Aggregation Of Graphene Oxide Research Articles

Superior strength and toughness of graphene/chitosan fibers reinforced by interfacial complexation

Graphene fibers have been fabricated by wet spinning, in which interfacial interactions including hydrogen bond, ionic bond, covalent bond and π-π interaction were usually constructed to achieve effective translation of the extraordinary properties inherent in graphene microscopic building blocks to macroscopic bulk materials. However, the interactions would induce the aggregation of graphene oxide (GO) spinning dope, leading to the local inhomogeneity and deteriorating mobility which was unfavorable for the translation of GO inherent performance. In order to overcome the contradiction, an interfacial polyelectrolyte complexation (IPC) spinning not involving in the usage of organic solvents was proposed to in-situ build a hierarchical assembly structure of well-ordered GO sheets and chitosan molecules by mimicking natural nacre. After partial reduction of GO fibers using HI, strong and tough graphene fibers with tensile strength of 875.5 MPa and toughness of 17.85 MJ/m3 were obtained due to strong ionic bond and hydrogen bond interactions between rGO and chitosan. Molecular dynamics simulation revealed the fracture mechanism of the graphene fibers that was the slippage of rGO sheets and chitosan molecules accompanied by the breaking and reforming of interfacial bonds. Combing the considerable electrical conductivity, the fibers exhibited a promising application potential as wearable electronics and conductors.

Antiangiogenic Effect of Graphene Oxide in Primary Human Endothelial Cells.

In this work, we exploited an integrated approach combining systematic analysis of cytotoxicity, angiogenic potential, and metabolomics to shed light on the effects of graphene oxide (GO) on primary human endothelial Huvec cells. Contrary to the outcomes observed in immortalized cell lines able to internalize a similar amount of GO, significant toxicity was found in Huvec cells at high GO concentrations (25 and 50 μg/mL). In particular, we found that the steric hindrance of GO intracellular aggregates perturbed the correct assembly of cytoskeleton and distribution of mitochondria. This was found to be primarily associated with oxidative stress and impairment of cell migration, affecting the formation of capillary-like structures. In addition, preliminary metabolomics characterization demonstrated that GO affects the consumption of niacinamide, a precursor of energy carriers, and several amino acids involved in the regulation of angiogenesis. Our findings suggest that GO acts at different cellular levels, both directly and indirectly. More precisely, the combination of the physical hindrance of internalized GO aggregates, induction of oxidative stress, and alteration of some metabolic pathways leads to a significant antiangiogenic effect in primary human endothelial cells.

Microstructural evolution and mechanical investigation of hot stretched graphene oxide reinforced polyacrylonitrile nanofiber yarns

To reveal the enhancement effect of graphene oxide (GO) in polymer nanofiber yarns, polyacrylonitrile (PAN)/GO nanofibers with different GO content (0.1‐0.5 wt%) were electrospun. The alignment of PAN chains and GO in nanofibers was enhanced by hot stretching of the yarn in dry conditions. The microstructure of the composite nanofiber yarns was investigated through X‐ray diffraction, polarized Fourier transform infrared spectroscopy and transmission electron microscopy. The results demonstrated that the hot stretching above Tg of PAN precursor lead to the increased orientation‐induced crystallization and alignment of PAN chain and GO. The yarn with 0.1 wt% GO and stretched by 4 times its length obtained the highest strength and modules (310.88 ± 24.68 MPa and 7.24 ± 0.55 GPa), which were 600% and 500% higher than those of the as‐electrospun pure PAN yarn. The most promising tensile properties found in hot stretched yarns with low GO content was because the strong interaction occurred between PAN molecules and oxygen‐containing functional groups. Indirect evidence of GO aggregation was also presented, which adversely affected the mechanical properties at higher GO content. Composite nanofiber yarns were sewable and weavable, and could be used as a new generation of composite reinforcement after pyrolysis.

A novel hollowed-out Si microsphere encapsulated by graphene oxide: a strong and reusable absorbent

Graphene oxide (GO) has been a promising adsorbent to remove dyes, heavy metals and other pollutants from aqueous solutions, due to its huge surface area and rich oxygenic groups. However, one of the major application challenges is the agglomeration of GO layers which greatly affect its adsorption capacity and recycle performance. In this paper, a novel hollowed-out Si microsphere encapsulated by GO layers was developed as a strong and reusable absorbent. Hollowed-out Si microsphere with a dendritic structure was obtained from micron Al–Si alloy powder via acid treatment. Then it can be encapsulated by GO layers via a hydrogen bonding interaction between oxygenic groups from GO and Si–OH/Al–OH from porous Si skeleton. The designed absorbent has excellent adsorption ability for methylene blue and heavy metal ions resulting from the large surface area and oxygen functional groups of unrolled GO sheets. Meanwhile, it shows good reusability for pollutants owing to the spread of GO sheets on the Si skeleton surface which could prevents the aggregation of GO during adsorption process.

General Strategy for Synthesis of Ordered Pt3M Intermetallics with Ultrasmall Particle Size

AbstractControllable synthesis of atomically ordered intermetallic nanoparticles (NPs) is crucial to obtain superior electrocatalytic performance for fuel cell reactions, but still remains arduous. Herein, we demonstrate a novel and general hydrogel‐freeze drying strategy for the synthesis of reduced graphene oxide (rGO) supported Pt3M (M=Mn, Cr, Fe, Co, etc.) intermetallic NPs (Pt3M/rGO‐HF) with ultrasmall particle size (about 3 nm) and dramatic monodispersity. The formation of hydrogel prevents the aggregation of graphene oxide and significantly promotes their excellent dispersion, while a freeze‐drying can retain the hydrogel derived three‐dimensionally (3D) porous structure and immobilize the metal precursors with defined atomic ratio on GO support during solvent sublimation, which is not afforded by traditional oven drying. The subsequent annealing process produces rGO supported ultrasmall ordered Pt3M intermetallic NPs (≈3 nm) due to confinement effect of 3D porous structure. Such Pt3M intermetallic NPs exhibit the smallest particle size among the reported ordered Pt‐based intermetallic catalysts. A detailed study of the synthesis of ordered intermetallic Pt3Mn/rGO catalyst is provided as an example of a generally applicable method. This study provides an economical and scalable route for the controlled synthesis of Pt‐based intermetallic catalysts, which can pave a way for the commercialization of fuel cell technologies.

General Strategy for Synthesis of Ordered Pt3 M Intermetallics with Ultrasmall Particle Size.

Controllable synthesis of atomically ordered intermetallic nanoparticles (NPs) is crucial to obtain superior electrocatalytic performance for fuel cell reactions, but still remains arduous. Herein, we demonstrate a novel and general hydrogel-freeze drying strategy for the synthesis of reduced graphene oxide (rGO) supported Pt3 M (M=Mn, Cr, Fe, Co, etc.) intermetallic NPs (Pt3 M/rGO-HF) with ultrasmall particle size (about 3 nm) and dramatic monodispersity. The formation of hydrogel prevents the aggregation of graphene oxide and significantly promotes their excellent dispersion, while a freeze-drying can retain the hydrogel derived three-dimensionally (3D) porous structure and immobilize the metal precursors with defined atomic ratio on GO support during solvent sublimation, which is not afforded by traditional oven drying. The subsequent annealing process produces rGO supported ultrasmall ordered Pt3 M intermetallic NPs (≈3 nm) due to confinement effect of 3D porous structure. Such Pt3 M intermetallic NPs exhibit the smallest particle size among the reported ordered Pt-based intermetallic catalysts. A detailed study of the synthesis of ordered intermetallic Pt3 Mn/rGO catalyst is provided as an example of a generally applicable method. This study provides an economical and scalable route for the controlled synthesis of Pt-based intermetallic catalysts, which can pave a way for the commercialization of fuel cell technologies.

Deep research about the mechanisms of graphene oxide (GO) aggregation in alkaline cement pore solution

In order to understand the fundamental mechanisms beneath the aggregation of Graphene Oxide (GO) in cement pore solution, deep research about the differences in the dispersion behavior of GO in Ca(OH)2, CaCl2, and NaOH solutions has been done. The results showed that the prime factor responsible for the immediate aggregation of GO in cement paste is the complexation of Ca2+, rather than the deoxygenation of GO in the alkaline environment. GO could disperse uniformly in NaOH solution (pH = 12.9) for a few hours. However, with the gradual deoxygenation of GO, small agglomerates formed after 6 h due to the weak electrostatic repulsion and less hydrophilic nature. The analysis of X-ray photoelectron spectroscopy (XPS) and hydration heat measurement showed that GO maintained a large number of functional groups (especially carboxyl groups), which could act as growth sites for hydration products and interact with them. Moreover, the dispersion capacity of GO in the alkaline environment with the presence of polycarboxylate superplasticizer (PC) was also studied and the results revealed that PC could adsorb on GO surface due to the entropy gain and delay the aggregation of GO through steric hindrance effect.

Size-dependent aggregation of graphene oxide

Graphene oxides (GO) of highly polydisperse size distribution were prepared by the Brodie method and their dispersion stability was characterized. Exfoliation and fractionation led to well-defined particle populations in the Nano, classical Colloidal (submicron) and Micrometer size ranges, as revealed by atomic force microscopy and light scattering measurements. Time-resolved dynamic light scattering experiments revealed that aggregation processes are fully impeded in the intermediate pH regime of 3–13 in the absence of electrolytes. While the resistance against salt-induced aggregation increases with the pH due to the progressive ionization of the surface functional groups of GO sheets, their dispersions are inherently unstable at supramillimolar concentrations of strong acids and submolar concentrations of bases, in line with the DLVO theory. However, the aggregation behavior quantified by the critical coagulation concentrations (CCCs) shows surprisingly substantial platelet size dependence. The CCC of Nano Brodie-GO reaches 360 mm at pH = 12, which is one of the highest values ever reported for GO aqueous dispersions. These results provide useful quantitative information to design processable GO dispersions of pH- and size-tunable stability for future applications.

New approach for the development of reduced graphene oxide/polyaniline nanocomposites via sacrificial surfactant-stabilized reduced graphene oxide

An aggregation of graphene oxide (GO) during its reduction to reduced graphene oxide (rGO) limits its performance in nanocomposites. The use of rGO stabilized by a sacrificial surfactant should overcome this limitation and it yields nanocomposites with enhanced properties. A new, simple and cost-effective approach for the synthesis of polyaniline (PANI) nanocomposites based on rGO stabilized by a sacrificial surfactant was developed. Two routes of synthesis, the in situ and ex situ reduction, were compared. The former involved the reduction of GO already coated by PANI, forming rGO/PANIin whereas the latter involved the reduction of GO in the presence of a sacrificial surfactant to well-exfoliated rGO sheets followed by polymerization of aniline, forming rGO/PANIex. Differences in morphology and physical properties between rGO/PANIin and rGO/PANIex correlated with their chemical structure were raised. Accordingly, rGO/PANIin exhibited higher thermal stability but lower electrical conductivity (0.01 S·cm−1) compared to neat PANI (0.11 S·cm−1), while rGO/PANIex demonstrated thermal properties comparable to those of PANI and remarkable electrical conductivity (∼ 280 S·cm−1). Mechanistic insights into the interactions between rGO and PANI are proposed.

Aggregation morphology of planar engineered nanomaterials

In this investigation, the utility of a static light scattering (SLS) technique to characterize aggregate morphology of two-dimensional engineered nanomaterials (2D ENMs) was systematically evaluated. The aggregation of graphene oxide (GO) and lithiated-molybdenum disulfide (Li-MoS2) were measured and compared to that of a spherical reference colloid, carboxylate-modified latex (CML) nanoparticles. The critical coagulation concentration (CCC) for all dispersions was determined via analysis of aggregation kinetics using time-resolved dynamic light scattering. This technique allowed for the elucidation of the transition from the reaction-limited aggregation (RLA) regime to diffusion-limited aggregation (DLA). The findings of this study support the aggregation trends predicted by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and recent computer simulations of aggregation kinetics. For all nanomaterials, as ionic strength increased towards the respective the CCC, fractal dimension decreased; any increase in ionic strength beyond the CCC did not yield significant change in fractal dimension. Across comparable primary particle sizes and using both carbonaceous (GO) and inorganic (Li-MoS2) 2D ENMs, this study further supports the use of SLS for the measurement of fractal dimension for 2D materials. To further support this claim, the aggregate morphology of GO in both RLA and DLA regimes was measured via cryogenic transmission electron microscopy.

The effect of water hardness on the toxicity of graphene oxide to bacteria in synthetic surface waters

Graphene oxide (GO), used in a wide variety of applications, is increasingly being introduced into aquatic environments; this situation calls for research on GO toxicity to assess its environmental risks. In this study, the toxic effect of GO to E.coli was studied before and after its aggregation equilibrium in the synthetic surface waters (the soft water, moderately hard water, and hard water) to reveal the effects of GO aggregation and solution hardness. The cytotoxicity of GO increased with increasing solution hardness while decreased after GO aggregation. The 3 h 50% inhibitory concentration (IC50) values of dispersed GO in the soft water, moderately hard water, and hard water were 12.2 ± 2.2, 8.5 ± 1.5, and 4.0 ± 1.0 mg/L, respectively. After 24 h shaking (aggregation equilibrium) in the synthetic surface waters, the dispersed GO aggregated and the 3 h IC50 values of GO aggregates in the three synthetic waters were 40.3 ± 6.9, 15.9 ± 2.2, and 7.5 ± 1.5 mg/L, respectively. The dispersed GO sheets wrapped E. coli cells and cut the cell membrane, resulting in the disruption of cell membrane and the cell inactivation. With increasing water hardness, the heteroaggregation between GO sheets/aggregates and E. coli cells was enhanced, resulting in the increase of toxic effect. The GO aggregates could also entrap E.coli cells while exhibited limited effect on cell membrane disruption without sharp edges, thereby causing the lower toxic effect compared with the dispersed GO sheets. These outcomes shed new light on the assessment of ecological effects of GO.

Facile fabrication of GO/Al composites with improved dispersion of graphene and enhanced mechanical properties by Cu doping and powder metallurgy

In recent years, graphene has attracted a lot of attention as a reinforcement to enhance aluminum (Al) matrix composites due to its two dimensional structure and excellent mechanical properties. However, there are still some obstacles that limit the performance of the composites, such as uniform distribution of graphene oxide (GO) and the interfacial bonding between GO and Al matrix. In this work, Cu2+ ions acting as a bonding bridge were added into GO/Al powder suspension to improve the distribution of GO sheets on Al powder via a simple electrostatic adsorption method. Then, vacuum hot-pressing sintering was applied to fabricate bulk composites, and their mechanical properties were investigated. The results showed that doping 3 wt% Cu2+ ions illustrated the best dispersion of GO on the surface of Al powder, meanwhile GO was maximally reduced. Compared with unreinforced Al, the hardness, compressive strength and flexural strength of the 3 wt% Cu-doped bulk composite were increased by 121%, 120% and 28%, respectively. However, the mechanical properties were deteriorated after doping 5 wt% Cu2+ ions, due to the aggregation of GO on the surface of Al powder. This was attributed that Cu2+ ions had a critical coagulation concentration in aqueous GO suspensions. It was expected this work could offer some useful information for preparing graphene reinforced metal composites.

Aggregation of Graphene Oxide in Natural Waters: Role of Solution Chemistry and Specific Interactions

AbstractIn this paper, the impact of pH, ionic strength, and dissolved organic matter [humic acid (HA) and tannic acid (TA)] on graphene oxide (GO) stability was investigated. The results showed th...

Fate and Transformation of Graphene Oxide in Estuarine and Marine Waters.

The possibility of graphene oxide (GO) exposure to the environment has spurred several studies investigating the fate of this nanoparticle (NP). However, there is currently little or no data on the fate of GO in estuarine and marine waters. This study investigated the aggregation, sedimentation, and transformation of GO in saline waters, considering the roles of salinity (0-50 ‰), light (visible light and solar irradiation), and aging, among others. The attachment efficiency of GO reached unity at 1.33 ‰. The sedimentation rate of GO increased with salinity up to 10 ‰ after which it decreased due to formation of ramified GO agglomerates and media density. On the basis of the sedimentation rate determined at 30 ‰ (0.121 m/d), the residence time of GO agglomerates in the euphotic zone of typical open oceans will exceed 500 days. Aging in the presence of visible light increased the relative abundance of the GO's aromatic (C-C/C=C) fraction, reducing the NP. Reduction of GO in visible light was confirmed via UV-vis and Raman spectroscopic techniques. Reduction of GO was faster under solar irradiation. This study demonstrates that when introduced into saline waters, GO will undergo a range of transformations affecting its fate and potential effects to aquatic organisms.

New insight into the aggregation of graphene oxide in synthetic surface water: Carbonate nanoparticle formation on graphene oxide

Graphene oxide (GO), used in a wide variety of applications, is increasingly being introduced into aquatic environments; this situation calls for research on GO aggregation and sedimentation to regulate the environmental behaviors and risks. Many studies have investigated the aggregation and the mechanism of GO in water with a single background salt (monosalt system); however, this may not reflect real water environments where multiple salts coexist (multisalt system). A typical synthetic surface water (soft water) with representative multisalts was therefore used to study the aggregation and sedimentation of GO. The GO concentration-dependent aggregation (low concentration aggregation, high concentration stability) was observed in the soft water, and this concentration-dependent aggregation is opposite to the aggregation in monosalt systems (NaCl or CaCl2 solutions). The presence of GO sheets induced the formation of amorphous CaMg(CO3)2 nanoparticles on the GO surfaces in the soft water, and the formed nanoparticles promoted the aggregation and sedimentation of low concentrations of GO through bridging action. Neutral and alkaline conditions were favorable for the formation of CaMg(CO3)2 nanoparticles and the induced GO aggregation. These findings show a new mechanism of GO aggregation in environmentally relevant waters and help us to better evaluate the environmental fate of GO.

Aggregation size effect of graphene oxide on its reinforcing efficiency to cement-based materials

Aggregation of graphene oxide (GO) in cement matrix greatly reduces its reinforcing efficiency to the mechanical properties of cement-based materials. However, current technology is too difficult to distinguish the aggregated GO from the complex components in hardened cement matrix, so how does the GO aggregation influence the mechanical properties of cement-based materials is still not clear. In this study, the aggregation size effect of GO on its reinforcing efficiency to cement-based materials was investigated by pre-aggregating GO in aqueous solutions by a novel extrusion method before mixing with cement particles. It was found that with the increasing aggregation size of GO from nanometers, micrometers to millimeters, its reinforcing efficiency to the compressive strength of cement paste was gradually reduced, because the intrinsic advantages of excellent mechanical properties and high specific surface area of GO faded away with the increasing size of GO. More importantly, the mechanism behind that the addition of GO always showed a stronger mechanical reinforcement to cement mortar than paste was revealed by measuring the particle size variation of aggregated GO in a simulated alkaline cementitious solution with and without sand under mixing process. The sand shearing effect on de-aggregation of GO by reducing their particle size, for the first time, was proposed. Compared with the aggregated GO with a larger size, the de-aggregated GO with a smaller size and larger specific surface area due to the sand shearing effect should be responsible for the stronger mechanical reinforcement to cement mortar. In conclusion, this study provides a new perspective to the GO reinforcing efficiency to the mechanical properties of cement-based materials in terms of aggregation size of GO and sand shearing effect.

Enhanced interlaminar shear strength of ramie fiber/polypropylene composites by optimal combination of graphene oxide size and content

To improve interlaminar shear strength (ILSS) of ramie fiber (RF) reinforced polypropylene (PP) composites, graphene oxide (GO) with different particle sizes (ca. 1, 5 and 40 μm) are employed to treat the RF surfaces. Untreated and GO-treated RF/PP composites are then prepared by hot press. The combined effect of the GO size and content is examined on the ILSS of the RF reinforced PP composites. Moreover, the wrinkle morphology of GO surfaces is considered to play a mechanical interlocking role in improving the interfacial bonding between RFs and PP matrix, thus enhancing stress transfer from the PP matrix to the RFs. Consequently, the observed maximum improvement of about 40% in the ILSS of the PP composite by GO treatment of appropriate size and content is reasonably interpreted mainly via GO aspect ratio, GO aggregation and PP crystallinity.

Porous crumpled graphene with improved specific surface area based on hydrophilic pre-reduction and its adsorption performance

Porous crumpled graphene (PCG) with high specific surface area was prepared by aerosol method. The combination of hydrophilic pretreatment of graphene oxide (GO) and subsequent NaOH activation was the key to improve the specific surface area of PCG. Hydrophilic pretreatment of GO by ammonia prevented GO from further reduction by NaOH before spray drying and thus alleviated the aggregation of GO induced by the reduction by NaOH; subsequently, the specific surface area was improved. The specific surface area of NaOH-GO-N-4 was up to 846.31 m2 g−1, which was two times as large as that of NaOH-GO-4. Due to the high specific surface area and spherical structure, a maximum rhodamine B (RB) adsorption capacity of 340.65 mg g−1 was obtained of NaOH-GO-N-4, which was much higher than that of activated carbon with the specific surface area of 1422 m2 g−1. The kinetic data fitted the pseudo-second-order model, and the equilibrium data were well fitted by the Langmuir isotherm. The thermodynamic analysis indicated that the adsorption of RB was a spontaneous and exothermic process.

Effects of Surfactant and Electrolyte Concentrations, Cation Valence, and Temperature on Graphene Oxide Retention and Transport in Saturated Porous Media

Environmental fate and impacts of graphene oxide (GO) nanoparticles are strongly influenced by their subsurface behaviors. The present work examined the aggregation and transport behaviors of GO in saturated sand columns under different temperature (6 and 24 °C), surfactant concentration (0.04% and 0.4%), cation valence, and electrolyte concentration conditions. In monovalent electrolyte (NaCl), although the presence of cationic surfactant (CTAB) notably increased GO stability and mobility, GO ripening happened due to their concurrent aggregation and transport in the columns. GO particles were more mobile at a lower temperature probably because the CTAB coating of GO increased with decreasing temperature, leading to stronger electrostatic repulsion. Furthermore, GO retention in the media increased with the increase of NaCl concentration due to the enhanced compression of the electric double layer. In multivalent electrolyte (CaCl2 or AlCl3), the presence of CTAB greatly improved GO stability and mobility and no deposition occurred in saturated porous media under all the tested conditions. This is because the CTAB coating of GO diminished the cation bridging effects in both GO-GO and GO-sand systems. Results from extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory considering steric repulsion suggest that secondary minimum aggregation and depositions were the main mechanisms of GO retention transport in monovalent electrolyte in saturated porous media.

Insights into aggregation and transport of graphene oxide in aqueous and saturated porous media: Complex effects of cations with different molecular weight fractionated natural organic matter

The stability of nanomaterials in aquatic environment is a critical factor that governs their fate and ecotoxicity. Meanwhile, the interaction between nanomaterials and ubiquitous natural organic matter (NOM) is a vital process that influences the transport and biological effects of nanomaterials in the environment. However, impacts of NOM on the aggregation and transport of two-dimensional nanomaterials, especially for the increasingly used graphene oxide (GO), are not well understood. Particularly, there is lack of exploration on potential impacts of the heterogeneous properties of NOM on GO behaviour, especially that induced by the wide molecular weight (MW) span of NOM. In this study, effects of several kinds of well-characterized MW fractionated Suwannee River NOM (Mf-SRNOMs) on the aggregation and transport of GO in aqueous media and saturated porous media were investigated. Our results suggest that the stability and migration capacity of GO under most investigated electrolyte conditions are promoted by all Mf-SRNOMs, and efficiencies of different Mf-SRNOMs are generally positively correlated with their MW. Primarily, mechanisms including MW-dependent steric hindrance and sorption of Mf-SRNOMs onto GO are critical in stabilizing GO, and thus facilitating its transport. However, the stronger sorption of higher Mf-SRNOMs onto the GO basal plane through π-π interaction further facilitated the cation bridging between both ends of Mf-SRNOM and GO, and resulted in heteroaggregation of NOM-GO. Moreover, the weight analysis indicated that despite the fact that high Mf-SRNOMs only occupied a small percentage of pristine-SRNOM, they showed a stronger contribution towards pristine-SRNOM's capacity in stabilizing GO, when compared with that of lower MW counterpart. These findings pointed out that complex effects of the heterogeneities of NOM and cations should be highly relevant when the aggregation and transport behaviour of two-dimensional nanomaterials is investigated, and NOM fractions that are highly aromatic and of a higher MW should receive greater attention.