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H-Bonding interactions in Carbon Nitride model compounds

Polymeric carbon nitride materials p-CN(H) are a new interesting class of metal-free organophotocatalysts. The melamine molecule, a building block of p-CN(H), was studied in the gas phase and in the solid state within a H-bonded network by XPS and XAS to identify the effect of the H-bonding interactions on the local electronic structure of N-containing molecular functional groups (amino group and pyridine-like N). With the support of DFT simulations, it was found that the H-bonds mainly affect the N1s level of the amino group, leaving that of the pyridine-like N mostly unperturbed. V. Lanzillotto et al. 10.1002/chem.201802435


Polymeric carbon nitride, p-CN(H), is a new interesting material able to (photo)catalyze a variety of important chemical processes, like the solar-driven water splitting reaction for hydrogen and oxygen production. Originally, p-CN(H) materials were modelled as binary CN sheets based on s-triazine or s-heptazine units connected through tertiary amino groups NC3 (Fig. 1a, b). Triamino-s-triazine (melamine) and triamino-s-heptazine (melem) can also be considered as building blocks of the different structures as shown in Fig. 1.
 

Figure 1.a), b) Proposed structure of binary CN materials based on melamine and melem building blocks. c), d) CNH graphitic structures g-CN(H) based on the melem unit. c) H-bonded 2D assembly of melon chains. d) 2D continuous network of PHI with H-bonded melamine molecules. H-bonds are marked by red ovals.[V. Lanzilotto et al., Chem. Eur. J. 24 (2018),14198; DOI:10.1002/chem.201802435]
 
In Fig. 1c, a linear polymer (melon) is connected by H-bonds, with s-heptazine units connected through NH bridges and one primary amino group per unit preserved. The chains are arranged in a zigzag-type fashion, which, through medium-strong H-bonding interactions (NH···N=C), stabilizes a close-packed 2D molecular network. In Fig. 1d instead, a continuous 2D heptazine network, based on covalent NH bridges (polyheptazine imide or PHI) is shown. The resulting large triangular voids can host a single melamine molecule anchored to the surroundings by multiple H-bonds. Accordingto theoretical studies, the water-splitting mechanism would be related to the H-
 

Figure 2.a) Experimental XPS spectra of the N 1s of triazine. Gas phase, thick film (6ML) and single layer (1.2 ML) films are shown. b) Computed N1s XP spectra in different conformations. c) Structural models used for the calculations [V. Lanzilotto et al., Chem. Eur. J. 24 (2018),14198; DOI:10.1002/chem.201802435]
 
bond formationbetween the H atoms of the water molecule and thelone pair of the aromatic N atom of this molecular structure. However, according to recent studies, also primary and secondary amines seem to be active sites, finding that heptazineoligomers have up to three times the activityof ill-defined p-CN(H) materials. For such oligomers, the amino groups are both prevalent andbetter exposed.
To understand how the intermolecular H-bonding interactions affect the local electronic state of the CN functional groups, we have characterized the melamine molecules in gas-phase and adsorbed on the Au(111) surface of gold, at monolayer and multilayer coverages, by X-ray photoelectron spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS). The H-bonds can in fact strongly affect the core-level binding energies of the involved heteroatoms. The comparison of gas phase and adsorbate studies allows to identify the modifications of the electronic structure induced by the adsorption process. In melamine there are two chemically non-equivalent N atoms: the N of the amino group (–NH2) and the pyridine-like N of the triazine ring (N=C). We found that the H-bonding interactions mainly modify the local 1s electronic state of the amino group, as seen by the change of the position of the XPS N 1s peak of the amino group in Fig. 2a. This results in a decreased binding energy (BE) shift between the two types of N with respect to the free molecule. To investigate the H-bonding effects on the local electronic state of both functional groups, we performed ab initio Density Functional Theory (DFT) simulations of the N1s core level BE position for gas-phase melamine and for H-bonded structures of an increasing number of melamine units (Fig. 2b). We considered a monomer, a dimer, a trimer, and a periodic hexagonal melamine layer (Fig.2c). The latter was both “floating” and adsorbed on Au(111), to mimic the melamine adsorbed multilayer and monolayer, respectively.
The calculations show that starting from the dimers, a new feature appears between the two main peaks of the amino and pyridine-like N, corresponding to a BE shift of the amino N involved in H-bonds with the pyridine N of the adjacent molecule. The 1s XPS peak of the pyridine N is instead only very slightly affected by the H-bonds, maintaining its energy position almost unchanged for all structures. The calculations are in full agreement with the measurements for the adsorbed layers, Fig. 2a), where the formation of H-bonds in the adsorbate layer is detectable by the shift to lower BEs of the amino peak, strongly indicating that the H-bonds are the dominant interaction even at monolayer coverage on the gold surface. This work has shown how the formation of a H-bonded network affects the electronic states of the amino N (the donor side of the H-bonding) and much less the pyridine N, providing a backbone for future characterizations aiming to describe the water splitting reaction on such photocatalysers.
 


 

 

Retrive article

H-Bonding interactions in Carbon Nitride model compounds Chemistry – A European Journal, Vol. 24 - 53, pp. 14198-14206 (2018)
doi: 10.1002/chem.201802435
V. Lanzilotto1,2, J. L. Silva1, T. Zhang1,3, M. Stredansky4, C. Grazioli5, K. Simonov1, E. Giangrisostomi6, R. Ovsyannikov5, M. de Simone7, M. Coreno5, C. Moyses Araujo1, B. Brena1, C. Puglia1 
 
 

 

Last Updated on Friday, 29 November 2019 17:16