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 via the hydrolysis of tantalum ethoxide, in water at a molar ratio of TMAOH/Ta ≥ 0.7. Despite the preparation at room temperature, the obtained TMA tantalate was a crystalline compound with a layered hexatantalate structure, TMA₄Ta₆O₁₇·nH₂O. 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 MTaO₃ (M⁺ = alkaline metal ion or H⁺), are known. Moreover, at TMAOH/Ta ≤ 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, TMA₄Ta₆O₁₇·nH₂O (TMA/Ta = 0.67).