Advanced Materials Research Vol. 650 (2013) pp 44-48 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.650.44
Production of Highly Efficient Photocatalytic TiO2 Powders by Mechanical Ball Milling Tugce Oztas1,a, Jongee Park*2,b and Abdullah Ozturk 3,c 1 2 3
AP Nanotechnology Co., R&D Division, Golbasi, Ankara 06830, Turkey
Atilim University, Metallurgical and Materials Engineering Department, Ankara 06836, Turkey
Middle East Technical University, Metallurgical and Materials Engineering Department, Ankara 06800, Turkey a
[email protected], b
[email protected], c
[email protected]
Keywords: Photocatalyst, TiO2, Ball Milling
Abstract. Highly efficient photocatalytic TiO2 powders were prepared using a conventional ball mill with various milling times of 0, 12, 24 and 48 h. The photocatalytic activity of the prepared TiO2 powders was evaluated using the decomposition rate obtained by methylene blue (MB) solution and acetic acid gas under UV light irritation. After 24 h milling, the particle size decreased from 555 nm to 122 nm without changing any of the crystal structure. The photocatalytic TiO2 powders prepared by 24 h milling decomposed 94% of the methylene blue solution while the nonmilled TiO2 powders provided only 61% decomposition. After the removal of acetic acid gas, it took 1.5 h for the 24h-milled powders to decompose 100%, while the non-milled TiO2 showed 73% decomposition with same UV illumination duration. Introduction TiO2 is a photocatalytic material and has been widely used because of its biological and chemical stability, non-toxicity, long lifetime, and low cost [1-4]. When it is exposed to the light, photocatalytic TiO2 oxidizes toxic substances and suspends them from the atmosphere by decomposing them into carbon dioxide, water, and other small molecules [3-6]. The activity of the photocatalyst depends upon several factors, including phase composition, crystallization level, the size of the crystal, and surface properties. Additionally, the activity of the photocatalyst is changeable depending upon the production method and the preparation conditions [7-9]. Basic research on and industrial development of a high-efficiency TiO2 photocatalyst are still attractive topics due to their prospective importance, and existing problems are still difficult to solve although TiO2 has been investigated for more than two decades [10-12]. Low photocatalytic efficiency is one of the main obstacles to the application of TiO2 powder. One of the common ways to enhance that efficiency is to load a noble metal or a non-metal onto the surface of the TiO2 powder without preventing the reaction on the surface of the photocatalyst [13-15]. However, this method of doping significantly increases the production costs. The present study was undertaken to produce a highly efficient photocatalytic TiO2 powder by mechanical ball milling. Ball milling is an effective process for size reduction of particles [16-18]. The final product contains nano-scale crystalline particles produced through a modification process using micron-sized starting material. After various milling durations, TiO2 powder was filtrated and dried for characterization. X-Ray powder diffraction (XRD) was employed to identify the crystalline phases that were present. Scanning Electron Microscope (SEM) and a particle size analyzer were used to examine the size reduction effect of the ball milling. The specific surface area was determined using the BET method. The photocatalytic activity of the pure and ball-milled TiO2 powders was investigated to assess the decomposition of methylene blue (MB) in aqueous solution and acetic acid gas under UV illumination.
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Experimental Procedure Preparation of the powder. A commercial photocatalytic TiO2 powder (NT-22) was supplied from Nano Co. in Korea and used as a starting material. This power has an anatase crystalline structure and its specific surface area is 101.5 m2/g. At first, 50 g of TiO2 powder and 250 ml of distilled water were placed into a 500 ml zirconia jar. The ratio of powder-to-ball was 1:15 during the milling process; furthermore, 5 mm-diameter zirconia balls were used as milling media. Milling was performed using 200 rpm for 0, 12, 24 and 48 h. According to the associated milling durations, the non-milled TiO2 powders and the milled TiO2 powders were designated as 0h-TiO2, 12h-TiO2, 24h-TiO2 and 48h-TiO2. After each mechanical milling, the mixture was taken from the jar and dried at 90°C for 3 h. Characterization. The phases of TiO2 powder were determined by X-Ray diffractometer (XRD) (Rigaku Geigerflex-DMAK/B). Each of the samples was scanned between 2θ of 20° and 90° with 2° velocity with periodical 0.02° risings for each minute. The microstructural morphology of the powder was observed using SEM (Nova Nanosem 430). Particle size measurements were made using a particle size analyzer (Malvern Mastersizer 2000). The specific surface area of the samples was calculated using the BET method. The photocatalytic activity of the TiO2 powder with different milling durations was defined using MB aqueous solution. The amount of MB in solution was 10 mg/l and this amount was fixed in all the measurements. A suspension was obtained by mixing 300 ml MB aqueous solution with 0.3 g of milled or non-milled TiO2 powder. Before the photo-radiation, this suspension was mixed continuously in the dark for 30 min using a magnetic mixer with a constant velocity of 500 rpm. Therefore, the TiO2 powder absorbed the MB solution completely and absorption-set back stability was provided. Afterwards, the suspension was put under the UV lamp (UVP Co., 360 nm), which was set at 100 Watt. In order to prevent any ultra-heating born of the UV lamp, the suspension was held in a water-cooling system during the test. In each 30 min increment, analytic samples were taken from the suspension and the UV-Vis absorption ratio of the solution was measured with 664 nm wavelength using UV-Vis Spectrophotometer (Sinco S-3100). The decomposition of the acetic acid vapor under UV light and the elimination of the environment amounts were evaluated with the aim of comparing the air-cleaning effects of milled and non-milled TiO2 powders. Next, 0.5 g photocatalytic powder was mixed with pure water and then placed into a 100 x 18 mm (diameter x depth) glass cap. Consequently, a smooth and homogeneous sheet was formed. This cap, which consists of TiO2, was placed into a 3-lt Tedlar bag that included 50 ppm of acetic acid gas. The variation of acetic acid concentration, which was held under UV light for 90 min, was measured every15 min with gas detector tubes (Gastec Co.) Results and Discussion As the milling time increased, the particle size decreased. Table 1 shows the changes in the average particle size with different milling durations. The average particle size of pure TiO2 was 555 nm. After 24 h milling, the particle size was shown to be a minimum of 122 nm. That size increased slightly to 128 nm within 48 h of milling, due to agglomeration. Table 1. Average particle size of TiO2 with different milling durations. Milling Time [h]
Average Particle Size [nm]
0 12 24 48
555 157 122 128
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The X-ray diffraction (XRD) patterns of the milled and non-milled TiO2 powders are shown in Fig. 1. Characteristic anatase (JCPDS #21-1272) peaks can be seen in all the diffraction patterns of the TiO2 powder. During the entire milling time, no change occurred in the crystalline patterns that describe the anatase phase. Furthermore, XRD analysis shows that no contamination was found. Therefore, it is obvious that no phase transformation or contamination took place during 48 h of mechanical milling.
Figure 1. XRD patterns TiO2 powders obtained after various milling durations The microstructural morphologies of the non-milled and milled-TiO2 powders are shown in Fig. 2. As can be seen, agglomeration and irregular pelleting was found in the non-milled TiO2 powders (Fig. 2(a)). However, as the duration of the milling process increased, the size of the TiO2 particles decreased and the distribution became homogeneous and regular. The specific surface area of the 24h-TiO2 sample showed a maximum of 186.5 m2/g, which can be compared to the surface area of the 0h-TiO2 sample (101.5 m2/g).
Figure 2. SEM micrographs of TiO2 powders obtained after various milling times; (a) 0 h, (b) 12 h, (c) 24 h, and (d) 48h.
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Fig. 3 shows a comparison of the decomposition rates of MB for the non-milled and 24h-milled TiO2 powders. Both of the samples showed photocatalytic activity under UV irradiation. However, when the milling timed increased, the photocatalytic activity was enhanced. Non-milled TiO2 decomposed 61% of MB, while 24h-milled TiO2 powders decomposed 94% of MB within 90 min of UV-irradiation. This suggests that a reduction in particle size increases the absorption capability of hydroxyl/water on the surface of TiO2, which serve as active redox sites in photocatalytic reactions [19]. Therefore, it can be said that mechanical ball milling is a practical way to increase photocatalytic activity in TiO2 powders.
Figure 3. Decomposition rate of MB solution Fig. 4 shows the decomposition rate of acetic acid gas by the 0h-TiO2 and 24h-TiO2 powder samples under UV illumination. After 90 min UV illumination, the 24h-TiO2 sample removed the acetic acid completely, but the 0h-TiO2 sample decomposed only 73% of the acetic acid gas. Similar to the results obtained from the decomposition of the MB solution, the milled TiO2 powders showed better photocatalytic activity than the pure powders with acetic acid gas due to the effect of the reduced particle size.
Figure 4. Decomposition rate of acetic acid gas Summary Photocatalytic TiO2 powders with a highly efficient activity were prepared with mechanical ball milling. When the milling period was increased, it was observed that the particle size of the TiO2 powders decreased without any phase change or contamination effect. Ball milling plays an important role in the photocatalytic activity of TiO2 powders. The TiO2 powders that were produced
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by ball milling showed better photocatalytic activity in decomposition of methylene blue and removal of acetic acid gas. Ball milling is a convenient technique for increasing the photocatalytic activity of TiO2 powder by decreasing its particle size. Acknowledgement The authors are very grateful for the financial support they received from Atilim University (Project No.: BAP-2010-04) and the Small and Medium Enterprises Development Organization of Turkey (KOSGEB). The second author of this paper is the corresponding author. References
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