Ferroelectric bismuth-titanate nanoplatelets and nanowires with a new crystal structure.

Two different morphologies of ferroelectric bismuth titanate (Bi 4 Ti 3 O 12 ) nanoparticles, i.e. , nanoplatelets and nanowires, were synthesized by changing the concentration of NaOH during a hydrothermal treatment of precipitated Ti 4+ and Bi 3+ ions. The nanoparticles' crystal structures were characterized using atomic-resolution imaging with a C S -probe-corrected scanning-transmission electron microscope in combination with X-ray diffractometry and Raman spectroscopy. The nanoplatelets (10 nm thick and from 50 nm to 200 nm wide) exhibit the Aurivillius-type layered-perovskite crystal structure that is characteristic of Bi 4 Ti 3 O 12 , whereas the nanowires (from 15 nm to 35 nm wide and from several hundreds of nm to several μm long) exhibit an entirely new structure with an orthorhombic unit cell ( a = 3.804(1) Å, b = 11.816(3) Å, and c = 9.704(1) Å). The nanowire structure is composed of two structural layers alternating along the orthorhombic c -direction: a structural layer composed of two parallel layers of Bi atoms that resembles the (Bi 2 O 2 ) 2+ layer of the Aurivillius structure, and a structural layer composed of two parallel layers of Ti atoms, where every sixth Ti is replaced with Bi. Observations of the ferroelectric domains with transmission electron and piezo-response force microscopy indicated the ferroelectric nature of both nanostructures. The nanowire structure is a metastable polymorph of the bismuth titanate stabilized at the nanoscale. With annealing at temperatures above 500 °C the nanowire structure topotactically transforms into the Aurivillius structure.