Organic Thin Film Formation Using Asymmetric Surface-Active

In the present study, asymmetric surface-active viologens were applied successfully for the thin film formation of organic pigments using the immersio...
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Langmuir 2007, 23, 1912-1916

Organic Thin Film Formation Using Asymmetric Surface-Active Viologens Nabeen K. Shrestha, Hirotaka Kobayashi, and Tetsuo Saji* Department of Chemistry & Materials Science, Tokyo Institute of Technology, 2-12-1 S1-44, O-okayama, Meguro-ku, Tokyo 152-8552, Japan ReceiVed October 12, 2006. In Final Form: NoVember 13, 2006 In the present study, asymmetric surface-active viologens were applied successfully for the thin film formation of organic pigments using the immersion plating technique. The influences of the hydrophobicity of the surfactants and the pH of the plating solution on the film formation were investigated. In addition, the interfacial chemistry and electrochemistry of the surfactants were studied, and the mechanism of the film formation has been proposed and discussed.

1. Introduction

2. Experimental Section

The interest in the deposition of well-defined organic thin films has been increasing due to their application in organic electronic devices, such as memory storage devices, organic solar cells, organic field effect transistors, sensors, etc. In some of these applications, a well-controlled crystallinity1,2 and film thickness3 are desired. Most of the methods used to deposit highly ordered film are dry processes, for example, CVD, PVD, etc.4 All of these methods are very expensive and time-consuming. In our previous papers, we presented a wet process called a micelle disruption method to deposit uniform organic thin film from an aqueous dispersion of the organic particles by the electrochemical or electroless oxidation or reduction of redoxactive surfactants containing ferrocenyl5-8 and azobenzene9-13 moieties. These surfactants lose their ability as a surfactant by oxidation or reduction because of the enhancement of their hydrophilic character. In the present study, we investigated the dicationic surfactants with a viologen moiety for the organic thin film formation. Unlike the surfactant with an azobenzene moiety, the hydrophilic character of these viologen surfactants decreases by reduction due to the decrease of their charge. This phenomenon enabled us the formation of organic thin film. Although extensive research on the electrochemical, electrochromic, and selfassembly properties of the viologen surfactant and compounds has been reported,14-16 viologen surfactants have not been investigated as an organic film formation device so far.

All chemicals were used without further purification. β-Type copper pthalocyanine (CuPc, particle size: 0.1-0.2 µm, Dainichiseika Color & Chemicals Mfg. Co., Ltd.) pigment was used as the film forming material. The asymmetric viologen surfactants (ASVm/n, Figure 1) were synthesized by the reaction of 4,4′-bipyridine with the short-chain bromoalkane (m number of carbon atoms in the alkyl chain) in acetonitrile for 20 h at room temperature and washing the precipitate with toluene followed by alkylation with the long-chain bromoalkane (n number of carbon atoms in the alkyl chain). The final pale yellow product was washed with chloroform and recrystallized in ethanol. The details of the synthesis can be found elsewhere.17 All of the experiments were performed at 25 °C. The surface tension of the surfactants was determined in aqueous buffer solution using the Wilhemy plate method. For the film formation, an aqueous dispersion of CuPc in buffer solution containing ASVm/n was prepared by agitating the above mixture ultrasonically (Ultrasonic Disruptor, UR-200P, Tomy Seiko Co., Ltd.) for 40 min and stirring the suspension for 24 h. An indium tin oxide (ITO)-coated glass substrate short-circuited with an aluminum plate was immersed into the above dispersion solution for 20 min, and the film was washed in distilled water by subsequently immersing and withdrawing the substrate gently 4-5 times. The amount of the film was determined colorimetrically by redispersing the film in an aqueous solution of 5 mM polyoxylethylene (23) lauryl ether (Brij-35, Kanto Kagaku, Japan). The maximum amount of film of a uniform film is considered here as the optimum condition for film formation. The adsorption isotherm of the surfactant on CuPc was studied by dispersing the given amount of CuPc in an aqueous buffer solution containing different concentrations of the surfactant and stirring the mixture for 24 h followed by separating the CuPc using a centrifugal machine. The amount of surfactant adsorbed on CuPc was determined by analyzing the amount of the surfactant left in the supernatant spectrophotometrically. The amount of viologen surfactant incorporated into the film was determined colorimetrically as follows: The film scraped from the substrate was transferred into the Soxhlet apparatus. The film material was extracted for 24 h with acetone. The acetone was evaporated, and the residue was dissolved in an aqueous solution of 0.1 M KCl. The electronic absorption spectrum of this solution was similar to that of the viologen surfactant dissolved in an aqueous solution of 0.1 M KCl with λmax ) 262 nm and molar absorption coefficient, 262 ) 8.8 × 103 M-1 cm-1. The amount was determined using the

* Corresponding author. Tel./fax: +81-3-5734-2627. E-mail: tsaji@ o.cc.titech.ac.jp. (1) Getzlaff, M.; Pascal, R.; Wiesendanger, R. Surf. Sci. 2004, 566-568, 236. (2) Hayashi, K.; Tachibana, T.; Kawakami, N.; Yokoto, Y.; Kobashi, K.; Ishihara, H.; Uchida, K.; Nippashi, K.; Matsuoka, M. Diamond Relat. Mater. 2006, 15, 792. (3) Ito, Y.; Mitsuo, K.; Asai, K.; Okura, I.; Saji, T. Chem. Lett. 2004, 33, 222. (4) Chopra, K. L. Thin Film Phenomena; McGraw-Hill: New York, 1969; p 10. (5) Saji, T.; Hoshino, K.; Aoyagui, S. J. Am. Chem. Soc. 1985, 107, 6865. (6) Hoshino, K.; Saji, T. J. Am. Chem. Soc. 1987, 109, 5881. (7) Hoshino, K.; Suga, K.; Saji, T. Chem. Lett. 1986, 979. (8) Saji, T.; Hoshino, K.; Ishiii, Y.; Goto, M. J. Am. Chem. Soc. 1991, 113, 450. (9) Saji, T.; Ebata, K.; Sugawara, K.; Liu, S.; Kobayashi, K. J. Am. Chem. Soc. 1994, 116, 6053. (10) Saji, T.; Igusa, Y.; Kobayashi, K.; Liu, S. Chem. Lett. 1995, 24, 401. (11) Ito, Y.; Saji, T. Langmuir 2002, 18, 6633. (12) Saji, T.; Shrestha, N. K. Electrochemistry 2005, 74, 868. (13) Shrestha, N. K.; Saji, T. J. Jpn. Soc. Colour Mater. 2000, 73, 227. (14) Kostela, J.; Elmgren, M.; Hansson, P.; Almgren, M. J. Electroanal. Chem. 2002, 536, 97. (15) Mortimer, R. J. Electrochim. Acta 1999, 44, 2971. (16) Park, Y.-S.; Jang, J.-M.; Lee, C.-W. J. Electroanal. Chem. 1999, 468, 70.

(17) Pileni, M.-P.; Braun, A. M.; Gratzel, M. Photochem. Photobiol. 1980, 31, 423.

10.1021/la063002r CCC: $37.00 © 2007 American Chemical Society Published on Web 12/24/2006

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Figure 1. Molecular structure of a surface-active asymmetric viologen (ASVm/n).

Figure 3. (a) Adsorption isotherm of the ASV2/18 on CuPc particles at 25 °C. (b) Plot of Ceq/Γ versus Ceq. The straight line shows the validity of the Langmuir-type monolayer adsorption of the ASV2/ 18 on CuPc particles.

Figure 2. SEM images of (a) surface and (b) cross section, of the β-CuPc particle film on ITO glass plate prepared by immersing the plate short-circuited with an Al-plate in an aqueous dispersion of 20 mM β-CuPc in pH 3 phthalate buffer containing 4.5 mM ASV2/ 18 for 20 min at 25 °C. calibration curve at λmax ) 262 nm on the assumption that the reduced species of the viologen undergoes a complete aerial oxidation. The morphology of the film was investigated using field effect scanning electron microscopy (FE-SEM, S-800, Hitachi).

3. Results and Discussion A uniform blue film was formed on an ITO plate short-circuited with the Al-plate when these plates were immersed into the aqueous dispersion of 20 mM CuPc in buffer solution (pH 3 phthalate buffer system) containing 4.5 mM ASV2/18 for 20 min. The FE-SEM image of the surface of the film (Figure 2a) reveals the dense and uniform distribution of the CuPc particles throughout the film. The cross-sectional image of this film (Figure 2b) showed the uniform thickness of approximately 1.5 µm. The size of the particles in the film (Figure 2a) is the same as that of CuPc particles used for the dispersion. On the other hand, the electronic absorption spectrum of an aqueous dispersion of the CuPc in Brij-35 solution was similar to that of the aqueous dispersion of the film prepared by redispersing the film in the aqueous solution of Brij-35. These results reveal that the film was composed of the same CuPc particles without any physical or chemical modification, as those used for their dispersion. This means the film was formed by the precipitation of the CuPc particles from the dispersion as in the micelle disruption method.5-12 In this method, the aqueous insoluble particles are dispersed by the adsorption of a surfactant. The surfactants near the substrate are then forced to lose their ability to disperse

Figure 4. Cyclic voltammogram of an aqueous solution of 5 mM ASV2/18 in pH 3 phthalate buffer under N2 atmosphere at 25 °C.

insoluble particles.5-9,18 Consequently, the dispersed particle is released and precipitated on the surface of the substrate to form the film. The critical micelle concentration (CMC) of the ASV2/18 in aqueous buffer solution of pH 3 was found to be 0.5 mM by the surface tension method. A typical adsorption isotherm of this surfactant on the CuPc particles is shown in Figure 3. This isotherm shows that the adsorption saturation of the surfactant on CuPc particles was close to 4.5 mM of the equilibrium concentration of the surfactant. The plot of Ceq/Γ versus Ceq in Figure 3b gave a straight line satisfying the Langmuir adsorption isotherm, Ceq/Γ ) (1/Γ∞)Ceq + [1 + Γ∞K]. This result reveals the Langmuir-type monolayer adsorption of the ASV2/18 surfactant on the CuPc particles. The saturated amount of the adsorption (Γ∞) and the adsorption equilibrium constant (Κ) were 14.4 µmol m-2 and 1.8 × 103 M-1, respectively. From this isotherm, the adsorption area per ASV2/18 molecule was found to be 11.6 Å2. Figure 4 shows the cyclic voltammogram (CV) of the ASV2/ 18 surfactant in an aqueous solution of pH 3 buffer. As can be seen in this voltammogram, there are two pairs of redox peaks. The first reduction peak at about -0.44 V vs Ag/AgCl can be (18) Liu, S.; Ban, S.; Saji, T. J. Jpn. Soc. Colour Mater. 1999, 72, 218.

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Figure 5. Dependency of film amount on electrolysis potential at 25 °C. WE, ITO; CE, Pt; dispersion composition was as in Figure 2.

ascribed to the reduction ASV2+ to ASV+, whereas the second reduction peak at about -0.8 V vs Ag/AgCl can be ascribed to the further reduction of ASV+ to ASV0. The peak current in the first reduction wave was found to be controlled by the diffusion process, whereas the current in the second reduction peak and the oxidation peak corresponding to the oxidation of ASV0 to ASV+ in the reversal sweep was found to be controlled by both adsorption and diffusion processes. The above adsorption peaks are due to the enhancement of hydrophobicity in the reduced states, ASV+ and ASV0, of the surfactant.19 Similarly, the current in the second oxidation peak for the oxidation of ASV+ to ASV2+ was controlled by both diffusion and adsorption processes. The open circuit potential of the ITO-Al short-circuited plates immersed into the above dispersion solution was about -0.70 V vs Ag/AgCl. This potential is more negative than the reduction potential of ASV2+. Therefore, when these short-circuited plates are immersed into the aqueous dispersion of CuPc in pH 3 buffer solution containing ASV2/18, the free ASV2/18 (ASV2+) molecules not adsorbed on the CuPc particles are reduced to ASV+ on the surface of the ITO plate. This reduction decreases the concentration of the free ASV2+ in the vicinity of the ITO plate, and thereby the adsorption equilibrium of the ASV2+ collapsed. To satisfy the adsorption equilibrium, the ASV2+ molecules desorb from the particle surface in the vicinity of the substrate. Consequently, these released particles deposit on the substrate and form the film. On the other hand, the reduction of free ASV2+ decreases the surface tension of the solution around the substrate because ASV in the reduced states is more hydrophobic. The voltammogram in Figure 4 exhibited the strong adsorption of the ASV+ on the electrode surface, and this adsorption phenomenon indicates the incorporation of the AVS+ into the film or deposition on the ITO plate. To confirm if some of the ASV+ molecules incorporated into the film, the Soxhlet extract of the film was investigated, which revealed the incorporation of 0.07 mole fraction of the ASV+. The measured amount of the incorporated ASV+ in the film is smaller than the expected value. It seems that the partial desorption of the incorporated ASV+ from the film took place during the further course of film formation. However, the above results revealed the decrease in the concentration of ASV2+ and ASV+ in the vicinity of the ITO plate. Therefore, although the hydrophobicity of the ASV increases after its reduction, the redispersion of the particles after the desorption of ASV2+ hardly takes place. Figure 5 shows the dependency of film formation on the electrolysis potential. The film started to form at -0.38 V vs Ag/AgCl of the electrolysis potential. This potential is very close to the reduction peak potential of ASV2+ into ASV+ exhibited (19) Anton, P.; Heinze, J.; Laschewsky, A. Langmuir 1993, 9, 77.

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by the voltammogram in Figure 4. These data support that the film is formed by the reduction of the surfactant. However, because of the larger concentration gradient and much smaller size, the free surfactant molecules diffuse faster than the particles with the surfactant molecules adsorbed on them. Therefore, only free surfactant gets reduced at the electrode surface, which leads to the film formation as described above. Even after the electrode was completely covered with a uniform film, the film continued to grow for a long time due to the continuous reduction of the free surfactant at the electrode surface, which arrived there by diffusion through the small spaces among particles in the film. This continuous film growth for a long time reveals that the film formation was due to the reduction of free surfactant and not due to the reduction of the surfactant adsorbed on the particle surface. Otherwise, the film growth must be ceased after a short time when the electrode surface is completely covered with a monolayer of the particle film. As shown in Figure 5, the film growth increased with respect to the electrolysis potential due to the higher rate of reduction of the free surfactant. However, at more negative electrolysis potential, the current efficiency decreased due to the evolution of hydrogen, and it destroyed the film. Consequently, the film growth gradually decreased as shown in Figure 5. To further confirm that the desorption of the adsorbed ASV2+ from the particles surface takes place only due to the reduction of the free ASV2+ at the electrode, the critical concentration of the free ASV2+ at film formation (Ceq) was estimated using the following equation, which was derived on the basis of the Nernst equation: Ef ) E0 + (RT/nF) ln[Ceq/(Ceq0 - Ceq)], where Ef is the critical (most positive) electrolysis potential where film formation begins, E0 is the standard reduction potential of free ASV2+, and Ceq0 is the concentration of free ASV2+ in the vicinity of the electrode before electrolysis, respectively. The value of Ceq0 was determined by separating the CuPc particles using a centrifugal machine followed by analyzing the supernatant for the amount of the surfactant, spectrophotometrically. As shown in Figure 5, Ef was found to be -0.38 V vs Ag/AgCl. From these data, the value of Ceq was calculated to be 0.37 mM, which is the concentration of the free surfactant in the vicinity of the electrode after the electrolysis. The estimated Ceq is less than the CMC of the ASV2+ (ASV2/18) surfactant. These results show the direct evidence that the electrolysis of ASV2+ decreases the free ASV2+ to less than the CMC, which leads to the desorption of the adsorbed ASV2+ from the particle surface. As described above, surfactant molecules get adsorbed to disperse the particles, and then these surfactant molecules get desorbed near the substrate from the particle surface to form the particle film. The extent of adsorption and desorption depends on the hydrophobicity of the surfactants and the particles. In the present study, the influence of hydrophobicity of the ASV m/n surfactants on the film formation was investigated using the surfactants with m ) 2, 3, 4; n ) 18, and m ) 2; n ) 14, 16, 18. The film formation was performed by immersing the ITO plate short-circuited with an Al-plate in a dispersion containing various concentrations of the ASV for 20 min. The amount of film formed on various concentrations of ASV is shown in Figure 6. However, due to the strong hydrophobicity, surfactant ASV4/ 18 was difficult to dissolve in aqueous buffer solution. Consequently, no satisfactory film was formed using ASV4/18. Therefore, the data of ASV4/18 have not been shown in Figure 6. As can be seen in this figure, there is a peak corresponding to the maximum amount of film in each case of the surfactants. The concentrations of the surfactants corresponding to these peaks were close to their saturated amount of adsorption in the adsorption

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Figure 8. Concentration of mono- and dianions of the phthalate buffer solution used in Figure 7. Figure 6. Film amount under various concentrations of the ASVm/n surfactants. Dispersion composition and plating conditions were as in Figure 2.

Figure 7. Film amount under various pH conditions. Dispersion composition and plating conditions were as in Figure 2.

isotherms on CuPc particles. This reveals that the maximum amount of film is formed when the particles are covered by the monolayer of the adsorbed surfactant. When the concentration of surfactants increases, the amount of the dispersed particles also increases. As a result, the amount of the film increased with the increment of the surfactant concentration as shown in Figure 6. However, when the concentration of the surfactant is relatively higher than that needed for the saturation adsorption, the free surfactant becomes barrier for the film formation because it redisperses the particles released by the desorption of surfactant in the vicinity of the substrate. Consequently, the amount of film decreased after the optimum concentration of the surfactant. The more hydrophobic surfactants need their smaller amount to disperse a given amount of the particles due to their larger adsorption area and larger adsorption tendency. Therefore, the more hydrophobic surfactants have smaller optimum concentration for the film formation. Among all of the surfactants investigated in the present study, ASV2/18 produced a very uniform film, and this surfactant was considered as the best one for the film formation in the present investigation. Unlike the azobenzene surfactants,11-13 no protons are utilized in the reduction process of the ASV surfactants.14 Therefore, the film formation must not depend on the pH of the solution. However, no film was formed when the pH of the solution was less than 2.5. In contrast, a uniform and dense film was formed when the pH of the solution was 2.5 and higher up to pH 5 (phthalate buffer system). Moreover, the dependency of the film amount on the pH was observed as shown in Figure 7. To examine this dependency, film formation was carried out using other buffer systems in the pH range of 2.5-5, but no satisfactory film was formed. This reveals that the film formation does not depend on the pH of the solution, but the dependency of film formation on

Figure 9. Proposed mechanism of the organic film formation using a surface-active viologen.

pH shown in Figure 7 might be due to the influence of the phthalate ion concentration in the buffer solution. To confirm the influence of the phthalate ion on the film formation, the concentration of mono- and dianions of the phthalate salt in the buffer solution with different pH was calculated, and the data were plotted in Figure 8. This figure shows that the dianions almost did not exist up to pH 3 and its amount started increasing after pH 3.5. Figures 7 and 8 revealed the maximum amount of the film when the concentration of the monoanion was 39 mM. At this concentration of monoanion, the pH of the solution was 3, whereas the amount of the dianion was almost negligible (Figure 8). Further increasing the pH of the solution above 3, the concentration of the dianions increases as shown in Figure 8, whereas the film amount decreased as shown in Figure 7. Therefore, these results reveal that the anion of phthalic acid is necessary for the film formation and it is favored by the presence of monoanions, but the existence of the dianions prevents the film formation. Further investigation was performed to examine if the monoanions of organic acids other than phthalic acid can produce the film. The presence of acetic acid and benzoic acid in the pH 3 maintained aqueous solution (with aqueous HCl and NaOH) produced a very uneven film. Moreover, the film was formed only on some part of the substrate. However, like the potassium hydrogen phthalate, the presence of maleic acid produced the uniform film, but the film was very thin. These results reveal that the monoanions of the diprotonic organic acids or their salts are necessary to produce the uniform film. These anions interact electrostatically with the

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cationic viologens. It seems that the negative charge density of the monoanions stabilizes the ASV+ state more than the ASV2+ state14 and produces the neutral salts, which get adsorbed on the substrate. In this way, the reduction of ASV2+ becomes easier than when dianions are present, and this phenomenon decreases the concentration of ASV2+ to less than CMC so that the ASV2+ adsorbed on the particles is desorbed. Consequently, the particles get deposited on the substrate to produce the film as shown by the proposed mechanism in Figure 9. However, in contrast to the monoanions, the dianions stabilize the ASV2+ state more than ASV+ state. Moreover, the interaction of dianions with ASV+ produces the salt with a net negative charge. Because of this negative charge, the ASV+ hardly get adsorbed on the substrate in the presence of dianions, and the formation of the film in this case hardly takes place. In the present investigation, the films produced under the optimum conditions were uniform and dense in all of the pH values ranging from 2.5 to 5.0. Moreover, using the ASV2/18 surfactant, large varieties of pigment particles were able to deposit on a number of metallic substrate including Zn and Fe, which have a higher tendency to undergo oxidation in lower pH.

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4. Conclusion The present investigation revealed that the surface-active viologens undergo electrochemical reduction from dicationic state to monocationic state, when a base metal substrate or ITO-Al coupled plates are immersed into an aqueous solution of the viologens. This reduction phenomenon leads to the formation of a thin film of particles on the substrate. The reduced species of the surfactant also get deposited to the substrate. Although there is no direct influence of the pH on the reduction process of the surfactant, film was able to produce only in the pH range of 2.5-5.0 with the maximum amount at pH 3.0. This dependency of the film amount on the pH was related here to the amount of the phthalate ions of the buffer solution. Supporting Information Available: Photographs of the CuPc film on ITO plates showing the effect of hydrophobicity of the ASVm/n surfactants and the pH on the film formation. This material is available free of charge via the Internet at http://pubs.acs.org. LA063002R