What does graphitic carbon nitride really look like?

Area of Science:

• Materials Science
• Computational Chemistry
• Photocatalysis

Background:

• Graphitic carbon nitrides (g-CNs) are widely used as light absorbers in photocatalytic water splitting.
• Experimental synthesis of g-CNs, often from melamine, results in incomplete polymerization and incorporated hydrogen.
• Previous theoretical studies primarily focused on fully polymerized graphitic carbon nitride (g-C3N4).

Purpose of the Study:

• To investigate the conformational stability of hydrogen-containing "melon" structures ([C6N9H3]n) using Density Functional Theory (DFT).
• To compare the stability and optical properties of different "melon" conformers.
• To correlate theoretical findings with experimental observations for photocatalytic applications.

Main Methods:

• Density Functional Theory (DFT) calculations were employed.
• B3LYP-dDsC and PBE-MBD functionals were used to assess conformational stability.
• Wannier-Mott-type approach was utilized to determine optical gaps.

Main Results:

• Several 3D "melon" conformers showed comparable stability, within 5 kJ mol-1 per tecton.
• Fina's orthorhombic melon structure was identified as the most stable conformer.
• Fina's and Lotsch's monoclinic melon structures exhibited the lowest optical gaps (2.8 eV), aligning with experimental values (2.7 eV).
• All studied conformers displayed optical gaps lower than the 2D melon sheet (3.2 eV), indicating potential Jelley-type aggregation effects.

Conclusions:

• The conformational flexibility of hydrogen-containing graphitic carbon nitrides significantly impacts their stability and electronic properties.
• The theoretical stability and optical gap of Fina's orthorhombic melon structure closely match experimental data, validating its relevance for photocatalysis.
• Aggregation effects in graphitic carbon nitrides can reduce the optical gap, enhancing their performance in light absorption for water splitting applications.