Carbon Nitride Photocatalysts for Water Splitting: A Computational Perspective

Abstract

We study the thermodynamic ability of carbon nitride materials to act as water splitting photocatalysts using a computational approach that involves a combination of density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on cluster models of both triazine-and heptazine-based structures. We first use TD-DFT to calculate the absorption spectra of the different cluster models and compare these spectra to those measured experimentally and then calculate using DFT and TD-DFT the reduction potentials of the free electron, free hole, and exciton in these models. We predict that all classes of carbon nitride structures considered should thermodynamically be able to reduce protons and oxidize water. We further provide evidence for the hypothesis that the experimental lack of overall water splitting activity for pure carbon nitride arises from the fact that water oxidation is a four-hole reaction and hence very susceptible to competition with electron−hole recombination. Finally, we propose that the recently reported overall water splitting activity of carbon nitride loaded with polypyrrole nanoparticles arises from a junction formed at the interface of both materials, which assists in keeping electrons and holes apart.

Shining Light on Carbon Nitrides: Leveraging Temperature To Understand Optical Gap Variations

“…This convergence at a small system size yields the important insight that the photoexcitation process in PCNs is a localized process, essentially involving the nearest neighbors. 7,21,23,28,29 This is consistent with the high exciton binding energies, characteristic of localized excitons, reported previously. 23 …”

“…The notion that PCNs' optical properties are largely local is supported in the literature. Recently, Merschjann et al 28 advanced experimental evidence for this notion, and Butchosa et al 29 contributed insights from theory. Disagreement exists as to what extent the optical nature of PCNs can be described in terms of a classical semiconductor's, and to what extent it should be considered as an oligomeric aromatic moiety embedded in a molecular solid.…”

Modelling materials for solar fuel synthesis by artificial photosynthesis; predicting the optical, electronic and redox properties of photocatalysts

“…This can be analysed in terms of the potentials associated with the solar fuel synthesis half reactions and those associated with free electrons, holes and excitons in the photocatalyst [35,36]. The half reactions for the free electrons, holes and excitons are: Figure 1.…”

“…However, such papers generally suggest ways of improving the optical gap without considering the effect on the thermodynamic driving force for solar fuel production. Calculation of the optical gap and the overall optical absorption spectrum can also provide insight in the dominant structural elements of an (amorphous) photocatalyst, for example, to understand [36] the difference between differently prepared carbon nitride photocatalysts [21,26].…”

Label-free photoelectrochemical cytosensing via resonance energy transfer using gold nanoparticle-enhanced carbon dots

“…The C-dot modified ITO electrode exhibited a small cathodic photocurrent of 23 nA (Fig. 41 AA acted as a nontoxic electron donor, scavenging some of the photo-generated holes located on the excited state C-dots, which inhibited the electron-hole recombination and the photocorrosion of the C-dots. The photoexcited C-dots generated an electron-hole pair, which accompanied the electron transfer between the valence band and the conduction band.…”