Few-layer Graphene Oxide Research Articles

Sodium carboxymethylcellulose induced engineering a porous carbon and graphene immobilized magnetite composite for lithium-ion storage

Immobilizing nanosized electrochemically active materials with supportive carbonaceous framework usually brings in improved lithium-ion storage performance. In this work, magnetite nanoparticles (Fe3O4) are stabilized by both porous carbon domains (PC) and reduced graphene oxide sheets (RGO) to form a hierarchical composite (Fe3O4@PC/RGO) via a straightforward approach. The PC confined iron nanoparticle intermediate sample (Fe@PC) was first fabricated, where sodium carboxymethylcellulose (Na-CMC) was employed not only as a cross-linker to trap ferric ions for synthesizing a Fe-CMC precursor sample, but also as the carbon source for PC domains and iron source for Fe nanoparticles in a pyrolysis process. The final redox reaction between Fe@PC and few-layered graphene oxide (GO) sheets contributed to the formation of Fe3O4 nanoparticles with reduced size, avoiding any severe aggregation or excessive exposure. The Fe3O4@PC/RGO sample delivered a specific capacity of 522.2 mAh·g−1 under a current rate of 1000 mA·g−1 for 650 cycles. The engineered Fe@PC and Fe3O4@PC/RGO samples have good prospects for application in wider fields.

Nanoscopic humidity-dependent adhesion behaviors of 2D materials

Two-dimensional materials have gained significant attention in the research community due to their unique physicochemical and mechanical properties. Regarding their applications in devices irrespective of the involved length scale, interfacial adhesion plays an important role in nanoscopic single-asperity and macroscopic multi-asperity contacts. Therefore, this study aims at studying the adhesion behaviors of few-layered Ti3C2Tx, graphene oxide and WSe2 nano-sheets as a function of the adjusted relative humidity by atomic force microscopy. Reference experiments are conducted with graphite and silicon surfaces. The experimental results for graphene oxide and WSe2 nano-sheets show a monotonous increase of the adhesion force with increasing humidity. In contrast, silicon reference and bi-layered Ti3C2Tx nano-sheets show an increasing adhesion with increasing humidity before reaching a maximum value at a critical humidity followed by a pronounced decrease. Based upon the obtained adhesion and characterization data, the measured water adsorption behavior, and the existing knowledge for silicon regarding their superficial water structures, the observed experimental trends were connected with the respective surface groups of the nanomaterials and their ability to adsorb water on their surfaces.

Fabrication and characterization of biodegradable zinc matrix composites reinforced by uniformly dispersed beta-tricalcium phosphate via graphene oxide-assisted hetero-agglomeration

The development of biodegradable Zn matrix composites has been considered a promising approach to achieving enhanced mechanical properties, controllable degradation rate, good biocompatibility, and good osseointegration as orthopedic implants. However, scant literature regarding Zn matrix composites has been reported because of the great difficulty in dispersing the nano-sized bioactive reinforcements uniformly within the Zn matrix. In the present study, a novel and effective method were employed to obtain Zn matrix composites reinforced by uniformly dispersed beta-tricalcium phosphate (β-TCP) via graphene oxide (GO)-assisted hetero-agglomeration and subsequent spark plasma sintering process. A very low-content (0.04 vol%) few-layered GO was used as a coupling reagent to connect the Zn matrix and nano-sized TCP particles. In an appropriate polarity solvent, the negatively charged GO sheets could combine with both the positively charged Zn powder and TCP particles by electrostatic attraction and charge neutralization. Due to the nature of hetero-agglomeration, the flexible GO sheet could adhere to the large Zn powder and attracted a certain amount of TCP particles to form a Zn/GO/TCP sandwich structure by charge neutralization thereby forming a uniform dispersion of TCP particles within Zn matrix. After the spark plasma sintering (SPS) process, the TCP particles incorporated with very thin ZnO layers (thickness of a few dozen nanometers) formed a homogeneous and unique 3D network-like distribution in as-sintered TCP/Zn composites. A unique “snap pea”-like structure was confirmed at the grain boundary of α-Zn grains, which consisted of the TCP particles as “pea” and thin ZnO layer as “pod”. Due to the uniform dispersion of bioactive TCP particles and unique structure of the TCP incorporating grain boundary, as-sintered 3TCP/Zn matrix composites possessed yield strength (YS) of 140.8 ± 7.7 MPa, failure strain of 36.0 ± 2.8%, the moderate degradation rate of 19.1 ± 3.3 μm·y−1 and good cytocompatibility to MC3T3-E1 cells. Moreover, osteogenic differentiation activity evaluation revealed that the addition of TCP could significantly improve the expressions of the osteogenic differentiation-related gene (ALP) in MC3T3-E1 cells, thereby resulting in improved osteogenic capability. Therefore, biodegradable 3TCP/Zn matrix composites fabricated by GO-assisted hetero-agglomeration and subsequent SPS process could be a promising material as orthopedic implants.

Influence of the ratio of organic solvent on the deposition of graphene oxide by spin-coating

In this work we investigate influence of the organic solvent addition on the deposition of graphene oxide solution by spin-coating. In order to overcome the poor wettability of polyethylene terephthalate substrate by water graphene oxide suspension we made series of multicomponent solution with different n-methylpyrrolidone to water ratio. We successfully showed possibility and good uniformity of graphene oxide films deposited from the multicomponent solution. According to AFM and Raman spectroscopy we achieved good local uniformity in all cases. The most uniform films were made with 2,35 mg/ml GO concentration. We also found that in case of 0,47 and 1,175 mg/ml concentrations we achieved few-layered graphene oxide with film thickness about 3 nm and less.

Improved Process Efficiency in Laser-Based Powder Bed Fusion of Nanoparticle Coated Maraging Tool Steel Powder.

Research and development in the field of metal-based additive manufacturing are advancing steadily every year. In order to increase the efficiency of powder bed fusion of metals using a laser beam system (PBF LB/M), machine manufacturers have implemented extensive optimizations with regard to the laser systems and build volumes. However, the optimization of metallic powder materials using nanoparticle additives enables an additional improvement of the laser–material interaction. In this work, tool steel 1.2709 powder was coated with silicon carbide (SiC), few-layer graphene (FLG), and iron oxide black (IOB) on a nanometer scale. Subsequently, the feedstock material and the modified powder materials were analyzed concerning the reflectance of the laser radiation and processed by PBF-LB/M in a systematic and consistent procedure to evaluate the impact of the nano-additivation on the process efficiency and mechanical properties. As a result, an increased build rate is achieved, exhibiting a relative density of 99.9% for FLG/1.2709 due to a decreased reflectance of this modified powder material. Furthermore, FLG/1.2709 provides hardness values after precipitation hardening with only aging comparable to the original 1.2709 material and is higher than the SiC- and IOB-coated material. Additionally, the IOB coating tends to promote oxide-formation and lack-of-fusion defects.

Mechanism of Cd(II) and Cu(II) Adsorption onto Few-Layered Magnetic Graphene Oxide as an Efficient Adsorbent.

Heavy metal contamination caused by industrial discharge is a challenging environmental issue. Herein, an efficient adsorbent based on few-layered magnetic graphene oxide (FLMGO) was fabricated, characterized, and utilized to remove aqueous Cd(II) and Cu(II). Results present that the two components graphene oxide (GO) and Fe3O4 of FLMGO promote mutually, enabling FLMGO to outperform either GO or Fe3O4. Specifically, FLMGO adsorbs Cd(II) and Cu(II) with adsorption quantities of 401.14 and 1114.22 mg·g–1 in 5 and 7 min, respectively. Moreover, FLMGO can be readily recovered via magnetic separation using a hand-held magnet. Adsorptions are spontaneous, endothermic, and entropy increasing, which are the best described by the Freundlich and pseudo-second-order model. The interaction mechanism is as follows: lone pair electrons in C=O- and C–O-related groups were coordinated toward Cd(II) and Cu(II) to induce chemical interaction. The high adsorption efficiency endows FLMGO with encouraging application potential in heavy metal remediation.

Properties of Silicone Rubber-Based Composites Reinforced with Few-Layer Graphene and Iron Oxide or Titanium Dioxide.

The increasing demand for polymer composites with novel or improved properties requires novel fillers. To meet the challenges posed, nanofillers such as graphene, carbon nanotubes, and titanium dioxide (TiO2) have been used. In the present work, few-layer graphene (FLG) and iron oxide (Fe3O4) or TiO2 were used as fillers in a room-temperature-vulcanized (RTV) silicone rubber (SR) matrix. Composites were prepared by mixing RTV-SR with nanofillers and then kept for vulcanization at room temperature for 24 h. The RTV-SR composites obtained were characterized with respect to their mechanical, actuation, and magnetic properties. Fourier-transform infrared spectroscopy (FTIR) analysis was performed to investigate the composite raw materials and finished composites, and X-ray photoelectron spectroscopy (XPS) analysis was used to study composite surface elemental compositions. Results showed that mechanical properties were improved by adding fillers, and actuation displacements were dependent on the type of nanofiller used and the applied voltage. Magnetic stress-relaxation also increased with filler amount and stress-relaxation rates decreased when a magnetic field was applied parallel to the deformation axes. Thus, this study showed that the inclusion of iron oxide (Fe3O4) or TiO2 fillers in RTV-SR improves mechanical, actuation, and magnetic properties.

Graphene films irradiated with safe low-power NIR-emitting diodes kill multidrug resistant bacteria

Bacterial adhesion to surfaces is the onset of biofilm formation and a hard problem to tackle, aggravated by the rise in drug-resistant bacteria, responsible for more than 500 000 deaths globally/year. This work reports few-layer graphene (FLG) and few-layer graphene oxide (FLGO) as stand-alone light-responsive platforms to develop smart antibacterial surfaces. Films exposure to low-intensity NIR drastically improve their ability to kill planktonic (up to ∼99%) and adherent (up to ∼85%) methicillin-resistant S. aureus and S. epidermidis. Upon irradiation, a mild photothermal effect is observed in supernatant, with temperature rising from 37.0 °C to 39.0 °C–42.0 °C, while surface temperature of non-oxidized FLG films increases to 51.3 °C versus 56.0 °C for oxidized films. Both films prompt total glutathione oxidation when irradiated, despite FLG films induce higher ROS generation than FLGO, suggesting antioxidants depletion occurs preferentially by ROS-dependent pathway (photodynamic effect) for FLG versus ROS-independent pathway for FLGO films. This proof-of-principle study demonstrates that safe, low-intensity NIR irradiation is a valuable and effective tool to boost graphene surfaces’ antibacterial performance through a synergistic photothermal and photodynamic effect. These stand-alone NIR-activated graphene-based platforms arise as simple and economical disinfection surfaces/systems, with widespread use in medical and non-medical applications.

Multi-Layer Graphene Oxide in Human Keratinocytes: Time-Dependent Cytotoxicity, Proliferation, and Gene Expression

Few-layer graphene oxide (GO) has shown no or very weak cytotoxicity and anti-proliferative effects in a wide range of cell lines, such as glioma cells and human skin HaCaT cells at concentrations up to 100 µg/mL. However, as multi-layer GO has hardly been explored in the biomedical field, in this study, this other type of GO was examined in human keratinocyte HaCaT cells treated with different concentrations, ranging from 0.01 to 150 µg/mL, for different periods of time (3, 12, and 24 h). The results revealed a time–concentration dependence with two non-cytotoxic concentrations (0.01 and 0.05 µg/mL) and a median effective concentration value of 4.087 µg/mL at 24 h GO exposure. Contrary to what has previously been reported for few-layer GO, cell proliferation of the HaCaT cells in contact with the multi-layer GO at 0.01 μg/mL showed identical proliferative activity to an epidermal growth factor (1.6-fold greater than the control group) after 96 h. The effects of the multi-layer GO on the expression of 13 genes (SOD1, CAT, MMP1, TGFB1, GPX1, FN1, HAS2, LAMB1, LUM, CDH1, COL4A1, FBN, and VCAN) at non-cytotoxic concentrations of GO in the HaCaT cells were analyzed after 24 h. The lowest non-cytotoxic GO concentration was able to upregulate the CAT, TGFB1, FN1, and CDH1 genes, which confirms multi-layer GO’s great potential in the biomedical field.

Fabrication of Silver-Decorated Graphene Oxide Nanohybrids via Pulsed Laser Ablation with Excellent Antimicrobial and Optical Limiting Performance.

The demand for metallic nanoparticle ornamented nanohybrid materials of graphene oxide (GO) finds copious recognition by virtue of its advanced high-tech applications. Far apart from the long-established synthesis protocols, a novel laser-induced generation of silver nanoparticles (Ag NPs) that are anchored onto the GO layers by a single-step green method named pulsed laser ablation has been exemplified in this work. The second and third harmonic wavelengths (532 nm and 355 nm) of an Nd:YAG pulsed laser is used for the production of Ag NPs from a bulk solid silver target ablated in an aqueous solution of GO to fabricate colloidal Ag-GO nanohybrid materials. UV-Vis absorption spectroscopy, Raman spectroscopy, and TEM validate the optical, structural, and morphological features of the hybrid nanomaterials. The results revealed that the laser-assisted in-situ deposition of Ag NPs on the few-layered GO surface improved its antibacterial properties, in which the hybrid nanostructure synthesized at a longer wavelength exhibited higher antibacterial action resistance to Escherichia coli (E. coli) than Staphylococcus aureus (S. aureus) bacteria. Moreover, nonlinear optical absorption (NLA) of Ag-GO nanohybrid was measured using the open aperture Z-scan technique. The Z-scan results signify the NLA properties of the Ag-GO hybrid material and have a large decline in transmittance of more than 60%, which can be employed as a promising optical limiting (OL) material.

Miscanthus as a carbon precursor for graphene oxide: A possibility influenced by pyrolysis temperature

This study investigates the potential of producing graphene oxide (GO) from biomass via green (comparatively) processing and the impact of graphitization temperature on GO quality. Our findings show that it is possible to convert biomass into highly pyrolytic biochar, followed by shear exfoliation to produce few-layer GO. However, pyrolysis temperature is key in ensuring that the biochar is suited for effective exfoliation. Low temperatures (<1000 °C) would preserve undesirable heterogenous, complex cellular structure of biomass whilst excessive temperatures (≥1300 °C) result in uncontrolled melting, coalescence and loss of functional groups. Results show 1200 °C to be the optimum graphitization temperature for miscanthus, where the resultant biochar is highly aromatic with sufficient functional groups to weaken van der Waals forces, thus facilitating exfoliation to form 6-layer GO with specific surface area of 545.3 m2g−1. This study demonstrates the potential of producing high quality, fit-for-purpose graphene materials from renewable sources.

Facile Synthesis of Few-Layer Graphene Oxide from Cinnamomum camphora

This study presents a facile synthesis technique to produce few-layer graphene oxide from Cinnamomum camphora (Camphor L.). Camphor upon carbonization and chemical oxidation leads to the formation of few-layer graphene oxide sheets of around ten layers with a lateral size of 4.14 nm and stacking height of 3.10 nm. AFM and SEM analysis results reveal the wrinkled morphology of the graphene oxide sheets formed. The sharp G band and the relative intensity of the defect to the graphitic band in the Raman spectrum indicate the formation of nanocrystalline graphene oxide sheets with fewer defects. The FTIR spectrum and the deconvoluted C 1s XPS spectrum of graphene oxide synthesized reveal the presence of predominant sp2 hybridized carbon along with carbon bound to various oxygen functionalities. In brief, the formation of high-quality few-layer graphene oxide from an abundant, low-cost, and readily available botanical precursor is herein reported.

Fabrication and characterization of few-layer graphene oxide reinforced magnesium matrix composites

Abstract In this study, few-layer graphene oxide (FLGO) reinforced Mg matrix composite was successfully fabricated by the hetero-agglomeration method and spark plasma sintering (SPS) process. The microstructure, mechanical properties, and strengthening behavior of these composites were comprehensively investigated. A unique amorphous MgO/ultrafine MgO + reduced FLGO/amorphous MgO sandwich-like grain boundary of the α-Mg matrix grain was confirmed in FLGO/Mg matrix composites. This kind of grain boundary indicated the strongly interfacial bonding between the reduced FLGO and Mg matrix. The low mismatch between the d{2 0 0}MgO and d{1 0 0}graphene and the instantaneous elevated temperature during the SPS process could contribute to the formation of these unique grain boundaries. The mechanical strength and Vickers hardness were apparently increased with increasing FLGO concentration. The 0.5 vol%FLGO/Mg composite exhibits the maximum increase in the mechanical properties. The uniaxial compressive strength (UCS) of 373.2 MPa, yield strength (YS) of 140.7 MPa, failure strain of 14.9% and Vickers hardness of 57.6 was achieved in this composite. Quantitative analysis of the strengthening behavior in these composite was performed, and a high strengthening efficiency was achieved in the FLGO/Mg matrix composites. Therefore, FLGO could be considered as a promising candidate as reinforcement for the Mg matrix.

Engineering zinc ferrite nanoparticles in a hierarchical graphene and carbon nanotube framework for improved lithium-ion storage

This work presents the successful fabrication of a composite made of multi-walled carbon nanotubes and reduced graphene oxide, with immobilized zinc ferrite nanoparticles (ZnFe2O4@CNT/RGO). Functionalized CNT (F-CNT) and few-layered graphene oxide (GO) not only works as a precursor for the hierarchical CNT/RGO skeleton, but also participates in the redox reactions with zinc and ferrous ions to synthesize the intermediate products ZnO@CNT and FeOOH@RGO, respectively. A ZnO@CNT/FeOOH@RGO composite is obtained by through the spontaneous assembly process between the above intermediate species, and the final ZnFe2O4@CNT/RGO composite is fabricated through a simple solid-state reaction. The ZnFe2O4@CNT/RGO composite delivers a reversible capacity of about 1250 mAh·g−1 after 100 cycles at a low current of 200 mA·g−1, about 1100 mAh·g−1 after 300 cycles at a high current of 1000 mA·g−1. It has been verified that an increase in battery performance can be attributed to the engineered hierarchical CNT/RGO supportive skeleton, the generation of smaller electrochemically active ZnO and Fe2O3 crystals, and pseudocapacitive behavior. The sample design and preparation method in this work are both economical and scalable, allowing further development and use in other applications.

Electrical Properties of Poly(Monomethyl Itaconate)/Few-Layer Functionalized Graphene Oxide/Lithium Ion Nanocomposites.

Poly(monomethyl itaconate) is outstanding because it is a glassy and dielectric polymer obtained from sustainable feedstock. Consequently, the study of the properties of its nanocomposites has gained importance. Herein, the electrical properties of nanocomposites based on poly(monomethyl itaconate) and functionalized few-layer graphene oxide (FGO) in the presence and absence of lithium ions (Li+) are studied. Not only did the electrical conductivities of the nanocomposites present values as high as 10−5 Scm−1, but also the dielectric permittivity of nanocomposites with (FGO) content lower than the percolation threshold was twice that of the pristine polymer, without presenting a drastic increase of the loss tangent. By contrast, nanocomposites containing Li+ ions presented significant increases of the permittivity with concomitant increases of the loss tangent. Moreover, it was determined that the presence of Li+ ions influenced the charge transport in the composites because of its ionic nature.

Construction of an epoxy composite with excellent thermal/mechanical properties using graphene oxide nanosheets reduced/functionalized by Tamarindus indiaca extract/zinc ions; detailed experimental and DFT-D computer modeling explorations

Few-layered graphene oxide (GO) nanosheets were obtained through the chemical reduction of the GO sheets via nature-based reagents, i.e., Tamarind (Tamarindus indiaca extract, TIE). The obtained rGO-TIE particles were then modified by zinc ions. The outcomes of the Raman spectroscopy, FT-IR, and X-ray diffraction analysis (XRD) revealed about 53% reduction in CO/CC ratio, 40% reduction in the 2ϴ corresponded to the (002) plane of GO, and a 60% increment of I2D/IG intensity (from Raman test results), indicating that the GO was successfully reduced and the interlayer d-spacing and the number of stacked layers decreased. The thermo-mechanical properties of the epoxy ester composite coatings containing 0.15 wt% of GO, rGO-TIE, and rGO-TIE-Zn particles were assessed by TGA-DTG, tensile, and DMTA analysis. From the DMTA outcomes, it was revealed that in the presence of rGO-TIE-Zn particles, the storage modulus (at 25 °C) and cross-linking density of the epoxy film were enhanced by 50% and 57%, respectively, compared to the unfilled coating. Tensile test results depicted about 58% and 41% degree of improvements in the energy at break and tensile strength of the epoxy film reinforced with rGO-TIE-Zn particles. FE-SEM analysis revealed that the better rGO-ESR-Zn(II) particles' dispersion/exfoliation level in the polymer matrix, as well as stronger polymer/particle interfacial bonding than GO, resulted in the higher thermo-mechanical properties. TGA results evidenced about 106 °C improvement in the T80% and 41% improvement in the thermal stability of the epoxy coating in the presence of rGO-ESE-Zn particles.

Graphene/polyamide-6 microsphere composites with high electrical and mechanical performance

The development of innovative methods to prepare graphene-based polymer composites with remarkable properties has become a priority due to the many practical applications of these microstructures. Herein, a new technology was used to fabricate composites by combining graphene materials and polymer microspheres. Polyamide 6 (PA6) microspheres were synthesised by reaction-induced phase separation method, and they were wrapped with few-layer graphene (FLG) or graphene oxide (GO) by exfoliated adsorption method, which allows FLG and GO sheets to form a three-dimensional network structure in the PA6 matrix. Electrical and mechanical characterisation led to a distinct improvement in the properties of PA6 composites, which were superior to traditional graphene/polymer composites. Moreover, the process proposed in this work proved to be an effective method to prepare graphene/polymer composites, suitable for industrial production, and can be extended to other kinds of composites with nanofillers.

Nose-to-Brain Translocation and Cerebral Biodegradation of Thin Graphene Oxide Nanosheets

Summary Understanding the interactions of graphene oxide (GO)-based materials with biological systems is critical due to the potential applications of these materials. Here, we investigate the extent to which single- to few-layer GO sheets of different controlled lateral dimensions translocate from the nose to the brain following intranasal instillation. We explore tissue location and in vivo biodegradability of the translocated materials using various techniques. Mass spectrometry and confocal Raman analyses indicate that trace amounts of GO undergo nose-to-brain translocation in a size-dependent manner. The smallest GO-sheet size category (us-GO, 10-550 nm) gains the greatest access to the brain in terms of quantity and coverage. Confocal Raman mapping and immunofluorescence combinations show that in vivo, us-GO resides in association with microglia. Point-and-shoot Raman spectroscopy shows that trace quantities of us-GO are maintained over 1 month, but undergo biodegradation-related changes. This study adds to growing awareness regarding the fate of graphene-based materials in biological systems.

Synthesis of a pomegranate shaped reduced graphene oxide stabilized secondary Si nanoparticles composite anode for lithium ion batteries

A 3D porous micro-size pomegranate shaped nano-silicon and reduced graphene oxide composite (Si/RGO-S) is synthesized in this work by spray drying and magnesiothermic reduction treatments of silicon dioxides (SiO2) nanospheres and few-layered graphene oxide sheets (GO). The SiO2 nanospheres convert to smaller Si secondary nanocrystals and GO sheets are deoxygenated to form a supporting RGO framework, respectively. Favorable void space is created between the secondary Si nanoparticles for the accommodation of the volume changes during lithium ion storage. The 3D porous RGO protecting skeleton not only improves the overall electronic and ionic conductivity, but also effectively immobilizes the inside Si nanoparticles. Under a current density of 500 mA g−1, the 3D Si/RGO-S composite exhibits a reversible capacity of 1200 mAh g−1 for 300 consecutive cycles, which is much higher than the controlled 2D sample (Si/RGO-F) prepared by a freeze drying and magnesiothermic reduction method as anode material for lithium ion batteries. The insight into the superior lithium ion storage performance of the Si/RGO-S has been given in this work.

An experimental investigation on the sulfur and nitrogen co-doping and oxidation of prepared graphene by electrochemical exfoliation of pencil graphite rods

In this research, the electrochemical exfoliation of pencil rods was carried out for the synthesis of graphene sheets in different mixtures of H2SO4 and HNO3 solution. Influence of different volume ratios of H2SO4: HNO3 at a low concentration (0.6 M) on the morphology and structure of prepared graphene sheets was investigated by field emission electron microscopy (FESEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), Raman spectroscopy and atomic force microscopy (AFM) analyses. Results indicated that the mixtures of few-layered graphene, multi-layered graphene, and graphene oxide were achieved by applying an anodic voltage of +10 V. However, the number of layers, degree of oxidation and defect density of the synthesized powders were different. By increasing the volume ratio of HNO3 in electrolytes, the degree of exfoliation of graphite decreased. On the other hand, the highest degree of oxidation was achieved for the prepared graphene by electrochemical method in the mixture of H2SO4: HNO3 with 1:1 ratio. EDS results of the prepared powders demonstrated the occurrence of nitrogen and sulfur co-doping of graphene sheets.