Photoeffects at Semiconductor-Electrolyte Interfaces - American

3 Rate of Reduction of Photogenerated, SurfaceConfined Ferricenium by Solution Reductants Derivatized n-Type Silicon Photoanode-Based Photoelectrochemical Cells

Downloaded by MICHIGAN STATE UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch003

N A T H A N S. LEWIS and MARK S. WRIGHTON

1

Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139

N-type semiconductors can be used as photoanodes in electrochemical cells (1, 2, 3), but photoanodic decomposition of the photoelectrode often competes with the desired anodic process (1, 4, 5). When photoanodic decomposition of the electrode does compete, the u t i l i t y of the photoelectrochemical device is limited by the photoelectrode decomposition. In a number of instances redox additives, A, have proven to be photooxidized at n-type semiconductors with essentially 100% current efficiency (1, 2, 3, 6, 7, 8, 9). Research in this laboratory has shown that immobilization of A onto the photo anode surface may be an approach to stabilization of the photo anode when the desired chemistry i s photooxidation of a solution species B, where oxidation of Β is not able to directly compete with the anodic decomposition of the "naked" (non-derivatized) photoanode (10, 11, 12). Photoanodes derivatized with a redox reagent A can effect oxidation of solution species Β according to the sequence represented by equations (1) - (3) (10-15).

+

Thus, A is oxidized by the photogenerated h , and the photogenerated A+surf in turn oxidizes B. By such a mechanism, the photooxidation of Β is possible for wavelengths of light that w i l l create e - h pairs in the n-type semiconductor (≥ E ) and for processes where the chemistry represented by equation (3) is spontaneous in a thermodynamic sense. The (A /A) reagent system must also result in a suppression of the anodic decomposition, equation (4), in order to achieve surf

-

+

g

+

surf

1

Address correspondence to this author. 0097-6156/81/0146-0037$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

38

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

a durable i n t e r f a c e f o r p h o t o o x i d a t i o n when the naked e l e c t r o d e undergoes photoanodic decomposition i n the presence of B . Large values of k are needed to achieve photoanode-based c e l l s having h i g h e f f i c i e n c y . Measurement of k can be made d i r e c t l y , s i n c e d e r i v a t i z e d photoanodes are two s t i m u l i response systems (10-16). O x i d a t i o n of A ^ r e q u i r e s 2l g l i g h t and some e l e c t r o d e p o t e n t i a l , E f . îîowever, r e d u c t i o n of ^surf. ^ y P semiconductor only r e q u i r e s a s u f f i c i e n t l y negative p o t e n t i a l . This two s t i m u l i response ( l i g h t and p o t e n t i a l ) allows e v a l u a t i o n of the r a t e constant k by measuring the time dependence of the A * f c o n c e n t r a t i o n i n the dark i n the presence o f Β (16) . The A+ ^. c o n c e n t r a t i o n can be monitored by e i t h e r a negative sweep or step"from the p o s i t i v e p o t e n t i a l needed i n the photogeneration of A ^ ^ f , equation (5) , and e t

e t

E

o

n

β

n _ t

e

e t

u r


U

e"

+ A

f

+

. — A . (5) surf. surf. i n t e g r a t i n g the charge passed i n the r e d u c t i o n o f A g . Separation o f the charge passed corresponding t o r e d u c t i o n o f A g f from that a s s o c i a t e d w i t h r e d u c t i o n of B+ can be accomplished by moving B away from the photoanode by s t i r r i n g . The key f a c t i s t h a t even though the p o s i t i v e l i m i t may be more p o s i t i v e than E°(A+ /A) . , regeneration o f A g ~ f after r e a c t i o n according to equation ( 3 ) , w i l l not occur i n the dark. Thus, the consumption o f Ag" f by Β i s d e t e c t a b l e by the d e c l i n e i n Ag" f c o n c e n t r a t i o n (16) measured by cathodic current a s s o c i a t e d w i t h i t s r e d u c t i o n a f t e r a r e a c t i o n time t-j_ a t s p e c i f i e d c o n d i t i o n s . D i r e c t monitoring of A g " f c o n c e n t r a t i o n i n t h i s sense i s not p o s s i b l e on a r e v e r s i b l e e l e c t r o d e , such as Pt o r Au, s i n c e the ( A / A ) f r a t i o depends only on Ef, I f ^ s u r f . does e f f e c t chemistry according t o equation ( 3 ) , the A|urf. i s regenerated t o an extent that depends only on Ef (16). However, i n d i r e c t procedures f o r e v a l u a t i n g k do e x i s t f o r r e v e r s i b l e e l e c t r o d e s , p a r t i c u l a r l y when the o x i d a t i o n o f Β a t the naked e l e c t r o d e occurs a t a n e g l i g i b l e r a t e compared to the r a t e a t an e l e c t r o d e d e r i v a t i z e d w i t h the ( A / A ) f system (17). I n photoelectrochemical c e l l s h i g h e f f i c i e n c y depends on having a h i g h quantum y i e l d f o r e l e c t r o n flow, Φ , a t a l l l i g h t i n t e n s i t i e s t o be used. I f k i s too small, Φ may be l e s s than u n i t y because back e l e c t r o n t r a n s f e r , equation ( 5 ) , can compete when E f i s s u f f i c i e n t l y negative. Negative values o f E f are d e s i r a b l e , s i n c e the extent to which A f -»• A+ p can be d r i v e n u p h i l l w i t h l i g h t , and here Β -> B , depends on E {!) . F u r t h e r , if k i s too s m a l l , d i r e c t e" - h recombination, equation ( 6 ) , + 6 e - h y heat and/or l i g h t (6) may occur when the A f Ag f conversion i s complete. When k i s l a r g e , back e l e c t r o n t r a n s f e r and recombination can s t i l l be competitive i f the c o n c e n t r a t i o n o f Β i s too low. I f B i s present back r e a c t i o n , equation ( 7 ) , can c o n t r i b u t e t o u r f

u r

β

+

Q11T

ur

f

β

ur

ur

β

ur

e

+

s u r

β

e t

+

s u r

e t

e

θ

s u r

#

ur;

+

f

+

e t

k

g u r

u r

e t

+

+

A

, + Β surf.

k

7

+

> A * + Β surf, A

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

(7)

3.

LEWIS AND WRIGHTON

39

Reduction of Ferricenium

a low v a l u e of Φ . L i g h t i n t e n s i t y i s a c o n s i d e r a t i o n f o r two reasons. F i r s t , the r a t e o f e" - h recombination i s a " b i m o l e c u l a r " process whereas the other e" o r h processes are "unimolecular"; Φ might be lower at h i g h e r l i g h t i n t e n s i t y . Second, when Β i s being consumed according to equation (3) i t can only be r e p l e n i s h e d at the i n t e r f a c e at a mass t r a n s p o r t c o n t r o l l e d r a t e ; the e x c i t a t i o n r a t e can exceed the mass t r a n s p o r t r a t e r e s u l t i n g i n a low s t e a d y - s t a t e value of Φ . In t h i s a r t i c l e we wish to a m p l i f y on our previous s t u d i e s (10-16) of d e r i v a t i z e d photoanode s u r f a c e s by r e p o r t i n g new r e s u l t s r e l a t e d to the measurement of k f o r η-type S i photoanodes d e r i v a t i z e d w i t h ( l , l - f e r r o c e n e d i y l ) d i c h l o r o s i l a n e , I . We r e p o r t that a number of s o l u t i o n reductants Β can be o x i d i z e d β

+

+

β

e t


f

Fe

SiCl

2

I by the photogenerated, s u r f a c e - c o n f i n e d f e r r i c e n i u m w i t h a value of k that exceeds 6 χ 10& cm^ mol"-*- s ~ l a t 298 K. Larger values of k cannot be measured owing to d i f f i c u l t i e s a s s o c i a t e d w i t h mass t r a n s p o r t c o n t r o l l e d r a t e s . This would correspond t o a homogeneous b i m o l e c u l a r r a t e constant o f >6 χ 10^ M""-*- s ~ l . In p r a c t i c a l terms t h i s means t h a t k i s l a r g e enough to y i e l d a good value of Φ at s o l a r i r r a d i a t i o n i n t e n s i t i e s and a t g e n e r a l l y a c c e s s i b l e concentrations of B. However, the extent to which the o x i d a t i o n of Β can be d r i v e n u p h i l l , Ey, i s g e n e r a l l y modest (0.4 - 0.5 V a t o p e n - c i r c u i t ) compared t o Eg = 1.1 eV f o r S i . Small values of Ey g i v e low o v e r a l l o p t i c a l energy conversion e f f i c i e n c y . e t

e t

e t

θ

Background and Working Hypothesis N-type S i d e r i v a t i z e d w i t h I i s b e l i e v e d to have the i n t e r f a c e s t r u c t u r e and e n e r g e t i c s represented by Scheme I (11). Taking E ° ( F e C p / 0 ) to be +0.43 V v s . SCE i n EtOH from measurements f o r Au or Pt e l e c t r o d e s d e r i v a t i z e d w i t h I , (18, 19) Ey f o r the ( F e C p ° ) . + ( F e C p ) s u r f . i d a t i o n can be up to -0.60 V (10). That i s , ' t h e value o f E of the photoanode where the ( ^ C P 2 ^ ) f r a t i o i s one i s ~0.60 V more negative than f o r Au or Pt i n the best cases. More t y p i c a l l y , the value of Ey i s 0.3-0,4 V as shown i n F i g u r e 1 f o r e l e c t r o d e s (Pt vs. i l l u m i n a t e d η-Si) c h a r a c t e r i z e d by c y c l i c voltammetry. +

2

surf#

+

2

surf

2

o x

f

e

+

s u r


PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

(-) C o n d u c t i o n Band Ε

=

-0.3V

CB


E°(FeCp^

/ 0

)

s u r f !

=

+0.43V

t (+) Scheme I

I n t e r f a c e e n e r g e t i c s f o r an η-type S i photoanode a t the f l a t - b a n d c o n d i t i o n showing the formal p o t e n t i a l f o r a s u r f a c e - c o n f i n e d f e r r i c e n i u m / f e r r o c e n e reagent r e l a t i v e to the p o s i t i o n of the top of the valence band,E^^,and the bottom o f t h e conduction band,E_ , at the i n t e r f a c e between the S i s u b s t r a t e and the r e d o x / e l e c t r o l y t e system. I n t e r f a c e e n e r g e t i c s apply to an EtOH/0.1 M [n-Bu^N^ClO^ e l e c t r o l y t e system. B

0.0

+0.4

P O T E N T I A L , V vs

SCE

Figure 1. Typical comparison of cyclic voltammetry (100 mV/s) for Pt vs. n-type Si in CH CN/0.1M [n-Bu^ClOj, derivatized with (1 J -ferrocenediyl)dichlorosilane. f

3

The Pt exhibits a reversible wave in the dark whereas the η-type Si exhibits no oxidation current unless illuminated with > E light (632.8 nm, ~ 50 mW I cm ). The photoanodic peak is more negative than the anodic peak on Pt, reflecting the extent to which ferrocene -> ferricenium can be driven uphill (380 mV in this case). For η-Si, ( ) represents the reduction when the light is not switched off at the +0.6 V positive limit; ( ) on the cathodic sweep corresponds to the dark reduction. 2

g


3.

LEWIS AND WRIGHTON

41


The main p o i n t i s that a t a photoanode p o t e n t i a l o f Ï~0,4 V v s . SCE the ( F e C p 2 / ° ) f . r a t i o i s t y p i c a l l y >10 and the a v a i l a b l e oxidant i s (FeCp2 )surf.· Thus, any m a t e r i a l Β t h a t i s o x i d i z a b l e w i t h (FeCp2: ) s o l u t i o n should be o x i d i z a b l e w i t h the η-type S i photoanode d e r i v a t i z e d w i t h I (10-16). N-type S i d e r i v a t i z e d w i t h I can be used i n ^ O / e l e c t r o l y t e s o l u t i o n , u n l i k e the naked η-type S i that i s r a p i d l y p a s s i v a t e d by photoanodic growth of an oxide l a y e r on the surface (10, 11, 12, 16). As a guide t o understanding the heterogeneous o x i d a t i o n o f Β by ( F e C p 2 ) u r f . > adopt the theory of Marcus (20, 21). However, we underscore the f a c t that redox r e a c t i o n o f the s u r f a c e - c o n f i n e d redox system, l i k e the s o l u t i o n system, i s accompanied by s o l v a t i o n changes (16). Heterogeneous e l e c t r o n t r a n s f e r a t a naked e l e c t r o d e need not i n v o l v e an e l e c t r o d e s o l v a t i o n term. Q u a l i t a t i v e l y , Marcus p r e d i c t s that k w i l l be l a r g e when the (B+/B) and FeCp2 /0) self-exchange are f a s t and when the d r i v i n g f o r c e i s l a r g e . +

sur

+

i

+

w

n

e


S

e t

+

Results Solvent Dependence of I O x i d a t i o n In an e a r l i e r study, (16), we found the r a t e law represented by equation (8) t o be a p p r o p r i a t e f o r B=I~ i n EtOH o r H2O s o l v e n t . Rate

=

k

e t

t(

F e C

2

P

+

B

>

2 1

U n i t s A n a l y s i s : Rate, molcm" s~ ; k r

[(FeCp

+ 2

)

s u r f e

L

molcm

( 8 )

rf.^ ^

S U

—2

1

, cm^ol^s" ;

^

; [ B ] , molcni

We assume the same r a t e law t o g e n e r a l l y apply when Β i s a one-electron reductant. The a b i l i t y to prove the r a t e law f o r B=I"~ stems from a r a t h e r low value o f k , Table I . I n t e r e s t i n the photochemical o x i d a t i o n o f I " t o I ^ ~ f o r energy storage purposes and the ~ 1 0 - f o l d d i f f e r e n c e i n k i n EtOH v s . H2O prompted us t o determine k f o r B=I"~ f o r s e v e r a l other s o l v e n t s . Values o f k , E ^ ( I ~ ) , and E ° ( F e C p 2 / ° ) f . i n the v a r i o u s s o l v e n t s used are given i n Table I . The E ^ ( I " " ) values are data from the l i t e r a t u r e (22) and our own measurements, and the E ° ( F e C p 2 ^ ) r f . from the c y c l i c voltammetry peak p o s i t i o n s f o r P t e l e c t r o d e s f u n c t i o n a l i z e d w i t h I i n the v a r i o u s s o l v e n t s . Together, these data provide i n f o r m a t i o n concerning the d r i v i n g f o r c e f o r the o x i d a t i o n o f I by (FeCp2 )surf. various solvents. Values o f k were determined as p r e v i o u s l y reported f o r H 0 o r EtOH s o l v e n t (16). The d e r i v a t i z e d e l e c t r o d e i s f i r s t c h a r a c t e r i z e d by c y c l i c voltammetry i n s o l v e n t / e l e c t r o l y t e s o l u t i o n without added I " . The value of k i s then determined from the time dependence of the s u r f a c e - c o n c e n t r a t i o n o f (FeCp2 )surf. i n the presence o f v a r i a b l e I " c o n c e n t r a t i o n and as a f u n c t i o n of s o l v e n t . The ( F e C p 2 ) u r f . generated i n a l i n e a r p o t e n t i a l sweep from -0.6 to +0.5 V vs. SCE w h i l e the e t

e t

e t

+

e t

o x d n

sur

O X ( i r i e

+

a

r

e

s u

+

i

n

t

n

e

e t

2

e t

+

+

i

s

S



42


Table I . Reduction of Surface-Confined Ferricenium by Iodide i n Various Solvents a t 298 K. P o t e n t i a l , V vs. SCE Solvent/Electrolyte EtOH/[n-Bu N]ci0

E°(FeCp

+/0,a ) 1

2

, -,b E^(I ) w

T

k

e t >

3^-1-1° cm mol s 4

0.43

0.42

3 χ 10 '

Η 0 (pH=1.3)/NaC10

0.35

0.30

Glacial Acetic Acid/[n-Bu N]C10

0,42

0.42

1 χ 10 4 3 χ 10

EtOH/Toltiene (1/1)/-

0.44

0.44

.4 3 χ 10

0.48

0.40

0.50

0,32

4

4

4

4

[n-Bu N]C10 4

4

CH Cl /£n-Bu N]C10 2

2

3

4

4

CH.CN/In-Bu.NJC10/

6 m 1Q 8 >6 χ 10 4

e

Formal p o t e n t i a l f o r surface-confined (FeCp ) as determined by slow sweep c y c l i c voltammetry f o r P t e l e c t r o d e s d e r i v a t i z e d w i t h ( 1,1 -ferrocenediyl)dichlorosilane. T

^Data are quarter wave p o t e n t i a l s f o r I*~ o x i d a t i o n reported i n r e f . 22 and measured a t P t , see Experimental. °Heterogeneous e l e c t r o n t r a n s f e r r a t e constant, see equation (8) i n t e x t , f o r I o x i d a t i o n determined as i n r e f . 16. ^Data from r e f , 16. e

+

Assuming [ ( F e C p ) 2

s

u

r

f ] = 10

mol/cm^.


3.

43


LEWIS AND WRIGHTON

e l e c t r o d e i s i l l u m i n a t e d a t a s u f f i c i e n t l y high 632.8 nm l i g h t i n t e n s i t y (-50 mW/cm ) t o i n s u r e that the ( F e C p 2 / 0 ) f ratio at +0.5 V vs. SCE i s >10/1. The l i g h t i s then switched off at the p o s i t i v e l i m i t . Concentration of (FeCp2 )surf. * » t±> i n the dark i s then determined by h o l d i n g the p o t e n t i a l at +0.5 V vs. SCE f o r a time, t and then sweeping the p o t e n t i a l a t >300 mV/s t o -0.6 V vs. SCE w h i l e monitoring cathodic current corresponding t o (FeCp2 )surf. r e d u c t i o n . Another way t o vary r e a c t i o n time, t-^, i s t o simply use no delay a t the p o s i t i v e l i m i t and to vary the sweep r a t e on the negative scan. This method has been used as our r o u t i n e method of determining k (16). E q u i v a l e n t data were a l s o obtained by measuring [ ( F e C p 2 ) f . ] by doing a p o t e n t i a l step from +0.5 V t o -0.6 V v s . SCE and i n t e g r a t i n g c u r r e n t . S o l u t i o n s are s t i r r e d t o remove o x i d i z e d product, Ιβ", from the v i c i n i t y of the e l e c t r o d e . +

s u r

+

i

v s

t i m e

5

+

e t

+


s u r

P l o t s of l n { [ F e C p 2 ] o / t P 2 ] t = t * ± with a slope = k b l [ l ~ ] . C o n s i s t e n t l y , the slopes are d i r e c t l y proportional to Since k = k [(FeCp f . L the value o f k i s given by d i v i d i n g k ^ ^ by s u r f a c e coverage. By h o l d i n g the e l e c t r o d e at +0.5 V vs. SCE and i r r a d i a t i n g w i t h a s u f f i c i e n t l y high l i g h t i n t e n s i t y t o i n s u r e that e x c i t a t i o n r a t e i s not r a t e l i m i t i n g , the s t e a d y - s t a t e photocurrent should be p r e d i c t a b l e u s i n g equation ( 8 ) . Indeed, using the surface coverage, t y p i c a l l y ~10~^ mol/cm^, and the k s given i n Table I c a l c u l a t e d and observed s t e a d y - s t a t e photocurrents are i n good agreement. For CH3CN solvent we can only p l a c e a lower l i m i t on the value of k . Using a d e r i v a t i z e d r o t a t i n g d i s k photoelectrode we f i n d that the s t e a d y - s t a t e photocurrent at +0.5 V vs. SCE i s d i r e c t l y p r o p o r t i o n a l t o ω^, the square-root of the r o t a t i o n v e l o c i t y . Such a dependence i s expected when the l i m i t i n g current i s c o n t r o l l e d by mass t r a n s p o r t (23, 24, 25) and not by the electron transfer rate ( i . e . k i s very l a r g e ) . For CH3CN s o l v e n t , the s t e a d y - s t a t e photocurrent i s independent of coverage of [ ( F e C p ) s u r f . ] 5 expected f o r a mass t r a n s p o r t c o n t r o l l e d r a t e . But f o r every other solvent system i n v e s t i g a t e d we f i n d values of k that are s m a l l compared to what would be expected f o r mass t r a n s p o r t l i m i t e d c u r r e n t s ; steady-state c u r r e n t s do not depend on ω or whether the s o l u t i o n i s s t i r r e d when the current i s not mass t r a n s p o r t l i m i t e d . U n f o r t u n a t e l y , the l a r g e value of k associated with I o x i d a t i o n i n CH3CN i s accompanied by an unstable i n t e r f a c e . For reasons that we do not p r e s e n t l y understand, the η-type S i e l e c t r o d e s d e r i v a t i z e d w i t h I are not durable enough i n CH^CN s o l u t i o n t o s u s t a i n I ~ o x i d a t i o n f o r prolonged periods o f time. However, the e l e c t r o d e s do s u r v i v e long enough t o e s t a b l i s h that the r a t e of I"" o x i d a t i o n i s l i m i t e d by mass t r a n s p o r t . At the highest ω from our 200p r.p.m. motor our s t r i c t l y l i n e a r p l o t s of l i m i t i n g current vs. ω e s t a b l i s h that the heterogeneous r a t e constant, k [(FeCp2 )surf.]> _>0.06 cm/sec (16). Thus, i f [(FeCp2 ) s u r f . ] ^ ^Vcm2, which approximates a 'monolayef' of reagent exposed to the s o l u t i o n , k i s ^ 6 χ 10 cm^mol~~^s~l. F e C

+

+

v s

t

a

r

e

l

i

n

e

a

r

t =

0

s v c

+

o b s v d

t

e t

0

2

s

u

r

s v

T

e t

e t

e t

+

a

s

2

e t

e t

2

+

i

s

e t

+

s

e t


44


Reduction of S u r f a c e - F e r r i c e n i u m by One-Electron Reductants I n our e a r l i e r work, we showed that a number o f reagents could be o x i d i z e d according to equations (1) - (3) where k is so l a r g e that the r a t e i s l i m i t e d by mass t r a n s p o r t (16), Thus, as f o r Β = IT i n CH CN,k i s >6 χ 1 0 cm^mol^is-l f o r an e f f e c t i v e s u r f a c e coverage of - 1 0 " mol/cm . The reagents p r e v i o u s l y s t u d i e d (16), Β = F e ( n - C H c ) , F e C n ^ ^ H ^ M e ) , F e ( n - C H ) ( n 5 - C H P h ) , Fe(r| -indenyl) , and [Fe(CO) ( n - C H ) ] , are a l l one-electron reductants that have f a s t self-exchange r a t e s and should, t h e r e f o r e , reduce ( F e C p 2 ) f . r a p i d l y . Table I I l i s t s some of our e a r l i e r data along w i t h i n f o r m a t i o n for s e v e r a l other systems t h a t we have now determined to have l a r g e values of k . F i g u r e 2 shows the s o r t of d i r e c t evidence that shows t h a t ( F e C p 2 ) f . can o x i d i z e s o l u t i o n reductants. The f i g u r e shows c y c l i c voltammograms f o r the d e r i v a t i z e d e l e c t r o d e i n the s t i r r e d EtOH/0.1 M n-Bu^N CIO4 e l e c t r o l y t e s o l u t i o n f i r s t i n the absence of Β and then i n the presence of Β = Co(bipy)3^ at 3.0 mM, The photocurrent a t tjie p o s i t i v e l i m i t of +0.3 V vs. SCE i s d i r e c t l y p r o p o r t i o n a l to i n the presence of 3.0 mM Co(bipy)3 . The c a t h o d i c current a s s o c i a t e d w i t h ( F e C p 2 ) f . ( P2°) urf. on the negative sweep i n the dark from +0.3 V v s . SCE i s completely absent i n the presence of 3.0 mM C o ( b i p y ) < ^ a t the scan r a t e used. The l a c k of a r e t u r n wave f o r the ( F e C p 2 ) f "** (FeCp °) s u r f , r e d u c t i o n d i r e c t l y evidences complete r e d u c t i o n of the (FeCp2^) urf. by C o ( b i p y ) 3 ^ . L i n e a r p l o t s of l i m i t i n g current vs. establish k to be >6 χ 10 cm m o l ' ^ s ' l . Representative data f o r e q u i l i b r a t i o n of (FeC]>2 )surf. s o l u t i o n ferrocene are shown i n F i g u r e 3 where the l i m i t i n g c u r r e n t v a r i e s l i n e a r l y w i t h s o l u t i o n ferrocene c o n c e n t r a t i o n a t a f i x e d ω and v a r i e s l i n e a r l y w i t h ur2 f o r a f i x e d s o l u t i o n ferrocene c o n c e n t r a t i o n . Such data have been obtained f o r a l l of the couples l i s t e d i n Table I I and f o r I " i n CH3CN. The measurement of the mass t r a n s p o r t r a t e constant by monitoring E ( E e C p 2 ) f . ] vs. t ^ i n the presence of the v a r i o u s f a s t reductants g e n e r a l l y gives a value that does not accord w e l l w i t h the s t e a d y - s t a t e photocurrents. This s i t u a t i o n r e s u l t s even though the s t e a d y - s t a t e photocurrent d e n s i t y f o r r o t a t i n g , d e r i v a t i z e d d i s k e l e c t r o d e s and the current d e n s i t y at a r o t a t i n g r e v e r s i b l e d i s k e l e c t r o d e (e.g. Pt) are n e a r l y the same f o r a l l reagents when c o r r e c t e d f o r minor d i f f e r e n c e s i n d i f f u s i o n constants. A r e p r e s e n t a t i v e s i t u a t i o n i s shown i n F i g u r e 4 where c y c l i c voltammetry of an η-type S i photoanode, d e r i v a t i z e d w i t h I , i s shown f o r s e v e r a l s i t u a t i o n s . Included are data f o r Β = ferrocene and 1 , l - d i m e t h y l f e r r o c e n e under i d e n t i c a l c o n d i t i o n s . These two s o l u t i o n reductants r e s u l t i n i d e n t i c a l s t e a d y - s t a t e photocurrents at +0.5 V v s . SCE, but as shown i n F i g u r e 4b vs. 4c the 0.5 mM 1,1'-dimethyIferrocene consumes more of the C(FeCp2 ) s u r f ] than does the 0.5 mM ferrocene under i d e n t i c a l c o n d i t i o n s . e t

s

3

et

1 0

2

5

2

5

5

2

5

5

5

5

5

4

2

5

5

+


s u r

et

+

s u r

+

+

FeC

s u r

s

+

+

s u r

2

+

t n e

S

e t

+

w

i

+

s u r

T

+


t

n

4


6

6

J

5

-C H )

2

5

5

5

4

2

5

5

4

+0.34

9

CH C1

2

CH C1

EtOH

EtOH

9

2

sulfolane

2

k

1

X

>6 >6

X

X

X

X

X

X

>6

>6

>6

>6

>6

>6

1

;

;

j

1

ίο

1&

io

{

io

ίο

1

ίο

io

io

10 X

>6 >6

10

X X

>6

, cn^mol^s" *

e t

See equation (8) o f t e x t . A l l k * s here a r e lower l i m i t s , s i n c e observed r a t e i s mass t r a n s p o r t l i m i t e d (e.g. F i g u r e 3 ) , see t e x t .

5

[Fe(CO)(n -C H )]

5

+0.17

5

+0.36

Fe(ri^-indenyl)

2

+0.22

CH C1

+0.46 2

EtOH

EtOH

2

2

H0

H0

EtOH

Solvent

+0.45

I r r e v . Oxdn @+0.2 @ P t

-0.20

+0.20

+0.37

+0.46

4

+

E°(B /B) , V v s . SCE

Fe(n -c H )(n -c H Ph)

5

Fe(n -C H Me)

Fe(n

2

5

2+

2+

(Me dtc)"

3

Ru(NH )

Fe(CN)

3

4

Co(bpy)

Reductant, Β

Table I I . Rate Constants f o r Reduction of S u r f a c e - F e r r i c e n i u m by V a r i o u s Reductants a t 298 K.




Figure 2.

Cyclic voltammetric (100 mV/s) characterization of an η-type Si elec trode. 9

2

(a) Derivatized with (1 ,V-ferrocenediyl)dichlorosilane (5 X 10~ molI cm ) in stirred EtOH/O.lM [n-Bu^ClO^. ( ) the dark current; ( ) the effect of illumination (632.8 nm, ~ 50 mW I cm ) from -0.5 V to +0.3 V. The light is switched off at +0.3 V showing that photogenerated ferricenium can be reduced in the dark on the negative sweep, (b) Same as in (a) except 3.0 m M Co(bipy) Cl is in the stirred electrolyte solu tion. Note enhanced photoanodic current indicating Co(bipy) -> Co(bipy) and the lack of a return wave for ferricenium -» ferrocene showing that Co(bipy) + formation occurs via surface ferricenium + Co(bipy) -» surface ferrocene + Co(bipy) *. 2

3

2

2+

3+

3

3

3

3

2+

3


3

3

LEWIS AND WRIGHTON


47


3.

ν

Figure 3. Photocurrent vs. ω * at +0.5 V vs. SCE from an η-type Si disk deriva tized with 5 X 10~ mol/cm of ferricenium/ferrocene from (1 ,r~ferrocenediyl)dichlorosilane. 9

2

The solution is EtOH/O.lM [n-Bu, N]ClO^/ferrocene and illumination is at 632.8 nm, ~50 W/cm . The strict linearity of the plots shows that oxidation of solution ferrocene is mass transport-limited for all ω used and for each ferrocene concentration used. t

2

American Chemical Society Library 1155 16th St., N.W. Washington, O.C. 20036 In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS


48

A T SEMICONDUCTOR-ELECTROLYTE INTERFACES

Figure 4. Cyclic voltammetry (100 mV/s) in stirred EtOH/O.lM [n-Bu,,N]ClO solutions for η-type Si derivatized with (1J -ferrocenediyl)dichlorosilane (5 X 10 mol/cm ) with illumination at 632.8 nm, ~ 50 mW/cm from negative initial poten tial to the positive limit at -\-0.5 V. h

f

2

9

2

Light is switched off at +0.5 V vs. SCE for the cathodic sweep. In (a) there is no added reductant; (b), (c), and (d) contain 0.5mM ferrocene, 1 ,Γ-dimethylferrocene, and acetylferrocene, respectively. Acetylferrocene does not attenuate the surface ferricenium -> surface ferrocene wave since it is not a sufficiently powerful reductant. Ferrocene and 1 ,Γ-dimethylferrocene both attenuate the surface ferricenium -» surface ferrocene wave. But 1 ,Γ-dimethylferrocene is more effective under identical conditions despite the fact that the same, mass transport-limited, steady-state photocurrent is found for these two reductants. These data suggest that after the light is switched off the reduction of surface ferricenium is controlled partially by mass transport and partly by the electron transfer rate (see text).


LEWIS AND WRIGHTON

3.

49


T

Since the 1,1 -dimethylferrocene consumes more of the (FeCp ) f we conclude that i t i s a f a s t e r reductant than ferrocene. C o n s i s t e n t l y , the r a t i o of cathodic peak areas f o r the reductants examined i s s t r i c t l y maintained f o r a v a r i e t y of scan r a t e s , pulse times t c o n c e n t r a t i o n s of reductant, and s t i r r i n g r a t e s . Using a s i m i l a r technique we order the r a t e s f o r s e v e r a l other reagents: +

2

s u r

β

i

?

F e ( - i n d e n y l ) > [ Fe(CO) (η - C H ) ] ~ Fe(n -C H Me) > 5

5

n

2

5

5

Fe(r|~*-C,_Hj-) £ ~ phenylferrocene

5

4

5

4

2

>> a c e t y l f errocene

The a c e t y l f e r r o c e n e does not consume the (FeCp2 ) urf.» F i g u r e 4d, because the r e a c t i o n i s not ther mo dynamically spontaneous. The c o n f l i c t i n our data i s that s t e a d y - s t a t e photocurrents f o r the f a s t reductants i s the same, but the ( F e C p 2 ) f . ^ consumed at d i f f e r e n t r a t e s i n the dark f o r the v a r i o u s f a s t reductants. The data demand the c o n c l u s i o n t h a t r e d u c t i o n of F e ( C p 2 ) f . can become l i m i t e d p a r t i a l l y by mass t r a n s p o r t and p a r t i a l l y by k a t some p o i n t i n the r e a c t i o n ( l a r g e f r a c t i o n a l consumption of ( F e C p 2 ) f . ) dark. This seems reasonable i n view of the expected r a t e law, equation ( 8 ) , the d e c l i n i n g ; ( F e C p ) s u r f . ] * a constant mass t r a n s p o r t r a t e of f r e s h s o l u t i o n reductant. +


S

+

c

a

n

e

s u r

+

s u r

e t

+

i

n

t

n

e

s u r

+

a n c

2

Discussion Data given i n Table I f o r mediated I ~ o x i d a t i o n r a t e , k , a n d f o r e n e r g e t i c s o f the process E°(FeCp2 /°) urf. - E ^ " ) seem to suggest that both s o l v e n t and d r i v i n g f o r c e f o r r e a c t i o n can i n f l u e n c e k . I n CH^CN where the d r i v i n g f o r c e i s greatest, we f i n d l a r g e values f o r k . However, i t does not seem that 0.18 V (CH CN) v s . 0.08 V (CH C1 ) would account f o r a f a c t o r o f 1 0 i n r a t e , and we conclude t h a t medium e f f e c t s can c o n t r i b u t e t o r a t e v a r i a t i o n as w e l l . I n support o f t h i s c o n c l u s i o n we note that H 0 y i e l d s a r a t e constant about a f a c t o r o f 10 lower than the other s o l v e n t s (EtOH, g l a c i a l a c e t i c a c i d , EtOH/toluene) where the d r i v i n g f o r c e i s even s l i g h t l y s m a l l e r . A c o m p l i c a t i o n i s that self-exchange r a t e s are not known f o r a l l of the media used. This precludes a d e t a i l e d i n t e r p r e t a t i o n o f the data w i t h i n the framework o f Marcus theory (20, 21). With respect t o s o l a r energy conversion and p h o t o e l e c t r o chemical s y n t h e s i s i n v o l v i n g I -> Iβ" o x i d a t i o n , i t i s noteworthy that s i g n i f i c a n t v a r i a t i o n i n k can be brought about by v a r i a t i o n o f the e l e c t r o l y t e s o l u t i o n . The data suggest that CH3CN c o u l d be a good s o l v e n t i n terms o f the measured v a l u e o f k , but the d u r a b i l i t y of the i n t e r f a c e i n CH3CN i s too poor. Perhaps a mixture o f CH3CN/H2O would prove workable. Two p o i n t s o f i n t e r e s t r e g a r d i n g d e r i v a t i z e d e l e c t r o d e s emerge from the data f o r mediated I " o x i d a t i o n . F i r s t , i t i s noteworthy that f e r r i c e n i u m i n H2O w i l l not o x i d i z e I ; the e t

+

v s

1

S

e t

e t

4

3

2

2

2

et

-


50


thermodynamics are such that the r e a c t i o n i s not spontaneous (26). However, we do f i n d that (FeCp2 )surf. f d e r i v a t i z a t i o n w i t h I does spontaneously o x i d i z e , a l b e i t s l o w l y , I " i n H2O. Apparently, the s u r f a c e - c o n f i n e d system has a s u f f i c i e n t l y more p o s i t i v e p o t e n t i a l that I"" can be o x i d i z e d . Since monosubstituted ferrocenes do not have a p o t e n t i a l i d e n t i c a l to that f o r ferrocene i t s e l f , the s u r f a c e reagent may be enough d i f f e r e n t that i t s o x i d i z i n g power can be greater. I t may a l s o be t h a t , d e s p i t e the general s i m i l a r i t y i n E° s f o r surface-bound and s o l u t i o n species (27), the d i f f e r e n c e i n E° brought about by b i n d i n g the reagent to the s u r f a c e i s s u f f i c i e n t to change the d i r e c t i o n of spontaneity f o r a given redox process. Second, the d i r e c t p r o p o r t i o n a l i t y of steady-state photocurrent to £ ( F e C p 2 ) f ] f o r k < (mass t r a n s p o r t l i m i t e d ) i n d i c a t e s that a l l surface-bound m a t e r i a l i s I " a c c e s s i b l e . This i s c o n s i s t e n t w i t h the a b i l i t y of s m a l l anions to penetrate throughout the redox polymer. Large anions have been e l e c t r o s t a t i c a l l y bound to p o l y c a t i o n i c m a t e r i a l confined to an electrode s u r f a c e (28), and such bound anions s l o w l y exchange w i t h smaller counterions. We do not f i n d evidence f o r slow I " or I^"" i o n d i f f u s i o n i n and out of polymer on e l e c t r o d e s d e r i v a t i z e d w i t h J . Larger c o u n t e r i o n s , however, do e f f e c t the e l e c t r o c h e m i c a l behavior as p r e v i o u s l y noted (19). As shown i n Table I I , there are a number of reagents that w i l l r a p i d l y reduce ( F e C p 2 ) f . · The value of k >_ 6 χ 1 0 cm^mol" s""l comes from the o b s e r v a t i o n f o r I ~ i n CH^CN and a l l reductants i n Table I I that photocurrent d e n s i t y i s d i r e c t l y p r o p o r t i o n a l to for r o t a t i n g disk electrodes. The h i g h e s t ω allows the c o n c l u s i o n that l ^ v g ^ >^ 0.06 cm/s. D i v i d i n g by a coverage of ~10~10 mol/cm gives k > 6 χ 10 cnr m o l " s ~ . We are not c e r t a i n that 1 0 ~ mol/cm i s the proper coverage to use; we take t h i s v a l u e because the data i n f a c t show l i m i t i n g photocurrent to be independent of coverage i n the range ~6 χ 10^° to 1 χ 10" mol/cm . The ~ 1 0 ~ mol/cm i s the l i k e l y coverage that would correspond to a monolayer of reagent derived from a molecule as l a r g e as I . But some c a u t i o n should be e x e r c i s e d i n i n t e r p r e t i n g k that i s assigned from k b p I n our e a r l i e r work (16) we used a coverage of -10"^ mol/cm to represent the a p p r o p r i a t e monolayer coverage but we b e l i e v e t h i s to be too h i g h , s i n c e _ 6 χ 1 0 cuAnol^s*" i n the u s u a l u n i t s f o r a bimolecular r a t e constant f o r homogeneous s o l u t i o n i s > 6 χ 105 M ~ l s ~ l , The ferrocene self-exchange constant i s 5 χ 1 0 M~T -1 (29). Various cross r e a c t i o n s of s u b s t i t u t e d ferrocenes and f e r r i c e n i u m d e r i v a t i v e s have b i m o l e c u l a r r a t e constants t h a t exceed 10^ M""^s""l where the e q u i l i b r i u m constant exceeds u n i t y (30). F u r t h e r , i n the cross r e a c t i o n s , the r a t e constants v a r i e d by almost two orders of magnitude f o r a change i n d r i v i n g f o r c e of -0.25 V (30). Thus, the data i n Table I I r e l a t i n g to the f e r r o c e n e - l i k e molecules i s reasonable. +

r

o

m


T

+

s u r

e t

+

sur

e t

8

2

e t

1

1

1 0

8

2

2

1 0

2

e t

0

s v c

2

2

8

1

e t

6

S


3.

LEWIS AND WRIGHTON

51


The cross r e a c t i o n r a t e s and self-exchange r a t e s p r e d i c t k > 6 χ 10^ M " ~ l s ~ l , provided that the s u r f a c e - c o n f i n e d reagent has the same a c t i v i t y as the s o l u t i o n s p e c i e s . While we have not ordered k s q u a n t i t a t i v e l y , the order of k ^ ' s f o r the f e r r o c e n e - l i k e reagents does appear to depend i n a systematic way on the d r i v i n g f o r c e f o r r e a c t i o n . C o ( b i p y ) self-exchange i s f a i r l y slow (31) , and we f e l t that t h i s species might reduce ( F e C p 2 ) f . slowly enough that k could a c t u a l l y be measured. However, i t too has too l a r g e a k t o measure. The dimethyldithiocarbamate, Me2dtc~, i s i r r e v e r s i b l y o x i d i z e d by (FeCp2 )surf. * again the r a t e constant i s too l a r g e t o measure. The product(s) from t h i s r e a c t i o n have not yet been i d e n t i f i e d . Ferrocene i n s u l f o l a n e i s reported t o have a low r a t e constant f o r heterogeneous exchange (32) ; we examined t h i s system w i t h the hope of being able t o measure k , but found that ferrocene again has too l a r g e a k t o measure. The aqueous R u i N ^ ) ^ and Fe(CN) ^"" show that water s o l u b l e redox reagents can be found that g i v e l a r g e values o f k . Indeed, the s i m i l a r i t y o f E° from I " / I " and F e ( C N ) ~ / ~ and t h e >10 change i n r a t e constant shows that the I ~ o x i d a t i o n has a k i n e t i c b a r r i e r i n ^ 0 . We could a t t r i b u t e t h i s k i n e t i c d i f f i c u l t y s o l e l y t o the two-electron process t o form I3"" v s . the one-e]ectron o x i d a t i o n of Fe(CN)^ . However, t h i s e x p l a n a t i o n i s not e n t i r e l y s a t i s f a c t o r y i n view of the r e s u l t s f o r I " i n CH3CN. The F e ( C N ) ^ ' ^ " system would seemingly be a good couple f o r a p h o t o e l e c t r o c h e m i c a l c e l l f o r t h e conversion o f l i g h t t o e l e c t r i c i t y but here long term d u r a b i l i t y i s again a s e r i o u s problem: F e ( C N ) " / ~ i s phot o s e n s i t i v e (33) and the long term d u r a b i l i t y of e l e c t r o d e s d e r i v a t i z e d w i t h I i n the presence of high concentrations of Fe(CN)^^~/^~ has not been demonstrated. S e v e r a l of the r e v e r s i b l e redox couples c o u l d , i n f a c t be used i n a c e l l f o r e l e c t r i c i t y generation. However, the output photovoltage, Ey, a s s o c i a t e d w i t h these η-type S i / f e r r i c e n i u m / ferrocene photoanodes i s only i n the 0.3-0.4 V range a t openc i r c u i t . Such i s l i k e l y too low t o be u s e f u l i n p r a c t i c a l schemes f o r s o l a r energy conversion. The approach o f dérivâtization, though, may be a p p l i e d t o other photoanode m a t e r i a l s t o r e a l i z e improved e f f i c i e n c y and d u r a b i l i t y . e t

1

e t

+

+

s u r

e t

e t


+

a n c

e t

e t

- 1 -

6

e t

3

3

4

5

6

3-

3

4

A

Summary Photogenerated, s u r f a c e - c o n f i n e d f e r r i c e n i u m can be reduced by a number o f reductants i n c l u d i n g I , C o ( b i p y ) ^ , ferrocene, l , l - d i m e t h y l f e r r o c e n e , phenylferrocene„ Fe(n^-indenyl)2? [ F e ( C O ) ( n - C H ) ] , R u ( N H ) ^ , F e ( C N ) ~ and d i m e t h y l d i t h i o carbamate. With the exception o f I"", a l l g e n e r a l l y reduce the surface-confined f e r r i c e n i u m w i t h an observed heterogeneous r a t e constant o f >0.06 cm/s which corresponds t o a b i m o l e c u l a r r a t e constant > 6 χ 10^ M ~ l s ~ l , assuming that a monolayer (~10-10 mol/cm ) o f the s u r f a c e - f e r r i c e n i u m p a r t i c i p a t e s i n the -

2 +

T

5

+

5

5

4

3

6

4

6

2


52

PHOTOEFFECTS AT

SEMICONDUCTOR-ELECTROLYTE INTERFACES


r e a c t i o n . While good k i n e t i c s f o r I " o x i d a t i o n can be obtained i n CH3CN, d u r a b i l i t y of the photoelectrode i s not good enough to promise long term o p e r a t i o n . For e l e c t r o d e / s o l v e n t / e l e c t r o l y t e / r e d o x couple combinations that are durable and where good k i n e t i c s o b t a i n , low output photovoltage remains a problem f o r η-type S i e l e c t r o d e s d e r i v a t i z e d w i t h ferrocene reagents. Acknowledgements We thank the United States Department of Energy, O f f i c e of B a s i c Energy Sciences, D i v i s i o n of Chemical Science f o r support of t h i s r e s e a r c h . N.S.L. acknowledges support from the John and Fannie Hertz Foundation 1977-present, and M.S.W. as a Dreyfus Teacher-Scholar Grant r e c i p i e n t , 1975-1980. We acknowledge the a s s i s t a n c e of Dr. A.B. B o c a r s l y i n some aspects of t h i s work. Experimental Chemicals Ferrocene ( A l d r i c h Chemical Co.) was p u r i f i e d by s u b l i m a t i o n . B i s i n d e n y l i r o n (34), phenylferrocene (35)> C ( n - C H ) F e ( C O ) ] 4 (36) Co(bipy)3Cl2 * 71^0 ( 3 7 ) , and sodium dimethyldithiocarbamate (_3δ) were prepared by l i t e r a t u r e methods. 1,1 -Dimethylferrocene ( P o l y s c i e n c e s ) , a c e t y l f e r r o c e n e , and l , l - d i a c e t y l f e r r o c e n e ( A l d r i c h ) were p u r i f i e d by chromatography on alumina w i t h hexane as e l u a n t . K^Fe(CN)^ was used as r e c e i v e d ( M a l l i n c k r o d t ) , as was anhydrous NaCK>4 (G. F r e d e r i c k Smith). R u ( N H 3 ) 6 was c o n v e n i e n t l y prepared by e l e c t r o c h e m i c a l r e d u c t i o n at -0.3 V vs. SCE at a Hg p o o l e l e c t r o d e of R u ( N H ) C l ( A l f a Ventron) i n pH = 4.0 HCIO4/O.I M NaClO^. The s o l u t i o n was then a c i d i f i e d to pH = 2.0 w i t h HCIO4 j u s t before use, and manipulated under Ar. P o l a r o g r a p h i c grade [n-Bu^NJClO^ (Southwestern A n a l y t i c a l Chemicals) was d r i e d at 353 Κ f o r 24 hours and s t o r e d i n a d e s s i c a t o r u n t i l use. [n-Bu^Njl (Eastman) was r e c r y s t a l l i z e d twice from a b s o l u t e EtOH. Absolute EtOH, s p e c t r o q u a l i t y i s o o c t a n e , CH2CI2, CH3CN, and toluene were used as r e c e i v e d , as was reagent grade a c e t i c a c i d and s u l f o l a n e . A l l aqueous s o l u t i o n s were prepared from doubly d i s t i l l e d d e i o n i z e d water. 5

5

5

T

!

2+

3

6

3

Electrodes S i n g l e - c r y s t a l Sb-doped, p o l i s h e d η-type S i wafers ((111) face exposed) were obtained from General Diode Co., Framingham, MA. The wafers were 0.25 mm t h i c k and had r e s i s t i v i t i e s of 4-5 ohm-cm. E l e c t r o d e s were fashioned as p r e v i o u s l y reported (12). Ohmic contacts were achieved by rubbing Ga-In e u t e c t i c onto the back s i d e of the e l e c t r o d e a f t e r a 48% HF etch and H2O r i n s e . The e l e c t r o d e was attached to a Cu w i r e w i t h Ag epoxy, and the Cu w i r e was passed through a 4 mm Pyrex tube. The e l e c t r o d e s u r f a c e was defined by i n s u l a t i n g a l l other surfaces w i t h o r d i n a r y epoxy, y i e l d i n g e l e c t r o d e s of areas 3-15 mm . 2



3.

LEWIS AND WRIGHTON

53


C i r c l e s o f 5 mm were cut u l t r a s o n i c a l l y from the o r i g i n a l S i wafers f o r use as r o t a t i n g d i s k e l e c t r o d e s . Mounting was c a r r i e d out as above i n a 6 mm o.d. c a p i l l a r y tube, except t h a t the Cu wire was r e p l a c e d by a Hg contact through the c a p i l l a r y . Ordinary epoxy i n s u l a t i o n was used s p a r i n g l y and care was taken t o maintain as f l a t a S i e l e c t r o d e surface as p o s s i b l e . For c a l i b r a t i o n purposes, r o t a t i n g P t d i s k s from c i r c l e s o f Pt f o i l were mounted i n e x a c t l y the same manner. R o t a t i n g d i s k e l e c t r o d e s were mounted v e r t i c a l l y and s t i r r e d by a v a r i a b l e speed motor from P o l y s c i e n c e s , Inc. R o t a t i o n v e l o c i t i e s were c a l i b r a t e d by two methods: (1) a s l i t t e d p i e c e o f cardboard was mounted on the d i s k s h a f t and the time response o f a photodiode was recorded on an o s c i l l o s c o p e , and (2) p l o t s o f the l i m i t i n g current as a f u n c t i o n o f K^Fe(CN)^ c o n c e n t r a t i o n i n 2 M KC1 y i e l d e d s t r a i g h t l i n e s whose slope i s uh (D f o r F e ( C N ) - i s 6.3 χ 1 0 ~ cm /sec) ( 3 9 ) . Agreement between the two methods was b e t t e r than 10%, and the motor was found t o be extremely s t a b l e over long p e r i o d s o f time. Befure use, a l l S i s u r f a c e s were etched i n concentrated HF and r i n s e d w i t h d i s t i l l e d H2O. E l e c t r o d e s to be d e r i v a t i z e d were then immersed i n 10 Έ NaOH f o r 60 seconds, washed w i t h H 0 followed by acetone, and then a i r d r i e d . D e r i v a t i z a t i o n was accomplished by exposing the p r e t r e a t e d e l e c t r o d e s t o d r y , degassed isooctane s o l u t i o n s o f ( 1 > 1 - f e r r o c e n e d i y l ) d i c h l o r o s i l a n e f o r 2-4 hours ( 1 9 ) . The e l e c t r o d e s were then r i n s e d w i t h isooctane f o l l o w e d by EtOH. 4

6

2

6

2

T

Electrochemistry A l l experiments were performed i n s i n g l e compartment Pyrex c e l l s equipped w i t h a s a t u r a t e d calomel reference e l e c t r o d e (SCE), P t wire c o u n t e r e l e c t r o d e , and the a p p r o p r i a t e working e l e c t r o d e . I r r a d i a t i o n was s u p p l i e d by a beam expanded He-Ne l a s e r o f -50 mW/cm (5 mW t o t a l ) a t 632.8 nm, Laser i n t e n s i t i e s were measured u s i n g a T e k t r o n i x J16 d i g i t a l radiometer equipped w i t h a J6502 probe, and were adjusted t o d e s i r e d i l l u m i n a t i o n l e v e l s by use o f Corning t r a n s m i s s i o n f i l t e r s . C y c l i c v o l t a mmetry measurements were obtained w i t h a P r i n c e t o n A p p l i e d Research Model 173 p o t e n t i o s t a t equipped w i t h a Model 179 d i g i t a l coulometer and d r i v e n by a Model 175 v o l t a g e programmer. P o t e n t i a l step experiments were performed u s i n g the same apparatus, and the coulometer readings were v e r i f i e d by o s c i l l o s c o p i c current-time t r a c e s on a T e k t r o n i x 564B storage o s c i l l o s c o p e w i t h Type 2B67 time base p l u g - i n . C y c l i c voltammetry t r a c e s were recorded on a Houston Instrument Model 2000 X-Y recorder and current-time p l o t s were obtained u s i n g a Hewlett-Packard s t r i p chart recorder. The supporting e l e c t r o l y t e s were 0.1 M [n-Bu N]C10 f o r CH3CN, EtOH, C H C 1 , EtOH/toluene (1:1 v / v ) . s u l f o l a n e , and g l a c i a l a c e t i c a c i d s o l v e n t s , and 1.0 M NaC10 /0.01-0.1 M HC10 f o r H 0. Ei^ values f o r I " o x i d a t i o n were obtained a t P t 2

4

2

4

2

4

4

2


54


r o t a t i n g d i s k e l e c t r o d e s a t 600 r.p.m.; e x c e l l e n t agreement w i t h l i t e r a t u r e values (22), where a v a i l a b l e , was obtained. P o t e n t i a l values f o r surface-attached m a t e r i a l were obtained by d e r i v a t i z i n g P t and Au surfaces as p r e v i o u s l y described (18, 1 9 ) , and t a k i n g E°(FeCp )surf. to be the a r i t h m e t i c mean of the anodic and cathodic peak p o s i t i o n s ; g e n e r a l l y peak-to-peak separations were l e s s than 10 mV a t a 20 mV/s scan r a t e . K i n e t i c Measurements For measurement of k , d e r i v a t i z e d e l e c t r o d e s were c y c l e d i n a s o l u t i o n o f appropriate solvent and e l e c t r o l y t e u n t i l s t a b l e c y c l i c voltammetric parameters were obtained a t 100 mV/sec scan r a t e . C y c l i c voltammograms a t s e v e r a l scan r a t e s were recorded w i t h i l l u m i n a t i o n f o r the anodic p o r t i o n of the scan, but the l i g h t was blocked a t the anodic l i m i t by a s o l e n o i d d r i v e n by the t r i g g e r output of the PAR 175 v o l t a g e programmer, Stock s o l u t i o n s of reductant were prepared as needed and a l i q u o t s i n j e c t e d i n t o the Pyrex e l e c t r o c h e m i c a l c e l l immediately p r i o r t o use. C y c l i c voltammetry data was then c o l l e c t e d f o r the same s e t o f scan r a t e s and i l l u m i n a t i o n c o n d i t i o n s i n the presence of s o l u t i o n reductant t o o b t a i n k i n e t i c data. The e l e c t r o d e s were r i n s e d w i t h solvent and checked f o r decay i n the absence o f reductant between every k i n e t i c measurement. A t l e a s t four d i f f e r e n t concentrations o f reductant were used f o r each s e t of data p o i n t s . From t h i s data, cathodic currents a s s o c i a t e d w i t h t h e r e d u c t i o n of surface-attached f e r r i c e n i u m were i n t e g r a t e d manually t o determine the time and c o n c e n t r a t i o n dependence o f the extent of consumption o f s u r f a c e - c o n f i n e d f e r r i c e n i u m . The r e a c t i o n time a s s o c i a t e d w i t h a c y c l i c voltam m e t r i c sweep was chosen as the p e r i o d from the anodic l i m i t t o the peak cathodic current i n the c y c l i c voltammogram. P o t e n t i a l step experiments were performed by scanning a n o d i c a l l y a t 500 mV/sec to the anodic l i m i t (+0.5 V v s . SCE), h o l d i n g at t h i s l i m i t i n the dark f o r a time t ^ , and then p u l s i n g c a t h o d i c a l l y back t o -0.6 V vs. SCE where the r e d u c t i o n was observed. +/0

2


e t

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Wrighton, M.S., Acc. Chem. Res., 1979, 12, 303. Nozik, A.J., Ann. Rev. Phys. Chem., 1978, 29, 189. Bard, A.J., Science, 1980, 207, 139. Gerischer, H. J. Electroanal. Chem., 1977, 82, 133. Bard, A.J.; Wrighton, M . S . , J. Electrochem. Soc., 1977, 124, 1706. E l l i s , A . B . ; Kaiser, S.W.; Wrighton, M.S., J. Am. Chem. Soc., 1976, 98, 1635, 6418, and 6855. Hodes, G . , Nature (London), 1980, 285, 29. Parkinson, B . A . ; Heller, Α.; M i l l e r , B . , Appl. Phys. L e t t . , 1978, 33, 521. Nakatani, K . ; Matsudaira, S.; Tsubomura, H . , J. Electrochem. Soc., 1978, 125, 406.


3. LEWIS AND WRIGHTON Reduction of Ferricenium 10.

11. 12.

13.


14. 15. 16. 17. 18.

19. 20. 21. 22. 23.

24. 25. 26. 27.

28. 29. 30. 31. 32. 33. 34. 35.

55

Wrighton, M.S.; Bocarsly, A . B . ; Bolts, J . M . ; Bradley, M.G.; Fischer, A . B . ; Lewis, N.S.; Palazzotto, M.C.; Walton, E.G., Adv. Chem. Ser., 1980, 184, 269. Bocarsly, A . B . ; Walton, E . G . ; Wrighton, M.S., J. Am. Chem. Soc., 1980, 102, 3390. Bolts, J . M . ; Bocarsly, A . B . ; Palazzotto, M.C.; Walton, E.G.; Lewis, N.S.; Wrighton, M . S . , J. Am. Chem. Soc., 1979, 101, 1378. Bolts, J . M . ; Wrighton, M.S., J. Am. Chem. Soc., 1979, 101, 6179. Bocarsly, A . B . ; Walton, E . G . ; Bradley, M.G.; Wrighton, M.S., J . Electroanal. Chem., 1979, 100, 283. Bolts, J . M . ; Wrighton, M.S., J. Am. Chem. Soc., 1978, 100, 5257. Lewis, N.S.; Bocarsly, A . B . ; Wrighton, M.S., J. Phys. Chem., 1980, 84, 2033. Murray, R.W., Acc. Chem. Res., 1980, 13, 135 and references therein. Wrighton, M.S.; Austin, R . G . ; Bocarsly, A . B . ; Bolts, J . M . ; Haas, O.; Legg, K . D . ; Nadjo, J.; Palazzotto, M. C., J . Electroanal. Chem., 1978, 87, 429. Wrighton, M.S.; Palazzotto, M.C.; Bocarsly, A . B . ; Bolts, J.M. Fischer, A . B . ; Nadjo, L., J. Am. Chem. Soc., 1978, 100, 7264. Marcus, R . A . , Ann. Rev. Phys. Chem., 1964, 15, 155. Marcus, R . A . , J . Chem. Phys., 1965, 43, 679. Janz, D. J.; Tomkins, R . P . T . , "Nonaqueous Electrolytes Handbook", Vol. 2, Academic Press: New York, 1972. Piebarski, S.; Adams, R . N . , in "Physical Methods of Chemistry", Part IIA, Weissberger, A. and Rossiter, B . , eds., Wiley-Intersciences: New York, 1971, Chapter 7. Galus, Ζ . , Adams, R . N . , J. Phys. Chem., 1963, 67, 866. Levich, V . G . , "Physicochemical Hydrodynamics", Prentice-Hall: Englewood C l i f f s , New Jersey, 1962. Yeh, P . ; Kuwana, T., J. Electrochem. Soc., 1976, 123, 1334. Lenhard, J . R . ; Rocklin, R.; Abruna, H . ; William, K . ; Kuo, K . ; Nowak, R.; Murray, R.W., J. Am. Chem. Soc., 1978, 100, 5213. Oyama, N . ; Anson, F.C., J. Electrochem. Soc., 1980, 127, 247. Yang, E . S . ; Chan, M . ; Wahl, A.C., J. Phys. Chem., 1975, 79, 2049. Pladziewicz, J . R . ; Espenson, J.H., J. Am. Chem. Soc., 1973, 95, 56. Farina, R.; Wilkins, R . G . , Inorg. Chem., 1968, 7, 514. Armstrong, N.R.; Quinn, R . K . ; Vanderborgh, N . E . , J. Electro chem. Soc., 1976, 123, 646. Balzani, V . ; Carassiti, V . , "Photochemistry of Coordination Compounds", Academic Press: New York, 1970. King, R . B . , "Organometallic Synthesis", Academic Press: New York, 1965, p. 73. Broadhead, G . ; Pauson, P . L . , J. Chem. Soc., 1955, 367.


56 36. 37. 38. 39.


King, R . B . , Inorg. Chem., 1966, 5, 2227. Burstall, F . H . ; Nyholm, R . S . , J. Chem. Soc., 1952, 3570. Delepine, M . ; Haller, Α . , Comptes Rendus de L'Academie des Sciences, 1907, 144, 1126. Adams, R . N . , "Electrochemistry at Solid Electrodes", Marcel Dekker: New York, 1969.


Received October 3, 1980.


Photoeffects at Semiconductor-Electrolyte Interfaces - American

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