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On the Factors Influencing the Preparation of Nanosized Titania Sols Jyh-Ping Hsu* and Anca Nacu Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 10617, R.O.C. Received December 23, 2002. In Final Form: March 4, 2003 Nanosized titania particles are prepared from titanium isopropoxide dissolved in alcohol and water under acidic conditions. The effects of the key parameters, including (alkoxide/water) ratio, HCl concentration, feed rate, and temperature, on the mean particle size and the standard deviation of size distribution of the final products are investigated through an experimental design technique. This technique can also be used to find the condition for obtaining transparent sols with mean particle size smaller than 50 nm and standard deviation smaller than 5 nm. We show that the main factors influencing the mean particle size and the standard deviation are (water/alkoxide) ratio, HCl concentration, and the interaction between these two factors. The present approach is applicable to finding the optimum conditions for preparation of titania with desired mean particle size and standard deviation.
1. Introduction Titania, one of the most common pigments, is widely used today for its humidity- and gas-sensitive behavior and excellent dielectric properties. Its unique photocatalytic properties make it suitable for the oxidation of organic pollutants and other contaminants from wastewater or for drinking water supplies.1-4 Thin porous films of titania are known for their catalytic properties5-7 or as electrodes for solar cells.8 For medical purposes, the use of titania as a coating for prosthetic applications is investigated9 and highly porous titania is used as a drug filler, additive, or carrier.10 Titania membranes with high chemical resistance are suitable for ultrafiltration processes and liquid and gas separation.11 Titania has three polyphorms: brookite, anatase, and rutile. Rutile is stable, and the other two are metastable. The properties of titania depend largely on its crystalline form. Typically, titania obtained through sol-gel hydrolysis is amorphous and crystallization can be induced by heat treatment. Usually, calcination results in an increase in its crystal size, and the redispersion in an aqueous medium can lead to further agglomeration. In practice, obtaining nanosized, transparent titania sols is preferred because they can be used in optical titania coatings, titania based glass, ceramic * To whom correspondence should be addressed. Fax: 886-223623040. E-mail:
[email protected]. (1) Fotou, G. P.; Pratsinis, S. E. J. Aerosol Sci. 1995, 26, 227. (2) Jung, K. Y.; Park, S. B.; Ihm, S. K. Appl. Catal. A 2002, 224, 229. (3) Bems, B.; Jentoft, F. C.; Schlo¨gl, R. Appl. Catal. B 1999, 20, 155. (4) Hermann, J. M.; Guillard, C.; Disdier, J.; Lehaut, C.; Malato, S.; Blanco, J. Appl. Catal. B 2002, 35, 281. (5) Zheng, Y. Y.; Pang, J. B.; Qiu, K. Y.; Wei, Y. Microporous Mesoporous Mater. 2001, 49, 189. (6) Zhao, J.; Wang, Z.; Wang, L.; Yang, H.; Zhao, M. Mater. Chem. Phys. 2000, 63, 9. (7) Moret, M.; Zallen, R.; Vijay, D. P.; Desu, S. B. Thin Solid Films 2000, 366, 8. (8) Grant, C. D.; Scwartzberg, A. M.; Smestad, G. P.; Kowalik, Y.; Tolbert, L. M.; Zhang, J. Z. J. Electroanal. Chem. 2002, 522, 40. (9) Manso, M.; Ogueta, S.; Garcia, P.; Perez-Riqueiro, J.; Jime´nez, C.; Martinez-Duart, J. M.; Langlet, M. Biomaterials 2002, 23, 349. (10) Gun’ko, V. M.; Vlasova, N. N.; Golovkova, L. P.; Stukalina, N. G.; Gerashchenko, I. I.; Zarko, V. I.; Tischenko, V. A.; Goncharuk, E. V.; Chuiko, A. A. Colloids Surf., A 2000, 167, 229. (11) Wu, L. Q.; Huang, P.; Xu, N.; Shi, J. J. Membr. Sci. 2000, 173, 263.
materials, UV screen cosmetics, and UV screening food packaging.12-14 The performance of a sol-gel process depends largely on the conditions such as water/alkoxide ratio, pH, temperature, nature of solvent and precursors, and additives. Wang and Ying15 prepared nanocrystalline anatase with a grain size in the range 5-100 nm via the sol-gel method first, followed by hydrothermal treatment in an acidic medium. They found that a high (water/ alkoxide) ratio has the effect of reducing the crystallite size in the calcined material. Tsevis et al.16 obtained titania powder in a fluidized-bed crystallizer packed with quartz glass beads using the sol-gel method. They concluded that the particle size of the product is influenced by the residence time, the length, the thickness of bed, and the presence of Li+, Nb5+, and W+. The influence of solvent on the photochemical properties of titania films obtained through the sol-gel method was investigated by Yoko et al.17 The sol-gel process involves the hydrolysis of titania alkoxide or salt, followed by condensation. That is,
Ti(OR)4 + 4H2O f Ti(OH)4 + 4ROH Ti(OH)4 f TiO2 + 2H2O Bogush and Zukoski18 found that the size and the distribution of silica particles obtained by the sol-gel method are influenced by five primary effects: concentrations of alkoxide, ammonia, and water; temperature; and alcohol. Park et al.19 determined the optimal conditions for the preparation of silica particles which yield simultaneously minimum particle size and size distribution smaller than (5 nm. The significance of the parameters (12) Yoldas, B. E. J. Mater. Sci. 1986, 21, 1087. (13) Yoldas, B. E. J. Non-Cryst. Solids 1984, 63, 145. (14) Yoldas, B. E. J. Non-Cryst. Solids 1980, 38/39, 81. (15) Wang, C. C.; Ying, J. Y. Chem. Mater. 1999, 11, 3113. (16) Tsevis, A.; Sotiropoulou, M.; Koutsoukos, P. G. Prog. Colloid Polym. Sci. 2000, 115, 151. (17) Yoko, T.; Hu, L.; Kozuka, H.; Sakka, S. Thin Solid Film 1996, 283, 188. (18) Bogush, G. H.; Zukoski, C. F. J. Non-Cryst. Solids 1988, 104, 95. (19) Park S. K.; Kim, K. D.; Kim, H. T. Colloids Surf., A 2002, 197, 7.
10.1021/la0270587 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/02/2003
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Figure 1. TEM and SEM pictures for sample no. 1 (a) and sample no. 4 (b). Table 1. Possible Combinations of Relevant Factors of a General 23-1 Factorial Design. +, Factor at a High Level, -, Factor at a Low Level A B C D ) ABC treatment run water/alkoxide feed rate HCl conc temp combinations 1 2 3 4 5 6 7 8
+ + + +
+ + + +
+ + + +
+ + + +
(1) ad bd ab cd ac bc abcd
Table 2. Operational Parameters for the Preparation of Titania Sol Experimental Design parameter H2O/Ti(OR)4 (mol/mol) feed rate (cm3/min) HCl conc (mol/L) temperature (°C)
low value high value (-) (+) 1.66 5.8 0.15 25
33.26 22.2 0.77 60
response 1. mean particle size 2. standard deviation
examined was found to follow the order temperature < (H2O/TEOS) ratio < ammonia concentration < reactant feed rate. The purpose of this work is to find appropriate conditions for the preparation of nanosized titania having a narrow size distribution using a sol-gel method. In particular, the effects of key parameters, including (water/alkoxide)
Table 3. Statistical Experiment Parameters and Results for the Preparation of Titania Sol exp. run water/ feed rate HCl conc temp no. alkoxide (cm3/min) (mol/L) (°C) 1 2 3 4 5 6 7 8
1.7 33.3 1.7 33.3 1.7 33.3 1.7 33.3
5.8 5.8 22.2 22.2 5.8 5.8 22.2 22.2
0.15 0.15 0.15 0.15 0.77 0.77 0.77 0.77
25 60 60 25 60 25 25 60
PS (nm)
SD (nm)
48.7 13.64 1571 257 37 1.2 1747 336.0 14.5 1.9 16.5 4.3 11 2.8 67.5 13.5
Table 4. Estimated Main Effects and Their Interactions for Particle Size (PS) and Standard Deviation (SD) estimated effect
PS (nm)
SD (nm)
lA lB lC lD lAB lAC lAD
822.7 52.9 -823.5 -33.2 79.0 817.9 -29.1
147.8 19.1 -146.3 -20.7 24.9 -140.5 -14.1
ratio, feed rate, HCl concentration, and temperature, on the properties of titania, are investigated. The nature of the problem under consideration suggests that an experimental design approach need be applied. For this purpose, a 2k-1 fractional factorial design is adopted, and
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Figure 2. Variation of mean particle size as a function of feed rate (B) (a), HCl concentration (C) (b), and temperature (D) (c), at different (water/alkoxide) ratios (A).
Design-Expert 5, a statistical experimental optimization program, is used for relevant statistical analyses. 2. Experimental Section The starting materials for the synthesis of titania were titanium isopropoxide (98% Acros Organics), ethanol (anhydrous, Acros Organics), and HCl (36 wt %, Shimakyu, extra pure). The
Figure 3. Plot of AB (a), AC (b), and AD (c) interactions for the standard deviation of particle size distribution. experiments were carried out in a double-walled glass reactor, thermostated by circulating water. A microfeed pump with a constant flow rate feeds the mixture of titanium isopropoxideethanol into the reactor that contains a solution of water, HCl, and ethanol under permanent stirring by a magnetically driven stirrer.
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Figure 4. Normal probability plots (a) and residuals plots (b) for particle size. Normal probability plots (c) and residuals plots (d) for the standard deviation of size distribution. Table 5. Estimated Values of the Coefficients in the Regression Model, Eqs (1) and (2). PS, Mean Particle Size, SD, Standard Deviation coefficient response
R2
constant
A
B
C
D
AB
AC
PS (nm) PS (nm) SD (nm) SD (nm)
1 0.9976 1 0.9785
-51.77 -68.06 2.89 -10.58
66.69 69.52 11.05 11.31
-0.34
102.90 102.90 16.32 16.34
-0.27
-0.072
-0.15
0.096
-89.09 -89.09 -14.42 14.42
-0.52
AD 0.091 -0.026
3. Experimental Design
(B), HCl concentration (C), and temperature (D). Assuming that the high order interactions between these factors are negligible, a fractional factorial design can be used to identify significant factors. The high and the low levels of each factor are coded by (+) and (-), respectively, and a linear response is assumed.20 The design is constructed by writing a basic 23 design for factors A, B, and C and setting the levels of the fourth factor, D ) ABC, as is summarized in Table 1. The present design has the advantages of using a small number of experimental runs and having the main effects not aliased with each other or with the two-factor interactions, but it has the
We are interested in examining four controllable variables or factors: (water/alkoxide) ratio (A), feed rate
(20) Montgomery, D. C. Design and Analysis of Experiments; John Wiley & Sons: New York, 2001.
The particle size was analyzed using a Malvern particle sizer 24 h after the preparation. The samples had to be diluted in alcohol, and in the case where they gelled overnight, they were also sonicated for 2 min. A transmission electron microscope, JEOL JSM, was used to acquire pictures of the titania particle in the mother liquor. After being left 45 days in the mother liquor, the dried particles were visualized using a Hitachi H-2400 scanning electron microscope. Figure 1 presents the TEM and SEM pictures for two characteristic samples: (a) the sample that remained a sol after 45 days and (b) the sample that became a gel after 45 days.
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Figure 5. Effects of (water/alkoxide) and HCl concentration on particle size (a) and standard deviation of size distribution (b).
disadvantage that the two-factor interactions are aliased with each other. The main effects can be estimated by
lA ) A + BCD lB ) B + ACD lC ) C + ABD lD ) D + ABC The interactions between two main factors can be calculated by
lAB ) AB + CD (aliased with CD) lAC ) AC + BD (aliased with BD) lAD ) AD + BC (aliased with BC) 4. Results and Discussions The operational parameters for our experimental design are described in Table 2. The preparation conditions and the experimental results for each experimental run in this experimental design are presented in Table 3. The main effects are calculated as indicated previously, and the estimated values are shown in Table 4. Note that if the interaction between two factors is significant, then
Hsu and Nacu
Figure 6. Effects of (water/alkoxide) ratio and feed rate on particle size (a) and standard deviation of size distribution (b).
the values of these factors may be meaningless. Table 3 reveals that there is a strong interaction between factors A and C, as can be seen also from Figures 2 and 3. In these figures the levels of B, C, and D are plotted against the levels of A. In Figures 2a and c and 3a and c the lines are almost parallel, indicating that there is no significant interaction between the factors. Figures 2b and 3b, however, indicate a strong interaction between factors A and C. This implies that, at a low (water/alkoxide) ratio, the particle size is influenced by neither the feed rate nor the temperature, while, at a high (water/alkoxide) ratio, an increase in the temperature leads to an increase in the particle size. Similarly, if the (water/alkoxide) ratio is low, the standard deviation of particle size distribution is insensitive to both the feed rate and the temperature, but it is sensitive to both of them when the (water/alkoxide) ratio is high. On the basis of Table 3 and Figures 2 and 3, we conclude that the significance of the influence of the parameters on the particle size follows the order feed rate (B) < temperature (D) < (water/alkoxide) < ratio (A), < HCl concentration (C). The interactions AD and AB are small compared to AC, which is significant. The standard deviation of particle size distribution is also influenced mostly by (water/alkoxide) ratio, acid concentration, and the interaction between them, in decreasing order. The effects of temperature and feed rate are very insignificant, compared to the previous three factors. Several methods are available for identifying significant effects.21 Among these, Montgomery recommends using normal probability
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Figure 8. Overlay of particle size and standard deviation of size distribution response surfaces.
Figure 7. Effects of (water/alkoxide) ratio and temperature on particle size (a) and standard deviation of size distribution (b).
plots.21 The normal probability plots of the effect estimates from our experiment are presented in Figure 4a and c. These figures indicate that the main effects are A (water/ alkoxide) ratio, C (HCl concentration), and AC interaction, because they deviate considerably from the line passing through the other points for both particle size and the standard deviation of size distribution. The plots of the residuals are presented in Figure 4b and d, and they appear to be reasonable. Regression models can be used to describe the dependence of the particle size (PS) and the standard deviation (SD) of size distribution on the relevant factors. A general regression model, which includes all the factors and their interactions, can be expressed as
PS ) R0 + R1A + R2B + R3C +R4D +R5AB + R6AC + R7AD (1) SD ) β0 + β1A + β2B + β3C + β4D + β5AB + β6AC + β7AD (2) However, since only eight experimental data are available, using a regression model, which involves less factors, is more realistic. As we showed previously, the most significant effects are A, C, and AC interaction. Therefore, for both PS and SD, we choose to work on the regression model, which includes only these factors. As can be seen (21) Montgomery, D. C.; Runger, G. C.; Hubele, N. F. Engineering Statistics; John Wiley & Sons: New York, 2001.
in Table 5, the performance of the model chosen is satisfactory. In the following discussions surface response plots are used to visualize the effect of the significant factors on the responses. Effect of Water and Acid Concentrations. As discussed previously, (water/alkoxide) ratio and acid concentration are the main factors influencing both particle size and the standard deviation of size distribution. This is also justified in Figure 5, where a high (water/ alkoxide) ratio or a high HCl concentration leads to a large particle size and a wide size distribution. Yoldas12 suggested using a (water/alkoxide) ratio in the range from 1.7 to 2. For (water/alkoxide) ratios smaller than 1.7, the solution did not deposit clear and continuous films, while, for (water/alkoxide) ratios larger than 2, apparition of gel was observed in time. Wang and Ying15 observed that increasing (water/alkoxide) ratio yields smaller crystals in the calcined material, supposedly because a more complete hydrolysis is ensured this way, favoring nucleation instead of particle growth. In our opinion the strong hydrogen bond between the increased number of nuclei leads to their agglomeration and, consequently, a larger particle size and a wider size distribution. The addition of acid (HCl or HNO3) is a necessity to ensure obtaining a clear sol. An increase in HCl concentration has the effect of decreasing not only the particle size but also the standard deviation of size distribution. The presence of acid promotes the hydrolysis versus the condensation reaction, acts as a catalyst, and prevents the particle agglomeration through electrostatic repulsions. If this is true, Yoldas’ observation12 that higher concentration of acid induces the cloudiness of a solution can be explained. Effects of Feed Rate and Temperature. The response surface plots presented in Figure 6 for particle size and the standard deviation of size distribution indicate that an increase in feed rate leads to a decrease in particle size and an increase in the standard deviation of size distribution. On the basis of the reaction between HCl and thiosulfate, LaMer and Dinegar22 developed a theory for monodispersed sols preparation. Their model comprises three stages: induction, in which the concentration of products builds up until it reaches a critical value; selfnucleation; and growth. If the beginning solutions are not dilute, as in the case of high feed rate, the concentration of product will exceed the critical concentration, which (22) LaMer, V. K.; Dinegar, R. J. Am. Chem. Soc. 1950, 72, 4847.
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results in rapid nucleation and growth, but repetitive nucleation cannot be avoided due to the high degree of supersaturation. The growth of the respective nuclei will depend on the time of their formation, leading to small particles with a wide size distribution. When the feed rate is low, the system under consideration is similar to a batch system if the beginning solutions are diluted. In this case the building up period is long and the nucleation period is short. The number of nuclei is small, but they grow uniformly, resulting in large particles with a uniform size distribution. Therefore, we conclude that, along with the (water/alkoxide) ratio, the concentration of the beginning solution (water/alcohol and alkoxide/alcohol) is a key factor to obtaining monodispersed titania sols. In our case, an increase in the temperature leads to a decrease in both the particle size and the standard deviation of size distribution, as can be seen in Figure 7. The former is not surprising, since an increase in the temperature leads to an increase in the number of nuclei in a short time. According to LaMer,22 the final particle size depends on both the number of nuclei and the diffusion coefficient of the product in the medium. The same quantity of titania to be deposited will be distributed to a larger number of particles, resulting in smaller particles with a narrow size distribution. Figure 8 shows the conditions for obtaining simultaneously a small particle size (
