Band structure and charge carrier dynamics in (W,N)-codoped TiO₂ resolved by electrochemical impedance spectroscopy combined with UV-vis and EPR spectroscopies

Semiconductor photocatalysis is on the verge of (probably) its most important deployment and boost since the pioneering paper of Fujishima and Honda in 1972. Photo-generation of unbound excitons, i.e. separated conduction band electrons and valence band positive holes, is the fundamental primary process triggering charge separation in solid semiconductors necessary to initiate their photocatalytic activity. Immediately after being generated, charge carriers can undergo processes like recombination, trapping in mid-band-gap states or, paramount for photocatalytic processes, transfer to species adsorbed on the solid semiconductor surface. In TiO₂ and doped TiO₂, interfacial charge transfers are the slowest amongst the primary processes; therefore, electron (and hole) transfer most likely occurs from single electron traps (i.e. involving radical species). We report here on an effective approach combining electrochemical impedance spectroscopy with other spectroscopic techniques such as UV-vis and electron paramagnetic resonance. This approach allows deriving important information about band structure and following electron dynamics triggered by photon absorption. The redox potentials of the band edges and the influence of the dopants on the band structure are elucidated by electrochemical impedance spectroscopy combined with UV-vis spectroscopy. Electron dynamics are then studied using electron paramagnetic resonance spectroscopy, to elucidate the photochemical reactions at the basis of the photo-generated electron-hole pairs, and subsequent trapping and/or recombination. Results of a TiO₂ sample containing W and N as dopants (0.1at.% of W) highlight a narrowing of the intrinsic band gap of about 0.12eV. The semiconductor visible light photochemistry is driven by diamagnetic donor states [N_iO]^-, and [N_iO]w^- (formally NO^3-), from which electrons can be excited to the conduction band, generating EPR active paramagnetic [N_iO] and [N_iO] _w states (formally NO^2-). The formation of W⁵⁺ electron trapping states, energetically more favourable than Ti³⁺ electron trapping states, is also identified.