Visible Photocatalytic Water Splitting and Photocatalytic Two-Electron Oxygen Formation over Cu- and Fe-Doped g-C3N4

Water splitting via two two-electron processes (the H2O first photocatalytically converted to H-2 and H2O2 under visible light irradiation and then the H2O2 disproportionation to H2O and O2 by a thermal catalytic process) has attracted extensive attention recently.1,2 Contrary to these reports, we found that not only the photocatalytic H-2 generation could be driven by visible light but also the two-electron H2O2 disproportionation to form H2O and O-2 could also be photocatalyzed by visible light over g-C3N4 catalysts. Photocatalytic H-2, O-2 generation, and simultaneous H2O2 formation in Cu/C3N4 and Fe/C3N4 dispersions were confirmed, about 2.1 and 1.4 mu mol of H-2 and 0.8 and 0.5 mu mol of O2 evolved over Cu/C3N4 and Fe/C3N4 in 12 h, respectively. To prove the photocatalytic process of H2O2 disproportionation, the H2O2 was added as a reagent in g-C3N4, Cu/C3N4, and Fe/C3N4 dispersions. The results showed that the activity of H-2 evolution decreased with the increase of H2O2 concentration; the corresponding AQEs of oxygen formation were 16.1%, 42.6%, and 78.5% at 400 nm, respectively. The remarkable increase of anodic photocurrents over Fe/C3N4/ITO and Cu/C3N4/ITO electrodes indicated that the two-electron H2O2 disproportionation was catalyzed via surface photocatalytic mechanism. The ESR results implied reaction occurred by O-2- radical path over g-C3N4 under irradiation.