Letter pubs.acs.org/NanoLett
High-Yield Seedless Synthesis of Triangular Gold Nanoplates through Oxidative Etching Lei Chen,† Fei Ji,† Yong Xu,† Liu He,†,‡ Yifan Mi,† Feng Bao,† Baoquan Sun,† Xiaohong Zhang,† and Qiao Zhang*,† †
Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123, People’s Republic of China Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
‡
S Supporting Information *
ABSTRACT: We demonstrate that monodispersed triangular gold nanoplates with high morphological yield (>90%) can be synthesized through a rapid one-pot seedless growth process. The edge length of triangular Au nanoplates can be readily tuned between 40 and 120 nm by varying the reaction parameters. Systematic studies reveal that distinct from previous hypothesis that the formation of nanoplates is mainly determined by the selective binding of iodide ions, our results show that iodide ions could have dual functions: it can selectively bind to the Au {111} facets and also selectively remove other less stable shape impurities through oxidative etching by forming tri-iodide ions (I3−), thus facilitating the formation of nuclei with dominant planar structure. This new synthetic route will not only help to better understand the growth mechanism of triangular gold nanoplates but also promote the research in anisotropic noble metal nanostructures. KEYWORDS: Gold nanoplates, seedless growth, surface plasmon resonance, SERS prepared large Au plates (edge length around 0.5−1.0 μm) by using the extract of the plant lemongrass.33 In 2005, Mirkin and co-workers successfully prepared colloidal gold nanoplates with edge length of around 100−200 nm through a seeded growth method.18 The as-prepared gold nanoplates show superior LSPR property with distinct dipolar and quadrupolar plasmon resonances. Since then, many synthetic strategies have been developed to synthesize gold nanoplates, including seededgrowth method,19,34,35 thermal reduction approach,36 electrochemical approach,37 and photocatalytic approach,38 among which the seeded growth method is still the most popular one because of its simple setup and relatively higher throughput. Despite the pioneering work in the synthesis of gold nanoplates, lots of challenges still remain, including the lack of a clear understanding of the growth mechanism, low morphological yield (lower than 70%), as well as the difficulty in synthesizing small nanoplates (edge length 90%) and broadly tunable edge length (from ∼45 to ∼120 nm) by harnessing the oxidative power of tri-iodide ions. A systematic study has been carried out to investigate the growth mechanism of gold nanoplates. Distinct from previously proposed mechanism that iodide ions act solely as the facet-selective species, we show that iodide ions act as a dual-functional reagent: not only can it facilitate the formation of nanoplates through selective binding on the Au {111} facets, but also it forms I3− to selectively remove other shaped impurities at an early stage through chemical etching, which is in line with the silver nanoplate synthesis where H2O2 has been used as an etchant to facilitate the formation of silver nanoplates.31 Compared to traditional seeded-growth protocols, this simple method has several advantages. First, the seedless growth process can be completed within 10 min, which is much faster than the seeded growth process that takes at least several hours. Second, the reproducibility of the seedless growth is much better than that of seeded growth process that has been limited by the multistep process. For example, it has been found that the yield of anisotropic nanostructures is determined by the quality of seeds, which is affected by various factors, such as the aging time, the injection rate of reducing agent (NaBH4 in most cases), the reaction temperature, the concentration of reagents, and so forth. Additionally, the injection rate of seed solution into growth solution and the interval between each seeded step are also critical to the quality of the final product.19 Third, all of the reported protocols can only yield less than 70% of Au nanoplates and a purification step is necessary to obtain monodispersed product.19,45 The proposed seedless growth process can produce more than 90% of monodispersed Au nanoplates and no purification step is needed. As a result, this rapid seedless growth process is more cost-effective toward practical applications. The seedless growth process is illustrated in Figure 1a. Hexadecyltrimethylammonium chloride (CTAC) is explored as the surfactant, and iodide ions are used as the shape-directing agent. Sodium hydroxide is used to adjust the pH of the growth solution. In a typical experiment, CTAC is first mixed with potassium iodide (KI), followed by the addition of sodium tetrachloroaurate solution (obtained by mixing chloroauric acid and NaOH in a 1:1 ratio). Upon the addition of ascorbic acid, Au3+ is quickly reduced to Au+, as evidenced by the color change of solution from light yellowish to colorless. Finally, a certain amount of NaOH is rapidly injected into the solution to initiate the reduction of Au+. The pH value is around 8.0. The color of the solution changes gradually from colorless to red, purple, and eventually blue, indicating the formation of anisotropic gold nanostructures. The blue colloidal solution is shown in the digital image (inset of Figure 1b). Neither precipitate nor strong scattering can be observed, implying the
Figure 1. (a) Schematic illustration of the synthesis process of Au nanoplates using a one-step seedless method; (b) a typical UV−vis spectrum of the obtained Au nanoplates. The inset in (b) shows a digital image of colloidal Au nanoplates solution. (c) The height profile of a gold nanoplate. The scale bar of the inset AFM image is 100 nm. (d) TEM image of the as-obtained Au nanoplates with average edge length of 67.30 ± 3.29 nm. (e) HRTEM image of Au nanoplate; the inset shows the SAED pattern.
small size of nanoparticles. The reaction can be completed within 10 min. UV/vis spectrometer has been used to characterize the obtained product. As shown in Figure 1b, the product has a sharp extinction band located around 655 nm and a weak shoulder around 570 nm, which can be attributed to the in-plane dipole and out-of-plane dipole resonance of gold nanoplates, respectively.18 No obvious peak around 530 nm can be observed, suggesting the high yield of anisotropic nanostructures, which has been further confirmed by the TEM characterization. It should be noted that the TEM image is obtained directly from the original product. No size selection or any other purification process has been applied. As shown in Figure 1d, monodispersed triangular gold nanoplates with sharp tips have been successfully prepared. The average edge length of the obtained nanoplates is 67.30 ± 3.29 nm. From both UV−vis spectrum and TEM characterization, it is determined that the morphological yield of triangular gold nanoplates is over 90%. To our best knowledge, this is the highest yield that can be obtained through a colloidal approach. High-resolution TEM (HRTEM) has been used to characterize the obtained Au nanoplates. As shown in Figure 1e, the lattice distance is 0.237 nm, which is in good agreement with bulk Au (111) spacing (0.236 nm), confirming that the planar facet is (111) facet. The selected area electron diffraction (SAED) pattern in the inset of Figure 1e confirms the as-prepared gold nanoplate is single crystalline. The appearance of 1/3{422} pattern can be attributed to the parallel stacking faults in the ⟨111⟩ direction.46 7202
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Figure 2. TEM images of Au nanoplates with tunable edge length. The average edge length of each sample is (a) 45.22 ± 3.38 nm; (b) 65.48 ± 2.92 nm; (c) 70.26 ± 3.07 nm; (d) 78.25 ± 3.16 nm; (e) 107.34 ± 8.01 nm; (f) 117.29 ± 5.78 nm. The scale bars are 100 nm.
Information (Table S1). The edge length can also be controlled by tuning the concentration of ascorbic acid. As shown in Figure 2b−d, the edge length of gold nanoplates can be well tuned from 65.48 ± 2.92 to 78.25 ± 3.16 nm when the concentration of ascorbic acid increases from 0.384 to 0.640 mM while the other conditions are kept identical. It is worth pointing out that the morphological yield of triangular gold nanoplates is over 90% in all of the reaction conditions, suggesting this seedless growth process is a robust method. Because the optical property of gold nanoplates is very sensitive to the physical dimensions, the SPR peak can therefore be tuned readily by controlling the edge length of gold nanoplates. As plotted in Supporting Information Figure S9, the SPR peak can be tuned within the range of 620 to 700 nm. No sharp peak around 530 nm has been observed in all the spectra, further confirming the high yield of triangular gold nanoplates. To figure out the growth mechanism of gold nanoplates, a systematic study has been carried out. Alkyltrimethylammonium halide surfactant (e.g., CTAC, CTAB) has been widely used to make anisotropic gold nanostructures as they can preferentially bind to certain gold facets and protect nanoparticles from aggregation. In the absence of CTAC, only big aggregates can be obtained, confirming the protective effect of CTAC. The optimum concentration of CTAC is around 15− 20 mM in the solution. The halide species also play an important role in this process. When CTAC is replaced by CTAB and the other conditions are kept the same, the reaction becomes much slower and takes almost 4 h. This phenomenon
The atomic force microscopy (AFM) has been used to evaluate the thickness of gold nanoplates, as shown in Figure 1c. The thickness profile in the AFM image is quite smooth, implying a flat surface. The average thickness is around 15 nm. Because the SPR position is highly sensitive to the aspect ratio, defined as the ratio between the edge length and the thickness of nanoplates, the theoretical absorption of Au nanoplates has been simulated by using the finite-difference time-domain (FDTD) method. As shown in Supporting Information Figure S1, the simulation data agree well with the experimental one in which a maximum absorption at 647 nm has been obtained when the edge length and thickness are set as 70 and 15 nm, respectively. The edge length of gold nanoplates can be easily tailored in the range of ∼40 to ∼120 nm with this seedless growth process by tuning the concentration of reagents, for example, ascorbic acid, potassium iodide, and NaOH. It is worth noting that the concentrations of CTAC and Au3+ are kept identical in all the experiments. It is well-known that fast nucleation can produce uniform and small-sized nanoparticles. As depicted in Figure 2a, because the reduction process is initiated by the addition of NaOH, gold nanoplates with the smallest edge length (45.22 ± 3.38 nm) can be obtained at high pH values when the added NaOH concentration is 0.20 mM, while gold nanoplates with the largest edge length (117.29 ± 5.78 nm, Figure 2f) can be obtained when the added NaOH concentration decreases to 0.10 mM and the other parameters are kept the same. The synthetic conditions have been detailed in the Supporting 7203
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can be attributed to the higher stability of Au−Br complex.47 Because the nucleation process is so slow, only large Au nanoplates (edge length >0.5 μm) with relatively low morphological yield (
