Nanoparticle Heat-Up Synthesis: In Situ X-ray Diffraction and Extension from Classical to Nonclassical Nucleation and Growth Theory

Abstract

Heat-up synthesis routes are very commonly used for the controlled large-scale production of semiconductor and magnetic nanoparticles with narrow size distribution and high crystallinity. To obtain fundamental insights into the nucleation and growth kinetics is particularly demanding, because these procedures involve heating to temperatures above 300 °C. We designed a sample environment to perform in situ SAXS/WAXS experiments to investigate the nucleation and growth kinetics of iron oxide nanoparticles during heat-up synthesis up to 320 °C. The analysis of the growth curves for varying heating rates, Fe/ligand ratios, and plateau temperatures shows that the kinetics proceeds via a characteristic sequence of three phases: an induction Phase I, a final growth Phase III, and an intermediate Phase II, which can be divided into an early phase with the evolution and subsequent dissolution of an amorphous transient state, and a late phase, where crystalline particle nucleation and aggregation occurs. We extended classical nucleation and growth theory to account for an amorphous transient state and particle aggregation during the nucleation and growth phases. We find that this nonclassical theory is able to quantitatively describe all measured growth curves. The model provides fundamental insights into the underlying kinetic processes especially in the complex Phase II with the occurrence of a transient amorphous state, the nucleation of crystalline primary particles, particle growth, and particle aggregation proceeding on overlapping time scales. The described in situ experiments together with the extension of the classical nucleation and growth model highlight the two most important features of nonclassical nucleation and growth routes, i.e., the formation of intermediate or transient species and particle aggregation processes. They thus allow us to quantitatively understand, predict, and control nanoparticle nucleation and growth kinetics for a wide range of nanoparticle systems and synthetic procedures.

Simulations to Aid in the Design of Microbes for Synthesis of Metallic Nanomaterials

“…and γ′ = γ 0 . 36 During heterogeneous nucleation, γ′ = γ where γ ≠ γ 0 . The free energy ΔG c driving nucleation is given by…”

“…12,23,36,40,42,84,85 For example, aggregation can play a critical role in nanomaterial formation yet only recently have efforts been made to explicitly incorporate them into models, albeit nonbiological. 36 Regarding nucleation, and the model laid out here, the precise change in the interfacial free energy γ/γ 0 due to the presence of stabilizing peptides or other biomolecules remains an unreported experimental quantity. Guided and informed by experiments, however, estimates can be made that recapitulate published data and capture the key qualitative outcomes.…”

“…A theoretical framework addressing the parameters affecting biogenic material synthesis, however, could significantly accelerate progress. Such frameworks exist to describe inorganic nanomaterial synthesis, most notably that of classical nucleation theory. − The nucleation and growth of nanoparticles was initially described by LaMer’s burst nucleation and by Ostwald’s ripening scheme. − Subsequent theories of varying scope have been developed − and strengthened by powerful in situ X-ray scattering experiments. − Alternatively, the ascension of computer simulations has made in silico methods of nanomaterial synthesis highly attractive. ,− In these simulations, distinct descriptions of chemical, nucleation, and growth processes can be integrated numerically to predict the outcome of nanoparticle synthesis experiments. ,− To date, however, few models or simulations exist describing the role of microbes in biogenic nanomaterial synthesis.…”

Using small-angle scattering to guide functional magnetic nanoparticle design

“…identifying transient amorphous phases and particle aggregation processes in the iron oleate heat-up synthesis, not accounted for in the classical description. 213 Post-processing steps may be required for purication and potentially phase transfer to polar solvents via ligand exchange. 214 Stable, aqueous dispersion of nanoparticles based on amphiphilic polymers are further functionalizable with selected macromolecules, 215 e.g.…”

Using small-angle scattering to guide functional magnetic nanoparticle design

“…identifying transient amorphous phases and particle aggregation processes in the iron oleate heatup synthesis, not accounted for in classical description. 213 Post-processing steps may be required for purification and potentially phase transfer to polar solvents via ligand exchange. 214 Stable, aqueous dispersion of nanoparticles based on amphiphilic polymers are further functionalizable with selected macromolecules, 215 e.g.…”