Valuable Carbon Nanomaterials Directly Prepared from CO2 via Sonication in Pure Water

Abstract

Carbon nano-onions (CNOs; also termed multi-walled nanospheres) are produced by one-pot sonication in water at room temperature from CaCO₃/Ca(OH)₂. The as-made material is a CNO-enriched mixed carbon, in which CNOs are frequently observed together with minor graphene-like sheets and filamentous objects; phase separation was not attempted at the present scale. Microscopy (TEM) and spectroscopy (Raman) corroborate 20–30 nm concentric shells; additional spectroscopic signatures indicate oxygen-containing surface functionalities. Carbon yield was quantified on clarified supernatants by total organic carbon (TOC = TC − IC), giving a lower-bound yield of 0.10% relative to input CaCO₃ and a lower-bound CO₂-to-carbon conversion of 0.8% (relative to carbon in CaCO₃). A CO₂-depleted control (N₂ headspace during Ca(OH)₂ pre-equilibration) suppressed precursor formation and is consistent with atmospheric CO₂ capture via CaCO₃ prior to sonication. This water-based, room-temperature route outlines a direct path from carbonate precursors to useful solid carbons; scope, limitations, and controls are discussed.