Facile green synthesis of oxygen-functionalized carbon nano-onion from lignin and biochar and its application in high-performance polylactic acid

Carbon nano-onion (CNO) is a new class of carbon nanomaterials with various applications such as electronics, photovoltaics, energy storage, biosensing, biomedicine, and catalysts. However, its widespread applications are hindered due to the high costs associated with hydrocarbon precursors and synthesis conditions involving hazardous chemicals, catalysts, non-atmospheric pressures, and prolonged processing time at high temperatures. Herein, we report a joule heating-based synthesis of oxygen-functionalized low-cost CNO using biomass residues without catalysts, chemicals, and solvents. It shows that while the crystalline structure and surface functional groups of the CNO are controllable, CNO with similar particle sizes (33–36 nm) and electrical conductivity (between 3.73 and 3.95 S/m) could be produced using lignins and biochar. The CNO also demonstrated excellent dispersibility in common solvents, attributed to their functional groups. When a new application of CNO as polymer additives was explored, 0.5% CNO could increase the tensile strength and modulus of polylactic acid (PLA) by 43.7% and 128.4%, flexural strength and modulus by 9.4% and 20%, and impact strength by 60.4%. Adding 0.5% CNO also increased the glass transition temperature and the thermal decomposition temperature of PLA by 6.3 and 13.7 °C, reducing oxygen and water vapor permeability by 67.4% and 48.4%. The comparison showed that the oxygen-functionalized CNO additive is superior to other nanocarbons in improving PLA performance. The energy consumption for synthesizing CNO from biochar was 15.6 MJ/kg, providing an attractive pathway to obtain low-cost green CNO for broad applications.