Jennifer A. Soltis

Oriented Aggregation: Formation and Transformation of Mesocrystal Intermediates Revealed

Oriented aggregation is a special case of aggregation in which nanocrystals self-assemble and form new secondary single crystals. This process has been suggested to proceed via an intermediate state known as the mesocrystal, in which the nanocrystals have parallel crystallographic alignment but are spatially separated. We present the first direct observations of mesocrystals with size and shape similar to product oriented aggregates by employing cryo-TEM to directly image the particles in aqueous suspension. The cryo-TEM images reveal that mesocrystals not only form but also transform to the final single crystal product while in the dispersed state. Further, high-resolution cryo-TEM images demonstrate that the mesocrystals are composed of spatially separated and crystallographically aligned nanocrystals.

Cation-Dependent Hierarchical Assembly of U60 Nanoclusters into Macro-Ion Assemblies Imaged via Cryogenic Transmission Electron Microscopy

Self-assembly of ([UO2(O2)OH]60)(60-) (U60), an actinide polyoxometalate with fullerene topology, can be induced by the addition of mono- and divalent cations to aqueous U60 solutions. Dynamic light scattering and small-angle X-ray scattering lend important insights into assembly in this system, but direct imaging of U60 and its assemblies via transmission electron microscopy (TEM) has remained an elusive goal. In this work, we used cryogenic TEM to image U60 and secondary and tertiary assemblies of U60 to characterize the size, morphology, and rate of formation of the secondary and tertiary structures. The kinetics and final morphologies of the secondary and tertiary structures strongly depend on the cation employed, with monovalent cations (Na(+) and K(+)) leading to the highest rates and largest secondary and tertiary structures.

A Perspective on the Particle-Based Crystal Growth of Ferric Oxides, Oxyhydroxides, and Hydrous Oxides

The iron oxides, oxyhydroxides, and hydroxides, which are commonly referred to as simply the iron oxides, are important materials at and near the Earth’s surface and in a wide range of industrial settings. The reactivity, phase transformations, and aggregation state of iron oxide minerals are fundamentally linked. The size, microstructure, and morphology of iron oxide crystals are path dependent, and specific features can potentially be linked directly to the crystal growth mechanism(s) that produced them. Many conclusions regarding crystal growth mechanism rely on characterization of the final crystals, and this approach has been fruitful. The iron oxides literature contains many reports of crystals with textures, morphologies, and microstructures that are consistent with particle-based crystal growth. However, multiple crystal growth mechanisms can operate simultaneously, which lead to erasure of features produced at earlier stages of crystal growth. Thus, time-resolved and in situ materials characterization is crucial to elucidating the crystal growth mechanisms of iron oxides.

Trace Uranium Partitioning in a Multiphase Nano-FeOOH System

The characterization of trace elements in minerals using extended X-ray absorption fine structure (EXAFS) spectroscopy constitutes a first step toward understanding how impurities and contaminants interact with the host phase and the environment. However, limitations to EXAFS interpretation complicate the analysis of trace concentrations of impurities that are distributed across multiple phases in a heterogeneous system. Ab initio molecular dynamics (AIMD)-informed EXAFS analysis was employed to investigate the immobilization of trace uranium associated with nanophase iron (oxyhydr)oxides, a model system for the geochemical sequestration of radiotoxic actinides. The reductive transformation of ferrihydrite [Fe(OH)3] to nanoparticulate iron oxyhydroxide minerals in the presence of uranyl (UO2)2+(aq) resulted in the preferential incorporation of U into goethite (α-FeOOH) over lepidocrocite (γ-FeOOH), even though reaction conditions favored the formation of excess lepidocrocite. This unexpected result is supported by atomically resolved transmission electron microscopy. We demonstrate how AIMD-informed EXAFS analysis lifts the strict statistical limitations and uncertainty of traditional shell-by-shell EXAFS fitting, enabling the detailed characterization of the local bonding environment, charge compensation mechanisms, and oxidation states of polyvalent impurities in complex multiphase mineral systems.

Effects of Ionic Strength, Salt, and pH on Aggregation of Boehmite Nanocrystals: Tumbler Small-Angle Neutron and X-ray Scattering and Imaging Analysis

The US government currently spends significant resources managing the legacies of the Cold War, including 300 million liters of highly radioactive wastes stored in hundreds of tanks at the Hanford (WA) and Savannah River (SC) sites. The materials in these tanks consist of highly radioactive slurries and sludges at very high pH and salt concentrations. The solid particles primarily consist of aluminum hydroxides and oxyhydroxides (gibbsite and boehmite), although many other materials are present. These form complex aggregates that dramatically affect the rheology of the solutions and, therefore, efforts to recover and treat these wastes. In this paper, we have used a combination of transmission and cryo-transmission electron microscopy, dynamic light scattering, and X-ray and neutron small and ultrasmall-angle scattering to study the aggregation of synthetic nanoboehmite particles at pH 9 (approximately the point of zero charge) and 12, and sodium nitrate and calcium nitrate concentrations up to 1 m. Although the initial particles form individual rhombohedral platelets, once placed in solution they quickly form well-bonded stacks, primary aggregates, up to ∼1500 Å long. These are more prevalent at pH = 12. Addition of calcium nitrate or sodium nitrate has a similar effect as lowering pH, but approximately 100 times less calcium than sodium is needed to observe this effect. These aggregates have fractal dimension between 2.5 and 2.6 that are relatively unaffected by salt concentration for calcium nitrate at high pH. Larger aggregates (>∼4000 Å) are also formed, but their size distributions are discrete rather than continuous. The fractal dimensions of these aggregates are strongly pH-dependent, but only become dependent on solute at high concentrations.

Cation-Dependent Hierarchical Assembly of U60 Nanoclusters into Blackberries Imaged via Cryogenic Transmission Electron Microscopy

The self-assembly of polyoxometalate nanoclusters into secondary and tertiary superstructures is of great interest to many fields, including sensing, catalysis, and biomedicine. Polyoxometalates often have unique physical properties that further heightens this appeal. When in the solution phase, these structures often have an overall negative charge and behave as dissolved species, leading to their description as macroanions. Their anionic character provides a facile route by which to tailor self-assembly behavior through the addition of positively charged species.

Correlative Microscopic, Spectroscopic, and Computational Analysis of the Nucleation and Growth of Europium (III) Oxalate Nanoparticles

The early stages of nanoparticle nucleation are not well understood, and yet increasing our understanding of these processes is fundamental to the production of monodisperse nanoparticles with targeted properties. Many technical challenges exist when trying to study nucleation events due to both the nuclei’s small size and rapid formation, and subsequent rapid growth into post-nucleation structures. Additionally, meaningful characterization of nucleation processes must be performed on samples in their in situ solution state. A multi-pronged characterization approach is critical, and experiments are ideally paired with computational modeling to form a more robust picture of these early stages of nanoparticle growth.