Environ. Sci. Technol. 2007, 41, 4441-4446
Influence of Preparation Method on Morphology and Photocatalysis Activity of Nanostructured TiO2 XIN JIANG* AND TING WANG College of Material and Chemical Engineering, Zhejiang University, Hangzhou 310027, China
Supported TiO2 photocatalysts on aerosol silica were prepared by adsorption phase reaction technique (APRT) and impregnation method (IMP) under different water concentrations. Energy dispersive X-ray microanalysis indicated that Ti concentration in samples prepared by both methods increased with the content of water rising. There was a sharp increase in Ti content curve of APRT samples, while Ti concentration of IMP samples changed slowly. The grain size of TiO2 by APRT was below 6 nm and TiO2 particles dispersed uniformly on surface of silica. But the size of TiO2 by IMP was larger than 20 nm and the number of large particles increased rapidly when water concentration increased. The photocatalysis activity of asprepared catalysts was characterized by methyl-orange photodegradation. The results showed that APRT photocatalysts had higher photocatalysis activity than IMP photocatalysts. Ti concentration and TiO2 crystalline form were two factors which affected photocatalysis activity the most. The grain size of TiO2 had little influence on photocatalysis activity, which might be counteracted by the decrease of anastase TiO2.
1. Introduction The application of semiconductor photocatalysis in wastewater treatment has been, and continues to be, a hot topic in studies of environmental catalysis. TiO2 nanoparticles are the most well-known heterogeneous photocatalyst because of its excellent properties such as high catalysis efficiency, no pollution, etc. (1). However, because nano TiO2 is easy to agglomerate in preparation and application, its photocatalysis activity usually decreases dramatically or even lost completely, which seriously limits its application and development. Supporting TiO2 on high dispersing supporters is an option to prevent agglomeration (2). There are many supporting methods in catalyst preparation and they provide different catalysts with variant structures, configurations, and properties (3). In this paper, we propose a new preparation method, called adsorption phase reaction technique (APRT) to prepare supported TiO2, and we compare it with a traditional preparation method. Nanoreactor is a new reaction technology for the preparation of nanoparticles; it provides a simple method to regulate particle size distribution by restricting reaction and the growth of the crystal nucleus in a narrow space. Microemulsions and inverse microemulsions are typical cases which have now been used widely. Adsorption phase reaction technique extends this idea and employs the adsorption layer * Corresponding author fax: +86 571 87951227; e-mail: jiangx@ zju.edu.cn. 10.1021/es070106v CCC: $37.00 Published on Web 05/17/2007
2007 American Chemical Society
on a supporter surface as a nanoreactor to prepare nanocomposites. Based on thermodynamics of multicomponent liquid mixtures, there is a tremendous distinction of adsorbents in attracting different kinds of solution molecules. When an adsorbent is dispersed in a binary liquid system, an adsorption layer, mainly consisting of one component, may be formed on its surface. This adsorption layer on the supporter surface is the nanoreactor used in adsorption phase reaction technique. De´ka´ny and co-workers (4) studied this adsorption phenomenon and prepared many nanoparticles with this technique (5). The authors (6) have also synthesized TiO2 nanoparticles with a 2∼7 nm diameter on the SiO2 surface. In our earlier research (6, 7), the volume of adsorption layer was estimated first by measuring the change of water concentration in bulk during adsorption. A solvent replacement experiment provided direct evidence for the first time that there was a water-rich adsorption layer, and the reaction took place in this layer. TiO2/SiO2 prepared by APRT shows some particular characteristics in shape and size distribution. These new features may effect photocatalysis. In this paper, we regulated the adsorption layer reactor to obtain different TiO2, and then compared them with samples prepared by a traditional impregnation method.
2. Experimental Section 2.1. Materials. Hydrophilic colloidal silica (SiO2) A-200 (average particle diameter is 12 nm) was obtained from Degussa (Germany). Tetrabutyl titanate was purchased from Lantian Chemical Plant of Changzhou and used as received. Ethanol was distilled and stored over a 0.4 nm molecular sieve (SCR 4A, China). 2.2. Sample Preparation. 2.2.1. Preparation of TiO2 by Adsorption Phase Reaction (APRT). One gram of hydrophilic, larger specific surface area SiO2 previously dehydrated and kept in a desiccator at 393 K for 2 h, 200 mL absolute alcohol together with different volume of water varied from 0 to 1.5 mL were added in a triflask to adsorb and react while stirring at 25 °C. Because of selective adsorption capacity of SiO2, a water-rich adsorption layer formed gradually on the supporter surface (Figure 1). After the adsorption equilibrium was attained (12 h), tetrabutyl titanate (2.15 g) dissolved in ethanol was added. The molecule of tetrabutyl titanate diffused from bulk to the supporter surface and reacted with water in adsorption layer. TiO2 particles formed in adsorption layer by hydroxylation and condensation reaction. After reacting for 5 h, the product was obtained by several centrifugation-redispersion-washing cycles, dried at 75100 °C, and calcined at 500 °C for 5 h. 2.2.2. Preparation of TiO2 by Impregnation Method (IMP). 200 mL of absolute alcohols and different volumes of water from 0 to 1.5 mL were added to a triflask. Then, tetrabutyl titanate dissolved in ethanol was added to react at 25 °C. After reacting for 4 h, 1 g of SiO2, previously kept in a desiccator at 393 K for 2 h, was added into the reaction system. After the reaction was complete (5 h), the resulting product was obtained by several centrifugation-redispersion-washing cycles, dried at 75-100 °C, and calcined at 500 °C for 5 h. 2.3. Photocatalysis Experiments. In these experiments, a stirred reactor equipped with a UV lamp (250 nm wavelength, 40 W) was used. 70 mg TiO2/SiO2 powder heat treated for 5 h at 500 °C, 200 mL of 4 mg/L methyl-orange aqueous solution and 10 mL buffer solution were added in the reactor while stirring at 30 °C. The sampled suspension was centrifuged at a different reaction time and the upper clear solution was extracted. Then, the change of concentraVOL. 41, NO. 12, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Scheme of preparation of TiO2 in adsorption layer at low water concentration: (a) Dispersion of supporter in binary liquids system; (b) Adsorption equilibrium; (c) Particles distribution after reaction.
TABLE 1. Relative Ti Content of the Different Samples from EDAX volume of water (mL)
relative content of mean mean Ti on SiO2 surface /% value % deviation
Ha
sample
0
0
APRT1 IMP1
5.79 5.24
5.78 3.96
5.79 4.60
0.005 0.64
0.5
4.40
APRT2 IMP2
9.60 5.85
10.05 5.16
9.83 5.51
0.225 0.345
1.0
8.79
APRT3 IMP3
15.23 8.27
15.17 10.98
15.20 9.63
0.03 1.355
1.1
9.67
APRT4 IMP4
20.52 12.54
19.78 13.56
20.15 13.05
0.37 0.51
1.2
10.55
APRT5 IMP5
20.32 12.23
21.99 8.61
21.16 10.42
0.835 1.81
1.5
13.19
APRT6 IMP6
29.30 15.30
28.94 14.74
29.12 15.02
0.18 0.28
a H is the molar concentration ratio of water to tetrabutyl titanate, H ) [H2O]/[Ti(OBu)4].
tion of methyl-orange in the solution was measured by 721 spectrophotometry. 2.4. Characterization. The morphology of the TiO2 particles on SiO2 surface was investigated with transmission electron microscope (TEM, JEM-200CX). The Ti concentration on SiO2 surface was measured by by energy dispersive X-ray microanalysis (EDAX, Genesis 4000). The crystallinity of TiO2 was was analyzed by XRD using D/max-rA X-ray diffraction instrument (XD-98) with Cu Ka radiation (1.5406A°). The accelerating voltage and the applied current were 40 kV and 30 mA, respectively. The crystal size of anatase (200) was determined using the Scherrer equation:
D)
Kλ β cos θ
(1)
Where k related to the crystallite shape; λ and θ are the radiation wavelength and Bragg’s angle, respectively; β ) B0 - b0, B0, and b0 is peak half-width of samples and standard separately. XRD analysis was carried out in two steps. First, we scanned a range of 2θ from 10 to 80° with a rate of 4°/min to observe the overall XRD peaks to identify the transformation of the crystal phase. Then, in the subsequent analysis, the scan, started from 45° and ended at 52° in a rate of 1°/min, was aimed to calculate the crystal size of the anatase (200).
3. Results and Discussion 3.1. Measure of Ti Content. The amount of water added into the reaction system will affect the reaction degree, which can be revealed by Ti content in the product. Table 1 is the Ti content measured by EDAX. 4442
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FIGURE 2. Dependence of Ti content on volume of water.
In measurement of EDAX, two areas were scanned and analyzed. The difference between two scan areas gives information of TiO2 distribution. It is found from Table 1 that the mean deviations of IMP sample are much larger than those of APRT sample. The distribution of TiO2 particles prepared by APRT is more homogeneous than those prepared by IMP. Figure 2 is the mean value of Ti content versus volume of water. It shows that Ti contents of the particles both prepared by APRT and IMP increase with the volume of water. And this common phenomenon, we think, results from the same reaction mechanism of tetrabutyl titanate in the adsorption layer and in bulk. It is generally accepted that the reaction of tetrabutyl titanate can be considered as two steps (8): hydrolysis and condensation, and the hydrolysis of tetrabutyl titanate is very quick:
Tetrabutyl titanate almost hydrolyzed to intermediateproduct soon after it was added to reaction system. Second step is the condensation and it is a slow-reaction:
The quantity and morphology of TiO2 depend on condensation, whereas the molar concentration ratio of water and tetrabutyl titanate (H ) [H2O]/[Ti(OBu)4]) determine the result and rate of condensation. In previous literature (11), TiO2 was precipitated by condensation of intermediateproduct when H > 3. In the experiment listed in Table 1, the ratio H is larger than 3 except APRT1 and IMP1, so the intermediate-product can condense to form TiO2 particles. According to the reaction mechanism of tetrabutyl titanate, condensation is a slow-reaction and its rate depends on the ratio H, which finally determines the Ti concentration. In the literature (11), at low molar concentration of tetrabutyl titanate ([Ti(OBu)4] ) 0.1 M) and H ) 15, the yield of Ti(OBu)4 after reacting for 5 h was 2.34% and it increased with water concentration. The concentration of Ti(OBu)4 in our experiments was about 0.025 M and all H ratios were less than 15 in IMP, so tetrabutyl titanate could not react completely in 5 h and Ti concentration of IMP samples analyzed by EDAX was less than or equal to 15% (theoretical value for complete reaction is about 45%). However the quantity curve in Figure 2 shows that Ti concentration of APRT samples is not only more than the IMP samples, but also increases dramatically when volume of water reaches
FIGURE 3. TEM photographs of the samples (a) APRT1; (b) APRT3; (c) APRT6; (d) IMP1; (e) IMP3; (f) IMP6.
FIGURE 4. Distribution of Ti element on SiO2 surface under different volume of water (a) APRT1; (b) APRT3; (c) APRT5; (d) IMP1; (e) IMP3; (f) IMP5. 1.1 mL We think these phenomena result from the adsorption layer used as a reaction region in APRT. Because the thickness of the adsorption layer is only several nanometers, water concentration in the adsorption layer is so high that the reaction of tetrabutyl titanate in the adsorption layer is much quicker than reaction in ethanol bulk in IMP. Therefore, Ti concentration of APRT samples is higher than IMP samples. In Figure 2, it is also found that there is a sharp increase in Ti concentration curve of APRT samples, and we think this sharp increase reflects the change of adsorption layer when water reaches 1.1 mL. It is well known that chemicaladsorption and physical-adsorption are the two forms of adsorption on the supporter surface. The chemical-adsorption layer is monolayer, and water molecules combine strongly with surface of SiO2 via chemical bond, so tetrabutyl titanate reacts slowly in chemical-adsorption layer; the physical-adsorption layer is a multimolecule layer, and water molecules are combined with each other by a weak molecular force (such as hydrogen bond and van der Waals force). The reaction of tetrabutyl titanate is quicker in the physicaladsorption layer. With the amount of water changing from 0 to 1.5 mL, the chemical-adsorption layer formed first on SiO2 surface. Then, only when water concentration was larger than the critical value (9) (like 1.1 mL water here), multimolecular physical-adsorption layer could be formed and
tetrabutyl titanate react more quickly in the adsorption layer; therefore, Ti content increases dramatically under this water concentration. Figure 2 shows that the concentration of Ti in APRT1 and IMP1 is only about 5%. The reason that Ti content appeared both in APRT1 and IMP1 is that some hydroxyl group on SiO2 surface can also react with tetrabutyl titanate:
Because the rate of this reaction is very small and SiO2 is added early in APRT, Ti concentration in APRT1 is a little higher than in IMP1. 3.2. Characterization of Morphologies of TiO2 on the SiO2 Surface. 3.2.1. Analysis by TEM and EDAX. Figure 3 is TEM picture of IMP and APRT. By comparing the picture at the first row, it is found that APRT1 and IMP1 manifest similar characteristics. The gray of both samples is somewhat darker than that of raw SiO2, which indicates some TiO2 is formed on SiO2 surface. Without water, tetrabutyl titanate can only react with hydroxyl group on SiO2 surface (eq 4), and the reaction in two methods is similar to each other. With water concentration increasing, different features appear in APRT and IMP samples. In APRT the gray becomes VOL. 41, NO. 12, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Dependence of grain size on volume of water.
FIGURE 5. XRD patterns of samples under different water concentration. darker and darker with an increase of water, and no obvious black spot is found. In IMP, the gray also darken and more and more black spots appear. The different reaction region in two preparation methods results in large variance of morphology of samples. In the APRT system, SiO2 surface was coated with water adsorption layer gradually when water was added. Adsorption layer became the place where reaction took place and particles grew, so it restrained the growth of the TiO2 particles effectively. The TiO2 coats evenly on surface of SiO2 and the morphology of samples in TEM are homogeneous. In IMP, the drop of water in alcohol bulk, and the bulk gradually becomes the major reaction region. Bigger TiO2 particles form in alcohol bulk and then adhere to the SiO2 (as shown especially in panels e and f). When water increases to 1.5 mL in APRT6, the remaining water in alcohol bulk after adsorption equilibrium is enough for the hydrolysis and condensation. Bigger TiO2 particles are formed and some black spots like that in IMP appear in panel c. The difference between preparation methods from TEM results is also confirmed by the Ti concentration analysis of EDAX (Figure 4). In the picture, the red points represent the Ti element and the deeper the red, the more Ti concentration. Overall, the Ti concentration on APRT sample is higher than that on IMP sample, which is quantitatively shown in Figure 2. Another important phenomenon we find in Figure 4 is the red point on APRT samples is more homogeneous than that on IMP samples. This phenomenon is especially obvious by comparing panel b with panel e. In the APRT picture, the red point is uniform and even. In the IMP picture, the red point concentrates in part of the region, whereas red points in other regions are correspondingly sparse. 3.2.2. Analysis by XRD. Samples were analyzed by XRD after sintering at 500 °C for 5 h. As shown in Figure 5, no crystalline TiO2 appeared in samples under 0 mL and 0.5 mL water and the possible reason is that high interaction -TiO-Si- or the Ti concentration is too low to form crystal. Except for these two conditions, different crystalline TiO2 appears in the samples. When water concentration changes from 1.0 to 1.2 mL, there is pure anatase TiO2 in all samples prepared by the two methods. It is well-known that pure anatase TiO2 transform to rutile begins at 400 °C after calcining 2 h (10). For SiO2-TiO2 nanocomposite materials, the transformation is restrained due to its structure of gel network.11 It is also shown in Figure 5 that there is a little rutile TiO2 appearing in IMP 6, whereas the crystalline TiO2 in APRT 6 remains pure anatase. We think this phenomenon is also the 4444
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result from the different reactions of the two methods. In IMP, TiO2 almost grew freely and combined with SiO2 via physical-adsorption or other molecular forces. So when volume of water reaches 1.5 mL, individual particles become so large and so numerous (as shown in panel f of Figure 3) that SiO2 could not restrain all the crystalline phase transmissions of TiO2 and a little rutile TiO2 appeared in IMP5. Whereas in APRT, TiO2 particles formed in the adsorption layer and combined with SiO2 by covalent bond; therefore, the inhibition of SiO2 is too strong for transformation to rutile TiO2, even at high Ti concentrations (such as APRT6). From Figure 5, rutile TiO2 only exists in IMP 6, and its peak in the XRD pattern is very weak. So, when studying the change of grain size with water concentration increasing, we neglect rutile TiO2 and only calculate the size of anatase TiO2. The peak at 48° in the XRD pattern was fitted via Guass equation, from which the grain size of anatase was calculated by the Scherrer equation. Figure 6 shows the dependence of grain size on water concentration. XRD results indicate there is no crystalline TiO2 in the samples under 0 and 0.5 mL water, and the experiments under 0.8 and 0.9 mL water were added to get a whole curve. As shown in Figure 6, grain size curves of samples by two preparations are obviously different: the grain sizes of all TiO2 gained by APRT are below 6 nm, whereas all IMP TiO2 particles are more than 20 nm. Combining with Ti quantity curve (Figure 2), Ti content of APRT samples is higher than in IMP samples under the same conditions, but the grain size of APRT samples is much smaller. That is to say, the distribution of TiO2 particles prepared by APRT is more homogeneous than those by IMP. Even at high Ti concentration, the dispersibility of particles also is good (such as APRT5 and APRT6). Therefore, the adsorption layer used as a nanoreactor in APRT can not only limit growth of particles effectively, but also maintains high dispersibility of particles. The grain-size curve of APRT also show that the crystalline size of TiO2 is about 3 nm and changes slowly when the volume of water is below 1.5 mL, and a little jump appears with a water level of 1.5 mL. Combining with results in Figure 5, we think that the jump also resulted from the increase of anatase TiO2 in APRT 6. According to the literature (11), SiO2 restrains not only the transformation from anatase to rutile, but also the crystallization of TiO2. And the higher the concentration of SiO2, the stronger the inhibition on crystallization of TiO2 becomes. Because of the strong covalent bond, SiO2 greatly restrained crystallization of TiO2 in APRT samples, and the peak intensity in XRD pattern of APRT samples is much lower than those of IMP samples (Figure 5). With Ti concentration increasing (Figure 2), the inhibition of SiO2 becomes weaker and the peak intensity in XRD pattern also becomes more and more obvious (Figure 5), that is to say, content of anatase TiO2 also increases, and under 1.5 mL water, Ti concentration becomes so high that more TiO2
FIGURE 7. Dependence of reaction rate constant of methyl-orange on volume of water.
transforms to anatase and the grain size of TiO2 in APRT 6 increases to 5.3 nm. 3.3. Photocatalytic Experiments. Photocatalytic activity of the prepared catalyst by IMP and APRT is examined by photo degradation of methyl orange. Methyl orange is widely used as model compound (12, 13), and its concentration can be monitored by spectrophotometry. Azo dyes, such as methyl orange, can be adsorbed onto catalysts from their aqueous solution. Hachen et al. (14) pointed out that the adsorption was quite fast and the equilibrium concentration could be reached within about 45 min. In our expriment, we found adsorption equilibrium was reached in 60 min under dark conditions, and the adsorption ratio of methyl orange of all catalysts was less than 1%. The effect of water volume in preparation on the decolorization ratio was studied at first. It was found the decolorization ratio of APRT catalysts is higher than those of IMP, and almost all methyl orange was decolorized completely by APRT 6 (decolorization ratio is 99.5%). To further compare the catalytic activity of catalysts by two methods, photodegradation reaction rate constant k is gained after fitting by power law equation. It is found that zeroorder reaction is most suitable to describe the evolution of the change of methyl-orange concentration: -dC/dt ) k, where k is the reaction rate constant. The change of k of different catalysts prepared under different conditions is shown in Figure 7. As shown in Figure 7, the catalytic activity is gradually enhanced with water increasing in the preparation. Under the same reaction condition, catalysts by APRT have a higher catalytic activity than those by IMP, especially APRT 6. Comparing this with Figure 2 shows the catalytic activity trend is similar to the Ti concentration. The effect of Ti Concentration is also reported by other researchers (15), and it is suggested that Ti concentration in a catalyst is a dominant factor in photocatalysis although the catalyst preparation is quite different. Comparing the curves in Figure 2 and Figure 7 in detail, the change of Ti concentration and reaction rate constant shows a little difference. When the volume of water is less than 1.0 mL, the Ti concentration curve in Figure 2 changes slightly without any turning point, whereas the reaction rate constant k in Figure 7 turns up at 0.5 mL of water, abruptly. It is suggested that there are other factors also affecting catalytic activity besides Ti concentration. According to studies on the mechanism of TiO2 photocatalysis (16), except Ti concentration, crystalline phase and morphology of TiO2 affect catalytic activity. It is generally accepted that crystalline has higher catalytic activity than amorphous TiO2, and TiO2 particles with smaller size also have higher catalytic activity. From XRD analysis (Figure 5), it can be found that there is
no crystalline TiO2 appearing in samples from 0 to 0.5 mL water, and anatase TiO2 forms gradually when the volume of water changes from 0.5 to 1.0 mL. The number of grains increase quickly (from the peak area of XRD pattern in Figure 5), although its size changes little (as shown in Figure 6). So it obviously indicates that the sharp increase of the reaction rate constant k is related to the gradual formation of anatase TiO2, and the crystalline of TiO2 is the other important factor influencing catalytic activity. From Figures 6 and 7, it is also found that, although the size of TiO2 particles prepared by APRT is much smaller than those prepared by IMP, the difference between two reaction rate constants is not larger except for the data at 1.5 mL water. Seemingly, the grain size of TiO2 shows little effect on catalytic activity, which conflicts with the obvious influence of grain size on photocatalysis activity concluded by other researcher (15). We think this apparent phenomenon probably results from the existence of much amorphous TiO2 with the lowest catalytic activity in APRT 1∼5. As mentioned above, the inhibition of SiO2 on crystalline phase transformation in APRT is so stronge that a lot of TiO2 in APRT 1∼5 still stays in amorphous form. So in Figure 5, the peak intensity in XRD pattern of APRT 3∼5 is weaker than those of IMP 3∼5. The existence of much amorphous TiO2 with the lowest catalytic activity largely counteracts the enhancement of catalytic activity from higher dispersivity and smaller size. The bonus from the size decrease is traded off by the decrease of anatase percent. When the amount of anatase TiO2 increases, the photocatalytic activity of the samples also becomes higher. So the catalytic activity of APRT 6 is the highest among the catalysts, although its grain size is larger than other APRT catalysts.
Acknowledgments Financial support from the National Science Foundation of China grants (contract 20476088), Zhejiang Provincial Natural Science Foundation of China (Y405125), and the Zhejiang Science & Technology Program (contract 2005C31027) is gratefully acknowledged.
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Received for review January 15, 2007. Revised manuscript received March 21, 2007. Accepted April 3, 2007. ES070106V
