Langmuir 1999, 15, 5535-5540
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Multistep Coating of Thick Titania Layers on Monodisperse Silica Nanospheres Xing-Cai Guo† and Peng Dong* National Laboratory of Heavy Oil Processing, Department of Chemical Engineering, University of Petroleum, Changping, Beijing, 102200, People’s Republic of China Received February 24, 1999. In Final Form: May 10, 1999 Titania coating on monodisperse silica spheres was carried out with a multistep method using titanium n-butoxide. Titania-coated silica spheres were characterized with transmission electron microscopy and energy-dispersive X-ray flourescence spectroscopy. Electrophoretic properties and size distributions of the particles were also measured. Starting from monodisperse silica spheres of 550 nm in mean diameter, the thickness of titania coatings achieved with five coating steps was up to 46 nm, or 125 monolayers of titania, equivalent to a titania weight loading of 54.7 wt %. The uniformity of the titania coating was confirmed by the precise agreement in the electrokinetic mobility of the resulting spheres with that of bulk titania particles. High monodispersity was maintained with a relative standard deviation in diameter of less than 5%. The aggregation extent of the coated spheres was only increased slightly from 11.8% for the silica spheres to 17.2% after three steps of titania coating.
1. Introduction Titania coated on monodisperse silica spheres is of great interest in potential applications as a catalyst,1 as a white pigment (whitener),2 as a photonic crystal,3 etc. Bulk titania, when used as a high surface area catalyst, is thermally unstable and readily loses its surface area, whereas titania dispersed on silica spheres was stable up to 1058 K and increased the reactivity for 1-propanol dehydration by over 2 orders of magnitude.1 When used as a whitener, titania powder is commonly produced by leaching ilmenite with sulfuric acid. Growing environmental concern with the acids makes it desirable to develop alternative procedures. One of such procedures is by coating titania on spherical silica particles with the lowest titania content possible.2 Because of their high refractive index, titania spheres may be good candidates for a photonic crystal with a complete band gap in the nearinfrared and visible regions if they are made uniform in size and packed orderly in structure.3 Since monodisperse silica spheres can be easily prepared with the Sto¨ber procedure4, it would be possible to coat titania on monodisperse silica spheres to large thickness while maintaining good monodispersity. This forms the objective of the present work. Several procedures have been reported in the literature to coat titania on monodisperse silica spheres. In one of the procedures, Srinivansan et al.1 added 270 nm (in diameter) silica spheres to a solution of titanium tertbutoxide in tetrahydrofuran under a dry nitrogen atmosphere. After the mixture was stirred for 0.25 h, the suspension was filtered under nitrogen, washed with tetrahydrofuran, and dried in a vacuum. Only up to a † Present address: Department of Materials Science and Engineering, University of Delaware, 301 Spencer Lab, Newark, DE 19716.
(1) Srinivasan, S.; Datye, A. K.; Hampden-Smith, M.; Wachs, I. E.; Deo, G.; Jehng, J. M.; Turek, A. M.; Peden, C. H. F. J. Catal. 1991, 131, 260. (2) Hsu, W. P.; Yu, R.; Matijevic, E. J. Colloid Interface Sci. 1993, 156, 56. (3) Mei, D.-B.; Dong, P.; Li, H.-Q.; Cheng, B.-Y.; Zhang, D.-Z. Chin. Phys. Lett. 1998, 15, 21. (4) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.
monolayer of titania could be coated with this method. In another procedure, Hsu et al.2 worked with silica spheres of different mean diameters from 0.40 to 1.3 µm and used a solution of titanyl sulfate in sulfuric acid as the starting material. The weight loading of titania thus prepared ranged from 7 to 33 wt %. In the preparation of particles containing more than 20 wt % titania, a two-step coating sequence was recommended. In the first step, one-half of the titanyl sulfate solution was added to react with the silica spheres. The suspension was then filtered and resuspended in the second step for reactions with the remaining half of the titanyl sulfate solution. Electrokinetic mobility of the resulting spheres approached those of bulk titania spheres but remained consistently lower than those of titania at pH values less than the isoelectric point, casting doubt about the uniformity of the titania coverage on silica spheres. Furthermore, the continued use of sulfuric acid and the low pH values (0.7-1.5) of the final suspension may not be suitable for some applications. Recently, Hanprasopwattana et al.5 have modified the procedure using titanium alkoxide as a precursor. In their procedure, an ethanol solution of titanium n-butoxide was refluxed after a certain amount of water was added. The final suspension was vacuum filtered, washed with ethanol, and dried in a vacuum. The titania loading obtained from 270 nm silica spheres was up to 36.9 wt %, corresponding to about 7 nm, or 20 monolayers if a titania monolayer was assumed to have a thickness of 0.355 nm (the interlayer spacing for the {101} plane of the anatase structure of titania). Complete coverage of silica with titania was demonstrated by the agreement between the BET surface area and the effective titania surface area derived from the reactivity of 2-propanol dehydration. The coating quality was found to be affected by the ratio of titanium alkoxide to water and the dilution of the reactant mixture in ethanol. When the alkoxide concentration was higher than 0.0091 M, second-phase titania particles precipitated rather than a uniform coating. Water concentrations higher than 0.32 M led to aggregated spheres connected with titania necks. (5) Hanprasopwattana, A.; Srinivasan, S.; Sault, A. G.; Datye, A. K. Langmuir 1996, 12, 3173.
10.1021/la990220u CCC: $18.00 © 1999 American Chemical Society Published on Web 06/18/1999
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Table 1. Diameter and Coating Thickness (nm) of Nanospheres Measured with TEM and Sedimentation Methods sample code A-1Si-0Ti A-1Si-1Ti A-1Si-5Ti B-1Si-0Ti B-1Si-1Ti B-1Si-3Ti C-0Si-1Ti
mean TEM diameter and its std deviationa (nm)
most populous Stokes diameter and its hwhma (nm)
coating thickness (nm)
545 ( 10 (1.8%) 593 ( 16 (2.7%) 637 ( 16 (2.5%) 559 ( 12 (2.1%)
549 ( 11 (2.0%)
0 24 46 0 9 13
588 ( 12 (2.0%) 262 ( 49 (19%)
637 ( 40 (6.3%) 551 ( 25 (4.5%) 568 ( 28 (4.9%) 577 ( 21 (3.6%) 255 ( 29 (8.7%)
aggregation extentb (%)
11.8 15.1 17.2 31.0
a The percentage in parentheses is the ratio of standard deviation or half width at half-maximum (hwhm) to diameter, giving an indicator for monodispersity. b The aggregation extent is defined here as the intensity ratio of the peak at large diameters to the peak at the Stokes diameter in the particle size distribution figure.
In these previous studies, no data were reported on the monodispersity of initial silica spheres and final titaniacoated spheres. On the other hand, monodispersity of the coated spheres, along with uniformity and thickness of the coating, is crucial to the objective of our present work. To monitor monodispersity, we have used a transmission electron microscope (TEM) and/or a particle size analyzer to measure particle size distributions before and after coating, which also provide information on the coating thickness. Titania weight loading is analyzed with energydispersive X-ray fluorescence spectroscopy (EDS). The procedure using titanium alkoxide precursor is adopted to ensure coating uniformity. In addition, electrokinetic mobility of titania-coated silica spheres is measured and compared with that of bulk titania spheres. To increase coating thickness and to minimize particle aggregation and second-phase formation, we have resorted to a multistep method to keep low the concentrations of alkoxide and water in each step. With all of these implemented, we are able to produce titania-coated silica nanospheres with large coating thickness and high monodispersity. The application of these coated spheres in studies of photonic band gaps is currently underway. 2. Experimental Section 2.1. Materials. The following materials have been used: tetraethyl orthosilicate (Si(OC2H5)4, >99%, Fluka Chemie AG, further purified by distillation), titanium n-butoxide (Ti(OC4H9)4, 98.0%, Beijing Jinlong Chemicals, used without further purification), ammonium hydroxide (25-28% NH3, Beijing Yili Fine Chemicals), ethanol (99.7%, Beijing Chemical Engineering), and a commercial sample of TiO2 particles (2% solids in water, Polysciences, Inc.). 2.2. Procedures. A. Monodisperse Silica Spheres. Monodisperse silica spheres were prepared with the procedure originally described by Sto¨ber et al.,4 i.e., hydrolysis of tetraethyl orthosilicate (TEOS) in an ethanol solution containing water and ammonia. In a typical experiment, a 5 mL ethanol solution of TEOS was added to a 25 mL ethanol solution of water and ammonia. The 30 mL mixture containing 0.22 M TEOS, 6 M H2O, and 2 M NH3 was stirred at 25 °C for 4 h. The resulting silica spheres were centrifugally separated from the suspension and ultrasonically washed with ethanol. For analysis purpose, the silica spheres were further washed with water. B. Titania Coating of Silica Spheres. Titania was coated on monodisperse silica spheres using a procedure originally reported by Hanprasopwattana et al.5 In one experiment, silica spheres dispersed in ethanol were mixed with a certain amount of titanium n-butoxide and water. More ethanol was added to make the total volume to 100 mL. The concentration of titanium n-butoxide was kept at 0.0091 M and that of water at 0.32 M. The mixture was refluxed and stirred for 1.5 h. The resulting titania-coated spheres were separated centrifugally and washed twice with ethanol. A small part of the sample was taken and washed with water for analysis. The above procedure was repeated several times in order to increase coating thickness. 2.3. Characterization. Zeta potentials of the nanoparticles were measured with a microelectrophoresis instrument (Pow-
Figure 1. Variation of zeta potential with pH for silica spheres without titania coating (sample A-1Si-0Ti), bulk titania spheres (C-0Si-1Ti), and titania-coated silica spheres (A-1Si-1Ti and A-1Si-5Ti). Titania coating has shifted the isoelectric point (IEP) from that of silica to that of bulk titania, indicating that titania is uniformly coated. ereach JS94E, Shanghai Jiecheng, Ltd.). An electrolyte solution (0.001 M KCl) was used to keep the ionic strength constant while the pH values was varied by adding 0.01 N HCl into the solution. Transmission electron micrographs and energy-dispersive X-ray fluorescence spectra were obtained on the same microscope (H-800, Hitachi) operated at 200 keV. Particles dried in air were supported on Formvar films mounted on 230 mesh, 3 mm TEM copper grids. Particle sizes were determined by direct measurements of particle images on the transmission electron micrographs as well as by the sedimentation method using the well-known Stokes formula
(F1 - F0)gd12 - 18ηυ1 ) 0
(1)
d1 ) [18ηυ1/(F1 - F0)g]1/2
(2)
or
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Figure 2. Energy-dispersive X-ray fluorescence spectra of (a) silica spheres before titania coating (sample A-1Si-0Ti), (b) silica spheres with one-step coating of titania (A-1Si-1Ti), (c) silica spheres with five-step coating of titania (A-1Si-5Ti), and (d) bulk titania spheres (C-0Si-1Ti). Assignments for various peaks are listed in the inset table and labeled for each element. where d1 is the diameter of a sphere with a uniform density F1 and a constant settling velocity υ1, and F0 and η are, respectively, the density and viscosity of fluid media, i.e., water in the present work. For a sphere coated with a layer of different density F2, the Stokes formula may be modified as
(F0 - F2)gd3 + 18ηυd(F1 - F2)gd13 ) 0
(3)
where d and υ are the diameter and settling velocity of the coated sphere, respectively. Let
a ) (F1 - F2)d13/2(F0 - F2)
(4)
b ) 6ηυ/g(F0 - F2)
(5)
and
Figure 3. Transmission electron micrographs of (a) silica spheres before titania coating (sample A-1Si-0Ti) and (b) five coating steps (Sample A-1Si-5Ti). Direct measurement of each sphere in the images gives the mean diameters (a) 545 nm and (b) 638 nm.
d ) [a + (a2 + b3)1/2]1/3 + [a - (a2 + b3)1/2]1/3
(6)
For example, a titania-coated silica sphere with d1 ) 549 nm, F1 ) 1.9 g cm-3, and F2 ) 2.8 g cm-3 settles at a velocity of 0.12 cm h-1. According to the modified Stokes equation, the particle diameter is calculated to be 637 nm, in agreement with that obtained from the TEM measurement (638 nm). In this situation, if one applies the Stokes formula regardless the differences in density, the obtained value will be 744 nm, 17% higher than the correct diameter. On the basis of the Stokes or modified Stokes formula, distributions in particle sizes were measured on a home-built particle size analyzer using a UV-vis spectrophotometer (model 721, Shanghai Third Analytical Instrument). UV light at a wavelength of 450 nm passes through a horizontal slit of 1 mm in width before reaching the suspension cell containing the particles. Transmission was monitored versus settling time which may be converted to particle sizes. Differentiation of transmission data gives the relative distribution in particle size.
3. Results and Discussion
then according to algebra the solution of eq 3 is
To provide an overview, the samples investigated are summarized in Table 1. Samples with an initial A or B in the sample code were prepared in this work, while sample
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Figure 4. Particle size distributions for (a) silica spheres without titania coating (sample B-1Si-0Ti), (b) with one coating step (B-1Si-1Ti), (c) with three coating steps (B-1Si-3Ti), and (d) the commercial bulk titania spheres (C-0Si-1Ti). See text for details.
C was purchased. The numbers in the sample code indicate preparation steps. For example, A-1Si-5Ti means a fivestep coating of titania on the silica spheres A-1Si-0Ti. Figure 1 shows the variations of zeta potentials with pH values for silica spheres (A-1Si-0Ti), titania-coated silica spheres (A-1Si-1Ti and A-1Si-5Ti), and bulk titania spheres (C-0Si-1Ti). The variations give an isoelectric point (IEP) at 3.17 for silica spheres and at 3.54 for titania spheres. The behavior is very similar to that in the literature, although the specific values are somewhat lower than those previously reported,2 possibly due to different heat treatments of the particles. No additional heat treatment or drying was carried out for the samples prepared in the present work. Nevertheless, it is clear
from Figure 1 that titania coating of silica spheres has systematically increased the electrophoretic mobility and shifted the IEP to the exact value of bulk titania spheres. It is also clear that one-step coating of titania is sufficient to alter the electrokinetic behavior completely, indicating that the uniformity of titania coating is excellent with the present procedure. Energy-dispersive X-ray fluorescence spectra were measured for the same samples and are illustrated in Figure 2. Assignments for peaks in the spectra are listed in the inset table. The two copper peaks in all spectra at 8.0 and 8.9 keV come from copper grids supporting the particles. A silicon peak at 1.7 keV shown in Figure 2a (Sample A-1Si-0Ti) is a combination of its KR1 at 1.740
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Figure 5. Transmission electron micrographs of (a) prepared titania-coated silica spheres (sample B-1Si-3Ti) and (b) commercial bulk titania spheres. Differences in monodispersity and aggregation extent are evident.
keV and KR2 at 1.739 keV and is obviously due to silica particles. No peak for other elements is visible in the spectrum. In Figure 2b (sample A-1Si-1Ti), besides the silicon and copper peaks, a titanium peak appears at 4.5 keV (KR1 + KR2) due to titania coatings. The intensity of the titanium peak increases for more coating steps, as shown in Figure 2c (sample A-1Si-5Ti) for five coating steps. An additional weak peak at 4.9 keV is the Kβ1 of titanium. The two titanium peaks coincide with those in Figure 2c (sample C-0Si-1Ti) for bulk titania particles, where the silicon peak is absent as it should be. Quantitative analysis of the spectra yields a titania weight loading of 5.3 wt % for sample A-1Si-1Ti and 64.1 wt % for sample A-1Si-5Ti, the latter is much higher than those
previously achieved (33-37 wt %).2,5 It should be mentioned, however, that the energy dispersive spectra were recorded for one particular particle in each sample. The weight loading thus obtained may vary somewhat from particle to particle, as shown early.1 Figure 3 shows transmission electron micrographs of two titania-coated samples, A-1Si-0Ti and A-1Si-5Ti. As can be seen, the titania-coated silica spheres are round and smooth, providing additional evidence for uniform coatings. Direct measurement of each sphere in the TEM images gives a mean diameter of (a) 545 nm with a standard deviation of 10 nm and (b) 638 nm with a standard deviation of 16 nm, a 93 nm increase due to five successive coating steps. The TEM mean diameters are
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in good agreement with those determined by sedimentation using the modified Stokes formula. For example, the Stokes diameter for particles in sample A-1Si-5Ti is 637 nm with a half-width-at-half-maximum (hwhm) of 40 nm (equivalent to a standard deviation of 34 nm). The reason for larger standard deviations in Stokes diameters is because sedimentation measurements sample a large number of particles, whereas only about 30 particles are measured in the TEM image. Sedimentation measurements were also performed for the starting silica sample (A-1Si-0Ti), giving a Stokes diameter of 549 nm. The coating thickness is estimated to be 24 nm after the first coating step and 46 nm after the fifth coating step, as listed in Table 1. On the basis of these data, equivalent titania loading are calculated to be 54.7 wt % for sample A-1Si-5Ti. Assuming a titania monolayer to have a thickness of 0.355 nm,5 equivalent titania monolayers may also be obtained, 125 monolayers for sample A-1Si-5Ti. Particle size distributions were measured for Sample B and C as shown in Figure 4, which illustrate minimal degredation in particle monodispersity and aggregation extent after titania coating. A lower concentration of titanium butoxide was used in the coating steps for sample B, resulting in thinner coatings per step (cf. Table 1). The size distribution of the starting silica spheres is shown in Figure 4a, which may be deconvoluted into two Gaussian peaks. The most populous peak exhibits a Stokes diameter of 551 nm with a hwhm of 25 nm, giving a relative monodispersity of 4.5% (cf. Table 1). The smaller and broader peak at larger diameters may be due to aggregated doublets of spheres, from which the aggregation extent is estimated to be 11.8%. The aggregation extent is defined here as the intensity ratio of the peak at large diameters to the peak at the Stokes diameter in the particle size distribution figure. Following the first step of titania coating, the mean particle diameter is increased by 9 nm, with a slight change in monodispersity (4.9%) and aggregation extent (15.1%). After the third coating step,
Guo and Dong
the monodispersity is maintained below 5%, and the aggregation extent is still low (17.2%). The two small, sharp peaks at large diameters in Figure 4c may be due to titania coatings with slight differences in thickness. For the purpose of comparison, Figure 4d shows the particle size distribution for a commercial titania sample (C-0Si-1Ti). Obviously, its monodispersity (8.7%) is poorer and aggregation (31.0%) is severer than those in the samples prepared in this work. The point is more clearly demonstrated by the TEM images in Figure 5. Figure 5a shows a typical image of 78 titania-coated silica spheres in sample B-1Si-3Ti, yielding a mean diameter of 580 nm with a standard deviation of 19 nm, whereas Figure 5b shows the image of 75 bulk titania spheres obtained commercially (C-0Si-1Ti) with a mean diameter of 262 nm and a standard deviation of 49 nm. The roughness of the titania spheres in Figure 5b is due to crystallization after high-temperature calcination. The effects of calcination on the morphology of titania-coated silica spheres have been investigated previously, showing that calcination at 773 K converts amorphous titania to crystallized anatase.5 No calcination was carried out for samples in the present work. 4. Conclusion From the results of the present study, it may be concluded that the multistep coating method using titanium n-butoxide can be successfully utilized to produce thick and uniform titania coatings on monodisperse silica spheres while maintaining high monodispersity and low aggregation extent. Further experiments are needed for a better control of the coating thickness. Acknowledgment. Financial support from the Chinese National Natural Science Foundation is gratefully acknowledged. LA990220U
