Straight and Branched Goethite Topology by Oriented Attachment at High pH

In this study, we report novel anisotropic nanostructures of goethite (alpha-FeOOH) nanocrystallites induced by oriented attachment at high pH in the absence of organic additives. Combined transmission electron microscopy, high-resolution transmission electron microscopy, and atomic force microscopy analyses shed new light on different stages of the oriented attachment process. It is found that a straight prerod intermediate was assembled by crystallographically aligned nanoparticles with a rectangular shape; a V-shaped structure with an angle of (118.5 ± 3.5)° was connected by apexes of two needle-shaped rods sharing three comparable dimensions, while a Y shape was constructed by a V-shaped rod and a straight one through planar stacking. This is the first example of such a straight and branched topology achieved at high pH without the assistance of organic additives. Our findings will be potentially applicable to a range of naturally abundant colloidal nanocrystallites (e.g., metal (oxyhydr)oxide and sulfide minerals) occurring in the evolution of hydro-, bio-, and geochemical systems. The results from this study could provide broad implications in self- and directed-engineering complex nanostructures through the oriented attachment mechanism.