Article pubs.acs.org/cm
Seedless Synthesis of Monodisperse Cuboctahedral Gold Nanoparticles with Tunable Sizes Aminah Umar, Jeeun Lee, Jahar Dey, and Sung-Min Choi* Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea S Supporting Information *
ABSTRACT: Polyhedral gold nanoparticles are of great current interest because of their unique optical and chemical properties which are attributable to their well-defined facets, corners, and size. While various polyhedral gold nanoparticles of different sizes mostly synthesized by the seed-mediated method have been reported, synthesis of gold cuboctahedra with tunable sizes still remains challenging. Here, we report for the first time a seedless method of synthesizing monodisperse gold cuboctahedra with finely tunable sizes ranging from 40 to 80 nm using cetyltrimethylammonium 4-vinylbenzoate (CTAVB) as a selective capping and mild reducing agent in the presence of a high concentration of HCl in aqueous solution. The HCl provides strong oxidative etching power to remove structural defects, resulting in single-crystal seeds, and significantly reduces the particle growth rate. This slow particle growth provides an easy and reliable way of tuning the particle size by stopping the reaction at different times and allowing sufficient time for the surface self-diffusion of Au atoms. Combined with the selective capping of {100} facets with CTA+, the surface selfdiffusion of Au atoms from {111} to {100} facets is considered to be the key mechanism for the formation of Au cuboctahedra and their stable growth without morphological deformation.
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INTRODUCTION Gold nanoparticles (Au NPs) are of great current interest because of their unique optical and chemical properties, which make them attractive for a wide range of applications, such as surface-enhanced Raman scattering,1−3 colorimetric sensing,4−6 catalysis,7,8 and drug delivery.9,10 Because the optical and chemical properties of Au NPs are strongly dependent on their shape and size, substantial research efforts have been made to synthesize Au NPs of different shapes and sizes. Recently, these efforts have focused on the synthesis of polyhedral Au NPs because of their unique properties induced by their well-defined facets and corners, as well as their size.11,12 The most widely used method for fabricating polyhedral Au NPs with desired shapes and sizes is seed-mediated synthesis, which has led to the fabrication of a wide range of nanostructures including cubes,13−17 octahedra,13,15,17 rhombic dodecahedra,13,16,17 cuboctahedra,15−19 icosahedra,20 plate and prisms,1,21 and more exotic shapes with high-index surface facets.17,22−24 The seed-mediated synthesis involves two steps: the preparation of seeds with a strong reducing agent and their growth in a growth solution containing a mild reducing agent. In this method, the particle shape is mainly controlled by the facet-selective capping of seeds and the supply rate of Au atoms, whereas the particle size is typically controlled by the concentration of seeds at a given concentration of gold precursor and the reduction rate, although it can also be controlled by the size of the seeds.25,26 Among polyhedral Au © XXXX American Chemical Society
NPs, cubes, octahedra, and cuboctahedra can be grown from single-crystal seeds containing {111} and {100} facets, and the shapes of the synthesized particles are determined by the relative growth rates of the {100} and {111} facets.27 Au cubes containing {100} facets only are synthesized by capping the {100} facets of seeds with bromide (Br−) or silver ions (Ag+) to retard the growth along the ⟨100⟩ direction, while allowing faster growth in the ⟨111⟩ direction; thus, eventually, the {111} facets disappear.27 Fairly monodisperse Au cubes of different sizes ranging from ca. 30 to ca. 200 nm (depending on the growth solution conditions) have been reported.13−16,18,19 Similarly, Au octahedra containing {111} facets only are synthesized by capping the {111} facets of seeds with polyvinylpryrrolidone or cetylpryridinium chloride, resulting in Au octahedra of different sizes ranging from ca. 15 to ca. 200 nm.13,17,19,28 Unlike cubes and octahedra that contain only one type of facet, cuboctahedra containing both {111} and {100} facets require a more subtle balance between the growth rates of the two facets. Although Au cuboctahedra of a few different sizes have been previously reported as an intermediate morphology in the transition between cubes and octahedra,15−19 in most of cases a rather broad size distribution or a limited size control has been the main issue, and thus, synthesis Received: March 28, 2016 Revised: June 16, 2016
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DOI: 10.1021/acs.chemmater.6b01238 Chem. Mater. XXXX, XXX, XXX−XXX
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Characterization of Cuboctahedral AuNPs. The progress of synthesis with time was monitored using an ultraviolet−visible (UV− vis) spectrophotometer (Perkin-Elmer Lambda 650), a transmission electron microscope (TEM, Tecnai F20, 200 kV), a scanning electron microscope (SEM, Magellan 400), and an inductively coupled plasma spectrometer (ICP-MS, Agilent ICP-MS 7700S). For the UV−vis measurements, 0.5 mL of sample was taken from the solution at each sampling time. For the TEM and SEM measurements, 2.0 mL samples were collected at each sampling time and centrifuged twice at 10 000 rpm (Eppendorf centrifuge 5415 D) for 10 min to remove excess CTAVB. For the ICP-MS measurements (to determine the amount of remaining gold ions in the solution at each reaction time), 2.0 mL of the sample taken at each sampling time was centrifuged, and 0.5 mL of the supernatant was mixed with 2.0 mL of aqua regia (HCl:HNO3 = 3:1) to suppress the reduction of the remaining gold ions in the supernatant. The crystallinity of the as-synthesized particles was examined using X-ray diffractometer (XRD, RIGAKU, D/MAX-2500, 40 kV, and 300 mA) with Cu Kα irradiation. The zeta potential of assynthesized cuboctahedral AuNPs was measured with a ZetaPlus particle size analyzer (Brookhaven Instruments Corporation).
of monodisperse Au cuboctahedra with controlled sizes has remained challenging. Furthermore, a few seedless synthesis methods for Au cubes and octahedra have been reported,29−32 but to the best of our knowledge, no seedless method is currently available for Au cuboctahedra. Here, we report a seedless method for the synthesis of highly monodisperse Au cuboctahedra with tunable sizes ranging from 40 to 80 nm by simply mixing cetyltrimethylammonium 4vinylbenzoate (CTAVB) and gold precursors in water in the presence of a high concentration of HCl. The CTAVB molecules, which are cationic surfactants with a benzoic acid derivative counterions, serve as both a selective capping and mild reducing agent. The high concentration of HCl provides strong oxidative etching power for the removal of structural defects, leaving only single-crystal seeds in the early stages, and also significantly reduces the particle growth rate. In this method, the enhanced surface self-diffusion of Au atoms, which is facilitated by the slow particle growth rate, is one of the key parameters to achieve the conditions for stable formation and growth of cuboctahedra. This slow particle growth rate also provides an efficient way to finely control the particle size by stopping the reaction at different times. To the best of our knowledge, this is the first report of the seedless synthesis of monodisperse Au cuboctahedra with tunable sizes. The different crystallographic facets of polyhedral Au NPs have different atomic configurations and surface energy which provide facet-dependent chemical activity or molecular affinity.33−37 Therefore, the monodisperse Au cuboctahedra (which are enclosed with well-defined surface fractions of {111} and {100} facets, 65% and 35%, respectively) with controlled size can be used as an excellent model system for quantitative comparative investigation of the facet-dependent chemical activities of Au NPs in combination with Au cubes and octahedra which contain {100} and {111} facets only, respectively. Furthermore, it can also be used as an ideal metallic NP for facet-selective molecular surface modifications which would direct the organization of NPs into a threedimensional array of Au NPs with precise positional and orientational control, providing highly tunable plasmonic coupling and absorption spectra.
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RESULTS AND DISCUSSION TEM and SEM measurements were performed for the Au NPs obtained at different reaction times (Figure 1). The TEM and SEM images clearly show that highly monodisperse cuboctahedral Au NPs were formed and their sizes increased with the reaction time. The TEM images show different projections of the cuboctahedral Au NPs depending on the relative orientation of particles to the substrate. Here, square or hexagonal projection images were most frequently observed, which is typical for cuboctahedral particles. It should be noted that all the images were taken using the as-synthesized Au NPs without any shape-selection treatment, and the morphological yield of cuboctahedral Au NPs was as high as ca. 95% for all cases. The size of cuboctahedral AuNPs obtained at 24 h of reaction time was 43 nm. When the reaction time was increased to 36, 48, 60, and 72 h, the average sizes of the particles monotonically increased to 56, 64, 73, and 79 nm, respectively. The size distributions of cuboctahedral Au NPs are presented in Figure S1. Here, the size of a cuboctahedron is presented as the edge length of the square projection images. The XRD measurements show that the cuboctahedral Au NPs have face-centered cubic crystalline symmetry with a dspacing of 0.236 nm across the {111} planes, which is consistent with the reported value (Figure S2).40 The crystallinity and structure of the cuboctahedral Au NPs was analyzed via electron diffraction. Figure 2 shows the diffraction patterns of two cuboctahedral Au NPs with their square or triangular facets oriented perpendicular to the electron beam. The square and hexagonal diffraction spots indicate that each cuboctahedral Au NP is a single crystal with its square facets being indexed with the {100} planes and its triangular ones being indexed with the {111} planes. To understand the growth process during synthesis, the samples were monitored by visual colorimetry and UV−vis spectroscopy over the reaction time (Figure 3). After the addition of HAuCl4 to the CTAVB solution at pH = 2, the color of the solution gradually changed from slightly yellow to colorless within 10 h, possibly indicating the reduction of gold ions from Au3+ to Au1+.41 After 15 h, the solution took on a pale pink color that intensified with time, indicating the formation of Au NPs. The UV−vis spectra over time show these changes quantitatively. A small localized surface plasmon resonance (LSPR) peak at about 535 nm starts to appear after 15 h, indicating the formation of gold seeds (Figure 3a, inset).
EXPERIMENTAL SECTION
Chemicals and Materials. Cetyltrimethylammonium bromide (CTAB, ≥98%), 4-vinyl benzoic acid (VBA, 97%), gold(III) chloride hydrate (HAuCl4, 99.99%), hydrochloric acid (HCl, 37%), nitric acid (HNO3, ≥65%), and an anion exchange resin (Dowex Monosphere 550A UPW) were purchased from Sigma-Aldrich. Cetyltrimethylammonium 4-vinylbenzoate (CTAVB) was synthesized using a previously reported method.38,39 Briefly, the bromide counterions of CTAB were replaced with hydroxide ions through the anion exchange method (using an anion exchange resin, Dowex Monosphere 550A UPW), resulting in cetyltrimethylammonium hydroxide (CTAOH). CTAVB was synthesized by neutralizing CTAOH in the presence of equal molar amounts of VBA followed by freeze-drying. Synthesis of Cuboctahedral Au NPs. The seedless synthesis of cuboctahedral Au NPs proceeded as follows: First, 81 mg of CTAVB was homogeneously dissolved in 25 mL of strongly acidic aqueous solution (10.0 mM HCl, pH = 2). Second, 357 μL of 7.0 mM HAuCl4 was added to the solution at 30 °C, resulting in total concentrations of CTAVB and HAuCl4 of 7.5 and 0.1 mM, respectively. This mixture was stirred for 1 h at 30 °C and then kept in a water bath at 42 °C for various reaction times to produce Au cuboctahedra of different sizes. To stop the reaction, the samples were immediately washed twice with water by centrifugation. B
DOI: 10.1021/acs.chemmater.6b01238 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 2. Electron diffraction patterns of cuboctahedral Au NPs for (a) a {100} facet and (b) a {111} facet perpendicular to the electron beam. The insets show the corresponding SEM images of the cuboctahedral Au NPs for clarity. Scale bars: 20 nm.
to a slight smoothing of the edges by oxidative etching induced by Cl−/O2 pairs (Figure S3). The growth process during synthesis was further characterized using TEM and SEM measurements (Figure 4). At 15 h of reaction time, polydisperse polyhedral or quasispherical particles with an average diameter of ca. 15 nm were observed. The high-resolution transmission electron microscopy (HRTEM) image and electron diffraction pattern show that the particles were single crystals (Figure 4a). Here, particles with twinned structures were not observed. At 17 h of reaction time, the average particle size increased to ca. 23 nm, and Au NPs with cuboctahedral shape became clearly visible, as evidenced by the SEM images; however, the particles were still polydisperse (Figure 4c). As the reaction time further increased, the cuboctahedral AuNPs continued to grow and became fairly monodisperse. At 24 h of reaction time, the size of cuboctahedral Au NPs was 43 nm with a polydispersity of 0.06 (standard deviation of size/average size). The size and polydispersity of the particles with the reaction time are summarized in Figure 5. During the seed-formation stage, the particles were fairly polydisperse. Then, up to 24 h, the particles grew relatively fast, and their polydispersity decreased steeply with time, from ca. 0.23 at 15 h to 0.06 at 24 h. Beyond 24 h, the particle growth became slower, and the polydispersity further decreased, resulting in a polydispersity of as low as ca. 0.04 at 48 h and beyond. Considering the clear size focusing with time, the growth mode can be identified as predominantly diffusion-controlled growth. In the diffusion-controlled growth mode, the increase rate of particle volume is equal to the diffusion rate of Au atoms from the solution to the particle surface, making the particle size growth rate inversely proportional to the particle size.42 In other words, the smaller particles grow faster than the larger ones. The time scale of size focusing depends on the concentration of Au atoms in the bulk, and it took about 9 h (from ca. 15 to ca. 24 h in the reaction time) since the reduction rate of Au ions is slow with the present method. At 72 h and beyond, the particle size was saturated. This is consistent with the saturation of the LSPR
Figure 1. TEM (left) and SEM (right) images of cuboctahedral Au NPs obtained at different reaction times: (a) 24 h, (b) 36 h, (c) 48 h, (d) 60 h, and (e) 72 h. Scale bars: 200 and 100 nm for TEM and SEM, respectively.
As the reaction time increases, the LSPR peak intensity increases, and the peak position becomes red-shifted (Figure 3b), indicating the growth of Au NPs. After reaching the maximum LSPR peak position at 60 h, the LSPR peak becomes slightly blue-shifted at 72 h, while the peak intensity remains relatively constant. The saturation of the peak intensity suggests that most of the gold precursors were consumed, terminating the growth of Au NPs. The slight blue-shift may be attributed C
DOI: 10.1021/acs.chemmater.6b01238 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 3. (a) UV−vis spectra of the reaction solution as a function of time (the inset shows the magnified spectra from 0 to 20 h for clarity), and (b) the LSPR peak position as a function of the reaction time.
has been used as a powerful etchant for both the nuclei and seeds of various metallic systems.27,45−47 Because Au has relatively low reactivity, the oxidative etching of an Au system requires a high concentration of Cl−.47 The presence of a high concentration of H+ (from HCl) enhances the oxidation power of O2 as can be seen from the higher standard electrical potential of O2 + 4H+ + 4e− → 2H2O (E0 = 1.229 V) as compared to that of O2 + 2H2O + 4e− → 4OH− (E0 = 0.401 V).48 Therefore, the high concentration of HCl (10.0 mM, pH = 2) in the present method was essential to remove all the structural defects through strong oxidative etching, producing high-purity single-crystal seeds in the early stage. The extensive oxidation of metallic Au atoms back to Au ions by the Cl−/O2 pairs can compete with the reduction of AuCl4−, decreasing the overall reduction rate of Au salt.27,47 This significantly slows the nucleation and particle growth rate. In the present method, the formation of seeds took approximately 15 h, and the full growth of particles took about 72 h, which is much slower than that at neutral pH. This slow particle growth provides a facile and robust way to finely control the particle size by stopping the reaction at different times. The extinction spectra of Au cuboctahedra produced in different batches for the same reaction time overlap fairly well, indicating good reproducibility of the present method (Figure S6). Single-crystal seeds typically have a truncated octahedron structure bound by the {100} and {111} facets to minimize the total surface free energy.27 These single-crystal seeds can grow into octahedra, cuboctahedra, cubes, or others depending on the relative growth rates along the ⟨100⟩ and ⟨111⟩ directions. Previous studies have shown that cuboctahedra are formed when the growth rate ratio is 0.87, whereas cubes and octahedra are formed when the growth rate ratios are 0.58 and 1.73, respectively.27 It should be noted that, in the present method, monodisperse Au cuboctahedra were formed and grew larger without altering their morphology. This indicates that a growth rate ratio of 0.87 is stably maintained. Although the growth rate of facets is determined by capping agents and the supply rate of Au atoms, it is difficult to quantitatively determine or predict the growth rate ratio based on the synthesis conditions. Nonetheless, to understand the formation of Au cuboctahedra in the present method, the preferential facet affinities of the
peak intensity at the same time point. The conversion of gold ions into Au NPs at different reaction times was monitored by measuring the concentration of remaining gold ions in the centrifuged supernatant via ICP-MS (Figure S4). These results show that the conversion percentages of gold ions to cuboctahederal Au NPs were 69% and 99% at 24 h (when the size of Au NP was 43 nm) and 72 h (when the size of Au NP was 79 nm), respectively. To understand the synthesis mechanism of cuboctahedral Au NPs in our method, the roles of each molecular component were investigated. CTAVB contains a long hydrocarbon chain with an ammonium headgroup (CTA+) and a counterion 4vinyl benzoate (VB−). CTA+ has been widely used in the synthesis of Au NPs as a capping agent to protect particles from aggregation, as shown in the use of CTAC or CTAB.1,14−16 Because benzoic acid derivatives have been used as mild reducing agents,41,43,44 we can expect that the counterion VB−, which is also a benzoic acid derivative, acts as a mild reducing agent. To confirm this, VBA (7.5 mM) was mixed with an aqueous solution of HAuCl4 (0.1 mM) at neutral pH in the absence of CTA+. This resulted in the formation of aggregated Au NPs with rather irregular shapes (Figure S5a). Here, the formation of particles began within 30 min, and the particle growth was completed in approximately 7 h. Thus, VB− does function as a mild reducing agent. A similar experiment was performed in the presence of CTA+ by mixing CTAVB (7.5 mM) with an aqueous solution of HAuCl4 (0.1 mM) at neutral pH. In this case, more regularly shaped Au NPs were formed within a similar time scale without any particle aggregation (Figure S5b). This indicates that CTA+ works as a capping agent. It is thus clear that CTAVB molecules act as reducing and capping agents, simultaneously, as expected. Considering that the zeta potential of the as-synthesized cuboctahedral Au NPs was +35.9 mV, the CTA+ should form a bilayer on the surface of the Au NPs. The HRTEM images of particles synthesized at neutral pH (with VBA or CTAVB) show that many particles possessed defects such as twinned structures or stacking faults (Figure S5). The particle defects, such as twins or stacking faults, have much higher energy than their single-crystal counterparts and are thus very susceptible to oxidative etching. The Cl−/O2 pair D
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Figure 5. Particle size and polydispersity as a function of the reaction time. The size distributions are presented in Figure S1.
direction than along the ⟨100⟩ direction, making the growth rate ratio less than 1. This is consistent with the growth rate ratio requirement for cuboctahedra, although the exact growth rate ratio of 0.87 could not be determined. The preferential adsorption of CTA+ on Au surfaces can be modified by its counterion.23 Although benzoic acid derivatives, such as salicylic acid, have been used as mild reducing agents in previous studies of Au NP synthesis, preferential capping on a specific Au facet has not been observed.41,43,44 This suggests that VB−, which is also a benzoic acid derivative, may not cause preferential capping. Because Cl− also has low affinity for gold surfaces,51 the selective adsorption of CTA+ on the high-index facets may not be altered by the presence of VB− and Cl−. The growth rate ratio of the facets also depends on the supply rate of Au atoms.26 When the reduction rate of gold precursors or the particle growth rate is very slow, it will allow sufficient time for the surface self-diffusion of Au atoms from one facet (of higher chemical potential, {111}) to another (of lower chemical potential, {100}), altering the growth rate ratio determined by the selective capping of facets.26,52,53 The surface self-diffusion distance of atoms, Δx, can be calculated as Δx = (2Dst)0.5, where Ds is the surface self-diffusion coefficient, and t is the time allowed for diffusion.52,53 The surface selfdiffusion distance of Au atoms over 24 h is around 130−400 nm, in which a typical Ds for Au atoms (10−14 to 10−15 cm2/s) was used.52 Considering the growth time scale (24−72 h) and the particle size (40−80 nm) in the present synthesis method, the contribution of atomic surface self-diffusion to the growth rate ratio along the ⟨100⟩ and ⟨111⟩ directions should be significant. Therefore, the surface self-diffusion of Au atoms from the {111} to {100} facets combined with the preferential capping of {100} facets by CTA+ may have played a key role in achieving the facets’ growth rate ratio close to 0.87 in the present synthesis method, thus inducing the formation of Au cuboctahedra and their stable growth without morphological deformation. The effects of HCl (pH), CTAVB, and HAuCl4 concentrations in the present synthesis method have been investigated by varying each parameter while the others were kept the same as the optimal condition for monodisperse cuboctahedra (HCl = 10.0 mM (i.e., pH = 2), CTAVB = 7.5 mM, and HAuCl4 =
Figure 4. (a) HRTEM image and its fast Fourier transform of Au NPs at 15 h (scale bar: 2 nm), and (b−e) TEM (left) and SEM (right) images of Au NPs as a function of the reaction time (15, 17, 20, and 24 h, respectively). Scale bars: 20 and 50 nm for TEM and SEM, respectively.
molecular components in the solution were explored. The large ammonium headgroup of CTA+ and its associated long alkyl chain can be more easily accommodated on the less closely packed {100} facets than on the close-packed {111} facets.23,49,50 This is because the Au atomic spacing of the high-index facets is more comparable to the size of the CTA+ than that of the low-index facet {111}. Therefore, the relatively weak adsorption of CTA+ on the {111} facets compared to that of the {100} facets leads to faster growth along the ⟨111⟩ E
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Figure 6. Schematic summary of the effects of HCl (pH), CTAVB, and HAuCl4 concentrations on the particle shape and size. In this figure, each synthesis condition is placed on the basis of its total reaction time until the completion of particle growth (in this case, the time at which the LSPR peak position is maximum). The green circles indicate the optimum condition for the synthesis of monodisperse cuboctahedra. The TEM and SEM images of the particles formed near the completion of particle growth with different HCl concentrations are included. The scale bars are 100 nm.
0.1 mM at T= 42 °C). The key results are summarized in Figure 6 as schematics together with representative TEM and SEM images for the HCl effects. In the schematics, each synthesis condition is placed according to its total reaction time until the completion of particle growth. As HCl concentration increases from 10.0 to 15.8 mM (i.e., pH decreases from 2.0 to 1.8), the total reaction time until the completion of particle growth increases (from 72 to 120 h) as expected, and the particle shape changes from cuboctahedron to truncated octahedron (Figure S7). At 15.8 mM HCl (pH = 1.8), the particles become a less truncated octahedron as compared to those at 12.6 mM HCl (pH = 1.9), and even a small fraction of octahedral particles are also found as shown in the inset of the SEM image. This shape transition with HCl concentration (from 10.0 to 15.8 mM) can be attributed to the longer time available for the surface self-diffusion of Au atoms from {111} to {100} facets, which makes the growth rate ratio along the ⟨100⟩ and ⟨111⟩ directions increase toward the value for the truncated octahedron (1.15)54 or octahedron (1.73).27 This is consistent with the proposed mechanism for the synthesis of Au cuboctahedra presented in this study. As HCl concentration decreases from 10.0 to 7.9 mM (i.e., pH increases from 2.0 to 2.1), the particles maintain their cuboctahedral shape but
become much smaller and more polydisperse. While the time available for the surface self-diffusion of Au atoms from {111} to {100} facets is reduced, it is still long enough to make the atomic surface self-diffusion effective. The decrease of particle size indicates that more seeds are formed with the decrease of HCl concentration (i.e., the increase of reduction power and the decrease of etching power). When the CTAVB and HAuCl4 concentrations are varied from the optimal condition, the particle shape mostly remains cuboctahedral (within the conditions summarized in Figure 6), but the particle size and polydispersity are changed as schematically shown in Figure 6 (Figures S8 and S9). In the case of 0.05 mM HAuCl4, the particle shape is slightly changed toward the truncated cuboctahderon due to the longer time available for atomic surface self-diffusion as discussed above. Since further variations of CTAVB and HAuCl4 (which is not as efficient as HCl for the reaction speed control, especially in making it slower while keeping particles in a good shape and size) make the particle polydisperse or too small to clearly identify their shape with SEM, more extensive variations were not pursued. The highly monodisperse cuboctahedral Au NPs with finely tunable sizes obtained in this work allowed us to systematically investigate their LSPR properties as a function of their size. F
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etching power for the removal of structural defects (forming single-crystal seeds) and significantly reduces the particle growth rate. This slow particle growth facilitates the easy and reliable tuning of particle size by stopping the reaction at different times. Combined with the selective capping of the {100} facets with CTA+, the significant surface self-diffusion of Au atoms from the {111} to {100} facets allowed by the slow growth is considered to be the key mechanism in the formation of Au cuboctahedra and their stable growth without morphological deformation. The approach reported here, which synergistically combines the uses of mild reducing agents and a high concentration of HCl to form single-crystal seeds and to enhance the contribution of atomic surface selfdiffusion in directing the particle shape, may be applicable for the synthesis cuboctahedra of other metals, such as platinum and silver for which monodisperse cuboctahedra with tunable sizes are not readily available, and also for synthesizing metallic NPs of other shapes.
Figure 7a shows the normalized UV−vis extinction spectra measured from aqueous solutions of Au cuboctahedra of
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01238. Experimental and characterization information, discussion on effects of different parameters, and related figures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by NRF grants funded by the MEST of the Korean government (No.2011-0031931 and 2014R1A2A1A05007109).
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Figure 7. (a) Normalized UV−vis spectra of cuboctahedral Au NPs of different sizes at different reaction times, and (b) the LSPR peak position vs particle size.
REFERENCES
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different sizes. The LSPR peaks display a red-shift from 540.5 to 550.5, 559.2, and 565.8 nm as the size of Au cuboctahedra increases from 43 to 56, 64, and 73 nm, respectively (Figure 7b). The LSPR peaks show a linear correlation with the size of the Au cuboctahedra, and the fitting curve can be expressed as λmax = 0.876d + 502.2 (R2 = 0.99), where λmax and d are the LSPR peak position and the particle size, respectively. Therefore, the size of Au cuboctahedra can be easily monitored during synthesis by simply measuring the LSPR peak position, making the fabrication of Au cuboctahedra of a specific size fairly reliable.
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CONCLUSION We have developed a novel, facile, and robust seedless method for the synthesis of monodisperse Au cuboctahedra with tunable sizes ranging from 40 to 80 nm by using CTAVB and HCl in aqueous solution. In this method, CTAVB serves as selective capping and mild reducing agent simultaneously, and the high concentration of HCl provides strong oxidative G
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Chemistry of Materials
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DOI: 10.1021/acs.chemmater.6b01238 Chem. Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.chemmater.6b01238 Chem. Mater. XXXX, XXX, XXX−XXX
