Modification of surface properties and photocatalytic performance of pure and oxygen-doped graphitic carbon nitride via DBD plasma treatment

Abstract

Graphitic carbon nitride (CN) is a non-metallic semiconductor with applications in photocatalysis, including the photocatalytic reduction of Cr(VI) under visible irradiation. To improve its intrinsic properties, two modification strategies were applied: i) oxygen doping by co-calcination of urea with two different amounts of oxalic acid, and ii) dielectric barrier discharge (DBD) plasma treatment. The plasma treatment was applied to pristine and previously oxygen-doped CNs. The properties of the photocatalysts were studied by XRD, FTIR, FESEM, EDS, PL and DRS analysis, as well as by determination of the number of acidic surface functional groups. Both modification methods increased the oxygen content: during oxygen doping, nitrogen was replaced by oxygen in the lattice, while during plasma treatment, oxygen-containing functional groups were introduced at the surface. Plasma treatment of oxygen-doped CNs facilitated surface functionalisation due to the open heptazine ring structure. Oxygen doping narrowed the band gap, which was further slightly reduced by subsequent plasma treatment, thereby lowering the oxidation and reduction potentials. Although the absorption of visible light was improved, the reduction of the band gap resulted in a reduced activity for the photocatalytic reduction of Cr(VI). The best results were obtained with plasma-treated pure CN, due to the increased content of oxygen-containing surface groups, which led to a slightly reduced recombination rate of charge carriers.