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J. Phys. Chem. C 2007, 111, 11216-11222
Template-Synthesized Cobalt Porphyrin/Polypyrrole Nanocomposite and Its Electrocatalysis for Oxygen Reduction in Neutral Medium Qin Zhou,† Chang Ming Li,*,† Jun Li,† Xiaoqiang Cui,† and Don Gervasio‡ School of Chemical and Biomedical Engineering & Center for AdVanced Bionanosystems, Nanyang Technological UniVersity, 70 Nanyang DriVe, Singapore 637457, Singapore, and Center for Applied Nanobioscience of the BioDesign Institute, Arizona State UniVersity, 1001 McAllister Road, Tempe, Arizona 85287 ReceiVed: March 27, 2007; In Final Form: May 12, 2007
For the first time, a linear J-aggregate of cobalt porphyrin and its templated, nanostructured cobalt porphyrin/ polypyrrole composite are formed in neutral aqueous solutions. With the assistance of ultrasonication under different preparation procedures, the nanocomposite can be electrochemically synthesized with uniform 2-D and 3-D nanostructures. The 3-D nanocomposite is synthesized by a three-step method and has an interesting regular nanoarray-structure: a vertical nanorod array with uniform diameters (∼50 nm). The resultant nanoarray catalyzes the oxygen reduction mainly through a four-electron pathway to form H2O in a neutral medium, exhibiting excellent electrocatalytic activity. The mechanisms of formation of the nanocomposite and its good electrocatalysis for oxygen reduction are suggested. The new nanocomposite could have broad potential applications in energy storage systems and electrochemical sensors, particularly in a neutral medium for the biofuel cells and biosensors.
Introduction Electrocatalysis of oxygen reduction is a highly interesting research area due to its great importance in fuel cells and sensors. Among all electrode materials reported,1-6 the noble metals are considered the best electrocatalysts for oxygen reduction. However, the disadvantages of high cost, poisoning by impurities, and biofouling in biological systems limit their applications in energy conversion and biosensing systems. Thus, metallomacrocyclic molecules, such as metalloporphyrins have been studied as alternative electrocatalysts in sensors and fuel cell applications.7-10 Further, the best electrocatalysis for oxygen reduction often occurs in strong acidic or alkaline mediums but does not in a neutral electrolyte, due to its involvement of H+ or OH- as a reactant. It is a great challenge to develop a good electrocatalyst of oxygen reduction in neutral electrolytes for applications in biofuel cells and biosensors. The common method to use the metalloporphyrin as an electrocatalyst is to preload it on a porous support, such as carbon particles, and then coat it on a bare electrode.11 The modified electrodes have been shown to be assor mores effective than Pt electrode, at least in a short term.10-12 Unfortunately, this kind of electrode cannot survive for the reasonably long time that is required for practical applications. An attractive approach for making an active and stable modified electrode is to incorporate the metalloporphyrins into conducting polymers such as polyaniline (PANI), polypyrrole (PPY), and polythiophene (PTh). It is known that conducting polymers have been applied in various electrochemical devices.13-18 Among all conducting polymers, PPY is most frequently used, mainly due to its high stability and conductivity in different aqueous * Corresponding author. Fax: 65 6791 1761. Tel.: 65 6790 4485. E-mail:
[email protected]. † Nanyang Technological University. ‡ Arizona State University.
and nonaqueous solvents. Catalytically active molecules such as metalloporphyrins can be doped into such a conducting polymer by entrapment16,17 or by direct electrosynthesis of the catalyst-modified monomers.18 Self-assembly of different molecular components into a large supermolecular structure is investigated primarily because of its involvement in many fundamental physicochemical and biological processes. The possibility of changing the mesoscopic structure of the resultant species through a proper choice of the molecular components opens a way to design and synthesize materials with specific properties and functions. From this point of view, porphyrins are good building blocks because, depending on their electronic and steric properties, they can spontaneously self-assemble into dimers or higher aggregates through noncovalent interactions. In particular, water-soluble porphyrins, such as 5,10,15,20-tetrakis(4-sulfophenyl)-21H,23H-porphine (TPPS), are very interesting candidates since an aggregation process can be conveniently controlled by screening the charge repulsion through changes of the ionic strength and pH.19-22 A variety of such self-assembled molecular aggregates are assumed to adopt a structure that can be classified as J-type or J-aggregate. It is reported that nanorod structures of polyethylenedioxathiophene (PEDOT) and PPY can be obtained by using TPPS aggregates as templates for electropolymerization.23 This gives us a clue to combine the electrocatalytic activity of the metalloporphyrin and its template effect to synthesize some novel, functional composites. However, there is no report about the J-aggregates of metalloporphyrins and their template effect on the electropolymerization of conducting polymers. Particularly, in a neutral electrolyte, it is very hard to synthesize the J-aggregates with a nonmetal porphyrin-based template effect by pH adjustment as reported.19-22 In all the metalloporphyrins, most research works have been carried out with cobalt porphyrin because of its high electrocatalytic activity.8 We report herein a new electrocatalyst for
10.1021/jp072390i CCC: $37.00 © 2007 American Chemical Society Published on Web 07/06/2007
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Figure 1. Synthesis of TPPS-Co.
the oxygen reduction in a neutral medium, which is based on the ultrasonic-assisted template-electropolymerization of a cobalt porphyrin/polypyrrole nanocomposite. The template effect of the cobalt porphyrin aggregates was explored by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) measurements. The electrocatalytic effect of the nanocomposite was studied and evaluated by comparison with that of gold and pure PPY. The performance and mechanism of the oxygen reduction on the nanocomposite was explored by the rotating ring-disc electrode (RRDE) method. Experimental Section Chemicals. All chemicals were of analytical grade. TPPS was purchased from TCI Co., Japan. Pyrrole and cobalt(II) acetate were purchased from Aldrich Co., U.S.A. The phosphate buffer solution (PBS, 0.01 M, pH 7.4) used in all experiments was prepared by dissolving PBS dry powder with the deionized water (DI water). All other chemicals were commercially available and used as received. DI water used in this study was produced by a Millipore water purification system (Q-Grad 1, Millipore Co., U.S.A.). All measurements were carried out at room temperature (20 ( 2 °C). UV-Vis Spectroscopy. The UV-vis spectroscopy was performed using a Hitachi U-2800 UV-vis spectrophotometer (Japan) equipped with a quartz cell (optical path length ) 1 cm). Electrochemical Measurements. All the electrochemical measurements were performed with CHI 760B bipotentiostat (CH Instruments, U.S.A.) in a three-electrode electrochemical cell. The working electrode (WE) was a gold disc electrode. Before each experiment, the WE surface was polished sequentially with fine grade alumina powders, from 1 µm, 0.3 µm, to 0.05 µm, to obtain a mirror-like surface. Then it was sonicated in order to remove traces of alumina from the surface, washed with DI water and acetone, and dried. The saturated calomel electrode (SCE) served as the reference electrode, and a platinum wire was employed as the counter electrode. In order to obtain an O2 saturated electrolyte solution, O2 gas (99.99%) was bubbled directly into the solution for 15 min prior to the electrochemical measurements, and it was flushed over the solution during the measurements. For baseline measurements, electrolyte solutions were deaerated by bubbling N2 gas (99.99%) for at least 15 min prior to the electrochemical measurements, and the gentle stream of N2 gas passed over the electrolyte solution during the whole experiment to exclude any oxygen. In particular, for the morphologic measurements, the silicon wafer coated with gold was chosen as the working
electrode, while the ITO glass was used as the working electrode for the UV-vis spectroscopy measurements. In addition, the oxygen reduction was studied by an RRDE (Pine Model AFMT28), of which the disc part was gold and the ring part was platinum. Specifically, in order to get the stable solution of the template, ultrasonic dispersion was used (Deltasonic sonicator, Meaux, France, 120 W, 42 kHz). AFM. An AFM instrument (Dimension 3100 SPM, Veeco, U.S.A.) was used to produce high-resolution surface images. The surfaces were scanned in the tapping mode with a commercial silicon probe (RTESP, Veeco, U.S.A.). Mica was chosen as the supporting substrate for aggregates of cobalt porphyrin. The samples of J-aggregates were formed by casting a droplet (20 µL) of the porphyrin solution onto the freshly cleaved mica. The drop covered ∼1 cm2 on the substrate surface, and it was removed after 1 min of adsorption. Then the sample was dried in ambient air. It was necessary to remove the drop in order to avoid formation of salt crystals on the surface. The I-V curves of the composite and PPY were studied by using the conductive atomic force microscope (C-AFM) module of Dimension 3100 SPM. The conductive diamond coated tip (DDESP, Veeco, U.S.A.) was used for the experiment. FESEM. The high-resolution scanning electron micrographs were obtained by using a JEOL JSM-6700F field emission scanning electron microscope. (Japan). Results and Discussion Synthesis of Cobalt Porphyrin and Characterization of Its J-Aggregates. Cobalt porphyrin (TPPS-Co) was prepared in our lab in the following way (Figure 1): TPPS was reacted with cobalt(II) acetate in refluxed methanol for 1 h. The resulted precipitate (TPPS-Co) was collected and washed with methanol, and then characterized by UV-vis absorption spectroscopy (shown in Figure 2a). TPPS had an intense absorption band at around 430 nm (Soret band) and weak absorption bands at 590 and 640 nm (Q bands). The change of the Soret band of TPPSCo indicated the incorporation of cobalt in the porphyrin. It was also found that two peaks appeared at around 490 and 710 nm after the metal cation was inserted into the porphyrin. It has been reported that porphyrins without the central metal ions tended to form J-aggregates at special conditions, and the appearance of the absorption peaks at around 490 and 710 nm could characterize the formation of J-aggregates.19-23 In order to verify the possible existence of J-aggregates in the cobalt porphyrin, we prepared an aqueous solution of TPPS-Co (1 mM in 0.01 M PBS, pH ) 7.4), dispersed it onto freshly cleaved mica, and characterized it by AFM measurements. It was shown
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Figure 2. (a) UV-vis absorption spectra of TPPS-Co (solid line) and TPPS (dashed line), and (b) AFM image of TPPS-Co aggregates on mica and the height profiles along line I and line II.
Figure 3. Structures of TPPS in the acid medium (a), TPPS-Co in the neutral medium (b), and the schematic formation of linear J-aggregate (c, d), data from ref 22.
clearly in the AFM image (Figure 2b) that there were threads and rods on the substrate. The threads could be the J-aggregates of TPPS-Co, and the large rod-like structures were possibly due to the macroaggregate of the TPPS-Co J-aggregates. Among all the reports about the J-aggregates of TPPS, the diprotonated form of TPPS has been proposed as being responsible for the formation of J-aggregates (Figure 3a). J-Aggregates of different geometrical arrangements have been hypothesized. The most relevant structure based on spectroscopic data and mathematical modeling supports the formation of linear J-aggregates. In our work, since the cobalt ion introduced into the porphyrin was a divalent cation, it could also give the porphyrin molecule a partial positive charge
localized at the center of the macro-ring to play the same role as the diprotonated form of TPPS (Figure 3b). Thus, we could expect that the linear J-aggregation would occur with TPPSCo in a neutral aqueous medium as well as with protonated TPPS in an acidic aqueous medium (Figure 3c,d): the polar groups (sulfonic groups) of one porphyrin molecule should have strong intermolecular interactions with the positive central metal ions of neighboring porphyrin molecules. Therefore, assuming that the J-aggregate has a linear structure, the expected AFM image of the J-aggregate on the substrate should be a thread with a diameter of approximate 1∼2 nm. In our study, as shown in Figure 2b, the diameter of the threads is indeed in the range of 1 to 2 nm, which is determined by the height profile analysis
Cobalt Porphyrin/Polypyrrole Nanocomposite
Figure 4. Cyclic voltammograms of the electrodepositions of TPPSCo/PPY composite (solid line) and pure PPY (dashed line); scan rate is 50 mV/s. The inset indicates the expansion figure around the onset potential.
of AFM image. This confirmed that TPPS-Co could form linear J-aggregates in the neutral aqueous solution. It has been reported that TPPS molecules cannot stack in neutral aqueous solution to form the J-aggregate because the negative charges at the sulfonic groups cause electrostatic repulsion. Only in an acidic medium can the J-aggregates form because of the formation of diprotonated species. In addition, previous studies reveal that the existence of organic or inorganic cations is essential for the J-aggregation of TPPS. We report here for the first time the efficient formation of J-aggregates of TPPS-Co in the 0.01 M PBS solution (pH ) 7.4), without any organic or inorganic cations. Apparently, the central metal ion in the macro-ring of the porphyrin molecule plays key role to form the nanostructured J-aggregation in a neutral solution as we discussed above. In addition to the expected threads we also observe rods with the diameter beyond 2 nm, which do not match the expected dimensions of the linear J-aggregates. In fact, although the diameter of the linear J-aggregates is supposed to be 1∼2 nm, the size of all reported J-aggregates is much larger than 2 nm. It can be explained that the J-aggregates can further assemble to form large clusters, the size of which depends on the concentration of porphyrin and other experimental conditions. Thus, in our case, we suggest that TPPS-Co self-organize into linear threads, in which the metalloporphyrins maintain the arrangement typical for the J-aggregates, and some of the threads combine into larger rod-like structures. Preparation of the Cobalt Porphyrin/Polypyrrole Composite for Different Nanostructures. Polypyrrole can be electrosynthesized on different electrode surfaces by cyclovoltammetric, galvanostatic, and potentiostatic deposition methods.24 Since TPPS-Co is soluble in water, it could be convenientlyincorporatedintoPPYbyelectrochemicalcopolymerization, and the J-aggregates of TPPS-Co could act as a template to form a TPPS-Co/PPY composite with special structures. In one approach of our work, the TPPS-Co/PPY composite was onestep electrochemically synthesized on a gold electrode by cyclic voltammetry (CV) from -0.2 to 1.0 V (vs SCE) in a threeelectrode electrochemical cell containing 0.1 M pyrrole + 1 mM TPPS-Co + 0.01 M PBS (pH ) 7.4). For comparison, pure PPY was also electrochemically polymerized under the same conditions in the absence of TPPS-Co in the pyrrole solution. As shown in Figure 4, the electrosynthesis of PPY in the presence of TPPS-Co is remarkably faster, starts at lower
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11219
Figure 5. UV-vis absorption spectra of TPPS-Co/PPY composite and pure PPY.
positive potential, and has much higher current. This demonstrates that the electrochemical copolymerization of TPPS-Co/ PPY has a much higher reaction rate than PPY polymerization. The results suggest that TPPS-Co is not only one of the reactants to participate in the reaction but also a good catalyst for the polymerization. To obtain evidence that the TPPS-Co aggregates are incorporated in the composite, we measured the UV-vis absorption spectra of TPPS-Co/PPY composite and pure PPY electrodeposited on the ITO electrodes, respectively. As shown in Figure 5, although both UV-vis spectra show the same shape of the PPY spectrum, one can clearly recognize the Sorest band (430 nm) and the characteristic peak of J-aggregates (490 nm) in the spectrum of TPPS-Co/PPY, while no obvious peak is seen in that of pure PPY. Hence, the J-aggregates of TPPS-Co exist in the composite. The influence of TPPS-Co aggregates on the morphology of the composite was studied by FESEM. As discussed above, there are J-aggregates in our synthesized cobalt porphyrin, in both thread- and rod-like structure, and these nanostructures could act as the templates for the synthesis of TPPS-Co/PPY composite. In order to confirm the template effect of TPPSCo, the FESEM images of both the TPPS-Co/PPY composite and the pure PPY were measured. As shown in Figure 6b, the one-step synthesized TPPS-Co/PPY composite is composed of many nanorods with high roughness and porosity, which is greatly different from the morphology of the TPPS-Co Jaggregate shown in Figure 2b and the PPY alone shown in Figure 6a. Obviously, the size of the TPPS-Co J-aggregates (Figure 2b) is generally much smaller than that of the composite rods, while the pure PPY is relatively compact and smooth with pebble-like masses (Figure 6a). The rough and porous nanocomposite could be attributed to the aggregation of TPPS-Co, that is, the cobalt porphyrin aggregates to form nanothreads and nanorods as templates to further produce larger nanorods with PPY deposition during the synthesis process of the TPPS-Co/ PPY nanocomposite. However, it should be admitted that the nanostructure of the composite with a lot of clusters is not as uniform as we expect. As discussed above, the TPPS-Co J-aggregates could self-assemble into large clusters, which would act as templates during the PPY deposition to directly affect the uniformity of the composite. In order to synthesize the nanocomposite with a uniform nanostructure, the self-assembling process of the templates that forms the large clusters must be restrained. Ultrasonic dispersion
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Figure 6. FESEM images of the pure PPY (a) and the TPPS-Co/PPY nanocomposites (b-d); details are described in the text.
is often used as an efficient method to make a uniform solutedistributed, stable solution. In our work, after TPPS-Co was dissolved in PBS, the solution was ultrasonicated for at least 3 h to have a uniform and stable solution of the templates. Then, the monomer of pyrrole was added in this solution, and the composite was electrosynthesized as the method described previously. The resulted composite was also tested by FESEM. Apparently, as shown in Figure 6c, the structure of this composite is significantly different from that in Figure 6b: all the nanorods are of the same size (diameter ∼ 50 nm) and are uniformly distributed in a two-dimensional nanostructure on the electrode surface. Thus, the ultrasonication is an efficient approach to disperse the porphyrin templates and to limit the formation of large clusters of J-aggregates for a more uniform nanorod-based nanostructure. From Figure 6c, it can be observed that most of the nanorods seem to grow from the substrate because one end is connected to the substrate and the other end is free. It can also be seen that there are some globular masses on the substrate, especially between the nanorods, which is the typical structure of pure PPY. On the basis of these results, a mechanism to form the nanorod-based composite can be suggested. The pure PPY is first eletrodeposited on the electrode surface, and the TPPS-Co J-aggregates assemble in the solution. It is very likely that the TPPS-Co J-aggregate attaches its negative sulfonic group onto the positively charged PPY through electrostatic attraction to form a template. Then, PPY is deposited on the template, and a nanorod-based composite structure is formed. Since PPY on both the nanorod and the substrate are positively charged, the other end of the assembled nanorod is pushed away from the substrate and becomes free. In succession, the TPPS-Co/PPY composite was synthesized by a three-step method in which pure PPY was electrodeposited on the electrode first and then this modified electrode was
immersed in the template solution for more than 6 h. After that the electrode was placed in a pyrrole solution for PPY electrosythesis again. The FESEM image of the resultant TPPSCo/PPY composite is shown in Figure 6d, which demonstrates an interesting nanostructrure with a regular nanoarray. The nanoarry consists of veritcal nanorods in uniform diameters (∼50 nm). The nanostructure produced by this approach is threedimensional rather than two-dimensional, as observed in Figure 6c. It is clear that there are more nanorods produced in this composite than that shown in Figure 6c. When the composite is synthesized by the three-step method, the formed TPPS-Co J-aggregates have enough time to react with the PPY-coated electrode surface and then to produce concentrated templates on the electrode surface for further PPY deposition. The dense pack of the templates would make a three-dimensional structure on the surface. These experimental results not only demonstrate that the nanostructure of the TPPS-Co/PPY composite can be tailored by preparation approaches but also further prove that the formation mechanism of the nanorod-based composite suggested above is reasonable. Electrocatalytic Property of the TPPS-Co/PPY Nanocomposite. For the composite represented by Figure 6d, the nanostructure is uniform with high density of the nanorods, which traps a high concentration of TPPS-Co and is supposed to have good electrocatalytic properties. Thus, the electrocatalytic behavior of the nanocomposite was tested. Parts a and b of Figure 7show the CVs of the nanocomposite-modified gold electrode in the O2- and N2-saturated PBS (0.01 M, pH ) 7.4), respectively. It is observed that there is a new and pronounced cathodic process over a wide range of potential in the O2saturated PBS. In addition, compared with the bare Au electrode in the O2-saturated PBS (Figure 7c), the peak potential (or the onset potential) of the oxygen reduction shifts positively and the peak current increases largely. These results are clear
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Figure 8. I-V curves of TPPS-Co nanocomposite (a) and pure PPY (b), obtained by C-AFM. Figure 7. Cyclic voltammograms of (a) the TPPS-Co/PPY nanocomposite-coated electrode in the O2-saturated PBS (solid line), (b) the TPPS-Co/PPY nanocomposite-coated electrode in the N2-saturated PBS (dashed line), and (c) the bare Au electrode in the O2-saturated PBS (dash-dotted line); scan rate is 50 mV/s.
indications for the occurrence of considerable electrocatalysis at the nanocomposite-modified electrode for the oxygen reduction. It is also reported that some conducting polymer by itself could electrocatalyze the oxygen reduction. In order to exclude the effect of PPY, the electrocatalytic activity of the pure PPY was checked by CVs in the O2- and N2-saturated PBS, respectively. The results show that there is no obvious electrocatalysis caused by pure PPY in the neutral medium (not shown here). On the basis of all these experimental results, it can be concluded that the TPPS-Co/PPY nanocomposite can be used as a good electrocatalyst for the oxygen reduction in the neutral medium. It might not be very difficult to understand the catalytic mechanism of this nanocomposite. TPPS-Co in the nanocomposite acts as the electroactive center for the oxygen reduction, and the high density of three-dimensional nanorods provides high concentration of the electroactive centers for the oxygen reduction. Additionally, the interaction between the π electron conjugated system of TPPS-Co and that of PPY enhances electron transfer through the nanocomposite, which can allow a rapid electron transfer from the electrode surface to the active metal centers. To ascertain the conductive nature of the TPPS-Co/PPY nanocomposite, I-V curve measurements are carried out using the C-AFM module in scanning probe microscopy. C-AFM has found applications in many areas during the past decade because it allows contact to be made easily with various substrates.25 The I-V measurements were carried out by imposing a bias between the gold-coated silicon wafer, modified by TPPS-Co/ PPY nanocomposite and pure PPY, respectively, and at room temperature in the air. The I-V curves were measured at different spots. The electrical properties of the nanocomposite and pure PPY were obtained by recording five sets of I-V curves, respectively. After averaging these curves, the I-V curves are shown in Figure 8. The results show that the electronic transport behavior of the pure PPY does not follow Ohm’s law, but it obviously illustrates a nonlinear electrical transport property of a typical semiconductor. This is in agreement with the literature.13 In addition, it is well-known that the metalloporphyrin is also a semiconduting material. However, the I-V curve of the TPPS-Co/PPY nanocomposite is essentially a straight line within the instrumental limits ((500 nA), which represents the typical I-V behavior of a sound
Figure 9. Rotating ring-disc voltammogram for oxygen reduction at the gold disc coated with TPPS-Co/PPY nanocomposite in O2-saturated PBS; rotation rate is 1000 rpm, and scan rate is 50 mV/s.
conductor. Hence, this is solid evidence that the TPPS-Co/PPY has much higher conductivity than both pure PPY and TPPSCo alone, and it can be a good electrocatalyst. In order to further analyze the mechanism of the oxygen reduction at the electrode modified by the TPPS-Co/PPY nanocomposite, RRDE experiments were carried out. The TPPSCo/PPY nanocomposite was electrodeposited on the disc electrode (Au). Two main mechanisms for the oxygen reduction process are reported. In one mechanism, oxygen is reduced to hydrogen peroxide as an intermediate through a two-electron transfer process followed by a further reduction to water. Alternatively, oxygen can be directly reduced to water through a four-electron transfer process. There is no doubt that the fourelectron transfer process without the intermediate is favored to have a much higher energy conversion efficiency. However, it only occurs on a few excellent electrocatalysts such as platinum black and in strong acidic or alkaline electrolyte as well. In our experiments, as the potential of the modified disc electrode was scanned from 0.6 V to -0.6 V, and the potential of the ring electrode (Pt) was fixed at 0.8 V to detect the oxidation current of H2O2 that could be generated as the intermediate during the oxygen reduction at the disc electrode if the reaction is a twoelectron transfer process. On the basis of the resultant RRDE voltammogram (as shown in Figure 9), a very small current at the ring electrode is detected, indicating only a very small
11222 J. Phys. Chem. C, Vol. 111, No. 30, 2007 amount of H2O2 produced. The number of electrons involved in the reduction process can be estimated by comparison of the ring and disc currents, taking into account the possible H2O2 diffusion difficulties with polymer-modified electrodes, by n ) 4 - 2(IR/IDN)26 (IR and ID are the ring and disc limitating currents and N is the collection factor of the ring-disc electrode used). Calculation of this ratio shows that 3.84 electron transfer is involved in the oxygen reduction at the modified electrode. This demonstrates that the oxygen reduction is mainly conducted through the four-electron transfer process, and the TPPS-Co/ PPY nanocomposite is a great electrocatalyst for the oxygen reduction in a neutral electrolyte. The mechanism of four-electron oxygen reduction by the TPPS-Co/PPY nanocomposite is likely related to its nanostructure. The mechanism of oxygen reduction depends on adsoption types of oxygen molecules on the electrocatalyst, including end,27 side,28 and bridge29,30 adsorptions. The bridge adsorption, M-O-O-M, where M represents the catalyst center, can efficiently weaken the O-O bond of oxygen and further split the two oxygen atoms for the four-electron transfer reduction. Apparently, the critical factor is the distance between two electrocatalytic centers that should be close to the size of the oxygen molecule. The polymer, PPY, used in the present study is not directly involved in the reaction other than providing conductivity and playing host to TPPS-Co on the electrode surface. The mechanism of electrocatalytic reduction by metalloporphyrin involves adsorption of oxygen molecule to the central metal atom. It has been reported that oxygen bound to two cobalt atoms in cofacial Co-porphyrins can make a fourelectron transfer process for oxygen reduction.30 In our work, the nanoscale linear J-aggregate array of TPPS-Co packs all TPPS-Co face to face in a nanoscale, which could provide neighboring Co sites for a bridge adsorption mode, leading to the four-electron transfer reduction process. For the application as a cathode catalyst in a fuel cell, there are a number of stringent requirements. The low overpotential and the four-electron reduction of oxygen are known to be the most important requirements. TPPS-Co/PPY nanocomposite shows excellent electrocatalysis that facilitates a fuor-electron process and relatively positive peak potential (or the onset potential) for oxygen reduction. In addition, most of the previously reported good electrode catalysts for the oxygen reduction perform only in strong acidic or alkaline medium. In our case, the electrocatalytic activity of the TPPS-Co/PPY nanocomposite is great even in a neutral medium. This favors extension of the applications of this cathode catalyst into the biological systems, such as the biofuel cells and biosensors. Conclusions In summary, the TPPS-Co/PPY nanocomposite has been synthesized, characterized, and used as the electrocatalyst for oxygen reduction in this study. For the first time, our work demonstrates that linear J-aggregates of TPPS-Co can form in a neutral aqueous solution and can further function as the templates to generate different nanostructured TPPS-Co/PPY composites. With the assistance of ultrasonic treatment, the nanocomposites are synthesized to have uniform 2-D nanorods and 3-D regular nanoarray-structure through different preparation approaches. The 3-D nanostructured array is synthesized by a three-step method. The electrode modified by this nano-
Zhou et al. composite shows excellent electrocatalytic activity for the oxygen reduction in a neutral aqueous solution. The peak potential (or the onset potential) of the oxygen reduction shifts positively, and the peak current increases remarkably in comparison to the electrode without the composite modification. The RRDE results indicate that the oxygen reduction at the modified electrode mainly occurs through a four-electron pathway to form H2O, which could be ascribed to the dense, arrayed nanostructure of the composite. This new electrocatalyst is very promising to have broad applications in biofuel cells and biosensors. Acknowledgment. The authors are grateful to the U.S.A. Army Research Office under contract No. W911NF-05-1-0303 for the financial support to this work. References and Notes (1) Yagi, I.; Ishida, T.; Uosaki, K. Electrochem. Commun. 2004, 6, 773. (2) Schmidt, T. J.; Stamenkovic, V.; Arenz, M.; Markovic, N. M.; Ross, P. N., Jr. Elecrtochim. Acta 2002, 47, 3765. (3) King, F.; Litke, C. D.; Tang, Y. J. Electroanal. Chem. 1995, 384, 105. (4) Gojkovic, S. L.; Zecevic, S. K.; Drazic, D. M. Elecrtochim. Acta 1994, 39, 975. (5) Dias, S. L. P.; Gushikem, Y.; Riberio, E. S.; Benvenutti, E. V. J. Electroanal. Chem. 2002, 523, 64. (6) Yang, J.; Xu, J. J. Electrochem. Commun. 2003, 5, 306. (7) Li, C. M.; Wang, Z.; Cha, C. S. J Wuhan UniV. Technol. 1983, 4, 129. (8) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 1537. (9) Wang, Z.; Deng, Z.; Li, C. M. J. Electrochem. Soc. 1986, 133 (3), C125-C125. (10) Chang, C. J.; Deng, Y.; Nocera, D. G.; Shi, C.; Anson, F. C.; Chang, C. K. Chem. Commun. 2000, 1355. (11) Deng, Z.; Wang, Z.; Li, C. M.; Cha, C. S. Acta Chim. Sinica. 1987, 45, 260. (12) Hutchison, J. E.; Postlethwaite, T. A.; Chen, C.; Hathcock, K. W.; Ingram, R. S.; Ou, W.; Linton, R. W.; Murray, R. W.; Tyvoll, D. A.; Chng, L. L.; Collman, J. P. Langmuir 1997, 13, 2143. (13) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers; Dekker: New York, 1998. (14) Dong, H.; Li, C. M.; Chen, W.; Zhou, Q.; Zeng, Z.; Luong, J. H. T. Anal. Chem. 2006, 8, 7424. (15) Li, C. M.; Chen, W.; Yang, X.; Sun, C. Q.; Gao, C.; Zheng, Z. X.; Sawyer, J. Frontiers Biosci. 2005, 10, 2518. (16) Paik, W.; Yeo, I. H.; Suh, H.; Kim, Y.; Song, E. Electrochim. Acta 2000, 45, 3833. (17) Kvarnstrom, C.; Neugebauer, H.; Blomquist, S.; Ahonen, H. J.; Kankare, J.; Ivaska, A. Electrochim. Acta 1999, 44, 2739. (18) Cosnier, S.; Gondran, C.; Wessel, R.; Montforts, F. P.; Wedel, M. J. Electroanal. Chem. 2000, 488, 83. (19) Koti, A. S. R.; Taneja, J.; Periasamy, N. Chem. Phys. Lett. 2003, 375, 171. (20) Schwab, A. D.; Smith, D. E.; Rich, C. S.; Young, E. R.; Smith, W. F.; De Paula, J. C. J. Phys. Chem. B 2003, 107, 11339. (21) Snitka, V.; Rackaitis, M.; Rodaite, R. Sensor. Actuat. B-Chem. 2005, 109, 159. (22) Rotomskis, R. R. A.; Snitka, V.; Valiokas, R.; Liedberg, B. J. Phys. Chem. B 2004, 108, 2833. (23) Hatano, T.; Takeuchi, M.; Ikeda, A.; Shinkai, S. Org. Lett. 2003, 5, 1395. (24) Li, C. M.; Sun, C. Q.; Chen, W.; Pan, L. K. Surf. Coat. Technol. 2005, 198, 474. (25) Cao, H.; Wang, L.; Qiu, Y.; Zhang, L. Nanotechnology 2006, 17, 1736. (26) Kingsborough, R. P.; Swager, T. M. Chem. Mater. 2000, 12, 872. (27) Pauling, L. Nature 1964, 203, 182. (28) Vaska, L. Science 1963, 140, 809. (29) Hibbert, D. B.; Tseung, A. C. J. Electrochem. Soc. 1981, 103, 160. (30) Collman, J. P.; Denisevich, P.; Konai, Y.; Marroco, M.; Roval, C.; Anson, F. J. Am. Chem. Soc.,1980, 102, 6027.
