Theoretical study of stable, defect-free (TiO2)n nanoparticles with n=10-16

Novel defect-free (TiO2)(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 TiO2)(n), nanoparticles with n = 10- 16 (similar to 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 (TiO2)(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 0 atoms. The new half-spherical (TiO2)(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 JiOA, 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 (TiO2)(n) clusters can significantly reduce the lowest vertical excitation energy to values significantly below the band gap of bulk TiO2 for clusters with odd n. If materials can be engineered to incorporate such JiOA, 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.