Abstract
We have monitored the hydrothermal crystallization of titania nanoparticles by in situ X-ray diffraction (XRD).
Using the refined average structures from the XRD measurements, we calculated potential energy variations
with particle size on periodic bulk structures using density functional theory (DFT). These variations cannot
account for the enthalpy required to stabilize anatase relative to rutile. Thus, the hypothesis that the strain of
the surface structure of nanoparticles accounts for the stabilization of anatase is not applicable to the growth
of titania in water. DFT calculations on model nanoparticles do generate lower surface energies for anatase
than for rutile that are large enough to explain the stability reversal in nanoparticles relative to the bulk
phase. Rather than arising from two-dimensional surface structure alone, as previously thought, the total
surface energies are critically dependent upon defects associated with edges and corners of nanocrystals at
particle sizes e3 nm (i.e., during the nucleation process). As the particles grow, the bulk free energy becomes
relatively more important, causing rutile to become stable at larger particle sizes. This study quantifies for
the first time the critical role of edge and vertex energies in determining the relative phase stabilities of TiO2
nanoparticles.
