Enhanced photo-reduction of chromium(VI) from aqueous solution by nanosheet hybrids of covalent organic framework and graphene-phase carbon nitride

Two new two-dimensional (2D) materials known as covalent organic framework (COF) and graphene phase carbon nitride nanosheets (CNNS) have induced great interests in photocatalysis due to their highly π-conjugate structure and wide light absorption. Herein, metal-free COF-CNNS hybrids were synthesized in-situ for photocatalytic reduction of Cr(VI). The COF-CNNS hybrids exhibit great enhancement of photocatalytic performance compared to COF or CNNS alone, mainly due to the formation of heterojunctions and the more stable photoelectrons. Static photocatalytic experiment reveals that the COF-CNNS hybrids can remove more than 99% Cr(VI) in 30 min, representing one of the most effective and the fastest photocatalysts reported recently. X-ray diffraction and Fourier transform infrared spectroscopy indicate that the COF-CNNS hybrids have excellent stability during the photocatalytic reduction process. X-ray photoelectron spectroscopy of the COF-CNNS hybrids before and after Cr(VI) removal confirm the photo-reduction of Cr(VI) to Cr(III). Moreover, for the first time, dynamic column experiment of Cr(VI) photocatalytic reduction was conducted to simulate the real industrial wastewater treatment, and the removal ratio as high as 80% for Cr(VI) was obtained. The present work provides new clues to develop metal-free photocatalyst for heavy metals removal.