J Sol-Gel Sci Technol DOI 10.1007/s10971-015-3950-7
BRIEF COMMUNICATION: NANO-STRUCTURED MATERIALS (PARTICLES, FIBERS, COLLOIDS, COMPOSITES, ETC.)
Synthesis of silica nanoparticles from sodium silicate under alkaline conditions Usama Zulfiqar1 • Tayyab Subhani1 • S. Wilayat Husain1
Received: 18 September 2015 / Accepted: 28 December 2015 Ó Springer Science+Business Media New York 2016
Abstract Compared to the synthesis of silica nanoparticles from tetraethyl orthosilicate, its synthesis from sodium silicate solution (SSS) in alkaline medium is less investigated. Herein, we present a study for the synthesis of nonagglomerated silica nanoparticles from SSS under alkaline conditions. SSS was diluted with water and slowly added in the mixture of ethanol and ammonia to form a sol which was aged and centrifuged to obtain silica nanoparticles. Effect of different ratios of ethanol and ammonia in the mixture on
the size of silica nanoparticles was studied, and these nanoparticles were characterized using scanning electron microscopy, Fourier transformation infrared spectroscopy and X-ray diffraction techniques. It was found that the produced silica nanoparticles were spherical in shape, nonagglomerated in nature and their size could be tailored with the change in the ratio of ammonia and ethanol. Graphical Abstract
Keywords Sodium silicate Alkaline medium Silica nanopartciles Dispersed systems
& Usama Zulfiqar
[email protected] 1
Department of Materials Science and Engineering, Institute of Space Technology, Islamabad, Pakistan
1 Introduction Silica nanoparticles are generally synthesized by tetraethyl orthosilicate (TEOS) as a precursor in alkaline medium [1]; spherical-shaped and non-agglomerated silica nanoparticles
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are usually produced [2, 3], and their particle size increases with an increase in the concentration of both TEOS and ammonia [4]. In addition to TEOS, sodium silicate solution (SSS) is another low-cost precursor which is used for the synthesis of silica nanoparticles. SSS offers typical advantages over TEOS including refined and uniform particle size along with the high concentration of nanoparticles [5]. The conventional synthesis of silica from SSS is carried out in acid medium [6, 7] which usually results in their agglomeration [2]. The use of SSS as a precursor in alkaline medium to produce non-agglomerated silica nanoparticles is less studied. However, colloidal silica has been reported to produce by passing diluted SSS through ion-exchange resin followed by titration with potassium hydroxide; it was found that the size of colloidal silica can be tailored by temperature, potassium hydroxide concentration and titration rate [8]. It is also reported that diluted SSS contains three size ranges of colloids, which are 0.5, 2.5–15 and 75–90 nm [9]. In the present study, the synthesis of silica nanoparticles from diluted SSS was studied in the mixture of ammonia and ethanol. SSS was diluted with distilled water and treated with the mixtures of ammonia and ethanol in their different ratios to observe the effect on the size, distribution and morphology of silica nanoparticles. Finally, the silica nanoparticles prepared from SSS were compared with those produced from TEOS in earlier reports.
2 Materials and methods 2.1 Materials SSS was procured from Merck Millipore having a pH value of 11.0–11.5. The solution contains 7.5–8.5 % Na2O and 25.5–28.5 % SiO2. Ammonia solution 33 % extra pure was purchased from Honeywell Burdick and Jackson. Ethanol Analar grade from BDH Laboratory Supplies was used in this study. Table 1 Quantities of the materials used for the synthesis of silica nanoparticles
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2.2 Methods A mixture was prepared with ammonia and ethanol (A/E) in equal amounts, i.e., 30 ml each. As a precursor medium of silica, 0.5 ml SSS was added in 7 ml of distilled water and added drop-wise to A/E mixture. After aging for 1 h, it was centrifuged and washed with distilled water and finally dried to obtain silica nanoparticles. Same procedure was repeated with six other A/E mixtures with different ratios of ammonia and ethanol, as given in Table 1. 2.3 Characterization Scanning electron microscopy (SEM) was performed by using Mira 3 TESCAN field emission gun scanning electron microscope to observe the particle size and morphology. X-ray diffraction (XRD) was performed by using X’Pert Pro diffractometer for the investigation of the amorphous nature of silica nanoparticles. Fourier transform infrared (FTIR) spectra of silica nanoparticles were acquired by using PerkinElmer Spectrum Two FTIR spectrometer.
3 Results and discussion Figure 1 shows SEM images of silica nanoparticles at two different magnifications produced from diluted SSS in different ratios of A/E. Figure 1a, b show the silica nanoparticles produced by adding diluted SSS in A/E of 1:1; the particle size is 94 ± 30 nm which decreased to 90 ± 21 nm (Fig. 1c, d) by changing the ratio to 1.5:1. The particle size further reduced to 84 ± 16 nm (Fig. 1e, f) by changing the ratio to 2:1. Still, a more reduction in the silica nanoparticle size, i.e., 60 ± 11 nm (Fig. 1g, h), was observed in A/E ratio of 3:1. It revealed that the concentration of ammonia had an inverse effect on the size of silica nanoparticles, i.e., particle size reduced gradually by systematically increasing the concentration of ammonia in A/E mixture. In addition to the effect of ammonia, the influence of ethanol on the particle size was also investigated. It was found that the size of silica nanoparticles in A/E ratio of
Ammonia/ ethanol ratio
Sodium silicate solution (ml)
Water (ml)
Ammonia (ml)
Ethanol (ml)
1:1
0.5
7
30
30
1.5:1
0.5
7
45
30
2:1
0.5
7
60
30
3:1
0.5
7
90
30
1:1.5
0.5
7
30
45
1:2
0.5
7
30
60
1:3
0.5
7
30
90
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Fig. 1 SEM images of silica nanoparticles produced by mixing diluted sodium silicate solution in different ratios of ammonia and ethanol. a and b, c and d, e and f, g and h represent the silica nanoparticles prepared in ammonia/ethanol ratios of 1:1, 1.5:1, 2:1 and 3:1, respectively
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Fig. 2 SEM images of silica nanoparticles produced by mixing diluted sodium silicate solution in different ratios of ammonia and ethanol. a and b, c and d, e and f, g and h represent the silica nanoparticles prepared in ammonia/ethanol ratios of 1:1, 1:1.5, 1:2 and 1:3, respectively
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Fig. 3 Different of particle sizes of silica nanoparticles with varying ratios of ammonia and ethanol mixtures
1:1.5 was 100 ± 31 nm (Fig. 2c, d) which is larger than the size achieved at A/E ratio of 1:1, i.e., 94 ± 30 nm (Fig. 2a, b). By increasing the ratio of ethanol in A/E mixture to 1:2, a further increase in the particle size was witnessed, i.e., 108 ± 33 nm (Fig. 2e, f). Still, a more increase in the quantity of ethanol in A/E mixture, i.e., ratio of 1:3, significantly increased the particle size to 134 ± 52 nm (Fig. 2g, h). It was found that the concentration of ethanol had a direct effect on the size of silica nanoparticles, i.e., an increase in the quantity of ethanol increased silica nanoparticle size. Figure 3 graphically shows the change in particle size with variation in quantities of ammonia and ethanol in
mixtures as observed in Fig. 1. It can be observed that not only the particle size but particle size distribution also decreased continuously with the increase in ammonia. On the contrary, the particle size along with particle size distribution gradually increased with increasing amount of ethanol in the mixture. FTIR results (Fig. 4a) of silica nanoparticles confirmed the presence of asymmetric vibration of Si–O–Si at 1094–1100 cm-1; the band at 466–470 cm-1 indicated the bending vibrations of Si–O–Si. Stretching of O–H appeared at 3426–3460 cm-1, while bending of O–H can be seen at 1637–1643 cm-1. The FTIR results confirmed the presence of silica network in the produced silica nanoparticles. The values in FTIR spectra of these nanoparticles are in good agreement with the literature [10–12]. The XRD results (Fig. 4b) of selected silica nanoparticles produced from A/E ratios of 1:1, 1.5:1, 2:1, 1:1.5 and 1:2 confirmed the presence of characteristic amorphous band of silica nanoparticles without any indication of crystallization. Previously it has been observed for the reaction system using TEOS as precursor of silica that the increased amount of ammonia increases the rate of hydrolysis and condensation, resulting in the formation of larger size particles [4, 10]. In the present reaction system consisting of diluted SSS, ammonia and ethanol, we did not observe increase in the particle size with increasing amount of ammonia; rather, a decrease in particle size with increasing amount of ammonia is noted. However, by keeping the amount of ammonia constant and increasing the ethanol content in the solvent system, the silica particle size increases, as shown in Fig. 2. The dilution of sodium silicate with water increases the size of pre-existing siliceous complexes behaving as
Fig. 4 a FTIR spectra of silica nanoparticles and b XRD patterns of selected silica nanoparticles
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colloids of size 0.6–0.8 nm; introduction of water first dissociates these neutral complexes into sodium ions and charged complexes followed by their condensation to larger colloids [13]. The introduction of ethanol and ammonia to diluted sodium silicate may lead to the formation of intermediate Si(OH)4 followed by condensation to form : Si–O–Si : [7, 14]. On higher pH with increased ammonia, the aggregation of primary particles is lowered due to increased electrostatic repulsion, thus producing small size silica particles. In contrast, at lower pH with increased ethanol, the electrostatic repulsions are reduced and primary particles aggregate rapidly forming the large size particles. The electrostatic repulsion of negatively charged particles at high pH was observed in TEOS system elsewhere to restrict the aggregation [15]. The formation mechanism of silica particles in the present system requires further investigation.
4 Conclusion Spherical-shaped, non-agglomerated and well-dispersed silica nanoparticles were produced from diluted SSS under alkaline conditions. It was observed that the size of silica nanoparticles decreased by increasing the amount of ammonia in the reaction system, while increasing the amount of ethanol increased the particle size along with particle size distribution. Fourier transformation infrared spectroscopy confirmed the presence of silica network, and X-ray diffraction indicated the amorphous nature of silica particles. It was found that silica nanoparticles can be produced from diluted SSS under alkaline conditions with similar chemical arrangement as found in particles produced from TEOS.
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