Photogenerated Charge Carriers and Paramagnetic Species in (W,N)-Codoped TiO2 Photocatalysts under Visible-Light Irradiation: An EPR Study

Abstract

(W,N)-codoped TiO2 has recently attracted interest due to substantial band gap narrowing in conjunction with good visible-light photocatalytic activities. A complete picture of the fundamental mechanism at the origin of their photoactivity is, however, far from being understood. We present an EPR study on (W,N)-codoped titanias by recording spectra in the dark and under 550 nm visible-light irradiation to identify the major species involved in the formation and migration of photogenerated charge carriers. Interstitial N–lattice O (formally NO2–) paramagnetic groups with and without a specific close range coupling interaction with a neighboring W, that is, [NiO]W● and [NiO]●, respectively, are observed when visible light excites electrons from diamagnetic intraband gap states located at ∼2.30 V versus RHE (formally NO3–) to the conductance band at around −0.05 V versus RHE. The [NiO]W● EPR signal, reported here for the first time, is characterized by a much stronger hyperfine interaction between the unpaired electron and the 14N nucleus than ordinary [NiO]● found in N-doped TiO2, leading to a much higher spin density on the N center. The overall signal also contains superhyperfine coupling of the unpaired electron with the 183W nuclide (natural abundance 14.31%), the only naturally occurring nuclide of W with a nonzero nuclear spin.