New Aspects of the Gold Nanorod Formation Mechanism via Seed

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New aspects of the gold nanorod formation mechanism via seed-mediated methods revealed by MD simulations José Silva, and Mario Roberto Meneghetti Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03703 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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New aspects of the gold nanorod formation mechanism via seed-mediated methods revealed by MD simulations

José Adriano da Silva and Mario R. Meneghetti* Grupo de Catálise e Reatividade Química - GCaR, Instituto de Química e Biotecnologia da Universidade Federal de Alagoas, Av. Lourival de Melo Mota, s/n, Cidade Universitária, CEP 57072-970, Maceió-AL, Brazil

[email protected]

ACS Paragon Plus Environment

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Abstract New aspects of the formation and growth mechanism of gold nanorods (AuNRs) during seedmediated colloidal synthesis are revealed from the results of molecular dynamics simulations. The model systems consist of cetyltrimethylammonium bromide (CTAB) units adsorbed on lowindex (Au(110), Au(100) and Au(111)) and high-index (Au(250)) gold surfaces. The CTAB units are adsorbed as adjacent cylindrical micelles when the relative number of adsorbed bromide ions is small. At later AuNR growth stages, the number of bromide ions increases as the [AuBr2]− species pass through the channels between the adsorbed micelles on the gold surface. Thus, the mature AuNRs have a high concentration of bromide ions at their surface, which appears to change the organization of the CTAB units on the particle surface from adsorbed micelles to a compact CTAB bilayer.

Keywords: molecular dynamics, nanoparticle formation mechanism, gold nanorods, cetyltrimethylammonium bromide, gold surface, micelles


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Introduction Anisotropic metal nanoparticles are characterized by several unique properties that originate from nanoparticle size effects due to their high surface areas and electron confinement. In particular, gold nanorods (AuNRs) exhibit remarkable properties and are promising for numerous applications.1 The most common and successful strategy for synthesizing AuNRs is thought to be seedmediated colloidal methods, which were originally used by Murphy et al. in 2001 to produce penta-twinned AuNRs.2 This method was later modified by El-Sayed et al. in 20033 to produce single-crystal AuNRs in higher yields (see Table 1).4 The crystallographic facet indices of the AuNRs obtained by both methodologies are depicted in Figure 1A.5,6 Table 1. General details for preparing AuNRs via seed-mediated synthesis. Synthesis

Seed solution

Seed shape

method Decahedral

Crystallinity

Aspect ratio

solution

of the AuNRs

(AR)

Ascorbic

Penta-twinned

6 to 20

ca. 15%

1.5 to 5.0

ca. 90%

Murphy and

NaBH4,

coworkers2

HAuCl4 and

acid,

with

citrate

HAuCl4 and

pentagonal

CTAB

cross-section

Ascorbic

Single-crystal

Cuboctahedral

El-Sayed and

NaBH4,

coworkers3

HAuCl4 and

acid,

with octagonal

CTAB

HAuCl4,

cross-section

CTAB and AgNO3 a

Selectivitya

Growth

Relative to the number of nanoparticles formed.


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Figure 1. (A) Crystallographic facets of different types of AuNRs: (1) intermediate growth stage of a single-crystal AuNR, (2) mature single-crystal AuNR, and (3) mature penta-twinned AuNR. (B) Illustration of the AuNR zipping-like growth mechanism. (C) Illustration of the role of the soft template during the growth process. Briefly, in the seed-mediated synthesis of AuNRs, the growth of these anisotropic nanoparticle is mediated by the addition of metallic gold atoms at the surface of previously prepared small gold nanoparticles (seeds), by a controlled reduction of Au(I) species at the


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surface of the seeds that growth in an anisotropic way due to the presence of CTAB as a growthdriving agent.2,3,7 Indeed, many studies of these standard synthesis methods and adaptations thereof have appeared in the literature. In particular, the effects of the seed size,8 cosurfactants,9 additives,10 temperature,11,12 surfactant chain length,13 and headgroup structure14 have been investigated. Because the properties of AuNRs depend on their shape, the particle symmetry breaking and anisotropic growth mechanism must be understood to optimize the synthesis conditions and thus improve the shape and size uniformity of the particles. Accordingly, several research groups are focused on understanding the synthesis mechanism. Despite advances in elucidating the formation mechanism of AuNRs, especially the development of mechanistic models that are consistent with the experimental data, some gaps in understanding still remain.4 In 2003, Murphy and coworkers15 proposed the first model explaining the origin of the anisotropic growth mechanism of colloidal AuNRs, i.e., the zipping-like growth mechanism. It states that the CTAB surfactants preferentially adsorb on the lateral facets of AuNRs. An AuNR begins to develop anisotropy when a CTAB bilayer forms on the lateral sides of the growing particle. The bilayer hinders the access of the Au(I) species to the gold surface, where they are reduced, whereas the Au(I) species can still diffuse to the tips of the AuNR (Figure 1B). Jana16 proposed another model for the anisotropic growth of AuNRs, namely, the soft template model. In this model, elongated CTAB micelles in the growth solution generate a template into which seeds with diameters of 1-3 nm can penetrate. Then, the shape of the micelle, i.e., the soft template, induces the anisotropy (Figure 1C). According to this model, the temperature and CTAB concentration during the synthesis must be within a certain range to generate elongated micelles that induce the formation of AuNRs. Additionally, additives that can


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promote micellar transitions to more elongated CTAB micelles, for example, salicylate aromatic additives,10 can be used to fabricate AuNRs with a higher AR. Thus, based on this model, seeds with diameters larger than 3.5 nm cannot generate AuNRs, explaining the observation that AuNRs cannot be fabricated from large seeds, because they are larger than the micelle template. It is essential to determine how CTAB is adsorbed and structurally organized on the surfaces of growing and mature AuNRs to understand the nanoparticle growth mechanism. Analogous CTAB surfactants with a shorter alkyl chain (CnTAB, 10 ≤ n < 16) were used to drive growth in the penta-twinned AuNR synthesis and led to AuNRs with smaller ARs. Using the zipping-like mechanism, these results were explained by the different capacities of these surfactants to generate a stable bilayer structure on the lateral surfaces of the AuNRs; the longer chains provided greater stability and thus generated AuNRs with higher ARs.13 Hafner and coworkers used surface-enhanced Raman spectroscopy (SERS) to confirm the existence of Au−Br− interactions and show that the arrangement of the adsorbed CTAB on the gold surface depends on its concentration.17 They verified that the adsorbed CTAB forms a “collapsed bilayer" at relatively low CTAB concentrations by detecting interactions between the alkane chains of the CTA+ moiety and the AuNR surface. In contrast, an organized bilayer pattern was proposed to exist at higher CTAB concentrations. It should be noted that separating and redispersing the AuNRs generated in this system in a dilute aqueous solution of CTAB resulted in the formation of a “collapsed bilayer” of CTAB on the particles. Gomez-Graña and coworkers used transmission electron microscopy (TEM), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) to measure the thickness of the CTAB structure adsorbed on single-crystal AuNR. The measured value of 3.2 ± 0.2 nm is


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significantly smaller than twice the extended surfactant chain length (4.34 nm), suggesting significant tilting or interdigitation of the hydrocarbon chains of the surfactants.18 In both seed-mediated synthesis protocols,2,3 the concentration of CTAB in the growth solution is generally 0.1 M; however, the CTAB second critical micelle concentration in an aqueous solution is 0.27 M,19 meaning cylindrical micelles are not present in the growth solution. Walsh and coworkers20 showed that when a standard synthesis protocol for single-crystal AuNRs is employed, i.e., when silver is present, the symmetry breaking only occurs in seeds that are 4-6 nm in diameter. In a 0.1 M CTAB solution, the micelles are ellipsoidal, with minor and major axes of 2.6 and 4.4 nm, respectively.21 Then, the micelles in the solution under standard synthesis conditions are smaller than or at least approximately the same size as the seed particles and cannot serve as templates during growth. Thus, evidence supporting the soft template mechanism is lacking, and it is not currently accepted as an explanation of the anisotropic growth. Vaia and coworkers mapped the crystallographic facets formed during single-crystal AuNR growth and divided the growth process of the particles into five stages based on the results.22 Thus, using the zipping-like growth mechanism as the main model, it is assumed that a bilayer exists on the lateral facets of the growing AuNR in all the stages. It should be noted, however, that this assumption is supported by experimental studies of mature AuNRs. In fact, the existence of a bilayer coating during all the growth stages has not been verified experimentally. The existence of a compact bilayer during the entire growth process conflicts with the experimental results, because growth occurs at the side facets, although to a lesser degree than at the tips, in the early stages.22 However, in the later stages of the growth process, the formation of a compact bilayer on the lateral facets appears to occur, making it difficult for the most abundant source of gold, i.e., [AuBr2]−, to pass through the hydrophobic core of the CTAB bilayer.23


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Sulpizi and coworkers24,25,26 used molecular dynamics (MD) simulations to identify the CTAB adsorption modes on low-index and high-index gold surfaces. They observed adjacent cylindrical micelles on all the surfaces, as well as channels (called intermicellar channels) that allow the molecular gold source to flow to the surface of the AuNRs. According to these researchers, the width of these channels depend on the facet on which the micelles are adsorbed and decreases in the following order: Au(111) ˃ Au(110) ˃ Au(100) ˃ Au(250). Thus, the Meena-Sulpizi growth mechanism states that a series of cylindrical CTAB micelles, not a CTAB bilayer, adsorb on all facets of the gold surface, resulting in the formation of channels filled with water molecules. Indeed, the presence of these channels is consistent with the growth process during the early stages of AuNR formation, because both the lateral facets and the tips of the AuNRs grow, although at different rates. However, the adsorption of adjacent cylindrical CTAB micelles observed in the simulations is contradictory to the experimental results,17,27,28 which confirm the existence of a bilayer structure. Nevertheless, it should be noted that all the experimental data confirming the presence of a compact bilayer of CTAB units were obtained using mature AuNRs. Furthermore, the number of bromide ions adsorbed on the surfaces in the Meena and Sulpizi simulations (Br/Au ratio of ca. 0.10) is smaller than that measured experimentally for AuNRs.24,25,26 Using XPS (X-ray photoelectron spectroscopy), Zhang and coworkers29 found that the Br/Au ratio on AuNR surfaces is ca. 0.17. This difference in the simulated and experimental Br/Au ratios motivated this study to determine the influence of this ratio on the formation of a bilayer of CTAB in MD simulations. Therefore, the adsorption of CTAB on gold surfaces is reexamined using MD simulations.30 Previous theoretical studies typically employed an initial configuration of the


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CTAB species in which the CTA+ cations are perpendicular to the gold surface and the bromide anions are on top of the CTA+ bilayer, dispersed in the aqueous solution.24,25,26 During the simulations, the migration of bromides to the gold surfaces is monitored. In this study, systems with a number of bromides inserted at the gold/CTA+ interface were constructed. In particular, the Br/Au ratio found by Meena and Sulpizi (0.1 Br/Au) and a slightly larger ratio than that measured experimentally (0.2 Br/Au) were employed. The systems were constructed with CTAB adsorbed on different surfaces, namely, the low-index Au(110), Au(100) and Au(111) surfaces and the high-index Au(250) surface, with bromides at the gold/CTA+ interface in ratios of 0.1 Br/Au (low concentration of bromides) and 0.2 Br/Au (high concentration of bromides). In this work, we demonstrate that the adsorption patterns observed in the simulations are consistent with the experimental measurements, and the results are used to propose the growth mechanisms of the penta-twinned and single-crystal AuNRs. It should be noted that the role of the silver, which is present during the production of single-crystal AuNRs, was not investigated in this study (silver was not included in the simulation models), which focuses on the gold surface/bromide/surfactant/bromide/water interactions. This analysis is reasonable because Funston and coworkers demonstrated that the presence of silver nitrate in the medium during the synthesis of single-crystal AuNRs only affects the symmetry-breaking event.31

Computational Methodology System setup


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The topology and atomic parameters for the CTA+ and Br− units32 used in the simulations were validated in our previews work;30 they were shown to lead to structures exhibiting CTAB/gold surface interactions and CTAB micelles in aqueous solution and to predict the counterion dissociation degree. The parameters for the gold surface33 were used in previous simulations (Table 2).24,25,26,30

Table 2. Lennard-Jones parameters used for the bromide ions and gold atoms. Species

C6 (kJ·mol−1·nm6) × 10−2

C12 (kJ·mol−1·nm6) × 10−5

Br−

5.8243749

5.10031

Au

2.92057

0.964326

The initial configurations (input) for all the simulations included: i) a gold surface, which could be any of the different facets, ii) a specific number of CTAB units, with the bromide ions distributed below or above of the CTA+ bilayer (Br-1 and Br-2, respectively), and iii) a suitable number of water molecules (see Figure 2). More specifically, for the simulations of the Au(250) surface, two different amounts of Br-1 (Br-1/Au molar ratios of 0.2 and 0.1) were placed on the gold surface. For the low-index Au(110), Au(100), and Au(111) surfaces, a 0.1 Br-1/Au ratio was employed, and for the Au(100) surface, a 0.2 Br/Au ratio was also used. The CTA+ packing density in the simulations was slightly higher than that calculated from the cross-sectional area of the CTA+ headgroup (0.32 nm2).34 Each system was filled with SPC water molecules35 and the number of the bromide ions (Br-2) required to neutralize the charged polar groups of the bilayers. The characteristics of the systems are summarized in Table 3.


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(A)

(B)

Figure 2. Initial configurations of the surfactant bilayer on the gold surfaces. (A) 0.1 Br/Au and (B) 0.2 Br/Au. Gold atoms (yellow), nitrogen atoms (blue), hydrophobic tail (gray), Br-1 (magenta) and Br-2 (green). The water molecules were omitted for clarity.

Table 3. Parameters of the simulated systems. Type of surface

Number of gold atoms

Numbers of bromide-1 and bromide-2a

Number of water molecules

CTA+ units

Box dimensions x/y/z (nm)

Au(250) Au(250) Au(100) Au(100) Au(111) Au(110)

1310 1310 1600 1600 1332 1400

40 and 156 80 and 116 40 and 156 80 and 116 40 and 156 40 and 156

4141 4181 3785 3785 3955 3782

196 196 196 196 196 196

4.00/4.07/16.39 4.00/4.07/16.39 4.10/4.10/15.61 4.10/4.10/16.61 4.10/4.10/15.60 4.08/4.08/15.90

a

The total number of bromide ions in all the calculations was 196.

Simulation procedures


Thickness of the gold surface (nm) 1.59 1.59 1.43 1.43 1.26 1.45

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The energies of the initial configurations were minimized by running 5000 steps of a steepest descent procedure. The systems were simulated under isothermal-isobaric NPT conditions, and periodic boundary conditions were applied to the rectangular boxes in all directions. During the equilibration and production phases, the leap-frog algorithm was used to integrate Newton’s equations of motion with a time step of 0.002 ps. Each simulation trajectory was 200 ns long. The center of mass motion was removed every 5 steps. The Berendsen thermostat was used to maintain the system temperature at 300 K by independently coupling the temperatures of the solute and solvent using a time constant of 0.4 ps.36 The pressure was maintained using the Parrinello-Rahman barostat by weakly coupling the particle coordinates and box dimensions to a pressure bath of 1.0 bar.37 Anisotropic coordinate scaling was used with a relaxation time of 0.4 ps and a compressibility of 4.5 × 10−5 bar−1, which is appropriate for water. No bond constraints were applied during the simulations. A generalized reaction field correction and a cutoff of 1.4 nm were used for both the van der Waals and long-range electrostatic interactions, and the permittivity (dielectric constant) was 66.38 In all the simulations, the pair lists for the short-range nonbonded and long-range electrostatic interactions were updated every five time steps. The configurations of the trajectory were recorded every 1 ps. The time-dependent distributions of nitrogen, bromide-1, bromide-2, the last carbon of the hydrophobic tail, and the water molecules were derived from the MD simulations and analyzed. GROMACS 4.5.5 was used for all the MD simulations and trajectory analyses.39 The VMD software version 1.9.1 was used to visualize the trajectories and prepare the figures.40

Results and Discussion


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Simulation of CTAB adsorption on the Au(250) surface with 0.1 Br/Au In this simulation, the initial configuration has a relatively low number of bromide ions (Br-1) near the gold surface, compared to the total number of bromide ions (Br-1 + Br-2). After the simulation begins, adjacent cylindrical micelles of CTAB are formed on the gold surface, and these structures are observed until the end of the simulation (see Figure 3A). Figure 3B shows the water molecule arrangement near the gold surface. Clearly, the space between the adsorbed CTAB micelles, i.e., the intermicellar channels, are filled with water molecules. The densities of nitrogen, bromide-1, bromide-2, the last carbon of the hydrophobic tail, and the water molecules were measured along the axis perpendicular to the surface (estimated for the last 50 ns of the simulation). These results are presented in Figure 3C. A density peak is observed for the last carbon of the CTA+ chain (indicated by the black circle), suggesting constant carbon-tail adsorption on the gold surface. This interaction was experimentally detected by Hafner and coworkers,17 and the related adsorption structure corresponds to the formation of adjacent cylindrical CTAB micelles at the surface, which the authors referred to as a "collapsed bilayer". The distribution of nitrogen atoms permeates the intermicellar channel walls. The Br-1 ions remain close to the surface, whereas the channel is filled with the Br-2 ions (Figures 3A and 3C). (A)

(B)


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(C) → Nitrogen Bromide-1 ∗ Bromide-2 Last carbon of the tail Water

Figure 3. (A) Snapshot of the MD simulation of CTAB adsorption on the Au(250) surface with 0.1 Br/Au. The yellow and dark-blue spheres represent gold and nitrogen atoms, respectively. The gray lines are the apolar tails. The bromide-1 and bromide-2 ions are represented by magenta and green spheres, respectively. The water molecules were omitted for clarity. (B) Snapshot of the MD simulation of water molecules on the Au(250) surface. (C) Measurements of the densities of nitrogen (blue), the last carbon of the hydrophobic tail (gray), bromide-1 (magenta), bromide-2 (green) and the water molecules (red and white spheres) relative to the gold surface.

Simulation of CTAB adsorption on the Au(250) surface with 0.2 Br/Au In the simulation of CTAB adsorption on the Au(250) surface with a relatively high number of bromide ions on the gold surface, a bilayer structure of CTAB units is observed on the gold


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surface (see Figure 4A). Figure 4B shows that only a few water molecules reach the gold surface. This bilayer adsorption pattern of the CTAB units is described in the literature.17,27,28 In this simulation, the densities of nitrogen, bromide-1, bromide-2, the last carbon of the hydrophobic tail, and the water molecules were measured along the axis perpendicular to the surface (estimated for the last 50 ns of the simulation). The distributions of these elements are shown in Figure 4C. The general distributions of the two types of bromide ions (Br-1 and Br-2) are nearly the same as those in the initial configuration (see Figure 2 and Table 3), i.e., no migration occurs between the two layers of bromide ions. Based on the difference between the two highest densities of nitrogen atoms indicated in Figure 4C, the thickness of the adsorbed surfactant bilayer is estimated to be 3.20 nm. This value is in agreement with the thickness on the surface of gold nanorods measured by SANS (3.2 ± 0.2 nm).18 (A) (B)


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(C) Nitrogen Bromide-1 ∗ Bromide-2 Last carbon of the tail Water

→

Figure 4. (A) Snapshot of the MD simulation of CTAB adsorption on the Au(250) surface with 0.2 Br/Au. The yellow and dark-blue spheres represent gold and nitrogen atoms, respectively. The gray lines are the apolar tails. The bromide-1 and bromide-2 ions are represented by magenta and green spheres, respectively. The water molecules were omitted for clarity. (B) Snapshot of the MD simulation of water molecules on the Au(250) surface. (C) Measured densities of nitrogen (blue), the last carbon of the hydrophobic tail (gray), bromide-1 (magenta), bromide-2 (green) and the water molecules (red and white spheres) relative to the gold surface.

Simulation of CTAB adsorption on the Au(110), Au(100) and Au(111) surfaces with 0.1 Br/Au In these simulations of a relatively low concentration of bromide ions on the gold surfaces, the adsorbed CTAB units are organized as adjacent cylindrical micelles, and intermicellar channels are observed between them. These channels are filled with water molecules, as shown in Figure 5.


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(A)

(B)

(C)

Figure 5. Intermicellar channels observed in the MD simulations of low-index surfaces with 0.1 Br/Au. The yellow spheres represent gold atoms. (A) Au(100), (B) Au(111), and (C) Au(110). The water molecules are represented by red and white spheres.

Indeed, a similar pattern of CTAB adsorption was also described by Meena and Sulpizi on low-index gold surfaces.24 They also observed a similar Br/Au ratio on the gold surfaces after simulation. Moreover, they observed that the channel thickness depends on the facet; it decreases in the following order: (110) > (100) > (111). In these simulations, alkyl chain interactions on the gold surface, which were not detected in previous studies, are observed (see the distribution of the last carbon of the hydrophobic tail along the axis perpendicular to the surface for the last 50 ns of the simulation in Figure S1).


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Simulation of CTAB adsorption on the Au(100) surface with 0.2 Br/Au A relatively high number of bromide ions were placed on the gold surface in the initial configuration for this simulation. Consequently, the CTAB units are arranged in a bilayer structure on this gold surface (see Figure 6A). As shown in Figure 6B, the water molecules do not reach the gold surface due to the formation of a compact bilayer of CTAB units. This pattern is also observed for the Au(250) system with a relatively high content of bromide ions (see Figure 4). The distributions of nitrogen, bromide-1, bromide-2, the last carbon of the hydrophobic tail, and the water molecules along the axis perpendicular to the surface (estimated for the last 50 ns of the simulation) are presented in Figure 6C. The general distribution of the Br-1 ions indicates that they remain at the gold/CTA+ interface, and likewise, the Br-2 ions remain near the CTA+/aqueous solution interface (see Figures 6A and 6C). Furthermore, as shown in this figure, the difference between the two nitrogen peaks indicates that the thickness of the adsorbed surfactant bilayer is 3.43 nm in this case. This distance is slightly larger than that observed in the simulation of the Au(250) surface under similar conditions. Figure 6C also shows that the nitrogen distribution relative to the gold surface exhibits two main peaks in the number density at approximately 0.5 and 3.9 nm and a third, smaller density peak at approximately 1.0 nm (indicated by the black circle). These features might be attributed to the weaker binding interactions between the Br-1 ions and the Au(100) surface, if compared with these interactions with the Au(250) surface (see Table S1 and Figures S2 and S3), which enable greater mobility of the CTA+ polar groups. Thus, it is suggested that the density of the bilayer depends on the strength of the bromide interaction with the gold surface.


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(A)

(B)

(C) Nitrogen Bromide-1 ∗ Bromide-2 Last carbon of the tail Water

→

Figure 6. (A) Snapshot of the MD simulation of CTAB adsorption on the Au(100) surface with 0.2 Br/Au. The yellow and dark-blue spheres represent gold and nitrogen atoms, respectively. The gray lines are the apolar tails. The bromide-1 and bromide-2 ions are represented by


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magenta and green spheres, respectively. The water molecules were omitted for clarity. (B) Snapshot of the MD simulation of water molecules on the Au(100) surface. (C) Measurements of the densities of nitrogen (blue), the last carbon of the hydrophobic tail (gray), bromide-1 (magenta), bromide-2 (green) and the water molecules (red and white) relative to the gold surface.

Proposed growth mechanism for penta-twinned AuNR Figure 7 shows a model of the evolution of the crystallographic facets during penta-twinned AuNR growth.6 In Stage I, the seed symmetry breaks, and anisotropy develops. Here, the pentatwinned AuNR must grow by the mechanism described by Meena and Sulpizi,24 in which adjacent cylindrical micelles are adsorbed on the lateral and tip facets. The [AuBr2]− species reach the surface through the intermicellar channels.

Figure 7. Illustration of the growth stages of a penta-twinned AuNR during seed-mediated colloidal synthesis.


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Previous studies of CTAB adsorption patterns on penta-twinned AuNRs revealed the presence of a bilayer.27 Our simulation model shows that a higher concentration of bromides on the gold surface is necessary for the formation of a bilayer on the lateral (100) facet of mature penta-twinned AuNRs (Figure 6). Thus, it is suggested that bromide must accumulate on the gold surface during the growth of the particle. An increase in the number of bromides on the gold surface induces the rearrangement of the CTAB units on the gold particle, causing the adsorbed cylindrical micelles (stages II-III) to transform into a compact bilayer (stage IV). This hypothesis is reasonable because the continuous reduction of the Au(I) ions of the [AuBr2]− species on the gold surface releases bromide ions in loco.23

Proposed growth mechanism for single-crystal AuNR Figure 8 shows the single-crystal AuNR growth stages.22 The growth begins with seed symmetry breaking (stage I) and the adsorption of the CTAB units via the bromide ions on the Au(100) and Au(111) surfaces. Vaia and coworkers22 reported growth on the lateral facets of AuNRs during the early growth stages, which is consistent with the presence of intermicellar channels on the lateral facets (stages II-IV). Thus, at these stages, due to the low concentration of bromide ions on the gold surfaces, the adsorbed CTAB molecules form adjacent cylindrical micelles on all the facets, based on the results of Meena and Sulpizi24 and this study (see Figure 5).


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Figure 8. Illustration of the growth stages of a single-crystal AuNR during seed-mediated colloidal synthesis.

Again, because the flux of [AuBr2]− to the tips of the AuNR and their subsequent rate of reduction is higher, many bromide ions are released at the tips. This excess of bromide ions must move from the surface of the tips (Au(111) facets) to the sides of the rods (Au(100) facets), due to the higher affinity of the latter facet for bromide ions.41 The increasing concentration of bromide ions on the lateral facets causes the intermicellar channels to close, which sustains the anisotropic growth (stages II-IV). This event is caused by the reorganization of the CTA+ ions as the concentration of bromide ions, on the gold surface, increases. Finally, the formation of the high-index Au(250) facet is induced in stage V. This transition from the Au(100) to the Au(250) crystallographic facet is probably due to the reorganization of the gold atoms at the surface to generate a crystalline facet with a greater ability to stabilize bromide ions. Studies of the CTAB arrangement on mature single-crystal AuNRs reported the presence of a CTAB bilayer.17,28 The simulation results indicate that this structure appears when the


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concentration of bromides on the gold surface is high (Figure 4). Therefore, the viability of the proposed growth mechanism, which assumes that a transition from cylindrical micelles to a bilayer occurs, relies on the assumption that bromide accumulates at the gold/CTA+ interface. This assumption must be verified by experimental evidence to confirm the proposed mechanism. It should be noted that in the synthesis of single-crystal AuNRs, the majority of the Au(I) ions present during the growth are not completely reduced. This result must be due to the formation of a compact bilayer of CTAB units on all the facets of mature AuNRs, thus preventing the Au(I) species from accessing the surface of the gold particle.42

Conclusions An MD simulation model that captures the different arrangements of CTAB units adsorbed on various crystallographic facets of AuNRs in an aqueous solution was constructed. Based on the model, a detailed mechanism for colloidal AuNR formation in an aqueous phase that accounts for the effect of the number of bromides at the gold surface on the CTAB adsorption structure, which can be anchored cylindrical micelles or a compact bilayer, was proposed. The formation of adsorbed cylindrical micelles with intermicellar channels allows the [AuBr2]− ions to reach the gold surface and is therefore proposed to be the driving force of the anisotropic growth, in accordance with the results of previous theoretical studies.24,25 This mechanism is consistent with the growth of the particles (length and width) in the early stages and with the zipping-like growth mechanism in the later stages of AuNR fabrication. The driving force that separates the early and later stages of the AuNR growth process appears to be related to the concentration of bromide ions adsorbed at the gold surface. A relatively small number of bromide ions on the gold surface leads to the formation of adsorbed adjacent micelles of CTAB


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on it. As the concentration of bromide ions at the gold surface increases during the later stages of particle growth, the arrangement of CTAB units on the gold particle changes to a bilayer structure. This mechanism can apply to both methods of AuNR synthesis, i.e., for the singlecrystal and penta-twinned methods. It is concluded that the concentration of bromide ions at the gold surface during the reduction of the [AuBr2]− species must be known to understand the AuNR growth process in aqueous solutions. Furthermore, the change in the CTAB adsorption pattern during the growth process provides insight into the role that the surfactant plays in causing anisotropic growth. This work hopefully contributes to the understanding of the growth mechanism of AuNRs and sheds more light on the fascinating studies of nanoparticle growth.

Supplementary Information

Supporting Information Available: Graph of the overlap o the last carbon of the tail on low-index surfaces with 0.1 Br/Au; MD simulations of bromide adsorption on Au(250) and Au(100) surfaces; and Model details of the gold surface and sodium bromide solution. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments The authors acknowledge Dr. Santosh Kumar Meena at Johannes Gutenberg University, Germany, for kindly providing the atomic coordinates of the gold surfaces and Prof. Dr. Paulo A. Netz at Federal University of Rio Grande do Sul for important contributions in solving technical problems with the simulations, as well as, Prof. Dr. Thereza A. Soares at Federal University of


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Pernambuco to encouraging us to carry out theoretical studies on field of nanoparticle syntheses. The authors are grateful for the financial support of the Brazilian funding agencies CNPq, Capes, Fapeal, and INCT-Catalise and for the computational resources provided by the CenapadUnicamp. MRM thanks CNPq for the research fellowships.


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Table of Contents Graphic

Illustration of surfactant adsorption patterns at different stages of AuNR formation in aqueous solutions.


New Aspects of the Gold Nanorod Formation Mechanism via Seed

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