High throughput exfoliation of graphene oxide from expanded graphite with assistance of strong oxidant in modified Hummers method

Research Status and Challenges of Mechanism, Characterization, Performance Evaluation, and Type of Nano-Pour Point Depressants in Waxy Crude Oil

Nano-Pour point depressants have great potential to improve the low-temperature fluidity of waxy crude oil. This Review reviews the recent research progress of nano-pour point depressants in the field of crude oil pour point reduction. The effect and mechanism of nanocomposite pour point depressants are analyzed; the preparation, modification, and microstructure characterization of nanocomposite pour point depressants are introduced; the three main types of nano-pour point depressants, namely, silicon-based, carbon-based, and magnetic metal-based, are introduced; the results of the current research are outlined; and the challenges of the current research and possible directions of future research are also pointed out. The in-depth analysis of nano-pour point depressants and their potential to improve the low-temperature fluidity of waxy crude oil are reviewed in order to thoroughly analyze the mechanism of nano-pour point depressants and to prepare nano-pour point depressants that are more suitable for reducing crude oil coagulation.

Synthesis of Graphene-Based Nanocomposites for Environmental Remediation Applications: A Review

The term graphene was coined using the prefix "graph" taken from graphite and the suffix "-ene" for the C=C bond, by Boehm et al. in 1986. The synthesis of graphene can be done using various methods. The synthesized graphene was further oxidized to graphene oxide (GO) using different methods, to enhance its multitude of applications. Graphene oxide (GO) is the oxidized analogy of graphene, familiar as the only intermediate or precursor for obtaining the latter at a large scale. Graphene oxide has recently obtained enormous popularity in the energy, environment, sensor, and biomedical fields and has been handsomely exploited for water purification membranes. GO is a unique class of mechanically robust, ultrathin, high flux, high-selectivity, and fouling-resistant separation membranes that provide opportunities to advance water desalination technologies. The facile synthesis of GO membranes opens the doors for ideal next-generation membranes as cost-effective and sustainable alternative to long existing thin-film composite membranes for water purification applications. Many types of GO-metal oxide nanocomposites have been used to eradicate the problem of metal ions, halomethanes, other organic pollutants, and different colors from water bodies, making water fit for further use. Furthermore, to enhance the applications of GO/metal oxide nanocomposites, they were deposited on polymeric membranes for water purification due to their relatively low-cost, clear pore-forming mechanism and higher flexibility compared to inorganic membranes. Along with other applications, using these nanocomposites in the preparation of membranes not only resulted in excellent fouling resistance but also could be a possible solution to overcome the trade-off between water permeability and solute selectivity. Hence, a GO/metal oxide nanocomposite could improve overall performance, including antibacterial properties, strength, roughness, pore size, and the surface hydrophilicity of the membrane. In this review, we highlight the structure and synthesis of graphene, as well as graphene oxide, and its decoration with a polymeric membrane for further applications.

Preparation and Characterization of Graphene Oxide for Pb(II) and Zn(II) Ions Adsorption from Aqueous Solution: Experimental, Thermodynamic and Kinetic Study

A thermodynamic and kinetic study of the adsorption process of Zn (II) and Pb (II) ions from aqueous solution on the surface of graphene oxide (GO) to establish the mechanisms of adsorbate–adsorbent interaction on this surface. The effect of pH on the retention capacity was studied and adsorption isotherms were determined from aqueous solution of the ions; once the experimental data was obtained, the kinetic and thermodynamic study of the sorption process was carried out. The data were fitted to the Langmuir, Freundlich, Dubinin-Raduskevich and Temkin isotherm models. The results showed that Zn(II) and Pb(II) on the GO adsorbing surface fitted the Langmuir model with correlation coefficients (R²) of 0.996. Kinetic models studied showed that a pseudo-second-order model was followed and thermodynamically, the process was spontaneous according to the values of Gibbs free energy (ΔG^o). N₂ adsorption isotherms were determined and modeled with the NLDFT (nonlocal density functional theory) and QSDFT (quenched solid density functional theory) kernels.

Two-dimensional graphene oxide-reinforced porous biodegradable polymeric nanocomposites for bone tissue engineering

This study investigates the mechanical properties and in vitro cytotoxicity of two-dimensional (2D) graphene oxide nanoribbons and nanoplatelets (GONRs and GONPs) reinforced porous polymeric nanocomposites. Highly porous poly(propylene fumarate) (PPF) nanocomposites were prepared by dispersing 0.2 wt % single- and multiwalled SONRs (SWGONRs and MWGONRs) and GONPs. The mechanical properties of scaffolds were characterized using compression testing and in vitro cytocompatibility was assessed using QuantiFlour assay for cellularity and PrestoBlue assay for cell viability. Immunofluorescence was used to assess collagen-I expression and deposition in the extracellular matrix. Porous PPF scaffolds were used as a baseline control and porous single and multiwalled carbon nanotubes (SWCNTs and MWCNTs) reinforced nanocomposites were used as positive controls. Results show that incorporation of 2D graphene nanomaterials leads to an increase in the mechanical properties of porous PPF nanocomposites with following the trend: MWGONRs > GONPs > SWGONRs > MWCNTs > SWCNTs > PPF control. MWGONRs showed the best enhancement of compressive mechanical properties with increases of up to 26% in compressive modulus (i.e., Young's modulus), ~60% in yield strength, and ~24% in the ultimate compressive strength. Addition of 2D nanomaterials did not alter the cytocompatibility of porous PPF nanocomposites. Furthermore, PPF nanocomposites reinforced with SWGONRs, MWGONRs, and GONPs show an improvement in the adsorption of collagen-I compared to PPF baseline control. The results of this study show that 2D graphene nanomaterial reinforced porous PPF nanocomposites possess superior mechanical properties, cytocompatibility, and increased protein adsorption. The favorable cytocompatibility results opens avenues for in vivo safety and efficacy studies for bone tissue engineering applications. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1143-1153, 2019.

Structure and Optical Features of Micro/Nanosized Carbon Forms Prepared by Electrochemical Exfoliation

Micro/nanosized carbon materials were prepared by electrochemical exfoliation method in the forms of the colloids and thin films. Scanning electronic microscopy, optical and luminescent microscopy, and Raman scattering and luminescent spectroscopy were applied for characterization of materials. The wide photoluminescence band in the visible spectral region was observed for each of the samples. The shape of the photoluminescence band depends on excitation wavelength and on the size of the particles. At least two components with maxima at ~580 and ~710 nm can be distinguished in the photoluminescence spectra. The relations between the photoluminescence properties and morphology of the samples have been described and discussed.

Reduced Graphene Oxide/Alumina, A Good Accelerant for Cellulose-Based Artificial Nacre with Excellent Mechanical, Barrier, and Conductive Properties

In this article, a simple strategy was employed to fabricate bioinspired hybrid composite with carboxymethyl cellulose (CMC), graphene oxide, and reduced graphene oxide/alumina (rGO/Al) by a facile solution casting method. The tensile strength and toughness of rGO/Al-CMC-GO can reach 586.6 ± 12 MPa, 12.1 ± 0.44 MJm^-3, respectively, due to the interface strengthening of alumina, which is 1.43 and 12 times higher than steel and about 4.3 and 6.7 times that of nature nacre. The artificial nacre hybrid composite is conductive due to the introduction of rGO/Al on the surface. Interestingly this structure can also be coated on the surface of cotton thread to give the thread good mechanical performance and conductivity. Additionally, the artificial nacre has better fire shielding and gas barrier properties. The oxygen permeability (OP) for 1% rGO/Al-CMC decreased from 0.0265 to 0.003 mLμm m^-2 day^-1 kpa^-1, the water vapor permeability (WVP) decreased from 0.363 to 0.205 gmmm^-2 day^-1 kpa^-1 when the concentration increased from 1% rGO/Al to 6% rGO/Al. It is believed this work provided a simple and feasible strategy to fabricate ultrastrong and ultratough graphene-based artificial nacre multifunctional materials.

A facile way to produce epoxy nanocomposites having excellent thermal conductivity with low contents of reduced graphene oxide

A well-dispersed phase of exfoliated graphene oxide (GO) nanosheets was initially prepared in water. This was concentrated by centrifugation and was mixed with a liquid epoxy resin. The remaining water was removed by evaporation, leaving a GO dispersion in epoxy resin. A stoichiometric amount of an anhydride curing agent was added to this epoxy-resin mixture containing the GO nanosheets, which was then cured at 90 °C for 1 h followed by 160 °C for 2 h. A second thermal treatment step of 200 °C for 30 min was then undertaken to reduce further the GO in situ in the epoxy nanocomposite. An examination of the morphology of such nanocomposites containing reduced graphene oxide (rGO) revealed that a very good dispersion of rGO was achieved throughout the epoxy polymer. Various thermal and mechanical properties of the epoxy nanocomposites were measured, and the most noteworthy finding was a remarkable increase in the thermal conductivity when relatively very low contents of rGO were present. For example, a value of 0.25 W/mK was measured at 30 °C for the nanocomposite with merely 0.06 weight percentage (wt%) of rGO present, which represents an increase of ~40% compared with that of the unmodified epoxy polymer. This value represents one of the largest increases in the thermal conductivity per wt% of added rGO yet reported. These observations have been attributed to the excellent dispersion of rGO achieved in these nanocomposites made via this facile production method. The present results show that it is now possible to tune the properties of an epoxy polymer with a simple and viable method of GO addition.

Bio-Based Artificial Nacre with Excellent Mechanical and Barrier Properties Realized by a Facile In Situ Reduction and Cross-Linking Reaction

Demands for high strength integrated materials have substantially increased across various kinds of industries. Inspired by the relationship of excellent integration of mechanical properties and hierarchical nano/microscale structure of the natural nacre, a simple and facile method to fabricate high strength integrated artificial nacre based on sodium carboxymethylcellulose (CMC) and borate cross-linked graphene oxide (GO) sheets has been developed. The tensile strength and toughness of cellulose-based hybrid material reached 480.5 ± 13.1 MPa and 11.8 ± 0.4 MJm^-3 by a facile in situ reduction and cross-linking reaction between CMC and GO (0.7%), which are 3.55 and 6.55 times that of natural nacre. This hybrid film exhibits better thermal stability and flame retardancy. More interestingly, the hybrid material showed good water stability compared to that in the original water-soluble CMC. This type of hybrid has great potential applications in aerospace, artificial muscle, and tissue engineering.

A Review on Graphene-Based Gas/Vapor Sensors with Unique Properties and Potential Applications

Graphene-based gas/vapor sensors have attracted much attention in recent years due to their variety of structures, unique sensing performances, room-temperature working conditions, and tremendous application prospects, etc. Herein, we summarize recent advantages in graphene preparation, sensor construction, and sensing properties of various graphene-based gas/vapor sensors, such as NH3, NO2, H2, CO, SO2, H2S, as well as vapor of volatile organic compounds. The detection mechanisms pertaining to various gases are also discussed. In conclusion part, some existing problems which may hinder the sensor applications are presented. Several possible methods to solve these problems are proposed, for example, conceived solutions, hybrid nanostructures, multiple sensor arrays, and new recognition algorithm.

Aqueous Colloidal Stability of Graphene Oxide and Chemically Converted Graphene

Graphene oxide (GO) was prepared by modified Hummer’s method, and chemically converted graphene (CCG) was prepared by further reduction of the aqueous GO colloid. The effect of pH on particle size, particle charge, and light absorption of the aqueous colloids of GO and CCG was studied with titration against HCl or NaOH, to find the ideal characteristics for a stable dispersion. The GO colloid was stable in the pH range of 4–11, whereas the CCG colloid gained stability at a relatively narrower pH range of 7–10. Poor stability of the colloids was observed for both GO and CCG colloids at both extremes of the pH scale. Both of the colloids exhibited average size of ~1 micron in the low pH range, whereas for higher pH the size ranged between 300 and 500 nm. The UV-Vis spectra showed absorption peak at 230 nm for GO colloids that shifted to 260 nm for the CCG colloid. Such shift can be ascribed to restoring of electronic conjugation of the C=C bonds in CCG.

Peculiar reduction of graphene oxide into graphene after diffusion in exponentially growing polyelectrolyte multilayers

In the present work, in situ reduction of graphene oxide (GO) into graphene was preformed, after diffusion in exponentially growing polyelectrolyte multilayers, using sodium citrate as the reducing agent. First, the graphene oxide was obtained by treating a commercial grade of Expanded Graphite (EG). Based on XRD and Raman spectroscopy results, a complete exfoliation of graphene nanopellets down to one layer was achieved during the oxidation process. Secondly, the diffusion of GO was carried out in an exponentially growing polyelectrolyte multilayer film made from poly(diallyldimethylammonium chloride) as the polycation and from poly(acrylic acid) as the polyanion. Electrical conductivity of the GO based films was measured during the reduction process as a function of time. The conductivity reached values of the order of 10(-4) S cm(-1), whereas the pristine polyelectrolyte multilayer was highly insulating (∼10(-8) S cm(-1)). The conductivity also reached a maximal value after about 24 h of reduction and decreased for longer reduction duration. Some tentative explanations for this peculiar finding will be given.

Preparation and examination of multilayer graphene nanosheets by exfoliation of graphite in high efficient attritor mill

Graphene multilayers have been prepared by mechanical method based on milling graphite in high efficient attritor mill. The results showed that the best dispersion media is ethanol, and 10 hours of intensive milling proved to be the most efficient way to separate the graphite layers as it was shown by scanning electron microscopy and X-ray diffraction measurements.