Photothermal Annealing-Enabled Millisecond Synthesis of Carbon Nanoonions and Simultaneous Single-Atom Functionalization

Carbon nanoonions (CNOs), known for their nanometer-scale surface curvature and potential for versatile functionalization, are widely used in energy and environmental applications. However, they face challenges from energy-intensive synthesis and time-consuming post-treatments, resulting in low yields and poor-quality sp² shells, which limit their commercial viability. In this study, we introduce a direct-contact annealing (DCA) platform reaching up to 3030 K within 1.4 ms (2.2 × 10⁶ K s^–1), utilizing black-colored photothermal agents for millisecond-scale synthesis of CNO under ambient air. Moreover, we demonstrate simultaneous in situ single-atom catalyst (SACs) functionalization with eight different metal elements on the outer surface of CNOs. A case study on Pt SAC-functionalized CNOs demonstrates outstanding hydrogen evolution reaction performance. This DCA platform provides a promising alternative to conventional harsh conditions for SAC/CNO electrocatalyst synthesis, enabling ultrafast and facile production of surface-functionalized catalysts with exceptional energy efficiency and scalability advantages for advanced energy applications.