Short-Lived Radicals at Photoactive Surfaces - Advances in Chemistry

10 Short-Lived Radicals at Photoactive Surfaces

Downloaded by KTH ROYAL INST OF TECHNOLOGY on February 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch010

Spin T r a p p i n g a n d Mechanistic

Consequences

M. L . HAIR and J. R. HARBOUR Xerox Research Centre of Canada, 2480 Dunwin Drive, Mississauga, Ontario, Canada L5L 1J9

The technique of spin trapping has been applied successfully to the study of radicals produced when photoactive particles are suspended in either aqueous or insulating fluids and irradiated in the presence of O . This trapping technique is reviewed with particular emphasis on the detection and identification of the superoxide anion and hydroxyl radicals. Results are interpreted within the framework of a simple band model for semiconductors. The effects of both anionic and cationic surfactants on the photoprocess are described. The addition of electron-donating molecules to the suspen sion results in a reaction (in the fluid) that is "pumped" by application of band gap radiation to the solid particle. The radicals that have been identified on irradiating several different photoactive particles are described. The ability to identify these radical intermediates is important in deter mining the exact reaction mechanism, as exemplified by a discussion of the photosynthesis of H O on zinc oxide. 2

2

2

T V J " a n y solar e n e r g y c o n v e r s i o n devices b a s e d u p o n t h e i n t e r a c t i o n o f light w i t h

semiconductors

have been

proposed.

These

include

p h o t o v o l t a i c d e v i c e s , p h o t o e l e c t r o c h e m i c a l cells t h a t c a n d i r e c t l y g e n erate e l e c t r i c i t y o r p r o d u c e a f u e l , a n d p i g m e n t d i s p e r s i o n s , w h i c h also can produce a fuel ( J ) or photodecompose a pollutant (2). I n systems w h e r e t h e s e m i c o n d u c t o r interfaces w i t h a s o l u t i o n , t h e proposed

photochemical

mechanisms

generally involve radical

inter

m e d i a t e s . H o w e v e r , there is v e r y l i t t l e e v i d e n c e of r a d i c a l p a r t i c i p a t i o n or i d e n t i f i c a t i o n . W e h a v e therefore b e g u n a n e x p e r i m e n t a l p r o g r a m a i m e d at i d e n t i f y i n g t h e r a d i c a l s p h o t o p r o d u c e d as a r e s u l t o f i r r a d i a t i o n 0-8412-0474-8/80/33-184-173$05.00/0 © 1980 American Chemical Society In Interfacial Photoprocesses: Energy Conversion and Synthesis; Wrighton, Mark S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

174

INTERFACIAL

of v a r i o u s p i g m e n t dispersions

(3).

PHOTOPROCESSES

A better u n d e r s t a n d i n g of

these

i n t e r m e d i a t e s a n d the factors t h a t influence t h e i r p r o d u c t i o n a n d d e s t r u c t i o n s h o u l d c o n t r i b u t e to t h e d e v e l o p m e n t of these types of solar e n e r g y converters.

T h e s e factors i n c l u d e a d e t a i l e d k n o w l e d g e of the r o l e t h a t

t h e s u r f a c t a n t p l a y s . T h e surfactant is a d d e d to p i g m e n t dispersions to prevent

flocculation

of the p a r t i c l e s . T h i s is i m p o r t a n t since s u c h

floccula-

t i o n causes a r e d u c t i o n i n surface a r e a as w e l l as a n increase i n t h e rate


of s e t t l i n g . H o w e v e r , i n a l l these p h o t o a c t i v e

systems the

surfactant

p l a y s a d u a l r o l e a n d a l w a y s affects the surface c h a r g e as i t stabilizes the system. I t is s p e c u l a t e d

that t h e p h o t o c h e m i s t r y

at a n i n t e r f a c e

occurs

t h r o u g h r a d i c a l i n t e r m e d i a t e s ( 4 ) . I n p r i n c i p l e , e l e c t r o n s p i n resonance ( E S R ) s p e c t r o s c o p y w o u l d b e the i d e a l m e t h o d for e x a m i n i n g t h i s t y p e of i n t e r f a c i a l p h o t o c h e m i s t r y .

U n f o r t u n a t e l y , the d i r e c t d e t e c t i o n

and

i d e n t i f i c a t i o n of r a d i c a l s b y this t e c h n i q u e is p o s s i b l e o n l y i f t h e r a d i c a l s are p r o d u c e d i n r e l a t i v e l y h i g h concentrations i n the E S R c a v i t y a n d are sufficiently l o n g - l i v e d to b e d e t e c t e d . I n most systems of p r a c t i c a l interest t h e r e w i l l b e r e l a t i v e l y l a r g e concentrations of b o t h 0 fore, there is a h i g h p r o b a b i l i t y t h a t s u p e r o x i d e ( 0 " ) 2

2

and H 0 . 2

There

o r h y d r o x y l (• O H )

radicals w i l l be formed under normal ambient conditions. T h e half-lives of these r a d i c a l s ( o r t h e i r s p i n l a t t i c e r e l a x a t i o n t i m e s T i ) are sufficiently short that d i r e c t d e t e c t i o n of t h e m is not a l w a y s possible.

I n o r d e r to

c i r c u m v e n t this p r o b l e m w e h a v e successfully a p p l i e d the t e c h n i q u e of s p i n t r a p p i n g to p h o t o a c t i v e p a r t i c u l a t e dispersions. T h e use of a r a d i c a l a d d i t i o n r e a c t i o n to detect s h o r t - l i v e d r a d i c a l s w a s first p r o p o s e d b y l a n z e n ( 5 ) i n 1965. E a r l y w o r k o n this t e c h n i q u e c e n t e r e d o n the interactions of nitrones w i t h r a d i c a l s a n d the c o n s e q u e n t p r o d u c t i o n of stable n i t r o x i d e s . T h e r e a d e r is r e f e r r e d to a r e v i e w

by

J a n z e n ( 6 ) w h i c h covers t h e d e v e l o p m e n t of the s p i n - t r a p p i n g reactions p r i o r to 1971. A m a j o r a d v a n c e i n the u t i l i t y of this t e c h n i q u e c a m e i n 1973 w h e n J a n z e n a n d L i u ( 7 )

d e s c r i b e d the use of a

five-membered

r i n g n i t r o n e 5 , 5 - d i m e t h y l - l - p y r o l i n e - l - o x i d e ( D M P O ) . T h i s a c t e d as a s p i n t r a p i n the f o l l o w i n g m a n n e r :

(1)

T h e s p i n a d d u c t of D M P O has the a d v a n t a g e that the h y p e r f i n e s p l i t t i n g constants are s t r o n g l y d e p e n d e n t

u p o n the n a t u r e of

the

complexed

r a d i c a l a n d are sufficiently separated that r e a d y i d e n t i f i c a t i o n of

In Interfacial Photoprocesses: Energy Conversion and Synthesis; Wrighton, Mark S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

the

10.

HAIR AND HARBOUR

Radicals

at Photoactive

175

Surfaces

r a d i c a l is g e n e r a l l y p o s s i b l e . H a r b o u r a n d B o l t o n (8,9)

have a p p l i e d this

s p i n - t r a p p i n g t e c h n i q u e to i n v i v o studies of chloroplasts a n d c h r o m a t o phores. and

T h e y f o u n d t h a t w h e n these systems w e r e i l l u m i n a t e d b o t h 0 ~ 2

- O H c o u l d b e i d e n t i f i e d f r o m the spectra of t h e r a d i c a l a d d u c t s .

A p p l i c a t i o n to p a r t i c u l a t e dispersions of p h o t o c o n d u c t i n g p a r t i c l e s w a s first r e p o r t e d i n 1977 b y H a r b o u r a n d H a i r ( 3 ) w h o s h o w e d t h a t w h e n a q u e o u s suspensions of c a d m i u m sulfide w e r e i r r a d i a t e d i n t h e presence of D M P O the 0 ~ s p i n a d d u c t w a s r e a d i l y o b s e r v e d . Downloaded by KTH ROYAL INST OF TECHNOLOGY on February 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch010

2

Experimental T h r e e types of p h o t o c o n d u c t i n g p a r t i c l e s h a v e b e e n u s e d i n t h i s w o r k . C a d m i u m sulfide, a n n - t y p e s e m i c o n d u c t o r , w a s o b t a i n e d f r o m F i s h e r a n d u s e d w i t h o u t f u r t h e r t r e a t m e n t ( 3 ) . I t c o n s i s t e d of p a r t i c l e s a p p r o x i m a t e l y 0.5 /xm i n d i a m e t e r w i t h a B E T ( N ) surface a r e a of 10 m / g . Metal-free phthalocyanine, an organic photoconducting pigment t h a t is often t a k e n as a n a n a l o g of c h l o r o p h y l l , w a s i n t h e x - c r y s t a l l i n e f o r m . T h i s i n s o l u b l e p o w d e r consisted of p a r t i c l e s less t h a n 1 / i m i n d i a m e t e r w i t h a B E T surface a r e a of 70 m / g . D i s t i l l e d w a t e r w a s r e d i s t i l l e d f r o m a n all-glass a p p a r a t u s . T h e s p i n t r a p D M P O w a s s y n t h e s i z e d and p u r i f i e d p r i o r to use b y b u l b - t o - b u l b d i s t i l l a t i o n o n a v a c u u m system and a d d e d d i r e c t l y to the d i s p e r s i o n ( ~ 0 . 1 M ) . I n a l l cases the p i g m e n t suspensions w e r e p r e p a r e d b y u l t r a s o n i c d i s p e r s i o n . T h e samples w e r e i l l u m i n a t e d i n s i t u w i t h a t u n g s t e n - q u a r t z i o d i d e l a m p d e s c r i b e d elsewhere ( J O ) or w i t h a H a n o v i a M o d e l 9 9 7 B 1KW H g - X e l a m p i n a Schoeffel M o d e l L H 1 5 1 N l a m p h o u s i n g w i t h a p p r o p r i a t e filters. T h e E S R spectra w e r e o b t a i n e d o n a V a r i a n E 1 2 E S R spectrometer. I n c e r t a i n cases either a c a t i o n i c surfactant, c e t y l t r i m e t h y l a m m o n i u m b r o m i d e ( C T A B ) f r o m S i g m a , or a n a n i o n i c s u r f a c t a n t , A e r o s o l O T ( A O T ) f r o m A m e r i c a n C y a n a m i d , w a s a d d e d to a i d d i s p e r s i o n a n d / o r to observe t h e effect of t h e a d s o r b e d m o l e c u l e s o n photochemistry. 2

2

2

Results and •OH

Discussion

Adduct.

T h e f o r m a t i o n a n d i d e n t i f i c a t i o n of t h e - O H a d d u c t

of D M P O w a s first r e p o r t e d b y H a r b o u r , C h o w , a n d B o l t o n i n 1974 T h e s e authors p r e p a r e d the aqueous H 0 2

2

(11).

- O H r a d i c a l b y U V p h o t o l y s i s of

dilute

solution.

2H 0 2

hv

>2 - O H

2

(2)

I n the presence of D M P O the s i g n a l s h o w n i n F i g u r e 1 w a s T h e signal was characterized by g = 14.9 G . T h e a c c i d e n t a l e q u a l i t y of a

2.0060 ± N

a n d ap

K

0.0002 a n d a

N

recorded. =

a^

H

=

gives rise to the 1 : 2 : 2 : 1

q u a r t e t . T h i s a s s i g n m e n t has b e e n c o n f i r m e d b y Sargent a n d G a r d y


(12)

INTERFACIAL


176

Figure 1.

The ESR spectrum of the

PHOTOPROCESSES

10G

OH adduct of DMPO

in water at

25°C

w h o p r e p a r e d - O H b y r a d i o l y s i s of d e o x y g e n a t e d w a t e r u s i n g 3 M e V electrons. T h e y o b t a i n e d a n i d e n t i c a l E S R s p e c t r u m u s i n g t h e s p i n t r a p DMPO. A s w i l l b e d i s c u s s e d i n m o r e d e t a i l l a t e r , b a n d g a p i r r a d i a t i o n of z i n c o x i d e s u s p e n d e d i n w a t e r gives rise to a s i g n a l i d e n t i c a l to t h a t s h o w n i n F i g u r e 1. 0 " Adduct. 2

T h e O " adduct was the major product observed a

H a r b o u r , C h o w , a n d B o l t o n ( 1 1 ) w h e n c o n c e n t r a t e d aqueous

by

solutions

of H 0 - c o n t a i n i n g D M P O w e r e p h o t o l y z e d . 2

2

•OH +

H 0 ->H 0 + 2

2

2

H0

(3)

2

T h e D M P O a d d u c t i n w a t e r gives a n E S R s p e c t r u m w i t h g = a

N

=

14.1 G ,

— 11.3 G a n d a

y

H

2.0061,

1.25 G .

I n aqueous systems the 0 ~ r a d i c a l is i n e q u i l i b r i u m w i t h t h e H 0 2

2

radical. H0

2

^± H

T h e p K f o r this e q u i l i b r i u m is 4.4 ± a

+

+

0

(4)

2

0.4 ( 1 3 ) .

H o w e v e r , the p K

a

of the

s p i n a d d u c t i o n i z a t i o n is not k n o w n . T h u s i t is n o t p o s s i b l e to d i s t i n g u i s h b e t w e e n 0 ~ a n d its p r o t o n a t e d f o r m w h e n t h e r a d i c a l s are i n c o r p o r a t e d 2

i n t o the D M P O a d d u c t . F u r t h e r p r o o f of the correct i d e n t i f i c a t i o n of this s p i n a d d u c t also has b e e n o b t a i n e d b y i n d e p e n d e n t l y g e n e r a t i n g 0 ~ b y s o l u b i l i z i n g p o t a s 2

s i u m s u p e r o x i d e w i t h the K - s e l e c t i v e 18-crown-6-ether +

( C E ) (14)

(see

Figure 2). K0

2

+

CE — CEK

+

+

0 2


(5)

10.

HAIR A N D HARBOUR

Radicals

at Photoactive

177

Surfaces


-10G-

Figure 2.

The ESR spectrum of 0 ' adduct in propylene 2

carbonate at

25°C

T h i s t e c h n i q u e has b e e n e x t e n d e d t o o t h e r solvents a n d t h e 0 ~ a d d u c t 2

has b e e n i d e n t i f i e d i n a series o f solvents w i t h p o l a r i t i e s r a n g i n g f r o m that of water to that of benzene. T h e nitrogen a n d ^ - h y d r o g e n splittings h a v e b e e n d e t e r m i n e d as a f u n c t i o n o f solvent p o l a r i t y . T h i s enables t h e extension o f the s p i n - t r a p p i n g t e c h n i q u e to a n y solvent system.

Further

e v i d e n c e t h a t 0 ~ w a s a c t u a l l y p r e s e n t i n those systems w a s o b t a i n e d 2

b y r a p i d freezing experiments i n the absence of the s p i n trap. A n E S R s i g n a l consistent w i t h the p r o d u c t i o n o f a n a x i a l l y s y m m e t r i c r a d i c a l w a s obtained ( g

M

= 2.08 a n d g

=

±

Application to Dispersions.

2.00). A q u e o u s dispersions of either c a d m i u m

sulfide o r x - p h t h a l o c y a n i n e g a v e n o E S R s i g n a l u p o n a d d i t i o n o f D M P O to t h e system. H o w e v e r , i l l u m i n a t i o n w i t h b a n d g a p i r r a d i a t i o n y i e l d e d a small E S R signal. T h i s was similar to the spectrum s h o w n i n F i g u r e 2 a n d is r e a d i l y i d e n t i f i e d as b e i n g t h a t of the D M P O / 0 ~ s p i n a d d u c t . T h e 2

s p i n a d d u c t does n o t f o r m i n t h e absence o f 0 is d e p e n d e n t u p o n t h e 0

2

p a r t i a l pressure.

2

a n d the a m o u n t o f p r o d u c t I t s f o r m a t i o n is consistent

w i t h a o n e - e l e c t r o n transfer f r o m t h e i r r a d i a t e d s o l i d t o t h e d i s s o l v e d 0 . 2

I n t h e absence o f surfactants o r other a d d i t i v e s , t h e i n t e n s i t y o f t h e

E S R s i g n a l is n e v e r v e r y great a n d thus t h e a p p a r e n t efficiency o f t h e p h o t o g e n e r a t i o n is q u i t e l o w .


178

INTERFACIAL

During

illumination i n oxygenated

non-aqueous

PHOTOPROCESSES

suspensions,

the

s i g n a l shows a c o n t i n u a l n a r r o w i n g of the s p e c t r a l l i n e s . T h i s is c o n sistent w i t h the r e m o v a l of 0

2

f r o m the s o l u t i o n t h r o u g h r e d u c t i o n of

to 0 ~ a n d s u b s e q u e n t t r a p p i n g b y the D M P O .

0

2

Also during illumination,

2

h o w e v e r , the increase i n s i g n a l i n t e n s i t y is f o l l o w e d b y a s l o w

decay.

A f t e r t u r n i n g off the l i g h t the s i g n a l shows a f u r t h e r d e c a y . T h i s i n d i c a t e s t h a t the 0 ~ s p i n a d d u c t is s o m e w h a t u n s t a b l e i n these systems, p r e v e n t 2

i n g a n a c c u r a t e q u a n t i t a t i v e d e t e r m i n a t i o n of the 0 ~ a d d u c t c o n c e n t r a Downloaded by KTH ROYAL INST OF TECHNOLOGY on February 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch010

2

tion.

The

- O H a d d u c t is m u c h m o r e

stable i n b o t h l i g h t a n d d a r k

c o n d i t i o n s . I n i t i a l experiments a i m e d at q u a n t i f y i n g r a d i c a l f o r m a t i o n i n the aqueous z i n c o x i d e system are d e s c r i b e d e l s e w h e r e

(15).

T h e results o b t a i n e d a b o v e are consistent w i t h a m e c h a n i s m s u c h as i n F i g u r e 3 ( 1 6 ) .

T h e i n c i d e n t p h o t o n creates a n e l e c t r o n - h o l e p a i r

a n d , i f a n a c c e p t o r state lies b e l o w the c o n d u c t i o n b a n d , e l e c t r o n t r a n s f e r to t h e a c c e p t o r l e v e l is t h e r m o d y n a m i c a l l y f a v o r a b l e .

T h e redox level

f o r t h e 0 / 0 " c o u p l e is b e l o w the c o n d u c t i o n b a n d f o r b o t h c a d m i u m 2

2

sulfide a n d x - p h t h a l o c y a n i n e , so f o r m a t i o n of 0 " is n o t u n e x p e c t e d . 2

T h e d i r e c t d e t e c t i o n of a n e l e c t r o n transfer r e a c t i o n i n t h e C d S - H 0 2

system c a n b e a c h i e v e d b y r e p l a c i n g m o l e c u l a r o x y g e n w i t h a m o l e c u l e whose

r e d u c e d f o r m is r e l a t i v e l y stable.

such a compound.

M e t h y l viologen

(MV

+ 2

)

is

It is w a t e r s o l u b l e , exists as a colorless c a t i o n w h i c h

has a r e d o x p o t e n t i a l of

—0.44

(vs. N H E ) a n d c a n b e r e d u c e d to a

stable b l u e c a t i o n r a d i c a l p r o v i d e d 0

2

is not present.

to a c a d m i u m sulfide d i s p e r s i o n u n d e r N i n d e e d g i v e rise to t h e M V

+

2

A d d i t i o n of

MV

+ 2

purging and illumination d i d

signal.

A

D

Figure 3. A photon (hv) of light causes the excitation of an electron from the valence band (VB) to the conduction band (CB). If the energy level of the acceptor (A) is below that of the CB then electron transfer can occur as indicated by the arrow. Similarly, if the donor (D) state is above that of the VB electron transfer to the hole can occur.


HAIR A N D HARBOUR

10.

A

Radicals

at Photoactive

179

Surfaces

d i r e c t c o n s e q u e n c e of the a b o v e m o d e l is that w h e n l i g h t is

a b s o r b e d a h o l e also m u s t b e c r e a t e d . I n i t i a l l y the h o l e m a y b e t r a p p e d i n the p h o t o c o n d u c t i n g p a r t i c l e . I n this case the p a r t i c l e m a y assume a r e v e r s e d c h a r g e a n d this t y p e of c h a r g e r e v e r s a l ( i n n o n a q u e o u s and

w i t h large a p p l i e d electric

field)

of e l e c t r o p h o t o g r a p h i c i m a g i n g ( 1 7 ) .

type

O v e r a p e r i o d of t i m e , h o w e v e r ,

the c a d m i u m sulfide w i l l u n d e r g o s e l f - o x i d a t i o n (18)

or the hole

react w i t h a n e l e c t r o n d o n o r i n the s u r r o u n d i n g s o l u t i o n . Downloaded by KTH ROYAL INST OF TECHNOLOGY on February 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch010

systems

f o r m s the basis of a n o v e l

e s t a b l i s h e d t h a t E D T A is p h o t o o x i d i z e d v e r y efficiently ( 1 9 ) .

will

It is w e l l Addition

of E D T A to the C d S - D M P O suspension g r e a t l y increases t h e E S R s i g n a l of the 0 ~ r a d i c a l a d d u c t . T h e c a d m i u m sulfide p a r t i c l e s are t h u s a c t i n g 2

as a p h o t o p u m p a n d d r i v i n g electrons f r o m E D T A to 0 .

A similar large

2

increase i n i n t e n s i t y of the s i g n a l d u e to the M V

+

radical cation was

o b s e r v e d w h e n E D T A w a s a d d e d to the i l l u m i n a t e d M V

+ 2

- C d S suspen

sion, thus p r o v i d i n g f u r t h e r c r e d i b i l i t y to t h e m e c h a n i s m . Effect of S u r f a c t a n t s .

F o r a n y p r a c t i c a l system i n v o l v i n g p h o t o

a c t i v e p i g m e n t dispersions i t is a l m o s t c e r t a i n that a surfactant w o u l d b e r e q u i r e d to p r e v e n t

flocculation

of the p a r t i c l e s . M a n y of the surfactants

u s e d i n aqueous systems are i o n i c a n d c a n c h a r g e the p a r t i c l e s e i t h e r p o s i t i v e l y or n e g a t i v e l y . T h e effect of a l t e r i n g the surface c h a r g e c a n b e p r e d i c t e d f r o m F i g u r e 3. T h e degree of b a n d b e n d i n g at t h e i n t e r f a c e defines the space c h a r g e r e g i o n w i t h i n the p h o t o c o n d u c t o r .

W h e n the

h o l e a n d e l e c t r o n are c r e a t e d b y a b s o r p t i o n of a p h o t o n , the holes a n d electrons m i g r a t e o p p o s i t e l y u n d e r the i n f l u e n c e of t h e field. I f a p o s i t i v e surface c h a r g e exists, electrons w i l l m i g r a t e m o r e r e a d i l y t o w a r d s

the

i n t e r f a c e a n d w i l l act m o r e efficiently as r e d u c i n g agents for t h e m o l e c u l a r o x y g e n . H o w e v e r , i f the surface is n e g a t i v e l y c h a r g e d , t h e opposite effect w o u l d b e a n t i c i p a t e d a n d e l e c t r o n transfer i m p e d e d .

T o test these p r e

d i c t i o n s s p i n - t r a p p i n g experiments h a v e b e e n p e r f o r m e d u s i n g p h t h a l o c y a n i n e suspensions w h i c h h a v e b e e n d i s p e r s e d b y a d s o r b e d m o n o l a y e r s of e i t h e r C T A B

( w h i c h adsorbs v i a the b u l k y h y d r o c a r b o n m o i e t y to

g i v e a p o s i t i v e surface) or A O T ( w h i c h adsorbs to g i v e a n e g a t i v e surface) (20).

O n i r r a d i a t i o n , the y i e l d of the 0 ~ a d d u c t is g r e a t l y i n c r e a s e d f o r 2

the system that has the i n c r e a s e d p o s i t i v e c h a r g e a n d is s i g n i f i c a n t l y d e c r e a s e d w h e n the surface assumes a n e g a t i v e charge.

T h e monolayer

of s u r f a c t a n t does n o t p r e v e n t the e l e c t r o n transfer f r o m o c c u r r i n g a n d the effect of the surface c h a r g e is m o r e n o t i c e a b l e i n the case of t h e p h t h a l o c y a n i n e t h a n the C d S . T h u s , i n these cases, t h e surface does not s i g n i f i c a n t l y alter the p r i m a r y p h o t o c h e m i s t r y .

charge

H o w e v e r , the

r o l e of s u r f a c t a n t is c r u c i a l a n d , as w i l l b e d e s c r i b e d e l s e w h e r e

(21),

a c o m b i n a t i o n of " r i g h t " p r o p e r t i e s of surfactant a n d e l e c t r o n

donor

can

b e u s e d to a c h i e v e photosynthesis of H 0 2

2

on a phthalocyanine

surface.


180

INTERFACIAL

A p p l i c a t i o n t o P h o t o s y n t h e s i s of H 0 . 2

PHOTOPROCESSES

W h e n z i n c o x i d e p o w d e r is

2

s u s p e n d e d i n H 0 a n d i r r a d i a t e d w i t h l i g h t of w a v e l e n g t h less t h a n 380 2

n m i n the presence of D M P O , a l a r g e E S R s i g n a l is o b s e r v e d .

T h i s is

i d e n t i c a l to t h a t s h o w n i n F i g u r e 1 a n d c a n therefore b e i d e n t i f i e d as t h e a d d u c t of D M P O a n d a n

-OH. H 0 2

is p h o t o g e n e r a t e d

2

under

these

e x p e r i m e n t a l c o n d i t i o n s a n d the efficiency of the p h o t o r e a c t i o n is i n creased b y t h e a d d i t i o n of c o m p o u n d s s u c h as f o r m a t e a n d oxalate

(4).


M a n y m e c h a n i s m s h a v e b e e n p r o p o s e d to a c c o u n t f o r this synthesis of H 0 , a n d a l t h o u g h r a d i c a l i n t e r m e d i a t e s are often p r o p o s e d , these s p i n 2

2

t r a p p i n g studies p r o v i d e the first d i r e c t e v i d e n c e f o r t h e i r presence. have recently concluded

a s t u d y o n the photosynthesis

of

We

hydrogen

peroxide on z i n c oxide c o m b i n i n g spin-trapping experiments, quantitative m e a s u r e m e n t of o x y g e n u p t a k e studies, a n d p e r o x i d e f o r m a t i o n i n a n a t t e m p t to define the r e a c t i o n p a t h . F u l l details are p u b l i s h e d e l s e w h e r e (15)

b u t a s u m m a r y is p e r t i n e n t because t h e s p i n - t r a p p i n g e x p e r i m e n t s

r e v e a l the m a j o r effect of the c a r b o x y l a t e - t y p e a d d i t i v e s o n t h e system. T h e salient p o i n t s are as f o l l o w s : (A)

A

quantitative comparison

between

product

formation

and

r a d i c a l c o n c e n t r a t i o n d e m o n s t r a t e d t h a t the r a d i c a l s w e r e m a j o r p a r t i c i p a n t s i n the r e a c t i o n m e c h a n i s m . ( B ) W h e n z i n c o x i d e suspensions are i l l u m i n a t e d i n the absence of a d d i t i v e s o n l y the - O H r a d i c a l a d d u c t is o b s e r v e d .

T h e 0 ~ a d d u c t is 2

n e v e r o b s e r v e d i n these e x p e r i m e n t s a n d therefore does not exist as a free e n t i t y i n the e x t e r n a l s o l u t i o n . ( A l t h o u g h this does not r u l e o u t its presence

as a surface species.)

T h e t i m e d e p e n d e n c e of the r a d i c a l

a d d u c t f o r m a t i o n is s h o w n i n F i g u r e 4.

T h e i n t e n s i t y peaks w i t h t i m e ,

p r o b a b l y c a u s e d b y p h o t o i n d u c e d d e s t r u c t i o n of the r a d i c a l a d d u c t since the i n t e n s i t y levels off w h e n i l l u m i n a t i o n is b l o c k e d . (C)

D e s p i t e the fact t h a t 0 " is n e v e r o b s e r v e d i n free s o l u t i o n the 2

k i n e t i c curves s h o w t h a t t h e rate of ' O H p r o d u c t i o n is d e p e n d e n t u p o n the 0

2

c o n c e n t r a t i o n i n t h e s o l u t i o n . M o r e o v e r , p r e v i o u s tracer studies

s h o w that the o x y g e n t h a t is i n c o r p o r a t e d i n t o H 0 2

molecular oxygen a n d not water (D)

comes f r o m

2

the

(22).

W h e n f o r m a t e is a d d e d to the aqueous z i n c o x i d e system a n d

i r r a d i a t e d i n the presence

of D M P O ,

the

- O H a d d u c t is n o

longer

o b s e r v e d , b u t is r e p l a c e d b y t h e l a r g e s i g n a l s h o w n i n F i g u r e 5. n e w s i g n a l has g — 2.006, a

N

— 15.6 G , a n d a^

u

This

— 18.7 G . B y a series of

e x p e r i m e n t s analogous to those d e s c r i b e d earlier f o r 0 ~ a n d * O H t h i s 2

s i g n a l c a n b e i d e n t i f i e d as b e i n g c a u s e d b y the D M P O / - C 0 " 2

a d d u c t . T h e l i m i t i n g c o n c e n t r a t i o n of H 0 2

1 0 " M to 8 X 4

2

radical

f o r m e d increases f r o m 1

10" M. 4


X


10.

HAIR A N D HARBOUR

i 12

i

i 36

Radicals

i

at Photoactive

i

i

60 Time (sec.)

Surfaces

i

181

1

84

1 108

Figure 4. The time dependence of the amplitude of the -OH adduct signal as a function of illumination. The field is fixed at the point indi cated by the arrow in the upper-right portion of the figure.

Figure 5.

The ESR spectrum of the 'C0 ~ adduct of DMPO at 25°C 2

in water


182

INTERFACIAL

PHOTOPROCESSES

T h e s e observations are consistent w i t h t h e f o l l o w i n g m e c h a n i s m . ZnO + hv^±

® + hole

T h e i n i t i a l step is a p h o t o r e d u c t i o n

e electron

(6)

of m o l e c u l a r o x y g e n , w h e r e

(s)

denotes a surface species. H e" -> 0 " . ) ^ H 0


+

0 .)

+

2 (

2

(

2

U

(7)

)

T h e s e m u s t b e surface species b e c a u s e t h e y are n e v e r d e t e c t e d b y t h e s p i n t r a p . T h i s p r o p o s a l is s u p p o r t e d b y t h e separate o b s e r v a t i o n t h a t the rate of 0

2

u p t a k e is a f u n c t i o n of the s q u a r e root of 0

concentration,

2

s u g g e s t i n g a surface effect. The H 0

2 (

, ) species c a n t h e n b e r e d u c e d b y a s e c o n d

photoproduced

e l e c t r o n to generate H 0 . 2

2

H e" - » H 0 ' ^ H 0 +

H0 A l t e r n a t i v e l y , since H 0

2

2

U

+

)

2

2

(8)

2

is k n o w n to d i s m u t a t e i n s o l u t i o n to g i v e H 0 , 2

2

t h e f o l l o w i n g r e a c t i o n c o u l d also o c c u r : 2 H0

2 ( 8 )

-> H 0 2

2

+ 0

(9)

2

F o r the c o n c u r r e n t o x i d a t i o n i t is c l e a r t h a t - O H r a d i c a l s m u s t b e p r o d u c e d , a n d this c a n o c c u r most easily b y either of the f o l l o w i n g r e a c t i o n s : (H 0)OH- + 2

®->

-OH

(10)

+

(11)

or Zn—OH , + ( i

If E q u a t i o n (10)

0 -> Z n

2 +

is o c c u r r i n g , the r e a c t i o n converts

c h e m i c a l free e n e r g y since t h e r e a c t i o n H 0 + 2

free e n e r g y ( + 2 5

-OH

kcal) (1).

0

2

solar e n e r g y

to

- » H 0 , has a p o s i t i v e 2

2

T h e o x i d a t i o n of z i n c o x i d e i t s e l f ( E q u a

t i o n 11) w o u l d b e a p h o t o c o r r o s i o n r e a c t i o n as d i s c u s s e d b y D i x o n a n d Healy (23).

T h e l i m i t i n g c o n c e n t r a t i o n of H 0 2

2

is t h e n p r o p o s e d

to

follow from the reaction H 0 („ + 2

2

- O H -> H 0 + 2

H0

(12)

2 ( a )

T h e f o r m a t e is p r o p o s e d to f u n c t i o n , at least i n p a r t , as a getter f o r t h e - O H r a d i c a l since n o

- O H adduct can be observed.

I t also c a n

f u n c t i o n as a r e d u c t a n t .


Radicals

HAIR A N D HARBOUR

10.

at Photoactive

H C 0 " -> - C 0

•OH +

2

2

- + H

183

Surfaces

2

(13)

0

or H C 0

2

-

+

®

^

-C0 " + 2

H

(14)

+

T h e experiments described here clearly demonstrate t h e utility of the s p i n - t r a p p i n g t e c h n i q u e as a m e t h o d f o r i d e n t i f y i n g r a d i c a l i n t e r mediates i n photoactive H 0


2

2

systems.

T h e study of t h e photosynthesis of

o n z i n c o x i d e reveals t h e d i s t i n c t l y different m e c h a n i s t i c p a t h w a y

w h i c h occurs i n t h e p r e s e n c e o f a d d i t i v e s . W e h a v e a p p l i e d t h e s p i n t r a p p i n g m e t h o d t o s i m p l e p h o t o p r o d u c e d r e a c t i o n s o c c u r r i n g across t h e s o l i d - l i q u i d interface i n photoactive p i g m e n t dispersions.

T h e method

is also c l e a r l y a p p l i c a b l e t o p h o t o e l e c t r o c h e m i c a l cells (18,24)

and any

o t h e r heterogeneous system i n v o l v i n g i n t e r f a c i a l c h a r g e transfer.

Literature

Cited

1. Rubin, T. R.; Calvert, J. G.; Rankin, G. T.; MacNevin, W. J. Am. Chem. Soc. 1953, 75, 2850. 2. Frank, S. N.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 303. 3. Harbour, J. R.; Hair, M. L. J. Phys. Chem. 1977, 81, 1791. 4. Freund, T.; Gomes, W. P. Catal. Rev. 1969, 3, 1. 5. Janzen, E. G. Chem. Eng. News 1965, 43, 50. 6. Janzen, E. G. Acc. Chem. Res. 1971, 4, 31. 7. Janzen, E. G., Liu, J. I-Ping. J. Mag. Reson. 1973, 9, 510. 8. Harbour, J. R.; Bolton, J. R. Biochem. Biophys. Res. Commun. 1974, 64, 803. 9. Harbour, J. R.; Bolton, J. R. Photochem. Bhotobiol., in press. 10. Warden, J. T.; Bolton, J. R. J. Am. Chem. Soc. 1973, 95, 6435. 11. Harbour, J. R.; Chen, V.; Bolton, J. R. Can. J. Chem. 1974, 52, 3549. 12. Sargent, F. P.; Gardy, E. M. Can. J. Chem. 1976, 54, 275. 13. Czapski, G.; Bielski, B. H. J. Phys. Chem. 1963, 67, 2180. 14. Harbour, J. R.; Hair, M. L. J. Phys. Chem. 1978, 82, 1397. 15. Harbour, J. R.; Hair, M. L. J. Phys. Chem., in press. 16. Gerischer, H . J. Electroanal. Chem. Interfacial Electrochem. 1975, 58, 263. 17. Weigl, J. W. Angew. Chem. Int. Ed. Engl. 1977, 16, 374. 18. Gerischer, H. In "Solar Power and Fuels"; Bolton, J. R., Ed.; Academic: New York, 1977; p. 77. 19. Markiewicz, S.; Chan, M. S.; Sparks, R. H.; Evans, C. A.; Bolton, J. R. International Conference on the Photochemical Conversion and Storage of Solar Energy, London, Ontario, Canada, 1976. 20. Harbour, J. R.; Hair, M. L. Photochem. Photobiol. 1978, 28, 721. 21. Harbour, J. R.; Hair , M. L.; Tromp, J., unpublished data. 22. Calvert, J. G.; Theurer, K.; Rankin, G. T.; MacNevin, W. M. J. Am. Chem. Soc. 1954, 76, 2575. 23. Dixon, D. R.; Healy, T. W. Aust. J. Chem. 1971, 24, 1193. 24. Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S. J. Am. Chem. Soc. 1976, 98, 6855. RECEIVED October 2, 1978.


Short-Lived Radicals at Photoactive Surfaces - Advances in Chemistry

Recommend Documents