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Metal oxide/water interfaces play a crucial role in many electrochemical and photocatalytic processes, such as photoelectrochemical water splitting, the creation of fuel from sunlight, and electrochemical $\mathrm{C}{\mathrm{O}}_{2}$ reduction. First-principles electronic structure calculations can reveal unique insights into these processes, such as the role of the alignment of the oxide electronic energy levels with those of liquid water. An essential prerequisite for the success of such calculations is the ability to predict accurate structural models of these interfaces, which in turn requires careful experimental validation. Here we report a general, quantitative validation protocol for first-principles molecular dynamics simulations of oxide/aqueous interfaces. The approach makes direct comparisons of interfacial x-ray reflectivity (XR) signals from experimental measurements and those obtained from ab initio simulations with semilocal and van der Waals functionals. The protocol is demonstrated here for the case of the ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}(001)$/water interface, one of the simplest oxide/water interfaces. We discuss the technical requirements needed for validation, including the choice of the density functional, the simulation cell size, and the optimal choice of the thermodynamic ensemble. Our results establish a general paradigm for the validation of structural models and interactions at solid/water interfaces derived from first-principles simulations. While there is qualitative agreement between the simulated structures and the experimental best-fit structure, direct comparisons of simulated and measured XR intensities show quantitative discrepancies that derive from both bulk regions (i.e., alumina and water) as well as the interfacial region, highlighting the need for accurate density functionals to properly describe interfacial interactions. Our results show that XR data are sensitive not only to the atomic structure (i.e., the atom locations) but also to the electron-density distributions in both the substrate and at the interface.

In situ surface x-ray diffraction during deposition of LaTiO3 on SrTiO3 (001) shows that a TiO2 adlayer participates in growth., In traditional models of heteroepitaxy, the substrate serves mainly as a crystalline template for the thin-film lattice, dictating the initial roughness of the film and the degree of coherent strain. Here, performing in situ surface x-ray diffraction during the heteroepitaxial growth of LaTiO3 on SrTiO3 (001), we find that a TiO2 adlayer composed of the (13×13)R33.7° and (2×2)R45.0° reconstructions is a highly active participant in the growth process, continually diffusing to the surface throughout deposition. The effects of the TiO2 adlayer on layer-by-layer growth are investigated using different deposition sequences and anomalous x-ray scattering, both of which permit detailed insight into the dynamic layer rearrangements that take place. Our work challenges commonly held assumptions regarding growth on TiO2-terminated SrTiO3 (001) and demonstrates the critical role of excess TiO2 surface stoichiometry on the initial stages of heteroepitaxial growth on this important perovskite oxide substrate material.

In recent years, the use of silver-based materials for selective and highly active ethanol reactivity in single atom catalysis and the ethanol oxidation reaction in direct fuel cells highlights the importance of silver (Ag) in an ethanol economy. Understanding the interaction of ethanol with Ag(111) and the natural defects found on extended Ag(111) is critical to the overall understanding of more complex catalytic processes including ethanol activation over Ag-based catalysts. The research herein aims to characterize the interaction of ethanol molecules on undercoordinated defect sites of Ag(111) to mimic active sites found on Ag nanoparticle catalysts. The interaction between ethanol and Ag(111) was studied using temperature programed desorption (TPD), x-ray photoelectron spectroscopy, and density functional theory (DFT). Molecular ethanol adsorption and desorption from Ag(111) and the distinction between undercoordinated Ag(111) adsorption sites were determined using TPD in correlation with DFT. Complete analysis of TPD data for ethanol adsorbed to terrace sites was used to calculate a kinetic prefactor (3.4 × 1015) and desorption energy (0.54 eV). A better understanding of defect-dependent behavior for ethanol on silver can lead to a greater insight into high surface area nanoparticle catalysts used in industries, catalytic converters, and photo-, electro-, and heterogeneous catalysis. The results suggest that ethanol preferentially adsorbs to undercoordinated sites on Ag(111), resulting in higher binding energies for these molecules (Redhead first order approximation for desorption energies is terrace, 0.54 eV; step edge, 0.57 eV; and kink sites, 0.61 eV). Furthermore, alteration of the silver surface can lead to a redistribution of these sites.

A major goal of energy research is to use visible light to cleave water directly, without an applied voltage, into hydrogen and oxygen. Although SrTiO3 requires ultraviolet light, after four decades, it is still the "gold standard" for the photo-catalytic splitting of water. It is chemically robust and can carry out both hydrogen and oxygen evolution reactions without an applied bias. While ultrahigh vacuum surface science techniques have provided useful insights, we still know relatively little about the structure of these electrodes in contact with electrolytes under operating conditions. Here, we report the surface structure evolution of a n-SrTiO3 electrode during water splitting, before and after "training" with an applied positive bias. Operando high-energy X-ray reflectivity measurements demonstrate that training the electrode irreversibly reorders the surface. Scanning electrochemical microscopy at open circuit correlates this training with a 3-fold increase of the activity toward the photo-induced water splitting. A novel first-principles joint density functional theory simulation, constrained to the X-ray data via a generalized penalty function, identifies an anatase-like structure as the more active, trained surface.

Reliable first-principles calculations of electrochemical processes require accurate prediction of the interfacial capacitance, a challenge for current computationally-efficient continuum solvation methodologies. We develop a model for the double layer of a metallic electrode that reproduces the features of the experimental capacitance of Ag(100) in a non-adsorbing, aqueous electrolyte, including a broad hump in the capacitance near the Potential of Zero Charge (PZC), and a dip in the capacitance under conditions of low ionic strength. Using this model, we identify the necessary characteristics of a solvation model suitable for first-principles electrochemistry of metal surfaces in non-adsorbing, aqueous electrolytes: dielectric and ionic nonlinearity, and a dielectric-only region at the interface. The dielectric nonlinearity, caused by the saturation of dipole rotational response in water, creates the capacitance hump, while ionic nonlinearity, caused by the compactness of the diffuse layer, generates the capacitance dip seen at low ionic strength. We show that none of the previously developed solvation models simultaneously meet all these criteria. We design the Nonlinear Electrochemical Soft-Sphere solvation model (NESS) which both captures the capacitance features observed experimentally, and serves as a general-purpose continuum solvation model., 7 pages, 4 figures, 2 tables; supplementary information (SI.pdf) in source archive

Solvation plays a key role in determining the capacitance behavior of the electrochemical interface, yet it presents significant challenges for computational modeling due to its complexity. We have recently demonstrated (doi: 10.1063/1.4976971) that the solvation models currently implemented in density functional theory (DFT) codes such as VASP perform poorly for the capacitance of aqueous metallic interfaces. In this talk, I identify solvation model characteristics that are necessary for capturing the nonlinear capacitance behavior of aqueous metallic interfaces.

Creating an artificial cathode-electrolyte interface (CEI) from self-assembled monolayers of phosphonic acids offers a promising avenue to prevent capacity loss in Li-ion batteries due to metal dissolution from the cathode. A complete theoretical description of the complex and inherently multi-scale interface between the battery electrode surface and an organic electrolyte can enable rational design of such functionalized cathode coatings. We first present a microscopically informed continuum model for organic electrolyte [1,2] which reproduces key solvation phenomena relevant to battery operation. We go on to predict how the structure and energetics of the metal oxide cathode in Li-ion batteries change due to the presence of liquid electrolyte. We find that compared to vacuum calculations [3], the voltage stability window of the Li-terminated (001) and (111) surfaces of the spinel LiMn2O4 (LMO) cathode in solution is enhanced. Furthermore, we demonstrate that our solvation model, in combination with density-functional theory and classical molecular dynamics, simultaneously captures both the formation of a stable artificial CEI on the LMO surface and the interaction of the CEI molecules with the electrolyte. Our theoretical description captures the experimentally observed trends in solubility and cyclic voltammetry of the coated LMO surface, demonstrating how adjusting the length and functionalization of the phosphonates can balance the competing needs of maximizing Li-ion conductivity but minimizing cathode dissolution. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [1] K. Letchworth-Weaver and T.A. Arias, Phys. Rev. B. 86, 075140 (2012). [2] D. Gunceler et al, Modelling Simul. Mater. Sci. Eng. 21,074005 (2013). [3] R. Warburton et al, ACS Applied Materials and Interfaces, 8 (17), 11108–11121 (2016).

Classical density-functional theory is the most direct approach to equilibrium structures and free energies of inhomogeneous liquids, but requires the construction of an approximate free-energy functional for each liquid of interest. We present a general recipe for constructing functionals for small-molecular liquids based only on bulk experimental properties and ab initio calculations of a single solvent molecule. This recipe combines the exact free energy of the non-interacting system with fundamental measure theory for the repulsive contribution and a weighted density functional for the short-ranged attractive interactions. We add to these ingredients a weighted polarization functional for the long-range correlations in both the rotational and molecular-polarizability contributions to the dielectric response. We also perform molecular dynamics calculations for the free energy of cavity formation and the high-field dielectric response, and show that our free-energy functional adequately describes these properties (which are key for accurate solvation calculations) for all three solvents in our study: water, chloroform and carbon tetrachloride., Comment: 13 pages, 7 figures

Solid-liquid interfaces are at the heart of many modern-day technologies and provide a challenge to many materials simulation methods. A realistic first-principles computational study of such systems entails the inclusion of solvent effects. In this work, we implement an implicit solvation model that has a firm theoretical foundation into the widely used density-functional code Vienna ab initio Software Package. The implicit solvation model follows the framework of joint density functional theory. We describe the framework, our algorithm and implementation, and benchmarks for small molecular systems. We apply the solvation model to study the surface energies of different facets of semiconducting and metallic nanocrystals and the SN2 reaction pathway. We find that solvation reduces the surface energies of the nanocrystals, especially for the semiconducting ones and increases the energy barrier of the SN2 reaction.

Delivering the full benefits of first principles calculations to battery materials demands the development of accurate and computationally-efficient electronic structure methods that incorporate the effects of the electrolyte environment and electrode potential. Realistic electrochemical interfaces containing polar surfaces are beyond the regime of validity of existing continuum solvation theories developed for molecules, due to the presence of significantly stronger electric fields. We present an ab initio theory of the nonlinear dielectric and ionic response of solvent environments within the framework of joint density-functional theory, with precisely the same optimizable parameters as conventional polarizable continuum models. We demonstrate that the resulting nonlinear theory agrees with the standard linear models for organic molecules and metallic surfaces under typical operating conditions. However, we find that the saturation effects in the rotational response of polar solvent molecules, inherent to our nonlinear theory, are crucial for a qualitatively correct description of the ionic surfaces typical of the solid electrolyte interface., 20 pages, 7 figures

Understanding the complex and inherently multi-scale interface between a charged electrode surface and a fluid electrolyte would inform design of more efficient and less costly electrochemical energy storage and conversion devices. Joint density-functional theory (JDFT) [1] bridges the relevant length-scales by joining a fully ab initio description of the electrode with a highly efficient, yet atomically detailed classical DFT description [2] of the liquid electrolyte structure, avoiding the costly statistical sampling of the liquid required by molecular dynamics calculations. Leveraging JDFT within our framework to treat charged systems in periodic boundary conditions [3], we then predict the voltage-dependent structure and energetics at the interface between a liquid electrolyte and single-crystalline metallic electrodes. We compare the JDFT-predicted interfacial water structure next to a metallic electrode with results obtained from classical and ab initio molecular dynamics simulations. We go on to elucidate the physical origin of the experimentally measured voltage-dependent differential capacitance of an Ag(111) electrode in aqueous electrolytes, examining the crucial role of ion desolvation and plating onto the electrolyte. Finally, we conclude with an exploration of how the choice of cation and anion in the electrolyte affects the atomically-detailed structure of the metal-liquid interface, yielding fundamental insight into processes in electrodeposition and corrosion. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [1] S.A. Petrosyan et al, Phys. Rev. B 75, 205105 (2007) [2] R. Sundararaman, K. Letchworth-Weaver, T. A. Arias, J. Chem. Phys. 140, 144504 (2014) [3] K. Letchworth-Weaver and T. A. Arias, Phys. Rev. B. 86, 075140 (2012)

In the Center for Electrochemical Energy Science (CEES) Energy Frontier Research Center, we are investigating a new class of energy storage materials – hybrid Li-ion/Li-oxygen materials – that combine transition metal and oxygen redox chemistries to deliver high energy storage capacity [1, 2, 3]. The electrochemical reactions, structural and electronic structure evolution of these materials are complex, requiring a combination of in-depth characterization and first principles modeling. In this talk, we will report on a comparison study of several examples of hybrid Li-ion/Li-oxygen materials using density functional theory calculations. We will discuss the structural, thermodynamic, and electronic structure reasons for differences in reaction pathways and competition between transition metal vs oxygen redox chemistries. Insight towards selecting and designing promising hybrid battery materials gleaned from the comparison will be discussed. References [1] L. Trahey, C. S. Johnson, J. T. Vaughey, S.-H. Kang, L. J. Hardwick, S. A. Freunberger, P. G. Bruce, M. M. Thackeray, “Activated Lithium-Metal-Oxides as Catalytic Electrodes for Li-O2 Cells,” Electrochemical and Solid-State Letters, 2011, 14, A64. [2] M. M. Thackeray, M. K. Y. Chan, L. Trahey, S. Kirklin, and C. Wolverton, “Vision for Designing High-Energy, Hybrid Li Ion/Li–O2 Cells,” Journal of Physical Chemistry Letters, 2013, 4, 3607. [3] Z. Yang, L. Trahey, Y. Ren, M. K. Y. Chan, C. Lin, J. Okasinski, and M. M. Thackeray, “In-Situ High-Energy Synchrotron X-ray Diffraction Studies and First Principles Modeling of α-MnO2 Electrodes in Li-O2 and Li-ion Coin Cells,” Journal of Materials Chemistry A, 2015, 3, 7389. Acknowledgement This work was supported as a part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under award number DE-AC02–06CH11. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Employing a classical density-functional description of liquid environments, we introduce a rigorous method for the diffusion quantum Monte Carlo calculation of free energies and thermodynamic averages of solvated systems that requires neither thermodynamic sampling nor explicit solvent electrons. We find that this method yields promising results and small convergence errors for a set of test molecules. It is implemented readily and is applicable to a range of challenges in condensed matter, including the study of transition states of molecular and surface reactions in liquid environments., Comment: 5 pages, 3 figures, accepted for publication in Physical Review B Rapid Communications

This work explores the use of joint density-functional theory, a new form of density-functional theory for the ab initio description of electronic systems in thermodynamic equilibrium with a liquid environment, to describe electrochemical systems. After reviewing the physics of the underlying fundamental electrochemical concepts, we identify the mapping between commonly measured electrochemical observables and microscopically computable quantities within an, in principle, exact theoretical framework. We then introduce a simple, computationally efficient approximate functional which we find to be quite successful in capturing a priori basic electrochemical phenomena, including the capacitive Stern and diffusive Gouy-Chapman regions in the electrochemical double layer, quantitative values for interfacial capacitance, and electrochemical potentials of zero charge for a series of metals. We explore surface charging with applied potential and are able to place our ab initio results directly on the scale associated with the Standard Hydrogen Electrode (SHE). Finally, we provide explicit details for implementation within standard density-functional theory software packages at negligible computational cost over standard calculations carried out within vacuum environments., Comment: 18 pages, 5 figures. Initially presented at APS March Meeting 2010. Accepted for publication in Physical Review B on Jul. 27, 2012

We present an accurate equation of state for water based on a simple microscopic Hamiltonian, with only four parameters that are well-constrained by bulk experimental data. With one additional parameter for the range of interaction, this model yields a computationally efficient free-energy functional for inhomogeneous water which captures short-ranged correlations, cavitation energies and, with suitable long-range corrections, the non-linear dielectric response of water, making it an excellent candidate for studies of mesoscale water and for use in ab initio solvation methods., Comment: 6 pages, 5 figures

Continuum solvation models enable electronic structure calculations of systems in liquid environments, but because of the large number of empirical parameters, they are limited to the class of systems in their fit set (typically organic molecules). Here, we derive a solvation model with no empirical parameters for the dielectric response by taking the linear response limit of a classical density functional for molecular liquids. This model directly incorporates the nonlocal dielectric response of the liquid using an angular momentum expansion, and with a single fit parameter for dispersion contributions it predicts solvation energies of neutral molecules with an RMS error of 1.3 kcal/mol in water and 0.8 kcal/mol in chloroform and carbon tetrachloride. We show that this model is more accurate for strongly polar and charged systems than previous solvation models because of the parameter-free electric response, and demonstrate its suitability for ab initio solvation, including self-consistent solvation in quantum Monte Carlo calculations., Comment: 5 pages, 4 figures