Copper-Ion-Assisted Growth of Gold Nanorods in Seed-Mediated

Nov 22, 2012 - In the well-developed seed-mediated growth of gold nanorods (GNRs), adding the proper amount of Cu2+ ions in the growth solution leads ...
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Copper-Ion-Assisted Growth of Gold Nanorods in Seed-Mediated Growth: Significant Narrowing of Size Distribution via Tailoring Reactivity of Seeds Tao Wen,†,‡ Zhijian Hu,† Wenqi Liu,†,‡ Hui Zhang,†,‡ Shuai Hou,†,‡ Xiaona Hu,†,‡ and Xiaochun Wu*,† †

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, Beijing 100190, P. R. China ‡ University of the Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: In the well-developed seed-mediated growth of gold nanorods (GNRs), adding the proper amount of Cu2+ ions in the growth solution leads to significant narrowing in the size distribution of the resultant GNRs, especially for those with shorter aspect ratios (corresponding longitudinal surface plasmon resonance (LSPR) peaks shorter than 750 nm). Cu2+ ions were found to be able to catalyze the oxidative etching of gold seeds by oxygen, thus mediating subsequent growth kinetics of the GNRs. At proper Cu2+ concentrations, the size distribution of the original seeds is greatly narrowed via oxidative etching. The etched seeds are highly reactive and grow quickly into desired GNRs with significantly improved size distribution. A similar mechanism can be employed to tune the end cap of the GNRs. Except for copper ions, no observable catalytic effect is observed from other cations presumably due to their lower affinity to oxygen. Considering the widespread use of seed-mediated growth in the morphology-controlled synthesis of noble metal nanostructures, the tailoring in seed reactivity we presented herein could be extended to other systems.

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INTRODUCTION

introducing various aromatic additives to the well-established protocols.12 Cu2+ ions have been suggested to tailor the shape of gold nanocrystals via selectively retarding the growth rate of {111} surfaces13 or help elongate Pd nanorods by periodic deposition and reoxidation of copper atoms on the growing rods.14 Herein, we report another role of copper ions in the growth of the GNRs: accelerating the growth rate of the GNRs. Under optimized conditions, significant narrowing in the size distribution of the GNRs is achieved, especially for the GNRs with short aspect ratios (longitudinal SPR maximum shorter than 750 nm). The fwhm (full width at half-maximum) minimum could be reduced down to 0.2 eV. Involvement of copper ions is highly compatible with current synthesis procedures of the GNRs, as demonstrated by changing various reaction parameters, such as the concentrations of reactants, pH, and silver ions. Furthermore, introduction of copper ions provides additional flexibilities in tailoring the growth kinetics for further optimization. The major roles of copper ions include narrowing the size distribution of the Au seeds and tailoring their reactivity via catalyzing the oxidative etching of the seeds by oxygen. To the best of our knowledge, this is the first report

Due to their unique shape-dependent optical properties, especially aspect ratios-controlled localized surface plasmon resonance (SPR) features in a wide spectral range, gold nanorods (GNRs) have received considerable attention during the past decade with promising potentials in plasmonics, optoelectronics, chemical or biological sensing, and disease theranostics.1 Seed-mediated growth, which was initially introduced by Murphy2 and later improved by El-Sayed and co-workers,3 is currently the most employed method to synthesize GNRs due to the simplicity of the procedure, high yield of nanorods, and flexibility in chemical modifications. For the well-developed procedure, Ag+ ions were used to adjust the aspect ratios (ARs) of the rods.3 Often a proper amount of acids4 was added to fine tune the ARs via further tailoring the growth kinetics of the rod. Yet, the size distribution of the as-prepared rods is relatively broad, especially for the GNRs with shorter ARs. Improvement of the GNRs quality is mainly via two ways: postsynthesis separations, such as repeated centrifugation, self-assembly,5 or density gradient centrifugation6 and finely tuning the growth kinetics,7,8 such as changing surfactants,9 adding a small amount of acid in the growth solution,10 or repeatedly adding a small amount of reducing agent.11 Recently, Murray et al. reported obvious narrowing in the size distribution of the GNRs by © 2012 American Chemical Society

Received: October 22, 2012 Revised: November 22, 2012 Published: November 22, 2012 17517

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that the reactivity of the Au seeds is finely tailored and results in significant improvement in the quality of the GNRs.

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EXPERIMENTAL SECTION

Materials. Sodium borohydride (NaBH4 ), chlorauric acid (HAuCl4·3H2O), cetyltrimethylammonium bromide (CTAB), silver nitrate (AgNO3), L-ascorbic acid (AA), and 2,6-pyridinedicarboxylic acid (PDCA) were purchased from Alfa Aesar and used as received. Sulfuric acid (H2SO4), copper(II) chloride dihydrate (CuCl2·2H2O), cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O), chromium(III) nitrate nonahydrate (Cr(NO3)3·9H2O), calcium chloride (CaCl2), nickelous nitrate (Ni(NO3)2·6H2O), zinc chloride (ZnCl2), lead(II) chloride (PbCl2), manganese(II) chloride (MnCl2·4H2O), ammonium iron(II) sulfate hexahydrate ((NH4)2Fe(SO4)2·6H2O), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O), and mercury(II) perchlorate (Hg(ClO4)2·6H2O) were at least analytical reagent grade and purchased from Beijing Chemical Reagent Co. (Beijing, China). Milli-Q water (18 MΩ cm) was used for all solution preparations. Typical Synthesis of Gold Nanorods (GNRs) with Cu2+ Ions. GNRs were synthesized using the well-developed seed-mediated growth method. Briefly, CTAB-capped Au seeds were synthesized by chemical reduction of HAuCl4 with NaBH4: CTAB (7.5 mL, 0.1 M) was mixed with HAuCl4 (100 μL, 25 mM) and diluted with water to 9.4 mL. Then, ice-cold NaBH4 (0.6 mL, 0.01 M) was added under magnetic stirring. The solution color turned immediately from bright yellow to brown, indicating formation of Au seeds. Au seeds were used within 2−5 h. For a typical preparation of the GNRs, in a growth solution consisting of a mixture of CTAB (10 mL, 0.1 M), HAuCl4 (200 μL, 25 mM), AgNO3 (70 μL, 10 mM), H2SO4 (100 μL, 1 M), and AA (80 μL, 0.1 M), a certain amount of CuCl2 was added just before adding seed solution (24 μL) to initiate growth of GNRs. The growth temperature was kept at 30 °C. Twelve hours later, the GNRs were purified by centrifuging the solution at 12 000 rpm for 5 min twice. The precipitate was collected and redispersed in deionized water. Note, instead of CuCl2, we used CuSO4 and Cu(NO3)2; no difference is observed, indicating that anions have no effect. Effects of Other Reaction Parameters on Growth of GNRs in the Presence of Cu2+ Ions. The effect of a single parameter on GNRs growth was done by keeping other growth conditions unchanged. For AA, its concentration range is tuned from 0.552 to 15 mM. For pH, the concentration of H2SO4 is changed from 0 and 125 mM. For seeds, the added volume is varied from 12 to 120 μL in a 10 mL reaction volume. The concentration of gold ions is tuned from 0.1 to 1.8 mM. Reshaping. Dog-bone-like GNRs, synthesized with a [AA]/[Au3+] ratio of 5, were employed to conduct reshaping experiments. Purified dog-bone-like GNRs were dispersed in 0.1 M CTAB aqueous solution. Different combinations of Cu2+ ions, H2SO4, and AA were used for reshaping at 30 °C. The concentrations are [CTAB] = 0.1 M, [Cu2+] = 100 μM, [H2SO4] = 10 mM, and [AA] = 750 μM. Effect of Other Cations on GNRs Growth. Apart from CuCl2, several other cations (Mn+) were chosen to test their effect on the growth of the GNRs. [Mn+] = 100 μM was used. HCl (100 μL, 2 M) is used to replace H2SO4. Characterization. UV−vis−NIR absorption spectra were recorded on a Varian Cary 50. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were obtained with a Tecnai G2 20 S-TWIN operating at an acceleration voltage of 200 kV and a Hitachi S-4800 operating at an acceleration voltage of 10 kV, respectively. Elemental analysis was performed with energy-dispersive X-ray (EDX) from SEM and inductively coupled plasma atomic emission spectrometry (ICP-AES) from IRIS Intrepid II.

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Figure 1. UV−vis−NIR absorption spectra of GNRs suspensions synthesized in the absence (dashed lines) and presence (solid lines) of Cu2+ ions. Concentrations of Ag+ are 40 (a), 70 (b), 100 (c), and 150 μM (d). Corresponding concentrations of Cu2+ are 160 (aCu), 100 (bCu), 60 (cCu), and 30 (dCu) μM, respectively. Size distribution histograms with Gaussian fitting (dark line) in the absence (B) and presence of 100 μM Cu2+ ions (C), [Ag+] = 70 μM.

the LSPR bandwidth and obvious reduction of long wavelength tailing, especially for GNRs with smaller ARs (see details in Table S1 and Figure S1, Supporting Information). For instance, GNRs prepared with [Ag+] = 70 μM, 100 μM Cu2+ resulted in fwhm of the LSPR band decreasing from 0.37 to 0.2 eV. The relative standard deviation (RSD) in AR is decreased from 22% to 9% from TEM measurements (using Gaussian fitting of size distribution histograms, RSD is from 18% to 7%). In addition, Cu2+ ions also lead to a blue shift in longitudinal SPR (LSPR) maximum: they affect GNRs with high ARs more strongly. For GNRs prepared with [Ag+] = 150 μM, addition of 30 μM Cu2+ gave a LSPR maximum of 800 nm, 111 nm blue shifted from that in the absence of Cu2+. Cu element was not found in the GNRs from energy-dispersive X-ray (EDX) analysis (Table S2 and Figure S2, Supporting Information) and inductively coupled plasma atomic emission spectrometry (ICP-AES) measurement (Table S3, Supporting Information). In contrast, ca. 3% Ag element is found for the obtained GNRs. The effect of Cu2+ concentration indicates the existence of an optimal concentration window for size narrowing. For the GNRs with [Ag+] = 70 μM, the concentration window is between 60 and 160 μM (Figure 2). The best concentration is around 100 μM. For [Ag+] = 150 μM, the window is between 30 and 90 μM. It seems that higher amounts of silver ions require less copper ions to achieve the best optimization (Figure S3, Supporting Information). We noticed that increasing Cu2+ leads to a nonlinear blue shift of LSPR maximum for all silver ions. 2,6-Pyridinedicarboxylic acid (PDCA), which is often chosen as a masking ligand to

RESULTS AND DISCUSSION

Improvement in the Quality of GNRs in the Presence of Cu2+ Ions. Figure 1A demonstrates the improved quality of the GNRs upon adding Cu2+. With a proper amount of copper ions, all four GNRs with different ARs exhibited narrowing of 17518

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accompanying an obvious reduction in the 400 nm absorbance (Figure S6, Supporting Information). Due to a small change in the 400 nm absorbance (corresponding to the total amount of Au atoms) in the second stage, such a large blue shift in the presence of Cu2+ ions is unusual. Considering the big influence of end-cap morphology on the LSPR maximum, we further checked the shapes of the GNRs from TEM images (Figure 4). Indeed, in the presence of Cu2+ ions, during the second stage the end caps change from dog bones to hemispheres. In contrast, in the absence of Cu2+ ions, no obvious end-cap change is observed. We also noticed the variations in nonrod nanoparticles (counted about 90): about 80% of them (Figure 4C) are cube-like, while 85% of them (Figure 4D) become obviously more round. FDTD (finite difference time domain) simulation (FDTD Solutions 6.0, Lumerical Solutions, Inc.) is consistent with our experimental results. Using the geometrical parameters of the rods from TEM measurement (Table S4, Supporting Information), we calculated the absorption spectra of the single rod by fixing the length and volume of the rod. Dog-bone shape (Figure 4e) shows a LSPR maximum at 756 nm, while it blue shifts to 690 nm for hemisphere-capped rods with w/2h = 1.3 (indicated in Figure 4), exhibiting a 66 nm blue shift. Corresponding scattering spectra show a similar trend (Figure S7, Supporting Information). The end-cap change is also observed at [Ag+] = 150 μM in the presence of Cu2+ ions (from rectangle shape to hemisphere-capped shape, Figure S8, Supporting Information). Thus, we propose that the large blue shift of the LSPR maximum is mostly due to the end-cap shape difference of the GNRs. In the presence of Cu2+ ions, during the second stage of GNRs growth, the average length decreases from 53 to 50 nm while the average diameter increases from 18 to 19 nm (Table S4, Supporting Information), indicating that the gold atoms may migrate from the end caps to the side facets of the rod: a reshaping process. Similar results are also observed at [Ag+] = 150 μM: the average length decreases from 55 to 52 nm, while the average diameter increases from 13 to 14 nm (Table S5, Supporting Information). Mechanism for End-Cap Morphology Variation. In the presence of copper ions, the end-cap morphology of the GNRs gradually changes. Thus, what is the mechanism behind it? Purified dog-bone GNRs were employed to clarify the role of Cu2+ ions in the second growth stage of the GNRs (Figure 5 and Figure S9, Supporting Information). Adding H2SO4 alone could lead to a gradual blue shift of the LSPR maximum (from 800 to 650 nm) during the first 21 h. Afterward, the position of the LSPR maximum remained nearly constant (from 21 to 32 h). Combination of H2SO4 and Cu2+ greatly accelerates the process. The LSPR maximum (from 800 to 630 nm) is achieved within 5 h, indicating the catalytic role of Cu2+. However, in the coexistence of AA and H2SO4, no obvious change in LSPR maximum happens. In the presence of AA (absorbance at 244 nm), dissolved oxygen preferentially oxidizes AA, thus protecting the GNRs. In contrast, combination of AA, Cu2+, and H2SO4 results in rapid blue shift but with a time lag. The time lag corresponds to the period of AA oxidation by dissolved oxygen under catalysis of copper ions (disappearance of AA absorbance at 244 nm, Figure S9A, Supporting Information). For the reshaped GNRs, after 21 h, dog bones change to cylinders with hemisphere caps. As the morphology in Figure 5D and 5E has no obvious difference, we believe that at acidic condition, dissolved oxygen can reshape

Figure 2. Effect of Cu2+ concentration on the quality of the GNRs with [Ag+] = 70 μM: vis−NIR absorption spectra (A) and LSPR maximum and fwhm vs Cu2+ concentration (B).

minimize the interference of Cu2+ for some metal ions assays due to its strong chelating capability for Cu2+,15 is employed herein to verify the role of Cu2+. After masking copper ions by PDCA, the quality of the GNRs shows no improvement, indicating the role of free copper ions (Figure S4, Supporting Information). Growth Kinetics in the Presence of Cu2+ Ions. In order to get more insights in size narrowing, we further monitored the growth kinetics (Figure 3). Growth can be divided into two stages: an initial rapid growth stage follows a subsequent slow growth stage. Taking [Ag+] = 70 μM as the example, in the absence of Cu2+ ions, it took 3 h to complete the first stage with a 93% yield (absorbance at 400 nm). The LSPR red shift is dominated, accompanying obvious narrowing in the bandwidth (fwhm from 0.49 to 0.35 eV). In the presence of Cu2+ ions, the first stage is shortened to 0.5 h with a yield of 88%. An initial red shift of the LSPR maximum is rapidly dominated by a large blue shift. In addition, an obvious decrease in bandwidth (fwhm from 0.29 to 0.24 eV) also occurs. In the second stage, the GNRs grown without Cu2+ ions exhibit a small blue shift (8 nm) accompanying a slight increase in the absorbance at 400 nm (7%). In contrast, the GNRs grown with Cu2+ ions show a quite large blue shift (74 nm) with a slight decrease in absorbance at 400 nm (9%). In addition, the fwhm shows a further slight improvement (to 0.21 eV). A similar scenario is observed at a silver ion concentration of 150 μM (Figure S5, Supporting Information). Obviously, Cu2+ ions accelerate growth of GNRs. In the case of [Ag+] = 150 μM, increasing Cu2+ ions (less than 200 μM) leads to a nonlinear increase in the growth rate of the GNRs. Higher than 200 μM Cu2+ ions, on the other hand, induces a decrease in growth rate, 17519

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Figure 3. Evolution of UV−vis−NIR absorption spectra of the GNRs in the absence (A) or presence (B) of Cu2+ ions, absorbance at 400 nm (C), LSPR maximum (D), and fwhm (E) versus growth time: dark squares and red stars represent without and with Cu2+ ions, respectively. [Ag+] = 70 μM.

Figure 4. Representative TEM images of the GNRs grown without Cu2+ ions (A, 3 h; B, 24 h) and with Cu2+ ions (C, 1.5 h; D, 24 h). Scale bar is 100 nm. Insets are an enlarged magnification, and scale bar is 20 nm. (E) Absorption spectra of single GNRs from FDTD calculations with five different end caps: (a) w/2h = 1, (b) w/2h = 1.3, (c) w/2h = 1.7, (d) cylinder, (e) dog bone.

the reported etching of the GNRs by dissolved oxygen (no copper ions) needs much harsher conditions (0.5 M HCl at 70 °C).16 In all, the end-cap change of the GNRs in the presence of copper ions can be understood as follows: fast growth kinetics of the GNRs leads to “pinning” of kinetically controlled morphology (such as dog bones and rectangles). There is a

the GNRs to a more stable morphology via partial oxidation of surface Au atoms (high surface energy sites) and subsequent migration of Au atoms on the surface. In addition, Cu2+ ions, acting as catalysts, accelerate this process. Additionally, in the presence of copper ions, we observed a growing decrease in the absorbance at 400 nm and more blue shifts of the LSPR maximum, indicating the appearance of GNRs etching. Note, 17520

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Figure 5. LSPR maximum vs reshaping time (A) and GNRs absorption spectra before and after reshaping (B): lines in A and B are (a) Cu2+, H2SO4 and AA, (b) Cu2+ and H2SO4, (c) H2SO4, and (d) H2SO4 and AA. TEM images: original dog-bone GNRs (C) and after 21 h reshaping with Cu2+, H2SO4, and AA (D) or H2SO4 only (E). Concentrations used for reshaping: [Cu2+] = 100 μM, [H2SO4] = 10 mM, [CTAB] = 0.1 M, and [AA] = 750 μM. Scale bar is 20 nm.

driving force to thermodynamically a more stable morphology upon aging. With the catalysis of copper ions, such transformation is easily realized by surface oxidative etching and subsequent migration of gold atoms. Role of Copper Ions in the Growth of Gold Nanorods. The role of copper ions on the end-cap variation of the GNRs should also work for the Au seeds. Indeed, the effect of copper ions on Au seeds can be directly visualized from the change in the absorption spectra of the seeds (Figure S11, Supporting Information). For 1 mL of as-prepared Au seed solution, the Au seeds themselves show no obvious variation with time during 12 h storage in a 30 °C water bath. Upon adding Cu2+ ions, an obvious change occurs for the seeds (Figure S11B, Supporting Information). The seeds are etched quickly within the first 4 min and then grow quickly with the appearance of a SPR band around 527 nm. In the coexistence of copper ions and acid, the whole process becomes even faster (Figure S11D, Supporting Information). We believe that Cu2+ ions catalyze oxidative etching of the seeds and lead to smaller seeds (see the new peak at 275 nm in Figure S11B and S11D, Supporting Information). The smaller seeds are more reactive and can grow faster via Ostwald ripening and become larger. Previously, in the polyol synthesis of high yields of uniform single-crystal Ag and Pd nanoparticles, oxidative etching by the Cl−/O2 pair has been employed to remove twinned particles by Xia et al.17,18 In order to demonstrate the role of copper ions better, we further measured the activation energy (Ea) of the GNRs formation both in the presence and in the absence of copper ions. Growth rates of the GNRs were obtained at different growth temperatures using the change of absorbance at 400 nm with time (Figure S10, Supporting Information). Using an Arrhenius plot, the activation energy of the GNRs growth is obtained as shown in Figure 6A. In the absence of copper ions, Ea(0) is estimated to be 76.4 ± 3.5 kJ/mol (R2 = 0.9916). In contrast, it decreases to 66.9 ± 1.0 kJ/mol (R2 = 0.9991) in the presence of copper ions (Ea(Cu)), indicating activation of the Au seeds by copper ions. The corresponding mechanism is shown

Figure 6. Arrhenius plot of the growth rate versus reciprocal of growth temperature for formation of GNRs from the Au seed in the absence (dark square) and presence (blue circle) of Cu2+ ions with [Ag+] = 70 μM (A), and schematic demonstration (B) of activation energy (Ea) change for GNR formation from Au seeds in the absence (Ea(0)) and presence (Ea(Cu)) of copper ions.

in Figure 6B: the as-prepared Au seeds have a relative large size distribution and thus expose different reactivities. In the subsequent growth, they exhibit different growth rates, thus leading to resulting GNRs with a broad size distribution (as 17521

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witnessed by the obvious long wavelength tailing in the absence of copper ions). Upon adding a proper amount of copper ions, the seeds are etched to a narrower size distribution with high reactivity. They have higher energy than the original seeds. These active seeds lead to fast formation of high-quality GNRs. The disappearance of some Au seeds due to oxidative etching is supported by the larger sizes of the GNRs obtained at higher Cu ion concentrations. For example, for GNRs with [Ag+] = 70 μM, the rod sizes (46 ± 5 nm long, 21 ± 2 nm wide) in the presence of 100 μM Cu2+ are larger than those (42 ± 6 nm long, 17 ± 2 nm wide) in the absence of copper ions (Table S1, Supporting Information). Effect of Other Factors in the Growth Solution. In order to test the compatibility of Cu2+ ions for the seedmediated growth of GNRs, the effects of other reaction parameters were also investigated. The amount of dissolved oxygen in the growth solution affects the quality of the obtained GNRs in the presence of Cu2+ ions, while there is no effect for the samples without adding Cu2+ ions (Figure S12, Supporting Information). Oxygen deficiency gave discounted improvement. For growth of the GNRs in the presence of Cu2+ ions, the concentrations of AA and acid also affect the size distribution and morphology of GNRs. At 1.5 ≤ [AA/Au3+] ≤ 3, the quality of the GNRs could be improved, more obviously at lower ratios (Figure S13, Supporting Information). The reason may be that the acceleration of copper ions is more easily exhibited at lower concentrations of AA. At 10 mM ≤ [H2SO4] ≤ 25 mM, nice GNRs could be obtained and the LSPR band is blue shifted by increasing acid concentration. At higher acid concentrations, the quality of the GNRs becomes worse or even could not be formed (Figure S14, Supporting Information). The reason is rapid oxidation of AA by dissolved oxygen in acidic condition under catalysis of copper ions. Thus, the amount of AA is not enough to reduce all gold ions to gold atoms. Interestingly, in the presence of Cu2+ ions, seed concentration can be utilized to tailor LSPR range while keeping the GNRs with a narrow size distribution (Figure S15, Supporting Information). TEM images show that the average diameter with more seeds is smaller than that with less seeds (Table S6, Supporting Information), thus providing a feasible way to tune the diameter of GNRs while keeping a narrow size distribution. As for HAuCl4, at its concentrations between 0.2 to 1.8 mM, the quality of the GNRs with copper ions is always better than those without copper ions. A small difference in LSPR maximum shows less dependence on HAuCl4 concentrations lower than 0.5 mM in the presence of copper ions (Figure S16, Supporting Information). Effect of Other Cations for GNRs Growth. In the polyol synthesis, Xia and co-workers observed that adding Fe2+ species facilitated growth of Ag nanoparticles and suggested that Fe2+ removed the adsorbed atomic oxygen from the surface of silver nanoparticles,19 which led to a faster growth rate. In our case, except for copper ions, other cations we tried exhibited a negligible effect in size distribution narrowing (Figure 7). We ascribed it to the high affinity of copper ions to oxygen atoms.

Figure 7. Effect of various cations on formation of the GNRs: UV− vis−NIR absorption spectra (A), fwhm (B), and LSPR maximum (C). [Mn+] is fixed at 100 μM.

Involvement of copper ions is highly compatible with the wellestablished protocol for GNRs and provides more flexibilities in tailoring growth kinetics for further optimization. Additionally, the similar role of copper ions can help reshape the GNRs to a more stable morphology, suggesting a simple way to obtain the GNRs with similar end caps. Apart from gold nanorods, seedmediated growth has been widely employed for synthesis of various noble metal nanocrystals with different shapes. Our findings on seed tailoring may be beneficial to these processes as well.

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ASSOCIATED CONTENT

S Supporting Information *

Details for UV−vis−NIR spectra of different GNRs solution, TEM images, and tables of characteristic parameters of the GNRs with different conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

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The authors declare no competing financial interest.

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CONCLUSION In this article, we achieved significant size narrowing for GNRs in the seed-mediated growth with the help of copper ions. The size distribution improvement mainly results from size narrowing of the Au seeds, owing to their optimal oxidative etching by oxygen catalyzed by copper ions. The etched seeds are more reactive, leading to fast growth of the GNRs.

ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (2012CB934001 and 2011CB932802) and National Natural Science Foundation of China (Grant Nos. 91127013 and 21173056) and Public Welfare Research Project (201210284). 17522

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