Synthesis of RGO/Cu8S5/PPy Composite Nanosheets with Enhanced Peroxidase‐Like Activity for Sensitive Colorimetric Detection of H2O2 and Phenol

In this paper, a novel strategy for the fabrication of reduced graphene oxide (rGO)/Cu₈S₅/polypyrrole (PPy) composite nanosheets with Cu₈S₅ nanoparticles and PPy layer anchored on the surface of rGO as peroxidase‐like nanocatalyst is reported. During the synthesis, graphene oxide (GO)/CuO composite nanosheets are prepared first and used as templates, then the sulfuration of CuO and polymerization of pyrrole are accompanied with the reduction of GO, resulting in ternary rGO/Cu₈S₅/PPy composite nanosheets. The synthesized Cu₈S₅ nanoparticles with a diameter in the range from tens to hundreds of nanometers are dispersed within PPy decorated rGO nanosheets. The resultant ternary rGO/Cu₈S₅/PPy composite nanosheets exhibit a higher peroxidase‐like catalytic activity toward the oxidation of 3,3′,5,5′‐tetramethylbenzidine in the presence of H₂O₂ than GO/CuO and rGO/CuS composite nanosheets, revealing a synergistic effect on their activity. The as‐prepared rGO/Cu₈S₅/PPy platform provides a simple colorimetric approach for the detection of H₂O₂ and phenol with a high sensitivity. This work offers a new way for the fabrication of rGO‐based nanocomposite with superior enzyme‐like activity, which displays great potential applications in biocatalysis and environmental monitoring.     

Graphene Quantum Dots from Polycyclic Aromatic Hydrocarbon for Bioimaging and Sensing of Fe3+ and Hydrogen Peroxide

An easy approach for large‐scale and low‐cost synthesis of photoluminescent (PL) graphene quantum dots (GQDs) based on the carbonization of commercially available polycyclic aromatic hydrocarbon (PAH) precursors with strong acid and followed by hydrothermal reduction with hydrazine hydrate is reported. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) characterizations indicate that the size and height of GQDs are in the range of 5–10 nm and 0.5–2.5 nm, respectively. PAH, which has more benzene rings, generally forms GQDs with relatively larger size. The GQDs show high water solubility, tunable photoluminescence, low cytotoxicity, and good optical stability, which makes them promising fluorescent probes for cellular imaging. In addition, the fluorescence of GQDs shows a sensitive and selective quenching effect to Fe³⁺ with a detection limit of 5 × 10^?9m . By combination with the Fe²⁺/Fe³⁺ redox couple, the PL GQDs are able to detect oxidant, using H₂O₂ as an example. This study opens up new opportunities to make full use of GQDs because of their facile availability, cost‐effective productivity, and robust functionality.     

Facile One‐Pot Conversion of Petroleum Asphaltene to High Quality Green Fluorescent Graphene Quantum Dots and Their Application in Cell Imaging

Benefiting from the natural nano‐size graphene‐structure in natural asphaltene material, a facile one‐pot route, mild chemical oxidation of low‐value petroleum asphaltene followed by routine ammonium neutralization, is presented to produce high quality graphene quantum dots (GQDs). The asphaltene‐derived GQDs possess a variety of oxygen‐containing and nitrogen‐containing functional groups such as carboxyl, hydroxyl, amine, and nitro groups. They present such excellent fluorescence properties as stable ability to retain strong green fluorescence within a relative broad excitation range in a bio‐suitable pH range of 4–7, high photoluminescence quantum yield of 18% and good fluorescent stability against photobleaching. And they are much smaller and thinner than most reported GQDs, displaying good biocompatibility with low cytotoxicity, effective cellular uptake, and excellent fluorescent probe performance for cancer cell imaging.     

Hydrothermal Preparation of Photoluminescent Graphene Quantum Dots Characterized Excitation‐Independent Emission and its Application as a Bioimaging Reagent

Zero‐dimensional photoluminescent (PL) graphene quantum dots (GQDs) that can be used as the cell‐imaging reagent are prepared by a hydrothermal route using the graphene oxide (GO) as the carbon source. Under the optimized hydrothermal conditions, an initial hydrogen peroxide concentration of 0.5 mg mL^?1 at 180 °C for 120 min, the GO sheets can be cut into nanocrystals with lateral dimensions in the range of 1.5–5.5 nm and an average thickness of around 1.1 nm. The as‐prepared GQDs exhibit an abundance of hydrophilic hydroxy and carboxyl groups and emit bright blue luminescence with up‐conversion properties in a water solution at neutral pH. Most interestingly, they indicate excitation‐independent emission characteristics, and the surface state is demonstrated to have a key role in the PL properties. The fluorescence quantum yield of the GQDs is tested to be around 6.99% using quinine sulfate as a standard. In addition, the as‐prepared GQDs can enter into HeLa cells easily as a fluorescent imaging reagent without any further functionalization, indicating they are aqueous stability, biocompatibility, and promising for potential applications in biolabeling and solution state optoelectronics.     

Flexible Graphene‐, Graphene‐Oxide‐, and Carbon‐Nanotube‐Based Supercapacitors and Batteries

Flexible energy‐storage devices increasingly attract attention owing to their advantages of providing lightweight, portable, wearable, or implantable capabilities. Many efforts are made to explore the structures and fabrication processes of flexible energy‐storage devices for commercialization. Here, the most recent advances in flexible energy‐storage devices based on graphene, graphene oxide (GO), and carbon nanotubes (CNTs), are described, including flexible supercapacitors and batteries. First, properties, synthesis methods, and possible applications of those carbon‐based materials are described. Then, the development of carbon‐nanotube‐based flexible supercapacitors, graphene/graphene‐oxide‐based flexible supercapacitors, and graphene‐ and carbon‐nanotube‐based flexible battery electrodes are discussed. Finally, the future trends and perspectives in the development of flexible energy‐storage devices are highlighted.     

Rapid,Microwave‐Assisted,and One‐Pot Synthesis of Magnetic Palladium–CoFe2O4–Graphene Composite Nanosheets and Their Applications as Recyclable Catalysts

Here, a microwave‐assisted approach has been demonstrated to rapidly prepare magnetic Pd–CoFe₂O₄–graphene (GE) composite nanosheets in ethylene glycol (EG) solvent. The generation of both Pd and CoFe₂O₄ nanoparticles is accompanied with the reduction process of graphene oxide (GO) by EG. The surface morphologies and chemical composition of the composite nanosheets are characterized by transmission electron microscopy (TEM), energy‐dispersive X‐ray spectrometer (EDS), powder X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) measurements. The as‐prepared Pd–CoFe₂O₄–GE composite nanosheets exhibit a remarkable catalytic activity towards the reduction of 4‐nitrophenol by sodium borohydride (NaBH₄) at room temperature. The apparent kinetic rate constant (K _app) of this catalytic reaction could reach about 11.0 × 10^?3 s^?1. Moreover, the CoFe₂O₄ component exhibits a magnetic property, which could make the Pd–CoFe₂O₄–GE composite nanocatalysts separated from the suspension system. The catalytic conversion of the 4‐nitrophenol to 4‐aminophenol could reach 87.2% after four cycles. This work presents a simple, rapid, and versatile method to fabricate both metal and spinel‐type complex oxides on GE nanosheets, providing a new opportunity for their applications in the recyclable catalytic reaction.     

Full Protection for Graphene‐Incorporated Micro‐/Nanocomposites Containing Ultra‐small Active Nanoparticles: the Best Li‐Storage Properties

In recent years, graphene‐incorporated micro‐/nanocomposites represent one of the hottest developing directions for the composite materials. However, a large number of active nanoparticles (NPs) are still in the unprotected state in most constructed graphene‐containing designs, which will seriously impair the effects of the graphene additives. Here, a fully protected Fe₃O₄‐based micro‐/nanocomposite (G/Fe₃O₄@C) is rationally developed by carbon‐boxing the common graphene/Fe₃O₄ microparticulates (G/Fe₃O₄). The processes and results of full protection are tracked in detail and characterized by X‐ray diffraction, X‐ray photoelectron spectroscopy, and nitrogen absorption–desorption isotherms, as well as scanning and transition electron microscopy. When used as the anode for lithium‐ion batteries, the fully protected G/Fe₃O₄@C exhibits the best lithium‐storage properties in terms of the highest rate capabilities and the longest cycle life compared to the common G/Fe₃O₄ composites and commercial Fe₃O₄ products. These much improved properties are mainly attributed to its novel structural features including complete protection of active Fe₃O₄ nanoparticles by the surface carbon box, a robust conductive network composed of nitrogen‐doped graphene nanosheets, ultra‐small Fe₃O₄ NPs of 4–5 nm, abundant mesopores to accommodate the volume variation during cycling, and micrometer‐sized secondary particles.     

Facile Fabrication of Bicomponent CoO/CoFe2O4‐N‐Doped Graphene Hybrids with Ultrahigh Lithium Storage Capacity

A facile strategy is developed to fabricate bicomponent CoO/CoFe₂O₄‐N‐doped graphene hybrids (CoO/CoFe₂O₄‐NG). These hybrids are demonstrated to be potential high‐performance anodes for lithium‐ion batteries (LIBs). The CoO/CoFe₂O₄ nanoplatelets are finely dispersed on the surface of N‐doped graphene nanosheets (CoO/CoFe₂O₄‐NG). The CoO/CoFe₂O₄‐NG electrode exhibits ultrahigh specific capacity with 1172 mA h g^?1 at 500 mA g^?1 and 970 mA h g^?1 at 1000 mA g^?1 as well as excellent cycle stability due to the synergetic effects of N‐doped graphene and CoO/CoFe₂O₄ nanoplatelets. The well‐dispersed bicomponent CoO/CoFe₂O₄ is responsible for the high specific capacity. The N‐doped graphene with high specific surface area has dual roles: to provide active sites for dispersing the CoO/CoFe₂O₄ species and to function as an electrical conducting matrix for fast charge transfer. This method provides a simple and efficient way to configure the hybridized electrode materials with high lithium storage capacity.     

Noble Metal-Free Surface-Enhanced Raman Scattering Enhancement from Bandgap-Controlled Graphene Quantum Dots

Graphene quantum dot (GQD) represents an emerging noble metal-free surface-enhanced Raman scattering (SERS)-active nanomaterials for applications such as optoelectronics, chemical sensing, and biomedical imaging and therapy. However, it lacks a scalable method to synthesize GQD with selective structures and the fundamental understanding of their SERS enhancement through charge transfer between GQD and probe molecules. Here a bottom–up liquid-phase synthesis of colloidal GQDs with selective bandgaps using atmospheric-pressure microplasmas is reported. Electron microscopic and optical spectroscopic characterizations suggest that highly crystalline GQDs with nanographene structures can be synthesized with ambient conditions using microplasmas. Moreover, the bandgaps of GQDs are tuned from 2.8 to 3.18 eV by controlling the size of organosulfate micelles. Raman spectroscopic study demonstrates that the as-synthesized GQDs exhibit a unique quantum dot bandgap-dependent SERS enhancement property with an improved charge transfer between the GQD and probe molecules. This study provides an insight into the fundamental of semiconductor-enhanced Raman scattering of GQDs and scalable production of structure-controlled GQDs using plasma-activated chemistry.     

Three‐Dimensional Multilayer Assemblies of MoS2/Reduced Graphene Oxide for High‐Performance Lithium Ion Batteries

Three‐dimensional (3D) multilayer molybdenum disulfide (MoS₂)/reduced graphene oxide (RGO) nanocomposites are prepared by a solution‐processed self‐assembly based on the interaction using different sizes of MoS₂ and GO nanosheets followed by in situ chemical reduction. 3D multilayer assemblies with MoS₂ wrapped by large RGO nanosheets and good interface are observed by transmission electron microscopy. The interaction of Na⁺ ions with oxygen‐containing groups of GO is also investigated. The measurement of lithium ion batteries (LIBs) shows that MoS₂/RGO anode nanocomposite with a weight ratio of MoS₂ to GO of 3:1 exhibits an excellent rate performance of 750 mAh g^?1 at 3 A g^?1 outperforming many previous studies and a high reversible capacity up to ≈1180 mAh g^?1 after 80 cycles at 100 mA g^?1. Good rate performance and high capacity of MoS₂/RGO with 3D unique layered‐structures are attributed to the combined effects of continuous conductive networks of RGO, good interface facilitating charge transfer, and strong RGO sheets preventing the volume expansion. Results indicate that 3D multilayer MoS₂/RGO prepared by a facile solution‐processed assembly can be developed to be an excellent nanoarchitecture for high‐performance LIBs.     

DFT study of graphene oxide reduction by a dopamine species

Recently a large interest has arisen for using less active reducers of graphene oxide, GO, that are friendly with the environment. In the present work, a DFT theoretical study on the reduction process of GO model surfaces is performed taking into account zwitterionic dopamine, ZDA, as reducing agent. Several periodic models representing epoxy and hydroxyl patches on GO basal plane are proposed. As the number of oxide groups in a patch of epoxies or hydroxyls on the surface of graphene increases from 1 to 5, these systems become more stable. Whereas the adsorption of ZDA on patches of GO with 5 epoxy groups is non-dissociative, that of ZDA on patches of GO with 5 hydroxyl groups is fundamentally dissociative, reducing the surface of graphene oxide. The H₂O molecule produced in the GO reduction becomes trapped to ZDA through a hydrogen bond. The ZDA binding to GO was analysed by considering electrostatic effects and attractive non-covalent contributions due to vdW interactions.     

Substrate‐Induced Synthesis of Nitrogen‐Doped Holey Graphene Nanocapsules for Advanced Metal‐Free Bifunctional Electrocatalysts

The development of efficient metal‐free electrocatalysts for oxygen electrocatalysis is of great significance for various energy conversion devices. Herein, novel nitrogen‐doped holey graphene nanocapsules (NHGNs) are reported prepared by self‐assembly of graphene oxide nanosheets on the surface of amino‐functionalized silica template and NH₃ activation with simultaneously enhanced nitrogen doping and etching of nanopores in graphene, followed by template etching. The silica template is demonstrated to show a substrate‐enhanced effect on nitrogen doping and etching of nanopores in graphene based on density functional theory calculations. Benefiting from the large surface area, unique pore distribution, and high surface functionality of nitrogen doping, the resulting NHGNs exhibit superior bifunctional electrocatalytic activity and durability for both oxygen reduction reaction and oxygen evolution reaction, which is similar to that of the commercial Pt/C and RuO₂ electrocatalysts, respectively. This work presents an advance in developing new nitrogen‐doped graphene species for highly efficient metal‐free electrocatalysis.     

Stress relaxation of GaN microstructures on a graphene‐buffered Al2O3 substrate

GaN microstructures were grown on c‐Al₂O₃ with a multi‐stacked graphene buffered layer using metal metal‐organic chemical‐vapor deposition. Under the same growth conditions, the nucleation of GaN was suppressed by the low surface energy of graphene, resulting in a much lower density of microstructures relative to those grown on c‐Al₂O₃. Residual stress in the GaN microstructures was estimated from the peak shift of the E₂ phonon using micro‐Raman spectroscopy. The results showed that the compressive stress of approximately 0.36 GPa in GaN on c‐Al₂O₃ caused by lattice mismatch and the difference in the thermal expansion coefficient was relaxed by introducing the graphene layer. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Liquid‐Crystal‐Mediated Self‐Assembly of Porous α‐Fe2O3 Nanorods on PEDOT:PSS‐Functionalized Graphene as a Flexible Ternary Architecture for Capacitive Energy Storage

A novel aqueous‐based self‐assembly approach to a composite of iron oxide nanorods on conductive‐polymer (CP)‐functionalized, ultralarge graphene oxide (GO) liquid crystals (LCs) is demonstrated here for the fabrication of a flexible hybrid material for charge capacitive application. Uniform decoration of α‐Fe₂O₃ nanorods on a poly(3,4‐ethylene‐dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)‐functionalized, ultralarge GO scaffold results in a 3D interconnected layer‐by‐layer (LBL) architecture. This advanced interpenetrating network of ternary components is lightweight, foldable, and possesses highly conductive pathways for facile ion transportation and charge storage, making it promising for high‐performance energy‐storage applications. Having such structural merits and good synergistic effects, the flexible architecture exhibits a high specific discharge capacitance of 875 F g^?1 and excellent volumetric specific capacitance of 868 F cm^?3 at 5 mV s^?1, as well as a promising energy density of 118 W h kg^?1 (at 0.5 A g^?1) and promising cyclability, with capacity retention of 100% after 5000 charge–discharge (CD) cycles. This synthesis method provides a simple, yet efficient approach for the solution‐processed LBL insertion of the hematite nanorods (HNR) into CP‐functionalized novel composite structure. It provides great promise for the fabrication of a variety of metal‐oxide (MO)‐nanomaterial‐based binder and current collector‐free flexible composite electrodes for high‐performance energy‐storage applications.     

Raman imaging on high‐quality graphene grown by hot‐filament chemical vapor deposition

We report the synthesis of high‐quality graphene on Cu foils using hot‐filament chemical vapor deposition technique and demonstrate that by suitably varying the CH₄ and H₂ flow rates, one can also obtain hydrogenated graphene. Micro‐Raman spectroscopy studies confirm the growth of monolayer graphene as inferred from the intensity ratio of 2D to G peak which is nearly four in unhydrogenated samples. Detailed Raman area mapping confirms the uniform coverage of monolayer graphene. The grown layer is also transferred onto a Si substrate over ~10 × 10 mm sq. area. The present results provide a leap in synthesis technology of high‐quality graphene and pave way for scaling up the process. Copyright © 2012 John Wiley & Sons, Ltd.     

Fractals of graphene quantum dots in photoluminescence of shungite

Viewing shungite as loosely packed fractal nets of graphene-based (reduced graphene oxide, rGO) quantum dots (GQDs), we consider photoluminescence of the latter as a convincing proof of the structural concept as well as of the GQD attribution to individual rGO fragments. We study emission from shungite GQDs for colloidal dispersions in water, carbon tetrachloride, and toluene at both room and low temperatures. As expected, the photoluminescence of the GQD aqueous dispersions is quite similar to that of synthetic GQDs of the rGO origin. The morphological study of shungite dispersions shows a steady trend of GQDs to form fractals and to drastically change the colloid fractal structure caused by the solvent exchange. Spectral study reveals a dual character of the emitting centers: individual GQDs are responsible for the spectra position while the fractal structure of GQD colloids ensures high broadening of the spectra due to structural inhomogeneity, thus causing a peculiar dependence of the photoluminescence spectra on the excitation wavelength. For the first time, photoluminescence spectra of individual GQDs were observed in frozen toluene dispersions, which paves the way for a theoretical treatment of the GQD photonics.     

H2S在氧化石墨烯表面的吸附和分解的理论研究

用周期性密度泛函方法对H₂S在氧化石墨烯(GO)上的吸附和分解进行了理论计算, 讨论了H₂S和GO上的羟基和环氧基团的反应过程．结果表明,反应过程是通过H₂S或-SH上的H转移使得GO的环氧基开环和羟基氢化,当GO相反面存在羟基时有助于环氧基团的开环和羟基氢化反应．H₂S在GO上吸附和分解到S原子的反应机理中引入了相应的中间态,计算两次脱氢过程能垒分别为3.2和10.4 kcal/mol,第二个H原子的转移是GO还原过程的速率决定步骤．结果还表明GO上的羟基和环氧基团有助于加强S原子和石墨烯的结合．     

Electron‐transfer transparency of graphene: Fast reduction of metal ions on graphene‐covered donor surfaces

The mechanism of charge transfer through nanomaterials such as graphene remains unclear, and the amount of charge that can be transferred from/to graphene without damaging its structural integrity is unknown. In this communication, we show that metallic nanoparticles can be decorated onto graphene surfaces as a result of charge transfer from the supporting substrate to an adjoining solution containing metal ions. Au or Pt nanoparticles were formed with relatively high yield on graphene‐coated substrates that can reduce these metal ions, such as Ge, Si, GaAs, Al, and Cu. However, metal ions were not reduced on graphene surfaces coated onto non‐reducing substrates such as SiO₂ or ZnO. These results confirm that graphene can be doped by exploiting charge transfer from the underlying substrate; thus graphene is not only transparent with respect to visible light, but also with respect to the charge transfer. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Metal Oxide Nanomaterials with Nitrogen‐Doped Graphene‐Silk Nanofiber Complexes as Templates

Fabricating electrode materials with superior electrochemical performance remains a challenge. Here, a simple but effective strategy for the formation of metal oxide nanomaterials with superior performance has been developed. Silk protein nanofibers adhered on reduced graphene oxide (rGO) sheets are used as templates to regulate the formation of nanostructured iron oxide composites, achieving porous nanorod structures that could not be attained in control experiments. These porous nanorods demonstrate superior electrochemical performance as electrodes with retention of a capacity of 1495 mAh g^?1 after 180 cycles at 0.2 C and a rate capability of 900 mAh g^?1 at 2 C discharge rate. These new rGO/silk composite templates provide a more controllable environment for Fe₂O₃ fabrication, resulting in improved nanostructures and superior electrical performance. The strategy developed here should also be more broadly applicable in the design of metal oxide nanomaterials with specialized structures and useful performance.     

Sonocatalytic degradation of methylene blue by a novel graphene quantum dots anchored CdSe nanocatalyst

Cadmium selenide/graphene quantum dots (CdSe/GQDs) nanocatalyst with small band gap energy and a large specific surface area was produced via a facile three-step sonochemical-hydrothermal process. The features of the as-prepared CdSe, GQDs and CdSe/GQDs samples were characterized by photoluminescence spectroscopy (PL), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), X-ray diffraction (XRD), Fourier transformed infrared (FT-IR), diffuse-reflectance spectrophotometer (DRS), and Brunauer–Emmett–Teller (BET) analysis. The sonocatalytic activity of the synthesized CdSe/GQDs was effectively accelerated compared with that of pure CdSe nanoparticles in degradation of methylene blue (MB). The influence of the CdSe/GQDs dosage (0.25–1.25 g/L), initial MB concentration (20–30 mg/L), initial solution pH (3–12), and ultrasonic output power (200–600 W/L) were examined on the sonocatalytic treatment of MB aqueous solutions. The degradation efficiency (DE%) of 99% attained at 1 g/L of CdSe/GQDs, 20 mg/L of MB, pH of 9, and an output power of 200 W/L at 90 min of ultrasonic irradiation. Furthermore, DE% increased with addition of K₂S₂O₈ and H₂O₂ as the enhancers via producing more free radicals. However, addition of sulfate, carbonate, and chloride as radical sweeper decreased DE%. Furthermore, well-reusability of the CdSe/GQDs sonocatalyst was demonstrated for 5 successive runs and some of the sonocatalytic generated intermediates were indicated by GC–MS analysis.