One key property distinguishing classicalpolymers from metals is their low electrical con-ductivity. A new class of organic polymerscapable of conducting electricity has recentlybeen developed (1, 2). These polymers becomeconductive upon partial oxidation or reduction,a process commonly referred to as ‘doping’. Theelectrical properties of conductive polymers canbe changed reversibly over the full range of con-ductivity from insulators to metallic conductors.Their potential as novel materials in value-addedindustrial and consumer products is opening upwholly new avenues of application for polymer-ic materials.

Among conducting plastics, polyaniline hasbecome a particular focus of interest because ofits environmental stability (3), controllable elec-trical conductivity (4), and interesting redoxproperties associated with the chain nitrogen (5).The electrical properties of the aniline polymerscan be improved substantially by secondary dop-ing (6). Polyaniline compounds can be designedto achieve the particular conductivity requiredfor a given application. The resulting blends canbe as conductive as silicon and germanium or as

insulating as glass. Additional advantages arethat the compound can be mixed simply withconventional polymers, and it is easy to fabricatepolyaniline products into specific shapes. Theconductivity of polyaniline makes it an idealshield against static electric discharges, and as aconsequence polyaniline compounds have beenused in the packaging of electronics products.Polyaniline compounds are being tested for useas protective materials against electromagneticradiation. Further, scientists hope that one dayprinted circuit boards, electrochromic windowsin houses and cars, and conductive fabrics willcontain polyaniline compounds.

The presence of a number of intrinsic redoxstates (Figure 1) has substantially increased thenumber of potential applications of aniline poly-mers for use in practical devices. The anilinepolymers have a general formula of the type[(–B–NH–B–NH–)y (–B–N=Q=N–)1–y]x, inwhich B and Q denote respectively the C6H4

rings in the benzenoid and the quinoid forms. Inpolyaniline, the neutral intrinsic redox states(Figure 1) can vary from that of the fully oxi-dised pernigraniline (PNA; y = 0), to that of the

3

Palladium-Polyaniline and Palladium-Polyaniline Derivative Composite MaterialsA BRIEF OVERVIEW OF THEIR PREPARATION AND POTENTIAL APPLICATIONS

By Kaushik MallickMolecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag 3, WITS, 2050, South Africa

Michael Witcomb*Electron Microscope Unit, University of the Witwatersrand, Private Bag 3, WITS, 2050, South Africa; *E-mail:

mikew@biology.biol.wits.ac.za

and Mike ScurrellMolecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag 3, WITS, 2050, South Africa

Palladium nanoparticles of different sizes and shapes combined with polyaniline and derivativesof polyaniline can give rise to a host polymer with interesting physical properties and importantpotential applications. The resulting composite can be produced in the form of nanofibres,nanorods, thin films, etc. Potential applications of this composite material for catalysis andsensor systems are discussed.

Platinum Metals Rev., 2007, 51, (1), 3–15

DOI: 10.1595/147106707X174203

fully reduced leucoemeraldine (LM; y = 1). The50% intrinsically oxidised polymer has beennamed emeraldine (EM; y = 0.5), and the 75%intrinsically oxidised polymer is named nigrani-line (NA; y = 0.75) (2). The synthesis andcharacterisation of polyaniline have beenreviewed previously by Geniès et al. (7), Lux (8)and Gospodinova et al. (9). Polyaniline is a mem-ber of the semi-flexible rod polymer family. Thesynthesis and characterisation of electroactivepolymers have become two of the most impor-tant areas of research into polymers, as well as inmaterials science over the past two decades.

Metal nanoparticles with different sizes andshapes can be combined with polyaniline toform hybrid materials. Hybrids represent a newclass of materials that may combine desirablephysical properties characteristic of both theirorganic and metallic components within a singlecomposite. The metallic portion offers thepotential for a wide range of electrical proper-ties, substantial mechanical hardness andthermal stability, whereas the polymer part canprovide high fluorescence efficiency, large polar-isability, plastic mechanical properties, ease ofprocessing and structural diversity.

The present article reviews both advances inmethods of synthesis and application-relatedperformance for various palladium-polyanilineand palladium-polyaniline derivative compositematerials systems.

Palladium-Polyaniline CompositeCoating for LD Polyethylene

Low-density polyethylene (LDPE) is a usefulsubstrate for a wide range of laboratory experi-ments as well as many industrial applications(10–13). Although LDPE is a relatively inertsubstrate, it can be graft-copolymerised withacrylic acid to enhance the growth and adhesionof polyaniline coatings so as to achieve a thinconductive surface layer. In an earlier work,Neoh and coworkers (14) reported the forma-tion of gold particles on the surface of apolyaniline film coated onto acrylic acid graft-copolymerised LDPE.

The thrust of this work was investigating howthe electroactive polymer substrate was affectedby the metal reduction process. The coating ofLDPE films with a polyaniline-palladium com-posite layer was studied by Wang et al. (15). Theyused two methods for the synthesis of thepolyaniline-palladium layer on the LDPE.Common to both was the initial step: the LDPEsurface was first graft-copolymerised withacrylic acid to enhance the adhesion of thepolyaniline-palladium layer. Subsequently, in thefirst method (Method 1 in Figures 2 and 3),polyaniline was deposited on the acrylic acidgraft-copolymerised LDPE. This was followedby a reaction with palladium nitrate which result-ed in a layer of palladium metal particles beingdeposited onto the polyaniline surface. In the

Platinum Metals Rev., 2007, 51, (1) 4

Leucoemeraldine (LM)

Emeraldine (EM)

Nigraniline (NA)

Pernigraniline (PNA)

Fig. 1 The various intrin-sic oxidation states ofpolyaniline. Reproducedfrom (2) with permissionfrom Elsevier

second method (Method 2 in Figures 2 and 3),polyaniline powder was first reacted with palla-dium nitrate. The powder was then treated withN-methylpyrrolidinone, after which it coatedthe acrylic acid graft-copolymerised LDPE. Thelatter method resulted in nanosized palladiummetal particles being distributed in the polyani-line coating, rather than being confined to thesurface of the polyaniline layer. In both meth-ods, the palladium metal particles conferredsurface conductivity on the LDPE substrate,even with the polyaniline in the undoped state.It was found that the polyaniline-palladiumcoating adhered excellently to the acrylic acidgraft-copolymerised LDPE substrate at low pal-ladium contents, but adhesion was weakenedsignificantly at high palladium contents due tothe palladium interfering with the interactionbetween the polyaniline and the acrylic acidgraft-copolymerised chains. The polyaniline wassynthesised in the reduced form, that is, leu-coemeraldine (LM). The advantage of using LM

is that it can very easily be oxidised, and palladi-um nitrate is reduced more rapidly.

X-ray photoelectron spectroscopy (XPS)analysis by Wang et al. (15) (Figure 2) of LM onacrylic acid graft-copolymerised LDPE afterreaction with Pd(NO3)2 revealed two Pd 3d5/2

peaks at 335 and 338 eV, which are the charac-teristic peaks for the Pd(0) and Pd(II) speciesrespectively. Initially, Pd(0) was the main oxida-tion state of palladium. For Method 1, theintensity of the Pd(II) peak increased with time,indicating that the Pd(II) species deposited onthe surface of polyaniline without further reduc-tion to Pd(0). By contrast, for the sample whichwas prepared using Method 2, XPS analysesconfirmed that the Pd 3d5/2 peak at 335 eV waspredominant throughout the reaction, while thepeak at 338 eV did not show any significantincremental change as the reaction progressed.It was concluded that Pd(0) remained the pre-dominant species on polyaniline for the samplewhich was prepared by Method 2. The disparity

Platinum Metals Rev., 2007, 51, (1) 5

Method 1 Method 2

Inte

nsity

332 335 338 341 344 332 335 338 341 344Binding energy, eV

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 2 XPS Pd 3d spectraof LM on acrylic acid-graftcopolymerised LDPE afterreaction by Method 1 for:(a) 10 min; (b) 60 min; (c)120 min; and (d) 180 min;and by Method 2 for: (e)10 min; (f) 60 min; and (g)180 min. Reproduced from(15) with permission fromElsevier

between the amounts of deposition of palladiumspecies for the two samples results from the dif-ference in the surface area and the availability ofthe reaction sites.

Figure 3 shows the scanning electronmicroscopy (SEM) images for the two methodscorresponding to the palladium(0)-polyanilinecomposite on the acrylic acid graft-copoly-merised LDPE film after different reactiontimes. In Figure 3 for Method 1, the average par-ticle size of 100 nm or less as estimated fromthese images does not increase substantially dur-ing the early stages, ~ 10 to 60 min (Figures 3(b)and 3(c)), of the reaction. After a reaction timeof 180 min or more, the surface of the LM filmwas covered with palladium (Figure 3(d)). Incontrast, for Method 2, during the initial stagesof the reaction (~ 10 min, Figure 3(f)), at aPd(II):N molar ratio of 1:5, the average particle

size was about 70 nm. After 180 min (Figure3(h)), the average particle size had increased bysome 300%. This increase in size was a directresult of the increase in size of the palladiumclusters accumulated on the surface of thepolyaniline powder as the reaction progressedand the subsequent dispersion of the particles inthe polyaniline-N-methylpyrrolidinone solution.

Chemical Deposition of Palladiumon Polyaniline

The reaction of polyaniline in its lowest oxi-dation state, LM, with palladium chloride andpalladium nitrate has been investigated by Wanget al. (16). Polyaniline was synthesised via theoxidative polymerisation of aniline by ammoni-um persulfate, and converted to emeraldine(EM) or 50% oxidised base by treatment withexcess 0.5 M sodium hydroxide. The fully

Platinum Metals Rev., 2007, 51, (1) 6

(c)

1 μμm

1 μμm

(b)(a)

1 μμm

(e)

1 μμm

(f)

0.2 μμm

(g) (h)

1 μμm

(d)

1 μμm

0.2 μμm

Fig. 3 SEM image ofLM on acrylic acid-graft copolymerisedLDPE after reaction byMethod 1 for: (a) 0min; (b) 10 min; (c) 60min; and (d) 180 min;and by Method 2 for:(e) 0 min; (f) 10 min;(g) 60 min; and (h)180 min. Reproducedfrom (15) with permis-sion from Elsevier

reduced form of the polyaniline, LM, wasobtained by the reduction of the EM base withanhydrous 98% hydrazine for 3 h, followed bythorough washing with deionised water. TheLM film obtained was then pumped dry underreduced pressure. Since the LM film is very eas-ily oxidised, the palladium uptake experimentswere conducted soon after the preparation ofthese films.

The LM film was used for the palladiumuptake experiments using palladium chlorideand palladium nitrate. The amounts of palladi-um deposited from the palladium precursors onthe LM film are indicated in Figure 4. This fig-ure shows that palladium nitrate reacts moreeffectively with LM than does palladium chlo-ride, a finding that Wang et al. confirmed wasreproducible. XPS measurements were per-formed to determine the state of the palladiumdeposited on the LM. Figure 5 shows the XPSPd 3d spectra of the LM films after reaction inpalladium chloride and palladium nitrate solu-tions for varying periods of time. For palladiumnitrate, it is quite clear that Pd(0) is the principalstate of the palladium deposited on the surface

of the LM film. For palladium chloride, whilePd(II) was the predominant state on the LMsurface at the beginning of the reaction, after300 min, the Pd(0) state became the predomi-nant species. Atomic force microscopy (AFM)images of the surface of the LM films after a 10min immersion in palladium chloride and palla-

Platinum Metals Rev., 2007, 51, (1) 7

Pd(NO3)2

PdCl2

0 50 100 150 200 250 300 350Reaction time, min

0.30

0.25

0.20

0.15

0.10

0.05Mol

ar ra

tio (P

d/N

) dep

osite

d

−•−

Fig. 4 Molar ratio of palladium deposited per mole ofLM at an initial molar ratio of Pd(II) in solution to LMof 1:4. Reproduced from (16) with permission from theRoyal Society of Chemistry

(a) (b)

(c) (d)

(e) (f)

332 335 338 341 344 332 335 338 341 344

Binding energy, eV

Inte

nsity

Fig. 5 XPS Pd 3d core-level spectra of LM basefilm after reaction withPdCl2 for: (a) 10 min; (b)30 min; and (c) 300 min;and after reaction withPd(NO3)2 for: (d) 10 min;(e) 30 min; and (f) 300min. Reproduced from (16)with permission from theRoyal Society of Chemistry

dium nitrate solutions are shown in Figures 6(a)and 6(b), respectively. A higher average rough-ness value, Ra, was achieved for the film when itwas treated in palladium nitrate, as a direct con-sequence of the larger amount of palladiumdeposited on this film.

Electrochemical Behaviour ofPolyaniline-Palladium CompositeFilms

The electrochemical behaviour of polyanilinefilms containing palladium nanoparticles wasinvestigated by Park et al. (17) when the filmswere immersed in a propylene carbonate solu-tion. To prepare the polyaniline-palladiumnanoparticle composite film, a preformedpoly(N-vinyl-2-pyrrolidone) stabilised palladiumnanoparticle colloid was dispersed in a N-methyl-2-pyrrolidone (NMP) solution. Then,polyaniline (36% oxidised state) was added slow-ly to the solution which was then stirred for 24 h.The resultant solution was cast on glassy carbonand dried under vacuum for 4 h at room temper-ature. This assembly was used as the workingelectrode. A platinum coil and Ag/Ag+ were usedas the counter and reference electrodes, respec-tively.

Figure 7 shows the electrochemical behaviourof polyaniline and polyaniline containing 20wt.% palladium nanoparticles. The cyclic voltam-mogram (CV) of the polyaniline film wascharacterised by the anodic peak near –0.5 V,which is associated with the transformation fromthe LM to the EM state. The second redox peakof polyaniline corresponds to the redox intercon-version of polyaniline from EM to PNA. Theanodic peak (~ –0.5 V) and cathodic peak (~–0.8 V) current of the polyaniline film containing

the palladium nanoparticles gradually decreasedwith successive cycles. In addition, the compos-ite film showed new anodic and cathodic peaks atabout –0.1 V and –0.3 V, respectively. The redoxpeak of the polyaniline-palladium nanoparticlecomposite film shifted to the positive potential

Platinum Metals Rev., 2007, 51, (1) 8

(a) Ra = 1.1 nm (b) Ra = 6.2 nm

100 nm 100 nm

0 0

5 μm 5 μm

Fig. 6 AFM images of LMfilms: (a) after reaction withPdCl2 and (b) after reactionwith Pd(NO3)2 for 10 min.Ra is the average roughnessvalue. Reproduced from(16) with permission fromthe Royal Society ofChemistry

(a)

50 μA

1

2

(b)

12

–1.2 –0.8 –0.4 0.0 0.4 0.8

E, V vs. Ag/Ag+

Fig. 7 Cyclic voltammograms of a polyaniline film: (a)without palladium nanoparticles, and (b) with palladiumnanoparticles (20 wt.%) on the glassy carbon electrodein 0.1 M LiClO4/propylene carbonate solution, measuredat a scan rate of 20 mV s–1. Reproduced from (17) withpermission from Elsevier

region in comparison with the polyaniline film.This result implies the presence of strong elec-trochemical interactions between polyaniline andpalladium nanoparticles. The complex formationprocess is readily evident in the CVs by the rapidgrowth of a pair of sharp redox waves at E1/2 =–0.2 V vs. Ag/Ag+ in the presence of palladiumnanoparticles.

The interaction between polyaniline and thepalladium nanoparticles was studied by UV-vis-ible spectroscopic measurements (Figure 8).Palladium nanoparticles, which were stabilisedand well dispersed in NMP solutions, showed acharacteristic plasmon absorption at 280 nm(Figure 8(a)). The absorption peaks observedfor polyaniline at 340 nm and 640 nm (Figure8(g)) indicate that the polyaniline was partiallyoxidised. Adding the polyaniline to a palladium-NMP solution caused a blue shift of the peaknear 640 nm resulting from the charge transferfrom the benzenoid to the quinoid (Figures8(b)–8(f)). The peak near 320 nm, which is dueto the π–π* transition of the phenyl ring, alsoblue shifted. These spectral changes indicatethat the coordination of the palladium particlesto the nitrogen atoms permitted the palladiumnanoparticles to interact with each otherthrough the π-conjugate chain. Such a blue shiftof the band at ~ 600 nm indicates the formationof a complex between the polyaniline and thepalladium nanoparticles.

Palladium and PolyanilineDerivative Composite Material

The derivatives of polyaniline have attractedgrowing scientific attention since their chemicalproperties are similar to those of polyaniline. Incomparison to the parent polymer, they exhibitbetter solubility in common organic solvents,which facilitates easier processing of these mate-rials.

Poly(3,5-dimethylaniline)-Palladium Nanofibre Composites

For the preparation of polymer-palladiumcomposite material an in situ synthesis approach ispreferable. Since the polymer and the nanosized

metal particles are produced simultaneously, thisis expected to yield a highly intimate contactbetween the two components.

An in situ chemical synthesis route was usedfor the synthesis of a poly(3,5-dimethylaniline)nanofibre and palladium nanoparticle (polymer-metal) composite material in which thenanoparticles were highly dispersed in the poly-mer fibre matrix (18). Transmission electronmicroscopy (TEM) images (Figure 9) illustratethe composite material at different magnifica-tions. Figure 9(a) shows an example of the largepopulation of polymer fibres with differentsizes, whereas Figure 9(b) images part of a sin-gle fibre, which is about 300 nm in diameter.The inset in Figure 9(b) shows the diffractionpattern from the fibre, revealing both diffusescattering from the amorphous polymer and dif-fraction rings. Dark field imaging confirms thatthe rings originate from the palladium nanopar-ticles. Figures 9(c) and 9(d) show TEM imagesof the surface morphology and internalmicrostructure of the polymer. It is clear thatthe surface is not smooth. On both the roughsurfaces and in the interior of the polymer, asshown by these and stereo pair images, there arehighly distributed dark regions of diameter

Platinum Metals Rev., 2007, 51, (1) 9

(g)

Abs

orba

nce

300 400 500 600 700 800Wavelength, nm

(f)

(e)

(d)

(c)

(b)

(a)

Fig. 8 UV-visible spectra obtained from palladiumnanoparticles (0.1 mM) dispersed in N-methyl-2-pyrroli-done solutions containing different concentrations ofpolyaniline: (a) 0 mM, (b) 0.02 mM, (c) 0.04 mM, (d)0.06 mM, (e) 0.08 mM and (f) 0.1 mM, (g) 0.12 mMpolyaniline in N-methyl-2-pyrrolidone solution.Reproduced from (17) with permission from Elsevier

about 2 nm. Electron energy loss spectroscopy(EELS) mapping for the palladium distributionhas confirmed that the dark spots are palladium.This is most clearly illustrated in Figure 10,which shows a fine strand of polymer compos-ite, in which the particles were not overlapping.Figure 10(a) is a zero-loss image of the strand.This is an energy-filtered image; that is, it isderived only from electrons which have retainedthe energy of the beam when passing throughthe thin sample. The image therefore containsno analytical information. In contrast, Figure10(b) is a palladium map from the same region.This palladium jump-ratio image was obtainedby dividing the Pd-N2,3 post-edge loss image bythe pre-edge loss image, thereby producing a sig-nal derived from electrons that have lost energyby generating Pd-N shell X-rays.

Palladium Nanoparticles in Poly(o-methoxyaniline)

Metal-polymer composites with a nano- ormicro-structured morphology have potentialapplications in various fields such as sensors,organic light-emitting diodes (OLEDs), field-effect transistors and nonlinear optics. Amicrostructured 3D rod-like morphology of apalladium-poly(o-methoxyaniline) compositematerial has recently been reported by our group(19). An in situ reaction between palladiumacetate and o-methoxyaniline was employed.Figure 11(a) is a SEM image showing the prod-uct, which consists of regular straightnanofibres. An image at higher magnification,Figure 11(b), reveals the 3D structure of thenanofibres, which were up to about 15 μm inlength, 0.5 μm in width and 0.25 μm thick. Allthe fibres were uniform in morphology and verystraight, suggesting a high degree of rigidity.Raman spectroscopy was employed to determinethe structural orientation of the polymer. Asseen in Figure 12, the benzene C–H bendingdeformation mode lies at 1140 to 1190 cm–1 forthe reduced semiquinone and quinoid ring struc-ture. The band at 1260 cm–1 can be assigned tothe C–N stretching mode of the polaronic units.The band at 1337 cm–1 corresponds to theC–N•+ stretching modes of the delocalised pola-ronic charge carriers, while the C–C stretchingof the benzenoid ring was observed at 1600cm–1. A small peak positioned at 1460 cm–1 cor-

Platinum Metals Rev., 2007, 51, (1) 10

(a) (b)

(c) (d)

5 μm 100 nm

20 nm 6 nm

Fig. 9 (a) TEM image showing an example of the vari-ous sizes of poly(3,5-dimethylaniline)-Pd nanofibresproduced; (b) TEM image of a single polymer fibre hav-ing a diameter ca. 300 nm. The diffraction pattern (inset)reveals diffuse scattering from the amorphous polymerand diffraction rings from the palladium nanoparticles;(c) shows the surface morphology; and (d) the ca. 2 nmsized dark spots are the palladium nanoparticles, whichshow a high dispersion throughout the polymer.Reproduced from (18) with permission from theAmerican Chemical Society

(a) (b)

10 nm 10 nm

Fig. 10 (a) Zero-loss image of a poly(3,5-dimethylani-line)-Pd composite strand about 10 nm in diameter; (b)Pd-N2,3 edge jump-ratio image of the area shown in (a).All of the dark regions in (a) can clearly be identified aspalladium nanoparticles. Reproduced from (18) with per-mission from the American Chemical Society

responds to the C=N stretching mode of thequinoid units. A band of moderate intensity at1500 cm–1 corresponds to the bending deforma-tion of the N•+–H unit.

A wide variety of methods have been appliedto the preparation of polyaniline or substitutedpolyaniline type compounds by oxidative poly-merisation of the monomer (20). We havesuggested (19) that the mechanism for the poly-merisation process involves the formation of aradical cation accompanied by the release of anelectron, this being the initiation process for thepolymerisation reaction. During the addition ofpalladium acetate, the pH of the reaction mix-ture solution dropped to ~ 5. Spectroscopicanalysis confirmed that the –OCH3 substitutedaniline oxidation product had only a head-to-tail(–N–Ph–N–Ph–) like arrangement rather than ahead-to-head (–Ph–N=N–Ph–) type. The N–Ncharacteristics arise from the head-to-head cou-pling only under neutral or basic pH conditions.On the other hand, aniline or substituted anilineoxidation products obtained in acid media havea predominantly head-to-tail arrangement (2).

The presence of the electron-donating group(–OCH3) in the o-methoxyaniline facilitates therelay of electrons through N••–H2, which formsa covalent bond with Pd(II) and attains a specieslike [(OCH3)Ph–N•+–H2]. Under acidic condi-tions, the [(OCH3)Ph–N•+–H2] species undergopolymerisation (Figure 13), which is an oxida-tion process. Each step of polymerisation isassociated with a release of an electron (21),leading to the reduction of Pd(II) to Pd(0).Subsequent coalescence of the atoms forms pal-ladium clusters which are stabilised by thepolymer. Figure 14(a) is a TEM image of this

material revealing a uniform size distribution ofpalladium nanoparticles which are highly dis-persed in the polymer matrix. A typical energydispersive X-ray (EDX) spot analysis confirmedthat dark regions in the polymer are palladiumnanoparticles, Figure 14(b).

Palladium Nanoparticles in Poly(o-aminophenol) Needles

Another example of an in situ chemical syn-thesis approach has been given by us (21) forthe preparation of a palladium-poly(o-aminophenol) composite material. Figure 15(a)shows a SEM image of the needle-like morphol-ogy of the composite material. TEMmicrographs, Figures 15(b) and 15(c), indicatethat the palladium nanoparticles are of the orderof 2 nm in diameter and are highly dispersedwithin the polymer matrix. An IR spectroscopicstudy was used to determine the chemical struc-ture of the polymer. In the IR spectrum, Figure16, the characteristic band at 1588 cm–1 can beassigned to the C=C stretching of the quinoidrings, while the two peaks at 1499 cm–1 and 1470cm–1 are the characteristic bands of the C=Cstretching vibration mode for benzenoid rings.

Palladium-Polyaniline Compositeas a Sensor

Athawale et al. (22) have shown that palladi-

Platinum Metals Rev., 2007, 51, (1) 11

(a)

10 μm

(b)

1 μm

Fig. 11 SEM images of poly(o-methoxyaniline): (a)image at low magnification of fibre-like metal-polymercomposite material; and (b) image at higher magnifica-tion showing the 3D morphology of the composite fibres.Reproduced from (19) with permission from EDPSciences

1600

1500

1460

1337

12601140

11901165

1100 1200 1300 1400 1500 1600 1700Wavenumber, R, cm–1

Rel

ativ

e In

tens

ity (a

.u.)

Fig. 12 Raman spectra of poly(o-methoxyaniline).Reproduced from (19) with permission from EDPSciences

um-polyaniline can be used as a methanol sen-sor. The nanocomposite material wassynthesised by using a thermal reflux methodfollowed by the oxidative polymerisation of ani-line by ammonium persulfate, Figure 17. Thesynthesised nanocomposite was exposed to dif-ferent aliphatic alcohol vapours such as

methanol, ethanol and isopropanol. The resultsshowed that the nanocomposite was highlyselective and sensitive to methanol vapours. Thesensor responded rapidly and reversibly in thepresence of different concentrations ofmethanol vapour. The selectivity of thenanocomposite was further investigated byexposing it to mixtures of methanol–ethanol andmethanol–isopropanol. Except for the responsetime, the nanocomposite was found to exhibit

Platinum Metals Rev., 2007, 51, (1) 12

Pd

0 1 2 3keV

PdO

PdCu

(b)

(a)

6 nm

Fig. 14 (a) TEM image of the composite materialpalladium-poly(o-methoxyaniline) at high magnification.The dark spots are the 2–3 nm sized palladium nanoparti-cles dispersed within the polymer matrix; (b) EDXspectrum from the area shown in (a). The presence of pal-ladium is clearly indicated. The copper peak originatesfrom scattering from the TEM copper mesh support grid.Reproduced from (19) with permission from EDP Sciences

(a)

(b) (c)

1 μm

20 nm 6 nm

Fig. 15 (a) SEM image of the composite materialpalladium-poly(o-aminophenol) showing a cluster oflarger polymer needles; (b) TEM images of the compos-ite made up of polymer nanoneedles and palladiumnanoparticles (dark spots); and (c) TEM image at highmagnification of the palladium nanoparticles in the poly-mer matrix. Reproduced from (21) with permission fromSpringer Science and Business Media

Resonating structure

Fig. 13 Formation of theradical-cation species andthe polymerisation pathwayof o-methoxyaniline.Reproduced from (19) withpermission from EDPSciences

an exactly identical response to that for puremethanol. The palladium-polyaniline nanocom-posite was quite stable and showed no effects ofageing after exposure to different concentra-tions of methanol.

Catalytic Activity for theHydrogenation of Nitrobenzene

The catalytic activities of palladium-contain-ing electroactive polymer microparticles,typically 1.5 μm diameter, in the reduction ofnitrobenzene to aniline have been studied byHuang and coworkers (23). Polyaniline was syn-thesised by adding ammonium persulfate tohydrochloric acid with vigorous stirring for 16h. The resultant solid particles were then washedwith excess hydrochloric acid and dried underreduced pressure. Polyaniline-SiO2 microparti-cles were also synthesised at ambienttemperature by adding colloidal silica to a solu-tion of ammonium persulfate in hydrochloricacid with constant stirring. Aniline was thenadded and the solution stirred vigorously for 16h. The colloidal suspension was then cen-trifuged twice and stored in 1 M HCl until used.The oxidation state of the resultant polyanilineand polyaniline-SiO2 was that of the emeraldineform (EM and EM-SiO2, respectively). The fullyreduced forms of polyaniline, LM, and polyani-line-SiO2 (LM-SiO2) were obtained aftersuccessive treatments with sodium hydroxideand hydrazine.

Palladium chloride was used as the precursorfor incorporating palladium in EM, EM-SiO2,LM and LM-SiO2. Figure 18 shows the decreasein the concentration of palladium chloride as afunction of reaction time. From Figure 18, it isapparent that LM and LM-SiO2 are more effec-tive in uptaking palladium from the solutionthan either EM or EM-SiO2. It is also clear thatSiO2 plays no direct role in the reaction with pal-ladium.

The catalytic activity of the palladium-con-taining LM-SiO2 was tested for thehydrogenation of nitrobenzene and it was foundthat there was an almost complete conversion toaniline (Figure 19) after 2 h at 30ºC, whereas thecatalytic activity of the LM without palladium inthe hydrogenation of nitrobenzene was shownto be negligible.

Huang et al. concluded that the reactions arerapid when these electroactive polymers are

Platinum Metals Rev., 2007, 51, (1) 13

800 1000 1200 1400 1600Wavenumber, cm–1

Tran

smitt

ance

848

879

1081 11

41

1269

1397

1470 14

9915

88

1218

Fig. 16 IR spectrum of palladium-poly(o-aminophenol)composite material. Reproduced from (21) with permis-sion from Springer Science and Business Media

100 nm

Fig. 17 TEM image and electron diffraction pattern ofpalladium-polyaniline composite. Reproduced from (22)with permission from Elsevier

Fig. 18 Palladium uptake by the various oxidation statesof polyaniline. Reproduced from (23) with permissionfrom the Royal Society of Chemistry

reduced to their lowest oxidation states. Theelectroactive polymers synthesised with SiO2

offer a larger specific surface area and a fasterreaction rate for uptake reactions with palladiumchloride than does the electroactive polymerwithout SiO2. XPS analysis confirmed that thepalladium accumulation was in the form ofPd(II) rather than elemental metallic palladium.

ConclusionPalladium-polyaniline and the derivatives of

polyaniline composite materials are a veryimportant addition to the repertoire of novelmaterials. From the standpoint of synthesistechniques, the morphology of these compositematerials can be in various forms such as thinfilms, nanorods and nanofibres. The in situ

chemical synthesis route is one of the most ver-satile approaches for the preparation ofmetal-polymer composite materials. By exploit-ing this approach, our group first reported apalladium-based polymeric hybrid material inwhich palladium nanoparticles of size ~ 3 nmwere uniformly dispersed and encapsulated inthe matrices of various derivatives of polyani-line. The matrices had a range of morphologies.Considering the progress of research in thisfield, it can be concluded that, in the near future,palladium-polyaniline composites will producefurther significant advances in the field of mate-rials science.

AcknowledgementsKaushik Mallick expresses thanks to the

University of the Witwatersrand and to theNational Research Foundation, South Africa, forthe award of postdoctoral fellowships. Theauthors thank the referees and the PlatinumMetals Review editorial staff for their constructivecontributions.

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(a) Calibration MixtureEthanol(20 cm3)

1 mmol 1 mmol

0.0 1.0 2.0 3.0 4.0Time, min

Res

pons

e, m

V

(b) Reaction Mixture

Fig. 19 Chromatograms showing: (a) a calibration mix-ture of 1 mmol of nitrobenzene and 1 mmol of aniline in20 cm3 ethanol, and (b) the composition of the reactionmixture containing 1 mmol of nitrobenzene and LM-SiO2

containing 4.3 wt.% Pd (Pd:nitrobenzene = 1:100 molebasis) in 20 cm3 ethanol after hydrogenation of nitroben-zene for 2 h. Reproduced from (23) with permission fromthe Royal Society of Chemistry

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Platinum Metals Rev., 2007, 51, (1) 15

Mike Scurrell hasPh.D. and D.Sc.degrees from theUniversity ofNottingham, U.K.,and is nowProfessor ofPhysical Chemistryat the University of

the Witwatersrand. He has beenfascinated by heterogeneous catalysis forthe past 35 years and continues to focuson structure-activity relationships in thisfield. Special current interests are incatalysis by gold and other preciousmetals, activation and conversion ofalkanes, and applications ofspectroscopic techniques to further ourunderstanding of surface chemistry andcatalysis.

Michael Witcombhas a Ph.D. degreefrom the Universityof Lancaster, U.K.,and now is Directorof the ElectronMicroscope Unit atthe University of theWitwatersrand,

where he holds a Personal Professorship.His current fields of research are thesynthesis and characterisation of metaland metal alloy nanoparticle-polymercomposites; precipitation processes inPt-C and Pd-C alloys; phase diagramstudies of intermetallic alloys; andmicrostructural characterisation ofhardmetals.

Kaushik Mallickobtained his Ph.D.degree from MagadhUniversity, India. Heis now apostdoctoralresearch fellow at theUniversity of theWitwatersrand in

Johannesburg, South Africa, working onmetal nanoparticle synthesis,characterisation and applications ofvarious homogeneous andheterogeneous catalytic systems. Hiscurrent research activities are centred onorganic electroactive materials with metalnanoparticles and inorganicsemiconductor composite materials forthe fabrication of various electronicdevices. He has published some 30research articles in various internationalpeer-reviewed journals.

The Authors