Materials Science Forum Vols. 675-677 (2011) pp 1233-1236 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.675-677.1233
Performance Improvement of TiO2/Ti Composite Photocatalyst Film by Heat Oxidation Treatment Yun Lu1, a, Hiroyuki Yoshida2, Keisuke Toh2, Liang Hao2 and Mitsuji Hirohashi1 1
Graduate school & Faculty of Engineering, Chiba University, Japan 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan 2 Graduate school, Chiba University, Japan a
[email protected]
Keywords: TiO2, composite photocatalyst, film, heat oxidation, mechanical coating technique, activity.
Abstract: In the present work, heat oxidation treatment was carried out for improving the photocatalyst activity of TiO2/Ti composite film. The microstructures and photocatalytic activity of the composite film were investigated. Effect of heat oxidation treatment was discussed. The results show that crystallinity of TiO2 in the composite film was improved by heat oxidation treatment. The photocatalyst activity was increased owing to the improvement of TiO2 crystallinity by heat oxidation treatment. Introduction Since 1970s, study and application of TiO2 photocatalyst have ever been paused for its high potential for environmental improvement [1-2]. Till now, film formation processes have been used to form photocatalysts with high performance [3]; besides, numerous techniques have been used in the formation/fabrication of TiO2 photocatalyst films on substrates of various materials with physical vapor deposition (PVD) and chemical vapor deposition (CVD) [4]. However, there are some problems existing in these techniques, for example, large and complicated equipments are required; besides, several of these techniques can be operated only in condition of high vacuum. In addition, it is very difficult to form a uniform film on spherical substrates such as balls or buttons. Therefore, a simple and economic film-coating technique is urgently expected. In this condition, we have proposed and carried out a new process, mechanical coating technique (MCT), to form TiO2 photocatalyst film. In the process of MCT, friction and abrasion are used effectively to form Ti film on the opposite conception of mixing powders in powder metallurgy process [5-6]. In first step, Ti film was formed on surface of Al2O3 balls using the friction and abrasion between Ti powder and the Al2O3 balls during the impact of balls/balls or balls/powder. Then TiO2 photocatalyst film was formed by oxidizing the Ti film at high temperature. From that work, although the resultant TiO2 photocatalyst film had rutile crystal form, it showed preferable photocatalytic activity. After that, we proposed and performed an advanced MCT, 2-step MCT, to form TiO2/Ti composite photocatalyst film with anatase TiO2 [7]. It is found that photocatalytic activity of the TiO2/Ti composite film by 2-step MCT strongly depends on crystallinity, volume and crystal form of TiO2. In the present work, heat oxidation treatment was carried out for improving crystallinity and increasing volume of TiO2 in TiO2/Ti composite film. The microstructures and photocatalytic activity of the composite film were investigated. Effect of heat oxidation treatment was also discussed. Experimental Formation of the Composite Film and Heat Oxidation Treatment. First, TiO2/Ti composite film was prepared by 2-step MCT proposed in the previous work [7] as shown in Fig.1. Ti film was formed on Al2O3 balls by MCT at the first step as the previous works [7-8]. Ti powder with 99.1% purity and an average diameter of 30 μm was used as the coating metal. Al2O3 balls with an average diameter of 1 mm were used as the substrates. A planetary ball mill (P5/4, Fritsch) was used for MCT. After that, the Al2O3 balls with
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1-step: forming Ti film Ti film and TiO2 powder (020-78675, Kishida Chemical Co. Ltd., Japan) with anatase crystal and an average diameter of 0.45 m were put Ti powder in + a pot in the second step. The received Al2O3 pot materials M10-Ti (Ti film) Planetary ball mill in the present work are listed in Table 1. Also, Al2O3 balls + to 2-step: forming TiO2/Ti film form strong composite film by high impact in the second step, Al2O3 or WC balls with a diameter of 10 mm were put in the pot. TiO2-S Al2O3 pot or Finally, heat oxid-ation treatments of the TiO2-K TiO2/Ti composite film were carried out in + Al2O3 balls with Al2O3 or WC balls air at 673, 773 and 873 K for 10 h. The TiO Planetary ball mill 2/Ti film sample markers for the above process are denoted as shown in Table 2, Oxidation treatment respectively. Fig.1 A schematic drawing of 2-step MCT. Analysis of the Microstructure and Photocatalytic Activity. The surface microstructure of the composite film formed by the above processes was examined by scanning electron microscope (S Table 1 The received materials in the present work. EM) (JEOL, JSM-5300LV) and crystal form Purity: 99.1％ was analyzed by X-ray diffraction (XRD) (J Ti powder Average diameter: 30 [μm] EOL, JDX-3530) with Cu-Kα radiations unSubstrate Al2O3 balls’ diameter: 1 [mm] der the condition of 30 kV and 20 mA, and TiO2 powder Average diameter: 0.45 [μm] spreading the Al2O3 balls coated with the (Anatase form) (TiO2-K) composite film on the holder. Table 2 Sample markers for TiO2/Ti composite Photocatalytic activity of the samples photocatalyst film. coated with the composite film was evaluated by measuring the decomposition TiO2 powder Impact ball Sample maker rate of methylene blue (MB) solution --CMxK-y (water solution) at room temperature. The TiO2-K Al2O3 ϕ10A-CMxK-y samples were spread uniformly on the WC ϕ10W-CMxK-y bottom of a cylinder-shaped cell with 18 Milling time x = 3, 6 [h] × 50 mm after washing the balls to remove Temperature of heat oxidation treatment y = 673, 773, 873 [K] adhesion substances adhered in the processes. To get the same starting condition of evaluating photocatalytic activity for all the samples, the pre-adsorption of MB was carried out on the samples using 3 ml MB solution with 20 where M = mol ･ l-1 before evaluating photocatalytic activity. In this step, the samples and MB solution were put into the cell and kept for over 12 h in a dark place. Then, the samples were again spread uniformly on the bottom of the cell and 7 ml MB solution with 10 was poured into the cell. After that, photocatalytic activity was evaluated under an intensity of 1 mW･cm-2 of the ultraviolet radiation for 24 h at room temperature. These conditions were referenced to Japanese industrial standard (JIS R 1703-2) [9]. The absorbance of MB solution was measured by a colorimeter (mini photo 10, Sanshin Industrial Co., Ltd.) with UV radiation with a wavelength of 660 nm, which is near the peak of absorption spectrum, 664 nm of MB solution used in the present work. The gradient, k nmol l-1 h-1 of the time-MB solution concentration curve was calculated by the least-squares method and the data from 1 to 12 h, and was used as the degradation constant.
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Results and Discussion TiO2/Ti Composite Film Formed by 2-step MCT and Heat Oxidation Treatment. A photograph of the samples coated with the composite film by 2-step MCT and heat oxidation treatment is shown in Fig.2. The samples lost the metallic luster and became bright comparing that of Ti film before heat oxidation treatment. Also, there are many different darkness areas and unevenness on surfaces of the balls. SEM micrographs of the samples' surfaces after heat oxidation treatment are shown in Fig.3. There was not an obvious
φ10ACM3K
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Fig.2 A photograph of the samples with TiO2/Ti composite photocatalyst film after heat oxidation treatment.
different area in the darkness and composition on the surfaces. In the case without heat oxidation treatment, there were light and dark areas which corresponded to Ti and TiO2 [7]. From the above comparison, it seems that 100μm (b) φ10W-CM3K-773 almost uniform composition of TiO2 on the surfaces was (a) φ10A-CM3K-773 obtained by heat oxidation treatment in present work. SEM micrographs of samples’ cross sections are shown in Fig.4. The film had a composite microstructure with two layers of TiO2 and Ti, which was TiO2/Ti composite film. The (d) φ10W-CM3K-873 (c) φ10A-CM3K-873 layer of TiO2 consisted of the piled TiO2 in the second Fig.3 SEM micrographs of the surfaces for TiO 2/Ti step and the oxidized TiO2 in heat oxidation treatment. It composite photocatalyst film after heat oxidation treatment.
hints that volume of TiO2 in the composite film was increased by heat oxidation treatment. XRD patterns of the TiO2 Al2O3 balls coated with TiO2/ Ti composite film by 2-step MCT adding the impact of large Al2O3 balls and heat Ti Al2O3 oxidized at 673 and 873 K are shown in Fig.5. Al2O3 30μm Ti peaks appeared from all the samples. TiO2 peaks were (a) φ10A-CM3K-873 (b) φ10W-CM3K-873 not detected from the samples ((a) and (d) in Fig.5) without Fig.4 SEM micrographs of cross-sections for TiO2/Ti composite photocatalyst film. heat oxidation treatment. It is because of still remaining the pulverization and decrease of TiO2 crystalline in the composite film owing to the impact of the large Al2O3 balls in the second step. On the other hand, anatase TiO2 peaks appeared due to improvement of TiO2 crystallinity in the composite film by heat oxidation treatment at 673 K from the samples ((b) and (e) in Fig.5). Besides, the peaks of anatase and rutile TiO2 appeared from the samples ((c) and (f) in Fig.5). It was due to the improvement of TiO2 crystallinity and the oxidation of Ti or phase transformation of TiO2 of the composite film in heat oxidation treatment at 873 K. Also, XRD patterns of the samples adding the impact of WC balls in the second step and heat-oxidized at 673 and 873 K are shown in Fig.6. The XRD patterns are similar with that of adding Al2O3 balls' impact as shown in Fig.5. However, improvement of TiO2 crystallinity in the composite film was not obvious like the case of adding Al2O3 balls' impact. TiO2(Anatase) TiO2(Rutile)
(f)φ10A-CM6K-873 (e)φ10A-CM6K-673 (d)φ10A-CM6K (c)φ10A-CM3K-873
X-ray intensity
X-ray intensity
Al2O3 Ti
Al2O3 Ti TiO2(Anatase) TiO2(Rutile) TiO2(Brookite)
(f)φ10W-CM6K-873 (e)φ10W-CM6K-673
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(a)φ10A-CM3K 30
40 50 2θ /deg Fig. 5 XRD patterns of TiO2/Ti composite film by 2-step MCT with Al2O3 balls and heat oxidation at 673 and 873 K.
(a)φ10W-CM3K
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Fig. 6 XRD patterns of TiO2/Ti composite film by 2-step MCT with WC balls and heat oxidation at 673 and 873 K.
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From the above results, the TiO2/Ti composite film with 300 300 anatase or anatase/rutile 200 200 TiO2 was obtained, especially, TiO2 100 100 φ10A-CM3K crystallinity of the 0 φ10A-CM6K 0 composite film was 300 400 500 600 700 800 300 400 500 600 700 800 improved by heat Temperature /K Temperature / K oxidation treatment. Fig. 7 Relationship between heat oxidation temperature and degradation constant of In evaluation of TiO2/Ti composite photocatalyst film by 2-step MCT and heat oxidation treatment. photocatalytic activity of the composite film, MB solution concentration decreased with UV irradiation time and photocatalytic activity appeared for all the samples. The degradation constants k, which is the gradient of the time-MB solution concentration curves, is shown in Fig. 7. It is found that the degradation constant was increased owing to improving TiO2 crystallinity by heat oxidation treatment at 673 K. However, the degradation constant was decreased in the cases of heat oxidation treatment over 673 K. It is probably resulted from decrease of the composite effect due to immoderate oxidation of Ti in the composite film. (a) With Al2O3 ball
400
(b) With WC ball
φ10W-CM3K φ10W-CM6K
k / nmol l-1h-1
400
Conclusion The TiO2/Ti composite film with anatase or anatase/rutile TiO2 was obtained; especially, TiO2 crystallinity in the composite film wasimproved by heat oxidation treatment. The degradation constant was increased owing to improvement of TiO2 crystallinity by heatoxidation treatment at 673 K. Beside, the degradation constant decreased in the cases of heat oxidation treatment over 673 K. It is probably resulted from decrease of the composite effect due to immoderate oxidation of Ti in the composite film. References [1] A. Fujishima and X. Zhang: C. R. Chimie Vol.6 (2006), p.750-760 [2] B. Ohtani: Journal of The Surface Finishing Society of Japan Vol.57 (2006), p.872-877 [3] Y. Zhao, X. Zhang, J. Zhai, J. He and A. Fujishima: Applied Catalysis B: Environmental, Vol.83 (2008), p. 24-29 [4] H. S. Nalwa: Handbook of thin film materials, (Academic Press, USA 2002). [5] Y. Lu, M. Hirohashi and S. Zhang: International Conference on Surfaces, Coatings and Nanostructured Materials (nanoSMat2005), Aveiro, PORTUGAL, 7-9th, September, 2005, Paper No. FP117. [6] H. Yoshida, Y. Lu and M. Hirohashi: The 5th International Forum on Advanced Materials Science and Technology (IFAMST 5), Zhangjiajie, China, 11-17th, June 2006, p.30. [7] Y. Lu, H. Yoshida, H. Nakayama, L. Hao and M. Hirohashi: 7th International Forum on Advanced Material Science and Technology (IFAMST 7), Dalian, China, 26-28th, June 2010, No.A-240. [8] H. Yoshida, Y. Lu, H. Nakayama and M. Hirohashi: Journal of Alloys and Compounds vol.475 (2009), p. 383-386 [9] Japanese Industrial Standard, JIS R 1703-2 (2007).
