Surface Phase Diagram Research Articles

Tracking Water Dissociation on RuO2(110) Using Atomic Force Microscopy and First-Principles Simulations.

The interaction between interfacial water and transition metal oxides is a primary enabling step for the oxygen evolution reaction (OER). RuO2 is a prototypical OER electrocatalyst whose ability to activate interfacial water molecules is essential to its OER activity. We image the dissociation of surface water into OH* and O* on RuO2(110), where * denotes adsorbed species, using atomic force microscopy. Starting from the surface-bound water molecules, which form a one-dimensional network along the rows of Ru surface sites, increasing the oxidative potential strips hydrogen away and transforms the water molecules into OH* and O*. This oxidative step changes the pattern of the adsorbates from one- to two-dimensional. First-principles calculations with interfacial polarization, capacitive charging, and adsorbate interactions attribute this evolution to the cooperative dehydrogenation of adsorbed water and OH* on RuO2. We use these results to map the surface phase diagram of RuO2(110) and provide a quantitative interpretation of its cyclic voltammetry. Our result provides the visualization of the water dissociation on a conductive oxide surface, a critical step in the OER, and demonstrates that the water activation is a collective phenomenon at RuO2(110) electrodes.

First-principles thermodynamical modeling of molecular reactions on α-U(001) and α-UH3(001) surfaces and their influence on hydrogen activation

Density functional theory calculations coupled with thermodynamic analysis have been performed to investigate the reactions of hydrogen and impurity gases including O2, CO2, CO, N2 on α-U(001) and α-UH3(001) surfaces. Binding strength of adsorbates on α-UH3 is calculated to be weaker than α-U, suggesting enhanced poisoning-resistant properties of U-hydride compared to its metallic state. Surface phase diagrams of U under binary H2-O2, H2-N2 and H2-CO gaseous environments show that while N and C elements prefer to stay in their hydrogenated states on the hydride surface, isolated O atoms favorably interact with both α-U(001) and α-UH3(001), indicating the heavily poisoning effect of impurities containing oxygen. Global optimization of partially oxidized α-U(001) slabs induces surface geometry reconstruction and the activity of oxidized surfaces toward hydrogen adsorption can be linearly correlated with local electronic and atomic properties of the adsorption site.

Investigation on thermodynamic stability and electronic structure properties of Bi2CrO6 (0 0 1) surface using density functional theory

A density functional theory (DFT)-based thermodynamic approach was applied to investigate the stability and analyze the surface electronic structures of potential termination structures for the Bismuth chromate (Bi2CrO6) (001) surface. Five Bi2CrO6 (001) surface terminations can be stabilized under certain thermodynamic equilibrium conditions, according to the constructed Bi-Cr-O surface phase diagrams, which are based on calculated surface Gibbs free energies. Analysis of the surface electronic structure shows that O-Bi surface termination has high potential for photocatalytic applications. Additionally, the work functions exhibit substantial differences across various Bi2CrO6 surface terminations, implying that by selecting the thermodynamically more stable surface termination under suitable conditions, the performance of the direct Z-scheme heterostructure based on Bi2CrO6 can be optimized. These findings offer a valuable approach for subsequent experimental research on Bi2CrO6-based photocatalysts and facilitate the investigation of the intrinsic characteristics of Bi2CrO6 surfaces.

Comparative analysis of surface phase diagrams in aqueous environment: Implicit vs explicit solvation models.

Identifying the stable surface phases under a given electrochemical conditions serves as the basis for studying the atomistic mechanism of reactions at solid/water interfaces. In this work, we systematically compare the performance of the two main approaches that are used to capture the impact of an aqueous environment, implicit and explicit solvent, on surface energies and phase diagrams. As a model system, we consider the magnesium/water interface with (i) Ca substitution and (ii) proton and hydroxyl adsorption. We show that while the implicit solvent model is computationally very efficient, it suffers from two shortcomings. First, the choice of the implicit solvent parameters significantly influences the energy landscape in the vicinity of the surface. The default parameters benchmarked on solvation in water underestimate the energy of the dissolved Mg ion and lead to spontaneous dissolution of the surface atom, resulting in large differences in the surface energetics. Second, in systems containing a charged surface and a solvated ion, the implicit solvent model may not converge to the energetically stable ionic charge state but remain in a high-energy metastable configuration, representing the neutral charge state of the ion. When these two issues are addressed, surface phase diagrams that closely match the explicit water results can be obtained. This makes the implicit solvent model highly attractive as a computationally-efficient surrogate model to compute surface energies and phase diagrams.

Critical-point wedge filling and critical-point wetting.

For simple fluids adsorbed at a planar solid substrate (modeled as an inert wall) it is known that critical-point wetting, that is, the vanishing of the contact angle θ at a temperature T_{w} lying below that of the critical point T_{c}, need not occur. While critical-point wetting necessarily happens when the wall-fluid and fluid-fluid forces have the same range (e.g., both are long ranged or both short ranged) nonwetting gaps appear in the surface phase diagram when there is an imbalance between the ranges of these forces. Here we show that despite this, the convergence of the lines of constant contact angle, 0<θ<π, to an ordinary surface phase transition at T_{c}, means that fluids adsorbed in wedges (and cones) always exhibit critical-point filling (wedge wetting or wedge drying) regardless of the range and imbalance of the forces. We illustrate the necessity of critical-point filling, even in the absence of critical-point wetting, using a microscopic model density functional theory of fluid adsorption in a right angle wedge, with dispersion and also retarded dispersionlike wall-fluid forces. The location and order of the filling phase boundaries are determined and shown to be in excellent agreement with exact thermodynamic requirements and also predictions for critical singularities based on interfacial models.

Critical behaviour of the contact angle within nonwetting gaps

Recent density functional theory and simulation studies of fluid adsorption near planar walls in systems where the wall–fluid and fluid–fluid interactions have different ranges, have shown that critical point wetting may not occur and instead nonwetting gaps appear in the surface phase diagram, separating lines of wetting and drying transitions, that extend up to the critical temperature Tc . Here we clarify the features of the surface phase diagrams that are common, regardless of the range and balance of the forces, showing, in particular, that the lines of temperature driven wetting and drying transitions, as well as lines of constant contact angle π>θ>0 , always converge to an ordinary surface phase transition at Tc . When nonwetting gaps appear the contact angle either vanishes or tends to π as t≡(Tc−T)/Tc→0 . More specifically, when the wall–fluid interaction is long-ranged (dispersion-like) and the fluid–fluid short-ranged we estimate π−θ∝t0.16 , compared with θ∝t0.77 when the wall–fluid interaction is short-ranged and the fluid–fluid dispersion-like, allowing for the effects of bulk critical fluctuations. The universal convergence of the lines of constant contact angle implies that critical point filling always occurs for fluids adsorbed in wedges.

Surface phase diagrams from nested sampling.

Studies in atomic-scale modeling of surface phase equilibria often focus on temperatures near zero Kelvin due to the challenges in calculating the free energy of surfaces at finite temperatures. The Bayesian-inference-based nested sampling (NS) algorithm allows for modeling phase equilibria at arbitrary temperatures by directly and efficiently calculating the partition function, whose relationship with free energy is well known. This work extends NS to calculate adsorbate phase diagrams, incorporating all relevant configurational contributions to the free energy. We apply NS to the adsorption of Lennard-Jones (LJ) gas particles on low-index and vicinal LJ solid surfaces and construct the canonical partition function from these recorded energies to calculate ensemble averages of thermodynamic properties, such as the constant-volume heat capacity and order parameters that characterize the structure of adsorbate phases. Key results include determining the nature of phase transitions of adsorbed LJ particles on flat and stepped LJ surfaces, which typically feature an enthalpy-driven condensation at higher temperatures and an entropy-driven reordering process at lower temperatures, and the effect of surface geometry on the presence of triple points in the phase diagrams. Overall, we demonstrate the ability and potential of NS for surface modeling.

Water adsorption at the (010) and (101) surfaces of CuWO4.

Copper tungstate (CuWO4) has attracted significant attention over the past two decades. However, the adsorption of water onto CuWO4, which plays a critical role in the photocatalytic water splitting process, has not been investigated in detail. In this study, we have employed density functional theory (DFT) calculations to investigate water adsorption onto the CuWO4 pristine (010) and reduced (101) surfaces. Surface phase diagrams as a function of temperature and partial pressure of H2O were also constructed to determine water coverage under particular environmental conditions. Our study provides a comprehensive understanding of the adsorption of water on the major CuWO4 surfaces, which is an important preliminary step in our investigation of photocatalytic water splitting over CuWO4.

Accelerating the prediction of inorganic surfaces with machine learning interatomic potentials.

The surface properties of solid-state materials often dictate their functionality, especially for applications where nanoscale effects become important. The relevant surface(s) and their properties are determined, in large part, by the material's synthesis or operating conditions. These conditions dictate thermodynamic driving forces and kinetic rates responsible for yielding the observed surface structure and morphology. Computational surface science methods have long been applied to connect thermochemical conditions to surface phase stability, particularly in the heterogeneous catalysis and thin film growth communities. This review provides a brief introduction to first-principles approaches to compute surface phase diagrams before introducing emerging data-driven approaches. The remainder of the review focuses on the application of machine learning, predominantly in the form of learned interatomic potentials, to study complex surfaces. As machine learning algorithms and large datasets on which to train them become more commonplace in materials science, computational methods are poised to become even more predictive and powerful for modeling the complexities of inorganic surfaces at the nanoscale.

Machine-learning-accelerated simulations to enable automatic surface reconstruction.

Understanding material surfaces and interfaces is vital in applications such as catalysis or electronics. By combining energies from electronic structure with statistical mechanics, ab initio simulations can, in principle, predict the structure of material surfaces as a function of thermodynamic variables. However, accurate energy simulations are prohibitive when coupled to the vast phase space that must be statistically sampled. Here we present a bi-faceted computational loop to predict surface phase diagrams of multicomponent materials that accelerates both the energy scoring and statistical sampling methods. Fast, scalable and data-efficient machine learning interatomic potentials are trained on high-throughput density-functional-theory calculations through closed-loop active learning. Markov chain Monte Carlo sampling in the semigrand canonical ensemble is enabled by using virtual surface sites. The predicted surfaces for GaN(0001), Si(111) and SrTiO3(001) are in agreement with past work and indicate that the proposed strategy can model complex material surfaces and discover previously unreported surface terminations.

Formation and stability of Fe-rich terminations of the Fe3O4(001) surface

Understanding how the structure of iron oxide surfaces varies with their environment is essential for rationalizing their role in (geo-)chemistry and optimizing their application in modern technologies. In this paper, we create Fe-rich terminations of Fe3O4(001) by depositing iron directly onto the ‘subsurface cation vacancy’-reconstructed surface, which is the most stable surface under ultrahigh vacuum conditions. Scanning tunneling microscopy and x-ray photoelectron spectroscopy data reveal that the excess iron is initially accommodated as two-fold coordinated adatoms and later incorporates into the subsurface cation vacancies. As the coverage increases, small patches of the octahedral pair termination (also known as the ‘Fe dimer’ termination) nucleate, eventually covering the entire surface after the deposition of 2 iron atoms per (√2×√2)R45° unit cell. This conclusion effectively rules out some existing models for the termination and provides support for the model proposed by Rustad et al (Surface Science 432, L583-L588, 1999), highlighting the need for further theoretical work to complete the Fe3O4(001) surface phase diagram. The octahedral pair termination is found to be unstable above 523 K and upon exposure to molecular O2 because the excess iron atoms agglomerate to form small FeOx clusters.

Surface Phase Diagrams for Wetting with Long-Ranged Forces.

Recent density functional theory and simulation studies of wetting and drying transitions in systems with long-ranged, dispersionlike forces, away from the near vicinity of the bulk critical temperature T_{c}, have questioned the generality of the global surface phase diagrams for wetting, due to Nakanishi and Fisher, pertinent to systems with short-ranged forces. We extend these studies deriving fully analytic results which determine the surface phase diagrams over the whole temperature range up to T_{c}. The phase boundaries, order of, and asymmetry between the lines of wetting and drying transitions are determined exactly showing that they always converge to an ordinary surface critical point. We highlight the importance of lines of maximally multicritical wetting and drying transitions, for which we determine the exact critical singularities.

Exploring the Redox Properties of the Low-Miller Index Surfaces of Copper Tungstate (CuWO4): Evaluating the Impact of the Environmental Conditions on the Water Splitting and Carbon Dioxide Reduction Processes.

Photocatalysis has gained significant attention and interest as an environmentally friendly and sustainable approach to the production of hydrogen through water splitting and the reduction and conversion of CO2. Copper tungstate (CuWO4) is a highly promising candidate for these applications owing to its appropriate bandgap and superior stability under different conditions. However, the redox behavior of the CuWO4 surfaces under different environments and their impact on the morphology of the material nanoparticles, as well as the electronic properties, remain poorly understood. In this study, we have employed density functional theory calculations to investigate the properties of the bulk and pristine surfaces of CuWO4 and how the latter are impacted by oxygen chemisorption under the conditions required for photocatalytic water splitting and carbon dioxide reduction processes. We have calculated the lattice parameters and electronic properties of the bulk phase, as well as the surface energies of all possible nonpolar, stoichiometric, and symmetric terminations of the seven low-Miller index surfaces and found that the (010) and (110) facets are the thermodynamically most stable. The surface-phase diagrams were used to derive the equilibrium crystal morphologies, which show that the pristine (010) surface is prominent under synthesis and room conditions. Our crystal morphologies suggest that the partially oxidized (110) surface and the partially reduced (011) surface may play an important role in the photocatalytic splitting of water and CO2 conversion, respectively. Our results provide a comprehensive understanding of the CuWO4 surfaces under the conditions of important photocatalytic applications.

Construction and analysis of surface phase diagrams to describe segregation and dissolution behavior of Al and Ca in Mg alloys

Construction and analysis of surface phase diagrams to describe segregation and dissolution behavior of Al and Ca in Mg alloys

Rapid mapping of alloy surface phase diagrams via Bayesian evolutionary multitasking

Surface phase diagrams (SPDs) are essential for understanding the dependence of surface chemistry on reaction condition. For multi-component systems such as metal alloys, the derivation of such diagrams often relies on separate first-principles global optimization tasks under different reaction conditions. Here we show that this can be significantly accelerated by leveraging the fact that all tasks essentially share a unified configurational search space, and only a single expensive electronic structure calculation is required to evaluate the stabilities of a surface structure under all considered reaction conditions. As a general solution, we propose a Bayesian evolutionary multitasking (BEM) framework combining Bayesian statistics with evolutionary multitasking, which allows efficient mapping of SPDs even for very complex surface systems. As proofs of concept, we showcase the performance of our methods in deriving the alloy SPDs for two heterogeneous catalytic systems: the electrochemical oxygen reduction reaction (ORR) and the gas phase steam methane reforming (SMR) reaction.

Stoichiometric and Nonstoichiometric Surface Structures of Pyrochlore Y2Zr2O7 and Their Relative Stabilities: A First-Principles Investigation

First-principle total energy calculations were performed to investigate the atomic structures and relative stabilities of two low miller-index surfaces of pyrochlore Y2Zr2O7. The stoichiometric Y2Zr2O7 (110) and (100) surfaces were predicted, with lowest formation energies of 1.20 and 1.47 J/m2, respectively. Based on a thermodynamic defect model, non-stoichiometric Y2Zr2O7 surface energies were further evaluated as a function of environmental oxygen partial pressure (pO2) and temperature (T). With all of the results, we were able to construct the surface phase diagrams for T = 300 and 1400 K. The strong correlation between the structural stabilities and the surface stoichiometry was revealed as varying T and pO2. At a given T, the most stable termination of the (110) surfaces would change from a (Y,Zr)−rich (ns−2Y2Zr6O) to O−rich ones (ns−4O_2 and ns−4O_1) as increasing pO2, while that of the (100) surfaces would change from the stoichiometric (stoi−1Y1Zr_1) to the O−rich one (ns−5O). The critical pO2 value for termination transition moves to its higher end as increasing T.

Surface phase diagrams of pristine and hydroxylated barium hexaferrite surfaces from first-principles atomistic thermodynamics

Barium hexaferrite (BHF) is a ferrimagnet, whose hexagonal unit cell presents five iron crystallographic sites along the c-axis. At the nanoscale, BHF nanoparticles grow in the form of platelets, characterized by a low thickness along its principal magnetization axis (c-axis), displaying uniaxial magnetic anisotropy with the easy axis pointing perpendicular to the platelet. This unique property of BHF nanoplatelets has lead to a variety of novel applications, which often require surface functionalization. However, it has been observed that the nanoplatelets display two different surface morphologies depending on the stage/conditions of preparation. To ground these experimental observations, we employ the ab initio thermodynamics framework to perform a systematic investigation of the thermodynamic stability of BHF bulk terminations under a wide range of chemical conditions. We calculate the surface phase diagrams of pristine and hydroxylated bulk terminations along the c-axis. For pristine terminations, two different iron terminated surfaces are preferred: the barium containing 2b termination (Ba-rich conditions) and the 4f2 iron terminated surface (Ba-poor conditions). In the presence of water, the hydroxylated oxygen-terminated surfaces (12k-O) are identified as the most stable ones at Ba-poor conditions and low pH values, whereas the hydroxylated 2b surface is preferred at high pH and Ba-rich conditions.

Temperature and hydrogen partial pressure dependence of zirconium hydride surface structures and stabilities

The surface structures and relative stabilities of low miller-index surfaces of ε-ZrH2 and γ-ZrH have been investigated as a direct function of environmental conditions (hydrogen partial pressure pH2 and temperature T) from the first-principles thermodynamic calculations. The results were used to construct their surface phase diagrams for a wide pH2 range at T = 300 and 900K. The γ-ZrH surfaces tend to be dominated by the Zr-rich (101) with the 101-ns-2Zr1H termination. The surface energy would depend on T and pH2, with a value of 1.2–1.3 J/m2. For pH2 > e−53 at 300K (or pH2 > e−5.3 at 900K), γ-ZrH can no longer be stable but have the tendency to transform into ε-ZrH2, by consistently absorbing H from the gaseous environment. The ε-ZrH2 surfaces tend to be dominated by the stoichiometric (111) facet with the 111-stoi-2H termination and the surface energy of 1.29 J/m2.

Surface Coassembly of Binary Mixed Polymer Brushes and Linear Block Copolymer Chains.

Binary mixed polymer brushes (BMPBs) are two different homopolymer chains that are covalently anchored to the solid surfaces at high grafting densities. One feature of the BMPBs is the unique ability to make surface phase separation under external stimuli. In this research, we demonstrate that different surface nanostructures can be fabricated by surface coassembly of BMPBs and free block copolymer (BCP) chains. Polystyrene/poly(2-(dimethylamino)ethyl methacrylate) (PS/PDMAEMA) BMPBs on silica particles (PS-PDMAEMA-SiO2) are synthesized by a two-step "grafting to" approach. PDMAEMA-b-PS block copolymer (BCP) chains and PS-PDMAEMA-SiO2 make surface self-assembly and a variety of surface nanostructures are formed in methanol. The grafting densities of PS and PDMAEMA brushes, solvent, and the BCP structures all exert significant influences on the surface morphology. With an increase in PDMAEMA grafting density, the surface structures change from perforated layers, to rods, and to spherical surface micelles (s-micelles). The PS grafting density also exerts an effect on the formation of the surface nanostructures. At low PS grafting density, sparsely distributed s-micelles are produced, and at high density, densely distributed s-micelles are observed. Based on transmission electron microscopy and scanning electron microscopy results, a surface phase diagram is constructed, which provides a guide to the surface morphology control.

Catalytically-Active Phases and Reaction Mechanism of Ni-Based and Co-Based Layered Double Hydroxides for the Oxygen Evolution Reaction

Electrochemical water splitting using renewable energy is a key route to generate green H2 fuel. In this process, however, O2 generation at the anode through the oxygen evolution reaction (OER) is inherently slower by over four orders of magnitude than H2 generation at the cathode. Thus, improving OER efficiency has been a majority effort in electrolysis. Ni-based and Co-based layered double hydroxides (LDHs) are among the most active and studied catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. Because it happens under extremely oxidative aqueous conditions, however, in-situ crystal structures of the OER active phase are still largely unknown, significantly hindering the establishment of structure-property relationships.In this talk, we provide the first direct atomic-scale evidence that, under applied anodic potentials, NiFe and CoFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions from carbonate to potassium. The calculated surface phase diagrams indicate that surface O sites are saturated with H by forming bridge OH, and coordinatively unsaturated metal sites are poisoned by OH adsorption under OER conditions. These structures, and the associated reaction free energies, suggest that the OER proceeds via a Mars van Krevelen mechanism, starting with the oxidation of bridge OH at the reaction centers with dual metal sites, i.e., M1-OH-M2.Our study suggests that the compound-dependent activity originates from the dual-metal site feature of the reaction centers. While this feature does not influence the OH-OOH scaling relationship, it leads to diverse OH-O scaling relationships, including those with near-zero slopes and negative slopes. Breaking OH-OOH scaling relationships were frequently discussed in the literature, as it determines the minimum overpotential. However, our study showed that, to approach the minimum overpotential dictated by a specific OH-OOH scaling relationship, the key is to break the OH-O scaling relationship. A possible route is to form binary metal oxyhydroxides with dual metal sites at the reaction centers or introduce a third element into NiFe LDH or CoFe LDH.