Discovery of quasi-carbon nanotube structures in tectonically deformed coals and their formation mechanisms

Objective Carbon nanotubes (CNTs) have attracted wide attention due to their unique structures and performance. However, their artificial synthesis requires stringent conditions, including high temperatures (700 ℃ to 2 300 ℃), stable carbon sources, and catalysts. Furthermore, these structures are rarely discovered in nature. As a carbon-rich medium, coals can form a variety of ordered carbon structures during their metamorphic and deformational processes, serving as an ideal carrier for exploring the formation mechanisms of natural CNTs.

Methods Typical tectonically deformed coals from the Pingdingshan and Huaibei mining areas were investigated in this study. After demineralization and chloroform extraction, coal samples were observed under the high-resolution transmission electron microscope (HRTEM), revealing the presence of abundant fibrous tubular structures in coals unexpectedly. These structures were systematically characterized using X-ray diffraction (XRD) and laser Raman spectroscopy (LRS), and their formation mechanisms were preliminarily explored.

Results This study revealed the presence of quasi-CNT structures in medium-rank tectonically deformed coals for the first time. These structures were largely characterized by a hollow structure and parallel tube walls, presenting a linear or curled morphology. They were dominated by multi-walled structures composed of 2‒18 carbon layers, with a diameter range of 0.4‒6.0 nm and an interlayer spacing of approximately 0.4 nm. The laser Raman spectra of the quasi-CNT structures showed G peaks at 1 595 cm⁻¹ and 1 599 cm⁻¹ and D peaks at 1 354 cm⁻¹ and 1 364 cm⁻¹, which are highly consistent with those of graphite. The XRD analysis indicates that the quasi-CNT structures had an intershell spacing (d002) range of 0.343‒0.348 nm, further confirming the presence of graphite-like crystal structures. Additionally, organic solvent extraction was found to enable the effective enrichment of quasi-CNT structures and enhance their observability.

Conclusions The analysis suggests that intense tectonic stress, especially shear stress, can significantly reduce the energy barriers against the ordering and curling of aromatic layers in coals through the synergistic effects of mechanochemistry, element migration, and frictional heat. Consequently, the tectonic stress drives organic carbon to form quasi-CNT structures under temperatures far below those required by traditional synthetic methods. The discovery of quasi-CNT structures in tectonically deformed coals enriches the system of natural carbon materials. More importantly, the discovery facilitates the introduction of another formation pathway of quasi-CNT structures, i.e., a tectonic stress-driven chemical process, which is independent of extreme thermo-catalytic conditions. This pathway provides a critical theoretical basis and a novel approach for developing green bionic preparation technology of CNTs based on stress adjustment. The key to future research lies in quantitatively revealing the mechanochemical process and dynamic mechanisms for the formation of coal-based quasi-CNT structures.