Title: Photocatalytic activity of graphitic carbon nitride with tunable morphology and bandgap

Abstract

Polymeric carbon nitride (C3N4) is an important photocatalyst due to its suitable band edge positions whose energies encompass both potentials of H+ reduction and H2O oxidation. However, the efficiency of photocatalysis is still quite low in solar energy converting due to the rapid recombination rate of photoinduced electron-hole pairs. Herein, we constructed heterojunction on the surface of C3N4 materials and studied the photocatalytic property. We presented a facile molten salt-assisted route to prepare red-color and water soluble g-C3N4 nanosheets with whole red-shift absorption and narrowed bandgap of 1.9 eV for sensitization of TiO2. Both experimental findings and theoretical calculations revealed that alkali heteroatoms modification led to the surface structure and electronic structure changes. The red-color carbon nitride showed enhanced visible-light absorption and charge transfer efficiency compared with general yellow-color carbon nitride. By hybridization with TiO2 photoanode, the modified TiO2 photoanode generates a photocurrent of approximately 2.33 mA cm-1 without any cocatalyst at 1.23 V versus reversible hydrogen electrode under Air Mass 1.5G illumination, which was 2.6 folds higher than that of bare TiO2 photo anode. Recently, we developed a new post-redox strategy to achieve reduced few-atom-thick C3N4(FAT C3N4)with controllable C-reduction and electron rich π-conjugated structure, which is different from existing exfoliation methods. Few atom-thick C3N4 and the as-prepared carbon reduced few-atom-thick C3N4 (CRed-FAT-C3N4) were obtained. The CRed-FAT-C3N4 possesses few number of periodic stacking layers, which suggests the transformation from thick C3N4 aggregates into apparent porous nanosheets, and the thickness of CRed-FAT-C3N4 is about 1.0 nm, corresponds to three and four single layers. CRed-FAT-C3N4 exhibits the supreme photocatalytic hydrogen evolution efficiency of 12.31 mmol h-1 g-1 (calculated from the first 5 h-cycle), which is about 17-fold enhancement of pristine C3N4 and much higher than that of FAT-C3N4 and CRed-C3N4.