Enhancing the photocatalytic performance of carbon nitrides through controlled local structure modification

Abstract text (incl. figure legends and references)

Carbon nitrides are the most studied sustainable metal-free photocatalysts: they can be synthesized by simple routes from abundant organic precursors, they are visible-light-active as well as thermally and chemically stable in photocatalytic reactions. However, most of them are poorly crystalline, which leads to a high recombination rate and difficulties in precise control and adjustment of their band structure.

Polyheptizine imides (M-PHIs) salts are one of the most crystalline carbon nitrides. They have high charge-separation efficiency, enabling storage of photogenerated electrons and the realization of so-called dark photocatalysis. In this work we show how the controlled modification of the local structure, obtained by varying the sizes and charges of metals as well as introducing a rotation between 2D heptazine layers, influences the energy band structure and allows to enhance the photocatalytic activity of M-PHIs. To realize this goal, we combined X-ray powder diffraction (XRD), low-dose HRTEM imaging, energy filtered electron radial distribution function (EF-eRDF) analysis obtained from rotational electron diffraction data and VEELS data with photocatalytic performance on M-PHI (M = Na, K, Mg, H) during the hydrogen evolution reaction (HER).

From low-dose HRTEM and XRD data and general assumptions about the structure from earlier works starting models for crystal structure refinement were proposed. PHI salts were found to crystalize in a trigonal unit cell; their idealized structure consists of heptazine units that are placed on top of each other in AAA stacking, forming continuous channels along the c direction. Based on XRD only we could not locate metal atoms uniquely in Mg-PHI and Na-PHI, thus we proposed 2 possible structural models. In order to find an unambiguous solution, we compared short-range order and the degree of crystallinity in different M-PHIs by performing an EF-eRDF analysis using rotational SAED data. The latter allowed to compensate for the preferential orientation of domains found in the M-PHI flakes. Fitting of EF-ePDF of Mg and Na salts showed that only one model with metal cations equally distributed between interlayer positions and bridging nitrogen atom positions in the channels is correct. The coherence lengths increase in the series H-, Na-, Mg-, K-PHI (reaching ~30 Å), indicatingan increase in crystallinity (Fig.1). The ePDF curves of Na- and Mg-PHI display broadening of the peaks beyond 8 Å pointing to a less pronounced fine structure compared to K-PHI, which is related with twists of layers found in these compounds.The rotation of layers by a random (usually only 2-8°) angle leads to the formation of "flower-like structures", while at specific rotation angles Moiré pattern lattices are formed. DFT calculations showed that both defects modify the band structure of these materials and should influence the HER.

Indeed, in tests as photocatalysts for the HER, Na- and Mg-PHI showed the best performance with the latter reaching an unprecedented quantum efficiency of 7.14%. This was attributed to the higher charge of Mg²⁺ compared to all other cations (Na⁺, K⁺, H⁺), which leads to a more efficient polarization of the heptazine backbone and the promotion of the cleavage of the H-O bond during the water splitting as well as a considerably faster interlayer charge transfer in the presence of rotational defects.

We gratefully acknowledge financial support by the Max Planck Society.