This research article studied the effect of pH on the properties of aqueous colloidal dispersions of graphene oxide (GO) and chemically converted graphene (CCG). GO was prepared using a modified Hummer's method and CCG was prepared by chemical reduction of GO. The particle size, zeta potential, and light absorption of GO and CCG colloids were analyzed as a function of pH achieved by titration with HCl or NaOH. The GO colloid was stable between pH 4-11, while the CCG colloid was stable in the narrower range of pH 7-10. Both colloids exhibited average particle sizes of ~1 micron at low pH and 300-500 nm at higher pH
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ISSN : 2321-0869 (O) 2454-4698 (P)
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Eco-friendly method for the estimation of cobalt (II) in real samples using 1...Innspub Net
An easy and quick spectrophotometric method is developed for the investigation of cobalt at trace level using 1-(2-Thiazolylazo)-2-naphthol (TAN) in presence of surfactant cetyltrimethylammonium bromide (CTAB) aqueous micellar solution. The cobalt forms bis [1-(2-Thiazolylazo)-2-naphthol] cobalt complex reacting with 1-(2-Thiazolylazo)-2-naphthol. Proposed method is of great importance because use of micellar system instead of solvent extraction steps that were toxic, expensive and time consuming. The method shows improved sensitivity, selectivity and molar absorption. The coefficient of molar absorption and Sandell’s sensitivity was found to be ε 1.89 × 104L mol-1 cm-1 and 3.1ngcm-2 at λmax 572.7nm. Graph of Linear concentration calibration was obtained in the range 0.02-9.0μgmL-1; stoichiometric metal ligand ratio was found 1:2 for the complex Co-[TAN]2 formation. The proposed method was applied for the investigation of cobalt from different alloys, biological, environmental and pharmaceutical samples. Full articles at https://lnkd.in/fbEHTJ6
Magnetic Fe3O4@MgAl–LDH composite grafted with cobalt phthalocyanine as an ef...Pawan Kumar
Magnetically separable layered double hydroxide MgAl–LDH@Fe3O4 composite supported cobalt
phthalocyanine catalyst was synthesized and used for the aerobic oxidation of mercaptans to corresponding
disulfides under alkali free conditions. The catalyst exhibited excellent activity for the oxidation of
mercaptans using molecular oxygen as an oxidant which can be effectively recovered by using an external
magnetic field. In addition, the covalent immobilization of cobalt phthalocyanine to MgAl–LDH@Fe3O4
support prevents the leaching of the catalyst and improves its activity and stability
Engineering Research Publication
Best International Journals, High Impact Journals,
International Journal of Engineering & Technical Research
ISSN : 2321-0869 (O) 2454-4698 (P)
www.erpublication.org
Eco-friendly method for the estimation of cobalt (II) in real samples using 1...Innspub Net
An easy and quick spectrophotometric method is developed for the investigation of cobalt at trace level using 1-(2-Thiazolylazo)-2-naphthol (TAN) in presence of surfactant cetyltrimethylammonium bromide (CTAB) aqueous micellar solution. The cobalt forms bis [1-(2-Thiazolylazo)-2-naphthol] cobalt complex reacting with 1-(2-Thiazolylazo)-2-naphthol. Proposed method is of great importance because use of micellar system instead of solvent extraction steps that were toxic, expensive and time consuming. The method shows improved sensitivity, selectivity and molar absorption. The coefficient of molar absorption and Sandell’s sensitivity was found to be ε 1.89 × 104L mol-1 cm-1 and 3.1ngcm-2 at λmax 572.7nm. Graph of Linear concentration calibration was obtained in the range 0.02-9.0μgmL-1; stoichiometric metal ligand ratio was found 1:2 for the complex Co-[TAN]2 formation. The proposed method was applied for the investigation of cobalt from different alloys, biological, environmental and pharmaceutical samples. Full articles at https://lnkd.in/fbEHTJ6
Magnetic Fe3O4@MgAl–LDH composite grafted with cobalt phthalocyanine as an ef...Pawan Kumar
Magnetically separable layered double hydroxide MgAl–LDH@Fe3O4 composite supported cobalt
phthalocyanine catalyst was synthesized and used for the aerobic oxidation of mercaptans to corresponding
disulfides under alkali free conditions. The catalyst exhibited excellent activity for the oxidation of
mercaptans using molecular oxygen as an oxidant which can be effectively recovered by using an external
magnetic field. In addition, the covalent immobilization of cobalt phthalocyanine to MgAl–LDH@Fe3O4
support prevents the leaching of the catalyst and improves its activity and stability
V mn-mcm-41 catalyst for the vapor phase oxidation of o-xylenesunitha81
The role of V and Mn incorporated mesoporous molecular sieves was
investigated for the vapor phase oxidation of o-xylene. Mesoporous monometallic
V-MCM-41 (Si/V = 25, 50, 75 and 100), Mn-MCM-41 (Si/Mn = 50) and bimetallic
V-Mn-MCM-41 (Si/(V ? Mn) = 100) molecular sieves were synthesized by
a direct hydrothermal (DHT) process and characterized by various techniques such
as X-ray diffraction, DRUV-Vis spectroscopy, EPR, and transmission electron
microscopy (TEM). From the DRUV-Vis and EPR spectral study, it was found that
most of the V species are present as vanadyl ions (VO2?) in the as-synthesized
catalysts and as highly dispersed V5? ions in tetrahedral coordination in the calcined
catalysts. The activity of the catalysts was measured and compared with each other
for the gas phase oxidation of o-xylene in the presence of atmospheric air as an
oxidant at 573 K. Among the various catalysts, V-MCM-41 with Si/V = 50
exhibited high activity towards production of phthalic anhydride under the experimental
condition. The correlation between the phthalic anhydride selectivity and
the physico-chemical characteristics of the catalyst was found. It is concluded that
V5? species present in the MCM-41 silica matrix are the active sites responsible for
the selective formation of phthalic anhydride during the vapor phase oxidation of
o-xylene.
Photocatalytic Properties of GO-(Cd0.8-Zn0.2)S Nanocomposites Prepared by Che...IJLT EMAS
Graphene oxide - (Cd0.8-Zn0.2)S nanocomposite
material was synthesized by the simple and economically viable
chemical precipitation method at different temperatures and its
photocatalytic properties were investigated. Measurement of
photocatalytic degradation of Rhodamine B dye was carried out
under visible light. The photocatalytic efficiency of the
synthesized nanocomposites was calculated and the effect of bath
temperature on the photocatalytic efficiency was studied. The
studies suggest that the prepared nanocomposites exhibit
reasonably good photocatalytic properties. Better photocatalysis
is observed at lower bath temperatures for preparation of the
nanocomposites. Photocatalytic efficiency close to 70% has been
obtained for the synthesised GO-(Cd0.8-Zn0.2) S nanocomposites
which can be further improved by optimizing the preparative
conditions
Examination of Adsorption Abilities of Natural and Acid Activated Bentonite f...AnuragSingh1049
The effect of the initial concentration of Cr, Co, Cu, Ni, and Pb metal ions from multicomponent solutions on the sorption capacity of natural and acid activated bentonite was examined in this paper. The acid activation was performed by using hydrochloric and sulfuric acid at different concentrations. The results of adsorption research have shown that bentonite can be effectively used as an adsorbent for the removal of metal ions from multicomponent solutions. Acid activation of bentonite changes the structure and content of individual oxides, increases the porosity and the number of available spots for the adsorption process. For this reason, the bentonite removal efficiency increased after acid activation for all heavy metals tested. With increased acid concentration, the degree of bentonite adsorption increased, and sulfuric acid rather than chloric acid showed better results in removal efficiency.
Since the Nobel prize for Physics was awarded to Andre Geim and Konstantin Novoselov “for groundbreaking experiments regarding the two-dimensional material graphene”, the eyes of the scientific world have been focused on this so-called miracle material.
IJERA (International journal of Engineering Research and Applications) is International online, ... peer reviewed journal. For more detail or submit your article, please visit www.ijera.com
The most difficult goal in the next few decades is the replacement of conventional petro-based fuels with more sustainable fuels that can be used in the existing infrastructure. By the use of Renewable energy or nuclear energy, CO2 and H2O can be recycled into liquid hydrocarbon fuels (the reverse of fuel combustion). Capture of CO2 from the atmosphere will form a close carbon-neutral fuel cycle loop. This article also reviews the aspects regarding thermodynamics involved, involved mechanisms and possible technological pathways for recycling CO2 into fuels using renewable energy. These pathways can be broken into three staged- CO2 capture from atmosphere, H2O and CO2 dissociation, and fuel synthesis.
Treatment of wastewater from chrome plating industry by ion exchange methodeSAT Journals
Abstract In the present work an attempt has been made to evaluate the efficiency of removal Cr(VI) from chrome plating industry wastewater by using fresh and reconditioned strong base anion exchanger Tulsion A-27(MP). The Experimental data is evaluated by plotting breakthrough curves and by employing Bohart-Adams Model. The maximum removal (80%) of Cr(VI) from plating wastewater is obtained at 14cm column bed height and flowrate 1mL/min for 1144.39mg/L of influent Cr(VI) concentration. The breakthrough curves for reconditioned and fresh resins indicates that breakthrough occurs early with decrease in bed height, increase in flow rate and influent Cr(VI) concentration. Regeneration of exhausted resin is also studied by using 2M NaCl and 2M HCl. Keywords: Chromium (VI) removal; Reused Anion-exchange resin; chrome plating industry wastewater;
We are a close-knit team from europe, middle east and asia who are able to translate client's ideas into successful
projects, whichwork both aesthetically and commercially. our designs and execution have integrity and our clients reap the rewards.
"we work from concept to completion.
V mn-mcm-41 catalyst for the vapor phase oxidation of o-xylenesunitha81
The role of V and Mn incorporated mesoporous molecular sieves was
investigated for the vapor phase oxidation of o-xylene. Mesoporous monometallic
V-MCM-41 (Si/V = 25, 50, 75 and 100), Mn-MCM-41 (Si/Mn = 50) and bimetallic
V-Mn-MCM-41 (Si/(V ? Mn) = 100) molecular sieves were synthesized by
a direct hydrothermal (DHT) process and characterized by various techniques such
as X-ray diffraction, DRUV-Vis spectroscopy, EPR, and transmission electron
microscopy (TEM). From the DRUV-Vis and EPR spectral study, it was found that
most of the V species are present as vanadyl ions (VO2?) in the as-synthesized
catalysts and as highly dispersed V5? ions in tetrahedral coordination in the calcined
catalysts. The activity of the catalysts was measured and compared with each other
for the gas phase oxidation of o-xylene in the presence of atmospheric air as an
oxidant at 573 K. Among the various catalysts, V-MCM-41 with Si/V = 50
exhibited high activity towards production of phthalic anhydride under the experimental
condition. The correlation between the phthalic anhydride selectivity and
the physico-chemical characteristics of the catalyst was found. It is concluded that
V5? species present in the MCM-41 silica matrix are the active sites responsible for
the selective formation of phthalic anhydride during the vapor phase oxidation of
o-xylene.
Photocatalytic Properties of GO-(Cd0.8-Zn0.2)S Nanocomposites Prepared by Che...IJLT EMAS
Graphene oxide - (Cd0.8-Zn0.2)S nanocomposite
material was synthesized by the simple and economically viable
chemical precipitation method at different temperatures and its
photocatalytic properties were investigated. Measurement of
photocatalytic degradation of Rhodamine B dye was carried out
under visible light. The photocatalytic efficiency of the
synthesized nanocomposites was calculated and the effect of bath
temperature on the photocatalytic efficiency was studied. The
studies suggest that the prepared nanocomposites exhibit
reasonably good photocatalytic properties. Better photocatalysis
is observed at lower bath temperatures for preparation of the
nanocomposites. Photocatalytic efficiency close to 70% has been
obtained for the synthesised GO-(Cd0.8-Zn0.2) S nanocomposites
which can be further improved by optimizing the preparative
conditions
Examination of Adsorption Abilities of Natural and Acid Activated Bentonite f...AnuragSingh1049
The effect of the initial concentration of Cr, Co, Cu, Ni, and Pb metal ions from multicomponent solutions on the sorption capacity of natural and acid activated bentonite was examined in this paper. The acid activation was performed by using hydrochloric and sulfuric acid at different concentrations. The results of adsorption research have shown that bentonite can be effectively used as an adsorbent for the removal of metal ions from multicomponent solutions. Acid activation of bentonite changes the structure and content of individual oxides, increases the porosity and the number of available spots for the adsorption process. For this reason, the bentonite removal efficiency increased after acid activation for all heavy metals tested. With increased acid concentration, the degree of bentonite adsorption increased, and sulfuric acid rather than chloric acid showed better results in removal efficiency.
Since the Nobel prize for Physics was awarded to Andre Geim and Konstantin Novoselov “for groundbreaking experiments regarding the two-dimensional material graphene”, the eyes of the scientific world have been focused on this so-called miracle material.
IJERA (International journal of Engineering Research and Applications) is International online, ... peer reviewed journal. For more detail or submit your article, please visit www.ijera.com
The most difficult goal in the next few decades is the replacement of conventional petro-based fuels with more sustainable fuels that can be used in the existing infrastructure. By the use of Renewable energy or nuclear energy, CO2 and H2O can be recycled into liquid hydrocarbon fuels (the reverse of fuel combustion). Capture of CO2 from the atmosphere will form a close carbon-neutral fuel cycle loop. This article also reviews the aspects regarding thermodynamics involved, involved mechanisms and possible technological pathways for recycling CO2 into fuels using renewable energy. These pathways can be broken into three staged- CO2 capture from atmosphere, H2O and CO2 dissociation, and fuel synthesis.
Treatment of wastewater from chrome plating industry by ion exchange methodeSAT Journals
Abstract In the present work an attempt has been made to evaluate the efficiency of removal Cr(VI) from chrome plating industry wastewater by using fresh and reconditioned strong base anion exchanger Tulsion A-27(MP). The Experimental data is evaluated by plotting breakthrough curves and by employing Bohart-Adams Model. The maximum removal (80%) of Cr(VI) from plating wastewater is obtained at 14cm column bed height and flowrate 1mL/min for 1144.39mg/L of influent Cr(VI) concentration. The breakthrough curves for reconditioned and fresh resins indicates that breakthrough occurs early with decrease in bed height, increase in flow rate and influent Cr(VI) concentration. Regeneration of exhausted resin is also studied by using 2M NaCl and 2M HCl. Keywords: Chromium (VI) removal; Reused Anion-exchange resin; chrome plating industry wastewater;
We are a close-knit team from europe, middle east and asia who are able to translate client's ideas into successful
projects, whichwork both aesthetically and commercially. our designs and execution have integrity and our clients reap the rewards.
"we work from concept to completion.
Presentación con consejos generales para redactar una demanda en el orden social de la jurisdicción. Tiene audios, pero en todo caso el vídeo tutorial completo está publicado en Youtube en la siguiente dirección: https://youtu.be/CEU2yWooS1k
Visible light assisted reduction of nitrobenzenes using Fe(bpy)3+2/rGOnanocom...Pawan Kumar
Visible-light-induced photocatalytic reduction of aromatic nitrobenzenes to the corresponding anilinesat room temperature using reduced graphene oxide (rGO) immobilized iron(II) bipyridine complex asphotocatalyst is described. The rGO-immobilized iron catalyst exhibited superior catalytic activity thanhomogeneous iron(II) bipyridine complex and much higher than metal free rGO photocatalysts. Theheterogeneous photocatalyst was found to be robust and could easily be recovered and reused for severalruns without any significant loss in photocatalytic activity.
Visible light assisted reduction of nitrobenzenes using Fe(bpy)3+2/rGOnanocom...Pawan Kumar
Visible-light-induced photocatalytic reduction of aromatic nitrobenzenes to the corresponding anilinesat room temperature using reduced graphene oxide (rGO) immobilized iron(II) bipyridine complex asphotocatalyst is described. The rGO-immobilized iron catalyst exhibited superior catalytic activity thanhomogeneous iron(II) bipyridine complex and much higher than metal free rGO photocatalysts. Theheterogeneous photocatalyst was found to be robust and could easily be recovered and reused for severalruns without any significant loss in photocatalytic activity
Visible light assisted reduction of nitrobenzenes using Fe(bpy)3+2/rGOnanocom...Pawan Kumar
Visible-light-induced photocatalytic reduction of aromatic nitrobenzenes to the corresponding anilinesat room temperature using reduced graphene oxide (rGO) immobilized iron(II) bipyridine complex asphotocatalyst is described. The rGO-immobilized iron catalyst exhibited superior catalytic activity thanhomogeneous iron(II) bipyridine complex and much higher than metal free rGO photocatalysts. Theheterogeneous photocatalyst was found to be robust and could easily be recovered and reused for severalruns without any significant loss in photocatalytic activity.
Nitrogen-doped graphene-supported copper complex: a novel photocatalyst for C...Pawan Kumar
A copper(II) complex grafted to nitrogen-doped graphene (GrN700–CuC) was synthesized and then
demonstrated as an efficient photocatalyst for CO2 reduction into methanol under visible light irradiation
using a DMF/water mixture. The chemical and microstructural features of GrN700–CuC nanosheets were
studied by FTIR, XPS, XRD and HRTEM analyses. Owing to its truly heterogeneous nature, GrN700–CuC
could be easily recovered after the photocatalytic reaction and showed efficient recyclability for
subsequent runs.
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erpublication.org,
Engineering Journal,
Science Journals,
Nitrogen-doped graphene-supported copper complex: a novel photocatalyst for C...Pawan Kumar
A copper(II) complex grafted to nitrogen-doped graphene (GrN700–CuC) was synthesized and then
demonstrated as an efficient photocatalyst for CO2 reduction into methanol under visible light irradiation
using a DMF/water mixture. The chemical and microstructural features of GrN700–CuC nanosheets were
studied by FTIR, XPS, XRD and HRTEM analyses. Owing to its truly heterogeneous nature, GrN700–CuC
could be easily recovered after the photocatalytic reaction and showed efficient recyclability for
subsequent runs.
SYNTHESIS AND CHARACTERIZATION OF KAOLINITE COATED WITH CU-OXIDE AND ITS EFFE...Premier Publishers
In this paper, a novel copper oxide coated kaolinite was prepared as an adsorbent of Hg(II) ions from aqueous media. The materials used for this study were synthesized, characterised and the product tested for mercury ion removal using standard laboratory procedures. Reactivity and removal kinetic models derived from Freundlich isotherm were used to investigate contact time and pH effects on the coefficient of protonation and rate of mass transfer of Hg(II) ions to the reactive sites, Proton coefficient of 0.89 indicated a decrease in proton consumption function when compared with uncoated kaolinite. At the 12th h reaction time, a maximum adsorption capacity of 85% was achieved. Mass transfer rates of 0.9359h-1 and 0.0748h-1 for the first and second reaction phases indicated a reduction when compared with uncoated kaolinite. These changes may be ascribed to masking of reaction sites and exposed surface area of the Cu-Oxide coated kaolinite.
SYNTHESIS AND CHARACTERIZATION OF KAOLINITE COATED WITH CU-OXIDE AND ITS EFFE...Premier Publishers
In this paper, the effect of copper oxide coated kaolinite on mercury (II) removal based on initial metal concentration, particle concentration, and prolonged residence time have been investigated. Experimental methods involved batch mode techniques at ambient temperature. The content was analysed for Hg(II) adsorbed, using standard laboratory procedures. Here, using empirical models derived from Freundlich isotherm, adsorption capacity increased from 300 to 400 mg/kg over the range of initial metal concentration investigated. The complex behavior of existing reactive sites and the generation of new active sites accounted for these changes in adsorption capacity. Increase in particle concentration led to decrease in adsorption. This was over the range of 2, 4, 6, 8 and 10 gL-1 solid concentration. Higher Cp reduced the specific surface area for adsorption. Hg(II) ions removal increased with increase in residence time. The most efficient ratio of Cu-Oxide coated kaolinite was at 0.002kgL-1 at 40mgL-1 mercury concentration. Values of between 600 to 900 mg/kg for Cu-Oxide coated kaolinite was higher when compared with the uncoated kaolinite. As ageing was increased, hydroxylation probably increased thus leading to the formation of new reactive sites.
cellulose, the most abundant natural biopolymer, has long been
investigated as a new green source to replace non-renewable materials and chemicals, but its highly ordered hydrogen bond network
and high crystallinity, which both detract cellulose reactivity and
solubility (Kondo, 1998), have made it difficult to exploit the full
potential of cellulose materials. To overcome these problems, many
new solvent systems have been studied to enable the homogeneous
modification of cellulose (Ramos, Frollini, & Heinze, 2005; Wu
et al., 2004). Many modifications are nevertheless still preferably
conducted heterogeneously in an aqueous medium, particularly
because of the advantages of this approach with regard to toxicity,
volatility and price.
One potential react
Graphene oxide grafted with iridium complex as a superior heterogeneous catal...Pawan Kumar
A novel graphene oxide (GO)-immobilized heteroleptic iridium complex was synthesized and demonstrated
as a first heterogenized homogeneous catalyst for the production of dimethylformamide (DMF)
from carbon dioxide, hydrogen, and dimethylamine. The synthesized hybrid catalyst showed comparable
activity as homogeneous heteroleptic iridium complex with additional benefits such as facile recovery
and recycling of the catalyst. After completion of the reaction, the heterogeneous catalyst was easily
recovered by filtration, and reused for subsequent recycling processes without any significant change in
the catalytic efficiency
Graphene oxide grafted with iridium complex as a superior heterogeneous catal...Pawan Kumar
A novel graphene oxide (GO)-immobilized heteroleptic iridium complex was synthesized and demonstrated
as a first heterogenized homogeneous catalyst for the production of dimethylformamide (DMF)
from carbon dioxide, hydrogen, and dimethylamine. The synthesized hybrid catalyst showed comparable
activity as homogeneous heteroleptic iridium complex with additional benefits such as facile recovery
and recycling of the catalyst. After completion of the reaction, the heterogeneous catalyst was easily
recovered by filtration, and reused for subsequent recycling processes without any significant change in
the catalytic efficiency.
Graphene oxide grafted with iridium complex as a superior heterogeneous catal...Pawan Kumar
A novel graphene oxide (GO)-immobilized heteroleptic iridium complex was synthesized and demonstrated
as a first heterogenized homogeneous catalyst for the production of dimethylformamide (DMF)
from carbon dioxide, hydrogen, and dimethylamine. The synthesized hybrid catalyst showed comparable
activity as homogeneous heteroleptic iridium complex with additional benefits such as facile recovery
and recycling of the catalyst. After completion of the reaction, the heterogeneous catalyst was easily
recovered by filtration, and reused for subsequent recycling processes without any significant change in
the catalytic efficiency.
Synthesis of new chelating ion exchange resins derived from guaran and diviny...IOSR Journals
New chelating ion exchange resin containing hydroximate of 5 amino salicylic acid was synthesized by incorporating it in divinylbenzene styrene (DVBS) copolymer. Hydrophilic polysaccharidematrix ofguaran was also used to incorporate hydroximate of 5 amino salicylic acid in it. The resin characteristics were studied by determining its bulk density, specific bulk volume, moisture content, degree of substitution and ion exchange capacity. The resin was characterized by means of IR spectra, Nitrogen content and pH titration. The distribution coefficient values of different metal ions namely Fe(II), Cu(II) , Zn(II) , Cd(II) , Co(II) , Ni(II), Ca(II), U(VI), Cr(VI) & W(VI) was carried out on these resins as a function of pH . Metal analysis was done by Atomic Absorption Spectrophotometer. The hydroximate derivative of guaran is found to be more selective than that of diving benzene co-polymer.
2. 2 Journal of Nanoparticles
graphene colloids over a range of pH. Both CCG and
GO sheets are negatively charged colloidal particles carry-
ing varying concentration of functional groups. Oxygen-
containing functional groups present on the graphene oxide
sheets make GO highly hydrophilic. On the other hand,
CCG, or partially reduced colloid of graphene oxide, uses
the remnant oxygen-containing groups for displaying the
stability of mildly oxidised graphene oxide sheets. As the
colloid is titrated with NaOH/HCl, the ensuing change in
the pH of the colloid leads to agglomeration of the graphene
based colloidal particles. With very few reports elucidating
the individual behaviour of CCG or GO colloids over size,
stability, or conjugation studies [18, 19], a comparative study
of both of the colloids on the said parameters has not been
reported. In the current work, the effect of pH on the zeta
potential, optical absorption, and equivalent particle size of
graphene oxide and chemically converted graphene colloids
is reported.
2. Experimental Details
Flake graphite (98%), sulphuric acid (98%), hydrochloric
acid (37%), hydrogen peroxide (30 wt%), potassium perman-
ganate, hydrazine monohydrate (98 wt% in water), ammonia
(35%), and sodium hydroxide were obtained from com-
mercial sources and used without any further purifica-
tion. Graphene oxide was prepared by modified Hummer’s
method as described by Chen et al. [20]. Graphite powder
(2 g) was poured into concentrated H2SO4 (46 mL) at 0∘
C
and stirred for about 30 min. Thereafter, KMnO4 (6 g) was
added gradually while stirring the batch for about 30 minutes.
Continuous cooling was ensured so that the temperature of
the mixture does not exceed 20∘
C. The reaction between
H2SO4 and KMnO4 leads to the formation of metal and
sulphate ions that help in exfoliating the graphite layers [21].
The mixture was then heated up to 35∘
C and was kept for
more than 30 min. Deionized water (92 mL) was gradually
added. After 60 min of further stirring, the reaction was
terminated by the addition of a large amount of deionized
water and 30% H2O2 solution (5 mL). This caused violent
effervescence and an increase in temperature (up to 100∘
C),
after which the color of the suspension changed to bright
yellow. The suspension was then washed with 1 : 10 HCl
solution (5 L) in order to remove metal ions by filter paper
and funnel. The paste collected from the filter paper is dried
at 60∘
C until it became agglomerated. This agglomerate was
dispersed into deionized water in static state for 2-3 h and
slightly stirred by a glass rod. The suspension was washed
with more deionized water several times until the pH reached
a value of 7. The collected paste was ultrasonicated for about
1 h for better dispersion in water. The obtained brown GO
hydrosol was then centrifuged at 4000 rpm for 30 min in
order to remove any unexfoliated GO. Scanning electron
microscopy of a drop of the colloid (after drying) exhibited
crumpled graphene oxide sheets.
Chemically converted graphene (CCG) was prepared as
per the process described by Li et al. [22]. In short, 5 mg
graphene oxide powder was dispersed in 100 mL of distilled
water by ultrasonication until a yellowish-brown colloid was
obtained. The GO colloid was then transferred to a 250 mL
round bottom flask and stirred for a few minutes. Subse-
quently, few drops of ammonia solution (35%) were added to
increase the pH up to 8 allowing stability of the sheets. 5 𝜇L
hydrazine hydrate (0.1 M) solution was added to the above
solution and refluxed at 98∘
C for 1 hour in a water bath.
The process afforded a black colloidal dispersion, hereafter
called CCG colloid. The GO colloid and the CCG colloid were
filtered and dried separately for additional characterizations.
The as-synthesized GO powder (0.2 mg/mL) was sus-
pended in distilled water by bath sonication for 30 minutes,
affording the formation of a uniformly dispersed yellowish-
brown colloid. The stable colloid had pH of around 6.7
indicating that the GO sheets bore strong –OH bond affinity
with the water molecules. At this stage, it can be assumed
that the GO colloid was devoid of any metallic impurity
or salt. The colloid was then divided into two halves. One
half of the GO colloid was titrated with 0.1 M HCl solution
to progressively decrease the pH from 6.7 to 1. The second
half was titrated with 0.1 M NaOH solution to increase the
pH from 6.7 to 13. Every integral change in the pH of GO
colloid was recorded by a pH meter (Mettler Toledo) and
collected in 5 mL vials. GO colloids with pH ranging from
1 to 12 were obtained for further characterization. Similar
procedures were followed to obtain CCG colloids with pH
from 1 to 12. To ensure a better dispersion, the samples were
sonicated for 15 minutes before each characterization.
Size and morphology of the GO powders were measured
by scanning electron microscopy (JEOL XL 30). Surface
charge of the colloidal dispersion (GO and CCG) was
measured by zeta potential meter (Zetasizer, Malvern, UK)
between the pH ranges of 2–12, and the average particle
size of the colloidal particles was measured by dynamic
light scattering technique. UV-Vis absorption spectra were
recorded by a spectrometer (Lambda 45, Perkin Elmer)
within the wavelength range of 200–900 nm.
3. Results and Discussion
X-ray diffraction (XRD) of the graphite precursors, GO,
and CCG was carried out over the range of 5 to 50∘
of
2Θ with a scan rate of 2∘
min−1
(cf. Figure 1). The XRD
pattern of graphite exhibited the characteristic peak at 26∘
corresponding to (200) reflections. The XRD pattern for
GO showed a peak at ∼11∘
, corresponding to an expanded
interlayer distance of 7.6 ˚A. This is due to the intercalation
of acidic functional groups in between the graphitic layers
that causes an increase in the interplanar distance. In such
a case, the (200) graphite peak shifts to lower 2Θ region
in the vicinity of 10∘
. However, if the graphene layers are
fully expanded and exfoliated to single layers, such a peak
may not be observed for graphene oxide. In addition, the
CCG powders (after filtration and drying) do not seem to
restack, which is evident by the absence of peaks for CCG
in the X-ray diffractogram (Figure 1). Single drops of the
as-prepared GO colloid and CCG colloid (at pH ∼7) were
cast on metallic stubs, and scanning electron microscopy was
performed. Figure 2(a) shows a representative image of the
GO sheets. Similarly, the morphology of CCG is shown in
3. Journal of Nanoparticles 3
Intensity(a.u.)
Graphite
Graphene oxide
Chemically converted graphene
(CCG)
10 20 30 40 50
2Θ (∘
)
Figure 1: X-ray diffraction patterns of graphite, graphene oxide, and
chemically converted graphene.
Figure 2(b). Subtle charging effects are seen from the SEM
images because the samples were not coated. Moreover, it
may be noted that we have taken the images (purposefully) at
20 kV, which is an exceedingly high voltage regime to image
carbon based materials. This high voltage and the sharp edges
of CCG exhibit subtle charging effects (chemically converted
graphene cannot have as high electrical conductivity as that
of pristine graphene; it may lie somewhere between that of
graphene oxide and graphene).
The zeta potential (𝜁) is an important factor for charac-
terizing the stability of colloidal dispersions. It is a measure
of the negative charge around the double layer associated
with the colloidal particle as a consequence of the ionization
of different functional groups. Generally, particles with zeta
potential in the range −30 mV to +30 mV are considered
stable due to electrostatic repulsion [23]. Our measurements
show that zeta potential for the GO colloids is pH sensitive,
and effective dispersion of the colloids occurred within the
pH range of 4–12 (cf. Figure 3). Experimentally, we observed
that and the highest magnitude of 𝜁 is obtained at pH 10
(−48.6 mV), although the pH range of 7–11 showed the highest
(negative) charge.
Dynamic light scattering (DLS) results for both GO and
CCG colloids showed similar trends with subtle differences.
It must be mentioned here that we do not attempt to
estimate the average particle size of GO and CCG. DLS is
a technique that is well suited for estimating the size of
spherical shaped particles. GO and CCG colloids, on the
contrary, are particulate systems that possess extremely large
ratios of length or breadth (few microns) to thickness (few
nanometers). Therefore, the DLS pattern of such a system
could provide a convoluted result, which is expected to be
close to the lateral dimensions of the GO and CCG platelets.
However, the objective here is to compare the relative change
in the platelet size as a function of pH. For the graphene
oxide sample at pH 7, the size distribution varied in the range
of 250 nm to 570 nm (cf. Figure 4). But, as the sample was
titrated with NaOH to a higher pH, the population lays in the
range of approximately 300 nm to 500 nm. The stability of the
GO colloid improves as NaOH acts as a hydrogenating agent
for graphene oxide [24]. Moreover, it has also been reported
that NaOH has an ability to successfully remove the oxygen
functionalities from the surface of graphene oxide to form
activated graphene sheets [25, 26]. These activated sheets
begin to stabilize themselves in the solution by reducing their
effective size. Thus, the change in color and stability of the
NaOH titrated samples could be due to the development of
small sized activated graphene sheets. GO colloids, on the
contrary, when titrated with HCl result in very large sheet size
and poor stability. This can be attributed to the increase in H+
ions in the solution, which gradually increase the sheet size
and reduced stability.
These observations reiterate our findings from the zeta
potential measurements that GO colloids are more stable in
the 3–11 pH range. The thickness of the GO sheets is normally
expected to increase with decrease in pH due to increasing
protonation of acidic groups (C–OH, COOH) present on
the sheets. Protonation decreases the negative charge on the
sheets, thereby decreasing electrostatic repulsion between
them, eventually resulting in the agglomeration of the sheets
and reduced colloidal stability. However, we observe that, in
the pH range of 6–9, the particle size distribution remains
almost the same, and the distributions for colloids of pH
10 and 11 exhibit tails towards higher size. The reason for a
slightly higher size distribution in pH 10 and pH 11 could
be ascribed to the salting out effect of the NaOH additives.
The CCG colloids, on the other hand, display sheets size
in the range of 200 nm to 700 nm at pH 7 (cf. Figure 5).
On titrating them with NaOH, the CCG sheets in the pH
range from 7 to 10 stabilize uniformly and lie in the range of
200 nm to 400 nm. But, above pH 10, the sheets agglomerate
and their size sharply increases over 1 micron, with very few
sheets in the nanometric range (not included in Figure 5).
This dramatic shift is supported by Fan et al. [27], who
comment that the stability of the CCG sheets is attributed
to a strengthened electrostatic stabilization under alkaline
conditions, as the repulsion between the negatively charged
sheets should increase at higher pH values. Thus, as the
repulsion lies within their threshold limits, the CCG is stable,
and above a threshold limit the sheets begin to agglomerate
and destabilize themselves.
Overall, the DLS spectra of GO and CCG colloids
exhibited size in the range of 200 nm to 1000 nm, with the
distribution tailing off at higher sizes for the pH range of 3–
10. The distributions for colloids of pH 1-2 and pH 10-11 were
completely different from the distributions in the pH range
of 3–9, the average being observed beyond 1 micron at the
pH extremes. This result closely matches with our common
observation of a highly agglomerated GO colloid that is not
transparent yellow but showed a muddy appearance.
UV spectroscopy was studied to observe the plasmon
transitions for the titrated GO and CCG colloids. Graphene
based colloids can exhibit interesting absorption patterns at
the wavelength regions of ∼230 nm, ∼260 nm, and ∼300 nm,
corresponding to transitions involving 𝜋, 𝜎, and n electrons
of different bond energies. The titrated GO colloids produced
a characteristic peak at ∼230 nm which is attributed to the
𝜋-𝜋∗
plasmon of the C=C bonds. This peak is similar for
4. 4 Journal of Nanoparticles
(a) (b)
Figure 2: Scanning electron micrographs of (a) graphene oxide and (b) chemically converted graphene.
Zetapotential(𝜁)(mV)
10
0
−10
−20
−30
−40
−50
pH
0 2 4 6 8 10 12
CCG
GO
Figure 3: Zeta potential of GO and CCG colloids at different pH.
samples in the pH range of 2 to 11 (cf. Figure 6). However, at
pH 1 and pH 12, the absorption pattern appeared completely
different, roughly indicating a continual decay of absorption
with wavelength. At pH 1, this arises as a result of high amount
of sedimentation that takes place due to the aggregation
and low repulsion within the sheets. As for pH 12, the
sedimentation occurs due to the salting out of the excess
sodium ions. The UV-Vis spectra for the CCG colloids,
on the other hand, displayed the plasmon peak at 260 nm,
which is a considerable peak shift to a higher wavelength (cf.
Figure 7). This is due to the reduction of the C=O groups
and restoration of conjugation in the graphene sheets that
lowers the energy required, thus increasing the wavelength of
the 𝜋-𝜋∗
transitions. This also predicts that CCG possesses
substantially decreased oxygen content and higher sheet size
for which the value of 𝜆max shifts to a higher wavelength.
With the restoration of the conjugation in the hexagonal
carbon sheets, the material is more graphene-like and is
expected to show high thermal and electrical conductivity
(also evident from the absorbing black color of the colloid).
Intensity(a.u.)
Particle size (nm)
0 200 400 600 800 1000 1200 1400 1600 1800 2000
pH 3
pH 4
pH 5
pH 6
pH 7
pH 8
pH 9
pH 10
pH 11
pH 12
Averagesize(nm)
1400
1200
1000
800
600
400
200
pH
2 4 6 8 10 12
Figure 4: Particle size distribution of GO colloids at different pH;
inset shows the average particle size.
For the GO colloids, we also observed a shoulder peak at
300 nm (cf. Figure 6). Literature suggests that there exists
a bond excitation value at ∼300 nm corresponding to the
n-𝜋∗
transition of the C=O chromophores for aldehyde and
ketone molecule [28]. However, this peak can shift slightly
due to the increased solvent polarity of graphene oxide as it is
titrated with NaOH/HCl. On the other hand, this peak almost
vanishes for the titrated CCG colloids indicating the absence
of the C=O chromophores in the colloid.
4. Summary
Aqueous dispersions of graphene oxide were found to be
stable in the pH range of 4–12, with excellent dispersibility
in the range of 7–11, as evidenced by highly negative zeta
potential. The colloidal stability of chemically converted
5. Journal of Nanoparticles 5
Intensity(a.u.)
Particle size (nm)
0 200 400 600 800 1000 1200 1400 1600 1800 2000
pH 3
pH 4
pH 5
pH 6
pH 7
pH 8
pH 9
pH 10
Averagesize(nm)
1000
800
600
400
200
0
pH
2 4 6 8 10 12
Figure 5: Particle size distribution of CCG colloids at different pH;
inset shows the average particle size.
Absorption(a.u.)
Wavelength (nm)
200 300 400 500 600 700 800 900
UV absorption spectra of graphene oxide
pH 1
pH 12
pH 1
pH 2
pH 3
pH 4
pH 5
pH 6
pH 7
pH 8
pH 9
pH 10
pH 11
pH 12
Figure 6: UV-Vis spectroscopy of GO colloids from acidic to basic
conditions.
graphene in aqueous media was also found to be similar
to that of graphene oxide colloids. UV-Vis spectroscopy of
the chemically converted graphene colloids indicated the
restoration of electronic conjugation in the graphene sheets.
These findings are useful in the fabrication of graphene based
composites and hybrids from aqueous processing methods.
Absorption(a.u.)
Wavelength (nm)
200 300 400 500 600 700 800 900
UV absorption of chemically converted graphene
pH 1
pH 12
pH 1
pH 2
pH 3
pH 4
pH 5
pH 6
pH 7
pH 8
pH 9
pH 10
pH 11
pH 12
Figure 7: UV-Vis spectroscopy of CCG colloids from acidic to basic
conditions.
Conflict of Interests
The authors declare that there is no conflict of interests
regarding the publication of this paper.
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