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Raman tensor analysis can be used to understand the surface chemistry of two-dimensional (2D) materials, which would enable the design of efficient MXene catalysts for electrochemical reactions. However, there are no reports of a detailed Raman tensor analysis on MXenes. In this work, we use density functional theory (DFT) Raman spectra calculations to provide insights into the surface chemistry of Ti2NTx MXene (Tx = −O– and/or −OH) by considering the influence of different Raman tensors on the spectra. We identify the Ti2NTx interlayer and surface termination as mostly −O– and −OH with small −F contributions based on optimized models, which influences MXene material reactivities. In this study we focus on the −OH termination group. Geometric tensor averages provide a better Raman spectra prediction for thick multilayer Ti2NTx MXene materials that match the experimental Raman spectra. In addition, we found the incident laser correction term to be essential for small wavenumber Raman intensities. Based on our Raman analysis, the surface termination coverages are either 25% −OH or 75% −OH. We found Raman intensities of 0%, 50%, and 100% −OH coverage to be negligible. Based on the simulated X-ray diffraction (XRD) results, the interlayer spacing is dictated by the interlayer −OH termination group. We also found that mismatched interlayer termination groups can restrict the interlayer distances. The present study provides insights into the role of the termination groups in stabilizing and influencing the structure of the MXene material, which, in turn, can be exploited to further enhance the application of MXenes in various energy conversion and storage systems.
Lai et al. (Thu,) studied this question.