Reduced Graphene Oxide Monolayers Research Articles

High‐Quality Monolayer Reduced Graphene Oxide Films via Combined Chemical Reduction and Ethanol‐Assisted Defect Restoration

AbstractHere, a facile two‐step reduction method is introduced to fabricate reduced graphene oxide (RGO) films with remarkable structural qualities. Many attempts are made to fabricate RGO with high structural qualities resembling pristine graphene. However, designing a reduction method to obtain high‐quality RGO more productively is highly desirable. To address this matter, in the present work, monolayer graphene oxide films are, at first, chemically reduced using a mixture of hydroiodic acid and trifluoroacetic acid vapor within a few minutes. Second, the films are reduced thermally at 800 °C for 30 min with ethanol, to repair the lattice vacancies. Through this method, the authors succeeded in obtaining RGO with high structural quality and electrical properties approaching pristine graphene. Raman spectroscopic results show that these RGO films have a very low ID/IG ratio (0.34), a high IG′/IG ratio (1.34), and sharp Raman peaks, which attribute to a large distance between defects (21 nm) and large crystallite size (56 nm). Further, these monolayer RGO films have high transparency (90%) and very low sheet resistance (0.46 kΩ sq−1). The present versatile two‐step reduction strategy is considered to provide practical advancements in fabricating RGO with high structural quality, as a means to produce graphene‐based electronic devices.

New Insight into the Characterization of Graphene Oxide and Reduced Graphene Oxide Monolayer Flakes on Si-Based Substrates by Optical Microscopy and Raman Spectroscopy

Graphene oxide (GO) and reduced GO (RGO) have attracted interest as economical alternatives for pristine graphene with controllable properties spanning their applications in many fields. Abundant lattice defects present in GO and RGO have made their physical and chemical properties largely diverse from that of pristine graphene. This makes their identification and characterization ambiguous as the rules applied for pristine graphene may not be practical for GO and RGO. Here, we report a new insight into the characterization of GO and thermally reduced GO monolayer flakes deposited on different commonly used Si-based substrates by Raman spectroscopy along with optical visualization to address this uncertainty. It was found that the dielectric substrates with a lower reflectance in the visible range give a better optical contrast under normal white light illumination and intense Raman signals owing to the multiple reflection and interference at the interfaces of the multilayer system. Accordingly, a Si substrate with a 90 nm SiO2 layer gives the highest IG′/IG ratio along with a better optical visibility for monolayer RGO flakes.

Structural Effect of Perylene Derivatives on Their Interaction with Reduced Graphene Oxide Monolayers

The elucidation of factors influencing energy transfer in hybrid systems based on organic sensitizers and two-dimensional carbon materials is an important problem of thin-film chemistry, whose solution will make it possible to rationalize the design of new type of optoelectronic devices. The role of substituents in a perylene molecule in the energy transfer between a chromophore and a reduced graphene oxide (GO) monolayer was eludicated by Raman spectroscopy. It was shown that the adsorption of a perylene derivative with a planar geometry on reduced GO leads to fluorescence quenching and enhances Raman scattering in the chromophore. At the same time, no Raman bands from perylene can be detected after a “planar” chromophore is immobilized on pure silicon or perylene with isopentyl substituents is immobilized on a reduced GO monolayer despite the presence of perylene fluorescence peaks in the spectra. The surface-enhanced Raman scattering is caused by strong interactions between the unsubstituted chromophore and reduced GO due to aromatic stacking, which interactions provide efficient energy transfer between the components. The interactions between the substituted chromophore and the carbon framework are weak due to steric hindrances, being expressed in the absence of Raman scattering enhancement.

Molecular dynamics simulation of the effect of oxygen-containing functional groups on the thermal conductivity of reduced graphene oxide

Models of reduced graphene oxide (RGO) with different numbers of layers and simulation sizes were established and molecular dynamics simulations were performed to study the influence of oxygen-containing functional groups on the thermal conductivity (TC) of RGO. The results show that the TCs of both monolayer RGO (MRGO) and bilayer RGO (BRGO) increase with the increasing simulation length; under the same simulation size, the TC of BRGO is always lower than that of the MRGO. With the increasing content of functional groups, TCs of both MRGO and BRGO increase firstly and then decline. For MRGO, the optimum content occurs at about 2%. The phonon resonant frequency of MRGO with content of functional groups lower than 2% remains constant, and shifts to a lower frequency when the content of functional groups is higher than 2%; this confirms the effects of functional groups and wrinkling structure with ripples on the phonon scattering. For BRGO, the optimum content occurs at about 1%, which is lower than that for MRGO, since the interlayer distance of BRGO increases linearly with the increasing content of functional groups. Moreover, the changing trends of interaction energy and interlayer thermal resistance of BRGO are consistent with the TC change of BRGO.

Two fluorescence lifetime components reveal the photoreduction dynamics of monolayer graphene oxide

Photoreduction of graphene oxide is a flexible and green method to produce high-quality large-scale reduced graphene oxide. However, the dynamics of photoreduction of monolayer graphene oxide on the substrate, which plays a key role in practical applications, is still unclear. Here we perform fluorescence lifetime measurements to develop and understand this process by using time-correlated single photon counting technology. Two fluorescence lifetime components have been determined to reveal the dynamics and structural evolutions of graphene oxide during laser irradiation. The component with shorter lifetime, the fluorescence intensity of which is barely dependent on the laser irradiation, is assigned to confined sp2 clusters. The component with longer lifetime that is strongly sensitive to the irradiation time is attributed to hydroxyl groups. The present study offers a useful guidance for optoelectronic devices based on monolayer reduced graphene oxide.

Structure-Dependent Electrochemistry of Reduced Graphene Oxide Monolayers

While graphene and other carbonaceous nanomaterials have shown promise in a variety of electrochemical applications, measurement of their intrinsic performance is often confounded with effects related to the complexities due to diffusion in a porous medium. To by-pass this limitation, we use effectively non-porous tiled monolayers of reduced graphene oxide as a model platform to study how rates of heterogeneous electron transfer evolve as a function of graphene structure/chemistry. A variety of electrochemical systems are investigated including the standard ferri/ferrocyanide redox probe, several common biomolecular redox systems as well as copper electrodeposition. We show that the rates of heterogeneous electron transfer can vary by as much as 3 orders of magnitude depending on the reduction or annealing conditions used and the redox system investigated. Performance changes are linked to graphene chemistry, and we show that the graphene oxide reduction procedure must be chosen judiciously to maximize the electrochemical performance for particular applications.

Mechanical Properties of Water-Assembled Graphene Oxide Langmuir Monolayers: Guiding Controlled Transfer.

Liquid-phase transfer of graphene oxide (GO) and reduced graphene oxide (RGO) monolayers is investigated from the perspective of the mechanical properties of these films. Monolayers are assembled in a Langmuir-Blodgett trough, and oscillatory barrier measurements are used to characterize the resulting compressive and shear moduli as a function of surface pressure. GO monolayers are shown to develop a significant shear modulus (10-25 mN/m) at relevant surface pressures while RGO monolayers do not. The existence of a shear modulus indicates that GO is acting as a two-dimensional solid driven by strong interaction between the individual GO sheets. The absence of such behavior in RGO is attributed to the decrease in oxygen moieties on the sheet basal plane, permitting RGO sheets to slide across one another with minimum energy dissipation. Knowledge of this two-dimensional solid behavior is exploited to successfully transfer large-area, continuous GO films to hydrophobic Au substrates. The key to successful transfer is the use of shallow-angle dipping designed to minimize tensile stress present during the insertion or extraction of the substrate. A shallow dip angle on hydrophobic Au does not impart a beneficial effect for RGO monolayers, as these monolayers do not behave as two-dimensional solids and do not remain coherent during the transfer process. We hypothesize that this observed correlation between monolayer mechanical properties and continuous film transfer success is more universally applicable across substrate hydrophobicities and could be exploited to control the transfer of films composed of two-dimensional materials.

Local charge transport properties of hydrazine reduced monolayer graphene oxide sheets prepared under pressure condition

Charge transport properties of chemically reduced graphene oxide (RGO) sheets prepared by treatment with hydrazine were examined using conductive atomic force microscopy. The current-voltage (I-V) characteristics of monolayer RGO sheets prepared under atmospheric pressure followed an exponentially increase due to 2D variable-range hopping conduction through small graphene domains in an RGO sheet containing defect regions of residual sp3 carbon clusters bonded to oxygen groups, whereas RGO sheets prepared in a closed container under moderate pressure showed linear I-V characteristics with a conductivity of 267.2−537.5 S/m. It was found that the chemical reduction under pressure results in larger graphene domains (sp2 networks) in the RGO sheets when compared to that prepared under atmospheric pressure, indicating that the present reduction of GO sheets under the pressure is one of the effective methods to make well-reduced GO sheets.

Chemically Bonded TiO2–Bronze Nanosheet/Reduced Graphene Oxide Hybrid for High-Power Lithium Ion Batteries

Although Li-ion batteries have attracted significant interest due to their higher energy density, lack of high rate performance electrode materials and intrinsic safety issues challenge their commercial applications. Herein, we demonstrate a simple photocatalytic reduction method that simultaneously reduces graphene oxide (GO) and anchors (010)-faceted mesoporous bronze-phase titania (TiO2-B) nanosheets to reduced graphene oxide (RGO) through Ti(3+)-C bonds. Formation of Ti(3+)-C bonds during the photocatalytic reduction process was identified using electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) techniques. When cycled between 1-3 V (vs Li(+/0)), these chemically bonded TiO2-B/RGO hybrid nanostructures show significantly higher Li-ion storage capacities and rate capability compared to bare TiO2-B nanosheets and a physically mixed TiO2-B/RGO composite. In addition, 80% of the initial specific (gravimetric) capacity was retained even after 1000 charge-discharge cycles at a high rate of 40C. The improved electrochemical performance of TiO2-B/RGO nanoarchitectures is attributed to the presence of exposed (010) facets, mesoporosity, and efficient interfacial charge transfer between RGO monolayers and TiO2-B nanosheets.

Transparent conductive thin film of ultra large reduced graphene oxide monolayers

There is a great interest in the use of graphene sheets in transparent electrodes with low cost and high transparency and conductivity. Here, the production of transparent conductive thin film of ultra large reduced graphene oxide monolayers was investigated. Graphene oxide monolayers were produced using a modified Hummers method. Then, the ultralarge monolayers of graphene oxide to about 70μm as liquid crystalline dispersion were used for production of thin films; which were reduced by hydriodic acid. The produced low cost thin films had high transparency up to 70%, electrical resistance as low as 200Ω/sq and high mechanical properties as 4.6GPa in Young's modulus. The improved properties suggest that the produced thin films could successfully be used as transparent electrodes for different applications.

Near room temperature reduction of graphene oxide Langmuir-Blodgett monolayers by hydrogen plasma.

Langmuir-Blodgett monolayer sheets of graphene oxide (GO) were transferred onto Si and SiO2/Si, and subjected to hydrogen plasma treatment near room temperature. GO monolayers were morphologically stable at low power (15 W) plasma treatment, for durations up to 2 min and temperatures up to 120 °C. GO monolayers reduced under optimized plasma treatment conditions (30 s duration at 50 °C) exhibit a sheet thickness of (0.5-0.6) nm, high sp(2)-C content (75%), a low O/C ratio (0.16) and a significant red-shift of Raman G-mode to 1588 cm(-1), indicating efficient de-oxygenation and a substantial decrease of defects. A study of the valence band electronic structure of hydrogen plasma reduced GO monolayers shows an increase of DOS in the vicinity of the Fermi level, due to the increase of C 2p-π states, and a substantial decrease of work function. These results, along with conductivity measurements and transfer characteristics, reveal the p-type nature of hydrogen plasma reduced GO monolayers, displaying a conductivity of (0.2-31) S cm(-1) and a field effect mobility of (0.1-6) cm(2) V(-1) s(-1). Plasma treatment at higher temperatures results in a substantial increase in sp(3)-C/damaged alternant hydrocarbon content and incorporation of defects related to the hydrogenation of the graphitic network, as evidenced by multiple Raman features, including a large red-shift of D-mode to 1331 cm(-1) and a high I(D)/I(G) ratio, and supported by the appearance of mid-gap states in the vicinity of the Fermi level.

Electronic structure of graphene oxide and reduced graphene oxide monolayers

Graphene oxide (GO) monolayers obtained by Langmuir Blodgett route and suitably treated to obtain reduced graphene oxide (RGO) monolayers were studied by photoelectron spectroscopy. Upon reduction of GO to form RGO C1s x-ray photoelectron spectra showed increase in graphitic carbon content, while ultraviolet photoelectron spectra showed increase in intensity corresponding to C2p-π electrons (∼3.5 eV). X-ray excited Auger transitions C(KVV) and plasmon energy loss of C1s photoelectrons have been analyzed to elucidate the valence band structure. The effective number of (π+σ) electrons as obtained from energy loss spectra was found to increase by ∼28% on reduction of GO.

Spectroscopic studies of large sheets of graphene oxide and reduced graphene oxide monolayers prepared by Langmuir–Blodgett technique

Graphene oxide (GO) sheets prepared by chemical exfoliation were spread at the air–water interface and transferred to silicon substrates by Langmuir–Blodgett technique as closely spaced monolayers of 20–40μm size. Hydrazine exposure followed by annealing in vacuum and argon ambient results in the formation of reduced graphene oxide (RGO) monolayers, without significantly affecting the overall morphology of the sheets. The monolayer character of both GO and RGO sheets was ascertained by atomic force microscopy. X-ray photoelectron spectroscopy supported by Fourier transform infrared spectroscopy revealed that the reduction process results in a significant decrease in oxygen functionalities, accompanied by a substantial decrease in the ratio of non-graphitic to graphitic (sp2 bonded) carbon in the monolayers from 1.2 to 0.35. Raman spectra of GO and RGO monolayers have shown that during the reduction process, the G-band shifts by 8–12cm−1 and the ratio of the intensities of D-band to G-band, I(D)/I(G) decreases from 1.3±0.3 to 0.8±0.2, which is in tune with the smaller non-graphitic carbon content of RGO monolayers. The significant decrease in I(D)/I(G) has been explained by assuming that substantial order is present in precursor GO monolayers as well as RGO monolayers obtained by solid state reduction.

One-step solid state preparation of reduced graphene oxide

We have developed an easy and scalable chemical reduction method assisted by microwave irradiation for the synthesis of reduced graphene oxide (RGO) nanosheets in solid state. The as-synthesized RGO is characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetry, X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy. It is revealed that the bulk of the oxygen-containing functional groups are removed from graphene oxide with this one-step reduction method and monolayer RGO sheets are got from its N,N-dimethyl formamide solution. It is found that the ammonium bicarbonate plays a key role in the preparation of RGO. Considering the analysis results, a mechanism for the formation of RGO is proposed. Besides being eco-friendly, when compared to previous chemical techniques, this process has several advantages like low cost, simplicity and short processing times, which may find practical applications in the preparation of graphene-based composites.

Reduced Graphene Oxide/Poly(3‐hexylthiophene) Supramolecular Composites

Poly(3-hexylthiophene) (P3HT) supramolecular structures are fabricated on P3HT-dispersed reduced graphene oxide (RGO) monolayers and surfactant-free RGO monolayers. P3HT is able to disperse RGO in hot anisole/N,N-dimethylformamide solvents, and forms nanowires on RGO surfaces through a RGO induced crystallization process. The TEM and AFM investigation of the resultant P3HT/RGO composites shows that P3HT nanowires grow from RGO, and connect individual RGO monolayers. Raman spectroscopy confirms the interaction between P3HT and RGO, which allows the manipulation of the RGO electrical properties. Such a bottom-up approach provides interesting graphene-based composites for nanometer-scale electronics.

Electrical and Spectroscopic Characterizations of Ultra-Large Reduced Graphene Oxide Monolayers

Ultra large and single-layer graphene oxide sheets (up to millimeter in lateral size) are obtained by a modified Hummers’ method, where we replace the first aggressive oxidation process with a short sonication step in H2SO4 solutions. The lateral size of obtained GO sheets can be adjusted by the sonication period: it decreases with the increasing sonication time. The thin-film electrodes made from ultra large reduced GO sheets exhibit lower sheet resistance compared with those from small-size reduced GO sheets. Moreover, the transistor devices made from these single-layer GO sheets after 800 °C thermal reduction exhibit the effective hole mobility ranged between 4 and 12 cm2/(V s). Raman spectroscopic results suggest that the enhancement in mobility at a higher-mobility regime is well explained by the graphitization of GO rather than the removal of functional groups. The ratio between the 2D and G peak areas, I(2D)/I(G), is well correlated to the effective hole mobility values in reduced GO sheets.