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, V_o) 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 V_o as responsible for visible light absorption. In contrast, at 550 °C, a high content of V_o 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.