Mesoporous Silica Spheres as Microreactors for Performing CdS

Mesoporous Silica Spheres as Microreactors for Performing CdS Nanocrystal Synthesis Yaoxia Li, Yihua Zhu,* Xiaoling Yang, and Chunzhong Li Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China UniVersity of Science and Technology, Shanghai 200237, P. R. China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4494–4498

ReceiVed May 3, 2008; ReVised Manuscript ReceiVed July 28, 2008

ABSTRACT: CdS nanoparticles were exclusively synthesized in the pores of mesoporous silica (MS) particles that had been coated with two bilayers of poly(allylamine hydrochloride) (PAH)/poly(styrene sulfonate) (PSS) via the layer-by-layer (LbL) self-assembly technique. The PAH/PSS multilyers film could prevent generation of nanocrystals outside the MS spheres. After removal of the silica cores, CdS nanocrystal encapsulated microcapsules were obtained confirming that numerous CdS nanocrystals were successfully formed. The sizes of the CdS nanocrystals were mainly determined by the pore size of the MS spheres. After being treated with a mixed solution of three salts including NaCl, LiCl, and KNO3, the MS spheres showed two different pore sizes of 3.2 and 9.7 nm. Thus, two differently sized (3.2 nm, 6.8 nm) CdS nanocrystals were formed in the pores of MS spheres. This result is believed to explain the photoluminescence (PL) band originating from band-to-band transition. Measurements and analysis of X-ray diffraction, transmission electron microscopy, high-resolution TEM, photoluminescence spectra and confocal laser scanning microscopy showed that the CdS nanocrystals were almost perfectly inserted into the pores of the MS spheres and had good crystallinity, which created a fluorescence emission. This approach can be used to prepare composite materials involving functional inorganic nanoparticles, which have potential application in biological immunoassay and photoelectronic fields. Introduction Recently, synthesis of inorganic nanoparticles inside the spatially confined volume of individual microreactors (emulsions, micelles, polyelectrolyte microcapsules, etc.) has been extensively investigated.1 Particular attention has been paid to polyelectrolyte (PE) capsules as confined microreactors with controlled shell permeability and the possibility of shell engineering on the nanolevel.1 Polyelectrolyte capsules are fabricated via the layer-by-layer (LbL) self-assembly technique. The LbL self-assembly technique for constructing hollow polyelectrolyte shells involves stepwise adsorption of oppositely charged polyelectrolytes onto functionalized colloidal templates, which are subsequently removed.2 In this way, the desired number of polyelectrolyte layers can be deposited, allowing control of the multilayer film thickness.2 This technique mainly utilizes the electrostatic attraction and complex formation between polyanions and polycations to form supramolecular multilayer assemblies of polyelectrolytes.2 Different materials, polyelectrolytes,2,3 proteins,4 inorganic and organic nanoparticles,5 lipids,6 etc., can be utilized as the shell component. The most important feature of the polyelectrolyte multilayer shells is the controllable wall permeability, which makes them promising as microcontainers for performing chemical reactions in restricted volumes. Basically, small molecules such as the decomposition products of the core or ions or dye molecules can readily penetrate though polyelectrolyte films, whereas large macromolecules are excluded or trapped.7 These hollow particles have recently been applied as nanoreactors for a variety of reactions.8-15 Shchukin and his co-workers investigated the formation of water-insoluble yttrium fluoride exclusively inside polyelectrolyte capsules containing an excess of poly(allylamine).8 And in the same year, CaCO3 was obtained exclusively inside the restricted volume of polyelectrolyte capsules.9 Urea hydrolysis, catalyzed by urease, led to the fermentative formation of CO32- ions and the precipitation of CaCO3, which completely * Corresponding author. Fax: +86-21-64250624. E-mail: [email protected].

filled the capsule interior. Later, Yu et al. synthesized CaCO3 particles inside the spatially confined volume of individual microreactors (polyelectrolyte capsules).10 The LbL approach was also adopted by Ghan et al. to polymerize phenols within polyelectrolyte microcapsules.11 These experiments show that the hollow capsules prepared by the LbL technique create many new possibilities for the synthesis of composite materials. Nanoparticles synthesized inside a confined multifunctional microreactor have several advantages: first, absence of particle aggregates; second, formation of amorphous or metastable crystal phases; third, unique composite inorganic/inorganic or inorganic/organic structures of the products.1 On the other hand, mesoporous and microporous materials are very useful hosts for the immobilization of various nanoparticles. Mesoporous silica (MS) spheres have extremely high surface areas, 3D pore networks, and pore sizes in the range of 2-50 nm.16 The ability to tune the pore size, the structure, and the topology of these materials makes them ideal nanostructured systems to host a large range of nanoscale objects possessing particular physical properties,17 especially to study the growth and the properties of quantum dots (QDs) confined on an inorganic porous matrix, which is expected to improve the quantum performance of QDs by imposing strict uniformity on size, shape, and organization. When semiconductor nanocrystals are embedded into the voids of carriers, the combination of electron and photon confinements can adjust the electron and photon density within one structure. Semiconductor nanomaterials often exhibit properties different from those of the corresponding bulk metals. Compared with their organic fluorophore counterparts, they exhibit high photobleaching threshold and high quantum yield and can be prepared with high chemical stability, while their spectral properties can be readily tuned by controlling their size.18 CdS is an important II-VI wide band gap semiconductor material (2.42 eV at room temperature)19 with tremendous potential applications in single photon detection,20 solar cells,21 and other optical devices based on its nonlinear properties. Especially, it holds immense promise as a versatile label for biological applications.

10.1021/cg800457u CCC: $40.75  2008 American Chemical Society Published on Web 10/24/2008

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Figure 1. Illustration of procedures for preparing CdS nanoparticles in the pores of the MS spheres and forming the CdS-filled microcapsules.

Recently, our group reported the confinement of CdSe nanoparticles into MS microspheres via direct reaction at low temperatures.22 The CdSe nanocrystals were almost embedded into the pores of the MS spheres and had good crystallinity. But nanocrystals outside the MS spheres, which were undesirable, also could be seen. In this paper, a PAH/PSS multilayer film, which has the properties of controllable wall permeability, was introduced to solve the problem and make the CdS nanoparticles be exclusively synthesized in the pores of MS microspheres. MS microspheres were also used as the host for immobilization of Cd ions bound to the Si-OH. To facilitate the incorporation of CdS nanoparticles, a mixture of three salts had been used to enlarge the pore size of the silica oxides before the immobilization of Cd ions. Then the Cd2+-absorbed MS spheres were alternately coated with two bilayers of poly(allylamine hydrochloride) (PAH)/poly(styrene sulfonate) (PSS) via the layer-by-layer (LbL) self-assembly technique. Sulfion from the surrounding solution could penetrate through polyelectrolyte multilayers into the pores of MS, so CdS nanoparticles were exclusively synthesized in the pores by the reaction between sulfion and Cd2+ adsorbed in silica hydroxyl groups. Finally, the mesoporous silica cores were removed and the CdSfilled microcapsules were obtained. Moreover, synthesis of CdS nanoparticles inside the pores of MS spheres without coating PAH/PSS multilayers was also carried out. Experimental Section Preparation of Mesoporous Silica Spheres. Mesoporous silica (MS) spheres were synthesized as described by the literature.23 The main procedure was as follows: 1 g of hexadecylamine was dissolved in a mixed solution including 100 mL of isopropanol and 90 mL of double distilled water to form a clear solution. Afterward 1.4 mL of ammonia followed by 12 mL of tetraethoxysilane (TEOS) was added, the mixture was homogenized and allowed to stand at ambient temperature overnight. The resulting solids were recovered by filtration of the reaction mixture, extensively washed with ethanol and the distilled water, and dried at ambient temperature. The templates were removed by calcination at 600 °C for 6 h in flowing air, and then the MS microspheres with a pore size of approximately 3.2 nm were obtained. To expand the pore size of MS microspheres, a mixed solution with three ingredients including NaCl, LiCl, and KNO3 was adopted. MS spheres were soaked in the above salts solution and dispersed with ultrasonic vibrations. Then the mixture was calcined at 300 °C for 2 h. Thus remodeled MS silica microspheres with a pore size of approximately 9.7 nm were produced.22 Immobilization of Cd2+ in Mesoporous Silica Spheres. Cd(NO3)2 solutions with a concentration of 0.01 M were used. The experiments typically involved mixing 6 mL of Cd2+ solution with 1 g of MS powder and shaking the mixture at room temperature for 24 h, followed by several washing cycles to separate the unabsorbed Cd2+ from the particles.

Coating the Cd2+-Absorbed Mesoporous Silica Spheres with Polyelectrolytes. A new type of core-shell spheres with confined internal volume were obtained by alternate adsorption of two bilayers of poly(allylamine hydrochloride) (PAH, MW ) 70 000)/poly(styrene sulfonate) (PSS, MW)70 000) onto the surface of Cd2+-absorbed MS particles via the LbL self-assembly technique. Typical adsorption conditions were 1 mg · mL-1 PAH in 0.5 mg · mL-1 NaCl, and 2 mg · mL-1 PSS in 0.5 mg · mL-1 NaCl. The PAH/PSS multilayer film is formed by the alternate adsorption pf poppositely charged polyions, beginning with deposition of the positively charged PAH onto the negatively charged MS particles. Approximately 1 mg of the Cd2+absorbed MS particles were dispersed in 10 mL of polyelectrolyte solution. The adsorption time was 15 min. After each adsorption step, the unadsorbed polyelectrolyte in solution is removed by repeated centrifugation and washing. Synthesis of CdS Nanoparticles in the Mesoporous Silica Microreactors. The confined pores of MS spheres can be used as the reactors for synthesis of nanoparticles. The Cd2+-absorbed MS particles were exposed to thioacetamide (0.01 M) solution, and then the mixture was dispersed with ultrasonic vibrations. After half an hour of reaction, the solutions turned yellow. So CdS nanoparticles were obtained in the pores of MS spheres. Dissolution of the Mesoporous Silica Core. The MS template cores were removed by exposure to a hydrofluoric acid (HF)/ammonium fluoride (NH4F) buffer (pH 5) for 5 min. Following several washing cycles, the decomposition products of the silica sphere were discarded, and then CdS-filled microcapsules were obtained. Characterization. Wide-angle (0-80°, 40 kV/200 mA) X-ray powder diffraction (XRD) data were recorded on a Rigaku D/max 2550 VB/PC diffractometer using nickel-filtered Cu KR radiation with wavelength λ ) 1.5406 Å. Transmission electron microscopy (TEM) was performed on a JEOL JEM 100CX transmission electron microscope at 100 kV that was used to examine particle size and shape. High-resolution electron microscopy (HRTEM, JEOL JEM-2100F) measurements were performed on CdS/silica samples that were dropcasted onto a carbon-coated Cu grid. The fluorescence experiment was carried out on a RF-5301PC spectrometer. Confocal laser scanning microscopy (CLSM) images of (PAH/PSS)2 capsules filled with CdS nanoparticles were obtained using a confocal laser scanning microscope (LSM 510, Carl-Zeiss Inc.).

Results and Discussion A typical procedure for the MS particles as microreactors for performing CdS nanocrystals synthesis exclusively inside the pores is shown in Figure 1. The CdS/MS (Figure 2a) and CdS/MS/PE (Figure 2b) composite particles were characterized by TEM observation. According to the TEM observations, both the composite particles had a uniform and spherical shape with an average particle diameter of approximately 1.2 µm. Some nanoparticles on the surface of the CdS/MS microspheres or in the surrounding solution can be clearly observed in Figure 2a, but nothing was present outside the CdS/MS/PE composite particles in Figure 2b. The nanoparticles formed outside the spheres were composed

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of small CdS particles. In fact, most of the CdS nanoparticles were synthesized in the pores of MS spheres. As illustrated in Figure 1, the pore-enlarged MS spheres were exposed to Cd(NO3)2 solution, resulting in immobilization of Cd2+ onto the Si-OH groups inside the MS microspheres to form Si-O-Cd bonds. The main driving force for Cd positive ions adsorption onto MS spheres is electrostatic interaction.24 It is known that the surface of MS is negatively charged above the isoelectric point (pH 2-3),25 while a Cd ion with two positive charges favors the adsorption process. Thus, Cd2+ is adsorbed to the Si-OH groups to form Cd-O-Si groups through electrostatic interaction. After they were coated with the PAH/PSS multilayers, the Cd2+-absorbed MS spheres of core-shell structure were dispersed to thioacetamide (TAA) solution to prepare the CdS nanoparticles. The CdS nanocrystals were grown in the pores of MS microspheres via the reaction between sulfion from the surrounding solution and Cd2+ adsorbed in silica hydroxyl groups. The possible reaction processes for the formation of CdS nanoparticles in the pores of MS can be summarized as follows:

Figure 2. TEM images for the CdS/MS (a), CdS/MS/PE (b) composite microspheres and polyelectrolyte capsules (c) filled with CdS nanoparticles.

CH3CSNH2 f CH3CN + H2S

(1)

Cd2+ + H2S f CdS + 2H+

(2)

nCdS f (CdS)n

(3)

During the chemical reaction, a large number of Si-O-Cd bonds would provide active positions for the formation of CdS nuclei. These freshly created nuclei would grow into large particles until they finally became stable. In this process, the Si-O-Cd bonds act as in situ spots and direct the growth of the primary CdS nanoparticles and their assembly into the MS spheres, similar to synthesis of CdSe nanoparticles in the pores of mesoporous silica microspheres.22 Synthesis of CdS nanoparticles can be exclusively carried out in the pores of MS spheres when the polyelectrolyte film is present, owing to its semipermeable properties (Figure 2b). CdS nanocrystals could be found outside the MS spheres when the MS sphere had not been coated with polyelectrolytes (Figure 2a). After removal of the porous silica cores, the CdS nanoparticles were trapped in the microcapsules (Figure 2c). The CdS nanoparticles could be seen as dark spots in Figure 2c. This result further confirmed that numerous CdS nanocrystals were successfully formed. Detailed structural investigations of the CdS nanoparticles were carried out via careful study of HRTEM images (Figure 3) and XRD patterns (Figure 4). From Figure 3, we can see the obvious crystal lattice structure of particles. The inset image in Figure 3 clearly demonstrates a single-crystalline structure with 0.35 nm lattice spacing, corresponding to the (100) interplanar distance of hexagonal CdS.26 Figure 4 shows the XRD patterns of CdS/MS (a) and CdS/MS/PE (b) composite microspheres. As can be seen from Figure 4a, five prominent peaks, which could be indexed to scattering from the (100), (002), (101), (110), and (112) planes for CdS,26 were detected in samples along with the broad feature at 2θ between 20° and 30° due to amorphous silica.17 This indicates that the CdS nanoparticles have the hexagonal crystal lattice, which is in good agreement with the HRTEM observation. The same information could be received from Figure 4b, but the CdS peaks were not clear due to the presence of the polyelectrolyte film outside the MS spheres. Maybe the polyelectrolytes have an influence on the diffractive peaks of CdS; the mechanism is not investigated in this paper. Figure 5 shows the fluorescence spectra of CdS/MS (Figure 5a) and CdS/MS/PE (Figure 5b) composite microspheres. In general for QDs, the optical properties mainly depend on the

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Figure 5. PL spectra of the CdS/MS (a) and CdS/MS/PE (b) composite microspheres.

Figure 3. HRTEM images of the (PAH/PSS)2 capsules filled with the CdS nanoparticles (inset, HRTEM image of the nanoparticles selected in the larger image).

Figure 4. XRD patterns of the CdS/MS (a) and CdS/MS/PE (b) composite microspheres.

distribution of particle size and shape and the nature of the QD surface.27 The sizes of the CdS nanocrystals were generally determined by the pore size of the MS spheres. It is known that the pore size of the pore-enlarged MS spheres consisted of two major pore sizes, 3.2 and 9.7 nm,22 so two different sizes of CdS nanocrystals could exist in the pores of the MS spheres. The diameters of CdS nanocrystals with different absorption peaks at 396 and 483 nm were calculated by the experiential equation28 as being 3.2 and 6.8 nm, respectively. The diameter of 3.2 nm most likely results from the small pores of 3.2 nm, and the diameter value of 6.8 nm most likely results from the large pores of 9.7 nm. The preparation of (PAH/PSS)2 microcapsules filled with CdS nanoparticles was visualized under a confocal microscope. The images (Figure 6a,b) reveal that CdS nanocrystals displayed fluorescence, and the aggregation of the capsules exhibited

Figure 6. Confocal laser scanning microscopy (CLSM) images of (PAH/PSS)2 capsules filled with CdS nanoparticles with good dispersion (a) or aggregation (b).

stronger fluorescence (Figure 6b) than the individuals with good dispersion (Figure 6a). Conclusion This work has described an effective and novel approach for carrying out inorganic nanoparticle synthesis exclusively inside

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the pores of MS spheres, owing to the controllable wall permeability of the polyelectrolyte multilayer, which makes them promising as the walls of microreactors. The CdS nanocrystals were almost perfectly inserted into the pores of the MS spheres and had good crystallinity, which created a fluorescence emission. Such composite semiconductor materials with fluorescence properties can be used in biological immunoassay and photoelectronic fields. Besides, both the CdS/MS/ PE composite particles and the CdS-filled capsules had a uniform and spherical shape. Further investigation may lead to the extension of this technique to prepare other composite materials or functional microcapsules involving functional inorganic nanoparticles that have uniform and spherical morphology. Acknowledgment. The authors gratefully acknowledge the National Natural Science Foundation of China (Grant 20676038), the Key Project of Science and Technology for Ministry of Education (Grant 107045), and the Shanghai Leading Academic Discipline Project (Project Number B502) for financial support.

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