What Does Graphitic Carbon Nitride Really Look Like?† Sigismund T.A.G. Melissen,a Tangui Le Bahers,b, Philippe Sautet,c;d, and Stephan N. Steinmann,∗b Graphitic Carbon Nitrides (g-CNs) have become popular light absorbers in photocatalytic water splitting cells. Early theoretical work on these structures focused on fully polymerized g-C3N4. Experimentally, it is known that the typically employed melamine polycondensation does not go toward completion, yielding structures with ∼15 at.% hydrogen. Here, we study the conformational stability of “melon”, with the [C6N9H3]n structural formula using DFT. Referencing to a 2D melon sheet, B3LYP-dDsC and PBE-MBD computations revealed the same qualitative trend in stability of the 3D structures, with several of them within 5 kJ/mol per tecton. Fina’s orthorhombic melon is the most stable of the studied conformers, with Lotsch’ monoclinic melon taking an intermediate value. Invoking a simple Wannier-Mott-type approach, Fina’s and Lotsch’ structures exhibited the lowest optical gaps (2.8 eV), within the error margin of the experimental value (2.7 eV). All conformers yielded gaps below that of the monolayer’s (3.2 eV), suggesting Jelley-type (“J”) aggregation effects. 1 Introduction provided. 13 As an alternative to photovoltaics, solar energy can be stored in n n n CN2H2 (l) ! C2N4H4 (l) ! C3N6H6 (l) (1a) the form of chemical bonds, counteracting the diurnal variation in 2 3 solar power. To design chemical schemes allowing such storage, 1 n ! [C N H ] (s) + NH (g) (1b) inspiration can be drawn from the photosynthetic process found 6 6 9 3 n 2 3 in nature. Such schemes are commonly referred to as “artificial 1 n photosynthesis”. 1 ! [C N ] (s) + NH (g) (1c) 3 3 4 n 6 3 An elegant route to perform artificial photosynthesis is by us- ing graphitic (g-) carbon nitride (CN), with general formula g- In Fig. 1 the key species discussed in this study are depicted. 2–8 CxNyHz, as photoharvester in photocatalytic water splitting The melamine core can be integrated directly as a tecton yielding cells. 9,10 Its synthesis from basic petrochemicals is cheap and gt-CNs (t for triazine). The melem core, however, yields the more straightforward, while allowing a wide structural variety. 11 Fur- stable gh-CNs (h for heptazine). thermore, putting such compounds to use in renewable energy Following pioneering theoretical work by Liu and Prewitt in the sources, instead of in fuels, is environmentally beneficial. 12 late 1980s, 14,15 many computational studies have been devoted 16 17–19 In Eq. 1 the formal reaction scheme from cyanamide to g-C3N4 is to fully deaminated CN. gt-C3N4, gh-C3N4, and other polymorphs 20,21 have been extensively studied, even though it is well known that these species are unstable, 22 decomposing at the a Université de Lyon, Université Claude Bernard Lyon 1, synthesis temperature that would typically be required to produce 23 CNRS, Institut Lumière Matière, F-69622 Lyon, France them. Furthermore, frequent presentation of these fully deam- b Université de Lyon, Université Claude Bernard Lyon 1, inated structures as flat, 18,24 rather than corrugated, 3,25 added Ecole Normale Supérieure de Lyon, CNRS, further confusion to the research field. Finally, the community in- 46 allée d’Italie, F-69007 Lyon Cedex. E-mail: [email protected] c Department of Chemical and Biomolecular engineering, University of California, Los vestigating the photocatalytic activity of these structures doubts Angeles, Los Angeles, CA 90095, USA that by increasing the degree of polycondensation, increasing d Department of Chemistry and Biochemistry, University of California, Los Angeles, Los photocatalytic activity should always be found: participation of Angeles, CA 90095, USA N lone pairs in denser p-aromatic networks reduces their ability † Electronic Supplementary Information (ESI) available: Energies for additional to bond to the noble metal cocatalyst that enables the half re- functionals and a comparison of computational unit cells to the experimental ones is provided. All structures are provided in POSCAR (Vienna Ab Initio Simulation Pack- action leading to H2, and defects - rather than always trapping 7,26–34 age (VASP)) format; subsequently in .gui (CRYSTAL14 format) and finally, for the 3D electrons or holes - possibly enhance the reaction. We re- structures only, in .cif (Crystallographic) format. See DOI: 10.1039/cXCP00000x/ Journal Name, [year], [vol.], 1–9 | 1 N NH 2 N N N N N N Cyanamide, CN2H2 H NN N N N N N N N N N H a H N N N N NH2 NNN N N N N N N N N N H N N H N N N N N N N N N N N N N H H N N H2N N NH2 N N N N N N N N N N N H Melamine, C3N6H6 N N N H N N N N N N N N N N N N H NH2 N N N H N N N N a N N N N N N N N H b b N N N N N N N NC N N N N N N H2N N N NH2 Melem, C6N10H6 Melon (gh-C6N9H3) gh-C3N4 Fig. 1 Overview of structures involved in the CN synthesis process. Melem’s core N atom is indexed C. ported earlier 22 that among the different polymeric species that sealed ampoule (Fina) or under ammonia (Lotsch). The influence 35 form during CN synthesis, gh-C6N9H3, known as “melon” is the of the synthesis conditions on the obtained polymorph and the most stable one. significant impact that it can have on photoactivity, has been de- 44 Melon can be synthesized using starting materials comprising a scribed before for other organic semiconductors. Furthermore, wider range of chemical elements than C, N and H alone, such as the structure analysis temperature could also affect the particular 36 37 crystal structure observed. 45 urea (CO(NH2)2) and thiourea (CS(NH2)2). Lotsch and co- workers 23,28,35 established the synthesis temperature range for In Fig. 2, the possible variations in stacking patterns studied here the typical melamine precursor. Polymerization onset 28,38 occurs are provided. Essentially (Fig. 2a), the shaded layer below the at 380 °C (in an open vessel). At the high end of this range crys- reference layer can shift in the hydrogen bonding direction (cor- talline 35 melon is produced (630 °C, closed vessel conditions). responding to b) by dH, and/or in the covalent bonding direction At higher temperatures the onset of material degradation is ob- by dCov. Second (Fig. 2b,c), the stacking pattern can be ABAB, served. 23,38 In practice, the experimental parameters are often or AAAA. In the latter case, AA-stacking does not mean that the compromises between these extremes. 38–40 layers are sandwich stacked. Rather, p-aromatic systems such as The stability of g-CN is largely due to the specific balance be- those discussed here tend to shift slightly with respect to each 46 tween steric repulsion, hydrogen bonding and the formation of other. Finally, variations in interaromatic distances (dAr-Ar), as a p-conjugated network. As can be understood from Fig. 1, the well as in the lattice vectors within the aromatic plane are possi- ble (arbitrarily referred to here as a, b), although these variations lone pairs in the hypothetical gh-C3N4 repel each other forcing a (partial) breaking of the overall p-conjugation, and causing the are typically smaller. We study four important conformers reported in the literature. material to be corrugated. The hydrogen atoms in gh-C6N9H3 stabilize several of these lone pairs, spatially separate others, and The first conformer is named “Lotsch”, representing the first allow the formation of a flat, aromatic network overall. 22 The melon characterized. It has an AA stacking pattern and dH = flat laminae themselves are interesting and can be integrated into 0. The structure determined from the aforementioned neutron other structures. Indeed, the Van der Waals structures they can scattering study is named “Fina”, with dH 6= dCov 6= 0 and AB- form are not just those that are created by stacking with itself, stacking. The third and fourth structures are included in Fina’s but also by interleaving other species. 41 Within this strategy, in- comparison and here the same labels are adopted, namely S1 for terleaving with molybdenum dichalcogenides to allow fast charge the alternative monoclinic structure and S2 for the alternative tri- separation is of particular experimental 42 and theoretical inter- clinic structure. S1 has an AA stacking pattern and dCov = 0. For est 43. Here, we focus on the question what particular lamination S2, like Fina, dH 6= dCov 6= 0, but the stacking pattern is AA. In- 22 pattern between gh-C N H layers yields the most stable form of cluded in the comparison are lastly 1D and 2D melon. 6 9 3 47 melon. A 2010 study that combined combined theory (molecular me- chanics level of theory 48) and dedicated Nuclear Magnetic Reso- In 2015, Fina et al. 40 published a neutron-scattering study of a nance (NMR) experiments 49 of melon conformers suggested that melon conformer with a different packing structure from the one configurations resembling Lotsch’ – with d = 0 – and Fina’s determined by Lotsch et al. in 2007. 35 Both Fina and Lotsch syn- Cov – with d >> d > 0 – to be the two prevalent ones within thesized the conformers studied here from melamine in a sealed H Cov melon macrostructures. These authors further proposed that the environment, the main differences in protocol being the temper- proximity in energies of the structures was the reason that ex- ature, 500 °C in Fina’s case and 630°C in Lotsch’ and working in a 2 | 1–9 Journal Name, [year], [vol.], Fig.