Carbon-based nanomaterials: Synthesis, atomic structure, and NMR overview

Carbon-based nanomaterials (CNMs) are among the most extensively researched materials due to their exceptional porosity, mechanical, thermal, electrical, and optical properties. This review provides a comprehensive overview of the major allotropes of carbon, including sp 3 -hybridized nanodiamond and sp 2 -hybridized fullerenes, carbon nanotubes, graphene, graphene oxide, reduced graphene oxide, and graphite. The review also presents a detailed discussion of top-down and bottom-up synthesis techniques for CNMs and their characterization by nuclear magnetic resonance (NMR). A background to the fundamental components of an NMR spectrometer, the principles of NMR, and the acquisition and assignment of 13 C NMR spectra are described. In addition, the review further examines the atomic-level structures of CNMs and discusses the major challenges associated with acquiring 13 C NMR spectra, such as low signal sensitivity, poor spectral resolution, and long spin-lattice relaxation times ( T 1 ), which lead to weak signals, poor signal-to-noise ratios, broad spectral lines, and extended experimental times. Strategies for overcoming these limitations are also discussed. Using 13 C-labeled isotopic enrichment during sample preparation and utilizing polarization transfer techniques such as cross-polarization (CP) and dynamic nuclear polarization (DNP) can enhance signal sensitivity and shorten effective relaxation times. Furthermore, poor resolution (broader lines) caused by internal interactions ( e.g., shift anisotropy and dipolar coupling) and shorter spin-spin relaxation times ( T 2 ) can be improved by ultrafast magic-angle spinning (MAS), proper shimming, high magnetic field strength, and variable temperature (VT) NMR experiments at elevated temperatures. Finally, the review concludes with a brief summary, states the broad applications of CNMs in energy, catalysis, medicine, environmental management, electronics, and optoelectronics, and outlines prospects for their continued development and application.