Surface Segregation Energies Research Articles

Cobalt Electrochemical Recovery from Lithium Cobalt Oxides in Deep Eutectic Choline Chloride+Urea Solvents.

Electrochemical recovery of the cobalt in deep eutectic solvent shows its promise in recycling and recovery of valuable elements from the spent lithium-ion battery due to its high selectivity and minimal environmental impacts. This work unveiled the roles of the substrates, applied potentials, and operating temperatures on the performance of cobalt electrochemical recovery in a deep eutectic choline chloride+urea solvent. The solvent contains cobalt and lithium ions extracted from lithium cobalt oxides - 3an essential lithium-ion battery cathode material. Our results highlight that the substrate predetermines the cobalt recovery modes via substrate-cobalt interactions, which could be predicted by the cobalt surface segregation energies and crystallographic misfits. We also show that a moderate cathode potential under -1.0 V vs. silver quasi-reference electrode at 94-104 °C is essential to ensure a selective cobalt recovery at an optimal rate. We also found that the stainless-steel mesh is an optimal substrate for cobalt recovery due to its relatively high selectivity, fast recovery rate, and easy cobalt collection. Our work provides new insights on metal recovery in deep eutectic solvents and offers a new avenue to control the metal electrodeposition modes via modulation of substrate compositions and crystal structures.

Machine Learning Prediction of Surface Segregation Energies on Low Index Bimetallic Surfaces

Surface chemical composition of bimetallic catalysts can differ from the bulk composition because of the segregation of the alloy components. Thus, it is very useful to know how the different components are arranged on the surface of catalysts to gain a fundamental understanding of the catalysis occurring on bimetallic surfaces. First-principles density functional theory (DFT) calculations can provide deeper insight into the surface segregation behavior and help understand the surface composition on bimetallic surfaces. However, the DFT calculations are computationally demanding and require large computing platforms. In this regard, statistical/machine learning methods provide a quick and alternative approach to study materials properties. Here, we trained previously reported surface segregation energies on low index surfaces of bimetallic catalysts using various linear and non-linear statistical methods to find a correlation between surface segregation energies and elemental properties. The results revealed that the surface segregation energies on low index bimetallic surfaces can be predicted using fundamental elemental properties.

Alloying effect in silver-based dilute nanoalloy catalysts for oxygen reduction reactions

To address the sluggish kinetics of the electrochemical oxygen reduction reaction (ORR), we consider a family of silver-based dilute nanoalloys by alloying the Ag(1 1 1) with one 3d transition metal atom M (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) as ORR catalysts in alkaline media. The M1Ag(1 1 1) dilute nanoalloys exhibit higher stability when M is on the subsurface (2L-M1Ag(1 1 1)) than when M is on the surface (1L-M1Ag(1 1 1)) of the Ag(1 1 1) facet as observed from surface segregation and mixing energies, and this subsurface stability is attributed to the relative positive charge transfer of surface Ag atoms as revealed from atomic charge analysis. Thus, 2L-M1Ag(1 1 1) catalysts are further investigated for their electronic structure and ORR activities. Particularly, 2L-Cu1Ag(1 1 1), 2L-Ni1Ag(1 1 1) and 2L-Zn1Ag(1 1 1) dilute nanoalloys exhibit a free-atom-like d state, and 2L-Mn1Ag(1 1 1) and 2L-Cu1Ag(1 1 1) dilute nanoalloys exhibit an ORR overpotential of 0.459 and 0.468 V, respectively. For the first time, the ternary 2L-Cu1Mn1Ag(1 1 1) is theoretically predicted with an overpotential of 0.450 V. Motivated by this prediction, we prepare a ternary CuMnAg nanoalloy by using the pulse laser deposition method. These ternary CuMnAg and dealloyed DE-CuMnAg catalysts exhibit an ORR overpotential of 0.50 and 0.47 V that is close to the predicted overpotential and that of a commercial Pt/C catalyst.

Atomistic Simulations of Carbon and Hydrogen Diffusion and Segregation in Alfa-Iron Deviant CSL Grain Boundaries

Polycrystalline materials’ mechanical properties and failure modes depend on many factors that include diffusion and segregation of different alloying elements and solutes as well as the structure of its grain boundaries (GBs). Segregated solute atoms to GB can alter the properties of steel alloys. Some of these elements lead to enhancing the strength of steel, on the other hand others can degrade the toughness of steel significantly. It is well known that carbon increases the cohesion at grain boundary. While the presence of hydrogen in steel have a drastic effects including blistering, flaking and embrittlement of steel. In practice during forming processes, the coincidence site lattice (CSL) GBs are experiencing deviations from their ideal configurations. Consequently, this will change the atomic structural integrity by superposition of sub-boundary dislocation networks on the ideal CSL interfaces. For this study, the ideal ∑3 (112) structure and its angular deviations in BCC iron within the range of Brandon criterion are studied comprehensively using molecular statics simulations. The GB and free surface segregation energies of carbon and hydrogen atoms will be quantified. Rice-Wang model is used to assess the strengthening/embrittlement impact variation over the deviation angles.

First-Principles Investigation of Intergranular Fracture in Copper by Grain Boundary Segregation of Sulfur

Grain boundary (GB) embrittlement by sulfur in fcc CuΣ5(012)[100] symmetrical tilt grain boundary (STGB) is simulated by first-principles calculations. The surface and grain boundary segregation energies are estimated by progressively placing solute atoms in the potential segregation sites in the boundaries. Based on the calculated segregation energies, the cohesive energy of the grain boundary is evaluated as a function of the sulfur atoms concentration. It is found that, when a two atomic layers’ concentration is attained, the cohesive energy is reduced by one order of magnitude compared to its value for the clean grain boundary.

Ab-initio study of surface segregation in aluminum alloys

We have calculated surface segregation energies of 41 impurities by means of density functional theory calculations. An interesting periodical variation tendency was found for surface segregation energies derived. For the majority of main group elements, segregation energies are negative which means solute elements enrichment at Al surface is energetically more favorable than uniformly dissolution. Half of transition elements possess positive segregation energies and the energies are sensitive to surface crystallographic orientations. A strong correlation is found between the segregation energies at the Al surface and the surface energ of solute elements.

Stabilization of MgAl2O4 spinel surfaces via doping

Surface structure of complex oxides plays a vital role in processes such as sintering, thin film growth, and catalysis, as well as being a critical factor determining the stability of nanoparticles. Here, we report atomistic calculations of the low-index stoichiometric magnesium aluminate spinel (MgAl2O4) surfaces, each with two different chemical terminations. High temperature annealing was used to explore the potential energy landscape and provide more stable surface structures. We find that the lowest energy surface is {100} while the highest energy surface is {111}. The surfaces were subsequently doped with three trivalent dopants (Y3+, Gd3+, La3+) and one tetravalent dopant (Zr4+) and both the surface segregation energies of the dopants and surface energies of the doped surface were determined. All of the dopants reduce the surface energy of spinel, though this reduction in energy depends on both the size and valence of the dopant. Dopants with larger ionic radius tend to segregate to the surface more strongly and reduce the surface energy to a greater extent. Furthermore, the ionic valence of the dopants seems to have a stronger influence on the segregation than does ionic size. For both undoped and doped spinel, the predicted crystal shape is dominated by {100} surfaces, but the relative fraction of the various surfaces changes with doping due to the unequal changes in energy, which has implications on equilibrium nanoparticle shapes and therefore on applications sensitive to surface properties.

Defect Chemistry of CeO2 Surfaces from First Principles and Space Charge Theory

CeO2 (ceria) is a diverse oxide with numerous technologically important functional properties. It is one of the most studied oxygen incorporation materials, and exhibits remarkable catalytic properties towards hydrocarbon and CO oxidation, and water splitting reactions.1-3 Further, acceptor doped ceria is of great interest as electrolyte for solid oxide fuel cells due to its considerable oxide ion conductivity at intermediate temperatures.4 Recently, nanocrystalline CeO2 ceramics were shown to display significant proton conduction even at room temperature,5 suggested to be due to surface, or sub-surface proton migration. The majority of these functional properties are affected by, or even originate from the rich surface, or interface defect chemistry of ceria, and corresponding space charge layers.3 , 6 In this contribution, we explore the defect chemistry of various surface terminations of CeO2 at finite temperatures, by combining first principles calculations and space charge theory. Plane-wave based electronic/atomic structure calculations at the GGA+U and hybrid HF-DFT levels are used to determine bulk defect formation energetics and the defects' surface segregation energies in CeO2. The first principles results are further combined with space charge theory through numerical evaluation of the space charge potential profiles and corresponding defect concentration profiles. The charged oxygen vacancies and protonic defects exhibit highly favorable surface segregation energies, and therefore tend to accumulate on CeO2 surfaces. Although such accumulation is partly charge compensated by oxide and hydroxide ad-ions and Ce3+ species at the surface, the surface turns out to be overall positively charged, and charge-compensated by a negative sub-surface space charge region, at least for low defect concentrations. Under humid conditions, especially at lower temperatures, the surfaces are saturated by protonic species, forming water layers. Finally, the effect of such surface saturation by protonic species, and also H2O monolayer and film formation on the sub-surface space charge potential and defect concentration profiles are discussed. 1. A. Trovarelli, Catalysis Reviews, 1996, 38, 439-520. 2. E. P. Murray, T. Tsai and S. A. Barnett, Nature, 1999, 400, 649-651. 3. Z. A. Feng, F. El Gabaly, X. Ye, Z.-X. Shen and W. C. Chueh, Nat Commun, 2014, 5. 4. H. L. Tuller and A. S. Nowick, Journal of The Electrochemical Society, 1975, 122, 255-259. 5. E. Ruiz-Trejo and J. Kilner, Journal of Applied Electrochemistry, 2009, 39, 523-528. 6. G. Gregori, M. Shirpour and J. Maier, Advanced Functional Materials, 2013, 23, 5861-5867.

Spontaneous formation of Au-Pt alloyed nanoparticles using pure nano-counterparts as starters: a ligand and size dependent process.

In this work we investigate the formation of PtAu monodisperse alloyed nanoparticles by ageing pure metallic Au and Pt small nanoparticles (sNPs), nanoparticle size <5 nm, under certain conditions. We demonstrate that those bimetallic entities can be obtained by controlling the size of the initial metallic sNPs separately prepared and by selecting their appropriate capping agents. The formation of this spontaneous phenomenon was studied using HR-STEM, EDS, ionic conductivity, UV-Vis spectroscopy and cyclic voltammetry. Depending on the type of capping agent used and the size of the initial Au sNPs, three different materials were obtained: (i) AuPt bimetallic sNPs showing a surface rich in Au atoms, (ii) segregated Au and Pt sNPs and (iii) a mixture of bimetallic nanoparticles as well as Pt sNPs and Au NPs. Surface segregation energies and the nature of the reaction environment are the driving forces to direct the distribution of atoms in the bimetallic sNPs. PtAu alloyed nanoparticles were obtained after 150 h of reaction at room temperature if a weak capping agent was used for the stabilization of the nanoparticles. It was also found that Au atoms diffuse towards Pt sNPs, producing a surface enriched in Au atoms. This study shows that even pure nanoparticles are prone to be modified by the surrounding nanoparticles to give rise to new nanomaterials if atomic diffusion is feasible.

Ab initio study of the surface properties of austenitic stainless steel alloys

Using ab initio calculations we investigated the surface energies of paramagnetic Fe1−c−nCrcNin random alloys within the concentration range of 0.12≤c≤0.32 and 0.04≤n≤0.32. These alloys crystallize mainly in the face centred cubic (fcc) structure and constitute the main building blocks of austenitic stainless steels. It is shown that all alloys have the lowest surface energies along the most close packed crystal orientation, namely the fcc (111) surfaces. The amount of Ni seems to have little effect on the surface energy, while almost all composition-driven change may be attributed to the changes in the Cr content. Within the studied compositional range, the change of the surface energy with the composition is of the order of 10%. Trends of the surface energy can be related to the magnetic structure of surfaces. Using the total energy as a function of the concentration, we determine the effective chemical potentials in bulk and at the surface, which can be used to estimate the surface segregation energies.

Density functional theory calculation of platinum surface segregation energy in Pt 3Ni (111) surface doped with a third transition metal

We have used density functional theory method to calculate the Pt surface segregation energy in the Pt 3Ni (111) surface doped with a third transition metal M and thus investigated the influence of component M on the extent of Pt segregation to the outermost layer of these Pt 3Ni/M (111) surface. As a third component in the Pt 3Ni/M (111) surface, V, Fe, Co, Mo, Tc, Ru, W, Re, Os, and Ir were predicted to lead to even more negative Pt surface segregation energies than that in the based Pt 3Ni (111) surface; Ti, Cr, Mn, Cu, Zr, Nb, Rh, Hf, and Ta would still retain the preference of Pt segregation to the surface but with less extent than the replaced Ni, while Pd, Ag, and Au would completely suppress the Pt segregation to the Pt 3Ni/M (111) surfaces. Furthermore, we examined the relation between the Pt surface segregation energy in the Pt 3Ni/M (111) surfaces and the material properties (lattice parameter, heat of solution, and Pt surface segregation energy) of binary alloys Pt 3M. It was found that the surface energy effect, strain effect, and heat of solution effect induced by the doped element M would collectively affect the Pt surface segregation energy in the Pt 3Ni/M (111) surfaces.

Point defects at the ice (0001) surface

Using density functional theory we investigate whether intrinsic defects in ice surface segregate. We predict that hydronium, hydroxide, and the Bjerrum L- and D-defects are all more stable at the surface. However, the energetic cost to create a D-defect at the surface and migrate it into the bulk crystal is smaller than its bulk formation energy. Absolute and relative segregation energies are sensitive to the surface structure of ice, especially the spatial distribution of protons associated with dangling hydrogen bonds. It is found that the basal plane surface of hexagonal ice increases the bulk concentration of Bjerrum defects, strongly favoring D-defects over L-defects. Dangling protons associated with undercoordinated water molecules are preferentially injected into the crystal bulk as Bjerrum D-defects, leading to a surface dipole that attracts hydronium ions. Aside from the disparity in segregation energies for the Bjerrum defects, we find the interactions between defect species to be very finely balanced; surface segregation energies for hydronium and hydroxide species and trapping energies of these ionic species with Bjerrum defects are equal within the accuracy of our calculations. The mobility of the ionic hydronium and hydroxide species is greatly reduced at the surface in comparison to the bulk due to surface sites with high trapping affinities. We suggest that, in pure ice samples, the surface of ice will have an acidic character due to the presence of hydronium ions. This may be important in understanding the reactivity of ice particulates in the upper atmosphere and at the boundary layer.

Surface segregation energies in low-index open surfaces of bimetallic transition metal alloys

We present a database of 24×24 segregation energies of single transition metal impurities in low-index surfaces of transition metal hosts, calculated using the localized self-consistent Green’s function (LSGF) method, in combination with the atomic sphere approximation including a multipole correction to the electrostatic potential and energy. The surface energies of {100} facets for fcc and bcc transition metals, and the more stable of the two {101¯0} facets of hcp transition metals are also calculated and compared with available theoretical results. Insights derived should be useful for determining the nature of active sites in a variety of catalytic reactions employing bimetallic catalysts.

Surface segregation of the metal impurity to the (100) surface of fcc metals

The surface segregation energies for a single metal impurity to the (100) surface of nine fcc metals (Cu, Ag, Au, Ni, Pd, Pt, Rh, Al and Ir) have been calculated using the MAEAM and molecular dynamics (MD) simulation. The results show that the effect of the surface is down to the fourth-layer and an oscillatory or monotonic damping (|E1| > |E2| > |E3| > |E4|) phenomenon in segregation energy has been obtained. The absolute value of the segregation energy E1 for a single impurity in the first atomic layer is much higher than that in the nether layers. Thus, whether the surface segregation will work or not is mainly determined by E1 which is in good relation to the differences in surface energy between the impurity and host crystals ΔQ = Qimp − Qhos. So we conclude that an impurity with lower surface energy will segregate to the surface of the host with higher surface energy.

Electrochemical dissolution of surface alloys in acids: Thermodynamic trends from first-principles calculations

A simple procedure is introduced to use periodic Density Functional Theory calculations to estimate trends in the thermodynamics of surface alloy dissolution in acidic media. With this approach, the dissolution potentials for solute metal atoms embedded in the surface layer of various host metals (referenced to the dissolution potential of the solute in its pure, metallic form) are calculated. Periodic trends in the calculated potentials are found to be related to trends in surface segregation energies of the various solute/host pairs. The effects of water splitting and concomitant hydroxyl adsorption on the dissolution potentials are also considered; these effects do not change the potentials for highly oxophilic solutes embedded in less active hosts, but they do decrease the dissolution potential for more inert solutes on oxophilic hosts. Finally, the dissolution of Pt “skin” layers from Pt 3X (X = Fe, Co, and Ni) bulk alloys is analyzed; the Pt skins are found to be stabilized compared to pure Pt.

Segregation at the PdAg(111) surface: Electronic structure calculations

An efficient procedure to calculate surface segregation profiles of substitutionally disordered binary alloys is presented. We show that a simple thermodynamic model with realistic atomic configurations at the surface region combined with the total energies obtained from exact muffin--tin orbitals calculations leads to accurate surface segregation profiles. We find that the calculated surface segregation energies in random alloys show significant dependence on the local environment of the atoms involved in the segregation process. Correspondingly, the alloy surface energy is significantly affected by the subsurface atomic layers. As an example the PdAg(111) surface is considered.

Impurities and nonstoichiometry in the bulk and on the (101̄4) surface of dolomite

Atomistic computer simulation methods have been used to model the nature of nonstoichiometry and impurity defects in the bulk and at the (101̄4) surface of dolomite (MgCa(CO 3) 2). Calcium and Mg in the bulk and at the surface have been replaced with divalent Ni, Co, Zn, Fe, Mn, and Cd. The results of these calculations indicate that in the bulk, these impurities will prefer to substitute at the Ca site rather than the Mg site. Ca excess in dolomite is most likely incorporated as basal stacking faults; this nonstoichiometry can influence the site distribution of impurities. The calculated surface segregation energies suggest that of all the impurities studied, only Cd will show a strong preference for the (101̄4) surface of dolomite.

Surface segregation of lanthanum and cerium ions in ceria/lanthana solid solutions: comparison between experimental results and a statistical–mechanical model

In correlating the composition of the mixed oxides of ceria and lanthana, Ce 1− x La x O 2− x/2 , with their catalytic activities, it is advantageous to model quantitatively the composition of the surface in terms of that of the bulk. An earlier paper presented the empirical results in terms of a Freundlich-style adsorption isotherm. Here, those results are shown to fit a statistical–mechanical model that includes the change in segregation energy with surface coverage due to solute–solute interactions. Lanthanum and cerium exhibit preferential segregation to the surface of their solid solutions in ceria and lanthana, respectively. From the most dilute solutions examined here the free energies of segregation are 34 and 11 kcals/mol, for lanthanum and cerium as solutes, respectively. Both surface segregation energies are coverage-dependent and obey a reciprocal dependence on the ratio of segregated solute/solvent ionic fractions falling asymptotically to 7 and 2 kcals/mol for lanthanum and cerium as solutes, respectively.

Steps, kinks, and segregation at metallic surfaces

We have used density functional theory to establish databases of surface energies for low index surfaces of monoatomic metals and of surface segregation energies of single transition metal impurities in transition metal hosts. The data form one consistent starting point for models of surface science phenomena.

Surface segregation energies in transition-metal alloys

We present a database of $24\ifmmode\times\else\texttimes\fi{}24$ surface segregation energies of single transition metal impurities in transition-metal hosts obtained by a Green's-function linear-muffin-tin-orbitals method in conjunction with the coherent potential and atomic sphere approximations including a multipole correction to the electrostatic potential and energy. We use the database to establish the major factors which govern surface segregation in transition metal alloys. We find that the calculated trends are well described by Friedel's rectangular state density model and that the few but significant deviations from the simple trends are caused by crystal structure effects.