Monolayer Graphitic Carbon Nitride as Metal-Free Catalyst with Enhanced Performance in Photo- and Electro-Catalysis

The g-C₃N₄ monolayer in the perfect 2D limit was successfully realized, for the first time, by the well-defined chemical strategy based on the bottom-up process. The most striking evidence was made from Cs-high resolution transmission electron microscopy measurements by observing directly the atomic structure of g-C₃N₄ unit cell, which was again supported by the corresponding high resolution transmission electron microscopy image simulation results. We demonstrated that the newly prepared g-C₃N₄ monolayer showed outstanding photocatalytic activity for H₂O₂ generation as well as excellent electrocatalytic activity for oxygen reduction reaction. The exfoliation of bulk graphitic carbon nitride (g-C₃N₄) into monolayer has been intensively studied to induce maximum surface area for fundamental studies, but ended in failure to realize chemically and physically well-defined monolayer of g-C₃N₄ mostly due to the difficulty in reducing the layer thickness down to an atomic level. It has, therefore, remained as a challenging issue in two-dimensional (2D) chemistry and physics communities. In this study, an "atomic monolayer of g-C₃N₄ with perfect two-dimensional limit" was successfully prepared by the chemically well-defined two-step routes. The atomically resolved monolayer of g-C₃N₄ was also confirmed by spectroscopic and microscopic analyses. In addition, the experimental Cs-HRTEM image was collected, for the first time, which was in excellent agreement with the theoretically simulated; the evidence of monolayer of g-C₃N₄ in the perfect 2D limit becomes now clear from the HRTEM image of orderly hexagonal symmetry with a cavity formed by encirclement of three adjacent heptazine units. Compared to bulk g-C₃N₄, the present g-C₃N₄ monolayer showed significantly higher photocatalytic generation of H₂O₂ and H₂, and electrocatalytic oxygen reduction reaction. In addition, its photocatalytic efficiency for H₂O₂ production was found to be the best for any known g-C₃N₄ nanomaterials, underscoring the remarkable advantage of monolayer formation in optimizing the catalyst performance of g-C₃N₄.