Nanoscience & Nanotechnology, 14 eds. E. Balabanova, E. Mileva, Sofia, 2014

120

THERMAL STABILITY OF RGO AND RGO/SiO2 NANOCOMPOSITE PREPARED BY SOL-GEL TECHNIQUE

A. Shalaby1,2, V. Yaneva2, A. Staneva2, L. Aleksandrov3, R. Iordanova3, Y. Dimitriev2

1 STCE – Science and Technology Center of Excellence, Cairo-Egypt 2 University of Chemical Technology and Metallurgy, 8 Kl. Ohridski blvd., 1756 Sofia, Bulgaria

3 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, G. Bonchev str., bld. 11, 1113 Sofia, Bulgaria

Abstract: Graphene the wonder material has attracted of a great deal of scientific interest in recent years. Graphene and its composites have been applied in the fields of nanoelectronic devices, transparent conductors, biosensors, photothermal therapy and drug delivery. Thermal stability is an important property of graphene required to be investigated due to the thermally induced changes of graphene or its composites. These transformations can significantly affect not only the electronic, chemical and geometric but also the heat transport properties of the carbon layer. Reduced graphene oxide (RGO) was prepared by Hummers method to produce graphite oxide and was finally exfoliated to RGO nanosheets by ultrasonication and reduction processes. RGO/silica composite was prepared by sol gel technique using tetraethoxysilane (TEOS) as precursor and small amount of RGO (2.5%). This study reports the thermal stability of RGO and RGO-Silica nanocomposite in both argon and air atmospheres. Characterization was performed by DTA analysis, X-ray phase analysis, TEM and SEM analysis. From the results, it was found that using sol-gel technique by mixing graphene and TEOS it is possible to obtain composite material which has thermal stability better than pure RGO.

Keywords: Reduced graphene oxide, TEOS, Nanocomposite, Sol-gel, Thermal stability

1. Introduction

Graphene is the first stable two-dimensional crystal composed of a single layer of sp2 network of carbon at-oms that was isolated in 2004 by Andre Geim & Kostya Novoselov, which received the Nobel Prize in 2010. Gra-phene has wonderful physicochemical properties such as high values of its Yong’s modulus, fracture strength, thermal conductivity, specific surface area, and electrical conductivity [1, 2]. These features have made graphene and graphene derivatives ideal for diverse applications such as nanoelectronic devices [ 3 ], transparent conduc-tors [ 4 ], transistors [5,6], energy-storage capacitors [7-9], biosensors [ 10, 11], dye-sensitized solar cells [12], photothermal therapy [13] and drug delivery [ 14 ]. The use of graphene has been extended to bacterial eradica-tion, including Escherichia coli, Staphylococus aureus, Pseudomonas aeruginosa, and Bacillus cereus [15–19]. Nevertheless, several of these applications are still not feasible because the large-scale production of unbroken pure graphene sheets remains challenging. Since No-voselov et al. in 2004 found a way to isolate individual graphene planes by using scotch tape [1] which is not a scalable technique; many researchers have attempted to create large scale graphene using various methods to pro-duce graphene sheets, Basically there are two different

approaches to preparing graphene, exfoliation and growth on Surfaces. The first approach which is more interesting for us included micromechanical cleavage, dispersion of graphite, graphite oxide exfoliation and substrate prepara-tion. In the second approaches, graphene layer can be grown directly on a substrate surface like epitaxial growth and chemical vapor deposition [20].

One of the methods considered as an effective route to synthesize graphene sheets due to its simplicity, reli-ability, ability for large-scale production and exception-ally low price is the isolation of graphene sheets from graphite by three steps. The first step is oxidation of graphite to graphite oxide [21- 22]. The second is sonica-tion of graphite oxide to obtain exfoliated graphene oxide layers [23, 24]. The last one is reduction the colloidal dispersions of graphene oxide to obtain graphene by chemical reducing agents, such as hydrazine [25], hydro-quinone [26], sodiumborohydride (NaBH4) [27, 28], ascorbic acid [29], vitamin C [30] hydriodic acid with acetic acid [31] etc. If there is no complete reduction which is difficult by chemical methods remain some oxy-gen on the graphene sheets so the correct name of these sheets are reduced graphene oxide (RGO) [32, 33]. In the past few years, significant efforts have been directed to-ward the preparation of graphene-based nanocomposites such as graphene/Bi2O3 [34], graphene/SnO2 [35], gra-phene/gold [36], Graphene/Silver [37], graphene/ Mn3O4 [38], Ag3VO4/TiO2/graphene [39], TiO2 / graphene [40, 41], ZnO/ graphene [42, 43] graphene/cobalt [44], etc. The unique properties of these materials have been dem-onstrated for a variety of catalysts, super capacitors and fuel cell batteries. It is indubitable that utilizing graphene-based nanocomposites in either of these technologies is a new endeavor, indicating that future research efforts will be abundant. One of the future challenges is to maintain the excellent physical properties of graphene in the proc-ess for synthesizing nanocomposites. [45]. In this study we prepared RGO/SiO2 nanocomposite which may find possible applications in the development of super capaci-tor device, drug delivery, biosensors and electrochemical sensor [46-47]. In this connection, Silica is very appro-priate compound due to its high chemical and thermal stability. Moreover by sol gel method it is easier to pre-pare homogenous composite with random distribution of RGO sheets in the volume of the materials. Outstanding thermal stability of RGO makes it a very attractive mate-rial for the fabrication of the composites with improved thermal stabilities. This is necessary to be taken in our

Thermal Stability of RGO and RGO/SiO2 Nanocomposite Prepared by Sol-Gel Technique 121

account especially for the fabrication of the composites due to the thermally induced changes of graphene or its composites which significantly affect the electronic, chemical, geometric and heat transport properties of the material [45, 48, 49]. This study reports the thermal sta-bility of RGO and RGO/SiO2 nanocomposite in both ar-gon and air atmospheres. Characterization was performed by DTA analysis, X-ray phase analysis, TEM and SEM analysis.

2. Experimental

2.1. Preparation of RGO and RGO/SiO2 composite In the first step, graphite oxide was prepared by

chemical exfoliation of purified natural graphite powder 99.9 % (Alfa Aesar Co.) by using a modified Hummers and Offeman method [50]. Graphite oxide was exfoliated to reduced graphene oxide (RGO) sheets by ultrasonica-

tion and reduction processes using sodium borohydride 98% (Alfa Aesar Co.) as shown in figure (1). The com-posite was prepared by sol gel technique using tetraethyl orthosilicate (TEOS) 98% (Aldrich Co.) as precursor. Mixture of few amount of RGO (2.5%) with deionized water was added to the solution of TEOS & ethanol under stirring for 1 hour at 110 ᵒC to obtain gel as shown in fig-ure (2). We selected low amount of RGO (2.5%) inside the composite according to some of literature data [51-54]. By this manner it is possible to avoid the aggregation process and to solve the problems connected to the exfo-liation and distributing the sheets inside the composites. It will be easy to cover separated sheets of RGO by silica matrix. This process will keep the stability of RGO sheets from the effect of air oxygen when heated the composite at different temperature.

Graphite flakes Grinding on ball milling

(1) H2SO4 +H3PO4

(2) NaNO3

(3) KMnO4 slowly adding after 30 min

Filtration

(4) Deionized H2Oslowly adding after 60 min

Deionized H2O + H2O2slowly adding

NaBH4 + NaOH

MixtureUnder cooled to 0C (ice bath) with vigorous agitation the color will convert from dark purple green to dark brown

Dark brown Mixture at room temperature

Yellow Brown filter cake Washing 3 times with (5 liter dist. Water &1M HCl) then drying 60 C for 12 h

Graphite Oxide Powder then ultrasonicated for 2 hours in

deionized water

Graphene Oxide SuspensionHeated in oil bath (100 C) for 24 h

Suspension Mixture

GrapheneWashing (5 x 100 ml dist water &

methanol) then drying 60 C for 12 h

Filtration

Figure 1. Scheme for preparation of RGO sheets by chemical exfoliation method

122 A. Shalaby, V. Yaneva, A. Staneva, L. Aleksandrov, R. Iordanova, Y. Dimitriev

Graphene + Deionized H2O

HCl Conc.

TEOS + Ethanol

Mixture

(Stirring for 1 hour at 110 C)

Gel(Drying 80 C for 6 hour under vacuum)

Composite

Figure 2. Scheme for preparation of RGO/SiO2 composite by sol gel method

2.2. Characterization The phase formation and structural transformation

were detected by X-ray phase analysis (Bruker D8 Ad-vance, Cu Kα radiation). The microstructure and morphol-ogy of the crystalline products were studied by scanning electron microscopy (SEM) (Jeol-357). Transmission elec-tron microscopy images were obtained by using (TEM JEOL 2100) at an accelerating voltage of 200 kV. The dif-ferential thermal analysis (DTA) in argon atmosphere (Ar-gon gas purity 99.999%) was carried out on LABSYSТМ EVO apparatus with Pt/Pt Rh thermocouples with a heat-ing rate of 10 K/s in argon flow, using Al2O3 as a reference material. While in air atmosphere by using differential thermal analysis Machine (STA PT1600 TG-DTA/DSC) STA Simultaneous Thermal Analysis, produced by LIN-SEIS Messgeräte GmbH, Germany.

3. Results and discussion

Figure (3a) shows the XRD patterns of graphite, graphite oxide (GO) and reduced graphene oxide (RGO). Pristine graphite exhibits a basal reflection (002) sharp peak at 2θ = 26.37ᵒ corresponding to d spacing of 0.337 nm which is compatible with the literature data (JCPDS 75-2078). Upon oxidation of pristine graphite, the (002) re-flection peak shifts to the lower angle at 2θ = 12.43ᵒ (d spacing = 0.711 nm) and remain small peak closed to the same position 2θ = 26.26ᵒ (d spacing = 0.339 nm). The in-crease in d spacing is due to the intercalation of water molecules and the formation of oxygen containing func-tional groups between the layers of the graphite [55]. In contrast to GO, all RGO have a broad peak centered at 2θ = 26.29ᵒ corresponding to d spacing of 0.338 nm which might be attributed to very thin RGO layers due to high degree of exfoliation. The close d-spacing of RGO to pris-tine graphite and disappearance of peak at 2θ = 12.43ᵒ in-dicates that the oxygen containing group of GO have been efficiently removed. Small amounts of amorphous-like structures around 2θ = ~23−25° explained by the complex folding structures, defects and impurities Furthermore, folding and unfolding processes depending on the presence of interlayer partial chemical and hydrogen bonds. These results are closed to other published data which indicated

that the graphene nanosheets are exfoliated into a monolayer or a few-layers and obtained a new lattice struc-ture, which is significantly different from the pristine graphite flakes and graphite oxide sheets [21, 56, 57].

Figure (3b) shows the XRD pattern of RGO/SiO2 nanocomposite. The amorphous nature of the obtained composite related to the amorphous or glassy nanoscale silica, which is the major component of the composite while, the amount of the RGO is too small (2.5%) to be detectable on the XRD pattern.

Figure 3a. XRD patterns of graphite, graphite oxide (GO)

and reduced graphene oxide (RGO)

Figure 3b. XRD pattern of RGO/SiO2 nanocomposite

Figure (4a) presents SEM image of RGO sheets which appeared as similar thin sheets randomly aggregated, with distinct edges, wrinkled surfaces, and folding. The sheets dimension of RGO was about 10 x 20 µm.

Figure (4b) presents SEM image of RGO/SiO2 nanocomposite which appeared as randomly aggregated of stack small sheets with the amorphous or glassy struc-ture of silica.

Figure (5a) presents TEM image shows few-layered RGO sheets. It was stacked on one another at lower reso-lution and appeared transparent. The number of sheets as appeared less than 5 sheets. This result is compatible with previously literature data [58, 59].

Figure (5b) shows the TEM image of RGO/SiO2 nanocomposite. It is clearly seen that the silica glassy na-ture is decorated by small RGO sheets which appeared like spots with small diameter about 10–30 nm.

Thermal Stability of RGO and RGO/SiO2 Nanocomposite Prepared by Sol-Gel Technique 123

Figure 4a. SEM image of RGO sheets

Figure 4b. SEM image of RGO/SiO2 nanocomposite

Figure 5a. TEM image of RGO sheets

Figure 5b. TEM image of RGO/SiO2 nanocomposite

Figures (6 & 7) present DTA images of RGO sheets and RGO/SiO2 nanocomposite under (a) air at-mosphere (b) argon atmosphere. The samples show significant weight loss with an onset temperature at slightly more than 100 °C, which was attributed to the elimination of interlamellar water, followed by re-moval of the labile oxygen-containing functional groups as CO, CO2 and H2O vapors from the sample at slightly higher temperatures. Obviously thermal stabil-ity in the air and argon atmospheres was drastically different. In argon atmosphere because the purity of argon gas was (99.999%) so we suggested that oxygen gets out from the samples itself. The small amount of oxygen evaporated from the sample itself wasn’t

enough to burn all amount of the carbon inside the sample so the mass lose after heating to 800 °C was 16.46 % and 13.68% from RGO and the composite re-spectively. While the mass losses in air atmosphere about 89% from RGO and 16% from the composite. According to literature data [60, 61] the silica pro-duced by sol gel technique is stable from 800 to 1000 ᵒC against crystallization. Amorphous silica ma-trix covered the sheets of RGO, by this manner the dif-fusion of the oxygen gas to the surface of RGO sheets is weak so the composite appear to be more stable than pure RGO sheets.

124 A. Shalaby, V. Yaneva, A. Staneva, L. Aleksandrov, R. Iordanova, Y. Dimitriev

Figure 6. TG/DTA images of RGO sheets for temperature up to 800 °C under (a) air atmosphere (b) argon atmosphere

Figure 7. TG/DTA images of RGO/SiO2 nanocomposite for temperature up to 800 °C under (a) air atmosphere (b) argon

atmosphere

4. Conclusions

In conclusion, we confirmed the possibility to ob-tain reduced graphene oxide sheets (RGO) by the exfoli-ated method. The sheets dimensions of RGO were about 10 x 20 µm and their number of stack small sheets ap-peared in TEM graph less than 5. It was established that using sol-gel technique by mixing RGO and TEOS it is possible to obtain composite materials. The thermal sta-bility of the composite is better than pure RGO sheets and the thermal stability in argon atmosphere is better than that in air atmosphere. These first results will encourage us to continue with increasing the amount of RGO in the future experiments and to understand more deeply the in-teraction between Silica and RGO particles.

Acknowledgements

The author Ahmed Shalaby thanks to the Erasmus Mundus MEDASTAR (Mediterranean Area for Science Technology and Research) Program.

References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y.

Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666-669.

[2] A.K. Geim, Science 324 (2009) 1530-1534. [3] A.K. Geim, K.S. Novoselov, Nat Mater 6 (2007) 183-191. [4] X. Li, X. Wang, L. Zhang, S. Lee, H. Dai, Science. 319

(2008) 1229-1232. [5] I. Meric, M.Y. Han, A.F. Young, B. Ozyilmaz, P. Kim,

K.L. Shepard, Nat Nano 3 (2008) 654-659. [6] P. Avouris, Z. Chen, V. Perebeinos, Nat Nano 2 (2007)

605-615. [7] M. Liang, L. Zhi, Journal of Materials Chemistry 19

(2009) 5871-5878. [8] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Nano

Letters 8 (2008) 3498-3502. [9] E. Yoo, J. Kim, E. Hosono, H.S. Zhou, T. Kudo, I. Honma,

Nano Lett. 8 (2008) 2277-2282. [10] M. Zhang, B.C. Yin, W. Tan, B.C. Ye, Biosens

Bioelectron. 26 (2011) 3260-3265. [11] J.D. Fowler, M.J. Allen, V.C. Tung, Y. Yang, R.B. Kaner,

B.H. Weiller, ACS Nano 3 (2009) 301-306. [12] M. SIMA, I. ENCULESCU, A. SIMA, Optoelectronics

and Advanced Materials – Rapid Communications 5 (2011) 414 – 418.

[13] S.P. Sherlock, S.M. Tabakman, L. Xie, H. Dai, ACS Nano 5 (2011) 1505-1512.

[14] Y. Sun, L. You, Y. Cao, J. Ma, Y. Guan, Y. Du, G. Lu, Sensor Letters 9 (2011) 856-860.

[15] W. Hu, C. Peng, W. Luo, M. Lv, X. Li, D. Li, Q. Huang, C. Fan, ACS Nano 4 (2010) 4317-4323.

[16] O. Akhavan, E. Ghaderi, ACS Nano 4 (2010) 5731-5736. [17] M.R. Das, R.K. Sarma, R. Saikia, V.S. Kale, M.V. Shelke,

Thermal Stability of RGO and RGO/SiO2 Nanocomposite Prepared by Sol-Gel Technique 125

P. Sengupta, Colloids Surf B Biointerfaces. 83 (2011) 16-22.

[18] S. Park, N. Mohanty, J.W. Suk, A. Nagaraja, J. An, R.D. Piner, W. Cai, D.R. Dreyer, V. Berry, R.S. Ruoff, Advanced Materials 22 (2010) 1736-1740.

[19] O. Akhavan, E. Ghaderi, The Journal of Physical Chemistry C 113 (2009) 20214-20220.

[20] N. Krane, Preparation of Graphene, Selected Topics in Physics: Physics of Nanoscale Carbon, 2011.

[21] L.Y. Meng, S.J. Park, Bull. Korean Chem. Soc. 33 (2012) 209-214.

[22] G. Shao, Y. Lu, F. Wu, C. Yang, F. Zeng, Q. Wu, Journal of Materials Science 47 (2012) 4400-4409.

[23] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nature 422 (2006) 282-286.

[24] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Advanced Materials 22 (2010) 3906-3924.

[25] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558-1565.

[26] G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu, J. Yao, The Journal of Physical Chemistry C 112 (2008) 8192-8195.

[27] Y. Si, E.T. Samulski, Nano Letters 8 (2008) 1679-1682. [28] H.-J. Shin, K.K. Kim, A. Benayad, S.-M. Yoon, H.K. Park,

I.-S. Jung, M.H. Jin, H.-K. Jeong, J.M. Kim, J.-Y. Choi, Y.H. Lee, Advanced Functional Materials 19 (2009) 1987-1992.

[29] V. Dua, S.P. Surwade, S. Ammu, S.R. Agnihotra, S. Jain, K.E. Roberts, S. Park, R.S. Ruoff, S.K. Manohar, Angewandte Chemie International Edition 49 (2010) 2154-2157.

[30] M.J. Fernandez-Merino, L. Guardia, J.I. Paredes, S. Villar-Rodil, P. Solis-Fernandez, A. Martinez-Alonso, J.M.D. Tascon, The Journal of Physical Chemistry C 114 (2010) 6426-6432.

[31] I.K. Moon, J. Lee, R.S. Ruoff, H. Lee, Nat Commun 1 (2010) 73.

[32] S. Pei, H.-M. Cheng, Carbon 50 (2011) 3210-3228. [33] S. Park, J. An, J.R. Potts, A. Velamakanni, S. Murali, R.S.

Ruoff, Carbon 49 (2011) 3019-3023. [34] K.R. Nemade, S.A. Waghuley, Solid State Sciences 22

(2013) 27-32. [35] F. Li, J. Song, H. Yang, S. Gan, Q. Zhang, D. Han, A.

Ivaska, N. Li, Nanotechnology 20 (2009) 455602. [36] A.R. Biris, S. Pruneanu, F. Pogacean, M.D. Lazar, G.

Borodi, S. Ardelean, E. Dervishi, F. Watanabe, A.S. Biris, International Journal of Nanomedicine 8 (2013 ) 1429 – 1438.

[37] S.V. Kumar, N.M. Huang, H.N. Lim, M. Zainy, I. Harrison, C.H. Chia, Sensors and Actuators B: Chemical 181 (2013) 885-893.

[38] Y. Wu, S. Liu, H. Wang, X. Wang, X. Zhang, G. Jin, Electrochimica Acta 90 (2013) 210-218.

[39] J. Wang, P. Wang, Y. Cao, J. Chen, W. Li, Y. Shao, Y. Zheng, D. Li, Applied Catalysis B: Environmental 136-137 (2013) 94-102.

[40] Y. Zhang, C. Pan, Journal of Materials Science 46 (2011) 2622-2626.

[41] J. Guo, S. Zhu, Z. Chen, Y. Li, Z. Yu, Q. Liu, J. Li, C. Feng, D. Zhang, Ultrasonics Sonochemistry 18 (2011) 1082-1090.

[42] X. Chen, Y. He, Q. Zhang, L. Li, D. Hu, T. Yin, Journal of Materials Science 45 (2010) 953-960.

[43] T. Xu, L. Zhang, H. Cheng, Y. Zhu, Applied Catalysis B: Environmental 101 (2011) 382-387.

[44] Z. Ji, X. Shen, Y. Song, G. Zhu, Materials Science and Engineering: B 176 (2011) 711-715.

[45] X. Wang, S. Chen, Graphene-Based Nanocomposites, Dr. Sergey Mikhailov (Ed.) ed., InTech 2011.

[46] M. Vallet-Regí, F. Balas, D. Arcos, Angewandte Chemie International Edition 46 (2007) 7548-7558.

[47] J.Q. Dalagan, E.P. Enriquez, L.J. Li, Journal of Materials Science 48 (2013) 3415-3421.

[48] G. Gedler, M. Antunes, V. Realinho, J.I. Velasco, Polymer Degradation and Stability 97 (2012) 1297-1304.

[49] R. Miranda, A.L. Vazquez de Parga, Nat Nano 4 (2009) 549-550.

[50] W.S. Hummers, R.E. Offeman, Journal of the American Chemical Society 80 (1958) 1339.

[51] Wang, K. Zhang, Journal of Molecular Catalysis A: Chemical 345 (2011) 101-107.

[52] S. Murali, Carbon based materials for electrodes in Electrical Double Layer Capacitors. Faculty of the Graduate School The University of Texas Austin, 2012, p. 169.

[53] Y.-Y. Yang, Z.-A. Hu, Z.-Y. Zhang, F.-H. Zhang, Y.-J. Zhang, P.-J. Liang, H.-Y. Zhang, H.-Y. Wu, Materials Chemistry and Physics 133 (2012) 363-368.

[54] H. Sun, S. Liu, S. Liu, S. Wang, Applied Catalysis B: Environmental 146 (2014) 162-168.

[55] J.I. Paredes, S. Villar-Rodil, A. Martínez-Alonso, J.M.D. Tascón, Langmuir 24 (2008) 10560-10564.

[56] S. Thakur, N. Karak, Carbon 50 (2012) 5331-5339. [57] S.H. Huh, Thermal Reduction of Graphene Oxide, Physics

and Applications of Graphene – Experiments, InTech, 2011.

[58] Y. Zhang, D. Li, X. Tan, B. Zhang, X. Ruan, H. Liu, C. Pan, L. Liao, T. Zhai, Y. Bando, S. Chen, W. Cai, R.S. Ruoff, Carbon 54 (2013) 143-148.

[59] M. Hassan, K.R. Reddy, E. Haque, A.I. Minett, V.G. Gomes, Journal of Colloid and Interface Science 410 (2013) 43-51.

[60] M.G. Garnica-Romo, J.M. Yañez-Limón, M. Villicaña, J.F. Pérez-Robles, R. Zamorano-Ulloa, J. González-Hernandez, Journal of Physics and Chemistry of Solids 65 (2004) 1045-1052.

[61] S.J. Darzi, A.R. Mahjoub, A.R. Nilchi, S.R. Garmarodi, IJMSE 8(2011)20-26.