Surfactant-Mediated Self-Assembly of Au Nanoparticles and Their

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Surfactant-Mediated Self-Assembly of Au Nanoparticles and Their Related Conversion to Complex Mesoporous Structures Yu Xin Zhang and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering, and Minerals, Metals, and Materials Technology Center, Faculty of Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ReceiVed October 10, 2007. In Final Form: NoVember 18, 2007 In this work, we report a hydrothermal method for self-assembly and organization of as-synthesized gold nanoparticles into various aggregative morphologies. Using the assembled gold nanoparticles as structural precursors, furthermore, mesoporous gold spheres in either discrete or interconnected form can be prepared at higher process temperatures through removal of bidentate organic linker molecules. Excellent product controllability and high morphological yield have been achieved Via tuning preparative parameters. Our preliminary investigations also show that the assembled gold nanoparticles and nanostructures can be used as building blocks for construction of three-dimensional networks as well as for fabrication of two-dimensional porous thin films. The present work confirms our earlier prediction that Ostwald ripening may also be operative for pre-organized organic capped nanocrystallites in producing hollow structures.

Introduction Gold nanoparticles (AuNPs) are among the most studied nanomaterials in recent years, owing to their outstanding properties in catalytic, electrical, optical, and biomedical applications.1-3 Accordingly, self-assembly and organization of nanostructures of this noble metal have also aroused significant attention, ranging from one-, two-, and three-dimensional (1D, 2D, and 3D) ordered arrays and superlattices to random aggregates and superstructures.1-3 While most of this research endeavor relies on the van der Waals interaction of surfactants adsorbed on the surfaces of AuNPs, increasing activities have been devoted to several types of constructional assemblies, in which assynthesized AuNPs were used as primary building units to grow * Tel: (65) 6516-2896, Fax: (65) 6779-1936, Email: [email protected]. (1) (a) Bae, A.-H.; Numata, M.; Hasegawa, T.; Li, C.; Kaneko, K.; Sakurai, K.; Shinkai, S. Angew. Chem., Int. Ed. 2005, 44, 2030-2033. (b) Deng, Z. X.; Tian, Y.; Lee, S.-H.; Ribbe, A. E.; Mao, C. D. Angew. Chem., Int. Ed. 2005, 44, 3582-3585. (c) Minko, S.; Kiriy, A.; Gorodyska, G.; Stamm, M. J. Am. Chem. Soc. 2002, 124, 10192-10197. (d) Burkett, S. L.; Mann, S. Chem. Commun. 1996, 321-322. (e) Correa-Duarte, M. A.; Liz-Marza´n, L. M. J. Mater. Chem. 2006, 16, 22-25. (f) Gao, X. Y.; Djalali, R.; Haboosheh, A.; Samson, J.; Nuraje, N.; Matsui, H. AdV. Mater. 2005, 17, 1753-1757. (g) Fresco, Z. M.; Freche´t, J. M. J. J. Am. Chem. Soc. 2005, 127, 8302-8303. (h) Huang, J. X.; Tao, A. R.; Connor, S.; He, R. R.; Yang, P. D. Nano Lett. 2006, 6, 524-529. (i) Guan, Y. F.; Pedraza, A. J. Nanotechnology 2005, 16, 1612-1618. (j) Yatsui, T.; Nomura, W.; Ohtsu, M. IEICE Trans. Electron. 2005, E88-C, 1798-1802. (k) Zubarev, E. R.; Xu, J.; Sayyad, A.; Gibson, J. D. J. Am. Chem. Soc. 2006, 128, 1509815099. (l) Podsiadlo, P.; Kaushik, A. K.; Arrud, E. M.; Waas, A. M.; Shim, B. S.; Xu, J. D.; Nandivada, H.; Pumplin, B. G.; Lahann, J.; Ramamoorthy, A.; Kotov, N. A. Science 2007, 318, 80-83. (m) DeVries, G. A.; Brunnbauer, M.; Hu, Y.; Jackson, A. M.; Long, B.; Neltner, B. T.; Uzun, O.; Wunsch, B. H.; Stellacci, F. Science 2007, 315, 358-361. (n) Xu, H.; Hong, R.; Wang, X. Y.; Arvizo, R.; You, C. C.; Samanta, B.; Patra, D.; Tuominen, M. T.; Rotello, V. M. AdV. Mater. 2007, 19, 1383-1386. (2) (a) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 1077-1080. (b) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635-8640. (c) Demers, L. M.; Park, S.-J.; Taton, A.; Li, Z.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 3071-3073. (d) Hassenkam, T.; Nørgaard, K.; Iversen, L.; Kiely, C. J.; Brust, M.; Bjørnholm, T. AdV. Mater. 2002, 14, 1126-1130. (e) Teranishi, T.; Hasegawa, S.; Shimizu, T.; Miyake, M. AdV. Mater. 2001, 13, 1699-1701. (f) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719-2724. (g) Kanehara, M.; Oumi, Y.; Sano, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 8708-8709. (h) Jin, R. C.; Egusa, S. J.; Scherer, N. F. J. Am. Chem. Soc. 2004, 126, 9900-9901. (i) Liz-Marza´n, L. M.; Mulvaney, P. J. Phys. Chem. B 2003, 107, 7312-7326. (j) Khanal, B. P.; Zubarev, E. R. Angew. Chem., Int. Ed. 2007, 46, 2195-2198. (3) Fan, H. Y.; Leve, E.; Gabaldon, J.; Wright, A.; Haddad, R. E.; Brinker, C. F. AdV. Mater. 2005, 17, 2587-2590.

Figure 1. TEM images for (a) as-synthesized AuNPs, (b) long pearl-chain-like Au aggregates, (c) short-chained 3D spongelike Au aggregates, and (d-f) spherical Au aggregates. All the Au aggregates (b-f) were prepared at 140 °C (Supporting Information SI-1).

larger monodisperse particles,4 and to construct continuous 3D networks under heating conditions.5 One important area remaining to be explored is whether these preassembled AuNPs can be utilized as structural precursors for fabricating other even more (4) (a) Maye, M. M.; Zhong, C. J. J. Mater. Chem. 2000, 10, 1895-1901. (b) Maye, M. M.; Zheng, W. X.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. Langmuir 2000, 16, 490-497. (c) Maye, M. M.; Lim, I.-I. S.; Luo, J.; Rab, Z.; Rabinovich, D.; Liu, T.; Zhong, C.-J. J. Am. Chem. Soc. 2005, 127, 1519-1529. (d) Halder, A.; Ravishankar, N. AdV. Mater. 2007, 19, 1854-1858. (5) Zhang, Y. X.; Zeng, H. C. J. Phys. Chem. C 2007, 111, 6970-6975.

10.1021/la7031253 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/04/2008

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complex Au nanostructures when surface organics are removed. Herein, we devise a new synthetic protocol, which combines both surfactant-assisted assembly and heat-activated attachment, to generate mesoporous gold nanostructures. In particular, we will use small AuNPs (2-5 nm in size) as starting units to fabricate several different kinds of complex mesoporous gold nanostructures in either discrete or interconnected forms with a high morphological yield of 100%. Experimental Section

Figure 2. Fine structures of as-synthesized AuNPs: (a) solid Au spheres (see Supporting Information SI-1, organized at 140 °C), and (b) solid Au spheres (see Supporting Information SI-1, organized at 180 °C).

The preparative scheme involved the synthesis of gold nanoparticles (AuNPs, 2-5 nm in diameter) and hydrothermal synthesis of hollow gold spheres. The preparation of the AuNPs suspension has been detailed in our previous report,5 after Brust’s two-phase approach with some minor modifications.6 Briefly, a hydrogen tetrachloroaurate trihydrate (HAuCl4‚3H2O, at 32.85 mM) aqueous solution (3.0 mL) was added to a tetraoctylammonium bromide (TOAB, at 49.5 mM) containing toluene solution (3.982 mL), and the resultant solution was thoroughly mixed, during which the aqueous phase turned from yellow to colorless while the organic phase turned orange as a result of the transformation of [AuCl4]- with TOAB cations. Under the stirring condition at room temperature, the solution was further mixed with 1-dodecanethiol (DDT, 0.1106 M in toluene, 0.4455 mL) for 15 min, followed by adding a freshly prepared sodium borohydride (NaBH4, at 0.4457 M in deionized water, 2.2 mL) solution. The above mixture immediately turned from orange to deep brown, and the resulting AuNPs suspension was continuously stirred for another 15 min. In a typical experiment, 0.2-4 mL of AuNPs suspension (in toluene phase) was added into 20 mL of toluene, followed by the addition of 1,9-nonanedithiol (0.04-0.8 mL, 0.11 M). The obtained mixture was then placed into a Teflonlined stainless steel autoclave, and the solvothermal synthesis was

Figure 3. Panoramic views of the prepared Au products (FESEM images): (a,b) pearl-chain-like Au aggregates (Figure 1b), (c,d) solid Au spheres (Figure 1d-f), and (e,f) porous Au spheres (Figure 5a,b).

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Figure 4. Time-dependent AuNPs aggregating and coarsening (TEM images): (a) 40 min, (b) 1 h, (c) 1.5 h, and (d) 4 h at 140 °C (i.e., the same sample used in Figure 1b). More details on this experiment can be found in Supporting Information SI-1 and SI-2.

Figure 5. TEM images for (a,b) porous Au spheres, (c,d) coreshell Au spheres, and (e,f) Au-thread-interconnected porous Au spheres. All these porous Au products were prepared at 180 °C (Supporting Information SI-1). conducted at 100-200 °C for 0.5-60 h in an electric oven. After the reactions, gold products were harvested by centrifuging and dissolved into ethanol solvent for their stabilization. Detailed preparative parameters adopted in the above experiments can be found in the Supporting Information (SI-1). The as-prepared gold nanomaterial products were characterized with various analytical techniques, which include high-resolution analytical transmission electron microscopy (TEM and HRTEM, JEM2010 and JEM2010F), selected area electron diffraction (SAED), field-emission scanning

Figure 6. FESEM images of Au core-shell structures (a,b), and TEM image of hollow Au spheres without cores (c); details on the preparation of these samples can be found in Supporting Information SI-1.

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Figure 7. (a) FTIR spectra for three different Au samples, (b) TGA scan and its DrTGA curve (porous Au spheres), and (c) a representative XRD pattern (porous Au spheres); details on the preparation of these samples can be found in Supporting Information SI-1. Color inset illustrates an assembly-then-attachment mechanism for total volume reduction (i.e., pore/void generation); individual AuNPs in this process are indicated as orange balls. electron microscopy (FESEM, JSM-6700F), scanning transmission electron microscopy (SEM, JSM-5600LV), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS, AXISHSi, Kratos Analytical), laser light scattering (LLS, BI 90 plus Particle Sizer, Brookhaven instruments), Fourier transform infrared spectroscopy (FTIR, Bio-Rad), surface area and porosity analysis (BET and BJH methods, NOVA-3000), powder X-ray diffraction (XRD, Shimadzu XRD-6000, Cu KR radiation), and thermogravimetric and differential thermogravimetric analysis (TGA and DrTGA, TA instrument, TA-2050).

Results and Discussion The as-synthesized freestanding AuNPs Via a two-phase route normally have a size distribution of 2 to 5 nm, as shown in Figure 1a. Using them as starting units, different modes of AuNPs assemblies can be obtained. By selecting a set of preparative parameters, for example, three major types of organizations have been made at 140 °C (Supporting Information, SI-1): (1) long pearl-chain-like 3D aggregates (Figure 1b), (2) short-chained 3D aggregates (Figure 1c), and (3) connected or discrete spherical aggregates (Figure 1d-f). In the first two 3D organizing modes, the primary AuNPs aggregate themselves into larger particles in the diameter range 50-150 nm, and the resultant particulates then connect one another into 3D networks, giving rise to a spongelike structure. In the third mode of assembly, the primary AuNPs simply produce a perfect spherical morphology (Figure 1d-f) with diameters in the range 400-500 nm, which is about 10 times larger than what had been achieved in a recent study using larger AuNPs (5-8 nm in diameter) as building blocks.7 (6) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (7) Hussain, I.; Wang, Z. X.; Cooper, A. I.; Brust, M. Langmuir 2006, 22, 2938-2941.

It should be emphasized that the AuNPs in these aggregative assemblies do not show severe crystallite coarsening, as detailed in Figure 1f and Figure 2, after the hydrothermal treatments at a relatively low temperature (e.g., 140 °C). The morphological yield for each type of product is extremely high at about 100%, and the panoramic views of some representative samples reported herein are displayed in Figure 3. Under identical conditions, the assembly process has been demonstrated in detail with our timedependent synthesis. As illustrated in Figure 4, when the process time is short (e.g., 1 h), the starting AuNPs in the initial suspension do not show appreciable aggregation. With an increase in time to 1.5 h, a three-dimensional porous network starts to take shape. It is clearly indicated that the assembled AuNPs are further concentrated in the junctures of the 3D network, while void spaces are being enlarged. More condensed aggregates are eventually produced, resulting in the same aggregative morphology displayed in Figure 1b. When this process is prolonged, large crystallites of Au are produced (6 h, Supporting Information SI-2). It has also been found that, in general, a higher content of 1,9-nonanedithiol favors the aggregation of AuNPs. As reported in Figure 1c and d, more spherical forms of aggregate can be obtained when 1,9-nonanedithiol is more abundant (Supporting Information SI-1). At higher temperatures, the growth of pristine AuNPs in the aggregates becomes appreciable (180 °C, Figure 2b), and more complex Au nanostructures can be made. Importantly, the hightemperature condition causes permanent engagement (or fusion) among the aggregated AuNPs, when organic surfactants (e.g., 1,9-nonanedithiol as linkers) are removed. As a result, porosity is generated within the aggregates. Figure 5 shows three representative porous Au products. The first one, freestanding Au nanospheres, was prepared by simply increasing the process

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Figure 8. XPS spectra for solid Au spheres (a-d) and porous Au spheres (e-h); details on the preparation of these samples can be found in Supporting Information SI-1.

temperature (180 °C in Figure 5a,b vs 140 °C in Figure 1). At this temperature, furthermore, complicated Au nanostructures could also be attained when an extra amount of tetraoctylammonium bromide (TOAB) was present during the hydrothermal treatment (Supporting Information SI-1). For instance, the second type of porous Au has a core-shell morphology (Figure 5c,d), noting that the Au core could also be removed, if desired; more detailed views on these hollow structures can be obtained from Figure 6. And in the third type of Au nanostructure, the coreshell Au spheres (type II) can be interlinked through thin gold threads, giving rise to an intricate hierarchical 3D network (Figure 5e,f) that is configured from two different building blocks (i.e., spheres and threads). It is worth mentioning that all types of solid AuNP assemblies and porous Au nanostructures were very stable in ethanol and did not show growth at room temperature for a period of several months. As noted earlier, the generation of interior porosity is apparently associated with depletion of anchored surfactants and direct

attachment among the AuNPs. To understand the mechanism of pore formation, different analytical techniques were further employed in this work. Our Fourier transform infrared spectroscopic (FTIR) study in Figure 7a reveals that hydrothermal process is indeed efficient in removing organic surfactants. For example, the intensities of the C-H vibrational modes located at 2920 and 2850 cm-1 [νas(CH2) and νs(CH2)] and two weaker modes at 2962 and 2878 cm-1 [νas(CH3) and νs(CH3)]8 of adsorbed surfactants have been reduced in the solid AuNPs aggregates (i.e., solid Au spheres) and eventually become undetectable in the porous Au spheres, indicating that the surfactants have been gradually removed after hydrothermal reactions at 140180 °C. Our thermogravimetric analysis (TGA) in Figure 7b also shows that about 10% of weight was still lost in the porous Au spheres over 200-500 °C. This weight loss centered at (8) Shon, Y.-S.; Gross, S. M.; Dawson, B.; Porter, M.; Murray, R. W. Langmuir 2000, 16, 6555-6561.

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Figure 9. (a) Isothermal nitrogen adsorption-desorption loop of porous Au spheres from which specific surface area and pore size distribution were derived (Supporting Information SI-4), (b) TEM image of two porous Au spheres, and (c,d) HRTEM images for two pores indicated in (b). Most lattice fringes observed belong to {111} plane spacing. Details on the preparation of this sample can be found in Supporting Information SI-1.

284 °C was a result of the combustion of adsorbed and/or trapped thiol surfactants in the surfaces of porous Au spheres; we expect that both 1-dodecanethiol and 1,9-nonanedithiol molecules are present on the surface. Furthermore, the result of our X-ray diffraction (XRD, Figure 7c) for the porous Au spheres shows well-defined characteristic metallic gold patterns (JCPDS card no. 04-0784), confirming an increase in the crystallite size of Au when surfactant organics were removed (also see our HRTEM data later). For example, the crystallite sizes of this sample determined by Scherrer’s equation are about 10 to 15 nm (e.g., 11.0 and 12.3 nm, based on the full-width-at-half-maximums (fwhms) of the (200) and (111) reflections, respectively). On the basis of the above TEM/FTIR/TGA/XRD investigations, Figure 7 (color inset) illustrates a formation mechanism of the porous Au spheres. In their as-prepared suspension, the AuNPs capped with 1-dodecanethiol surfactant molecules are discrete and freestanding (step i). Under hydrothermal conditions, some of the monodentate 1-dodecanethiol molecules are replaced by bidentate 1,9-nonanedithiol, which brings neighboring AuNPs together, producing solid Au aggregates (step ii). At an even higher temperature, the aggregative processes can be accelerated, during which both the monothiol and dithiol linkers could be further eliminated. As a result, direct contact among the AuNPs takes place and the total volume of the solid phase is reduced (step iii). Due to the agglomeration of AuNPs in this process, the development of pore/void space is expected, noting that, despite reduction of total gold surface, the final mesoporous structures are still covered with organic surfactants, as detected in our TGA measurements (Figure 7b). To acquire further information on the surfactant depletion, we carried out an X-ray photoelectron spectroscopy (XPS) investigation. In Figure 8a, the main peaks of C 1s at 284.6 eV can be assigned to aliphatic hydrocarbon chains of the surfact-

ants (i.e., thiols) and the smaller peaks at higher binding energies (BEs) of 285.5-285.7 eV are assigned to C-O-C and C-OH species, which resulted from a small degree of surface oxidation of the hydrocarbon chains.5,9,10 The peaks of O 1s at 531.6-531.3 eV can be attributed to surface-adsorbed hydroxyl groups and the small peak of O 1s at 533.2 eV to the entrapped/adsorbed water molecules in the solid AuNPs aggregates (Figure 8b), while the peak at 529.7 eV can be assigned to oxygen ions in sulfate ions (see the S 2p assignment below).5,9,10 The results of our S 2p spectra are also in good agreement with the FTIR findings. For the solid spheres, the S 2p3/2 and 2p1/2 at 162.2 and 163.5 eV have the typical BEs reported for adsorbed thiols on Au.5,9,10 These two peaks are significantly reduced in the porous Au spheres, while sulfate ions (S 2p3/2 and 2p1/2 at 168.8 and 170.2 eV) become pronounced,5,9 because some of the thiols were oxidized during the hydrothermal processes. Consistently, Au 4f7/2 and 4f5/2 photoelectrons also have typical BEs of alkanethiol-capped Au0 at 84.0-83.7 eV and 87.7-87.4 eV.5,9,10 In this agreement, a decrease in BEs of Au 4f in the porous Au spheres is observed, because of the increase in gold crystallite sizes and weaker interaction between the surfactant ad-layer and Au substrate. Furthermore, the tiny Au 4f components at 84.8-84.5 eV and 88.2-88.0 eV indicate a small degree of charge transfer from the gold to the thiol ends.5,9,10 There was no Br 3d photoelectron peak in both samples (Supporting Information SI-3), which confirms that the TOAB in the as-prepared AuNPs have been replaced by 1,9-nonanedithiol during the aggregative processes. In relation to the structural tailoring, Figure 9a reports our Brunauer-Emmett-Teller (BET) measurement for the (9) Zhang, Y. X.; Zeng, H. C. J. Phys. Chem. B 2006, 110, 16812-16815. (10) Li, J.; Zeng, H. C. Angew. Chem., Int. Ed. 2005, 44, 4342-4345.

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Figure 10. SEM image of the two-dimensional metal gold network on a quartz plate. The 2D porous structure was synthesized from the assembly of as-prepared solid Au spheres; details on the sample preparation can be found in Supporting Information SI-1.

porous Au spheres. The N2 adsorption-desorption isotherms show a hysteresis loop that belongs to type H3.11 The loop shape observed is normally attributed to slit-shaped pores given by assembly of platelike particles.11 In this connection, the {111} facets of gold metal are expected to be the predominant surfaces bordering these type of pores, since they are thermodynamically more stable. Indeed, our HRTEM examination in Figure 9b-d reveals that the pores in the spheres are mainly faceted with the {111} surfaces, as resolved in lattice fringes of d111 (0.24 ( 0.02 nm) along the pore spaces.5,9 Moreover, coherent crystalline grains in these porous spheres are greater than 10 nm, which indicates that the oriented attachment was a growth mechanism of Au when surfactants were removed.12-14 The specific surface area in this sample is 11.4 m2/g, and the average pore diameter determined by the standard BarrettJoyner-Halenda method is about 12.4 nm (Supporting Infor(11) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603-619. (12) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969-971. (13) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930-5933. (14) Zeng, H. C. J. Mater. Chem. 2006, 16, 649-662.

Zhang and Zeng

mationSI-4). As a point of reference, reported specific surface areas of Au particles with diameters of 20-100 nm are in the range of about 1-3 m2/g.15 In comparison, the specific surface area of our porous Au should be considered high, despite their large sphere size (averaged at ca 300 nm; based on our laser light scattering measurements; Supporting Information SI-5). Importantly, these mesoporous Au structures can be easily separated after use, which is a clear advantage over their unorganized counterparts concerning their future applications. More interestingly, our preliminary synthetic experiments reported in Figure 10 indicate that the AuNP aggregates or Au nanostructures reported herein can be used as building precursors for fabrication of conductive thin films (e.g., on SiO2 substrate). The resultant porous two-dimensional networks of gold allow light to go through their 2D pore spaces, and thus they can serve as optical transparent film electrodes for solar cells, light-emitting diodes, display devices, and other optoelectronic applications. Further work on these potential applications is in progress. Finally, one important finding we wish mention herein is that the present work confirms one of our earlier predictions that Ostwald ripening may also be operative for pre-organized organiccapped nanocrystallites in producing hollow structures.14 As demonstrated in this work, bidentate surfactants can be utilized to control an organization of the primary crystalline units (AuNPs), after which the elimination of surface-capping organics and permanent engagement among the primary crystallites can be further obtained. Upon this ripening process, interior spaces with architectural design are generated.

Conclusions In summary, for the first time, we have devised a hydrothermal method for self-assembly and organization of as-synthesized gold nanoparticles into various aggregative morphologies. Using the assembled gold nanoparticles as structural precursors, furthermore, mesoporous gold spheres in either discrete or interconnected forms can be prepared at higher process temperatures through removal of bidentate linker molecules. Excellent product controllability and high morphological yield have been achieved Via tuning preparative parameters. These preliminary investigations also show that the assembled gold nanoparticles and nanostructures can be used as building blocks for construction of three-dimensional networks as well as for fabrication of twodimensional porous thin films. Acknowledgment. The authors gratefully acknowledge the financial support of the Ministry of Education, Singapore. Supporting Information Available: Experimental details, TEM, XPS, BJH, and LLS results. This material is available free of charge via the Internet at http://pubs.acs.org. LA7031253 (15) Note for specific surface areas of gold nanoparticles: http://www.americanelements.co.uk/ aunp.html.