Impregnation versus Bulk Synthesis : How the Synthetic Route Affects the Photocatalytic Efficiency of Nb/Ta:N Codoped TiO2 Nanomaterials

Selecting the proper doping strategy is essential to controlling the photocatalytic activity of TiO2-based nanomaterials. In this work, we compare impregnation and bulk synthesis methods for feeding titania nanocrystals with either Nb or Ta as transition metal dopant and N as nonmetal codopant. The resulting photocatalytic efficiency was tested toward ethanol degradation under either UV or simulated solar irradiation. Microstructure, morphology, and electronic properties at various length scales were deeply investigated and compared with DFT simulations. Instead, under UV irradiation, impregnated samples performed better than bulk synthesis ones, with Ta-doped powders being more efficient than Nb-doped and undoped TiO2. Under simulated solar irradiation, bulk synthesis Nb-doped materials were the most active ones, while all the impregnated samples were even less performing than the undoped TiO2 reference. On the basis of XPS, EPR, DRS, and XRPD results, such differences were attributed to the bulk synthesis approach producing a more homogeneous distribution of guest dopants within the grains, in conjunction with a higher amount of intrinsic defects (such as O vacancies). Implications of these findings on the engineering of efficient titania photocatalysts are discussed.