Journal of Colloid and Interface Science 416 (2014) 147–150
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Short Communication
A facile template synthesis of asymmetric gold silica heteronanoparticles Ryo Miyanohata, Taro Matsushita, Takaaki Tsuruoka, Hidemi Nawafune, Kensuke Akamatsu ⇑ Frontiers of Innovative Research in Science and Technology (FIRST), Konan University, 7-1-20 Minatojima-minamimachi, Chuo-ku, Kobe 650-0047, Japan
a r t i c l e
i n f o
Article history: Received 9 July 2013 Accepted 22 October 2013 Available online 8 November 2013 Keywords: Asymmetric nanoparticles Gold nanoparticles Heterostructure
a b s t r a c t Silica hemispheres containing gold nanoparticle cores have been synthesized via immobilization of gold nanoparticles on a substrate and site-selective growth of silica followed by removal of the hemispherical particles. The structure of these asymmetric heteronanoparticles allows selective etching or overgrowth of the core gold seeds, which results in the respective formation of hemispherical capsules or gold homodimers. Ó 2013 Elsevier Inc. All rights reserved.
Asymmetric nanoparticles consisting of metal nanoparticles connected with different constituents, such as polymers, organic molecules, and other inorganic materials, have become a subject of active research in recent years, because they exhibit unique optical properties and catalytic activity due to a synergetic effect between each component in the heterostructure [1]. These nanoparticles are also used to promote directed assembly into low-dimensional nanostructures for the realization of nanoscale optical, electrical and sensing materials. One of the most effective ways to achieve the desired nano-assemblies is mono-functionalization of the nanoparticles with protective molecules, by which several types of gold nanoparticle nano-assemblies could be synthesized, such as dimers [2–7], satellites [8,9], and chains [10]. Such low-dimensional gold nanoparticle structures have been reported to exhibit plasmon hybridization due to coupled dipole moments in individual nanoparticles [11], and to be effective catalysts for the guided growth of inorganic nanowire assemblies [12]. Since these properties depend on an interparticle separation as well as nanoparticle size, a facile and effective ways to control these nanostructures need to be developed. To date, several approaches toward the synthesis of mono-functionalized nanoparticles have been developed, including immobilization of nanoparticles onto planar substrates [3–5,13], polymerization of surface-bound protective molecules on nanoparticle surfaces [10], and synthesis of Janus-type nanoparticles with different protective molecules [14– 17]. Partial surface coating of nanoparticles with other inorganic or organic materials also yields heteronanoparticles with asymmetrically exposed surfaces, which are then available for partial ⇑ Corresponding author. Fax: +81 78 303 1495. E-mail address: [email protected] (K. Akamatsu). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.055
functionalization. The heterodimer is one of the successful examples of such heterostructures, which include metal–metal [18,19], metal–polymer [20,21], metal–semiconductor [22–24], semiconductor–semiconductor [25,26], and metal–oxide heterodimers [27–29]. The surface coverage of the primary core of these nanoparticles with secondary materials is important for the construction of low-dimensional nanostructures. Many synthetic methods based on solution processes have been reported that yield such heterostructures, where almost half of the core nanoparticle surface is covered with different materials (these structures are thus referred to as ‘‘heterodimers’’). However, the synthesis of quasi-core/shell heteronanoparticles where most of core nanoparticle surface is covered, but only a small portion of the surface is exposed, remains challenging [21]. These new types of structures are expected to be used as effective building blocks for low-dimensional nanostructures because directional assembly of the heteronanostructures is much more effectively possible due to the local functionality of the core nanoparticle surface. Here, we report a facile approach (Fig. 1) for the synthesis of asymmetric gold/silica heteronanoparticles through the immobilization of gold nanoparticles on silver-coated glass substrates followed by site-selective growth of silica shells. Desorption of the heteronanoparticles from the substrate yields silica hemispheres containing gold nanoparticle cores with partially exposed surfaces. We also demonstrate the synthesis of hollow silica hemispheres and gold dimers from these heteronanoparticles by the selective etching of the gold nanoparticle cores and by the catalytic growth of additional gold nanoparticles, respectively. Gold nanoparticles (ca. 20 nm) are immobilized on thiolterminated silver thin films deposited onto a glass substrate by immersion of the substrates into an aqueous solution of citratestabilized gold nanoparticles. The glass substrate with the gold
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Fig. 1. Schematic of the synthesis of silica hemispheres containing gold nanoparticles. (A) TEM image of citrate-stabilized 20 nm gold nanoparticles. Scale bar: 50 nm. (B) SEM image of gold nanoparticles immobilized on 1,10-dodecanedithol-functionalized silver thin film on a glass substrate. The sample was obtained after immersion of the substrate into a solution of gold nanoparticles for 10 min. Scale bar: 200 nm. (C) Cross-sectional TEM image of gold nanoparticles coated with silica shells after hydrolysis reaction with TEOS for 15 min. Scale bar: 50 nm.
nanoparticle monolayer is then immersed into an ethanol solution of aminopropyltrimethoxysilane (APTMS) for surface modification of the immobilized gold nanoparticles. The nanoparticles on the substrate are then subjected to the selective growth of silica shells by ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS). Subsequent ultrasonication of the substrates in ethanol yields heteronanoparticles dispersed into solution. The formation of gold nanoparticle monolayers on the substrate was confirmed using scanning electron microscopy (SEM), and it was found that the amount of immobilized gold nanoparticles increased almost proportionally with the reaction duration up to 15 min (Fig. S1). For example, the substrate with an average coverage of gold nanoparticles at 135 ± 20 particle/lm2 was achieved after reaction for 10 min (Fig. 1B). The gold nanoparticles were then subjected to silica coating because the average
interparticle distance between adjacent gold nanoparticles (65 nm, calculated by assuming simple square arrangement for 20 nm gold nanoparticles) was appropriate for the growth of silica shells. The evolution of silica shell growth on the gold nanoparticles was monitored by taking samples at 10, 15, 20 min after immersion of the substrate with immobilized gold nanoparticles. An SEM image of the sample during the early growth stage (10 min) reveals the growth of silica on the immobilized gold nanoparticles (Fig. 2A), which is promoted by modification of the gold nanoparticle surface with APTMS. A cross-sectional TEM image of the sample directly reveals the selective growth of shells only on the exposed surface of the immobilized gold nanoparticles (Fig. 1C). As the reaction proceeds, the thickness of the silica shells increases, and some interconnected structures are observed (Fig. 2B and C).
Fig. 2. (A)–(C) SEM images of immobilized gold nanoparticles obtained after silica coating for 10, 15, and 20 min, respectively. (D)–(F) Corresponding TEM images of the gold nanoparticles covered with silica hemispheres shown in (A)–(C) after desorption by ultrasonication and purification using density-gradient centrifugation. Insets in (D)–(F): schematics of the gold nanoparticles coated with different thickness silica hemispheres.
R. Miyanohata et al. / Journal of Colloid and Interface Science 416 (2014) 147–150
149
Fig. 3. Photographs of ethanol solutions containing gold silica core/shell nanoparticles (A) and silica hemispheres containing gold nanoparticles (B) before (top) and after (bottom) addition of aqua regia. Final concentration of aqua regia is 5%. Corresponding TEM images for samples obtained after addition of aqua regia are also shown. Insets in TEM images: schematics of the corresponding structures.
Ultrasonication of the substrate in ethanol resulted in the desorption of immobilized particles from the substrate and the hetronanoparticles were obtained as a dispersion in ethanol. A TEM image of the sample obtained 10 min after silica shell growth indicates successful dispersion of isolated gold/silica heteronanostructures with a silica shell thickness of ca. 8 nm (Fig. 2D). Hemispheres containing gold nanoparticles were also evident. Tilting experiments during TEM observation confirmed that almost all of the nanoparticles observed in the images were isolated hemispheres containing single gold nanoparticles. The unwanted interconnected structures and particles containing multiple gold nanoparticles could be removed by density gradient centrifugation using a high molecular weight poly(ethylene glycol)/ethanol solution (see Supplementary Information). Although immobilized gold nanoparticles with no silica shells were not desorbed from the substrate by ultrasonication due to strong binding between the gold nanoparticles and the silver film on the substrate through dithiol molecules, the deposition of silica shells on the gold nanoparticles enabled desorption of the heteronanostructures. This is because the desorption efficiency under ultrasonication is increased as the total size of the particles (and thus total weight) increases with silica shell growth, as observed for different-sized gold nanoparticles immobilized on a glass substrate [30]. The formation of gold/silica heteronanoparticles with different thickness silica shells was investigated. The reaction time for silica shell growth was varied, so that heteronanoparticles with mean diameters of ca. 50 and 90 nm (corresponding to silica shell thicknesses of 15 and 35 nm, respectively) were also extracted (Fig. 2E and F). The TEM images of the products confirm the presence of hemispherical features, regardless of the core particle size. Therefore, the present process enabled the preparation of different sized silica hemispheres containing same-sized gold nanoparticles as well as same-sized hemispheres containing different-sized gold nanoparticles as probed by using larger (50 nm) gold nanoparticles (Fig. S2). UV–vis spectroscopy was also used to characterize the heterostructures. Gold/silica core shell nanoparticles have a typical surface plasmon resonance (SPR) that is red-shifted from that of the corresponding core gold nanoparticles [31]. In the present case, 50 and 90 nm silica hemispheres with 20 and 50 nm gold nanoparticle cores have SPR maxima at 530 and 545 nm, which are 10 and 25 nm red-shifted, respectively, from those of the gold nanoparticles without silica coating (Fig. S3). Partial exposure of the gold nanoparticle surface was confirmed by two independent experiments. Firstly, the dissolution of the gold nanoparticles from the heteronanoparticles using aqua regia was compared with that of independently-prepared gold nanoparticles that were completely surrounded with silica shells. When aqua
regia was added to the gold/silica core/shell nanoparticles, the color of the solution remained unchanged, as a consequence of core stabilization though completely surrounding silica shells (Fig. 3A). On the other hand, for the silica hemispheres containing gold nanoparticles, the color instantly changed from red to colorless, which indicated the rapid dissolution of the gold nanoparticle cores (Fig. 3B). Dissolution was evident using TEM; after the core gold nanoparticles were oxidatively dissolved, hollow silica hemispheres with open holes could be observed. The second experiment involved self-catalyzed electroless gold deposition through exposed core nanoparticle surfaces in the heteronanoparticles. When Au precursor (HAuCl4, 10 mM) and aminoethanethiol (200 mM) as a mild reducing agent were added into the ethanol solution containing heteronanoparticles (36 nm silica hemispheres containing 20 nm gold cores) at 60 °C, the color of the solution gradually changed from red to purple. Time dependent UV–vis spectra show a gradual increase in intensity of the plasmon resonance at longer wavelength (Fig. 4A). These changes can be attributed to the overgrowth of gold from the exposed core gold surface in the heteronanoparticles, forming interconnected gold homodimers as evident in the TEM images (Fig. 4B and C). In the extinction spectra, transverse and longitudinal bands for plasmon resonance modes are not clear, presumably due to very low aspect ratio of the present interconnected gold homodimers [32,33]. The spectra are rather similar in form with polymer stabilized gold nanoparticle dimers [34]. After electroless gold deposition for 15 min, gold homodimers were obtained, where the gold nanoparticles that complete formation of the dimer were almost the same size as the original gold core; half of the dimer is surrounded by the silica shell. Typically, 85% yield was achieved for formation of the gold homodimers when the purified gold/silica heteronanoparticles prepared by density gradient centrifugation were used as initial catalytic cores. These observations led us to conclude that the present heteronanostructures consist of silica hemispheres with gold nanoparticle cores and small open holes in the center of their flat plane sites. In conclusion, we have demonstrated a facile method for the synthesis of asymmetric heteronanostructures that consist of silica hemispheres containing gold nanoparticles through a simple surface immobilization approach. The obtained heteronanoparticles have a small open site on their surface, thereby allowing rapid dissolution of core gold nanoparticles by oxidative etching to form hemispherical silica capsules and site selective growth of gold nanoparticles to form gold homodimers. We envisage that this general strategy will be applied to synthesize nanoparticle-based building blocks to construct a variety of complex and functional nanostructures.
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Fig. 4. (A) Optical extinction spectra of silica hemispheres containing gold nanoparticles obtained before and after electroless deposition of gold. Inset: photographs of the solution. (B) and (C) TEM images of gold homodimers obtained after electroless deposition of gold onto the heteronanoparticles for (B) 10 and (C) 15 min using HAuCl4 and 2aminoethanethiol at 60 °C. Inset: schematics of gold homodimers with silica hemispheres.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.055.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Short Communication
A facile template synthesis of asymmetric gold silica heteronanoparticles Ryo Miyanohata, Taro Matsushita, Takaaki Tsuruoka, Hidemi Nawafune, Kensuke Akamatsu ⇑ Frontiers of Innovative Research in Science and Technology (FIRST), Konan University, 7-1-20 Minatojima-minamimachi, Chuo-ku, Kobe 650-0047, Japan
a r t i c l e
i n f o
Article history: Received 9 July 2013 Accepted 22 October 2013 Available online 8 November 2013 Keywords: Asymmetric nanoparticles Gold nanoparticles Heterostructure
a b s t r a c t Silica hemispheres containing gold nanoparticle cores have been synthesized via immobilization of gold nanoparticles on a substrate and site-selective growth of silica followed by removal of the hemispherical particles. The structure of these asymmetric heteronanoparticles allows selective etching or overgrowth of the core gold seeds, which results in the respective formation of hemispherical capsules or gold homodimers. Ó 2013 Elsevier Inc. All rights reserved.
Asymmetric nanoparticles consisting of metal nanoparticles connected with different constituents, such as polymers, organic molecules, and other inorganic materials, have become a subject of active research in recent years, because they exhibit unique optical properties and catalytic activity due to a synergetic effect between each component in the heterostructure [1]. These nanoparticles are also used to promote directed assembly into low-dimensional nanostructures for the realization of nanoscale optical, electrical and sensing materials. One of the most effective ways to achieve the desired nano-assemblies is mono-functionalization of the nanoparticles with protective molecules, by which several types of gold nanoparticle nano-assemblies could be synthesized, such as dimers [2–7], satellites [8,9], and chains [10]. Such low-dimensional gold nanoparticle structures have been reported to exhibit plasmon hybridization due to coupled dipole moments in individual nanoparticles [11], and to be effective catalysts for the guided growth of inorganic nanowire assemblies [12]. Since these properties depend on an interparticle separation as well as nanoparticle size, a facile and effective ways to control these nanostructures need to be developed. To date, several approaches toward the synthesis of mono-functionalized nanoparticles have been developed, including immobilization of nanoparticles onto planar substrates [3–5,13], polymerization of surface-bound protective molecules on nanoparticle surfaces [10], and synthesis of Janus-type nanoparticles with different protective molecules [14– 17]. Partial surface coating of nanoparticles with other inorganic or organic materials also yields heteronanoparticles with asymmetrically exposed surfaces, which are then available for partial ⇑ Corresponding author. Fax: +81 78 303 1495. E-mail address: [email protected] (K. Akamatsu). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.055
functionalization. The heterodimer is one of the successful examples of such heterostructures, which include metal–metal [18,19], metal–polymer [20,21], metal–semiconductor [22–24], semiconductor–semiconductor [25,26], and metal–oxide heterodimers [27–29]. The surface coverage of the primary core of these nanoparticles with secondary materials is important for the construction of low-dimensional nanostructures. Many synthetic methods based on solution processes have been reported that yield such heterostructures, where almost half of the core nanoparticle surface is covered with different materials (these structures are thus referred to as ‘‘heterodimers’’). However, the synthesis of quasi-core/shell heteronanoparticles where most of core nanoparticle surface is covered, but only a small portion of the surface is exposed, remains challenging [21]. These new types of structures are expected to be used as effective building blocks for low-dimensional nanostructures because directional assembly of the heteronanostructures is much more effectively possible due to the local functionality of the core nanoparticle surface. Here, we report a facile approach (Fig. 1) for the synthesis of asymmetric gold/silica heteronanoparticles through the immobilization of gold nanoparticles on silver-coated glass substrates followed by site-selective growth of silica shells. Desorption of the heteronanoparticles from the substrate yields silica hemispheres containing gold nanoparticle cores with partially exposed surfaces. We also demonstrate the synthesis of hollow silica hemispheres and gold dimers from these heteronanoparticles by the selective etching of the gold nanoparticle cores and by the catalytic growth of additional gold nanoparticles, respectively. Gold nanoparticles (ca. 20 nm) are immobilized on thiolterminated silver thin films deposited onto a glass substrate by immersion of the substrates into an aqueous solution of citratestabilized gold nanoparticles. The glass substrate with the gold
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Fig. 1. Schematic of the synthesis of silica hemispheres containing gold nanoparticles. (A) TEM image of citrate-stabilized 20 nm gold nanoparticles. Scale bar: 50 nm. (B) SEM image of gold nanoparticles immobilized on 1,10-dodecanedithol-functionalized silver thin film on a glass substrate. The sample was obtained after immersion of the substrate into a solution of gold nanoparticles for 10 min. Scale bar: 200 nm. (C) Cross-sectional TEM image of gold nanoparticles coated with silica shells after hydrolysis reaction with TEOS for 15 min. Scale bar: 50 nm.
nanoparticle monolayer is then immersed into an ethanol solution of aminopropyltrimethoxysilane (APTMS) for surface modification of the immobilized gold nanoparticles. The nanoparticles on the substrate are then subjected to the selective growth of silica shells by ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS). Subsequent ultrasonication of the substrates in ethanol yields heteronanoparticles dispersed into solution. The formation of gold nanoparticle monolayers on the substrate was confirmed using scanning electron microscopy (SEM), and it was found that the amount of immobilized gold nanoparticles increased almost proportionally with the reaction duration up to 15 min (Fig. S1). For example, the substrate with an average coverage of gold nanoparticles at 135 ± 20 particle/lm2 was achieved after reaction for 10 min (Fig. 1B). The gold nanoparticles were then subjected to silica coating because the average
interparticle distance between adjacent gold nanoparticles (65 nm, calculated by assuming simple square arrangement for 20 nm gold nanoparticles) was appropriate for the growth of silica shells. The evolution of silica shell growth on the gold nanoparticles was monitored by taking samples at 10, 15, 20 min after immersion of the substrate with immobilized gold nanoparticles. An SEM image of the sample during the early growth stage (10 min) reveals the growth of silica on the immobilized gold nanoparticles (Fig. 2A), which is promoted by modification of the gold nanoparticle surface with APTMS. A cross-sectional TEM image of the sample directly reveals the selective growth of shells only on the exposed surface of the immobilized gold nanoparticles (Fig. 1C). As the reaction proceeds, the thickness of the silica shells increases, and some interconnected structures are observed (Fig. 2B and C).
Fig. 2. (A)–(C) SEM images of immobilized gold nanoparticles obtained after silica coating for 10, 15, and 20 min, respectively. (D)–(F) Corresponding TEM images of the gold nanoparticles covered with silica hemispheres shown in (A)–(C) after desorption by ultrasonication and purification using density-gradient centrifugation. Insets in (D)–(F): schematics of the gold nanoparticles coated with different thickness silica hemispheres.
R. Miyanohata et al. / Journal of Colloid and Interface Science 416 (2014) 147–150
149
Fig. 3. Photographs of ethanol solutions containing gold silica core/shell nanoparticles (A) and silica hemispheres containing gold nanoparticles (B) before (top) and after (bottom) addition of aqua regia. Final concentration of aqua regia is 5%. Corresponding TEM images for samples obtained after addition of aqua regia are also shown. Insets in TEM images: schematics of the corresponding structures.
Ultrasonication of the substrate in ethanol resulted in the desorption of immobilized particles from the substrate and the hetronanoparticles were obtained as a dispersion in ethanol. A TEM image of the sample obtained 10 min after silica shell growth indicates successful dispersion of isolated gold/silica heteronanostructures with a silica shell thickness of ca. 8 nm (Fig. 2D). Hemispheres containing gold nanoparticles were also evident. Tilting experiments during TEM observation confirmed that almost all of the nanoparticles observed in the images were isolated hemispheres containing single gold nanoparticles. The unwanted interconnected structures and particles containing multiple gold nanoparticles could be removed by density gradient centrifugation using a high molecular weight poly(ethylene glycol)/ethanol solution (see Supplementary Information). Although immobilized gold nanoparticles with no silica shells were not desorbed from the substrate by ultrasonication due to strong binding between the gold nanoparticles and the silver film on the substrate through dithiol molecules, the deposition of silica shells on the gold nanoparticles enabled desorption of the heteronanostructures. This is because the desorption efficiency under ultrasonication is increased as the total size of the particles (and thus total weight) increases with silica shell growth, as observed for different-sized gold nanoparticles immobilized on a glass substrate [30]. The formation of gold/silica heteronanoparticles with different thickness silica shells was investigated. The reaction time for silica shell growth was varied, so that heteronanoparticles with mean diameters of ca. 50 and 90 nm (corresponding to silica shell thicknesses of 15 and 35 nm, respectively) were also extracted (Fig. 2E and F). The TEM images of the products confirm the presence of hemispherical features, regardless of the core particle size. Therefore, the present process enabled the preparation of different sized silica hemispheres containing same-sized gold nanoparticles as well as same-sized hemispheres containing different-sized gold nanoparticles as probed by using larger (50 nm) gold nanoparticles (Fig. S2). UV–vis spectroscopy was also used to characterize the heterostructures. Gold/silica core shell nanoparticles have a typical surface plasmon resonance (SPR) that is red-shifted from that of the corresponding core gold nanoparticles [31]. In the present case, 50 and 90 nm silica hemispheres with 20 and 50 nm gold nanoparticle cores have SPR maxima at 530 and 545 nm, which are 10 and 25 nm red-shifted, respectively, from those of the gold nanoparticles without silica coating (Fig. S3). Partial exposure of the gold nanoparticle surface was confirmed by two independent experiments. Firstly, the dissolution of the gold nanoparticles from the heteronanoparticles using aqua regia was compared with that of independently-prepared gold nanoparticles that were completely surrounded with silica shells. When aqua
regia was added to the gold/silica core/shell nanoparticles, the color of the solution remained unchanged, as a consequence of core stabilization though completely surrounding silica shells (Fig. 3A). On the other hand, for the silica hemispheres containing gold nanoparticles, the color instantly changed from red to colorless, which indicated the rapid dissolution of the gold nanoparticle cores (Fig. 3B). Dissolution was evident using TEM; after the core gold nanoparticles were oxidatively dissolved, hollow silica hemispheres with open holes could be observed. The second experiment involved self-catalyzed electroless gold deposition through exposed core nanoparticle surfaces in the heteronanoparticles. When Au precursor (HAuCl4, 10 mM) and aminoethanethiol (200 mM) as a mild reducing agent were added into the ethanol solution containing heteronanoparticles (36 nm silica hemispheres containing 20 nm gold cores) at 60 °C, the color of the solution gradually changed from red to purple. Time dependent UV–vis spectra show a gradual increase in intensity of the plasmon resonance at longer wavelength (Fig. 4A). These changes can be attributed to the overgrowth of gold from the exposed core gold surface in the heteronanoparticles, forming interconnected gold homodimers as evident in the TEM images (Fig. 4B and C). In the extinction spectra, transverse and longitudinal bands for plasmon resonance modes are not clear, presumably due to very low aspect ratio of the present interconnected gold homodimers [32,33]. The spectra are rather similar in form with polymer stabilized gold nanoparticle dimers [34]. After electroless gold deposition for 15 min, gold homodimers were obtained, where the gold nanoparticles that complete formation of the dimer were almost the same size as the original gold core; half of the dimer is surrounded by the silica shell. Typically, 85% yield was achieved for formation of the gold homodimers when the purified gold/silica heteronanoparticles prepared by density gradient centrifugation were used as initial catalytic cores. These observations led us to conclude that the present heteronanostructures consist of silica hemispheres with gold nanoparticle cores and small open holes in the center of their flat plane sites. In conclusion, we have demonstrated a facile method for the synthesis of asymmetric heteronanostructures that consist of silica hemispheres containing gold nanoparticles through a simple surface immobilization approach. The obtained heteronanoparticles have a small open site on their surface, thereby allowing rapid dissolution of core gold nanoparticles by oxidative etching to form hemispherical silica capsules and site selective growth of gold nanoparticles to form gold homodimers. We envisage that this general strategy will be applied to synthesize nanoparticle-based building blocks to construct a variety of complex and functional nanostructures.
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Fig. 4. (A) Optical extinction spectra of silica hemispheres containing gold nanoparticles obtained before and after electroless deposition of gold. Inset: photographs of the solution. (B) and (C) TEM images of gold homodimers obtained after electroless deposition of gold onto the heteronanoparticles for (B) 10 and (C) 15 min using HAuCl4 and 2aminoethanethiol at 60 °C. Inset: schematics of gold homodimers with silica hemispheres.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.055.
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