Aqueous Graphene Research Articles

Fluid and heat transfer characteristics of aqueous graphene nanoplatelet (GNP) nanofluid in a microchannel

In the present work, thermo-physical properties of aqueous Graphene Nanoplatelet (GNP) at various mass concentrations of GNPs was experimentally measured. An experimental investigation was conducted to quantify the heat transfer coefficient, friction factor, pressure drop value, pumping power and thermo-hydraulic performance index of the nanofluid within a microchannel at various heat flux and Reynolds number. Results showed that GNP/water nanofluid can plausibly enhance the heat transfer coefficient and the Nusselt number by ~80%. In addition, a small increase in the friction factor and the pressure drop value was seen, which was attributed to the augmentation in the friction forces. The maximum increase in the pressure drop was 18.3% recorded at the highest Reynolds number and the highest mass concentration of the nanofluid. Also, despite the augmentation in the pressure drop value, the thermal performance of the system increased by 76% showing the great potential of the GNP/water nanofluid cooling and/or heating applications despite ~20% augmentation in the pumping power at Reynolds number > 1376. The enhancement in the thermal performance of the system was attributed to the thermophoresis effect, Brownian motion and the enhancement in the thermal conductivity of the nanofluid due to the presence of the GNP nanoplatelets.

Prestasi Pemangkin Pt/rGO Menggunakan Teknik Susunan Berlapis untuk Elektrod Lawan bagi Sel Suria Terpeka Pewarna

The stacked layer technique of platinum (Pt) and reduce graphene oxide (rGO) counters electrode thin film fabricated by using doctor blade method was prepared. The first layer with direct intact on fluorine doped tin oxide (FTO) glass substrate was graphene thin film and second layer on top of graphene layer was Pt thin film. X-ray diffraction (XRD) and atomic force microscopy (AFM) were performed on the thin films to determine the formation of crystallite structure and the surface roughness of the thin films, respectively. The crystallite size was determined from XRD data and it shows that Pt/rGO-10 thin film has the suitable crystal size for a better catalytic activity. As for surface roughness analysis from AFM images, Pt and Pt/rGO thin films exhibit the rougher surface compared to rGO thin film. The thin films were further analysed using field emission scanning electron microscopy (FESEM) to observe the adhesion of Pt and rGO on FTO glass substrate in nanoscale image. The catalytic activity of each thin film was measured by cyclic voltammetry (CV). The Pt/rGO counter electrode of 10μl aqueous graphene oxide denoted as Pt/rGO-10 has high catalytic activity compare to Pt. The Pt/rGO-10 recorded the highest current density at -3.075 mA/cm-2 indicate a high catalytic activity at the counter electrode. Thus, Pt/rGO-10 counter electrode thin film deemed as comparable to conventional counter electrode material which is Pt. Therefore, Pt/rGO-10 counter electrode is expected to improve the performance of Dye-sensitized solar cell.

Surface Decoration and Dispersibility of Graphene Nanoplatelets in Aqueous Surfactant Solution

Preparing highly homogeneous and stable aqueous graphene nanoplatelets (GNPs) suspension plays a significant role in practical applications. To achieve good dispersion for the GNPs, a method utilizing ultrasonic processing and dispersants was employed. In this study, four dispersants, including gum arabic (GA), polyvinylpyrrolidone (PVP), polyoxyethylene (40) nonylphenylether (Igepal, CO890) and cetyltrimethyl ammonium bromide (CTAB), were used individually and in combination at given concentrations for dispersing the GNPs in aqueous solution. The dispersing performance for GNPs suspension was characterized by standing test and UV-vis absorbance. The maximum achievable dispersing status for GNPs in aqueous solution was demonstrated by optical microscope and transmission electron microscopy (TEM) from both macro and micro aspects. Some experimental results demonstrated that the best dispersion was achieved with CTAB addition of 0.3 g/L, meaning that the optimum concentration ratio of GNPs to CTAB was 1:3. In addition, the dispersing mechanism was also assessed using Fourier transform Infrared (FTIR) spectroscopy. The CTAB molecules were adsorbed on the surface of GNPs, which prevented aggregation of GNPs by electrostatic repulsion.

Preparation of stable dispersion of graphene using copolymers: dispersity and aromaticity analysis

ABSTRACTIn this study, effectiveness of non-ionic block copolymers such as Lugalvan BNO12 and Triton X series (Triton X100 & Triton X405) has been reported for graphene dispersion in aqueous solutions. Stability of the aqueous graphene dispersions is investigated using UV–visible spectroscopy, Rheological, and Conductivity studies. Adsorption isotherms are constructed to determine the amount of polymers adsorbed on the surface of graphene by the spectroscopic analysis. Lugalvan BNO12 has been found to be adsorbed in higher amounts on the graphene surface compared to the Triton X series polymers. Thermogravimetric analysis (TGA) and Fourier Transform Infrared (FTIR) Spectroscopy investigations indicated grafting of polymers chains to the graphene surfaces. The dispersions prepared with optimum concentrations (as determined from adsorption isotherms) of polymers have shown lower viscosity and conductivity values. Lugalvan BNO12 has been found to be a better stabilizer for graphene than the Triton X series dispersants because the former contains two aromatic rings in its structure that acts as an anchoring group and helps in the stabilization of graphene dispersion in comparison to the single aromatic group in the Triton X series. The experimental results reported have shown that the aromaticity of polymeric dispersants plays significant role in the aqueous graphene dispersions. The non-ionic block copolymers that assisted dispersed graphene are potential candidates for the fabrication of various devices such as sensors, batteries, and supercapacitors applications.

Highly thermoconductive fluid with aqueous compatible graphene

Heat transfer of nanofluids is a complex phenomenon, where many factors like viscosity, surface charge, composition, and fluid stability of the dispersed nanoplatelets will affect the final thermal performance. Various layered nanoplatelets have been applied to fabricate composite nanofluids for applications on thermal management systems. Up to date, heat transfer fluids with high thermal and electrical properties have not yet been synthesized. In this study, functionalized graphene nanoplatelets (FGNs) with 1-5 atomic-thick were synthesized via a green steam exfoliated technique. Such the FGNs can generate the highly stable aqueous suspensions with high shelf life. The FGNs-water nanofluid exhibits an unprecedented thermal conductivity of 2.52 W mK−1 at 20 vol% loading of nanoplatelets, suggesting it has superb potential application on various thermal fields.

Initial Studies Directed toward the Rational Design of Aqueous Graphene Dispersants.

This study presents preliminary experimental data suggesting that sodium 4-(pyrene-1-yl)butane-1-sulfonate (PBSA), 5, an analogue of sodium pyrene-1-sulfonate (PSA), 1, enhances the stability of aqueous reduced graphene oxide (RGO) graphene dispersions. We find that RGO and exfoliated graphene dispersions prepared in the presence of 5 are approximately double the concentration of those made with commercially available PSA, 1. Quantum mechanical and molecular dynamics simulations provide key insights into the behavior of these molecules on the graphene surface. The seemingly obvious introduction of a polar sulfonate head group linked via an appropriate alkyl spacer to the aromatic core results in both more efficient binding of 5 to the graphene surface and more efficient solvation of the polar head group by bulk solvent (water). Overall, this improves the stabilization of the graphene flakes by disfavoring dissociation of the stabilizer from the graphene surface and inhibiting reaggregation by electrostatic and steric repulsion. These insights are currently the subject of further investigations in an attempt to develop a rational approach to the design of more effective dispersing agents for rGO and graphene in aqueous solution.

Stabilization of aqueous graphene dispersions utilizing a biocompatible dispersant: a molecular dynamics study.

Flavin mononucleotide sodium (FMNS) was recently reported as a highly efficient dispersant for the exfoliation of defect-free, few-layer, stabilized aqueous graphene dispersions. Most importantly, FMNS is innocuous and eco-friendly and can facilitate biomedical applications of graphene. Complementing those experimental studies, the influence of FMNS molecules on the aggregation behavior of graphene flakes in solution is investigated via all-atom molecular dynamics simulations. The stabilizing role of FMNS is demonstrated by the potential of mean force calculations for pairs of graphene flakes covered by FMNS molecules. These results indicate that the optimal amount ratio between FMNS molecules and carbon atoms in monolayer graphene is about 0.026 leading to a surface coverage of 0.34 FMNS molecules per nm2 on the graphene flakes. Overall the simulations support the high efficiency of FMNS as a surfactant compared to other surfactants.

Achieving high yield of graphene nanoplatelets in poloxamer-assisted ultrasonication of graphite in water

The role of surfactant (Pluronic® F 127) concentration on the yield and morphological characteristics of graphene nanoplatelets (GNPs) produced from the sonication of aqueous graphene suspensions is investigated in this work. By employing a wide surfactant concentration range (0.1−15 wt%) and sonication power densities up to 420 W L−1 we identify two graphene exfoliation regimes: the first occurs at low sonication power densities (<340 W L−1) and produces GNPs with sizes 200–300 nm, aspect ratios between 70 and 100, and concentrations up 1 mg mL−1. In that regime, the surfactant concentration has no effect on the exfoliation results. In the second exfoliation regime (>340 W L−1), surfactant concentrations greater than 10 wt% produce dramatic increases in GNP yields, namely up to 3.0 mg mL−1, and overall larger GNPs (350–500 nm) with smaller aspect ratios (5–60). We attribute these changes to the onset of a more energy intensive mechanism, termed cleavage. Cleavage involves the separation of graphite clusters in sub-bulk multi-layered graphene entities, as opposed to exfoliation, which involves the separation of individual or few-layer GNPs. Choosing an exfoliation regime by tuning simple process parameters enables control over the yield, size and morphology of the produced GNPs.

Light actuated swarming and breathing-like motion of graphene oxide colloidal particles

Synthetic microparticles that can be propelled under light stimulus and generate collective behaviors via interaction among these particles may lead to applications in numerous fields. Here we show that aqueous graphene oxide (GO) particles can move autonomously to a light source. These self-propelled multilayer GO particles swarm but periodically “stop to take a breath” under continuous light stimulus. UV light causes a movement velocity of approximately 25 μm s−1 and oscillating frequency of approximately two times per minute for multilayer GO particles. Light with a wavelength longer than green light causes neither locomotion nor oscillation. Only multilayer GO particles exhibit the oscillating behavior. This unusual oscillating mode suggests that multilayer GO particles may undergo non-equilibrium dynamic processes in their light-actuated collective motion.

Stabilization of aqueous graphene oxide with acetone under γ-ray/UV irradiation

Graphene oxide (GO) is a kind of water soluble two-dimensional materials containing a large amount of oxygen-containing groups which infuse GO with water solubility, biocompatibility and functionality, etc. But GO can be easily reduced by losing oxygen-containing groups under some circumstances such as irradiation of $\gamma$ -ray or ultraviolet (UV). In this work, we found that acetone can significantly slow down the reduction process of GO under the irradiation of either $\gamma$ -ray or UV, which was supported by analysis results with UV-visible (UV-Vis) absorption spectra, X-ray photoelectron spectroscopy, etc. Acetone can capture and remove strongly reducible hydrated electrons generated under $\gamma$ -irradiation. GO reduction by UV also involves electron transfer process which can be affected by the presence of acetone. Hence, acetone can be used to stabilize, adjust the radiation reduction process of GO. This would be interesting not only in radiation and radiation protection, but also in understanding the redox properties of GO.

Green and Effective Removal of Aqueous Graphene Oxide under UV-Light Irradiation

The potential extensive application of graphene oxide (GO) in various fields results in the possibility of its release into the natural environment with negative impacts on humans and the ecosystem. The UV-induced removal behavior of aqueous GO was evaluated in this study, and the effect of various parameters (including initial GO concentration, initial solution pH and co-existing ions) on removal rate of GO were investigated in detail. The results showed that UV-light induced a maximum removal rate of GO of 99.1% after 32 h irradiation without any additives, and that the photo-induced removal process in all cases fitted well with pseudo-first-order kinetics. Under optimal conditions, GO was completely removed, with initial GO concentrations of 10 mg/L while adjusting solution pH to 3 or adding Ca2+-containing salt. The GO and photoreduced graphene oxide (prGO) were characterized using High-resolution Transmission Microscopy (HRTEM), X-ray Photoelectron Spectroscopy (XPS), and Fourier-transform Infrared Spectroscopy (FT-IR). The radical species trapping experiments and Electron Spin Resonance (ESR) tests indicated that self-reduction of GO upon UV-light exposure could be achieved via photogenerated electrons from a GO semiconductor. Further mechanism study showed that the high efficiency of UV-induced GO removal came from UV-induced photoreduction, and pH-induced or cation-induced coagulation. This study provided a green and effective method to remove GO from aqueous solutions.

Degradation of Single-Layer and Few-Layer Graphene by Neutrophil Myeloperoxidase.

Biodegradability of graphene is one of the fundamental parameters determining the fate of this material in vivo. Two types of aqueous dispersible graphene, corresponding to single-layer (SLG) and few-layer graphene (FLG), devoid of either chemical functionalization or stabilizing surfactants, were subjected to biodegradation by human myeloperoxidase (hMPO) mediated catalysis. Graphene biodegradation was also studied in the presence of activated, degranulating human neutrophils. The degradation of both FLG and SLG sheets was confirmed by Raman spectroscopy and electron microscopy analyses, leading to the conclusion that highly dispersed pristine graphene is not biopersistent.

Degradation of Single‐Layer and Few‐Layer Graphene by Neutrophil Myeloperoxidase

AbstractBiodegradability of graphene is one of the fundamental parameters determining the fate of this material in vivo. Two types of aqueous dispersible graphene, corresponding to single‐layer (SLG) and few‐layer graphene (FLG), devoid of either chemical functionalization or stabilizing surfactants, were subjected to biodegradation by human myeloperoxidase (hMPO) mediated catalysis. Graphene biodegradation was also studied in the presence of activated, degranulating human neutrophils. The degradation of both FLG and SLG sheets was confirmed by Raman spectroscopy and electron microscopy analyses, leading to the conclusion that highly dispersed pristine graphene is not biopersistent.

Cement-Induced Coagulation of Aqueous Graphene Oxide with Ultrahigh Capacity and High Rate Behavior.

Graphene oxide (GO) has excellent physicochemical properties and is used in multiple areas. However, the potential toxicity and environmental problems associated with GO increase its risk to the ecological system. In this study, cement was employed as a coagulant to eliminate GO from aqueous solutions. The effects of the cement dosage, the contact time, and the concentration and volume of the aqueous GO solution on the GO coagulation capacity were investigated in detail. The results showed that the dosage of cement had a significant effect on the coagulation process, and coagulation equilibrium was achieved in less than 1 h. Compared to coagulants used to remove GO from water in other reports, cement exhibited an ultrahigh coagulation capacity of approximately 5981.2 mg/g with 0.4 mg/mL GO solution. The kinetic analysis showed that the GO removal behavior could be described by a pseudo second-order model. The in-depth mechanism of GO coagulation using cement included Ca2+-induced coagulation of GO and adsorption by the hydrated product of cement paste. The present study revealed that cement could be a very cheap and promising material for the efficient elimination of GO from aqueous solutions.

(Battery Division Student Research Award sponsored by Mercedes-Benz Research & Development) Advanced Electrochemical Energy Storage Systems: Material Development, Alternative Electrode Processes & Planar Devices

Future advancements in energy storage systems (lithium-ion batteries/LIBs and beyond) rely on the development of novel materials, innovative electrode/device architectures and scalable processing techniques or a combination thereof. In this talk, I will discuss my work on each of these three aspects including in situ characterization of planar devices as well as the development of nanomaterials and additive-free electrode architectures via alternative manufacturing processes such as dry/cold pressing and extrusion printing, respectively. Operando studies can elucidate the reaction behavior of battery electrodes during operation; however, specialized electrochemical cells are required to study these materials in their natural environment (in situ). In this study, a planar microscale battery with an open cell configuration was designed to investigate the structural evolution and concomitant solid electrolyte interphase formation of model electrode-electrolyte systems, such as Na-MoS2.1 By coupling an atomic force microscope with the planar electrochemical cell platform, real-time topographical observations of MoS2 electrodes during sodiation are readily achieved, which leads to crucial information regarding battery operation at the nanoscale. The use of advanced in situ/operando techniques with this newly developed planar battery configuration can elucidate the electrochemical behavior of numerous electrode-electrolyte systems, particularly for alkali-metal-ion batteries, during cycling. The drive towards lighter electric vehicles with longer ranges requires the development of higher energy density systems beyond conventional LIBs, such as lithium-oxygen (Li-O2) batteries. To increase battery performance for practical applications, high mass loading electrodes are advantageous as long as optimal battery reactions are maintained throughout the entirety of the “thick” electrode. Here, I present a facile, alternative electrode processing method known as dry/cold pressing, where a highly porous and compressible carbon nanomaterial (holey graphene or hG) enables the formation of mechanically robust, high mass loading electrodes for Li-O2 batteries.2-3 Dry pressing hG, the compressible matrix, with incompressible materials also enables the preparation of unique mixed and stacked/sandwich electrode architectures under binder- and solvent-free conditions.3 The enhancements in electrochemical performance demonstrate the promise of dry pressed electrode architectures and the reported additive-free processing method for advanced electrochemical energy storage systems. Additive manufacturing (AM) techniques also show promise towards the fabrication of complex 3D designs for advanced electrochemical devices. In particular, the development of inexpensive, sustainable (aqueous), and easily processable material ink systems and the use of a facile and low-cost fabrication process, such as extrusion-based 3D printing, are advantageous for scalable battery manufacturing. In this work, hierarchically porous electrode architectures are readily extruded using additive-free, aqueous graphene oxide-based (GO) ink compositions and demonstrated as the first 3D printed Li-O2 cathodes.4 The development of a nanoporous GO material enabled trimodal porosity (nano-micro-macropores) within the 3D printed mesh architecture, which provides facile mass/ionic transport pathways and enhances active-site utilization during battery operation. The results demonstrate the potential of AM techniques towards the scalable fabrication and improvement of advanced energy storage devices through structurally conscious designs as well as tailored material compositions.

Interaction of Zwitterionic and Ionic Monomers with Graphene Surfaces.

Measurement of the interaction force between two materials provides important information on various properties, such as adsorption, binding, or compatibility for coatings, adhesion, and composites. The interaction forces of zwitterionic and ionic monomers with graphite platelets (G) and reduced graphene oxide (rGO) surfaces were systematically investigated by atomic force microscopy (AFM) in air and water. The monomers examined were 2-(methacryloyloxy)ethyl 2-(trimethylammonio)ethyl phosphate (MPC), [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBE), [2-(acryloyloxy)ethyl]trimethylammonium chloride (ATC), and 2-methyl-2-propene-1-sulfonic acid sodium (MSS). The AFM studies revealed that MSS and SBE monomers with sulfonate units have stronger interaction forces with G surface in air and that MPC and ATC monomers with quaternary ammonium units have higher interaction forces in water. In the case of rGO surface, the monomers with quaternary ammonium units showed stronger interactions regardless of the medium. These interactions could be rationalized by the interaction mechanism between the monomers with graphene surfaces, such as cation-π for MPC and ATC and anion-π for MSS and SBE. Overall, cation-π interactions were effective in water, whereas anion-π interactions are effective in air with G surface. The adhesion values of MPC, SBE, ATC, and MSS on rGO were lower than the values measured on G surface. Among the monomers, MPC showed the highest dispersibility for aqueous graphene dispersions. Further, the adsorption of MPC on G and rGO surfaces was verified by high-resolution transmission electron microscopy and X-ray diffraction patterns.

PEGlated graphene as nanoadditive for enhancing the tribological properties of water-based lubricants

Dispersion of Graphene in water is conducive to their practical applications as lubricating additive in water-based fluids due to their safety, low cost and environmental-friendliness. But owing to its hydrophobicity, it is difficult to achieve stable aqueous graphene suspension. Here, we use the Toluene-2,4-diisocyanate as bridging agent between hydrophilic Poly (ethylene glycol) (PEG) and ethanolamine to prepare the amino-containing PEG (PEG-NH2), followed by covalently grafting PEG-NH2 onto the surface of carboxylaed graphene (G-COOH) nanosheets to synthesize the PEGlated graphene (PEG-G) via facile amidation under mild conditions. The physical, chemical properties of product were characterized by Fourier transformed infrared spectra (FTIR), Raman spectra, X-ray photo electron spectroscopy (XPS) and so on. Transmission electron microscopy (TEM) and scanning probe microscope (SPM) were utilized to observed the morphology of G-COOH and PEG-G. The frictional properties of PEG-G as nanoadditive of water-based lubricant were characterized by UMT-3 tribometer, following with analyzes on wear surfaces by 3D laser scanning confocal microscopy. Based on the results of characterization, the PEG-G was prepared successfully and the dispersion shows superior friction-reducing and anti-wear properties owing to the formation of fluid adhesive film and carbon protective film, especially the concentration of PEG-G in water is 0.05 wt%.

Dispersion and Rheological Properties of Aqueous Graphene Suspensions in Presence of Nanocrystalline Cellulose

The possibility and feasibility of nanocrystalline cellulose (NCC) for dispersing hydrophobic graphene in aqueous solutions were studied in the present work. The dispersion properties of graphene particles in aqueous media as a function of various amounts of NCC were investigated. The rheological behavior of NCC-dispersed graphene suspensions was studied, and a polynomial model was proposed and employed to simulate the obtained shear flow of graphene suspensions. The results of UV–Vis analysis and sedimentation test suggested that the increased NCC addition significantly led to the improved dispersion properties of graphene suspensions. Rheological behavior measurements demonstrated that graphene suspensions exhibited a reduction trend in the shear viscosity, shear stress, and viscoelasticity with the increased amount of NCC addition. Meanwhile, the shear viscosity and shear stress of graphene suspensions were found to show a dependency on the process temperature. The above results supported the conclusion that NCC is a promising candidate for improving the dispersion and flowability of graphene in aqueous media.

A water-based green approach to large-scale production of aqueous compatible graphene nanoplatelets

The unique properties of graphene are highly desired for printing electronics, coatings, energy storage, separation membranes, biomedicine, and composites. However, the high efficiency exfoliation of graphene into single- or few-layered nanoplates remains a grand challenge and becomes the bottleneck in essential studies and applications of graphene. Here, we report a scalable and green method to exfoliate graphene nanoplatelets (GNPs) from nature graphite in pure water without using any chemicals or surfactants. The essence of this strategy lies in the facile liquid exfoliation route with the assistance of vapor pretreatment for the preparation of edge hydroxylated graphene. The produced graphene consisted primarily of fewer than ten atomic layers. Such the water soluble graphene can be stored in the form of dispersion (~0.55 g L−1) or filter cake for more than 6 months without the risk of re-stacking. This method paves the way for the environmentally friendly and cost-effective production of graphene-based materials.

Filling the gap between transient and steady shear rheology of aqueous graphene oxide dispersions

Even though the rheological behavior of aqueous graphene oxide (G-O) dispersions has been shown to be strongly time-dependent, only few transient measurements have been reported in the literature. In this work, we attempt to fill the gap between transient and steady shear rheological characterizations of aqueous G-O dispersions in the concentration range of 0.004 < ϕ < 3.5 wt%, by conducting comprehensive rheological measurements, including oscillatory shear flow, transient shear flow, and steady shear flow. Steady shear measurements have been performed after the evaluation of transient properties of the G-O dispersions, to assure steady-state conditions. We identify the critical concentration ϕ c = 0.08 wt% (where G-O sheets start to interact) from oscillatory shear experiments. We find that the rheology of G-O dispersions strongly depends on the G-O concentration ϕ. Transient measurements of shear viscosity and first normal stress difference suggest that G-O dispersions behave like nematic polymeric liquid crystals at ϕ/ϕ c = 25, in agreement with other work reported in the literature. G-O dispersions also display a transition from negative to positive values of the first normal stress difference with increasing shear rates. Experimental findings of aqueous graphene oxide dispersions are compared and discussed with models and experiments reported for nematic polymeric liquid crystals, laponite, and organoclay dispersions.