Efficient Water Cleavage by Visible Light in ... - ACS Publications

7 Efficient Water Cleavage by Visible Light in Colloidal Solutions of Bifunctional Redox Catalysts MICHAEL GRÄTZEL

Downloaded by PURDUE UNIV on May 25, 2016 | http://pubs.acs.org Publication Date: February 12, 1982 | doi: 10.1021/bk-1982-0177.ch007

Ecole Polytechnique Fédérale de Lausanne, Institut de Chimie Physique, 1015 Lausanne, Switzerland

Cleavage of water by four quanta of v i s i b l e l i g h t into hydrogen and oxygen is achieved i n aqueous solutions of RuO and Pt cosupported by c o l l o i d a l TiO p a r t i c l e s . The only other component present i s a s e n s i t i z e r . No electron relay compound is required to accomplish the photolysis. Amphiphilic surfactant derivatives of Ru(bipy) exhibit astonishingly high a c t i v i t y in promoting the water cleavage process. Adsorption of the sensitizer at the TiO surface i s evoked to explain the observations. Exposure to UV r a d i ation leads to e f f i c i e n t water cleavage in the absence of a s e n s i t i z e r . 2

2

2+

3

2

In the present area of dwindling fuel reserves the development of alternative energy supplies has become a research subject of high p r i o r i t y Q-.I). topic that has intrigued scientists from many different f i e l d s i s the photolysis of water using solar radiation (8-15). The p r a c t i c a l potential of devices achieving t h i s process would be enormous i f s u f f i c i e n t l y high conversion efficiencies could be obtained. Figure 1 shows a plot of the maximum conversion efficiency of a threshold absorber that would use a l l the photons i n the solar spectrum below an onset wavelength λ. In view of the thermodynamic requirements for the s p l i t t i n g of water into hydrogen and oxygen by a k photon process 0 n e

2H 0 2

^

V

> 2H + 0 2

2

(Δ6 = 1.23eV « 1000)

(1)

and unavoidable losses, a threshold wavelength of 600nm i s proba bly a r e a l i s t i c estimate for a water cleavage device. This would correspond to an upper l i m i t i n the conversion efficiency of ca. 20$. According to more conservative estimates a λβ% y i e l d ap pears to be feasible (16). S t i l l , such systems could make a tremendous contribution to satisfy future energy demands. 0097-6156/82/0177-0113$06.25/0 © 1982 American Chemical Society Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


114

INORGANIC REACTIONS IN ORGANIZED MEDIA

g

20

h

H 10

200

600

1000

1400

X(nm) Figure 1. Optical conversion efficiency for solar energy as a function of threshold wavelength of the absorber. Curve I is a plot of the fraction of incident solar power (percent) available at various threshold wavelengths; Curve II is a plot of the thermodynamic conversion efficiencies under optimal rates of energy conversion.

Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

7.

GRATZEL

Efficient Water Cleavage by

Visible Light

115

In order t o achieve the g o a l o f decomposing water i n t o hydro gen and oxygen by v i s i b l e l i g h t , d i f f e r e n t s t r a t e g i e s have been a p p l i e d . A s t r a i g h t f o r w a r d s o l u t i o n which s u f f e r s from h i g h cost would be the c o u p l i n g o f a p h o t o v o l t a i c device w i t h a water e l e c t r o l y z e r . An a l t e r n a t i v e approach i s based on the concept o f wet p h o t o v o l t a i c s , i . e . , the i l l u m i n a t i o n o f a semiconductor/electro l y t e j u n c t i o n . Honda and c o l l a b o r a t o r s have shown t h a t photoe l e c t r o l y s i s o f water can indeed be achieved w i t h T i 0 as a photoanode (17). However, t h i s m a t e r i a l absorbs only UV l i g h t , which makes i t u n s u i t a b l e f o r s o l a r a p p l i c a t i o n . T h i s l e c t u r e deals with y e t another concept o f photoinduced water cleavage based on microheterogeneous photochemistry and redox c a t a l y s i s . Assemblies o f minute dimensions w i l l be d e s c r i b e d which through s u i t a b l e mo l e c u l a r engineering accomplish the d i f f i c u l t t a s k o f l i g h t - i n d u c e d charge s e p a r a t i o n coupled t o energy c o n v e r s i o n . Among the v a r i e t y o f systems p r e s e n t l y under i n v e s t i g a t i o n one can d i s t i n g u i s h t h r e e d i f f e r e n t c a t e g o r i e s . The f i r s t com p r i s e s p h o t o s y n t h e t i c h y b r i d systems, F i g . 2. Here, c h l o r o p l a s t s or i n d i v i d u a l photosystems are employed as l i g h t h a r v e s t i n g u n i t s . The o b j e c t i v e i s t o e x p l o i t the h i g h e f f i c i e n c y o f the primary photosynthetic redox events without attempting t o s y n t h e s i z e c a r bohydrates from C0 . Instead hydrogen g e n e r a t i o n from water i s achieved by i n t e r c e p t i n g the e l e c t r o n t r a n s f e r c h a i n o f photosystem I w i t h a s u i t a b l e e l e c t r o n r e l a y such as methyl v i o l o g e n ( M V ) . The l a t t e r couples the photosynthetic e l e c t r o n flow t o a c a t a l y s t a f f o r d i n g water r e d u c t i o n t o hydrogen. Great progress has r e c e n t l y been made i n t h i s domain w i t h the advent o f s y n t h e t i c c a t a l y s t s such as u l t r a f i n e Pt p a r t i c l e s (18) or Pt on P t 0 (19) which r e p l a c e the n a t u r a l enzyme hydrogenase. Water o x i d a t i o n i s c a r r i e d out by the water s p l i t t i n g enzyme o f photosystem I I . Attemps are a l s o made t o r e p l a c e the l a t t e r by h i g h l y a c t i v e oxy gen g e n e r a t i n g c a t a l y s t s such as c o l l o i d a l Ru0 (20).


2

2

2+

2

2

The second approach i s t o employ s y n t h e t i c molecular assem b l i e s such as m i c e l l e s , microemulsions and v e s i c l e s as r e a c t i o n systems. S t r u c t u r a l f e a t u r e s o f these molecular o r g a n i z a t i o n s are o u t l i n e d i n F i g u r e 3. These aggregates simulate the m i c r o e n v i r o n ment present i n b i o l o g i c a l systems. Hydrophobic host molecules p a r t i c i p a t i n g i n photoredox r e a c t i o n s may be i n c o r p o r a t e d i n t h e i r l i p i d - l i k e i n t e r i o r and the charged l i p i d / w a t e r i n t e r f a c e may be e x p l o i t e d t o c o n t r o l k i n e t i c a l l y the e l e c t r o n t r a n s f e r events. Thus, i n the case o f an i o n i c m i c e l l e , the l o c a l e l e c t r o s t a t i c f i e l d present at the surface o f the aggregate can r e a d i l y exceed 10° V/cm and t h i s microscopic e l e c t r o s t a t i c b a r r i e r can be used t o achieve l i g h t - i n d u c e d charge s e p a r a t i o n (21). The s i t u a t i o n i s analogous t o e l e c t r o n - h o l e s e p a r a t i o n i n the space charge l a y e r o f a semiconductor. In the case o f the photοinitiated



116


Figure 2.

Schematic illustration of a cell-free hybrid system for the biophotolysis of water.

Figure 3.

Structural features of colloidal assemblies employed in light-induced charge separation.


7.

GRÄTZEL

Efficient Water Cleavage by Visible Light

117

redox r e a c t i o n between a s e n s i t i z e r (S) and an e l e c t r o n r e l a y (R) the goal i s t o enhance t h e r a t e o f the forward r e a c t i o n


S + R

S

+

+ R~

(2)

and at t h e same time r e t a r d t h a t o f the backward e l e c t r o n t r a n s fer. T h i s can be achieved by u s i n g s o l u t i o n s o f simple (22 ) o r f u n c t i o n a l ( 2 3 ) m i c e l l a r aggregates as r e a c t i o n medium. The l a t t e r type o f s u r f a c t a n t s are d i s t i n g u i s h e d by t h e f a c t that they are chemically l i n k e d t o moieties p a r t i c i p a t i n g themselves i n the redox events. Amphophilic redox r e l a y s have a l s o been successf u l l y employed f o r l i g h t - i n d u c e d charge s e p a r a t i o n . Here the a d d i t i o n o f an e l e c t r o n t o the molecule d r a s t i c a l l y s h i f t s i t s hydrophobic/hydrophilic balance. Apart from m i c e l l e s , molecular assemblies such as microemulsions (2k) and v e s i c l e s (2%) deserve p a r t i c u l a r a t t e n t i o n i n t h e context o f photoredox r e a c t i o n s i n biomimetic aggregates. C a l v i n and co-workers have f o r t h e f i r s t time i l l u s t r a t e d l i g h t - i n d u c e d e l e c t r o n t r a n s f e r across t h e b i l a y e r o f liposomes ( 2 6 ) . The same group has a l s o performed elegant s t u d i e s with i n v e r t e d m i c e l l e s . The whole domain o f biomimetic systems i s e x p e r i e n c i n g p r e s e n t l y a r a p i d growth and e x c i t i n g d i s c o v e r i e s manifest t h e a s t o n i s h i n g progress i n t h i s area. L i g h t Harvesting

Units i n A r t i f i c i a l

Water S p l i t t i n g Systems

A t h i r d category which subsequently w i l l be t r e a t e d i n more d e t a i l i s that o f t o t a l l y a r t i f i c i a l systems. These show no apparent s i m i l a r i t y w i t h t h e i r n a t u r a l counterpart. Figure k summarizes the mechanism o f l i g h t h a r v e s t i n g and energy convers i o n o p e r a t i v e i n three d i f f e r e n t c o n f i g u r a t i o n s t y p i c a l l y employed here. A s e n s i t i z e r / r e l a y p a i r i s used i n t h e f i r s t device. L i g h t - i n d u c e d e l e c t r o n t r a n s f e r produces t h e r a d i c a l ions S and R~ which are subsequently employed t o o x i d i z e and reduce water, r e s p e c t i v e l y . Thus, l i g h t functions here as an e l e c t r o n pump o p e r a t i n g against a gradient o f chemical p o t e n t i a l . Such photoinduced redox r e a c t i o n s have been s t u d i e d i n great d e t a i l both from the experimental ( 2 £ ) as w e l l as t h e o r e t i c a l p o i n t o f view. The r a t e o f e l e c t r o n t r a n s f e r between e x c i t e d s e n s i t i z e r and r e l a y i s expected t o approach the d i f f u s i o n c o n t r o l l e d l i m i t as soon as t h e d r i v i n g force f o r the r e a c t i o n exceeds a few hundred millivolts. Conversely, the backward e l e c t r o n t r a n s f e r between S and R~ which i s thermodynamic a l l y s t r o n g l y favored i s almost always d i f f u s i o n c o n t r o l l e d . T h i s poses a severe problem f o r t h e use o f such systems i n energy conversion devices as i t l i m i t s t h e l i f e t i m e o f t h e r a d i c a l ions t o a t most s e v e r a l m i l l i s e c o n d s under s o l a r l i g h t i n t e n s i t y . +


118



Figure 4.

Light harvesting and catalytic units for light-induced water decomposition.


7.

GRÀTZEL


119

With regards t o the choice o f the s e n s i t i z e r and the r e l a y , compounds have t o be found t h a t are s u i t a b l e both from the viewp o i n t s o f l i g h t a b s o r p t i o n and redox p o t e n t i a l s and undergo no chemical s i d e r e a c t i o n s i n t h e o x i d a t i o n s t a t e s o f i n t e r e s t . The s e n s i t i z e r should have good absorption f e a t u r e s w i t h respect t o the s o l a r spectrum. A l s o , i t s e x c i t e d s t a t e should be formed w i t h h i g h quantum y i e l d , have a reasonably l o n g l i f e t i m e and the e l e c t r o n t r a n s f e r r e a c t i o n must occur w i t h h i g h e f f i c i e n c y , v i z . good s o l v e n t cage escape y i e l d o f the redox products. The redox p r o p e r t i e s o f the donor-acceptor r e l a y must o b v i o u s l y be tuned t o the f u e l - p r o d u c i n g t r a n s f o r m a t i o n envisaged. I n t h e case o f wat e r cleavage by l i g h t the thermodynamic requirements are such t h a t E ° ( S / S ) > 1.23V (NHE/ and E°(R/lT) > OV under standard conditions. In the design o f s e n s i t i z e r / r e l a y couples s u i t a b l e f o r photoinduced water decomposition c o n s i d e r a b l e progress has been made over the l a s t few years (28). A number o f systems have been explored c o n v e r t i n g more than 90% o f the t h r e s h o l d l i g h t energy r e q u i r e d f o r e x c i t a t i o n o f the s e n s i t i z e r i n t o chemical p o t e n t i a l . Porphyrines appear t o be p a r t i c u l a r l y promising i n t h i s r e s p e c t . In s e v e r a l cases t h e reduced r e l a y and o x i d i z e d s e n s i t i z e r are thermodynamically capable o f generating H2 and O2 from water:


+

R~ + H 0 -> 2

2S

+

+ H0 2

I

H

2

+ OH" + R

I

0

2

+ 2H

+

+ 2S

(3) (k)

Noteworthy examples are s e n s i t i z e r s such as R u i b i p y ) ^ (29) » p o r phyrine d e r i v a t i v e s (30) and a c r i d i n e dyes, e.g., p r o f l a v i n (31). Among the e l e c t r o n r e l a y compounds i n v e s t i g a t e d i t i s worth ment i o n i n g the viologens (32) E u , V and t h e i r r e s p e c t i v e s a l i c y l a t e complexes (j£), u ( b i p y ) | (3k) and c o b a l t complexes (35). Thus, at present a c o n s i d e r a b l e choice o f s e n s i t i z e r / r e l a y p a i r s i s a v a i l a b l e t h a t f u l f i l l t h e photochemical and thermodynamic requirements f o r water decomposition. The next step i s t o combine t h e photoprocess w i t h t h e genera t i o n o f hydrogen u s i n g water as an e l e c t r o n source. T h i s p r e sents a formidable problem s i n c e water o x i d a t i o n as w e l l as reduc t i o n are m u l t i s t e p e l e c t r o n t r a n s f e r processes t h a t proceed through stages o f h i g h l y r e a c t i v e and e n e r g e t i c i n t e r m e d i a t e s . The success i n t h i s domain has t h e r e f o r e l a r g e l y been determined by the development o f redox c a t a l y s t s mediating hydrogen and oxygen formation and thus a v o i d i n g these r a d i c a l i n t e r m e d i a t e s . I n f a c t , only t h r o u ^ i d r a s t i c improvement o f p r e v i o u s l y known and discovery o f new redox c a t a l y s t s has t h e design o f a c y c l i c water decomposition system o p e r a t i n g on f o u r quanta o f v i s i b l e l i g h t become f e a s i b l e . The performance o f these c a t a l y s t s has t o 3+

R

3 +

+


120


s a t i s f y the f o l l o w i n g c o n d i t i o n s : a) The c a t a l y s t s have t o i n t e r c e p t the thermal back r e a c t i o n which occurs i n the m i c r o - t o m i l l i s e c o n d time domain, b) The water r e d u c t i o n c a t a l y s t 2 must compete with oxygen r e d u c t i o n by R~ which i s expected t o occur at a d i f f u s i o n c o n t r o l l e d r a t e . T h i s sets the r a t e l i m i t r e q u i r e d f o r H2 generation at s e v e r a l microseconds, c) The i n t e r v e n t i o n o f the c a t a l y s t s has t o be s p e c i f i c i n o r d e r t o a v o i d s h o r t - c i r c u i t r y o f the back r e a c t i o n , d) I n o r d e r t o achieve high quan tum y i e l d s i t i s b e n e f i c i a l t o keep the oxygen concentration i n s o l u t i o n as low as p o s s i b l e . Hence, i t i s d e s i r a b l e to have an oxygen c a r r i e r present i n s o l u t i o n t h a t absorbs the 0 produced during p h o t o l y s i s . T h i s allows a l s o f o r H 2 / O 2 separation.


2

Development o f Highly E f f i c i e n t Hydrogen Producing C a t a l y s t s A few years ago i t would have seemed impossible t o overcome a l l o f the d i f f i c u l t i e s a s s o c i a t e d w i t h requirements (a-d). A t t h a t time there was even no 0 producing c a t a l y s t a v a i l a b l e . T h i s was d i s c o v e r e d i n 1978 i n our l a b o r a t o r y (36) i n the form o f noble metal oxides such as P t 0 , I r 0 and Ru0 . The l a t t e r has been most widely i n v e s t i g a t e d since then (37). Platinum has been known f o r a l o n g time t o mediate water r e d u c t i o n by agents such as V , C r and a l s o reduced viologens. However, i t r e q u i r e d s e v e r a l years o f research t o develop a Pt c a t a l y s t t h a t would s a t i s f y c o n d i t i o n s (a-d) i n d i c a t e d above, i . e . , produce H i n the microsecond time domain at reasonably low Pt concentrations. Our s t r a t e g y here was t o develop P t c o l l o i d s o f minimum p a r t i c l e s i z e which would render the water r e d u c t i o n by the e l e c t r o n r e l a y R~ p a r t i c u l a r l y e f f e c t i v e . The goal was t o o b t a i n hydrogen e v o l u t i o n rates i n the microsecond time domain which would com pete w i t h back e l e c t r o n t r a n s f e r . F i r s t a c e n t r i f u g e d Pt s o l p r o t e c t e d by p o l y v i n y l a l c o h o l was employed which gave c l e a r and almost c o l o r l e s s aqueous suspensions even at higgi P t concentra t i o n s (38). F l a s h p h o t o l y s i s technique was a p p l i e d f o r the f i r s t time t o study the dynamics o f i n t e r v e n t i o n o f the P t p a r t i c l e s i n the water reduction process. Methylviologen ( M V ) was reduced by the e x c i t e d s t a t e o f R u ( b i p y ) : 2

2

2 +

2

2

2 +

2

2+

2 +

*Ru(bipy)

2 +

+ MV

2+

+ MV

+

+ Ru(bipy)|

+

(5)

+

and the b e h a v i o r o f MV analyzed by absorbance technique. The r a t e constant f o r water r e d u c t i o n :

MV

+

I H + OH" + M V

+ H 0 2

2+

2

(6)

was found t o be 6 χ lO^S" at 10" M Pt c o n c e n t r a t i o n . The high a c t i v i t y o f t h i s c a t a l y s t was a l s o demonstrated i n a c l a s s i c a l 1

3


7.

GRATZEL


121

photochemical r e a c t i o n , i . e . , t h e p h o t o l y s i s o f benzophenone i n a water/alcohol mixture. Here i n the presence o f c o l l o i d a l Pt t h e k e t y l r a d i c a l s were d i r e c t e d towards water r e d u c t i o n (39) Φ c - OH + H 0

(7)


2

i n s t e a d o f d i m e r i z a t i o n o r d i s p r o p o r t i o n a t i o n . Thus the pathway o f the p h o t o r e a c t i o n i s t o t a l l y a l t e r e d i n t h e presence o f these c a t a l y s t s . I n s t e a d o f the photoreduction o f "benzophenone t o benzpinakol one observes s e n s i t i z e d conversion o f isopropanol i n t o acetone and hydrogen, which i s an energy s t o r i n g process. For f u r t h e r refinement o f the P t c a t a l y s t we made use o f Turkevich's method t o prepare u l t r a f i n e and monodisperse Pt s o l s (40). Employing polymer s u r f a c t a n t s as p r o t e c t i v e agents a f i r s t s u c c e s s f u l attempt was made t o render t h e i n t e r v e n t i o n o f these p a r t i c l e s s p e c i f i c . Thus p o l y v i n y l p y r i d i n e absorbs w e l l t o the 30 A platinum p a r t i c l e s r e n d e r i n g t h e i r surface a m p h i p h i l i c . Hydrophobic r e l a y s such as l o n g chain s u b s t i t u t e d v i o l o g e n r a d i c a l s are r e a d i l y t r a p p e d by these p a r t i c l e s and subsequently a f f e c t water r e d u c t i o n . In contrast t h e o x i d i z e d s e n s i t i z e r i s r e j e c t e d from the P t surface by hydrophobic and e l e c t r o s t a t i c i n t e r a c t i o n s . Thus, s h o r t - c i r c u i t r y o f the back r e a c t i o n i s avoided. A f u r t h e r remarkable i n c r e a s e i n a c t i v i t y i s observed when these u l t r a f i n e Pt aggregates are d e p o s i t e d on a c o l l o i d a l semi conductor such as T i O 2 . In t h i s case t h e c r o s s - s e c t i o n o f t h e r e a c t i o n o f the reduced r e l a y R with the c a t a l y s t i s g r e a t l y i n c r e a s e d as the support i t s e l f can a c t as an e l e c t r o n acceptor (FigureS). The e l e c t r o n i s i n j e c t e d i n t o the conduction band o f the semiconductor from where i t migrates t o P t s i t e s where hydro gen generation from water takes p l a c e . By u s i n g l a s e r p h o t o l y s i s technique, i t was p o s s i b l e t o d i r e c t l y monitor t h e k i n e t i c s o f hydrogen formation from reduced methyl v i o l o g e n and water i n such a system (kV). We observed a r e a c t i o n r a t e constant o f k = 3 x 10 S at 20mg P t / l . The k i n e t i c events are i l l u s t r a t e d i n Figure 6, which i l l u s t r a t e s t h e time course o f MV decay (602nm) and R u ( b i p y ) * b l e a c h i n g (U70nm) a f t e r l a s e r e x c i t a t i o n . In t h e absence o f c a t a l y s t t h e r e i s back r e a c t i o n between these two species w i t h i n a time domain o f s e v e r a l hundred microseconds. I n t r o d u c t i o n o f the Pt/Ti02 c a t a l y s t i n t h e s o l u t i o n causes a dramatic i n c r e a s e i n the r a t e o f MV decay, which i s due t o charge i n j e c t i o n i n t o t h e c o l l o i d a l T i 0 and subsequent hydrogen formation. In contrast t h e R u ( b i p y ) * b l e a c h i n g i s r e t a r d e d i n the presence o f the c a t a l y s t showing t h a t the p a r t i c l e s i n t e r a c t s e l e c t i v e l y w i t h the MV , w h i l e no o r very slow r e a c t i o n o c c u r r e d 5

- 1

+

2

+


122


electron energy k

Ε·(Η,/Η ) A +

hv

iH+OH

S «- S Downloaded by PURDUE UNIV on May 25, 2016 | http://pubs.acs.org Publication Date: February 12, 1982 | doi: 10.1021/bk-1982-0177.ch007

+

Figure 5. Mediator function of the colloidal TiO particle, loaded with Pt in the light-induced H generation from water. Electron injection from the reduced relay into the Ti0 conduction band. g

g

2

X = 602nm

100 200 300 400 500 TIME (microseconds) 800

100 1000

r

200 300 400 500 TIME (microseconds)

h

ο 800 g

600

Ι

400

5 5

200

Ο

100 200 300 400 500 TIME (microseconds)

200 400 600 800 1000 TIME (microseconds) 4

Figure 6. Oscilloscope traces obtained from the laser photolysis of 10' M Ru(bipy)$ * and 2 X 10 M MV * in deaerated aqueous solution at pH 5. Key: a,b, without catalyst; c d> catalyst Pt (40 mg/L) or RuO (8 mg/L) loaded on 500 mg/L TiO . 3

3

2

t

t

t


7.

GRÀTZEL

123


+

with the o x i d i z e d s e n s i t i z e r R u ( b i p y ) * . This f i n d i n g i s o f c r u c i a l importance f o r the design o f a c y c l i c water decomposition system where s p e c i f i c i t y and h i g h r a t e s o f i n t e r a c t i o n are r e q u i r e d as p o i n t e d out above. Development o f Highly E f f i c i e n t Oxygen Generating C a t a l y s t s The photodecomposition o f water became f e a s i b l e i n a regener a t i v e way only a f t e r oxygen g e n e r a t i n g c a t a l y s t s had been developed. In 1978, we showed (36) t h a t noble metal oxides such as P t 0 , I r 0 and Ru0 i n macrodisperse o r c o l l o i d a l form are capab l e o f mediating water o x i d a t i o n by agents such as Ce*"", R u ( b i p y ) * and F e C b i p y ) ^ . The development o f Ru0 has advanced r a p i d l y s i n c e then. An impression o f the improvement o f the act i v i t y o f Ru0 based c a t a l y s t s may be gained from the f o l l o w i n g comparison: Three years ago, i n order t o e f f e c t water o x i d a t i o n by R u ( b i p y ) * 2

2

2

1

1

+


2

2

+

URu(bipy)?

+

+ 2H 0 + URu(bipy)* 2

3

+

+ 0

3

2

+ UH

(8)

+

w i t h i n a time domain o f s e v e r a l minutes we r e q u i r e d l g / l Ru0 powder. Today, by u s i n g an u l t r a f i n e deposit o f Ru0 on c o l l o i dal T i 0 p a r t i c l e s a h a l f l i f e t i m e o f 5 t o 10ms can be o b t a i n e d (jj2) with o n l y kmg Ru0 /l. Thus, by decreasing the p a r t i c l e s i z e o f Ru0 and s t a b i l i z i n g the c a t a l y s t on a s u i t a b l e c a r r i e r a more than 10 f o l d increase i n the c a t a l y t i c a c t i v i t y has been achieved. A p p l i c a t i o n o f combined f l a s h p h o t o l y t i c and f a s t conductometric technique made i t p o s s i b l e t o probe the mechanistic det a i l s o f the oxygen e v o l u t i o n r e a c t i o n . Thus, R u ( b i p y ) | was produced v i a the photoredox r e a c t i o n : 2

2

2

2

2

6

+

2Ru(bipy)*

+

2

+ S0~

2Ru(bipy)*

2

+

+ 2S0J"

(9)

and the k i n e t i c s o f oxygen p r o d u c t i o n v i a r e a c t i o n (8) were s t u d i e d i n the presence o f a c a t a l y s t c o n s i s t i n g o f a transparent T i 0 s o l loaded w i t h Ru0 . A comparison o f the temporal behavior o f the R u ( b i p y ) * absorption decay and the i n c r e a s e i n conductivi t y a s s o c i a t e d w i t h water o x i d a t i o n was made. Results obtained from 530nm l a s e r p h o t o l y s i s experiments are i l l u s t r a t e d i n Figure 7. The temporal behavior o f the R u ( b i p y ) | a b s o r p t i o n a t 6^0nm i s juxtaposed t o t h a t o f the s o l u t i o n conductance. The lower o s c i l l o g r a m shows t h a t the decrease i n the absorbance occurs concomitantly w i t h an i n c r e a s e i n the c o n d u c t i v i t y . T h i s i n d i c a t e s that h o l e t r a n s f e r from R u ( b i p y ) * t o Ru0 i s immediately f o l l o w ed b y r e l e a s e o f protons and oxygen from water. Thus the c o l l o i d a l c a t a l y s t p a r t i c l e couples r e d u c t i o n o f R u ( b i p y ) g t o water 2

2

+

+

+

2

+


124



TIME (microseconds) A

Figure 7. Laser photolysis experiments with solutions containing 10" M Ru(bipy) and 2 X 10 M S 0 "; catalyst TiO (200 mg/L) loaded with RuO, (4 mg/L). Simultaneous observation of transient conductance and absorbance. 2+

s

s

2

t

6

t


7.

125


GrätZEL

o x i d a t i o n . T h i s experimental observation provides a d i r e c t p r o o f t h a t t h e concept o f redox c a t a l y s i s , c o n s i d e r i n g the Ru0 c o l l o i d as microelectrodes, i s v a l i d f o r water oxidation, as w e l l as f o r water r e d u c t i o n . Moreover, The Ru0 p a r t i c l e s can be regarded as a r t i f i c i a l analogues o f the water s p l i t t i n g enzyme i n photosystem I I , present i n c h l o r o p l a s t s . Both are capable o f t r a n s f e r r i n g four o x i d a t i o n equivalents from a s u i t a b l e s o l u t e t o water r e l e a s i n g oxygen and protons. The Ru0 mediator a f f o r d s water o x i dation w i t h i n m i l l i s e c o n d s at s u r p r i s i n g l y low c o n c e n t r a t i o n . T h i s proved t o be extremely valuable i n energy conversion systems where water i s the source o f photodriven u p h i l l e l e c t r o n flow. 2

2

2


C y c l i c Water Cleavage by V i s i b l e L i g h t +

+

Since photo generated MV and R u ( b i p y ) g can be used f o r wat e r r e d u c t i o n and o x i d a t i o n r e s p e c t i v e l y , i t i s tempting t o examine a system where the two c a t a l y t i c processes can take p l a c e simultaneously f o l l o w i n g photoinduced e l e c t r o n t r a n s f e r . As was p o i n t e d out above, the Ru0 and Pt c a t a l y s t s have t o be a c t i v e enough t o i n t e r c e p t the back r e a c t i o n . A l s o , t h e i r i n t e r v e n t i o n has t o be s p e c i f i c i n t h a t MV reacts s e l e c t i v e l y with the P t p a r t i c l e s while R u ( b i p y ) | i n t e r a c t s with Ru0 . Cross r e a c t i o n s have t o be avoided s i n c e they l e a d t o s h o r t - c i r c u i t r y o f the back reaction. A f i r s t s u c c e s s f u l attempt t o s p l i t water photochemically t h i s way was made by us i n 1979 ( ^ 3 ) » A copolymer o f maleic anhydride and styrene was used as a p r o t e c t i v e agent f o r the P t s o l . This i s s u i t a b l e t o achieve s e l e c t i v i t y s i n c e i t provides funct i o n s with pronounced hydrophobicity. Of the redox products formed i n the ligjht r e a c t i o n MV i s r e l a t i v e l y hydrophobic and w i l l t h e r e f o r e i n t e r a c t w i t h the Pt. R u ( b i p y ) ^ on the other hand i s prone t o i n t e r a c t w i t h the h y d r o p h i l i c and n e g a t i v e l y charged Ru0 s u r f a c e . One disadvantage o f t h i s system i s t h a t the quantum y i e l d o f water s p l i t t i n g i s s m a l l (M).l#) and t h a t the photo r e a c t i o n stops i n a c l o s e d v e s s e l a f t e r a few hours o f i r r a d i a t i o n . One encounters here a fundamental problem which i s inherent t o a l l devices t h a t attempt t o produce p h o t o l y t i c a l l y H and 0 without l o c a l separation: The presence o f oxygen w i l l s e v e r e l y l i m i t the quantum y i e l d o f water s p l i t t i n g as both depol a r i z a t i o n o f the c a t h o d i c a l l y tuned Pt p a r t i c l e s as w e l l as r e o x i d a t i o n o f the reduced r e l a y according t o 2

+

2

+

2

2

2

R" + 0

2

•+ 0

+ R

2

(10)

w i l l i n t e r f e r e w i t h hydrogen generation. Using computer simulat i o n , I n f e l t a (hh) has e l a b o r a t e d the d e t a i l e d k i n e t i c s o f the processes o c c u r r i n g i n the Ru(bipy )* /MV system under +

2+



126

i l l u m i n a t i o n . By t a k i n g i n t o account t h e r a t e parameters f o r a l l r e l e v a n t r e a c t i o n s i n c l u d i n g c a t a l y t i c H and 0 p r o d u c t i o n , he a r r i v e s at t h e c o n c l u s i o n t h a t water s p l i t t i n g w i l l cease once the oxygen concentration b u i l d s up i n s o l u t i o n . T h i s problem has been overcome only r e c e n t l y through the development o f b i f u n c t i o n a l redox c a t a l y s t s The l a t t e r are d i s t i n g u i s h e d by t h e f a c t t h a t Pt and Ru0 are loaded onto t h e same mineral c a r r i e r p a r t i c l e . C o l l o i d a l T i 0 was the f i r s t ma t e r i a l t o be used as a support. I t f u l f i l l s f o u r d i f f e r e n t func t i o n s i n t h e water s p l i t t i n g system (Figure 8): 1) I t serves as a c a r r i e r f o r Pt and Ru0 and maintains these c a t a l y s t s i n a h i g h l y d i s p e r s e d s t a t e . 2) The T i 0 conduction band accepts e l e c t r o n s from the reduced r e l a y o r the e x c i t e d s e n s i t i z e r . These are channeled t o Pt s i t e s where hydrogen generation occurs. As t h e whole T i 0 p a r t i c l e i s r e a c t i v e t h e c r o s s - s e c t i o n and, hence, the r a t e o f e l e c t r o n capture i s g r e a t l y i n c r e a s e d w i t h respect t o systems i n which polymer p r o t e c t e d P t p a r t i c l e s are used as c a t a l y s t s . 3) Ru0 c a t a l y z e s oxygen production from water, k) T i 0 serves as an adsorbent f o r 0 produced during the p h o t o l y s i s . Some adsorption w i l l take p l a c e spontaneously, however, t h e main p a r t i s photoinduced: E l e c t r o n s i n j e c t e d i n t o the conduction band are used t o reduce 0 t o 0 which i s s t r o n g l y attached t o t h e T i 0 s u r f a c e . Assuming monolayer coverage l g o f T i 0 w i t h a surface area o f 200m can adsorb ca. 50ml o f 0 . Through t h i s mechanism the amount o f 0 i n s o l u t i o n i s kept very low, which b e n e f i t s g r e a t l y t h e e f f i c i e n c y o f water p h o t o l y s i s . 2

2

2

2

2


2

2

2

2

2

2

2

2

2

2

2

2

A c o r r e l a t i o n has meanwhile been e s t a b l i s h e d between t h e wa t e r cleavage e f f i c i e n c y and t h e c a p a c i t y f o r oxygen b i n d i n g by Ti0 . The l a t t e r i s favorably i n f l u e n c e d by surface hydroxyl groups. Thus, a good support m a t e r i a l i s a f u l l y hydroxylated anatase with a high surface area. Flame hydrolyzed T i 0 has a s m a l l number o f surface hydroxyl groups and, hence, a low a f f i n i ty for 0 binding. When charged w i t h P t and Ru0 i t shows only s m a l l c a t a l y t i c a c t i v i t y i n water decomposition systems. The b i n d i n g o f 0 t o the support has been demonstrated unambiguously by exposing a photolyzed s o l u t i o n t o a high concentration o f oxyanions. T h i s provokes r e l e a s e o f 0 from the T i 0 s u r f a c e , which can be r e a d i l y analyzed (k6). D e t a i l e d i n v e s t i g a t i o n s have meanwhile been c a r r i e d out w i t h the T i 0 based redox c a t a l y s t u s i n g the R u ( b i p y ) 3 / M V couple as a s e n s i t i z e r / r e l a y p a i r . Apart from the composition o f the c a t a l y s t (η-doping, Ru0 and P t loading) the quantum y i e l d o f water s p l i t t i n g depends s t r o n g l y on T i 0 c o n c e n t r a t i o n , pH and tempera t u r e (Jtl). Under optimum c o n d i t i o n s , the e f f i c i e n c y f o r hydrogen production (φ(Η )) i s 6% (T5°C). A study o f the k i n e t i c s o f H and 0 generation showed t h a t over t h e i n i t i a l p e r i o d o f 10-20h i r r a d i a t i o n time the gas r e l e a s e d from the s o l u t i o n i s pure 2

2

2

2

2

2

2

+

2+

2

2

2

2

2


2


7.

GRÀTZEL

127


ELECTRON ENERGY

Figure 8. Schematic illustration of the intervention of a colloidal TiO -based bifunctional redox catalyst in the cleavage of water by visible light. t



128

hydrogen, oxygen being r e t a i n e d i n the s o l u t i o n through absorp t i o n on Ti02. The k i n e t i c s of H2 production from i r r a d i a t i n g a s o l u t i o n c o n t a i n i n g Ru(bipy)23 as a s e n s i t i z e r are i l l u s t r a t e d i n F i g u r e 9. One observes a l i n e a r i n c r e a s e i n the amount of hydro gen generated during the f i r s t 10 to 20h of i r r a d i a t i o n time. T h e r e a f t e r the r a t e of H2 p r o d u c t i o n decreases. This i s a t t r i b u ted to the i n c r e a s e i n oxygen c o n c e n t r a t i o n i n the s o l u t i o n which lowers the quantum y i e l d of hydrogen formation. A f t e r f l u s h i n g the systems with argon the H2 generation resumes a t the i n i t i a l r a t e . T h i s f i n d i n g i s important i n that i t p o i n t s a t a way to separate hydrogen from oxygen which presents a problem f o r p r a c t i c a l a p p l i c a t i o n s of such systems. The c a p a c i t y of the c a r r i e r must be made high enough to absorb the q u a n t i t y of oxygen produc ed from one day of s o l a r i r r a d i a t i o n . In such a system d a y l i g h t production of hydrogen would a l t e r n a t e with O2 r e l e a s e during the night. C y c l i c water cleavage by v i s i b l e l i g h t was a l s o achieved i n e l e c t r o n r e l a y f r e e systems (48). In t h i s case the f r a c t i o n of s e n s i t i z e r that i s absorbed onto the p a r t i c l e s u r f a c e i s photo a c t i v e and e l e c t r o n i n j e c t i o n occurs d i r e c t l y from i t s e x c i t e d s t a t e i n t o the TiO2 conduction band. Using the s u r f a c t a n t r u t h e nium complex d e p i c t e d i n F i g u r e 10, a quantum y i e l d of 7% was ob tained f o r the water s p l i t t i n g process. A t h i r d type of water p h o t o l y s i s system i s based on band gap e x c i t a t i o n of c o l l o i d a l semiconductors as depicted i n F i g u r e 4c. Photoinduced e l e c t r o n / h o l e s e p a r a t i o n i s followed by H2 produc t i o n from conduction band e l e c t r o n s c a t a l y z e d by Pt. Holes i n the valence band are used to generate oxygen. Previous s t u d i e s have been c a r r i e d out with TiO2 or SrTi03 as support m a t e r i a l (49-54). However, UV i r r a d i a t i o n i s r e q u i r e d to e x c i t e these p a r t i c l e s and e f f i c i e n c i e s are u s u a l l y s m a l l . An exception i s made by the b i f u n c t i o n a l redox c a t a l y s t which s p l i t s water with s u r p r i s i n g e f f i c i e n c y under near UV i l l u m i n a t i o n (λ > 300nm). R e s u l t s are shown i n F i g u r e 11 which i l l u s t r a t e s the amount of hydrogen and oxygen produced as a f u n c t i o n of i l l u m i n a t i o n time. The s o l u t i o n (25ml) contained 12.5mg TiO2 loaded with lmg Pt and 0.025mg Ru02« I n i t i a l l y one obtains H2 at a r a t e of 2ml/h, which corresponds to a quantum y i e l d of ca. 30%. The r a t e decreases as the pressure of H2 and O2 b u i l d s up i n s o l u t i o n . A f t e r f l u s h i n g with argon the H2 generation resumes a t the i n i t i a l r a t e . A key r o l e i n a c h i e v i n g t h i s high e f f i c i e n c y i s played by the Ru02 d e p o s i t on the Ti02 p a r t i c l e , which g r e a t l y f a c i l i t a t e s the t r a n s f e r of holes from the valence band of the semiconductor to the s o l u t i o n bulk. T h i s c a t a l y t i c e f f e c t of Ru02 has been e x p l o i t e d r e c e n t l y to s t a b i l i z e small band gap semiconductor p a r t i c l e s which from t h e i r a b s o r p t i o n p r o p e r t i e s are more s u i t a b l e f o r s o l a r energy conversion than T1O2. An u n d e s i r a b l e property of these m a t e r i a l s i s that they undergo photocorrosion under i l l u m i n a t i o n . Holes produced i n the valence band migrate to the s u r f a c e where photoc o r r o s i o n occurs, i . e . ,


+


GrätZEL


129


7.

2

Figure 9. Cyclic water cleavage by visible light in the presence of Ru(bipy) * as a sensitizer and afunctional redox catalyst as an electron relay. s




130

Figure 10. Processes involved in the photodecomposition of water in a relay-free system. An amphiphilic Ru(bipy) derivative is used as a sensitizer. 2+

3


GRÄTZEL

131



7.

TIME[h] Figure 11. UV irradiation (Pyrexfiltered)of the afunctional TiO (anatase) catalyst, 500 mg Ti0 /L doped with 0.4% Nb 0 loaded with 0.1% RuO and 40 mg Pt, pH 4.5 adjusted with HCl. Triangles indicate the volume of oxygen in mL. t

2

2

5


t


132

CdS + 2

+

+ Cd

2 +

+ S

(11)

Recently, we d i s c o v e r e d (55) that l o a d i n g o f c o l l o i d a l or macrod i s p e r s e CdS p a r t i c l e s w i t h an u l t r a f i n e d e p o s i t of Ru(>2 prevents photodecomposition through promotion o f water o x i d a t i o n according (Ru0 ) ^ 2

+

4h (CdS) + 2H 0 2

0

£

+ 4H

( 1 2 )

Sustained water cleavage by v i s i b l e l i g h t i s observed when CdS p a r t i c l e s loaded simultaneously w i t h Pt and Ru0 are used as p h o t o c a t a l y s t . Again, hydrogen and oxygen are generated by conduction band e l e c t r o n i c s and valence band h o l e s , r e s p e c t i v e l y , produced by band gap e x c i t a t i o n . Thus i r r a d i a t i o n of a 25ml s o l u t i o n c o n t a i n i n g 2.5mg c o l l o i d a l CdS loaded w i t h 0.5mg Pt and 0.2mg Ru02 y i e l d s 2.8ml H and 1.4ml 0 a f t e r 44 hours of i r r a d i a t i o n with the v i s i b l e output of a 450W Xenon lamp. The quantum y i e l d i s s t i l l s i g n i f i c a n t l y below that observed with the T i C ^ / Pt/Ru0 s o l i n the presence of a s u i t a b l e s e n s i t i z e r and f u r t h e r development i s r e q u i r e d to improve the performance of t h i s system.


2

2

2

2

Cleavage of Hydrogen S u l f i d e by V i s i b l e L i g h t The a b i l i t y of Ru0 t o promote h o l e t r a n s f e r from the conduct i o n band of CdS to s o l u t i o n s p e c i e s can be e x p l o i t e d f o r processes other than water o x i d a t i o n . Thus, CdS suspension loaded with RuÛ2 decompose very e f f i c i e n t l y H S i n t o hydrogen and s u l f u r under v i s i b l e l i g h t i r r a d i a t i o n (56). 2

2

HS 2

H

2

+ S

(13)

F i g u r e 12 i l l u s t r a t e s the amount of H produced by i l l u m i n a t i n g a suspension of 25mg CdS loaded w i t h 0.1% Ru0 i n 25ml water of pH3. The hydrogen formation r a t e i s constant up to more than 90% consumption of H S. Rates improve with pH and Ru02 l o a d i n g . For example, H2 i s produced a t 9ml/h (ca. 35% quantum y i e l d ) a t pH 13 w i t h the same amount of CdS loaded by 3% Ru02- The r o l e of Ru02 i n t h i s system i s to enhance the r a t e of s u l f i d e o x i d a t i o n by h o l e s produced i n the v a l e n c e band through band gap e x c i t a t i o n . Thus e l e c t r o n - h o l e recombination i s i n t e r p r e t e d e f f i c i e n t l y and high quantum y i e l d s are obtained. E l e c t r o n s i n the conduction band are used to reduce protons to hydrogen ( F i g u r e 13). Important with r e s p e c t to p r a c t i c a l a p p l i c a t i o n i s the f a c t that no Pt c a t a l y s t i s r e q u i r e d to promote hydrogen formation. T h i s i s due to a c a t h o d i c s h i f t of the f l a t b a n d p o t e n t i a l of CdS caused by a b s o r p t i o n of s u l f i d e i o n s . Thereby the d r i v i n g f o r c e f o r water r e d u c t i o n to hydrogen i s i n c r e a s e d making the use o f a Pt c a t a l y s t superfluous. The composition of hydrogen s u l f i d e i s an energy s t o r i n g process that could become an important a l t e r n a t i v e to 2

2

2


GRÄTZEL



7.


133


134


Figure 13.

Schematic illustration of elementary processes involved in the CdSsensitized decomposition of H S. g


7.

GRATZEL


135

water cleavage as a source for hydrogen production from sunlight. is an abundant waste product in coal and petrol-related industry that could be made use of in this way.


Conclusions Colloidal semiconductors, molecular assemblies such as micelles or vesicles and ultrafine redox catalysts provide suit able microscopic organization to accomplish the difficult task of light-induced water cleavage. Work in the future will be directed to improve the efficiency of such devices by identifying new photocatalysts and solving the problem of hydrogen/oxygen separation. Colloidal semiconductors will certainly play a primordial role in this development. Their key advantage over other functional organizations is that light-induced charge separation and catalytic events leading to fuel production can be coupled without intervention of bulk diffusion. Thus a single colloidal semiconductor particle can be treated with appropriate catalysts so that different regions function as anodes and cathodes. It appears that this wireless photoelectrolysis could be the simplest means of large scale solar energy harnessing and conversion. Acknowledgement s The author wishes to express his deep gratitude to his collaborators as well as to his Italian colleagues Prof. E. Pelizzetti and Dr. M. Visca, whose inspired and enthusiastic efforts has made possible the success of this work. Financial assistance of the Swiss National Science Foundation, Ciba Geigy, Engelhard Industries and the United States Army Procurement Agency, Europe is also gratefully acknowledged. LITERATURE CITED 1.

Balzani, V.; Moggi, L.; Manfrin, M.F.; Boletta, F.; Gleria, M. Science 1975, 189, 852.

2.

Calvin, M. Photochem. Photobiol. 1976, 23, 425.

3. Porter, G.; Archer, M.D. Interdisc. Sci. Rev. 1976, 1, 119. 4.

Bolton, J. Science 1978, 202, 705.

5.

Harriman, Α.; Barber, j. "Photosynthesis in Relation to Model Systems"; Barber, J. (Ed.) Elsevier: Amsterdam, 1979.


136 6.


Schumacher, E. Chimia 1978, 32, 194.


7. Grätzel, M. Ber. Bunsenges. Phys. Chem. 1980, 84, 981. 8.

Paleocrassas, S.N. Solar Energy 1974, 16, 45.

9.

Claesson, S. (Ed.) "Photochemical Conversion and Storage of Solar Energy"; Swedish National Energy Board Report: Stockholm, 1977.

10.

Tomikwicz, M.; Fay, H. Appl. Phys. 1979, 18, 1.

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