In-Plane Structural Fluctuations in Differently Condensed Graphitic Carbon Nitrides

Graphitic carbon nitrides (g-CNxHy) are an important class of materials for driving energy conversion processes such as photocatalysis and fuel cell reactions. However, a poor understanding of structural disorder in this system impedes the ability to predict structure-property relationships and direct synthesis routes toward the most active forms. Both the bulk and local in-plane structures need to be described for multiple states of polymer condensation. Low-fluence-rate transmission electron microscopy (TEM) has been applied to explore the in-plane structures occurring in differently-condensed g-CNxHy. By extracting local Patterson functions, fluctuations in the nearest-neighbor environment of the planar heptazine building blocks are revealed. This analysis points to a trend in the extent of planar structural ordering that is consistent with polymer chain lengths of only two units up to tens of nanometers in length. The bulk hydrogen content of the various g-CNxHy structures also suggests that shorter chains are associated with higher amine content (in which N-H2 defects terminate the chains). Additionally, investigation of the 3D stacking sequence, via X-ray diffraction and TEM, indicates the presence of turbostratic stacking disorder. With these insights, more accurate structure models of g-CNxHy may be constructed to predict the (photo)catalytically relevant properties and accelerate rational engineering of these materials.