Visible Light Absorption Is Not Always Related to N‑Doped TiO2 and High Photocatalytic Activity in Materials Synthesized by the Sol–Gel Method Using Urea and Ammonia as Precursors

For years, the sol–gel synthesis of visible-light absorbing TiO2 modified with ammonia and urea has been considered to generate nitrogen doping and low photocatalytic activity. Herein, it was found that urea modification of TiO2 at 450 °C led to the generation of g-C3N4/TiO2 type II heterojunctions containing, in addition, single-electron trapped oxygen vacancies (SETOV, Vo) and Ti3+ species all responsible for visible light absorption. On the other hand, ammonia-modified TiO2 may lead, at 450 °C, to generating NO-TiO2 surface complexes and Vo as responsible for visible light absorption. In contrast, at 550 °C, a high content of Vo would exclusively be responsible for light harvesting. It is suggested that the ammonia presence in both synthesis methods would participate in the generation of defective surface TiO2. By ESR-spin trapping, it was found that all the synthesized materials exhibited photogeneration of •OH radicals only under UV irradiation, and their photocatalytic activity was evaluated under UV- and blue-LED irradiation in the malachite green (MG) solutions. The presence of g-C3N4/TiO2 type II heterojunctions caused a detrimental effect on the UV-LED photocatalytic activity, while under blue-LED irradiation, a discoloration close to 58% was observed. Ammonia-modified TiO2 materials did not exhibit interesting photocatalytic activity under blue-LED irradiation.