Low temperature reduction of free-standing graphene oxide papers with metal iodides for ultrahigh bulk conductivity

Scalable synthesis of high-quality, reduced graphene oxide with a large C/O ratio and its dispersion in a chemically modified polyimide matrix for electromagnetic interference shielding applications

High-purity reduced graphene oxide (RGO or rGO) with appreciable conductivity is a desired conductive filler for lightweight polymer composites used in coatings, electronics, catalysts, electromagnetic interference (EMI) shielding, and energy storage devices. However, the intrinsic conductivity and the uniform dispersion of RGO in relatively polar matrices are challenging, leading to poor overall conductivity and performance of the composite material. The reported study improved the RGO intrinsic conductivity by increasing its C/O ratio while also simultaneously enhancing its compatibility with the polyimide (PI) matrix through ester linkages for better dispersion. A two-step reduction method drastically increased the number of structural defects and carbon content in the resulting RGO, corresponding to a maximum ID/IG and C/O of 1.54 and ∼87, respectively. Moreover, the 2D nanosheets with limited hydroxyl (-OH) groups effectively interacted with anhydride-terminated polyamic acid (AT-PAA) through chemical linkages to make high-performance RGO/PI nanocomposites. Consequently, the polymer matrix composites possessed the highest direct current conductivity of 15.27 ± 0.61 S cm-1 for 20 wt% of the prepared RGO. Additionally, the composite material was highly stiff (3.945 GPa) yet flexible (easily bent through 180°), lightweight (∼0.34 g cm-3), and capable of forming thin films (162 ± 15 μm). Unlike most polymer matrix composites, it showcased one of its class's highest thermal stabilities (a weight loss of only 5% at 638 °C). Ultimately, the composite performed as an effective electromagnetic interference (EMI) shielding material in the X-Band (8 to 12 GHz), demonstrating outstanding shielding effectiveness (SE), shielding effectiveness per unit thickness (SEt), specific shielding effectiveness (SSE), and absolute shielding effectiveness (SSEt) of 46 dB, 2778 dB cm-2, 138 dB cm3 g-1, and 8358 dB cm2 g-1, respectively. As a consequence of this research, the high-purity RGO and its high-performance PI matrix nanocomposites are anticipated to find practical applications in conductive coatings and flexible substrates demanding high-temperature stability.

The Influence of Lateral Size and Oxidation of Graphene Oxide on Its Chemical Reduction and Electrical Conductivity of Reduced Graphene Oxide

The chemical reduction efficiencies of graphene oxide (GO) are critically important in achieving graphene-like properties in reduced graphene oxide (rGO). In this study, we assessed GO lateral size and its degree of oxidation effect on its chemical reduction efficiency in both suspension and film and the electrical conductivity of the corresponding rGO films. We show that while GO-reduction efficiency increases with the GO size of lower oxidation in suspension, the trend is opposite for film. FESEM, XRD, and Raman analyses reveal that the GO reduction efficiency in film is affected not only by GO size and degree of oxidation but also by its interlayer spacing (restacking) and the efficiency is tunable based on the use of mixed GO. Moreover, we show that the electrical conductivity of rGO films depends linearly on the C/O and Raman I_D/I_G ratio of rGO and not the lateral size of GO. In this study, an optimal chemical reduction was achieved using premixed large and small GO (L/SGO) at a ratio of 3:1 (w/w). Consequently, the highest electrical conductivity of 85,283 S/m was achieved out of all rGO films reported so far. We hope that our findings may help to pave the way for a simple and scalable method to fabricate tunable, electrically conductive rGO films for electronic applications.

Tuning the Oxygen Content of Reduced Graphene Oxide and Effects on Its Properties

The need to recover the graphene properties in terms of electrical and thermal conductivity calls for the application of reduction processes leading to the removal of oxygen atoms from the graphene oxide sheet surface. The recombination of carbon-carbon double bonds causes a partial recovery of the original graphene properties mainly limited by the presence of residual oxygen atoms and lattice defects. However, the loss of polar oxygen-based functional groups renders the material dispersibility rather complicated. In addition, oxygen-containing functional groups are reaction sites useful to further bind active molecules to engineer the reduced graphene sheets. For these reasons, a variety of chemical processes are described in the literature to reduce the graphene oxide. However, it is greatly important to select a chemical process enabling a thin modulation of the residual oxygen content thus tuning the properties of the final product. In this work, we will present a chemical-processing technique based on the hydroiodic acid to carefully control the degree of residual oxidation. Graphene oxides were reduced using hydroiodic acid with concentrations from 0.06 to 0.95 mol L-1. Their properties were characterized in detail and tested, and the results showed that their oxygen content was finely tuned from 33.6 to 10.7 atom %. This allows carefully tailoring the material properties with respect to the desired application, which is exemplified by the variation of the bulk resistance from 92 Ω to 14.8 MΩ of the film from the obtained rGO.

rGO Functionalized ZnO–TiO2 Core-Shell Flower-Like Architectures for Visible Light Photocatalysis

Core-shell heterostructures with a complex, flower-like morphology, comprising a ZnO core and a TiO2 shell decorated with reduced graphene oxide (rGO) sheets by hydrothermal wrapping, are reported to extend the absorption properties of the semiconductors toward the visible light range. The ternary photocatalysts were characterized by X-ray diffraction, field emission scanning electron microscopy, Raman spectroscopy, diffuse reflectance UV–Vis, and attenuated total reflectance-Fourier transform infrared spectroscopy. Its photocatalytic performance was evaluated under visible light irradiation using methylene blue dye as a model pollutant. The rGO-modified ZnO–TiO2 photocatalyst exhibited superior photoactivity compared to that of the parent ZnO–TiO2 core-shell structures, which was dependent on its graphene content. The enhanced photocatalytic response was attributed to the higher absorption in the visible light range, as well as the pronounced electron and hole separation in the ternary system.

Synthesis of CoO-Decorated Graphene Hollow Nanoballs for High-Performance Flexible Supercapacitors

The formation of thin and uniform capacitive layers for fully interacting with an electrolyte in a supercapacitor is a key challenge to achieve optimal capacitance. Here, we demonstrate a binder-free and flexible supercapacitor with the electrode made of cobalt oxide nanoparticle (CoO NP)-wrapped graphene hollow nanoballs (GHBs). The growth process of Co(OH)₂ NPs, which could subsequently be thermally annealed to CoO NPs, was monitored by in situ electrochemical liquid transmission electron microscopy (TEM). In the dynamic growth of Co(OH)₂ NPs on a film of GHBs, the lateral formation of fan-shaped clusters of Co(OH)₂ NPs spread over the surface of GHBs was observed by in situ TEM. This CoO-GHBs/CC electrode exhibits high specific capacitance (2238 F g^-1 at 1 A g^-1) and good rate capability (1170 F g^-1 at 15 A g^-1). The outstanding capacitive performance and good rate capability of the CoO-GHBs/CC electrode were achieved by the synergistic combination of highly pseudocapacitive CoO and electrically conductive GHBs with large surface areas. A solid-state symmetric supercapacitor (SSC), with CoO-GHBs/CCs used for both positive and negative electrodes, exhibits high power density (6000 W kg^-1 at 8.2 Wh kg^-1), high energy density (16 Wh kg^-1 at 800 W kg^-1), cycling stability (∼100% capacitance retention after 5000 cycles), and excellent mechanical flexibility at various bending positions. Finally, a serial connection of four SSC devices can efficiently power a red light-emitting diode after being charged for 20 s, demonstrating the practical application of this CoO-GHBs/CC-based SSC device for efficient energy storage.

Enhancing Peroxidase Activity of Cytochrome c by Modulating Interfacial Interaction Forces with Graphene Oxide

Graphene oxide (GO) has drawn worldwide attention in various biomedical fields because of its unique properties, and great progress has been made in the past years. Probing the interaction between GO and proteins, understanding and evaluating potential impact of GO on the protein structure and function, is of significant importance for design and optimization of functional interfaces and revealing the bioeffect of GO materials. Cytochrome c (cyt c), one of the key components of respiratory chain, has played important roles in energy generation/consumption and many cellular processes including growth, proliferation, differentiation, and apoptosis. In this study, by combination of solution chemistry and spectroscopy, we systematically studied the interfacial interaction between GO and cyt c. Results suggest that GO could slightly perturb the active site of cyt c, enhancing its peroxidase activity. Structure of the active site is obviously changed with elapsed time, which in turn reduces peroxidase activity. Further study suggests that adsorption of cyt c on GO and the resulted structure change is a complex process resulting from the cooperation of various interaction forces. Hydrophobic interaction and π-π stacking, as well as electrostatic attraction, only slightly perturb the microenvironment of the active site of cyt c while hydrogen-bonding interaction is the main driving force for the structural change of the active site. Furthermore, long range electrostatic attraction between GO and cyt c may facilitate the short range hydrogen-bonding interaction, which intensifies the hydrogen-bonding-induced structural change. In addition, cyt c is partially reduced by GO in an alkaline environment. Based on the understanding of interfacial interaction mechanism between GO and cyt c, stable nanocomposites with enhanced peroxidase activity are successfully constructed by modulating the interfacial interaction forces. This work not only deepens the understanding of interaction between GO and functional protein, but also is of great importance for designing and applying of GO-based biomaterials.

Tunable electronic, electrical and optical properties of graphene oxide sheets by ion irradiation

The tunable electronic, electrical and optical properties of graphene oxide (GO) sheets were investigated using a controlled reduction by 500 keV Ar⁺-ion irradiation. The carbon to oxygen ratio of the GO sheets upon the ion beam reduction has been estimated using resonant Rutherford backscattering spectrometry analyses and its effect on the electrical and optical properties of GO sheets has been studied using sheet resistance measurements and photoluminescence (PL) measurements. The restoration of sp ²-hybridized carbon atoms within the sp ³ matrix is found to be increases with increasing the Ar⁺-ion fluences as evident from Fourier transform infrared, and x-ray absorption near-edge structure measurements. The decrease in the number of disorder-induced local density of states (LDOSs) within the π-π* gap upon the reduction causes the shifting of PL emission from near infra-red to blue region and decreases the sheet resistance. The improved electrical and optical properties of GO sheets were correlated to the decrease in the number of LDOSs within the π-π* gap. Our experimental investigations suggest ion beam irradiation is one of an effective approaches to reduce GO to RGO and to tailor its electronic, electrical and optical properties.

Green and Mild Oxidation: An Efficient Strategy toward Water-Dispersible Graphene

Scalable fabrication of water-dispersible graphene (W-Gr) is highly desirable yet technically challenging for most practical applications of graphene. Herein, a green and mild oxidation strategy to prepare bulk W-Gr (dispersion, slurry, and powder) with high yield was proposed by fully exploiting structure defects of thermally reduced graphene oxide (TRGO) and oxidizing radicals generated from hydrogen peroxide (H₂O₂). Owing to the increased carboxyl group from the mild oxidation process, the obtained W-Gr can be redispersed in low-boiling solvents with a reasonable concentration. Benefiting from the modified surface chemistry, macroscopic samples processed from the W-Gr show good hydrophilicity (water contact angle of 55.7°) and excellent biocompatibility, which is expected to be an alternative biomaterial for bone, vessel, and skin regeneration. In addition, the green and mild oxidation strategy is also proven to be effective for dispersing other carbon nanomaterials in a water system.

Nanoscaled self-alignment of Fe3O4 nanodiscs in ultrathin rGO films with engineered conductivity for electromagnetic interference shielding

Ultrathin (∼2 μm) reduced graphene oxide (rGO) film embedded with self-aligned Fe3O4 nanodiscs were successfully fabricated through the filtration-assisted self-assembly method. In the as-fabricated hybrid film, Fe3O4 nanodiscs with thin thickness (26 nm) and high aspect ratio (∼9) were readily self-assembled and aligned in rGO intersheets under the assistance of hydrostatic forces. Compared with spherical Fe3O4 nanoparticles, introducing the Fe3O4 nanodiscs into rGO paper could not only offer high magnetic permeability and magnetic loss in a broad frequency range at the gigahertz level, but also increase the electrical conductivity of rGO film by means of improving the surface roughness without disrupting the conductive network of the rGO layers. Due to the above advantages, the free-standing rGO/Fe3O4 nanodisc magnetic hybrid film (56 wt%) exhibited an EMI shielding effectiveness (SE) of around 11.2 dB in the frequency range of 2-10 GHz, which is about 50% and 72% higher than that of neat rGO film and rGO/Fe3O4 nanosphere hybrid films (with similar particle size and loading weight fraction) prepared under the same conditions, respectively. Furthermore, compared with non-magnetic neat rGO film, the outstanding magnetic properties of the rGO/Fe3O4 nanodisc film paves the way for it to be used as a multifunctional material that can be controlled by magnetic fields. Additionally, the moderate thermal reduction temperature (420 °C) would be meaningful for large scale fabrication. Meanwhile, the strategy of achieving good alignment at the nanoscale could shed light on developing heterogeneous structures with self-aligned two-dimensional (2D) (magnetic or non-magnetic) nano-inclusions for various applications.

Highly proton conducting MoS2/graphene oxide nanocomposite based chemoresistive humidity sensor

This paper reports the development of MoS₂/GO nanocomposite based sensing layers for resistive humidity sensors.

New approach for the reduction of graphene oxide with triphenylphosphine dihalide

We developed a one-flask method for the thermal reduction of graphene oxide (GO) with triphenylphosphine dihalide (Ph₃PX₂) at 180 °C.

Bulk synthesis of highly conducting graphene oxide with long range ordering

A two step mild oxidation process instead of extensive oxidation of graphite based on Hummers' method (H-GO) preserves the honeycomb graphene sheet structures in the range of 51 Å without reduction of mGO.