Exfoliation of graphite oxide in propylene carbonate and thermal reduction of the resulting graphene oxide platelets

A hybrid nanocomposite based on CuFe layered double hydroxide coated graphene oxide for photocatalytic degradation of trimethoprim

Photocatalytic activation of persulfate (PS) has recently been considered an effective and environmentally friendly approach for antibiotic decomposition due to its high treatment efficiency, low energy consumption, and high reliability. The development of safe and high-performance catalysts is important for PS-based advanced oxidation processes. In this study, a CuFe-layered double hydroxide (LDH) coated graphene oxide (CuFe-LDH/GO) composite was constructed as a photocatalyst for trimethoprim (TMP) decomposition. The CuFe-LDH/GO catalyst was prepared via the co-precipitation method and characterized through Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and X-ray electron microscopy (XPS) techniques. Characterization results revealed that GO was entirely covered by LDH platelets which also kept its hydrotalcite structure in the as-prepared nanocomposite. The average crystallite size of CuFe-LDH/GO was 28.22 nm. The results confirmed that CuFe-LDH/GO exhibited excellent performance for the PS activation with a TMP removal efficiency of 90.8% under UV-light irradiation. Compared with pristine CuFe-LDH, the rate constant of TMP degradation of CuFe-LDH/GO was doubled. The results also indicated that acidic and alkaline conditions were not favorable for TMP degradation, and the catalytic activity of the used photocatalyst has not decreased significantly after 720 h of continuous recycling. Overall, CuFe-LDH/GO could be a promising photocatalyst for the treatment of wastewater containing antibiotics.

Green and Sustainable Ultrasound-Assisted Anodic Electrochemical Preparation of Graphene Oxide Dispersions and Their Antioxidant Properties

A fast method for preparing aqueous graphene oxide (GO) dispersions by electrochemical oxidation of a graphite anode without preliminary intercalation with oxidizing agents is proposed. Ultrasonic probing was used in the modulation mode of ultrasonic waves (work/rest) for more efficient graphite oxidation-exfoliation. It is shown that the 4/2 s mode of ultrasonic modulation is the most effective due to the probe material's low corrosion while maintaining the optimum synthesis temperature not exceeding 30-35 °C and achieving the best characteristics of the resulting product. Three cases of anodic oxidation of graphite to obtain graphene oxide were considered: (1) a combined cathode-anode compartment, (2) a split cathode-anode salt-bridged compartment, and (3) separated anode compartment with a 3.5 kDa dialysis membrane. It was determined that the approach to synthesis with a divided cathode-anode compartment makes it possible to obtain GO sheets with fewer defects compared to chemical methods or methods with a combined cathode-anode compartment and makes it possible to control the oxidation degree of the material (C:O ratio) by varying the current density. The prepared samples showed good stability for more than six months. The spectral and morphological characteristics were studied. Using chemiluminometry in the luminol/Co(II)/H₂O₂ system, the antioxidant properties concerning three key reactive oxygen species (H₂O₂, superoxide anion radical, and hydroxyl radical) were demonstrated. It was also shown that the prepared GO dispersions do not induce lipid and phospholipid peroxidation.

Electro-stimulated drug release by methacrylated hyaluronic acid-based conductive hydrogel with enhanced mechanical properties

Recently, the design of stimuli-responsive hydrogels for controlled drug delivery systems has been extensively investigated to meet therapeutic needs and optimize the release pattern of the drug. Being a natural polyelectrolyte, hyaluronic acid (HA) is excellent potential to generate new opportunities for electro-responsive drug carrier applications. In the current study, HA-based electroconductive hydrogel was developed as a novel smart drug carrier for anti-inflammatory drug release by the combination of in-situ and post polymerization mechanisms. HA was modified through methacrylation reaction to introduce photocrosslinkable groups into its structure and then reduced graphene oxide (rGO) was encapsulated into methacrylated HA (HA/MA) hydrogel by using the photopolymerization technique. In the post polymerization process, polyaniline (PANI) was incorporated/loaded into HA/MA-rGO polymeric network produced in previous step. The produced HA/MA-rGO-PANI hydrogel exhibited sufficient electrical conductivity providing the desirable electro-responsive ability for Ibuprofen (IBU) release. Furthermore, it has superior mechanical performance compared to pure (HA/MA) and rGO containing (HA/MA-rGO) hydrogels. IBU release from the hydrogel was successfully triggered by electrical stimulation and the cumulative drug release also enhanced by increasing of the applied voltage. These results highlighted that the novel HA/MA-rGO-PANI hydrogel could be a promising candidate for electrical-stimulated anti-inflammatory release systems in neural implant applications.

A Comprehensive Compilation of Graphene/Fullerene Polymer Nanocomposites for Electrochemical Energy Storage

Electricity consumption is an integral part of life on earth. Energy generation has become a critical topic, addressing the need to fuel the energy demands of consumers. Energy storage is an offshoot of the mainstream process, which is now becoming a prime topic of research and development. Electrochemical energy storage is an attractive option, serving its purpose through fuel cells, batteries and supercapacitors manipulating the properties of various materials, nanomaterials and polymer substrates. The following review presents a comprehensive report on the use of carbon-based polymer nanocomposites, specifically graphene and fullerene-based polymer nanocomposites, towards electrochemical energy storage. The achievements in these areas, and the types of polymer nanocomposites used are listed. The areas that lack of clarity and have a dearth of information are highlighted. Directions for future research are presented and recommendations for fully utilizing the benefits of the graphene/fullerene polymer nanocomposite system are proposed.

Nitrogen-Doped Porous Core-Sheath Graphene Fiber-Shaped Supercapacitors

In this study, a strategy to fabricate nitrogen-doped porous core-sheath graphene fibers with the incorporation of polypyrrole-induced nitrogen doping and graphene oxide for porous architecture in sheath is reported. Polypyrrole/graphene oxide were introduced onto wet-spun graphene oxide fibers by dip-coating. Nitrogen-doped core-sheath graphene-based fibers (NSG@GFs) were obtained with subsequently thermally carbonized polypyrrole/small-sized graphene oxide and graphene oxide fiber slurry (PPY/SGO@GOF). Both nitrogen doping and small-sized graphene sheets can improve the utilization of graphene layers in graphene-based fiber electrode by preventing stacking of the graphene sheets. Enhanced electrochemical performance is achieved due to the introduced pseudo-capacitance and enhanced electrical double-layered capacitance. The specific capacitance (38.3 mF cm⁻²) of NSG@GF is 2.6 times of that of pure graphene fiber. The energy density of NSG@GF reaches 3.40 μWh cm⁻² after nitrogen doping, which is 2.59 times of that of as-prepared one. Moreover, Nitrogen-doped graphene fiber-based supercapacitor (NSG@GF FSSC) exhibits good conductivity (155 S cm⁻¹) and cycle stability (98.2% capacitance retention after 5000 cycles at 0.1 mA cm⁻²).

An Update on Graphene Oxide: Applications and Toxicity

Graphene oxide (GO) has attracted much attention in the past few years because of its interesting and promising electrical, thermal, mechanical, and structural properties. These properties can be altered, as GO can be readily functionalized. Brodie synthesized the GO in 1859 by reacting graphite with KClO3 in the presence of fuming HNO3; the reaction took 3-4 days to complete at 333 K. Since then, various schemes have been developed to reduce the reaction time, increase the yield, and minimize the release of toxic byproducts (NO2 and N2O4). The modified Hummers method has been widely accepted to produce GO in bulk. Due to its versatile characteristics, GO has a wide range of applications in different fields like tissue engineering, photocatalysis, catalysis, and biomedical applications. Its porous structure is considered appropriate for tissue and organ regeneration. Various branches of tissue engineering are being extensively explored, such as bone, neural, dentistry, cartilage, and skin tissue engineering. The band gap of GO can be easily tuned, and therefore it has a wide range of photocatalytic applications as well: the degradation of organic contaminants, hydrogen generation, and CO2 reduction, etc. GO could be a potential nanocarrier in drug delivery systems, gene delivery, biological sensing, and antibacterial nanocomposites due to its large surface area and high density, as it is highly functionalized with oxygen-containing functional groups. GO or its composites are found to be toxic to various biological species and as also discussed in this review. It has been observed that superoxide dismutase (SOD) and reactive oxygen species (ROS) levels gradually increase over a period after GO is introduced in the biological systems. Hence, GO at specific concentrations is toxic for various species like earthworms, Chironomus riparius, Zebrafish, etc.

Two-dimensional layered carbon-based catalytic ozonation for water purification: Rational design of catalysts and an in-depth understanding of the interfacial reaction mechanism

This review renewed insight into the existing complex and contradictory mechanisms of catalytic ozonation by two-dimensional layered carbon-based materials (2D-LCMs) for degradation toxic refractory organics in aqueous solution. Migration and capture of active electrons are central to catalytic ozonation reactions, which was not studied or reviewed more clearly. Based on this perspective, the catalytic ozonation potential of 2D-LCMs synthesized by numerous methods is firstly contrasted to guide the design of subsequent carbon based-catalysts, and not limited to 2D-LCMs. Matching ROS to active sites is a key step in understanding the catalytic mechanism. The structure-activity relationships between reported numerous active sites and ROS evolution is then constructed. Result showed that OH could be produced by -OH, -C=O, -COOH groups, defective sites, immobilized metal atoms, doped heteroatoms and photo-induced electrons; and O₂^- could be produced by -OH groups and sp²-bonded carbon. The normalized model further be used to visually compare the contribution degree of various regulatory methods to performance improvement. More importantly, this review calls for 2D-LCMs-based catalytic ozonation to be studied without circumventing the issue of structural stability, which would lead to many proposals of catalysts and its involved catalytic reaction mechanism being meaningless.

High Performance of Functionalized Graphene Hydrogels Using Ethylenediamine for Supercapacitor Applications

High-performance supercapacitor (SC) electrodes typically require excellent rate capabilities, long cycle life, and high energy densities. In this work, ethylenediamine (EDA) functionalized graphene hydrogels (FGHs) with a high capacitor performance were prepared from graphene oxide (GO) dispersions using a two-step hydrothermal method. In addition, we used a very small amount of EDA to achieve the partial reduction and functional modification of GO, and the synthesized FGH-4 binder-free electrodes exhibited a high specific capacitance of −240 F/g at 1 A/g. We also successfully fabricated a symmetric SC device based on the FGH-4 electrode, with a wide voltage window of 3.0 V. More importantly, the as-assembled symmetric SC delivered a high specific energy of 39 Wh/kg at a specific power of 749 W/kg, while still maintaining its superior cycle life (retaining 88.09% of its initial capacitance after 10,000 cycles).

Heat- and freeze-tolerant organohydrogel with enhanced ionic conductivity over a wide temperature range for highly mechanoresponsive smart paint

Binary solvent-based fabrication permits the conductive organohydrogel to function well at low-temperature environments. However, the deep cryogenic and high temperatures are still threatening the performance of conductive organohydrogels in the application of stretchable electronics, biosensors, and intelligent coatings. Here, a radically new method is developed to introduce propylene and carbonate cellulose nanofibrils into freeze tolerance polymer matrix, and fabricate an antifreezing/antiheating organohydrogel integrated a high mechanical strength (1.6 MPa) and high level of ionic conductivity (4.2 S cm^-1) over a wide temperature range (-40 to 100 °C). In this designed system, the propylene carbonate with low freezing point and high boiling point was shown to enhance antifreezing (-40 °C) and antiheating (100 °C) performance of organohydrogel. Furthermore, negative charge-rich cellulose nanofibrils (CNFs) were served as an ion transport channel and nanoreinforcements to boost the conductive and mechanical properties of the organohydrogel. In particular, Molecular Dynamics (MD) simulations reveal that propylene carbonate with high dielectric constant is capable of generating ion migration-facilitated effects, enabling the high ionic conductivity of organohydrogel. Tapping into these attributes, potential applications in mechanoresponsive smart coating have been demonstrated utilizing the appealing organohydrogel as a paint, rendering unprecedented protection and monitoring performance.

Soft Actuators Based On Carbon Nanomaterials

Inspired by the sophisticated design of biological systems, interest in soft intelligent actuators has increased significantly in recent years, providing attractive strategies for the design of elaborate soft mechanical systems. For the construction of those soft actuators, carbon nanomaterials were extensively and successfully explored for the properties of highly conductive, electrothermal, and photothermal conversion. This review aims to trace the recent achievements for the material and structural design as well as the general mechanisms of the soft actuators, paying particular attention to the contribution of carbon nanomaterials resulted from their diversified interplaying properties, which realized the flexible and dexterous deformation responding to various environmental stimuli, including light, electricity and humidity. The properties and mechanisms of soft actuators are summarized and the potential for future applications and research are presented.

Design of a Superhydrophobic Strain Sensor with a Multilayer Structure for Human Motion Monitoring

A flexible strain sensor is of significant importance in wearable electronics since it can help monitor the physical signals from the human body. Among various strain sensors, the polyurethane (PU)-based ones have received widespread attention owing to their excellent toughness, large working range, and nice gas permeability. However, the hydrophobicity of these sensors is not good enough, which may affect their use life and sensitivity. In this work, a high-performance strain sensor composed of PU, reduced graphene oxide (rGO), polydopamine (PDA), and 1H,1H,2H,2H-perfluorodecane-thiol (PFDT) was designed and prepared. The results revealed that this PU/rGO/PDA/PFDT device possessed good superhydrophobicity with a water contact angle of 153.3°, a wide working strain range of 590%, and an outstanding gauge factor as high as 221 simultaneously. Because of these above advantages, the sensor worked effectively in detecting both subtle and large human movements (such as joint motion, finger motion, and vocal cord vibration) even in a high humidity environment. This strain sensor with high sensitivity, wide working range, and suitable modulus may have great potential in the field of flexible and wearable electronics in the near future.

Graphene Synthesis: Method, Exfoliation Mechanism and Large-Scale Production

Graphene is a unique attractive material owing to its characteristic structure and excellent properties. To improve the preparation efficiency of graphene, reduce defects and costs, and meet the growing market demand, it is crucial to explore the improved and innovative production methods and process for graphene. This review summarizes recent advanced graphene synthesis methods including “bottom-up” and “top-down” processes, and their influence on the structure, cost, and preparation efficiency of graphene, as well as its peeling mechanism. The viability and practicality of preparing graphene using polymers peeling flake graphite or graphite filling polymer was discussed. Based on the comparative study, it is potential to mass produce graphene with large size and high quality using the viscoelasticity of polymers and their affinity to the graphite surface.

Graphene Family Nanomaterials (GFN)-TiO2 for the Photocatalytic Removal of Water and Air Pollutants: Synthesis, Characterization, and Applications

Given the industrial revolutions and resource scarcity, the development of green technologies which aims to conserve resources and reduce the negative impacts of technology on the environment has become a critical issue of concern. One example is heterogeneous photocatalytic degradation. Titanium dioxide (TiO2) has been intensively researched given its low toxicity and photocatalytic effects under ultraviolet (UV) light irradiation. The advantages conferred by the physical and electrochemical properties of graphene family nanomaterials (GFN) have contributed to the combination of GFN and TiO2 as well as the current variety of GFN-TiO2 catalysts that have exhibited improved characteristics such as greater electron transfer and narrower bandgaps for more potential applications, including those under visible light irradiation. In this review, points of view on the intrinsic properties of TiO2, GFNs (pristine graphene, graphene oxide (GO), reduced GO, and graphene quantum dots (GQDs)), and GFN-TiO2 are presented. This review also explains practical synthesis techniques along with perspective characteristics of these TiO2- and/or graphene-based materials. The enhancement of the photocatalytic activity by using GFN-TiO2 and its improved photocatalytic reactions for the treatment of organic, inorganic, and biological pollutants in water and air phases are reported. It is expected that this review can provide insights into the key to optimizing the photocatalytic activity of GFN-TiO2 and possible directions for future development in these fields.

Features and Consequences of Isopropanol Burning off PTFE-rGO Aerogels

An improved procedure for the preparation of aerogel granules of polytetrafluoroethylene-graphene oxide (PTFE-GO) with a composition of 50:50 (in wt %) and a specific density of 35 ± 2 mg/cm³ is described. The technique practically excludes the granule cracking. The specific density of the pellets after reduction using hydrazine vapor and annealing at 370 °C decreased to 29 ± 2 mg/cm³. The PTFE-reduced GO (rGO) pellets obtained were tested as a recyclable sorbent for isopropyl alcohol (IPA) in sorption/combustion cycles. It has been shown that the aerogel sorption capacity for IPA increases from 35.6 to 39.3 g/g as a result of alcohol burning off. During the combustion of IPA, the temperature of an individual pellet can exceed 300 °C. When several contingent pellets are burned, the temperature of their heating increases. The fine-pored structure of the near-surface layer of the granule is destroyed during the alcohol burning, the internal structure with larger pores is exposed, and the relative proportion of PTFE on the surface of the granules decreases. It was also shown that the specific surface area of PTFE-rGO increases from 26 to 49 m²/g during cycling.

Controlling pore structure and conductivity in graphene nanosheet films through partial thermal exfoliation

Thermal exfoliation is an efficient and scalable method for the production of graphene nanosheets or nanoplatelets, which are typically re-assembled or blended to form new macroscopic "graphene-based materials". Thermal exfoliation can be applied to these macroscopic graphene-based materials after casting to create internal porosity, but this process variant has not been widely studied, and can easily lead to destruction of the physical form of the original cast body. Here we explore how the partial thermal exfoliation of graphene oxide (GO) multilayer nanosheet films can be used to control pore structure and electrical conductivity of planar, textured, and confined GO films. The GO films are shown to exfoliate explosively when the instrument-set heating rates are 100 K/min and above leading to complete destruction of the film geometry. Textured films with engineered micro-wrinkling and crumpling show similar thermal behavior to planar films. Here, we also demonstrate a novel method to produce fairly large size intact rGO films of high electrical conductivity and microporosity based on confinement. Sandwiching GO precursor films between inert plates during partial exfoliation at 250°C produces high conductivity and porosity material in the form of a flexible film that preserves the macroscopic structure of the original cast body.

Facile synthetic route to exfoliate high quality and super-large lateral size graphene-based sheets and their applications in SERS and CO2 gas sensing

A combination of low-cost synthetic route and simplified exfoliation technique to develop high-quality graphene-based sheets with very large lateral dimensions, which are viable to scale up, remains a challenging problem. Herein, super-large graphene oxide (GO) sheets with lateral size up to 104 μm with a surface area of 6831 μm2 have been developed based on a simple approach using mild heating conditions, and subsequent deoxygenation yields reduced graphene oxide (rGO) sheets. With the decrease in number of layers (<10, <5, bi-layer and mono-layer) in GO, the Raman intensity ratio, I D/I G value increases systematically from 0.73 to 0.97. The efficacy of reducing oxygen-containing functional groups from GO to rGO is confirmed from Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV-visible absorption spectroscopy, photoluminescence, and thermogravimetric analysis. Current-voltage measurements revealed substantial improvement of current by three orders of magnitude upon reduction of GO to rGO, which is consistent with the significant decrease in charge transfer resistance in rGO, as revealed from the electrochemical impedance spectra. The large-area GO and rGO sheets when applied in surface-enhanced Raman scattering (SERS) exhibited a large enhancement factor of 104 and high detection capability down to a concentration of 10 nM for Rhodamine B. Furthermore, the rGO incorporated hybrid rGO-SnO2 demonstrated ∼50% improvement in sensitivity for CO2 gas sensing as compared to the commercial SnO2 based gas sensor. The higher sensitivity in the rGO case is ascribed to its high surface area, as revealed from the BET analysis. Therefore, the present simplified and economical approach of large-area graphene oxide could potentially open up a new strategy for industrial-scale production in the future.

Aggregation-induced negative differential resistance in graphene oxide quantum dots

Negative differential resistance (NDR) devices have attracted considerable interest due to their potential applications in switches, memory devices, and analog-to-digital converters. Modulation of the NDR is an essential issue for the development of NDR-based devices. In this study, we successfully synthesized graphene oxide quantum dots (GOQDs) using graphene oxide, cysteine, and H2O2. The current-voltage characteristics of the GOQDs exhibit a clear NDR in the ambient environment at room temperature. A peak-to-valley ratio as high as 4.7 has been achieved under an applied voltage sweep from -6 to 6 V. The behavior of the NDR and its corresponding peak-to-valley ratio can be controlled by adjusting the range of applied voltages, air pressure, and relative humidity. Also, the NDR is sensitive to the the concentration of H2O2 added in the synthesis. The charge carrier injection through the trapping states, induced by the GOQD aggregation, could be responsible for the NDR behavior in GOQDs.

The effect of graphene oxide cross-linking and ionization on the crystallization behavior, and the electrical and thermal conductivity of a nylon-66 nanocomposite

A GO–PPD/NAAK/PA-66 composite with excellent electrical and thermal conductivity was obtained through a step-by-step chemical surface modification method involving GO.

Carbon Black and Reduced Graphene Oxide Nanocomposite for Binder-Free Supercapacitors with Reduced Graphene Oxide Paper as the Current Collector

Reduced graphene oxide (rGO) is an ideal candidate for the improvement of supercapacitor (SC) performances due to its industrial-ready manufacturing process and ease of processing. In this work, rGO was used as an active binder for the manufacture of carbon black (CB) and rGO-based SCs. Being able to form a stable suspension in water, graphene oxide (GO) was initially exploited as a dispersing agent to fabricate a homogeneous slurry with CB having exclusively water as a low-cost and environment-friendly solvent. After casting on a suitable substrate, the material was subjected to thermal treatment allowing the reduction of GO to rGO, which was successively confirmed by chemical-physical analysis. An innovative current collector, consisting of high-quality rGO paper, was also proposed ensuring an improved adhesion between the active material and the substrate and a reduction of the whole weight with respect to devices fabricated using common metallic current collectors. Due to the interesting electrochemical performances, with a high specific power of 32.1 kW kg^-1 and a corresponding specific energy of 8.8 Wh kg^-1 at a current of 1 A g^-1, and the improved manufacturing process, the described "all-graphene-based" device represents a valuable candidate for the future of SCs.

Swelling properties of graphite oxides and graphene oxide multilayered materials

Graphite oxide (GtO) and graphene oxide (GO) multilayered laminates are hydrophilic materials easily intercalated by water and other polar solvents. By definition, an increase in the volume of a material connected to the uptake of a liquid or vapour is named swelling. Swelling is a property which defines graphite oxides and graphene oxides. Less oxidized materials not capable of swelling should be named oxidized graphene. The infinite swelling of graphite oxide yields graphene oxide in aqueous dispersions. Graphene oxide sheets dispersed in a polar solvent can be re-assembled into multilayered structures and named depending on applications as films, papers or membranes. The multilayered GO materials exhibit swelling properties which are mostly similar to those of graphite oxides but not identical and in some cases surprisingly different. Swelling is a key property of GO materials in all applications which involve the sorption of water/solvents from vapours, immersion of GO into liquid water/solvents and solution based chemical reactions. These applications include sensors, sorption/removal of pollutants from waste waters, separation of liquid and gas mixtures, nanofiltration, water desalination, water-permeable protective coatings, etc. Swelling defines the distance between graphene oxide sheets in solution-immersed GO materials and the possibility for penetration of ions and molecules inside of interlayers. A high sorption capacity of GO towards many molecules and cations is defined by swelling which makes the very high surface area of GO accessible. GtO and GO swelling is a surprisingly complex phenomenon which is manifested in a variety of different ways. Swelling is strongly different for materials produced using the most common Brodie and Hummers oxidation procedures; it depends on the degree of oxidation, ad temperature and pressure conditions. The value of the GO interlayer distance is especially important in membrane applications. Diffusion of solvent molecules and ions is defined by the size of "permeation channels" provided by the swelled GO structure. According to extensive studies performed over the last decade the exact value of the inter-layer distance in swelled GO depends on the nature of solvent, temperature and pressure conditions, and the pH and concentration of solutions and exhibits pronounced aging effects. This review provides insight into the fundamental swelling properties of multilayered GO and demonstrates links to advanced applications of these materials.

Rheological and Morphological Properties of Non-Covalently Functionalized Graphene-Based Structural Epoxy Resins with Intrinsic Electrical Conductivity and Thermal Stability

In this paper, a non-covalent π–π interaction between graphene nanoparticles (G) and a pyrene-based molecule (py) has been successfully accomplished to give the functionalized nanofillers (G-py). The proposed modification has proven to be a winning solution aimed at safeguarding the graphene’s notable electronic properties, while promoting a more effective nanofiller dispersion attributable to a decrease in viscosity with consequent improvement of the rheological properties of the formulated nanocomposites filled with G-py. The electrical current maps of the G-py based epoxy composites, loaded with filler weight percentages both above and below the electric percolation threshold (EPT), were obtained by tunneling atomic force microscopy (TUNA) technique. The possibility to detect low currents also for the sample at lower concentration (0.1 wt%) confirms the good electrical performance of the nanocomposites and, consequently, the successful performed functionalization. The non-covalent modification significantly improves the thermal stability of the unfunctionalized G of about 70 °C, thus causing an increase in the composite oxidative thermostability since the evolution of CO₂ shifts to higher values. Moreover, non-covalent functionalization proved to be impactful in imparting an overall enhancement of the nanocomposite mechanical properties due to good bonding between graphene and epoxy matrix, also showing a greater roughness which is decisive in influencing the interface adhesion efficiency.

A Holey Graphene Additive for Boosting Performance of Electric Double-Layer Supercapacitors

We demonstrate a facile and effective method, which is low-cost and easy to scale up, to fabricate holey graphene nanosheets (HGNSs) via ultrafast heating during synthesis. Various heating temperatures are used to modify the material properties of HGNSs. First, we use HGNSs as the electrode active materials for electric double-layer capacitors (EDLCs). A synthesis temperature of 900 °C seems to be optimal, i.e., the conductivity and adhesion of HGNSs reach a compromise. The gravimetric capacitance of this HGNS sample (namely HGNS-900) is 56 F·g⁻¹. However, the volumetric capacitance is low, which hinders its practical application. Secondly, we incorporate activated carbon (AC) into HGNS-900 to make a composite EDLC material. The effect of the AC:HGNS-900 ratio on the capacitance, high-rate performance, and cycling stability are systematically investigated. With a proper amount of HGNS-900, both the electrode gravimetric and volumetric capacitances at high rate charging/discharging are clearly higher than those of plain AC electrodes. The AC/HGNS-900 composite is a promising electrode material for nonaqueous EDLC applications.

0D-1D Hybrid Silicon Nanocomposite as Lithium-Ion Batteries Anodes

Lithium ion batteries (LIBs) are the enabling technology for many of the societal changes that are expected to happen in the following years. Among all the challenges for which LIBs are the key, vehicle electrification is one of the most crucial. Current battery materials cannot provide the required power densities for such applications and therefore, it makes necessary to develop new materials. Silicon is one of the proposed as next generation battery materials, but still there are challenges to overcome. Poor capacity retention is one of those drawbacks, and because it is tightly related with its high capacity, it is a problem rather difficult to address with common and scalable fabrication processes. Here we show that combining 0D and 1D silicon nanostructures, high capacity and stability can be achieved even using standard electrode fabrication processes. Capacities as high as 1200 mAh/g for more than 500 cycles at high current densities (2 A/g) were achieved with the produced hybrid 0D/1D electrodes. In this research, it was shown that while 0D nanostructures provide good strain relaxation capabilities, 1D nanomaterials contribute with enhanced cohesion and conductive matrix integrity.

Synthesis of Few-Layer Graphene Sheets from Waste Expanded Polystyrene by Dense Fe Cluster Catalysis

Graphene sheets have a vast number of potential applications due to their excellent properties. However, poor quality and harsh preparation conditions restrict their application. Here, few-layer graphene (FLG) sheet powder with high quality has been synthesized from waste expanded polystyrene (EPS) at low temperature by dense Fe cluster catalysis. The micron-sized FLG sheets comprising about three layers show high crystallinity and good electrical conductivity that are comparable to those of the shear-exfoliated graphene nanoplatelets. More than 70% carbon yield of FLG sheets from cheap EPS and their safe, controllable synthesis conditions make it easy to expand production. The catalytic formation mechanism of FLG sheets is studied.

Direct Reduction of Graphene Oxide/Nanofibrillated Cellulose Composite Film and its Electrical Conductivity Research

With the rapid development of wearable and portable electronic devices, it is increasingly important to develop conductive paper-like films (CPFs) with the characteristics of light, thin and self-supporting. In this paper, nanofibrillated cellulose (NFC) was used as reinforcing phase of film-forming to combine with graphene oxide (GO). Then graphene-based CPFs were prepared by directly reducing the GO/NFC composite film without any additional adhesives, which effectively avoided the difficulties of dispersion and combination with other materials caused by direct using of high content graphene. Meanwhile, three representative reduction methods for direct reduction of GO/NFC composite films were also compared. The results show that 450 °C thermal reduction and hydroiodic acid reduction were more effective than ascorbic acid reduction. On this basis, hydroiodic acid reduction and thermal reduction were used to discuss the effect of NFC addition to the conductivity of the film. This occured when increasing the content of NFC from 10% to 50%, the electrical conductivity of the composite film by hydroiodic acid reduction decreased from 153.8 S/m to 22.2 S/m. While the conductivity of composite film increased first and then decreased after thermal reduction both at 450 °C and 550 °C. What's more, when NFC content was about 16.6% the electrical conductivity reached a high level which was 86.21 S/m and 168.9 S/m, respectively. This study provides a groundwork for the further development of graphene-based CPFs with low square resistance and high conductivity in large-scale preparation.

Green Reduction of Graphene Oxide using Kaffir Lime Peel Extract (Citrus hystrix) and Its Application as Adsorbent for Methylene Blue

Green reduction of graphene oxide (GO) by phytochemicals was explored using the aqueous extract of kaffir lime peels. The research methods included preparation of extracts, preparation of GO, preparation and characterization of reduced-GO (RGO) using Fourier Transform Infrared (FTIR), X-ray diffraction (XRD), and UV-Vis spectroscopy, as well as methylene blue (MB) adsorption test using RGO. The RGO characterization showed that GO was successfully reduced by a C=C group restoration. The MB adsorption kinetics profile in RGO is more suitable for the pseudo-second-order model, whereas for the adsorption isotherm it is more suitable for the Langmuir model with a maximum adsorption capacity (q_max) of 276.06 mg/g at room temperature. The best ratio of GO: kaffir lime peel extract used to prepare RGO was at a ratio of 1: 2. Based on the ΔG, ΔH, and ΔS values, the adsorption of RGO-MB was defined as spontaneous and endothermic process. The results promise the potential application of RGO derived via green route to remove cationic dye in wastewater.

Nanoscale chemical mapping of oxygen functional groups on graphene oxide using atomic force microscopy-coupled infrared spectroscopy

The unambiguous determination of the chemical functionality over graphene oxide (GO) is important to unleash its potential applications. However, the mapping of oxygen functionalities distribution remains to be unequivocally determined because of highly inhomogeneous non-stoichiometric structures and ultra-thin layers of GO. In this study, we report an experimental observation of the spatial distribution of oxygen functional groups on monolayer and multilayer GO using AFM-IR, atomic force microscopy coupled with infrared spectroscopy. Overcoming conventional IR diffraction limit for several micrometers, the novel AFM-IR reaches high spatial resolution ∼20 nm and could detect IR absorption on ∼1 nm thickness of monolayer GO. With nanoscale chemical mapping, the distribution of different oxygen functional groups is distinguished with AFM-IR over the GO surface. It allows us to observe that these oxygen functional groups prefer to sit on the fold areas, in discrete domains and on the edges of GO, which gave more insights into its chemical nature. The determination of the position of functional groups through precise imaging contributes to our understanding of GO structure-properties relations and paves the way for targeted tethering of polymers, biomaterials, and other nanostructures.

Self-Assembled Functionally Graded Graphene Films with Tunable Compositions and Their Applications in Transient Electronics and Actuation

The facile fabrication of functionally graded all-graphene films using a single-step casting process is reported. The films consist of a self-assembled graphene oxide (GO) precursor that can be reduced to different levels on an active metal substrate. Control of processing conditions such as the underlying substrate metal and the film-drying environment results in an ability to tailor the internal architecture of the films as well as to functionally grade the reduction of GO. A gradient arrangement within each film, where one side is electrically conductive reduced GO (rGO) and the other side is insulating GO, was confirmed by scanning electron microscopy, Raman, X-ray diffraction, Fourier transform infrared, and X-ray photoelectron spectroscopy characterization studies. All-graphene-based freestanding films with selectively reduced GO were used in transient electronic applications such as flexible circuitry and RFID tag antennas, where their decommissioning is easily achieved by capitalizing on GO's ability to readily dissociate and create a stable suspension in water. Furthermore, the functionally graded structure was found to exhibit differential swelling behavior, and its potential applications in graphene-based actuators are outlined.

Ion chromatographic determination of hydrazine in excess ammonia for monitoring graphene oxide reduction reaction

A new ion chromatography method has been developed to study graphene oxide (GO) reduction by monitoring hydrazine concentration in the GO suspension. The method is based on ion chromatographic separation of hydrazine (from excess ammonia) and its selective determination by electrochemical detection. The developed analytical protocol overcame the significant practical challenges of atmospheric hydrazine oxidation and minimised the matrix interference in both separation and detection which result from the excess of ammonium with respect to hydrazine (up to 5.8 × 10⁴ times) in GO reduction experiments. Chromatographic separations were achieved using a high capacity IonPac CS16 cation-exchange column with a 30 mM methanesulfonic acid (MSA) eluent, within an analysis time of less than 20 min. Detection of hydrazine as hydrazinium ion using electrochemical detector was linear between 10 μM and 4 mM, with LOD and LOQ values of 3 μM and 10 μM, respectively. Standard additions confirmed 103 ± 0.8% recovery. The developed method was successfully used to determine the point of complete GO reduction with hydrazine. Reaction curves for GO reduction generated using the method were compared to results from Fourier-transform infrared spectroscopy and Raman spectroscopy to verify the utility of the approach.

Advances in Microwave-Assisted Production of Reduced Graphene Oxide

Efficient reduction of graphene oxide to obtain high-quality graphene nanosheets is desirable for energy storage, catalysis, electronics and environmental remediation. In this brief review, we mainly focus on the microwave-assisted production of reduced graphene oxide in three categories: (1) microwave-assisted chemical reduction of graphene oxide; (2) microwave-assisted thermal reduction of graphene oxide; (3) microwave-assisted simultaneous thermal exfoliation & thermal reduction of graphite oxide. We also summarize common techniques for characterizing reduction efficiency and quality of as-obtained rGO.

Laser fabrication of graphene-based electrothermal actuators enabling predicable deformation

Electrothermal actuators (ETAs) that can convert electric energy into mechanical works have been extensively studied for their great potential in artificial muscles and robotics. However, the production of ETAs that enable complex and predictable deformation is still challenging. In this Letter, an ETA based on reduced graphene oxide (RGO) and polyethylene (PE) bimorph is developed through a facile laser-scribing method. Since the laser-scribing technology permits flexible patterning, conductive RGO electrodes with complex circuit patterns can be readily produced on a thermally active PE film, forming an ETA capable of fast and reversible deformation. In addition, the laser-scribed ETA demonstrated orientation-defined bending performance, enabling more sophisticated deformation control. The laser scribing of graphene oxide has opened up a new way to produce ETAs towards cutting-edge applications such as soft robotics and intelligent systems.