Synthesis of Complex Nanoparticle Geometries via pH-Controlled Overgrowth of Gold Nanorods.

We show that many complex gold nanostructures such as the water chestnut, dog bone, nanobar, and octahedron, which are not easily accessible via a direct seed-growth synthesis approach, can be prepared via overgrowth of the same gold nanorods by varying pH and Ag concentrations in the growth solution. Overgrown nanostructures' shapes were determined by the rate of gold atom deposition, which is faster at higher pH. In the presence of AgNO3, codeposition of gold and silver atoms affects the shapes of overgrown nanostructures, particularly at high pH.

Introduction

Gold nanoparticles are known for their original optical, electronic, and catalytic properties, which differ from those of the bulk metal.[1] Such properties arise from the collective oscillation modes of their conduction electrons. These oscillation modes are known as surface plasmons.[1,2] For small spherical nanoparticles (diameters below 50 nm), plasmon resonance happens at ∼530 nm, and the resonance is nearly independent of its size. For elongated particles such as nanorods, electron oscillations can happen either along the principal axis or perpendicular to it, producing two different plasmonic modes, longitudinal and transverse plasmon.[3] Longitudinal plasmon resonance can be tuned over a broad spectral range from visible to NIR wavelengths by carefully controlling the aspect ratio of a nanorod.[4]

Recent advances in wet-chemical nanoparticle synthesis have significantly improved our ability to tune plasmonic properties of gold nanoparticles. Many different nanoparticle shapes with their plasmon resonance tuned over the entire visible and NIR spectral range have been synthesized.[5] Typically, a seed-mediated nanocrystal growth method is preferred since it provides robust control over nanoparticles’ size and/or shapes under mild experimental conditions.[6−9] Many different nanoparticle shapes such as the sphere, rod, cube, triangular prism, and nanostar have been synthesized by varying concentrations of silver nitrate, ascorbic acid, halide ions, and surfactants.[6,10−22]

Controlled overgrowth of already synthesized nanoparticles provides an alternate method for tuning the nanoparticle’s geometry and optical properties.[23] In this method, already synthesized nanoparticles are used as seeds for further growth. Many different nanostructures, which are not easily accessible via the original seed-growth method, such as nanodogbones,[24−26] nanodumbbells,[25,26] arrowheads,[25,27] octahedrons,[26,28,29]and nanobars[30] can be synthesized by overgrowing already synthesized nanoparticles by varying chemical parameters of the growth medium. Formation of these different geometries is driven by thermodynamic and kinetic parameters.[31] Surface capping agents are known to chemisorb to a particular facet of the growing nanocrystals thereby reducing the surface free energy of that particular facet.[31−33] They also hinder the deposition of atoms onto these facets and hence allow the incoming atoms to deposit on the other facets, which are either free or have less coverage from the capping agent. Similarly, foreign ions such as Ag+, Cu+, I–, Br–, etc. may influence the growth mechanism by blocking some facets and thereby facilitating growth from other facets.[25,30] Reaction kinetics also plays an important role in determining the final shapes of overgrown nanoparticles.[31] Atoms are initially deposited on high-energy facets and then migrate to thermodynamically more stable low-energy facets. However, in the case of inadequate surface migration of atoms, the nanoparticle may be trapped in a thermodynamically unfavorable geometry leading to the formation of kinetically controlled products. For such products, overgrown nanoparticle shapes are determined by the rates of atom deposition and surface migration. Various experimental methods have been developed to carefully tune these rates to prepare various kinetically controlled geometries. Zhang et al. have varied the concentration of one of the reactants among silver nitrate, ascorbic acid, or CTAB while keeping all other parameters same to get shapes like the dog bone, arrow-headed NRs (AHNR), and many other intermediate structures.[25] Khlebtsov et al. had used a binary surfactant and ascorbic acid as a reducing agent to show that slow growth of gold nanorods leads to the formation of cigar-like nanorods, whereas faster growth leads to the formation of dogbones.[34]

Here, we show that various kinetically controlled nanostructure geometries such as the water chestnut, dog bone, nanobar, icosahedron, and octahedron can be formed by overgrowing gold nanorods at different pH values of the growth medium. Our method utilizes the pH-dependent reduction potential of hydroquinone. At high pH (pH ≥ 4), fast deposition of gold atoms leads to the formation of kinetically controlled geometries. Meanwhile, at low pH (pH = 2), the overall nanorod shape remains unchanged as the slow deposition rate allows adequate surface migration of deposited atoms. We also note that, in the presence of AgNO3, overgrown nanostructures’ geometries were further influenced by Ag/Au codeposition, which is prominent at high pH.

Results and Discussion

Gold nanorods (AuNRs) were prepared by a modified seed-mediated growth method using binary surfactant CTAB and sodium oleate in the presence of silver nitrate and ascorbic acid.[35]Figure shows the extinction spectra and the SEM image of the prepared AuNR. The longitudinal plasmon resonance (LSPR) appears at 666 nm in water (Figure a). The nanorods formed were uniform with the average length and width of 58 ± 7 nm and 25 ± 2 nm, respectively (Figure c,d). Figure S1 shows the HRTEM image of a [110]-orientated particle. The image indicates the orientations of the {111} and {200} planes that are aligned closest with the particle edges. It is not possible to conclude the crystallographic nature of the particle facet simply based on a single two-dimensional projection image; however, our results resemble the result shown by other groups including Wang and co-workers who identified the presence of {111}/{110} at corners, {100} at tips, and {110}/{100} at the lateral facet.[24,27,36,37,42] However, possibly due to the lack of favorably oriented nanorods (standing), we did not find any additional facet such as the {250} facet reported by the group of Liz-Marzán.[38]

Figure 1

(a) Extinction spectra of the gold nanorod prepared using the seed-mediated method with binary surfactant CTAB and sodium oleate with the LSPR maximum at 666 nm. (b) SEM image of the AuNR. (c, d) Histogram distribution of measured widths and lengths of AuNR with an average width and average length of 25 ± 2 nm and 58 ± 7 nm, respectively.

These AuNRs were then used as seeds for further growth. All the reaction conditions and concentrations of CTAB (0.089 M), silver nitrate (0.354 M), HAuCl4 (0.089 M), and hydroquinone (9 mM) were kept constant, and the only pH of the reaction was varied from 1 to 8 either by adding 0.2 M HCl or 0.1 M NaOH in the reaction medium as mentioned in the Experimental Details. The overgrowth of the AuNRs was monitored via vis–NIR spectroscopy.

Extinction spectra of the overgrown nanoparticles at different pH values are shown in Figure a. A clear difference in extinction spectra indicates that different structures were formed at different pH values. This is also evident from the SEM images (Figure b–e). At pH 2, nanorods retain their overall shape. However, with an increase in pH, a significant deviation from the nanorod shape was seen. At pH 4, dog-bone-shaped nanoparticles were formed, whereas at pH 6, overgrown nanoparticles had an octahedral shape. At an even higher pH of 8, overgrown nanoparticles had largely deformed shapes, which resemble the shape of a water chestnut. A summary of various geometries obtained by varying the pH of the growth solution and their dimensions is given in Table S1.

Figure 2

(a) Normalized extinction spectra of starting AuNR (black) and overgrown nanostructures at different pH values: magenta (pH = 8), red (pH = 6), blue (pH = 4), and green (pH = 2), after 12 h of overgrowth. (b–e) SEM images of the overgrown nanostructures at pH 8 (b), pH 6 (c), pH 4 (d), and pH 2 (e). Representative 3D geometries are shown with their SEM images. (f) Atomic percentage of Ag% present in overgrown nanostructures at different pH values calculated through EDS.

To understand the reasons behind the formation of different nanoparticle geometries from the same starting nanorod, we monitored the growth kinetics at different pH values. Figure a–d shows extinction spectra of gold nanorods during overgrowth at pH 8.0, pH 6.0, pH 4.0, and pH 2.0, respectively. We notice that both the rate and the directionality of the overgrowth process change significantly with pH of the growth medium. Overgrowth is slow at low pH but becomes much faster with the increase of pH of the growth medium. This is evident from the rate of change of LSPR intensities as a function of time (Figure e) for different pH values. At pH 2, the LSPR intensity increases only by a factor of 1.2 over a time period of 190 min. In the same time interval, LSPR intensity increased by 2.7 times when the pH of the growth medium was 8. A faster overgrowth rate at higher pH is also supported by inductively coupled plasma–optical emission spectroscopy (ICP-OES, Figure S3).

Figure 3

(a–d) Extinction spectra of gold nanoparticles during the overgrowth process at different pH values of the growth medium (a: pH 8, b: pH 6, c: pH 4, and d: pH 2) for the initial 190 min in the growth process. (e, f) LSPR and TSPR intensities as a function of time at different pH values (magenta: pH 8, red: pH 6, blue: pH 4, and green: pH 2). (g) LSPR peak position as a function of time during the overgrowth process. The color code is the same as for (e)–(g).

Monitoring the LSPR wavelengths during the growth process reveals that the directionality of the overgrowth process is also different at different pH values. At pH 2, LSPR wavelengths show a small redshift of 9.5 nm followed by an even smaller blueshift of 7 nm. This small LSPR shift indicates that the overall aspect ratio of the nanorods remains unchanged, and therefore the overgrowth happens uniformly from all sides of the nanorod. This is consistent with Figure e, which shows that overgrown nanostructures have a nanorod shape. At pH 4, the large redshift of LSPR by 86 nm was observed indicating that growth is preferred from the longitudinal direction. It is important to mention that a new plasmonic mode at 620.5 nm also appears during the overgrowth. The emergence of a new plasmon mode indicates a deviation from the rod shape. At even higher pH of 6 and 8, the nanoparticles’ LSPR shows an initial redshift of 21.5 nm at pH 6 and 26 nm at pH 8 followed by a large blueshift of 90.5 nm at pH 6 and 31 nm at pH 8. At pH 6, the overgrown nanostructures’ LSPR overlaps with TSPR and results in one peak in their extinction spectrum (Figure b).

We will now discuss the probable factors behind the formation of different nanoparticle geometries from the same nanorod seeds. For noble metals like Au with an fcc structure, low-index facets are known to be thermodynamically more stable than high-index facets.[31] However, the energies of these facets are different in the presence of a surfactant or other foreign ions.[31,32] In the presence of CTAB, the stability order of the facet reverses due to the preferential binding of CTAB on (110) greatly reducing its surface energy.[29,32] This coverage is responsible for variation in diffusion flux of various ions present in the growth solution toward each surface.[31] It has been reported that, during nanoparticle growth, the Au atoms first deposit (underpotential deposition) at the (111) facet and then diffuse to the more thermodynamically stable (100) facet, and the final nanoparticle geometry is determined by the relative rates of these two processes.[31] When the rate of atom deposition (overgrowth) is slow, as in pH 2, deposited gold atoms are expected to diffuse to the thermodynamically more stable (100) and (110) facets resulting in uniform growth of nanorods. This is indeed what we saw at pH 2 (Figure e) where the overall nanorod shape is preserved albeit with a small decrease of the aspect ratio. With the increasing rate of deposition, it is more likely that the deposited atoms will not be able to diffuse to thermodynamically more stable states, and therefore the growth will be predominant along the (111) plane. This is indeed the scenario at pH 4 where dog-bone structures were formed (Figure d). The formation of dog-bone nanostructures via overgrowth along {111} facets was also reported previously.[24,30,34,39] However, the growth mechanism seems to be more complex at higher pH values of 6 and 8. Initial redshifts of nanorod plasmon (Figure a,b) indicate a preferential growth along the nanorods’ tips at the early stage of growth. However, that directionality was lost in the later stage of growth as evident from the large blueshift of the nanorod plasmon. We believe that such a growth mechanism is a result of Ag/Au codeposition along with previously discussed underpotential deposition of Ag. Such codeposition of Ag/Au was found to be responsible for the formation of different nanostructures, particularly at high Ag/Au concentrations.[25,26] In our study, concentrations of Ag and Au remain unchanged; however, the effective reduction potential of the reducing agent changes with the pH of the medium.[36] The reduction potential of hydroquinone is higher at the acidic condition and decreases with the increase in pH of the medium (Figure S4). In a basic medium, hydroquinone can directly reduce Au(III) to Au(0) unlike in an acidic condition where hydroquinone first reduces Au(III) to Au(I), and further reduction of Au(I) to Au(0) can happen only on the gold seed surface.[37,40,41] In fact, at pH 8, we clearly see the formation of gold nanoparticles in the absence of gold nanorod seeds (Figure S5). Contrary to the underpotential deposition pathway, Au/Ag codeposition happens preferentially along the lateral side facets leading to an overall decrease of the aspect ratio (Table S1). This is also consistent with the blueshift of the longitudinal plasmon resonance (Figure a) for the overgrown nanostructures at pH 6 and 8. We believe that the water chestnut-shaped structure formation is due to fast Au/Ag codeposition resulting in a rather irregular growth with the presence of void spaces. The codeposition of Ag is supported by a large atomic percentage of silver on overgrown nanostructures at high pH calculated through energy dispersive X-ray spectroscopy (EDS, Figure f) and ICP-OES (Figure S6).

To understand the effect of the rate of atom depositions on the final shape of overgrown nanostructures, we monitored overgrowth at three different Au(III) concentrations in growth solution while keeping the ratio between Ag(I) and Au(III) unchanged. The pH of the growth solution was kept at 8. Figure summarizes the result of this study. To get a clear distinction in the spectra obtained on varying pH, we have shown normalized spectra in Figure a. At pH 8, with the decrease of Au(III) concentrations from 90 to 10 mM, the rate of the reaction decreases, which is evident from the change of LSPR intensity versus time plot (Figure b), and overgrowth leads to the formation of different nanostructures (Figure c–e). At 90 mM HAuCl4, overgrown nanostructures have a water-chestnut geometry (same as in Figure b). A threefold decrease in Au(III) concentration in growth solution leads to the formation of dog-bone-shaped nanoparticles (Figure d), whereas a ninefold decrease leads to rodlike nanostructures (Figure e). Interestingly, similar structures were obtained while the pH of the growth solution was varied (Figure b,d,e). This indicates that the rate of deposition of the Au/Ag atoms dictates the shape of overgrown nanostructures.

Figure 4

(a) Extinction spectra of starting gold nanorods (black) and overgrown gold nanostructures at different concentrations of HAuCl4 (red: 90 mM, magenta: 32 mM, and green: 10 mM). The corresponding AgNO3 concentration was kept at 357.46 mM (red), 128.24 mM (magenta), and 39.52 mM (green) to keep the ratio of Ag/Au at four. The pH of the solution was kept at 8. (c–e) SEM image of the overgrown nanostructures at 90, 32, and 10 mM HAuCl4, respectively.

Further, we probe the role of AgNO3 in determining the final shape of overgrown nanostructures. We repeated the overgrowth process at different pH values but in the absence of AgNO3, and the results are shown in Figure . The extinction spectra of the overgrown nanostructures are significantly different at different pH values, indicating the formation of different nanostructures (Figure a). The formation of different nanostructures was confirmed from SEM images (Figure b–e). At pH 2, icosahedron-shaped nanostructures are formed (Figure e). At pH 4, overgrown nanostructures retain overall nanorod shapes, but the tips seem to have become irregular (Figure d). At higher pH of 6 and 8, dog-bone-shaped nanostructures were formed (Figure c and b). A summary of various geometries obtained in the absence of silver nitrate by varying the pH of the growth solution and their dimensions is given in Table S2.

Figure 5

(a) Normalized extinction spectra of starting gold nanorods (black) and overgrown nanostructures at different pH values of the growth medium in the absence of silver nitrate (magenta: pH 8, red: pH 6, blue: pH 4, and green: pH 2). (b–e) SEM images of the overgrown nanostructures at pH 8, pH 6, pH 4, and pH 2, respectively.

The formation of different nanostructures in the absence of AgNO3 can be explained in a similar fashion as in the presence of AgNO3 by considering the rate of Au atom deposition and their diffusion to thermodynamically more favorable facets. When the deposition rate is less (at pH 2), the deposited Au atoms readily move to thermodynamically more favorable (100) facets thereby favoring growth from those facets leading to the formation of icosahedron structures. Predominant growth along (100) is supported by the fact that, at pH 2, the LSPR wavelength shows a gradual blueshift and eventually overlaps with the TSPR mode (Figure d). Note that, at same pH values, overgrown nanoparticles in the presence of 357.5 mM AgNO3 had rod shapes (Figure e). This shape difference can be attributed to Ag(I) ions’ affinity to selectively bind to (100) facets of the gold nanorod and thereby restricting growth from these facets.[24,32] To verify this, we have studied overgrowth at the same pH but with increasing AgNO3 concentrations from 0 mM to 0.4 M (Figure S7). With increase in the Ag(I) concentration at pH 2, the shape of overgrown nanostructures changed from an icosahedron (in the absence of AgNO3) to a nanorod shape. This change of shape supports our assessment that Ag(I) ions are restricting growth along the (100) plane.

Figure 6

(a–d) Extinction spectra of gold nanoparticles during the overgrowth process at different pH values of the growth medium (a: pH 8, b: pH 6, c: pH 4, and d: pH 2) in the absence of silver nitrate for the initial 190 min in the growth process. (e, f) LSPR and TSPR intensities as a function of time at different pH (magenta: pH 8, red: pH 6, blue: pH 4, and green: pH 2). (g) LSPR peak position as a function of time during the overgrowth process. The color code is the same for (e)–(g).

When the rate of Au deposition is very fast (as in the case of pH 8 and 6) compared to atom migration, growth happens along (111) facets resulting in a dog-bone geometry (Figure b,c). This growth process is also consistent with a large redshift of nanorod LSPR during growth (Figure g). However, at the same pH (6 and 8), octahedral and water chestnut-shaped nanostructures were formed in the presence of AgNO3 (Figure b,c), highlighting that codeposition of Au and Ag is crucial for the formation of those geometries. To further ascertain the Ag(I) ion’s role, we studied overgrowth at pH 4, 6, and 8 at different AgNO3 concentrations from 0 to 0.4 M. Increasing AgNO3 concentration from 0 to 0.4 M resulted in a change in the overgrown nanostructures’ shape from nanorod to dog bone at pH 4 (Figure S8), dog bone to octahedral at pH 6 (Figure S9), and dog bone to water chestnut at pH 8 (Figure S10). The different shapes and dimensions obtained by varying silver nitrate concentrations at pH 2, pH 4, pH 6, and pH 8 are summarized in Tables S3, S4, S5, and S6, respectively. These findings establish that the formation of octahedral and water chestnut shapes are due to silver codeposition.

We note that Cl– ions did not play any significant role in the overgrowth process. This was verified by replacing HCl with HBr to adjust the growth solution’s pH to 2 and 4. Extinction spectra and the SEM images of overgrown nanostructures in the HBr medium are shown in Figure S11. The overgrown nanostructures’ shapes were found to be similar to HCl: dog-bone shape at pH 4 and nanorod shape at pH 2.

Conclusions

In conclusion, we demonstrated that different complex nanostructures geometries such as the water chestnut, dog bone, nanobar, and octahedron can be made via overgrowth of gold nanorods by controlling the pH of the growth medium and concentration of Ag while keeping all other reactant concentrations unchanged. In the absence of AgNO3, overgrown nanostructures’ shapes were defined by the growth rate, which is faster at higher pH. In the presence of AgNO3, overgrown nanostructures’ geometries were influenced by different growth rates at different pH values as well as Ag/Au codeposition, particularly at high pH. Our study provides a simple approach to prepare many complex nanostructures, which are not easily accessible via a direct seed-growth synthesis approach.

Experimental Details

Materials

l-Ascorbic acid (>99% crystalline), gold(III) chloride trihydrate (HAuCl4), sodium borohydride (NaBH4), and silver nitrate (AgNO3) were bought from Sigma-Aldrich. Hexadecyltrimethylammonium bromide (CTAB, 98%) and sodium oleate (>97%) were purchased from TCI, U.S.A.. Hydroquinone (HQ, 99%) was purchased from Alfa Aesar, sodium hydroxide pellets were purchased from SDFCL, and hydrochloric acid(35–37%) was purchased from Rankem. All chemicals were used as received. Ultrapure water (18.2 MΩ cm–1) was used during all syntheses.

Synthesis of Gold Nanorods

(a) Preparation of seeds: 5 mL of 0.5 mM HAuCl4 was added to 5 mL of 0.2 M CTAB and the solution was stirred for 2 min. Six hundred microliters of 0.01 M NaBH4 was diluted to 1 mL and was then quickly added to the solution under vigorous stirring. The color of the solution changed to brown instantly. The seed solution was then kept undisturbed for 30 min. (b) Preparation of growth solution: a CTAB (1.4 g) and sodium oleate (0.2468 g) mixture was dissolved in 50 mL of warm Milli-Q water. The growth solution was prepared by adding 550 μL of 40 mM AgNO3 to 50 mL of the CTAB and sodium oleate mixture. The solution was stirred for a minute and then kept undisturbed for 15 min. Five hundred microliters of 100 mM HAuCl4 was added to the solution and the solution was stirred till the solution turns colorless. One hundred microliters of HCl (11.3 M) was added to the solution. The solution was stirred for 15 min. Two hundred fifty microliters of 64 mM ascorbic acid was then added to the solution followed by the addition of 80 μL of the seed solution. The mixture was stirred for a few minutes and then kept overnight undisturbed at 28 °C.

Overgrowth of Gold Nanorods

The growth solution was prepared by adding 50 μL of 40 mM AgNO3 in 5 mL of 0.1 M CTAB, and the solution was kept undisturbed for 15 min. Five microliters of 100 mM HAuCl4 was added to the above solution and sonicated until the solution became clear. Different volumes of 0.2 M HCl or 0.1 M NaOH were added to vary the pH of the growth solution from 2 to 8. Five hundred microliters of 0.1 M hydroquinone was added to the growth solution, which changes the solution to colorless. The prepared growth solution (1980 μL) was mixed with 20 μL of the prepared AuNRs and was kept undisturbed.

Characterization

All extinction spectra were acquired with an Analytik Jena Specord 210 Plus UV–visible spectrophotometer using a 1 cm quartz cuvette. All spectra were collected at room temperature (25–28 ° C). Scanning electron microscopy (SEM) micrographs were acquired using SEM (JSM 7600FJEOL) at 10 kV. For each sample, a minimum of 100 particles was measured to obtain the average size and the size distribution. Transmission electron micrographs were acquired using 300 kV FEI, Tecnai G2, F30 where the sample was drop-casted on a 300-mesh copper grid. ICP-OES data were acquired using Avio 200-ICP-OES by PerkinElmer.