Abstract
Phase transformation and crystal growth in nanoparticles may happen via mechanisms distinct from those in bulk materials. We combine experimental studies of as-synthesized and hydrothermally coarsened titania (TiO2) and zinc sulfide (ZnS) with thermodynamic analysis, kinetic modeling and molecular dynamics (MD) simulations. The samples were characterized by transmission electron microscopy, X-ray diffraction, synchrotron X-ray absorption and scattering, and UV-vis spectroscopy. At low temperatures, phase transformation in titania nanoparticles occurs predominantly via interface nucleation at particle–particle contacts. Coarsening and crystal growth of titania nanoparticles can be described using the Smoluchowski equation. Oriented attachment-based crystal growth was common in both hydrothermal solutions and under dry conditions. MD simulations predict large structural perturbations within very fine particles, and are consistent with experimental results showing that ligand binding and change in aggregation state can cause phase transformation without particle coarsening. Such phenomena affect surface reactivity, thus may have important roles in geochemical cycling.
Highlights
The transformation of structure between stable or metastable phases reveals basic thermodynamical information about the state of matter
We show how classical thermodynamics can be combined with microscopic predictions from molecular dynamics (MD) simulations to consider the energetics and phase stability of two nanoparticle systems
We summarized special pathways for crystal growth and phase transformations in nanocrystalline materials using TiO2 and zinc sulfide (ZnS) as examples
Summary
The transformation of structure between stable or metastable phases reveals basic thermodynamical information about the state of matter. Investigations into the phase stability of nanoparticles have given some of the clearest demonstrations of the differences between nanoscale and bulk material of the same stoichiometry. It is clear that the phase diagrams for materials can be size dependent.[11] Using the concepts of classical thermodynamics, observed size dependencies can be associated with the large surface area present in nanoscale materials. The excess energy of a nanoparticles relative to that of the bulk material (normalized by the surface area) is defined as the surface energy (usually in J m22). Many of the observed data can be explained once surface energy contributions are considered. This quantity is difficult to measure accurately, and depends on the details of both surface and interior structure.
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