Formation and Stability of Gold Nanoflowers by the Seeding Approach

Aug 27, 2009 - University of Arkansas. ... The attachment of the small particles on the seed surface contributed to the growth of the nanoflowers. The...
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J. Phys. Chem. C 2009, 113, 16645–16651

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Formation and Stability of Gold Nanoflowers by the Seeding Approach: The Effect of Intraparticle Ripening Lili Zhao,† Xiaohui Ji,† Xuejiao Sun,† Jun Li,† Wensheng Yang,*,† and Xiaogang Peng*,‡ State Key Laboratory for Supramolecular Structures and Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China, and Department of Chemistry & Biochemistry, UniVersity of Arkansas, FayetteVille, Arkansas 72701 ReceiVed: June 22, 2009; ReVised Manuscript ReceiVed: August 1, 2009

The seeding approach for preparation of gold nanoflowers in which 25 nm gold nanoparticles were used as the seeds and a mixture of HAuCl4 and hydroxylamine as growth solution was investigated systematically. It is revealed that the formation and stability of the nanoflowers were affected greatly by the intraparticle ripening induced by the chlorine ions that existed in the reaction system. In this seeding approach, hydroxylamine promoted the rapid reduction of HAuCl4 and thus rapid formation of small Au particles with a diameter around 3 nm in the growth solution. The attachment of the small particles on the seed surface contributed to the growth of the nanoflowers. The branch length of the nanoflowers increased with the increased pH of the growth solution due to the suppressed ripening at higher pH. The stability of the nanoflowers can be improved by increasing the pH of the storing solution and/or removal of the chlorine ions. Introduction Gold nanocrystals have been extensively studied as active components in a wide variety of fundamental researches and technical applications due to their unique optical, electric, and catalytic properties.1,2 Great efforts have been devoted to control the size and shape of gold nanocrystals3-20 since it is well documented that their properties are both size- and shapedependent. Among them, gold nanoflowers prepared by the seeding approach have attracted special interest in recent years21-23 due to the electromagnetic field effect located around the branch of the nanoflowers, which may provide us a new chance to extend the applications of gold nanocrystals in surfaceenhanced Raman scattering (SERS)24-27 and catalysis.28 For the seeding approach carried out in aqueous solution, HAuCl4 and hydroxylamine/ascorbic acid are the most used gold precursor and reductant in the growth solution, and introduction of surfactants is known to be profitable to promote the growth of the nanoflowers.7,29-32 Many perspectives about the growth mechanism and stability of the gold nanoflowers by the seeding approach have been proposed in the literature. Some researches considered that the preferential bonding of surfactants on a given facet of a nanocrystal surface promoted the growth of the uncovered surface and resulted in the formation of the branches of the nanoflowers.21,22,26 Kuo et al. reported that the increased reaction rate with the increased concentration of ascorbic acid facilitated the formation of the branches of the nanoflowers.33 Some other publications implied that the reactivity of the ascorbic acid reductant was improved by increasing the pH of the reaction solution, leading to the fast reduction of gold salts and formation of the gold nanoflowers.23,31 Another important concern for the nanoflowers is their poor stability.33 Wu et al. found that the as-prepared anisotropic gold nanoflowers transformed into isotropic spherical gold nanopar* To whom correspondence should be addressed. E-mail: wsyang@ jlu.edu.cn and [email protected]. † Jilin University. ‡ University of Arkansas.

ticles within 1 h after the reaction attributed to their thermodynamically unstable character.34 Another publication showed that the gold nanoflowers could only be kept for a longer time when the storing solution was put in a refrigerator.35 Similar poor stability has also been reported for the nanoflowers of other noble metals. For example, Yang et al. reported the transformation of platinum nanoflowers into spherical nanoparticles and interpreted it by Ostwald ripening.36 It is deduced that the formation and stability of the nanoflowers may be dominated by the same factor since the transformation of the nanoflowers into spherical nanoparticles can be considered as a “reverse” process of the growth from spherical seeds to the nanoflowers. In our previous work, we studied the reaction of the reduction of HAuCl4 by citrate quantitatively and revealed that there are two different pathways for the growth of the gold nanoparticles dependent on the pH of the reaction sysytem.37 This encouraged us to make an attempt to reveal the key factor that dominates the formation and stability of the gold nanoflowers based on a quantitative investigation. In this work, we selected the growth of gold nanoflowers by the seeding approach as the model system, in which 25 nm gold nanoparticles were used as seeds and the mixture of HAuCl4 and hydroxylamine was used as the growth solution. The growth kinetics of the gold nanoflowers was investigated quantitatively over a wide range of pH (from 4.2 to 11.0). It is addressed that the ripening induced by the chlorine ions that existed in the system has a dominant effect on both the formation and stability of the nanoflowers. The ripening rate is identified to be related to the pH of the solution. Thus the branch length of the nanoflowers can be tuned by simply changing the pH of the growth solution and their stability can be improved greatly by increasing the pH or removal of the chlorine ions from the storing solution. Experimental Section Preparation of Gold Seeds. The 25 nm spherical gold seeds were synthesized according to the citrate reduction approach.37 A 100 mL sample of aqueous HAuCl4 (0.25 mM) was put in a 250 mL flask. After the solution was brought to a boil under

10.1021/jp9058406 CCC: $40.75  2009 American Chemical Society Published on Web 08/27/2009

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SCHEME 1: Schematic Illustration of the Formation and Stability of the Gold Nanoflowers and Their Morphology Change in the Presence or Absence of Chlorine Ions

stirring, 1 mL of 5% sodium citrate solution was added. The reaction was allowed to run until the solution became wine red in color. Growth of Gold Nanoflowers. The route for syntheses for the gold nanoflowers was modified from the literature.3,4,30,38 First, 3.7 mL of HAuCl4 aqueous solutions were adjusted to different pH values (4.2, 4.6, 5.6, 7.0, 10.8, and 11.0) by 1 mol/L NaOH solution (the final concentration of HAuCl4 is 0.25 mM). Then the mixture of 0.03 mL of NH2OH · HCl solution (40 mM) and 0.3 mL of gold seeds (25 nm, 0.25 mM) was added into the gold salt solution at 25 °C (the molar ratio of HAuCl4, NH2OH, and gold seeds is 14:17:1). With mild shaking, the color of the solution turned from pale pink to blue green, depending on the pH value of the growth solution. Detection of Gold Monomers. To determine the consumption of the gold monomer, an aliquot of the reaction solution was taken out at a definite time interval and then quenched immediately by adding it into a prepared solution of NaCl (0.9 M) and HCl (pH 1). After being kept for 24 h at room temperature, the absorbance of the supernatant was measured by UV-visible spectrophotometer to determine the concentration of the gold monomers according to the standard curve.37 Instrumentation. UV-visible spectra were measured with a CARY 100 UV-vis spectrophotometer. Temporal evolutions of the UV-visible spectra were characterized by an Ocean Optics HR4000CG-UV-NIR high-resolution spectrophotometer. TEM images were acquired by using a JEOL JEM-2010 electron microscope with an operating voltage of 200 kV. Highresolution TEM images were characterized by a JEOL JEM3010 electron microscope operated at an acceleration voltage of 300 kV. Point-to-point resolution of the HRTEM was 0.14 nm. Samples for JEOL JEM-2010 were prepared by dipping a drop of the colloidal solution (the aliquots taken during the course of a reaction) onto Formvar-coated copper grids. Specimens for HRTEM were prepared by depositing a drop of colloidal solution onto holey carbon-coated Cu grids.

the seeding approach are illustrated in Scheme 1. Gold nanoflowers are formed due to the suppressed ripening if the reaction being carried out under basic condition (pH 7.0-11.0). The nanoflowers are transformed into spherical nanocrystals attributed to the accelerated ripening related to the chlorine ions in the solution. The stability of the nanoflowers is improved greatly after removal of the chlorine ions. Transmission electron microscope (TEM) images of the gold nanocrystals synthesized with the growth solutions with different pH values by using 25 nm gold nanocrystals as seeds are summarized in Figure 1. When the growth solution was acidic, i.e., pH 4.2, the size and standard deviations of the spherical nanocrystals increased from 25 nm ((8%) (seeds, Figure 1a) to 35 nm ((11%) after the reaction (Figure 1b). This result was consistent with the classic seeding growth of spherical gold nanocrystals reported in the literature.3,4,30,38 When the pH of the growth solution was basic, the obtained gold nanocrystals were identified to present a flower-like structure. The branch length of the nanoflowers increased with the elevated pH of the growth solutions (Figure 1c,d). The average sizes and standard deviations of the nanoflowers were 40 nm ((20%) at pH 7.0 and 53 nm ((15%) at pH 11.0 as indicated by the TEM observations (at least 100 particles were measured to calculate their average sizes and standard deviations). UV-visible spectra were carried out to characterize the gold nanocrystals prepared with the growth solutions of different pH

Results and Discussion The recipe for the formation of gold nanoflowers was modified from the classical seeding approach for growth of spherical gold nanocrystals.3,4,30,38 For clarity, the concentration of HAuCl4 in the growth solution and the molar ratio of HAuCl4, NH2OH, and the seeds were kept unchanged (14:17:1) unless stated. The synthesis of the spherical nanocrystals by the seeding approach was usually carried out under acidic condition (pH 4.2-5.6). The formation and stability of the nanoflowers by

Figure 1. TEM image of (a) the gold seeds and the nanocrystals prepared from the growth solutions with pH (b) 4.2, (c) 7.0, and (d) 11.0.

Formation and Stability of Gold Nanoflowers

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Figure 2. (a) UV-visible spectra of the gold nanocrystals prepared by the seeding approach from the growth solutions with pH 4.2, 4.6, 5.6, 7.0, 10.8, and 11.0 at 25 °C. Inset: Photos of the corresponding Au hydrosols prepared at different pH values. (b) Temporal evolution of the Au(III) ion concentrations of the reactions with the growth solutions of pH 4.2, 7.0, and 11.0. (c) Temporal evolution of Au(III) ion concentrations and (d) UV-visible spectra of the gold nanocrystals for the reactions with the growth solutions of pH 4.2, 7.0, and 11.0. The concentrations of hydroxylamine contained in the growth solutions used were 0.70, 0.45, and 0.30 mM respectively to keep the consumption rate of Au(III) ion unchanged under different pH values. Inset of panel d: TEM images of the nanocrystals prepared in the growth solutions with pH 4.2, 7.0, and 11.0. The scale bar was 50 nm.

values (Figure 2a). The plasmon band of the as-prepared gold nanocrystals experienced a gradual red-shift from 526 to 595 nm corresponding to a color change from pale pink to blue green (inset of Figure 2a) when the pH was increased from 4.2 to 11.0. The changes in the optical properties of the nanocrystals shown in Figure 2a are consistent with the shape evolution of the nanocrystals observed by TEM (Figure 1b-d). Therefore, it is reasonable to represent the branch length of the nanoflowers by using their maximal extinction wavelengths. The temporal evolution of the monomer concentration during the growth of the nanocrystals was followed to elucidate the growth mechanism of the gold nanoflowers. The consumption of HAuCl4 monomers in the growth solutions of different pH values was measured as given in Figure 2b. For all three reactions, the concentration of HAuCl4 decreased sharply at the initial stage of the reactions (0-5 s) and then showed almost no consumption of the monomers at the latter stage of the reactions (5 s-3 min). It is noted that the concentration of the residual gold ions in the reactions decreased with the increased pH. When the pH value was 11.0, the gold ions in the reaction solution were exhausted almost completely. When the pH was 4.2 and 7.0, about 44% and 32% of the monomers remained after the reactions. It is known that the reductive activity of hydroxylamine increases with elevated pH value.39 Our experimental results showed that the stoichiometric reaction ratio of NH2OH to HAuCl4 decreased from 2.7 to 1.2 when the pH increased from 4.2 to 11.0 (see Figure S1 in the Supporting Information). It is plausible that the increased branch length of the nanoflowers was related to the faster consumption of the gold ions under higher pH. A series of control experiments, in which the pH was fixed at 11.0 and the consumption of the gold ions was mediated by varying the concentration of NH2OH, were carried out to further check the relation between the branch length of the nanoflowers and the consumption rate of the gold ions (see Figure S2 in the Supporting Information). The maximal extinction wavelengths

in UV-visible spectra were used to represent the branch length of the gold nanoflowers. Both UV-visible spectra and TEM observations indicated that the branch length of the nanoflowers increased with the increased consumption rate of the gold ions. It is also plausible to conclude that the branch length of the gold nanoflowers was related to the consumption rate of the gold ions. Another series of control experiments were carried out with the growth solutions with different pH values, in which the consumption rate of the gold ions was kept unchanged by changing the concentration of NH2OH (Figure 2c). However, the resulting nanocrystals obtained at different pH also showed different UV-visible spectra and morphologies although the consumption rate of the gold ions was kept unchanged (Figure 2d). The branch length of the nanoflowers increased with the increased pH value, the same as those shown in Figure 1. These results imply that there should be an unknown effect related to the pH that resulted in the pH-dependent branch length of the gold nanoflowers besides the consumption rate of the gold ions. To further understand the growth mechanism of the gold nanoflowers, the reactions carried out at pH 4.2, 7.0, and 11.0 were stopped at different reaction times by adding abundant mercaptoacetic acid to the reaction solutions. The reactions were quenched immediately since no further changes in the UV-visible spectra were observable even after 5 min (see Figure S3 in the Supporting Information). For the reaction carried out at pH 4.2, surface of the seeds became rough after 1 s of reaction (Figure 3a). For the reactions carried out at pH 7.0 and 11.0, gold nanoflowers were observed after 1 s of reaction (Figure 3b,c). It is noted that small particles with a diameter around 3 nm were observable for all three reactions, which were proven to be gold nanoparticles with single crystal structure as observed by high-resolution TEM (Figure 3d). A different orientation of the lattice fringes was observable at the end of the branch of the gold nanoflowers under high-resolution TEM (Figure 3e), indicating that the growth of the branches was not dominated

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Figure 3. TEM images of the intermediate products in the growth solutions with pH (a) 4.2, (b) 7.0, and (c) 11.0 at the reaction time of 1 s. The reaction conditions were the same as those in Figure 2d. (d) High-resolution TEM image of the small particles. (e) High-resolution TEM image for the end of the branch of the intermediate product (nanoflowers) obtained at pH 11.0. The lattice plane with a d spacing of 0.236 nm is attributed to the (111) lattice plane of the face-centered cubic gold.

by the epitaxial mechanism. Thus it is supposed that the growth of the branch resulted from the random attachment of the small gold particles on the seed surface. The intermediates at different time for the reaction at pH 11.0 were followed to elucidate this assumption (see Figure S4 in the Supporting Information). After 1 s of reaction, there were great number of small Au particles around the nanoflowers. As the reaction proceeded, the number of small Au particles decreased and disappeared almost completely after 3 s of reaction, accompanied by an increased size of the nanoflowers. These results indicated that the small gold particles were the intermediates of the reaction, which were subsequently attached on the seed surface to contribute to the growth of the gold nanoflowers. Distinctly, the growth of the nanoflowers at different pH adopted the same aggregation mechanism but not the epitaxial one. There is no difference in the growth mechanism of the nanoflowers obtained with the growth solutions of different pH values. Temporal evolution of UV-visible spectra was recorded for the above three reactions carried out at different pH values (Figure 4). Generally, there were two distinguishable stages in each reaction. First, a rapid growth process was identified for all the reactions as indicated by the rapid red-shift of the maximal extinctions. For example, the maximal extinctions shifted from 526 nm to 565 nm, 586 nm, and 598 nm respectively after 2, 5, and 10 s for the reactions carried out at pH 4.2, 7.0, and 11.0. Subsequently, the maximal extinctions underwent a blue-shift and the full width at half-maximum (fwhm) of the bands also became narrower in the latter stage of the reactions, especially for the reaction carried out at pH 4.2. For the reaction at pH 4.2, the maximal extinction experienced a blue-shift from 565 to 532 nm in 8 s (2-10 s). For the reaction at pH 7.0, the maximal extinction shifted from 587 to 557 nm after 290 s (10-300 s). However, there was almost no blue-shift observable in 300 s for the reaction at pH 11.0. The blue-shift of the maximal extinction was related to the shape evolution of the gold nanocrystals40-42 induced by Ostwald ripening or intraparticle ripening. It is obvious that the rate of the ripening decreased with the increased pH. At the pH 4.2, the seeds grew into spherical nanocrystals but not nanoflowers due to the rapid ripening. At pH 7.0, the ripening rate became slower and the product was nanoflowers with a short branch. When the pH increased to 11.0, there was almost no blue-shift of the maximal extinction at the latter stage of the reaction, indicating the very slow ripening rate of the nanoflowers. The ripening process can also be evidenced by following the color change of the as-prepared gold nanoflowers under different pH values. For example, the blue color of the gold nanoflowers

Figure 4. Temporal evolution of UV-visible spectra of the three reactions with the different pH values shown in Figure 2d.

prepared at pH 11.0 can be maintained for about 30 min after the reaction (data not shown). However, when the pH was mediated to 4.2, the color of the nanoflowers was changed from blue to pink red after 25 s (Figure 5a) and the maximal extinction wavelength shifted from 600 to 538 nm accordingly (Figure 5b). TEM observations showed that the gold nanocrystals evolved from flower-like into quasi-spherical in shape after 25 s, which was consistent with the color and spectral changes. Such spectral change and shape evolution of the gold nanocrystals is attributed to the ripening in an intraparticle manner but not interparticle way since no monomer was detectable during the shape evolution of the nanocrystals. Thus it is reasonable to represent the intraparticle ripening rate of the nanoflowers by the blue shift of the maximal extinction in the UV-visible spectra. The conceivable chemical species that may affect the intraparticle ripening of the nanoflowers were investigated by

Formation and Stability of Gold Nanoflowers

Figure 5. Temporal evolution of (a) colors, (b) UV-visible spectra, and (c) TEM images of the nanocrystals when the pH of the as-prepared gold nanoflowers was changed from 11.0 to 4.2 by addition of 1 mol/L HCl solution.

Figure 6. Temporal evolution of the maximal extinctions in UV-visible spectra of the nanoflowers when the pH was mediated to 4.2 by 1 mol/L HCl. (a) Nanoflowers purified by centrifugation and redispersed in citrate solution (0.125 mM). (b) Addition of supernatant solution of the dotted seeds into the solution of spectrum a. (c) Addition of hydroxylamine (0.3 mM) into the solution of spectrum a. (d) Addition of NaCl (1.3 mM) into the solution of spectrum a.

monitoring the shift of the maximal extinction in UV-visible spectra as shown in Figure 6. The gold nanoflowers prepared at pH 11.0 were purified by centrifugation and then redispersed in the citrate solution with pH 4.2. The maximal extinction underwent a blue shift from 600 to 535 nm after 15 min, accompanied by transformation of the nanocrystals from flowerlike into quasi-spherical in shape. Among the three conceivable species that existed in the reaction system, hydroxylamine and supernatant solution of the spherical seeds containing citrate and its oxidation products were found to suppress the intraparticle ripening of the nanoflowers to some extent (Figure 6b,c). Whereas the introduction of sodium chloride accelerated the ripening obviously (Figure 6d), it is likely that the intraparticle ripening of the nanoflowers was related to the chlorine ions. A control experiment was carried out to clarify the effect of chlorine ions on the intraparticle ripening of the gold nanoflowers. The pH of the purified gold nanoflowers was mediated to 4.2 with 1 mol/L HNO3 instead of HCl in the presence of NaNO3 and NaCl with the same concentration (1.3 mM). As shown in Figure 7a, the UV-visible spectra of the nanocrystals exhibited a blue-shift from 600 to 535 nm in 5 min in the presence of NaCl. TEM observation proved that the nanoflowers transformed into quasi-spherical nanocrystals after 5 min as a result of the intraparticle ripening (Figure 7b). However, the nanoflowers were quite stable in the presence of NaNO3. There

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Figure 7. (a) Temporal evolution of UV-visible spectra of the purified gold nanoflowers dispersed in Na3Ct (0.125 mM) solution with pH 4.2 tuned by 1 mol/L HNO3 in the presence of 1.3 mM NaCl. (b) Corresponding TEM image of the nanocrystals taken after 5 min. (c) Temporal evolution of UV-visible spectra of the purified gold nanoflowers dispersed in Na3Ct (0.125 mM) solution with pH 4.2 tuned by 1 mol/L HNO3 in the presence of 1.3 mM NaNO3. (d) Corresponding TEM image of the nanocrystals taken after 5 min.

was almost no blue-shift of the maximal extinction (Figure 7c) and the nanocrystals kept their flower-like shape after 5 min (Figure 7d). It is reliable to conclude that the chlorine ions have a key effect on the intraparticle ripening process of the nanoflowers. It is known that during the intraparticle ripening, the atoms move from one facet to another facet in an intraparticle manner.40 It is likely that the chlorine ions may bind with the gold atoms on the surface of the nanoflowers and thus facilitate the movement of the gold atoms during the ripening. At high pH, the binding of the chlorine ions with the gold atoms will be suppressed to some extent due to the increased concentration of the hydroxyl groups. Such an assumption can be supported by control experiments. No ripening is observable even in the presence of chlorine ions when a stronger ligand, mercaptoacetic acid (MPA), was introduced after the formation of the nanoflowers. The ripening will be accelerated if the same concentration of bromide ion, a stronger chelating agent with the gold atoms, was added after the formation and purification of the nanoflowers. Thus it is possible to address the stability puzzle of the gold nanoflowers based on these results. The nanoflowers underwent rapid intraparticle ripening under low pH value (4.2) in the presence of chlorine ions (Figure 8a). After the removal of chlorine ions, the stability of the nanoflowers was improved greatly. The nanoflowers showed pretty good stability even at pH 4.2 in the absence of chlorine ions (Figure 8b). If the pH of the storing solution was increased to 11.0, the nanoflowers retained their original optical properties and shape even after two weeks at room temperature (Figure 8c). It is obvious that the effect of pH on the intraparticle ripening became less prominent after the removal of the chlorine ions. On the basis of the above results and discussion, the formation and stability of the gold nanoflowers by the seeding approach is summarized as follows. At the initial stage of the reaction, a large number of the small gold particles with a diameter around 3 nm are produced due to the strong reductive activity of hydroxylamine to HAuCl4. Subsequently, the small gold particles attached to the surface of the spherical

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Figure 8. Temporal evolution of UV-visible spectra of the purified gold nanoflowers dispersed in Na3Ct solution (0.125 mM) with different pH tuned by 1 mol/L HCl (a), HNO3, (b), and NaOH (c).

gold seeds, resulting in the formation of gold nanoflowers. The branch length of the nanoflowers increases with increased pH value of the growth solutions attributed to the suppressed intraparticle ripening under higher pH. The formation of the nanoflowers is fairly correlated with the pH thus the intraparticle ripening rate of the reactions in the presence of the chlorine ions. Obviously, the chlorine ions play a key role in the intraparticle ripening process of the gold nanoflowers. The stability of the nanoflowers can be improved greatly by removal of chlorine ions. The ripening phenomenon during the syntheses of semiconductor nanocrystals has been reported in many literatures.40-42 However, the effect of ripening on the formation and stability of noble metal nanocrystals has been studied rarely up until now.37 The SERS activity of the nanoflowers was evaluated by using 4-mercaptobenzoic acid (p-MBA) as the probe molecule (see Figure S5 in the Supporting Information). The intensity of the SERS signal at 1581 cm-1 and 1070 cm-1 assigned to aromatic ring vibrations of p-MBA43 increased with the increased branch length of the gold nanoflowers, meaning an increased local electromagnetic field for the nanoflowers with longer branch.27,35 Conclusion In summary, for the reactions to synthesize gold nanoflowers by the seeding approach, the intraparticle ripening induced by chlorine ions is likely to affect both the formation and stability of the nanoflowers. The strong reductive activity of hydroxylamine resulted in the rapid formation of small gold particles with a diameter around 3 nm in the growth solution. Subsequent attachment of the small Au particles onto the seed surface contributed to the growth of the nanoflowers. The branch length of the nanoflowers is determined by the competition between the growth and ripening of the nanocrystals, which is tunable by simply changing the pH of the growth solutions. The stability of the nanoflowers can be improved by increasing the pH of the storing solution or removal of the chlorine ions. This understanding on the intraparticle ripening effect on the formation and stability of the nanoflowers is expected to be helpful for rational design and synthesis of gold nanocrystals with the desired shape and optical properties. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20803029, 20773053, 50825202), the National Research Fund for Fundamental Key Project (2009CB939701), and the Program for NCET in

Supporting Information Available: Figure S1 showing the stoichiometric ratios for the reactions of NH2OH and HAuCl4 obtained at different pH, Figure S2 showing variation of the maximal absorption wavelengths in UV-vis spectra and TEM images of the nanocrystals prepared in the growth solutions with different initial concentrations of NH2OH, Figure S3 showing UV-visible spectra of the reaction carried out with the growth solution of pH 11.0, Figure S4 showing TEM images of the intermediates obtained with the growth solution of pH 11.0 stopped by the addition of abundant mercaptoacitic acid after different times, and Figure S5 showing SERS spectra of p-MBA adsorbed on the gold nanoflowers prepared from the growth solutions with different pH values. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (3) Jana, N. R. Small 2005, 1, 875. (4) Murphy, C. J.; San, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (5) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389. (6) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414. (7) Gou, L. F.; Murphy, C. J. Chem. Mater. 2005, 17, 3668. (8) Rai, A.; Singh, A.; Ahmad, A.; Sastry, M. Langmuir 2006, 22, 736. (9) Sharma, J.; Vijayamohanan, K. P. J. Colloid Interface Sci. 2006, 298, 679. (10) Chu, H. C.; Kuo, C. H.; Huang, M. H. Inorg. Chem. 2006, 45, 808. (11) Gao, S. Y.; Zhang, H. J.; Liu, X. D.; Wang, X. M.; Ge, L. H. J. Colloid Interface Sci. 2006, 293, 409. (12) Chen, J. Y.; McLellan, J. M.; Siekkinen, A.; Xiong, Y. J.; Li, Z. Y.; Xia, Y. N. J. Am. Chem. Soc. 2006, 128, 14776. (13) Zhang, H.; Xu, J. J.; Chen, H. Y. J. Phys. Chem. C 2008, 112, 13886. (14) Niu, J. L.; Zhu, T.; Liu, Z. F. Nanotechnology 2007, 18. (15) Wang, W.; Yang, X.; Cui, H. J. Phys. Chem. C 2008, 112, 16348. (16) Jana, N. R.; Peng, X. G. J. Am. Chem. Soc. 2003, 125, 14280. (17) Camargo, P. H. C.; Xiong, Y. J.; Ji, L.; Zuo, J. M.; Xia, Y. N. J. Am. Chem. Soc. 2007, 129, 15452. (18) McLellan, J. M.; Geissler, M.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 10830. (19) Lu, X. M.; Yavuz, M. S.; Tuan, H. Y.; Korgel, B. A.; Xia, Y. N. J. Am. Chem. Soc. 2008, 130, 8900. (20) Niu, W. X.; Zheng, S. L.; Wang, D. W.; Liu, X. Q.; Li, H. J.; Han, S. A.; Chen, J.; Tang, Z. Y.; Xu, G. B. J. Am. Chem. Soc. 2009, 131, 697. (21) Lu, L. H.; Ai, K.; Ozaki, Y. Langmuir 2008, 24, 1058. (22) Yuan, H.; Ma, W. H.; Chen, C. C.; Zhao, J. C.; Liu, J. W.; Zhu, H. Y.; Gao, X. P. Chem. Mater. 2007, 19, 1592. (23) Wang, C. G.; Wang, T. T.; Ma, Z. F.; Su, Z. M. Nanotechnology 2005, 16, 2555. (24) Hao, F.; Nehl, C. L.; Hafner, J. H.; Nordlander, P. Nano Lett. 2007, 7, 729. (25) Wang, Y. L.; Camargo, P. H. C.; Skrabalak, S. E.; Gu, H. C.; Xia, Y. N. Langmuir 2008, 24, 12042. (26) Zou, X. Q.; Ying, E. B.; Dong, S. J. Nanotechnology 2006, 17, 4758. (27) Bakr, O. M.; Wunsch, B. H.; Stellacci, F. Chem. Mater. 2006, 18, 3297. (28) Rashid, M. H.; Bhattacharjee, R. R.; Kotal, A.; Mandal, T. K. Langmuir 2006, 22, 7141. (29) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782. (30) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726. (31) Chen, S. H.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (32) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (33) Kuo, C. H.; Huang, M. H. Langmuir 2005, 21, 2012. (34) Wu, H. Y.; Liu, M.; Huang, M. H. J. Phys. Chem. B 2006, 110, 19291.

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