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Supramolecular photocatalyst of RGO-cyclodextrin-TiO2
Journal of Alloys and Compounds 580 (2013) 239–244
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Supramolecular photocatalyst of RGO-cyclodextrin-TiO2 Jianfeng Shen, Na Li, Mingxin Ye ⇑ Center of Special Materials and Technology, Fudan University, Shanghai, China
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Article history: Received 1 March 2013 Received in revised form 11 May 2013 Accepted 14 May 2013 Available online 23 May 2013 Keywords: Graphene b-cyclodextrin TiO2 Photocatalyst
a b s t r a c t Reduced graphene oxide (RGO)/b-cyclodextrin (b-CD)/titanium oxide (TiO2) supramolecular photocatalyst was synthesized with a one-pot hydrothermal method. The reducing process was accomplished with the attaching of b-CD and generation of TiO2. b-CD acted as a linker between RGO and monodisperse TiO2 nanoparticles. The structure and composition of the hybrid had been characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, thermal gravimetric analysis, X-ray diffraction and Transmission electron microscopy. The as-prepared RGO-CD-TiO2 showed significant enhanced performance for phenol and Cr (VI) removal, due to the effective transfer of photo-generated electron from TiO2 to RGO and improved absorbance performance of the hybrid. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphene, sheet of carbon only one atom thick, has attracted numerous attention and investigations because of its superior properties [1–6]. Several procedures, including mechanical exfoliation, liquid exfoliation, chemical vapor deposition and the chemical method, have been developed for the preparation of graphene [7–11]. Among them, the chemical method is superior to the others because of its simplicity and mass productivity [12]. Generally, graphite oxide (GO) can form stable dispersion in water because of its high rate of oxygen-containing groups. However, if the oxygen functionalities are removed by reduction to yield reduced graphene oxide (RGO), it would immediately lose its water dispersibility, then aggregate, and eventually precipitates due to the prominent interlayer conjugate interaction of RGO. Thus, it is essential to modify the GO and RGO surfaces to enhance the dispersion of them in a variety of solvents [13]. To obtain stable RGO colloidal dispersions, either an organic stabilizer (polymer or surfactant) or exact control of pH value of the dispersions is usually needed [14–16]. Recently, it has been found that attaching of cyclodextrin (CD) is also a good method to stable RGO. CDs can be attached on the surface of graphene sheets with strong hydrogen bonding to make graphene more hydrophilic [17,18]. b-CD, a cyclic oligosaccharide, possesses an electronic and hydrophobic interior microenvironment in its cavity structure, which allows hydrophobic molecules to be easily trapped into its cavity by displacing water. Since it has a lot of hydroxyl groups, the exterior of its cavity is hydrophilic. This unique structure makes it easy to bind inorganic, organic and biological ⇑ Corresponding author. Tel./fax: +86 021 55664094. E-mail address: [email protected] (M. Ye). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.090
molecules into its cavity, thus forming supramolecular hybrid without structural changes [19]. Nanocrystalline titanium oxide (TiO2) is a well-known wideband gap semiconductor and has been investigated as an excellent photocatalyst, owing to its outstanding properties such as nontoxicity, low cost and energy applications [20]. In addition, well-prepared TiO2 also shows good adsorption activity, which is widely used to remove heavy metals [21]. Graphene/inorganic composites, derived from the decoration of graphene with nanoparticles, are attracting more and more attention. Several attempts in using RGO for modification of TiO2 for photocatalytic degradation of organics have been reported. For example, Pu and coworkers synthesized graphene oxide-TiO2 hybrids by a one-pot microwave-assistant combustion method. The hybrids showed improved photocatalytic properties than neat TiO2 [22]. Anandan and coworkers developed a simple approach to fabricate graphene-loaded TiO2 thin films on glass substrates by the spin-coating technique. They found that the enhanced photocatalytic activity of the films is attributed to its efficient charge separation, owing to electrons injection from the conduction band of TiO2 to graphene [23]. On the basis of the above considerations, combining the selfassembly behavior of b-CD and its interaction with RGO, we suspect that the supramolecular structure of RGO-CD-TiO2 will offer the possibility to increase the separation and transportation of photogenerated charges. In this paper, we present a novel approach for the preparation of RGO-CD-TiO2 hybrid in water, using graphene oxide as a precursor of RGO and tetrabutyl titanate as the source precursor of TiO2. The method used in this study has several novelties and advantages: (1) b-CD acted as a linker between RGO and monodispersed TiO2 nanoparticles since it can interact with both RGO and TiO2 nanoparticles. (2) The whole
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process is simple and industrially compatible. Glucose was used as the reducing agent and its reducing ability is greatly enhanced under the hydrothermal condition, which is strong enough to reduce the graphene oxide. (3) The new supramolecular nanocomposites possess unique properties of high surface area and host–guest recognition, thus can be used to enrich ad detect inorganic, organic and biological molecules. 2. Experimental 2.1. Materials Pristine graphite was purchased from Qingdao BCSM, CO., Ltd. Tetrabutyl titanate, b-CD and glucose were supplied by Shanghai Chemical Reagent Company. All other reagents were at least of analytical reagent grade and used without further purification.
microscopy (TEM) was performed with a JEOL JEM-2100F. Scanning electron microscopy (SEM) was performed with a Philips XL30 FEG FE-SEM instrument at an accelerating voltage of 25 kV. The UV–Vis diffuse reflectance spectra were conducted on a UV-3600 UN-VIS–NIR spectrophotometer using an integrating-sphere accessory.
2.4. Photocatalytic activity and adsorption measurements Photocatalytic and adsorption experiments were conducted to remove phenol and Cr (VI) in water. In a typical procedure, RGO-CD-TiO2 nanocomposite (10 mg) was added into 500 mL of phenol or Cr (VI) solution (10 mg/L). The mixtures were stirred moderately on a magnetic stirring apparatus at room temperature and then exposed to a UV light lamp which was placed 20 cm above the beaker. Samples were withdrawn regularly from the reactor and immediately centrifuged to separate any suspended solid before analysis. The concentration of phenol or Cr (VI) in the solution was monitored by measuring the absorbance of the solution with a UV–Vis spectrophotometer. RGO-CD, TiO2 and RGO-TiO2 were adopted as the reference to compare the photocatalytic activity with that of RGO-CD-TiO2 nanocomposites under the same experimental conditions.
2.2. Preparation of RGO-CD-TiO2 GO was obtained by the modified Hummers method [24]. 100 mg of tetrabutyl titanate was added to 5 mL of ethanol. The above mixture was slowly dropped into the mixture of 4 mg ammonium chloride/5 mL water to form part A. On the other hand, 100 mg of GO was added to 100 mL water. The mixture was sonictaed for 30 min, followed by high-speed stirring for further 1 h. 100 mg glucose, 200 mg b-CD and 1 mL ammonium hydroxide was added to the GO solution to get part B. Subsequently, part A and part B were mixed. The mixture was put into an autoclave and heated at 160 °C for 4 h. When the reduction reaction was finished, the as-synthesized product (RGOCD-TiO2-1) was isolated by centrifugation, washed with pure water and ethanol several times, and dried at 90 °C for 12 h. The whole fabrication process is outlined in Fig. 1. RGO-CD-TiO2-2 and RGO-CD-TiO2-3 were got with the similar method by adding 200 mg and 400 mg of tetrabutyl titanate. In addition, RGO-CD, TiO2 and RGO-TiO2 were synthesized in water in the same way as the composites in the absence of tetrabutyl titanate, GO, and CD, respectively. 2.3. Characterization Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a NEXUS 670 spectrometer. Raman spectra were recorded on a Dilor LABRAM-1B multi-channel confocal microspectrometer with 514 nm laser excitation. Thermogravimetric analysis (TGA) was accomplished with Netzsch TG 209F1 that was fitted to a nitrogen purge gas at 10 °C/min heating rate. X-ray diffraction (XRD) was taken on D/max-rB diffractometer using Cu Ka radiation. Transmission electron
3. Results and discussion One purpose of this study is to find an efficient and green way to prepare the hybrid of RGO and TiO2. We used GO as the precursor of RGO in this research. It is now well known that heavy oxygenation of graphite results in multiform functional groups on the basal planes and at the edge of sheets, which makes GO strongly hydrophilic and easily dispersible in water. These decorated functional groups are able to reduce the interplanar forces and promote complete exfoliation of single-layer graphene oxide sheets in aqueous media. In the present work, the starting material graphene oxide is synthesized by oxidation of graphite with a modified Hummers method. In addition, we judiciously introduced CD molecules into GO before it was fully reduced, and the resulted RGO remain soluble in water and do not aggregate for a long time. The color of the suspension shifts from brown to black, which further indicates the change from GO to RGO during the reduction process [25]. Fig. 2 shows IR spectra of GO (a), TiO2 (b), RGO-CD (c) and RGOCD-TiO2s (d–f). In the spectrum of GO, it shows a broad absorption band at 3440 cm1, which is related to the OAH groups. The
Fig. 1. Experimental procedure used in this study to prepare RGO-CD-TiO2.
J. Shen et al. / Journal of Alloys and Compounds 580 (2013) 239–244
Fig. 2. FTIR spectra of GO (a), TiO2 (b), RGO-CD (c) and RGO-CD-TiO2s (d–f).
absorption band at 1650 cm1 is a typical peak of carboxyl group and the CAO vibration of the epoxy group in GO appears at 1200 cm1. In the spectrum of TiO2 (b), peaks at 780 and 1100 cm1 are assigned to TiAOATi and TiAO stretching. As to RGOCD (c), the aromatic ring stretching frequency of the RGO results in the bands at 1630, 1450, 1100, 1000, and 875 cm1. After GO was chemically reduced, the C@O vibration band (1650 cm1) almost disappears, but the broad OAH stretching band still remains, presumably due to inter- and intramolecular hydrogen bonding [26]. In addition, compared with GO, the spectrum of RGO-CD shows two bands between 1155 and 1040 cm1, corresponding to the antisymmetric glycosidic CAOAC vibration, and the coupled CAC/CAO stretching vibration. As to RGO-CD-TiO2s (d–f), there still exhibit typical CD absorption features of the ring vibrations at 600, 700, 760, and 960 cm1, the coupled CAOAC stretching and OAH bending vibrations at 1150 cm1, the coupled CAO/CAC stretching/OAH bending vibrations at 1030 and 1080 cm1, CH2 stretching vibrations at 2930 cm1, and OAH stretching vibrations at 3450 cm1. This clearly confirms that CD molecules are still attached to the surface of RGO. According to a previous report, multiple hydrogen bondings could be strong enough to construct a complex structure, even under sonication [27]. Raman spectroscopy is one of the most widely used techniques to characterize the structural and electronic properties of graphene-based materials. Generally, the Raman spectrum of graphene
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is characterized by three main features. The G peak, which is due to the E2g vibration mode of sp2 bonded carbon, usually observed at 1575 cm1; the D peak, which is arising from the doubly resonant disorder-induced mode, can be found at 1350 cm1; and the 2D peak at 2700 cm1, which is a second order vibration caused by the scattering of phonons at the zone boundary [28]. In addition, ratio of the intensities of the D and G band (ID/IG) can be used to indicate the level of chemical functionalization in a carbon material. High ID/IG ratio indicates a high degree of disorder. The results of GO, TiO2, RGO-CD and RGO-CD-TiO2s are shown in Fig. 3. The spectrum of GO shows broadened D-band (1330 cm1) and the G-band at 1570 cm1. When GO was chemically converted to RGO-CD, its Raman spectrum also contains both G and D bands, however, with an increase in the ID/IG ratio. This change suggests that the average size of the sp2 domains is decreased upon reduction of the GO sheets. New graphitic domains are created, which are smaller in size than the ones present in GO before reduction, but more numerous in number. Moreover, after RGO was functionalized with b-CD, the G-band shifts to 1597 cm1. This tendency is due to an increase in the number of sp3 carbons that were formed on the graphene during chemical functionalization [28]. Besides these Raman features, 2D (D + D0 ) peaks at 2640 and 2910 cm1 are displayed. The position and shape of the 2D peak can be used to distinguish the layer numbers of graphene [29]. The broader 2D peak located at 2640 cm1 of RGO-CD indicated that the sample is probably between 2 and 5 layers. In the spectra of RGO-CD-TiO2s, we can find that the characteristic peaks of TiO2 and RGO-CD still exist. Another evidence of successful preparation of RGO-CD-TiO2s came from TGA analysis. As shown in Fig. 4, comparing with raw graphite, GO was thermally unstable and more than 25% of its weight loss took place below 200 °C, which was assigned to the decomposition of the labile oxygen-containing functional groups. However, as to RGO-CD, it showed weight loss less than 10% below 250 °C. This indicates that most of the labile groups had been removed via reduction. In addition, a major weight loss of 10% in the range from 250 to 500 °C followed, which is because of the decomposition of CDs on the RGO sheets. Finally, there was a very slow weight-loss process of RGO-CD, from 500 to 700 °C, which was due to the partial decomposition of RGO sheets itself. TGA data reveal that RGO could load a number of CD molecules, which can greatly enhance the supramolecular function between them. This phenomenon is probably caused by the synergetic effects from RGO (high conductivity and high surface area) and CDs (host–guest recognition and enrichment). From the TGA curve of RGO-CD-TiO2-
Fig. 3. Raman spectra of GO, TiO2, RGO-CD and RGO-CD-TiO2s.
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Fig. 4. TGA curves of graphite (a), GO (b), RGO-CD (c), TiO2 (d) and RGO-CD-TiO2s (e–g).
s, comparing with GO and RGO-CD, we can also find that their weight losses are greatly restricted. It can be seen that after TiO2 nanoparticles were bonded onto RGO-CD, the total weight loss decreased to about 18%, 16% and 14%, indicating the weight percentage of TiO2 nanoparticles of RGO-CD-TiO2s are about 20 wt%, 28 wt% and 36 wt%. XRD patterns are used to further study the changes in structure. Fig. 5 shows XRD results of raw graphite, GO and RGO-CD-TiO2-1. Raw graphite (Fig. 5a) showed a very strong 002 peak at 26.44°. As to GO (Fig. 5b), while a small change in the position of the principal reflection is observed with oxidation, the most striking difference is the intensity and broadness of the peak observed at 2h = 10.5°, corresponding to an average interlayer spacing of 0.8 nm. Since the presence of oxygen-containing groups on both sides of the graphene sheets, individual graphene oxide sheet is expected to be thicker than individual pristine graphene sheet. In addition, the broad diffraction peak of GO powder hinted that the process can influence the crystallization of the samples in the functionalization process. Both the d-spacing value and broadness of this reflection in GO are typical for randomly ordered graphitic platelets. After reduction and decoration of CD and TiO2 (Fig. 5c), we discern a gradual change in the patterns to finally achieve a randomly ordered carbonaceous layered solid, with basal spacing of 0.34 nm instead of 0.78 nm for the parent GO, indicating that the bulk of the
Fig. 5. XRD patterns of raw graphite (a), GO (b), and RGO-CD-TiO2-1 (c).
oxygen-containing groups is removed from GO. This is consistent with the results of FTIR analysis. Moreover, the 002 reflection in the sample is very broad suggesting that the samples are very poorly ordered along the stacking direction. This is an indication that the sample comprises mainly of single layer sheets. In addition, the broad diffraction peak hinted that the process could influence the crystallization of the samples in the functionalization process, implying that the extensive conjugated sp2 carbon network is restored. Besides, the major diffraction lines can be indexed to the anatase phase (JCPDS 21-1272). The broadened diffraction peaks of low intensity imply that the particles are of nanoscale size. The average particle size of the prepared TiO2 nanoparticles was calculated to be ca. 10 nm based on Scherrer’s equation. The investigation of the structure of GO and RGO-CD-TiO2-1 had been performed by TEM. As Fig. 6a shows, most parts of the GO sheets are homogeneous and wrinkled. However, the surfaces of the RGO-CD-TiO2-1 become crumpled (Fig. 6b). Upon addition of CD, besides the corrugated layers, the sheets tend to form small aggregates, owing to the cross-linked RGO sheets with CD molecules [30]. SEM images of fractured surface also shows a wrinkled topography for GO (Fig. 6c). In addition, the distribution of TiO2 particles on RGO is relatively uniform (Fig. 6d), while RGO shows a crumpled structure. TiO2 is a well-known and the most investigated functional material in semiconductor photocatalysis. Because of the unique properties of graphene, it can not only improve the separation and transport of photocarriers, but also might cause a higher conduction band position with a stronger reductive power. On the other hand, one of the key chemical properties of CDs is their ability to form inclusion complexes with a wide variety of molecules that is to accommodate guest molecules into their inner cavity. In addition, as to the removal of heavy metal ions, with the linker of CD, the high surface of RGO can also help to greatly improve the adsorption ability of TiO2s. We speculate that the prepared RGOCD-TiO2s may have a synergistic effect toward its photocatalytic properties. We carried out the experiments of removal and adsorption of phenol and Cr (VI) in aqueous solutions under UV light irradiation to study the photocatalytic properties of RGO-CD-TiO2s. Fig. 7 presents the phenol and Cr (VI) concentration ratio (Ct/C0) of different samples and those of the direct photolysis process with irradiation time, respectively. We can find that the phenol conversion over the as-prepared catalyst within 120 min decreased in the order of RGO-CD-TiO2-1 RGO-CD-TiO2-2 > RGO-CD-TiO2-3 > RGO-TiO2 > TiO2 > RGO-CD (Fig. 7a). Similar results were also observed in the adsorption removal of Cr (VI) under UV light irradiation (Fig. 7b). It is known that during photocatalysis, the adsorption capacity, enhanced light absorption, charge transfer and separation are crucial factors [31–33]. Thus, the enhancement of the photocatalytic performance should be ascribed to the increase of the light absorption intensity and range, and the reduction of electron–hole pair recombination [34]. Based on the experimental data, we suspect that the surface structures of RGO-CD may be well responsible for its improved activity. The reasons underlying the improved photo-catalytic performance may be as follows: (1) the increase in utilization efficiencies for TiO2 catalysts on these RGO supports can be attributed to the increased surface areas and the well-dispersed nature of the prepared RGO. Owing to the oxygen-containing groups on the GO sheets (such as hydroxyl, carbonyl, and epoxy groups), the GO surface can interact strongly with coated species and easily absorb positive ions to form different composites. Simultaneously, GO were converted to RGO. In addition, it has been proved that RGO, with small amount of oxygen-containing groups bound to the edges of the basal planes, can form well-dispersed aqueous colloids, of which the surface charge is highly negative when dispersed in water. The synergetic
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Fig. 6. TEM and SEM images of GO (a and c) and RGO-CD-TiO2-1 (b and d).
Fig. 7. Photocatalytic and adsorption curves under UV irradiation of the samples for phenol (a) and Cr (VI) (b).
effect of RGO, b-CD and TiO2 makes its adsorption ability highly increased, which is useful for removal of heavy metal ions. (2) The role of RGO-CD could act as a macromolecular ‘‘photosensitizer’’ for TiO2, hence making it exhibit higher photoactivity. The photocatalytic process for RGO-CD-TiO2 under UV light irradiation seems to be very similar to the well-known strategy of photosensitization of semiconductors by the matched adsorption of organic dyes, by which the photo-response of a wide band gap semiconductor can be extended to the visible light region [35]. (3) The enhanced photocatalytic activity for RGO-CD-TiO2 composites could be
attributed to the ability of RGO to capture and transport electrons, and to promote charge separation. It is known that the higher separation efficiency of electron–hole pairs will enhance photocatalytic activity and results in a large number of holes participated in the photocatalytic process [36]. However, excessive addition of black color RGO-CD will lower the light intensity through the depth of reaction solution, thus decreasing its photoactivity. According to the results of photocatalytic analysis and adsorption experiments, we propose that the mechanism as follows: (1) It is reported that the nucleation and growth mechanism depends
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on the degree of oxygen functionalization on the surface of graphene sheets. The nucleation of nanoparticles at GO surfaces is mainly governed by the presence of oxygen groups at GO [37]. Aqueous solution of graphene oxide was selected as the precursor for the deposition of TiO2 nanoparticles. Oxygen-containing groups can coordinate Ti4+ and provide reactive sites for the nucleation and growth of TiO2. During this process, in the first step, tetrabutyl titanate was attached on graphene oxide sheets, in which monolayer graphene oxide were homogeneously dispersed in water. Therefore, the mixing of tetrabutyl titanate and graphene oxide can be considered at molecular level. In the second step, graphene oxide was reduced to RGO nanosheets and TiO2 nanoparticles generated simultaneously. This process can ensure the in situ formation of TiO2 nanoparticles and graphene nanosheets simultaneously, with an advantage to prevent serious stacking of RGO nanosheets. (2) The presence of remained oxygen functionalities (mainly carboxyl groups) at the RGO surface continually provides reactive sites for the nucleation of TiO2 nanoparticles. (3) One of the key chemical properties of cyclodextrins is their ability to form inclusion complexes with a wide variety of molecules that is to accommodate guest molecules that is to accommodate guest molecule into their inner cavity. With the synergistic effect of CD and RGO, pollutant molecules can be quickly caught. However, the dependence of physical properties of TiO2-RGOs is very complex. Further study will be necessary to elucidate the precise mechanism in the future.
4. Conclusions In this work, we demonstrate an effective one-pot hydrothermal route for the synthesis of RGO-CD-TiO2 nanocomposite. The hydrothermal process provides a straightforward and facile approach to deposit TiO2 nanoparticles onto the RGO sheets. Interestingly, it was also found that the photocatalysis performance and adsorption capacity of as-prepared nanocomposite could be enhanced by the interaction between RGO, CD and TiO2. In addition, the properties of the hybrids may be controlled with different ration of TiO2 and RGO. This method may be readily extended to the preparation of other classes of hybrids based on RGO sheets for technological applications.
Acknowledgements We gratefully acknowledge the financial sponsor of National Natural Science Foundation of China (51202034) and ‘‘Chen Guang’’ Project (12CG02) supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Supramolecular photocatalyst of RGO-cyclodextrin-TiO2 Jianfeng Shen, Na Li, Mingxin Ye ⇑ Center of Special Materials and Technology, Fudan University, Shanghai, China
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Article history: Received 1 March 2013 Received in revised form 11 May 2013 Accepted 14 May 2013 Available online 23 May 2013 Keywords: Graphene b-cyclodextrin TiO2 Photocatalyst
a b s t r a c t Reduced graphene oxide (RGO)/b-cyclodextrin (b-CD)/titanium oxide (TiO2) supramolecular photocatalyst was synthesized with a one-pot hydrothermal method. The reducing process was accomplished with the attaching of b-CD and generation of TiO2. b-CD acted as a linker between RGO and monodisperse TiO2 nanoparticles. The structure and composition of the hybrid had been characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, thermal gravimetric analysis, X-ray diffraction and Transmission electron microscopy. The as-prepared RGO-CD-TiO2 showed significant enhanced performance for phenol and Cr (VI) removal, due to the effective transfer of photo-generated electron from TiO2 to RGO and improved absorbance performance of the hybrid. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphene, sheet of carbon only one atom thick, has attracted numerous attention and investigations because of its superior properties [1–6]. Several procedures, including mechanical exfoliation, liquid exfoliation, chemical vapor deposition and the chemical method, have been developed for the preparation of graphene [7–11]. Among them, the chemical method is superior to the others because of its simplicity and mass productivity [12]. Generally, graphite oxide (GO) can form stable dispersion in water because of its high rate of oxygen-containing groups. However, if the oxygen functionalities are removed by reduction to yield reduced graphene oxide (RGO), it would immediately lose its water dispersibility, then aggregate, and eventually precipitates due to the prominent interlayer conjugate interaction of RGO. Thus, it is essential to modify the GO and RGO surfaces to enhance the dispersion of them in a variety of solvents [13]. To obtain stable RGO colloidal dispersions, either an organic stabilizer (polymer or surfactant) or exact control of pH value of the dispersions is usually needed [14–16]. Recently, it has been found that attaching of cyclodextrin (CD) is also a good method to stable RGO. CDs can be attached on the surface of graphene sheets with strong hydrogen bonding to make graphene more hydrophilic [17,18]. b-CD, a cyclic oligosaccharide, possesses an electronic and hydrophobic interior microenvironment in its cavity structure, which allows hydrophobic molecules to be easily trapped into its cavity by displacing water. Since it has a lot of hydroxyl groups, the exterior of its cavity is hydrophilic. This unique structure makes it easy to bind inorganic, organic and biological ⇑ Corresponding author. Tel./fax: +86 021 55664094. E-mail address: [email protected] (M. Ye). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.090
molecules into its cavity, thus forming supramolecular hybrid without structural changes [19]. Nanocrystalline titanium oxide (TiO2) is a well-known wideband gap semiconductor and has been investigated as an excellent photocatalyst, owing to its outstanding properties such as nontoxicity, low cost and energy applications [20]. In addition, well-prepared TiO2 also shows good adsorption activity, which is widely used to remove heavy metals [21]. Graphene/inorganic composites, derived from the decoration of graphene with nanoparticles, are attracting more and more attention. Several attempts in using RGO for modification of TiO2 for photocatalytic degradation of organics have been reported. For example, Pu and coworkers synthesized graphene oxide-TiO2 hybrids by a one-pot microwave-assistant combustion method. The hybrids showed improved photocatalytic properties than neat TiO2 [22]. Anandan and coworkers developed a simple approach to fabricate graphene-loaded TiO2 thin films on glass substrates by the spin-coating technique. They found that the enhanced photocatalytic activity of the films is attributed to its efficient charge separation, owing to electrons injection from the conduction band of TiO2 to graphene [23]. On the basis of the above considerations, combining the selfassembly behavior of b-CD and its interaction with RGO, we suspect that the supramolecular structure of RGO-CD-TiO2 will offer the possibility to increase the separation and transportation of photogenerated charges. In this paper, we present a novel approach for the preparation of RGO-CD-TiO2 hybrid in water, using graphene oxide as a precursor of RGO and tetrabutyl titanate as the source precursor of TiO2. The method used in this study has several novelties and advantages: (1) b-CD acted as a linker between RGO and monodispersed TiO2 nanoparticles since it can interact with both RGO and TiO2 nanoparticles. (2) The whole
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process is simple and industrially compatible. Glucose was used as the reducing agent and its reducing ability is greatly enhanced under the hydrothermal condition, which is strong enough to reduce the graphene oxide. (3) The new supramolecular nanocomposites possess unique properties of high surface area and host–guest recognition, thus can be used to enrich ad detect inorganic, organic and biological molecules. 2. Experimental 2.1. Materials Pristine graphite was purchased from Qingdao BCSM, CO., Ltd. Tetrabutyl titanate, b-CD and glucose were supplied by Shanghai Chemical Reagent Company. All other reagents were at least of analytical reagent grade and used without further purification.
microscopy (TEM) was performed with a JEOL JEM-2100F. Scanning electron microscopy (SEM) was performed with a Philips XL30 FEG FE-SEM instrument at an accelerating voltage of 25 kV. The UV–Vis diffuse reflectance spectra were conducted on a UV-3600 UN-VIS–NIR spectrophotometer using an integrating-sphere accessory.
2.4. Photocatalytic activity and adsorption measurements Photocatalytic and adsorption experiments were conducted to remove phenol and Cr (VI) in water. In a typical procedure, RGO-CD-TiO2 nanocomposite (10 mg) was added into 500 mL of phenol or Cr (VI) solution (10 mg/L). The mixtures were stirred moderately on a magnetic stirring apparatus at room temperature and then exposed to a UV light lamp which was placed 20 cm above the beaker. Samples were withdrawn regularly from the reactor and immediately centrifuged to separate any suspended solid before analysis. The concentration of phenol or Cr (VI) in the solution was monitored by measuring the absorbance of the solution with a UV–Vis spectrophotometer. RGO-CD, TiO2 and RGO-TiO2 were adopted as the reference to compare the photocatalytic activity with that of RGO-CD-TiO2 nanocomposites under the same experimental conditions.
2.2. Preparation of RGO-CD-TiO2 GO was obtained by the modified Hummers method [24]. 100 mg of tetrabutyl titanate was added to 5 mL of ethanol. The above mixture was slowly dropped into the mixture of 4 mg ammonium chloride/5 mL water to form part A. On the other hand, 100 mg of GO was added to 100 mL water. The mixture was sonictaed for 30 min, followed by high-speed stirring for further 1 h. 100 mg glucose, 200 mg b-CD and 1 mL ammonium hydroxide was added to the GO solution to get part B. Subsequently, part A and part B were mixed. The mixture was put into an autoclave and heated at 160 °C for 4 h. When the reduction reaction was finished, the as-synthesized product (RGOCD-TiO2-1) was isolated by centrifugation, washed with pure water and ethanol several times, and dried at 90 °C for 12 h. The whole fabrication process is outlined in Fig. 1. RGO-CD-TiO2-2 and RGO-CD-TiO2-3 were got with the similar method by adding 200 mg and 400 mg of tetrabutyl titanate. In addition, RGO-CD, TiO2 and RGO-TiO2 were synthesized in water in the same way as the composites in the absence of tetrabutyl titanate, GO, and CD, respectively. 2.3. Characterization Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a NEXUS 670 spectrometer. Raman spectra were recorded on a Dilor LABRAM-1B multi-channel confocal microspectrometer with 514 nm laser excitation. Thermogravimetric analysis (TGA) was accomplished with Netzsch TG 209F1 that was fitted to a nitrogen purge gas at 10 °C/min heating rate. X-ray diffraction (XRD) was taken on D/max-rB diffractometer using Cu Ka radiation. Transmission electron
3. Results and discussion One purpose of this study is to find an efficient and green way to prepare the hybrid of RGO and TiO2. We used GO as the precursor of RGO in this research. It is now well known that heavy oxygenation of graphite results in multiform functional groups on the basal planes and at the edge of sheets, which makes GO strongly hydrophilic and easily dispersible in water. These decorated functional groups are able to reduce the interplanar forces and promote complete exfoliation of single-layer graphene oxide sheets in aqueous media. In the present work, the starting material graphene oxide is synthesized by oxidation of graphite with a modified Hummers method. In addition, we judiciously introduced CD molecules into GO before it was fully reduced, and the resulted RGO remain soluble in water and do not aggregate for a long time. The color of the suspension shifts from brown to black, which further indicates the change from GO to RGO during the reduction process [25]. Fig. 2 shows IR spectra of GO (a), TiO2 (b), RGO-CD (c) and RGOCD-TiO2s (d–f). In the spectrum of GO, it shows a broad absorption band at 3440 cm1, which is related to the OAH groups. The
Fig. 1. Experimental procedure used in this study to prepare RGO-CD-TiO2.
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Fig. 2. FTIR spectra of GO (a), TiO2 (b), RGO-CD (c) and RGO-CD-TiO2s (d–f).
absorption band at 1650 cm1 is a typical peak of carboxyl group and the CAO vibration of the epoxy group in GO appears at 1200 cm1. In the spectrum of TiO2 (b), peaks at 780 and 1100 cm1 are assigned to TiAOATi and TiAO stretching. As to RGOCD (c), the aromatic ring stretching frequency of the RGO results in the bands at 1630, 1450, 1100, 1000, and 875 cm1. After GO was chemically reduced, the C@O vibration band (1650 cm1) almost disappears, but the broad OAH stretching band still remains, presumably due to inter- and intramolecular hydrogen bonding [26]. In addition, compared with GO, the spectrum of RGO-CD shows two bands between 1155 and 1040 cm1, corresponding to the antisymmetric glycosidic CAOAC vibration, and the coupled CAC/CAO stretching vibration. As to RGO-CD-TiO2s (d–f), there still exhibit typical CD absorption features of the ring vibrations at 600, 700, 760, and 960 cm1, the coupled CAOAC stretching and OAH bending vibrations at 1150 cm1, the coupled CAO/CAC stretching/OAH bending vibrations at 1030 and 1080 cm1, CH2 stretching vibrations at 2930 cm1, and OAH stretching vibrations at 3450 cm1. This clearly confirms that CD molecules are still attached to the surface of RGO. According to a previous report, multiple hydrogen bondings could be strong enough to construct a complex structure, even under sonication [27]. Raman spectroscopy is one of the most widely used techniques to characterize the structural and electronic properties of graphene-based materials. Generally, the Raman spectrum of graphene
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is characterized by three main features. The G peak, which is due to the E2g vibration mode of sp2 bonded carbon, usually observed at 1575 cm1; the D peak, which is arising from the doubly resonant disorder-induced mode, can be found at 1350 cm1; and the 2D peak at 2700 cm1, which is a second order vibration caused by the scattering of phonons at the zone boundary [28]. In addition, ratio of the intensities of the D and G band (ID/IG) can be used to indicate the level of chemical functionalization in a carbon material. High ID/IG ratio indicates a high degree of disorder. The results of GO, TiO2, RGO-CD and RGO-CD-TiO2s are shown in Fig. 3. The spectrum of GO shows broadened D-band (1330 cm1) and the G-band at 1570 cm1. When GO was chemically converted to RGO-CD, its Raman spectrum also contains both G and D bands, however, with an increase in the ID/IG ratio. This change suggests that the average size of the sp2 domains is decreased upon reduction of the GO sheets. New graphitic domains are created, which are smaller in size than the ones present in GO before reduction, but more numerous in number. Moreover, after RGO was functionalized with b-CD, the G-band shifts to 1597 cm1. This tendency is due to an increase in the number of sp3 carbons that were formed on the graphene during chemical functionalization [28]. Besides these Raman features, 2D (D + D0 ) peaks at 2640 and 2910 cm1 are displayed. The position and shape of the 2D peak can be used to distinguish the layer numbers of graphene [29]. The broader 2D peak located at 2640 cm1 of RGO-CD indicated that the sample is probably between 2 and 5 layers. In the spectra of RGO-CD-TiO2s, we can find that the characteristic peaks of TiO2 and RGO-CD still exist. Another evidence of successful preparation of RGO-CD-TiO2s came from TGA analysis. As shown in Fig. 4, comparing with raw graphite, GO was thermally unstable and more than 25% of its weight loss took place below 200 °C, which was assigned to the decomposition of the labile oxygen-containing functional groups. However, as to RGO-CD, it showed weight loss less than 10% below 250 °C. This indicates that most of the labile groups had been removed via reduction. In addition, a major weight loss of 10% in the range from 250 to 500 °C followed, which is because of the decomposition of CDs on the RGO sheets. Finally, there was a very slow weight-loss process of RGO-CD, from 500 to 700 °C, which was due to the partial decomposition of RGO sheets itself. TGA data reveal that RGO could load a number of CD molecules, which can greatly enhance the supramolecular function between them. This phenomenon is probably caused by the synergetic effects from RGO (high conductivity and high surface area) and CDs (host–guest recognition and enrichment). From the TGA curve of RGO-CD-TiO2-
Fig. 3. Raman spectra of GO, TiO2, RGO-CD and RGO-CD-TiO2s.
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Fig. 4. TGA curves of graphite (a), GO (b), RGO-CD (c), TiO2 (d) and RGO-CD-TiO2s (e–g).
s, comparing with GO and RGO-CD, we can also find that their weight losses are greatly restricted. It can be seen that after TiO2 nanoparticles were bonded onto RGO-CD, the total weight loss decreased to about 18%, 16% and 14%, indicating the weight percentage of TiO2 nanoparticles of RGO-CD-TiO2s are about 20 wt%, 28 wt% and 36 wt%. XRD patterns are used to further study the changes in structure. Fig. 5 shows XRD results of raw graphite, GO and RGO-CD-TiO2-1. Raw graphite (Fig. 5a) showed a very strong 002 peak at 26.44°. As to GO (Fig. 5b), while a small change in the position of the principal reflection is observed with oxidation, the most striking difference is the intensity and broadness of the peak observed at 2h = 10.5°, corresponding to an average interlayer spacing of 0.8 nm. Since the presence of oxygen-containing groups on both sides of the graphene sheets, individual graphene oxide sheet is expected to be thicker than individual pristine graphene sheet. In addition, the broad diffraction peak of GO powder hinted that the process can influence the crystallization of the samples in the functionalization process. Both the d-spacing value and broadness of this reflection in GO are typical for randomly ordered graphitic platelets. After reduction and decoration of CD and TiO2 (Fig. 5c), we discern a gradual change in the patterns to finally achieve a randomly ordered carbonaceous layered solid, with basal spacing of 0.34 nm instead of 0.78 nm for the parent GO, indicating that the bulk of the
Fig. 5. XRD patterns of raw graphite (a), GO (b), and RGO-CD-TiO2-1 (c).
oxygen-containing groups is removed from GO. This is consistent with the results of FTIR analysis. Moreover, the 002 reflection in the sample is very broad suggesting that the samples are very poorly ordered along the stacking direction. This is an indication that the sample comprises mainly of single layer sheets. In addition, the broad diffraction peak hinted that the process could influence the crystallization of the samples in the functionalization process, implying that the extensive conjugated sp2 carbon network is restored. Besides, the major diffraction lines can be indexed to the anatase phase (JCPDS 21-1272). The broadened diffraction peaks of low intensity imply that the particles are of nanoscale size. The average particle size of the prepared TiO2 nanoparticles was calculated to be ca. 10 nm based on Scherrer’s equation. The investigation of the structure of GO and RGO-CD-TiO2-1 had been performed by TEM. As Fig. 6a shows, most parts of the GO sheets are homogeneous and wrinkled. However, the surfaces of the RGO-CD-TiO2-1 become crumpled (Fig. 6b). Upon addition of CD, besides the corrugated layers, the sheets tend to form small aggregates, owing to the cross-linked RGO sheets with CD molecules [30]. SEM images of fractured surface also shows a wrinkled topography for GO (Fig. 6c). In addition, the distribution of TiO2 particles on RGO is relatively uniform (Fig. 6d), while RGO shows a crumpled structure. TiO2 is a well-known and the most investigated functional material in semiconductor photocatalysis. Because of the unique properties of graphene, it can not only improve the separation and transport of photocarriers, but also might cause a higher conduction band position with a stronger reductive power. On the other hand, one of the key chemical properties of CDs is their ability to form inclusion complexes with a wide variety of molecules that is to accommodate guest molecules into their inner cavity. In addition, as to the removal of heavy metal ions, with the linker of CD, the high surface of RGO can also help to greatly improve the adsorption ability of TiO2s. We speculate that the prepared RGOCD-TiO2s may have a synergistic effect toward its photocatalytic properties. We carried out the experiments of removal and adsorption of phenol and Cr (VI) in aqueous solutions under UV light irradiation to study the photocatalytic properties of RGO-CD-TiO2s. Fig. 7 presents the phenol and Cr (VI) concentration ratio (Ct/C0) of different samples and those of the direct photolysis process with irradiation time, respectively. We can find that the phenol conversion over the as-prepared catalyst within 120 min decreased in the order of RGO-CD-TiO2-1 RGO-CD-TiO2-2 > RGO-CD-TiO2-3 > RGO-TiO2 > TiO2 > RGO-CD (Fig. 7a). Similar results were also observed in the adsorption removal of Cr (VI) under UV light irradiation (Fig. 7b). It is known that during photocatalysis, the adsorption capacity, enhanced light absorption, charge transfer and separation are crucial factors [31–33]. Thus, the enhancement of the photocatalytic performance should be ascribed to the increase of the light absorption intensity and range, and the reduction of electron–hole pair recombination [34]. Based on the experimental data, we suspect that the surface structures of RGO-CD may be well responsible for its improved activity. The reasons underlying the improved photo-catalytic performance may be as follows: (1) the increase in utilization efficiencies for TiO2 catalysts on these RGO supports can be attributed to the increased surface areas and the well-dispersed nature of the prepared RGO. Owing to the oxygen-containing groups on the GO sheets (such as hydroxyl, carbonyl, and epoxy groups), the GO surface can interact strongly with coated species and easily absorb positive ions to form different composites. Simultaneously, GO were converted to RGO. In addition, it has been proved that RGO, with small amount of oxygen-containing groups bound to the edges of the basal planes, can form well-dispersed aqueous colloids, of which the surface charge is highly negative when dispersed in water. The synergetic
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Fig. 6. TEM and SEM images of GO (a and c) and RGO-CD-TiO2-1 (b and d).
Fig. 7. Photocatalytic and adsorption curves under UV irradiation of the samples for phenol (a) and Cr (VI) (b).
effect of RGO, b-CD and TiO2 makes its adsorption ability highly increased, which is useful for removal of heavy metal ions. (2) The role of RGO-CD could act as a macromolecular ‘‘photosensitizer’’ for TiO2, hence making it exhibit higher photoactivity. The photocatalytic process for RGO-CD-TiO2 under UV light irradiation seems to be very similar to the well-known strategy of photosensitization of semiconductors by the matched adsorption of organic dyes, by which the photo-response of a wide band gap semiconductor can be extended to the visible light region [35]. (3) The enhanced photocatalytic activity for RGO-CD-TiO2 composites could be
attributed to the ability of RGO to capture and transport electrons, and to promote charge separation. It is known that the higher separation efficiency of electron–hole pairs will enhance photocatalytic activity and results in a large number of holes participated in the photocatalytic process [36]. However, excessive addition of black color RGO-CD will lower the light intensity through the depth of reaction solution, thus decreasing its photoactivity. According to the results of photocatalytic analysis and adsorption experiments, we propose that the mechanism as follows: (1) It is reported that the nucleation and growth mechanism depends
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on the degree of oxygen functionalization on the surface of graphene sheets. The nucleation of nanoparticles at GO surfaces is mainly governed by the presence of oxygen groups at GO [37]. Aqueous solution of graphene oxide was selected as the precursor for the deposition of TiO2 nanoparticles. Oxygen-containing groups can coordinate Ti4+ and provide reactive sites for the nucleation and growth of TiO2. During this process, in the first step, tetrabutyl titanate was attached on graphene oxide sheets, in which monolayer graphene oxide were homogeneously dispersed in water. Therefore, the mixing of tetrabutyl titanate and graphene oxide can be considered at molecular level. In the second step, graphene oxide was reduced to RGO nanosheets and TiO2 nanoparticles generated simultaneously. This process can ensure the in situ formation of TiO2 nanoparticles and graphene nanosheets simultaneously, with an advantage to prevent serious stacking of RGO nanosheets. (2) The presence of remained oxygen functionalities (mainly carboxyl groups) at the RGO surface continually provides reactive sites for the nucleation of TiO2 nanoparticles. (3) One of the key chemical properties of cyclodextrins is their ability to form inclusion complexes with a wide variety of molecules that is to accommodate guest molecules that is to accommodate guest molecule into their inner cavity. With the synergistic effect of CD and RGO, pollutant molecules can be quickly caught. However, the dependence of physical properties of TiO2-RGOs is very complex. Further study will be necessary to elucidate the precise mechanism in the future.
4. Conclusions In this work, we demonstrate an effective one-pot hydrothermal route for the synthesis of RGO-CD-TiO2 nanocomposite. The hydrothermal process provides a straightforward and facile approach to deposit TiO2 nanoparticles onto the RGO sheets. Interestingly, it was also found that the photocatalysis performance and adsorption capacity of as-prepared nanocomposite could be enhanced by the interaction between RGO, CD and TiO2. In addition, the properties of the hybrids may be controlled with different ration of TiO2 and RGO. This method may be readily extended to the preparation of other classes of hybrids based on RGO sheets for technological applications.
Acknowledgements We gratefully acknowledge the financial sponsor of National Natural Science Foundation of China (51202034) and ‘‘Chen Guang’’ Project (12CG02) supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation.
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