Multistep Coating of Thick Titania Layers onMonodisperse 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 titaniumn-butoxide. Titania-coated silica spheres were characterized with transmission electron microscopy andenergy-dispersive X-ray flourescence spectroscopy. Electrophoretic properties and size distributions of theparticles were also measured. Starting from monodisperse silica spheres of 550 nm in mean diameter, thethickness 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 confirmedby the precise agreement in the electrokinetic mobility of the resulting spheres with that of bulk titaniaparticles. High monodispersity was maintained with a relative standard deviation in diameter of less than5%. The aggregation extent of the coated spheres was only increased slightly from 11.8% for the silicaspheres to 17.2% after three steps of titania coating.

1. Introduction

Titania coated on monodisperse silica spheres is of greatinterest in potential applications as a catalyst,1 as a whitepigment (whitener),2 as a photonic crystal,3 etc. Bulktitania, when used as a high surface area catalyst, isthermally unstable and readily loses its surface area,whereas titania dispersed on silica spheres was stable upto 1058 K and increased the reactivity for 1-propanoldehydration by over 2 orders of magnitude.1 When usedas a whitener, titania powder is commonly produced byleaching ilmenite with sulfuric acid. Growing environ-mental concern with the acids makes it desirable to developalternative procedures. One of such procedures is bycoating titania on spherical silica particles with the lowesttitania content possible.2 Because of their high refractiveindex, titania spheres may be good candidates for aphotonic crystal with a complete band gap in the near-infrared and visible regions if they are made uniform insize and packed orderly in structure.3 Since monodispersesilica spheres can be easily prepared with the Stoberprocedure4, it would be possible to coat titania onmonodisperse silica spheres to large thickness whilemaintaining good monodispersity. This forms the objectiveof the present work.

Several procedures have been reported in the literatureto coat titania on monodisperse silica spheres. In one ofthe procedures, Srinivansan et al.1 added 270 nm (indiameter) silica spheres to a solution of titanium tert-butoxide in tetrahydrofuran under a dry nitrogen atmo-sphere. After the mixture was stirred for 0.25 h, thesuspension was filtered under nitrogen, washed withtetrahydrofuran, and dried in a vacuum. Only up to a

monolayer of titania could be coated with this method. Inanother procedure, Hsu et al.2 worked with silica spheresof different mean diameters from 0.40 to 1.3 µm and useda solution of titanyl sulfate in sulfuric acid as the startingmaterial. The weight loading of titania thus preparedranged from 7 to 33 wt %. In the preparation of particlescontaining more than 20 wt % titania, a two-step coatingsequence was recommended. In the first step, one-half ofthe titanyl sulfate solution was added to react with thesilica spheres. The suspension was then filtered andresuspended in the second step for reactions with theremaining half of the titanyl sulfate solution. Electro-kinetic mobility of the resulting spheres approached thoseof bulk titania spheres but remained consistently lowerthan those of titania at pH values less than the isoelectricpoint, casting doubt about the uniformity of the titaniacoverage on silica spheres. Furthermore, the continueduse of sulfuric acid and the low pH values (0.7-1.5) of thefinal suspension may not be suitable for some applications.Recently, Hanprasopwattana et al.5 have modified theprocedure using titanium alkoxide as a precursor. In theirprocedure, an ethanol solution of titanium n-butoxide wasrefluxed after a certain amount of water was added. Thefinal suspension was vacuum filtered, washed withethanol, and dried in a vacuum. The titania loadingobtained from 270 nm silica spheres was up to 36.9 wt %,corresponding to about 7 nm, or 20 monolayers if a titaniamonolayer was assumed to have a thickness of 0.355 nm(the interlayer spacing for the {101} plane of the anatasestructure of titania). Complete coverage of silica withtitania was demonstrated by the agreement between theBET surface area and the effective titania surface areaderived from the reactivity of 2-propanol dehydration. Thecoating quality was found to be affected by the ratio oftitanium alkoxide to water and the dilution of the reactantmixture in ethanol. When the alkoxide concentration washigher than 0.0091 M, second-phase titania particlesprecipitated rather than a uniform coating. Water con-centrations higher than 0.32 M led to aggregated spheresconnected with titania necks.

† Present address: Department of Materials Science and En-gineering, University of Delaware, 301 Spencer Lab, Newark, DE19716.

(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.

(5) Hanprasopwattana, A.; Srinivasan, S.; Sault, A. G.; Datye, A. K.Langmuir 1996, 12, 3173.

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10.1021/la990220u CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 06/18/1999

In these previous studies, no data were reported on themonodispersity of initial silica spheres and final titania-coated spheres. On the other hand, monodispersity of thecoated spheres, along with uniformity and thickness ofthe coating, is crucial to the objective of our present work.To monitor monodispersity, we have used a transmissionelectron microscope (TEM) and/or a particle size analyzerto measure particle size distributions before and aftercoating, which also provide information on the coatingthickness. Titania weight loading is analyzed with energy-dispersive X-ray fluorescence spectroscopy (EDS). Theprocedure using titanium alkoxide precursor is adoptedto ensure coating uniformity. In addition, electrokineticmobility of titania-coated silica spheres is measured andcompared with that of bulk titania spheres. To increasecoating thickness and to minimize particle aggregationand second-phase formation, we have resorted to amultistep method to keep low the concentrations ofalkoxide and water in each step. With all of theseimplemented, we are able to produce titania-coated silicananospheres with large coating thickness and highmonodispersity. The application of these coated spheresin studies of photonic band gaps is currently underway.

2. Experimental Section2.1. 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 purifica-tion), ammonium hydroxide (25-28% NH3, Beijing Yili FineChemicals), ethanol (99.7%, Beijing Chemical Engineering), anda commercial sample of TiO2 particles (2% solids in water,Polysciences, Inc.).

2.2. Procedures. A. Monodisperse Silica Spheres. Mono-disperse silica spheres were prepared with the procedureoriginally described by Stober et al.,4 i.e., hydrolysis of tetraethylorthosilicate (TEOS) in an ethanol solution containing waterand ammonia. In a typical experiment, a 5 mL ethanol solutionof TEOS was added to a 25 mL ethanol solution of water andammonia. The 30 mL mixture containing 0.22 M TEOS, 6 MH2O, and 2 M NH3 was stirred at 25 °C for 4 h. The resultingsilica spheres were centrifugally separated from the suspensionand ultrasonically washed with ethanol. For analysis purpose,the silica spheres were further washed with water.

B. Titania Coating of Silica Spheres. Titania was coatedon monodisperse silica spheres using a procedure originallyreported by Hanprasopwattana et al.5 In one experiment, silicaspheres dispersed in ethanol were mixed with a certain amountof titanium n-butoxide and water. More ethanol was added tomake the total volume to 100 mL. The concentration of titaniumn-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 resultingtitania-coated spheres were separated centrifugally and washedtwice with ethanol. A small part of the sample was taken andwashed with water for analysis. The above procedure wasrepeated several times in order to increase coating thickness.

2.3. Characterization. Zeta potentials of the nanoparticleswere measured with a microelectrophoresis instrument (Pow-

ereach JS94E, Shanghai Jiecheng, Ltd.). An electrolyte solution(0.001 M KCl) was used to keep the ionic strength constant whilethe pH values was varied by adding 0.01 N HCl into the solution.

Transmission electron micrographs and energy-dispersiveX-ray fluorescence spectra were obtained on the same microscope(H-800, Hitachi) operated at 200 keV. Particles dried in air weresupported on Formvar films mounted on 230 mesh, 3 mm TEMcopper grids.

Particle sizes were determined by direct measurements ofparticle images on the transmission electron micrographs as wellas by the sedimentation method using the well-known Stokesformula

or

Table 1. Diameter and Coating Thickness (nm) of Nanospheres Measured with TEM and Sedimentation Methods

sample codemean TEM diameter

and its std deviationa (nm)most populous Stokes

diameter and its hwhma (nm)coating

thickness (nm)aggregationextentb (%)

A-1Si-0Ti 545 ( 10 (1.8%) 549 ( 11 (2.0%) 0A-1Si-1Ti 593 ( 16 (2.7%) 24A-1Si-5Ti 637 ( 16 (2.5%) 637 ( 40 (6.3%) 46B-1Si-0Ti 559 ( 12 (2.1%) 551 ( 25 (4.5%) 0 11.8B-1Si-1Ti 568 ( 28 (4.9%) 9 15.1B-1Si-3Ti 588 ( 12 (2.0%) 577 ( 21 (3.6%) 13 17.2C-0Si-1Ti 262 ( 49 (19%) 255 ( 29 (8.7%) 31.0

a The percentage in parentheses is the ratio of standard deviation or half width at half-maximum (hwhm) to diameter, giving an indicatorfor monodispersity. b The aggregation extent is defined here as the intensity ratio of the peak at large diameters to the peak at the Stokesdiameter in the particle size distribution figure.

Figure 1. Variation of zeta potential with pH for silica sphereswithout titania coating (sample A-1Si-0Ti), bulk titania spheres(C-0Si-1Ti), and titania-coated silica spheres (A-1Si-1Ti andA-1Si-5Ti). Titania coating has shifted the isoelectric point (IEP)from that of silica to that of bulk titania, indicating that titaniais uniformly coated.

(F1 - F0)gd12 - 18ηυ1 ) 0 (1)

d1 ) [18ηυ1/(F1 - F0)g]1/2 (2)

5536 Langmuir, Vol. 15, No. 17, 1999 Guo and Dong

where d1 is the diameter of a sphere with a uniform density F1and a constant settling velocity υ1, and F0 and η are, respectively,the density and viscosity of fluid media, i.e., water in the presentwork. For a sphere coated with a layer of different density F2, theStokes formula may be modified as

where d and υ are the diameter and settling velocity of the coatedsphere, respectively. Let

and

then according to algebra the solution of eq 3 is

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 cmh-1. According to the modified Stokes equation, the particlediameter is calculated to be 637 nm, in agreement with thatobtained from the TEM measurement (638 nm). In this situation,if one applies the Stokes formula regardless the differences indensity, the obtained value will be 744 nm, 17% higher than thecorrect diameter.

On the basis of the Stokes or modified Stokes formula,distributions in particle sizes were measured on a home-builtparticle size analyzer using a UV-vis spectrophotometer (model721, Shanghai Third Analytical Instrument). UV light at awavelength of 450 nm passes through a horizontal slit of 1 mmin width before reaching the suspension cell containing theparticles. Transmission was monitored versus settling time whichmay be converted to particle sizes. Differentiation of transmissiondata gives the relative distribution in particle size.

3. Results and Discussion

To provide an overview, the samples investigated aresummarized in Table 1. Samples with an initial A or B inthe sample code were prepared in this work, while sample

Figure 2. Energy-dispersive X-ray fluorescence spectra of (a)silica spheres before titania coating (sample A-1Si-0Ti), (b) silicaspheres with one-step coating of titania (A-1Si-1Ti), (c) silicaspheres with five-step coating of titania (A-1Si-5Ti), and (d)bulk titania spheres (C-0Si-1Ti). Assignments for various peaksare listed in the inset table and labeled for each element.

(F0 - F2)gd3 + 18ηυd(F1 - F2)gd13 ) 0 (3)

a ) (F1 - F2)d13/2(F0 - F2) (4)

b ) 6ηυ/g(F0 - F2) (5)

Figure 3. Transmission electron micrographs of (a) silicaspheres before titania coating (sample A-1Si-0Ti) and (b) fivecoating steps (Sample A-1Si-5Ti). Direct measurement of eachsphere 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)

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C was purchased. The numbers in the sample code indicatepreparation steps. For example, A-1Si-5Ti means a five-step coating of titania on the silica spheres A-1Si-0Ti.

Figure 1 shows the variations of zeta potentials withpH values for silica spheres (A-1Si-0Ti), titania-coatedsilica spheres (A-1Si-1Ti and A-1Si-5Ti), and bulk titaniaspheres (C-0Si-1Ti). The variations give an isoelectric point(IEP) at 3.17 for silica spheres and at 3.54 for titaniaspheres. The behavior is very similar to that in theliterature,althoughthespecificvaluesaresomewhat lowerthan those previously reported,2 possibly due to differentheat treatments of the particles. No additional heattreatment or drying was carried out for the samplesprepared in the present work. Nevertheless, it is clear

from Figure 1 that titania coating of silica spheres hassystematically increased the electrophoretic mobility andshifted the IEP to the exact value of bulk titania spheres.It is also clear that one-step coating of titania is sufficientto alter the electrokinetic behavior completely, indicatingthat the uniformity of titania coating is excellent with thepresent procedure.

Energy-dispersive X-ray fluorescence spectra weremeasured for the same samples and are illustrated inFigure 2. Assignments for peaks in the spectra are listedin the inset table. The two copper peaks in all spectra at8.0 and 8.9 keV come from copper grids supporting theparticles. A silicon peak at 1.7 keV shown in Figure 2a(Sample A-1Si-0Ti) is a combination of its KR1 at 1.740

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.

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keV and KR2 at 1.739 keV and is obviously due to silicaparticles. No peak for other elements is visible in thespectrum. In Figure 2b (sample A-1Si-1Ti), besides thesilicon and copper peaks, a titanium peak appears at 4.5keV (KR1 + KR2) due to titania coatings. The intensity ofthe titanium peak increases for more coating steps, asshown in Figure 2c (sample A-1Si-5Ti) for five coatingsteps. An additional weak peak at 4.9 keV is the Kâ1 oftitanium. The two titanium peaks coincide with those inFigure 2c (sample C-0Si-1Ti) for bulk titania particles,where the silicon peak is absent as it should be. Quan-titative analysis of the spectra yields a titania weightloading 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 men-tioned, however, that the energy dispersive spectra wererecorded for one particular particle in each sample. Theweight loading thus obtained may vary somewhat fromparticle to particle, as shown early.1

Figure 3 shows transmission electron micrographs oftwo titania-coated samples, A-1Si-0Ti and A-1Si-5Ti. Ascan be seen, the titania-coated silica spheres are roundand smooth, providing additional evidence for uniformcoatings. Direct measurement of each sphere in the TEMimages gives a mean diameter of (a) 545 nm with astandard deviation of 10 nm and (b) 638 nm with astandard deviation of 16 nm, a 93 nm increase due to fivesuccessive coating steps. The TEM mean diameters are

Figure 5. Transmission electron micrographs of (a) prepared titania-coated silica spheres (sample B-1Si-3Ti) and (b) commercialbulk titania spheres. Differences in monodispersity and aggregation extent are evident.

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in good agreement with those determined by sedimenta-tion using the modified Stokes formula. For example, theStokes diameter for particles in sample A-1Si-5Ti is 637nm with a half-width-at-half-maximum (hwhm) of 40 nm(equivalent to a standard deviation of 34 nm). The reasonfor larger standard deviations in Stokes diameters isbecause sedimentation measurements sample a largenumber of particles, whereas only about 30 particles aremeasured in the TEM image. Sedimentation measure-ments were also performed for the starting silica sample(A-1Si-0Ti), giving a Stokes diameter of 549 nm. Thecoating thickness is estimated to be 24 nm after the firstcoating step and 46 nm after the fifth coating step, aslisted in Table 1. On the basis of these data, equivalenttitania loading are calculated to be 54.7 wt % for sampleA-1Si-5Ti. Assuming a titania monolayer to have athickness of 0.355 nm,5 equivalent titania monolayers mayalso be obtained, 125 monolayers for sample A-1Si-5Ti.

Particle size distributions were measured for SampleB and C as shown in Figure 4, which illustrate minimaldegredation in particle monodispersity and aggregationextent after titania coating. A lower concentration oftitanium butoxide was used in the coating steps for sampleB, resulting in thinner coatings per step (cf. Table 1). Thesize distribution of the starting silica spheres is shown inFigure 4a, which may be deconvoluted into two Gaussianpeaks. The most populous peak exhibits a Stokes diameterof 551 nm with a hwhm of 25 nm, giving a relativemonodispersity of 4.5% (cf. Table 1). The smaller andbroader peak at larger diameters may be due to aggregateddoublets of spheres, from which the aggregation extent isestimated to be 11.8%. The aggregation extent is definedhere as the intensity ratio of the peak at large diametersto the peak at the Stokes diameter in the particle sizedistribution figure. Following the first step of titaniacoating, the mean particle diameter is increased by 9 nm,with a slight change in monodispersity (4.9%) andaggregation extent (15.1%). After the third coating step,

the monodispersity is maintained below 5%, and theaggregation extent is still low (17.2%). The two small,sharp peaks at large diameters in Figure 4c may be dueto titania coatings with slight differences in thickness.

For the purpose of comparison, Figure 4d shows theparticle size distribution for a commercial titania sample(C-0Si-1Ti). Obviously, its monodispersity (8.7%) is poorerand aggregation (31.0%) is severer than those in thesamples prepared in this work. The point is more clearlydemonstrated by the TEM images in Figure 5. Figure 5ashows a typical image of 78 titania-coated silica spheresin sample B-1Si-3Ti, yielding a mean diameter of 580 nmwith a standard deviation of 19 nm, whereas Figure 5bshows the image of 75 bulk titania spheres obtainedcommercially (C-0Si-1Ti) with a mean diameter of 262nm and a standard deviation of 49 nm. The roughness ofthe titania spheres in Figure 5b is due to crystallizationafter high-temperature calcination. The effects of calcina-tion on the morphology of titania-coated silica sphereshave been investigated previously, showing that calcina-tion at 773 K converts amorphous titania to crystallizedanatase.5 No calcination was carried out for samples inthe present work.

4. Conclusion

From the results of the present study, it may beconcluded that the multistep coating method usingtitanium n-butoxide can be successfully utilized to producethick and uniform titania coatings on monodisperse silicaspheres while maintaining high monodispersity and lowaggregation extent. Further experiments are needed fora better control of the coating thickness.

Acknowledgment. Financial support from the Chi-nese National Natural Science Foundation is gratefullyacknowledged.

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