Size controllable synthesis of single-crystal ferroelectric Bi₄Ti₃O₁₂ nanosheet dominated with {001} facets toward enhanced visible-light-driven photocatalytic activities

Highly crystallized single crystal Bi₄Ti₃O₁₂ nanosheets with dominant {001} facets were synthesized by heating a stoichiometric composition of α-Bi₂O₃ and TiO₂ in molten NaCl-KCl at 800°C for 2h. Effects of the amount of the added molten salts (their mole ratio (M) to nominal Bi₄Ti₃O₁₂ varied from 4 to 60 and the samples were denoted as BTO-M) on the size distribution and photoactivity of the resulting products were systematically evaluated. The side length of as-prepared Bi₄Ti₃O₁₂ gradually decreases by increasing the addition of molten salts, and the minimal side length is achieved in the sample BTO-50. As a result, the sample BTO-50 shows the best photocatalytic kinetics in photodegradation of rhodamine B (RhB) under visible-light irradiation, which is about 8.65 times faster than that of the sample BTO-0 obtained by a traditional solid-state reaction method, demonstrating the superiority of the present molten salts synthesis for bismuth titanate. The improved photocatalytic kinetics of BTO-M is a concert of several factors including the highly faceted surfaces, thin flakes that shorten the charge transport distance to the surface, as well as the internal electric fields produced by the spontaneous polarization in the BTO-50 sample that facilitate the separation of photoinduced charges as evidenced by Kelvin probe force microscopy. Electrochemical impedance spectroscopy further reveals that under visible-light, the electron transfer resistance of the BTO-50 decreases to nearly 50% of that of the BTO-0 sample.