Rong Zhou

Despite numerous efforts, the mechanism behind multicolor photoluminescence (PL) in graphene quantum dots (GQDs) is still controversial. A deep insight into the origin of the multicolor emissions in GQDs is quite necessary for modulating their luminescence to facilitate the better use of this fluorescent material. Herein, GQDs with amino, carboxyl, and ammonium carboxylate groups were synthesized. The as-prepared GQDs exhibited intriguing excitation- and concentration-dependent multicolor PL characteristics. By regulating the excitation wavelength or concentration of GQDs, specific luminescence colors including blue, cyan, green, yellow, and even orange can be obtained. Systematic structural and optical studies indicated that the graphene basal plane and different functional groups dominantly exhibited nN 2P-σ*, π-π*, nO 2p-π* (-COOH), nO 2p-π* (-COO-) and nN 2p-π* electronic transitions, which appeared as multi-fluorescent centers that gave rise to the excitation-dependent multicolor PL. The occurrence of different types of electronic transitions and their color emissions were proved by pH-dependent PL measurements. In addition, systematic optical and morphology analyses revealed that GQDs could self-assemble into J-type aggregates with different morphologies and sizes as the concentration increased, and the observed concentration-dependent multicolor PL can be ascribed to aggregation-mediated energy level reconstruction in GQDs. Our findings further suggest that the competition among various fluorescent centers and self-aggregation processes dominated the luminescent properties of GQDs. This work will contribute to understand the origins of excitation- and concentration-dependent multicolor emissions in GQDs, which is also highly instructive for broadening the application fields of GQDs.

Waterborne polyurethane has drawn extensive attention due to its environmental friendliness and is widely used in many areas. However, it is still a great challenge to synthesize waterborne polyurethanes with flame retardancy and fast room-temperature self-healing ability, along with excellent mechanical performance and emulsion stability due to the mutually contradictory nature of these properties. Herein, waterborne polyurethanes containing organic selenium (SWPU-x) from 0.67 to 3.28 wt % were synthesized, which could simultaneously realize flame retardancy and self-healing ability based on the ability to scavenge active free radicals at high temperature and the dynamic switch of diselenide. All these SWPU-x films self-extinguished within 1 s after the ignition in the vertical combustion tests. The limiting oxygen index of SWPU-4 was improved to 28.5% with excellent UL-94 level (V-0) and self-healing efficiency (91.25%, after being healed in the photoreactor for 30 min at room temperature), together with high mechanical properties (tensile strength was 18.5 MPa and elongation at break was 869.63%), and the total heat release (THR) for SWPU-4 (49.28 MJ/m2) could decrease to 23.80% of the THR for the original waterborne polyurethane WPU (64.67 MJ/m2). This work discovered a new flame-retardant element (organic selenium) and studied its flame-retardant behaviors and self-healing function simultaneously, which would extremely extend the application of waterborne polyurethanes.

It is very challenging to achieve polymers that are mechanically robust and fast self-healable at ambient conditions, which are highly desirable for smart materials of the next-generation. Herein, combining dynamic ditelluride bonds and 2-ureido-4[1H]-pyrimidinone (UPy) moieties in the main chains, a novel type of visible-light-induced self-healing water-dispersed supramolecular polymers (DTe-WSPs) with outstanding healing properties were developed. The prepared DTe-WSPs emulsions showed excellent emulsion stability, and highly transparent DTe-WSPs films obtained from these emulsions exhibited much improved mechanical properties and fast recoverability after the incorporation of UPy groups, owing to the physical cross-links formed by quadruple hydrogen-bonded UPy moieties. Supertoughness (105.2 MJ m–3) and fast self-healability under visible light (healing efficiency of 85.6% within 10 min) could be achieved simultaneously with the adjustment of the ditelluride content and the UPy content, and the toughness of our polymers is higher than those of the reported ambient temperature self-healable polymers. The visible-light-induced ditelluride metathesis is a predominant factor in the healing process of DTe-WSPs, and the ditelluride metathesis triggered by photothermy and hydrogen bonding could also afford the ultimate healing result. Meanwhile, DTe-WSPs can be reprocessed using visible light, providing a facile way to process polymers at mild conditions. To our surprise, the “living” DTe-WSPs exhibited the ability to initiate the polymerization of vinyl monomers under visible light, which is first reported for water-dispersed self-healing polymers. We considered the elaborated design philosophy, based on the readily available, clean, safe, and easily manipulated visible light, which can not only provide inspiration for preparing fast ambient temperature self-healing and reprocessing polymer materials with robust mechanical properties but also develop a new macroinitiator to initiate the ambient temperature polymerization of vinyl monomers.