Site-dependent photoinduced charge carrier dynamics in nitrogen/fluorine doped TiO2 nanoparticles

Abstract

Nitrogen/fluorine doping of TiO₂ nanoparticles (NPs) serves to play an important role with regard to visible light absorption and charge carrier dynamics. However, it is largely unknown how nitrogen/fluorine doping retards electron–hole recombination. We carried out density functional theory and nonadiabatic molecular dynamics to demonstrate that the dopant levels, formation energies, and charge carrier dynamics depend strongly on the doping site. N-doping introduces both a hole and an electron trap without the presence of oxygen vacancies, while F-doping introduces only an electron trap. Quantum dynamics simulations show that holes are trapped within several picoseconds; N-doped NPs requires longer hole-trapping time than F-doped NPs due to the presence of an extra hole trap originating from the N 2p state. Electrons are trapped at a longer time scale than holes. F-doped NPs required longer electron-trapping time due to more strongly localized electron densities than N-doped TiO₂ NPs. In general, Nitrogen and fluorine doping suppressed electron–hole recombination compared to pristine NP. NPs doped at the tip domain showed different charge recombination pathways by passing several hole traps (N-doping) or by dual channel routes (F-doping). NPs doped at edge domains showed relatively shorter excited state lifetime due to greater electronic coupling between trapped holes and electrons. NPs with doping at the inside domain effectively suppressed charge recombination due to a moderate energy gap and a reduced propensity to electron/hole localization. The diverse charge recombination scenarios revealed by nonadiabatic molecular dynamics simulations provided guidelines for the rational design of nanoscale metal oxides for solar energy harvesting and utilization.