The impact of tilt grain boundaries on the thermal transport in perovskite SrTiO3 layered nanostructures. A computational study

Stacking of interfaces at different length-scales affect the lattice thermal conductivity of strontium titanate layered nanostructures improving their thermoelectric performance

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 <i>c</i>-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

Structure and Properties of Some Layered U₂O₅ Phases: A Density Functional Theory Study

U₂O₅ is the boundary composition between the fluorite and the layered structures of the UO_2→3 system and the least studied oxide in the group. δ-U₂O₅ is the only layered structure proposed so far experimentally, although evidence of fluorite-based phases has also been reported. Our DFT work explores possible structures of U₂O₅ stoichiometry by starting from existing M₂O₅ structures (where M is an actinide or transition metal) and replacing the M ions with uranium ions. For all structures, we predicted structural and electronic properties including bulk moduli and band gaps. The majority of structures were found to be less stable than δ-U₂O₅. U₂O₅ in the R-Nb₂O₅ structure was found to be a competitive structure in terms of stability, whereas U₂O₅ in the Np₂O₅ structure was found to be the most stable overall. Indeed, by including the vibrational contribution to the free energy using the frequencies obtained from the optimized unit cells we predict that Np₂O₅ structured U₂O₅ is the most thermodynamically stable under ambient conditions. δ-U₂O₅ only becomes more stable at high temperatures and/or pressures. This suggests that a low-temperature synthesis route should be tested and so potentially opens a new avenue of research for pentavalent uranium oxides

Water Adsorption on AnO₂ {111}, {110}, and {100} Surfaces (An = U and Pu): A Density Functional Theory + <i>U</i> Study

The interactions between water and the actinide oxides UO₂ and PuO₂ are important both fundamentally and when considering the long-term storage of spent nuclear fuel. However, experimental studies in this area are severely limited by the intense radioactivity of plutonium, and hence, we have recently begun to investigate these interactions computationally. In this paper, we report the results of plane-wave density functional theory calculations of the interaction of water with the {111}, {110}, and {100} surfaces of UO₂ and PuO₂, using a Hubbard-corrected potential (PBE + <i>U</i>) approach to account for the strongly correlated 5f electrons. We find a mix of molecular and dissociative water adsorption to be most stable on the {111} surface, whereas the fully dissociative water adsorption is most stable on the {110} and {100} surfaces, leading to a fully hydroxylated monolayer. From these results, we derive water desorption temperatures at various pressures for the different surfaces. These increase in the order {111} < {110} < {100}, and these data are used to propose an alternative interpretation for the two experimentally determined temperature ranges for water desorption from PuO₂

Ab Initio Investigation of the UO₃ Polymorphs: Structural Properties and Thermodynamic Stability

Uranium trioxide (UO₃) is known to adopt a variety of crystalline and amorphous phases. Here we applied the Perdew–Burke–Ernzerhof functional + U formalism to predict structural, electronic, and elastic properties of five experimentally determined UO₃ polymorphs, in addition to their relative stability. The simulations reveal that the methodology is well-suited to describe the different polymorphs. We found better agreement with experiment for simpler phases where all bonds are similar (α- and δ-), while some differences are seen for those with more complex bonding (β-, γ-, and η-), which we address in terms of the disorder and defective nature of the experimental samples. Our calculations also predict the presence of uranyl bonds to affect the elastic and electronic properties. Phases containing uranyl bonds tend to have smaller band gaps and bulk moduli under 100 GPa contrary to those without uranyl bonds, which have larger band gaps and bulk moduli greater than 150 GPa. In line with experimental observations, we predict the most thermodynamically stable polymorph as γ-UO₃, the least stable as α-UO₃, and the most stable at high pressure as η-UO₃

Toward Modeling Clay Mineral Nanoparticles: The Edge Surfaces of Pyrophyllite and Their Interaction with Water

The basal surfaces of phyllosilicate minerals have been widely studied, whereas the edge surfaces have received little attention. However, in order to simulate complete clay particles at the atomic level, the modeling of edge surfaces becomes crucially important, and such surfaces are likely to be far more active. We used a combination of quantum and potential based techniques to evaluate the structure of the edge surfaces of pyrophyllite and their interaction in an aqueous environment. These include {110}, {100}, {010}, {1̅10}, {130}, and {1̅30}. We found that the CLAYFF force field is an effective model for reproducing the DFT results. Furthermore, the results show that, for this notorious natural hydrophobic clay, all edge surfaces show hydrophilic behavior and that the precise structure of water above these surfaces is influenced by both the presence of hydroxyl groups and under-coordinated surface Al atoms; this will impact both geological processes where natural clays are involved and processes where such clays act as primary retention barriers to the dispersion of contaminants

The Shape of TiO₂‑B Nanoparticles

The shape of nanoparticles can be important in defining their properties. Establishing the exact shape of particles is a challenging task when the particles tend to agglomerate and their size is just a few nanometers. Here we report a structure refinement procedure for establishing the shape of nanoparticles using powder diffraction data. The method utilizes the fundamental formula of Debye coupled with a Monte Carlo-based optimization and has been successfully applied to TiO₂-B nanoparticles. Atomistic modeling and molecular dynamics simulations of ensembles of all the ions in the nanoparticle reveal surface hydroxylation as the underlying reason for the established shape and structural features

Cationic Surface Reconstructions on Cerium Oxide Nanocrystals: An Aberration-Corrected HRTEM Study

Instabilities of nanoscale ceria surface facets are examined on the atomic level. The electron beam and its induced atom migration are proposed as a readily available probe to emulate and quantify functional surface activity, which is crucial for, for example, catalytic performance. <i>In situ</i> phase contrast high-resolution transmission electron microscopy with spherical aberration correction is shown to be the ideal tool to analyze cationic reconstruction. Hydrothermally prepared ceria nanoparticles with particularly enhanced {100} surface exposure are explored. Experimental analysis of cationic reconstruction is supported by molecular dynamics simulations where the Madelung energy is shown to be directly related to the binding energy, which enables one to generate a visual representation of the distribution of “reactive” surface oxygen

Cationic Surface Reconstructions on Cerium Oxide Nanocrystals: An Aberration-Corrected HRTEM Study

Instabilities of nanoscale ceria surface facets are examined on the atomic level. The electron beam and its induced atom migration are proposed as a readily available probe to emulate and quantify functional surface activity, which is crucial for, for example, catalytic performance. <i>In situ</i> phase contrast high-resolution transmission electron microscopy with spherical aberration correction is shown to be the ideal tool to analyze cationic reconstruction. Hydrothermally prepared ceria nanoparticles with particularly enhanced {100} surface exposure are explored. Experimental analysis of cationic reconstruction is supported by molecular dynamics simulations where the Madelung energy is shown to be directly related to the binding energy, which enables one to generate a visual representation of the distribution of “reactive” surface oxygen

Strain and Architecture-Tuned Reactivity in Ceria Nanostructures; Enhanced Catalytic Oxidation of CO to CO₂

Atomistic simulations reveal that the chemical reactivity of ceria nanorods is increased when tensioned and reduced when compressed promising strain-tunable reactivity; the reactivity is determined by calculating the energy required to oxidize CO to CO₂ by extracting oxygen from the surface of the nanorod. Visual reactivity “fingerprints”, where surface oxygens are colored according to calculated chemical reactivity, are presented for ceria nanomaterials including: nanoparticles, nanorods, and mesoporous architectures. The images reveal directly how the nanoarchitecture (size, shape, channel curvature, morphology) and microstructure (dislocations, grain-boundaries) influences chemical reactivity. We show the generality of the approach, and its relevance to a variety of important processes and applications, by using the method to help understand: TiO₂ nanoparticles (photocatalysis), mesoporous ZnS (semiconductor band gap engineering), MgO (catalysis), CeO₂/YSZ interfaces (strained thin films; solid oxide fuel cells/nanoionics), and Li-MnO₂ (lithiation induced strain; energy storage)