ISSN 1061�933X, Colloid Journal, 2010, Vol. 72, No. 2, pp. 158–162. © Pleiades Publishing, Ltd., 2010.

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1 INTRODUCTION

In the past decade, semiconductor nanoparticleshave gained increasing attention due to their uniqueoptical properties [1–4]. Among other semiconductornanoparticles, the CdS ones represent one of the mostimportant groups of II�TV semiconductors [5]. Cad�mium sulfide is luminescent material and, therefore,can serve as a luminescent probe of macromolecules[6, 7], including biological ones, such as bovine serumalbumin [8–10]. However, CdS nanoparticles are eas�ily decomposed, and the substance is poisonous. Toovercome these limitations, the surface modificationof CdS nanoparticie is required [11–15]. SiO2 is con�sidered as one of the most suitable materials for pro�tecting of CdS particles due to its chemical stability,biocompatibility, and reactivity with various couplingagents [16–19]. Silica�coated semiconductor quan�tum dots, such as cadmium sulfide and cadmiumselenide, have showed high stability, chemical versatil�ity, and biocompatibility that are crucial for many bio�medical applications [20, 21]. Therefore, CdS nano�particles coated with silica (assigned as CdS–SiO2) areof considerable interest in both fundamental studiesand applied research [22–25].

The preparation methods can be classified into twomajor groups: the Stöber process�based approaches[26] and the reverse (water�in�oil) microemulsionmethod [27–30]. The silica�coated CdS particles havebeen reported in many papers. For example, Correa�Duartetal et al. [31] has introduced a method of add�ing 3�(mercaptopropyl)trimethoxysilane as a coupling

1 The article is published in the original.

agent to incorporate the semiconductor nanocrystalsinto silica. Asher et al. [32] have prepared monodis�perse colloidal silica�CdS by water�in�oil microemul�sions. Wang et al. [33] have synthesized monodis�persed CdS nanoparticles through γ�irradiatingCdCl2, Na2S2O3, and polyvinylpyrrolidone aqueoussolution at room temperature. With these monodis�persed CdS nanoparticles, CdS–SiO2 core–shellstructures were prepared through the hydrolysis of tet�raethylorthosilicate (TEOS).

In this paper, we present a relatively simple two�step process approach for preparing silica�encapsu�lated CdS nanoparticles. At the first step, CdS nano�particles were synthesized in the AOT/heptane/H2Oreverse micelles, since the reverse micelles allow forcontrolling the particle size distribution [34–37].Then, core–shell CdS–SiO2 nanoparticles were pre�pared through the hydrolysis and condensation ofTEOS in the absence of a coupling agent. The spectro�scopic properties, size distribution and morphology ofobtained CdS–SiO2 nanoparticles were studied indetails. The investigation of photodegradation ofnanoparticles upon UV irradiation showed thatCdS⎯SiO2 nanoparticles are more stable than CdSnanoparticles. That is, silica coating efficiently inhib�its the photodegradation of the CdS core.

EXPERIMENTAL

Materials

Cadmium nitrate (Cd(NO3)2 ⋅ 4H2O), sodium sul�fide (Na2S ⋅ 9H2O), ammonium hydroxide (NH3 ⋅H2O), TEOS, C2H5OH, n�heptane, and polyethylene

Synthesis, Characterization and Stability of Multicore�Shell CdS⎯SiO2 Nanoparticles1

Chao Weia, Weiqiang Zhoua, Yukou Dua, Jingkun Xub, and Ping Yanga

a Department of Chemistry and Chemical Engineering, Suzhou University, Suzhou 215123, P R. Chinab Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University,

Nanchang, 330013, P R. Chinae�mail: duyk@suda.edu.cn

Received March 27, 2009

Abstract—Multicore�shell CdS⎯Si02 nanoparticles were synthesized in AOT/heptane/H2O reverse micellesat room temperature. CdS⎯SiO2 nanoparticles were characterized by UV�vis spectroscopy, TEM, and SEMmethods. The results show that multicore�shell composites were formed. Fluorescence properties of compos�ites were also investigated; the data imply that fluorescence properties of the silica�coated CdS nanoparticleswere significantly improved when compared to those of the non�coated CdS nanoparticles. The stability ofmulticore�shell CdS⎯SiO2 nanoparticles upon the UV irradiation was higher than that of non�coated CdSnanoparticles.

DOI: 10.1134/S1061933X1002002X

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SYNTHESIS, CHARACTERIZATION AND STABILITY OF MULTICORE�SHELL 159

glycol (PEG, MW = 20000) were purchased fromSinopharm chemical reagent Co., Ltd. Sodium bis(2�ethylhexyl)sulfosuccinate (AOT, MW = 444.55) waspurchased from Aifa Aesar, a Johnson Matthey Com�pany. All the chemicals were of analytical grade andused without further purification.

Instruments

Transmission electron microscopy (TEM) mea�surements were performed on FEI Tecnai�G20 elec�tron microscope operated at 200 kV. Samples for thesemeasurements were prepared by depositing a drop ofcolloidal solution onto copper grid covered withFormvar and further air�drying. UV�Vis absorptionspectra were recorded on UV�1800 SPC spectropho�tometer. The surface morphology was analyzed with aHitachi S�570 scanning electron microscope (SEM).The fluorescence spectra were obtained with a HitachiF�4500 fluorescence spectrophotometer. The lightsource of the photodegradation was a 70 W high�pres�sure mercury lamp (365 nm). The irradiation intensitywas measured by a Handy UV�A radiometer.

Preparation of Nanoparticles

CdS nanocrystals were synthesized throughAOT/heptane/H2O reverse micelle method [38]. Thesolution of 0.005 mol AOT in 100 ml n�heptane was pre�pared. The aqueous solutions of Cd(NO3)2 (0.2 mol/L)and Na2S (0.2 mol/L) were separately added to the twoportions of AOT containing n�heptane solution. Themixed solutions were stirred for two hours until opticaltransparency was achieved. Then both solutions weremixed together and stirred for another 1 h for obtain�ing CdS nanoparticles. However, CdS nanoparticlesprepared by this method represent the small CdSseeds. Therefore, a certain amount of Na2S�contain�ing micelles was further added into the reverse micellesolution containing these seeds. After stirring for 3 h,the equal amount of Cd(NO3)2 micelle solution wasadded into the reverse micelle solution. After the seedgrowth procedure, we obtained the larger CdS nano�particles.

For the synthesis of CdS–SiO2 nanoparticles, theammonia solution (10 μL, 25 wt %) was added as a cat�alyst into the obtained CdS solution. After 10 min, the0.96 mL TEOS was introduced under vigorous mag�netic stirring. The reacting system was stirred at aroom temperature for 24 h. Silica shell was formed onthe surface of CdS nanoparticles through hydrolysisand condensation of TEOS.

The solid SiO2 nanoparticles were also prepared bythe procedure described in the literature [39] to com�pare them with the core�shell CdS⎯SiO2 nanoparti�cles.

Stability Measurements

The stability of CdS–SiO2 was measured in a courseof the photodegradation upon UV irradiation. Thesource was hanged in a dark box and fixed at about 10 cmabove the colloid solution. The irradiation intensity onthe solution surface was of 0.387 mW/cm2 as measuredby a Handy UV�A radiometer. Upon stirring, a smallamount of CdS–SiO2 was taken every hour for analysiswith a UV�vis spectrophotometer. The concentrationof CdS–SiO2 was determined by monitoring the absor�bance at 450 nm. The stability of CdS was measured ina similar fashion.

RESULTS AND DISCUSSION

UV�vis Spectroscopy

Figure 1 shows the UV�vis absorption spectra ofsmall CdS seeds and CdS nanoparticles after the seed�ing growth procedure. The characteristic absorptionbands of small CdS seeds and CdS nanoparticles wereobserved at 420 and 450 nm, respectively. The absorp�tion band of the obtained CdS nanoparticles was red�shifted compared to that of CdS seeds because of thequantum confinement effect. The results imply thatthe size of CdS seeds increases in a course of the seed�ing growth and, therefore, can be easy controlled.

Scanning Electron Microscopy

Figure 2 shows the SEM image of the CdS–SiO2nanoparticles. As illustrated by the image, the sug�gested procedure allows for forming uniformly dis�persed CdS⎯SiO2 particles with spherical morphologyand narrow size distribution.

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Fig. 1. UV�vis spectra of CdS nanoparticles (1) before and(2) after the seeding growth procedure.

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Transmission Electron Microscopy

TEM micrographs of the CdS (a), CdS–SiO2nanoparticles (b) and solid SiO2 nanoparticles (c) areshown in Fig. 3. As determined from the images, theaverage size of spherical CdS is ca. 3–4 nm (Fig. 3a).The core�shell structure of CdS⎯SiO2 nanoparticleswith an average size of 90�95 nm (Fig. 3b) wasobserved and compared with that of solid SiO2 nano�particles of almost similar size (Fig. 3c). As seen fromFigs. 2 and 3, multiple CdS nanoparticles of 4–5 nmin diameter are immobilized in one silica particle thatindicates the formation of many multicore compos�ites.

Fluorescence Spectroscopy

Figure 4 shows the fluorescence spectra of smallCdS seeds (curve 1) and CdS nanoparticles after theseed growth procedure (curve 2); the excitation wave�length is of 380 nm. As seen from the graphs, strongemission bands at 569 nm are observed in both spectra.Moreover, the fluorescence spectra of CdS semicon�ductor nanocrystals consist of two parts: one is attrib�uted to the recombination of electrons and holes, and

another—to the recombination of surface state anddefect [40]. We suggested that the band at 569 nmoccurred due to the recombination of surface state anddefect. The emission peak of the CdS nanoparticlesafter the seeding growth procedure became weakerwhen compared to CdS seeds. The effect originatesfrom the presence of S2– on a surface of the CdS nano�particles after the seeding growth. The number ofdefects formed by sulfur vacancies is decreased, that,in turn, decreases the intensity of the peak [41].

Figure 5 shows the fluorescence spectra of CdS(curve 1) and CdS–SiO2 (curve 2) nanoparticles. Asillustrated by Fig. 4, the emission peak at 569 nmobserved for CdS nanoparticles is shifted to 574 nm inthe spectrum of the CdS–SiO2 sample. The fluores�cence intensity in the case of the CdS–SiO2 nanopar�ticles was stronger than that for the non�coated CdSnanoparticles. This effect suggests that the core—shellCdS–SiO2 nanoparticles can be potentially useful toimprove luminescence performance in certain systemsdue to the protection of luminescent core by the SiO2shell [42].

Figure 6 shows the time�dependent UV�vis spectraof CdS nanoparticles. Upon the UV�light irradiation,

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Fig. 2. SEM microphotograph of the core–shell CdS–SiO2 nanoparticles.

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Fig. 3. TEM microphotographs of (a) CdS nanoparticles after the seeding growth procedure, (b) CdS–SiO2 core–shell nanopar�ticles, and (c) homogeneous SiO2 nanoparticles.

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the intensity of absorption gradually decreases withthe exposition time. The time�dependent UV�visspectra of CdS⎯SiO2 are shown in Fig. 7. Remarkably,the data suggest that CdS–SiO2 is a very stable com�posite, which undergoes negligible degradation uponUV�light irradiation. The stabilities of CdS and CdS–SiO2 are compared in Fig. 8, where C0 and C are theconcentrations of CdS or CdS–SiO2 before and afterthe UV treatment, respectively. The photodegradationcurves (Fig. 8) suggest that more than 70% of unpro�tected CdS nanoparticles decomposed after UV irra�diation (5 hours), whereas only minor amount(ca. 0.2%) of CdS–SiO2 degraded within the similar

period of time. The results demonstrated that CdS–SiO2 system exhibits noticably higher stability than theCdS nanoparticles.

CONCLUSIONS

In summary, monodispersed CdS nanoparticleswere synthesized in AOT/heptane/H2O reverse micellesystem. Using these CdS nanoparticles, we preparedCdS⎯SiO2 multicore–shell structure in the absence ofa coupling agent. The obtained nanoparticles werecharacterized by UV�vis spectroscopy, SEM andTEM. The data show that the spherical multicore

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Fig. 4. Fluorescence spectra of (1) CdS seeds and (2) CdSnanoparticles after the seeding growth.

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Fig. 5. Fluorescence spectra of (1) CdS and (2) CdS⎯SiO2nanoparticles.

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Fig. 7. Time�dependent UV�vis spectra of CdS⎯SiO2nanoparticles upon UV irradiation (λ = 365 nm).

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CdS–SiO2 nanoparticles were formed. The intensity offluorescence of the silica�coated CdS nanoparticleswas significantly stronger than that of the non�coatedCdS nanoparticles.

Moreover, the CdS–SiO2 showed increased stabil�ity upon UV irradiation compared to that of CdSnanoparticles. The suggested method of protectingCdS nanoparticles by the formation of monodispersedmulticore–shell CdS–SiO2 nanoparticles is thereforepotentially promising with respect of preventing semi�conductor decomposition, improvement of lumines�cence characteristics and further functionalization ofparticle surface.

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Fig. 8. The comparison of stabilities of (2) CdS and(2) CdS–SiO2 nanoparticles upon the UV irradiation.