Sara E. Mason

First-principles extrapolation method for accurate CO adsorption energies on metal surfaces

We show that a simple first-principles correction based on the difference between the singlet-triplet CO excitation energy values obtained by density-functional theory ~DFT! and high-level quantum chemistry methods yields accurate CO adsorption properties on a variety of metal surfaces. We demonstrate a linear relationship between the CO adsorption energy and the CO singlet-triplet splitting, similar to the linear dependence of CO adsorption energy on the energy of the CO 2p* orbital found recently @Kresse et al., Phys. Rev. B 68, 073401 ~2003!#. Converged DFT calculations underestimate the CO singlet-triplet excitation energy DES-T , whereas coupled-cluster and configuration-interaction ~CI! calculations reproduce the experimental DES-T . The dependence of Echem on DES-T is used to extrapolate Echem for the top, bridge, and hollow sites for the ~100! and ~111! surfaces of Pt, Rh, Pd, and Cu to the values that correspond to the coupled cluster and CI DES-T value. The correction reproduces experimental adsorption site preference for all cases and obtains Echem in excellent agreement with experimental results.

Structure and polarization in the high Tc ferroelectric Bi(Zn,Ti)O3-PbTiO3 solid solutions.

Theoretical ab initio and experimental methods are used to investigate the [Bi(Zn1/2Ti1/2)O3]x[PbTiO3]1-x solid solution. We find that hybridization between Zn 4s and 4p and O 2p orbitals allows the formation of short, covalent Zn-O bonds, enabling favorable coupling between A-site and B-site displacements. This leads to unusually large polarization, strong tetragonality, and an elevated ferroelectric to paraelectric phase transition temperature.

Sustainable Nanotechnology: Opportunities and Challenges for Theoretical/Computational Studies

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.

Comparative DFT study of inner-sphere As(III) complexes on hydrated α-Fe2O3(0001) surface models

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-Fe(2)O(3)(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.

Contaminant adsorption on nanoscale particles: structural and theoretical characterization of Cu2+ bonding on the surface of Keggin-type polyaluminum (Al30) molecular 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 Cu(2+) 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 Cu(2+) and SO4(2-) 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 Cu(2+) or SO4(2-) adsorption. The associated DFT energetics and charge density analyses suggest that strong electrostatic interactions between SO4(2-) and the beltway of Al30 play a vital role in governing where Cu(2+) 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.

Establishing trends in ion adsorption on the aqueous aluminium hydroxide nanoparticle Al30

The fact that chemical reactions at environmental interfaces are becoming accessible to quantum mechanical computational studies provides geochemical researchers with a new means to predict properties that cannot readily be measured and to develop molecular-level understanding of geochemical model systems. Recent computational studies of Cu2+ and adsorption onto the Keggin-based aqueous aluminium nanoparticle (), or Al30, revealed opposing trends in adsorption site preference as a function of molecule surface topology. Specifically, the adsorption site favourable for the inner-sphere adsorption of Cu2+ is on the caps of Al30 while outer-sphere prefers adsorption in the so-called beltway region of the molecule. When co-adsorbed, it is predicted that both species adsorb in the beltway, consistent with an experimental crystal structure. Here, we discuss results for individual cation and anion adsorption to Al30. Our goals are to better understand how the adsorbate properties govern interactions with Al30 and to assess whether generalisations can be formed. We test the reactivity of cations (Cu2+, Pb2+, Zn2+) and anions (, Cl− ) to aqueous Al30 by using density functional theory modelling. It is determined that all the cations favour the adsorption sites on the caps of Al30 and both anions favour outer-sphere adsorption in the beltway region. The results are discussed in terms of the electrostatic potential of Al30 and three-dimensional induced charge density mapping.

Characterization of Phosphate and Arsenate Adsorption onto Keggin-Type Al30 Cations by Experimental and Theoretical Methods.

Keggin-type aluminum oxyhydroxide species such as the Al30 (Al30O8(OH)56(H2O)26(18+)) 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.

Systematic density functional theory study of the structural and electronic properties of constrained and fully relaxed (0 0 1) surfaces of alumina and hematite

Abstract Owing to their widespread use in a variety of technological applications, as well as their prevalence as naturally occurring phases in the environment, there is a prolific amount of computational research devoted to the surfaces of metal oxides. However, there is no standard approach for how to best represent the surface structurally in quantum mechanical modeling, specifically in the standard supercell slab geometry that is amenable to density functional theory calculations that employ periodic boundary conditions. There is a choice in both slab thickness and in how the atomic positions are treated during geometry optimisations; the atomic coordinates can either be fully relaxed or partially fixed. Constraining the atomic positions of select layers of the slab can decrease overall computational cost and is often reported to have a minimal effect on the details of the optimised geometries. In this study, we compare fully relaxed structures of alumina () and hematite () (0 0 1) surfaces to two slab models in which either one or two stoichiometric layers, denoted as trilayers, are constrained to bulk positions. We go on to study the electronic structure of the slab models, and we also assess how modeled reactivity is affected through studies of atomic chemisorption on the slab models. Our results suggest that while structural differences between partially constrained or fully relaxed slab models may be subtle, both the electronic structure and modeled reactivity can vary significantly in quantitative and qualitative ways.

Influence of nickel manganese cobalt oxide nanoparticle composition on toxicity toward Shewanella oneidensis MR-1: redesigning for reduced biological impact

Lithium nickel manganese cobalt oxide (LixNiyMnzCo1−y−zO2, 0 < x, y, z < 1, also known as NMC) is a class of cathode materials used in lithium ion batteries. Despite the increasing use of NMC in nanoparticle form for next-generation energy storage applications, the potential environmental impact of released nanoscale NMC is not well characterized. Previously, we showed that the released nickel and cobalt ions from nanoscale Li1/3Ni1/3Mn1/3Co1/3O2 were largely responsible for impacting the growth and survival of the Gram-negative bacterium Shewanella oneidensis MR-1 (M. N. Hang et al., Chem. Mater., 2016, 28, 1092). Here, we show the first steps toward material redesign of NMC to mitigate its biological impact and to determine how the chemical composition of NMC can significantly alter the biological impact on S. oneidensis. We first synthesized NMC with various stoichiometries, with an aim to reduce the Ni and Co content: Li0.68Ni0.31Mn0.39Co0.30O2, Li0.61Ni0.23Mn0.55Co0.22O2, and Li0.52Ni0.14Mn0.72Co0.14O2. Then, S. oneidensis were exposed to 5 mg L−1 of these NMC formulations, and the impact on bacterial oxygen consumption was analyzed. Measurements of the NMC composition, by X-ray photoelectron spectroscopy, and composition of the nanoparticle suspension aqueous phase, by inductively coupled plasma-optical emission spectroscopy, showed the release of Li, Ni, Mn, and Co ions. Bacterial inhibition due to redesigned NMC exposure can be ascribed largely to the impact of ionic metal species released from the NMC, most notably Ni and Co. Tuning the NMC stoichiometry to have increased Mn at the expense of Ni and Co showed lowered, but not completely mitigated, biological impact. This study reveals that the chemical composition of NMC nanomaterials is an important parameter to consider in sustainable material design and usage.

Systematic Study of Aluminum Nanoclusters and Anion Adsorbates

The interactions between aqueous aluminum (Al) nanoclusters and ions in solution influence the reactivity of nanomaterials in natural waters and are crucial to the targeted syntheses of aluminum oxides. To contribute to the fundamental understanding of how both anion and Al-nanocluster properties affect the interactions, we carry out systematic modeling studies that employ density functional theory calculations embedded in a continuum solvent model. Energetic and electronic structure analysis is applied toward delineating the interactions of a range of probe adsorbate anions with Al nanoclusters to elucidate how small molecules may react with naturally occurring nanomaterials. The study spans seven small molecules on three model Al nanoclusters. Using this ion set, we correlate the size, shape, and formal charge of the adsorbate to the trends in adsorption energies. A key finding is that the collective effects of exposed oxygen functional groups, i.e., the distribution of functional groups, dictates the electrostatic potential of the nanocluster surface, which, in turn, controls trends in anion adsorption. The computed adsorption and deprotonation trends are correlated to known synthetic routes of Al-nanocluster formation and subsequent crystallization to give insight into the potential optimization of synthetic conditions.