Ultrasound assisted simultaneous reduction and direct functionalization of graphene oxide with thermal and cytotoxicity profile

The new sonochemical approach for simultaneous reduction and direct functionalization of graphene oxide (GrO) has been developed. The GrO was functionalized with 2-Aminobenzoxazole (2-ABOZ) in twenty min with complete deletion of hazardous steps. The significance of ultrasound was exemplified with the comparative conventional methods. The newly prepared f-(2-ABOZ)GrO was extensively characterized with near edge X-ray absorption fine structure (NEXAFS) spectroscopy, ¹³C solid state NMR, XPS, XRD, HRTEM, SAED, AFM, Raman, UV–vis, FTIR and TGA. The thermal stability of f-(2-ABOZ)GrO was confirmed with total percentage weight loss in TGA. The biological activity of f-(2-ABOZ)GrO was explored with MCF-7 and Vero cell lines. The inherent cytotoxicity was evaluated with SRB assay at 10, 20, 40 and 80 μg mL⁻¹. The estimated cell viabilities were >78% with f-(2-ABOZ) GrO. A high cytocompatibility of f-(2-ABOZ)GrO was ensured with in vitro evaluation on living cell lines, and low toxicity of f-(2-ABOZ)GrO was confirmed its excellent biocompatibility. The morphological effect on Vero cell line evidently supports the formation of biocompatible f-(2-ABOZ)GrO. Therefore, f-(2-ABOZ)GrO was emerged as an advanced functional material for thermally stable biocompatible coatings.     

Instantaneous integration of magnetite nanoparticles on graphene oxide assisted by ultrasound for efficient heavy metal ion retrieval

We fabricated a magnetite nanoparticle-graphene oxide (GO) hybrid via a non-chemical and one-step process assisted by ultrasound in an aqueous solution where the nanoparticle attached to the hydrophobic region on graphite oxide (multi-layered GO) which, at the same time, was exfoliated. Unlike chemical methods such as precipitation, oxygen-containing functional groups on GO have not been consumed or reduced during the hybridization, leading that this hybrid exhibited good water solubility and high adsorption capacity for heavy metal ions such as Pb(II) and Au(III). After the adsorption, the hybrid was instantly collected using a magnet. This method can be useful for hybridizing various nanoparticles with GO.     

Experimental study on the reducibility of graphene oxide by hydrazine hydrate

In this paper, we focus on the reducibility of graphene oxide by hydrazine hydrate. The main emphasis is placed on the problem of the dosage of hydrazine hydrate and the reaction time on the reduction of chemical groups. This paper describes a system for the analysis of FT-IR. A model is developed for the FT-IR analysis using exponential decay function (Y=Y₀e^−RX). The height of the peaks in FT-IR spectra of the chemical groups was fitted with exponential decay function. Decay constant R is given the name of Reducibility Factor R. Emphasis is placed on the decay constant R in the fitting functions by which the reducibility of the chemical groups on graphene oxide is evaluated. It is found that R is different for different chemical groups. The sequence is R_OH>R_C=O>R_C−O−C. This means that C−O−C is the most difficult one to be reduced, OH is the most easy, and C=O is between C−O−C and OH. The experimental results reveal a basis for the research of the application of graphene oxide.     

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.     

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.     

Sonochemical synthesis of amide-functionalized metal-organic framework/graphene oxide nanocomposite for the adsorption of methylene blue from aqueous solution

Graphene oxide-[Zn₂(oba)₂(bpfb)]·(DMF)₅ metal-organic framework nanocomposite (GO-TMU-23; H₂oba = 4,4′-oxybisbenzoic acid, bpfb = N,N′-bis-(4-pyridylformamide)-1,4-benzenediamine, DMF = N,N-dimethylformamide) is prepared through a simple and large-scale sonochemical preparation method at room temperature. The obtained nanocomposite is characterized by Field Emission Scanning Electron Microscopy (FE-SEM), powder X-ray diffraction (PXRD) and FT-IR spectroscopy. Additionally, the absorption ability of GO-TMU-23 nanocomposite toward cationic dye methylene blue was also performed. Significantly, GO-TMU-23 nanocomposite exhibits remarkably accelerated adsorption kinetics for methylene blue in comparison with the parent materials. The adsorption process shows that 90% of the dye has been removed and the equilibrium status has been reached in 2 min by using the nanocomposites as the adsorbent.     

Laser effects on graphene oxide irradiated in high vacuum

Graphene oxide (GO) foils were irradiated by using different fluences of an infrared nanosecond pulsed laser and characterized before and after the laser irradiation. The produced laser ablation was investigated as well as the generated plasma. Optical and AFM microscopies, mass quadrupole spectrometry, Rutherford backscattering analysis and X-ray photoelectron spectroscopy were used to analyze the irradiated GO foils. Results demonstrated that the GO loses oxygen with the laser irradiation becoming richer in sp²-hybridized carbon content.     

Effect of copper oxide on the resistive switching responses of graphene oxide film

The role of CuO films in meliorating resistive switching behavior of graphene oxide (GO) in CuO/GO/CuO memory structure was investigated. An increase in the set voltage from 1.3 to 3.0 V and a step-like switching current was clearly observed when the GO film was sandwiched between two CuO layers. It is attributed to the fact that the set voltage of GO is lower than that of CuO and accumulated charge carriers located at the interface of GO and CuO can pass through CuO abruptly at set voltage of 3.0 V. Our results suggested that designed sandwich structure of materials with different set voltage enables to amend resistive switching response characteristics.     

Ultrasound irradiation: A robust approach for direct functionalization of graphene oxide with thermal and antimicrobial aspects

Sonochemical waves as mechanochemical energy was employed to exfoliate graphite oxide and functionalized graphene oxide (GrO), through a reaction of solvent and accountable for top-down and bottom-up approach respectively. The in situ formation of ester intermediate was inferred and a polymeric surface of GrO was further functionalized with 6-Aminoindazole (6-AIND) through sonochemical nucleophilic substitution reaction. As compared to conventional method the effect of ultrasound was verified for the direct functionalization of GrO. The conventional hazardous acylation step for functionalization of GrO was deleted in ultrasound assisted formation of f-(6-AIND) GrO nanocomposite, prepared by stereoselective exploitation of carboxyl groups at edges of GrO. The characterization has ascertained a covalent attachment of 6-AIND onto GrO surface with ATR-FTIR, XPS, SSNMR, TGA, DSC, XRD, AFM, RAMAN, EDX, SEM, BET and elemental analyzer. A weight loss in TGA depicts enhanced thermal stability of f-(6-AIND) GrO and a thermally sensitive behavior. The f-(6-AIND) GrO was studied for in vitro antimicrobial activity to ensure health and environmental safety. Antibacterial activity was identified against human pathogenic gram-positive (Staphylococcus aureus; ATCC 25923) and gram-negative bacteria (Escherichia coli; ATCC 25922). The antifungal activity was observed against Candida albicans (ATCC 10231).     

Ultrasound coupled with supercritical carbon dioxide for exfoliation of graphene: Simulation and experiment

Ultrasound coupled with supercritical CO₂ has become an important method for exfoliation of graphene, but behind which a peeling mechanism is unclear. In this work, CFD simulation and experiment were both investigated to elucidate the mechanism and the effects of the process parameters on the exfoliation yield. The experiments and the CFD simulation were conducted under pressure ranging from 8 MPa to 16 MPa, the ultrasonic power ranging from 12 W to 240 W and the frequency of 20 kHz. The numerical analysis of fluid flow patterns and pressure distributions revealed that the fluid shear stress and the periodical pressure fluctuation generated by ultrasound were primary factors in exfoliating graphene. The distribution of the fluid shear stress decided the effective exfoliation area, which, in turn, affected the yield. The effective area increased from 5.339 cm³ to 8.074 cm³ with increasing ultrasonic power from 12 W to 240 W, corresponding to the yield increasing from 5.2% to 21.5%. The pressure fluctuation would cause the expansion of the interlayers of graphite. The degree of the expansion increased with the increase of the operating pressure but decreased beyond 12 MPa. Thus, the maximum yield was obtained at 12 MPa. The cavitation might be generated by ultrasound in supercritical CO₂. But it is too weak to exfoliate graphite into graphene. These results provide a strategy in optimizing and scaling up the ultrasound-assisted supercritical CO₂ technique for producing graphene.     

Investigation on fluorescence quenching of dyes by graphite oxide and graphene

Rhodamine B (Rh B), eosin (E) and methylene blue (MB) were used as a probe to investigate the molecular structure and charge of the dyes on the sensitized efficiency of graphite oxide (GO) and graphene (G). The structure of the prepared GO and G were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. To study the electron transfer between dyes and GO or G, UV-vis absorption spectra (UV-vis), steady state fluorescence spectra (FL) and time resolved fluorescence spectra have been determined. It has been found that the electron transfer from the excited dyes to G was more efficient than to GO, and the transfer from excited MB to G was easier than to Rh B and E, because of the different electrostatic attraction between the dye and G.     

Ultrasonic assisted synthesis of Ni3(VO4)2-reduced graphene oxide nanocomposite for potential use in electrochemical energy storage

In the present study, Ni₃(VO₄)₂-reduced graphene oxide (NV/RGO) nanocomposite was synthesized for energy storage purpose. To this end, a mixture containing RGO nanosheets, Ni (CH₃COOH)₂ and Na₃VO₄ mixture was prepared under probe-type ultrasonic irradiation with frequency of 20 KHz and the optimized power of 100 W. The Raman and energy-dispersive X-ray spectroscopies confirmed the presence of RGO nanosheets, nickel and vanadium elements in the NV/RGO, respectively. In addition, field emission-scanning electron microscopy (FESEM) data showed the formation of the NV nanoparticles on the RGO nanosheets. NV/RGO nanocomposite was pasted on nickel foam (NF) and its performance was investigated in energy storage using a three-electrode cell containing 6 M KOH. In cyclic voltammogram of NV/RGO/NF, redox peaks for Ni (II)/Ni (III) with intensities higher than that for NV/NF were observed which confirms the synergistic effect of RGO on the performance of NV. Chronopotentiometry data revealed that the NV/RGO/NF electrode exhibits high capacity of 117.22 mA h g⁻¹ at 2 A g⁻¹. Electrochemical impedance spectroscopy also demonstrated an improvement in the electrical conductivity and electrochemical behavior of NV/RGO/NF nanocomposite compared to the RGO/NF and NV/NF. Furthermore, NV/RGO/NF electrode reserved about 88% of its initial capacity after 1000th potential cycle at 50 mV s⁻¹. Overall, the results of our study suggest that the NV/RGO nanocomposite prepared in the presence of ultrasonic irradiation might be regarded as a suitable active material for energy storage systems.     

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.     

In situ sonochemical reduction and direct functionalization of graphene oxide: A robust approach with thermal and biomedical applications

The rapid, robust, scalable and non-hazardous sonochemical approach for in situ reduction and direct functionalization of graphene oxide has been developed for non-toxic biomedical applications. The graphene oxide (GrO) was directly functionalized with tryptamine (TA) without using any hazardous acylating and coupling reagents. The reaction was completed within 20 min. An impact of ultrasound was inferred for a direct functionalization with other conventional methods. The evolved electronic states were confirmed with near edge X-ray absorption fine structure (NEXAFS). The direct covalent functionalization and formation of f-(TA) GrO was proven with FTIR, ¹³C solid state NMR, XPS, XRD, Raman‚ HRTEM, AFM and TGA. The total percentage weight loss in TGA confirms an enhanced thermal stability of f-(TA) GrO. The f-(TA) GrO was further explored for an investigation of in vitro antimicrobial activity to ensure the health and environmental safety. An outstanding antibacterial activity of f-(TA) GrO was found against gram positive Staphylococcus aureus at MIC 128 mg mL⁻¹. It confirms a suitability of f-(TA) GrO for thermally stable antibacterial coating. The f-(TA) GrO showed 39.14–48.9% antioxidant activities, evaluated with 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical assay. The inherent cytotoxicity of f-(TA) GrO was evaluated with SRB assay to living cells, MCF-7 and Vero. The estimated cell viabilities were >80% upon addition of f-(TA) GrO over a wide concentration range of 10–80 μg mL⁻¹. The high cytocompatibility of f-(TA) GrO confirms the low toxicity and an excellent biocompatibility. The morphological effect on Vero cell line, evidently confirmed the biocompatibility of f-(TA) GrO. Therefore, f-(TA) GrO was emerged as an advanced functional biomaterial for thermal and biomedical applications.     

State of the art and recent advances in the ultrasound-assisted synthesis,exfoliation and functionalization of graphene derivatives

Sonochemistry, an almost a century old technique was predominantly employed in the cleaning and extraction processes but this tool has now slowly gained tremendous attention in the synthesis of nanoparticles (NPs) where particles of sub-micron have been produced with great stability. Following this, ultrasonication techniques have been largely employed in graphene synthesis and its dispersion in various solvents which would conventionally take days and offers poor yield. Ultrasonic irradiation allows the production of thin-layered graphene oxide (GO) and reduced graphene oxide (RGO) of up to 1 nm thickness and can be produced in single layers. With ultrasonic treatment, reactions were made easy whereby graphite can be directly exfoliated to graphene layers. Oxidation to GO can also be carried out within minutes and reduction to RGO is possible without the use of any reducing agents. In addition, various geometry of graphene can be produced such as scrolled graphene, sponge or foam graphene, smooth as well as those with rough edges, each serving its own unique purpose in various applications such as supercapacitor, catalysis, biomedical, etc. In ultrasonic-assisted reaction, deposition of metal NPs on graphene was more homogeneous with custom-made patterns such as core-shell formation, discs, clusters and specific deposition at the edges of graphene sheets. Graphene derivatives with the aid of ultrasonication are the perfect catalyst for various organic reactions as well as an excellent adsorbent. Reactions which used to take hours and days were significantly reduced to minutes with exceedingly high yields. In a more recent approach, sonophotocatalysis was employed for the combined effect of sonication and photocatalysis of metal deposited graphene. The system was highly efficient in organic dye adsorption. This review provides detailed fundamental concepts of ultrasonochemistry for the synthesis of graphene, its dispersion, exfoliation as well as its functionalization, with great emphasis only based on recent publications. Necessary parameters of sonication such as frequency, power input, sonication time, type of sonication as well as temperature and dual-frequency sonication are discussed in great length to provide an overview of the resultant graphene products.     

Difference of dispersion behavior between graphene oxide and oxidized carbon nanotubes in polar organic solvents

This study examined the dispersion behavior of graphene oxide (GO) and oxidized carbon nanotubes (o-CNT) in a polar solvent, as well as the differences in the behavior related to the Hansen solubility parameter windows. In polar aprotic solvents, GO and o-CNT showed similar dispersion behavior. On the other hand, in polar protic solvents, such as ethanol and isopropanol, GO did not show dispersion stability whereas the o-CNTs did. This difference in the dispersion behavior between GO and o-CNTs resulted from the stronger hydrogen bonding between the GO interlayer induced by a large amount of oxygen functional groups and flexible two-dimensional morphology with a large surface area.     

不同基底石墨烯涂层的层间滑移减磨性能研究

石墨烯薄膜作为一种二维材料,是提高微/纳机电系统(MEMS/NEMS)摩擦力学性能的优异润滑剂.为了探究基底材料和石墨烯层数对其减磨性能的影响,本文通过在不同基底制备了不同层数的石墨烯涂层,利用原子力显微镜(AFM)实验和分子动力学(MD)仿真结合的方法,研究了石墨烯层数对减磨效应的影响.并且通过建立不同层数石墨烯涂层的摩擦性能分析模型,探究出石墨烯层间滑移是产生减磨的主要因素.结果表明:在不同载荷下,石墨烯涂层对硅基底和铜基底均有优异的减磨效果,摩擦力随着石墨烯层数的增加逐渐降低,当石墨烯层数大于10层时,达到最优99.3%的减磨效果.通过仿真分析发现,随着层数增加,石墨烯与基底的干摩擦转变为石墨烯的层间摩擦,并产生层间剪切滑移,石墨烯层间滑移是导致多层石墨烯优异减磨性能的主要因素.     

Impact of synthesis routes on the chemical,optical, and electrical properties of graphene oxides and its derivatives

Solution-processed graphene oxides in their reduced forms are prominent prospective functional materials for organic optoelectronics. For graphene oxide synthesis, several methods have been developed, which induce varying properties in their products. However, the dependence of the graphene oxide properties on their synthesis methods has rarely been studied, hindering the selection of the optimum synthesis route for a target application. In this study, we report our study results on the properties of synthesized graphene oxides and their reduced forms created using several synthesis methods, such as the modified Hummers' method, the improved method, and the Staudenmaier's method as well as from two commercial sources, Angstron Material, Inc. and Graphos, Inc. Focusing on the transparent electrode application, the properties of thin films were investigated using UV–visible spectroscopy, Hall measurements, atomic force microscopy, Raman spectroscopy, work function measurements, and X-ray photoelectron spectroscopy. Our results reveal significant morphological, elemental, structural, and optoelectrical property variations among the as-prepared and reduced thin films of graphene oxides by their synthesis methods. In addition, the results show that the graphene oxides synthesized using the modified Hummers' method and the product from Angstron Material, Inc. are the most suitable materials for the transparent electrode application.     

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.