Hierarchical rutile TiO2 mesocrystals (RTMs) were synthesized on a large scale by a one-step hydrothermal method under a weak acid assistant. The structure and morphologies of these RTMs are investigated by detailed transmission electron microscopy (TEM). It is demonstrated that the branched structures of RTMs exhibit a sword-like shape at the end with exposed {101} facets, and a twinned {200} plane is found in each branched nanorod. Additionally, another {101} twinned plane is formed at the interface of the “trunk” and “branch” with a 65° angle. Interestingly, it has been found that homogeneous nanocrystals with diameters below 14 nm are epitaxially attached on the surface of the {110} planes with a lower surface energy at an angle of about either 113° or 67°. Correspondingly, a model of crystal planes as building blocks of RTMs is proposed to illustrate the complex structure of the RTMs. The growth mechanism of 3D RTMs is ascribed to the nanocrystals-oriented attachment and the formation of rutile {101} twinned structures.
