93 Electro-Synthesis and Characterization of Polythiophene Nano

Electro-synthesis and characterization of polythiophene nano-wires/platinum nano-particles

УДК 544.653.1

M.A. DEL VALLE1, M. GACITUA1, F.R. DIAZ1, J.F. ARMIJO1, J.P. SOTO2

ELECTRO-SYNTHESIS AND CHARACTERIZATION OF POLYTHIOPHENE NANO- WIRES/PLATINUM NANO-PARTICLES COMPOSITE ELECTRODES; ELECTRO- CATALYTIC STUDY OF THE FORMIC ACID OXIDATION

1Pontificia Universidad Catolica de Chile, Macul 2Pontificia Universidad Catolica de Valparaiso, Valparaiso, Chile

We report the successful electro-synthesis of a composite made of polythiophene nano-wires and platinum nano-particles. Their application to the development of fuel cells was studied by measuring the electro-catalytic activity to formic acid electro-oxidation. The results were promising. The composite was analyzed by transmission electron microscopy, confirming the expected morphology.

Introduction matrix the new composite should present higher metal Fuel cells are a good choice for designing cheaper, dispersion and, as a consequence, better formic acid more efficient and non-contaminating energy sources. oxidation catalytic performance. These devices collect the energy output from the Experimental electro-catalyzed oxidation of small molecules. One of Mesoporous Silica Template the most employed fuels is formic acid, HCOOH. Silica films were potentiostatically deposited on a Since this phenomenon depends on the specific fuel- non-annealed Pt disc electrode (2 mm diameter). substrate interaction, electrode material (typically Pt, Working solution details, electrochemical methods and Pd or combinations) [1] choice is an important issue characterization of the formed silica film are reported to be taken into account. elsewhere [10]. One approach to improve the electro-catalytic Polythiophene Nanowires performance of the electrode is to prepare surfaces A silica modified Pt electrode was used as working with enhanced active area. There have been reports on electrode for potentiostatic electro-polymerization of the preparation of a composite consisting on dispersing polythiophene, obtaining a Pt|PTh nano-wire modified metals onto a conducting polymer matrix. On 1995, electrode (Pt|PTh-nw) as reported elsewhere [9]. All for the first time, we have successfully dispersed platinum potentials quoted in this paper are referred to a Ag|AgCl on a polythiophene (PTh/Pt) electrode employing its electrode in tetramethylammonium chloride to match doping properties [2,3]. Basically, it consist on modifying the potential of a saturated calomel electrode (SCE) PTh matrix by doping it with a Pt-containing anionic at room temperature (200C) [11]. A Pt gauze of species, applying an anodic p-doping potential, and large geometrical area, separated from the cell main then reducing it with a cathodic pulse, electro- compartment by a fine glass sinter, was used as counter crystallizating Pt metallic particles. These substrates electrode. 1⋅10–3 mol⋅L–1 thiophene, Th (monomer, –1 –1 proved to have a better formic acid electro-oxidation Aldrich, 99%) and 1⋅10 mol⋅L TBAPF6 in performance when compared to a bare Pt electrode. CH3CN was used as working solution [12]. To assess An alternative approach could be to additionally the effect of the nano-wire on electrode performance control the polymer matrix morphology at nano-scale. another modified electrode without silica template was Recent works have been published reporting that assembled, i.e., a conventional platinum polythiophene electrochemically prepared nano-structured conducting electrode, Pt|PTh. polymers present improved sensing properties [4–8]. Electrode characterization was accomplished by These results can be attributed to a higher exposed studying its electrochemical behavior in solutions electrodic surface when using a nanostructured containing only the background electrolyte. conducting polymer layer. In an earlier communication Formic Acid Electrooxidation we reported the successful electro-synthesis of Preparation of platinum|polythiophene nano-wires/ polythiophene ~5.7 nm width nano-wires (PTh-nw) platinum nano-particles electrodes (Pt|PTh-nw/ on a fluorine doped tin oxide glass electrode (FTO) Pt-np) was achieved by cycling the Pt|PTh-nw –3 –1 –1 [9] previously modified with a silica mesoporous film electrode in a 1⋅10 mol⋅L H 2 PtCl6+0.1 mol⋅L as template [10]. Therefore, if the Pt dispersion is KPF6 aqueous solution. The dispersion of Pt-np in combined with the nano-wire morphology of the polymer the polymeric matrix was performed by a platinization

ISSN 0321-4095. Вопросы химии и химической технологии, 2011, №4(1) 93 M.A. del Valle, M. Gacitua, F.R. Diaz, J.F. Armijo, J.P. Soto cycle (PC) based on the p-doping process of the acid electro-oxidation [2,3] was accomplished. After polymer, applying a potential step from 0.80 V for 6 applying platinization cycles, PC, to both Pt|PTh and min to –0.50 V for 10 s. The PC was also applied Pt|PTh-nw electrodes, their electrochemical response to the Pt|PTh electrode affording a platinum|poly- was recorded in the presence of HCOOH, Fig. 2. thiophene/platinum nano-particles modified electrode (Pt|PTh/Pt-np). The electro-catalytic activity of these electrodes for formic acid electro-oxidation was determined by cycling them in a 1⋅10–1 mol⋅L–1 HCOOH+1⋅10–1 –1 mol⋅L KPF6 aqueous solution between –0.20 and 1.00 V at 0.10 Vs–1. All the electrochemical experiments were conducted at room temperature on a PALM SENS portable potentiostat. TEM images were obtained on a JEOL/ JEM 1200 EX II microscope at an acceleration voltage of 80 kV. Samples for TEM measurements were prepared by mechanically removing (scraping) Fig. 2. Formic acid oxidation. (a) bare Pt electrode. (b) and some pieces of the deposit which were directly supported (c) Pt|PTh/Pt-np and Pt|PTh-nw/Pt-np respectively after on a nickel grid. one PC, and (d) Pt|PTh-nw/Pt-np after two PC. Results and discussion –1 –1 –1 –1 Working solution, 1⋅10 mol⋅L KPF6, 1⋅10 mol⋅L As reported, an appropriate silica modification of HCOOH in H O. Cycle n° 3. Scan rate, 0.10 Vs–1 platinum was performed applying a potentiostatic pulse 2 of –0.90 V for 5 s [10]. Silica film Pt modified With the experimental methods employed the electrodes were then employed for polythiophene electro- Pt|PTh/Pt-np electrode exhibited no appreciable polymerization. A potential step program was selected electro-catalytic activity for formic acid oxidation, when to form the Pt|PTh-nw electrode, mainly because a compared to the response at a bare Pt electrode, potentiostatic pulse modifies the electrode completely Fig. 2 (a) and (b). In contrast, the Pt|PTh-nw/ faster and good results have been previously reported Pt-np electrode response, (c), presents a noticeable [9]. A potential step from 0.30 V for 10 s to difference. Here the voltammogram resembles the 1.85 V for 240 s was selected for nano-wire preparation. typical response obtained for formic acid electro-oxidation Its electrochemical response was studied in a solution at a polycrystalline Pt electrode. When two PCs are containing only supporting electrolyte, Fig. 1. applied (d) current and general behavior are more A considerable charge difference between Pt|PTh pronounced. The reaction mechanism for this case has and Pt|PTh-nw response, Fig. 1, can be observed for been fully characterized [1]. Comparison of charge the p-doping/undoping process of polythiophene. Also when one or two PCs were applied, (b) and (d) the Pt|PTh-nw p-doping electrode process occurs at respectively, presents a considerable current increase in a less anodic value than Pt|PTh, meaning that just the the second. Hence, PTh nano-wire architecture morphology has an intrinsic electro-catalytic effect improved the electrode performance. concerning the anion movement through the polymeric These observations were confirmed by examining matrix. the deposits on a Transmission Electron Microscope (TEM), Fig. 3.

Fig. 1. Electrochemical response of Pt|PTh (- -) and –1 –1 Pt|PTh-nw (–) electrode on 1⋅10 mol⋅L TBAPF6 in –1 Fig. 3. TEM micrographs of the deposits CH3CN. Scan Rate: 0.01 V⋅s These new features represent an improvement of Fig. 3,a shows PTh-nw deposits directly scraped the polymer doping properties accomplished by just from the electrode confirming previously reported nano- changing its morphology at a nano-metric scale. In wire morphology [9] with an average width of ~5 nm order to take advantage of this characteristic a study in a brush-like disposition. Fig. 3,b to (d) shows concerning the dispersion of platinum nano-particles images for different fragments of the PTh-nw/Pt-np on PTh and its electro-catalytic capacity towards formic composite from where, in general, the Pt nanoparticles

94 ISSN 0321-4095. Вопросы химии и химической технологии, 2011, №4(1) Electro-synthesis and characterization of polythiophene nano-wires/platinum nano-particles of different size can be observed. (b) presents a dark REFERENCES field image of a fragment where Pt-np sparkles are observed. The inset shows the diffraction pattern of 1. Lu G.Q., Crown A., Wieckowski A. Formic Acid the sample indicating a polycrystalline structure, Decomposition on Polycrystalline Platinum and Palladized corroborating thus the assumption made from the Platinum Electrodes // J. Phys. Chem. B. – 1999. – № formic acid oxidation profile. From (c) the presence 103. – P.9700-9711. of ~1300 np per mm2 evenly dispersed over the 2. Preparation of polythiophene-modified electrodes by polymer was quantified. This nano-particle density electrodeposition of Pt and Pt+Pb - application to formic- covered the polythiophene fragment almost completely, acid electrooxidation / Schrebler R., del Valle M.A., Gomez leaving only some places where the nano-wires H., Veas C., Cordova R. // J. Electroanal Chem. – 1995 – morphology can be identified (circled areas). Finally № 380. – P.219-227. from (d) an average nano-particle diameter of 3. Polythiophene, polyaniline and polypyrrole electrodes ~20 nm can be estimated. modified by electrodeposition of Pt and Pt+Pb for formic acid Therefore, the novel PTh nano-wire doping electrooxidation / del Valle M.A., Diaz F.R., Bodini M.E., properties presented in the current communication enables Pizarro T., Cordova R., Gomez H., Schrebler R. // J. Appl. the appropriate formation of Pt nano-particles to the Electrochem. – 1998. – № 28. – P.943-946. PTh-nw modified electrode for its employment on the 4. Nanowire-based electrochemical biosensors / formic acid oxidation. Such modification conferred to A.K. Wanekaya, W. Chen, N.V. Myung, A. Mulchandani the polymer/metal based composite polycrystalline // Electroanal. – 2006. – № 18. – P.533-550. character, which represents an important outcome 5. Dielectrophoretically assembled polymer nanowires considering that only electrochemical techniques were for gas sensing / Dan Y.P., Cao Y.Y., Mallouk T.E., John- used. son A.T., Evoy S. // Sensor Actuat B-Chemical. – 2007. – Conclusions № 125. – P.55-59. PTh-nw voltammetric studies performed in 6. Ghanbari K., Bathaie S.Z., Mousavi M.F. supporting electrolyte solution revealed a higher current Electrochemically fabricated polypyrrole nanofiber-modified for the doping/undoping couple than for a regular electrode as a new electrochemical DNA biosensor // Biosens polythiophene electrode indicating that an increase in Bioelectron – 2008. – № 23. – P.1825-1831. electro-active surface occurred, at least to the ion 7. Rajesh, Ahuja T., Kumar D. Recent progress in the exchange process. development of nano-structured conducting polymers/ A very promising polymer/metal composite nanocomposites for sensor applications // Sensor Actuat B- electrode was obtained which electro-catalytical activity Chemical. – 2009. – № 136. – P.275-286. for formic acid oxidation rose above the one of bare 8. Liu C.J., Hayashi K., Toko K. Electrochemical Pt and regular polythiophene platinum nano-particles deposition of nanostructured polyaniline on an insulating substrate electrode, Pt|PTh. In fact the cyclic voltammogram // Electrochem Commun. – 2010. – № 12. – P.36-39. resembles the one of formic acid oxidation over 9. Electrosynthesis of polythiophene nanowires via polycrystalline bare Pt electrode. mesoporous silica thin film templates / del Valle M.A., Gaci- PTh-nw deposits analyzed by TEM presented tua M.A., Diaz F.R., Armijo J.F., del Rio R.R. // the expected nano-wire structure. In addition, the Electrochem Commun. – 2009. – № 11. – P.2117-2120. composite showed platinum nano-particles dispersion 10. Electrochemically assisted self-assembly of mesoporous with a polycrystalline diffraction pattern, confirming silica thin films / A. Walcarius, E. Sibottier, M. Etienne, the formic acid electro-catalysis outcome. J. Ghanbaja // Nat Mater. – 2007. – № 6. – P.602-608. Acknoledgements 11. East G.A., del Valle M.A. Easy-to-Make The authors thank CONICYT-Chile for financial Ag/AgCl Reference Electrode // J. Chem. Educ. – 2000. support through grant Fondecyt nє 1100055. M. – № 77. – P.97. Gacitua thanks CONICYT for doctoral scholarship 12. Oligomer chain length effect on the nucleation and nє 24090059. growth mechanisms (NGM) of polythiophene / del Val- le M.A., Gacitua M.A., Canales L.I., Diaz F.R. // J. Chil. Chem. Soc. – 2009 – № 54, – P.260-266.

Received 07.04.11

ISSN 0321-4095. Вопросы химии и химической технологии, 2011, №4(1) 95