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Diane Haiber1 Peter Crozier1

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

Layered carbon nitrides have recently emerged as metal-free, visible-light absorbing semiconductors with growing interest as photocatalysts for hydrogen production1. While transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) are powerful tools to characterize nanoscale features in (photo)catalysts, their application to layered carbon nitrides is difficult due to radiation damage. Graphitic carbon nitride (g-CNxHy) compounds, based on layers of amine-bridged heptazine (C6N7) chains2, differentiated by their residual H-content, represent a wide-ranging class of material with ill-defined variation in structure. Molten salt synthesis routes have yielded crystalline, layered carbon nitrides based on triazine (C3N3) motifs with correspondingly less H-content3. These are referred to as poly(triazine imide) with intercalated halide ions (such as Li and Cl), or PTI/LiCl. Amine (e.g. N-Hx) content in both g-CNxHy and PTI/LiCl are correlated to changes in photocatalytic hydrogen evolution under visible-light, suggesting these defects can regulate optical absorption or charge transfer kinetics depending on how the host structure is modified. While infrared (IR) spectroscopy enables comparison of H-content, it lacks the spatial resolution needed for unambiguously correlating defects with catalytically relevant sites.

Here, we utilize vibrational EELS to locally probe bonding in g-CNxHy and PTI/LiCl, which are expected to contain a high and low degree of chemical heterogeneity, respectively. A radiolytic damage mechanism involving loss of amine groups is proposed based on variable dose-rate TEM imaging and core-loss EELS observations. To mitigate radiolysis, an ‘aloof-beam’ configuration is employed, meaning the electron beam is placed several nanometers outside the specimen by a distance called the ‘impact parameter’. By comparing to photon-based vibrational spectroscopies (e.g. IR absorption and Raman), the aloof-beam EELS resembles a broadened IR absorption spectrum with two major peaks attributed to C-N ring and N-Hx vibrations. Two types of heterogeneity in the vibrational EELS is observed: (i) an extra, weak peak at 265-meV associated with cyano (C≡N) defects and (ii) variation in the amine content defined as the ‘N-H’ to ‘C-N’ peak intensity ratio. By considering the consistency in probed-volume and lack of correlation with impact parameter, it is concluded that amine and cyano defects spatially-fluctuate throughout the g-CNxHy structure. On the other hand, PTI/LiCl does not exhibit spatially-varied cyano defects and the increased type-(ii) heterogeneity was attributed to radiolysis.

[1] X. Wang et al. Nat. Mater. 2009, 8, 76-80. [2] B. Lotsch et al. Chem. Eur. J. 2007, 13, 4969-80. [3] E. Wirnhier et al. Chem. Eur. J. 2011, 13, 3213-21. [4] We gratefully acknowledge the support of DOE grant DE-SC0004954, ASU’s John M. Cowley Center for High Resolution Electron Microscopy and ASU’s Center for Solid State Science.

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