Hierarchy in Au Nanocrystal Ordering in Supracrystals: III. Competition

Article pubs.acs.org/Langmuir

Hierarchy in Au Nanocrystal Ordering in Supracrystals: III. Competition between van der Waals and Dynamic Processes Nicolas Schaeffer,†,‡ Yanfen Wan,†,‡ and Marie-Paule Pileni*,†,‡,§ †

Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, MONARIS, 75005, Paris, France CNRS, UMR 8233, MONARIS, 75005, Paris, France § CEA/IRAMIS, CEA Saclay, 91191, Gif-sur-Yvette, France ‡

S Supporting Information *

ABSTRACT: Au nanocrystals coated with thiol derivatives of varying chain sizes ranging from C12 to C16 were produced; two different size nanocrystals have been synthesized (5 and 7 nm in diameter) for each coating agent. All of those specimens are characterized by a low size distribution (below 7%). Those Au nanocrystals were used as building blocks to grow larger self-assembled crystalline structures or supracrystals. These crystalline growths were carried out by slow and controlled solvent evaporation at different temperatures and under nonnull partial solvent vapor pressure (Pt). We show that the order within the supracrystals is temperature-dependent when they are made of hexadecanethiol-coated gold nanocrystals, regardless of the size of the nanocrystals. The interparticle distances within the various supracrystals that were produced were determined by small-angle X-ray diffraction (SAXRD). We demonstrate that the interparticle distance is controlled not only by the presence of physisorbed thiol residues, as previously reported, but also, at higher temperatures, by the dynamics of the organic chains and the van der Waals forces involved between the metallic cores of the nanocrystals forming the structure.

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prepared using various experimental conditions.14,15 We showed that the overall structure of the supracrystals and also their internal structures (that is the tridimensional arrangements of the nanocrystals and their distances within the supracrystals) could be easily controlled through variations in the experimental setup during the growth stage and/or after this process. We also showed that the interparticle distances within the supracrystals could be tuned through control of the solvent vapor pressure during the growth process; the interparticle distance following a linear dependency with respect to the alkyl chain length of the coating agent when the solvent pressure is null and reaching a plateau when in the presence of a toluene atmosphere within the growth chamber. Furthermore, we linked those data to a theoretical model (the overlapping cones model or OCD) developed by Vlugt et al.17−19 and demonstrated that, at room temperature, the variations in the interparticle distances within a supracrystal were dependent upon the coating agent chain length when the vapor pressure is kept null during the growth process; however, when the supracrystals are grown under non-null vapor pressure, it becomes a key factor in the control of the interparticle distance, with a contribution of foreign molecules (free alkenthiols, etc.) that may remain during growth.14,15

INTRODUCTION Over the last few decades, there has been a tremendous effort made to understand and control not only the synthesis and formation of discrete nanostructures of controlled sizes, size dispersions, shapes, and intrinsic crystalinity but also their selfassembly into larger meta-materials in view of studying their physical properties that often differ greatly from the bulk.1−6 One of these types of structures, the periodic two-dimensional (2D) or three-dimensional (3D) arrangements of nanocrystals into larger crystals or supracrystals is of interest because of the possible collective properties originating from the nanocrystals that they are made of, with these properties being highly dependent upon the fine internal structure of the nanocrystals forming the supracrystal. Hence, a fine control over the final structure of supracrystals is needed to allow one to study these effects and ultimately integrate them into more complex devices.5−17 Thus, it is crucial to obtain an in-depth understanding of the various parameters governing the final internal structure of supracrystals and to develop new synthetic ways in view of the design of new functional supracrystal materials, to allow one to control and tune their intrinsic physical properties through variations of their internal structure. Supracrystals are generally produced by slow and controlled solvent evaporation of nanocrystal suspensions. Recently, we reported the formation of gold supracrystals made of nanocrystals of varying sizes and coated with organic thiolcontaining molecules of different alkyl chain lengths and © 2014 American Chemical Society

Received: March 6, 2014 Revised: May 22, 2014 Published: May 22, 2014 7177

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average radius of the Au nanocrystals determined from TEM images. The width of the reflections is usually resolution-limited (except for Au5C16 and Au7C16 at room temperature), indicating average supracrystal dimensions of at least a few tenths of a micrometer. The average interparticle distance, δpp, is calculated from the d111 spacing.

Here, we extend this study to the influence of the temperature over the fine internal structure of supracrystals and show that thermal variations during supracrystal growth also have a drastic effect on the crystal growth and the final interparticle distances and, in some cases (when using longer coating agents), the supracrystal overall structure itself.

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RESULTS AND DISCUSSION In the case of 5 nm Au nanocrystals coated with various alkyl chains, called Au5C12, Au5C14, and Au5C16 for simplicity, it is observed that, at the end of the solvent evaporation process at room temperature and under a toluene vapor pressure Pt value equal to 39%, Au5C12 and Au5C14 nanocrystals self-assemble into film supracrystals ordered in face-centered cubic (fcc) superlattices, as already observed previously.14,15 At room temperature, the interparticle distances of Au5C12 and Au5C14 supracrystals remain unchanged with increasing the alkyl chain length (Table 1), as already described.15 This effect

EXPERIMENTAL SECTION

The procedure for the production of 5 and 7 nm nanocrystals (Au5 and Au7) coated with various thiol derivative alkyl chain lengths (C12, C14, and C16) has been extensively described in our previous papers.14,15 In brief, two solutions are used; in the case of 5 nm gold nanoparticles (Au5), the first solution consists of 0.25 mmol of chlorotriphenylphosphine Au(I) (STREM) dissolved in 25 mL of toluene (Riedel de Haën), to which 2 mmol of the desired thiol derivative is added. The second solution is made of 5 mmol of tertbutylamine borane complex (Aldrich) dissolved in 2 mL of toluene. The two solutions are placed in an oil bath at 100 °C and stirred until all of the products are completely dissolved. Then, the two colorless and clear solutions are mixed together. Upon reduction, the mixture turns rapidly to brown and, ultimately, reaches a dark red color. The so-formed colloidal solutions are dried under a nitrogen flow and redispersed in ethanol. After vigorous stirring, the solution is centrifuged down and the nanocrystals that form a black precipitate are separated from the supernatant. The black precipitate is dried under a nitrogen flow to eliminate the remaining traces of ethanol. The Au nanocrystals are then redispersed in toluene and centrifuged at 5000 rpm for 5 min. Larger aggregates that may have formed during the synthesis are removed, and the resulting pure colloidal suspension is then obtained. All of the experimental steps are carried out under a nitrogen atmosphere. For 7 nm Au nanocrystals (Au7), a procedure similar to the procedure described is used; however, in this case, the amount of thiol derivative in solution is lowered to 1 mmol and the second solution contains 2.5 mmol of tert-butylamine borane complex dissolved in 15 mL of toluene. The three thiol derivatives used throughout this study to coat the gold nanocrystals are dodecanethiol (C12H25−SH), tetradecanthiol (C14H29−SH), and hexadecanethiol (C16H33−SH), all of them purchased from Aldrich. Hence, we will describe two sizes of nanocrystals that are Au5 and Au7, both of them coated with three different coating agents. For simplicity, all of these specimens will be referred to as Au5C12 and Au7C12, Au5C14 and Au7C14, and Au5C16 and Au7C16, respectively, throughout this report. The average sizes and size distributions of the various nanocrystals produced were determined from transmission electron microscopy (TEM) images acquired using a JEOL M1011 operated at a 100 kV acceleration voltage. TEM specimens were prepared by placing a drop of colloidal suspension onto a carbon-coated copper mesh grid and wiping out the excess liquid. Average sizes and size distributions were calculated by measuring at least 500 nanocrystals. TEM images and size distributions are shown in Figure S1 of the Supporting Information. The toluene vapor pressure, Pt, was evaluated using the principle of a wet−dry hygrometer as described elsewhere.15 The supracrystalline samples were prepared as follows: A total of 100 μL of a colloidal Au nanocrystal suspension was deposited in a glass container (volume = 0.82 mL) into which a piece of silicon wafer (5 × 5 mm2) had been placed and allowed to dry out at room temperature or at 50 °C under dry nitrogen. The evaporation times and the partial toluene vapor pressures Pt when the substrate temperature was fixed at room temperature and 50 °C were measured to be 9 and 5 h and 39 and 25%, respectively. After solvent evaporation, the supracystals were imaged by scanning electron microscopy (SEM) on a JEOL JSM-5510LV operating at a 20 kV acceleration voltage. The structural information for these supracrystalline assemblies was deduced from small-angle X-ray diffraction (SAXRD). A rotating copper anode operating at 40 kV and 20 mA provided the X-ray beam that was focused with a Kirkpatrick−Baez optics. The edge−edge interparticle spacing δpp is given by δpp = D − 2R, with R being the

Table 1. Values of the Interparticle Distances (δpp) within the Various Supracrystals Prepared Using Different Growth Conditionsa nanocrystal size (nm) 5

5

temperature (°C) RT

50

Pt value (%)

a

nanocrystal size (nm) 7

7

temperature (°C) RT

50

Pt value (%)

coating

39

25

39

25

C12H25SH (δpp, nm) C14H29SH (δpp, nm) C16H33SH (δpp, nm)

3.1 3.0 A

3.0 2.7 2.5

2.9 2.9 A

2.5 2.8 2.8

RT, room temperature; A, amorphous.

was attributed to the fact that the partial solvent vapor pressure during the supracrystal growth is one of the key factors in controlling the interparticle distance within the supracrystal. We showed previously that the interparticle distance within a supracrystal is governed by the coating agent chain length only when the partial vapor pressure is kept null during the growth process; however, a relatively more complex growth process occurs in the presence of solvent molecules within the growth atmosphere (i.e., when vapor pressure is not equal to 0), and in this situation, the interparticle distance is not governed by the coating agent chain length. These observations were rationalized as being due to the influence of depletion forces that occur when in the presence of thiol-containing molecules physisorbed on the coating molecules on the internal structure of these supracrystals.15 In the case of Au5C16 nanocrystals (Figure 1a and corresponding inset), the film produced is amorphous. By increasing the substrate temperature to 50 °C during the growth process, the partial vapor pressure Pt reaches a value of 25% (compared to 39% at room temperature). In these conditions, the films of Au5C12 and Au5C14 nanocrystals are highly homogeneous (panels a and b of Figure 2) compared to the supracrystals obtained at room temperature (see Figure S2 of the Supporting Information). The diffraction patterns (inset in panels a and b of Figure 2) show the formation of fcc supracrystals. Similarly, Au5C16 nanocrystals form fcc film supracrystals (Figure 1c and the inset). Hence, when the temperature is increased from room to 50 °C, the film deposited onto the substrate at the end of evaporation evolved from amorphous to highly ordered supracrystals in the case of 7178

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Figure 1. SEM images of amorphous structures made of C16H33SH coated (a) 5 nm and (b) 7 nm prepared at room temperature (Pt = 39%) and film supracrystals made of C16H33SH coated (c) 5 nm and (d) 7 nm prepared at 50 °C (Pt = 25%). (Insets) Corresponding SAXRD patterns.

Figure 2. SEM images of supracrystals made (a and b) 5 nm and (c and d) 7 nm nanocrystals coated with (a) C12H25SH and (c) C14H29SH and grown at 50 °C (Pt = 25%). (Insets) Corresponding SAXRD patterns.

Au5C16. Furthermore, the interparticle distance within Au5 supracrystals differing by the length of the alkyl chains of their respective coating agents markedly drop when increasing the length of these alkyl chains (Table 1). Such a feature could

be related to the fact that, even though the chain lengths are predominantly in the all-trans zigzag conformation, the endgauche defects markedly increase with increasing the alkyl chain length, whereas no internal (surface) gauche defects are 7179

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detected.20−24 The long alkyl chains can rotate more freely, leading to an increase in the residence time upon increasing the temperature above the melting temperature of the organic chains.25 Hence, large amplitude motions about the long axes of the chains and molecular mobility increase toward the unbound ends.21,24 The increase in the alkyl chain length induces an increase in the population of gauche conformer and, consequently, a higher mobility of the chains. Upon increasing the substrate temperature from room temperature to 50 °C, the alkyl chains bound to the Au surfaces can form bundles during the evaporation time and, consequently, when the dry supracrystals are formed. The formation of bundles along the alkyl chains is likely to be favored by the end-gauche defects. Upon increasing the alkyl chain length, the defects move along the chain as well the bundles formed. Hence, the number of carbon atoms involved in the bundles increases with the length of the alkyl chains. Consequently, the distance between nanocrystals decreases with increasing the length of the alkyl chains. Upon increasing the size of the nanocrystal to 7 nm at room temperature and partial solvent vapor pressure Pt value equals to 39%, the films of Au7C12 and Au7C14 nanocrystals are ordered in fcc superlattices and the interparticle distance remains unchanged.14 With Au7C16 nanocrystals, the film remains amorphous (Figure 1b and the inset), similar to what has been observed in the case of Au5C16 nanocrystals. However, upon increasing the substrate temperature to 50 °C (partial solvent vapor pressure Pt = 25%), the SEM images of Au7C12 and Au7C14 nanocrystals produced at the end of the solvent evaporation show the formation of well-defined shaped aggregates (panels c and d of Figure 2). The SAXRD patterns (insets in panels c and d of Figure 2) show highly ordered aggregates with fcc crystalline structures [note that the (222) diffraction peak is very well-defined]. With Au7C16 nanocrystals, a “broken” film is produced (Figure 1d). The SAXRD pattern (inset in Figure 1d) shows yet another well-ordered film with fcc structures. The average interparticle distance between nanocrystals determined from the SAXRD pattern markedly increases when increasing the alkyl chain length from C12 to C14 and remains the same between C14 and C16 (Table 1 and Figure 3). Such behavior is the opposite of what was observed with Au5 nanocrystals at the same growth temperature. In the case of Au5 nanocrystals, the interparticle distance markedly dropped upon increasing the length of alkyl chains, whereas it increases to reach a plateau with Au7 nanocrystals (Figure 3).

Note that these data are highly reproducible. The only difference between these two processes is related to the core size of nanocrystals. To explain such a difference, one could assume that the increase in the van der Waals attractive forces because of an increase in the nanocrystal metallic core size leads to a reduction of the chain dynamics and, consequently, a decrease in the formation of bundles. In fact, one could assume that both the van der Waals interactions and the dynamic processes of the chains control the interparticle distance. Hence, the interparticle distance within Au5 and Au7 nanocrystals can be assumed to be governed by both the dynamics of the coating agent chains and the van der Waals attractive forces occurring between the nanocrystals. When at 50 °C, these interparticle distances are controlled almost exclusively by the van der Waals forces when in the case of larger particles. From this set of data, it seems reasonable to claim that, even though some general rules can be drawn, the interparticle distance remains very challenging to predict because it is dependent upon many experimental parameters. This confirms our previous studies, carried out using the same nanocrystals, and from which it was found that the vapor pressure, the residual surfactant molecules, and other remaining molecules were also key parameters in the interparticle distance.

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CONCLUSION From these sets of data, it was shown that the flexibility and dynamics of the ligand chains that can be controlled through variations in the temperature during the growth process of a supracrystal are yet another key parameter that control the supracrystal overall structure. This effect is less pronounced for 7 nm particles than for 5 nm; this difference can be explained by the fact that the conformation of the ligands on the metallic surface is screened by the influence of the van der Waals interaction forces that occur between the metallic cores.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of the vartious nanocrystal specimens used throughout this study (Figure S1) as well as SEM of Au5C12, Au5C14, Au7C12, and Au7C14 supracrystals (and their respective SAXRD patterns) (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. P.-A. Albouy for fruitful discussions and SAXRD data. This work has been financially supported by the Advanced Grant 267129 of the European Research Council.

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Figure 3. Plot of the δpp values at 50 °C (Pt = 25%) versus the number of carbons in the alkyl chain of the various thiol-containing coating agents. 7180

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