A scalable, solution-phase processing route to graphene oxide and graphene ultralarge sheets

Synthesis of ultra-large diameter graphene oxide flakes from natural flake graphite

Graphene and its derivatives are widely used in various fields due to their unique two-dimensional lamellar structure. This study aims to synthesize ultra-large graphene oxide (GO) sheets from natural flake graphite and investigate the factors influencing their size. Using a two-intercalation method based on the modified Hummers' method, we address the challenge of intercalating large-diameter graphene oxide by employing a secondary intercalation technique. Three different approaches were explored to control the size of the produced GO sheets. The results revealed that the completeness of the expansion graphite structure after initial intercalation significantly influenced the final GO sheet size, with more complete expansion leading to larger sheets. Optimal processing conditions were identified, involving soaking natural flake graphite in a mixed solution (H2SO4:H2O2 = 4:1), followed by drying at 60 °C for 24 h. Under these conditions, ultra-large GO sheets were predominantly monolayer with an average size of 220.99 μm and a maximum size of 438 μm. These monolayer GO sheets can be chemically reduced to graphene, making them promising for applications in transparent conductive films, optoelectronic devices, and aligned graphene composites.

Optimizing Carbon Structures in Laser-Induced Graphene Electrodes Using Design of Experiments for Enhanced Electrochemical Sensing Characteristics

In this study, we explored the morphological and electrochemical properties of carbon-based electrodes derived from laser-induced graphene (LIG) and compared them to commercially available graphene-sheet screen-printed electrodes (GS-SPEs). By optimizing the laser parameters (average laser power, speed, and focus) using a design of experiments response surface (DoE-RS) approach, binder-free LIG electrodes were achieved in a single-step process. Traditional trial-and-error methods can be time-consuming and may not capture the interactions between all variables effectively. To address this, we focused on linear resistance and substrate delamination to streamline the DoE-RS optimization process. Two LIGs, designated LIG A and LIG B, were fabricated using distinct and optimized laser settings, which resulted in a sheet resistance of 25 ± 2 Ω/sq and 21 ± 1 Ω/sq, respectively. These LIGs, characterized by scanning electron microscopy, Raman spectroscopy, and contact angle analysis, exhibited a highly porous morphology with 13% pore coverage and a contact angle <50°, which significantly increased their hydrophilicity when compared to the GS-SPE. For the electrochemical studies, the oxidation of NO2- ion by the graphene-based working electrodes was investigated, as it allowed for the direct comparison of the LIGs to the GS-SPE. These included cyclic voltammetry, electrochemical impedance spectroscopy, and differential pulsed voltammetry studies, which revealed that LIG electrodes displayed a remarkable 500% increase in peak current during NO2- oxidation compared to the GS-SPE. The LIGs also demonstrated improved stability and sensitivity (420 ± 30 and 570 ± 10 nAμM-1 cm-2) compared to the GS-SPE (73 ± 4 nAμM-1 cm-2) in the oxidation of NO2- ions; however, LIG B was more susceptible to ionic interference than LIG A. These findings highlight the value of applying statistical approaches such as DoE-RS to systematically improve the LIG fabrication process, enabling the rapid production of optimized LIGs that outperform conventional carbon-based electrodes.

Quaternized Graphene for High-Performance Moisture Swing Direct Air Capture of CO2

Nowadays, moisture-swing adsorption technology still relies on quaternary ammonium resins with limited CO2 capacity under ambient air conditions. In this work, a groundbreaking moisture-driven sorbent is developed starting from commercial graphene flakes and using glycidyltrimethylammonium chloride for incorporation of CO2-sensitive quaternary ammonium functional groups. Boasting an outstanding CO2 capture performance under ultra-diluted conditions (namely, 3.24 mmol g-1 at CO2 400 ppm and 20% RH), the functionalized sorbent (fGO) features clear competitive advantages over current technologies for direct air capture. Notably, fGO demonstrated unprecedented moisture-swing capacity, ease of regenerability, versatility, selectivity, and longevity. These distinctive features position the fGO as an advanced and promising solution, showcasing its potential to outperform existing methods for moisture-swing direct air capture of CO2.

Advances in Two-Dimensional Ion-Selective Membranes: Bridging Nanoscale Insights to Industrial-Scale Salinity Gradient Energy Harvesting

Salinity gradient energy, often referred to as the Gibbs free energy difference between saltwater and freshwater, is recognized as "blue energy" due to its inherent cleanliness, renewability, and continuous availability. Reverse electrodialysis (RED), relying on ion-selective membranes, stands as one of the most prevalent and promising methods for harnessing salinity gradient energy to generate electricity. Nevertheless, conventional RED membranes face challenges such as insufficient ion selectivity and transport rates and the difficulty of achieving the minimum commercial energy density threshold of 5 W/m2. In contrast, two-dimensional nanostructured materials, featuring nanoscale channels and abundant functional groups, offer a breakthrough by facilitating rapid ion transport and heightened selectivity. This comprehensive review delves into the mechanisms of osmotic power generation within a single nanopore and nanochannel, exploring optimal nanopore dimensions and nanochannel lengths. We subsequently examine the current landscape of power generation using two-dimensional nanostructured materials in laboratory-scale settings across various test areas. Furthermore, we address the notable decline in power density observed as test areas expand and propose essential criteria for the industrialization of two-dimensional ion-selective membranes. The review concludes with a forward-looking perspective, outlining future research directions, including scalable membrane fabrication, enhanced environmental adaptability, and integration into multiple industries. This review aims to bridge the gap between previous laboratory-scale investigations of two-dimensional ion-selective membranes in salinity gradient energy conversion and their potential large-scale industrial applications.

The Influence of Reaction Conditions on the Properties of Graphene Oxide

The present study focuses on correlations between three parameters: (1) graphite particle size, (2) the ratio of graphite to oxidizing agent (KMnO4), and (3) the ratio of graphite to acid (H2SO4 and H3PO4), with the reaction yield, structure, and properties of graphene oxide (GO). The correlations are a challenge, as these three parameters can hardly be separated from each other due to the variations in the viscosity of the system. The larger the graphite particles, the higher the viscosity of GO. Decreasing the ratio of graphite to KMnO4 from 1:4 to 1:6 generally leads to a higher degree of oxidation and a higher reaction yield. However, the differences are very small. Increasing the graphite-to-acid-volume ratio from 1 g/60 mL to 1 g/80 mL, except for the smallest particles, reduced the degree of oxidation and slightly reduced the reaction yield. However, the reaction yield mainly depends on the extent of purification of GO by water, not on the reaction conditions. The large differences in the thermal decomposition of GO are mainly due to the bulk particle size and less to other parameters.

Multifunctional graphene oxide nanoparticles for drug delivery in cancer

Recent advancements in nanotechnology have enabled us to develop sophisticated multifunctional nanoparticles or nanosystems for targeted diagnosis and treatment of several illnesses, including cancers. To effectively treat any solid tumor, the therapy should preferably target just the malignant cells/tissue with minor damage to normal cells/tissues. Graphene oxide (GO) nanoparticles have gained considerable interest owing to their two-dimensional planar structure, chemical/mechanical stability, excellent photosensitivity, superb conductivity, high surface area, and good biocompatibility in cancer therapy. Many compounds have been functionalized on the surface of GO to increase their biological applications and minimize cytotoxicity. The review presents an overview of the physicochemical characteristics, strategies for various modifications, toxicity and biocompatibility of graphene and graphene oxide, current trends in developing GO-based nano constructs as a drug delivery cargo and other biological applications, including chemo-photothermal therapy, chemo-photodynamic therapy, bioimaging, and theragnosis in cancer. Further, the review discusses the challenges and opportunities of GO, GO-based nanomaterials for the said applications. Overall, the review focuses on the therapeutic potential of strategically developed GO nanomedicines and comprehensively discusses their opportunities and challenges in cancer therapy.

Graphene oxide enhances thermal stability and microwave absorption/regeneration of a porous polymer

Microwave regeneration of adsorbents offers several advantages over conventional regeneration methods; however, its application for microwave transparent adsorbents such as polymers is challenging. In this study, hypercrosslinked polymer/graphene oxide (GO) nanocomposites with large surface area and enhanced microwave absorption ability were synthesized. Polymers of 4, 4´-bis ((chloromethyl)-1, 1´-biphenyl- benzyl chloride) were hypercrosslinked through the Friedel-Crafts reactions. GO sheets were synthesized through the Hummer's method. Nanocomposites with different GO contents (1-8 wt%) were synthesized by solution mixing method. Thermogravimetry analysis revealed a large enhancement in the thermal stability of GO-filled nanocomposites compared to pristine polymer. N₂ adsorption isotherm analysis showed 7% and 10% reduction in BET surface area and total pore volume of the nanocomposite with 8 wt% GO. Compared to the pristine polymer, the dielectric constant and dielectric loss factor increased from 5 to 17 and 0.05-1.6, respectively, for the nanocomposites with 8 wt% GO. Microwave-assisted desorption of toluene from samples revealed more than 160 ºC and 4 times improvement in the desorption temperature and desorption efficiency, respectively, by addition of 4 wt% GO to the polymer. This study showed the important role of GO addition for efficient microwave-assisted regeneration of polymer adsorbents.

The influence of laser-induced alignment on Z-scan properties of 2D carbon nanomaterials suspension dependent on polarization

The Z-scan technique uses a single beam that can be used for observing the nonlinear or optical limiting properties of materials. For the first time, the Z-scan properties dependent on the polarization of 2D carbon nanomaterial suspension were experimentally investigated using optical Z-scan technology. The Z-scan curves of graphene and graphene oxide (GO) in N-methyl-2-pyrrolidinone suspensions exhibited strong polarization-dependent characteristics. In paper, a reverse saturated absorption (RSA) dip surrounded the lens focus when the horizontal polarized beam was focused in the suspension, and two saturated absorption (SA) peaks appeared adjacent to the dip. However, for the vertical polarized beam, only one RSA dip surrounded the lens focus, and the threshold was higher than the SA for a horizontally polarized beam. The transmission of RSA for the GO suspension was evidently lower than that of the graphene suspension. The polarization-dependent characteristic can be ascribed to the laser-induced alignment in case the suspension is moved in or out of the beam focal point. Furthermore, the polarization-dependent 2D carbon nanomaterial suspension can be applied in several practical purposes such as 2D material-based optical and opto-fludic devices.

The Removal of Pb2+ from Aqueous Solution by Using Navel Orange Peel Biochar Supported Graphene Oxide: Characteristics, Response Surface Methodology, and Mechanism

The value-added utilization of waste resources to synthesize functional materials is important to achieve the environmentally sustainable development. In this paper, the biochar supported graphene oxide (BGO) materials were prepared by using navel orange peel and natural graphite. The optimal adsorption parameters were analyzed by response surface methodology under the conditions of solution pH, adsorbent dosage, and rotating speed. The adsorption isotherm and kinetic model fitting experiments were carried out according to the optimal adsorption parameters, and the mechanism of BGO adsorption of Pb2+ was explained using Scanning Electron Microscope (SEM-EDS), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR). Compared with virgin biochar, the adsorption capacity of Pb2+ on biochar supported graphene oxide was significantly increased. The results of response surface methodology optimization design showed that the order of influence on adsorption of Pb2+ was solution pH > adsorbent dosage > rotating speed. The optimal conditions were as follows: solution pH was 4.97, rotating speed was 172.97 rpm, and adsorbent dosage was 0.086 g. In the adsorption−desorption experiment, the desorption efficiency ranged from 54.3 to 63.3%. The process of Pb2+ adsorption by BGO is spontaneous and endothermic, mainly through electrostatic interaction and surface complexation. It is a heterogeneous adsorption process with heterogeneous surface, including surface adsorption, external liquid film diffusion, and intra-particle diffusion.

Laser-induced dynamic alignment and nonlinear-like optical transmission in liquid suspensions of 2D atomically thin nanomaterials

Nonlinear optical property of atomically thin materials suspended in liquid has attracted a lot of attention recently due to the rapid development of liquid exfoliation methods. Here we report laser-induced dynamic orientational alignment and nonlinear-like optical response of the suspensions as a result of their intrinsic anisotropic properties and thermal convection of solvents. Graphene and graphene oxide suspensions are used as examples, and the transition to ordered states from initial optically isotropic suspensions is revealed by birefringence imaging. Computational fluid dynamics is performed to simulate the velocity evolution of convection flow and understand alignment-induced birefringence patterns. The optical transmission of these suspensions exhibits nonlinear-like saturable or reverse saturable absorptions in Z-scan measurements with both nanosecond and continuous-wave lasers. Our findings not only demonstrate a non-contact controlling of macroscopic orientation and collective optical properties of nanomaterial suspensions by laser but also pave the way for further explorations of optical properties and novel device applications of low-dimensional nanomaterials.

Graphene Oxide/Nanofiber-Based Actuation Films with Moisture and Photothermal Stimulation Response for Remote Intelligent Control Applications

The rapid development of intelligent technology and industry has induced higher requirements for multifunctional materials, especially intelligent materials with stimulus-responsive self-actuation behavior. In this study, a Cu@PVA-co-PE/GO composite actuation film, with an asymmetric sandwich structure, was prepared by attaching graphene oxide (GO) to the surface of a polyvinyl alcohol ethylene copolymer (PVA-co-PE) nanofiber composite film containing copper nanoparticles (Cu) through layer-on-layer adsorption. This unique structural design endowed the composite film with not only excellent structural stability but also different bending directions (in response to moisture and infrared light). The actuation performance shows that when the adsorption time was 4 h, the maximum bending angle of the Cu@PVA-co-PE/GO composite film was up to 90° within 5.99 s. Furthermore, the actuation behavior was stable after 100 cycles of reversible moisture stimulation. Additionally, the maximum actuation strain of the composite film was up to 1.35 MPa during the illumination time of 6.8 s and maintained an excellent stability for 400 s under continuous infrared stimulation of 0.53 W/cm². The rapid and sensitive stimulus response of the Cu@PVA-co-PE/GO composite film exhibited self-actuation behavior under the remote control of moisture and infrared light. This, in turn, suggests prospects for wide applications in emerging technologies, such as intelligent switches, artificial muscles, intelligent medical treatment, and flexible robots.

Microstructure and electrochemical properties of high performance graphene/manganese oxide hybrid electrodes

Hybrids consisting of 2D ultra-large reduced graphene oxide (RGO) sheets (∼30 μm long) and 1D α-phase manganese oxide (MnO2) nanowires were fabricated through a versatile synthesis technique that results in electrostatic binding of the nanowires and sheets. Two different hybrid (RGO/MnO2) compositions had remarkable features and performance: 3 : 1 MnO2/RGO (75/25 wt%) denoted as 3H and 10 : 1 MnO2/RGO (90/10 wt%) denoted as 10H. Characterization using spectroscopy, microscopy, and thermal analysis provided insights into the microstructure and behavior of the individual components and hybrids. Both hybrids exhibited higher specific capacitance than their individual components. 3H demonstrated excellent overall electrochemical performance with specific capacitance of 225 F g-1, pseudocapacitive and electrochemical double-layer capacitance (EDLC) contributions, charge-transfer resistance <1 Ω, and 97.8% capacitive retention after 1000 cycles. These properties were better than those of 10H; this was attributed 3H's more uniform distribution of nanowires enabling more effective electronic transport. Thermal annealing was used to produce reduced graphene oxide (RGO) that exhibited significant removal of oxygen functionality with a resulting interlayer spacing of 0.391 nm, higher D/G ratio, higher specific capacitance, and electrochemical properties representing more ideal capacitive behavior than GO. Integrating ultra-large RGO with very high surface area and MnO2 nanowires enables chemical interactions that may improve processability into complex architectures and electrochemical performance of electrodes for applications in electronics, sensors, catalysis, and deionization.

Nanofluidic Membranes to Address the Challenges of Salinity Gradient Power Harvesting

Salinity gradient power (SGP) has been identified as a promising renewable energy source. Reverse electrodialysis (RED) and pressure retarded osmosis (PRO) are two membrane-based technologies for SGP harvesting. Developing nanopores and nanofluidic membranes with excellent water and/or ion transport properties for applications in those two membrane-based technologies is considered viable for improving power generation performance. Despite recent efforts to advance power generation by designing a variety of nanopores and nanofluidic membranes to enhance power density, the valid pathways toward large-scale power generation remain uncertain. In this review, we introduce the features of ion and water transport in nanofluidics that are potentially beneficial to power generation. Subsequently, we survey previous efforts on nanofluidic membrane synthesis to obtain high power density. We also discuss how the various membrane properties influence the power density in RED and PRO before moving on to other important aspects of the technologies, i.e., system energy efficiency and membrane fouling. We analyze the importance of system energy efficiency and illustrate how the delicately designed nanofluidic membranes can potentially enhance energy efficiency. Previous studies are reviewed on fabricating antifouling and antimicrobial membrane for power generation, and opportunities are presented that can lead to the design of nanofluidic membranes with superior antifouling properties using various materials. Finally, future research directions are presented on advancing membrane performance and scaling-up the system. We conclude this review by emphasizing the fact that SGP has the potential to become an important renewable energy source and that high-performance nanofluidic membranes can transform SGP harvesting from conceptual to large-scale applications.

Construction of stable bio-Pd catalysts for environmental pollutant remediation

It has been reported that Pd nanoparticles were a little weak to bind to the dried microbial (yeast) surface, leading to the poor stability of the bio-supported catalysts. The objectives of the study are to construct stable Pd nanocatalysts supported on the dried yeast surface with the help of a tiny amount (<0.1 wt%) of reduced graphene oxide (Pd/yeast/rGO) and apply the catalysts in environmental pollutant remediation. The characterizations of the as-obtained Pd/yeast/rGO catalysts showed that reduced GO could cover Pd/yeast materials and prepare 15–21 nm Pd nanoparticles under acid and base media. The catalytic performance of the Pd/yeast/rGO catalyst was compared with that of control Pd/yeast catalysts without GO. The results revealed the kinetic constant K_app in the reduction of 4-nitrophenol of Pd/yeast/rGO catalysts could reach 3.6 × 10⁻² s⁻¹ without stirring during the reaction, which was 2.4 times higher than that of Pd/yeast catalysts, and the Pd/yeast/rGO catalysts kept a good stability even after being reused in seven cycles. Furthermore, the catalysts also showed quite good catalytic activities on CO oxidation and decolorization of dye methylene blue (MB). Thus, Pd/yeast/rGO catalysts were proven to be highly active and stable for environmental remediation and have the advantage that they can prevent the loss of noble metals and be prepared conveniently from discarded microorganisms.

Stable bio-supported Pd/yeast/rGO catalysts were prepared by covering with a tiny amount (<0.1 wt%) of GO based on a non-enzyme reduction method.

Green Carbon Material for Organic Contaminants Adsorption

Eco-friendly and economical adsorbents are desirable for removing organic pollutants from the environment. Herein, a kind of green carbon material, electrolytic carbon (EC) prepared by the electrochemical conversion of greenhouse gas (CO₂) in molten carbonate, is verified as an effective adsorbent for aniline and other small aromatic organic molecules. The EC consists of nanoparticles and nanoflakes, featuring the specific surface area of ∼641 m²/g with an enriched micropore structure. It exhibits a large adsorption capacity (Q_max > 114.1 mg/g) for aniline, especially in water with a lower contamination level. The adsorption conforms to the pseudo-second-order equation kinetically and the Freundlich model thermodynamically in the temperature range of 303-323 K. Moreover, it is found that the adsorption performance of the material can be further improved through reducing surface oxygen functional groups by a simple thermotreatment. Its adsorption capacity for aniline is enhanced by 1.7 times, demonstrating that the π-π dispersive interaction plays a primary role for the efficient adsorption. This adsorption mechanism is further confirmed by the excellent adsorption performance of the carbon materials for other analogue aromatic compounds (phenol, nitrobenzene). The super performance of the CO₂-derived carbon adsorbents will be helpful for capturing CO₂ as well as for removing organic pollutants.

Controlled assembly of artificial 2D materials based on the transfer of oxo-functionalized graphene

Functionalized 2D materials have unique properties, but are currently not used for the assembly of van der Waals heterostructures. Here, we present the controlled transfer of artificially synthesized, polar and highly transparent oxo-functionalized graphene, which can decouple graphene layers.

Efficient Production of Multi-Layer Graphene from Graphite Flakes in Water by Lipase-Graphene Sheets Conjugation

Biographene was successfully produced in water from graphite flakes by a simple, rapid, and efficient methodology based on a bioexfoliation technology. The methodology consisted in the application of a lipase, with a unique mechanism of interaction with hydrophobic surfaces, combined with a previous mechanical sonication, to selectively generate lipase-graphene sheets conjugates in water at room temperature. The adsorption of the lipase on the graphene sheets permits to keep the sheets separated in comparison with other methods. It was possible to obtain more than 80% of graphene (in the form of multi-layer graphene) from low-cost graphite and with less damage compared to commercial graphene oxide (GO) or reduced GO. Experimental analysis demonstrated the formation of multi-layer graphene (MLG) mainly using lipase from Thermomyces Lanuginosus (TLL).

Graphene oxide nanocomposite hydrogel based on poly(acrylic acid) grafted onto salep: an adsorbent for the removal of noxious dyes from water

In this study, a graphene oxide nanocomposite hydrogel (GONH) based on poly(acrylic acid) grafted onto a natural salep polysaccharide was synthesized and investigated as an adsorbent for the removal of cationic dye from aqueous solution.

Millstone Exfoliation: a True Shear Exfoliation for Large-Size Few-Layer Graphene Oxide

A millstone (MS) was introduced in the production of large-size few-layer-graphene oxide (FLGO) via true shear exfoliation in order to minimize fragmentation. The MS was constructed with two glass plates, where the top plate was designed to rotate against the stationary bottom plate, thereby generating true shear force. Mildly oxidized graphite (MOG) was used for MS exfoliation in order to obtain both good property and high yield. The rpm of rotation (10, 20, 30, 40, and 50), solution concentration (0.5, 1, and 2 mg/ml), and number of exfoliation (1, 2, and 3) were optimized by measuring the UV-vis absorption, and the effect of oxidation time (30, 60, and 90 min) was studied under the given optimum conditions. Next, the FLGO was isolated by centrifugation and characterized by TEM and AFM. The FLGO obtained was as large as ~ 10 μm in size, which was slightly smaller than the pristine graphite, suggesting a possibility of slight fragmentation. But it was still much larger than the FLGO obtained via sonication (< 1 μm), demonstrating successful MS exfoliation.

New insight of high temperature oxidation on self-exfoliation capability of graphene oxide

The preparation of graphene oxide (GO) via Hummers method is usually divided into two steps: low temperature oxidation at 35 °C (step I oxidation) and high temperature oxidation at 98 °C (step II oxidation). However, the effects of these two steps on the exfoliation capability and chemical structure of graphite oxide remain unclear. In this study, both the functional group content of graphite oxide and the entire evolution of interlayer spacing were investigated during the two steps. Step I oxidation is a slowly inhomogeneous oxidation step to remove unoxidized graphite flakes. The prepared graphite oxide can be easily self-exfoliated but contains a lot of organic sulfur. During the first 20 min of step II oxidation, the majority of organic sulfur can be efficiently removed and graphite oxide still remains a good exfoliation capability due to sharp increasing of carboxyl groups. However, with a longer oxidation time at step II oxidation, the decrease of organic sulfur content is slowed down apparently but without any carboxyl groups forming, then graphite oxide finally loses self-exfoliation capability. It is concluded that a short time of step II oxidation can produce purer and ultralarge GO sheets via self-exfoliation. The pure GO is possessed with better thermal stability and liquid crystal behavior. Besides, reduced GO films prepared from step II oxidation show better mechanical and electric properties after reducing compared with that obtained only via step I oxidation.

Oxygen functional groups improve the energy storage performances of graphene electrochemical supercapacitors

Graphene is a promising electrode material for supercapacitors due to its superior physical and chemical properties, but the influence of its oxygen functional groups on capacitive performance still remains somewhat uncertain. In this work, graphene sheets with different oxygen content have been prepared through thermal reduction in argon. Furthermore, oxidation and pore-forming treatment of graphene annealed at 800 °C are also performed to explore the important effect of oxygen functional groups. The effects of disorder degree, surface area and oxygen functional groups on the specific capacitance were explored systematically. The content and species of oxygen functional groups are found to be significant factors influencing the electrochemical supercapacitor performance of graphene electrodes. The specific capacitances of graphene annealed at 200, 400 and 800 °C are 201, 153 and 34 F g⁻¹, respectively. However, the specific capacitance of graphene reduced at 800 °C can be increased to 137 F g⁻¹ after nitric acid oxidation treatment, and is only 39 F g⁻¹ after pore forming on graphene surface, demonstrating that the oxygen functional groups can improve the capacitive performances of graphene electrochemical supercapacitors.

The content and species of oxygen functional groups are significant factors influencing the electrochemical supercapacitor performance of graphene electrodes.

In situ preparation of Fe3O4 in a carbon hybrid of graphene nanoscrolls and carbon nanotubes as high performance anode material for lithium-ion batteries

A new conductive carbon hybrid combining both reduced graphene nanoscrolls and carbon nanotubes (rGNSs-CNTs) is prepared, and used to host Fe₃O₄ nanoparticles through an in situ synthesis method. As an anode material for LIBs, the obtained Fe₃O₄@rGNSs-CNTs shows good electrochemical performance. At a current density of 0.1 A g^-1, the anode material shows a high reversible capacity of 1232.9 mAh g^-1 after 100 cycles. Even at a current density of 1 A g^-1, it still achieves a high reversible capacity of 812.3 mAh g^-1 after 200 cycles. Comparing with bare Fe₃O₄ and Fe₃O₄/rGO composite anode materials without nanoscroll structure, Fe₃O₄@rGNSs-CNTs shows much better rate capability with a reversible capacity of 605.0 and 500.0 mAh g^-1 at 3 and 5 A g^-1, respectively. The excellent electrochemical performance of the Fe₃O₄@rGNSs-CNTs anode material can be ascribed to the hybrid structure of rGNSs-CNTs, and their strong interaction with Fe₃O₄ nanoparticles, which on one hand provides more pathways for lithium ions and electrons, on the other hand effectively relieves the volume change of Fe₃O₄ during the charge-discharge process.

Formation of toroids by self-assembly of an α-α corner mimetic: supramolecular cyclization

An α-α corner mimetic self-assembles to form a rod-like supramolecular structure which bends and closes end-to-end like a cyclization reaction to form uniform toroids. Each peptide fragment containing l-leucine, α-aminoisobutyric acid (Aib) and l-tyrosine forms rigid 3₁₀ helical structures stabilized by multiple intramolecular N-HO hydrogen bonds. Two 3₁₀ helices are connected by the spacer 3-aminomethyl-benzylamine and maintain an angular distance of 120° and therefore mimic the α-α corner motif of a protein super secondary structure. The individual α-α corner subunits are themselves regularly interlinked through multiple water mediated intermolecular hydrogen-bonding interactions to form the rod-like supramolecular structure and toroids. The formation of the supramolecular structure has been proven with X-ray crystallography and other spectroscopic techniques. The cyclization of the supramolecular structure and toroid formation were studied by optical microscope, AFM and FE-SEM experiments. Despite other assignments such as exfoliation of graphene from graphite, the compound exhibits significant memory to finally produce the toroids.

Resistive Switching Performance Improvement via Modulating Nanoscale Conductive Filament, Involving the Application of Two-Dimensional Layered Materials

Reversible chemical and structural changes induced by ionic motion and reaction in response to electrical stimuli leads to resistive switching effects in metal-insulator-metal structures. Filamentary switching based on the formation and rupture of nanoscale conductive filament has been applied in non-volatile memory and volatile selector devices with low power consumption and fast switching speeds. Before the mass production of resistive switching devices, great efforts are still required to enable stable and reliable switching performances. The conductive filament, a bridge of microscopic metal-insulator-metal structure and macroscopic resistance states, plays an irreplaceable part in resistive switching behavior, as unreliable performance often originates from unstable filament behavior. In this Review, departing from the filamentary switching mechanism and the existing issues, recent advances of the switching performance improvement through the conductive filament modulation are discussed, in the sequence of material modulation, device structure design and switching operation scheme optimization. In particular, two-dimensional (2D) nanomaterials with excellent properties including and beyond graphene, are discussed with emphasis on performance improvement by their active roles as the switching layer, insertion layer, thin electrode, patterned electrode, and edge electrode, etc.

Self-assembling graphene-anthraquinone-2-sulphonate supramolecular nanostructures with enhanced energy density for supercapacitors

Boosting the energy density of capacitive energy storage devices remains a crucial issue for facilitating applications. Herein, we report a graphene-anthraquinone supramolecular nanostructure by self-assembly for supercapacitors. The sulfonated anthraquinone exhibits high water solubility, a π-conjugated structure and redox active features, which not only serve as a spacer to interact with and stabilize graphene but also introduce extra pseudocapacitance contributions. The formed nest-like three-dimensional (3D) nanostructure with further hydrothermal treatment enhances the accessibility of ion transfer and exposes the redox-active quinone groups in the electrolytes. A fabricated all-solid-state flexible symmetric device delivers a high specific capacitance of 398.5 F g^-1 at 1 A g^-1 (1.5 times higher than graphene), superior energy density (52.24 Wh kg^-1 at about 1 kW kg^-1) and good stability (82% capacitance retention after 10 000 cycles).

Distinguishing thermal lens effect from electronic third-order nonlinear self-phase modulation in liquid suspensions of 2D nanomaterials

The interaction of light with atomically thin nanomaterials has attracted enormous research interest in order to understand two-dimensional (2D) electron systems and develop novel opto-electronic devices. The observations of spatial self-phase modulation and the associated multiple diffraction ring patterns in liquid suspensions of 2D nanomaterials are believed to be excellent examples of strong laser interaction with 2D nanomaterials and this phenomenon has been attributed to their large electronic third-order susceptibilities. By performing a series of control experiments with liquid suspensions of graphene and graphene oxide flakes in different solvents at various temperatures under an increasing modulation frequency of laser illumination, we first show that the diffraction ring pattern has little dependence on the type of nanomaterial but strongly depends on the duration of laser illumination. A laser induced local refractive index change is then monitored by a weaker probe beam, resulting in the divergent diffraction of the probe beam that indicates a lower self-induced refractive index in the center of the pump laser beam than at its periphery: a clear signature of the thermal lens effect. Finally, we use computational fluid dynamics to simulate laser induced temperature and index changes of the suspensions. The evolution of diffraction rings is well correlated to the transient temperature distribution. Our understanding of complex laser interactions with nanomaterial suspensions and the associated thermal lens effect paves the way for further basic studies and fluid opto-electronic applications of 2D nanomaterials.

Green and Mild Oxidation: An Efficient Strategy toward Water-Dispersible Graphene

Scalable fabrication of water-dispersible graphene (W-Gr) is highly desirable yet technically challenging for most practical applications of graphene. Herein, a green and mild oxidation strategy to prepare bulk W-Gr (dispersion, slurry, and powder) with high yield was proposed by fully exploiting structure defects of thermally reduced graphene oxide (TRGO) and oxidizing radicals generated from hydrogen peroxide (H₂O₂). Owing to the increased carboxyl group from the mild oxidation process, the obtained W-Gr can be redispersed in low-boiling solvents with a reasonable concentration. Benefiting from the modified surface chemistry, macroscopic samples processed from the W-Gr show good hydrophilicity (water contact angle of 55.7°) and excellent biocompatibility, which is expected to be an alternative biomaterial for bone, vessel, and skin regeneration. In addition, the green and mild oxidation strategy is also proven to be effective for dispersing other carbon nanomaterials in a water system.

Conductive graphene oxide hydrogels reduced and bridged by l-cysteine to support cell adhesion and growth

l-Cysteine reduces and bridges graphene oxide into a network, yielding conductive hydrogels nicely supporting cell adhesion and growth.

Synthesis and reduction of large sized graphene oxide sheets

Graphene oxide (GO) can be considered as one of the most visible outcomes of graphene research in terms of large scale production and commercialization prospects.

Hierarchical porous MnO/graphene composite aerogel as high-performance anode material for lithium ion batteries

MnO/graphene composite anode material with hierarchical pore structure shows high capacity, excellent rate capability and stability.

Surface Modification of C3N4 through Oxygen-Plasma Treatment: A Simple Way toward Excellent Hydrophilicity

We developed a universal method to prepare hydrophilic carbon nitrogen (C₃N₄) nanosheets. By treating C₃N₄ nanosheets with oxygen plasma, hydroxylamine groups (N-OH) with intense protonation could be introduced on the surface; moreover, the content of N-OH groups increased linearly with the oxygen-plasma treatment time. Thanks to the excellent hydrophilicity, uniformly dispersed C₃N₄ solution were prepared, which was further translated into C₃N₄ paper by simple vacuum filtration. Pure C₃N₄ paper with good stability, excellent hydrophilicity, and biocompatibility were proved to have excellent performance in tissue repair. Further research demonstrated that the oxygen-plasma treatment method can also introduce N-OH groups into other nitrogen-containing carbon materials (NCMs) such as N-doped graphene, N-doped carbon nanotube, and C₂N, which offers a new perspective on the surface modification and functionalization of these carbon nanomaterials.

Prospects of Supercritical Fluids in Realizing Graphene-Based Functional Materials

Supercritical-fluids science and technology predate all the approaches that are currently established for graphene production by several decades in advanced materials design. However, it has only recently been proposed as a plausible approach for graphene processing. Since then, supercritical fluids have emerged into contention as an alternative to existing technologies because of their scalability and versatility in processing graphene materials, which include composites, aerogels, and foams. Here, an overview is presented of such materials prepared through supercritical fluids from an advanced materials science standpoint, with a discussion on their fundamental properties and technological applications. The benefits of supercritical-fluid processing over conventional liquid-phase processing are presented. The benefits include not only better performances for advanced applications but also environmental issues associated with the synthesis process. Nevertheless, the limitations of supercritical-fluid processing are also stressed, along with challenges that are still faced toward the achievement of the great expectations from graphene materials.

Tailoring the Oxygen Content of Graphite and Reduced Graphene Oxide for Specific Applications

Graphene oxide (GO) is widely recognized as a promising material in a variety of fields, but its structure and composition has yet to be fully controlled. We have developed general strategies to control the oxidation degree of graphene-like materials via two methods: oxidation of graphite by KMnO4 in H2SO4 (oGO), and reduction of highly oxidized GO by hydrazine (rGO). Even though the oxygen content may be the same, oGO and rGO have different properties, for example the adsorption ability, oxidation ability, and electron conductivity. These differences in property arise from the difference in the underlying graphitic structure and the type of defect present. Our results can be used as a guideline for the production of tailor-made graphitic carbons. As an example, we show that rGO with 23.1 wt% oxygen showed the best performance as an electrode of an electric double-layer capacitor.

Sheet Size-Induced Evaporation Behaviors of Inkjet-Printed Graphene Oxide for Printed Electronics

The size of chemically modified graphene nanosheets is a critical parameter that affects their performance and applications. Here, we show that the lateral size of graphene oxide (GO) nanosheets is strongly correlated with the concentration of graphite oxide present in the suspension as graphite oxide is exfoliated by sonication. The size of the GO nanosheets increased from less than 100 nm to several micrometers as the concentration of graphite oxide in the suspension was increased up to a critical concentration. An investigation of the evaporation behavior of the GO nanosheet solution using inkjet printing revealed that the critical temperature of formation of a uniform film, T(c), was lower for the large GO nanosheets than for the small GO nanosheets. This difference was attributed to the interactions between the two-dimensional structures of GO nanosheets and the substrate as well as the interactions among the GO nanosheets. Furthermore, we fabricated organic thin film transistors (OTFTs) using line-patterned reduced GO as electrodes. The OTFTs displayed different electrical performances, depending on the graphene sheet size. We believe that our new strategy to control the size of GO nanosheets and our findings about the colloidal and electrical properties of size-controlled GO nanosheets will be very effective to fabricate graphene based printed electronics.