Ordered Array of Gold Nanoshells Interconnected with Gold Nanotubes Fabricated by Double Templating**

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1 DOI: /adma Ordered Array of Gold Nanoshells Interconnected with Gold Nanotubes Fabricated by Double Templating** By Wen Dong, Han Dong, Zhenlin Wang,* Peng Zhan, Ziqin Yu, Xiaoning Zhao, Yongyuan Zhu, and Naiben Ming Metal nanostructures are of great interest because of their important applications in catalysis, [1] sensing, [2] surface-enhanced Raman scattering (SERS), [3] optoelectronics, [4] information storage, [5] and optics. [6] Further processing of these nanostructures into ordered arrays or entities with a hollow interior is technically important as it could lead to a significant improvement of their optical, catalytic, biosensing, or SERS performances. [6 9] For example, ordered metal microstructures have been demonstrated to exhibit a photonic bandgap in which the propagation of electromagnetic waves is prohibited. [7] In another example, Halas and co-workers have demonstrated that the surface plasmon resonance (SPR) of gold nanoshells can be tuned from the visible to near-infrared region of the electromagnetic spectrum. [10] For potential applications, it is highly desirable to develop new strategies to arrange hollow metal units into ordered structures and to allow control of their separation on a nanometer scale. Hollow metal nanostructures are often prepared by templating against sacrificial templates, such as colloidal microspheres, [11 18] or channels in anodized aluminum oxide and track-etched polycarbonate membranes. [19,20] A convenient method to prepare ordered metal nanomaterials is to deposit metal against colloidal crystal templates. [21 23] Although new strategies have been developed to fabricate isolated units of metallic hollow nanostructures, [24] there are few methods that can allow the preparation of highly monodisperse hollow metal nanostructures and their ordered arrays. [12,15,25,26] One of the successful methods is the so-called lost-wax approach demonstrated by Colvin and co-workers. [27] This method uses a silica colloidal crystal as the starting template to create a macroporous polymer membrane. Highly monodisperse and ordered inorganic, polymeric, and metallic hollow nanostructures can be generated within the uniform voids in the membranes. [27] [*] Prof. Z. Wang, Dr. W. Dong, H. Dong, Dr. P. Zhan, Z. Yu, X. Zhao, Prof. Y. Zhu, Prof. N. Ming National Laboratory of Solid State Microstructures Nanjing University Nanjing (P.R. China) zlwang@nju.edu.cn [**] This work was supported by a grant for the State Key Program for Basic Research of China and by NSFC under Grant Nos , , and Z. L. Wang is grateful for the Distinguished Youth Foundation of NSFC. In this communication, the preparation of a novel ordered gold network with hollow interiors by a two-step replication procedure is reported. A non-close-packed (NCP) silica colloidal crystal, first introduced by Fenollosa and Meseguer, [28] is used as the primary template. Macroporous polystyrene (PS) membranes are prepared by replication of the NCP silica opals, which contain spherical voids that are interconnected with nanochannels. Monodisperse hollow gold spheres interconnected with gold nanonecks with a hollow interior are electroless-plated within the PS membranes. More importantly, an indirect seeding method is adopted so as to confine plating mainly to the void surface of the PS template. A wide reflectance minimum band is observed in the near-infrared specular reflectivity spectra of the gold nanoshell/nanotube networks. It is believed that these highly ordered nanostructures may have applications in areas such as plasmonics, biophysics, and nanophotonics. The method to fabricate a NCP gold nanoshell network involves several distinct steps, which are summarized in Figure 1. First, a high quality silica colloidal crystal is prepared. Several successful approaches can be used to self-organize monodisperse silica microspheres into a colloidal crystal. [29] Recently, Ozin and co-workers [29c] developed an isothermal heating evaporation-induced self-organization technique for the assembly of large silica microspheres. Here, a modification of a microchannel method is adopted [29a] to prepare monolayer and multilayer colloidal crystals with large single domains from silica spheres with diameters near or larger than one micrometer. Second, NCP silica templates are fabricated, following the method reported by Fenollosa and Meseguer. [28] In short, the close-packed silica colloidal crystal is first sintered at a high temperature and then chemically etched with HF acid. After that, a thin layer of gold of about 5 8 nm in thickness is thermally evaporated onto the surfaces of the NCP template in step c. After the seeding process, a freestanding macroporous PS membrane is prepared by encapsulating the NCP silica template with PS, followed by removal of the silica template. The prepared PS membranes have uniform spherical pores that are interconnected with nanochannels, due to the presence of silica nanonecks between adjacent silica spheres. Another important feature of the PS template is that, after this replication, the gold nanoseeds are transferred from the surface of the NCP silica template onto the inside walls of the polymer film. These as-deposited metal nanoparticles are accessible Adv. Mater. 2006, 18, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 755

2 Figure 1. Schematic of the fabrication of a gold nanoshell/nanotube network. Figure 2. SEM images of the templates obtained in the experiment. a) 2D close-packed silica colloidal crystal (spheres of 1550 nm in diameter). b) NCP silica colloidal crystal. c) Macroporous polystyrene membrane containing spherical voids interconnected via channels. The voids have small openings on the membrane surface, which corresponds to the bottom side where the spheres are in contact with the substrate. from the openings on the surface of the PS membrane and act as reactive sites during the electroless-plating process. In step e, the polymer membrane is immersed in an electroless plating bath. Gold is electroless-deposited on the void surface of the PS template. [19] Continuous gold shells can be formed within the pores of the polymer membrane. Finally, the whole structure is immersed in a solution of chloroform, wherein the PS membrane is dissolved, and a gold nanoshell/ nanotube network is left. Figure 2 shows the scanning electron microscopy (SEM) images of the templates obtained at three distinct stages. Figure 2a is a two-dimensional (2D) close-packed 1550 nm diameter silica colloidal crystal template in which the adjacent silica microspheres are in touch with each other. Shown in Figure 2b is a typical sample of an NCP silica colloidal crystal template that is obtained by sintering the close-packed crystal at 1000 C for 3 h and after etching in an aqueous solution of 1 % HF for 15 min. It is seen that narrow necks are formed between neighboring nanospheres after etching. Silica nanonecks can be seen clearly in the SEM image of the 2D NCP colloidal crystal under higher magnification (Fig. 2b inset). Since chemical etching is isotropic for silica spheres, the spherical shape of the parent silica beads is well preserved after etching (Fig. 2b). This allows the preparation of NCP templates with different structure parameters by controlling the sintering temperature and etching time. Figure 2c shows the SEM image of a macroporous PS membrane with its bottom-side up. It is seen that an ordered array of small circular openings is created on the surface of the PS nanomold, where the original silica spheres were in contact with the silicon substrate. Both the void size and the dimensions of the windows that join adjacent voids are dictated by the NCP silica template. The spatial period of the opening array (Fig. 2c) has been measured and a 4 5 % shrinkage has been found after removal of the NCP silica WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18,

3 Figure 3. SEM images of 2D gold nanoshell/nanotube networks. The corresponding NCP silica colloidal crystal templates were obtained from identical close-packed silica templates with the same sintering time (t sint = 3 h) but different sintering temperatures (T sint ), followed by different etching durations (t etch ). The measured diameter (d shell ) of the gold nanoshells and the length (l tube )anddiameter(d tube ) of the gold nanotubes are: a) d shell =1338± 33 nm, l tube =136± 12 nm, and d tube =238± 15 nm for T sint = 950 C and t etch =10 min,b) d shell =1305± 28 nm, l tube =172± 15 nm, and d tube =185± 16 nm for T sint = 950 C and t etch =15 min, and c) d shell =1280± 29 nm, l tube =206± 13 nm, and d tube =202± 18 nm for T sint =1000 Candt etch = 15 min. The upper inset in (a) shows a higher-magnification SEM image and the bottom inset shows a transmission electron microscopy (TEM) image of the bottom side of the same sample. template, as compared with that of the NCP silica colloidal crystal (Fig. 2b). Figures 3a c show the SEM images of three hollow gold shell/tube networks with different structure parameters. The samples are prepared by electroless plating for the same duration, but macroporous PS membranes with different void sizes are used. It is seen that the replicated gold nanoshells and nanotubes have a continuous and uniform wall and that these networks have a highly ordered structure. Shown in the upper inset of Figure 3a is the SEM image of the bottom side of the same sample, from which it is seen that each gold nanoshell has a small opening, as do the voids in the PS matrix. The hollow nature of the gold necks can be seen from the transmission electron microscopy (TEM) image presented in the bottom inset of Figure 3a, in which the pale region along the neck as well as in the central parts of spheres forms a large contrast relative to their dark edges. The wall thickness of the gold structure is estimated to be about 40 nm by TEM analysis. Thus, the hollow gold nanospheres in the 2D metal networks are not complete shells but look like nanocups that are interconnected with nanotubes at their equator. This kind of geometry may allow these microstructures to be used as nanocontainer arrays for the storage of nanomaterials, which could be useful for parallel analysis in the field of biophysics. [30] The PS nanomolds used for the synthesis of the samples in Figure 3a c, notated as S1, S2, and S3, respectively, are replicated from three NCP silica templates with different structure parameters. For sample S1 shown in Figure 3a, the NCP silica template is obtained by first sintering a 2D array of a closepacked silica sphere (1550 nm in diameter) template at a temperature of T sint = 950 C for 3 h, followed by etching in HF for t etch = 10 min. A two-step replication produced a network of NCP gold nanoshells with a diameter d shell = 1338 nm and nanotubes with a length l tube = 136 nm and a diameter d tube = 238 nm. When the duration of etching is increased to 15 min while keeping the other treatment conditions the same as for S1, double replication of the resultant NCP template leads to the product S2 with d shell = 1305 nm, l tube = 172 nm, and d tube = 185 nm. For NCP opals, the longer the etching time, the smaller the silica nanospheres and nanonecks in diameter, and the longer the remaining silica nanonecks. [28] As a consequence, a longer time of etching results in a simultaneous decrease in the diameters of the gold nanoshells and nanotubes, whereas it leads to an increase in the nanotube length after replications. On the other hand, when the sintering temperature is increased from 950 to 1000 C while keeping the other conditions the same as for S2, double replication produced a NCP gold nanoshell network (S3) with an increase in the dimensions of the nanotubes (l tube = 206 nm and d tube = 202 nm) but a small decrease in the diameter of the nanoshells (d shell = 1280 nm). It is believed that sintering a close-packed silica template at a higher temperature will enhance the interpenetration of adjacent nanospheres. Therefore, for the same etching time, the NCP silica template has a wider and longer neck compared with that obtained at a lower sintering temperature. The method mentioned above has also been used to fabricate multilayered NCP gold nanoshells. Figure 4 presents the SEM image of one of the samples, which is a double layer of interconnected hollow gold nanospheres. As in the monolayer Adv. Mater. 2006, 18, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 757

4 Figure 4. SEM image of a multilayered array of NCP gold nanoshells interconnected with gold nanotubes. case, nanoshells in the top layer have a small opening. The wall thickness of the fabricated hollow gold particles is relatively uniform. However, some particles in the second layer are observed to have a larger opening. It is suggested that this could be due to a very low density of nanoparticle seeds in the corresponding area of the NCP silica templates. It has been observed that thermal sputtering results in a highly non-uniform distribution of metal particles on the silica sphere surface, due to a nanosphere masking effect. [25] In this case, the sputtered gold layer is very thin, with a thickness less than 5 8 nm on top of the NCP silica microspheres. Thus, the presence of nanoparticle seeds should be extremely sparse on the opposite sides of the NCP silica template. It is expected that the implementation of uniform seeding methods [10a,12c,19] on the colloidal particle surface could allow improvement in the homogeneity of the prepared gold nanoshells. Another possible reason for the incomplete shell structure in the second layer could be due to a low transport rate of reaction ions to the deep voids, as these voids are only accessible through nanowindows to those cavities in the top layer, which have relatively larger openings on the membrane surface. The prepared hollow gold networks exhibit strong optical scattering effects under illumination. Some preliminary optical measurements of one of the 2D gold nanoshell/nanotube ordered networks have been made. Near-infrared specular reflectance is measured over the wavelength range from 1400 to 2600 nm for two small angles of incidence, h= 10 and 20 (Fig. 5a,b). Measurements are made using S- and P-polarized light, of which the electromagnetic (EM) wave electric field is perpendicular to and parallel with the plane of incidence, respectively. All spectra are normalized with that of a silver mirror. The reflectivity shows a remarkable modulation in the spectrum of interest at a wavelength that is comparable with the spatial period of the network. Under the off-normal incidence, a reflectance minimum with a wide band is observed in the data for both polarizations. The effect is related to the collective EM wave scattering and propagation of surface plasmons in these metal microstructures. At h = 10, the minimum is at 1920 nm for S-polarization and 1930 nm for P-polarization. This small difference in wavelength location is consistent with the hexagonal symmetry of the structure that is nearly Figure 5. Near-infrared specular reflectance spectra of a 2D gold nanoshell/nanotube network under a) S-polarization and b) P-polarization at two off-normal incidence angles: h = 10 and 20. The sample is the same as that shown in Figure 3b. isotropic in the x y plane for a small angle of incidence. However, when the angle of incidence is increased to h = 20, different effects are observed for the two polarizations. For the S-state, the minimum location in the transmittance is almost unchanged and this angle change only leads to a red-shift of about 10 nm in the spectrum. For the P-state, however, the previously observed minimum is shifted to a longer wavelength of 2000 nm with a 70 nm red-shift. It is also noted that a new weak reflectance band evolves at 1650 nm for the P- state. Numerical simulation that takes scattering and surface plasmon excitation in the gold nanoshells and nanotubes into account is needed in order to explain the observed strong modulations of the reflectivity and its different response with the angle of incidence. In summary, a method for the fabrication of ordered hollow gold networks composed of interconnected gold nanoshells and nanotubes is demonstrated by using a combination of double templating and electroless plating. It is believed that the above double-templating method can be applied to the synthesis of other metal hollow networks that have a similar microstructure. The prepared samples may find potential applications as substrates for the enhancement of Raman scattering. They can also serve as physical systems to study plasmon excitation in such an ordered metal microstructure. [31] Other possible applications could include realization of a tunable photonic bandgap in the infrared. [32] WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18,

5 Experimental Commercial hydrogen tetrachloroaurate(iii) (HAuCl 4, 99.9 %), sodium hydroxide (NaOH, 99.5 %), sodium chloride (NaCl, 99.5 %), tartaric acid (C 4 H 6 O 6, 99.5 %), chloroform (CHCl 3, 99.0 %), hydrofluoric acid (HF, 40 %), toluene (C 6 H 5 CH 3, 99.5 %), ethanol, and deionized water were used in the experiment. Monodisperse silica microspheres with a diameter of 1550 nm (size dispersion of 1.9 %) were purchased from Duke Company. Prior to use, the aqueous dispersion of silica beads was diluted with deionized water. The ordered close-packed silica colloidal crystal was prepared by infiltrating a solution of the colloidal dispersion into a horizontally placed channel that was formed between two parallel slides, using a silicon wafer as the bottom substrate. The colloidal dispersion solution was allowed to dry in air. Large-area, highly ordered, close-packed silica colloidal crystals were grown on the silicon substrate. The closepacked template was sintered at a high temperature and then etched in a 1 % aqueous solution of HF. NCP silica colloidal crystals were prepared in this way. After these processes, a thin layer of gold nanoparticles with a thickness of 5 8 nm was sputtered onto the surface of the NCP silica template. A few drops of toluene solution of polystyrene were then dropped into the gold-nanoparticle-seeded NCP silica template. After evaporation of the toluene, the silica template was embedded within a matrix of polystyrene. The composite structure was further shifted to a solution of HF to etch the silica. After removal of the silica microspheres, freestanding macroporous PS membranes were obtained that contained NCP spherical voids interconnected with narrow windows. Subsequently, the PS nanomold was dipped into a plating solution that was prepared by mixing solutions A and B in a volume ratio of 10:3. 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