Tribology Improvement of Graphene-Oxide/Polyamide-Imide Composite Coating: Experiment and Simulation Investigation

The hydrothermal process was used to prepare Mn3O4/x%GO nanocomposites (NC’s) having different ratios of the Mn3O4 nanoparticles (NP’s) on the surface of graphene oxide (GO) sheet. SEM image showed that the Mn3O4 NP’s were distributed over the surface of GO sheet. HRTEM images exhibited the lattice fringe arising from the (101) plane of the Mn3O4 NP’s having the interplanar d-spacing of 0.49 nm decorating on the surface of GO. The electronic absorption spectra of Mn3O4/x%GO NC’s also show broad bands from 250 to 550 nm. These bands arise from the d–d crystal field transitions of the tetrahedral Mn3+ species and indicate a distortion in the crystal structure. Photo-catalytic activity of spinel ferrite Mn3O4 NP’s by themselves was low but photo-catalytic activity is enhanced when the NP’s are decorating the GO sheet. Moreover, the Mn3O4/10%GO NC’s showed the best photo-catalytic activity. This result comes from the formation of Mn–O–C bond that confirm by FT-IR. This bond would facilitate the transfer of the photoelectrons from the surfaces of the NP’s to the GO sheets. PL emission which is in the violet–red luminescent region shows the creation of defects in the fabricated Mn3O4 NP’s nanostructures. These defects create the defect states to which electrons in the VB can be excited to when the CB. The best degradation efficiency was achieved by the Mn3O4 NP’s when they were used to decorate the GO sheets in the Mn3O4/10%GO NC’s solution.HighlightsLattice fringe of Mn3O4 with an interplanar d-spacing of 0.49 nm for (101) plane.Photocatalytic activity of spinel ferrite Mn3O4 nanoparticles by itself is low.Number of photoelectrons created depends on number of Mn3O4 on a given area of GOThe bonding of the Mn3O4 to the GO sheet would be though a Mn–O–C junction.The degradation processes were accelerated by Mn3O4/10%GO nanocompositesGraphic abstract

Graphene oxide (GO) is a promising precursor to produce graphene-family nanomaterials for various applications. Their potential health and environmental impacts need a good understanding of their cellular interactions. Many factors may influence their biological interactions with cells, and the lateral dimension of GO sheets is one of the most relevant material properties. In this study, a model bacterium, Escherichia coli ( E. coli ), was used to evaluate the antibacterial activity of well-dispersed GO sheets, whose lateral size differs by more than 100 times. Our results show that the antibacterial activity of GO sheets toward E. coli cells is lateral size dependent. Larger GO sheets show stronger antibacterial activity than do smaller ones, and they have different time- and concentration-dependent antibacterial activities. Large GO sheets lead to most cell loss after 1 h incubation, and their concentration strongly influences antibacterial activity at relative low concentration (<10 μg/mL). In contrast, when incubating with small GO sheets up to 4 h, the inactivation rate of E. coli cells continues increasing. The increase of small GO sheet concentration also results in persistent increases in their antibacterial activity. In this study, GO sheets with different lateral sizes are all well dispersed, and their oxidation capacity toward glutathione is similar, consistent with X-ray photoelectron spectroscopy and ultraviolet-visible absorption spectroscopy results. This suggests the lateral size-dependent antibacterial activity of GO sheets is caused by neither their aggregation states, nor oxidation capacity. Atomic force microscope analysis of GO sheets and cells shows that GO sheets interact strongly with cells. Large GO sheets more easily cover cells, and cells cannot proliferate once fully covered, resulting in the cell viability loss observed in the followed colony counting test. In contrast, small GO sheets adhere to the bacterial surfaces, which cannot effectively isolate cells from environment. This study highlights the importance of tailoring the lateral dimension of GO sheets to optimize the application potential with minimal risks for environmental health and safety.

Large-area sheets are highly desirable for fundamental research and technological applications of graphene. Here we introduce a modified chemical exfoliation technique to prepare large-area graphene oxide (GO) sheets. The maximum area of the GO sheets obtained can reach ∼40000 μm(2). We found that the GO area is strongly correlated with the C-O content of the graphite oxide, which enables the area of the synthesized GO sheets to be controlled. By simply changing oxidation conditions, GO sheets with an average area of ca. 100-300, ca. 1000-3000, and ∼7000 μm(2) were selectively synthesized. For transparent conductive film applications, thin GO films were fabricated by self-assembly on a liquid/air interface and reduced by HI acid. We found that the sheet resistance of the reduced GO (rGO) films decreases with increasing sheet area at the same transmittance because of the decrease in the number of intersheet tunneling barriers. The rGO film made from GO sheets with an average area of ∼7000 μm(2) shows a sheet resistance of 840 Ω/sq at 78% transmittance, which is much lower than that (19.1 kΩ/sq at 79% transmittance) of a rGO film made from small-area GO sheets of ca. 100-300 μm(2), and comparable to that of graphene films grown on Ni by chemical vapor deposition.

Lithium – sulfur (Li-S) battery, with highest predicted theoretical capacity of ~1675 mAh/g, is a promising technology meeting the demands of next-generation electric vehicles. Abundance of sulfur and environmental friendliness are major factors that make Li-S batteries interesting and important. However, Li-S battery is plagued by its own limitations, which include low electrical conductivity of pure sulfur cathode and loss of active material due to dissolution of intermediate polysulfides from the cathode during reduction reactions. These factors limit the performance of Li-S batteries. Developing superior cathode structures to mitigate these problems has been the major focus of research in recent times. Carbon-based materials such as fibers, nanospheres and nanotubes embedded with sulfur are being explored as cathodes for Li-S batteries. In particular, graphene, which has superior electrical conductivity and mechanical flexibility, has been studied as potential candidate for cathodes and cathode supports in Li-S batteries. However, economical synthesis of pure graphene is a major hurdle for their use in commercially-viable Li-S batteries. Graphene oxide – sulfur composite based structures are viable alternatives that could provide required electrical conductivity, based on the concentration of functional groups (epoxy and hydroxyl). Additionally, graphene oxides are believed to act like anchors to sulfur/polysulfides particles and help reduce their dissolution, leading to significantly improved performance of Li-S batteries. However, leveraging the advantages of enhanced electrical conductivity and reduced polysulfide shuttle requires tuning of the functional groups present in the graphene oxide. In an effort to determine the most favorable graphene oxide structure, we performed molecular dynamics (MD) simulations to calculate the mobility of polysulfide species in the vicinity of different graphene oxide structures with varying concentration of functional groups. Diffusion coefficients of polysulfides were calculated along the surface of these graphene oxide sheets using MD simulations. Initially graphene oxide sheets with pure epoxy and hydroxyl functional groups were simulated. Partial charge on carbon atoms in graphene oxide with epoxy and hydroxyl groups were evaluated from the optimized structures obtained using Density Functional Theory (DFT) calculations. Bond, angle and dihedral parameters along with partial charges on sulfur particles in polysulfides S8 2- were evaluated from the optimized structure using DFT. A standard electrolyte DME – DOL in 1:1 v/v ratio, modeled using OPLS force filed parameters, was used in all the MD simulations. The density of equilibrated solvent was within 5% of the experimental value. A system comprising graphene oxide solvated with electrolyte containing polysulfides was simulated at 300K. Overall charge neutrality of the system was maintained by incorporating counter ions, while using specific strategy to screen out long range electrostatic interactions between the polysulfides and counter ions. The surface diffusion coefficients of polysulfide was evaluated in the vicinity of graphene oxide and compared to that near pure graphene. Our simulations evaluated the effectiveness of various groups in reducing dissolution of polysulfides. The concentration of polysulfides in the vicinity of the graphene/graphene oxide structures tend to increase from graphene to graphene oxide with pure epoxy groups and maximizes for graphene oxide with pure hydroxyl functional groups. The relatively greater binding of polysulfides to graphene oxide structures leads to reduced surface diffusion coefficients for polysulfides on graphene oxide compared to that near graphene interfaces.

This work aimed to interrogate the role that the starting graphitic material played on the physicochemical properties of graphene oxide (GO) sheets and their impact on mammalian cell viability following exposure to those flakes. Three different GO thin sheets were synthesised from three starting graphite material: flakes (GO-f), ground (GO-g) and powder (GO-p) using a modified Hummers’ method. The synthetic yield of this methodology was found to differ according to type of starting material, with GO-p resulting in most efficient yields. Structural and morphological comparison of the three GO sheet types were carried out using transmission electron microscopy and atomic force microscopy. Optical properties were measured using UV/visible and fluorescence spectroscopy. Surface characteristics and chemistry were determined using a battery of techniques. Exposure to human cells was studied using the human A549 lung epithelial cultures. Our results revealed that all three GO samples were composed of few-layer sheets with similar physicochemical and surface characteristics. However, significant differences were observed in terms of their lateral dimensions with GO-p, prepared from graphite powder, being the largest among the GOs. No cytotoxicity was detected for any of the GO samples following exposure onto A549 cells up to 48 h. In conclusion, the form and type of the starting graphite material is shown to be an important factor that can determine the synthetic yield and the structural characteristics of the resulting GO sheets.

We present a comparative experimental and molecular dynamics (MD) simulation study to elucidate the role of oxygen functionalization on aqueous dispersibility of graphene oxide (GO). Our experimental results indicated better aqueous dispersibility of hydroxyl-rich GO due to distribution of hydroxyl groups on GO basal plane, covering more GO surface area than the edge carboxyl groups. Photoluminescence and UV absorbance results indicated that hydroxyl-rich GO consists of maximum number of well-exfoliated GO layers, which leads to the formation of more stable GO dispersions. Further, MD simulations and thermodynamic calculations clearly indicated that the potential of mean force is most repulsive for hydroxyl-modified GO sheets, in comparison to epoxy- and carboxyl-modified GO sheets. An increase in the number of hydrogen bonds between GO and water molecules was observed with increased functionalization. The present experimental study, underpinned by MD simulations, suggests that concentration, distribution, and chemical nature of individual oxygen functional groups present on GO surface determine the strength and nature of interfacial interactions between GO and water, which in turn decides the stability of aqueous GO dispersions. Thus, our results provide a mechanistic insight into the stability of GO dispersions and a guide for controlling GO aqueous dispersibility by custom functionalization.

Graphene oxide (GO), the oxidized state of graphene, has a similar chemical structure to graphene, except for the existence of many defects and oxygen-containing functional groups. GO could be photoreduced under light irradiation, and it is crucial to study and understand the changes of chemical structure and photoluminescence during this photoreduction process. Herein, by using single-molecule optical microscopy, the photoreduction of three different GO sheets were monitored in situ. The photoluminescence from highly oxidized GO sheets changed continuously under light irradiation, and many blinking spots showed up on this GO sheet surface. The appearance of these blinking events is highly related to the stacking of the GO sheets. Much less blinking could be observed on stacked GO sheets. It was found that the reduction speed of GO in water is much higher than that in air, and much less blinking could be observed when GO was reduced in water. Moreover, this is also highly related to the chemical structures of the GO sheets used for photoreduction. No blinking could be observed on less oxidized GO sheets. This work provides an effective approach for in situ study of the optical properties of GO sheets, boosting a deeper understanding of GO photoreduction. Moreover, this method could also be extended for in situ studying the optical properties of other materials.

Effects of probe and bath ultrasonic treatments on graphene oxide structure

Due to their unique surface-active functionalities, graphene and its derivatives, i.e., graphene oxide (GO) and reduced graphene oxide (rGO), have received enormous research attention in recent decades. One of the most intriguing research hot spots is the integration of GO and rGO coatings on textiles through dyeing methods, e.g., dip-pad-dry. In general, the GO sheets can quickly diffuse into the fabric matrix and deposit onto the surface of the fibers through hydrogen bonding. The GO sheets can be conformally coated on the fiber surface, forming strong adhesion as a result of the high flakiness ratio, mechanical strength, and deformability. Moreover, multiple functions with application significance, e.g., anti-bacteria, UV protection, conductivity, and wetting control, can be achieved on the GO and rGO-coated fabrics as a result of the intrinsic chemical, physical, electronic, and amphiphilic properties of GO and rGO. On the other hand, extrinsic functions, including self-cleaning, self-healing, directional water transport, and oil/water separation, can be achieved for the GO and rGO coatings by the integration of other functional materials. Therefore, multi-scale, multifunctional, smart fabrics with programmable functions and functional synergy can be achieved by the design and preparation of the hybrid GO and rGO coatings, while advanced applications, e.g., healthcare clothing, E-textiles, anti-fouling ultrafiltration membranes, can be realized. In this review, we aim to provide an in-depth overview of the existing methods for functionalizing fabrics with graphene-based coatings while the corresponding functional performance, underlying mechanisms and applications are highlighted and discussed, which may provide useful insights for the design and fabrication of functional textiles and fabrics for different applications.

Optimization for size separation of graphene oxide sheets by flow/hyperlayer field-flow fractionation

Microstructure, deposition mechanism and tribological performance of graphene oxide reinforced Fe composite coatings by electro-brush plating technique

Separation of graphene oxide by density gradient centrifugation and study on their morphology-dependent electrochemical properties

Objectives Graphene oxide (GO) membranes are promising materials in many fields such as supercapacitors1, gas and liquid separation2, sensing3, and acoustic transduction4. For these applications, a high efficiency, scalable method for preparing GO membranes without compromising mechanical properties are required.To date, the most commonly used technique to manufacture GO membranes is vacuum filtration. By filtering the GO suspension, GO sheets are stacked and assembled into membranes. However, this technique has several shortcomings. Firstly, this method cannot be used to control the ordering of GO sheets – a property that determines the microstructures and directly influences interactions between sheets. Secondly, this technique is time- and energy-consuming, and the size of the GO membranes produced is limited by the size of the filter5. New Results We report a new method for preparing large scale GO membranes with tunable microstructures and controllable mechanical properties. This method involves injecting the GO suspension through a thin nozzle into a coagulation bath for promoting the gelation of the GO into a membrane. The nozzle moves at a speed coordinated with the injection flow rate to prepare uniformly thick membranes (Fig. 1(a)). The flow rate in the nozzle induces shear stress, which in turn induces the alignment of GO sheets. Therefore, by adjusting the flow rate, the alignment of GO sheets is tunable, and the mechanical property is controllable. By using this technique, we achieved a long continuous GO membrane with high ultimate tensile strength (UTS) of 67-160 MPa at preparation speeds of up to 160 cm/min. As a comparison, the vacuum filtration method takes a day to prepare a single 47 mm diameter GO membrane with a UTS of 70-125 MPa6. Conclusions In this study, we designed a highly efficient flow injection method that allows us to prepare large-scale GO membranes with tunable microstructures rapidly. This in turn allows us to influence interactions between sheets and, most importantly, manipulate the mechanical properties. In this paper, we show the vast improvement in UTS measurements for GO membranes prepared using the flow injection method as opposed to the vacuum-filtration. This technique will enable fast and controllable production of GO membranes for various fields which call for membranes with good mechanical performance.Fig. 1. (a) Schematically drawing of the process of the injection method for GO membranes preparation; (b) UTS of GO membranes prepared under different flow rates Acknowledgments This research is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the McGill Engineering Doctoral Awards (MEDA).

Polystyrene particles “armoured” with nanosized graphene oxide (GO) sheets have been prepared by aqueous miniemulsion polymerization of styrene, exploiting the amphiphilic properties of GO in the absence of conventional surfactants. The nanoscale GO sheets were prepared from graphite nanofibers of diameter approximately 100 nm based on a novel procedure, thus effectively ensuring the absence of larger sheets. Polymerization proceeded to high conversion with minor coagulation, with final number‐average particle diameters of approximately 500 nm, but relatively broad particle size distributions. Scanning electron microscopy analysis revealed particles with a textured surface, consistent with the expected morphology. Interestingly, analysis of GO sheets recovered from the polymerization revealed that the GO sheets are partially reduced during the polymerization—approximately 50% of the initial carboxyl groups of the GO were lost, consistent with some loss in colloidal stability at high conversion. The overall approach offers a convenient and attractive synthetic route to novel graphene‐based polymeric nanostructures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Graphene oxide (GO) membranes become emerging efficient filters for molecular or ionic separation due to their well-defined two-dimensional nanochannels formed by closely spaced GO sheets and tunable physicochemical properties. The stability of GO membranes in aqueous solutions is a prerequisite for their applications. Here we show a novel and easy strategy for fabricating GO membranes with strong stability in aqueous solutions and controllable lamellar spacing by simply doping with partially reduced graphene oxide (prGO) sheets. With our prGO-doping strategy, the interlayer stabilizing force in GO membranes is enhanced due to the weakened repulsive hydration and enhanced π-π attraction between GO sheets; as a result, the fabricated GO membranes are featured with controllable lamellar spacing and extraordinary stability in water or even strong acid and base solutions as well as strong mechanical properties, which will expand the application scope of GO membranes and provide ever better performances in their applications with aqueous solution environments.