The presence of oxygen vacancies and its role as a double-edged sword in anatase TiO2/polymerized carbon nitride composites for oxidative and reductive photocatalytic reactions

Abstract

In-depth characterization of TiO2@PCN composites (PCN, polymeric carbon nitride) prepared with a urea precursor (T/U30 % T/U80 %) and their photocatalytic activities in nitrobenzene (NB) reduction and malachite green dye (MGD) oxidation allowed to evaluate the role of oxygen vacancies in the photocatalytic activity. Materials T/U30 % and T/U80 % exhibited low amounts of PCN (<2.3 % wt.), a significant presence of Ti3+ sites related to deep electron traps and visible light absorption. Continuous wave electron spin resonance (CW-ESR) spectra at − 196 ◦C during in-situ UV and visible light irradiation showed that the intensity of signals attributed to deep electron traps increased in the composites. Regarding photocatalytic activity, T/U30 % and T/U80 % materials were more efficient than pristine TiO2 and PCN in MGD oxidation, whereas their capacity to reduce NB was strongly affected under UVA- and blue-LED irradiation. We concluded that this synthetic method using urea as a PCN precursor led to the generation of TiO2@PCN composites with visible light absorption, direct Zschemes, low amounts of PCN, and high presence of oxygen vacancies acting as recombination centers, which caused a detrimental effect on the photocatalytic reductive properties of T/U30 % and T/U80 % composites. In addition, interaction with NB promoted the formation of nitrosobenzene and azoxybenzene instead of aniline, as occurred with pure PCN. Computational calculations demonstrated that NB interacts differently with PCN, TiO2, and oxygen-deficient TiO2. Finally, the visible-light absorption of T/U30 % and T/U80 % samples could not be exclusively attributed to PCN’s presence but also to oxygen vacancies generating midgap states.