Silver Ions Direct Twin-Plane Formation during the Overgrowth of Single-Crystal Gold Nanoparticles.

It is commonly agreed that the crystalline structure of seeds dictates the crystallinity of final nanoparticles in a seeded-growth process. Although the formation of monocrystalline particles does require the use of single-crystal seeds, twin planes may stem from either single- or polycrystalline seeds. However, experimental control over twin-plane formation remains difficult to achieve synthetically. Here, we show that a careful interplay between kinetics and selective surface passivation offers a unique handle over the emergence of twin planes (in decahedra and triangles) during the growth over single-crystalline gold nanoparticles of quasi-spherical shape. Twinning can be suppressed under conditions of slow kinetics in the presence of silver ions, yielding single-crystalline particles with high-index facets.

Introduction

The mechanistic understanding of the emergence of twin planes is of great importance in nanoparticle synthesis because the presence of defects can alter the physical properties of nanostructures, thereby opening new avenues toward the design of novel systems.[1−7] Recently, experimental monitoring of twin emergence has attracted the attention of several research groups. For example, Mirkin and co-workers showed that single-crystalline gold nanoparticles can develop twin defects during their growth with silver, to yield core–shell nanoparticles with a twinned structure.[8] Although the initial single-crystal particles used in their work were prepared by conventional surfactant-assisted methods, the emergence of twin planes originated from the photochemical reduction of silver in the presence of additives that are not commonly used in established methods. Thus, the need for further experimental advancement is more than justified, which will not only ensure generalization of the current growth models but also open up opportunities for new synthetic pathways and crystal diversity at the nanoscale.

It is commonly agreed that reaction kinetics and selective surface passivation are the primary factors behind nanoparticle growth mechanisms.[9,10]Kinetic effects dominate at high rates of metal reduction and usually result in thermodynamically unstable products (displaying high-index facets). On the contrary, at slow reduction rates, thermodynamically more stable particles, which display low-index facets, emerge. Selective surface passivation, on the other hand, involves the use of chemical additives. Silver ions (Ag+) are known to bind preferentially onto certain gold facets, which can result in anisotropic crystal growth through the well-known underpotential deposition (UPD) mechanism. UPD occurs under conditions of seed-mediated synthesis in which ascorbic acid (AA) cannot reduce Ag+ in bulk, thereby facilitating the formation of a silver (sub)monolayer on a gold surface and hindering further deposition of Au atoms. As a result, particle growth slows down, affecting the final nanoparticle shape and crystalline structure. At high Ag+ concentrations, seeded growth has been reported to lead to anisotropic shapes with high-index facets.[11] Although these design considerations—kinetic- and surface-controlled growth—were established from single-crystalline nanoparticles, further experimental work is required to understand how twinned structures fit within this context.[12]

Results and Discussion

We define experimental conditions for the emergence of twin planes during the overgrowth of single-crystalline nanoparticles in a typical seeded-growth synthesis. We observed that relatively fast kinetics and high concentration of silver ions are crucial to forming twinned particles (decahedra, triangles) with low-index facets from single-crystalline seeds (10 nm). Our findings indicate that kinetic- and surface-controlled growth does not necessarily apply to the formation of twinned particles, as recently proposed for single-crystalline analogues.

It is well known that very small seeds can significantly change in solution because of the spontaneous ripening processes at room temperature.[13] We, therefore, decided to use larger seeds, which are more stable but still maintain a single-crystalline structure. By following a protocol recently reported by Xia’s group,[14] we synthesized 10.0 ± 0.3 nm spherical particles in the presence of the cationic surfactant cetyltrimethylammonium chloride (CTAC) and AA (reducing agent). Detailed inspection of the nanocrystals obtained using high-resolution transmission electron microscopy (HRTEM) revealed that 93% of the particles were monocrystalline (Figures and 2d). Importantly, extended storage of the 10 nm nanoparticles in the surfactant solution did not affect the particle size, suggesting higher stability than their 3 nm analogues commonly used in seed-mediated synthesis.

Figure 1

Transmission electron microscopy (TEM) (a) and HRTEM (b) images of single-crystalline gold nanoparticles used as seeds in overgrowth experiments. The average diameter was 10.0 ± 0.3 nm; 93% were single crystals, and 7% were polycrystalline.

Figure 2

Effect of selective surface passivation. (a–c) Effect of silver ions on the final nanoparticle shape: twinned particles formed at [AgNO3] = 0.12 mM. (d) Population of single- and polycrystalline particles in seeds and particles grown at selected silver concentrations; diameters of the seeds and final products. UV–vis spectra of the corresponding solutions are available in Figure S1.

Because our aim was to evaluate the role of silver in selective surface passivation and the emergence of twin planes, we tried to keep our system as simple as possible, using chemicals that contain chloride counterions only:[11] the cationic surfactant benzyldimethylhexadecylammonium chloride (BDAC), gold precursor (AuCl4–), and hydrochloric acid (HCl). Note that Cl– is the halide with the lowest binding affinity to gold surfaces, as compared with bromide or iodide,[15] which minimizes the risk of undesired symmetry breaking in the growing particles. Therefore, the growth over the crystals is dominated by the presence of Ag+.[16] In a typical experiment, the solution containing the initial nanoparticle seeds was mixed with a growth solution containing AuCl4– (0.5 mM), AA (0.067 mM), BDAC (100 mM), and different concentrations of silver nitrate (AgNO3) and HCl. The solution was stored at 30 °C for 2 h to ensure complete reduction.

Mirkin and co-workers reported that silver ions drive the formation of single-crystal particles with high-index facets by adsorption on the particle surface.[9] We indeed confirmed this when increasing the concentration of silver nitrate up to 0.04 mM. The resulting particles had the shape of concave cubes (Figure b). Supported by previous studies,[10] we assume that these particles are of single-crystalline nature. Overgrowth in the absence of silver nitrate rendered nearly spherical nanoparticles, thus preserving the shape of the initial seeds (Figure a). Interestingly, when increasing the Ag+ concentration to 0.12 mM, twinned nanoparticles were obtained with either decahedral (34%) or triangular (66%) shapes. The absence of spherical nanoparticles and the similar dimensions of triangles and decahedra (46.8 ± 3.4 nm for decahedra; 47.9 ± 4.6 nm for triangles) excluded a scenario involving the nucleation of new particles away from the seed surface. We, therefore, postulate that the twinned particles emerged from single-crystal seeds under these conditions. To provide further arguments, we found that the volume of decahedra and triangles is ∼70-fold higher than in the initial seeds. Thus, the size of final particles that contained twin defects is sufficient to accommodate 10 nm seeds between the boundaries of twin planes.

Interestingly, the formation of twinned particles from single-crystal seeds can be suppressed by adding extra HCl. At a low HCl concentration (2.5 mM), nanoparticles with a variety of poorly defined shapes formed (Figure b). Further increase in HCl concentration up to 19 mM rendered particles with unconventional shapes (Figure c). High-resolution electron microscopy, electron diffraction, and tomography confirmed the single-crystalline nature of the nanoparticles prepared at higher HCl content (Figures , S2, and S3). The morphology of these particles, with dominating high-index surface facets, {311}, {110}, and {100}, is close to that of the truncated ditetragonal prisms (DTP).[17] UV–vis spectroscopy showed that the localized surface plasmon resonance (LSPR) position blue-shifted from 620 to 555 nm when the amount of acid was increased, suggesting the formation of more isotropic nanoparticles (Figure e). We should, however, note that, at a higher acid concentration, a new plasmon band emerged at 846 nm. Scanning electron microscopy (SEM) analysis revealed the presence of bipyramids (Figures S4 and S5). The population of bipyramids (∼10%) is in good agreement with the initial population of twinned seeds.

Figure 3

Effect of pH on shape evolution. (a–c) Effect of HCl concentration on the final shape of the nanoparticles. (d) UV–vis–NIR spectra of solutions prepared at different HCl concentrations.

Figure 4

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of a single DTP particle, indicating the absence of twin planes. The insets show HRTEM of a selected area and the corresponding Fourier transform, which confirms the monocrystalline structure.

The above results suggest that the correct combination of HCl and Ag+ as additives defines the shape and crystallinity of the final particles. To better understand the role of additives in the crystal growth, we evaluated the reduction kinetics for selected experimental conditions. We carried out time-dependent UV–vis analysis of the reactions during the formation of (1) spheres (no Ag+, no HCl), (2) twinned nanoparticles with mixed shapes (Ag+, no HCl), and (3) DTP nanoparticles (Ag+ and HCl). The results are summarized in Figure a. Whereas the formation of spheres was fast (10 min), 70 min was needed to complete the reduction in the presence of Ag+ (0.12 mM), which yielded twinned products. The presence of both Ag+ and HCl slowed the kinetics further down so that 160 min was required to obtain the final single-crystalline DTP nanoparticles. Control experiments showed that the addition only of acid yielded spherical single-crystalline nanoparticles (Figure S6), but when doubling the concentration of AA—keeping HCl (0.19 mM) and Ag+ (0.12 mM) constant—we could again obtain twinned particles (Figure S7). These results show that kinetics is, indeed, the main parameter behind the formation of twinned particles, but only in the presence of a surface passivation/stabilizing agent—silver ions.

Figure 5

Kinetics and surface passivation effects. (a) Kinetic traces for three different samples, as labeled. (b) Ag/Au ratio for each particle type, as obtained from ICP-MS analysis (grey), and estimated values for monolayer coverage of Ag (red).

We analyzed further the amount of silver ions and their distribution in particles of different crystallinity. Mirkin and co-workers showed that the number of Ag atoms that can deposit onto a particular facet is equivalent to the number of surface atoms for that facet.[11] Thus, it is possible to estimate the Ag/Au ratio in nanoparticles of a given shape and crystallinity. Comparison of the calculated Ag/Au ratio with the experimental one obtained by elemental analysis using inductively coupled plasma mass spectrometry (ICP-MS, Table S1) allowed us to estimate the silver incorporation in the nanoparticles.

The surface energy of twinned particles (decahedra, triangles) is lower than that of DTPs. Twinned particles display low-index {111} surface facets, with a greater number of exposed surface atoms than higher-index facets in DTPs. We, thus, hypothesize that twinned nanoparticles can accommodate a larger amount of silver atoms,[9] which was confirmed by comparing the experimental and calculated Ag/Au ratios for three different samples: (1) spheres prepared in the absence of silver, (2) mixture of twinned nanoparticles, and (3) DTPs (for theoretical estimation of the Ag/Au ratio, see Supporting Information). To our surprise, the calculated and experimental Ag/Au ratios for twinned and DTP nanoparticles are nearly identical and equal to 0.04 and 0.013, respectively (Figure b). The amount of silver in the spherical nanoparticles is negligible (Ag/Au = 0.0003). The relatively good agreement between the experimental and calculated Ag/Au ratios suggests that (1) silver locates only on the surface of the nanoparticles and (2) twinned nanoparticles (decahedra and triangles) carry triple the amount of silver than single-crystalline nanoparticles, which is related to their surface facets.

Next, we evaluate the effect of the particle size. It has been suggested that gold nanoparticles exhibit different structural motifs and twinning as a function of particle size.[18] Thus to evaluate the size effect, we used conventional seeds (1–3 nm) prepared by fast reduction with borohydride and subjected them to overgrowth in the presence of Ag+ and/or HCl. We observed a similar trend in the morphologies of the final products for smaller seeds as those obtained using the 10 nm ones, but with less-defined shapes. Twinned particles emerged in the presence of Ag, whereas single-crystal particles were obtained in the presence of both Ag+ and HCl (Figure S8). This finding, therefore, shows that controlled twin engineering is possible for different size ranges.

We also evaluated the influence of the nature of the surfactant as well as the counterion. For instance, it has been demonstrated that the presence of bromide is essential for symmetry breaking in the synthesis of gold nanorods. Thus, we evaluated the influence of the surfactant by employing CTAC instead of BDAC. We again observed a similar trend in the final products (twinned particles in the presence of silver ions) by using CTAC in the growth solution (Figure S9). Interestingly, the surfactant counterion clearly plays an important role. When the synthesis was performed in cetyltrimethylammonium bromide (CTAB), irregular shapes were obtained regardless of the experimental conditions used (Figure S10).

We finally discuss the origin of twin planes. Our results show that when the Ag/Au ratio in the growth solution is 0.08, the particles adopt a concave cubic shape. Silver UPD has been proposed as a key mechanism to control the final nanoparticle shape while retaining the single-crystalline structure.[11] When the Ag/Au ratio is increased to 0.24, that is 3-fold higher, twinned particles are formed (Figure c). This seems to indicate that Ag+ ions determine the formation of twins under our experimental conditions: Ag+ (0.12 mM); Cl– (100 mM from BDAC). Note that the formation of argentichloride ions[19,20] (AgCl2–) and insoluble AgCl is favorable under these conditions. It is, thus, reasonable to assume that small clusters of silver chloride can coprecipitate on the surface of gold particles during growth, facilitating the formation of defects. This reasoning is supported by the fact that there is a low-energy barrier to the formation of twin defects.[21] Once the twin is formed, the final morphology will be determined by a balance between the surface energy and the elastic strain needed to form the non-space-filling structure.[22,23] Therefore, increasing the content of low-energy {111} surface facets could also drive the formation of twins by minimizing the overall surface energy. The previous assumption is supported by the fact that an increase in chloride concentration up to 19 mM produced a transition from twinned to single-crystalline particles enclosed by high-index facets (Figure c). It has been reported that high chloride concentrations increase the solubility of AgCl because the formation of AgCl–( complexes becomes favorable.[24] Such an increase in the solubility of silver chloride could inhibit twin formation. Additionally, the presence of HCl reduces the reduction kinetics. On the other hand, twinned particles were also formed when doubling the concentration of AA (keeping HCl constant) (Figure S7), thus supporting the kinetically controlled nature of the process. Bearing all this in mind, we propose a mechanism in which, at low pH and in the presence of Ag+ ions, the UPD mechanism dominates the growth of the particles leading to a thermodynamically controlled process.[25] At high silver ion concentrations in the absence of HCl, the higher pH increases the redox potential of AA, favoring the reduction of the gold precursor, in such a way that the process is kinetically controlled.

In the latter case, we should also consider that the Ag/Au ratio in the final product is 0.04, which corresponds to the estimated value of monolayer coverage of Ag. Therefore, silver could be reduced on the gold surface via the UPD mechanism, selectively stabilizing the {111} facets and the entire structure by favoring twin formation.

Conclusions

In conclusion, we have determined the experimental conditions required to obtain twinned nanoparticles with mixed shapes (decahedra, triangles), using 10 nm spherical single-crystalline particles as seeds. We showed that not only a fast reduction rate but also the presence of a surface passivation agent—silver ions—is necessary to induce the formation of twin planes. Our results open up possibilities for stepwise monitoring of emerging twin planes on a single-particle level that may contribute to further development of twin engineering at the nanoscale.

Experimental Section

Chemicals

Gold(III) chloride trihydrate (HAuCl4·3H2O), sodium borohydride (NaBH4), AA, CTAC, BDAC, AgNO3, and HCl were purchased from Sigma-Aldrich and used without further purification. Milli-Q water was used in all experiments.

Synthesis of 10 nm Single-Crystal Gold Nanoparticles[14]

Initial seeds were prepared by a reduction of HAuCl4 (5 mL, 0.25 mM) with freshly prepared NaBH4 (0.3 mL, 10 mM) in an aqueous CTAC solution (100 mM). A brown solution formed upon the addition of NaBH4. The mixture was left undisturbed at 27 °C for 2 h to ensure complete decomposition of NaBH4. An aliquot of the seed solution (0.11 mL) was added to a solution containing CTAC (20 mL, 200 mM), HAuCl4 (20 mL, 0.5 mM), and AA (15 mL, 100 mM). The mixture was left undisturbed at 27 °C for 30 min. The solution was then centrifuged (1 h, 14 500 rpm) to remove excess reagents and redispersed in water to obtain a final concentration of gold, equal to 2 mM. The 10 nm gold nanospheres were used as seeds in further growth experiments.

Main Synthesis

In a typical experiment, a solution of 10 nm gold seeds (0.04 mL) was added under vigorous stirring to an aqueous growth solution containing BDAC (10 mL, 100 mM), HAuCl4 (0.1 mL, 50 mM), and AA (0.075 mL, 100 mM), and desired amounts of Ag+ or HCl (vide infra) at 30 °C. The mixture was left undisturbed at 30 °C for 2 h. The solution was centrifuged twice (6500 rpm, 30 min) to remove excess reagents and redispersed in Milli-Q water.

Effect of Ag+ and HCl

To study the effect of silver ions, additional experiments were performed in the presence of different amounts of 10 mM AgNO3 (0.04 and 0.12 mL). Similarly, to study the effect of HCl, additional experiments were performed with 1 M HCl (0.025 and 0.19 mL) in the presence of Ag+ (0.12 mM).

Characterization

Transmission electron microscopy (TEM) images were obtained with a JEOL JEM-1400PLUS transmission electron microscope operating at an acceleration of 120 kV. UV–vis spectra were measured with an Agilent 8453 UV–vis spectrophotometer. SEM images were obtained using a JEOL JSM-6700F FEG microscope operating at 3.0 kV for secondary electron imaging. HAADF-STEM images and electron tomography tilt series were acquired using a double aberration corrected cubed FEI Titan 50-80 electron microscope operated at 200 kV. For the reconstruction of the series, we used the SIRT algorithm, as implemented in the ASTRA toolbox.