Synthesis of photocatalytic TiO₂ nanoparticles at low cost

LIU Qing-ju(柳清菊)¹, ZHOU Mi(周  密)², LIU Qiang(刘  强)¹,

ZHANG Jin(张  瑾)¹, ZHU Zhong-qi(朱忠其)¹

1. Department of Material Science and Engineering, Yunnan University, Kunming 650091,China;

2. Joinyou Fengyu Science and Technology Company Limited, Kunming 650101,China

Received 10 April 2006; accepted 25 April 2006

Abstract: Anatase TiO₂ nanoparticles were synthesized by using HTiO₃ as raw material and adopting TiOSO₄ solution neutralizing process. The results show that the hydrolytic reaction temperature and time, molar ratio of urea to Ti precursors (R), volume ratio of distilled water to the TiOSO₄ solution (F) determine the yield of TiO₂; the average crystalline size of TiO₂ nanoparticles can be controlled through the sintering temperature and the optimum temperature is about 600 ℃; the TiO₂ particles have good photocatalytic activity for decolorization of methyl orange.

Key words: synthesis; hydrolytic efficiency; particle size; photocatalytic activity; TiO₂; TiOSO₄

1 Introduction

Anatase TiO₂ has been widely used for different environmental applications, such as photocatalytic purification and treatment of water and air[1,2], sterilization, deodorization[3-7] and water photosplitting into H₂ and O₂[8]. The photocatalytic activity of TiO₂ nanoparticles is strongly dependent on the size and the extensive application is up to the production cost. In order to get TiO₂ nanoparticles, several preparation methods have been reported, such as sol-gel method, chemical vapour deposition, precipitation and mechanical alloying[9]. However, the preparation of TiO₂ nanoparticles by using cheap HTiO₃ as raw material and adopting TiOSO₄ solution neutralizing process has not been reported yet. This work is devoted to study the influence of different parameters such as reaction and sintering temperature, quantity of urea and water, on the preparation of TiO₂. The nanoparticles were characterized with an X-ray diffractometer(XRD), the photocatalytic activity of these materials was evaluated using aqueous methyl orange as pollutant.

2 Experimental

The preparation of TiO₂ nanoparticles was described as follows: first, HTiO₃ was dissolved in H₂SO₄ solution and TiOSO₄ was generated, then distilled water was added to the solution at volume ratios of distilled water to TiOSO₄ solution(F) from 10 to 100. The mixtures were stirred at room temperature for about 2 h, and then urea ((NH₂)₂CO) was directly added to the aqueous solution at molar ratios of urea to Ti precursors from 0.5 to 2.5, stirred vigorously at 70-90 ℃, and the hydrolytic reaction took place and the precipitation TiO(OH)₂ was produced slowly. After a certain time, the precipitation TiO(OH)₂ was separated and sintered at 500-700 ℃ for 2 h in air using an electric furnace.

The crystalline phase was determined with XRD (type D/maxⅢ, Japan) using Cu K_α radiation. The acce- lerating voltage was 35 kV and the applied current was 25 mA. The average crystallite size of TiO₂ particles was determined according to the Scherrer equation using the full width of half maximum data.

The photocatalytic activities of TiO₂ nanoparticles were examined by using methyl orange degradation decolorization described below. The same mass TiO₂ particles and TiO₂-P25 (purity 99.8%, Degussa Corporation) were added in aqueous methyl orange with an initial content of 25 mg/L in quartz cells, respectively. An ultra-violet lamp (30 W) was used as the light source. The average intensity of UV radiation was 78 μW/cm² measured with an UV irradiance meter. Its wavelength range was 320-400 nm, and peak wavelength was 360 nm. The density of methyl orange was determined with a spectrometer (type UV-VIS8500, China).

The yield of TiO(OH)₂ from TiOSO₄ in the solution was determined by the hydrolytic efficiency η. The larger the η, the higher the TiO₂ yield. Absorbance A can be measured according to the following formular:

                      (1)

where  A_initial is the absorbance of TiOSO₄ solution before adding urea, A_end is the absorbance of the solution after adding urea and finishing the reaction. Both the above solutions contain 50%(volume fraction) H₂O₂. The absorbance was determined with an UV-Vis spectrophotometer (UV-VIS8500, China) at 411 nm.

The hydrolytic reaction in TiOSO₄ aqueous solutions containing urea generated at 70, 80 and 90 ℃, respectively. The effect of the temperature on the hydrolytic efficiency η is shown in Fig.1. It is found that the hydrolytic efficiency is too low at 70 ℃, but η achieves 97% at 90 ℃. Therefore, the hydrolytic reaction temperature should be higher than 90 ℃. However, the temperature has little influence on the average crystallite size of TiO₂ particles. At 70, 80 and 90 ℃, the corresponding crystallite size (sintered at 550 ℃ for 2 h) is 19.1, 14.9 and 16.5 nm, respectively.

Fig.1 Effect of temperature on hydrolytic efficiency

Fig.2 shows the hydrolytic efficiency and average crystalline size at different R. It can be seen that the hydrolytic efficiencies are all above 95% and the crystalline sizes are all less than 20 nm while R varies from 0.5 to 2.5. The minimal size of TiO₂ crystalline can be obtained at R=1.5.

The change of the crystalline size with R can be explained in terms of two contrary factors: the precipitant’s increasing and decreasing. On the one hand, the precipitant (NH₄OH) increases with the increase of urea ((NH₂)₂CO), resulting in TiO₂ crystalline growing larger, on the other hand, however, if there is too much urea, the density of precipitant (NH₄OH) generated by the urea hydrolytic reaction decreases, which causes the inhibition of TiO₂ crystalline growing. In this work R is around 1.5, which makes the crystalline size turn from decrease to increase.

Fig.2 Hydrolytic efficiency (a) and average crystalline size (b) at different molar ratios of urea to Ti precursors (R)

Fig.3 shows the hydrolytic efficiency and average crystalline size at different F. It is observed that the hydrolytic efficiencies rise from below 10% to over 90%,

Fig.3 Hydrolytic efficiency (a) and average crystalline size (b) at different volume ratios of distilled water to TiOSO₄ solution (F)

but the sizes reduce from about 20 nm to below 10 nm. It is due to that more NH₄OH yield and TiO₂ crystalline particles can not easily grow bigger in relatively dilute TiOSO₄ aqueous solution containing urea.

Fig.4 shows the hydrolytic efficiency after different reaction time. It indicates that the hydrolytic efficiencies increase obviously in the first 2 h and up to 95% after 3 h. Therefore, the reaction time should be longer than 3 h to improve the production of TiO₂ nanoparticles.

Fig.4 Effect of reaction time on hydrolytic efficiency

Fig.5 shows the average crystalline sizes of TiO₂ sintered for 2 h at 500, 550, 600, 650 and 700 ℃, respectively. It can be seen that the average size increases with the increase of the sintering temperature. The XRD pattern for the above TiO₂ powders is confirmed that the crystal form of TiO₂ sintered at 500 ℃ is the mixture of amorphous and anatase TiO₂, and the crystal form sintered at 550 and 650 ℃ are all of anatase TiO₂. However, the crystal form sintered at 700 ℃ is the mixture of anatase and rutile TiO₂. Because the sulfur in the precipitation can be eliminated at 600 ℃ and anatase TiO₂ assumes good photocatalytic activity, so the sintering temperature is chosen as 600 ℃.

Fig.5 Effect of sintering temperature on average crystalline size

Fig.6 shows the results of photocatalytic decolorization of methyl orange by the TiO₂ particles (the hydrolytic reaction temperature is 90 ℃, R =1.5,

F=80) and TiO₂-P25, respectively. After illuminating with ultra-violet light for 10 min, the decolorization rate of methyl orange by the TiO₂ and TiO₂-P25 is 42.8% and 31.6%, respectively. After illuminating with ultra-violet light for 60 min, both of the decolorization rates are above 98%.

Fig.6 Decolorization of methyl orange degraded by TiO₂ and TiO₂-P25

TiO₂ nanoparticles are prepared by using cheap HTiO₃ as raw material and adopting TiOSO₄ solution neutralizing process. The yield of TiO₂ is determined by the hydrolytic reaction temperature and time, the molar ratio of urea to Ti precursors (R) and volume ratio of distilled water to the TiOSO₄ solution (F). The average crystalline size of TiO₂ nanoparticles can be controlled by adjusting R, F and the sintering temperature. The as-prepared TiO₂ particles have good photocatalytic activity for decolorization of methyl orange.

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(Edited by CHEN Wei-ping)

Foundation item: Project(NCET-04-0915) supported by Program for New Century Excellent Talents in Umiversity; Project(2005E0007M) supported by the Natural Science Foundation of Yunan Province

Corresponding author: LIU Qing-ju; Tel: +86-871-5035376; Fax: +86-871-5035376; E-mail: qjliu@ynu.edu.cn