Effect of Cationic Dopant on the Physicochemical Properties and Visible Light Photocatalytic Degradation Activity of (M,N) Codoped TiO₂ Nanoparticles (M = Nb, Ta, W)

(M,N) codoped TiO 2 nanoparticles (with M = Nb, Ta, or W) were synthesized via a sol–gel process, followed by a thermal nitridation in ammonia. The doping cation role in determining the codoped materials' physicochemical properties and their photocatalytic activity was studied. In situ temperature-dependent XRD analysis indicated an inhibition of rutile formation due to cation doping for all codoped TiO 2 nanomaterials. Depending on the thermal nitridation conditions, two types of absorption behaviors have been identified for each (M,N) codoped sample family: mild nitridation induced an absorption increase between 400 and 550 nm, while harsh nitridation induced additional absorption at a longer wavelength (>550 nm). Furthermore, the presence of W in the TiO 2 lattice has been revealed to be more favorable than Nb and Ta to promote nitrogen insertion. A similar charge compensation mechanism, involving defects such as oxygen vacancies, was revealed for (Nb,N) and (Ta,N) codoped materials: low defect concentrations were achieved by the mild nitridation, while the harsh nitridation created a large amount of Ti 3+ acting as charge recombination centers. On the opposite end, the defect concentration for (W,N) codoped TiO 2 nanoparticles remains low under both mild and harsh nitridation conditions. Additionally, for (W,N) codoped TiO 2 samples, W 5+ was created by the harsh nitridation condition instead of Ti 3+, resulting in an improved visible light photocatalytic activity.