Preparation of Carbon-Coated TiO 2 at Different Heat Treatment Temperatures and Their Photoactivity

Carbon Science Vol. 7, No. 4 December 2006 pp. 259-265 Preparation of Carbon-Coated TiO 2 at Different Heat Treatment Temperatures and Their Photoactivity Ming-Liang Chen 1, Jang-Soon Bae 2 and Won-Chun Oh 1, 1 Department of Advanced Materials & Science Engineering, Hanseo University, Chungnam 356-706, Korea 2 Department of Industrial Chemistry, Dankook University, Chungnam 330-714, Korea e-mail: wc_oh@hanseo.ac.kr (Received September 26, 2006; Accepted December 11, 2006) Abstract Carbon-coated TiO 2 was prepared by CCl 4 solvent mixing method with the different heat treated temperatures (HTTs). Since the carbon layers derived from pitch on the TiO 2 particles were porous, the carbon-coated TiO 2 sample series showed a good adsorptivity. The values of BET surface areas measured were shown independently on the HTTs. The surface states by SEM present to the characterization of porous texture on the carbon-coated TiO 2 sample and carbon distributions on the surfaces. From XRD data, PT700 and PT750 were shown the X-ray diffraction patterns of the anatase TiO 2, but PT800 and PT850 were kept anatase-type structure even after heating at 800 o C, though small amount of the rutile-type structure appears. The results of EDX microanalyses were observed for each sample show the spectra corresponding to almost all samples similar to C, O and Ti elements with an increase of HTTs. Finally, the excellent photoactivity of carbon-coated TiO 2 (especially, PT700 and PT750) could be attributed to the homogeneous coated carbon on the external surface and the structural anatase phase. Keywords : Carbon-coated TiO 2, BET, SEM, XRD, EDX, Photoactivity 1. Introduction Environmental pollution by various organic compounds in water has caused various serious problems for human health. TiO 2 has been extensively studied as an influential material for environmental pollutants removal for the past few decades because of its distinguished photocatalytic activity. Especially, anatase-type TiO 2 has attracted great attention because of its excellent photocatalytic activity [1, 2]. The anatase-type structure transforms to rutile-type at a high temperature, the photocataytic effect of which is much lower in most applications. This transformation temperature was studied to depend strongly upon the preparation conditions of TiO 2 crystals [3]. The common forms of TiO 2 are anatase, rutile and brookite; the last is relatively less studied [4]. The photocatalytic reactions of organic compounds over anatase and rutile surfaces have been studied. Carbon coating of photocatalyst anatase-type titanium dioxide was found to give many advantages such as high photosensitivity, high photocatalytic activity and high adsorptivity. And, carboncoated TiO 2 was applied to the oxidation of different organic components in wastewater [5]. Former studies mainly focused on coating TiO 2 on fixed supports, such as fibers, glass, stainless steal and quartz, etc. [6, 7]. According to previous study [8], it suggested that carbon-coated TiO 2 was completely different from TiO 2 mounted carbons in the positional relation between TiO 2 and carbon. In this carbon-coated TiO 2, the contaminant molecules have to be adsorbed into the carbon layer that covers the TiO 2 particle, diffuse through the carbon layer to reach the surface of the TiO 2 photocatalyst and then be decomposed under UV irradiation. We also have been interested in the carbon-titania studies and their technologies, and have found through our investigation that it is important to quantify the impact that the carboncoated TiO 2 particles give adsorption ability to the catalyst particles, to transfer the adsorbates to the surface of the TiO 2. In this study, we have prepared pitch based carbon-coated TiO 2 photocatalysts through carbon tetrachloride solvent method. The properties of pitch based carbon-coated anatase-type titanium dioxide photocatalysts are investigated through the preparation from the different heat treatment temperatures (HTTs) and the determination of their photoactivity. The carbon coated catalysts were characterized by BET surface area, X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray (EDX) and UV/VIS spectrophotometer. 2. Experimental 2.1. Materials The petroleum pitch was used carbon precursor for preparation of carbon-coated TiO 2 photocatalysts. The granular

260 M.-L. Chen et al. / Carbon Science Vol. 7, No. 4 (2006) 259-265 Table 1. Nomenclatures of Samples Prepared at Different Heat Treatment Temperature of Carbon-Coated Anatase Preparation method HT at 700 o C HT at 750 o C HT at 800 o C HT at 850 o C HT: Heat treatment Nomenclatures PT700 PT750 PT800 PT850 2.3. Photocatalytic effect In order to reveal the photocatalytic effect of the prepared samples, the decomposition reaction of MB in water was followed. Powdered samples of 0.05 g were dispersed in ultra sonicate for 3 min. For UV irradiation the UV lamp (20 W, 365 nm) was used at the distance of 100 mm from the solution in darkness box. By sampling 3mL of solution after removal of dispersed powders through centrifuge, the concentration of MB in the solution was determined as a function of irradiation time from the absorbance change at a wavelength of 660 nm. pitch was supplied from Jungwoo Chemical Co. (Korea). The TiO 2 photocatalysts was commercially available (Duk- San Pure Chemical Co., Korea), which was composed of a single phase of anatase with secondary particles of about 80-150 μm aggregated from the primary particles of about 30-50 μm. This anatase-type titanium dioxide powder had a relatively large BET surface area of about 132 m 2 /g. For the melting of pitch, carbon tetrachloride (Dae-Jung Chemical Co., Korea) was used as solvent. After melting of pitch in CCl 4 solution, TiO 2 powder was mixed with pitch-ccl 4 solution. The powder mixtures with 50 : 50 mixing ratio of anatase to carbon precursor (pitch) were heated at various heat treated temperatures for 1 h. Before heat treatment, the solvent in the mixtures was vaporized at 353 K for 6 hours. The nomenclatures of prepared samples were listed in Table 1. 2.2. Characterization For the physical parameter measurement, the BET specific surface areas were measured using a Quantachrome Autosorb Automated Gas System (Quantachrome Corporation, U.S.A) at 77 K. Scanning electron microscopy (SEM, JSM-5200 JOEL, Japan) was used to observe the surface state and structure of carbon-coated TiO 2 transformed through the various heat treatment. For the elemental analysis in carboncoated TiO 2, energy dispersive X-ray analysis (EDX) was also used. X-ray diffraction patterns were taken using an X- ray generator (Shimatz XD-D1, Japan) with Cu Kα radiation. As one of the analysis of photocatalytic activity, UV/VIS spectrophotometer (Genspec III (Hitachi), Japan) was used to characterize of catalytic efficiency of TiO 2 photocatalysts. Characterization of methylene blue (C 16 H 18 N 3 S, MB) in water was determined by the following procedure. A carboncoated TiO 2 powdered sample of 0.05 g was dispersed in an aqueous solution with a concentration of 1.0 10 4 mol/l in the dark atmosphere at room temperature. Each concentration was measured as a function of UV irradiation time from the absorbance at the range of 250-800 nm wavelength of MB measured by UV/VIS spectrophotometer. 3. Results and Discussion 3.1. Surface properties In Table 2, textural properties of the pristine TiO 2 and carbon-coated TiO 2 are presented against heat treated temperature of the samples prepared through different HTTs. The BET surface areas measured were shown an apparent values for the powder samples composed carbon and TiO 2. The results of BET surface area were shown independently on the HTTs. It was very difficult to evaluate the surface area of TiO 2 particles and carbon layer by variation of HTTs on these carbon-coated particles. In case of using ceramic substrate, carbon coating was experimentally exposed that the carbon formed on most ceramic surface from carbon precursors was very microporous [9]. It is plausible assume that thin carbon layers derived from pitch on TiO 2 particles are responsible for the BET surface area. According to the former study [10], it was obtained that the BET surface area for the carbon layer in the sample increases to increasing with pitch contents. From the point of different HTTs, the structure of carbon-coated TiO 2 samples can be affected to the amount of decomposition of MB in an aqueous solution. Fig. 1 shows SEM micrographs of carbon-coated TiO 2 sample series as a function of different HTTs. The surface structure of the carbon-coated TiO 2 samples was studied, and the Table 2. Textural Properties of Pristine Materials and Carbon- Coated TiO 2 samples Sample S BET (m 2 /g) Parameter Total Pore Volume (cm 3 /g) Average Pore Diameter (Å) As-received TiO 2 132 PT700 326 0.0481 5.88 PT750 317 0.0591 4.24 PT800 354 0.0427 4.51 PT850 299 0.0404 4.27

Preparation of Carbon-Coated TiO2 at Different Heat Treatment Temperatures and Their Photoactivity 261 SEM micrographs of carbon-coated TiO2 prepared at different heat treatment temperatures; (a) Pristine TiO2 ( 200), (b) Pristine TiO2 ( 200), (c) PT700 ( 500), (d) PT700 ( 2000), (e) PT750 ( 500), (f) PT750 ( 2000), (g) PT800 ( 500), (h) PT800 ( 2000) (i) PT850 ( 500), (j) PT850 ( 2000). Fig. 1. relationship between carbon deposition and pristine TiO2, and change of their structure were investigated. These figures present results from the porous textural properties on the carbon-coated TiO2 sample and carbon distributions on the surfaces for all the materials used. SEM pictures of carboncoated TiO2 sample provide information about the distribution of carbon on the TiO2 surface. A homogeneous distribution of carbon with providing the information of increased surface area can be promoting the photocatalytic efficiency for the removal of MB in aqueous solution. Carbons derived from pitch were homogeneously covered on the partial surface of the TiO2. Each of these analyses showed that carbons on the TiO2 are uniformly distributed in the rounded and edges. Compare to former study [9], carbon coating on the TiO2

262 M.-L. Chen et al. / Carbon Science Vol. 7, No. 4 (2006) 259-265 Fig. 1. Continued. was not obtained homogeneous results. In our case, however, homogeneously carbon-coated TiO2 samples were obtained from CCl4 solvent method as shown in Fig. 1. 3.2. Structural and chemical properties The XRD patterns of the samples heated at different temperatures are shown Fig. 2. In Fig.2, PT700 and PT 750 were shown the X-ray diffraction patterns of the anatase TiO2. The major peaks at 25.3, 37.8, 48.0, 53.8, 54.9 and 62.5 degrees are reflections from (101), (004), (200), (105), (211) and (204) planes of anatase, indicating the TiO2 developed existed in the anatase state. But, PT800 and PT850 were kept anatase-type structure even after heating at 800oC, though small amount of the rutile-type structure appears. On the other hand, PT800 and PT850 have an anatase-rutile mixture structure, partly transformed to rutiletype structure above 800oC. The identification of rutile peaks appearing in Fig. 2 could not be performed mainly because of its formation in only small amounts, which is reasonably supposed to be formed by the heat treatment of carboncoated TiO2 sample. The crystallinity of the all sample is improved by heating above 700oC, as indicated by sharpen- XRD patterns of carbon-coated TiO prepared at different heat treatment temperatures. Fig. 2. 2 ing of all the diffraction lines. For the elemental microanalysis of carbon-coated TiO2 sample as a function of different HTTs, these samples were analyzed by EDX. Fig. 3 shows the EDX spectra of carboncoated TiO2 sample. These spectra show the presence of C,

Preparation of Carbon-Coated TiO 2 at Different Heat Treatment Temperatures and Their Photoactivity 263 Fig. 3. EDX elemental microanalysis of carbon-coated TiO 2 prepared at different heat treatment temperatures; (a) PT700, (b) PT750, (c) PT800 and (d) PT850. O and S with strong Ti peaks, and some kinds of metals such as Si, Al and Cu. Most of these samples are richer in carbon and major Ti metal than any other elements. The quantitative elemental contents of heat treated carbon-coated TiO 2 sample series were compiled in Table 3. In the case of most of the samples, carbon and Ti were present as major elements in the carbon-coated TiO 2 sample. These results were observed for each sample show the spectra corresponding to almost all

264 M.-L. Chen et al. / Carbon Science Vol. 7, No. 4 (2006) 259-265 Table 3. EDX Elemental Microanalysis of Samples Prepared at Different Heat Treatment Temperature of Carbon-Coated Anatase Samples C O Ti Cu Others PT700 20.7 21.1 46.5 1.17 1.37 PT750 21.2 21.2 48.4 0.83 1.19 PT800 21.5 23.2 43.7 0.77 1.24 PT850 22.4 23.6 45.8 1.03 1.37 samples similar to C, O and Ti elements with an increase of HTTs. It should be note that the adsorption and photoactivity may be responsible for the transformation to from anatase to rutile by different HTTs. 3.3. Photoactivity In Fig. 4, absorbance of MB solution under UV irradiation, which is due to the adsorption and photoactivity of structural transformation of MB into the sample particles, was shown on the samples prepared with the different HTTs under various time conditions. As can be seen from the figure, the absorbance mixima for the all samples slowly decrease with increase of UV irradiation time. This implies that the transparent of the MB concentration highly increase by photocatalytic effect of carbon-coated TiO 2. These results clearly show two types of degradation of MB into carbon-coated TiO 2 particles; rapid adsorption and slow photocatalytic performance. The former is the adsorption of MB into micropore of the carbon coated layer which is supposed to have a high surface area. The latter is observed on the particles of all carbon-coated TiO 2 series degraded slowly by photocataytic reactivity. Fig. 4 shows the changes in relative concentration (c/c 0 ) of MB in the aqueous solution (c/c 0 ) on time of UV irradiation for the carbon-coated TiO 2 prepared at the different heat treatment temperatures. The changes in the relative concentration of MB in the aqueous solution with time of UV irradiation are plotted for the sample series. The relationship was shown approximately linearity, as reported on similar modified TiO 2 samples [11]. Because the present the carbon-coated TiO 2 samples had an adsorptivity, as above mentioned, it is considered that the decrease of MB concentration in the aqueous solution can be occurred in two physical phenomena such as adsorption by carbon and photocatalytic decomposition. The improvement in the crystalinity of anatase was reported to be advantageous for MB decomposition, and the decrease in specific surface area due to the grain growth and also phase transformation to rutile by heating were also reported to be unfavorable for MB decomposition [12]. According to earlier workers [13], MB Fig. 4. UV/VIS spectra of MB concentration against the carbon-coated TiO 2 prepared at different heat treatment temperatures under various time conditions; (a) PT700, (b) PT750, (c) PT800 and (d) PT850.

Preparation of Carbon-Coated TiO 2 at Different Heat Treatment Temperatures and Their Photoactivity 265 Fig. 5. Dependence of relative concentration (1.0 10 4 mol/l) of MB in the aqueous solution c/c 0 on time of UV irradiation for the carbon-coated TiO 2 prepared at different heat treatment temperatures. molecules absorbed energy from irradiation, thereby shifting their delocalized electrons from bonding to antibonding orbital. Since MB adsorption likely occurs via π-π interactions between its delocalized electrons and the carbon s graphene layers, it is reasonable that shifts in its electron orbitals would alter adsorption. Because the photocatalytic reaction is light excited, carbon deep inside TiO 2 is not easily accessible to light because of enhanced reflection and scattering by the support and the long traveling distance. In this study, the excellent photocatalytic activity of carbon-coated TiO 2 (especially, PT700 and PT750) could be attributed to the homogeneous coated carbon on the external surface and the structural anatase phase. Removal of MB in the solution was measured periodically over 50 min. For the all carbon-coated TiO 2 samples prepared from the different HTTs, slope relationship between relative concentration (c/c o ) of MB and t were observed at 1.0 10 4 mol/l of MB concentration. At slope relationship between c/c o of MB and t, it was obtained that removal effects of MB for the carbon-coated TiO 2 (especially, PT700 and PT750) are better excellent then that of pristine TiO 2. Based on these observations, we therefore can conclude that the degradation of MB concentration should be attributed to the both effects between photocatalysis of the supported anatase typed TiO 2 and adsorptivity of the carbon-coated particles derived from pitch. 4. Conclusion In this study, the properties of pitch based carbon-coated anatase-type titanium dioxide photocatalysts are investigated through the preparation from the different HTTs and the determination of their photoactivity. The developed photocatalysts were characterized with surface properties, surface structure, crystallinity between carbon and TiO 2, elemental analysis and photoactivity. The BET surface area for carboncoated TiO 2 prepared at various HTTs give a common independence on HTT. The SEM results present to the characterization of porous texture on the carbon-coated TiO 2 sample and carbon distributions on the surfaces. From XRD data, PT700 and PT750 were shown the X-ray diffraction patterns of the anatase TiO 2, but PT800 and PT850 were kept anatasetype structure even after heating at 800 o C, though small amount of the rutile-type structure appears. The results of EDX microanalyses were observed for each sample show the spectra corresponding to almost all samples similar to C, O and Ti elements with an increase of HTTs. Finally, the excellent photoactivity of carbon-coated TiO 2 (especially, PT700 and PT750) could be attributed to the homogeneous coated carbon on the external surface and the structural anatase phase. References [1] Ollis, D. F.; Pelizzetti, E.; Serpone, N. Photocatalysis fundamentals and applications, ed. Serpone, N.; Pelizzetti, E.; Wiley: New York, 1989. [2] Datye, A. K.; Riegel, G.; Bolton, J. R.; Huang, M.; Prairie, M. R. J. Solid State Chem. 1995, 115, 236. [3] Inagaki, M.; Hirose, Y.; Matsunaga, T.; Tsumura, T.; Toyoda, M. Carbon 2003, 2619, 2624. [4] Reztsova, T.; Chang, C. H.; Koresh, J.; Idriss, H. J. Catalysis 1999, 223, 235. [5] Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 69, 96. [6] Peill, N. J.; Hoffman, M. R. Environ. Sci. Technol. 1996, 2806, 2812. [7] Ferdinandez, A.; Lassaletta, G.; Jimennez, V. M.; Justo, A.; Gonjalez-Elipe, A. R. Appl. Catal. B. 1995, 49, 63. [8] Inagaki, M.; Kojin, F.; Tryba, B.; Toyoda, M. Carbon 2005, 1652, 1659. [9] Inagaki, M.; Kobayashi, S.; Kojin, F.; Tanaka, N.; Morishita, T.; Tryba, B. Carbon 2004, 3153, 3158. [10] Chen, M. L.; Bae, J. S.; Oh, W. C. Analytical science & Technology 2006, 301, 308. [11] Zhang, X.; Zhou, M.; Lei, L. Carbon 2006, 325, 333. [12] Inagaki, M.; Nakazawa, Y.; Hirano, M.; Kobayashi Y.; Toyoda, M. Int. J. Inorg. Mater. 2001, 809, 811. [13] Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental organic chemistry, 2nd ed. John Wily and Sons, 2002.