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Synthesis of polymer-embedded noble metal clusters by thermolysis of mercaptides dissolved in polymers.

INTRODUCTION

Surface and confinement effects produce in nanosized metals novel optical, magnetic, and electronic properties that can be exploit for technological applications simply by embedding these nanostructures (quantum-dots) into polymeric matrices (e.g., optical plastics, conductive polymers, resins, etc.) [1-4]. A number of advanced devices can be fabricated on the basis of such special nanocomposite materials [5-9]. For example, plasmon devices (e.g., optical limiters, polarizers, optical sensors, etc.), nonlinear optical devices, photo- and electro-luminescent materials, and new magnetic materials (e.g., magneto-optical modulators, super-paramagnetic plastics, etc.) are only some of the possible applications of polymer-embedded nanosized metals.

To investigate the technological potentialities of metal--polymer nanocomposites, the availability of a general preparative technique that may allow to produce combinations of a variety of polymers and metal clusters is strictly required. At present, only a limited number of synthesis routes have been developed [10-18], and these techniques may work only for specific metal-polymer combinations.

Cluster of noble metals can be produced by thermal decomposition of their mercaptides at moderately low temperatures (180-250[degrees]C). Mercaptides of noble metals are covalent organic salts characterized by: (i) high chemical compatibility with techno-polymers, (ii) simple synthesis, and (iii) adequate thermolysis characteristics. The use of such thermal decomposition reactions for the preparation of organic compounds (disulfides and sulfides) is well known in organic chemistry [19], but the same process has been only occasionally exploited to produce metals and metal sulfides (e.g., noble metal coatings on ceramic substrates [20-24]).

Here, a preparative scheme for the synthesis of noble-metal/polymer nanocomposites based on the controlled thermal decomposition of their homoleptic mercaptides dissolved in polymer at temperatures compatible with polymer stability is described. This process results of a certain importance from an industrial point of view, since thermoplastic polymers are usually manufactured by hot-processing technologies (e.g., extrusion, injection-molding, hot-pressing, etc.). Therefore, most of these technologies can be used for nanocomposite preparation through a reactive processing stage. To avoid the formation of gaseous by-products, which may cause film foaming during the annealing treatment, dodecyl-mercaptides have been used as metal precursor in the nanocomposite preparation (in addition, dodecyl-mercaptides have a polymeric structure, consequently their synthesis is much simplified since they precipitate from the alcoholic medium where the compound is prepared).

EXPERIMENTAL

Usually, homoleptic mercaptides are not commercially available compounds, but their synthesis is very simple and requires common chemical reagents. In particular, mercaptides of different noble metals (e.g., Pd, Pt, Au, Ag, etc.) were synthesized according to the following general scheme. A few milliliters of dodecane thiol ([C.sub.12][H.sub.25]SH. Aldrich, 98%) were dissolved in ethanol and added under stirring to a stoichiometric amount of metal salt solution in ethanol. Metal salts quite soluble in alcohols were selected for this reaction. The reaction was performed at room temperature and reagents were used without purification. The homoleptic mercaptide precipitates during the thiol addition. Owing to the reducing nature of thiols, changes in the metal oxidation number were also possible before precipitation. When there were no changes in the metal oxidation number (for example with [Ag.sup.+] and [Pd.sup.2+] ions), the precipitation promptly occurred; but precipitation required some time to happen if a variation in the metal oxidation number occurred (e.g., Au(I)-S-R formation from Au(III) salts). Metal thiolates were separated by vacuum filtration and washed several times by acetone, then they were purified by dissolution in hot chloroform (50[degrees]C) and reprecipitated by ethanol addition.

To prepare mercaptide-polymer blends, the mercaptide powder was dissolved or dispersed in chloroform and mixed with a chloroform solution of polystyrene (Aldrich, [M.sub.w] = 230,000 g/mol). After homogenization, the system was cast onto a glass substrate and solvent slowly evaporated. Usually, a mercaptide content ranging from 5 to 10% by weight was used. Annealing treatments were performed at temperatures ranging from 180 to 250[degrees]C for times of a few minutes. The films of mercaptide/polymer blends were heated on a hot-plate and to reduce the polymer oxidation reaction, their surface was protected by covering with a glass plate.

In the preparation of TEM specimens, the nanocomposite material was dissolved in heptane and the solution was dropped on a Formovar covered copper grid. Then, the dry films were coated with graphite by sputtering. Transmission electron micrographs were obtained by a Philips EM208S microscope, using an accelerating voltage of 100 kV.

When the metal clusters were characterized by surface plasmon absorption, they were identified by UV--Visible spectroscopy. Optical spectra were recorded by an UV--Visible--NIR spectrophotometer (HP-8453 UV--Vis Spectrophotometer) and a spectrofluorimeter (PerkinElmer, LS-55).

RESULTS

The microstructure of obtained nanocomposite films was imaged by Transmission Electron Microscopy (TEM). Figure la shows the inner structure of a nanocomposite sample obtained by annealing a Pd(S[C.sub.12][H.sub.25])[.sub.2]/PS blend for 5 min at 170[degrees]C. As visible, a uniform, contact-free distribution of metal particles resulted inside the films. Clusters were spherical, quite monodispersed (with an average size of 10 nm), and aggregates were not present. However, as shown in Fig. lb, a special inner morphology characterized films of gold clusters embedded in polystyrene. In these nanocomposite films, clusters were organized in two-dimensional aggregates of different sizes. Probably, thiolate molecules generated during the annealing treatment absorbed on the surface of gold, producing thiol-derivatized clusters, which may strongly interact by interdigitation of thiolate chains [25]. Consequently, the thiolate coating caused particle close-packing with uniform spacing.

[FIGURE 1 OMITTED]

The dependence of clusters size distribution on the experimental conditions (i.e., amount of mercaptide precursor and annealing time) was studied for the polystyrene/silver system. Precursor films were thermally annealed at 200[degrees]C for time periods of 30 and 180 s. Three different compositions of AgS[C.sub.12][H.sub.25]/polystyrene blends were analyzed: 5, 10, and 15% by weight. Because of the low solubility of silver mercaptide in polystyrene matrices, higher concentrations were not investigated. TEM micrographs showing the microstructure of resulting silver/polystyrene nanocomposites is given in Fig. 2. As visible, for a short annealing time (30 s), the average cluster size was quite independent on the precursor amount and clusters with a size of 2-3 nm resulted for each composition. In particular, the average cluster size decreased a little with increasing of mercaptide concentration (a size of ca. 3.2 nm resulted for 5% by weight of mercaptide, a size of ca. 2.1 nm for 10%, and a size of 1.9 nm for 15%). The size of silver clusters was strictly related to the annealing time, and it increased with increasing of annealing time. For example, for a composition of 10% by weight of mercaptide, the average size increased of 35% with increasing of annealing time of 150 s. Silver particles resulted quite monodispersed for short annealing treatments, and particle size distribution broadened with increasing of annealing time. In particular, after thermal annealing of 30 s, the standard deviation value was of 0.6, while a standard deviation value close to 1 was observed after longer annealing time (180 s). The very small cluster size and monodispersed nature after a short annealing time was probably related to the absence of a growth stage for clusters, which was caused by the high viscosity of the polymeric medium. The monodispersed nature of silver phase was rapidly lost with increasing of annealing time, probably because nucleation and growth of silver particles were not separated stages. In addition, coarsening mechanisms (Ostwald ripening) were probably involved.

[FIGURE 2 OMITTED]

Figure 3a shows the XRD pattern (Rigaku DMAX-IIIC, Cu K[alpha]) of a AgS[C.sub.12][H.sub.25]/polystyrene blend annealed at 200[degrees]C for about 180 s. This spectrum included a large signal at 21.0[degrees], corresponding to the amorphous polymer matrix, and four distinct diffraction peaks at 37.8[degrees], 44.1[degrees], 64.2[degrees], and 77.2[degrees], corresponding respectively to (111), (200), (220), and (311) crystalline planes of cubic silver. The broadening of diffraction signals, as compared to bulk silver, is related to the nanometric size of crystals. The dimension of silver crystals as measured by the Scherrer's equation resulted of about 8.2 nm. The significant difference with crystallite size value determined by TEM should be related to the negligible contribution that very small crystals give to X-ray scattering. The length of lattice constant, estimated from XRD peaks, was a = 4.1188 [Angstrom], which is quite close to the value of bulk silver (a = 4.0862 [Angstrom], JCPDS N[degrees] 4-0783). Inset of Fig. 3a shows the XRD-spectrum of AgS[C.sub.12][H.sub.25]/polystyrene blend before thermal treatment. The presence of equally spaced low-angle diffraction peaks was related to the layered structure of silver dodecyl-mercaptide. Since the mercaptide pattern disappeared after thermal annealing, a complete precursor decomposition and nanosized silver phase formation was involved in this process. Figure 3b shows the X-ray powder diffraction spectrum of the gold mercaptide/polystyrene blend (20% by weight of mercaptide) annealed at 250[degrees]C. The formation of a crystalline zero-valence gold phase was proved by the presence of gold pattern in the XRD spectrum. In particular, the spectrum included the broad peak attributed to the diffraction of noncrystalline polystyrene matrix (2[theta] = 21.0[degrees]) and some low angle peaks due to by-products of mercaptide decomposition reaction. The spectrum revealed four evident but significantly broad scattering peaks of low intensity at 38.2[degrees], 44.6[degrees], 64.0[degrees], and 78.0[degrees] that can be assigned to the gold (111), (200), (220), and (311) planes, respectively. The obtained pattern indicated that gold particles were in the face-centered cubic (fcc) structure. The value of the lattice constant calculated from the XRD pattern was a = (4.073 [+ or -] 0.002) [Angstrom], which was consistent with the value for pure bulk gold. The significant broadness of the scattering peaks was related to the very small size of crystallites. The average size of Au crystals was calculated by the Scherrer's equation and it resulted in 9.1 [+ or -] 0.03 nm for a sample with 20% by weight of gold dodecylmercaptide. The XRD estimation of average crystal size gave an average cluster dimension larger than TEM values, probably because of the negligible contribution that small crystals (molecular clusters) may give to X-ray diffraction.

When metals characterized by a surface plasmon absorption (e.g., gold and silver) were generated inside the polystyrene matrix by this technique, the nanocomposite films developed the characteristic color of the nanosized metal (see Fig. 4). In particular, polystyrene films containing silver mercaptide (AgS[C.sub.12][H.sub.25]) developed a yellow color during the thermal annealing that changed to brown by cooling down the films. Such color variation was probably produced by a multiple splitting of the surface plasmon absorption of silver consequent to a decrease in the interparticle distance that causes dipole-dipole interactions among particles. In other words, when silver particles were close enough to each other (as it happens at temperatures inferior to the polystyrene glass transition temperature, [T.sub.g] = 98[degrees]C), the system showed a broad absorption in the whole visible spectra. When particles were insulated, because the interparticle distance resulted large enough (temperatures higher than the PS [T.sub.g]), the spectrum contained only the tight surface plasmon absorption band of single spherical silver nanoparticles centered at 430 nm. A significant change in the nanocomposite coloration was not observed with further decreasing of temperature below [T.sub.g]. Such thermochromism of polystyrene films filled by silver clusters can have important technological application for example in the area of thermal sensors.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The obtained Au/polystyrene nanocomposites showed a variety of interesting optical properties. In particular, for blends containing a high percentage of mercaptide, long annealing treatments produced perfectly transparent nanocomposite films that were characterized by the characteristic purple coloration of thiol-derivatized gold clusters. According to UV-visible characterization, such coloration was due to the surface plasmon absorption of large thiol-derivatized gold clusters (maximum absorption frequency of 560 nm) (see Fig. 4). However, as visible in Fig. 5, these nanocomposite films were also able to strongly reflect red light and infrared radiation. Such important characteristic of gold/polystyrene nanocomposites allows to exploit these materials for the fabrication of heat mirrors (i.e., thermal reflectors). After short annealing treatments, the films that resulted were perfectly transparent and colorless. When observed under a long waves UV-light source (e.g., 364 nm, Spectroline UV-lamp), these films appeared luminescent (see Fig. 6) with high quantum-efficiency and an emission frequency depending on the average size of clusters produced in the polymer film during the thermal annealing. Usually, the emitted light had a color ranging from orange to red, depending on the gold cluster size. The observed nanocomposite luminescence should be related to the presence of molecular gold clusters uniformly distributed in the material. Molecular gold clusters are difficult to be synthesized using conventional solution-chemistry methods because very strong capping agents must be used to inhibit cluster growth. However, these systems are spontaneously generated by such in-situ technique. Probably, during the cluster formation process, the polymeric medium plays an essential role both in controlling growth of molecular gold clusters and preventing clusters from aggregation. The formation of molecular clusters is probably related to the very low diffusion rate, which characterizes gold atoms into a high-viscous polymeric medium. In particular, during thermal annealing at moderately high temperature, the mercaptide precursor rapidly decomposes producing a large amount of Au atoms, but owing to the low diffusion rate of generated atoms, a high supersaturation level is maintained for some time at the beginning of the process. During this stage, nucleation cannot start since atom clustering is difficult to happen, and the system contains only small embryos (i.e., molecular clusters) in addition to single metal atoms. If the thermal annealing treatment is stopped before phase separation by cooling down the system at room temperature, embryos are frozen in a glassy matrix and they may live for a long time in such metastable state.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

DISCUSSION

Homoleptic mercaptides of noble metals are organic compounds characterized by a combination of chemical and physical properties that make them as really adequate for the generation of nanosized metal inclusions in polymers. In particular, these chemical compounds have a covalent nature because the metallic atom is bonded to one or more sulfur atoms through a covalent-polar bond. Usually, the ionic contribution to the Me-S bond is low and the presence of long-chain alkyl groups gives a quite hydrophobic character to these molecules, making possible dissolution in typical non-polar organic solvents (e.g., hydrocarbons, ethers, chlorine solvents, etc.). Consequently, these compounds are also highly compatible with hydrophobic polymers (technopolymers) and their blends with polymer are usually homogeneous solid phases also at quite high concentrations (ca. 30%). To avoid the foaming of nanocomposite films, during the annealing treatment caused by the production of volatile by-products, high-molecular weight alkanethiol derivatives (e.g., dodecyl derivatives) are used.

The metal-sulfur bond energy is usually quite low (200-400 kJ/mol) [26] and the cleavage reactions may happen at moderately low temperatures (180-250[degrees]C). However, these compounds are stable enough at room-temperature and therefore, they can be handled and stored without special care. The mechanism for metal cluster formation probably involves the homolysis of Me-S bonds, with formation of zero-valence metal atoms and sulfur radicals, SR. When the polymer phase results saturated by noble metal atoms, clusters are generated. Sulfur radicals may remove hydrogen from the polymer matrix producing thiol molecules, which in some cases (e.g., silver, gold) partially absorb on the cluster surface with formation of cluster compounds (e.g., [Ag.sub.n](SR)[.sub.m]) and gaseous hydrogen. The residual free-fraction of thiol leaves dissolved in the polymer or may evolve when the annealing treatment is done at very high temperatures. Benzyl radicals are preferentially generated because of their stabilization by resonance (phenyl and other radical

species are high thermodynamically unstable); then, these carbon radicals are combined together, crosslinking the polystyrene chains.

In the case of mercaptides of non-noble metals (e.g., mercaptides of Cd, Zn, Pb, and Cu), the thermolysis process also produces nano-sized clusters but they are based on the corresponding sulfide molecules (e.g., Pb(SR)[.sub.2] [right arrow] PbS + R-S-R). Consequently, this method based on the thermolysis of mercaptide/polymer blends can also be used to produce a variety of polymer-embedded calcogenides. Probably, the formation of sulfide molecules is related to the ability of [alpha]-carbon to functions an electrophilic center toward the nucleophilic sulfur atom [27]. Red-ox reaction involving the as-generated zero-valence metal and disulfide molecules (e.g., Pb(SR)[.sub.2] [right arrow] Pb + RS-SR [right arrow] PbS + R-S-R).

CONCLUSIONS

The preparation of noble-metal/polymer nanocomposites by thermolysis of corresponding homoleptic mercaptides dissolved in polymer has shown to be a very effective and general approach. In particular, this method makes possible to generate contact-free dispersions of monodisperse nanoparticles in the polymeric matrix or aggregate topologies with sizes and concentrations that can be accurately tuned simply by varying the amount of dissolved mercaptide precursor and the annealing time.

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G. Carotenuto, L. Nicolais

Institute of Composite and Biomedical Materials, National Research Council, Piazzale V. Tecchio 80, 80125 Napoli, Italy

P. Perlo

FIAT Research Center, Strada Torino 50, 10043 Orbassano (Torino), Italy

Correspondence to: Gianfranco Carotenuto; e-mail: giancaro@unina.it
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Author:Carotenuto, G.; Nicolais, L.; Perlo, P.
Publication:Polymer Engineering and Science
Date:Aug 1, 2006
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