High Yield Seedless Synthesis of High-Quality Gold Nanocrystals with

Feb 21, 2014 - In this Article, high-quality gold nanocrystals (Au NCs) with various shapes including concave cubic, trisoctahedral, cubic, rod-like, ...
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High Yield Seedless Synthesis of High-Quality Gold Nanocrystals with Various Shapes Jihui Zhang,† Chunxiao Xi,† Cong Feng,† Haibing Xia,*,† Dayang Wang,‡ and Xutang Tao† †

State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, People’s Republic of China Ian Wark Research Institute, University of South Australia, Adelaide, SA 5095, Australia

‡

S Supporting Information *

ABSTRACT: In this Article, high-quality gold nanocrystals (Au NCs) with various shapes including concave cubic, trisoctahedral, cubic, rod-like, and quasi-spherical have been successfully produced in high yield via adding a trace amount of NaBH4 solution into growth solutions mainly composed of HAuCl4, ascorbic acid, and surfactants. The sizes and shapes of as-prepared Au NCs can be tuned by the compositions of the growth solutions and the amount of NaBH4 added. The electrocatalytic performance of differently shaped Au NCs for methanol oxidation was studied; as-prepared trisoctahedral or concave cubic Au NCs are more highly active electrocatalysts for methanol oxidation due to the presence of high-index facets on their surface.

1. INTRODUCTION The research interest of gold nanocrystals (Au NCs) over the past decades has been stemming from their peculiar localized surface plasmon resonance (SPR) behavior,1−6 which has promised innovative ways of using the Au NCs for sensing, diagnostics, and photothermal treatments especially in biomedicines. This SPR behavior of Au NCs is strongly dependent on both their sizes and their shapes. Although Au NCs are commonly thought chemically inert, increasing evidence has recently revealed that they can become highly catalytically active when their sizes are significantly reduced and, most importantly, their shapes are deliberately tailored to introduce high-index facets on the surfaces.7 In this context, it remains one of the most important endeavors in the research field of Au NCs to develop simple, reliable, and easily accessible methods to efficiently control and manipulate the shapes of Au NCs. Up to date, the seed-mediated growth method is the dominant workhorse for synthesis of Au NCs with different shapes. This method is a two-step process, including preparation of Au seeds and addition of the seed dispersions into growth solutions. To produce high-quality Au NCs with a given shape, one has to meticulously adjust the experimental condition of the growth solutions such as the nature and purity of the reactants8−12 and especially that of the seed dispersions such as the size and size distribution of Au seeds and their activity. For instance, because the activity of 3−5 nm Au seeds obtained via NaBH4 reduction of HAuCl4 is expected to considerably vary during aging, the change in the seed aging time from 5 min to 3 h may cause a noticeable change in the aspect ratio of the Au nanorods (NRs) obtained.13,14 This deliberate adjustment of a number of experimental parameters © 2014 American Chemical Society

makes currently available seed-mediated methods less accessible. The two-step seed-mediated growth process13,14 has been recently simplified by adding the trace amount of NaBH4 instead of preformed Au seeds into the growth solution.15,16 It is well-known that NaBH4 is a very strong reducing agent and can directly reduce Au3+ into Au0. The addition of NaBH4 into the growth solution is expected to form very small Au NCs, which can act as seeds for growth of large Au NCs. Up to date, this seedless growth method, which was first reported by Jana17 et al. and shortly after by Zijlstra18 et al., is used for production of Au NRs,15,16,19,20 high aspect ratio Au NRs with assistance of paradioxybenzene,21 nanoprisms,16 trisoctahedral Au NCs,22 and highly anisotropic, twisted single crystalline Au NCs recently;23 the quality and production yield of the NRs obtained are not so good as compared to those produced via optimized seed-mediated growth methods.13,14,24 In some cases, furthermore, the auxiliary tailored devices such as flow reactors are necessary.19,20 Thus, to address this challenge, the present work aims to meticulously explore the seedless growth of Au NCs via addition of NaBH4 into the growth solution. It was believed that the key factor for formation of anisotropic nanoparticles is the seeds with sizes smaller than 3 nm and the relative rates of nucleation and growth, which determine the aspect ratio of the final nanoparticle. However, in conventional solution chemical methods, the nucleation rate is slow, and any anisotropic nanoparticles formed at early stages of growth can grow further and be converted into more Received: November 29, 2013 Revised: February 21, 2014 Published: February 21, 2014 2480

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reaction mixture with the aid of centrifugation (11000 rcf × 6 min). These NCs with sizes of 63.7 ± 4.5 nm were redispersed into water and centrifuged two more times to remove the excess reactants. When the concentrations of NaBH4 in the final reaction media were increased from 0.18 to 0.36 μM, the sizes of the resulting cubic Au NCs were decreased from 63.7 ± 4.5 to 42.9 ± 3.2 nm. 2.4. Synthesis of Trisoctahedral Gold Nanocrystals (TOH Au NCs). The growth solutions for synthesis of TOH Au NCs were prepared according to literature.30,31 Typically, 5.0 mL of the aqueous solution of CTAC (40 mM), 100 μL of the aqueous solution of HAuCl4 (25 mM), and 50 μL of the aqueous solution of AA (0.10 M) were sequentially added in a 10 mL glass vial under stirring at room temperature to form the growth solution. After 10 μL of the freshly prepared ice-cold NaBH4 solution (25 μM) was added to the growth solution, followed by vigorous stirring (the stirring speed is about 1200 rpm by IKA Multiposition digital magnetic stirrer RO 15), a series of experiments were also performed by the same procedure except that the stirring time was changed from 10 to 15, 25, 30, and 40 s, respectively. Our results showed that the optimal stirring time was in the range of 25−30 s; we usually choose about 25 s. The results (Supporting Information Table S2) also indicated that the optimal stirring time was about 25−30 s. The resulting mixture was placed in a water bath for aging at 28 °C. After aging of 3 h, the resulting TOH Au NCs with size of 236 ± 25 nm along the longest axis were separated from the reaction mixture with the aid of centrifugation (8000 rcf × 6 min). These NCs were redispersed into water and centrifuged two more times to remove the excess reactants. The influence of the NaBH4 concentration on the size of the resulting TOH Au NCs was studied, as summarized in Supporting Information Table S3. Note that the values listed in Supporting Information Table S3 represent the concentrations in the final reaction media. 2.5. Synthesis of Gold Nanorods (Au NRs). The growth solutions for synthesis of Au NRs were prepared according to the literature.13 Typically, 5.0 mL of the aqueous solution of CTAB (0.10 M), 100 μL of the aqueous solution of HAuCl4 (25 mM), 50 μL of the aqueous solution of AgNO3 (10 mM), and 30 μL of the aqueous solution of AA (0.10 M) were sequentially added into a 10 mL glass vial at room temperature under stirring to form the growth solution. Subsequently, 40 μL of freshly prepared, ice-cold, aqueous solution of NaBH4 with a given concentration (0.2 mM) was added into the growth solution, followed by vigorous stirring at a speed of about 1200 rpm (IKA Multiposition digital magnetic stirrer RO 15). A series of experiments were performed by the same procedure except that the stirring time was changed from 10 to 15, 25, 30, and 40 s, respectively. Our results showed that the optimal stirring time was in the range of 25−30 s; we usually choose about 25 s. The results (Supporting Information Table S4) also indicated that the optimal stirring time was about 25−30 s. After that, the resulting reaction mixture was placed in a water bath for aging at 28 °C. After aging of 3 h, the resulting Au NRs with dimensions were separated from the reaction mixture with the aid of centrifugation (13 000 rcf × 6 min). These NRs were redispersed into water and centrifuged two more times to remove the excess reactants. The influence of the concentrations of NaBH4, AgNO3, and AA on the aspect ratios and sizes of Au NRs was investigated in detail, as summarized in Supporting Information Table S5. Note that the values listed in Supporting Information Tables S4 and S5 represent the concentrations in the final reaction media. 2.6. Synthesis of Quasi-Spherical Au NCs. The procedure of the synthesis of quasi-spherical Au NCs with sizes ranging from 7.0 ± 1.1 to 27.2 ± 2.4 nm was the same as that of the synthesis of TOH Au NCs except that after addition of NaBH4 into the growth solutions, the resulting reaction mixtures were placed in a water bath for aging at 28 °C under vigorous stirring (the stirring speed is about 1200 rpm by IKA Multiposition digital magnetic stirrer RO 15). After aging overnight, the resulting Au NCs were separated from the reaction mixture with the aid of centrifugation (15 000 rcf × 40 min for Au NCs with sizes equal to or smaller than 11 nm; 14 000 rcf × 25 min for Au NCs with sizes bigger than 11 nm). These NCs were redispersed into water and centrifuged one more time to remove the excess reactants. The sizes of quasi-spherical Au NCs were reduced with the

symmetric shapes. In addition, the sizes of the seeds after 3 h of aging used in two-step seed-mediated growth method are about 3.5 nm,25,26 while the sizes of in situ seeds by NaHB4 are about 1.5 nm.17,26 Therefore, seedless method with size of about 1.5 nm is in favor for the synthesis of anisotropic Au NCs.26 In the present work, we have demonstrated that after addition of freshly prepared ice-cold aqueous solution of NaBH4 into growth solutions, high-quality Au NCs with designed shapes (including sphere, rod, cube, concave cube, and trisoctahedron) can be produced in high yield by properly increasing the stirring time of the reaction media before aging at room temperature. Dependent on the compositions and concentrations of the growth solutions (such as CTAB and CTAC) and the NaBH4 amount used, the shapes and sizes of asprepared Au NCs can be easily tuned.

2. EXPERIMENTAL SECTION 2.1. Materials. Cetyltrimethylammonium bromide (CTAB, 99%, catalog no.: 30037416), cetyltrimethylammonium chloride (CTAC, 99%, catalog no.: 30232128), hydrogen tetrachloroaurate tetrahydrate (HAuCl4·4H2O, 99%, catalog no.: 10010711), sodium borohydride (NaBH4, 99%, catalog no.: 80115862), L-ascorbic acid (AA, 99%, catalog no.: 10004014), and hydrochloride acid (HCl, catalog no.: 10111061) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Silver nitrate (AgNO3, 99+%, catalog no.: 43087) was purchased from Alfa Aesar (Tianjin, China). Milli-Q water with a resistance of 18 MΩ cm was used for all experiments. Glassware was thoroughly cleaned with aqua regia and washed with water. Caution: Aqua regia solutions are dangerous and should be used with extreme care; never store these solutions in closed containers. 2.2. Synthesis of Concave Cubic Au NCs. The growth solutions for the synthesis of concave cubic Au NCs were prepared according to the literature.27 Typically, 5.0 mL of the aqueous solution of CTAC (0.10 M), 100 μL of the aqueous solution of HCl (1.0 M), 100 μL of the aqueous solution of HAuCl4 (25 mM), 25 μL of the aqueous solution of AgNO3 (10 mM), and 50 μL of the aqueous solution of AA (0.10 M) were sequentially added into a 10 mL glass vial at room temperature under stirring to form the growth solution. 40 μL of the freshly prepared ice-cold NaBH4 solution (25 μM) was then added to the growth solution, followed by vigorously stirring (the stirring speed is about 1200 rpm by IKA Multiposition digital magnetic stirrer RO 15). A series of experiments were performed by the same procedure except that the stirring time was changed from 10 to 15, 25, 30, and 40 s, respectively. Our results showed that the optimal stirring time was in the range of 25−30 s; we usually choose about 25 s. The results (Supporting Information Table S1), as will be shown below, indicated that the optimal stirring time was about 25−30 s. The resulting mixture was placed in a water bath for aging at 28 °C. After aging overnight, the resulting concave cubic Au NCs with sizes of 62.3 ± 6.0 nm were separated from the reaction mixture with the aid of centrifugation (8000 rcf × 6 min). These NCs were redispersed into water and centrifuged two more times to remove the excess reactants. The edge length of as-prepared concave cubic Au NCs was increased from 62.3 ± 6.0 to 70 ± 6.5 and 173 ± 13 nm when the concentrations of NaBH4 in the final reaction media were decreased from 0.19 to 0.14 and 0.048 μM. 2.3. Synthesis of Cubic Au NCs. The growth solutions for the synthesis of cubic Au NCs were prepared according to the literature.28,29 Typically, 5.0 mL of the aqueous solution of CTAB (20 mM), 45 μL of the aqueous solution HAuCl4 (25 mM), and 530 μL of the aqueous solution of AA (0.10 M) were sequentially added into a 10 mL glass vial under stirring at room temperature to form the growth solution. After 20 μL of the freshly prepared ice-cold NaBH4 solution (50 μM) was added to the growth solution, followed by vigorous stirring (the stirring speed is about 1200 rpm by IKA Multiposition digital magnetic stirrer RO 15) for 25 s, the resulting reaction mixture was placed in a water bath for aging at 28 °C. After aging overnight, the resulting cubic Au NCs were separated from the 2481

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increasing concentrations of NaBH4, as indicated in Supporting Information Table S6. 2.7. Characterization. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) images were obtained with a JEOL JEM 2100F TEM at an acceleration voltage of 200 kV. A total of 300 particles from TEM image were counted to calculate the average dimension/size or aspect ratio of Au NCs of each shape. The error bars in the sizes correspond to one standard deviation in each case. In addition, production yield in the present work was defined as the yield of Au NCs with target shapes versus byproducts (such as spherical and other irregular shapes). UV−vis spectroscopy was implemented with a Cary 50 spectrophotometer by using a 10 mm path length quartz cuvette at room temperature. Scanning electron microscopy (SEM) images were obtained with a Hitachi S-4800 SEM operating at 10 kV. 2.8. Electrochemical Measurements. Cyclic voltammograms (CV) measurements were carried out in a three-electrode cell using a CHI660D electrochemical analyzer. A platinum wire and Ag/AgCl electrode were used as the counter and reference electrode, respectively. A glassy carbon electrode (diameter = 3.0 mm) was used as a working electrode and carefully polished and washed prior to each measurement. After as-prepared Au NCs were carefully purified from the reaction residuals via water washing with the aid of centrifugation, their aqueous suspensions were dropped onto the surfaces of the glassy carbon electrodes and dried at room temperature. The deposited NC layer was then coated with 8.0 μL of the Nafion solution (0.20 wt %). An aqueous solution of H2SO4 (0.50 M) was employed as the electrolyte solution. It was purged with high-purity nitrogen (99.99%) for 30 min before electrochemical measurement. In addition, as-prepared Au NCs with different shapes were utilized for electrochemical catalysis of methanol oxidation, and their cyclic voltammograms were recorded in 0.10 M KOH solutions containing 1.0 M methanol. The solutions were purged with highpurity nitrogen (99.99%) for 30 min prior to the tests. The voltage scan rates in H2SO4 and KOH solutions were 50 and 20 mV s−1, respectively.

Figure 1. TEM images of the concave cubic Au NCs prepared by seedless growth method. After addition of NaBH4 solution into the growth solution, the reaction mixture was vigorously stirred for 10 s (a), 15 s (b), 25 s (c), and 40 s (d). The concentrations of NaBH4, HAuCl4, CTAC, HCl, AgNO3, and AA in the reaction mixture are 0.19 μM, 0.47 mM, 96 mM, 19 mM, 47 μM, and 94 mM.

and Supporting Information Table S1). When the stirring time increases to 25 s, few byproduct NCs are visible (Figure 1c and Supporting Information Table S1). In the current work, we have found that the optimal stirring time is in the range of 25− 30 s. The stirring of the reaction mixtures after NaBH4 addition was expected to mainly improve the efficiency and yield of formation of Au seeds in situ and have little impact on the crystal growth of Au NCs; therefore, this optimal stirring time range for synthesis of concave cubic Au NCs should be applied for the synthesis of Au NCs of other shapes. For instance, high yields of TOH Au NCs (Supporting Information Figures S2 and S3) and Au NRs (Supporting Information Figures S4 and S5) in high quality were also obtained under this optimal stirring time range of 25−30 s. The considerably improved quality and product yield of the resulting concave cubic Au NCs should be due to the fact that before the reaction mixture is submitted for aging, the longer stirring time can facilitate the complete consumption of NaBH4 to reduce HAuCl4 to form active nuclei for growth of concave cubic Au NCs and in turn avoid secondary nucleation during aging. However, the byproducts reappear when the stirring time is further increased over 30 s (Figure 1d and Supporting Information Table S1). This should be because a too long stirring time may destroy the formation of the stagnant phase (the area right next to the surface of a crystal) between the bulk solution and the newly formed Au seeds, which favors the kinetic-controlled growth of anisotropic NCs.32 On the other hand, however, inhibiting formation of the stagnant phase may be favorable for growth of isotropic NCs; we have observed formation of quasi-spherical NCs under continuous stirring during aging of the reaction mixture at room temperature after NaBH4 addition. 3.2. Synthesis of Concave Cubic Au NCs. Concave cubic Au NCs are fairly interesting to synthesize as they have higher index facets on their surfaces.27 By introducing HCl into the growth solution as suggested in literature, we also succeeded in growth of concave cubic Au NCs via the present seedless

3. RESULTS AND DISCUSSION 3.1. Influence of Stirring Time on Quality and Production Yield of Au NCs of Various Shapes. Twostep seed-mediated growth strategies require deliberate control of various experimental parameters at the same time, which are obviously less accessible to researchers from disciplines with limited chemistry-training. In the conventional two-step seedmediated growth method,13,14 adding freshly prepared Au seeds into growth solutions is necessary to activate the growth of Au NRs as AA in the growth solutions is able only to reduce Au3+ into Au+ ions. Adding a trace amount of NaBH4 instead of Au seeds can obviate the preparation of the Au seeds, the quality and chemical nature of which considerably affect the quality and production yield of the Au NRs obtained thereof. In the currently accessible seedless growth process, nucleation takes places soon when a trace amount of NaBH4 is added in the growth solution. The subsequent growth stage of Au NRs is expected to be fairly similar to that embodied in conventional seed-mediated growth processes, which involves a few second stirring of the reaction mixture and then aging at room temperature. However, the quality and production yield of concave cubic Au NCs obtained via currently available seedless growth methods are relatively poor as compared to those obtained via conventional seed-mediated growth methods.27 A noticeable amount of Au NCs with other shapes coexists with concave cubic Au NCs (Figure 1a and Supporting Information Table S1). In our work, surprisingly, the number of byproduct NCs has been found to decrease with the increase of the stirring time of the reaction mixture prior to aging (Figure 1b 2482

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NCs. The edge length of as-prepared concave cubic Au NCs can be reduced from 173 ± 13 to 70 ± 6.5 nm with the increase of the NaBH4 concentration in the reaction mixture from 0.048 to 0.14 μM. HRTEM image confirms that the facets of the concave cubic Au NCs are mostly high-index {720} plane (Figure 2d). The lattice fringe spacing of the concave cubic NCs is about 0.201 nm, which is consistent with the spacing between the {100} planes of face-centered cubic Au. The corresponding FFT analysis of the HRTEM images indicates that concave cubic Au NCs were aligned along the [001] zone axis. In addition, {720} facets can be regarded as a composition of {310} and {410} subfacets (Figure 2e). The UV−vis spectra of as-prepared concave cubic Au NCs are shown in Supporting Information Figure S6. Small concave cubic Au NCs (with edge lengths of 62.3 ± 6.0 nm) show a broader SPR band centered at ca. 647 nm, which red-shifts about 95 nm in comparison with those of cubic Au NCs with similar edge length (Supporting Information Figure S7). The result is in good agreement with theoretical simulation that the SPR band for cubic Au NCs with concave surfaces is expected to red-shift into the long wavelength range as compared to that of cubic Au NCs with flat surfaces because the concave cubic NCs have sharper tips.33 When the sizes of the concave cubic Au NCs are relatively large (with edge lengths of 173 ± 13 nm for instance), the SPR bands of the NCs become rather broad with a pronounced absorption in the wavelength range from 650 to 1000 nm. 3.3. Synthesis of Cubic Au NCs. Cubic Au NCs are particularly interesting because they are bounded entirely by {100} facets, which is usually difficult to be realized. After the growth solution was modified according to seed-mediated growth methods reported in the literature,29 monodisperse cubic Au NCs could be produced in high yield by adding a trace amount of NaBH4 into the growth solution. The edge length of as-prepared cubic Au NCs can be tuned from 63.7 ± 4.5 to 42.9 ± 3.2 nm with increase of the NaBH4 concentration from 0.18 to 0.36 μM (Figure 3a and b); the preparation of uniform cubic Au NCs with the edge length over than 70 nm is still a challenge for the present seedless growth approach. The HRTEM image reveals a lattice fringe spacing of 0.201 nm (inset in Figure 3a), which is consistent with the spacing between the {100} planes of face-centered cubic Au. With the size decrease from 63.7 ± 4.5 to 42.9 ± 3.2 nm, the SPR bands of the cubic Au NCs blue-shift from 552 to 535 nm (Figure 3c). 3.4. Synthesis of Trisoctahedral Au NCs. In general, the high-index facets of a single crystal possess a high density of low-coordinated atoms such as steps, edges, and kinks, which can be served as highly active sites for adsorption and even catalysis. In comparison with those with {hk0} (h > k > 0) facets, Au NCs with {hhl} (h > l > 0) facets are rarely prepared.30,31,34 Thus, it is technically imperative for applications in catalysis and sensing to synthesize Au NCs with tailored high-index facets on their surfaces in large quantity. Here, we have successfully produced uniform trisoctahedral (TOH) Au NCs by adding the trace amount of NaBH4 into the growth solution (Figures 4 and 5). Despite the presence of byproducts such as pentagonal bipyramids, the production yield of TOH Au NCs is >85% (versus other products). The overall quality and yield of the TOH Au NCs obtained via the present seedless growth method are comparable to or even better than those derived from the seed-mediated growth method.31 As shown in Figure 4, the

growth method; the production yield was >70%. The SEM image clearly reveals that each face of the Au cubes is concave rather than flat (Figure 2a and b). This concave feature is also exemplified by the TEM images (insets in Figure 2a and b), in which the middle regions of the Au cubes are obviously darker than their edges. Figure 2c shows a set of TEM images of a concave cube recorded at different tilting angles with x as the axis of rotation from 0° to 15° and 30° and their corresponding schematic models. When TEM images are taken at different tilting angles from 0° to 15° and 30°, the edge regions are gradually enlarged, which further confirmed concave cubic shape of as-prepared Au

Figure 2. SEM images (a and b) of the concave cubic Au NCs prepared at the NaBH4 concentration of 0.048 μM (a) and 0.14 μM (b), respectively. The concentrations of HAuCl4, CTAC, HCl, AgNO3, and AA in the reaction mixture are 0.47 mM, 94 mM, 19 mM, 47 μM, and 0.94 mM, respectively. A set of TEM images (c) of a single concave cubic Au NC with the edge length of 173 ± 13 nm, recorded at the tilting angle changed from 0° to 15° and 30° with x as the axis of rotation (upper panel) and their corresponding schematic model images (lower panel). (d) HRTEM image of a concave cubic NCs with the edge length of 173 ± 13 nm, viewed along the ⟨001⟩ direction. The inset in (d) is the corresponding FFT image. (e) Atomic model of the {720} planes projected from the ⟨001⟩ direction. 2483

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Figure 4. SEM images (a−c) and UV−vis spectra (d) of the TOH Au NCs produced at the NaBH4 concentrations of 0.048 μM (a, black curve), 0.096 μM (b, red curve), and 0.19 μM (c, green curve). The concentrations of HAuCl4, CTAC, and AA in the reaction mixture are 0.48, 38, and 0.96 mM, respectively.

Figure 5. The geometrical model (a), SEM image (b), and TEM image (c) of a single TOH NC with average size of 236 ± 25 nm, viewed along the ⟨110⟩ direction. The measured projection angles are marked. (d) A table to summarize the calculated values of the angles α, β, and γ when the NC is bounded by different crystallographic facets (left panel) and an atomic model of the {331} planes projected from the [110] zone axis (right panel). The {331} planes are made up from a (111) terrace of two atomic width with one (110) step.

Figure 3. TEM images (a and b) and UV−vis spectra (c) of the cubic Au NCs produced at the NaBH4 concentration of 0.18 μM (a, black curve) and 0.36 μM (b, red curve). The inset in (a) is the HRTEM image. The concentrations of HAuCl4, CTAB, and AA in the reaction mixture are 0.20, 18, and 9.5 mM, respectively.

spectra of as-prepared TOH Au NCs. The SPR peaks of the NCs gradually red-shift from 547 to 603 and 630 nm and become broader when their sizes along the longest axis increase from 57.2 ± 3.5 to 124 ± 10 and 236 ± 25 nm. The SPR band broadening may be due to the sharpness increase of the vertexes of the TOH Au NCs as the NC size increases, or excitation of multipolar modes.31,35 As compared to spherical Au NCs of comparable sizes, the red-shift of the SPR bands

average sizes of as-prepared TOH Au NCs along the longest axis can be tuned from 236 ± 25 to 124 ± 10 and 57.2 ± 3.5 nm with the increase of the NaBH4 concentration from 0.048 to 0.096 and 0.19 μM. This size increase causes little change in the shape and shape distribution of as-prepared TOH NCs (Supporting Information Table S3). Figure 3d shows the SPR 2484

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to the Au NRs prepared by the two-step seed-mediated growth method. The lattice fringe spacing of as-prepared Au NRs in the highresolution TEM (HRTEM) image (inset in Supporting Information Figure S10a) is about 0.234 nm, which is consistent with the spacing between the {111} planes of facecentered cubic Au. As shown in Supporting Information Figure S10a−d and Table S5, the size and shape of the resulting Au NRs can be varied with the NaBH4 concentration in a rather linear way. When the NaBH4 concentration is increased from 0.096 to 0.19, 0.38, and 1.5 μM, the length of the Au NRs is decreased from 83.9 to 66.4, 55.6, and 46.1 nm (about 45% reduction in length), and the width is decreased from 45.7 to 25.6, 16.5, and 11.5 nm (about 75% reduction in width). The aspect ratios of the resulting NRs are accordingly increased from 1.8 to 2.6, 3.3, and 4.0, respectively (Supporting Information Table S5). The UV−vis spectra of the resulting Au NRs are shown in Supporting Information Figure S10e. The longitudinal SPR bands of the Au NRs substantially red-shift from 627 to 827 nm with their aspect ratio increase. In contrast, the transverse SPR bands of the Au NRs also blue-shift slightly from 520 to 509 nm due to increasing aspect ratio.42 Spalla and co-workers have performed a quantitative analysis of object populations obtained by TEM images for the classical scheme of aqueous seedless synthesis of Au NRs. They demonstrated the repartition of different crystallographic faces on stable nuclei or on the newly formed particles at the critical size of about 5 nm. When the sizes of the stable nuclei or newly formed particles were below the critical sizes, Au NRs were dominantly formed. Otherwise, globular NCs were favorable to form.43 Recently, in situ small-angle X-ray scattering and X-ray absorption techniques have been utilized to quantitatively assess the growth of Au NRs, suggesting that the dominant growth mechanism is surface reduction of Au(I) at nuclei or newly formed primary crystals.44 In seed-mediated growth processes, the product yield of Au NRs is obviously controlled by the size and shape distribution of preformed seeds.26 In our work, accordingly, the growth of Au NRs should be controlled via optimization of the NaBH4 concentration, which affects the formation of the nuclei (in situ seeds with size of about 1.5 nm26). For instance, the aspect ratios of the resulting Au NRs have been found to be affected by the concentration of Ag+ ions in the reaction mixtures (Supporting Information Figures S12 and S13). The AA concentration also affects the shape of Au NRs obtained via the seedless growth method (Supporting Information Figure S14) because Au seeds, in situ formed upon addition of NaBH4 solution into the growth solution, have higher activity and are more sensitive to the growth kinetics. In practical applications in biomedicine, for instance, when Au NRs with the same aspect ratios are utilized for photothermal tumor therapy under irradiation of near-infrared light, the dimension of the NRs, length and width, is expected to considerably affect their in vivo circulation half-life.45 Thus, it becomes desirable to synthesize Au NRs with varied length and width but fixed aspect ratios. According to the studies of seed-mediated growth methods,13,14 the dimensions of Au NRs strongly depend on the total amount of gold ions in the growth solution, and in turn both the concentration of Au seeds and the concentration of Ag+ ions in the growth solution due to the consumption kinetics of Au3+ ions. For example, when the concentrations of both Au3+ and Ag+ ions in the reaction mixtures are reduced and, at the same time, the concentration of NaBH4 is increased,

with the NC size should be attributed to the sharp extremities of the TOH Au NCs, which intensify the polarization.31,33,36 The typical TEM image of the resulting TOH Au NCs is shown in Supporting Information Figure S8. When a set of TEM images of the same TOH Au NC is recorded at different tilting angles with x as the axis of rotation from −15° to 0° and 15°, the different trisoctahedral shapes are clearly revealed under TEM observation (Supporting Information Figure S8b− d). We believed that the different shapes observed on the TEM images (Supporting Information Figure S8a) and SEM images (Figure 4a−c) are due to the different tilting angles of each single TOH NC on the substrates. The geometrical model, SEM, and TEM images of a single TOH NC with the average size of 236 ± 25 nm along the longest axis are shown in Figure 5a−c, respectively. In the selective TEM image of TOH Au NCs projected along ⟨110⟩ direction, the projection angles are measured and marked. The values of α, β, γ angles are consistent with those estimated from the Miller indices of the {331} and {441} facets (Figure 5d), respectively. Supporting Information Figure S9 shows the HRTEM of one edge-on facet of the TOH NC projected along ⟨110⟩ direction, which indicates that the atomic arrangement is periodic with two atomic width of (111) terraces, followed by one atomic width of a (110) step, which confirm the edge-on facet of {331}. Fast Fourier transform (FFT) analysis of the HRTEM images also indicates that TOH Au NCs are aligned along the [110] zone axis, which is consistent with the observations reported in literature.31 Because of the high surface energy of high-index facets, crystal-growth rates in directions perpendicular to high-index facets are usually much faster than those in directions perpendicular to low-index ones. Thus, Au NCs with thermodynamically controlled facets such as {111} or {100} facets would be obtained in general, while Au NCs with kinetically controlled {hhl} (h > l > 0) facets are rarely prepared.30,31,34 TOH Au NCs, obtained via seed-mediated growth methods under the same reaction conditions, dominantly have the edge-on facets of {221}. In contrast, we found that the present seedless growth yielded TOH Au NCs with the edge-on facets of {331}. This is possibly due to the fact that the nucleation, in situ initiated by NaBH4, and the following crystal growth in our seedless growth method, are much faster than those involved in conventional seed-mediated growth methods, thus enabling us to kinetically freeze thermodynamically less stable crystallographic facets on the surfaces of as-prepared NCs. This underlines the advantage of the present seedless method over two-step seed-mediated growth in terms of production of Au NCs with high-index crystallographic facets. 3.5. Synthesis of Au NRs. Au NRs exhibit a strong and polarized SPR, which is useful for photothermal effect and twophoton luminescence.37,38 There is much speculation about promising application of Au NRs in many disparate fields including biomedicine and sensing.39−41 When the concentration of NaBH4 in the reaction mixture is in the range of 0.096−1.5 μM and the time of stirring the reaction mixture prior to aging at room temperature is optimized to be ca. 25 s, the production yield of uniform Au NRs obtained via seedless growth method is ca. 90% (Supporting Information Figure S10 and Table S5), which is comparable to that via seed-mediated growth method.13,14 When the concentration of NaBH4 was smaller than 0.048 μM or larger than 3 μM, nonuniform Au NRs were obtained in coexistence with Au NCs with other shapes (Supporting Information Figure S11), which was similar 2485

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to study the plasmonic properties and have been widely utilized in many technical applications such as photonics, catalysis, electronics, and biomedicine.47,48 Up to date, there are a number of protocols available to produce quasi-spherical Au NCs in a rather reliable way.49,50 To test its applicability and generality, the present seedless growth method was applied to synthesize quasi-spherical Au NCs. Note that it is usually difficult to produce large uniform quasi-spherical Au NCs via NaBH4 reduction in water. Here, we found that adding a trace amount of NaBH4 into the growth solution, usually prepared for synthesis of TOH Au NCs, was able to yield quasi-spherical Au NCs when the reaction mixture with increased NaBH4 concentrations was aged for a longer period of time (>12 h) at room temperature under continuous stirring. As shown in Figure 7, as-prepared Au NCs are quasi-spherical and their sizes

the dimensions of as-prepared Au NRs can be tuned with little change in the NR aspect ratio (Figure 6 and Table 1), and both

Figure 6. TEM images (a−d) of the Au NR with fixed aspect ratios but varied dimensions (length × width). The NRs shown in (a) and (b) are obtained at the concentrations of NaBH4 of 4.0 and 0.23 μM, respectively, the NRs shown in (c) and (d) are obtained at the concentrations of NaBH4 of 2.0 and 0.76 μM, respectively, the concentrations of HAuCl4 are 0.39 mM (a and c) and 0.48 mM (b and d), and the concentrations of AgNO3 are 58 μM (a and c) and 96 μM (b and d), respectively. The concentration of CTAB is always 96 mM. The concentration of AA is slightly increased from 0.54 mM (a and c) to 0.58 mM (b and d).

the length and the width of the NRs increase with the decrease of the NaBH4 concentration. It was reported in the literature14 that the rod dimension strongly depends on the total amount of gold ions and the concentration of seeds in the solution. In general, the increase in the amount of gold ions and the decrease in the concentration of seeds can lead to short and thick Au NRs. It was also reported13 that the increase of silver ions below critical concentration (120 uM) can increase the aspect ratios of Au NRs. According to the recent literature,46 the AgUPD layer, which is highly mobile in 0.10 M CTAB, expands to cover a larger area with the same facet structure. Thus, an increase in the concentration of Ag+ ions leads to Au NRs with large aspect ratios as the NRs with large aspect ratios have large surface areas relative to their volumes. In our work, therefore, the concentration of silver ions was increased to adjust their growth at longitudinal and transverse axes to increase the same aspect ratio of Au NRs. 3.6. Synthesis of Quasi-Spherical Au NCs. Among differently shaped Au NCs, quasi-spherical Au NCs are the most intensively studied, and they are important model systems

Figure 7. TEM images of quasi-spherical Au NCs obtained at the NaBH4 concentrations of 0.77 μM (a), 0.96 μM (b), 1.2 μM (c), 2.4 μM (d), 4.8 μM (e), and 30 μM (f). The concentrations of HAuCl4, CTAC, and AA in the reaction mixture are 0.48, 38, and 0.96 mM, respectively.

are fairly uniform; the size deviation is less than 10%, and NCs in other anisotropic shapes are hardly visible. The production yield of quasi-spherical Au NCs was almost 100%. The NC sizes can be readily tuned from 27 ± 2.4 to 22 ± 2.2, 18 ± 1.6, 14 ± 1.3, 11 ± 1.1, and 7.0 ± 1.1 nm with the increase of the NaBH4 concentration in the reaction mixture from 0.77 to 0.96, 1.2, 2.4, 4.8, and 30 μM. When the concentration of NaBH4 is further increased, one can produce very small Au NCs of 3.0 ± 0.5 nm in size (Supporting Information Figure S15), which are regarded as “seeds” and utilized for seeded growth of Au NCs. The HRTEM image shows the lattice fringe spacing of 0.236 nm (inset in Figure 7a), which is the characteristic of the {111} plane of Au NCs. Most of these Au NCs are single crystalline,

Table 1. Summary of the Dimensions (Length × Width), Aspect Ratios, and Production Yield of the Au NRs Synthesized under Different Reaction Conditions samples shown in Figure no.

HAuCl4 (mM)

AgNO3 (μM)

AA (mM)

CTAB (mM)

NaBH4 (μM)

6a 6b 6c 6d

0.39 0.48 0.39 0.48

58 96 58 96

0.54 0.58 0.54 0.58

96 96 96 96

4.0 0.23 2.0 0.76

length (nm) × width (nm)a 18.5 63.6 30.9 50.4

± ± ± ±

1.6 3.3 2.5 3.1

× × × ×

5.9 ± 1.2 22.1 ± 2.6 8.5 ± 1.4 13.9 ± 1.3

yield (%) >80 >95 >85 >95

aspect ratiosa 3.0 3.0 3.6 3.6

± ± ± ±

0.17 0.15 0.26 0.16

a

A total of 300 particles from TEM image were counted to determine the average dimensions and aspect ratios of Au NCs of each shape. The error bars in the sizes correspond to one standard deviation in each case. 2486

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located around 1.13 ± 0.002 V (I) and a weaker oxidation peak located around 1.34 ± 0.003 V (II) (cyan curve). These results demonstrate that Au NCs with high-index facets are distinctively different from those with low-index {111} and {100} facets. One may expect that the sequence of chemical activity of asprepared Au NCs is as follows due to their different facets on the surfaces: {331} facets on TOH NCs > {720} facets on concave cubic NCs ≈ {110} and {100} facets on NRs > {100} facets on cubic NCs ≈ {111} and {100} facets on spherical NCs. Here, we further studied the electrocatalytic activity of quasi-spherical Au NCs, cubic Au NCs, Au NRs, concave cubic Au NCs, and TOH Au NCs, and by utilizing them as electrocatalysts for methanol oxidation. Figure 8b shows the electrocatalytic performance of the Au NCs for methanol oxidation in 0.10 M KOH aqueous solution containing 1.0 M methanol. The oxidation peak positions with maximum current intensity of as-prepared TOH, concave cubic, NRs, cubic, and spherical Au NCs are located around 0.26 ± 0.005, 0.28 ± 0.005, 0.28 ± 0.01, 0.31 ± 0.005, and 0.31 ± 0.005 V, respectively. Thus, this suggests that the TOH or concave cubic Au NCs are more active for electrocatalysis of methanol oxidation,55 which can be easily rationalized as a result of the presence of high-index facets in the former two NCs,56 which are in good agreement with previous results.27,31 The electrocatalytic performance (mass activity, specific activity, stability, and durability) for the Au NCs of different shapes is also size-dependent. Therefore, our future work will further systematically investigate the electrocatalytic performance of each Au NC with different sizes.

which are different from those obtained in CTAB or citrate solution. As-prepared quasi-spherical Au NCs exhibit fairly symmetric SPR bands, and the absorption maximum hardly red-shifts with the NC size increase (Supporting Information Figure S16). This further confirms the narrow size distribution and the uniform spherical shape of the Au NCs obtained. 3.7. Electrochemical Measurement of As-Prepared Au NCs with Different Shapes. Cyclic voltammetry (CV) has been commonly used to characterize the activity of the surface facets of as-prepared Au NCs with different shapes.30,31,51−53 As shown in Figure 8a, as-prepared quasi-spherical Au NCs display

4. CONCLUSIONS In summary, we demonstrate that seedless growth methods, adding a trace amount of NaBH4 into the growth solution, allow high yield production of high-quality Au NCs with various shapes, including quasi-spherical, rod-like, cubic, concave cubic, and trisoctahedral. We have found that upon NaBH4 addition, the stirring time of the reaction mixture before aging is crucial for the quality and yield of Au NCs, which should be properly prolonged to produce high-quality NCs. After the stirring time in our work is optimized in the range of 25−30 s, the sizes and structural features of as-prepared Au NCs can be easily tuned by the composition of the reaction mixtures and the concentrations of various reactants, mainly NaBH4. Thus, our work opens a general, facile, efficient, and, more importantly, easily accessible strategy for the production of uniform Au NCs directly in water, which should be important for further exploitation in different technical applications. For instance, Au NCs with high-index facets on their surfaces, such as trisoctahedral and concave cubic NCs, can be easily produced and exhibit higher activity for electrocatalysis of methanol oxidation.

Figure 8. (a) Cyclic voltammograms of quasi-spherical Au NCs (black curve), cubic Au NCs (red curve), Au NRs (green curve), concave cubic Au NCs (blue curve), and TOH Au NCs (cyan curve) in 0.50 M H2SO4. The scan rate is 50 mV s−1. (b) Cyclic voltammograms of methanol oxidation catalyzed by quasi-spherical Au NCs (black curve), cubic Au NCs (red curve), Au NRs (green curve), concave cubic Au NCs (blue curve), and TOH Au NCs (cyan curve) in 0.10 M KOH solutions containing 1.0 M methanol. The scan rate is 20 mV s−1. The quasi-spherical NCs, cubic NCs, Au NRs, concave cubic NCs, and TOH NCs are shown in Figures 7d, 3a, Supporting Information S10c, 2b, and 4c, respectively.

only one oxidation peak located around 1.37 ± 0.003 V (black curve). However, cubic NCs have no remarkable oxidation peaks (red curve), which are located at about 1.37 ± 0.005 V (I), 1.25 ± 0.005 V, and 1.16 ± 0.004 V (II), respectively.54 In contrast, the oxidation peaks of Au NRs (green curve) are located at about 1.38 ± 0.003 V (I) and 1.18 ± 0.005 V (II), respectively, and the latter intensity is higher than the former. Similarly, concave cubic NCs also exhibit two distinct oxidation peaks (blue curve), the oxidation peaks of concave cubic NCs are located at about 1.09 ± 0.003 V (I) and 1.29 ± 0.005 V (II), respectively, and the latter intensity is higher than the former. As-prepared TOH Au NCs display a stronger oxidation peak

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ASSOCIATED CONTENT

S Supporting Information *

Additional TEM images and UV−vis spectra of the Au NCs with different shapes synthesized under different reaction conditions; Tables S1, S2, S3, S4, S5, and S6 that summarize the structure features and yield of concave cubic Au NCs, trisoctahedral Au NCs, Au NRs, and quasi-spherical Au NCs produced under different reaction conditions. This material is available free of charge via the Internet at http://pubs.acs.org. 2487

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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 is financially supported by the Natural Science Foundation of China (51172126, 51002086, 51227002, and 51272129), 973 program (2010CB630702), and Shandong Provincial Natural Science Foundation (ZR2010EM006). H.X. is grateful to the Program for New Century Excellent Talents in University (NCET-10-0553), Independent Innovation Foundation of Shandong University (2010JQ013), and Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, for financial support. D.W. thanks the Australian Research Council for financial support (DP 110104179 and DP 120102959).

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dx.doi.org/10.1021/la404602h | Langmuir 2014, 30, 2480−2489