Dean C. Sayle

Converting Ceria Polyhedral Nanoparticles into Single-Crystal Nanospheres

Ceria nanoparticles are one of the key abrasive materials for chemical-mechanical planarization of advanced integrated circuits. However, ceria nanoparticles synthesized by existing techniques are irregularly faceted, and they scratch the silicon wafers and increase defect concentrations. We developed an approach for large-scale synthesis of single-crystal ceria nanospheres that can reduce the polishing defects by 80% and increase the silica removal rate by 50%, facilitating precise and reliable mass-manufacturing of chips for nanoelectronics. We doped the ceria system with titanium, using flame temperatures that facilitate crystallization of the ceria yet retain the titania in a molten state. In conjunction with molecular dynamics simulation, we show that under these conditions, the inner ceria core evolves in a single-crystal spherical shape without faceting, because throughout the crystallization it is completely encapsulated by a molten 1- to 2-nanometer shell of titania that, in liquid state, minimizes the surface energy. The principle demonstrated here could be applied to other oxide systems.

The induction of angiogenesis by cerium oxide nanoparticles through the modulation of oxygen in intracellular environments

Angiogenesis is the formation of new blood vessels from existing blood vessels and is critical for many physiological and pathophysiological processes. In this study we have shown the unique property of cerium oxide nanoparticles (CNPs) to induce angiogenesis, observed using both in vitro and in vivo model systems. In particular, CNPs trigger angiogenesis by modulating the intracellular oxygen environment and stabilizing hypoxia inducing factor 1α endogenously. Furthermore, correlations between angiogenesis induction and CNPs physicochemical properties including: surface Ce(3+)/Ce(4+) ratio, surface charge, size, and shape were also explored. High surface area and increased Ce(3+)/Ce(4+) ratio make CNPs more catalytically active towards regulating intracellular oxygen, which in turn led to more robust induction of angiogenesis. Atomistic simulation was also used, in partnership with in vitro and in vivo experimentation, to reveal that the surface reactivity of CNPs and facile oxygen transport promotes pro-angiogenesis.

Self‐Assembly of Cerium Oxide Nanostructures in Ice Molds

The formation of nanorods, driven by the physicochemical phenomena during the freezing and after the aging of frozen ceria nanoparticle suspensions, is reported. During freezing of a dilute aqueous solution of CeO2 nanocrystals, some nuclei remain in solution while others are trapped inside micro- and nanometer voids formed within the growing ice front. Over time (2-3 weeks) the particles trapped within the nanometer-wide voids in the ice combine by an oriented attachment process to form ceria nanorods. The experimental observations are consistent with molecular dynamics simulations of particle aggregation in constrained environments. These observations suggest a possible strategy for the templated formation of nanostructures through self-assembly by exploiting natural phenomena, such as voids formed during freezing of water. This research suggests a very simple, green chemical route to guide the formation of one- and three-dimensional self-assembled nanostructures.

Ionic conductivity in nano-scale CeO2/YSZ heterolayers

CeO2 based materials are promising candidates as solid oxide electrolytes within fuel cell systems. In this capacity, the oxygen anion conductivity is pivotal. Sata et al. [Nature, 2000, 408, 946–949] demonstrated the ability to ‘fine tune’ conductivities in BaF2 and CaF2 by generating BaF2/CaF2 heterolayers with different nanoscale film thicknesses. The resulting fluoride ion conductivities were found to be orders of magnitude higher compared with the component BaF2 and CaF2 materials. Similarly, it may be possible to fabricate CeO2 thin films with tuneable conductivities. In this study, we explore this possibility using atomistic simulation. In particular, simulated amorphisation and recrystallisation was used to generate an atomistic model for a CeO2/YSZ (yttrium stabilised zirconia) heterolayered system and, using this model, the ionic diffusivity, conductivity and associated activation energy barriers were calculated. However, in contrast to the BaF2/CaF2 system, the heterolayered CeO2/YSZ system did not exhibit exemplary transport properties compared with the parent materials. This study describes a framework simulation procedure, which can be used in partnership with experiment, to explore a variety of microstructural features that may facilitate an increase in the ionic conductivity of heterolayered systems.

Oxygen transport in unreduced, reduced and Rh(III)-doped CeO2 nanocrystals

Ceria, CeO2, based materials are a major (active) component of exhaust catalysts and promising candidates for solid oxide fuel cells. In this capacity, oxygen transport through the material is pivotal. Here, we explore whether oxygen transport is influenced (desirably increased) compared with transport within the bulk parent material by traversing to the nanoscale. In particular, atomistic models for ceria nanocrystals, including perfect: CeO2; reduced: CeO1.95 and doped: Rh0.1Ce0.9O1.95, have been generated. The nanocrystals were about 8 nm in diameter and each comprised about 16,000 atoms. Oxygen transport can also be influenced, sometimes profoundly, by microstructural features such as dislocations and grain-boundaries. However, these are difficult to generate within an atomistic model using, for example, symmetry operations. Accordingly, we crystallised the nanocrystals from an amorphous precursor, which facilitated the evolution of a variety of microstructures including: twin-boundaries and more general grain-boundaries and grain-junctions, dislocations and epitaxy, isolated and associated point defects. The shapes of the nanocrystals are in accord with HRTEM data and comprise octahedral morphologies with {111} surfaces, truncated by (dipolar) {100} surfaces together with a complex array of steps, edges and corners. Oxygen transport data was then calculated using these models and compared with data calculated previously for CeO1.97/ YSZ thin films and the (bulk) parent material, CeO197. Oxygen transport was calculated to increase in the order: CeO2 nanocrystal < (reduced) CeO1.95 nanocrystal approximately Rh0.1Ce0.9O1.95 nanocrystal < CeO1.97/YSZ thin film < (reduced) CeO1.97 (bulk) parent material; the mechanism was determined to be primarily vacancy driven. Our findings indicate that reducing one- (thin film) or especially three- (nanocrystal) dimensions to the nanoscale may prove deleterious to oxygen transport. Conversely, we observed dynamic evolution and annihilation of surface vacancies via surface oxygens migrating to the bulk of the nanocrystal; the vacancies left are then filled by other oxygens moving to the surface. Coupled with previous simulation studies, in which we calculated that oxygen extraction from the surface of a ceria nanocrystal was energetically easier compared with the bulk surface, our calculations predict that ceria nanocrystals would facilitate effective oxidative catalysis. This study describes framework simulation procedures, which can be used in partnership with experiment, to explore transport in nanocrystalline ionic systems, which include complex microstructures. Such data can provide predictions for experiment or help reduce the number of experiments required.

Application of molecular dynamics DL_POLY codes to interfaces of inorganic materials

Three recent applications of the DL_POLY molecular dynamics code are described, which demonstrate the flexibility and viability of the code for extending our understanding of the structure, stability and reactivity of ceramics and minerals at the atomic level. The first is an investigation into differences in oxygen atom mobility in bulk and at the most stable {111} surface of ceria. The results show enhanced surface transport but that it is via subsurface oxygen. Secondly, we investigate how polychloro-dibenzo-pdioxins (PCDDs) molecules might adsorb on clay surfaces. The resulting adsorption energies show a clear relationship with chlorine content of the molecule. Finally, we apply DL_POLY to comparing the aggregation of magnesium oxide and calcium carbonate nanoparticles. We find that very small calcium carbonate nanoparticles are amorphous and their aggregation shows no preferred orientation in contrast to magnesium oxide, which remain highly crystalline and combine in a highly structural specific way.

Elastic Deformation in Ceria Nanorods via a Fluorite-to-Rutile Phase Transition

Atomistic simulations reveal that ceria nanorods, under uniaxial tension, can accommodate over 6% elastic deformation. Moreover, a reversible fluorite-to-rutile phase change occurs above 6% strain for a ceria nanorod that extends along [110]. We also observe that during unloading the stress increases with decreasing strain as the rutile reverts back to fluorite. Ceria nanorods may find possible application as vehicles for elastic energy storage.

Generating structural distributions of atomistic models of Li2O nanoparticles using simulated crystallisation

Simulated crystallisation has been used to predict that Li2O nanoparticles comprise octahedral morphologies bounded by {111} and truncated by {100} with inverse fluorite crystal structure. We observe that by changing the temperature of the (simulated) crystallisation, changes in the microstructure can be realised, such a strategy facilitates the generation of full atomistic models with microstructural distributions similar to the structural diversity observed synthetically.

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 oxide nanoparticles were constructed using simulated amorphisation and crystallisation and used to populate a library of oxide nanoparticles (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 He transport in UO(2), which is an important material with respect to both civilian and military applications. We found that He transport proceeds much faster through the grain-boundary and grain-junction network compared with intracrystalline UO(2) regions, in accordance with experiment.