Synthesis of layered tantalate nanocrystals by aqueous process at room temperature

Colloidal suspensions of highly water-dispersible tantalate were prepared by the acid-base reaction of tetramethylammonium hydroxide (TMAOH) and tantalic acid, which was formed viathe hydrolysis of tantalum ethoxide, in water at a molar ratio of TMAOH/Ta greater than or equal to 0.7. Despite the preparation at room temperature, the obtained TMA tantalate was a crystalline compound with a layered hexatantalate structure, TMA sub(4)Ta sub(6)O sub(17 ).nH sub(2)O. Many of the tantalate nanocrystals were about 50 nm in lateral size and consisted of 6 to 12 tantalate layers. The tantalate structure was independent of the TMAOH/Ta ratio although other layered tantalates, such as MTaO sub(3) (M super(+) = alkaline metal ion or H super(+)), are known. Moreover, at TMAOH/Ta less than or equal to 0.6, the prepared sols were turbid and the precipitates were amorphous. Thus, the TMAOH/Ta ratio required for the formation of the highly water-dispersible tantalate nanocrystals likely has a relation to the chemical composition, TMA sub(4)Ta sub(6)O sub(17 ).nH sub(2)O (TMA/Ta = 0.67).