Pd Vacancies Research Articles

First-principles calculation of monolayer PdSe2 with Se and Pd vacancy and its effect on quantum capacitance

Abstract The emergence of 2-D materials such as graphene has caught the attention of the scientific community. 2-D materials have a higher surface area per unit mass, which is ideal for electrodes. Another popular material among researchers is the monolayer palladium diselenide or PdSe2, a semiconductor with a tunable bandgap energy. Since there are fewer states in the fermi level of PdSe2, the quantization effect is more prevalent, hence, it would most likely draw its capacitance from its electronic configuration. In this study, the electronic properties such as band structure, density of states, and quantum capacitance of pristine monolayer PdSe2, PdSe2 with Se, and Pd vacancy were calculated based on density functional theory via Quantum Espresso. The formation energies of all systems were energetically favourable. The system with Se vacancy has the highest formation energy with a value of -3.47 eV. The density of states of all systems were observed to have a local minima at the fermi level. More occupiable states around the valence band were observed for the systems with vacancy. Bader charge analysis showed a notable decrease of charge in Se atoms near the Pd vacancy, while the Pd atoms in the Se vacated system showed stronger charge transfer between Pd and another Se atom. The quantum capacitance and surface charge values were calculated using the density of states. Higher surface charge values at the negative voltage were observed for the systems with induced vacancies. Based on the results, the systems with vacancies have improved the quantum capacitance and surface charge at the negative potential.

Realizing new 2D spintronic materials from the non-magnetic 1T-PdO2 monolayer through vacancy defects and doping.

In this work, vacancy- and doping-based magnetism engineering in a non-magnetic 1T-PdO2 monolayer is explored in order to realize new two-dimensional (2D) spintronic materials. The pristine monolayer is an indirect gap semiconductor with a band gap of 1.45 (3.20) eV obtained using the PBE (HSE06) functional. Half-metallicity with a total magnetic moment of 3.95 μB is induced by creating a single Pd vacancy, where the magnetic properties are produced mainly by O atoms around the vacancy site. In contrast, the non-magnetic nature is preserved under the effects of a single O vacancy, however a band gap reduction in the order of 37.93% is achieved. Further doping with transition metals (TMs = V, Cr, Mn, and Fe) in the Pd sublattice and with non-metals (B, C, N, and F) in the O sublattice is investigated. TM impurities lead to the emergence of a diluted magnetic semiconductor nature, where total magnetic moments of 1.00, 2.00, and 3.00 μB are obtained in the V-, Cr(Fe)-, and Mn-doped systems, respectively. In these cases, the TMs' 3d electrons mainly originate the system's magnetism. Significant magnetization of the PdO2 monolayer is also achieved by doping with B, N, and F atoms, where either half-metallic or diluted magnetic semiconductor natures are induced. Herein, electronic and magnetic properties are regulated mainly by the interactions between the 2p orbital of the dopant, 4d orbital of the first neighbor Pd atoms, and 2p orbital of the second neighbor O atoms. Meanwhile, C impurity induces no magnetism in the PdO2 monolayer because of the strong electronic hybridization with their neighbor atoms. Results presented herein may introduce efficient approaches to engineer magnetism in a non-magnetic PdO2 monolayer, such that the functionalized systems are further recommended for prospective spintronic applications.

Hetero-phase Pd4S metallene nanoribbons with Pd-rich vacancies for sulfur ion degradation-assisted hydrogen production

Vacancy defects and crystal-phase are essential parameters that affect surface electronic structure. It is still a great challenge to introduce both vacancy defects and unconventional crystal-phase into electrocatalysts to investigate their effects on the electrocatalytic performance. Herein, we in-situ constructed ultrathin Pd4S metallene nanoribbon with Pd-rich vacancies (VPd-Pd4S MNRs) via hydrothermal sulfuration for outstanding hydrogen evolution reaction (HER) and sulfion oxidation reaction (SOR) performance. Based on the metallene morphology, synergistic effect of Pd vacancies and crystal-phase, VPd-Pd4S MNRs exhibits excellent activity and long-term stability, reaching 100 mA cm−2 at 0.776 V to accomplish the simultaneous hydrogen production and sulfur ion degradation. As confirmed by density functional theory (DFT) simulations, the vacancies and S atoms affect electronic structure of catalyst surface, modulating the d-band center of Pd and thus optimizing the adsorption/dissociation during catalytic process. This work presents a promising method to simultaneously construct metal vacancies and crystal-phase in metallene electrocatalysts.

Interplay of hydrogen and point defects in B2-type PdCu: A density functional theory study

To improve the performance of hydrogen-permeable alloys, quantitatively elucidating the metal–hydrogen interactions and identifying the controlling factors that govern the bottleneck step of the hydrogen permeation process are essential. In this study, based on the density functional theory, we characterized the interactions of hydrogen with point defects (such as antisite atoms and vacancies) in B2-type non-stoichiometric PdCu alloys, which are promising candidate materials for low-temperature working membranes for hydrogen purification. We proposed a “double defect” mechanism that produces a new type of defect complex with an atomic configuration similar to the Cu vacancy in B2-type PdCu alloys. We found that both Cu vacancies and double defects can act as strong hydrogen-trapping sites and are thus responsible for the marked decrease in hydrogen diffusivity. In contrast, antisite Cu atoms only repel H atoms and do not significantly hinder hydrogen diffusion. Whereas the binding energy of hydrogen to Cu vacancies and double defects exceeded 0.3 eV when relatively few (one or two) H atoms were trapped, the binding energy of hydrogen to Pd vacancies was substantially smaller (0.02 eV at most). Therefore, in Cu-rich non-stoichiometric PdCu alloys, the double defects may be the major hydrogen trapping sites. Furthermore, the effects of hydrogen on the formation of Pd vacancies, Cu vacancies, and double defects in B2-type PdCu alloys were clarified. Notably, double-defect formation from Pd vacancies in B2-type PdCu was significantly enhanced by the presence of nearby H atoms. This phenomenon, in which vacancies in B2-type intermetallic compounds transform into defect complexes in the presence of hydrogen, is postulated for the first time in this study. The findings of this study contribute to further understanding the driving forces for the formation of vacancy-hydrogen complexes in PdCu alloys at the atomic level and provide theoretical support for identifying the lattice defects that inhibit hydrogen permeability in Pd-based membranes.

Effects of 3d transition metal impurities and vacancy defects on electronic and magnetic properties of pentagonal Pd2S4: competition between exchange splitting and crystal fields

In this paper, we first investigate the electronic properties of the two-dimensional structure of dichalcogenide Pd2S4. These properties strongly depend on the crystal field splitting which can change by atomic vacancies (S and Pd vacancies). The main purpose of the present paper is to create remarkable magnetic properties in the system by adding 3d transition metal atoms where the presence of Mn, Cr, and Fe creates the exchange interaction in the system as well as change in the crystal field. The created magnetic properties strongly depend on the competition between exchange interaction and crystal field to separate the levels of d orbitals. In addition, the presence of the transition metals in the structures with S and Pd vacancy has been investigated carefully. The calculations demonstrate that we can achieve an extensive range of magnetic moment up to 3.131 {mu }_{B}. The maximum one is obtained in the presence of Mn and absence of sulfur while some of the doped structures does not have magnetic moment. Our results show that Pd vacancy in the presence of Cr, Mn and Fe metals increases the magnetic property of the Pd2S4 structure. The extensiveness and variety of the obtained properties can be used for different magnetic and non-magnetic applications.

Defect induced electrocatalytic hydrogen properties of pentagonal PdX2 (X = S, Se).

Searching for catalysts of hydrogen evolution reaction (HER) that can replace Pt is critical. Here, we investigated the HER electrocatalytic activity of pentagonal PdS2 (penta-PdS2) and PdSe2 (penta-PdSe2) by first-principles calculations. Three types of vacancies (VS/Se, VPd, DVS/Se) were constructed to activate the inert basal planes of PdS2 and PdSe2. The results show that S/Se and Pd vacancies significantly improve HER performance, and the Gibbs free energy (ΔGH) of systems can be further regulated by vacancy concentration. Particularly, PdS2 with 2.78% VS, 50% VPd and PdSe2 with 12.5% VSe display the optimal ΔGH value and the highest exchange current density. Further analysis of charge transfer and band structures were described that the introduce of vacancies efficiently regulates the electronic properties, resulting in the diminution of bandgap, and accelerates the charge transfer, thereby contributing to an enhanced electron environment for HER process. Our results provide a theoretical guidance for the applications of pentagonal transition-metal dichalcogenides as catalysts of hydrogen evolution reaction.

Crystal growth and metal-insulator transition in two-dimensional layered rare-earth palladates

We synthesized high-quality, single-crystalline thin films of layered rare-earth (R) palladates R${}_{2}{\mathrm{PdO}}_{4}$ ($R$ = La, Nd, and Sm) and Ce substituted ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ by reactive molecular beam epitaxy. For ${\mathrm{La}}_{2}{\mathrm{PdO}}_{4}$, ${\mathrm{Nd}}_{2}{\mathrm{PdO}}_{4}$, and ${\mathrm{Sm}}_{2}{\mathrm{PdO}}_{4}$, we find that the electronic conduction is independent of R elements. Doping charge carriers into the ${\mathrm{PdO}}_{2}$ planes of ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ increases the electronic conduction and this effect is enhanced by vacuum annealing. The enhanced electronic conduction originates not solely from the doped charge carriers but is superimposed by Pd vacancies. X-ray photoelectron spectroscopy combined with inductively coupled plasma mass spectrometry revealed Pd deficiencies in ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ and these defects play a crucial role for the electronic conduction. We observe a monotonic enhancement of the electronic conduction in ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ thin films induced by Ce substitution and vacuum annealing. The estimated charge carrier concentration necessary for metallic conduction ($x\ensuremath{\approx}0.45$) at $T=0$ K is far beyond the experimentally accessible solubility limit.

Antiferromagnetic Kondo lattice in the layered compoundCePd1−xBi2and comparison to the superconductorLaPd1−xBi2

The layered compound ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{2}$ with the tetragonal ${\mathrm{ZrCuSi}}_{2}$-type structure was obtained from excess Bi flux. Magnetic susceptibility data of ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{2}$ show an antiferromagnetic ordering below 6 K and are anisotropic along the $c$ axis and the $ab$ plane. The anisotropy is attributed to crystal-electric-field (CEF) effects and a CEF model which is able to describe the susceptibility data is given. An enhanced Sommerfeld coefficient $\ensuremath{\gamma}$ of 0.191 J mol $\mathrm{Ce}{}^{\ensuremath{-}1} \mathrm{K}{}^{\ensuremath{-}2}$ obtained from specific-heat measurement suggests a moderate Kondo effect in ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{2}$. Other than the antiferromagnetic peak at 6 K, the resistivity curve shows a shoulderlike behavior around 75 K which could be attributed to the interplay between Kondo and CEF effects. Magnetoresistance and Hall-effect measurements suggest that the interplay reconstructs the Fermi-surface topology of ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{2}$ around 75 K. Electronic structure calculations reveal that the Pd vacancies are important to the magnetic structure and enhance the CEF effects which quench the orbital moment of Ce at low temperatures.

Physical properties of the InPd intermetallic catalyst

The intermetallic phase InPd is a candidate material for the use as a catalyst in the methanol steam reforming process. To study the connection between the catalytic properties of the surface and the structural and electronic properties of the bulk, we have grown single crystals of the InPd phase by the Czochralski method and determined their electronic, thermal, magnetic and hydrogen-absorption properties. By growing crystals from a high-temperature solution, we could crystallize a slightly off-stoichiometric In-rich composition In1.04Pd0.96, which contained a significant amount of constitutional defects in the lattice (Pd vacancies on the Pd sublattice) to retain the CsCl-type structure. The strongly inhomogeneously broadened 115In NMR spectrum and the high residual (T → 0) electrical resistivity confirmed the presence of constitutional defects. Single crystals of InPd do not absorb hydrogen, as requested for a good hydrogenation catalyst material. Calculated electronic density of states (DOS) shows large contribution of Pd(d) states at the Fermi level. Application of the electron localizability indicator reveals ionic and multi-centre In–Pd interactions stabilizing the crystal structure. The electrical and thermal conductivities of InPd show metallic character, whereas the thermoelectric power and the Hall coefficient both show positive sign, revealing that InPd is a predominant hole-type conductor. The calculated electronic DOS at the Fermi energy is in a good agreement with the experimental value determined from the low-temperature specific heat. Magnetic measurements have shown that InPd is a diamagnet. All results are compared to the chemically related intermetallic compound GaPd. The active–site-isolation concept for increased catalytic selectivity is discussed in relation to the InPd and GaPd structures.

Palladium Clusters Anchored on Graphene Vacancies and Their Effect on the Reversible Adsorption of Hydrogen

The hydrogen storage capacity of nanoporous carbons can be enhanced through metal doping, for instance doping with palladium. However, there are two problems that may limit the positive effect of metal doping on the reversible storage capacity. First, clustering of the metal atoms decreases its effectiveness, which is largest for maximum dispersion. A second problem that is often overlooked is that the desorption of metal–hydrogen complexes may compete favorably with the desorption of hydrogen molecules. Desorption of complexes would spoil the reversible storage of hydrogen in the material. Both problems can be avoided by firmly anchoring the metal atoms and clusters to defects of the carbon substrate, for instance vacancies. With this goal in mind, we have performed density functional calculations to investigate the desorption of hydrogen and of Pd–hydrogen complexes from Pd atoms and clusters supported on pristine graphene and on graphene layers with vacancies. We show that palladium atoms bind much stronger to graphene vacancies, binding energy Eb = 5.13 eV, than to pristine graphene, Eb = 1.09 eV. The Pd atoms tend to nucleate and form clusters around the vacancies and the small Pdn clusters (n = 2–6) also bind strongly, Eb ≈ 5 eV, to the vacancies. However, the Pd–Pd interaction is much smaller than the Pd–vacancy interaction, and therefore, the vacancies favor the dispersion of palladium on the graphene layer. For hydrogen adsobed on Pd atoms and clusters supported on pristine graphene, desorption of Pd–hydrogen complexes competes with desorption of molecular hydrogen. However, for hydrogen adsobed on a Pd atom anchored on a graphene vacancy, the desorption of the PdH2 complex costs 4.2 eV, and therefore, it does not compete with the desorption of molecular hydrogen, which takes place with an energy cost of only 0.2 eV. This shows the beneficial effect that anchoring Pd atoms and clusters to graphene vacancies has on the reversible adsorption/desorption of hydrogen.

Superconductivity and strong intrinsic defects in LaPd1−xBi2

Two new phases LaPd${}_{1\ensuremath{-}x}$Bi${}_{2}$ and CePd${}_{1\ensuremath{-}x}$Bi${}_{2}$ were obtained by growing single crystals in Bi flux. They adopt the tetragonal ZrCuSi${}_{2}$-type structure and feature Bi-square nets and PbO-type PdBi layers with significant partial Pd occupancy. Bulk superconductivity at 2.1 K and metallic behavior above ${T}_{c}$ are observed in LaPd${}_{1\ensuremath{-}x}$Bi${}_{2}$. A small residual resistance ratio (RRR) indicates a strong scattering effect induced by the Pd vacancies, which implies an $s$-wave pairing symmetry in LaPd${}_{1\ensuremath{-}x}$Bi${}_{2}$. The broadening of the resistivity transition was measured under different magnetic fields demonstrating a high upper critical field of 3 T. Hall effect measurements reveal dominantly electron-like charge carriers and single-band transport behavior in LaPd${}_{1\ensuremath{-}x}$Bi${}_{2}$. The paramagnetic CePd${}_{1\ensuremath{-}x}$Bi${}_{2}$ is nonsuperconducting but shows antiferromagnetic ordering below 6 K.

First-principles evaluation of carbon diffusion in Pd and Pd-based alloys

The absorption and diffusion of carbon in Pd and Pd-based alloys can be important in applications of these materials as catalysts or membranes, but little is known about these processes. We used first principles calculations to characterize the absorption of C in pure Pd and PdM alloys with $M=\text{Ag}$, Au, and Cu. Our calculations show that the preferred configuration of C in Pd is as an interstitial atom; effects from Pd vacancies are minimal and substitutional sites are much less favorable. When the effects of thermal lattice expansion are included, our calculations predict C diffusivities in good agreement with experimental data, which is only available at elevated temperatures. Characterizing the binding energy and hopping energies of interstitial C in terms of lattice expansion or contraction is also a useful way to understand the effect of small amounts of $M=\text{Ag}$, Au, or Cu in PdM alloys. We also examined the properties of interstitial C in ${\text{Pd}}_{77.7}{\text{Ag}}_{22.3}$. Our calculations predict that the diffusivity of C in this alloy is two to three orders of magnitude lower than in pure Pd.

First-principles study of vacancy-hydrogen interaction in Pd

Hydrogen absorption in face-centered-cubic palladium is studied from first principles, with particular focus on interaction between hydrogen atoms and vacancies, formation of hydrogen-vacancy complexes, and multiple hydrogen occupancy of a Pd vacancy. Vacancy formation energy in the presence of hydrogen, hydrogen trapping energy, and vacancy formation volume have been calculated and compared to existing experimental data. We show that a vacancy and hydrogen atoms form stable complexes. Further we have studied the process of hydrogen diffusion into the Pd vacancy. We find the energetically preferable position for hydrogen to reside in the palladium unit cell in the presence of a vacancy. The possibility of the multiple hydrogen occupancy (up to six hydrogen atoms) of a monovacancy is elucidated. This theoretical finding supports experimental indication of the appearance of superabundant vacancy complexes in palladium in the presence of hydrogen.

The hydrogen interaction in an FCC FePd alloy with a vacancy

The absorption of hydrogen in the ordered face-centered cubic FePd alloy is investigated using a density functional calculation method. Changes in the electronic structure and bonding after introducing an Fe or Pd vacancy are analysed. H locates close to a tetrahedral site and the H–metal bond is achieved at the expense of the interfacial Fe–Pd bond.

First-principles study of constitutional and thermal point defects in B2 PdIn

We performed first-principles calculations to investigate the point defect structure of B2 PdIn. In agreement with experiments, our calculations show that Pd vacancies and Pd antisites are the constitutional point defects in In-rich and Pd-rich B2 PdIn, respectively. To predict the thermal defect concentrations at finite temperatures, we adopted the statistical-mechanical Wagner–Schottky model in combination with our first-principles calculated point defect formation enthalpies. Our calculations suggest that the predominant thermals defects in B2 PdIn are of triple-defect type and not of Schottky type.

Surface structure of Al-Pd-Mn quasicrystals: Existence of supersaturated bulk vacancy concentrations

We identified the presence and chemical nature of bulk vacancies in Al-Pd-Mn quasicrystals by measuring the structure and composition of two- and fivefold cleavage surfaces of different preannealed quasicrystals subjected to postcleavage heat treatments using scanning electron microscopy and Auger electron spectroscopy. A strong dependence of the surface structure from the preannealing is observed and explained by varying concentrations of bulk vacancies. The analysis of the data shows that as-grown Al-Pd-Mn quasicrystals contain Al, Pd, and Mn vacancies in supersaturated but near stoichiometric concentrations, while long-term preannealed samples contain a much lower vacancy concentration, with a tendency to have excess Al vacancies. We found that Al and Mn vacancies are more mobile at lower temperatures than Pd vacancies, and that the diffusion of Mn vacancies is directly coupled to the mobility of Al vacancies. The results demonstrate that the evolution of the surface structure is primarily affected by bulk vacancies migrating toward the surface during heat treatments and provide a new methodology to characterize the vacancies in intermetallic alloys.

Structural and optical properties of Pd 1− xIn x thin films

Pd 1− x In x (0.4 < x < 0.56) thin films have been prepared by RF sputtering from a multi-zone target. Their structural and optical properties have been studied by X-ray diffractometry, near normal incidence optical reflectivity and ellipsometry. Both structural and optical properties exhibit composition dependent characteristics. Indium-deficient films include Pd in antisite positions. Indium-rich films incorporate Pd vacancies. The optical spectrum shows both free electron-like properties and interband transitions giving rise to absorption peaks around 2.7 and 4.8 eV. A rigid band model is not sufficient to explain the optical properties of the alloys. The effects of Pd antisite atoms and Pd vacancies on the band structure must be taken into account to explain the evolution of the optical parameters.

Point defects and texture of Pd1 − xInx sputtered intermetallic thin films

Pd1 − xInx thin films (0.4 < x < 0.56) were prepared by radio frequency sputtering from a multi-zone target. The properties of these Hume-Rothery alloys were studied by X-ray diffractometry, electron probe microanalysis and scanning tunneling microscopy. The diffraction spectra were analyzed to obtain the intensity ratio of the (100) superlattice line to the (200) normal line, together with the variations of the lattice constant. The results are explained quantitatively by a model based on point defects, i.e. Pd vacancies in In-rich films and Pd antisite atoms in Pd-rich films. In-rich films grow preferentially in the [100] direction while Pd-rich films grow preferentially in the [110] direction. The grains in indium-rich sputtered films appear to be enclosed in an atomically thick, indium-rich layer. The role of texture and the influence of point defects on electrical resistivity is also reported.

Sn119Mössbauer-effect studies in Heusler alloysPd1+zMnSb1−xSnx—Compositional and heat-treatment effects

M\ossbauer-effect experiments on samples of ${\mathrm{Pd}}_{2}$Mn${\mathrm{Sb}}_{0.99}$${\mathrm{Sn}}_{0.01}$ performed systematically as a function of heat treatment have revealed the existence of three $^{119}\mathrm{Sn}$ magnetic field sites characterized by high-, intermediate-, and low-field values. A high-field site of 210\ifmmode\pm\else\textpm\fi{}5 kOe is identified with Sn replacing Sb in the ordered $L{2}_{1}$ structure of ${\mathrm{Pd}}_{2}$MnSb. An intermediate-field site of about 100 kOe is identified with a defect Sn environment which probably consists of 4 Pd vacancies in the near-neighbor coordination of the regular high-field site. The identification of the intermediate-field site is suggested by a set of measurements performed as a function of Pd composition $z$ in ${\mathrm{Pd}}_{1+x}\mathrm{Mn}{\mathrm{Sb}}_{0.995}{\mathrm{Sn}}_{0.005}$ samples, which revealed the Sn field in the $C{1}_{b}$ structure of PdMnSb to be 100 kOe. Finally, a low-field site (\ensuremath{\sim}0 kOe) is observed on prolonged annealing ${\mathrm{Pd}}_{2}$Mn${\mathrm{Sb}}_{0.99}$${\mathrm{Sn}}_{0.01}$ samples. This site is shown to be formed irreversibly due to the presence of oxygen contamination and represents a defect Sn site whose structure is less certain. The $^{119}\mathrm{Sn}$ field results in PdMnSb and ${\mathrm{Pd}}_{2}$MnSb hosts are compared with estimates of these fields based on the Jena-Geldart model.