Photoelectrolysis of Aqueous Solutions to ... - ACS Publications

15 Photoelectrolysis of Aqueous Solutions to Hydrogen—An Chemistry for Energy Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 02/27/18. For personal use only.

Approach to Solar Energy Storage FRANK R. SMITH Chemistry Department, Memorial University of Newfoundland, St. John's, Newfoundland, Canada A1B 3X7

I t i s g e n e r a l l y a g r e e d t h a t e l e c t r o l y s i s o f aqueous s o l u t i o n s o f f e r s t h e b e s t p r o s p e c t f o r t h e p r o d u c t i o n o f h y d r o g e n from w a t e r , b e c a u s e o f t h e e a s y s e p a r a t i o n o f t h e H2 a n d Ο 2 p r o d u c t s and b e c a u s e o f t h e r e l a t i v e l y l o w e n e r g y c o n s u m p t i o n i f c a t a l y t i c a l l y a c t i v e metal electrodes are used. T h u s , t h e minimum energy r e q u i r e m e n t s a r e those f o r w h i c h w a t e r , hydrogen and o x y g e n , each a t 1 atmosphere p r e s s u r e , a r e i n e q u i l i b r i u m : H 0(1)

=

2

H (g) 2

h 0 (g)

+

This equilibrium e.m.f. pressures: Ε

rev

=

1.23

2

V +

'

3

E

2

r

e

v

e

r

s

i

b

l

e

= 1.23

V at

298 Κ

i n c r e a s e s w i t h hydrogen and oxygen p a r t i a l

3

R

° F

T

log^

^10

p

p^

TT

H

2

*0

at

298

Κ.

(1)

2

The e n e r g y r e q u i r e m e n t o f 1.23 eV p e r h a l f - m o l e c u l e o f H2 t r a n s l a t e s t o a minimum o f 1 0 . 6 MJ p e r c u b i c m e t r e a t S . T . P . o f h y d r o g e n , w h e r e a s p r a c t i c a l e l e c t r o l y s e r s o p e r a t e somewhere b e t w e e n 14 a n d 25 MJ m (_1) . T h i s i s b e c a u s e o f t h e n e e d t o c a r r y o u t e l e c t r o l y s i s a t a f i n i t e r a t e , t h a t i s t o say a t p o t e n t i a l d i f f e r e n c e s g r e a t e r than the e q u i l i b r i u m c o n d i t i o n s r e f e r r e d to above. The v o l t a g e s a r e w a s t e d i n two p r i n c i p a l w a y s : o v e r v o l t a g e s a s s o c i a t e d w i t h hydrogen and oxygen e v o l u t i o n , η and η , r e s p e c t i v e l y , and r e s i s t i v e l o s s e s . S i n c e the exchange c u r r e n t d e n s i t i e s f o r oxygen e v o l u t i o n , i , are i n general s m a l l e r than those f o r hydrogen e v o l u t i o n , i , the o v e r v o l t a g e s a s s o c i a t e d w i t h the former are l a r g e r than tSose o f the l a t t e r . Both increase i n a logarithmic fashion with current density, i : 3

1

n

η

c

a

= η

= η

Η

2

=

b log

=

b

1

(i/i )

(b

(i/i') J-U ο

(b

1 ( )

log

1 A

o

1

~

30

mV a t

300K)

(2)

~

60

mV a t

300K)

(3)

This chapter not subject to U . S . Copyright. Published 1979 American C h e m i c a l Society.

Chemistry for Energy Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 02/27/18. For personal use only.

222

CHEMISTRY

FOR

E N E R G Y

The r e s i s t i v e l o s s e s ( t h e i R d r o p s i n t h e s o l u t i o n b e t w e e n t h e e l e c t r o d e s ) a r e more s i g n i f i c a n t b e c a u s e t h e y i n c r e a s e l i n e a r l y w i t h c u r r e n t d e n s i t y . B r i n g i n g the electrodes c l o s e r together should diminish these losses, i n p r i n c i p l e , but e l e c t r o l y s i s a t h i g h p r e s s u r e i s e v e n more a d v a n t a g e o u s i n d e c r e a s i n g r e s i s t i v e l o s s e s ( e . g . a t 100 a t m o s p h e r e s by 0.4 V) b e c a u s e t h e gas b u b b l e s a r e s m a l l e r , t h i s d e c r e a s e more t h a n o f f s e t t i n g t h e 0.1 V i n c r e a s e in Ε (2). Iron or n i c k e l - p l a t e d s t e e l electrodes are usually u s e d ^ ? h e s e m e t a l s b e i n g q u i t e good c a t a l y s t s , e v e n i f t h e y a r e i n f e r i o r t o t h e much more e x p e n s i v e p l a t i n u m . R e c e n t l y , t h e u s e o f a t h i n s o l i d i o n - e x c h a n g e r e s i n e l e c t r o l y t e and f i n e l y d i v i d e d p l a t i n u m cathode has been advocated f o r l o w e r i n g the energy consumption i n p r a c t i c a l e l e c t r o l y t i c hydrogen g e n e r a t i o n (3). e

P h o t o - e f f e c t s a t M e t a l s and

Semiconductors

I n o r d e r t o d i m i n i s h t h e e n e r g y w h i c h has t o be s u p p l i e d f r o m a n e l e c t r i c a l power s o u r c e t o l i b e r a t e h y d r o g e n f r o m w a t e r , one may l o o k f o r a s s i s t a n c e i n t h e f o r m o f l i g h t e n e r g y . Photoe f f e c t s have been o b s e r v e d a t metal-aqueous s o l u t i o n i n t e r f a c e s , e.g. a t m e r c u r y , b u t t h e s e i n v o l v e p r o d u c t i o n o f h y d r a t e d e l e c t r o n s and t h e i r subsequent r e a c t i o n s w i t h e l e c t r o n s c a v e n g e r s (4). They a r e n o t l i k e l y t o be o f i n t e r e s t i n t h e p r e s e n t context. I n s t e a d , one m u s t c o n s i d e r s e m i c o n d u c t o r - a q u e o u s s o l u t i o n i n t e r f a c e s , w h e r e p h o t o - c u r r e n t s a r e known t o o c c u r , enabling electrochemical reactions to occur i n i l l u m i n a t i o n which h a r d l y o c c u r a t a l l i n t h e d a r k . Thus, hydrogen e v o l u t i o n a t p - t y p e Ge (J5,6) a n d a t p - t y p e Se (_7) o c c u r s w i t h i n c r e a s e d c u r r e n t d e n s i t y ( r a t e ) o r w i t h d e c r e a s e d o v e r v o l t a g e upon i l l u m i n a t i o n . In the case o f selenium, green l i g h t diminished the overvoltage much more t h a n d i d r e d l i g h t , o n l y t h e f o r m e r b e i n g o f e n e r g y > t h e b a n d gap o | t h e s e m i c o n d u c t o r (1.8 eV ( 8 ) ) . Whereas t h e r e d u c t i o n o f H was shown t o r e q u i r e t r a n s f e r o f c o n d u c t i o n b a n d e l e c t r o n s f r o m η-type Ge i n t h e d a r k o r o f p h o t o - g e n e r a t e d e l e c t r o n s ( f r o m t h e c o n d u c t i o n band) o f i l l u m i n a t e d p - t y p e Ge, t h e r e d u c t i o n o f Ο2 r e q u i r e d h o l e t r a n s f e r t o t h e v a l e n c e b a n d (9). A l t h o u g h o x y g e n l i b e r a t i o n was n o t d e m o n s t r a t e d u s i n g germanium e l e c t r o d e s , because o f the i n t e r v e n t i o n o f anodic d i s s o l u t i o n o f the semiconductor, a process i n v o l v i n g h o l e t r a n s f e r from the v a l e n c e b a n d , i t i s e v i d e n t t h a t t h e r e v e r s e r e a c t i o n t o Ο2 r e d u c t i o n a l s o i n v o l v e s h o l e t r a n s f e r , t h a t i s t o say i t would o c c u r a t a p - t y p e s e m i c o n d u c t o r l i k e germanium i n t h e d a r k , o r a t t h e same η-type s e m i c o n d u c t o r when i l l u m i n a t e d . I n f a c t , i n 1972, F u j i s h i m a and Honda (10) d e m o n s t r a t e d t h a t O2 e v o l u t i o n o n η-type Τ1Ο2 o c c u r s a s a p h o t o c u r r e n t , p r o p o r t i o n a l t o t h e l i g h t i n t e n s i t y ( F i g u r e 1) o f w a v e l e n g t h s l e s s t h a n 415 nm, i . e . f o r p h o t o n e n e r g i e s e q u a l t o o r g r e a t e r t h a n t h e b a n d gap o f Τ1Ο2 : 3.0 eV. I n t h i s w o r k a n d t h a t o f O h n i s h i e t a l . (11) a p l a t i n u m b l a c k m e t a l c a t h o d e was c o n n e c t e d i n a n e x t e r n a l c i r c u i t t o an i n d i u m c o n t a c t on t h e back s i d e o f t h e photo-anode (see

15.

of

Photoelectrolysis

S M I T H

to

Solutions

223

Hydrogen

F i g u r e 2 ) . H y d r o g e n was e v o l v e d a t t h e c a t h o d e , t h e o v e r - a l l process being the decomposition of water. This process, involving a n a p p l i e d p o t e n t i a l d i f f e r e n c e b e t w e e n t h e anode and c a t h o d e , we s h a l l r e f e r t o as p h o t o - a s s i s t e d e l e c t r o l y s i s .


Energy C o n d i t i o n s f o r P h o t o - a s s i s t e d

Electrolysis

R e f e r r i n g t o F i g u r e 3, e v i d e n c e e x i s t s f o r p l a c e m e n t o f t h e Fermi l e v e l s (chemical p o t e n t i a l s ) of the redox r e a c t i o n s i n v o l v i n g H 2 , H2O and Ο2 r o u g h l y a t t h e p o s i t i o n s shown r e l a t i v e t o t h e e n e r g i e s o f t h e c o n d u c t i o n b a n d minimum and v a l e n c e b a n d maximum o f t h e s e m i c o n d u c t o r , Ε and Ε , r e s p e c t i v e l y . This p i c t u r e t a k e s t h e e l e c t r o n i n a vacuum a t i n f i n i t y a s t h e z e r o o f energy. On t h i s b a s i s , t h e F e r m i l e v e l f o r t h e r e a c t i o n H

+

+ e" vac

aq

=

h a s t h e v a l u e E° ,„

h H_ (g) 2

.

s

F(H20/H2) Λ

^

Ζ

-4.5

eV f o r u n i t a c t i v i t y o f a l l

the

s p e c i e s ( 1 2 ) . T h i s E° i s t h e s t a n d a r d c h e m i c a l p o t e n t i a l p e r e l e c t r o n t r a n s f e r r e d , i . e . AG /nN , f o r t h e r e a c t i o n , w h e r e AG i s t h e s t a n d a r d G i b b s f r e e e n e r g y o f t h e r e a c t i o n , η t h e number o f e l e c t r o n s t r a n s f e r r e d and Ν A v o g a d r o s number. On t h e same b a s i s , t h e r e d u c t i o n o f oxygen under s t a n d a r d c o n d i t i o n s 1

k Ο

(g) +

h H 0(1)

+ e~ vac ,„ Ζ

λ

2

h a s a F e r m i l e v e l , E° .

F(Ο2/Η2Ο) Λ ν

0H~

=

-5.73

aq eV, o r 1.23

eV

below

t h a t f o r t h e h y d r o g e n i o n . Changes i n c o n c e n t r a t i o n s o f r e a c t a n t s and p r o d u c t s c h a n g e t h e F e r m i l e v e l s f o r r e d o x r e a c t i o n s i n accordance w i t h a m o d i f i e d form o f N e r n s t e q u a t i o n , e.g. f o r t h e h y d r o g e n r e a c t i o n , !

E

F(H 0/H ) 2

2

=

E

F(H 0/H2) 2

+

*

T

l

n

Ί^Τ"

( 4 )

At e q u i l i b r i u m i n the dark, the Fermi l e v e l s f o r electrons and h o l e s i n a p a r t i c u l a r s a m p l e o f s e m i c o n d u c t o r a r e c o i n c i d e n t , b e i n g n e a r t h e t o p o f t h e f o r b i d d e n gap i n an η-type and n e a r t h e b o t t o m o f t h e gap i n a p - t y p e s e m i c o n d u c t o r . F i g u r e 4 i l l u s t r a t e s t h e r e g i o n n e a r t h e s u r f a c e o f an η-type s e m i c o n d u c t o r , t h e F e r m i l e v e l s f o r e l e c t r o n s and h o l e s , c o i n c i d e n t up t o t h e e l e c t r o d e - s o l u t i o n i n t e r f a c e , b e i n g d e n o t e d E_ and E_ η F ρ F respectively. I n F i g u r e 3 t h e F e r m i l e v e l s f o r e l e c t r o n s and h o l e s a r e s e p a r a t e d as a r e s u l t o f the a b s o r p t i o n o f photons o f e n e r g y , hV, g r e a t e r t h a n t h e s e m i c o n d u c t o r b a n d gap. This a b s o r p t i o n o f e n e r g y l e a d s t o t h e g e n e r a t i o n o f e l e c t r o n s and h o l e s (one o f e a c h f o r e a c h p h o t o n a b s o r b e d ) , i n t h e v o l u m e o f the semiconductor p e n e t r a t e d by the l i g h t . The d e g r e e o f


224

CHEMISTRY

F O R

E N E R G Y

Nature

Figure 1. Photocurrent/electrode potential curves for η-type TiO single crystal with ohmic-indium contact on back side. Potentials measured with respect to saturated KCl calomel electrode with a platinum black counter electrode. Light intensity increasing in order 3,2,1. Wavelength, 415 nm or less. Exposed surface of TiO crystal: (001) (10). t

t

Verlag Chemie GmbH

Figure 2. Cell and circuit used in experiments like those in Figure 1. (1)Illumi nated TiOg electrode; (2) platinum counter electrode in the dark; (3) reference electrode (SCE); (4) buffered electrolyte solution; (5) quartz window for UV light; (A) ammeter; (V) voltmeter (11).

15.

S M I T H

Photoelectrolysis

of Solutions

to

Hydrogen

225


separation o f the quasi-Fermi l e v e l s ,

E* a n d E* i n c r e a s e s w i t h η F Ρ F the l i g h t i n t e n s i t y , as F i g u r e 5 i l l u s t r a t e s . I t might be thought t h a t p h o t o g e n e r a t i o n o f e l e c t r o n s and h o l e s i n a s e m i c o n d u c t o r w i t h n o r m a l l y a s m a l l p o p u l a t i o n o f one o r o t h e r (or both) charge c a r r i e r would r e s u l t i n b o t h hydrogen and o x y g e n p r o d u c t i o n a t t h e i n t e r f a c e , t h r o u g h t h e r e a c t i o n s : H + e ( c o n d u c t i o n band) = h H (g) +

+

OH~ + h ( v a l e n c e band) = h 0 ( g ) + h H 0 ( 1 ) aq 2. 2. G e r i s c h e r (13) h a s t e r m e d s u c h a p r o c e s s " p h o t o c a t a l y t i c a c t i o n o f a semiconductor e l e c t r o d e " . The r e a s o n t h a t s u c h p r o c e s s e s h a r d l y occur i s t h a t they a r e i n t e r f a c i a l r e a c t i o n s and, i n t r a v e l l i n g from t h e r e g i o n beneath t h e s u r f a c e t o t h e i n t e r f a c e , t h e e l e c t r o n s a n d h o l e s h a v e many e n c o u n t e r s , t h e s e e n c o u n t e r s l e a d i n g t o r e c o m b i n a t i o n and r e - e m i s s i o n o f l i g h t . Referring to F i g u r e 3, n e c e s s a r y c o n d i t i o n s f o r t h e r e a c t i o n s a b o v e a r e f u l f i l l e d when, r e s p e c t i v e l y , 1) t h e F e r m i l e v e l f o r e l e c t r o n s l i e s a b o v e t h a t o f t h e H 2 O / H 2 redox system and i i ) t h e Fermi l e v e l f o r h o l e s l i e s below t h a t o f t h e O2/H2O system. Unfortunately, these conditions are not s u f f i c i e n t . It i s n e c e s s a r y t o f i n d some way t o s e p a r a t e t h e c h a r g e c a r r i e r s because t h e i r i n t e r a c t i o n i s so s t r o n g and t h e i r r e c o m b i n a t i o n so r a p i d t h a t t h e y m u s t n o t b e p e r m i t t e d t o o c c u p y t h e same r e g i o n of the c r y s t a l . T h i s i s n o t s u c h a d i f f i c u l t c o n d i t i o n t o meet as m i g h t be a t f i r s t t h o u g h t . F i g u r e 4 and F i g u r e 5 a r e i l l u s t r a t i v e o f o n e a p p r o a c h , t h a t o f u s i n g a n η-type s e m i conductor w i t h a surface having a considerably decreased e l e c t r o n population, a so-called depletion layer. T h i s m a t e r i a l would f o r m t h e anode f o r o x y g e n g e n e r a t i o n , h y d r o g e n g e n e r a t i o n o c c u r r i n g a t t h e metal cathode. These a r e t h e c o n d i t i o n s a l r e a d y e x e m p l i f i e d b y t h e w o r k o f F u j i s h i m a a n d Honda (10) i n F i g u r e s 1 and 2. Two a l t e r n a t i v e a p p r o a c h e s a r e p o s s i b l e , u s e o f a p - t y p e s e m i c o n d u c t o r c a t h o d e a n d a m e t a l anode o r u s e o f a p - t y p e s e m i c o n d u c t o r c a t h o d e a n d a n η-type anode i n c o m b i n a t i o n , i n each case t h e semiconductor having a d e p l e t i o n l a y e r a t i t s s u r f a c e . Each approach w i l l be o u t l i n e d , b e g i n n i n g w i t h t h e η-type a n o d e . o

η-type S e m i c o n d u c t o r

i n t h e D a r k a n d Under

Illumination

F i g u r e s 4 a n d 5 show a n η-type s e m i c o n d u c t o r w i t h a s u r f a c e d e p l e t i o n l a y e r , t h e e n e r g y b a n d s b e n d i n g upwards a s t h e s u r f a c e i s approached from t h e i n t e r i o r . U n d e r i l l u m i n a t i o n ( F i g u r e 5) the band bending i s d i m i n i s h e d because o f t h e p h o t o - g e n e r a t i o n

226

CHEMISTRY

F O R ENERGY

Academic Press

Figure 3. Schematic of a semiconduc tor-aqueous electrolyte solution inter face, ignoring band bending. E and E are the band edges of the conduction and valence bands, respectively. E H O/H2) and E /H o) are the Fermi levels in the solution for the redox reactions indi cated. The quasi-Fermi levels with illu mination by light of energy hv are designated E * and P E F * respectively, for electrons and holes (13).


C

E (H Q/H ) F

F(

N

2

i.23eV

2

E (0 F

g

F(0X

2

V

2

/H 0) 2

F

Semiconductor-Aqueous Electrolyte Solution

DARK E (H 0/H ) F

Academic Press

"0,

Figure 4. Semiconductor-electrolyte so lution interface in the dark. An n-type semiconductor with a depletion layer at the surface is illustrated. Ε is electron energy, E and E are the equal Fermi levels for electrons and holes at equi librium, other symbols as in Figure 3 (13). N

F

P

2

2

tr^TJ^^IL ! 01

E (0 /H 0) F

2

2

F

JF(H ) 2

pEf""

-F(0 /H 0) 2

at dark

2

1

E;

F(0 ) 2

moderate illumination

intense illumination Academic Press

Figure 5. η-Type semiconductor-electrolyte solution interface with a surface depletion layer, in the dark and with two intensities of illumination. Symbols as in Figure 3 and 4 with E * and E * the band edges of the conduction and valence bands, respectively, under illumination, and E ) and E ) abbreviations for C

V

F(H2

E O/H ) F(HJS

2

and

E /H o), F(0I

2

respectively.

The

quasi-Fermi

F(0M

levels

E*

N

F

and

V

E

F

*

are at different positions in the surface region than in the bulk as a result of the limited penetration of light into the interior. Fermi levels in solution as in Fig ures 3 and 4 (IS).

15.

SMITH

Photoetectrolysis

of

Solutions

to

Hydrogen

227


of c a r r i e r s i n the r e g i o n near the surface. The b a n d gap r e m a i n s t h e same: Ε* - Ε* = Ε - Ε . Whereas i n t h e d a r k t h e c ν c V c o n c e n t r a t i o n s o f e l e c t r o n s , n , and o f h o l e s , p , a r e r e l a t e d t o t h e ( e q u a l ) numbers o f e l e c t r o n s and h o l e s i n a n i n t r i n s i c s e m i c o n d u c t o r , n^, b y t h e mass a c t i o n l a w : η ρ

=

n^

(5)

under c o n d i t i o n s o f i l l u m i n a t i o n the c o n c e n t r a t i o n s a r e i n c r e a s e d t o v a l u e s n* a n d p * where η* - η a n d ρ* - ρ d e p e n d o n t h e l i g h t i n t e n s i t y and t h e q u a s i - F e r m i l e v e l s d i f f e r f r o m t h e e q u i l i b r i u m (dark) v a l u e s : Ε* - Ε n F n F

=

kT I n — η

(6)

Ε* - E„ p F p F

=

kT I n 2 ρ

(7)

F o r a n η-type s e m i c o n d u c t o r t h e q u a s i - F e r m i l e v e l f o r e l e c t r o n s w i l l n o t s h i f t v e r y much e x c e p t a t t h e s u r f a c e w h i c h f o r m e r l y was d e p l e t e d o f e l e c t r o n s , b e c a u s e e l s e w h e r e n * w i l l n o t be g r e a t l y i n e x c e s s o f n. F o r h o l e s , h o w e v e r , t h e c o n c e n t r a t i o n p* w i l l be much l a r g e r t h a n ρ w h e r e l i g h t p e n e t r a t e s i n t o t h e semiconductor. For t h i s reason, the quasi-Fermi l e v e l f o r holes E*, d e p a r t s m a r k e d l y f r o m i t s f o r m e r v a l u e when l i g h t o f l u f f i c i e n t l y h i g h f r e q u e n c y i s i n c i d e n t upon t h e m a t e r i a l . P h o t o - a s s i s t e d E l e c t r o l y s i s o f W a t e r U s i n g η-type Anode a n d M e t a l Cathode F o u r c r i t e r i a h a v e t o be met f o r s u c c e s s f u l p h o t o - a s s i s t a n c e w i t h t h e e l e c t r o l y s i s o f w a t e r t o h y d r o g e n and o x y g e n , a s s u m i n g t h a t t h e s e m i c o n d u c t o r b a n d gap e x c e e d s 1.23 eV. F i r s t , t h e r e m u s t be upward b e n d i n g o f t h e e n e r g y b a n d s a t t h e i n t e r f a c e , even under i n t e n s e i l l u m i n a t i o n . Second, t h e q u a s i - F e r m i l e v e l f o r e l e c t r o n s must l i e above t h a t f o r t h e H2O/H2 r e d o x system. T h i r d , t h e l i g h t i n t e n s i t y m u s t be h i g h enough t o s p l i t s u f f i c i e n t l y t h e q u a s i - F e r m i l e v e l s f o r e l e c t r o n s and h o l e s , i . e . b y more t h a n 1.23 eV, s u c h t h a t t h e l e v e l f o r h o l e s a t t h e i n t e r f a c e i s b e l o w t h a t f o r t h e Ο2/Η2Ο r e d o x s y s t e m , a s shown i n F i g u r e 6. F o u r t h , t h e f a v o u r e d a n o d i c r e a c t i o n a t t h e s e m i c o n d u c t o r m u s t be Ο2 e v o l u t i o n f r o m w a t e r , r a t h e r t h a n some a n o d i c d i s s o l u t i o n p r o c e s s i n w h i c h t h e s e m i c o n d u c t o r b r e a k s down, a s happens w i t h Ge ( 9 ) , GaP (14,15,16) CdS o r e v e n ZnO ( 1 3 ) . I n F i g u r e 6, t h e c a s e o f η-type Τ1Ο2 and a m e t a l c a t h o d e i s d e p i c t e d , w i t h a n a p p l i e d p o t e n t i a l d i f f e r e n c e (E^, - E ' ) / e , e i t h e r t o ensure t h a t the Fermi l e v e l of e l e c t r o n s i n the metal i s h i g h e r t h a n t h e H2O/H2 r e d o x system so t h a t hydrogen

228

CHEMISTRY

FOR

ENERGY

e v o l u t i o n occurs a t a l l , o r t o i n c r e a s e the r a t e o f such e v o l u t i o n (and t h a t o f Ο2 a s w e l l ) . Holes, generated near the s e m i c o n d u c t o r - e l e c t r o l y t e i n t e r f a c e , t r a v e l towards t h i s i n t e r f a c e , some b e i n g l o s t b y r e c o m b i n a t i o n w h i l e t h e r e m a i n d e r r e a c t :


+

0H~ + h ( T i 0 _ , v a l e n c e ) = h Ο.(g) + h H.0(1) aq 2 2 2 E l e c t r o n s , generated near the s e m i c o n d u c t o r - e l e c t r o l y t e i n t e r f a c e a r e unable t o s t a y i n t h i s r e g i o n because o f the e l e c t r i c f i e l d t h e r e w h i c h d r i v e s them i n t o t h e b u l k o f t h e T1O2 c r y s t a l , o u t t h r o u g h t h e m e t a l l i c c o n t a c t , t h e e x t e r n a l c i r c u i t (where t h e p h o t o - c u r r e n t may be measured) and i n t o t h e c a t a l y t i c a l l y a c t i v e metal. A t the i n t e r f a c e o f t h i s metal w i t h the e l e c t r o l y t e solution, reaction occurs: H*

+

e" ( m e t a l )

=

% H. (g)

ct.vj

Ζ

A c c o r d i n g t o N o z i k (16) t h e e n e r g y a v a i l a b l e f o r e l e c t r o l y s i s when a n η-type anode and m e t a l c a t h o d e a r e u s e d i s : E

gap "

V

B

"

( E

c

"

V

' ά -

(

A

G

/

F

+

\

+

n

c

+

i

R

+

V

( 8 )

Ο

where t h e terms on t h e l e f t - h a n d s i d e r e f e r t o t h e s e m i c o n d u c t o r and i t s e l e c t r o n i c e q u i l i b r i u m , whereas t h o s e on t h e r i g h t r e f e r t o t h e two e l e c t r o c h e m i c a l c h a r g e - t r a n s f e r r e a c t i o n s . The minimum e n e r g y o f l i g h t e x p e c t e d t o be e f f e c t i v e i s hv = Ε = Ε - Ε . v i s t h e amount o f b a n d b e n d i n g , e q u a l gap c ν Β t o Ε ( s u r f a c e ) - Ε ( b u l k ) s o t h a t t h e l e f t - h a n d s i d e may be s i m p l i f i e d t o E - E ( s u r f a c e ) . On t h e r i g h t - h a n d s i d e , t h e s y m b o l s η and Ν a r e t h e number o f e l e c t r o n s t r a n s f e r r e d i n a s i n g l e s t e p o f ? h e r e a c t i o n and A v o g a d r o s number, r e s p e c t i v e l y . The f i r s t t e r m i n p a r e n t h e s e s i s t h e G i b b s f r e e e n e r g y c h a n g e i n t h e o v e r a l l r e a c t i o n , t h e second and t h i r d a r e t h e a n o d i c ( e q u a t i o n ( 3 ) ) and c a t h o d i c ( e q u a t i o n ( 2 ) ) o v e r v o l t a g e s , r e s p e c t i v e l y , f o r o x y g e n and h y d r o g e n e v o l u t i o n , t h e f o u r t h i s t h e i R drop ( r e s i s t i v e l o s s e s ) term,which i n the case o f semi conductor e l e c t r o d e s i n c l u d e s l o s s e s from charge passage through t h e s o l i d p h a s e a s w e l l a s i n s o l u t i o n , and t h e l a s t t e r m embraces the p o t e n t i a l drops across the s o l u t i o n double l a y e r s a t the anode a n d c a t h o d e . U s i n g a s e m i c o n d u c t o r anode i s l i k e l y t o d r i v e up t h e o v e r v o l t a g e o f t h e r e a c t i o n o c c u r r i n g a t t h a t e l e c t r o d e (η ) compared w i t h t h e o v e r v o l t a g e t o be e x p e c t e d i f a m e t a l were used i n s t e a d . The d o u b l e l a y e r p o t e n t i a l d r o p s i n s o l u t i o n a r e l i k e l y t o be s m a l l i f m o d e r a t e l y c o n c e n t r a t e d s o l u t i o n s a r e employed, b u t t h e i R drop i n the s e m i c o n d u c t o r e l e c t r o d e may be i n c o n v e n i e n t l y l a r g e u n l e s s h e a v i l y d o p e d m a t e r i a l (of h i g h e r c o n d u c t i v i t y ) i s used. V

p

1


15.

S M I T H

Photoelectrolysis

of Solutions

to

229

Hydrogen

Academic Press

Figure 6. Schematic showing energy correlations for photoassisted electrolysis of water using n-type TiO as a photoanode and a metal cathode. Symbols as in Figures 3, 4, and 5, except E is Fermi level for metal contact to TiO and E/ is higher Fermi level in metal cathode, polarized by an external source to a potential negative to the semiconductor anode. E ) and E o ) & e abbrevi ated forms for Fermi energies for redox systems of Figure 3 (13). x

F

x

F(ffg

'metal

η-type Ti0

2

E (H 0/H ) F

2

2

[E (0 /H 0)| F

2

2

electrolyte solution

F(

2

f

Academic Press

Figure 7. Schematic showing energy correlations for photoelectrolysis of water without external power source. An ntype semiconductor is depicted with a metal contact and connection through an external circuit to a catalytically active metal with E/ = E . Other symbols as in Figures 5 and 6 (13). F

230

CHEMISTRY

F O R

E N E R G Y


P h o t o - e l e c t r o l y s i s W i t h o u t A p p l i e d P o t e n t i a l U s i n g η-type Anode and M e t a l C a t h o d e Provided t h a t l i g h t o f s u f f i c i e n t l y short wavelength i s used, p h o t o - a s s i s t e d e l e c t r o l y s i s s h o u l d be r e l a t i v e l y easy t o a c h i e v e w i t h a r a n g e o f η-type s e m i c o n d u c t o r s . Τ 1 Ο 2 , SnOz, WO3, t i t a n a t e s and t a n t a l a t e s have been used, these m a t e r i a l s h a v i n g b a n d g a p s o f 2.7 t o 3.5 eV (20, 2 1 , 2 2 ) . The m o s t i n t e r e s t i n g c a s e , however, i s t h a t o f a c h i e v i n g hydrogen and oxygen g e n e r a t i o n by a c t i o n o f l i g h t a l o n e , i . e . w i t h o u t a n a p p l i e d p o t e n t i a l . This has b e e n a c h i e v e d w i t h t h e η-type anode / m e t a l c a t h o d e c o n f i g u r a t i o n i n o n l y one c a s e , t h a t o f S r T i 0 3 , w h i c h has a band gap o f 3.2 eV b u t a g r e a t e r amount o f b a n d b e n d i n g a t t h e i n t e r f a c e than Τ1Ο2, e s p e c i a l l y i n strongly a l k a l i n e s o l u t i o n . The d i f f e r e n c e b e t w e e n Τ 1 Ο 2 a n d S r T i 0 3 i n b a n d b e n d i n g c a n b e e x p r e s s e d b y q u o t i n g t h e f l a t - b a n d p o t e n t i a l s o f t h e two materials: Ο V for T i 0 (23) a n d -0.25 V f o r S r T i 0 (17, 2 0 ) , b o t h w i t h r e f e r e n c e t o a s t a n d a r d h y d r o g e n e l e c t r o d e . The consequence o f t h i s i n c r e a s e d band b e n d i n g i s t h a t t h e charge s e p a r a t i o n o f e l e c t r o n s and holes i n t h e s u r f a c e r e g i o n which i s n o t p o s s i b l e f o r Τ 1 Ο 2 when i l l u m i n a t e d i n t h e a b s e n c e o f a n a p p l i e d v o l t a g e m a k i n g E^, h i g h e r t h a n Ε (as i n F i g u r e 6 ) , becomes p o s s i b l e f o r S r T i 0 3 (17, 1 8 ) . This s i t u a t i o n i s i l l u s t r a t e d i n F i g u r e 7. S t r o n t i u m t i t a n a t e i s s t a b l e u n d e r c o n d i t i o n s o f u s e a s a n anode i n aqueous a c i d o r b a s e . Photoc u r r e n t s o b s e r v e d i n one s t u d y a t z e r o a p p l i e d p o t e n t i a l ranged f r o m 0.5 mA t o 1.5 mA cm" i n 9.1 M NaOH, u s i n g a 200 W s u p e r p r e s s u r e m e r c u r y lamp f o c u s s e d o n t h e p h o t o - a n o d e ( 1 8 ) . P o t a s s i u m t a n t a l a t e , KTaO3, h a s a s i m i l a r d e g r e e o f b a n d b e n d i n g ( f l a t b a n d p o t e n t i a l o f -0.2 V) b u t i t s b a n d g a p i s e v e n l a r g e r ( 3 . 5 eV) ( 2 0 ) . 2

3

2

P h o t o - e l e c t r o l y s i s W i t h Semiconductor Cathodes L o g i c a l l y , t h e use o f a p-type cathode f o r e v o l v i n g hydrogen and a m e t a l anode i s c o m p l e m e n t a r y t o t h e s y s t e m s j u s t d i s c u s s e d . D i f f i c u l t i e s a r i s e i n t h i s c a s e , however, because o f t h e h i g h o v e r p o t e n t i a l s needed t o d r i v e hydrogen e v o l u t i o n on s e m i c o n d u c t o r s s u c h a s S i a n d GaAs. G a l l i u m p h o s p h i d e i s a f a v o u r a b l e i n s t a n c e , however, p h o t o - e v o l u t i o n o f hydrogen a t i l l u m i n a t e d p-type m a t e r i a l o c c u r r i n g a t a higher p o s i t i v e p o t e n t i a l than e x p e c t e d ( 1 4 ) , s e e e . g . F i g u r e 10. Nozik has proposed t h e combination o f a p-type cathode w i t h a n η-type anode a s a means o f u t i l i s i n g l i g h t e n e r g y a t b o t h e l e c t r o d e s i n s t e a d o f a t j u s t o n e o f them ( 1 6 ) , i . e . a t w o - p h o t o n photo-electrolysis. I n t h i s p r o p o s a l i t s h o u l d be p o s s i b l e t o u t i l i s e s e m c o n d u c t o r s w i t h s m a l l e r b a n d g a p s t h a n t h e 3.0 - 3.2 eV o f Τ 1 Ο 2 a n d S r T i 0 3 , b e c a u s e i t i s o n l y n e c e s s a r y t h a t E * ( n ) be b e l o w t h e o x y g e n F e r m i l e v e l a n d t h a t E * ( p ) be a b o v e Ihe h y d r o g e n Fermi l e v e l . The e n e r g y c o n d i t i o n s anâ t h e i n f l u e n c e o f l i g h t

15.

Photoelectrolysis

SMITH

of Solutions

to

231

Hydrogen

i r r a d i a t i o n o n them a r e i l l u s t r a t e d i n F i g u r e s 8 & 9. The e n e r g y a v a i l a b l e f o r e l e c t r o l y s i s ( c o r r e c t i n g a n e r r o r o f s i g n i n ( 1 6 ) ) when u s e i s made o f a p - t y p e c a t h o d e a n d a n η-type anode i s :


E

(n)+

gap Vp

(P)

"V

n) +

V

p)

"V

E

n) +

V

n ) +E

v

(p)

" V

p)

n

w h i c h s i m p l i f i e s t o (E^(n) ( ' s u r f a c e ) ) + (Ε^(ρ, s u r f a c e ) (p))· This corresponds t o the e q u i l i b r i u m s i t u a t i o n depicted i n F i g u r e 8. T h i s a v a i l a b l e e n e r g y i s e q u a t e d t o t h e t e r m s o n t h e right-hand side o f equation (8), so that v

E

F

( E _ ( n ) - Ε (η, s u r f a c e ) ) + (Ε ( p s u r f a c e ) - Ε (ρ)) Jî V C F #

=

— ^ — (AG/F + η + η + i R + ν ) (9) nN a c H ο M a x i m i z a t i o n o f t h e a v a i l a b l e energy f o r e l e c t r o l y s i s w i t h a g i v e n s e m i c o n d u c t o r seems t o r e q u i r e u s e o f t h e m o s t h e a v i l y doped m a t e r i a l t o g e t h e r w i t h a m i n i m a l band b e n d i n g c o n s i s t e n t w i t h e f f i c i e n t charge s e p a r a t i o n , so t h a t i n t h e n-semiconductor, E^, (n) i s a s h i g h a s p o s s i b l e w h i l e Ε ( n , s u r f a c e ) i s a s l o w a s p o s s i b l e a n d i n t h e p - s e m i c o n d u c t o r £p(p) i s a s l o w a s p o s s i b l e and Ε (ρ, s u r f a c e ) a s h i g h a s p o s s i b l e . C a u t i o n must be e x e r c i s e d , h o w e v e r , b e c a u s e a h e a v i l y d o p e d s e m i c o n d u c t o r may become d e g e n e r a t e , i . e . m e t a l l i c i n b e h a v i o u r a n d , a s we h a v e a l r e a d y n o t e d , i t i s n e c e s s a r y t o have s u i t a b l e band bending t o separate the charges. N o z i k s i d e a h a d a l r e a d y b e e n t r i e d b y Yoneyama e t a l . ( 1 9 ) . F i g u r e 10 shows t h e p h o t o - c u r r e n t s o b s e r v e d b y t h e s e w o r k e r s w i t h a p - t y p e GaP c a t h o d e a n d a n n - t y p e T 1 O 2 anode i n H2S0t». Figures 11 a n d 12 show t h e s e w o r k e r s ' d e t e r m i n a t i o n s o f t h e f l a t b a n d p o t e n t i a l s o f Τ 1 Ο 2 a n d GaP i n t h e same e l e c t r o l y t e s o l u t i o n , b o t h i n t h e d a r k a n d w i t h UV i l l u m i n a t i o n . The p o s i t i v e p o t e n t i a l s h i f t upon i l l u m i n a t i o n o f Τ1Ο2 and t h e n e g a t i v e p o t e n t i a l s h i f t a t GaP a r e i n t h e d i r e c t i o n s e x p e c t e d , b u t i t was t h o u g h t t h a t t h e p h o t o - c u r r e n t s o f F i g u r e 10 w o u l d h a v e b e g u n a t t h e f l a t b a n d p o t e n t i a l f o r t h e i l l u m i n a t e d c a s e , w h e r e a s i t a c t u a l l y commenced a t b o t h m a t e r i a l s a t p o t e n t i a l s c l o s e r t o t h e "dark" f l a t band value. T h i s appears t o be a f a v o u r a b l e c i r c u m s t a n c e , perhaps, however, b r o u g h t about by d i f f e r i n g i n t e n s i t i e s o f i l l u m i n a t i o n i n t h e two t y p e s o f e x p e r i m e n t . S u f f i c e t o s a y , t h e f l a t b a n d p o t e n t i a l i s a f f e c t e d by t h e i n t e n s i t y o f l i g h t , i n c r e a s e d i n t e n s i t y producing greater p o t e n t i a l s h i f t s , as expected theoretically. F i g u r e 13 shows p h o t o - c u r r e n t s o b s e r v a b l e a t p - t y p e GaP a n d n - t y p e T 1 O 2 i n NaOH, w h e r e a g a i n t h e f l a t b a n d p o t e n t i a l s d i d n o t q u i t e c o i n c i d e w i t h t h e commencement o f t h e p h o t o - c u r r e n t . C u r r e n t s i n t h e d a r k f o r a l l o f t h e s e c a s e s were r e p o r t e d t o be negligibly small. 1


232

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E N E R G Y

E (P) C

E (H 0/H F

2

2

Photoelectrolysis of Aqueous Solutions to ... - ACS Publications

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