Fast Energy Relaxation by Trap States Decreases Electron Mobility in TiO2 Nanotubes: Time-Domain Ab Initio Analysis

Highly ordered TiO2 nanotube arrays can be grown by simple electrochemical anodization of a titanium metal sheet, stimulating intense research and applications to solar cells and fuels. TiO2 nanotubes were expected to exhibit better electron transport than nanocrystal films. However, experiments showed that nanotubes provided little advantage over nanoparticles. Using nonadiabatic molecular dynamics, we demonstrate that oxygen vacancies, which are common in TiO2, accelerate nonradiative energy losses by an order of magnitude. Oxygen vacancies produce localized Ti(3+) states hundreds of millielectronvolts below the TiO2 conduction band. The states lower the nanotube band gap, trap excited electrons, induce stronger electron-phonon couplings, and facilitate the relaxation. Our results rationalize the unforeseen experimental observations and provide the atomistic basis for improving the structure and charge transport by TiO2 nanotubes.