Tungsten-catalyzed microwave-irradiation synthesis of carbon nano-onions from nanodiamonds: Structure-property relationships and oxygen reduction electrocatalysis

ABSTRACT We report an ultrafast method for producing metal-free carbon nano-onions (CNOs) by tungsten-assisted microwave (MW) irradiation of nanodiamond (ND) precursors. In contrast to conventional pyrolysis, which typically requires temperatures above 1650 °C, complete ND-to-CNO conversion is achieved within 45 s at 700 W. Tungsten serves as a plasma initiator and field-localization element, enabling rapid graphitization without detectable tungsten incorporation into the final material. Structural analyses confirm the formation of quasi-spherical multilayer CNOs with predominantly sp 2 -hybridized C atoms. At the same time, the layers' regularity and defect density depend strongly on the surface chemistry of the starting NDs. Broadband dielectric spectroscopy shows that the resulting CNOs are up to 8 orders of magnitude more conductive than the parent NDs and CNOs obtained by the high-temperature pyrolysis, and exhibit enhanced dielectric response, indicating efficient charge transport and charge accumulation. Nitrogen sorption reveals moderate surface areas and predominantly interparticle mesoporosity. In 0.1 M NaCl, all CNOs display electric double-layer capacitive behavior, with specific capacitances of 146.6, 76.2, and 45.4 F g −1 for CNO Molto, CNO Amine, and CNO Vox, respectively, demonstrating that capacitive performance is governed by layer ordering, conductivity, and pore accessibility rather than by surface area alone. In an alkaline electrolyte, the CNOs also exhibit oxygen-reduction activity characteristic of defect-containing sp 2 carbon frameworks. These results establish tungsten-assisted MW processing as a rapid route to structurally tunable CNOs with coupled electrical, capacitive, and electrocatalytic functionality.