Yiyong Mai

Two-Dimensional Soft Nanomaterials: A Fascinating World of Materials

The discovery of graphene has triggered great interest in two-dimensional (2D) nanomaterials for scientists in chemistry, physics, materials science, and related areas. In the family of newly developed 2D nanostructured materials, 2D soft nanomaterials, including graphene, Bx Cy Nz nanosheets, 2D polymers, covalent organic frameworks (COFs), and 2D supramolecular organic nanostructures, possess great advantages in light-weight, structural control and flexibility, diversity of fabrication approaches, and so on. These merits offer 2D soft nanomaterials a wide range of potential applications, such as in optoelectronics, membranes, energy storage and conversion, catalysis, sensing, biotechnology, etc. This review article provides an overview of the development of 2D soft nanomaterials, with special highlights on the basic concepts, molecular design principles, and primary synthesis approaches in the context.

Controlled Incorporation of Particles into the Central Portion of Vesicle Walls

Vesicles have attracted considerable attention recently because of many potential applications as well as intrinsic interest in the structures. The incorporation of various particles into vesicle walls has also received attention. One of the unsolved problems, in this context, is the controlled incorporation of particles into only the central portion of the vesicle walls, i.e. approximately halfway between the external and internal interfaces. In this paper, we describe a general method for the incorporation of particles into only the central portion, i.e. central 10-20%, of the vesicle walls. The strategy involves the use, as coatings on the particles, of diblock copolymers of a structure similar to that of the vesicle formers, which allows the particles to be preferentially localized in the central portion of the walls.

Selective Localization of Preformed Nanoparticles in Morphologically Controllable Block Copolymer Aggregates in Solution

The development of nanodevices currently requires the formation of morphologically controlled or highly ordered arrays of metal, semiconducting, or magnetic nanoparticles. In this context, polymer self-assembly provides a powerful bottom-up approach for constructing these materials. The self-assembly of block copolymers (BCPs) in solution is a facile and popular method for the preparation of aggregates of controllable morphologies, including spherical micelles, cylindrical micelles, vesicles (or polymersomes), thin films, and other complex structures that range from zero to three dimensions. Researchers can generally control the morphology of the aggregates by varying copolymer composition or environmental parameters, including the copolymer concentration, the common solvent, the content of the precipitant, or the presence of additives such as ions, among others. For example, as the content of the hydrophilic block in amphiphilic copolymers decreases, the aggregates formed from the copolymers can change from spherical micelles to cylindrical micelles and to vesicles. The aggregates of various morphologies provide excellent templates for the organization of the nanoparticles. The presence of various domains, such as cores, interfaces, and coronas, in BCP aggregates allows for selective localization of nanoparticles in different regions, which may critically affect the resulting properties and applications of the nanoparticles. For example, the incorporation of quantum dots (QDs) into micelle cores solves many problems encountered in the utilization of QDs in biological environments, including enhancement of water solubility, aggregation prevention, increases in circulation or retention time, and toxicity clearance. Simultaneously it preserves the unique optical performance of QDs compared with those of organic fluorophores, such as size-tunable light emission, improved signal brightness, resistance against photobleaching, and simultaneous excitation of multiple fluorescence colors. Therefore, many studies have focused on the selective localization of nanoparticles in BCP aggregates. This Account describes the selective localization of preformed spherical nanoparticles in different domains of BCP aggregates of controllable morphologies in solution, including spherical micelles, cylindrical micelles, and vesicles. These structures offer many potential applications in biotechnology, biomedicine, catalysis, etc. We also introduce other types of control, including interparticle spacing, particle number density, or aggregate size control. We highlight examples in which the surface coating, volume fraction, or size of the particles was tailored to precisely control incorporation. These examples build on the thermodynamic considerations of particle-polymer interactions, such as hydrophobic interactions, hydrogen bonding, electrostatic interactions, and ligand replacement, among others.

Honeycomb-structured microporous films made from hyperbranched polymers by the breath figure method.

Honeycomb-structured microporous films were self-assembled from a new type of multiarm copolymer, hyperbranched poly(3-ethyl-3-oxetanemethanol)-star-polystyrene (HBPO-star-PS). The precursor consisting of an HBPO core and a number of PS arms was synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization. The microporous film was prepared by the evaporation of a chloroform solution of the precursor in a humid atmosphere (the so-called breath figure method). Compared to our former work, the hexagonally packed pores in the film were not interpenetrated and isolated from one another by the walls. The size of the pores could be controlled easily by changing the casting volume of the solution, the molecular weight and concentration of the polymer, and so forth. The water contact angle on the film surface indicated that the hydrophobicity of the film surface was significantly enhanced as a result of the formation of the porous structure.

Superhydrophobic and superoleophilic graphene aerogel prepared by facile chemical reduction

A superhydrophobic neat graphene aerogel was fabricated for the first time by the facile chemical reduction of a graphene oxide dispersion. The chemical reduction of graphene oxide was confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and thermogravimetric analysis (TGA). The resulting graphene aerogel showed complete water repellency, superoleophilicity, ultra-low density, large specific surface area, high absorption capacity of oils and organic solvents, superior absorption recyclability, and excellent mechanical properties. Its absorption capacity was higher than 100 g g−1 for all the utilized common organic solvents. In addition, the fabrication procedure was simple, scalable, and environmental friendly. Thus, it displays promising application prospects in the field of oil-absorption and oil–water separation.

Patterning two-dimensional free-standing surfaces with mesoporous conducting polymers.

The ability to pattern functional moieties with well-defined architectures is highly important in material science, nanotechnology and bioengineering. Although two-dimensional surfaces can serve as attractive platforms, direct patterning them in solution with regular arrays remains a major challenge. Here we develop a versatile route to pattern two-dimensional free-standing surfaces in a controlled manner assisted by monomicelle close-packing assembly of block copolymers, which is unambiguously revealed by direct visual observation. This strategy allows for bottom-up patterning of polypyrrole and polyaniline with adjustable mesopores on various functional free-standing surfaces, including two-dimensional graphene, molybdenum sulfide, titania nanosheets and even on one-dimensional carbon nanotubes. As exemplified by graphene oxide-based mesoporous polypyrrole nanosheets, the unique sandwich structure with adjustable pore sizes (5–20 nm) and thickness (35–45 nm) as well as enlarged specific surface area (85 m2 g−1) provides excellent specific capacitance and rate performance for supercapacitors. Therefore, this approach will shed light on developing solution-based soft patterning of given interfaces towards bespoke functions.

Metal–Nitrogen Doping of Mesoporous Carbon/Graphene Nanosheets by Self‐Templating for Oxygen Reduction Electrocatalysts

We demonstrate a general and efficient self-templating strategy towards transition metal-nitrogen containing mesoporous carbon/graphene nanosheets with a unique two-dimensional (2D) morphology and tunable mesoscale porosity. Owing to the well-defined 2D morphology, nanometer-scale thickness, high specific surface area, and the simultaneous doping of the metal-nitrogen compounds, the as-prepared catalysts exhibits excellent electrocatalytic activity and stability towards the oxygen reduction reaction (ORR) in both alkaline and acidic media. More importantly, such a self-templating approach towards two-dimensional porous carbon hybrids with diverse metal-nitrogen doping opens up new avenues to mesoporous heteroatom-doped carbon materials as electrochemical catalysts for oxygen reduction and hydrogen evolution, with promising applications in fuel cell and battery technologies.

Bio-based green composites with high performance from poly(lactic acid) and surface-modified microcrystalline cellulose

Bio-based green composites with high performance were prepared from poly(lactic acid) (PLA) and microcrystalline cellulose (MC) fibers grafted with L-lactic acid oligomers (g-MC). The chemical structure of g-MC was characterized by fourier transform infrared (FTIR) and NMR methods, which indicate that L-lactic acid oligomers were successfully grafted onto MC. The grafting percentage of L-lactic acid oligomers is ca. 3.4%, and the average degree of polymerization of grafted L-lactic acid oligomers is ca. 10%. The improved compatibility between g-MC and PLA, caused by the grafting, results in an excellent dispersion of g-MC in the composites, and consequently a considerably improved transparence of the g-MC/PLA composites compared with that of the MC/PLA composites. In addition, due to the improved compatibility between g-MC and PLA, the g-MC/PLA composites exhibit better mechanical properties than pure PLA, with a high tensile strength of 70 MPa and a higher elongation at breakage. The enhanced properties, coupled with the excellent biocompatibility and degradability, offer the bio-based composites potential applications in biomedical fields and the packaging industry.

Dual-Template Synthesis of 2D Mesoporous Polypyrrole Nanosheets with Controlled Pore Size.

The first synergistic dual-template self-assembly approach is presented for bottom-up construction of 2D mesoporous polypyrrole nanosheets based on different supramolecular assemblies, which feature a double-layered architecture, controlled pore sizes, ultrathin thickness, and large surface area. The unique structure rends them with superior reversible discharge capability, rate performance, and stable cyclability when serving as the cathode materials for Na-ion batteries.

Nitrogen-enriched hierarchically porous carbon materials fabricated by graphene aerogel templated Schiff-base chemistry for high performance electrochemical capacitors

This article presents a facile and effective approach for synthesizing three-dimensional (3D) graphene-coupled Schiff-base hierarchically porous polymers (GS-HPPs). The method involves the polymerization of melamine and 1,4-phthalaldehyde, yielding Schiff-base porous polymers on the interconnected macroporous frameworks of 3D graphene aerogels. The as-synthesized GS-HPPs possess hierarchically porous structures containing macro-/meso-/micropores, along with large specific surface areas up to 776 m2 g−1 and high nitrogen contents up to 36.8 wt%. Consequently, 3D nitrogen-enriched hierarchically porous carbon (N-HPC) materials with macro-/meso-/micropores were obtained by the pyrolysis of the GS-HPPs at a high temperature of 800 °C under a nitrogen atmosphere. With a hierarchically porous structure, good thermal stability and a high nitrogen-doping content up to 7.2 wt%, the N-HPC samples show a high specific capacitance of 335 F g−1 at 0.1 A g−1 in 6 M KOH, a good capacitance retention with increasing current density, and an outstanding cycling stability. The superior electrochemical performance means that the N-HPC materials have great potential as electrode materials for supercapacitors.