5228
Chem. Mater. 2005, 17, 5228-5230
Formation of Gold Nanoparticles in a Side-Chain Liquid Crystalline Network: Influence of the Structure and Macroscopic Order of the Material I. Gascon, J.-D. Marty,* T. Gharsa, and C. Mingotaud Laboratoire IMRCP, UMR 5623 CNRS, UniVersite´ Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse, France ReceiVed May 24, 2005 ReVised Manuscript ReceiVed August 26, 2005
Metal nanoparticles are well-known for their unique size-dependent optical and electronic properties with potential applications in different fields such as microelectronics, catalysis, and magnetic or optical devices.1,2 Incorporation of nanoparticles in a polymer matrix provides materials that possess the properties of the metal clusters as well as the processing and handling advantages of bulk materials.3-5 Although there are a variety of ways to achieve nanoparticle-polymer composites, two different approaches dominate. The first one involves polymerization of the matrix around preformed particles.6 The second one, most frequently used, consists of the in situ synthesis of the nanoparticles in the polymer matrix (acting as a template) either by reduction of the metal salts dissolved in that matrix or by evaporation of the metal on the heated polymer surface.7 In that case, the influence of the polymer matrix organization and functionalities on the growth and distribution of nanoparticles should be analyzed. This requires a series of materials with tunable properties. For this aim, mesomorphous polymers and networks,8 which mainly consist of substances with either thermotropic or lyotropic liquid crystalline (LC) behavior, could be a very useful tool. Furthermore, polymer LC nanocomposites are quite attractive materials because of their LC features (self-ordering, optic anisotropy, ability to orient under the action of external magnetic or electric fields, etc.). Only few thermotropic LC metal-containing nanocomposites were studied to date,9,10 and all these studies focused only on the modification of the mesomorphic properties in the presence of metal particles (lowering of the clearing temperature) and on the nature of * To whom correspondence should be addressed. E-mail: chimie.ups-tlse.fr.
marty@
(1) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (2) Rotello, V. Nanoparticles: buiding blocks for nanotechnology; Kluwer Academic/Plenum Publishers: New York, 2004. (3) Gangopadhyay, R.; Amitabha, D. Chem. Mater. 2000, 12, 608-622. (4) Corbierre, M. K.; Carmeron, N. S.; Sutton, M.; Mochrie, S. G. J.; Lurio, L. B.; Ru¨hm, A.; Lennox, R. B. J. Am. Chem. Soc. 2001, 123, 10411-10412. (5) Aymonier, C.; Bortzmeyer, D.; Thomman, R.; Mu¨lhaupt, R. Chem. Mater. 2003, 15, 4874-4878. (6) Cardenas, G.; Munoz, C.; Carbacho, H. Eur. Polym. J. 2000, 10911099. (7) Schmid, G. Clusters and Colloids; From Theory to Applications; VCH: Weinheim, 1994. (8) Demus, D., Goodby, J., Grey, G. W., Spiess, H. W., Eds. Handbook of liquid crystals; Wiley-VCH: New York, 1998; Vol. I-IV. (9) Barmatov, E. B.; Pelbak, D. A.; Barmatova, M. V. Langmuir 2004, 20, 10868-10871. (10) Lee, J. W.; Jin, J.-I. J. Nanosci. Nanotechnol. 2003, 3, 219-221.
chemical interactions between these particles and the polymer matrix. However, the influence of the mesophase on the formation of particles is yet to be understood. Here, we demonstrate that the size of particles grown in the LC matrix is strongly linked to the degree of molecular ordering of the mesophase. Liquid Crystalline Polymers (LCP) Formation. For this study, side chain LC polysiloxane elastomers were synthesized (Figure 1).11,12 More precisely, two kinds of samples were elaborated with the same chemical structure but different macroscopic organization of the mesogenic pendant groups. These groups could be either macroscopically disoriented in the LC state, polydomain sample (denoted EP), or macroscopically ordered, monodomain sample (denoted EM). LCP synthesis is based on a spin-cast technique developed by Ku¨pfer and Finkelmann12 and followed by a one-step (polydomain) or two-step (monodomain) hydrosilylation reaction, between the silane functions of the polymer chain and the vinyl end groups of the mesogenic groups and cross-linkers, catalyzed by dichloro(dicyclopentadienyl) platinum(II) (see Supporting Information). Before use, all the samples were previously washed so that only residual traces of unlinked molecule remained in the polymer matrix. The absence of the Si-H stretching vibration band at 2160 cm-1 in Fourier transform infrared spectroscopy and of the corresponding 1H NMR peak at 4.7 ppm showed that no unsubstitued silane functions remained in the final networks. The proportion of the substitued silane functions was checked by 1H high-resolution magic angle spinning NMR spectroscopy and was found to be in good agreement with the expected values (i.e., 80% of the silane functions substituted by mesogenic pendant groups). Scanning electron microscopy revealed a dense material with no macro- or microporosity. As demonstrated by differential scanning calorimetry (DSC), EP and EM exhibited a mesomorphous behavior until 75 °C and 95 °C, respectively (Table 1). The larger mesomorphous domain of EM could be explained by the better organization of the mesogen induced during synthesis with the slower cross-linking process. As demonstrated by X-ray diffraction (XRD) experiments, networks showed a smectic phase with a layer spacing (d) around 34.5 Å. This value was less than twice the length of the mesogenic moiety in its most extended conformation plus the backbone thickness (i.e., 19 Å). It should, therefore, correspond to a partial bilayer structure. Formation of the Gold Nanoparticles. Networks (EP or EM) were swollen by a solution of gold chloride (AuCl3) in acetonitrile (5 × 10-3 mol‚L-1) during 24 h at room temperature in the dark. Samples were then removed from gold solution, and acetonitrile was allowed to fully evaporate under the same conditions. Although the mesomorphous (11) Marty, J.-D.; Tizra, M.; Mauzac, M.; Rico-Lattes, I.; Lattes, A. Macromolecules 1999, 32, 8674-8677. (12) Ku¨pfer, J.; Finkelmann, H. Makromol. Chem., Rapid Commun. 1991, 12, 717-726.
10.1021/cm051099h CCC: $30.25 © 2005 American Chemical Society Published on Web 09/28/2005
Communications
Chem. Mater., Vol. 17, No. 21, 2005 5229
Figure 1. Chemical structure of the side-chain LC networks.
Figure 2. Absorption spectra of NP,S, NM,S, NP,I, and NM,I at 25 °C.
Table 1. Elastomer and Nanocomposite Characterization polydomain samples Tga (°C) TLC-Ia (°C) db (nm) P2b sizec (nm) λmaxd
monodomain samples
EP
NP,S
NP,I
EM
NM,S
NM,I
-6.5 74.9 34.1 0
0.6 75.9 34.1 0 17 ( 7 556
0.2 78.5 34.1 0 25 ( 10 wide peak
-2.9 94.3 34.5 0.67
1.3 93.3 34.5 0.45 3(1 550
2.7 93.1 34.5 0.57 50 ( 15 wide peak
a Deduced from DSC measurements. b Deduced from XRD experiments. Evaluated from TEM measurements. d Plasmon band maximum obtained from UV-visible absorbance measurements.
c
organization was theoretically inscribed inside the final network, swelling of the samples was restrained to avoid distortion of the structure or even damage of the material. Acetonitrile was chosen as a solvent for its low swelling ratio (ratio of the weight of the swollen network to the weight of the dry network around 1.3). Gold ions were then thermally reduced. Nanocomposites NP and NM were obtained from EP and EM, respectively (gold content was estimated below 5% approximately). To check the influence of the mesophase on particle formation, nanocomposites were formed either in the smectic phase at 60 °C (NP,S and NM,S) or in the isotropic state at 100 °C (NP,I and NM,I). Characterization of these nanocomposites is given in Table 1. XRD patterns of the composites showed peaks that correspond to the ordering of the side mesogenic groups, as well as four (strong) Bragg reflections associated to the [111], [200], [220], and [311] reflections of the face-centered cubic gold structure. The lattice constant for the cubic cell was calculated to be 4.0 Å, which is in good agreement with the reported data for pure gold. XRD analysis of the peaks related to the mesogen organization allowed us to evaluate the order parameter P2 of the mesomorphous organization of EM, NM,S, and NM,I.13 As expected, the formation of nanoparticles induced a decrease of P2 as a result of local distortion of the netwoks. An increase of the glass transition temperature and a small variation of the clearing temperature observed by DSC demonstrated a slight decrease of the overall mesophase stability. These results are a clear indication that the LC properties were maintained in the nanocomposite despite the presence of gold particles dispersed in the polymer matrix. In the absorption spectra of the composites, a characteristic band at 550 nm corresponding to a surface plasmon resonance was recorded (Figure 2). Absorption peaks associated with NP,S and NM,S were more intense and better defined than those arising from NP,I and NM,I. Moreover, (13) Davidson, P.; Levelut, A. M. Liq. Cryst. 1992, 11, 469-517.
Figure 3. TEM micrograph and histogram (ca. 300 nanoparticles analyzed) illustrating the particle size distribution of gold nanoparticles embedded in composites formed in the smectic phase (NM,S and NP,S).
nanocomposites formed at 20 °C presented absorption spectra identical to those of NP,S and NM,S (see Supporting Information). So, the observed variations were not related to the difference in temperature formation of particles. This suggests a dramatic change in size and repartition density of the particles in material when the formation of particles occurred in the smectic or isotropic state. This was confirmed by transmission electron microscopy (TEM) analysis of the gold nanocomposites that showed nanoparticles with mainly a circular shape and a size distribution close to Gaussian (Table 1 and Figure 3). The particles obtained were smaller with a narrower distribution, when growth of the particles occurred in the smectic phase rather than in the isotropic state (3 nm for NM,S and 50 nm for NM,I). Moreover, TEM micrographs of NP,S and NM,S show a rather uniform distribution of particles throughout the sample NP,S, whereas this distribution is localized in a specific area in the sample NM,S (see Supporting Information). NP,S are characterized by an approximatively five-times larger size of nanoparticles and a broader distribution. To understand theses results we can assume that the main factor controlling the formation of gold particles was the diffusion of gold ions through the samples. Increasing the number of defects (by using a polydomain sample or a monodomain sample in the isotropic state) facilitated diffusion of gold ions and then increased the size of the nanoparticles. In conclusion, these experiments demonstrated that the size of the nanoparticles can be controlled by the macroscopic organization of the polymer matrix (from 3 to 50 nm). They point out the importance of the mesophase structure on the elaboration of nanoparticles. Further experiments are now
5230 Chem. Mater., Vol. 17, No. 21, 2005
in progress to allow a precise control of the size and shape of nanoparticles. Acknowledgment. I.G. acknowledges a postdoctoral fellowship from Ministerio de Educatio´n y Cienca of Spain. The authors thanks H. Gornitzka for XRD experiments.
Communications Supporting Information Available: Formation and characterization of the gold nanocomposites. This material is available free of charge via the Internet at http://pubs.acs.org. CM051099H
