nanoparticules d’or fonctionnalisées avec des
thioalcanes
Controlled 2D organization of gold nanoparticles in block
copolymer monolayers
Résumé
L’incorporation de nanoparticules d’or décorées de ligands organiques à l’intérieur d’une monocouche de PS-
b-PMMA est étudiée. L’emplacement des particules dans la matrice semble dépendre de la taille des
nanoparticules et de la longueur des ligands aliphatiques. Dans le cas de ligands relativement courts, les particules se comportent à la manière de sphères dures et leur incorporation dans la matrice polymère peut être expliquée qualitativement par des explications de nature entropique. Il existe trois morphologies observées. Les particules petites par rapport au rayon de giration du polymère hôte se dispersent de manière égale à l’intérieur des domaines de PS, alors que les plus volumineuses des particules étudiées forment des agrégats à la forme d’îlot. Les particules de taille intermédiaire présentent la morphologie la plus impressionnante, c’est-à-dire qu’elles sont ségréguées à l’interface PS-PMMA pour former des structures en anneaux. Comme ces particules ont suffisamment tendance à s’assembler à l’interface PS-PMMA la morphologie de la monocouche est modifiée à forte concentration en particules afin d’en permettre l’inclusion à cet endroit spécifique. Ainsi, les micelles de surface s’allongent afin de former des nano-brins avec l’augmentation de la concentration en particules.
Abstract
The organization of organic-capped gold nanoparticles in PS-b-PMMA monolayers is investigated. The preferred location of the particles within the block copolymer template is found to depend on both nanoparticle size and the length of the aliphatic capping agent. In the case of relatively short ligands, the particles behave as hard spheres and their incorporation in the polymer matrix can be qualitatively rationalized by entropic considerations. Three distinct arrangements are observed. Particles that are small, relative to the radius of gyration of the host polymer, evenly disperse within the PS domains, whereas the largest particles are considered form ordered, island-like aggregates. Particles of intermediate size exhibit the most striking arrangement, being relegated to the PS-PMMA interface to form organized ring structures. The tendency of these particles to assemble at the interface is sufficiently strong to force a modification of the polymer morphology to accommodate the particles at higher loadings. As the number of particles is increased, the
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Introduction
Block copolymers are known to undergo phase separation at the molecular level to form ordered arrays of self- assembled nanodomains. Block copolymer self-assembly has been extensively studied, and the ability to control the resulting morphology by tailoring the length and chemical nature of the blocks is well-documented. Although the majority of the scientific literature is dedicated to bulk samples, self-assembly at the air−water interface has also been investigated, particularly in the case of amphiphilic block copolymers. Several interesting monolayer morphologies have been reported, including that arising from the formation of surface micelles.4, 43 Surface micelles are formed by the aggregation of hydrophobic blocks to form hemispherical
domains, around which the surface active blocks spread to form a corona. The resulting well-defined, periodic structures have been investigated for several systems, including polystyrene-b-polymethylmethacrylate (PS-b- PMMA)8, 9 which forms surface micelles with a PS core and a PMMA corona.
In this paper, we report the organization of metallic nanoparticles (NPs) within PS-b-PMMA monolayers. Importantly, we demonstrate that the precise spatial distribution of the particles can be modified through particle size and the choice of capping ligand. The ability to control the location of metallic NPs within polymer matrices is relevant to several applications. For example, the introduction of metallic NPs into organic solar cells has been shown to significantly increase light harvesting efficiency as the result of plasmonic scattering.50, 52, 53, 68 The influence of the particles on device performance will clearly depend on the precise
location of the particles within these multicomponent systems. A second example of the importance of ordered NP assemblies involves plasmon coupling. The photonic properties of such assemblies depend both on the distance between neighboring particles and the symmetry of the array. Particularly pertinent to the results presented here is the appearance of new coupled modes, including collective magnetic modes, for particles arranged in rings.69, 70 These novel magnetic modes correspond to a resonant circulation of displacement
current and open the door to the possible development of negative refractive index materials, or so-called metamaterials, of interest for revolutionary applications such as superlens and cloaking.71
Although mixtures of self-assembling block copolymers and inorganic NPs have been studied in film form,72-78
few experiments of this sort have been conducted at the monolayer level on a Langmuir trough.19, 21 The
present study differs from previous reports in that the impact of varying NP size and NP loading is considered in a systematic way. Importantly, these parameters are found to influence particle distribution and a number of well-defined particle assemblies are identified. Through this work, we hope to develop a new effective way to pattern NPs in a hierarchical bottom-up manner via Langmuir−Blodgett transfer using the subjacent copolymer nanostructured monolayer as a template. On a more fundamental level, the results presented here are not restricted to Langmuir−Blodgett assemblies but could also be relevant to composite copolymer nanostructures in general.
Experimental Section
Gold Core Synthesis and Surface Passivation
Gold nanoparticles of three different sizes were obtained via different synthetic routes. The smallest NPs of diameter 2.0 ± 1.0 nm were prepared by the Brust-Schiffrin phase transfer method.66 Gold(III) chloride
trihydrate, HAuCl4·3H2O, (100 mg) was dissolved in 9 mL of Nanopure water (18.2 mΩ·cm) and added to an
organic phase composed of 0.5 g of tetraoctyl ammonium bromide (TOABr) and excess alcanethiol ligand (about 0.1 mL) dissolved in chloroform. Under magnetic stirring, a solution of 100 mg of sodium borohydride (NaBH4) in 4 mL of water was added. Stirring was maintained overnight. The organic phase was isolated and
washed twice with 0.1 M sulphuric acid and twice with Nanopure water in a separatory funnel. The NPs were isolated from the organic phase by addition of methanol followed by centrifugation (17 000 rpm). Particles were washed several times by suspension in a minimal quantity of chloroform, followed by the addition of methanol and centrifugation.
Particles of intermediate size were obtained via a modified version of the above procedure.11 The same
quantities of gold salt, phase-transfer agent (TOABr), and reducing agent (NaBH4) were employed. The
alcanethiol ligand, however, was introduced by ligand exchange only after the reduction and washing of the particles. Ligand exchange was carried out by the addition of excess alcanethiol to a chloroform suspension of the particles. NPs were washed as described above. With this synthesis, NP diameter and polydispersity varied slightly from one sample to another and with the different capping agents. For the sake of clarity, we will refer to these particles as having a mean diameter of 5.8 ± 2.3 nm, which is calculated from the ensemble of intermediate size samples.
The third NP sample was synthesized using oleylamine (OLA) as both the reducing and capping agent.79, 80
Gold(III) chloride trihydrate (20 mg) was dissolved in 50 mL of Nanopure water (18.2 mΩ·cm) with magnetic stirring. The solution temperature was adjusted to 50 °C with an oil bath. Oleylamine (250 μL) was then added and the temperature raised to 80 °C for 2 h. NPs were extracted from the aqueous phase by the addition of 250 mL of chloroform and 8 mL of concentrated NaOH to the reaction mixture. The organic phase was isolated with a separatory funnel. The NPs were isolated by the addition of methanol followed by centrifugation (12 000 rpm) and washed as described above. Ligand exchange was achieved by the addition of excess alcanethiol to a concentrated NP suspension in chloroform. The mixture was stirred overnight. NPs were washed as described above. NPs with a diameter of 10 ± 2 nm were obtained.
Ligand Selection
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ligands for the preparation of stable suspensions, 5.8 nm diameter NPs could not be dried and redispersed when capped with butanethiol. Similarly, 10 nm NPs could only be redispersed when coated with octadecanethiol. In all, six different NP populations, illustrated in Figure 37, were available for study.
Figure 37. Schematic representation of the six nanoparticle populations investigated in this work. Ligand contour lengths are drawn to scale relative to particle size.
Thermogravimetric Analysis
The proportion of organic ligand relative to the inorganic core was measured by thermogravimetric analysis. A TA Instruments model Q5000 was used for the TGA measurements. Thermograms were recorded under nitrogen flow. Samples exhibited a ligand footprint of 16 ± 3 nm Å2/ligand, which is in agreement with literature
values for spherical gold NPs of this size.81 This indicates that the surfaces of the particles are saturated with
closepacked aliphatic chains and that no free residual thiol is present in the samples.
Polymers
PS-b-PMMA samples were obtained either commercially from Polymer Source Inc. (Montréal, Canada) or by in-house synthesis.82 Characteristics of the three near-symmetrical block copolymers investigated are provided
in Table 12. The majority of experiments were carried out with the 104 000 g/mol copolymer and, unless otherwise specified, all references to PS-b-PMMA indicate this sample. The two other copolymers are denoted by mention of their total molecular weight. A PMMA homopolymer from Polymer Source with a mean molecular mass of 45 500 g/mol was used in contact angle experiments.
Table 12. Characteristics of the PS-b-PMMA Samples
Total PS block PMMA block
28 000 13 200 14 800 1.1 synthesis
104 000 50 000 54 000 1.1 Polymer Source
220 000 112 000 108 000 1.1 synthesis
Polydispersity index Mean molecular weight (g/mol)
Composite Langmuir−Blodgett Films
Monolayers were spread on KSV 3000 Langmuir trough from chloroform solutions containing both PS-b- PMMA and NPs. Solutions were made using HPLC grade chloroform (Laboratoires MAT Inc., Québec, Canada). A PS-b-PMMA concentration of 1 mg/mL was employed for all samples. Except for the samples depicted in Figure 39 and Figure 43, a NP concentration of 0.5 mg/mL was used for the 2.0 nm NPs and of 1 mg/mL for the other particles. Solutions were deposited drop by drop in a grid pattern on Nanopure water surface (18.2 mΩ·cm). Five minutes were allowed before compression for the solvent to evaporate. The surface film was symmetrically compressed by mobile barriers advancing at a speed of 10 mm/min. Monolayer films were transferred at a constant surface pressure of 17 mN/m to carbon-coated TEM grids glued on glass substrate with a lift-off speed of 0.5 mm/min.
Transmision Electron Microscopy
TEM images were obtained with a JEOL 1230 microscope (80 kV). Particle counting and size analysis was carried out with ImageJ analysis software.
Atomic Force Microscopy
AFM analysis was carried out with a Digital Instruments NanoScope MultiMode in tapping mode. Silicon cantilevers (MikroMasch) with a characteristic resonance frequency of 325 kHz and a force constant of 40 N/m were used. Tips had a radius of curvature of 10 nm.
Contact Angle Measurements
Contact angles of a sessile water drop deposited on LB films transferred to glass substrates were measured with a FTA200 model goniometer by First Ten Angstrom. Seven replicates were measured for each sample to obtain a mean contact angle value.
Results and Discussion
NPs Capped with C8SH
TEM images of PS-b-PMMA surface micelles, containing gold NPs coated with octanethiol are shown in Figure 38. The distribution of the particles within the block copolymer matrix depends on particle size. The smaller (2.0 nm) particles are found to disperse evenly within the PS cores (Figure 38a,b), whereas the larger (5.8 nm) particles tend to segregate to the PS/PMMA/air triple junction interface (Figure 38c,d).
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Figure 38. TEM images of C8SH stabilized gold NPs incorporated in a PS-b-PMMA monolayer. Gold core diameter is 2.0 nm in
panels a and b and 5.8 nm in panels c and d.
NP Size and Entropy
At first view, the dependence of particle distribution within the polymer film on particle size illustrated in Figure 38 could be explained by entropic considerations.72, 74, 83 The incorporation of NPs within the PS domains
requires the deformation of the polymer chains as they stretch around the particles. This results in a loss in conformational entropy that is dependent on particle size; the smaller the particles, the smaller the entropic loss. This line of thought could explain why the Brust-Schiffrin NPs can be evenly dispersed within the PS domains whereas the larger, 5.8 nm, NPs are excluded to the PS-PMMA interface as the solvent evaporates from the swollen PS domains. As discussed below, however, particle size is not the only parameter that determines the distribution of the NPs within the copolymer matrix.
Effect of NP Loading in the Case of Interfacial Assembly
The regular assembly of the C8SH-capped 5.8 nm gold NPs at the interface between the PS core and the
PMMA corona of the surface micelles is arguably the most striking feature of this study. We investigated this system further by modifying particle loading in the composite Langmuir monolayers. The TEM images of Figure 39 indicate that, as loading is increased, a further interesting phenomenon is observed. The initially circular PS domains deform to nanostrands. The NPs effectively play the role of a surfactant in reducing the interfacial tension between the PS and PMMA blocks. The tendency of particles to assemble at the interface is sufficiently strong to force a modification of the polymer morphology to accommodate the particles.
Figure 39. TEM images C8SH-capped 5.8 nm gold NPs incorporated in a PS-b-PMMA monolayer at different NP loadings.
Surface concentrations are 320 NPs/µm2 in panel a, 730 NPs/µm2 in panel b, and 1900 NPs/μm2 in panel c.
NPs Capped with C18SH
TEM images of composite films containing NPs coated with a longer chain ligand (octadecanethiol) are presented in Figure 40. In this case, three different particles sizes can be considered since the C18 chain is
long enough to stabilize suspensions of the largest (10 nm) particles. When capped with C18SH, the gold
nanoparticles are incorporated within the PS domains for all three particle sizes considered here. Differences are, however, observed in the exact way in which the NPs are distributed. In the case of the smaller gold particles (2.0 and 5.8 nm), all of the PS domains are occupied by a similar number of particles and no particle ordering is observed. In the case of the larger 10 nm particles, the distribution of the gold particles among the PS domains is less regular. Some domains contain a relatively large number of particles, whereas many PS domains remain empty. This pattern is similar to what is observed by Li et al. for a similar system.21
Furthermore, within the more highly occupied PS domains, the 10 nm gold nanoparticles are observed to form ordered arrays.
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NP Size and Aggregation Due to van der Waals Interactions
The formation of 2D close-packed hexagonal aggregates, as illustrated in Figure 40f, can be attributed to interparticle van der Waals attractions. These forces increase with increasing particle size. The ordered arrays observed for the 10 nm NPs indicate that, for particles of this size, van der Waals attractions are present at distances extending beyond the C18SH ligands. In fact, similar aggregates have been reported for similar
particles spread alone (in the absence of copolymer) at the air−water interface.13 When cospread with the
block copolymer, NP aggregation probably occurs upon deposition of the spreading solution. The copolymer molecules then self-assemble to form empty surface micelles or to surround preexisting NP aggregates. This process most likely begins as early as solvent evaporation and continues through barrier compression as the film become denser. In this case, there is no apparent interaction between the PS-b-PMMA matrix and the particles; NP aggregation is independent of monolayer formation. The conclusion that NP ordering occurs before that of the copolymer is supported by the observation that the occupied island-like PS domains are significantly larger than the unoccupied self-assembled surface micelles.
Ligand Length Effect on Interaction with PS from the Matrix
The length of the alkyl chain of the capping ligand also influences the distribution of NPs within the polymer matrix.84 This is illustrated by comparing the results obtained with the 5.8 nm diameter gold cores capped with
either C8SH or C18SH (Figure 38c,d and Figure 39c,d). Although, in both cases the NPs are evenly distributed
among the PS domains, their location within the domains is very different. C8SH-capped particles are
relegated to the periphery of the PS domains whereas C18SH capped particles are dispersed, in a disordered
fashion, within them. This difference can be attributed to the increased free volume at the surface of the thicker capping layer. In the case of a spherical particle, the effective grafting density decreases with distance r from the particle surface according to σeff = σo(rc/r)2 where σo is the grafting density at the particle surface and rc is the core radius.67 As the length of the ligand alkyl chain is increased, the decrease in lateral packing density
permits increased interaction with the polystyrene chains of the matrix. Whereas in the case of hard spheres, entropy considerations predominate leading to exclusion of the metal nanoparticles, the longer ligand allows for more favorable interaction with the matrix and NPs tend to be more randomly dispersed within PS domains. Densely packed C8SH ligands can thus be considered to form a relatively hard organic shell around the NP,
while C18SH ligands constitute a softer coating.85 Therefore, in order to isolate the effect of NP size alone on
the nature of particle integration in the polymer matrix, it is important to compare particle populations with dense lateral packing of the ligands. This can be achieved by considering only the shortest possible ligand for each particle population. In the present case, a maximum ratio of gold core radius to ligand contour length of about 3 is found. With a higher ratio (larger particles or shorter ligands), NPs coalesce irreversibly upon drying
under vacuum and cannot be redispersed in CHCl3. This limit corresponds to C18SH ligands for the 10.0 nm
NPs and to C8SH for the 5.8 nm NPs.
NPs Caped with C4SH
To complete the series, composite monolayers were prepared from 2.0 nm NPs capped with C4SH. TEM
images of the resulting films are provided in Figure 41.
Figure 41. TEM images of C4SH-stabilized gold NPs incorporated in a PS-b-PMMA monolayer. Gold core diameter is 2.0 nm in
panels a and b.
With a diameter of 2.0 nm, these NPs are smaller than the thickness of the PS domains, and it is thus unclear, from TEM images alone, whether they are located at the PS/air interface or dispersed within the PS domains. Discrimination between the two possibilities, illustrated in the schematic representation of Figure 42, is nontrivial because of the very small dimensions involved. AFM measurements indicate that the PS domains have a height of 6 nm and there is thus only 3 nm between the domain center and PS/air interface.
Figure 42. Schematic cross-sectional representation of a PS domain illustrating the two possible locations for the 2 nm gold NPs: at the domain surface or dispersed within it.
Two indirect experimental methods were employed to probe NP location. In the first, NP loading was varied to establish its effect of the morphology of the copolymer matrix. If the NPs were preferentially located at the PS/air interface, one would predict an increase in the surface density of NPs on top of the PS domains with no change in the dimensions of the surface micellar morphology. In the case of incorporation of the NPs within the PS, the size of the domains can be expected to increase with increased particle loading. The results are