Solvothermal synthesis and controlled self-assembly of monodisperse titanium-based perovskite colloidal nanocrystals

Abstract

The rational design of monodisperse ferroelectric nanocrystals with controlled size and shape and their organization into hierarchical structures has been a critical step for understanding the polar ordering in nanoscale ferroelectrics, as well as the design of nanocrystal-based functional materials which harness the properties of individual nanoparticles and the collective interactions between them. We report here on the synthesis and self-assembly of aggregate-free, single-crystalline titanium-based perovskite nanoparticles with controlled morphology and surface composition by using a simple, easily scalable and highly versatile colloidal route. Single-crystalline, non-aggregated BaTiO3 colloidal nanocrystals, used as a model system, have been prepared under solvothermal conditions at temperatures as low as 180 °C. The shape of the nanocrystals was tuned from spheroidal to cubic upon changing the polarity of the solvent, whereas their size was varied from 16 to 30 nm for spheres and 5 to 78 nm for cubes by changing the concentration of the precursors and the reaction time, respectively. The hydrophobic, oleic acid-passivated nanoparticles exhibit very good solubility in non-polar solvents and can be rendered dispersible in polar solvents by a simple process involving the oxidative cleavage of the double bond upon treating the nanopowders with the Lemieux-von Rudloff reagent. Lattice dynamic analysis indicated that regardless of their size, BaTiO3 nanocrystals present local disorder within the perovskite unit cell, associated with the existence of polar ordering. We also demonstrate for the first time that, in addition to being used for fabricating large area, crack-free, highly uniform films, BaTiO3 nanocubes can serve as building blocks for the design of 2D and 3D mesoscale structures, such as superlattices and superparticles. Interestingly, the type of superlattice structure (simple cubic or face centered cubic) appears to be determined by the type of solvent in which the nanocrystals were dispersed. This approach provides an excellent platform for the synthesis of other titanium-based perovskite colloidal nanocrystals with controlled chemical composition, surface structure and morphology and for their assembly into complex architectures, therefore opening the door for the design of novel mesoscale functional materials/nanocomposites with potential applications in energy conversion, data storage and the biomedical field.

Manifestation of dipole-induced disorder in self-assembly of ferroelectric and ferromagnetic nanocubes

“…Ferroelectricity evolves via extended coop- erative ordering of charge dipoles from local titanium off-center displacements within the Ti-O 6 octahedra. Previous high-energy XRD and atomic PDF studies indicated a progressive decay of ferroelectric coherence across the free standing nanocrystals in the diminished size regime 33,[56][57][58][59] , whereas the tetragonal distortion itself persists down to sub-10 nm scale 4,56,57,59 . In turn, high fidelity mapping of ferroelectric structural distortions in discrete sub-10 nm nanocubes of BaTiO 3 via aberration-corrected transmission electron microscopy confirmed a coherent ferroelectric monodomain state with a stable net polar ordering 33 .…”

“…Recent advances in chemical synthesis 4,15,[26][27][28] have begun to deliver bulk quantites of nearly monodisperse highly crystalline singledomain nanocubes of some ABO 3 perovskites (incl. BaTiO 3 4,28 , SrTiO 3 27 and a range of solid solutions), which are ideal spacefilling building blocks with a 100% theoretical packing efficiency, and enabled solution processing via a simple, easily scalable and highly versatile colloidal route to produce superlattice assemblies 4,9,29 and more complex hierarchies 4,9,29 for integration into nanostructured ferroelectric devices 5,30,31 . Meanwhile, understanding and excercising precise control over the local behaviour of anisometric nanocrystals is absolutely imperative for fabricating highly-ordered assemblies 29 , e.g.…”

A numerical method for deriving shape functions of nanoparticles for pair distribution function refinements

“…Barium titanate (BaTiO 3 ) nanocrystals can be synthesized with a variety of sizes from $8 to 100 nm using solvothermal synthesis and are of interest for applications such as data storage, energy-storage applications and polymer composites (Caruntu et al, 2015). They are also of interest in the mission to understand the fundamental lower size limit of ferroelectricity and how different factors such as shape (e.g.…”

“…Nanomaterials are at the forefront of many technologically important and scientifically interesting material advances. For example, oxide nanoparticles are of great interest for catalysis (Yang et al, 2008), for lithium-ion battery anode materials , as self-assembled thin films (Mimura & Kato, 2013) and as dielectric media in composites (Parizi et al, 2014;Adam et al, 2014;Caruntu et al, 2015). A thorough description of a nanoparticle's structure is key for developing structure-function relationships.…”

A facile one step conversion of the sub-micrometer to uniform nanopowder in tetragonal BaTiO3 via a surface active etching salt

“…Notably, the most important problems caused via soft chemistry still emanate as much from low crystallinity and tetragonality of BaTiO 3 nanoproducts as decreased size, resulting in gradually decreasing spontaneous polarization and T c . In the case of high basic solution, problems usually arise from hydroxylation incorporated in the oxygen sub-lattice, which stabilizes in a metastable cubic phase with non-ferroelectric properties [24,25]. The limitation of the chemical method motivates the search for other processes that produce high performing BaTiO 3 nanoparticles.…”