Size Of Graphene Oxide Sheets Research Articles

Linear relationship between lateral size of reduced graphene oxide (RGO) and water vapor barrier property in RGO/PEI composite membrane

The wide distribution of the lateral size of lab-prepared graphene oxide (GO) sheets often complicates the measurement of accurate average size. Herein, a series of reduced-GO/polyethyleneimine (RGO/PEI) composite membranes with compact brick-wall structures were successfully prepared. The large lateral size of RGO sheets in the membrane benefited gas barrier properties, including oxygen, nitrogen, and water vapor. The RGO sheets acted as impermeable inorganic filler to prolong the diffusion path, forcing gas molecules to wiggle through the permeable and tortuous channels of the PEI/water mixture. The water vapor transmission rate of RGO/PEI composite membranes was relatively high, and a linear correlation was established between the length of PEI/water channels and the water vapor barrier property. The length of PEI/water channels under the same membrane thickness was determined by the lateral size of RGO sheets. Therefore, the statistic averages of lateral size of RGO sheets (from 2.48μm to 44.35μm) showed a linear correlation with the water vapor barrier property, as confirmed by the positron annihilation technique and molecular dynamics simulations. In sum, the linear correlation could be used to practicably measure the statistic averages of lateral size of GO sheets according to water vapor permeability measurements.

Chlorambucil-Loaded Graphene-Oxide-Based Nano-Vesicles for Cancer Therapy.

In this study, the authors have designed biocompatible nano-vesicles using graphene oxide (GO) for the release of chlorambucil (CHL) drugs targeting cancerous cells. The GO sheets were first sulfonated and conjugated with folic acid (FA) molecules for controlled release and high loading efficiency of CHL. The chlorambucil (CHL) drug loading onto the functionalized GO surface was performed through π-π stacking and hydrophobic interactions with the aromatic planes of GO. The drug loading and "in vitro" release from the nano-vesicles at different pH were studied. The average particle size, absorption, and loading efficiency (%) of FA-conjugated GO sheets (CHL-GO) were observed to be 300 nm, 58%, and 77%, respectively. The drug release study at different pH (i.e., 7.4 and 5.5) showed a slight deceleration at pH 7.4 over pH 5.5. The amount of drug released was very small at pH 7.4 in the first hour which progressively increased to 24% after 8 h. The rate of drug release was faster at pH 5.5; initially, 16% to 27% in the first 3 h, and finally it reached 73% after 9 h. These observations indicate that the drug is released more rapidly at acidic pH with a larger amount of drug-loading ability. The rate of drug release from the CHL-loaded GO was 25% and 75% after 24 h. The biotoxicity study in terms of % cell viability of CHL-free and CHL-loaded GO against human cervical adenocarcinoma cell line was found to have lower cytotoxicity of CHL-loaded nano-vesicles (IC50 = 18 μM) as compared to CHL-free (IC50 = 8 μM). It is concluded that a high drug-loading efficiency and controlled release with excellent biotoxicity of CHL-GO offers an excellent application in the biomedical field.

Mechanical and Water Absorption Properties of Waterborne Polyurethane/Graphene Oxide Composites

Nanocomposites based on waterborne polyurethane (WPU) and graphene oxide (GO) have been synthesized and characterized. It was found that after the incorporation of GO, WPU films became mechanically more rigid, and the Young's modulus increased by almost six times. It is shown that the lateral size of GO sheets influences the mechanical properties of WPU/GO composites. In particular, composites with larger lateral size of GO sheets have higher values of Young's modulus. Additionally, if the mechanical properties are improved with the addition of GO additive, then water absorption decreases for WPU modified with small GO sheets whereas it increases for WPU modified with large GO sheets. Possible reasons for this behavior are discussed.

Photo-thermal characteristics of water-based graphene oxide (GO) nanofluids at reverse-irradiation conditions with different irradiation angles for high-efficiency solar thermal energy harvesting

In this paper, the optical absorption properties and photo-thermal conversion characteristics of GO-water nanofluids were experimentally studied and compared. With the increase of GO mass fraction or the decrease of GO sheet size, the optical absorption capacity of nanofluids can be enhanced, and a thin layer of nanofluid can absorb most of the solar irradiation. As for a direct absorption solar collector (DASC), the temperature rise increased with the irradiation angle at the reverse-irradiation mode because of the natural convection induced temperature uniformity improvement. There existed optimal GO concentrations with respect to the maximum temperature rises of GO nanofluids, and the optimal value is GO sheet size dependent. For GO sheet sizes of 50–200 nm, 200–500 nm and >500 nm, the optimal mass fractions are 150, 180 and 220 ppm, respectively, and the corresponding temperature rises were increased by 9.2 °C (28.2%), 7.9 °C (24.1%), and 6.9 °C (21.0%) as compared with that of water at the reverse irradiation angle of 45°. A 3D numerical simulation was performed to understand the irradiation angle induced natural convection contribution to the photo-thermal conversion performance, and an agreement was achieved with the experiment.

THE EFFECT OF LASER ENERGY DENSITY ON THE PROPERTIES OF GRAPHENE DOTS

The structural and optical properties of dots based on graphene oxide (GO) obtained by laser ablation method with various energy densities of radiation were studied. It was shown that after laser ablation, the average lateral size of GO sheets was decreased from 1280–1900 nm to 230±95 nm and 110±42 nm for samples prepared at a laser radiation with energy density of E~15 and 21 mJ/cm2, respectively. Raman spectroscopy datashowed that after ablation, the ID/IGratio decreased from 1.04 to 0.97, indicating an increase in the number of sp2-hybridized domains in GO after ablation. A change in the energy density has practically no effect on a change in its functionalization, aswell as the ordering of sp2domains inside a GO sheet. The optical density of GO dispersions and the intensity of their fluorescence depend on the ablation conditions. For non-ablated dispersions along with the luminescence band at 450 nm, additional bandappears in the spectrum with a maximum at about 600 nm. After ablation the distribution of particles becomes more uniform, as evidenced by both the change in the shape of the GO luminescence band and the constancy of the fluorescence lifetime upon registration at different wavelength.

Aspect ratio dependent viscoelastic properties of graphene oxide liquid crystals

We present a systematic investigation of the size-dependent viscoelastic properties of aqueous suspensions of graphene oxide (GO). The study is performed on three different size graphene oxide dispersions with an average sheet surface area of 22.89, 126.42 and 173.83 μm2. The viscoelastic properties of the GO suspensions are found to be strongly dependent on the size of GO sheets. Irrespective of the size, all the GO dispersions revealed an isotropic phase at low volume fractions and a nematic liquid crystalline phase above critical volume fractions. The formation of nematic liquid crystalline phase strongly depends on the size of GO sheets. The steady-state flow measurements confirmed the nematic liquid crystalline phase formation at a lower fraction (φ = 0.24%) for ultra-large GO sheet dispersion than small area graphene oxide dispersion (φ = 0.45%). Further, frequency sweep measurements of ultra-large GO suspension exhibit higher storage and loss moduli than small area graphene oxide dispersion for a particular GO concentration. The study also confirmed the gel-like phase formation at a higher GO fraction (φ > 0.73%), where both storage and loss moduli are nearly independent of frequency. The reported size-dependent viscoelastic properties of GO liquid crystals are very important for the fabrication of GO-based materials for various potential applications.

Влияние лазерной обработки на структуру и спектрально-люминисцентные свойства графеновых точек

The structure and optical properties of nanodots based on graphene oxide (GO) obtained by ablation by laser radiation with different wavelengths were studied. It was shown that after laser ablation, the average lateral size of GO sheets decreases from 820±120 nm to 204±40 nm and 105±23 nm for samples prepared at λgen=355 and 532 nm, respectively. In this case, a change in the intensities of the 2D and G bands was observed, which indicates a decrease in the number of layers in the GO sheets. The optical density of GO dispersions and the intensity of fluorescence depend on the ablation conditions. After ablation, the optical density of GO increased by ~13% for samples obtained at λgen=355 nm and by 20% for λgen=532 nm. The fluorescence intensity of GO ablated at λgen=532 nm increased by 57% relative to the value registered for GO before ablation. For 355 nm, the fluorescence intensity was changed by 7%.

Effect of laser treatment on the Structure and Spectral-Luminescent Properties of Graphene Dots

The structure and optical properties of nanodots based on graphene oxide (GO) obtained by ablation by laser radiation with different wavelengths were studied. It was shown that after laser ablation, the average lateral size of GO sheets decreases from 820±120 nm to 204± 40 nm and 105± 23 nm for samples prepared at λgen=355 and 532 nm, respectively. In this case, a change in the intensities of the 2D and G bands was observed, which indicates a decrease in the number of layers in the GO sheets. The optical density of GO dispersions and the intensity of fluorescence depend on the ablation conditions. After ablation, the optical density of GO increased by ~ 13% for samples obtained at λgen=355 nm and by 20% for λgen=532 nm. The fluorescence intensity of GO ablated at λgen=532 nm increased by 57% relative to the value registered for GO before ablation. For 355 nm, the fluorescence intensity was changed by 7%. Keywords: graphene oxide, graphene dots, ablation, structure, optical properties.

Dynamic light scattering analysis of size-selected graphene oxide 2D colloids fractioned via liquid crystal phase separation.

Exfoliated platelets of graphene oxide (GO) can be considered as polydisperse 2D colloids that form nematic colloidal liquid crystal phases in aqueous suspension even at very low concentrations thanks to their extremely high aspect ratios. However, with the rapidly emerging scientific interest in these GO-based liquid crystals, it became clear that the precise analysis and control of the GO sheet size distribution is essential, both for their scientific understanding and for potential applications, e.g., in optoelectronic devices, nanocomposites, or catalysis. In this work, we show that the mean effective (hydrodynamic) GO platelet width can be determined from the translational diffusion coefficient with depolarized dynamic light scattering by using a model for circular, infinitely thin disks. We further studied the phase separation process of biphasic isotropic-nematic GO dispersions and developed a simple fractionation protocol, which can be used to prepare relatively monodisperse fractions of GO sheets with widths ranging from 2.0-12.4 μm. Overall, we expect that the combined application of these relatively simple fractionation and analysis methods will advance the fabrication of well-defined and size-selected GO-based systems.

Modification of structure and optical properties of graphene oxide dots, prepared by laser ablation method

Modification of structure, optical and luminescent properties of graphene oxide (GO) by laser ablation at 532 nm during various times was studied. It was shown that after 30 minutes of laser ablation, the average lateral size of GO sheets decreases from 272±120 nm to 62±29 nm. Studies of structural properties by Raman and FTIR spectroscopy and microscopy indicate the reduction and graphitization of GO at laser irradiation times greater than 20 minutes, which leads to an increase in the average crystallite size of sp2 carbon domains and a decrease in the number of GO layers in the resulting particles. The optical density of the resulting GO solutions and the intensity of their fluorescence are also time dependent. After ablation, the GO fluorescence intensity increased by 10, 25, and 36% for 10, 20 and 30 minutes, respectively. In the long-lived luminescence spectrum of GO, a broad emission band was recorded after ablation, whereas for GO before ablation long-lived luminescence was not exhibited.

Electrochemical collision of single graphene oxide sheets at ultramicroelectrodes and its usage as substrate for Pt nanoparticle deposition

AbstractRevealing the physiochemical properties of individual graphene oxide (GO) sheets is highly desired for both fundamental research interests and practical applications, especially for their surface functionalization and modification toward hybrid composites with high performance. Herein, we report three different strategies including surface blockage, capacitive charging, and electrochemical reduction to detect and study single GO sheets using single‐entity electrochemical methods. The size, surface charge, dielectric, and electrochemical features of single GO sheets can then be revealed and quantified from the transient current signals. Moreover, the adsorbed GO can confine the electrode surface for metal deposition and produce single Pt nanoparticles, as indicated by the discrete reductive spikes shown in the deposition processes.

Structural Complexity of Graphene Oxide: The Kirigami Model.

Investigation of highly oxidized graphene oxide (GO) by solid-state nuclear magnetic resonance (NMR) spectroscopy has revealed an exceptional level of hitherto undiscovered structural complexity. A number of chemical moieties were observed for the first time, such as terminal esters, furanic carbons, phenolic carbons, and three distinct aromatic and two distinct alkoxy carbon moieties. Quantitative one-dimensional (1D) and two-dimensional (2D) 13C{1H} NMR spectroscopy established the relative populations and connectivity of these different moieties to provide a consistent "local" chemical structure model. An inferred 2 nm GO sheet size from a very large (∼20%) edge carbon fraction by NMR analysis is at odds with the >20 nm sheet size determined from microscopy and dynamic light scattering. A proposed kirigami model where extensive internal cuts/tears in the basal plane provide the necessary edge sites is presented as a resolution to these divergent results. We expect this work to expand the fundamental understanding of this complex material and enable greater control of the GO structure.

Size effect of graphene oxide sheets on enantioseparation performances in membrane separation

Recently, graphene oxide (GO)-based membranes have been demonstrated to be a potential candidate for gas purification and liquid separation techniques owing to their large surface area, tunable interactive sites, and adjustable interlayer spacing. Notably, GO-based membranes functionalized with chiral selectors have been demonstrated to possess high enantiomer permeability and impressive permeation selectivity toward enantiomeric target guests. However, the influence of the GO sheet size on the separation performance remains unclear. Here, we investigated the effect of the lateral size on the enantioseparation performances by modifying a chiral selector onto GO flakes with different sizes, considering that GO membranes possess an inherently high throughput character. In this work, three GO sheets with mean lateral sizes (~60, 650 and 4100 nm) were prepared, hereafter defined as GO1, GO2, and GO3, respectively. Then, a chemical modification of GO1, GO2 and GO3 with a chiral selector, L-Phenylalanine (L-Phe), yielded the corresponding L-Phe grafted GO sheets, L-Phe-GO1, L-Phe-GO2 and L-Phe-GO3. Finally, L-Phe-GO1, L-Phe-GO2 and L-Phe-GO3 membranes derived from a simple vacuum filtration method were employed to separate D-/L-phenylalanine. Results show that the L-Phe-GO3 based membrane allows for a remarkable chiral separation capacity, having a greatest enantioselectivity among three L-Phe-GO membranes with different lateral GO sizes. Our findings illustrate that the sheet size of GOs plays a dominant role in the enantioselectivity of membrane separation.

The Origin of the Sheet Size Predicament in Graphene Macroscopic Papers.

The larger size of graphene sheets should intuitively generate higher overall properties of their macroscopic materials. However, this intuitive notion still remains ambiguous. Here, we uncover that the wrinkle formation causes the counterintuitive size predicament of graphene sheets in their macroscopic materials. In the model of graphene oxide assembled papers, we reveal that the giant size of graphene oxide sheets aggravates the formation of larger wrinkles and more microvoids, causing the negative size effect in mechanical strength. A major microscopic origin of the size predicament is the skin wrinkling in the drying process, and the wrinkling behavior follows a general rule of deformation of an elastic thin plate. We use a wrinkle-engineering strategy to depress the spontaneously formed large wrinkles and succeed in the resolution of the size predicament. After wrinkle modulation, an authentically positive size effect reversely appears in which giant graphene sheets generate ultrahigh mechanical strength and superior functionalities of graphene papers. The origin of the size predicament reminds us of the hidden importance of modulating wrinkles for graphene macroscopic materials and provides a guidance of wrinkle engineering for graphene materials with advanced performances.

Optical properties of ablated graphene oxide in aqueous dispersions

The effect of laser radiation on the structural and optical properties of graphene oxide dispersed in water was studied. It was shown that under laser ablation a significant reduction in the size of graphene oxide sheets can be achieved. In this case, the resulting main parts of particles have a size of about 110–120 nm, and are similar to graphene quantum dots. The Raman spectra indicate the reduction of graphene oxide during laser radiation. The thickness of the formed particles practically was not changed, since the ID/IG ratio has close values. The prepared dispersions of graphene oxide exhibit wide luminescence bands in the region of 400–600 nm with a maximum of about 450 nm and a lifetime of 1.6 ns. It was shown that by laser ablation it is possible to achieve a significant increasing in the luminescent ability of graphene oxide in an aqueous solution. In this case, the luminescence intensity increased by almost 2 times, while the optical density of the solution was increased by only 5 % relative to the initial dispersion. The results can be used to create organic luminescent materials, in optical nanotechnology, as well as in photovoltaics, biophysics and bioimaging.

Probing the influence of graphene oxide sheets size on the performance of label-free electrochemical biosensors

The integration of graphene materials into electrochemical biosensing platforms has gained significant interest in recent years. Bulk quantities of graphene can be synthesized by oxidation of graphite to graphite oxide and subsequent exfoliation to graphene oxide (GO). However, the size of the resultant GO sheets changes from the parent graphite yielding a polydispersed solution of sizes ranging from a few nanometers to tens of micrometers. Here, we investigate the direct effect of GO sheets sizes on biosensor performance. We separated different GO sheets sizes, and we characterized them via atomic force, scanning electron, Raman and X-ray photoelectron spectroscopies and solid state nuclear magnetic resonance (NMR). As proof of concept, the sensing performance of these GO samples was probed using a well-known ssDNA aptasensor against microcystin-LR toxin and an immunosensor against β-lactoglobulin. The resulting aptasensors and immunosensors are fabricated by using covalent attachment and physical adsorption. We found that the aptasensors fabricated using physical adsorption, the binding signal variation was dramatically increased with increasing the GO sheet size. In contrast, for the aptasensor fabricated using covalent immobilization, the binding signal variation decreased with increasing GO sheet size. However, for the β-lactoglobulin immunosensors, the optimum signals were observed at intermediate GO sheet size. GO sheet size could enhance or inhibit the sensitivity of the graphene-based electrochemical sensors. Our results demonstrate that controlling the size of GO sheets may have a profound impact in specific biosensing applications.

Microwave-assisted exfoliation and tearing of graphene oxide in the presence of TiO2 nanoparticles

In this paper, we present a facile approach to reduce, exfoliate, and tear graphene oxide (GO) sheets, which can be used to produce graphene quantum dots. The prepared GO sheets were easily exfoliated and teared under UV and microwave (MW) irradiation in the presence of TiO2 nanoparticles (NPs). Results show that the GO sheets can be reduced/fragments by MW depending on the irradiation power. We found that; GO sheets are partially reduced under MW irradiation with a power of 90 W and crushed to nanosheets by increasing the MW power up to 450 W. In addition, the photoluminescence peak is broadened, confirming a decrease in the size of GO sheets. The optical bandgap of the nanosheets was measured about 3 eV in agreement with the results of the tight-binding calculation. In addition, HR-TEM results show the size of prepared nanosheets is about 20 nm, while X-ray photoelectron spectroscopy shows a bond formation between residual TiO2 NPs and the prepared nanosheets.

Formation and trapping of CO2 due to the decomposition of amide solvents during the chemical reduction of graphene oxide by using the solvothermal method

We report on the chemical reduction (cR) of graphene oxide (GO) by the solvothermal method at 180 °C using two similar amide solvents, dimethylformamide (DMF) and dimethylacetamide (DMA). One of the differences between the obtained cRGO nanostructures is their solubility in polar solvents. This can be related to the quantity of decomposed amide molecules into carbon monoxide (CO), a reducing agent, and amine molecules of the employed solvents. The amount of reactive agent generated with DMA during the solvothermal reduction process is lower than those resulting from the use of DMF. Thus, a large (small) quantity of oxygen groups in GO sheets is removed using DMF (DMA) as reducing agent. The removal of oxygen groups from GO sheets, due to interaction with CO, creates CO2 molecules and some of them are trapped between cRGO nanocrystals. Despite vacancy creations in graphene sheets, via CO2, the average size of GO sheets is preserved during the reduction process, indicating the tendency of CO to heal vacancies in the graphene lattice. Moreover, different GO concentrations have been used to study the reduction efficiency of these solvents. Average sizes of cRGO sheets as well as their graphitization are similar for both solvents, showing that the influence of GO concentrations in the cRGO crystallinity is limited.

Size separation of graphene oxide via multi-layer filtering by silica gel column

The properties and applications of graphene-based materials largely depend on the lateral size and size distribution of graphene and its derivatives sheets. In this work, a novel size separation method was developed for graphene oxide (GO) sheets using a multi-layer filtering of silica gel column. GO sheets were separated into three groups with narrowed size distributions after filtering from the multi-scale holes that were formed due to the packing of irregular silica gel particles. This economical and continuous-operational separation method shows promising future to realize the size separation in industrial sector. The size separation mechanism was considered that the size of GO sheets affected the flow speed, owing to the friction force between the GO sheet and silica gel particle surface. In addition, the related reduced graphene oxide (rGO) was obtained via the separated GO. The size impact of rGO sheets on the electrochemical properties were studied using cyclic voltammetry (CV), galvanostat charge/discharge (GC/D) and electrochemical impedance spectroscopy (EIS).

The transmittance and sheet resistance of chemically and heat reduced graphene oxide film

The graphene oxide (GO) sheets were prepared from Hummer’s method. The reduced process is important to graphene related materials for widely functional use in many photoelectric fields. Chemically and heat reduced treatment are carried out in this research and the electrical and optical properties of reduced GO films are measured. The size of GO sheets was examined by transmission electron microscopy with a size of about 5–6 µm. The chemically converted graphene (CCG) film are made by spin coating method. We used different GO concentration and different spin coating times to investigate the properties of graphene transparent conductive films. As the decrease of the GO concentration of solution and the times of spin coating, the transmittance is higher. The electrical property of the mixing of GO and CCG is more stable than the GO sheets only, this is discussed in this research and it is cause by the stacking condition of sheets. The conductivity of reduced graphene oxide film come from GO is lower than that come from CCG, we suppose that is because that the overlapping is less (i.e. film-forming ability) in the former, the transmittance and sheet resistance are 56 T% and 50 kohm/sq.