Green and scalable production of colloidal perovskite nanocrystals and transparent sols by a controlled self-collection process

Abstract

Colloidal perovskite oxide nanocrystals have attracted a great deal of interest owing to the ability to tune physical properties by virtue of the nanoscale, and generate thin film structures under mild chemical conditions, relying on self-assembly or heterogeneous mixing. This is particularly true for ferroelectric/dielectric perovskite oxide materials, for which device applications cover piezoelectrics, MEMs, memory, gate dielectrics and energy storage. The synthesis of complex oxide nanocrystals, however, continues to present issues pertaining to quality, yield, % crystallinity, purity and may also suffer from tedious separation and purification processes, which are disadvantageous to scaling production. We report a simple, green and scalable "self-collection" growth method that produces uniform and aggregate-free colloidal perovskite oxide nanocrystals including BaTiO3 (BT), Ba(x)Sr(1-x)TiO3 (BST) and quaternary oxide BaSrTiHfO3 (BSTH) in high crystallinity and high purity. The synthesis approach is 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, 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 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 inks, suitable for printing or spin/spray coating, demonstrating great capabilities of this process for fabrication of high performance dielectric 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.

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

“…A well-defined size and shape of BaTiO 3 nanoparticles could be obtained from the direct reaction of precursor ions through the hydrothermal method [9,10], while the solvothermal technique can produce more uniform nanoparticles, due to complex organic solvents preventing their agglomeration [11][12][13]. The sol-gel process shows outstanding control of a very small particle size (~10-50 nm) by using complex compound solution [14,15]. Co-precipitation via oxalate [16] and citrates [17] can produce BaTiO 3 at a lower temperature easier than other methods.…”

Interface structure, precursor rheology and dielectric properties of BaTiO3/PVDF–hfp nanocomposite films prepared from colloidal perovskite nanoparticles

“…12 However, with the decrease of particle size, the volume fraction of the lowpermittivity organic coating layer increases sharply, which would have an increasingly dominant role in the properties and thus reduce the permittivity of the nanocomposite. 13 In a different approach to conventional coating/mixing of commercially available nanopowders, in this study, we take advantage of the synthetic process of colloidal nanoparticles, 14 which possess highly reactive hydroxyl ligand naturally, to fabricate 0-3 ceramic-organic nanocomposite lms directly by mixing the as-synthesized BT nanoparticles with the polymer sol. In this study, poly(vinylidene uoride-co-hexauoro propylene), PVDF-hfp, which is a ferroelectric polymer that possesses a relative high electric displacement polarization, is chosen as the matrix phase to fabricate the nanocomposite lms with comprehensive high dielectric constant and low dielectric mismatching between the two phases.…”

The modification of surface, size and shape of barium zirconate powder via salt flux

“…Acceleration of the nucleation rate can be realized with a high degree of supersaturation in supercritical water, thus providing higher crystallinity with uniform particle size distribution, but this synthetic process has to be done with expensive equipment and a difficult operational system . The sol‐gel synthesis method is a powerful one for obtaining more homogeneous powder products . The complex isopropoxide [M(OiPr) 2 , M = Ba, Zr and iPr = CH(CH 3 ) 2 ] framework was synthesized from metal precursors before calcination of the hydrolyzed gel at >400°C in order to remove organic traces.…”

“…22,23 The sol-gel synthesis method is a powerful one for obtaining more homogeneous powder products. 24,25 The complex isopropoxide [M(OiPr) 2 , M = Ba, Zr and iPr = CH(CH 3 ) 2 ] framework was synthesized from metal precursors before calcination of the hydrolyzed gel at >400°C in order to remove organic traces. Although the sol-gel process shows outstanding control of tiny and uniform particles (~10-50 nm), it also requires the use of highly purified precursors in complicated experiments and long synthesis times.…”