Three-dimensional imaging of dislocation dynamics during the hydriding phase transformation.

Crystallographic imperfections significantly alter material properties and their response to external stimuli, including solute-induced phase transformations. Despite recent progress in imaging defects using electron and X-ray techniques, in situ three-dimensional imaging of defect dynamics remains challenging. Here, we use Bragg coherent diffractive imaging to image defects during the hydriding phase transformation of palladium nanocrystals. During constant-pressure experiments we observe that the phase transformation begins after dislocation nucleation close to the phase boundary in particles larger than 300 nm. The three-dimensional phase morphology suggests that the hydrogen-rich phase is more similar to a spherical cap on the hydrogen-poor phase than to the core-shell model commonly assumed. We substantiate this using three-dimensional phase field modelling, demonstrating how phase morphology affects the critical size for dislocation nucleation. Our results reveal how particle size and phase morphology affects transformations in the PdH system.

Avalanching strain dynamics during the hydriding phase transformation in individual palladium nanoparticles.

Phase transitions in reactive environments are crucially important in energy and information storage, catalysis and sensors. Nanostructuring active particles can yield faster charging/discharging kinetics, increased lifespan and record catalytic activities. However, establishing the causal link between structure and function is challenging for nanoparticles, as ensemble measurements convolve intrinsic single-particle properties with sample diversity. Here we study the hydriding phase transformation in individual palladium nanocubes in situ using coherent X-ray diffractive imaging. The phase transformation dynamics, which involve the nucleation and propagation of a hydrogen-rich region, are dependent on absolute time (aging) and involve intermittent dynamics (avalanching). A hydrogen-rich surface layer dominates the crystal strain in the hydrogen-poor phase, while strain inversion occurs at the cube corners in the hydrogen-rich phase. A three-dimensional phase-field model is used to interpret the experimental results. Our experimental and theoretical approach provides a general framework for designing and optimizing phase transformations for single nanocrystals in reactive environments.

Total-reflection inelastic X-ray scattering from a 10-nm thick La0.6Sr0.4CoO3 thin film.

To study equilibrium changes in composition, valence, and electronic structure near the surface and into the bulk, we demonstrate the use of a new approach, total-reflection inelastic x-ray scattering, as a sub-keV spectroscopy capable of depth profiling chemical changes in thin films with nanometer resolution. By comparing data acquired under total x-ray reflection and penetrating conditions, we are able to separate the O K-edge spectra from a 10 nm La0.6Sr0.4CoO3 thin film from that of the underlying SrTiO3 substrate. With a smaller wavelength probe than comparable soft x-ray absorption measurements, we also describe the ability to easily access dipole-forbidden final states, using the dramatic evolution of the La N4,5 edge with momentum transfer as an example.

Comparative density functional study of methanol decomposition on Cu4 and Co4 clusters.

A density functional theory study of the decomposition of methanol on Cu(4) and Co(4) clusters is presented. The reaction intermediates and activation barriers have been determined for reaction steps to form H(2) and CO. For both clusters, methanol decomposition initiated by C-H and O-H bond breaking was investigated. In the case of a Cu(4) cluster, methanol dehydrogenation through hydroxymethyl (CH(2)OH), hydroxymethylene (CHOH), formyl (CHO), and carbon monoxide (CO) is found to be slightly more favorable. For a Co(4) cluster, the dehydrogenation pathway through methoxy (CH(3)O) and formaldehyde (CH(2)O) is slightly more favorable. Each of these pathways results in formation of CO and H(2). The Co cluster pathway is very favorable thermodynamically and kinetically for dehydrogenation. However, since CO binds strongly, it is likely to poison methanol decomposition to H(2) and CO at low temperatures. In contrast, for the Cu cluster, CO poisoning is not likely to be a problem since it does not bind strongly, but the dehydrogenation steps are not energetically favorable. Pathways involving C-O bond cleavage are even less energetically favorable. The results are compared to our previous study of methanol decomposition on Pd(4) and Pd(8) clusters. Finally, all reaction energy changes and transition state energies, including those for the Pd clusters, are related in a linear, Brønsted-Evans-Polanyi plot.

Carbon ad-dimer defects in carbon nanotubes.

The adsorption of carbon dimers on carbon nanotubes leads to a rich spectrum of structures and electronic structure modifications. Barriers for the formation of carbon dimer induced defects are calculated and found to be considerably lower than those for the Stone-Wales defect. The electronic states introduced by the ad-dimers depend on defect structure and tube type and size. Multiple carbon ad-dimers provide a route to structural engineering of patterned tubes that may be of interest for nanoelectronics.

Quantum chemical study of mechanisms for oxidative dehydrogenation of propane on vanadium oxide.

We have carried out a hybrid density functional study of mechanisms for oxidative dehydrogenation of propane on the (010) surface of V2O5. The surface was modeled using both vanadium oxide clusters and a periodic slab. We have investigated a Mars-van Krevelen mechanism that involves stepwise adsorption of the propane at an oxygen site followed by desorption of a water molecule and propene, and subsequent adsorption of an oxygen molecule to complete the catalytic cycle. The potential energy surface is found to have large barriers, which are lowered somewhat when the possibility of a triplet state is considered. The barriers for propane adsorption and propene elimination are 45-60 kcal/mol. The highest energy on the potential energy surface at the B3LYP/6-31G* level of theory is about 80 kcal/mol above the energy of the reactants and corresponds to formation of an oxygen vacancy after water elimination. Subsequent addition of an oxygen molecule to fill the vacancy is predicted to be energetically downhill. The reactions of propane at a bridging oxygen site and at a vanadyl site have similar energetics. The key results of the cluster calculations are confirmed by periodic calculations. Factors that may lower the barriers on the potential energy surface, including the interaction of vanadium oxide clusters with a support material and a concerted reaction with O2, are discussed.

Modeling the Morphology and Phase Stability of TiO2 Nanocrystals in Water.

The potential of titanium dioxide nanoparticles for advanced photochemical applications has prompted a number of studies to analyze the size, phase, and morphology dependent properties. Previously we have used a thermodynamic model of nanoparticles as a function of size and shape to predict the phase stability of titanium dioxide nanoparticles, with particular attention given to the crossover of stability between the anatase and rutile phases. This work has now been extended to titanium dioxide nanoparticles in water, to examine the effects of various adsorption configurations on the equilibrium shape and the phase transition. Density functional calculations have been used to accurately determine surface energies and surface tension of low index hydrated stoichiometric surfaces of anatase and rutile, which are presented along with a brief outline of the surface structure. We have shown that morphology of TiO2 nanocrystals is affected by the presence of water, resulting in variations in the size of the (001) and (001̄) truncation facets in anatase, and a reduction in the aspect ratio of rutile nanocrystals. Our results also highlight that the consideration of hydrated nanocrystal surfaces is necessary to accurately predict the correct size dependence of the anatase to rutile phase transition.

A model for the phase stability of arbitrary nanoparticles as a function of size and shape.

A thermodynamic model describing relative stability of different shapes for nanoparticles as a function of their size was developed for arbitrary crystalline solids and applied to group IV semiconductors. The model makes use of various surface, edge and corner energies, and takes into account surface tension. Approximations and importance of each term of the model were analyzed. The predictions for clean and hydrogenated diamond nanoparticles are compared to explicitly calculated density functional results. It is shown that diamond nanocrystal morphology is markedly different from silicon and germanium.

Charge distribution and stability of charged carbon nanotubes.

Density-functional calculations of charge distribution on negatively and positively charged nanotubes result in charge density profiles characterized by a significant increase of charge density at the tube ends. These results are in quantitative agreement with classical electrostatic analysis, which assumes constant electrostatic potential on the conductive tube surface. At high charging levels, the tube ends are observed to be unstable due to Coulomb repulsion. By combining ab initio calculations with classical electrostatics, we determine, as a function of tube length and geometry of the tube end, the critical voltage beyond which nanotubes are unstable.