Gel collection is the name we gave to our in-house synthesis methods used to prepare nanocrystalline oxides. It's a combination of air-sensitive chemistry (glovebox, Schlenk-line), nanoparticle nucleation/growth, sol-gel and hydrothermal. Sounds more fancy than it is - sometimes you make complex fully crystallized oxides at room temperature with the right mixing and stirring!
Gel collection in action: (a) Photo images, (b) XRD patterns and yields (inset) of BST nanocrystals evolution with the crystallization time at 55 deg. C, respectively. (c) TEM image of BST nanocrystals (inset: high-resolution image of a particle oriented along the [111] direction). (d) XRD of various BT-based perovskite nanocrystals. The stick patterns for BaTiO3 (ICSD #029147, in black) and SrTiO3 (ICSD# 080874, in red) are included in (b) and (d).
Printable BST nanocrystal solution (a) ethanol as solvent (40 mg/ml) and (b) FA as solvent (50 mg/ml), a red (633 nm) laser beam travelling through the transparent solution, showing the Tyndall effect and confirming a uniform colloidal dispersion of nanocrystals; (c) cross-section SEM image of a BST/PFA nanocomposite thin film, and (d) dielectric properties of a BST/PFA thin film.
It has been observed by our group that a series of ABO3-type transition metal complex oxides can be prepared at near room temperature in meaningful (> 1g) quantities, with > 99% crystallinity and a relatively high monodispersity (typically 8, 10, 15 or 25 nm ± 1nm). The following compounds have been successfully prepared and reported: BaTiO3, Ba(Fe,Ti)O3, (Ba,Sr)TiO3 e.g. Ba0.7Sr0.3TiO3, (Ba,Sr)(Ti,Hf)O3 e.g. Ba0.65Sr0.35Ti0.5Hf0.5O3. The processing method relies on dissolving Ba, Sr and Ti metal alkoxides in an anhydrous alcohol solvent to achieve a critical concentration of metal organic sources, at which point reaction can occur. The method is highly reproducible and provides a high degree of satisfaction, given the notorious difficulty in identifying reliable procedures through materials chemistry. The synthesis approach can be described as solution processed, based on the sol-gel transformation of metal alkoxides in alcohol solvents with controlled or stoichiometric amounts of water and in the stark absence of surfactants and stabilizers, providing pure colloidal nanocrystals in a remarkably low temperature range (15 ˚C -55 ˚C). Under a static condition (no stirring), a particular set of observations can be made: the solution (which starts as macroscopically turbid) undergoes a transformation in which a white rod starts to “self-collect” until a complete (but delicate) monolithic structure is formed. Characterization reveals that the nanoscale hydrolysis of the metal alkoxides accomplishes a complete transformation to fully crystallized single domain perovskite nanocrystals with a passivated surface layer of hydroxyl/alkyl groups, such that the as-synthesized nanocrystals can exist in the form of super-stable and transparent sol, or self-accumulate to form a highly crystalline solid gel monolith of nearly 100% yield for easy separation/purification. The process produces high purity ligand-free nanocrystals but with excellent dispersibility in polar solvents, with no impurity remaining in the mother solution other than trace alcohol byproducts (such as isopropanol). The afforded stable and transparent suspension/solution can be treated as an “ink”, suitable for printing or spin/spray coating, demonstrating great capabilities of this process for fabrication of thin films. The simple “self-collection” strategy can be described as green and scalable due to the simplified procedure from synthesis to separation/purification, minimum waste generation, and near room temperature crystallization of nanocrystal products with tunable sizes in extremely high yield and high purity.
Related References
1. Pearsall, F., A.; Lombardi, J.; O’Brien, S. “Monomer Derived Poly(Furfuryl)/BaTiO3 0–3 Nanocomposite Capacitors: Maximization of the Effective Permittivity Through Control at the Interface.” ACS Applied Materials & Interfaces, 2017 9 (46), 40324-40332. DOI: 10.1021/acsami.7b13879.
2. Hao, Y. N.; Bi, K.; O’Brien, S.; X. H. Wang, X. H.; Lombardi, J.; Pearsall, F.; Li, W. L.; Lei, M.; Wu, Y.; Li, L. T. “Interface structure, precursor rheology and dielectric properties of BaTiO3/PVDF-hfp nanocomposite films prepared from colloidal perovskite nanoparticles” RSC Advances, 2017, 7(52), 32886-32892.
3. Lombardi, J.; Pearsall, F.; Li, W.; O'Brien, S. “Synthesis and dielectric properties of nanocrystalline oxide perovskites, [KNbO3]1−x[BaNi0.5Nb0.5O3−δ]x, derived from potassium niobate KNbO3 by gel collection” J. Mater. Chem. C, 2016, 4, 7989-7998.
4. Van Tassell, B.; Yang, S.; Le, C.; Liu, S.; Huang, L.; Chando, P.; Liu, X.; Byro, A.; Gerber, D.; Leland, E.; Sanders, S.; Kinget, P.; Kymissis, I.; Steingart, D.; O’Brien, S. “Metacapacitors: Printed thin-film, flexible capacitors for power conversion applications,” Power Electronics, IEEE Transactions on. 2016, 31, 4, 2695 – 2708. DOI: 10.1109/TPEL.2015.2448529.
5. Hao, Y. N.; Wang, X. H.; O'Brien, S; Lombardi, J.; Li, L. T. “Flexible BaTiO3/PVDF gradated multilayer nanocomposite film with enhanced dielectric strength and high energy density” J. Mater. Chem. C, 2015, 3, 9740-9747. DOI: 10.1039/C5TC01903F.
6. Liu, S.; Huang, L.; Wanlu Li, W.; Xiaohua Liu, W.; Shui, J.; Li, J. and O'Brien, S. “Green and scalable production of colloidal perovskite nanocrystals and transparent sols by a controlled self-collection process”, Nanoscale, 2015, 7, 11766. DOI: 10.1039/c5nr02351c.
7. Liu Shuangyi, L.; Li, W.; Li J., O'Brien S., "Electrical Properties of New Hollandite Complex Oxide Nanocrystals." Journal of nanoscience and nanotechnology 2015, 15 (9), 7074-80.
8. Liu, S.; Akbashev, A. R.; Yang, X.; Liu, X.; Li, W.; Zhao, L.; Li, X.; Couzis, A.; Han, M.-G.; Zhu, Y.; Krusin-Elbaum, L.; Li, J.; Huang, L.; Billinge, S. J. L.; Spanier, J. E.; O’Brien, S. “Hollandites as a new class of multiferroics,” Nature Scientific Reports, 2014, doi:10.1038/srep06203.
9. Huang, L.; Liu, S.; van Tassel, B.; Liu, X.; Byro, A.; Zhang, H.; Akins; D. L.; Steingart, D. A.; Li, J.; O’Brien, S. “Structure and Performance of Dielectric Films based on Self-Assembled High Dielectric Constant Nanocrystals”, Nanotechnology, 2013, 24, 415602 doi:10.1088/0957-4484/24/41/415602.
10. Liu, X.; Liu, S.; Han, M.-G.; Zhao, L.; Deng, H.; Li, J.; Zhu, Y.; Krusin-Elbaum, L.; O’Brien, S. “Magnetoelectricity in CoFe2O4 Nanocrystal-P(VDF-HFP) Thin-Films”, Nanoscale Research Letters, 2013, 8, 374.
11. Yang, S.Y.; Kymissis, I.; Leland, E.S.; Liu, S.Y.; O'Brien, S. “Influence of electromigration on the maximum operating field of (Ba,Sr)TiO3/parylene-C composite capacitors” Journal of Vacuum Science and Technology B. 2013, 31, 6. doi:10.1116/1.4828365.
12. Yang, S.; Tull, B. R.; Pervez, N. K.; Huang, L.; Leland, E. S.; Steigart, D. A.; O'Brien, S.; Kymissis, I. “Asymmetric Leakage in (Ba, Sr)TiO3 Nanoparticle/Parylene-C Composite Capacitors”, Journal of Polymer Science B: Polymer Physics, 2012 DOI: 10.1002/polb.23156.
13. Nguyen-Thanh, D.; Frenkel, A. I.; Wang, J.; O’Brien, S.; Akins, D. L. "Cobalt-Polypyrrole-Carbon Black (Co-PPY-CB) “Electrocatalysts for the Oxygen Reduction Reaction (ORR) in Fuel Cells: Composition and Kinetic Activity” Applied Catalysis B: Environmental, 2011, 105, 50-60.
14. Huang, L.; Zhang, J.; Kymissis, I.; O’Brien, S. “High K thin films built from uniform Barium Titanate Nanocrystals in the Superparaelectric Limit” Adv. Func. Mater. 2010, 20, 554-560.