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Synthesis and characterization of CaCO3@SiO2 core–shell nanoparticles
Powder Technology 141 (2004) 75 – 79 www.elsevier.com/locate/powtec
Synthesis and characterization of CaCO3@SiO2 core–shell nanoparticles Shicheng Zhang *, Xingguo Li State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, PR China Received 18 February 2003; received in revised form 11 February 2004; accepted 18 February 2004
Abstract This paper reports on a new type of composite nanoparticle consisting of a calcite (CaCO3) core and a silica (SiO2) shell. The cubic core – shell particles were created using a surface precipitation procedure using cubic calcite as seeds on which a silica layer was grown. Moreover, the calcite of core – shell nanoparticles can be dissolved leaving silica shells. The characterizations of composite and silica shells were made by using TEM, BET, XRD, IR, XRF, and TGA. The results show that the silica layer was continuously coated on the surface of CaCO3 core with a thickness of about 5 nm. After coated with silica shells, the thermal stability decreases for its reaction with silica. The shape and size of pores of silica shells were similar to that of the CaCO3 cores. D 2004 Elsevier B.V. All rights reserved. Keywords: Core – shell; Nanoparticles; Calcite; Silica
1. Introduction Recently, a lot of interest in core – shell nanoparticles has arisen due to their importance in various fields of science and technology, such as biological labels [1,2], optical resonances [3,4], catalysis [5,6], magnetics [7], ceramics [8,9], and pigments [10]. By building up such shells, various surface properties of dispersed matter can be altered in order to meet some required specifications. Some examples of surface properties that can be improved or modified are flowability, dispersibility, solubility, wettability (hydrophilic/hydrophobic properties), electrostatic, electric, magnetic, optical, color, flavor, taste, particle shape/sphericity, sinterability, and solid phase reactivity [11]. Furthermore, an important extension of core –shell particles is the subsequent removal of the core by either dissolution or decomposition to produce hollow shells [12,13]. Various methods, which yielded core – shell nanoparticles, have been reported in the literatures, such as electroless plating [14,15], precipitation [16], sonochemical deposition [17 – 19], reverse micelles [20,21], sol – gel [22,23], and layer-by-layer (LbL) technique [24,25]. Generally, these methods can be divided into four categories:
* Corresponding author. Tel.: +86-10-6277-6886. E-mail address: [email protected] (S. Zhang). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.02.018
surface reaction, surface precipitation, precipitation heterocoagulation, and the layer-by-layer (LbL) technique. CaCO3 is widely used in various industries (e.g. paper, rubber, plastics and paint industries as a coating pigment, filler or extender; food and horticulture industries) [26]. In this paper, the core –shell CaCO3@SiO2 nanoparticles were produced by using surface precipitation of silica on the surface of CaCO3 cores. Subsequently, the silica shells were created by dissolving the CaCO3 cores. The detailed structures of the core –shell CaCO3@SiO2 nanoparticles and the silica shells were investigated.
2. Experimental section 2.1. Sample preparation The synthetic procedures for the CaCO3@SiO2 core – shell nanoparticles are briefly summarized in Scheme 1 . CaO, Na2SiO39H2O, and HCl used in the experiments were of analytical purity and were purchased from Beijing Chemical Factory (China). CO2 was of chemical purity and was purchased from Beijing Alcohol Factory (China). De-ionized water was used in our experiments. The preparation of CaCO3 nanoparticles was carried out in a beaker using the reaction system Ca(OH)2 – H2O – CO2 such as that described in the literature [27]. Bulk CaO was slaked into lime milk in deionized water at 80 jC. The lime milk was cooled to room
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S. Zhang, X. Li / Powder Technology 141 (2004) 75–79
Scheme 1. The synthetic procedures for the CaCO3@SiO2 core – shell nanoparticles.
temperature (20 jC), and its density was measured by titration and adjusted to 0.5 mol/l. The CO2 gas was blown with a flow rate of 120 ml/min into the lime milk from the bottom of the breaker with vigorous stirring. The pH value and electric conductivity of the reaction solution were inspected on-line with both a pH-meter and a conductometer. When the pH value reached 9 and the electric conductivity showing a sharp decrease, the reaction was completed, and the CO2 flow was stopped. CaCO3 nanoparticles were obtained after the slurry was washed, filtered, and dried. For preparation of CaCO3@SiO2 core –shell nanoparticles, the Na2SiO3 solution was added into the slurry of CaCO3 nanoparticles with the mole ratio of SiO2/CaCO3 1:5. The mixture was heated up to 80 jC, and into which the CO2 gas was blown with a flow rate of 120 ml/min. After the pH value reached 7, the slurry was aged for 2 h. The CaCO3@SiO2 core – shell nanoparticles were obtained after the slurry was washed, filtered, and dried. Hollow silica shells were obtained after dissolving the CaCO3 core using a mineral acid. Hydrochloric acid (0.25 mol/l) was added to a dilute water suspension of CaCO3@SiO2 core – shell nanoparticles (1 g particles per 100 ml water) with moderate stirring. The dissolution process was carried out for a period of 7 days. 2.2. Sample characterization The structure and morphology of the samples were investigated using transmission electron microscopy (JEM100 CX, TEM) and X-ray diffraction (Rigaku D/MAX-RA diffractometer with CuKa radiation). The interfacial structure is also checked by a Fourier transform infrared spectrophotometer (Shimadzu FTIR-8400). The composition was detected by using fluorescent X-ray spectrometry (BrookHaven Instruments AXS). Thermogravimetric analysis (TGA) was carried out using Du Pont thermal analysis system (Dupont 1090B TGA 951 Thermogravimetric Analyzer) with a heating rate 10j/min in air. Pore size distribution was measured using a COULTER SA3100 surface area analyzer (COULTER).
obtained. The particle size distribution of CaCO3 nanoparticles was counted from TEM images for about 150 particles (Fig. 1). CaCO3 nanoparticles were directly coated with a silica layer by hydrolysis and condensation of Na2SiO3 in an aqueous solution. The silica layer, created by the Sto¨ber syntheses, is porous with a pore size about 3 ˚ , and this size is sufficiently large to allow free diffusion of A Zn2 + and S2 ions across the silica layer [28,29]. The H+ ions also can diffuse through the silica layer created by our method and this makes the core CaCO3 dissolve. Fig. 2 shows the TEM images of CaCO3 nanoparticles, CaCO3@SiO2 core – shell nanoparticles, and silica shells. From the image of CaCO3@SiO2 core –shell nanoparticles, it can be shown that the silica layer is continuously coated on the core particle surface. The shape and size of the hollows of silica shells are similar to that of CaCO3 nanoparticles, which also confirms that the silica layer is continuously coated on the surface of core CaCO3 nanoparticles. The thickness of the silica layer is estimated at about 5 nm. From the pore size distribution (Fig. 3) of three kinds of samples, we can also find the same results. The pore size distributions of CaCO3 and CaCO3@SiO2 core – shell nanoparticles are similar, which means that the silica created by hydrolysis of Na2SiO3 in an aqueous solution are all coated on the surface of CaCO3 and a minimal amount of silica exits freely. The pore size distribution of the silica shells is smaller than the particle size distribution of CaCO3 nanoparticles and the pore size of silica shells measured by TEM. This is caused by the deference of the preparation method of samples for TEM and BET. For TEM, the sample is prepared by putting a drop of the slurry of the silica shell on copper grid coated with carbon film, and then is dried in ambient conditions. The sample for BET is dry powder, which is prepared by filtering and drying the slurry of the silica shells. During the filtering and drying process, the silica shells may shrink, and the pore size become smaller. Even though this reason, form the pore size distributions of CaCO3 nanoparticles, CaCO3@SiO2 core –shell nanopar-
3. Results and discussions By use of the above-described procedure, cubic CaCO3 nanoparticles with a particle size of 30 – 60 nm were
Fig. 1. Particle size distribution of CaCO3 nanoparticles.
S. Zhang, X. Li / Powder Technology 141 (2004) 75–79
77
Fig. 2. Morphologies of CaCO3 nanoparticles (a), CaCO3@SiO2 core – shell nanoparticles (b), and silica shell (c, d).
ticles, and silica shell, it can also confirm that the hollows of silica shells are created by dissolving the core CaCO3 nanoparticles. Fig. 4 shows the X-ray diffraction results of the samples. The synthesized CaCO3 nanoparticles and the CaCO3@SiO2 core –shell nanoparticles are calcite phase and silica layer is amorphous. The coating of silica layer on CaCO3 surface do not change the crystal structure of CaCO3 core. After heat processing under 500 jC for 2 h, the crystal structure was still unchanged. The results of FTIR (Fig. 5) show that the IR absorption of the CaCO3@SiO2 core –shell nanoparticles is similar to the combination of absorptions of CaCO3 and silica shell.
The composition of CaCO3@SiO2 core –shell nanoparticles was detected by fluorescent X-ray spectrometry. The mole ratio of SiO2/CaCO3 is 17.1%. So the efficiency of silica can be characterized by using the mole ratio of deposited silica to total one, and can be calculated according to the Eq. (1):
where Esilica is the efficiency of silica, Ru is the mole ratio of SiO2/CaCO3 in ultimate products, and Rp is the mole ratio of SiO2/CaCO3 in the precursor.
Fig. 3. Pore size distributions of CaCO3 nanoparticles, CaCO3@SiO2 core – shell nanoparticles, and silica shell.
Fig. 4. X-ray diffraction patterns for samples: (a) silica shell; (b) CaCO3; (c) CaCO3@SiO2; (d) CaCO3@SiO2 with further heat processing under 500 jC for 2 h.
Esilica ¼
Ru 17:1% ¼ 85:5% ¼ Rp 20:00%
ð1Þ
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S. Zhang, X. Li / Powder Technology 141 (2004) 75–79
Fig. 5. FTIR analysis of CaCO3 nanoparticles, CaCO3@SiO2 core – shell nanoparticles, and silica shell.
Fig. 6 shows the TGA measurements on CaCO3 nanoparticles and CaCO3@SiO2 core – shell nanoparticles. It can be seen that the CaCO3 nanoparticle lost its weight in one step at around 640 jC due to the decomposition of CaCO3.
The CaCO3@SiO 2 core – shell nanoparticle lost its weight in three steps: first step at around 110 jC, the second step at 440 jC, and the third step at 665 jC. The first step of the thermal decomposition is the loss of water. The second weight loss is caused by the reaction of calcite and amorphous silica according to Eq. (2) [30], and followed by the decomposition of the rest CaCO3. SiO2 þ CaCO3 ! CaSiO3 þ CO2 ðgasÞ
(a)
ð2Þ
From the X-ray diffraction result of the CaCO3@SiO2 core – shell nanoparticle after heat processing at 500 jC for 2 h (Fig. 4d), the phase of CaSiO3 cannot be found. This means the CaSiO3 is amorphous, or is coated on the surface of CaCO3 and cannot be detected by X-ray diffraction [31]. The third step of the thermal decomposition is the decomposition of core CaCO3. The decomposition temperature of core CaCO3 is higher about 25 jC than that of CaCO3 nanoparticles. It is suggested that the silica shell inhibited the decomposition of core CaCO3.
4. Conclusions
(b)
Fig. 6. Thermogravimetric analysis of CaCO3 nanoparticles (a) and CaCO3@SiO2 core – shell nanoparticles (b).
CaCO3@SiO2 core – shell nanoparticles were prepared by surface deposition. The silica layers were coated continuously on the surface of CaCO3 cores with a thickness of about 5 nm. The efficiency of silica in our experiment is about 85.5%. After being coated with silica shells, the thermal stability decreases for its reaction with silica. The CaCO3 cores can be dissolved to produce hollow silica shells. The shape and size of pores of silica shells are similar to that of the CaCO3 cores. If the shape or size of CaCO3 cores is changed, the silica shells with a different shape or size of pores can be gained. The CaCO3@SiO2 core – shell nanoparticles can be used in paper, rubber, plastics and paint industries. The silica shells can be used as template for the
S. Zhang, X. Li / Powder Technology 141 (2004) 75–79
synthesis of other nanoparticles. The further studies on their applications are being performed.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 20025103, 20221101 and 10335040), the Ministry of Science and Technology of China (Grant No. 2001CB610503), and the China Postdoctoral Science Foundation.
References [1] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013. [2] W.C.W. Chan, S. Nie, Science 281 (1998) 2016. [3] P. Viravathana, D.W.M. Marr, J. Colloid Interface Sci. 221 (2000) 301. [4] S.J. Oldenburg, R.D. Averitt, S.L. Westcott, N.J. Halas, Chem. Phys. Lett. 288 (1998) 243. [5] H. Tamai, T. Ikeya, F. Nishiyama, H. Yasuda, K. Iida, S. Nojima, J. Mater. Sci. 35 (2000) 4945. [6] Y. Jiang, S. Decker, C. Mohs, K.J. Klabunde, J. Catal. 180 (1998) 24. [7] J.J. Schneider, Adv. Mater. 13 (2001) 529. [8] W.J. Walker Jr., M.C. Brown, V.R.W. Amarakoon, J. Eur. Ceram. Soc. 21 (2001) 2031. [9] E.P. Luther, F.F. Lange, D.S. Pearson, J. Am. Ceram. Soc. 78 (1995) 2009. [10] R.L. DeColibus, U.S. Patent 3,928,057. [11] R. Pfeffer, R.N. Dave, D. Wei, M. Ramlakhan, Powder Technol. 117 (2001) 40.
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[12] F. Caruso, R.A. Caruso, H. Mohwald, Science 282 (1998) 1111. [13] F. Caruso, Chem. Eur. J. 6 (2000) 413. [14] L.N. Xu, K.C. Zhou, H.F. Xu, H.Q. Zhang, L. Huang, J.H. Liao, A.Q. Xu, N. Gu, H.Y. Shen, J.Z. Liu, Appl. Surf. Sci. 183 (2001) 58. [15] V. Kobayashi, V. Salgueirin˜o-Maceira, L.M. Liz-Marza´n, Chem. Mater. 13 (2001) 1630. [16] I. Haq, E. Matijevic, J. Colloid Interface Sci. 192 (1997) 104. [17] V.G. Pol, D.N. Srivastava, O. Palchik, V. Palchik, M.A. Slifkin, A.M. Wiess, A. Gedanken, Langmuir 18 (2002) 3352. [18] M.L. Breen, A.D. Dinsmore, R.H. Pink, S.B. Qadri, B.R. Ratna, Langmuir 17 (2001) 903. [19] Y. Mizukoshi, T. Fujimoto, Y. Nagata, R. Oshima, Y. Maeda, J. Phys. Chem., B 104 (2000) 6028. [20] C.J. O’Connor, C.T. Seip, E.E. Carpenter, S.C. Li, V.T. John, Nanostruct. Mater. Part A 12 (1999) 65. [21] W.L. Zhou, E.E. Carpenter, J. Lin, A. Kumbhar, J. Sims, C.J. O’Connor, Eur. Phys. J., D 16 (2001) 289. [22] R.A. Caruso, M. Antonietti, Chem. Mater. 13 (2001) 3272. [23] D.S. Bae, D.J. Kim, K.S. Han, J.H. Adair, J. Ceram. Proc. Res. 3 (2002) 38. [24] F. Caruso, M. Spasova, Adv. Mater. 13 (2001) 1090. [25] R.A. Caruso, A. Susha, F. Caruso, Chem. Mater. 13 (2001) 400. [26] M. Vucˇak, M.N. Pons, J. Pericˇ, H. Vivier, Powder Technol. 97 (1998) 1. [27] S.C. Zhang, Y.X. Han, J.H. Jiang, H.K. Wang, J. Northeast. Univ. (Nat. Sci.) 21 (2000) 169. [28] K.P. Velikov, A. van Blaaderen, Langmuir 17 (2001) 4779. [29] A. Walcarius, C. Despas, J. Bessie’re, Microporous Mesoporous Mater. 23 (1998) 309. [30] K. Lee, W.M. Sackett, Deep-Sea Res., Part 1, Oceanogr. Res. Pap. 45 (1998) 1015. [31] Y.C. Xie, Y.Q. Tang, Spontaneous monolayer dispersion of oxides and salts onto surfaces of supports: applications to heterogeneous catalysis, in: D.D. Eley, H. Pines, P.B. Weisz (Eds.), Advances in Catalysis, vol. 37, Academic Press, San Diego, CA, 1990, pp. 1 – 43.
Synthesis and characterization of CaCO3@SiO2 core–shell nanoparticles Shicheng Zhang *, Xingguo Li State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, PR China Received 18 February 2003; received in revised form 11 February 2004; accepted 18 February 2004
Abstract This paper reports on a new type of composite nanoparticle consisting of a calcite (CaCO3) core and a silica (SiO2) shell. The cubic core – shell particles were created using a surface precipitation procedure using cubic calcite as seeds on which a silica layer was grown. Moreover, the calcite of core – shell nanoparticles can be dissolved leaving silica shells. The characterizations of composite and silica shells were made by using TEM, BET, XRD, IR, XRF, and TGA. The results show that the silica layer was continuously coated on the surface of CaCO3 core with a thickness of about 5 nm. After coated with silica shells, the thermal stability decreases for its reaction with silica. The shape and size of pores of silica shells were similar to that of the CaCO3 cores. D 2004 Elsevier B.V. All rights reserved. Keywords: Core – shell; Nanoparticles; Calcite; Silica
1. Introduction Recently, a lot of interest in core – shell nanoparticles has arisen due to their importance in various fields of science and technology, such as biological labels [1,2], optical resonances [3,4], catalysis [5,6], magnetics [7], ceramics [8,9], and pigments [10]. By building up such shells, various surface properties of dispersed matter can be altered in order to meet some required specifications. Some examples of surface properties that can be improved or modified are flowability, dispersibility, solubility, wettability (hydrophilic/hydrophobic properties), electrostatic, electric, magnetic, optical, color, flavor, taste, particle shape/sphericity, sinterability, and solid phase reactivity [11]. Furthermore, an important extension of core –shell particles is the subsequent removal of the core by either dissolution or decomposition to produce hollow shells [12,13]. Various methods, which yielded core – shell nanoparticles, have been reported in the literatures, such as electroless plating [14,15], precipitation [16], sonochemical deposition [17 – 19], reverse micelles [20,21], sol – gel [22,23], and layer-by-layer (LbL) technique [24,25]. Generally, these methods can be divided into four categories:
* Corresponding author. Tel.: +86-10-6277-6886. E-mail address: [email protected] (S. Zhang). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.02.018
surface reaction, surface precipitation, precipitation heterocoagulation, and the layer-by-layer (LbL) technique. CaCO3 is widely used in various industries (e.g. paper, rubber, plastics and paint industries as a coating pigment, filler or extender; food and horticulture industries) [26]. In this paper, the core –shell CaCO3@SiO2 nanoparticles were produced by using surface precipitation of silica on the surface of CaCO3 cores. Subsequently, the silica shells were created by dissolving the CaCO3 cores. The detailed structures of the core –shell CaCO3@SiO2 nanoparticles and the silica shells were investigated.
2. Experimental section 2.1. Sample preparation The synthetic procedures for the CaCO3@SiO2 core – shell nanoparticles are briefly summarized in Scheme 1 . CaO, Na2SiO39H2O, and HCl used in the experiments were of analytical purity and were purchased from Beijing Chemical Factory (China). CO2 was of chemical purity and was purchased from Beijing Alcohol Factory (China). De-ionized water was used in our experiments. The preparation of CaCO3 nanoparticles was carried out in a beaker using the reaction system Ca(OH)2 – H2O – CO2 such as that described in the literature [27]. Bulk CaO was slaked into lime milk in deionized water at 80 jC. The lime milk was cooled to room
76
S. Zhang, X. Li / Powder Technology 141 (2004) 75–79
Scheme 1. The synthetic procedures for the CaCO3@SiO2 core – shell nanoparticles.
temperature (20 jC), and its density was measured by titration and adjusted to 0.5 mol/l. The CO2 gas was blown with a flow rate of 120 ml/min into the lime milk from the bottom of the breaker with vigorous stirring. The pH value and electric conductivity of the reaction solution were inspected on-line with both a pH-meter and a conductometer. When the pH value reached 9 and the electric conductivity showing a sharp decrease, the reaction was completed, and the CO2 flow was stopped. CaCO3 nanoparticles were obtained after the slurry was washed, filtered, and dried. For preparation of CaCO3@SiO2 core –shell nanoparticles, the Na2SiO3 solution was added into the slurry of CaCO3 nanoparticles with the mole ratio of SiO2/CaCO3 1:5. The mixture was heated up to 80 jC, and into which the CO2 gas was blown with a flow rate of 120 ml/min. After the pH value reached 7, the slurry was aged for 2 h. The CaCO3@SiO2 core – shell nanoparticles were obtained after the slurry was washed, filtered, and dried. Hollow silica shells were obtained after dissolving the CaCO3 core using a mineral acid. Hydrochloric acid (0.25 mol/l) was added to a dilute water suspension of CaCO3@SiO2 core – shell nanoparticles (1 g particles per 100 ml water) with moderate stirring. The dissolution process was carried out for a period of 7 days. 2.2. Sample characterization The structure and morphology of the samples were investigated using transmission electron microscopy (JEM100 CX, TEM) and X-ray diffraction (Rigaku D/MAX-RA diffractometer with CuKa radiation). The interfacial structure is also checked by a Fourier transform infrared spectrophotometer (Shimadzu FTIR-8400). The composition was detected by using fluorescent X-ray spectrometry (BrookHaven Instruments AXS). Thermogravimetric analysis (TGA) was carried out using Du Pont thermal analysis system (Dupont 1090B TGA 951 Thermogravimetric Analyzer) with a heating rate 10j/min in air. Pore size distribution was measured using a COULTER SA3100 surface area analyzer (COULTER).
obtained. The particle size distribution of CaCO3 nanoparticles was counted from TEM images for about 150 particles (Fig. 1). CaCO3 nanoparticles were directly coated with a silica layer by hydrolysis and condensation of Na2SiO3 in an aqueous solution. The silica layer, created by the Sto¨ber syntheses, is porous with a pore size about 3 ˚ , and this size is sufficiently large to allow free diffusion of A Zn2 + and S2 ions across the silica layer [28,29]. The H+ ions also can diffuse through the silica layer created by our method and this makes the core CaCO3 dissolve. Fig. 2 shows the TEM images of CaCO3 nanoparticles, CaCO3@SiO2 core – shell nanoparticles, and silica shells. From the image of CaCO3@SiO2 core –shell nanoparticles, it can be shown that the silica layer is continuously coated on the core particle surface. The shape and size of the hollows of silica shells are similar to that of CaCO3 nanoparticles, which also confirms that the silica layer is continuously coated on the surface of core CaCO3 nanoparticles. The thickness of the silica layer is estimated at about 5 nm. From the pore size distribution (Fig. 3) of three kinds of samples, we can also find the same results. The pore size distributions of CaCO3 and CaCO3@SiO2 core – shell nanoparticles are similar, which means that the silica created by hydrolysis of Na2SiO3 in an aqueous solution are all coated on the surface of CaCO3 and a minimal amount of silica exits freely. The pore size distribution of the silica shells is smaller than the particle size distribution of CaCO3 nanoparticles and the pore size of silica shells measured by TEM. This is caused by the deference of the preparation method of samples for TEM and BET. For TEM, the sample is prepared by putting a drop of the slurry of the silica shell on copper grid coated with carbon film, and then is dried in ambient conditions. The sample for BET is dry powder, which is prepared by filtering and drying the slurry of the silica shells. During the filtering and drying process, the silica shells may shrink, and the pore size become smaller. Even though this reason, form the pore size distributions of CaCO3 nanoparticles, CaCO3@SiO2 core –shell nanopar-
3. Results and discussions By use of the above-described procedure, cubic CaCO3 nanoparticles with a particle size of 30 – 60 nm were
Fig. 1. Particle size distribution of CaCO3 nanoparticles.
S. Zhang, X. Li / Powder Technology 141 (2004) 75–79
77
Fig. 2. Morphologies of CaCO3 nanoparticles (a), CaCO3@SiO2 core – shell nanoparticles (b), and silica shell (c, d).
ticles, and silica shell, it can also confirm that the hollows of silica shells are created by dissolving the core CaCO3 nanoparticles. Fig. 4 shows the X-ray diffraction results of the samples. The synthesized CaCO3 nanoparticles and the CaCO3@SiO2 core –shell nanoparticles are calcite phase and silica layer is amorphous. The coating of silica layer on CaCO3 surface do not change the crystal structure of CaCO3 core. After heat processing under 500 jC for 2 h, the crystal structure was still unchanged. The results of FTIR (Fig. 5) show that the IR absorption of the CaCO3@SiO2 core –shell nanoparticles is similar to the combination of absorptions of CaCO3 and silica shell.
The composition of CaCO3@SiO2 core –shell nanoparticles was detected by fluorescent X-ray spectrometry. The mole ratio of SiO2/CaCO3 is 17.1%. So the efficiency of silica can be characterized by using the mole ratio of deposited silica to total one, and can be calculated according to the Eq. (1):
where Esilica is the efficiency of silica, Ru is the mole ratio of SiO2/CaCO3 in ultimate products, and Rp is the mole ratio of SiO2/CaCO3 in the precursor.
Fig. 3. Pore size distributions of CaCO3 nanoparticles, CaCO3@SiO2 core – shell nanoparticles, and silica shell.
Fig. 4. X-ray diffraction patterns for samples: (a) silica shell; (b) CaCO3; (c) CaCO3@SiO2; (d) CaCO3@SiO2 with further heat processing under 500 jC for 2 h.
Esilica ¼
Ru 17:1% ¼ 85:5% ¼ Rp 20:00%
ð1Þ
78
S. Zhang, X. Li / Powder Technology 141 (2004) 75–79
Fig. 5. FTIR analysis of CaCO3 nanoparticles, CaCO3@SiO2 core – shell nanoparticles, and silica shell.
Fig. 6 shows the TGA measurements on CaCO3 nanoparticles and CaCO3@SiO2 core – shell nanoparticles. It can be seen that the CaCO3 nanoparticle lost its weight in one step at around 640 jC due to the decomposition of CaCO3.
The CaCO3@SiO 2 core – shell nanoparticle lost its weight in three steps: first step at around 110 jC, the second step at 440 jC, and the third step at 665 jC. The first step of the thermal decomposition is the loss of water. The second weight loss is caused by the reaction of calcite and amorphous silica according to Eq. (2) [30], and followed by the decomposition of the rest CaCO3. SiO2 þ CaCO3 ! CaSiO3 þ CO2 ðgasÞ
(a)
ð2Þ
From the X-ray diffraction result of the CaCO3@SiO2 core – shell nanoparticle after heat processing at 500 jC for 2 h (Fig. 4d), the phase of CaSiO3 cannot be found. This means the CaSiO3 is amorphous, or is coated on the surface of CaCO3 and cannot be detected by X-ray diffraction [31]. The third step of the thermal decomposition is the decomposition of core CaCO3. The decomposition temperature of core CaCO3 is higher about 25 jC than that of CaCO3 nanoparticles. It is suggested that the silica shell inhibited the decomposition of core CaCO3.
4. Conclusions
(b)
Fig. 6. Thermogravimetric analysis of CaCO3 nanoparticles (a) and CaCO3@SiO2 core – shell nanoparticles (b).
CaCO3@SiO2 core – shell nanoparticles were prepared by surface deposition. The silica layers were coated continuously on the surface of CaCO3 cores with a thickness of about 5 nm. The efficiency of silica in our experiment is about 85.5%. After being coated with silica shells, the thermal stability decreases for its reaction with silica. The CaCO3 cores can be dissolved to produce hollow silica shells. The shape and size of pores of silica shells are similar to that of the CaCO3 cores. If the shape or size of CaCO3 cores is changed, the silica shells with a different shape or size of pores can be gained. The CaCO3@SiO2 core – shell nanoparticles can be used in paper, rubber, plastics and paint industries. The silica shells can be used as template for the
S. Zhang, X. Li / Powder Technology 141 (2004) 75–79
synthesis of other nanoparticles. The further studies on their applications are being performed.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 20025103, 20221101 and 10335040), the Ministry of Science and Technology of China (Grant No. 2001CB610503), and the China Postdoctoral Science Foundation.
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