Kan-Sen Chou* and Yu-Chieh Lu Department of Chemical Engineering

National Tsing Hua University Hsinchu, Taiwan 30013

*corresponding author: kschou@che.nthu.edu.tw

Abstract—Nanoshells composed of close-packed silver nanocrystals have been fabricated on silica spheres by electroless plating technique. By controlling the solution’s pH in acidic condition during the synthesis of SiO2-Ag core-shell particles, the homogeneous deposition resulted in thin and uniform silver shells. The deposition process was carried out in two steps: the first step was the modification of silica surface with the adsorption of 3-(2-Aminoethylamino) propyltrimethoxysilane (APTS), and the second step was the deposition of silver by reacting AgNO3, dextrose and acetic acid. The reaction was performed at high temperature to accelerate the deposition process. The resulting silica-silver core-shell colloids were characterized with XRD, high-resolution TEM and SEM, and UV-visible spectroscopy. It was found that the core-shell structures, such as shell thickness and shell continuity, influenced significantly the absorption spectra. Moreover, the thin shell at about 3nm was very sensitive to the temperature, and the core-shell structure collapsed after 200 treatment for 30 minutes probably due to the coalescence between adjacent Ag particles. When heating temperature was over than 300 the Ag particles began to move and leave the SiO2 surface, causing the corlour of particles to fade gradually.

Keywords-SiO2-Ag core-shell particle; electroless plating; acidic condition

I. INTRODUCTION

Due to its very high surface area, materials in nano-scale possess chemical and physical properties that are distinct from both the bulk phase and individual molecules. Among the different types of nanoparticles widely used, metals are attracting a great deal of attention due to their special properties on optics and electronics. Recently, lots effort has been made to create novel classes of materials through either surface modification or manipulation of compositions. In particular, the fabrication of core-shell particles is currently an attractive area of investigation because of their applications in the fields of surface enhanced Raman scattering(SERS) [1], magnetics [2], biochemistry [3] and catalysis [4]. Especially, spheres consist of a dielectric or semiconducting core coated with metal nanoshell manifest a particular optical resonance, and that is dependent on the dielectric core diameter to

metallic shell thickness ratio. Varying this ratio allows tuning of the peak position from the UV to the IR range [5]. A number of reports have been published on the preparation of SiO2-Ag core-shell nanoparticles by electroless plating technique [6-11], however, in most of them, the Ag coating is non-uniform and/or the degree of surface coverage is low, owing to the poor control over reaction rate. In general, the reduction reaction of metallic ions is sensitive to the solution’s pH. Based on our prior work, the reduction of silver ion was vigorous at high pH but not at low one. Unlike the violent precipitation at high pH, creating manu silver particles, the reduction of Ag+ ion to Ag solid occurred on the surface of existing colloids with slower reaction rates [12]. In addition, electroless plating method was usually performed in alkaline condition and without any protective agents, for that the irregular agglomeration of Ag particles became inevitable and resulted in nonuniform deposited morphologies. In this work, we propose a method to prepare Ag-coated SiO2 spheres in acidic condition to obtain uniform and dense nanoshells. We’ll investigate the optical and thermal behaviour of Ag nanoparticles adhering on the SiO2 surface. And moreover, the investigation of reaction kinetics of the deposition will also be preformed.

II. EXPERIMENTAL

A. Synthesis of SiO2 spheresMonodispersive submicron SiO2 spheres were prepared

through the procedure originally described by Stöber et al [13]. In order to increase the stability of Ag particles deposited on SiO2 spheres, 3-(2-Aminoethylamino) propyltrimethoxysilane (APTS) as a surfactant was introduced in this work. It acts like a connection in that the bifunctional molecules of APTS would form chemical bonding with both Ag and SiO2particles respectively. Alternative modification reagents can be found elsewhere [6-10, 14]. Two different sizes of Stöber SiO2 spheres modified by APTS were used in this work. After several times of washing, the particles were re-dispersed in deionized water by sonication for 1 hour. The actual SiO2content in the working suspension was determined by

Preparation and Characterization of Ag Nanoshell on SiO2 Spheres via Electroless Plating Technique in

Acidic Conditions

ICONN 20061-4244-0453-3/06/$20.00 2006 IEEE 46

TABLE I. EXPERIMENTAL CONDITIONS FOR THE PREPARATION

Alkaline condition (a) (Reaction temperature 60 )Reagents amount

SiO2a (size=570nm) 0.824g

AgNO3 0.1M Dextrose 0.2M NH4OH Proper amount to adjust the pH value to 10

Deionized water Proper amount to the final volume is 50ml Acidic condition (b) (Reaction temperature 60 )

SiO2a(size=230nm) 0.745g AgNO3 0.1M

Dextrose 0.2M CH3COOH Proper amount to adjust the pH value to 4

Deionized water Proper amount to the final volume is 50ml Acidic condition (c) (Reaction temperature 80 )

SiO2a (size=230nm) 0.542g

AgNO3 0.1M Dextrose 0.2M

CH3COOH Proper amount to adjust the pH value to 4 Deionized water Proper amount to the final volume is 50ml

a. silica colloids have been surface pre-modified by APTS.

calculating the weight difference between the original suspension and final remnant after solvent had been evaporated.

B. Preparation of Ag nanoshell on modified SiO2 spheresSiO2 suspension was mixed thoroughly by stirrer at

constant speed of 500r.p.m. with a stock solution, including AgNO3 (Kojima, Japan) as silver ion precursor and dextrose C6H12O6 (T e d i a , USA) as reducing agent. The reaction apparatus was covered to avoid loss of water during the reaction period and the reaction temperature was kept constant by a water bath. In order to compare the influence of pH value on the morphology of deposition of Ag nanoparticles, two pH conditions (pH=4 and 10) were tested. The pH of original solution was about 6.5 and it could be adhusted by adding either CH3COOH (Union Chemical, Taiwan) or NH4OH (Union Chemical, Taiwan). The concentrations of various reagents were listed in Table 1. The Ag-coated SiO2 colloids were then centrifuged at 5000rpm and washed with deionized water twice to remove the unreacted Ag+ ions, finally redispersed in deionized water to obtain the working colloids. The actual content of colloids was determined by weight difference before and after solvent evaporation.

C. Thermal Treatment A glass slide (cut to 26mm × 26mm, thickness 1.2-1.4mm,

Leatec Fine Ceramics Co. Ltd., Taiwan) was first immersed in HNO3 solution, then moved into acetone in order to remove any contaminants on the surface, and finally dried in an oven before use. 0.3ml Ag-coated SiO2 colloids were dropped onto the slide and dispersed uniformly to the whole surface by a tip. By the natural sedimentation method, spheres deposition on the slide formed a multi-layer close packing structure after drying at room temperature overnight. These samples were subsequently heated to various temperatures, 200,300,400 and 500 , for 30 minutes to investage the thermal behaviour of these particles.

(a) (b) Figure 1. Typical SEM images of SiO2-Ag core-shell spheres prepared under different pH conditions for 2 hours. (a) pH=10, (b) pH=4.

(a) (b) Figure 2. Typical TEM images of SiO2-Ag core-shell spheres with different magnification. (core diameter/shell thickness ratio was 230/5 in this case).

D. Characterization As for the examination of the core-shell structure, surface

morphologies and Ag shell thickness were observed by both Scanning electron microscopy (SEM, S-4700I, Hitachi, Japan) and Transmission electron microscopy (TEM, TECNAI 20, Philips, Netherlands). The X-ray diffraction patterns of the product were measured with a powder X-ray diffractometer (MXP18, MAC Science, Japan) using CuK =1.5418Å radiation. Absorption spectroscopy of colloidal solution was carried out on an UV-3000 (Hitachi, Japan) spectrophotometer to observe the simultaneous changes in optical absorption during the synthesis process, in addition, an UV-2450 (Shimadzu, Japan) spectrophotometer was used to observe the absorption characterization of the powders treated with different temperatures. As for kinetics studies, reaction has been performed in two temperatures, 60 and 80 . To follow the reaction kinetics, a silver electrode (Cole-Parmer, USA) was adopted.

A. Electron Microscopy Studies (SEM & TEM) The SEM micrographs of the Ag-coated SiO2 spheres in

different pH conditions (pH=4 and 10) were shown in Fig.1.

Figure 3. Ag+ ion concentration and corresponding coating thickness were recorded with reaction in acidic condition.

III. RESULTS AND DISCUSSIONS

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2 CuK

10 20 30 40 50 60 70 80 90

Inte

nsity

(111)

(200)

(220) (311)

Figure 4. Typical XRD patterns of Ag-coated SiO2 powders.

When the preparation made under alkaline condition of pH=10, the reduction rate of Ag+ ion was so fast that the deposition was irregular and some aggregated Ag particles resulting in rough morphologies formed on the SiO2 surface,exhibited in Fig.1(a). Otherwise, in Fig.1(b), due to mild reduction rate of Ag+ ion under acidic condition of pH=4, smooth morphologies of Ag deposition can be observed evidently. From another image obtained by TEM, exhibited in Fig.2, the continuous Ag nanoshell formed after 18 hours at the temperature of 80 was illustrated, and about 5nm in nanoshell thickness can be observed.

B. Kinetic analysis According to our previous work [12], the deposition

(reaction) rate of Ag+ ion in the growth period followed the [Ag+]3/2 kinetics with an apparent activation of about 15kcalmol-1. The coating density can be estimated if the amount of existing SiO2 spheres and the difference between residual and initial Ag+ ion concentration in the colloids has been known. Fig. 3 showed measured kinetic results of [Ag+]as well as corresponding coating thickness with time at two different reaction temperatures in acidic condition. The kinetics fitting and the derived activation energy, 15.8kcal/mol by the Arrhenius equation was consistent with our previous results [12]. Therefore, the deposited rate can be controlled by various parameters, such as initial Ag+ ion concentration, reaction temperature, reducing agent and so on. And for that the coating density was predictable based on our kinetic model.

Wavelength (nm)

300 350 400 450 500 550 600 650 700 750 800

Abs

orba

nce(

A.U

.)

time

t = 0 min

t = 10 min

t = 0.5 h

t = 1 ht = 2 h

t = 4 ht = 6 ht = 10 h

t = 1 8h

Figure 5. The optical absorption of SiO2 colloids varied with the deposited Ag amount as function as time.

Wavelength (nm)

300 400 500 600 700 800

Abs

orpt

ion

(A.U

.)

(a)

(b)

(c)

Figure 6. The optical absorption of SiO2-Ag core-shell powders with different core diameter/shell thickness ratios.(a) 230/10; (b)230/5; (c) 230/3.

C. X-ray Diffraction (XRD) Fig.4 showed the typical X-ray diffraction patterns of Ag-

coated SiO2 spheres. For Ag coated SiO2 powders with core/shell ratio: 230/5, four main diffraction peaks related to the (111), (200), (220), and (311) planes of face-centered cubic Ag were obvious, indicating that the Ag deposited layer was well crystalline.

D. UV-Visible Absorption Spectra The UV-visible absorption spectra of the Ag-coated SiO2

colloids were recorded as function of reaction time, shown in Fig.5. The absorption peaks of SiO2 changed after 10 minutes meaning that the tiny amount of Ag deposition started on SiO2surface. Based on the statement of Oldenburg et al. [5] that the initial red shift in the peak absorbance was resulted from the coalescence of metallic layer, and once the shell was complete, that is shifted to shorter wavelengths. The color of

Wavelength (nm)

250 300 350 400 450 500 550 600 650 700 750 800

Abs

orba

nce(

A.U

.)

original

200

300

400

500

(a)

Wavelength (nm)

250 300 350 400 450 500 550 600 650 700 750 800

Abs

orba

nce(

A.U

.)

Original

200300

400

500

(b) Figure 7. The optical absorption of SiO2-Ag core-shell powders with two core diameter/shell thickness ratios: (a)230/3; (b)230/5, varied with heat treatment with different temperatures.

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colloids changed from white to light yellow, dark yellow and finally gray green during the course of the reaction. Fig.6 showed the absorption spectra of SiO2 cores coated with different shells in thickness. First it can be noted that the absorption peaks for the same powders existing in different surroundings are different. According to Mie scattering theory, the absorption of core-shell particles depend upon the both refractive indexes and the existing surrounding such as water, air, etc., and that is why the absorption peaks are different. Otherwise, SiO2–Ag core-shell structure have optical resonances shifted into the infrared spectral range as the core-shell ratio increased can be observed in this work. The results were similar to that proposed by Oldenburg et al., in which SiO2–Au core-shell structure had also optical resonances shifted into the infrared spectral range as the core-shell ratio increased. Hence we believe the peak absorbance in the spectral range of long wavelength was resulted from the interaction between SiO2 core and Ag shell.

Fig. 7 exhibited the UV-visible spectra of Ag-coated SiO2powders after the heat treatment, not only intensity decreased with the increase of heating temperature but shape and position of absorption peaks also varied. Especially in case of 200 treatment, the original absorption in long spectral range disappeared abruptly, suggesting the nanoshell was not continuous any more probably due to the collapse of nanoshell resulted form the coalescence between adjacent Ag et al. [15] on the sintering effect on nano-Ag particle films.

Moreover, colours of appearance were also exhibited in Fig.8. The changes in colour can inform us of changes occurred during the heating process. Owing to surface plasmon resonance, yellow color was often observed for individual nano-silver coloids. Yet, for the original core-shell particles of silica-silver, the color is green. Yellow and orange in appearances of powders were shown after respective heating to 200 and 300 , and colour faded gradually due to movement of Ag when heating temperature was over 300 .Up to 500 , the treated powders were nearly white but not as pure as simple SiO2 spheres. We believe that some Ag particles might have diffused into the interior of SiO2 after this heat treatment. In summary, the changes in colour clearly indicated that the optical properties were deeply influenced by the dispersed morphology of Ag onto SiO2 spheres. In addition, after heat treatment the glass substrate was cleaned with sonication in methanol to remove the overlying SiO2powders, and subsequently examined by UV-visible spectrophotometry. The spectra indicated to us the adhesion between Ag and glass slide were very poor if the heating temperature was as only 400 . The specific absorption peak of Ag (420nm) began to appear until heating temperature was up to 480 , suggestion that some silver colloids might remain on glass slide after heating to this temperature.

In this work, submicron SiO2 with uniform Ag nanoshell have been successfully prepared by electroless plating technique in acidic condition due to mild precipitation rate of silver. The kinetics fitting and the activation energy derived were consistent with our precious kinetic model. Owing to very thin thickness of Ag nanoshells, it was very sensitive to the temperature, and the core-shell structure collapsed after 200 treatment for 30 minutes probably because of the coalescence between adjacent Ag particles. The Ag particles might start to move off the SiO2 surface with its corlour faded when heating temperature was over than 300 .

ACKNOWLEDGMENT

The authors wish to thank National Science Council for financial support of this work. (Grant number NSC 93-2214-E007-011).

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original 200 300 400 500 Pure SiO2

Figure 8. Colours of Ag-coated SiO2 powders after heating with different temperatures for 30 minutes.

CONCLUSION

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