Oxyfluoride Photoelectrodes for the Photodecomposition of Water by

9 Oxyfluoride Photoelectrodes for the Photodecomposition of Water by Solar Energy A.

WOLD

and K .

DWIGHT

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch009

Department of Chemistry, B r o w n University, Providence, R I 02912

Several oxides—for

example,

TiO ,

SrTiO3,

2

SnO2, and

—have been used as anodes for the photoelectrolytic position

of water by solar energy.

made to conduct ever, oxygen the

chemical oxygen

method

substitution vacancies

stable electrodes

of oxygen

undoubtedly

instability

An alternative

All these oxides must

by the creation

deficiencies

long-term

of the

should toward

result oxygen

defects.

are

electrodes

of enhancing

of oxygen

WO3 decom-

responsible studied

conductivity

by fluorine. in the in

be

Howto

for date.

is to

use

The absence

formation

of

of

more

solution.

T n m o s t of the p r e v i o u s i n v e s t i g a t i o n s t h a t h a v e dealt w i t h n - t y p e o x i d e A

electrodes for t h e p h o t o e l e c t r o l y s i s of w a t e r , i n c r e a s e d c o n d u c t i v i t y

w a s a c h i e v e d b y t h e p r o d u c t i o n of o x y g e n deficiencies.

T h i s early w o r k

o n n - t y p e anodes s u c h as T i 0 - a > i n d i c a t e d t h a t t h e d e f e c t 2

were

stable.

H o w e v e r , recent

evidence

compounds

has s h o w n t h a t these

(1,2)

c o m p o u n d s d o not s h o w l o n g - t e r m s t a b i l i t y i n t h e p r e s e n c e of o x y g e n at t h e i r surfaces. A n a l t e r n a t i v e m e t h o d of p r o d u c i n g c o n d u c t i n g electrodes is to substitute fluorine f o r o x y g e n r a t h e r t h a n to create o x y g e n v a c a n c i e s . B o t h methods

r e s u l t i n the f o r m a t i o n of

3d

1

titanium, w h i c h w o u l d

account for the relatively h i g h conductivity obtained. I n a recent p u b l i c a t i o n D e r r i n g t o n et a l . (3) 3 x

r e p o r t e d o n t h e p h o t o e l e c t r o l y t i c b e h a v i o r of

Pure W 0 , prepared b y completely o x i d i z i n g tungsten foil,

W0 . F . x

3

c o u l d best b e i n d e x e d o n a t r i c l i n i c system s i m i l a r to t h e one r e p o r t e d b y R o t h a n d W a r i n g (14).

I t t r a n s f o r m s to a m o n o c l i n i c p h a s e u p o n t h e

r e m o v a l of s m a l l a m o u n t s of o x y g e n (15). (O ^

x ^

W h e r e a s the system W 0 . 3

1) is m o n o c l i n i c t h r o u g h o u t t h e e n t i r e r a n g e (5)

i

WO3.fl.F-p p r e p a r e d i n t h e s t u d y of

D e r r i n g t o n et a l . (3)

f

t h e system undergoes

p r o g r e s s i v e s t r u c t u r a l m o d i f i c a t i o n s , w h i c h are s u m m a r i z e d i n T a b l e I . 0-8412-0472-l/80/33-186-161$05.00/l © 1980 A m e r i c a n C h e m i c a l S o c i e t y

Holt et al.; Solid State Chemistry: A Contemporary Overview Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

162

SOLID S T A T E

CHEMISTRY: A CONTEMPORARY OVERVIEW

Table I. Structural x

a (A)

b(A)

0 0.0079 ± 0.0005 0.0177 ± 0.001 0.0663 ± 0.005

7.306(1) 7.301(1) 7.311(1) 7.369(1)

7.527(1) 7.527(1) 7.545(1) 7.482(1)

T h e s t r u c t u r e r e m a i n s t r i c l i n i c f o r v e r y s m a l l (x = substituted stituted

fluorine

fluorine

i n t h e system W03. F . X

X

0.0079) a m o u n t s of

W h e n t h e a m o u n t of s u b

is 0.0177, t h e s t r u c t u r e has u n d e r g o n e t h e t r a n s i t i o n t o

the monoclinic phase, a n d w h e n x =

0.0663, t h e r e s u l t i n g p h a s e i s


orthorhombic. T h e resistivity of the pure W 0 cm.

3

samples w a s a p p r o x i m a t e l y 1 0 O • 6

T h e resistivities of t h e W O s . * samples r a n g e d b e t w e e n 1.2 X 1 0

a n d 7 X 10" fi • c m , a n d t h e resistivities of t h e W0 . F 1

3 x

4

samples r a n g e d

x

b e t w e e n 100 a n d 5 O • c m . A b s o r p t i o n m e a s u r e m e n t s i n d i c a t e d t h a t t h e b a n d g a p of W 0

3

a n d a l l W 0 . « F . samples w a s 2.65 ± 3

consistent w i t h p r e v i o u s i n v e s t i g a t i o n s ( 3 ,

0.10 e V . T h i s is

5-12).

T h e p h o t o c u r r e n t s versus a p p l i e d v o l t a g e are p l o t t e d i n F i g u r e 1 f o r several W 0 . 3

samples. M e a s u r e m e n t s w e r e m a d e w i t h t h e e l e c t r o l y t e i n

x

e q u i l i b r i u m w i t h a i r . A s c a n b e seen, t h e largest p h o t o c u r r e n t i s r e a c h e d for the W 0 . 3

s a m p l e h a v i n g t h e l o w e s t resistance, w i t h t h e p h o t o c u r

x

rents of t h e r e m a i n i n g samples d e c r e a s i n g as t h e resistance increases. T h e s e results are consistent, w i t h t h e o n l y effect b e i n g a c h a n g e i n t h e o v e r a l l c e l l resistance. T h e p h o t o c u r r e n t s f o r t w o t r i c l i n i c samples of WO . F s x

x

are shown

versus a p p l i e d bias i n F i g u r e 2, t h e m e a s u r e m e n t s b e i n g m a d e w i t h t h e electrolyte i n e q u i l i b r i u m w i t h a i r . A l t h o u g h p h o t o c u r r e n t s w e r e o b s e r v e d f o r a l l o f t h e t u n g s t e n oxyfluorides s t u d i e d , t h e m o n o c l i n i c a n d o r t h o r h o m b i c c o m p o s i t i o n s (x ^

0.0177) s h o w m o r e c o m p l e x b e h a v i o r .

T h e s p e c t r a l responses of t h e W O ^ F a . samples s h o w n i n F i g u r e 3 w e r e o b t a i n e d w i t h a n a p p l i e d bias of 0.5 V . T h e p h o t o c u r r e n t s p l o t t e d here were n o r m a l i z e d for clarity b y taking the ratio of the photocurrent at a g i v e n w a v e l e n g t h to t h e m a x i m u m p h o t o c u r r e n t o b t a i n e d ( t h a t i s , at 4 0 0 n m ) . T h e a c t u a l p h o t o c u r r e n t s at 400 n m are f o r x =

0.0079, I

— 16.06 fjA • ( c m ) " , a n d f o r x — 0.0083, I — 6.07 pA • ( c m ) " . T h e 2

1

2

1

colors of t h e m a t e r i a l s v a r i e d f r o m a l i g h t g r e e n f o r t h e x = 0.0079 s a m p l e to d a r k e r g r e e n f o r t h e x =

0.0083 s a m p l e .

T h e s t a b i l i t y of t h e W0 .

3 x

a n d W O ^ F a , samples w a s i n v e s t i g a t e d

b y three p r o c e d u r e s : s t a b i l i t y against r e o x i d a t i o n , s t a b i l i t y against h y d r o l ysis o r d i s s o l u t i o n , a n d s t a b i l i t y i n a w o r k i n g c e l l a r r a n g e m e n t . F i g u r e 4 shows t h e results o f t h e r e o x i d a t i o n e x p e r i m e n t s o n samples o f a b o u t 175 mg. While W 0 . 3

f

r e a d i l y o x i d i z e s at 2 5 0 ° - 3 0 0 ° C , t h e W O g ^ F * s a m p l e


9.

WOLD A N D DWIGHT

Oxyfluoride

163

Photoelectrodes

Properties of WO3-3F4. C

a

3.854(1) 3.856(1) 3.851(1) 3.848(1)

88°43' ± 2' 88°48'±2' 90° 90°

y

90°17' ± 2' 90° 17' ± 2 ' 90°51' 90°

90°39' ± 2' 90°35' ± 2' 90° 90°

is stable to 6 0 0 ° C , i m p l y i n g a n i n c r e a s e d s t a b i l i t y to r e o x i d a t i o n . A l s o , the t h e r m a l g r a v i m e t r i c d a t a s h o w n i n F i g u r e 4 i n d i c a t e s t h a t t h e c o m p o s i t i o n of t h e oxyfluoride samples p r e p a r e d i n this s t u d y c a n b e r e p r e sented b y t h e f o r m u l a W0 . F . I f t h e c o m p o s i t i o n of t h e samples h a d b e e n W0 . ¥ i n s t e a d of W 0 - * F « , t h e n there w o u l d h a v e b e e n a g a i n i n w e i g h t r e c o r d e d e q u i v a l e n t to t h e v a l u e x — y. F r o m F i g u r e 4 w e see t h a t t h e s e n s i t i v i t y of this m e t h o d is s u c h t h a t values of x (represent i n g a n o x y g e n d e f i c i e n c y ) i n the system W0 . can be determined, where values o f x are less t h a n 0.01. 3 x

3 x


/3

y

x

3

3 x

H y d r o l y s i s a n d d i s s o l u t i o n experiments w e r e m a d e b y p l a c i n g s a m p l e s of W O 3 - S a n d W0 . F i n 0 . 2 M H S 0 a n d l e a v i n g these at 9 0 ° C f o r 350 h r . N e i t h e r s a m p l e s h o w e d a n y h y d r o l y s i s o r d i s s o l u t i o n . 3 x

x

2

4

S t a b i l i t y i n a w o r k i n g c e l l , w i t h t h e electrolyte i n e q u i l i b r i u m w i t h air, w a s d e t e r m i n e d b y b i a s i n g t h e electrode at 0.5 V w i t h respect t o t h e

PHOTOCURRENT

UF WD

-+a

D.7

OHM-CM

+

16

OHM-CM

•

ISO

OHM—CM

•

520

OHM-CM

•

I EDO

OHM-CN

1

-a.s

I

0.S

B1R5 Figure I .

Photocurrent versus applied in 0.2M NaC H 0 2

s

2

l

1

l

•

I

1

3-X

»

i >

I

VQLTREE

Inorganic Chemistry

bias for several W 0 . (pH 7.8) (2) 5

x

samples


164

SOLID S T A T E C H E M I S T R Y : A C O N T E M P O R A R Y O V E R V I E W

PHDTDCURRENT

DF WD

F

J x x I . 1. . . . I I . . . I . . . . I•

5

H

D

X a

+

X n

0.0D73C2) 0083(2)

«

3


2 --

-4-0.5

0.5

I

1.5

BIH5 VDLTREE Inorganic Chemistry

Figure 2.

Photocurrent versus applied bias for several W0 _ F in 0.2M NaC H O (pH 7.8) (2) S

2

s

X

X

samples

g

p l a t i n u m c a t h o d e , i l l u m i n a t i n g t h e n w i t h t h e f u l l o u t p u t of a 1 5 0 - W x e n o n l a m p a n d m o n i t o r i n g the changes i n p h o t o c u r r e n t w i t h t i m e . W h i l e t h e s l i g h t l y r e d u c e d W 0 . , films g a v e stable p h o t o c u r r e n t s , t h e m o r e r e d u c e d samples w e r e less stable. T h e most r e d u c e d s a m p l e (x « 0.03) was very unstable, w i t h t h e photocurrent decreasing b y 3 0 % over a p e r i o d of 2 h r . T h i s is consistent w i t h t h e results f o u n d b y H a r d e e a n d B a r d ( I I ) . O n t h e other h a n d , t h e t r i c l i n i c samples o f W O a . ^ F ^ gave stable p h o t o c u r r e n t s o f 2 m A • ( c m ) " for p e r i o d s o f u p to 46 h r ( a b o u t 7 0 0 ° C ) . I n a d d i t i o n , there w a s n o v i s i b l e c h a n g e o n the surface o f the electrode. 3

2

1

A s a r e s u l t o f the w o r k o n the W0 _ ¥ system, i t w a s e x p e c t e d t h a t m e m b e r s o f t h e system T i 0 - . F also w o u l d s h o w i n c r e a s e d s t a b i l i t y over T i O o . , since a l l the a n i o n sites w o u l d b e o c c u p i e d . 3

2

a

x

x

a 7

x

S a m p l e s o f Ti0 . Fx w e r e p r e p a r e d b y t h e fluorination o f T i 0 w a f e r s ( c u t f r o m single crystals o b t a i n e d f r o m N a t i o n a l L e a d ) , u s i n g a fluorinating system s h o w n i n F i g u r e 5. H y d r o g e n fluoride w a s g e n e r a t e d by the thermal decomposition of potassium bifluoride at 260°C. T h e T i 0 wafer was positioned w i t h i n a sample tube a n d centered w i t h respect t o t h e h o t z o n e o f a f u r n a c e . F i g u r e 5 d e p i c t s the a r r a n g e m e n t of the furnaces a n d gas t r a i n u s e d f o r t h e p r e p a r a t i o n o f the T i 0 . . F samples. A gas m i x t u r e of 8 5 % a r g o n a n d 1 5 % h y d r o g e n w a s d r i e d b y p a s s i n g i t t h r o u g h a p h o s p h o r u s p e n t o x i d e d r y i n g t u b e , flowing i t o v e r 2 x

2

2

2


a

a r

9.

WOLD A N D DWIGHT


I

Oxyfluoride

165

Photoelectrodes

5PECTRRL RESPONSE or WD F -a—i—i—i—i—i—J—i—i—i—|—i—i—i—i—|-

0 4—i—i—i—*—i—i—i—i—i—i—i—i—*—*• H00

HS0

500

550

WRVELENETH ( N M ) Inorganic Chemistry

Figure 3.

Normalized spectral reponse of W O _ x F in 0.2M Eg = optical band gap (2.7 eV) (2). s

x

NaC H 0 ; 2

s

2

the p o t a s s i u m b i f l u o r i d e ( w h i c h w a s k e p t at 2 6 0 ° C ) , p a s s i n g i t over t h e s a m p l e , a n d finally e x i t i n g i t t h r o u g h a s o d i u m h y d r o x i d e b u b b l e r . S a m p l e s w e r e fluorinated at t e m p e r a t u r e s b e t w e e n 575° a n d 7 0 0 ° C . A t t e m p t s to fluorinate a T i 0 w a f e r at 550° f a i l e d to y i e l d a h o m o g e n e o u s p r o d u c t . T h e p r o d u c t s p r e p a r e d i n this m a n n e r a p p e a r e d p a l e b l u e t o b l a c k i n color. T h e d a r k e r colors r e s u l t e d f r o m h i g h e r - t e m p e r a t u r e preparations. 2

S a m p l e s of T i 0 . F . w e r e p r e p a r e d f o r analysis b y g r i n d i n g a p i e c e of e a c h fluorinated w a f e r to g i v e a p p r o x i m a t e l y 100 m g of s a m p l e . T h e finely d i v i d e d p o w d e r w a s h e a t e d i n a n o x y g e n atmosphere f r o m r o o m t e m p e r a t u r e to 1000°C, a n d changes i n t h e w e i g h t w e r e r e c o r d e d u s i n g a C a h n electrobalance ( m o d e l R G ) a n d a c h a r t recorder. N o n e o f t h e samples i n t h e series s h o w e d a n y m e a s u r a b l e w e i g h t c h a n g e , w h i c h 2

a ?

a


166

SOLID S T A T E C H E M I S T R Y :

THERMRL

i . . . .

0.E • ID

hX ID Ld 2

0.S

-

A CONTEMPORARY

5TRBILITY

i . . . .

i • . . .

OVERVIEW

i

WD.

a.B7 run X

WD,

v

-

Ln

ui u

EC

£50

B00

E50

700

750

FLUOR INHTI ON TEMPERRTURE ( D E E C ) Inorganic Chemistry

Figure

6.

Variation

of resistivity with the fluorination TiO . F electrodes (13) s

X

temperature

of

X

T h i s r e s u l t e d i n a r e p r o d u c i b l e area 2.25 m m i n d i a m e t e r i r r a d i a t e d w i t h a p p r o x i m a t e l y 50 m W determined

for e a c h

(16-junction

of t o t a l p o w e r . curve

Coblentz-type).

by

T h e instantaneous

power

using a calibrated E p p l e y

Anodic

bias w a s

was

thermopile

applied via a

voltage

f o l l o w e r h a v i n g a n o u t p u t i m p e d a n c e less t h a n 0.1 O, a n d t h e r e s u l t i n g response w a s

measured

w i t h a current

amplifier, w h i c h

inserted

a

n e g l i g i b l e p o t e n t i a l d r o p (less t h a n 1 /xV) i n t h e e x t e r n a l c i r c u i t . T h e v a r i a t i o n of p h o t o c u r r e n t w i t h a n o d e p o t e n t i a l ( m e a s u r e d

against

S C E ) is s h o w n i n F i g u r e 7 for samples f l u o r i n a t e d at several temperatures (T ) F

b e t w e e n 575° a n d 7 0 0 ° C .

T h e indicated photocurrent

h a v e a l l b e e n n o r m a l i z e d to a t o t a l i r r a d i a t i o n of 12.5 m W corresponding

densities

• (mm )" , 2

1

to 50 m W i n c i d e n t o v e r the i l l u m i n a t e d area o f 4 m m . 2

I t is e v i d e n t t h a t the s a t u r a t i o n p h o t o c u r r e n t p o t e n t i a l of 0.6 V )

increases

(measured

significantly w i t h decreasing

at a n a n o d e fluorination

t e m p e r a t u r e o v e r this range. T h e essentially l i n e a r n a t u r e of this r e l a t i o n -



9.

WOLD A N D DWIGHT

Oxyfluoride

169

Photoelectrodes

RNDM: POTENT IRL

Y5

5CE

(VOLTS) Inorganic Chemistry

Figure 7. Dependence of photocurrent upon anode potential (SCE reference) for Ti0 - F electrodes fluorinated at various temperatures T for white xenon arc irradiation of 1.25 W • (cm )' (13) 2 x

x

F

2

s h i p c a n b e seen i n F i g u r e 8. (cm )" 2

1

T h e p e a k p h o t o c u r r e n t o f 17.9 m A •

o b t a i n e d here is a p p r o x i m a t e l y t w i c e t h e m a x i m u m [9.2 m A •

( c m ) " ] f o u n d w i t h u n f l u o r i n a t e d Ti0 2

1

1

under similar conditions ( 1 4 ) .

2 x

T h e l a t e r a l shift of t h e c u r v e s p r e s e n t e d i n F i g u r e 7 results f r o m a systematic v a r i a t i o n o f t h e

flat-band

p o t e n t i a l (Ufb).

Values for I7

fb

( m e a s u r e d against S C E ) w e r e o b t a i n e d f r o m t h e l i n e a r d e p e n d e n c e of the square root of t h e p h o t o c u r r e n t u p o n a n o d e p o t e n t i a l f o r s m a l l v a l u e s of t h e c u r r e n t ( v a l u e s less t h a n 0.5 m A • ( c m ) " 2

shown plotted against

fluorination

1

i n F i g u r e 7 ) . T h e y are

t e m p e r a t u r e i n F i g u r e 9. T h e o b s e r v e d

increase i n flat-band p o t e n t i a l corresponds t o a decrease i n its energy. T h i s decrease i n e n e r g y appears a n o m a l o u s f o r a n increase i n c a r r i e r concentration, w h i c h w o u l d be expected

t o raise t h e F e r m i l e v e l , b u t

s i m i l a r b e h a v i o r has b e e n r e p o r t e d elsewhere ( 8 ) . T h e d e t e r m i n i n g f a c t o r m a y w e l l b e a l o w e r i n g o f t h e v a l e n c e b a n d because o f t h e greater e l e c t r o n e g a t i v i t y of t h e s u b s t i t u t e d

fluorine

(10,15).


170

SOLID S T A T E

CHEMISTRY: A

T l D

2

20

N

Z

15

--

IB

-

CONTEMPORARY OVERVIEW

-x x F

\


rx z

EC Z3

•

— I • X 0_

-h 5512!

500

E5B

70B

75B

FLUOR INRTIDN TEMPERRTURE ( D E E C )

Inorganic Chemistry

Figure 8. Variation of saturation photocurrent, measured at an anode potential of 0.6 V, with the fluorination temperature of Ti0 _ F electrodes for white xenon arc irradiation of 1.25 W • (cm )' (13) 2

2

T h e s p e c t r a l p h o t o response of the

fluorinated

X

X

1

electrodes w a s s t u d i e d

b y i n s e r t i n g a n O r i e l m o n o c h r o m e t e r ( m o d e l 7240) i n p l a c e of the 5 - m m aperture.

T h e c u r v e s s h o w n i n F i g u r e 10 w e r e o b t a i n e d at a n

anode

p o t e n t i a l of 0.6 V w i t h a slit w i d t h of 0.5 m m , w h i c h g a v e a s p e c t r a l r e s o l u t i o n of 4 n m . T h e p h o t o c u r r e n t s h a v e b e e n n o r m a l i z e d so as to y i e l d i n t e g r a t e d outputs c o r r e s p o n d i n g to the v a l u e s g i v e n i n F i g u r e 8. T h e s e d a t a c a n also be expressed i n terms o f q u a n t u m efficiency trons p e r p h o t o n )

(elec

b y d i v i d i n g the o b s e r v e d p h o t o c u r r e n t (electrons p e r

s e c o n d ) b y the i n c i d e n t r a d i a t i o n ( p h o t o n s

per second).

The

curves

p r e s e n t e d i n F i g u r e 11 h a v e n o t b e e n c o r r e c t e d f o r a n y a b s o r p t i o n i n t h e e l e c t r o l y t e or c e l l w i n d o w , n o r f o r r e f l e c t i o n f r o m t h e s a m p l e surface. F r o m F i g u r e s 10 a n d 11 w e see that the increase i n o b s e r v e d current w i t h decreasing

fluorination

photo

t e m p e r a t u r e arises f r o m i n c r e a s e d

r e s p o n s i v i t y at the l o n g e r w a v e l e n g t h s .

These wavelengths

m o r e d e e p l y i n t o t h e electrodes, a n d t h e i r p h o t o - g e n e r a t e d

penetrate

electron-hole


9.

WOLD AND DWIGHT

Oxyfluoride

T

-0.3

1 I

in

• >

I

171

Photoelectrodes

l

I

W x

[

I

I

I

I

I

1 1

•0.35--

Ld in -H.M >


± -0.H5 +

u

n m -0.55 I -0.B

-f-

-h

550

£00

E50

700

750

FLUDRINRTDN TEMPERRTURE ( D E B c) Inorganic Chemistry

Figure

9.

Variation of flat-band potential fluorination temperature of Ti0 _jF 2

x

(SCE reference) with electrodes (13)

the

p a i r s w i l l b e c o m e s e p a r a t e d o n l y i f this p e n e t r a t i o n is less t h a n the w i d t h of the d e p l e t i o n l a y e r . T h u s t h e i m p r o v e d response o b s e r v e d at l o w T

F

m a y b e a t t r i b u t e d to the increase i n d e p l e t i o n l a y e r w i d t h r e s u l t i n g f r o m the increase i n r e s i s t i v i t y seen i n F i g u r e 6. T h e s p e c t r a l p h o t o response of a s a m p l e of T i 0 - a , r e d u c e d at 6 0 0 ° C 2

also has b e e n m e a s u r e d . i n t e g r a t e d o u t p u t (13)

T h e results, n o r m a l i z e d t o 9 . 2 - m A - ( c m ) "

1

2

i n F i g u r e 12. I t is e v i d e n t t h a t the a b l y greater response

2

are c o m p a r e d w i t h those o b t a i n e d for T i 0 . » F « fluorinated

m a t e r i a l gives a n a p p r e c i

at t h e l o n g e r w a v e l e n g t h s , w h i c h effect c a n

a t t r i b u t e d to a n absence of v a c a n c i e s i n t h e

fluorinated

samples.

be The

solar s p e c t r u m f a l l s off m u c h m o r e r a p i d l y b e l o w 400 n m t h a n t h e x e n o n arc s p e c t r u m . T h u s the d i s p a r i t y b e t w e e n the o u t p u t s of

fluorinated

and

r e d u c e d r u t i l e w o u l d b e g r e a t l y e n h a n c e d u n d e r solar i r r a d i a t i o n . T h e l o n g - t e r m s t a b i l i t y of electrodes p r e p a r e d f r o m these same t w o samples has b e e n d e t e r m i n e d u n d e r 12.5 m W • ( m m ) " 2

1

irradiation w i t h

a n a p p l i e d a n o d i c b i a s of 1.5 V . T h e r e s u l t i n g d e c a y of p h o t o c u r r e n t w i t h


172

SOLID S T A T E

5PECTRRL

CHEMISTRY:

A CONTEMPORARY

O F

RE5PDN5E

T I D

2

_

X

OVERVIEW

F

X

0.2S f

0.2

•

+


•

£

S7S

F

SBH

p

z:

S

T =

* T = +

T

=

BSD

•

0.1

x + x * * °

0.05

° +

-+250

300

350

H00

WBVELENGTH ( N M )

H50 Inorganic Chemistry

Figure 10. Spectral photo response of Ti0 .J? for electrodes fluorinated at various temperatures T , normalized to white xenon arc irradiation of 1.25 W • (cm )- (13) 2

x

F

T

,

D

2

* >z UJ ^

- x

x

2

1

F

x O

0.H

0.5

0. H

0.2

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--

+

+

•

• $ o *

.

n

a

+

n

OT

n

R

=

+ * n

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T = SBB

P

T

+

x

F

=

7DQ

•+-

250

300

350

400

H50

WAVELENGTH C N M ) Inorganic Chemistry

Figure 11. Quantum efficiency in electrons per photon as a function of excitation wavelength for TiO _ F electrodes fluorinated at various temperatures T (13) g

X

X

F


9.

WOLD A N D DWIGHT

Oxyfluoride

173

Fhotoeleqtrodes

5PECTRRL

RESPONSE

0.25 • f

0

0.2


n

I

0 . 15

E

0.1

• o zn

0.05

XX

+

X i

250

1 . 300

•

•

•

1

• • • « T 9

350

e 450

400

WRVELEN5TH ( N M ) Inorganic Chemistry

Figure 12. Comparison between the spectral photo response of TiO _ F electrodes fluorinated at 575°C and that of Ti0 _ electrodes reduced at 600°C, normalized to white xenon arc irradiation of 1.25 W • (cm )' (13) z

2

x

x

x

2

1

t i m e is s h o w n i n F i g u r e 13. T h e fluorinated electrode w a s f o u n d t o b e a p p r e c i a b l y less stable t h a n the r e d u c e d electrode i n 0 . 2 M s o d i u m acetate, p r e s u m a b l y because of h y d r o l y s i s o f t h e fluorine ions. S t i c h h y d r o l y s i s s h o u l d b e suppressed b y a sufficient c o n c e n t r a t i o n o f fluoride ions i n t h e electrolyte. M e a s u r e m e n t s w e r e m a d e i n a l u c i t e c e l l h a v i n g a fluorite w i n d o w 2 m m t h i c k w i t h a 0 . 2 M s o l u t i o n of p o t a s s i u m b i f l u o r i d e b u f f e r e d to a p H o f 6 w i t h p o t a s s i u m h y d r o x i d e a n d w i t h a 0 . 2 M s o l u t i o n o f u n b u f f e r e d p o t a s s i u m b i f l u o r i d e ( p H = 3 . 5 ) . A s seen i n F i g u r e 13, t h e h y d r o l y s i s w a s p a r t i a l l y a n d c o m p l e t e l y suppressed, r e s p e c t i v e l y , b y these t w o electrolytes. I n t h e latter case t h e l o n g - t e r m s t a b i l i t y w a s i m p r o v e d over t h a t of u n f l u o r i n a t e d r u t i l e . I n s u m m a r y , t h e s u b s t i t u t i o n o f s m a l l amounts of fluorine f o r o x y g e n i n W 0 increases t h e s t a b i l i t y of t h e p h o t o a n o d e a n d i n T i 0 increases the p h o t o c u r r e n t o u t p u t , w h i c h arises f r o m i n c r e a s e d response at l o n g e r w a v e l e n g t h s . T h e i m p r o v e d response m a y b e p a r t i a l l y a t t r i b u t e d t o a n increase i n t h e w i d t h of t h e c h a r g e d e p l e t i o n l a y e r . T h e increase i n s t a b i l i t y is a r e s u l t of t h e l a c k o f defects i n t h e n - t y p e o x y f l u o r i d e elec trodes. F u r t h e r w o r k is necessary i n o r d e r t o d e t e r m i n e i f a n o x y f l u o r i d e electrode c a n b e p r e p a r e d t h a t c o m b i n e s i n c r e a s e d s t a b i l i t y w i t h a suffi cient p h o t o response necessary f o r a s u i t a b l e p h o t o anode. 3

2


174

SOLID S T A T E C H E M I S T R Y : A

CONTEMPORARY OVERVIEW


5TRBILITY

2

1 i i i i ii ii i | i i i i | i i i i Ii i i i 0

I

2

3

H

ELRP5ED TIME

S

1 i i i i I i i i ii

H

7

(HOURS) Inorganic Chemistry

Figure 13. Comparison between the decay of photocurrent with time for Ti0 . electrodes in 0.2M sodium acetate and that for TiO . F electrodes fluorinated at 575°C in 0.2M potassium bifluoride, 0.2M potassium bifluoride buffered with potassium hydroxide, and 0.2M sodium acetate. Measurements were made with 1.5 V of anodic bias under white xenon arc irradiation of 1.25 W • (cm )' (13). 2 x

s x

2

x

1

Acknowledgment T h e authors w o u l d l i k e to a c k n o w l e d g e the Office of N a v a l R e s e a r c h , A r l i n g t o n , V A , f o r t h e i r s u p p o r t of K i r b y D w i g h t . A c k n o w l e d g m e n t is also m a d e to the N a t i o n a l Science F o u n d a t i o n , W a s h i n g t o n , D . C . , N o . G H 37104. I n a d d i t i o n , t h e authors w o u l d also l i k e to a c k n o w l e d g e the support

of

the

Materials

Research

Laboratory

Program

at

Brown

University.

Literature Cited 1. Harris, L. A.; Wilson, R. H. J. Electrochem. Soc. 1976, 123, 1010. 2. Harris, L. A.; Gross, D. R.; Gerstner, M . E. J. Electrochem. Soc. 1977, 124, 839. 3. Derrington, C. E.; Godek, W. S.; Castro, C. A.; Wold, A. Inorg. Chem. 1978, 17, 977.



9.

WOLD

A N D DWIGHT

Oxyfluoride

Photoelectrodes

175

4. Roth, R. S.; Waring, J. L. J. Res. Natl. Bur. Stand., Sec. A 1966, 70(4), 281. 5. Gebert, E.; Ackermann, R. J. Inorg. Chem. 1966, 5, 136. 6. Butler, M. A.; Nasby, R. D.; Quinn, R. K. Solid State Commun. 1976, 19, 1011. 7. Deb, S. K. Philos. Mag. 1973, 27, 801. 8. DeWald, J. F. J. Phys. Chem. Solids 1960, 14, 155. 9. Gissler, W.; Memming, R. Electrochem. Soc., Virginia Conf., Airlie, VA, May 1977. 10. Gomes, W. P.; Cardon, F. "Proceedings of Conference on Electrochemistry and Physics of Semiconductor-Liquid Interfaces Under Illumination"; Heller, A., Ed.; Electrochem. Soc.: Princeton, NJ, 1977; 120. 11. Hardee, K. L.; Bard, A. I. J. Electrochem. Soc. 1977, 124, 215. 12. Hodes, G.; Cahen, D.; Manassen, J. Nature (London) 1786, 26, 312. 13. Van der Pauw, L. J. Phillips Tech. Rev. 1958, 20, 220. 14. Subbarao, S. N . ; Yun, Y. H.; Kershaw, R.; Dwight, K.; Wold, A., unpub lished data. 15. McCauldin, J. O.; McGill, T. C. "Electrochem. Soc. Monograph on Thin Films and Interfaces"; Mayer et al., Eds.; 1977; in press. RECEIVED September 13, 1978.


Oxyfluoride Photoelectrodes for the Photodecomposition of Water by

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