B. Kianpour et al. / Journal of Chemical and Petroleum Engineering, 50 (2), Feb. 2017 / 37-45
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Evaluating the Effect of Graphite Source and Operating Conditions on the Synthesis of Graphene Oxide Bahareh Kianpour1, Akram Ebrahimi1, Zeinab Salehi2* and Shohreh Fatemi3 1. M.Sc. Student, Dept. of Chemical Eng., University of Tehran, Tehran, Iran. 2. Assistant Professor, Dept. of Chemical Eng., University of Tehran, Tehran, Iran. 3. Professor, Dept. of Chemical Eng., University of Tehran, Tehran, Iran. (Received 25 February 2016, Accepted 4 September 2016)
Abstract
Keywords
In this research graphene oxide was synthesized by two methods. These methods were achieved by changing the improved Hummers’ and modified Hummers’ methods. Structure of graphene oxide was characterized by scanning electron microscopy (SEM) images, X-ray diffraction (XRD) patterns, Raman spectroscopy and Fourier transform infrared (FTIR) spectra. According to SEM image, the thickness of graphene oxide sheets prepared by improved Hummers’ method is about 66 nm. In improved Hummers’ method excluding NaNO3 from reacting gel and performing reaction in a 9:1 volume ratio of concentrated H2SO4/H3PO4 mixture improved the oxidation process by elimination of toxic gases, finally the prepared GO contains well-oxidized carbon materials. XRD results implied more oxidation for synthesized GO by improved Hummers’ based method. Importance of graphite source was shown in synthesis of pure GO. Two sources of graphite supplied by Daejung and Fluka Companies were used to synthesis GO in improved Hummers’ method. According to SEM images and XRD patterns, the graphite source prepared by Fluka Co. was more efficient towards production of pure GO than other graphite source. The results also indicated that temperature and mixing condition are two important factors for synthesis of GO.
Graphite source; Graphene oxide; Operating condition; Synthesis.
1. Introduction
G
raphite is a crystalline form of carbon. In each layer of graphite, the carbon atoms are arranged in a honeycomb lattice with the interval of 0.1421 nm, and the distance between Introduction * Corresponding Author. E-mail:
[email protected] (Z. Salehy)
planes is 0.335 nm [1]. Bonding between layers is via weak van der Waals bonds, which allows layers of graphite to be easily separated, or to slide over each other. After graphite layer separation, each layer is named graphene, a two-dimensional structure, with single layer of sp2-hybridized carbon atoms in a honeycomb crystal lattice. Graphene is one of the most exciting materials that is being investigated today.
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B. Kianpour et al. / Journal of Chemical and Petroleum Engineering, 50 (2), Feb. 2017 / 37-45
In 2004, for the first time, graphene was introduced to scientific community [2, 3]. Due to its unique structure and geometry, graphene possesses remarkable physicochemical properties, including a high Young’s modulus, high fracture strength, and excellent electrical and thermal conductivity, fast mobility of charge carriers, large specific surface area and biocompatibility [4]. These properties enable graphene to be used in many fields such as quantum physics, nano-electronics, energy research, catalysts and engineering of nanocomposites and biomaterials [5, 6]. Graphene is highly hydrophobic and this property limits its applications in many fields such as biological science and nanomedicine. Despite, GO shows excellent hydrophilicity because of large amount of oxygen containing functional groups (i.e., carboxyl, hydroxyl, epoxy, carbonyl), and is relatively, easily dispersed in water [7-9]. Graphene oxide aggregates in physiological buffers in the presence of salts. Therefore, to achieve desired GO, it is needed to perform surface modification [10]. The most common method is the use of strong oxidizing agents to have graphene oxide (GO) as the hydrophilic carbon material. Interlayer spacing for graphite increases from 0.335 nm to more than 0.625 nm after oxidation of graphene disrupting the lattice [11, 12]. Using an oxidizing agent, sheets of graphite are separated easily because of weak van der Waals bonds and carbon atoms bind with O and H, to form graphene oxide (GO). Although the exact structure of GO is unclear, but the basal planes of the graphene oxide sheets contain mostly epoxide and hydroxyl groups, also carbonyl and carboxyl groups are in the structure, mostly decorated at the edges [13]. The first one who demonstrated the synthesis of GO was Brodie [14]. The synthesis was achieved by adding a portion of potassium chlorate to graphite in fuming nitric acid [14]. Staudenmaier [15] improved this protocol, using concentrated sulfuric acid and fuming nitric acid and adding the chlorate to multiple aliquots. This change in the procedure resulted production of highly oxidized GO in a vessel of single reaction significantly more practical. Hummers and Offeman [16] reported a procedure that most commonly used today. They oxidized graphite in sulfuric acid by adding KMnO4 and NaNO3 as oxidizing agents. These procedures commonly produce toxic gases like NO2 or N2O4 because of using the fuming nitric acid or sodium nitrate. Marcano and co-workers, reported an improved synthesis of GO. They used the mixture of H2SO4/H3PO4 in a 9:1 volume ratio, and elimi-
nated using of NaNO3 and increased the amount of KMnO4 [12] By removing sodium nitrate and fuming nitric acid during these procedures, production of toxic gases like NO2 and N2O4 was inhibited, also in this procedure the yield of GO was increased. Based on the 3 g of graphite, Hummers method yields the production of 4.2 g graphene oxide while the yield by improved method was 5.8 g graphene oxide. This oxidation procedure (KMnO4 and a 9:1 mixture of concentrated H2SO4/H3PO4), called the “improved method” which could be used to prepare improved GO (IGO), containing fewer defects in the basal plane comparing with GO prepared by the Hummers’ method [12]. By electrical reduction of graphene oxide, the formed reduced graphene oxide (rGO) is similar to graphene but contains some oxygen and other heterogeneous atoms [17]. GO and rGO have great applications in nano composite materials [18, 19], polymer composite materials [18], energy storage [19], biomedical applications [20-22], catalysis [23], and as a surfactant [24]. Cote and co-workers exposed that GO sheets are amphiphilic with hydrophobic and hydrophilic domains that distributed in the edge to center of GO sheets [25]. Therefore GO can attach to interfaces, playing role of surfactant. Understanding this new property of GO can help to identify the solution properties of GO which can originate new material assembly and processing method such as for manufacturing thin film with controllable micro-structures and segregation GO sheets of different sizes. Further, GO can be used as a surfactant sheet to emulsify organic solvent with water and disperse insoluble materials such as graphite and carbon nanotubes in water, that opens up chances for developing functional hybrid materials of graphene and other π-conjugated systems [25]. This study focuses on comparison between two synthesis procedures of graphene oxide, considering oxidation degree, purity of the synthesized product and evaluation of the two graphite sources (Daejung and Fluka CO.). As well, the effects of temperature and mixing in oxidation process have been investigated.
2. Structural Features
Aside from the oxidative mechanisms, the chemical structure of GO has been the subject of considerable debate over the years, even to this day no clear model has been suggested. Considerable efforts have been done successfully to understand the structural features of GO. Hofmann and Holst,
the formed is similar to oxygen and 7]. GO and s in nano ], polymer rgy storage s [20-22], t [24]. d that GO hydrophobic istributed in sheets [25]. interfaces, nderstanding an help to ies of GO al assembly ch as for controllable n GO sheets can be used sify organic se insoluble and carbon up chances
B. Kianpour et al. / Journal of Chemical and Petroleum Engineering, 50 (2), Feb. 2017 / 37-45
Ruess, Scholz and Boehm, Nakajima and Matsuo studied on the structure of GO [11]. The latticebased model was rejected by most of the recent models, and a non-stoichiometric, amorphous alternative have been investigated more. The most well-known model is the one prepared by He and co-workers. (Fig. 1) used solid state nuclear magnetic resonance (NMR) spectroscopy to characterize the material and structural features [[11, 26]. In addition to other ketone groups, carboxylic acid groups were present in very low quantities at the periphery of the graphitic sheets [11].
3. Experimental Details
3.1. Materials and reagents Two graphite sources as powders (Fluka, purity 99.99% and Daejung), sulfuric acid (Merck, 98 wt. %), orthophosphoric acid (Merck, 85 wt. %), potassium permanganate (Merck 99.99%), Hydrogen peroxide (Mojallali, 37 wt. %), and sodium nitrate (Merck 99.99%) were prepared and used without any further treatment.
3.2. Synthesis procedure 3.2.1. First method This synthesis method was achieved by changing the improved Hummers’ method. A mixture of 9:1 concentrated acids, H2SO4/H3PO4 (40.5:4.5 ml) was added to graphite powder (1.0 g). KMnO4 (6.5 g) was gradually added during one hour, at the controlled temperature below 20°C. The reaction by most theheated recentupmodels, andand a mixed nonmixture wasofthen to 37±2°C stoichiometric, amorphous alternative have using a mechanical agitator (home made agitator) been investigated more. was Thesonicated most wellfor two days. Then the mixture (Digital ultrasonic cleaner, 4820, 170 W)by forHe 30and min, known model is theCD one prepared inco-workers. order to exfoliate oxidestate to graphene (Fig. 1)graphite used solid nuclear oxide. Thereafter, the reaction was cooled magnetic resonance (NMR)mixture spectroscopy to down to room temperature. Diluted H2O2, a 4:1 characterize the material and structural (v/v) mixture of de-ionized water/H2O2, was add[[11, In addition to iceother edfeatures drop wise to the26]. graphene oxide in an bath ketone groups, carboxylic acid groups were in order to remove heat generated from reaction. Addition H2O2very was applied to convert remained presentof in low quantities at the
periphery of the graphitic sheets [11].
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MnO2 to Mn+2. The best way for treating GO is to add diluted H2O2 drop wise while controlling the temperature below 20°C, otherwise there will be greenish yellow solution that is due to reduction of produced GO and existence of both rGO (reduced GO) and GO in final product. Then the mixture was centrifuged (7000 rpm) and washed repeatedly with de-ionized water and ethanol (96% v) until the supernatant becomes neutralized to prevent any inorganic ion sedimentation. The obtained solid was vacuum-dried for one day at 50 oC to obtain GO-A.
3.2.2. Second method Second method was achieved by changing the modified Hummers’ method. For this method, concentrated H2SO4 (45 mL) was added to a mixture of graphite powder (1.0 g) and NaNO3 (0.5 g). Same as previous procedure, KMnO4 (6.0 g) was added slowly in portions controlling temperature below 20°C. Afterward, the mixture was heated up to 37±2°C, and mixed using mechanical agitator for two days. Then the mixture was cooled down to room temperature and sonicated (170 W). Similar to previous method, diluted H2O2, a 4:1 (v/v) mixture of deionized water/H2O2, was added drop wise to the graphene oxide in an ice bath. At this time, the color of the mixture changed to bright yellow. Then the mixture was centrifuged and washed repeatedly with de ionized water and ethanol similar to the previous procedure. The obtained product was vacuum-dried for one day at 50°C to obtain GO-B. 3.3. Characterization X-ray powder diffraction (XRD) patterns were obtained by D8 ADVANCE X-Ray (Bruker AXS) diffractometer (Cu Kα radiation). Fourier transform infrared (FTIR) spectra were carried out using Perkin-Elmer spectrometer in frequency range of 8004000 cm-1. Scanning electron microscope (SEM) images were recorded with Hitachi-S4160 microscope operating at 30 kV. Sample morphologies were investigated by transmission electron microscopy (TEM) images with Philips-cm30 apparatus and acceleration voltage of 150 kV. Raman spectroscopy was performed on a Renishaw Raman microscope, with an excitation wavelength at 785 nm.
4. Results and Discussions
Fig. 1. proposedstructural structural model of [11]. GO [11] Figure 1. The The proposed model of GO
3. Experimental Details 3.1. Materials and reagents Two graphite sources as powders (Fluka, purity 99.99% and Daejung), sulfuric acid (Merck, 98 wt. %), orthophosphoric acid (Merck, 85 wt. %), potassium
4.1. Stages of synthesis In this research, the proposed structure for graphene oxide considering very well oxidation is COH0.5, by regarding absence of carboxylic acid
Mn2+ can be removed by washing with water or alcohol in the final step. 40
4.2. and Evaluation of graphite source B. Kianpour et al. / Journal of Chemical Petroleum Engineering, 50 (2), Feb. 2017 / 37-45
groups in He’s model, assuming O:H ratio as two, and C:O ratio as one. The stoichiometry for reaction, considering full oxidation, can be considered as below: 3C + 1.5KMnO4 + 0.75H2SO4 → 3COH0.5 + 1.5MnO2 + 0.75K2SO4
(1)
Considering the obtained stoichiometry, 6.5 gram potassium permanganate is necessary to fully oxidize 1 gram of graphite powder. H2SO4 diffuses between layers of graphite to generate graphite intercalate compound (GIC). In this step, the layers of graphite are relatively separated and KMnO4 as oxidizing agent easily diffuses between layers and oxidizes graphite [27]. Structure of IGO is a more regular structure; it may be due to using phosphoric acid and formation of five-membered cyclic phosphate groups that is formed between phosphoric acid and two vicinal diols on the graphite basal plane [12, 28]. After completion of reaction, GO is synthesized and MnO2 is produced during synthesis. Treating with H2O2 is an important step to remove MnO2 and formation of soluble Mn2+, preventing any inorganic ion sedimentation: MnO2 + H2O2 + 2H+ → Mn2+ + 2H2O + O2
(2)
influence First method was applied for synthesis of treating with H2O2 there were no black particles GO with the same operating conditions but left in the final solution. Enhanced oxidation of G-F twoproduction graphiteofsources supplied are from two and GO-F nano-sheets obviously companies (Daejung G-D and Fluka G-F). concluded from Fig. 3b against Fig. 2b. XRD patterns andwere SEM images show that The XRD analysis applied on sources and products andofthey are observed in Fig 4afactor and b. the source graphite is an important XRD patterns pure of G-DGO. and GO-D are presented in Fig. to produce 4a. In this pattern, a typical peaksynthesized around 2θ=26°, SEM images of G-D and GOis observed due to the graphite phase structure. The from this GO-D, are shown in Fig. specific peaksource, of GO can be observed at 2θ=10.32° 2a d-spacing and b, respectively. Comparison of Fig. and about 8.56 Å� .As seen in Fig. 4a, pres2a and 2b reveals that small changes ence of graphite impurity is observed in thehave product of GO-D,on asthe wellinitial some graphite other impurities can it be occurred source and observed around and is 2θ=36°. The original means that the2θ=20° graphite not completely G-D graphite and has also impurities is apoxidized thesome layers are and not it well peared in the pattern of GO-D. separated. Fig. 4b presents XRD patterns of G-F and GOthe synthesis witha this and F.During XRD pattern of GO-F shows peaksource on 2θ=10.5° , some the after treating with 2O2the with d-spacing about 8.6 Å�H , and peak of of graphite suspended graphite material was observed is completely eliminated. Therefore it can be concluded a qualified product of GO can synin thethat final solution. However, the be black thesized an appropriate of graphite. particlesfromwere removed source by 1000 rpm centrifugation. 4.3. Comparison of two GO synthesis methods SEM images of Fluka graphite source, G-F Graphite powder was oxidized to graphene oxides and synthesized graphene oxide from this source (GO-F) are shown in Fig. 3a and b.
Fig. 2. SEM Daejung (G-D so
Fig. 3b illu laid on each thickness o GO synthes were no b solution. E production obviously c Fig. 2b. The XRD a and produc 4a and b. X are present typical pea due to the specific pe 2θ=10.32° seen in F impurity is D, as well observed ar original G impurities a of GO-D.
Mn2+ can be removed by washing with water or alcohol in the final step.
4.2. Evaluation of graphite source influence First method was applied for synthesis of GO with the same operating conditions but two graphite sources supplied from two companies (Daejung G-D and Fluka G-F). XRD patterns and SEM images show that the source of graphite is an important factor to produce pure GO. SEM images of G-D and synthesized GO from this source, GO-D, are shown in Fig. 2a and b, respective2+ ly.Mn Comparison of Fig. 2a and 2b that with small can be removed byreveals washing changes have occurred on the initial graphite source water or alcohol in the final step. and it means that the graphite is not completely oxidized and the layers are not well separated. 4.2. Evaluation of graphite During the synthesis with thissource source and after influence treating with H2O2, some of the suspended graphmethod applied for final synthesis of iteFirst material was was observed in the solution. However, were removedbut by GO withthe theblack sameparticles operating conditions 1000 centrifugation. tworpm graphite sources supplied from two SEM images(Daejung of Fluka graphite and companies G-D andsource, FlukaG-F G-F). synthesized graphene oxide from this source (GOXRD patterns and SEM images show that F) are shown in Fig. 3a and b. the of graphite is an Fig.source 3b illustrates that the GOimportant layers are factor laid on to produce pure GO. each other. According to this image, thickness of SEMisimages G-D and GO synthesized GO sheets about 66ofnm. During synthesis, after
from this source, GO-D, are shown in Fig. 2a and b, respectively. Comparison of Fig. 2a and 2b reveals that small changes have occurred on the initial graphite source and it means that the graphite is not completely oxidized and the layers are not well separated.
Figure images, a) a) Graphite source fromfrom Daejung Fig.2.2.SEM SEM images, Graphite source (G-D), b) Synthesized from graphite of Daejung Daejung (G-D), b) GO Synthesized GOsource from graphite (GO-D). source of Daejung (GO-D)
Fig. 3b illustrates that the GO layers are laid on each other. According to this image, thickness of sheets is about 66 nm. During GO synthesis, after treating with H2O2 there were no black particles left in the final solution. Enhanced oxidation of G-F and
5
2θ=10.32° and d-spacing about 8.56 Å.As seen in Fig. 4a, presence of graphite impurity is observed in the product of GOD, as well some other impurities can be observed around 2θ=20° and 2θ=36°. The original G-D graphite has also some B.impurities Kianpour et al.and / Journal Chemical and Engineering, 50 (2), Feb. 2017 it is ofappeared inPetroleum the pattern of GO-D. 120
Graphene Oxide Daejung Graphite
100
Intensity
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(a)
80 60 40
20 0
120 120
Fig. 3. SEM images, a) Graphite source of Fluka (G-F), b) Synthesized GO from graphite source of Fluka (GO-F)
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20
25
2θ (deg.)
30
35
Graphene Oxide Oxide Graphene Fluka Graphite Daejung Graphite
100 100
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(b) (a)
Intensity Intensity
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Fig. 4b presents XRD patterns of G-F and GO-F. XRD pattern of GO-F shows a peak on 2θ=10.5° with d-spacing about 8.6 Å, and the peak of graphite is completely eliminated. Therefore it can be concluded that a qualified product of GO can be synthesized from an appropriate source of graphite.
60 60 40 40 20 20
00
55
10 10
15 15
20 20
25 25
2θ (deg.)
30 30
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40 40
1204. X-ray diffractogram, a) G-D and GO-D, b) Fig. Graphene Oxide a) G-D and GO-D, b) (b) Figure 4. X-ray diffractogram, G-F and G-F and GO-F. Fluka Graphite GO-F.
Fig. 3.3.SEM images,a)a)Graphite Graphite source of Fluka Figure SEM images, source of Fluka (G-F), b) Synthesized GO from graphite b) (G-F), Synthesized GO from graphite source of Flukasource (GO-F).
4.3. Comparison of two GO synthesis of Fluka (GO-F) methods Graphite powder XRD was oxidized Fig. 4b presents patterns to ofgraphene G-F and using the two mentioned methods. XRD oxides using the two mentioned methods. GO-F. XRD pattern of GO-F shows aanalysis peak was performed figure out the resulting materiXRD analysisto was performed to figure out on 2θ=10.5° with d-spacing about 8.6 Å, als. The XRD spectra of GO-A and GO-B are shown the resulting materials. The XRD spectra of of interlayer graphite spacing is completely inand Fig. the 5. Forpeak GO, the (d) of the GO-A and GO-B are shown in 5. For eliminated. Therefore it can beFig. concluded product is proportional to the degree of oxidation GO, interlayer (d)two of the that16]. athe qualified of are GO can be [12, As shown inproduct Fig.spacing 5, there diffracproduct is proportional to the degree tion peaks for from GO-B an at appropriate 2θ=11.7° (d=7.5 Å� ) of and synthesized source of 2θ=25.5° The As weaker peakin with d=3.4 oxidation [12,Å� ).16]. shown Fig. 5, Å� graphite.(d=3.4 corresponds to the normal graphite there are two diffraction peaks forspacing. GO-B This at peak reveals that graphite was remained in final 2θ=11.7° (d=7.5 Å) andGO 2θ=25.5° (d=3.4 4.3. Comparison of two synthesis product. Å). The weaker peak with d=3.4 Å methods The stronger peak with d=7.5 Å� corresponds corresponds to the normal graphite spacing. powder was peak oxidized toGraphite the typical diffraction of GO.toOngraphene the other This peak reveals that graphite wasat hand, there are just diffraction peak for GO-A oxides using the atwo mentioned methods. remained in final product. 2θ=10.5° (d=8.6 Å�was ) thatperformed corresponds the typical XRD analysis to tofigure out diffraction peak of GO (Fig. 5). the resulting materials. The XRD spectra of Compared with GO-B, the interlayer spacing of GO-A and GO-B are shown in Fig. 5. For GO-A is larger, implying more extensive oxidation GO,As the spacing (d)is shifted of theto [29]. well, interlayer diffraction peak of GO-A product is proportional to the degree of left in comparison to GO-B, because of more oxidaoxidation [12, 16]. As shown in Fig. 5, tion that it is obvious in Fig 5. there are two diffraction peaks for GO-B at 4.4. Structural characterization of GO-A(d=3.4 2θ=11.7° (d=7.5 Å) and 2θ=25.5° XRD analysis and SEM images clarified effective Å). The weaker peak with d=3.4 Å corresponds to the normal graphite spacing. This peak reveals that graphite was remained in final product.
10
100
The80 stronger peak with d=7.5 Å corresponds to the typical diffraction peak 60 graphite source, protocol of GO. On the synthesis other hand, thereand areoperating just a conditions. Synthesized GO-A using Fluka graphite 40 diffraction peak for GO-A at 2θ=10.5° source and operating conditions was characterized (d=8.6 Å) that corresponds to the typical with 20complementary analysis such as Raman specdiffraction peakand of TEM. GO (Fig. 5). spectroscopy is troscopy, FTIR, Raman Intensity
5
5
0 used to characterize the structural and elecwidely 10 15 20 25 30 35 40 tronic 5properties of carbon-based materials. The 2θ (deg.) Raman spectrum of GO-A is shown in Fig. 6. The Fig. 4.features X-ray diffractogram, a) spectra G-D andof GO-D, b) main in the Raman graphitic G-F and GO-F. carbon-based materials are the G and D peaks and their overtones [30]. The G peak at 1581 cm-1, and -1 TheD peak stronger peak with d=7.5 the at 1335 cm are recorded (Fig. 6). Å The Gcorresponds peak corresponds the bond stretchingpeak of sp2 to thetotypical diffraction carbon rings and chains. Thejust D peak of GO.pairs On in theboth other hand, there are a represents the breathing mode of aromatic rings diffraction peak for GO-A at 2θ=10.5° arising due to the defect in the sample. The D peak (d=8.6 Å) that corresponds to athe typical intensity is therefore often used as measure tool diffraction peak of GO [31, (Fig.32]. 5).Higher disorder for the degree of disorder 5. X-ray diffractogram GO-Aofand GO-B inFig. graphite leads to a broad of D-band higher relative intensity compound to that of the G-band [33]. The ID/IG is awith measure of disorder. a result, ID/ Compared GO-B, the Asinterlayer Ispacing >1 shows great disorder in aromatic of aGO-A is larger, implyingstructure more G because of attachment of functional groups on the extensive oxidation [29]. As well, carbon [29].
diffraction peak of GO-A is shifted to left in comparison to GO-B, because of more
6
Fig. 5. X-ray diffractogram of GO-A and GO-B
s of G-F and shows a peak about 8.6 Å, s completely be concluded GO can be ate source of
Intensity
Inten
XRD analysis and SEM images clarified bending vibration of OH groups of water 60 effective graphite source, synthesis protocol molecules adsorbed on graphene oxide. The 40 and operating conditions. Synthesized GOabsorption peak at 1092 cm-1 can be attributed to epoxy groups of GO (Fig. 7). A20 using Fluka graphite source and operating conditions was characterized with 0 complementary analysis such as Raman 5 10 15 20 25 30 35 40 spectroscopy, FTIR, and TEM. Raman 2θ (deg.) B. Kianpour et al. / Journal of Chemical and Petroleum Engineering, 50 (2), Feb. 2017 / 37-45 42 spectroscopy is widely used to characterize Fig. 4. X-ray diffractogram, a) G-D and GO-D, b) the structuralG-F andand electronic properties of GO-F. 1397 1732 FTIR spectrum has been recorded wave ecules adsorbed on graphene oxide. The1092 absorpcarbon-based materials. The in the Raman -1 -1 number range of 800-4000 cm , to further investion peak at 1092 cm can be attributed to epoxy spectrum of GO-A is shown in Fig. 6. The The stronger peak with d=7.5 Å 1629 tigation of graphene oxide chemical structure (Fig. groups of GO (Fig. 7). main features the Raman spectra corresponds to thein typical diffraction peakof 7). The broad peak at 3425 cm-1 and the sharp The absorption peaks at 1732 cm-1 and 1397 graphitic carbon-based materials are theaG of GO. On the other hand, there are just -1 -1 peak at 1629 cm are attributed to the stretching cm are attributed 3425 to stretching vibration of C=O andbending D peaks andfor their overtones [30]. diffraction peak at of2θ=10.5° and vibration of-1GO-A OH groups waterThe moland C‒OH deformation of carboxylic acid and car4000 groups 3600 3200 2800 2400 2000edges 1600 of 1200 800 peak at G peak 1581 cm , and tothetheD typical (d=8.6 Å)atthat corresponds bonyl presented at the graphene -1 Wave number (cm-1) 1335 cm are recorded (Fig. 6). The G peak oxide (Fig. 7). Therefore, oxygen functional groups diffraction peak of GO (Fig. 5). Fig. 7. FTIR spectra ofby GO-A of graphene oxide investigated FTIR spectrum, corresponds to the bond stretching of sp2 showed that graphene oxide has strong-1 hydrocarbon pairs in both rings and chains. The The property absorption at 1732 cmthe and philic [30,peaks 34]. TEM image of GO-A D peak represents the breathing mode of -1 attributed to stretching is1397 showncm in Fig.are 8. Sheets of graphene oxide are aromatic rings arising due to the defect in clearly shown in thisand image. vibration of C=O C‒OH deformation of the sample. The D peak intensity is carboxylic acid and carbonyl groups therefore often used as a measure tool for 4.5. Effect of temperature presented at the edges of graphene oxide the degree of disorder [31, 32]. Higher To study the effect of temperature, GO was pre(Fig. using 7). first Therefore, functional pared method oxygen at room temperature. disorder in graphite leads to a broad D-band groups of graphene investigated At the instant of addingoxide KMnO4 in solution,by the of higher relative intensity compound to FTIR spectrum, showed that color was converted to green, because graphene of one step that of the G-band [33]. The ID/IG is a reduction permanganate (MnO4property -) to manganite oxide hasof strong hydrophilic [30, measure of disorder. As a result, ID/IG>1 (MnO -2) on image the surface of GO-A graphite. process 34]. 4TEM of the is The shown in shows a great disorder inof aromatic structure ofFig. reduction in presence of oxidant was carried Fig. 5. X-ray diffractogram GO-A and GO-B 8. Sheets of graphene oxide are clearly Figure 5. X-ray and GO-B. groups because of diffractogram attachmentofofGO-A functional out. After four days, the color of the mixture was shown in this image. changed to brown because of formation of MnO2. on the carbon [29].GO-B, the interlayer Compared with
ynthesis
d to graphene ned methods. to figure out RD spectra of n Fig. 5. For (d) of the he degree of n in Fig. 5, for GO-B at 25.5° (d=3.4 h d=3.4 Å phite spacing. raphite was
spacing of GO-A is larger, implying more extensive oxidation [29]. As well, I /I = 1.37 diffraction peak of1335GO-A is shifted to left in comparison to GO-B, because of more
Fig 5.
tion of GO-A mages clarified thesis protocol nthesized GOsource and racterized with ch as Raman TEM. Raman to characterize c properties of The Raman in Fig. 6. The an spectra of rials are the G ones [30]. The the D peak at 6). The G peak etching of sp2 nd chains. The thing mode of o the defect in k intensity is easure tool for 1, 32]. Higher a broad D-band
1581
chemical structure (Fig. 7). The broad peak at 3425 cm-1 and the sharp peak at 1629 cm1 are attributed to the stretching and 550 1150 2350 bending 850 vibration of1450 OH 1750 groups2050 of water Wave number (cm-1) molecules adsorbed on graphene oxide. The Fig. 6. Raman spectra of GO-A Figure 6. Raman spectra absorption peak ofatGO-A. 1092 cm-1 can be attributed to epoxy GO (Fig.in7).the FTIR spectrum hasgroups been of recorded wave number range of 800-4000 cm-1, to further investigation of graphene oxide
1397
1732
Intensity
6
Raman intensity
D G
1092
1629
3425
4000
3600
3200
2800
2400
2000
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Fig. 7. FTIR spectra of GO-A
Figure 7. FTIR spectra of GO-A.
The absorption peaks at 1732 cm-1 and 1397 cm-1 are attributed to stretching vibration of C=O and C‒OH deformation of carboxylic acid and carbonyl groups presented at the edges of graphene oxide (Fig. 7). Therefore, oxygen functional
Hence, increasing the reaction temperature is useful for rising rate of reaction. The effect of high temperature is also significant towards GO synthesis, heating up the mixture causes reduction of graphene oxide. Therefore, temperature of the media should be adjusted at a specific level. In this work, the temperature was set at 37±2°C. Addition of H2O2 was required to reduce MnO2 to colorless Mn+2, whereas during addition of H2O2 the temperature of solution was increased and the color was changed to greenish yellow, which means that the reverse reaction (reduction of graphene oxide) is taking place and some of MnO2 is converted to MnO4-2. Thus, temperature must be controlled bellow 20°C.image of GO-A Fig. 8. TEM
4.6. ofof mixing 4.5.Effect Effect temperature To study the effect of mixing, GO-A was prepared using magnetic stirrer (Heidolph, D-91126 schwabach). By adding potassium permanganate after a 7 while, viscosity of the reaction mixture began to increase. So that, magnet stirrer was not able to mix the whole mixture uniformly. After three days, color of the mixture was changed to brown. After graphite oxidation process, H2O2 solution was added to the mixture. At this time, the color of the mixture was changed to greenish yellow. This indicates that un-oxidized graphite and oxidant was remained in reaction media because of inappropriate and non-uniform mixing. It can be concluded that uniform mixing with appropriate round per
ure tool for 32]. Higher oad D-band mpound to ID/IG is a ult, ID/IG>1 ic structure onal groups
presented at the edges of graphene oxide (Fig. 7). Therefore, oxygen functional groups of graphene oxide investigated by FTIR spectrum, showed that graphene oxide has strong hydrophilic property [30, 34]. TEM image of the GO-A is shown in B.Fig. Kianpour et al. / Journal of Chemical and Petroleum Engineering, 50 (2), Feb. 2017 8. Sheets of graphene oxide are clearly shown in this image.
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ish yellow that shows reduction of GO and existence of both GO and reduced GO. In addition, the results show that temperature and mixing are two important factors for synthesis of graphene oxide.
ID/IG = 1.37
Nomenclature
2050
SEM
Scanning electron microscopic
NMR
Nuclear magnetic resonance
FTIR IGO
2350
O-A
Figure 8. TEM image of GO-A. Fig. 8. TEM image of GO-A
ded in the 00 cm-1, to hene oxide
4.5. Effect of temperature
7
minute is one of the prominent points in synthesis of graphene oxide.
5. Conclusions
In this study, the graphene oxide was prepared by oxidizing graphite powder using two methods based on modified Hummers and Improved methods. The XRD spectra shows that interlayer spacing of GO-A (d=8.6 Å� ) is higher than GO-B (d=7.5 Å� ). Therefore oxidation level of GO-A is more than GO-B. As well, diffraction peak of graphite (2θ=25.5°) was revealed for GO-B, but this peak was disappeared for GO-A. It can be concluded that GO-A is more pure than GO-B. Hence, first method can be considered as promising method for synthesis of graphene oxide. The Raman spectrum of GO-A shows that ID is higher than IG. Therefore, a great defect has been created in aromatic structure of GO-A. The FTIR spectrum shows that functional groups (hydroxyl, carboxyl, epoxy, and carbonyl) have been created in GO-A. Additionally, source of graphite is an important factor for synthesis of pure GO. Some of graphite sources that have impurities and ashes would not allow the exfoliation and complete oxidation of graphite layers. According to the preformed analysis, XRD patterns and SEM images, although un-reacted graphite was separated by centrifugation, some of the un-reacted materials were remained in the product. As well treating with H2O2 is an important step. The best way for treating is adding diluted H2O2 drop wise and controlling temperature below 20°C. In this way color of GO would be bright yellow that shows existence of GO. While in any other treating procedures the color of solution is green-
G-D
GO-D d
XRD GO
rGO GIC G-F
GO-F θ
Fourier transform infrared Improved graphene oxide
Graphite source supplied from Daejung Company Synthesized graphene oxide from G-D
The distance between similar atomic planes in a mineral (� ) X-ray diffraction Graphene oxide
Reduced graphene oxide
Graphite intercalate compound
Graphite source supplied from Fluka Company Synthesized graphene oxide from G-F Angle of diffraction (degree)
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