Solving the Hydrogen and Lithium Substructure of Poly(triazine imide)/LiCl Using NMR Crystallography

Abstract

Poly(triazine imide) with incorporated lithium chloride has recently attracted substantial attention due to its photocatalytic activity for water splitting. However, an apparent H/Li disorder prevents the delineation of structure-property relationships, for example, with respect to band-gap tuning. Herein, we show that through a combination of one- and two-dimensional, multinuclear solid-state NMR spectroscopy, chemical modelling, automated electron diffraction tomography, and an analysis based on X-ray pair distribution functions, it is finally possible to resolve the H/Li substructure. In each cavity, one hydrogen atom is bound to a bridging nitrogen atom, while a second one protonates a triazine ring. The two lithium ions within each cavity are positioned between two nitrogen atoms of neighbouring triazine rings. The thereby induced local dipole moments cause slight buckling of the framework and lateral displacements of the Cl ions at a coherence length below 2 nm. Nevertheless, the average structure conforms to space group P2 2 2 . In this way, we demonstrate that, in particular, the above-mentioned techniques allow for smart interplay in delineating the real structure of PTI/LiCl.

Carbon nitrides: synthesis and characterization of a new class of functional materials

“…That suggestion is consistent with results of detailed structural analysis of PTI materials. 26,27 This effect is also noted for the Br 3d region, that contains two peaks due to spin-orbit (I = 3/2, 5/2) coupling. The C1s spectrum of PTIÁLiBr is different from those for polymeric to graphitic C x N y H z materials.…”

“…. [25][26][27] It is interesting to note that the PTI layers have composition C 2 N 3 H, equivalent to that of Liebig's melon (Fig. 2).…”

“…13). [25][26][27] However, in these compounds it is important to note that most of the XRD peak intensity is derived from scattering by the ''heavy'' elements Cl and Br, that are maintained in their crystalline positions by the carbon nitride layered structure and thus act as a proxy for that arrangement. It is also important to note that the heavy anion sites may not be completely filled, but as long as their scattered X-rays interfere coherently over an appropriate length scale they give rise to ''crystalline'' diffraction lines.…”

“…25,26 Additional information on the site occupancies of some of the elements can also be obtained from Rietveld analysis of the X-ray diffraction patterns, and more recently neutron scattering data, and from quantitative NMR measurements. 27 …”

“…4c). 27 It should be noted that all PTI materials examined to date maintain a significant H concentration as an essential component of their structure, as the substitution by Li is always only partial. 25,26,68,69 3.…”