Unraveling the quantum dynamics origin of high photocatalytic activity in nitrogen-doped anatase TiO2: time-domain ab initio analysis

A wide bandgap and short-lived charge carrier constitutes two major issues for restricting anatase titanium dioxide (TiO2) photocatalytic activity in the ultraviolet light region of the solar spectrum. Interestingly, experiments reported that anatase TiO2 doping with substitutional N can achieve a high visible-light photocatalytic activity but the mechanism remains controversial and unclear yet. N substituting oxygen creates a mid-gap state, which typically acts as a nonradiative charge recombination center. Using the nonadiabatic (NA) molecular dynamics, we demonstrate that charge carrier lifetimes of N-doped anatase TiO2 are notably prolonged regardless of the oxidation states of the N dopant, but it operates by different mechanisms. The neutral and negatively charged N dopant reduces the bandgap by creating either an electron or a hole trap state prior to recombination with free holes and free electrons, respectively. While the direct recombination of free electrons and free holes, bypassing the electron trap state, dominates the nonradiative relaxation in the neutral N-doped TiO2 and extends the charge carrier lifetimes over 4-fold compared to the pristine anatase TiO2; the hole-trap-assisted electron-hole recombination beats the direction pathway in the negatively charged N-doped TiO2 system, delaying the nonradiative charge recombination over (22 times slower) the pristine system. Our simulations point out to changes in the relative values of NA coupling, decoherence times and energy gaps as determinants of the aforementioned changes in the carrier lifetimes. In this way, our study rationalizes the long-term debate on the enhanced visible-light photocatalytic activity of the TiO2 doping with N and, therefore, it contributes to rational defect engineering for design of high performance of photocatalytic and optoelectronic devices based on TiO2 and other metal oxides.