Thermodynamic modelling of nanomorphologies of hematite and goethite

Iron oxide and oxyhydroxide nanoparticles are among the most important mobile and catalytic agents in a variety of biogeochemical environments, and are being increasingly synthesized for energy, electronic, catalyst, environmental and medical applications. The morphologies at nanoscale are relevant to the control of shapes and sizes, surface chemistry, and performance of these nanoparticles, as well as our understanding of naturally occurring processes. Therefore, we have begun to develop this understanding by studying the relationship between size, shape, and thermodynamic stability of unpassivated hematite (α-Fe₂O₃) and goethite (α-FeOOH) nanoparticles, using a robust thermodynamic morphology model with input parameters from reliable first-principles calculations and thermochemical data. The results revealed the thermodynamic stable shapes of hematite and goethite nanoparticles, and demonstrated that the phase transformation from goethite to hematite is highly dependent on the particle size and temperature. Goethite nanoparticles are thermodynamically stable with small sizes, compared to hematite, but the equilibrium transformation temperature increases rapidly with decreasing particle size. The morphology sensitive phase transformation predicted by our model is a step further towards a nanophase diagram of iron oxides and oxyhydroxides.