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| Semicond. Sci. Technol. 20 No 8 (August 2005) L36-L39 |
| doi:10.1088/0268-1242/20/8/L03 |
| PII: S0268-1242(05)98255-2 |
A simple new way to prepare anatase TiO2 hydrosol with high photocatalytic activity
Jing-yu Wang, Jun-xia Yu, Zhi-hong Liu, Zhi-ke He and Ru-xiu Cai
Department of Chemistry, Wuhan University, Wuhan 430072, People's Republic of China
Email: zhhliu.whu@163.com
Received 13 April 2005, in final form 24 May 2005
Published 18 July 2005
| Abstract. Anatase TiO2 hydrosol was prepared by hydrolyzation of tetrabutyl titanate followed by acidic peptization of the precipitate under 70 °C. The method was highly simplified and the growth of TiO2 nanocrystallites that bothered traditional preparations was avoided. The photocatalytic activity of the hydrosol was confirmed through the photocatalytic degradation of methyl blue dye under visible light irradiation. |
Contents
1. Introduction
Anatase TiO2 semiconductor is the most widely used photocatalyst for oxidative decomposition of organic compounds nowadays [1]. The configurations of titanium oxide researched and reported have mainly been powders or films based on microporous materials [2-4]. The process of converting titanium alkoxides to titanium oxide powders or titanium oxide films has been widely studied as a common preparation method during the past decade. Normally, calcination or other pyrochemical treatment is involved in these preparation processes. A notable problem connected with these traditional preparations is the growth of TiO2 nanocrystallites during the calcining process at high temperature. The serious aggregation of prepared nanoparticles when dispersed in aqueous solution is also very troublesome. Therefore it is highly desirable to find some new ways that are capable of overcoming the above problems to prepare crystal structure TiO2. Many attempts have been made in this field during the past few years. The fabrication of sol-gel-derived anatase nanocomposite SiO2-TiO2 film at low temperature was studied by Matsuda et al [5]. And most recently, the preparation of TiO2 sol by a chemical coprecipitation-peptization method at low temperature was reported [6, 7]. In the present work, a much simpler method to directly prepare anatase TiO2 hydrosol at low temperature (70 °C) was proposed. It was based on a thermal hydrolyzation of tetrabutyl titanate followed by an acidic peptization of the precipitate, in which only two simple steps were involved. To our knowledge, such works have not yet been reported. The photocatalytic activity of the newly obtained photocatalyst was also investigated.
2. Experimental details
A typical synthesis of the TiO2 hydrosol can be described as follows. 3 ml tetrabutyl titanate was dissolved in 25 ml absolute ethanol, and then the solution was added dropwise into 30 ml doubly distilled deionized water under a vigorous stir. After complete addition, the suspension was kept stirring under 70 °C for about 45 min to ensure complete hydrolysis. In this way, the acquired amorphous TiO2 was well dispersed in the water. 80 ml 0.04 M HNO3 was then added. Subsequently, the mixture was continuously stirred under 70 °C for 4 h in an airproof condition. The thus obtained semitransparent hydrosol was diluted with water to 100 ml. It is stable in the long term without sedimentation or delamination in aqueous solutions given that the pH value of the solution is less than 5.0.
For the characterization of the product, XRD patterns were obtained on an XRD-6000 diffractometer (Shimadazu Corporation, Japan) using graphite monochromatic copper radiation (Cu Kα) at 40 kV, 30 mA. The TiO2 xerogel powder used for the XRD characterization was prepared by centrifugation and vacuum drying of the hydrosol sample at 40 °C for 4 h. And TEM images were taken on a JEOL JEM 2010HT microscope and a JEOL JEM 2010FEF microscope operating at an accelerating voltage of 200 kV.
Catalytical degradation of methyl blue dye (MB) was employed to investigate the photocatalytic activity of the synthesized hydrosol. Parallel experiments were performed with commercial Degussa P-25 TiO2 powder (obtained from Degussa AG Company, Germany, with 80% anatase, 20% rutile and BET area of about 50 m2 g - 1, CP > 98.0%) for comparison. A 200 ml beaker was filled with 30 ml diluted hydrosol photocatalyst (or the P-25 TiO2 suspension) and 0.24 ml of 2 mg ml - 1 MB. The mixtures were irradiated by a 500 W halogen-tungsten lamp with UV and IR cut-off filter, and were sampled every 5 min to determine the photodegradation rate of MB by measuring its absorbance at 560 nm. The absorption spectra of MB were measured on a TU 1900 UV-VIS spectrophotometer.
3. Results and discussion
To determine the crystal phase composition of the prepared photocatalyst, x-ray diffraction (XRD) measurements were carried out at room temperature over the diffraction angle (2θ) 20°-60°. To trace the time course of phase transition, samples were taken and analysed at different stages during the preparation process. Upon the complete addition of tetrabutyl titanate to water (figure 1, curve a), and even after the 45 min of vigorous stirring under 70 °C (i.e., after a complete hydrolysis, curve b), the precipitate obtained was amorphous and no crystal TiO2 was detected. The phase-transfer was found to occur within the process of peptization either by water (curve c) or by acid (curve d). The pattern of the hydrosol samples showed the presence of peaks (2θ = 25.38°, 37.98°, 47.66° and 54.82°), which was regarded as an attributive indicator of anatase phase TiO2 crystallites [8]. Careful comparison between the two XRD spectra (curves c and d) suggested that the role of acid in the peptization process was to catalyze the formation of nanocrystalline anatase and to disperse TiO2 particles. Therefore, the significant and crucial factor of phase-transformation at low temperature must be acid peptization through a different mechanism (from that of calcinations), which is a low-activation energy process.
| Figure 1. X-ray diffraction patterns of samples taken at different stages: (a) upon the complete addition of tetrabutyl titanate to water; (b) after vigorous stirring at 70 °C for 45 min; (c) after the peptization of precipitate by water; (d) after the peptization of precipitate by HNO3. |
Though the relatively low signal-to-noise ratio of the XRD patterns did not allow a reliable quantification of the percentage of anatase in the sample, we could qualitatively conclude that nearly no rutile TiO2 was produced. The rather strong background was probably due to the presence of a few amorphous parts of the sample. The crystallite sizes of samples were calculated from the half-height width of different diffraction peaks of anatase using the Scherrer equation. High-purity silicon powder (99.9999%) was used as an internal standard to account for the instrumental line broadening effect during crystal size estimation. Results of the XRD analysis were in good agreement with the transmission electron microscopy (TEM) micrograph, which showed small crystalline anatase titanium oxide particles of 5-8 nm diameter (figure 2(a)). The diffraction rings, which are indexed in table 1, showed the corresponding selected area electron diffraction (SAED) and confirmed the presence of crystal structure TiO2 according to Pattern: 21-1272 (figure 2(b)). As further evidences, figures 2(c) and (d) showed lattice fringes and Miller indices which were the characteristic values of the anatase phase. The TEM images also showed a good dispersibility of the crystalline particles.
| Figure 2. The TEM images of the TiO2 hydrosol: (a) TEM image on a JEM 2010HT microscope; (b) the diffraction rings; ((c) and (d)) HRTEM images from different angles on a JEM 2010FEF microscope. |
| Table 1. The diffraction rings indexed according to Pattern: 21-1272. |
| Number | d | i | h | k | l |
| 1 | 3.520 00 | 100 | 1 | 0 | 1 |
| 2 | 2.431 00 | 10 | 1 | 0 | 3 |
| 2.378 00 | 20 | 0 | 0 | 4 | |
| 2.332 00 | 10 | 1 | 1 | 2 | |
| 3 | 1.892 00 | 35 | 2 | 0 | 0 |
| 4 | 1.699 90 | 20 | 1 | 0 | 5 |
| 5 | 1.666 50 | 20 | 2 | 1 | 1 |
| 6 | 1.493 00 | 4 | 2 | 1 | 3 |
| 1.480 80 | 14 | 2 | 0 | 4 | |
| 7 | 1.364 10 | 6 | 1 | 1 | 6 |
| 8 | 1.337 80 | 6 | 2 | 2 | 0 |
| 9 | 1.279 50 | 2 | 1 | 0 | 7 |
| 1.264 90 | 10 | 2 | 1 | 5 |
The photocatalytic activity of the TiO2 hydrosol was investigated through the photodegradation of MB and was compared with that of a commercial Degussa P-25 TiO2 powder. For a convictive comparison of their catalytic activity, the mass of TiO2 in the hydrosol sample, which was calculated according to the preparation method, was approximately equal to that of the commercial TiO2 powder. The time courses of MB reduction rate (A/A0 ~ t) were obtained and are illustrated in figure 3. Obviously, the synthesized TiO2 hydrosol showed higher photocatalytic activity than P-25 TiO2 powder in this situation. As a common opinion, the photodegradation of MB under visible light irradiation should have followed the sensitized photocatalysis mechanism [9, 10]. The process involves the excitation of MB dye molecules with visible light and the subsequent electron injection or electron transfer from excited dye molecules to the conduction band of semiconductor TiO2. The well-dispersed sol nanoparticles with high surface area allow good access of the reactants to the TiO2 surface in photocatalytic system. In this way, both the photon absorbing (excitation of MB molecules) and the electron transfer procedures have benefited greatly from the homogeneous property of the TiO2 hydrosol. This might be the main cause of the higher catalytic efficiency of the synthesized TiO2 hydrosol than the TiO2 powder. Besides its satisfying photocatalytic activity, another potential merit of the newly prepared TiO2 hydrosol is that it would facilitate the research of the photodegradation mechanism, which is still a puzzling question today. Because, when combined with some analytical techniques such as flow injection or stopped-flow analysis, the photodegradation process is able to be kinetically monitored, and many reactive intermediates can optically be detected on-line in aqueous media, which is essential to revealing the molecular mechanism of photodegradation reactions [11].
Figure 3. The time courses of MB degradation rate under visible light irradiation. ( -  - ) Only methyl blue dye in acidic solution as blank; ( -  - ) P-25 powder; ( - • - ) TiO2 hydrosol. |
Acknowledgment
Authors want to thank Dr Jin-cai Zhao and Dr Wan-hong Ma in the Key Laboratory of Photochemistry Institute of Chemistry, Chinese Academy of Sciences, Beijing, for their kindly technical assistance. This work was financially supported by Dr Zhen-liang Ma, Power Dekor, Hubei Ltd.
References
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