Plasma surface modification of TiO 2 photocatalysts for improvement of catalytic efficiency

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1 Surface & Coatings Technology 200 (2005) Plasma surface modification of TiO 2 photocatalysts for improvement of catalytic efficiency Chung-Kyung Jung *, I.-S. Bae, Y.-H. Song, J.-H. Boo * Department of Chemistry, Sungkyunkwan University, Suwon , Korea Available online 30 August 2005 Abstract We have deposited titanium dioxide (TiO 2 ) thin films on glass using titanium (IV) isopropoxide (Ti[OCH(CH 3 ) 2 ] 4, 97%) by sol gel processing. In order to elevate photocatalytic activity of the as-grown TiO 2 films, argon and oxygen plasmas ignited by radio-frequency (RF) and microwave (MW) under both vacuum and atmosphere conditions were also used in the range of W within 30 min at room temperature. Photocatalytic activity was evaluated by the measurements of the contact angle, UV/vis irradiation, and toluene removal test. In this work, the effect of the plasma with photocatalyst (TiO 2 ) on the improvement of hydrophilic properties has mainly been investigated. A superhydrophilic property and surface morphology change appeared in the light irradiation with O 2 plasma treatment. Based on this work, we confirmed that the plasma treatment method was very reliable method for the synthesis of TiO 2 thin films with high catalytic performance. D 2005 Elsevier B.V. All rights reserved. Keywords: TiO 2 -based photocatalysts; Surface modification; RF and MW plasma treatment 1. Introduction Titanium dioxide (TiO 2 ) has many unusual properties that make it suitable for a variety of thin film applications. Moreover, TiO 2 film has been known as a useful photocatalytic material because it is stable, photosensitive, and low cost [1]. TiO 2 is being widely commercialized due to high photocatalytic efficiency, chemical stability and wide range of application [2 4]. Photocatalyst absorbs light and causes various chemical reactions. Under the condition of light energy, which is bigger than band gap energy (<380 nm) of the TiO 2, it occurs photo-oxidation, photo-reduction, and hydrophilic reaction [5 7]. This reactivity can be used for a variety of uses. The sol gel technique is relatively low cost and deposition parameters, including composition, are easily modified, thus offering a promising alternative to vacuum deposition [8 10]. Especially, plasma treatment for surface modification is used to produce hydrophobic or hydrophilic surfaces on metals, plastics or polymers [11].To * Corresponding authors. Tel.: ; fax: addresses: ckjung7818@skku.edu (C.-K. Jung), jhboo@skku.edu (J.-H. Boo). make the best photocatalyst with high catalytic efficiency and chemical stability as well as long lifetime, in this paper, we propose and demonstrate a surface modification process of TiO 2 photocatalyst, which can offer higher mechanical durability to water-affinity thin films. This process consists of TiO 2 sol coating and the following O 2 plasma treatment by RF and MW plasma system. The optical properties of the water-affinity thin films prepared using TiO 2 thin films with/without oxygen-plasma treatment method are also investigated for comparative study. Also, there is no detailed mechanism study on the photocatalytic reaction with TiO 2 sol treated by various plasmas. Therefore, this study is about photocatalyst TiO 2 thin films grown on glass by sol gel method with/without RF and MW plasma treatment to understand the role of the difference photocatalytic reaction mechanism as well as improvement technology of activity by surface modification. 2. Experimental TiO 2 thin films were deposited on glass by sol gel method with a titanium isopropoxide sol (TiO 2 coating sol /$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi: /j.surfcoat

2 C.-K. Jung et al. / Surface & Coatings Technology 200 (2005) %), in which precursor was used as that of Refs. [9,10]. A coating sol for TiO 2 -based photocatalysts was also prepared by the sol gel method, using titanium tetrisopropoxide, acid (HCl, HNO 3, HF etc.) and water. Titanium tetraisopropoxide was slowly dropped into the 0.4% nitric acid solution after stirring it vigorously for 2 h at room temperature, and heated at 80 -C for 24 h. During the reaction, isopropanol was removed by distillation and then the milky-bulk reaction solution was gradually changed to blue fine-milky solution. For the deposition of TiO 2 photocatalyst, the glass substrates were in first degreased, cleaned thoroughly and dried before deposition. The substrate was dipped into the viscous TiO 2 precursor sol and pulled out with a uniform pulling rate of 5 mm/s, and then dried at room temperature for 6 h. The plasma-treated TiO 2 samples were prepared by radio-frequency (RF, MHz) discharge under 25 Pa of O 2 plasma at the power range of W at 295 K for 30 min and by microwave (MW, 2.45 GHz) discharge under 25 Pa of O 2 plasma at the power range of W at 295 K for 5 min. After plasma treatment of the dip-coated TiO 2 thin films, their optical and structural properties were characterized with contact angle as well as scanning electron microscope (SEM). The surface composition following oxygen plasma exposure has been characterized by X-ray photoelectron spectroscopy (XPS). To make the best photocatalysts by surface modification, moreover, we also performed the phenol and toluene removal test. Especially, in order to aging effect, we have measured their contact angles again after 1 year to check durability of photocatalytic TiO 2 films, respectively. 3. Results and discussion Photocatalytic TiO 2 thin films have been characterized by using contact angles. Fig. 1 shows contact angles for TiO 2 films deposited on glass using TiO 2 sol with various conditions. Fig. 1 shows the variation of contact angles as a Contact angle ( 0 ) S1 S2 S3 S4 S5 S6 Sample Conditions Fig. 1. Contact angle of water droplet on TiO 2 films produced by sol gel method. Photos of water droplets on the surface of glass, asdeposited film, RF discharge at 50 W, RF discharge at 200 W, MW discharge at 50 W, and (f) MW discharge at 200 W. (f) Fig. 2. Contact angle images of TiO 2 thin films; as-deposited TiO 2 without sunlight, as-deposited TiO 2 with sunlight, and with sunlight after 1 year, respectively. function of sol coating, RF power, and MW power with/ without O 2 plasma treatment. Fig. 1 and show the contact angle obtained from TiO 2 coating layers on glass substrates with/without TiO 2 sol. The result of contact angle in Fig. 1 provides a gradual hydrophilic effect rather than only in glass (Fig. 1). From Fig. 1 to (f), contact angle of O 2 plasma-treated TiO 2 surfaces within 200 W RF and MW power decreased from an initial value of 16- to 5-. It means that the contact angle of TiO 2 surface treated with O 2 plasma decreased more rapidly than without O 2 plasma treatment under the same conditions. Moreover, we have also measured quite low contact angle even after 1 year, signifying that our photocatalysts with O 2 plasma treatment will have long lifetime and durability. Fig. 2 shows the real image of contact angles taken by CCD camera with the TiO 2 films without light 2 as well as with light 2 and 2 after 1 year, respectively. Fig. 2 shows a contact angle image of TiO 2 film taken with O 2 plasma treatment, indicating strong hydrophilicity and long durability. The contact angle of TiO 2 surface treated with RF and MW using O 2 plasma decreased more rapidly than without RF and MW under the same conditions. It means that more effective photocatalytic TiO 2 films with hydrophilic surface and high surface energy could be obtained using O 2 plasma treatment with RF and MW system rather than without plasma treatment. Moreover, we have measured quite low contact angle even after 1 year,

3 1322 C.-K. Jung et al. / Surface & Coatings Technology 200 (2005) signifying that our photocatalysts with O 2 plasma treatment will have long lifetime and durability. Fig. 3 shows the typical scanning electron microscopy (SEM) images of the obtained TiO 2 films grown on only sol coating and MW 50 W treated TiO 2, respectively. It can be seen clearly that the small grain size was grown on the glass and the surface of deposited TiO 2 catalyst. Moreover, the grain sizes of TiO 2 are decreased with O 2 plasma treatment both RF plasma and MW plasma. The surface of TiO 2 films grown on glass has grain size of 40 nm using only sol coating and 10 nm using O 2 plasma treatment, respectively. The surface of treated TiO 2 density was decreased with RF and MW 50 W plasma power, however, the surface morphologies were gradually changed by the development of typically quite smooth surfaces and high density with increasing RF and MW 200 W plasma power. In conclusion, higher density and smaller grain size of TiO 2 films were obtained from RF and MW 200 W plasma power rather than RF and MW 50 W plasma power. Fig. 4 shows typical XPS spectra obtained from as-deposited TiO 2 films and treated TiO 2 films that are grown at room temperature under 25 Pa of O 2 plasma at the power range of W and different treatment time using RF and MW plasma system. Fig. 4 shows the Ti 2p and O 1s peak measured by regional scans. Different XPS profile Intensity (cps) Intensity (cps) no treat RF 50 W MW 50 W RF 200 W MW 200 W 2p 1/ () ( ) ( ) a Binding Energy (ev) 2p 3/2 O 1s Ti 2p O1s Binding Energy (ev) Fig. 4. High-resolution XP spectra of TiO 2 photocatalysts synthesized with/ without RF and MW plasma treatment; Ti 2p and O 1s. Fig. 3. Typical SEM images of photocatalytic TiO 2 films; as-deposited TiO 2 and MW discharge at 50 W. results for O 2 plasma-treated and untreated TiO 2 samples were obtained. For the TiO 2 films treated by O 2 plasma treatment, the Ti 2p peaks are decreased while RF and MW power is increased. This means that the amounts of Ti atoms in the film surface region are lower than O atoms. Therefore, the Ti 2p peak intensity is decreased with increasing plasma power. In the spectrum of O 1s (Fig. 4), the spectrum can be seen with two components located at ev and ev. These two O states are attributed to OH species, and show an increasing tendency of binding energy with increasing RF and MW power. Especially, with increasing RF and MW power 200 W, OH species of TiO 2 films were obtained from RF plasma power rather than MW 200 W plasma power, respectively. Fig. 5 also shows the time course of the phenol and toluene concentration for various treated TiO 2 surfaces using UV irradiation. Plots denote the experimental data. There is a general agreement that this behavior is related to the accumulation of strongly absorbed toluene on the active

4 C.-K. Jung et al. / Surface & Coatings Technology 200 (2005) Concentration (ppm) Concentration (ppm) 200 Phenol min. 60 min. 90 min. 120 min. Toluene Time 0 min. 5 min. 15 min. 30 min. 60 min. Time a a a a a a a TiO2 coating RF plasma treatment MW plasma treatment Fig. 5. Changes of concentrations with time obtained during photocatalytic decomposition reactions of phenol and toluene solutions, respectively, using TiO 2 photocatalysts synthesized with/without RF and MW plasma treatment. sites [12]. The phenol in the solution was quickly decomposed with RF plasma treatment higher than with only coating and MW plasma treatment. This result suggested that the catalytic property of the TiO 2 for degradation of organic compound appeared by RF and MW plasma treatment. The degradation rate of phenol increased with RF plasma treatment, because catalytic surface area working for the radical formation by photocatalytic reaction was increased. Especially, we have obtained for the photocatalytic oxidation of toluene degradation. Fig. 5 shows photocatalytic oxidation data obtained in the presence of surfactant and shows loss of toluene concentration during the photocatalytic reaction. As can be seen from the results, increasing reaction time lengthens with decreasing toluene concentration after reaching the concentration of about 0% for 60 min with/ without TiO 2 coating. However, the amount of absorbed toluene was higher for treated TiO 2 coating than for untreated TiO 2 coating. This means that based on the difference of initial slope of each photocatalysts, the TiO 2 film with O 2 plasma treatment has 1.5 times higher photocatalytic activity than that without O 2 plasma. This result also indicates that increase of UV irradiation time in aqueous causes increase of removal degree of toluene with RF plasma treatment rather than with MW plasma treatment. 4. Conclusions Photocatalytic TiO 2 films have been deposited on glass substrates using titanium isopropoxide different plasma treatment in the range of W by RF and MW plasma system after sol gel processing. The RF and MW plasma-treated TiO 2 photocatalyst shows much more catalytic efficient than that of TiO 2 coating layer without plasma treatment. The contact angle of water decreases as oxygen plasma treatment power increased since hydrophilic functional groups such as non-stoichiometric TiO x were introduced on the TiO 2 surface by oxygen radicals in the plasma. Moreover, photocatalytic TiO 2 films with RF plasma treatment have maintained low contact angle even after 1 year. We have also obtained about 10 nm grain sizes with plasma treatment rather than without plasma treatment by SEM analysis. In order to better understand the differences between RF plasma treatment and TiO 2 sol or MW plasma treatment in terms of surface parameters, we have undertaken their characterization by XPS analysis. The degradation of phenol and toluene by UV irradiation in the presence of TiO 2 was investigated. With increasing O 2 plasma treatment, the degradation of phenol and toluene increased. Moreover, RF plasma-treated TiO 2 showed excellent photocatalytic activity on phenol and toluene rather than TiO 2 sol coating or MW plasma-treated TiO 2 surface, respectively. Acknowledgements Support of this research by the Ministry of Science and Technology in Korea is gratefully acknowledged. This work was also supported by the BK21 project of the Ministry of Education, Korea, and by the Center for Advanced Plasma Surface Technology at the Sungkyunkwan University. References [1] N. Tohge, G. Zhao, F. Chiba, Thin Solid Films 351 (1999) 85. [2] I. Sopyan, M. Watanabe, S. Murasawa, K. Hashimoto, A. Fujishima, J. Electroanal. Chem. 415 (1996) 183. [3] T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima, K. Hashimoto, Thin Solid Films 351 (1999) 260. [4] C.H. Ao, S.C. Lee, J.Z. Yu, J.H. Xu, Appl. Catal., B Environ. 54 (2004) 41. [5] J.C. Kennedy, A.K. Datye, J. Catal. 179 (1998) 375. [6] V. Augugliaro, E. Garcia Lopez, V. Loddo, G. Marci, L. Palmisano, M. Schiavello, J. Soria Ruiz, Catal. Today 54 (1999) 245.

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