Surface charge modification via protonation of graphitic carbon nitride (g-C3N4) for electrostatic self-assembly construction of 2D/2D reduced graphene oxide (rGO)/g-C3N4 nanostructures toward enhanced photocatalytic reduction of carbon dioxide to methane

In this work, we reported a 2D/2D hybrid heterojunction photocatalyst with effective interfacial contact by incorporating reduced graphene oxide (rGO) and protonated g-C3N4 (pCN) synthesized by a novel combined ultrasonic dispersion and electrostatic self-assembly strategy followed by a NaBH4-reduction process. The resulting 2D rGO-hybridized pCN (rGO/pCN) nanostructures formed an intimate contact across the heterojunction interface as supported by the electron microscopy analysis. The rGO/pure g-C3N4 (rGO/CN) developed without the modification of surface charge on g-C3N4 has also been prepared for comparison. Compared with pure g-C3N4 and rGO/CN, the rGO/pCN photocatalysts demonstrated a remarkable enhancement on the CO2 reduction in the presence of H2O vapor to CH4 under a low-power energy-saving daylight bulb at ambient temperature and atmospheric pressure. The optimized 15 wt% rGO/pCN (15rGO/pCN) exhibited the highest CH4 evolution of 13.93 µmol gcatalyst−1 with a photochemical quantum yield of 0.560%, which was 5.4- and 1.7-folds enhancement over pCN and 15rGO/CN samples, respectively. This was ascribed to the addition of rGO with pCN in a controlled ratio as well as sufficient interfacial contact between rGO and pCN across the rGO/pCN heterojunction for efficient charge transfer to suppress the recombination of electron–hole pairs as evidenced by the electron microscopy, zeta potential and photoluminescence studies. In addition, the 15rGO/pCN possessed a moderately high stability after three successive cycles with no obvious change in the production of CH4 from CO2 reduction. Lastly, a visible-light photocatalytic mechanism associated with rGO/pCN hybrid nanoarchitectures was presented.