Abstract
Understanding inorganic nanocrystal (NC) growth dynamic pathways under their native fabrication environment remains a central goal of science, as it is crucial for rationalizing novel nanoformulations with desired architectures and functionalities. We here present an in-situ method for quantifying, in real time, NCs’ size evolution at sub-nm resolution, their concentration, and reactants consumption rate for studying NC growth mechanisms. Analyzing sequential high-resolution liquid-state 19F-NMR spectra obtained in-situ and validating by ex-situ cryoTEM, we explore the growth evolution of fluoride-based NCs (CaF2 and SrF2) in water, without disturbing the synthesis conditions. We find that the same nanomaterial (CaF2) can grow by either a particle-coalescence or classical-growth mechanism, as regulated by the capping ligand, resulting in different crystallographic properties and functional features of the fabricated NC. The ability to reveal, in real time, mechanistic pathways at which NCs grow open unique opportunities for tunning the properties of functional materials.
Highlights
Understanding inorganic nanocrystal (NC) growth dynamic pathways under their native fabrication environment remains a central goal of science, as it is crucial for rationalizing novel nanoformulations with desired architectures and functionalities
Prior to conducting HR-NMR studies on water-dispersed, small-sized CaF2 NCs capped with 2-aminoethyl phosphate (AEP, at pH7, molecular structure, Fig. 1a), they were synthesized and fully characterized with complementary techniques
The fluorite, faced cubic centered, crystal structure of the obtained AEP-CaF2 NCs was confirmed by powder X-ray diffraction (XRD, Supplementary Fig. 1b) and their relatively small size as well as their monodispersity were preserved in water, as revealed by dynamic light scattering (DLS, Supplementary Fig. 1c)
Summary
HR-NMR accurately determines the diameter of NCs in solution. Prior to conducting HR-NMR studies on water-dispersed, small-sized CaF2 NCs capped with 2-aminoethyl phosphate (AEP, at pH7, molecular structure, Fig. 1a), they were synthesized and fully characterized with complementary techniques. To demonstrate the robustness and reproducibility of this approach, we applied it to evaluate the sizes of various nanofluoride (CaF2 or SrF2) fabrications (Fig. 1f and Supplementary Fig. 3) from their HR 19F-NMR spectra and found them to be in good correlation (r2 = 0.989) with the sizes obtained from HR-TEM images This ability to accurately determine the average size of a variety of small-sized NCs demonstrates the uniqueness and the potential variability of the proposed HRNMR-based method. Magic-angle spinning (MAS) solid-state 19F-NMR of the same synthetic AEP-CaF2 colloids’ powder showed a similar 19F-NMR spectrum (Supplementary Fig. 4a) to that obtained using high-resolution 19F-NMR (Supplementary Fig. 4b), from which similar NC diameters were calculated This observation confirms that water-dispersed AEPCaF2 NCs tumble fast enough in solution so as to average out the intrinsic homonuclear dipolar interactions, allowing to directly detect their fluoride content with HR liquid-state 19F-NMR setups. We validated this possibility by synthesizing nanofluorides in an NMR tube under ambient conditions while acquiring consecutive 19F-NMR spectra from reaction initiation to final formation, a b
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