4184
J . Phys. Chem. 1989, Y3. 4184-4188
Electrochemical Characterization of Phthalocyanine Thin Films Prepared by the Electrolytic Micelle Disruption Method Yutaka Harima* and Kazuo Yamashita Faculty of Integrated Arts and Sciences, Hiroshima Unicersitj,, N a k a - k u , Hiroshima 730, Japan (Receiued: August 2, 1988: In Final Form: October 26, 1988)
Phthalocyanine (Pc) t h i n films prepared by the electrolytic micelle disruption (EMD) method on indium tin oxide (ITO) electrodes were investigated mainly by use of an electrochemical technique. It was found from a scanning electron microscope and X-ray diffraction studies that the Pc films thus prepared consisted of rodlike particles of a-type 3000 A in length which were folded loosely with each other. Such a structure of the Pc layer led to a high electrolyte permeability. Because of this, ITO/Pc electrodes had two different sites exhibiting Faradaic activity: one was the IT0 surface and the other was the surface of Pc particles deposited on the ITO. Depending on the Faradaic activity of the respective sites against electroactive species in solution, both or either of the active sites was involved in the electrode reaction on the ITO/Pc electrode. Pc particles on the IT0 were well characterized as a p-type semiconductor having a leaky nature for reduction processes. The results obtained can provide a criterion of which redox couple can be used in photoelectrochemical cells comprising the ITO/Pc electrodes prepared by the EMD method.
Introduction Recently, a new technique to prepare organic thin films on conductive substrates has been proposed.'-4 The film formation consists of the following successive stages: (1) solubilization (or dispersion) of a water-insoluble chemical by a surfactant with ferrocenyl moiety, (2) electrochemical oxidation of the ferrocenyl moiety, followed by breakup of the micelle, (3) release of molecules (or particles) from the aggregates, and then (4) deposition of the water-insoluble substance on the electrode surface. The technique based on electrochemistry is getting known to be applicable to a variety of water-insoluble organic compounds, including viologen,' polymers,* 1 -phenylaz0-2-naphthoI,~and p h t h a l ~ c y a n i n e . ~ A successful preparation of metal-free phthalocyanine (Pc) thin films using a nonionic surfactant is of special interest because of a high stability of the phthalocyanine and its related compounds, allowing their practical use as functional substances. In fact, a number of documents have been focused on phthalocyanine thin films as active elements for c a t a l y ~ i s , ~e l-e~c t r o p h o t ~ g r a p h y , ~ - ' ~ solar-energy conversion,'3-20 and so Performance of an organic thin film depends greatly on its chemical structure, and this explains the importance of a molecular design technology. Besides this, of course, a change of a crystal form has a sizable influence on physicochemical properties of ( I ) Hoshino, K.; Saji, T . Chem. Lett. 1987, 1439. (2) Hoshino, K.; Goto, M.; Saji, T. Chem. Letr. 1988, 547. (3) Hoshino, K.; Saji, T . J . A m . Chem. SOC.1987. 109, 5881. (4) Saji, T. Chem. Lett. 1988, 893. ( 5 ) Boucher, L. J. In Coordination Chemistry of Macrocyclic Compounds; Melson, G. A,, Ed.; Plenum: New York, 1979; p 461. (6) Meier, H.; Tschirwitz, U.; Zimmerhackl, E.; Albrecht, 4 . ; Zeitler. G. J . Phys. Chem. 1977, 81, 8 . (7) Appleby, A . J.; Fleisch, J.; Savy, M . J . Catal. 1976, 44, 281 (8) Elzing, A.; Putten, A. V. D.; Visscher, W.; Barendrecht, E. J . Elecfroanal. Chem. 1987, 233, 113. (9) Hackett. C . F. J . Chem. Phys. 1971, 55, 3178. ( I O ) Arishima. K.; Hirazuka, H . ; Tate, A.; Okada, T . .4ppl. Phys. Lett. 1982, 40, 279. ( 1 1 ) Kakuta, A.; Mori, Y.; Takano, S.; Sawada. M.; Shibuya, I . Imaging Terhnol. 1985, 1 1 , 7 . (12) Khe, Ir;. C.; Aizawa, M . Nippon Kagaku Kaishi 1986, 3, 393. (13) Ghosh, A. K . ; Morel, D. L.; Feng, T.: Shaw, R . T.; Rowe. C. A , . J r . J . Appl. Phys. 1974, 45, 230. (14) Fan, F.-R.; Faulkner, L . R . J . Chem. Phys. 1978, 69, 3341. ( 1 5 ) Loutfq, R . 0.;Sharp, J. H . J . Chem. Phys. 1979, 7 1 , 121 1 (16) Loutfy, R. 0.;Mclntyre, L. F . Can. J . Chem. 1983. 61, 72. (17) Leempoel, P.; Fan, F.-R.; Bard, A. J . J . Phys. Chem. 1983, 87, 2948. (18) Harima. Y.; Yamashita, K.; Suzuki, H . Appl. Phj,r. Letr 1984, 45, 1144.
(19) Belanger, D.; Dodelet, J. P.; Dao. L. H . ; Lombos, B. A. J . Phys. Chem. 1984, 88, 4288. (20) Perrier, G . ; Dao, L. H . J . Electrochem. Soc. 1987, 134. 1148. ( 2 1 ) Kasuga. K : Tsutsui. M . Coord. Chem. R w . 1980. 32. 67.
0022-3654/89/2093-4l84$01 SO/O
molecular solids and a similar influence in the case of inorganic crystals. In the former case, molecules are combined with each other by a weak van der Waals force and they exist in various polymorphic forms arising from different molecular stacking arrangements.22-2' Likewise, organic thin films can consist of crystallites having various crystallinity or morphology. They can also affect the performance of devices based on organic stuffs, although its control, in general, is quite difficult. In a preceding letter,26 we have reported that a metal-free phthalocyanine film prepared by the electrolytic micelle disruption (EMD) method is 1-2 orders of magnitude more efficient in photoelectrochemical performance than that from an ordinary vacuum deposition or physical vapor deposition (PVD) technique. Evidently, the difference in photoactivity between the two sorts of films results from structures of these films obtained from the separate techniques based on entirely different mechanisms. So far, phthalocyanine films have been prepared almost exclusively by use of PVD and a variety of fundamental studies to characterize the Pc films thus prepared have been made by use of X-ray d i f f r a c t i ~ n , ~ ~ ,optical ~ ' ~ ' ' absorption spectroscopy,28electron mic r o ~ c o p y ,c~o~n d ~ c t i v i t y , ~ ~and - ~ ' electrochemical measurem e n t ~ . ~ On ? , ~the ~ other hand, except for their photoactivity, little is known about characteristics of the Pc films prepared by the newly developed E M D method, although the understanding of the Pc films by the E M D is an important subject. On these bases, w e describe in this article some electrochemical observations of Pc electrodes prepared by the EMD together with measurements of X-ray diffraction and scanning electron microscopy to characterize the Pc(EMD) films. Experimental Section Metal-free phthalocyanine (Pc) from Tokyo Kasei Co. was used without further purification. The nonionic surfactant used to Sharp. J. H.; Lardon, M . J . Phys. Chem. 1968, 72, 3230. Iwatsu, F.; Kobayashi, T.; Uyeda, N. J . Phys. Chem. 1980,84, 3223. Hor, 0. M.; Loutfy, R . 0. Thin Solid Films 1983, 106, 291. Takano. S.: Enokida, T.; Kakuta, A.; Mori, Y. Chem. Lett. 1984,
(26) Harima, Y.; Yamashita, K.; Saji, T. Appl. Phys. Lerr. 1988, 52, 1542. (27) Taomoto. A.: Machida, Y.; Nichogi, K.; Asakawa, S. h'ippon Kaguku Knish; 1987, 11, 2025. ( 2 8 ) Wagner. H. J.: Loutfy. R . 0 . ;Hsiao, C.-K. J . Mater. Sci. 1982, 17, 27Sl. (29) Abkowitz, M. A.; Lakatos, A. I . J . Chem. Phys. 1972, 57, 5033. (30) Sakai. Y.: Sadaoka, Y.; Yokouchi, H . Bull. Chem. Soc. Jpn. 1974, 47. 1886. ( 3 I ) Kitamura, T . ; Yoshida, M.; Imamura, S. Denshi Shashin Gakkaishi 1985. 24, 21 ( 3 2 ) Tachikawa, H.; Faulkner, L . R. J . Am. Chem. SOC.1978, 100,4379. ( 3 3 ) Fan, F.-R.: Faulkner, L. R . J . A m . Chem. Soc. 1979, 101, 4779.
E I989 American Chemical Society
Characterimtion of Phthalocyanine Thin Films
The Journal of Physical Chemistry, Vol. 93, No. IO. 1989 4185
a
b Figure 1. Scanning electron microscope pictures of a sublimed Pc layer
on a glass substrate: (a) top view and (b) side view.
disperse Pc is (1 1-ferrocenyl)undecyl tridecaethylene glycol ether (FPEG) from Dojin Lab. Lithium bromide from Kanto Chemical Co. was used as a supporting electrolyte in preparing Pc films. Indium tin oxide (ITO) electrodes were obtained from Matsuiaki Shinku Co. (IO Q/cm2). Platinum electrodes were prepared by sputtering on Pyrex glasses. p-Hydroquinone (HQ) from Katayama Chemicals was used without purification. Potassium hexacyanoferrate(l1) ([ Fe(CN),]") was from Hayashi Pure Chemical Industry Ltd. Potassium tris(oxalato)ferrate(I l l ) ([ Fe(C204)3]3-) and EDTA chelate of iron( 111) (Fe'I'EDTA) were prepared according to literature methods.33 1\11 other chemicals were of reagent grade and used without further purification. Electrochemical preparation of Pc films is similar to that for other organic thin films,'-3 except for the use of the nonionic surfactant (FPEG) replacing ionic surfactants. Forty milligrams of Pc powder was dispersed in a SO-mL aqueous solution containing 2 mM FPEG and 0.2 M LiBr. The solution was sonicated for 30 min and was stirred for a couple of days with occasional sonications. The resulting solution was centrifuged at 2000 rpm for 30 min, and then a supernatant solution was subjected to electrolysis. Controlled-potential electrolysis of the solution at the IT0 or Pt electrode was made at 0.5.V versus SCE under nitrogen atmosphere while the solution was stirred slowly. After a passage of a given amount of electricity, the electrode was taken out of the solution, dried overnight under ambient atmosphere, and then rinsed with ethanol and distilled water to remove surfactants and supporting electrolyte. The thickness of Pc films prepared in this manner was evaluated approximately by measuring the optical density of the film. For the purpose of comparison, Pc films were also prepared by the PVD method. Pc was sublimed on I T 0 at pressures less than Pa from a quartz
a
b Figure 2. Scanning electron microscope pictures of a
Pc layer prepared by the EMD method on IT0 of 0.1 pm in thickness: (a) top view and (b) side view.
crucible heated with a furnace of coiled tungsten. Rate of sublimation was at around 100 A min-' when measured by an Inficon digital film thickness monitor. In both cases, after a Pc film formation on a conductive substrate ( I T 0 or Pt), the contact with the substrate electrode was made with a copper wire and a silver conducting paint. An insulating epoxy resin was used to cover the contact and to define a 1 -cm2 active area in the Pc electrodes. Electrochemical measurements were made with a Hokuto Denko HAR- 15 1 potentiostat combined with a function generator. The current-potential curves were recorded on a Riken Denshi F-5C X-Y recorder. A coiled Pt wire was used as a counter electrode, and a saturated calomel electrode (SCE) served as a reference. All measurements were performed at room temperature in the dark. X-ray diffraction patterns were obtained by a Rigaku Denki Model RAD-I B diffractometer with the use of Cu Kcu radiation. Results and Discussion Figures 1 and 2 display scanning electron micrographs at 19500X magnification for Pc films prepared by PVD and by
EMD, respectively. Thicknesses of the films are 0.6 pm for Pc(PVD) and 0.8 pm for Pc(EMD), when estimated from the cross sections of Figures 1 b and 2b. The difference in morphology between the films is quite clear from both of their top and side views. Figure 1 for the Pc(PVD) film shows that needlelike crystals of 400 A in diameter grow perpendicular to the surface of a glass substrate and some of them aggregate on the Pc surface. A similar growing nature of Pc crystals in sublimed films has already been reported.27 It is found also that the crystal diameter increases with a decrease in the rate of ~ u b l i m a t i o n . An ~ ~ X-ray
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The Journal of Physical Chemistry. Vol. 93, .Yo. 10, 1980
Figure 3. Cyclic voltammograms of (a) bare ITO. (b) ITO/Pc(PVD), and (c-f) ITO/Pc(EMD) electrodes in a 0.1 M KCI solution of I m M [Fe(CN)6]4-,Dotted curves in (c-f) are obtained in a 0.1 M KCI solution containing no electroactive species. Potential-sweep rate 50 m V s-'. Numerical values on curves denote thicknesses of Pc layers.
diffraction study indicates that each particle consists of an oriented a-tqpe In contrast to crystal shapes seen with the Pc(PVD) film, the S E M pictures of Figure 2 for the Pc(EMD) film show first that the film consists of rodlike particles as large as 3000 8, in length and second that they are folded loosely. S E M pictures of other sections of this film were almost identical with Figure 2 . An X-ray diffraction study was carried out on the Pc(EMD) film. The diffraction pattern of the Pc(EMD) layer on I T 0 showed clearly that the film was made of a-type crystallites with no orientation, although the diffraction intensity was weak compared with that of a Pc powder. Figure 3 depicts cyclic voltammograms of 1 mM [Fe(CN)6J4on ITO. ITO/Pc(PVD), and ITO/Pc(EMD) electrodes. Curve a for the bare I T 0 electrode exhibits a reversible one-electrontransfer step, when judged from the conventional reversibility criteria of peak separation and peak current ratio. In addition, the value of a current density for the anodic peak corresponds well to that calculated assuming a diffusion-controlled oxidation of [ Fe(CN),]'-. These indicate that the I T 0 electrode. which is a degenerate n-type s e m i c o n d ~ c t o rbehaves ,~~ like a metal electrode u.ith sufficient electron as well as hole pools. Zs i h shown in Figure 3b, only one anodic peak accompanied with no reduction peak is observed at 0.6 V versus SCE when an ITO!Pc(PVD) electrode is used in the same solution. The peak potential and the peak current were almost independent of thickness of sublimed Pc layers in the range of 1000-4000 8,. The results are in good agreement, except for a slight shift in the anodic peak potential to a positive direction, with a previous study by Fan and Faulkncr using gold in place of I T 0 as a substrate The contact of IT0 with a sublimed Pc film was found to be Ohmic from no rectifying behavior observed on the I-Vcurve of an ITO/Pc/Au sandwich-type cell as well as from no photoactkit) Lit a n interface of ITO/Pc(PVD). On these bases. the irreversible oxidation of [ Fe(CV),lJ- on the ITO/Pc(PVD) can be cxplaincd in terms of the p-type semiconduction of Pc(PVD) film M hose Flat-band potential is around 0.5 V or more cathodic. 1.ac.k of dependence of film thickness on the peak potential and current is consistent with that a charge-transfer reaction does occur ; i t rhc surface of Pc(PVD) contacting solution. I n addition, no redox peak :it the standard potential ( E e q ) of [ Fe(CU)6]3-/4implie\ a negligible amount of pinholes in the Pc(PVD) films, through hich an electron exchange on the IT0 can take placc. Cqclic Loltaminograms of Figure 3c-f for the ITO/Pc(EMD) electrodes Mere in contrast to I-V characteristics on ITO/Pc( P V D ) .although both Pc(EMD) and Pc(PVD) particles are of it-t)pe Figure 3c \ h o w that a n I-Vcurve for rhe thinnest Pc( 3 4 ) U ' i h k r e n . h : : V e w k i r k . A . E . J . C'hent. P h j y . 1961, 34. 2 1 8 1 j 1 5 ) )I\hid,:. 21 B i t / / . Chonr Sor J p n 1966. 3Y. 2625. 2632. 136) Buchan.in. M , Webb. _I. B.: William\. D F. 4 p p l . PAL'
