A simple method of preparing Ag nanoparticles coated silica colloidal crystals and polymer-ag nanoparticles composite macroporous films

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1 Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) A simple method of preparing Ag nanoparticles coated silica colloidal crystals and polymer-ag nanoparticles composite macroporous films Zhimin Chen, Tian Gang, Yanfei Wang, Xin Chen 1, Cheng Guan, Junhu Zhang, Zhiqiang Sun, Kai Zhang, Bing Zhao, Bai Yang Key Laboratory for Supramolecular Structure and Materials, College of Chemistry Jilin University, Changchun , PR China Received 26 August 2005; received in revised form 31 October 2005; accepted 2 November 2005 Available online 27 December 2005 Abstract Using the Tollens-soaked sintered silica colloidal crystals as template, we developed a simple method of preparing Ag nanoparticles coated silica colloidal crystals (ANCSCC) and polymer-ag nanoparticles composite macroporous films. In our experiment, the Ag nanoparticles are mainly distributed on the surface of the silica spheres instead of in the voids of the colloidal crystals due to the Ag (NH 3 ) 2 + ions are mainly coming from the silica microspheres. As one of the application of such composite colloidal crystals, the ANCSCC are used as the surface-enhanced Raman scattering substrates and large enhancements were observed. The ANCSCC also can be used as template to construct more complex structures of polymer-ag nanoparticles composite macroporous films. It can be anticipated that except these samples can be used as Raman analysis and Raman image, our method introduced here will also provide a powerful platform for patterning other metal particles (such as Au or platinum) sensitized to the colloidal particles used in the colloidal crystals or preparing other novel composite structures due to its convenience and controllability Elsevier B.V. All rights reserved. Keywords: Ag nanoparticles; Silica microspheres; Composite macroporous films 1. Introduction During the past few years, colloidal crystals have been a topic of growing interest due to their potential applications as photonic crystals (PCs) [1]. In addition to their optical characters, colloidal crystals also have unique structural properties such as three-dimensional periodicity and large surface area, which make them useful for optical filter, switches, sensor, cavities, catalysis, as well as others in wide scientific and technical areas [1 3]. The character of the colloidal crystals is mainly based on the natural property of the constructional colloidal particles. Monodispersed silica and polystyrene (PS) microspheres can self-assemble into face-centered-cubic (fcc) or body-centeredcubic (bcc) ordered structure in a large area, so they are widely used in the construction of colloidal crystals. Although these structures exhibit interesting optical properties [4], they are mostly used as starting point for the preparation of more sophis- Corresponding author. Tel.: ; fax: address: byangchem@jlu.edu.cn (B. Yang). 1 Present address: Max Planck Institute of Microstructure Physics, Weinberg 2, D Halle/Saale, Germany. ticated systems, allowing the modification of their properties almost at will. For example, by voids infiltration, various macroporous and mesoporous materials have been made, including carbon [5], chalcogenides [6], NaCl [7], polymers [8], and silicagold [9]. Silica and PS spheres containing a metallic or magnetic core can be synthesized, allowing coupling the metallic and/or magnetic properties with the colloidal crystals [10]. Using patterned surfaces as substrates, the fabrication of prisms [11] and mcirofibers [12] made of PCs is possible. By infiltration of different materials in a multilayered way, Garcia-Santamaria has shown that band engineering is possible [13]. By use of twophoto polymerization [14], laser microannealing [15], and combined method of connective colloidal self-assembly and oxide infiltration by chemical vapor deposition [16], the introduction of controlled defects in colloidal crystals has been realized. Nanosized metal particles have unique electronic and optical properties that are substantially distinct from both the bulk phase and individual molecules, these nanoparticles would be potentially applied in biological labeling [17], surface-enhanced Raman [18], solar cell [19], as well as others [20]. The extension of diverse materials patterning method to metal nanoparticles is particularly attractive, as metals with ordered microstructures /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.colsurfa

2 38 Z. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) exhibit unique optical, electrical, magnetic, and catalytic properties. For example, silver nanoparticles filled colloidal crystal films have been used as surface enhanced scattering substrates and very large enhancements were observed [21]. Asher had reported their results of synthesis and utilization of monodisperse superamagnetic colloidal particles for magnetically controllable photonic crystals a few years ago [22]. Although the adaptation of the templating method to preparation of metal or metal and colloidal particles composite colloidal crystals is limited, several strategies have been reported as following: in the preparation of porous metal films, including precipitation/chemical conversion [23], direct penetration of metal nanocrystals [24], electroless deposition [25], electrochemical reduction method [26] and electrodeposition of materials in colloidal crystal template [27] have been used. By using layerby-layer (LBL) technique, Caruso has fabricated composite photonic crystals from semiconductor nanocrystals/polyelectrolytecoated colloidal spheres [28]. Xia has showed Asymmetric dimers formed by dewetting half-shells of gold deposited on the surfaces of spherical oxide colloids [29]. Using chemical vapor deposition method, macroporous platinum and palladiumplatinum alloy films have been formed by using polystyrene spheres as template [30]. Recently, an electroless plating method have been reported to fabricate two- and three-dimensional ordered structures of hollow silver spheres prepared by colloidal templating [31]. Previously we have prepared Ag midnanoparticles on Tollens-soaked silica spheres [32]. Here, we developed a simple method for preparation of Ag nanoparticle-coated silica colloidal crystals (ANCSCC) and polymer-ag nanoparticles composite macroporous films. We believe our method and structure have distinct difference from others as the following: (a) our method is relative simple, low cost and environment begin due to without complex steps, hydrophobic solution and expensive instruments during the whole process. (b) Using Tollens process and annealed silica colloidal crystals as template, robust multilayered ordered composite structures were achieved. (c) Ag nanoparticles can be formed mainly on the surface of the silica microspheres instead of in the voids of the colloidal crystals [33] due to the redox process mainly occurred on the microspheres. (d) The composite colloidal crystals can be further used as template to construct more complex structures of polymer-ag nanoparticles composite macroporous films. (e) Ag nanoparticles produced by our method in the composite macroporous films are mainly distributed around the inner pores instead of in the pore wall like most of other methods [9,27,33-34]. Due to the advantages and distinct structure mentioned above, we believe that our method will provide a powerful platform for patterning other metal particles (such as Au or platinum) sensitized to the colloidal particles used in the colloidal crystals. 2. Experimental 2.1. Materials Ag nitrate (AgNO 3 ; 99.8%), ammonium hydroxide (NH 4 OH; 25 wt% in water), sulfuric acid (H 2 SO 4 ) and glucose were analytical grade and used as received. Styrene was distilled under reduced pressure before use. 4,4 -isopropylidenediphenol bimethacrylate (BVA) was used as cross-linker and prepared in our laboratory. Azobisisobutyronitrile (AIBN) was recrystallized before used as initiator. Silica spheres were synthesized by the well-known Stöber method and washed two times with centrifugation/dispersion procedure. In all preparations absolute ethanol and Milli-Q water were used Preparation of stable silica colloidal crystals Silicon and cover glass wafers (8 mm 25 mm 1 mm) were boiled in an H 2 SO 4 and H 2 O 2 (7:3, v/v) mixture for 10 min to create a clean hydrophilic surface. A piece of pretreated silicon wafer and cover glass wafer were vertically placed together and then dipped into an 8 ml vessel containing 4 ml silica spheres (2.0 wt%) dispersed in ethanol. Following the evaporation of the ethanol, the colloids were packed orderly into colloidal crystals under induction of the capillary force. After the colloidal crystals have dried completely, the cover glass wafer was peeled off carefully and the colloidal crystals were almost entirely left on the silicon wafer perhaps due to their different surface properties. Then the colloidal crystals on the silicon wafer was sintered Fig. 1. Sketch of preparing ANCSCC and polymer-ag nanoparticles composite macroporous films with Ag nanoparticles distributed around the inner pores. Using the Tollens-soaked sintered silica colloidal crystals as template, ANCSCC were fabricated in step A. In step B, styrene was infiltrated into the voids of the colloidal crystals and polymerized. After etching away the silica particles, the polymer-ag nanoparticles composite macroporous films were formed in step C.

3 under 750 C for 6 h, this results in the formation of small necks between neighboring spheres. This pretreatment step makes the template very stable during the material patterning process Preparation of ANCSCC The as prepared stable colloidal crystals were immersed into 0.3 M aqueous solution of Ag (NH 3 ) 2 + ions for 2 h to ensure that the Ag (NH 3 ) 2 + ions permeated into the voids of the template and absorbed onto the silica spheres to the maximum extent. Then the colloidal crystals were directly rinsed several times by water to remove the excess Ag (NH 3 ) 2 + ions not absorbed to the silica spheres. After that, the colloidal crystals were put into 0.5 M glucose aqueous solution for 6 h to complete the redox reaction. Finally, the ANCSCC were formed Preparation of polymer-ag nanoparticles composite macroporous film Z. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) g BVA and g AIBN were dissolved into 5 ml styrene in a 20 ml vessel. Then the ANCSCC were immersed into the solution for 3 h. After that, the colloidal crystals were firstly prepolymerized under 50 C for 3 h and then continuously polymerized for 5 h at 75 C. When the polymerization has completed, put the composite colloidal crystals into an aqueous hydrogen fluoride solution (HF) for 4 h to remove the silica spheres. The obtained polymer macroporous film was then immersed into the Fig. 3. Photograph of colloidal crystals deposited on silicon wafers. From left to right, the samples are respectively corresponding to the silica colloidal crystals, the sintered silica colloidal crystals and the ANCSCC. Milli-Q water to remove the remained HF solution and dried at ambient temperature before characterization Characterization X-ray photoelectron spectroscopy (XPS) of Ag nanoparticles coated silica colloidal crystals on a silicon wafer was performed using a VG ESCALAB MKII spectrometer with an Fig. 2. XPS spectra of ANCSCC, which confirmed the formation of Ag nanoparticles on the microspheres of the colloidal crystals. Fig. 4. UV vis absorption spectra of the silica colloidal crystals deposited on the quartz wafer.

4 40 Z. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) Al K monochromatized X-ray source. The morphologies of Ag nanoparticles coated silica spheres were investigated with a JEOL FESEM JEM-6700F field emission scanning electron microscope (FESEM) with primary electron energy of 3 kv. The AFM images of ANCSCC were recorded with a commercial instrument (Digital Instruments, Nanoscope IIIa, Dimension 3100) in the tapping mode under ambient conditions at room temperature. UV vis absorption spectra were obtained on a Shimadzu 3100 UV vis near-ir recording spectrophotometer. For SERS measurements, 1,4-bis [2-(4-pyridyl)ethenyl]- benzene (BPENB) was selected as a model compound. The procedure to modify the ANCSCC substrates with BPENB is as follows. First, substrates were immersed in a mol solution of BPENB in absolute methanol for 20 min. Then, the substrates were taken out and dried under a stream of nitrogen. Raman spectra were measured in ambient air with a Reninshaw 1000 model confocal microscopy Raman spectrometer with a charged-coupled device detector and a holographic notch filter. The SERS excitation was provided by the nm line of a Coherent Radiation Innova Ar + laser. 3. Results and discussion Fig. 1 outlines the procedure of preparing the ANCSCC and the polymer-ag nanoparticles composite macroporous films. Before step A, silica colloidal particles with an average diameter of 220 nm firstly crystallized between the gap of the silicon wafer and the cover glass. In order to enhance the stability of the colloidal crystals during the material patterning process and easily remove the excess Ag (NH 3 ) 2 + ions in the following steps, we carefully peeled off the cover glass and sintered the colloidal crystals as described in the Section 2. Because the sintered step, the colloidal crystals still kept bright color in the whole process, indicating that the large area ordered structure was preserved. After that, the colloidal crystals were immersed into Ag (NH 3 ) 2 + ions solution and directly rinsed two times by water, then a redox reaction occurs in step A involving the reduction of Ag (NH 3 ) 2 + ions to Ag nanoparticles and oxidation of glucose to its corresponding acid. As a result, colloidal crystals turns from green to pale yellow, which implies Ag nanoparticles were formed on the silica spheres from the reduction of Ag (NH 3 ) 2 + ions. The formation of Ag nanoparticles also can be confirmed by X-ray photoelectron spectroscopy (XPS) as shown in Fig. 2. The Ag 3d binding energy peaks are centered at and ev, which is close to the literature value for Ag powders [35]. Then the ANCSCC were immersed into styrene solution and then polymerized to construct the composite structure, which has been clearly shown in step B. When the polymerization has been completed, put the composite colloidal crystals into an aqueous HF solution to remove the silica spheres as shown in step C. The Fig. 5. SEM images of the ANCSCC: (a) The packing of the ANCSCC; (b) a high-magnification image for a detail view of the ANCSCC, the insert shows the further high-magnification image of the ANCSCC; (c) the cleaved edge of the ANCSCC and (d) a high-magnification image for a detail view of the cleaved edgeof the ANCSCC.

5 Z. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) obtained polymer macroporous films were then immersed into the Milli-Q water to remove the remained HF solution and dried at ambient temperature. Fig. 3 shows the digital image of the samples prepared in the process outlined in Fig. 1. From left to right, the samples are respectively corresponding to the silica colloidal crystals, the sintered silica colloidal crystals and the ANCSCC. From Fig. 3, we can see that a uniform colloidal crystals film was formed on the silicon wafer after the cover glass was peeled off. The bright color of the colloidal crystals indicates the 3D ordered structures in the resulted colloidal crystals, which also can be proved by the normal incidence UV vis absorption spectra of the colloidal crystals deposited on the quartz wafer in Fig. 4. The color change of the colloidal crystals before and after the sintered procedure is mainly because the shrinkage of the silica spheres during the sintered process. Due to the formation of the Ag nanoparticles, the composite silica colloidal crystals show pale yellow. Fig. 5a shows the scanning electro microscopy (SEM) image of the resultant ANCSCC after step A in Fig. 1. Except for the existence of several cracks in the composite colloidal crystals, which probably were caused during the drying and sintered procedure, most of the Ag nanoparticles coated silica spheres assembled in an ordered hexagonal array. The ordered structure of the composite microspheres also can be confirmed in Fig. 5b and by AFM in Fig. 6a, illustrating that after the sintered process, the stability of the colloidal crystals is highly enhanced and the ordered structures were almost preserved in the whole experiment procedure. Although the composite microspheres in Fig. 5b shows an ordered array, we find that the silica microspheres in the colloidal crystals are not very uniform, which undoubtedly will affect the ordered structure and result some defects in the colloidal crystals as shown in Figs. 5a and 6a. The inset of Fig. 5b is a high-magnification image of the ANCSCC. Fig. 6b is the 3D view of AFM height image of the ANCSCC. We find that the surface roughness of each silica sphere is obviously increased due to the formation of about 20 nm Ag nanoparticles. These Ag nanoparticles distribute on the silica microspheres and form a similar structure like strawberry. Fig. 5c shows the cleaved edge of the ANCSCC, Fig. 5d is a high-magnification image of Fig. 5c. We could clearly see that the Ag nanoparticles were formed on the silica microspheres in the entire 3D ordered composite colloidal crystals. Unlike other methods, the nanoparticles are mainly deposited in the voids of the colloidal crystals. The Ag nanoparticles prepared in our experiment are mainly distributed on the silica spheres. The results are based on the following two factors: firstly, the silica microspheres are negative charged and Ag (NH 3 ) 2 + ions can be easily absorbed onto these microspheres. When the excess Ag (NH 3 ) 2 + ions were removed by the washing procedure, the Ag nanoparticles formed in the later experiment are mainly coming from the silica microspheres, which effectively avoid the formation of Ag nanoparticles in the voids of the templates and so produced Ag nanoparticles mainly on the silica spheres. In this regard, the present strategy for metal patterning is different from the precipitation/chemical Fig. 6. AFM images of the ANCSCC obtained in tapping mode: (a) Height image of the ANCSCC and (b) 3D height image of the ANCSCC. Fig. 7. Raman spectra of BPENB obtained on three different kinds of substrates with the same laser power of about 40 W: ANCSCC substrate (line a), Ag mirror film (line b) and Ag foil film (line c).

6 42 Z. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) Fig. 8. SEM images of the polymer-ag nanoparticles composite macroporous films: (a) the surface morphology of the polymer-ag nanoparticles composite macroporous films; (b) a detail view of the polymer-ag nanoparticles composite macroporous films and (c) Cleaved edge of the polymer-ag nanoparticles composite macroporous films. conversion method [23], the direct penetration of metal nanoparticles [24], or the electrochemical deposition method [26]. Use of the latter three methods invariably leads to a complete filtration of the voids in the template. Secondly, sintered silica colloidal crystals are used in the experiments. This procedure preserves the ordered structure during the entire fabrication process. The later factor is also important for the formation of the ANCSCC. This kind of metal dielectric film could also be prepared by the synthesis of metal-coated dielectric particles followed by their organized into an ordered structure by a self-assembly method. However, on the one hand, due to the large increase in the surface roughness after metal coating, organization of these composite core-shell particles into an ordered structure with a large area is difficult. On the other hand, the preparation of the stabilized nanoparticles and the composite core-shell particles has been very complicated [36]. The fabricated ANCSCC could be used as SERS substrates. To evaluate the SERS effect of the ANCSCC, silver mirror and silver foil were also selected as SERS substrates and modified with BPENB by the same treatment as described in the Section 2 for ANCSCC. The spectra derived from the three kinds of substrates are shown in Fig. 7. The peaks at 1180 and 1595 cm 1 are the characteristic peaks of BPENB. As we known, with the same experiment conditions, the stronger the spectrum is, the higher the SERS enhancement ability is. Therefore, the enhancement ability of the ANCSCC was estimated to be about 5 times larger than the Ag foil film and about 3.5 times larger than the Ag mirror film. The larger signal enhancement of the ANCSCC could be attributed to three possible reasons. The first is the large surface area of the ANCSCC film. As the ANCSCC is a three-dimensional composite colloidal crystal film, the surface area is much larger than that of a flat film. As a result, the number of molecules absorbed on the surface of the ANCSCC surface is larger than that absorbed on the other two substrates. A second reason should lie in the special structure properties of the ANCSCC film. As we can see that the basic framework of the ANCSCC film is a collection of close-packed spheres, which naturally form many crevices in the film. Crevices of this type are very efficient for the enhancement of Raman scattering according to the theoretical result reported by García-Vidal and Pendry [37]. Additionally, a third reason is that the spheres in the ANCSCC film have surfaces with roughness on the nanometer scale after the immobilization of metal nanoparticles. Such roughness can also enhance the Raman scattering [38,39]. The ANCSCC also can be used as template to construct the polymer-ag nanoparticles composite macroporous films as shown in Fig. 8. Although the uniformity of the pores is obviously decreased, the structure also shows interconnected

7 Z. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) macropores characteristic of the inverse opals. We attribute the ununiformity of the pores to two reasons. On the one hand, the ununiformity of the silica microspheres will decrease the uniformity of the polymer pores. On the other hand, the existence of the Ag nanoparticles will result the polymer infiltrated into the voids of the template becomes discontinuous in a certain degree and decrease the strength of the polymer films. So some polymer will detach from the films during the etching and washing procedure and decreased the uniformities of the pores and the channels. The detachment of the polymer also results the pores on different layers shown in the same image, and even if the pores are on the same layer, the open degree are not the same, so the pores displayed in Fig. 8a shows disordered. But from Fig. 8b, we find the whole composite macroporous pores films are still kept in an ordered hexagonal array in a certain degree. The Ag nanoparticles in Fig. 8b are mainly individually distributed around the inner pores except a little party of Ag nanopartilces aggregate together, illustrating that after etching away the silica particles, the Ag nanoparticles also can be stabilized by the pore wall. Fig. 8c shows the cleaved edge of the composite macroporous films. In the pores of the cleaved edge, we could still clearly observe some Ag nanoparticles. This illustrates that the Ag nanoparticles are distributed around the pore wall in the entire 3D macroporous composite films. 4. Conclusion Using the Tollens-soaked sintered silica colloidal crystals as template, we developed a simple method to fabricate ANCSCC using glucose solution as reducing agent. The fabricated ANC- SCC can be used as SERS substrate and large enhancement have been observed. 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