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Synthesis of Spongy Gold Nanocrystals with Pronounced Catalytic Activities Md. Harunar Rashid, Rama Ranjan Bhattacharjee, Atanu Kotal, and Tarun K. Mandal* Polymer Science Unit, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed April 7, 2006. In Final Form: July 3, 2006 A simple and easier chemical method for preparing spongy gold nanocrystals has been developed on the basis of a modified-citrate reduction technique of the corresponding gold salt at 25 °C in the absence of template. These nanocrystals possessed autocatalytic behavior and exhibited pronounced catalytic activity in the borohydride reduction of 4-nitrophenol due to their unique spongy morphology.
The morphological control of metal and semiconductor nanoparticles (NPs) has attracted a great deal of attention in nanotechnology and materials science because of the unusual optical,1 electronic,2 catalytic,3 and magnetic4 properties possessed by such materials. Especially, anisotropic metal NPs have unique optical and electronic properties that make them desirable for emerging applications in biolabeling,5 chemical sensing,6 and surface-enhanced Raman scattering.7 Consequently, intensive approaches have been devoted to the synthetic control of the morphologies of metal NPs. For example, metal NPs of various morphologies such as triangle, plate, cube, cage, self-assembled wire, quasi-spherical, rod, and wire have been synthesized using a variety of methods.3,7-9 Apart from these, the synthesis of dendritic-shaped NPs of gold (Au), silver (Ag), and platinum (Pt) have also been reported, and these are interesting materials because of their unique structure, large surface area, and high potential in catalytic applications.10-13 The preparation of colloidal Au NPs by the citrate reduction technique has been extensively studied by a number of researchers, who focused mainly on the size and polydispersity of spherical NPs as well as two-dimensional networked nanostructures at high temperature.14,15 Herein, we report for the first time a very simple chemical method to synthesize spongy gold nanocrystals (NCs) in aqueous solution using ammonium bismuth citrate (ABC) as a reducing as well as stabilizing agent at 25 °C. Recently, several catalytic reactions have been investigated using a number of noble metal NPs.3,16-18 In case of Au NPs, * Corresponding author. E-mail:
[email protected]. Fax: 91-33-2473 2805. (1) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668-677. (2) Zhang, Z.; Sun, X.; Dresselhaus, M. S.; Ying, J. Y.; Heremans, J. Phys. ReV. B 2000, 61, 4850-4861. (3) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 1266312676. (4) Mattei, G.; de Julian, F. C.; Mazzoldi, P.; Sada, C.; De, G.; Battaglin, G.; Sangregorio, C.; Gatteschi, D. Chem. Mater. 2002, 14, 3440-3447. (5) Han, M.; Gao, X.; Su, J. Z.; Nie, S. Nat. Biotechnol. 2001, 19, 631-635. (6) Kim, Y.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165-167. (7) Zhang, J.; Li, X.; Sun, X.; Li, Y. J. Phys. Chem. B 2005, 109, 1254412548. (8) Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. Chem. Mater. 2005, 17, 566-572. (9) 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. (10) Pang, S.; Kondo, T.; Kawai, T. Chem. Mater. 2005, 17, 3636-3641. (11) Lee, G.-J.; Shin, S.-I.; Oh, S.-G. Chem. Lett. 2004, 33, 118-119. (12) Teng, X.; Liang, X.; Maksimuk, S.; Yang, H. Small 2006, 2, 249-253. (13) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; van Swol, F.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635-645. (14) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55-75. (15) Pei, L.; Mori, K.; Adachi, M. Langmuir 2004, 20, 7837-7843.
the particles supported by either polymers or resin beads are generally used as catalysts.19 Remarkably, we have found that our spongy Au NCs show pronounced catalytic activity toward the reduction of 4-nitrophenol by sodium borohydride (NaBH4) and also show autocatalytic behavior toward the further growth of Au and Ag NCs from the surface of these spongy NCs. The catalytic activity could be a consequence of the spongy nature of the Au NCs. Although a number of researchers have been extensively studied, the catalytic reduction of 4-nitrophenol by NaBH4 using polymer/resin bead-supported metal NPs,16-18 the synthesis of spongy Au NCs and the use of these NCs without any support for this catalytic purpose is completely new. The synthesis of spongy Au NCs was carried out by simple aqueous-phase mixing of ABC and HAuCl4 at 25 °C (see Supporting Information for details). The color change from yellow to reddish purple indicated the formation of colloidal Au NCs. Figure 1 shows the transmission electron microscopic (TEM) images of the as-prepared Au NCs, with the left inset showing an enlarged view of a single Au NC. This image clearly indicates the formation of polydisperse porous spongy Au NCs within the size range of 20-140 nm and does not show any other morphology. A possible reason for such high polydispersity in size is the difficulty in controlling the nucleation and continuous growth of NCs throughout the entire process. The crystalline nature of the product was revealed from the corresponding electron diffraction (ED) pattern (right inset in Figure 1) of a single Au NC. The observed diffraction spots indicate that the product is a single crystal and may have grown preferentially along the (111) direction, as concluded by other researchers.11,20 The X-ray diffraction (XRD) pattern of the spongy Au NCs shows a single intensive peak centered at 2θ ) 38.15, which corresponds to the (111) lattice plane of the face-centered cubic structure of Au crystals (JCPDF No. 4-0784), while the peaks belonging to other lattice planes are quite weak (Figure 2). Jiang et al. observed similar types of XRD patterns for Ag nanoplates and Au nanorings.20 In our case, the presence of the single intense (111) crystal plane indicates that the Au NCs are arranged unidirectionally in the spongy nanostructures, as indicated in the ED pattern (right inset in Figure 1). The Brunauer-Emmett-Teller (BET) specific surface area measurement of the dried Au NCs based on N2 gas adsorption(16) Esumi, K.; Isono, R.; Yoshimura, T. Langmuir 2004, 20, 237-243. (17) Praharaj, S.; Nath, S.; Ghosh, S. K.; Kundu, S.; Pal, T. Langmuir 2004, 20, 9889-9892. (18) Hayakawa, K.; Yoshimura, T.; Esumi, K. Langmuir 2003, 19, 55175521. (19) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (20) Jiang, L.-P.; Xu, S.; Zhu, J.-M.; Zhang, J.-R.; Zhu, J.-J.; Chen, H.-Y. Inorg. Chem. 2004, 43, 5877-5883.
10.1021/la060939j CCC: $33.50 © 2006 American Chemical Society Published on Web 07/15/2006
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Figure 1. TEM micrograph of spongy Au NCs. The left and right insets show an enlarged view and an ED pattern of a single Au NC, respectively.
Figure 3. UV-vis absorption spectra of a colloidal spongy Au NC’s suspension recorded after 2 h.
Figure 2. XRD pattern of spongy Au NCs.
Figure 4. Successive UV-vis absorption spectra of the reduction of 4-nitrophenol by NaBH4 in the presence of spongy Au NCs. The inset shows the plot indicating the variation of ln A vs time.
desorption shows a hysteresis loop (Figure S1 in the Supporting Information) with a BET specific surface area value of 42 m2/g. The literature survey shows that there is no reported value of the surface area of gold NPs. However, this value is much higher than that reported for Ag and Pt NPs.11,12 The pore diameter of these spongy Au NCs ranges from 2.4 to 6.0 nm, with a maximum population of pore sizes at ∼6 nm, as measured by the standard Barrett-Joyner-Halenda method (Figure S2 in the Supporting Information). The absorption spectra of the colloidal Au NCs suspension exhibited a clear surface plasmon resonance (SPR) band located at 545 nm along with a broad band originating from ∼800 nm (Figure 3). The first peak corresponds to the transverse component of the SPR band of Au NCs, and the band at the higher wavelength region is probably due to the unidirectional growth of Au NCs. Lee et al. and a few others have reported the appearance of a similar type of broad band at higher wavelength regions and claimed that this band is due to the unidirectional growth of dendritic nanostructures.10,11 To explore whether other metal salts also have a similar response to ABC, we performed the reduction of AgNO3 by ABC in aqueous medium. Flower-like silver NCs were obtained with an SPR absorption band at 425 nm, which is characteristic
of colloidal Ag NCs (Figures S3 and S4 in the Supporting Information). To study the catalytic activity of the spongy Au NCs, we have chosen the reduction of 4-nitrophenol by NaBH4 as a model reaction. The details of the catalytic experiments are given in the Supporting Information. In the absence of spongy Au NCs, the aqueous mixture of 4-nitrophenol and NaBH4 shows an absorption maximum at 400 nm, which corresponds to 4-nitrophenolate ion in alkaline conditions. This peak remains unaltered with time, which suggests that the reduction did not proceed in the absence of a catalyst as reported in the literature.18 However, the addition of a small amount of purified spongy Au NCs (0.006 g) with stirring causes fading and ultimate bleaching of the yellow color of the reaction mixture in quick succession. Time-dependent ultraviolet-visible (UV-vis) absorption spectra of this catalytic reaction mixture shows the disappearance of the 400 nm peak and the gradual development of a new peak at 300 nm (Figure 4). These results indicate that spongy gold NCs catalyze the reduction process. Esumi et al. and Pal et al. have also reported a similar kind of spectral change of 4-nitrophenol using dendrimerand resin bead-supported Au NPs as catalysts. The rate constant of this catalytic reaction in the presence of spongy Au NCs is
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2.1 × 10-3 sec-1, as measured from the plot of ln A (A ) absorbance at 400 nm) vs time (inset in Figure 4). This value is very close to the reported value for Au NPs supported by poly(amidoamine) (PAMAM) dendrimer, but a little lower than that supported by poly(propyleneimine) (PPI) dendrimer.18 Again, our rate constant value is higher than that of Ag NPs but is one order of magnitude lower than that of Pt and Pd NPs supported on PAMAM dendrimers.16 However, Pt and Pd NPs usually have higher catalytic activities than do Au NPs. According to Esumi et al. and Pal et al., the dendrimers and resin beads (respectively) that are used as supports for the Au NPs effect the diffusion of 4-nitrophenol to the surface of Au NPs and thus enforce the catalytic properties.17,18 This might be the reason for the slightly lower value of our rate constant than that of PPIsupported Au NPs. In our case, the spongy Au NCs have no such host materials, and hence the pronounced catalytic activity shown by these nanostructures is solely due to their unique spongy (porous) nature with high surface area (42 m2/g), and we believe this is one of the important merits of our spongy Au NCs for using them as catalysts or autocatalysts. We have also studied the autocatalytic activity of the spongy Au NCs. In a typical reaction, an aqueous solution of 0.050 mL of 1 × 10-3 M HAuCl4 or AgNO3 was separately added to 1 mL of the redispersed colloidal spongy Au NCs suspension obtained by repeated centrifugation and washing of the as-prepared sol. The absorption spectra of the resultant colloids were recorded with time using the original redispersed sample as a blank for monitoring the further growth of Au and Ag NCs. The spectra show only one broad band at 604 nm in the case of the addition of fresh HAuCl4 solution but shows two clearly distinguishable peaks at 485 and 640 nm after the addition of AgNO3 solution (Figure S5A,B in the Supporting Information). In the case of HAuCl4 addition, the increase of absorbance in the wavelength range of 495-710 nm is a clear signature of the formation of new Au NCs. Similarly, the increase in absorbance in the wavelength range of 350-590 nm in the case of AgNO3 addition indicates the formation of Ag NCs on the surface of spongy gold NCs. The energy-dispersive X-ray analysis of the resultant alloy (Au-Ag) shows the presence of both Ag and Au (Figure S6 in the Supporting Information). The integrated absorbance between the above-mentioned two wavelength ranges of the corresponding SPR bands was plotted with time (Figure 5). The data show a gradual increase in integrated absorption, which clearly indicates the gradual growth of metal (Au or Ag) NCs on the surface of spongy Au NCs (Figure 5A,B). Excess growth and a change in the shape of the Au NCs were also observed through TEM (Figure S7A,B in the Supporting Information).3 It has been well studied that the citrate reduction of metal salts results in either spherical or networked metal NPs. However, in our case, we observed the formation of Au NCs with spongy morphology. It has also been reported in the literature that the presence of foreign ions can change the shape of Au NCs.21 Thus, we assumed that the NH4+ or Bi3+ ions present in the solution might be responsible for the growth of such spongy Au NCs. To investigate which component played the key role in directing the shape of these NCs, we have performed the reduction of HAuCl4 using triammonium citrate. In this case, under the same reaction conditions, a mixture of spherical and other shaped Au NPs was obtained along with some tiny spongy nanostructures (Figure S8 in the Supporting Information). The absorption spectrum of such colloidal Au NCs exhibits only one SPR band centered at 541 nm (Figure S9 in the Supporting Information). We thought that these tiny spongy structures might act as seeds (21) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957-1962.
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Figure 5. Plot indicating the variation of the integrated absorbance of the SPR band of a suspension of Au NCs with time, after the addition of (A) 50 µL of 1 × 10-3 M HAuCl4 and (B) 50 µL of 1 × 10-3 M AgNO3 to the redispersed spongy Au NC sol.
for the further growth of larger spongy structures in the case of reduction by ABC. However, we did not observe any prominent spongy Au nanostructures in the TEM micrograph (Figure S8 in the Supporting Information). These results might suggest that NH4+ ions alone are not sufficient to control the further growth of spongy Au NCs. Again, the role of Bi3+ ions in directing the anisotropic growth of metal NPs has already been repoted.22 Thus, we believe that the presence of both NH4+ and Bi3+ ions in ABC simultaneously enhances the seeding and autocatalytic dendritic growth of Au crystals along the (111) direction and ultimately results in spongy-shaped Au NCs. A similar kind of seeding and autocatalytic growth has also been reported for Pt NCs.13 Detailed mechanistic studies of the formation of such spongy and flower-like metal NCs are the present topic of our research. The Au colloids synthesized by ammonium citrate reduction did not show autocatalytic behavior. This result strongly indicates that the Au NCs synthesized by ABC act as autocatalysts because of their typical spongy structure. We are unable to perform experiments with other Bi3+ salts because such salts undergo hydrolysis in aqueous solution. In summary, we have successfully prepared spongy Au NCs by modifying the citrate reduction technique at 25 °C. The asprepared spongy gold NCs are highly porous and surface active. These spongy Au NCs showed pronounced catalytic activity in the reduction of 4-nitrophenol by NaBH4. The spongy Au NCs also showed autocatalytic behavior in the further growth of Au and Ag NPs from its surface. Acknowledgment. Md.H.R. thanks the CSIR, Government of India, for providing Fellowship. This research was supported by grants from the Department of Biotechnology, New Delhi, and partially by the Nanoscience and Nanotechnology Initiatives, DST, New Delhi. Supporting Information Available: Experimental details for the synthesis of spongy Au NCs, the synthesis of Ag NCs, the study of catalytic activity of Au NCs, and Figures S1-S9. This material is available free of charge via the Internet at http://pubs.acs.org. LA060939J (22) Chen, R.; So, M. H.; Che, C.-M.; Sun, H. J. Mater. Chem. 2005, 15, 4540-4545.
