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Ind. Eng. Chem. Res. 2008, 47, 8228–8232
Coating of TiO2 Thin Films on the Surface of SiO2 Microspheres: Toward Industrial Photocatalysis Gang Li,† Renbi Bai,‡ and X. S. Zhao*,† Department of Chemical and Biomolecular Engineering and DiVision of EnVironmental Science and Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576
A core/shell SiO2/TiO2 photocatalyst was prepared using a liquid-phase deposition (LPD) method. Zetapotential measurement showed that deposition of a layer of polyelectrolyte on the surface of SiO2 microspheres was a prerequisite for subsequent deposition of the TiO2 shell with a controllable and uniform thickness. The photocatalytic activity of the core/shell SiO2/TiO2 catalyst for decomposition of Orange II in liquid phase was observed to be comparable with that of P25 (a commercial TiO2 product of Degussa). Experimental data also showed that the SiO2/TiO2 core/shell nanostructured photocatalyst can be easily separated from the reaction medium by sedimentation, and the solid can be recycled and reused. Thus, the photocatalyst described in this work represents a new catalyst system with a high potential for practical applications in treating wastewater. 1. Introduction TiO2 has received much attention in the fields of solar energy conversion and heterogeneous photocatalysis because of its excellent optical transmittance, high refractive index, and chemical stability.1 In treating pollutants present in water, a slurry reactor is the most commonly used operation mode.2,3 However, due to the spontaneous aggregation of TiO2 nanoparticles in the reaction system, the effective surface area of the photocatalyst is decreased, resulting in a rapid decay of the photocatalytic activity.4 In addition, use of such ultrafine particles gives rise in separation problems.5,6 To solve the above-mentioned problems, attempts have been made to disperse TiO2 nanoparticles on porous solids, such as microporous zeolites7 and mesoporous silicas.7-9 Recent research interest has been focused on nonporous solid supports because light cannot penetrate into the pore surface to access the TiO2 particles dispersed on the pore surface.3,10-14 Our previous work9 showed that it is the external surface of the porous support on which supported TiO2 is effective in photocatalytic decomposition of organic compounds. This is because the external surface is easily accessible to light and the target contaminants experience less mass transfer resistance to active site. It turns out that a large portion of the TiO2 supported on the internal surface of the porous support shows poor photocatalytic activity. Thus, one of the motivations of this work was to use nonporous submicrometer-sized SiO2 spheres as the support for TiO2 photocatalyst. Several strategies have been described for coating of TiO2 films on SiO2 materials including sol-gel,7 chemical vapor deposition (CVD),15-18 and liquid-phase deposition (LPD) methods.19-21 While the sol-gel method is widely used to prepare supported catalysts, it is fairly difficult to obtain TiO2 films of uniform thickness using this method because of the fast hydrolysis rate of most titanium precursors. The CVD method is effective for coating TiO2 on a substrate with a flat smooth surface. However, it is ineffective to deposit TiO2 on a highly curved surface like the surface of microspheres. In contrast, the LPD method is considered as a promising route to * To whom correspondence should be addressed. Tel.: (65)65164727. Fax: (65)-67791936. E-mail:
[email protected]. † Department of Chemical and Biomolecular Engineering. ‡ Division of Environmental Science and Engineering.
synthesizing high-quality thin films with a well-controlled composition.21 The main characteristic of the LPD method is the use of metal-fluoride complexes whose hydrolysis in water is modulated by adding boric acid. The fluoride ligand provides a slower and more controllable hydrolysis rate, while the boric acid functions as an F- scavenger.20 When industrial heterogeneous photocatalysis for water purification operated in a plug-flow reactor is considered, separation of solid catalysts from the reaction medium and reuse of the solid catalysts are two important parameters.3,10,22,23 Therefore, another motivation of this work was to prepare SiO2/ TiO2 core/shell photocatalyst, which can be separated from the reaction system by simple means, such as sedimentation. In this work, SiO2 spheres of 800 and 1300 nm in diameter were used to support TiO2 thin films using the LPD method. The effect of surface functionality of the silica spheres on the formation and morphology of the coated TiO2 films was evaluated. The photocatalytic activity of the SiO2/TiO2 core/ shell materials for decomposition of Orange II dye in the liquid phase was measured using a semibatch swirl flow reactor.7 The SiO2/TiO2 core/shell photocatalyst was recycled by sedimentation, and reuse of the photocatalyst was evaluated. 2. Experimental Section 2.1. Catalyst Preparation. Silica microspheres were prepared as described previously.24 Modification of the surface of silica spheres using polyelectrolyte poly allylamine hydrochloride (PAH, Mw ≈ 70 000, Aldrich) was conducted using a layerby-layer (LbL) technique.21 About 20 mL of a PAH solution containing 1 g/L of PAH was added to 30 mL of a colloidal suspension containing 0.5 g of silica spheres under magnetic stirring. After 0.5 h, the spheres were collected by centrifugation followed by washing using deionized (DI) water three times to remove residual PAH. The PAH-functionalized silica spheres were dispersed in 40 mL of DI water. Then, a given amount of 0.6 mol/L (NH4)2TiF6 solution was added to the dispersion followed by ultrasonication for 1 h and adjustment of pH to 2.85 using an l M HCl solution. Finally, a H3BO3 acid solution of 0.6 mol/L was added under ultrasonication for 9 h. The spheres were collected by centrifugation, washed with DI water, and dried at 60 °C overnight. The sample thus obtained is denoted as TS-X-Y, where X and Y stand for the average
10.1021/ie800561y CCC: $40.75 2008 American Chemical Society Published on Web 09/27/2008
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Figure 1. Zeta-potential profiles of silica spheres before and after various modifications.
Figure 2. Wide-angle X-ray diffraction patterns of (a) SiO2 spheres, (b) photocatalyst TS-1300-170, and (c) Degussa P25.
diameter in nanometers of the silica spheres and the thickness in nanometers of the coated titania film estimated using the scanning electron microscope technique, respectively. For comparison purposes, a SiO2/TiO2 core/shell sample was also prepared following the same procedure as described above except without using PAH to modify the surface of silica spheres. 2.2. Characterization of Catalysts. Zeta-potential profiles of the bare and polyelectrolyte-coated silica spheres were determined using a Zetaplus 100. A powdery sample was suspended in ultrapure water, and the pH of the suspension was adjusted using 1 mol/L NaOH or HCl. The crystalline structure of the catalysts was characterized using X-ray diffraction (XRD) (XRD-6000, Shimadzu, Japan) with Cu KR radiation (λ ) 1.5418 Å). A field-emission scanning electron microscopy (FESEM) (JEOL JSM-6700F) was used to observe the morphology of the titania films on the silica spheres. 2.3. Evaluation of Photocatalytic Activity. The photocatalytic activity of the SiO2/TiO2 core/shell photocatalyst was evaluated by measuring the degradation rate of Orange II under UV light illumination. Orange II is a nonbiodegradable synthetic dye with a molecular formula of C16H11N2NaO4S, widely used in the textile industry. The photocatalytic test was carried out in a semibatch swirl flow reactor. An aqueous solution containing Orange II was pumped tangentially into the reactor from a reservoir using a peristaltic pump. The reservoir was jacketed and maintained at a temperature of 25 °C throughout the experiment. A high-pressure mercury vapor lamp (Phillips HPR 125 W) was used as the UV light source with a wavelength of 365 nm and an incident light intensity of 110 W/m2. Light intensity was measured using a digital radiometer (model UVX-
Figure 3. FESEM images of silica spheres and TS particles: (a) 1.3 µm SiO2 spheres, (b) 0.8 µm SiO2 spheres, (c and d) TS-1300-170, (e) TS-1300-50, (f) TS-800-60, (g and h) crushed TS-1300-170, (i) a sample of TiO2-coated silica spheres without using PAH to modify the silica sphere surface, and (j) a sample prepared under magnetic stirring instead of ultrasonication during deposition of TiO2.
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Figure 4. Dependence of TiO2 shell thickness on the concentration of (NH4)2TiF6.
Figure 5. Photocatalytic activity of TS-1300-170, TS-1300-50, TS-80060, and P25 Degussa in degradation of Orange II.
36, UVP). The catalyst dosage in solution is 0.5 g/L, and the catalyst was recycled from solution by sedimentation after the Orange II was completely decomposed. The Orange II concentrations before and after reaction were measured using a UV-vis spectrophotometer (UV-1601, Shimadzu). Turbidity of SiO2/ TiO2 and P25 slurry was measured using a turbidimeter (Hach 2100N) after Orange II was completely decomposed. 3. Results and Discussion 3.1. Characterization of TS Catalysts. Figure 1 shows the Zeta-potential profiles of the silica spheres at the different catalyst preparation stages. As seen from Figure 1a, the surface of the SiO2 spheres was negatively charged in the whole pH range. As expected, upon coating a layer of PAH on the surface of the SiO2 spheres, they became positively charged (see Figure 1d). The positively charged surface of the PAH-modified silica microspheres facilitated the subsequent deposition of the TiO2 shell by offering a strong electrostatic interaction. An obvious charge reversal can be observed after adsorption of [TiF6]2anions on the PAH-functionalized silica spheres (see Figure 1b). After adding H3BO3, the electrostatically adsorbed [TiF6]2species were converted to TiO2 and fluoride ions were consumed by boric acid following reactions 1 and 2 + TiF26 + 2H2O S TiO2 + 6F + 4H
(1)
+ BO33 + 4F + 6H f BF4 + 3H2O
(2)
and the resultant solid displayed a zeta-potential profile as shown in Figure 1c, which is similar to that of dispersed titania. It is obvious that the first layer of the titania film formed on the silica spheres is due to electrostatic attractions between the PAHfunctionalized silica spheres and [TiF6]2- anions. For the subsequent layers of TiO2, van der Waals interactions are the main forces driving continuous deposition of TiO2 on the surface of the silica spheres.21 Figure 2 shows the wide-angle XRD patterns of the silica spheres, sample TS-1300-170, and Degussa P25. As expected, the silica spheres displayed a featureless XRD pattern because of the amorphous nature of the silica. Both samples TS-1300170 and P25 exhibited five characteristic peaks at 2θ ) 25.3°, 37.8°, 48.3°, 54.8°, and 63.4°, which are assigned to anatase titania.25 It is interesting to note that in comparison with the conventional methods of preparing TiO2 photocatalysts, which require high-temperature calcination to obtain anatase phase,26-28 the preparation method described in this work allowed us to directly obtain anatase phase without using high-temperature treatments. This is attributed to orientated deposition of TiO2
Figure 6. Turbidity against sedimentation time after photocatalytic reaction of two photocatalysts.
nanoparticles on the SiO2 microspheres from supersaturated aqueous solutions rather than irregular aggregation of TiO2 particles. Once SiO2 spheres are completely covered by a TiO2 film, growth along the c axis can only proceed on crystallites which have a c-axis orientation perpendicular to the SiO2 surface, leading to a columnar growth of the film with a preferred crystallographic orientation.20,21 It has also been reported that crystalline titania thin films were obtained on glass and various kinds of organic substrates at 40 - 70 °C by deposition from aqueous solutions containing titanium precursor.29 Figure 3 shows the FESEM images of silica spheres before and after TiO2 coating. As is seen from Figure 3a and 3b, the silica spheres synthesized in this work possessed a uniform size and smooth surface. With the assistance of a layer of PHA, uniform titania shells of different thicknesses were obtained (see Figures 3c and 3e). This surface coating of TiO2 can be accomplished on silica spheres of different diameters (see Figures 3e and f). As clearly seen from Figure 3d, the TiO2 film consisted of TiO2 nanoparticle with particle sizes in the range of 10-20 nm. In order to observe the microstructure of the TiO2 layer, sample TS-1700-170 was mechanically crushed to break up the TiO2 layer. As shown in Figure 3g and 3h, the titania shell was separated from the silica core. The smooth and uniform TiO2 shell is obvious. However, without using PAH to modify the surface of silica spheres, irregular patches instead of a uniform shell of TiO2 was obtained (see Figure 3i), showing the essence of surface modification of the silica spheres by PAH to form a uniform TiO2 shell. Figure 3j is the FESEM image of a sample which was synthesized under magnetic mixing instead of ultrasonication. As can be seen, the silica spheres agglomer-
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Figure 7. Sedimentation profiles of catalysts P25 and TS-1300-170 at times of (a) 0 and (b) 150 min.
ated to form sphere clusters and only the external surface of the aggregates was covered by titania, indicating that a good dispersion of SiO2 spheres is a prerequisite to prepare SiO2/ TiO2 core/shell structures. As only the outmost layer of the coated TiO2 is available for photocatalytic reactions, efforts were made to control the thickness of the titania shell in this work. Figure 4 shows the dependence of TiO2 shell thickness on the concentration of the titanium precursor used during the LPD step. It can be seen that the thickness of the titania shell increased with the concentration of (NH4)2TiF6. Thus, the thickness of the TiO2 shell can be controlled by varying the titanium precursor concentration. 3.2. Photocatalytic Activity. Figure 5 shows the photocatalytic properties of the SiO2/TiO2 core/shell photocatalysts and P25. All photocatalytic activity tests were carried out in solutions of different Orange II concentrations with pH values between 5.5 and 6.5. A blank experiment in the absence of a photocatalyst but with a radiation of 365 nm wavelength showed no obvious conversion of Orange II. The photocatalytic properties of photocatalysts with the same diameter of SiO2 core but different thicknesses of TiO2 shell (e.g., samples TS-1300-50 and TS-1300-170) and with a similar thickness of TiO2 shell but different diameters of SiO2 core (e.g., samples TS-1300-50 and TS-800-60) were evaluated. It can be seen from Figure 5 that the photocatalytic activities of catalysts TS-1300-50 and TS-1300-170 are similar with a less than 5% variation, indicating the thickness of coated TiO2 shell does not have an obvious effect on the photocatalytic activity. On the other hand, the photoactivity of catalyst TS-800-60 is slightly higher than that of catalyst TS-1300-50. This may be attributed to the slightly higher specific surface area of the former because of its smaller particle size. It is also seen from Figure 5 that both catalysts TS-1300-170 and TS-800-60 displayed a lower photocatalytic activity than that of P25, which again is believed to be due to the smaller specific surface areas of the former two samples (their particle sizes are larger than that of P25). While agglomeration of P25 nanoparticles under the experimental conditions was indeed observed to form larger particles of diameters in the range of 200-300 nm, the average size of the agglomerated P25 particles is still smaller than that of samples TS-1300-170 and TS-800-60. 3.3. Catalyst Decantation. In addition to its photocatalytic performance, the decantation property (turbidity) of a photocatalyst for water treatment is another important factor in terms of the “practical efficiency”.30 Therefore, the turbidity of the reaction system was measured after Orange II had been
completely decomposed. Figure 6 compares the turbidity as a function of time after photocatalytic reactions of two photocatalysts. As can be seen, the turbidity of the reaction system catalyzed by TS-1300-170 dropped rapidly. In contrast, the turbidity of the reaction system of catalyst P25 changed insignificantly within 200 min. As clearly seen from Figure 7, solid catalyst TS-1300-170 completely precipitated after 150 min, yielding a clear solution on the top. Separation of the solid from the liquid by decanting yielded about 95% of solid TS1300-170, while the other 5% of solid catalyst was lost due to both sampling and adherence to the wall of the container. These observations demonstrate that the core/shell SiO2/TiO2 photocatalyst can be easily separated from the reaction system simply by sedimentation, while this is unachievable with P25. The photocatalytic activity of the recycled catalyst TS-1300170 was further evaluated, and the results are shown in Figure 8. It can be seen that the recycled solid retained good photocatalytic activity in spite of a slight decrease in activity compared with fresh TS-1300-170. With increasing the runs of recycling, the photocatalytic activity of the solid began to decrease quickly. Because of the electrostatic interaction between the TiO2 shell and the SiO2 core via the polyelectrolyte layer, peeling off of the TiO2 shell from the SiO2 core is unlikely. Thus, the observed decrease in catalytic activity is believed to be due to adsorption of some organic intermediates on the catalyst surface, which were formed due to decomposition of Orange II.7 An element analysis for the recycled photocatalyst indeed confirmed the presence of carbon species on the surface of the recycled TS-1300-170 photocatalyst. The major intermediate products include phenol, oxalate, malonate, and dibutyl
Figure 8. Photocatalytic activities of catalyst TS-1300-170 in the (a) fresh, (b) first, (c) second, (d) third, and (e) fourth runs.
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phthalate.7 After four runs of recycling, the recovered solid retained less than 50% of the photoactivity of the fresh catalyst. 4. Conclusions With the assistance of polyelectrolyte poly allylamine hydrochloride, uniform anatase titania films with controllable thickness have been coated on the surfaces of silica spheres of micrometer size and characterized. The polyelectrolyte modification of the silica spheres is a prerequisite to obtain a uniform TiO2 shell on the surface of silica spheres. The core/shell SiO2/ TiO2 materials were observed to possess a slightly poorer photocatalytic activity than a commercial photocatalyst, P25. The unique feature of the core-shell photocatalyst described in this paper lies in the fact that they can be easily separated from the reaction medium simply by sedimentation, thus facilitating industrial applications. Acknowledgment Financial support from a AcRF Tier 1 grant of the Ministry of Education of Singapore with an account number of RP279000235112 is acknowledged. Literature Cited (1) Liu, Z. Y.; Quan, X.; Fu, H. B.; Li, X. Y.; Yang, K. Effect of embedded-silica on microstructure and photocatalytic activity of titania prepared by ultrasound-assisted hydrolysis. Appl. Catal. B: EnViron. 2004, 52, 33. (2) Crittenden, J. C; Liu, J. B.; Hand, D. W.; Perram, D. L. Photocatalytic oxidation of chlorinated hydrocarbons in water. Water Res. 1997, 31, 429. (3) Lee, S.-W.; Drwiega, J.; Wu, C.-Y.; Mazyck, D.; Sigmund, W. M. Anatase TiO2 nanoparticle coating on barium ferrite using titanium bisammonium lactato dihydroxide and its use as a magnetic photocatalyst. Chem. Mater. 2004, 16, 1160. (4) Li, G. Performance improvement of TiO2 catalysts supported on adsorbents Ph.D thesis, National University of Singapore, Singapore, 2007. (5) Yi, D. K.; Lee, S. S.; Ying, J. Y. Synthesis and application of magnetic nanocomposite catalysts. Chem. Mater. 2006, 18, 2459. (6) Yuan, R. S.; Zheng, J. T.; Guan, R. B.; Zhao, Y. C. Surface characteristics and photocatalytic activity of TiO2 loaded on activated carbon fibers. Colloid Surf. A 2005, 254, 131. (7) Bhattacharyya, A.; Kawi, S.; Ray, M. B. Photocatalytic degradation of organic II by TiO2 catalysts supported on adsorbents. Catal. Today 2004, 98, 431. (8) Van Grieken, R.; Aguado, J.; Lo´pez-Mun˜oz, M. J.; Maruga´n, J. Synthesis of size-controlled silica-supported TiO2 photocatalyst. J. Photochem. Photobiol. A 2002, 148, 315. (9) Li, G.; Zhao, X. S. Characterization and photocatalytic properties of titanium-containing mesoporous SBA-15. Ind. Eng. Chem. Res. 2006, 45, 3569. (10) Toyoda, M.; Nanbu, Y.; Kito, T.; Hirano, M.; Inagaki, M. Preparation and performance of anatase-loaded porous carbons for water purification. Desalination 2003, 159, 273. (11) Zhou, J.; Chen, M.; Qiao, X. G.; Wu, L. M. Facile preparation method of SiO2/PS/TiO2 multilayer core-shell hybrid microspheres. Langmuir 2006, 22, 10175.
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ReceiVed for reView April 8, 2008 ReVised manuscript receiVed July 2, 2008 Accepted August 24, 2008 IE800561Y
