Chemically Converted Graphene Research Articles

Folding of graphene into elastic nanobelts

Here we report the finding of a new crumpled graphene structure – folded graphene belts (FGBs) – generated by means of shock cooling of an aqueous chemically converted graphene (CCG) dispersion. Unlike the traditional tubular hollow structures such as CNTs or CNSs, the as-made FGBs feature an accordion-like geometry in which the 2D graphene sheets were folded along multiple parallel axes. In situ scanning electron microscope (SEM) measurements revealed that the prepared FGBs were highly elastic and can keep their shape under repeated large strains. The formation and growth of ice crystals during the shock cooling step in liquid nitrogen are believed to be the driving force for the formation of such unique folded graphene structures.

Graphene–BODIPY as a photocatalyst in the photocatalytic–biocatalytic coupled system for solar fuel production from CO2

The utilization of CO2 for production of solar fuels/chemicals is gaining increasing importance due to worldwide fossil-fuel shortage and global warming. As a means to achieve this, we herein report on the synthesis and development of a graphene-based visible light active photocatalyst (CCG–BODIPY) which is chemically converted graphene (CCG) covalently bonded to a light harvesting BODIPY molecule (1-picolylamine-2-aminophenyl-3-oxy-phenyl-4,4′-difluoro-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene-triazine). The photocatalyst-biocatalyst coupled system developed using CCG–BODIPY as photocatalyst functions in a highly efficient manner, leading to high NADH regeneration (54.02 ± 0.61%), followed by its consumption in exclusive formic acid production (144.2 ± 1.8 μmol) from CO2. The present research endeavour highlights the development and application of a graphene based photocatalyst for direct solar fuel formation from carbon dioxide.

Determination of melamine based on electrochemiluminescence of Ru(bpy)32+ at chemically converted graphene-modified glassy carbon electrode

In this paper, electrochemiluminescence (ECL) of Ru(bpy)32+ at chemically converted graphene (CCG)-modified glassy carbon (GC) electrode has been employed for the determination of melamine in milk with a remarkable detection limit of 5.0 × 10−15 mol L−1.

Optical Characterisation of Non-Covalent Interactions between Non-Conjugated Polymers and Chemically Converted Graphene

Optical characterisation using dye molecules as probes was used to study the non-covalent interactions between chemically converted graphene (CCG) and non-conjugated, water soluble polymers in aqueous solution. The strong adsorption of non-conjugated polymers such as poly(ethylene oxide) (PEO) and poly(vinyl alcohol) (PVA) on CCG is observed by fluorescence and ultraviolet-visible spectroscopy and atomic force microscopy, and this leads to desorption of π-conjugated molecules from CCG. Such adsorption/desorption behaviour can be tailored by modifying the molecular weight of polymers and the chemistry of graphene. This finding provides a facile and non-covalent approach to the functionalisation of CCG and opens up new opportunities for the fabrication of graphene/polymer nanocomposites.

Supercapacitance of chemically converted graphene with composite pores

We report the synthesis and characterization of chemically converted graphene (CCG) with composite pores that has exhibited high specific capacitance (up to 350Fg−1). The porous CCG was obtained by photocatalytic reaction aided by ZnO quantum dots and it has both macropores (>50nm) and meso/micro-pores (<10nm). The introduction of macropores allowed penetration of electrolyte into the inter-layer space of CCG while the meso/micro-pores increased the adsorption of charges. The supercapacitor made from such porous CCG also exhibited excellent rate performance and cyclic stability.

Formation of Regular Stripes of Chemically Converted Graphene on Hydrophilic Substrates

Chemically converted graphene (CCG), from a chemistry point of view, is a giant molecule with a unique two-dimensional (2D) configuration. The availability of CCG dispersion provides a range of scalable methods to assemble graphene-based materials but brings the challenge of understanding and control of the CCG morphology in solution processing. In this study, we found that, similar to conventional colloidal systems (e.g., spherical particles or polymers), a 2D sheet of CCG can be transferred from its aqueous dispersion to solid substrates in the form of highly regular stripe patterns by evaporation-driven deposition. The width and spacing can be defined by the concentration of the CCG dispersion and the properties of the substrate (e.g., roughness and surface charge). Furthermore, the high resolution AFM images illustrate that both 2D flattened and highly wrinkled CCG can be formed in each individual stripe, depending on the location across the stripe. The in situ optical observation of the stripe formation indicates that the morphological change of CCG may occur in the crowded meniscus of the drying front.

Black Carbon-Mediated Reduction of 2,4-Dinitrotoluene by Dithiothreitol

By using various types of black carbon (BC), including chemically converted graphene (CCG), multiwalled carbon nanotubes (MWCNT), and granular activated carbon (GAC), BC-mediated reduction was investigated with 2,4-dinitrotoluene (DNT), a model nitroaromatic compound. We hypothesized that by providing sorption and electron transfer sites, BC can be used as a catalyst to accelerate DNT reduction by dithiothreitol (DTL), a thiol reductant. Results from batch experiments showed that CCG, MWCNT, and GAC could promote reduction of DNT by DTL. The yield ratio of the two aminonitro intermediates was approximately 1:1, which was consistent with that in a graphite system. However, fullerene did not significantly enhance the reduction of DNT, likely due to being a π acceptor. Kinetic data analysis showed that removal of DNT in the presence of BC and DTL was linearly proportional to the electrical conductivity of BC, suggesting that the graphitic structure of BC may be responsible for DNT removal. Our results indicate that the presence of BC materials may affect the fate of nitroaromatic compounds under electron-rich conditions.

Ultrathin Graphene Nanofiltration Membrane for Water Purification

AbstractA method of fabricating ultrathin (≈22–53 nm thick) graphene nanofiltration membranes (uGNMs) on microporous substrates is presented for efficient water purification using chemically converted graphene (CCG). The prepared uGNMs show well packed layer structure formed by CCG sheets, as characterized by scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The performance of the uGNMs for water treatment was evaluated on a dead end filtration device and the pure water flux of uGNMs was high (21.8 L m−2 h−1 bar−1). The uGNMs show high retention (&gt;99%) for organic dyes and moderate retention (≈20–60%) for ion salts. The rejection mechanism of this kind of negatively charged membranes is intensively studied, and the results reveal that physical sieving and electrostatic interaction dominate the rejection process. Because of the ultrathin nature of uGNMs, 34 mg of CCG is sufficient for making a square meter of nanofiltration membrane, indicating that this new generation graphene‐based nanofiltration technology would be resource saving and cost‐effective. The integration of high performance, low cost, and simple solution‐based fabrication process promises uGNMs great potential application in practical water purification.

Liquid phase co-exfoliated MoS2–graphene composites as anode materials for lithium ion batteries

We report a facile method to prepare the MoS2–graphene composites by liquid phase co-exfoliation of commercial MoS2 with graphene nanoplatelet (GnP) and chemically converted graphene (CCG) in N-methyl pyrrolidone with the aid of ultrasonication. The combination of SEM, XRD and BET results reveal that the MoS2–graphene composites are mesoporous materials consisting of very thin MoS2 flakes and graphene sheets. The prepared MoS2–graphene composites display good performance as anode materials for lithium ion batteries. Exfoliation of the MoS2 is the key factor for high electrochemical lithiation/delithiation performance. The MoS2/CCG composite exhibits the best reversible capacity (up to 750 mA h g−1 after 50 cycles), excellent cycling stability and high rate capability. The reversible capacity at a rate of 2000 mA g−1 is over 350 mA h g−1 and recovers to as high as 720 mA h g−1 at a rate of 100 mA g−1.

A remarkably simple characterization of glassy carbon-supported films of graphite, graphene oxide, and chemically converted graphene using Fe(CN)3−6/Fe(CN)4−6 and O2 as redox probes

This study investigated the behavior of thin films of graphene oxide (GO) and chemically converted graphene (CCG) on a glassy carbon surface in the presence of two redox probes (Fe(CN)3−6/Fe(CN)4−6 and O2), employing cyclic voltammetry, electrochemical impedance spectroscopy, and hydrodynamic voltammetry as a simple procedure for characterizing these films. The feasibility of using these electrochemical techniques for this purpose opens up the possibility of applying them to biosensors and electrocatalysts using surface-supported GO and CCG materials. Raman spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and scanning electron microscopy were employed to characterize GO and CCG.

Interaction of Cy3 dye with CCG and its application for BSA detection.

Chemically converted graphene (CCG) was found to effectively quench the fluorescence emission of Cy3 dye 1 (the intensity is down to 1/38 of 1 alone) in aqueous solution, due to the formation of a CCG-1 hybrid. After the addition of a certain amount of bovine serum albumin (BSA) into the above-mentioned system, about 60 times fluorescence enhancement was observed for the hybrid CCG-1. This was employed to discriminate BSA: the fluorescence intensity was found to be proportional to the BSA added in the concentration range from 0 to 8 × 10-6 M, and the BSA detection limit was down to 5 × 10-8 M.

Self-assembly to monolayer graphene film with high electrical conductivity

Monolayer chemically converted graphene (CCG) nanosheets can be homogeneously self-assembled onto silicon wafer modified by 3-aminopropyl triethoxysilane (APTES) to form very thin graphene film. The CCG film was characterized by FT-IR, XRD, SEM, TEM and AFM. Results show that CCG sheets formed monolayer film after assembled onto silicon wafer and there is a very tight chemical bond between sheets and wafer. Furthermore, the electrical measurements revealed that the monolayer graphene film has an excellent electrical conductivity.

Direct catalytic oxidation of benzene to phenol over metal-free graphene-based catalyst

We report an efficient, highly selective, and low temperature graphene-catalyzed reaction process for one-step oxidation of benzene to phenol with hydrogen peroxide as the oxidant. The chemically converted graphene (CCG) from small graphite was used as the catalyst. The conversion of benzene reaches 18%, with phenol being the sole product. The catalyst was reusable and very stable. By XPS, C K-edge X-ray absorption spectra, benzene-TPD, and kinetic measurements, it was concluded that the moderate H2O2 activation rate, good benzene adsorption ability, and balanced kinetic control over the oxidation reaction are responsible for the outstanding catalytic performance of the metal-free catalyst.

Enhanced photoresponse of large-sized photoactive graphene composite films based on water-soluble conjugated polymers

Composite films of chemically converted graphene (CCG) and water-soluble polythiophenes (P3TOPS and P3TOPA) were prepared by a LBL method using a suspension of negatively charged CCG-P3TOPS sheets and a solution of positively charged P3TOPA. The composite films show enhanced photoresponse due to photoinduced electron transfer from the polythiophenes to CCG.

Stitching Chemically Converted Graphene on Solid Surfaces by Solvent Evaporation

The suspension of chemically converted graphene (CCG) provides a cost-effective and facile approach to construct graphene-based materials. However, wrinkles and aggregates usually occur when transferring graphene from suspension to solid-state, which significantly alter the optical, electrical, and electrochemical properties of deposited graphene. Our effort is devoted to the control of the morphology of individual graphene sheet deposited on solid surfaces by the solvent evaporation. Here we have studied the effects of additional components (e.g., organic solvent and electrolyte) in the CCG suspension on the CCG morphology. It was found that the CCG sheets could be stitched together and the graphene monolayer could be flattened by the addition of appropriate additives to the CCG suspension.

Multiple pH-responsive graphene composites by non-covalent modification with chitosan

Multiple pH-sensitive composites have been prepared through non-covalently functionalizing chemically converted graphene (CCG) with chitosan. Chtiosan exhibits as polybases and CCG shows characteristics of polyacids. Owing to the synergistic effects of chitosan and CCG, chitosan decorated graphene (CS-G) presents a multiple pH-responsive behavior that it can be dispersed well whether in acidic or in basic solution but aggregated in near-neutral solution. After CS-G was modified through a controlled deposition and cross-linking process of chitosan, the resultant cross-linked chitosan decorated graphene (CLCS-G) can be converted to a different pH-sensitive material that disperses only in acidic solution. Both CS-G and CLCS-G present a reversible switching between dispersed and aggregated states with pH as a stimulus. The unique pH response mechanisms of CS-G and CLCS-G have been further investigated by zeta potential analysis. Based on the unique pH-responsive property of CS-G, a stable and repeatable pH-driven switch was developed for monitoring pH change.

Photocatalytic Performance of a Ag/ZnO/CCG Multidimensional Heterostructure Prepared by a Solution-Based Method

The photocatalytic performance of a multidimensional heterojunction composed of decorated Ag nanoparticles on ZnO nanorods vertically grown on a chemically converted graphene (CCG) was investigated. The combined heterojunction helps to improve photocatalytic activity by increasing light absorption, preventing photoinduced electron–hole recombination, and providing a carrier pathway for the giant π-conjugated system of CCG. A significant finding was that the low work function value of Ag (−4.74 eV) at the (111) surface makes possible the transfer of electrons from CCG to Ag. Consequently, the degree of photodegradation by Ag/ZnO/CCG is much higher than the sum of photodegradations by Ag/ZnO and ZnO/CCG samples, which indicates that the combination of metal, CCG, and semiconductor provides enhanced photocatalytic activity through the double transference of electrons.

Preparation and Photophysical and Photoelectrochemical Properties of a Covalently Fixed Porphyrin–Chemically Converted Graphene Composite

Chemically converted graphene (CCG) covalently linked with porphyrins has been prepared by a Suzuki coupling reaction between iodophenyl-functionalized CCG and porphyrin boronic ester. The covalently linked CCG-porphyrin composite was designed to possess a short, rigid phenylene spacer between the porphyrin and the CCG. The composite material formed stable dispersions in DMF and the structure was characterized by spectroscopic, thermal, and microscopic measurements. In steady-state photoluminescence spectra, the emission from the porphyrin linked to the CCG was quenched strongly relative to that of the porphyrin reference. Fluorescence lifetime and femtosecond transient absorption measurements of the porphyrin-linked CCG revealed a short-lived porphyrin singlet excited state (38 ps) without yielding the porphyrin radical cation, thereby substantiating the occurrence of energy transfer from the porphyrin excited state to the CCG and subsequent rapid decay of the CCG excited state to the ground state. Consistently, the photocurrent action spectrum of a photoelectrochemical device with a SnO(2) electrode coated with the porphyrin-linked CCG exhibited no photocurrent response from the porphyrin absorption. The results obtained here provide deep insight into the interaction between graphenes and π-conjugated systems in the excited and ground states.

Synthesis of highly concentrated suspension of chemically converted graphene in organic solvents: Effect of temperature on the extent of reduction and dispersibility

We report the effect of temperature on the extent of graphene oxide reduction by hydrazine and the dispersibility of the resulting chemically converted graphene (CCG) in polar organic solvents. The extent of graphene oxide reduction at high temperatures was only slightly higher than at low temperatures (30–50 °C), while the dispersibility of the resulting CCG in organic solvents decreased markedly with increasing temperature. The low dispersibility of CCGs prepared at high temperatures was greatly affected by reduction and influenced by the formation of an irreversible agglomerate of CCG at high temperatures. The reduction of graphene oxide at low temperatures is necessary to prepare highly dispersible CCG in organic solvents. CCG prepared at 30 °C is dispersible in N-methyl-2-pyrrolidone concentrations as high as 0.71 mg/mL. The free-standing paper made of this CCG possessed an electrical conductivity of more than 22,000 S/m, one of the highest values ever reported.

Generalized syntheses of nanocrystal–graphene hybrids in high-boiling-point organic solvents

Nanocrystal-graphene have been proposed as a new kind of promising hybrid for a wide range of application areas including catalysts, electronics, sensors, biomedicine, and energy storage, etc. Although a variety of methods have been developed for the preparation of hybrids, a facile and general synthetic approach is still highly required. In this study, nanocrystal-graphene hybrids were successfully synthesized in high-boiling-point organic solvents. Graphene oxide (GO) nanosheets were modified by oleylamine (OLA) to form a OLA-GO complex in order to be readily incorporated into hydrophobic synthesis. A rich library of highly crystalline nanocrystals, with types including noble metal, metal oxide, magnetic material and semiconductor were successfully grown on chemically converted graphene (CCG), which is simultaneously reduced from GO during the synthesis. High boiling-point solvents afford sufficient thermal energy to assure the high-quality crystalline nature of NCs, therefore the post-annealing process is obviated. Controlled experiments revealed that OLA-GO triggers heterogeneous nucleation and serves as excellent nuclei anchorage media. The protocol developed here brings one step closer to achieve "unity in diversity" on the preparation of nanocrystal-graphene hybrids.