We use GGA + U methodology to model the bulk and surface structure of varying stoichiometries of the (001) surface of LiCoO2. The DFT energies obtained for these surface-slab models are used for two thermodynamic analyses to assess the relative stabilities of different surface configurations, including hydroxylation. In the first approach, surface free energies are calculated within a thermodynamic framework, and the second approach is a surface-solvent ion exchange model. We find that, for both models, the −CoO–H1/2 surface is the most stable structure near the O-rich limit, which corresponds to ambient conditions. We find that surfaces terminated with Li are higher in energy, and we go on to show that H and Li behave differently on the (001) LiCoO2 surface. The optimized geometries show that terminal Li and H occupy nonequivalent surface sites. In terms of electronic structure, Li and H terminations exhibit distinct bandgap characters, and there is also a distinctive distribution of charge at the surface. We go on to probe how the variable Li and H terminations affect reactivity, as probed through phosphate adsorption studies.

We report the bulk properties and ab initio thermodynamics surface free energies for α-Fe2O3(0001) using density functional theory (DFT) with calculated Hubbard U values for chemically distinct surface Fe atoms. There are strong electron correlation effects in hematite that are not well-described by standard DFT. A better description can be achieved by using a DFT + U approach in which U represents a Hubbard on-site Coulomb repulsion term. While DFT + U calculations result in improved predictions of the bulk hematite band gap, surface free energies using DFT + U total energies result in surface structure predictions that are at odds with most experimental results. Specifically, DFT + U predictions stabilize a ferryl termination relative to an oxygen termination that is widely reported under a range of experimental conditions. We explore whether treating chemically distinct surface Fe atoms with different U values can lead to improved bulk and surface predictions. We use a linear response technique to derive specific Ud values for all Fe atoms in several slab geometries. We go on to add a Coulomb correction, Up, to better describe the hybridization between the Fe d and oxygen p orbitals, accurately predicting the structural and electronic properties of bulk hematite. Our results show that the site-specific Ud is a key factor in obtaining theoretical results for surface stability that are congruent with the experimental literature results of α-Fe2O3(0001) surface structure. Finally, we use a model surface reaction to trace how the various DFT + U methods affect the surface electronic structure and heterogeneous reactivity.

For assistance in the design of the next generation of nanomaterials that are functional and have minimal health and safety concerns, it is imperative to establish causality, rather than correlations, in how properties of nanomaterials determine biological and environmental outcomes. Due to the vast design space available and the complexity of nano/bio interfaces, theoretical and computational studies are expected to play a major role in this context. In this minireview, we highlight opportunities and pressing challenges for theoretical and computational chemistry approaches to explore the relevant physicochemical processes that span broad length and time scales. We focus discussions on a bottom-up framework that relies on the determination of correct intermolecular forces, accurate molecular dynamics, and coarse-graining procedures to systematically bridge the scales, although top-down approaches are also effective at providing insights for many problems such as the effects of nanoparticles on biological membranes.

The structural chemistry of Group 13 polyoxometalates lags far behind related negatively charged transition metal species and limits the development of advanced materials. A novel heterometallic cluster [Ga2Al18O8(OH)36(H2O)12]8+ (Ga2Al18) has been isolated using a supramolecular approach and structurally characterized using single-crystal X-ray diffraction. Ga2Al18 represents the Wells–Dawson structure polycations and variations in the structural topology may be related to the initial stabilization of the Keggin isomer. DFT calculations on the related ε-Keggins (GaAl12 and Al13), Ga2Al18, and theoretical Al2Al18 clusters reveal similar features of electronic structure, suggesting additional heteroatom substitution in other isostructural clusters should be possible.

Keggin-type aluminum oxyhydroxide species such as the Al30 (Al30O8(OH)56(H2O)2618+) polycation can readily sequester inorganic and organic forms of P(V) and As(V), but there is a limited chemical understanding of the adsorption process. Herein, we present experimental and theoretical structural and chemical characterization of [(TBP)2Al2(μ4-O8)(Al28(μ2-OH)56(H2O)22)]14+ (TBP = t-butylphosphonate), denoted as (TBP)2Al30-S. We go on to consider the structure as a model for studying the reactivity of oxyanions to aluminum hydroxide surfaces. Density functional theory (DFT) calculations comparing the experimental structure to model configurations with P(V) adsorption at varying sites support preferential binding of phosphate in the Al30 beltway region. Furthermore, DFT calculations of R-substituted phosphates and their arsenate analogues consistently predict the beltway region of Al30 to be most reactive. The experimental structure and calculations suggest a shape–reactivity relationship in Al30, which counters predictions based on oxygen functional group identity.

•Molecular studies of environmental interfaces use geochemical models.•Chemical information about environmental interfaces is accessible through DFT.•Theoretical geochemical surface science is an emerging field.The fact that essential chemical information about environmental interfaces is becoming accessible through density functional theory (DFT) studies provides researchers with a means to interpret experimental information, to predict properties that cannot be measured, and to develop conceptual, molecular-level understanding of structure–property relationships of these systems. Molecular studies of environmental interfaces require the use of structurally well-defined geochemical models, and in here we discuss the reactivity of mineral–water interfaces and aqueous aluminum hydroxide nanoparticles with ionic species. These adsorption processes are a major factor controlling pollutant transport and transformation. In the examples we highlight, DFT studies are used to extract new conceptual understanding of the underlying physical factors of the substrates that dictate reactivity. In particular, we review DFT studies used to rationalize an empirically determined reactivity trend of hydrated alumina and hematite towards Pb(II), and later review how DFT calculations of aqueous aluminum hydroxide nanoparticles along with experimental structural information helped to identify the electrostatic potential as a key factor in understanding particle reactivity towards cationic and anionic species.

The adsorption of contaminants onto metal oxide surfaces with nanoscale Keggin-type structural topologies has been well established, but identification of the reactive sites and the exact binding mechanism are lacking. Polyaluminum species can be utilized as geochemical model compounds to provide molecular level details of the adsorption process. An Al30 Keggin-type species with two surface-bound Cu2+ cations (Cu2Al30-S) has been crystallized in the presence of disulfonate anions and structurally characterized by single-crystal X-ray diffraction. Density functional theory (DFT) calculations of aqueous molecular analogues for Cu2Al30-S suggest that the reactivity of Al30 toward Cu2+ and SO42– shows opposite trends in preferred adsorption site as a function of particle topology, with anions preferring the beltway and cations preferring the caps. The bonding competition was modeled using two stepwise reaction schemes that consider Cu2Al30-S formation through initial Cu2+ or SO42– adsorption. The associated DFT energetics and charge density analyses suggest that strong electrostatic interactions between SO42– and the beltway of Al30 play a vital role in governing where Cu2+ binds. The calculated electrostatic potential of Al30 provides a theoretical interpretation of the topology-dependent reactivity that is consistent with the present study as well as other results in the literature.

This study reports on density functional theory (DFT) modeling of antimony adsorption at model environmental interfaces. Both Sb(III) and Sb(V) surface complexes were studied on hydrated alumina and hematite, at varying adsorption sites and with varying distal Sb coordination. We calculate the DFT reaction energies of a number of hypothetical heterogeneous interconversion reactions of Sb surface complexes with gas-phase water and oxygen, which predict that an octahedral Sb(V) surface complex is overall the most favorable. Additionally, the results suggest that several different heterogeneous pathways starting from distinct metastable Sb surface complexes are possible, including an antimonyl (SbO) surface complex. A total of 28 Sb surface complexes are found through DFT geometry optimizations, the structural and energetic details of which are reported to guide future experimental studies and to form a basis for ongoing theoretical work including dynamic simulations.Graphical abstractHighlights► Sb(III) and Sb(V) surface complexes are modeled using density functional theory. ► Several Sb distal coordinations at varying surface sites are modeled. ► Hematite is predicted to be more reactive towards Sb than isostructural alumina. ► DFT reaction energies predict that surfaces promote Sb oxidation. ► Several Sb surface complexes are predicted as potential reaction intermediates.

The long-recognized risk to human health arising from arsenic-contaminated waters is known to be linked to partitioning reactions between arsenic and natural solid phases. Currently, the ability to predict As surface complexation is limited by the lack of molecular-level understanding of As-solid interactions. In the present study, we use density functional theory (DFT) to model mono-, bi-, and tri-dentate As(III) surface complexes on different (previously proposed) structural models for hydrated hematite, modeled as α-Fe2O3(0001)–water interfaces. One of the modeled hematite–water interfaces is terminated entirely by hydroxyl surface functional groups, comprised of hematite lattice oxygen atoms. The other hematite–water interface is an iron-terminated model in which the outermost oxygen functional groups are water (and water dissociation product) ligands. We report the DFT trends in adsorption energies in terms of As-hematite coordination, hematite surface geometry/stoichiometry, and oxygen functional group identity. The DFT energetics predict that a monodentate As(III) surface complex is preferred on both hematite–water structures, suggesting that the two structural models here employed do not sufficiently represent the true surface structure to reproduce the experimental observation of As(III) bidentate coordination. However, the results do elucidate fundamental concepts of interface reactivity: A key result, supported by electronic structure analysis, is that ligand oxygen functional groups cannot be treated on equal ground with true surface oxygen functional groups. For the systems modeled here the distinction between surface and ligand functional groups supersedes the differences in oxygen coordination with surface Fe. We discuss the impact of this finding on the application of bond-valence-based predictions of mineral–water reactivity, and use the results of this study to pose questions and directions for ongoing modeling efforts aimed at linking macroscopic reactivity with molecular-level understanding.

We report on a density functional theory study aimed at comparing the reactivity of differently structured α-Al2O3−water interfaces as probed through adsorption of the Pb(II) cation. We assign the Pb−O bonding in Pb(II)/Al2O3 geometries to in-plane and out-of-plane orbital contributions. From our analysis, the empirically known greater Pb(II) reactivity of α-Al2O3(11̅02) over α-Al2O3(0001) is ascribed to the ability of oxygen functional groups in the corrugated (11̅02) interface to hybridize more effectively with Pb(II) electronic states than oxygen functional groups in the topographically flat (0001) interface. The theoretical evidence of a Pb−O hybridization−reactivity relationship goes beyond bond-valence predictions that cite oxygen functional group coordination as a key predictor of mineral−water interface reactivity. We also report the details of adsorption-induced surface relaxations, including an example of surface hydrogen bond rearrangement, as well as evidence of long-range Pb−O interaction. To further assess the bonding saturation of lead in the optimized Pb(II)/Al2O3 structures, molecular H2O adsorption studies are carried out and shown to support that the cation coordination is largely satisfied through interactions with surface functional groups.

The long-recognized risk to human health arising from arsenic-contaminated waters is known to be linked to partitioning reactions between arsenic and natural solid phases. Currently, the ability to predict As surface complexation is limited by the lack of molecular-level understanding of As-solid interactions. In the present study, we use density functional theory (DFT) to model mono-, bi-, and tri-dentate As(III) surface complexes on different (previously proposed) structural models for hydrated hematite, modeled as α-Fe2O3(0001)–water interfaces. One of the modeled hematite–water interfaces is terminated entirely by hydroxyl surface functional groups, comprised of hematite lattice oxygen atoms. The other hematite–water interface is an iron-terminated model in which the outermost oxygen functional groups are water (and water dissociation product) ligands. We report the DFT trends in adsorption energies in terms of As-hematite coordination, hematite surface geometry/stoichiometry, and oxygen functional group identity. The DFT energetics predict that a monodentate As(III) surface complex is preferred on both hematite–water structures, suggesting that the two structural models here employed do not sufficiently represent the true surface structure to reproduce the experimental observation of As(III) bidentate coordination. However, the results do elucidate fundamental concepts of interface reactivity: A key result, supported by electronic structure analysis, is that ligand oxygen functional groups cannot be treated on equal ground with true surface oxygen functional groups. For the systems modeled here the distinction between surface and ligand functional groups supersedes the differences in oxygen coordination with surface Fe. We discuss the impact of this finding on the application of bond-valence-based predictions of mineral–water reactivity, and use the results of this study to pose questions and directions for ongoing modeling efforts aimed at linking macroscopic reactivity with molecular-level understanding.

The structural chemistry of Group 13 polyoxometalates lags far behind related negatively charged transition metal species and limits the development of advanced materials. A novel heterometallic cluster [Ga2Al18O8(OH)36(H2O)12]8+ (Ga2Al18) has been isolated using a supramolecular approach and structurally characterized using single-crystal X-ray diffraction. Ga2Al18 represents the Wells–Dawson structure polycations and variations in the structural topology may be related to the initial stabilization of the Keggin isomer. DFT calculations on the related ε-Keggins (GaAl12 and Al13), Ga2Al18, and theoretical Al2Al18 clusters reveal similar features of electronic structure, suggesting additional heteroatom substitution in other isostructural clusters should be possible.