Anal. Chem. 2007, 79, 5188-5195
Sol-Gel-Derived Composite Antimony-Doped, Tin Oxide-Coated Clay-Silicate Semitransparent and Conductive Electrodes A. Sadeh,† S. Sladkevich,† F. Gelman,† P. Prikhodchenko,† I. Baumberg,‡ O. Berezin,‡ and O. Lev*,†
The Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel, and ELM, Electroluminescence Industries, Hamarpe 8, Jerusalem, 91450, Israel
A new form of conductive and transparent porous composite electrode is introduced. The electrode material is composed of antimony-doped, tin oxide (ATO)-coated mica platelets imbedded in sol-gel-derived silicate or methyl silicate network. The platelet clays self-align in a layered structure within the silicate film, an anisotropic construction that minimizes the ATO loading required to achieve electric percolation. Transparency and resistance as a function of clay loading is reported with typical values of 100 kΩ/square and 1.5 OD for a 20-µm-thick film. The transparency is lower as compared to sputtered ATO glasses, but this is, as far as we know, the best method for the low-temperature preparation of transparent, porous, and electrically conductive (as opposed to the amply reported ionically conductive) electrode materials. Permselectivity induced by the silicate and clay ingredients is demonstrated by permeation of positively charged methyl viologen compared to negatively charged ferricyanide. Prussian blue-modified ATO-coated platelets dispersed in sol-gel-derived silicate were used to demonstrate feasibility of a transparent and electrically conductive porous electrochromic material. Electric conductivity and transparency are two desirable properties of electrode materials. The two are almost mutually exclusive when it comes to low-temperature preparation of porous film-modified electrodes. Thus, for the most part, modified electrodes are made of thin, permeable films of dielectric material, which ensure rapid access of bulk analytes to an underlying conductive electrode material where charge transport takes place. When transparency is unimportant, then carbon pastes or ceramic electrodes (CCEs)1-4 and metal composite electrodes5-9 can be * To whom correspondence should be addressed. E-mail:
[email protected]. † The Hebrew University of Jerusalem. ‡ ELM, Electroluminescence Industries. (1) Wang, J. Electroanalysis 2005, 17, 7-14. (2) Svancara, I.; Vytras, K.; Barek, J.; Zima, J. Crit. Rev. Anal. Chem. 2002, 31 (4), 311-345. (3) Walcarius, A.; Mandler, D.; Cox, J. A.; Collinson, M.; Lev, O. J. Mater. Chem. 2005, 15, 3663-3689. (4) Rabinovich, L.; Lev, O. Electroanalysis 2001, 13, 265-275. (5) Liu, S. Q.; Leech, D.; Ju, H. X. Anal. Lett. 2003, 36, 1-19. (6) Fishelson, N.; Shkrob, I.; Lev, O.; Gun, J.; Modestov, A. D. Langmuir 2001, 17, 403-412. (7) Bharathi, S.; Nogami, M.; Lev, O. Langmuir 2001, 17, 2602-2609.
5188 Analytical Chemistry, Vol. 79, No. 14, July 15, 2007
used to attain high surface area of the charge-transfer interface within the electrode. Metal ceramic and metal hydrogel composites as well as thin metal impregnated layers are increasingly used, partly owing to the evolution of nanomaterial electrochemistry, which brought about a plethora of new ways to prepare nanoparticles, functionalize them, and produce intricate constructions of metallic nanoparticles by sol-gel or other binding gels or by bridging molecules. However, when it comes to transparency, metals are much inferior to some oxides and polymers. Two other alternative directions, which are widely used to acquire conductivity while still maintaining the transparency of hydrogels, involve the use of conductive polymers or doped tin oxide (SnO2) matrixes. Polyaniline and polythiophene derivatives (and to some extent even the less transparent polypyrrole) based electrodes found many applications in optically active electrodes and solar cells.10-13 However, the electrochemical working window of the polymer electrodes in aqueous solutions is rather narrow, and therefore, their application for electrochemical and electroanalytical applications is confined to niche fields. Doped tin oxides are by far the transparent conductive electrode of choice with an unsurpassed combination of transparency, stability, and conductivity, but despite the tremendous progress in their preparation, particularly in the past decade or so, their manufacture still requires prolonged high-temperature annealing, long exposure to ultraviolet light, or both.14-19 Here we show that by using the anisotropy of mica flakes it is possible to produce transparent and conductive permeable, solgel-derived silicate-mica clay electrodes at (or close to) room temperature. The starting material for electrode construction is a (8) Willner, I.; Riklin, A.; Shoham, B.; Rivenzon, D.; Katz, E. Adv. Mater. 1993, 5, 912-915. (9) Katz, E.; Willner, I.; Wang, J. Electroanalysis 2004, 16, 19-44. (10) Spanggaard, H.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2004, 83, 125146. (11) MacDiarmid, A. G. Synth. Met. 1997, 84, 27-34. (12) Argun, A. A.; Cirpan, A.; Reynolds, J. R. Adv. Mater. 2003, 15 (16), 13381341. (13) Yang, Y.; Heeger, A. J. Appl. Phys. Lett. 1994, 64 (10), 1245-1247. (14) Puetz, J.; Gasparro, G.; Aegerter, M. A. Thin Solid Films 2003, 442, 4043. (15) Al-Dahaudi, N.; Aegerter, M.A.; J. Sol-Gel Sci. Technol. 2003, 26, 693-697. (16) Batzill, M.; Diebold, U. Prog. Surf. Sci. 2005, 79, 47-154. (17) Granqvist, C. G.; Hultaker, A. Thin Solid Films 2002, 411, 1-5. (18) Lewis, B. G.; Paine, D. C. MRS Bull. 2000, 25, 22. (19) Hartnagel, H. L.; Dawar, A. L.; Jain, A. K.; Jagadish, C. Semiconducting Transparent Thin Films; IOP Publishing: Bristol, 1995. 10.1021/ac070165r CCC: $37.00
© 2007 American Chemical Society Published on Web 06/08/2007
dispersion of antimony-doped, tin oxide-coated mica flakes (commercially available) in sol-gel-derived silicate or methyl silicate binder. Clay-modified electrodes are by no means new to electrochemistry. The first works on the subject date back to the seminal papers of Bard’s team in the mid 1980s.20 The field is still vivid and attracts considerable scientific attention, as is apparent from recent comprehensive reviews.21,22 The driving forces for the fast proliferation of this scientific field are the simplicity of electrode preparation (by spray coating, dip coating, electrophoretic deposition, Langmuir-Blodget, etc.), the cation-exchange capacity of the clays, their ability to preconcentrate analytes, and the porosity of the film-coated electrodes. The latter entails an ability to immobilize catalysts, biocatalysts, reagents, and mediators in the porous clay-containing films. Of importance, though not directly relevant to the results demonstrated at the current stage of the research, is the ability to intercalate electrochemical and chemical agents and basal or edge plane adsorbents in these electrodes. Xiang and Villemure23 demonstrated long-range charge transport by a hopping mechanism through Fe sites in the clay lattice. Several patents have indicated ways to prepare antimony-doped tin oxide (ATO)-coated mica (ATO/mica)24 and needle clays,25 and commercial ATO-coated mica is available from Merck (Darmstadt, Germany). In view of the large scientific efforts in the fields of sol-gel electrodes3 and clay-modified electrodes,21 it is not surprising that several authors have already studied sol-gel encapsulation of synthetic and natural clays in sol-gel silicates. Esterification of the silanols on the tetrahedral lattices of clays and the sol-gel silicate precursors provide seamless integration of the dopant and host matrix. The first article on the use of sol-gel binding of clays was published by the Coche-Guerente and Cosnier groups,26 demonstrating modified electrodes made by delaminated Laponite clay sol and octakis(3-aminopropylsilasesquioxane). The work was then followed by a demonstration of the electroanalytical applications (see for example ref 27), and recently Walcarius and coworkers demonstrated biosensing with enzyme doped sol-gel clay composites.28 The latter article also presents a comprehensive review of clay-modified biosensors. Despite this activity, until today, there has been no report on electrically conductive clayssol-gel composites, let alone their electrochemical potential. EXPERIMENTAL SECTION Chemicals. Methyltrimethoxysilane (MTMOS), tetramethoxysilane (TMOS), methyl viologen dichloride hydrate, cetyltrimethylammonium bromide (CTAB), and sodium dodecyl sulfate (SDS) were purchased from Aldrich (Steinheim, Germany). Poly(vinylidene fluoride) (PVDF) was purchased from Dyneon 3M Co. (Deutschland, Germany), Elvacite 2014 acrylic resin was (20) Ghosh, P. K.; Bard, A. J. J. Am Chem. Soc. 1983, 105, 5691-5693. (21) Mousty, C. Appl. Clay Sci. 2004, 27, 159-177. (22) Navratilova, Z.; Kula, P. Electroanalysis 2003, 15, 837-846. (23) Xiang, Y.; Villemure, G. J. Electroanal. Chem. 1995, 381, 21-27. (24) Motohiko, Y.; Kuniaki, W. Patent JP 01175104, 1989. (25) Koichi, Y.; Mitsutoshi, M.; Osamu, T. Patent JP 62086066, 1987. (26) Coche-Guerente, L.; Cosnier, S.; Desprez, V.; Labbe, P.; Petridis, D. J. Electroanal. Chem. 1966, 401, 253-260. (27) Ozsoz, M.; Erdem, A.; Ozkan, D.; Kerman, K.; Pinnavaia, T. J. Langmuir 2003, 19, 4728-4732. (28) Mbouguen, J. K.; Ngameni, E.; Walcarius, A. Anal. Chim. Acta 2006, 578, 145-155.
purchased from Lucite International (Southampton, United Kingdom), dimethylformamide, chloroform, methanol, and ethanol were purchased from JT Baker (Deventer, Holland). Sodium hydroxide and KCl were from Frutarom (Haifa, Israel). Triton X100 was purchased from Rohm and Haas Co. (Arnsberg, Germany). KNO3 was purchased from Merck KGaA (Darmstadt, Germany). K4Fe(CN)6‚3H2O was purchased from Mallinckrodt (St. Louise, MO). FeSO4‚7H2O was purchased from BDH Chemicals Ltd. (Poole, England). All chemicals were analytical grade. Solutions were prepared with triply distilled water (10 mV/s) the current is almost independent of the scan rate. The behavior
Figure 8. Cyclic voltammetry of 1 mM hexcyanoferrate in 0.1 M KNO3, pH 6.5 as a function of scan rate on (A) silicate-based ATO/ mica electrode (curves correspond to scan rates 1, 5, 10, 20, 30, 50, and 100 mV/s in intuitive order) and (B) methyl silicate-based electrode. (scan rates: 5, 10, 15, 25, 50, 75, and 100 mV/s).
is typical of ensembles of microelectrodes.30,31 The methyl silicate binder repels water and leaves wetted only isolated spots of uncoated ATO surfaces near the surface of the electrode. For fast scan rates, the diffusion layers around the exposed ATO surfaces do not overlap, and the exposed ATO sites behave as an ensemble of separate microelectrodes all exposed to the same potential constraint. At the low scan rate range (
