Ionic liquid mediated synthesis of graphene–TiO2 hybrid and its photocatalytic activity

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Ionic liquid mediated synthesis of graphene–TiO2 hybrid and its photocatalytic activity

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Van Hoa Nguyen a,b,∗ , Jae-Jin Shim a,1 a b

School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Republic of Korea Department of Chemistry, Nha Trang University, 2 Nguyen Dinh Chieu, Nha Trang, Viet Nam

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Article history: Received 19 June 2013 Received in revised form 3 October 2013 Accepted 23 October 2013 Available online xxx

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Keywords: Graphene TiO2 nanoparticles Ionic liquid Photocatalyst Solvothermal method

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1. Introduction

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A graphene–TiO2 hybrid was synthesized by a solvothermal microwave-assisted method in a mixture of two green solvents: water and an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF4 ]). Graphene oxide (GO) could be easily reduced under microwave irradiation without any additional reducing reagent. Titanium (IV) isopropoxide was used as a starting material for the growth of TiO2 nanoparticles on the graphene sheets accompanied by the reduction of GO. The structure and morphology of the as-prepared hybrid were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and thermogravimetric analysis. The graphene–TiO2 hybrid had a high surface area and exhibited high photocatalytic degradation of methylene blue. © 2013 Elsevier B.V. All rights reserved.

Graphene has been considered an ideal component for composite materials due to its superior carrier transportability, high mechanical stiffness, extremely large surface area and fine thermal/chemical stability [1–5]. Many composites based on graphene have been developed for potential applications in lithium batteries, solar cells, electronic devices, and sensors [6–18]. Graphene sheets can be prepared by several methods such as mechanical cleavage from graphite [19], epitaxial growth [20], bottom-up synthesis [21] and supercritical fluid processing [22,23]. However, the as-prepared graphene and graphene itself prepared by using these methods are insoluble and intractable and cannot be shaped into the desired structures of composites by common material processing techniques [24]. To overcome this problem, reduced graphene oxide (RGO) sheets have been prepared by the chemical reduction of GO sheets. GO sheets can be dispersed very well in water because of their oxygen containing surface functionalities such as hydroxyl, carboxylic, carbonyl, and epoxide groups. Although chemical reduction of GO removes most of the oxygenated groups of GO sheets, residual functional groups remain on

∗ Corresponding author at: Department of Chemistry, Nha Trang University, 2 Q2 Q3 Nguyen Dinh Chieu, Nha Trang, Viet Nam. Tel.: +84 966337972; fax: +84 583831147. E-mail addresses: [email protected], hoa [email protected] (V.H. Nguyen), [email protected] (J.-J. Shim). 1 Tel.: +82 53 810 2587; fax: +82 53 810 4631.

the surface of RGO sheets which provides new properties or functions [6,14,24,25]. In comparison to graphene, RGO sheets have less room temperature conductivity and carrier mobility, but their solution processability over large areas offers new direction in flexible optoelectronics [26]. Up to date, there are huge interests in the preparation of graphene/TiO2 hybrids via various conventional methods for photocatalytic applications [27–33]. In other hand, a number of different ways of preparing of RGO by chemical reduction using hydrazine, sodium borohydride, hydroquinone, or strong alkaline solution as reducing agents have been reported [34]. However, these processes often require a large amount of hazardous organic chemicals, lengthy reduction, stabilizers, and a complicated approach that are harmful to our environmental and require a highcost process. To solve this problem, it is necessary to develop a fast, simple and efficient method for preparing soluble RGO in bulk quantities with relatively low cost. Also, 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 solvent power, designable structures, high ionic conductivity, wide electrochemical window, and excellent microwave absorbing ability [35]. Many nanocomposites have been prepared using ILs as green solvents [36,37]. In previous works, we have successfully prepared polymers and composites in ILs [38,39]. Microwave-assisted preparation is considered to be faster, cleaner and more cost effective than the conventional solvothermal

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or hydrothermal methods for the synthesis of metal oxide nanoparticles (NPs) [40]. In addition, the microwave-assisted process, taking advantage of both microwave radiation and solvothermal effect, produces graphene nanosheets within a short reaction time of 5–15 min below 300 ◦ C, and therefore offers possibilities of easy scale up and reduction in manufacturing cost [10]. Recently, Ding et al. [41] successfully synthesized size-controlled anatase nanocrystals in [bmim][BF4 ] via a microwave-assisted process. In this work, TiO2 and SnO2 NPs of different sizes and shapes were easily obtained. This route was carried out under atmospheric pressure in a domestic microwave oven. No high-pressure or high temperature apparatus was necessary and the size of the NPs could be easily controlled. More recently, Shen et al. [24] has reported a simple method for preparing RGO–TiO2 composites in [bmim][PF6 ] under solvothermal conditions with ascorbic acid as a reducing agent. They reported the successful synthesis of the soluble composite by one-step method, although high-temperature autoclaving, long reaction time, and a reducing agent were required. Herein, we report a simple, fast and efficient one-step route to synthesize a graphene–TiO2 hybrid with uniform-sized and shaped TiO2 NPs under microwave irradiation in a mixture of two green solvents, water and [bmim][BF4 ]. This method minimized the use of organic solvents. The obtained samples were characterized by Brunauer–Emmett–Teller (BET) surface area measurement, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). This procedure can be performed under atmosphere pressure in a domestic microwave oven without any high-pressure or high-temperature apparatus and the size of the TiO2 particles can be controlled easily. No additional reducing agent is necessary for the reduction of GO. Moreover, the catalytic efficiency of the hybrid is high due to the photoelectron degradation of methylene blue (MB).

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2. Experimental

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2.1. Materials

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Graphite powder (99.995%, Alfa Aesar) and titanium (IV) isopropoxide (TTIP) (99.999%, Aldrich) was used as received. [bmim][BF4 ] (>98%, Ionic Liquids Tech., Germany) was kept in a vacuum oven at 100 ◦ C for 24 h to remove volatile impurities before use. Other reagents were of analytical grade and were used without further purification. Water was deionized in the laboratory.

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2.2. Synthesis of GO

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GO was synthesized from graphite powder by a modified Hummers method [1]. Typically, 2 g graphite powder was added in 50 ml concentrated H2 SO4 along with 2 g NaNO3 in a flask at 0 ◦ C in an ice-bath. Afterwards, 6 g KMnO4 was slowly added to the solution while maintaining vigorous stirring below 20 ◦ C. The ice-bath was then removed and the flask was put into another water bath at 35 ◦ C, while keeping the mixture stirred for 3 h. The color of the resulting paste became brownish gray. It was diluted with 100 ml deionized water and stirred for 2 h. A 10 ml H2 O2 (30 wt.%) solution was slowly added into the suspension along with 100 ml HCl (10 v/v %) solution. The mixture was centrifuged and washed with deionized water until the decanted solution was neutral. The product was dried at room temperature under vacuum for 24 h.

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2.3. Synthesis of the graphene–TiO2 hybrid

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In a typical reaction, 0.015 g GO was dispersed in a mixture of 5 ml deionized water and 10 g [bmim][BF4 ] by ultrasonication

for 2 min. Then, 0.15 g TTIP was added into that suspension and sonicated for another 2 min. The suspension was sealed and kept stationary for the hydrolysis of TTIP at room temperature for 2 h. The mixture was then irradiated in a microwave oven at 130 W for 10 min. After being cooled to room temperature, the collected products were washed with water and centrifuged three times, and then washed with ethanol and centrifuged three times to remove IL, followed by drying in vacuum at 40 ◦ C for 12 h. For comparison, pure TiO2 NPs and bare RGO were prepared at the same condition. 2.4. Characterization The RGO–TiO2 hybrid was characterized by SEM (Hitachi, S4200), TEM (Philips, CM-200) at an acceleration voltage of 200 kV, XRD (PANalytical, X’Pert-PRO MPD) with Cu K␣ radiation, and XPS (ULVAC-PHI electron spectrometer, Quantera SXM) with an Al Xray source. FT-IR spectra were recorded over 400–4000 cm−1 using an Excalibur Series FTS 3000 (BioRad) spectrometer at a resolution of 16 cm−1 within 32 scans using KBr pellets. Raman spectra were recorded on a confocal micro-Raman spectrometer (LabRAM ARAMIS, Horiba JobinYvon) with 532 nm laser excitation. UV–vis diffuse-reflectance spectra were recorded over 200–800 nm using a UV–vis–NIR spectrophotometer (Cary 5000, Varian). TGA was performed on a simultaneous TGA/DSC analyzer (SDT Q600, TA Instrument) from 30 to 600 ◦ C at a heating rate of 10 ◦ C/min under nitrogen atmosphere. The Brunauer–Emett–Teller (BET) surface area of the composites was evaluated from nitrogen adsorption isotherms at 77 K using a BET surface Analyzer (Gemini 2357, Germany). 2.5. Photocatalytic study The photocatalytic activity of the graphene–TiO2 hybrid was investigated for the MB decomposition under UV illumination. In a typical test, a 10 mg of the RGO–TiO2 hybrid was added into a 10 ml MB solution (c0 = 10−5 M). Prior to illumination, the suspensions were sonicated for 10 min and then magnetically stirred in dark for 30 min to get desorption–adsorption equilibrium. The mixture was irradiated by UV light (using aVL-4.LC 8 W lamb) at 365 nm. The lamp was used at a distance of 10 cm from the solution in a dark box. The solution was sampled regularly from the vessel at certain times, centrifuged, and the MB concentration was determined by UV spectroscopy at 654 nm. 3. Results and discussion RGO–TiO2 hybrids of different compositions were obtained by simple solvothermal treatment of TTIP and GO in the mixed solvents of water and [bmim][BF4 ] as shown in Scheme 1. By this treatment the process under microwave irradiation, GO could be easily reduced to RGO, while TiO2 NPs were simultaneously grown on the RGO sheets. The advantage of this approach was that no additional chemicals such as a reducing agent were used. The rapid absorption of microwave by GO in polar solvents with a subsequent increase in temperature and pressure facilitated the reduction of GO. The reduction of GO is thermal-based and a small part is by the isopropanol that released from titanium (IV) isopropanol [42]. On the other hand, [bmim][PF6 ] has an excellent microwave absorbing ability [35], which may enhance the reduction of GO. Fig. 1 shows photographs of the suspension of GO and TTIP and the mixture of TTIP and GO before and after microwave treatment. The color of the RGO suspension and the RGO–TiO2 hybrid changed from grayish to black. A similar phenomenon was observed in previous works [17,24]. Fig. 2 presents SEM images of GO, RGO, TiO2 , and RGO–TiO2 . GO and RGO appeared as similar thin sheets, with distinct edges,

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Scheme 1. Schematic illustration of the synthesis of graphene–TiO2 hybrid.

Fig. 1. Photographs of the suspension of GO and TTIP and their mixture of TTIP and GO before and after microwave treatment.

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wrinkled surfaces, and folding. TiO2 showed a structure of agglomerated NPs. The surfaces of the hybrid’s graphene sheets were clearly decorated with TiO2 NPs. To further characterize the hybrid morphology more clearly, the RGO sheets and RGO–TiO2 hybrids were characterized by TEM (Fig. 3). Sheets with wrinkled surfaces and folding at the edges were clearly visible (Fig. 3(a) and (b)).

In Fig. 3(b), the multi-layered RGO sheets are mostly covered by TiO2 NPs. Fig. 3(c) shows an enlarged image of Fig. 3(b). It can be observed that TiO2 NPs are mostly distributed homogeneously on the RGO sheets. The RGO–TiO2 hybrid showed uniformly sized TiO2 NPs with diameters of ca. 5.5 nm dispersed on the surfaces of the RGO sheets (Fig. 3(b) and (c)). Analysis of the 2D lattice fringes of

Fig. 2. SEM images of (a) GO, (b) bare RGO, (c) pure TiO2 , and (d) RGO–TiO2 hybrid.

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Fig. 3. TEM images of (a) RGO, (b) and (c) RGO-TiO2 at different magnifications, and (d) the diffraction pattern of RGO–TiO2 .

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individual small crystallites in the high-resolution image of TiO2 ˚ which is in good (Fig. 3(c)) showed the lattice spacing of 3.5 ± 0.1 A, ˚ The agreement with the anatase (1 0 1) lattice spacing of 3.52 A. electron diffraction studies indicate that TiO2 is highly crystalline, as the pattern could be indexed to the anatase phase only (Fig. 3(d)). The XRD patterns were recorded of graphite, GO, RGO, TiO2 , and RGO-TiO2 (Fig. 4). The graphite had a very strong peak at 26.5◦ , GO a sharp peak at 11.85◦ , and RGO a broad peak at around 26.0◦ . The diffraction peaks of TiO2 at 2 of 25.33◦ , 38.04◦ , 47.95◦ , 54.42◦ , 62.67◦ , 69.65◦ , and 75.24◦ with hkl values of (1 0 1), (0 0 4), (2 0 0), (1 0 5) & (2 1 1), (2 0 4), (1 1 6) & (2 2 0), and (2 1 5) are representative of the anatase form of TiO2 , and are in good agreement with the literature [43]. The hybrid showed all the diffraction peaks belonging to the anatase phase of TiO2 , and the characteristic peak of GO at 2 of 11.85◦ was absent. The average crystallite sizes, D, were calculated using the Debye–Scherrer formula, D = K·/(ˇcos), where K is the Scherrer constant,  the X-ray wavelength, ˇ the peak width at half-maximum, and  the Bragg diffraction angle. The peak at 2 = 25.33◦ implies a crystallite size of ca. 5.4 nm, consistent with the TEM results. The chemical structure of GO, RGO, and RGO-TiO2 was characterized by FTIR (Fig. 5(a)). Compared with the spectrum of GO, the representative peaks at 1721 cm−1 (carboxyl C O stretching vibration), 1410 cm−1 (C OH stretching vibration), and 1042 cm−1 (C O stretching vibration) were decreased significantly or completely absent after reduction, confirming the almost total decomposition of oxygen-containing functional groups. These peaks were also largely decreased in the spectrum of RGO–TiO2 due to the reduction of GO, which suggests the strong interactions between the TiO2 NPs and the remaining surface hydroxyl O atoms. The new

strong absorption around 500 cm−1 was attributed to the vibration of the Ti O Ti bonds in TiO2 . The crystallinity of graphite, GO, and RGO–TiO2 was assessed by Raman spectroscopy (Fig. 5(b)). Each showed two prominent peaks: a D band around 1350 cm−1 and a G band around 1590 cm−1 . The D band represents edges, other

Fig. 4. XRD patterns of graphite, GO, RGO, and RGO–TiO2 hybrid.

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Fig. 5. (a) FT-IR spectra of GO, RGO, RGO–TiO2 hybrid and (b) Raman spectra of graphite, GO, RGO, RGO–TiO2 hybrid.

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defects, disordered carbon due to the vibration of sp3 -bonded carbon atoms, and impurities. The G band represents the zone center E2g mode, corresponding to the ordered sp2 -bonded carbon atoms. The spectra show that the carbon framework underwent significant structural changes during the modification from graphite to RGO-TiO2 . Oxidation and reduction made the D band signal stronger and broader, and the ratio of the D and G bands’ intensities (the R-value) also significantly increased. The R-value represents the degree of disorder and the average size of the sp2 domains. The observed increase in the R-values indicates increased disorder of the graphene layers and an increased number of defects. The G-bands of GO and RGO-TiO2 broadened and shifted to higher wavenumbers compared with that of graphite, indicating the decreased in-plane size of graphene. RGO–TiO2 showed a slightly

greater R-value than GO did, indicating a smaller average size after the reduction of the exfoliated GO. The surface information of the samples was characterized by XPS (Fig. 6). Fig. 6(a) shows the XPS survey of GO, RGO, and the RGO–TiO2 . The observed peaks of Ti 2p, O 1s and C 1s indicated the presence of TiO2 on the RGO surfaces in the hybrid sample. The core-level C 1s and O 1s peaks are shown in Fig. 6(b) and (c), respectively. After the microwave irradiation, the shape of both peaks was changed. It has been reported that the C 1s peak of GO can be deconvoluted into four peaks at binding energies (BEs) of 284.5 (C C), 285.4 (C O), 286.8 (C O), and 288.5 eV (O C = O), respectively [45]. The core-level O 1s at BE of 532.5 eV corresponded to the hydroxyl groups and surface adsorbed water. The intensity of the C O, C O, and O 1s peaks decreased after the microwave

Table 1 BET surface areas of bare RGO, pure TiO2 , and RGO–TiO2 hybrids. Sample

Composition (RGO:TiO2 )

Single point BET surface area (m2 /g)

Total pore volume (cc/g)

TiO2 RGO RGO–TiO2 RGO–TiO2 RGO–TiO2

0:1 1:0 1:2 1:3 1:10

15.67 231.15 149.69 109.23 65.25

0.01 0.11 0.08 0.07 0.03

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Fig. 6. XPS results of (a) survey spectra, and the core-levels of (b) C 1s (c) O 1s and (d) Ti 2p.

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treatment, confirming the reduction of GO. The high core-level Ti 2p spectrum presented BEs of 459.1 and 465.3 eV, that were attributed to the Ti 2p3/2 and Ti 2p1/2 peaks, respectively (Fig. 6(d)). The positions of these peaks were in agreement with a previous work [45]. Fig. 7(a) shows the TGA results of GO, RGO, and RGO-TiO2 under nitrogen atmosphere. All of the samples exhibited similar decomposition curves. However, GO had much lower thermal stability than RGO and the RGO-TiO2 , which suggested the pyrolysis of the thermally stable oxygen-containing functional groups. The weight reduction of RGO observed at temperatures between 300–400 ◦ C was consistent with the degradation of some remaining oxygen functional groups. The existence of these groups such as–COOH and -OH on the RGO surface not only enhanced the dispersibility but also anchored the metal oxide NPs onto the surface [24]. The hybrid began to gradually degrade at a temperature around 110 ◦ C and attained the final residual mass at around 450 ◦ C, which was attributed to the evaporation of the absorbed solvent and the decomposition of residual oxygen groups on the RGO surface. The TGA results in Fig. 7 also suggested that RGO–TiO2 had a higher thermal stability than GO and RGO, with a lower weight loss than that of RGO. This may be due to the strong interactions between RGO and TiO2 , which impose a restriction on the decomposition of the remaining oxygen groups on the RGO surface. It has also been reported that these interactions can result in homogeneous heating and the avoidance of heat concentration [44]. The UV–vis absorption spectra of GO and RGO–TiO2 were recorded (Fig. 7(b)). The GO spectrum shows two absorption bands at 234 and 304 nm due to the excitation of the ␲–␲* transition of aromatic C C and the n-␲ transition of C O [46], respectively. The spectrum of RGO–TiO2 has intense absorption edges at about 394 nm. The optical absorption near the band edge follows the formula Eg = 1240/␭, where Eg , and  are the absorption

band gap energy and absorption edge. So, the estimated band gap energies of the hybrid is about 3.15 eV. The compositions, BET surface areas, and micropore volumes of pure TiO2 , bare RGO, and RGO–TiO2 are shown in Table 1. The hybrid’ BET surface area of RGO–TiO2 was higher than that of pure TiO2 but lower than that of bare RGO. The BET surface area of the hybrid was decreased with increasing amount of TiO2 precursor because the micropores were blocked by the TiO2 NPs formed on the RGO surfaces during microwave irradiation. The photocatalytic activities of the pure TiO2 and different RGO–TiO2 hybrids were examined by degrading aqueous MB as a model pollutant under UV irradiation. The photocatalytic efficiencies on the decomposition of MB were investigated over irradiation durations of up to 60 min at fixed pH (7.0), hybrid concentration (1 g/L) and MB concentration (10−5 M). The concentration of MB (c) is proportional to the maximum absorbance at 654 nm (A). The change of concentration (c/c0 ) was determined from the variation of absorbance (A/A0 ), where c0 and A0 are the initial concentration and absorbance of MB, respectively. The photodegradation results are shown in Fig. 8. All RGO–TiO2 hybrids showed a significantly improved photodegradation efficiency to MB compared with pure TiO2 , achieving a degradation percentage of more than 80% in 60 min. In comparison with pure TiO2 , the RGO–TiO2 hybrid exhibited the following advantages: (i) an increased adsorptivity of pollutants, (ii) improved light absorption intensity and extended light absorption range, and (iii) enhanced transportation of photogenerated charge carriers. Moreover, the enhanced photocatalytic activity was due to the good distribution of TiO2 particles and the unique properties of graphene. According to two previous reports [47,28], graphene acts as an electron-acceptor material to effectively hinder the electron–hole pair recombination, and a photosensibilizer to enhance the utilization of visible

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light in photocatalysis. The effect of the weight ratio of graphene to TiO2 in the hybrid on the photodegradation of MB was studied. The sample prepared with more RGO showed a better photocatalytic activity than that with a lower amount of RGO. However, increasing the amount of RGO did not induce any further improvement in the photocatalytic activity as expected. It has been reported that a large amount of graphene exhibited a strong absorption to light, thus reducing the light absorption on TiO2 surface and thereby decreasing the number of photoexcited electrons [47]. In contrast, the addition of a large amount of TiO2 precursor caused the agglomeration of TiO2 NPs, which reduced the good distribution of the TiO2 particles on the hybrid. We have previously reported a similar observation on CNT–TiO2 [39].

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In this study, a RGO–TiO2 hybrid was efficiently prepared using GO and tetrabutyl titanate as starting materials in a solvent of IL–water. The BET surface areas of the RGO–TiO2 hybrid decreased with increasing TiO2 content. XRD analysis confirmed the anatase structure of the TiO2 NPs. SEM and TEM analyses indicated that the TiO2 NPs had a well-mixed on the RGO surface. An investigation of the photocatalytic behaviors of the RGO–TiO2 hybrid under different conditions revealed that they exhibited much higher stability and activity than those of the bare TiO2 . The RGO–TiO2 hybrid prepared with RGO and TiO2 at a ratio of 1:3 showed excellent photocatalytic activity. The method reported here is a green route suitable for large-scale production.

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Acknowledgment

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This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012009529).

Fig. 7. (a) TGA results of GO, RGO, and RGO–TiO2 and (b) UV–vis spectra of GO and RGO–TiO2 .

Fig. 8. Effect of the relative concentration of MB (c/c0 ) on UV irradiation time for various hybrids.

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