Highly dispersed spherical Bi 3.25 La 0.75 Ti 3 O 12 nanocrystals via topotactic crystallization of aggregation-free gel particles from an effective inverse miniemulsion sol–gel approach

Aggregation-free spherical lanthanum-doped bismuth titanate (Bi 3.25 La 0.75 Ti 3 O 12 , BLT) gel particles with an average size of about 150 nm were successfully obtained from an inverse miniemulsion sol–gel process, with Span-80 acting as surfactant, n -butanol as co-surfactant, cyclohexane as continuous phase, and submicro-droplets of aqueous solution containing Bi 3+ , La 3+ and Ti 4+ ions as dispersed phase, and then topotactically transformed into highly dispersed spherical BLT nanocrystals after an in situ crystallization at 600 °C for 8 h. It has been found that the BLT gel particles can be obtained via a moderate sol–gel reaction inside the miniemulsion droplets at 65 °C, but their morphology and aggregation degree are strongly affected by the relative amounts of Span-80 and n -butanol. The perfect spherical BLT gel particles with no aggregation can be achieved only under the condition of 3 wt% n -butanol relative to the mass of cyclohexane, with excessive amount of n -butanol leading to the formation of ill-gelled particles with irregular shapes, while insufficient addition of n -butanol resulting in terrible aggregation of gel particles. To understand the formation of aggregation-free spherical BLT gel particles, a tentative mechanism is proposed and discussed, which reveals that a well-coordinated oil–water interfacial film made up of Span-80 and n -butanol molecules and the appropriately enhanced evaporation of water from such interfaces should be responsible for the formation of aggregation-free spherical BLT gel particles. Graphical Abstract Aggregation-free spherical BLT (Bi 3.25 La 0.75 Ti 3 O 12 ) gel particles can be prepared from an effective inverse miniemulsion sol–gel process, and subsequently topotactically transformed into spherical BLT nanocrystals through an in situ crystallization.