Xiaogong Wang

Mechanically Tough Large‐Area Hierarchical Porous Graphene Films for High‐Performance Flexible Supercapacitor Applications

Mechanically tough large-area hierarchical porous graphene films are fabricated by blade-casting of graphene oxide hydrogel and postcasting reduction. The as-prepared graphene films, which consist of well-exfoliated graphene nanosheets, possess interpenetrating 3D hierarchical porous structures, high strength and modulus, large specific area, and high electrical conductivity. Flexible film supercapacitors fabricated with the graphene electrodes show superior areal capacitance, good rate performance, and excellent mechanical stability.

Graphene Functionalized with Azo Polymer Brushes: Surface‐Initiated Polymerization and Photoresponsive Properties

Since it was fi rst successfully prepared as a true two-dimensional nanomaterial, graphene has attracted tremendous attention for its unique structure and outstanding properties. [ 1 , 2 ] Graphene is one of the best candidates for numerous applications in functional devices such as gas sensors, [ 3 ] photovoltaics, [ 4 ] and fi eld-effect transistors. [ 5 ] The versatile approach of chemical modifi cation on the graphene basal planes or edges affords nanomaterials with additional novel functions. [ 6 ] If chemically modifi ed graphene is used as a fi ller, the electrical, thermal, and mechanical properties of polymer-based nanocomposites can be dramatically enhanced at a very low fi ller content. [ 7 , 8 ] Recently, the optical properties of single-layered graphene have aroused considerable research interest. [ 9–13 ] Although the refl ection and transmission of graphene have been intensively studied, little is known about the effect of graphene on the optical properties of the nanocomposites. Graphene has a two-dimensional (2D) ordered structure with unique electronic band structure, in which the conduction band touches the valence band at two points in the Brillouin zone. [ 9 , 10 ] Electrons in 2D structures behave like massless Dirac fermions and could show unique interactions with electromagnetic fi elds. Graphene possesses visual transparency, defi ned by the fi ne structure constant, with high transparency (97.7%) and refractive index (2.6–3) in the visible light range. [ 9–13 ] In view of these unique characteristics, the interaction between light and the nanocomposite fi lms could be a subject with great scientifi c signifi cance, and the results could be used to improve upon conventional polymers and other organic materials for optical applications. In this Communication, we report the preparation of singlelayered graphene grafted with azo polymer brushes and its interesting use to signifi cantly enhance the diffraction effi ciency (DE) of photoinduced surface-relief gratings (SRGs) on azo molecular glass fi lms. The chemical structures of the functionalized graphene (G-PCN) and azo molecular glass (TrAz-CN) used in this work are shown in Figure 1 . Azobenzene and its derivatives are well-known for their reversible trans–cis isomerization induced by light irradiation. [ 14 ] Polymers containing azobenzene or its derivatives (azo polymers for short)

Micron-sized liquid crystalline elastomer actuators

Over the last few years, following a trend to miniaturization found in many fields of materials science, micron-sized liquid crystalline elastomer (LCE) actuators have been described, with the aim to apply them in such domains like responsive surfaces, microfluidic devices or microsensors. The preparations of the micron-sized LCE actuators involve plenty of cutting-edge, advanced technologies such as soft lithography, microfluidic, microprinting or photomasking. In this highlight, we describe the most striking results obtained in the domain of micron-sized LCE-based responsive systems.

Epoxy-based azo polymers: synthesis, characterization and photoinduced surface-relief-gratings

Abstract Epoxy-based azo polymers with three kinds of backbones, different types of azo chromophores, and selected degrees of functionalization (DFs), were synthesized through a post-polymerization azo-coupling scheme. The T g s of the azo polymers were found to be considerably influenced by the substituents on the 4-positions of the azobenzene units and increased with the increase of the DFs of the polymers. Surface-relief-gratings (SRGs) were induced on the polymer films by interferenced Ar + laser beams. With the same irradiating light intensity and approximately equal film thickness, the rates of SRG formation were found to be significantly dependent on the electron-withdrawing groups on the 4-positions of the azobenzene units. For the polymers with the same type of azo chromophores and backbone structure, the inscription rates increased with the increase of the DFs. Meanwhile, the backbone structure also exhibited some effect on the inscription rates through the ways such as changing chromophore loading density or main-chain rigidity.

Photo-switched wettability on an electrostatic self-assembly azobenzene monolayer

A simple electrostatic self-assembly technique was used to fabricate a photo-switched azobenzene monolayer, on which superhydrophobicity and a large reversible CA change could be realized.

Fast Photoinduced Large Deformation of Colloidal Spheres from a Novel 4-arm Azobenzene Compound

A novel 4-arm shaped amphiphilic azobenzene compound was synthesized. The tetraphenylethylene (TPE) core precursor was prepared and further modified by azo coupling reaction at the four peripheral groups. Colloidal spheres could be directly prepared by self-assembly of the prepared amphiphilic azobenzene compound in selective solvents (THF/H2O), which were characterized by using transmission electron microscopy. The colloid diameters could be controlled by adjusting the initial compound concentration and water-adding rate in the preparation processes. By irradiation with visible linearly polarized LED light (450 nm), fast photoinduced deformation of the colloidal spheres along the polarization direction was observed. A very large deformation degree (l/d > 4) could be easily obtained.

Light-responsive wires from side-on liquid crystalline azo polymers

The pioneering work of Professor de Gennes in the field of liquid crystalline polymers and elastomers is outlined with an emphasis on artificial muscles. To illustrate the presentation, results on light-responsive nematic liquid crystalline elastomer fibres are reported. Two kinds of light-responsive wires were fabricated from an azobenzene-containing liquid crystalline copolymer with, or without cross-linking. Cross-linked wires were obtained by the reaction between the hydroxyl groups of the liquid crystalline copolymer and the diisocyanate groups of the cross-linker. The wires showed light-responsive bending towards the direction of the incident UV light at room temperature. Uncross-linked wires, drawn from the pure azobenzene copolymer, presented the same UV light-induced bending properties.

Hollow microspheres of amphiphilic azo homopolymers: self-assembly and photoinduced deformation behavior

A unique way to fabricate hollow microspheres by using an amphiphilic azo homopolymer (BP-AZ-CA) and photoinduced deformation behavior of the hollow structures in the solid state are reported.

Photofabrication of two-dimensional quasi-crystal patterns on UV-curable molecular azo glass films.

In this work, two-dimensional surface quasi-crystal patterns were developed by using a novel azobenzene-containing amorphous material (IAC-4), which was newly synthesized for the application. IAC-4 contains a core of isosorbide moiety and two push-pull type azo chromophores as the inner part. The periphery of IAC-4 is functionalized with four cinnamate groups, which can undergo [2+2] photocycloaddition reaction upon UV light irradiation. The molecular design can allow IAC-4 to readily form surface relief structures upon Ar+ laser irradiation, and the formed structures can be further stabilized through a photo-cross-linking reaction induced by UV light irradiation. On the basis of the material, two-dimensional (2D) quasi-crystal structures with different rotation symmetries were successfully fabricated on the IAC-4 films by using the dual-beam multiple exposure technique. In contrast to the approach using photoresist, the quasi-crystal structures were fabricated through the photoinduced mass migration, and no subsequent wet-etch or dry-etch step was required in the process. The quasi-crystal structures with rotation symmetry as high as 60-fold could be feasibly fabricated through this approach. The surface patterns and fabrication method can be potentially applied in areas such as optics, communications, and security inspection.

Self-assembled multilayer films of sulfonated graphene and polystyrene-based diazonium salt as photo-cross-linkable supercapacitor electrodes.

Photo-cross-linkable multilayer films composed of sulfonated reduced graphene oxide (SRGO) and polystyrene-based diazonium salt (PSDAS) were fabricated by electrostatic layer-by-layer (LbL) self-assembly. Polystyrene with narrow molecular weight distribution was synthesized by atom transfer radical polymerization (ATRP), and cationic PSDAS was prepared through nitration, reduction, and diazotization reactions. Negatively charged SRGO nanosheets were prepared through prereduced by NaBH4, modified by diazonium salt of sulfanilic acid, and then further reduced by hydrazine. The multilayer films were obtained by alternately dipping substrates in the PSDAS solution and SRGO dispersion in acidic conditions. The cross-linking between the components occurred during the multilayer formation process and was further achieved by the UV light irradiation after the film preparation. The assembling process and surface morphology of LbL multilayer films were monitored by UV-vis spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM). The cross-linking between SRGO and PSDAS was verified by attenuated total reflectance FTIR (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle measurement. The graphene nanosheets were found to be homogeneously distributed in the cross-linked network of the films. The large accessible surface area of graphene nanosheets and the cross-linking structure afforded the LbL films with high specific capacitance and excellent cyclic stability when used as supercapacitor electrodes. At a sweeping rate of 10 mV/s, the film with nine bilayers exhibited a specific capacitance of 150.4 F/g with ideal rectangular cyclic voltammogram. Large capacitance retention of 97% was observed after 10 000 charge-discharge cycles under the scanning rate of 1000 mV/s. This new approach for preparing graphene-containing multilayer films can be used to develop supercapacitor electrodes and other functional devices.