Effects of surfactant concentration on formation of high-aspect-ratio gold nanorods

Journal of Colloid and Interface Science 407 (2013) 265–272

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Effects of surfactant concentration on formation of high-aspect-ratio gold nanorods Yoshiko Takenaka a,⇑, Youhei Kawabata b, Hiroyuki Kitahata c,d, Masaru Yoshida a, Yoko Matsuzawa a, Takuya Ohzono a a

Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8565, Japan Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan Department of Physics, Graduate School of Science, Chiba University, Chiba 263-8522, Japan d PRESTO, JST, 4-1-8 Hon-cho, Kawaguchi, Saitama 332-0012, Japan b c

a r t i c l e

i n f o

Article history: Received 24 April 2013 Accepted 5 June 2013 Available online 19 June 2013 Keywords: High-aspect-ratio gold nanorod Hexadecyltrimethylammonium bromide (HTAB) Micellar structure Small-angle X-ray scattering (SAXS) Secondary nucleation

a b s t r a c t The effects of surfactant concentration in a growth solution on the elongation of gold nanorods were examined. Gold nanorods were synthesized in solutions with different concentrations of hexadecyltrimethylammonium bromide (HTAB): 100, 200, 300, 400, 500, and 600 mM. The nanorods grown in a solution with higher surfactant concentrations were longer (aspect ratio 30) than those grown in that with lower concentrations (aspect ratio <10). The self-assembled surfactant structures in the solutions were analyzed using viscosity measurement and small-angle X-ray scattering. These results showed a decrease in the inter-micellar distance with increasing surfactant concentration. Taking the chemical equilibrium for the complex formation between Au ions and HTAB micelles into account, we found that the free Au ion concentration decreases accompanied with the increase in the surfactant concentration. This decrease in the free Au ion concentration suppresses undesirable secondary nucleation of gold crystals in a growth solution, resulting in gold nanorod elongation. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Gold nanorods, i.e., one-dimensional gold crystals, are being actively studied because they are interesting in terms of the basic science of crystal growth [1] and are useful for a wide range of applications [2]. Gold nanorods with an aspect ratio (AR = length/ width) of less than 10 (low-AR nanorods) have been used as chemical sensors [3] or as medical probes to kill cancer cells in photothermal therapy, based on the absorption of the surface plasmon at around 900 nm, because this wavelength is safe for a human body [4]. On the other hand, high-AR (AR > 20) gold nanorods [5– 10] are expected to act as electrodes [11], nanogap electrodes [12,13], and nanorod arrays [14–18]. However, the industrial applications of high-AR gold nanorods are less advanced than those of the low-AR gold nanorods. It is because the method on controlling size and the high-yield synthesis of high-AR nanorods has not been enough established [5,7], compared to the case of low-AR gold nanorods [19]. It is therefore important to achieve both size-controlled high-AR nanorods and their high-yield synthesis for the ⇑ Corresponding author. Fax: +81 29 861 6236. E-mail addresses: [email protected] (Y. Takenaka), [email protected] (Y. Kawabata), [email protected] (H. Kitahata), [email protected] (M. Yoshida), [email protected] (Y. Matsuzawa), [email protected] (T. Ohzono). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.06.008

industrial application. In this paper, we mainly focus on the former, i.e., size control especially length control, to reveal how the nanorods with ARs of up to 30 can be achieved. It is well known that gold nanorods are synthesized with the reduction of Au ions in a surfactant solution [19]. In the growth solution, Au ions are anions and surfactant micelles are cations, and thus, they make complexes together by the electrostatic interaction. When gold nanorods grow, they elongate by the collision between the complexes and Au seeds or growing nanorods. To control the length of gold nanorods, we should investigate the following two issues relating the above process. First, the state of Au in a growth solution should be examined; for instance, the study about the complex formation between Au ions and surfactant micelles falls under this category. Second, the crystallization process should be examined; for instance, the studies about the collision between Au ions and gold nanorods, about the selective adsorption of chemicals on some specific facets of growing nanorods and about the templating effects of surfactant micelles fall under this category. Specific examples of studies focused on the state of Au in a growth solution are as follows [20,21]. Perez-Juste et al. suggested that the strong binding of Au ions and surfactant micelles is critical for gold nanorod formation [22]. Sharma et al. noted that a Au ion which exists as [AuBr4], instead of [AuCl4], is beneficial to form longer gold nanorods [21]. They also suggested that the complex

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formation of Au ions with larger micelles prevents the immediate reduction of Au ion and results in the elongation of gold nanorods [21]. Murphy et al. and we suggested that the number of Au seeds included in a growth solution affects the nanorod length [23,24]. There are also many reports about the crystallization process. Perez-Juste et al. demonstrated with the numerical model that the enhanced electric field at the tips of the gold nanorods and the slow collision frequency between Au ions and gold nanorods allow site-selective reduction of Au ions, which lead to the nanorod formation [22]. The site-selective adsorption of chemicals on some specific facets of growing nanorods also affects the nanorod size. Koeppl et al. reported that the nanorod growth in (1 1 0) direction is induced by the adsorption of hexadecyltrimethylammonium bromide (HTAB) molecules, which is usually used in a growth solution of gold nanorods, to specific crystal facets of nanorods [25]. Si et al. suggested that the extra amount of Br ions hinders the anisotropic growth of nanoparticles and proposed that the critical [Br]/ [Au3+] ratio to achieve nanorods exists [26]. Some researchers indicated the importance of the surfactant-micellar templates [27–29]. For instance, Jana suggested that the aspect ratio of gold nanorods was closely related to that of surfactant rod-like micelles in a growth solution [27]. Recently, some reports indicated another role of the surfactant self-assembly besides the role of the direct template for shapes of gold nanorods. For instance, Koeth et al. reported that gold nanorods can grow in the tubular network structure of surfactants [30]. These results suggest that surfactant self-assemblies in a growth solution intricately play an important role on the formation of gold nanorods. Another factor which affects the crystallization process is a small amount of additives [31–33]. Ha et al. reported the derivation of gold nanocrystal shapes including nanorods with the addition of a small amount of halide ion [31]. Smith et al. reported that the impurity in the surfactant strongly affects the gold nanorod formation [32,33]. As we mentioned above, there are many factors which should be considered for nanorod growth. Among them, we will focus on the state of Au in a growth solution, especially, the complex formation between Au ions and surfactant micelles in a growth solution. In the present paper, we have carefully investigated the effect of surfactant concentration on the complex formation. We synthesized gold nanorods at various concentrations of hexadecyltrimethylammonium bromide (HTAB) surfactant. The results will be discussed from the viewpoint of the chemical equilibrium of the complex formation between Au ions and HTAB micelles in a growth solution. Taking the chemical equilibrium into account, we indicate that the number of free Au ions which do not bind to surfactant micelles decreases following the increase in the surfactant concentration in a growth solution. We demonstrate experimentally that the secondary nucleation of seeds, which is originated from free Au ions, affects the length of gold nanorods. At last, we conclude that when the amount of free Au ions is small, the secondary nucleation of seeds is well suppressed, leading to the formation of high-AR gold nanorods.

2. Experimental part 2.1. Materials and equipment We prepared six solutions with different concentrations of HTAB (C16H33N(CH3)3Br) as the surfactant: X mM (X = 100, 200, 300, 400, 500, and 600). We measured the viscosity using HTAB supplied by Wako Pure Chemical Industries (Japan), and we used HTAB supplied by Tokyo Chemical Industry (Japan) for other experiments such as the synthesis of gold nanorods. SEM (Hitachi S-4800T, Japan), small-angle X-ray scattering (SAXS), ultraviolet–visible (UV–vis) (Shimadzu UV-2450, Japan), and

UV–vis-near-infrared (IR) (Jasco V-670, Japan) measurements were performed. (Note that the use of different companies as suppliers has no effect on the experimental results.) Pure water (resistivity: 18.20 MX cm) was produced by Nihon Millipore (Yamato Auto Pure WT 100, Japan). Gold nanorod synthesis, viscosity measurements, and SAXS observations were performed at 30 °C, which is above the Krafft temperature of HTAB (26 °C); HTAB solutions were in the micellar state according to the phase diagram [34,35]. 2.2. Gold nanorod synthesis We synthesized gold nanorods using a modified seeding method with HTAB as a surfactant. Before synthesis, each surfactant solution was kept at 60 °C so that the solution would remain well dissolved. Critical micelle concentration (CMC) of HTAB at room temperature is around 1 mM [36] and its temperature dependence [37] is reported by Zielinski et al. According to their studies, CMC of HTAB at 60 °C is 15 mM, which is much lower than 100 mM used in our study. Thus, the CMC at 30 °C in gold nanorod synthesis is also guessed to be enough lower than 100 mM, and gold nanorods are considered to be synthesized in the solution with surfactant micelles. A seed solution was prepared by mixing 1.875 mL of 100 mM aqueous HTAB solution, 62.50 lL of 10 mM tetrachloroauric acid (HAuCl44H2O; Nacalai, Japan) solution, and 150 lL of 10 mM sodium borohydride (NaBH4; Nacalai, Japan) solution, in this order. The final concentrations of tetrachloroauric acid, NaBH4, and the surfactant in the seed solution were 0.30, 0.72, and 90 mM, respectively. As soon as NaBH4 was added, the solution was rapidly mixed for 2 min, and the evolved gas was allowed to escape. The resulting brown suspension of gold seeds was stored at room temperature (25 °C) for 2 h. The gas is considered to be completely deaerated for 2 h. For the formation of gold nanorods, 9.5 mL of the surfactant solution, 400 lL of 10 mM tetrachloroauric acid solution, and 64 lL of 100 mM ascorbic acid (C6H8O6; Nacalai, Japan) solution were mixed. Here, ascorbic acid is a reductant agent for Au ions from Au(III) to Au(I) [19]. The obtained solution was clear. The final concentrations of tetrachloroauric acid, ascorbic acid, and the surfactant in the growth solution were respectively 0.40, 0.64, and 0.95X mM, where X is the millimolar concentration of the prepared HTAB solution. Next, 17 lL of the prepared seed suspension were added, and the solution was mixed gently for 10 s. The solution was then kept at 30 °C, using a thermostat bath, overnight. 2.3. SEM observations For SEM observations of the gold nanorods, we stabilized the nanorods by adding 500 lL of 1% dodecanethiol (C12H25SH; Wako Pure Chemical Industries, Japan) in ethanol solution to the aqueous gold-nanorod suspensions described above [24]. (The nanostructures did not change after the dodecanethiol treatment. This was spectroscopically confirmed from the UV–vis spectra of nanorod samples before and after the dodecanethiol treatment, as shown in Fig. S1 in the Supporting information). Then, 1400 lL of the resulting gold-nanorod suspension was centrifuged at 5000 rpm for 5 min, and the transparent supernatant portion was removed. The residual portion was redispersed in 700 lL of pure water and re-centrifuged at 5000 rpm for 5 min. After the supernatant portion was removed, the residual dense suspension was used to prepare SEM samples. For the SEM samples, 15 lL of the suspension was placed for 1 min on a Si substrate, with a hydrophobic surface treatment [29]. After the removal of excess suspension, the substrate was spun-dry at 5000 rpm for 30 s and then used for SEM observations.

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2.4. Viscosity measurements Viscosity measurements were performed using a Couette cell (Physica MCR 301, Anton Paar, Austria) with Couette geometry. Before the measurements, each surfactant solution was kept at 70 °C to induce the isotropic condition. The surfactant solution was then placed in the rheometer and kept at 30 °C for 5 min. The measurement started with the shear rate c changing from 10 to 100 s1, where the surfactant solution was in the linear flow region, i.e., it can be regarded as a Newtonian fluid. 2.5. SAXS observations SAXS measurements were performed at BL6A at the Photon Factory, High Energy Accelerator Research Organization (KEK), Japan. The incident X-rays were monochromatized by Ge (1 1 1) at a wavelength of 1.5 Å. The samples were prepared at 60 °C and then kept in a cell at 30 °C for 1 h before the observations. The q (wave number) range was 0.03 < q < 0.25 Å1. 2.6. UV–vis spectroscopy The measurements were done to examine which working hypotheses are feasible: (A) no secondary nucleation occurs or (B) secondary nucleation occurs. See Section 3.4 for details. 2.6.1. For conditions with working hypothesis (A) Solutions (200 lL) of the nanorod suspensions after growth were mixed with mixtures of pure water and 100 mM HTAB solution in the proportions shown in Table 1. Note that the surfactant concentrations of all samples were adjusted to be equal using this procedure, i.e., 100 mM. All the samples should therefore have identical viscosities and should be affected in the same way by centrifugation (5000 rpm, 5 min). The supernatant portions were used directly for the measurements. The residual portions were redispersed in 1 mL of 100 mM HTAB solution and measured. Note that all the measured samples should have the same concentration of surfactant, i.e., 100 mM. The measurements were performed at room temperature. 2.6.2. For conditions with working hypothesis (B) NaBH4 (0.10 mM, 10 lL) was added to one-fifth of the amounts of the six growth solutions described in Section 2.2 without seeds and mixed rapidly. After 10 min at room temperature, the absorption intensity at 550 nm was monitored. 3. Results and discussion 3.1. SEM images of gold nanorods Fig. 1 shows SEM images of gold nanorods grown in surfactant solutions with different concentrations. The percentage yields of nanorods for these samples estimated from SEM images are shown in the Supporting information (Table S1). The total yields were low or moderate (12–43%) for all samples, in contrast to our previous results [8]. One reason for the low yield could be the absence of additives in this case, because the additives such as nitrate [5] or

Fig. 1. Gold nanorods grown in surfactant (hexadecyltrimethylammonium bromide) solutions with different concentrations. (a) Scanning electron microscopy (SEM) images; the scale bar is 1 lm. The inserted values are the surfactant concentration in the growth solutions. The nanorods grown at higher concentration are longer than those grown at lower concentration. (b) Average lengths and standard deviations of nanorods, obtained from the SEM images shown in (a). We measured at least 100 nanorods for each sample.

nitric acid [7] are known to increase the nanorod yield. However, changes in the nanorod length, depending on the surfactant concentration, were clearly observed. For example, the surfactant solution with the highest concentration (600 mM) mainly gave high-AR (ca. 30) nanorods, in contrast to formation of a limited number of relatively short nanorods at the lowest concentration (100 mM). By measuring at least 100 nanorods from each sample, we obtained the average lengths and the standard deviations. The results showed that the nanorods grown at higher surfactant concentration are longer by ca. 500 nm than those grown at lower concentration. In the present synthesis, the surfactant should form micelles

Table 1 Sample preparation for spectroscopy. Surfactant concentration in gold nanorod suspension (mM)

100

200

300

400

500

600

Gold nanorod suspension (mL) HTAB 100 mM (mL) Pure water (mL) Total (mL)

0.2 1.2 0 1.4

0.2 1.0 0.2 1.4

0.2 0.8 0.4 1.4

0.2 0.6 0.6 1.4

0.2 0.4 0.8 1.4

0.2 0.2 1.0 1.4

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in the growth solutions because the growth temperature is above the Krafft temperature, as mentioned before. In order to evaluate the dispersion state of the micelles in the solutions, next we measured the viscosities of the surfactant solutions. 3.2. Viscosity measurements Generally, the viscosity reflects the dispersion state and the morphology of a colloid in a solution [38]. We measured the viscosity of the surfactant solutions to analyze the dispersion state and the morphology of colloids (surfactant micelles) in the solutions. Fig. 2 shows the relative viscosities gr (=g/g0) of the six surfactant solutions with different concentrations, where g and g0 are the viscosities of the sample and water, respectively. It is seen that the relative viscosity gr increased from 2 to 1200 with increasing surfactant concentration. The change in gr is linear up to 300 mM, in line with Einstein’s law, but exceeds a linear relationship above 400 mM. The nonlinear increase indicates that, at higher concentrations, the micelles would be cylindrical or worm-like and would be densely packed [39,40]. However, the surfactant concentration was too high to determine the dispersion state in more detail using viscosity measurements. 3.3. SAXS observations In order to examine the microscopic state of the six surfactant solutions in more detail, we performed SAXS measurements. Fig. 3a shows the one-dimensional SAXS profiles of the solutions. At higher concentrations of surfactant solutions, the position of the broad peak between 0.060 and 0.080 Å1 shifts to a higher q position. As shown in our previous work [29], the broad peaks represent the center-to-center distance between micelles. The solid curves in Fig. 3a are the fitting results with the core–shell model of micelles [41], taking into account the inter-particle structure factor based on repulsive forces [42], which describes the broad peaks at q 0.060–0.080 Å1. (The detailed fitting procedure is shown in the Supporting information). The fitting curves indicate that the space between micelles become smaller in solutions with higher concentration. Fig. 3b shows the characteristic center-tocenter distances between micelles in a real space, represented by the broad peaks shown in Fig. 3a. At the lowest (100 mM) and highest (600 mM) surfactant concentrations, the distances are 120 and 80 Å, respectively. This difference is relatively large compared with the diameter of a micelle (ca. 25 Å), as shown in Fig. 3b; the change in the inter-micellar distance could therefore strongly

Fig. 3. Experimental data obtained from small-angle X-ray scattering (SAXS) observations. (a) One-dimensional SAXS profiles for surfactant (hexadecyltrimethylammonium bromide, HTAB) solutions. Profiles are fitted (red solid line) with the core–shell model of micelles, taking into account the interparticle structure factor based on repulsive forces. (The details of the fitting analysis are described in the Supporting information.) At higher concentration of surfactant, the broad peaks that indicate the inter-micellar distance are shifted toward a wider angle. (b) The center-to-center distance between micelles was calculated from (a). The dotted line approximately displays the diameter of an HTAB micelle. (The cross-section of a micelle is shown in Fig. 3b; the detailed micellar structure, i.e., spherical, cylindrical, or worm-like, was not determined by the SAXS observations and the fitting analysis.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

influence the volume fraction of micelles. It might affect the complex formation of Au ions and surfactant micelles since Au ions would find binding sites more easily. (Note that there are no significant differences between the SAXS profiles of surfactants with and without Au ions, as shown in Fig. S3 in the Supporting information). 3.4. Discussion about the parameter a

Fig. 2. Dependence of relative viscosity of surfactant solution gr (=g/g0) on surfactant (hexadecyltrimethylammonium bromide) concentration; gr changes linearly at lower concentration but exceeds a linear relationship at higher concentration.

The present experimental results show that gold nanorods grown in a surfactant solution with higher concentration are longer than those with lower concentration. Why does the sample with a dense surfactant solution provide longer gold nanorods than those with a dilute surfactant solution; nevertheless, all of the samples contain the same amount of Au ions which are substances of gold nanorods? Here, we discuss how the surfactant concentration affects the elongation of gold nanorods from the viewpoints of the complex formation between Au ions and surfactant micelles and the secondary nucleation of the seeds. (Some reports showed that the high Br-ion concentration and the CTAB (=HTAB) absorption on gold surface hinder the anisotropic growth of nanoparticles [25,26]. In the present experiment, however, the similar trend was not observed because the experimental conditions were quite

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different, i.e., the large difference in the concentrations of HAuCl4 and AgNO3, and because the effects might not be observed due to the strong counter-influence of the secondary nucleation which we will discuss hereafter.) As we described, all six samples with different surfactant concentrations contain the same amounts of Au ions and ascorbic acid, and we added there the same amount of seed particles. The number of initial seeds we added is introduced to be NIS. In addition, no Au ions remain in the growth solutions after nanorod growth, as reported in our previous study of numerical modeling of the growth process [24]. In the present synthesis, the growth solutions were kept at 30 °C overnight, and this is enough time to complete the growth. In other words, all of Au ions contained initially in sample solutions must be exhausted completely and used for the growth of gold nanocrystals including nanorods as shown in Fig. 1a. Taking these facts into account, we can derive two equations for the following two issues: (1) the total volume of gold in the growth solution and (2) the relationship between the number of initial seed particles NIS and that of resulting nanorods NNR.

NNR LS ¼ mN0 ;

ð1Þ

aNIS ¼ NNR ;

ð2Þ

where N0, L, S, v, and a are the total number of gold atoms, the average nanorod length, the average nanorod cross-section, the volume of one gold atom, and the ratio of NNR to NIS, respectively. From Eqs. (1) and (2), we get

aNIS LS ¼ mN0 :

ð3Þ

Here, NNR, NIS, N0, S, and v are constants. Note that the parameter a is ideally unity when one nanorod grows from one seed particle. In this case, L is fixed to be a constant. However, the experimental results clearly demonstrated that L changes depending on the surfactant concentration in growth solutions (Fig. 1b). Thus, for Eq. (3) to hold, the parameter a should vary, also depending on the surfactant concentration. Here, we can consider the following two alternative situations where a is not unity: (A) no secondary nucleation occurs or (B) secondary nucleation occurs. When no secondary nucleation occurs, there are remaining seeds which cannot grow to nanocrystals including nanorods in a growth solution: a < 1. In this case, only a small portion of initial seeds is considered to grow to nanorods, and the other seeds must remain in a growth solution as small particles. On the other hand, when the secondary nucleation occurs, the number of resulting gold nanorods NNR is clearly larger than that of initial seeds NIS a > 1. First, we experimentally examined the working hypothesis (A). To generate longer nanorods, more Au ions should be distributed to one seed. Thus, the number of the seeds which can grow to nanorods is smaller in the sample which generates longer nanorods. Taking the present experimental results into account, the number of seeds which can grow to nanorods is smaller in a dense surfactant solution; the number of seeds which have not grown to nanorods in a dense surfactant solution is larger than in a dilute solution. Hence, more seeds must remain as small particles in the growth solution with a dense surfactant concentration than that with a dilute one. This remaining small particles should be detectable by UV–vis spectroscopy, because small gold particles have an absorption at around 550 nm due to surface plasmons, and because the absorption intensity obviously has a positive relationship with the amount of small nanoparticles. (Small particles are larger than 5 nm to at most several tens of nanometers. For reference [24], the seed particles are of size around 5 nm.) To separate the small particles from the grown crystals, including spherical and rod shapes, as shown in Fig. 1a, the sample solutions were centrifuged and the supernatant portions were analyzed by UV–vis spectroscopy (see the Experimental Section 2.6.1). According to the

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working hypothesis (A), the absorption intensity of the dense surfactant solution should be larger than that of the dilute one, because small particles cannot be removed by standard centrifugation at 5000 rpm for 5 min and thus remain in the supernatant portion. (Note that we integrated the absorption-intensity profiles from 300 to 800 nm to rule out the effects of peak shifts.) Fig. 4 shows the relationship between the surfactant (HTAB) concentration in the just-grown gold-nanorod suspension and the integrated absorption intensity. The results suggest that the supernatant portion in the denser surfactant solutions contain smaller amounts of small nanoparticles. In contrast to the working hypothesis (A), these results indicate that larger numbers of remaining seeds exist as small particles in a dilute solution than in a dense solution. Thus, the working hypothesis (A) is not feasible. We should therefore consider the working hypothesis (B). Based on the working hypothesis (B), the secondary nucleation occurs in a growth solution. To discuss the kinetics of the secondary nucleation, we focus on how Au ions exist in the growth solution. It has been reported that complexes consisting of Au ions and HTAB micelles contribute to nanorod growth [22]. If we set formal 1:1 complexation between a Au ion and an HTAB micelle, the chemical equilibrium is written as

Au ion þ HTAB micelle¡complex of Au ion and HTAB micelle:

ð4Þ

(Here, the equilibrium constant K = [Au ion][HTAB micelle]/[complex of Au ion and HTAB micelle], is quite low; the detailed value is not known.) In a growth solution, the excess NaBH4 in the seed solution would result in secondary nucleation during the rod growth. NaBH4 can reduce free Au ions although it cannot reduce the Au ions bound to HTAB micelles [22]. Thus, secondary nucleation occurs if NaBH4, whose amount seems to be quite small, incidentally meets free Au ions that are not bound to HTAB micelles. As shown in Fig. 3b, the inter-micellar distance decreases with increasing surfactant concentration, suggesting that the number of micelles per unit volume increases with increasing surfactant concentration. (We did not determine the exact structure of the micelles, so we do not refer to the total number of micelles. Nevertheless, it can be confirmed from Fig. 3b that the effective number of micelles bound to Au ions increases with increasing surfactant concentration.) Following the increase in the surfactant concentration,

Fig. 4. Relationship between surfactant (hexadecyltrimethylammonium bromide) concentration in the just-grown gold-nanorod suspension (before centrifugation) and the integrated absorbance of the residual and supernatant portions of the resulting growth suspensions (after centrifugation). The absorption intensity was integrated in the range from 300 to 800 nm to avoid the effects of peak shifts. The supernatant portions (circle) in the dense surfactant solutions contain smaller amounts of nanoparticles than in the dilute solutions, whereas all the lower portions (square) contain almost the same amounts of nanoparticles.

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the position of the equilibrium in (4) then shifts to the right, and the amount of free Au ions decreases. Accordingly, secondary nucleation is assumed to occur more difficult in a dense surfactant solution than in a dilute one. Therefore, more seeds must be generated by secondary nucleation in a dilute solution and they exist as small particles in a growth solution. To confirm this working hypothesis (B), we measured the absorption intensity at 550 nm of the six growth solutions without seeds 10 min after adding NaBH4 (see the Experimental Section 2.6.2). As Jana reported, when the small particles are organized by the reduction with NaBH4, the just-organized small particles will grow to larger particles which absorb at 550 nm, because NaBH4 is a strong reductant agent [27]. Fig. 5 demonstrates the decline of the absorbance at 550 nm accompanied with the increase in the surfactant concentration, in accordance with the working

Fig. 5. Absorbance at 550 nm of the six growth solutions without seeds 10 min after adding NaBH4. The figure shows that smaller amounts of nanoparticles are organized in a dense surfactant (hexadecyltrimethylammonium bromide) solution.

hypothesis (B). It means that Au ions are more fixed by the micelles and are not available for the reduction process by NaBH4 (for secondary nucleation) at higher surfactant concentration. Of course, this only means that the effective Au ion concentration for secondary nucleation is lower at higher surfactant concentration, but not assure that the rest Au ions will predominantly form nanorods. Nevertheless, we can understand all the results reasonably if we assume that the rest Au ions predominantly form nanorods. Thus, the results can suggest that secondary nucleation of seeds in a growth solution occur, and the rate of nucleation decreases with increasing surfactant concentration. As a consequence of the increase in the number of seeds in a dilute solution, the number of Au ions per nanorod decreases and therefore the length of the gold nanorods decreases. We can summarize the elongation mechanism of gold nanorods using schematic illustrations for dilute and dense surfactant solutions shown in Fig. 6. Here, we show the unit area of HTAB micelles against one binding Au ion for simplicity. In both cases (dense and dilute surfactant solutions), the growth solutions contain the same amounts of Au ions. When the solution is dilute, some Au ions are free and the others bind to HTAB micelles. By the increase in the surfactant concentration, the number of HTAB micelles in the left hand side of the chemical equilibrium increases and then the equilibrium shifts to the right hand side. As a result, the number of free Au ions that are not bound to HTAB micelles decreases with increasing the complexes in the right hand side. Onset of the addition of seeds, small amounts of reductant agents would be added at the same time. Secondary nucleation would occur when the reductant agents incidentally meet free Au ions, and therefore, the resulting number of seeds in a dilute solution is larger than in a dense solution. The number of Au ions per seed in a dilute solution decreases and the nanorods become shorter. In contrast, one seed can be associated with more Au ions in a dense solution than in a dilute solution; therefore, the seeds can grow to longer gold nanorods.

Fig. 6. Schematic illustrations of the present mechanism. When the solution is dilute, some Au ions are free and the others bind to HTAB micelles. By the increase in the surfactant (HTAB) concentration, the number of HTAB micelles increases and the equilibrium shifts to the right hand side. As a result, the number of free Au ions that are not bound to HTAB micelles decreases. Onset of the addition of seeds, small amounts of reductant agents would be added at the same time. Secondary nucleation would occur when the reductant agents incidentally meet free Au ions, and therefore, the resulting number of seeds in a dilute solution is larger than in a dense one. In other words, one seed can be associated with more Au ions in a dense solution than in a dilute one; therefore, the seeds can grow to long gold nanorods in a dense solution. Here, we only show the cross-section of HTAB micelles because the exact structure (spherical, cylindrical, or worm-like) was not determined in this study.

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3.5. Nanorod growth below the Krafft temperature In general, for cationic surfactants, the intrinsic Krafft temperature is the critical parameter in forming a micellar structure or a hydrated solid structure in a solution. In the present study, the synthesis and observations were made at 30 °C, which is slightly higher than the Krafft temperature of HTAB (26 °C). We discuss whether or not the present mechanism is appropriate to previous studies performed below the Krafft temperature of the surfactant used in the growth solutions [8,29,43]. We have already reported that high-AR gold nanorods, which have a long-axis length of 1000 nm and an AR of 50, can grow with high yields (>90%) in mixed HTAB and OTAB surfactant solutions below the Krafft temperature, even without additives such as nitrates [8,29,43]. In such a case, the structure of the surfactant self-assembly is a hydrated solid, which forms a lamellar structure with periodic stacking of bilayer membranes with water layers between them [43]. When the surfactants form hydrated solid structure in a growth solution of gold nanorods, hydrophilic Au ions should exist in the water layer between the bilayer membranes. Thus, it seems that they might have many chances to bind surfactant molecules similar to the situation as the denser surfactant solution in the present experiments. Assuming that the complex of Au ions and surfactant molecules is thermodynamically stable even when the hydrated solid structure appears, a similar mechanism to that proposed in this study, i.e., suppression of secondary nucleation by the tight adsorption of Au ions on the surface of the surfactant hydrated solid, could explain previous results [8,29,43]. Although the assumption is feasible, it is too hasty to conclude from the present results whether or not the phase transition of a surfactant solution and the difference between the self-assembled structures are effective for complex formation of Au ions and HTAB molecules, and eventually for nanorod elongation. The validity of this assumption needs to be explored in future studies.

4. Conclusion In this study, we examined how the surfactant concentration in a growth solution affects the elongation of gold nanorods. To reveal the growth mechanism of gold nanorods, many factors have been studied [5,7,21–33]. Among them, here we have focused on the complex formation of Au ions with surfactant micelles in a growth solution. We have synthesized gold nanorods in solutions with different concentrations of HTAB. The self-assembled HTAB structures in the solutions were analyzed using both viscosity measurement and small-angle X-ray scattering. They revealed that the volume fraction of micelles increases in a denser HTAB solution, and it suggests that the frequent complex formation of Au ions and HTAB micelles might affect the nanorod growth. Taking the chemical equilibrium of the complex formation between Au ions and HTAB micelles into account, we presented the following mechanism on how the surfactant concentration affects the nanorod elongation. The amount of free Au ions which is not bound to HTAB micelles in a growth solution with a high HTAB concentration decreases as a result of enhancement of the complex formation with increasing the number of HTAB micelles, according to a simple chemical equilibrium between Au ions and HTAB micelles. In a dense growth solution, secondary nucleation hardly occurs and therefore the number of seeds which can grow to nanorods is smaller than in a dilute solution. Consequently, the long gold nanorods (AR  30) can grow in a dense surfactant solution. Hence, we concluded that the prohibition of secondary nucleation of seeds in a growth solution effectively promotes the formation of high-AR gold nanorods.

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