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Electrochromism of colloidal tungsten oxide
Solid State Ionics 9 & 10 (1983) 357-362 North-Holland Publishing Company
357
ELECTROCHROMISM
OF C O L L O I D A L
A. C h e m s e d d i n e ,
R. M o r i n e a u
TUNGSTEN
OXIDE
and J. L i v a g e
L a b o r a t o i r e de S p e c t r o c h i m i e du Solide, LA 302, U n i v e r s i t 4 P. et M. C u r i e - 4 p l a c e J u s s i e u - T.44, 2e @t. 75230 Paris C e d e x 05 - F r a n c e A c i d i f i c a t i o n of t u n g s t a t e s o l u t i o n s leads to the f o r m a t i o n of t u n g s t i c acids that p o l y m e r i z e , t h r o u g h a p o l y c o n d e n s a t i o n process, g i v i n g rise to c o l l o i d a l t u n g s t e n oxide. R e v e r s i b l e d i s p l a y d e v i c e s can be o b t a i n e d by e l e c t r o c h e m i c h r o m i s m . The c o l l o i d a l s o l u t i o n is then d e p o s i t e d onto a c o n d u c t i v e t r a n s p a r e n t e l e c t r o d e and p l a c e d into an e l e c t r o c h e m i c a l cell c o n t a i n i n g a l i q u i d e l e c t r o l y t e such as L i C I O a in p r o p y l e n e c a r b o n a t e . W i t h such a device, an o p t i c a l d e n s i t y -d.o = 0.3 is o b t a i n e d w i t h i n 0.3s by a p p l y i n g a n e g a t i v e v o l t a g e (-4V). The blue c o l o r a t i o n r e m a i n s a f t e r the v o l t a g e has been removed. O p t i c a l s p e c t r o scopy and E S R i n d i c a t e that the blue layer c o n t a i n s r e d u c e d W(V) ions.
I.
INTRODUCTION
E l e c t r o c h r o m i c d i s p l a y d e v i c e s b a s e d on a m o r p h o u s WO. thin film have b e e n ext e n s i v e l y s t u d i e d (I) (2) (3). A b o u t ten y e a r s ago, Deb (4) s h o w e d that such films c o u l d e x h i b i t two stable states. One is t r a n s p a r e n t and h i g h l y r e s i s t i v e w h i l e the second one is deep blue and m u c h less resistive. C o l o r a t i o n may be o b t a i n e d by UV i r r a d i a t i o n or by a p p l i c a t i o n of an e l e c t r i c f i e l d . B o t h proc e s s e s a p p e a r to be q u i t e slow and t h e r e f o r e not u s e f u l l for display. Fast r e v e r s i b l e c o l o r a t i o n can on the o t h e r h a n d be o b t a i n e d if the a m o r p h o u s oxide is d e p o s i t e d onto a c o n d u c t i v e e l e c t r o de and p l a c e d into an e l e c t r o c h e m i c a l cell (5). Such d e v i c e s e x h i b i t a clear image, i n s e n s i t i v e to the v i e w i n g angle and keep a l o n g - t e r m o p e n c i r c u i t m e m o ry. They open n e w p o s s i b i l i t i e s in the f i e l d of d i g i t a l d i s p l a y d e v i c e s (6). A m o r p h o u s W O 3 thin films are u s u a l l y o b t a i n e d by v a c u u m e v a p o r a t i o n (7) or s p u t t e r i n g (8). No a t t e m p t has yet been done to m a k e e l e c t r o c h r o m i c thin films by the s o l - g e l technology. This m e t h o d first c a l l e d the "gel route" by Roy (9) has g r o w n c o n t i n u o u s l y in r e c e n t years. It is n o w well k n o w n that i n o r g a n i c gels can be u s e d as s t a r t i n g m a t e r i a l s in o r d e r to m a k e g l a s s e s or c e r a m i c s at low t e m p e r a t u r e , well b e l o w their melting p o i n t (10). O x i d e thin films can be e a s i l y o b t a i n e d by d e p o s i t i o n of a c o l l o i d a l s o l u t i o n onto a s u b s t r a t e (11). The sol-gel p r o c e s s has r e c e n t l y been u s e d to m a k e s e m i c o n d u c•t i n g V~O~b layers (12) or a n t i - r e f l e c t l n g c o a t i n g s (13). In this paper, we shall d e s c r i b e the e l e c t r o c h r o m i c p r o p e r t i e s of tungsten o x i d e layers o b t a i n e d by d e p o s i tion from t u n g s t e n o x i d e colloids.
0 167-2738/83/0000-0000/$ 03.00 © 1983 North-Holland
2.
EXPERIMENTAL
P o l y m e r i z a t i o n of t u n g s t a t e anions upon a c i d i f i c a t i o n of a q u e o u s solutions leads to the f o r m a t i o n of t u n g s t i c acids (14) or t u n g s t e n t r i o x i d e h y d r a t e s W O ~ , n H 2 0 (15). D e p e n d i n g on the e x p e r i mefital-procedure d i f f e r e n t h y d r a t e s m a y be o b t a i n e d the n a t u r e of w h i c h sometimes r e m a i n s a m a t t e r of c o n t r o v e r s y (16). N e v e r t h e l e s s , the e x i s t e n c e and s t r u c t u r e of the m o n o and d i h y d r a t e s seem to be n o w well e s t a b l i s h e d (17). More recently, a n e w WOq, I/3 H20 was o b t a i n e d by h y d r o t h e r m a l t r e a t m e n t at 120°C of an a q u e o u s s u s p e n s i o n of e i t h e r t u n g s t i c acid gel or c r y s t a l l i z e d dih y d r a t e (18). U p o n d e h y d r a t i o n , this c o m p o u n d leads to a n e w h e x a g o n a l t u n g sten o x i d e W O 3 (19). C o l l o i d a l t u n g s t e n o x i d e is o f t e n obt a i n e d as a t r a n s i e n t i n t e r m e d i a t e d u r i n g the p o l y m e r i z a t i o n p r o c e s s of t u n g s t i c acids (15). This c o l l o i d a l p h a s e is n e v e r t h e l e s s not stable and p r e c i p i t a t i o n occurs l e a d i n g to h y d r a ted t u n g s t e n oxides. Such o x i d e s do not e x h i b i t any p h o t o c h r o m i c or e l e c t r o c h r o mic property. M o r e stable c o l l o i d a l solutions can be o b t a i n e d by ion exc h a n g e from s o d i u m t u n g s t a t e s o l u t i o n s (20) or by a d d i n g o r g a n i c s o l v e n t s (21). Such a p r o c e d u r e was f o l l o w e d in our e x p e r i m e n t s . A 0.5M s o d i u m t u n g s t a t e (Na2WO~, 2H~O) s o l u t i o n was p a s s e d t h r o u g h a D ~ w e x 50 WX2, 100-200 m e s h resin, l e a d i n g to a c o l l o i d a l s o l u t i o n that r e m a i n s stable for a few days. A few drops of this c o l l o i d a l s o l u t i o n w e r e then deposited, o n t o a c o n d u c t i v e t r a n s p a r e n t e l e c t r o d e (Indium Tin O x i d e c o a t e d glass substrate). The s o l v e n t r e a d i l y e v a p o r a t e s and a r a t h e r h a r d
358
A. Chemseddine et al. / Electrochromism o f colloidal tungsten oxide
c o a t i n g is obtained. B e t t e r r e s u l t s are o b t a i n e d if the g l a s s p l a t e is h e a t e d d u r i n g a p p l i c a t i o n w i t h an i n f r a - r e d lamp a n d p l a c e d on a q u i c k l y r o t a t i n g device. The t h i c k n e s s of the layers w a s m e a s u r e d by o p t i c a l i n t e r f e r o m e t r y . A c c o r d i n g to the e x p e r i m e n t a l p r o c e d u r e (tungstate c o n c e n t r a t i o n , a m o u n t of c o l l o i d a l solution) the t h i c k n e s s can be v a r i e d b e t w e e n 0.1 ~m and 10 ~m. L a y e r s of a b o u t 0.5 ~m u s u a l l y give a g o o d c o m p r o m i s e b e t w e e n fast r e s p o n s e a n d high o p t i c a l d e n s i t y w h e n u s e d for display. X - r a y d i f f r a c t i o n e x p e r i m e n t s show that the c o l l o i d a l t u n g s t e n o x i d e r e m a i n s a m o r p h o u s d u r i n g the w h o l e process.
tj m A
decoloral 0.2
-2 3.
RESULTS
3.1
Electrochemical
1
V vs AcffAg ÷
measurements
The I.T.O g l a s s electrode, c o a t e d w i t h c o l l o i d a l t u n g s t e n o x i d e layer, w a s p l a c e d into an e l e c t r o c h e m i c a l cell cont a i n i n g a IM L i C I O 4 s o l u t i o n in p r o p y lene c a r b o n a t e as an e l e c t r o l y t e . A p t p l a t e was u s e d as a c o u n t e r e l e c t r o d e in a c l a s s i c a l t h r e e - e l e c t r o d e s p o t e n t i o s t a t i c cell, w i t h the A g - A g C I O , (I0-2M) in P.C. as the r e f e r e n c e 4 e l e c trode. E l e c t r i c a l a n d o p t i c a l m e a s u r e m e n t s w e r e c a r r i e d out s i m u l t a n e o u s l y as d e s c r i b e d by B o h n k e e t al (22). C y c l i c voltammetric experiments were performed at scan rates of 50 m V / s b e t w e e n -2 v o l t s and +1.5 volts. B e l o w -2.9 Volts, an i r r e v e r s i b l e e l e c t r o c h e m i c a l d e p o s i tion of l i t h i u m f o l l o w e d by a r e a c t i o n w i t h i n d i u m o x i d e w o u l d occur, l e a d i n g to the d e s t r u c t i o n of the e l e c t r o d e . The a n o d i c d o m a i n is l i m i t e d by the o x i d a t i o n of C I O ~ at 2 V o l t s (23). A t y p i c a l c u r v e i s - s h o w n in fig(q). A p p l i c a t i o n of a c a t h o d i c p o t e n t i a l r e s u l t s in e l e c t r o c h e m i c a l l i t h i u m i n s e r t i o n and is a s s o c i a t e d w i t h the c o l o r a t i o n process. A n o d i c c u r r e n t on the o t h e r h a n d c o r r e s p o n d s to the b l e a c h i n g p r o cess. S u c c e s s i v e c y c l e s are s u p e r i m p o s e d one over the o t h e r s and a single w a v e is o b s e r v e d d u r i n g b o t h p r o c e s s e s show i n g that the e l e c t r o c h e m i c a l i n s e r t i o n is r e v e r s i b l e . E l e c t r o c h e m i c h r o m i c p r o p e r t i e s of c o l l o i d a l t u n g s t e n o x i d e layers w e r e s t u d i e d in a cell h a v i n g the f o l l o w i n g c o n f i g u r a t i o n : ITO/WOq/LiCIO4(M)-PC/Pt. W i t h such a d e v i c e 7 a b l u ~ c o l o r a t i o n is o b t a i n e d w i t h i n 0.3s by a p p l y i n g a n e g a t i v e v o l t a g e of -4 volts. The b l u e c o l o r a tion r e m a i n s for a b o u t 12 h o u r s a f t e r the v o l t a g e has b e e n removed. The i n i t i a l t r a n s p a r e n t state c a n be rest o r e d w i t h i n 0.4s u p o n a p p l i c a t i o n of a r e v e r s e v o l t a g e (+ 4 volts). U n d e r t h e s e c o n d i t i o n s , the e n e r g y c o n s u m p t i o n for c o l o r a t i o n and b l e a c h i n g
v= 50 inV. s"1
.1
Figure I : Cyclic voltammetric curve ITO - c o l l o i d a l WO~ e l e c t r o d e in I M LiCIO4-Pc electrolyte.
of
is a b o u t the same and c o r r e s p o n d s to 8 m C / c m 2 for an o p t i c a l d e n s i t y of 0.3. The d e v i c e a p p e a r s to be q u i t e r e v e r sible, c o l o r i n g and b l e a c h i n g cycles have b e e n r e p e a t e d over 105 times w i t h out failure. 3.2 E l e c t r o n
Spin
Resonance
An E.S.R. study of the layers was perf o r m e d w i t h an X - b a n d V a r i a n s p e c t r o meter. No signal w a s o b s e r v e d on the t r a n s p a r e n t c o l l o i d a l t u n g s t e n oxide, even at l i q u i d h e l i u m t e m p e r a t u r e , ind i c a t i n g that all t u n g s t e n ions are in t h e i r h i g h e r o x i d a t i o n states W(VI) . An E S R signal is on the o t h e r h a n d o b s e r v e d w h e n the layers have b e e n blue c o l o r e d in an e l e c t r o c h e m i c a l cell. Such a spectrum, r e c o r d e d at 100K is shown in fig(2). It e x h i b i t s two signals. A small one, on the l o w f i e l d side, cent e r e d a r o u n d g = 1.91, c o r r e s p o n d i n g to Mo(V) ions o f t e n f o u n d as i m p u r i t i e s in t u n g s t e n o x i d e (24). A s t r o n g e r signal a p p e a r s u p o n c o l o r a t i o n on the h i g h field side of the spectrum. Its i n t e n sity i n c r e a s e s w i t h the o p t i c a l density.
A. Chemseddine et al. / Electrochromism o f colloidal tungsten oxide
359
Absorption
Me (V) 34'00
W (V) 36~00
3800
H (Gauss) 400
860
1260
16'00 l(nm)
F i g u r e 2 : E.S.R. s p e c t r u m of a c o l l o i dal t u n g s t e n o x i d e layer a f t e r e l e c t r o chemical coloration;recording temperature 100K.
Figure 3 : O p t i c a l s p e c t r u m of a c o l l o i dal t u n g s t e n o x i d e layer. a. b e f o r e c o l o r a t i o n b. after e l e c t r o c h e m i c a l c o l o r a t i o n .
It is t y p i c a l of an u n p a i r e d e l e c t r o n (S = I/2) in an axial c r y s t a l f i e l d and can be d e s c r i b e d by the Zeeman H a m i l t o nian : = g~BHzS z + g±8(HxS x + Hy SY ) w i t h g] = 1.72 and g± = 1.79.
c o l o r a t i o n , a b r o a d a b s o r p t i o n band, w i t h a m a x i m u m a r o u n d I eV, appears. It grows in i n t e n s i t y d u r i n g the c o l o r a tion c y c l e and d i s a p p e a r s a f t e r b l e a ching. Such a band is t y p i c a l of o p t i cally activated intervalence transfers of u n p a i r e d e l e c t r o n s b e t w e e n t u n g s t e n ions in d i f f e r e n t v a l e n c e states W ( V ) - W ( V I ) . Its m a x i m u m o c c u r s at a s o m e w h a t s m a l l e r e n e r g y than for evap o r a t e d a m o r p h o u s w e 3 thin films (25).
Such g v a l u e s are t y p i c a l of W(V) ions a r i s i n g from the e l e c t r o c h e m i c a l r e d u c tion of the layer. They are c l o s e to t h o s e a l r e a d y found in a m o r p h o u s WO. thin films o b t a i n e d by v a p o r d e p o s i t i o n or s p u t t e r i n g (25). Some c h a n g e s can be o b s e r v e d on the ESR s p e c t r u m w h e n the r e c o r d i n g t e m p e r a t u r e increases. The low f i e l d signal c o r r e s p o n d i n g to Me(V) ions does not v a r y w h i l e the l i n e w i d t h of the high f i e l d signal increases. A b o v e 200K, the E S R signal c o r r e s p o n d i n g to W(V) ions b e c o m e s too b r o a d to be seen anymore.
4.
DISCUSSION
E.S.R. e x p e r i m e n t s show the p r e s e n c e of W(V) ions in the blue layers. The a m o u n t of r e d u c e d t u n g s t e n ions i n c r e a ses upon c o l o r a t i o n and d e c r e a s e s upon b l e a c h i n g . This agrees w i t h the g e n e rally accepted electrocoloration m e c h a n i s m (28) i n v o l v i n g the s i m u l t a neous d o u b l e i n j e c t i o n of ions and e l e c t r o n s into W O 3 to form a b r o n z e : +
3.3 O p t i c a l
spectroscopy
we 3 + xM +
The o p t i c a l a b s o r p t i o n s p e c t r a of the layers w e r e s t u d i e d at r o o m t e m p e r a t u r e on a B e c k m a n 5240 s p e c t r o m e t e r . F i g u r e (3a) shows the o p t i c a l s p e c t r u m of a t r a n s p a r e n t layer o b t a i n e d by d e p o s i t i o n of a t u n g s t e n o x i d e c o l l o i d a l solution. No a b s o r p t i o n o c c u r s in the v i s i b l e r e g i o n w h i l e a large b a n d can be seen on the U . V side of the spectrum. It c o r r e s p o n d s to c h a r g e t r a n s f e r t r a n s i tions f r o m o x y g e n to t u n g s t e n W(VI) ions and i n d i c a t e s an o p t i c a l gap of a b o u t 3.3 eV as in e v a p o r a t e d a m o r p h o u s we 3 thin films (25) upon e l e c t r o c h e m i c a l
+ xe +
> MxWO 3 +
where M = H or Li a c c o r d i n g to the e l e c t r o l y t e . It has b e e n shown h o w e v e r that w i t h L i C i O 4 - p r o p y l e n e c a r b o n a t e + w a t e r e l e c t r o l y £ e s , both Li + ions and H ions c o u l d take p a r t to the b r o n z e form a t i o n (22). Even w h e n dried, c o l l o i d a l t u n g s t e n oxide, still c o n t a i n s some w a t e r m o l e c u l e s and should be d e s c r i b e d as W O ,nH~O. W e must then a d m i t that some 3watlr r e m a i n s in the l a y e r and should take part to the e l e c t r o c h r o m i c process. The blue layers a p p e a r to be m i x e d v a l e n c e t u n g s t e n oxide. At low t e m p e r a -
A. Chernseddine et al. / Electrochromism o f colloidal tungsten oxide
360
ture (100K), the u n p a i r e d e l e c t r o n rem a i n s l o c a l i z e d on a single t u n g s t e n site g i v i n g rise to p a r a m a g n e t i c W(V) ions. A t h e r m a l l y a c t i v a t e d small p o l a ron h o p p i n g p r o c e s s o c c u r s at h i g h e r t e m p e r a t u r e l e a d i n g to a d e l o c a l i z a t i o n of the u n p a i r e d e l e c t r o n s a m o n g t u n g s ten sites (29). This r e s u l t s in a b r o a d e n i n g of the h i g h field E S R signal c o r r e s p o n d i n g to W(V). No b r o a d e n i n g of course is o b s e r v e d on the E S R signal c o r r e s p o n d i n g to Mo(V) impurities. The b l u e c o l o r a t i o n of the r e d u c e d layers a r i s e s from o p t i c a l l y a c t i v a t e d small p o l a r o n s h o p p i n g (29) l e a d i n g to the b r o a d a b s o r p t i o n b a n d c e n t e r e d a r o u n d I eV.
4.
Deb, S.K., 801-22.
5.
Hersh, H.N., Kramer, W.E. Mc Gee, J.E., Appl. Phys. 27 (1975) 646-8.
6.
Chang, I.F., G i l b e r t , B . L . and Sun, T.I., J. E l e c t r o c h e m . Soc. 122 (1975) 955-62.
7.
Yamaka, K., Japan. J. of A p p l i e d Phys. 19 (1980) L 517-8.
8.
Deneuville, A., G@rard, P. and Billat, R., Thin solid films, 70 (1980) 203-33.
C o m p a r e d w i t h usual a m o r p h o u s WO 3 thin films used in o t h e r e l e c t r o c h r o m l c display devices, c o l l o i d a l t u n g s t e n o x i d e e x h i b i t s two main advantages. The first one is due to the special n a t u r e of the c o l l o i d a l state. Thin layers can be very e a s i l y m a d e at low t e m p e r a t u r e . The sol-gel p r o c e s s should then be m u c h c h e a p e r than v a p o r d e p o s i tion or sputtering. The s e c o n d one a r i s e s from the fact that c o l l o i d a l t u n g s t e n o x i d e e x h i b i t s a large s u r f a c e area and c o n t a i n s w a t e r molecules. A c c o r d i n g to the literature, the c h a r a c t e r i s t i c s of WO. thin films , . J are v e r y s e n s ± t l v e to the m e t h o d of p r e p a r a t i o n . It has been o b s e r v e d for i n s t a n c e that it is m u c h m o r e d i f f i c u l t to c o l o r and b l e a c h s p u t t e r e d films than e v a p o r a t e d ones. This is p r o b a b l y due to the smaller a m o u n t of w a t e r cont a i n e d into s p u t t e r e d films (23). K n o w l e s et al. (27) have shown that some w a t e r has to be i n c o r p o r a t e d into the WO_ film in o r d e r to o b t a i n any c o l o r a tion. It thus a p p e a r s that the p r e s e n c e of w a t e r in the e l e c t r o c h r o m i c cell may g r e a t l y improve its c h a r a c t e r i s t i c s .
9.
Roy, R., J. Am. Ceram. (1969) 344-5.
ACKNOWLEDGEMENT
18. Gerand, B., N o w o g r o c k i , G. and Figlarz, M., J. of S o l i d State Chem. 38 (1981) 312-20.
A u t h o r s are i n d e b t e d to O. B o h n k e electrochemical measurements.
for
REFERENCES I.
Colton, R.J., Guzman, A.M. and Rabalais, J.W., A c c o u n t s of Chem. Res. 11 (1978) 170-6.
2.
Faugham, B.W. and Crandall, R.S., in " D i s p l a y Devices" J. P a n k o v e Ed. (1980) p.181. S p r i n g e r V e r l a g B e r l i n and N e w York.
3.
Beni, G. and Shay, L. in Adv. in Image P i c k u p and Display, 5 (1982) 83-136, B. K a z a n Ed., A c a d e m i c Press N e w York.
Phil. Mag.
27
(1973)
and Lett.
Soc.
52
10. Mukherjee, S.P., J. of N o n - C r y s t . Solids, 42 (1980) 477-88. 11. Dislich, H. and Hinz, P., J. of NonCryst. Solids, 48 (1982) 11-16. 12. Sanehez, C., Babonneau, F., Morineau, R. and Livage, J., Phil. Mag. B, 47 (1983) 279-90. 13. Brinker, C.J. and M u k h e r j e e , S.P., Thin Solid Films, 77 (1981) 141-8. 14. Freedman, M.L., J. Am. Chem. 81 (1959) 3834-9. 15. C h a t t e r j e e , S.N., 13 (1958) 61-6.
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16. Ramans, G.M., G a b r u s e n o k s , J.V. and Veispals, A.A., Phys. Stat. Sol. 74a (1982) K41-4. 17. Gunter, J.R., J. S o l i d State Chem. 5 (1972) 354.
19. Gerand, B., N o w o g r o c k i , G., Guenot, J. and Figlarz, M., J. of S o l i d State Chem. 29 (1979) 429-34. 20. Richardson, E., J. Inorg. Chem. 12 (1959) 79-83.
Nucl.
21. Lemerle, J. and Lefebvre, J., C a n a d i a n J. of Chem., 55 (1977) 3758-62. 22. Bohnke, O., Bohnke, C., C a r q u i l l e , B. and Robert, G., S o l i d State Ionics, 6 (19829 121-8.
A. Chemseddine et al. / Electrochromism o f colloidal tungsten oxide
23. Bohnke, O. and Robert, G., Solid State Ionics, 6 (1982) 115-20. 24. Deb, S.K., Phys. Review B 16 (1977) 1020-24. 25. G4rard, P., Deneuville, A. and Courths, R., Thin Solid Films, 71 (1980) 221-36. 26. Nakamura, A. and Yamada, S., Appl. Phys. 24 (1981) 55-9.
27. Knowles, T.J., Hersh, H.N. and Kramer, W., in: 19th Electronic Materials Conf. Aime, Corneil, N.Y. (1977). 28. Reichman, B. and Bard, A.J., J. Electrochem. Soc. 126 (1979) 583. 29. Austin, I.G. and Mott, N.F., Adv. Phys. 18 (1968) 41-102.
361
357
ELECTROCHROMISM
OF C O L L O I D A L
A. C h e m s e d d i n e ,
R. M o r i n e a u
TUNGSTEN
OXIDE
and J. L i v a g e
L a b o r a t o i r e de S p e c t r o c h i m i e du Solide, LA 302, U n i v e r s i t 4 P. et M. C u r i e - 4 p l a c e J u s s i e u - T.44, 2e @t. 75230 Paris C e d e x 05 - F r a n c e A c i d i f i c a t i o n of t u n g s t a t e s o l u t i o n s leads to the f o r m a t i o n of t u n g s t i c acids that p o l y m e r i z e , t h r o u g h a p o l y c o n d e n s a t i o n process, g i v i n g rise to c o l l o i d a l t u n g s t e n oxide. R e v e r s i b l e d i s p l a y d e v i c e s can be o b t a i n e d by e l e c t r o c h e m i c h r o m i s m . The c o l l o i d a l s o l u t i o n is then d e p o s i t e d onto a c o n d u c t i v e t r a n s p a r e n t e l e c t r o d e and p l a c e d into an e l e c t r o c h e m i c a l cell c o n t a i n i n g a l i q u i d e l e c t r o l y t e such as L i C I O a in p r o p y l e n e c a r b o n a t e . W i t h such a device, an o p t i c a l d e n s i t y -d.o = 0.3 is o b t a i n e d w i t h i n 0.3s by a p p l y i n g a n e g a t i v e v o l t a g e (-4V). The blue c o l o r a t i o n r e m a i n s a f t e r the v o l t a g e has been removed. O p t i c a l s p e c t r o scopy and E S R i n d i c a t e that the blue layer c o n t a i n s r e d u c e d W(V) ions.
I.
INTRODUCTION
E l e c t r o c h r o m i c d i s p l a y d e v i c e s b a s e d on a m o r p h o u s WO. thin film have b e e n ext e n s i v e l y s t u d i e d (I) (2) (3). A b o u t ten y e a r s ago, Deb (4) s h o w e d that such films c o u l d e x h i b i t two stable states. One is t r a n s p a r e n t and h i g h l y r e s i s t i v e w h i l e the second one is deep blue and m u c h less resistive. C o l o r a t i o n may be o b t a i n e d by UV i r r a d i a t i o n or by a p p l i c a t i o n of an e l e c t r i c f i e l d . B o t h proc e s s e s a p p e a r to be q u i t e slow and t h e r e f o r e not u s e f u l l for display. Fast r e v e r s i b l e c o l o r a t i o n can on the o t h e r h a n d be o b t a i n e d if the a m o r p h o u s oxide is d e p o s i t e d onto a c o n d u c t i v e e l e c t r o de and p l a c e d into an e l e c t r o c h e m i c a l cell (5). Such d e v i c e s e x h i b i t a clear image, i n s e n s i t i v e to the v i e w i n g angle and keep a l o n g - t e r m o p e n c i r c u i t m e m o ry. They open n e w p o s s i b i l i t i e s in the f i e l d of d i g i t a l d i s p l a y d e v i c e s (6). A m o r p h o u s W O 3 thin films are u s u a l l y o b t a i n e d by v a c u u m e v a p o r a t i o n (7) or s p u t t e r i n g (8). No a t t e m p t has yet been done to m a k e e l e c t r o c h r o m i c thin films by the s o l - g e l technology. This m e t h o d first c a l l e d the "gel route" by Roy (9) has g r o w n c o n t i n u o u s l y in r e c e n t years. It is n o w well k n o w n that i n o r g a n i c gels can be u s e d as s t a r t i n g m a t e r i a l s in o r d e r to m a k e g l a s s e s or c e r a m i c s at low t e m p e r a t u r e , well b e l o w their melting p o i n t (10). O x i d e thin films can be e a s i l y o b t a i n e d by d e p o s i t i o n of a c o l l o i d a l s o l u t i o n onto a s u b s t r a t e (11). The sol-gel p r o c e s s has r e c e n t l y been u s e d to m a k e s e m i c o n d u c•t i n g V~O~b layers (12) or a n t i - r e f l e c t l n g c o a t i n g s (13). In this paper, we shall d e s c r i b e the e l e c t r o c h r o m i c p r o p e r t i e s of tungsten o x i d e layers o b t a i n e d by d e p o s i tion from t u n g s t e n o x i d e colloids.
0 167-2738/83/0000-0000/$ 03.00 © 1983 North-Holland
2.
EXPERIMENTAL
P o l y m e r i z a t i o n of t u n g s t a t e anions upon a c i d i f i c a t i o n of a q u e o u s solutions leads to the f o r m a t i o n of t u n g s t i c acids (14) or t u n g s t e n t r i o x i d e h y d r a t e s W O ~ , n H 2 0 (15). D e p e n d i n g on the e x p e r i mefital-procedure d i f f e r e n t h y d r a t e s m a y be o b t a i n e d the n a t u r e of w h i c h sometimes r e m a i n s a m a t t e r of c o n t r o v e r s y (16). N e v e r t h e l e s s , the e x i s t e n c e and s t r u c t u r e of the m o n o and d i h y d r a t e s seem to be n o w well e s t a b l i s h e d (17). More recently, a n e w WOq, I/3 H20 was o b t a i n e d by h y d r o t h e r m a l t r e a t m e n t at 120°C of an a q u e o u s s u s p e n s i o n of e i t h e r t u n g s t i c acid gel or c r y s t a l l i z e d dih y d r a t e (18). U p o n d e h y d r a t i o n , this c o m p o u n d leads to a n e w h e x a g o n a l t u n g sten o x i d e W O 3 (19). C o l l o i d a l t u n g s t e n o x i d e is o f t e n obt a i n e d as a t r a n s i e n t i n t e r m e d i a t e d u r i n g the p o l y m e r i z a t i o n p r o c e s s of t u n g s t i c acids (15). This c o l l o i d a l p h a s e is n e v e r t h e l e s s not stable and p r e c i p i t a t i o n occurs l e a d i n g to h y d r a ted t u n g s t e n oxides. Such o x i d e s do not e x h i b i t any p h o t o c h r o m i c or e l e c t r o c h r o mic property. M o r e stable c o l l o i d a l solutions can be o b t a i n e d by ion exc h a n g e from s o d i u m t u n g s t a t e s o l u t i o n s (20) or by a d d i n g o r g a n i c s o l v e n t s (21). Such a p r o c e d u r e was f o l l o w e d in our e x p e r i m e n t s . A 0.5M s o d i u m t u n g s t a t e (Na2WO~, 2H~O) s o l u t i o n was p a s s e d t h r o u g h a D ~ w e x 50 WX2, 100-200 m e s h resin, l e a d i n g to a c o l l o i d a l s o l u t i o n that r e m a i n s stable for a few days. A few drops of this c o l l o i d a l s o l u t i o n w e r e then deposited, o n t o a c o n d u c t i v e t r a n s p a r e n t e l e c t r o d e (Indium Tin O x i d e c o a t e d glass substrate). The s o l v e n t r e a d i l y e v a p o r a t e s and a r a t h e r h a r d
358
A. Chemseddine et al. / Electrochromism o f colloidal tungsten oxide
c o a t i n g is obtained. B e t t e r r e s u l t s are o b t a i n e d if the g l a s s p l a t e is h e a t e d d u r i n g a p p l i c a t i o n w i t h an i n f r a - r e d lamp a n d p l a c e d on a q u i c k l y r o t a t i n g device. The t h i c k n e s s of the layers w a s m e a s u r e d by o p t i c a l i n t e r f e r o m e t r y . A c c o r d i n g to the e x p e r i m e n t a l p r o c e d u r e (tungstate c o n c e n t r a t i o n , a m o u n t of c o l l o i d a l solution) the t h i c k n e s s can be v a r i e d b e t w e e n 0.1 ~m and 10 ~m. L a y e r s of a b o u t 0.5 ~m u s u a l l y give a g o o d c o m p r o m i s e b e t w e e n fast r e s p o n s e a n d high o p t i c a l d e n s i t y w h e n u s e d for display. X - r a y d i f f r a c t i o n e x p e r i m e n t s show that the c o l l o i d a l t u n g s t e n o x i d e r e m a i n s a m o r p h o u s d u r i n g the w h o l e process.
tj m A
decoloral 0.2
-2 3.
RESULTS
3.1
Electrochemical
1
V vs AcffAg ÷
measurements
The I.T.O g l a s s electrode, c o a t e d w i t h c o l l o i d a l t u n g s t e n o x i d e layer, w a s p l a c e d into an e l e c t r o c h e m i c a l cell cont a i n i n g a IM L i C I O 4 s o l u t i o n in p r o p y lene c a r b o n a t e as an e l e c t r o l y t e . A p t p l a t e was u s e d as a c o u n t e r e l e c t r o d e in a c l a s s i c a l t h r e e - e l e c t r o d e s p o t e n t i o s t a t i c cell, w i t h the A g - A g C I O , (I0-2M) in P.C. as the r e f e r e n c e 4 e l e c trode. E l e c t r i c a l a n d o p t i c a l m e a s u r e m e n t s w e r e c a r r i e d out s i m u l t a n e o u s l y as d e s c r i b e d by B o h n k e e t al (22). C y c l i c voltammetric experiments were performed at scan rates of 50 m V / s b e t w e e n -2 v o l t s and +1.5 volts. B e l o w -2.9 Volts, an i r r e v e r s i b l e e l e c t r o c h e m i c a l d e p o s i tion of l i t h i u m f o l l o w e d by a r e a c t i o n w i t h i n d i u m o x i d e w o u l d occur, l e a d i n g to the d e s t r u c t i o n of the e l e c t r o d e . The a n o d i c d o m a i n is l i m i t e d by the o x i d a t i o n of C I O ~ at 2 V o l t s (23). A t y p i c a l c u r v e i s - s h o w n in fig(q). A p p l i c a t i o n of a c a t h o d i c p o t e n t i a l r e s u l t s in e l e c t r o c h e m i c a l l i t h i u m i n s e r t i o n and is a s s o c i a t e d w i t h the c o l o r a t i o n process. A n o d i c c u r r e n t on the o t h e r h a n d c o r r e s p o n d s to the b l e a c h i n g p r o cess. S u c c e s s i v e c y c l e s are s u p e r i m p o s e d one over the o t h e r s and a single w a v e is o b s e r v e d d u r i n g b o t h p r o c e s s e s show i n g that the e l e c t r o c h e m i c a l i n s e r t i o n is r e v e r s i b l e . E l e c t r o c h e m i c h r o m i c p r o p e r t i e s of c o l l o i d a l t u n g s t e n o x i d e layers w e r e s t u d i e d in a cell h a v i n g the f o l l o w i n g c o n f i g u r a t i o n : ITO/WOq/LiCIO4(M)-PC/Pt. W i t h such a d e v i c e 7 a b l u ~ c o l o r a t i o n is o b t a i n e d w i t h i n 0.3s by a p p l y i n g a n e g a t i v e v o l t a g e of -4 volts. The b l u e c o l o r a tion r e m a i n s for a b o u t 12 h o u r s a f t e r the v o l t a g e has b e e n removed. The i n i t i a l t r a n s p a r e n t state c a n be rest o r e d w i t h i n 0.4s u p o n a p p l i c a t i o n of a r e v e r s e v o l t a g e (+ 4 volts). U n d e r t h e s e c o n d i t i o n s , the e n e r g y c o n s u m p t i o n for c o l o r a t i o n and b l e a c h i n g
v= 50 inV. s"1
.1
Figure I : Cyclic voltammetric curve ITO - c o l l o i d a l WO~ e l e c t r o d e in I M LiCIO4-Pc electrolyte.
of
is a b o u t the same and c o r r e s p o n d s to 8 m C / c m 2 for an o p t i c a l d e n s i t y of 0.3. The d e v i c e a p p e a r s to be q u i t e r e v e r sible, c o l o r i n g and b l e a c h i n g cycles have b e e n r e p e a t e d over 105 times w i t h out failure. 3.2 E l e c t r o n
Spin
Resonance
An E.S.R. study of the layers was perf o r m e d w i t h an X - b a n d V a r i a n s p e c t r o meter. No signal w a s o b s e r v e d on the t r a n s p a r e n t c o l l o i d a l t u n g s t e n oxide, even at l i q u i d h e l i u m t e m p e r a t u r e , ind i c a t i n g that all t u n g s t e n ions are in t h e i r h i g h e r o x i d a t i o n states W(VI) . An E S R signal is on the o t h e r h a n d o b s e r v e d w h e n the layers have b e e n blue c o l o r e d in an e l e c t r o c h e m i c a l cell. Such a spectrum, r e c o r d e d at 100K is shown in fig(2). It e x h i b i t s two signals. A small one, on the l o w f i e l d side, cent e r e d a r o u n d g = 1.91, c o r r e s p o n d i n g to Mo(V) ions o f t e n f o u n d as i m p u r i t i e s in t u n g s t e n o x i d e (24). A s t r o n g e r signal a p p e a r s u p o n c o l o r a t i o n on the h i g h field side of the spectrum. Its i n t e n sity i n c r e a s e s w i t h the o p t i c a l density.
A. Chemseddine et al. / Electrochromism o f colloidal tungsten oxide
359
Absorption
Me (V) 34'00
W (V) 36~00
3800
H (Gauss) 400
860
1260
16'00 l(nm)
F i g u r e 2 : E.S.R. s p e c t r u m of a c o l l o i dal t u n g s t e n o x i d e layer a f t e r e l e c t r o chemical coloration;recording temperature 100K.
Figure 3 : O p t i c a l s p e c t r u m of a c o l l o i dal t u n g s t e n o x i d e layer. a. b e f o r e c o l o r a t i o n b. after e l e c t r o c h e m i c a l c o l o r a t i o n .
It is t y p i c a l of an u n p a i r e d e l e c t r o n (S = I/2) in an axial c r y s t a l f i e l d and can be d e s c r i b e d by the Zeeman H a m i l t o nian : = g~BHzS z + g±8(HxS x + Hy SY ) w i t h g] = 1.72 and g± = 1.79.
c o l o r a t i o n , a b r o a d a b s o r p t i o n band, w i t h a m a x i m u m a r o u n d I eV, appears. It grows in i n t e n s i t y d u r i n g the c o l o r a tion c y c l e and d i s a p p e a r s a f t e r b l e a ching. Such a band is t y p i c a l of o p t i cally activated intervalence transfers of u n p a i r e d e l e c t r o n s b e t w e e n t u n g s t e n ions in d i f f e r e n t v a l e n c e states W ( V ) - W ( V I ) . Its m a x i m u m o c c u r s at a s o m e w h a t s m a l l e r e n e r g y than for evap o r a t e d a m o r p h o u s w e 3 thin films (25).
Such g v a l u e s are t y p i c a l of W(V) ions a r i s i n g from the e l e c t r o c h e m i c a l r e d u c tion of the layer. They are c l o s e to t h o s e a l r e a d y found in a m o r p h o u s WO. thin films o b t a i n e d by v a p o r d e p o s i t i o n or s p u t t e r i n g (25). Some c h a n g e s can be o b s e r v e d on the ESR s p e c t r u m w h e n the r e c o r d i n g t e m p e r a t u r e increases. The low f i e l d signal c o r r e s p o n d i n g to Me(V) ions does not v a r y w h i l e the l i n e w i d t h of the high f i e l d signal increases. A b o v e 200K, the E S R signal c o r r e s p o n d i n g to W(V) ions b e c o m e s too b r o a d to be seen anymore.
4.
DISCUSSION
E.S.R. e x p e r i m e n t s show the p r e s e n c e of W(V) ions in the blue layers. The a m o u n t of r e d u c e d t u n g s t e n ions i n c r e a ses upon c o l o r a t i o n and d e c r e a s e s upon b l e a c h i n g . This agrees w i t h the g e n e rally accepted electrocoloration m e c h a n i s m (28) i n v o l v i n g the s i m u l t a neous d o u b l e i n j e c t i o n of ions and e l e c t r o n s into W O 3 to form a b r o n z e : +
3.3 O p t i c a l
spectroscopy
we 3 + xM +
The o p t i c a l a b s o r p t i o n s p e c t r a of the layers w e r e s t u d i e d at r o o m t e m p e r a t u r e on a B e c k m a n 5240 s p e c t r o m e t e r . F i g u r e (3a) shows the o p t i c a l s p e c t r u m of a t r a n s p a r e n t layer o b t a i n e d by d e p o s i t i o n of a t u n g s t e n o x i d e c o l l o i d a l solution. No a b s o r p t i o n o c c u r s in the v i s i b l e r e g i o n w h i l e a large b a n d can be seen on the U . V side of the spectrum. It c o r r e s p o n d s to c h a r g e t r a n s f e r t r a n s i tions f r o m o x y g e n to t u n g s t e n W(VI) ions and i n d i c a t e s an o p t i c a l gap of a b o u t 3.3 eV as in e v a p o r a t e d a m o r p h o u s we 3 thin films (25) upon e l e c t r o c h e m i c a l
+ xe +
> MxWO 3 +
where M = H or Li a c c o r d i n g to the e l e c t r o l y t e . It has b e e n shown h o w e v e r that w i t h L i C i O 4 - p r o p y l e n e c a r b o n a t e + w a t e r e l e c t r o l y £ e s , both Li + ions and H ions c o u l d take p a r t to the b r o n z e form a t i o n (22). Even w h e n dried, c o l l o i d a l t u n g s t e n oxide, still c o n t a i n s some w a t e r m o l e c u l e s and should be d e s c r i b e d as W O ,nH~O. W e must then a d m i t that some 3watlr r e m a i n s in the l a y e r and should take part to the e l e c t r o c h r o m i c process. The blue layers a p p e a r to be m i x e d v a l e n c e t u n g s t e n oxide. At low t e m p e r a -
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ture (100K), the u n p a i r e d e l e c t r o n rem a i n s l o c a l i z e d on a single t u n g s t e n site g i v i n g rise to p a r a m a g n e t i c W(V) ions. A t h e r m a l l y a c t i v a t e d small p o l a ron h o p p i n g p r o c e s s o c c u r s at h i g h e r t e m p e r a t u r e l e a d i n g to a d e l o c a l i z a t i o n of the u n p a i r e d e l e c t r o n s a m o n g t u n g s ten sites (29). This r e s u l t s in a b r o a d e n i n g of the h i g h field E S R signal c o r r e s p o n d i n g to W(V). No b r o a d e n i n g of course is o b s e r v e d on the E S R signal c o r r e s p o n d i n g to Mo(V) impurities. The b l u e c o l o r a t i o n of the r e d u c e d layers a r i s e s from o p t i c a l l y a c t i v a t e d small p o l a r o n s h o p p i n g (29) l e a d i n g to the b r o a d a b s o r p t i o n b a n d c e n t e r e d a r o u n d I eV.
4.
Deb, S.K., 801-22.
5.
Hersh, H.N., Kramer, W.E. Mc Gee, J.E., Appl. Phys. 27 (1975) 646-8.
6.
Chang, I.F., G i l b e r t , B . L . and Sun, T.I., J. E l e c t r o c h e m . Soc. 122 (1975) 955-62.
7.
Yamaka, K., Japan. J. of A p p l i e d Phys. 19 (1980) L 517-8.
8.
Deneuville, A., G@rard, P. and Billat, R., Thin solid films, 70 (1980) 203-33.
C o m p a r e d w i t h usual a m o r p h o u s WO 3 thin films used in o t h e r e l e c t r o c h r o m l c display devices, c o l l o i d a l t u n g s t e n o x i d e e x h i b i t s two main advantages. The first one is due to the special n a t u r e of the c o l l o i d a l state. Thin layers can be very e a s i l y m a d e at low t e m p e r a t u r e . The sol-gel p r o c e s s should then be m u c h c h e a p e r than v a p o r d e p o s i tion or sputtering. The s e c o n d one a r i s e s from the fact that c o l l o i d a l t u n g s t e n o x i d e e x h i b i t s a large s u r f a c e area and c o n t a i n s w a t e r molecules. A c c o r d i n g to the literature, the c h a r a c t e r i s t i c s of WO. thin films , . J are v e r y s e n s ± t l v e to the m e t h o d of p r e p a r a t i o n . It has been o b s e r v e d for i n s t a n c e that it is m u c h m o r e d i f f i c u l t to c o l o r and b l e a c h s p u t t e r e d films than e v a p o r a t e d ones. This is p r o b a b l y due to the smaller a m o u n t of w a t e r cont a i n e d into s p u t t e r e d films (23). K n o w l e s et al. (27) have shown that some w a t e r has to be i n c o r p o r a t e d into the WO_ film in o r d e r to o b t a i n any c o l o r a tion. It thus a p p e a r s that the p r e s e n c e of w a t e r in the e l e c t r o c h r o m i c cell may g r e a t l y improve its c h a r a c t e r i s t i c s .
9.
Roy, R., J. Am. Ceram. (1969) 344-5.
ACKNOWLEDGEMENT
18. Gerand, B., N o w o g r o c k i , G. and Figlarz, M., J. of S o l i d State Chem. 38 (1981) 312-20.
A u t h o r s are i n d e b t e d to O. B o h n k e electrochemical measurements.
for
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