Nitrogen vacancy engineered graphitic C3N4-based polymers for photocatalytic oxidation of aromatic alcohols to aldehydes

- The defective g-C3N4 nanosheets were successfully synthesized by high-temperature thermal condensation of nitric acid-pretreated melamine as a precursor.
- The nitrogen vacancies were proved to be located at the uncondensed terminal NHx lattice sites.
- The defective g-C3N4 photocatalyst exhibited greatly enhanced photocatalytic activity and excellent stability.
- The photocatalytic oxidation mechanism of defective g-C3N4 system were investigated.

Vacancy defect in the semiconductors plays an important role in the improvement of the electronic structure and the increase of specific reaction sites for reactant molecules, and consequently enhancing the photocatalytic activity of semiconductor photocatalysts. Through high-temperature thermal condensation of a nitric acid-pretreated melamine precursor, nitrogen vacancies were successfully introduced in the framework of g-C3N4. The nitrogen vacancies located at the uncondensed terminal NHx lattice sites were conducive to the enhancement of optical absorption, the improvement of the separation efficiency of the photogenerated charge carrier and the increase of surface area, which was beneficial to the photocatalytic oxidation process. More significantly, the novel CNNA(X) were used as efficient photocatalysts in the green process of aromatic aldehydes from the photocatalytic selective oxidation of aromatic alcohols and the photocatalytic degradation of organic pollutants. CNNA(X) samples exhibited enhanced photocatalytic activity and excellent recyclability and stability. The 68.3% benzyl alcohol conversion and almost 100% selectivity was observed for the CNNA (0.9) photocatalyst, higher than that of pure g-C3N4. Meanwhile, CNNA (0.9) showed superior photocatalytic degradation performance of organic dyes (RhB and MO). Furthermore, the underlying photocatalytic oxidation mechanism was proposed by the controlled experiments using radical scavengers.

171Nitrogen vacancies were successfully introduced in the framework of g-C3N4 by high-temperature thermal condensation of nitric acid-pretreated melamine as precursors. The nitrogen vacancies are proved to be located at the uncondensed terminal NHx lattice sites, which is beneficial to the rapid separation of photogenerated charge carrier, the enhancement of optical absorption and the increase of surface area. Meanwhile, the CNNA(X) samples exhibited enhanced photocatalytic activity and excellent stability for the photocatalytic selective oxidation of aromatic alcohols and the photocatalytic degradation of organic pollutants (such as RhB and MO).