A facile hydrothermal synthesis route to N and S, N co-doped graphene quantum dots (GQDs) was developed by using citric acid as the C source and urea or thiourea as N and S sources. Both N and S, N doped GQDs showed high quantum yield (78% and 71%), excitation independent under excitation of 340-400 nm and single exponential decay under UV excitation. A broad absorption band in the visible region appeared in S, N co-doped GQDs due to doping with sulfur, which alters the surface state of GQDs. However, S, N co-doped GQDs show different color emission under excitation of 420-520 nm due to their absorption in the visible region. The excellent photocatalytic performance of the S, N co-doped GQD/TiO2 composites was demonstrated by degradation of rhodamine B under visible light. The apparent rate of S, N:GQD/TiO2 is 3 and 10 times higher than that of N:GQD/TiO2 and P25 TiO2 under visible light irradiation, respectively.

Co‐electrospinning of core–shell polymer nanofibers (see Figure) is introduced. This process can be used for manufacturing of coaxial nanofibers made of pairs of different materials. Non‐spinnable materials can be forced into 1D arrangements by co‐electrospinning using a spinnable shell polymer. The method results in a novel two‐stage approach for fabrication of nanotubes instead of the previously used three‐stage process.

As a new member of the carbon material family, carbon dots (CDots) endow carbon materials with luminescent property and expand their application in fluorescent field. Because of their unique optical property, they have attracted much more attention for their potential applications in the photoelectrical conversion, [1] photo catalysis, [2] bioimaging systems, [3] and light-emittaing devices (LEDs) [4] since CDots were discovered in 2004. [5] In early research, CDots had low photoluminescence (PL) quantum yield (QY) and the emission was limited at blue light emission. Over a decade, researchers have put much more effort to improve their PL QY through surface passivation and heteroatom doping methods to induce better charge/carrier transport. Since then, the PL QY of CDots can reach up to 94% for strong blue emission [6] and ≈60% for green emission. [7] This progress leads to potential applications for CDots in bioimaging, optoelectronic devices, and LEDs due to their low toxicity, biocompatibility, and excellent photostability. [3e,8] However, efficient red emissive CDots are still highly desired because red light has deep tissue penetration for bioimaging and is one of the primary colors for white LEDs. There are a few reports on the synthesis of red emissive CDots. For example, using expensive polythiophene phenylpropionic acid as a precursor, Wang and co-workers developed hydrothermal reaction processes to prepare red emissive CDots with low PL QY of 2.3%. [9] Our group reported synthesis of red emissive CDots with 8% PL QY using citric acid (CA) and thiourea, which results in S-and N-doped CDots with enhanced electron delocalization. [10] Lin and co-workers demonstrated synthesis of red emissive CDots with PL QY of 26%. The CDots were prepared through solvothermal route using phenylenediamine isomers. [11] Xiong and co-workers reported the red-emitting CDots with PL QY of ≈24%. The red emission was achieved by tuning the surface state from precursors p-phenylenediamine and urea that were prepared carefully through silica column chromatography. [12] Recently, Yang's group used dopamine and o-phenylenediamine as precursors and obtained near infrared emissive CDots by generating large sp 2 domains. The final QY of the CDots can reach to ≈30%. [13] Despite these efforts, the final materials are expensive for practical applications and lack of clear understanding of luminescence mechanism. Thus, Multiple-color-emissive carbon dots (CDots) have potential applications in various fields such as bioimaging, light-emitting devices, and photocatalysis. The majority of the current CDots to date exhibit excitation-wavelengthdependent emissions with their maximum emission limited at the blue-light region. Here, a synthesis of multiple-color-emission CDots by controlled graphitization and surface function is reported. The CDots are synthesized through controlled thermal pyrolysis of citric acid and urea. By regulating the thermal-pyrolysis temperature and ratio of reactants, the maximum emission of the resulting CDot...

Photoluminescent graphene quantum dots (GQDs) have received enormous attention because of their unique chemical, electronic and optical properties. Here a series of GQDs were synthesized under hydrothermal processes in order to investigate the formation process and optical properties of N-doped GQDs. Citric acid (CA) was used as a carbon precursor and self-assembled into sheet structure in a basic condition and formed N-free GQD graphite framework through intermolecular dehydrolysis reaction. N-doped GQDs were prepared using a series of N-containing bases such as urea. Detailed structural and property studies demonstrated the formation mechanism of N-doped GQDs for tunable optical emissions. Hydrothermal conditions promote formation of amide between –NH2 and –COOH with the presence of amine in the reaction. The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework. Further, the pyrrolic N transformed to graphite N under hydrothermal conditions. N-doping results in a great improvement of PL quantum yield (QY) of GQDs. By optimized reaction conditions, the highest PL QY (94%) of N-doped GQDs was obtained using CA as a carbon source and ethylene diamine as a N source. The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.