pubs.acs.org/Langmuir © 2010 American Chemical Society
Electrophoretic Deposition of Phthalocyanine in Organic Solutions Containing Trifluoroacetic Acid Nabeen K. Shrestha, Hideki Kohn, Mitsuharu Imamura, Kazunobu Irie, Hitoshi Ogihara, and Tetsuo Saji* Department of Chemistry & Materials Science, Tokyo Institute of Technology, 2-12-1 S1-9, O-okayama, Meguro-ku, Tokyo 152-8552 Received June 1, 2010. Revised Manuscript Received August 27, 2010 The absorption spectra of copper phthalocyanine (CuPc) 1,2-dichloroethane (DCE) solutions containing trifluoroacetic acid (TFAA) shows that the number of protons coordinating to the CuPc molecule was 1 and 2 for the first and second proton adducts, respectively, which indicates the formations of CuPcHþ and CuPcH22þ. This CuPc molecule may act as a catalyst to dissociate TFAA into trifluoroacetate anion (A-) and Hþ and form the proton adducts. The electrical conductivity dependence of the solution on CuPc concentration also supports this mechanism. A dense film of CuPc was deposited on an indium tin oxide cathode plate by electrophoresis of the solution. Similar dense films of a wide variety of phthalocyanines (MPc; M = Cu, H2, Fe, Ni, Zn, Pb, VO) were also deposited using this method. Similar films of CuPc were also formed using dichloromethane (DCM) and 1,1,1-trichloroethane (TCE) in place of DCE. Depositions are ascribed to the migration of positively charged monomers (i.e., protonated MPc). Scanning electron microscopy revealed that these films are composed of fibrous crystallites, size of which was found to increase with the electrophoresis time, the strength of the applied electrical field and the concentration of CuPc in the bath. The influence of the dielectric constant of the organic solvent on the film growth is discussed.
1. Introduction Research on phthalocyanines (MPc, Figure 1)) has greatly increased since their synthesis, due to their potential application in various fields. MPc are chemically stable and have strong color and, therefore, are used as dyes and pigments.1,2 MPc are also used as catalysts, photovoltaic materials, sensitizers, and gas sensors,2,3 and they are practically applicable to many devices as thin films. However, they have poor solubility in aqueous and organic solvents. Therefore, the preparation of MPc thin films from MPc monomers is limited to vacuum sublimation. Previously, we reported wet processes for the thin film formation of MPc by the micelle disruption method using redox-active surfactants,4-6 and by the electrophoretic deposition method using an I2/2-butanone system.7 These wet processes form the films by depositing the particles of MPc from the dispersion. It has been reported that some MPc could be dissolved in some organic solvents containing various kinds of acids (e.g., Cl3CCOOH, CF3SO3H) and such dissolution is due to the stepwise protonation of peripheral aza nitrogen atoms of the MPc.8,9 In our previous study,10 we demonstrated the electrophoretic deposition *Corresponding author. E-mail:
[email protected]. Tel/Fax: þ81-35734-2627. (1) Zollinger, H. Color Chemistry; VCH Publishers: Weinheim, New York, 1987; pp 77-82. (2) Phthalocyanines: Properties and Applications; Leznoff, C. C.; Lever, A. B. P., Eds.; VCH Publishers: New York, 1989. (3) Pizzini, S.; Timo, G. .L.; Beghi, M.; Butta, N.; Mari, C. M. Sens. Actuators 1989, 17, 481. (4) Saji, T.; Ishii, Y. J. Electrochem. Soc. 1989, 136, 2953. (5) Saji, T.; Hoshino, K.; Ishii, Y.; Goto, M. J. Am. Chem. Soc. 1991, 113, 450. (6) Saji, T.; Ebata, K.; Sugawara, K.; Liu, S.; Kobayashi, K. J. Am. Chem. Soc. 1994, 116, 6053. (7) Sato, N.; Saji, T. Chem. Lett. 1998, 647. (8) Iodko, S. S.; Kaliya, O. L.; Lebedev, O. L.; Luk’yanets, E. A. Kood. Khim. 1979, 5, 611. (9) Iodko, S. S.; Kaliya, O. L.; Kondratenko, N. V.; Luk’yanets, E. A.; Popov, V. I.; Yagupol’skii, L. M. Zh. Obshch. Khim. 1983, 53, 901. (10) Yamanouchi, H.; Irie, K.; Saji, T. Chem. Lett. 2000, 10.
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of a CuPc film from monomers of protonated CuPc using a trifluoroacetic acid (TFAA)/dichloromethane (DCM) solution. Takada et al. later reported that CuPc films prepared using this method exhibits a p-type semiconducting behavior.11 Recently, Su et al. reported that CuPc films prepared using electrophoretic deposition of CuPc film from TFAA/nitromethane solution.12 Komino et al. reported spin-coated films of MPc (M = H2 and Cu) from TFAA/chlorobenzene solution.13 We have reported the dyeing of cotton with unsubstituted MPc using a TFAA/organic solvent solution.14 In this paper, we report details of the electrophoretic deposition of MPc from monomeric protonated adducts in TFAA/organic solvent. The mechanism of film formation is discussed on the basis of the equilibrium between CuPc and protons as determined from absorption spectra and the electrical conductivity of the TFAA/organic solvent solution containing CuPc.
2. Experimental Section All chemicals used in the present investigation were reagent grade and were used without further purification. As a solvent, three different kinds of organic solvents viz. dichloromethane (DCM, Kanto Chemicals, Japan), 1,2-dichloroethane (DCE, Kanto Chemicals, Japan), and 1,1,1-trichloroethane (TCE, Kanto Chemicals, Japan) were used. A known amount of MPc was dissolved in the organic solvents containing trifluoroacetic acid (TFAA, Kanto Chemicals, Japan) by stirring the mixture for about 30 min. The electrophoretic cell consisted of a copper plate (20 40 mm2) as an anode and an indium tin oxide (ITO) plate (15 40 mm2) as a cathode. These electrodes were fixed vertically at a parallel distance of 10 mm, and this cell was immersed in the (11) Takada, M.; Yoshioka, H.; Tada, H.; Matsushige, K. Jpn.. J. Appl. Phys., Part. 2; Lett. 2002, 41, L73. (12) Su, J. L.; Min, Z.; Ma, N.; Sheng, Q. R.; Zhang, Q.; Yan, G. Sci. China, Ser. B: Chem. 2009, 52, 911. (13) Komino, T.; Matsuda, M.; Tajima, H. Thin Solid Films 2009, 518, 688. (14) Saji, T.; Shimomura, A. J. Jpn. Soc. Color, Mater. 2005, 78, 412.
Published on Web 10/01/2010
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Figure 1. Molecular structures of phthalocyanines (MPc). above solution. A potential of 10-100 V was applied using a dc power regulator (B418A-125, Metronix Japan) for 5-60 s. The films were dried naturally in air. The amount of the film was determined by the following method. The film was dissolved in DCE containing 1 M TFAA. Absorbance of this solution at 732 nm was observed using UV/vis spectrophotometry. The amount of the film was determined using this absorbance and the calibration curve. Surface and cross-sectional morphologies of the deposits were analyzed using a field emission scanning microscope (FE-SEM, S-800, Hitachi, Japan). Electrical conductivity of the solution was measured in a cell with a cell constant of 9.26 m-1 using a Kohlrausch bridge, which was coupled to an oscilloscope. Most of the experiments for the equilibrium studies were performed using DCE as the organic solvent due to its higher boiling point (83 °C) than that of DCM (40 °C). All experiments were performed at 25 °C.
3. Results and Discussion 3.1. Equilibrium between CuPc and Protons. The color of a dispersion of β-CuPc particles in DCE was a blue; however, the addition of a small amount of TFAA changed solution into a green to purple color. This change in color after addition of TFAA can also be ascertained from the electronic absorption spectra of these two different solutions (Figure 2). The appearance of isosbestic points revealed that CuPc in these two solutions existed as two different species. The broad absorption peaks of β-type CuPc (β-CuPc) particles in DCE without TFAA supports a simple dispersion of β-CuPc particles.16 When CuPc is dissolved as a monomer in an organic solvent (e.g., 1-chloronaphthalene), the spectrum exhibits sharp peaks.16-18 In TFAA/DCE solution, CuPc is dissolved to form a complex by the protonations of peripheral aza nitrogen atoms of CuPc.8,9 The absorption spectra of this solution at different TFAA concentrations shows a bathochromic shift of the absorption band (Figure 2). At lower concentrations of TFAA (1 M), the two isosbestic points are observed at 694 and 724 nm. The isosbestic points reveal the existence of two species of CuPc that are protonated to different degree. Iodko et al.8,9 reported that for trichloroacetic acid, the two successive protonation steps of the CuPc molecule can be represented by the following equilibrium reactions. K1
CuPc þ mHA T ½CuPc 3 ðHÞm mþ ;mA -
ð1Þ
K2
½CuPc 3 ðHÞm mþ ;mA -þ nHA T ½CuPc 3 ðHÞm þ n ðmþnÞþ ;ðm þ nÞAð2Þ
(16) Heimer, G. H.; Warfield, G. J. Chem. Phys. 1963, 38, 893. (15) Riddick, J. A.; Bunger, W. B. Organic Solvents, 3rd ed.; Wiley-Interscience Inc.: New York, 1970; Vol. II. (17) Bernauer, K.; Fallab, S. Helv. Chim. Acta 1961, 44, 1287. (18) Saji, T. Chem. Lett. 1988, 693.
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Figure 2. Electronic absorption spectra of DCE solutions containing 0.01 mM CuPc and 0.25-2.0 M TFAA.
where HA represents the TFAA and m and m þ n are the number of protons coordinating to the CuPc molecule for the first and second proton adducts, respectively. Assuming that most of proton adducts form ion pairs with A-, the equilibrium constants K1 and K2 may be expressed as K1 ¼ C1 =ðCHA - mC1 Þm
ð3Þ
K2 ¼ C2 =½ðC1 - C2 ÞðCHA - mC1 - nC2 Þn
ð4Þ
where the activity of CuPc was assumed to be unity due to the insolubility of CuPc in DCE and C1, C2, and CHA are the concentrations of the first and second proton adducts and TFAA, respectively. CHA is significantly larger than C1 and C2; therefore, eqs 3 and 4 can be approximately expressed as log C1 ¼ m log CHA þ log K1
ð5Þ
log C2 =ðC1 - C2 Þ ¼ n log CHA þ log K2
ð6Þ
From plots of log C1 vs log CHA and log C2/(C1 - C2) vs logCHA, the values of m and n were determined to be 1.01 and 1.25, respectively (Figure 3). These values must be integers; both n and m are determined to be 1. From these values, the number of protons coordinating to peripheral aza nitrogen atoms of the CuPc molecule was 1 and 2 for the first and the second proton adducts, respectively, which indicate the formations of CuPcHþ and CuPcH22þ, respectively. These numbers of protons coordinating to CuPc agree with those reported by Su et al., as estimated by the split and shift of the Q-band.12 The equilibrium constants K1 and K2 were calculated to be 76 M-1 and 1.0 M-2, respectively. However, these numbers of protons do not agree with those reported by Iodko et al. for MPc derivatives with Cl3COOH and Cl2CHCOOH (m = n = 2),8 and this may be due to the difference in the solvent and acid used in these studies.19 Figure 4 shows the electrical conductivity of the DCE solution containing 1 M TFAA at different concentrations of CuPc. The electrical conductivity of the solution without CuPc is very low, (19) In our previous paper,10 we calculated the equilibrium constants of the first and second steps assuming the number of protons to be 2 and 4, respectively.
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Figure 4. Electrical conductivity of a DCE solutions containing 1 M TFAA and various amount of CuPc.
Figure 3. (a) Plots of log C1 vs log CHA and (b) log C2/(C1 - C2) vs log CHA.
and despite the strong acidic behavior of TFAA in aqueous solution, no free protons appear to be present. This electrical conductivity increased gradually until the concentration of CuPc reached 1 mM. The saturation may be due to the constant activity of CuPc molecules dispersing as solid. The excess CuPc molecules do not contribute to the conductivity in Figure 4. A similar result was observed for H2Pc in TFAA/DCM (Figure S1, Supporting Information). The increase in electrical conductivity by the addition of CuPc also supports the enhanced ionization of TFAA in TFAA/DCE solution. The CuPc molecule may make the ionization equilibrium change to dissociate TFAA into A- and Hþ and form the protonated CuPc species. While most of these proton adducts may form ion pairs with A- in the solvent with such a low dielectric constant, the dissociation of these ion pairs may result in an increase of the electrical conductivity of the solution. 3.2. Electrophoretic Deposition of MPc. A dense CuPc blue film was formed within a few seconds on an ITO cathode by application of 100 V cm-1 (104 V m-1) between two plates using a DCE solution containing 1 mM CuPc and 3 M TFAA. Similar blue films of CuPc were formed using DCM and TCE in place of DME, and similar blue films of other MPc (M = H2, Fe, Ni, Zn, Pb, VO) were also formed using the same method (Figure S2, Supporting Information). The deposition of MPc on the cathode indicates that the positively charged species generated by the dissociation of ion pairs of the protonated species migrate to the cathode. From the equilibrium constants, the positively charged species in the 1 mM CuPc elelctrophoretic bath containing 3 M TFAA may be mainly the CuPc diprotonated species. The adherence of these films to the substrate was as strong as the film prepared by the vacuum sublimation technique, and the degree of adherence was satisfactory as estimated by the cross-cut tape-test method (ASTM D 3359 standard). The electronic absorption spectrum of the CuPc film (Figure 5) shows that the relative 17026 DOI: 10.1021/la102172t
Figure 5. Electronic absorption spectra of CuPc films on ITO plate.
intensity of the Q-bands and their positions are similar to those of the R-type CuPc film and different from those of the β-type CuPc film. To determine whether the above film of CuPc prepared in the present investigation consisted of R-type CuPc, XRD analysis of this film was performed (Figure S3, Supporting Information), which revealed one major peak at 2θ = 6.81° together with a minor peak at 2θ = 15.70°. The XRD pattern is different from that of the β-type CuPc film prepared by electrophoresis with β-CuPc particles using I2/2-butanone system.7 Several researchers20-22 have reported that the major peak in the electric absorption spectrum of the CuPc film prepared in this study is due to R-type CuPc. All of the experimental evidence reveals that the film consists mainly of R-type CuPc, which was different from the β-type CuPc used in the electrophoretic bath, and this conclusion is in agreement with that reported by Su et al.12 FE-SEM images of the surfaces and cross sections of the CuPc and PbPc films prepared in the present investigation are shown in Figure 6. These micrographs show that the films are composed of fibrous crystallites, the shapes of which are different from those used for preparation of the electrophoretic bath. Generally, MPc crystallites prepared by the sublimation technique have been reported to be granular or columnar.23 The shape and peaks of (20) (21) (22) (23)
Saji, T.; Ishii, Y. J. Electrochem. Soc. 1989, 136, 2953. Sharp, J. H.; Abkowitz, M. J. Phys. Chem. 1973, 77, 477. Lucia, E. A.; Verderame, F. D. J. Chem. Phys. 1968, 48, 2674. Debe, M. K.; Poirier, R. J. Thin Solid Films 1990, 186, 327.
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Figure 6. FE-SEM images of the surface and cross section of MPc films on an ITO plate prepared by electrophoretic deposition. (a), (b) CuPc film: DCE solution containing 1 mM CuPc and 3 M TFAA, 100 V cm-1 for 1 m. (c), (d) PbPc film: DCM solution containing 1 mM PbPc and 1 M TFAA, 40 V cm-1 for 30 s.
the absorption spectrum (Figure 5) is not in complete agreement with that of R-type CuPc and seem to be mixed with β-type CuPc. This difference can be ascribed to the growth process during film formation. The present method enables the preparation of a film within a few seconds (e.g., 5 s), which is much shorter than that of the sublimation technique. However, the rapid formation may result in an imperfect film crystalline structure. This imperfect crystalline may lead to R-type f β-type phase transformation. Such a heat and solvent transformation has been reported by some researchers.24-26 The cross-sectional SEM images of the films shown in Figure 6b,d reveal the uniform distribution of these fibrous crystallites throughout the film. The size of these crystallites was found to increase with the applied electrical field and electrolysis time (Figure S4, Supporting Information). The amount of the film was also found to increase with the applied electric field (Figure 7a), electrolysis time (Figure 7b), and concentration of CuPc (Figure S4, Supporting Information). The linear relationship between amount of CuPc film and electrolysis time (applied potential) indicates that the film was formed by the electrophoretic migration of the positively charged monomeric MPc species.27 The mechanism for film formation can be explained as follows: MPc is first dissolved as proton adducts by the protonation of the peripheral aza nitrogen atoms of MPc in a TFAA/organic solvent solution. These positively charged proton adducts of MPc migrate to the cathode under an electric field and protons are reduced to yield the MPc monomer and hydrogen at the cathode surface. The amount of deposit is dependent on the number of proton adducts migrated to the cathode, which increases with the applied electrical field and electrolysis time in the bath. The order of the amount of CuPc film for three different solvents as shown in Figure 7 was as follows: DCE > DCM > TCE The difference is ascribed to the different degree of ion pair dissociation in these solvents. Although the dissociation constants of these ion pairs in the solvents could not be determined in the present study, it is believed they are in the order given above, because the dielectric constants of the solvents were in the same (24) (25) (26) (27)
Suito, E.; Uyeda, N. Kolloidz. Z. Polym. 1963, 193, 7. Hamm, F.; Norman, E.-V. J. Appl. Phys. 1968, 19, 1097. Iwatsu, F.; Kobayashi, T.; Uyeda, N. J. Phys. Chem. 1980, 84, 3223. Hamaker, H. C. Trans. Faraday Soc. 1940, 36, 279.
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Figure 7. Amount of CuPc film vs (a) applied electric field (30 s) and (b) electrolysis time (100 V cm-1). Electrophoretic bath: DCE solution containing 1 mM CuPc and 3 M TFAA.
order; dielectric constants for DCE, DCM, and TCE are 10.0, 8.9, and 7.5, respectively.15 The degree of dissociation may depend on the solvation energy of ions.28 The solvation energy of an ionic species bearing charge z (S) is expressed by the Born equation:29 S ¼ - NðZeÞ2 ð1 - 1=Ds Þ=2r where N is the Avogadro number, Z is the charge of ionic species, Ds is the static dielectric constant of the solvent, and r is the radius of the ionic species. Based on this equation, the absolute solvation energy of the ionic species may increase with the dielectric constant of the solvent.
4. Conclusions There are two types of CuPc proton adducts, CuPcHþ and CuPcH22þ, in a CuPc solution containing TFAA. Dissociation of the ion pairs with A- may contribute to the electrical conductivity of this solution. The electrophoretic deposition proceeds by migration of these positively charged monomeric proton adducts of CuPc in an organic solvent containing TFAA. The amount of CuPc film depends on the dielectric constant of the organic solvents and was ascribed to the difference in the degree of ion pair dissociation. A large variety of MPc films were successfully deposited on the ITO plate. Supporting Information Available: Electrical conductivity of a DCE solution containing 1 M TFAA vs concentration of H2Pc, photographs of MPc films, XRD pattern of CuPc film, and amount of CuPc film vs concentration of CuPc in DCE electrophoretic bath. This information is available free of charge via the Internet at http://pubs.acs.org/. (28) Peover, M. E.; Davies, J. D. J. Electroanal. Chem. 1963, 6, 46. (29) Hush, N. S.; Blackledge, J. J. Chem. Phys. 1955, 23, 514.
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