Electronic and Optical Properties of Doped and Undoped (TiO2)n Nanoparticles

The geometry and electronic structure of bare and anion-doped nanoscale (TiO2)(n) clusters, where n = 1, 2, 3, 4, 5, 6, 7, 10, and 13, were calculated using density functional theory (DFT) and lime dependent density functional theory ODDITY Initial (TiO2) structures were chosen from the global minima obtained using an evolutionary algorithm. With increasing nanoparticle size we find the HOMO-LUMO transition energy saturates toward bulk values for surprisingly small values of n. For the C-, N-, and S-substitutional doped nanoparticles, all dopant formation energies are lower than those calculated for bulk rutile. Both C- mid N-dopants prefer to reside on the 4-fold oxygen site, maximizing their coordination, while the S-dopants reside on 2-fold sites On the exterior of the cluster. The anion dopant with the lowest formation energy is sulfur, then nitrogen, with carbon having the largest formation energy. All of the dopants reduce the transition energy, with nitrogen-doping giving a transition energy that is too low (at approximately 1.0 eV) for water-splitting applications. Carbon and sulfur dopants give transition energies that are close to the peak in the solar spectrum (similar to 2.5 eV), and thus are more efficient at photoconversion than undoped nanopartieles. Through analysis of the frontier molecular orbitals and determination of the optical spectra (within TDDFT) we conclude that carbon is the optimum dopant for maximizing the pholoactivity of subnanometer (TiO2)(n) nanoparticles.