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Derivatization of platinum electrodes by photoactive surface active porphyrins
J. Electroanal. Chem., 95 (1979) 113--116
113
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary note DERIVATIZATION OF PLATINUM ELECTRODES BY PHOTOACTIVE SURFACE ACTIVE PORPHYRINS
YOSHIO UMEZAWA and TAKESHI YAMAMURA
Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo 113 (Japan) (Received 14th August, 1 9 7 8 ; i n revised form 10th September, 1978)
INTRODUCTION
Derivatization of the electrode surface by chemically important materials has become very popular in a past few years [1]. Two methods have been used to make derivatized electrodes [2]: (1) strong adsorption, and (2) covalent chemical bonding. Obviously, each approach has its advantages and disadvantages: In the case of the covalent attachment, although the surface synthetic process takes place fairly selectively, the yield of the synthesis is not necessarily high. For the method (1), strong adsorption, the surface coverage can be made sufficiently high and the attachment is relatively non-specific which should allow modification Of virtually any electrode surface [3, 4]. In the present study, an attachment of photoactive porphyrins onto the platinum electrode has been performed by strong adsorption using surface active porphyrins. With this interesting approach, a sufficiently stable porphyrin electrode was constructed so as to perform the conversion of visible light to electricity.
EXPERIMENTAL
The compounds used are meso-tetrakis-(4-methylpyridinium)porphine magnesium tetrakis-tetraphenylborate-tetrahydrate, MgMe4PyP(B@4)44H20, and its free base, H2Me4PyP(B04)4" 4H20, and meso-tetrakis-(4-N-stearylpyridinium)porphine magnesium tetraiodide-monohydrate, MgSt4PYPL" H20. The synthetic procedure for these compounds was described elsewhere [5]. For a typical experiment, about 1 X 10 is molecules of each compound are deposited on a clean polished Pt disk (0.2 cm 2 in area) sealed in soft glass by allowing 10 pl of a 5 X 10 -4 M solution of each compound in acetonitrile [for MgMeaPYP(B04)4" 4H20 and H2Me4PyP(BO4)4.4H20] or in CH3OH/CHC13 [ 1 : 2 vol.%, for MgSt4PYPI4" 4H20] to evaporate on the surface of the Pt disk. The Pt-MgMe4PyP(B~4)4, Pt-H2MeaPyP(Bq)4)4, and Pt-MgSt4PyPL electrodes thus constructed were immersed in 0.1 M aqueous solution of tetraethylammonium perchlorate (TEAP). Sample solutions were exhaustively degassed by argon bubbling. The reference electrode was a saturated calomel electrode (SCE) with an agar KC1 salt
114
bridge. A P t coil was used as the auxiliary electrode. Photocurrents were measured with a PAR potentiostat Model 173 and an electrometer Model TR-8651 of Takeda Riken Co. The light source was a 750-W tungsten lamp. The action spectra of the photocurrent was measured with a grating m o n o c h r o m e t e r Model 338607 of Bansch & L o m b Co. The spectral distribution of the light source and the actual light intensities on the electrode surface were determined with a silicon photocell Model $642 from Hamamatsu TV Co. RESULTS AND DISCUSSION
Fig.1 shows the pH and applied potential dependence of steady photocurrents with Pt-MgMe4PyP(Bq~4), and Pt-H2Me4PYP(Bq)4)4 electrodes. As shown in Fig.l, the reproducibility for photocurrents/applied potential curves are not necessarily satisfactory mainly due to the dissolution of the adsorbed MgMe4PyP(B04)4 and H2Me4PYP(B(P4)4 layers into the solution phase. This tendency is generally observed at the above two electrodes regardless of pH and applied potential values for each measurement. This p h e n o m e n o n is quite a disadvantage when we try to construct a stable wet photocell and also to study fundamental processes occur-
10 7
I/A *0.5
/
/! /',
:
: i I
;,
t-Z ILl n*" re'
u0 0 -r
- 0.5
Fig.1. Hysteresis o f p h o t o c u r r e n t vs. a p p l i e d p o t e n t i a l curves. (a) Pt-MgMe4PYP(B04) 4 e l e c t r o d e s t a r t e d f r o m 0 V. T b e p H v a l u e was c o n t r o l l e d at p H 10.28. (b) P t - M g M e 4 P y P ( B 0 4 ) 4 e l e c t r o d e p r e - i l l u m i n a t e d a t 0 V t h r o u g h - 2 5 0 m V f o r a b o u t 2 h a n d t h e n s t a r t e d f r o m 0 V. T h e decay o f p h o t o c u r r e n t s as c o m p a r e d w i t h t h e curve a was i n d i c a t e d b y d o t t e d arrows, p H = 10.28. (c) Pt-H2Me4PyP(B04) 4 electr6de, p H -- 7.50. T h e curves a. b a n d c were r e c o r d e d u s i n g a single s e p a r a t e e l e c t r o d e s p e c i m e n , respectively. A b o u t 15 m i n were n e e d e d t o o b t a i n t h e s t e a d y p h o t o c u r r e n t s a t e a c h p o i n t in this figure. W h i t e light was used: S a m p l e s o l u t i o n : 0.1 M T E A P a n d n o r e d u c i n g a n d o x i d i z i n g agents were added. T h e p H was a d j u s t e d u s i n g HC104 a n d NaOH. T = 20 + 0.5°C.
115
ring at the surface of the porphyrin electrode. In order to overcome the abovementioned problem, the porphyrin should be made completely water insoluble and also surface active so that it can be attached permanently at the surface of the electrode. Therefore, we used MgSt4PYPL" H:O which was found to exhibit surface active nature [5]. This particular c o m p o u n d was put in the same way as MgMe4PyP(B04)4 and H:Me4PYP(B@4)4 onto the platinum electrode and the photoelectrochemical behavior was examined as shown in Fig.2. In contrast with the result of Fig.l, the reproducibility for the photocurrent/applied potential profiles was sufficiently good, indicating that the dissolution of the immobilized porphyrin in this case is negligible and the stability of this porphyrin electrode is better by far. The electrode thus constructed provided a steady photocurrent for at least several hours without decay. The current flow during this time period is well over that equivalent to the charge for the total decomposition of surface layer. Photocurrent spectra (photocurrent vs. wavelength curve, i.e., Action spectra) before and after such illumination in acidic (pH 2.7) and alkaline (pH 10 7 I/A .0.2--
.0.1
/;0
Z, LU rr
0 ..r n
• 100
*200
E
mV(SCE)
-0.1
Fig.2. D e p e n d e n c e o f s t e a d y p h o t o c u r r e n t s o n t h e a p p l i e d p o t e n t i a l a n d p H values w i t h a Pt-MgSt4PyPI 4 electrode. T h e p H values were c o n t r o l l e d at p H 1 0 . 3 2 a n d 4 . 1 2 for t h e curves a a n d b, respectively. B o t h curves a a n d b were o b t a i n e d u s i n g t h e same e l e c t r o d e s p e c i m e n . Because it t o o k ca. 15 m i n t o o b t a i n t h e s t e a d y p h o t o c u r r e n t s a t e a c h p o i n t , a b o u t 7 h were n e e d e d t o c o m p l e t e b o t h t h e c u r v e a a n d b. O t h e r e x p e r i m e n t a l c o n d i t i o n s were t h e same t o t h o s e o f Fig.1.
116 9.3) solution showed no change. These results indicate that the adsorbed layer is stable chemically and physically and any change such as demetallization or dissolu tion need not be taken into consideration. The observed steady photocurrent must be due to reactions of photo-produced cation and anion radicals of the Mg(II) porphyrin with solution species such as O H - , H +, and 02, and the details of this interesting photoelectrochemical reaction will be published elsewhere [6]. The film thickness is readily adjustable simply by changing the amount of the volume of the porphyrin CH3OH/CHC13 solution to be p u t on the electrode surface. It was found that the photocurrent reached a maximum with 15 × 1014 molecules/cm 2 on the Pt electrode, equivalent to ca. 30 molecular layers. In contrast with the photo-sensitization p h e n o m e n a at the semiconductor electrode where just a monomolecular layer of a dye is sufficient to observe the appreciable photocurrent, some multi-molecular layers are needed for the metal electrode to obtain observable photocurrents. This is simply due to the difference in the mechanism of the electron transfer between attached dye molecules and substrate electrodes depending on semiconductor or metal electrodes [ 7 ]. We have also performed similar experiments with surface active Mn(III), Co(II), Zn(II), and metal flee porphyrins: The results were very similar to the Mg(II) porphyrin with respect to stability, and general photoelectrochemical behavior. In conclusion, the present approach using the surface active porphyrin is extremely advantageous for the construction of a stable photoactive porphyrin electrode. The results presented here further imply that the adsorption of surface active porphyrins on the electrode surface will provide a variety of applications to catalysis, and selective synthesis as well as photoelectrochemistry. ACKNOWLEDGEMENT
The authors express their deep gratitude to Profs. S. Fujiwara and Y. Sasaki for their continuing interest and support.
REFERENCES 1 W.R. H e i n e m a n a n d P.T. K i s s i n g e r , A n a L C h e m . , 5 0 ( 1 9 7 8 ) 1 6 6 R . 2 R . F . L a n e a n d A.T. H u b b a r d , J. P h y s . C h e m . , 77 ( 1 9 7 3 ) 1 4 0 1 , 1 4 1 1 ; 8 1 ( 1 9 7 7 ) 7 3 4 ; B . F . W a t k i n s , J . R . B e h l i n g , E. K a r i v a n d L.L. Miller, J. A m . C h e m . S o c . , 9 7 ( 1 9 7 5 ) 3 5 4 9 ; R.J. R e n h a r d a n d R.W. M u r r a y , J. E l e c t r o a n a l . C h e m . , 78 ( 1 9 7 7 ) 1 9 5 ; M. F u j i h i r a , A. T a m u r a a n d T. Osa, C h e m . L e t t . , 3 6 1 ( 1 9 7 7 ) ; G.J. L e i t h a n d C.J. P i c k e t , J. C h e m . S o c . , D a l t o n T r a n s . , 1 7 9 7 ( 1 9 7 7 ) ; M.T. H e n n e , G.P. R o y e r , J . F . E v a n s a n d T. K u w a n a , J. E l e c t r o a n a l . C h e m . , 8 0 ( 1 9 7 7 ) 4 0 9 ; A. Diaz, J. A m . C h e m . S o c . , 9 9 ( 1 9 7 7 ) 5 8 3 8 ; A.P. B r o w n , C. K o v a l a n d F.C. A n s o n , J. E l e c t r o a n a l . C h e m . , 7 2 ( 1 9 7 6 ) 3 7 9 ; N. O y a m a , A.P. B r o w n a n d F.C. A n s o n , J. E l e c t r o a n a l . C h e m . , 8 7 ( 1 9 7 8 ) 4 3 5 ; K . S . V . S a n t h a n a m , J. J e s p e r s e n a n d A.J. B a r d , J. A m . C h e m . S o c . , 9 9 ( 1 9 7 7 ) 2 7 4 ; J . F . E v a n s , T. K u w a n a , A n a l . C h e m . , 49 ( 1 9 7 7 ) 1 6 3 2 a n d references therein. 3 A. Merz a n d A.J. B a r d , J. A m . C h e m . S o c . , 78 ( 1 9 7 8 ) 3 2 2 2 . 4 M.R. V a n d e r M a r k a n d L . L . Miller, J. A m . C h e m . S o c . , 7 8 ( 1 9 7 8 ) 3 2 2 3 . 5 T. Y a m a m u r a , C h e m . L e t t . , 7 7 3 ( 1 9 7 7 ) . 6 Y. U m e z a w a a n d T. Y a m a m u r a , s u b m i t t e d . 7 H. G e r i s c h e r , J. E l e c t r o c h e m . S o c . , 1 2 5 ( 1 9 7 8 ) 2 1 8 C .
113
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary note DERIVATIZATION OF PLATINUM ELECTRODES BY PHOTOACTIVE SURFACE ACTIVE PORPHYRINS
YOSHIO UMEZAWA and TAKESHI YAMAMURA
Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo 113 (Japan) (Received 14th August, 1 9 7 8 ; i n revised form 10th September, 1978)
INTRODUCTION
Derivatization of the electrode surface by chemically important materials has become very popular in a past few years [1]. Two methods have been used to make derivatized electrodes [2]: (1) strong adsorption, and (2) covalent chemical bonding. Obviously, each approach has its advantages and disadvantages: In the case of the covalent attachment, although the surface synthetic process takes place fairly selectively, the yield of the synthesis is not necessarily high. For the method (1), strong adsorption, the surface coverage can be made sufficiently high and the attachment is relatively non-specific which should allow modification Of virtually any electrode surface [3, 4]. In the present study, an attachment of photoactive porphyrins onto the platinum electrode has been performed by strong adsorption using surface active porphyrins. With this interesting approach, a sufficiently stable porphyrin electrode was constructed so as to perform the conversion of visible light to electricity.
EXPERIMENTAL
The compounds used are meso-tetrakis-(4-methylpyridinium)porphine magnesium tetrakis-tetraphenylborate-tetrahydrate, MgMe4PyP(B@4)44H20, and its free base, H2Me4PyP(B04)4" 4H20, and meso-tetrakis-(4-N-stearylpyridinium)porphine magnesium tetraiodide-monohydrate, MgSt4PYPL" H20. The synthetic procedure for these compounds was described elsewhere [5]. For a typical experiment, about 1 X 10 is molecules of each compound are deposited on a clean polished Pt disk (0.2 cm 2 in area) sealed in soft glass by allowing 10 pl of a 5 X 10 -4 M solution of each compound in acetonitrile [for MgMeaPYP(B04)4" 4H20 and H2Me4PyP(BO4)4.4H20] or in CH3OH/CHC13 [ 1 : 2 vol.%, for MgSt4PYPI4" 4H20] to evaporate on the surface of the Pt disk. The Pt-MgMe4PyP(B~4)4, Pt-H2MeaPyP(Bq)4)4, and Pt-MgSt4PyPL electrodes thus constructed were immersed in 0.1 M aqueous solution of tetraethylammonium perchlorate (TEAP). Sample solutions were exhaustively degassed by argon bubbling. The reference electrode was a saturated calomel electrode (SCE) with an agar KC1 salt
114
bridge. A P t coil was used as the auxiliary electrode. Photocurrents were measured with a PAR potentiostat Model 173 and an electrometer Model TR-8651 of Takeda Riken Co. The light source was a 750-W tungsten lamp. The action spectra of the photocurrent was measured with a grating m o n o c h r o m e t e r Model 338607 of Bansch & L o m b Co. The spectral distribution of the light source and the actual light intensities on the electrode surface were determined with a silicon photocell Model $642 from Hamamatsu TV Co. RESULTS AND DISCUSSION
Fig.1 shows the pH and applied potential dependence of steady photocurrents with Pt-MgMe4PyP(Bq~4), and Pt-H2Me4PYP(Bq)4)4 electrodes. As shown in Fig.l, the reproducibility for photocurrents/applied potential curves are not necessarily satisfactory mainly due to the dissolution of the adsorbed MgMe4PyP(B04)4 and H2Me4PYP(B(P4)4 layers into the solution phase. This tendency is generally observed at the above two electrodes regardless of pH and applied potential values for each measurement. This p h e n o m e n o n is quite a disadvantage when we try to construct a stable wet photocell and also to study fundamental processes occur-
10 7
I/A *0.5
/
/! /',
:
: i I
;,
t-Z ILl n*" re'
u0 0 -r
- 0.5
Fig.1. Hysteresis o f p h o t o c u r r e n t vs. a p p l i e d p o t e n t i a l curves. (a) Pt-MgMe4PYP(B04) 4 e l e c t r o d e s t a r t e d f r o m 0 V. T b e p H v a l u e was c o n t r o l l e d at p H 10.28. (b) P t - M g M e 4 P y P ( B 0 4 ) 4 e l e c t r o d e p r e - i l l u m i n a t e d a t 0 V t h r o u g h - 2 5 0 m V f o r a b o u t 2 h a n d t h e n s t a r t e d f r o m 0 V. T h e decay o f p h o t o c u r r e n t s as c o m p a r e d w i t h t h e curve a was i n d i c a t e d b y d o t t e d arrows, p H = 10.28. (c) Pt-H2Me4PyP(B04) 4 electr6de, p H -- 7.50. T h e curves a. b a n d c were r e c o r d e d u s i n g a single s e p a r a t e e l e c t r o d e s p e c i m e n , respectively. A b o u t 15 m i n were n e e d e d t o o b t a i n t h e s t e a d y p h o t o c u r r e n t s a t e a c h p o i n t in this figure. W h i t e light was used: S a m p l e s o l u t i o n : 0.1 M T E A P a n d n o r e d u c i n g a n d o x i d i z i n g agents were added. T h e p H was a d j u s t e d u s i n g HC104 a n d NaOH. T = 20 + 0.5°C.
115
ring at the surface of the porphyrin electrode. In order to overcome the abovementioned problem, the porphyrin should be made completely water insoluble and also surface active so that it can be attached permanently at the surface of the electrode. Therefore, we used MgSt4PYPL" H:O which was found to exhibit surface active nature [5]. This particular c o m p o u n d was put in the same way as MgMe4PyP(B04)4 and H:Me4PYP(B@4)4 onto the platinum electrode and the photoelectrochemical behavior was examined as shown in Fig.2. In contrast with the result of Fig.l, the reproducibility for the photocurrent/applied potential profiles was sufficiently good, indicating that the dissolution of the immobilized porphyrin in this case is negligible and the stability of this porphyrin electrode is better by far. The electrode thus constructed provided a steady photocurrent for at least several hours without decay. The current flow during this time period is well over that equivalent to the charge for the total decomposition of surface layer. Photocurrent spectra (photocurrent vs. wavelength curve, i.e., Action spectra) before and after such illumination in acidic (pH 2.7) and alkaline (pH 10 7 I/A .0.2--
.0.1
/;0
Z, LU rr
0 ..r n
• 100
*200
E
mV(SCE)
-0.1
Fig.2. D e p e n d e n c e o f s t e a d y p h o t o c u r r e n t s o n t h e a p p l i e d p o t e n t i a l a n d p H values w i t h a Pt-MgSt4PyPI 4 electrode. T h e p H values were c o n t r o l l e d at p H 1 0 . 3 2 a n d 4 . 1 2 for t h e curves a a n d b, respectively. B o t h curves a a n d b were o b t a i n e d u s i n g t h e same e l e c t r o d e s p e c i m e n . Because it t o o k ca. 15 m i n t o o b t a i n t h e s t e a d y p h o t o c u r r e n t s a t e a c h p o i n t , a b o u t 7 h were n e e d e d t o c o m p l e t e b o t h t h e c u r v e a a n d b. O t h e r e x p e r i m e n t a l c o n d i t i o n s were t h e same t o t h o s e o f Fig.1.
116 9.3) solution showed no change. These results indicate that the adsorbed layer is stable chemically and physically and any change such as demetallization or dissolu tion need not be taken into consideration. The observed steady photocurrent must be due to reactions of photo-produced cation and anion radicals of the Mg(II) porphyrin with solution species such as O H - , H +, and 02, and the details of this interesting photoelectrochemical reaction will be published elsewhere [6]. The film thickness is readily adjustable simply by changing the amount of the volume of the porphyrin CH3OH/CHC13 solution to be p u t on the electrode surface. It was found that the photocurrent reached a maximum with 15 × 1014 molecules/cm 2 on the Pt electrode, equivalent to ca. 30 molecular layers. In contrast with the photo-sensitization p h e n o m e n a at the semiconductor electrode where just a monomolecular layer of a dye is sufficient to observe the appreciable photocurrent, some multi-molecular layers are needed for the metal electrode to obtain observable photocurrents. This is simply due to the difference in the mechanism of the electron transfer between attached dye molecules and substrate electrodes depending on semiconductor or metal electrodes [ 7 ]. We have also performed similar experiments with surface active Mn(III), Co(II), Zn(II), and metal flee porphyrins: The results were very similar to the Mg(II) porphyrin with respect to stability, and general photoelectrochemical behavior. In conclusion, the present approach using the surface active porphyrin is extremely advantageous for the construction of a stable photoactive porphyrin electrode. The results presented here further imply that the adsorption of surface active porphyrins on the electrode surface will provide a variety of applications to catalysis, and selective synthesis as well as photoelectrochemistry. ACKNOWLEDGEMENT
The authors express their deep gratitude to Profs. S. Fujiwara and Y. Sasaki for their continuing interest and support.
REFERENCES 1 W.R. H e i n e m a n a n d P.T. K i s s i n g e r , A n a L C h e m . , 5 0 ( 1 9 7 8 ) 1 6 6 R . 2 R . F . L a n e a n d A.T. H u b b a r d , J. P h y s . C h e m . , 77 ( 1 9 7 3 ) 1 4 0 1 , 1 4 1 1 ; 8 1 ( 1 9 7 7 ) 7 3 4 ; B . F . W a t k i n s , J . R . B e h l i n g , E. K a r i v a n d L.L. Miller, J. A m . C h e m . S o c . , 9 7 ( 1 9 7 5 ) 3 5 4 9 ; R.J. R e n h a r d a n d R.W. M u r r a y , J. E l e c t r o a n a l . C h e m . , 78 ( 1 9 7 7 ) 1 9 5 ; M. F u j i h i r a , A. T a m u r a a n d T. Osa, C h e m . L e t t . , 3 6 1 ( 1 9 7 7 ) ; G.J. L e i t h a n d C.J. P i c k e t , J. C h e m . S o c . , D a l t o n T r a n s . , 1 7 9 7 ( 1 9 7 7 ) ; M.T. H e n n e , G.P. R o y e r , J . F . E v a n s a n d T. K u w a n a , J. E l e c t r o a n a l . C h e m . , 8 0 ( 1 9 7 7 ) 4 0 9 ; A. Diaz, J. A m . C h e m . S o c . , 9 9 ( 1 9 7 7 ) 5 8 3 8 ; A.P. B r o w n , C. K o v a l a n d F.C. A n s o n , J. E l e c t r o a n a l . C h e m . , 7 2 ( 1 9 7 6 ) 3 7 9 ; N. O y a m a , A.P. B r o w n a n d F.C. A n s o n , J. E l e c t r o a n a l . C h e m . , 8 7 ( 1 9 7 8 ) 4 3 5 ; K . S . V . S a n t h a n a m , J. J e s p e r s e n a n d A.J. B a r d , J. A m . C h e m . S o c . , 9 9 ( 1 9 7 7 ) 2 7 4 ; J . F . E v a n s , T. K u w a n a , A n a l . C h e m . , 49 ( 1 9 7 7 ) 1 6 3 2 a n d references therein. 3 A. Merz a n d A.J. B a r d , J. A m . C h e m . S o c . , 78 ( 1 9 7 8 ) 3 2 2 2 . 4 M.R. V a n d e r M a r k a n d L . L . Miller, J. A m . C h e m . S o c . , 7 8 ( 1 9 7 8 ) 3 2 2 3 . 5 T. Y a m a m u r a , C h e m . L e t t . , 7 7 3 ( 1 9 7 7 ) . 6 Y. U m e z a w a a n d T. Y a m a m u r a , s u b m i t t e d . 7 H. G e r i s c h e r , J. E l e c t r o c h e m . S o c . , 1 2 5 ( 1 9 7 8 ) 2 1 8 C .