06 polyaniline

Journal of Colloid and Interface Science298(2006)87–93/locate/jcisCoating of zinc ferrite particles with a conducting polymer,polyaniline Jaroslav Stejskal a,∗,Miroslava Trchováa,Jitka Brodinováb,Petr Kalenda b,Svetlana V.Fedorova c,Jan Prokešd,Josef Zemek ea Institute of Macromolecular Chemistry,Academy of Sciences of the Czech Republic,16206Prague6,Czech Republicb Faculty of Chemical Technology,University of Pardubice,53210Pardubice,Czech Republicc Lumex,St.Petersburg198005,Russiad Faculty of Mathematics and Physics,Charles University in Prague,12116Prague2,Czech Republice Institute of Physics,Academy of Sciences of the Czech Republic,16253Prague6,Czech RepublicReceived3June2005;accepted15December2005Available online18January2006AbstractParticles of zinc ferrite,ZnO·Fe2O3,were coated with polyaniline(PANI)phosphate during the in situ polymerization of aniline in an aqueous solution of phosphoric acid.The PANI–ferrite composites were characterized by FTIR spectroscopy.X-ray photoelectron spectroscopy was used to determine the degree of coating with a conducting polymer.Even a low content of PANI,1.4wt%,resulted in the45%coating of the particles’surface.On the other hand,even at high PANI content,the coating of ferrite surface did not exceeded90%.This is explained by the clustering of hydrophobic aniline oligomers at the hydrophilic ferrite surface and the consequent irregular PANI coating.The conductivity increased from 2×10−9to6.5S cm−1with increasing fraction of PANI phosphate in the composite.The percolation threshold was located at3–4vol%of the conducting component.In the absence of any acid,a conducting product,1.4×10−2S cm−1,was also obtained.As the concentration of phosphoric acid increased to3M,the conductivity of the composites reached1.8S cm−1at10–14wt%of PANI.The ferrite alone can act as an oxidant for aniline;a product having a conductivity0.11S cm−1was obtained after a one-month immersion of ferrite in an acidic solution of aniline.©2005Elsevier Inc.All rights reserved.Keywords:Conductivity;Conducting polymer;Ferrite;FTIR spectra;Percolation;Polyaniline;Surface coating1.IntroductionMaterials comprising a ferromagnetic component and a con-ducting polymer have recently been investigated in several stud-ies that concentrated on the preparation of nano-colloidal iron-oxides,γ-Fe2O3[1,2]and Fe3O4[3–11],and their subsequent modification with conducting polymers,specifically polyani-line(PANI)or polypyrrole.Most of the papers have reported the magnetic and electrical properties of the resulting nano-sized composite materials.Less attention has been paid to the coating of larger micro-meter-sized magnetic particles.Several studies only concerned the surface modification of ferrites with conducting polymers.*Corresponding author.Fax:+420296809410.E-mail address:stejskal@imc.cas.cz(J.Stejskal).Ferrites have the general formula M O·Fe2O3,where M is an el-ement in a bivalent state,e.g.,M2+=Fe2+,Co2+,Ni2+,Mn2+, Zn2+,Mg2+,etc.,or a combination of them.Nickel–cobalt ferrites have been coated electrochemically with polypyrrole and used as composite electrodes for hydrogen peroxide for-mation[12].Manganese–zinc ferrite particles of micrometer size have recently been coated with PANI by using a chemi-cal oxidation of aniline[13].It was observed that the magnetic properties of a ferrite were influenced by a coating of a conduct-ing polymer.This was explained as the result of an electronic interaction occurring at the interface between the conducting polymer and the ferromagnetic material.The structure of the interface is thus of interest,and is investigated in the present paper.Conducting polymers,viz.PANI,may improve the corrosion protection of metals[14].Iron oxides,e.g.,γ-Fe2O3,Fe3O4,or ferrites,have fundamental importance in the study of iron corro-0021-9797/$–see front matter©2005Elsevier Inc.All rights reserved. doi:10.1016/j.jcis.2005.12.03488J.Stejskal et al./Journal of Colloid and Interface Science298(2006)87–93sion.They are currently used as components in formulations ofcorrosion-protective materials.The coating of iron oxides withconducting polymers is a logical extension in the design of newanti-corrosion pigments.Inorganic oxides have often been coated with conductingpolymers.Silica gel[15],titanium dioxide[16,17],aluminumoxide[18],cerium oxide[19],and copper oxide[20]may serveas examples.The coating is produced in situ during the poly-merization of aniline[15],and this technique is also applicableto iron oxides and ferrites.In this study,the coating of zinc fer-rite powder with PANI is reported and analyzed.The materialsso formed are potential candidates in the corrosion protection ofmetals.More detailed information on the nature of the coating,an interface between ferrite and conducting polymer,is neededfor the understanding of its performance.2.Experimental2.1.Preparation of ferriteAn equimolar mixture of zinc and ferric oxides was thor-oughly homogenized in an agate mortar and calcinated inunglazed corundum pot in an electric high-speed furnace op-erating at1100◦C for2h at ambient atmosphere.The cal-cinated zinc ferrite,ZnO·Fe2O3,was leached with water andwet-ground in a vibration ball-mill for5h at390rpm,followedby washing with water,and drying in a desiccator.2.2.Coating of ferrite with polyanilineIn thefirst series of experiments,the PANI–ferrite com-posites of various compositions were prepared.To portions offerrite(2g),various volumes(5,10,20,30,50,75,100,and200ml)of freshly prepared reaction mixture(0.1M aniline,0.125M ammonium peroxydisulfate in0.4M phosphoric acid)were added at20◦C.The mixture was stirred during the poly-merization of aniline,which was completed within1h.Nextday,the ferrite coated with PANI was separated on a paperfil-ter,rinsed with0.4M phosphoric acid and with acetone,anddried at60◦C in a vacuum oven.In the second series,the effect of the acidity of the reactionmedium was assessed.The ferrite(2g)was coated with PANIin50ml of reaction mixtures based on0–3M phosphoric acid.The precipitates were similarly separated,rinsed with the cor-responding acid,then acetone,and similarly dried.2.3.CharacterizationThe content of PANI in the coated ferrites was deter-mined as an ash.The weight fractions of PANI were con-verted into volume fractions by using densities of zinc ferrited f=4.90g cm−3,and PANI phosphate,d PANI=1.45g cm−3. Due to a large difference in the densities of the components,the volume fraction of PANI in the composite is considerablylarger than the corresponding weight fraction.Infrared spectra in the range of400–4000cm−1wererecorded at64scans per spectrum at2cm−1resolution using a fully computerized Thermo Nicolet NEXUS870FTIR spec-trometer(Nicolet,USA)with a DTGS TEC detector.Samples were dispersed in potassium bromide and compressed into pel-lets.Spectra were corrected for the moisture and carbon dioxide in the optical path.The X-ray photoelectron spectroscopic(XPS)measurements were carried out in an ADES-400spectrometer(VG Scientific, UK)equipped with a hemispherical angle-resolved energy an-alyzer operating with pass energy of100eV and using Mg Kα(1253.6eV)and Al Kα(1486.6eV)excitation sources.Wide-survey and narrow-scan spectra(C1s,O1s,Fe2p,Zn2p,N1s, S2p,and P2p)were recorded for qualitative and quantitative analysis,respectively.All spectra were measured at the normal emission angle.To avoid the expected ion-beam-induced mod-ifications of composition and bonding in the analyzed volume, the sample surfaces were analyzed without any cleaning.The energy positions were referenced to the Cu2p peak at932.6eV and Au4f peak at84.0eV binding energy.The overall energy resolution for the XPS measurements reached0.8eV.Atomic concentrations were determined from background-subtracted peak areas corrected for the photoelectron cross-sections[21], the inelastic mean free paths[22],and the experimentally de-termined transmission function of the hemispherical energy-analyzer[23].Conductivity was determined by a four-point van der Pauw method using a Keithley237High-V oltage Source Measure-ments Unit and a Keithley2010Multimeter equipped with a 2000-SCAN10Channel Scanner Card on samples compressed into pellets,13mm in diameter and ca.1mm thick.The density of the PANI was evaluated by weighing the pellets in air and immersed in decane with a Sartorius R160P bal-ance.3.ResultsThe oxidation of aniline in an acidic aqueous medium yields PANI(Fig.1)[24,25].When the reaction mixture contains a ferrite powder,composite materials of both components are ob-tained(Fig.2).Phosphoric acid has been selected as a medium for the present study because a phosphate counter-ion in the PANI is beneficial in anti-corrosion applications[14].The con-ductivity and electrical stability of PANI phosphate are also good[26].The dependence of the composite density on the volume fraction of PANI in the composite is linear,as expected,con-firming the internal integrity of the experimental data(Fig.3). The FTIR spectra prove that PANI has been produced.The typ-ical bands of protonated PANI phosphate located at1561cm−1 (with a second maximum at1574cm−1and a shoulder at 1603cm−1),1483,1304,1242,1130,879,and802cm−1are observed(Fig.4).Strong bands located in ferrite spectra at551 and430cm−1decrease as the fraction of the PANI in the com-posites increases.The peaks are visible even at a high content of PANI,e.g.,at41wt%PANI,because the penetration depth of the IR radiation is of the order of micrometer,i.e.,it is much larger than the thickness of the PANI coating,which is ex-pected to be of the order of100nm and lower[27,28].ThisJ.Stejskal et al./Journal of Colloid and Interface Science 298(2006)87–9389Fig.1.Polyaniline (emeraldine)protonated with an acid,HA.Fig.2.SEM micrographs of (a)original ferrite and ferrite containing (b)1.8wt%(45%coating)and (c)22.7wt%(79%coating)of PANI at low (left)and large (right)magnifications.also suggests that the coating of ferrite with PANI need not be complete.The shape of the quinone-ring-deformation mode in PANI phosphate changes from a single peak at 1583cm −1(with a shoulder at 1603cm −1)for a low fraction of PANI phos-phate (2wt%)to typical two maxima at 1561and 1574cm −1(again with a shoulder at 1603cm −1)for a high content of PANI phosphate (41wt%).The position of the benzene-ring-deformation mode in PANI phosphate is red-shifted from 1498to 1483cm −1.A similar shift has been observed during the conversion of the non-conducting PANI base to a protonated conducting emeraldine form of PANI [29,30].The degree of the surface coverage of ferrites with PANI was quantified by analysis of the Fe 2p peak shape and the shape of extended background behind the peak.The method relies on the fact that the energy distribution of emitted electrons depends on the in-depth concentration profile and is able to differentiate among thin and thick homogeneous overlayers,buried layers,three-dimensional island structures on different substrates,etc.The theoretical framework of the technique is described in de-tail in the literature [31].The validity of the technique has been established through systematic experimental investigations and comparison to measurements on the same samples by Ruther-ford backscattering spectrometry,ion scattering spectrometry,and atomic force microscopy [31–34].The corresponding spec-tra processing is presently facilitated by the commercial soft-ware package QUASES-Tougaard [35].The technique is de-90J.Stejskal et al./Journal of Colloid and Interface Science 298(2006)87–93Fig.3.The densities of composites containing various volume fractions of PANIphosphate.Fig.4.FTIR spectra of ferrite coated with PANI phosphate.The content of PANI phosphate (wt%)is denoted at each spectrum.scribed in more detail elsewhere [36].Typical Fe 2p spectra with an extended inelastic background are shown in Fig.5.From the QUASES-Tougaard analysis it follows that (1)sur-faces under study are partially covered by PANI,and (2)the PANI thickness exceeds the information depth of the method.For the Fe 2p photoelectrons with kinetic energy of 540eV trav-eling through the PANI overlayer,their inelastic mean free path (IMFP)reaches 1.7nm [37].Therefore,the PANI overlayer thickness exceeds 5.1nm.In other words,the Fe 2p photoelec-trons originate from PANI uncovered surfaces.The results of QUASES-Tougaard analysis are shown in Fig.5together with the results of nitrogen atomic percentage calculated by quanti-tative analysis from photoelectron spectra described above.The results of conductivity measurement are summarized in Figs.5and 6,and discussedbelow.Fig.5.The Fe 2p and Fe LMM electron spectra recorded for uncoated and partly PANI-coated (79%and 91%)ferrites.Fig.6.The dependence of the degree of ferrite-surface coating and nitrogen atomic content at the surface on the composition of the PANI–ferrite composite.4.Discussion4.1.The coating of ferrite with polyanilineAny surface in contact with the reaction mixture used for the polymerization of aniline becomes coated with a thin PANI film.It is assumed that aniline oligomers are produced at first [28].Because of their reduced solubility in water,they ad-sorb at the available interfaces.The adsorbed species start the growth of PANI chains,and that is why the polymerization at the interface is preferred to the corresponding process in the whole volume of the reaction mixture [30,38].The polymeriza-tion of aniline is auto-accelerated [39],i.e.,PANI formation isJ.Stejskal et al./Journal of Colloid and Interface Science298(2006)87–9391preferred at the spots where some PANI has already been pro-duced.This leads to a proliferation of the PANIfilm nucleated at the interface[28].This result is consistent with the QUASES-Tougaard analysis of PANI coverage and also with the results of quantitative analysis of nitrogen content.The latter is pro-portional to the PANI coverage.When a powdered material is introduced into the reaction mixture,the surfaces of the individual particles serve as tem-plate areas for the growth of PANI.This is the case with fer-rite particles.The degree of coating is suitably evaluated by XPS(Fig.6)from the decrease of the signal assigned to iron atoms.A low amount of conducting component PANI,1.8wt% PANI(=4.5vol%PANI)results in45%coating of the ferrite surface.Obviously,the coating is thin and is not discernible in the SEM micrographs(Fig.2).When the amount of PANI becomes greater,the degree of coating increases but is never complete(Fig.6).A similar observation has been made when silica particles[36,40]or organic polymer microparticles[41–43]have been used as templates for coating.It seems that hydrophilic objects are more difficult to coat completely with conducting polymers,than hydrophobic surfaces[44].This is probably connected with the better and uniform adsorption of hydrophobic oligomers at the hydrophobic substrate surface.At hydrophilic surfaces,oligomers are likely to produce droplet-like clusters,resulting in the subsequent patchy coating of the surface with a conducting polymer.The granular PANI deposits are visible at the surface of ferrite particles at a larger content of PANI(Fig.2c),probably because of the secondary nucleation of PANI growth on the already existing PANI[28].Some aggre-gates of ferrite particles,held together with PANI,are also seen in SEM micrographs(Fig.2c).The presence of PANI precip-itate in the composites has not been found.This indicates that the growth of PANI at the surface of particles is preferred to a precipitation polymerization of aniline,in accordance with the earlier results obtained with the coating of silica particles[15].4.2.Conductivity of PANI–ferrite compositesThe conductivity of surface-modified ferrites is important for electric applications,and electromagnetic-radiation shield-ing by the composites[13],while for the corrosion protection the conductivity is of secondary interest.The zinc ferrite is non-conducting,its conductivity being2×10−9S cm−1.The conductivity of a PANI-coated ferrite exhibits typical percola-tion behavior(Fig.7).This means that the conductivity starts to grow exponentially,after the volume fraction of conducting component exceeds a so-called percolation threshold,up to the conductivity of neat PANI prepared in0.4M phosphoric acid, 6.5S cm−1.The percolation limit is estimated to be located at 3–4vol%of PANI.A quantitative analysis could not be per-formed,because the samples of low PANI content,specially prepared for this purpose,could not be compressed into pellets, so their conductivity could only be estimated on powders.The percolation limit for polymer microparticles coated with a con-ducting polymer has often been found to be below10vol%of the conducting component[40,44,45].The experimental values of the percolation parameters for the PANI–ferritecomposite Fig.7.The conductivity of PANI–ferrite composites containing various volume fractions of PANI phosphate.Table1The properties of ferrite coated with polyaniline in aqueous solutions of phos-phoric acid of various molar concentrations,[H3PO4]a[H3PO4](mol L−1)Composition(wt%PANI)Density(g cm−3)Conductivity(S cm−1)Coating(%) 010.23.980.01475.1 0.212.73.820.3380.6 0.412.03.610.1372.60.613.93.701.0590.11.011.73.831.0193.11.514.43.611.5582.32.012.93.701.6986.03.012.43.681.8388.5a0.1M aniline was oxidized at20◦C with0.125M ammonium peroxydisul-fate.We refer to the PANI prepared in the presence of phosphoric acid as to PANI phosphate.This need not be precise,because PANI can be protonated with dihydrogen phosphate counter-ions,as many indications suggest.found in the present case thus confirm the coating of the parti-cles;the percolation threshold for mixtures of ferrite and PANI would be considerably higher[46].4.3.The coating of ferrite in media of various aciditiesThe acidity of the reaction medium affects the polymer-ization of aniline;for this reason,this parameter has been taken into account in the coating of ferrites.Aqueous solutions of phosphoric acid of various concentrations were therefore used as reaction media.In the absence of acid,the polymer-ization proceeds surprisingly well,with good yield and coat-ing level(Table1).The conductivity of the composite was 1.4×10−2S cm−1,i.e.,the same as that of neat PANI prepared under the same reaction conditions in the absence of ferrite[26]. When phosphoric acid was present in the reaction mixture,the effect of the acidity was relatively weak.The content of PANI in the composite and the degree of coating are comparable in all cases.The conductivity increased with increasing acid concen-tration to the level of1–2S cm−1.This is a relatively high value, considering the10–14wt%content of PANI in the composite.92J.Stejskal et al./Journal of Colloid and Interface Science298(2006)87–93The conductivity of neat PANI prepared in1.2M phosphoric acid was4.8S cm−1[26].4.4.Coating without an external oxidantCoating the ferrites with a conducting polymer can also be achieved also without using a conventional oxidant,like am-monium peroxydisulfate.Iron(III)compounds themselves act as oxidants,the oxidation of pyrrole with iron(III)chloride to polypyrrole being an example[10,47].The self-induced poly-merization of pyrrole on iron(III)-oxide particles,α-Fe2O3,has also been reported[48].When a zinc ferrite powder was immersed in a solution of aniline in1M hydrochloric acid in the absence of ammonium peroxydisulfate,and the suspension was occasionally shaken, the ferrite powder darkened in a period of days.In the case of polypyrrole,Partch et al.[48]and later Chen et al.[10] concluded that the polymerization was initiated directly at the oxide surface.This seems to be also the case with PANI,where the polymerization at surfaces is preferred to the same process in the surrounding aqueous phase[38].After one month,the coated ferrite was separated on afilter and dried.It had a good conductivity,0.11S cm−1.Although the polymerization was slow,yet it is feasible.A green PANIfilm also developed on the glass walls of the reaction vessel,demonstrating that iron(III) ions were present in the liquid phase,i.e.,the ferrite partly dis-solved in the acid.The ferrite mass-loss was marginal,and was compensated by the deposited PANI.5.ConclusionsEven a low amount of PANI,1.8wt%,produced on the zinc ferrite particles during the in situ polymerization of ani-line,results in a high degree of coating,45%.On the other hand,the coverage was always incomplete,even at high PANI content>80vol%.This is likely to be beneficial for the appli-cation of the surface-modified ferrite in corrosion protection, because of the potential synergism of ferrite and conducting polymer.The conductivity of composites exhibits percolation behavior,the threshold being located at ca.3–4vol%of the conducting component.Despite incomplete coating with PANI, the conductivity of PANI–ferrite composites was of the order of 10−4–10−1S cm−1above10vol%PANI content.A composite having a conductivity of1.4×10−2S cm−1 was produced when aniline was oxidized in the absence of any acid.Increasing the concentration of phosphoric acid in the reaction mixture to3M increased the conductivity of the com-posite to1.8S cm−1at12wt%content of PANI.The ferrite itself can act as an oxidant of aniline,and the PANI coating is slowly obtained on its surface in strongly 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