Harmonic NaNbO3 and KNbO3 nanoparticles via solvothermal synthesis in glycols

Abstract

Soft chemical methods for the synthesis of alkali niobates are so far very limited due to the insufficient knowledge of the underlying niobium chemistry and the influences of the solvents used. With the aim of synthesizing particles of high quality without the use of water as an additional solvent, a set of glycols was used here. Glycols combine several benefits as they possess a large dipole moment, a relatively high boiling point, are inexpensive and pose little toxicological risk. We show that solvothermal synthesis in glycols without the addition of water or surfactants produces small particles of alkali niobates in the size range from above 100 nm down to about 5 nm, depending on the glycol used, with a relatively narrow size distribution. NaNbO₃ and KNbO₃ crystallize as orthorhombic nanocrystals. The KNbO₃ obtained, however, can also be interpreted as monoclinic, especially if the particle size is small. While there is no obvious size dependence in the corresponding band gaps, smaller particles show a lower harmonic response than larger ones.