Synthesis of Nanosized Anatase Particles from Commercial Rutile Powder by Using Hydrothermal Method

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1 Chiang Mai J. Sci. 2008; 35(1) CE Chiang Mai J. Sci. 2008; 35(1) : Contributed Paper Synthesis of Nanosized Anatase Particles from Commercial Rutile Powder by Using Hydrothermal Method Ubonwan Chutiphunphinyo [a], Siriporn Larpkiattaworn [b] and Pornapa Sujaridworakun* [a] [a] Research Unit of Advanced Ceramics, Department of Materials Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand. [b] Thailand Institute of Scientific and Technological Research (TISTR), 35 Klong Luang, Pathum Thani 12120, Thailand. *Author for correspondence; pornapa.s@chula.ac.th Received: 20 September 2007 Accepted: 15 October 2007 ABSTRACT Nanosized anatase particles with size ranging from 5.73 nm to 8.26 nm were prepared by low temperature hydrothermal process (150 o C) using commercial rutile powder as a starting material. Various parameters such as ph, hydrothermal temperature and reaction time were studied. It was found that, the degree of crystallinity, crystallite size, and specific surface area of anatase powder can be controlled by varying these parameters. The synthesized powders were characterized by XRD, SEM, BET and UV-VIS spectrophotometer. The results show that ph value affected obviously on the photocatalytic activity of anatase. The anatase obtained under ph7, hydrothermal at C for 6h has crystallite size about 8.26 nm and specific surface area m 2 /g which its photocatalytic activity closes to that of commercial photocatalyst (ST-01). Keywords: Nanosized, anatase, rutile, hydrothermal. 1. INTRODUCTION Titanium dioxide (TiO 2 ) is one of the basic materials in everyday life. It has three main structures (brukite, anatase and rutile), each with their own physicals properties. Low cost TiO 2 in rutile form has been widely used as white pigment in paints, cosmetics, foodstuffs, etc. On the other hand, most researchers had been reported that anatase crystallites have relative high photocatalytic activity than other forms of TiO 2 [1-3]. Recently, most study has been focused on the synthesis of nanosized TiO 2 anatase due to its strong photocatalytic activity when irradiated with UV light. This photocatalytic reaction resulted from the production of an excited electron in the conduction band of the nanosized TiO 2, along with a corresponding positive hole in the valence band after the absorption of UV radiation. These excited species play an important role in giving rise to various chemical reactions because the resulted radical species exist on the surfaces of the nanosized TiO 2. In addition, because of their relatively long lifetime, the electrons and holes can be allowed to travel to the crystallite surface. Plenty of pollutants, such as NO x,

2 2 Chiang Mai J. Sci. 2008; 35(1) SO x and other toxic organic compounds can be detoxified by these species existing on the surface of the nanosized TiO 2 photocatalysts. As a result, there has been growing interest in preparing nano-tio 2 powders used as excellent photocatalysts in detoxifying the waste water derived from the industry production. Various methods have been used to prepare of TiO 2 nanopowders such as solgel, homogenous precipitation, flame synthesis etc. However, these methods required post calcinations at high temperature to decompose organic precursor, and to crystallize TiO 2. Moreover, most research used alkoxide as a precursor which is high cost and it needs tight control experimental conditions [1,3, 4]. In contrast, hydrothermal method is a genuinely low temperature technique for TiO 2 synthesis. It is a single step and well established process to prepare single crystals and fine powders [5]. In this study, we had an attempt to synthesize and characterize nanosized anatase from low cost commercial rutile powder by simple hydrothermal method, which has not been found report working on this. 2. MATERIALS AND METHODS Commercial rutile powders (98% TiO 2 ) with approximate 0.2 micron in size and surface area of about m 2 /g, was used as a precursor. It was mixed and continuously stirred in concentrated sulfuric acid (95% AR grade, Merck) under heating until clear solution was obtained. Then, it was adjusted ph (3-7) with NH 3 solution (25% AR grade, Merck) and poured in Teflon container which was sealed inside stainless steel autoclaves. Next, it was heated in oven at 120 and C for 6-24 h. After cooling down in air, the products were neutralized by distilled water, centrifuged to separate the powders and dried at 80 o C in an oven for a night. Finally, the obtained powder was characterized for phase structure, specific surface area and morphology by XRD (Bruker, D8 Advance), BET (Coulter SA 3100) and SEM (JEOL JSM 6400), respectively. The crystallite sizes were calculated by Scherrer equation : D c = Kλ/B cos θ, Where D c accounts for the average crystal size, K = 0.89 is the Scherrer coefficient, λ is nm, which is the x-ray wavelength, B is the full width at half-maximum (FWHM) and θ is the diffraction angle. The photocatalytic activity was evaluated through the degradation of methyleneblue solution by using 0.01 g catalyst sample suspended in 20 ml of 0.02 mm methyleneblue solution. The suspension was first stirred in dark condition for 1 h before irradiation under black lamp (NEC, FL10BL- B,intensity 2 mw/cm 2 ). The irradiated suspensions were centrifuged to separate catalyst powder from solution, only clear solution was used for analyzing photocatalytic degradation of methylene blue using UV-VIS spectrophotometer (Perkin Elmer inst., Lambda 35). 3. RESULTS AND DISCUSSION XRD patterns of starting material and the synthesized powders were given in Figure1. It showed that the starting material was pure rutile phase and the powders synthesized under our hydrothermal conditions, ph 3-7, temperature of o C, reaction period for 6-24 h consisted of only anatase phase. In general, the crystallite size is regarded as the main factor that causes peak broadening, the smaller size gives broader peak. Thus the broadening peak of anatase was caused by its small crystallite size whereas large crystal size of rutile precursor showed sharp peaks with high intensity. It can be noticed that the crystallinity of anatase phase obtained at different preparation conditions was slightly different which determined by the intensities and half widths of the main diffraction peak

3 Chiang Mai J. Sci. 2008; 35(1) 3 of anatase at 2 = 25.2 (Figure 1). It was found that the anatase crystallite size calculated from scherrer equation increased when preparation ph increased from 3 to 7 as shown in table 1. Moreover, higher temperature and longer reaction period enhanced the growth of anatase crystal. The results suggested that at the same synthesis temperature and time, ph value had an obvious effect on the crystallite size and surface area of powders. Surface area of synthesized powder was ranging from to m 2 /g which were much higher than that of starting material. Figure1. XRD patterns of synthesized TiO 2 and commercial rutile powders (starting material). Table 1. Crystallite sizes, phases and surface area of synthesized TiO 2 at various hydrothermal synthesis conditions. No. ph value temperature ( 0 C) time (h) crystallite sizes (nm)* phases surface area (m 2 /g) anatase anatase anatase anatase anatase * calculated by Scherrer equation. The morphology of starting powder and synthesized powders observed through SEM was shown in Figure 2. SEM images show well defined spherical morphology of rutile particles with a diameter range of submicron, whereas the synthesized powders consisted of aggregated nanocrystalline anatase. Due to its very small in size of anatase, nanoscale, each particle had high driving force to agglomerate, thus the morphology shown by SEM was secondary particles of agglomerated nanosized anatase. The accurate crystallite size observed by TEM could be used to confirm the value calculated by Scherer equation.

4 4 Chiang Mai J. Sci. 2008; 35(1) (a) (b) (c) (d) Figure 2. SEM micrographs of synthesized TiO 2 at (a) ph C for 6h (b) ph C for 6h (c) ph C for 6h (d) starting rutile powder Figure 3. Decreasing in absorbance at wavelength 664 nm of methylene blue in suspensions with irradiation time of starting rutile powder, synthesized anatase (150 o C for 6 h) and commercial photocatalyst ST-01.

5 Chiang Mai J. Sci. 2008; 35(1) 5 The photocatalytic activity of the anatase synthesized at various conditions comparing with the starting rutile powder and commercial photocatalyst (ST-01) investigated by decreasing in absorbance at wavelength 664 nm of methylene blue solution was presented in Fig. 3. It showed that rutile precursor had very poor photocatalytic activity after irradiated under UV lamp comparing with other synthesized anatase. It can be seen that the efficiency of photocatalytic degradation of methylene blue of anatase synthesized at ph 7 was better than that of ph 5 and ph 3, respectively, which was close to the photocatalytic activity of commercial photocatalyst (ST-01). On the other hand, we found that the increase in reaction time did not promote the photocatalytic activity of anatase synthesized at ph 3. From these results, it proved that the preparation ph value was a significant factor influenced the photocatalytic efficiency. Generally large surface area tends to exhibit better photocatalyst activity, due to more active sites for adsorbing methyleneblue solution. However, large surface area does not always give better photocatalytic activity as shown by this study. It has been proven that the low activity could be due to its poor degree of crystallinity, which is not compensated by the positive effect given by its high surface area. This is because of a low degree of crystallinity would imply a great amount imperfections and defects in the TiO 2 structure which could act as recombination center for e-hole pairs and lead to lower efficiency for photocatalytic process[ 6 ]. As a result, an enhancing in photocatalytic activity with the increase in preparation ph observed in this study is presumably due to the increase in the degree of crystallinity. 4. CONCLUSIONS Nanosized anatase TiO 2 with high efficiency photocatalytic activity has been synthesized by a simple low temperature hydrothermal method using commercial rutile powder as a precursor. This study has showed that the preparation conditions, especially ph, has obviously affected the physical properties of synthesized powder and influenced on its photocatalytic activity. It can be suggested that the photocatalytic activity of anatase TiO 2 is dependent on the compromised factors between crystallinity and surface area. ACKNOWLEDGEMENTS This work was supported by the CU. Graduate School Thesis Grant and the Research Unit of Advanced Ceramics, Department of Materials Science, Faculty of Science, Chulalongkorn University, Thailand. The authors would like to thank Mr. Wittaya Panpa for his help in the experiment and using characterization equipments. REFERENCES [1] Spicera P.T., Chaoulb O., Tsantilisc S. and Pratsinisc S.E., Titania Formation by TiCl 4 Gas Phase Oxidation, Surface Growth and Coagulation, J. Aeros. Sci., 2002; 33: [2] Yang S., and Gao L., Preparation of Titanium Dioxide Nanocrystallite with High Photocatalytic Activities, J. Am. Ceram. Soc., 2005; 88: [3] Tang W. S., Wan L., Wei K. and Li D., Preparation of Nano-TiO 2 photocatalyst by Hydrolyzation-precipitation Method with Metatitanic Acid as the Precursor, J. Mater. Sci., 2004; 39: [4] Sivakumar S., Krishna Pillai P., Mukundan P., and Warrier K.G.K., Sol-gel Synthesis of Nanosized Anatase from Titanyl Sulfate, Material Letters., 2002; 57: [5] Kolen Y.V., Churagulov B.R., Kunst M., Mazerolles L. and Justin C.C., Photocatalytic Properties of Titania Powders Prepared by Hydrothermal Method, Appl.Catal.B., 2004; 54: [6] Sakthivel S., Hidalgo M.C., Bahnemann D. W., Geissen S.U., Murugesan V. and Vogelpohl A. Appl. Catal B., 2006; 63: 31-40