Void space and secondary oriented attachment mechanisms in cerium oxide nanorods

Two new processes occurring during nanorod annealing of ceria are disclosed, both belonging to the wider “oriented attachment” (OA) scheme of crystal growth, which commonly drives nanoparticles growing into nanorods. The first new process is an inversion of the standard OA, where the usual solid particles are replaced by well-facetted shape-equilibrated voids inside larger single crystals. The internal facetted voids are then found to aggregate during dry heat treatment into rod-shaped elongated voids growing eventually towards nanotubes. For the case of CeO ₂, a perfect equivalence is found between positive OA, involving cuboctahedral {111}/{100} nanoparticles turning into {110}/{100} nanorods, and negative or void space OA, where cuboctahedral voids turn into negative tubular rods of same indexing. The second OA process (“secondary OA”) concerns aggregation of small nanorod segments into larger, double, or quadruple sized nanorods, with perfection of alignment and bonding exceeding simple van der Waals forces. Eventually, the new rods merge into single crystal grain boundary–free larger rods, but with an external shape, including double-ended rods, indicating their origin from several rods. Both processes are found on identical samples and occur in parallel.