Controllable Synthesis of Stable Urchin-like Gold Nanoparticles Using

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Controllable Synthesis of Stable Urchin-like Gold Nanoparticles Using Hydroquinone to Tune the Reactivity of Gold Chloride Jing Li,† Jie Wu,† Xue Zhang,† Yi Liu,† Ding Zhou,† Haizhu Sun,†,‡ Hao Zhang,*,† and Bai Yang† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, People's Republic of China ‡ College of Chemistry, Northeast Normal University, Changchun 130024, People's Republic of China

bS Supporting Information ABSTRACT: We reported an aqueous synthesis of urchin-like gold nanoparticles (NPs) in the presence of hydroquinone through a seed-mediated growth approach. By altering the feed ratio of hydroquinone, seeds, and additional HAuCl4, the diameters of urchin-like NPs were tunable from 55 to 200 nm. Accordingly, the centers of surface plasmon resonance absorption shifted from 555 to 702 nm. Systematical analysis revealed that the generation of urchin-like particles as well as their size evolution strongly depended on the reactivity of gold ions, mainly controlled by the concentration of hydroquinone. At low hydroquinone concentration, only spherical particles were achieved. The increase of the hydroquinone concentration promoted a kinetics-favored deposition of gold atoms on the (111) lattice planes and thereby the growth of branches. Moreover, the as-prepared urchin-like particles possessed good structural stability, which could be kept in the growth solution for more than 10 days without morphology variation.

’ INTRODUCTION Gold nanomaterials have attracted great interests for a long time because of their excellent functions in catalysis, photonics, surface-enhanced Raman spectroscopy (SERS), and biological applications.1-14 Recently, nonspherical gold nanoparticles (NPs) have become the focus of this field because the optical and electronic properties of NPs are not only size-dependent but also shape-dependent.15,16 Gold nanomaterials have been produced to different shapes, such as rods, prisms, cubes, plates, and various branched particles.17,18 They exhibit unique SERS and catalysis behaviors in comparison to spherical particles due to the anisotropic distribution of the electromagnetic field near the surface of nonspherical particles. In particular, both theoretical calculations and experimental results indicate that a large electromagnetic field enhancement exists at the tips of branched particles, leading to stronger SERS activity relative to nonbranched ones.19-22 Accordingly, the reports for synthesizing branched gold particles with different sizes and more branches are fast emerging in the last 5 years.23-25 Branched gold particles are mostly synthesized through a seeding growth approach, namely, growth of larger particles from smaller seeds through the epitaxial deposition of atoms.26,27 In this method, the preformed seeds are put into a growth solution containing HAuCl4, reductants, and sometimes shape-directing agents to make particles grow.28,29 Two factors are dominant for a successful synthesis. First, reducibility of the reductant used. In r 2011 American Chemical Society

the case of a strong reductant, such as NaBH4, the fast reduction of AuIII to Au0 makes the secondary nucleation unavoidable. The products are the mixtures of particles with different sizes and shapes. Consequently, mild reductants, such as citrate,26 ascorbic acid,30,31 hydroxylamine and the derivatives,32,33 and hydrogen peroxide,19,34 are used in seeding growth. In these examples, the seeds can act as surface-catalyzed centers, favoring exterior gold atoms to reduce on the seed surface, thereby avoiding secondary nucleation.35 Thus, homogeneous particles are obtained. Second, preferential deposition of atoms on certain facets, whereas suppressing the growth of other facets is required for the epitaxial growth of branches. Several approaches have been applied to achieve this goal: (1) preferential adsorption of surfactants on given facets to favor the growth of uncovered facets,36 (2) addition or removal of ions in the reaction system to tune the activity of specific facets,32 (3) application of macromolecules to act as both reductants and shape-directing agents,22,37,38 (4) coalescence of neighboring crystallographically oriented particles via an oriented attachment process,39 and (5) spontaneous anisotropic growth by tuning the reactivity of gold ions.40,41 In any case, the formation of branched particles is driven by kinetic factors, provisionally created in the reaction system. The as-prepared Received: December 15, 2010 Revised: January 25, 2011 Published: February 14, 2011 3630

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The Journal of Physical Chemistry C branched particles are thermodynamically unstable and therewith tend to transform to thermodynamically stable spherical particles during storage.42 Although the mechanism for the growth of branched particles is becoming clear, fine-tuning the structures and sizes of branched particles is still challenging. Significant progress has been achieved in controlling the number and length of branches. For example, gold monopod, bipod, tripod, tetrapod, and urchinlike particles have been successfully synthesized using different epitaxial growth approaches.43 Particles with longer branches are also synthesized by using Agþ to increase facet selectivity, thus broadening the range of plasmon resonance absorption.44,45 However, it is less capable to continuously control the diameters of branched particles as that for spherical particles,26 which is another important means for tailoring plasmon resonance absorption. This disadvantage is attributed to the difficulty in tuning the reactivity of gold ions and therewith maintaining a kinetics-favored growth with facet selectivity all along the synthesis. As mentioned above, the reducibility of reductants strongly influences the reduction rate of AuIII to Au0 as well as the reduction process. It also implies that the reactivity of gold ions might be tuned by choosing the proper reductant or reductant mixture. In this scenario, the reduction of AuIII to Au0 referred to the stepwise reduction of AuIII to AuI and AuI to Au0. AuI is the middle state. Because the latter reduction directly relates to the seeding growth of particles, AuI is considered as the direct source for NP growth. Thus, the key to tuning the reactivity of gold ions is choosing a reductant to selectively reduce AuI. Most recently, hydroquinone is proved to have high selectivity in reducing AuI and used for synthesizing spherical gold particles.35 Herein, we demonstrate the synthesis of urchin-like gold particles using a reductant mixture of citrate and hydroquinone. AuIII is reduced to AuI by citrate, while AuI is further reduced to Au0 by hydroquinone. Thus, by altering the feed ratio of various raw materials, the diameters of urchin-like gold particles are continuously tunable from 55 to 200 nm.

’ EXPERIMENTAL SECTION Materials. The analytically pure HAuCl4 reagent (47.8%) was purchased from Alfa Aesar. Hydroquinone (98%) and sodium citrate (99.0%) were analytical grade and used as received. In all preparations, deionized water was used. Preparation of Gold Seeds. The spherical gold seeds were synthesized according to the citrate reduction approach.46 The 100 mM HAuCl4 aqueous solution was foremost prepared by dissolving 1 g of HAuCl4 into 25 mL of deionized water. A 75 μL HAuCl4 solution was put into a flask with 30 mL of deionized water under vigorous stirring and heated to boil. As soon as the solution was boiling, 900 μL of 1 w/v% sodium citrate aqueous solution was added and kept at boiling until the solution became wine red in color. The solution was then cooled under stirring to obtain gold seeds. The seeds were used within 1 day after preparation. Synthesis of Urchin-like Gold NPs. The 30 mM hydroquinone aqueous solution was foremost prepared by dissolving 33 mg of solid hydroquinone in 10 mL of deionized water and used within 1 day. For a typical synthesis of urchin-like NPs, 25 μL of aqueous HAuCl4 (100 mM) was put into 9.6 mL of deionized water under vigorous stirring. Subsequently, 50 μL of gold seeds, 22 μL of 1% sodium citrate, and 1000 μL of 30 mM hydroquinone was added one by one. The solution was kept under stirring at room temperature for 30 min to obtain urchin-like NPs.

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Scheme 1. Stepwise Reduction of AuIII to AuI by Citrate (Equation 1) and AuI to Au0 by Hydroquinone (Equation 2)

Control Experiments. The effect of the hydroquinone amount was studied by altering the amount of hydroquinone from 50, 100, 120, 150, and 500 μL to 1000 μL, whereas the amount of seeds, HAuCl4, and sodium citrate was fixed at 50, 25, and 22 μL, respectively. To investigate the effect of seeds, NPs were synthesized by altering the amount of gold seeds from 15, 50, 100, 200, and 300 μL to 400 μL, whereas the amount of HAuCl4, sodium citrate, and hydroquinone was fixed at 25, 22, and 1000 μL, respectively. In the study of the effect of HAuCl4, urchin-like NPs were synthesized by altering the amount of HAuCl4 from 10, 15, 20, 25, and 30 μL to 40 μL, whereas the amount of seeds, sodium citrate, and hydroquinone was fixed at 15, 22, and 1000 μL, respectively. The effect of sodium citrate was studied by altering the amount of citrate from 11, 22, and 66 μL to 88 μL, whereas the amount of seeds, HAuCl4, and hydroquinone was fixed at 50, 25, and 1000 μL, respectively. The growth duration for all samples was 30 min. Characterization. UV-visible absorption spectra were obtained using a Lambda 800 UV-vis spectrophotometer at room temperature under ambient conditions. Transmission electron microscopy (TEM) was conducted using a Hitachi H-800 electron microscope at an acceleration voltage of 200 kV with a CCD camera. High-resolution TEM (HRTEM) imaging was implemented by a JEM-2100F electron microscope at 300 kV. The X-ray powder diffraction (XRD) investigation was carried out by using a Siemens D5005 diffractometer.

’ RESULTS AND DISCUSSION The current synthesis of urchin-like gold NPs involved the preparation of gold seeds and subsequent NP growth in a growth solution containing HAuCl4, sodium citrate, and hydroquinone. Gold seeds with a diameter of about 20 nm were prepared according to a typical citrate reduction method in boiling water (Figure S1, Supporting Information), in which citrate acted as a ligand cum reductant.17,47,48 The growth of urchin-like NPs was performed in water, but at room temperature. Citrate also acted as the ligand, and both citrate and hydroquinone acted as reductants. They played different roles in the reduction of gold ions. As indicated in Scheme 1, the formation of urchin-like gold NPs included a stepwise reduction of AuIII to AuI and AuI to Au0. Because of the weak reducibility of sodium citrate at room temperature, it could only reduce AuIII to AuI rather than Au0 as that at elevated temperature (Figure S2, Supporting Information). Meanwhile, hydroquinone had a high selectivity in reducing AuI to Au0 on the gold seed surface.35 It has been reported that the standard reduction potential was 1.002 V in the presence of seeds, whereas it was -1.5 V in reducing isolated AuI to Au0.35 Consequently, hydroquinone led to a preferential reduction of AuI on the seed surface, directly contributing to the formation of urchin-like NPs. 3631

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Figure 1. Influence of hydroquinone amount on the optical observation (a) and UV-vis absorption spectra (b) of the as-prepared gold NPs. The amount of 30 mM hydroquinone was altered from 50, 100, 120, 150, and 500 μL to 1000 μL, from left to right, whereas the amount of seeds, HAuCl4, and sodium citrate was fixed at 50, 25, and 22 μL, respectively. The growth duration for all samples was 30 min.

Experimentally, the influence of citrate concentration on the properties of NPs was studied by altering the amount of additional sodium citrate. As shown in Figure S3 (Supporting Information), the increase of citrate from 11 to 66 μL had little effect on the size, morphology, and the position of the plasmon absorption of NPs. A further increase of citrate to 88 μL even led to a blue shift of the plasmon absorption. This observation was consistent with the finding of Peng et al., namely, that the high citrate concentration suppressed the growth of gold NPs.25 It revealed that citrate possessed two opposite effects on the growth of NPs. On the one hand, citrate reduced AuIII to AuI (Figure S2, Supporting Information). High citrate concentration favored the growth of NPs by supplying AuI. On the other hand, citrate also coordinated with AuI and/or adsorbed on NPs.47,48 High citrate concentration lowered the reactivity of gold ions as depositing on seeds and therewith suppressed the growth of bigger NPs. Note that hydroquinone might also adsorb on NPs, which was in competition with citrate. The adsorption of hydroquinone would be weakened with increasing citrate concentration, which, in return, influenced the process of NP growth by altering the ability of the coalescence of gold ions with NPs. Nevertheless, citrate possessed stronger coordination with gold NPs than hydroquinone, acting as the primary ligand.48

’ INFLUENCE OF HYDROQUINONE Figure 1 indicated the effect of hydroquinone concentration on the properties of as-prepared NPs. With increasing the amount of 30 mM hydroquinone from 50 to 1000 μL, the apparent color of the NP solution gradually turned from pink to green-pink (Figure 1a), revealing a size and/or morphology variation of NPs. Accordingly, the plasmon absorption of NPs shifted from 555 to 652 nm (Figure 1b), typical for an increase of NP diameters. TEM images exhibited that the products from the addition of 50, 100, and 120 μL

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Figure 2. Influence of hydroquinone amount on the morphologies of the as-prepared gold NPs. The amount of hydroquinone was altered from 50 (a), 100 (b), 120 (c), 150 (d), and 500 μL (e) to 1000 μL (f), whereas the amount of seeds, HAuCl4, and sodium citrate was fixed at 50, 25, and 22 μL, respectively.

of hydroquinone were spherical particles with the diameters slightly increased from 100 to 110 nm. When the amount of hydroquinone reached 150 μL, urchin-like NPs with more than 10 branches were observed. The size of NPs also increased to 150 nm with the addition of more hydroquinone (Figure 2). The obvious variation of NP size and morphology by controlling the hydroquinone concentration clearly presented that the supply of more Au0 in the reaction system was the key for generating urchin-like NPs. As indicated in Scheme 1, Au0 was supplied by the reduction of AuI using hydroquinone. The increase of hydroquinone was thought to promote the transformation of AuI to Au0 and, therefore, a fast growth of NPs. If the growth rate was high enough, urchin-like NPs formed. Contrarily, only spherical particles were obtained at low hydroquinone concentration. Although the monitoring of the AuI concentration was difficult due to the difficulty in distinguishing aqueous AuIII and AuI, an indirect estimation of its variation was available by measuring the pH alteration of the solution. The consumption of hydroquinone in reducing AuI generated quinone and Hþ (Scheme 1), and a high reduction rate of AuI would surely accelerate the generation of Hþ. As hydroquinone increased from 50 to 1000 μL with a fixed growth duration of 30 min, the pH of the NP solution decreased from 3.65 to nearly 3.40 (Table 1). The results were consistent with our consideration. Note that the pH of 1000 μL of 30 mM hydroquinone in 9.6 mL of water was 6.34, less acidic than the resultant NP solution, firmly presenting that the decrease of pH resulted from the reductive reaction rather than the ionization of hydroquinone. Besides, the pH with the addition of 1000 μL of hydroquinone was slightly higher than that of adding 500 μL of hydroquinone, 3.41 for the former and 3.35 for the latter. Also, as shown in Figure 1b, 3632

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Table 1. pH versus the Amount of Hydroquinonea hydroquinone (μL) pH

50 3.65

100 3.44

120 3.39

150 3.36

500 3.35

1000 3.41

The amount of seeds, HAuCl4, and sodium citrate was fixed at 50, 25, and 22 μL, respectively. The pH was measured after the addition of hydroquinone for 30 min to achieve reaction equilibrium. a

Figure 4. Influence of seed amount on the morphologies of the asprepared gold NPs. The amount of gold seeds was altered from 15 (a), 50 (b), 100 (c), 200 (d), and 300 μL (e) to 400 μL (f), whereas the amount of HAuCl4, sodium citrate, and hydroquinone was fixed at 25, 22, and 1000 μL, respectively. Figure 3. Influence of seed amount on the optical observation (a) and UV-vis absorption spectra (b) of the as-prepared gold NPs. The amount of gold seeds was altered from 15, 50, 100, 200, and 300 μL to 400 μL, from left to right, whereas the amount of HAuCl4, sodium citrate, and hydroquinone was fixed at 25, 22, and 1000 μL, respectively. The growth duration was 30 min.

although the plasmon absorption of the as-prepared NPs from the addition of 1000 μL of hydroquinone generally located at longer wavelength than that of 500 μL, the opposite result was occasionally observed. It meant that 1000 μL of hydroquinone was enough for the current reduction. Excess hydroquinone might adsorb on the surface of NPs, suppressing the growth of bigger NPs, though the deposition of Au0 on the seeds was highly favored. This result implied the multifunction of hydroquinone, acting both as a reducing agent and as a ligand. The role of ligand was exhibited only at extremely high hydroquinone concentration and was adverse for the growth of urchin-like NPs. The aforementioned results demonstrated the possibility to tune the size as well as the morphology of as-prepared NPs by altering the reduction rate of AuI to Au0. Because the amount of seeds was fixed at 50 μL, the enhanced Au0 concentration, in return, increased the Au0/seed ratio, which represented the reactivity of gold ions. This reminded us to investigate the influence of other experimental variables that could improve the Au0/seed ratio, which also allowed for tailoring the properties of urchin-like NPs.

’ INFLUENCE OF SEEDS In this section, the influence of seed concentration on the properties of as-prepared NPs was studied. As the amount of seeds increased from 15 to 400 μL, with the amount of HAuCl4,

sodium citrate, and hydroquinone kept fixed at 25, 22, and 1000 μL, the color of the solutions turned from green-pink to pink (Figure 3a), revealing a size decrease of NPs. Meanwhile, a blue shift of the plasmon absorption from 676 to 559 nm was measured (Figure 3b), which was consistent with the optical observation. Under TEM, a clear decrease of the diameters of urchin-like NPs was found (Figure 4). Obviously, the addition of fewer seeds led to a higher Au0/seed ratio, providing more Au0 to supply the growth of each NP. As a result, the formation of bigger NPs was favored. On the other aspect, a higher Au0/seed ratio facilitated a branched growth rather than an isotropic one, which also depended on the amount of hydroquinone (Figures 4 and S5 (Supporting Information)). A control experiment indicated that, at low hydroquinone concentration, such as 100 μL, although the NP size also increased with the decrease of seeds, the as-prepared NPs were mainly spherical rather than branched (Figure S5, Supporting Information). Slightly branched NPs were prepared only at an extremely low seed concentration, for instance 15 μL of seeds (Figure S5a, Supporting Information). This meant that a fast reduction of AuI to Au0 was achievable only at high hydroquinone concentration, which led to a high reactivity of gold ions. It, in return, led to the rapid deposition of Au0 on the seeds due to hydroquinone-induced preferential reduction of AuI on the particle surface.35 If the concentration of excess Au0 was high enough in the reaction system, the deposition rate of Au0 on higher energy facets would be faster than on lower ones. It was deduced as a kinetics-favored process, having been observed for the anisotropic growth of both metal and semiconductor NPs.31,49 As a result, urchin-like NPs formed. In comparison, the slow reduction of AuI to Au0 at low hydroquinone concentration only maintained a low Au0 concentration, making it less 3633

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Figure 5. Influence of HAuCl4 amount on the optical observation (a) and UV-vis absorption spectra (b) of the as-prepared gold NPs. The amount of 100 mM HAuCl4 was altered from 10, 15, 20, 25, and 30 μL to 40 μL, from left to right, whereas the amount of seeds, sodium citrate, and hydroquinone was fixed at 15, 22, and 1000 μL, respectively. The growth duration was 30 min.

capable for a selective deposition of Au0 on high-energy facets. Therefore, the growth of spherical particles was favored. The aforementioned results firmly proved that, to obtain urchin-like NPs, a high hydroquinone concentration should be maintained in the reaction system, which kept a high reactivity of gold ions.

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Figure 6. Influence of HAuCl4 amount on the morphologies of the asprepared gold NPs. The amount of HAuCl4 was altered from 10 (a), 15 (b), 20 (c), 25 (d), and 30 μL (e) to 40 μL (f), whereas the amount of seeds, sodium citrate, and hydroquinone was fixed at 15, 22, and 1000 μL, respectively.

’ INFLUENCE OF HAUCL4 Another factor that increased the Au0/seed ratio was the increase of HAuCl4. As shown in Figure 5a, the increase of the HAuCl4 amount from 10 to 40 μL led to the alteration of the apparent color of the NP solution from blue-pink to green-pink. Accordingly, the peak position of the plasmon absorption shifted from 608 to 702 nm (Figure 5b). The TEM observation indicated an increase of urchinlike NP diameters from 85 to 195 nm (Figure 6). Undoubtedly, the increase of HAuCl4 with a fixed citrate concentration promoted the reduction of AuIII to AuI and, subsequently, a reduction of AuI to Au0 by providing more AuI (Scheme 1). Consequently, the reactivity of gold ions was improved. Note that a low concentration of seeds, but a high amount of hydroquinone, was also adopted in this investigation. These factors maintained a high reactivity of gold ions (Figures 1-4); thus, the morphology of the as-prepared NPs was branched.

Figure 7. Comparison of the UV-vis absorption spectra of the asprepared gold NPs with a growth duration of 2 (solid) and 30 min (dash). Inset: corresponding TEM images. The amount of seeds, HAuCl4, sodium citrate, and hydroquinone was 50, 25, 22, and 100 μL, respectively.

’ PROFILE OF KINETICS-FAVORED GROWTH As mentioned above, the formation of urchin-like NPs was facilitated by increasing gold reactivity, which was deduced as a kinetics-favored process. We failed to catch the time-dependent size increase and plasmon absorption red shift of urchin-like NPs, which represented the gradual growth of NPs, because the formation of urchin-like NPs was completed within seconds after the addition of hydroquinone. Nevertheless, this rapid growth of urchin-like NPs revealed the profile of kinetics-favored growth. Besides, a slight blue shift of the plasmon absorption of

about 18 nm was observed as maintaining the growth duration from 2 to 30 min (Figure 7), which kept constant during prolonged growth. Meanwhile, the morphology of NPs altered from urchin-like particles to spherical ones simultaneously with the diameters decreasing from 125 to 120 nm (Figure 7, inset). These results were consistent with the property of anisotropic NPs prepared via kinetics-favored growth.50-52 Namely, the kinetics-favored formation of anisotropic NPs should be thermodynamically unstable and tended to transform to isotropic particles.42 Consequently, in the aforementioned studies of the influence of hydroquinone, seeds, and HAuCl4, the duration of 3634

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Figure 8. UV-vis absorption spectra of the as-prepared gold NPs by stepwise addition of 1000 μL of hydroquinone. One-off addition (solid), and two-step addition with the time interval of 2 (dash) and 30 min (dot) between the primary and secondary additions. Inset: corresponding TEM images. The amount of hydroquinone was 100 μL, followed by 900 μL (a), and 500 μL, followed by 500 μL (b), whereas the amount of seeds, HAuCl4, and sodium citrate was fixed at 50, 25, and 22 μL, respectively.

30 min was adopted to achieve the growth equilibrium (Figures 1-6). After 30 min, a thermodynamic equilibrium of the NP surface structure was built up through the reorganization of the surface atoms, which, in return, suppressed the further transformation of the NP morphology and thereby the blue shift of absorption spectra. The kinetics-favored profile of NP growth was further proved by a stepwise addition of hydroquinone. In general, the morphology of as-prepared NPs was determined by the hydroquinone amount of the primary addition, which did not relate to the total amount of hydroquinone (Figure 8). Meanwhile, the size of NPs via stepwise addition of hydroquinone was smaller than those via one-off addition, which decreased from 150 to 95 nm in Figure 8a and from 150 to 130 nm in Figure 8b. These revealed that the formation of urchin-like NPs was mostly favored by an instantaneous high concentration of Au0 via hydroquinone reduction rather than the total concentration during the reaction. The instantaneous high concentration of Au0 aided the preferential deposition of Au0 on higher energy facets. This was the characteristic of kinetics-favored growth, which had been revealed in many previous investigations.31,49 Interestingly, the time interval in between the two addition steps also influenced the plasmon absorption and size of the as-prepared NPs. The secondary addition of hydroquinone after 2 min generated the absorption peaks with longer wavelengths and NPs with bigger sizes than those after 30 min. This was attributed to the different facet activity of NPs after 2 and 30 min of growth. As mentioned above, the buildup of the thermodynamic equilibrium of the NP surface structure completed after 30 min of growth, whereas the

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Figure 9. (a) XRD pattern of urchin-like NPs. (b) TEM image of an enlarged NP. (c) HRTEM image of one branch of the urchin-like NP.

Figure 10. TEM images of freshly prepared urchin-like NPs (a) and the NPs after 11 days of storage at room temperature in the growth solution (b). In the preparation of NPs, the amount of seeds, HAuCl4, sodium citrate, and hydroquinone was 15, 25, 22, and 1000 μL, respectively.

NPs with 2 min of growth possessed higher facet activity. It soundly benefited the growth of bigger and more branched NPs via a kinetics-favored process. Similarly, it was found that the use of seeds stored within 1 day after preparation resulted in bigger and more branched NPs than that over 1 day. The freshly prepared seeds possessed active facets with higher surface energy, facilitating the deposition of Au0 along these facets and, therefore, the branched growth of NPs. These highly active facets were affirmed by the powder XRD and HRTEM observations of urchin-like NPs (Figure 9). XRD indicated that the lattice parameters of urchin-like NPs fitted well to the cubic structure of bulk gold crystal, represented by the 2θ degrees at 38.4, 44.6, 64.8, and 77.7° of (111), (200), (220), and (311) planes (Figure 9a).27 The appearance of these peaks meant that the urchin-like NPs were polycrystalline. Besides, the urchin-like NPs 3635

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The Journal of Physical Chemistry C possessed the strongest (111) diffraction peak, indicating that the branched growth of NPs might be through the rapid deposition of Au0 on (111) lattice planes. This consideration was further proved by HRTEM observation of the branched parts (Figure 9c). Under HRTEM, two interplanar distances of 0.236 and 0.204 nm were measured, which were consistent with the (111) and (200) planes of gold crystal.42 For all analyzed branches, the growth direction was vertical to the (111) plane, indicating that this plane was the high-energy facet and allowed the preferential deposition of Au0. Furthermore, as nanometer-sized building blocks, the as-prepared urchin-like NPs were stable in the growth solution, because the thermodynamic equilibrium of the NP surface structure was built up with 30 min of growth. The branched structures could be maintained for over 10 days at room temperature without morphology variation (Figures 10 and S6 (Supporting Information)). In comparison, similar urchin-like NPs that were prepared by other approaches were stable only at 4 °C. The storage of them at room temperature resulted in the morphology variation toward spherical particles.42 Thus, the current synthetic approach would benefit the technical applications of urchin-like NPs in view of the further assembly and conjugation with other objects.

’ CONCLUSION In summary, we demonstrated a seed-mediated growth of urchin-like gold NPs by modulating the reactivity of gold ions, making it possible to tune the diameters of as-prepared NPs from 55 to 200 nm. Among various experimental variables, the presence of hydroquinone was dominant. Urchin-like NPs were synthesized only at high hydroquinone concentration. Systematical studies presented that hydroquinone could enhance the reactivity of gold ions by preferential reduction of AuI to Au0 on the seed surface. A high concentration of hydroquinone led to excess Au0 in the reaction system, promoting the rapid deposition of Au0 on the highly active (111) planes via a kineticsfavored process and, therefore, the branched growth. Meaningfully, the as-prepared urchin-like NPs were stable in the aqueous solution, which could be stored for more than 10 days without morphology variation. The current acquirement will facilitate the synthesis of various branched metal NPs and thereby promote the technical applications in SERS detection and highly efficient catalysis. ’ ASSOCIATED CONTENT

bS

Supporting Information. Characterization of gold seeds, the evolution of the HAuCl4 solution in the presence of citrate, effect of citrate concentration, effect of seed amount at low hydroquinone concentration, and the stability of urchin-like NPs. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: þ86 431 85193423. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the NSFC (20974038, 20921003, 50973039, and 20804008), the 973 Program of China (2007CB936402 and 2009CB939701), the FANEDD of China (200734),

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the Special Project from MOST of China, and the Program for New Century Excellent Talents in University.

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