DOI: 10.1002/adma.200502105 Ordered Silica Microspheres Unsymmetrically Coated with Ag Nanoparticles, and Ag-Nanoparticle-Doped Polymer Voids Fabricated by Microcontact Printing and Chemical Reduction** By Zhimin Chen, Tian Gang, Xin Yan, Xiao Li, Junhu Zhang, Yanfei Wang, Xin Chen, Zhiqiang Sun, Kai Zhang, Bing Zhao, and Bai Yang* [*] Prof. B. Yang, Dr. Z. M. Chen, T. Gang, X. Yan, X. Li, Dr. J. H. Zhang, Y. F. Wang, Dr. X. Chen, [+] Z. Q. Sun, K. Zhang, Prof. B. Zhao Key Laboratory for Supramolecular Structure and Materials College of Chemistry, Jilin University Changchun, 130012 (P.R. China) E-mail: byangchem@jlu.edu.cn [+] Present address: Max Planck Institute of Microstructure Physics, Weinberg 2, 06120, Halle/Saale, Germany. [**] This work was supported by the National Nature Science Foundation of China (Grant No. 90401020, 20534040) and the program for Changjiang Scholars and Innovative Research Team in University (No. IRT0422). Supporting Information is available online at Wiley InterScience or from the author. Colloid particles usually have charges or nanoparticles uniformly distributed over their surface. The design and preparation of unsymmetrically coated colloid particles have been a long-standing challenge in surface and colloid science. [1 7] These particles would remedy some limitations of their spherical counterparts for potential applications in modeling the behaviors of highly irregular colloids that are more commonly found in industrial products, [8] in the fields that require lattices with lower symmetries and high complexities, and as potential building blocks in generating three-dimensional (3D) photonic crystals with complete bandgaps. [9] However, due to the thermodynamic limitations of the reaction, there have been only a few reports about the preparation of microspheres unsymmetrically coated with nanoparticles, which typically involve Langmuir Blodgett techniques, [1] the evaporation of metals on colloidal particles, [2 4] controlled phase separation, [5] and using the gas/liquid and liquid/solid interface action. [6,7] Although some of these methods are effective to fabricate unsymmetrically coated particles, challenges in this field still exist. For example, the assembly of these particles into ordered arrays, enhancement of the unsymmetrical coating density, and precise control of the tropism of these particles have not been well developed. Two-dimensional (2D) structured arrays and patterns are also important due to their potential applications in engineering microelectronic and optoelectronic devices, [10,11] the fabrication of biological and chemical sensors, [12,13] and for controlled crystallization. [14] Existing, elegant approaches that involve lithography, imprinting, and soft lithography techniques have been successfully applied to create patterned surfaces in microelectronic and plastic electronics. Among them, soft lithography [15] encompasses a set of flexible methods for patterning materials. As a branch of soft lithography, microcontact printing (lcp) has also been used to modify solid surfaces with different properties, such as charge nature [16,17] and wettability, [18 20] to direct the deposition of colloidal microspheres on special regions of substrates. Recently, lift-up soft lithography [21] and modified microcontact printing methods have been developed [22] to pattern colloidal crystals. In this communication, the fabrication of ordered silica microspheres unsymmetrically coated with Ag nanoparticles using lift-up soft lithography and chemical reduction is reported. Taking advantage of the flexibility of lcp, these microspheres are easily transferred onto polymer-coated solid substrates and precisely realize a tropism conversion. By etching away the silica microspheres, ordered Ag-nanoparticle-doped polymer voids are obtained. These silica microspheres unsymmetrically coated with Ag nanoparticles and Ag-nanoparticle-doped polymer voids can also be used as templates to fabricate ordered Ag-nanoparticle-doped polymer and gold composite voids with different morphologies. Compared with previous methods, it is believed that some progress in preparing ordered microspheres unsymmetrically coated with nanoparticles and nanoparticle-doped composite voids has been made, in that: a) the unsymmetrical coating density on the microspheres is increased, b) the ordered array of these unsymmetrically coated microspheres has been realized, c) the tropism of these ordered unsymmetrically coated microspheres can be well controlled, and d) ordered nanoparticle-doped polymer or polymer and metal composite voids with different morphologies can be easily obtained. Due to this progress, this method will provide a powerful platform for fabricating ordered, versatile, colloidal microspheres unsymmetrically coated with nanoparticles, and hybrid patterns. Figure 1 outlines the procedure for preparing ordered silica microspheres unsymmetrically coated with Ag nanoparticles and Ag-nanoparticle-doped polymer voids. First, monodisperse silica microspheres are assembled into colloidal crystals on a silicon wafer. Using the lift-up soft lithography technique, [21] a single layer of close-packed silica microspheres are then transferred onto the surface of a poly(dimethylsiloxane) (PDMS) stamp. After depositing Ag nanoparticles on these microspheres by chemical reduction [23] and spin-coating a thin 924 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18, 924 929
Figure 1. A schematic illustration of the procedure for the preparation of ordered silica microspheres unsymmetrically coated with Ag nanoparticles and Ag-nanoparticle-doped polymer voids. poly(vinyl alcohol) (PVA) film on a planar substrate, the PDMS stamp with ordered silica microspheres unsymmetrically coated with Ag nanoparticles is brought into contact with the PVA film under a certain pressure and heated at 100 C for 1.5 h. Careful peeling away of the PDMS stamp leaves the microspheres on the substrate. By etching away the silica microspheres with HF, ordered Ag-nanoparticle-doped polymer voids are finally obtained. Figure 2a shows the scanning electron microscopy (SEM) image of the ordered silica microspheres on the PDMS stamp transferred by the lift-up method. A single layer of hexagonally arrayed microspheres are clearly observed. These microspheres have a smooth surface and an average diameter of about 520 nm. Figure 2b is an SEM image of ordered silica microspheres unsymmetrically coated with Ag nanoparticles on the PDMS stamp. These silica microspheres are uniformly coated with Ag nanoparticles and also adopt an ordered hexagonal array, which illustrates that the silica microspheres on the PDMS stamp are robust and the ordered structure is perfectly preserved during the entire chemical reduction procedure. Due to a small portion of the microspheres being embedded in the PDMS stamp during the lift-up procedure and COMMUNICATIONS Figure 2. SEM images of a) a monolayer of silica microspheres on a PDMS stamp, b) silica microspheres unsymmetrically coated with Ag nanoparticles on a PDMS stamp; the inset is a magnified SEM image of the silica microspheres unsymmetrically coated with Ag nanoparticles. c) Raman spectra of 1,4-bis[2-(4-pyridyl)ethenyl]benzene obtained on the ordered silica microspheres unsymmetrically coated with Ag nanoparticles (lines I), Ag mirror film (line II), and Ag foil film (line III)with the same laser power. Adv. Mater. 2006, 18, 924 929 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de 925
the hydrophobic property of the PDMS, the [Ag(NH 3 ) 2 ] + ions are likely to concentrate at the part of the microspheres left outside, which results in a higher unsymmetrical coating density of Ag nanoparticles on the silica microspheres during the chemical reduction procedure. The inset of Figure 2b is a magnified SEM image of the microspheres. It can be clearly seen that the Ag nanoparticles, with diameters ranging from 30 to 40 nm, are homogeneously distributed on the silica microspheres and form a structure similar to a strawberry, which causes the surface roughness of the silica microspheres to increase. Due to the uniformity of the Ag nanoparticles and the ordered arrays of the composite microspheres, these ordered microspheres can be used as substrates for surface-enhanced Raman scattering (SERS). Figure 2c shows the Raman spectra of 1,4-bis[2-(4-pyridyl)ethenyl]benzene (BPENB) obtained on the composite microspheres, on an Ag mirror, and on Ag foil films with the same laser power of about 20 mw. In the figure, the ordered silica microspheres unsymmetrically coated with Ag nanoparticles provide the strongest Raman enhancement with the smallest background signal. The intensity derived from these silica microspheres unsymmetrically coated with Ag nanoparticles (lines I) is about 10 times larger than that from the Ag mirror film (line II), and about 15 times larger than that from the Ag foil film (line III). In addition to the great enhancement of the Raman signal, the dispersion of the signal measured at different points is very small (see lines I). This means that the same results can be obtained at any point on the substrate, which is very important for techniques such as SERS imaging. [24] Figure 3a is an SEM image of the silica microspheres unsymmetrically coated with Ag nanoparticles that have been transferred onto a polymercoated surface using the developed lcp method. [22] The Agnanoparticle-coated sides are embedded into the polymer film and the uncoated sides are left outside. Except for some Ag nanoparticles that are closely distributed around the silica microspheres, which can be attributed to the detachment of some Ag nanoparticles when the composite microspheres are pressed into the polymer film, no Ag nanoparticles are observed on the upper surface of the silica microspheres. This proves that the method is efficacious to produce ordered silica microspheres unsymmetrically coated with Ag nanoparticles, and the tropism of these particles can be perfectly transformed. Because the PDMS stamp must be swollen before the transferring procedure, which facilitates the procedure and quality, the unsymmetrically coated microspheres observed in Figure 3a are non-close packed. If the PVA film spin-coated onto the planar substrate is thin, or the pressure applied between the PDMS and the planar substrate is low during the procedure of transferring the composite microspheres to the polymer film, the silica microspheres unsymmetrically coated with Ag nanoparticles will not be completely transferred onto the PVA film. Figure 3b shows the SEM image of silica microspheres unsymmetrically coated with Ag nanoparticles that are transferred onto the PVA-coated silicon wafer under low pressure. Compared with the microspheres in Figure 3a, except for some composite microspheres that are not transferred Figure 3. a) SEM image of the silica microspheres unsymmetrically coated with Ag nanoparticles that have been transferred onto the PVA spin-coated silicon wafer, the inset shows the morphological details of these transferred composite microspheres. b) SEM image of the silica microspheres unsymmetrically coated with Ag nanoparticles that were transferred onto the PVA spin-coated silicon wafer under low pressure, the insert is a magnified SEM image of the sub-micrometer-sized Ag dots. onto the PVA film, the transferred microspheres are only stuck onto and not embedded in the PVA film. On the surface of the PVA film not covered in microspheres, there are a lot of sub-micrometer-sized dots with an average diameter of about 270 nm, which are clearly shown in the insert of Figure 3b. These dots are composed of Ag nanoparticles and have a periodicity of about 520 nm. From these dots, it can be estimated that although some of the ordered silica microspheres unsymmetrically coated with Ag nanoparticles are not transferred onto the PVA film during the experimental procedure, these microspheres still make contact with the substrate and leave Ag nanoparticles on them. Figure 4a is the SEM image of the ordered Ag-nanoparticle-doped polymer voids after the silica microspheres in Figure 3a have been etched away. The insert of Figure 4a presents the morphological details of these composite voids. It can be seen that except for where more Ag nanoparticles are located at the edge, which accords with the result in Figure 3a, the Ag nanoparticles in the whole voids are uniformly distributed. This illustrates that these composite voids are the exact replica of the 926 www.advmat.de 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18, 924 929
Figure 4. a) SEM image of the ordered Ag-nanoparticle-doped PVA voids; the inset shows the distribution of the Ag nanoparticles in the voids. b) 2D AFM height image and the cross-section analysis of the ordered Ag-nanoparticle-doped PVA voids. composite microspheres. The relative 2D atomic force microscopy (AFM) image and the corresponding cross-section analysis in Figure 4b shows that the depth of the composite voids is about 310 nm, which suggests that the majority of each microsphere is embedded into the polymer film. In addition to constructing the Ag-nanoparticle-doped polymer voids, these ordered composite microspheres and voids can be further used as templates to fabricate more complex structures. Figure 5a displays the ordered Ag-nanoparticle-doped PVA and gold composite voids obtained by using the patterns in Figure 4a as a template and after evaporation of a layer of gold Figure 5. SEM images of a) the ordered Ag-nanoparticle-doped PVP and gold composite voids obtained by directly evaporating a layer of gold film onto the Ag-nanoparticle-doped PVP voids in Figure 4a. b) The silica microspheres unsymmetrically coated with Ag nanoparticles that have been transferred onto the PVA spin-coated silicon wafer and then coated with an evaporated layer of gold film. The insert shows the morphological details of the evaporated gold film. c) The ordered Ag-nanoparticle-doped PVP and gold composite voids obtained by etching away the silica microspheres in (b); the inset shows the morphological details of the gold film collapsed into the voids. film. The insert of Figure 5a shows the morphological detail of the corresponding voids. In comparison with Figure 4a, except for a dense layer of gold film uniformly distributed in and out of the voids, the morphology of the patterns is almost unchanged. Figure 5b shows the microspheres in Figure 3a after being coated with an evaporated layer of gold film. The Adv. Mater. 2006, 18, 924 929 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de 927
newly formed film is composed of gold nanoparticles with diameters ranging from 15 to 30 nm and uniformly spreads over the entire microsphere and PVA film, which is clearly seen in the insert of Figure 5b. If the silica microspheres are etched away, the gold film on the silica microspheres in Figure 5b collapses and falls into the voids. Figure 5c is an SEM image of the ordered composite voids obtained under these conditions. Compared with the voids in Figure 5a, the gold film in the voids observed here is not uniform. The insert of Figure 5c is a magnified SEM image of these composite voids. It can be seen that most of the gold film in the voids is cracked and segmented. This is mainly because some of the gold film on the silica microspheres has collapsed and fallen into the voids during the etching procedure; other areas of the gold film on the silica microspheres have cracked and dispersed into the solution. However, the gold film between the composite voids remains intact, which illustrates that the collapse of the gold film is mainly due to the lack of support from the microspheres. With respect to the different morphologies of the gold film in Figures 5a,c, the Ag nanoparticles are still only distributed in the voids instead of over the whole patterned film, which indicates that Ag-nanoparticle-doped composite films with different morphologies can be obtained through simply changing the experimental procedure. Recently, although versatile nanopatterned surfaces have been obtained by the colloidal-assisted capillary nanofabrication method, [25] and ordered composite voids have been achieved by the colloidal-crystal-assisted imprint method, [26] to realize the deposition of metal nanoparticles only into the voids and the easy tuning of the morphology of the film in the voids is not possible. In conclusion, using a modified lcp method and chemical reduction, ordered silica microspheres unsymmetrically coated with Ag nanoparticles and Ag-nanoparticle-doped polymer and gold composite voids have been prepared. In the experiment, not only are the unsymmetrical coating densities of the Ag nanoparticles on the silica microspheres enhanced, the tropism of these microspheres can also be well controlled. A non-close-packed array of spheres with designable lattice structures on solid substrates has recently been successfully fabricated by using solvent-swelling and mechanical deformation behaviors of a PDMS stamp: [27] the arrays of silica microspheres unsymmetrically coated with Ag nanoparticles and Ag-nanoparticle-doped polymer and gold composite voids can also be easily tuned. Although only silica microspheres unsymmetrically coated with Ag nanoparticles are presented here, this method undoubtedly can be extended to other nanoparticles or catalysts. These functionally patterned structures may have a variety of potential applications in SERS, nanofabrication, nanosensors, and the control of crystallization. Experimental Monodisperse silica spheres with an average diameter of 520 nm were prepared in ethanol according to the Stöber method [28]. The silica spheres were centrifuged and redispersed in ethanol. The colloidal crystals were formed by the evaporation of a suspension, which was based on the method described by Micheletto and collaborators [29]. The deposition of Ag nanoparticles onto the silica microspheres was similar to our previous work [23]. More specifically, the PDMS stamp containing the transferred silica microspheres was immersed in a 0.3 M [Ag(NH 3 ) 2 ] + ion solution for 4 h and dried under N 2 flow. The substrate was then immersed in a 0.5 M glucose solution for 6 h at 40 C to reduce the [Ag(NH 3 ) 2 ] + ions to Ag nanoparticles. After that, the PDMS stamp containing the silica microspheres unsymmetrically coated with Ag nanoparticles was brought into contact with the PVA-coated substrate under a pressure of about 0.2 10 5 Pa and the system was heated at 100 C for 1.5 h. The PDMS stamp was then carefully peeled away and the silica microspheres unsymmetrically coated with Ag nanoparticles were transferred onto the polymer-coated substrate. To remove the silica microspheres and form the ordered Ag-nanoparticle-doped polymer voids, the substrate was immersed in a HF solution (3 wt.-%) at ambient temperature for 20 min To construct the ordered Ag-nanoparticle-doped PVP and gold composite voids, the silica microspheres unsymmetrically coated with Ag nanoparticles or the Ag-nanoparticle-doped PVP voids were coated with an evaporated layer of gold film. The morphologies of the silica microspheres unsymmetrically coated with Ag nanoparticles and the ordered Ag-nanoparticle-doped polymer voids were investigated with a JEOL FESEM JEM-6700F field-emission scanning electron microscope with a primary electron energy of 3 kv. The AFM images of the samples were recorded with a commercial instrument (Digital Instruments, Nanoscope IIIa, Multimode) in the tapping mode under ambient conditions at room temperature. For SERS measurements, BPENB was selected as a model compound. The procedures to modify the silica microspheres unsymmetrically coated with Ag nanoparticles, the Ag mirror, and the Ag foil substrates with BPENB were as follows. First, substrates were immersed in a 1.0 10 4 mol solution of BPENB in absolute methanol for 20 min. The substrates were then taken out and dried under a stream of nitrogen. 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