Evidence of linear lattice expansion and covalency enhancement in rutile TiO2 nanocrystals

Lattice variations and bonding characteristics in rutile TiO2 nanocrystals were examined by x-ray diffraction and x-ray photoelectron spectroscopy. With a reduction in the physical dimensions, rutile TiO2 nanocrystals show a linear lattice expansion and an anomalous covalency enhancement in apparent contradiction to the ionicity increase in BaTiO3 and CuO nanocrystals as reported recently by S. Tsunekawa [Phys. Rev. Lett. 2000, 85, 3440] and V. R. Palkar [Phys. Rev. B 1996, 53, 2167]. A surface defect dipole model is proposed to explain these physical phenomena in terms of the strong interactions among the surface dipoles that produce an increased negative pressure. The covalency enhancement is interpreted according to the critical properties of the increased Ti-O bond lengths in the expanded lattice. (C) 2004 American Institute of Physics.