Origin of electrochemical activity in nano-Li2MnO3; Stabilization via a 'point defect scaffold'

Molecular dynamics (MD) simulations of the charging of Li2MnO3 reveal that the reason nanocrystalline-Li2MnO3 is electrochemically active, in contrast to the parent bulk-Li2MnO3, is because in the nanomaterial the tunnels, in which the Li ions reside, are held apart by Mn ions, which act as a pseudo 'point defect scaffold'. The Li ions are then able to diffuse, via a vacancy driven mechanism, throughout the nanomaterial in all spatial dimensions while the 'Mn defect scaffold' maintains the structural integrity of the layered structure during charging. Our findings reveal that oxides, which comprise cation disorder, can be potential candidates for electrodes in rechargeable Li-ion batteries. Moreover, we propose that the concept of a 'point defect scaffold' might manifest as a more general phenomenon, which can be exploited to engineer, for example, two or three-dimensional strain within a host material and can be fine-tuned to optimize properties, such as ionic conductivity

Environment-mediated structure, surface redox activity and reactivity of ceria nanoparticles

Nanomaterials, with potential application as bio-medicinal agents, exploit the chemical properties of a solid, with the ability to be transported (like a molecule) to a variety of bodily compartments. However, the chemical environment can change significantly the structure and hence properties of a nanomaterial. Accordingly, its surface reactivity is critically dependent upon the nature of the (biological) environment in which it resides. Here, we use Molecular Dynamics (MD) simulation, Density Functional Theory (DFT) and aberration corrected TEM to predict and rationalise differences in structure and hence surface reactivity of ceria nanoparticles in different environments. In particular we calculate reactivity 'fingerprints' for unreduced and reduced ceria nanoparticles immersed in water and in vacuum. Our simulations predict higher activities of ceria nanoparticles, towards oxygen release, when immersed in water because the water quenches the coordinative unsaturation of surface ions. Conversely, in vacuum, surface ions relax into the body of the nanoparticle to relieve coordinative unsaturation, which increases the energy barriers associated with oxygen release. Our simulations also reveal that reduced ceria nanoparticles are more active towards surface oxygen release compared to unreduced nanoceria. In parallel, experiment is used to explore the activities of ceria nanoparticles that have suffered a change in environment. In particular, we compare the ability of ceria nanoparticles, in an aqueous environment, to scavenge superoxide radicals compared to the same batch of nanoparticles, which have first been dried and then rehydrated. The latter show a distinct reduction in activity, which we correlate to a change in the redox chemistry associated with moving between different environments. The reactivity of ceria nanoparticles is therefore not only environment dependent, but is also influenced by the transport pathway or history required to reach the particular environment in which its reactivity is to be exploited. © 2013 The Royal Society of Chemistry

Mechanical properties of mesoporous ceria nanoarchitectures

Architectural constructs are engineered to impart desirable mechanical properties facilitating bridges spanning a thousand meters and buildings nearly 1 km in height. However, do the same 'engineering-rules' translate to the nanoscale, where the architectural features are less than 0.0001 mm in size? Here, we calculate the mechanical properties of a porous ceramic functional material, ceria, as a function of its nanoarchitecture using molecular dynamics simulation and predict its yield strength to be almost two orders of magnitude higher than the parent bulk material. In particular, we generate models of nanoporous ceria with either a hexagonal or cubic array of one-dimensional pores and simulate their responses to mechanical load. We find that the mechanical properties are critically dependent upon the orientation between the crystal structure (symmetry, direction) and the pore structure (symmetry, direction). This journal i

Mechanical properties of ceria nanorods and nanochains; The effect of dislocations, grain-boundaries and oriented attachment

We predict that the presence of extended defects can reduce the mechanical strength of a ceria nanorod by 70%. Conversely, the pristine material can deform near its theoretical strength limit. Specifically, atomistic models of ceria nanorods have been generated with full microstructure, including: growth direction, morphology, surface roughening (steps, edges, corners), point defects, dislocations and grain-boundaries. The models were then used to calculate the mechanical strength as a function of microstructure. Our simulations reveal that the compressive yield strengths of ceria nanorods, ca. 10 nm in diameter and without extended defects, are 46 and 36 GPa for rods oriented along [211] and [110] respectively, which represents almost 10% of the bulk elastic modulus and are associated with yield strains of about 0.09. Tensile yield strengths were calculated to be about 50% lower with associated yield strains of about 0.06. For both nanorods, plastic deformation was found to proceed via slip in the {001} plane with direction ã??110ã?? - a primary slip system for crystals with the fluorite structure. Dislocation evolution for the nanorod oriented along [110] was nucleated via a cerium vacancy present at the surface. A nanorod oriented along [321] and comprising twin-grain boundaries with {111} interfacial planes was calculated to have a yield strength of about 10 GPa (compression and tension) with the grain boundary providing the vehicle for plastic deformation, which slipped in the plane of the grain boundary, with an associated ã??110ã?? slip direction. We also predict, using a combination of atomistic simulation and DFT, that rutile-structured ceria is feasible when the crystal is placed under tension. The mechanical properties of nanochains, comprising individual ceria nanoparticles with oriented attachment and generated using simulated self-assembly, were found to be similar to those of the nanorod with grain-boundary. Images of the atom positions during tension and compression are shown, together with animations, revealing the mechanisms underpinning plastic deformation. For the nanochain, our simulations help further our understanding of how a crystallising ice front can be used to 'sculpt' ceria nanoparticles into nanorods via oriented attachment. © 2011 The Royal Society of Chemistry

Sequential measurement of δ15N, δ13C and δ34S values in archaeological bone collagen at the Scottish Universities Environmental Research Centre (SUERC): a new analytical frontier

Rationale: The use of multi‐isotopic analysis (δ15N, δ13C and δ34S values) of archaeological bone collagen to assist in the interpretation of diet, movement and mobility of prehistoric populations is gradually increasing, yet many researchers have traditionally avoided investigating sulphur due to its very low concentrations (<0.3%) in mammalian collagen. For this reason, and as a consequence of analytical detection limits, sulphur is usually measured separately from carbon and nitrogen, which leads to longer analytical times and higher costs. Methods: A Thermo ScientificTM EA IsoLinkTM IRMS system, with the ability to rapidly heat a gas‐chromatography (GC) column and concentrate the sample gas online without cryo‐trapping, was used at the Radiocarbon Laboratory at SUERC. Optimisation of the GC temperature and carrier gas flow rate in the elemental analyser resulted in improved signal‐to‐noise ratio and sensitivity for SO2. This allowed for routine sequential N2, CO2 and SO2 measurements on small samples of bone collagen. Results: Improvements in sample gas transfer to the mass spectrometer allows for sequential δ15N, δ13C and δ34S values to be measured in 1–1.5 mg samples of bone collagen. Moreover, the sensitivity and signal‐to‐noise ratio of the sample gas, especially SO2, is improved, resulting in precisions of ±0.15‰ for δ15N values, ±0.1‰ for δ13C values and ±0.3‰ for δ34S values. Previous instrumentation allowed for the analysis of ~30 unknown samples before undertaking maintenance; however, ~150 unknown samples can now be measured, meaning a 5‐fold increase in sample throughput. Conclusions: The ability to sequentially measure δ15N, δ13C and δ34S values rapidly in archaeological bone collagen is an attractive option to researchers who want to build larger, more succinct datasets for their sites of interest, at a much‐reduced analytical cost and without destroying larger quantities of archaeological material

The Structure of Surface Entrance Sites for Li-intercalation into TiO2Nanoparticles, Nanosheets and Mesoporous Architectures with Application for Li-ion Batteries

Power output is central to the viability of a Li-ion battery, and is, in part, dependent upon the activation energy barrier associated with Li intercalation/deintercalation into the host lattice (electrode). The lower the energy barrier, the faster the intercalation reaction rate and greater the power. The activation energy is governed by the atomistic structure(s) of the entrance sites for Li intercalation. Accordingly, a first step in optimising battery power via structural manipulation of entrance sites, is to understand the structure of these entrance sites. However, HRTEM is (presently) unable to characterise the structures of entrance sites with atomistic resolution. Accordingly, we generate models of the entrance sites using Molecular Dynamics. In particular, we simulate the synthetic protocol used to fabricate nanostructured TiO2 experimentally. The resulting atomistic models reveal a highly complex and diverse structural distribution of entrance sites, which emanate from the surface curvature of the nanostructured material. In particular, we show how nanostructuring can be used to change profoundly the nature and concentration of such entrance sites

Is Geometric Frustration-Induced Disorder a Recipe for High Ionic Conductivity?

Ionic conductivity is ubiquitous to many industrially important applications such as fuel cells, batteries, sensors, and catalysis. Tunable conductivity in these systems is therefore key to their commercial viability. Here, we show that geometric frustration can be exploited as a vehicle for conductivity tuning. In particular, we imposed geometric frustration upon a prototypical system, CaF2, by ball milling it with BaF2, to create nanostructured Ba1–xCaxF2 solid solutions and increased its ionic conductivity by over 5 orders of magnitude. By mirroring each experiment with MD simulation, including “simulating synthesis”, we reveal that geometric frustration confers, on a system at ambient temperature, structural and dynamical attributes that are typically associated with heating a material above its superionic transition temperature. These include structural disorder, excess volume, pseudovacancy arrays, and collective transport mechanisms; we show that the excess volume correlates with ionic conductivity for the Ba1–xCaxF2 system. We also present evidence that geometric frustration-induced conductivity is a general phenomenon, which may help explain the high ionic conductivity in doped fluorite-structured oxides such as ceria and zirconia, with application for solid oxide fuel cells. A review on geometric frustration [ Nature 2015, 521, 303] remarks that classical crystallography is inadequate to describe systems with correlated disorder, but that correlated disorder has clear crystallographic signatures. Here, we identify two possible crystallographic signatures of geometric frustration: excess volume and correlated “snake-like” ionic transport; the latter infers correlated disorder. In particular, as one ion in the chain moves, all the other (correlated) ions in the chain move simultaneously. Critically, our simulations reveal snake-like chains, over 40 Å in length, which indicates long-range correlation in our disordered systems. Similarly, collective transport in glassy materials is well documented [for example, J. Chem. Phys. 2013, 138, 12A538]. Possible crystallographic nomenclatures, to be used to describe long-range order in disordered systems, may include, for example, the shape, length, and branching of the “snake” arrays. Such characterizations may ultimately provide insight and differences between long-range order in disordered, amorphous, or liquid states and processes such as ionic conductivity, melting, and crystallization

Amorphisation and recrystallisation study of lithium intercalation into TiO 2 nano-architecture.

Titanium dioxide is playing an increasingly significant role in easing environmental and energy concerns. Its rich variety of polymorphic crystal structures has facilitated a wide range of applications such as photo-catalysis, photo-splitting of water, photoelectrochromic devices, insulators in metal oxide, semiconductors devices, dye sensitized solar cells (DSSCs) (energy conversions), rechargeable lithium batteries (electrochemical storage). The complex structural aspects in nano TiO 2 , are elucidated by microscopic visualization and quantification of the microstructure for electrode materials, since cell performance and various aging mechanisms depend strongly on the appearance and changes in the microstructure. Recent studies on MnO 2 have demonstrated that amorphisation and recrystallisation simulation method can adequately generate various nanostructures, for Li-ion battery compounds. The method was also previously employed to produce nano-TiO 2 . In the current study, the approach is used to study lithiated nanoporous structure for TiO 2 which have been extensively studied experimentally, as mentioned above. Molecular graphic images showing microstructural features, including voids and channels have accommodated lithium’s during lithiation and delithiation. Preliminary lithiation of TiO 2 will be considered

Deciphering diet and monitoring movement: multiple stable isotope analysis of the Viking Age settlement at Hofstaðir, Lake Mývatn, Iceland

Objectives: A previous multi-isotope study of archaeological faunal samples from Skútustaðir, an early Viking age settlement on the southern shores of Lake Mývatn in north-east Iceland, demonstrated that there are clear differences in δ34S stable isotope values between animals deriving their dietary protein from terrestrial, freshwater, and marine reservoirs. The aim of this study was to use this information to more accurately determine the diet of humans excavated from a nearby late Viking age churchyard. Materials and Methods: δ13C, δ15N, and δ34S analyses were undertaken on terrestrial animal (n = 39) and human (n = 46) bone collagen from Hofstaðir, a high-status Viking-period farmstead ∼10 km north-west of Skútustaðir. Results: δ34S values for Hofstaðir herbivores were ∼6‰ higher relative to those from Skútustaðir (δ34S: 11.4 ± 2.3‰ versus 5.6 ± 2.8‰), while human δ13C, δ15N, and δ34S values were broad ranging (−20.2‰ to −17.3‰, 7.4‰ to 12.3‰, and 5.5‰ to 14.9‰, respectively). Discussion: Results suggest that the baseline δ34S value for the Mývatn region is higher than previously predicted due to a possible sea-spray effect, but the massive deposition of Tanytarsus gracilentus (midges) (δ34S: −3.9‰) in the soil in the immediate vicinity of the lake is potentially lowering this value. Several terrestrial herbivores displayed higher bone collagen δ34S values than their contemporaries, suggesting trade and/or movement of animals to the region from coastal areas. Broad ranging δ13C, δ15N, and δ34S values for humans suggest the population were consuming varied diets, while outliers within the dataset could conceivably have been migrants to the area

Structure-Activity Map of Ceria Nanoparticles, Nanocubes and Mesoporous Architectures

Structure-activity mapping is central to the exploitation and optimisation of nanomaterial catalysts in a variety of technologically important heterogeneous reactions, such as automotive catalysis and water gas shift reactions. Here, we present a catalytic activity map for nanoceria, calculated as a function of shape, size, architecture and defect content, using atom-level models. The activity map reveals that as oxygen is gradually depleted from the nanoceria catalyst, so it becomes energetically more difficult to extract further oxygen. We propose that the oxygen storage capacity (OSC) of ceria corresponds to the level of oxygen depletion where it becomes thermodynamically prohibitive to extract further oxygen from the material (positive free energy). Moreover, because the reaction enthalpy contributes to the free energy, we predict that the OSC is influenced by the particular reaction being performed. Specifically, the more negative the reaction enthalpy, the higher the potential OSC (notwithstanding entropic contributions). The decrease in catalytic activity during an oxidation reaction - emanating from the increase in energy required to extract oxygen - suggests that there exists a ‘window of catalytic operation’, where the activity of the catalyst can be controlled by operating at different points within this window. We show experimentally, how the activity can be modified by engineering the oxygen vacancy concentration and hence the oxygen content of the catalyst to facility tunable activity. In addition to the defect content, we find that size (particle diameter, mesoporous wall thickness) and nanostructuring (particle, cube, mesoporous architecture, morphology and surfaces exposed) are key drivers of catalytic activity. To generate the atom-level models of ceria nanostructures, we use non-equilibrium Molecular Dynamics to simulate the self-assembly of mesoporous ceria from amorphous nano-building blocks, followed by a (simulated) crystallisation step; the latter evolves the crystal structure and microstructural features such as grain-boundaries and dislocations. Our simulated crystallisations emanate wholly from a multitude of ‘random’ atom collisions, which result in the spontaneous evolution of a crystalline seed that nucleates crystallisation of the whole system. The atomistic models generated by ‘simulating synthesis’ are shown to be in quantitative structural agreement with experiment