Ind. Eng. Chem. Res. 2006, 45, 8043-8048
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MATERIALS AND INTERFACES Analysis of Parameters and Interaction between Parameters in Preparation of Uniform Silicon Dioxide Nanoparticles Using Response Surface Methodology Hui-Chun Wang, Cheng-Yuan Wu, Chin-Chun Chung, Ming-Hong Lai,† and Tsair-Wang Chung* Department of Chemical Engineering/R&D Center for Membrane Technology, Chung-Yuan UniVersity, Chungli, Taiwan 320, Republic of China
This research makes use of tetraethylortho-silicate (TEOS) to synthesize silicon dioxide (SiO2) nanoparticles with the sol-gel process. During the discussions of reactants (TEOS, NH3, H2O, solvent) and reaction conditions (temperature, reaction time), the size of the silicon dioxide particle will be subjected to change with the variation of these preparation parameters. When the concentrations of TEOS and NH3 are reduced and the reaction temperature is increased, the particle size diminishes immediately. The particle size rises first and then declines along with the water concentration increment. Because of the rise in the value of the solvent’s dielectric constant, the synthesized silicon dioxide particles are smaller. The concentrations of NH3 and H2O increment will achieve a more uniform particle size. To synthesize the silicon dioxide particles in the nano class, the concentrations of TEOS, H2O, and NH3 need to be reduced. The preparation parameters should be chosen under the relatively higher reaction temperature, and solvents with lower dielectric constants should not be chosen. The aim of this experiment is to understand the influence of particular factors (NH3 concentration, H2O concentration, reaction temperature) on the target function (particle size and its distribution) and to determine the interaction between factors by using the response surface methodology (RSM). It is shown that the above three factors have notable influences on particle size and that the concentration of NH3 has a more notable influence on the particle size distribution. Introduction The physical and chemical characteristics of nanomaterials are different from those of bulk materials. Many scholars have already studied nanomaterials and nanotechniques in the past decade, including the manufacturing of nanostructure,1 nanoparticle,2 and nano thin film.3 Nanoparticles are most commonly used as the original material, and research on making nanoparticles is the root of the application of nanomaterials. Therefore, we attempt to synthesize silica dioxide nanoparticles with the sol-gel process in this study. However, controlling the preparation parameters is of great importance to engineering applications. The effect of preparation parameters on the particle size and its distribution are analyzed in the experiment. Silicon dioxide was discovered in 1946 by Ebelman and Graham, who used tetraethylortho-silicate (TEOS) as the starting raw material from which to manufacture silicon dioxide. After 1960, the importance of the sol-gel method was gradually accepted by chemists. They focused on the phenomena of the reaction, the applications of solid catalysts, and research on commercial products. The standard procedures in the production of silica spheres of predetermined size were studied by Sto¨ber et al.4 In their study, the particle size could be controlled to be monodisperse in a suspension range from 0.05 to 2 µm in diameter. Generally speaking, making silicon dioxide nanoparticles is similar to making traditional silicon dioxide. Both procedures * To whom correspondence should be addressed. Tel.: 886-32654125. Fax: 886-3-2654199. E-mail address:
[email protected]. † Current address: Taiwan Police College, Taipei, Taiwan, R.O.C.
are carried on with the simple sol-gel process.5 Traditional silicon dioxide can be synthesized with the hydrolysis and the polycondensation reactions.6 The intermediate of silane oxide is generated by hydrolysis first, followed by the condensation reaction to form a three-dimensional net structure gradually, which will limit the action of molecules. After producing a homogeneous silicon dioxide sample, the processes of drying and calcinations are carried on immediately. This produces a tougher net colloid product. If we consider this synthesis with different raw materials, the product of silicon dioxide will present dissimilarities in the apparent characteristics. When the particle diameter of silicon dioxide is smaller than 100 nm, the apparent energy of the particle will become large. It will be accumulated by the weak force very easily, but it is also very easy to break the accumulation. In addition, at the nano size, the particles will have the characteristics of high specific surface area, chemical stability, low coefficient of expansion, fire resistance, good reactivity, and low refractive index. In making silicon dioxide particles, the particle diameter and its distribution will be subjected to changes with the effects of many preparation parameters during the sol-gel process. The discussions on the effects of the TEOS concentration,7 water quantity,8,9 catalyst concentration,10,11 type of solvent,12 reaction temperature,13,14 and reaction time15,16 are found in the literature. Because the sol-gel process can acquire nanopowders easily at normal temperature and pressure, it is applied in this study to make silicon dioxide nanoparticles. The response surface methodology is a more systematic method with which to design and discuss the experiment. Also, it can facilitate investigation of whether the preparation parameters have alternative influences or not.
10.1021/ie060299f CCC: $33.50 © 2006 American Chemical Society Published on Web 10/25/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006
Table 1. Selected Variables and Coded Levels of This Study by Box-Behnken Experimental Design
Table 2. Box-Behnken Experimental Design and Results of This Study
coded level
factor
respones
variable
symbol
-1
0
+1
trial no.
X1
X2
X3
Y1a (nm)
Y2b (%)
NH3 (mole) H2O (mole) reaction temperature (°C)
X1 X2 X3
0.15 0.50 20
1.08 1.75 40
2.00 3.00 60
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0 + 0 0 + 0 0 + 0 + 0
+ + 0 0 0 0 + + 0 0 0 -
0 0 + + 0 0 + 0 0 + 0 -
210.40 51.95 248.80 151.10 54.66 23.53 94.80 232.40 185.00 50.55 209.00 159.10 161.00 26.96 114.70
26.72 42.36 19.34 32.40 30.12 29.02 22.96 27.93 27.46 35.53 29.23 33.19 14.99 40.49 17.35
Experimental Procedure The procedure of this research was to change a series of the parameters used in making the silicon dioxide nanoparticles by the experimental design methodology. The influence of the preparation parameters on the particle diameter and its distribution was observed. Using the data regression principle and the experiment design to analyze the experiments in a more systematic way to understand the relationship between the preparation parameters and the particle characteristics, the importance of each parameter, and the interaction between parameters will compensate for the deficiency of the traditional one-factor-at-a-time method. It makes the research more complete and systematic. The initial raw material of the reaction was TEOS. NH3 was regarded as the catalyst. Water and adequate alcohol were regarded as the solvents. Almost equal amounts of TEOS and alcohol were blended for 10 min first, producing the A solution. The NH3, water, and alcohol were blended for 10 min, producing the B solution. Then, the two cups of solutions A and B were mixed at a particular temperature and at a fixed stirring speed (450 rpm). When the reaction was completed, about 5 mL of the silicon dioxide solution was removed for analysis of the particle diameter and measurement of the interface electric potential by a Zatasizer. The laser beam in the Zatasizer is used to get the dynamic light scattering of particles in the solution to calculate the particle size and its distribution. To increase the accuracy of the analysis in the Zatasizer, each sample was tested 3 times and 10 to 15 subruns were conducted in each test. The TEOS, the catalyst, water, the type of alcohol, and the reaction temperature are discussed in the one-factor-at-a-time method first. The TEOS concentration was controlled in the range of 0.02-0.67 mol, the concentration of the catalyst NH3, in the range of 0.26-1.31 mol, and the concentration of the water, in the range of 0.55-11.10 mol. Four different solvents, methanol, ethanol, propanol, and butanol, were chosen for evaluation. The reaction temperature varied from 20 to 40 °C. The influence of the five different parameters on the particle diameter and its distribution was observed. The pH values of the solution were maintained in the range of 8-11, which maintained the sol solution in a more stable condition. Since the effect of the amount of TEOS on the particle size distribution is not significant under 0.8 mol in this study, this factor is not included in the response surface methodology as a preparation factor. This research adopted three factors and three levels of the response surface methodology17 to design the experiments. In the process of making the silicon dioxide nanoparticles, we chose the NH3 concentration, X1, in the range of 0.15-2 mol, the water concentration, X2, in the range of 0.5-3 mol, and the reaction temperature, X3, at 20-60 °C as three preparation factors and selected the average particle diameter, Y1, and the relative standard deviation of the particle diameter, Y2, (regarded as the particle diameter distribution) to be the target functions. Each preparation factor in the experiment scope was established and coded into levels of -1 to +1, as shown in Table 1. Depending on the response surface methodol-
a
Y1: particle diameter (nm). b Y2: standard deviation (nM).
ogy with which the experiment was designed, as in Table 2, only 15 experimental runs were needed. All experimental data regression and analysis in this research were performed with the software JMP of the SAS (statistical analysis system) program. Results and Discussion This research studied the influence of the starting material (TEOS), solvent (H2O and alcohol), catalyst (NH3), reaction temperature, and reaction time on the size of silicon dioxide nanoparticles. At the same time, it presents an explanation of the particle size distribution. It is hoped that this research will further the understanding of the preparation parameters in the synthesis of silicon dioxide nanoparticles. TEOS (Si(C2H5O)4) is the source of the silicon (Si) in the silicon dioxide. As shown in Figure 1 a, when the quantity of the TEOS increases considerably, the diameter of the particle also increases gradually. In this study, the relative standard deviation (RSD) is selected to represent the particle diameter distribution. Its calculation method is as follows:
RSD )
x
n
(Vi - Vj)2/(n - 1) ∑ i-1 Vj
× 100%
where Vi is the measured particle diameter (nm), Vj is the average value of all particle diameters (nm), and n is the taken number of particles. Assume 100 particles are taken to perform the calculation each time. Figure 1a shows that the TEOS quantity has a significant influence on the particle diameter distribution of the silicon dioxide. Figure 1b shows that the particle size rises first and then declines along with the water concentration increment. A similar phenomenon was reported in the study by Bogush et al.18 Figure 1c reveals that the size of the particle increases with the NH3 concentration increment. In the particle diameter distribution aspect, the particle diameter distribution is more uniform along with the increment of the NH3 quantity. The influence of NH3 on the particle size and its distribution has been discussed in the literature.11 Table 3 shows the experimental data of the particle diameter and the particle size distribution according to the variation of each preparation parameter. To keep the reaction in a uniform phase, solvent was added to the experiment to prevent the phenomenon of phase separa-
Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006 8045 Table 3. Experimental Data of This Study in Different Preparation Conditions
Figure 1. Effects of the amounts of TEOS, H2O, and NH3 on the particle diameter and the relative standard deviation of particle diameter.
tion. This research adopts methanol, ethanol, propanol, and butanol, four different solvents, respectively, to draw a comparison and to observe the influence on the particle size. Because these four solvents have different dielectric constants (methanol ) 32.6, ethanol ) 24.3, propanol ) 20.1, butanol ) 17.8), this affected the particle in the nucleating process of accumulation. The static repulsive force between particles is greater than the van der Waard attractive force when the solvent’s dielectric constant is larger. Because it prevents the gathering of nuclei, the formation of smaller particles is easier. Therefore, in choosing the solvent with which to make nanomaterials, solvents with low dielectric constants are not considered. In the synthesis of silicon dioxide, the ethanol will be formatted, and in the literature, there are also many studies that have used ethanol as the solvent. Therefore, ethanol was chosen as the solvent in the follow-up experiments, in which silicon dioxide nanoparticles were synthesized.
no.
temp (°C)
TEOS (mol)
NH3 (mol)
H2O (mol)
solvent
particle diam (nm)
RSD (%)
TEOS1 TEOS2 TEOS3 TEOS4 TEOS5 H1 H2 H3 H4 H5 N1 N2 N3 N4 N5 S1 S2 S3 S4 T1 T2 T3
40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 20 40 60
0.02 0.07 0.22 0.45 0.67 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.05 0.05 0.05
0.52 0.52 0.52 0.52 0.52 0.52 0.52 0.52 0.52 0.52 0.26 0.52 0.78 1.05 1.31 0.52 0.52 0.52 0.52 2.00 2.00 2.00
8.32 8.32 8.32 8.32 8.32 0.55 2.77 5.55 8.32 11.10 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 1.75 1.75 1.75
ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol methanol ethanol propanol butanol ethanol ethanol ethanol
181.10 249.70 400.30 525.20 882.70 52.59 87.20 141.00 125.30 107.00 23.80 52.59 63.00 71.20 82.80 11.00 52.59 484.30 1740.00 248.80 231.10 161.00
19.06 20.96 18.46 20.63 33.00 25.12 29.32 28.81 20.96 17.02 29.66 25.12 22.85 20.49 18.26 32.91 25.12 21.12 15.73 19.34 26.21 14.99
Tan13 found that the size of the nucleating zone is increased and the particle diameter is smaller, when the reaction temperature is increased. In the experiment, the equilibrium solubility of the silicon dioxide synthesized at 60 °C was higher than that synthesized at the reaction temperature of 20 °C. The result is that the particle synthesized at the higher temperature will grow faster than that synthesized at the lower temperature and that the particle reaches the stop-growth period faster. Furthermore, the increase of temperature will cause a higher nucleating speed, representing obstruction of the growth of the nucleus. Therefore, different temperatures have an apparent influence on particle size. The higher reaction temperature will lead to a smaller particle diameter. The formation of silicon dioxide goes through the steps of the induction period, nucleating period, and nucleus accumulation to produce the small particles, and these small particles subsequently grow into large particles. With increasing reaction time, the particle will gradually increase in size and grow more completely. From the reaction mechanism of silicon dioxide, the relationship between the reaction time and the growth behavior is observed. The particles will join with each other, making larger and larger particles, until the reactants are eliminated and the reaction is terminated completely. Under the experimental conditions, the particle diameters exhibit no significant further changes after synthesis for 30 min, which agrees with the results in the literature. In the synthesis of silicon dioxide nanoparticles, changing the preparation parameters is the most direct and the most convenient and efficient way. From the variation of the abovementioned single parameter, if the concentrations of TEOS, H2O, and NH3 are reduced, and a solvent with a low dielectric constant is chosen, it is easy to make silicon dioxide nanoparticles (particles with diameter T
model error total
9 5 14
81 793.10 2 257.49 84 050.59
9088.12 451.50
20.13
0.0021
a
R2 ) 0.97.
Table 5. Analysis of the Model of the Relative Standard Deviation of the Particle Diametera source
DF
sum of squares
mean square
F ratio
prob > T
model error total
9 5 14
781.25 70.51 851.76
86.8054 14.1025
6.16
0.0297
a
R2 ) 0.92.
Table 6. Analysis of Variances for the Response of the Particle Diametera factor
coefficient
std. error
T value
prob > T
intercept X1b X2c X3d X12 X22 X32 X1X2 X1X3 X2X3
165.07 86.76 23.54 -37.33 -15.30 -19.69 -27.77 -0.40 -14.17 -12.86
12.27 7.51 7.51 7.51 11.06 11.06 11.06 10.62 10.62 10.62
13.46 11.55 3.13 -4.97 -1.38 -1.78 -2.51 -0.04 -1.33 -1.21
