Greener Electrochemical Synthesis of High Quality Graphene Nanosheets Directly from Pencil and its SPR Sensing Application

A green, simple, and cost effective electrochemical method to synthesize pure graphene oxide (GO) and graphene nanosheets (GNs) using pencil in ionic liquid medium is reported. The morphology and microstructure of prepared GNs and GO are examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X‐ray diffraction (XRD), and Raman spectroscopy; the experiments confirm the formation of high quality graphene. The synthesized GO is used for the real‐time and label‐free surface plasmon resonance (SPR) sensing of the biological warfare agent Salmonella typhi.     

Self‐Propagating Domino‐like Reactions in Oxidized Graphite

Graphite oxide (GO) has received extensive interest as a precursor for the bulk production of graphene‐based materials. Here, the highly energetic nature of GO, noted from the self‐propagating thermal deoxygenating reaction observed in solid state, is explored. Although the resulting graphene product is quite stable against combustion even in a natural gas flame, its thermal stability is significantly reduced when contaminated with potassium salt by‐products left from GO synthesis. In particular, the contaminated GO becomes highly flammable. A gentle touch with a hot soldering iron can trigger violent, catastrophic, total combustion of such GO films, which poses a serious fire hazard. This highlights the need for efficient sample purification methods. Typically, purification of GO is hindered by its tendency to gelate as the pH value increases during rinsing. A two‐step, acid–acetone washing procedure is found to be effective for suppressing gelation and thus facilitating purification. Salt‐induced flammability is alarming for the fire safety of large‐scale manufacturing, processing, and storage of GO materials. However, the energy released from the deoxygenation of GO can also be harnessed to drive new reactions for creating graphene‐based hybrid materials. Through such domino‐like reactions, graphene sheets decorated with metal and metal oxide particles are synthesized using GO as the in situ power source. Enhanced electrochemical capacitance is observed for graphene sheets loaded with RuO₂ nanoparticles.     

Electrical Conductivity and Ferromagnetism in a Reduced Graphene–Metal Oxide Hybrid

The rare coexistence of ferromagnetism and electrical conductivity is observed in the reduced graphene oxide–metal oxide hybrids, rGO‐Co, rGO‐Ni, and rGO‐Fe, using chemical reduction with hydrazine or ultraviolet photoirradiation of the graphene oxide–metal complexes, GO‐Co, GO‐Ni, and GO‐Fe. The starting and final materials are characterized by X‐ray photoelectron spectroscopy, transmission electron microscopy (TEM), elemental analysis, Mössbauer spectroscopy, and Raman spectroscopy. In contrast to graphene, where the electrical conductivity and magnetic properties are controlled by carrier (electron or hole) doping, those of graphene oxide can be controlled by complexation with Co²⁺, Ni²⁺, and Fe³⁺ cations through the strong electrostatic affinity of negatively charged graphene oxide towards metal cations. The presence of ferromagnetism and electrical conductivity in these hybrids can promote significant applications including magnetic switching and data storage.     

Self‐Assembling TiO2 Nanorods on Large Graphene Oxide Sheets at a Two‐Phase Interface and Their Anti‐Recombination in Photocatalytic Applications

TiO₂ nanorods are self‐assembled on the graphene oxide (GO) sheets at the water/toluene interface. The self‐assembled GO–TiO₂ nanorod composites (GO–TiO₂ NRCs) can be dispersed in water. The effective anchoring of TiO₂ nanorods on the whole GO sheets is confirmed by transmission electron microscopy (TEM), X‐ray diffraction (XRD), Fourier transform IR spectroscopy (FTIR), and thermogravimetric analysis (TGA). The significant increase of photocatalytic activity is confirmed by the degradation of methylene blue (MB) under UV light irridiation. The large enhancement of photocatalytic activity is caused by the effective charge anti‐recombination and the effective absorption of MB on GO. The effective charge transfer from TiO₂ to GO sheets is confirmed by the significant photoluminescence quenching of TiO₂ nanorods, which can effectively prevent the charge recombination during photocatalytic process. The effective absorption of MB on GO is confirmed by the UV‐vis spectra. The degradation rate of MB in the second cycle is faster than that in the first cycle because of the reduction of GO under UV light irradiation.     

Electroluminescence and Photoluminescence from a Fluorescent Cobalt Porphyrin Grafted on Graphene Oxide

A new graphene oxide–cobalt porphyrin (GO–CoTPP) hybrid material has been used as an emissive layer in organic light-emitting diodes (OLEDs). Devices with fundamental structure of indium-doped tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, 45 nm)/polyvinylcarbazole (PVK):2-(4-biphenyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD):GO–CoTPP (70 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene (TPBI, 20 nm)/Al (150 nm) were fabricated. A red electroluminescence (EL) was obtained from thin-film PVK:PBD:CoTPP at 70 nm thickness. When CoTPP was covalently grafted on graphene oxide (GO) sheets, near-white EL was obtained. The white emission, which was composed of bluish green and red, is attributed to electroplex formation at the GO–CoTPP/PBD interface. Such electroplex emission between electrons and holes is a reason for the low turn-on voltage of the GO–CoTPP-based OLED. Maximum luminance efficiency of 1.43 cd/A with Commission International de l’Eclairage coordinates of 0.33 and 0.40 was achieved at current of 0.02 mA and voltage of 14 V.     

A Controllable Self‐Assembly Method for Large‐Scale Synthesis of Graphene Sponges and Free‐Standing Graphene Films

A simple method to prepare large‐scale graphene sponges and free‐standing graphene films using a speed vacuum concentrator is presented. During the centrifugal evaporation process, the graphene oxide (GO) sheets in the aqueous suspension are assembled to generate network‐linked GO sponges or a series of multilayer GO films, depending on the temperature of a centrifugal vacuum chamber. While sponge‐like bulk GO materials (GO sponges) are produced at 40 °C, uniform free‐standing GO films of size up to 9 cm² are generated at 80 °C. The thickness of GO films can be controlled from 200 nm to 1 µm based on the concentration of the GO colloidal suspension and evaporation temperature. The synthesized GO films exhibit excellent transparency, typical fluorescent emission signal, and high flexibility with a smooth surface and condensed density. Reduced GO sponges and films with less than 5 wt% oxygen are produced through a thermal annealing process at 800 °C with H₂/Ar flow. The structural flexibility of the reduced GO sponges, which have a highly porous, interconnected, 3D network, as well as excellent electrochemical properties of the reduced GO film with respect to electrode kinetics for the [Fe(CN)₆]^3?/4? redox system, are demonstrated.     

Non‐Invasive High‐Throughput Metrology of Functionalized Graphene Sheets

The utilization of fluorescence quenching microscopy (FQM) for quick visualization of chemical functionalization in relatively large regions of graphene, grown via chemical vapor deposition (CVD), is discussed. Through reactive ion plasma etching, patterns of p‐type CVD‐grown graphene functionalized with fluorine are generated. 4‐(dicyanomethylene)‐2‐methyl‐6‐(4‐dimethylaminostyryl)‐4H‐pyran (DCM) is used as the fluorescent agent. The emission of DCM is quenched to a different extent by fluorinated and pristine graphene, which provides the fluorescence‐imaging contrast essential for this metrology. To probe the functionalized surface patterns with DCM, the dye is dispersed in polymethylmethacrylate (PMMA) then the graphene surface is coated, forming a 30‐nm‐thick DCM‐PMMA layer. Fluorescence images of dye‐coated graphene distinctly reveal the difference between the chemically treated and as‐grown regions. The pristine graphene quenches the DCM emission more efficiently than the fluorinated graphene. Therefore, the regions with pristine graphene appear darker on the fluorescence images than the regions with fluorinated graphene, enabling large‐scale mapping of the functionalized regions in CVD grown graphene sheets Due to its simplicity and consistent results, FQM is now poised for widespread adoption by graphene manufacturers as a basis for facile and high throughput metrology of large‐scale graphene sheets.     

Direct Observation of Conducting Nanofilaments in Graphene‐Oxide‐Resistive Switching Memory

Determining the presence of conducting filaments in resistive random access memory with nanoscale thin films is vital to unraveling resistive switching mechanisms. Bistable resistive switching within graphene‐oxide (GO)‐based resistive memory devices, recently developed by many research groups, has been generally explained by the formation and rupture of conducting filaments induced by the diffusion of metal or oxygen ions. Using a low‐voltage spherical aberration‐corrected transmission electron microscopy (TEM), we directly observe metallic nanofilaments formed at the amorphous top interface layer with the application of external voltages in an Al/GO/Al memory system. Atomic‐resolution TEM images acquired at an acceleration voltage of 80 kV clearly show that the conducting nanofilaments are composed of nanosized aluminum crystalline within the amorphous top interface layer after applying a negative bias (ON state). Simultaneously, we observe the change in the crystallinity of GO films by the back‐diffusion of oxygen ions. The oxygen‐deficient regions are clearly confirmed by energy‐filtered TEM oxygen elemental mapping. This work could provide strong evidence to confirm the resistive switching mechanism previously suggested by our group.     

Bioinspired Fabrication of Superhydrophobic Graphene Films by Two‐Beam Laser Interference

Reported here is a bioinspired fabrication of superhydrophobic graphene surfaces by means of two‐beam laser interference (TBLI) treatment of graphene oxide (GO) films. Microscale grating‐like structures with tunable periods and additional nanoscale roughness are readily created on graphene films due to laser induced ablation effect. Synchronously, abundant hydrophilic oxygen‐containing groups (OCGs) on GO sheets can be drastically removed after TBLI treatment, which lower its surface energy significantly. The synergistic effect of micro‐nanostructuring and the OCGs removal endows the resultant graphene films with unique superhydrophobicity. Additionally, dual TBLI treatment with 90° rotation is implemented to fabricate superhydrophobic graphene films with two‐dimensional grating‐like structures that can effectively avoid the anisotropic hydrophobicity originated from the grooved structures. Moreover, the superhydrophobic graphene films become conductive due to the laser reduction effect. Unique optical characteristics including transmission diffraction and brilliant structural color are also observed due to the presence of periodic microstructures. As a mask‐free, chemical‐free, and cost‐effective method, the TBLI processing of GO may open up a new way to biomimetic graphene surfaces, and thus hold great promise for the development of novel graphene‐based microdevices.     

Ultrafast Fabrication of Covalently Cross‐linked Multifunctional Graphene Oxide Monoliths

Stable graphene oxide monoliths (GOMs) have been fabricated by exploiting epoxy groups on the surface of graphene oxide (GO) in a ring opening reaction with amine groups of poly(oxypropylene) diamines (D₄₀₀). This method can rapidly form covalently bonded GOM with D₄₀₀ within 60 s. FTIR and XPS analyses confirm the formation of covalent C‐N bonds. Investigation of the GOM formation mechanism reveals that the interaction of GO with a diamine cross‐linker can result in 3 different GO assemblies depending on the ratio of D₄₀₀ to GO, which have been proven both by experiment and molecular dynamics calculations. Moreover, XRD results indicate that the interspacial distance between GO sheets can be tuned by varying the diamine chain length and concentration. We demonstrate that the resulting GOM can be moulded into various shapes and behaves like an elastic hydrogel. The fabricated GOM is non‐cyctotoxic to L929 cell lines indicating a potential for biomedical applications. It could also be readily converted to graphene monolith upon thermal treatment. This new rapid and facile method to prepare covalently cross‐linked GOM may open the door to the synthesis and application of next generation multifunctional 3D graphene structures.     

Ethylene glycol assisted hydrothermal synthesis of graphene sheets supporting CdS nanospheres for quenched photoluminescence

In this paper, we report the obtention of graphene–cadmium sulfide (G/CdS) nanocomposites were successfully synthesized by the ethylene glycol assisted hydrothermal method. The structure and composition of the obtained nanocomposites were confirmed by means of X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) measurements. XRD patterns showed that the CdS nanospheres belong to hexagonal structure. SEM and TEM images suggested a homogeneous distribution of CdS nanospheres coated on the graphene sheets successfully. FT-IR and XPS analyses indicated that GO has been simultaneously reduced to graphene nanosheets during the deposition of CdS nanocomposite. Moreover, PL investigations demonstrated that the G/CdS nanocomposites displayed significant decrease in PL emission compared with the corresponding sphere-like CdS nanoparticles. The investigation gave a promise to the development of original yet highly efficient graphene oxide-based novel electrode material in optical detectors.     

A Graphene Nanoprobe for Rapid,Sensitive, and Multicolor Fluorescent DNA Analysis

Coupling nanomaterials with biomolecular recognition events represents a new direction in nanotechnology toward the development of novel molecular diagnostic tools. Here a graphene oxide (GO)‐based multicolor fluorescent DNA nanoprobe that allows rapid, sensitive, and selective detection of DNA targets in homogeneous solution by exploiting interactions between GO and DNA molecules is reported. Because of the extraordinarily high quenching efficiency of GO, the fluorescent ssDNA probe exhibits minimal background fluorescence, while strong emission is observed when it forms a double helix with the specific targets, leading to a high signal‐to‐background ratio. Importantly, the large planar surface of GO allows simultaneous quenching of multiple DNA probes labeled with different dyes, leading to a multicolor sensor for the detection of multiple DNA targets in the same solution. It is also demonstrated that this GO‐based sensing platform is suitable for the detection of a range of analytes when complemented with the use of functional DNA structures.     

Rapid microwave-assisted synthesis of silver decorated-reduced graphene oxide nanoparticles with enhanced photocatalytic activity under visible light

A facile, fast, and scalable microwave irradiation (MWI) method for the synthesis of Ag nanoparticles (Ag NPs) dispersed on graphene sheets has been developed. The reduction of graphene oxide takes place in ethanol solution within 2 min of MWI without any additional reducing agent or complicated treatment. The morphology and microstructure of the as-prepared hybrid were characterized by Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) images. The result showed the Ag NPs with an average size of 5–10 nm decorated on the rGO sheets. X-ray powder diffraction (XRD) determined that the crystallographic structure of Ag is face-centered cubic and there was a strong interaction between Ag NPs and rGO sheets. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) showed that GO had been reduced to rGO in our hybrid. Moreover, visible photocatalytic activity of the rGO–Ag nanocomposites was tested using Rhodamine B (RhB) as the model contaminant. This result indicates that rGO–Ag nanocomposites display distinctly enhanced photocatalytic activities. The investigation gave a promise to the development of original yet highly efficient graphene-based photocatalysts that utilize visible light as an energy source.     

Study on the fluorescence quenching of ZnO by graphene oxide

Zinc oxide (ZnO) microrod arrays were synthesized on Si substrate by a vapor phase transport (VPT) method in a tube furnace. The obtained ZnO microrods are characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The photoluminescence (PL) measurement indicates that the ZnO microrods have a strong ultraviolet (UV) emission centered at ~391 nm and a defect-related emission centered at ~530 nm. After the microrods were coated with graphene oxide (GO), the PL intensity of the hybrid microstructure is quenched compared with that of the bare one at the same excitation condition, and the PL intensity changes with the concentration of the GO. The fluorescence quenching mechanism is also discussed in this work.     

Bioinspired Nanocomposites: Ordered 2D Materials Within a 3D Lattice

Composites, materials composed of two or more materials—metallic, organic, or inorganic—usually exhibit the combined physical properties of their component materials. The result is a material that is superior to conventional monolithic materials. Advanced composites are used in a variety of industrial applications and therefore attract much scientific interest. Here the formation of novel carbon‐based nanocomposites is described via incorporation of graphene oxide (GO) into the crystal lattice of single crystals of calcite. Incorporation of a 2D organic material into single‐crystal lattices has never before been reported. To characterize the resulting nanocomposites, high‐resolution synchrotron powder X‐ray diffraction, electron microscopy, transmission electron microscopy, fluorescence microscopy and nanoindentation tests are employed. A detailed analysis reveals a layered distribution of GO sheets incorporated within the calcite host. Moreover, the optical and mechanical properties of the calcite host are altered when a carbon‐based nanomaterial is introduced into its lattice. Compared to pure calcite, the composite GO/calcite crystals exhibits lower elastic modulus and higher hardness. The results of this study show that the incorporation of a 2D material within a 3D crystal lattice is not only feasible but also can lead to the formation of hybrid crystals exhibiting new properties.     

Multilayer Hybrid Films Consisting of Alternating Graphene and Titania Nanosheets with Ultrafast Electron Transfer and Photoconversion Properties

Alternating graphene (G) and titania (Ti_0.91O₂) multilayered nanosheets are fabricated using layer‐by‐layer electrostatic deposition followed by UV irradiation. Successful assemblies of graphene oxide (GO) and titania nanosheets in sequence with polyethylenimine as a linker is confirmed by UV–vis absorption and X‐ray diffraction. Photocatalytic reduction of GO into G can be achieved upon UV irradiation. Ultrafast photocatalytic electron transfer between the titania and graphene is demonstrated using femtosecond transient absorption spectroscopy. Efficient exciton dissociation at the interfaces coupled with cross‐surface charge percolation allows efficient photocurrent conversion in the multilayered Ti_0.91O₂/G films.     

Ultrathin MnO2/Graphene Oxide/Carbon Nanotube Interlayer as Efficient Polysulfide‐Trapping Shield for High‐Performance Li–S Batteries

Ultrathin MnO₂/graphene oxide/carbon nanotube (G/M@CNT) interlayers are developed as efficient polysulfide‐trapping shields for high‐performance Li–S batteries. A simple layer‐by‐layer procedure is used to construct a sandwiched vein–membrane interlayer of thickness 2 µm and areal density 0.104 mg cm^?2 by loading MnO₂ nanoparticles and graphene oxide (GO) sheets on superaligned carbon nanotube films. The G/M@CNT interlayer provides a physical shield against both polysulfide shuttling and chemical adsorption of polysulfides by MnO₂ nanoparticles and GO sheets. The synergetic effect of the G/M@CNT interlayer enables the production of Li–S cells with high sulfur loadings (60–80 wt%), a low capacity decay rate (?0.029% per cycle over 2500 cycles at 1 C), high rate performance (747 mA h g^?1 at a charge rate of 10 C), and a low self‐discharge rate with high capacity retention (93.0% after 20 d rest). Electrochemical impedance spectroscopy, cyclic voltammetry, and scanning electron microscopy observations of the Li anodes after cycling confirm the polysulfide‐trapping ability of the G/M@CNT interlayer and show its potential in developing high‐performance Li–S batteries.     

Towards Novel Multifunctional Pillared Nanostructures: Effective Intercalation of Adamantylamine in Graphene Oxide and Smectite Clays

Multifunctional pillared materials are synthesized by the intercalation of cage‐shaped adamantylamine (ADMA) molecules into the interlayer space of graphite oxide (GO) and aluminosilicate clays. The physicochemical and structural properties of these hybrids, determined by X‐ray diffraction (XRD), Fourier transform infrared (FTIR), Raman and X‐ray photoemission (XPS) spectroscopies and transmission electron microscopy (TEM) show that they can serve as tunable hydrophobic/hydrophilic and stereospecific nanotemplates. Thus, in ADMA‐pillared clay hybrids, the phyllomorphous clay provides a hydrophilic nanoenvironment where the local hydrophobicity is modulated by the presence of ADMA moieties. On the other hand, in the ADMA‐GO hybrid, both the aromatic rings of GO sheets and the ADMA molecules define a hydrophobic nanoenvironment where sp³‐oxo moieties (epoxy, hydroxyl and carboxyl groups), present on GO, modulate hydrophilicity. As test applications, these pillared nanostructures are capable of selective/stereospecific trapping of small chlorophenols or can act as cytotoxic agents.     

Surface Chemistry Routes to Modulate the Photoluminescence of Graphene Quantum Dots: From Fluorescence Mechanism to Up‐Conversion Bioimaging Applications

The bandgap in graphene‐based materials can be tuned from 0 eV to that of benzene by changing size and/or surface chemistry, making it a rising carbon‐based fluorescent material. Here, the surface chemistry of small size graphene (graphene quantum dots, GQDs) is tuned programmably through modification or reduction and green luminescent GQDs are changed to blue luminescent GQDs. Several tools are employed to characterize the composition and morphology of resultants. More importantly, using this system, the luminescence mechanism (the competition between both the defect state emission and intrinsic state emission) is explored in detail. Experiments demonstrate that the chemical structure changes during modification or reduction suppresses non‐radiative recombination of localized electron‐hole pairs and/or enhances the integrity of surface π electron network. Therefore the intrinsic state emission plays a leading role, as opposed to defect state emission in GQDs. The results of time‐resolved measurements are consistent with the suggested PL mechanism. Up‐conversion PL of GQDs is successfully applied in near‐IR excitation for bioimaging.     

2D and 3D Hybrid Systems for Enhancement of Chondrogenic Differentiation of Tonsil‐Derived Mesenchymal Stem Cells

2D/3D hybrid cell culture systems are constructed by increasing the temperature of the thermogelling poly(ethylene glycol)‐poly(l ‐alanine) diblock copolymer (PEG‐l ‐PA) aqueous solution in which tonsil tissue‐derived mesenchymal stem cells and graphene oxide (GO) or reduced graphene oxide (rGO) are suspended, to 37 °C. The cells exhibit spherical cell morphologies in 2D/3D hybrid culture systems of GO/PEG‐l ‐PA and rGO/PEG‐l ‐PA by using the growth medium. The cell proliferations are 30%–50% higher in the rGO/PEG‐l ‐PA hybrid system than in the GO/PEG‐l ‐PA hybrid system. When chondrogenic culture media enriched with TGF‐β3 is used in the 2D/3D hybrid systems, cells extensively aggregate, and the expression of chondrogenic biomarkers of SOX 9, COL II A1, COL II, and COL X significantly increases in the GO/PEG‐l ‐PA 2D/3D hybrid system as compared with the PEG‐l ‐PA 3D systems and rGO/PEG‐l ‐PA 2D/3D hybrid system, suggesting that the GO/PEG‐l ‐PA 2D/3D hybrid system can be an excellent candidate as a chondrogenic differentiation platform of the stem cell. This paper also suggests that a 2D/3D hybrid system prepared by incorporating 2D materials with various surface biofunctionalities in the in situ forming 3D hydrogel matrix can be a new cell culture system.