Multitwinned Spinel Nanowires by Assembly of Nanobricks via Oriented Attachment: A Case Study of Zn2TiO4

Nanowires with twinned morphology have been observed in many cubic-phase materials including spinel. We study systematically the formation of multitwinned Zn2TiO4 nanowires based on a solid-solid reaction of ZnO nanowires with a conformal shell of TiO2, which is deposited by atomic layer deposition (ALD). By varying the solid-state reaction temperature, reaction time, and TiO2 shell thickness, the formation process is carefully analyzed with the help of transmission electron microscopy. It is found that the multitwins develop through an oriented attachment of initially separated spinel nanobricks and a simultaneous Ostwald ripening process. The oriented assembly of the individual bricks is strongly dependent on annealing conditions, which is required to favor the motion and interaction of the bricks. This mechanism differs dramatically from those proposed for twinned nanowires grown with the presence of metal catalysts. Our result provides new insights on controlling the morphology and crystallinity of designed 1-D nanostructures based on a solid-state reaction route.