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

Abstract

Water splitting via two two-electron processes (the H2O first photocatalytically converted to H2 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 H2 generation could be driven by visible light but also the two-electron H2O2 disproportionation to form H2O and O2 could also be photocatalyzed by visible light over g-C3N4 catalysts. Photocatalytic H2, O2 generation, and simultaneous H2O2 formation in Cu/C3N4 and Fe/C3N4 dispersions were confirmed, about 2.1 and 1.4 μmol of H2 and 0.8 and 0.5 μmol 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 H2 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 O2–· radical path over g-C3N4 under irradiation.