SrTiO3/Nb2O5 Composites via Sol–Gel Synthesis: Structural, Optical, Dielectric and Photocatalytic Properties Under UV and Visible Light

SrTiO3/Nb2O5 composite materials with 1 and 10 wt.% Nb2O5 were prepared by a sol–gel route and characterized by XRD, Raman spectroscopy, SEM, BET, UV-Vis DRS, photoluminescence, and dielectric spectroscopy. The photocatalytic activity was evaluated via degradation of rhodamine B and tetracycline under UV and visible light. The addition of Nb2O5 resulted in a significant reduction in specific surface area (from 22.7 to 3.1 m2/g), a narrowing of the optical band gap (from 3.22 to 2.49 eV), and a decrease in photoluminescence intensity. Despite these changes, the photocatalytic performance decreased with increasing Nb2O5 content. For rhodamine B degradation, the UV rate constant fell from 0.0136 min−1 for pristine SrTiO3 to 0.0035 min−1 for SrTiO3/10% Nb2O5. The lower activity is mainly ascribed to the loss of active sites, surface carbonate formation, and enhanced non-radiative recombination at interface defects, which suppress charge transfer to the surface. The results demonstrate that careful control of the composite microstructure is essential for achieving efficient photocatalysis, even when heterojunction formation is thermodynamically favorable.