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

Abstract

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 sub-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 UV excitation but also green two-photon luminescence (TPL), 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.