Comparative studies between synthetic routes of SiO2@Au composite nanoparticles

Article abstract of DOI:10.1016/j.materresbull.2006.11.019

We studied two different methods for the deposition of Au nanoparticles (Au NPs) onto the functionalized silica microspheres. One method was to mix the two kinds of particles together and react at room temperature overnight. The other one was to reduce hydrochloroauric acid to Au NPs with sodium citrate in the presence of the functionalized silica microspheres (one for the naked silica microspheres, the other for the Au-attached silica microspheres). We investigated the morphologies of SiO₂@Au composite nanoparticles synthesized by these two methods, and found that the latter achieved a denser Au coverage on silica microspheres. Furthermore, we studied the effect of the pH values in a wide range, respectively, for the two methods. A possible mechanism was put forward to interpret the formation of SiO₂@Au composite nanoparticles and the effects of the synthetic routes and pH values on the morphologies and optical properties.

Full text of DOI:10.1016/j.materresbull.2006.11.019

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Materials Research Bulletin 42 (2007) 1457–1467
www.elsevier.com/locate/matresbu
Comparative studies between synthetic routes of SiO2@Au
composite nanoparticles
Lu Zhang a,b, Yong-Gang Feng a, Li-Ying Wang a, Jun-Yan Zhang b,
a
Meng Chen a,b, , Dong-Jin Qian
*
a Department of Chemistry and Laboratory of Advanced Materials, Fudan University, 220 Handan Rd., Shanghai 200433, PR China
b State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science,
Lanzhou 730000, PR China
Received 7 September 2006; received in revised form 27 October 2006; accepted 4 November 2006
Available online 15 December 2006
Abstract
We studied two different methods for the deposition of Au nanoparticles (Au NPs) onto the functionalized silica microspheres.
One method was to mix the two kinds of particles together and react at room temperature overnight. The other one was to reduce
hydrochloroauric acid to Au NPs with sodium citrate in the presence of the functionalized silica microspheres (one for the naked
silica microspheres, the other for the Au-attached silica microspheres). We investigated the morphologies of SiO2@Au composite
nanoparticles synthesized by these two methods, and found that the latter achieved a denser Au coverage on silica microspheres.
Furthermore, we studied the effect of the pH values in a wide range, respectively, for the two methods. A possible mechanism was
put forward to interpret the formation of SiO2@Au composite nanoparticles and the effects of the synthetic routes and pH values on
the morphologies and optical properties.
# 2006 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures; B. Chemical synthesis; C. Electron microscopy
1. Introduction
Metal nanoparticles have always been attracting extensive attention due to their unusual electrical [1], optical [2–
4], magnetic [5,6] and catalytic [7,8] properties. They exhibit more excellent properties when they are assembled into
different structures. So far, a series of nanostructures, such as nanowires [9,10], nanorods [11–13], nanotubes [14–16]
nanotriangles [17] and nanoribbons [18] have been successfully synthesized. Properties of these nanoscale materials
are dependent not only on the nature of nanoparticles, but on their shape and size as well.
The core-shell nanomaterials, as one of the complex nanostructures, have been widely studied. Inorganic
nanoparticles can obtain improved chemical stability and size monodispersity through deposition onto the inert supports
in this type of nanostructure. Recently, there are many reports on the deposition of various inorganic nanoparticles onto
silica microspheres, including metals [19–21], metal oxides [22–24] and sulfides [25,26]. Au nanoparticles (Au NPs) are
widely used metal nanoparticles for the deposition onto silica microspheres. The typical method contains three steps: the
* Corresponding author at: Department of Chemistry and Laboratory of Advanced Materials, Fudan University, 220 Handan Rd., Shanghai
200433, PR China. Tel.: +86 21 55664181; fax: +86 21 55664192.
E-mail address: chenmeng@fudan.edu.cn (M. Chen).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.11.019

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synthesis of silica microspheres; the surface functionalization of silica microspheres with silane coupling agents;
depositionofAu NPsontothefunctionalizedsilica microspheresbysimplymixingthetwo kinds ofparticles. Obviously,
the three-step method has been widely used for its convenient manipulation. However, the process is tedious, and the Au
coverage on silica microspheres is not dense. Halas and co-workers [27] already made a detailed study for the three-step
method. They found that the surface functionalization of silica microspheres with different terminal groups had a
significant influence on the Au coverage. The hydrophilic groups, such as amino (NH2) and mercapto (SH) groups
promotedtheattachmentof AuNPs, whilethehydrophobicgroups, suchas methyl (CH3) and diphenylphosphine(PPh2)
groups could not favor the attachment of Au NPs. They also studied the impact of mixture of organosilanes and mixture
of solvents on the coverage and morphologies of theattached Au NPs. Peopleare now making greateffort to optimize the
reaction conditions of the three-step method and to explore novel methods to improve the Au coverage on silica
microspheres. In addition, sonochemical deposition [20], electroless deposition [28] and seeded growth [29] have been
successfully used for the deposition of Au NPs onto silica microspheres.
In the present work, we used a simplified method [30] to prepare SiO2@Au core-shell composite nanoparticles, and
compared it with the three-step method. Firstly, silica microspheres were functionalized with the silane coupling
agent. Secondly, hydrochloroauric acid was reduced to Au NPs with sodium citrate in the presence of the
functionalized silica microspheres. By contrast with the general three-step method, the procedure was simplified, the
reaction time was remarkably shortened, and the Au coverage on silica microspheres was denser shown by scanning
electron microscope (SEM). We also studied, respectively, the influence of the pH values in a wide range on the Au
coverage in these two methods. The results indicated that the pH values had significant influence on the Au coverage
on silica microspheres.
2. Experimental
2.1. Reagents
Tetraethoxylsilane (TEOS), aminopropyltriethylsilane (APTES), hydrochloroauric acid and sodium citrate were
purchased from Shanghai Chemical Reagent Co. Other reagents were of analytical grade. All the chemicals were used
without further purification.
2.2. Synthesis and functionalization of silica microspheres
Silica microspheres were prepared using the Stober method [31]. Briefly speaking, 2 mL of 28% ammonia hydrate
was mixed with 20 mL of absolute ethanol. The solution was stirred overnight after 1 mL of TEOS was added through
the syringe dropwise. The produced silica microspheres were centrifuged from the solution and redispersed in ethanol
at least three times to remove excess reactants, then dispersed in 20 mL of absolute ethanol.
Forthesurfacefunctionalizationofsilicamicrospheres,0.1 mLofAPTESwasaddeddropwiseto20 mLoftheobtained
silica colloidal suspension with vigorous stirring after the injection of 0.5 mL of ammonia. The silica microspheres were
centrifuged and redispersed in 20 mL of absolute ethanol after stirringfor4 h. Theabove procedurewas repeated twice to
make sure the surface functionalization was complete. The final products were centrifuged and redispersed in ethanol at
least three times to remove excess reactants, then dispersed in 20 mL of distilled water for further use.
2.3. Deposition of Au NPs onto silica microspheres I
In our work, the deposition of Au NPs onto silica microspheres was carried out using three different strategies. In the
generally three-step method (Fig. 1), the amino-terminated silica colloidal suspension was simply mixed with the Au
hydrosol. The Au hydrosol prepared using the following steps: 5 mL of the aqueous solution of hydrochloroauric acid
(10 mM) was diluted to 50 mL with distilled water, and heated to 100 8C in the oil bath under refluxing. Then, 5 mL of
the aqueous solution of sodium citrate (38.8 mM) was rapidly injected into the solution. The mixture was allowed to
reflux for further 15 min when its color turned deep red. The obtained Au hydrosol was cooled to room temperature
without further purification. In the next step, 1 mL of the amino-terminated silica colloidal suspension was added to
10 mL of the Au hydrosol, and the mixture was stirred under room temperature overnight. The final products were
centrifuged and redispersed in distilled water for several times, then dispersed in 10 mL of distilled water.

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Fig. 1. Schematic illustration of the deposition of Au NPs onto silica microspheres I.
2.4. Deposition of Au NPs onto silica microspheres II
The second strategy was a simplified one (Fig. 2). HAuCl4 was reduced to Au NPs with sodium citrate in the
presence of the functionalized silica microspheres. In this method, 2.5 mL of hydrochloroauric acid (10 mM) was
added to 22.5 mL of the dilute functionalized silica colloidal suspension, and the solution was heated to 100 8C in the
oil bath under refluxing. Then, 2.5 mL of sodium citrate (38.8 mM) was rapidly injected into the solution. The mixture
was allowed to reflux for further 15 min when its color turned deep red. The obtained products were centrifuged and
redispersed in distilled water for several times, then dispersed in 20 mL of distilled water.
2.5. Deposition of Au NPs onto silica microspheres III
The third strategy was derived from the second simplified one (Fig. 3). We first prepared the SiO2@Au composite
nanoparticles using the three-step method, and substituted them for the naked functionalized silica microspheres to be
mixed with HAuCl4 in the aqueous solution. Sodium citrate was rapidly injected to the solution when it was heated to
100 8C under refluxing. The mixture was allowed to reflux for further 15 min. The obtained products were centrifuged
and redispersed in distilled water for several times, then dispersed in 20 mL of distilled water.
A series of experiments were designed to explore the effect of the pH values on the Au coverage deposited on silica
microspheres. In the three-step method, the pH values of the treatment solutions were adjusted in a wide range prior to
stirring overnight. The adjusted pH values included 3.77, 4.65, 5.51, 6.30, 7.79, 9.08, 9.55 and 10.17. In the simplified
method, the pH values of the dilute silica colloidal suspensions were adjusted in a wide range prior to addition of
HAuCl4. The pH values were adjusted as follows: 3.85, 4.86, 5.38, 7.89 and 8.90. All the pH adjustments were carried
out with the aqueous solutions of hydrochloric acid and sodium hydrate.
2.6. Characterization
The UV–vis absorption spectra were taken at room temperature on a UV-3150 spectrophotometer (Shimadzu,
Japan) with a variable wavelength between 250 and 700 nm using a glass cuvette with 1-cm optical path. The
morphologies of SiO2@Au samples were observed using a Shimadzu SSX-550 scanning electron microscope. The
morphologies of Au NPs were observed using a Hitachi H-600 transmission electron microscope (TEM), which was
operated at 75 kV with samples deposited onto a 230-mesh copper grid covered with Formvar. Particle size analysis
was carried out by manually digitzing the TEM and SEM images with Image Tool, from which the average size and
Fig. 2. Schematic illustration of the deposition of Au NPs onto silica microspheres II.

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Fig. 3. Schematic illustration of the deposition of Au NPs onto silica microspheres III.
standard deviation of particles were generated. The Au contents of the SiO2@Au samples were measured using a
Z-500 atom absorption spectrometer (AAS).
3. Results and discussion
3.1. SiO2@Au composite nanoparticles I
Silica microspheres and the Au hydrosol were prepared, respectively. The naked silica microspheres with smooth
surfaces (Fig. 4a) had an average diameter of 227 nm and a narrow size distribution of 9.2% (Fig. 4b). Au NPs were
basically spherical as shown in Fig. 4c, which had an average diameter of 14 nm and the size distribution of 13.6%
(Fig. 4d). The SiO2@Au composite nanoparticles was prepared using the general three-step method. The SEM images
revealed the morphologies of the SiO2@Au composite nanoparticles, which showed distinctly that Au NPs were
attached to the surfaces of the functionalized silica microspheres (Fig. 5). At lower pH values, sparse Au NPs were
distributed on the functionalized silica microspheres (Fig. 5a–d). As the pH values were adjusted to lower levels
Fig. 4. (a) SEM image of the naked silica microspheres, and the inset is TEM image of a single naked silica microsphere; (b) histogram for the size
distribution of silica microspheres; (c) TEM image of Au NPs; (d) histogram for the size distribution of Au NPs.