Carbon nitride

Carbon nitride (C₃N₄), like elemental carbon, has several stable allotropes. The two most common are β-C₃N₄ (partial structure shown), which is analogous to diamond, and g-C₃N₄, whose properties are more like graphite.

In the late 1980s, theoretical physicists Amy Y. Liu at the University of California, Berkeley, and Marvin L. Cohen at Lawrence Berkeley Laboratory predicted that β-C₃N₄ would have short, strong bonds and could be harder than diamond. The material proved to be difficult to synthesize; but in 1993, Chunming Niu, Yuan Z. Lu, and Charles M. Lieber* at Harvard University (Cambridge, MA) created thin films of β-C₃N₄ using pulsed laser ablation of graphite targets combined with an intense atomic nitrogen source.

Also in 1993, Wolfgang Schnick at the University of Bayreuth (Germany) reviewed the theoretical and synthetic literature on β-C₃N₄; and, like Liu and Cohen, he surmised that the substance would be harder than diamond. But 30 years later, this prediction has not been demonstrated. It has been shown, however, that the bulk modulus (resistance to compression) of β-C₃N₄ is similar to that of diamond.

g-C₃N₄, as might be expected, consists largely of planar molecular units, mostly heptazines (condensed s-triazines). It usually contains a small amount of hydrogen at the peripheries of these structures. In 2008, Markus Antonietti and coauthors at the Max Planck Institute of Colloids and Interfaces (Golm, Germany) and the Fritz Haber-Institute of the Max Planck-Society (Berlin) reported that g-C₃N₄ can be made by polymerizing small C–N molecules such as cyanamide, dicyandiamide, and melamine. The researchers showed that the graphitic compound’s semiconductor properties give it “unexpected” catalytic activity for a variety of organic reactions.

Subsequently, several research teams found that g-C₃N₄ is a powerful photocatalyst. Much of this work is summarized in excerpts from the 2021 book Materials Science in Photocatalysis. If you want to learn even more, read “A Tour-Guide through Carbon Nitride-Land” (2021) by Vincent Wing-hei Lau at Korea University (Seoul) and Bettina V. Lotsch at the Max Planck Institute for Solid State Research (Stuttgart, Germany).