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, highresolution 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.