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C: Physical Processes in Nanomaterials and Nanostructures

Growth Behavior of Gold Nanorods Synthesized by the SeedMediated Method: Tracking of Reaction Progress by TimeResolved X-Ray Absorption Near-Edge Structure, SmallAngle X-Ray Scattering, and Ultraviolet-Visible Spectroscopy Yoshikiyo Hatakeyama, Koh Sasaki, Ken Judai, Keiko Nishikawa, and Kazuyuki Hino J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00016 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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The Journal of Physical Chemistry

March 19, 2018

Growth Behavior of Gold Nanorods Synthesized by the Seed-mediated Method: Tracking of Reaction Progress by Time-resolved X-ray Absorption Near-edge Structure, Small-angle X-ray Scattering, and Ultraviolet-visible Spectroscopy

Yoshikiyo Hatakeyama,*,†,‡ Koh Sasaki,§ Ken Judai,† Keiko Nishikawa,|| and Kazuyuki Hino*,§

†

College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku,

Tokyo 156-8550, Japan §

Faculty of Education, Aichi University of Education, 1 Hirosawa, Igaya, Kariya, Aichi

448-8542, Japan ||

Graduate School of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522,

Japan

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ABSTRACT: Gold nanorods (GNRs) are attractive and important nanomaterials that show enormous promise for a wide range of applications. To investigate the formation process of GNRs generated by the seed-mediated method, we tracked the growth of GNRs with aspect ratios of 2 (GNR2s), 4 (GNR4s), and 6 (GNR6s) using time-resolved X-ray absorption near-edge structure (XANES). Moreover, for GNR6s, additional measurements by small-angle X-ray scattering and ultraviolet-visible spectroscopy were carried out for longer reaction times of up to 20,000 s. Cetyltrimethylammonium bromide (CTAB) was used as the surfactant in the generation of GNR2s and GNR4s, while benzyldimethylhexadecylammonium chloride (BDAC) plus CTAB was used for GNR6s. The three analysis methods used provided consistent and compensatory results. It was found that GNR2s and GNR4s finish growing by 2,000 s, consuming all the Au in the solutions, and that GNR6s keeps growing for more than 20,000 s by a different formation process. From these comprehensive results, it was revealed that severe competition for existence among the GNRs occurs in all solutions. The seed particles added to the solutions of GNR2s and GNR4s start growing but the whole seed particles cannot mature into GNRs. Conversely, in the solution for GNR6s, some of the already-grown GNRs release Au atoms, allowing the growth of further GNRs, making the formation process slow, unique, and complex. The growth of GNR6s coordinated by CTAB and BDAC in appropriate proportions continues more than 20,000 s. In particular, back-and-forth growth of GNR6s is first observed by tracking the growth solution using time-resolved XANES.


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1. INTRODUCTION Among the various metal nanomaterials, gold nanorods (GNRs) have attracted particular attention for more than two decades.1–4 This is because their attractive properties may be applied in a wide range of fields,5–8 including catalysis,9–12 optical devices,13,14 and cancer treatment.15,16 An example of their characteristic properties is their distinctive surface plasmon resonance, which is individually localized along the major and transversal axes.1,17 The properties and functions of GNRs are strongly dependent on their size and aspect ratio (AR),18–22 which are controllable by altering their synthesis procedure and conditions.23 Among the various methods for the synthesis of GNRs,2,3 the development and improvement of the seed-mediated method has proved particularly valuable for facilitating their study.24–27 In early studies on this method, the effect of seed concentration,25 type of surfactant,26 and optimum concentrations of Au and surfactant27 were investigated, and later the roles and effects of Ag ions,28–30 surfactants,31 impurities,32 and seed age33 were also revealed. A practical guide for the synthesis of GNRs has been reported, allowing for the identification of unresearched aspects of their preparation.34 As well as the development of applications for GNRs, a great deal of research effort has been focused on investigating and developing their complex formation process. Understanding the GNR growth process is essential to refine current preparation procedures and develop new ones, and this may only be achieved by observing the growth of GNRs very carefully. This has been attempted through the use of a range of methods, including ultraviolet-visible spectroscopy (UV-Vis),35 transmission electron microscopy (TEM),36,37 small-angle X-ray scattering (SAXS),38,39 and X-ray absorption fine structure (XAFS).40



TEM observation provides information not only on the size and shape of GNRs, but also on their formation yields.25,41,42 UV-Vis is used to confirm the formation of GNRs because the spectra change enormously depending on the AR of the GNRs. The use of SAXS and XAFS is less convenient because synchrotron radiation is necessary in most cases. However, SAXS provides a considerable amount of information regarding the size, shape, and especially the maximum length of GNRs.33,39,43,44 Moreover, the development of the Pilatus range of high-performance two-dimensional detectors by DECTRIS Ltd.45,46 has facilitated the performance of time-resolved SAXS measurements. Additionally, from the results of XAFS measurements, and particularly from changes in the X-ray absorption near-edge structure (XANES), we can obtain information on the growth of GNRs,47–51 and pioneering research using XANES on the growth progress and reduction of Au+ to Au therein has been reported.40 Needless to say, these measurement methods are far more effective when used in combination. In the current study, we investigated the formation of GNRs with ARs of 2, 4, and 6 prepared according to the seed-mediated method.24 These particles are termed GNR2s, GNR4s, and GNR6s, respectively, hereafter. We have already reported a SAXS and UV-Vis study on the growth processes of GNR2s, GNR4s, and GNR6s from the start of the reaction to the apparent stopping point of 4000 s.39 In the present study, we performed time-resolved quick XAFS (QXAFS) measurements to track the generation of the three types of GNRs. QXAFS measurements revealed that GNR2s and GNR4s stop growing by ca. 2,000 s, and that only GNR6s continues to grow for a longer time, a few or several days. Consequently, we undertook


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SAXS study of GNR6s for 20,000 s. The SAXS data for GNR2 and GNR4 in this report are those reported previously.39 With both the XAFS and SAXS measurements, UV-Vis spectra were recorded simultaneously to monitor of the progress of the reaction. However, simultaneous XAFS and SAXS analyses were not performed because it is extremely difficult to obtain both data at the same time with satisfactory accuracy.52 Time-resolved XANES, SAXS, and UV-Vis analyses were used to obtain the reaction-time dependences of Au consumption rates in the growth solutions, the maximum lengths of the GNRs, and their ARs. Finally, combining these results, the structural changes, number of GNRs, and growth process for GNR6s were discussed using the data for GNR2s and GNR4s as a reference.

2. EXPERIMENTAL SECTION 2.1 GNR Synthesis GNRs were prepared using the seed-mediated growth method developed by Jana et al.24,25,53 and improved by Nikoobakht and El-Sayed.27 The method enables us to control the length of the GNRs by changing the concentration of Ag ions in the growth solution. Ag is thought to have several roles,31 and it is needed to generate GNRs in any case. It is known that Ag ions are reduced and located on the surface of the GNRs,29 and they also form complexes with the surfactant and encourage anisotropic growth of the Au seeds.4 To generate GNRs with ARs of 2 (GNR2s) and 4 (GNR4s), we prepared a growth solution containing 5.0 mL of HAuCl4 (1.0 mM) and 5.0 mL of cetyltrimethylammonium bromide (CTAB, 0.20 M). To this solution, we added 50 and 200 µL of AgNO3 (4.0 mM)



for GNR2s and GNR4s, respectively. Then, we mixed 110 µL of ascorbic acid (78.8 mM) and 100 µL of seeds (0.236 mM as Au content) into the respective solutions, which is somewhat different from the procedure in the literature.27 To generate GNRs with an AR of 6 (GNR6s), we prepared a growth solution of 5.0 mL of benzyldimethylhexadecylammonium chloride (BDAC) plus CTAB as surfactant (in a BDAC/CTAB molar ratio of 2.5), to which we added 5.0 mL of HAuCl4 (1.0 mM) and 200 µL of AgNO3 (4.0 mM). During synthesis, the temperature of the sample was kept at 27.0 ± 0.2 °C by circulating temperature-controlled water. These synthesis methods are identical to those reported in our previous study.39 Figure 1 shows typical TEM images of the GNRs synthesized by these procedures. Their average lengths (more than 100 particles) were 27.5 nm (GNR2s), 33.0 nm (GNR4s), and 41.5 nm (GNR6s) with the standard deviations, σ, of 4.8 nm, 4.1 nm, and 4.8 nm respectively. Moreover their average ARs were 2.2 (σ = 0.4), 3.4 (σ = 0.3), and 5.6 (σ = 0.9).

2.2 XANES Measurements The XAFS measurements were performed using the apparatuses at the BL-7C, -9C and -12C,54–57 Photon Factory (PF) operated at 2.5 GeV and 450 mA, in Institute for Materials Structure Science, the High Energy Accelerator Research Organization (IMSS-KEK), Tsukuba. Au L3-edge XANES spectra were recorded in a fluorescence mode using a Lytle-type detector. The X-ray radiation was monochromatized using a Si(111) double crystal. The monochromator was rotated and QXAFS spectra were obtained at 30 s per


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spectrum for the Au L3-edge region from 11,870 to 12,050 eV. It took a further 2–3 s to return the monochromator to the original position. Thus, a QXAFS measurement was carried out every 32–33 s. The sample holder had an inner diameter of 10 mm and a thickness of 30 mm. An illustration of the sample holder is shown in Figure S1 in the supporting information (SI). The edge energy was calibrated with Au foil

several times

during the measurements. The ion chamber for the incident intensity and the Lytle-type detector for the fluorescence X-ray were N2 and Ar, respectively. As a pre-analysis step, the XANES spectra were averaged every five spectra. The XANES data were analyzed using the XAFS analysis package REX2000 coded by the Rigaku Corporation.56–58

2.3 SAXS Measurements We have previously reported results for the growth process of GNR2s, GNR4s, and GNR6s using time-resolved SAXS.39 In the current study, time-resolved SAXS measurements were carried out to track the growth of GNR6s for 20,000 s with considerable accuracy because only GNR6s continue to grow for this length of time, as discussed later. The SAXS experiments were performed using the apparatus at the BL-6A station, PF at KEK, Tsukuba. A two-dimensional Pilatus 1M detector (DECTRIS Ltd.) was used to acquire the SAXS intensities. The camera length was determined to be 2,520 mm by the diffraction of silver behenate. The observable q-region was 0.07–2.76 nm−1, with scattering parameter q = 4πsinθ/λ, where 2θ is the scattering angle and λ is the wavelength of the X-rays. The accumulation time for each exposure was 30 s, and it took a further 0.3 s to transfer the data to the control computer. Thus, SAXS measurements were carried out every



30.3 s. The intensities of the incident and transmitted X-rays were also measured to obtain the absorption coefficient of the sample using an in situ beam monitor apparatus.61 Determination of the precise absorption factor is important because it seriously affects the precision of the background and absorption corrections. X-ray profiles from the sample holder filled with water only were used to determine the absorption contribution and eliminate the background intensity. The path length of the sample was 1.0 mm. Before analysis, every two SAXS patterns were averaged.

2.4 UV-Vis Measurements As previously mentioned, UV-Vis spectroscopy is a convenient method to identify the growth of GNRs because they have two particular plasmon bands, i.e., the transverse and longitudinal plasmon bands. In this study, the simultaneous measurement of UV-Vis spectra was used to confirm the progress of the reaction during XAFS and SAXS measurements. UV-Vis light emitted by the light source (Hamamatsu Photonics, L10671; output range 200–1,100 nm) was guided to the sample holder by an optical fiber system. Parallel lenses were set at both the incident and output sides of the sample holder. The output signal was guided to a charge-coupled device array spectrometer (B&W TEK, Inc., BRC642F; resolution of 1.7 at 546 nm), and continuously recorded every 30 s. The path length of the sample was 1.0 mm. We ensured that the probe light did not affect the GNR6s synthesis. Moreover, the spectral changes during XANES and SAXS measurements were consistent. Thus, it was confirmed that X-ray irradiation did not significantly affect the growth of GNR6s. For the actual analysis, the absorption spectra measured at the same time during


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the SAXS measurements were used because the spectra were recorded for a longer time and had a better signal-to-noise (S/N) ratio than that of the data obtained from the XANES experiments. Other experimental details of the simultaneous measurements of UV-vis spectra are the same as those reported in our previous paper.39

3. RESULTS AND DISCUSSION 3.1 Change of XANES with Reaction Time Figure 2 shows the time-resolved XANES spectra of the GNRs where all spectra are normalized by edge height. The first-order derivatives of the spectra are taken to make their changes easy to compare (See Figure S2 in the SI). A XANES spectrum of Au foil and its differential spectrum are shown in Figure S3 in the SI as references. The two peaks for GNR2s and GNR4s at ca. 11.94 and 11.97 keV, which appear immediately after the start of the reaction, are smaller than those for the Au foil (see Figure S3a in the SI), and the intensities get closer to those of Au foil as time advances. In the case of GNR6s, the reaction progress is much slower than those of the other two types of GNRs. This difference is evident in the differential XANES spectra shown in Figure S2 in the SI. While peaks are observed around 11.93 keV for GNR2s and GNR4s, the corresponding peak is not present in the spectra of GNR6s. (see Figure S2 and S3b in the SI). This slow growth trend is consistent with our previous research using time-resolved SAXS.39 In order to discuss the progress of the reaction in more depth, the XANES spectra were separated into two contributions using the spectrum of the growth solution (Figure S4a) and the spectrum of Au foil as standards, which are shown in Figure S3a in the SI.



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Because it is possible to substitute the spectrum of Au foil for that of the GNRs,40 we can discriminate between the Au atoms in the growth solution (AuGS) and Au atoms in the GNRs (AuRod). Figure 3 shows the ratios of AuGS to AuRod versus the reaction time obtained by fitting analysis and R factors defined by, R = Σ{aobs(E) − acalc(E)}2/Σ{acalc(E)}2

(1)

where aobs(E) and acalc(E) are the observed X-ray absorption coefficient and calculated one, respectively. The R factors are as small as the one in the literature.47 It will be possible to say that such good agreements between XANES spectra of the determined compositions and the experimental data assure the appropriate assumption and the proper analysis in this study. In the case of GNR2s and GNR4s, the Au atoms in the growth solutions are consumed rapidly and the reactions complete in 2,000 s. GNR2s complete their growth faster than GNR4s. Conversely, GNR6s grow slowly, and only the back-and-forth reaction occurs after 4,000 s. Although, in our previous study by SAXS,39 we did not obtain precise intensities after 4,000 s, we realized that the growth velocity would decrease. The only difference between the solutions for GNR4 and GNR6 is the surfactant. The presence of BDAC in the GNR6 system causes the effective anisotropic growth of particles and the decrease of the reaction velocity. Although we could possibly consider the influence of Ag atoms existing on the surface of the GNRs on the Au spectrum,29 it is thought that they do not cause any problems in the analysis because the ratio of Au atoms on the surface is small. This assumption is reasonable because the XANES spectra are well separated, as shown in Figure S4b in the SI. The important matter is that the ratios of Au existing as AuGS or AuRod can be investigated. Here, at the start of the reaction, most of the Au comes from HAuCl4 and the 10 ACS Paragon Plus Environment

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amount of Au in the seeds is less than 1% of the total amount. In the case of GNR6, half of the Au atoms existing as AuGS are used in the early stage of growth up to 4,000 s. Specifically, when the concentration of AuGS decreases to half, the growth of GNR6 stops.

3.2 Growth in Length of GNR6 The size of the GNRs, especially the maximum length, was determined from analysis of the SAXS patterns. Figure 4a displays the time-resolved SAXS patterns for GNR6 against q after corrections for the intensity fluctuation of incident X-rays, background intensities, and absorption effects.62 The increase of scattering intensity in Figure 4a stops at 4,000–8,000 s, which indicates that the consumption of AuGS in the GNR6s solution has stopped. This reflects the results obtained using time-resolved XANES measurements. Distance distribution functions (DDFs), P(r), were derived by the Fourier transformation of SAXS patterns to estimate the maximum length of the GNR6s.63 P(r) gives reliable information on the lengths of the GNRs because neither approximation nor assumption is used in the function. The results are shown in Figure 4b. In the DDF analysis, the value of r at which P(r) = 0 corresponds to the maximum length of the particles. Because there were no byproduct particles larger than the GNRs in the system (as shown by the TEM observations in Figure 1), the maximum length obtained from the DDF analysis corresponds to the length of the GNRs.63 Figure 5 shows DDFs from every ca. 1,700 s. It is clear that the GNRs grow rapidly in the early stage of the reaction, and grow slowly after the temporal backset of the growth. In the DDF curves, based on previous reports, we observed shoulders around r =



25 nm after 12,000 s, which can be attributed to the formation of considerably large spherical and cube-shaped particles.39,63 In fact, particles other than GNRs are frequently observed in the TEM images, as shown in Figure 1. The reaction time dependence of the maximum lengths extracted from the DDFs for GNR6s is shown in Figure 6. The length obtained from DDF contains analytical error of ±0.2 nm. The growth in length proceeds rapidly until ca. 4,000 s and then plateaus. These phenomena correspond well to the data for the consumption of Au in the growth solution (Figure 2). After 4000 s, the curve of the maximum length shows a slight but gradual increase. Here, we must convey uncertainty of the DDF analysis. As mentioned above, the DDF is the Fourier transform analysis in which neither approximation nor assumption is used. However, there is a weak point. Namely, while the result of the analysis is correct for the system composed of dispersed particles with a relatively uniform size, DDF value is strongly affected by the contribution from larger particles for the system of particles with various sizes. This is because SAXS intensity increases in proportion to not only the number of particles but also the square of the volume of particles. As a result, when larger particles exist, even if few, they give a huge contribution to the SAXS intensity. In the present system, it is known that the size distribution of the rod becomes larger as the reaction time as can be seen in Figure 1. Namely, the maximum length in the present study does not correspond to the one of the greatest number of the rods but include the contribution of few larger rods. The gradual increase of the maximum length after 4000 s shown in Fig. 6 may imply the slight increase of the size as a whole, and an extreme


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growing of few rods.

3.3 Determination of the AR for GNR6 The reaction time dependence of the AR for GNR6s was elucidated from the time-resolved UV-Vis data. The UV-Vis spectra shown in Figure 7 change significantly immediately after the beginning of the reaction, much like the XAFS spectra and SAXS patterns. First, the positions of the longitudinal plasmon bands around 750–900 nm and those of the transverse plasmon bands around 500 nm are determined. Another absorption band due to byproducts appears at the longer wavelength side of the transverse plasmon band around 520 nm,22,34,64 so each peak was separated by a combination of damping and Gaussian functions65 as shown in Figure S5 in the SI. Although it is known that large cubic Au particles exhibit complex plasmon bands,66,67 we treated the plasmon bands as single peaks in this analysis because they are smaller than 30 nm.67–70 The peak positions obtained in this method are shown in Figure 8a. As the reaction progresses, the peak position of the longitudinal plasmon band shifts to longer wavelength, while the peak positions of the transverse plasmon band and the byproducts are mostly unchanged. A shift of the longitudinal plasmon band is well known to be proof of increasing AR. The ARs of the GNRs were determined from the peak positions of the longitudinal plasmon bands22 and are shown in Figure 8b. As with the data obtained by XANES and SAXS, the UV-Vis spectra show that AR of the GNRs increase until 4,000 s and then stabilize at about 4.9.



3.4 Growth of GNRs Considering the results presented in sections 3.1, 3.2, and 3.3, we can discuss the growth process of the three kinds of GNRs. For all the XANES spectra, there is no characteristic peak for Au+ ions40,47 around the Au L3-edge, even in the early stage of the reaction. In the case of Au+ ions, a distinct peak is observed as shown in Figure S6 in the SI. Like in the case of the spherical Au nanoparticles,47,48 the reduction of Au+ ions occurs in the very early stage of the reaction, even using only ascorbic acid, as in this study. Therefore, it is thought that Au in the growth solution exists as atoms or small clusters coordinated by surfactants and Ag ions. For GNR6s, the main question is: do all of the seeds grow into GNRs? This can be answered by analyzing the change of Au concentration in the GNR6s solution. As shown in Figure 3b, GNR6s grow until 4,000 s at an approximately constant rate, consuming half the quantity of Au. As the maximum length of GNR6s at 4,000 s is determined to be 33 nm (Figure 6) and the AR is 4.9 (Figure 8b), the diameter is 6.7 nm. Since half the Au is used for growth of GNRs until 4,000 s in this system, the number of GNRs is calculated to be 2.3 × 1013. If we assume that this is equal to the number of seed particles, their size is estimated to be 2.6 nm from the Au molar quantity of 23.6 × 10−9 mol in the seed solution. The size of the seed particles is reported to be 2−3 nm.4,39,71 This is definite evidence for the fact that all the seeds grow until 4,000 s in the GNR6 solution. However, this is not the case for GNR2s and GNR4s. The quantity of seed solution added to the growth solution was the same for all GNRs. Thus, the numbers of seed particles in the solutions were equal at the starting point. In our previous research, the


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final size of GNR2s was 35 nm in length and 13 nm in diameter, and that of GNR4s was 41 nm in length and 10 nm in diameter.39 Thus, there are not enough Au atoms in the solutions for all the seeds to grow into GNR2s or GNR4s with these sizes. Although their growth reactions appear to be simple and to proceed smoothly, there is probably competition for existence between different GNRs in this system. From calculations using the sizes above, less than 54% and 74% of the total number of seeds poured into the solutions form GNR2s and GNR4s, respectively. In practice, the percentages are smaller than those because byproducts are generated in the solutions. Thus, by TEM, we observe only the “winners” that survive the competition. It is difficult to know exactly what happens to the losing seeds, but it is possible that they are consumed by Ostwald ripening71,72 or remain as unchanged seeds or aggregates in the solutions.

3.5 Unique Growth Process of GNR6 In this section, we analyze the growth process of GNR6s in detail, focusing on the amount of Au existing in the growth solution. From the above results, their growth process can be summarized as follows: (1) GNR6s grow until 4,000 s consuming half the Au in the growth solution. By 4,000 s, they reach 33 nm in length with an AR of 4.9 (Figure 6 and Figure 8b). (2) Their growth stagnates at around 4,000 s once, although all of the seed particles have grown and a considerable amount of Au remains in the solution. Although the growth is very slow, they grow in both the longitudinal and transverse directions, maintaining an AR of 4.9 (Figure 6 and Figure 8b). After 4000 s, their growth restarts, maintaining the AR of 4.9. The length of the largest GNRs reaches 58 nm at 20,000 s (Figure 6). This growth



process is illustrated in Figure S7. Although the number of the larger rods may be few, they increase in volume more than five times from 4,000 s to 20,000 s. As with the other two GNR systems, the amount of Au in the solution is not enough for all the seeds to grow to these final sizes. As a result, it must be concluded that competition for survival among the GNRs was also carried out in the solution of GNR6s after 4,000 s. The oscillation in the ratios of the Au atoms in the growth solution and in rods (Fig. 3(b)) seems to imply the competition. For clarity, we have divided the growth process into two time domains where it is thought that the different processes occur, i.e., Stage 1: 0–4,000 s and Stage 2: 4,000– 20,000 s (see Figure 6). In Stage 1, all the seeds grow together, especially in the longitudinal direction. The question to be answered here is: why does the growth of GNR6s proceed slowly compared with that of GNR4s or GNR2s? The difference between GNR4s and GNR6s is only in the composition of the surfactants. This difference has been already discussed and described in detail.31 Briefly, if BDAC is present in the solution with CTAB at an adequate concentration27 from the beginning of the reaction,23 the micelles formed by the two surfactants are smaller than the ones formed by CTAB only because of the nonpolarity of the benzyl ring.73–77 As is the case in the preparation using CTAB alone, these micelles work as templates to produce anisotropic particle growth. These small micelles inhibit growth in the transverse direction allowing GNR6s to grow mainly in the longitudinal direction.31 For this reason, GNR6s grow slowly and more anisotropically than the others, and only GNRs coordinated by CTAB and BDAC in appropriate proportions grow smoothly. In fact, when prepared using pure BDAC, only spherical particles are


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generated.27 In Stage 2, where it looks as if the reaction has stagnated once, what is happening? As described above, until reaching Stage 2, almost all the seeds grow favorably, as shown in Figure 6. At 4,000 s, the growth of the GNRs appears to stagnate, despite the fact that approximately half of the Au remains in the growth solution. Thus, we suggest that there is a threshold Au concentration for further growth, and that there is a watershed moment in the growth of the GNRs. The GNRs, which grow simultaneously until 4,000 s, are divided into two groups at the watershed; GNRs in one group continue growing and those in the other group start supplying Au to the former by ejection of complex Au ions.36 In Stage 2, a competition for which group a particular GNR belongs to occurs constantly. By the ejection of Au from the GNRs of the latter group, the Au concentration in the growth solution crosses the threshold concentration and a little growth of the GNRs in the former group can occur. It is thought this repeated Au supply to the solution and consumption from the solution are observed as the back-and-forth change of the Au concentration shown in Figure 3b. In Stage 2, the GNRs restart their growth in both the longitudinal and transverse directions. At the beginning of Stage 2, the GNR6s of the two above-mentioned groups are in competition. It is thought that the competition is expected to continue, and that the size-distribution of the rods widens as shown in the increase of the maximum length (Fig. 6). This growth goes on until at least 20,000 s and will continue further until there is not enough Au in the solution. It is necessary that over 60% of the total number of GNRs that grow until 4,000 s stop growing and support the growth of the others as Au suppliers. Only



GNRs coordinated by CTAB and BDAC in the appropriate proportion will continue growing to the end. Why do they then grow in both directions maintaining their ARs? This regrowth can be explained by the Au concentration in the growth solution. In the seed-mediated method, longer GNRs than the present ones can be generated by the gradual addition of the growth solution to the GNR solution.27,40 If enough growth solution is added, GNRs more than 100-times larger in volume than those in the present study can be produced.40 There is no doubt that GNRs will continue to grow anisotropically with the presence of an adequate amount of Au in the solution. Under the present conditions, the anisotropic growth seems to be temporarily complete by 4,000 s due to the lack of Au in the solution, and the AR of 4.9 is maintained. However, some further limited anisotropic growth continues, indicated by the AR of GNR6s finally reaching 6. Although it is hard to track the entire growth of GNR6s and consumption of Au, we speculate that the supply of Au from the GNRs and limited anisotropic growth continue by the above-mentioned complex Au ejection throughout Stage 2. Finally, we briefly consider the shape change of the GNRs. In the case of GNRs with comparatively small ARs,36,37 the shape changes from the dumbbell- or bow-tie-shaped to a hemispherically capped cylinder. As discussed above, in the preparation of GNRs using only CTAB, GNRs with thick sections at both the ends of the longitudinal axis are observed. This is because most active growth occurs on the (111) facets, which are exposed at both ends of the GNRs. Conversely, for GNR6s, dumbbell- or bow-tie-shaped GNRs are not observed in the TEM observation at any point. Although we


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do not yet know the full details of the shape of GNR6s, it is thought that growth in the longitudinal direction is strongly promoted while growth in a transversal direction is inhibited by the use of CTAB and BDAC. Therefore, they retain their hemispherically capped shape from the early stages of growth until completion. It is seen from TEM images that the particles other than GNRs are generated in the growth solution, which is clear from the DDFs after 12,000 s as mentioned in section 3.2. It is thought that these other particles, especially the cubic particles, are generated from poorly coordinated seeds or the supplier particles. In the former case, it is apparent that the growth occurs on the (100) facets and that the seeds mature into cubic particles from studies on the effects of CTAB concentration.8,37 In the latter case, where GNRs release Au atoms from themselves, it is reasonable that some of the supplier particles would stop releasing Au and mature into cubic particles. Moreover, GNRs that keep releasing Au further may be left as spherical particles. In fact, small GNRs and cubic and spherical particles are observed in the GNR6s growth solution, as shown in Figure 1 and Figure S8 in the SI. From these TEM observations, it is supposed that the release of Au is also occurring anisotropically. It is thought that the anisotropic release of Au from the both ends of the GNRs occurs as the back reaction of their growth. It is thought that growth or release occurs depending on the relative surface energy of the facets.36 To summarize the discussion so far, GNR6s are considered to grow as illustrated in Figure 9. In Stage 1, i.e., until 4,000 s, all seeds in the solution grow simultaneously with increasing AR. In Stage 2, the GNRs divide into two groups; the GNRs in one group continue growing and the GNRs in the other group start supplying Au to the former group



by ejecting complex Au ions. The GNR6s of the above-mentioned two groups exist in competition. Then, the GNRs coordinated by CTAB and BDAC in the appropriate proportions will keep growing to completion. It is thought such changes in the growing pattern are influenced by the balance of the surface energy for the growing facets, and the chemical potential of Au in the growth solution, which is determined by its concentration.

Conclusion Using three time-resolved methods of measurements, we tracked the growth behaviors of three kinds of GNRs with different ARs and discussed their formation processes. From the comprehensive results of XANES, SAXS, and UV-Vis analyses, we obtained the amounts of Au consumed in the GNR solutions, the elongation rates of the GNRs, and dependence of the AR of the GNRs on reaction time. Our results revealed the growth process for GNRs from the viewpoints of changes in the concentration of Au and the number of generated GNRs. We summarize the results as follows: 1) The reduction of Au+ ions occurs in the very early stage of the reaction in all solutions. Using reduced Au atoms, GNR2s and GNR4s grow rapidly until completion. 2) GNR6s grow extremely slowly, unlike GNR2s and GNR4s. This is because the anisotropic growth is strongly encouraged by the coordination of CTAB and BDAC. 3) In the cases of GNR2s and GNR4s, all seeds cannot grow to be final products. Less than 54% and 74% of the seed particles mature into GNRs in the solution of GNR2s and GNR4s, respectively. 4) The volume of GNR6s at 4,000 s (1.1 × 103 nm3) is smaller than those of GNR2s (4.1 ×


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103 nm3) and GNR4s (3.0 × 103 nm3). The volume of GNR6s gradually increases after 4,000 s to 5.9 × 103 nm3 at 20,000 s. 5) In the case of GNR6s, GNRs once generated from the seeds are divided into growing GNRs and GNRs that act as suppliers by ejecting Au. The growth of the former GNRs occurs in both longitudinal and transverse directions after 4,000 s because of the decrease in the concentration of Au. It is thought that GNR6s grow quite slowly and slightly anisotropically into GNRs with an AR of 6 at the end. In this study, we investigated the formation process of GNRs with three different ARs using XANES, SAXS, UV-Vis, and TEM. The slow growth of GNR6s is divided into three stages and discussed in detail. Particularly, fluctuating growth is first observed by tracking the Au concentration in the growth solution.

Associated Content Supporting Information Diagrammatic illustration of sample holder. XANES and differential XANES spectra of reference materials and examples of the curve-fitting results of UV-Vis and XANES. Diagrammatic illustration of growth process estimated from only the size of GNR6s. TEM image of GNR6s obtained after the measurements. This material is available free of charge via the Internet at http://pubs.acs.org.



Author Information Corresponding Author *Y.H.: Tel&Fax: +81-277-30-1359. E-mail: [email protected]. *K.H.: Tel&Fax: +81-566-26-2351. E-mail: [email protected].

Present Address ‡

Yoshikiyo Hatakeyama: Graduate School of Science and Technology, Gunma University,

Tenjin-cho, Kiryu, Gunma 376-8516, Japan.

Notes The authors declare no competing financial interest.

Acknowledgment We are grateful to the PF Advisory Committee of KEK for approval of XANES and SAXS measurements (Proposal Nos. 2010G146, 2010G600, 2012G676, 2015G582). We would like to thank Prof. K. Asakura of Hokkaido University for helpful discussions. We gratefully acknowledge the assistance of Dr. Takeshi Morita, Mr. Kei Onishi, and Mr. Jun-ichi Kato of Chiba University with SAXS and XANES measurements. This study was supported by JSPS KAKENHIs (Grant-in-Aid for Young Scientists (B) Nos. 16710082 and 23710127 for K. H. and Grant-in-Aid for Young Scientists (B) No. 15K17812 for Y. H.).


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Figure 1 Typical TEM images of GNRs in the final stages with aspect ratios of 2, 4, and 6: (a) GNR2s, 27.5 nm in average length (σ = 4.8) and 2.2 in aspect ratio (σ = 0.4), (b) GNR4s, 33.0 nm in average length (σ = 4.1) and 3.4 in aspect ratio (σ = 0.3), and (c) GNR6s, 41.5 nm in average length (σ = 4.8) and 5.6 in aspect ratio (σ = 0.9).



Figure 2. Time-resolved XANES spectra at the Au L3-edge.

Figure 3. Ratios of Au atoms in the growth solution (AuGS) and in the rods (AuRod). (a) For the GNR2 and GNR4 solutions. (b) For the GNR6 solution.

Figure 4. (a) Time-resolved SAXS patterns and (b) distance distribution functions for


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GNR6s.

Figure 5. Distance distribution functions recorded every ca. 1,700 s. A shoulder caused by non-rod-like particles became conspicuous after 12,000 s.

Figure 6. Maximum length of GNR6s.



Figure 7. Time-resolved UV-Vis spectra of GNR6s.

Figure 8. (a) Peak positions of the longitudinal plasmon band, the transverse plasmon band, and the band from the byproduct. (b) Aspect ratio of GNR6s estimated from the plasmon bands. 36 ACS Paragon Plus Environment

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Figure 9. Formation process for GNR6s. The blue particles indicate suppliers. The red particles are the cubic or spherical particles derived from suppliers.



Figure 1 Typical TEM images of GNRs in the final stages with aspect ratios of 2, 4, and 6: (a) GNR2s, 27.5 nm in average length (σ = 4.8) and 2.2 in aspect ratio (σ = 0.4), (b) GNR4s, 33.0 nm in average length (σ = 4.1) and 3.4 in aspect ratio (σ = 0.3), and (c) GNR6s, 41.5 nm in average length (σ = 4.8) and 5.6 in aspect ratio (σ = 0.9). 42x11mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. Time-resolved XANES spectra at the Au L3-edge. 160x129mm (300 x 300 DPI)



Figure 3. Ratios of Au atoms in the growth solution (AuGS) and in the rods (AuRod). (a) For the GNR2 and GNR4 solutions. (b) For the GNR6 solution. 150x94mm (300 x 300 DPI)


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Figure 4. (a) Time-resolved SAXS patterns and (b) distance distribution functions for GNR6s. 160x73mm (300 x 300 DPI)



Figure 5. Distance distribution functions recorded every ca. 1,700 s. A shoulder caused by non-rod-like particles became conspicuous after 12,000 s. 76x71mm (300 x 300 DPI)


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Figure 6. Maximum length of GNR6s. 75x73mm (300 x 300 DPI)



Figure 7. Time-resolved UV-Vis spectra of GNR6s. 76x69mm (300 x 300 DPI)


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Figure 8. (a) Peak positions of the longitudinal plasmon band, the transverse plasmon band, and the band from the byproduct. (b) Aspect ratio of GNR6s estimated from the plasmon bands. 154x72mm (300 x 300 DPI)



Figure 9. Formation process for GNR6s. The blue particles indicate suppliers. The red particles are the cubic or spherical particles derived from suppliers. 80x60mm (300 x 300 DPI)


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65x44mm (300 x 300 DPI)


Growth Behavior of Gold Nanorods Synthesized by ... - ACS Publications

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