The Initial Transformation Mechanism of Gold Seeds on Indium Tin

DOI: 10.1021/cg070474a. Publication Date (Web): February 9, 2008. Copyright © 2008 American Chemical Society. * Corresponding author. Tel: +86 592 ...
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The Initial Transformation Mechanism of Gold Seeds on Indium Tin Oxide Surfaces Zhi-jie Lin,† Xiao-mei Chen,† Zhi-min Cai,† Munetaka Oyama,§ Xi Chen,*,†,‡ and Xiao-ru Wang†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 863–868

Department of Chemistry and Key Laboratory of Analytical Sciences of the Ministry of Education, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China, State Key Laboratory of Marine EnVironmental Science, Xiamen UniVersity, Xiamen 361005, China, and DiVision of Research InitiatiVes, International InnoVation Center, Kyoto UniVersity, Nishikyo-ku, Kyoto, 615-8520, Japan ReceiVed May 23, 2007; ReVised Manuscript ReceiVed December 10, 2007

ABSTRACT: In this study, the initial transformation of gold seed on indium tin oxide (ITO) surfaces was investigated. Gold nanoparticles (AuNPs) grew on ITO surfaces using a seed-mediated method. The reduction rate of gold(I) salt and the shapes of AuNPs were affected by the concentration of cetyltrimethylammonium bromide (CTAB). Gold nanorods (AuNRs) appeared when the CTAB concentration was above 75 mM, and the shapes agreed with the dispersing state of CTAB micelles in solution. CTAB was replaced by cetyltrimethylammonium chloride (CTAC) in the growth procedure. Only small gold nanospheres (AuNSs) appeared when the CTAC concentration was increased. This result agreed with the dispersing state of CTAC micelles in solution and implied that gold salts were not free in either CTAB or CTAC solution. A red shift of 14 nm in the UV–vis spectra of AuBr4- was found in the presence of CTAB, and furthermore, AuBr2- became stable. This indicated that the gold salt strongly bound to cetyltrimethylammonium ion (CTA+) and was capped with CTAB micelles. As a result, we postulated that the growth of symmetric gold seeds into asymmetric AuNPs were caused by the initial transformation of gold seeds into asymmetric ones with the assistance of the CTA+ micelles, which capped the gold salts. Introduction Nanoscale materials have exhibited excellent size- and shapedependent properties in fields including electronics,1,2 optics,3–5 and catalysis.6 Because of these properties, great efforts have been made to synthesize various shapes of nanoparticles such as rods,3,4 cubes,7 and wires.8 Esumi et al.9 first reported that gold nanorods were prepared under UV irradiation in the presence of a cationic surfactant, but the ability to synthesize relatively monodispersed gold nanorods seemed to be lacking. Wang et al.10 developed an electrochemical method to synthesize high-yield gold nanorods with a cationic surfactant. The Murphy3 and El-Sayed groups4a intensively studied a wet chemical synthesis of gold nanorods and successfully achieved controllable synthesis of gold nanorods with various aspect ratios (Scheme 1). The cationic surfactant plays an important role in nanorod formation, and studies of the functioning of the cationic surfactant have given a profound perspective of rod formation. Jana et al.3a demonstrated that the cationic surfactant not only functioned as a stabilizer but also functioned as a soft template. More evidence was found by Nikoobakht et al.4b to suggest that there was a more defined bilayer of the surfactant on the surface of AuNRs than on the gold nanospheres (AuNSs). They attributed this phenomenon to the presence of the nanorods’ {110} surface and implied that the surfactant might direct the growth of the gold nanorods. Gai et al.11 found a defective structure on the gold nanorods {110} facet and proved that the surfactant favored binding on that facet so as to stabilize * Corresponding author. Tel: +86 592 2184530; fax: +86 592 218 4530; e-mail: [email protected]. † Department of Chemistry and Key Laboratory of Analytical Sciences of the Ministry of Education, Xiamen University. ‡ State Key Laboratory of Marine Environmental Science, Xiamen University. § Division of Research Initiatives, International Innovation Center, Kyoto University.

Scheme 1. Proposed Growing Process of AuNPsa

a Procedure 1: CTAB micelles diffuse onto seeds and cover the seeds. Procedure 2: Transformation of symmetric seeds into asymmetric rods. Procedure 3: Growth of gold nanoparticles into rods or spheres under the effect of the electric field. b: gold seed and gold nanoparticles. O: CTAB micelles that cap the gold salt.

the high energy surface. According to these results, Murphy et al.3d postulated that the gold nanoseed first experienced a single crystalline form and developed the initial {110} facet, before the surfactant was bound to the Au {110} facet and blocked the transportation of gold salt toward this facet, thus causing the anisotropic growth of gold nanoparticles (AuNPs), which led to rod formation. Pérez-Juste et al.12 considered that there was a local static electric field around cetyltrimethylammonium bromide (CTAB)-capped AuNPs, which was caused by positivecharged CTAB adsorbed on the metal nanoparticles’ surfaces. The static electric field decayed more rapidly at the tip of the ellipsoid, and, as a result, the gold salt was easier to be transported to the tips, which promoted rod formation. Although this consideration provided a feature of rod formation, it aroused the question as to how the {110} facets or the ellipsoid tip originally came into being from the symmetric spherical nanoseed, since the charged sphere contributed a symmetric electric field distribution. Although many studies have been conducted concerning the growth mechanism of AuNPs, not much attention has been paid to the study of the intermediate gold salt before it was reduced to gold. In the present study, we investigated the initial transformation of gold seed into asymmetric particles and paid

10.1021/cg070474a CCC: $40.75  2008 American Chemical Society Published on Web 02/09/2008

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particular attention to the gold intermediates in the growth solution and proposed a possible structure of the intermediate. This is because we found that the state of the intermediate played a key role in the growth of AuNPs. Because it is hard to obtain the initial state of AuNPs in solution, in this work the method developed by Zhang et al.1 was employed to grow the AuNPs directly onto the ITO surface. Since the AuNPs were grown on the ITO surface and the ITO surface is conductive, it was more convenient to capture the initial state of the AuNPs under controlled conditions using scanning electron microscopy (SEM), which presents a more direct understanding of the growth of AuNPs. On the basis of this study, we proposed a possible transformation path for gold seed and give a possible growth mechanism for AuNPs based on Mulvaney’s theory. Experimental Section Materials. Hydrogen tetrachloroaurate (HAuCl4 · 3H2O) was purchased from Sigma Aldrich (USA); sodium tetraborohydride (NaBH4), trisodium citrate, and ascorbic acid were from WAKO Chemical Company (Japan); CTAB was from Shanghai National Reagents Company; and cetyltrimethylammoium chloride (CTAC) was from Nanjing Xuanguang Science and Technology Company (China). All the other chemicals were of analytical reagent grade. All solutions were prepared with ultrapure water obtained from a Millipore purification system. ITO-sputtered glass (ITO) was purchased from CBC Ings Ltd. (Japan). Typical Preparation Procedure of AuNPs Attached ITO. The modified ITO was prepared in a typical procedure reported by Zhang et al.1 The procedures involved seed and growth solutions, as well as seeding and growing steps. The seed solution was prepared by adding 0.5 mL of 0.01 M HAuCl4, 0.5 mL of 0.01 M trisodium citrate, and 0.5 mL of 0.1 M NaBH4 in sequence into 18 mL of water while stirring. The solution was aged for 2 h, and then a bare ITO was immersed in it. After a 1 h immersion, the ITO was rinsed with ultrapure water and dried in nitrogen gas. The seeded ITO was then immersed in the growth solution for 6 h. The growth solution was prepared by adding 0.1 mL of 0.1 M ascorbic acid and 0.025 mL of 0.1 M NaOH to a solution containing 4.5 mL of 0.1 M CTAB and 0.125 mL of 0.01 M HAuCl4. All the experiments were carried out in a sealed dark case with a constant temperature of 28 °C. For comparison, a controlled experiment was performed. The ITO without seeding was immersed in the same growth solution for 6 h. Cyclic Voltametry Behavior of Ascorbic Acid on a Seeded ITO Electrode and a Bare ITO Electrode. A 1 mM ascorbic acid solution was prepared using 0.05 M Clark-Lubs buffer solution (pH ) 3.60) containing 0.3 M KNO3. The ITO electrode was connected to a strip of copper adhesive tape attached to an electrical wire. Then, a piece of a waterproof adhesive tape with a 2 mm diameter circular window cut on it was attached to the electrical face of the ITO electrode. For electrochemical measurements, the working surface contact with the solution was thus the exposed area of 3.14 mm2. Three-electrode system including saturated calomel electrode as the reference electrode, and platinum as the counter electrode, was applied. The seeded and bare ITO electrode was taken as the working electrode. Cyclic voltametry (CV) behavior of the ascorbic acid was studied in the range of 0 to 1.5 V under a scan rate of 0.1 V/s. Time-Dependent Growth. The growth solution was prepared by adding 0.1 mL of 0.1 M ascorbic acid and 0.025 mL of 0.1 M NaOH to a solution containing 4.5 mL of 0.056 M CTAB and 0.125 mL of 0.01 M HAuCl4 for this experiment. The seeded ITOs were immersed in the same growth solution for different times: 30, 60, 120, 240, 360, and 480 min, respectively. The ITOs were collected and given a UV–vis absorption test to investigate the relationship between UV–vis absorbance of the modified ITO and the amount of deposited AuNPs. Effect of CTAB Concentration. In this experiment, different volumes of CTAB stock solution (0.1 M) were used: 0.5, 1.5, 2.5, 3.5, and 4.5 mL, and then they were diluted to 4.5 mL separately using ultrapure water. The growth solution was prepared by adding 0.125 mL of 0.01 M HAuCl4, 0.1 mL of 0.1 M ascorbic acid, and 0.025 mL of 0.1 M NaOH to the above freshly prepared CTAB solutions in sequence. ITO was immersed in the growth solutions for 6 h. The ITO

Figure 1. SEM images of ITO surfaces: (a) seeded ITO surface; (b) bare ITO (no seeding) immersed in growth solution for 6 h; (c) seeded ITO immersed in growth solution for 6 h. Scale: (a) 100 nm; (b) 1 µm, (c) 1 µm. was collected and given a UV–vis absorption test. For comparison, CTAB was replaced by CTAC with the other factors unchanged. Identification of the Growth Solution Intermediate. Test solutions for UV–vis measurements were prepared as follows: S1 solution was prepared by mixing 0.030 mL of 0.01 M HAuCl4 and 2 mL of 0.1 M KBr; S2a, S2b, S2c, and S2d solutions were obtained by adding 0.030 mL of 0.01 M HAuCl4 into a 2 mL solution prepared by mixing 0.1 M KBr and 0.1 M CTAB (the CTAB volume varied from 0.03 to 0.12 mL, 0.15 and 0.30 mL in sequence); S2e solution contained 0.030 mL of 0.01 M HAuCl4 and 2.0 mL of 0.1 M CTAB; S3 solution was prepared by adding 0.030 mL of 0.01 M HAuCl4, 0.2 mL of 0.1 M KBr and 1.8 mL of ethanol; and in the S4 solution, 0.030 mL of 0.01 M HAuCl4, 0.2 mL of 0.1 M KBr and 1.8 mL of acetone were used. The precipitates from S2a, and S2b solutions were collected and analyzed by EDS. Apparatus. UV–vis spectra were obtained using a DU-7400 UV–vis spectrophotometer (Beckman, USA). SEM and energy dispersive spectroscopy (EDS) observations of the ITO surface were made using a LEO-1530 SEM (Oxford Co., UK) with an accelerating voltage at 20 kV.

Results and Discussion Preparation of AuNPs Attached ITOs and the Role of Seeds. AuNPs attached ITOs were prepared by a seed-mediated method. In the seeding process, the ITOs were immersed in a seed solution containing 3-4 nm gold seeds. Consequently, the gold seeds were attached on the ITO surface. It can be clearly found from Figure 1a that 3-4 nm seeds (marked with red circles) attach to the ITO surface. Then the seeded ITOs were immersed in the growth solution for the further growth of gold seeds. As a result, AuNPs were attached to the ITO surface (Figure 1c) tightly. In the control experiment, bare ITO and seeded ITO were immersed in the same growth solution for 6 h. SEM images clearly presented that few AuNPs attached to the bare ITO surface (Figure 1b); in contrast, the seeded ITO grew high density AuNPs (Figure 1c). It indicated that the seeds attached onto ITO facilitate the growth of AuNPs on the ITO surface. As shown in Figure 2, CV results showed that there was a dramatic decrease (about 300 mV) of oxidation potential of ascorbic acid on seeded ITO electrodes (dash line) compared to on the bare ones (solid line) and an increase of peak current. The CV results showed that gold salts were more easily reduced on the gold seed than on the ITO surface, for the oxidation potential of ascorbic acid was greatly decreased. Because the reductant was the ascorbic acid in the experiment, it indicated

Growth Mechanism of Gold Seeds on ITO Surfaces

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Figure 2. CV profiles of 1 mM ascorbic acid on bare ITO electrode (solid line) and seeded ITO electrode (dash line).

Figure 4. SEM images for AuNPs growing on ITO in the same growth solution, but with different immersion times: (a) 30 min; (b) 60 min; (c) 120 min; (d) 240 min; (e) 360 min; (f) 480 min.

Figure 3. UV–vis spectra for AuNPs growing on ITO in the same growth solution, but with different immersion times: (a) 30 min; (b) 60 min; (c) 120 min; (d) 240 min; (e) 360 min; (f) 480 min.

that the gold seed functioned as a catalyst to help AuNPs to grow on the ITO surface. Time-Related UV–vis Absorbance of AuNPs Attached on ITO. We varied the growing time of seeded ITO in the same growth solution to verify the relationship between UV–vis absorbance of gold-nanoparticle-attached ITO and the deposited amount of AuNPs. The experimental result showed that the UV–vis profiles only presented one strong absorbent peak (around 540 nm) for the ITO and that the absorbent peak gradually increased with the increase in growth time (Figure 3). But a relatively small absorption band (around 700 nm) could be observed after 120 min growth and become strong with further growing. The SEM images of the ITO shown in Figure 4 clearly indicated the relatively multidispersed state of the AuNPs. The main shape was spherical, the minor rod-shape and a little aggregation appeared after 120 min of growth. It has been reported that UV–vis absorption spectra of AuNPs is size and shape dependent,3,4 AuNRs present another plasma absorption band at longer wavelength to AuNSs. We inferred that the peak at 540 nm was mainly contributed by AuNSs, and the peak at 700 nm was contributed by AuNRs and the aggregates absorption. As a result, the strong peak (540 nm) in the UV–vis profiles and the peak intensity could be a feature of the deposited amount of AuNPs in the experiment. It could be concluded that the increase of the UV–vis absorbance around

Figure 5. CTAB concentration related UV–vis absorbance of AuNPs attached to an ITO surface. The result was the average value of three duplicate experimental results.

540 nm was caused by the increase in the amount of AuNPs deposited on the ITO. Deposited Amount and Shape of AuNPs in Relation to CTAB Concentration. It has been reported that CTAB forms a bilayer4b on the AuNPs defect faces11 and functions as a soft template. As a result, the growing rate of AuNPs is different along the long axis and the short axis, producing relatively more rod-like AuNPs in the solution.3,4a Since CTAB would greatly affect the growth of the AuNPs, we varied the CTAB concentration in the growth solution and found that the CTAB concentration affected both the amount and the shape of the AuNPs deposited. As shown in Figure 5, the amount of AuNPs deposited on the ITO surface in 6 h initially increased with an increase in the CTAB concentration, but decreased with further increase in the CTAB concentration. This indicated that the larger micelle volume with a higher CTAB concentration could help to concentrate the ascorbic acid by hydrophobic interaction or electrostatic interaction,13 resulting in accelerating the reaction rate. Further increase of the CTAB concentration, on one hand, would lead to a decrease in the concentration of gold(I) salt in

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Figure 6. SEM images for AuNPs growing on ITO in growth solutions with different CTAB concentrations. Scale: 200 nm. (a) 0.5 mL of 0.1 M CTAB (about 10 mM); (b) 2.5 mL of 0.1 M CTAB (about 50 mM); (c) 3.5 mL of 0.1 M CTAB (about 75 mM); (d) 4.5 mL of 0.1 M CTAB.

the CTAB micelles and probably increase the numbers of empty CTAB micelles. Because the aggregation number of the micelles was maintained relatively low under the second critical micelle concentration (CMC),14 the increase of the CTAB concentration would increase the amount of micelles. On the other hand, the absorption equilibrium was shifted to a more stable double layer on gold seeds surface with an increase of CTAB concentration, and thereby passive the gold seeds surface. As a result, the apparent reaction rate was decreased. From the SEM images of the ITO surface shown in Figure 6, we see that the dominant shape was spherical when the CTAB concentration was under the second CMC. With an increase in the CTAB concentration AuNRs appeared and the amount increased. However, when the concentration of CTAB was above 75 mM (3.5 mL of CTAB, Figure 6c,d), the dominant shape remained spherical. This result corresponded well with the dispersal state of CTAB micelles in the solution.14 It has been reported that most CTAB micelles were spherical below the second CMC (0.25 mol/kg), but some nonspherical micelles, such as rods, formed when the CTAB concentration was above this value. For comparison, when CTAC was selected to replace CTAB, only small AuNSs could be observed with an increase in the CTAC concentration (Figure 7). Quirion et al.15 pointed out that the bromide ion had a greater effect on the aggregation number and shape of cetyltrimethylammonium ion (CTA+) micelles than did the chloride ion. The CTA+ micelles would grow larger and transform to a cylindrical from a spherical shape with an increase in the bromide ion concentration, while with an increase in the chloride ion concentration, the size and shape of the CTA+ micelles remained the same. The experimental results showed that the deposited amount of AuNPs was affected by the concentration of CTAB and the shape of AuNPs was related to the shape of CTA+ micelles. It indicated that the gold(I) salt was transported to the surface of gold seed by the CTA+ micelles. At this point, we postulated that gold salts were not free in solution and capped inside the CTAB micelles. It was possible that the initial shape of the CTAB micelles greatly affected the final shape of the AuNPs. Identification of the Intermediate in the Growth Solution. It was important to investigate the intermediate in the growth solution before the growth mechanism is discussed, since CTAB greatly affected the deposited amount and shape of AuNPs. During the procedure of preparing the growth solution, the color changed obviously from light yellow to dark yellow while CTAB stock solution was added to the HAuCl4

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Figure 7. SEM images for AuNPs growing on ITO in growth solutions with different CTAC concentrations. Scale: 200 nm. (a) 0.5 mL of 0.1 M CTAC (about 10 mM); (b) 2.5 mL of 0.1 M CTAC (about 50 mM); (c) 3.5 mL of 0.1 M CTAC (about 75 mM); (d) 4.5 mL of 0.1 M CTAC.

Figure 8. UV–vis spectra of different AuBr4- solutions. Solvent for S1: ultrapure water; for S2c: a mixture of 1.85 mL of 0.1 M KBr and 0.15 mL of 0.1 M CTAB; for S2d: a mixture of 1.70 mL of 0.1 M KBr and 0.30 mL of 0.1 M CTAB; for S2e: 0.1 M CTAB; for S3: ethanol; for S4: acetone.

solution (S2e solution). This indicated that a new gold complex had formed in the solution. To keep the same anion, Br-, as that of CTAB, we selected KBr and kept the concentration of the Br- anion relatively constant in the experiment. Interestingly, a similar phenomenon was observed when the CTAB was partially (S2c and S2d solutions) or totally (S1 solution) replaced by the same concentration of KBr. Although the complex has been deduced as AuCl4- binding onto the CTAB micelle,12 we prefer to infer that this phenomenon arose by the formation of AuBr4- in S2e solution, because the Br- ion was more concentrated than Cl- in the experimental condition. As shown in Figure 8, in the UV–vis profiles, the spectra revealed a maximum absorption wavelength at 396 nm for S2c, S2d, and S2e solutions, but 382 nm for S1. A clear 14 nm red shift of the wavelength could be observed when CTAB was present. According to previous work16a,b on AuBr4-, there was a charge transfer from A1g (σ orbital) to A2u (π orbital) recognized as the ligand to metal charge transfer (LMCT) at 382 nm in an aqueous solution, and this indicated the formation of AuBr4- in the S1 solution. In the experiment, precipitation was observed if the molar ratio of CTAB/HAuCl4 was lower than 50:1 (V/V, S2a and S2b solutions), but the precipitates would

Growth Mechanism of Gold Seeds on ITO Surfaces Chart 1. Possible Structure of CTAB Capped AuBr4- and AuBr2-a

a

O: AuBr4- or AuBr2-; b: headgroup of CTAB.

gradually be redispersed into solution if more CTAB was added. This accorded well with earlier results.9,12 The precipitates were collected and analyzed by EDS, and the analytical result showed that C, Br, and Au were in the ratio of 18.02:3.40:1. The result confirmed that the C16TA+[AuBr4-] was formed under this condition. The maximum absorption bands of S2c, S2d, and S2e were also caused by the LMCT of AuBr4-. In the S1, S2c, S2d, and S2e solutions, the solvents were all selected to be ultrapure water and the absorption bands were caused by the LMCT of AuBr4-; the same UV–vis spectra for S1 and S2c, S2d, S2e were expected, but their UV–vis spectra were clearly different. As shown in Figure 8, a clear 14 nm red shift was found when CTAB was present. This indicated that the microenvironment was different when CTAB was present. It has been reported that the charge transition of AuBr4- in 2-CH3THF- CH3OH (v/v, 2:1) at 300 K was about 402 nm.16c We inferred that solvent effects caused the red shift of S2c, S2d, and S2e relative to S1. For σ orbital, energy was decreased in a more polar solvent while π orbital energy remained almost unchanged. When the solvent polarity was adjusted by ethanol (S3) and acetone (S4), the spectra red-shifted as the decrease of the solvent polarity was observed as in Figure 8 inset. CTAB would strongly ionize in an aqueous solution and form micelles above the first CMC (8 × 10-4 M).13 In this experiment, the CTAB concentration above the first CMC was selected. The only way to decrease the solvent polarity was by the formation of CTAB micelle capped AuBr4-, which was strongly bound to CTA+ since there was no other solvent added into S2c, S2d, and S2e. The possible structure is presented in Chart 1. The addition of ascorbic acid to S1, S2c, S2d, and S2e caused the solution to change color from dark yellow to colorless. A similar phenomenon was observed when ascorbic acid was added to the solution containing precipitates (S2a and S2b). The precipitates were analyzed by EDS, and the experimental results showed that the ratio of C, Br, and Au was 14.84:1.37:1, which indicated that the reduced product was C16TA+[AuBr2-]. Since the AuBr4- ions had been capped inside the CTAB micelles before its reduction, its reduced product, AuBr2- ions, could also be capped by the CTAB micelles. Further addition of the proper amount of NaOH to S1, S2c, S2d, and S2e solutions caused a gray precipitate in the S1 solution while S2c, S2d, and S2e

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solutions remained unchanged. This confirmed that the AuBr2ions were protected by the CTAB micelles and not simply absorbed on the micelle surface by electrostatic interaction, and that the final form of gold might be the CTAB capped AuBr2-. Possible Growth Mechanism of AuNPs on the ITO Surface. The growth mechanisms of AuNRs formed in solution have been intensively studied.3,4,12 Murphy et al.3d postulated a two step mechanism: (1) single crystalline seeds which would lead to the formation of facets; (2) the binding groups bind to certain crystal facets of the seeds, thus causing an anisotropic growth rate on different facets and helping rod formation. We have presented the spectral difference of the AuBr4- ions and the reactivity toward the base of AuBr2- ions in the presence and absence of CTAB. It could be postulated that gold salts (AuBr4- and AuBr2- ions) were not free in the solution when CTAB was present. Hence, we considered that the crystalline process of the seeds might be affected by the aggregation state of the gold salts. Mulvaney et al.12 also postulated a mechanism for AuNRs formation. They stated that there was an electric field around the ellipsoidal seeds and that the electric field decreased rapidly at the ellipsoidal tip, so that the micelles bound with gold salt were transported to the tip easily. As a result of the anisotropic growth, rods formed. However, they failed to give an explanation of the seeds’ initial tips. It would seem difficult to form an ellipsoidal shape from the spherical seed under their theory, because the spherical seed contributed a symmetric electrical field. We had indicated that the AuBr2ion was capped by the CTAB micelles and that the shapes of AuNPs accorded well with the dispersal state of CTAB micelles in solution, and so we postulated that the growing process might involve two stages. In the first stage, the CTAB micelles that capped some AuBr2- ions diffused onto the seed surface, and thus capped on the seed surface. As a result, the gold(I) salts in the micelles were reduced to Au0 by the catalysis of the gold seeds, so forming a larger AuNPs shape as the micelles the newly formed nanoparticles and the CTAB micelles capping on them functioned as rigid templates. This step could cause the symmetric gold seed sphere to transform into asymmetric gold rods if the CTAB micelle was rod-like. In the second stage, the electric field around the AuNPs12 would greatly affect the further diffusion of CTAB micelles onto the AuNPs’ surfaces. If the newly formed particles were rod shaped, the electric field would decay more rapidly at the tip, and hence it would be easier for the CTAB micelles to approach at the tip than at the side,12 and, as a result, a rod-like nanoparticle would be formed. On the basis of the above deduction, the number of rodshaped AuNPs increased, but their size was relatively small when the CTAB concentration was increased to the second CMC, and the immersion time in the solution was 6 h. As a result of the high CTAB concentration, there were more empty micelles than in the previous experiment, and the viscosity of the solution was also higher. Both of these factors blocked the reaction rate, and so the reaction might stop at the first stage and, as a result, we might capture the initial shape of the gold template. Figure 9a presents the SEM image of a seeded ITO growing in a growth solution containing 0.5 M CTAB for 6 h. Although we could not observe rod-like gold particles, numerous pairs of AuNPs are presented (circled) in the images. All the pairs are close to each other and the distance from the head of one to the tail of the other is around 20-30 nm. This distance is slightly longer than the rod-like CTAB micelle formed in pure 0.5 M CTAB solution.14 Since the salt effect14 would increase the aggregation number, and the rod-like micelles might be slightly longer in the growth solution, we inferred that the

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were capped by CTAB micelles. The CTAB micelles capped the seed when the seeded ITO was placed in the growth solution, and then the gold salt was reduced to Au0 forming new AuNPs shaped as micelles. As a result, the CTAB micelle played an important role in the initial transformation of symmetric gold seed into asymmetric AuNPs. A method based on the seedmediated method developed by Zhang et al. was applied to grow AuNPs directly on the ITO surface, which was then applied to capture the early growth stage of AuNPs. The results gave support to our deduction that the transformation was caused by CTAB micelles. After asymmetric transformation into the rod shape, the electric field around the AuNPs changed and then the AuNPs were induced to grow into rods as postulated by Mulvaney et al. Acknowledgment. This research work was financially supported by the National Nature Scientific Foundation of China (NSFC, No. 20775064), key project of NSFC (20735002), NFFTBS (No. J0630429), the Program for New Century Excellent Talents in University of China (NCET) and the Japan Society for Promotion of Science (JSPS), which are gratefully acknowledged. Furthermore, we would like to extend our thanks to Professor John Hodgkiss of The University of Hong Kong for his assistance with English. Note Added after ASAP Publication. The caption to Figure 7 was modified in a new version of this paper.

References Figure 9. SEM images of AuNPs growing on ITO in different solutions. (a) 4.5 mL of 0.5 M CTAB + 0.125 mL of 0.01 M HAuCl4; (b) 4.5 mL of 0.5 M CTAB + 0.375 mL of 0.01 M HAuCl4. Scale: 200 nm.

pairs were formed either in the same rod-like micelle or in a large micelle that covered two nearby seeds, but that they could not develop into a rod-shape due to the insufficiency of gold salt. The pairs might also be formed from two spherical micelles by diffusion onto the two nearby gold seeds, but it seemed to be impossible to form all the rods in this way since there was a considerable number of such pairs (Figure 9a). The results shown in Figure 9b confirmed the deduction that rod shapes would be formed when the amount of HAuCl4 was increased, since relatively small rod-shape particles and even small rod aggregates were seen when the volume of HAuCl4 used increased to 0.375 mL. These results implied that the initial transformation of symmetric gold seed to asymmetric AuNPs might be caused by the covering of CTAB micelles, since the micelle shape greatly affected the anisotropic transformation of gold seed and the final shape of the AuNPs. Conclusions In this study, we presented a possible transformation pathway of symmetric gold seeds into asymmetric ones. We investigated the growth solution intermediate and found that the gold salts

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