Nucleation, growth, and aggregation are interconnected processes which control the formation nanoparticles within aqueous environments and the development of nanoparticle structures. These processes are of fundamental engineering importance for the development of new nanoparticle synthesis methodologies and the creation of hierarchical self-assembled structures. They are also fundamentally important concepts for describing how nanoparticles and minerals are formed and distributed throughout the natural environment. The development of more sophisticated models for nucleation, growth, and aggregation within the aqueous environment has become a pressing scientific need, because nanoparticle size, shape, and aggregate structure are known to impact particle reactivity, bioavailability, transport and fate within the environment. In recent years, it has become apparent that complex interactions may exist between the processes of nucleation, growth, and aggregation, and these interactions are especially important during the formation of nanometer scale particles. For example, aggregation has been shown to serve as a mechanism for the growth of nanocrystals, and as a potential driver for phase transformations. However, the details of these interactions are not fully understood.
In this work, a combination of advanced in situ characterization techniques, including cryogenic transmission electron microscopy (cryo-TEM) and small angle x-ray scattering (SAXS), have been combined to better understand the development of nanoscale structures in aqueous systems. These techniques are complementary. Cryo-TEM provides new capabilities for nanoscale imaging of particles in aqua. It is especially useful for imaging fragile aggregate structures, which cannot be dried without damage, and for obtaining snapshots of reactive solutions that are evolving over time. Furthermore, cryo-TEM can be used to produce three-dimensional tomographic reconstructions (cryo-ET), providing structural models for particles and nanoparticles. However, TEM methodologies are limited by sampling statistics and are not ideal for determining the kinetics of structural change. SAXS is a complementary method that allows suspension properties such as particle size and aggregate structure to be characterized in a time-resolved fashion. SAXS has the potential to provide more statistically robust measurements, and to provide detailed reaction kinetics. However, SAXS data interpretation requires some level of a priori knowledge of the structure being characterized. Thus, by combining SAXS with cryo-TEM, a structurally accurate and statistically robust description of nanoparticle aggregate structure can be obtained.
This dissertation consists of four studies, which seek explain how iron oxyhydroxide nanoparticles nucleate and develop new structures via aggregation, within the aqueous environment.
The aim of the first study is to determine the structure of ferrihydrite nanoparticle aggregates in aqua. This is achieved using complimentary cryo-TEM and SAXS methodologies. Ferrihydrite nanoparticles are known to form complex aggregate structures. Interpretation of SAXS data is difficult due to suspension polydispersity. Cryo-ET is used to obtain three-dimensional images of the nanoparticle suspensions. A variety of aggregate structures are observed, with branched networks of linear chains of particles being prevalent in most suspensions. The tomographic structural models are processed to determine aggregate fractal dimensions, using an autocorrelation function based approach. These results are combined with SAXS data to obtain a more comprehensive understanding of the suspension complexity. The networks of linear chains are shown to possess low fractal dimensions, between 1.0 and 1.4; significantly lower than would be expected from traditional models for aggregation. This has important consequences for the aggregate's physical behavior, and allows very large aggregates to exist in stable colloidal suspension without flocculation.
The second study addresses how the ferrihydrite aggregate structure responds to changes in the ionic strength of the suspension, and how low-dimensional aggregate structures may influence nanoparticle transport through subsurface environments. Introducing ferrihydrite particle aggregates into solutions of 2 mM to 50 mM NaNO3 is shown to induce aggregate collapse, with more salt leading to the formation of denser aggregate structures and eventual flocculation. Complementary experiments show that millimolar quantities of NaNO3 induce a fundamental change in nanoparticle transport through a saturated quartz sand column. In deionized water, where low fractal dimension aggregates are stable, nanoparticles deposit evenly throughout the column, which soon saturates with particles so that subsequent injections are transported freely. When conditions favor aggregate collapse, dense localized accumulations occur and more nanoparticles can be deposited within the column. These deposits may be mechanically unstable, leading to irregular transport behavior.
In the third study, the relationship between aggregation and iron oxyhydroxide phase transformations is explored. Previous researchers have found that akaganeite (β-FeOOH) nanoparticles transform to create hematite (α-Fe2O3) nano-spindles in response to hydrothermal aging, but the mechanism of transformation is unknown. Some researchers have proposed a process based on the aggregation of hematite precursors, while others advocated for dissolution and re-precipitation mechanisms. In this study, the kinetics of the phase transformation from akaganeite to hematite is studied, and cryo-TEM is used to characterize the aggregate structures in the transforming suspension. The hematite spindles are shown to be nanoporous, while akaganeite nanoparticles display a tendency for oriented aggregation. Hematite spindles are frequently found in intimate contact with akaganeite nanoparticle aggregates during the process of phase transformation, suggesting a model for phase transformation in which the dehydration of akaganeite to form hematite is enhanced by aggregation.
In the final study, the nucleation and growth of akaganeite nanoparticles from acidic (pH 1.5-3) FeCl3 solutions is tracked with in situ small angle x-ray scattering (SAXS). The hydrothermal precipitation process studied can generate highly monodisperse particles, whose size, shape, and nucleation rate can be tuned by varying solution saturation and temperature.
Classical nucleation modeling is applied to determine new values for the interfacial energy of ferric oxyhydroxide clusters. The interfacial energy (interfacial tension) of the nucleus is shown to be pH dependent and ranges from 0.06 to 0.12 J/m2 within the range of experimental conditions. The interfacial tension decreases with decreasing pH. At the onset of nucleation, this corresponds to very small critical nuclei, containing just 4 to 30 iron atoms. The free energy of the early critical nuclei (40-70 kJ/mol) is found to be small relative to the effective activation energy for particle growth (140-200 kJ/mol). This suggests a situation where differences in growth kinetics may be as important for determining the first formed phase as differences in precipitate solubility or interfacial energy.
A thermodynamic construction for the free energy of an embryonic cluster is presented that can be extended to clusters of arbitrarily small size, including iron monomers. This construction can be used to define the interfacial tension of dissolved species, and determine this interfacial tension from readily available solubility data. The interfacial tension of the monomer is shown to closely track the experimentally determined interfacial tension of the critical nucleation clusters, suggesting a new method for estimating oxyhydroxide interfacial tensions when direct experimental measurements are unavailable.
In combination, these studies reveal the wide array of structures and behaviors that can occur in aqueous suspensions of ferric oxyhydroxide nanoparticles. In aqua methodologies with nanoscale resolution have allowed novel nanoparticle structures to be observed (e.g. linear particle chains and nanoporous hematite), and have been used to show the impact of nanoparticle aggregation on a variety of important physical processes (e.g. nanoparticle transport and phase transformation). These in aqua methods are also powerful tools for quantitative characterization of fundamental processes such as nanoparticle nucleation and growth; allowing important material properties (i.e. interfacial energy) that were previously unknown to be obtained. This type of information will allow for the refinement of existing iron oxyhydroxide synthesis approaches, to provide better control over particle size, shape, and phase, and will allow scientists to predict where nanoparticles may form within the environment.
