Photoelectrochemical Systems Involving Solid ... - ACS Publications

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Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 10, 2015 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch015

Photoelectrochemical Systems Involving SolidLiquid Interfacial Layers of Chlorophylls TSUTOMU MIYASAKA and KENICHI HONDA Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan

Photoelectrochemical conversion from visible light to elec tric and/or chemical energy using dye-sensitized semiconductor or metal electrodes is a promising system for the in vitro simula tion of the plant photosynthetic conversion process, which is con sidered one of the fundamental subjects of modern and future photoelectrochemistry. Use of chlorophylls(Chls) and related compounds such as porphyrins in photoelectric and photoelectro chemical devices also has been of growing interest because of its close relevance to the photoacts of reaction center Chls in photo synthesis. Although Chl, as well as most of biological pigments isolated from living organisms, is unstable under ambient conditions, the usefulness of Chl as a photoreceptor in in vitro studies is im portant for the following reasons: (a) its capability of utilizing red incident light, (b) strong redox reactivities in its excited states, (c) the occurrence of highly efficient energy migration among excited molecules, (d) the ability to form a wide variety of photoactive derivatives absorbing in far red region, such as Chl-nucleophile aggrega tes, and (e) the availability of its surfactant structure which enables the ideal incorporation of the molecule into membrane struc tures. Based on these characteristics, a number of investigations of the photoelectrochemical as well as the photovoltaic effects of in vitro Chl have appeared during the last decade. Besides developing an efficient model system, the goal of light conversion studies using in vitro Chl is to obtain as much useful information about the photobehavior of Chl as possible in order to identify the in vivo reaction mechanisms involved. To this end, various photoelectrochemical approaches have emphasized the molecular configuration and geometry of Chl. Although studies in this field are currently progressing rapidly, this paper will review the photoelectrochemical behavior of Chl from the physical and photochemical perspectives. 0097-6156/81/0146-0231$05.25/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

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1.

Morphological

Aspects of C h l o r o p h y l l I n t e r f a c i a l Layers

We are p r i m a r i l y concerned with s e v e r a l t y p i c a l modes of the Chl i n t e r f a c i a l l a y e r r e l e v a n t to p h o t o e l e c t r i c and p h o t o e l e c t r o chemical systems. Since the Chl molecule, a s u r f a c t a n t pigment c o n s i s t i n g of a p a r t i a l l y h y d r o p h i l i c porphyrin r i n g and a t o t a l l y hydrophobic p h y t o l chain, i s i n s o l u b l e i n water, i t i s general l y employed f o r p h o t o e l e c t r i c and photoelectrochemical measure ments i n the form of a s o l i d (or q u a s i - s o l i d ) f i l m deposited on an e l e c t r o d e s u r f a c e . Such Chl f i l m s are d i v i d e d i n t o three t y p i c a l modes according to molecular c o n f i g u r a t i o n . 1) Amorphous Films. An amorphous f i l m i s g e n e r a l l y pre pared by solvent evaporation of a dry organic s o l u t i o n of Chl on a s o l i d substrate s u r f a c e . The vacuum sublimation technique, which i s widely employed f o r most s y n t h e t i c dyes, i s not a p p l i c a b l e to Chl due to p o s s i b l e thermal degradation of the pigment. The red absorption peak of a dry amorphous Chl a f i l m i s around 675-680 nm. This i s r e d - s h i f t e d from that of monomeric Chl a (660 nm) i n organic s o l u t i o n (1,2), i n d i c a t i n g that aggregated forms of Chl a such as dimers and oligomers (absorbing i n the red at 670-680 nm (2 3^) ) are i n v o l v e d . 9

2) M a c r o c r y s t a l l i n e C h l o r o p h y l l and I t s F i l m s . The c r y s t a l l i z a t i o n e f f e c t i s a c h a r a c t e r i s t i c of Chl. Jacob et at.(5) e s t a b l i s h e d evidence f o r the s i g n i f i c a n t r o l e of water i n the f o r mation of m a c r o c r y s t a l l i n e Chl. L a t e r , B a l l s c h m i t e r and Katz (6) explained t h i s m i c r o c r y s t a l l i n e form of Chl a, absorbing i n the red around 740 nm, i n terms of (Chl α - Η 2 θ ) adduct. A great deal of study has focused on the s t r u c t u r a l and photochemical charac t e r i z a t i o n of v a r i o u s Chl-I^O aggregate species (2,3,4,6-13). The relevance of these f a r - r e d absorbing aggregates as s t r u c t u r a l models to the photosynthetic r e a c t i o n center Ρ700 (14), which i s b e l i e v e d to i n v o l v e a s p e c i a l p a i r of Chl a, has a l s o been a sub j e c t of intense study (3,4,7,9,10,11,13); f o r example, s t r u c t u a l proposals f o r Chl α - Η 0 species have suggested that the water molecule i s bound to Chl a with i t s oxygen coordinated to a cen t r a l magnesium and with i t s hydrogen to the C-9 keto carbonyl group (_7>13) or the C-10 keto carbonyl (9_,10) i n the porphyrin r i n g . I t i s known that other n u c l e o p h i l e s can a l s o a s s o c i a t e to form aggregates s i m i l a r to Chl-H 0 species (8). I t should be noted that the formation of these hydrated Chl species as w e l l as t h e i r l i g h t r e a c t i o n s may be involved more or l e s s i n the other types of Chl f i l m s prepared on e l e c t r o d e s , with the extent depending s i g n i f i c a n t l y on the method of preparing the f i l m as w e l l as the e x p e r i mental c o n d i t i o n s . Chl α - Η 0 m i c r o c r y s t a l s (745 nm) (5,6,10,11) and other hy drated Chl species are r e a d i l y obtained by allowing a small η

2

2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


15.

MiYASAKA AND HONDA

Interracial Layers of Chlorophylls

233

amount of water to remain i n s o l u t i o n s of Chl a d i s s o l v e d i n s u i t able nonpolar organic solvents. Films can be formed on the s o l i d ( e l e c t r o d e ) s u r f a c e by solvent evaporation. A Chl a-H2Û macrocryst a l l i n e f i l m can be prepared conveniently by the e l e c t r o d e p o s i t i o n technique e s t a b l i s h e d by Tang and Albrecht (15); the m i c r o c r y s t a l s ( p o s i t i v e l y charged) i n 3-methylpentane suspension are deposited on the e l e c t r o d e substrate (cathode) under a p p l i e d f i e l d s (about 1 kV/cm ), allowing f o r uniform f i l m s as t h i c k as s e v e r a l hundred monolayers. This m i c r o c r y s t a l l i n e f i l m (740-5 nm) can e a s i l y be converted to an anhydrous and amorphous form (675 nm) by heating the f i l m below 70°C. 3) Monolayer Assemblies o f C h l o r o p h y l l Monolayer and m u l t i l a y e r f i l m s of Chls have been studied i n d e t a i l since 1937 (16,17). From the b i o l o g i c a l point of view, Chl monolayer arrays are of i n t e r e s t because they may be important i n the s t r u c t u r e of the c h l o r o p l a s t - l a m e l l a e or s o - c a l l e d t h y l a k o i d membrane (18,19) Due to t h e i r s u r f a c t a n t s t r u c t u r e s , Chls are among the i d e a l and s t a b l e monolayer-forming pigments, and surface pressure-molecular area (Π-Α) isotherms of t h e i r monolayers formed a t air-water i n t e r f a c e s have been w e l l c h a r a c t e r i z e d (20). I t i s b e l i e v e d that at the water surface Chl molecules are o r i e n t e d with the porphyrin r i n g s and p h y t o l chains d i r e c t e d upward and the h y d r o p h i l i c e s t e r linkages downward. Monolayers and mixed monolayers of Chls a r e prepared on a n e u t r a l agueous b u f f e r surface and can be deposited at a c o n t r o l l e d surface concentration onto a s o l i d substrate by the Langmuir-Blodgett technique (21). Such preparation and deposi t i o n techniques are s p e c i f i e d i n l i t e r a t u r e s (20,22,23). As a t y p i c a l f e a t u r e , a monolayer f i l m of pure Chl a possesses absorp t i o n peaks a t 675-680 nm and 435-440 nm with corresponding o p t i c a l d e n s i t i e s of O.008-O.01 and O.01-O.013 i n the red and blue bands, r e s p e c t i v e l y (24,25). The r e d - s h i f t s observed i n the absorption peaks with respect to Chl a i n s o l u t i o n may r e s u l t from d i p o l e d i p o l e i n t e r a c t i o n between Chl molecules (20). 9

Various i n e r t compounds such as f a t t y a c i d s , f a t t y a l c o h o l s , and l i p i d s behave as two-dimensional d i l u e n t s f o r Chl monolayers and lead to the formation of homogeneously mixed monolayers (20). These d i l u e n t s have f a c i l i t a t e d the study o f Chl-Chl energy t r a n s f e r w i t h i n a two-dimensional plane as a f u n c t i o n of the intermol e c u l a r Chl separation (26,27). In s u f f i c i e n t l y d i l u t e mixed monolayers, a majority of the Chl molecules are thought to e x i s t i n the monomeric s t a t e , with t h e i r mutual aggregations e f f e c t i v e l y suppressed w i t h i n the geometrically c o n t r o l l e d , ordered c o n f i g u r a t i o n . M u l t i l a y e r s ( b u i l t - u p monolayers) of Chl a have a l s o been studied (23) and u t i l i z e d f o r p h o t o v o l t a i c studies (see the next s e c t i o n ) . The molecular o r i e n t a t i o n i n such Chl a m u l t i l a y e r s has been ascertained from the observed dichroism i n s p e c t r o p o l a r i z a t i o n measurements with respect to absorption (23) and emission (28). I t has been reported that Chl a i n both m i c r o c r y s t a l l i n e f i l m s (5,10) and monolayer assemblies (23) are f a i r l y s t a b l e over long periods of time.



234


was 2%. S i m i l a r d i f f e r e n c e s between c r y s t a l l i n e and amorphous f i l m s were reported e a r l i e r by Corker and Lundstrom (42). However, they mentioned that the photοconductive and r e c t i f y i n g nature of the s o l i d Chl a l a y e r i s not a t t r i b u t a b l e to the postulated organic semiconductivity of Chl a, but more l i k e l y to an oxide l a y e r e x i s t ing on the metal surface which functions as a r e c t i f y i n g b a r r i e r and to a l a r g e number of trapped e l e c t r o n s which act l i k e dopants w i t h i n the l a y e r . Such a concept i s compatible with McCree's p o s t u l a t i o n (34), although there are d i f f e r e n c e s i n the f i l m c o n f i g u r a t i o n s i n t h e i r systems. Whether or not Chl i s regarded i n t r i n s i c a l l y as an organic semiconductor, the s o l i d Chl l a y e r i n contact with a metal does d i s p l a y a p-type p h o t o v o l t a i c e f f e c t , and i t s e f f i c i e n c y depends s i g n i f i c a n t l y on the morphology of the Chl l a y e r as w e l l as the nature of the metal. The e f f e c t corresponding t o a p-type photoconductor can a l s o be expected at the j u n c t i o n of a metal /Chl/ l i q u i d i n a photoelectrochemical system. Such a presumption i s i n f a c t compatible with the photoelectrochemical behavior observed f o r most of Chl-coated metal e l e c t r o d e s , as w i l l be shown l a t e r . 3.

E l e c t r o c h e m i c a l Energetics of C h l o r o p h y l l

The ground s t a t e o x i d a t i o n and reduction p o t e n t i a l s of Chl a and b i n v a r i o u s organic s o l u t i o n s have been measured by a number of i n v e s t i g a t o r s . These experimental data as w e l l as other impor tant energy parameters f o r Chls have been covered i n a recent review by Seely (43). I t i s noted that the p o t e n t i a l values r e ported by various i n v e s t i g a t o r s are not i n good agreement. This might r e f l e c t the d i f f i c u l t y (due to j u n c t i o n p o t e n t i a l s ) i n cor r e l a t i n g the observed redox p o t e n t i a l s (versus, e.g., Ag) i n organic media to those i n aqueous media r e f e r r e d to the SCE (saturated calomel electrode) or the NHE, r a t h e r than r e s u l t s from solvent e f f e c t s . Taking the various sources of data i n t o account, onee l e c t r o n o x i d a t i o n and r e d u c t i o n p o t e n t i a l s f o r Chl a are +O.52 to +O.62 V v s . SCE and -1.08 to -1.25 V v s . SCE, r e s p e c t i v e l y ; those f o r Chl b are +O.56 to +O.72 V v s . SCE and -1.03 t o - 1 . 2 9 V v s . SCE, r e s p e c t i v e l y . We r e c e n t l y measured the redox p o t e n t i a l s f o r Chls by c y c l i c voltammetry (mainly i n DMF medium), employing ferrocene as a standard redox sample to c o r r e c t f o r the reference e l e c t r o d e p o t e n t i a l . The r e s u l t s obtained were +O.59 V and -1.17 V v s . SCE f o r Chl a o x i d a t i o n and reduction (one-electron), and +O.63 V and'v-1.1 V v s . SCE f o r Chl b o x i d a t i o n and r e d u c t i o n (44). A l l are w i t h i n the range of p r e v i o u s l y reported values. Using s i n g l e t and t r i p l e t e x c i t a t i o n energies reported f o r Chl a, 1.85 eV (45) and 1.33 eV (46), and f o r Chl b, 1.91 eV (45) and 1.39 eV (46), r e s p e c t i v e l y , o x i d a t i o n p o t e n t i a l s of Chls i n the ground and e x c i t e d s t a t e s are presented i n Figure 1 and compared with redox p o t e n t i a l s of other b i o l o g i c a l l y important reagents. Another t o p i c of i n t e r e s t i s the r e a c t i o n of e x c i t e d s t a t e Chl a with water. This occurs as a r e s u l t o f the primary photo-



15.

MiYASAKA AND HONDA

2.

I n t r i n s i c P h o t o e l e c t r i c a l Behavior of S o l i d C h l o r o p h y l l Films

Interfacial Layers of Chlorophylls

235

Since photoelectrochemical measurements deal mostly with s o l i d i n t e r f a c i a l l a y e r s of Chl i n contact with e l e c t r o d e s , i t i s of prime importance to understand the p h o t o e l e c t r i c p r o p e r t i e s of Chl i n the s o l i d s t a t e . P r i o r to any photoelectrochemical s t u d i e s , a number of i n v e s t i g a t i o n s of the photoconductive behav i o r of Chl were performed based on the hypothesis (29,30) that charge separation i n photosynthetic primary r e a c t i o n s may r e s u l t from the photoconduction i n Chl systems. Among the e a r l y s t u d i e s of Chl photophysics were those by Nelson (31) and Arnold and Macley (32) who measured p h o t o c o n d u c t i v i t i e s of solvent-evaporated Chl f i l m s on platinum g r i d s and of monolayer assemblies on c o l l o i dal g r a p h i t e g r i d s , r e s p e c t i v e l y . Therein and co-workers (33) studied the p h o t o c o n d u c t i v i t y of m i c r o c r y s t a l l i n e Chl l a y e r s sand wiched between two e l e c t r o d e s and c h a r a c t e r i z e d the s o l i d Chl l a y e r as a p-type organic semiconductor. On the other hand, a l a t e r study by McCree (34) pointed out that monolayer assemblies of Chl are undoubtedly i n e f f i c i e n t p h o t o c o n d u c t o r s . A similar conclusion was made by Reucroft and Simpson (35) u s i n g Chl a m u l t i l a y e r s coated on Sn02 or Sn e l e c t r o d e s . However, the photo v o l t a i c e f f e c t of Chl a was enhanced s i g n i f i c a n t l y by superimposed monolayers of an e l e c t r o n acceptor or doner. This e f f e c t o f the added acceptor l a y e r s has a l s o been e s t a b l i s h e d by Shkuropatov et al. (36) and Janzen and Bolton (37) u s i n g amorphous f i l m s and monolayers of Chl a , r e s p e c t i v e l y , placed on A l e l e c t r o d e s . High power e f f i c i e n c i e s of p h o t o v o l t a i c e f f e c t s due to metal / Chl l a y e r contacts have r e c e n t l y been reported using metal / Chl a I metal sandwich-type dry c e l l s . Meilanov et al. (38) reported quantum e f f i c i e n c i e s as high as 10% f o r the charge gen e r a t i o n i n amorphous Chl a f i l m s between A l e l e c t r o d e s . Tang and Albrecht (39,40), employing the e l e c t r o d e p o s i t i o n technique, have e x t e n s i v e l y studied the metal / m i c r o c r y s t a l l i n e Chl a I metal sand wich c e l l s with respect to v a r i o u s combinations of metals with d i f f e r e n t work f u n c t i o n s . In t h e i r systems, m i c r o c r y s t a l l i n e Chl a (the hydrated form absorbing at 745 nm) behaved as an e f f i c i e n t p-type semiconductor, and the highest power conversion e f f i c i e n c y , 5 χ 1 0 ~ % at 745 nm (O.7% photocurrent quantum e f f i c i e n c y ) , was obtained i n the c e l l mode, Cr / Chl a I Hg (40). They a t t r i b u t e d the p h o t o v o l t a i c e f f e c t to a p o t e n t i a l b a r r i e r (Schottky b a r r i e r ) formed at the contact of the low work-function metal (e.g.,Cr) and the p-type organic semiconductor ( c r y s t a l l i n e Chl a). More r e c e n t l y Dodelet et al. (41) studied A l / Chl a I Ag p h o t o v o l t a i c c e l l s (the A l / Chl a contact i s photoactive) where the p h o t o c o n d u c t i v i t i e s of m i c r o c r y s t a l l i n e l a y e r s and amorphous (anhydrous) l a y e r s were compared. The power conversion e f f i c i e n c y i n the former U O.2%) was much higher than i n the l a t t e r U O.04%), which supports the contention that the c r y s t a l l i n e form i s a b e t t e r photoconductor than the amorphous form. In t h i s study, the maximum photocurrent quantum e f f i c i e n c y f o r the c r y s t a l l i n e Chl a 2


PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE


236

Si 1

D */Dt s

\

Ti

1

D* /Dt T

^ 2

_

MV^MV* NADP/NADPH

650 nm

Q/H Q 2

Fe(CN) +O.5 So Ό

1M

Chl b

2

670nm

!

O

INTERFACES

3 _ A 6

"

-0 /0H

D / Dt

2

pH 7

Chl α

Figure 1. Correlation of one-electron oxidation potentials for Chl a and Chl b in the ground (S ) and excited singlet (SJ and triplet (T ) states with redox potentials of some donor and acceptor species at pH 7 ((MV) methylviologen; (H Q) hydroquinone) 0

t

2



15.

MIYASAKA AND

HONDA

Interfacial Layers of Chlorophylls

237

s y n t h e t i c processes i n terms of the enzyme-assisted o x i d a t i o n of water by the e x c i t a t i o n of photosystem I I r e a c t i o n center Chl a (P680) and reduction of NADP (or protons i n the presence of hydrogenase) by the e x c i t a t i o n of photosystem I r e a c t i o n center Chl a (P700). From Figure 1, one can see that the e x c i t e d s i n g l e t s t a t e of in vitro Chl a, as w e l l as Chl b, can reduce water on an energ e t i c b a s i s since an o v e r p o t e n t i a l of about O.7 V i s a v a i l a b l e . However, i t i s known (47,48) that e x c i t e d Chl a i n s o l u t i o n undergoes e l e c t r o n t r a n s f e r r e a c t i o n s with redox species e x c l u s i v e l y v i a a l o n g - l i v e d t r i p l e t s t a t e ( l i f e t i m e , ca. 2 ms (49)). Since the o x i d a t i o n p o t e n t i a l s of ground and t r i p l e t e x c i t e d s t a t e Chls are s i t u a t e d c l o s e to the o x i d a t i o n (+O.57 V v s . SCE) and reduction (-O.66 V v s . SCE) p o t e n t i a l s of water, r e s p e c t i v e l y , i n n e u t r a l s o l u t i o n , the photodecomposition of water by t r i p l e t e x c i t e d Chls, namely, Chl*

+

Chl"*" +

H

+

0H~

^

Chl"*" +

1/2

Chl

H

+

+

H +

(1)

2

1/2

0

2

(2)

may not be e f f i c i e n t at pH 7. On the other hand, e i t h e r r e a c t i o n might be promoted when the pH of s o l u t i o n (i.e.,redox p o t e n t i a l of water) i s v a r i e d so as to favour the r e a c t i o n . This conjecture i s c o n s i s t e n t with the demonstration of water s p l i t t i n g e i t h e r i n an a c i d i c e l e c t r o l y t e ( f o r the r e a c t i o n 1) or i n an a l k a l i n e e l e c t r o l y t e ( f o r the r e a c t i o n 2) as w i l l be described i n the next s e c t i o n . In connect i o n with r e a c t i o n 2, Watanabe and Honda (50) r e c e n t l y found that the r a t e constant f o r the r e a c t i o n of the Chl a c a t i o n r a d i c a l with water i n a c e t o n i t r i l e medium i s extremely low. However, the redox p o t e n t i a l as w e l l as the r e a c t i o n behavior i n v e s t i g a t e d i n organic s o l u t i o n s may not be d i r e c t l y a p p l i c a b l e to the energetics of a s o l i d Chl layer-aqueous s o l u t i o n i n t e r f a c e , the general c o n d i t i o n i n a photoelectrochemical system. 4.

Photoelectrochemical Systems Involving Semiconductor and Metal Electrodes

Chlorophyll-Coated

1) General Technique f o r C h l o r o p h y l l U t i l i z a t i o n . In photoe l e c t r o c h e m i c a l measurements, the methods commonly employed f o r preparing Chl i n t e r f a c i a l l a y e r s on e l e c t r o d e substrates are solvent evaporation, e l e c t r o d e p o s i t i o n ( f o r c r y s t a l l i n e Chl), and monolayer d e p o s i t i o n techniques, as o u t l i n e d p r e v i o u s l y . Chl i s p u r i f i e d from c h l o r o p l a s t e x t r a c t s , u s u a l l y obtained from spinach leaves, by dioxane p r e c i p i t a t i o n method (51) and conventional sugar column chromatography (52). For r a p i d and easy preparation, the method r e c e n t l y developed by Omata and Murata (53) i s s a t i s f a c t o r y ; s y n t h e t i c (DEAE-) Sepharose i s s u b s t i t u t e d f o r sugar on the column. Besides using spectroscopic c r i t e r i a (52), the p u r i t y of Chl samples can be checked r e a d i l y by means of s i l i c a - g e l t h i n l a y e r chromatography (54). C o l o r l e s s contaminations i n Chl



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preparations can be determined by measuring Chl monolayer IT - A isotherms (20). S p e c t r a l parameters f o r Chl a , such as e x t i n c t i o n c o e f f i c i e n t , i n various p o l a r and nonpolar organic solvents have been i n v e s t i g a t e d by Seely (1). 2) Chlorophyll-Coated Semiconductor E l e c t r o d e s . Chl has f i r s t been employed by T r i b u t s c h and C a l v i n (55^56) i n dye s e n s i t i z a t i o n studies of semiconductor e l e c t r o d e s . Solvent-evapo rated f i l m s of Chl a Chl b and b a c t e r i o c h l o r o p h y l l on n-type semiconductor ZnO electrodes ( s i n g l e c r y s t a l ) gave anodic s e n s i t i z e d photocurrents under p o t e n t i o s t a t i c conditions i n aqueous e l e c t r o l y t e s . The photocurrent a c t i o n spectrum obtained f o r Chl a showed the red band peak at 673 nm corresponding c l o s e l y to the amorphous and monomeric s t a t e of Chl a. The a d d i t i o n of supers e n s i t i z e r s (reducing agents) increased the anodic photocurrents, and a maximum quantum e f f i c i e n c y of 12.5% was obtained f o r the photocurrent i n the presence of phenylhydrazine. Based on the observed strong s u p e r s e n s i t i z a t i o n e f f e c t , generation of the anodic photocurrent was explained i n terms of an e l e c t r o n t r a n s f e r mechanism i n v o l v i n g s e v e r a l p o s s i b l e pro cesses. For example, e l e c t r o n i n j e c t i o n from e x c i t e d Chl a ( s i n g l e t and/or t r i p l e t ) i n t o the conduction band of ZnO could take place followed by a r a p i d reduction of the Chl a r a d i c a l c a t i o n by a reducing agent, and/or Chl a could undergo photoreduction by the reducing agent to produce a r a d i c a l anion which subsequently i n j e c t s an e l e c t r o n to the conduction band. P a r t i c i p a t i o n of both s i n g l e t and t r i p l e t e x c i t e d s t a t e s of Chl a i n the above e l e c t r o n t r a n s f e r processes was proposed. Knowing that the s i n g l e t s t a t e donor l e v e l of Chl a (around -1.3 V vs. SCE) i s located consider ably above the conduction band edge of ZnO (ca. -O.75 V vs. SCE at pH 7 (57) ) while the t r i p l e t donor l e v e l (around -O.75 V v s . SCE) i s s i t u a t e d very c l o s e to the ZnO l e v e l , i t seems reasonable to assume that the d i r e c t e l e c t r o n i n j e c t i o n from the e x c i t e d Chl a should occur predominantly v i a the s i n g l e t s t a t e . Since only a short time (ca. 10" s (58) ) i s necessary f o r e l e c t r o n i n j e c t i o n from a s e n s i t i z i n g dye to a semiconductor, e l e c t r o n t r a n s f e r from an e x c i t e d s i n g l e t s t a t e of Chl a ( l i f e t i m e ca. 6 χ 10~ s (59) ) must be efficient. In c o n t r a s t , another p o s s i b l e process i n i t i a t e d by the reduction of an e x c i t e d Chl α by a reducing agent, i f i n v o l v e d , i s considered favourable to the t r i p l e t s t a t e of Chl a, i n view of the f a c t that the p h o t o i o n i z a t i o n of Chl a by the r e a c t i o n with redox species has so f a r been found only v i a t r i p l e t e x c i t a t i o n (43,47,48). Besides ZnO e l e c t r o d e , i t was a l s o found that n-type CdS can be s e n s i t i z e d by Chls. We have studied the photoelectrochemical behavior of Chl a and Chl b on an n-type Sn0 (60) o p t i c a l l y transparent e l e c t r o d e (OTE; thickness of the Sn0 l a y e r on the g l a s s substrate, ca. 2000 A; donor d e n s i t y , 1 0 c m " ) . Chl monolayer assemblies, deposited by means of the Langmuir-Blodgett technique (20,21), were employed. The use of such monolayer assemblies as i n t e r f a c i a l dye l a y e r s


3

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MiYASAKA A N D HONDA

Ititerfacial Layers of Chlorophylls

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i n photoelectrochemical s t u d i e s has the f o l l o w i n g advantages : (a) Strong attachment of dye l a y e r s to the e l e c t r o d e surface can be e s t a b l i s h e d v i a h y d r o p h i l i c and/or hydrophobic i n t e r a c t i o n s , without accompanying chemical r e a c t i o n s of the dye, (b) two-dimensional homogeneous dye l a y e r s can be formed on the electrode surface, (c) the surface concentration as w e l l as the mean i n t e r m o l e c u l a r distance of the dye can be c o n t r o l l e d , and (d) the t o t a l thickness of the dye l a y e r s can be p r e c i s e l y con t r o l l e d at the molecular l e v e l . Besides these p r a c t i c a l advantages, i t i s of b i o l o g i c a l importance to study the photoeffects of Chl i n an ordered s t r u c t u r e , because such a s t r u c t u r e i s a c r u c i a l f a c t o r i n r e g u l a t i n g energy migra t i o n among Chls as w e l l as promoting e l e c t r o n t r a n s f e r processes i n photosynthetic organisms. Photoredox r e a c t i o n s i n v o l v i n g Chl incorporated i n t o a r t i f i c i a l membranes have long been studied by the use of b i l a y e r l i p i d membranes (BLM) (61). BLM studies are based on photogalvanic e f f e c t s caused by e x c i t e d Chl and mediated v i a the s o l u t i o n , while our membrane-electrode system deals with the d i r e c t capture of an e l e c t r o n (or hole) from photoexcited Chl at the e l e c t r o d e - s o l u t i o n i n t e r f a c e . Monolayer and mixed monolayers of Chl a and/or Chl b were deposited on a Sn0 OTE at a constant surface pressure of 10-20 dyn/cm. This r e s u l t e d i n a c l o s e l y packed, ordered f i l m on the electrode. The high transmittance of the OTE allowed f o r the d i r e c t measurement of the absorbance of a monolayer at the electrode-electrolyte interface. Quantum e f f i c i e n c i e s f o r the generation of photocurrents were c a l c u l a t e d using t h i s information. Monolayers of Chl a as w e l l as Chl b gave anodic s e n s i t i z e d photo currents under p o t e n t i o s t a t i c c o n d i t i o n s i n aqueous e l e c t r o l y t e containing a reducing agent (hydroquinone, H Q) (25). The s p e c t r a l dependence of the photocurrent matched the absorption spectrum of Chl at a S n 0 2 - e l e c t r o l y t e i n t e r f a c e . The spectra are shown i n Figure 2; the photoresponse of Sn0 i t s e l f i s n e g l i g i b l e over the v i s i b l e region due to i t s l a r g e band gap (3.5-3.8 eV). Under open c i r c u i t c o n d i t i o n , Chl α-coated Sn0 developed negative photovoltages

Photoelectrochemical Systems Involving Solid ... - ACS Publications

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