Hydrothermal formation of bismuth-titanate nanoplatelets and nanowires: the role of metastable polymorphs

Understanding the processes involved in the formation of nanoparticles is a prerequisite for the control of their morphology. In this investigation we reveal the processes leading to the formation of two distinct morphologies of bismuth-titanate (Bi4Ti3O12) nanoparticles, i.e., nanoplatelets and nanowires, during hydrothermal treatment of precipitated Bi3+ and Ti4+ ions in a mineralizer hydroxide aqueous solution. Washed precipitates of the two ions in a stoichiometric ratio were suspended in hydroxide (NaOH or KOH) and hydrothermally treated for different times at temperatures up to 220 degrees C. The product was analysed with a combination of X-ray diffractometry and different electron-microscopy techniques using a transmission and an aberration-corrected scanning-transmission electron microscope. The hydrothermal treatment for 38 hours at 200 degrees C led to the formation of nanowires with a metastable orthorhombic structure and nanoplatelets with an equilibrium Aurivillius layered-perovskite structure at lower (<= 1 mol L-1) and higher (>1 mol L-1) NaOH concentrations, respectively. At the initial stage of the hydrothermal synthesis, a mixture of two metastable polymorphs was formed: the nanowires and the highly defective perovskite phase. The perovskite phase in the form of globular aggregates of nanocrystallites only contained Bi3+ and Ti4+ cations (Na+, K+ and Ti3+ were not detected). In the continuation of the hydrothermal treatment, the aggregates of perovskite nanocrystals dissolved at lower NaOH concentrations, whereas at higher NaOH concentrations the nanowires dissolved while the Aurivillius nanoplatelets grew from the surfaces of the perovskite aggregates.