Liquid crystal seed nucleates liquid–solid phase change in ceria nanoparticles

Molecular dynamics (MD) simulation was used to explore the liquid–solid (crystal) phase change of a ceria nanoparticle. The simulations reveal that the crystalline seed, which spontaneously evolves and nucleates crystallisation, is a liquid rather than a solid. Evidence supporting this concept includes: (a) only 3% of the total latent heat of solidification had been liberated after 25% of the nanoparticle had (visibly) crystallised. (b) Cerium ions, comprising the (liquid) crystal seed had the same mobility as cerium ions comprising the amorphous regions. (c) Cerium ion mobility only started to reduce (indicative of solidification) after 25% of the nanoparticle had crystallised. (d) Calculated radial distribution functions (RDF) revealed no long-range structure when 25% of the nanoparticle had (visibly) crystallised. We present evidence that the concept of a liquid crystal seed is more general phenomenon rather than applicable only to nanoceria

Accommodation of the misfit strain energy in the BaO(100)/MgO(100) heteroepitaxial ceramic interface using computer simulation techniques

Static atomistic simulation techniques have been employed to investigate the accommodation of the misfit strain energy in the BaO(100)/MgO(100) interface. The materials return to their natural (bulk) lattice parameters a few planes from the interface, while maintaining expanded or contracted lattice parameters at the interface to ensure charge matching of counter ions. BaO also forms three-dimensional islands when grown on MgO(100), in accordance with molecular beam epitaxy results. This behaviour is attributed to the instability of a monatomic BaO layer on MgO compared with a BaO bilayer

Simulation study of copper(I) and copper(II) species in ZSM-5 zeolite

Low energy configurations of CuI and CuII species in the ZSM-5 zeolite, probed by energy minimisation techniques, are found to be bound strongly to framework aluminium or copper species

Ab Initio Calculation of the Lattice Distortions induced by Substitutional Ag- and Cu- Impurities in Alkali Halide Crystals

An ab initio study of the doping of alkali halide crystals (AX: A = Li, Na, K, Rb; X = F, Cl, Br, I) by ns2 anions (Ag- and Cu-) is presented. Large active clusters with 179 ions embedded in the surrounding crystalline lattice are considered in order to describe properly the lattice relaxation induced by the introduction of substitutional impurities. In all the cases considered, the lattice distortions imply the concerted movement of several shells of neighbors. The shell displacements are smaller for the smaller anion Cu-, as expected. The study of the family of rock-salt alkali halides (excepting CsF) allows us to extract trends that might be useful at a predictive level in the study of other impurity systems. Those trends are presented and discussed in terms of simple geometric arguments.Comment: LaTeX file. 8 pages, 3 EPS pictures. New version contains calculations of the energy of formation of the defects with model clusters of different size

High-pressure crystallisation of TiO2 nanoparticles

The full atomistic structure of a TiO2 nanocrystal, about 7 nm in diameter and comprising 16,000 atoms, has been generated using simulated melting and crystallisation, performed under high-pressure. Specifically, the nanoparticle was heated to 6000 K after which the molten nanoparticle was crystallised at 2000 K under 20 GPa pressure. The resulting nanocrystal comprises rutile- and alpha-PbO2-structured domains (alpha-PbO2 has been identified experimentally as a high-pressure phase of TiO2) expresses (111), (010), (001), and (110) surfaces facilitating a polyhedral morphology and includes grain-boundaries and grain- junction. Molecular graphics images of the various microstructural features are presented together with snapshots of the crystallisation

Nanopolycrystalline materials; A general atomistic model for simulation

We present a general strategy for generating full atomistic models of nanopolycrystalline materials including bulk and thin film. In particular, models for oxidenanoparticles were constructed using simulated amorphisation and crystallisation and used to populate a library of oxidenanoparticles (amorphous and crystalline) with different radii. Nanoparticles were then taken from this library and positioned, within a specific volume, using Monte Carlo techniques, to facilitate a tight-packed structure. The grain-size distribution of the polycrystalline material was controlled by selecting particular sized nanoparticles from the library. The (randomly oriented) grains facilitated a polycrystalline oxide, which comprised a network of general grain-boundaries. To help validate the model, gas diffusion through the (polycrystalline) oxide material was then simulated and the activation energy calculated directly. Specifically, we explored Hetransport in UO2, which is an important material with respect to both civilian and military applications. We found that Hetransport proceeds much faster through the grain-boundary and grain-junction network compared with intracrystalline UO2 regions, in accordance with experiment

Atomistic Models and Molecular Dynamics

Here we show how atomistic computer simulation can help experiment unravel the rich structuralcomplexity of oxide nanomaterials and, ultimately, aid the fabrication of nanomaterials withimproved, tuneable or indeed new properties. We first explore the simulation methodologies:energy minimisation, monte-carlo, genetic algorithms and molecular dynamics together with thepotential models used to describe the interactions between metal and oxide ions. These tools can beused to generate realistic structures that include all the essential microstructural features observedexperimentally, such as surface structure (morphology, surface energy, faceting, surface steps,corners and edges), grain-boundaries and dislocations, intrinsic and extrinsic point defects andepitaxy. We show how the theoretician is able to capture all these (experimentally observed)structural details by attempting to simulate crystallisation. Equipped with realistic models,important properties can be calculated, including: electronic, chemical (catalytic activity, ionicdiffusion and conductivity) and mechanical (hardness, elastic constants). This is illustrated bycalculating the ease of oxygen extraction from the surface of a CeO2 nanocrystal compared with thebulk parent material with implications for oxidative catalysis. Throughout this chapter weemphasise the importance of molecular graphics - a much maligned and underrated tool - butwithout which, the generation of much of the simulation and experimental data would not havebeen possible

Visualizing The Enhanced Chemical Reactivity of Mesoporous Ceria; Simulating Templated Crystallization in Silica Scaffolds at the Atomic Level

Unique physical, chemical, and mechanical properties can be engineered into functional nanomaterials via structural control. However, as the hierarchical structural complexity of a nanomaterial increases, so do the challenges associated with generating atomistic models, which are sufficiently realistic that they can be interrogated to reliably predict properties and processes. The structural complexity of a functional nanomaterial necessarily emanates during synthesis. Accordingly, to capture such complexity, we have simulated each step in the synthetic protocol. Specifically, atomistic models of mesoporous ceria were generated by simulating the infusion and confined crystallization of ceria in a mesoporous silica scaffold. After removing the scaffold, the chemical reactivity of the templated mesoporous ceria was calculated and predicted to be more reactive compared to mesoporous ceria generated without template; visual “reactivity fingerprints” are presented. The strategy affords a general method for generating atomistic models, with hierarchical structural complexity, which can be used to predict a variety of properties and processes enabling the nanoscale design of functional materials

Structure, epitaxial growth and nucleation of CaO/SrO interfaces using energy minimisation, molecular dynamics and computer graphics

Atomistic models of CaO/SrO interfaces have been constructed via the sequential deposition of CaO species to SrO surfaces in conjunction with energy minimisation and dynamics. Using this approach the nucleation and growth mechanisms of CaO on the SrO support can be explored. The calculations suggest that the surface of the SrO substrate exacts a critical influence on the structure of the thin film. In particular, surface steps and roughness lead to incoherent films which are difficult to characterise with implications for the surface preparation of support materials. Conversely, the CaO species deposited may also degrade the SrO support: both charged and charge neutral surface vacancies on the SrO surface are created and surface steps are eroded during the deposition process. The results also suggest that the lattice misfit between the two materials is accommodated via defects and dislocations within the thin film in addition to considerable structural relaxation of the overlying thin film and the support

The predicted 3-D atomistic structure of an interfacial screw-edge dislocation

Atomistic computer simulation techniques have been employed to generate an amorphous SrO thin film on an MgO(100) substrate, which, under dynamics simulation, recrystallises revealing misfit induced structural modifications including mixed screw-edge dislocations, lattice slip and twist boundaries