Graphene Oxide Aqueous Solution Research Articles

The Electrical conductivities and Tribological properties of Vacuum Hot-Pressed Cu/Reduced Graphene Oxide Composite

The nanoscale reduced graphene oxide/copper (rGO/Cu) pellets were synthesized by reducing aqueous solution of graphene oxide and CuSO4. The Cu/rGO composite disks with high conductivity were fabricated by a vacuum hot-pressing sintering method. The composites were characterized by transmission electron microscopy, field emission scanning electron microscopy, x-ray diffraction. The results show that the vacuum hot-pressing sintering process can produce Cu/rGO composite successfully. The conductivities and wear resistance of the sintered Cu/rGO disks are improved, respectively, as rGO nanosheets being added into the matrix and/or increasing sintering temperatures, which are attributed to the destruction of the pores and the increasing compactness.

Steric stabilization of graphene oxide in alkaline cementitious solutions: Mechanical enhancement of cement composite

Graphene oxide (GO) with extraordinary properties has been regarded as a good candidate to reinforce cementitious materials. Due to a variety of oxygen functionalities, GO can be well dispersed in aqueous solutions but unstable in the presence of alkaline cementitious solutions, which restricts the fully utilization of GO to the mechanical enhancement of cementitious materials. In this study, a stabilized GO aqueous solution by pre-adding polycarboxylate-ether (PCE) copolymer was obtained before mixing with cement. The experimental results indicated that PCE can significantly improve the dispersion and stability of GO in alkaline cementitious solutions due to the stronger steric repulsion and hydrogen bonding. However, GO suddenly agglomerated when it was firstly mixed with cement by a rapid physical and chemical interaction, and post-addition of PCE has negligible helps to de-flocculate the GO. In terms of mechanical properties, cement paste with the stabilized GO-PCE showed higher flexural and compressive strength in comparison to that without PCE due to the better dispersion of GO in the hardened cement matrix. In conclusion, the prerequisite to make GO contribute more to the mechanical enhancement of cement paste is to ensure the good dispersion of GO in alkaline cementitious solutions achieved by pre-adding PCE.

Improved photocatalytic properties of ZnS/RGO nanocomposites prepared with GO solution in degrading methyl orange

Zinc sulfide/Reduced graphene oxide (ZnS/RGO) nanocomposites were synthesized via one-step solvothermal method with commercial graphene oxide (GO) aqueous solution as precursor. The crystal structure and morphology of the ZnS/RGO nanocomposites were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. During the process GO was reduced into RGO and ZnS nanoparticles with a diameter of about 5 nm were uniformly loaded on the surface of graphene sheets. The smaller size and uniform distribution of ZnS nanoparticles benefited from the higher purity and smaller size of GO in aqueous solution. The photocatalytic activity of the nanocomposites was investigated through the photocatalytic degradation of methyl orange in aqueous solution. The ZnS/RGO nanocomposites exhibited higher photocatalytic activity compared to pure ZnS; furthermore, the surface reaction rate constant of the nanocomposites was six times higher than that of the pure ZnS.

Self-assembled graphene film to enable highly conductive and corrosion resistant aluminum bipolar plates in fuel cells

We report in this paper a novel method to form protective graphene film on aluminum substrate, which is particularly applicable to bipolar plates in proton exchange membrane (PEM) fuel cells. By simply immersing an aluminum sheet in an aqueous solution of graphene oxide (GO), a layer of cross-linked GO gel forms on the aluminum sheet, taking advantage of dissociated aluminum ions as a cross-linker. Then the cross-linked GO is converted to graphene at 400 °C in hydrogen atmosphere. The chemistry of the self-assembled GO layer and its conversion to graphene film is revealed by FTIR and XPS. Under simulated fuel cell environment the graphene coated aluminum sheet shows a corrosion current density of <1 × 10−6 A/cm2, which is around four orders of magnitude lower than a bare aluminum sheet. Meanwhile, the graphene film on aluminum results in a much lower and more stable interfacial contact resistance (ICR) of <5 mΩ cm2. These enable the graphene coated aluminum sheet to meet the U.S. DOE targets of 2020 for bipolar plates in terms of both the corrosion and electrical resistance. Thus the proposed method is very promising for protecting aluminum bipolar plates in PEM fuel cells.

Three-Dimensional Reduced Graphene Hydrogels Using Various Carbohydrates for High Performance Supercapacitors.

In present work, three-dimensional (3D) reduced graphene hydrogels (RGHs) are prepared through an efficient and facile strategy by employing three types of carbohydrates (glucose, fructose and sucrose) as reducing agents in aqueous solution of graphene oxide (GO) with ammonia. The formation of RGHs could be confirmed by X-ray powder diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). The structures and porosity were characterized by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and N2 sorption experiments. Benefiting from the abundant porous architectures as fast ionic channels for electrochemical energy storage, the prepared RGHs exhibited a high specific capacitance up to 153.5, 145.0 and 150.3 F g−1 at 0.3 A g−1 for FRGHs (fructose), GRGHs (glucose) and SRGHs (sucrose), which can be maintained for 61.4, 61.5 and 46.9% as the discharging current density was increased up to 20 A g−1. Moreover, it also showed that the electrode based on RGHs has good electrochemical stability and high degree of reversibility in the charge/discharge cycling test.

Wettability effects of graphene oxide aqueous solution in photodetectors based on graphene oxide/silicon heterojunctions via ultraviolet ozone treatment

We report the wettability effects of graphene oxide (GO) aqueous solution on the electrical and photoresponse properties of GO-silicon (Si) photodetectors via ultraviolet ozone (UVO) treatment. GO-Si vertical heterojunctions were fabricated using a spray-coating method. In the GO/p-Si heterojunctions without UVO treatment (GO/p-Si), an island-like assembly of GO sheets was formed, indicating that a GO stacking structure may have an island shape on the substrate due to partial surface coverage. In contrast, for those with UVO treatment (GO/UVO-p-Si), a relatively more close-packed GO deposition may occur due to the more hydrophilic property of the substrate surface, leading to the full surface coverage of the p-Si surface. Photodetectors based on GO/p-Si and GO/UVO-p-Si heterojunctions exhibit distinct electrical and photoresponse properties. The GO/UVO-p-Si-based devices showed a higher photo-to-dark current ratio, higher turn-on voltage, and lower ideality factor compared to the GO/p-Si-based devices. The transport mechanism in the photodetectors can be explained in terms of space-charge-limited conduction and the tunneling of charge carriers through the GO layer.

Aggregation, Adsorption, and Morphological Transformation of Graphene Oxide in Aqueous Solutions Containing Different Metal Cations.

The colloidal behavior of graphene oxide (GO) has been extensively studied in the presence of common environmental cations, but the aggregation, adsorption, and morphological transformation of GO under heavy metal ions have not been investigated. We observed that heavy metal cations (Cr3+, Pb2+, Cu2+, Cd2+, Ag+) destabilized GO suspension more aggressively than common cations (Ca2+, Mg2+, Na+, K+). In addition to electric double-layer (EDL) suppression, heavy metal cations can easily cross the EDL, bind to GO surface, and then change the surface potential, which is a more efficient pathway for GO aggregation. According to aggregation kinetics, the destabilizing ability of cations follows the order of Cr3+ ≫ Pb2+ > Cu2+ > Cd2+ > Ca2+ > Mg2+ ≫ Ag+ > K+ > Na+. The destabilizing capability of metal cations is consistent with their adsorption affinity with GO, which is determined by their electronegativity and hydration shell thickness. GO nanosheets can be transformed to 1D tube-like carbon material, 2D multiple overlapped GO plane, and 3D sphere-like particles during aggregation, thereby combined to form a sphere-like aggregated GO, which is for the first time observed by TEM and AFM images. Therefore, the aggregation of GO 2D nanosheets follows the Schulze-Hardy rule, which is usually used for spherical particles. An integrative process of adsorption-transformation-aggregation is proposed to better understand the nanomaterial (e.g., GO) colloidal behavior, environmental risk, self-assembly process, and application as a novel adsorbent.

A combined self-assembly and calcination method for preparation of nanoparticles-assembled cobalt oxide nanosheets using graphene oxide as template and their application for non-enzymatic glucose biosensing

Cobalt oxide (Co3O4)-based materials have been extensively investigated as efficient electrocatalysts for non-enzymatic glucose biosensing. The proper tuning the structure of Co3O4-based materials could improve sensing performances for glucose detection. Herein, we have developed a combined self-assembly and calcination method to prepare nanoparticles-assembled Co3O4 nanosheets, which exhibit good sensing performances for glucose biosensing. The Co3O4 nanosheets were prepared by the following three steps: (i) the Co2+ modified graphene oxide (GO) was prepared by the self-assembly of Co2+ and GO in aqueous solution firstly, and then Co2+ modified GO film was obtained by vacuum filtration method; (ii) after calcination of Co2+ modified GO film in N2 atmosphere, Co3O4 nanoparticles modified reduced GO (RGO) was obtained; (iii) finally, nanoparticles-assembled Co3O4 nanosheets were obtained by calcination of Co3O4 modified RGO in air to remove RGO template. Most importantly, Co3O4 nanosheets exhibit good electrocatalytic activity for oxidation of glucose, leading to high-performance glucose sensor. The detection limit and linear range of Co3O4 nanosheets-based glucose sensor in the present work are 0.15μM, and 1–50μM, respectively. Furthermore, such sensor also shows excellent selectivity for glucose detection, compared to the commonly interfering species including dopamine, and uric acid.

Superior capacitive performance of reduced graphene hydrogels via dimethyl ketoxime

Three-dimensional (3D) reduced graphene hydrogels (RGHs) have been prepared by using dimethyl ketoxime as reducing agents in aqueous solution of graphene oxide (GO) with ammonia. The structure and surface chemistry were analyzed by scanning electron microscopy, Raman, X-ray diffraction, and X-ray photoelectron spectroscopy. The capacitive performance of the RGHs materials are studied in 6M KOH electrolyte. Benefitting from the 3D porous structures and heteroatom-doped polar pore surface, the as-prepared RGHs materials exhibit high specific capacitances up to 159.8, 215.1 and 163.4Fg−1 at 1Ag−1 for RGHs-1, RGHs-2 and RGHs-5, respectively. More importantly, the materials can maintain high capacitances of 95.6, 155.2 and 118.9Fg−1 at a very high current density of 20Ag−1, the retention rates are 59.8, 72.2 and 72.8% for RGHs-1, RGHs-2 and RGHs-5, respectively.

Novel green nano composites films fabricated by indigenously synthesized graphene oxide and chitosan

Graphene oxide (GO) was indigenously synthesized from graphite using standard Hummers method. Chitosan-graphene oxide green composite films were fabricated by mixing aqueous solution of chitosan and GO using dilute acetic acid as a solvent for chitosan. Chitosan of different viscosity and calculated molecular weight was used keeping amount of GO constant in each composite film. The structural properties, thermal stability and mechanical properties of the composite films were investigated using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and tensile test. FTIR studies revealed the successful synthesis of GO from graphite powder and it was confirmed that homogenous blending of chitosan and GO was promising due to oxygenated functional groups on the surface of GO. XRD indicated effective conversion of graphite to GO as its strong peak observed at 11.06° as compared to pristine graphite which appeared at 26°. Moreover, mechanical analysis confirmed the effect of molecular weight on the mechanical properties of chitosan-GO composites showing that higher molecular weight chitosan composite (GOCC-1000) showed best strength (higher than 3GPa) compared to other composite films. Thermal stability of GOCC-1000 was enhanced for which residual content increased up to 56% as compared to the thermal stability of GOCC-200 whose residue was restricted to only 24%. The morphological analysis of the composites sheets by SEM was smooth having dense structure and showed excellent interaction, miscibility, compatibility and dispersion of GO with chitosan. The prepared composite films find their applications as biomaterials in different biomedical fields.

Electrochemical capacitors based on the composite of graphene and nickel foam

An improved Hummers method was developed for the simple and efficient production of high-quality graphene oxide (GO), and the composite of GO and nickel foam (NF) (GO/NF) was fabricated by ultrasonication-vacuum-assisted deposition of an aqueous solution of GO on NF. After chemical or thermal reduction, the composite of reduced GO and nickel foam (rGO/NF) was obtained. The electrochemical capacitance performance of rGO/NF was investigated using cyclic voltammetry and galvanostatic charge/discharge measurements. The chemically reduced rGO/NF composite (C-rGO/NF) exhibited high specific capacitance of 379 F/g at 1.0 A/g and 266.5 F/g at 10 A/g. We also prepared thermally reduced graphene oxide at 473 K in order to illuminate the difference in effect between the chemical and low-temperature thermal reduction methods on electrochemical properties. The cycling performance of thermally reduced rGO/NF composite (T-rGO/NF) and C-rGO/NF had ~91% and ~95% capacitance retention after 2000 cycles in a 6 mol/L KOH electrolyte, respectively. Electrochemical experiments indicated that the obtained rGO/NF has very good capacitive performance and could be used as a potential application of electrochemical capacitors. Our work revealed high electrochemical capacitor performance of rGO/NF composite and provided a facile method of rGO/NF preparation.

Enhanced photothermal effect of surface oxidized silicon nanocrystals anchored to reduced graphene oxide nanosheets

We demonstrate the coupling of the photothermal effects of silicon nanocrystals and graphene oxide (GO) dispersed in water. Using laser irradiation (532nm or 355nm) of suspended Si nanocrystals in an aqueous solution of GO, the synthesis of surface oxidized Si-reduced GO nanocomposites (SiOx/Si-RGO) is reported. The laser reduction of GO is accompanied by surface oxidation of the Si nanocrystals resulting in the formation of the SiOx/Si-RGO nanocomposites. The SiOx/Si-RGO nanocomposites are proposed as promising materials for photothermal therapy and for the efficient conversion of solar energy into usable heat for a variety of thermal and thermomechanical applications.

Carbohydrazide-dependent reductant for preparing nitrogen-doped graphene hydrogels as electrode materials in supercapacitor

Three-dimensional (3D) nitrogen-doped graphene hydrogels (NGHs) are designed and synthesized in an efficient and fast way by using a strong reductant of carbohydrazide as reducing and doping agent in an aqueous solution of graphene oxide (GO). The transformation of GO suspension to the hydrogels can be completed in 1h, which can be confirmed by X-ray powder diffraction (XRD), Raman spectroscopy, and Fourier transform infrared spectroscopy (FT-IR). With adding different amounts of carbohydrazide, the obtained NGHs behave different doping of N and unlike performances in supercapacitors, which can be demonstrated by elemental analysis and X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), N2 sorption experiments, and electrochemical measurements, respectively. According to the network architectures, the NGHs all exhibited high specific capacitance, NGHs-1, NGHs-2, NGHs-5 and NGHs-10 showed specific capacitance at 167.7, 156.8, 140.4 and 119.3Fg−1 at 1Ag−1 in KOH electrolyte. The specific capacitance can still be maintained for 80.5, 79.5, 80.3 and 78.6% with an increase of the discharging current density of 10Ag−1, respectively. More interestingly, the NGHs-1 based supercapacitor also exhibited good electrochemical stability and high degree of reversibility in the long-term cycling test (81.5% retention after 4000 cycles).

A sedimentation study of graphene oxide in aqueous solution using gradient differential centrifugation.

This work involved the study of sedimentation of graphene oxide (GO) in aqueous solution by gradient differential centrifugation. GO sheets of size varying from 400 nm to 1100 nm were separated with layer numbers ranging from 2-17. Semiquantitative analysis of FT-IR spectra was conducted based on statistical variance, in which relative oxidation and hydration degrees were numeralized. Combining XRD, optical microscopy and particle size measurements, the dominant effects of hydration and d-spacing on GO sediment in aqueous solution were observed. However, lateral particle shifting showed a relatively insignificant influence even though it has much larger effects on the GO weight compared with the GO thickness. Highly oxidized GO sheets were highly hydrated and had the tendency to face more severe resistance in aqueous sedimentation. Larger d-spacing allowed more water molecules to enter into interlayers and thus improved the degree of hydration, the lower density and the lower sedimentation velocity of GO sheets. However, fast sedimentation could be found in both large and small GO sheets due to the relatively non-dominant effect from lateral size shifting. The underlying mechanism was further probed by the mathematical modeling process. Our study reveals the potential limitations of current theory for explaining GO sedimentation and also demonstrates the effectiveness of gradient differential centrifugation for sorting GO sheets varying in hydration degree and thickness.

Layer-by-layer films and colloidal dispersions of graphene oxide nanosheets for efficient control of the fluorescence and aggregation properties of the cationic dye acridine orange

Chemically derived graphene oxide (GO) nanosheets have received great deal of interest for technological application such as optoelectronic and biosensors. Aqueous dispersions of GO become an efficient template to induce the association of cationic dye namely Acridine Orange (AO). Interactions of AO with colloidal GO was governed by both electrostatic and π–π stacking cooperative interactions. The type of dye aggregations was found to depend on the concentration of GO in the mixed ensemble. Spectroscopic calculations revealed the formation of both H and J-type dimers, but H-type aggregations were predominant. Preparation of layer-by-layer (LbL) electrostatic self-assembled films of AO and GO onto poly (allylamine hydrochloride) (PAH) coated quartz substrate is also reported in this article. UV–Vis absorption, steady state and time resolve fluorescence and Raman spectroscopic techniques have been employed to explore the detail photophysical properties of pure AO, AO/GO mixed solution and AO/GO LbL films. Scanning electron microscopy was also used for visual evidence of the synthesized nanodimensional GO sheets. The fluorescence quenching of AO in the presence of GO in aqueous solution was due to the interfacial photoinduced electron transfer (PET) from photoexcited AO to GO i.e. GO acts as an efficient quenching agent for the fluorescence emission of AO. The quenching is found to be static in nature. Raman spectroscopic results also confirmed the interaction of AO with GO and the electron transfer. The formation of AO/GO complex via very fast excited state electron transfer mechanism may be proposed as to prepare GO-based fluorescence sensor for biomolecular detection without direct labeling the biomolecules by fluorescent probe.

Photoluminescence of Graphene Oxide in Visible Range Arising from Excimer Formation

Graphene oxide (GO) has attracted considerable attention due to its interesting structure and properties. The photoluminescence (PL) of GO is much stronger than that of graphene owing to the opening of an energy band gap. However, the origin of the PL bands in the ultraviolet and visible ranges remains controversial. In this paper, we report the dependence of the PL spectrum of GO on the pH value and concentration of GO aqueous solutions. It was discovered that PL in the visible range becomes prominent when the pH value is low and/or the GO concentration is high. As revealed by the time-resolved photoluminescence, the lifetime of the PL in the visible range is longer than that in the UV range. These results evidence the formation of excimers and prove that the PL band at the long wavelength is caused by the GO excimers.

Reduction Kinetics of Thermally Reduced Graphene Oxide Thin Films

We report an experimentalstudy of thermal reduction of graphene oxide (GO) under nitrogen flow.Ultrathin films of GO were prepared on quartz glass substrates by spin-coatingfrom 3 mg/ml aqueous solutions of GO. These films were annealed at 50 °C under vacuum overnight before thermal reduction.The films were exposed to temperatures of 100 °C, 150 °C, 200 °C, 300 °C, 400 °C, 500 °C and 800 °C under nitrogen flowing. We used UV-Vis absorption spectroscopy tomonitor the change of absorption spectra caused by thermal reduction. We observed a significant change ofabsorption spectra due to formation of reduced graphene oxide (RGO). We found that thermal reduction startedalready at 100 °C, but proceeded significantly faster at heating temperatures higherthan 200 °C.

Three dimensional reduced graphene hydrogels with tunable pore sizes using thiourea dioxide for electrode materials in supercapacitors

In present work, we demonstrate a rapid and easy approach to fabricate three-dimensional (3D) reduced graphene hydrogels (RGHs) by using thiourea dioxide as reducing agents in an aqueous solution of graphene oxide (GO) with ammonia. The transformation of GO suspension to the hydrogels can be confirmed by X-ray powder diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. The hierarchical porosity, structure and surface chemical properties can be demonstrated by N2 sorption experiments, scanning electron microscopy and X-ray photoelectron spectroscopy. With adding different amounts of thiourea dioxide, the obtained RGHs behave different degree of reduction, controlled specific surface area and pore size distribution, and unlike performances in supercapacitors. Benefiting from well-defined and cross-linked 3D porous network architectures, the supercapacitors based on the RGHs in KOH electrolyte exhibited a high specific capacitance of 258.6, 167.3 and 198.3 F g−1 at 0.1 A g−1 for RGHs-1, RGHs-2 and RGHs-5, respectively. Furthermore, this capacitance also showed good electrochemical stability and a high degree of reversibility in the repetitive charge/discharge cycling test.

One-pot solution-phase preparation of a MoS2/graphene oxide hybrid

Hybrids of graphene and other two-dimensional materials have shown extraordinary properties. However, the current preparation method, chemical vapor deposition, suffers from the disadvantages of low throughput and high cost, and the produced solid hybrids are unsuitable for those applications in aqueous solution. Therefore, it is very important to find a facile method to prepare these hybrids in solution. Here, a hybrid of MoS2 and graphene oxide (GO) (MoS2/GO) is prepared by a one-pot liquid-phase-exfoliation process. The exfoliated MoS2 by ultrasound waves is stabilized by amphiphilic GO in aqueous solution. The size of GO plays an important role in tuning the structure of MoS2/GO. Ultraviolet–visible absorption spectrum, dynamic light scattering, scanning electron microscope, transmission electron microscope and atomic force microscope are used to characterize the structure of MoS2/GO produced by fractional centrifugation after exfoliation of MoS2 in GO suspension. The results indicate that GO is adhered to the surface of MoS2 to stabilize MoS2/GO in solution. Furthermore, this preparation method can also be used to prepare the hybrids of GO and other two-dimensional materials.

Contesting the two-component structural model of graphene oxide and reexamining the chemistry of graphene oxide in basic media

The remaining ambiguity in the chemical structure of graphene oxide (GO) inspires new research, but also gives rise to numerous misconceptions that are based on the misinterpretation of experimental data. Recently, the two-component GO model was proposed and widely accepted by the scientific community. Here we discuss this model, demonstrate its insurmountable contradictions, and propose an alternative explanation for experimental observations that served as the model’s foundation. We demonstrate that so-called oxidation debris, allegedly preexisting on all the GO samples, is in fact derived from GO during the strong-base-treatment. The base-treatment of aqueous GO solution hydrolyses organic sulfates and triggers a chain of reactions that result in C–C bond cleavage with the formation of ketones at newly formed edges. The ketones further transform into carboxylate ions. The ability of GO to delocalize and to accumulate a negative charge is a necessary condition and the driving force of these reactions. We stress that the absorbance band near 1620cm−1 in GO FTIR spectra is originated by the bending modes of water molecules, but not by the CC bond stretch as commonly interpreted. Our findings provide new insights into GO chemistry, and reduce the confusion present in the modern GO field.