David A. Tompsett

Rutile (β-)MnO2 Surfaces and Vacancy Formation for High Electrochemical and Catalytic Performance

MnO2 is a technologically important material for energy storage and catalysis. Recent investigations have demonstrated the success of nanostructuring for improving the performance of rutile MnO2 in Li-ion batteries and supercapacitors and as a catalyst. Motivated by this we have investigated the stability and electronic structure of rutile (β-)MnO2 surfaces using density functional theory. A Wulff construction from relaxed surface energies indicates a rod-like equilibrium morphology that is elongated along the c-axis, and is consistent with the large number of nanowire-type structures that are obtainable experimentally. The (110) surface dominates the crystallite surface area. Moreover, higher index surfaces than considered in previous work, for instance the (211) and (311) surfaces, are also expressed to cap the rod-like morphology. Broken coordinations at the surface result in enhanced magnetic moments at Mn sites that may play a role in catalytic activity. The calculated formation energies of oxygen vacancy defects and Mn reduction at key surfaces indicate facile formation at surfaces expressed in the equilibrium morphology. The formation energies are considerably lower than for comparable structures such as rutile TiO2 and are likely to be important to the high catalytic activity of rutile MnO2.

Surface properties of α-MnO2: relevance to catalytic and supercapacitor behaviour

Hollandite (α-)MnO2 gives superior performance compared to other MnO2 polymorphs in surface sensitive applications in supercapacitors and catalysis. However, a thorough understanding of its atomic-scale surface properties is lacking, which we address here using density functional theory (DFT). A Wulff construction based upon relaxed surface energies demonstrates that the equilibrium morphology expresses the low index (100), (110) and (111) surfaces as well as the high index (211) and (112) surfaces. The predicted morphology exhibits clear elongation along the c-axis which is consistent with the large number of nanorod type structures that are obtainable experimentally. The surface structures expressed in the morphology are discussed in detail and it is found that α-MnO2 gives rise to larger surface relaxations than are observed for the less open rutile structured MnO2. Enhanced magnetic moments at surface sites are rationalised by a crystal field argument. Experimental studies consistently find that α-MnO2 has higher catalytic activity than other polymorphs of MnO2. In this work, calculated formation energies for oxygen vacancy defects at the expressed surfaces are demonstrably lower, by ∼1 eV, than for rutile MnO2 surfaces [Tompsett et al., JACS, 2014, 136, 1418]. The lowest vacancy formation energy occurs at the (112) surface, which despite its relative high Miller index constitutes 17% of the surface area of the calculated morphology. This may play a key role in the favourable catalytic performance observed for α-MnO2 in a broad range of applications.

Atomistic Insights into the Oriented Attachment of Tunnel-Based Oxide Nanostructures

Controlled synthesis of nanomaterials is one of the grand challenges facing materials scientists. In particular, how tunnel-based nanomaterials aggregate during synthesis while maintaining their well-aligned tunneled structure is not fully understood. Here, we describe the atomistic mechanism of oriented attachment (OA) during solution synthesis of tunneled α-MnO2 nanowires based on a combination of in situ liquid cell transmission electron microscopy (TEM), aberration-corrected scanning TEM with subangstrom spatial resolution, and first-principles calculations. It is found that primary tunnels (1 × 1 and 2 × 2) attach along their common {110} lateral surfaces to form interfaces corresponding to 2 × 3 tunnels that facilitate their short-range ordering. The OA growth of α-MnO2 nanowires is driven by the stability gained from elimination of {110} surfaces and saturation of Mn atoms at {110}-edges. During this process, extra [MnOx] radicals in solution link the two adjacent {110} surfaces and bond with the unsaturated Mn atoms from both surface edges to produce stable nanowire interfaces. Our results provide insights into the controlled synthesis and design of nanomaterials in which tunneled structures can be tailored for use in catalysis, ion exchange, and energy storage applications.

LixFeF6 (x = 2, 3, 4) battery materials: structural, electronic and lithium diffusion properties

Lithium iron fluoride materials have attracted recent interest as cathode materials for lithium ion batteries. The electrochemical properties of the high energy density Li(x)FeF6 (x = 2, 3, 4) materials have been evaluated using a combination of potential-based and DFT computational methods. Voltages of 6.1 V and 3.0 V are found for lithium intercalation from Li2FeF6 to α-Li3FeF6 and α-Li3FeF6 to Li4FeF6 respectively. The calculated density of states indicate that Li2FeF6 possesses metallic states that become strongly insulating after lithium intercalation to form α-Li3FeF6. The large energy gain associated with this metal-insulator transition is likely to contribute to the associated large voltage of 6.1 V. Molecular dynamics simulations of lithium diffusion in α-Li3FeF6 at typical battery operating temperatures indicate high lithium-ion mobility with low activation barriers. These results suggest the potential for good rate performance of lithium iron fluoride cathode materials.

Electronic structure and phonon instabilities in the vicinity of the quantum phase transition and superconductivity of ( Sr , Ca ) 3 Ir 4 Sn 13

The nature of the lattice instability connected to the structural transition and superconductivity of (Sr,Ca)3 Ir 4 Sn 13 is not yet fully understood. In this work density functional theory (DFT) calculations of the phonon instabilities as a function of chemical and hydrostatic pressure show that the primary lattice instabilities in Sr 3 Ir 4 Sn13 lie at phonon modes of wave vectors q=(0.5,0,0) and q=(0.5,0.5,0). Following these modes by calculating the energy of supercells incorporating the mode distortion results in an energy advantage of -14.1 and -9.0 meV per formula unit, respectively. However, the application of chemical pressure to form Ca3 Ir 4 Sn 13 reduces the energetic advantage of these instabilities, which is completely removed by the application of a hydrostatic pressure of 35 kbar to Ca3 Ir 4 Sn 13. The evolution of these lattice instabilities is consistent with the experimental phase diagram. The structural distortion associated with the mode at q=(0.5,0.5,0) produces a distorted cell with the same space-group symmetry as the experimentally refined low-temperature structure. Furthermore, calculation of the deformation potential due to these modes quantitatively demonstrates a strong electron-phonon coupling. Therefore, these modes are likely to be implicated in the structural transition and superconductivity of this system.

Experimental Determination of the Fermi Surface of Sr3Ir4Sn13

The stannide family of materials

Experimental determination of the Fermi surface ofSr3Ir4Sn13

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Atomistic Exploration of the Surface-Sensitive Oriented Attachment Growth of a-MnCh Nanowires and the Formation of Defective Interface with 2×3 and 2×4 Tunnel Intergrowth

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Electrochemistry of hollandite α-MnO2:Li-ion and Na-ion insertion and Li2O incorporation

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Nanostructuring of β-MnO2:The important role of surface to bulk ion migration

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