Surface Segregation Effects Research Articles

Hydrogen and NH3 co-adsorption on Pd–Ag membranes

Ammonia (NH3) represents a carbon-free hydrogen carrier that can be used as a zero-emission fuel in the maritime and heavy transport sector with hydrogen fuel cells. To use ammonia as feedstock, the hydrogen must be recovered through NH3 decomposition into H2 and N2. Ammonia decomposition by a membrane-enhanced reactor would inherently produce high-purity H2 through the membrane avoiding the need for a costly hydrogen separation/purification unit. Pd-based membrane reactors can obtain full ammonia decomposition and show significantly higher conversion than conventional reactors. However, the H2 permeability is found to be inhibited in the presence of NH3. A further fundamental understanding of the long-term stability and performance of the Pd-based membranes under exposure to NH3 is therefore required. In the current work, the adsorbate-adsorbate interactions during co-adsorption, the influence the presence of NH3 has on the hydrogen dissociation kinetics, and surface segregation effects in the presence of NH3 and/or hydrogen on the surface of Pd and Pd3Ag are investigated in detail using density functional theory calculations. We find that both the hydrogen surface coverage and dissociation kinetics are hindered by the presence of NH3 on the surface, and possible segregation of Ag towards the surface in the presence of NH3, which could explain the reduced hydrogen permeation in NH3.

Single-atom alloy Pd1Ag10/Al2O3 catalyst: Effect of CO-induced Pd surface segregation on the structure and catalytic performance in the hydrogenation of C2H2

The effective surface concentration of Pd1 sites composed of individual palladium atoms isolated from each other by Ag atoms in the Pd1Ag10/Al2O3 single-atom alloy (SAA) catalyst can be significantly enhanced by the adsorption-induced surface segregation upon a special pretreatment of the catalyst in a CO-containing atmosphere at 200 °C. The catalyst activity towards acetylene hydrogenation gets tripled without sacrificing selectivity for ethylene. The segregated surface of the catalyst is thus preserved under target reaction conditions.

Electrochemically Assisted Construction of a La2NiO4+δ@Pt Core-Shell Structure for Enhancing the Performance and Durability of La2NiO4+δ Cathodes.

Ruddlesden-Popper oxide La2NiO4+δ (LNO) has a high ionic conductivity and good thermal match with the electrolyte of solid oxide fuel cells (SOFCs); however, LNO suffers from performance decay owing to the La surface segregation under the operation conditions of SOFCs. Herein, we report an in situ electrochemical decoration strategy to improve the electrocatalytic activity and durability of LNO cathodes. We show that the electrochemical polarization leads to in situ construction of the LNO@Pt core-shell structure, significantly suppressing the detrimental effect of La surface segregation on the LNO cathode. The initial peak power density of a single cell with the LNO cathode is 0.71 W cm-2 at 750 °C, increasing to 1.39 W cm-2 by the in situ construction of the LNO@Pt core-shell structure after polarization at 0.5 A cm-2 for 20 h. The LNO@Pt core-shell structure is also highly durable without noticeable performance degradation over the duration of the test for 180 h. The findings shed light on the design and fabrication of highly active and durable LNO-based cathodes for SOFCs.

Phase separation paths in metastable Zr1-xAlxN monolithic layers compared to multilayers with TiN: Growth versus annealing temperatures

Metastable super-saturated Zr1−xAlxN alloys tend to phase separate into the equilibrium cubic (c) ZrN and wurtzite (w) AlN due to a deep miscibility gap. Transformation is shown here to follow distinctly different paths depending on if Zr1−xAlxN (x = 0.3 and 0.6) is sputter deposited as a single layer or multi-layered with TiN, and further varied by post-deposition annealing. Using in situ high-energy synchrotron wide-angle X-ray scattering and analytical transmission electron microscopy, surface segregation effects are compared to secondary phase transformations occurring in as-deposited layers during thermal annealing up to 1000°C. For the primary phase transformation from the vapor phase, w-AlN nucleates and an AlN-ZrN labyrinthine structure evolves at elevated deposition temperature with semi-coherent interfaces over several nanometers, where the higher Al content narrows the structure in both single and multilayers. Transformation in thinner alloy layers is governed by epitaxial forces which stabilize single-phase c-Zr0.4Al0.6N, which enables c-Zr0.4Al0.6N/TiN superlattice growth at temperatures as low as 350°C. Regardless of the decomposition route, the formation of c-AlN is impeded and w-AlN instantaneously forms during transformation. In contrast, isostructural decomposition into w-AlN and w-Zr(Al)N occurs in w-Zr0.4Al0.6N alloys during annealing.

Simultaneously enhanced separation and antifouling properties by synergistic effect of pore-formation and surface segregation through incorporating bowl-like amphiphiles

Herein, we propose a new strategy to improve the separation and antifouling properties of ethylene vinyl alcohol (EVAL) membranes by a dual pore formation mechanism modified with natural macrocyclic amphiphiles cyclodextrin (CDs). The amphiphilic bowl-like geometry of CDs, consisting of a hydrophobic cavity and hydrophilic rim, induces spontaneous surface segregation during the non-solvent induced phase separation to form a CD-enriched layer. Here, CDs play multiple roles, including pore-forming and antifouling enhancement agents, by modifying the morphological, physical, permeation, and separation properties of the membranes. The three-dimensional honeycomb sponge-like structure was determined by SEM analysis, and the elemental composition was examined by SEM-EDX analysis. The O element indicated a gradient decrease distribution from the top to the bottom layer of the cross-section, concomitant with the combination of pore-forming and surface segregation functions. The introduction of amphiphilic CDs increases the roughness, hydrophilicity, and porosity of the membranes. Furthermore, the CDs-modified EVAL (d-M) membranes have excellent mechanical properties, a high water flux (148 L/m2h) with superior rejection to BSA (98%), and can be used for dye separation (Congo Red: 99.7%, Coomassie brilliant blue: 97.5%, Trypan blue: 92.4%). Moreover, the modified EVAL with the best performance has excellent antifouling capacity and long-term stability. This work demonstrates the effect of the addition of CDs and octanol as dual pore formers on the fabrication of hydrophilic antifouling membranes for efficient separation.

The Correlation between Structure, Surface, and Performance in Fe- and Mg-Substituted LiNi0.5Mn1.5O4 Cathodes

A need for lithium-ion cathodes that have a reduced dependence on scarce elements, such as cobalt, has been prompted by surge in eco-consciousness over the past decade. The spinel-type LiNi0.5Mn1.5O4 (LNMO) cathodes, with high operating voltages (ca. 4.7V vs Li+/Li) and energy densities (ca. 650 Wh kg-1), have received particular attention for this reason.1 Poor cycling stability, however, which arises from structural instabilities, electrolyte degradation, surface densification and loss of active material through transition metal dissolution, limits their widespread adoption.2 Elemental substitution has been employed to overcome such limitations, in which improved cycling stability has been observed. In such samples, the segregation of substituents to the cathode surface has been reported, yet the effect of surface segregation on the local atomic structure and surficial reactivity of the cathodes is under-researched.3 To further explore these areas, Fe- and Mg-substituted LNMO samples have been synthesised. Despite lower initial capacity, Mg-substituted LNMO exhibits significantly improved cycling stability over 300 cycles at 50°C. A combination of x-ray and neutron diffraction, neutron pair distribution function analysis, and x-ray absorption and photoelectron spectroscopy are used to explore the complex relationships between bulk structure, local structure, surface reactivity and performance in Fe- and Mg-substituted LNMO cathodes. References Liang, G., Peterson, V.K., See, K.W., Guo, Z., and Pang, W.K. (2020). Developing high-voltage spinel LiNi 0.5 Mn 1.5 O 4 cathodes for high-energy-density lithium-ion batteries: current achievements and future prospects . J. Mater. Chem. A.Ma, J., Hu, P., Cui, G., and Chen, L. (2016). Surface and Interface Issues in Spinel LiNi0.5Mn1.5O4: Insights into a Potential Cathode Material for High Energy Density Lithium Ion Batteries. Chem. Mater. 28, 3578–3606.Shin, D.W., Bridges, C.A., Huq, A., Paranthaman, M.P., and Manthiram, A. (2012). Role of Cation Ordering and Surface Segregation in High-Voltage Spinel LiMn LiMn 1.5Ni 0.5-xM xO 4(M = Cr, Fe, and Ga) Cathodes for Lithium-Ion Batteries. Chem. Mater. 24, 3720–3731. Figure 1

Moderate Surface Segregation Promotes Selective Ethanol Production in CO2Hydrogenation Reaction over CoCu Catalysts

AbstractCobalt‐copper (CoCu) catalysts have industrial potential in CO/CO2hydrogenation reactions, and CoCu alloy has been elucidated as a major active phase during reactions. However, due to elemental surface segregation and dealloying phenomena, the actual surface morphology of CoCu alloy is still unclear. Combining theory and experiment, the dual effect of surface segregation and varied CO coverage over the CoCu(111) surface on the reactivity in CO2hydrogenation reactions is explored. The relationship between C−O bond scission and further hydrogenation of intermediate *CH2O was discovered to be a key step to promote ethanol production. The theoretical investigation suggests that moderate Co segregation provides a suitable surface Co ensemble with lateral interactions of co‐adsorbed *CO, leading to promoted selectivity to ethanol, in agreement with theory‐inspired experiments.

Moderate Surface Segregation Promotes Selective Ethanol Production in CO2 Hydrogenation Reaction over CoCu Catalysts.

Cobalt-copper (CoCu) catalysts have industrial potential in CO/CO2 hydrogenation reactions, and CoCu alloy has been elucidated as a major active phase during reactions. However, due to elemental surface segregation and dealloying phenomena, the actual surface morphology of CoCu alloy is still unclear. Combining theory and experiment, the dual effect of surface segregation and varied CO coverage over the CoCu(111) surface on the reactivity in CO2 hydrogenation reactions is explored. The relationship between C-O bond scission and further hydrogenation of intermediate *CH2 O was discovered to be a key step to promote ethanol production. The theoretical investigation suggests that moderate Co segregation provides a suitable surface Co ensemble with lateral interactions of co-adsorbed *CO, leading to promoted selectivity to ethanol, in agreement with theory-inspired experiments.

Stable Bismuth-Doped Lead Halide Perovskite Core-Shell Nanocrystals by Surface Segregation Effect.

Lead halide perovskite nanocrystals (NCs) exhibit excellent optoelectronic performance, however, the broad application is limited by their poor stability. Herein, a strategy for stable core-shell structured bismuth-doped lead halide perovskite NCs is reported. The stable core-shell perovskite NCs are prepared based on heterovalent substitutions and surface segregation effect. Core-shell features are revealed through advanced characterization and structure analyses. Meanwhile, the transfer of carriers between the core and the shell is observed by ultrafast transient absorption spectroscopy. The core-shell structured perovskite NCs exhibit outstanding structure stability and retain 97% of the original photocatalytic efficiency after cycle experiments under moisture ambient and light irradiation. Such a core-shell structure constructs gradient energy levels. These findings are expected to facilitate the development of stable lead halide perovskite devices.

Different Hydrogen Bond Changes Driven by Surface Segregation Behavior of Imidazolium-Based Ionic Liquid Mixture at the Liquid-Vacuum Interface.

In this work, molecular dynamics (MD) simulations were carried out to study the behaviors of a binary ionic liquid (IL) mixture consisting of equimolar [C2C1Im][BF4] and [C4C1Im][BF4], as well as two corresponding pure ILs, at the liquid-vacuum interface. Our simulation results show that the competition of nonpolar interactions between different alkyl chains of two cations results in an obvious surface segregation behavior of the IL mixture at the interface, indicating an enhanced aggregation of the [C4C1Im]+ cations but a weakened aggregation of the [C2C1Im]+ cations at the outermost surface. More interestingly, different hydrogen bond (HB) changes between two imidazolium cations at the interface can be driven by such surface segregation behavior, where the [C2C1Im]+ cations rather than the [C4C1Im]+ ones have more and stronger HBs with the [BF4]- anions by comparison with the corresponding pure ILs at the interface. Meanwhile, it is interesting to find that such a stronger HB would lower the rotations of the imidazolium rings of interfacial [C2C1Im]+ cations. By contrast, the [C4C1Im]+ cations at the outermost surface rotate faster owing to their weaker HB. In addition, the orientation analysis uncovers that there is a major decrease for the orderliness of interfacial [C2C1Im]+ cations, but a minor decrease for that of interfacial [C4C1Im]+ cations, from the pure IL to the IL mixture. Such distinct results are closely related to the surface segregation between the [C2C1Im]+ and [C4C1Im]+ cations in the IL mixture and their interfacial HB properties. Thus, our simulation results afford a deep insight into the surface segregation effect on the HB behavior of the imidazolium-based IL mixture at liquid-vacuum interface.

Surface segregation effect for prevention of oxidation in Ni‐X (X=Sn, Sb) alloy by in situ photoelectron spectroscopy

Ni‐base alloys has been widely used for chemical plants because of their high strength and excellent oxidation resistance. In particular, the addition of Sn and Sb in Ni‐base alloys significantly improves the metal dusting resistance. It is indicated that Sn and Sb are segregated on the alloy surface in the metal dusting environment; however, the details have not been clarified yet. The behavior of the Ni‐Sn and Ni‐Sb alloys under a high‐temperature oxidation environment was investigated by in situ X‐ray photoelectron spectroscopy (XPS). It was confirmed that Sn and Sb have been segregated at the surface of their alloys during oxidation in low oxygen potential, PO2, environment. These results indicate that Sn and Sb segregation improves the metal dusting resistance.

Sintering reaction and microstructure of MAl (M = Ni, Fe, and Mg) nanoparticles through molecular dynamics simulation* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11572124 and 51871096) and the Natural Science Foundation of Hunan Province of China (Grant Nos. 2018JJ4044 and 2018JJ3100).

The sintering–alloying processes of nickel (Ni), iron (Fe), and magnesium (Mg) with aluminum (Al) nanoparticles were studied by molecular dynamics simulation with the analytic embedded-atom model (AEAM) potential. Potential energy, mean heterogeneous coordination number , and surface atomic number N surf–A were used to monitor the sintering–reaction processes. The effects of surface segregation, heat of formation, and melting point on the sintering–alloying processes were discussed. Results revealed that sintering proceeded in two stages. First, atoms with low surface energy diffused onto the surface of atoms with high surface energy; second, metal atoms diffused with one another with increased system temperature to a threshold value. Under the same initial conditions, the sintering reaction rate of the three systems increased in the order MgAl < FeAl < NiAl. Depending on the initial reaction temperature, the final core–shell (FeAl and MgAl) and alloyed (NiAl and FeAl) nanoconfigurations can be observed.

Size effect of electronic properties in highly arsenic-doped silicon nanowires

The unique electrostatic properties of semiconductor nanowires enable the realization of novel transistor types by the possibility to use surround gate architectures resembling ideal gate electrostatic control. Nevertheless one fundamental issue of semiconducting nanowire channels is the reliable control of doping to adjust the charge carrier concentration. Indeed, as dimensions scale down the surrounding media and the interfaces become more important. In this study we experimentally investigate the role of surface depletion and dielectric mismatch on the electronic charge transport of highly arsenic doped and bottom-up grown silicon nanowires. Electrical characterization of silicon nanowires (SiNWs) synthesized by Au catalyzed vapour-liquid-solid (VLS) growth and in-situ arsine (AsH3) doping is reported for the first time. We demonstrate that high n-type doping is possible by adjusting the dopant precursor flow ratio during growth. Based on electrical measurements of individual nanowires, reproducible donor concentrations of up to 5.2 × 1019 cm−3 could be revealed. By measuring the electrical characteristics for individual nanowires in dependence of their radius, we show that the electrically active carrier density drastically reduces for small nanowires at radii much larger than those at which quantization or dopant surface segregation effects are expected to occur. Furthermore, enhancement of the contact transparency for small radii nanowires is demonstrated through dopant segregation upon metal silicidation. Size dependent measurement of electrical characteristics revealed improved contact resistivities as low as 1.4 × 10−11 Ωm2.

Using density functional calculations to elucidate atomic ordering of Pd-Rh nanoparticles at sizes relevant for catalytic applications

Pd-Rh nanoparticles are known to easily undergo surface restructuring in reactive environment. This study quantifies, with the help of density functional (DFT) calculations and a novel topological approach, atomic ordering and surface segregation effects in Pd-Rh particles with compositions 1:3, 1:1 and 3:1 containing up to 201 atoms (ca. 1.7 nm). The obtained data are used to reliably optimise energetically preferred atomic orderings in inaccessible by DFT Pd-Rh particles containing thousands of atoms and exhibiting sizes exceeding 5 nm, which are typical for catalytic metal particles. It is outlined, how segregation effects on the surface arrangement of Pd-Rh nanoalloy catalysts induced by adsorbates can be evaluated in a simple way within the present modelling setup. Catalytically active Pd-Rh nanoparticles adapt their surface to chemical environment. Density functional modelling identifies surface composition of Pd-Rh particles containing thousands atoms and paves the way for evaluating surface segregation caused by adsorbed reactants.

Segregation at interfaces in (GaIn)As/Ga(AsSb)/(GaIn)As- quantum well heterostructures explored by atomic resolution STEM

Surface segregation and interaction effects of In and Sb in (GaIn)As/Ga(AsSb)/(GaIn)As- “W”-type quantum well heterostructures (“W”-QWHs) are investigated by high angle annular dark field scanning transmission electron microscopy with atomic resolution. “W”-QWHs are promising candidates for type-II laser applications in telecommunications. In this study, independent (GaIn)As and Ga(AsSb) quantum wells as well as complete “W”-QWHs are grown by metal organic vapour phase epitaxy on GaAs substrate. The composition is determined with atomic resolution by comparison of the experimental data to complementary contrast simulations. From concentration profiles, an altered segregation in “W”-QWHs in comparison to single (GaIn)As and Ga(AsSb) quantum wells grown on GaAs is detected. In and Sb are clearly influencing each other during the growth, including blocking effects of In incorporation by Sb and vice versa. Especially, growth rate and total amount of Sb incorporated into Ga(AsSb) are decreased by In being present.

Chromium segregation in Cr-doped TiO2 (rutile): impact of oxygen activity

This work considers the effect of chromium surface segregation for polycrystalline Cr-doped TiO2 on surface vs. bulk defect disorder. It is shown that annealing of Cr-doped TiO2 (0.04 at% Cr) in the gas phase of variable oxygen activity at 1273 K results in a gradual transition in the valence of chromium at the surface from predominantly Cr3+ species in reduced conditions, p(O2) = 10−12 Pa, to comparable concentrations of both Cr3+ and Cr6+ species in oxidising conditions, p(O2) = 105 Pa. The reported data is considered in terms of defect equilibria leading to the formation of positively and negatively charged chromium in both the cation sub-lattice and interstitial sites. The derived theoretical models represent the effect of oxygen activity on the surface charge and the resulting electric field leading to migration mechanism of charged chromium species.

Tailoring the switching performance of resistive switching SrTiO3 devices by SrO interface engineering

Redox-based resistive switching is one of the most-promising concepts in the focus of research to meet the ever-growing demand for faster and smaller non-volatile memory devices. In this work we present detailed studies of the impact of cation stoichiometry and surface segregation effects on the performance of the valence change memory model material SrTiO3. In order to clarify if the enhanced switching performance of Sr-rich SrTiO3 devices can be attributed to SrO segregation or to the formation of Sr-rich extended defects, we artificially engineered the formation of SrO islands by depositing additional SrO on top of stoichiometric SrTiO3. We thereby unravel that the enhanced switching performance is solely accounted for by the formation of SrO islands and not influenced by extended defects. Consequently following our findings, we design devices with a further improved retention by tailoring the amount of SrO on the surface.

Zwitterionic Membrane via Nonsolvent Induced Phase Separation: A Computer Simulation Study.

Dissipative particle dynamics (DPD) was adopted to study the nonsolvent induced phase separation (NIPS) process during a pH-responsive poly(ether sulfone) membrane preparation with a zwitterionic copolymer poly(ether sulfone)- block-polycarboxybetaine methacrylate (PES-b-PCBMA) as the blending additive. The membrane formation process and final morphology were analyzed. Simulation results show that the hydrophilic PCBMA segments enrich on the membrane surface by surface segregation and exhibit pH-responsive behavior, which is attributed to the deprotonation of the carboxylic acid group. With the polymer concentration increasing, both the shrinkage of the membrane and the flexibility of the system decrease, which also reduce the effect of surface segregation. By adjusting the blend ratio of PES-b-PCBMA with PES from 5% to 15%, the surface coverage of PCBMA segments on the membrane can be regulated. This work contributes to a better understanding on the mechanism of NIPS and can serve as a guide for the design of the polymer blend membrane.

Evolution kinetics of microgravity facilitated spherical macrosegregation within immiscible alloys

The evolution kinetics of microgravity facilitated spherical macrosegregation within model Fe-Cu immiscible alloys has been systematically investigated by drop tube experiments and 3D phase-field simulations. In microgravity environment, liquid phase separation of Fe-Cu alloys induced the appearance of spherical macrosegregation patterns with various core-shell structures. The formation probability and evolution characteristics of these phase-separated core-shell morphologies depended on the volume fraction of surface active Cu-rich liquid together with the cooling rate. As the cooling rate decreased, the duration time of phase separation extended and the formed dispersed structures showed a tendency to form as core-shell structures influenced by the effects of Marangoni convection and surface segregation. Meanwhile, core grew larger while the shell became thinner. The occurrence probability of spherical macrosegregation firstly increased and then decreased with the rise in the copper concentration, and core-shell morphologies changed from three-layer to two-layer which was accompanied by core shrinkage and shell thickening.

Surface-Segregation-Induced Nanopapillae on FDTS-Blended PDMS Film and Implications in Wettability, Adhesion, and Friction Behaviors.

Polymer composites have been extensively used to tune the surface property (e.g., wettability, friction, and adhesion) for its advantages of cost-effectiveness, high efficiency, and ease of fabrication. In this work, different amount of trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FDTS) was added into poly(dimethylsiloxane) elastomer to prepare polymer composite films and were selected as a model to illustrate the effects of surface segregation on surface topology, wettability, friction, and adhesion. The results show that the added FDTS forms aggregations and increasing the content of FDTS leads to the difficulty of air bubble elimination, increase in viscosity, and drop in transparency. Driven by the differences of chemical potential, FDTS aggregations migrate to the air-polymer interface, resulting in surface enrichment and formation of nanopapillae (1-200 nm). This phenomenon becomes more significant with the increment in FDTS. The change in surface composition and structure generates profound effects on wettability, friction, and adhesion. The addition of FDTS makes the surface relatively oleophobic and further increasing the content of FDTS does not helpful in improving the oleophobicity due to the notable aggregation. Friction forces first grow with the increasing content of FDTS and then decline after the maximum point at 1.0 wt % of FDTS, which is attributed to the generated regular larger nanopappillae at high concentration. However, these larger nanopapillae lead to the increase in adhesion because more interactions are formed. The findings demonstrate the behaviors of FDTS in polymer composites and provide important guidance for controlling the formation of nanostructures via aggregation and phase segregation and exploring their implications on surface properties.