Hydrothermal Synthesis of Octadecahedral Hematite (α-Fe2O3) Nanoparticles: An Epitaxial Growth from Goethite (α-FeOOH)

Driven by the demand for shape-controlled synthesis of alpha-Fe2O3 nanostructures and the understanding of their growth mechanism and shape-dependent properties, we report the synthesis of octadecahedral alpha-Fe2O3 nanocrystals with a hexagonal bipyramid shape by introducing F- anions in the solution. The hydrothermal growth process from hydrolysis of Fe3+ precursors involves three steps: the nucleation of akaganeite (beta-FeOOH) nanorods, followed by the formation of goethite (alpha-FeOOH) crystals with acicular and twinned shapes, and a subsequent transformation into hematite (alpha-Fe2O3) nanoparticles. The phase transformation and growth of alpha-Fe2O3 particles from alpha-FeOOH follows dissolution of goethite and reprecipitation as hematite process. The initial nucleation of alpha-Fe2O3 particles was found to form epitmially on goethite {001} surfaces due to a perfect lattice match between goethite {001} surface and hematite {001} planes. The structural relationship between goethite and hematite is G(020)//H(030) with G[100]//H[100]. The obtained alpha-Fe2O3 hexagonal bipyramid particles are enclosed by 12 {113} planes and six {104} facets. Since the twinned alpha-FeOOH particles are one of the typical shapes of intermediate goethite crystals, the nucleation of hematite particles on two twinned arms gives rise to the formation of twinned hematite particles. F- anions play an important role in the formation of alpha-Fe2O3 particles with a hexagonal bipyramid shape because high concentration of F- anions can stabilize the exposed {113} surfaces. The controlled synthesis of alpha-Fe2O3 nanoparticles with defined surfaces not only provides significant information on hematite surface structures and energies but also is critical to give the structure-property relationship for the application of hematite materials.