Vacuum heat-treatment of carbon nitride for enhancing photocatalytic hydrogen evolution

Polymeric carbon nitride prepared by thermal condensation of cyanuric chloride with melamine was post-treated under vacuum conditions at different temperatures in order to study in depth the structure–performance relationship. The structure, composition, photoelectric and photocatalytic properties of the resulting samples were characterized in detail by physicochemical means, such as X-ray diffraction, thermogravimetry, elemental analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, photocurrent response, electrochemical impedance spectroscopy photocatalytic hydrogen production, etc. The results revealed that the untreated polymeric carbon nitride was not a single phase but a mixture consisting of unequal-sized particles with different degrees of polymerization and chemical structures containing both s-triazine and tri-s-triazine ring building blocks. This makes it possible to modify polymeric carbon nitride through a post-treatment. Tremendous changes in the C/N ratio, IR spectra, photoresponse, morphology, and photocatalytic activity occurred mainly in two temperature ranges of 300–500 °C and 500–600 °C. Among all samples, the carbon nitride treated at 500 °C showed the highest photocatalytic activity for production of hydrogen from water, as a result of the higher content of the tri-s-triazine phase, better lamellar morphology, wider photoabsorption, and smaller electrochemical impedance. The vacuum heat-treatment at temperature above 500 °C gave rise to the broken structure, and consequently the photoactivity was reduced.