Tailoring the Oxygen Content of Graphite and Reduced Graphene Oxide for Specific Applications

Oxygen plasmas: a sharp chisel and handy trowel for nanofabrication

Although extremely chemically reactive, oxygen plasmas feature certain properties that make them attractive not only for material removal via etching and sputtering, but also for driving and sustaining nucleation and growth of various nanostructures in plasma bulk and on plasma-exposed surfaces. In this minireview, a number of representative examples is used to demonstrate key mechanisms and unique capabilities of oxygen plasmas and how these can be used in present-day nano-fabrication. In addition to modification and functionalisation processes typical for oxygen plasmas, their ability to catalyse the growth of complex nanoarchitectures is emphasized. Two types of technologies based on oxygen plasmas, namely surface treatment without a change in the size and shape of surface features, as well as direct growth of oxide structures, are used to better illustrate the capabilities of oxygen plasmas as a powerful process environment. Future applications and possible challenges for the use of oxygen plasmas in nanofabrication are discussed.

Improved thermomechanical and electrical properties of reduced graphene oxide reinforced polyaniline - dodecylbenzenesulfonic acid/divinylbenzene nanocomposites

HYPOTHESIS

Various efforts are going on to improve the electrical properties of carbon fiber reinforced polymer (CFRP) composites. Conducting polymer is one the promising material to achieve the desired electrical properties of CFRP composites without compromising the mechanical properties as a lighting sticking material.

EXPERIMENTS

In present study, in addition to conducting polymer polyaniline (PANI), another conducting phase reduced graphene oxide (RGO) was incorporated in PANI based system. The RGO was synthesized and incorporated in different weight (0-0.5 wt%) fraction in dodecylbenzenesulfonic acid (DBSA) doped PANI-divinylbenzene (DVB) polymer to get PANI-DBSA/DVB nanocomposite. The mechanical and interfacial interaction was analyzed by universal testing machine (UTM) and transmitted electron microscopy (TEM).

FINDINGS

The addition of optimum 0.3 wt% RGO improved flexural strength and modulus of PANI-DSBA/RGO-DVB composite by 153% and 32% respectively over neat PANI-DBSA/DVB nanocomposite. The maximum electrical conductivity 0.301 S/cm, glass transition temperature (T_g) and thermal stability of nanocomposite realized at 0.3 wt% of RGO. Raman spectroscopy and HRTEM confirmed the improvement of interfacial bonding by H-bonding and π-π interaction. For the 1st time we are reporting RGO utilisation for the improvement of thermomechanical and electrical interfacial properties of PANI-DBSA/DVB nanocomposite for the structural applications.

Shungite Carbon as Unexpected Natural Source of Few-Layer Graphene Platelets in a Low Oxidation State

The paper reports on the feasibility of obtaining graphene nanomaterials with remarkable structural and chemical features from shungite rocks. The investigation of the composition and structural modifications induced in the pristine, natural C-containing mineraloid by a specifically designed physicochemical purification treatment is performed by a combined use of several techniques (scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman and X-ray photoelectron spectroscopies). The adopted material processing enables efficient extraction of the C phase in the form of thin polycrystalline platelets of a few hundred nanometers sizes, and formed by 6-10 graphene sheets. About 80% of such nanostructures are characterized by a regular sp² C honeycomb lattice and an ordered stacking of graphene layers with a d-spacing of ∼0.34 nm. The low oxygen content (∼5%), mainly found in the form of hydroxyl functional groups, provides the graphene platelets (GP) with a chemistry strictly close to that of conventional rGO materials. Such a feature is supported by the high conductivity value of 1.041 × 10³ S cm^-1 found for pelletized GP, which can be considered a valuable active material for a wide spectrum of advanced applications.

Enhanced Osteogenic Differentiation of Stem Cells on Phase-Engineered Graphene Oxide

Graphene oxide (GO) has attracted significant interest as a template material for multiple applications due to its two-dimensional nature and established functionalization chemistries. However, for applications toward stem cell culture and differentiation, GO is often reduced to form reduced graphene oxide, resulting in a loss of oxygen content. Here, we induce a phase transformation in GO and demonstrate its benefits for enhanced stem cell culture and differentiation while conserving the oxygen content. The transformation results in the clustering of oxygen atoms on the GO surface, which greatly improves its ability toward substance adherence and results in enhanced differentiation of human mesenchymal stem cells toward the osteogenic lineage. Moreover, the conjugating ability of modified GO strengthened, which was examined by auxiliary osteogenic growth peptide conjugation. Overall, our work demonstrates GO's potential for stem cell applications while maintaining its oxygen content, which could enable further functionalization and fabrication of novel nano-biointerfaces.

A Highly Efficient Adsorbent Cu-Perusian Blue@Nanodiamond for Cesium in Diluted Artificial Seawater and Soil-Treated Wastewater

A new adsorbent Cu-Perussian blue@Nanodiamond (Cu-PB@DND) for Cs⁺ removal was prepared and characterized with IR, SEM, X-ray diffraction, particle size analysis, and zeta-potential. The adsorbent consists of a core of aggregated detonation nanodiamond (DND) particles with the surface treated with Cu-PB. Cesium adsorption was studied in two modes; a co-precipitation mode and a batch mode. In the co-precipitation mode, DND, CuCl₂, and K₄[Fe(CN)₆] were added sequentially to a Cs⁺ solution in diluted artificial seawater. In the batch mode, adsorbent Cu-PB@DND was dispersed into a Cs⁺ solution with stirring. The distribution coefficient (K_d) of the co-precipitation mode was 8.8 × 10⁷ (mL/g) at Cs⁺ 6.6 ppm in 0.07% seawater. The K_d value of the batch mode was 1.3 × 10⁶ (mL/g). Precipitation of Cs⁺-incorporated particles was complete, and post filtration was not necessary. Excess copper and iron ions were completely removed and were not detected in the supernatant. The adsorption data for Cu-PB@DND were analyzed by assuming Langmuir isotherm and a good fit was obtained with a maximum adsorption capacity Q_max of 759 mg/g. The co-precipitation method was also applied to soil-treated wastewater.

Metallic Ti3C2Tx MXene Gas Sensors with Ultrahigh Signal-to-Noise Ratio

Achieving high sensitivity in solid-state gas sensors can allow the precise detection of chemical agents. In particular, detection of volatile organic compounds (VOCs) at the parts per billion (ppb) level is critical for the early diagnosis of diseases. To obtain high sensitivity, two requirements need to be simultaneously satisfied: (i) low electrical noise and (ii) strong signal, which existing sensor materials cannot meet. Here, we demonstrate that 2D metal carbide MXenes, which possess high metallic conductivity for low noise and a fully functionalized surface for a strong signal, greatly outperform the sensitivity of conventional semiconductor channel materials. Ti₃C₂T_x MXene gas sensors exhibited a very low limit of detection of 50-100 ppb for VOC gases at room temperature. Also, the extremely low noise led to a signal-to-noise ratio 2 orders of magnitude higher than that of other 2D materials, surpassing the best sensors known. Our results provide insight in utilizing highly functionalized metallic sensing channels for developing highly sensitive sensors.

Advanced Energy Storage Devices: Basic Principles, Analytical Methods, and Rational Materials Design

Tremendous efforts have been dedicated into the development of high-performance energy storage devices with nanoscale design and hybrid approaches. The boundary between the electrochemical capacitors and batteries becomes less distinctive. The same material may display capacitive or battery-like behavior depending on the electrode design and the charge storage guest ions. Therefore, the underlying mechanisms and the electrochemical processes occurring upon charge storage may be confusing for researchers who are new to the field as well as some of the chemists and material scientists already in the field. This review provides fundamentals of the similarities and differences between electrochemical capacitors and batteries from kinetic and material point of view. Basic techniques and analysis methods to distinguish the capacitive and battery-like behavior are discussed. Furthermore, guidelines for material selection, the state-of-the-art materials, and the electrode design rules to advanced electrode are proposed.

Surface modification effects on the tensile properties of functionalised graphene oxide epoxy films

Graphene oxide (GO) is a candidate for nanofillers to improve the mechanical and thermal stability of nanocomposites. In order to determine the molecular interaction to improve the mechanical properties of GO–epoxy resin composites, we investigated the relationship between GO oxidation properties and the tensile strength of the epoxy resin. With respect to GO preparation, graphite was oxidised by the Brodie or Hummers method, and the oxidised GO was reduced or chloride substituted. The X-ray photoelectron spectroscopy (XPS) spectral patterns indicate that a shorter Brodie oxidation method GO (B-GO) is associated with a higher proportion of hydroxyl groups. The oxidised GO materials, with the exception of the sample produced by the 54 h Brodie oxidation method, improved the tensile strength of the composites while the epoxy resin with reduced or chlorinated GO did not increase the tensile strength of the film. Based on XPS and elemental analyses, the improvement in the tensile strength is due to the presence of O atom based functional groups, such as hydroxyl groups, on the GO surface. The interaction between the epoxy resin and O atom based functional groups on the GO contributes to improving the tensile strength of the composites.

In order to determine the molecular interaction to improve the mechanical properties of graphene oxide (GO)–epoxy resin composites, we investigated the relationship between GO oxidation properties and the tensile strength of the epoxy resin.

Real-time and stepwise deoxidization processes to tune the photoluminescence properties of graphene oxide using EC-SPR spectroscopy

The development of a stepwise deoxidized process and real-time monitoring of the large-scale mass production of electrochemically reduced graphene oxide (ErGO) sheets are important issues.

Pseudocapacitive Desalination of Brackish Water and Seawater with Vanadium-Pentoxide-Decorated Multiwalled Carbon Nanotubes

A hybrid membrane pseudocapacitive deionization (MPDI) system consisting of a hydrated vanadium pentoxide (hV₂ O₅ )-decorated multi-walled carbon nanotube (MWCNT) electrode and one activated carbon electrode enables sodium ions to be removed by pseudocapacitive intercalation with the MWCNT-hV₂ O₅ electrode and chloride ion to be removed by non-faradaic electrosorption of the porous carbon electrode. The MWCNT-hV₂ O₅ electrode was synthesized by electrochemical deposition of hydrated vanadium pentoxide on the MWCNT paper. The stable electrochemical operating window for the MWCNT-hV₂ O₅ electrode was between -0.5 V and +0.4 V versus Ag/AgCl, which provided a specific capacity of 44 mAh g^-1 (corresponding with 244 F g^-1 ) in aqueous 1 m NaCl. The desalination performance of the MPDI system was investigated in aqueous 200 mm NaCl (brackish water) and 600 mm NaCl (seawater) solutions. With the aid of an anion and a cation exchange membrane, the MPDI hybrid cell was operated from -0.4 to +0.8 V cell voltage without crossing the reduction and oxidation potential limit of both electrodes. For the 600 mm NaCl solution, the NaCl salt adsorption capacity of the cell was 23.6±2.2 mg g^-1 , which is equivalent to 35.7±3.3 mg g^-1 normalized to the mass of the MWCNT-hV₂ O₅ electrode. Additionally, we propose a normalization method for the electrode material with faradaic reactions based on sodium uptake capacities.

An Affordable Wet Chemical Route to Grow Conducting Hybrid Graphite-Diamond Nanowires: Demonstration by A Single Nanowire Device

We report an affordable wet chemical route for the reproducible hybrid graphite-diamond nanowires (G-DNWs) growth from cysteamine functionalized diamond nanoparticles (ND-Cys) via pH induced self-assembly, which has been visualized through SEM and TEM images. Interestingly, the mechanistic aspects behind that self-assembly directed G-DNWs formation was discussed in details. Notably, above self-assembly was validated by AFM and TEM data. Further interrogations by XRD and Raman data were revealed the possible graphite sheath wrapping over DNWs. Moreover, the HR-TEM studies also verified the coexistence of less perfect sp² graphite layer wrapped over the sp³ diamond carbon and the impurity channels as well. Very importantly, conductivity of hybrid G-DNWs was verified via fabrication of a single G-DNW. Wherein, the better conductivity of G-DNW portion L2 was found as 2.4 ± 1.92 × 10⁻⁶ mS/cm and revealed its effective applicability in near future. In addition to note, temperature dependent carrier transport mechanisms and activation energy calculations were reported in details in this work. Ultimately, to demonstrate the importance of our conductivity measurements, the possible mechanism behind the electrical transport and the comparative account on electrical resistivities of carbon based materials were provided.

Oil Palm Waste-Based Precursors as a Renewable and Economical Carbon Sources for the Preparation of Reduced Graphene Oxide from Graphene Oxide

Herein, a new approach was proposed to produce reduced graphene oxide (rGO) from graphene oxide (GO) using various oil palm wastes: oil palm leaves (OPL), palm kernel shells (PKS) and empty fruit bunches (EFB). The effect of heating temperature on the formation of graphitic carbon and the yield was examined prior to the GO and rGO synthesis. Carbonization of the starting materials was conducted in a furnace under nitrogen gas for 3 h at temperatures ranging from 400 to 900 °C and a constant heating rate of 10 °C/min. The GO was further synthesized from the as-carbonized materials using the ‘improved synthesis of graphene oxide’ method. Subsequently, the GO was reduced by low-temperature annealing reduction at 300 °C in a furnace under nitrogen gas for 1 h. The I_G/I_D ratio calculated from the Raman study increases with the increasing of the degree of the graphitization in the order of rGO from oil palm leaves (rGOOPL) < rGO palm kernel shells (rGOPKS) < rGO commercial graphite (rGOCG) < rGO empty fruit bunches (rGOEFB) with the I_G/I_D values of 1.06, 1.14, 1.16 and 1.20, respectively. The surface area and pore volume analyses of the as-prepared materials were performed using the Brunauer Emmett Teller-Nitrogen (BET-N₂) adsorption-desorption isotherms method. The lower BET surface area of 8 and 15 m² g⁻¹ observed for rGOCG and rGOOPL, respectively could be due to partial restacking of GO layers and locally-blocked pores. Relatively, this lower BET surface area is inconsequential when compared to rGOPKS and rGOEFB, which have a surface area of 114 and 117 m² g⁻¹, respectively.

Comparative in vitro study of single and four layer graphene oxide nanoflakes - Cytotoxicity and cellular uptake

In recent years, graphene and its derivatives have been extensively investigated because of their unique properties, which can be used in many fields including biomedical applications. Therefore, detailed biological study is required. In the current paper the detailed toxicological studies on single and four layer graphene oxide (GO) nanoflakes is presented. The morphology and size of the nanomaterials were characterized via atomic force microscopy. Cytotoxicity, proliferation and internalization study were performed using various methods, including optical, confocal and Raman microscopy imaging, flow cytometry analysis, colorimetric and luminescent cell assays. Our first findings undeniably show that the nanomaterials' functionalization has a considerable impact on their behavior in a biological environment. The cytotoxicity assay confirmed comparable, dose dependent cytotoxicity of single and four layers GO flakes. The differences between these two nanomaterials became more distinct during cell proliferation study and ROS detection. Namely, markedly stronger inhibition of cell proliferation and higher ROS generation by one-layer GO-PEG than four-layer GO-PEG were observed. Cell imaging revealed efficient internalization of the both GO nanoflakes in a time dependent manner. These findings emphasize the role of number of layer and functionalization in GO toxicological characteristics and may provide helpful information for their further biomedical applications.

Microwave reduction of graphene oxide rationalized by reactive molecular dynamics

Obtaining graphene (GRA) in industrial quantities is among the most urgent goals in today's nanotechnology. Elegant methods involve the oxidation of graphite with its subsequent solvent-assisted exfoliation. The reduction of graphene oxide (GO) is challenging leading to a highly-disordered oxygen-rich material. A particularly successful microwave-induced reduction of GO was reported recently (Science, 2016, 353, 1413-1416). We mimic the experiment by reactive molecular dynamics and establish the molecular mechanisms of reduction and their time scales as functions of temperature. We show that the rapid removal of oxygen groups achieved by microwave heating leaves GRA sheets intact. The epoxy groups are most stable within GO. They can rearrange into the carbonyl groups upon quick heating. It is important to avoid creating holes upon graphite oxidation. They cannot be healed easily and undermine GRA thermal stability and electronic properties. The edge oxygen groups cannot be removed by irradiation, but their effect is marginal on the properties of μm GRA sheets. We demonstrate that different oxygen groups are removed from GO at drastically different temperatures. Therefore, it is possible to obtain separate fractions, e.g. carbonyl-, hydroxyl- and carboxyl-free partially reduced GO. Our results guide the improvement of the GO reduction methods and can be tested directly by experiment.

Carbocatalytic reductive coupling reactions via electron transfer from graphene to aryldiazonium salt

A reductive coupling reaction using two-dimensional nanocarbon, i.e., reduced graphene oxide (rGO), as a carbocatalyst and/or a reaction initiator was developed.

Accelerated evaporation of water on graphene oxide

The evaporation rate of water on patterned GO with different degrees of oxidation.

Derivatization and interlaminar debonding of graphite–iron nanoparticle hybrid interfaces using Fenton chemistry

Physico-chemical phenomena endure in the nanoscale domains of organic–inorganic interfaces for exfoliation, interfacial debonding and cracking of the graphite sheets.

Tuning graphitic oxide for initiator- and metal-free aerobic epoxidation of linear alkenes

Graphitic oxide has potential as a carbocatalyst for a wide range of reactions. Interest in this material has risen enormously due to it being a precursor to graphene via the chemical oxidation of graphite. Despite some studies suggesting that the chosen method of graphite oxidation can influence the physical properties of the graphitic oxide, the preparation method and extent of oxidation remain unresolved for catalytic applications. Here we show that tuning the graphitic oxide surface can be achieved by varying the amount and type of oxidant. The resulting materials differ in level of oxidation, surface oxygen content and functionality. Most importantly, we show that these graphitic oxide materials are active as unique carbocatalysts for low-temperature aerobic epoxidation of linear alkenes in the absence of initiator or metal. An optimum level of oxidation is necessary and materials produced via conventional permanganate-based methods are far from optimal.

Graphitic oxide is readily produced by the oxidation of graphite. Here the authors show that the amount and nature of the oxidant can be used to tune the properties of graphitic oxide, and furthermore report a carbocatalyst for alkene epoxidation without the need for metal or initiators.

Chemical reactivity of graphene oxide towards amines elucidated by solid-state NMR

Graphene oxide (GO) is an attractive nanomaterial for many applications. Controlling the functionalization of GO is essential for the design of graphene-based conjugates with novel properties. But, the chemical composition of GO has not been fully elucidated yet. Due to the high reactivity of the oxygenated moieties, mainly epoxy, hydroxyl and carboxyl groups, several derivatization reactions may occur concomitantly. The reactivity of GO with amine derivatives has been exploited in the literature to design graphene-based conjugates, mainly through amidation. However, in this study we undoubtedly demonstrate using magic angle spinning (MAS) solid-state NMR that the reaction between GO and amine functions occurs via ring opening of the epoxides, and not by amidation. We also prove that there is a negligible amount of carboxylic acid groups in two GO samples obtained by a different synthesis process, hence eliminating the possibility of amidation reactions with amine derivatives. This work brings additional insights into the chemical reactivity of GO, which is fundamental to control its functionalization, and highlights the major role of MAS NMR spectroscopy for a comprehensive characterization of derivatized GO.

Concurrent Formation of Carbon-Carbon Bonds and Functionalized Graphene by Oxidative Carbon-Hydrogen Coupling Reaction

Oxidative C-H coupling reactions were conducted using graphene oxide (GO) as an oxidant. GO showed high selectivity compared with commonly used oxidants such as (diacetoxyiodo) benzene and 2,3-dichloro-5,6-dicyano-p-benzoquinone. A mechanistic study revealed that radical species contributed to the reaction. After the oxidative coupling reaction, GO was reduced to form a material that shows electron conductivity and high specific capacitance. Therefore, this system could concurrently achieve two important reactions: C-C bond formation via C-H transformation and production of functionalized graphene.