Graphitic carbon nitrides form a popular family of materials, particularly as photoharvesters in photocatalytic water splitting cells. Recently, relatively ordered g-C3N4 and g-C6N9H3 were characterized by X-Ray diffraction and their ability to photogenerate excitons was subsequently estimated using Density Functional Theory and the Wannier Mott (WM) model. The ability of triazine-based g-C3N4 and g-C6N9H3 to photogenerate excitons was studied using self-consistent GW computations followed by solving the Bethe-Salpeter Equation (BSE). In particular, monolayers, bilayers and 3D-periodic systems were characterized.&lt;span style="font-size:10.8333px"&gt; &lt;/span&gt;In agreement with previous studies, flat gt-C3N4 has a lower band gap than corrugated gt-C3N4 and gt-C6N9H3 has a higher band gap than both of the others. However, the fundamental band gaps are about 2 eV higher according to scGW than what has been predicted by HSE06. Furthermore, the simple WM model is unable to reliably capture the effect of exfoliation, which leaves the optical gap unchanged, while the fundamental gap increases dramatically. However, in good agreement with the WM model, the exciton binding energy is very high in CNs, making spontaneous exciton dissociation unlikely at room temperature. In comparison with experiment, the computed optical gaps are higher by at least 1 eV for the triazine based CNs. We suggest that the experimentally determined gap either corresponds to heptazine based CNs, patches of material where the C:N ratio is higher than on average, or that a small percentage of defects is responsible for the onset of the optical response, which would explain the discrep- ancy between experiment and scGW for the perfectly crystalline triazine-based CNs.

Challenges in calculating the bandgap of triazine-based carbon nitride structures

253rd National Meeting (2017)

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