2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO)

The International Conference Centre Birmingham

20-23 August 20112, Birmingham, United Kingdom

Clean Synthesis of Reduced Graphene-Ti02 Composites in Ionic

Liquid

Van Hoa Nguyen and Jae-Jin Shim

Abstract- A facile and fast method of preparing composites

of graphene and Ti02 nanoparticles via a microwave-assisted

synthesis in an ionic liquid ([bmim)[BF4D was reported.

Titanium (IV) isopropoxide (Ti(OiPr)4) was used as precursor

for forming of Ti02 nanoparticles onto graphene sheets by the

thermal decomposition under microwave irradiation. The

as-prepared composites were characterized by BET surface

area, X-ray diffraction (XRD), scanning electron microscopy

(SEM), and transmitted electron microscopy (TEM) and cyclic

voltammetry (CV). The Ti02 nanoparticles were found to be

anatase of 9 nm average size and the obtained composites had

high surface areas and high photocatalytic degradation of

methylene blue.

I. INTRODUCTION

GraPhene, a one-atom-thick two-dimensional single layer of Sp

2 -bonded carbon, has received a rapidly growing

research interest. Graphene has reported as a good substrate for fonning composites with a variety of materials because of its high conductivity and good mechanical properties, and high surface area [1]. In other hand, graphene-based materials can be easily obtained by simple chemical processing of graphite [2]. For various applications, many efforts have been made to assemble metal or metal oxide nanoparticles on graphene sheets, which have synergistic contribution of two or more components [3].

Two polymorphs of Ti02 (anatase and rutile) are suitable for use in composites because of their photocatalytic properties, relative nontoxlclty, and long-term thermodynamic stability. Anatase has been reported to exhibit a significantly higher photocatalytic activity than rutile, and has been widely used in the photocatalytic degradation of organic pollutants, Combining the advantages of graphene and Ti02 nanoparticles has been attempted through the preparation of graphene-Ti02 nanocomposites for specific applications [4]

Microwave-assisted approach offers several advantages compared to the conventional solvothennal or hydrothennal methods. This method is considered to be faster, cleaner and more cost effective than the other conventional and wet

Manuscript received June 20, 2012. This work is supported by Grant No. RTl04-01-04 from the Regional Technology Innovation Program of the Ministry of Knowledge and Economy (MOKE).

J. J. Shim is with Yeungnam University, Gyeongsan, Gyeoungbuk, 712-749, Republic of Korea (corresponding author to provide phone: +82-53-810-2587; fax: +82-53-810-4631; e-mail: jjshim@ yu.ac.kr).

V. H. Nguyen is with Yeungnam University, Gyeongsan, Gyeoungbuk, 712-749, Republic of Korea (e-mail: hoadhnt@gmail. com).

chemical methods for the synthesis of metal oxide nanoparticles. Recently, Ding et al. successfully synthesized size-controlled anatase nanocrystals in [bmimHBF4] via a microwave-assisted process [5]. Ti02 and Sn02 nanoparticles of different sizes and shapes can be easily obtained in this way. This route was carried out under atmospheric pressure in a domestic microwave oven; no high-pressure and high temperature apparatus is needed; the size of Ti02 nanoparticles can be easily controlled

The use of green solvents, such as supercritical carbon dioxide and ionic liquids (ILs), should be developed to overcome these environmental concerns. ILs have unique properties such as extremely low volatility, wide liquid temperature range, good thermal stability, good dissolving ability, designable structures, high ionic conductivity, wide electrochemical window, and excellent microwave absorbing ability [6].

In this study, graphene-Ti02 composites were successfully prepared via a simple, fast and efficient one-step route in [bmimHBF4] under microwave irradiation. The obtained samples were characterized by BET surface area, X-ray diffraction (XRD), cyclic voltammetry (CV), scanning electron microscopy (SEM), and transmitted electron microscopy (TEM). Moreover, the catalytic efficiency of the composites was investigated through the photoelectrodegradation of methylene blue (MB).

II, MATERIALS METHODS

A. Materials

Graphite powder (99.995%, Alfa Aesar) and Titanium (IV) isopropoxide (TTIP) (99.999%, Aldrich) was used as received. The ionic liquid (IL), I-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), provided by Ionic Liquids Technologies (>98%, Gennany) was kept in a vacuum oven at lOO°C for 24 h to remove volatile impurities such as water and organic substances remained in the IL before use. Other reagents were of analytical grade and used without further purification.

B. Synthesis ofgraphene oxide (GO)

Graphene oxide was synthesized from graphite powder by a modified Hummers method [1]. Typically, 2g of graphite powder was added in 50 ml of concentrated H2S04 along with 2g of NaN03 in a flask at O°C in an ice-bath. Afterwards, 6g of KMn04 was slowly added to the solution while maintaining vigorous stirring at below 20°C. The ice-bath

was then removed and the mixture was stirred at 35 °C for 30 minutes and it became pasty brownish grey in color. The paste was diluted with 100 ml of deionized water and stirred for 2 h. A 10 ml of H202 (30 wt. %) solution was slowly added into the suspension along with 100 ml of HCI ( l0 v/v %) solution. The mixture was centrifuged and washed with deionized water until the decant solution was neutral. The product was dried at room temperature under vacuum for 24 h.

C. Synthesis of the graphene-Ti02 composites

The hydrothermal deposition of anatase-type Ti02 on the graphene surface was carried out in a vial of 25 ml according to the modified procedure [5]. In a typical reaction, 0.015 g of graphene oxide was dispersed in a mixture of 5 ml anhydrous ethanol and 10 g [bmimHBF4] under sonication for 2 min. Then, 0.15 g of TTIP was added into that suspension and sonicated for another 2 min. Afterwards, the suspension was sealed and kept static for hydrolysis of TTIP at room temperature for 2h. The mixture was microwave-irradiated (l26W) to evaporate ethanol from the solution for 10 min. After cooled to room temperature, ethanol was added to the mixture to remove IL from the composite by centrifugation, and the collected products were washed repeat with ethanol for 6 times and dried in vacuum at 40 °C for 12 h. As a comparison, Ti02 nanoparticles were prepared without graphene oxide under exactly the same condition. (Caution: Keep the vial open during microwave radiation in case explosion occurring).

D. Characterization of the graphene-Ti02 composites

The FT-IR spectra were measured using an Excalibur Series FTS 3000 (Bio-Rad) spectrometer with KBr pellet method. The TEM images were obtained on a transmission electron microscope (Philips, CM-200) operated with an accelerating voltage of 200 kY. The XRD patterns were collected on a powder X-ray diffractometer (PANalytical, MPD) with Cu Ka radiation. Cyclic voltammetry experiments were carried out with a CH760C apparatus (Shanghai Instruments, Ltd., Shanghai, China). A conventional three-electrode system that consisted of a working electrode, a platinum wire auxiliary electrode, and an Ag/ AgCI reference electrode was used.

E. Photocatalytic study

The photocatalytic activity of the graphene-Ti02 nanocomposites was investigated by methylene blue (MB) decomposition under UV illumination. In a typical test, 10 mg of the composite was added in 10 ml of MB 1 x 10-5M solution (co). The mixture was kept in the dark for absorption of MB onto the surface of the composite for 2 h. Then, the mixture was irradiated under UV lamp (VL-4.LC 8W) at 365 llill . The lamp was used at the distance of 10 cm from the solution in darkness box. The solution was withdrawn regularly from the vessel at certain time, centrifuged, and determined the MB concentration by UV spectroscopy at 654 llill .

III. RESUL TS AND DISCUSSION

Fig. 1 and Fig. 2 show the SEM and TEM images of bare Ti02, graphene oxide, and graphene-Ti02 composites. The morphology of the bare graphene was significantly different from that of the composites. Clusters of Ti02 particles were observed, with their abundance increasing with increasing Ti02 content. It can be explained that graphene oxide were functionalized with negative carboxylic groups which had affmity for the positive Ti

4+ ions. Ti(OH)4 was then formed

during hydrolysis. After microwave irradiation, Ti02 nanoparticles were formed on the surface of the graphene. Graphene was homogeneously coated by well-dispersed nanoparticles with few Ti02 aggregates. The TEM images show that the Ti02 nanoparticles obtained here were cube-like in shape with average size of ca. 5.0 llill .

The XRD patterns of graphene and graph en-Ti02 composites indicated that graphene oxide was converted to graphene and Ti02 anatase was formed.

Fig. 1. SEM image of (a) bare Ti02 and (b) graphene-Ti02 composites

The diffraction peaks of graphene correspond to the ( l00) band. The diffraction peaks at 28 of 25.33°, 38.04°, 47.95°,

54.42°, 62.67°, 69.65°, and 75.24° with hkl values of (101), (004), (200), (105) & (211), (204), (116) & (220), and (215) are representative of anatase form of TiOz, in good agreement with the literature [6]. The average crystallite size, D, of the Ti02 particles was calculated using the Debye-Sherrer formula: D = K)J(jJcosB), where K is the Sherrer constant, /L is the X-ray wave length, jJ is the peak width at half-maximum, and () is the Bragg diffraction angle. This equation gave a crystallite average size from the peak at 25.35° of 5.1 nm, consistent with the TEM results. The C peaks were not clearly observable in the XRD pattern of the composite due to its amorphous structure and low C content.

500nm -

Fig. 2. TEM image of (a) graphene oxide and (b) graphene-Ti02 composites

Graphene-Ti02

10 20 30 40 50 60 70 80

28(deg)

Fig. 3. XRD patterns of graphene and graph ene-TiOz composites

Photocatalytic activity tests were carried out by degrading aqueous methylene blue under UV irradiation. The effect of the composites' graphene contents on the decomposition was investigated for irradiation of duration up to 60 min at fixed pH (7.0), composite concentration (1 giL) and MB concentration (1 x 10-5 M). From the results of MB solution degradation, it was shown that the graphene-Ti02 composites has a more significant degradation effect than that of pristine Ti02 and the decomposition of MB increased with increasing TiOz content. It can be seen that the degradation of MB was ahnost removed by 85% after 2 min. But the degradation of pristine TiOz sample was only achieved at 7%. The decrease in MB concentration in the aqueous solution was due to adsorption by the graphene and photocatalytic decomposition by the Ti02. Graphene acted as electron sensitizers and donors in the graphene-TiOz composite photocatalysts.

1.0

0.9

0.8

? 0.7 � I: 0.6 0

� � 0.5 C ., 0.4 " I:

--Ti02 0 0.3 " --- Graphene- Ti02 al :=; 0.2

0.1

0.0 0 10 20 30 40 50 60

UV irradiation time (min)

Fig. 4. Effect of the relative concentration of MB (clco) on UV irradiation time with bare TiOz and the composite.

IV. CONCLUSIONS

In this study, the graphene-Ti02 nanocomposites were efficiently prepared in an ionic liquid, [bmim HBF 4]. BET surface areas of the graphene-Ti02 composites decreased with increasing Ti02 content. XRD analysis confIrmed the Ti02 nanoparticles as anatase. SEM and TEM analyses indicated that the Ti02 nanoparticles were cube-like in shape and well mixed on the surface of graphene nanosheets. The composites' photocatalytic behaviors were found that the degradation of MB was almost removed by 85% after 2 min. The method reported here is a green route suitable for large-scale production.

REFERENCES

[I] H. L. Wang, J. T. Robinson, G. Diankov, H. Dai, "Nanocrystal growth on graphene with various degrees of oxidation". J. Am. Chern. Soc. 2010, 132, 3270-3271.

[2] R. Ruoff, "Graphene: Calling all chemists". Nat. Nanotechnol. 2008, 3, 10-11.

[3] M. H. Liang, B. Luo, L. J. Zhi, "Application of graphene and graph ene-based materials in clean energy-related devices". into J. Energy Res. 2009, 33,1161-1170

[4] Y. W. Zhu , S. T. Murali , W. W. Cai , X. S. Li , J. W. Suk , J. R. Potts, R. S. Ruoff, "Graphene and Graphene Oxide: Synthesis, Properties, and Applications" Adv. Mater. 2010, 22, 3906-3924.

[5] K. Ding, Z. 1. Miao, Z. Liu, Z. Zhang, B. Han, G. An, S. Miao, Y. Xie, "Facile synthesis of high quality Ti02 nanocrystals in ionic liquid via a microwave-assisted process", J. Am. Chern. Soc., 2007, 129, 6362-6363.

[6] M. Antonietti, D. Kuang, B. Smarsly, Y. Zhou, "Ionic liquids for the convenient synthesis of functional nanoparticles and other inorganic nanostructures", Angew. Chern. into Ed., 2004, 43, 4988-4992.

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