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9 Electrochemical Behavior and Surface Structure of Gallium Phosphide Electrodes Y. N A K A T O , A . TSUMURA, and H . TSUBOMURA

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch009

Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, 560 Japan

The p r a c t i c a l success of semiconductor e l e c t r o c h e m i c a l photo c e l l s depends on how to prevent the photo-corrosion of the e l e c trode m a t e r i a l s . The v a r i o u s e l e c t r o c h e m i c a l processes a t the surface of semiconductor photo-electrodes e l e c t r o n t r a n s f e r as w e l l as decomposition r e a c t i o n s have been discussed so f a r mainly by taking account of the s t a t i c e l e c t r o n i c energy l e v e l s of the semiconductors and the s o l u t i o n ; that i s to say, the conduction band edge, E , the valence band edge, E , the redox p o t e n t i a l s of the redox couple i n the s o l u t i o n , E°(Ox/R), the decomposition p o t e n t i a l , E^; e t c . ( j L - 6 ) . However, the competition between the e l e c t r o n t r a n s f e r process and the decomposition r e a c t i o n paths should b e t t e r be understood from a k i n e t i c point of view (4). Namely, i f the r a t e of the former i s f a s t e r than the l a t t e r , the photoanode i s maintained s t a b l e . A l s o , i t seems v i t a l l y important to take i n t o account the presence of the surface s t a t e whose energy and s t r u c t u r e may be dynamically changed by the e l e c t r o d e reactions. In t h i s paper, we w i l l r e p o r t our experimental f i n d i n g s on the photo-anodic behavior of η-type g a l l i u m phosphide (GaP) i n aqueous e l e c t r o l y t e and d i s c u s s them based on a p i c t u r e of the r e a c t i o n intermediates, which are to play an important r o l e on the r e a c t i o n pathway as w e l l as on the c r e a t i o n of photo-voltages and photoc u r r e n t s . The main point i s that the surface band energies (de p i c t e d f o r an η-type semiconductor i n F i g . 1 ) , which play the most important r o l e i n the e l e c t r o d e processes, by no means remain con stant, although t h i s has been t a c i t l y assumed to be the case i n many previous papers, but change during the photoelectrode pro cesses by the accumulation of surface intermediates and of surface charge ( 7 , 8 ) . For l a t e r d i s c u s s i o n s , we a l s o d e f i n e a p o t e n t i a l U , which i s a p o t e n t i a l a t which the i n v e r s e square of the d i f f e r e n t i a l capacitance 1/C^ tends to zero as determined from the 1/C^ v s p o t e n t i a l p l o t (Mott-Schottky p l o t ) . I t i s r e l a t e d to E§ i n the f o l l o w i n g way: c

v

s

0097-6156/81/0146-0145$05.00/0 © 1981 American Chemical Society Nozik; Photoeffects at Semiconductor-Electrolyte Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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AT SEMICONDUCTOR-ELECTROLYTE

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Journal of the Electrochemical Society

Figure 1. Schematic of the band structure and energy terms of a semiconductor Π)


Figure 2.

Current-potential curve for the illuminated (lll)-face of an n-GaP electrode in aO.lM NaOH solution (1 )

Nozik; Photoeffects at Semiconductor-Electrolyte Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

9.

NAKATO E T A L .

E

c

-

"

e

U

GaP Electrode

s+

147

Δ

where e i s the elementary charge and Δ a small energy d i f f e r e n c e between the conduction band edge i n the bulk and the Fermi l e v e l . Experimental The η-type GaP used was a s i n g l e c r y s t a l i n the form of wafers, 99.999%pure and doped with s u l f u r to the c o n c e n t r a t i o n of 2 to 3 χ 1 0 cm~3 (Yamanaka Chemical I n d u s t r i e s L t d . ) . The p-type GaP used was doped with z i n c to 3.7 χ 10 cm~3 (Sanyo E l e c t r i c Co., L t d . ) . Both were cut perpendicular to the [ l l l ] - a x i s . The ohmic contact was made by vacuum d e p o s i t i o n of indium on one face of the c r y s t a l , followed by heating a t ca. 500 °C f o r 10 min. The s i d e connected with a wire was covered with epoxy r e s i n . Before the experiment, the c r y s t a l s were p o l i s h e d and_etched with warm aqua r e g i a . The (111)-face (Ga face) and the ( l l l ) - f a c e (P face) were d i s t i n g u i s h a b l e by microscopic i n s p e c t i o n of the etched s u r f a c e s , the former very rough and the l a t t e r smooth. The c u r r e n t - p o t e n t i a l curve was obtained with a Hokutodenko HA-101 p o t e n t i o s t a t . The d i f f e r e n t i a l capacitance was measured mostly with a Yokogawa-Hewlett-Packard U n i v e r s a l Bridge 4265B, having a modulation frequency of 1 kHz and a modulation amplitude of 20 mV (peak to peak). The frequency dependence of Mott-Schottky p l o t s were checked by connecting a f u n c t i o n generator t o the bridge. The e l e c t r o d e was i l l u m i n a t e d through a quartz window with a 250 watt high pressure mercury lamp. The l i g h t i n t e n s i t y was attenuated by use of n e u t r a l density f i l t e r s made of metal nets. Solutions were prepared from deionized water by using reagent grade chemicals, i n most cases, without f u r t h e r p u r i f i c a t i o n . They were deaerated by bubbling n i t r o g e n and s t i r r e d with a magnetic s t i r r e r . The pH of the s o l u t i o n s i n the range of 3 to 11 was con t r o l l e d by using b u f f e r mixtures; acetate, phosphate, borate, or carbonate, each a t about 0.01 M, where M means mol/drn^. T h i s a b b r e v i a t i o n i s used throughout the present paper. 1 7


7

Results and Discussions 1. The E f f e c t of I l l u m i n a t i o n . In an a l k a l i n e s o l u t i o n , an n-GaP e l e c t r o d e , (111) surface, under i l l u m i n a t i o n shows an anodic photocurrent, accompanied by q u a n t i t a t i v e d i s s o l u t i o n of the e l e c trode. The c u r r e n t - p o t e n t i a l curve shows considerable h y s t e r i s i s as seen i n F i g . 2; the anodic c u r r e n t , scanned backward, (toward l e s s p o s i t i v e p o t e n t i a l ) begins to decrease at a p o t e n t i a l much more p o s i t i v e than the onset p o t e n t i a l of the anodic current f o r the forward scanning, the l a t t e r being s l i g h t l y more p o s i t i v e than the U value i n the dark, U ( d a r k ) . These r e s u l t s suggest that the GaP surface with backward scanning develops an o x i d i z e d s t r u c t u r e , which i s a c t i n g as a pre cursor, or precursors, to the anodic d i s s o l u t i o n r e a c t i o n s . s

s

American Chemical Society Library 1155 16th St., N.W.

Nozik; Photoeffects at Semiconductor-Electrolyte Washington, D.C. 20036 Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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We have found that a good l i n e a r Mott-Schottky p l o t can be obtained f o r the e l e c t r o d e not only i n the dark but a l s o under weak i l l u m i n a t i o n ( F i g . 3). This means that, even under anodic p o l a r i z a t i o n and i l l u m i n a t i o n , the s t a t e of the surface i s main tained at a constant c o n d i t i o n . The U value determined from the i n t e r c e p t of the p l o t with the a b s c i s s a i s somewhat l e s s negative than the U value determined s i m i l a r l y i n the dark. The U values i n the dark and under i l l u m i n a t i o n , r e s p e c t i v e l y , at v a r i o u s pH are given i n F i g . 4 f o r the (111) face of GaP, together with the value, the l a t t e r defined as the p o t e n t i a l at zero current f o r backward scanning (see F i g . 2). The l i n e a r dependence of U ( d a r k ) on pH i s understood, as i s g e n e r a l l y the case f o r some semiconduc tors (9,10), to be the r e s u l t of the acid-base e q u i l i b r i u m on the s u r f a c e . The d e v i a t i o n of U ( i l l . ) (the U value under i l l u m i n a t i o n ) from U ( d a r k ) increases s l i g h t l y with the i l l u m i n a t i o n i n t e n s i t y , and i s explained by assuming the accumulation of s u r f a c e intermediates. That i s , when the e l e c t r o d e i s under i l l u m i n a t i o n , the holes generated by i l l u m i n a t i o n are drawn to the s u r f a c e by the b u i l t - i n p o t e n t i a l gradient, and cause anodic decomposition of the e l e c t r o d e . As Gerischer suggested f o r Ge or GaAs e l e c t r o d e s (5,9,11), intermediates are formed as the precursors i n the decom p o s i t i o n or d i s s o l u t i o n path: g

s

s

g


s

g

s

-P

Ρ-

v

w

-P -Ga -OH

P - Ga"

Ρ

Ga-OH

P - Ga— OH

Ga- P—

OH

The extent of the accumulation of such intermediates depends on t h e i r r a t e s of the formation and those of the ensuing decomposi t i o n (or d i s s o l u t i o n ) r e a c t i o n s . I f the l a t t e r are not high, the t o t a l d e n s i t y of such surface intermediates becomes so h i g h that an a p p r e c i a b l e s u r f a c e p o t e n t i a l Δψ i s created by the e l e c t r i c double l a y e r formed by the charge unbalance of these intermediates, as w e l l as by the approach of counter ions and the h y d r a t i o n around these intermediates. The experimentally obtained d i f f e r e n c e between U ( d a r k ) and U ( i l l . ) can be a t t r i b u t e d to t h i s Δψ. The surface p o t e n t i a l p o s t u l a t e d above should a f f e c t the U and the U Q mentioned p r e v i o u s l y by an equal magnitude. However, the s h i f t of from U ( d a r k ) as seen i n F i g . 4 i s much l a r g e r than that of U ( i l l . ) . T h i s l a r g e s h i f t of the former may be ex p l a i n e d by assuming that the surface intermediates capture e l e c trons i n the conduction band and act e f f e c t i v e l y as recombination centers. G

g

S

g

s

2. The Surface P o t e n t i a l a r i s i n g from the I n t e r a c t i o n between the Surface " S t a t e s " and the Redox Couples i n the S o l u t i o n . When the f e r r i c y a n i d e / f e r r o c y a n i d e redox couple i s present i n a 0.1 Ν NaOH s o l u t i o n , the dark cathodic current of the n-GaP (111)-face sets out at — 1 . 1 V (SCE) , showing t h a t an e l e c t r o n t r a n s f e r occurs


ΝAKATO E T A L .


GaP Electrode

Figure 4. The U (dark), U (illuminated), and U for the (lll)-face of n-GaP in solutions of 0.05M Να 30^ and buffers. The anodic photocurrentsflowingduring measurements are shown in units of liAcm' . 8

s

0

a

2

2


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from the conduction band of n-GaP to the f e r r i c y a n i d e . T h i s onsetp o t e n t i a l i s ca. 0.6 V more anodic than the onset p o t e n t i a l f o r the cathodic current which corresponds to H 2 e v o l u t i o n observed i n a s o l u t i o n l a c k i n g the redox couple ( F i g . 5), the l a t t e r being c l o s e to U ( d a r k ) . T h i s q u i t e l a r g e anodic d e v i a t i o n i s analogous to the anodic s h i f t of U Q from U ( d a r k ) mentioned above, and i n d i c a t e s the formation of surface intermediates which a c t as an e f f e c t i v e e l e c t r o n t r a n s f e r mediator as w e l l as being r e s p o n s i b l e f o r s h i f t i n g the s u r f a c e band energy E^. As F i g . 6 shows, the dark cathodic current f o r a p-GaP e l e c t r o d e i n the presence of the f e r r i c y a n i d e / f e r r o c y a n i d e couple sets out at ca. 0 V (SCE), i n d i c a t i n g that hole i n j e c t i o n to the valence band of GaP occurs from the f e r r i c y a n i d e . T h i s suggests that holes are i n j e c t e d by f e r r i cyanide i n t o GaP and supports the above idea that holes are chem i c a l l y relaxed at the surface to form surface intermediates. s


s

A good l i n e a r Mott-Schottky p l o t was obtained f o r the (111)face of n-GaP i n a s o l u t i o n of f e r r i c y a n i d e / f e r r o c y a n i d e , as shown i n F i g . 7. The U values determined from such p l o t s , i n e l e c t r o l y t e s o l u t i o n s e i t h e r with the absence or with the presence of the redox couple ( U and U ( r e d o x ) , r e s p e c t i v e l y ) , are p l o t t e d at v a r i o u s pH ( F i g . 8). Although the U changes l i n e a r l y with pH, the U ( r e d o x ) stays constant from pH 5 to pH 10, y i e l d i n g a constant d i f f e r e n c e of 1.7 V with the observed redox p o t e n t i a l E(0x/R) of the f e r r i c y a n i d e / f e r r o c y a n i d e couple. This r e s u l t can be under stood by again assuming that a surface p o t e n t i a l , a r i s i n g from the accumulated surface intermediates, ( i n c l u d i n g s u r f a c e trapped h o l e s ) , brings down the e l e c t r o n i c energy of the surface-trapped hole (as w e l l as the surface band energies) to the l e v e l of the redox p o t e n t i a l of the redox couple i n the s o l u t i o n and achieves e l e c t r o n exchange e q u i l i b r i u m . The surface trapped hole may be v i s u a l i z e d as the e l e c t r o n d e f i c i e n t Ga-P bond, s t a b i l i z e d by the d i s t o r t i o n of the surface Ga-P framework and the h y d r a t i o n or the approach of OH" ions i n the s o l u t i o n . There a l s o might p o s s i b l y be c o n t r i b u t i o n s from the intermediates formed by the chemical r e l a x a t i o n processes thereof. However, i n the present d i s c u s s i o n , we t e n t a t i v e l y assume that only one species a surface trapped hole i s r e s p o n s i b l e f o r the e l e c t r o c h e m i c a l behavior. This s p e c i e s , and the unreacted Ga-P bonds, would c o n s t i t u t e a redox system which i s c h a r a c t e r i z e d by a redox p o t e n t i a l Eft r e l a t e d to the s u r f a c e p o t e n t i a l ψ: G

S

S

G

G

kT

E

h

=

E£

+ -£±-lnC

h

+

ψ

+

const.

where C^ i s the d e n s i t y of the surface trapped hole which i s under r e v e r s i b l e e l e c t r o n i c e q u i l i b r i u m with the s o l u t e . In most cases, the e f f e c t of ψ on E^ i s much stronger than the second term of the above equation. With t h i s view, the energy i n t e r v a l s between the E^, E^ and -eEj^ becomes n e a r l y constant, because these l e v e l s a l l change e q u a l l y with ψ. We can determine t h e i r r e l a t i v e p o s i t i o n s by the


9.

NAKATO E T A L .

151

GaP Electrode

U / V vs SCE


-2.0

-1.5

-1.0

0


Figure 5. Current-potential curves for the (lll)-face of n-GaP in aO.lM NaOH solution, in the absence (a) and the presence (b) of 0.05M potassium ferrocyanide and 0.05M potassium ferricyanide (1 ).

Figure 6. Dark cathodic currents at a p-GaP electrode in solutions of 0.05M potassium ferricyanide and 0.05M potassium ferrocyanide


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U / V vs SCE

Figure 7. Mott-Schottky plots for the (lll)-face of an n-GaP electrode in solu tions containing 0.05M potassium ferricyanide and 0.05M potassium ferrocyanide

Figure 8. U values for the (lll)-face of n-GaP (dark) at various concentrations of the ferricyanide/ferrocyanide couple (equal concentrations): (Φ) OM; (O) 0.005M; (A) 0.05M; (A) 0.4M; E(Ox/R) redox potential of the redox couple determined by the cyclic voltammetry; (U ) the U for a p-GaP in the absence of the redox couple. s

p

s

s


9.

NAKATO E T A L .

153

GaP Electrode

f o l l o w i n g argument. As mentioned before, at the e q u i l i b r i u m with the f e r r i c y a n i d e / f e r r o c y a n i d e couple the d i f f e r e n c e between U and E(Ox/R) i s 1 . 7 V. Then, by taking Δ to be 0 . 1 V, the d i f f e r e n c e between and E ( 0 x / R ) becomes 1 . 8 V. I f we set E = Ε(ferricy anide/f errocyanide) , which i s observed to be 0 . 2 V (SCE), then from the band gap, Ε , of GaP ( 2 . 3 V ) , E^ i s c a l c u l a t e d to be 0 . 5 V below Ε^. As an example, the U of n-GaP at pH 6 with no redox couple present i n the s o l u t i o n i s — 1 . 6 V (SCE). T h i s shows that Ε^ without a redox couple i s ca. + 0 . 1 V (SCE). I t i s then under stood that the i n t e r a c t i o n of a f e r r i c y a n i d e / f e r r o c y a n i d e couple s h i f t s Eft, as w e l l as U , by 0 . 1 V to the anodic d i r e c t i o n at pH 6 ( F i g . 1 1 ) . At pH 1 0 , the s h i f t i s 0 . 4 V. The breakdown at a pH higher than 1 0 of the p a r a l l e l i s m be tween the U (redox) and the redox p o t e n t i a l of the redox couple i n s o l u t i o n can be explained by assuming that the r a t e of the d i s s o l u t i o n r e a c t i o n , caused by the attack of H 0 or OH" on the sur face trapped hole, i s so high i n t h i s pH range that the e l e c t r o n exchange e q u i l i b r i u m at the i n t e r f a c e i s no longer achieved. For the ( Ϊ 1 Ϊ ) face of n-GaP, the measured U (redox) changed almost l i n e a r l y with the pH, showing that the surface-trapped hole i s l e s s s t a b l e than that f o r the case of the ( 1 1 1 ) f a c e above men tioned. In the presence of a f e r r i c / f e r r o u s ( F e ^ / F e ) couple, and that of tetraammine copper (II) Cu(NH3)^^ i o n , the measured U values a l s o s h i f t e d . The redox p o t e n t i a l f o r the f e r r i c / f e r r o u s couple, both i n equal concentrations, i s + 0 . 5 V (vs SCE) i n a low pH r e g i o n , and that f o r the tetraammine copper (II)/(IŒ) couple i s 0 to — 0 . 2 V, depending on the c o n c e n t r a t i o n of ammonia. In the presence of a vanadate/vanadite ( V ^ / V ) couple at pH < 3, whose formal redox p o t e n t i a l i s very h i g h l y negative ( — 0 . 5 V (SCE)), the U s h i f t e d very l i t t l e and was the onset p o t e n t i a l f o r the cathodic c u r r e n t . I t was pointed out p r e v i o u s l y ( 1 2 , 1 3 ) that n-GaP emitted luminescence under cathodic p o l a r i z a t i o n i n contact with a f e r r i c y a n i d e or other oxidant s o l u t i o n . We have a l s o measured the electrochemiluminescence spectrum i n the presence of the f e r r i c y a n i d e / f e r r o c y anide couple ( F i g . 9). The spectrum i s s i m i l a r to that observed by P e t t i n g e r , Schoppel and G e r i s c h e r , while that measured by Beckman and Memming showed two peaks. The luminescence peak measured by us i s at 1 . 6 eV, which i s i n rough agreement with the energy d i f f e r ence between E | and E^ ( 1 . 8 eV). The luminescence can be observed only when the cathodic current i s f a i r l y strong and only when the redox couple i s present. Hence the luminescence can be probably assigned to an e l e c t r o n i c t r a n s i t i o n from the conduction band to the s u r f a c e trapped hole. s

h

s

g


s

2

g

+

2 +

+

g

+

2+

g

3. S t a b i l i t y of I l l u m i n a t e d n-GaP i n Redox S o l u t i o n s . F i g u r e 1 0 shows the current-potential curves f o r the n-GaP e l e c t r o d e under i l l u m i n a t i o n , i n the presence of f e r r o u s oxalate F e ^ 2 0 4 ) 2 ^ i f e r r o c y a n i d e Fe(CN) ^~, together with that f o r the s o l u t i o n without a

6


n

(

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154

Figure 10. Current-potential curves for the (lll)-face of n-GaP under illumination at pH 6.0 in the presence of 0.05M ferrous oxalate (a) and 0.05M ferrocyanide (b), together with that of a solution containing only 0.05M Na SO as a supporting electrolyte 2


k


9.

NAKATO E T A L .

GaP Electrode

155

a reducing agent. For the case of f e r r o u s oxalate, (pH 6.0), the onset p o t e n t i a l f o r the photoanodic current i s l a r g e l y moved to the negative, and the i - U curve has l o s t i t s strong h y s t e r e t i c be havior, i n d i c a t i n g that the surface trapped holes are e f f e c t i v e l y quenched by f e r r o u s oxalate and that the e l e c t r o d e i s kept from c o r r o s i o n . The sharp increase of the cathodic current a t — 1 . 3 V i s undoubtedly due to hydrogen e v o l u t i o n . For the case of f e r r o cyanide, the onset p o t e n t i a l of photoanodic current i s somewhat more cathodic but the h y s t e r e s i s s t i l l remains, i n d i c a t i n g that there are s t i l l some surface trapped h o l e s . The e l e c t r o d e i s , however, kept i n t a c t from c o r r o s i o n , the photocurrent showing no decay even at the magnitude of 100 μΑ/cm , i n c o n t r a s t to the case of the s o l u t i o n c o n t a i n i n g no reducing agent, where the photocur rent decays q u i c k l y by the oxide f i l m formation i f the i n i t i a l value of the photocurrent i s higher than 30 μΑ/cm . A l l these r e s u l t s can be explained i n terms of the model pro posed above ( c f . F i g . 11). Namely, with f e r r o u s oxalate having a standard redox p o t e n t i a l E° (Ox/R) of — 0 . 2 V (SCE), which i s a l i t t l e more negative than the E^ of the surface trapped hole l o cated ca. 0.5 V above E^, the surface trapped hole i s e f f e c t i v e l y quenched by the r a p i d r e d u c t i o n , and the photoanodic current flows without decomposition. With f e r r o c y a n i d e , having an Ε(Ox/R) of 0.2 V (SCE), which i s more p o s i t i v e than the E^ of the surface trapped hole, the surface trapped holes are accumulated to the extent that the surface p o t e n t i a l created w i l l l e v e l i t down to the Ε(Ox/R) of the redox couple. At t h i s p o i n t , the r a t e s of nuc l e o p h i l l i c a t t a c k of E^O and OH" to the surface trapped holes are s t i l l low and the e l e c t r o d e decomposition i s prevented. Concluding Remarks In the d i s c u s s i o n s by many authors of the energy conversion e f f i c i e n c y of semiconductor photoelectrochemical systems, i t has been t a c i t l y assumed that the maximum t h e o r e t i c a l photovoltages produced i s the d i f f e r e n c e between E | ( i n u n i t s of eV) and Ε(Ox/R). The best conversion e f f i c i e n c y should then be obtained with a redox couple whose standard redox p o t e n t i a l i s as low as p o s s i b l e , with a reasonable margin x, say 0.3 V, above E ^ ( F i g . 11). From t h i s i t f o l l o w s that the maximum photovoltage obtainable i s equal to the band gap, E , i n an eV u n i t , minus a small margin χ plus Δ. It has been pointed out by some authors (1,2) that f o r a semi conductor having a thermodynamic decomposition p o t e n t i a l , E^, i n between E and E^, a redox couple with a standard redox p o t e n t i a l , E°, more negative than E^ i s needed i n order to operate the photoanode without decomposition. Then, the maximum photovoltage a t t a i n a b l e i s U - E , which i s o f t e n much lower than Ε - Δ - χ . For GaP, t h i s i s only 0.8 V (_4) ( F i g . 11). In t h i s regard, the main c o n c l u s i o n of the present paper i s as f o l l o w s : 1) The n-GaP photoanode can be operated i n a s t a b l e c o n d i t i o n with g

c

s

d



156

PHOTOEFFECTS

n-GaP


INTERFACES

Electrolyte (pH = 6.0)

Figure 11. Energy diagram of the interface between n-GaP and an electrolyte solution of pH 6.0


9.

NAKATO E T A L .

GaP Electrode

157

a reducing agent (e.g., f e r r o c y a n i d e ) having E° more p o s i t i v e than the decomposition p o t e n t i a l E^. This means that the t h e o r e t i c a l l i m i t of the output photovoltage can be higher than U - E^. 2) The best photovoltage can be obtained with a redox couple having E° s l i g h t l y higher (say — 0 . 2 V) than the E ^ of surface trapped hole (0.1 V), e.g., f e r r o u s oxalate. The use of a redox couple having E° much more p o s i t i v e than that has no merit because i n that case the surface trapped holes a r e accumulated and the sur face p o t e n t i a l moves up so that not only E^ but a l s o U are shifted more p o s i t i v e , thus reducing the t h e o r e t i c a l l i m i t of the photovoltage. F i n a l l y i t i s pointed out that these conclusions f o r n-GaP can be extended to other v a r i o u s η-type semiconductors f o r general c r i t e r i a of the performance of photoelectrochemical systems. s


s

Abstract The current-potential curves of the n-GaP electrode were studied in aqueous solutions in the dark and under illumination, in connection with the surface conduction band energy Esc of the GaP, which is equivalent to the electrode potential U determined from the Mott-Schottky plots. From the hysteresis in the currentpotential curves, and the change of U under anodic polarization caused by the action of light or by an oxidizing agent in the solu tion, it has been concluded that the surface trapped holes or sur face intermediates of the anodic decomposition reactions are rather stable in the region of pH between 5 and 10, and, by the accumula tion of these species, a surface potential is built up, causing the shift of U to the positive. In such a pH region, the U values are fixed by the presence of a redox couple, e.g., ferricyanide/ ferrocyanide. It is deduced that the redox couple is in an elec tron transfer equilibrium with the surface trapped holes having a 'standard'redox potential E°(trapped hole) of 0.5 eV above the valence band edge. Electrochemiluminescence spectrum was observed and attributed to the electron transition from the conduction band to the surface trapped hole. Based on these results, it has been theoretically concluded that a photoelectrochemical cell can be operated as a stable system when a redox couple is present in the aqueous phase which has E°(redox) slightly more negative than E° (trapped hole). This conclusion has been experimentally varified. The photovoltage of such a photocell can have the theoretical limit defined by the difference between U and E°(trapped hole), which is much higher than those previously quoted rather pessimistically, on the basis of the thermodynamic decomposition potentials. s

s

s

s

s

Literature Cited 1. Gerischer, H. J. Electroanal. Chem., 1977, 82, 133. 2. Bard, A. J.; Wrighton, M. S. J. Electrochem. Soc., 1977, 124, 1706.



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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

3. Fujishima, Α.; Inoue, T.; Watanabe, T.; Honda, K. Chem. Lett. 1978, 375; Inoue, T.; Watanabe, K.; Fujishima, Α.; Honda, K. Bull. Chem. Soc. Jpn., 1979, 52, 1243. 4. Memming, R. J. Electrochem. Soc., 1978, 125, 117. 5. Gerischer, H.; Ed. "Topics in Applied Physics, Solar Energy Conversion"; vol. 31, Springer: Berlin, Heidelberg, New York, 1979. 6. Yoneyama, H.; Tamura, H. Chem. Lett., 1979, 457. 7. Nakato, Y.; Tsumura, Α.; Tsubomura, H. J. Electrochem. Soc., 1980, 127, 1502. 8. Nakato, T.; Tsumura, Α.; Tsubomura, H. ibid., submitted for publication. 9. Gerischer, H.; Hoffmann-Perez, M.; Mindt, W. Ber. Bunsenges. phys. Chem., 1965, 69, 130. 10. Lohmann, F., ibid., 1966, 70, 428. 11. Gerischer, H.; Mindt, W. Electrochim. Acta, 1968, 13, 1329; Gerischer, H. Surf. Sci., 1969, 13, 265. 12. Beckmann, Κ. H.; Memming, R. J. Electrochem. Soc., 1969, 116, 368. 13. Pettinger, B.; Schöppel, H. -R.; Gerischer, H. Ber. Bunsenges. phvs. Chem., 1976, 80, 849.

RECEIVED October

3, 1980.


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