Oxygen-doped protonated C3N4 nanosheet as particle electrode and photocatalyst to degrade dye by photoelectrocatalytic oxidation process

Three-dimensional photoelectrochemical oxidation (PECO) process could be a novel technology to boost degradation efficiency by adding photoactive particle electrodes (PEs) in solution. In this work, oxygen-doped protonated C3N4 (OP-C3N4) nanosheets derived from HNO3 treatment is testified as a new type of visible light responsive PEs for degradation of rhodamine B (RhB) and methylene blue (MEB) in NaCl and NaNO3 solutions, respectively. The resultant OP-C3N4 nanosheets show superior dispersion stability, high specific surface area, positive zeta potential, despite of a slightly lower separation efficiency of electrons and hole due to presence of defects, which leads to an improved dye photodegradation efficiency, i.e., ca. 40 % and ca. 30 % increase for RhB and MEB, respectively, compared to that of pristine g-C3N4 in salt solution. Density Function Theory (DFT) calculations reveal that the promoting effect can be attributed to the alter of electrostatic potential, which favors adsorption of anionic ions and promotes formation of radicals. Electrochemical oxidation (EO) experiment verifies the potential of OP-C3N4 nanosheet as PEs for degradation of these dyes. The reaction rate constant (kRhB = 0.0125 min−1, kMEB = 0.0082 min−1) reaches about 1/3 that in the photocatalytic process. Meanwhile, a high degradation effect was found in NaCl solution due to the electrolysis of Cl− over OP-C3N4. Finally, the three-dimensional PECO process using the dispersed OP-C3N4 in solution was evaluated. For NaCl solution, the reaction rate constant (kRhB = 0.054 min−1) equals to the sum of that in the photo and EO process, but it is lower for NaNO3 solution. Meanwhile, the degradation mechanism of RhB over OP-C3N4 in salt solution was also analyzed, which is similar to that in water. The current investigation extends the application of OP-C3N4 and the three-dimensional PECO technology using visible light photoactive PE.