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shell nanospheres
Materials Letters 66 (2012) 285–288
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation and application of magnetic cobalt/SiO2 core/shell nanospheres Mengshi Xu a, Ru Qiao a,⁎, Xiao Li Zhang c, Young Soo Kang b,⁎, Liang-Chao Li a a b c
College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China Department of Chemistry, Sogang University, Seoul 121-742, South Korea Department of Materials Engineering, Monash University, Clayton VIC, Australia
a r t i c l e
i n f o
Article history: Received 19 June 2011 Accepted 1 September 2011 Available online 7 September 2011 Keywords: Co/SiO2 nanospheres Magnetic materials Hollow spheres CdS Nanocomposites
a b s t r a c t Homogeneous SiO2-coated cobalt nanospheres with tunable silica shell thickness from 21.7 nm to 4.5 nm were synthesized by using modified Stöber method. These nanocomposites were used as source materials to prepare SiO2 semi-hollow and hollow nanospheres by partially and completely etching cobalt cores, respectively. A proposed formation mechanism of these Co/SiO2 nanospheres with a core/shell structure was presented in this paper, which is also important for the rational design and synthesis of other monodisperse core/shell nanoarchitectures with uniform size and shape. Furthermore, these Co/SiO2 nanospheres were also used as a substrate for the deposition of CdS nanocrystals to prepare magnetic luminescent Co/SiO2/CdS nanocomposites. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanoparticles (NPs) used in medicine bioengineering field [1–3] commonly need to be embedded in a nonmagnetic matrix to avoid aggregation and sedimentation and endow them with particular surface properties. Among various processing methods, coating NPs with silica is becoming a promising approach [4] because the inert silica shell could screen the magnetic dipolar attraction between magnetic NPs and provide these particles with a chemical hydrophilicity [5,6]. The existence of silanol groups also makes the exterior surface easy to link various functional groups. Differing with coating on metal oxides [5,7], silica deposition on pure metal particles is more complicated because of the lack of hydroxyl groups on metal surface. So it is necessary to use a primer to make the surface vitreophilic [8]. Moreover, how to control the silica shell thickness is also a problem that must be resolved because excessive amorphous silica can deleteriously affect magnetic properties of the products. Xu and co-workers once synthesized Co–SiO2 spheres by reversed micelle techniques [9]. Herein, we report a preparation of Co/SiO2 core/shell nanospheres by modified Stöber method, using 3-aminopropyl-trimethoxysilane (APS) and tetraethyl orthosilicate (TEOS) as surface primer of cobalt NPs and silica precursor, respectively. Furthermore, the fabrication of hollow structures, such as cobalt nanocages [10], Au–ZnO, Pt–ZnO hollow particles [11], rattle-type Au@mSiO2, Fe2O3@mSiO2 hollow capsules [12,13], and nickel silicate hollow spheres [14], via sacrificial chemical etching
⁎ Corresponding authors. Tel.: +86 15888928162; fax: +86 579 82282269. E-mail addresses: [email protected] (R. Qiao), [email protected] (Y.S. Kang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.004
has attracted great interest because of its facile synthesis process. In this paper, we also prepared SiO2 hollow and magnetic semihollow nanospheres by a selective acid etching strategy. Because magnetic NPs combined with semiconductor nanocrystals would lead to a special functionalized magnetic luminescent composite, we also used Co/SiO2 nanospheres as a substrate for the deposition of CdS nanocrystals to prepare Co/SiO2/CdS nanocomposites. 2. Experimental 2.1. Preparation of Co/SiO2 nanospheres 0.5 mL of 0.04 M CoCl2 solution was added to 50 mL of aqueous solution including 4.4 mM NaBH4 and 8 × 10 − 6 M citric acid with nitrogen bubbling under stirring. It was followed by rapid addition of 200 mL of ethanol containing APS (4 μL) and TEOS. After 2 h, the precipitates were collected and redispersed in ethanol. 2.2. Preparation of Co/SiO2/CdS nanocomposites 40 mL of water solution including CdCl2 (1 mmol), thioglycolic acid (1.5 mmol, TGA) and Na2S (1 mmol) was transferred into a stainless steel autoclave with a Teflon liner of 50 mL capacity, and heated at 180 °C for 4 h. After the autoclave was air-cooled to room temperature, the resulting TGA-capped CdS precipitates were collected and dried at 60 °C. The obtained Co/SiO2 nanospheres were dispersed in 30 mL of ethanol including 50 μL APS, stirring at 60 °C for 12 h. The composites were collected and washed to remove the remained APS on SiO2 surface, and then redispersed in ethanol followed by addition of CdS NPs
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Fig. 1. TEM images of (a) cobalt NPs (Inset: SAED pattern), and (b) Co/SiO2 nanospheres (Sample 1). (c) FT-IR spectrum and (d) HRTEM image of Sample 1.
under stirring for 2 h. Finally, the Co/SiO2/CdS products were separated by a magnet.
Emmett–Teller (BET) surface area of SiO2 hollow spheres was calculated from adsorption–desorption isotherm of nitrogen gas. 3. Results and discussion
2.3. Characterization The products were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). Magnetic measurements were carried out using a Quantum Design superconducting quantum interference device (SQUID) magnetometer at 300 K. Brunauer–
Uniform cobalt/SiO2 nanospheres were prepared using amorphous cobalt NPs (main particle size distribution ~ 40 nm, Fig. 1a). Citric acid presented in solution is used as a stabilizer via the chemisorption of carboxylic acid groups onto the basic particle surfaces to prevent further growth of metallic Co colloids through double-layer repulsion [15]. The absorption peak at 1405.7 cm− 1 in FT-IR spectrum of Co/SiO2
Fig. 2. (a) XRD patterns of Sample 1 (i) and its annealed products at 400 °C (ii) and 600 °C (iii). (b) Hysteresis loops recorded at 300 K of Sample 1(i), its annealed products at 400 °C (ii) and 600 °C (iii), and Sample 4 (iv).
M. Xu et al. / Materials Letters 66 (2012) 285–288
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Fig. 3. TEM images of (a) composite semi-hollow spheres and (b) silica hollow spheres. (c) N2 adsorption–desorption isothermal curve of SiO2 hollow spheres. The inset shows the corresponding pore size distribution.
nanospheres (Fig. 1c) can be assigned to symmetric stretching vibration of carboxylate groups in citric acid. When using 42.5 μL of TEOS as silica precursor, Co/SiO2 nanospheres with a shell thickness of ~21.7 nm (Sample 1, Fig. 1b) were obtained. HRTEM image in Fig. 1d shows that cobalt core particles were composed of smaller clusters (indicated by white arrows). The XRD pattern (Fig. 2a i) indicates that the sample is amorphous, and its saturation magnetization (Ms) was only 1.2 emu g− 1 (Fig. 2b i). High-temperature annealing of the particles drove the solid-state crystallization of the cores. A peak attributed to metallic cobalt was observed at 44.0° after annealing at 400 °C although the crystallinity was not very good (Fig. 2a ii), and Ms increased to 5.2 emu g− 1 (Fig. 2b ii). Increasing annealing temperature to 600 °C, the characteristic peaks (111), (200), (220) of metallic cobalt were found at 44.0°, 51.3°, and 75.7° (Fig. 2a iii). Though the crystallinity of core materials was improved, the relative Ms had no much increase (5.5 emu g− 1, Fig. 2b iii). So it is confirmed that the silica shell is the dominant factor on affecting magnetic properties of these core/shell nanostructures after the metal materials grow from amorphous to crystal structure. Compared to conventional methods [16,17], SiO2 hollow spheres could easily be derived by etching away cobalt cores from these
core/shell nanospheres. With mild etching in 0.5 wt.% HCl solution for 5 min, it was possible to have partial removal of the cobalt core, producing Co/SiO2 semi-hollow nanospheres (Fig. 3a). Increasing HCl concentration to 1 wt.%, SiO2 hollow spheres were obtained after 5 min of etching (Fig. 3b). The porous structure of the hollow spheres was revealed by the N2 adsorption–desorption isothermal curve (Fig. 3c). The curve shows a hysteresis loop that belongs to type H3. And the BET surface area of the product is 64.5 m 2·g − 1, the pore size distribution mainly ranges in 1.8–3.7 nm (Inset of Fig. 3c). As described above, APS is used as an indispensable surface primer because cobalt surface does not form a passivating oxide film and has very little electrostatic affinity for silica. During this process, APS monolayer is allowed to adsorb onto the cobalt surface, with the silanol groups pointing into solution. It was verified by FT-IR analysis as shown by APS characteristic peaks at 1626.9 cm − 1 and 784.3 cm − 1 (Fig. 1c), corresponding to N–H bending. Hydrolysis of the surfacebonded siloxane moieties to form silane triols occurs within minutes. It is followed by condensation between APS and TEOS to form a relatively homogeneous silica layer around the particles. If APS was absent in the experiment and other terms were same as Sample 1, as
Fig. 4. TEM images of Co/Silica nanocomposites (a) Sample 2, (b) Sample 3, (c) Sample 4, and (d) Sample 5. (e) XRD pattern of CdS NPs (Inset: TEM image of CdS NPs). (f) HRTEM image of Co/SiO2/CdS nanocomposite.
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indicated in Fig. 4a, most of produced silica did not deposit onto cobalt surface (Sample 2). The amount of TEOS is a critical factor for determining the silica shell thickness. By reducing TEOS content from 42.5 μL to 20 μL (Sample 3), and then to 7.5 μL (Sample 4) while other terms being same as Sample 1, SiO2 shell thickness can be decreased to 8.6 nm and 4.5 nm, respectively (Fig. 4b and c). Comparing with Sample 1, the Ms value of Sample 4 increases to 8.5 emu g − 1 (Fig. 2b iv), which means that the nonmagnetic silica shell thickness is the dominant factor to determine magnetic properties of these Co/silica nanospheres, maintaining the magnetic cores with a constant particle size. When raising CoCl2 content from 0.5 mL to 1.0 mL while other terms being same as Sample 3, average particle size of the cobalt cores increases to ~ 110 nm, which is caused by further coalescence of cobalt clusters with a higher concentration. In the meantime the silica shell thickness can also decrease to 5.4 nm (Sample 5, Fig. 4d). By using Sample 5 as a substrate, Co/SiO2/CdS nanocomposites can be achieved. As shown in Fig. 4e, CdS nanocrystals exhibit a hexagonal phase (JCPDS card no. 41-1049). During the deposition of CdS onto silica surface, the electrostatic repulsion cannot be ignored because both silica surface and TGA-capped CdS carry negative charges. As a result, CdS NPs cannot be deposited on the SiO2 surface. Following this mechanism, APS was used to modify SiO2 surface because the condensation between siloxane moieties of APS and Si–OH groups of SiO2 results in the decrease of the surface charge density. And the hydrogen bonds between –NH2 group and Si–OH group also have the same effect. Therefore, CdS nanocrystals can be deposited onto SiO2 surface, though the deposition is not evenly (Fig. 4f). 4. Conclusions Co/SiO2 core/shell nanospheres have been synthesized with wellcontrolled shell thickness. The use of APS primer is critical for deposition of silica onto cobalt particles to form homogeneous shells. Both
TEOS and CoCl2 contents are dominant to determine the final silica shell thickness. These nanospheres were successfully used as templates for preparation of SiO2 semi-hollow and hollow spheres. And they could also be used as a substrate for fabrication of magnetic luminescent nanomaterials. Acknowledgments This work was supported by the Natural Science Foundation of Zhejiang Province (Grant No. Y4090636), Sci-Tech Research Project of Jinhua (Grant No. 2010-1-069) and ZJNU Students' Project of Research Study and Creative Experiments. References [1] Fu A, Hu W, Xu L, Wilson RJ, Yu H, Osterfeld SJ, et al. Angew Chem Int Ed 2009;48: 1620. [2] Herdt AR, Kim BS, Taton TA. Bioconjugate Chem 2007;18:183. [3] Jun Y, Seo J, Cheon J. Acc Chem Res 2008;41:179. [4] Li D, Teoh WY, Selomulya C, Woodward RC, Amal R, Rosche B. Chem Mater 2006;18:6403. [5] Vestal CR, Zhang ZJ. Nano Lett 2003;3:1739. [6] Deng YH, Wang CC, Hu JH, Yang WL, Fu SK. Colloid Surface A 2005;262:87. [7] Yi DK, Lee SS, Papaefthymiou GC, Ying JY. Chem Mater 2006;18:614. [8] Ung T, Marzán LML, Mulvaney P. Langmuir 1998;14:3740. [9] Umegaki T, Yan JM, Zhang XB, Shioyama H, Kuriyama N, Xu Q. J Power Sources 2010;195:8209. [10] Wang X, Fu H, Peng A, Zhai T, Ma Y, Yuan F, et al. Adv Mater 2009;21:1636. [11] Zeng H, Cai W, Liu P, Xu X, Zhou H, Klingshim C, et al. ACS Nano 2008;2:1661. [12] Wang TT, Chai F, Wang CG, Li L, Liu HY, Zhang LY, et al. J Colloid Interf Sci 2011;358:109. [13] Chen Y, Chen H, Zeng D, Tian Y, Chen F, Feng J, et al. ACS Nano 2010;4:6001. [14] Wang Y, Tang C, Deng Q, Liang C, Ng DHL, Kwong F, et al. Langmuir 2010;26: 14830. [15] Kobayashi Y, Horie M, Konno M, González BR, Marzán LML. J Phys Chem B 2003;107:7420. [16] Lu Y, McLellan J, Xia Y. Langmuir 2004;20:3464. [17] Fujiwara M, Shiokawa K, Tanaka Y, Nakahara Y. Chem Mater 2004;16:5420.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation and application of magnetic cobalt/SiO2 core/shell nanospheres Mengshi Xu a, Ru Qiao a,⁎, Xiao Li Zhang c, Young Soo Kang b,⁎, Liang-Chao Li a a b c
College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China Department of Chemistry, Sogang University, Seoul 121-742, South Korea Department of Materials Engineering, Monash University, Clayton VIC, Australia
a r t i c l e
i n f o
Article history: Received 19 June 2011 Accepted 1 September 2011 Available online 7 September 2011 Keywords: Co/SiO2 nanospheres Magnetic materials Hollow spheres CdS Nanocomposites
a b s t r a c t Homogeneous SiO2-coated cobalt nanospheres with tunable silica shell thickness from 21.7 nm to 4.5 nm were synthesized by using modified Stöber method. These nanocomposites were used as source materials to prepare SiO2 semi-hollow and hollow nanospheres by partially and completely etching cobalt cores, respectively. A proposed formation mechanism of these Co/SiO2 nanospheres with a core/shell structure was presented in this paper, which is also important for the rational design and synthesis of other monodisperse core/shell nanoarchitectures with uniform size and shape. Furthermore, these Co/SiO2 nanospheres were also used as a substrate for the deposition of CdS nanocrystals to prepare magnetic luminescent Co/SiO2/CdS nanocomposites. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanoparticles (NPs) used in medicine bioengineering field [1–3] commonly need to be embedded in a nonmagnetic matrix to avoid aggregation and sedimentation and endow them with particular surface properties. Among various processing methods, coating NPs with silica is becoming a promising approach [4] because the inert silica shell could screen the magnetic dipolar attraction between magnetic NPs and provide these particles with a chemical hydrophilicity [5,6]. The existence of silanol groups also makes the exterior surface easy to link various functional groups. Differing with coating on metal oxides [5,7], silica deposition on pure metal particles is more complicated because of the lack of hydroxyl groups on metal surface. So it is necessary to use a primer to make the surface vitreophilic [8]. Moreover, how to control the silica shell thickness is also a problem that must be resolved because excessive amorphous silica can deleteriously affect magnetic properties of the products. Xu and co-workers once synthesized Co–SiO2 spheres by reversed micelle techniques [9]. Herein, we report a preparation of Co/SiO2 core/shell nanospheres by modified Stöber method, using 3-aminopropyl-trimethoxysilane (APS) and tetraethyl orthosilicate (TEOS) as surface primer of cobalt NPs and silica precursor, respectively. Furthermore, the fabrication of hollow structures, such as cobalt nanocages [10], Au–ZnO, Pt–ZnO hollow particles [11], rattle-type Au@mSiO2, Fe2O3@mSiO2 hollow capsules [12,13], and nickel silicate hollow spheres [14], via sacrificial chemical etching
⁎ Corresponding authors. Tel.: +86 15888928162; fax: +86 579 82282269. E-mail addresses: [email protected] (R. Qiao), [email protected] (Y.S. Kang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.004
has attracted great interest because of its facile synthesis process. In this paper, we also prepared SiO2 hollow and magnetic semihollow nanospheres by a selective acid etching strategy. Because magnetic NPs combined with semiconductor nanocrystals would lead to a special functionalized magnetic luminescent composite, we also used Co/SiO2 nanospheres as a substrate for the deposition of CdS nanocrystals to prepare Co/SiO2/CdS nanocomposites. 2. Experimental 2.1. Preparation of Co/SiO2 nanospheres 0.5 mL of 0.04 M CoCl2 solution was added to 50 mL of aqueous solution including 4.4 mM NaBH4 and 8 × 10 − 6 M citric acid with nitrogen bubbling under stirring. It was followed by rapid addition of 200 mL of ethanol containing APS (4 μL) and TEOS. After 2 h, the precipitates were collected and redispersed in ethanol. 2.2. Preparation of Co/SiO2/CdS nanocomposites 40 mL of water solution including CdCl2 (1 mmol), thioglycolic acid (1.5 mmol, TGA) and Na2S (1 mmol) was transferred into a stainless steel autoclave with a Teflon liner of 50 mL capacity, and heated at 180 °C for 4 h. After the autoclave was air-cooled to room temperature, the resulting TGA-capped CdS precipitates were collected and dried at 60 °C. The obtained Co/SiO2 nanospheres were dispersed in 30 mL of ethanol including 50 μL APS, stirring at 60 °C for 12 h. The composites were collected and washed to remove the remained APS on SiO2 surface, and then redispersed in ethanol followed by addition of CdS NPs
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Fig. 1. TEM images of (a) cobalt NPs (Inset: SAED pattern), and (b) Co/SiO2 nanospheres (Sample 1). (c) FT-IR spectrum and (d) HRTEM image of Sample 1.
under stirring for 2 h. Finally, the Co/SiO2/CdS products were separated by a magnet.
Emmett–Teller (BET) surface area of SiO2 hollow spheres was calculated from adsorption–desorption isotherm of nitrogen gas. 3. Results and discussion
2.3. Characterization The products were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). Magnetic measurements were carried out using a Quantum Design superconducting quantum interference device (SQUID) magnetometer at 300 K. Brunauer–
Uniform cobalt/SiO2 nanospheres were prepared using amorphous cobalt NPs (main particle size distribution ~ 40 nm, Fig. 1a). Citric acid presented in solution is used as a stabilizer via the chemisorption of carboxylic acid groups onto the basic particle surfaces to prevent further growth of metallic Co colloids through double-layer repulsion [15]. The absorption peak at 1405.7 cm− 1 in FT-IR spectrum of Co/SiO2
Fig. 2. (a) XRD patterns of Sample 1 (i) and its annealed products at 400 °C (ii) and 600 °C (iii). (b) Hysteresis loops recorded at 300 K of Sample 1(i), its annealed products at 400 °C (ii) and 600 °C (iii), and Sample 4 (iv).
M. Xu et al. / Materials Letters 66 (2012) 285–288
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Fig. 3. TEM images of (a) composite semi-hollow spheres and (b) silica hollow spheres. (c) N2 adsorption–desorption isothermal curve of SiO2 hollow spheres. The inset shows the corresponding pore size distribution.
nanospheres (Fig. 1c) can be assigned to symmetric stretching vibration of carboxylate groups in citric acid. When using 42.5 μL of TEOS as silica precursor, Co/SiO2 nanospheres with a shell thickness of ~21.7 nm (Sample 1, Fig. 1b) were obtained. HRTEM image in Fig. 1d shows that cobalt core particles were composed of smaller clusters (indicated by white arrows). The XRD pattern (Fig. 2a i) indicates that the sample is amorphous, and its saturation magnetization (Ms) was only 1.2 emu g− 1 (Fig. 2b i). High-temperature annealing of the particles drove the solid-state crystallization of the cores. A peak attributed to metallic cobalt was observed at 44.0° after annealing at 400 °C although the crystallinity was not very good (Fig. 2a ii), and Ms increased to 5.2 emu g− 1 (Fig. 2b ii). Increasing annealing temperature to 600 °C, the characteristic peaks (111), (200), (220) of metallic cobalt were found at 44.0°, 51.3°, and 75.7° (Fig. 2a iii). Though the crystallinity of core materials was improved, the relative Ms had no much increase (5.5 emu g− 1, Fig. 2b iii). So it is confirmed that the silica shell is the dominant factor on affecting magnetic properties of these core/shell nanostructures after the metal materials grow from amorphous to crystal structure. Compared to conventional methods [16,17], SiO2 hollow spheres could easily be derived by etching away cobalt cores from these
core/shell nanospheres. With mild etching in 0.5 wt.% HCl solution for 5 min, it was possible to have partial removal of the cobalt core, producing Co/SiO2 semi-hollow nanospheres (Fig. 3a). Increasing HCl concentration to 1 wt.%, SiO2 hollow spheres were obtained after 5 min of etching (Fig. 3b). The porous structure of the hollow spheres was revealed by the N2 adsorption–desorption isothermal curve (Fig. 3c). The curve shows a hysteresis loop that belongs to type H3. And the BET surface area of the product is 64.5 m 2·g − 1, the pore size distribution mainly ranges in 1.8–3.7 nm (Inset of Fig. 3c). As described above, APS is used as an indispensable surface primer because cobalt surface does not form a passivating oxide film and has very little electrostatic affinity for silica. During this process, APS monolayer is allowed to adsorb onto the cobalt surface, with the silanol groups pointing into solution. It was verified by FT-IR analysis as shown by APS characteristic peaks at 1626.9 cm − 1 and 784.3 cm − 1 (Fig. 1c), corresponding to N–H bending. Hydrolysis of the surfacebonded siloxane moieties to form silane triols occurs within minutes. It is followed by condensation between APS and TEOS to form a relatively homogeneous silica layer around the particles. If APS was absent in the experiment and other terms were same as Sample 1, as
Fig. 4. TEM images of Co/Silica nanocomposites (a) Sample 2, (b) Sample 3, (c) Sample 4, and (d) Sample 5. (e) XRD pattern of CdS NPs (Inset: TEM image of CdS NPs). (f) HRTEM image of Co/SiO2/CdS nanocomposite.
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indicated in Fig. 4a, most of produced silica did not deposit onto cobalt surface (Sample 2). The amount of TEOS is a critical factor for determining the silica shell thickness. By reducing TEOS content from 42.5 μL to 20 μL (Sample 3), and then to 7.5 μL (Sample 4) while other terms being same as Sample 1, SiO2 shell thickness can be decreased to 8.6 nm and 4.5 nm, respectively (Fig. 4b and c). Comparing with Sample 1, the Ms value of Sample 4 increases to 8.5 emu g − 1 (Fig. 2b iv), which means that the nonmagnetic silica shell thickness is the dominant factor to determine magnetic properties of these Co/silica nanospheres, maintaining the magnetic cores with a constant particle size. When raising CoCl2 content from 0.5 mL to 1.0 mL while other terms being same as Sample 3, average particle size of the cobalt cores increases to ~ 110 nm, which is caused by further coalescence of cobalt clusters with a higher concentration. In the meantime the silica shell thickness can also decrease to 5.4 nm (Sample 5, Fig. 4d). By using Sample 5 as a substrate, Co/SiO2/CdS nanocomposites can be achieved. As shown in Fig. 4e, CdS nanocrystals exhibit a hexagonal phase (JCPDS card no. 41-1049). During the deposition of CdS onto silica surface, the electrostatic repulsion cannot be ignored because both silica surface and TGA-capped CdS carry negative charges. As a result, CdS NPs cannot be deposited on the SiO2 surface. Following this mechanism, APS was used to modify SiO2 surface because the condensation between siloxane moieties of APS and Si–OH groups of SiO2 results in the decrease of the surface charge density. And the hydrogen bonds between –NH2 group and Si–OH group also have the same effect. Therefore, CdS nanocrystals can be deposited onto SiO2 surface, though the deposition is not evenly (Fig. 4f). 4. Conclusions Co/SiO2 core/shell nanospheres have been synthesized with wellcontrolled shell thickness. The use of APS primer is critical for deposition of silica onto cobalt particles to form homogeneous shells. Both
TEOS and CoCl2 contents are dominant to determine the final silica shell thickness. These nanospheres were successfully used as templates for preparation of SiO2 semi-hollow and hollow spheres. And they could also be used as a substrate for fabrication of magnetic luminescent nanomaterials. Acknowledgments This work was supported by the Natural Science Foundation of Zhejiang Province (Grant No. Y4090636), Sci-Tech Research Project of Jinhua (Grant No. 2010-1-069) and ZJNU Students' Project of Research Study and Creative Experiments. References [1] Fu A, Hu W, Xu L, Wilson RJ, Yu H, Osterfeld SJ, et al. Angew Chem Int Ed 2009;48: 1620. [2] Herdt AR, Kim BS, Taton TA. Bioconjugate Chem 2007;18:183. [3] Jun Y, Seo J, Cheon J. Acc Chem Res 2008;41:179. [4] Li D, Teoh WY, Selomulya C, Woodward RC, Amal R, Rosche B. Chem Mater 2006;18:6403. [5] Vestal CR, Zhang ZJ. Nano Lett 2003;3:1739. [6] Deng YH, Wang CC, Hu JH, Yang WL, Fu SK. Colloid Surface A 2005;262:87. [7] Yi DK, Lee SS, Papaefthymiou GC, Ying JY. Chem Mater 2006;18:614. [8] Ung T, Marzán LML, Mulvaney P. Langmuir 1998;14:3740. [9] Umegaki T, Yan JM, Zhang XB, Shioyama H, Kuriyama N, Xu Q. J Power Sources 2010;195:8209. [10] Wang X, Fu H, Peng A, Zhai T, Ma Y, Yuan F, et al. Adv Mater 2009;21:1636. [11] Zeng H, Cai W, Liu P, Xu X, Zhou H, Klingshim C, et al. ACS Nano 2008;2:1661. [12] Wang TT, Chai F, Wang CG, Li L, Liu HY, Zhang LY, et al. J Colloid Interf Sci 2011;358:109. [13] Chen Y, Chen H, Zeng D, Tian Y, Chen F, Feng J, et al. ACS Nano 2010;4:6001. [14] Wang Y, Tang C, Deng Q, Liang C, Ng DHL, Kwong F, et al. Langmuir 2010;26: 14830. [15] Kobayashi Y, Horie M, Konno M, González BR, Marzán LML. J Phys Chem B 2003;107:7420. [16] Lu Y, McLellan J, Xia Y. Langmuir 2004;20:3464. [17] Fujiwara M, Shiokawa K, Tanaka Y, Nakahara Y. Chem Mater 2004;16:5420.