Several Shapes of Single Crystalline Gold Nanomaterials Prepared in the Surfactant Mixture of CTAB and Pluronics.

Twin structures in gold nanomaterials are destined because they reduce the severe strains in the misfit region of nanostructures. Defect-free single crystalline plasmonic nanomaterials gain interests these days as the integration of plasmonic materials or plasmons into electronic devices and circuits becomes more common. In this study, without subtle experimental adjustments, such as pH or halide additives, several shapes of single crystalline gold nanoparticles (NPs) are prepared in the surfactant mixture of cetyltrimethylammonium bromide (CTAB) and Pluronic triblock copolymers. The synthesized NPs are primarily composed of {100} planes with small numbers of particles possessing a [110] zone axis. Pluronic copolymers with low number average molecular weights (M n), such as L-31 (M n ≈ 1100) and L-64 (M n ≈ 2900), prefer anisotropic nanorods with the aspect ratios of 4.3 and 3.0, respectively, while Pluronics with high M n values, such as F-68 (M n ≈ 8400) and F-108 (M n ≈ 14 600), favor more concentric and isotropic cube-like NPs. Extended micelles are believed to form in Pluronics with low M n values in which hydrophobic cores are merged with the increase of temperature, while the corona regions that are composed of long tails of PEO prevent the merge of hydrophobic cores, and the growth of the micelles is limited in Pluronic copolymers with high M n values. The catalytic degradation reactions of methyl orange are conducted, and rather than isotropic particles, gold nanorods exhibit better catalytic performances. More hydrophilic environment and the steric alignment of rigid aromatic structures of methyl orange along the nanorods are thought to contribute to the catalytic activities. Overall, highly confined geometries of the appropriately swollen micellar templates of Pluronics and CTAB, which is not so hydrophobic as for the formation of contracted deswollen templates and for the inhibition of the growth of NPs, and which is not so hydrophilic as for the formation of coarse templates and for the formation of isotropic spheres with varying sizes, are believed as the main factor for the formation of single crystalline gold NPs.

Introduction

The templated growth method, whether it is a hard template composed of metal oxides or a soft template prepared by an amphiphilic surfactant in aqueous solution, has been known efficient to obtain one-dimensional nanomaterials. Soft micelles are one of them, and they can be constructed into anisotropic templates under external conditions. For example, the micelles of Pluronic copolymers, which are constituted of PEO (polyethylene oxide)–PPO (polypropylene oxide)–PEO triblocks, are typically organized into elongated micelles with the increase of the temperature.[1] This is due to the dehydration of hydrophobic cores[2] and the merge of hydrophobic cores with the increase of the temperature. Though they have the ability to guide the anisotropic growth, these soft templates can hardly control the crystallinity of nanomaterials. In this study, the soft micellar templates prepared from the surfactant mixture of Pluronics and cetyltrimethylammonium bromide (CTAB) are shown to govern the growth of single crystalline gold nanoparticles (NPs). In general, without the energy input from the surrounding environment, nanosize gold cannot endure the strains that are caused by the deformation in the misfit regions, and it becomes twinned structures. Though these twin structures are reflected as twin boundaries on the surface, the twins start to form at the early stages of the crystal growth and three–four layers below the surface of nanomaterials.[3] While it is a debate whether the single crystalline products are necessarily formed from the single crystalline seeds,[4−7] it is important to prepare single crystalline gold NPs from the synthetic point of view as many optical, electrical, and mechanical properties change according to the crystallinity of materials. For example, the yield strengths of gold nanowires are enhanced due to the decrease in the number of defect sites in highly restrained anisotropic structures.[8] The splitting of quadrupolar vibrations are observed in single crystalline gold NPs, while they are absent in twinned NPs.[9] It was also observed that the structural imperfections in twinned gold structures reduce the efficiency of individual circuits in plasmonic nanocircuitry.[10]

Among several shapes of single crystalline gold nanomaterials, gold nanorods with the aspect ratio of 4.3 are notable. With the relatively unstable {100} planes as the dominant lattice fringes, this structural stability is attributed to the confined micellar structure of CTAB and Pluronics that surrounds gold nanorods. Pluronics in this study is highly concentrated, for example, with the concentration of Pluronic L-64 (17.9%) being 62.1 mM and with that of F-108 (17.9%) being 12.2 mM, which is above the critical micelle concentrations of Pluronic L-64 (0.48 mM at body temperature)[11] and F-108 (2.2 × 10–2 mM at body temperature).[11] Phase diagrams indicate that lamellar mesophases are generated when Pluronics are most concentrated,[12] and these lamellar mesophases are thought to create highly confined geometry. Gold nanorods with single crystallinity have been prepared in several methods. The early synthetic achievement of gold nanorods in electrochemical methods produces single crystalline structures.[13,14] Silver(I)-mediated growth in pH-controlled conditions,[15] the change of CTAB to similar alkyl ammonium surfactants,[16] or the addition NaBr[17] or substituted aromatics such as salicylic acid or benzoic acid,[18] also produce single crystalline structures. The current work demonstrates another example, while no pH adjustment, nor halide additives, nor seed controls are necessary, and it focuses on more macroscopic controls using the surfactant mixture of CTAB and Pluronics.

Results and Discussion

Scheme represents the formation of elongated micelles from the mixture of CTAB and Pluronic copolymers. With small tails of PEO units in L-31 (EO2PO16EO2)[19] and L-64 (EO13PO30EO13),[20] the hydrophobic cores merge with each other as the temperature increases due to the dehydration of water in the core PPO region and due to the increased interaction of hydrophobic PPO cores. The hydrophobicity of the solution also increases through the segregation and depletion of water in the cores. It is known that in the mixture of Pluronics and charged surfactants, amphiphilic surfactants facilitate the formation of micelles, or destruct the micellar structure by the saturation of ionic surfactants in the PPO region.[21] CTAB has strong hydrophobic interaction with PPO blocks of Pluronics with no interaction with PEO blocks;[22] thus, the formation of extended micelles is facilitated by the hydrophobic interaction between PPO blocks and alkyl chains of the cationic CTAB surfactant. When the tails of PEO units are long as in the case of Pluronic F-68 (EO76PO29EO76)[11] and F-108 (EO132PO50EO132),[20] they cover the hydrophobic cores, and the interaction of these PPO cores between each other is prohibited by the presence of thick PEO coronas. In addition, the growth of micelles proceeds more isotropically with weak hydrophobic interactions due to the crowded PEO coronas.

Scheme 1

Illustration of the Formation of Single Crystalline Gold Nanorods and Cuboidal Shapes

Nanorods were prepared in the presence of Pluronic L-31 (aspect ratio of 3.0) and L-64 (aspect ratio of 4.3); cuboidals were prepared in the presence of Pluronic F-68 and F-108.

As for the growth of gold NPs in Pluronic copolymers, previous studies are rather varied; as in some studies, the reduction of Au(III) and the nucleation and growth are asserted to occur in the hydrophilic corona areas,[23] or in the surface cavities around the micelles.[24] In other studies, the reduction of Au(III) happens mostly in the PEO region, while PPO cores are increasingly important as the nucleated seeds are surrounded by PPO blocks and the growth proceeds mainly in PPO regions.[25] The fact of the remarkably slow growth of Au NPs in the presence of polyethyleneglycole without PPO blocks was ascertained for the roles of PPO blocks during the nucleation and growth of gold NPs.[25] When CTAB is added, the formation mechanism is thought to follow differently due to the large amount of CTAB (0.2 M, 5.0 mL), which increases the hydrophobicity and also assist the growth of anisotropic micelles. There is a significant interaction between CTAB and Pluronics and consequently they form mixed micelles with the alkyl chains of CTAB being self-assembled with the methyl groups of PPO and with the ammonium head group being placed on the interface between PPO cores and PEO coronas.[26] There are in fact several studies about the interaction between the charged surfactants and Pluronic copolymers. Mostly, Pluronic micelles become disintegrated and they are stretched as micellar aggregates. In the presence of anionic surfactants, both the suppression of micellization[27,28] and the formation of the mixed micelles[29] are observed. In the case of a cationic CTAB surfactant, it is known that the electronegative PEO units undergo synergistic interactions with the cationic CTAB surfactant,[30] and the mixed micelles are formed.

CTAB alone can organize into micelles, for example, cylindrical micelles or rod-like micelles,[31] which are used as the soft template to make mesoporous silica. In the case of gold nanorods, however, they are constructed into bilayers with the significant contribution from hydrophobic interactions between alkyl chains of the two layers.[32] In the bilayer model, hydrophilic head groups of CTAB are placed toward gold surfaces, and even at 350 °C the disruption does not severely occur and the large fraction of ammonium head groups are still close to the gold surfaces.[32] The bilayer formation of CTAB plays a significant role for the anisotropic growth, and in this study, it is proposed that initially the mixed micelles from CTAB and Pluronics are formed, in which CTAB bilayers are gradually constructed as the growth of gold nanorods progresses through the stabilized complex between CTAB and Ag(I).[33] It is also believed that the growth proceeds in the micellar cores of Pluronics, as hydrophobic cores are not perfectly dried but they also contain around 20% of aqueous solution.[34]

When Pluronic L-31 (17.9%) and AgNO3 (200 μL, 4.0 mM) were used, gold nanorods with the average aspect ratio of 3.04 (±0.39) were obtained (Figure a–g), while Pluronic L-64 (17.9%) and AgNO3 (200 μL, 4.0 mM) produced gold nanorods with the average aspect ratio of 4.26 (±0.56) (Figure h–o). Optical absorption spectra show two different shapes of these nanorods with the longitudinal resonance modes at 716 nm (Pluronic L-31, Figure g) and 792 nm (Pluronic L-64, Figure o). The Fourier-filtered diffraction pattern in the specified areas indicate that they are composed of the [100] zone axis (Figure d) or [110] zone axes (Figure f,k,m). Nanorods that are prepared with Pluronic L-64 are more uniform. The structural integrity of extended micelles are affected more by the short PEO chains of Pluronic L-31 (EO2PO16EO2), and the structure of micelles is considered not as rigid as those obtained with Pluronic L-64 (EO13PO30EO13).

Figure 1

(a) TEM image, (b) high-angle annular dark-field (HAADF) image, (c, e) high-resolution TEM images, (d, f) FFT patterns of rectangular regions in (c) and (e), respectively, and (g) the absorption spectrum of gold nanorods prepared with Pluronic L-31 (17.9%). (h) TEM image, (i) HAADF image, (j, l) high-resolution TEM images, (k, m) FFT patterns of rectangular regions in (j) and (l), respectively, and (o) the absorption spectrum of gold nanorods prepared with Pluronic L-64 (17.9%).

The small seeds that do not participate in the growth process are believed to be covered by hydrophobic PPO blocks (Figure c), which obstruct the adsorption of gold ions on the surface of seed particles. PEO blocks enable the reduction of Au(III) ions, while PPO blocks adsorb on the surface of NPs. Competition happens between the nucleation of Au as a result of the reduction of Au(III) and the nanoparticle growth that is hindered by PPO blocks. Small particles were entrapped by PPO blocks, and as the number of micelles increases, the number of small particles also increases. The merge of the hydrophobic cores happen, which acts as the micellar template for anisotropic growth, while segregated water during the dehydration also makes each micelle being apart. During the growth of gold nanorods with Pluronic L-31 (17.9%), these small NPs were not observed as much, and it is thought that the corona areas built from the small PEO blocks of Pluronic L-31 do not efficiently prevent the approach of Au ions on the seed NPs. The seed particles are not entirely single crystalline, but contain NPs with twin boundaries. The specified NP in Figure c is single crystalline with a ⟨110⟩ zone axis (inset in Figure c). It is not certain that this small portion of twinned seeds leads to the formation of spherical particles and that single crystalline nanorods were prepared from single crystalline seeds. Single-crystal gold nanorods grow along ⟨001⟩ directions (Figure a). Close examination of the facets reveal that several high index crystalline planes exist. The average aspect ratio is 4.3 (±0.58) with the dimension (width × length) of 17.7 nm (±1.32) × 74.9 nm (±6.77). The width itself and the standard deviation are quite narrow, and this implies that the mixed micelles are elongated into 1D with the width of micelles being barely changed.

Figure 2

(a) High-resolution TEM image of gold nanorods prepared with Pluronic L-64 (17.9%). (b) Reconstructed image after filtering the FFT pattern of the rectangular region in (a) showing a defect-free single crystalline structure. (c) TEM image of small NPs that were observed during the synthesis of gold nanorods in the presence of Pluronic L-64 (17.9%). The inset image is the electron diffraction pattern of the square region in NPs.

When Pluronics with high number average molecular weights (Mn) were used along with AgNO3 (200 μL), more concentric and rectangular shapes were obtained (Figure ). Diffraction patterns indicate that the structures are single crystalline with the zone axis of [100] or [110]. Optical absorption spectra exhibit the major longitudinal modes at 682, 683, and 693 nm. Two transverse modes were observed in all three NPs, with 513 and 593 nm (Figure p), with 515 and 586 nm (Figure q), and with 519 and 590 nm (Figure r). These peaks with low absorption intensities are believed to originate from the higher order resonance modes. In the case of silver nanocubes, several quadrupole resonance modes occur, for example, at 433, 441, 443 nm in 60 nm Ag nanocubes with a major dipole mode around at 500 nm.[35] However, in gold nanocubes, usually a single plasmon resonance mode occurs.[36] A concave structure with high index facets is different with surface plasmons concentrated at the corners and extended along the edges, showing quadrupolar and octupolar plasmon modes.[37] It is believed that for relatively small particles in this study, for example, 26.1 nm × 52.9 nm (width × length) in Figure a, the dipole longitudinal mode is strong with little retardation effect and shows at around 690 nm, while the peak at around 515 nm is from the edge quadrupolar mode, and the peak at around 590 nm is from the corner quadrupolar mode of concave particles.[37] In high-resolution TEM images and diffraction patterns, concave gold NPs with fourfold symmetry also indicate single crystalline structures with the ⟨100⟩ zone axis (Figure ).

Figure 3

(a, d, f) TEM images, (e, g) FFT patterns of rectangular regions in (d) and (f), respectively, and (p) the absorption spectrum of gold NPs prepared with Pluronic F-68 (17.9%, Au3). (b, h, j) TEM images, (i, k) FFT patterns of rectangular regions in (h) and (j), respectively, and (q) the absorption spectrum of gold NPs prepared with Pluronic F-108 (17.9%, Au4). (c, l, n) TEM images, (m, o) FFT patterns of rectangular regions in (l) and (n), respectively, and (r) the absorption spectrum of gold NPs prepared with Pluronic F-127 (17.9%, Au5).

Figure 4

(a) TEM image and (b, e) HRTEM images of concave gold NPs with 4-fold symmetry, which was prepared with Pluronic F-68 (17.9%). (c, f) Reconstructed images after filtering FFT patterns of (b) and (e), respectively. (d, g) FFT patterns of the images (b) and (e), respectively.

As was studied in gold nanorods with the aspect ratio of 4.3 in Figure h, which were prepared with Pluronic L-64 (17.9%) and AgNO3 (200 μL, 0.40 mM), the variation of the concentration of Pluronic L-64 and the amount of AgNO3 were investigated. Mixtures of the major nanorods and minor spheres with a small amount of cubes were obtained when Pluronic L-64 (35.9%) and AgNO3 (200 μL) were used (Figure a,b). Rectangular-shaped gold NPs with minor cubes and spheres were observed when Pluronic L-64 (17.9%) and AgNO3 (100 μL) were used (Figure f,g) When the nanorods were only counted, the aspect ratio is 5.0 (±0.80), which is higher than when prepared with Pluronic L-64 (17.9%) and AgNO3 (200 μL). Spherical particles are twinned structures. The aspect ratio of the rectangles is 2.24 (±0.53). In the absorption spectrum of gold NPs with an aspect ratio of 5.0 (Figure e), the longitudinal plasmon mode appears at 924 nm with the transverse mode at 512 nm. Applied Mie calculation shows the longitudinal modes at 868 and 917 nm with the aspect ratios of 5.0 and 5.5, respectively,[38] and the spectra in Figure e is more close to nanorods with an aspect ratio of 5.5. The two peaks at 557 and 596 nm are from the plasmon resonances of truncated or faceted structures of spherical particles. More intriguing is the absorption spectra of rectangular particles (Figure j). Four plasmon peaks were observed. The peak at 914 nm is the longitudinal mode of a small number of nanorods with the aspect ratios over 5.0. The three peaks at 695, 603, and 518 nm are very similar to the absorption spectra of particles when Pluronics with high Mn values were used (Figure ).

Figure 5

(a, b) TEM images, (c) HRTEM image, (d) the FFT pattern of image (c), and (e) the absorption spectrum of gold NPs prepared with Pluronic L-64 (35.9%) and AgNO3 (200 μL). (f, g) TEM images, (h) an HRTEM image, (i) FFT pattern of the rectangular region in image (h), and (j) the absorption spectrum of gold NPs prepared with Pluronic L-64 (17.9%) and AgNO3 (100 μL).

When AgNO3 (300 μL) and aqueous Pluronic L-64 (17.9% or 35.9%) were used, the products include spherical-shaped particles (Figure ). These spheres are twinned structures with twin boundaries shown on the surface of particles. Rod-shaped particles exhibit single crystallinity with the [100] zone axis. Absorption spectra indicate the longitudinal resonance peaks at 783 (Figure d) and 872 nm (Figure h). The peaks at lower wavelengths around 600 nm derive from the mixture of spheres and cubes. Twin boundaries in spheres are in fact protruded into certain directions and they are not exactly spherical shapes. The increased amount of AgNO3 (300 μL) preferably produces more isotropic NPs. In general, the increase of the amount or the concentration of AgNO3 provides longer nanorods.[39,40] Whether it is underpotential deposition[15] or the complex formation between CTAB and Ag(I),[41] the increased amount of AgNO3 is believed to inhibit the access of Au ions on the side walls of gold nanorods. One reason that more isotropic NPs are formed with AgNO3 (300 μL) is thought to be the increase of hydrophobicity by the salt effect.[42] With the increase of hydrophobicity, micelles are contracted with the depletion of water in the cores and the micellar structure is loosened.[43] This is reflected on the smaller size, for example, 20.9 nm (width) × 67.5 nm (length) in Figure a, and the unjustifiable shape of gold nanorods with the bodies not smoothly grown.

Figure 6

(a, b) TEM images, (c) the FFT pattern of image (b), and (d) the absorption spectrum of gold NPs prepared with Pluronic L-64 (17.9%) and AgNO3 (300 μL). (e, f) TEM images, (g) the FFT pattern of image (f), and (h) the absorption spectrum of gold NPs prepared with Pluronic L-64 (35.9%) and AgNO3 (300 μL).

Pluronic F-68, which is highly hydrophilc (EO76PO29EO76, Mn ≈ 8400), was investigated for the comparison with Pluronic L-64. TEM images in Figure a−c indicate the formation of rectangles, spheres, and bipyramids (sample Au10) when Pluronic F-68 (17.9%) and AgNO3 (100 μL) were used. Pluronic F-68 (17.9%) and AgNO3 (300 μL) produce concave NPs (sample Au11, Figure e–g). Twin defects are hardly found in those bipyramids and concave structures. Rectangles with sharp edges were obtained with a higher concentration of Pluronic F-68 (35.9%) with AgNO3 (100 μL) (sample Au12, Figure i,j), while Pluronic F-68 (35.9%) and AgNO3 (300 μL) produce a mixture of spheres and concave-shaped NPs (sample Au13, Figure m,n). Optical spectra are similar with each other according to the amount of AgNO3 (Figure d,l and Figure h,p). The major peaks at 568 nm in rectangular NPs are the dipole modes of the corners, while the shoulder peaks at 655 and 708 nm in Figure d are from bipyramids and rectangles, respectively. The optical spectra of concave NPs are similar to those in Figure when high Mn value Pluronic polymers were used. The major peaks at 689 (Figure h) and 714 nm (Figure p) are the dipole modes along the long axis of the NPs, and several peaks at 520–620 nm are the higher order resonance modes along the transverse axis of the NPs.

Figure 7

(a, b, c) TEM images and (d) the absorption spectrum of gold NPs prepared with Pluronic F-68 (17.9%) and AgNO3 (100 μL). (e, f, g) TEM images and (h) the absorption spectrum of gold NPs prepared with Pluronic F-68 (17.9%) and AgNO3 (300 μL). (i, j) TEM images, (k) the diffraction pattern of image (j), and (l) the absorption spectrum of gold NPs prepared with Pluronic F-68 (35.9%) and AgNO3 (100 μL). (m, n) TEM images, (o) the diffraction pattern of image (n), and (p) absorption spectrum of gold NPs prepared with Pluronic F-68 (35.9%) and AgNO3 (300 μL).

It is recently studied by theoretical calculations that the deposition of Au precursors into Au nanorods is prohibited more on the side walls of nanorods than on the tip surfaces.[44] Therefore, it is thought in this study that in the cubes, which were initially formed in the presence of Pluronic F-68, the symmetric deposition into the corners of the cubes leads to the formation of concave structures, while assymetric deposition causes anisotropic growth and the formation of gold bipyramids (Scheme ). Shape change into a dog bone in gold nanorods is also considered to be caused by the further growth through the differential deposition of gold precursors into corners.

Scheme 2

Description of the Formation of Concave Gold NPs by the Symmetric Growth of the Corners of Cubes and of the Formation of Bipyramids by the Assymetric Deposition of Gold Ions at the Initial Stage of the Growth

Catalytic degradation reactions of methyl orange were conducted with Au NPs as catalysts in the presence of aqueous NaBH4 solution. Photocatalytic degradation is a popularly used method,[45,46] and metal-catalyzed reactions without UV or visible light irradiation are investigated actively for the decay of azo dyes.[47,48] While photocatalytic degradation proceeds through the attack of a hydroxyl group to aromatic rings,[49] degradation by NaBH4 through metal catalysts proceeds to azo bond cleavage, leading to the intermediates of 4-amino-N,N-dimethylaniline and sodium 4-aminobenzenesulfonate (Figure ).[50] Methyl orange has a strong absorption peak at 464 nm stemming from the azo moiety (Figure a). The intensity decreases slowly in the presence of NaBH4 (100 μL, 0.634 M) before adding Au NPs (Figure b). With NaBH4 (100 μL, 0.634 M) and CTAB (10 μL, 0.2 M), the peak shift from 464 to 423 nm was observed at the initial stage of degradation, and the reaction proceeds quite well even without Au NPs (Figure c). In regards with the peak shift, it is all seen in Au-catalyzed reactions in this study (Figure ). A peak shift in the absence of Au NPs and only after the addition of CTAB, and the immediate shift are all cumbersome, since not many studies show a similar peak shift even with Au NPs.[51] It is only presumed that the change of an azo group to diamine happens from the start of the reaction. In addition, catalytic activities of CTAB are also noted, and similar micellar catalysis without any solid catalysts was also observed in the reduction of 4-nitrophenol.[52] Usually the reaction of methyl orange with NaBH4 proceeds through the electron transfer from borohydride to methyl orange, but the redox potential difference between two species is large, and this energy barrier is reduced by metal catalysts.[53,54] CTAB may act as a catalyst by changing the redox potentials of methyl orange on the surface of CTAB micelles.[55] In fact, CTAB adopts anisotropic rod-like micelles in hexagonal arrays when the concentration is high enough over the critical micelle concentrations.[56] At low concentrations, micelles are deeply hydrated and the spherical micelles are formed, while with the increase of concentration, rod-shaped micelles are formed with the elongation depending on the concentration. The correct alignment of methyl orange along the rod-shaped CTAB micelles, which is assisted by the rigid structure of methyl orange, is thought to occur through hydrophobic interaction between the long alkyl chains of CTAB and the aromatic rings of methyl orange.

Figure 8

Time-dependent absorption spectra of (a) methyl orange; (b) the mixture of methyl orange and NaBH4 (0.4 mL, 0.675 M); and (c) the mixture of CTAB (10 μL, 0.2 M), NaBH4 (0.4 mL, 0.675 M), and methyl orange (2.0 mL, 0.27 mM).

Figure 9

Time-dependent absorption spectra for the catalytic degradation reactions of methyl orange in the presence of (a) gold nanorods with the aspect ratio of 4.3 (Au2); (b) gold nanorods with the aspect ratio of 5.0 with minor spherical particles (Au6); (c) mixture of spheres, rectangles, and bipyramids (Au 10); and (d) gold rectangular NPs (Au12).

Degradation reactions using Au NPs as catalysts proceed differently depending on Au NPs (Figure ). The fast reaction with Au2 completes within 7 min (Figure a), while the reaction with Au10 completes within 15 min (Figure c). The ratio between the number of moles of NaBH4 and methyl orange is 317, and it is not much different from other studies. For Au2, which are gold nanorods prepared by L-64 (17.9%) and AgNO3 (200 μL), the rate constant is 0.01058 s–1. For Au6, which are the mixture of spheres and rods prepared with L-64 (35.9%) and AgNO3 (200 μL), the rate constant is 0.00668 s–1. Au10 and Au12, which are prepared by F-68 (17.9%) with AgNO3 (100 μL) and F-68 (35.9%) with AgNO3 (100 μL), respectively, show the rate constants of 0.00455 s–1 and 0.00605 s–1. The size and shape of NPs affect the rate constants, and more importantly, the hydrophobicity surrounding gold NPs and Mn values of Pluronics are thought to affect the kinetics of catalytic reactions. As seen in Figure , the catalytic reactions do not commence abruptly after the addition of Au NPs when the hydrophobicity surrounding Au NPs increases, which implies that it takes time for methyl orange and NaBH4 to reach the surface of metal NPs through the coronas of Pluronics. This retardation occurs similarly in Au10 and Au12, and the rate constant is higher when 35.9% solution rather than 17.9% solution of Pluronic F-68 was used. This discrepancy is believed to be due to the surface planes of particles, as bipyramids in Au10 are constructed by stable {111} planes, and therefore the catalytic activities are reduced, while in Au12, rather unstable {100} planes are the surface planes of rectangles. Another aspect to consider is the roles of CTAB/Pluronics as surfactants to affect the redox potentials of metal NPs.[57] It is also suggestive that not just as the steric interferers by thick coronas in thecase of Pluronic F-68 but also the change of redox potentials of the surface of Au NPs exhibit different catalytic performances. Overall, the rigid structure along two aromatic rings connected by the azo group of methyl orange match sterically well with gold nanorods (Au2); this arrangement enables the concentrated alignment of methyl orange, and their surfactants are hydrophilic enough for the substrates to approach metal NPs quite easily to begin catalytic reactions without delays.

Figure 10

Plots of ln (C/C0) against the reaction time for the catalytic reaction of methyl orange in the presence of Au nanomaterials.

Conclusions

For the synthesis of gold nanorods, Ag(I) concentration has been known critical to the control of the aspect ratio. It is demonstrated in this study that Pluronic triblock copolymers with different Mn values and hydrophilicity can also determine the aspect ratio of gold nanorods. In addition, it is proposed that rather than the previously suggested mechanism of the evolution of cuboctahedral seeds with {111} planes into single crystalline rods by symmetry breaking, single crystalline gold nanorods and rectangles with sharp edges in this study are constructed from the cubes that are surrounded by {100} planes. Pluronic polymers in their highest concentration in aqueous solution prefer lamellar mesophases, and these layered lamellar templates are thought to contribute to the formation of the single crystalline particles with {100} planes in their confined geometry, while CTAB bilayers and Ag(I) are thought to guide the anisotropic growth. The degradation reactions of methyl orange dyes with these gold nanomaterials indicate the combining factors of the shape of gold nanoparticles, whether they are correctly aligned with the rigid aromatics of methyl orange, and the hydrophilicity and Mn values of Pluronic polymers which surround gold nanoparticles.

Experimental Section

Commercial chemicals including Pluronic L-31 (Mn ≈ 1100), L-64 (Mn ≈ 2900), F-68 (Mn ≈ 8400), F-127 (Mn ≈ 12 600), and F-108 (Mn ≈ 14 600) were bought from Sigma–Aldrich and were used without further purification. A JAC ultrasonic 1505 (150 W, 40 KHz) was used for sonication. TEM images were obtained with a Talos F200x G2 (FEI, Thermo Scientific) with an operating voltage of 200 keV. For measuring optical absorbance spectra of gold NPs (250–1200 nm), a JASCO V-770 spectrophotometer was used with a 1 cm cell path length and with a UV/vis bandwidth of 2.0 nm and an NIR bandwidth of 8.0 nm.

Preparation of Gold Nanomaterials

CTAB (5.0 mL, 0.2 M) and the corresponding aqueous Pluronic solution (5.0 g) according to Table are mixed together, and the total volume of the mixture will be near 10 mL. The mixture of two surfactants was vortex-mixed for 15 s, followed by sonication at 23 °C for 10 min. Then, the solution was placed in the oven for 1 h while the temperature was kept at 30 °C. The corresponding amount of AgNO3 (0.4 mM) according to Table was added, and the mixture was gently mixed by hands for 30 s. HAuCl4·3H2O (0.50 mL, 0.036 M) was added, and after around 1 min, ascorbic acid (0.050 mL, 0.568 M) was added and the mixture was gently mixed until the solution turns clean. The seeds were prepared by the literature method using NaBH4 as the reducing agent,[40] with the exception of the synthetic temperature at 23 °C. After 10 min of the aging, the seeds (10 μL) were added to the growth solution and the mixture was placed in the oven operating at 30 °C for 1 h. Gold nanomaterials were collected by centrifugation at 4000 rpm for 3 min for TEM sample preparation, or they were used as-synthesized for the catalytic reactions. Summarized synthetic conditions for each gold NP is shown in Table .

Table 1

Experimental Conditions for the Synthesis of Au Nanomaterials

sample	Pluronic copolymers	amount of AgNO3 (4.0 mM)	shape and dimension of Au nanomaterials (width × height)
Au1	L-31 (17.9%)	200 μL	rods, 19.5 nm (±2.35) × 59.1 nm (±8.75)
Au2	L-64 (17.9%)	200 μL	rods, 17.7 nm (±1.33) × 74.9 nm (±6.77)
Au3	F-68 (17.9%)	200 μL	rectangles, 26.1 nm (±3.66) × 52.9 nm (±5.82)
Au4	F-108 (17.9%)	200 μL	rectangles , 27.5 nm (±3.44) × 56.3 nm (±4.64)
Au5	F-127 (17.9%)	200 μL	rectangles, 28.6 nm (±4.09) × 54.3 nm (±4.92)
Au6	L-64 (35.9%)	200 μL	rods, 17.1 nm (±1.96) × 84.8 nm (±7.78)
Au7	L-64 (17.9%)	100 μL	rods, 33.8 nm (±6.04) × 73.2 nm (±9.64)
Au8	L-64 (17.9%)	300 μL	mixture of spheres and rods, rods have dimensions of 20.9 nm (±1.64) × 67.5 nm (±4.89)
Au9	L-64 (35.9%)	300 μL	mixture of spheres and rods, rods have dimensions of 26.3 nm (±3.41) × 86.5 nm (±9.28)
Au10	F-68 (17.9%)	100 μL	mixture of spheres, rectangles, and bipyramids
Au11	F-68 (17.9%)	300 μL	mixture of spheres and concave particles
Au12	F-68 (35.9%)	100 μL	rectangles, 47.6 nm (±5.38) × 67.1 nm (±7.13)
Au13	F-68 (35.9%)	300 μL	mixture of spheres and concave particles

Catalytic Degradation of Methyl Orange

The catalytic reaction of methyl orange with NaBH4 solution under Au NPs was monitored in a standard quartz cell with a path length of 1 cm. Aqueous Au NPs (10 μL), which was dispersed in CTAB and Pluronics in the as-prepared state after the synthesis before centrifugation, was added to aqueous methyl orange solution (2.0 mL, 0.10 mM) in a quartz cell. Then, freshly prepared aqueous NaBH4 solution (100 μL, 0.634 M) was added to start the reaction. The absorption spectra of methyl orange during the reaction were recorded every 60 s in the range from 200 to 600 nm at 23 °C.