Selective Fragmentation through C-N Bond Cleavage of Carbon Nitride Framework for Enhanced Photocatalytic Hydrogen Production

A simple, practical approach for the structural modification of bulk g-C3N4 employing high-pressure NH3 and H2O formed by the polycondensation of urea is reported. The high-pressure processes the planarization of carbon nitride sheets that is disruptive because of structural distortion or defects, thus creating non-crystalline lines with highly reactive carbon species. The reaction of these carbon species with NH3 leads to highly selective and oriented fragmentation of the carbon nitride framework, which is entirely different from previous reports, producing nanofragments with a very small density of defects. The high pressure proceeds the sheet planarization and the structural condensation of nanofragments, resulting in very high crystallinity. The fragmentation also creating a high concentration of functional groups (-NH2 and -OH) on the edge of C3N4 sheets with a suitable proportion, constructing a large optimized hydrogen-bond network across intra- and interplanes that further enhance the crystallinity of the formed nanofragments. The high crystallinity, especially the strong planarization of carbon nitride sheets, significantly speeds up the charge separation and transfer, while the functional groups on the edge of sheets result in an excellent charge drive. Also, these groups simultaneously shift the conduction band to a higher level and improve proton adsorption and activation. As such, the as-prepared nanofragment photocatalyst exhibits a photocatalytic hydrogen production rate that is nearly five times increased, as compared to that of the bulk g-C3N4, with a high quantum efficiency of 12.3% at 420 nm.