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Facile synthesis of surfactant-free SiO2 nanoparticles via emulsion method
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Facile synthesis of surfactant-free SiO2 nanoparticles via emulsion method
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Sachin N. Bramhe, Ja Young Park, Seon-Ae Hwangbo, Min Cheol Chu ⇑
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Center for New Functional Materials Metrology, Korea Research Institute of Standards and Science, 267 Gajeong-ro, Yuseong-gu, Daejeon 305-340, South Korea
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Article history: Received 22 July 2015 Received in revised form 28 July 2016 Accepted 16 September 2016 Available online xxxx Keywords: Emulsion Surfactant free Silica nanoparticles Monodisperse
a b s t r a c t Silica (SiO2) nanoparticles have wide potential applications in many advanced areas. In this paper, we report an easy method for the synthesis of monodisperse SiO2 nanoparticles by a simple emulsion method. The novelty of this method lies in the synthesis of nanoparticles without the traditional use of any surfactants and toxic solvents. The phase purity of the synthesized SiO2 nanoparticles were determined by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). Morphology and nanoparticle size was determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of the concentration of tetraethyl orthosilicate (TEOS), ammonia and water on the final size of the nanoparticles was observed. By precisely controlling the concentration of ammonia and water, SiO2 nanoparticles ranging between 30 and 90 nm were synthesized without the use of any surfactants. The results presented a simple, convenient and non-toxic method for the synthesis of SiO2 nanoparticles on a large-scale with precise control of the particle size. Ó 2016 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan.
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1. Introduction SiO2 is the one of the most abundant substances available on our planet. Useful properties of nanosized SiO2 have led to a marked increase in applications of SiO2 nanoparticles. Easy preparation methods and expedient properties have led to applications in areas such as biomedicine, catalysis, sensors, coatings, electronic and thermal insulators and thin film substrates [1–5]. However, many of these applications are heavily dependent on the size of the nanoparticles. Stöber et al. were the first to synthesize SiO2 microparticles utilizing esters of silicic acid as a silica precursor along with ammonia as a catalyst [6]. Over the years, many research groups have used modified versions of the Stöbers method to synthesize silica nanoparticles [7–9]. The general results of these reports is hydrolysis and condensation of various silicon alkoxides. The water in oil emulsion method was also developed for manufacturing SiO2 nanoparticles, which gave precise control over the size of nanoparticles [10–12]. The water particles in the oil phase provide site for hydrolysis and condensation of silicon alkoxide, thereby forming SiO2 nanoparticles. However, these reverse micro-emulsion methods employ surfactants for stabilizing the metastable water nanoparticles and subsequently increase the hydrophilicity of the
⇑ Corresponding author. E-mail address: [email protected] (M.C. Chu).
synthesized particles [13]. Many of the solvents used for dispersion of these surfactants are toxic in nature. For certain applications, it is necessary to remove any surfactants before the nanoparticles can be used. Owing to its large-scale application, it is imperative to find a method to synthesize SiO2 nanoparticles without the use of surfactants and toxic solvents. Recently, Satoshi et al. were able to synthesize hollow SiO2 nanoparticles by surfactant free water in oil emulsions [14]. However, toxic solvents were employed in preparing the emulsions. In this study, we demonstrate a facile method for the synthesis of surfactant-free SiO2 nanoparticles using nanodisperser for generating a stable emulsion. We used nontoxic and inexpensively accessible ethyl acetate as oil phase. By this simple method, we were able to control the size of the synthesized nanoparticles from 35 to 90 nm by controlling its various parameters.
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2. Materials and methods
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Ethyl acetate (OCI Company Ltd.), tetraethyl orthosilicate (TEOS) (Aldrich), ammonia solution (28–30%, Junsei) and ethyl alcohol anhydrous (Carlo Erba) were all purchased and used as received. Water was purified via Milli Q system and had an electrical resistance of 18.4 MX cm. To synthesize water nanoparticles in the ethyl acetate phase without the use of any surfactants, a nanodisperser was used. A nanodisperser is a high speed homogenizer that gives better results and ultrasonication than the conventional homogenizer for synthesizing nanoparticles.
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http://dx.doi.org/10.1016/j.apt.2016.09.019 0921-8831/Ó 2016 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan.
Please cite this article in press as: S.N. Bramhe et al., Facile synthesis of surfactant-free SiO2 nanoparticles via emulsion method, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.09.019
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Typically, 1 mL of DI water is added to 99 mL ethyl acetate to give 1% (vol/vol) DI water solution which is then passed through the nanodisperser (Model No. ISA-NLM100, SufLux Ltd.) at 900 bar pressure. The fluid passes an orifice module of the nanodisperser through a high speed intensifier pump. Supersonic speed is generated by this pump that creates a powerful shear force caused by the viscosity and surface tension of the liquid and frictional force of the orifice module. Because of the sudden decrease in pressure cavitation occurs, which on bursting collide with other particles and helps in the breakdown of larger particles. The combination of shear force, impact and cavitation inside the nanodisperser at high pressure conditions aids in the formation of the emulsion. However, we found that the resulting emulsion is not always monodisperse. To that end, we kept the emulsion obtained from the nanodisperser, at rest for a period of 24 h in a graduated cylinder to allow the larger particles to settle down. After the 24 h mark, the lower 50 mL of the emulsion containing unstable larger particles was discarded. To the remaining 50 mL emulsion solution, 0.045 M TEOS and 300 lL ammonia solution was added and continuously stirred for 24 h. The concentrations were calculated based on its final concentration in the reaction mixture. Then the emulsion was broken by adding ethyl alcohol anhydrous and centrifuged to collect the synthesized nanoparti-
Fig. 1. Schematic procedure of synthesis of SiO2 nanoparticles.
cles. Experiments were carried out with varying amounts of TEOS, ammonia and DI water to determine its effect on the particle size. Scanning electron microscopy images were taken on FE-SEM, Hitachi, Model No.: S-4800. EDX analysis was done on scanning electron microscope equipped with EDX detector. X-ray diffraction (XRD) pattern of the synthesized powder was analyzed with Cu Ka radiation (k = 1.5406 Å) on a SmartLab X-ray diffractometer (Rigaku). TEM images were taken by JEM-2100F, Jeol.
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3. Results and discussion
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Synthesizing nanoparticles via emulsion method involves two main steps. First, the synthesis of water nanoparticles in the oil phase followed by hydrolysis and condensation of tetraethyl orthosilicate (TEOS). In an aqueous solution, TEOS undergoes hydrolysis to form silicon tetra hydroxide, which condenses to form nanoparticles of silica. The following reaction takes place during the conversion of TEOS to SiO2:
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SiðOC2 H5 Þ4 þ 2 H2 O ! SiO2 þ 4 C2 H5 OH An illustration of the steps involved in the synthesis of SiO2 nanoparticles is given in Fig. 1. Ammonia serves as a catalyst while water particles react with TEOS to give SiO2 nanoparticles. The main parameters that affect the size of SiO2 nanoparticles: (i) TEOS concentration (ii) ammonia concentration and (iii) water concentration. TEOS, being the source of SiO2, plays an important role in determining the concentration of primary particles of SiO2. After nucleation, the primary particles aggregate to form the stable secondary particles. After this, any new nuclei or primary particles will dissolve and deposit on the secondary particles due to Ostwald ripening until a stable system is obtained [15]. As can be observed in Fig. S1, optimized SiO2 nanoparticles were formed when 0.045 M of TEOS concentration was added while keeping the ammonia solution and water concentration constant at 300 lL and 1% respectively. At a higher concentration the polydispersity index increased, whereas at a lower concentration the yield was relatively low. Particles obtained at 0.045 M TEOS concentration were spherical and nearly monodisperse. First, the powder synthesized by adding 0.045 M TEOS was analysed by an X-ray diffraction to check its phase purity. As observed in Fig. 2(a), the absence of any secondary peaks confirms that only SiO2 phase is obtained in the powder. The peak at 2h = 23.61was in accordance with JCPDS file no. 01-082-1563. To check the spectrum of mineral crusts, energy dispersive X-ray spectroscopy (EDX) was used. The results indicated in Fig. 2(b)
Fig. 2. (a) XRD analysis showing pure SiO2 phase; (b) EDX analysis showing only Si and O peaks.
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show peaks pertaining to Si and O only, confirming that only SiO2 phase is present in the synthesized powder. Fig. S2 shows the EDS element mapping of the synthesized SiO2 nanoparticles. Next, we checked the effects of ammonia on the size of nanoparticles while keeping the concentration of TEOS and water constant at 0.045 M and 1% respectively. Fig. 3 shows the SEM and TEM images of SiO2 synthesized with the addition of varying concentration of ammonia. As can be seen in SEM analysis and its accompanying size distribution (shown in inset), a general trend of increase in particle size with increase in the concentration of ammonia is observed. Similar results were also obtained by Rahman et al. [16]. Ammonia tends to increase the rate of hydrolysis of TEOS and the condensation rate of the synthesized monomers while promoting the rate of polymerization [17–20]. Hence, we observe an increase in size of nanoparticles with the increase in concentration of ammonia. Size distribution analysis in Fig. 4 shows the general trend observed with the increase in concentration of ammonia [21]. The lowest concentration of ammonia at 200 lL resulted in the synthesis of 47 nm nanoparticles, while the largest amount of ammmonia at 600 lL resulted in 90 nm particles. Moreover, a marked increase in the standard deviation of the mean size of the particles was observed as the concentration of ammonia solution added was increased. This might be because ammonia solution provides excess water molecules. These excess water molecules might increase the size of the emulsion, thus increasing the standard deviation from the mean value. Fig. 3(f) shows the TEM image of SiO2 nanoparticles synthesized by the
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Fig. 4. Effect of ammonia and water concentration on final size of SiO2 nanoparticles.
addition of 200 lL ammonia. As can be observed, highly monodisperse and spherical nanoparticles of size 45 nm were observed. We also checked the effect of DI water concentration on the relative size of silica nanoparticles while keeping TEOS and ammonia
Fig. 3. Effect of change of ammonia concentration on final size of SiO2 nanoparticles. (a) 200 lL, (b) 300 lL, (c) 400 lL, (d) 500 lL, (e) 600 lL; and (f) TEM image of SiO2 nanoparticles synthesized by typical addition of 200 lL ammonia solution.
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Fig. 5. Effect of change of DI water concentration on size of SiO2 nanoparticles (a) 0.5%, (b) 0.75%, (c) 1.25%, (d) 1.5% DI water concentration.
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concentration constant at 0.045 M and 300 lL respectively. Fig. 5 shows the SEM images of SiO2 nanoparticles synthesized by the addition of varying concentrations of DI water. Particle size of SiO2 increases with the increase in water concentration. With the increase in water concentration, the rate of nucleation also increases, which ultimately results in a large number of particle synthesis [21]. At a higher water concentration, the SiO2 subparticles produced have stronger hydrogen bonds due to excess water which causes aggregation [17]. This leads to the formation of larger nanoparticles and a deviation from the mean particle size. Size distribution analysis in Fig. 4 shows the general trend observed with the increase in concentration of DI water from 0.5% to the highest checked concentration of 1.5%. As seen in Fig. 4 and 05% water concentration leads to formation of particles sized at 35 nm while the highest concentration of 1.5% gave particles sized at 86 nm. However, when the concentration was decreased to 0.25% the particle size obtained was 57 nm. The drop size distribution and stability of emulsion is dependent on breakage rate and coalescence of the dispersed phase [22]. The water concentration of 0.5% to 1.5% results in breakage and coalescence for formation of stable emulsion with increasing size, however the water concentration of 0.25% results in formation of stable emulsion of higher size. This might be because of lower rate of breakage at 0.25% resulting in larger emulsion size or strong attraction force between the smaller water particles causing an increase in size [23]. With the increase in concentration of ammonia and water, we observed an increase in size of resulting SiO2 nanoparticles. Both these results show that by controlling the rate of nucleation we can control the size of the end product. By not using any surfactants or toxic solvents, we were able to synthesize SiO2 nanoparticles of various sizes by controlling various parameters.
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4. Conclusions
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In conclusion, we have demonstrated a facile and an inexpensive method for the synthesis of monodisperse SiO2 nanoparticles without using any surfactants or toxic solvents. These monodisperse particles have the potential application in drug delivery and as a catalyst. Easy tunability of particle size from 30 to
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90 nm means that these nanoparticles could well be used as certified reference materials. This inexpensive and convenient method can enable large-scale manufacturing of SiO2 nanoparticles without causing any obtrusive toxic effects on the individual or the environment.
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Acknowledgement
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This work was supported by Nano Material Technology Development Program (2014M3A7B6020163) of MSIP/NRF.
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Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apt.2016.09.019.
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References
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[1] Y. Zhong, F. Peng, F. Bao, S. Wang, X. Ji, L. Yang, Y. Su, S.T. Lee, Y. He, Large-scale aqueous synthesis of fluorescent and biocompatible silicon nanoparticles and their use as highly photostable biological probes, J. Am. Chem. Soc. 135 (2013) 8350–8356. [2] L. Pavesi, D.L. Negro, C. Mazzoleni, G. Franzo, F. Priolo, Optical gain in silicon nanocrystals, Nature 408 (2000) 440–444. [3] X.J. Wu, D. Xu, Formation of yolk/SiO2 shell structures using surfactant mixture as template, J. Am. Chem. Soc. 131 (2009) 2774–2775. [4] C.J. Jia, M. Schwickardi, C. Weidenthaler, W. Schmidt, S. Korhonen, B.M. Weckhuysen, F. Schüth, CO3O4-SiO2 nanocomposite: a very active catalyst for CO oxidation with unusual catalytic behavior, J. Am. Chem. Soc. 133 (2011) 11279–11288. [5] S. Zhang, Z. Chu, C. Yin, C. Zhang, G. Lin, Q. Li, Controllable drug release and simultaneously carrier decomposition of SiO2 drug composite nanoparticles, J. Am. Chem. Soc. 135 (2013) 5709–5716. [6] W. Stöber, A. Fink, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69. [7] H.S. Jung, Y.J. Kim, S.W. Ha, J.K. Lee, White light-emitting diodes using thermally and photochemically stable fluorescent silica nanoparticles as colorconverters, J. Mater. Chem. C 1 (2013) 5879–5884. [8] H.K. Young, D.K. Lee, H.G. Cha, C.W. Kim, Y.S. Kang, Synthesis and characterization of antibacterial Ag-SiO2 nanocomposite, J. Phys. Chem. C 111 (2007) 3629–3635. [9] J.W. Choi, H. Kang, M. Lee, J.S. Kang, S. Kyeong, J.K. Yang, J. Kim, D.H. Jeong, Y.S. Lee, Y.E. Sung, Plasmon-enhanced dye-sensitized solar cells using SiO2 spheres decorated with tightly assembled silver nanoparticles, RSC Adv. 4 (2014) 19851–19855.
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[10] R.P. Bagwe, C. Yang, L.R. Hilliard, W. Tan, Optimization of dye-doped silica nanoparticles prepared using a reverse microemulsion method, Langmuir 20 (2004) 8336–8342. [11] F.J. Arriagada, K. Osseo-Asare, Controlled hydrolysis of tetraethoxysilane in a nonionic water-in-oil microemulsion: a statistical model of silica nucleation, Colloids Surf. A 154 (1999) 311–326. [12] Y. Naka, Y. Komori, H. Yoshitake, One-pot synthesis of organo-functionalized monodisperse silica particles in W/O microemulsion and the effect of functional groups on addition into polystyrene, Colloids Surf. A 361 (2010) 162–168. [13] F.J. Arriagada, K. Osseo-Asare, Synthesis of nanosize silica in a nonionic waterin-oil microemulsion: effects of the water/surfactant molar ratio and ammonia concentration, J. Colloid Interface Sci. 211 (1999) 210–220. [14] S. Horikoshi, Y. Akao, T. Ogura, H. Sakai, M. Abe, N. Serpone, On the stability of surfactant-free water-in-oil emulsions and synthesis of hollow SiO2 nanospheres, Colloids Surf. A 372 (2010) 55–60. [15] G.H. Bogush, C.F. Zukoski IV, Uniform silica particle precipitation: an aggregative growth model, J. Colloid Interface Sci. 142 (1991) 19–34. [16] I.A. Rahman, P. Vejayakumaran, C.S. Sipaut, J. Ismail, M. Abu Bakar, R. Adnan, C. K. Chee, An optimized sol-gel synthesis of stable primary equivalent silica particles, Colloids Surf. A 294 (2007) 102–110.
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[17] S.K. Park, K.D. Kim, H.T. Kim, Preparation of silica nanoparticles: determination of the optimal synthesis conditions for small and uniform particles, Colloid Surf. A 197 (2002) 7–17. [18] S.S. Kim, H.S. Kim, S.G. Kim, W.S. Kim, Effect of electrolyte additives on solprecipitated nano silica particles, Ceram. Int. 30 (2004) 171–175. [19] T. Matsoukas, E. Gulari, Monomer-addition growth with a slow initiation step: a growth model for silica particles from alkoxides, J. Colloid Interface Sci. 132 (1989) 13–21. [20] D.L. Green, S. Jayasundara, Y.F. Lam, M.T. Harris, Chemical reaction kinetics leading to the first Stober silica nanoparticles – NMR and SAXS investigation, J. Non-Cryst. Solids 315 (2003) 166–179. [21] K.S. Rao, K. El-Hami, T. Kodaki, K. Matsushige, K.J. Makino, A novel method for synthesis of silica nanoparticles, J. Colloid Interface Sci. 289 (2005) 125–131. [22] A.N. Sathyagal, D. Ramkrishna, Droplet breakage in stirred dispersions. Breakage functions from experimental drop-size distributions, Chem. Eng. Sci. 51 (1996) 1377–1391. [23] P. Pattekari, Z. Zheng, X. Zhang, T. Levchenko, V. Torchilin, Y. Lvov, Top-down and bottom-up approaches in production of aqueous nanocolloids of low solubility drug paclitaxel, Phys. Chem. Chem. Phys. 13 (2011) 9014–9019.
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18 October 2016 Advanced Powder Technology xxx (2016) xxx–xxx 1
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
2
Rapid Communication
6 4 7 5
Facile synthesis of surfactant-free SiO2 nanoparticles via emulsion method
8
Sachin N. Bramhe, Ja Young Park, Seon-Ae Hwangbo, Min Cheol Chu ⇑
9
Center for New Functional Materials Metrology, Korea Research Institute of Standards and Science, 267 Gajeong-ro, Yuseong-gu, Daejeon 305-340, South Korea
11 10 12 1 2 4 6 15 16 17 18 19 20 21 22 23 24 25
a r t i c l e
i n f o
Article history: Received 22 July 2015 Received in revised form 28 July 2016 Accepted 16 September 2016 Available online xxxx Keywords: Emulsion Surfactant free Silica nanoparticles Monodisperse
a b s t r a c t Silica (SiO2) nanoparticles have wide potential applications in many advanced areas. In this paper, we report an easy method for the synthesis of monodisperse SiO2 nanoparticles by a simple emulsion method. The novelty of this method lies in the synthesis of nanoparticles without the traditional use of any surfactants and toxic solvents. The phase purity of the synthesized SiO2 nanoparticles were determined by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). Morphology and nanoparticle size was determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of the concentration of tetraethyl orthosilicate (TEOS), ammonia and water on the final size of the nanoparticles was observed. By precisely controlling the concentration of ammonia and water, SiO2 nanoparticles ranging between 30 and 90 nm were synthesized without the use of any surfactants. The results presented a simple, convenient and non-toxic method for the synthesis of SiO2 nanoparticles on a large-scale with precise control of the particle size. Ó 2016 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan.
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1. Introduction SiO2 is the one of the most abundant substances available on our planet. Useful properties of nanosized SiO2 have led to a marked increase in applications of SiO2 nanoparticles. Easy preparation methods and expedient properties have led to applications in areas such as biomedicine, catalysis, sensors, coatings, electronic and thermal insulators and thin film substrates [1–5]. However, many of these applications are heavily dependent on the size of the nanoparticles. Stöber et al. were the first to synthesize SiO2 microparticles utilizing esters of silicic acid as a silica precursor along with ammonia as a catalyst [6]. Over the years, many research groups have used modified versions of the Stöbers method to synthesize silica nanoparticles [7–9]. The general results of these reports is hydrolysis and condensation of various silicon alkoxides. The water in oil emulsion method was also developed for manufacturing SiO2 nanoparticles, which gave precise control over the size of nanoparticles [10–12]. The water particles in the oil phase provide site for hydrolysis and condensation of silicon alkoxide, thereby forming SiO2 nanoparticles. However, these reverse micro-emulsion methods employ surfactants for stabilizing the metastable water nanoparticles and subsequently increase the hydrophilicity of the
⇑ Corresponding author. E-mail address: [email protected] (M.C. Chu).
synthesized particles [13]. Many of the solvents used for dispersion of these surfactants are toxic in nature. For certain applications, it is necessary to remove any surfactants before the nanoparticles can be used. Owing to its large-scale application, it is imperative to find a method to synthesize SiO2 nanoparticles without the use of surfactants and toxic solvents. Recently, Satoshi et al. were able to synthesize hollow SiO2 nanoparticles by surfactant free water in oil emulsions [14]. However, toxic solvents were employed in preparing the emulsions. In this study, we demonstrate a facile method for the synthesis of surfactant-free SiO2 nanoparticles using nanodisperser for generating a stable emulsion. We used nontoxic and inexpensively accessible ethyl acetate as oil phase. By this simple method, we were able to control the size of the synthesized nanoparticles from 35 to 90 nm by controlling its various parameters.
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Ethyl acetate (OCI Company Ltd.), tetraethyl orthosilicate (TEOS) (Aldrich), ammonia solution (28–30%, Junsei) and ethyl alcohol anhydrous (Carlo Erba) were all purchased and used as received. Water was purified via Milli Q system and had an electrical resistance of 18.4 MX cm. To synthesize water nanoparticles in the ethyl acetate phase without the use of any surfactants, a nanodisperser was used. A nanodisperser is a high speed homogenizer that gives better results and ultrasonication than the conventional homogenizer for synthesizing nanoparticles.
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http://dx.doi.org/10.1016/j.apt.2016.09.019 0921-8831/Ó 2016 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan.
Please cite this article in press as: S.N. Bramhe et al., Facile synthesis of surfactant-free SiO2 nanoparticles via emulsion method, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.09.019
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Typically, 1 mL of DI water is added to 99 mL ethyl acetate to give 1% (vol/vol) DI water solution which is then passed through the nanodisperser (Model No. ISA-NLM100, SufLux Ltd.) at 900 bar pressure. The fluid passes an orifice module of the nanodisperser through a high speed intensifier pump. Supersonic speed is generated by this pump that creates a powerful shear force caused by the viscosity and surface tension of the liquid and frictional force of the orifice module. Because of the sudden decrease in pressure cavitation occurs, which on bursting collide with other particles and helps in the breakdown of larger particles. The combination of shear force, impact and cavitation inside the nanodisperser at high pressure conditions aids in the formation of the emulsion. However, we found that the resulting emulsion is not always monodisperse. To that end, we kept the emulsion obtained from the nanodisperser, at rest for a period of 24 h in a graduated cylinder to allow the larger particles to settle down. After the 24 h mark, the lower 50 mL of the emulsion containing unstable larger particles was discarded. To the remaining 50 mL emulsion solution, 0.045 M TEOS and 300 lL ammonia solution was added and continuously stirred for 24 h. The concentrations were calculated based on its final concentration in the reaction mixture. Then the emulsion was broken by adding ethyl alcohol anhydrous and centrifuged to collect the synthesized nanoparti-
Fig. 1. Schematic procedure of synthesis of SiO2 nanoparticles.
cles. Experiments were carried out with varying amounts of TEOS, ammonia and DI water to determine its effect on the particle size. Scanning electron microscopy images were taken on FE-SEM, Hitachi, Model No.: S-4800. EDX analysis was done on scanning electron microscope equipped with EDX detector. X-ray diffraction (XRD) pattern of the synthesized powder was analyzed with Cu Ka radiation (k = 1.5406 Å) on a SmartLab X-ray diffractometer (Rigaku). TEM images were taken by JEM-2100F, Jeol.
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3. Results and discussion
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Synthesizing nanoparticles via emulsion method involves two main steps. First, the synthesis of water nanoparticles in the oil phase followed by hydrolysis and condensation of tetraethyl orthosilicate (TEOS). In an aqueous solution, TEOS undergoes hydrolysis to form silicon tetra hydroxide, which condenses to form nanoparticles of silica. The following reaction takes place during the conversion of TEOS to SiO2:
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SiðOC2 H5 Þ4 þ 2 H2 O ! SiO2 þ 4 C2 H5 OH An illustration of the steps involved in the synthesis of SiO2 nanoparticles is given in Fig. 1. Ammonia serves as a catalyst while water particles react with TEOS to give SiO2 nanoparticles. The main parameters that affect the size of SiO2 nanoparticles: (i) TEOS concentration (ii) ammonia concentration and (iii) water concentration. TEOS, being the source of SiO2, plays an important role in determining the concentration of primary particles of SiO2. After nucleation, the primary particles aggregate to form the stable secondary particles. After this, any new nuclei or primary particles will dissolve and deposit on the secondary particles due to Ostwald ripening until a stable system is obtained [15]. As can be observed in Fig. S1, optimized SiO2 nanoparticles were formed when 0.045 M of TEOS concentration was added while keeping the ammonia solution and water concentration constant at 300 lL and 1% respectively. At a higher concentration the polydispersity index increased, whereas at a lower concentration the yield was relatively low. Particles obtained at 0.045 M TEOS concentration were spherical and nearly monodisperse. First, the powder synthesized by adding 0.045 M TEOS was analysed by an X-ray diffraction to check its phase purity. As observed in Fig. 2(a), the absence of any secondary peaks confirms that only SiO2 phase is obtained in the powder. The peak at 2h = 23.61was in accordance with JCPDS file no. 01-082-1563. To check the spectrum of mineral crusts, energy dispersive X-ray spectroscopy (EDX) was used. The results indicated in Fig. 2(b)
Fig. 2. (a) XRD analysis showing pure SiO2 phase; (b) EDX analysis showing only Si and O peaks.
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show peaks pertaining to Si and O only, confirming that only SiO2 phase is present in the synthesized powder. Fig. S2 shows the EDS element mapping of the synthesized SiO2 nanoparticles. Next, we checked the effects of ammonia on the size of nanoparticles while keeping the concentration of TEOS and water constant at 0.045 M and 1% respectively. Fig. 3 shows the SEM and TEM images of SiO2 synthesized with the addition of varying concentration of ammonia. As can be seen in SEM analysis and its accompanying size distribution (shown in inset), a general trend of increase in particle size with increase in the concentration of ammonia is observed. Similar results were also obtained by Rahman et al. [16]. Ammonia tends to increase the rate of hydrolysis of TEOS and the condensation rate of the synthesized monomers while promoting the rate of polymerization [17–20]. Hence, we observe an increase in size of nanoparticles with the increase in concentration of ammonia. Size distribution analysis in Fig. 4 shows the general trend observed with the increase in concentration of ammonia [21]. The lowest concentration of ammonia at 200 lL resulted in the synthesis of 47 nm nanoparticles, while the largest amount of ammmonia at 600 lL resulted in 90 nm particles. Moreover, a marked increase in the standard deviation of the mean size of the particles was observed as the concentration of ammonia solution added was increased. This might be because ammonia solution provides excess water molecules. These excess water molecules might increase the size of the emulsion, thus increasing the standard deviation from the mean value. Fig. 3(f) shows the TEM image of SiO2 nanoparticles synthesized by the
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Fig. 4. Effect of ammonia and water concentration on final size of SiO2 nanoparticles.
addition of 200 lL ammonia. As can be observed, highly monodisperse and spherical nanoparticles of size 45 nm were observed. We also checked the effect of DI water concentration on the relative size of silica nanoparticles while keeping TEOS and ammonia
Fig. 3. Effect of change of ammonia concentration on final size of SiO2 nanoparticles. (a) 200 lL, (b) 300 lL, (c) 400 lL, (d) 500 lL, (e) 600 lL; and (f) TEM image of SiO2 nanoparticles synthesized by typical addition of 200 lL ammonia solution.
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Fig. 5. Effect of change of DI water concentration on size of SiO2 nanoparticles (a) 0.5%, (b) 0.75%, (c) 1.25%, (d) 1.5% DI water concentration.
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concentration constant at 0.045 M and 300 lL respectively. Fig. 5 shows the SEM images of SiO2 nanoparticles synthesized by the addition of varying concentrations of DI water. Particle size of SiO2 increases with the increase in water concentration. With the increase in water concentration, the rate of nucleation also increases, which ultimately results in a large number of particle synthesis [21]. At a higher water concentration, the SiO2 subparticles produced have stronger hydrogen bonds due to excess water which causes aggregation [17]. This leads to the formation of larger nanoparticles and a deviation from the mean particle size. Size distribution analysis in Fig. 4 shows the general trend observed with the increase in concentration of DI water from 0.5% to the highest checked concentration of 1.5%. As seen in Fig. 4 and 05% water concentration leads to formation of particles sized at 35 nm while the highest concentration of 1.5% gave particles sized at 86 nm. However, when the concentration was decreased to 0.25% the particle size obtained was 57 nm. The drop size distribution and stability of emulsion is dependent on breakage rate and coalescence of the dispersed phase [22]. The water concentration of 0.5% to 1.5% results in breakage and coalescence for formation of stable emulsion with increasing size, however the water concentration of 0.25% results in formation of stable emulsion of higher size. This might be because of lower rate of breakage at 0.25% resulting in larger emulsion size or strong attraction force between the smaller water particles causing an increase in size [23]. With the increase in concentration of ammonia and water, we observed an increase in size of resulting SiO2 nanoparticles. Both these results show that by controlling the rate of nucleation we can control the size of the end product. By not using any surfactants or toxic solvents, we were able to synthesize SiO2 nanoparticles of various sizes by controlling various parameters.
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4. Conclusions
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In conclusion, we have demonstrated a facile and an inexpensive method for the synthesis of monodisperse SiO2 nanoparticles without using any surfactants or toxic solvents. These monodisperse particles have the potential application in drug delivery and as a catalyst. Easy tunability of particle size from 30 to
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90 nm means that these nanoparticles could well be used as certified reference materials. This inexpensive and convenient method can enable large-scale manufacturing of SiO2 nanoparticles without causing any obtrusive toxic effects on the individual or the environment.
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Acknowledgement
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This work was supported by Nano Material Technology Development Program (2014M3A7B6020163) of MSIP/NRF.
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Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apt.2016.09.019.
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