Theoretical Study of Stable, Defect-Free (TiO2)n Nanoparticles with n = 10−16

Novel defect-free (TiO 2 ) n nanoparticles can be very useful for mechanistic studies of photooxidation of water and organic pollutants on nanostructured oxide materials involving surface-trapped electrons and holes. In this study, the electronic structure and stability of (TiO 2 ) n nanoparticles with n = 10−16 (∼1 nm in diameter) have been investigated using the density functional B3LYP/LANL2DZ method to find stable, Ti O defect-free structures. The even- n (TiO 2 ) n clusters tend to form compact (rather than linear or cyclic) covalent networks consisting of only 4-coordinated Ti (4) and 2-coordinated O (2) atoms, while odd- n clusters tend to form more ionic structures with additional highly coordinated Ti and O atoms. The new half-spherical (TiO 2 ) 15 15b cluster represents the smallest defect-free rutile nanocrystal. Strong vibrational bands around 770 and 820 cm - 1 are found to be useful for identifying surface O (2) and O (3) species, respectively. Consistent with the odd−even oscillation of structural features of stable (TiO 2 ) n nanoparticles, various electronic properties, including cluster formation energies, HOMO−LUMO gaps, vertical excitation energies, and ionization potentials, also show odd−even oscillations. Strong interatomic O(2p) lone pair interactions that destabilize the highest occupied molecular orbitals of the (TiO 2 ) n clusters can significantly reduce the lowest vertical excitation energy to values significantly below the band gap of bulk TiO 2 for clusters with odd n . If materials can be engineered to incorporate such (TiO 2 ) n nanoparticles, they could exhibit visible light photoactivity. The electron holes are shown to be localized mainly on surface O (1) and O (2) rather than O (3) sites, suggesting that a recently proposed water photooxidation mechanism [Nakamura et al. J. Am. Chem. Soc. 2005, 127, 12975] should be revised.