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Functionalized Mesoporous Silica Films as a Matrix for Anchoring Electrochemically Active Guests Dina Fattakhova Rohlfing,* Jirˇ´ı Rathousky´,† Yven Rohlfing, Oliver Bartels, and Michael Wark* Institute of Physical Chemistry and Electrochemistry, University Hanover, Callinstrasse 3-3a, 30167 Hanover, Germany Received June 16, 2005. In Final Form: September 12, 2005 Mesoporous silica thin films were shown to be an appropriate matrix for immobilization of discrete electroactive moieties, yielding uniform transparent thin film electrodes with defined texture and enhanced electrochemical activity. The mesoporous silica films prepared on conducting FTO-coated glass substrate were postsynthetically functionalized. Alkoxysilanes were used as precursors for subsequent grafting via ionic or covalent bonds of representative electroactive species, such as polyoxometalate PMo12O403-, hexacyanoferrate(III), and ferrocene. The electrochemically active concentration within the silica-based composite electrodes achieves 90, 260, and 60 µmol cm-3 for polyoxometalate, hexacyanoferrate(III), and ferrocene, respectively. The amount of molecules involved in the charge-transfer sequence is proportional to the film thickness and comparable to the total amount of embedded guests. Thus, eventually the whole bulk volume of the modified silica films is electrochemically accessible. Immobilization in the chemically modified silica matrix alters the redox potential of the electroactive molecules. Electron exchange between the adjacent redox centers (electron hopping) is proposed as a possible charge propagation pathway through the insulating silica matrix, which is supported by the fact that the high charge uptake is observed also for the hybrid electrodes with the covalently anchored redox guests.
Introduction Preparation of electrochemically active films on the electrode surface is required in numerous electrochemical applications such as electrocatalysis (including biomimicking electrocatalytic processes), energy storage, photoand optovoltaics, sensors, and electroanalysis.1,2 While for some of the redox materials such continuous films can be easily prepared (e.g., for crystalline metal oxides and polymers such as polythiophene or polypyrrole),3 for the majority of electroactive compounds with discrete character, such as redox enzymes, it is not possible. Therefore, suitable methods for assembling the discrete molecules into electroactive layers should be developed, which is however often complicated and remains a challenging task * To whom correspondence should be addressed. E-mail:
[email protected],
[email protected]. † J. Heyrovsky ´ Institute of Physical Chemistry, Dolejsˇkova 3, 18223 Prague 8, Czech Republic. (1) For an overview, see: Murray, R. W. In Chemically Modified Electrodes in Electroanalytical Chemistry, A Series of Advances; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13. (2) (a) Bedioui, F.; Villeneuve, N. Electroanalysis 2003, 15, 5-18. (b) Yoon, D. S.; Roh, J. S.; Baik, H. K.; Lee, S. M. Crit. Rev. Solid State Mater. Sci. 2002, 27, 143-226. (c) Krasnov, A. N. Prog. Cryst. Growth Charact. Mater. 1998, 37, 123-167. (d) Aurbach, D. J. Power Sources 2000, 89, 206-218. (e) Granqvist, C.G.; Azens, A.; Hjelm, A.; Kullman, L.; Niklasson, G.A.; Ronnow, D.; Mattsson, M. S.; Veszelei, M.; Vaivars, G. Solar Energy 1998, 63, 199-216. (f) Malinauskas, A.; Garjonyte, R.; Mazeikiene, R.; Jureviciute, I. Talanta 2004, 64, 121-129. (g) Abe, T.; Kaneko, M. Prog. Polym. Sci. 2003, 28, 1441-1488. (h) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693-3723. (i) Prodromidis, M. I.; Karayannis, M. I. Electroanalysis 2002, 14, 241-261. (g) Ugo, P.; Moretto, L. M. Electroanalysis 1995, 7, 11051113. (h) Bartlett, P. N.; Cooper, J. M. J. Electroanal. Chem. 1993, 362, 1-12. (3) (a) Dorey, R.A.; Whatmore, R.W. J. Electroceram. 2004, 12, 1932. (b) Aita, C. R. J. Vac. Sci. Technol. A, Part 1 1998, 16, 1303-1310. (c) Hiratsuka, A.; Karube, I. Electroanalysis 2000, 12, 695-702. (d) Li, L.; Yan, F.; Xue, G. J. Appl. Polym. Sci. 2004, 91, 303-307. (e) Eftekhari, A. J. Electrochem. Soc. 2004, 151, A1816-A1819. (f) Villullas, H. M.; Mattos-Costa, F. I.; Nascente, P. A. P.; Bulhoes, L. O. S. Electrochim. Acta 2004, 49, 3909-3916.
in different applications. Immobilization of the discrete redox species by self-assembly (e.g., by grafting to electrode surface, Langmuir-Blodgett techniques, etc.) is often hardly controllable and mostly undefined or the achieved monolayer coverage is not sufficient for practical applications. However, the texture of the resulting films has a decisive influence on the electrochemical performance. Common alternatives to the self-assembled layers are supported electrodes, where the electroactive sites are immobilized in an appropriate host matrix via physical entrapment or covalent and noncovalent binding. For the supported electrodes choice of the host material becomes a crucial point in the general performance of the host/ guest system. The matrix should provide the mechanical stability of the whole ensemble and serve as a structuredetermining framework, which should not hinder the charge transfer to the embedded redox sites. In this respect, ordered mesoporous metal oxides MxOy (M ) Si, Ti, Zr, etc.), generally prepared by templateassisted procedures exploiting self-assembling surfactants, block copolymers, etc.,4 are nearly ideally suited for application as support matrix. They offer a number of highly attractive features such as (i) chemical and mechanical stability of the inorganic materials, (ii) highly ordered porous structure with a large surface area, (iii) pore size ranging from 1.5 to 10 nm, which allows embedding the large molecules and provides diffusion pathways for the charge-balancing ions, and (iv) possibility (4) (a) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552. (b) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 356, 152-155. (c) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813-2826. (d) Alberius, P. C. A.; Frindell, K. L.; Hayward, R. C.; Kramer, E. J.; Stucky, G. D.; Chmelka, B. F. Chem. Mater. 2002, 14, 3284-3294. (e) Choi, S. Y. C.; Mamak, M.; Coombs, N.; Chopra, N.; Ozin, G. A. Adv. Funct. Mater. 2004, 14, 335-344. (f) Grosso, D.; Cagnol, F.; Soller-Illia, A. A.; Crepaldi, E. L.; Amenitsch, H.; Brunet-Bruneau, A.; Bourgeois, A.; Sanchez, C. Adv. Funct. Mater. 2004, 14, 309-322. (g) Soller-Illia, A. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. Rev. 2002, 102, 4093-4138.
10.1021/la051616a CCC: $30.25 © 2005 American Chemical Society Published on Web 10/13/2005
Functionalized Mesoporous Silica Films
of easy modification with the various guests by either onepot synthesis5 or postsynthetic grafting,6 which makes those materials especially suitable as a universal substrate for immobilization of the broad range of desired functionalities demanded by the application needs. Use of porous oxides in electrochemistry is still viewed with some skepticism due to the fact that those materials are insulators or semiconductors and thus cannot provide direct electron transfer to the immobilized electroactive sites. This concerns mostly zeolite- and microporous silicate and alumosilicate-supported materials,7,8 for which just the species located in the intimate vicinity to the electron conductor are considered to be electrochemically active. However, regarding the mesoporous metal oxides, the already published studies demonstrate that the rich variety of electroactive guests ranging from enzymes to metal cations can be embedded in such materials, mostly silica,9-15 titania,16 or their mixed oxides with Zr, Sb, Sn, etc.,17-19 and that despite the nonconductive character of the mesoporous matrix the entire amount of immobilized guests is electrochemically accessible. The reported examples concern mostly powder materials, which need to be mixed with a binder or a conductive matrix for obtaining the final modified electrodes. Mesoporous films as the supports for electroactive species are almost uncharted land, although they are highly desirable in applications such as membrane electrocatalysis, sensors, or electronic devices.16,20 Metal oxides can be prepared as crack-free films of controllable thickness, texture, and porosity. Another attractive feature of films of the metal oxides, in contrast to conductive metal and (5) (a) Cagnol, F.; Grosso, D.; Sanchez, C. Chem. Commun. 2004, 1742-1743. (b) Fun, H. Y.; Lu, Y. F.; Stump, A.; Reed, S. T.; Baer, T.; Schunk, R.; Perez-Luna, V.; Lopez, P. G.; Brinker, C. J. Nature 2000, 405, 56-60. (c) Liu, N.; Assink, R. A.; Smarsly, B.; Brinker, C. J. Chem. Commun. 2003, 1146-1147. (6) (a) Angelome, P. C.; Soler-Illia, G. de A. A. Chem. Mater. 2005, 17, 322-331. (b) Cagnol, F.; Grosso, D.; Sanchez, C. Chem. Commun. 2004, 1742-1743. (c) Yoshida, W.; Castro, R.; Jou, J.; Cohen, Y. Langmuir 2001, 17, 5882-5888. (7) (a) Bedioui, F.; Devynck, J. J. Phys. Chem. 1996, 100, 86078609. (b) Senaratne, C.; Zhang, J.; Baker, M. D.; Bessel, C. A.; Rolison, D. R. J. Phys. Chem. 1996, 100, 5849-5862. (c) Rolison, D. R.; Bessel, C. A.; Baker, M. D.; Senaratne, C.; Zhang, J. J. Phys. Chem. 1996, 100, 8610-8611. (8) For a comprehensive overview of the electrochemistry of silicabased porous materials, see: Walcarius, A. In Handbook of Zeolite Science and Technology; Auerbach, S. M., Carrado, K. A., Dutta, P. K, Eds.; Marcel Dekker: New York, 2003; pp 721-783. (9) Diaz, I.; Garcia, B.; Alonzo, B.; Casado, C.; Moran, M.; Losada, J.; Perez-Pariente, J. Chem. Mater. 2003, 15, 1073-1079. (10) Walcarius, A.; Etienne, M.; Sayen, S.; Lebeau, B. Electroanalysis 2003, 15, 414-421. (11) Li, W.; Li, L.; Wang, Z.; Cui, A.; Sun, C.; Zhao, J. Mater. Lett. 2001, 49, 228-234. (12) Li, L.; Li, W.; Sun, C.; Li, L. Electroanalysis 2002, 14, 368-375. (13) (a) Zheng, S.; Gao, L.; Guo, J. J. Solid State Chem. 2000, 152, 447-452. (b) Alvaro, M.; Ferrer, B.; Garcia, H.; Lay, A.; Trinidad, F.; Valenciano, J. Chem. Phys. Lett. 2002, 356, 577-584. (c) Villemure, G.; Pinnavaia, T. G. Chem. Mater. 1999, 11, 789-794. (d) Jiang, Y.; Song, W.; Liu, Y.; Wei, B.; Cao, X.; Xu, H. Mater. Chem. Phys. 2000, 62, 109114. (e) Domenech, A.; Alvaro, M.; Ferrer, B.; Garcia, H. J. Phys. Chem. B 2003, 107, 12781-12788. (f) Dai, Z.; Xu, X.; Ju, H. Anal. Biochem. 2004, 332, 23-31. (14) Walcarius, A.; Bessiere, J. Chem. Mater. 1999, 11, 3009-3011. (15) (a) Bond, A. M.; Miao, W.; Smith, T. D.; Jamis, J. Anal. Chim. Acta 1999, 396, 203-213. (b) Hajek, J.; Kumar, N.; Ma¨ki-Arvela, P.; Salmi, T.; Murzin, D. Yu.; Paseka, I.; Heikkila¨, T.; Laine, E.; Laukkanen, P.; Va¨yrynen, J. Appl. Catal. A: Gen. 2003, 251, 385-396. (16) Choi, S. Y.; Mamak, M.; Coombs, N.; Chopra, N.; Ozin, G. A. Nanoletters 2004, 4, 1231-1235. (17) Cardoso, W. S.; Francisco, M. S. P.; Lucho, A. M. S.; Gushikem, Y. Solid State Ionics 2004, 167, 16-173. (18) Zaitseva, G.; Gushikem, Y.; Ribeiro, E. S.; Rosatto, S. S. Electrochim. Acta 2002, 47, 1496-1474. (19) Ribeiro, E. S.; Rosatto, S. S.; Gushikem, Y.; Kubota, L. T. J. Solid State Electrochem. 2003, 7, 665-670.
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carbon materials, is their transparency, important for applications utilizing both electrochemical and optical properties. This paper is the first systematic study into the use of mesoporous silica films as a support for immobilization of redox guests in order to obtain nanocomposite mesostructured electrodes with high electrochemical activity and leaching stability. In our approach, first surfactanttemplated silica mesostructures in the form of thin films are prepared by dip coating. After removal of the template, the open pore structure becomes accessible to silylation agents with functional groups. The latter serve as precursors for anchoring of redox species, which are finally immobilized via ionic or covalent bonds. The electrochemical activity of the redox guests in mesoporous silica films was supposed to be established via two major pathways, namely, (i) direct electron transfer from the conductive electrode surface to electrochemically active species due to their physical mobility (diffusion or migration), which requires desorption of the immobilized species from their positions in the matrix, and (ii) longrange electron transfer from the electrode surface through the immobilized redox centers via electron exchange between the adjacent redox moieties (electron hopping) presuming that the conditions for electron exchange are provided.21 In this respect, the species with the fast electronexchange kinetics were chosen as representative redox functionalities. The other criteria for the selection of electroactive guests were (i) simple and reversible redox behavior to allow straightforward interpretation of electrochemical properties of composite electrodes, (ii) demonstration of the effect of differing types of immobilization, and (iii) availability. Therefore, for ionic anchorage commercially available anions, namely, Keggin-type molybdenum polyoxometalate PMo12O403- (Mo-POM) and hexacyanoferrate(III) anion Fe(CN)63-, were used. Linking of ferrocenecarboxylic acid is an example of covalent bonding to the amino- and iodo-functionalized silica films via peptide and ester bonds. Experimental Section Materials. Mesoporous films were prepared from tetraethylorthosilicate (TEOS, Aldrich) with block copolymers Pluronic F127 (HO(CH2CH2O)106(CH2CH(CH3)O)70(CH2CH2O)106H, BASF) or Brij76 (HO(CH2CH2O)10C18H37, BASF) as templates. The mesoporous films were silylated with N-trimethoxysilylpropylN,N,N-trimethylamonium chloride (MAPTMS, 50% in methanol, ABCR), (3-aminopropyl)triethoxysilane (APTES, Aldrich), and (3-iodopropyl)trimethoxysilane (IPTMS, ABCR) in dichloromethane (300 mV) for high scan rates approaching reversible values (
