High-Resolution Microstructural Analyses of Polymeric Carbon Nitride Incorporated with Graphitic Carbon

<br/>Graphitic carbon nitride (GCN) is a polymeric semiconductor having a layered structure. Owing to its desirable characteristics for solar water splitting, the GCN attracts much attention as a sustainable metal-free photocatalysis.^1,2 Among the polymorphs in GCN, heptazine-based structure aligned along the in-plane film direction is well-known as a stable one. Recently, research reports demonstrated that the GCN nanosheet incorporated with graphitic carbon can enhance the lifetime of photoexcited electrons due to its high electron-hole separation ability and increase the absorption under visible light.² Accordingly, such the C-incorporated GCN is expected to be applied for not only a high-efficiency photocatalysis but also the photovoltaic material promising to various next-generation optoelectronic devices. However, nanosheets are not appropriate for optoelectronics. We have succeeded in fabricating centimeter-scale GCN films that are applicable to optoelectronic devices.³ The purpose of this study is to clarify the structural feature of the aforementioned polymer, which is still unclear.<br/>Here we show microstructural analyses of C-GCN films using the high-resolution transmission electron microscopy (HREM) technique. Our HREM observations demonstrate the heptazine structure of GCN in the real-space, which is also confirmed by superstructural reflection in the corresponding Fast Fourier Transform (FFT) pattern. Both real-space images and corresponding FFT clearly show the super structure with a long period revealing the polymeric heptazine. Meanwhile, the primitive structure with hexagonal symmetry and a short period is observed and assigned to well-known graphitic carbon, which is clearly distinct from polymeric heptazine. The present study based on the HREM observations provides fundamental microstructural information of C-GCN film, that contributes to a practical design of polymer-based optoelectronic devices.<br/><br/>*This work was supported in part by Grants-In-Aid for Scientific Research (A) (Grant No. 19H00660) and Research Activity Start-up (Grant No. 22K20363) from the Japan Society for the Promotion of Science (JSPS) and the Japan Science and Technology Agency (JST) CREST program (Grant No. JPMJCR1874, JPMJCR20T1), Japan.<br/><br/>References<br/>1. X. Wang <i>et al</i>., <i>Nat. Mater.</i> 8, 76 (2009).<br/>2. W. Che <i>et al</i>., <i>J. Am. Chem. Soc.</i> 139, 3021 (2017).<br/>3. H. Arazoe<i> et al</i>., <i>Nat. Mater.</i> 15, 1084 (2016)