General Synthesis of 2D Ordered Hollow Sphere Arrays Based on

Feb 4, 2010 - Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, ... A general strategy, nonshadow deposition dominated coll...
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General Synthesis of 2D Ordered Hollow Sphere Arrays Based on Nonshadow Deposition Dominated Colloidal Lithography Guotao Duan,* Fangjing Lv, Weiping Cai,* Yuanyuan Luo, Yue Li, and Guangqiang Liu Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, Anhui, P.R. China Received October 29, 2009. Revised Manuscript Received December 22, 2009 A general strategy, nonshadow deposition dominated colloidal lithography (NSCL), was proposed for the synthesis of twodimensional (2D) ordered hollow sphere arrays of conductive materials. Gold, polypyrrole, CdS, and ZnO were taken as model materials to demonstrate the NSCL strategy, and built as 2D hollow sphere arrays successfully. In this strategy, a thin gold coating is first introduced on a polystyrene sphere (PS) colloidal monolayer via ion-sputtering deposition, and a hollow sphere array can thus be obtained by further electrochemical deposition on such a monolayer and by subsequent removal of PSs. The proposed strategy is flexible and facile to control the microstructure and size of the hollow sphere array, and the features are as follows: (i) controllable shell of the hollow sphere from single-layer to multilayer with single or multiple compositions, (ii) tunable morphology from simple structure to hierarchical micro/nanostructure, and (iii) changeable arrangement of hollow spheres from close-packing to non-close-packing. Besides these, the hollow sphere size and the shell thickness can also be controlled by changing the colloidal sphere and deposition time, respectively. Further investigation indicates that the success of NSCL should be owed to a key step, that is, an ion-sputtering induced nonshadow deposition surrounding the whole surfaces of colloidal spheres. This allows an equipotential face and thus homogeneous deposition surrounding the surfaces of PSs in an electrochemical deposition process, and final formation of hollow sphere structure. The 2D ordered hollow sphere arrays with controllable microstructure and size could exhibit importance both in fundamental research and in practical applications.

Introduction Hollow spheres are of great interest in micro/nanofabrication because of their novel interior geometry and surface *To whom correspondence should be addressed. E-mail: [email protected]. [email protected]. (1) (a) Zhu, Y.; Shi, J.; Shen, W.; Dong, X.; Feng, J.; Ruan, M.; Li, Y. Angew. Chem., Int. Ed. 2005, 44, 5083–5087. (b) Chen, J. F.; Ding, H. M.; Wang, J. X.; Shao, L. Biomaterials 2004, 25, 723–727. (2) Shchukin, D. G.; Sukhorukov, G. B.; M€ohwald, H. Angew. Chem., Int. Ed. 2003, 42, 4472–4475. (3) (a) Shchukin, D. G.; Patel, A. A.; Sukhorukov, G. B.; Lvov, Y. M. J. Am. Chem. Soc. 2004, 126, 3374–3375. (b) Chen, J.; Saeki, F.; Wiley, B. J.; Cang, H.; Cobb, M. J.; Li, Z. Y.; Au, L.; Zhang, H.; Kimmey, M. B.; Li, X. D.; Xia, Y. Nano Lett. 2005, 5, 473–477. (4) (a) Shchukin, D. G.; Caruso, R. A. Chem. Mater. 2004, 16, 2287–2292. (b) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642–7643. (c) Li, B.; Xie, Y.; Jing, M.; Rong, G.; Tang, Y.; Zhang, G. Langmuir 2006, 22, 9380– 9385. (d) Li, Y.; Kunitake, T.; Fujikawa, S. J. Phys. Chem. B 2006, 110, 13000–13004. (5) (a) Ma, H.; Cheng, F. Y.; Chen, J.; Zhao, J. Z.; Li, C. S.; Tao, Z. L.; Liang, J. Adv. Mater. 2007, 19, 4067–4070. (b) Lou, X. W.; Wang, Y.; Yuan, C.; Lee, J. Y.; Archer, L. A. Adv. Mater. 2006, 18, 2325–2329. (6) Zhang, Q.; Zhang, T.; Ge, J.; Yin, Y. Nano Lett. 2008, 8, 2867–2871. (7) (a) Martinez, C. J.; Hockey, B.; Montgomery, C. B.; Semancik, S. Langmuir 2005, 21, 7937–7944. (b) Zhang, H. G.; Zhu, Q. S.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B. Adv. Funct. Mater. 2007, 17, 2766–2771. (8) (a) Pordan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. Science 2003, 302, 419–422. (b) Lu, L.; Capek, R.; Kornowski, A.; Gaponik, N.; Eychm€uller, A. Angew. Chem., Int. Ed. 2005, 44, 5997–6001. (c) Schwartzberg, A. M.; Oshiro, T. Y.; Zhang, J. Z.; Huser, T.; Talley, C. E. Anal. Chem. 2006, 78, 4732–4736. (9) (a) Luo, Y.; Duan, G.; Li, G. Appl. Phys. Lett. 2007, 90, 201911. (b) Kamata, K.; Lu, Y.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 2384–2385. (10) Dai, Z. F.; M€ohwald, H.; Tiersch, B.; D€ahne, L. Langmuir 2002, 18, 9533– 9538. (11) (a) Caruso, F.; Caruso, R. A.; M€ohwald, H. Science 1998, 282, 1111–1114. (b) Caruso, F. Chem.;Eur. J. 2000, 6, 413–419. (c) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W.; Giersig, M.; Liz-Marzan, L. M. Chem. Mater. 2005, 17, 3268–3272. (d) Hosein, I. D.; Liddell, C. M. Langmuir 2007, 23, 2892–2897. (e) Wolosiuk, A.; Armagan, O.; Braun, P. V. J. Am. Chem. Soc. 2005, 127, 16356–16357. (f) Sun, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 3827–3831. (g) Lou, X. W.; Archer, L. A.; Yang, Z. C. Adv. Mater. 2008, 20, 3987–4019. (12) (a) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711–714. (b) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 16744–16746. (c) Liang, H. P.; Zhang, H. M.; Hu, J. S.; Guo, Y. G.; Wan, L. J.; Bai, C. L. Angew. Chem., Int. Ed. 2004, 43, 1540–1543. (d) Xie, L.; Zheng, J.; Liu, Y.; Li, Y.; Li, X. G. Chem. Mater. 2008, 20, 282–286.

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functionality.1-14 This allows their widespread use in applications such as drug delivery,1 food and cosmetic industries,2 biotechnology,3 catalysis,4 battery materials,5 micro/nanoreactors,6 sensing,7 and photonic devices.8 Success of these applications strongly relies on material category and size of the hollow sphere but also on the microstructure of the spherical shell.9 Up to date, there have been many available cases for the fabrication of hollow sphere structures.11-14 In past years, patterned arrays induced by colloidal lithography have received much attention due to their potential applications, such as catalysis, photonic crystals, and optoelectronic devices.15-23 It is important for novel micro/nanodevices and practical applications to build hollow spheres into an ordered array, since such an organized system combines the merits of hollow building blocks and patterned array. Work by Lu et al. has (13) (a) McDonald, C. J.; Bouck, K. J.; Chaput, A. B.; Stevens, C. J. Macromolecules 2000, 33, 1593–1605. (b) Wang, H. R.; Song, Y. J.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2006, 128, 9284–9285. (14) (a) Im, S. H.; Jeong, U. Y.; Xia, Y. N. Nat. Mater. 2005, 4, 671–675. (b) Lou, X. W.; Wang, Y.; Yuan, C.; Lee, J. Y.; Archer, L. A. Adv. Mater. 2006, 18, 2325–2329. (c) Li, J.; Zeng, H. C. J. Am. Chem. Soc. 2007, 129, 15839–15847. (15) (a) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599–5611. (b) Zhang, X.; Whitney, A. V.; Zhao, J.; Hicks, E. M.; Van Duyne, R. P. J. Nanosci. Nanotechnol. 2006, 6, 1920–1934. (c) Hulteen, J. C.; Van Duyne, R. P. J. Vac. Sci. Technol., A 1995, 13, 1553–1558. (d) Li, Y.; Cai, W.; Duan, G. Chem. Mater. 2008, 20, 615–624. (16) (a) Ghanem, M. A.; Bartlett, P. N.; de Groot, P.; Zhukov, A. Electrochem. Commun. 2004, 6, 447–453. (b) Che, X.; Chen, Z.; Fu, N.; Lu, G.; Yang, B. Adv. Mater. 2003, 15, 1413–1417. (c) Bartlett, P. N.; Baumberg, J. J.; Coyle, S.; Abdelsalam, M. E. Faraday Discuss. 2004, 125, 117–132. (d) Wang, X. D.; Graugnard, E.; King, J. S.; Wang, Z. L.; Summers, C. J. Nano Lett. 2004, 4, 2223–2226. (17) (a) Kosiorek, A.; Kandulski, W.; Chudzinski, P.; Kempa, K.; Giersig, M. Nano Lett. 2004, 4, 1359–1363. (b) Kosiorek, A.; Kandulski, W.; Glaczynska, H.; Giersig, M. Small 2005, 1, 439–444. (18) (a) Netti, M. C.; Coyle, S.; Baumberg, J. J.; Ghanem, M. A.; Birkin, P. R.; Bartlett, P. N.; Whittaker, D. M. Adv. Mater. 2001, 13, 1368–1370. (b) Jang, S. G.; Yu, H. K.; Choi, D.-G.; Yang, S.-M. Chem. Mater. 2006, 18, 6103–6105. (c) Li, Y.; Sasaki, T.; Shimizu, Y.; Koshizaki, N. J. Am. Chem. Soc. 2008, 130, 14755–14762. (19) Lu, L.; Eychm€uller, A.; Kobayashi, A.; Hirano, Y.; Yoshida, K.; Kikkawa, Y.; Tawa, K.; Ozaki, Y. Langmuir 2006, 22, 2605–2609.

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demonstrated that Au/Ag hollow sphere arrays show strong surface-enhanced Raman scattering (SERS) effects correlated with their structures.19 Hollow sphere arrays or films have found important advantages in lithium-ion batteries as anode materials, which can lead to a much improved cycle life and rate capability due to their enhanced capacity retention by reversibly accommodating large volume changes (i.e., to enhance “breathability”).5 Hollow sphere arrays have also exhibited novel optical properties and thus beneficial device-application.21 Our previous work demonstrated that a 2D hollow sphere array shows an equivalent angle-independent photonic stop band, owing to its special hollow interior and spherical symmetry.21b In addition, hollow sphere arrays or their extension structures (such as core-shell sphere arrays prepared without removal of colloidal sphere templates) have found use in electrochemical catalysis and sensors,24 surperhydrophobic films,25 solar cells,26 photocatalysis,4d and micro/nanoreactors.27 In general, a hollow sphere array can be prepared by templating against a polystyrene sphere (PS) (or silica sphere) template.28,29 Chen et al. prepared 2D and 3D ordered silver hollow sphere arrays by combining a seeded growth technique for metal coatings on isolated colloids in solution and the confined template-directed synthesis method for material patterning.28a Sun and Yu a fabricated 2D gold hollow sphere array by photochemical deposition surrounding a floating colloidal monolayer on the surface of the precursor solution.29b In addition, our group fabricated Ni and Ni(OH)2 monolayer hollow sphere arrays via a preferential electrochemical deposition of materials surrounding the surfaces of PSs at special synthesis conductions.21b,30 Even though success was achieved in these cases, the controllable fabrication of hollow sphere arrays still encounters great challenges. First, conventional methods are materialdependent, such as using material-correlated surface modification, which limits their applications for other materials. Second, the microstructure control of hollow sphere arrays is of difficult realization, for example, how to control the shell of a hollow sphere from single-layer to multilayer, from a single material to multiple materials, and from single structure to hierarchical (20) (a) Yang, S.-M.; Jang, S. G.; Choi, D.-G.; Kim, S.; Yu, H. K. Small 2006, 2, 458–475. (b) Duan, G.; Cai, W.; Luo, Y.; Li, Y.; Lei, Y. Appl. Phys. Lett. 2006, 89, 181918. (c) Duan, G.; Cai, W.; Luo, Y.; Li, Z.; Li, Y. Appl. Phys. Lett. 2006, 89, 211905. (d) Duan, G.; Cai, W.; Luo, Y.; Li, Z.; Lei, Y. J. Phys. Chem. B 2006, 110, 15729– 15733. (e) Sun, F.; Cai, W.; Li, Y.; Cao, B.; Lu, F.; Duan, G.; Zhang, L. Adv. Mater. 2004, 16, 1116–1121. (21) (a) Blanco, A.; Chomsk, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; van Driel, H. M. Nature 2000, 405, 437–440. (b) Duan, G.; Cai, W.; Luo, Y.; Sun, F. Adv. Funct. Mater. 2007, 17, 644–650. (22) (a) Zhang, G.; Wang, D.; M€ohwald, H. Angew. Chem., Int. Ed. 2005, 44, 7767–7770. (b) Zhang, G.; Wang, D.; M€ohwald, H. Nano Lett. 2005, 5, 143–146. (c) Zhang, G.; Wang, D.; M€ohwald, H. Nano Lett. 2007, 7, 127–132. (d) Zhang, G.; Wang, D.; M€ohwald, H. Nano Lett. 2007, 7, 3410–3413. (e) Zhang, G.; Wang, D. J. Am. Chem. Soc. 2008, 130, 5616–5617. (23) (a) Jiang, P.; McFarland, M. J. J. Am. Chem. Soc. 2005, 127, 3710–3711. (b) Jiang, P. Angew. Chem., Int. Ed. 2004, 43, 5625–5628. (c) Jiang, P. Langmuir 2006, 22, 3955–3958. (d) Sun, C.-H.; Linn, N. C.; Jiang, P. Chem. Mater. 2007, 19, 4551–4556. (24) Huang, X.-J.; Li, Y.; Im, H.-S.; Yarimaga, O.; Kim, J.-H.; Jang, D.-Y.; Cho, S.-O.; Cai, W.-P; Choi, Y.-K. Nanotechnology 2006, 17, 2988–2993. (25) Li, Y.; Huang, X. J.; Heo, S. H.; Li, C. C.; Choi, Y. K.; Cai, W. P.; Cho, S. O. Langmuir 2007, 23, 2169–2174. (26) Yang, S.-C.; Yang, D.-J.; Kim, J.; Hong, J.-M.; Kim, H.-G.; Kim, I.-D.; Lee, H. Adv. Mater. 2008, 20, 1059–1064. (27) Zhou, Q.; Zhao, J.; Xu, W.; Zhao, H.; Wu, Y.; Zheng, J. J. Phys. Chem. C 2008, 112, 2378–2381. (28) (a) Chen, Z.; Zhan, P.; Wang, Z.; Zhang, J.; Zhang, W.; Ming, N.; Chan, C. T.; Sheng, P. Adv. Mater. 2004, 16, 417–422. (b) Yang, S.; Cai, W.; Yang, J.; Zeng, H. Langmuir 2009, 25, 8287–8291. (29) (a) Han, S.; Shi, X.; Zhou, F. Nano Lett. 2002, 2, 97–100. (b) Sun., F.; Yu, J. C. Angew. Chem., Int. Ed. 2007, 46, 773–777. (c) Sen, T.; Tiddy, G. J. T.; Casci, J. L.; Anderson, M. W. Angew. Chem., Int. Ed. 2003, 42, 4649–4653. (30) Duan, G.; Cai, W.; Li, Y.; Li, Z.; Cao, B.; Luo, Y. J. Phys. Chem. B 2006, 110, 7184–7188.

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structure. Obviously, controllability is of great importance for novel properties and practical applications. Third, accurate size-control of structural parameters, such as the diameter of a hollow sphere and the thickness of a shell, in a facile strategy still remains unresolved. In this Article, nonshadow deposition dominated colloidal lithography (NSCL) was proposed and used as a general strategy for the synthesis of 2D ordered hollow sphere arrays of conductive materials. Gold, polypyrrole, CdS, and ZnO were taken as model materials to demonstrate this NSCL strategy, and built as hollow sphere arrays successfully. Figure 1 demonstrates the whole NSCL strategy. First, a hexagonally close-packed colloidal monolayer is synthesized and transferred onto a conductive ITO substrate (Figure 1A), as previously depicted.20e Second, a thin gold layer was introduced to surround the whole surfaces of PSs in the colloidal monolayer via nonshadow deposition (Figure 1B, and details in the following section). Lastly, a hollow sphere array can thus be obtained by further electrochemical deposition on such a template and subsequent removal of PSs (Figure 1C and D). Obviously, the second step is of most importance and some difficulty, which will bring an equipotential face surrounding the surface of PSs and thus a final homogeneous deposition of the hollow sphere shell. In fact, this strategy is proposed due to an interesting finding in our recent experiment. We find the second step, that is, nonshadow deposition, can be easily realized by not using vacuum thermal evaporation deposition but by using ion-sputtering deposition. The proposed strategy is of facile controllability both in microstructure and in size. The details for the controlled synthesis of a 2D hollow sphere array based on NSCL strategy will be depicted as follows.

Experimental Section Materials. All of the chemicals used were of reagent grade or better. PS (2 μm in diameter) suspensions (2.5 wt % in water) were bought from Alfa Aesar Corporation. Commercial glass slides coated with a layer of 50 nm indium-tin oxide (ITO-glass) were used as conductive substrates. The sheet resistance was about 50 Ω/sq. Water (18.2 MΩ/cm) was obtained from an ultrafiltration system (Milli-Q, Millipore, Marlborough, MA). Preparation of Templates. An ordered colloidal monolayer was prepared as previously described.20e Briefly, ordinary glass substrates (1.5 cm  1.5 cm) were strictly ultrasonically cleaned, in turn, in acetone and ethanol solution, piranha solution (3:1 concentrated H2SO4/30% H2O2), and a mixture of 5:1:1 H2O/ NH4OH/30% H2O2 (all in volume ratio). Then 10 μL of PS latex was dropped on the cleaned glass substrate, which was put onto a home-built spin-coater, followed by spinning at a proper speed (about 50-140 round/min in our experiments). A circular hexagonally close-packed colloidal monolayer with centimetersquared size was thus formed on the glass substrate. ITO substrates were cleaned in acetone, ethanol, and distilled water for 30 min each. The monolayer on the glass substrate was then integrally transferred onto a cleaned ITO substrate using a monolayer-floating strategy, as illustrated previously.20e The ITO substrate with a close-packed colloidal monolayer was heated at 110 °C in an oven for 15 min, followed by etching in an argon plasma cleaner (PDC-32G-2) at a pressure of 0.2 mbar and an input power of 100 W for 45 min, and the nonclosed packed colloidal monolayer was thus obtained. The spacing between the etched PSs depends on the etching time. Both the close-packed colloidal monolayer and the non-close-packed one on ITO substrates were coated with a gold layer by ion-sputtering deposition before their following applications in electrochemical deposition. The sputtering deposition was carried out on an ion coater (EIKO, IB-3) in 0.2 Torr vacuum. The deposition thickness Langmuir 2010, 26(9), 6295–6302

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Figure 1. Schematic illustration of the nonshadow deposition dominated colloidal lithography strategy. (A) PS colloidal monolayer on ITO substrate, (B) introducing a 10 nm gold layer on colloidal monolayer by ion-sputtering deposition (nonshadow deposition), (C) ordered array of spherical structure with a core of PS and a shell of conductive material by electrochemical deposition, and (D) hollow sphere array after removal of PSs. can be controlled by sputtering time, and it was estimated on the basis of the data provided by the manufacturer of the coater. In our case, the thickness was about 10 nm. Electrochemical Deposition. All electrochemical experiments were carried out using galvanostatic deposition on a three-electrode cell at a current density depending on the deposited material. A colloidal monolayer on ITO substrate coated with a gold layer was used as working electrode. A platinum plate and a saturated calomel electrode (SCE) were used as auxiliary and reference electrodes, respectively. The distance between the working electrode and auxiliary electrode was about 6 cm. The electrolytes for Au, polypyrrole (PPy), and CdS were, respectively, a solution composed of HAuCl4 (12 g/L), EDTA (5 g/L), Na2SO3 (160 g/L), and K2HPO4 (30 g/L) with pH = 5;20b a solution composed of 0.1 M pyrrole and 0.1 M dodecylbenzene sulfonate (DBSA);16a and a solution composed of 0.05 M CdCl2 and 0.1 M thioacetamide (TAA) with pH = 4.31 ZnO was deposited in 0.05 M Zn(NO3)2/ammonia solution. The 0.05 M Zn(NO3)2/ ammonia solution was prepared by adding ammonia dropwise to the 0.05 M Zn(NO3)2 aqueous solution at 70 °C under continuous stirring until it turned clear.32 The deposition was carried out at 25 °C for Au and PPy, while it was performed at 70 °C for ZnO and CdS. The electrochemical deposition current density is -0.4 mA/cm2 for Au, 0.5 mA/cm2 for PPy, -0.025 mA/ cm2 for CdS, and -1.0 mA/cm2 for ZnO. After deposition of the desired materials, the PS colloidal monolayer was removed by dissolution in CH2Cl2, and the hollow sphere arrays were thus obtained. Characterization. All the as-prepared samples were examined by field-emission scanning electron microscopy (FESEM, Sirion (31) (a) Yamaguchi, K.; Yoshida, T.; Sugiura, T.; Minoura, H. J. Phys. Chem. B 1998, 102, 9677–9686. (b) Xu, D.; Xu, Y.; Chen, D.; Guo, G.; Gui, L.; Tang, Y. Adv. Mater. 2000, 12, 520–522. (32) (a) Xu, L.; Chen, Q.; Xu, D. J. Phys. Chem. C 2007, 111, 11560–11565. (b) Cao, B.; Li, Y.; Duan, G.; Cai, W. Cryst. Growth Des. 2006, 6, 1091–1095.

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200), and some samples were tilted on the work table for side imaging. For some samples, the films of hollow sphere arrays were scraped down slightly and transferred to a copper grid for transmission electron microscopy (TEM) examination (JEM200CX). X-ray diffraction (XRD) was measured on a Philips X’Pert instrument using a Cu KR line (0.15419 nm).

Results and Discussion Colloidal Monolayer Template Coated with a Gold Layer. The PS colloidal monolayer was synthesized and transferred onto an ITO substrate.20e After coating a 10 nm gold layer on the colloidal monolayer by ion-sputtering deposition, a template for further use was thus prepared. Figure 2A shows the FESEM image of the template with a PS size of 2000 nm in diameter. It can be seen that a hexagonally orderly structure is kept integrally without breakage. Figure 2B is a corresponding tilted image after removal of most of the colloidal spheres by slight scraping. From color contrast and the relative position of one leaving PS, we can identify the substrate as three regions: the black dot regions are of the contact area between PSs and substrate, which cannot be coated with gold, obviously; the ringlike regions around the black dots are coated with a thin gold layer; and the bright region is a gold layer on the substrate. As is imagined, the regions of the PSs’ undersides and their facing regions in the ITO substrate should not be deposited with gold because of the shadow effect. However, in our case, the diameter of the PSs is larger than that of the black dot region; that is, the deposited gold regions on the ITO substrate are far larger than those without the sheltering effect by PSs, indicating a nonshadow deposition coming into being. Because of the similar position between the PSs’ underside and their facing regions in the ITO substrate, we believe that the PSs’ undersides are also coated with a gold layer; that is, the whole PS surface was deposited with a gold layer. DOI: 10.1021/la904116p

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Figure 4. TEM images of PPy hollow sphere arrays. Electrochemical deposition times are (A) 60 s, (B) 120 s, and (C, D) 200 s.

Figure 2. (A) Typical SEM image of a colloidal monolayer on ITO substrate coated with a 10 nm gold layer. (B) Corresponding tilted SEM image of (A) after removal of most PSs by slight scraping.

Figure 3. (A) SEM image of single-layer-shell gold hollow sphere array; inset: tilted image in the edge region with broken spheres. (B) TEM image of double-layer-shell (gold inner layer and PPy outer layer) hollow sphere array; inset: local magnification. (C) SEM image in the edge region for a double-layer-shell (PPy inner layer and gold outer layer) hollow sphere array; inset: image of a broken hollow sphere. (D) Tilted SEM image in the edge region for threelayer-shell (gold inner layer, PPy middle layer, and gold outer layer) hollow sphere array; inset: image of a broken hollow sphere. In all cases, the electrochemical deposition times are 10 min for the gold layer and 40 s for the PPy layer.

Single-Layer-Shell Hollow Sphere Array. Based on the template shown in Figure 2A, and using electrochemical deposition in gold electrolyte, a 2D gold hollow sphere array was thus fabricated after removal of PSs by dissolution in CH2Cl2 solvent. Figure 3A shows the tilted FESEM image of the as-prepared sample. It can be seen that the spherelike particles are uniformly packed into an array with hexagonal symmetry. The periodicity, 6298 DOI: 10.1021/la904116p

that is, the central distance between adjacent microparticles in the array, is still 2000 nm, equal to that in a PS colloidal monolayer (see Figure 2A and Supporting Information Figure S1A). However, the microparticles were enlarged in size compared to PSs, showing an effective deposition of gold on the template. From the inset in Figure 3A, a broken sphere in the edge region of the sample shows the feature of the hollow interior clearly; it can be found that a layer of gold was also grown onto ITO substrate, in addition to that onto the PS surface. Due to the spherical feature, the shell thickness can be determined theoretically as m = (D - d)/2, where m is the shell thickness and D and d are the outer and inner diameters of the hollow sphere, respectively. Based on this, it can be estimated that the shell thickness for the gold hollow sphere array is about 47 nm (including 10 nm inner gold layer, see Supporting Information Figure S1A). It should be noted that an accurate measurement of the shell thickness must be carried out via TEM, while it is difficult to do so for the material of gold. Besides gold, other materials can also be synthesized into 2D ordered hollow sphere array structures based on this strategy. For example, PPy and CdS (a conductive polymer and a semiconductor) hollow sphere arrays were also synthesized by electrochemical deposition on the templates. As is reported,16a a PPy film can be grown onto a conductive substrate by electrochemical deposition in a solution containing 0.1 M pyrrole and 0.1 M DBSA. Based on the template shown in Figure 2A and the proposed NSCL strategy, PPy hollow sphere arrays were obtained (Figure 4 and Supporting Information Figure S2). It can be seen, the thickness of PPy spherical shells can be controlled easily by varying electrochemical deposition time (Figure 5). As for CdS, the deposition mechanism is different from the ordinary electrochemical deposition process, which was reported systematically by Yamaguchi and et al.31a Briefly, CdS is deposited through the decomposition of the Cd-TAA complex at the surface of the substrate. Herein, the electrochemical reaction, that is, electroreduction of protons, is used only to maneuver the chemical formation of CdS to take place preferentially at the substrate surface while prohibiting the contribution of the solution phase reaction to the film growth.31a In addition, a CdS film grown by electrochemical deposition is dependent on fabrication conditions such as cathodic current density, temperature, pH value, and deposition time. Figure 6 shows the SEM image and XRD pattern for the as-prepared CdS hollow sphere array with the deposition time of 3 h based on the proposed NSCL strategy. From the SEM image, it can be seen that the spheres are hexagonally packed in the array, the same as the gold hollow sphere array. The shell Langmuir 2010, 26(9), 6295–6302

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Figure 5. Curve of PPy hollow spherical shell thickness versus electrochemical deposition time.

Figure 6. Typical (A) SEM image and (B) XRD pattern of asprepared CdS hollow sphere array. Electrochemical deposition time is 3 h.

thickness was estimated to be approximately 50 nm. In addition, the spherical shells consist of many nanoscaled grains. The corresponding XRD pattern shows that the as-deposited layer is hexagonal R-CdS. Further experiments demonstrate that the deposition time is important for the formation of a continuous CdS shell layer. When the deposition time was decreased to 2 h, the skeleton of the array collapsed after removal of the PSs in CH2Cl2 solvent. When the deposition time was increased (such as Langmuir 2010, 26(9), 6295–6302

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4 h), the CdS hollow sphere array was still synthesized while both the shell thickness and the grain size increased. Multilayer-Shell Hollow Sphere Array. According to the proposed NSCL strategy, if a multistep electrochemical deposition (i.e., layer-by-layer deposition) is employed, we may synthesize the hollow sphere array with a multilayer-shell structure. The designed experiments confirmed this speculation. Figure 3B and C shows two typical double-layer-shell hollow sphere arrays. By first deposition in gold electrolyte and second deposition in PPy electrolyte, the hollow sphere array with the double-layer-shell structure of gold inner layer and PPy outer layer was thus prepared after removal of PSs (Figure 3B and Supporting Information Figure S1B). From the TEM image, it can be clearly seen that the PPy was formed uniformly onto the surface of the as-deposited gold spherical shell (inset in Figure 3B). The inner gold layer is of nanoparticle film with rough surface, while the outer PPy layer is smooth comparatively. In addition, the PPy can be deposited onto the interstitial sites between gold nanoparticles, showing a nice compatibility between these two materials. If the deposition order is changed, a double-layer-shell structure of PPy inner layer and gold outer layer can also be prepared (Figure 3C and Supporting Information Figure S1), showing the universality of this strategy. As is shown, the inner layer and outer layer can easily be indexed and of continuity from the broken hollow sphere (see arrows in the inset in Figure 3C). Further, if a three-step deposition is employed, a three-layer-shell hollow sphere array can thus be obtained (Figure 3D and Supporting Information Figure S1D). From the tilted image in the edge region (inset in Figure 3D), we can identify the three layers of the hollow sphere shell clearly. The square, ellipse, and arrow index the outer, middle, and inner layer, respectively. Each layer equally covers the PS surface, indicating an effective deposition and a strong mechanical intension of the layer. Hierarchically Micro/Nanostructured Array. Besides a single-layer-shell or multilayer layer structure, the hollow sphere array can also have a diverse surface morphology. In a special electrolyte and with an appropriate synthesis condition, a complex hierarchically micro/nanostructured array can be prepared. Figure 7 shows the as-prepared ZnO sample based on the NSCL strategy and electrochemical deposition in 0.05 M Zn(NO3)2/ ammonia solution. It can be seen that the array has a hexagonal symmetry, with the building blocks of a spherical outline (i.e., microcell). Each microcell in the array is of a hierarchical micro/nanostructure, which consists of many nanorods with their growth orientation nearly vertical to the shell surface. Thus, we call the as-prepared sample a hierarchically micro/nanostructured array. Obviously, the formation of a hierarchically micro/nanostructured array is correlated with the desired material. Electrochemical nucleation and growth mechanism is the deciding factor for morphology control. As for ZnO, the formation mechanism of the nanorods has been clearly understood.32 Up to the present, much work on electrochemical crystallization of nanomaterials has been carried out by many groups. By combining these available results with the NSCL strategy, more hierarchically micro/nanostructured arrays should be prepared according to requirement. Non-Close-Packed Hollow Sphere Array. As is known, a non-close-packed colloidal monolayer can be fabricated by many methods, such as plasma etching on the original close-packed colloidal monolayer.22e After argon plasma etching on a PS (2000 nm in diameter) colloidal monolayer for 45 min, followed by surface coating of a 10 nm gold layer, we can obtain another kind of template for further electrochemical deposition. Figure 8A shows the corresponding morphology of the template. It can be DOI: 10.1021/la904116p

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Figure 9. Typical SEM image of ordered triangular nanoparticle array prepared by vacuum thermal evaporation deposition (or “nanosphere lithography”). The film thickness (i.e., the height of nanoparticle) is controlled at 70 nm.

Figure 7. Typical SEM images of a ZnO hierarchically micro/ nanostructured array. Electrochemical deposition time is 1 h.

Figure 8. (A) SEM image of surface gold-coated non-close-packed colloidal monolayer on ITO substrate. Inset: tilted image in an edge region. (B) SEM image of gold non-close-packed hollow sphere array prepared on the template shown in (A). Inset: tilted image in an edge region. Electrochemical deposition time for Au is 10 min.

seen that PSs were completely isolated from each other with their size reduced to 1620 nm in diameter. It can be understood that the spacing between the etched PSs (or size of the etched PSs) can be controlled by varying the etching time. 6300 DOI: 10.1021/la904116p

If using the template of a non-close-packed one, the corresponding non-close-packed hollow sphere array should be prepared by using NSCL strategy. Figure 8B shows the morphology of the as-prepared gold sample by this method. From the tilted image in the edge region, it is found that both the PS surfaces and the ITO substrate were deposited with a homogeneous gold layer. By comparing this with the above results, it can be seen that there is no obvious difference between the formation of a hollow sphere array in the case of close-packing and that in the case of non-closepacking. This indicates that the hollow sphere array can also be controlled in a pattern (or arrangement) according to requirement. In addition, the size of the hollow sphere can be controlled by etching time and further deposition time. Formation Mechanism. Now, let us discuss the mechanism for NSCL strategy briefly. In our case, formation of a hollow sphere array is dependent on two key steps: first, introducing a gold layer surrounding PSs’ surfaces by ion-sputtering deposition; second, subsequent homogeneous electrochemical deposition surrounding PS surfaces based on this conductive gold layer (see Figure 1). In fact, the second step can easily be understood and achieved. After success of the first step, the whole surface for each PS is of conductivity and of equipotential face. This will easily lead to a homogeneous growth of designed material surrounding PS surfaces during the electrochemical deposition process, and then the hollow sphere array will be formed after removal of PSs. However, the first step seems difficult and incomprehensible. Based on a close-packed PS colloidal monolayer, using the thermal evaporation deposition in a 10-7 Torr vacuum with the depositing orientation vertical to the substrate, a triangular nanoparticle array can be formed after removal of PSs (Figure 9). This has also been noted in the famous case of “nanosphere lithography”.15 In this case, both the underside of PS and its opposite region in the substrate cannot be coated with depositing material due to a shadow effect. Though there is a same shelter of PS in the case of ionsputtering deposition, an interesting nonshadow deposition was found in our case (Figure 2B). The deep mechanism for this nonshallow deposition should ask for careful understanding of the physical process. Briefly, the tremendous difference between above two results should be because of the different deposition processes for ion-sputtering and vacuum thermal evaporation.33 As for vacuum thermal evaporation deposition, gold atoms were (33) Ohring, M. Materials Science of Thin Film: Deposition and Structure, 2nd ed.; Academic Press: San Diego, 2002; pp 95-202.

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Figure 10. Geometrical sketch of three neighboring hollow spheres derived from a close-packed colloidal monolayer. The top-right corner shows the corresponding appearance. Here, m, d, and D represent the spherical shell thickness, the inner diameter (i.e., diameter of the used PSs), and the outer diameter of hollow sphere, respectively.

first formed by thermal evaporation with a low energy of about 0.1-0.2 eV and a long atomic mean free path (larger than the evaporant-substrate spacing, no gas collisions in vacuum), and then moved vertically onto PS coated ITO substrate without scattering.33 Then the upside of PS would be covered with the gold atoms directly. Because of the shelter of PS, the gold atoms can only pass through the triangular channels among PSs and then reach the ITO substrate vertically. Because of the low energy of gold impinging atoms onto the substrate, they would be easily captured by the substrate and thus the triangular nanoparticles were thus formed after removal of PS colloidal monolayer.15 As for ion-sputtering deposition, gold atoms were formed by sputtering with a high energy of about 3-10 eV and a short atomic mean free path (less than the target-substrate spacing, many gas collisions).33 When the part of gold atoms pass through the triangular channels, they cannot easily be captured by the substrate due to their high energy. The gold atoms will scatter and diffuse in any direction due to collisions. This gives an invalidation of PS shelters. The nonshadow deposition was thus achieved and the whole PS surfaces were coated with a gold layer. Obviously, the gold layer is not coated on the whole PS surface isotropically. There should be a difference in thickness between the obverse side and the reverse side of the sphere. Fortunately, such an inhomogeneous gold layer surrounding whole PS surface can still bring an equipotential face and homogeneous deposition in the electrochemical deposition process. As is known, PS can be easily dissolved in CH2Cl2 solvent even with a surface complete coating.28,29b Both ion-sputtering and electrochemical deposition can bring many slight gaps among deposited grains. This is advantageous to the dissolution of PSs. Then, the formation of the hollow sphere array is understood. It should be noted that the nonshallow deposition is an irreplaceable step. If using a template Langmuir 2010, 26(9), 6295–6302

of a colloidal monolayer on ITO substrate coated with a gold layer by vacuum thermal evaporation deposition, only a bowl-like pore array can be prepared after electrochemical deposition and removal of PSs. This result is similar to that obtained by using a colloidal monolayer directly without coating of a gold layer.20e It should be mentioned here, from another point of view, the formation of hollow spheres indicates that there must be nonshadow deposition in the ion-sputtering process. Otherwise, only a bowl-like pore array would be obtained. A question will be brought forward here: why not form a hollow sphere array by such a nonshadow deposition using ionsputtering method directly (i.e., not using the step of further electrochemical deposition). This can also be easily understood. As is known, deposition by such a physical method usually leads to a noncompact film (with porosity) consisting of ultrathin nanoparticles or grains with a weak interbinding energy.34 Our further experiments demonstrated that the skeleton was collapsed entirely for a gold coated colloidal monolayer on ITO substrate prepared by ion-sputtering deposition after removal of PSs. Unlike that in the case of ion-sputtering deposition, the hollow spheres prepared by the NSCL strategy are robust enough to maintain their configuration upon preparation, characterization, and strong shaking, which should be because of the electrochemical deposition induced strong interbinding energy between asdeposited grains. The size and microstructure control of hollow sphere arrays are very important for their application, which can easily be realized for the NSCL strategy in our case. For any hollow sphere array, the periodicity is determined by the diameter of the used PSs, and (34) Sun, F.; Cai, W.; Li, Y.; Duan, G.; Nichols, W. T.; Liang, C.; Koshizaki, N.; Fang, Q.; Boyd, I. W. Appl. Phys. B: Lasers Opt. 2005, 81, 765–768.

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the thickness of the spherical shell is determined by the deposition time. In the case of a close-packed array, the inner diameter of the hollow sphere is equal to that of the used PSs; while in the case of a non-close-packed array, the inner diameter is less than that of the used PSs and can be controlled by plasma etching time. The microstructure of the hollow sphere can be controlled by layer-bylayer deposition in a different electrolyte from a single-layer-shell to a multilayer-shell, and from unitary material to multiple materials. Also various hierarchically micro/nanostructured arrays can be obtained by controlling electrochemical deposition parameters (including electrolyte and current density). An important parameter for hollow sphere array is noted here. As is shown, the triangular channel among three hollow spheres in array is correlated with the spherical shell in size (Figure 10). The channel even disappears as the shell thickness is large enough. In many cases, the channel is important for transportation and diffusion. Thus, the thickness should be controlled within a limit. As calculated from geometry, when m ≈ 0.077d, the channel disappears, where m and d are the thickness of the shell and the inner diameter of the hollow sphere, respectively. In the case of a hollow sphere array based on a 2000 nm PS close-packed colloidal monolayer, m should be limited to within 154 nm to maintain the channel.

Conclusions In summary, nonshadow deposition dominated colloidal lithography (NSCL) was proposed and used as a general strategy for the synthesis of 2D ordered hollow sphere arrays of conductive materials. Au, PPy, CdS, and ZnO were taken as model materials to demonstrate the whole synthesis strategy. Using this strategy,

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the microstructure of the hollow sphere was easily controlled from single-layer-shell to multilayer-shell, from unitary material to multiple materials, and from simple structure to hierarchically micro/nanostructure; the whole array was controlled from closepacking to non-close-packing; the size of hollow sphere and the thickness of the shell were controlled by changing the PS and the deposition time, respectively. Two-dimensional ordered hollow sphere arrays synthesized by NSCL strategy have special geometric morphology, high surface area, enhanced capacity retention, and large diffusion and transportation channels. They could exhibit importance both in fundamental research and in practical applications, such as surface science, catalysis, SERS, micro/nanoreactors, electrochemistry, and optoelectronic devices. As a demonstration of a general synthesis strategy, a detailed description for each hollow sphere array is beyond the scope of this article. Further work on desired hollow sphere arrays and their properties and applications is in process. Acknowledgment. The authors acknowledge financial support from the Natural Science Foundation of China (Grant Nos. 10704075, 10974203, and 50831005) and the Major State research program of China “Fundamental Investigation on Micro-Nano Sensors and Systems based on BNI Fusion” (Grant No. 2006CB300402). Supporting Information Available: SEM images for hollow sphere arrays. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(9), 6295–6302