Nanocomposites, Metal Filled conductive 23p9u234

484 NANOCOMPOSITES, METAL-FILLED Vol. 10

NANOCOMPOSITES, METAL-FILLEDIntroduction

Atomic clusters of transition metals embedded into polymeric matrices repre-sent a very attractive class of materials which combine properties belongingto the nanometer-sized metal phase (such as certain magnetic, electronic, opti-cal, or catalytic properties) and the polymer phase (such as processability andlm-forming properties) (1). By the careful selection and combination of bothcomponents, tailor-made functional materials exhibiting unique characteristics(2) can be obtained, with the advantage of using the well-established, low costtechnologies available for polymer processing, such as printing, spraying, orspin-coating.

Polymeric dispersions of nanometer-sized metal particles offer the possibil-ity of functionalizing the polymer by properties coming from the large numberof surface atoms (3) and the quantum-size effects (4). Nanometric metals showproperties that differ signicantly from that of bulk metals, which makes thesenanocomposite systems intriguing for scientic study and potentially useful for anumber of technological applications (512). The control of nanoparticle morphol-ogy becomes a very important aspect, since morphology profoundly inuences thematerial performance. As a long-term goal the ability to control and vary particlesize, distributions, shapes, and composition independently from one another isvery desierable, in order to allow the tuning of nanocomposite properties. A broadarea of nanoparticle features, ranging from nanosized single crystals to somewhatlarger (in the range of about 100200 nm) yet well-stabilized nonagglomerates,gain signicant technological importance.

Polymer-embedded nanostructures are potentially useful for a num-ber of technological applications, especially as advanced functional materials(eg, high energy radiation shielding materials, microwave absorbers, optical lim-iters, polarizers, sensors, hydrogen storage systems) (512). In addition to theintrinsic nanoscopicmaterial properties, the presence of a very large interface areain these polymer-based nanocomposites can affect signicantly polymer charac-teristics (eg, glass-transition temperature, crystallinity, free volume content, igni-tion temperature), allowing the appearance of further technologically exploitablemechanical and physical properties (eg, re-resistance, low gas diffusivity).

Historical Background

The fundamental knowledge on the preparation and nature of metal/polymernanocomposites looks back at a long history which is connected to the namesof many illustrious scientists (2).

The oldest technique for the preparation of metal/polymer nanocompositesthat can be found in the literature was described in detail in an abstract in 1835(13). The original article appeared in 1833 (J. Erdmann, p. 22). In an aqueoussolution, a gold salt was reduced in the presence of gum-arabic, and subsequentlya nanocomposite material was obtained in the form of a purple solid simply bycoprecipitation with ethanol.

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Vol. 10 NANOCOMPOSITES, METAL-FILLED 485

Around 1900, widely forgotten reports indicated the preparation of polymernanocomposites with uniaxially oriented inorganic particles, and their remark-able optical properties (14,15). Dichroic plants and animal brils (eg, linen, cotton,spruce, or chitin, amongst others) were prepared by impregnation with solutionsof silver nitrate, silver acetate, or gold chloride, followed by reduction of the cor-responding metal ions under the action of light (16).

Dichroic lms were also obtained using gold chloride treated gelatin whichwas subsequently drawn, dried, and nally exposed to light (17). Similar resultswere obtained when gelatin was mixed with colloidal gold before drying and draw-ing (18). The gold or silver content in such systems amounted typically to ca 1%by weight (19).

In 1904, Kirchner and Zsigmondy (Nobel Laureate in Chemistry, 1925) re-ported that nanocomposites of colloidal gold and gelatin reversibly changed colorfrom blue to red upon swelling with water (20). In order to explain the mechanismof nanocomposite color change, they suggested that the material absorption mustalso be inuenced by the distance between the embedded particles. In addition,around the same time, the color of gold particles embedded in dielectric matriceswas subject of detailed theoretical analyses by Maxwell Garnett who explainedthe color shifts upon variation of particle size and volume fraction in a medium(21,22).

During the following three decades, dichroic bers were prepared with manydifferent elements (ie, Os, Rh, Pd, Pt, Cu, Ag, Au, Hg, P, As, Sb, Bi, S, Se, Te,Br, I) (2327). The dichroism was found to depend strongly on the employed ele-ment, and optical spectra of dichroic nanocomposites, made of stretched poly(vinylalcohol) lms containing gold, silver, or mercury, were presented in 1946 (however,the preparative scheme used is not really clear) (28). It was assumed already inthe early reports that dichroismwas originated by the linear arrangement of smallparticles (29) or by polycrystalline rod-like particles (30) located in the uniaxiallyoriented spaces present in the bers. An electron micrograph depicted in 1951showed that tellurium needles with typical dimensions of ca 5 50 nm werepresent inside a dichroic lm made of stretched poly(vinyl alcohol), however alsoin this case, just a limited number of details were given about the technique usedfor sample preparation (31).

In 1910, Kolbe was the rst to prove that dichroic nanocomposite samplesbased on gold contained the metal indeed in its zero-valence state. Such afrma-tion was conrmed a few years later by X-ray scattering. In particular, it wasshown that zero-valence silver and gold were present in the respective nanocom-posites made with oriented ramie bers, and the ring-like interference patternsof the metal crystallites showed that the individual primary crystallites were notoriented (32). Based on Scherrers equation, which was developed just in this pe-riod, the average particle diameter of silver and gold crystallites was determinedin bers of ramie, hemp, bamboo, silk, wool, viscose, and cellulose acetate to bebetween 5 and 14 nm (33).

Basic Concepts

A nanocomposite is a material made of two or more phases one of which has atleast one dimension in a nanometric size range. Metal/polymer nanocomposites


Fig. 1. Structure of atomic clusters of metals: (a) elemental cluster, (b) small cluster, (c)large cluster, (d) thiol-derivatized cluster, and (e) alloyed cluster.

are made of a continuous polymeric matrix embedding nanosized metal do-mains. Thermoplastic polymers, elastomers, and thermosetting resins can be po-tentially used as matrix, but amorphous linear polymers, like optical plastics(eg, polystyrene, poly(methylmethacrylate)) and conductive polymers are themostfrequently used. About the metallic ller, very small clusters and nanoparticleswith a pseudospherical shape are frequently employed (see Fig. 1). The use ofnanorods and nanowires has also been described (2).

In these materials, the polymer has the function of protecting the nanostruc-tures and allowing their manipulation, whereas the nanosized ller provides thepolymericmatrixwith unique properties coming from small-size effects (ie,meso-scopic properties). To allow the appearance of mesoscopic properties, the nanopar-ticle size is required to be very small; usually a dimension less than 30 nm isnecessary for most metallic materials. In particular, the metal domain size, whereelectrons move, needs to be comparable to the critical lengths of physical phe-nomena (eg, wavelength of the electrons at the Fermi edge, mean free paths ofelectrons or phonons, coherency length, screening length). Such phenomenon isnamed electron connement (4). In addition, on this size scale, also phenomenarelated to the high percentage of surface atoms appear in the material (3).

Different topologies can result in the preparation of polymer-embeddedmetalclusters. To prepare materials characterized by the properties of surface atoms orwith characteristics coming from the connement effect, a contact-free disper-sion of clusters must result in the polymer matrix, since only in this case thelarge amount of surface atoms present allows the surface properties of matter toprevail on that of bulk. Both regular (eg, uniaxially oriented pearl-necklace typeof arrays of nanoparticles) and irregular metal cluster distributions are used intechnological applications.

The properties of nanometric particles strictly depend on their microscopicalstructure (ie, chemical composition, shape, size, percentage of defects, microstrainconcentration, etc). For example, the characteristic surface plasmon absorption ofa system of metal nanoparticles dispersed into a dielectric matrix is related to theparticle shape and size (34). To prepare a color lter, identical particles should beused, otherwise the material will appear black. The presence of a single type ofmicroscopic structure allows each particle to provide the same contribution to thecomposite properties. From a theoretical point of view, an ideal nanostructuredcomposite should be made of identical metal domains uniformly dispersed intothe polymeric matrix. However, since it is very difcult to prepare a sample of


identical metal grains, the only practically possible situation is a system of singlecrystals with a narrow size distribution.

Because of the high surface chemical reactivity, metal clusters can be easilyoxidized or contaminated by nucleophilicmolecules. The surface reactivity of smallclusters is so high that even noble metals (eg, Pt, Pd, Ag) can be oxidized by air.Since small molecules present in the air (eg, oxygen, water, sulfur dioxide) aredissolved in polymers and can diffuse through it, the polymeric matrix cannotprevent metal from surface reactions. Finally, the polymer matrix has only thefunction to freeze particles, avoiding their diffusion and aggregation by sintering.For such a reason the nanoparticles need to be passivated before their embeddingin polymers. The surface treatment (eg, thiol-derivatization, see Fig. 1d) preventsparticle aggregation by short-range steric repulsion and stabilizes the metal coreprincipally by electronic effects. Surface passivation offers also the possibility todisperse ller into most of liquid monomers and organic solvents where polymersare soluble.

Classication

Nanocomposites are biphasic materials that can be classied on the basis of theirmicrostructure by the self-connectivity concept, that is the number of space di-rections (X, Y, Z) in which each phase inside the composite physically contactsitself. Composite classication based on self-connectivity has been proposed forthe rst time by Newnham in 1978 (35). Because of the discontinuous nature ofmost nanocomposite materials, the self-connectivity concept needs to be appliedto a limited but representative composite portion (ie, a local self-connectivity def-inition should be used). In particular, like in macro- and microcomposite systems,each phase in a nano-composite material can be locally self-connected in zero, one,two, or three dimensions. It is natural to conne the attention to three perpen-dicular axes since all property tensors are generally referred to such a system. Ingeneral, for a n-phase system the number of possible self-connectivity patternsis given by (n + 3)!/3!n!. Therefore, inside a biphasic system (n = 2) there are10 different self-connectivity patterns: (00), (10), (20), (30), (11), (21),(31), (22), (32), and (33). The rst number in the notation represents thephysical connectivity of one of the two phases and the second number refers toconnectivity of the other one. A schematic representation of these 10 types of self-connectivities is given in Figure 2, using a cube as the basic building block. Arrowsare used in Figure 2 to indicate the connected directions, and an asterisk has beenintroduced in the symbol to indicate the local connectivity.

Properties of Nanosized Metals

Nanosized metals are characterized by novel thermodynamic, chemical, catalytic,optical, magnetic, and transport properties which are much different from thoseof corresponding massive metals. For this reason a 3-D periodic table of ele-ments has been frequently proposed by chemists working in this material scienceeld (36).


Fig. 2. The connectivity patterns of a biphasic solid (35).

The thermodynamic properties of matter are classically described as naturalconstants; however, they change signicantly when the dimension approaches afew nanometers (37). The rst small-size effect which has been observed for met-als is the change in the melting point (3843). When a solid is heated, it melts, andthe melting temperature is normally considered one of the most important charac-teristics of a material. Nanometer-size crystals melt at much lower temperaturesthan extended ones. This can be understood in two ways. First, the cohesive en-ergy of a crystal arises by the sum of all pairwise interactions between the atoms.In a very small crystal the number of surface atoms is large, and therefore the co-hesive energy per atom has not yet converged on the bulk value. A second pictureis more thermodynamic: as a solid is heated, melting takes place at the temper-ature where the chemical potentials of solid and liquid are equal. In addition tothe usual term in the chemical potential, a very small crystal also has a term init for the surface energy. In general, the surface energy of liquids is less than thatof solids, because a liquid can readily assume the lowest surface area shape, asphere. As a consequence, the smaller the crystal the lower is the melting temper-ature, and the reduction in the melting temperature is proportional to the surfaceto volume ratio, or inversely proportional to the nanocrystal radius. This scalinglaw for melting temperature reduction has been veried in many metals (37).

Several interesting chemical properties arise as the grain size of a metal isdecreased to a nanometer-size range (eg, enhanced reactivity, stoichiometric be-havior of heterogeneous reactions, new reaction routes). Theoretical computations


(37) showed that the fraction of metal atoms residing on the surface of a 100-nmgrain increases from 12% up to 8595% for grain sizes in the range of 12 nm.The importance of this observation is that the chemistry of such surfaces is muchdifferent from that of the same atoms contained within larger grains. If the size ofa cubic crystal is decreased from 100 nm down to the 35 nm range, the fractionof atoms contained in edge sites as compared to those contained in basal planesincreases to 70%. The consequence of this observation is that as the crystal sizedecreases, the fraction of atoms located in low coordination sites increases sharply,which inherently imparts a higher chemical reactivity to suchmaterials (activatedmetals). High surface areas and intrinsically high surface reactivities of hyper-ne reactants allow surface reactions to approach stoichiometric conversion. Withthe development of hyperne powders, the heterogeneous phase reaction can becarried out much faster and at lower temperatures (44,45). Finally, gassolid re-actions and liquidsolid reactions take on a new dimension, and hyperne solidsenter now in the chemists arsenal as novel chemical reagents.

The properties of nanosized metals in heterogeneous catalysis are well es-tablished, since heterogeneous catalysts represent one of the pioneering elds ofnanotechnology (3). The increasing portion of surface atomswith decreasing parti-cle size, comparedwith bulkmetals,makes smallmetal particles as highly reactivecatalysts, since surface atoms are the active centers for catalytic elementary pro-cesses. Among the surface atoms, those sitting on the crystal edges and corners(see Fig. 3) are more reactive than those on basal planes. The percentage of edge

Fig. 3. SEM micrograph of 3-D self-organized poly(methyl methacrylate) particles. Ifbeads are considered as atoms, the picture can be used to visualize the different catalyticsites on the surface of a metal crystal.


and corner atoms also increases with decreasing size and this is the reason forthe enhanced catalytic activity (ie, supercatalytic effect) and different selectivityof very small metal catalysts.

If metal particles become very small, reaching the nanometer-size scale, acolor may occur. This is a typical phenomenon of nanometric metals. Actually, op-tical absorption may result in the ultraviolet or visible part of the spectrum, andthis arises from a surface plasmon resonance. This is due to a collective electronplasma oscillation (plasmon) that is coupled to an external transverse electro-magnetic eld through the particle surface. It is possible to quantitatively relatethe absorption coefcient to the wavelength of the exciting radiation by the Mietheory for spherical inclusions in a dielectric matrix (34). Far-IR luminescence isanother optical phenomenon frequently observed with nanosized metals (46).

New magnetic properties appear in metals when the size approaches thenanometric regime (47). The so-called oddeven effect is a phenomenon observedwhen diamagnetic metals are reduced to a nanometric-size regime. In particular,diamagnetic materials have only spin-paired electrons. However, in practice itcannot be assumed that a macroscopic piece of a diamagnetic metal does nothave one or more unpaired electrons. This cannot be measured because of theeffectively innite number of atoms and electrons. However, if the particle sizeis small enough to make one unpaired electron measurable, the oddeven effectshould become visible. Among small diamagnetic metal particles there should bean equal percentage of odd and even numbers of electrons.

For magnetic materials such as Fe, Co, and Ni, the magnetic properties aresize-dependent (47). In particular, the coercivity force Hc needed to reverse aninternal magnetic eld within the particle changes with particle size and it ismaximum for single-domain particles. Further, the strength of the internal mag-netic eld of a single particle can be size-dependent.

Giant magnetostriction, magnetoresistivity, and magnetocaloric effects rep-resent further examples of new properties arising from the small size of magneticdomains (48).

Also transport properties, which are strictly related to the electronic struc-ture of a metal particle, critically depend on size (37). For small particles, theelectronic states are not continuous, but discrete, because of the connement ofelectron wave function. Consequently, also properties like electrical and thermalconductivity may exhibit quantum-size effects.

Most of these unique chemical and physical characteristics of nanosized met-als can be used for the functionalization of plastic materials.

Preparation Methods

A limited number of methods have been developed for the preparation ofmetal/polymer nanocomposites. Usually, such techniques consist of highly spe-cic approaches, which can be classied as in situ and ex situ methods. In the insitu methods two steps are needed: rstly, the monomer is polymerized in solu-tion, with metal ions introduced before or after polymerization. Secondly, metalions in the polymer matrix are reduced chemically, thermally, or by UV/ - irradia-tion. In the ex situ processes, the metal nanoparticles are chemically synthesized,


and their surface is organically passivated. The derivatized nanoparticles are dis-persed into a liquid monomer which is then polymerized.

In situ Methods. A simple, direct, and versatile in situ method was pro-posed by Watkins and McCarthy (49). In this method, an organometallic precursoris dissolved in supercritical uid (SCF) carbon dioxide and infused into a solidpolymer as SCF solution. Chemical or thermal reduction of the precursor to thezero-valence metal either in presence of SCF or subsequent to its removal pro-duces metal domains within the solid polymer matrix. Such technique has beenused in the synthesis of nanoscale platinum clusters embedded into poly(4-methyl-1-pentene) and poly(tetrauoroethylene), using dimethyl(cyclooctadiene)Pt(II) asmetal precursor. There are multiple advantages associated with this approach:rst, the high permeation of CO2 in virtually all polymers and the wide range oforganic and organometallic reagents which are soluble in CO2 render this tech-nique a generally useful scheme for the synthesis of polymer composites. Neitherthe polymer substrate nor the reaction product needs to be soluble in CO2. Second,the sorption of CO2 is a very fast process which signicantly enhances the kinet-ics of the penetrant absorption. The degree of polymer swelling, diffusion rateswithin the substrate, and the partitioning of penetrates between the SCF andthe swollen polymer can be controlled by density mediated adjustments of solventstrength via changes in temperature and pressure. Coupled with manipulation ofreaction rates, SCFs offer unprecedented control over composite composition andmorphology. In addition, SCFs such as CO2 are gases at ambient conditions andthe solvent dissipates rapidly upon the release of pressure. In the case of metala-tion using dimethyl(cyclooctadiene)Pt(II), the process efuent consists of CO2 andlight hydrocarbons (methane and cyclooctane) derived from the precursor ligands.

Bronstein and co-workers (50) reported an in situmethod for the preparationof polymer-cobalt nanocomposites by mixing CO2(CO)8 with a polyacrylonitrilecopolymer or an aromatic polyamide in dimethylformamide (DMF). The cobaltcarbonyl interactswithDMFgiving the complex [Co(DMF)6]2+[Co(CO)4]2 , whichis then converted to nanodispersed Co particles by thermolysis.

Analogously, silver/polyimide nanocomposites have been prepared by Fra-gala` and co-workers (51) by thermolysis of a mixture of a metallorganic silver com-plex and polyamidic acid (PPA). The PAA precursor transforms in polyimide (PI)at a curing temperature compatible with the metallorganic precursor reductiontemperature, so that at the same time the precursor decomposes, giving metallicsilver particles, and the PAA transforms itself in PI, allowing the silver particlesto remain trapped in the polymeric cage.

Copper/polymer nanocomposites have been prepared in the solid state byLyons and co-workers (52). A soluble precursor was synthesized by complexingpoly(2-vinylpyridine) with copper formate (ie, Cu(HCO2)2) in methanol. The ther-mal decomposition of the complex results in a redox reaction whereby Cu(II) isreduced to copper metal and the formate anion is oxidized to CO2 and H2. By incor-porating a reducing agent into the complex, the thermal decomposition reactionis not diffusion-limited, and nanocrystalline copper particles can be prepared inthe solid state.

Chen and co-workers (53) describe the synthesis of iron nanoparticles dis-persed in poly(4-vinylpyridine) homopolymer and vinylstyrene-4-vinylpyridinecopolymers by chemical reduction of thin lms of iron chloride/polymer


nanocomposite precursors swollen by DMF. Idrazine was used as reducing agent.Such approach can be used because the metal ions have a high tendency to formionic clusters in a polymeric matrix. The produced microscopic phases, which areseparated by the hydrophobic portions of the polymer, have a diameter rangingfrom 2 to 10 nm. The chemical reduction of the dispersed ionic metal domainsproduces ultrane metal particles.

Huang and co-workers (54) prepared copper/poly(itaconic acid-co-acrylicacid) nanocomposites via in situ chemical reduction of the Cu2+polymer com-plex by hydrazine hydrate aqueous solution.

Metal/polymer nanocomposites were prepared by Chen and co-workers (55)using dispersion of metal chlorides in polyurethane. Both polyurethane and metalsalts were dissolved in N,N-dimethylacetamide, followed by lm casting and re-duction of themetal salts by sodiumborohydrate. Themetal particle size dependedon the type of metal salt used and on its concentration.

Zhu and co-workers (56) developed a method for the preparation ofmetal/polymer nanocomposites based on -irradiation. In this method, the metalsalt was dissolved in the organic monomer, and the formation of nanocrystallinemetal particles andmonomer polymerizationwas obtained simultaneously in solu-tion by irradiation with a 60Co -ray source, leading to a homogeneous dispersionof nanocrystalline metal particles in the polymer matrix. Also UV-irradiation hasbeen used for the reduction of metal ions dispersed in polymer matrices (57).

Wizel and co-workers (58) have described the use of ultrasounds in thein situ synthesis of composite materials made of polystyrene and iron. The prop-agation of ultrasound waves through a uid causes the formation of cavita-tion bubbles. The collapse of these bubbles, described as an implosion in thehot-spot theory, is the origin of extreme local conditions: high temperatures(500025,000 K) and high pressures (1000 atm). The cooling rates obtained duringthe bubble collapse are greater than 107 K s1. These high cooling rates havebeen utilized in the sonication of Fe(CO)5 as a neat liquid or in solution, to produceamorphous iron nanoparticles. In addition, ultrasound radiation has been widelyused for the preparation of polymers without the use of initiators (59). Thesetwo chemical processes have been combined for the fabrication of metal/polymernanocomposites by sonication of styrene solution of iron carbonyl.

Ex Situ Methods. Because of the high optical purity that can be achievedin the nal product, the ex situ synthesis of metal/polymer nanocomposites is avery attractive technique, especially in the preparation of materials for opticalapplications. The particulate material of the required size, as obtained by a so-lution chemistry route, is stabilized by legand chemisorption (eg, thiols) in orderto reduce their surface reactivity and tendency to agglomeration, and then it isincorporated into a castable polymeric matrix. Usually, the passivated nanoparti-cles are dispersed into a liquid monomerinitiator mixture (eg, styrene or methylmethacrylate activated by benzoyl peroxide), which is then thermally polymer-ized. Because a little amount of ller is required, the polymerization behavior ofthemonomer is not signicantly inuenced by the presence of passivated nanopar-ticles (the reaction rate is only slightly decreased (60)). Gonsalves and co-workers(60,61) have prepared surface-derivatized gold particles by phase-transfer reac-tion of gold ions with dodecanthiol and used these particles for the synthesis ofpoly(methyl methacrylate) based nanocomposites. Carotenuto (62) has developed


a new technique for the preparation of bulk quantities of atomic clusters of gold,derivatized by thiols by a modication of the well-known Polyol Process technique(63). The ex situ approach has been used also in the synthesis of silver/polyethylenenanocomposites (64,65). A variety of high nuclearity metal cluster compoundswhich can be used for the polymeric nanocomposite preparation have been re-cently synthesized and studied (6676).

Clusters of noble and easily reducible metals (eg, Ag, Au, Pd) can be ob-tained by alcoholic reduction of metal ions in presence of polymeric stabilizers.Poly(vinylpyrrolidinone), poly(vinyl alcohol), poly(methylvinyl ether), etc are usu-ally utilized. High quality polymer-protected metal clusters can be obtained atthe end of reaction simply by addition of a nonsolvent liquid which causes thenanocomposite separation from the reactive mixture (coprecipitation) (7779).However, the use of such nanocomposites is limited because of their excessivemoisture sensitivity.

Characterization Techniques

For the comprehension of mechanisms involved in the appearance of novel prop-erties in polymer-embedded metal nanostructures, their characterization repre-sents the fundamental starting point. The microstructural characterization ofnanollers and nanocomposite materials is performed mainly by transmissionelectron microscopy (TEM), large-angle X-ray diffraction (XRD), and optical spec-troscopy (UVvis). These three techniques are very effective to determine particlemorphology, crystal structure, composition, and grain size (48).

Of the many techniques which have been used to study the structure ofmetal/polymer nanocomposites, TEM has undoubtedly been the most useful. Thistechnique is currently used to probe the internal morphology of nanocomposites.As visible in Figure 4, high quality images can be obtained because of the presence,in the sample, of regions that do not allow the high voltage electron beam passage(ie, the metallic domains) and region perfectly transparent to the electron beam(ie, the polymeric matrix). High resolution TEM (HRTEM) allows morphologicalinvestigations with a resolution of 0.1nm, and thus this technique makes possibleto accurately image nanoparticle sizes, shapes, and in some cases even inner atoms(80) (see Fig. 5).

Large-angle X-ray powder diffraction (XRD) has been one of the most ver-satile techniques utilized for the structural characterization of nanocrystallinemetal powders. The modern improvements in electronics, computers, and X-raysources have allowed it to become an indispensable tool for identifying nanocrys-talline phases as well as crystal size and crystal strain (see Fig. 6). The comparisonof the crystallite size obtained by the XRD diffractogram using the Scherrer for-mula with the grain size obtained from the TEM image allows to establish if thenanoparticles have a mono- or polycrystalline nature (11).

Metal clusters are characterized by the surface plasmon resonance, which isan oscillation of the surface plasma electrons induced by the electromagnetic eld,and consequently their microstructure can be indirectly investigated by opticalspectroscopy (UVvis spectroscopy). The characteristics of this absorption (shape,intensity, position, etc) are strictly related to the nature, structure, topology, etc


Fig. 4. TEM micrograph of metal gold clusters embedded in poly(vinylpyrrolidinone).

of the cluster system. In fact, the absorption frequency is a ngerprint of the par-ticular metal, the eventual peak splitting reects aggregation phenomena, theintensity of the peak is related to the particle size, the absorption wavelengthis related to the particle shape, the shift of the absorption with increasing oftemperature is indicative of a cluster melting, and so on. For bimetallic particlesinformation about inner structure (intermetallic or core/shell) and compositioncan be obtained from the maximum absorption frequency (81). Differently fromout-line techniques (eg, TEM, XRD) this method allows on-line and in situ clustersizing and monitoring of morphological evolution of the system (see Fig. 7). Thismethod has been used also in the study of cluster nucleation and growth mech-anisms (82,83). In addition to multielectron transition (ie, plasmon absorption)also single-electron transition (interband transition) can be detected and studiedby optical spectroscopy in order to obtain important microstructural information.

This outline of the principal characterization techniques for nanocompositematerials and nanosized metal llers is far from being complete. Advances inRaman spectroscopy, energy dispersive spectroscopy, infrared spectroscopy, andmany other techniques are of considerable importance as well. In fact, the successthat nanostructured materials are having in the last few years is strictly relatedto the advanced characterization techniques which are available today.


Fig. 5. HRTEM image of a very small metal cluster.

Fig. 6. XRD pattern of nanosized gold clusters.


Fig. 7. UVvis spectra of a colloidal gold suspension during the cluster growth. Temper-ature = 60C; 5 min, 10 min, - - - - 15 min.

Applications

Applications of metal/polymer nanocomposites have already been made in differ-ent technological elds. However the use of a much larger number of devices basedon these materials is predicted for the near future.

Because of the plasmon surface absorption band, atomic clusters of metalscan be used as pigments for optical plastics. The color of the resulting nanocom-posites is light-fast and intensive, and these materials are perfectly transparent,since the cluster size is much lower than light wavelength. Gold, silver, and coppercan be used for color lter application. Also UV absorbers can be made for exampleby using Pd clusters. The plasmon surface absorption frequency is modulated bymaking intermetallic particles (eg, Pd/Ag, Au/Ag) of adequate composition.

As shown in Figure 8, polymeric lms containing uniaxially oriented pearl-necklace type of arrays of nanoparticles exhibit a polarization-dependent and tun-able color (64,65,8486). The color of these systems is very bright and can changestrongly, modifying the light polarization direction. These materials are obtainedby dispersing metal nanoparticles in polymeric thin lms and subsequently reor-ganizing the dispersed phase into pearl-necklace arrays by solid-state drawing attemperature below the polymer melting point. The formation of these arrays inthe lms is the cause of a strong polarization-direction-dependent color which can


Fig. 8. (a) UVvis spectra of drawn polyethylene/silver nanocomposites containing 4 wt%dodecanethiol coated silver nanoparticles in linearly polarized light. (b) Absorption spectraof drawn polyethylene/silver nanocomposite lm annealed for 15 h at 180C. The anglebetween the polarization direction of the light and the drawing direction of the lm isgiven (64).

be used in the fabrication of liquid-crystal color display (see Fig. 9) and specialelectrooptical devices (see Fig. 10).

Surface plasmon resonance has been used to produce awide variety of opticalsensors, eg, systems which are able to change their color in presence of specicanalytes. These devices can be used as sensors for immunoassay, gas, and liquid(see Fig. 11) (48).

Metals are characterized by ultrahigh/low refractive indices and thereforethey can be used to modify the refractive index of optical plastics (8789). Ultra-high/low refractive index optical plastics can be used in the waveguide technology(eg, planar waveguides and optical bers).

Plastics doped by atomic clusters of ferromagnetic metals show magneto-optical properties (ie, when subject to a strong magnetic eld, they can rotatethe vibration plane of a plane-polarized light) and therefore they can be used

Fig. 9. Schematic representation of (a) a conventional color twisted nematic liquid-crystaldisplay; and (b) display set-up containing a color polarizing lter (64).


Fig. 10. Twisted nematic liquid-crystal cell equipped with a polyethylene/silver nanocom-posite lter (65).

as Faraday rotators (90,91). These devices have a number of important opticalapplications (eg, magneto-optic modulators, optical isolators, optical shutters).

Nanosized metals (eg, gold, silver) have attracted much interest because ofthe nonlinear optical polarizability, which is caused by the quantum connementof the metals electron cloud (92) (see NONLINEAR OPTICAL PROPERTIES). When ir-radiated with light above a certain threshold power, the optical polarizabilitydeviates from the usual linear dependence on that power. By incorporating theseparticles into a clear polymeric matrix, nonlinear optical devices can be made ina readily processable form (61). These materials are used to prepare a number ofdevices for photonics and electrooptics (see ELECTROOPTICAL APPLICATIONS).

Contact-free dispersion of noble metal clusters in polymer can be also used asnonporous catalytic membranes (9395). Traditionally, nonporous catalytic sep-aration layers are composed of palladium or palladium alloy foils. Reduction in

Fig. 11. Nanosized gold-based optical sensor for biological assay.


thickness by metal lm deposition on special supports increases the hydrogenpermeability and reduces the costs for the precious metal. Polymeric matrix lledby a catalytic nanosized metal offers in addition to the above advantages, thepossibility of using the catalytic properties of nanoscale metal catalysts.

One of the most important characteristics of polymers lled by metal mi-croclusters is their ability to absorb microwave radiation, producing heat (96).These plastic materials can be processed by using new technologies based onthe microwave heating and can be used for food packaging, microwave shielding,RADAR camouage, plastic welding by microwaves, etc.

Metal/polymer nanocomposites can have many other important applications.For example, nanoparticles embedded into poly(vinylpyrrolidinone) can be usedfor the electroless plating of polymeric, ceramic, and semiconductor substrates(9398). These materials have also been used for the preparation of smart sys-tems that experience a reversible alteration of their properties upon exposure tolight. They are used as infrared barriers against exposures to intense solar lightor res (99).

Summary

Metal clusterpolymer systems are receiving increased attention because of theinteresting properties and large potentialities for technological applications. Inparticular, these nanocomposite systems can provide simple, low cost options forobtaining tailored materials with high promise for various catalytic, optical, mag-netic, and electronic applications. Usually, the control of the nanoparticle mor-phology, ie, particle size, size distribution, shape, and composition, is of main in-terest. Nanoparticle features can be varied by selection of preparation method andvariation of experimental conditions. In addition to the versatility of the nanopar-ticle features and the polymer morphologies, options can be provided to tailor thetopology of the metalpolymer systems. This includes for example the uniaxiallyoriented pearl-necklace type distribution of metal nanoparticles within the poly-mer matrix. For a variety of applications special topologies are important anddetermining factors for tuning the composite material performance.

In conclusion, further research activity in this eld is extremely importantfor the development of advanced devices for functional applications based on thesematerials.

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G. CAROTENUTOL. NICOLAISInstitute for the Composite Material Technology,National Research Council

NANOCOMPOSITES, POLYMERCLAY. See Volume 3.

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