Powder Technology 214 (2011) Contents lists available at SciVerse ScienceDirect. Powder Technology

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1 Powder Technology 214 (2011) Contents lists available at SciVerse ScienceDirect Powder Technology journal homepage: Synthesis of TiO 2 nanoparticles by a combined sol gel ball milling method and investigation of nanoparticle size effect on their photocatalytic activities Mansoor Farbod, Marzieh Khademalrasool Department of physics, Shahid Chamran University, Ahvaz, Iran article info abstract Article history: Received 12 April 2011 Received in revised form 27 June 2011 Accepted 28 August 2011 Available online 2 September 2011 Keywords: Titania Congo red Sol gel Ball milling Photocatalysis TiO 2 nanoparticles were synthesized by a sol gel method and the effect of ball milling of dried gel on the particle size has been investigated. The results show that the ball milling has a crucial role in preparation of nanosized TiO 2 powder. Also thermal treatment at different temperatures can control the size of prepared nanoparticles. The photocatalytic activities of TiO 2 nanoparticles with different sizes were studied by photocatalytic degradation of Congo red dye using a homemade photoreactor. The photo degradation of Congo red dye was monitored by UV Vis absorption measurements. It was found that the nanopowders which were synthesized at 550 C with average particle size of 55 nm show the maximum photocatalytic activities. These nanopowders which were a mixture of anatase and rutile phases did not show the same photocatalytic effect on Eriochrome black, Bromocresol green, Methyl orange and Rose Bengal dyes Elsevier B.V. All rights reserved. 1. Introduction Environmental contaminations due to dyestuff from textile and painting industries are a major source of environmental pollutions. Many attempts have been made in recent years to find suitable ways for removal of these pollutants from environment [1]. Utilization of photocatalytic properties of some semiconducting nanoparticles such as TiO 2 and ZnO, is a common way in this area [2]. Nanoparticles of TiO 2 (titania), due to their unique chemical, mechanical, optical and electrical properties have been widely used in variety of applications such as pigments, photocatalysis detectors, solar cells, purification of water and air [3 6]. Different methods have been employed to prepare titania nanostructures. Among them the sol gel technique is a powerful method which is employed by researchers to prepare titania in the form of powder, thin films and porous materials [7 11]. The photocatalytic property of TiO 2 was discovered in In this process the TiO 2 nanoparticles are illuminated with UV radiation and some electron-hole pairs are produced. Most of the pairs were recombined but the remaining by use of oxidation reduction reactions, produce OH(hydroxide) and O 2 (anion peroxide) radicals. These radicals which are themselves very powerful oxidant react with organic and toxic compounds, converting them to water, carbon dioxide and other substances [12 16]. In this work, TiO 2 nanoparticles were synthesized by sol gel method using titanium alkoxides tetra n-butyl titanate. In order to investigate the photocatalytic activities of TiO 2 nanoparticles, Congo red dye was chosen as the model pollutant. Congo red with the molecular formula of C 32 H 22 N 6 Na 2 S 2 O 6 is a diazo dye which is soluble in water and is used in textile and paper industries (Fig. 1) [16]. 2. Materials and methods 2.1. TiO 2 nanoparticle preparation The nanoparticles of titanium dioxide were synthesized by sol gel method in which the sol was obtained through hydrolysis of tetra n- butyl titanate. The tetra n-butyl titanate was added dropwise into the deionized water while it was vigorously stirred by a magnetic stirrer. The volume ratio of tetra n-butyl titanate to DI water was chosen 1:10. After dissolving the tetra n-butyl titanate, the stirring was continued for 30 min. The ph of the solutions was fixed at 7 by adding ammonia. A yellowish precipitate was obtained and then dried at Corresponding author. Tel./fax: address: farbod_m@scu.ac.ir (M. Farbod). Fig. 1. Chemical structures of Congo red dye /$ see front matter 2011 Elsevier B.V. All rights reserved. doi: /j.powtec

2 M. Farbod, M. Khademalrasool / Powder Technology 214 (2011) The dried powders divided in two parts. The first part (sample I) was ball milled with a lab-scale planetary mill for 30 min using a ball to powder mass ratio of 10:1. The second part (sample II) was left without milling. Both parts were then subjected to the same thermal treatments at different temperatures of 350, 400, 550, 600, 700, 800 and 900 C for 2 h Sample characterization Characterization of samples was performed by measuring SEM and EDX using a Leo, 1455 VP scanning electron microscope, TEM using a Leo, 906E transmission electron microscope and XRD (PW, 1840). UV Vis absorption measurement was done by use of a GBC, Cintra 101 spectrophotometer. The direct observation of color change was a supplementary method to ensure decolorization of dyes Photocatalytic activity measurements Fig. 2. XRD patterns of nanocrystalline TiO 2 at different calcinations temperatures. 50 C in 10 h. The formation of TiO 2 nanoparticles through hydrolysis take place based on the following reactions [17]: mti þ 4 þ 4mH 2 O mtiðohþ 4 þ 4mH þ ð1þ mtiðohþ 4 mtio 2 þ 2m H 2 O: ð2þ In order to measure the photocatalytic activity of different size nanoparticles, a dye solution was prepared with concentration of 20 mgl 1, by dissolving the dye powder in DI water. About 100 cm 3 of this solution was put in the photoreactor and stirred gently in darkness for 15 min afterwards the TiO 2 nanoparticles with concentration of 1 gl 1 were added to the solutions. After this stage the stirring was continued in dark for another 15 min with the intention of eliminating the pre produced electrons and holes due to the environmental light sources which can change the degradation rate. Also in order to measure how much dye adsorbs onto the surface of TiO 2 nanoparticles, the dark adsorption experiment was performed. In this experiment after stirring the mixture of dye and nanoparticles in dark for Fig. 3. SEM and TEM images of the TiO 2 nanopowders, (I) with milling and (II) without milling, calcined at different temperatures: (a) 400, (b) 550 and (c) 800 C.

3 346 M. Farbod, M. Khademalrasool / Powder Technology 214 (2011) min and separating the nanoparticles by centrifuging, the dye concentration was determined by taking the UV absorption spectra. The photoreactor consist of a magnetic stirrer with two 15 W UV lamps and the possibility of sampling and measuring the ph and temperature of the solutions. The dye solution with nanoparticles was illuminated 210 min while stirring and the sampling was taken in 30 min interval. The samples were kept in a quite dark place to prevent them from the other UV sources like sun and fluorescent lamps. By separating of the TiO 2 nanoparticles from the solution by centrifuging using a 10,000 rpm centrifuge, the UV visible absorption spectra of the clear solutions were then taken. Besides the change in the UV visible absorption spectra, the change in the solution color was observed. This was a supplementary test for the decomposition of the Congo red dye The dark adsorption experiment Before the photocatalytic activity measurement, it is necessary to distinguish the effect of dye adsorption by TiO 2 nanoparticles from their photocatalytic activities. The experiment was performed as explained in Section 2.3. Fig. 4 shows the UV Vis spectrum of different dyes before and after addition of TiO 2 nanoparticles. As can be seen the adsorption is not the same for different dyes. All the photocatalytic measurements were corrected for this effect Photocatalytic activity results The decomposition of dye solutions was performed by TiO 2 nanoparticles of different sizes. Fig. 5(a, b and c) shows the UV Vis 3. Results and discussion 3.1. XRD, TEM and SEM measurements It is well known that the titanium dioxide can be formed in anatase, rutile and brookite phases. The formation of these phases depends on the sintering conditions. Fig. 2 shows the XRD patterns of the TiO 2 nanopowders sintered at temperatures 350, 400, 550, 600, 700, 800 and 900 C. By comparing these patterns with the TiO 2 reference patterns, one can conclude that the samples synthesized at 350 and 400 C are pure anatase type. But those synthesized at 550, 600 and 700 C are a mixture of anatase and rutile types. The percentages of the anatase and rutile phases were calculated [18]. The anatase phase percentage in the sample synthesized at 550 C was 71.5% and for 600 and 700 C was at 61.5% and 5.9% respectively. The pure rutile powder was obtained by sintering the dry gel at 800 and 900 C. So a phase transformation from anatase to rutile occurs at C. We did not observe any change in XRD patterns of the samples I (milled before sintering) and samples II (without milling). The SEM and TEM images of the samples synthesized at 400, 550 and 800 C are shown in Fig. 3. The images which are denoted by (I) are the milled samples and those which are denoted by (II) are the samples without milling. Comparing the SEM and TEM images of I and II samples as the scale bars show, the particle size of milled samples are much smaller than that of the samples without any milling and shows the effect of ball milling of dry gel on the particle sizes. The particle sizes of two sets of samples were measured using microstructure measurement computer software. The averaged sizes are listed in Table 1. Table 1 also shows the grain size of samples I and II which are synthesized at temperatures 400, 550 and 800 C. The crystalline grain sizes of the samples were calculated using XRD data and the Debye Sherer formula. It can be observed that in both sets of samples by increasing the sintering temperature the grain size and particle size increased. But the sizes are much smaller for those samples that were ball milled before sintering. Table 1 Grain size and particle size of samples I and II, synthesized at temperatures of 400, 550 and 800 C. Temperature ( C) Without milling Crystalline grain size(nm) Average particle size(nm) With milling Crystalline grain size(nm) Average particle size(nm) Fig. 4. UV Vis spectrum of different dyes before (a) and after (b) addition of nanoparticles, exhibiting the effect of dye adsorptions.

4 M. Farbod, M. Khademalrasool / Powder Technology 214 (2011) Fig. 6. Normalized dye concentration of Congo red versus reaction time for different sized TiO 2 nanoparticles. Fig. 5. UV Vis spectrum of Congo red dye solutions in 30 min intervals. The decomposition was performed by different sized TiO 2 nanoparticles, a) 40, b) 55, c) 135 nm. spectrum of Congo red dye solutions which were taken at 30 min intervals which have been prepared at 400, 550 and 800 C and had average particle size of 40, 55 and 135 nm respectively. It can be seen that by increasing the UV illumination time, the absorbance is reduced. This means the dye concentration in the solutions has been reduced due to decomposition of the dye by TiO 2 nanoparticles. Also the decomposition rates are not the same for different size TiO 2 nanoparticles. Fig. 6 shows the normalized dye concentration versus reaction time for all different sized TiO 2 nanoparticles. One can observe that the degradation rates are not the same for the TiO 2 nanoparticles with different sizes and those which have been synthesized at 550 C are more efficient. Based on the XRD patterns these nanoparticles with an average size of 55 nm have a mixture of anatase (71.5%) and rutile (28.5%) phases. Fig. 7 shows the comparison of photocatalytic degradation of Congo red for different sized TiO 2 nanoparticles after 30 min UV illumination. It shows that there is an optimum nanoparticle size for which the photocatalytic efficiency is a maximum. The decrease in the photocatalytic behavior of smaller nanoparticles could be due to the increase in the surface recombination of photo generated electrons and holes. We tried also to investigate whether this optimum sized nanoparticles with maximum photocatalytic property have the same effect on other dyes, so the degradation rates of Bromocresol green, Rose bengal, Eriochrome black T and Methyl orange dyes were measured using the optimal sized nanoparticles and are shown in Fig. 8. This figure clearly shows the different effects of TiO 2 nanoparticles of the same size on different dyes. Fig. 9 shows the comparison of the photocatalytic degradation of different dyes using the optimum sized TiO 2 nanoparticles. The results in Fig. 9 are corrected for dye adsorption of nanoparticles. One can observe that the photocatalytic efficiency is not the same for different dyes. In fact many parameters like surface chemistry and morphology can affect the photocatalytic properties of a substance like TiO 2. But the degradation rate can be affected by chemical composition of the pollutant as well. Zeng et al. [19] have investigated the photodegradation of Rhodamine B by ZnO nanodisks and nanowires. They showed that the different morphologies have no the same photocatalytic activities due to the formation of different by-products with certain radicals on the surfaces of different morphologies which can change the reaction constant. In our experiment although the morphology of the different sized TiO 2 nanoparticles is assumed to be the same, but because the dyes are not the same and definitely have dissimilar radicals, the reaction mechanisms and times would not be the same. 4. Conclusion TiO 2 nanoparticles with different sizes were synthesized using sol gel method. The results showed the milling of dry gel is necessary to prepare nanosized particles. Also the sintering temperature could affect the particle size and crystal structure of samples. We found also that the photocatalytic properties of different sized nanoparticles Fig. 7. Degradation percentage of Congo red dye versus average particle size after 30 min UV illumination.

5 348 M. Farbod, M. Khademalrasool / Powder Technology 214 (2011) Fig. 8. UV Vis spectrum of (a) Eriochrome black T, (b) Bromocresol green, (c) Methyl orange and (d) Rose Bengal dyes solutions at 30 min intervals. Fig. 9. Comparison of photocatalytic degradation of different dyes using the optimal sized nanoparticles. are different and nanoparticles prepared at 550 C with an average particle size of 55 nm were more efficient in eliminating the dyes from water. These particles were a mixture of anatase (71.5%) and rutile (28.5%) phases. It was deduced that the TiO 2 nanoparticles with the same size did not have the same photocatalytic effect on different dyes. References [1] Q. Liu, X. Wu, B. Wang, Q. Liu, Preparation and super-hydrophilic properties of TiO 2 /SnO 2 composite thin films, Materials Research Bulletin 37 (2002) [2] S.N. Frank, A.J. Bard, Heterogeneous photocatalytic oxidation of cyanide and sulfite in aqueous solutions at semiconductor powders, The Journal of Physical Chemistry 81 (1977) [3] Y. Chen, A. Lin, F. Gan, Preparation of nano-tio 2 from TiCl 4 by dialysis hydrolysis, Powder Technology 167 (2006) [4] K.K. Akurati, A. Vita, U.E. Klotz, B. Bommer, T. Graule, M. Winterer, Synthesis of non-aggregated titania nanoparticles in atmospheric pressure diffusion flames, Powder Technology 165 (2006) [5] A.R. Gandhe, J.B. Fernandes, A simple method to synthesize N-doped rutile titania with enhanced photocatalytic activity in sunlight, Journal of Solid State Chemistry 178 (2005) [6] A.P. Ccaricato, M.G. Manera, M. Martino, R. Rella, F. Romano, J. Spadavecchia, T. Tunno, D. Valerini, Uniform thin films of TiO 2 nanoparticles deposited by matrix-assisted pulsed laser evaporation, Applied Surface Science 253 (2007) [7] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Formation of titanium oxide nanotube, Langmuir 14 (1998) [8] S. Sivakumar, P.K. Pillai, P. Mukundan, K.G.K. Warrier, Sol gel synthesis of nanosized anatase from titanyl sulfate, Materials Letters 57 (2002) [9] U.G. Akpan, B.H. Hameed, The advancements in sol-gel method of doped-tio 2 photocatalysts, Applied catalysis A: General 375 (2010) [10] B. Li, X. Wang, M. Yan, L. Li, Preparation and characterization of nano-tio 2 powder, Materials Chemistry and Physics 78 (2002) [11] T.V.D. Meulen, A. Mattson, L. Österlund, A comparative study of the photocatalytic oxidation of propane on anatase, rutile, and mixed-phase anatase-rutile TiO 2 nanoparticles: role of surface intermediates, Journal of Catalysis 251 (2007) [12] R. Benedix, F. Dehn, J. Quaas, M. Orgass, Application of titanium dioxide photocatalysis to create self-cleaning building materials, LACER No. 5 (2000) [13] G. Balasubramanian, D.D. Dionysiou, M.T. Suidan, I. Baudin, J.-M. Laˆıné, Evaluating the activities of immobilized TiO 2 powder films for the photocatalytic degradation of organic contaminants in water, Applied Catalysis B: Environmental 47 (2004) [14] M.E. Pena, G.P. Korfiatis, M. Patel, L. Lippincott, X. Men, Adsorption of As(V) and As(III) by nanocrystalline titanium dioxide, Water Research 39 (2005) [15] Z. Li, W. Shen, W. He, X. Zu, Effect of Fe-doped TiO 2 nanoparticle derived from modified hydrothermal process on the photocatalytic degradation performance on methylene blue, Journal of Hazardous Materials 155 (2008) [16] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, Ch. Guillard, J.-M. Herrmann, Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania, Applied Catalysis B: Environmental 39 (2002) [17] T. Sano, E. Puzenat, Ch. Guillard, Ch. Geantet, S. Matsuzawa, Degradation Of C 2 H 2 with modified-tio 2 photocatalysts under visible light irradiation, Journal of Molecular Catalysis A: Chemical 284 (2008) [18] R.A. Spurr, H. Myers, Quantitative analysis of anatase-rutile mixtures with an X-ray diffractometer, Analytical Chemistry 29 (1957) 760. [19] J.H. Zeng, B.B. Jin, Y.F. Wang, Facet enhanced photocatalytic effect with uniform single-crystalline zinc oxide nanodisks, Chemical Physics Letters 472 (2009)