Article pubs.acs.org/Langmuir
Control of Gold Nanoparticle Superlattice Properties via Mesogenic Ligand Architecture Wiktor Lewandowski, Kamil Jatczak, Damian Pociecha,* and Jozef Mieczkowski Pasteura 1, Faculty of Chemistry, University of Warsaw, 02-093 Warsaw, Poland S Supporting Information *
ABSTRACT: Hybrid structures made of metal nanoparticles with liquid crystalline coating attract considerable attention due to their conspicuous self-assembly and potential synergistic properties. Here we report on a new structural parameter that can be used to control the formation of hybrid gold nanoparticles superlattice. A series of Au nanoclusters covered with mixed monolayers of alkyl and liquidcrystalline ligands were obtained. For the first time in such systems the lengths of both alkyl ligands and mercapto-functionalized alkyl spacers of the promesogenic molecules were varied. The physicochemical properties of the obtained materials were investigated by different instrumental techniques, such as X-ray photoelectron spectroscopy (XPS), small-angle X-ray diffraction (SAXRD), and transmission electron microscopy (TEM). Interestingly, the applied variations of the grafting layer composition enabled the formation of 1D (lamellar) and 3D long-range ordered structures with systematically changing thermal stability range. Such behavior is explained based on the structural parameters of the hybrid nanoparticles, namely the separation of the cores and ligand flexibility. This work gives some new insights into the nanoparticle self-assembly subject and points out the critical parameters controlling the degree of order within the self-assembled superstructures.
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INTRODUCTION The bottom-up approach to the nanoparticle (NP) synthesis is a powerful tool to control the matter at nanometer scale. Within this strategy a plethora of methods, utilizing atomic or molecular species as substrates and yielding NPs with predefined size, shape, constitution and consequently controlled properties, have been developed.1,2 Over the past few years it was shown that the NPs themselves can be used as building blocks in the bottom-up approach toward obtaining larger scale structures.3 This fact opened an access to the ordered arrays of nanoparticles, termed nanoparticle superlattices (NP-SL), which are currently a hot topic in the nanoscience research due to their unique, collective properties3 and perspective applications in electronic and optoelectronic devices.4 Because of their high metal density, it has been proposed that the superlattices could find applications as metamaterials.5 Noticeably, the properties of the NP-SL depend not only on the shape and size of the building blocks but also on the configuration of the assembly6 and can be controlled via a rational design of the nanoparticles. Various methods have been employed for preparing ordered nanoparticle structures.3,7 For small monodisperse particles the close-packed structures, fcc and hcp, of NP-SL are most commonly observed.8 Nevertheless, new strategies are still being sought for preparing superlattices with desirable properties such as thermal stability or reconfigurability.9 This is why recently the scientists attention has turned to the idea of equipping the NP soft corona with liquid crystalline molecules, © 2013 American Chemical Society
as it may be the key element for enabling changes in the overall NP morphology leading to anisotropic aggregate formation. To this day, various 2D and 3D structures made of 2−11 nm diameter spherical gold nanoparticles (AuNP) have been obtained by nanoparticle surface decoration with mesogenic or promesogenic molecules;10−19 some of them exhibited polymorphism of the long-range ordered structures.12,20,21 Thus, the NPs with mesogenic coating are considered as potential candidates for applications such as reconfigurable metamaterials.22 Although the introduction of mesogens to a NP ligand shell is such promising and versatile strategy, the factors influencing the order in obtained assemblies are yet not fully characterized. Until now, only the impacts of the mesogenic ligand architecture,11 alkyl coligand length,11,12 and relative stoichiometry of the ligands16 on the nanoparticle positional order have been studied. No factors influencing the orientational order of mesogenic ligands have been discussed yet. To address the above-mentioned problem of tailoring NP assembly modes by carefully designing their (pro)mesogenic surface coating, we have focused on a new variable parameter the protrusion of mesogenic molecule cores beyond alkyl thiol grafting layer. It is known that the nanoparticle self-assembly mode depends on the ligand dimensions;23,24 e.g., the tendency Received: October 31, 2012 Revised: February 15, 2013 Published: February 19, 2013 3404
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solutions (Figure 2)the A4, A8, and A12 gold nanoparticles had the average gold core diameter of ∼2.2 nm.
of gold nanoparticles to form 3D superlattices was shown to decrease with the elongation of the n-alkylthiol capping.25,26 Therefore, we decided to systematically change the length of the alkyl spacer between the metallic cluster and the rigid core of the secondary ligand as well as to vary the length of the primary alkyl coligands to hinder or promote interactions between mesogenic units. Ultimately, the introduced changes allowed to systematically modify the NP cores separation and organic corona flexibility. Also, it should be noted that the molecular architecture of the ligands included 1,3-disubstituted phenyl ring, resulting in ca. 120° angle between the rigid core of the mesogenic molecules and the spacer unit. Thus, the studied ligands can be viewed as complementary to the up-to-date investigated lateral10,11 and side-on12,13,15 attachment of the mesogens. This research resulted in the finding of a new parameter that can be used to control superlattices of gold nanoparticles, giving rise to a wide variety of structuresfrom short-range-ordered to 3D superlattices. Furthermore, we show that changes which we have introduced in NPs grafting layers correlate well with thermal stability of the ordered structures. In the near future these studies will significantly help in the preparation of self-assembling, nanoparticulate hybrid materials with predefined properties.
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Figure 2. A Guinier plot for A8 particles dissolved in toluene. The calculated curve (red line) was obtained assuming noninteracting spherical metal particles with the mean radius of 1.1 nm and the standard deviation of 0.13 nm. In the inset, the diffraction pattern for the same nanoparticles in the condensed state is presented. The sample was drop-casted on a Kapton foil. The peak position corresponds to the mean nanoparticle diameter including the organic coating, which is equal to 3.2 nm.
EXPERIMENTAL SECTION
Ligands Synthesis. The synthesis of promesogenic ligands, denoted as Ln (Figure 1), is described in detail in the Supporting Information (Figure S1).
Hybrid Gold Nanoparticles Synthesis. The A4, A8, and A12 gold nanoparticles were used as substrates for preparation of nine hybrid materials, denoted as AkLn (Figure 3). The ligand exchange reaction
Figure 1. General structure of promesogenic ligands Ln, n = 5 (L5), 10 (L10), or 15 (L15). Gold Nanoparticle Synthesis. A series of gold clusters were synthesized according to a modified Brust−Schiffrin protocol27 using butane-, octane-, and dodecanethiols as surface ligands, yielding A4, A8, and A12 nanoparticles, respectively (referred to as Ak in general). An aqueous solution of hydrogen tetrachloroaurate (90 mL, 30 mmol dm−3) was extracted three times, each time with 200 mL of methyltrioctylammonium chloride (5.57 g, 1.38 mmol)−toluene solution to transfer all tetrachloroaurate ions into the organic layer. The toluene layer was separated, and the proper alkanethiol was added to the organic solution (2 mol equiv with respect to AuCl4−). The mixture was stirred for 15 min at room temperature. Then, under vigorous stirring, a freshly prepared aqueous solution of sodium borohydride (1.40 g, 30 mmol in 10 mL of cold H2O) was quickly added. Immediately, the solution turned dark brown and the evolution of gas was observed. After further stirring for 3 h the organic phase was separated, washed with deionized water (2 × 50 mL), concentrated to 5 mL using a rotary evaporator, and mixed with 200 mL of absolute ethanol to precipitate nanoparticles. The mixture was kept for 12 h at −4 °C. The dark brown precipitate was sonificated for 60 s and centrifuged (5 min, 13 000 rpm). The precipitate was dissolved in a small amount of toluene (5 mL), precipitated again with ethanol (100 mL), and centrifuged. The procedure was repeated until no trace of excess thiol was found, as determined by 1H NMR spectra and TLC. Finally, all samples were dissolved in 20 mL of toluene and centrifuged (30 min, 13 000 rpm) to remove the formatted aggregates. The yield of final NPs was 80−85%. The size of the nanoparticles was determined from the TEM and SAXS measurements of nanoparticle
Figure 3. Schematic picture of the AkLn hybrid nanoparticle structure; parameters k and n correspond to the number of carbon atoms in the alkyl thiols and in the spacers of the promesogenic ligands, respectively. was performed following a Murray procedure.28 In each synthesis of the hybrid, to 20 mg of Ak nanoparticles dissolved in 10 mL of toluene 50 μmol of Ln ligand was added. The reaction proceeded at room temperature for 72 h. No precipitation or change of color occurred. After 72 h the reaction mixture was concentrated to ca. 2 mL; the nanoparticles were precipitated with 20 mL of acetone and centrifuged (13 000 rpm, 5 min). This washing procedure was repeated until no traces of free mesogenic ligand were present, as determined by TLC. Then, all samples have been dissolved in toluene (20 mL) and centrifuged (13 000 rpm, 5 min) to remove the formed aggregates. The precipitate was removed, and the solution was concentrated. This procedure afforded final hybrid NPs with 35−45% yield. Material Characterization. The 1H NMR spectra were recorded on either a 200 or 500 MHz NMR Varian Unity Plus spectrometer. 3405
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The 13C NMR spectra were recorded on a NMR Varian Unity Plus 200 MHz spectrometer. X-ray photoelectron spectroscopy (XPS) experiments were performed in a PHl 5000 VersaProbe-Scanning ESCA microprobe (ULVAC-PHI, Japan/USA) instrument at a base pressure below 5 × 10−9 mbar. The X-ray beam of monochromatic Al Kα radiation, focused to a diameter of 100 μm, was used at an operating power of 25 W, and the 250 × 250 μm sample area was scanned. Photoelectron survey spectra were acquired using a hemispherical analyzer at pass energy 117.4 eV with a 0.4 eV energy step. Core-level spectra were obtained at pass energy 23.5 eV with a 0.1 eV energy step. All spectra were acquired with the 90° angle between X-ray source and the analyzer, with the use of low-energy electrons. Also, low-energy argon ions were used for the charge neutralization. After the subtraction of the Shirley-type background,29 the core-level spectra were deconvoluted into their components with mixed Gaussian−Lorentzian (30:70) shape lines using the CasaXPS software. Quantification calculations were conducted with using sensitivity factors supplied by PHI. The spectra were calibrated against 284.6 eV for C 1s region. The small-angle X-ray diffraction (SAXRD) patterns for the powder as well as partially aligned samples were obtained with the Bruker Nanostar system utilizing Cu Kα radiation. The patterns were registered with an area detector VANTEC2000. The temperature of the sample was controlled with precision of 0.1 K. The nanoparticles were drop-casted onto Kapton foil, first heated up to 170 °C and then aligned by shearing at ca. 80 °C. The same Bruker Nanostar system was also used for the scattering experiments (SAXS). The scattering data from the nanoparticle solution in toluene were analyzed using NANOFIT software, assuming spherical form factor for noninteracting gold particles (structure factor S = 1) and Schultz distribution of the particle sizes. The optical birefringence of the samples was checked by polarizing optical microscopy (Zeiss AxioImager A2m). Transmission electron microscopy (TEM) was performed using Zeiss Libra 120 instrument, with LaB6 cathode, equipped with OMEGA internal columnar filters and a CCD camera.
Figure 4. Representative curve fitting analysis for high-resolution XPS scans of (a) Au 4f, (b) S 2p, and (c) Cl 2p regions of hybrid nanoparticles (A12L15).
metallic gold,30 while the second pair of Au 4f signals, at higher BE values (84.6 and 88.3 eV), have been associated with Au atoms covalently bound to sulfur.30 S 2p region exhibited a doublet with binding energies 162.0 and 163.2 eV, typical for sulfur atoms directly bound to the gold surface.30 In the Cl 2p region a double signal was present (BE = 200.7 and 202.3 eV). The two major conclusions that should be drawn from XPS studies are the number of the NP surface ligands and their relative stoichiometry. To make sure that it is possible to perform such calculations, we checked whether all layers of gold atoms that constitute the nanoparticle have been sampled. According to Volkert et al.,31 the electron escape depth was estimated, escape depth = λ cos(θ), where λ is the inelastic mean free path (IMFP) and θ is the angle between the surface normal and the direction of the emitted electron. In our experiments θ is 0 and λ is 1.78 ± 0.002 nm. The total number of atomic layers in 2.2 nm diameter gold nanoparticle is 11.00 Å/2.88 Å ∼ 4 layers, where 2.88 Å is the gold atom radius. The number of sampled layers is ∼6, as calculated in ref 31; therefore, we can say that all atoms of nanoparticles used in this study are probed. To determine the number of the NP surface ligands, the Au 4f and S 2p peak areas were converted to Au:S atomic ratio, which in all studied samples was close to 3.6. From theoretical calculations we know that a 2.2 nm diameter gold nanoparticle is composed of ∼310 gold atoms,32 which mean there are ∼86 ligands bound to the nanoparticle surface. Our findings related to the surface coverage are in good agreement with the data reported previously.33 Analogous calculations were performed to establish S:Cl atomic ratio, which was found to be 5:3. Since each promesogenic ligand has one Cl atom per molecule and both ligands are monothiols, 60% of surface ligands can be identified as Ln, which correlates well with the NMR results. In conjunction with the abovementioned theoretical predictions of the NP composition and NMR data, it means that from the average ∼90 ligands bound to the NP surface ∼55 are mesogens. Self-Assembly of Hybrid Nanoparticles. The selfassembling of hybrid nanoparticles was studied by the temperature-dependent small-angle X-ray diffraction method. Out of nine hybrids only one, A12L5, did not form long-range
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RESULTS AND DISCUSSION Hybrid Nanoparticles Composition. The ligand exchange reaction conditions used to prepare the hybrid gold nanoparticles were carefully controlled to ensure a comparable number of mesogenic ligands attached to the nanoparticle surface. This is especially important since the additional variables would distort the self-assembly picture of the synthesized series of hybrid nanoparticles. To evaluate the exact ratio of promesogenic ligands (Ln) to alkyl thiols on the nanoparticle surface, 1H NMR studies were used (Figure S2, Supporting Information). The observed peak at 0.88 ppm was attributed to the terminal methyl group of both primary and secondary ligands, and the peak at 3.92 ppm was assigned to the OCH2 moiety from Ln. By the comparison of the integral peak intensities, the contribution of each ligand could be estimated (for details see the Supporting Information). For all samples the Ln population was found to be in 60−65% range. 1 H NMR spectra also indicated the absence of free ligands.10 The composition of organic coating in hybrids was further investigated by X-ray photoelectron spectroscopy (XPS) measurements. A representative survey analysis of A12L15 (Figure S3) revealed gold, carbon, sulfur, chloride, oxygen, and silicon (coming from the substrate) atoms present in the sample as expected. No other elements were detected. Highresolution scans of Au 4f, S 2p, and Cl 2p regions were collected (Figure 4 and Table S1). It was found that the Au 4f spectrum results from two pairs of spin−orbit components. The Au 4f7/2 and 4f5/2 peaks found at BE = 84.0 and 87.7 eV, respectively, are attributed to the 3406
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centered unit cell (filled with two objects). The structure can be therefore seen as columnar (column axis along c-direction), in which the NPs within the neighboring columns are at the same level or, alternatively, as ABA stacking of the 2D ordered layers (ac-planes). The latter seems to be in-line with orientation of the XRD patterns with respect to the shearing directionfor all 3D structures the main diffraction signals appear in the same azimuthal direction as for smectic phases. The calculated lattice parameters ratio, b/a ∼ √3, suggests that the metal spheres are arranged into a slightly distorted hexagonal lattice in the abplane with the mean distance between particles ∼50 Å and promesogenic molecules nearly homogenously distributed between them (Figure 6). Along the c-direction the NPs are
ordered structures; its diffraction pattern exhibited a single broad peak centered at ∼5 nm. The lack of long-range order occurs most probably due to the entrapping of promesogenic ligands within the n-alkylthiol corona and shielding of their interactions; if the parameter k is substantially larger than n, then the organic corona is not flexible enough to change its overall shape. The XRD patterns for all other hybrid nanoparticles decorated with ligands L5 and L10 were typical for lamellar structures;10 they showed a series of commensurate, sharp Bragg peaks along the longitudinal direction, corresponding to ∼8 and ∼9 nm periodicity, respectively, and a diffuse reflection in the equatorial direction at ∼0.3−0.4 nm (Figure 5a). Based on the XRD results, the phase structure can
Figure 5. 2D SAXRD patterns for (a) A12L10, (b) A12L15, (c) A8L15, and (d) A4L15. All samples were aligned by shearing. For integrated profiles see Figure S4.
be identified as a stacking of the layers rich of metal spheres separated by organic sublayers, which thickness is roughly defined by double promesogenic ligand length. Lamellar organization of hybrid nanoparticles can be obtained via rearrangement of ligands on the surface of metallic clusters, changing the overall shape of the object from spherical to ellipsoidal.10 As in our previous studies on other lamellar systems, the NPs spatial arrangement is only weakly temperature-dependent.10 With the increasing promesogenic ligand spacer length more ordered structures were found. For all AkL15 samples characteristic X-ray patterns were observed, which can be indexed assuming orthorhombic crystallographic unit cell (Figure 5 and Table 1). The absence of (100) and (010) signals and the presence of (001) signal indicates a base-
Figure 6. Schematic picture of NPs arrangement into 3D basecentered orthorhombic structure. For clarity, only the ab-plane is shown; along the c-direction objects are stacked one above the other. Sizes of the metallic cores and ligands are not to scale; also, only a limited number of ligands are presented.
separated mainly by alkylthiols; the related interparticle distance is ∼33−38 Å. The presence of signals indexed with l ≠ 0 evidence the 3D long-range positional order. The material A4L15 is exceptional since it forms 3D lattice with a noticeably larger unit cell, with the volume roughly doubled with respect to A8L15 and A12L15. Based on the calculated unit cells volumes (see Table 1) and assuming a similar density for all AkL15 materials, it can be deduced that the crystallographic unit cell of A4L15 must be filled with four objects. On the basis of the available data, we are not able to get conclusive information about the exact structure of the phase displayed by A4L15 hybrid and determine the type of additional modulation, which makes the every second object along the a-axis different. Transmission electron microscopy was utilized to confirm the proposed packing of the NPs. The main interparticle distance measured from the TEM image for A8L15 is 5.1 nm
Table 1. Calculated Dimensions and Volumes of BaseCentered Crystallographic Unit Cells Obtained from SAXRD Patterns for AkL15 Hybrids NPs (T [°C])
a [Å]
b [Å]
c [Å]
V [×103 Å3]
A12L15 (80 °C) A8L15 (96 °C) A4L15 (96 °C)
55.5 55.8 130.0
117.6 109.6 90.0
33.6 33.0 38.6
219 201 451 3407
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(Figure 7), which correlates well to the mean distance between columns formed by the nanoparticles, deduced from XRD.
Figure 7. TEM image of a sheared thin film sample of A8L15 hybrid. The monodomain section is highlighted. The scale bar shows 10 intercolumnar distances.
Finally, polarizing optical microscopy (POM) revealed an optical birefringence of A4L15 hybrid sample (Figure 8), which
Figure 9. Types of structure formed by self-assembled hybrid nanoparticles (a) and the lattice melting temperature (b) as a function of the structural parameters of the surface ligands, n and k.
redistribution of the mesogenic cores around the metallic core, in order to change the overall particle shape from spherical to anisotropic, which is more compatible with the lamellar or columnar organization.10,18 Noticeably, in the studied systems the tendency of hybrid NPs to form more ordered structures grows with the increasing length of the ligands, in contrast to the previously reported Au NPs covered with n-alkylthiols.25,26 This further supports the idea that flexibility of the organic corona of the investigated NPs is the determining parameter for the self-assembly mode. Superlattice Melting Temperatures. By analyzing the evolution of XRD patterns with temperature (Figure S5), we could derive more details of the superlattice properties. First of all, we could see that the superlattices, once formed, are stable over a wide temperature range, and their structural parameters are only weakly temperature dependent, suggesting that the lowest energy spatial arrangement of the nanoparticles is observed. The lattice melting temperatures, at which the longrange structure of the NPs disappears, clearly depend on both the length of the primary grafting layer thiols and the length of the mesogenic ligand spacer (Figure 9 and Table S2). The highest transition temperature (∼190 °C) is observed for A4L5 having both the shortest primary alkyls and the shortest ligand spacer length and, conversely, the lowest for A12L15 (∼95 °C). Such a dependence is in line with previously studied case of nalkylthiol-capped gold nanoparticles,34 for which it was shown that the longer the alkane chain, the lower is the superlattice melting temperature. For all of the studied hybrid nanoparticles the ordered superlattices are recovered after cooling NPs below the lattice melting temperature. The observed regularity of the transition temperatures shows that the interactions between NPs’ cores responsible for the formation of ordered structures are weakened with increased separation between the metallic spheres. In the earlier works, a
Figure 8. Polarizing optical microscope image of A4L15 hybrid after shearing of a thin film sample, with shearing direction inclined (a) or along (b) polarizers directions (marked as arrows). The change of transmission with the rotation of the sample is due to the birefringence of the material.
comes from the orientational order of the mesogenic cores of the ligands. It is noticeable that the birefringence is observed for the NPs exhibiting the highest flexibility of promesogenic ligands (the longest alkyl spacer of the ligand and the shortest alkyl thiol). An additional experiment with the lambda plate inserted into optical path between crossed polarizers was performed to determine the sign of the birefringence; unfortunately, due to small value of birefringence and strong absorption of the sample, results were not conclusive. On the basis of the SAXRD and POM measurements, we can reason that by controlling the flexibility of the promesogenic ligands we are able to control the degree of the order within the self-assembled samples (Figure 9). The flexibility depends primarily on the length of the alkyl spacer of the mesogenic molecule (n parameter), influencing the degree of reorganization available to those ligands. Long spacers allow the 3408
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Union Regional Development Fund. Authors acknowledge the TEAM program from Foundation for Polish Science (Project TEAM/2010-5/4). The TEM images were obtained using the equipment purchased within CePT Project No.: POIG.02.02.00-14-024/08-00.
simple parametrization for the softness of the ensuing organic corona was proposedthe length of the ligand should be at least 0.7 times greater than the nanoparticle diameter to enable the formation of structures other than close-packed.23,24 Here, both lengths of mesogenic ligands and n-alkylthiols influence the melting point.
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CONCLUSIONS In summary, a series of gold nanoparticles grafted with nalkylthiols and promesogenic ligands were synthesized. The variation of two architecture parameters, namely the promesogenic ligand spacer length (n) and the alkyl coligand length (k), enabled the preparation of different self-assembled structures: 1D (lamellar) and 3D superlattices with base-centered orthorhombic unit cells. We have shown that the longer the mesogenic molecule spacer the more ordered assemblies of the NPs are observed. Thus, we can deduce that the flexibility of the ligand (reflected by the n parameter) is the one of the factors one can use to control the degree of order within the self-assembled structures. Additionally, the promesogenic units having the longest spacer are able to correlate their directions in space, leading to the optical birefringence of the sample. Such deformation susceptibility of the organic layer is an important factor for the future preparation of metamaterials. Noticeably, the systematic lowering of the melting temperatures, correlated to growing separation between the metal cores of the hybrids was also observed. The melting, or more generally, the phase transition temperatures (in reconfigurable systems), at which the distribution of the liquid-crystalline ligands around the metallic core is altered, are important parameters from the application point of view. We have shown that by the n and k parameters variation we were able to increase the structure stability by ∼100 K. Overall, for the first time the variation of a distance between ligand mesogenic core and a nanoparticle surface was shown to be an important parameter influencing the type and order of the NPs superlattices, giving a new insight into the factors controlling the NP self-assembly modes. This is an important step on the way to the “smart design” process in anisotropic NP−SL preparation.
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ASSOCIATED CONTENT
S Supporting Information *
Details of mesogenic ligands synthesis, NMR analysis of hybrid nanoparticles composition, enumerative data for XPS measurements, and SAXRD results. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by the Polish National Science Center (Project N204533439) and Foundation for Polish Science MPD Programme cofinanced by the European 3409
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dx.doi.org/10.1021/la3043236 | Langmuir 2013, 29, 3404−3410
