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Electrochemical Synthesis and Characterization of Redox Polymer Nanostructures I. Tchepournaya,† S. Vasilieva,† S. Logvinov,† A. Timonov,*,† R. Amadelli,‡ and D. Bartak§ Herzen State Pedagogical University of Russia, Department of Chemistry, Moyka 48, St. Petersburg, Russia, 191186, ISOF-CNR (sez.Ferrara), Dipartimento di Chimica, Universita` di Ferrara, via L.Borsari, 46, 44100 Ferrara, Italy, and University of Northern Iowa, Department of Chemistry, MSH, Cedar Falls, Iowa 50614 Received February 18, 2003. In Final Form: June 10, 2003 Template-based electrochemical methods for the synthesis of nanostructures on the basis of new redox polymers poly[M(Schiff)] (M ) Ni, Pd; Schiff ) tetradentate Schiff bases) with nanowire diameters in the range of 20-200 nm were developed. Polymeric nanostructures were characterized by means of electron microscopy, cyclic voltammetry, and dc-conductometry. Elaboration of data from all techniques revealed that nanostructures based on redox polymers have conductivities that are more than an order of magnitude greater than those of bulk samples of the same polymers when these polymers function in the electrochemical system (redox conductivity) and in the dry state (polaron conductivity).
Introduction The synthesis of new nanostructured materials and the study of their properties (electrochemical, catalytical, and optical), compared to those of macroscopic samples of the same materials, are attracting increasing interest in modern chemical science. The transition between bulk and molecular scales often leads to dramatic changes in the properties of a material, which can be interesting for the practical applications in catalytic and analytical systems, sensors, and optoelectronic devices.1-3 One of the most well developed and promising methods for producing nanostructured materials is that of template synthesis, which was first introduced by the research group of C. R. Martin.4 This method typically entails the chemical or electrochemical synthesis of the desired material within the pores of micro- and nanoporous templates, most often membranes. This method is very general: to date there have been reports on obtaining micro- and nanostructures of metals, electronically conducting and electronically insulating polymers, semiconductors, carbons, and other materials in the form of nanotubules and nanowires of different diameters and lengths. A broad choice of template membranes with a uniform distribution of pores of equal size is commercially available, and their use permits the formation of nanomaterials with a high degree of spatial regularity. It is possible to use nanomaterials inside the matrix (so-called nanoelectrode ensembles) as well as after dissolution (so-called nanobrushes). We report herein on the results that represent, to the best of our knowledge, one of the first attempts to synthesize and characterize new nanostructured materials based on polymeric transition metal complexes poly[M(Schiff)]
where M ) Ni, Pd and Schiff ) tetradentate N2O2 Schiff * Corresponding author. E-mail: amt@ mail.lanck.net. † Herzen State Pedagogical University of Russia. ‡ Universita ` di Ferrara. § University of Northern Iowa.
base ligands: N,N′-ethylene-bis(salicylidenimine) (SalEn) and N,N′-ethylene-bis(3-methoxysalicylidenimine) (CH3OSalEn). In our previous work, we described methods for the synthesis of the series of [M(Schiff)] complexes and presented results of systematic investigations on polymers obtained via oxidative electrochemical polymerization of these complexes on inert conventional electrodes.5,6 Polymer-transition metal complexes with Schiff base ligands consist of stacks that are formed via donor-acceptor interactions between a metal center of one monomer fragment and a phenyl ring of a ligand that is part of another monomer unit.5,7-9 These polymers possess a number of unique properties, such as high directional redox conductivity, electrochromic behavior, and selective catalytic activity in heterogeneous reactions including electrocatalysis,10 that make them suitable for application in a variety of systems. It is possible to change the properties of electropolymerized [M(Schiff)] complexes by changing the substituents in the ligand environment, the chemical nature of the metal center, the solvent, and the supporting electrolyte.7,8 The key property of conducting polymers that determines their application in different devices is the rate of charge transport which, in turn, correlates with such important characteristics of polymer-metal complexes as stability and kinetics of formation. Poly[M(Schiff)] complexes possess two types of conductivity: (i) redox conductivity, which has paramount importance for conductivity at the solution interface; (ii) polaron conductivity, which is responsible for conductivity in the dry state. In solutions, charge transport takes place along polymer (1) Bracht, H.; Nichols, S. P.; Walukiecwicz, W.; Silveira, J. P.; Briones, F.; Haller, E. E. Nature 2000, 408, 67. (2) Paolo, U.; Moretto, L. M.; Bellomi, S.; Menon, V. P.; Martin, C. R. Anal. Chem. 1996, 68, 4160. (3) Malinauskas, A. Synth. Met. 1999, 107, 75. (4) Hulteen, J. C.; Martin, C. R. J. Mater. Chem. 1997, 7, 1075. (5) Popeko, I. E.; Vasiliev, V. V.; Timonov, A. M.; Shagisultanova, G. A. Russ. J. Inorg. Chem. 1990, 35, 933. (6) Vasilieva, S. V.; German, N. A.; Gamankov, P. V., Timonov, A. M. Russ. J. Electrochem. 2001, 37, 363. (7) Vasilieva, S. V.; Balashev, K, P.; Timonov, A. M. Russ. J. Electrochem. 1998, 34, 1090. (8) Vasilieva, S. V.; Balashev, K, P.; Timonov, A. M. Russ. J. Electrochem. 2000, 36, 85. (9) Tchepournaya, I. A.; Gamankov, P. V.; Rodyagina, T. Y.; Vasilieva, S. V.; Timonov, A. M. Russ. J. Electrochem. 2003, 39, 348. (10) Dahm, C. E.; Peters, D. G. J. Electroanal. Chem. 1996, 406, 119.
10.1021/la030060t CCC: $25.00 © 2003 American Chemical Society Published on Web 09/16/2003
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Table 1. Characteristics of the Membranes Used
pore diameter, nm
pore surface area in the planar section of the membrane, cm2, referred to 1 cm2 of membrane visible surface area
average center-to center distance between individual pores, nm
200 100 40 20
0.189 0.047 0.008 0.002
200 300 360 380
stacks via electron hopping between adjacent metal centers through the system of conjugated π-bonds in the ligand environment (redox conductivity).11 Twisting and breaking of individual stacks as well as possible interactions between stacks inhibit the conductivity of redox metal polymers. It has been shown4 that the synthesis of nanostructured organic conducting polymers provides materials with new and improved characteristics. In particular, Martin et al.12 have shown earlier that in nanostructured organic conductive polymers, such as polypyrrole, the electronic conductivity is higher than in bulk samples of the same polymer because in the first case the polymer material is aligned and ordered; there are also fewer defects in the polymer structure. There is then reason to expect that in the template-synthesized poly[M(Schiff)] redox polymers the structure will be more regular and an interaction between individual polymer stacks will be weaker. Based on Martin’s work, a significant increase in the rate of charge transport as well as changes in the electrochemical and catalytic properties should be observed in nanostructured redox polymers. In the following we discuss, in particular, templatebased methods for the synthesis of nanostructures of different redox polymers and analyze results of studies on the processes of charge transport in a nanostructured polymer-modified electrode in the solution phase and in the dry state. Experimental Section Materials. The complexes N,N′-ethylene-bis(salicylideniminato)nickel(II), N,N′-ethylene-bis(salicylideniminato)palladium(II), N,N′-ethylene-bis(3-methoxysalicylideniminato)nickel(II), and N,N′-ethylene-bis(3-methoxysalicylideniminato)palladium(II) were synthesized according to the procedures described in the literature.5,7 Acetonitrile (Aldrich) was refluxed over CaH2 before use. TBABF4 and TBAPF6 (both Fluka) were used as received and dried in an oven at 60 °C prior to use. Alumina membranes were obtained from Anopore. Filters with pore diameters of 20, 40, 100, and 200 nm and a pore density of 6 × 108 pores/cm2 were used to make nanostructured polymer-metal complexes. Table 1 contains information on the porous structure of the alumina membranes used in this work. Instrumentation. Electrochemical measurements were performed using a SVA-1B potentiostat. Nanostructured polymers were synthesized in a quartz electrochemical cell (Figure 1). A membrane with a gold layer sputtered on one of its surfaces was used as a cell wall. The electrochemical cell for electrochemical measurements was a standard three-electrode cell with separated compartments for the working, auxiliary, and reference electrodes. Either a Pt gauze or a piece of glassy carbon with a 6-cm2 surface area was used as an auxiliary electrode. All potentials refer to a Ag/AgCl (saturated aqueous NaCl) reference electrode. Cyclic voltammograms were recorded in either 0.1 M TBABF4 or TBABF6 solutions in acetonitrile. A scanning electron microscope, CamScan 4-88 DV 100 from Cambridge Instruments, was used to provide information on the morphology of the nanostructured polymer-metal complexes. (11) Vasilieva, S. V.; Tchepournaya, I. A.; Logvinov, S. A.; Gamankov, P. V.; Timonov, A. M. Russ. J. Electrochem. 2003, 39, 344. (12) Cai, Z.; Lei, J.; Liang, W.; Menon, V.; Martin, C. R. Chem. Mater. 1991, 3, 960.
Figure 1. Schematic view of an electrochemical cell for the formation of nanostructured materials. RE, reference electrode; AE, auxiliary electrode; WE, working electrode (template membrane with a deposited gold contact layer). The resolution of the instrument was approximately 50 Å. The cross sections of the membranes were examined to determine the length of the nanowires. The nanostructured electrode surface was studied after the template membrane was dissolved. For all the samples with different pore diameters, the length of the polymer nanowires was defined with a precision of 100 nm. The conductivity of nanostructured redox polymers in the dry state was measured with the Keithley 6517 electrometer. The conductivity was measured in the cross direction using a twoprobe method. Gold-covered contact electrodes with a surface area of either 1 cm2 or 0.25 cm2 were used. The reproducibility of results was higher than 90%. Gold was evaporated on the membrane surface using the Edwards S-150 sputter coater. The thickness of the resulting gold layer was from 30 ( 1 nm up to 500 ( 50 nm. Procedures. A thin gold layer was sputtered on one of the membrane surfaces. This metal layer was used to make electrical contact with the system (served as a working electrode). Polymermetal complexes were deposited inside the pores of the template membrane from 0.1 M TBABF4 solutions in acetonitrile containing 10-3 mol dm-3 of the initial monomer complex. Polymerization was carried out under both static and dynamic conditions. In the static regime, polymers were obtained by applying a constant potential Ef to the working electrode during the period of time τf. The applied potential was chosen so as to provide the maximum rate of polymer formation; its value depends on the nature of the monomer (poly[Ni(SalEn)], +1.10 V; poly[Ni(CH3O-SalEn)], +1.05 V; poly[Pd(SalEn)], +1.08 V; poly[Pd(CH3O-SalEn)], +0.98 V).6 The dynamic regime entailed cycling the potential of the working electrode between 0 and 1.1 V; the scan rate Vs was 0.05 V s-1. In both regimes, the formation time τf depended on the required length of the polymer nanowires (from 100 nm to 4 µm) and varied from 10 min to 48 h. The correlation between τf and the length of the nanowires was determined on the basis of the results of electron microscopy measurements. In a series of experiments, polymer nanowires were formed on the Pt nanowires preliminarily deposited into the pores of the membrane. In this case, each nanowire consisted of a metal and a polymer part, and the metal part was used to make electrical contact with the system. A gold layer was sputtered on the membrane surface as described above. Pt nanowires were deposited into the pores via cycling the potential of the working electrode between -0.3 and +0.4 V at the scan rate 0.05 V s-1. A solution containing 10-2 mol dm-3 H2PtCl6 in 0.5 mol dm-3 H2SO4 was used. The deposition time varied from 36 to 40 h depending on the required length of the nanowires, which was from 56 to 60 µm (thickness of the membrane). After the formation of Pt nanowires, the cell with the membrane mounted on it was rinsed with double-distilled water and dry CH3CN and then dried under
Redox Polymer Nanostructures a vacuum. Subsequently, polymer nanowires were formed on top of the metal nanowires according to the procedure described above. All electrochemical measurements were started after at least a 15-min interval from the moment when the solution was poured into the cell. This period of time was essential for the membrane to become completely wetted by the electrolyte. Following determination of the length of the polymer nanowires from electron micrographs, a 5 mm × 5 mm piece of the template membrane with the polymer nanowires deposited into the pores was fixed to an 8 mm × 100 mm strip of a copper foil using silver conductive adhesive (“KEEP”; silver-based acrylic varnish, art.7/ 188), with the gold layer facing the foil. This was done to increase the mechanical stability of the composite membrane. Except for the outside membrane surface, the electrode was isolated with a silicone paste (Bison, Italy) whose chemical stability in CH3CN was tested at room temperature during 24 h by means of UV-vis spectroscopy. The area of the noninsulated part of the membrane was measured. The membrane was then dissolved by immersing it in 2 M NaOH for 5 min. The electrode was very carefully rinsed with ethanol and dry CH3CN and transferred while wet into a standard three-electrode electrochemical cell. Thus, the working part of the composite electrode contained a gold layer (and sometimes Pt nanowires) responsible for the electrical contact to the system and nanowires of redox polymers distributed across the surface of this gold layer. When the nanostructured polymer electrode ensemble was dried, the reproducibility of the results decreased dramatically, most likely because of the partial destruction of the nanowires. Nanostructured polymer samples for conductivity measurements in the dry state were prepared as follows. Nanobrush Pt was synthesized inside the membrane pores according to the procedure described above so that Pt fills the pores not less than 90%. These Pt nanowires were used as working electrodes to grow the polymer throughout the remaining thickness of the membrane. The electropolymerization process was terminated when the polymer appeared at the membrane surface. The polymer film was removed from the outer membrane surface with a 3M Scotch brand Magic tape (No. 810);13 the integrity of the conducting nanostructures inside the membrane, after removing the excess protruding from the surface, was verified by electron microscopy measurements. The samples were then rinsed with CH3CN and stored in a drybox filled with argon for 24 h. After that, the gold-free side of the membrane surface was covered with a thin layer of colloidal graphite/2-propanol solution (BIORAD Microscience Division), which produces a thin highly conductive film on the membrane surface upon evaporation of the solvent. The samples that were dried in an argon atmosphere for 24 h were placed between two flat gold-covered electrodes and pressed at 3.45 MPa. The surface contact area was chosen to provide measured resistance in the range from 5 to 100 Ω. The length of the polymer part of the nanowires was determined by examining the cross section of the membrane by means of scanning electron microscopy after all the conductivity measurements were made. Comparison between nanostructured and conventional redox polymers was conducted on the basis of 1 cm2 of surface area occupied by the polymer. This was done on the basis of the data, given in Table 1, on the porous structure of the template membranes used in this work.
Results and Discussion Electrochemical Polymerization. Figure 2 shows cyclic voltammetry curves recorded for the anodic polymerization of 1 mM [Pd(CH3O-SalEn)] inside the pores of an alumina membrane (20 nm pore diameter) in acetonitrile containing 0.1 M TBABF4. Curve 1 shows a typical voltammogram for the first few potential cycles, and curve 2 refers to the voltammogram recorded at the end of the planned potential cycling experiment. Because of the high ohmic resistance of the solution inside the pores of the membrane, a significant shift of the cathodic and anodic peaks and a distortion of the shape of the curves were observed compared to the analogous voltammograms for the growth of a conventional polymer on the electrode.6 (13) Menon, V.; Martin, C. R. Anal. Chem. 1995, 67, 1920.
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Figure 2. Cyclic voltammogram showing the anodic polymerization of 1 mmol dm-3 [Pd(CH3O-SalEn)] in 0.1 mol dm-3 TEABF4/CH3CN inside the 20-nm-diameter pores of the alumina membrane, between 0.0 and 1.2 V at 0.05 V s-1: (1) 5th cycle; (2) last, 580th cycle.
Figure 3. SEM image of the cross section of an alumina membrane with 20-nm-diameter pores. There are three layers in the picture: Pt nanowires that are used to make electrical contact to the system (1); 600-nm-long nanofibrils of poly[Ni(SalEn)] (2); fragments of the polymer protruding from the membrane surface (3).
In both cases, one observes a constant increase in the peak currents for consecutive cycles and, in accord with our previous work on the oxidation behavior of transition metal complexes with Schiff base ligands,5,7 we conclude that the product of [Pd(CH3O-SalEn)] oxidation is a polymer whose formation takes place even inside the pores with the smallest diameter, for the whole range of pore sizes used in this study. As mentioned above, scanning electron microscopy can be used to estimate the length of the polymer nanowires obtained inside the pores of the template membranes, and for this purpose it is most convenient to examine cross sections. Figure 3 shows a typical image of a nanostructured poly[Ni(SalEn)] inside a porous membrane with 20nm-diameter pores. The length of these polymer nanowires is 0.6 ( 0.1 µm; nanowires of the same length were obtained in the case of the poly[Pd(CH3O-SalEn)] discussed above. Dissolving the alumina membrane in 2 M NaOH evidences a “brush” nanostructured polymer that extends throughout the thickness of the membrane. Polymer nanowires obtained in a 100-nm pore diameter membrane are 3 µm in length and are shown in the SEM micrograph of Figure 4.
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Figure 4. SEM image of poly[Ni(CH3O-SalEn)] nanowires (diameter of nanowires, 100 nm; nanowire length, 3 µm).
The electronic conductivity of polymer complexes is crucial for their electrochemical formation inside the narrow pores of templates. We were able to prepare only poly[M(Schiff)] nanostructures that possess maximal electronic conductivity. Attempts to produce nanowires of polymeric metal complexes with such ligands as N,N′ethylene-bis(5-chlorosalicylidenimine) and N,N′-ethylenebis(5-bromosalicylidenimine) failed because of the comparatively low electronic conductivity of these polymers.11 Cyclic Voltammetry Studies. Figure 5 shows typical electrochemical responses of nanostructured polymers compared with the nonstructured analogues in the supporting electrolyte solutions. Parts a and b of Figure 5 depict cyclic voltammograms for poly[Pd(CH3O-SalEn)] and poly[Ni(SalEn)] synthesized inside membranes with pore diameters ranging from 20 to 200 nm and on the surface of a macroelectrode, respectively. In experiments with nanostructured polymer electrodes in acetonitrile containing 0.1 M TBABF4, the measured parameters remain fairly stable throughout the experiment (1-2 h) except for the first cycle which is usually different from the subsequent scans.10 The voltammograms shown in Figure 5 were recorded after constant current conditions were achieved; each curve is an average of at least four parallel experiments. The reproducibility of results was not less than 75%, the lowest being observed in the case of small-diameter nanowires possibly due to their relatively low mechanical stability. In this context, the data for electrodes obtained from 40-nm-diameter and especially from 20-nm-diameter pore templates can be considered as the lowest possible limit for electrochemical use because partial loss of the electroactive compound during dissolution of the template membrane, rinsing, and transferring the electrodes is highly possible. All voltammograms analogous to those of Figure 5 feature an increase of the peak currents when the nanowire diameter decreases. Figure 6 shows the dependence of the anodic maxima in the voltammograms for poly[Pd(CH3O-SalEn)] on the diameter of the nanowires. Two voltammetry peaks are seen during the oxidation of this polymer in the supporting electrolyte solution (Figure 5a)
Figure 5. Cyclic voltammograms of a poly[Pd(CH3O-SalEn)] nanostructure (a) and a poly[Ni(SalEn)]-modified electrode (b) in 0.1 mol dm-3 TEABF4/CH3CN between 0.0 and 1.2 V at 0.1 V s-1 compared with a nonstructured polymer-modified electrode (Qf g 0.14 C cm-2 in all cases): (1) nonstructured polymer; nanowires with diameter of 200 nm (2), 100 nm (3), 40 nm (4), and 20 nm (5).
which, on the basis of our previous work,8 can be assigned
Redox Polymer Nanostructures
Figure 6. Dependence of anodic peak currents for cyclic voltammograms of poly[Pd(CH3O-SalEn)] nanowire modified electrodes (Figure 5a) on the diameter of polymer nanowires: (1) first anodic maximum (0.3 V); (2) second anodic maximum (0.7 V). Conditions for polymer formation and registration of voltammograms are the same as in Figure 5.
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Figure 8. Dependence of the cathodic peak currents on the square root of the rate of potential scanning for cyclic voltammograms of a poly[Ni(SalEn)] nanowire modified electrode; the diameter of the nanowires is 40 nm. Conditions for polymer formation and registration of voltammograms are the same as in Figure 5. Table 2. Experimental Values of Charge Diffusion Coefficients in Nanostructured Polymer Complexes charge diffusion coefficient, cm2/s (×1010), for the following nanowire diameters (nm)
complex poly[Ni(SalEn)] poly[Ni(CH3O- SalEn)] poly[Pd(SalEn)] poly[Pd(CH3O-SalEn)] average values a
Figure 7. Schematic view of a polymer nanostructure modified electrode used for cyclic voltammetry studies: (1) common gold film; (2) polymer nanowires; h is the segment of the nanowires that takes part in redox processes during potential scanning under semi-infinite diffusion conditions.
to the removal of one electron from the metal-centered molecular orbital of the polymer (peak I) and to electron transfer from the ligand-centered orbitals (peak II). As a general feature, peak currents increase most significantly when the diameter of nanowires becomes smaller than 100 nm, even not considering that the obtained data represent lower limiting values due to the partial destruction of nanowires in the samples. When voltammograms are characterized by wellresolved anodic and cathodic waves, a decrease in the diameter of the nanowires causes the difference between anodic and cathodic peak potentials to become smaller, indicating that the reversibility of charge transfer in the system increases. The case of poly[Ni(SalEn)], for example, falls into this type of behavior since this complex has ligand and metal-centered orbitals that are close in energy and, as a result, the potentials corresponding to the two maxima almost completely overlap.7 In all cases examined here, oxidation and reduction of the polymer take place in a regime of semi-infinite diffusion,14 meaning that during the potential scan only polymer layers adjacent to the electrode surface are oxidized and reduced (Figure 7) while remote parts of the polymer stacks do not participate in redox processes. The concentration of redox sites in these layers remains constant and does not depend on potential. To make sure (14) Khannanov, N. K.; Yatsun, T. F.; Shafirovich, V. Y.; Strelets, V. V. Acad. Sci. SSSR News, Chem. Ser. 1983, 20, 1282.
nonstructured polymera 2.5 ( 0.5 5.5 ( 0.5 1.5 ( 0.5 2.0 ( 0.5
200
100
40
20
5(1 7(1 6(1 5(1 6(2
10 ( 1 12 ( 1 11 ( 1 12 ( 1 11 ( 2
25 ( 4 24 ( 4 26 ( 4 24 ( 4 25 ( 5
36 ( 6 38 ( 6 33 ( 6 35 ( 6 36 ( 9
Reference 11.
that all electrochemical measurements were made under semi-infinite diffusion conditions, we plotted the dependence of anodic and cathodic peak currents on the square root of the scan rate Vs1/2 for each polymer and pore size value.14 Optimization of redox processes was achieved by preparing polymer nanowires with maximum length and by using high rates of potential scanning. The amount of charge passed through the system during oxidative polymerization of corresponding monomers was not less than 0.14 C cm-2 for all cases. Figure 8 shows an example of the dependence of cathodic peak currents versus Vs1/2 for the case of nanostructured poly[Ni(SalEn)] with the 40-nm-diameter nanowires. Similar linear plots were obtained for all studied cases, which indicates semi-infinite diffusion control and allows us to use the data shown in Figure 5 to calculate the charge transport diffusion coefficients, Dct, from the Randles-Sevcik equation.15 In Table 2, we compare the obtained values with analogous values for macrosamples of the same polymer. The values of charge diffusion coefficients (Dct) for macrosamples of different polymers differ by over a factor of 3, while for nanostructured polymers the charge diffusion coefficient seems to depend only on the diameter of nanowires and not on the chemical structure of the polymer. As the diameter of nanowires decreases, the values for Dct increase significantly for all cases investigated. Charge diffusion coefficients Dct reflect either the rate of electron transfer in the polymer, De, or the rate of migration of charge-compensating counterions, Di, depending on the limiting stage of charge transfer in the system. We determined the nature of the limiting stage of charge transport in the nanostructured polymers by (15) Nicholson, R. S.; Shain, I. Anal. Chem. 1965, 37, 178.
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Figure 9. Cyclic voltammograms for a poly[Ni(SalEn)] nanowire modified electrode (diameter of nanowires is 40 nm) in 0.1 mol dm-3 TEABF4/CH3CN (1) and 0.1 mol dm-3 TEAPF6/CH3CN (2) between 0.0 and 1.2 V at 0.1 V s-1.
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studying the influence of different anions of the supporting electrolyte (BF4- and PF6-, with diameters 3.33 and 4.35 Å, respectively16) on the value of Dct. For all nanostructured polymers and all template pore diameters used in this study, the rate of charge transport in the polymer decreases when the size of charge-compensating anions increases. Figure 9 shows voltammograms of nanostructured poly[Ni(SalEn)] recorded in acetonitrile containing TBABF4 and TBAPF6 as supporting electrolytes. The dependence of voltammetric characteristics on the size of the counteranions of the supporting electrolyte indicates that the charge transfer in nanostructured polymers is limited by migration of counterions, and the values of Dct calculated above characterize the rate of this process. The higher values of Di for nanostructured polymers in comparison with macrosamples of the same polymers, and the significant increase of Di with decreasing nanowire diameter, can be explained by the fact that in the bulk polymer ions migrate in narrow spaces between individual stacks, with the average distances between stacks being
Figure 10. Scheme illustrating the diffusion conditions of charge-compensating anions in conventional polymers (a), in redox polymer nanowires (b), and with randomly oriented conducting nanowires (c).
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Figure 11. SEM image showing nonideal orientation of nanowires of poly[Pd(SalEn)]; the diameter of nanofibrils is 100 nm; the length is 2.5 µm.
comparable to the diameter of the stack (Figure 10a). In the nanostructured polymer, there is a possibility for counterions to move in the spaces between polymer nanowires that are separated by significant distances, ranging from 200 to 380 nm (Table 1, Figure 10b). When redox processes occur within the inner stacks, ions can move in and out of the nanowire through the side surface. We did not expect that migration of ions would remain the limiting stage of charge transport even for polymers with the smallest diameter of nanowires, where the distances that the ions travel inside the nanowire are not big. One can predict instead that for the case of nanostructures, the charge transfer in the polymer should be limited by the transport of electrons along the polymer stack. The reason for the observed phenomenon lies most likely in the nonideal structure of the nanostructured polymer such as, for example, the nonperpendicular orientation to the electrode found in long nanowires (Figure 10c, Figure 11). The data that we obtained to date do not allow us to draw ultimate conclusions on the change of the rate of electron transport between the polymer redox centers in nanowires because the rate of charge transfer is limited by the rate of diffusion of charge-compensating counterions. However, we can provide some indirect proof of an increase in the rate of electron transfer in the investigated stacked redox polymer nanostructures. Thus, for example, earlier we established that for nonstructured poly[Ni(SalEn)] the rates of electron and ion transport are very close.11 For nanostructured poly[Ni(SalEn)], we observe an over 10-fold increase of Di (Table 2) and, at the same time, the diffusion of counterions remains the limiting stage of charge transfer. This clearly indicates that also the rate of electron transport increases in nanostructured redox polymers compared to the nonstructured materials. Studies of the Conductivity of Redox Polymers in the Dry State. In the dry state, the polymer-metal complexes studied in this work possess polaron conductivity due to the presence of conjugated π-bonds in the ligand environment. Previous studies concerning the influence of the preparation method of organic polymer
Figure 12. Schematic side view of a sample containing redox polymer nanostructures used for measuring conductivity of polymers in the dry state.
nanostructures on their conductivity17 show that the increased supramolecular order of nanostructured systems, due to the better alignment of nanowires in the nanopores of the template membrane, together with a less likely twisting and breakage of the polymeric chains, leads to better π-conjugation of individual fragments of polymeric chains. The last factor explains the increase of polaron conductivity in nanostructured organic polymers.12,17 The most dramatic increase occurs for polymers deposited into the pores of the smallest diameter template (
