On the entangled growth of NaTaO3 cubes and Na2Ti3O7 wires in sodium hydroxide solution

Abstract

The entangled growth of sodium titanate Na2Ti3O7 nanowires and sodium tantalate NaTaO3 cubes was investigated with electron microscopy, X-ray diffraction, and UV diffuse reflectance spectroscopy. Depending on the composition of the Ta2O5- and TiO2-particle-based powder mixtures, which served as educts, we observed different types of hybridization effects. These include the titanium-induced contraction of the NaTaO3 perovskite-type unit cell and the generation of electronic defect states in NaTaO3 that give rise to optical subbandgap transitions and tantalum-induced limitations of the Na2Ti3O7 nanowire growth. The transformation from Ta2O5 to NaTaO3 occurs through a dissolution-recrystallization process. A systematic analysis of the impact of different titanium sources on NaTaO3 dispersion and, thus, on the properties of the entangled nanostructures revealed that a perfect intermixture of cubes and nanowires can only be achieved when titanate nanosheets emerge during transformation as reaction intermediates and shield nucleation and growth of isolated NaTaO3 cubes. The here demonstrated approach can be highly instrumental for understanding the nucleation and growth of composite and entangled nanostructures in solution and--at the same time--provides an interesting new class of photoactive composite materials.

Keywords: crystal growth; doping; nanostructures; optical properties; tantalum.