88 research outputs found

    Fully coordinated silica nanoclusters: (SiO2)(N) molecular rings

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    A new form of finite silica with edge-sharing SiO2 units connected in a ring is proposed. High-level density-functional calculations for (SiO2)(N), N = 4-14, show the rings to be energetically more stable than the corresponding (SiO2)(N) linear chains for N > 11. The rings display frequency modes in remarkable agreement with infrared bands measured on dehydrated silica surfaces indicating their potential as models of strained extended silica systems. Silica rings, if synthesized, may also be useful precursors for new bulk-silica polymorphs with tubular or porous morphologies

    The effect of particle size on the optical and electronic properties of magnesium oxide nanoparticles

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    The quasiparticle states, fundamental gaps, optical gaps, exciton binding energies and UV-vis spectra for a series of cuboidal nanoparticles of the prototypical oxide magnesium oxide (MgO), the largest of which has 216 atoms and edges of 1 nm, were predicted using many-body perturbation theory (evGW-BSE). The evolution of the properties with the particle size was explicitly studied. It was found that, while the highest occupied and lowest unoccupied quasiparticle states and fundamental gap change with the particle size, the optical gap remains essentially fixed for all but the smallest nanoparticles, in line with what was previously observed experimentally. The explanation for these observations is demonstrated to be that, while the optical gap is associated with an exciton that is highly localised around the particle's corner atoms, the highest occupied and lowest unoccupied quasiparticle states, while primarily localised on the oxygen corner atoms (hole) and magnesium corner atoms (electron), show significant delocalisation along the edges. The strong localisation of the exciton associated with the optical gap on the corner atoms is argued to also explain why the nanoparticles have much smaller optical gaps and red-shifted spectra compared to bulk MgO. Finally, it is discussed how this non-quantum confinement behaviour, where the properties of the nanoparticles arise from surface defects rather than differences in localisation of quasiparticle or exciton states, appears typical of alkaline earth oxide nanoparticles, and that the true optical gap of bulk crystals of such materials is also probably the result of surface defects, even if unobservable experimentally

    Apparent Scarcity of Low-Density Polymorphs of Inorganic Solids

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    For most inorganic solids, very few dense polymorphs and no low-density polymorphs are observed. Taking a wide range of tetrahedrally-coordinated binary solids (e.g., ZnO, GaN) as a prototypical system, we show that the apparent scarcity of low-density polymorphs is not due to significant structural or energetic limitations. Using databases of periodic networks as sources of novel crystal structures, followed by ab initio energy minimization, we predict a dense spectrum of low-density low-energy polymorphs. The diverse range of materials considered indicates that this is likely to be a general phenomenon

    The Role of Computational Chemistry in Discovering and Understanding Organic Photocatalysts for Renewable Fuel Synthesis

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    In this review the role computational chemistry plays in helping to rationalize the ability of organic materials, such as conjugated polymers, to drive photocatalytic water splitting and carbon dioxide reduction, and the discovery of new organic photocatalysts, is reviewed. The ways in which organic photocatalysts differ from their inorganic counterparts, the mechanism by which such materials, when illuminated, reduce protons or CO2 and oxidize water or sacrificial donors, and how this can be studied using computational methods, as well as the high-throughput virtual screening of organic materials as photocatalysts, are discussed. Finally, the current opportunities and challenges associated with studying photocatalysts computationally, are examined

    Polymeric watersplitting photocatalysts; a computational perspective on the water oxidation conundrum

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    A computational scheme to predict the thermodynamic ability of photocatalysts to drive both of the watersplitting half reactions, proton reduction and water oxidation, is discussed, and applied to a number of polymeric systems to explain their apparent inability to oxidise water. We predict that the poly(p-phenylene) (PPP) is thermodynamically unable to oxidise water and that PPP is hence unlikely to split water in the absence of an external electrical bias. For other polymers, however, for example carbon nitride, the lack of oxygen evolution activity appears kinetic in origin and hence a suitable co-catalyst could potentially transform them into true watersplitting photocatalysts

    Validating a Density Functional Theory Approach for Predicting the Redox Potentials Associated with Charge Carriers and Excitons in Polymeric Photocatalysts

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    We compare, for a range of conjugated polymers relevant to water-splitting photocatalysis, the predictions for the redox potentials associated with charge carriers and excitons by a total-energy ΔDFT approach to those measured experimentally. For solid-state potentials, of the different classes of potentials available experimentally for conjugated polymers, the class measured under conditions which are the most similar to those during water splitting, we find a good fit between the ionization potentials predicted using ΔB3LYP and those measured experimentally using photoemission spectroscopy (PES). We also observe a reasonable fit to the more limited data sets of excited-state ionization potentials, obtained from two-photon PES, and electron affinities, measured by inverse PES, respectively. Through a comparison of solid-state potentials with gas phase and solution potentials for a range of oligomers, we demonstrate how the quality of the fit to experimental solid-state data is probably the result of benign error cancellation. We discuss that the good fit for solid-state potentials in vacuum suggests that a similar accuracy can be expected for calculations on solid-state polymers interfaced with water. We also analyze the quality of approximating the ΔB3LYP potentials by orbital energies. Finally, we discuss what a comparison between experimental and predicted potentials teaches us about conjugated polymers as photocatalysts, focusing specifically on the large exciton-binding energy in these systems and the mechanism of free charge carrier generation

    Polymer photocatalysts for water splitting: insights from computational modeling

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    Based on insights from computational chemistry calculations, the ability of polymers to act as water splitting photocatalysts for the production of renewable hydrogen from water and sunlight is discussed. Specifically, the important role of exciton dissociation in these materials is highlighted, as well as the possible microscopic origins of the experimentally observed changes in the photocatalytic activity of a polymer with increasing chain length or changing chemical composition. The reason why water oxidation, with polymeric photocatalysts, is difficult, and which polymer properties to target when developing new polymers for water splitting photocatalysis are, finally, also discussed

    Structure prediction of (BaO)n nanoclusters for n⩽24 using an evolutionary algorithm

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    Knowing the structure of nanoclusters is relevant to gaining insight into their properties for materials design. Computational studies predicting their structure should aim to reproduce experimental results. Here, barium oxide was chosen for its suitability for both computational structure prediction and experimental structure determination. An evolutionary algorithm implemented within the KLMC structure prediction package was employed to find the thermodynamically most stable structures of barium oxide nanoclusters (BaO)n with n=4-18and24. Evolutionary algorithm runs were performed to locate local minima on the potential energy landscape defined using interatomic potentials, the structures of which were then refined using density functional theory. BaO clusters show greater preference than MgO for adopting cuts from its bulk phase, thus more closely resemble clusters of KF. (BaO)4, (BaO)6, (BaO)8, (BaO)10 and (BaO)16 should be magic number clusters and each are at least 0.03 eV/BaO more stable than all other PBEsol local minima clusters found for the same size

    Synthesis target structures for alkaline earth oxide clusters

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    Knowing the possible structures of individual clusters in nanostructured materials is an important first step in their design. With previous structure prediction data for BaO nanoclusters as a basis, data mining techniques were used to investigate candidate structures for magnesium oxide, calcium oxide and strontium oxide clusters. The lowest-energy structures and analysis of some of their structural properties are presented here. Clusters that are predicted to be ideal targets for synthesis, based on being both the only thermally accessible minimum for their size, and a size that is thermally accessible with respect to neighbouring sizes, include global minima for: sizes n = 9, 15, 16, 18 and 24 for (MgO)n; sizes n = 8, 9, 12, 16, 18 and 24 for (CaO) n ; the greatest number of sizes of (SrO) n clusters (n = 8, 9, 10, 12, 13, 15, 16, 18 and 24); and for (BaO) n sizes of n = 8, 10 and 16
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