Abstract
The g-C(3)N(4) 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-C(3)N(4) unit cell, which was again supported by the corresponding high resolution transmission electron microscopy image simulation results. We demonstrated that the newly prepared g-C(3)N(4) monolayer showed outstanding photocatalytic activity for H(2)O(2) generation as well as excellent electrocatalytic activity for oxygen reduction reaction. The exfoliation of bulk graphitic carbon nitride (g-C(3)N(4)) 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-C(3)N(4) 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-C(3)N(4) with perfect two-dimensional limit" was successfully prepared by the chemically well-defined two-step routes. The atomically resolved monolayer of g-C(3)N(4) 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-C(3)N(4) 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-C(3)N(4), the present g-C(3)N(4) monolayer showed significantly higher photocatalytic generation of H(2)O(2) and H(2), and electrocatalytic oxygen reduction reaction. In addition, its photocatalytic efficiency for H(2)O(2) production was found to be the best for any known g-C(3)N(4) nanomaterials, underscoring the remarkable advantage of monolayer formation in optimizing the catalyst performance of g-C(3)N(4).