Synthesis of graphitic carbon nitride under low ammonia partial pressure

Abstract

Graphitic carbon nitride (g-C3N4) is a very attractive potential solar-light-activated photocatalyst for hydrogen evolution. Substantial effort has been devoted to improving the photoactivity of g-C3N4, but to date most g-C3N4 prepared by the thermal polymerization of melamine are the linear polymer carbon nitride “melon”. We thus explored a new way of synthesizing g-C3N4 under low ammonia partial pressure using an approach similar to train sublimation. The obtained product (p-CN) is reddish-orange in color and has a different chemical composition and different optical properties from those of melon and melon-based materials. These differences were characterized by ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction measurements and X-ray photoemission spectroscopy. Our findings show that p-CN is nano-scale g-C3N4. In addition, theoretical calculations using density functional theory, as well as direct observation of the characteristic electronic structure around the energy gap of p-CN using ultraviolet photoemission spectroscopy combined with inverse photoemission spectroscopy, provide evidence that p-CN has the g-C3N4 structure. These findings shed light on the synthesis of g-C3N4 composed of stacked two-dimensional layers of heptazine networks. Furthermore, the fundamental insights into the electronic structures of melem, melon and p-CN are particularly significant for research in the field of visible light-responsive photocatalysts.