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The Journal of Physical Chemistry, Vol. 83, No. 10, 1979
for support of R.J.L., the Foxboro Company for support Of p*s*w*' and the Science Foundation for support of M.W.P.H. and DEM. References and Notes (1) J. H. Purnell and J. M. Vargas de Andrade, J. Am. Chem. Soc., 97, 3585, 3590 (1975). (2) R. J. Laub and J. H. Purnell, J. Am. Chem. Soc., 98, 30, 35 (1976). (3) R. J. Laub and C. A. Wellington in "Molecular Association", Vol. 2, R. Foster, Ed., Academic Press, London, In press. (4) R. J. Laub and R. L. Pecsok, "Physicochemical Applications of Gas Chromatography", Wiley-Interscience, New York, 1978, Chapter 6. (5) G. R. Primavesi, Nature (London), 184, 210 (1959). (6) G. P. Hildebrand and C. N. Reilley, Anal. Chem., 36, 47 (1964). (7) J. Klein and H. Widdecke, J. Chromatogr., 147, 384 (1978). (8) R. J. Laub, J. H. Purnell, D. M. Summers, and P. S. Williams, J . Chromatogr., 155, 1 (1978). (9) R. J. Laub and J. H. Purnell, J . Chromafogr., 161, 59 (1978). (10) G. M. Janlni and D. E. Martire, J . Chem. Soc., Faraday Trans. 2 , 70, 837 (1974). (11) D. E. Martire, Anal. Chem., 46, 1712 (1974); 48, 398 (1976). (12) G. M. Janini, J. W. King, and D. E. Martire, J. Am. Chem. Soc., 96, 5388 (1974). (13) D. E. Martire, J. P. Sheridan, J. W. King, and S.E. O'Donnell, J. Am. Chem. Soc.. 98. 3101 11976). (14) R. J. Laub, 0.' E. Martire, and J.' H. Purnell, J. Chem. Soc., Faraday Trans. 7 , 73, 1686 (1977). (15) R. J. Laub, D. E. Martire, and J. H. Purnell, J. Chem. Soc.,Faraday Trans. 2 , 74, 213 (1978). (16) W. Hayduk and S . C. Cheng, Can. J . Chem. Eng., 48, 93 (1970). (17) Alternatively, the data of Nitta, Tatsuishi, and Katayama ( J . Chem. Eng. Jpn., 6, 475 (1973)) for nitrogen with isooctane/cyclohexane mixtures at 25 OC show a positive deviation of ca. 5% at 4 = 0.5; cf. Figure 6.18 of ref 4. (18) A. J. Ashworth and D. M. Hooker, J. Chromatogr., 131, 399 (1977). (19) J. F. Parcher and T. N. Westlake, J. Phys. Chem., 81, 307 (1977). (20) S. D. Christian, E. E. Tucker, and A. Mltra, J. Chem. Soc., Faraday Trans. 7 , 73, 537 (1977). (21) E. E. Tucker and S.D. Christian, J. Am. Chem. Soc., 100, 1418 (1978). (22) R. J. Laub, J. H. Purnell, P. S. Williams, M. W. P. Harbison, and D. E. Martire, J. Chromatogr., 155, 233 (1978). (23) A. J. Ashworth, J. Chem. Soc., Faraday Trans. 7 , 89, 459 (1973). (24) R. J. Laub and J. H. Purnell, J. Chromatogr., 112, 71 (1975). (25) R. J. Laub and J. H. Purnell, Anal. Chem., 48, 799, 1720 (1976).
V. R. Shepard and N. R. Armstrong (26) R. J. Laub, J. H. Purnell, and P. S. Williams, J . Chromatogr., 134, 249 (1977). (27) W. K. AI-Thamir, R. J. Laub, and J. H. Purnell, J . Chromatogr., 142, 3 I19771 - \ ' - - - I -
(28) R. J. Laub, J. H. Purnell, and P. S. Williams, Anal. Chim. Acta, 95, 135 (1977). (29) We thank J. A. Ballantine, University College of Swansea, Swansea, Wales, for this analysis. (30) D. E. Martlre and P. Riedl, J. Phys. Chem., 72, 3478 (1968). (31) R. R. Dreisbach, "Physical Properties of Chemical Compounds", American Chemical Society, Washington, D.C., Vol. I,1955; Vol. 11. 1959. (32) M: L. McGlashan and D. J. B. Potter, Proc. R. SOC. London, Ser. A , 267, 478 (1962). (33) E. A. Guggenheim and C. J. Wormald, J . Chem. Phys., 42, 3775 (1965). (34) Y . B. Tewari, D. E. Martire, and J. P. Sheridan, J. Phys. Chem., 74, 2345 (1970). (35) D. D. Deshpande, D. Patterson, H. P. Schreiber, and C. S. Su, Macromolecules, 7, 530 (1974). (36) Self-evidently, applicatlon of precise treatments to the present data is somewhat less than satisfactory given the amount of impurities in DNP. However, the point here is comparison of conventional theories with eq 1, which requires analysis of interaction parameters as described. In addition, BDH DNP Is often employed "as received" in analytical GLC, for which purposes the present study also has relevance. (37) A. J. Ashworth and D. M. Hooker, J . Chem. Soc., Faraday Trans. 7 , 72, 2240 (1976). (38) R. J. Laub and R. L. Pecsok, Anal. Chern., 46, 1214 (1974). (39) J. H. Purnell and 0. P. Srlvastava, Anal. Chem., 45, 1111 (1973). (40) R. J. Laub, J. H. Purnell, P. S. Williams, M. W. P. Harbison, and D. E. Martire, to be submitted for publication. (41) T. L. Hill, "Introduction to Statistlcal Thermodynamics", AddisonWesley, Reading, Mass., 1960, Chapter 21. (42) D. Patterson, Macromolecules, 2, 672 (1989). (43) D. Patterson, Pure Appl. Chem., 31, 133 (1972). (44) M. M. Kopecni, S. K. Milonjic, and N. M. Djordjevic, J. Chromatogr., 139, 1 (1977). (45) M. M. Kopecni, 2. E. Ilic, and S.K. Milonjic, J . Chromafogr. Scl., in press. (46) Dj. M. Petkovic, J . Inorg. Nucl. Chem., 30, 603 (1968). (47) Yu. N. Bogoslovsky, V. M. Sakharov, and I.M. Shevchuk, J . Chromafogr., 69, 17 (1972). (48) A. Poczynailo, P. R. Danesi, and G. Scibona, J . Inorg. Nucl. Chem., 35, 3294 (1973). (49) P. S.Williams, Ph.D. Thesis, University of Wales, 1978; to be published.
Electrochemical and Photoelectrochemical Studies of Copper and Cobalt Phthalocyanine-Tin Oxide Electrodes V. Rogers Shepard, Jr.,t and Neal R. Armstrong*+ Department of Chemistiy, Michigan State University, East Lansing, Michigan 48824 (Received May 15, 19 78; Revised Manuscript Received December 11, 1978) Publicafion costs assisted by the Natlonal Science Foundation
Tetrasulfonated copper and cobalt phthalocyanines have been bound to SnOz electrode surfaces modified by either (y-aminopropy1)triethoxyilaneor 3-(N-(2-aminoethyl)aminopropyl) trimethoxysilanes. The electrochemical behavior of the surface bound dye was compared with that of the solution and absorbed forms at SnOzin Me,SO and aqueous media. A stable reversible redox couple was seen on the cobalt phthalocyanine electrodes/SnOz in aqueous media. Concentrations of surface bound phthalocyanines were easily detected by monitoring the Cu(2p1/,,,/,) and N( 1s) X-ray photoelectron transitions. SnOz electrodes with bound phthalocyanine and SnOz electrodes in contact with solutions of phthalocyanines showed similar photoelectrochemical response to the oxidation of oxalate or ascorbate. Introduction
We consider it desirable to be able to covalently
0022-3654/79/2083-1268$01.00/00 1979 American Chemical Society
Copper and Cobalt Phthalocyanine-Tin Oxide Electrodes
use of various phthdocyanine-coated electrodes to sensitize both oxidation and reduction reactions to visible wavelength light (507 nm). The ability to catalyze oxygen reduction in both dark and light is also a well-understood property of the phthalocyanine~.~-l~ Murray arid c ~ - w o r k e r s ~and ~ J Anson ~ and co-workers16 have recently reported the attachment of various porphyrin molecules to carbon and metal oxide electrodes via the formation of amide linkages to the surface coupling agents or through adsorption. We have attempted several different covalent attachment schemes for the phthalocyanines all initiated with either (yaminopropy1)triethoxysilane (pr-silane) or 3-(N-2-aminoethyl)aminopropyltrimethoxysilane (en-silane). Attachment of the phthalocyanine to these surface-bound silanes was designed to be nccompllished by the formation of sulfonamide linkages to tetrasulfonated phthalocyanines, or coupling of tetrasulfonamide phthalocyanines with cyanuric chloride and coupling of this reagent to the S n 0 2 electrode, modified with the en-silane. We report here on various electrochemical characterizations of the phthalocyanine/Sn02 electrodes compared with thin, sublimed films of the phthalocyanines on Sn02. X-ray photoelectron spectroscopic (XPS) data for both the sublimed and covalently attached copper phthalocyanines are also presented to show the relative surface concentrations obtainable by each attachment technique and to verify the chemical state of the central metal atom in the attached state. The goal of most chemical modifications of electrode surfaces is to enhancle the charge transfer rates of a solution species in a chemically specific fashion.1E-21Photoelectrolysis sensitized by dyes adsorbed to electrodes is thought to involve the excitation of the dye at the electrode surface, oxidation or reduction of the excited dye molecule, and regeneration of the ground state of the dye by another reducing or oxidizing agent (supersensitizer)in the solution adjacent to the electrode surface (eq 1). I
I
cathode
anode
This process is shown schematically for the sensitization of photooxidation of a reducing agent (R) by a phthalocyanine molecule (M-Pc) and the reduction of H+ to hydrogen in the adjacent cell. Several photooxidation reactions have been designed to sensitize the production of hydrogen in aqueous media by visible wavelength light.1J7J8 Oiur own results discussed here and elsewhere3 indicate that it may be possible to carry out these reactions with the dye molecule bound to the electrode surface rather than in solution form adjacent to the electrode surface. Experiment a1 Section Copper phlthalocyanines (CuPc) and cobalt phthalocyanines (CoPc) were obtained from Eastman Kodak. 4,4',4",4"'-Tetrasulfonated copper and cobalt phthalocyanines (CuTSPc and CoTSPc) were synthesized according to the procedure of Weber and Busch.17 The monosodium salt of 4-sulfophthalic acid, ammonium chloride, urea, ammoinium molybdate, and copper or cobalt sulfate (hydrated form) were condensed in nitrobenzene and the crude product was then washed with methanol and
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1 N hydrochloric acid saturated with sodium chloride, dissolved in 0.1 N sodium hydroxide, heated to 80 "C, and filtered. Sodium chloride was added to precipitate the solid product. The pure blue product was filtered, washed with aqueous ethanol, and dried in vacuo overnight. Purities of all phthalocyanines were verified spectrophotometrically and electrochemically. Tetrasulfonated copper phthalocyanines with varying amounts of cyanuric chloride attached were obtained as samples from American Cyanamide and CIBA-Geigy. Spectrophotometric grade dimethyl sulfoxide (Me2SO) was obtained from Burdick and Jackson Labs and was further purified by passing through an activated alumina column and then stored over activated 4 A molecular sieves. Aqueous solutions were prepared from purified reagents and water from a Milli Pore Milli Q system containing an anion exchanger, cation exchanger, and activated charcoal. Reagent grade tetraethylammonium perchlorate (TEAP) was recrystallized from ethanol or H2O and dried at 80 "C overnight. Fluoride-doped S n 0 2 on glass was manufactured by Pittsburgh Plate Glass Co. The electrodes were cleaned in an ultrasonic bath, using successive washings with detergent, ethanol, and distilled water. The electrodes were then vacuum-dried and stored in an inert atmosphere. Potentiostats and electrochemical cells used were of conventional design. Photoelectrochemical studies were conducted with a 450-1000 W xenon arc lamp source with appropriate optical filters or monochromator. The source was chopped at 13 Hz and the photocurrent detected with a lock-in amplifier. Thin CuPc and CoPc films were sublimed onto Sn02a t torr. The optimum sublimation time was ca. 5 X determined experimentally (5-15 min at 250-270 " C ) . Electrodes were stored in a nitrogen atmosphere. The thickness of the sublimed films were assayed by spectrophotometric means or by XPS analysis. For covalent modification, the Sn02 electrodes were refluxed in a 1 4 % silane-dry toluene solution under nitrogen for 1-12 h after which the excess silane was removed by refluxing the electrodes in dry toluene for 1 The silanized electrodes were stored in dry toluene in an inert atmosphere. To yield SnOz electrodes with CuTSPc or CoTSPc attached after a sulfonamide synthesis route 0.1 g of C U P C ( S O ~ - N ~or+ C ) ~O P C ( S O ~ - N ~20 + ) mL ~ , of thionyl chloride, 30 mL of dry toluene, and 2 drops of dimethylformamide were refluxed with stirring for 48 h under nitrogen. The dye was then filtered and washed with dry benzene until the filtrate gave a negative AgCl tests. The pr-SnOz or en-Sn02 electrodes, 0.1 g of CUPC(SO~C or~COPC(SO~C~)~, )~ and 100 mL of dry toluene were refluxed under nitrogen for 72 h. The chemically modified electrodes were washed in a Sohxlet extractor with a dry toluene and then water for 24 h, vacuum-dried, and stored in a nitrogen atmosphere. Some en-Sn02 electrodes were refluxed with 0.1 g of MX-G from CIBA-Geigy in dry benzene for 1 2 h. This copper phthalocyanine dye contained, on the average, 2.6 free sulfonic acid groups and 1,4 dichlorocyanuric groups per molcule of dye. These chemically modified electrodes were washed in a Sohxlet extractor with benzene and Me2S0 for 1 2 h, vacuum-dried, and stored in a nitrogen atmosphere. All forms of the chemically modified electrodes were subjected to XPS analysis. Electrodes biased at different potentials past the various redox peaks of the dye, as well as unused electrodes, were analyzed. All manipulations
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V. R. Shepard and N. R. Armstrong
SULFONAMIDE
-
Sn-OH
s n - 0 ~+
(E?O)~-SI-~HZ
+ S0,Cl
Sn -OH
SOzCl
SO2CI
Figure 1. Synthesis schemes for attachment of phthalocyanines to SnO, surfaces.
of the electrodes following these studies were done in a glove box under purified, dry nitrogen. The electrodes were rinsed thoroughly with ethanol, vacuum-dried, and then mounted onto appropriate holders for surface analysis. The XPS data were obtained using a Physical Electronics, Inc. (PHI) Model 548 ESCA/Auger spectrometer which was equipped with a Mg source (Mg Kas2) operated at a power of 400 W. The pressure in the analyzer chamber was maintained at less than lo3 torr during analysis. Binding energies of the XPS transitions were corrected for charging effects by referencing to the Sn(3d) peak of the S n 0 2 electrodes which was assumed to have a BE of 486.2 eV.21 Data acquisition, storage, and processing, particularly for obtaining signal averaged XPS spectra and deconvolution of spectral components, were accomplished using previously described techniquesq21
TABLE I: Cyclic Voltammetric Data for CuPc and CoPc at or on SnO, Electrodes
Results and Discussion Electrochemical Studies. Figure 1 shows the reactions which would be involved in the covalent attachment of the copper and cobalt phthalocyanines (CuTSPc and CoTSPc) to the Sn02surfaces. We have irreversibly bound CuTSPc via the sulfonamide (a) and cyanuric chloride (b) routes at spectrophotometrically detectable levels on glass and SnOz. CoTSPc has been bound using the sulfonamide route (a). Transmission spectrophotometry showed absorbance increases of ca. 0.005 at the wavelength maxima (620 and 690 nm (CuPc) and 630 and 687 nm (CoPc)) for glass or S n 0 2electrodes subjected to the synthesis routes in Figure la,b. These absorbance changes correspond to the addition of approximately one monolayer, 5.2 X lOI3 molecules/cm2 of each phthalocyanine, assuming the projected surface area of each molecule to be ca. 160 A2.22 Control studies were carried out to ascertain the extent of true covalent attachment of the phthalocyanines to the S n 0 2 electrodes. Electrodes subjected to the synthetic routes (a) or (b) without prior attachment of a silane coupling agent showed no detectable levels of bound phthalocyanine by either spectrophotometric assay or electrochemical or photoelectrochemical experiments (see below). We have recent evidence, however, which indicates that the phthalocyanines may be attached via adsorption to the silane in addition to a true covalent attachmentaZ3 We will henceforth refer to the phthalocyanines which have been irreversibly attached through the synthesis routes (a) or (b) as surface bound. Abbreviations such a CuTSPcPr-Sn02 or CoTSPc-en-SnOz will refer to surface-bound
38 mV/s. 100 mV/s. Ag/AgCl for aqueous media.
Epeak,C
CoTSPc/Me,SO at SnOZa CoTSPc-pr-SnO,/Me,SO* CoTSPc-pr-SnO,/pH 4* CuTSPc/Me,SO at SnOza CuTSPc-pr-SnO,/Me,SO*
*
CuTSPc-pr-SnO,/pH 4
-0.54 -1.18 - 1.25 t 1.10 -0.45 t0.94 -0.62 -1.10 t 1.20 -1.05 t 1.10 -0.38 -0.57d
t1.15 a
*
Vs. AgRE for Me,SO and Desorption process.
phthalocyanines attached via a propylamine or ethylenediaminesilane. We will not attempt, at this time, to distinguish between irreversible adsorption and covalent attachment as the mode of surface modification. The electrochemical reactions of both CuTSPc and CoTSPc in both Me2S0 and aqueous pH 4 buffer were first explored in order to estimate the position of the redox levels with respect to the conduction and valence band edges of SnOz and to anticipate the type of redox chemistry that might be expected from the surface-bound material. Millimolar solutions of these compounds in Me2S0 showed behavior similar to that previously reportedSz4A comparison of the cyclic voltammetric behavior of CuTSPc on platinum and Sn02electrodes in Me2S0 is shown in Figure 2. The formation and oxidation of the monoanion (peaks a/a') and dianion (peaks b/b') of CuTSPc are chemically reversible, diffusion-controlled processes on both Sn02and platinum. The one-electron oxidation appears to be less chemically reversible on Pt than on Sn02for reasons which are not yet clear. Oxidation of CuTSPc on both SnOz and platinum followed by a cathodic scan also leads to an additional reduction (peaks e/e'). Apparently, the product of the chemical reaction following the one-electron oxidation is reducible in a chemically reversible fashion. Voltammetry of CoTSPc also was in agreement with previous studies.24 The first and second one-electron reductions appear to show slower charge transfer rates a t SnOz than did the CuTSP,.
The Journal of Physical Chemistry, Vol. 83, No. 10, 1979
Copper and Cobalt Phthalocyanine-Tin Oxide Electrodes /L
li"
Figure 2. Cyclici voltammetry of CuTSPc at SnOl and Pt in 0.1 M TEAP/Me,SO. Initlai potential 0.0 V vs. Ag/AgCI.
The positions of the CuPc and CoPc redox potentials relative to the conduction and valence band edges of Sn02 and the AgRE in Me2S0 are shown in Figure 3. The upper energy of the valence band, E,, and the lower edge of the conduction band, E,, of Sn02were determined from differential capacitance measurements by established procedures.25 Also shown are the redox levels at a platinum electrode and the peak potential of the one-electron, reversible ferrocene oxidation in Me2S0 as a potential of reference. It is clear that the one-electron oxidationreduction reactions are all within the band gap region of Sn02 in Me2S0. Excitation by a visible wavelength photon of either phthalocyanine from the Fermi level represented by the one-electron oxidation potential (CuPc P CuPc+ + e-) is not capable of bringing an electron to an energy above the conduction band edge of SnOz in Me2S0. Since the Sn02electrodes are highly doped, electron transfer by tunneling cannot be ruled out, and it may not be necessary to invoke strong energy overlap of the excited state manifold of the dye and the conduction band of the SnOz electrode.lv2 Cyclic voltammetric studies of the redox behavior of CuTSPc and CoTSPc in pH 4 buffer a t S n 0 2 electrodes gave poorly defined current/voltage curves. Apparent adsorption of the dye molecule complicated the cur-
rent-voltage curves, but we were able to observe an irreversible oxidation wave for both molecules with a peak potential of ca. 0.8-1.0 V vs. Ag/AgCl. This is near the potential noted for the oxidation of CoTSPc adsorbed to graphite electrodes in 1 N H2S04.10Capacitance studies showed the apparent positions of the conduction and valence band edges to be -1.45 and +2.05 V vs. Ag/AgCl, respecti~ely.~~ As in Me2S0,light excitation a t ca. 670 nm (1.85 eV) of either dye molecule must occur from a potential no greater than ca. +0.4 V vs. Ag/AgCl in order for electron transfer to the conduction band to be f e a ~ i b l e ' ~ ~ ~ (see Discussion). Following these experiments, a comparison of the electrochemical activity of all forms of surface-bound phthalocyanines was conducted. Cyclic voltammetric studies of S n 0 2 electrodes with sublimed films of CuPc and CoPc (unsulfonated) were conducted in both Me2S0 and H 2 0 (pH 7, phosphate buffer). 10-50 monolayer thickness films of both compounds showed recognizable electrochemical behavior in Me2S0. Reversible cyclic voltammetric behavior of the CuPc films was observed for the first reduction wave (Figure 4a) similar to that observed for solutions of the sulfonated dyes. The fact that normal, diffusion-controlled cyclic voltammetric behavior was observed for this CuPc film was surprising, and may indicate an unusual conductivity of the phthalocyanine film.26 However, if the S n 0 2 electrode was polarized to sufficiently negative values, a desorption was observed at potentials near the reduction peak potential for the formation of the dianion. Further cycling of the electrode potential between the anodic and cathodic limits, shown in Figure 4, resulted in a permanent irreversible reduction/oxidation reaction (Figure 4b). XPS studies, shown below, indicate the presence of a tightly bound form of CuPc permanently retained on the electrode surface following voltammetric cycling. Analogous results to these were obtained for thin deposits of CoPc on S n 0 2electrodes. In aqueous media, very poorly defined electrochemical behavior was observed for both the CuPc and CoPc sublimed films. Irreversible reduction and oxidation of the surface-adsorbed material was observed, but retention of the dye was still possible as discussed in the XPS studies below. It is clear that the sublimed films of CuPc and CoPc are sensitive to redox changes within the phthalocyanine molecule and have a
Sn02 Redox P o t e n t i a l CuTSPc
--
'$ FL-
1271
Pt
Redox P o t e n t i a l
Redox P o t e n t i a l
Redox P o t e n t i a l
CoTSPc
CuTSPc
CoTSPc
-1 * 74 v
- 1 -08 v coPc=/coPc-
-1.05 v CoPc'lCoPc-
-0.9 v CuPc'/cuPc-0.5 v CUPC-/CUPC
-0.54
V
COPC-/COPC
CuPc=/CuPc
-0.53
V
-0.15
v cuPc-/cuPc
-0.5
v COPC-/COPC(~'
-_____-______ _ --_
1.L!.uAQ%
_ _ - I - - - - - _ - - - - _ _ _ _ _ -
e5
v F r C k 0.8 v CoPct/CoPc(a)
1.1 v CUPC+C& 1.45 v CUPC+/CUPC EV
1.76 v
Figure 3. Energy diagram of CuTSPc and CoTSPc at SnOl and Pt in 0.1 M TEAP/Me,SO. Ferrocene (Fc) at Pt is shown for reference. (a) ,Ewk for irreversible wave.
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The Journal of Physical Chemlsfry, Vol. 83,No. 10, 1979
H
a)
500mv
V. R. Shepard and N. R. Armstrong
16.3pA/cm2
30 m v / s e c
b)
500 m v ] 6 3 p A / c m 2 75 mv/sec
Figure 4. Cyclic voltammetry of sublimed CuPc (-30 monolayers) on SnO, in 0.1 M TEAP/Me,SO. Scan rate 38 mV/s. Initial Dotential 0.0 V vs. Ag/AgCI.
H
A,WI
C’.,.,
/
-
’
/
- f ”’
/’
Figure 5. Cyclic voltammetry (scan rate 100 mV/s) and differential pulse voltammetry (scan rate 5 mV/s, 0.5-s pulse, and 5-mV modulation amplitude) of CoTSPc-pr-SnO, electrode in 0.1 M TEAP/Me,SO. Initial potential 0.0 V vs. Ag/AgCI.
limited potential range in nonaqueous media that they are stable. pr-SnO, and en-SnO, electrodes with surface-bound CuTSPc and CoTSPc gave much more reasonable electrochemical results. An example of the cyclic voltammetric and differential pulse voltammetric behavior of CoTSPc-pr-Sn02electrodes in MezSO is shown in Figure 5. It is interesting to note that the first reduction wave does not always appear for each electrode used. The reason for this phenomenon is not yet clear. Cyclic voltammetric oxidation of the surface-bound CoTSPc gave no current voltage curves in MezSO as well-defined as the reduction waves. Cyclic voltammetric oxidation was barely discernible and apparently irreversible, consistent with solution behavior. Successive excursion of the CoTSPcpr-SnO, electrode into the anodic potential region eventually degraded all of the electrochemical response of the electrode which would be expected from the irreversible oxidation of the dye. The electrochemical behavior of CuTSPc-pr-SnO, or CuTSPc-en-Sn02electrodes in Me2S0
Flgure 6. Cyclic voltammetry of CuTSPc-pr-SnO, electrode in pH 4 aqueous media. Scan rate 100 mV/s. Initial potential 0.0 V vs. Ag/AgCI.
was similar to the CoTSPc-pr-SnO, electrodes; however, the reduction waves were not as clearly defined and reversible in nature. Reduction current densities comparable to the CoTSPc-pr or en-SnOz electrodes were observed for the CuTSPc-pr or en-Sn02 electrodes following several cyclic voltammetric scans. In aqueous pH 4 media, both the CuTSPc-pr or en-Sn02 and CoTSPc-pr or en-Sn02 electrodes showed reasonable electrochemical behavior. Cyclic voltammetric studies of CuTSPc-pr-SnOzelectrodes are shown in Figure 6. If the potential was scanned to values of ca. -0.500 V vs. Ag/AgCl and returned, a reversible electrochemical process with cathodic and anodic peak potentials of ca. -0.350 V vs. Ag/AgCl was observed. If the potential was kept positive of -0.5 V, the reduction process was unaffected by repetitive cyclic voltammetric scans. If the potential was scanned to more negative potentials, a pronounced desorption current was observed and further cyclic voltammetric scans showed only a barely discernible reduction of the surface-bound CuTSPc. The fact that desorption of a major portion of the CuTSPc was produced by a sufficiently cathodic polarization may indicate an incomplete coupling reaction to the electrode surface. Anodic polarization of the CuTSPc-pr-Sn02electrodes did not cause desorption of the phthalocyanine and a small oxidation wave was noted with E , , = +1.13 V vs. Ag/AgCl. Similar voltammetric results were obtained for CuTSPc attached to en-Sn02electrodes via the cyanuric chloride attachment route.23 The CoTSPc-pr or en-SnO, electrodes showed more pronounced cyclic voltammetric behavior than the CuTSPc-pr or en-Sn02 electrodes using the same voltammetric procedures. As shown in Figure 7, a reversible = reduction was observable with Ep,c= -0.475 and 0.435 V. The first cyclic voltammetric scan also showed an additional small reduction wave, scan A, which was barely resolvable a t potential scan rates in excess of 100 mV/s. Differential pulse voltammetric experiments, also shown in Figure 7, scan A, more clearly resolve this reduction wave. Following the first cyclic scan, the more cathodic reduction wave disappeared. After as many as 100 cyclic voltammetric scans, the peak current densities for the remaining reduction wave on the CoTSPc-pr or en-SnO, electrodes remained unchanged and both the cyclic voltammetric and differential pulse voltammetric experiments revealed only one reduction process in that potential range. Voltammetric scans of the anodic potential region after several (ca. 100) cyclic voltammetric
The Journal of Physical Chemistry, Vol. 83, No.
Copper and Cob,alt Phthallocyanine-Tin Oxide Electrodes
A
I
300 cis/sec
IO, 1979 1273
I300 cts/sec 300 c w s e c
11"
-
-
I
cts/sec
I
+
I I
(f)
I I
Figure 7. Cycllc voltammetry (scan rate 100 mV/s) and differential pulse voltammetry (scan rate 5 mV/s, 0 . 5 s pulse, and 5-mV modulation amplitude) of CoTSPc-pr-SnO, electrode in pH 4, aqueous media. Initial potential 0.0 V vs. AglAgCI.
scans of the CoTSPc reduction process revealed a small, irreversible oxidation process a t ca. +0.90 V vs. Ag/AgCl. The peak current for this process was only ca. 10-25% of that observed for the reduction process. The integrated charge observed for the reduction wave was consistent with the presence of one monolayer of surface-bound CoTSPc (- 10 pC/cm2, 1 X 1O1O molecules/cm2 if n = 1). Additions of small amounts of oxygen to the electrolyte showed only a small catalysis for oxygen reduction over that observed on clean SnOz surfaces.23 The reduction potential observed for the CoTSPc electrodes is consistent with the transition from a Co"' Co" valence d a t e for cobalt in a strongly complexed environment." This reduction potential is at variance with that recently reported for CoTSPc adsorbed to graphite surfaces (+O.El V vs. SCE).'O The small anodic wave observed on the CoTSPc-pr, en-Sn02 electrodes is in agreement with the redox process noted on graphite surfaces. We cannot discount the possibility that the Co' tranreduction wave we observe is due to a Co" sition. Liquid chromatography and spectrophotometric assay of the CoTSPc product prior to attachment shows that the majority of the cobalt is in the (111) valence form.23 S n 0 2 electrodes were examined by these same voltammetric procedures which had no silane coupling agent attached prior to exposure to the phthalocyanine. Following extraction in the normal solvents, no electrochemical activity was observed. The silane must be present for binding of the phthalocyanine molecule. The voltammetric results for both the CuTSPc and CoTSPc electrodes suggest that the phthalocyanines can be bound to the electrode surface in more than one fashion with subsequent variations in stability and electrochemical behavior. In the case of CuTSPc, it is apparent that a large fraction of the bound material is not stable to electrochemical reduction in aqueous media a t potentials more cathodic than -0.5 V vs. Ag/AgCl. Since a fully formed sulfonamide linkage is expected to be electrochemically stable, we may infer that CuTSPc is present primarily in an adsorbed vs. chemically bound form on the S n 0 2
-
-
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-1
960
940
920'410
I
400
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390
BINDING ENERGY (ev)
Flgure 8. XPS, Cu(Pp,,,,,,,) and N(1s) spectra of (a) CuSO,/SnO, powder pressed pellet, (b) sublimed CuPc film on SnO,, (c) sublimed CuPc film on SnO, electrochemically treated in Me,SO, (d) sublimed CuPc film on SnO, electrochemicallytreated in H,O, (e) CuTSPc-pr-SnO, electrode, (f) CuTSPcen-SnOPelectrode, (9)cyanuric chloride coupled CuTSPc-pr-SnO, electrode.
electrode surface. The voltammetric results for the CoTSPc-pr-Sn02 electrodes are more encouraging. Excellent reproducibility of the electrochemical behavior confirms a stronger bonding of the CoTSPc molecule. The additional transient reduction wave at more cathodic potentials on the CoTSPc-pr-Sn02electrodes may indicate that an adsorbed form of CoTSPc is also initially present which is reduced at more cathodic potentials and upon reduction becomes solubilized and leaves the electrode surface.23 More detailed studies of the effect of solvent and type of silane-amino coupling agent on the voltammetric behavior of the chemically modified Cu-TSPc and CoTSPc/Sn02 electrodes are underway. There are indications that complexation/adsorption effects are very important in the surface bonding of these molecules. Relative surface concentrations of CoTSPc can be increased by the use of higher molecular weight silanes.23 XPS Analysis of Modified Sn02Electrodes. Surface analysis of the various phthalocyanine electrodes was conducted using XPS. Typical spectra for copper phthalocyanine electrodes are shown in Figure 8. Comparison of spectra 8a and 8b indicate general agreement of the C U ( ~ P ~2) , transitions ~,~ in a CuSO', standard and a sublimed huPc film on Sn02 (-10 monolayers). Multiplet splitting was observed in both samples consistent with a Cu(I1) oxidation state in the phthalocyanine. In all of the CuPc electrodes studied, the binding energies of the Cu(2p312)peaks (933.8 eV) and the energy difference between this peak and the Cu XPS Auger transition (C~(2p~/~)-Cu(Auger) = 595.9 eV) were consistent with a fully oxidized copper.28i29 The ratio of
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Cu(2p3p) to N(1s) intensities, corrected for differences in photoionization cross section, were 0.46 f 0.05, in good agreement with a previously reported ratio, 0.43.30 XPS spectra 8c and 8d represent S n 0 2with thin sublimed films of CuPc (approximately near the initial concentration of the film from Figure 4b) which have been electrochemically polarized in Me2S0 (prior to the desorption wave) and HzO, respectively (see above discussion). Clearly, no appreciable changes occurred in the surface composition of these films other than to change the intensity of the Cu(2pIizai2) multiplets. Further experiments of this type must be undertaken with strict control over transfer of the electrode from the electrochemical cell to assess the possible valence state changes experienced by the bound dye during electrochemical treatment. Spectrum 8e shows the Cu(2~1/2,~/~) and N(1s) transitions observed for surface-bound CuTSPc on a pr-Sn02 electrode (via the sulfonamide synthetic route, Figure 1). The intensity of the C U ( ~ P peak ~ / ~ )is consistent with a monolayer concentration of the dye. A similar conclusion was drawn by measuring the attentuation of the Sn(3d6/,) signal, from the S n 0 2 electrode, caused by the attachment of the silane and the CuTSPc dye.3320Sn02 electrodes with CuTSPc bond via the sulfonamide synthesis to en-Sn02electrodes and via the cyanuric chloride route to pr-SnOz electrodes (Figure lb) were also examined by XPS (Figure 8f and 8g). C U ( ~ P , ~intensities ,) from those electrodes were very near those seen for XPS spectrum 8e. We can infer that all three synthetic routes yield roughly comparable surface concentrations of the phthalocyanine in agreement with the electrochemical results. Little change in signal intensity was observed for these electrodes following polarization to potentials positive of the desorption potentials. The N(1s) spectra for the native phthalocyanine (spectra 8b) always show a main peak with a binding energy of 398.0 eV and a small satellite peak with binding energy of 399.7 eV.2s The S n 0 2 electrodes with covalently attached CuTSPc also show N( 1s) peaks corresponding to the nitrogen in the coupling agents (spectra 8e-g). Nitrogen in the y-aminopropyl- and ethylenediaminesilanes has an N(1s) binding energy of 400.1 eV, while that in the cyanuric chloride groups has an N(1s) binding energy of 399.4 eV.31 Resolution of the spectra in Figures 8e-g gave near the predicted intensities for nitrogen in its various forms as part of the coupling agents. As a test for the completeness of the coupling reaction and hydrolysis of residual chlorine from the tetrasulfonated dye (SOpCl form) and the cyanuric chloride, the XPS spectrum for Cl(2p) was sought on the modified SnO, electrodes. No detectable chlorine was seen on any of the SnOz electrodes following chemical modification and extraction to remove the excess dye. The XPS spectra for the CoPc electrodes was similar to that of the CuPc electrodes. Monolayer concentrations of the dye were observed although quantitation of the cobalt XPS transitions was more difficult because of the lower sensitivity of the 2plf2,3f2transitions for cobalt as opposed to copper.32 A more complete XPS study of CoTSPc electrodes is underway with special emphasis on valence states of cobalt.23 Photoelectrochemical Studies. The preliminary studies of the redox behavior of the CuPc/Sn02 electrodes were complemented by an assessment of the ability of the bound dyes to sensitize anodic photocurrents of SnOz to visible wavelength light. It was of interest to compare the photoelectrochemical response of a CuPc or CoPc/SnOP electrode with that response observed for an electrode immersed in a dilute aqueous solution of the tetra-
V. R. Shepard and N. R. Armstrong
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Copper and Cobalt Phthalocyanine-Tin Oxide Electrodes
The Journal of Physical Chemistry, Vol. 83, No. 10, 1979
4275
(Figure 10). Apparently, irreversible oxidation of the surface-bound dye occurs at the higher concentrations of ascorbate. These preliminary results are encouraging, however, because of the comparable photocurrent magnitudes obtained for both adsorbed or surface-bound phthalocyanines.
~
,
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Ascorbic Acid Concentrotion
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Figure 10. Phoiocurrent response (at 13 Hz) vs. (a) oxalate concentration (pH 4) for two sublimed CuPc films (-5 monolayers) on SnO,, (b) oxalate concentration (pH 4) for two CuTSPc-pr-SnO, electrodes, (c) ascorbic acid concentration (pH 4) for a CuTSPc-pr-SnO, electrode, and (d) oxalate concentration (pH 4) for two CoTSPc-pr-SnO, electrodes.
current response observed (Figure 9). Addition of reducing agents to the dye solution helps to regenerate the ground state of the dye molecule, completing a catalytic cycle for -~~~' its oxidation (leq 1). As observed p r e v i ~ u s l y , ~ addition of oxalate or ascorbate to the solution up to concentrations of M sharply increased the photocurrent response of the dye a t the S n 0 2 electrode (Figure 9b). Ascorbate showed greater photocurrent response per unit concentration than oxalate at all concentrations. Figure 10 shows the photocurrent response of S n 0 2 electrodes with sublimed multilayers and surface-bound monolayers of' CuTSPc with the same light excitation as that for Figure 9 as a function of the concentration of the reducing agent (either oxalate or ascorbate). The photocurrent response for the monolayer, surface-bound dyes is clearly comparable in magnitude to that of the photocurrents seen with Sn02electrodes immersed in solutions of the dye and with S n 0 2 electrodes which have 5-10 monolayers of the unsulfonated dyes sublimed to the electrode surface. The Sn02electrodes with the sublimed dyes show nearly linear dependence of photocurrent response with oxalate concentration. The electrodes with various forms of bound dye all show a distinct saturation of photocurrent response with oxalate concentration, possibly indicating a limit to the rate at which the catalytic cycle can be completed in the presence of oxalate. In the presence of ascorbate, the S n 0 2 electrodes with surfacebound dyes show a sharper increase of photocurrent activity with increased ascorbate concentration. Once the ascorbate concentration has been raised to ca. M, however) a drop in photocurrent response was observed and all subsequent observations of the photocurrent activity by any ascorbate solution gave a lower response
Conclusions The experiments described here and elsewhere3indicate the feasibility of photoassisted redox reactions with a surface-bound mediator. Although the current densities observed are low, they still demonstrate the catalyzed oxidation of a solution species at bias potentials where this species is normally electroinactive. The capacitance data discussed above indicated that light excitation of the attached or adsorbed dye may produce an energy state with unfavorable overlap of the conduction band edge of the e l e ~ t r o d e . ' J ~More ~ ~ ~precise experiments must be done to fix the position of the conduction and valence band edges of Sn02in the aqueous medium, with respect to the true redox levels of the ground state (Pc e Pc+ + e-) and excited state (Pc* + Pc*+ + e-) dye molecules. The equilibrium potential for the ground and excited state dye is not as yet known. I t is clear that electron transfer from the dye to the electrode occurs, and that this process must involve transfer to the conduction band or a surface state of the S n 0 2 electrode or electron tunneling to the highly doped semiconductor. Surface spectroscopies, such as characteristic energy loss (CLS) and UV-photoelectron spectroscopies (UPS), may be capable of detecting important changes in surface energy states of the S n 0 2 electrode caused by chemical m~dification.~~ The above data have led to further research on the phthalocyanine/SnOz electrodes. Recent results indicate that binding of the CoTSPc and FeTSPc molecules to S n 0 2 is facilitated by use of polyamine silane^.^^ Enchanced cyclic voltammetric curves for CoTSPc reduction) small oxygen reduction catalysis, and enhanced anodic photocurrent efficiencies can be obtained by varying the chemistry of the coupling agent. The stabilities of the bound CoTSPc and CuTSPc molecules on Sn02electrodes may also facilitate their utilization in photoelectrochemical hydrogen production. Since no dye solution is needed for photosensitization) multilayered stacks of the chemically modified electrodes on optically transparent substrates may provide higher light capture efficiencies than electrodes in contact with a concentrated dye solution. Acknowledgment. The authors gratefully acknowledge T. Kuwana and A. Lin for the use of the XPS instrumentation at Ohio State University.
References and Notes (1) H. Gerischer and F. Willig, Top. Current Cbem., 61, 31 (1976). (2)H. Kim and H. A. Laitlnen, J. Electrochem. SOC.,122, 53 (1975). (3)D. Hawn and N. R. Armstrong, J. Pbys. Chem., 82, 1288 (1978). (4) M. Fujihara and T. Osa, Nature (London), 64, 349 (1976). (5) S. Meshitsuka and K. Tamura, J. Cbem. Soc., 73, 705 (1977). (6)G. G. Kommissarov, Russ. J. Pbys. Chem., 7, 927 (1973). (7)G. G. Kommissarov, Y. S. Shumov, and 0. L. Morozova, Biofizika, 15, 1163 (1970). (8)J. Manassen and A. Bar-Ilan, J. Catal., 17, 86 (1970). (9) A. J. Appelby and M. Savy, Nectrocbim. Acta, 21, 567 (1976). (10) J. Zagal, R. K. Sen, and E. Yeager, J. Nectroanal. Cbem., 83, 207 (1977). (11) S. Andrusera, K. A. Radyushkina, and M. Rarasevich, Electrokbimiya, 13, 483 (1977). (12)M. Brezina, W. Khalil, J. Koryta, and M. Musilova, J. Electroanal. Chem., 77, 237 (1977). (13) Y. S.Shumov and M. Heyrovsky, J . Nectroanal. Cbem., 65, 469 (1975). (14) J. C.Lennox and R. W. Murray, J. Eh?cmna/. Chem., 78,395(1977). (15) D. 0. Davis and R. W. Murray, Anal. Cbem., 49, 194 (1977).
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(16) A. P. Brown, C. Koval, and F. C. Anson, J . Electroanal. Chem., 72, 379 (1976). (17) M. T. Spitler and M. Calvin, J . Chem. Phys., 66, 4294 (1977). (18) P. D. Fleischauer and J. K. Allen, J. Phys. Chem., 82, 432 (1978). (19) P. R. Moses, L. M. Wier, J. C. Lennox, H. D. Finklea, J. R. Lenhard, and R. W. Murray, Anal. Chem., 50, 576 (1978). (20) P. R. Moses and R. W. Murray, Anal. Chem., 47, 1882 (1975). (21) N. R. Armstrong, A. W. C. Lin, M. Fujihara, and T. Kuwana, Anal. Chem., 48, 741 (1976); A. W. C. Lin, N. R. Armstrong, and T. Kuwana, ibid., 49, 1228 (1977). (22) F. H. Moser and A. L. Thomas, ACS Monogr., No. 157 (1963). (23) R. Shepard, C. Linkous, and N. R. Armstrong, unpublished results. (24) L. D. Rollman and R. T. Iwamot, J. Am. Chem. Soc., 90, 1455 (1968).
Keiser et al. (25) S. N. Frank and A. J. Bard, J . Am. Chem. Soc., 97, 7427 (1975), and references therein. (26) H. Tachikawa and L. Faulkner, J. Am. Chem. Soc., 100, 4379 (1978). (27) N. Maki and N. Tanka, “Cobalt”, in “Encyclopedia of Electrochemistry of the Elements”, A. J. Bard, Ed., Vol. 111, Marcel Dekker, New York, 1973, p 43. (28) N. S. McIntyre and M. G. Cook, ‘Anal. Chem., 7, 2208 (1975). (29) G. Schon, Surf. Scl., 35, 96 (1973). (30) Y. Hiwa, H. Kobayashi, and T. Tsuchiya, J . Chem. Phys., 60, 799 (1974). (31) A. M. Yacynych and T. Kuwana, Anal. Chem., 50, 640 (1978); A. W. C. Lin, Ph.D. Dissertation, Ohio State University, 1978. (32) J. H. Scofield, J . Electron Spectrosc., 8, 129 (1976).
Detergentless Water/Oil Microemulsions Composed of Hexane, Water, and 2-Propanol. 2. Nuclear Magnetic Resonance Studies, Effect of Added NaCI1 Bruce A. Kelser,2 Davld Varie, Roland
E. Barden, and
Smlth L. HOW2
Department of Chemistty, The University of Wyoming, Laramie, Wyoming 82071 (Received July 26, 1978; Revised Manuscript Received February 15, 1979)
A pseudophase diagram for ternary compositions of n-hexane,water and 2-propanol has been published previously. Some of the compositions exhibited the physical characteristics of a water/oil (w/o) microemulsion even though a detergent was not present. We have now determined the effect of ionic materials on the system by replacing water with NaCl solutions; Addition of NaCl caused significant changes in the location of region boundaries. The microemulsion was stabilized while a region of small H-bonded aggregates appeared to be destabilized by the added NaCl. An investigation of these systems by lH NMR, with or without added NaC1, showed that “bulk” water was present in the detergentless microemulsion. In addition, ‘H NMR was found to be a useful tool for locating region boundaries and for elucidating the nature of compositions assigned to these regions.
Introduction Certain compositions of n-hexane, water, and 2-propanol display the physical characteristics of microemulsions even in the absence of detergenta3 The ternary pseudophase diagram for this system, constructed on the basis of conductivity and ultracentrifugation experiments, shows four region^.^ In an attempt to ascertain the effect of various concentrations of sodium chloride on the ternary diagram of this system, we have undertaken an extensive series of conductivity and ultracentrifugation measurements. In addition, recent studies”’ have utilized lH NMR to locate region boundaries and obtain structural information about detergent stabilized microemulsions. We wished to ascertain whether lH NMR would be equally useful in characterizing detergentless ternary solutions, in the presence and absence of NaC1. The data presented here show that lH NMR detects transitions between regions of common structure in these systems and that the boundaries are located in the same place as those constructed from conductivity data. Experimental Section Reagent grade n-hexane and 2-propanol were purified by treatment with silica gel, then distilled, and stored over 4A molecular sieves. The 2-propanol was subsequently redistilled just prior to use so that the conductivity was less than 0.02 pmho/cm. Water was twice distilled in a Pyrex still. Sodium chloride was purchased from Mallinckrodt Chemicals and rhodamine 6G from Eastman *Address correspondence to Dr. Smith L. Holt, Department of Chemistry, The University of Georgia, Athens, GA 30602. 0022-365417912083-1276$01.0010
Kodak. Both were used without further purification. The ternary pseudophase diagrams were constructed on the basis of room temperature physical properties of various compositions of n-hexane, water, and 2-propanol. Initially, each solution contained 10 mL of n-hexane and an aliquot of water which was then titrated with 2-propanol in a manner similar to that reported by Bowcott and Schulmann8 Visual observations and conductivity measurements were made after each successive addition of 2-propanol. Ultracentrifugation and lH NMR studies were carried out on selected compositions of n-hexane, water, and 2-propanol. Conductivity measurements were made with a YSI Model 31 conductivity bridge utilizing a Leeds and Northup insertion-type cell ( h = 0.1, series 4905). The ultracentrifugation studies were carried out on a Beckman L5-75 preparative instrument using polyallomer tubes. Rhodamine 6G was used to stain the water-rich phase of the w/o microemulsions, as described by Smith et aL3 The lH NMR spectra were taken on a Varian 100-MHz NMR using benzene (7.2 6) as an external standard. Diamagnetic susceptibility corrections were made by comparing the major resonance of n-hexane in the solutions with that of pure n-hexane (1.21 6). Results and Discussions Ternary Pseudo-Phase Diagrams. A ternary pseudophase diagram for the system n-hexane-water-2-propanol has been described by Smith, Donelan, and Bardena3 Recent improvements in technique, data interpretation, and solvent purification have led to slight modifications of this ternary diagram. The corrected diagram appears 0 1979 American Chemical Society
