Influence of the ultrasound cavitation intensity on reduced graphene oxide functionalization

Graphene is a valuable and useful nanomaterial due to its exceptionally high tensile strength, electrical conductivity and transparency, as well as the ability to tune its materials properties via functionalization. One of the most important features needed to integrate functionalized graphene into products via scalable processing is the effectiveness of graphene dispersion in aqueous and organic solvents. In this study, we aimed to achieve the functionalization of reduced graphene oxide (rGO) by sonication in a one-step process using polyvinyl alcohol (PVA) as a model molecule to be bound to the rGO surface. We investigated the influence of the sonication energy on the efficacy of rGO functionalization. The correlation between the performance of the high-intensity ultrasonic horn and the synthesis of the PVA functionalized rGO was thoroughly investigated by TGA coupled with MS, and IR, Raman, XPS, Laser diffraction, and SEM analysis. The results show that the most soluble PVA-functionalized rGO is achieved at 50% of the ultrasonic horn amplitude. Analysis of cavitation dynamics revealed that in the near vicinity of the horn it is most aggressive at the highest amplitude (60%). This causes rGO flakes to break into smaller domains, which negatively affects the functionalization process. On the other hand, the maximum of the pressure pulsations far away from the horn is reached at 40% amplitude, as the pressure oscillations are attenuated significantly in the 2-phase flow region at higher amplitudes. These observations corelate well with the measured degree of functionalization, where the optimum functionalized rGO dispersion is reached at 50% horn amplitude, and generally imply that cavitation intensity must be carefully adjusted to achieve optimal rGO functionalization.     

Reduced graphene oxide foils for ion stripping applications

ABSTRACT

Reduced graphene oxide (rGO) films can be employed as ion strippers in an accelerator. They show some advantages with respect to the graphite foils, due to their high thermal and electrical conductivity, low density, high mechanical resistance and high stability. Thin graphene oxide (GO) films with a sub-micron thickness have been synthesized and transformed into reduced GO (rGO) by ion beam irradiations. Physical characterizations of the pristine and ion irradiated GO films have been performed. Measurements of stripping efficiency have been carried out by using helium, lithium, carbon and oxygen ion beams. The rGO stripper films demonstrate a significantly high charge production, comparable to that of the graphite films but with the advantage of a longer lifetime.     

Investigation of Raman and photoluminescence studies of reduced graphene oxide sheets

In this paper, we are investigating the Raman and photoluminescence properties of reduced graphene oxide sheets (rGO). Moreover, graphene oxide (GO) sheets are synthesized using Hummer’s method and further reduced into graphene sheets using D-galactose. Both GO and rGO are characterized by UV-vis spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Thermogravimetric (TGA) analysis. Raman analysis of rGO shows the restoration of graphitic domains in GO after reduction. The photoluminescence of rGO showed emission in the UV region which is blue shifted along with luminescent quenching as compared to GO. This blue shift and quenching in photoluminescence arises due to the newly formed crystalline sp² clusters in rGO which created percolation pathways between the sp² clusters already present.     

Density functional study of the reaction of O2 with a single site on the zigzag edge of graphene

The reaction between molecular oxygen and an isolated zigzag graphene edge has been studied using density functional theory at the B3LYP/6-31G(d) level of theory. The initial reaction forms a peroxide, ΔH = −135 kJ mol⁻¹. If the graphene edge is pre-oxidised, the dangling peroxy atom can (E_a = 91 kJ mol⁻¹) migrate across contiguous ketone groups until finding another vacant site and stabilizing as a ketone. However, if no further vacant sites are available, the peroxy oxygen has a number of other possibilities open to it, including desorption of an oxygen atom (E_a = 140 kJ mol⁻¹), migration via the basal plane to form a lactone (E_a = 147 kJ mol⁻¹), and direct interaction with an adjacent oxide to form the lactone or a carbonate (E_a = 146 kJ mol⁻¹). The combination of thermal energy and the heat released in the initial formation of the peroxy adduct is likely to be sufficient to overcome these secondary barriers at modest temperatures.Transfer of the dangling peroxy O to the basal plane produces an epoxide that is mobile on the basal surface (E_a = 40–80 kJ mol⁻¹) but that is transferred back to the edge upon coming into proximity of either a vacant edge site or ketone. The instability of the edge epoxide structure implies that it cannot play a significant role in carbon gasification through promoting the reactivity of ketones, contrary to earlier suggestions.The desorption of an oxygen atom creates a very active species capable of reacting with basal or edge sites as well as with oxygen complexes. The reaction of ketone + O has been reported to yield a five-membered ring + CO₂, leading to an overall stoichiometry which is consistent with the observed oxyreactivity of carbon surface oxides identified in isotopic labelling studies in which one O atom is gasified and the other forms a new surface oxide.     

Fabrication of ultrasound-mediated tunable graphene oxide nanoscrolls

In this work, a cost-effective and facile method was adopted for the fabrication of graphene oxide nanoscrolls (GONS) by low frequency (20 kHz) ultrasonication with tunable dimensions. The graphene oxide (GO) was synthesized by modified Hummer’s method using synthetic graphite as a base material. Later, GO suspension (0.05 g L⁻¹) were made using methanol as solvent and subjected to different ultrasonication conditions. It was found that GO sheets curls themselves into nanoscrolls by overcoming the energy barrier for scrolling with the help of bubble cavitation energy provided by ultrasonication. Also, the effect of ultrasonication power (100–150 W) for irradiation time (0.5–3 h) over the GONS dimensions were investigated. The spiral wounded GONS structures were shown using electron microscopy. Raman Spectroscopy, Thin-film X-Ray Diffraction, Energy Dispersive X-Ray, FT Infrared Spectroscopic analysis were also done to endorse GONS formation. Factors affecting GONS formation such as sonication power and solvent selection were studied as scrolling of GO sheets are strongly dependent on sonication parameters and solvent characteristics. It was found that GONS length varies inversely with irradiation time for identical power density. Also, a solvent with relatively large Hansen solubility parameter, lower dipole movement and less negative value of zeta potential support GONS formation of longer length. Raman analysis overlays the rapid oxygen-defect site cleavage mechanism. The obtained GONS unlocks further developments in various engineering applications like adsorption, drug delivery and filtration membrane.     

Preparation of graphene/PMMA composites with assistance of ultrasonic wave under supercritical CO2 conditions

The two-dimensional material graphene has many excellent physicochemical properties such as large specific surface area, high electron migration rate, good chemical properties, good thermal conductivity, high elastic modulus and mechanical strength that make it very valuable for theoretical research and application in the preparation of graphene/polymer composites. In this paper, the effects of ultrasonic intensity and reaction time on the molecular weight and yield of PMMA under supercritical CO₂ conditions were investigated. It was found that there are threshold and optimal values of ultrasonic intensity for initiating the reaction in supercritical CO₂ system. The threshold value is 150 W/cm² and the optimal ultrasonic intensity value is 225 W/cm². There is also an optimal value of ultrasonic initiation time for ultrasonic initiation polymerization. Combining the reaction yield and the molecular weight of the product, 2 h of ultrasonic initiation is a suitable initiation reaction time. Based on the synthesis of PMMA by ultrasonic excitation, the preparation of Graphene/PMMA composites by ultrasound assistance was also investigated. The TG and DTG characterization of PMMA and complex materials prepared by ultrasonic excitation showed that the radicals generated by ultrasonic excitation were uniformly distributed and did not generate unsaturated double bonded polymers. The electrical conductivity of the Graphene/PMMA composites prepared by ultrasonic excitation at a graphene content of 1 wt% increased to 1.13 × 10^-1 S/cm, which is better than that of the Graphene/PMMA prepared by in situ polymerization. This may be attributed to the ultrasound-assisted supercritical CO₂ fluid action that resulted in a more uniform distribution of Graphene mixed with PMMA in the prepared composites. Therefore, it is of important practical significance for the preparation of Graphene/PMMA composites by ultrasound-induced polymerization.     

Synthesis and characterization of graphene oxide,reduced graphene oxide and their nanocomposites with polyethylene oxide

This work describes the synthesis of GO, rGO and their nanocomposites with PEO. GO and rGO were prepared by the modified Hummers method and in-situ reduction of GO utilizing green reductant L (+) Ascorbic acid. The nanocomposites were characterized by Fourier-transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Thermogravimetric Analysis (TGA), and Universal Testing Machine (UTM). FT-IR and XRD confirmed the synthesis of GO and rGO. FE-SEM confirmed the uniformly exfoliated GO and rGO nanosheets in the polymer matrix. Hydrogen bonding was the main interaction mechanism for GO with PEO while no interaction was detected by FT-IR for rGO. Enhanced thermal stability was observed for both GO/PEO and rGO/PEO nanocomposites. The mechanical analysis showed an increase in Young's modulus, tensile strength, and elongation at break for GO/PEO nanocomposites, which is attributed to the homogeneous dispersion and hydrophilic hydrogen bonding interaction of GO with PEO.     

Improvement of water softening efficiency in capacitive deionization by ultra purification process of reduced graphene oxide

Capacitive deionization (CDI) is the next generation of water desalination and softening technology by using relatively low capacitive current of electrochemical double layer. Among various carbon-based materials used for making electrode, reduced graphene oxide (rGO) has been intensively studied due to its excellent electrical conductivity and high surface area. Although Hummer method for making graphene oxide (GO) and rGO is a simple process, it remains some impurities in inherent GO and rGO which affect negatively to the CDI performance. In this work, we successfully prepared ultra purified GO and rGO by modifying Hummer method in order to remove entirely excess elements degrading the CDI performance. The electrosorption capacity of ultra purified rGO is considerably better than that of previous rGO, and maximum removal achieves 3.54 mg g⁻¹ at applied voltage of 2.0 V. Thus, this result could be comparable to other researches in CDI process.     

Enhanced catalytic hydrogenation activity of Ni/reduced graphene oxide nanocomposite prepared by a solid-state method

A simple solid-state method has been applied to synthesize Ni/reduced graphene oxide (Ni/rGO) nanocomposite under ambient condition. Ni nanoparticles with size of 10–30 nm supported on reduced graphene oxide (rGO) nanosheets are obtained through one-pot solid-state co-reduction among nickel chloride, graphene oxide, and sodium borohydride. The Ni/rGO nanohybrid shows enhanced catalytic activity toward the reduction of p-nitrophenol (PNP) into p-aminophenol compared with Ni nanoparticles. The results of kinetic research display that the pseudo-first-order rate constant for hydrogenation reaction of PNP with Ni/rGO nanocomposite is 7.66 × 10^?3 s^?1, which is higher than that of Ni nanoparticles (4.48 × 10^?3 s^?1). It also presents superior turnover frequency (TOF, 5.36 h^?1) and lower activation energy (E_a, 29.65 kJ mol^?1) in the hydrogenation of PNP with Ni/rGO nanocomposite. Furthermore, composite catalyst can be magnetically separated and reused for five cycles. The large surface area and high electron transfer property of rGO support are beneficial for good catalytic performance of Ni/rGO nanocomposite. Our study demonstrates a simple approach to fabricate metal-rGO heterogeneous nanostructures with advanced functions.

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Tuned CuCr layered double hydroxide/carbon-based nanocomposites inducing sonophotocatalytic degradation of dimethyl phthalate

This study is the first to explore the possibility of utilizing CuCr LDH decorated on reduced graphene oxide (rGO) and graphene oxide (GO) as sonophotocatalysts for the degradation of dimethyl phthalate (DMP). CuCr LDH and its nanocomposites were successfully fabricated and characterized. Scanning electron microscopy (SEM) along with high-resolution transmission electron microscope (HRTEM) both evidenced the formation of randomly oriented nanosheet structures of CuCr LDH coupled with thin and folded sheets of GO and rGO. The impact of diverse processes on the degradation efficiency of DMP in the presence of the so-prepared catalysts was compared. Benefiting from the low bandgap and high specific surface area, the as-obtained CuCr LDH/rGO represented outstanding catalytic activity (100 %) toward 15 mg L⁻¹ of DMP within 30 min when subjected to light and ultrasonic irradiations simultaneously. Radical quenching experiments and visual spectrophotometry using an O-phenylenediamine revealed the crucial role of hydroxyl radicals compared to holes and superoxide radicals. Overall, outcomes disclosed that CuCr LDH/rGO is a stable and proper sonophotocatalyst for environmental remediation.     

Simultaneous reduction and sulfonation of graphene oxide for efficient hole selectivity in polymer solar cells

We developed sulfonated, reduced graphene oxide (S-RGO) through fuming/concentrated sulfuric acid treatment of graphene oxide (GO) in ambient conditions. It was demonstrated that the optical band gap and electrical conductivity of S-RGO are easily tunable, and depend on the level of reduction and sulfonation of GO. Whereas, reduction and sulfonation were found dependent on SO₃ content, acid strength, and gas tightness of the reaction mixture. It's actually the water content of oleum that determines the nature of the final product. The easily adjustable band gap and electrical conductivity suggest that S-RGO can be employed as a potential hole extraction layer (HEL) material for several donor-acceptor systems. For P3HT:PC₆₁BM based inverted polymer solar cells, it was observed that the shape of the J–V curve is tailorable with the choice of HEL. Compared to a 2.75% power conversion efficiency (PCE) attained with PEDOT:PSS, a PCE of 2.80% was achieved with tuned S-RGO. Our results imply that an S-RGO of sufficiently high band gap and conductivity can replace some of the state of the art HEL materials for a host of device applications.     

Synthesis of graphene oxide and graphene quantum dots from miscanthus via ultrasound-assisted mechano-chemical cracking method

Whilst graphene materials have become increasingly popular in recent years, the followed synthesis strategies face sustainability, environmental and quality challenges. This study proposes an effective, sustainable and scalable ultrasound-assisted mechano-chemical cracking method to produce graphene oxide (GO). A typical energy crop, miscanthus, was used as a carbon precursor and pyrolysed at 1200 °C before subjecting to edge-carboxylation via ball-milling in a CO₂-induced environment. The resultant functionalised biochar was ultrasonically exfoliated in N-Methyl-2-pyrrolidone (NMP) and water to form GOs. The intermediate and end-products were characterised via X-ray diffraction (XRD), Raman, high-resolution transmission electron microscopy (HR-TEM) and atomic force microscopy (AFM) analyses. Results show that the proposed synthesis route can produce good quality and uniform GOs (8–10% monolayer), with up to 96% of GOs having three layers or lesser when NMP is used. Ultrasonication proved to be effective in propagating the self-repulsion of negatively-charged functional groups. Moreover, small amounts of graphene quantum dots were observed, illustrating the potential of producing various graphene materials via a single-step method. Whilst this study has only investigated utilising miscanthus, the current findings are promising and could expand the potential of producing good quality graphene materials from renewable sources via green synthesis routes.     

A simple approach to synthesize nanosized sulfur/graphene oxide materials for high-performance lithium/sulfur batteries

We report on a simple and facile synthesis route for the sulfur/graphene oxide composite via ultrasonic mixing of the nano-sulfur and graphene oxide aqueous suspensions followed by a low-temperature heat treatment. High-resolution transmission and scanning electronic microscopy observations revealed the formation of a highly porous structure consisting of sulfur with uniform graphene oxide coating on its surface. The resulting sulfur/graphene oxide (S/GO) composite exhibited high and stable specific discharge capacities of 591 mAh g^?1 after 100 cycles at 0.1 C and good rate capability. This enhanced electrochemical performance could be attributed to the effective confining the polysulfides dissolution and accommodation of the volume changes during the Li-S electrochemical reaction by the functional groups on the graphene oxide coating layer. Furthermore, the highly developed porous structure of S/GO composite favors the enhanced ion transport and electrolyte diffusion.     

Sono-synthesis approach of reduced graphene oxide for ammonia vapour detection at room temperature

In this paper, we report the sono-synthesis of reduced graphene oxide (rGO) using polyethyleneimine (PEI), and its performance for ammonia vapour detection at room temperature. Graphene oxide (GO) and reduced graphene oxide (rGO) were prepared by sonication method by using low-frequency ultrasound under ambient condition and films were deposited by Doctor Blade method. The rGO, which has vapour accessible structure showed a good sensing response with a minimum detection limit of 1 ppm and the detection range from 1 ppm to 100 ppm. The sensing response was found to be 2% at 1 ppm and 34% at 100 ppm of ammonia and the developed sensor operated at room temperature. The sensor displays a response time of 6 s and a recovery time of 45 s towards 100 ppm of ammonia vapour. The source for the highly sensitive, selective and stable detection of ammonia with negligible interference from other vapours is discussed and reported. We believe reduced graphene oxide (rGO) could potentially be used to manufacture a new generation of low-power portable ammonia sensors.     

Mechanical properties of thin films of graphene materials: A study on their structural quality and functionalities

Several studies have been done on physiochemical properties of thin films of graphene materials, but less on their mechanical properties. The mechanical properties such as tensile and storage modulus of films of graphene oxide (GO), different reduced graphene oxides (rGO), functionalised reduced graphene oxide (frGO) and a few layers graphene (graphene) were analysed in this study. During syntheses processes, a range of variations occurs due to different reducing agents and functionalising components used; this affects or changes the mechanical properties of the materials. In addition, it has become vital to comprehend the mechanical properties of these films as the potential applications such as sensor and electrodes demand extended life cycles or lifetime. It has been found that the ultimate tensile strength (UTS), tensile modulus, and storage modulus vary across all the samples that highly depend on nature/efficiency of reducing agent used, amount of impurities such as oxygen functional groups and defect density such as discrepancies/holes in the aromatic structure. The highest UTS and modulus have been identified with a few layers graphene and with hydroiodic acid reduced GO among the rGOs. The frGO shows almost similar properties to that of graphene.     

Hydrothermal method for the production of reduced graphene oxide

Reduced graphene oxide, RGO (also called chemically modified graphene, CMG) was synthesized by a simple hydrothermal method, with graphite oxide (GO), prepared by the modified Hummers method, served as the raw material. Structural and morphological studies indicate the degree of reduction is dependent on the temperature, which is also verified by Raman analysis. The variation in interlayer distance and the intensity ratio of the D to G Raman modes (I_D/I_G) indicates higher reaction temperature can accelerate the reduction of GO. The conductivity also varies with the degree of reduction, as verified by electrochemical analyzer. Moreover, the reaction process affects organic functional groups, the mechanism during the reaction process is discussed.     

Facile preparation of graphene nitride by irradiating MHz ultrasound

The present study aimed at developing a simple sonochemical method to prepare graphene nitride from the mixture of graphite and aqueous ammonia solution. Ultrasound of 1.6 MHz was irradiated to the sample in a fabricated sonoreactor at predetermined ultrasonic power and duration. The one-pot method succeeded in the preparation of graphene nitride. The generation was proven by XPS analysis in finding N1S peak in the spectrum. Detail analysis of N1s peak suggested that the major nitrogen species was pyrrolic type. Furthermore, the presence of CO bond proved the oxidation by OH radical. The reaction product had the value of N/C as high as 0.08, which is comparable to reported values for ultrasonic preparation of graphene nitride. The fact indicates that the significance of chemical effects of MHz range ultrasound, and the finding of the simple preparation method will accelerate practical application of graphene nitride.     

NMR of localized magnetic states in graphene

The NMR relaxation rate is studied on the magnetic states of an impurity in bilayer graphene within a tight-binding scenario. The dependencies of the relaxation rate on temperature, interlayer interaction and also the chemical potential have been considered. Although for low temperatures we observe the usual Korringa relation, a characteristic of the conventional fermions, the rate increases with the increase in temperature and tends to saturate for high temperatures. For small interlayer interactions (t_⊥) the system can be either magnetic or non-magnetic. However for higher t_⊥ we observe the existence of only a pure magnetic state. In graphene this transition is also observed with two cusps related to the magnetic to non-magnetic transition, which modifies to a single hump for higher t_⊥, where the system is purely magnetic for any value of chemical potential.     

Low intensity-ultrasonic irradiation for highly efficient,eco-friendly and fast synthesis of graphene oxide

High quality graphene oxide (GO) with low layer number (less than five layers) and large inter-layer space was produced via a new and efficient method using environmentally friendly, fast and economic ultrasonic radiation. The ultrasonic method neither generated any toxic gas nor required any NaNO₃, which have been the main drawbacks of the Hummers methods. The major obstacles of the recently reported improved Hummers method for GO synthesis, such as high reaction temperature (50 °C) and long reaction time (12 h), were successfully solved using a low intensity-ultrasonic bath for 45 min at 30 °C, which significantly reduced the reaction time and energy consumption for GO synthesis. Furthermore, ultrasonic GO exhibited higher surface area, higher crystallinity and higher oxidation efficiency with many hydrophilic groups, fewer sheets with higher spaces between them, a higher sp³/sp² ratio, and more uniform size distribution than classically prepared GO. Therefore, the new ultrasonic method could be applicable for the sustainable and large-scale production of GO. The production yield of the ultrasonic-assisted GO was 1.25-fold greater than the GO synthesized with the improved Hummers method. Furthermore, the required production cost based on total energy consumption for ultrasonic GO was only 6.5% of that for classical GO.     

Dynamics of Dirac quasi-particles in lattice vibration and anomalous phonon frequency shift of graphene

By exact solution of time-dependent Schrödinger equation of electron in graphene under interaction with E_2g phonons, we investigate the dynamical behavior of Dirac quasi-particle in the process of lattice vibration. Due to the global geometric phases acquired by electron during lattice vibration, an anomalous shift of the vibration frequency is obtained. We calculate the Fermi energy dependence of frequency shift which is in consistence with experiment in case of small doping density.