Fig. 1. Sketch of synthesis and structure of the different CN materials.

Graphical sketch of the synthesis of the various CN structures, with the associated photograph of the as-obtained powdery material. Standard thermal conditions applied to melamine lead to synthesis of g-CN, where the morphology is typically described by melem units connected in-plane; oxidative treatment presumably introduces small amounts of oxygenated functional groups on the surface (ox-CN), while reductive treatment partly removes N atoms, creating planar vacancies (red-CN); higher-temperature thermal treatment under inert atmosphere generates partially amorphous structure by misalignment of CN planar domains (am-CN). Note that the graphic rendering of the structure is only an idealized depiction used for the benefit of discussion. Real CN structures are much more complex. Photo credit: Francesco Longobardo, University of Trieste, Italy.