Impact of Solution Chemistry and Particle Anisotropy on the Collective Dynamics of Oriented Aggregation

Although oriented aggregation of particles is a widely recognized mechanism of crystal growth, the impact of many fundamental parameters, such as crystallographically distinct interfacial structures, solution composition, and nanoparticle morphology, on the governing mechanisms and assembly kinetics are largely unexplored. Thus, the collective dynamics of systems exhibiting OA has not been predicted. In this context, we investigated the structure and dynamics of boehmite aggregation as a function of solution pH and ionic strength. Cryogenic transmission electron microscopy shows that boehmite nanoplatelets assemble by oriented attachment on (010) planes. The coagulation rate constants obtained from dynamic light scattering during the early stages of aggregation span 7 orders of magnitude and cross both the reaction-limited and diffusion-limited regimes. Combining a simple scaling analysis with calculations for stability ratios and rotational/translational diffusivities of irregular particle shapes, the effects of orientation for irregular-shaped particles on the early stages of aggregation are understood via angular dependencies of van der Waals, electrostatic, and hydrodynamic interactions. Using Monte Carlo simulations, we found that a simple geometric parameter, namely, the contact area between two attaching nanoplatelets, presents a useful tool for correlating nanoparticle morphologies to the emerging larger-scale aggregates, hence explaining the unusually high fractal dimensions measured for boehmite aggregates. Our findings on nanocrystal transport and interactions provide insights toward the predictive understanding of nanoparticle growth, assembly, and aggregation, which will address critical challenges in developing synthesis strategies for nanostructured materials, understanding the evolution of geochemical reservoirs, and addressing many environmental problems.

Synthesis and Adsorption Properties of Hierarchically Ordered Nanostructures Derived from Porous CaO Network

Using the porous framework of CaO as templates and reagents, we explored a surfactant-free and economical method for preparing calcium silicate hydrate (CSH) hierarchically ordered nanostructures. Incorporation of SiO₂ nanoparticles into the CaO framework, followed by a reaction assisted by hydrothermal treatment, resulted in the formation of CSH with well-defined morphologies. The structural features of CSH were characterized by 3-D hierarchical networks, wherein nanofibers assembled to form nanosheets, and nanosheets assembled to form hierarchically ordered structures. Investigation of the crystal growth mechanism indicated that the key to forming the CSH ordered assembly structure was confining the Ca/Si ratio within a small range. Nonclassic oriented aggregation mechanism was used to describe the crystal growth of nanosheets, while the porous CaO framework served as template/reagents responsible for the formation of hierarchical structures. The resulting CSH adsorbent exhibited better performance in removing Pb(II) compared with other types of random CSH adsorbents. Additionally, the hierarchical structure of CSH provided more pores and active sites as support for other active functional materials such as zerovalent iron (Fe⁰). As-produced CSH@Fe nanocomposite with self-supported structures displayed high capacities for removal of Pb(II) after five adsorption-desorption cycles, and high capacities for other heavy metal ions (Cu²⁺, Cd²⁺, and Cr₂O₇^2-) and organic contaminants.

BaTiO3 supercages: unusual oriented nanoparticle aggregation and continuous ordering transition in morphology

Here we report the organic-free mesocrystalline superstructured cages of BaTiO3, i.e., the BaTiO3 supercages, which are synthesized by a one-step templateless and additive-free route using molten hydrated salt as the reaction medium. An unusual three-dimensional oriented aggregation of primary BaTiO3 nanoparticles in the medium of high ionic strength, which normally favors random aggregation, is identified to take place at the early stage of the synthesis. The spherical BaTiO3 aggregates further experience a remarkable continuous ordering transition in morphology, consisting of nanoparticle faceting and nanosheet formation steps. This ordering transition in conjunction with Ostwald ripening-induced solid evacuation leads to the formation of unique supercage structure of BaTiO3. Benefiting from their structure, the BaTiO3 supercages exhibit improved microwave absorption property.