²⁺) decreases with increasing laser pulse energy; therefore, for colloidal nanoparticles synthesized at the highest laser pulse energy approximately all Pd²⁺ ions have been reduced. Hence, the gasochromic response for this sample is nearly reversible in all cycles, whereas those due to other samples are not reversible in the first cycle.     

First-principles study of small palladium clusters on NiAl(1 1 0) alloy surface

Theoretical calculations focused on the geometry, stability, electronic and magnetic properties of small palladium clusters Pd_n (n=1–5) adsorbed on the NiAl(1 1 0) alloy surface were carried out within the framework of density functional theory (DFT). In agreement with the experimental observations, both Ni-bridge and Al-bridge sites are preferential for the adsorption of single palladium atom, with an adsorption energy difference of 0.04 eV. Among the possible structures considered for Pd_n (n=1–5) clusters adsorbed on NiAl(1 1 0) surface, Pd atoms tend to form one-dimensional (1D) chain structure at low coverage (from Pd₁ to Pd₃) and two-dimensional (2D) structures are more stable than three-dimensional (3D) structures for Pd₄ and Pd₅. Furthermore, metal-substrate bonding prevails over metal–metal bonding for Pd cluster adsorbed on NiAl(1 1 0) surface. The density of states for Pd atoms of Pd/NiAl(1 1 0) system are strongly affected by their chemical environment. The magnetic feature emerged upon the adsorption of Pd clusters on NiAl(1 1 0) surface was due to the charge transfer between Pd atoms and the substrate. These findings may shade light on the understanding of the growth of Pd metal clusters on alloy surface and the construction of nanoscale devices.     

Adsorption behaviour of SF6 decomposed species onto Pd4-decorated single-walled CNT: a DFT study

Metal nanocluster decorated single-walled carbon nanotubes (SWCNT) with improved adsorption behaviour towards gaseous molecules compared with intrinsic ones, have been widely accepted as a workable media for gas interaction due to their strong catalysis. In this work, Pd₄ cluster is determined as a catalytic centre to theoretically study the adsorption property of Pd₄-decorated SWCNT upon SF₆ decomposed species. Results indicate that Pd₄-SWCNT possessing good responses and sensitivities towards three composed species of SF₆ could realise selective detection for them according to the different conductivity changes resulting from the varying adsorption ability. The response of Pd₄-SWCNT upon three molecules in order is SOF₂ > H₂S > SO₂, and the conductivity of the proposed material is about to increase in SOF₂ and H₂S systems, while declining in SO₂ system. Such conclusions would be helpful for experimentalists to explore novel SWCNT-based sensors in evaluating the operating state of SF₆ insulation devices.     

Predicting experimental signatures for the oxidation of magnesia supported palladium clusters by density functional theory

We showed in a recent density functional study that small palladium cluster on a MgO surface with F-centers can be oxidized to epitaxial Pd_xO_y nano-oxides below room temperature [1]. Here, we employ density functional theory in order to explore different methods for an experimental verification of the Pd_xO_y formation. The electronic density of states (DOS) of bare, O₂-decorated and of oxidized palladium cluster was calculated. For many cluster sizes a clear difference in the DOS could be observed allowing for a detection of the oxidation with surface sensitive spectroscopic methods. In addition, adsorption sites and stretch frequencies of a single CO molecule on bare and oxidized Pd₄ clusters were calculated. While CO prefers hollow sites on Pd₄, top adsorption sites are found for Pd₄O₂. Markedly different CO stretch frequencies indicate a possible discrimination of bare clusters and oxides by Fourier transform infrared spectroscopy.     

Geometries,stabilities, electronic and magnetic properties of small PdnIr (n = 1–8) clusters from first-principles calculations

The geometrical structures, relative stabilities, electronic and magnetic properties of small Pd_nIr (n = 1–8) clusters have been systematically investigated using density functional theory at the B3PW91 level. The optimised geometries show that the lowest-energy structures of Pd_nIr clusters prefer a three-dimensional configuration. The relative stability of these clusters was examined by analysis of the binding energies per atom, fragmentation energies, the second-order difference of energies and the HOMO–LUMO energy gaps as a function of cluster size. The obtained results exhibit that the Pd₂Ir, Pd₃Ir and Pd₅Ir clusters are more stable than their neighbouring clusters. The energy gap of the Pd₂Ir cluster is the largest of all the clusters (2.258 eV). In addition, the charge transfers, vertical ionisation potentials, vertical electron affinities and chemical hardness were calculated and discussed. The magnetism calculations indicate that the total magnetic moment of Pd_nIr clusters is mainly localised on the iridium atom for Pd_1–6Ir clusters. Meanwhile, the 5d orbital plays the key role in the magnetic moment of the iridium atom.     

Segregation at the surfaces of CuxPd1 − x alloys in the presence of adsorbed S

The influence of adsorbed S on surface segregation in Cu_xPd_1 ? x alloys (S/Cu_xPd_1 ? x) was characterized over a wide range of bulk alloy compositions (x = 0.05 to 0.95) using high-throughput Composition Spread Alloy Film (CSAF) sample libraries. Top-surface and near-surface compositions of the CSAFs were measured as functions of bulk Cu composition, x, and temperature using spatially resolved low energy ion scattering spectroscopy (LEISS) and X-ray photoemission spectroscopy (XPS). Preferential segregation of Cu to the top-surface of the S/Cu_xPd_1 ? x CSAF was observed at all bulk compositions, x, but the extent of Cu segregation to the S/Cu_xPd_1 ? x surface was lower than the Cu segregation to the surface of a clean Cu_xPd_1 ? x CSAF, clear evidence of an S-induced “segregation reversal.” The Langmuir–McLean formulation of the Gibbs isotherm was used to estimate the enthalpy and entropy of Cu segregation to the top-surface, ΔH_seg(x) and ΔS_seg(x), at saturation sulfur coverages. While Cu segregation to the top-surface of the clean Cu_xPd_1 ? x is exothermic (ΔH_seg < 0) for all bulk Cu compositions, it is endothermic (ΔH_seg > 0) for S/Cu_xPd_1 ? x. Segregation to the S/Cu_xPd_1 ? x surface is driven by entropy. Changes in segregation patterns that occur upon adsorption of S onto Cu_xPd_1 ? x appear to be related to formation of energetically favored PdS bonds at the surface, which counterbalance the enthalpic driving forces for Cu segregation to the clean surface.     

Temperature-programmed surface reaction (TPSR) of CH4 synthesis by PdxNi100−x nanoparticles

A series of Pd_xNi_100−x nanoparticles were prepared by the co-precipitation method and analyzed using a temperature-programmed surface reaction (TPSR) of their methanation reactions. ESCA measurement suggested that the as-prepared Pd-Ni alloys had Pd-core/Ni-shell structure. Surface Pd segregation occurred during H₂ reduction and resulted in a surface composition close to the nominal value. The TPSR experiments were performed by pre-adsorption of CO with H₂ to form methane. The peak temperature of methanation increased as Pd content increased, indicating that a methanation reaction is favored on Ni and Ni-rich alloy nanoparticles. For physical mixtures of Pd and Ni nanoparticles, methanation behaviors is similar to those of alloy nanoparticles; but the methanation temperatures of physical mixtures are always higher than those of alloy nanoparticles. This may be due to the formation of a Pd-enriched alloy surface layer during reduction in H₂ at 400 °C, or because the CO molecules adsorbed on the Pd sites spill over onto the Ni sites for methanation. Using TPSR technique and measuring methanation temperature, the top-most surface of such bimetallic nanoparticles can be probed.     

Size and shape-dependent melting mechanism of Pd nanoparticles

Molecular dynamics simulation was employed to understand the thermodynamic behavior of cuboctahedron (cub) and icosahedron (ico) nanoparticles with 2–20 number of full shells. The original embedded atom method (EAM) was compared to the more recent highly optimized version as inter-atomic potential. The thermal stability of clusters were probed using potential energy and specific heat capacity as well as structure analysis by radial distribution function (G(r)) and common neighbor analysis (CNA), simultaneously, to make a comprehensive picture of the solid-state and melting transitions. The result shows ico is the only stable shape of small clusters (Pd₅₅–Pd₃₀₉ using original EAM and Pd₅₅ using optimized version) those are melting uniformly due to their small diameter. An exception is cub Pd₃₀₉ modeled via optimized EAM that transforms to ico at elevated temperatures. A similar cub to ico transition was predicted by original EAM for Pd₉₂₃–Pd₂₀₇₅ clusters, while for the larger clusters both cub and ico are stable up to the melting point. As detected by \(G(r)\) and CNA, moderate and large cub clusters were showing surface melting by nucleation of the liquid phase at (100) planes and growth of liquid phase at the surface before inward growth. While diagonal (one corner to another) melting was dominating over ico clusters owing to their partitioned structure, which retarded the growth of the liquid phase. The large ico clusters, using optimized EAM, presented a combination of surface and diagonal melting due to the simultaneous diagonal melting started from different corners. Finally, the melting temperature as well as latent heat of fusion were calculated and compared with the available models and previous studies, which showed, unlike the present result, the models failed to predict size-dependent motif cross-over.     

Calculation of chemisorption and absorption induced surface segregation

Using a pair bond type model for the interatomic interactions, we determine surface segregation on clean, H, O and CO covered surfaces of various alloys. Furthermore, we study surface segregation caused by bulk hydrogen absorption. Numerical results are presented for Pd_x Zr_1?x H_y We find, that strong surface segregation may result from chemisorption of O and CO and from absorption of H in the bulk of an alloy.     

An ab initio potential energy surface for the C2H2–N2 system

An ab initio potential energy surface determined at the CCSD(T) level of theory is presented for the van der Waals complex C₂H₂–N₂. Additional calculations performed with the HF- and DFT- SAPT methods compare well with the CCSD(T) results and allow a better understanding of the main features of this interaction potential surface. An expansion of this surface over spherical harmonics has also been performed. The global energy minimum of the complex is obtained for the linear conformation. The T conformations are the least attractive. Such characteristics mainly arise because of the variation, in sign and in absolute value of the electrostatic energy between all these conformations. The specific role of the quadrupole–quadrupole interaction which involves two moments of opposite signs is therefore examined. The main features derived from the present surface are compared and discussed according to the following relevant systems: N₂–H₂, C₂H₂–H₂, C₂H₂–C₂H₂ and N₂–N₂. Calculated rotational constants for selected conformations of the C₂H₂–N₂ dimer are found to be in good agreement with available values.     

Effects of CeO2-ZrO2 present in Pd/Al2O3 catalysts on the redox behavior of PdOx and their combustion activity

The ceria-zirconium-modified alumina-supported palladium catalysts are prepared using impregnation method with H₂PdCl₄ as Pd source, hydrazine hydrate as reducing agent. The physicochemical properties of these catalysts are characterized by BET surface area (BET), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction (H₂-TPR) and temperature programmed oxidation (O₂-TPO) techniques, and their catalytic activities for the combustion of methane are examined. The results show that the palladium mainly exist in a highly dispersed PdO species on Ce-Zr-rich grains as well as Al₂O₃-rich grains surfaces, and a stable PdO species due to the strong interaction between PdO and CeO₂-ZrO₂ on the Ce-Zr/Al₂O₃ surfaces. The catalytic activity is strongly related to the redox behavior of PdO species highly dispersed on Ce-Zr-rich grains and Al₂O₃-rich grains surfaces, and the higher the reducibility of the PdO species, the higher the catalytic activity. The presence of Ce-Zr in Pd/Al₂O₃ catalyst would inhibit the site growth of PdO_x particles and decomposition of PdO to Pd⁰, and the reoxidation property of Pd⁰ to PdO_x is significantly improved, which obviously increases thermal stability and catalytic activity of Pd/Ce-Zr/Al₂O₃ catalyst for the methane combustion.     

Density functional study of ethylene adsorption on small gold,palladium and gold-palladium binary clusters

Density functional theory calculations were performed to investigate the structural and energetic properties of chemisorbed Au _n -(C₂H₄) _x and Pd _n -(C₂H₄) _x complexes, with n = 2 ? 4 and x = 1 ? 4. Adsorption in π-bonded mode dominates in Au _n -(C₂H₄) _x species irrespective of x while the di-σ mode can be formed in the most stable Pd _n -(C₂H₄) _x species. The adsorption energy decreases as the number of C₂H₄ molecules increases in Au _n -(C₂H₄) _x with n ? 4 and Pd _n -(C₂H₄) _x with n ? 3. The adsorption of one C₂H₄ molecule on bimetallic Au/Pd clusters has also been studied. The C₂H₄ molecule prefers binding to Pd atoms in π-bonded configuration on Au/Pd clusters. The net charge transfer and the typical shift in the vibrational frequencies of C₂H₄ have also been determined and discussed.     

Process of direct copper plating on ABS plastics

The processes of direct copper plating on ABS plastics were investigated by atomic force microscopy (AFM), ultraviolet-visible absorption spectrometry (UV-vis) and X-ray fluorescence spectroscopy (XRF) techniques. The substrates were etched by CrO₃/H₂SO₄ solution containing Pd²⁺ ions, catalyzed by Pd/Sn colloids solution and accelerated in an alkaline solution containing copper ions. The Pd²⁺ ions in etching solution can reduce the surface roughness and enhance the colloids adsorption. The good dispersivity colloids have excellent catalysis and its UV-vis peaks broaden. After acceleration, when the stability of Cu²⁺-complex is relatively low, Sn²⁺ was oxidized by Cu²⁺ in the alkaline solution meanwhile Cu₂O can be formed. The disproportionation reaction of Cu₂O will proceed and metallic copper forms between the Pd particles, so the conductivity of ABS surface increased. The copper particles play an important role in determining the uniformity of the propagation of copper plating. The particles of copper plating layer were uniformity and fine. The atomic concentration and the thickness of copper layer were analyzed by XRF.     

Solubility and diffusion of H in ordered (L12) and disordered Pd3Mn

Hydrogen isotherms have been measured from 423 to 573 K for the disordered and L1₂ ordered forms of Pd₃Mn and, from these equilibrium isotherms, Δ_HH$\mathrm{\Delta }{H}_{\mathrm{H}}$ and Δ_SH$\mathrm{\Delta }{S}_{\mathrm{H}}$ have been determined over a range of r values where r  =H-to-metal, atom ratio. Δ_HH$\mathrm{\Delta }{H}_{\mathrm{H}}$ values are significantly more negative for the L1₂ ordered form than for the disordered form. H diffusion constants have been determined for the L1₂ and disordered forms of Pd₃Mn from gas phase H permeation measurements through Pd₃Mn membranes (423–573 K). The activation energy for diffusion of H in the L1₂ form is 35.2 kJ/mol H which is more reasonable than the value of 76.3 kJ/mol H previously reported. The diffusion constant is greater for the disordered than for the L1₂ form.     

Hydrogen diffusion constants in stainless steel and Pd60Ag40 alloy, and permeability of diaphragms made of these metals

Hydrogen permeability through diaphragms made of 12X18H12T stainless steel and Pd₆₀Ag₄₀ alloy under electrolytic hydrogen saturation has been studied with an electrolytic cell with a vacuum chamber. Hydrogen diffusion constants D _H = 3.86 × 10⁻¹⁰ cm² s⁻¹ for stainless steel and D _H = 4.36 × 10⁻⁸ cm² s⁻¹ for Pd₆₀Ag₄₀ alloy have been determined at a temperature of 40°C using the Berrer relations.     

Geometries,stabilities and electronic properties of small-sized Pd2-doped Sin (n = 1–11) clusters

The geometries, growth patterns, relative stabilities and electronic properties of small-sized Pd₂Si_n and Si_n+2 (n = 1–11) clusters are systematically studied using the hybrid density functional theory method B3LYP. The optimised structures revealed that the lowest energy Pd₂Si_n clusters are not similar to those of pure Si_n clusters. When n = 9, one Pd atom in Pd₂Si₉ completely falls into the centre of the Si outer frame, forming metal-encapsulated Si cages. On the basis of the optimised structures, the averaged binding energy, fragmentation energy, second-order energy difference and highest occupied–lowest unoccupied molecular orbital energy gap are calculated. It is found that the Pd₂Si₅ and Pd₂Si₇ clusters have stronger relative stabilities among the Pd₂Si_n clusters. Additionally, the stabilities of Si_n+2 clusters have been reduced by the doping of Pd impurity. The natural population and natural electronic configuration analysis indicated that the Pd atoms possess negative charges for n = 1–11 and there exist the spd hybridisation in the Pd atom. Finally, the chemical hardness, chemical potential, electrostatic potential and polarisability are discussed.     

Fabrication of BixPdy bimetallic materials characterized by catalytic activity at low temperature: Nitro reduction and Suzuki−Miyaura coupling reactions under green conditions

A simple method for synthesizing the Bi_xPd_y bimetallic particles is described. The structure, composition distribution and size of synthesized Bi_xPd_y bimetallic particles were characterized using a number of analytical techniques. The Bi:Pd atomic ratio (x:y) of the nanoparticles was determined to be approximately 1:3 (Bi₂₄Pd₇₆), 1:1 (Bi₅₄Pd₄₆) and 3:1 (Bi₇₄Pd₂₆). The (111) diffraction peaks within the X-ray diffraction patterns of the bimetallic nanoparticles shifted from 39.9° to 38.5° as the Bi content increased from 0% to 75%. The d-spacings calculated from the 2θ data of (111) planes were 2.33, 2.34, 2.32 and 2.26 nm for nanoparticles with a Bi:Pd atomic ratio of 3:1, 1:1, 1:3 and 0:1 respectively. The crystalline properties of the surface of the Bi_xPd_y bimetallic nanoparticles were observed in high-resolution transmission electron microscopy analysis. The d-spacings between the adjacent lattice planes were measured on the surface of Bi_xPd_y bimetallic nanoparticles by averaging 10 lattice fringes distance. A regular face-centered cubic lattice was observed throughout the prepared Bi_xPd_y bimetallic nanoparticles. The lattice d-spacing of the Bi₃Pd₁, Bi₁Pd₁ and Bi₁Pd₃, bimetallic nanoparticles was approximately 2.34, 2.33 and 2.32 Å, respectively, which can be indexed to the (111) planes. These measurements correspond to the values calculated using the Bragg equation (d = nλ/2sinθ). The catalytic activity of Bi_xPd_y bimetallic nanoparticles was determined for the nitro compound reduction and Suzuki-Miyaura coupling reactions under green conditions (in an aqueous solution). Bi₁Pd₃ nanoparticles were shown to provide the best catalytic performance during both reactions, resulting in a yield of 98% in both cases.     

A density functional theory study of small bimetallic PdnAl (n=1–8) clusters

The geometries, stabilities, and magnetic properties of Pd_nAl (n=1–8) neutral clusters are studied using density functional theory with generalized gradient approximation. The growth pattern for different sized Pd_nAl (n=1–8) clusters is Al-substituted Pd_n+1 clusters and it keeps the similar framework of the most stable Pd_n+1 clusters except n=6 and 8. Al atoms in the ground state Pd_nAl isomers tend to occupy the most highly coordinated position. The analysis of stabilities shows that doping an Al atom can enhance the stabilities of the host Pd clusters and the magic number characteristic of Pd₄ cluster cannot be changed, the Pd₃Al cluster has a higher stability. Charges are transferred from Al atom to Pd atoms in all Pd_nAl clusters, so the Al atom is the electron donor, and Pd atoms are the electron accepters. Doping an Al atom decreases the average atomic magnetic moments of the host Pd clusters.