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ELECTRON EXCHANGES AT THE INTERFACE, PORPHYRIN/SEMI CONDUCTOR OR INSULATOR
OXIDES, STUDIED BY PAS AND ”MIRAGE EFFECT” DETECTION
Antoine Merle, A. Cherqaoui, C. Giannotti
To cite this version:
Antoine Merle, A. Cherqaoui, C. Giannotti. ELECTRON EXCHANGES AT THE INTERFACE, PORPHYRIN/SEMI CONDUCTOR OR INSULATOR OXIDES, STUDIED BY PAS AND ”MI- RAGE EFFECT” DETECTION. Journal de Physique Colloques, 1983, 44 (C6), pp.C6-291-C6-296.
�10.1051/jphyscol:1983646�. �jpa-00223205�
JOURNAL DE PHYSIQUE
Colloque C6, supplkment a u nO1O, Tome 44, octobre 1983 page C6- 291
A.M. Merle, A. Cherqaoui and C. ~ i a n n o t t i ~
?hotopkysique Pkotochimie MoZEcuZaire*, IJniaersitE de Bordeaux I , 33405 TaZence, Prance
+ I n s t i t u t des Substances NntureZZes, 91 190 Gif sur Y v e t t e , Prance
I I KesumP
-
Les echannes d ' e l e c t r o n s 3 l ' i n t e r f a c e n o r o h y r i n e de c o b a l t (Co TPP)/oxydes
semi-conducteurs i s o l a n t s o n te t e
e t u d i e s o a r SPA. On a observe une monooxydation e n (CoPYITPP)+ l o r s o u e l a o o r n h y r i n e e s t adsorbee s u r Nb205, T i 0 2 , Sr T i O3,
ZnO, Sn02, W03. I 1 s ' a n i t d ' u n e p h o t o o x y d a t i o n dans l e cas de l ' a d s r p t i o n s u r S i 0 2 e t Zr02.-Pu c o n t r a i r e , l a n o r o h > l r i n e r e s t e l ' e t a t r e d u i t l o r s o u ' e l l e e s t adsorbee s u r Ta2 O5 ou Al2Q3. Ces mesures o n t permis de p l a c e r l e p o t e n t i e l d ' o x y d a t i o n de l a o o r p h y r i n ea
l ' e t a t adsorbe
a
peu prPs 0,8 \I au-dessus de c e l u i en s o l u t i o n s o i t 5,3 V/vide A f i n d ' e t u d i e r l ' e f f e t d ' u n @ l e c t r o l y t e s u r de t e l s echanoesd 1 6 l e c t r o n s , nous avons r e a l i s @ une e x p e r i e n c e d ' e f f e t mirape e t nous avons e t u d i e i n s i t u l ' 6 v o l u t i o n du s p e c t r e de l a t e t r a n h e n y l p o r p h y r i n e adsorbee s u r ? e l de s i l i c e en f o n c t i o n de l ' a c i d i t e du m i l i e u a y e u x o u i l a baipne.
P l o r s q u ' e n absence d ' e l e c t r o l y t e l a o r o t o n a t i o n de l a o o r n h y r i n e e s t immediate e t s t a b l e , e l l e depend de l ' a c i d i t e du m i l i e u e t de l a oresence dloxy?Pne dans l e cas d ' u n c o n t a c t avec un e l e c t r o l y t e .
A b s t r a c t
-
P h o t o a c o u s t i c spectroscopy has shown t h a t , dependin? on t h e o x i d e- -.-
wTere i t i s adsorbed COIITPP may e i t h e r be o x i d i z e d i n i t s ~ r o u n d s t a t e ( c o I I I T P P ) ~ p h o t o o x i d i z e d o r r e r a i n i n t h e reduced f o r v . These o b s e r v a t i o n s have been r e l a t e d w i t h t h e band p o s i t i o n s of t h e o x i d e s and have a l l o w e d t o p o s i t i o n t h e o x i d a t i o n o o t e n t i a l o f adsorbed CO-ITPP r o u o h l y 0.8 V h i p h e r t h a n i n s o l u t i o n t h a t i s 5.3 V/vacuum.
I n o r d e r t o s t u d y t h e i n f l u e n c e o f an e l e c t r o l y t e on t h e e l e c t r o n exchanoes o f p o r p h y r i n we have i n v e s t i c a t e d by " m i r a q e e f f e c t d e t e c t i o n " (beam d e f i e c t i o n t e c h n i q u e ) , t h e e v o l u t i o n o f t h e a b s o r ~ t i o n spectrum o f t e t r a p h e n y l p o r p h y r i n as a f u n c t i o n o f t h e a c i d i t y o f t h e s u r r o u n d i n o aqueous medium.
INTRODUCTION
- -
D h o t o c a t a l y t i c e f f e c t s o f semi c o n d u c t o r s have been under i n v e s t i o a t i o n f o r l o n g t i m e because o f t h e i r ~ o t e n t i a l a ~ p l i c a t i o n t o d i r e c t c o n v e r s i o n o f s o l a r e n e r g y i n t o chemical eneroy ( 1 ) . The i r r a d i a t i o n o f t h e semi c o n d u c t o r w i t h an enerfly h i g h e r t h a n t h e band-gap i s accomoanied by t h e p r o m o t i o n o f an e l e c t r o n from t h e v a l e n c e band t o t h e c o n d u c t i o n band. E i t h e r t h e h o l e l e f t i n t h e v a l e n c e band o r t h e e l e c t r o n may be used f o r f u r t h e r chemical r e a c t i o n ( 2 ) o r c u r r e n t g e n e r a t i o n ( 3 ) .
One o f t h e problems o f t h e p h o t o e l e c t r o c h e m i c a l c e l l s f o r s o l a r energy s t o r a g e i s t h e c h o i c e o f t h e semi c o n d u c t o r . When narrow band gap semi c o n d u c t o r s which a r e uood absorbers o f s o l a r spectrum a r e used such as CdS, a s t r o n c anodic decomnosition i s observed which degrades t h e semi c o n d u c t o r . On t h e o t h e r hand, t h e i n c o n v e n i e n c e o f t h e wide band qao semi c o n d u c t o r s such as
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983646
C6-292 JOURNAL DE PHYSIQUE
Ti02, S r T i 0 3 which a r e s t a b l e a n a i n s t p h o t o a n o d i c decomposition i s t h a t t h e y a r e p o o r absorbers o f t h e s o l a r spectrum.
I n o r d e r t o remedy t h i s s i t u a t i o n i t has been tempted t o s e n s i t i z e t h e semi c o n d u c t o r . One p o s s i b i l i t y c o n s i s t s i n t h e a d s o r p t i o n of v i s i b l e l i p h t a b s o r b i n g o r g a n i c m a t e r i a l a t t h e s u r f a c e o f t h e semi conductor. Most o f t h e compounds which have been used i n such an aim a r e dyes ( 4 ) . However p w p h y r i n s a r e o t h e r o r g a n i c compounds which a r e good c a n d i d a t e s f o r s e n s i t i z a t i o n o f semi conductors. They s t r o n g l y absorb t h e v i s i b l e l i g h t and a r e a b l e t o undergo a wide v a r i e t y o f e l e c t r o n exchanaes. They have been t e n t a t i v e l y used d i r e c t l y f o r w a t e r p h o t o o x i d a t i o n ( 5 ) a n d f o r photoinduced redox r e a c t i o n s i n aqueous m i c e l l e s ( 6 ) . When b e i n g d e p o s i t e d on metal e l e c t r o d e s t h e i r b e h a v i o r may be d e s c r i b e d by a m e t a l - i n s u l a t o r - s e m i c o n d u c t o r S c h o t t k y b a r r i e r where t h e p o r p h y r i n behaves as a p - t y p e semi c o n d u c t o r ( 7 ) .
The s e n s i t i z a t i o n o f semi c o n d u c t o r e l e c t r o d e s by p o r p h y r i n s has a l s o been i n v e s t i g a t e d ( 8 ) . Photochemical measurements have shown t h a t a n - t y p e Sn02 semi c o n d u c t o r e l e c t r o d e a c t s as a photo cathode when s e n s i t i z e d by a l a y e r o f meso-tetraphenyl p o r p h y r i n ( 9 ) i n s t e a d o f t h e usual p h o t o anodic b e h a v i o r .
However, i t i s t o be n o t e d t h a t t h e i n t e r f a c e p o r p h y r i n / e l e c t r o d e has never been s t u d i e d d i r e c t l y as m o s t l y e l e c t r o c h e m i c a l t e c h n i q u e s have been used t o i n v e s t i g a t e t h e s e n s i t i z e d e l e c t r o d e . One o f t h e reasons f o r such a l a c k i s t h e d i f f i c u l t y i n a p p l y i n g c l a s s i c a l s p e c t r o s c o u i c t e c h n i q u e s t o t h e i n v e s t i a a t i o n o f an opaque i n t e r f a c e . P h o t o a c o u s t i c spectroscopy can overcome t h i s p r o b l e m as i t a l l o w s t h e d i r e c t o b s e r v a t i o n o f t h e a b s o r p t i o n spectrum o f opaque samples (10, 11,
12).
It has been r e c e n t l y a p p l i e d t o p h o t o e l e c t r o c h e m i c a l i n v e s t i g a t i o n s . The change i n t h e a b s o r p t i o n spectrum o f a ZnO e l e c t r o d e upon e l e c t r o l y s i s has been i n v e s t i g a t e d ( 1 3 ) . S e n s i t i z a t i o n s t u d i e s have j u s t s t a r t e d : dye o x i d a t i o n has been observed a t f i x e d wavelength as a f u n c t i o n o f t h e a p p l i e d p o t e n t i a l on an Sn02 e l e c t r o d e ( 14)
.
I n t h i s work, we have a p p l i e d p h o t o a c o u s t i c spectroscopy t o t h e
i n v e s t i g a t i o n o f t h e i n t e r f a c e o b t a i n e d by a d s o r p t i o n o f c o b a l t t e t r a p h e n y l porphy- r i n on v a r i o u s semi c o n d u c t o r o x i d e s and i n s u l a t o r s . I n o r d e r t o s t u d y t h e e l e c t r o n exchanges a t t h i s i n t e r f a c e we have determined t h e o x i d a t i o n s t a t e o f t h e adsorbed p o r p h y r i n by o b s e r v i n g i t s p h o t o a c o u s t i c spectrum dependino on t h e s u b s t r a t e and i r r a d i a t i o n .
The disadvantage o f c l a s s i c a l p h o t o a c o u s t i c spectroscopy i s t h a t o n l y a s o l i d - g a z i n t e r f a c e can be s t u d i e d whereas t h e most i n t e r e s t i n g i n t e r f a c e f o r p h o t o - e l e c t r o c h e m i c a l purposes i s t h e e l e c t r o d e - e l e c t r o l y t e one t h a t i s a s o l i d - l i q u i d i n t e r f a c e . F o r t h i s reason, we have s e t up a "miraae e f f e c t e x p e r i m e n t "
(15,16) and we have i n v e s t i g a t e d t h e e v o l u t i o n o f t e t r a p h e n y l p o r p h y r i n adsorbed on s i l i c a g e l as a f u n c t i o n a c i d i t y o f t h e surnagent water.
Commercialsemi c o n d u c t o r o x i d e s p r o v i d e d by Merck have been used. I n o r d e r t o e l i m i n a t e w a t e r and oxygen e f f e c t s t h e y have been heated a t 130°C d u r i n g 12 hours, degassed up t o t o r r and k e p t under d r y n i t r o g e n u n t i l used. The p o r p h y r i n s have been p r e p a r e d a c c o r d i n g t o t h e l i t e r a t u r e (17) and f u r t h e r p u r i f i e d by t h i n l a y e r chromatography.
The a d s o r p t i o n o f t h e p o r p h y r i n on t h e o x i d e s was r e a l i z e d by a l l o w i g p a m e t h y l e n c h l o r i d e s o l u t i o n o f p o r p h y r i n t o c o n t a c t t h e degassed o x i d e . A 10- P s o l u t i o n c o n c e n t r a t i o n was chosen as b e i n q a pood compromise f o r good s i g n a l t o n o i s e r a t i o and no a g g r e g a t i o n e f f e c t s . A f t e r o b t e n t i o n o f t h e a d s o r p t i o n e q u i 1 i b r i u m t h e surnagent s o l u t i o n was t h e n e l i m i n a t e d by f i l t r a t i o n , t h e o x i d e
was vacuum d r i e d and f i n a l l y t r a n s f e r r e d under n i t r o g e n p r e s s u r e i n t h e p h o t o - a c o u s t i c c e l l . E l u t i o n o f t h e samples and o x i d e s s u r f a c e s measurements have shown t h a t t h e s u r f a c e c o n c e n t r a t i o n o f t h e samples i s l e s s t h a n a monolayer and t h u s t h a t t h e p o r p h y r i n i s adsorbed i n monomeric form.
The p h o t o a c o u s t i c s p e c t r a have been t a k e n i n a EDT i n s t r u m e n t . We have checked ( 2 0 ) t h a t t h e r e was no s a t u r a t i o n a t t h e f r e q u e n c y o f 80 Hz w h i c h has been used f o r a l l t h e s p e c t r a . The e x p e r i m e n t a l s p e c t r a l r e s o l u t i o n was 4 nm w h i c h was n o t a l i m i t a t i o n f o r t h e o b s e r v a t i o n s .
I n s o l u t i o n , t h e v a l e n c y o f t h e c o b a l t atom i n c o b a l t t e t r a p h e n y l p o r p h y r i n i s 11. The o p t i c a l spectrum, o f COII TPP c o n s i s t s i n a S o r e t band a t 415 nm and a broad v i s i b l e band a t 540 nm (18). Eas o x i d a t i o n may o c c u r t h r o u g h v a l e n c y change o f t h e c e n t r a l metal atom. The ( C o l ~ f TPP)+ has a r e d s h i f t e d o p t i c a l spectrum w i t h a S o r e t band a t 438 nm and two v i s i b l e bands a t 550 nm and 590 nm ( 19).
The p h o t o a c o u s t i c spectrum o f CoTPP adsorbed on t h e v a r i o u s o x i d e s i s t h e s u p e r p o s i t i o n o f t h e p o r p h y r i n and s u b s t r a t e a b s o r p t i o n s . We have observed t h a t , i n t h e adsorbed s t a t e , t h e c o b a l t p o r p h y r i n has t h e same b e h a v i o r as i n s o l u t i o n : o x i d a t i o n may t a k e p l a c e v i a t h e m e t a l atom. The COII TPP spectrum i s observed when t h e p o r p h y r i n i s adsorbed on A1203 and Ta 0 I t c o n s i s t s i n a S o r e t band a 418 nm and a v i s i b l e band a t 540nm ( f ~ g . 1 ) . on2 5 ' ~ i 0 and ZrO s u b s t r a t e s ,
f f
t h Co ITPP spectrum i s i n i t i a l l y observed. However, p h o t o - o x i g a t i o n o c g u r s as Co ITPP t r a n s f o r m s i n ( c o I I I T P P ) + upon e x c i t a t i o n o f t h e p o r p h y r i n as made e v i d e n t on f i g . 2. The i r r a d i a t i o n has been done by 5s s t e p s w i t h an XBO 450 W lamp f i l t e r e d i n such a way t h a t o n l y l i g h t a t h i g h e r wavelength t h a t 400 nm c o u l d pass.
When t h e p o r p h y r i n i s adsorbed on a l l t h e o t h e r o x i d e s t h a t i s : SnO Nb205. Ti02. ZnO, S r T i O3 b o t h t y p e s p e c t r a ( ~ 0 1 1 TPP and ~ 0 1 1 1 TPP+) a r e con&- m i t a n t l y observed ( f i g . 3). I n a l l t h e s e cases p h o t o - o x i d a t i o n i s n o t observed.
We have tempted t o i n t ~ r n r e t e t h e o b s e r v a t i o n s by r e l a t i n g t h e e n e r a y l e v e l of t h e semi c o n d u c t o r s w i t h t h e o x i d a t i o n p o t e n t i a l o f t h e p o r p h y r i n .
The bands e n e r g i e s o f t h e o x i d e s a r e i n d i c a t e d on f i g .
+
The f i r s t o x i d a t i o n p o t e n t i a l o f t h e c o b a l t p o r p h y r i n measured i n 0 . 1 M n-Bu NBF /benzo- n i t r i l e s o l u t i o n and c o r r e c t e d f o r e l e c t r o l y t e e f f e c t (19) i s 5.27 4/vac!um (0.52 V/SCE).
T h i s p o t e n t i a l l i e s a t l o w e r e n e r o y t h a n t h e c o n d u c t i o n ban o f a l l t h e I ?
o x i d e s ( f i g . 4 ) . I t i s t h u s d i f f i c u l t t o a c c o u n t f o r o x i d a t i o n o f Co TPP w h i c h i s usual!y i n t e r p r e t e d as i s o e n e r g e t i c e l e c t r o n t r a n s f e r f r o m t h e p o r p h y r i n t o t h e semi c o n d u c t o r c o n d u c t i o n band. Thus we propose, i n o r d e r t o i n t e r p r e t e t h e o b s e r v a t i o n s t h a t t h e o x i d a t i o n p o t e n t i a l o f t h e cobal t - p o r p h y r i n i n adsorbed s t a t e l i e s a t h i g h e r energy t h a n i n s o l u t i o n .
We have t e n t a t i v e l y p o s i t i o n e d t h i s o x i d a t i o n p o t e n t i a l w i t h t h e f o l l o w i n g hypotheses :
-
t h e p o r p h y r i n o x i d a t i o n p o t e n t i a l i n adsorbed s t a t e does n o t depend on t h e o x i d e ,-
bhe o x i d a t i o n p o t e n t i a l o f t h e e x c i t e d p o r p h y r i n d i f f e r s f r o m t h e ground s t a t e one o n l y by t h e energy d i f f e r e n c e between t h e ground s t a t e and t h e f i r s t e x c i t e d s i n g l e t s t a t e ,-
ground s t a t e o x i d a t i o n as w e l l as e x c i t e d s t a t e o x i d a t i o nJOURNAL DE PHYSIQUE
I . . . . . L I .
5a) 400 5cll
fm
nm 400 500 hOO..
F i g . 1 PA Spe.c&~m 04 Co TPP Fig. 2 : P h o t o o x i d a ~ o n
0 3
Co TPPadhotbed o n A l p O3 when adbotbed on Sio2
o c c u r v i a an i s o e n e r g e t i c t r a n s f e r from t h e p o r p h y r i n t o t h e c o n d u c t i o n band o f t h e semi c o n d u c t o r a t t h e eneray of h i a h e s t d e n s i t y of s t a t e s ,
-
no band bendinp i s c o n s i d e r e d as t h e o b s e r v a t i o n s have been done i n absence o f e l e c t r o l y t e .W i t h these hypotheses and c o n s i d e r i n a t h a t Co TPP i s p h o t o o x i d i z e d when adsorbed on SiO and ZrO
,
we may p o s i t i o n t h e o x i d a t i o n p o t e n t i a l o f e x c i t e d Co TPP a t 2.5 ~ / v a c u $ m . ~ a k i n g i n t o account t h a t t h e eneroy d i f f e r e n c e between t h e ground and t h e e x c i t e d s t a t e o f Co TPP i s 2 e.V. we may p o s i t i o n t h e ground s t a t e o x i d a t i o n p o t e n t i a l of Co TPP a t 4.5 e.V./vacuum which i s a b o u t 0.8 e.V.h i g h e r t h a n i n s o l u t i o n . I t i s known t h a t chanoes i n s o l v a n t p o l a r i t y may i n d u c e s h i f t s i n t h e s o l u t i o n o x i d a t i o n p o t e n t i a l up t o 0.5 V. Thus i t i s n o t s u r p r i s i n n t o f i n d such an i m p o r t a n t chanae when a o i n o from s o l u t i o n t o adsorbed s t a t e . The ground s t a t e o x i d a t i o n e v a l u a t e d i n such a way l i e s a t h i a h e r enerav t h a n t h e c o n d u c t i o n l e v e l o f SnO
.
Nb20S,kTiOT, ZnO, S r T i 0 3 and 1~10~. I t i s t h u su n d e r s t a n d a b l e t h a t o x i d a t i g n may a e p ace i n t h e "round s t a t e when t h e p o r p h y r i n i s adsorbed on t h o s e s u b s t r a t e s . However, i t i s n o t complete and i s a l ~ o s t i n e x i s - t a n t i n t h e case o f Ta205 as t h e c o n d u c t i o n band o f t h i s l a t t e r l i e s a t s l i g h t l y h i @ h e r enerpy.
a u J m
@" SCE
1.
----CoTPP4
O d S O - t d
- - - -
-
- . - -- CoTFP
I
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--- - - - - - - - - - -CB c o w6
- - -
- - -- - - sdhon
2. - - . - - - - -.-FL
7. - -
3. 3
-
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-A -VB
F i g . 4
YU) 400 500 600 n m
The A1 0 c o n d u c t i o n band l i e s a t such a h i g h eneroy ( f i n . 4) t h a t n e i t h e r e x c i t e d s t a t e o x i & a $ i o n n o r ( a f o r t i o r i ) ground s t a t e o x i d a t i o n o f t h e p o r p h y r i n i s p o s s i b l e .
C l a s s i c a l P A S a l l o w e d us t o d i r e c t l y i n v e s t i p a t e t h e process o f s e n s i - t i z a t i o n o f an e l e c t r o d e by a p o r n h y r i n . However, t h i s t e c h n i q u e does n o t a p p l y f o r t h e s t u d y o f t h e e l e c t r o d e i n t h e r e a l e l e c t r o c h e m i c a l c o n d i t i o n s t h a t i s , i n presence o f an e l e c t r o l y t e . F o r such an aim, m i r a o e e f f e c t d e t e c t i o n i s much more a p p r o p r i a t e (15, 16, 20) as i t a l l o w s U i n s i t u " o b s e r v a t i o n o f t h e s e n s i t i z e d e l e c t r o d e .
11
- !odlflcatlon-of-?dsorkd-I1HH2 g2 as_-a-funct~on-of-theeacIilty-of-Lhe
surnanent e l e c t r o l y t e
---
The samples have be n p r e p a r e d i n t h e same way a s f o r t h e PPS i n v e s t i - g a t i o n s on Co T P P , u s i n p a 10-
5
M t e t r a p h e n y l p o r p h y r i n l m e t h y l e n c h l o r i d e s o l u t i o n i n c o n t a c t w i t h a s i l i c a a e l powder. A f t e r t h e a d s o r p t i o n was completed, t h e powder was d e p o s i t e d i n 1 m l a y e r i n a 2 cm o p t i c a l c u v e t t e and covered by a 5 mm l a y e r o f e l e c t r o l y t e .WNSlTCU P . Ro.. B l n g o l ) ABSORPTION SPECTRUM
I !
J?:
a c i d i c mediwnJOURNAL DE PHYSIQUE
We have used a commercial P.A. E.D.T. i n s t r u m e n t f o r i r r a d i a t i o n and s i g n a l t r e a t m e n t and j u s t r e p l a c e d t h e PA c e l l by a home made m i r a a e
e f f e c t system w i t h a LS 52 l a s e r o p t r o n i c double p h o t o d i o d e p o s i t i o n d e t e c t o r and a 1 mV He Ne l a s e r .
I n o r d e r t o v e r i f y t h e dependence o f t h e m i r a a e s i g n a l on t h e a b s o r p t i o n c o e f f i c i e n t o f an heterogeneous sample, we have f i r s t i n v e s t i a a t e d " s e n s i t o x powder" w h i c h i s o b t a i n e d by c o v a l e n t l y a t t a c h i n ? Rose Benoal on a non a b s o r b i n g polymer powder. F i g . 5 shows t h e i d e n t i t y o f t h e s p e c t r a o f d r y s e n s i t o x s t u d i e d by PAS and o f s e n s i t o x i n w a t e r s t u d i e d by m i r a s e e f f e c t . Such s p e c t r a a r e v e r y s i m i l a r t o t h o s e observed f o r Rose Bengal i n s o l u t i o n .
On t h e c o n t r a r y , i n t h e case o f t e t r a p h e n y l p o r p h y r i n , d r a s t i c change o c c u r s when g o i n g f r o m m e t h y l e n c h l o r i d e s o l u t i o n t o adsorbed s t a t e on s i l i c a g e l as shown on f i g . 6. T h i s change i s i n t e r p r e t e d as a p r o t o n a t i o n o f t h e p o r p h y r i n due t o t h e a c i d i c groups o f t h e s i l i c a g e l . Such a p r o t o n a t i o n l e a d i n g t o t h e d i c a t i o n H T P P ~ + has been observed when t h e p o r p h y r i n i s d i s s o l v e d i n a c i d i c s o l u t i o n s ( f i g . 87 o r adsorbed on m e n t m o r i l l a n i t e ( 2 1 ) . I t i s d e t e c t e d by t h e r e d s h i f t and an i n t e n s i t y decrease o f t h e S o r e t band and t h e development o f a broad v i s i b l e band around 660 nm. The d i c a t i o n formed when a d s o r b i n g TTPH2 on s i l i c a p e l i s v e r y s t a b l e a g a i n s t oxygen and l i g h t d e g r a d a t i o n .
When t h e TTPHP/silica g e l samble ( f i g 7 A ) . i s immersed i n d i s t i l l e d v:ater- d e p r o t o n a t i o n i m m e d i a t e l y o c c u r s as snow,, on f i g . 7 B . When a c i d i f y i n g t h e water, t h e p r o t o n a t e d form i s s t i l l observed. However, due t o t h e occurrence o f b o t h p o r p h y r i n / s u b s t r a t e and p o n p h y r i n / e l e c t r o l y t e i n t e r a c t i o n s t h e spectrum i s broadened and s h i f t e d r e l a t i v e l y t o t h e d i c a t i o n s o l u t i o n spectrum. Furthermore, i n t h e s e c o n d i t i o n s the c a t i o n i s r a p i d l y d e s t r o y e d i n presence o f oxyaen. (fig7C,7D) CONCLUSION
P h o t o a c o u s t i c spectroscopy has a l l o w e d t o i n v e s t i g a t e t h e o x i d a t i o n p o t e n t i a l o f a s e n s i t i z e r (Co TPP) adsorbed on an e l e c t r o d e and t h i s i t appears as a u s e f u l spectroscopy f o r s e n s i t i z a t i o n s t u d i e s . However, "miraqe e f f e c t d e t e c - t i o n " appears as a more p r o m i s i n g t e c h n i q u e as i t a p p l i e s t o " i n situ"measurements i n presence of an e l e c t r o l y t e .
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