Charge carrier dynamics and visible light photocatalysis in vanadium-doped TiO2 nanoparticles

Vanadium-doped TiO2 nanoparticles (V-TiO2 NPs) 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 -NO2 reduction and photoelectrochemical water splitting were induced by V-doping in the visible spectral range lambda > 450 nm, where undoped TiO2 NPs 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. lambda(e) = 330, 400, and 530 nm. Only in V-doped NPs, the photoexcitation of electrons into the conduction band by sub-bandgap irradiation (lambda(e) = 530 nm) was detected by TA spectroscopy. This observation was associated with electronic transitions from an infra-gap level localized on V4+ cations. The photoexcited electrons subsequently relaxed, with characteristic times of 200-500 ps depending on lambda(e), into Ti-related surface traps that possessed suitable energy to promote -NO2 reduction. 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 infra-gap level was estimated as 2.2 eV below the conduction band minimum.