Unraveling the Cooperative Mechanism of Visible-Light Absorption in Bulk N,Nb Codoped TiO2 Powders of Nanomaterials

N,Nb-codoping has recently been proposed as a promising strategy to enhance the activity of nanostructured TiO2 under visible irradiation. Here, we suggest a possible electronic mechanism to account for the observed visible absorption improvement. The effects of N and Nb species on the electronic, crystallographic, and morphological properties of TiO2 were deeply investigated both experimentally (HR-XRPD, EXAFS, EDX, BET, SEM, EPR, and DRS) and theoretically (DFT). We found a significant synergism between N and Nb species, while EXAFS, HR-XRPD, and DFT simulations provided compelling evidence for the Nb substitutional position in anatase. At variance with interstitial, substitutional Nb can transfer an electron to low-energy valence states of the N codopant near the valence band. This intrinsic charge compensation mechanism is substantiated by EPR, which shows a reduction of the paramagnetic bulk N species signal in N,Nb-codoped samples. DRS analysis of N,Nb-codoped samples shows a slight reduction of the apparent band gap and a significantly increased visible-light absorbance. This effect is due to the shallow midgap states created by Nb (below conduction band) and N (above valence band). DFT results suggest that substitutional Nb ions transfer electrons to low-lying guest N states within the band gap, eventually enhancing the light absorption.