Yiyu Feng

Functionalized few-walled carbon nanotubes for mechanical reinforcement of polymeric composites.

Compared to single-walled carbon nanotubes (SWNTs) and more defective multiwalled carbon nanotubes (MWNTs), the thin few-walled carbon nanotubes (FWNTs) are believed to have extraordinary mechanical properties. However, the enhancement of mechanical properties in FWNTs-polymer composites has remained elusive. In this study, free-standing carbon nanotubes (CNTs)/polymer composite films were fabricated with three types (SWNTs, FWNTs, MWNTs) of functionalized CNTs. The mechanical properties of composite films have been investigated. It is observed that the Youngs modulus of composite films with only 0.2 wt % functionalized FWNTs shows a remarkable reinforcement value of dY/dV(f) = 1658 GPa, which is approximately 400 GPa higher than the highest value (dY/dV(f) = 1244 GPa) that was previously reported. In addition, the Youngs modulus increased steadily with the increased concentration of FWNTs. The results indicated that FWNTs are practically the optimum reinforcing filler for the next generation of carbon nanotube-based composite materials.

Two-Dimensional Fluorinated Graphene: Synthesis, Structures, Properties and Applications

Fluorinated graphene, an up‐rising member of the graphene family, combines a two‐dimensional layer‐structure, a wide bandgap, and high stability and attracts significant attention because of its unique nanostructure and carbon–fluorine bonds. Here, we give an extensive review of recent progress on synthetic methods and C–F bonding; additionally, we present the optical, electrical and electronic properties of fluorinated graphene and its electrochemical/biological applications. Fluorinated graphene exhibits various types of C–F bonds (covalent, semi‐ionic, and ionic bonds), tunable F/C ratios, and different configurations controlled by synthetic methods including direct fluorination and exfoliation methods. The relationship between the types/amounts of C–F bonds and specific properties, such as opened bandgap, high thermal and chemical stability, dispersibility, semiconducting/insulating nature, magnetic, self‐lubricating and mechanical properties and thermal conductivity, is discussed comprehensively. By optimizing the C–F bonding character and F/C ratios, fluorinated graphene can be utilized for energy conversion and storage devices, bioapplications, electrochemical sensors and amphiphobicity. Based on current progress, we propose potential problems of fluorinated graphene as well as the future challenge on the synthetic methods and C‐F bonding character. This review will provide guidance for controlling C–F bonds, developing fluorine‐related effects and promoting the application of fluorinated graphene.

Covalent functionalization of graphene by azobenzene with molecular hydrogen bonds for long-term solar thermal storage

Reduced graphene oxide-azobenzene (RGO-AZO) hybrids were prepared via covalent functionalization for long-term solar thermal storage. Thermal barrier (ΔEa) of cis to tran reversion and thermal storage (ΔH) were improved by molecular hydrogen bonds (H-bonds) through ortho- or para-substitution of AZO. Intramolecular H-bonds thermally stabilized cis-ortho-AZO on RGO with a long-term half-life of 5400 h (ΔEa = 1.2 eV), which was much longer than that of RGO-para-AZO (116 h). RGO-para-AZO with one intermolecular H-bond showed a high density of thermal storage up to 269.8 kJ kg−1 compared with RGO-ortho-AZO (149.6 kJ kg−1) with multiple intra- and intermolecular H-bonds of AZO according to relaxed stable structures. Thermal storage in experiment was the same order magnitude to theoretical data based on ΔH calculated by density functional theory and packing density. Photoactive RGO-AZO hybrid can be developed for high-performance solar thermal storage by optimizing molecular H-bonds.

Organic solar cells using few-walled carbon nanotubes electrode controlled by the balance between sheet resistance and the transparency

Organic photovoltaic devices (OPD) using high conductive transparent few-walled carbon nanotubes (FWNT) film prepared by spraying was fabricated as a selective hole collection. Photovoltaic response with different sheet resistance (Rs) and the transparency (T) of FWNT film was investigated. Maximum efficiency of OPD up to 0.61% with the structure of FWNT (T=70%, Rs=86 Ω/◻)/poly(3-hexylthiophene): [6-6]phenyl-C61-butyric acid methyl ester/Al demonstrates a promising alternative of ITO (0.68%) with almost identical operation. The performance improvement results from the optimal balance between sheet resistance and transparency with three-dimensional network interface between nanotubes and polymers.

Hierarchical graphene oxide/polyaniline nanocomposites prepared by interfacial electrochemical polymerization for flexible solid-state supercapacitors

Hierarchical graphene oxide/polyaniline (GO/PANI) nanocomposites deposited on flexible substrates were synthesized by one-step interfacial electrochemical polymerization. PANI nanorods were coated on the surface of GO to form a three-dimensional nanostructure, which was prepared by the polymerization at the water/chloroform interface. The current density for interfacial electrochemical polymerization was up to 0.5 mA cm−2, which is one-fold higher than that used in conventional electrochemical methods for nanostructures. Fourier transform infrared spectra, Raman and ultraviolet-visible absorption spectra indicated that PANI in a highly doped state showed a strong π–π electronic interaction with GO, resulting in an increased degree of conjugation. The flexible all-solid-state supercapacitors using GO/PANI nanocomposites exhibited a high specific capacitance of 1095 F g−1 at 1 A g−1, good rate capability and cycling performance, thus showing an energy density of 24.3 W h kg−1 and the maximum power density of 28.1 kW kg−1. This performance outperforms numerous previous solid-state supercapacitors that used carbon-based/PANI nanocomposites because of the synergistic effect of GO and PANI based on hierarchical nanostructures controlled by interfacial electrochemical polymerization.

Optical and electrical characterizations of nanocomposite film of titania adsorbed onto oxidized multiwalled carbon nanotubes

Composite film containing titania electrostatically linked to oxidized multiwalled carbon nanotubes (TiO2-s-MWNTs) was prepared from a suspension of TiO2 nanoparticles in soluble carbon nanotubes. The structure of the film was analysed principally by Fourier transform infrared spectroscopy, scanning electron micrography and x-ray diffraction. The optical and electrical characterizations of the film were investigated by UV–vis spectrum, photoluminescence and photoconductivity. The enhancement of photocurrent in the TiO2-s-MWNT film is discussed by taking the photoinduced charge transfer between the MWNT and TiO2 into consideration.

Nitrogen and fluorine co-doped graphene as a high-performance anode material for lithium-ion batteries

Nitrogen and fluorine co-doped graphene (NFG) with the N and F content as high as 3.24 and 10.9 at% respectively was prepared through the hydrothermal reaction of trimethylamine tri(hydrofluoride) [(C2H5)3N·3HF] and aqueous-dispersed graphene oxide (GO) as the anode material for lithium ion batteries (LIBs). The N and F co-doping in graphene increased the disorder and defects of the framework, enlarged the space of the interlayer, wrinkled the nanosheets with many open-edge sites, and thus facilitated Li ion diffusion through the electrode compared with sole-N or F doped graphene. X-ray photoelectron spectroscopy (XPS) analysis of NFG demonstrated the presence of active pyridine and pyrrolic type N, and highly electrically conductive graphitic N and the semi-ionic C–F bond in the structure. The N and F doping content and the component types of N and F functional groups could be controlled by the hydrothermal temperature. The NFG prepared at 150 °C exhibited the best electrochemical performances when tested as the anode for LIBs, including the high coulombic efficiency in the first cycle (56.7%), superior reversible specific discharge capacity (1075 mA h g−1 at 100 mA g−1), excellent rate capabilities (305 mA h g−1 at 5 A g−1), and outstanding cycling stability (capacity retention of ∼95% at 5 A g−1 after 2000 cycles), which demonstrated that NFG was a promising candidate for anode materials of high-rate LIBs.

Infrared-Actuated Recovery of Polyurethane Filled by Reduced Graphene Oxide/Carbon Nanotube Hybrids with High Energy Density

Optically actuated shape recovery materials receive much interest because of their great ability to control the creation of mechanical motion remotely and precisely. An infrared (IR) triggered actuator based on shape recovery was fabricated using polyurethane (TPU) incorporated by sulfonated reduced graphene oxide (SRGO)/sulfonated carbon nanotube (SCNT) hybrid nanofillers. Interconnected SRGO/SCNT hybrid nanofillers at a low weight loading of 1% dispersed in TPU showed good IR absorption and improved the crystallization of soft segments for a large shape deformation. The output force, energy density and recovery time of IR-triggered actuators were dependent on weight ratios of SRGO to SCNT (SRGO:SCNT). TPU nanocomposites filled by a hybrid nanofiller with SRGO:SCNT of 3:1 showed the maximum IR-actuated stress recovery of lifting a 107.6 g weight up 4.7 cm in 18 s. The stress recovery delivered a high energy density of 0.63 J/g and shape recovery force up to 1.2 MPa due to high thermal conductivity (1.473 W/mK) and Youngs modulus of 23.4 MPa. Results indicate that a trade-off between the stiffness and efficient heat transfer controlled by synergistic effect between SRGO and SCNT is critical for high mechanical power output of IR-triggered actuators. IR-actuated shape recovery of SRGO/SCNT/TPU nanocomposites combining high energy density and output forces can be further developed for advanced optomechanical systems.

A high energy density azobenzene/graphene hybrid: a nano-templated platform for solar thermal storage

Effective conversion of light into heat is an emerging field showing great potential for large-scale applications, markedly driven by novel molecules and structures. Unfortunately, until now, it is still hindered by a low storage capacity and short-time storage. A nano-template for covalently attaching new azobenzene chromophores on graphene as solar thermal fuels is presented here, in which the intermolecular hydrogen bond and proximity-induced interaction, resulting from a high functionalization density and inter-planar bundling interaction, remarkably improve both the storage capacity and lifetime. This nanoscopic template exhibits a high energy density up to 112 W h kg−1 and long-term storage with a half-life of more than one month (33 days), which are also confirmed by the calculations using density functional theory, simultaneously maintaining an excellent cycling stability tuned by visible light for 50 cycles. Our work develops a promising class of solar thermal fuels with high energy density, which outperform previous nano-materials and are comparable to commercial soft-packing Li-ion batteries.

Enhanced Reversible Photoswitching of Azobenzene-Functionalized Graphene Oxide Hybrids

An enhanced photoinduced reversible switching of graphene oxide-azobenzene (GO-AZO) hybrid was investigated as a highly sensitive photoswitch. The internal short-range ordered crystalline structure of GO-AZO hybrid was advantageous to charge transfer. The AZO moieties on GO underwent a rapid trans-cis photoisomerization upon ultraviolet irradiation due to the electron interaction between AZO and GO. The GO-AZO hybrid film showed an enhanced reversible photoswitching performance with high on/off ratio of 8 and fast response time less than 500 ms. The high sensitivity of GO-AZO switch arises from the intramolecular donor-acceptor architecture with efficient charge transfer.