ORIENTED ATTACHMENT AND GROWTH, TWINNING, POLYTYPISM, AND FORMATION OF METASTABLE PHASES - INSIGHTS FROM NANOCRYSTALLINE TIO2

Atomic-resolution transmission electron micrographs show that nanocrys talline TiO2 coarsens by oriented attachment and growth under hydrothe rmal conditions. In addition to forming homogeneous single crystals, a ttachment at anatase surfaces leads to twinning on {112} and intergrow ths on (001) and {001}. Brookite, a polytype of anatase, occurs at som e {112} twin surfaces. Alternating two octahedra-wide structural slabs in brookite are shared with the two adjacent anatase twin domains. Be cause {112} anatase twin interfaces contain one unit cell of brookite, we propose that brookite may nucleate at twin planes and grow at the expense of anatase. Alternatively, anatase-brookite interfaces may for m by oriented attachment of primary brookite and anatase {112} crystal lites. In this case, three unit cell-wide lamellae of brookite are int erpreted as remnants of larger crystals that partly converted to anata se by propagation of the anatase-brookite interface. Which phase is st able is unclear over this particle size range, and products of random thermal fluctuations may be preserved by quenching. Regardless of reac tion direction, polytypic interconversion of anatase and brookite esse ntially involves displacement of Ti (by c/4 brookite) into adjacent oc tahedral sites in one of the pair of two octahedra-wide structural sla bs. The results have broad relevance for nucleation and growth models as they suggest that twinning and polytypism in macroscopic crystals c an originate at oriented interfaces between primary nanocrystalline pa rticles early in their crystallization history.