© Copyright 2001 by the American Chemical Society
VOLUME 105, NUMBER 29, JULY 26, 2001
LETTERS Controlled Synthesis of CdS Nanoparticles inside Ordered Mesoporous Silica Using Ion-Exchange Reaction Zongtao Zhang,† Sheng Dai,*,† Xudong Fan,‡ Douglas A. Blom,| Stephen J. Pennycook,‡ and Yen Wei§ Chemical Technology, Solid State, and Metal & Ceramic DiVisions, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and Department of Chemistry, Drexel UniVersity, Philadelphia, PennsylVania 19104 ReceiVed: February 12, 2001; In Final Form: April 12, 2001
A new method of preparing CdS nanoparticles inside MCM-41 is described. This method makes uses of the unique interfacial properties of selectively functionalized ordered mesoporous materials to deliver cadmium ions inside the mesopores of MCM-41 via an ion-exchange reaction, thereby minimizing uncontrolled precipitation reactions of cadmium ions outside the mesopores. The general methodology can also be used to prepare other sulfide, telluride, and oxide nanoparticles.
The structure of MCM-41 consists of a hexagonal array of one-dimensional channels of uniform mesopores with pore diameters in the range of 25-100 Å.1 The perfect periodic nanostructures of these mesoporous oxides suggest that they could serve as ideal hosts for rational nanomanufacturing. The objective of several recent studies has been to develop a synthetic methodology of using ordered nanoporous oxide materials as generic nanoscale reactors for manufacturing and replicating technologically important nanomaterials.2-5 Two methodologies have been developed to transport precursor molecules or ions for assembly of nanoparticles or nanorods inside the channels of MCM-41 and related mesoporous oxides. The first methodology involves the direct impregnation of mesoporous materials with precursor molecules or ions,2-4 while the second one makes use of functional ligands to randomly functionalize both internal and external surfaces of mesoporous oxides, followed by loading of the precursor compounds through affinity interaction between the functional ligands and metal ions.5 Notably, Yang and co-workers4 have reported facile * To whom all correspondence should be addressed. † Chemical Technology Division. ‡ Solid State Division. | Metal & Ceramic Division. § Department of Chemistry.
assembly of silver nanorods inside SBA-15 through the direct impregnation and evaporation of silver ion sources. Ying and co-workers2 developed a novel gas-phase transport methodology to load MCM-41 with cluster compounds. The key drawback associated with these methodologies is the difficulty in controlling the location of the growth of nanoparticles and preventing their uncontrolled aggregation on external surfaces of MCM41. In this letter, we describe a precisely regulated method of assembling nanoparticles inside the mesopores of externally functionalized MCM-41 materials through the controlled transport of metal ions via ion-exchange reactions. This new methodology makes use of the unique mesoscopic structure of as-synthesized MCM-41, whose external surfaces can be selectively passivated with inert organic groups to eliminate the external nucleation sites but still retain the ion exchangeability of the internal pore surfaces. The ion-exchange reaction, eq 1, has been used extensively to prepare novel intercalate materials.6,7 The general reaction mechanism can be expressed as follows:
m(S-O-)M1+ + M2m+ ) (S-O-)mM2m+ + mM1+ (1) where (S-O-) is a surface anionic group in inorganic materials, such as the galleries of pillared materials, M1+ is a singly
10.1021/jp010541q CCC: $20.00 © 2001 American Chemical Society Published on Web 07/03/2001
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Figure 1. Schematic representation of a mesopore in as-synthesized ordered mesoporous materials.
charged cation in as-synthesized materials, and M2m+ is a new cationic species that needs to be intercalated into the preceding materials. This type of ion-exchange reaction has been previously used to organize CdS and ZnS clusters in crystalline zeolites with diameters less than 10 Å.8,9 Ordered mesoporous materials synthesized by the use of cationic surfactants have the same structural feature as those of ionic intercalate materials (Figure 1). In this case, the (S-O-) groups correspond to the SiO- anions on the nanopore surface, while M1+ is a cationic surfactant ion. The cationic surfactant ions are organized in the form of a cylindrical micellar structure, with hydrophilic positive ends Coulombicly interacting with negatively charged silica pore surfaces. The weak Coulombic interaction can be easily broken or replaced by another cation through ion exchange. In fact, this exchange reaction has been employed to remove surfactant molecules from the ordered inorganic materials synthesized through the surfactant-templating method.10 The surfactant ions can be removed using HCl, NaCl, NH4Cl, etc. For example, Badiei and Bonneviot have utilized the ion-exchange reaction between cetyltrimethylammonium (CTA+) and a copper complex to load Cu2+ into as-synthesized mesoporous silica.10a We have recently demonstrated the use of the ion-exchange reactions to deliver molecular imprints inside the mesopores.11 Therefore, the surfactant ions in the mesopores can be used as a unique vehicle to transport metal ions inside the nanoreactors. Accordingly, metal ions, such as Cu2+, Ni2+, Cd2+, Zn2+, Hg2+, and Ag+, can be loaded inside the nanoreactors through this ionexchange reaction. The unique property of this ion-exchange
Letters technique is that metal ions are transported only inside the pores. Very few metal ions are expected on the outer surfaces of the oxide substrates because there are no cationic surfactants in these places. Juan and Ruiz-Hitzky reported a novel method to selectively functionalize the external surfaces of MCM-41 materials with inert functional group, such as phenyl groups.12 This functionalization method leaves the inner wall of MCM41 materials intact because of the protection from surfactant cation (cetyltrimethylammonium-CTA+) while the external surface is capped with the inert hydrophobic phenyl groups. This capping of the external surface eliminates the possibility of adsorption of hydrophilic metal ions on the external surfaces during the ion-exchange reaction, thereby directing charged hydrophilic species into the mesopore channels. The transported metal ions can then be used as sources for nanoparticle assemblies inside the mesopores. Since the metal ions are located inside the nanopores, the sizes of the nanoparticles assembled are precisely controlled by the pore diameters. This nanoparticle growth can be viewed as a template growth process.13,14 Figure 2 shows a schematic diagram of the protocol used in such synthesis. Here, the formation of CdS nanoparticles via this method is reported. The ordered as-synthesized mesoporous silica hosts (MCM41) were prepared by a surfactant assembly pathway based on charge matching between surfactant and inorganic silica precursors.1,2 The typical procedure15 involves mixing cetryltrimethylammonium bromide (CTAB), water, and a base (NaOH). The molar ratio of CTAB: H2O: NaOH is 0.12: 130: 0.7. To this solution, tetraethyl orthosilicate (TEOS) was added at room temperature. The mixture was then heated at 100 °C for 24 h. The solid product was recovered by filtration. The powder X-ray diffraction scattering shows a peak at around 2θ ) 2°. The assynthesized MCM-41 was washed with excess ethanol five times for 5 h at room temperature in order to remove the water. The external surfaces of MCM-41 were then functionalized by immersion of MCM-41 in a phenyltrimethoxysilane ethanol solution (ethanol: phenyltrimethoxysilane ) 1:1) for 6 h at room temperature. Room temperature was used to ensure that the surfactant molecules were intact inside the mesopores during the functionalization of the external surfaces. Subsequently, the solid phase was washed with excess ethanol four times and dried.16 The loading of Cd2+ was achieved through the ionexchange reaction. The cadmium(II) methanol solution was prepared by dissolving 1.25 g of Cd(OAc)2•H in 100 mL of
Figure 2. Schematic diagram of transportation of metal ions into a mesopore and space confined growth of nanocrystals.
Letters
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Figure 3. Small-Angle XRD patterns of MCM-41 at various preparation stages: (A) as-synthesized MCM-41, (B) externally functionalized MCM41 (PhMCM-41), (C) Ph-MCM-41 loaded with Cd2+ via ion-exchange reaction (Cd-PhMCM-41), (D) PhMCM-41 loaded with CdS nanocrystals (CdS-PhMCM-41).
Figure 4. Wide-angle XRD pattern of CdS nanocrystals inside PhMCM-41.
MeOH. The resulting solution was added to 1.00 g of the modified as-synthesized MCM-41 material with the external surface functionalized with phenyltrimethoxysilane. The slurry of the salt precursor and MCM-41 was refluxed while being stirred for 4 h. The ion-exchanged MCM-41 was recovered by filtration and washed with heated MeOH and distilled water. The product was then dried at 100 °C for 4 h. Finally, treatment with H2S yielded the desired CdS nanoparticles inside MCM41. The powder X-ray diffraction patterns of the samples were recorded using a SIEMENS D5005 X-ray diffractometer, where Cu target KR-ray (λ ) 0.154 nm and operating at 40 kV and 40 mA) was used as the X-ray source. UV-vis diffuse reflectance spectra were measured on a Cary-14 scanning spectrophotometer converted by the on-line instrument system (OLIS) for data acquisition and analysis using a PC. Highresolution transmission electron microscopy (HRTEM) image analyses were performed using a HF-2000 electron microscope operating at 200 kV. Variations of small-angle powder X-ray diffraction (XRD) patterns for MCM-41 at various stages of preparation are shown in Figure 3. The XRD patterns for all three samples display four nice reflection peaks 100, 110, 200, and 210, which are characteristic of MCM-41 materials. Therefore, the hexagonal ordered structure was maintained during the formation process of CdS nanoparticles. Compared with the modified MCM-41 sample, all peaks shift slightly to lower angles for the nanoparticle-containing samples. This is probably due to enlargement
Figure 5. Diffuse-reflectance UV-vis absorption spectra of CdSPhMCM-41.
of the frameworks that may occur as a result of the ion-exchange reaction and the formation of CdS nanoparticles. The wideangle XRD pattern (Figure 4) for the sample containing CdS nanoparticles shows broad characteristic peaks belonging to the CdS nanoparticles.17,18 On the basis of the width of the diffraction peaks, the average size of the nanoparticles should be smaller than 2.5 nm. The UV-vis diffuse reflectance spectrum for MCM-41 containing the nanoparticles is shown in Figure 5. The onset of absorption is close to 450 nm, which
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Letters impregnation,2-5 which normally result in precipitation of impregnated materials on external surfaces of MCM-41 materials. This facile method of preparing CdS nanoparticles through ion-exchange reaction should be also applicable in synthesizing other semiconductor nanoparticles formed and dispersed in the pores of the MCM-41 materials. Other mesoscopic materials, such as as-synthesized MCM-48, can also be employed as the hosts for nanomanufacturing. Acknowledgment. This work was conducted at the Oak Ridge National Laboratory and supported by the Division of Materials Science, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC05-00OR22725 with UT-Battelle, LLC. References and Notes
Figure 6. TEM image of CdS-PhMCM-41.
is significantly blue-shifted relative to that of the bulk CdS. This increase in the band gap arises from the quantumconfinement effect. On the basis of the λonset-2R relationship,17,18 the average diameter of the CdS nanoparticles is estimated to be less than 2.5 nm. This is consistent with the estimation based on the width of the XRD peaks and the pore diameter (∼3.6 nm) of the MCM-41 host. Accordingly, CdS nanoparticles should be assembled inside the channels of MCM-41.19 By the reaction of the organosilane reagent with the assynthesized MCM-41, the external surface of the as-synthesized MCM-41 was functionalized by the inert phenyl group. Therefore, the affinity of this hydrophobic surface toward hydrophilic inorganic cations should be significantly reduced. This ensures that the loading of Cd2+ is achieved through the ion-exchange reaction of Cd2+ with CTA+ in the internal surfaces of the mesopores. This is consistent with the TEM investigation, which reveals that no CdS nanoparticles were found on the external surfaces. Figure 6 shows a typical TEM image of the MCM-41 loaded with CdS nanoparticles. Our controlled method for the assembly of nanoparticles is in sharp contrast to those of direct
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