Microwave assisted ultrafast synthesis of graphene oxide based magnetic nano composite for environmental remediation

We have successfully synthesized multi-layer graphene oxide and graphene oxide based magnetic nanocomposite (M/GO) by microwave-assisted modified Hummers’ method for removal of toxic lead (Pb²⁺) and cadmium (Cd²⁺) ions from aqueous solution. The X-ray diffraction spectra of synthesized graphene oxide and M/GO confirm increased interlayer spacing along c-axis. Raman spectra revealed the good quality of synthesized GO and M/GO. The wrinkles were seen in the SEM images of synthesized graphene oxide. The presence of conjugated double bond (CC) and carbonyl (CO) were confirmed by using the UV–Vis spectroscopic spectra. Brunauer–Emmett–Teller (BET) analysis showed high (126 m²/g) surface area M/GO composite which accounts for large number of active binding sites for the adsorption of heavy metal ions. The adsorption studies revealed that Pb²⁺ ions were efficiently adsorbed on GO sheets. Interestingly, M/GO showed better adsorption for cadmium ions.     

Fabrication of a hybrid graphene/layered double hydroxide material

A hybrid graphene/Ni²⁺–Fe³⁺ layered double hydroxide material has been fabricated by the hydrothermal treatment of a mixed suspension of the exfoliated graphite oxide, Ni(NO₃)₂ 6H₂O, Fe(NO₃)₃·9H₂O, urea and trisodium citrate. The Ni²⁺–Fe³⁺ layered double hydroxide platelets are first homogeneously grown on the surface of GO nanosheets which are then reduced to graphene under a mild hydrothermal treatment. In the hybrid graphene/Ni²⁺–Fe³⁺ layered double hydroxide material, the restacking of graphene nanosheets is effectively prevented by the formation of Ni²⁺–Fe³⁺ layered double hydroxide platelets, and the graphene nanosheets exist in a complete exfoliation state.     

A green strategy for the synthesis of graphene supported Mn3O4 nanocomposites from graphitized coal and their supercapacitor application

A by-product free strategy based on modified Hummers method was proposed to synthesize graphene/Mn₃O₄ composites without any additional manganese source. Coal-derived graphite (CDG) was used as carbon source instead of conventional natural graphite flakes and MnSO₄ produced from the modified Hummers was in situ transformed into Mn₃O₄ by precipitation in air. After reduction with hydrazine, the reduced coal-derived graphene oxide/Mn₃O₄ (RCDGO/Mn₃O₄) was obtained and employed as the electrode material for the supercapacitors. In addition, K₂SO₄ produced from the modified Hummers was used as electrolyte, as a result, residual-free was achieved during the whole process, and the atom utilization was calculated as high as about 97%. A maximum specific capacitance of 260 F g⁻¹ was achieved for RCDGO/Mn₃O₄ composite with 86% Mn₃O₄ in saturated K₂SO₄ electrolyte solution based on the synergetic effects between coal-derived graphene and attached Mn₃O₄ nanoparticles. Its specific energy density reached 8.7 Wh kg⁻¹ at a current density of 50 mA g⁻¹ when used as a symmetrical supercapacitor. The good capacitance retention (92–94%) was also observed after 1000 continuous cycles of galvanostatic charge–discharge.     

Polydopamine-assisted formation of Co3O4-nanocube-anchored reduced graphene oxide composite for high-performance supercapacitors

Tailoring transition-metal oxide nanoparticles with two-dimensional carbon has become a favorite way to improve their electrochemical performance. In this study, a composite of reduced graphene oxide was anchored by Co₃O₄ nanocubes and easily prepared with the assistance of polydopamine (PDA), using a combination of hydrothermal reaction and pyrolysis (Co₃O₄@PDA-rGO). Polydopamine, which possesses abundant catechol and amine groups, could be easily grafted onto graphene oxide to reduce the aggregation of graphene particles. Furthermore, PDA provided active sites, i.e., catechol and amine groups, which coordinated with Co²⁺, enabling enrichment of metal ions on the surface of graphene. After the pyrolysis of Co²⁺-containing PDA-grafted graphene at 400 °C, the Co²⁺ ions were converted into Co₃O₄ nanocubes, while the PDA carbonized to form N-doped porous carbon on the surface of graphene. The resulting product, Co₃O₄@PDA-rGO, demonstrated extraordinary supercapacitive behavior with good cycling stability owing to its unique porous structure as well as the intimate contact between Co₃O₄ and the carbon matrix.     

Laser-induced anchoring of WO3 nanoparticles on reduced graphene oxide sheets for photocatalytic water decontamination and energy storage

In this work, the synthesis of tungsten oxide/reduced graphene oxide (WO₃-rGO) nanocomposite, using a simple method of pulsed laser ablation in liquids (PLAL) is reported. The pulsed laser beam of 355 nm wavelength carries out two simultaneous processes: the reduction of graphene oxide and at the same time the anchoring of nanostructured WO₃ on reduced graphene oxide. In the photo-catalytic application, WO₃-rGO shows much better visible light absorption and less photo-generated charge recombination than pure WO₃, as indicated by optical absorption and photoluminescence spectra. These improved features in WO₃-rGO significantly enhanced the photo-catalytic decontamination of methylene blue (MB) dye in the water, compared to the use of pure WO₃ as a photocatalyst. A Poly 2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPS) based electrolyte together with the high electrical conductance and porosity of rGO which were produced after anchoring WO₃ on the graphene oxide, were harnessed for the energy storage application using this material for a supercapacitor. The specific capacitance for WO₃-rGO based device is achieved to be 577 F g⁻¹ measured by the galvanostatic charge-discharge (GCD) method. Also, at a power density of 1000 W kg⁻¹, the as-synthesized WO₃-rGO demonstrated a large energy density value of 76.3 Wh Kg⁻¹ that is much larger than obtained, using WO₃ alone. Besides these photocatalytic and energy storage performance evaluation of WO₃-rGO, the optical, morphological and elemental characteristics of synthesized WO₃-rGO were also investigated to study the improved performance of the nanocomposite in these two applications.     

ION EXCHANGE PROPERTIES OF BiO(NO3) 0.5H2O TOWARDS FLUORIDE IONS

ABSTRACT

We studied the ion exchange behavior of the inorganic anion exchanger BiO(NO₃)0.5H₂O with regard to fluoride ions. The ion exchange reaction was rapid at pH 1, 6.6, and 12. The mechanisms of ion exchange reactions at pH 1 and pH 2-12 were studied in a solution with fluoride ions excess to BiO(NO₃)0.5H₂O. A mixture of the β-phase and an unknown phase was produced in the solution at pH 1. BiOF was produced at pH 2-12. Fluoride ions did not react at pH 13, due to the decomposition of BiO(NO₃) 0.5H₂O at pH 13 to yield Bi₂O₃ (major) and Bi₂O₂CO₃ (minor). The structure of the reaction products depended on the solution pH, mole ratios of BiO(NO₃)0.5H₂O to F’, and the reaction time. We observed that BiO(NO₃)0.5H₂O is capable of removing 99% of the fluoride ions from the solution at pH 1-12 under optimal conditions. The ion exchange reaction of BiO(NO₃)0.5H₂O with fluoride ions was studied under the co-existence of both Cl and Br^?at pH 1, 6.6, 12, and 13. The order of decreasing affinity was found to be (Br^?, Cl^?)> F^?. The reaction product was not a simple mixture of BiOCl, BiOBr, and bismuth oxide fluorides, but an unknown compound.     

Graphene non-covalently tethered with magnetic nanoparticles

We describe a novel approach for coupling pristine graphene with superparamagnetic iron oxide nanoparticles to create dispersed, magnetically responsive hybrids. The magnetic iron oxide (Fe₃O₄) nanoparticles are synthesized by a co-precipitation method using ferric (Fe³⁺) and ferrous (Fe²⁺) salts and then grafted with polyvinylpyrrolidone (PVP). These PVP-grafted Fe₃O₄ nanoparticles are then used to stabilize colloidal graphene in water. The PVP branches non-covalently attach to the surface of the pristine graphene sheets without functionalization or defect creation. These Fe₃O₄–graphene hybrids are stable against aggregation and are highly responsive to external magnetic fields. These hybrids can be freeze-dried to a powder or magnetically separated from solution and still easily redisperse while retaining magnetic functionality. At all stages of synthesis, the Fe₃O₄–graphene hybrids display no coercivity after being brought to magnetic saturation, confirming superparamagnetic properties. Microscopy and light scattering data confirm the presence of pristine graphene sheets decorated with Fe₃O₄ nanoparticles. These materials show promise for multifunctional polymer composites as well as biomedical applications and environmental remediation.     

Process intensification of uniform loading of SnO2 nanoparticles on graphene oxide nanosheets using a novel ultrasound assisted in situ chemical precipitation method

In this paper, a novel ultrasound assisted, solution-based chemical synthesis method for the preparation of SnO₂–graphene nanocomposite is presented. Graphene oxide (GO) was prepared by the modified Hummers–Offeman method in presence of ultrasonic irradiation. Further loading of SnO₂ on GO was carried out with an ultrasound assisted solution-based synthesis route. The prepared GO and SnO₂–graphene nanocomposite were characterized by XRD, TEM, FTIR spectra, TGA and DTA analysis in order to confirm the formation of graphene–SnO₂ nanocomposite. TEM analysis of ultrasonically prepared graphene–SnO₂ composite shows the uniform and fine loading of SnO₂ particles (3–5 nm) on graphene nanosheets. However agglomerated morphology was observed in case of conventionally prepared graphene–SnO₂ composite. The cavitational effects generated due to the ultrasonic irradiations during the synthesis of graphene–SnO₂ composite improve the fine and uniform loading of SnO₂ on graphene nanosheets by oxidation–reduction reaction between GO and SnCl₂·2H₂O compared to conventional synthesis methods. The formed material was used for the preparation of anode in lithium ion batteries and its electrochemical performance was characterized by cyclic voltammetry and charge/discharge cycles. It is found that the capacity of SnO₂–graphene nanocomposite based Li-battery is stable for around 120 cycles, and the battery could repeat stable charge–discharge reaction.     

Scavenging Ratios in an Urban Area in the Spanish Basque Country

For a period of six years (1995–2000) the scavenging ratio, which is the ratio of a pollutant's concentration in water to its concentration in air, collected at an urban site in the Spanish Basque Country was studied. The aerosol is characterized by SO₄ ^2? and NO₃? with 1.79 and 1.61 μg m^?3, respectively. Greater fractions of SO₄ ^2?, NO₃?, and NH₄+ ions were present in the fine particle range, while greater fractions of other ions appeared in the coarse range. The most important species found in the precipitation is SO₄ ^2? with 3.0 mg l^?1. NO₃?, Ca²⁺, and Cl^? are the second most important ions. The volume-weighted mean concentration of H⁺ is 4.6 μg l^?1 (pH = 5.3). The concentration of all analyzed ions (except H⁺) decreases throughout the rain event, showing the washout phenomenon of the rainwater. The scavenging ratio for the anthropogenic ions NO₃?, SO₄ ^2?, NH₄+, and K⁺ is lower than the scavenging ratio for the marine-terrigenous ions, Cl^?, Na⁺, and Ca²⁺.     

Studies on synergetic effect of rGO-NiCo2S4 nanocomposite as an effective counter electrode material for DSSC

Nanocomposites of reduced graphene oxide (rGO) and NiCo₂S₄ with different amount of graphene oxide (GO) are synthesized through a one- step solvothermal method and their catalytic activity towards I^-/I₃^- redox electrolyte for dye-sensitized solar cell (DSSC) application are reported. The growth mechanism of the pristine hierarchical marigold like microspheres of NiCo₂S₄ that formed without rGO and the nanocomposite of rGO-NiCo₂S₄ are also proposed. Electrochemical studies confirmed the synergetic effect of nickel and cobalt ions with the high electrical conductive rGO networks that enhance the electrocatalytic activity of NiCo₂S₄ nanostructures. The synergistic effect between NiCo₂S₄ and rGO may be attributed to the higher conductivity of rGO and the inverse spinel crystal structure of NiCo₂S₄ that have more octahedral catalytic active sites of Co³⁺. The amount of graphene oxide plays the important role of controlling the DSSC performance and the power conversion efficiency. The efficiency achieved for the rGO-NiCo₂S₄ counter electrode (CE) based DSSC is 8.15%, which is remarkably higher than that of pristine NiCo₂S₄ (7.36%), and Pt (7.23%) under the same experimental conditions.     

Preparation and characterization of tin oxide,SnO2 nanoparticles decorated graphene

SnO₂ nanoparticles/graphene (SnO₂/GP) nanocomposite was synthesized by a facile microwave method. The X-ray diffraction (XRD) pattern of the nanocomposite corresponded to the diffraction peak typical of graphene and the rutile phase of SnO₂ with tetragonal structure. The field emission scanning electron microscope (FESEM) images revealed that the graphene sheets were dotted with SnO₂ nanoparticles with an average size of 10 nm. The X-ray photoelectron spectroscopy (XPS) analysis indicated that the development of SnO₂/GP resulted from the removal of the oxygenous groups on graphene oxide (GO) by Sn²⁺ ions. The nanocomposite modified glassy carbon electrode (GCE) showed excellent enhancement of electrochemical performance when interacting with mercury(II) ions in potassium chloride supporting electrolyte. The current was increased by more than tenfold, suggesting its potential to be used as a mercury(II) sensor.     

Influences of pH on the structure,morphology and dielectric properties of bismuth titanate ceramics produced by a low-temperature self-combustion synthesis without an additional fuel agent

Bismuth titanate (BIT) ceramics were prepared by incorporating low-temperature self-combustion synthesis and pH modification. The pH value of the initial precursor was adjusted to 3, 5 and 7 by the addition of ammonium hydroxide (NH₄OH) with different amount. The reaction between ammonium ions (NH₄⁺) and nitrate ions (NO₃^?) induced the formation of ammonium nitrate (NH₄NO₃), in turn to favor the combustion by enhancing the decomposition rate. Excessive hydroxyl ions (OH^?) at higher pH value dominated the chelating of the metal carboxylate and the metal ions, resulting in a strong hybridization between bismuth (Bi) and oxygen (O), and also the suppression of the independent volatility of Bi and bismuth oxide (Bi₂O₃). Such conditions contributed to the formation of pure BIT via the low-temperature self-combustion synthesis without the use of an additional fuel agent. A BIT ceramic with high relative density (91.35%) that exhibited a high dielectric constant of ～340 and a low dissipation factor ～0.028 was obtained by the synthesis method at the neutral condition. Furthermore, it offers ability for the use in high temperature applications up to 675 °C.     

Preparation of graphene/carbon hybrid nanofibers and their performance for NO oxidation

A series of novel microdomain-graphitized polyacrylonitrile (PAN)-based nanofibers were prepared by adding varied amounts of graphene oxide into the precursor via the electrospinning method. These hybrid electrospun nanofibers with were stabilized in ambient atmosphere, carbonized in nitrogen atmosphere and treated in NH₃ atmosphere for NO oxidation with low concentration (50 ppm) at room temperature. The samples were characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and nitrogen adsorption at 77 K. Oxidation of NO into NO₂ at room temperature was investigated in a fiber fixed-bed. The results demonstrated that the reduced graphene oxide sheets provide catalytic active sites embedded in the PAN-based nanofibers. In addition it was determined that nitrogen-containing functional groups played important roles in the enhancement of the catalytic oxidation of NO to NO₂. The samples with 5 wt.% GO exhibit the most catalytic oxidation of NO into NO₂.     

Highly selective removal of Ga3+ ions from Al3+/Ga3+ mixtures using graphite oxide

In this paper, we present a simple solution to a major technological issue, the extraction of gallium from bauxite ore and aluminium–gallium separation. Due to its extremely large surface area and oxygen functional groups, graphite oxide was used for the highly selective sorption of Ga³⁺ from Al³⁺/Ga³⁺ mixtures. Graphite oxide prepared according to the Hummers method has various oxygen functionalities which form coordination bonds with metal ions. Sorption capacity and selectivity of graphite oxide towards Al³⁺, Ga³⁺ and their mixtures were investigated. Since the results of these experiments revealed much higher affinity of graphite oxide to Ga³⁺ ions, we tested the procedure on aluminium–gallium oxide prepared by Bayer process from real bauxite ore. In this case too, the results showed that Ga³⁺ accumulated on the graphite oxide surface. This suggests that high sorption selectivity can be used for the extraction of gallium on an industrial scale and, thus, as a rich source of gallium for the semiconductor industry.     

Preparation and electrochemical characterization of MnOOH nanowire–graphene oxide

MnOOH nanowire–graphene oxide composites are prepared by hydrothermal reaction in distilled water or 5% ammonia aqueous solution at 130 °C with MnO₂–graphene oxide composites which are synthesized by a redox reaction between KMnO₄ and graphene oxide. Powder X-ray diffraction (XRD) analyses and energy dispersive X-ray analyses (EDAX) show MnO₂ is deoxidized to MnOOH on graphene oxide through hydrothermal reaction without any extra reductants. The electrochemical capacitance of MnOOH nanowire–graphene oxide composites prepared in 5% ammonia aqueous solution is 76 F g⁻¹ at current density of 0.1 A g⁻¹. Moreover, electrochemical impedance spectroscopy (EIS) suggests the electrochemical resistance of MnOOH nanowire–graphene oxide composites is reduced when hydrothermal reaction is conducted in ammonia aqueous solution. The relationship between the electrochemical capacitance and the structure of MnOOH nanowire–graphene oxide composites is characterized by cyclic voltammetry (CV) and field emission scanning electron microscopy (FESEM). The results indicate the electrochemical performance of MnOOH nanowire–graphene oxide composites strongly depends on their morphology.     

TiO2 nanoparticles assembled on graphene oxide nanosheets with high photocatalytic activity for removal of pollutants

A novel method was developed to synthesize graphite oxide/TiO₂ composites as a highly efficient photocatalyst by in situ depositing TiO₂ nanoparticles on graphene oxide nano-sheets by a liquid phase deposition, followed by a calcination treatment at 200 °C. The two-dimensional porous graphene oxide/TiO₂ composites had specific surface area of 80 m² g⁻¹ being considerably larger than that of P25 and the similarly prepared neat TiO₂ particles without using graphene oxide. The composites exhibited excellent photocatalytic activity, being influenced by post-calcination temperature, graphene oxide content and solution pH. Under optimal conditions, the photo-oxidative degradation rate of methyl orange and the photo-reductive conversion rate of Cr(VI) over the composites were as high as 7.4 and 5.4 times that over P25, respectively. The excellent enhancing effect of graphene oxide nano-sheets on the photocatalytic properties of TiO₂ was attributed to a thin two-dimensional sheet support, a large surface area and much increased adsorption capacity, and the strong electron transfer ability of the thermally reduced graphene oxide in the composite.     

Electrochemical and spectroscopic characterization of a tungsten electrode as a sensitive amperometric sensor of small inorganic ions

Cyclic voltammetry was used to investigate the electrochemical behaviour of the tungsten oxide films toward the electroreduction of BrO₃⁻, ClO₂⁻ and NO₂⁻ ions in acidic medium. The effects of the temperature, scan rate, pH, chemical composition of the electrolytic solutions, were investigated and the reduction mechanism was critically discussed.The reduction currents, evaluated in cyclic voltammetry and measured at −0.250 V versus SCE, increased linearly on increasing analyte concentration up to 20, 55 and 45 mM for nitrite, chlorite and bromate ions, respectively. The detection limits, evaluated in cyclic voltammetry, were 0.1, 0.4 and 0.7 mM for BrO₃⁻, ClO₂⁻ and NO₂⁻, respectively.The tungsten oxide film was successfully characterized as an amperometric sensor for the analytical determination of BrO₃⁻, ClO₂⁻ and NO₂⁻ ions in flowing stream. Operating under constant applied potential of −0.3 V versus Ag/AgCl the good reproducibility of the peak height and background current level during consecutive injections, indicates the absence of fouling effects and the potential applicability of the amperometric sensor for the routine analytical determination of the investigated inorganic ions. Considering the low values of the background currents (ca. 1.1 ± 0.1 μA) obtained in acidic and not deoxygenated carrier electrolyte, the tungsten sensing electrode seems to compete favourably with other common sensors for the amperometric determination of electroactive molecules under cathodic conditions.The X-ray photoelectron spectroscopy technique (XPS) was used in order to evaluate the chemical composition of the tungsten film upon electrochemical treatment in 0.1 M H₂SO₄ solution. Independently of the electrochemical treatment in acid solution, the tungsten surface electrode is generally composed by 50-60% of W⁰, 35-40% of W⁶⁺ and traces of W²⁺ oxide species.     

One-step electrosynthesis of MnO2/rGO nanocomposite and its enhanced electrochemical performance

We present a facile one-step electrochemical approach to generate MnO₂/rGO nanocomposite from a mixture of Mn₃O₄ and graphene oxide (GO). The electrochemical conversion of Mn₃O₄ into MnO₂ through potential cycling is expedited in the presence of GO while the GO is reduced into reduced graphene oxide (rGO). The MnO₂ nanoparticles are evenly distributed on the rGO nanosheets and act as the spacer to prevent rGO nanosheets from restacking. This unique structure provides high electroactive surface area (1173?m² g^?1) that improves ions diffusion within the MnO₂/rGO structure. As a result, the MnO₂/rGO nanocomposite exhibits high specific capacitance of 473?F?g^?1 at 0.25?A?g^?1, which is remarkably higher (3 times) than the Mn₃O₄/GO prior conversion. In addition, the electrosynthesized nanocomposite shows higher conductivity and excellent potential cycling stability of 95% at 2000 cycles.     

Graphene-nanocrystalline metal sulphide composites produced by a one-pot reaction starting from graphite oxide

Graphene-nanocrystalline metal sulphide composites were prepared by a one-pot reaction. A dispersion of graphite oxide layers in an aqueous solution of metal ions (Cd²⁺/Zn²⁺) was reacted with H₂S gas, which acts as a sulphide source as well as a reducing agent, resulting in the formation of metal sulphide nanoparticles and simultaneous reduction of graphite oxide sheets to graphene sheets. The surface defect related emissions shown by free metal sulphide particles are quenched in the composites due to the interaction of the surface of the nanoparticles with graphene sheets.     

Ethylammonium nitrate enhances the extraction of transition metal nitrates by tri-n-butyl phosphate (TBP)

Several molecular polar solvents have been used as solvents of the more polar phase in the solvent extraction (SX) of metals. However, the use of hydrophilic ionic liquids (ILs) as solvents has seldomly been explored for this application. Here, the hydrophilic IL ethylammonium nitrate (EAN), has been utilized as a polar solvent in SX of transition metal nitrates by tri-n-butyl phosphate (TBP). It was found that the extraction from EAN is considerably stronger than that from a range of molecular polar solvents. The main species of Co(II) and Fe(III) in EAN are likely [Co(NO₃)₄]²⁻ and [Fe(NO₃)₄]⁻, respectively. The extracted species are likely Fe(TBP)₃(NO₃)₃ and a mixture of Co(TBP)₂(NO₃)₂ and Co(TBP)₃(NO₃)₂. The addition of H₂O or LiCl to EAN reduces the extraction because the metal cations coordinate to water molecules and chloride ions stronger than to nitrate ions. This study highlights the potential of using hydrophilic ILs to enhance SX of metals.