Photo Responses of Pure and Doped Rutile - Advances in Chemistry

6 Photo Responses of Pure and Doped Rutile JOHN

B.

GOODENOUGH

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Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3 Q R E n g l a n d

Photoconductivity, (ESR),

and

photosensitized

photosensitized

electron

pure and doped rutile are summarized. urements

have revealed

localized

states—both

—relative

spin

electrochemical

resonance

reactions

The first two

the energies of metastably native-defect

to the band edges.

occupied,

and impurity

Electrochemical

3d

states

n

studies

sug-

gest the general shape of the density of donor surface in reduced

rutile;

sec)

and a slow (t = 10 sec)

anodic

they also reveal

in rutile photosensitized related

to the

questions

of

1/2

[Ru(bipy) ]Cl2. 3

localized

possible

versus

h

b

17,

~

h

c o n v e n t i o n a l b a n d t h e o r y m u s t b e m o d i f i e d b y t h e i n t r o d u c t i o n of s t r o n g c o r r e l a t i o n energies a n d e l e c t r o n - l a t t i c e c o u p l i n g . T h e locations of the d

n

and f

energies r e l a t i v e to E

n

and E

c

v

deter

m i n e w h a t f o r m a l v a l e n c e states are a v a i l a b l e to t r a n s i t i o n m e t a l or r a r e e a r t h cations.

T h e r a r e e a r t h oxides h a v e 5 d b a n d s t h a t o v e r l a p t h e 6s

a n d 6p b a n d s , so E m a y b e the edge of a 5 d r a t h e r t h a n a 6s b a n d . S u c h c

is the case i n E u O , f o r e x a m p l e ( 3 ) .

If t h e e n e r g y of a 4 f

w i t h i n the energy gap E =

v

g

n

m a n i f o l d lies

( E — E ) , then two formal valence c

states

are p o s s i b l e f o r the r a r e e a r t h c a t i o n ; t h e y c o r r e s p o n d to t h e c o n f i g u r a tions 4 f

and 4 f

n

n_1

.

I n E u O the E u : 4 f 2 +

7

b e l o w E , a n d o x i d a t i o n to create E u : 4 f 3 +

c

6

e n e r g y l e v e l lies a b o u t 1.1

eV

configurations is p o s s i b l e .

On

the other h a n d , the energies 17 for 4f electrons are g e n e r a l l y U > E , a n d g

c o m m o n l y no 4f

n

c o n f i g u r a t i o n has a n e n e r g y w i t h i n E . I n this case o n l y g

ine 4f configuration can be obtained. F o r example, only the G d : 4 f n

3 +

configuration

is f o u n d .

S u b s t i t u t i o n of

gadolinium for

7

core

europium

in

E u i . a . G d a . 0 results i n t h e G d c o n f i g u r a t i o n 4 f 5 d , w h e r e t h e 5d* state is a 7

1

s h a l l o w d o n o r state just b e l o w E . ( I n this o x i d e . s h a l l o w d o n o r states are c

p o s s i b l e because E the G d : 4 f 3 +

7

c

is at the b o t t o m of a 5 d b a n d ) .

O n the o t h e r h a n d ,

l e v e l lies w e l l b e l o w E , so i t is i m p o s s i b l e to o x i d i z e G d 0 v

so as to create a G d : 4 f 4 +

6

2

3

level.

T h e free i o n energies U for t r a n s i t i o n m e t a l d s m a l l e r t h a n those f o r r a r e e a r t h 4 f

n

n

configurations

are

configurations, a n d i n a c r y s t a l , c o v a l -

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

116

SOLID S T A T E

CHEMISTRY: A

CONTEMPORARY OVERVIEW

ent m i x i n g w i t h n e i g h b o r i n g l i g a n d s p r o d u c e s

a l a r g e r r e d u c t i o n of a

t r a n s i t i o n m e t a l U f r o m its free i o n v a l u e . T h e r e f o r e , a t r a n s i t i o n m e t a l U < Eg =

— E ) is c o m m o n , e s p e c i a l l y w h e r e E

(E

c

v

c

is the b o t t o m of a

c a t i o n s b a n d . C o n s e q u e n t l y , at least one, a n d p e r h a p s s e v e r a l , d

config

n

u r a t i o n has its e n e r g y w i t h i n a g i v e n E . T h i s s i t u a t i o n m a k e s m u l t i p l e g

f o r m a l v a l e n c e states p o s s i b l e . I n t h e case of v a n a d i u m , f o r e x a m p l e , t h e oxides V O , V 0 , V 0 , a n d V 0 2

3

2

separations of the V

2 +

2

:d , V 3

3 +

5

are a l l k n o w n , w h i c h i m p l i e s e n e r g y

: d , a n d V : d * configurations t h a t are less 2

4 +

t h a n 3 e V . S i n c e these ions c a n h a v e d e l e c t r o n b a n d w i d t h s a p p r o a c h i n g 2 e V i n oxides, the c o n d i t i o n U ~

w

h

is f u l f i l l e d . T h e p e c u l i a r c h e m i c a l

a n d p h y s i c a l p r o p e r t i e s of v a n a d i u m oxides reflect this s i t u a t i o n ( 2 ) . Downloaded by CORNELL UNIV on October 26, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch006

I f t w o t r a n s i t i o n m e t a l ions are present i n the same c r y s t a l , the r e l a t i v e energies of t h e i r d

n

m a n i f o l d s d e t e r m i n e t h e f o r m a l v a l e n c e states o n

the cations. F o r e x a m p l e , F e V 0

3

could be F e

2 +

V

4 +

0

3

or F e

3 +

V

3 +

0 .

With

3

the first set of v a l e n c e states, o r d e r i n g of c a t i o n s w i t h i n t h e c o r u n d u m s u b a r r a y is e x p e c t e d to g i v e the i l m e n i t e s t r u c t u r e of F e T i 0 . I n fact, t h e 3

ions r e m a i n d i s o r d e r e d a n d M o s s b a u e r spectroscopy confirms a n F e configuration

3 +

:d

5

(4).

A k n o w l e d g e of the energies o f l o c a l i z e d e l e c t r o n m a n i f o l d s a n d t h e energies U t h a t separate t h e m , o f t h e p o s i t i o n s of the b a n d edges E

c

and

E , a n d of the e n e r g y d i s t r i b u t i o n of b u l k states associated w i t h n a t i v e v

defects is necessary f o r a n y systematic d e s i g n of s o l i d state e l e c t r o n i c devices.

I n a d d i t i o n , t h e d e v i c e e n g i n e e r m a y n e e d to c o n t r o l surface

states. I n e l e m e n t a l s e m i c o n d u c t o r s these h a v e energies n e a r t h e center of the g a p E — E c

v

unless a c t i v e i n c h e m i s o r p t i o n ; i n p o l a r

the surface states for e a c h species t e n d to b e d i s p l a c e d a b o v e or b e l o w ( a n i o n i c ) t h e center of the g a p .

compounds (cationic)

C h e m i s o r p t i o n occurs

where

surface state o r b i t a l s i n t e r a c t w i t h a t o m i c or m o l e c u l a r o r b i t a l s of a c h e m i c a l species a d s o r b e d o n the surface; t h e i n t e r a c t i o n p r o d u c e s a c h e m i c a l b o n d b e t w e e n t h e s o l i d a n d the adsorbate. C h e m i c a l a c t i v i t y at t h e s u r face d e p e n d s o n the p o s i t i o n s of t h e surface states r e l a t i v e to t h e b u l k states a n d t h e i r e l e c t r o n o c c u p a n c y

before a n d after c h e m i s o r p t i o n or

physisorption. T h i s c h a p t e r s u m m a r i z e s some e x p e r i m e n t s d e s i g n e d to p r o v i d e i n f o r m a t i o n a b o u t b u l k a n d surface states i n T i 0 . R u t i l e has b e e n extensively 2

s t u d i e d as a p o s s i b l e a n o d e f o r 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 b y s u n l i g h t , a t o p i c d e a l t w i t h i n a n o t h e r c h a p t e r of this v o l u m e

Dopant

3d

n

(5).

Configurations

Rutile crystallizes i n a tetragonal structure w i t h T i ions strings of e d g e - s h a r e d o c t a h e d r a p a r a l l e l to t h e c-axis t h a t are b y s h a r i n g c o m m o n o c t a h e d r a l site corners; see F i g u r e 1 . E a c h b r i d g i n g i n o n e s t r i n g a n d n o n b r i d g i n g i n the o t h e r ; e a c h 4 +

i n simple connected O " i o n is has t h r e e


2


Figure 1.

Tetragonal

structure of

Ti0

2

c o p l a n a r , n e a r e s t - n e i g h b o r cations a n d a p,r o r b i t a l d i r e c t e d p e r p e n d i c u lar to the p l a n e t o w a r d a v a c a n t o c t a h e d r a l site. T h i s s t r u c t u r e gives rise to e n e r g y b a n d s h a v i n g E

v

as the t o p of the 0 ":2p7r b a n d a n d E 2

2

c

as t h e

b o t t o m of t h e e m p t y T i : 3 d b a n d s ; see F i g u r e 2. T h e c u b i c c o m p o n e n t of 4 +

t h e c r y s t a l l i n e fields splits t h e m o r e stable t

2 g

fivefold-degenerate

3 d o r b i t a l s i n t o three

a n d t w o less stable e orbitals. T h e T i g

4 +

—O "—Ti 2

inter

4 +

actions are strong e n o u g h to t r a n s f o r m the e orbitals i n t o o n e - e l e c t r o n o-* g

o r b i t a l s a n d t w o of the t

2 g

orbitals i n t o one-electron ?r* o r b i t a l s ; T i


4 +

—Ti

4 +

118

SOLID S T A T E

CHEMISTRY: A


interactions p a r a l l e l to the c-axis t r a n s f o r m the r e m a i n i n g t

orbital into a

2 g

o n e - d i m e n s i o n a l b a n d a l l o w i n g c o n d u c t i v i t y a l o n g the c-axis. T h e p h o t o c o n d u c t i v i t y of s i n g l e - c r y s t a l T i 0 E

=

g

(E

2

exhibits a l a r g e a n i s o t r o p y at t h r e s h o l d ,

= 3.0 e V ; the c o n d u c t i v i t y a l o n g the c-axis is m o r e t h a n

— E)

c

y

t w o orders of m a g n i t u d e l a r g e r t h a n t h a t i n t h e b a s a l p l a n e ( 6 ) , w h i c h places E

c

at t h e b o t t o m of the o n e - d i m e n s i o n a l 3 d b a n d , as i l l u s t r a t e d i n

F i g u r e 2. E l e c t r o c h e m i c a l measurements locate the H / H +

2

l e v e l of a n aqueous

s o l u t i o n of p H = 1 at 4.5 e V b e l o w t h e v a c u u m l e v e l , a n d measurements of t h e f l a t - b a n d p o t e n t i a l of n - t y p e T i 0

s u c h a s o l u t i o n suggest a n e l e c t r o n affinity Downloaded by CORNELL UNIV on October 26, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch006

E

F

a b o u t 0.4 e V b e l o w E

r e l a t i v e to the H / H +

2

x

~ 4 e V (7).

level i n

2

( A F e r m i energy

is e s t i m a t e d i n these e x p e r i m e n t s , w h e r e E

c

F

w a s a b o u t o p t i m a l f o r the p h o t o e l e c t r o l y s i s of w a t e r . )

T h i s measurement,

together w i t h E ,

and E

g

p r o v i d e s absolute energies f o r E

c

v

n e u t r a l a d s o r b e d l a y e r at the surface o c c u r r i n g f o r p H = 5.7 I n i t i a l attempts to measure the energies of 3 d t r a n s i t i o n m e t a l cations s u b s t i t u t e d i n t o T i 0 empirical arguments

(9).

e l e c t r o n s p i n resonance figurations lithium.

I n these

2

n

in T i 0 , a 2

(8).

m a n i f o l d s of s e v e r a l

relied heavily on

semi-

experiments the intensities of

( E S R ) signals associated w i t h different 3 d

n

the con

w e r e m o n i t o r e d as a f u n c t i o n of the c o n c e n t r a t i o n of i n t e r s t i t i a l U n f o r t u n a t e l y , i t w a s not possible to d e t e r m i n e a c c u r a t e l y t h e

p o s i t i o n of the F e r m i e n e r g y i n the b a n d g a p as a f u n c t i o n of t h e L i i o n +

c o n c e n t r a t i o n . T h e r e f o r e , a c o m b i n a t i o n of p h o t o c o n d u c t i v i t y a n d p h o t o sensitive E S R measurements w a s m a d e at the U n i v e r s i t y of T o k y o

to

o v e r c o m e this difficulty ( 1 0 ) . S u r f a c e p h o t o c u r r e n t versus w a v e l e n g t h A ( A A ^ 10 n m ) of the i n c i d e n t l i g h t , c h o p p e d at 300 H z for i m p r o v e d d e t e c t i o n , w a s m e a s u r e d at 77 K (to r e d u c e d a r k c u r r e n t s ) o n as-sliced or o x i d i z e d samples.

Figure

3 ( a ) shows the results b e f o r e a n d after exposure to a n intense w h i t e l i g h t f o r a c r y s t a l slab as s l i c e d f r o m a n o r i g i n a l , u n d o p e d ingot.

Electrons

e x c i t e d b y the w h i t e l i g h t at 77 K b e c o m e t r a p p e d i n m e t a s t a b l e states h a v i n g energies w i t h i n E ;

these are i n t e r p r e t e d to b e p r i m a r i l y surface

g

states. A r o o m t e m p e r a t u r e a n n e a l returns the s a m p l e to its e q u i l i b r i u m distribution.

T h e s t r o n g a b s o r p t i o n b e l o w 415 n m is d u e to e x c i t a t i o n

across the 3.0 e V b a n d gap. F i g u r e 3 ( b )

shows t y p i c a l results for s i m i l a r

e x p e r i m e n t s o n o x i d i z e d c r y s t a l slabs s u b s t i t u t i o n a l l y d o p e d w i t h V , C r , M n , a n d F e . D o p i n g to 0 . 0 0 1 - 0 . 0 1 % levels w a s a c c o m p l i s h e d b y h e a t i n g s i n g l e - c r y s t a l slabs of T i 0

2

w i t h the t r a n s i t i o n m e t a l p o w d e r

at

400°-

8 0 0 ° C f o r 1 h r i n a sealed t u b e a n d s u b s e q u e n t l y o x i d i z i n g i n o x y g e n at 8 5 0 ° - 1 0 0 0 ° C f o r 1 d a y . T h e results w e r e i n s e n i t i v e to the d o p a n t i o n ; the i n c r e a s e d densities of states a p p e a r to be b u l k states associated w i t h c a t i o n v a c a n c i e s c r e a t e d b y o x i d a t i o n (see ments b e l o w ) .

d i s c u s s i o n of p o l a r i z e d l i g h t e x p e r i

A s expected, the d e n s i t y of d e e p a c c e p t o r states associated



6.

GOODENOUGH

Photo Responses of Pure and Doped Rutile

Energy 7

1

30

b

T

hv

(eV)

1

2-5

119

i

2-0

1-5

3

f

\ l

l

b

)

C D

U

V^(a)

1

\

O

o a-

0

I

400

1

500

I

700

600

Wavelength X

r^fl—i— 800

900

(mu)

Figure 3. Surface photocurrent at 77 K vs. wavelength \ of incident monochromatic light (A\ ^ 10 nm): (%) before; (O), after exposure to intense white light for (a) as-sliced and (b) doped, followed by oxidation single-crystal Ti0 (10). 2


120

SOLID S T A T E

CHEMISTRY:

A CONTEMPORARY

OVERVIEW

f^-^EW'/Fe *) 2

..y ::3d . o 3

2

-^3d! o E

E

( M n

( V

>

4

V v

/ M n

^

2-

}

^EW'/Mn *) •55TE°(VW) Downloaded by CORNELL UNIV on October 26, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch006

3

!£E°(Cr 7Cr *) ^-E°(Fe W) 4

3

4

Figure 4. Schematic summary of band gap states metastably occupied at 77 K as revealed by surface photoconductivity and photosensitive ESR measurements

Density of States.

w i t h c a t i o n v a c a n c i e s extends a b o u t 1 e V a b o v e E

v

. Before irradiation

w i t h w h i t e l i g h t , f e w of the a c c e p t o r states are o c c u p i e d b y electrons, c o n sistent w i t h a f e r m i e n e r g y E

F

near E . v

A f t e r exposure of t h e d o p e d samples to w h i t e l i g h t , t h e p h o t o c u r r e n t w a s m o n i t o r e d as a f u n c t i o n of t h e t i m e of exposure to t h e m o n o c h r o m a t i c l i g h t . T h e p h o t o c u r r e n t d e c r e a s e d w i t h t i m e f o r A ^ 500 n m , as m i g h t b e expected

i f the monochromatic light returns the electron distribution

t o w a r d e q u i l i b r i u m . H o w e v e r , f o r A ^ 450 n m t h e p h o t o c u r r e n t increases w i t h t i m e , w h i c h i n d i c a t e s t h a t t h e m o n o c h r o m a t i c r a d i a t i o n is either p u m p i n g electrons o u t of a c c e p t o r

states t h a t a r e b e i n g r e p o p u l a t e d

t h e r m a l l y b y electrons o r i n c r e a s i n g t h e surface m o b i l i t y of t h e electrons b y p o p u l a t i n g surface t r a p s w i t h electrons e x c i t e d f r o m t h e b u l k . p h e n o m e n a c a n b e e x p e c t e d f o r b u l k a c c e p t o r states b e l o w E w o u l d place E

F

w i t h i n 0.3 e V of E

v

F

Both

, which

i n t h e o x i d i z e d samples.

I n s u m m a r y , t h e p h o t o c u r r e n t m e a s u r e m e n t s c o n f i r m a b a n d g a p of Eg

3.0 e V , a n d t h e y r e v e a l a n a r r o w b a n d of l o w d e n s i t y of states ( d e s

i g n a t e d E i states i n F i g u r e 4 ) e x t e n d i n g f r o m t h e center of t h e b a n d g a p to a b o u t 0.5 e V a b o v e E , a n d i n o x i d i z e d samples a d e n s i t y of a c c e p t o r v

states e x t e n d i n g a f u l l 1 e V a b o v e E

v

with an E

F

a b o u t 0.3 e V a b o v e E . v

T h e E states are a s s u m e d t o b e surface states t h a t are p r i m a r i l y of a n i o n i c x

c h a r a c t e r ; t h e b u l k a c c e p t o r states ( d e s i g n a t e d E i n F i g u r e 4 ) a p p e a r t o 2

be centered at cation vacancies. I n contrast t o t h e p h o t o c o n d u c t i v i t y m e a s u r e m e n t s , p h o t o s e n s i t i v e E S R s p e c t r a a t 77 K , o b s e r v e d w i t h a c o n v e n t i o n a l X b a n d spectrometer,


6.

GOODENOUGH

121

Photo Responses of Pure and Doped Rutile

w e r e d o p a n t - i o n specific. that were performed:

(a)

F i g u r e 5 i l l u s t r a t e s three types of e x p e r i m e n t s the constant-intensity dark signal

undergoes

a c h a n g e A l t h a t v a r i e s w i t h t h e t i m e ti after exposure to m o n o c h r o m a t i c l i g h t ; ( b ) t h e A l f r o m a d a r k s i g n a l t h a t has b e e n c h a n g e d b y p r e v i o u s w h i t e l i g h t exposure at 77 K is m o n i t o r e d as a f u n c t i o n of t h e t i m e t

2

after

exposure to m o n o c h r o m a t i c l i g h t ; a n d ( c ) t h e A l f r o m a d a r k s i g n a l t h a t has b e e n c h a n g e d b y exposure a t 77 K to w h i t e l i g h t f o l l o w e d b y 4 1 5 - n m l i g h t is m o n i t o r e d as a f u n c t i o n of the t i m e t

3

after exposure t o

c h r o m a t i c l i g h t . F i g u r e s 6 a n d 7 s h o w A l versus A f o r t = x

mono

2 min and t

2


Monochromatic Light on

(a)

l v.Dark

i i i i 0 2 4 6 min.

*t,

Monochromatic Light on

(b)

1

1 Dark

1

1 i i i ^«-2 0 2 4 6 min. Dark

Intense White Light

Monochromatic Light on 1

V v

^

V*—1

i Dark

i i i i

~

0 2 4 6 min. Dark

Intense 415mu White Light Light

Figure 5. Definition of experimental times (a) t (b) t , and (c) t , and schematic representation of typical variations of ESR signal intensity with times at 77 K (10). (Differential peak oc intensity) l9

2


3

—

122

SOLID

400

500

—i

1—


30

400

STATE CHEMISTRY:

600

400

i 500

700

OVERVIEW

800 —I

"T—

20

2-5

4

A CONTEMPORARY

1-5 eV (b)

i 600

700

600

700

500

^ •' 800 m\i

800 mil (c) Fe * 3

7

30

25

1-5 eV

20 i

(d)

^^^^ • • Mn * 4

i

400

500

600 ^"'700

800 m\L

It

•«!

Mn --3

30 i

25 i

Wavelength

20 .. _ i monochromatic

1-5

light

eV

(m^)

Figure 6. Spectra at 77 K of the change A l in ESR signal intensity from t = 0 to t = 2 min for four impurity cations in Ti0 : (a-c) oxidized; (d) reduced (10). 1

t

2



Figure 7. Spectra at 77 K of the change A l in ESR signal intensity from t =0 to t = 2 min for the same impurity-cation states in the same TiO crystals as in Figure 6: (a-c) oxidized; (d) reduced (10). 2

2


s

124

SOLID S T A T E

CHEMISTRY: A CONTEMPORARY OVERVIEW

2 m i n , r e s p e c t i v e l y . A l l signals w e r e n o r m a l i z e d to t h e i n t e n s i t y of t h e m o n o c h r o m a t i c l i g h t , a n d b e t w e e n e a c h m e a s u r e m e n t t h e i n f l u e n c e of t h e p r e v i o u s exposure t o m o n o c h r o m a t i c l i g h t w a s r e m o v e d b y e i t h e r a room temperature anneal ( F i g u r e 6)

or a r e i r r a d i a t i o n b y w h i t e l i g h t

(Figure 7). For

e a c h d o p a n t a n E S R s i g n a l is associated w i t h a g i v e n f o r m a l

v a l e n c e state M

m +

: 3 d ; t h e i n t e n s i t y of t h e s i g n a l d e p e n d s u p o n t h e n u m n

b e r of d o p a n t ions i n the M

m

+

state. C h a n g e s i n t h e 3 d

c a u s e d b y l i g h t - i n d u c e d e l e c t r o n transfer to or f r o m t h e M


M M

:3d*+ le-»M

w +

m

+ : 3 d

M

m+

The E i and E

2

n _

( m

-

:3d

1 ) +

m

+

-» M

C

m

_

1

)

ions:

+

(2) (3)

1

+ M

+

m

n + 1

l e ^ M ^ ^ d " -

+ M

p o p u l a t i o n are

n

(

w

+

1

)

(4)

+

states as w e l l as t h e v a l e n c e b a n d m a y serve as sources or

sinks f o r electrons, a n d e l e c t r o n or h o l e t r a n s f e r t h r o u g h t h e c r y s t a l occurs via the conduction a n d valence bands, respectively. A s indicated i n F i g ures 6 a n d 7, t h e E S R signals m o n i t o r e d w e r e f r o m t h e V ^ r S d ,

the

1

C r : 3 d , the F e : 3 d , a n d the M n : 3 d 3 +

3

3 +

5

4 +

3

ions.

I n t e r p r e t a t i o n of the s p e c t r a of F i g u r e s 6 a n d 7 is b a s e d o n t h e a s s u m p t i o n t h a t electrons, o n c e e x c i t e d f r o m E

E , or 3 d

l9

conduction band by monochromatic

2

w

states t o t h e

l i g h t of w a v e l e n g t h A, c a n m o v e

t h r o u g h t h e c r y s t a l e i t h e r t o b e c o m e r e t r a p p e d at a d i s t a n t , l o c a l i z e d e l e c t r o n center or to r e c o m b i n e w i t h a v a l e n c e b a n d h o l e .

Moreover,

m o n o c h r o m a t i c l i g h t h a v i n g A < 450 n m m a y create m o b i l e v a l e n c e b a n d holes e i t h e r i n d i r e c t l y — b y e x c i t i n g electrons f r o m a n E

2

state t h a t b e

comes t h e r m a l l y r e p o p u l a t e d f r o m t h e v a l a n c e b a n d — o r d i r e c t l y i f A < 415 n m . M e t a s t a b l e electrons c a n b e t r a p p e d at E

l9

In

oxidized T i 0 : V 2

valence V :3d°. 5+

monitored. T h e V

/V

5 +

2

n

centers.

m o s t of t h e v a n a d i u m ions h a v e t h e f o r m a l

T h e E S R signal from a V 4 +

E , or d

4 +

:3d

x

i o n is s h a r p a n d easily

r a t i o is so l o w i n o x i d i z e d samples t h a t a n y V

3 +

i o n p o p u l a t i o n c a n b e i g n o r e d a n d a n y changes A l versus tx i n t h e E S R s i g n a l represent a n increase i n t h e V

4 +

i o n p o p u l a t i o n f r o m its e q u i l i b r i u m

v a l u e . T h e s p e c t r u m of F i g u r e 6 ( a ) s h o w s A l > 0 o n l y f o r A < 600 n m . From Figure 3(b)

the d e n s i t y of o c c u p i e d states f o r the v i r g i n d a r k

state is n e g l i g i b l e f o r A > 700 n m , a n d w e w o u l d e x p e c t a t w o - w a y ex c h a n g e of electrons b e t w e e n 3 d c o n f i g u r a t i o n s a n d E i or E n

t h e e n e r g y hv & [ E hv > [ E — E ( V c

4 +

c

— E(V

4 +

2

states w h e r e

) ] . H o w e v e r , a A l > 0 can be expected for

) ] , since a m e t a s t a b l e e l e c t r o n d i s t r i b u t i o n b e c o m e s

p o s s i b l e u n d e r these c o n d i t i o n s . T h i s r e a s o n i n g places t h e V

4 +

:3d

1

level

a b o u t 2.1 e V b e l o w E , a n a s s i g n m e n t t h a t a p p e a r s t o b e c o n f i r m e d b y c

Figure 7(a).

A f t e r exposure t o w h i t e l i g h t , t h e V : d * p o p u l a t i o n is 4 +


6.

GOODENOUGH

Photo Responses of Pure and Doped

125

Rutile

n e a r l y d o u b l e d , a n d m o n o c h r o m a t i c l i g h t tends to r e t u r n this p o p u l a t i o n to its e q u i l i b r i u m v a l u e , m a k i n g A l < 0, i f hv « m a x i m u m A l at a b o u t 450 n m for b o t h tx a n d t from V

[E

— E(V

c

)].

4 +

ions to m o b i l e holes c r e a t e d b y t h e r m a l e x c i t a t i o n to E

4 +

The

reflects e l e c t r o n transfer

2

states

2

near E . v

I n oxidized T i 0 : C r , the C r 2

t h a n the V

/V

4 +

5 +

t h a t the C r : 3 d 3 +

3

p o p u l a t i o n r a t i o is m u c h l a r g e r

4 +

T h i s observation

2

and 7(b),

c

T i 0 : V places the C r 2

3 +

1

indicates

l e v e l at 2.1

eV

r e a s o n i n g s i m i l a r to t h a t for

: 3 d l e v e l a b o u t 2.7 e V b e l o w E . A l a r g e c r y s t a l 3

c

splitting places the high-spin C r

v a l e n c e state n e e d not b e Downloaded by CORNELL UNIV on October 26, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch006

/Cr

l e v e l lies d i s t i n c t l y b e l o w t h e V ^ d

below E . F r o m Figures 6(b) field

3 +

ratio i n oxidized T i 0 : V .

2 +

: 3 d l e v e l w e l l a b o v e E , so this 4

c

considered.

I n t h e case of o x i d i z e d T i 0 : F e , essentially a l l t h e i r o n ions are i n 2

the F e : 3 d 3 +

state at e q u i l i b r i u m , a n d a n y A l at t m u s t b e n e g a t i v e .

5

level may lie below E

c

2 +

Ti

0

4 +

indicates

3

that the

Fe :3d 2 +

6

i n T i 0 , at least i f o c c u p i e d , so a A l < 0 c a n b e 2

p r o d u c e d either b y excitations f r o m E near E

The

±

existence of t h e i l m e n i t e p h a s e F e

or E

x

2

states to F e : 3 d 2 +

levels

6

or b y t h e r e a c t i o n

c

2Fe if the empty F e : 3 d 3 +

5

hv

> Fe

3 +

2 +

+

Fe

(5)

4 +

l e v e l lies a b o v e E . A s seen i n F i g u r e 6 ( c ) , v

failure

to observe at A l < 0 u n t i l A < 650 n m seems to p l a c e t h e e m p t y F e : 3 d 2 +

6

l e v e l a b o u t 0.1 e V a b o v e E . H o w e v e r , l a t t i c e r e l a x a t i o n a b o u t a n o c c u c

pied F e : 3 d 2 +

6

l e v e l w o u l d s t a b i l i z e this b y a b o u t 0.3 e V , t h e r e b y

t h e o c c u p i e d l e v e l a b o u t 0.2 e V b e l o w E Fe :3d 2 +

6

ions i n t h e p r e s e n c e of T i

placing

and allowing formation

c

of

ions. T h i s d e d u c t i o n a p p e a r s t o b e

4 +

c o n f i r m e d b y the o b s e r v a t i o n of A l at t , F i g u r e 7 ( c ) , a n d at 1 (10). 2

As

3

A - » 4 l 5 n m , R e a c t i o n 5 a p p e a r s to p r o d u c e a s h a r p increase i n |Al|.

It

also is e n h a n c e d f o r A < 450 n m b y t h e r m a l e x c i t a t i o n of v a l e n c e b a n d electrons to e m p t i e d E level.

2

states w i t h s u b s e q u e n t h o l e c a p t u r e at a n F e : 3 d 3 +

A l t h o u g h the o c c u p i e d F e : 3 d 3 +

5

b e l o w , E , the e m p t y l e v e l is l i f t e d a b o v e E v

the F e : 3 d 4 +

E

v

in T i 0

2

4

i o n . T h u s t h e filled F e : 3 d 3 +

a n d the empty F e : 3 d 2 +

6

5

l e v e l a p p e a r s to b e at, or just 5

v

b y lattice relaxation about

l e v e l a p p e a r s to l i e just b e l o w

l e v e l just a b o v e E , so o c t a h e d r a l site c

i r o n i n oxides has a n e n e r g y U =

S(Fe ) 2 +

#(Fe ) ~ 3 +

3 eV

(6)

S i n c e this m a r k s a d d i t i o n of the first e l e c t r o n after t h e h a l f - f i l l e d , h i g h s p i n 3 d s h e l l , i t is a r e l a t i v e l y l a r g e f r e e - i o n U. S u c h a s m a l l v a l u e f o r the c r y s t a l l i n e U is q u i t e consistent w i t h t h e r e l a t i v e l y h i g h s u p e r e x c h a n g e interactions between F e

3 +

ions i n oxides

(J).


126

SOLID S T A T E

Since F e V O

contains F e

s

CHEMISTRY: A

and V

3 +

ions r a t h e r t h a n F e

3 +

ions, w e m u s t p l a c e the o c t a h e d r a l site V level, a n d hence below E

in T i 0 .

c


3 +

:3d

and V

2 +

level below the F e

2

2 +

4 +

:3d

6

T h i s d e d u c t i o n i m p l i e s t h a t for o c t a

2

h e d r a l site v a n a d i u m i n oxides, E (V ) -

U =

3 +

E (V ) £ 2 eV

(7)

4 +

w h i c h is consistent w i t h t h e o b s e r v a t i o n of a s e m i c o n d u c t o r - m e t a l t r a n s i tion i n V 0

c h a r a c t e r i s t i c of a U «

2

(1).

w

h

I n the case of T i 0 : M n , o x i d i z e d samples c o n t a i n n e a r l y a l l t h e 2


m a n g a n e s e ions i n t h e M n v i c i n i t y of t h e V

4 +

the ilmenite M n

2 +

:3d Ti

:3d

state, i n d i c a t i n g a M n

3

c

:3d

4

level i n the

c

0

indicates that the M n : 3 d 2 +

3

B e c a u s e t h e rate of d e c a y of a m e t a s t a b l e M n

E.

3 +

l e v e l , w h i c h is 2.1 e V b e l o w E . T h e existence of

1

4 +

4 +

l e v e l also lies b e l o w

5

i o n to a M n

3 +

state

4 +

i n o x i d i z e d T i 0 : M n is too r a p i d f o r c o n v e n i e n t m o n i t o r i n g of t h e E S R 2

signal from M n ures 6 ( d )

f e r m i energy E Mn :3d . 4 +

4 +

:3d

3

F

ions, t h e p h o t o s e n s i t i v e E S R signals s h o w n i n F i g

3

and 7(d)

are f o r r e d u c e d T i 0 : M n .

I n reduced T i 0 : M n the

2

2

lies close to E , a n d v e r y f e w of the m a n g a n e s e ions are c

T h i s finding is also consistent w i t h a M n c

for the measured M n ions, w h e r e the M n population.

:3d

4

l e v e l at least

E S R s i g n a l represents a n e g a t i v e A l f o r t h e M n

4 +

2 +

3 +

a n d 7 ( d ) , a positive A l

2 e V b e l o w E . I n t h e s p e c t r a of F i g u r e s 6 ( d )

i o n p o p u l a t i o n m u s t be i n c l u d e d i n t h e M n

Raising E

above the M n

F

3 +

:3d

4

level i n reduced

3 +

3 +

ion

Ti0 :Mn 2

also r e d u c e s t h e c o n c e n t r a t i o n of c a t i o n v a c a n c i e s a n d m a y i n t r o d u c e anion vacancies.

T h e r e f o r e , t h e d e n s i t y of E

states s h o u l d b e s m a l l e r ,

2

a n d a c o r r e s p o n d i n g d e n s i t y of d o n o r states b e l o w E m a y b e i n t r o d u c e d . c

H o w e v e r , t h e r e is n o e v i d e n c e f o r a m e t a s t a b l e c a p t u r e of electrons i n c a t i o n i c states. S i n c e t h e p o p u l a t i o n of M n

ions is v e r y s m a l l i n r e d u c e d T i 0 : M n ,

4 +

o n l y a A l > 0 c a n o c c u r for t

2

The reaction

lt

hp

2Mn s h o u l d i n t r o d u c e at A l > places the M n

3 +

:3d

4

3 +

—-» M n

0 for t

4 +

+ Mn

w h e r e hv ^

u

(8)

2 +

— E(Mn

[E

c

3 +

) ] , which

l e v e l a b o u t 1.9 e V b e l o w E . A f t e r exposure to w h i t e c

l i g h t , t h e m e t a s t a b l e e l e c t r o n d i s t r i b u t i o n leaves a l a r g e r p o p u l a t i o n of Mn t

2

4 +

i o n s — a n d p r e s u m a b l y of M n

is p o s s i b l e hv

4 +

ions the s p e c t r a

A l for S i g n a l I a n d A l < A l 0

S i g n a l I I , w h e r e A Z is the i s o t r o p i c s i g n a l of F i g u r e 7.

For Fe

0

S i g n a l I has AZ
0

A l just the reverse 0

0

for

ions

3 +

(10).

T h e s e a n i s o t r o p i c s i m p o s e severe constraints o n a n y m o d e l of the b u l k n a t i v e d e f e c t r e s p o n s i b l e for t h e E

2

states.

T h e o b s e r v e d anisotropics reflect different v a l e n c e state p o p u l a t i o n s at P a n d Q cations after p h o t o e x c i t a t i o n b y the p o l a r i z e d m o n o c h r o m a t i c light.

E s s e n t i a l l y o n e - d i m e n s i o n a l e l e c t r o n transfer a l o n g the c-axis is

possible b e c a u s e E F i g u r e 2.

c

is at the b o t t o m of a o n e - d i m e n s i o n a l 3 d b a n d ;

charge transfer f r o m E l e v e l lies a b o v e the E

2

2

defects to P cations ( S i g n a l I ) , since the V ^ i S d

by a V

2

5

state b e l o w E ( V

4 +

) may be followed

i o n d e c a y , a d i r e c t c h a r g e transfer to t h a t E

neighboring V

4 +

ion.

d e c a y makes AZ

0. P r e f e r e n t i a l p h o t o e x c i t a t i o n f r o m E

2

0 and

states to a P

l i n e , b u t e q u a l d e c a y f r o m P a n d Q lines, w o u l d p r o d u c e a AZ > A Z for 0

S i g n a l I a n d a AZ
o cr

3*6

UJ

z

L i g h t of e n e r g y hv >

T

4-5eV

l-23eV

o cr LU

Layer n-type

Figure 8.

Aqueous Solution

Metal

One-electron energies versus distance x for the semiconductorliquid-^metal interfaces of a photoelectrolysis cell


E

g

130

SOLID S T A T E

CHEMISTRY: A

i n c i d e n t o n t h e s e m i c o n d u c t i n g electrode


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

I f l i g h t is a b s o r b e d i n the d e p l e t i o n l a y e r near t h e l i q u i d - s o l i d i n t e r f a c e , s e p a r a t i o n of the electrons a n d holes c a n b e a c c o m p l i s h e d b y the i n t e r n a l electric

field—proportional

to

o r the

dEJdx,

e l e c t r o n - h o l e r e c o m b i n a t i o n occurs.

band

bending—before

The band bending shown i n Figure

8 has a s i g n that d r i v e s holes to the s e m i c o n d u c t o r - l i q u i d i n t e r f a c e a n d electrons a w a y f r o m i t to the p l a t i n u m - l i q u i d i n t e r f a c e . A t the p l a t i n u m electrode, electrons c o m b i n e w i t h H ions t o cause t h e e v o l u t i o n of H . F o r +

2

t h i s t o o c c u r t h e F e r m i e n e r g y E — c o m m o n to b o t h s h o r t e d

electrodes

F

— m u s t be at the H / H +


at p H =

1. If E

2

l e v e l i n the l i q u i d , w h i c h is 4.5 e V b e l o w v a c u u m

is 0.3-0.4 e V b e l o w E , b a n d b e n d i n g of t h e correct

F

c

s i g n r e q u i r e s a n e l e c t r o n affinity x
R u L

t r a n s i t i o n excites a surface state e l e c t r o n i n t o t h e c o n d u c t i o n b a n d .

3

2 +

In

a n y of these processes n o net c h a r g e transfer to or f r o m t h e s o l i d takes place.

H o w e v e r , t h e r e is a c h a n g e i n t h e c h a r g e d i s t r i b u t i o n at t h e

surface d u e to transfer o f electrons to the b u l k . T h e b u i l d u p of p o s i t i v e c h a r g e at the surface is e q u i v a l e n t to s h i f t i n g the a p p l i e d v o l t a g e to a m o r e n e g a t i v e v a l u e , w h i c h increases e x p e r i m e n t a l l y t h e o n g o i n g d a r k current.

T h e c a t h o d i c p h o t o c u r r e n t thus represents a l i g h t m o d u l a t i o n

of the d a r k c a t h o d i c c u r r e n t . W i t h this m e c h a n i s m the t i m e constant f o r e s t a b l i s h i n g a steady state c o n d i t i o n d e p e n d s o n the rate of r e c o m b i n a t i o n of t h e c o n d u c t i o n b a n d e l e c t r o n a n d t h e surface state h o l e c r e a t e d . A n i m p o r t a n t f u t u r e step is to a t t a c h the sensitizer i o n d i r e c t l y to t h e semiconductor

surface to see h o w

changes

i n the surface states a n d

c h a r g e transfer processes alter the s i t u a t i o n . P r e l i m i n a r y e x p e r i m e n t s are encouraging. Glossary of Symbols E

g

=

e n e r g y g a p , e q u a l to (E

E

c

=

e n e r g y at b o t t o m of the t i t a n i u m 3 d c o n d u c t i o n b a n d

E

v

=

Ei, E ,E = 2

3

c

— E ) v

energy at t o p of the o x y g e n v a l e n c e b a n d energies of b a n d s of e n e r g y g a p states: E i p r e s u m a b l y refers to surface states, E

2

to c a t i o n v a c a n c i e s , E

anion vacancies


3

to

136

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

E ( d * ) , E ( f ) = energies of l o c a l i z e d d o r f n

w

V

0

A CONTEMPORARY

OVERVIEW

configurations

n

= isolated anion vacancy

R = distance from a V

G

center to a n e a r e s t - n e i g h b o r c a t i o n

e = m a g n i t u d e of t h e e l e c t r o n c h a r g e w = bandwidth h

U = intraatomic coulomb E(d»

+ 1

energy

)orE(f»)fromE(f»

+ 1

sphtting E ( d )

from

n

)

X — e l e c t r o n affinity, t h a t i s , s e p a r a t i o n of E f r o m v a c u u m c

energies by = resonance ( o r e l e c t r o n t r a n s f e r ) i n t e g r a l b e t w e e n o r


bitals o n n e i g h b o r i n g l i k e cations at p o s i t i o n s R f a n d e , t g

2 g

= o n e - e l e c t r o n d o r b i t a l s i n a c u b i c c r y s t a l l i n e field

h, h, h = times d e f i n e d b y F i g u r e 5 A l = c h a n g e i n i n t e n s i t y of X b a n d E S R s i g n a l n o r m a l i z e d to t h e i n t e n s i t y of t h e i n c i d e n t m o n o c h r o m a t i c l i g h t hv, A = energy, w a v e l e n g t h of the i n c i d e n t

monochromatic

light A A = change i n wavelength S i g n a l I , I I — X b a n d E S R s i g n a l f r o m P a n d Q strings o f cations ( F i g u r e 1 ) i f e x t e r n a l H || [110] a n d m o n o c h r o m a t i c l i g h t is p o l a r i z e d w i t h E v e c t o r 11 H . H = a p p l i e d m a g n e t i c field E = e l e c t r i c - f i e l d v e c t o r of m o n o c h r o m a t i c , p o l a r i z e d l i g h t L = 2,2'-bipyridyl fc (sec ) g

= r a t e of d e c a y f r o m e x c i t e d state * R u L

-1

state R u L

3

3

2 +

to g r o u n d

2 +

• s e c " ) = e l e c t r o n transfer v e l o c i t y f r o m e x c i t e d * R u L

k(cm

1

3

to the

2 +

anode D(cm

2

• sec" ) = d i f f u s i o n coefficient for R u L 1

3

i n t h e electrolyte

x = d i s t a n c e into electrolyte f r o m t h e a n o d e surface a,b = c o n c e n t r a t i o n s of R u L =

3

2 +

and * R u L

3

2 +

e x t i n c t i o n coefficient f o r l i g h t a b s o r b e d b y R u L

3

2 +

z = n u m b e r of l i k e nearest n e i g h b o r s ^ E

= p e r t u r b a t i o n of o n e - e l e c t r o n p o t e n t i a l at site Kj F

= f ermi energy level

E S R = e l e c t r o n s p i n resonance S S E = s t a n d a r d A g / A g C l electrode 26

Acknowledgments I w o u l d l i k e t o express m y t h a n k s t o K . M i z u s h i m a , M . P . D a r e E d w a r d s , a n d R . D . W r i g h t f o r p e r m i s s i o n to discuss t h e i r e x p e r i m e n t s


6.

GOODENOUGH

Photo RespQnses of Pure and Doped

Rutile

137

before p u b l i c a t i o n , a n d to A . H a m n e t t for d i s c u s s i o n o f his q u a n t i t a t i v e analyses of t h e p h o t o c u r r e n t m e c h a n i s m s o c c u r r i n g i n p h o t o s e n s i t i z e d TiO . z


Literature

Cited

1. Goodenough, J . B . Prog. Solid State Chem. 1971, 5, 145. 2. Goodenough, J . B . Mater. Res. Bull. 1971, 6, 967. 3. Goodenough, J . B . "Defects and Transport i n Oxides"; Setzer, M. S., Jaffee, R. I., E d s . ; P l e n u m : N e w York, 1974; 55. 4. Cox, D. E.; T a k i n , W . J.; Shirane, G. J. Phys. Chem. Solids 1962, 23, 863. 5. W o l d , A . Chapter 4 i n this book. 6. Parker, R. A.; Wasilik, J . H. Phys. Rev. 1960, 120, 1631. 7. Mavroides, J. G.; Kafalas, J. A.; Kolesar, D . F . Appl. Phys. Lett. 1976, 29, 10. 8. Butler, M. A.; Ginley, D. S. J. Electrochem. Soc. 1978, 125, 228. 9. M i z u s h i m a , K.; Tanaka, M.; Iida, S. J. Phys. Soc. Jpn. 1972, 32, 1519. 10. Mizushima, K . ; Tanaka, M.; Asai, A.; Iida, S.; Goodenough, J. B . J. Phys. Chem. Solids 1979, 40, 1129. 11. Fujishima, A.; H o n d a , K . Nature 1972, 238, 37. 12. Mavroides, J. G.; Tchernev, D . I.; Kafalas, J. A.; Kolesar, D . F . Mater. Res. Bull. 1975, 10, 1023. 13. Clark, W . D. K.; Sutin, N. J. Am. Chem. Soc. 1977, 99, 4677. 14. Hamnett, A.; D a r e - E d w a r d s , M. P . ; W r i g h t , R. D.; Seddon, K . R.; G o o d enough, J . B . J. Phys. Chem. 1979, 83, 3280. 15. W i l s o n , R. H. J. Appl. Phys. 1977, 48, 4292. RECEIVED

September 21, 1978.


Photo Responses of Pure and Doped Rutile - Advances in Chemistry

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