Preparation of Fullerene/TiO 2 Composite and Its Photocatalytic Effect

Transcription

1 J. Ind. Eng. Chem., Vol. 13, No. 7, (2007) Preparation of Fullerene/TiO 2 Composite and Its Photocatalytic Effect Won-Chun Oh, Ah-Reum Jung, and Weon-Bae Ko* Department of Advanced Materials & Science Engineering, Hanseo University, Chungnam , Korea *Department of Chemistry, Sahmyook University, Seoul , Korea Received October 17, 2007; Accepted December 9, 2007 Abstract: A Fullerene/TiO 2 composite photocatalyst was prepared with titanium (IV) n-butoxide (TNB) by using a MCPBA oxidation method. Since fullerene has absorptive and semiconducting properties, the fullerene/tio 2 composite revealed a sound photo-degradation activity. From the XRD data, a strong carbon peak of fullurene graphene remained and anatase and rutile peaks were observed in the X-ray diffraction patterns for the fullerene/tio 2 composite. The surface properties seen by SEM present a characterization of the texture on the fullerene/tio 2 composite and homogenous compositions in the particles for the titanium sources that were used. For the elemental identification, the EDX spectra showed the presence of C and O with strong Ti peaks. From the MALDI-TOF mass spectrum of the fullerene/tio 2 composite, the observation of the peak due to titanium trace appeared at and m/z. This is also consistent with the elemental compositions of C 60 (TiO 2 ) 2 and C 60 O(TiO 2 ) 2. From the photocatalytic results, the excellent activity of the fullerene/tio 2 composites for organic dye and UV irradiation time could be attributed to both the effects between the photocatalysis of the supported TiO 2 and the absorptivity of the fullerene. Keywords: fullerene, titanium (IV) n-butoxide, XRD, SEM, EDX, MALDI-TOF mass, photocatalysis Introduction 1) While a number of remarkable investigations on the preparation of fullerene derivatives have been made in the new field of material science associated with fullerene chemistry [1-3], the interesting higher fullerenes have received special attention. The main discoveries of C 60 molecular and its bulk have laid the foundation for a vast new field of studies on the subject of condensed matter materials. Several researchers have investigated charge transfer properties, photochemical and photophysical properties of fullerene in the derivative media [4-6]. The interest in understanding photophysico-chemical properties stems from its potential application in developing photoconductive, photovoltaic and optical devices [7,8]. Another interesting feature of fullerenes is the charge transfer with the host material. Photocatalytic properties such as TiO 2 have been shown to participate in the surface photochemical processes resulting in the oxidation of adsorbed substrate [9]. TiO 2 is considered to To whom all correspondence should be addressed. ( wc_oh@hanseo.ac.kr) be a promising material for the treatment of gas and liquid pollutants such as NO, VOCs, HNO 3, and NH 3 compounds including sulfur because of its strong oxidative ability. Anatase TiO 2 requires UV light to initiate bandgap excitation due to its large bandgap semi-conducting properties. Many researchers agree that TiO 2 currently has the best photocatalytic properties for the removal of pollutants due to its many advantages such as its high activity, high stability for light illumination, low cost and its non-toxicity. Recently, carbon/tio 2 composites are one of the most common and widely investigated in the field because of their preparation method [10,11] and characterizations [12,13]. The composite products of carbon and TiO 2 photocatalysts could perhaps offer the merits of an adsorption effect on the porous structure and the photo-degradation by light excitation for the chemical dyes and pollutants. Much of the interest in this form of carbon originates from the subsequent key point of photocatalytic effects when C 60 is treated with TiO 2 [14,15]. Among the same types of carbon supports, fullerene is very promising for synergistic reasons; its energy sensitizer for improving the quantum efficiency and an increase of charge transfer between the fullerene

2 Preparation of Fullerene/TiO 2 Composite and Its Photocatalytic Effect 1209 and TiO 2 by acidification of surface groups. This study investigated a method for the preparation of fullerene/ TiO 2 composites involving the m-chloroperbenzoic acid (MCPBA) oxidation method. The MCPBA method is expected to have some advantages due to the high diffusivity with a homogenous distribution of oxyrane groups on the fullerene surface. This study presents the preparation and characterization of fullerene/tio 2 composites that have been synthesized by an improved oxidation method. The structural variations, surface state, mass transformation and photocatalytic performance are investigated by preparing oxidized fullerene [C 60 ] and fullerene/tio 2 composites sequentially after oxidation is compared to pristine fullerene [C 60 ]. X-ray diffraction (XRD), scanning electron microscopy (SEM), Maldi-TOF-Mass spectrometry, energy dispersive X-ray (EDX) spectroscopy and UV/VIS spectroscopy were used to characterize these new complexes. Experimental Methods Materials Crystalline C 60 powder of 99.9 wt% purity from TCI (Tokyo Kasei Kogyo Co. Ltd., Japan) was used as a starting material. Reagents (benzene and ethyl alcohol) were purchased as reagent-grade from Duksan chemical Co and Daejung cheimical Co. and were used without further purification unless otherwise stated. Evaporation and concentration in a vacuum were carried out at a water aspirator pressure and compounds were dried at 1.33 Pa. In order to make a comparison, commercially available TiO 2 photocatalysts were used, supplied by the Duk-San Pure Chemical Co., Korea. These were composed of a single phase of anatase with secondary particles of approximately µm aggregated from the primary particles of approximately µm. This anatase-type titanium dioxide powder has a relatively large BET surface area of about 125 m 2 /g. The titanium (IV) n-butoxide (TNB, C 16 H 36 O 4 Ti) as a titanium source for the preparation of fullerene/tio 2 composites was purchased as reagent-grade from Acros Organics, USA. Chemical Oxidation on the Fullerene Surface M-chloroperbenzoic acid (ca g) was suspended in 60 ml benzene as a solvent, and fullerene [C 60 ] (ca. 40 mg) was added. The mixture was then refluxed in an air atmosphere for 6 h. The solvent was subsequently dried at the boiling point of benzene ( K). The solid precipitates were transformed to a dark brown color. After completion, dark brown precipitates were washed with ethyl alcohol and then dried at 363 K. Preparation of Fullerene/TiO 2 Composite Surface oxidized fullerene slurry solutions were prepared with pristine concentrations of 20 ml of TNB for the preparation of fullerene/tio 2 composites. For the preparation, 0.5 g of powdered surface oxidized fullerene was mixed into 20 ml of TNB aqueous solution and stirred for 5 h at 333 K. The powder mixtures of TNB with surface oxidized fullerene were then heated at 973 K for 1 h. Before heat treatment, the solvent in the mixtures was vaporized at 343 K for 1 h. Finally, these samples were dried at 368 K for 72 h in an air atmosphere. Characterization of Fullerene/TiO 2 Composites For the measurements of structural variations, XRD patterns were taken using an X-ray generator (Shimatz XD-D1, Japan) with Cu Kα radiation. SEM (JSM-5200 JOEL, Japan) was used to observe the surface state and structure of the oxidized fullerene [C 60 ] and the fullerene [C 60 ] that was treated with TNB sequentially after an oxidation. This was ten compared to the pristine fullerene [C 60 ]. EDX spectra were also used for the elemental analysis of the samples. As one of the analyses of mass transformation for the sample, Maldi-TOF Mass spectroscopy (Voyager, DE-STR, USA) was used in order to characterize the structural transformation for the fullerene/tio 2 composites. The fullerene derivative was examined with a cyano-4-hydroxyl sinamic acid as a matrix using the spectroscopy. The thermal analyses were carried out in a Schimahzu thermoanalyzer (TG- 50, Japan) under dry nitrogen, using platinum crucibles and a constant heating rate of 10 K/min up to 1273 K. Fine alumina powder was used as a reference substance. For the measurement of photo-degradation effects, UV- VIS spectra of fullerene/tio 2 composites were recorded using a Genspec III (Hitachi, Japan) spectrometer. Analysis of Photocatalytic Effect In order to analyze the photocatalytic effect, the decomposition reaction of MB in water was used. Powdered samples of 0.05 g were dispersed in the MB solution under ultra sonicate for 3 min. For UV irradiation the UV lamp (20 W, 365 nm) was used at a distance of 100 mm from the solution in the dark box. The suspension was irradiated with ultraviolet light as a function of the irradiation time. Samples were then withdrawn regularly from the reactor and removal of dispersed powders was carried out through centrifuge. The clean transparent solution was analyzed by UV/VIS spectroscopy. The concentration of MB in the solution was determined as a function of irradiation time from the absorbance region at a UV wavelength line of 256 nm.

3 1210 Won-Chun Oh, Ah-Reum Jung, and Weon-Bae Ko Figure 1. XRD patterns of fullerene/tio 2 composite. Results and discussion Structural Analysis The XRD technique was used in order to determine the crystallographic structure of the inorganic constituent of the composite. The result is shown in Figure 1. The XRD result indicates that the phase transition from TNB to the anatase and rutile phases has taken place at 973 K with the formation of titania crystalline. According to former studies for the TiO 2 formation from TNB [16,17], the XRD results showed that in the increasing temperature process the anatase phase has not appeared and the rutile phase has occurred at relatively a low temperature. In another study [18] however, the XRD lines for the anatase type structure were sharpened and the separation of these diffraction peaks became clear. However, at the same time a rutile phase appeared from the carbon coated samples. It was considered that the crystallinity of the fullerene/tio 2 composite sample is improved by heating to around 973 K, as indicated by the distinguishable sharpness of all the diffraction lines. In the XRD patterns for the fullerene/tio 2 composite, the diffraction peaks of fullerene were clearly observed because of the high carbon contents and the high crystallinity. The results from Maldonado-Hodar and coworkers [19], showed that the XRD patterns of TiO 2 /carbon composites formed anatase crystallites at low temperatures. However, with an increase in the pyrolysis temperature, the patterns show the presence of mixtures of anatase and rutile structures. This implies that the increase in the heat-treated temperature is included in the phase transition from the pure anatase phase to the mixture phase of anatase and rutile or to the rutile phase. In this study, for the fullerene/tio 2 composite, the TiO 2 structure shown is that of the anatase and rutile mixed type. After the heat treatment at 973 K for 1 h, the main crystalline phase has not transformed to a pure rutile structure. The major peaks are diffractions from (101), Figure 2. SEM micrographs of fullerene derivatives; (a) oxidized fullerene, (b) fullerene/tio 2 composite (over-all scale) and (c) fullerene/tio 2 composite (close-up). (004), (200) and (204) planes of anatase and (101), (103), (112), (105), (211), (204), (116), and (220) plans of rurile, indicating that the TiO 2 prepared existed in the anatase and rutile mixture state. Surface Characteristics The surface characteristics of the fullerene/tio 2 com-

4 Preparation of Fullerene/TiO 2 Composite and Its Photocatalytic Effect 1211 Figure 3. EDX elemental microanalysis of fullerene/tio 2 composite. posite and the oxidized fullerene derivative are shown in Figure 2. The change in the morphology of the oxidized fullerene particles is found on a non-homogenous surface due to surface oxidation, as shown in Figuire 2(a). Figures 2(b) and 2(c) illustrate the fullerene/tio 2 composite, where it can be clearly seen that the TiO 2 particles prepared at a source of the temperature of 973 K, showed a great distribution of dimensions with particle sizes ranging from small nm to large µm. It was observed that fullerene was covered with TiO 2 particles. These TiO 2 particles were regularly dispersed on the fullerene surface and some large clusters were found with an irregular agglomerate dispersion. It can be seen that the high-quality dispersion of small particles on the fullerene surface could provide improved information on the existence of more reactive sites for the TNB reactants on the oxidized fullerene surface than that of non-oxidized fullerene. Moreover, an obvious advantage of the oxidized fullerene particles was that a considerable portion of TiO 2 would be enclosed in the three-dimensional matrix. Accordingly, a high photocatalytic yield is expected for a homogenous and small TiO 2 particle distribution. Therefore, a higher photocatalytic activity of the fullerene/tio 2 composite might be attributed to the homogenous distribution of titanium complexes. This would include titanium dioxide and the energy sensitizer for improving the quantum efficiency and an increase of charge transfer of the fullerene. Elemental Analysis A quantitative microanalysis of C and Ti, as major elements for the fullerene/tio 2 composite, was performed by EDX. The EDX spectrum of fullerene/tio 2 composite is shown in Figure 3. The carbon coated solid particles and composite type carbon for the elemental analyses has been reported on the various types of titania [12,13]. In this study, the spectrum shows the presence as major elements of C and O with strong Ti peaks. The numerical result of EDX quantitative microanalysis of the fullerene/tio 2 composite revealed the ratios of 47.5 : 31.2 : 21.3 for components of C : Ti : O. In the case of most samples, carbon and titanium were present as ma- Figure 4. MALDI-TOF mass spectrum of fullerene/tio 2 composite. jor elements with small quantities of oxygen elements in the fullerene/tio 2 composite. In order to obtain further information, the composition of these prepared materials was needed for the MALDI- TOF mass spectroscopical measurements. The MALDI- TOF mass spectrum of fullerene/tio 2 composite is shown in Figure 4. The result for the measurements gave additional evidence for the difference between oxidized fullerene and fullerene/tio 2 composite. Details of the obtained results are, however, rather surprising, as in the MALDI-TOF mass spectra the fullerene/tio 2 composite are observed as compared to the material transformed with oxidation [2]. The traditional mass spectrum of pure fullerene [C 60 ] has been reported in former studies [20,21]. The mass spectrum shows major peaks for pristine fullerene [C 60 ] at 720 m/z and oxidized fullerene [C 60 O] by oxidation at m/z. It was confirmed that the O signals of oxidized fullerene [C 60 O] are , , , , and m/z. These results indicate that these dominating peaks of the spectrum correspond to the increasing O elements in the pristine fullerene [C 60 ] due to the structural transformation. From the MALDI-TOF mass spectrum of fullerene/tio 2 composite in Figure 4, the observation of the peak due to a titanium trace appeared at and m/z. It is also consistent with the elemental compositions of C 60 (TiO 2 ) 2 and C 60 O (TiO 2 ) 2. The titanium dioxide has a strong tendency to form stable compounds using heat treatment, which as functional groups on the surface layer presents the direct interaction between fullerene [C 60 ] or oxidized fullerene [C 60 O] and treated metallic titanium. It can be concluded that the differences in the spectra recorded on two kinds of fullerene composites are due to the oxidation including chemical bonding and interposing of a titanium dioxide in the fullerene [C 60 ] or oxidized fullerene [C 60 O]. The spectrum recorded on two kinds of fullerene composites illustrates the influ-

5 1212 Won-Chun Oh, Ah-Reum Jung, and Weon-Bae Ko Figure 5. UV/VIS spectra of MB concentration against the fullerene/tio 2 composite under various irradiation time conditions. ence by the formation of O group on fullerene layers with the oxidation and transformation by interaction of carbon and titanium. These influences have their origin in the different chemical reactivity between the substrate fullerene and the reactant (titanium dioxide). Due to the pristine fullerene, [C 60 ] is very chemically stable. However, it may not be reacted with any chemical reactants without oxidation process [22]. Photocatalytic Decomposition of MB From the results of the UV/VIS spectra for the MB solution after photolysis, the relative yields of the photoproducts formed at different irradiation time functions were compared. The UV/VIS spectra of MB concentration against the fullerene/tio 2 derivative under various time conditions are shown in Figure 5. As shown earlier [23,24], if the MB concentration used is higher (eg mol/l), the intensity of the absorbance maxima values could be estimated by a photoproduct formed as a function of irradiation time. However, if the concentration were lower (eg mol/l), it would not be possible to positively estimate the maxima values. It is considered that the formation of absorbance maxima is proportional to the concentration decreased of the transient formed after the UV light excitation. In this study, the trial absorption from the 260 nm to the 550 nm regions with MB products was tried for photodegradation by fullerene/tio 2 derivative. At that time, the concentration of MB was mol/l. It was shown that the maxima values moved into a long wavelength and decreased to lower absorbance region with increasing irradiation time. This implies that the light transparent of the MB concentration increases significantly due to the photocatalytic degradation effect. Figure 6 shows changes by color disappearance in relative MB degradation concentrations under UV light irra- Figure 6. Dependence of relative concentration (-ln c/c 0) of MB in the aqueous solution on time of UV irradiation for the fullerene/tio 2 composite. diation in the solution as a function of time. The MB degradation with fullerene/tio 2 composite is carried out to observe the UV photolysis effect. The changes by color shade are presented on the logarithm scale of relative concentration (-ln (c/c 0 )) of MB degraded in the aqueous solution as a function of UV irradiation time. The MB solution is quite unstable with variations of concentration when it is irradiated under UV light with fullerene/tio 2 composites. The relationship of the -ln (c/c 0 ) of the MB solution products as a time function under UV irradiation revealed linearity properties depending on the kinetic first-order. As mentioned above, fullerene had an energy sensitizer for improving the quantum efficiency and an increase of charge transfer. It is believed that the degradation of MB concentration in the aqueous solution can occur as two physical phenomena such as quantum efficiency by fullerene and photocatalytic decomposition by TiO 2. According to earlier researchers [25], MB molecules absorbed energy from irradiation, thereby shifting their delocalized electrons from a bonding to an antibonding orbital. Since MB adsorption likely occurs via π-π interactions between its delocalized electrons and the graphene layers of the carbon, it is reasonable to expect that shifts in its electron orbital would alter adsorption. It is considered that the TiO 2 deposited on the fullerene surface can retain its photo-degradation activity. In this paper, the photocatalytic activity of fullerene/tio 2 composites could be attributed to the homogeneous distribution of TiO 2 on the external surface by the MCPBA oxidation method. The synergetic effect between fullerene and TiO 2 could be caused by the enhanced degradation of the MB as pollutants on fullerene/tio 2, followed by a transfer through an interphase to titania where it is photo-degraded. In earlier studies [10-13], the synergitic effects for the MB degradation should be attributed to both effects between photo-

6 Preparation of Fullerene/TiO 2 Composite and Its Photocatalytic Effect 1213 catalysis of the supported TiO 2 and to the adsorptivity of the carbons. From the results of the MB solution degraded during 50 min, it was shown that the kinetics for the fullerene/tio 2 resulted in a more significant degradation effect with an increase of -ln (c/c 0 ) values over those of the original TiO 2. In the case of photocatalytic performance, the slope relationship between -ln (c/c 0 ) for MB and UV irradiation time were measured at mol/l of initial MB concentration. From the photo-degradation results between c/c 0 for MB and UV irradiation time, we therefore can conclude that the decrease of MB concentration should be considered in both effects between photocatalysis of the supported TiO 2 and the charge transfer of the fullerene. The photodecomposition of pollutants occurs on TiO 2 particles, even on those where it was coated on the carbon, and that light absorption was active on the carbon particles.however, carbon deep inside TiO 2 is not easily accessible to light with excitation due to the long traveling distance and reflection and scattering by the support. Conclusion This study presented the preparation and characterization of fullerene/tio 2 composites synthesized by an improved oxidation method. The structural variations, surface state and mass transformation were sequentially investigated through the preparation of oxidized fullerene [C 60 ] and fullerene/tio 2 composites subsequent to a comparison made between oxidation and pristine fullerene [C 60 ]. In the XRD patterns for the fullerene/tio 2 composite, the diffraction patterns are clearly observed where there are peaks of fullerene due to the high carbon content and high crystallinity with a mixed phase of anatase and rutile. From the SEM results, the TiO 2 particles were regularly dispersed on the fullerene surface and some large clusters were found with an irregular agglomerate dispersion. The EDX spectrum revealed the presence as major elements of C and O with strong Ti peaks. From the MALDI-TOF mass spectrum of fullerene/tio 2 composite, the observation of the peak due to titanium trace appeared at and m/z. This is consistent with the elemental composition of C 60 (TiO 2 ) 2 and C 60 O (TiO 2 ) 2. According to the photocatalytic results, the excellent activity of the fullerene/tio 2 composites between organic dye and UV irradiation time could be attributed to synergitic effects between the supported TiO 2 and the fullerene. References 1. C. D. Stevenson, J. R. Noyes, and R. Reiter, J. Am. Chem. Soc., 122, (2000). 2. W. C. Oh, A. R. Jung, and W. B. Ko, Analytical science & Technology, 20, 2, 124 (2007). 3. W. C. Oh, A. R. Jung, and W. B. Ko, Preparation and characterization of Zn-containing fullerene, Proceeding of International conference on Carbon (CARBON 2007), CD-rom, p.284, 15th-20th July 2007, Seattle, U.S.A. 4. B. Sun, M. Li, H. Luo, and Z. Shi, Z. Electrochimica Acta, 47, 3545 (2002). 5. F. Langa, P. Cruz, J. L. Delgado, E. Espildora, M. J. Gomez-Escalonilla, and A. Hoz, J. Mater. Chem., 12, 2130 (2002). 6. M. S. Dresselhaus, G. Dresselhaus, and P. C. Eklund, Academic press, USA, p (1996). 7. H. Dang, M. Levitus, and M. A. Garcia-Garibay, J. Am. Chem. Soc., 124, 136 (2002). 8. M. Drees, K. Premaratne, W. Graupner, and J. R. Heflin, App. Phy. Lett., 81, 24, 4607 (2002). 9. M. Kaneko and I. Okura, Photocatalysis: Science and Technology, Kodansha & Springer, p.124, (1999). 10. W. C. Oh, M. L. Chen, and C. S. Lim, Journal of Ceramic Processing Research, 8, 2, 119 (2007). 11. W. C. Oh, J. S. Bae, and M. L. Chen, Carbon Science, 7, 4, 259 (2006). 12. W. C. Oh, J. S. Bae, and M. L. Chen, Korean Chem. Soc., 27, 9, 1423 (2006) 13. W. C. Oh, J. S. Bae, M. L. Chen, and Y. S. Ko, Analytical Science & Technology, 19, 5, 376 (2006). 14. T. Akiyama, A. Miyazaki, M. Sutoh, I. Ichinose, T. Kunitake, and S. Yamada, Colloides and Surfaces, 169, 137 (2000). 15. P. V. Kanmat, M. Gevaert, and K. Vinodgopal, J. Phys. Chem. B, 101, 4422 (1997) 16. K. Yu, J. Zhao, Y. Tian, M. Jiang, X. Ding, Y. Liu, Y. Zhu, and Z. Wang, Materials Letters, 59, 3563 (2005). 17. W. C. Oh and M. L. Chen, Carbon Science, 8, 2, 108 (2007). 18. T. Tsumura, N. Kojitani, I. Izumi, N. Iwashita, M. Toyoda, and M. Inagaki, J. Mater. Chem., 12, 1391 (2002). 19. F. J. Maldonado-Hodar, C. Moreno-Castilla, and J. Rivera-Utrilla, Applied Catalysis A: General, 203, 151 (2000). 20. B. Burger, H. Kuzmany, T. M. Nguyen, H. Sitter, M. Walter, K. Martin, and K. Kullen, Carbon, 36, 661 (1998). 21. Z. Gu, L. Zhang, J. L. Margrave, V. A. Daveydov, A. V. Rakhmanina, V. Agafonov, and V. N. Khabashesku, Carbon, 43, 2989 (2005). 22. M. Baibarac, L. Mihut, N. Preda, I. Baltog, J. Y. Mevellec, and S. Lefrant, Carbon, 43, 1 (2005).

7 1214 Won-Chun Oh, Ah-Reum Jung, and Weon-Bae Ko 23. M. L. Chen, Y. S. Ko, and W. C. Oh, Carbon Science, 8, 1, 6 (2007). 24. W. C. Oh, S. B. Han, and J. S. Bae, Analytical science & Technology, 20, 4, 279 (2007). 25. R. P. Schwarzenbach, P. M. Gschwend, and D. M. Imboden, Environmental organic chemistry, 2nd Ed., John Wily and Sons (2002).