Langmuir 1998, 14, 6347-6349
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Electrochemistry of Langmuir-Blodgett Films Based on Prussian Blue S. Ravaine,* C. Lafuente, and C. Mingotaud Centre de Recherche Paul Pascal, C.N.R.S., Avenue A. Schweitzer, F-33600 PESSAC, France Received July 2, 1998. In Final Form: September 7, 1998 The voltammetric, spectroelectrochemical, and photoelectrochemical behaviors of new Prussian blue/ dimethyldioctadecylammonium bromide Langmuir-Blodgett films are described. The results concerning redox and ion-transport processes compare favorably with those of published studies based on electrogenerated Prussian blue films or Prussian blue particles mechanically attached to conductive electrodes. The voltammetric response, the visible absorption, and the photoresponse of the hybrid Langmuir-Blodgett films are found to be proportional to the number of deposited layers, suggesting that these inorganic/ organic lamellar materials are good candidates to construct a new kind of molecular device whose properties could be perfectly controlled.
Introduction It has long been known that highly effective devices whose organization has to be controlled at the molecular level can be elaborated using the Langmuir-Blodgett (LB) technique.1 In the last two decades, the preparation of a large number of gas sensors,2,3 biosensors,4 photodiodes,5 or electrochromic devices6,7 based on LB films of various organic molecules has been reported. More recently, the formation of inorganic/organic LB films has been developed.8-11 One of the advantages of modeling such hybrid lamellar materials is the possibility of incorporating typically inorganic solid-state properties into the multilayers. We have just reported the elaboration of new LB films based on the absorption of Prussian blue (PB), whose composition is expressed as FeIII4[FeII(CN)6]3, along dimethyldioctadecylammonium (DODA) Langmuir monolayers.12 These films, whose lamellar structure can be described as a single layer of cyanide-bridged irons intercalated between two layers of DODA, undergo a magnetic ordering transition at 5.7 K to a ferromagnetic state,12 similarly to the PB powder.13 Besides, it is wellknown that PB exhibits other interesting properties14,15 which make PB-modified electrodes potentially applicable * To whom correspondence should be addressed. (1) Ulman, A. An introduction to ultrathin organic films: from Langmuir-Blodgett to self-assembly; Academic Press: New York, 1991. (2) Gu, C.; Sun, L.; Zhang, T.; Li, T. Thin Solid Films 1996, 284285, 863. (3) Lloyd, J. P.; Pearson, C.; Petty, M. C. Thin Solid Films 1988, 160, 431. (4) Reichert, W. M.; Bruckner, C. J.; Joseph, J. Thin Solid Films 1987, 152, 345. (5) Nishikata, Y.; Fukui, S.; Kakimoto, M.; Imai, Y.; Nishiyama, K.; Fujihira, M. Thin Solid Films 1992, 210-211, 296. (6) Lukas, B.; Lovett, D. R.; Silver, J. Thin Solid Films 1992, 210211, 213. (7) Petty, M.; Lovett, D. R.; Townsend, P.; O’Connors, J. M.; Silver, J. J. Phys. D: Appl. Phys. 1989, 22, 1604. (8) Clemente-Leon, M.; Mingotaud, C.; Agricole, B.; Gomez-Garcia, C. J.; Coronado, E.; Delhaes, P. Angew. Chem., Int. Ed. Engl. 1997, 36, 1114. (9) Seip, C. T.; Granroth, G. E.; Meisel, M. W.; Talham, D. R. J. Am. Chem. Soc. 1997, 119, 7084. (10) Seip, C. T.; Byrd, H.; Talham, D. R. Inorg. Chem. 1996, 35, 3479. (11) Aiai, M.; Ramos, J.; Mingotaud, C.; Amiell, J.; Delhaes, P.; Jaiswal, A.; Singh, R. A.; Singh, B.; Singh, B. P. Chem. Mater. 1998, 10, 728. (12) Mingotaud, C.; Lafuente, C.; Amiell, J.; Delhaes, P. Langmuir, submitted. (13) Herren, F.; Fisher, P.; Ludi, A.; Ha¨lg, W. Inorg. Chem. 1980, 19, 956. (14) Itaya, K.; Uchida, I.; Neff, V. D. Acc. Chem. Res. 1986, 19, 162.
for energy storage,16,17 sensors,18 and electrochromic display devices.19-22 The present work focuses on the voltammetric, spectroelectrochemical, and photoelectrochemical behaviors of the new PB/DODA LB films in aqueous potassium chloride electrolytes. Our particular interest is the comparison of data obtained with this new kind of hybrid materials to those when a PB film is electrogenerated in a ferric-ferricyanide solution,23-30 particularly with respect to transport of ions. Reversible redox reactions are observed, suggesting that potassium ions can easily diffuse through the hybrid LB films. A linear dependence of the voltammetric response, the visible absorption, and the photoresponse of the PB/DODA LB films on the number of deposited layers is shown and allows one to believe that these new inorganic/organic materials can be used as display devices or sensors. Experimental Section Reagents and Materials. Prussian blue was purchased from Aldrich, and dimethyldioctadecylammonium bromide (DODA) (99%), from Kodak. Indium tin oxide (ITO) glass slides were obtained from Thomson (sheet resistance ) 20 Ω per square; area ) 2.5 cm2) and were cleaned by sequential sonication in acetone and distilled water prior to each experiment. Procedures. Spreading solutions were prepared from HPLC grade chloroform (Prolabo) and were kept at -18 °C between (15) Joseph, J.; Gomathi, H.; Prabhakara Rao, G. Bull. Electrochem. 1992, 8, 86. (16) Kaneko, M.; Okeda, T. J. Electroanal. Chem. 1988, 255, 45. (17) Neff, V. D. J. Electrochem. Soc. 1985, 132, 1382. (18) Deakin, M. R.; Byrd, H. Anal. Chem. 1989, 61, 290. (19) Carpenter, M. K.; Conell, R. S. J. Electrochem. Soc. 1990, 137, 2464. (20) Duek, E. A. R.; De Paoli, M. A.; Mastragostino, M. Adv. Mater. 1992, 4, 287. (21) Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Electrochromism: Fundamentals and Applications; VCH: Weinheim, 1995; Chapter 6. (22) Kulesza, P. J.; Zamponi, S.; Malik, M. A.; Miecznikowski, K.; Berrettoni, M.; Marassi, R. J. Solid State Electrochem. 1997, 1, 88. (23) Neff, V. D. J. Electrochem. Soc. 1978, 125, 886. (24) Itaya, K.; Ataka, T.; Toshima, S. J. Am. Chem. Soc. 1982, 104, 4767. (25) Kulesza, P. J.; Chelmecki, G.; Galadyk, B. J. Electroanal. Chem. 1993, 347, 417. (26) Itaya, K.; Uchida, I.; Toshima, S.; De La Rue, R. M. J. Electrochem. Soc. 1984, 131, 2086. (27) Itaya, K.; Akahoshi, H.; Toshima, S. J. Electrochem. Soc. 1982, 129, 1498. (28) Itaya, K.; Uchida, I. Inorg. Chem. 1986, 25, 389. (29) Roig, A.; Navarro, J.; Garcia, J. J.; Vicente, F. Electrochim. Acta 1994, 39, 437. (30) Roig, A.; Navarro, J.; Tamarit, R.; Vicente, F. J. Electroanal. Chem. 1993, 360, 55.
10.1021/la9808014 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/09/1998
6348 Langmuir, Vol. 14, No. 22, 1998
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Cyclic Voltammetry. A typical cyclic voltammetric response of a nine-layer DODA/BP LB film in a 1 M KCl aqueous solution (pH ) 4.0) is shown in Figure 1 A. Two well-defined sets of reversible peaks are observed, consistent with the known redox behavior of PB films31 or PB particles.32 The signals for the reduction of PB which occur at 165 mV vs SCE are sharper than those for the processes
associated with the oxidation response which occur at 900 mV vs SCE. The voltammetric response remained unchanged after dozens of potential cycles, so that any desorption of PB from the multilayer assembly or any oxidation of the reduced form of PB by oxygen was not observed. The dependence of the cathodic peak current around 165 mV vs SCE on the number of deposited layers is shown in Figure 1B. A linear behavior is observed and indicates that the transfer of PB is homogeneous during the LB film elaboration. Furthermore, it shows that the oxidation-reduction of the PB occurs even in the presence of the insulating lipid layers, which may be correlated to the presence of holes in the DODA layers. Spectroelectrochemistry. The four-color polyelectrochromicity of PB electrodeposited films is wellknown.21,33 To study the electrochromic behavior of the PB/DODA LB films, visible absorption spectra were first registered at various applied potentials. The visible absorption of the PB/DODA LB films at 0.55 V was found to be linearly dependent on the number of deposited layers, which confirms the homogeneity of the transfer of PB during the fabrication of the hybrid materials. On increasing from 0.55 V to more oxidizing potentials (until 1.2 V), the absorption of the internal charge-transfer band of PB at 700 nm continuously decreased, while the absorption at 430 nm steadily increased, following the oxidation of PB. Upon application of potentials from 0.55 V to -0.2 V, the characteristic band at 700 nm decreased and practically disappeared, following the formation of the colorless reduced form of PB. These results are consistent with the spectroelectrochemical behavior of electrodeposited PB films.21,34 The reversibility and the stability of the electrochromic properties associated with the oxidation and the reduction of PB/DODA films were determined by spectroelectrochemical potential-step experiments. Typical patterns of absorbance changes of a 19-layer LB film at 700 and 430 nm induced by repeated switching of the applied potential are shown in Figure 2. In all cases, coloration and bleaching of the multilayer assemblies occur very quickly and are perfectly reversible over a period of 30 min. These results demonstrate that potassium ions can easily penetrate inside the lamellar structure of the LB films to provide the charge compensation required for the electrochromic redox reactions.21 Photoelectrochemistry. The cyclic voltammograms of a 29-layer PB/DODA LB film recorded in the dark and under illumination with a polychromatic light are shown in Figure 3. Both anodic peaks are slightly shifted under illumination to potentials higher by about 20 mV than those in the dark, and the cathodic peaks to lower potential. These results show that the irradiation induces a shift of the equilibrium of the electrochemical process of the PB/ DODA LB films. The photoresponse of a 19-layer PB/DODA LB film at an applied potential of 1 V is shown in Figure 4. A stable photocurrent is induced repeatedly and reversibly by switching the irradiation on and off. The direction of the photocurrent is dependent on the applied potential, as shown in Figure 5. An anodic photocurrent is obtained at potentials higher than approximately 0 V, while a cathodic photocurrent is generated at potentials lower than this value. These results are consistent with those reported by Kaneko and co-workers about the photoresponse of
(31) Upadhyay, D. N.; Kolb, D. M. J. Electroanal. Chem. 1993, 358, 317. (32) Dostal, A.; Meyer, B.; Scholz, F.; Schro¨der, U.; Bond, A. M.; Marken, F.; Shaw, S. J. J. Phys. Chem. 1995, 99, 2096.
(33) Mortimer, R. J.; Dillingham, J. L. J. Electrochem. Soc. 1997, 144, 1549. (34) Yano, J.; Terayama, K.; Yamasaki, S. J. Mater. Sci. 1996, 31, 4785.
Figure 1. (A) Cyclic voltammogram of a nine-layer PB/DODA LB film in aqueous 1 M KCl (pH ) 4.0). Scan rate ) 25 mV/s. Area of the electrode ≈ 1 cm2 (+: starting point of scanning). (B) Dependence of the cathodic current at about 165 mV vs SCE on the number of deposited layers. Supporting electrolyte: 1 M KCl (pH ) 4.0). Scan rate ) 100 mV/s. Area of the electrode ≈ 1 cm2. experiments to limit solvent evaporation. An appropriate amount of the DODA solution was carefully spread onto a 10-6 M Prussian blue aqueous solution such that the initial area per molecule was close to 200 Å2, and the spreading solvent was allowed to evaporate for 10 min prior to compression. The Langmuir monolayers were compressed at 20 ( 1 °C using a continuous barrier speed of 4 Å2 molecule-1 min-1. The Y-type DODA/PB LB films were obtained by the vertical lifting method with a transfer ratio close to unity, at a target pressure of 25 mN/m, with a dipping speed of 5 mm/min. After each cycle, the substrate was allowed to dry for 10 min in air. Instrumentation. The LB experiments were carried out with a KSV Instruments 5000 system. The area of the Teflon trough was 707 cm2. Surface pressure was measured with a platinum Wilhelmy plate suspended from a KSV microbalance. A Millipore purification system produced water with a resistivity higher than 18 MΩ‚cm. All the electrochemical experiments were conducted at the ambient laboratory temperature 20 ( 1 °C, in KCl aqueous solutions that had been bubbled with nitrogen for at least 20 min, with an Autolab PGSTAT 20 potentiostat from EcoChemie, computer controlled by their General Purpose Electrochemical System software. The pH of the supporting electrolyte solution was adjusted to 4.0 by addition of hydrochloric acid. The cyclic voltammetry and photoelectrochemical experiments were carried out in a three-electrode conventional cell. Potentials were measured with respect to a saturated calomel electrode (SCE). For the spectroelectrochemical measurements, an ITO glass plate was fixed in a cuvette with a platinum counter electrode and a SCE. Spectra were recorded during the electrochemical experiments with an Unicam UV 4 spectrophotometer. For the photoelectrochemical experiments, the ITO electrode was irradiated with white light from a KL 1500 halogen lamp (Schott; 150 W).
Results and Discussion
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Langmuir, Vol. 14, No. 22, 1998 6349
Figure 4. Current changes induced by switching on and off the irradiation of a 19-layer PB/DODA LB film. Supporting electrolyte: 0.5 M KCl (pH ) 4.0). Applied potential ) 1 V vs SCE. Area of the electrode ≈ 1 cm2.
Figure 5. Dependence of the photocurrent of a 19-layer PB/ DODA LB film on the applied potential. Supporting electrolyte: 0.5 M KCl (pH ) 4.0). Area of the electrode ≈ 1 cm2.
Figure 2. Changes in the absorbance of a 19-layer PB/DODA LB film (A) at 700 nm in response to potential steps between 0.55 and 1.2 V, (B) at 700 nm in response to potential steps between 0.55 V and -0.2 V, and (C) at 430 nm in response to potential steps between 1.2 and 0.55 V. Supporting electrolyte: 1 M KCl (pH ) 4.0). Area of the electrode ≈ 1 cm2.
Figure 6. Dependence of the photocurrent at 0.8 V vs SCE on the number of deposited layers. Supporting electrolyte: 0.5 M KCl (pH ) 4.0). Area of the electrode ≈ 1 cm2.
LB films (see Figure 6). These results demonstrate once more that the redox behavior of PB trapped within the LB films is not affected by the presence of the DODA layers. Figure 3. Cyclic voltammogram of a 29-layer PB/DODA LB film in the dark (dashed line) and under illumination (solid line) in aqueous 0.5 M KCl (pH ) 4.0). Scan rate ) 100 mV/s. Area of the electrode ≈ 1 cm2 (+: starting point of scanning).
PB35 or bilayer membranes of PB and polymer-pendant Ru(bpy)32+ (refs 36 and 37) coated over basal plane pyrolitic graphite. Furthermore, the photocurrent was observed to be proportional to the number of layers of the PB/DODA (35) Kaneko, M.; Hara, S.; Yamada, A. J. Electroanal. Chem. 1985, 194, 165. (36) Kaneko, M. J. Macromol. Sci., Chem. 1987, A24, 357. (37) Kaneko, M.; Yamada, A. Electrochim. Acta 1986, 31, 273.
Conclusion Our study has emphasized that PB/DODA LB films exhibit reversible redox properties. It has been shown that the voltammetric response, the visible absorption, and the photoresponse of the new hybrid LB films are proportional to the number of deposited layers. Potential applications of the easily prepared, stable lamellar materials that are described in this study may include the fashioning of display devices or sensors. Further studies based on other polynuclear transition metal cyanometalates are currently underway in our laboratory. LA9808014
