Preparation and Enhanced Visible-Light Photocatalytic H₂-Production Activity of Graphene/C₃N₄Composites

Abstract

Graphene and graphitic carbon nitride (g-C3N4) composite photocatalysts were prepared by a combined impregnation−chemical reduction strategy involving polymerization of melamine in the presence of graphene oxide (precursors) and hydrazine hydrate (reducing agent), followed by thermal treatment at 550 °C under flowing nitrogen. The resulting graphene/g-C3N4 composite photocatalysts were characterized by X-ray diffraction, transmission electron microscopy, UV−visible spectrophotometry, nitrogen adsorption, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy. The transient photocurrent response was measured for several on−off cycles of intermittent irradiation. The effect of graphene content on the rate of visible-light photocatalytic hydrogen production was studied for a series of graphene−graphitic carbon nitride composite samples containing Pt as a cocatalyst in methanol aqueous solutions. This study shows that graphene sheets act as electronic conductive channels to efficiently separate the photogenerated charge carriers and, consequently, to enhance the visible-light photocatalytic H2-production activity of g-C3N4. The optimal graphene content was determined to be ∼1.0 wt %, and the corresponding H2-production rate was 451 μmol h−1 g−1, which exceeded that of pure g-C3N4 by more than 3.07 times. The proposed mechanism for the enhanced visible-light photocatalytic activity of g-C3N4 modified by a small amount of graphene was further confirmed by photoluminescence spectroscopy and transient photocurrent response. The metal-free graphene/g-C3N4 composites showed high visible-light photocatalytic activity, which makes them promising nanomaterials for further applications in water treatment and dye-sensitized solar cells.