Youfu Wang

Carbon quantum dots: synthesis, properties and applications

Carbon quantum dots (CQDs, C-dots or CDs), which are generally small carbon nanoparticles (less than 10 nm in size) with various unique properties, have found wide use in more and more fields during the last few years. In this feature article, we describe the recent progress in the field of CQDs, focusing on their synthetic methods, size control, modification strategies, photoelectric properties, luminescent mechanism, and applications in biomedicine, optronics, catalysis and sensor issues.

Preparation of carbon nanodots from single chain polymeric nanoparticles and theoretical investigation of the photoluminescence mechanism

Even after several years of research, controlled synthesis of photoluminescent carbon nanodots (C-dots) still constitutes a major challenge, and investigation of their photoluminescence (PL) mechanism remains elusive. Various top-down and bottom-up approaches have been reported lately. However, these methods usually suffer from limited control over the major factors that dictate the PL behaviour of these fascinating carbon materials. To this end, we discover a new approach to prepare C-dots from size-tunable single chain polymeric nanoparticles. Taking advantage of the state of the art living radical polymerization technique and unique features of Bergman cyclization, narrowly dispersed C-dots are prepared in a straightforward manner. PL study shows that the optimal emission wavelength of C-dots red-shifts when the size of C-dots decreases, which is different from the trends typically found in semiconductor quantum dots and C-dots prepared from graphitized materials. To clarify the PL mechanism of C-dots prepared from different sources, a theoretical study based on density functional theory is performed. Two series of model compounds, fused aromatic rings and cyclo-1,4-naphthylenes, are chosen for C-dots with different microstructures. The calculation data indicate that PL energy of C-dots is dictated by the size and microstructure of the sp2 carbon core. For a C-dot with a graphitized core, the smaller the size of the core, the higher the PL energy, while for a C-dot with an amorphous core, an inverse trend is revealed. Surface reduction experiments further show that the quantum yield of C-dots is controlled by the surface chemistry.

Embedding Co3O4 nanoparticles in SBA-15 supported carbon nanomembrane for advanced supercapacitor materials

A three dimensional porous carbon nanomembrane (CNM), silica-supported CNM (SS-CNM), is developed by formation of a self-assembled monolayer of an enediyne compound on the surface of mesoporous silica (SBA-15) followed by Bergman cyclization and carbonization. The SS-CNM is applied as a conductive support for the electroactive material Co3O4 to fabricate advanced supercapacitors. A large fraction of Co3O4 clusters (66% of total weight) are impregnated in the SS-CNM host to form regularly packed nanorods with diameters of 7 nm. The specific capacitance of the supercapacitor electrode material reaches 1086 F g−1 (1645 F g−1 based on Co3O4) with aqueous electrolyte. This extraordinary high performance of the electrode material is attributed to the unique pore structure of the SS-CNM support that enhances the use of active material and shortens the transport path of ions and electrons.

Practical access to bandgap-like N-doped carbon dots with dual emission unzipped from PAN@PMMA core–shell nanoparticles

As emergent nanolights for bioimaging, catalysis, sensors, and photoelectronics, carbon dots (C-dots) have attracted much research attention. However, the practical and scalable preparation of C-dots has been less explored, even after various top-down and bottom-up approaches being reported recently. To this end, we discover a new approach to prepare C-dots by simply unzipping core–shell polymeric nanoparticles, prepared by a microemulsion polymerization. Uniformly distributed N-doped C-dots are prepared by the pyrolysis of PAN@PMMA core–shell nanoparticles at different temperatures, followed by dialysis. TEM analysis shows that many of the C-dots of 2–3 nm size are unzipped from each polymeric particle. All the purified C-dots show a bandgap-like photoluminescence (PL) behaviour, with dual emission and a stable PL between pH 5–12.

Size-Tunable Polymeric Nanoreactors for One-Pot Synthesis and Encapsulation of Quantum Dots

Hydrophilic polymeric nanoparticles are synthesized through a Bergman cyclization- mediated intramolecular chain collapse of structurally well-defined linear polymers, and then used as size-tunable nanoreactors to fabricate and encapsulate quantum dots in a one-pot reaction. Crystalline quantum dots are formed in all of these nanoreactors and visualized by transmission electron microscopy. Smaller nanoreactors produce one quantum dot each while larger nanoreactors form a number, resulting in fluorescence quenching. By controlling the molecular weight of the linear polymer precursor, a variable number of nanocrystals are fabricated and assembled in a single nanoreactor.

Study on the relation between pore size and supercapacitance in mesoporous carbon electrodes with silica-supported carbon nanomembranes

Electrochemical capacitors (ECs) have traditionally been considered as standing at the opposite end against batteries in energy–power diagram. They charge and discharge faster than batteries but are limited by much lower energy density. By optimizing the pore structure of porous electrode materials, the performance of ECs could overcome this limitation. However to date, no study has addressed the complex relationship between the texture parameters of the electrode materials and the supercapacitance of ECs. Using silica-supported carbon nanomembranes, four electrode materials with similar pore geometry are generated. The electrodes with a pore size of 4.14 nm shows the highest capacitance of 305 F g−1 in aqueous electrolytes. A new model is developed to simulate the accommodation of the solvated ions at the electrode surface. The simulation reveals that the optimal capacitance of ECs can be achieved using porous carbon electrode materials with open pores of 3.0–5.0 nm.

Functionalization of Pristine Graphene with Conjugated Polymers through Diradical Addition and Propagation

Hanging on: Pristine graphene was grafted with conjugated polymers through addition and propagation of diradicals generated from Bergman cyclization of enediyne-containing dendrimers. The surface functionalization was confirmed with TGA, FTIR and Raman spectroscopy, and AFM analysis. The sp(2) network of graphene is only slightly destroyed, as revealed by conductivity measurements.

Co-sensitization of N719 with polyphenylenes from the Bergman cyclization of maleimide-based enediynes for dye-sensitized solar cells

The co-sensitization of N719 with conjugated polyphenylenes for dye-sensitized solar cells (DSSCs) was demonstrated. The conjugated polyphenylenes (6-BC and 7-BC), which contain two triphenylamine (TPA) groups as electron donors and carboxylic acids as adsorption groups, were prepared from thermally triggered Bergman cyclization polymerization of two maleimide-based enediynes, 6 and 7. Both the short-circuit photocurrent density (JSC) and the open-circuit voltage (VOC) of the co-sensitized DSSCs were enhanced due to the increase in light harvesting and the retardation of electron recombination and dye aggregation when N719 was co-sensitized with the conjugated polyphenylenes. The co-sensitized (7-BC + N719) DSSC showed a high efficiency of 9.68% (VOC of 750 mV, JSC of 18.18 mA cm−2 and FF of 0.71), exhibiting an improvement of 16% compared to the devices sensitized with N719 (PCE of 8.34%) alone under identical conditions.

Preparation of hierarchically porous carbon nanofoams for electrode materials of supercapacitors

Hierarchically porous carbon nanofoams (CNFs) with uniform cavity and highly porous skeleton have been prepared via the formation of core–shell organosilica nanoparticles in one-pot reaction and subsequent Friedel–Crafts chemistry. The porosity of the CNFs, which have macroporous cavity (∼50 nm), mesoporous windows (∼12 nm) between adjacent cavities and mesoporous skeleton (∼3 nm), were facially tuned by varying the amount of phenyltriethoxysilane (PTES) used in the first step. CNFs with high surface area (>1100 m2 g−1), large pore volume (∼2 cm3 g−1) and partially graphitized skeleton exhibited good performances as supercapacitor electrode materials. The specific capacitance of the porous CNFs reached 170 F g−1 at a current density of 0.5 A g−1 in an aqueous electrolyte.

Synthesis of carbon nanomembranes through cross-linking of phenyl self-assembled monolayers for electrode materials in supercapacitors

A bottom-up synthesis of three-dimensional (3D) carbon nanomembranes (CNMs) was developed through Friedel–Crafts cross-linking of phenyl self-assembled monolayers (SAMs) on silica nanospheres (SNSs) followed by high temperature treatment and template removal. The CNMs show a hierarchically 3D connected porous structure and the pore sizes are facilely tuned by varying the sizes of SNS templates. CNMs with high surface area (>1500 m2 g−1), large pore volume (∼3 cm3 g−1) and partially graphitized frameworks exhibited good performances as supercapacitor electrode materials. The specific capacitance of the porous 3D CNMs reached 202 F g−1 at a current density of 0.5 A g−1 in an aqueous electrolyte.