Porous Gold Nanobelts Templated by Metal−Surfactant Complex

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Porous Gold Nanobelts Templated by Metal-Surfactant Complex Nanobelts Lianshan Li, Zhijian Wang, Teng Huang, Jinglin Xie, and Limin Qi* Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry, Peking University, Beijing 100871, People’s Republic of China Received April 20, 2010. Revised Manuscript Received May 18, 2010 Unique, porous gold nanobelts consisting of self-organized nanoparticles were synthesized in a high yield by morphologypreserved transformation from metal-surfactant complex precursor nanobelts formed by a bolaform surfactant dodecane1,12-bis(trimethylammonium bromide) (N-C12-NBr2) and HAuCl4. It was revealed that the precursor nanobelts of the stoichiometric N-C12-N(AuCl4)2 complex formed through electrostatic combination of the positively charged quaternary ammonium headgroups of N-Cn-NBr2 and the negatively charged AuCl4- ions. They were subsequently converted into porous gold nanobelts with shrunken sizes upon reduction by NaBH4. The morphology of the produced gold nanostructures could be adjusted by changing the mixing ratio between N-C12-NBr2 and HAuCl4 in the reaction solution. It was found that the obtained porous Au nanobelts exhibited enhanced catalytic activity toward reduction of 4-nitrophenol compared with solid gold nanobelts, probably owing to their larger surface area and more active sites.

Introduction Gold nanostructures have attracted significant interest in recent years owing to their fascinating optical, electronic, and chemical properties as well as promising application potential in nanoelectronics, optics, sensing, catalysis, and biomedicine.1,2 Since the properties and applications of gold nanostructures largely depend on their size, shape, and pattern, remarkable efforts have been devoted to the morphology-controlled synthesis of gold nanostructures in the past decade.3 In particular, a variety of colloid chemical methods have been developed to fabricate Au nanostructures with tunable shapes such as rods,2 wires,4 belts,5 plates/ prisms,6 polyhedra,7 cages,1b stars/flowers,8 branched particles,9 *To whom correspondence should be addressed: e-mail liminqi@ pku.edu.cn; Fax þ86-10-62751708.

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and dendrites.10 Of all the possible nanoparticle shapes, onedimensional (1D) gold nanostructures are especially intriguing as they offer strong plasmonic fields while exhibiting excellent tenability and biocompatibility; in this regard, 1D gold nanostructures are attractive candidates as optical sensors for biological and medical applications and as robust interconnects in electronic or electromechanic devices.2,4 On the other hand, nanoporous gold structures have attracted considerable attention because they are promising for a broad range of applications such as catalysis,11 sensors,12 actuators,13 and surface-enhanced Raman scattering and fluorescence.14 While organic templates15 and supraspherical nanoparticle aggregates16 have been employed for the preparation of nanoporous gold microspheres and monoliths, the chemical/ electrochemical dealloying technique has been widely used to fabricate various nanoporous gold structures with well-defined three-dimensional bicontinuous nanopores.11-14,17 Particularly, porous gold nanowires18 and nanotubes19 were successfully fabricated by the dealloying process combined with the porous (10) (a) Qin, Y.; Song, Y.; Sun, N.; Zhao, N.; Li, M.; Qi, L. Chem. Mater. 2008, 20, 3965. (b) Huang, T.; Meng, F.; Qi, L. Langmuir 2010, 26, 7582. (11) (a) Zielasek, V.; J€urgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; B€aumer, M. Angew. Chem., Int. Ed. 2006, 45, 8241. (b) Xu, C. X.; Su, J. X.; Xu, X. H.; Liu, P. P.; Zhao, H. J.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 42. (c) Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; B€aumer, M. Science 2010, 327, 319. (12) (a) Huang, J.-F.; Sun, I.-W. Adv. Funct. Mater. 2006, 15, 989. (b) Liu, Z.; Searson, P. C. J. Phys. Chem. B 2006, 110, 4318. (13) (a) Kramer, D.; Viswanath, R. N.; Weissmuller, J. Nano Lett. 2004, 4, 793. (b) Biener, J.; Wittstock, A.; Zepeda-Ruiz, L. A.; Biener, M. M.; Zielasek, V.; Kramer, D.; Viswanath, R. N.; Weissm€uller, J.; Baumer, M.; Hamza, A. V. Nat. Mater. 2009, 8, 47. (14) (a) Qian, L. H.; Yan, X. Q.; Fujita, T.; Inoue, A.; Chen, M. W. Appl. Phys. Lett. 2007, 90, 153120. (b) Lang, X. Y.; Guan, P. F.; Zhang, L.; Fujita, T.; Chen, M. W. J. Phys. Chem. C 2009, 113, 10956. (c) Lang, X. Y.; Guan, P. F.; Zhang, L.; Fujita, T.; Chen, M. W. Appl. Phys. Lett. 2010, 96, 073701. (15) Shchukina, D. G.; Caruso, R. A. Chem. Commun. 2003, 1478. (16) Klajn, R.; Gray, T. P.; Wesson, P. J.; Myers, B. D.; Dravid, V. P.; Smoukov, S. K.; Grzybowski, B. A. Adv. Funct. Mater. 2008, 18, 2763. (17) (a) Ding, Y.; Kim, Y.-J.; Erlebacher, J. Adv. Mater. 2004, 16, 1897. (b) Snyder, J.; Asanithi, P.; Dalton, A. B.; Erlebacher, J. Adv. Mater. 2008, 20, 4883. (18) (a) Ji, C. X.; Searson, P. C. J. Phys. Chem. B 2003, 107, 4494. (b) Laocharoensuk, R.; Sattayasamitsathis, S.; Burdick, J.; Kanatharana, P.; Thavarungkul, P.; Wang, J. ACS Nano 2007, 1, 403. (19) Shin, T. Y.; Yoo, S. H.; Park, S. Chem. Mater. 2008, 20, 5682.

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alumina membrane templates. Recently, porous gold nanowires/ nanrods were synthesized by employing Te nanowires20 and Ni nanorods21 as sacrificial templates through gradual galvanic replacement. Such 1D Au nanostructures with nanopores are appealing materials since they may combine the advantages of both 1D structures and nanoporous structures. However, the current synthetic routes to nanoporous, 1D Au nanostructures have been limited to a few morphologies and are usually accompanied by the removal of the other component(s) and the possible leave of contamination. It would be of great interest to develop facile, cleaner, templating approaches toward nanoporous, 1D Au nanostructures with tailored morphologies. It has been shown that 1D nanostructures of reactive precursors can be employed as effective sacrificial templates for the controlled synthesis of 1D inorganic nanostructures with desired compositions. In this regard, a variety of 1D inorganic nanostructures have been converted into target 1D nanostructures through galvanic replacement,20,21 ion exchange,22 chemical combination,23 and thermal decomposition.24 Meanwhile, relatively flexible 1D nanostructures of metal-organic complexes have turned out to be promising reactive templates for the fabrication of 1D nanostructures with tunable morphologies. For example, porous CuS nanotubes were fabricated by using Cu(I)-thioacetamide and Cu(I)-thiourea complex precursor nanowires as self-sacrificial templates.25 Long CdTe nanotubes with tunable diameters were synthesized by using diameter-tunable nanowires made of Cd-thioglycolic acid coordination polymer as the 1D reactive templates,26 while a series of lead chalcogenide nanotubes were fabricated by templating against precursor nanowires composed of Pb(II)-cysteine complexes.27 Moreover, 1D helical nanostructures based on Zn(II)-cholate complexes were recently employed as reactive templates to prepare helical ZnS nanotubes.28 It is noteworthy that Shimizu et al. have designed a number of lipid molecules that can self-assemble into hollow cylindrical structures and applied the 1D aggregates doped with metal ions as 1D templates to create unique 1D nanostructures, such as 1D helical arrays of CdS nanocrystals,29a fluorescent CdSembedded hybrid nanotubes,29b and antimicrobial Ag-embedded hybrid nanotubes.29c Besides the small biomolecules, normal surfactant molecules can form complexes with suitable metal ions, which can also be used as 1D reactive templates for producing corresponding 1D nanostructures. For example, surfactantmetal ion complex nanofibers formed by hexadecyltrimethylammonium hydroxide and H2PtCl6 on graphite surfaces were
(20) Sanchez-Iglesias, A.; Grzelczak, M.; Rodrı´ guez-Gonz
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an, L. M.; Kotov, N. A. Langmuir 2009, 25, 11431. (21) Mohl, M.; Kumar, A.; Reddy, A. L. M.; Kukovecz, A.; Konya, Z.; Kiricsi, I.; Vajtai, R.; Ajayan, P. M. J. Phys. Chem. C 2010, 114, 389. (22) (a) Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L.-W.; Paul Alivisatos, A. P. Science 2007, 317, 355. (b) Luther, J. M.; Zheng, H.; i Sadtler, B.; Alivisatos, A. P. J. Am. Chem. Soc. 2009, 131, 16851. (c) Wang, H.; Qi, L. Adv. Funct. Mater. 2008, 18, 1249. (d) Shim, H.-S.; Shinde, V. R.; Kim, J. W.; Gujar, T. P.; Joo, O.-S.; Kim, H. J.; Kim, W. B. Chem. Mater. 2009, 21, 1875. (23) (a) Jeong, U.; Camargo, P. H. C.; Lee, Y. H.; Xia, Y. J. Mater. Chem. 2006, 16, 3893. (b) Huang, T.; Qi, L. Nanotechnology 2009, 20, 025606. (24) Tian, L.; Zou, H.; Fu, J.; Yang, X.; Wang, Y.; Guo, H.; Fu, X.; Liang, C.; Wu, M.; Shen, P. K.; Gao, Q. Adv. Funct. Mater. 2010, 20, 617. (25) (a) Yao, Z.; Zhu, X.; Wu, C.; Zhang, X.; Xie, Y. Cryst. Growth Des. 2007, 7, 1256. (b) Mao, J.; Shu, Q.; Wen, Y.; Yuan, H.; Xiao, D.; Choi, M. M. F. Cryst. Growth Des. 2009, 9, 2546. (26) Niu, H.; Gao, M. Angew. Chem., Int. Ed. 2006, 45, 6462. (27) Tong, H.; Zhu, Y.; Yang, L.; Li, L.; Zhang, L. Angew. Chem., Int. Ed. 2006, 45, 7739. (28) Qiao, Y.; Lin, Y.; Wang, Y.; Yang, Z.; Liu, J.; Zhou, J.; Yan, Y.; Huang, J. Nano Lett. 2009, 9, 4500. (29) (a) Zhou, Y.; Ji, Q.; Masuda, M.; Kamiya, S.; Shimizu, T. Chem. Mater. 2006, 18, 403. (b) Zhou, Y.; Kogiso, M.; He, C.; Shimizu, Y.; Koshizaki, N.; Shimizu, T. Adv. Mater. 2007, 19, 1055. (c) Zhou, Y.; Kogiso, M.; Asakawa, M.; Dong, S.; Kiyama, R.; Shimizu, T. Adv. Mater. 2009, 21, 1742.

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Article Scheme 1. Chemical Structure of the Bolaform Surfactant N-C12-NBr2

applied to prepare nanofibers containing Pt nanoparticles.30 Interestingly, Xia et al. reported a facile method for preparing ultrathin Au nanowires using 1D polymer strands composed of (oleylamine)AuCl complex chains formed through aurophilic attraction.31 These results demonstrate the great potential of the use of metal-organic complex precursors as 1D reactive templates in the morphology-tailored synthesis of 1D nanostructures although the achieved morphologies remain to be extended. Quaternary ammonium salt cationic surfactants have been widely used as directing agents in the shape-controlled synthesis of gold nanostructures. In particular, cetyltrimethylammonium bromide (CTAB) is the most frequently used surfactant for the synthesis of Au nanorods in aqueous solution.2 The effects of the tail length32 and headgroup size33 of the cationic ammonium surfactants on the synthesis of Au nanorods have been systematically investigated. When cetylpyridinium chloride that contains a pyridinium-type headgroup was used as the surfactant, selective synthesis of single-crystalline rhombic dodecahedral, octahedral, and cubic gold nanocrystals was achieved.34 However, little attention has been given to the possible effect of the number of the quaternary ammonium headgroups on the synthesis of gold nanostructures. Bolaamphiphiles are bolaform surfactants containing two hydrophilic headgroups connected by a hydrophobic spacer, which have attracted considerable attention because of their novel properties and useful applications.35 It would be worthwhile to explore the application of bolaform surfactants containing two quaternary ammonium headgroups in the synthesis of gold nanostructures. Herein, we report a facile, high-yield synthesis of unprecedented porous gold nanobelts by transformation from metalsurfactant complex precursor nanobelts formed by hydrochloroauric acid and a bolaform surfactant containing two quaternary ammonium headgroups. In the presence of HAuCl4, the surfactant dodecane-1,12-bis(trimethylammonium bromide) (designated as N-C12-NBr2, Scheme 1) self-assembled into beltlike nanostructures by electrostatic combination of negatively charged AuCl4groups with positively charged trimethylammonium headgroups, which were converted to porous gold nanobelts with a curled morphology upon chemical reduction by NaBH4. It was shown that the obtained porous Au nanobelts exhibited enhanced catalytic activity toward reduction of 4-nitrophenol (4-NP) compared with solid gold nanobelts. (30) Kawasaki, H.; Uota, M.; Yoshimura, T.; Fujikawa, D.; Sakai, G.; Arakawa, R.; Kijima, T. Langmuir 2007, 23, 11540. (31) Lu, X.; Yavuz, M. S.; Tuan, H.-Y.; Korgel, B. A.; Xia, Y. J. Am. Chem. Soc. 2008, 130, 8900. (32) Gao, J. X.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9065. (33) (a) Kou, X. S.; Zhang, S. Z.; Tsung, C. K.; Yeung, M. H.; Shi, Q. H.; Stucky, G. D.; Sun, L. D.; Wang, J. F.; Yan, C. H. J. Phys. Chem. B 2006, 110, 16377. (b) Kou, X.; Zhang, S.; Tsung, C.-K.; Yang, Z.; Yeung, M. H.; Stucky, G. D.; Sun, L.; Wang, J.; Yan, C. Chem.;Eur. J. 2007, 13, 2929. (34) Niu, W.; Zheng, S.; Wang, D.; Liu, X.; Li, H.; Han, S.; Chen, J.; Tang, Z.; Xu, G. J. Am. Chem. Soc. 2009, 131, 697. (35) (a) Fuhrhop, J.-H.; Wang, T. Chem. Rev. 2004, 104, 2901. (b) Shen, S.; Garcia-Bennett, A. E.; Liu, Z.; Lu, Q.; Shi, Y.; Yan, Y.; Yu, C.; Liu, W.; Cai, Y.; Terasaki, O.; Zhao, D. J. Am. Chem. Soc. 2005, 127, 6780. (c) Nilsson, M.; Valente, A. J. M.; Olofsson, G.; S€oderman, O.; Bonini, M. J. Phys. Chem. B 2008, 112, 11310.

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Experimental Section Materials. 1,12-Dibromododecane was purchased from Sigma-Aldrich. Dodecyltrimethylammonium bromide (DTAB or C12-NBr) was obtained from Alfa Aesar. Hydrochloroauric acid trihydrate (HAuCl4 3 3H2O, 99.9%), sodium borohydride (NaBH4), 4-nitrophenol (4NP), trimethylamine, and ethanol were of analytical grade and obtained from Beijing Chemical Reagents Co. The water used was deionized. Synthesis of N-C12-NBr2. The bolaform surfactant Br(CH3)3N-(CH2)12-N(CH3)3Br (N-C12-NBr2) was prepared following the procedure reported by Menger and Wrenn36 with minor modification. Typically, stoichiometric amounts of 1,12dibromododecane and trimethylamine were refluxing in ethanol for 24 h. The obtained yellow mixture was rotary-evaporated to remove the solvent. The resulting solid product was recrystallized from ethanol/ethyl acetate for three times until all of the impurities were removed. The purity of N-C12-NBr2 was checked by 1H NMR spectrometry and elemental analysis. 1H NMR (CDCl3, ppm): δ 3.29 (4H, t, -CH2-N-), 3.09 (18H, s, -N(CH3)3), 1.77 (4H, t, -CH2-CH2-N-), 1.35-1.29 (16H, m, -CH2-). Elemental analysis: Anal. Calcd for (N-C12-N)Br2 (%): C, 48.45; H, 9.42; N, 6.28; Br, 35.84. Found (%): C, 48.53; H, 9.45; N, 6.01; Br, 36.01. Synthesis of Porous Gold Nanobelts. In a typical synthesis, 25 μL of N-C12-NBr2 (50 mM) and 100 μL of HAuCl4 (10 mM) solutions were first added to 4.38 mL of water at room temperature, leading to the formation of a yellow precipitate. Then, 500 μL of NaBH4 solution (0.1 M) was added, and the mixture was kept unstirred for 24 h, resulting in the formation of a black precipitate, which was collected by centrifugation, washed thoroughly with water, and dried under ambient conditions. Characterization. The products were characterized by scanning electron microscopy (SEM, Hitachi S4800, 15 kV), environmental SEM (ESEM, FEI Quanta 200FEG, 5 kV), transmission electron microscopy (TEM, JEOL JEM-200CX, 160 kV), high resolution TEM (HRTEM, FEI Tecnai F30, 300 kV), X-ray diffraction (XRD, Rigaku Dmax-2000, Ni-filtered Cu KR radiation), X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra spectrometer with monochromatized Al KR radiation), Fourier transform IR spectroscopy (FTIR, Nicolet Magna-IR 750), and UV-vis spectroscopy (Perkin-Elmer Lambda 35). The BET surface area was measured using Micromeritics ASAP 2010. For the XRD measurements, the gold product was dispersed in water and several drops of the suspension were dropped on a clean glass slide, followed by drying naturally in the air. For the TEM and SEM measurements, the suspension was dropped onto a Formvar-covered copper grid and a silicon wafer, respectively, followed by drying naturally. Catalytic Study. The catalytic reduction reaction of 4NP was conducted in aqueous solution in a standard quartz cell with 1 cm path length. The reaction procedure was as follows: 0.1 mL of 1.2 M NaBH4 was mixed together with 2.9 mL of 0.11 mM 4NP in the quartz cell, leading to a color change from light yellow to yellow-green. Immediately after addition of 0.12 mL of a 1 g/L Au nanoparticle aqueous dispersion, the UV-vis absorption spectra were recorded with a time interval of 120 s in a scanning range of 200-700 nm at ambient temperature (20 ( 2 °C).

Results and Discussion Figure 1 presents typical SEM images of the porous gold nanobelts obtained by adding NaBH4 to a solution containing 0.25 mM N-C12-NBr2 and 0.2 mM HAuCl4. The low-magnification SEM image shown in Figure 1a suggests that the product consists predominantly of 1D structures with lengths about several micrometers. An enlarged image is shown in Figure 1b, which suggests that the 1D structures are actually curled, porous (36) Menger, F. M.; Wrenn, S. J. Phys. Chem. 1974, 78, 1387.

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Figure 1. SEM images of porous gold nanobelts obtained from metal-surfactant complex precursor formed from 0.25 mM N-C12-NBr2 and 0.2 mM HAuCl4.

Figure 2. TEM (a-c) and HRTEM (d) images of porous gold nanobelts obtained from metal-surfactant complex precursor formed from 0.25 mM N-C12-NBr2 and 0.2 mM HAuCl4. Inset shows the related ED pattern.

nanobelts with widths typically ranging from 100 to 200 nm. From Figure 1c, it can be seen that the gold nanobelts curl up into a channel-like structure and exhibit a thickness about 10-20 nm. A high-magnification image shown in Figure 1d indicates that the gold nanobelts consist of primary nanoparticles and many nanopores existed between the nanoparticles. Figure 2a gives a representative TEM image of the curled Au nanobelts, which shows that the two ends of the nanobelts are relatively thinner than the middle part. An enlarged image shown in Figure 2b confirms that the nanobelts with widths about 100-200 nm contain many inner pores. The fine structure of the nanobelts is clearly visible in the high-magnification TEM image shown in Figure 2c, which indicates that the porous nanobelts are actually made of gold nanoparticles with the diameters ranging from less than 10 nm to about 20 nm. The related electron diffraction (ED) pattern shows sharp rings that can be indexed as face-centered-cubic gold. The HRTEM image shown in Figure 2d exhibits clear lattice fringes with the d spacing of 0.24 nm, corresponding to the (111) lattice spacing of cubic gold crystals. The XRD pattern of the porous gold nanobelts is given in Figure 3a, which exhibits reflections characteristic of the cubic gold (JCPDS No. 04-0784), indicating that the porous nanobelts are pure gold crystals. The energy-dispersive X-ray spectroscopy (EDS) spectrum shown in Figure 3b shows only the Au signals Langmuir 2010, 26(14), 12330–12335

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Figure 3. XRD pattern (a) and EDS spectrum (b) of porous gold nanobelts.

Figure 5. ESEM images of metal-surfactant complex precursor formed from 0.25 mM N-C12-NBr2 and 0.2 mM HAuCl4.

Figure 4. SEM images of gold products obtained from metalsurfactant complex precursors formed at different N-C12-NBr2 concentrations: (a) 0, (b) 0.1 mM, (c) 2.5 mM, and (d) 5 mM. [HAuCl4] = 0.2 mM.

except the Cu signals arising from the copper grids supporting the gold nanobelts, confirming that the nanobelts are composed of pure gold. These results indicate that the porous Au nanobelts are composed one or two layers of assembled Au nanoparticles with sizes ranging from several nanometers to about 20 nm. It was found that the concentration of the bolaform surfactant N-C12-NBr2 greatly influenced the morphology of the obtained gold products. Only irregular aggregates of nanoparticles instead of porous gold nanobelts were obtained in the absence of N-C12-NBr2 under otherwise similar synthesis conditions (Figure 4a). At a N-C12-NBr2 concentration of 0.1 mM, porous nanobelts were obtained in addition to the irregular nanoparticle aggregates (Figure 4b). When the N-C12-NBr2 concentration was increased to 0.25 mM, pure porous nanobelts about several micrometers in length were formed as shown in Figure 1. If the N-C12-NBr2 concentration was increased 2.5 mM, shorter porous gold nanobelts (less than 1 μm in length) with relatively rougher surface and edges were obtained (Figure 4c). Irregular 1D assemblies of nanoparticles were formed as the N-C12-NBr2 concentration was further increased to 5 mM (Figure 4d). Moreover, the concentration of HAuCl4 also considerably affected the morphology of the gold product obtained in the presence of N-C12-NBr2. While porous gold nanobelts were still obtained when the HAuCl4 concentration was decreased from 0.25 to 0.02 mM, a small amount of porous gold nanobelts coexisted with a large amount of irregular aggregates of gold nanoparticles when the HAuCl4 concentration was increased to 1 mM (Figure S1, Supporting Information). To reveal the special role of the bolaform surfactant N-C12NBr2 with two quaternary ammonium headgroups in the formation of the porous gold nanobelts, the synthesis of gold nanostructures in the presence of the cationic surfactant C12-NBr with the same alkyl chain length but a single quaternary ammonium Langmuir 2010, 26(14), 12330–12335

headgroup was examined. When C12-NBr was used instead of N-C12-NBr2 under otherwise the similar synthesis conditions, irregular aggregates of nanoparticles were always obtained instead of porous gold nanobelts (Figure S2, Supporting Information). It was observed that the solution containing C12-NBr and HAuCl4 remained clear before the addition of the NaBH4 solution, whereas a yellow precipitate formed immediately after the N-C12-NBr2 solution was mixed with the HAuCl4. This comparison indicates that the bolaform surfactant with two quaternary ammonium headgroups could form a complex precursor with HAuCl4 before the reduction by NaBH4 due to the much stronger interaction between the two cationic headgroups and the AuClh4 anions, which then played a key role in the formation of the porous gold nanobelts. Therefore, it is worthwhile to pay attention to the complex precursor formed between N-C12-NBr2 and HAuCl4. The yellow precipitate formed from the aqueous solution containing 0.25 mM N-C12-NBr2 and 0.2 mM HAuCl4 under ambient conditions was separated and characterized by environmental SEM (ESEM), XRD, EDS, XPS, and FTIR. Figure 5 shows typical ESEM images of the obtained metal-surfactant complex precursor, which suggest that the precursor consisted predominantly of curled nanobelts about 500 nm in width, 50-100 nm in thickness, and several micrometers in length. It may be noted that, compared with the final porous gold nanobelts, the precursor had a similar beltlike morphology but was considerably larger in width and thickness, indicating that the precursor nanobelts were converted to the shrunken, porous gold nanobelts after reduction into metallic gold. The XRD pattern of the obtained metal-surfactant complex precursor is presented in Figure 6, which can be indexed by using the structure-analysis program Powder-X.37 The crystal structure was readily identified as having monoclinic P symmetry, with lattice parameters of a = 11.7084 A˚, b = 10.3462 A˚, and c = 7.9011 A˚ (Part S1, Supporting Information), which suggests the formation of well-crystallized metal-surfactant complex crystals. Our preliminary experimental results showed that the XRD patterns of the metal-surfactant complexes formed at different N-C12-NBr2/HAuCl4 ratios could be ascribed to essentially the same crystal structure, indicating that the metal-surfactant (37) (a) Zhang, X. J.; Zhang, X. H.; Zou, K.; Lee, S. T. J. Am. Chem. Soc. 2007, 129, 3527. (b) Yu, H.; Qi, L. Langmuir 2009, 25, 6781.

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Figure 6. XRD pattern of metal-surfactant complex precursor formed from 0.25 mM N-C12-NBr2 and 0.2 mM HAuCl4.

complex could be a stoichiometric complex irrespective of the initial mixing ratio between the metal ions and the surfactant. The XPS analysis revealed that the ratio between N and Au in the metal-surfactant complex was 1.05:1, in good agreement with the stoichiometric N-C12-N(AuCl4)2 within experimental error. This result indicated that each cationic (N-C12-N)2þ ion could combine with two anionic AuCl4- ions to form the metal-surfactant complex N-C12-N(AuCl4)2. This deduction was supported by the FTIR analysis of the metal-surfactant complex together with the pure surfactant N-C12-NBr2 (Figure S3, Supporting Information). The IR bands assignable to C-N stretching at 1480 cm-1 became much stronger in the complex than in the N-C12-NBr2 surfactant, which could be attributed to the dipole change caused by the combination of the quaternary ammonium headgroups and the AuCl4- anions. In addition, the number of the peaks below 1000 cm-1 was decreased in the complex due to the restriction of vibration modes, which could be ascribed to more compact arrangement of the N-C12-N backbones in the complex. It was previously reported that crystalline nanofibers of the metal-surfactant complex (C16TA)2(PtCl6) could form on a highly oriented pyrolytic graphite (HOPG) surface from mixed solutions of hexadecyltrimethylammonium hydroxide (C16TAOH) and H2PtCl6.30 In the present situation, well-crystallized precursor nanobelts of the stoichiometric metal-surfactant complex N-C12-N(AuCl4)2 were formed in the mixed solution of N-C12NBr2 and HAuCl4 due to the strong interaction between the quaternary ammonium headgroups and the AuCl4- anions. Nevertheless, further investigation is needed to elucidate the detailed crystal structure of the metal-surfactant complex. Accordingly, the formation process of the porous gold nanobelts in the presence of the bolaform surfactant N-C12-NBr2 is tentatively proposed as follows (Figure 7). When 0.25 mM N-C12NBr2 was mixed with 0.2 mM HAuCl4, the stoichiometric N-C12N(AuCl4)2 complex formed immediately via electrostatic interaction as well as van der Waals interaction between the positively charged quaternary ammonium headgroups and the negatively charged AuCl4- ions. This metal-surfactant complex then quickly self-assembled and crystallized into curled nanobelts about 500 nm in width, 50-100 nm in thickness, and several micrometers in length. As the reductant NaBH4 was added into the suspension of the complex nanobelts, the N-C12-N(AuCl4)2 complex was reduced to gold nanocrystals, resulting in the formation of porous gold nanobelts with shrunken sizes (i.e., about 100-200 nm in width and less than 20 nm in thickness) 12334 DOI: 10.1021/la1015737

Figure 7. Schematic illustration of the proposed mechanism for the formation of porous gold nanobelts via metal-surfactant complex precursor template.

because of the presence of a large amount of organic species inside the complex precursor nanobelts. If the N-C12-NBr2 concentration was decreased to 0.1 mM, there could be some soluble AuCl4- ions coexisting with the complex precursor nanobelts in the solution because of the relatively large solubility of the N-C12N(AuCl4)2 complex. Therefore, porous nanobelts together with irregular nanoparticle aggregates were obtained after the reduction by NaBH4, as shown in Figure 4b. On the other hand, if the N-C12-NBr2 concentration was increased from 0.25 mM to 2.5 mM, there could be a large amount of excess N-C12-NBr2 molecules in the solution, leading to the formation of shorter porous gold nanobelts with relatively rougher surface and edges (Figure 4c). If the N-C12-NBr2 concentration was further increased to 5 mM, the formation of the complex precursor nanobelts could be significantly inhibited, resulting in the formation of irregular 1D assemblies of gold nanoparticles (Figure 4d). It may be noted that N-C12-NBr2 does not form metal-surfactant complexes with normal metal cations due to the absence of strong electrostatic attraction, and hence porous nanobelts of the corresponding metals cannot be obtained in this way. Moreover, our preliminary experiment showed that no metal-surfactant complex formed when N-C12-NBr2 was simply mixed with the PtCl62anions, resulting in failure to obtain porous Pt nanobelts. This result indicated that there could be a specific interaction between the quaternary ammonium headgroups and the AuCl4- anions, which is worthy of further investigation. It has been reported that porous Au nanorods provide a new form of unsupported catalysts in contrast to traditional supported nanoparticle catalysts and can be separated easily from the reaction mixtures without aggregation.21 Hence, it can be expected that the current porous Au nanobelts also provide a promising candidate for unsupported catalysts that can be easily separated. A typical catalytic reaction, i.e., reduction of p-nitrophenol (4NP) to p-aminophenol by NaBH4,38 was used to (38) Huang, T.; Meng, F.; Qi, L. J. Phys. Chem. C 2009, 113, 13636.

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Article

400 nm peak and the gradual development of a new peak at 300 nm, indicating that the solid gold nanobelts catalyzed the reduction process. The rate constant of this catalytic reaction was estimated to be 3.72  10-3 s-1 according to the plot of ln(c/c0) vs time (Figure 8c). Figure 8b shows the time-dependent UV-vis absorption of the reaction mixture catalyzed by porous gold nanobelts with the same concentration (i.e., 0.2 mM). The decrease of the 400 nm peak became much faster compared with the reaction catalyzed by solid gold nanobelts. The rate constant of this reaction catalyzed by the porous Au nanobelts was 9.99  10-3 s-1, as measured from the plot of ln(c/c0) vs time shown in Figure 8c, which is nearly 2.7 times as high as that for solid gold nanobelts. On the one hand, this enhanced catalytic activity could be attributed to the higher surface areas of the porous nanobelts compared with the solid nanobelts. The BET surface area of the obtained porous nanobelts was measured to be ∼8.8 m2 g-1, which was higher than the surface area calculated for the solid gold nanobelts with an average thickness ∼18 nm and an average width ∼150 nm (namely, 6.4 m2 g-1). On the other hand, the enhanced catalytic activity may be related to the considerably more active sites on the porous nanobelts since the porous nanobelts were assemblies of primary nanoparticles with a curved surface, which had a high density of low-coordinate surface sites, such as step and kink atoms.11a Considering that the obtained porous Au nanobelts can be easily separated from the reaction solution without aggregation, they may represent a new kind of easy-to-handle, unsupported catalysts with good catalytic activity.

Conclusions

Figure 8. Successive UV-vis absorption spectra of the reduction of 4-nitrophenol by NaBH4 with different catalysts: (a) solid Au nanobelts; (b) porous Au nanobelts. (c) Plot of ln(c/c0) of 4NP against time for the solid Au nanobelts and porous Au nanobelts.

determine the catalytic activity of the obtained porous gold nanobelts. It is known that this redox reaction is catalyzed by metals, and an absorption peak at 400 nm provides an easy way to monitor the reaction spectroscopically.39,40 In the current study, solid gold nanobelts with a similar thickness (about 15-20 nm) were synthesized in solutions of mixed surfactants following the reported procedure (Figure S4, Supporting Information)5b and employed as a control sample. Figure 8a shows the successive UV-vis absorption spectra of the reduction of 0.1 mM 4NP by 37 mM NaBH4 in the presence of the solid Au nanobelts (0.2 mM) as catalyst. Time-dependent UV-vis absorption spectra of this catalytic reaction mixture showed the disappearance of the (39) Qin, G. W.; Pei, W.; Ma, X.; Xu, X.; Ren, Y.; Sun, W.; Zuo, L. J. Phys. Chem. C 2010, 114, 6909. (40) Zeng, J.; Zhang, Q.; Chen, J.; Xia, Y. Nano Lett. 2010, 10, 30.

Langmuir 2010, 26(14), 12330–12335

A facile, high-yield synthesis of unique, porous gold nanobelts consisting of self-organized nanoparticles was achieved by transformation from metal-surfactant complex precursor nanobelts formed by the bolaform surfactant N-Cn-NBr2 and HAuCl4. It was revealed that the precursor nanobelts of the stoichiometric N-C12-N(AuCl4)2 complex formed through electrostatic combination of the positively charged quaternary ammonium headgroups of N-Cn-NBr2 and the negatively charged AuCl4- ions when N-C12-NBr2 was mixed with HAuCl4. They were subsequently converted into porous gold nanobelts with shrunken sizes upon reduction by NaBH4. It was found that the obtained porous Au nanobelts exhibited enhanced catalytic activity toward reduction of 4-nitrophenol compared with solid gold nanobelts, probably owing to their larger surface area and more active sites. These porous Au nanobelts might be able to find widespread use as catalysts in a number of industrial applications. They could also be ideal candidates for investigating the electrical properties of porous 1D metal nanobelts. This work demonstrated for the first time that metal-surfactant complex precursors could be used as reactive templates for the controlled synthesis of 1D beltlike, porous metal nanostructures. This synthetic strategy may open a new route for the mild fabrication of various metal nanostructures with novel morphologies and useful applications. Acknowledgment. Financial support from NSFC (Grants 20873002, 20633010, and 50821061), MOST (Grant 2007CB936201), and SRFDP (Grant 20070001018) is gratefully acknowledged. Supporting Information Available: SEM images of additional gold products and FTIR spectra of the bolaform surfactant and the complex precursor. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la1015737

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