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

Abstract

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.

Connecting energetics to dynamics in particle growth by oriented attachment using real-time observations

“…We calculate k 0 as 73.5 nm 2 s −1 , which results in W st = k 0 /k 11 = 48.0. Since the stability ratio can span as many as six orders of magnitude for a single colloidal system, depending on the surface and solution chemistry 45 , this low-to-intermediate value further supports the conclusion of a negligible energy barrier for particle attachment and thus a diffusion-limited process. This analysis also shows that the effect of the membrane is simply to impose a high viscosity that resists particle motion, thus rendering it mass transport limited.…”

“…To determine the extent to which the experimentally observed coagulation rate compares to the ideal rate for non-interacting particles, we derived the stability ratio (W st ) which equals unity in the ideal case and increases as the strength of the repulsive interparticle interaction increases 45 . For the case of particles diffusing in quasi-2D, the ideal coagulation rate (k 0 ) is expressed as:…”

Interplay between Short‐ and Long‐Ranged Forces Leading to the Formation of Ag Nanoparticle Superlattice

“…Accomplishing this goal requires an in‐depth understanding of the dynamic behavior of NPs in solutions over various time and length scales, its correlation to interparticle forces, and resultant superlattice structures . The NP self‐assembly processes involve various short‐ and long‐ranged forces, including Brownian force ( F Br ), van der Waals attractive force ( F vdW ), hydrodynamic force ( F D ), electrostatic force ( F elec ), hydration force ( F hyd ), and steric hindrance force ( F Sh ) . Recent advances in liquid cell transmission electron microscopy (LC‐TEM) enable us to understand the dynamics of NPs, behaving either individually or in cluster form, to delineate the competition of interparticle forces in the nonequilibrium state.…”

Building advanced materials via particle aggregation and molecular self-assembly

“…These materials have been applied in important technological fields such as energy, catalysis, optical devices, water purification, pollutant removal, CO 2 sequestration, and biomedicine [3,4,5,6,7]. Particle-based crystallization and selfassembly of molecules are important pathways to synthesize advanced materials of complex structures [8,9,10,11]. Unlike monomer-by-monomer addition or Ostwald ripening, particlebased crystallization occurs via particle-by-particle addition, to form larger crystals or clusters [8,12].…”