Microwave-assisted Exfoliation Research Articles

Microwave-exfoliated 2D oligo-layer MoO3-x nanosheets with outstanding molecular adsorptivity and room-temperature gas sensitivity on ppb level

Large specific surface area and oxygen defects of metal oxides are the dominant factors to affect performances related to surface adsorption processes, and have been fully considered in the synthesis of materials. Understanding completely the effect of oxygen vacancies on the structure and the interaction with reactants is still under process. Here, for the first time, oxygen deficient two-dimensional (2D) oligo-layer molybdenum trioxide (MoO3-x) nanosheets with large aspect ratio of ∼670, were fabricated via a convenient microwave-assisted exfoliation method. The effect of oxygen vacancies on the surface adsorption of molecules was comprehensively investigated with experiments and DFT calculations. From the analysis of molecular dynamics and molecular electrostatic potential results, the enlarged interlayer spacings and oxygen vacancies working as the capture centers for negatively charged groups made 2D oligo-layer MoO3-x nanosheets promising for the adsorption for various organic molecules. Especially, a high value of ∼1290 mg/g rarely reported before was achieved for the adsorption of methylene blue. Benefited from the enlarged adsorption energies to gas molecules and the preferential adsorption of oxygen molecules in ambient atmosphere, p-type gas sensors based on 2D oligo-layer MoO3-x nanosheets were fabricated, which exhibited superior gas sensing performance to ethanol, acetone and hydrogen sulfide at room temperature. This work proposed an effective approach for synthesis of 2D nanomaterials, and underscored the importance of oxygen vacancies into 2D structures for different applications.

Visible-Light-Assisted Photoelectrochemical Biosensing of Uric Acid Using Metal-Free Graphene Oxide Nanoribbons.

In this study, we demonstrate the visible-light-assisted photoelectrochemical (PEC) biosensing of uric acid (UA) by using graphene oxide nanoribbons (GONRs) as PEC electrode materials. Specifically, GONRs with controlled properties were synthesized by the microwave-assisted exfoliation of multi-walled carbon nanotubes. For the detection of UA, GONRs were adopted to modify either a screen-printed carbon electrode (SPCE) or a glassy carbon electrode (GCE). Cyclic voltammetry analyses indicated that all Faradaic currents of UA oxidation on GONRs with different unzipping/exfoliating levels on SPCE increased by more than 20.0% under AM 1.5 irradiation. Among these, the GONRs synthesized under a microwave power of 200 W, namely GONR(200 W), exhibited the highest increase in Faradaic current. Notably, the GONR(200 W)/GCE electrodes revealed a remarkable elevation (~40.0%) of the Faradaic current when irradiated by light-emitting diode (LED) light sources under an intensity of illumination of 80 mW/cm2. Therefore, it is believed that our GONRs hold great potential for developing a novel platform for PEC biosensing.

N-doped graphene foam obtained by microwave-assisted exfoliation of graphite

The synthesis of metal-free but electrochemically active electrode materials, which could be an important contributor to environmental protection, is the key motivation for this research approach. The progress of graphene material science in recent decades has contributed to the further development of nanotechnology and material engineering. Due to the unique properties of graphene materials, they have found many practical applications: among others, as catalysts in metal-air batteries, supercapacitors, or fuel cells. In order to create an economical and efficient material for energy production and storage applications, researchers focused on the introduction of additional heteroatoms to the graphene structure. As solutions for functionalizing pristine graphene structures are very difficult to implement, this article presents a facile method of preparing nitrogen-doped graphene foam in a microwave reactor. The influence of solvent type and microwave reactor holding time was investigated. To characterize the elemental content and structural properties of the obtained N-doped graphene materials, methods such as elemental analysis, high-resolution transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy were used. Electrochemical activity in ORR of the obtained materials was tested using cyclic voltamperometry (CV) and linear sweep voltamperometry (LSV). The tests proved the materials’ high activity towards ORR, with the number of electrons reaching 3.46 for tested non-Pt materials, while the analogous value for the C-Pt (20 wt% loading) reference was 4.

Microwave-assisted exfoliation and tearing of graphene oxide in the presence of TiO2 nanoparticles

In this paper, we present a facile approach to reduce, exfoliate, and tear graphene oxide (GO) sheets, which can be used to produce graphene quantum dots. The prepared GO sheets were easily exfoliated and teared under UV and microwave (MW) irradiation in the presence of TiO2 nanoparticles (NPs). Results show that the GO sheets can be reduced/fragments by MW depending on the irradiation power. We found that; GO sheets are partially reduced under MW irradiation with a power of 90 W and crushed to nanosheets by increasing the MW power up to 450 W. In addition, the photoluminescence peak is broadened, confirming a decrease in the size of GO sheets. The optical bandgap of the nanosheets was measured about 3 eV in agreement with the results of the tight-binding calculation. In addition, HR-TEM results show the size of prepared nanosheets is about 20 nm, while X-ray photoelectron spectroscopy shows a bond formation between residual TiO2 NPs and the prepared nanosheets.

A comparative study for producing few-layer graphene sheets via electrochemical and microwave-assisted exfoliation from graphite powder

Graphene’s astonishing properties drew attention of many scientists to dedicate a lot of their time to find out more about this extraordinary material. However, challenges continue to produce high-quality graphene in large quantities by using inexpensive and readily available methods. In this study, three different graphite powders have been used as starting materials to produce few-layer graphene sheets, which are pure natural graphite (NGr) and two different electrochemically treated expanded graphite EE1 and EE2. Two simple and time-effective techniques have been applied on the samples interchangeably to investigate the order effect on producing graphene sheets in few-layer form. These techniques are sonication in dimethylformamide (DMF) for one hour and rapid microwave irradiation for 30 s. The study suggests that if the graphite powder is treated first with a strong exfoliation reagent followed by microwave irradiation, the obtained graphene will be high-quality few-layer (~ 5 layers). Sonication in DMF has worked to increase the inter-planar spacing between graphite layers, while microwave irradiation has worked to decrease the defect density ratio that resulted after sonication process. Our work suggests a novel route to prepare high-quality few-layer graphene sheets, not only with time efficiency, low-cost, and without using harmful chemicals, but also an adequate method for large-scale high-efficiency production of graphene materials.

Thermally Activated Upconversion Near-Infrared Photoluminescence from Carbon Dots Synthesized via Microwave Assisted Exfoliation.

Upconversion near-infrared (NIR) fluorescent carbon dots (CDs) are important for imaging applications. Herein, thermally activated upconversion photoluminescence (UCPL) in the NIR region, with an emission peak at 784 nm, which appears under 808 nm continuous-wave laser excitation, are realized in the NIR absorbing/emissive CDs (NIR-CDs). The NIR-CDs are synthesized by microwave-assisted exfoliation of red emissive CDs in dimethylformamide, and feature single or few-layered graphene-like cores. This structure provides an enhanced contact area of the graphene-like plates in the core with the electron-acceptor carbonyl groups in dimethylformamide, which contributes to the main NIR absorption band peaked at 724 nm and a tail band in 800-850 nm. Temperature-dependent photoluminescence spectra and transient absorption spectra confirm that the UCPL of NIR-CDs is due to the thermally activated electron transitions in the excited state, rather than the multiphoton absorption process. Temperature dependent upconversion NIR luminescence imaging is demonstrated for NIR-CDs embedded in a polyvinyl pyrrolidone film, and the NIR upconversion luminescence imaging in vivo using NIR-CDs in a mouse model is accomplished.

A facile co-solvent strategy for preparation of graphene nanoplatelet powder: An industrially viable innovative approach

We report an industrially viable promising approach to produce micrometer-sized multilayer graphene nanoplatelet powder (MGNP) in a scalable quantity via microwave-assisted exfoliation of graphite (MEG) and fragmented into MGNP through liquid-phase exfoliation in the co-solvent mixture by kitchen mixer (KM). KM allows rapid delaminating MEG into MGNP by shear force dominated exfoliation. Majority of MGNPs are with a diameter of few micrometers and thickness is in nanometers. MGNP are crystalline with very limited defects was confirmed by Raman measurements and transmission electron microscopy. This process transforms, more than 86% of graphite flakes into MGNP. This advanced approach opens a new pathway to produce MGNP in bulk quantity as it is feasible, rapid, and cost-effective.

Influence Factors of Exfoliation Synthesis Exfoliated Graphite from Vietnamese Natural Graphite Flakes Using Microwave Irradiation

In this study, we prepared the exfoliated graphite (EG) using microwave-assisted exfoliation synthesis exfoliated graphite from the inexpensive natural Vietnamese graphite flakes as the precursors. The exfoliation of graphite was conducted employing intercalation method using H2O2 as oxidizing agent and H2SO4 as intercalating agent. The influent of the precursors (VH2O2/VH2SO4) (1.0/20 – 2.0/20), pH value of EG (1-6), time intercalation (70-120 min), microwave power (180-720 W), and time microwave (10-60 second) in the exfoliation volume of EG was investigated.

Microwave-assisted exfoliation strategy to boost the energy storage capability of carbon fibers for supercapacitors

Carbon fibers, with reduced oxidized graphite layers on the surface obtained using a facile method, were developed as one-dimensional electrodes. The oxidized layers directly formed from the surface of the carbon fibers had a strong interconnectivity with the fiber core, and their conductivity was apparently improved after reduction by hydrazine hydrate. With an abundance of oxygen functional groups induced by oxidation, the one-dimensional electrodes showed a specific capacitance of 213.3 F g−1 at a current density of 0.1 A g−1, which was forty times more than that of the original carbon fibers when tested in a three-electrode configuration. The innately stability in aqueous solutions and the graphite layers from the carbon fibers themselves made the electrodes based on the carbon fibers endowed optimal cycling ability without a decline in specific capacitance after 10000 cycles. The device made of CF-R had a good energy density performance. This strategy provided a facile way to obtain a one-dimensional electrode with a high specific capacitance and good durability for energy storage systems.

Microwave Exfoliation of Graphite Oxides in H2S Plasma for the Synthesis of Sulfur-Doped Graphenes as Oxygen Reduction Catalysts.

Tuning the electronic and chemical properties of graphene can be carried out through heteroatomic doping, enabling its use as an electrocatalyst. Sulfur-doped graphene has been suggested to be a viable alternative to traditional Pt-based catalysts for oxygen reduction under alkaline conditions. Herein we present a fast and efficient route to synthesize S-doped graphenes through the microwave-assisted exfoliation and reduction of three different graphite oxides in the presence of hydrogen sulfide. The materials obtained were characterized using scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, combustion elemental analysis, and voltammetry. These S-doped graphenes were found to have good electrochemical performance and were active in the catalysis of the oxygen reduction reaction.

Mass production of highly-porous graphene for high-performance supercapacitors.

This study reports on a facile and economical method for the scalable synthesis of few-layered graphene sheets by the microwave-assisted functionalization. Herein, single-layered and few-layered graphene sheets were produced by dispersion and exfoliation of functionalized graphite in ethylene glycol. Thermal treatment was used to prepare pure graphene without functional groups, and the pure graphene was labeled as thermally-treated graphene (T-GR). The morphological and statistical studies about the distribution of the number of layers showed that more than 90% of the flakes of T-GR had less than two layers and about 84% of T-GR were single-layered. The microwave-assisted exfoliation approach presents us with a possibility for a mass production of graphene at low cost and great potentials in energy storage applications of graphene-based materials. Owing to unique surface chemistry, the T-GR demonstrates an excellent energy storage performance, and the electrochemical capacitance is much higher than that of the other carbon-based nanostructures. The nanoscopic porous morphology of the T-GR-based electrodes made a significant contribution in increasing the BET surface as well as the specific capacitance of graphene. T-GR, with a capacitance of 354.1 Fg−1 at 5 mVs−1 and 264 Fg−1 at 100 mVs−1, exhibits excellent performance as a supercapacitor.

Vacuum-assisted microwave reduction/exfoliation of graphite oxide and the influence of precursor graphite oxide

One-step synthesis of high quality graphene at gram-scale quantities is important for industrial applications, e.g. in electrochemistry for sensing and energy storage. Currently, thermal reduction/exfoliation of graphite oxide (GO) is a typical method of choice. However, it has the drawback of requiring specialized equipment for rapid thermal shock. A recent alternative method, microwave-assisted exfoliation, usually suffers from poor reduction of graphite oxide and thus low C/O ratios. Herein we show that vacuum-assisted microwave reduction/exfoliation of graphite oxide in a closed system leads to high C/O ratios and partial hydrogenation of graphene (2.6at.% of H). Microwave irradiation of graphite oxide in vacuum leads to outgassing from GO and the creation of plasma which aids temperature distribution and hydrogenation. This plasma is quickly extinguished by further dramatic evolution of gases from GO and consequent pressure increase. We assess the influence of precursor graphite oxide, prepared by Hummers, Staudenmaier, and Hofmann methods, upon the materials properties of microwave exfoliated graphene. We show that microwave-exfoliated graphenes prepared from different graphite oxides show very fast heterogeneous electron transfer rates, with similar electrochemical behaviour to thermally reduced graphene oxide.

Microwave-assisted exfoliation method to develop platinum-decorated graphene nanosheets as a low cost counter electrode for dye-sensitized solar cells

Graphene nanosheets (GNs) are prepared from natural graphite by a simple ecofriendly microwave-assisted exfoliation technique.

Electrochemical studies of few-layered graphene as an anode material for Li ion batteries

Few-layered graphene (FLG) with specific surface area of only ~8.2 m2 g−1 was synthesized from graphene oxide (GO) using microwave-assisted exfoliation. GO was prepared using modified Hummers method. Few-layered nature of the exfoliated material was confirmed by electron microscopy, X-ray and electron diffraction, and Raman spectroscopy. Coin cells were fabricated using FLG as an anode and lithium metal as a counter electrode. The cells were tested using cyclic voltammetry and galvanostatic cycling techniques. FLG showed reversible capacity values of ~400 and ~250 mAh g−1 at current rates of 0.1 and 1 C, respectively. Columbic efficiency was more than 98 % while cycle to cycle capacity fading was less than 2 %. Maximum discharge or charging capacity was below 0.3 V, a preferable characteristic for achieving ideal anodic behavior.

Effect of Microwave Irradiation on Exfoliation of Graphene Oxide

Graphene oxide has been synthesized by microwave-assisted exfoliation of graphite oxide prepared by modified Hummers method. Graphite was oxidized in a solution of <TEX>$H_2O_2$</TEX> and <TEX>$KMnO_4$</TEX> at <TEX>$65{\sim}80^{\circ}C$</TEX>, followed by 10 % <TEX>$H_2O_2$</TEX> solution treatment at <TEX>$80{\sim}90^{\circ}C$</TEX>. The graphite oxide was exfoliated under microwave irradiation of 1 kW and was reduced to graphene effectively by hydrazine hydrate (<TEX>$H_4N_2{\cdot}H_2O$</TEX>) treatment. The exfoliation of graphene oxide was significantly affected by the microwave irradiation on (heating)/off (cooling) period. An on/off period of 10 s/20 s resulted in much more effective exfoliation than that of 5 s/10 s with the same total treatment time of 10 min. This can be explained by the higher exfoliation temperature of 10 s/20 s. Repetition of the graphite oxidation and exfoliation processes also enhanced the exfoliation of graphene oxide. The thickness of the final graphene products was estimated to be several layers. The D band peaks of the Raman spectra of the final graphene products were quite low, suggesting a high crystal quality.

Rapid carbon activation via microwave irradiation of nongraphitic carbon doped with metallic potassium and tetrahydrofuran (THF)

We report here a rapid process to synthesize microporous carbon from nongraphitic carbon source through microwave-assisted exfoliation in a non-inert environment. A ternary compound of potassium-nongraphitic carbon-tetrahydrofuran (THF) was prepared and then heating by microwave irradiation for 2 min widens the interstices between adjacent graphene layers in ternary compound, inducing microporous texture with a large amount of ultramicropores. Exploiting microwave heating allows for efficient and rapid synthesis of activated carbon compared with commonly used chemical activation process. After microwave irradiation on nongraphitic carbon, the constituent stacked graphene layers were broken and the specific surface area of 563 m2 g−1 was developed. The feasibility of an electrode material for supercapacitor was estimated by cyclic voltammetry and galvanostatic charge/discharge cycling. The specific areal capacitance reveals as high as 16.4 μF cm−2 in 1 M NaNO3 aqueous solution, which is significantly larger than values found in traditional activated carbons made for use in electrochemical double layer capacitors. Without restrictive processing conditions of chemical activation, microporous structure carbon can be efficiently and rapidly synthesized via microwave irradiation for possible electrochemical capacitor electrode.

Repeated microwave-assisted exfoliation of expandable graphite for the preparation of large scale and high quality multi-layer graphene

The increasing demand for graphene's industrial application requires a new route for its mass production with high quality. Here, we report a facile, green, highly efficient and cost effective method for preparing a large amount of high quality graphene flakes, which is by the repeated microwave assisted exfoliation of expandable graphite (EG). The successful exfoliation of graphite is realized through the intercalation and decomposition of eco-friendly chemicals with the assistance of a microwave source. The chemical morphology and electrocatalytic performance analyses reveal that the graphene flakes are of high quality with little degradation.