Morphology-controlled nonaqueous synthesis of anisotropic lanthanum hydroxide nanoparticles

Abstract

The preparation of lanthanum hydroxide and manganese oxide nanoparticles is presented, based on a nonaqueous sol–gel process involving the reaction of La(O iPr) 3 and KMnO 4 with organic solvents such as benzyl alcohol, 2-butanone and a 1:1 vol. mixture thereof. The lanthanum manganese oxide system is highly complex and surprising results with respect to product composition and morphology were obtained. In dependence of the reaction parameters, the La(OH) 3 nanoparticles undergo a shape transformation from short nanorods with an average aspect ratio of 2.1 to micron-sized nanofibers (average aspect ratio is more than 59.5). Although not directly involved, KMnO 4 plays a crucial role in determining the particle morphology of La(OH) 3. The reason lies in the fact that KMnO 4 is able to oxidize the benzyl alcohol to benzoic acid, which presumably induces the anisotropic particle growth in [0 0 1] direction upon preferential coordination to the ±(1 0 0), ±(0 1 0) and ±(−110) crystal facets. By adjusting the molar La(O iPr) 3-to-KMnO 4 ratio as well as by using the appropriate solvent mixture it is possible to tailor the morphology, phase purity and microstructure of the La(OH) 3 nanoparticles. Postsynthetic thermal treatment of the sample containing La(OH) 3 nanofibers and β-MnOOH nanoparticles at the temperature of 800 °C for 8 h yielded polyhedral LaMnO 3 and worm-like La 2O 3 nanoparticles as final products.