XPS and structural studies of high quality graphene oxide and reduced graphene oxide prepared by different chemical oxidation methods

XPS and structural studies of high quality graphene oxide and reduced graphene oxide prepared by different chemical oxidation methods

Ceramics International xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate...

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Ceramics International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

XPS and structural studies of high quality graphene oxide and reduced graphene oxide prepared by different chemical oxidation methods R. Al-Gaashania,b,∗, A. Najjarc, Y. Zakariaa, S. Mansoura, M.A. Atieha a

Qatar Environment and Energy Research Institute (QEERI), Hamad Bin Khalifa University (HBKU), Qatar Foundation, PO Box 5825, Doha, Qatar Physics Department, Faculty of Education, Thamar University, Dhamar, Republic of Yemen c College of Life and Health Sciences, Hamad Bin Khalifa University (HBKU), PO Box 34110, Doha, Qatar b

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene oxide Reduced graphene oxide Graphite Chemical method XPS

High quality graphene oxide (GO) and reduced graphene oxide (rGO) have been synthesized by chemical oxidation of graphite flakes via three modified Hummers methods using a mixture of sulfuric acid (H2SO4), phosphoric acid (H3PO4) and nitric acid (HNO3) as intercalating agents and potassium permanganate (KMnO4) and hydrogen peroxide (H2O2) as oxidizing agents. In this study the production of dangerously explosive gases was avoided. The temperature was carefully controlled using ice baths, ensuring the temperature was kept at the minimum during the reaction. The prepared samples were extensively characterized using various techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results indicated greater presence of oxygen containing groups and an increase in the (C/O) ratio, both of which served as reliable indicators of high quality graphene oxide. Atomic ratio of carbon to oxygen (C/O) quantified by XPS was calculated to be 25.67, 1.81, 1.63, and 2.77 of graphite, GO-I, GO-II, and GO-III, respectively. As revealed by FTIR analysis, the GO-I had more hydrophilic oxygen functional groups compared to GO-II and GO-III.

1. Introduction Since its discovery in 2004 [1] graphene has attracted much attention in various fields of scientific research and applications ranging from electronics, where it is used in the manufacture of graphene field effect transistors [2] to sensors [3] and clean energy devices [4] towards water purification and sterilization of waste water [5]. The immense interest from such wide array of scientific disciplines arose from the fundamentally unique and spectacular properties of graphene. Chemically, graphene is two-dimensional nanomaterial and is composed solely of a single layer of arranged carbon atoms. The resulting honeycomb lattice is held together by σ bonds [2]. There are several ways in which graphene oxide (GO) can be produced; each have their own advantages and limitations. Mechanical exfoliation [6], whilst attractive for producing high quality GO is limited in the way of large scale production feasibility [7]. Both mechanical exfoliation (by means of ultrasonication or stirring) and epitaxial growth on silicon carbide (SiC) wafer methods have been shown to produce low yields [8]. Thermal exfoliation and chemical vapor

deposition (CVD) [9] are other methods that have been employed for producing GO from graphite. However, chemical oxidation, remains the most commonly favored approach in GO production [10]. Work done by Brodie in the 1859 produced the first ever GO using potassium chlorate, graphite powder (usually graphite natural flakes) and concentrated fuming nitric acid [11]. The method was improved by Staudenmaier in 1898 where he included the use of concentrated sulfuric acid in addition to the fuming nitric acid [12]. This modification saw an increase in the amount of oxidized graphene oxide produced whilst making the method more facile. Hummers and Offeman published their paper in 1958 where they described the preparation of what they called graphitic oxide using potassium permanganate (KMnO4) and sodium nitrate (NaNO3) in the presence of sulfuric acid [13]. This became known as the Hummers method and is the most favored method used today [14]. A common feature of all three methods is the ultimate oxidation of graphite to different degrees. During the process of oxidizing graphite, functional groups such as hydroxyl groups (OH) containing oxygen were introduced. This sees the expansion of the d-spacing [8] of GO whereby interlayer interactions are restricted. These

∗ Corresponding author. Qatar Environment and Energy Research Institute (QEERI), Hamad Bin Khalifa University (HBKU), Qatar Foundation, PO Box 5825, Doha, Qatar. E-mail address: [email protected] (R. Al-Gaashani).

https://doi.org/10.1016/j.ceramint.2019.04.165 Received 17 February 2019; Received in revised form 20 March 2019; Accepted 19 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: R. Al-Gaashani, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.04.165

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Fig. 1. Processes used in this study for synthesis of graphene oxide (method 1).

some cases chlorine dioxide (ClO2) is released as a byproduct. This product, along with N2O2 are both explosive and hence render the experiments hazardous [14]. The Hummers method has seen many modifications and improvements [16], where different inorganic acids have been used in variable ratios and proportions [17]. Different oxidizing agents in different molar ratios have also been tested in order to further optimize the procedure of producing GO [14]. One of the most important issues relating to the largescale production of GO is the safety aspect of the method. Indeed, the traditional Hummers method has the inherent risk of explosion due to the buildup of manganese heptoxide (Mn2O7) [18]. Despite attempts to reduce the temperature of the reactor [16] the control of the temperature throughout the experiment has proven troublesome and impractical [10]. Other studies have trailed the production of GO by means where no toxic gases were produced [14]. Namely, Marcano et al., 2010 doubled the mass of KMnO4 whilst using a combination of sulfuric and phosphoric acid in a 9:1 ratio. However, the rapid temperature increase was not addressed in this study and therefore the need for a safer and temperature controlled method remains of utmost importance when producing GO. It is also worth noting that lower temperature has been implicated in producing GO with lower defect [10]. GO with oxygen functional groups can be formed either by oxidized graphene sheets or graphene oxide sheets that have their basal planes adhered mostly with hydroxyl, epoxide, carbonyl, and carboxyl groups. As reported by Lerf–Klinowski, oxygen functional groups localize presumably at the edges and transform the GO layer surfaces to high hydrophilic surfaces while water molecules can readily be inserted into the interlayer galleries [19]. Recently, a large number of studies have revolved around the production of GO for use as a nanofiller for polymeric membranes. The use of GO was seen to enhance membrane performance attributes such as a reduction in biofouling processes and increased water flux [20]. In one such study GO sheets were synthesized for studying the possible antibacterial effects of GO. The authors fabricated GO using a modified version of the Hummer's method, however the processing temperature was not maintained below 5 °C [21]. The use of different inorganic acids were also not employed, rather only 98% sulfuric acid was used for the production of GO [21]. The demand for high-grade GO over the years has greatly increased. This is due to the versatility of GO in a wide number of applications. Recently, GO was reported to find its use in biomedical applications alongside reduced graphene and zinc oxide nanoparticles [22]. More recently, a facile synthesis method of GO which possessed significant enhanced properties for optoelectronic and energy devices was

Fig. 2. Shows XRD patterns of graphite (a), graphene oxide (GO-I) prepared using acids mixture of H2SO4, H3PO4, and HNO3 (70: 20:10) (b), graphene oxide (GO-II) prepared using acids mixture of H2SO4, and H3PO4 (90:10) (c), and graphene oxide (GO-III) and rGO prepared using ultrasonic bath (d).

polar oxygen-containing groups also result in the GO being hydrophilic, with the added benefit of being easily dispersed in water and exfoliated in a wide range of solvents [15]. The downside of all three methods described above involves the evolution of toxic gases such as nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O2), both of which are harmful to the environment. In 2

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Fig. 3. SEM images of graphite (a and b), GO-I graphene oxide (c and d), GO-II graphene oxide (e and f) (the inset in Fig. 3 (f) shows high magnification SEM image (1 μm)), and GO-III graphene oxide (g and h).

requirements. In this study we present findings of fully characterized GO, which have been prepared in a safe manner without the use of oxidizing agents, which cause explosive or toxic gaseous products. Furthermore,

reported. The study used a one-step approach employing electrolysis technique. The GO sheets obtained showed superior characteristics in terms of thermal stability [23]. However, it should be noted that electrolysis manufacturing processes are known for their energy intensive 3

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Fig. 4. EDS spectra of graphite (a), GO-I graphene oxide (b), GO-II graphene oxide (c), and GO-III graphene oxide (d).

2. Experimental

Table 1 EDS elemental analysis of graphite, GO-I graphene oxide, GO-II graphene oxide, and GO-III graphene oxide. Small Sepeaks appeared in GO-I and GO-II samples due to treatment process with H2SO4. Sample

Elements

Weight (Wt. %)

Atom. conc. (at. %)

Graphite

C O Total C O S Total C O S Total C O Total

98.72 01.28 100 51.10 47.43 01.47 100 54.52 43.81 01.67 100 72.57 27.43 100

99.03 0.97 100 58.56 40.81 0.63 100 61.93 37.36 0.71 100 77.90 22.10 100

GO-I

GO-II

GO-III

2.1. Materials Natural graphite flake (−10 mesh, purity 99.9%, manufacturer: Alfa Aesar, Germany) was used as a raw material to produce graphene oxide. A mixture of nitric acid (HNO3) (67–69%, RomilSpA limited™, UK), hydrochloric acid (HCl) (37%, Sigma-Aldrich®, USA), phosphoric acid (H3PO4) (85 wt %, Honeywell Fluka®, India), and sulfuric acid (H2SO4) (95%, VWR Chemicals®, France) were used as intercalating agents. Hydrogen peroxide (H2O2) (30%, VWR Chemicals®, France) and potassium permanganate (KMnO4) (≥99%, Honeywell Fluka®, India) were used as strong oxidizing agents. All chemicals were of analytical reagent grades and used as received, without further purifications. The aqueous solutions were prepared in deionized water. GO and rGO have been produced by oxidizing the graphite flakes by three modified Hummers methods using different acids mixtures.

2.2. Graphene oxide (GO-I) preparation (method 1)

the approaches utilized in this study demonstrate a cost-effective and facile procedure for producing high-grade GO, while keeping the process time to a minimum. The temperature was maintained below 5 °C during the addition of different reactants with the use of ice baths and chilled deionized water. The results indicate an improvement in the quality of GO produced as well as a significant increase in oxygen containing functional groups. The C/O ratio was also higher than that reported in many of the previous studies [24].

In a typical experiment, 1 g of the graphite flakes was dispersed in a ratio of 70: 20: 10 acids mixtures of H2SO4, H3PO4, and HNO3, respectively, in a 500 ml beaker. The mixture was stirred at a medium rpm for uniform dispersion. 6 g of KMnO4 was slowly added to the mixture while maintaining the temperature below 5 °C using ice bath. The solution was then maintained at 45 ± 5 °C for 2 h using an oil bath. The beaker was then placed back into the ice bath and 100 ml of deionized water was added slowly to avoid rapid increase in temperature. The beaker was then placed in the oil bath whilst maintaining the 4

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Fig. 5. TEM images of graphene oxide prepared using acids mixture of H2SO4, H3PO4, and HNO3 (70:20:10) (a and b), graphene oxide prepared using acids mixture of H2SO4, and H3PO4 (90:10) (c and d), and graphene oxide (GO-III) prepared using ultrasonic bath (e and f).

was repeated many times using deionized water to wash out the acid. The pH was checked continually and the washing was stopped once the solution reached a neutral pH. The solution was placed in glass beakers and incubated overnight in an oven at 85 °C. The preparation steps are outlined in Fig. 1.

temperature at 85 °C for 1 h. The mixture was stirred at a moderate rate throughout the experiment. The experiment was brought to an end by the simultaneous addition of 120 ml of deionized water and 15 ml H2O2 (30%). The reduction of the permanganate and manganese dioxide could be easily observed by the color change from caramel brown to a greenish yellow solution. The solution was left to reach room temperature before being washed with an additional 25 ml of 9:1 deionized water to HCl acid to remove metal ions and acids. The mixture was centrifuged at 5000 rpm for 30 min at room temperature. The washing

2.3. Graphene oxide (GO-II) preparation (method 2) Graphene oxide (GO-II) was prepared using the same experimental 5

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steps detailed in the previous section of GO-I preparation with a change of acid mixture to be H2SO4 (90%) and H3PO4 (10%) (90: 10). 2.4. Reduced graphene oxide (rGO) and graphene oxide (GO-III) preparation (method 3) The effect of ultrasonication on preparation of GO-III was studied using an ultrasonic bath to produce rGO. The acid mixture with graphite and KMnO4 were placed in an ultrasonic water bath at room temperature and subjected to 1 h of ultrasound treatment in the ultrasonic bath before adding 120 ml of deionized water and 15 ml H2O2 (30%) to end the reaction. Then, the same experimental steps used to prepare graphene oxide (GO-II) were applied for the remainder of the experiment. 2.5. Characterization of samples A Bruker D8 Advance X-Ray diffractometer (XRD) using Cu-Kα radiation source (λ = 0.15418 nm), operated at 40 KV and 15 mA was used to study the prepared GO samples. The morphology of samples was observed using high-resolution transmission electron microscopy (HRTEM) (FEI Talos200X) and Quanta 650 scanning electron microscope (SEM) equipped with Bruker energy dispersive X-ray spectrometer (EDS). Thermo Scientific NICOLET iS50 FT-IR spectrometer was

Fig. 6. Shows FT-IR spectra of graphite (a), graphene oxide (GO-I) prepared using acids mixture of H2SO4, H3PO4, and HNO3 (70:20:10) (b), graphene oxide (GO-II) prepared using acid mixture of H2SO4, and H3PO4 (90:10) (c), and graphene oxide (GO-III) prepared using ultrasonic bath (d).

Table 2 XPS compositional analysis, fitted parameters, and C/O atomic ratio of the C 1s, and O 1s spectra of the graphite and the as-prepared samples GO- (I, II, and III). Samples

Peaks

Binding energy (eV)

Peak area

FWHM (eV)

Atomic conc. (%)

(C/O)a (C/O)b

Phase/Groups

Graphite

(1) (2) (3) (4) (5) (1) (2) (3) (4)

C 1s C 1s C 1s C 1s C 1s O 1s O 1s O 1s O 1s

284.76 287.00 288.28 289.50 291.28 531.22 532.45 533.29 533.95

70009.30 1000.31 875.27 875.27 3500.56 2325.42 2959.49 421.95 879.40

0.59 1.15 1.35 1.35 2.25 1.20 1.20 0.77 1.25

(25.67)a

CeC CeO C=O OeC=O π- π* Satellite OeC=O C=O CeOH CeOeC

(1) (2) (3) (4) (5) (1) (2) (3) (4)

C 1s C 1s C 1s C 1s C 1s O 1s O 1s O 1s O 1s

284.82 286.62 287.78 289.12 290.90 531.50 532.34 533.10 534.07

17653.27 39809.74 22473.69 7042.23 681.97 14782.08 51706.23 45584.04 19384.42

1.42 1.42 1.42 1.41 1.42 1.35 1.23 1.23 1.35

(1.81)a (1.43)b

CeC CeO C=O OeC=O π- π* Satellite OeC=O C=O CeOH CeOeC

(1) (2) (3) (4) (5) (1) (2) (3) (4)

C 1s C 1s C 1s C 1s C 1s O 1s O 1s O 1s O 1s

284.80 286.82 288.30 289.70 292.51 530.97 531.77 532.60 533.33

22161.80 39354.78 5254.50 1067.14 1192.73 4688.07 17690.59 65830.99 22833.94

1.11 1.12 1.44 1.44 1.55 0.96 0.96 1.19 1.44

(1.63)a (1.66)b

CeC CeO C=O OeC=O π- π* Satellite OeC=O C=O CeOH CeOeC

(1) (2) (3) (4) (5) (1) (2) (3) (4)

C 1s C 1s C 1s C 1s C 1s O 1s O 1s O 1s O 1s

284.83 286.99 288.43 289.50 291.20 531.19 532.26 533.01 533.80

37636.94 20739.10 5382.79 2413.86 1363.45 4911.12 18976.21 28848.95 8390.21

1.24 1.15 1.35 1.35 1.88 1.35 1.35 1.25 1.44

92.61 1.33 1.16 1.16 – 1.32 1.68 0.24 0.50 100 13.06 29.46 16.63 5.21 – 4.01 14.01 12.36 5.26 100 20.24 35.95 4.80 0.97 – 1.61 6.06 22.55 7.82 100 41.76 23.03 5.98 2.68 – 2.13 8.24 12.53 3.65 100

(2.77)a (3.52)b

CeC CeO C=O OeC=O π- π* Satellite OeC=O C=O CeOH CeOeC

Total GO-I

Total GO-II

Total GO-III

Total a b

Atomic ratio of carbon to oxygen quantified by XPS. Atomic ratio of carbon to oxygen quantified by EDS. 6

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of the graphite particles before acid treatment, where rough surfaces resembling leaf-like structures were observed. However, post acid treated graphite samples; Fig. 3 (c to h) displayed the expected smooth surfaces with wrinkles and folded regions. These observations could be accounted for by the Sp3 carbons and formation of oxygen-containing functional groups in the basal planes and various GO structural defects [26,27]. From the SEM images (Fig. 3(g) and h), the degree of aggregation of rGO is clearly higher than those of the GO samples shown in Fig.3 (c to f). The reduction of oxygen containing functional groups located in the basal plane of the Sp2 carbon permitted the lamellas of the reduced GO sheets to be held together via weak van der Waal forces. Consequently, rGO sheets were found to be highly aggregated with crumpled features, as shown in their SEM images (Fig. 3 (g) and (h)) [26]. The elemental analysis of graphite as well the prepared GO samples was investigated using EDS (Fig. 4 and Table 1). The O/C atomic percentage (at. %) of the oxygen contents increased from 0.98% for the raw graphite (Fig. 4 (a)) to ∼ 41% for GO-I and 38% for GO-II (Fig. 4 (b)). However GO-III samples showed a lower oxygen content with 22.10% (at. %) compared to GO-I. This can be attributed to the fact that a portion of GO-III was reduced to rGO when exposed to ultrasonic treatment for 1 h, as confirmed by XRD (Fig. 2 (d)). Fig. 5 depicts the TEM images of the three types of the graphene oxide nanosheets (GO I, II and III). The presence of reduced graphene oxide is seen in Fig. 5 (e and f) confirms the results seen in the XRD and XPS analyses. All TEM images show clear folding, indicating the formation of oxygen functional groups, as confirmed by the FTIR (Fig. 6) and XPS (Table 2) data. FTIR was used to determine the formed functional groups on the graphene oxide samples. Fig. 6 represents the FTIR spectra of the (a) graphite powder used as a raw material and the prepared graphene oxide (GO-I) (b-d). The FTIR spectrum of GO shows a peak at 3425 cm−1, which represents the OeH stretching of the COOH group while the bond centered at 2972 cm−1 is attributed to CeH stretching. The peaks at approximately 1735 and 1635 cm−1 represent carboxyl/ carbonyl (C=O) and aromatic (C=C) stretching groups, respectively. The broad peak occurring at 1450 cm−1 is due to OeH deformations of CeOH groups, and the sharp peaks at 1083 and 1050 cm−1 represent CeO stretching [28] while the peak at about 800 cm−1 is attributed to = CeH bond. The absorption band centered approximately at 2340 cm−1 (Fig. 6 (c)) can be assigned to atmospheric CO2 (O=C=O). During the ultrasonic treatment, some amount of carbon-oxygen functional groups in the GO-III sample were reduced, resulting in reduced graphene oxide yields as shown in Fig. 6 (c) and XRD pattern (Fig. 2 GO-III). The results of the FTIR spectra indicated that the GO-I possessed the highest content of oxygen functional groups compared to GO-II and GO-III samples. Fig. 6 (d) clearly shows the reduction of the carbon-oxygen functional groups in the GO-III sample, indicating that part of GO was reduced to rGO as confirmed by XRD.

Fig. 7. XPS survey spectra of graphite (a), graphene oxide (GO-I) prepared using acids mixture of H2SO4, H3PO4, and HNO3 (70:20:10) (b), graphene oxide (GO-II) prepared using acid mixture of H2SO4, and H3PO4 (90:10) (c), and graphene oxide (GO-III) prepared using ultrasonic bath (d).

used to measure IR spectra. The chemical states of composing elements of GO's were investigated by X-ray photoelectron spectroscopy (XPS) using ThermoFisher ESCALAB 250i system. The binding energy values of XPS lines were calibrated using the C 1s peak of adventitious carbon at 284.8 eV as a reference. 3. Results and discussion 3.1. Structural characteristics of the samples The graphite powder and synthesized GOs samples were analyzed by XRD to study structural changes before and after acid treatments, in particular, the interlayer distances. Fig. 2(a) shows XRD pattern of graphite powder. The very sharp diffraction peaks observed at 26.50 and 54.64 2θ- Bragg angle, corresponds to (002) and (004) reflections of graphite hexagonal structure, respectively. Fig. 2(b–d) shows the XRD patterns of the prepared graphene oxides. The diffraction peak detected at 2θ of about 10.9° can be attributed to the (001) plane of the hexagonal crystalline structure of graphene oxide. For GO-I (Fig. 2 (b)), the corresponding d (001)-spacing at 2θ = 10.9° was calculated using Bragg's law to be 8.15 Å. However, the d (001) - spacing at 2θ = 11.2 and 2θ = 12.6° was calculated to be 7.94 and 7.07 Å of GO-II and GO-III (Fig. 2 (c) and (d), respectively). The hydrophilic oxygen-containing functional groups formed in the basal plane of GO sheets absorbed water molecules, thereby increasing the d-spacing. It has been reported that the interlayer distance of GO changes with the quantity of absorbed water molecules in the basal plane galleries and structural defects to be in the range of 6.1 Å for dry GO to 12 Å for hydrated GO [19]. Fig. 2 (d) also displays a wide diffraction peak (200) located at 2θ = 25.4° corresponding to rGO with d-spacing of 3.46 Å which is almost close to the typical thickness of pure single-layer graphene flake (3.4 Å) [25]. The XRD results indicated that, the GO-I had a higher quantity of hydrophilic oxygen-containing functional groups compared to GO-II and GO-III, due to higher inter-planar spacing. The absorbed water molecules and the oxygen functional groups in the basal plane hold the GO sheets together with an increased d-spacing. The XRD results (Fig. 2 (d)) also confirmed the reduction portion of GO to rGO when exposed to ultrasound treatment for 1 h. Morphological properties of the raw graphite and prepared GO samples were studied by SEM. Fig. 3 (a and b) display the SEM images

3.2. Compositional studies of the synthesized samples The synthesized samples were further analyzed by X-ray photoelectron spectroscopy (XPS) to further evaluate the chemical states of various elements and the presence of functional groups. The survey scan spectra of all samples showed mainly the presence of carbon and oxygen with trace amount of sulfur (Fig. 7 (b) and (c)), which was a result of process contamination. Fig. 8 shows high-resolution XPS spectra of C 1s region. The deconvoluted high resolution XPS C 1s region spectra of graphite used as raw material is shown in Fig. 8 (a). The deconvoluted C 1s peaks 1, 2, 3, 4, and 5, as shown in Fig. 8 (a), showed peak binding energies of 284.76, 287.00, 288.28, 289.50, and 291.28 eV which correspond to CeC (92.61 at.%), CeO (1.33 at.%), C=O (1.16 at.%), OeC=O (1.16 at.%), and π- π* satellite bonds, respectively. However, after acid treatments, the deconvoluted C 1s peaks 1, 2, 3, 4, and 5 with binding 7

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Fig. 8. Typical high-resolution XPS spectra of C 1s region of graphite (a), graphene oxide (GO-I) prepared using acids mixture of H2SO4, H3PO4, and HNO3 (70:20:10) (b), graphene oxide (GO-II) prepared using acid mixture of H2SO4, and H3PO4 (90:10) (c), and graphene oxide (GO-III) prepared using ultrasonic bath (d).

534.07 eV represent the OeC=O (4.01 at.%), C=O (14.01 at.%), CeOH (12.36 at.%), and CeOeC (5.26 at.%), groups, respectively, for the graphene oxide phase prepared using method (1) (Fig. 9 (b)). However, the deconvoluted O 1s peaks 1, 2, 3, and 4 for the graphene oxide phase prepared using method 2 have binding energies at 530.97, 531.77, 532.60, and 533.33 eV which are attributed to the OeC=O (1.61 at.%), C=O (6.06 at.%), CeOH (22.55 at.%), and CeOeC (7.82 at.%), respectively, (Fig. 9 (c)). The high-resolution XPS spectrum of O 1s region of GO and rGO prepared by method 3 was fitted to peaks 1, 2, 3, and 4 with binding energies of 531.19, 532.26, 533.01, and 533.80 eV which represent the OeC=O (2.13 at.%), C=O (8.24 at.%), CeOH (12.53 at.%), and CeOeC (3.65 at.%), respectively, (Fig. 9 (d)) [31,33–35]. The quantified atomic concentration of all phases and functional groups was calculated using XPS analysis and shown in Table 2. The atomic ratio of carbon to oxygen (C/O) quantified by XPS was calculated to be 25.67, 1.81, 1.63, and 2.77 for graphite, GO-I, GOII, and GO-III samples, respectively, (Table 2). These ratios are in close agreement with values calculated using EDS analysis as shown in Tables 1 & 2 and Fig. 4. From the XPS analysis, the atomic concentration (at.%) of the oxygen-containing moieties, such as carbonyl, and carboxyl

energies of 284.82, 286.62, 287.78, 289.12, and 290.90 eV, assigned to the CeC (13.06 at.%), CeO (29.46 at.%), C=O (16.63 at.%), OeC=O (5.21 at.%), and π- π* satellite groups, respectively, of the graphene oxide phase (Fig. 8 (b)). Similarity, the deconvoluted C 1s peaks 1, 2, 3, and 4 with binding energies of 284.80, 286.82, 288.30, 289.70, and 292.51 eV represent the CeC (20.24 at.%), CeO (35.95 at.%), C=O (4.80 at.%), OeC=O (0.97 at.%), and π- π* satellite bonds, respectively, for the graphene oxide phase prepared using method 2 (Fig. 8 (c)). On the other hand, the deconvoluted C 1s peaks 1, 2, 3, 4 and 5 with binding energies of 284.83, 286.99, 288.43, 289.5, and 291.2 eV are related to the CeC (41.76 at.%), CeO (23.03 at.%), C=O (5.98 at. %), OeC=O (2.68 at.%), and π- π* satellite bonds, respectively, for the graphene oxide and reduce graphene oxide phases prepared using method 3 (Fig. 8 (d)) [19,29–32]. Fig. 9 shows high-resolution XPS spectra of O 1s region. The deconvoluted O 1s peaks 1, 2, 3, and 4 with binding energies of 531.22, 532.45, 533.29, and 533.95 eV represent the OeC=O (1.32 at.%), C=O (1.68 at.%), CeOH (0.24 at.%), and CeOeC (0.50 at.%), respectively, of graphite phase (Fig. 9 (a)). The deconvoluted O 1s peaks 1, 2, 3, and 4 with binding energies of 531.50, 532.34, 533.10, and 8

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Fig. 9. Typical high-resolution XPS spectra in O 1s region of graphite (a), graphene oxide (GO-I) prepared using acids mixture of H2SO4, H3PO4, and HNO3 (70:20:10) (b), graphene oxide (GO-II) prepared using acid mixture of H2SO4, and H3PO4 (90:10) (c), and graphene oxide (GO-III) prepared using ultrasonic bath (d).

surfaces of GO-I and GO-II were more hydrophilic than that of graphite and GO-III, while the sp2 hybridized zone on GO-I and GO-II were much less than that of graphite and GO-III samples. FTIR results further confirmed that, the GO-I samples contained more oxygen functional groups, compared to the other GO prepared samples. This further increased the (O/C) ratio in the samples, rendering them higher-grade graphene oxide. According to the quantified XPS data, GO-I sample were found to possess the highest atomic ratio of carbon to oxygen (C/ O). The high quality graphene oxide produced has many applications and uses in a number of scientific fields, such as in the design of membranes incorporating GO as nanofillers for water treatment applications. GO with such high oxygen content would render the membrane strongly hydrophilic. This would improve membrane characteristics, including reduction in biofouling and increased flux. The demand for high quality GO makes the samples produced and analyzed in this study a promising way forward in GO production.

groups of the GO-I and GO-II, considerably increased, showing that the surfaces of GO-I and GO-II are more hydrophilic than that of graphite and GO-III, while the sp2 hybridized zone on GO-I and GO-II are much less than that of graphite and GO-III [36]. After exposing GO-III to ultrasound for 1 h, the C: O ratio of GO significantly increased from 1.63 to 2.77, indicating that part of GO was reduced to rGO as confirmed by XRD and FTIR data. 4. Conclusions We have reported facile and safe methods to synthesize high quality graphene oxide (GO) and reduced graphene oxide (rGO) without the production of toxic and explosive gases, whilst maintaining a low production temperature. The morphological, structural and compositional analyses showed that the synthesized materials were strongly dependent on the mixture of acids. The structural analyses of all samples revealed the presence of a crystalline GO phase and confirmed the reduction portion of GO to rGO when exposed to ultrasound treatment for 1 h. The XPS and EDS analyses confirmed the atomic concentration (at. %) of oxygen-containing moieties, such as carbonyl, and carboxyl groups of the GO-I and GO-II, considerably increased, showing that

Acknowledgments The authors would like to thank the support of Qatar Environmental and Energy Research Institute and Hamad Bin Khalifa University. The 9

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authors would also like to acknowledge the characterization support by QEERI's Core Labs as well as the help of the following Core Labs members; Dr. Dhanasekaran Thirunavukkarasu (FTIR), Dr. Akshath Raghu Shetty (XRD), Mr. Janarthanan Ponraj (TEM) and Mr. Mohamed I. Helal (SEM).

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