Twinning Driven Growth of Manganese Oxide Hollow Cones through Self-Assembly of Nanorods in Water

Hollow gamma-MnO2 nanocones are synthesized by precipitation in water at 95 degrees C through a "one-pot" procedure. Characterization of the particles is carried out using powder XRD, electron diffraction, FESEM, and TEM. The hexagonal-shaped base is shown to originate from specific {021} twinning. The growth mechanism is investigated and proposed to proceed through two steps. The first stage involves the growth of pyramidal seeds that direct the final shape of the particles. The second stage proceeds through oriented attachment of primary alpha-MnOOH nanorods on previously formed gamma-MnO2 cones. Attachment is accompanied by phase transformation of the oxyhydroxide to the oxide by a topotactic reaction, because of the structural relations between both structures. This study reports on the first demonstration of specific twinning from oriented attachment. This also provides a deep insight into the role of structural defects for further design of complex and novel architectures. Additionally, the procedure developed herein occurs in water without any surfactant. It is therefore low cost and environmentally friendly.