Charge carrier dynamics and visible light photocatalysis in vanadium-doped TiO2nanoparticles

Vanadium-doped TiO2nanoparticles (V-TiO2NPs) with a V/Ti ratio of 3.0 at. % were prepared by gas-phase condensation and subsequent oxidation at elevated temperature. Both photocatalytic activity for -NO2reduction and photoelectrochemical water splitting were induced by V-doping in the visible spectral range λ> 450 nm, where undoped TiO2NPs are completely inactive. The photocatalytic properties were correlated with the ultrafast dynamics of the photoexcited charge carriers studied by femtosecond transient absorption (TA) spectroscopy with three different excitation wavelengths, i.e. λe= 330, 400, and 530 nm. Only in V-doped NPs, the photoexcitation of electrons into the conduction band by sub-bandgap irradiation (λe= 530 nm) was detected by TA spectroscopy. This observation was associated with electronic transitions from an intra-gap level localized on V4+cations. The photoexcited electrons subsequently relaxed, with characteristic times of 200–500 ps depending on λe, into Ti-related surface traps that possessed suitable energy to promote -NO2reduction. The photoexcited holes migrated to long-lived surface traps with sufficient overpotential for the oxidization of both 2-propanol and water. On the basis of TA spectroscopy and photocurrent measurements, the position of the dopant-induced intra-gap level was estimated as 2.2 eV below the conduction band minimum.