Versatile, Aqueous Soluble C2N Quantum Dots with Enriched Active Edges and Oxygenated Groups.

C2N has emerged as a new family of promising two-dimensional (2D) layered frameworks in both fundamental studies and potential applications. Transforming bulk C2N into zero-dimensional quantum dots (QDs) could induce unique quantum confinement and edge effects that produce improved or new properties. Despite their appealing potential, C2NQDs remain unexplored, and their intriguing properties and a fundamental understanding of their prominent edge effects are still not well understood. Here, we report the first synthesis of water-soluble C2NQDs via a top-down approach without any foreign stabilizer and exploit their linear/nonlinear optical properties and unique edge-preferential electrocatalytic activity toward polysulfides for versatile applications. The resultant dispersant-free C2NQDs with an average size of less than 5 nm feature rich oxygen-carrying groups and active edges, not only enabling excellent dispersion in water but also creating interesting multifunctionality. They can emit not only blue one-photon luminescence (OPL) under ultraviolet (UV) excitation but also green two-photon luminescence (TPL) with a wide near-infrared (NIR) excitation range of 750-900 nm, enabling their use as a new fluorescent ink. Interestingly, when C2NQDs are introduced to modify commercial separators, they can function as new metal-free catalysts to boost polysulfide redox kinetics and endow Li-S batteries with excellent cycling stability, high rate capability, and large areal capacity (7.0 mA h cm-2) at a high sulfur loading of 8.0 mg cm-2. Detailed theoretical and experimental results indicate that the edge of C2N is more favorable for trapping and catalyzing the polysulfide conversion than the terrace and that the synergy between the active edges and oxygenated groups enriched in C2NQDs remarkably improves polysulfide immobilization and catalytic conversion.