Growth of Gold Nanorods and Bipyramids Using CTEAB Surfactant

The growth of gold nanorods in CTEAB solutions takes 5−10 h, more than 5 times ... seeds gives both gold bipyramids and a small percentage of gold n...
0 downloads 0 Views 592KB Size
Download PDF
J. Phys. Chem. B 2006, 110, 16377-16383

16377

Growth of Gold Nanorods and Bipyramids Using CTEAB Surfactant Xiaoshan Kou,† Shuzhuo Zhang,†,‡ Chia-Kuang Tsung,†,§ Man Hau Yeung,† Qihui Shi,§ Galen D. Stucky,§ Lingdong Sun,‡ Jianfang Wang,*,† and Chunhua Yan‡ Department of Physics, Chinese UniVersity of Hong Kong, Shatin, Hong Kong, China, State Key Lab of Rare Earth Materials Chemistry and Applications, Peking UniVersity, Beijing 100871, China, and Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106 ReceiVed: June 22, 2006; In Final Form: July 11, 2006

Gold nanorods and bipyramids have been synthesized using the seed-mediated approach in aqueous cetyltriethylammonium bromide (CTEAB) solutions in the presence of silver nitrate. Gold nanoparticle seeds that are stabilized with either CTEAB or sodium citrate have been used. The use of the CTEAB-stabilized seeds gives gold nanorods in high yield in one step with the longitudinal plasmon wavelength ranging from 750 to 1030 nm, depending on the amount of the seeds. The longitudinal plasmon wavelength can be extended to 1100 nm by the use of a two-step growth method. The growth of gold nanorods in CTEAB solutions takes 5-10 h, more than 5 times slower than that in cetyltrimethylammonium bromide solutions at the same concentration of surfactants. The use of the citrate-stabilized seeds gives both gold bipyramids and a small percentage of gold nanorods. The longitudinal plasmon wavelength of the bipyramids is tunable from 700 to 1100 nm by varying the amount of the citrate-stabilized seeds. The growth of gold bipyramids takes more than 1 day. Transmission electron microscopy characterizations reveal that the gold nanorods grown from both types of gold nanoparticle seeds are single-crystalline and that the gold bipyramids are penta-twinned.

Introduction Gold nanostructures have received attention because of their shape- and size-dependent surface plasmon-related optical properties. Nonspherical gold nanostructures generally exhibit more than one surface plasmon-resonance mode, which result from dipolar and multipolar electron oscillations along different directions. For example, gold nanorods exhibit two surface plasmon-resonance modes, which are called the transverse and longitudinal plasmon modes, corresponding to electron oscillations perpendicular and parallel to the rod length axis, respectively. The longitudinal plasmon wavelength (LPW) of gold nanorods is highly dependent on their length-to-diameter aspect ratio in a linear relationship so that a slight deviation from the spherical geometry can induce distinct color changes.1,2 Gold nanostructures have potential applications in photonics, optoelectronics, and biotechnology. For example, closely spaced gold nanoparticles that exhibit surface plasmon coupling can function as sub-wavelength plasmonic waveguides.3 Chains of gold nanoparticles embedded in dielectric silica nanofibers exhibit reversible photosensitive resistance changes, which arise from the excitation of the surface plasmon of the embedded gold nanoparticles.4 These one-dimensional hybrid nanostructures can therefore be used as optical switches. Gold nanostructures also exhibit several advantages for biotechnological applications, including chemical inertness, biological compatibility, rich surface functionalization chemistry, and tunable surface plasmon-related optical properties. It has been demonstrated that gold nanostructures can function as contrast agents for optical coherence tomography,5 two-photon chromophores for luminescence imaging of blood vessels,6 biological sensors,7 * Corresponding author. E-mail: [email protected]. † Chinese University of Hong Kong. ‡ Peking University. § University of California, Santa Barbara.

carriers for gene delivery,8 and therapeutic agents for photothermal cancer treatment.9 For in vivo biological sensing, imaging, and photothermal therapy, gold nanostructures that exhibit surface plasmon-resonance modes in the near-infrared (NIR) spectral region are strongly desired because NIR light in the spectral region of 800-1300 nm can penetrate biological tissues with relatively high transmittivity.10 In addition, gold nanostructures with their surface plasmon wavelength in the region of 1300-1600 nm are also desired for the fabrication of photonic and optoelectronic devices for telecommunication applications. Gold nanorods and elongated nanostructures are particularly suitable for photonic, optoelectronic, and biotechnological applications in the NIR region because of the strong dependence of their LPW on the aspect ratio.1,2 One additional advantage of gold nanorods is that light emitted from or scattered off gold nanorods is strongly polarized along the rod length axis, making them an ideal orientation probe.11 Several methods have been developed for preparing gold nanorods, including electrochemical deposition in templates12 and solutions,13 photochemical synthesis,14 and seed-mediated chemical reduction.15,16 Using the seed-mediated synthetic method, the aspect ratio of the gold nanorods can be controlled by varying the amount ratio of spherical gold nanoparticle seeds to the gold precursor. Moreover, gold nanorods can be produced in nearly quantitative yields when AgNO3 is used as an additive.17-20 The seed-mediated synthesis of gold nanorods in the presence of AgNO3 thus promises to be a practical method in terms of both yield and size uniformity. Cationic ammonium surfactants have been used as the directing agent in the synthesis of gold nanorods in aqueous solutions. The most widely used surfactant is cetyltrimethylammonium bromide (CTAB). Although a complete understanding of the CTAB-directed growth mechanism has remained

10.1021/jp0639086 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/03/2006

16378 J. Phys. Chem. B, Vol. 110, No. 33, 2006 elusive, it is generally believed that CTAB plays two roles during the growth of gold nanorods. First, CTAB surfactants adsorb to gold nanorods in a bilayer fashion with the trimethylammonium headgroups in the first monolayer facing the gold surface.21 CTAB adsorption makes gold nanorods positively charged and thus stabilizes them in aqueous growth solutions. The growth of gold nanorods is probably governed by preferential adsorption of CTAB surfactants to different crystal facets during the growth. Second, CTAB surfactants form micelles in aqueous growth solutions. The negatively charged complex ions of Au(III) and Au(I) (a reduction intermediate) bind to the positively charged headgroups around the micelles to establish an association-dissociation equilibrium, which could alter the kinetics and energetics of the redox reaction and the growth of gold nanorods.22 While previous experiments have investigated the effect of the tail length of alkyltrimethylammonium surfactants on the growth of gold nanorods,23 little attention has been given to the possible effect of the surfactant headgroup, even though the headgroup plays an important role in the micellization behavior of cationic ammonium surfactants24 and in the assembly of surfactants and silica precursors to form mesoporous silica materials with varying pore structures.25 In this paper, we use cetyltriethylammonium bromide (CTEAB) to grow gold nanorods and bipyramids. Because both adsorption of cationic surfactants to the surfaces of gold nanorods and binding of the complex ions to micelles involve the surfactant headgroup, it is expected that a change of the headgroup will alter the growth behavior of gold nanorods. Although previous experiments have demonstrated the synthesis of gold nanorods with large aspect ratios in CTAB solutions using citrate-stabilized seeds without the presence of AgNO3,16,22 we focus on the synthesis of gold nanorods and bipyramids in CTEAB solutions using the seed-mediated method in the presence of AgNO3 to compare the growth behavior in CTEAB solutions with that in CTAB solutions because the seed-mediated method in the presence of AgNO3 in CTAB solutions can give gold nanorods in nearly quantitative yields.19,20,26 We use two types of gold nanoparticle seeds. One is stabilized by CTEAB, and the other is stabilized by citrate. The former seeds are positively charged, and the latter seeds are negatively charged. The use of the CTEAB-stabilized seeds gives gold nanorods in high yields, and the use of the citrate-stabilized seeds gives gold bipyramids as a major product. For both types of seeds, we vary the amount ratio of seeds to the gold precursor during growth, monitor the extinction spectra of growth solutions as a function of time, and characterize in detail the crystal structures of gold nanorods and bipyramids. Experimental Procedures Materials. 1-Bromohexadecane, triethylamine, HAuCl4‚ 3H2O, NaBH4, ascorbic acid, AgNO3, and sodium citrate dihydrate were purchased from Sigma-Aldrich. Acetonitrile and ether were purchased from Lab-Scan. Deionized water was used in the preparation of gold nanoparticle seeds, nanorods, and bipyramids. Synthesis of CTEAB. CTEAB was prepared by refluxing stoichiometric amounts of 1-bromohexadecane and triethylamine in acetonitrile for 24 h, as described previously.24 The volume ratio of the total reactant to the solvent was 1:2.2. The yellow two-phase mixture remaining after reflux was rotary-evaporated to remove the solvent. The resulting solid product was recrystallized from dry ether 1-3 times until all of the impurities were removed. The purity of CTEAB was checked by mass spectrometry.

Kou et al. Preparation of CTEAB-Stabilized Gold Nanoparticle Seeds. The CTEAB-stabilized seeds were prepared following the method developed previously for the preparation of CTABstabilized seeds.19a Briefly, 0.25 mL of an aqueous 0.01 M HAuCl4 solution was added into 7.5 mL of an aqueous 0.1 M CTEAB solution in a plastic tube. After the solution was mixed by inversion, 0.6 mL of an ice-cold, freshly prepared aqueous 0.01 M NaBH4 solution was added all at once, followed by rapid inversion mixing for 2 min. The resulting seed solution was kept at room temperature and was used 2-5 h after its preparation. One-Step Growth of Gold Nanorods. The synthesis procedure of gold nanorods in CTAB solutions developed previously19a,26 was used to grow gold nanorods in CTEAB solutions. Briefly, multiple aliquots of the nanorod growth solution were prepared. For each aliquot, 0.3 mL of 0.01 M HAuCl4 and 0.045 mL of 0.01 M AgNO3 were added into 7.125 mL of 0.1 M CTEAB in a plastic tube, followed by gentle inversion mixing. A total of 0.048 mL of 0.1 M freshly prepared ascorbic acid was further added, and the resulting solution was mixed. Finally, a varying amount of the CTEAB-stabilized seed solution was added into each aliquot of the growth solution. The reaction solution was mixed by gentle inversion for 10 s and then left undisturbed or used for the time-dependent measurements of extinction spectra. Two-Step Growth of Gold Nanorods. Gold nanorods exhibiting a certain LPW were first grown following the onestep growth procedure. A fresh growth solution prepared by mixing together 7.125 mL of 0.1 M CTEAB, 0.3 mL of 0.01 M HAuCl4, 0.045 mL of 0.01 M AgNO3, and 0.048 mL of 0.1 M ascorbic acid was then directly added into the as-grown nanorod solution at a volume ratio of 1:1. The reaction solution was mixed by gentle inversion for 10 s and then left undisturbed or used for the time-dependent measurements of extinction spectra. Preparation of Citrate-Stabilized Gold Nanoparticle Seeds. The citrate-stabilized seeds were prepared via the procedure developed previously.15 Briefly, 0.125 mL of 0.01 M HAuCl4 and 0.25 mL of 0.01 M sodium citrate were first added into 9.625 mL of deionized water, and then 0.15 mL of ice-cold, freshly prepared 0.01 M NaBH4 was added under vigorous stirring. The resulting seed solution was kept for at least 2 h before use. Growth of Gold Bipyramids. Multiple aliquots of the bipyramid growth solution were prepared. Each aliquot was composed of 7.125 mL of 0.1 M CTEAB, 0.3 mL of 0.01 M HAuCl4, 0.045 mL of 0.01 M AgNO3, and 0.048 mL of 0.1 M ascorbic acid. A varying amount of the citrate-stabilized seed solution was then added into each aliquot of the growth solution. The reaction solution was mixed by gentle inversion for 10 s and then left undisturbed or used for the time-dependent measurements of extinction spectra. Instrumentation. Extinction spectra of the solutions were measured using a Hitachi U-3501 UV-vis/NIR spectrophotometer. Low-magnification transmission electron microscopy (LMTEM) images were acquired with a FEI CM120 microscope at 120 kV. Particle yields and sizes were determined from LMTEM images. High-resolution transmission electron microscopy (HRTEM) images were recorded with a TECNAI 20 ST microscope at 200 kV. For TEM sample preparation, 6 mL of an as-grown solution was centrifuged at 17 600g for 15 min. The precipitate was redispersed in 6 mL of deionized water, centrifuged again at 17 600g for 15 min, and finally redispersed in 0.4 mL of deionized water. A total of 8 µL of this solution

Nanorod and Bipyramid Growth Using CTEAB Surfactant

Figure 1. (A) Extinction spectra of the gold nanorods grown with varying amounts of the CTEAB-stabilized seed solution. The volumes of the seed solution are (a) 0.060 mL, (b) 0.048 mL, (c) 0.032 mL, (d) 0.015 mL, (e) 0.010 mL, (f) 0.0075 mL, (g) 0.0065 mL, and (h) 0.0050 mL. The spectra were taken at least 1 day after the addition of the seed solution. (B) LMTEM image of the nanorods grown with 0.032 mL of the seed solution. (C) LMTEM image of the nanorods grown with 0.0065 mL of the seed solution. (D) HRTEM image of one gold nanorod oriented in the [100] direction. The aspect ratio of the rod is 9.6. The upper inset shows the LMTEM image of the same rod, and the lower inset is the convergent-beam electron diffraction pattern taken on a different [100] oriented nanorod. (E) HRTEM image of one nanorod oriented in the [110] direction. The aspect ratio of the rod is 7.6. The upper inset shows the LMTEM image of the same rod, and the lower inset is the convergent-beam electron diffraction pattern taken on a different [110] oriented nanorod. The nanorods shown in panels D and E were grown with 0.0065 mL of the seed solution.

was drop-cast onto a lacey-Formvar TEM grid stabilized with carbon and allowed to dry in the open atmosphere overnight. Results and Discussion Growth from the CTEAB-Stabilized Seeds. One-Step Growth. Figure 1A shows the extinction spectra of the gold nanorods grown with varying amounts of the CTEAB-stabilized seed solution. As the amount of the seed solution decreased, the LPW of the gold nanorods first red-shifted from 750 to 1030 nm and then shifted back. There exists an upper limit at 1030 nm for the LPW that can be achieved during growth by varying

J. Phys. Chem. B, Vol. 110, No. 33, 2006 16379 the amount of the seed solution. In comparison, the LPW of the gold nanorods grown in CTAB solutions at the same surfactant concentration red-shifts slightly with increasing amounts of the CTAB-stabilized seed solution,19a and it is limited below ∼850 nm even if the amounts of HAuCl4, AgNO3, and CTAB-stabilized seeds are varied.17 The increase in the upper limit of the LPW for growth in the CTEAB solution might be related to the larger headgroup of the surfactant, which could change the binding strength between surfactant and gold nanorods and thus alter the growth behavior along the longitudinal and transverse directions of the gold nanorods. In addition, because both the average aggregation number and the fractional counterion association value for CTEAB micelles have been found to be smaller than those for CTAB micelles,24 the binding and unbinding behavior of the negatively charged Au(III) and Au(I) complex ions with CTEAB micelles will also appear different from that with CTAB micelles. Figure 1B,Cs show the LMTEM images of two nanorod samples grown with 0.032 and 0.0065 mL of the CTEABstabilized seed solution, respectively. The growth gives high yields of gold nanorods, together with a small number percentage of roughly spherical particles. The nanorods are relatively uniform in both diameter and length. The average diameter, length, and aspect ratio of the nanorods grown with 0.0065 mL of the seed solution are all larger than those of the nanorods grown with 0.032 mL of the seed solution (Table 1). These gold nanorods are single-crystalline, as revealed by HRTEM images (Figure 1D,E and Supporting Information Figures S1 and S2). They are oriented either in the [100] or in the [110] direction on the TEM grids. A majority of the nanorods that were grown with 0.032 mL of the seed solution are oriented along the [110] zone axis (35 out of 40 nanorods). This observation is in agreement with the previous HRTEM results on the relatively short gold nanorods that were grown either electrochemically27 or using the seed-mediated method in the presence of AgNO3.28 Interestingly, a majority of the nanorods that were grown with 0.0065 mL of the seed solution are oriented in the [100] direction (10 out of 15 nanorods). It is unclear whether there exists a transformation process for shorter gold nanorods that are dominated by the {110} facets to become dominated by the {100} facets when they grow longer. Temporal evolution of the extinction spectra (Figure 2 and Supporting Information Figure S3) was monitored during growth to compare the growth kinetics of gold nanorods in CTEAB solutions with that in CTAB solutions. Previous experiments have shown that the growth of gold nanorods in 0.1 M CTAB solutions takes ∼1 h.19a In comparison, the nanorod growth in 0.1 M CTEAB solutions in our experiments continues for 5-10 h, depending on the amount of the CTEAB-stabilized seed solution added to the growth solution. The more seed solution that is added, the faster the nanorod growth. For example, for the growth with 0.0065 mL of the seed solution, the LPW first red-shifts, reaches a maximum at ∼1070 nm after 5 h, then blue-shifts, and stabilizes at ∼1030 nm after 10 h (Figure 2). During the entire growth process, the extinction of the longitudinal plasmon-resonance peak increases steadily and becomes stabilized at 1.75 after 6 h. The red-shift of the LPW suggests that the growth is preferentially along the ends of the gold nanorods at the early stage, and the blue-shift suggests that the growth along the length direction slows down at the late stage. For the growth with 0.032 mL of the seed solution, the LPW red-shifts first and then blue-shifts slightly, while the extinction increases gradually. It takes about 5 h for both of them to become stabilized at 880 nm and 2.0, respectively (Figure S3).

16380 J. Phys. Chem. B, Vol. 110, No. 33, 2006

Kou et al.

TABLE 1: Yields and Average Sizes of Gold Nanorods and Bipyramids growtha

yieldb

Nc

D (nm)d

L (nm)d

σd

0.032 mL of seed I, one-step 0.0065 mL of seed I, one-step 0.032 mL of seed I, two-step 0.0065 mL of seed I, two-step 0.12 mL of seed II, one-step

NRs, 84 NRs, 90 NRs, 87 NRs, 88 BPs, 37 NRs, 10 BPs, 29 NRs, 23 BPs, 32 NRs, 6.4

400 300 400 300 960

8.2 (0.9) 9.9 (0.9) 9.8 (1.1) 13.8 (1.7) 21.4 (1.0) 11.3 (1.2) 25.2 (1.2) 14.3 (2.3) 37.2 (1.9) 21.2 (2.6)

27.5 (3.6) 63.0 (10.9) 37.2 (5.2) 72.5 (13.1) 49.9 (3.0) 44.7 (11.1) 66.8 (2.8) 58.8 (12.4) 111.3 (4.3) 80.9 (11.2)

3.4 (0.7) 6.5 (1.4) 3.8 (0.9) 5.5 (1.8) 2.3 (0.1) 4.1 (1.5) 2.7 (0.1) 4.2 (1.3) 3.0 (0.2) 3.9 (0.8)

0.048 mL of seed II, one-step 0.015 mL of seed II, one-step

1060 630

a Seed I: CTEAB-stabilized seed solution and seed II: citrate-stabilized seed solution. b NRs: nanorods and BPs: bipyramids. Product yield is defined as (number of particular particles)/(total number of particles) × 100%. c N: total number of particles counted to calculate the yield for each synthesis. d D: diameter; L: length; and σ: aspect ratio, defined as L/D. The number of the particles measured to obtain the average size is 150 for each sample. The numbers in parentheses represent standard deviations.

Figure 2. (A) Extinction spectra of the nanorod growth solution taken as a function of time after the addition of 0.0065 mL of the CTEABstabilized seed solution. (B) Variation of the LPW vs the growth time. (C) Variation of the longitudinal plasmon-peak extinction vs the growth time. The data points in panels B and C were extracted from the curves shown in panel A.

The slight blue-shift observed at the late growth stage in the case of 0.032 mL of the seed solution is due to that the decrease in the aspect ratio for shorter nanorods is smaller than that for longer nanorods if the diameter is increased by an equal amount. This growth behavior can also be used to explain the presence of an upper limit for the LPW of the gold nanorods grown with varying amounts of the CTEAB-stabilized seed solution in onestep growth (Figure 1A). The growth along the length axis slows down at the late stage of the growth relative to the growth on the sides of nanorods, and the aspect ratio decreases. Because the decrease in the aspect ratio for longer nanorods is more sensitive to the increase in the diameter than that for shorter nanorods, the aspect ratio of gold nanorods stops increasing and becomes smaller after the seed solution decreases below a certain amount. Two-Step Growth. A two-step growth was carried out to obtain longer gold nanorods. In the first step, gold nanorods exhibiting a certain LPW were grown by using an appropriate amount of the seed solution. Freshly prepared growth solutions were then directly added to the as-grown nanorod solutions at a volume ratio of 1:1 in the second step. Figure 3A shows the extinction spectra recorded after the addition of the growth solution for varying time. The gold nanorod solution was made in the first step with 0.0065 mL of the seed solution, and it was kept for 8 h before the second-step growth. The peak extinction value drops from 1.75 to 0.88 (Figure 3A) right after the addition of the growth solution because the nanorod concentration was half diluted. The LPW red-shifts, and the extinction value increases steadily with the growth time. After ∼130 min, the LPW reaches a maximum at ∼1100 nm and then blue-shifts.

Figure 3. (A) Extinction spectra taken as a function of time after the growth solution was added to a gold nanorod solution at a volume ratio of 1:1 for the second-step growth. The nanorod solution was taken directly from the first-step growth, for which 0.0065 mL of the CTEABstabilized seed solution was used, and the growth time was 8 h. Spectrum a (dashed line) was taken right before the second-step growth. Spectra b-g were taken 10 min, 70 min, 130 min, 190 min, 12 h, and 36 h, respectively, after the addition of the growth solution for the second-step growth. (B) LMTEM image of the gold nanorods obtained from the two-step growth process. (C) TEM image of a single nanorod obtained from the two-step growth process. (D-G) HRTEM images recorded from the boxed areas 1-4 shown in panel C, respectively.

After ∼12 h, it becomes even shorter than that after the firststep growth, while the extinction value is very close to that after the first-step growth. This experimental observation again suggests a preferential end growth at the early stage, which slows down at the late growth stage. For the growth with 0.0065 mL of the seed solution, the number percentage of the nanorods after the second-step growth determined from LMTEM images (Figures 3B and Supporting Information Figure S4A) is the same as that after the first-step growth within the experimental error. The average diameter and length of the nanorods after the

Nanorod and Bipyramid Growth Using CTEAB Surfactant second-step growth are larger than those after the first-step growth, while the average aspect ratio of the former is smaller than that of the latter (Table 1). Therefore, the LPW of the former is shorter. The volume of the average-sized nanorod after the second-step growth is about 2.2 times that after the firststep growth. Because the extinction coefficient of gold nanorods is roughly proportional to their volume and the total solution volume was doubled for the second-step growth, the similar extinction values exhibited by the nanorod solution after both the first-step and the second-step growth indicate that no new nuclei are formed during the second-step growth. The two-step process was also applied to the gold nanorods that were grown with 0.032 mL of the CTEAB-stabilized seed solution. The extinction spectra show a similar time-varying behavior after the addition of the growth solution, first red-shifting and then blue-shifting (Supporting Information Figure S5A). LMTEM images (Figure S5B) show that the number percentage of the nanorods after the second-step growth remains approximately the same. The average diameter, length, and aspect ratio all become larger after the second-step growth. Such growth behavior observed in our experiments for the two-step growth is similar to that observed previously during the growth of AuAg core-shell nanorods19c and dogbone-like gold structures,19d but it is different from the previously reported overgrowth of gold shells on electrochemically prepared gold nanorods, where growth primarily occurs on the side surfaces close to the ends of the nanorods and the length of the nanorods remains essentially constant.29 It can be seen that to grow nanorods with even larger aspect ratios, the growth system will have to be modified so that the preferential growth along the ends of the nanorods occurs until the gold precursor is completely consumed. On the other hand, the growth of gold nanorods can be arrested at intermediate stages to obtain nanorods of desired sizes and LPWs by appropriate treatment with sulfide salts or thiol molecules, as demonstrated previously.28,30 We carried out TEM studies on the gold nanorods that were grown with the two-step process and with the use of 0.0065 mL of the CTEAB-stabilized seed solution in the first step. Most of these nanorods exhibit nonuniform diameters along the rod length direction (Figure 3B,C and Supporting Information Figure S4A,B). Their ends are fatter than their central region, and the enlarged segments cover a substantial fraction of the entire rod length. Although these nanorods have nonuniform width, HRTEM imaging (Figure 3D-G and Supporting Information Figure S4C-H) reveals that they are single-crystalline over the entire length. Of 15 gold nanorods imaged, they are all oriented in the [110] direction. Moreover, the {110} facets of the gold nanorods grown with the two-step process are unstable under electron beam irradiation during HRTEM imaging. After electron beam irradiation for ∼0.5 h, irregular saw-tooth structures are formed on the {110} facets (Supporting Information Figure S4C,D). This surface reconstruction results in the formation of small {111} facets because gold has a facecentered-cubic structure and the {111} facets are the most stable surfaces. Similar surface reconstruction behavior has also been observed previously on the gold nanorods prepared electrochemically.31 In contrast, the gold nanorods grown with the onestep process in our experiments are stable under electron beam irradiation. Their surfaces, either {110} or {100} facets, exhibit no reconstruction even after ∼3 h of electron beam irradiation. Growth from Citrate-Stabilized Seeds. To explore the effect of different seeds on the growth products, we carried out the growth in CTEAB solutions in the presence of AgNO3 using

J. Phys. Chem. B, Vol. 110, No. 33, 2006 16381

Figure 4. (A) Extinction spectra of the gold bipyramids grown with varying amounts of the citrate-stabilized seed solution. The volumes of the seed solution were (a) 0.12 mL, (b) 0.048 mL, (c) 0.032 mL, (d) 0.015 mL, (e) 0.010 mL, (f) 0.0075 mL, and (g) 0.0065 mL. The spectra were taken at least 1 day after the addition of the seed solution. (B and C) TEM images of the product grown with 0.12 mL of the seed solution at two different magnifications. (D and E) LMTEM images of the products grown with 0.048 and 0.015 mL of the seed solution, respectively.

citrate-stabilized gold nanoparticle seeds. Previous TEM characterization has shown that the citrate-stabilized seeds are 3 nm in diameter and multiply twinned crystals.28 Figure 4A shows the extinction spectra of the growth products made with varying amounts of the citrate-stabilized seed solution. The LPW redshifts with decreasing amount of the seed solution, with the lower and upper limits achieved in our experiments being 700 and 1100 nm, respectively. The longitudinal plasmon-resonance peaks are very sharp, with full widths at half-maximum ranging from 40 to 110 nm. The extinction value at the longitudinal plasmon-resonance peak is comparable to that at the shorterwavelength peak. As the amount of the seed solution decreases, both extinction peaks decrease in intensity, and the shorterwavelength peak also red-shifts from 530 to 640 nm. In comparison, when the CTEAB-stabilized seeds were used, the longitudinal plasmon-resonance peaks are much broader, with full widths at half-maximum ranging from 150 to 310 nm. The extinction value at the longitudinal plasmon-resonance peak is approximately twice that at the transverse plasmon-resonance peak (Figure 1A). In addition, as the amount of the seed solution decreases, the longitudinal plasmon-resonance increases slightly in intensity, and the transverse plasmon wavelength remains at ∼520 nm. LMTEM images (Figure 4B-E) show that the growth product with the citrate-stabilized seeds contains bipyramids, nanorods, and irregularly faceted particles, with the yield of bipyramids being about one-third and that of irregularly faceted particles being about one-half, irrespective of the amount of the seed solution (Table 1). Since the bipyramids are faceted and have a nonuniform width along the length direction, the pyramid base width was measured and approximated as the

16382 J. Phys. Chem. B, Vol. 110, No. 33, 2006

Kou et al.

Figure 5. (A) Extinction spectra of the growth solution taken as a function of time after the addition of 0.032 mL of the citrate-stabilized seed solution. (B) Variation of the longitudinal (squares) and transverse (circles) plasmon wavelength vs the growth time. (C) Variation of the longitudinal (squares) and transverse (circles) plasmon-peak extinction vs the growth time. The data points in panels B and C were extracted from the curves shown in panel A.

diameter. The length of the bipyramids was measured between the two apexes. As the amount of the seed solution decreased, the average diameter, length, and aspect ratio for both bipyramids and nanorods all increased (Table 1). Moreover, the standard deviation of the aspect ratio of the gold bipyramids was much smaller than that of the gold nanorods, whether they were grown with the CTEAB-stabilized seeds or with the citratestabilized seeds. The narrow size distribution of the bipyramids suggests that the sharp longer-wavelength extinction peak is predominantly due to the contribution from the gold bipyramids. In contrast, the shorter-wavelength extinction peak should mainly be contributed by the transverse plasmon resonance of the bipyramids and various plasmon modes of the irregularly faceted particles. The successful synthesis of gold bipyramids with narrow size distributions and tunable LPWs might be important for their photonic, optoelectronic, and biotechnological applications because the apexes of gold bipyramids exhibit a very large local electrical field enhancement.32 Temporal evolution of the extinction spectra shows that the growth with the citrate-stabilized seeds is much slower than with the CTEAB-stabilized seeds. Figure 5A shows the timedependent extinction spectra taken after 0.032 mL of the citratestabilized seed solution was added into the growth solution. The wavelength (Figure 5B) and extinction value (Figure 5C) of both the longer- and shorter-wavelength peak increases steadily as a function of time. It takes ∼30 h for the growth to be completed. The slower growth of the gold bipyramids than that of the gold nanorods is probably inherently determined by the penta-twinned crystal structure of the bipyramids, as shown next. Selected-area electron diffraction and HRTEM imaging were performed to investigate the crystal structure of the gold bipyramids. The electron diffraction patterns (Figure 6A) recorded under appropriate tilt conditions can only be indexed as a superposition of the two crystallographic zones, 〈110〉 and 〈111〉, of the face-centered-cubic structure, indicating the existence of twinning in the gold bipyramids. HRTEM images (Figure 6B and Supporting Information Figure S6) provide further evidence of the twinned structure, with the twining planes running parallel to the direction of elongation. Similar electron diffraction patterns and HRTEM images have been observed previously on penta-twinned gold nanorods grown without the presence of AgNO3.33 We thus believe that the gold bipyramids grown in our experiments also have the penta-twinned crystal structure, as schematically shown in Figure 6C. Each bipyramid

Figure 6. (A) Selected-area electron diffraction pattern of a gold bipyramid. The pattern is a superposition of the 〈110〉 and 〈111〉 zones of the face-centered-cubic structure. The pattern indicated by the dashed line corresponds to the [11h0] zone axis and can be indexed as (a) (1h1h1h), (b) (002h), (c) (111h), (d) (2h2h0), (g) (113), and (h) (220). The pattern indicated by the solid line corresponds to the [11h1h] zone axis and can be indexed as (d) (2h2h0), (e) (2h02h), (f) (022h), and (h) (220). Some spots, for example, spots d and h, belong to both patterns simultaneously. (B) HRTEM image of a gold bipyramid, with one domain oriented in the [110] direction. The inset is the LMTEM image of the same bipyramid. (C) Schematic illustration of the penta-twinned structure of the gold bipyramids. The right side is the base cross-section, showing the {111} twinning planes, {110} planes, and stepped {100} facets. (D) HRTEM image of a gold nanorod oriented in the [110] direction. The inset is the LMTEM image of the same rod. The bipyramid shown in panel B and the nanorod shown in panel D were from the same sample, which was grown with 0.12 mL of the citrate-stabilized seed solution.

has five domains. For penta-twinned gold nanorods, the side surfaces are the {100} facets, running parallel to the length direction. For the bipyramids, the side {100} facets have to be stepped periodically along the direction of elongation to fit the geometry. The growth with the citrate-stabilized seeds in the CTEAB solution also yields a small amount of gold nanorods. In contrast, growth with the citrate-stabilized seeds in aqueous CTAB solutions gives only bipyramids and irregularly faceted particles.15,28 The gold nanorods grown together with the bipyramids in our experiments are single-crystalline and oriented predominantly along the [110] zone axis (Figure 6D). This result suggests that the geometrical shape and crystal structure of growth products depend on both the crystal structure of seeds and the surfactant in growth solutions. Conclusion This paper describes the use of CTEAB as the stabilizing and structure-directing agent to grow gold nanorods and bipyramids using the seed-mediated method in the presence of silver nitrate. The aim is to explore the effect of the surfactant headgroup size on the growth of gold nanostructures. Both CTEAB-stabilized and citrate-stabilized seeds were used. The growth with the CTEAB-stabilized seeds takes 5-10 h and gives single-crystalline gold nanorods in high yields. The longitudinal plasmon wavelength of the nanorods is tunable from 750 to 1030 nm by varying the amount of the seed solution and can

Nanorod and Bipyramid Growth Using CTEAB Surfactant be further increased to 1100 nm by use of the two-step growth process. Time-evolution studies indicate that the growth of gold nanorods occurs preferentially along the ends at the early stage and that the end growth slows down relative to the growth on the sides at the late stage. In comparison, the growth of gold nanorods in CTAB solutions under similar conditions is more than 5 times faster, and the LPW of the gold nanorods is limited below 850 nm. The growth with the citrate-stabilized seeds takes ∼30 h and yields in the same growth batch both penta-twinned gold bipyramids and a small percentage of single-crystalline gold nanorods. The gold bipyramids exhibit narrow size distributions and sharp extinction peaks. Their longitudinal plasmon wavelength is tunable from 700 to 1100 nm by simply varying the amount of the citrate-stabilized seeds. These gold bipyramids are attractive for technological applications because of the very large local electrical field enhancement associated with the sharp apexes. Acknowledgment. This work was supported by start-up funding for J.W. from CUHK, by the Institute for Collaborative Biotechnologies through Grant DAAD19-03-D-0004 from the U.S. Army Research Office, and by the U.S. National Science Foundation through Grant DMR 02-33728. Supporting Information Available: LMTEM and HRTEM images of gold nanorods that were grown with the one-step process and those grown with the two-step process, timedependent extinction spectra of the nanorod growth using 0.032 mL of the CTEAB-stabilized seed solution for both one-step and two-step processes, and HRTEM images of the gold bipyramids. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (b) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L. F.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (c) Murphy, C. J.; Sau, T. K.; Gole, A.; Orendorff, C. J. MRS Bull. 2005, 30, 349. (d) Pe´rez-Juste, J.; Pastoriza-Santos, I.; Liz-Marza´n, L. M.; Mulvaney, P. Coord. Chem. ReV. 2005, 249, 1870. (2) (a) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (b) Yan, B. H.; Yang, Y.; Wang, Y. C. J. Phys. Chem. B 2003, 107, 9159. (c) Brioude, A.; Jiang, X. C.; Pileni, M. P. J. Phys. Chem. B 2005, 109, 13138. (3) (a) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. AdV. Mater. 2001, 13, 1501. (b) Maier, S. A.; Kik, P. G.; Atwater, H. A.; Meltzer, S.; Harel, E.; Koel, B. E.; Requicha, A. A. G. Nat. Mater. 2003, 2, 229. (c) Ozbay, E. Science 2006, 311, 189. (4) Hu, M.-S.; Chen, H.-L.; Shen, C.-H.; Hong, L.-S.; Huang, B.-R.; Chen, K.-H.; Chen, L.-C. Nat. Mater. 2006, 5, 102. (5) (a) Chen, J. Y.; Wiley, B.; Li, Z.-Y.; Campbell, D.; Saeki, F.; Cang, H.; Au, L.; Lee, J.; Li, X. D.; Xia, Y. N. AdV. Mater. 2005, 17, 2255. (b) Chen, J. Y.; Saeki, F.; Wiley, B. J.; Cang, H.; Cobb, M. J.; Li, Z.-Y.; Au, L.; Zhang, H.; Kimmey, M. B.; Li, X. D.; Xia, Y. N. Nano Lett. 2005, 5, 473.

J. Phys. Chem. B, Vol. 110, No. 33, 2006 16383 (6) (a) Imura, K.; Nagahara, T.; Okamoto, H. J. Phys. Chem. B 2005, 109, 13214. (b) Wang, H. F.; Huff, T. B.; Zweifel, D. A.; He, W.; Low, P. S.; Wei, A.; Cheng, J.-X. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 15752. (c) Bouhelier, A.; Bachelot, R.; Lerondel, G.; Kostcheev, S.; Royer, P.; Wiederrecht, G. P. Phys. ReV. Lett. 2005, 95, 267405. (7) (a) Li, C.-Z.; Male, K. B.; Hrapovic, S.; Luong, J. H. T. Chem. Commun. 2005, 3924. (b) Liao, H. W.; Hafner, J. H. Chem. Mater. 2005, 17, 4636. (c) Sudeep, P. K.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005, 127, 6516. (8) Takahashi, H.; Niidome, Y.; Yamada, S. Chem. Commun. 2005, 2247. (9) Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13549. (10) Weissleder, R. Nat. Biotechnol. 2001, 19, 316. (11) So¨nnichsen, C.; Alivisatos, A. P. Nano Lett. 2005, 5, 301. (12) van der Zande, B. M. I.; Bo¨hmer, M. R.; Fokkink, L. G. J.; Scho¨nenberger, C. J. Phys. Chem. B 1997, 101, 852. (13) Yu, Y.-Y.; Chang, S.-S.; Lee, C.-L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (14) (a) Kim, F.; Song, J. H.; Yang, P. D. J. Am. Chem. Soc. 2002, 124, 14316. (b) Niidome, Y.; Nishioka, K.; Kawasaki, H.; Yamada, S. Chem. Commun. 2003, 2376. (c) Miranda, O. R.; Ahmadi, T. S. J. Phys. Chem. B 2005, 109, 15724. (15) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389. (16) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (b) Busbee, B. D.; Obare, S. O.; Murphy, C. J. AdV. Mater. 2003, 15, 414. (c) Wu, H.-Y.; Chu, H.-C.; Kuo, T.-J.; Kuo, C.-L.; Huang, M. H. Chem. Mater. 2005, 17, 6447. (17) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (18) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (19) (a) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414. (b) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633. (c) Huang, C.-C.; Yang, Z. S.; Chang, H.-T. Langmuir 2004, 20, 6089. (d) Gou, L. F.; Murphy, C. J. Chem. Mater. 2005, 17, 3668. (20) Jana, N. R. Small 2005, 1, 875. (21) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368. (22) Pe´rez-Juste, J.; Liz-Marza´n, L. M.; Carnie, S.; Chan, D. Y. C.; Mulvaney, P. AdV. Funct. Mater. 2004, 14, 571. (23) Gao, J. X.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9065. (24) (a) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1982, 88, 594. (b) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236. (25) (a) Huo, Q. S.; Margolese, D. I.; Ciesla, U.; Feng, P. Y.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. (b) Huo, Q. S.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P. Y.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (c) Huo, Q. S.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (26) Tsung, C.-K.; Kou, X. S.; Shi, Q. H.; Zhang, J. P.; Yeung, M. H.; Wang, J. F.; Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 5352. (27) Wang, Z. L.; Mohamed, M. B.; Link, S.; El-Sayed, M. A. Surf. Sci. 1999, 440, L809. (28) Liu, M. Z.; Guyot-Sionnest, P. J. Phys. Chem. B 2005, 109, 22192. (29) Song, J. H.; Kim, F.; Kim, D.; Yang, P. D. Chem.sEur. J. 2005, 11, 910. (30) Zweifel, D. A.; Wei, A. Chem. Mater. 2005, 17, 4256. (31) Wang, Z. L.; Gao, R. P.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 5417. (32) Wang, H.; Brandl, D. W.; Le, F.; Nordlander, P.; Halas, N. J. Nano Lett. 2006, 6, 827. (33) (a) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765. (b) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 771.