Middle East Journal of Applied Sciences ISSN 2077-4613

Volume : 05 | Issue : 05 | Oct.-Dec. | 2015 Pages: 23-30

Corresponding Author: A.M. Bakr, National Research Center, Al-Behooth St., Cairo, Egypt, Email: ahmedbakr_8@yahoo.com

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Thermal Degradation Studies of Graphene Oxide Polymer Composite 1 M. A. Moharram, 2K. M. T. Ereiba, 1Walid El Hotaby and 1A.M. Bakr

1 Spectroscopic Department, Physics Division. National Research Center Cairo, Egypt. 2Physics Department, Faculty of Science, El Azhar University, Cairo, Egypt.

ABSTRACT The solid beads shape composites of graphene oxide (GO) and biopolymers have a great potential in the efficient removal of hazardous pollutants from wastewater. In this work, graphene oxide (GO) was firstly prepared using a modified Hummers method from natural graphite powder. Graphene oxide porous crosslinked cellulose nanocomposites (GOCB) have been prepared based on GO and cellulose acetate. Thermo-decomposition studies were carried on 25–600 ᴼC range at rate of 10 ᴼC min -1. The resulting composite samples were characterized by using scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). TGA analysis showed that the thermal stability of the composites was influenced by increasing GO ratio in the composite. GO powder underwent a two step thermal degradation process, which was distinctly different from the graphite curve. The 1st step was attributed to the loss of H2O at ~100 ᴼC, and the 2nd step appeared at ~200 ᴼC for the loss of oxygen-containing functional groups and residues. In contrast, natural graphite showed no weight loss until 600 ᴼC. Independent GO nanosheets are observed through TEM micrographs and it was clear that the addition of GO to the composites increases folding, roughness and groves that what would lead to increasing in the surface area and consequently enhancing the adsorption capacities of the composites. The FTIR spectra of GOBC proved that there is a chemical interaction between GO and the cellulose chains via hydrogen bonding. The XRD pattern of graphite and GO revealed that the original graphite powders had almost been completely oxidized and GO has been exfoliated successfully. Key words: Graphene oxide, Cellulose, microspheres, thermal analysis, characterization.

1. Introduction Graphene, a 2D crystal composed of monolayers of carbon atoms arranged in a honeycombed network with six-membered rings, has unique physical and chemical properties which make it ideal candidate for extended applications such as, nanoelectronics ( Zhang et al., 2009), supercapacitors (Zhu et al., 2011), sensors (Garaj et al., 2010), hydrogen storage (Paul et al., 2010), drug delivery (Yang et al., 2013), and nanocomposites (Moharram et al., 2015 ; Shahil and Balandin, 2012). Graphene has become a sparkling star on the horizon of materials science due to its extraordinary electrical, thermal, and mechanical properties. Graphene has a large theoretical specific surface area of about 2.6 × 103 m2 g–1 , high thermal conductivity(3 ×103 W m–1 K–1) and Young’s modulus (1.06 ×103 GPa). What’s more, the stable six-membered ring structure endows graphene with excellent electrical conductivity. The electrical transfer rate could reach 1.5 × 104 cm2 V–1 s–1 in RT (Jiang, 2011; Guo and Dong, 2011). With respect to its unique physical, chemical and mechanical properties, graphene oxide (GO) has gained an explosion of interest and opened up a new research area for material science. Recent researches have indicated that GO proved to be a promising material to adsorb metal ions due to their extraordinary mechanical strength and relatively large specific area (Moharram et al., 2015 ; Willi Paul and Sharma, 2011). Dramatically, it is of great significance to develop adsorptive composites from biopolymers for the removal of organic hazardous pollutants in water. First of all, biopolymers are non-toxic and nonhazardous to the environment, animals and human beings. In addition, biopolymers are degradable in the natural system (S Ray and Bousmin, 2005; Eckenfelder, 2000; Yang et al., 2001; Bose et al., 2002; Wingenfelder et al., 2005; Shammas, 2004; Semerjian and Ayoub, 2003; Licskó 1997, Ayoub et al., 2001; Metcalf and Eddy 2003; Jüttner et al., 2000). Among various biopolymers, cellulose is the most abundant one on earth. Moreover, it can be converted into various derivatives through chemical reactions. The interest in cellulose and its modification as cellulose-based composite has been exponentially increasing. During the last decades, cellulose and cellulose-based composite have been extensively designed for many applications (Zhou et al., 2011). Therefore, in our laboratory, efforts have been made to fabricate adsorptive graphene oxide porous crosslinked cellulose microspheres and investigate its thermal decomposition and chemical structural.

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2. Materials and experimental methods: 2.1. Materials: Graphite powder with a size less than 20µm and Cellulose acetate (Mn~30,000) were purchased from Sigma-Aldrich, USA. Hydrogen peroxide was bought from ROTH, Germany. Sulphuric acid (98%) was obtained from Fisher. Sodium nitrate (99%) was purchased from ACROS, Belgium. Dimethyl sulfoxide (DMSO) was obtained from TEDIA. All the reagents were of analytical grade or highest purity available, and used without further purification. Doubly distilled water was used throughout the experiments. 2.2 Synthesis of graphene oxide (GO): Purified graphite was used to prepare GO according to the well-known Hummer’s method with some modification (Hummers and Offeman 1958). Firstly, 5.0 g of graphite powder and 115 mL 98% H2SO4 were put into a 5 L flask. The mixture was kept at an ice-water bath and the temperature was below 0°C under stirring until the graphite full dissolved. Then 2.5 g of NaNO3 was added, after 10 min 15 g of KMnO4 was added by batch addition while stirring. The flask was kept at 35°C for 3 h. After 3h, 230 ml of distilled water was slowly added, and the temperature of the mixture was kept at 98 °C for 20 min. Then, the reaction was stopped by adding distilled water (700 mL) and H2O2 (50 mL, 30 %). The solution's color transformed to brilliant yellow. The obtained powder was washed, filtered, and dried at 60 °C for 24h. 2.3 Preparation of graphene oxide porous crosslinked cellulose beads GOCB: Crosslinked porous graphene oxide cellulose beads (GOCB) were prepared as follows. Graphene oxide powder with certain amount was dissolved in 40 ml of dimethyl sulfoxide (DMSO) and treated by ultrasonic for 15 min at room temperature to form a homogeneous suspension. Then 4 g of cellulose acetate was added slowly while stirring until full dissolving. Then 0.4 g of sodium bicarbonate and 1.6 g of anhydrous sodium sulfate were added, and blended adequately with a magnetic stirrer. After that, the mixture was added dropwise into an acid coagulation bath (500 ml of 0.015 M HCl) using a 0.8 mm diameter injector. The injection speed rate was about 150 ml/h. The prepared beads were collected by filtration. Then, beads were immerged in large quantities of hot water to dissolve and remove sodium sulfate in order to form plenty interior pores in the beads. Also, in order to initiate symmetrical pore in the surface layer of the beads, the beads were immersed into 120 ml sodium hydroxide solution (2 M) and stirred gently at 40 °C for 1.5 h, and at the same time 25 ml of epichlorohydrin was added into the reactor for a latter-crosslinking. Finally, the beads were rinsed thoroughly with distilled water till pH reach 7, then the beads were isolated by filtration and were left to air-dry in Petri dish for two days then collected and saved in closed vile. A series of graphene oxide / cellulose composites coded as A5, A10, A15, and A20 were prepared by altering the weight of GO into the composite as 0.2105 g, 0.4445 g, 0.7059 g and 1 g, respectively. It is obvious that the weight percentage of GO in the composites are 5 wt%, 10 wt%, 15wt % and 20 wt%. In addition, cross-linked cellulose, coded as A0, was also prepared according to the same method without adding GO. 2.4. Characterizations: The thermal analysis of the prepared beads powder were analyzed with SDT Q600 V20.9 Build 20 DSC-TGA apparatus under nitrogen atmosphere (50ml/min) in alumina pan increasing the temperature (10 Co/minute) between 25-600Co. Vertex 70 system (Bruker optik-Germany) FT/IR Spectrometer was used for recording the IR spectra. Spectra were recorded in a spectral range of 4000–400 cm-1, resolution 2 cm-1 and scan speed 2mm/sec using kBr disc technique. The structure of GO nanocomposites are investigated by Broker D8 Advance, Germany X-ray diffraction (XRD) with CuKα 40kV/40mA and an incidence angle of 0.5° with wavelength equal to 1.5418 A°. The diffraction patterns were recorded automatically with a scanning rate of 2θ=2 (deg/min). SEM characterization was carried out using a Quanta FEG 250 type instruments in vacuum environment.

Results and Discussion 3.1. Thermal properties: 3.1.1. Thermal gravimetric analysis of graphene oxide porous crosslinked cellulose composites (GOCB): Thermogravimetric curves TGA and differential thermal gravimetric analysis DTG for raw graphite and GO are presented in Figures 1. While TGA and DTG curves of the prepared GOCB composites are presented in figure 2, the decomposition temperature range, temperature related to DTG peak maximum values, and obtained mass loss values are listed in table 1.

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100 200 300 400 500 600

0

10

20

30

40

50

60

70

80

90

100

100 200 300 400 500

-0.6

-0.4

-0.2

0.0

Temprature (oC)

W/

T

Weig

ht (%

)

T em pera ture ( C )

graphite

G O

Fig. 1: TGA and DTG curves of graphite and GO. Raw graphite powder almost displays no mass loss all over the temperature range (mass loss about 1.39 %). GO shows about 17.5 % mass loss below 160°C with maximum decomposition rate at 50 C0, resulting from the evaporation of adsorbed water. Then there is a rapid mass loss (20 %) from 160 to 240 °C with maximum decomposition rate at 210 C0 owing to the removal of the oxygen-containing functional groups. After 240 °C, GO displays a gradual mass loss, which is due to the further removal of functional groups.

100 200 300 400 500

0

20

40

60

80

100

100 200 300 400 500

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

B

Temprature (0C)

W

/ T

A0

A5

A10

A15

A20

Weig

ht (%

)

Temperature (C)

A0A5

A10

A15

A20

Fig. 2: TGA and DTG curves of GOCB nanocomposites. As it shown in figure 2, porous crosslinked cellulose is degraded in two steps. The first step from the room temperature to 120ᴼC represents the evaporation of residual absorbed water with maximum decomposition rate at 50 ᴼC and shows about 10 % mass loss. The second step starts at about 280°C and ends at 400°C with maximum decomposition rate at 341 C0 (mass loss about 84%), and represents the main thermal degradation of the cellulose chains. The TGA and DTG curves of GO porous crosslinked cellulose beads show that the composites are less thermally stable than the porous crosslinked cellulose. The GO curve shows that the GO is degraded into lower temperatures than cellulose, thus, the inclusion of GO in the polymer network makes a proportionate decrease in the thermal stability of the composites. DTG curves reflected this behavior, the maximum decomposition rates appear at 341°C, 340°C, 338°C, 334°C, and 332 °C for samples A0, A5, A10, A15 and A20 respectively. Also there is proportional emersion of a new peak at about 173°C due to the degradation of GO and this peak enlarges as the GO concentration increases. This event may be related to the O-H bond present in the composites. The total mass loss were 94%, 77.5%, 75%, 67% and 64% for the samples A0, A5, A10, A15 and A20 respectively.

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Table 1:Thermal analysis data of GO and GOCB composites.

sample Temperature range (°C)

DTG peak temperature (°C)

Mass loss (%) Total mass loss (%)

GO 25-120 120-160 160-240

50 -

210

15 2.5 20

38.5

A0 25-120 280-400

50 341

10 84

94

A5 25-120 120-270 270-400

50 -

340

7.5 5

65 77.5

A10 25-120 120-270 270-400

50 -

338

7.5 6.5 61

75

A15 25-120 120-270 270-400

50 -

334

10 7

50 67

A20 25-120 120-270 270-400

50 173 332

12 8

44 64

3.2. Morphology and structure properties of graphene oxide porous crosslinked cellulose microspheres (GOCB) :

The morphology and structure of GO nanosheets were investigated through SEM observation. Figure 3 showed that the GO nanosheet appears flat and transparent, with some wrinkles and folding on the surface, which may be attributed to of deformation upon the exfoliation and restacking processes of graphite. Independent GO nanosheets are observed through TEM micrographs (Fig. 4). The individual nanosheets have sizes extending from tens to several hundreds of square nanometers, which are due to the sonication process that destroyed the van der Waals interactions between GO layers, and the existence of a large amount of oxygen-containing functional groups on the surface of GO nanosheets.

Fig. 3: HR-SEM micrograph of GO powder.

Fig 4: TEM micrograph of GO. The SEM micrographs of the surface morphologies of GOCB (samples A10, and A20) are presented in figures 5 and 6 respectively. The images demonstrate that the graphene oxide sheets are fully exfoliated and clearly well-dispersed in the cellulose matrix. GOCB have regular pores with smaller pore sizes. Also, it can be observed that there are much more homogenous porous structures.

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Fig. 5: SEM micrograph of GO cellulose nanocomposite (A10)

Fig. 6: SEM micrograph of GO cellulose nanocomposite (A20). 3.3. The Infrared spectra of graphene oxide porous crosslinked cellulose composites (GOCB): FTIR analysis has often been used as a useful tool in determining specific functional groups or chemical bonds that exist in a material. Figure (7) shows the FTIR spectra of cellulose acetate powder (CA).

4000 3000 2000 1000 0

Wavenumber [cm-1

]

Cellulose acetate

Abs

Fig. 7: FT-IR spectra of graphene oxide and cellulose acetate powder. The following illustration explicates the position and significance of each peak pertaining to the CA skeletal structure; these peaks have been distinctly marked in the upper IR spectrum (CA) in Fig (7). The spectrum shows strongly broad absorption band at 3444 cm-1, which is attributed to the stretching vibration of O – H. There are a peak and small shoulder located at 2929 cm−1and 2885 cm−1 respectively which are attributed to stretching vibration of C ̶ H and C ̶ H2 groups respectively. Strong and characteristic peak at 1750 cm-1 represents C=O stretching mode of ester group. Small peak at 1431 cm-1 is symmetric ̶ CH2 or asymmetric ̶ CH3 stretching mode. The peaks at 1380 and 1245 cm-1 represent the symmetric ̶ CH, carbohydrate C ̶ O. The band

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positioned at 1640 cm−1 relates to the bending mode of the naturally absorbed water. The C−O−C functional group appears at 1040cm-1 (Azzaouia et al., 2015; John et al., 2012; Zhou et al., 2011, and Liu and Bai, 2005).

4 0 0 0 3 5 0 0 3 0 0 0 1 5 0 0 1 0 0 0 5 0 0

W a v e n u m b er [c m-1

]

A 0

A 5

A 1 0

A 1 5

A 2 0

A b s

Fig. 8: FT-IR spectra of graphene oxide cellulose nanocomposites. Figure (8) shows the FTIR spectra of the crosslinked porous cellulose beads (A0) and GOCB samples (A5, A10, A15 and A20). In comparison with the starting material CA, the absorption peaks of the stretching vibration of the C=O for CA should be at 1740 cm-1. This absorption peak disappeared absolutely in the IR spectra of the crosslinked porous cellulose beads. This indicates that the cellulose acetate has completely hydrolyzed to the porous cellulose beads. The broadening as well as the splitting of the C−O−C band at about 1040 cm-1 was caused by the incorporation into the chains. Another important feature of the spectra is the broadening of the absorption at 3400 cm-1, resulting from the increase in hydroxyl groups of the carboxylic acids in the crosslinked structure. The Go porous crosslinked cellulose composite spectra are shown in figure (8). In fact, these spectra are almost indistinguishable from those of crosslinked porous cellulose composite, while The main vibration modes of stretching vibration of O – H (3400cm−1), C–H2 stretching vibration(2891 cm-1), O–H bending mode (1647cm−1) and C−O−C stretching (1061cm−1) have been systematically shifted to shorter wavenumbers as the GO concentrations increase. This trend suggests that there is a chemical interaction between GO and the cellulose chains via hydrogen bonding. (Moharram et al., 2015; Krishnamoorthy et al., 2013; Han et al., 2011, and Wang et al., 2009). 3.4. X-ray diffraction of GOCB: Figure (9) presents XRD patterns of the pure graphite powder and GO obtained from the oxidation and exfoliation of graphite. Careful analysis of the figure reveals that the XRD pattern of the pure graphite exhibits a high intensity peak around 26.7° corresponding to the graphitic structure (002). After the oxidation and exfoliation process, this peak disappears in the XRD pattern of the GO and a new peak is observed at 2θ = 11.4◦, corresponding to the (0 0 1) plane reflection of GO, which indicates that graphite was converted into GO. The calculated interlayer spacing of GO is 0.798 nm, which is much larger than that of graphite (0.336 nm). The larger interlayer distance of GO could be attributed to the oxide induced oxygen containing functional groups such as hydroxyl, epoxy and carboxyl.

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

2

G r a p h i t e

G r a p h e n e

O x i d e

A 0A 5

A 1 0A 1 5

A 2 0

I n t e n s i t y

Fig. 9: XRD patterns of graphite, graphene oxide and graphene oxide cellulose acetate samples.

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As are shown in figure (9), all of GO cellulose composites samples and cellulose sample exhibit a broad peak at 2θ = 21° due to the amorphous state of the cellulose. Apparently, the diffraction peaks of GO are not seen in the XRD peak of GO cellulose composites samples, indicating that GO was well exfoliated with cellulose in GO cellulose composites and the amorphous structure of cellulose dominated at GO . Besides, the diffraction intensity of cellulose samples is of a little stronger intensity compared to GO cellulose composites, implying to increase in the crystalline degree after combining GO with cellulose. The good compatibility and mixability between GO and cellulose in the blend was reflected in the XRD results, because, if GO and cellulose had low compatibility in the blend, each component would show its own Crystal region in the blend and the X ray diffraction patterns should show a simply mixed pattern of GO and cellulose. These results are in good agreement with the results reported by Taha et al. (2012).

Conclusion: Due to its large specific surface area, abundant functional groups, good dispersibility in water, relatively easy preparation method and better biocompatibility, GO demonstrate the possibility of applying as an excellent water treatment agent. From the view of environment and economy, biopolymers attract a strong interest as a more environmental and cost effective alternative. GO powder underwent a two step thermal degradation process, which was distinctly different from the graphite curve. The 1st step was attributed to the loss of H2O at ~100 ᴼC, and the 2nd step appeared at ~200 ᴼC for the loss of oxygen-containing functional groups and residues. In contrast, natural graphite showed no weight loss until 600 ᴼC. TGA analysis showed that the thermal stability of the composites influenced by the addition of GO. Independent GO nanosheets are observed through TEM and SEM micrographs. The individual nanosheets have sizes extending from tens to several hundreds of square nanometers. Also it is noticeably that the addition of GO to cellulose matrix increases folding, and roughness that in turn lead to increasing in the surface area and consequently enhancing the adsorption capacities of the composites. The GO showed well exfoliated within chitosan matrix. The XRD pattern of graphite and GO revealed that the original graphite powders had almost been completely oxidized and GO has been exfoliated successfully also the XRD results reflected the good compatibility and mixability between GO and the cellulose. The applicable form of GO-nanocomposites beads prepared as spherical carriers with high porosity structure is easily swellable in water and demonstrates a great potential in the efficient removal of heavy metals ions from wastewater. From the view of environment and economy, biosorbents attract a strong interest as a more environmental and cost effective alternative, this method of preparation is simple, economical and environmental friendly.

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