ABSTRACT
The engineering of epitaxial, two-dimensional (2D) nano-heterostructures has stimulated great interest owing to an expectation of better functional properties (e.g., photocatalytic, piezoelectric). Hydrothermal topotactic epitaxy is one of the promising synthetic approaches for their preparation, particularly the formation of a highly ordered, epitaxial interface and possibilities for the preparation of anisotropic nanostructures of symmetrical materials. The present study highlights the key parameters when steering the alkaline, hydrothermal, topochemical conversion process from Bi4Ti3O12 nanoplatelets to the intermediate, epitaxial, SrTiO3/Bi4Ti3O12 nano-heterostructures and the final SrTiO3 nanoplatelets by balancing the lattice mismatch and the supersaturation. An atomic-scale examination revealed the formation of an ordered epitaxial SrTiO3/Bi4Ti3O12 interface with the presence of dislocations. The SrTiO3 grows in islands for a stoichiometric amount of Sr (Sr/Ti = 1) and the growth resembles a layer-by-layer mode for surplus Sr content (Sr/Ti ≥ 12). The latter enables SrTiO3 overgrowth of the Bi4Ti3O12 basal surface planes, protecting them against dissolution from the top and consequently ensuring the preservation of the platelet morphology during the entire transformation process, the kinetics of which is controlled by the base concentration. A developed understanding of this particular transformation provides the guiding principles and ideas for designing other defined or complex epitaxial heterostructures and structures under low-temperature hydrothermal conditions.
ABSTRACT
Low-temperature hydrothermal epitaxial growth and topochemical conversion (TC) reactions offer unexploited possibilities for the morphological engineering of heterostructural and non-equilibrium shape (photo)catalyst particles. The hydrothermal epitaxial growth of SrTiO3 on Bi4Ti3O12 platelets is studied as a new route for the formation of novel nanoheterostructural SrTiO3/Bi4Ti3O12 platelets at an intermediate stage or (100)-oriented mesocrystalline SrTiO3 nanoplatelets at the completed stage of the TC reaction. The Bi4Ti3O12 platelets act as a source of Ti(OH)62- species and, at the same time, as a substrate for the epitaxial growth of SrTiO3. The dissolution of the Bi4Ti3O12 platelets proceeds faster from the lateral direction, whereas the epitaxial growth of SrTiO3 occurs on both bismuth-oxide-terminated basal surface planes of the Bi4Ti3O12 platelets. In the progress of the TC reaction, the Bi4Ti3O12 platelet is replaced from the lateral ends toward the interior by SrTiO3, while Bi4Ti3O12 is preserved in the core of the heterostructural platelet. Without any support from noble-metal doping or cocatalysts, the SrTiO3/Bi4Ti3O12 platelets show stable and 15 times higher photocatalytic H2 production (1265 µmol·g-1·h-1; solar-to-hydrogen (STH) efficiency = 0.19%) than commercial SrTiO3 nanopowders (81 µmol·g-1·h-1; STH = 0.012%) in pH-neutral water/methanol solutions. A plausible Z scheme is proposed to describe the charge-transfer mechanism during the photocatalysis.
ABSTRACT
Plate-like Bi4Ti3O12 particles were synthesized using a one-step, molten-salt method from Bi2O3 and TiO2 nanopowders at 800 °C. The reaction parameters that affect the crystal structure and morphology were identified and systematically investigated. The differences between various Bi4Ti3O12 plate-like particles were examined in terms of the ferroelectric-to-paraelectric phase transition and the photocatalytic activity for the degradation of Rhodamine B under UV-Alight irradiation. The results encouraged us to conduct further testing of the as-prepared Bi4Ti3O12 plate-like particles as templates for the preparation of plate-like SrTiO3 perovskite particles using a topochemical conversion under hydrothermal conditions. The characteristics of the Bi4Ti3O12 plates and the reaction parameters for which the SrTiO3 preserved the shape of the initial Bi4Ti3O12 template particles were determined.
