Electrochemistry of conductive polymers XXXV

Electrochimica Acta50(2005)3085–3092Electrochemistry of conductive polymers XXXV:Electrical and morphological characteristics of polypyrrolefilms prepared in aqueous media studied by current sensing atomic force microscopyDong-Hun Han,Hyo Joong Lee,Su-Moon Park∗Department of Chemistry and Center for Integrated Molecular Systems,Pohang University of Scienceand Technology,Pohang,Gyeongbuk790-784,KoreaReceived24August2004;received in revised form19October2004;accepted22October2004Available online11April2005AbstractElectrical and morphological characteristics of polypyrrole(PPy)films electro-deposited in four different aqueous electrolyte solutions including p-toluenesulfonate,dodecylsulfate,poly(styrenesulfonate),and perchlorate have been investigated using current-sensing atomic force microscopy.Experimental parameters including the electrolyte,applied current density,and deposition time were shown to affect the morphologies and current images of PPyfilms thus prepared.The PPyfilms were galvanostatically prepared under the identical conditions except for different electrolytes;the globular-shaped topographic structures with the unusually large sizes were obtained in dodecylsulfate. The p-toluenesulfonate doped PPyfilm showed the highest average conductivity whereas the perchlorate doped one showed the lowest of all thefilms prepared.©2005Elsevier Ltd.All rights reserved.Keywords:Polypyrrole;Current sensing AFM;Electrolyte effects;Conductivity1.IntroductionSince polyacetylene was shown to have high electrical conductivities when properly doped[1],␲-conjugated poly-mers have been studied extensively from both fundamental and practical points of view because these classes of poly-mers showed possibilities of a wide range of applications [2].Recently,they are regaining a great deal of attention owing to growing interests in“organic electronics”includ-ingfield effect transistors[3],light-emitting diodes[4],solar cells[5],electrochromic devices[6],electronic circuits[7], sensors[8],and other devices[9]because of their favorable processibility,reasonable stability,low cost,and possibilities of tailoring the structures on the molecular scale.Most of the␲-conjugated(conducting)polymers are straightforwardly prepared by chemical and/or electrochem-∗Corresponding author.Tel.:+82542792102;fax:+82542793399.E-mail address:smpark@(S.-M.Park).ical methods and their electronic properties can be reversibly changed between insulating and conducting states by chemi-cal and/or electrochemical doping reactions[2].PPy is one of the most studied conducting polymers because of its rather straightforward preparation methods and reasonable stabil-ities in air and aqueous media.However,it is well known from a number of experimental studies that various param-eters such as the solvent,electrolyte,electrode,deposition time,temperature,current density,and applied voltage all af-fect both its structures and electroactivities[2].Thus far,the conductivity data obtained from macroscopic experiments along with spectroscopic data have been used to describe its various electrical states depending on its preparation meth-ods and doping levels[2].Thus,primarily the bulk electrical properties were measured and averaged out over many inho-mogeneous regions.However,in order to ultimately control the electrical properties of conducting polymerfilms and to use those in organic electronics on nanometer scales,it is necessary to make the electrical measurements on nanome-0013-4686/$–see front matter©2005Elsevier Ltd.All rights reserved. doi:10.1016/j.electacta.2004.10.0853086 D.-H.Han et al./Electrochimica Acta50(2005)3085–3092ter scales.Also,as the dimension of devices becomes smaller, the change in electrical properties and its measurements on nanometer scales would be increasingly important.Atomic force microscopy(AFM)is a widely used sur-face characterization tool with nanometer scale resolution, some modifications of which can give a variety of otherwise impossible valuable information[10].The current-sensing AFM(CS-AFM)with its conducting tip allows not only the topographical and current images to be obtained simultane-ously but also current–voltage traces to be recorded on se-lected spots of the image,where one wants to study elec-trical characteristics on a nanometer scale[11].This tech-nique has been applied to various nanostructures such as single molecules[12],biomolecules[13],carbon nanotubes [14],self-assembled monolayers[15],quantum dots[16],and other nanostructures[17]because it allows easy and repro-ducible contacts to be made with various substances without having to use the difficult lithography processes.Thus,the CS-AFM has a number of advantages in studies of electri-cal properties of conducting polymerfilms.Recently,we and other investigators applied this technique successfully to the studies of conducting polymerfilms of various doping lev-els,proving its usefulness in the study of doping distributions by obtaining two-dimensional current images and nanoscale electrical properties by measuring the current–voltage char-acteristics[18,19].In the present study,we demonstrate that the electrical properties of PPyfilms are visualized by obtaining the two-dimensional current images under various preparation con-ditions and the two-dimensional current images offer useful data in following and understanding the changes of electri-cal properties of conducting polymerfilms depending on the preparation conditions and doping levels.2.Experimentalp-Toluenesulfonic acid(p-TS:Aldrich,98.5%),sodium dodecylsulfate(SDS:Fluka,99%),poly(styrene sulfonic acid)(PSS:Alfar Aesar,M w:70,000),and lithium perchlo-rate(LiClO4:Aldrich,99.99%)were used as received.Pyr-role(Aldrich,98%)was used after distillation over zinc powder and stored in the dark under the nitrogen atmo-sphere.PPyfilms were electrochemically prepared on gold-on-silicon electrodes(with chromium adhesive layers,LGA Films),after the solutions were de-aerated with the nitro-gen gas(99.0%,BOC Gases).The gold-on-silicon electrodes were annealed for5min with a hydrogenflame prior to their use as a working electrode after they had been rinsed thor-oughly with methanol and deionized water.A platinum foil was used as a counter electrode and an Ag|AgCl(in sat-urated KCl)electrode as a reference electrode.PPyfilms were grown galvanostatically with an EG&G model273 potentiostat–galvanostat after the aqueous solution contain-ing0.10M pyrrole and0.10M electrolyte was purged with nitrogen for1h through an aqueous solution.The solution containing SDS was not purged due to the bubbles gener-ated during the purging process and,thus,the dodecylsulfate (DS−)doped PPyfilm was grown immediately after prepar-ing the solution.After electrochemical synthesis,thefilms were rinsed thoroughly with deionized water to remove ex-cess monomer,oligomer,and electrolyte molecules,and dried under vacuum at room temperature.A Molecular Imaging’s PicoSPM AFM with a current-sensing tip,CS-AFM,was used in a contact mode to simultaneously obtain topographical and current im-ages.The platinum/iridium-coated cantilevers(spring con-stant,0.31–0.41N/m)were purchased from nanosensors (/).The load force was main-tained below6nN to avoid damages of the tip and thefilms. During current–voltage measurements at given spots of the film,a load force was always maintained at6nN.A bias voltage between the substrate(gold electrode)and the con-ducting cantilever(which is grounded)was50mV during all the imaging experiments.Before imaging the PPyfilm cov-ered surfaces,they were purged with the high purity nitrogen gas(99.999%,BOC Gases)to minimize the moisture and water,and all the AFM experiments were carried out under the controlled nitrogen atmospheric environment.3.Results and discussionFig.1shows the topographic(top)and current(bot-tom)images obtained simultaneously for an identical area of1␮m×1␮m on a PPyfilm surface,which was galvano-statically grown by applying an anodic current of0.10mA for100s(=0.238mA/cm2or23.8mC/cm2)in aqueous so-lution containing0.10M p-TS(a),SDS(b),PSS(c),and LiClO4(d).The globular-shaped surface morphologies of the PPyfilms grown in aqueous and non-aqueous solutions are typical[20],although their sizes and shapes are slightly different from each other depending on growth conditions [18a,21].The sizes of DS−-doped PPy globules were signif-icantly larger than those on the other PPyfilm surfaces as can be seen from Fig.1b.Naoi et al.[22]reported columnar structures with globular surfaces when the PPyfilms were grown while the concentrations of the surfactants and anodic charges were varied.Thus,the large globules shown in Fig.1b were formed due to the interaction between DS−micelles and pyrrole monomers[22].The critical micelle concentration for SDS is8mM and large amounts of the micelles must have been formed in the SDS concentration used here[22].The current images shown in Fig.1,which were obtained at a50mV bias voltage,showed bright(high current-flowing) and dark(low current-flowing)regions;so the doping pro-cess of PPyfilms during their preparation in aqueous media is not as homogeneous as in acetonitrile solutions[18a].Sur-face potential images for polybithiophene and PPy recorded by the Kelvin probe method[23]and the current images of doped polyaniline[18b]recorded by CS-AFM also showed inhomogeneity of conducting polymers,but our recent re-D.-H.Han et al./Electrochimica Acta50(2005)3085–30923087Fig.1.Concurrently obtained topographic(top)and current(bottom)images with their cross-sectional analyses for the PPyfilms galvanostatically grown by applying0.1mA for100s(=23.8mC/cm2)in aqueous solution containing0.1M p-TS(a),SDS(b),PSS(c),and LiClO4(d).The scan area was1␮m×1␮m.sults have shown that relatively homogeneous surfaces were obtained for PPy when prepared in acetonitrile[18a].The PPyfilms shown in Fig.1are all inhomogenous and it ap-pears very difficult to obtain homogeneousfilms in aqueous media.In general,the conductivities of PPyfilms prepared in aqueous solutions are lower than those obtained in organic solvents[18a,24].p-TS-doped PPy,however,exhibited sig-nificantly high conductivities even when prepared in aqueous electrolyte solutions[24].The average conductance values obtained from the cross-sectional analyses of the current im-ages in Fig.1are4.76×10−6,1.46×10−6,1.41×10−6,and 4.10×10−9S for p-TS,SDS,PSS,and LiClO4,respectively, when calculated using Ohm’s law from the average currents read and the bias voltage of50mV.The average currents were calculated from those read across the cross sectional current profiles given under the current images,which were drawn using256data points across a1␮m distance.Thus each of these currents was obtained at every about4nm.In order to obtain the vertical conductivity of each PPy film,the contact area of the probe with thefilm was calculated using the Hertz theory[18b,25].The contact area between the tip and the sample was then calculated to be37.4nm2 under our experimental conditions,assuming the Poisson’s ratio of PPy as0.38just as for platinum and Young’s mod-ulus of all PPyfilms as the value(1.1GPa)of p-TS-doped PPy[26].Thefilm thickness was measured from the cross-sectional view of the SEM image,and the average thicknesses are101.7nm(±11.6)for p-TS,239.9nm(±59.7)for SDS,114.0nm(±26.8)for PSS,and53.1nm(±8.9)for LiClO4 doped PPy.The conductivities of the PPyfilms were calcu-lated from these thicknesses and the contact area to be129.4, 102.1,43.0,and5.8×10−2S/cm for p-TS-,SDS-,PSS-,and LiClO4-doped PPyfilms,respectively.These conductivities were calculated with an assumption that the currentflows vertically in one dimension from the substrate to the tip. While the path for the currentflow may not be exactly one-dimension,we believe this assumption is reasonable as the currentflows through the least resistive path of the shortest distance.In calculating the conductivity,the contact resis-tance between the tip and thefilm was not taken into account as not much is known about it.For this reason,the conductivi-ties we present here would be a bit smaller than the true values obtained with a four-probe method.Unfortunately,however, it is not possible to obtain conductivities in the vertical di-rection with the four-probe method.The bulk conductivities of PPy prepared in the aqueous solutions have been reported by Warren and Anderson[24a]to increase in the order of p-TS>SDS>PSS>LiClO4,which is in excellent agreement with our results.One more point to make here is that the thickness of the polymer prepared in these electrolyte so-lutions is in the order of SDS>PSS>p-TS>ClO4−for the identical amount of anodic charges(23.8mC/cm2)used for the preparation.This indicates how important the electrolyte molecules are in making up the polymer matrix.It is thus expected that the amount of the dopant material(counter an-ions in this case)included in the polymer matrix would be3088 D.-H.Han et al./Electrochimica Acta50(2005)3085–3092Fig.2.Current–voltage curves obtained at the highest current-flowing re-gions in the each current image of the PPyfilms in Fig.1.in the same order of the thickness as listed above unless the current efficiencies for polymer synthesis are affected by the electrolyte,which is highly unlikely.Fig.2shows the current–voltage(I–V)curves obtained at the highest current-flowing regions in each current image. The slopes of the I–V curves were5.56×103,5.25×103, 4.63×103,and7.84×102nA/V for p-TS-,SDS-,PSS-,and LiClO4-doped PPyfilms,respectively.The slope of the I–V curve for the ClO4−-doped PPy is much smaller than the others as can be seen in Fig.2.This agrees with the report that the PPyfilms prepared in a solution containing small dopant anions have a low conductivity with a tendency to lose their conductivities easily by exposure to the laboratory atmosphere[24a].A similar observation has been reported on a polyanilinefilm prepared in the perchloric acid electrolyte [18b].The effects of growth time,i.e.,the amount of anodic charges,on topographic and electrical properties have been studied when the PPyfilms were galvanostatically grown in0.10M p-TS at a constant applied current of0.10mA. Fig.3shows topographic(top)and current(bottom)images of PPyfilms galvanostatically grown for different growth pe-riods or different amounts of charges.As shown in Fig.3a, the surface of the gold-on-silicon electrode is almost fully covered with insulating thin layers at the veryfirst stage (at4.76mC/cm2)of the PPyfilm formation,although the current image shows some conductive areas.So,the elec-trode is probably covered with un-doped PPy or pyrrole oligomers at this stage,which is usually observed during PPy synthesis[27],with some conductive PPy islands.In efforts to verify this,we doped thefilm further at0.80and 0.90V for20s and recorded the topographical and current im-ages.The result was that the current increased significantly when doped at0.80V without noticeable changes in topo-graphical image.However,the current was decreased when doped at0.90V suggesting the PPyfilm was over-doped. This indicates that thefilm is covered with poorly doped PPy and relatively larger oligomer molecules rather than small oligomers,which would dissolve away when oxidized in so-lution.After40s(=9.52mC/cm2),a PPyfilm with more conduc-tive islands is obtained as shown in rge aggregates of grains whose areas and heights are broader(a few hundred nm)and higher(as high as about20nm)are seen to form at this stage.Interestingly,the aggregates of grains sticking up are shown to be less conductive than the lower valley re-gions.The grains might have grown on the more conductive spots of the PPyfilm in thefirst stage but the newly formed grains may not be doped with the limited amount of charges, as most of the anodic charges must have been used up for the growth of the grains.As the charge is further increased to16.7mC/cm2(electrolysis time,70s),the large aggregates of grains shown in Fig.3b start to disappear and much of the anodic charges are now used to dope the rather evenly grown film,resulting in larger average conductive areas across the film,as can be seen in Fig.3c.Note that the amount of charges used for thefilm shown in Fig.1a,which is23.8mC/cm2,is between those for Figs.3c and d under the otherwise the same experimental conditions.As more charges are spent(Fig.3d, 47.6mC/cm2),the conductive regions increase with an in-crease in the amount of anodic charges.The line average currents obtained from cross sectional profiles in the current images of Figs.1a and3are14.1,95.2,120.2,283.4,and 651.51nA for an applied bias voltage of50mV at20,40,70, 100,and200s,respectively.This indicates that the amount of doped PPy increases,as the deposition time is increased. Our results shown in Fig.3describe the PPy growth process as:(1)the electrode surface is covered with a thin insulat-ing layer of PPy during the very early stage of the growth with occasional conductive PPy islands(Fig.3a);(2)the PPy then grows laterally at these islands resulting in broadening of the insulating grains of neutral PPy to cover a large areas of the surface with a rather insulating PPyfilm(Fig.3b)and the grain boundaries eventually disappear(Fig.3c);and(3)finally the charges are used to dope the PPyfilm to make it more conductive(Figs.1a and3d).Thus,an appropriate time longer than200s is needed for the preparation of an evenly conductive PPyfilm.Depending on the applied current,a different overpoten-tial is applied to the electrode during the polymerization re-action and the doping level of PPy is changed accordingly. The PPyfilms shown in Figs.1d and4were prepared us-ing an identical charge density(=23.8mC/cm2)by applying0.10mA for100s(Fig.1d),0.5mA for20s(Fig.4a),and1.0mA for10s(Fig.4b).By applying these currents during the polymerization reaction,steady-state potentials of0.65, 0.80,and1.0V were measured,respectively,at the electrode. The cross-sectional line average currents were0.21,5.1,and 34.7nA for thefilms shown in Figs.1d,4a and b,respectively. These results indicate that a more conductivefilm is obtained when a higher current is used even though exactly the same amount of anodic charges is used.Boxall et al.reported that the PPyfilms required an increment in an applied voltage of 72±8mV to induce a tenfold increase in conductivity[28].D.-H.Han et al./Electrochimica Acta 50(2005)3085–30923089Fig.3.Topographic (top)and current (bottom)images (1␮m ×1␮m)with their cross-sectional analyses for the PPy films galvanostatically grown by applying 0.1mA for 20s (=4.76mC/cm 2)(a),for 40s (=9.52mC/cm 2)(b),for 70s (=16.66mC/cm 2)(c),and for 200s (=47.6mC/cm 2)(d)in aqueous solution containing 0.1M p -TS and 0.1M pyrrole.Our PPy films were prepared under different experimental conditions from those used by Boxall et al.and the increase in conductivity for the potential change was not the same as those expected from their relation,but our results indi-cate that a high oxidation overpotential is necessary to obtain a highly doped PPy film.The doping current and potential should also be optimized to obtain better conductive PPy films.Large globules are formed when a surfactant,SDS,is used as an electrolyte as shown in Figs.1b and 5.It is interesting to note from the current images shown in Figs.1b and 5a that the topologically lower borders of these globules arebetterFig.4.Current images (1␮m ×1␮m)with their cross-sectional analyses for the PPy films galvanostatically grown by applying 0.5mA for 20s (=23.8mC/cm 2)(a)and 1.0mA for 10s (=23.8mC/cm 2)(b)in aqueous solution containing 0.1M LiClO 4and 0.1M pyrrole.3090 D.-H.Han et al./Electrochimica Acta 50(2005)3085–3092Fig.5.PPy films galvanostatically grown by applying 0.1mA for 100s in the aqueous solution containing 0.1M SDS (Fig.1b and then de-doped at −0.6V for 5min (a)and for 15min (b)in the 0.1M SDS solution.Topographic (top)and current (bottom)images (1␮m ×1␮m)simultaneously obtained and their cross-sectional analyses.doped than their tops.When the PPy film was reduced at −0.60V for 5min,the current image shown in Fig.5a is very similar to that in Fig.1b,indicating that the borders were still in the doped state.The amount of current flowing,however,is significantly reduced compared with that of the as prepared PPy film shown in Fig.1b.The DS −-doped PPy is almost fully de-doped when reduced at −0.60V for 15min and almost no current flows over the entire area except for a few spots as can be seen in Fig.5b.In DS −-doped poly-mers,the cations,not anions,are known to be inserted/de-inserted during the doping/de-doping process [29].Hallik et al.reported that DS −ions could be easily removed from PPy in alcohol and acetonitrile,but the electrochemical re-duction in aqueous solution was not as effective in reducing the doping level [30].When compared with PPy prepared in acetonitrile [18a],the de-doping process of DS-doped PPy shown in Fig.5is very slow.The topographic images shown in Figs.1b and 5display that the sizes of PPy globulesare significantly reduced as the polymer is thoroughly de-doped (Fig.5b).This suggests that probably the large DS anions leave the PPy matrix upon slowly de-doping the poly-mer as in the case of non-aqueous media [30],which might have led to the collapse of the PPy structure resulting in changes in both the morphology and the electrical conductiv-ity;however,the counter cations of dodecylsulfate are known to be inserted during the de-doping in aqueous solutions [29].4.ConclusionsOur study demonstrates that morphological and electrical properties of PPy films electrochemically prepared in aque-ous media are affected by the solvent,electrolyte,current density,and deposition time (amount of charge spent).Of PPy films thus prepared,the p -TS-doped PPy film showedD.-H.Han et al./Electrochimica Acta50(2005)3085–30923091the highest conductivity while the ClO4−doped one the low-est.The PPyfilms prepared in aqueous electrolyte solutions were in general inhomogeneous in their electrical properties showing much poorer conductivities than those prepared in acetonitrile[18a].The electrolyte plays a very important role in determining the morphological and electrical properties of the PPyfilms.For example,SDS,which forms large micelles in aqueous media,helps form large globules with smaller do-mains not well connected with each other.On the other hand, ClO4−is homogeneously dispersed in the polymer matrix due to its lowest basicity,perhaps providing a high density polymerfilm.The p-TS-doped PPyfilm has the highest con-ductive state due most likely to its relatively high basicity in comparison with that of ClO4−and the high polarizability of the p-TS group.The relatively high basicity would allow the radical cations,i.e.,polarons,of PPy to associate with the dopants rather well,stabilizing the conductive states.These explain the extremes in morphological and electrical proper-ties of this series of PPyfilms we have seen in this study.The growth time dependence boils down to the 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