Diane Haiber1 Michael Treacy1 Peter Crozier1

1, Arizona State University, Tempe, Arizona, United States

Graphitic carbon nitrides are layered, polymeric compounds that are widely studied as a semiconducting support for visible-light driven photocatalytic water splitting1. However, structural disorder in these materials is poorly understood due to the difficulty of performing high spatial resolution characterization through transmission electron microscopy (TEM) on these beam-sensitive materials. Fully-condensed g-C3N4 contains hexagonally periodic heptazine (C6N7) or triazine (C3N3) motifs, bridged by 3-fold coordinated N-atoms, creating a graphite-like structure with regularly-spaced voids. Synthesis involves calcination of N-rich precursors to yield yellow powders of varying paleness. In this way, g-CNxHy compounds are formed with a sub-stoichiometric C/N ratio at the expense of residual hydrogen preventing complete structural condensation. In 2007, nuclear magnetic resonance and electron diffraction were combined to solve the average in-plane structure of a g-CNxHy compound which consisted of amine (N-Hx) -bridged heptazine molecules arranged in zig-zag chains3. Based on x-ray and neutron diffraction, the bulk stacking order was determined giving a 3D structural description4. While these works set the foundation for understanding the structure of g-CNxHy’s, low-dose TEM may be applied to directly image variations of the in-plane structure and obtain a more precise description of structural disorder.

Using an aberration-corrected TEM operating at 300-kV under low dose rate conditions (e.g. ~20 e-2/s) coupled with a direct electron detector, we have acquired images from three g-CNxHy specimens with different degrees of presumed structural disorder. So far, several techniques are being considered to elucidate (in-plane) disorder based on TEM images taken with the basal plane perpendicular to the beam. Firstly, rotationally-averaged Fourier transforms (FT’s) of large areas (e.g. (50 nm)2) show the improvement in the in-plane scattering information when compared to powder XRD and maintain a similar trend in diffraction broadening. Scanned FT’s could be used to determine local variations in layer buckling, chain separation, and azimuthal orientation of the heptazine building blocks. Autocorrelations have been applied to the TEM images to generate pseudo Patterson functions and radial distribution functions, which may be used to compare the real-space extent of structural correlations. Correlographs, azimuthal auto-correlations performed on an FT, are another tool being used to evaluate symmetry axes and are particularly useful in FT’s containing speckled rings.

[1] X. Wang et al. Nat. Mater. 2009, 8, 76-80. [2] B.V. Lotsch et al. Chem. Eur. J. 2007, 13, 4969-80. [3] F. Fina et al. Chem. Mater. 2015, 27, 2612-18. [4] We gratefully acknowledge the support of DOE grant DE-SC0004954, ASU’s John M. Cowley Center for High Resolution Electron Microscopy, ASU's Center for Solid State Science and Gatan Inc. for the use of a K2-IS direct electron detector.