414
J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY
6. J. R. Lenhard and R. W. Murray, ibid., 78, 195
(1977).
7. J. R. Lenhard, R. D. Rocklin, H. Abruna, K. W.
Willman, K. N. Kuo, and R. W. Murray, J. Am.
Chem. Soc., 190, 5213 (1978).
8. P. R. Moses and R. W. Murray, ibid., 98, 7435
(1976).
9. P. R. Moses, L. M. Wier, J. C. Lennox, H. O. Finklea, J. R. Lenhard, and R. W. Murray, Anal.
Chem., 50, 576 (1978).
10. M. S. Wrighton, R. G. Austin, A. B. Bocarsly, J. M.
Bolts, O. Haas, K. D. Legg, L. Nadjo, and M. C.
Palazzotto, J. Electroanal. Chem., 87, 429 (1978).
11. R. J. Burt, G. J. Leigh, and C. J. Pickett, Chem.
Commun., 940 (1976).
12. A. Diaz, J. Am. Chem. Soc., 99, 5838 (1977).
13. D. Galizzioli, F. Tantardini, and S. Trasatti, J.
Appl. Electrochem., 4, 57 (1975); 5, 203 (!975).
14. W. O'Grady, C. Iwakura, J. Huang, and E. Yeager,
"Electrocatalysis," M. W. Breiter, Editor, p. 286,
The Electrochemical Society ~oftbound Symposium Series, Princeton, N.J. (1974).
15. T. Arikado, C. Iwakura, and H. Tamura, Electrochim. Acta, 22, 513 (1977).
16. L. D. Burke, O. 5. Murphy, and J. F. O'Neill, J.
Electroanal. Chem., 81, 394 (1977).
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327 (1977).
18. S. Trasatti and G. Buzzanca, J. EIectroanal. Chem.,
29, App. 1 (1971).
19. S. Pizzini, G. Buzzanca, C. Mari. L. Rossi, and S.
Torchio, Mater. Res. Bull., 7, 449 (1972).
20. A. T. Kuhn and C. J. Mortimer, This Journal, 120~
231 (1973).
21. K. L. Hardee and A. J. Bard, ibid., 122, 739 (1975).
22. J. Horkans and M. W. Sharer, ibid., 124, 1202
(1977).
March 1979
23. C. Creutz and N. Sutin, J. Am. Chem. See., 99, 241
(1977).
24. M. E. Peover, "Electrochemistry of Aromatic Hydrocarbons and Related Substances," in Vol. 2,
"Electroanalytical Chemistry," A. J. Bard, Editor,
pp. 9-11, Marcel Dekker, inc., New York (1967).
25. C. M. Hughes and D. J. Macero, Inorg. Chem., 15,
2O40 (1976).
26. L. Meites and P. Zuman, "Electrochemical Data,"
Part 1, Vol. A, Wiley-Interscience, New York
(1974).
27. "Techniques of Chemistry," Vol. 1, "Physical Methods of Chemistry," Part IIa, A. Weissberger and
B. W. Rossiter, Editors, p. 591, Wiley-Interscience, New York (1977).
28. R. S. Nicholson and I. Shain, Anal. Chem., 36, 706
(1964).
29. D. L. Coffen, J. Q. Chambers, D. R. Williams, P. E.
Garrett, and N. D. Canfield, J. Am. Chem. Soc.,
93, 2258 (1973).
30. G. Gruver, T. Osa, and T. Kuwana, U.S. Army Research Symposium, Durham, N.C. (1968).
31. L. M. Wier, Ph.D. Dissertation, University of North
Carolina (1977).
32. K. S. Kim and N. Winograd, J. Catal., 35, 66 (1974).
33. J. Augustynski, L. Balsenc, and J. Hinden, This
Journal, 125, 1093 (1978).
34. C. Iwakura, H. Tada, and H. Tamura, Electrochim.
Acta., 22, 217 (1977).
35. J. H. Scofield, Lawrence Livermore Laboratory,
Report No. UCRL-51326.
36. T. A. Carlson, "Photoelectron and Auger Spectroscopy," p. 358, Plenum Press, New York (1975).
37. L. Ramquist, K. Hamrin, G. Johansson, A. Fahlmen,
and C. Nording, J. Phys. Chem. Solids, 30, 1835
(1969); V. J. Nefecov, Y. V. Salyn, A. A. Chertkov, and L. N. Padurets, Russ. J. Inorg. Chem.,
(English translation), 19, 785 (1974).
Electrogenerated Chemiluminescence
34. Photo-Induced Electrogenerated Chemiluminescence and
Up-Conversion at Semiconductor Electrodes
J. D. Luttmer* and Allen J. Bard**
Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712
ABSTRACT
Photosensitized oxidations or reductions at p- and n-type GaAs and InP
semiconductor electrodes of aromatic species in aprotic solvents leading to
electrogenerated chemiluminescence (ECL) are described. The energy of the
irradiating light equal to the bandgap energy of the semiconductor and the
electrical energy input couple to produce excited states which eventually produce emission at shorter wavelengths. Intense ECL is observed by alternately
generating radical cations and radical anions at an illuminated semiconductor
electrode with smaller potential excursions than required on metal (platinum)
electrodes.
Photoassisted electrode processes of a number of
species (e.g., aromatic hydrocarbons and heterocyclic
compounds) at semiconductor electrodes in aprotic
solvents have been described (1-5). These studies have
demonstrated that electrochemical oxidations at n-type
semiconductors occur at less positive potentials (or
reductions at p-type semiconductors at less negative
potentials) than are required to carry out the same
process at an inert metal (e.g., Pt) electrode. These
processes thus involve the partial conversion of light
energy to chemical energy in the electrogenerated
species. When the electrogenerated species (e.g., radi* Electrochemical Society Student Member.
** Electrochemical SocietyActive Member.
visible, veltammetry, energy
Key words: photoluminescence,
conversion.
ions) are capable of undergoing homogeneous
electron transfer reactions near the electrode surface
to produce excited states, electrogenerated chemiluminescence (ECL) results. Hence the over-all scheme involves coupling of the input light energy and electrical
energy to produce an output emission of higher energy
(or "up-conversion"). Such a scheme was initially
suggested by Nicholson (6), but prior to this report
was never reduced to practice. A previous report
from this laboratory described a similar experiment
where the photoinduced reduction of 9,10-dichloro9,10-dihydro-9,10 diphenyl-anthracene (DPAC12) on
p-Si irradiated with red light in acetonitrile (7) produced violet emission. There, however, emission is observed only upon reduction of DPACI2 via a chemicalcal
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VoL I26, No. 3
ELECTROGENERATED CHEMILUMINESCENCE
electrochemical mechanism (8), and much of the energy of the emitting light may be attributed to the
chemical energy of DPACI~ moiety itself, which is not
regenerated in the over-all process.
The ECL scheme usually involves production and
annihilation of radical ion species at metal electrodes
by an electron-transfer mechanism (9, 10)
(cathodic step)
A + e ~ A-'
(E~
~)
[1]
+
(anodic step)
A ~" +
D -- e c - D "
D*' --> ~A* (or 1A*)
(E~
+ D
[2]
[3]
8A* + 8A* --> 1A* + A
[4]
1A* -+ A -t- h~
[5]
where A represents an electron acceptor (e.g., an aromatic hydrocarbon) and D represents an electron donor
(e.g., an aromatic hydrocarbon or amine). Formation
of excited states of D and exciplexes are also possible
during the ion-annihilation reaction, Eq. [4], but are
omitted, since they are not important in the reactions
to be described here. When the ECL reaction is carried
out on a semiconductor electrode, two new features
emerge. First, the semiconductor/liquid interface behaves like a Schottky junction and shows diode-like
behavior. This results in anodic reactions occurring
positive of the flatband potential, Vfb, being blocked
at n - t y p e materials and cathodic reactions occurring
negative of Vfb being blocked at p - t y p e materials (11).
Thus irradiation of the electrode is required to allow
generation of both A - and D + and observe emission.
Secondly, irradiation of the semiconductor photoassists the electrochemical process, so that oxidations
at irradiated n - t y p e electrodes occur at potentials less
positive than E ~
and reductions at irradiated ptype electrodes occur at potentials less negative than
Details of ECL experiments at several semi-
E~
conductor electrodes incorporating these features are
reported here.
Experimental
Acetonitrile (ACN; Speetroquality, Matheson, Coleman and Bell) was refluxed over fresh Call2 powder
for 8 hr followed by fractional distillation under dry
nitrogen. The middle fraction was transferred to a
vacuum line and subjected to four freeze-pump-thaw
cycles to remove dissolved gases. The solvent was then
vacuum distilled into a storage flask containing molecular sieves (Linde 4A) activated by heating under
high vacuum at 400~ for 72 hr. Benzene (BZ; Spectroquality, Matheson, Coleman and Bell) was refluxed
over potassium for 24 hr under dry nitrogen, distilled,
and subjected to four f r e e z e - p u m p - t h a w cycles. The
solvent was vacuum distilled into a storage flask containing N a - K alloy.
T e t r a - n - b u t y l a m m o n i u m perchlorate (TBAP; Southwestern Analytical Chemicals) and rubrene (RUB;
Aldrich) were purified as previously described (8).
Chrysene (CHR; Aldrich) was treated with maleic anhydride, chromatographed on an alumina column, and
recrystallized repeatedly from benzene. Fluoranthene
(FLU; Zone Refined, Aldrich) and perylene (PER;
Gold Label, Aldrich) were used as received. 9,10-Diphenylanthracene (DPA; Gold Label, Aldrich) was
sublimed and zone refined. 10-Methylphenothiazine
(10-MP; Eastman) and tetracene (TET; Eastman)
were purified by multiple recry~tallizations from benzene. Pyrene (PYR; Eastman) was recrystallized from
ethanol and sublimed twice. N,N,N',N'-tetramethyl-pphenylenediamine (TMPD) was prepared and purified
as previously reported (i2). p - ( 9 - A n t h r a n y l ) - N , N - d i methylaniline (DMAA), generously provided by Dr.
Kingo Itaya, was used as received.
415
N- and p - t y p e InP and GaAs semiconductor electrodes were prepared as previously reported (13, 14).
Vfb of the semiconductor electrodes was determined by
photocurrent onset measurements and Schottky-Mott
plots. GaAs and InP semiconductor electrodes were
etched in H2SO4-30% H202-H20 (3:1:1) for 5-10 sec
and then in 6M HC1 for 30 sec (GaAs) or 5 sec (InP).
The electrodes were then rinsed with H~O and ethanol,
and dried immediately under vacuum at room temperature.
The electrochemical ECL cell was of conventional
design (12) and incorporated semiconductor and Pt
wire-working electrodes. The Ag wire quasireference
electrode was shielded in a Teflon jacket having a
porous Vycor plug. The auxiliary electrode consisted
of a large Pt foil in the bulk solution. The cell, containing radical ion precursor compounds and supporting electrolyte, was assembled and evacuated 5-18 hr
at high vacuum. Solvent was added by vacuum distillation. Solutions typically contained 1-3 mM A and D
and 0.1M TBAP as supporting electrolyte. The electrochemical and spectroscopic apparatus and techniques
utilized have previously been reported (12, 13). Illumination of the semiconductor electrodes was accomplished with either a 3 mW He-Ne laser or a 650W
tungsten-halogen lamp incorporating Ditric Company
infrared pass (<1.7 eV) filters. A CuSO4 solution filter
was typically used to eliminate stray infrared light
incident on the photomultiplier tube.
Results
The semiconductors studied and their properties are
given in Table T, and the spectroscopic and electrochemical properties of the donor and acceptor molecules of the ECL systems are given in Table Ii.
N-type semiconductors.~Typical electrochemical behavior on n - t y p e semiconductor electrodes is given in
Fig. 1 and 2. in the dark, virtually no current flows at
potentials positive of Vfb. At potentials negative of
V~b, however, the semiconductor electrode becomes degenerate (11) and shows cyclic voltammetric (cv) behavior similar to that on a metal electrode. Thus redox
couples located negative of V~b (e.g., DPA, FLU) show
reversible cv waves. When the n - t y p e semiconductor
Table I. Semiconductor electrode properties
Electrode
E~ ( e V ) *
V~b**
A r e a ( c m ~)
n-InP
n-GaAs
p-InP
p-GaAs
1.27
1.43
1.27
1.43
- 1.0 -- 0.2
- 1.0 ---+0.2
+ 0.0 -- 0.2
+ 0.1 • 0.2
0.22
0.12
0.15
0.11
* Bandgap energy.
* * F l a t b a n d p o t e n t i a l , v o l t s vs. SCE in a c e t o n i t r i l e .
Table II. Electrochemical and spectroscopic properties
Compound
N,N,N',N'-tet ramethyl
p-phe~ylenediarnine
(TIVlPD)
1O-methylpheno-thiaz m e (10-MP)
9,10-diphenyl-anthracene (DPA)
fluoranthene (FLU)
rubrene (RUB)
p- ( 9 - a n t h r a n y l ) -N,Ndimethylaniline
(DMAA)
perylene (PER)
pyrene (PYR)
t e t r a c e n e (TET)
chrysene (CHR)
( R / R .+) *
0.20, 0.77
0.79
(R/R~) *
Es (eV)
E~ (eV)
--
3.1 (15)
2.38 (15)
--
2.9 (15)
2.4 (15)
1.28
-0.94**
-1.84
--1.74
-1.43"*
3.06 (16)
3.4 (17)
2.36 (18)
1.77 (17)
2.30 (17)
1.2 (18)
0.75
1.14
1.31
1.02"*
1.56
-2.00
-1.63
-2.05
-1.58"*
-2.25
2.52 (19)
2.85 (17)
3.34 (17)
2.60 (17)
3.4 (17)
1.52
2.10
1.27
2.48
(17)
(17)
(17)
(17)
* V o l t s vs. SCE in a c e t o n i t r i l e u n l e s s o t h e r w i s e noted.
** A c e t o n i t r i l e - b e n z e n e m i x e d s o l v e n t .
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J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y
416
March 1979
b
a
S t"
EC,
~.................
.-B. ~
hv
E~'"':: ................
VN
."
~"
n - TYPE
~ ~ C
a
b
C,B.
,
,
;
,
_;
,
hv
Volts vs SCE
V.B.
Fig. 1. Cyclic voltammetry of ,-,3 mM 10 MP and 3 mM FLU in
0.1M TBAP/ACH; scan rate, 200 mY/see. (o) On platinum; (b) an
n-lnP in the dark; (c) an illuminated (3 mW He-He laser) n-lnP.
)- TYPE
j//~a
I
150 pA
~
I
C
~
i
O.5
i
O.O
i
-0.5
Fig. 3. Shift of conduction and valence band energies upon
film formation: (a) prior to film formation; (b) film formation allowing electron transfer upon photoexcitation of the semiconductor
electrode.
i
-1.O
Volts vs SCE
I
-1.5
b
I
-2.0
Fig. 2. Cyclic voltammetry of ,-,2.5 mM TMPD and 1 mM DPA in
0.1M TBAP/ACN; scan rate, 200 mV/sec. (a) On platinum; (b) on
n-GaAs in the dark; (c) on illuminated (3 mW He-Ne laser)
n-GaAs.
nation of the n - t y p e semiconductor electrode w i t h
b a n d g a p e n e r g y light results in the o x i d a t i o n of the
solution species D at less positive potentials t h a n those
w h e r e this species is r e v e r s i b l y oxidized on m e t a l electrodes. This "negative o v e r p o t e n t i a l " effect (7, 14) is
the basis for solar e n e r g y conversion utilizing e l e c t r o chemical systems.
A typical e x p e r i m e n t involving photoinduced ECL
at an n - t y p e semiconductor electrode is shown in Fig.
4. The initial p o t e n t i a l is established w h e r e no c u r r e n t
flows until the electrode is illuminated, w h e r e u p o n a n
anodic c u r r e n t flows and the oxidized species is p r o duced at the electrode. S t e p p i n g the potential to n e g a tive values to form the r e d u c e d species results in i n -
I
electrode is i l l u m i n a t e d w i t h light of g r e a t e r e n e r g y
than the b a n d g a p e n e r g y (Eg), electrons a r e excited
into the conduction b a n d and holes p r o d u c e d in the
valence b a n d can be p o p u l a t e d by electron t r a n s f e r
f r o m suitable solution species. This process is most
efficient at potentials positive of Vfb w h e r e the photop r o d u c e d holes a c c u m u l a t e at the Semiconductor-solution interface. The photoinduced oxidation of species
with solution e n e r g y levels located at potentials m o r e
positive t h a n the valence b a n d edge (Ev) is not e x pected since a hole p r o d u c e d at e n e r g y E~ w o u l d not
be sufficiently energetic. Such a p h e n o m e n o n was o b s e r v e d here, however, A similar photoeffect on o x i d a tions occurring n e g a t i v e of Vfb was found in a p r e v i ous s t u d y at GaAs electrodes (14) and was ascribed to
production of a film on the electrode surface upon r e duction which effectively f o r m e d a solid/solid j u n c t i o n
adding a bias u n d e r illumination. Such an effect cannot be occurring here, since a photoeffect is found for
+
the D / D " wave, b u t not at the A - / A one. An a l t e r nate e x p l a n a t i o n rests on a change in the surface of
the electrode which shifts the value of Vfb t o w a r d
m o r e positive values (Fig. 3). Effectively then, i l l u m i -
il
2 sec.
I
Potential
J
Ilium.
On
Ilium.
Off
I
9
t
~
Current
ia
l
O
~-
9
_~
Total
Emission
Fig. 4. Potential-, current-, and emission-time profiles illustrating
photoinduced oxidation of 10 MP and reduction of FLU on n-lnP
yielding ECL. Illumination of n-lnP with 3 mW He-He laser.
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Vol. 126, No. 3
I
3,50
!
400
ELECTROGENERATED
I
L
450
CHEMILUMINESCENCE
417
6bo
.qO0
x(.m)
Fig. 5. Uncorrected ECL spectrum of ,~2 mM TMPD and I mM
DPA on n-lnP illuminated with 3 mW He-Le laser. Potential pulsed
from --0.05 to --2.0V vs. SCE at ! Hz.
tense ECL as the r a d i c a l ions a n n i h i l a t e in the diffusion l a y e r a d j a c e n t the electrode. Pulsing the semiconductor electrode p o t e n t i a l b e t w e e n these p o t e n t i a l
limits results in pulses of ECL emission w h e n the
electrode was i l l u m i n a t e d w i t h b a n d g a p e n e r g y light
(>1.4 eV). W i t h these same p o t e n t i a l excursions, no
emission was o b s e r v e d on a p l a t i n u m electrode, b e cause the p o t e n t i a l limits w e r e insufficient to p r o d u c e
/
I
I
I"'
0
I
I
-1
+
Lo
I
-2
V o l t s vs S C E
D 9 and A with photoassistance. A t y p i c a l ECL spect r u m o b t a i n e d b y scanning the emission w a v e l e n g t h
d u r i n g r e p e t i t i v e pulsing u n d e r i r r a d i a t i o n is shown
in Fig. 5.
The potential d e p e n d e n c e of the emission was asc e r t a i n e d b y e m p l o y i n g anodic l i n e a r potential sweeps
f r o m potentials w h e r e the r e d u c e d species is f o r m e d
t o w a r d positive values, as p r e v i o u s l y d e s c r i b e d (13)
(Fig. 6). U n d e r d a r k conditions, v i r t u a l l y no emission
is observed at the semiconductor u n t i l the e l e c t r o d e
p o t e n t i a l reaches v e r y positive potentials. The semiconductor electrode i l l u m i n a t e d with b a n d g a p e n e r g y
light, however, gives emission at potentials less positive t h a n those necessary for emission on platinum.
The ECL emission observed in all of the cases e x a m i n e d (Table I I I ) was quite bright, visible to the eye
in a w e l l - i l l u m i n a t e d room, and c o m p a r a b l e to the
emission o b s e r v e d on p l a t i n u m electrodes w h e n using
l a r g e r p o t e n t i a l excursions. A l t h o u g h the ECL of systems utilizing easily oxidizable species (e.g., TMPD)
was quite stable w i t h continuous potentiostatic pulsing
and illumination, less s t a b i l i t y was found w i t h species
oxidized at m o r e positive potentials. F o r example, although an initial ECL pulse w i t h RUB on n - I n P was
intense (and easily visible), the i n t e n s i t y r a p i d l y d e cayed upon f u r t h e r pulsing, p r o b a b l y because of oxi-
Fig. 6. ECL- and current-potential curves observed by anodic
linear potential sweep voltammetry at 2 V/sec with 3 mM TMPD
and 1 mM DPA in 0.1M TBAP/ACN. (e) ECI. intensity; and (b)
current obtained on n-lnP illuminated with hz, ( 1.7 eV.; (c) ECL;
and (d) current obtained on platinum.
dation of t h e semiconductor lattice (14). Electrode
s t a b i l i t y was s i m i l a r l y a d v e r s e l y affected b y traces of
H20 w h e n less r i g o r o u s l y d e h y d r a t e d solvent was
used. U n d e r these conditions, the p h o t o i n d u c e d o x i d a tion of the semiconductor lattice was found to occur at
less positive potentials and resulted in the a c c u m u l a tion of an electrically insulating film on the semiconductor electrode surface.
P-type semiconductors.--The electrochemical b e h a v i o r at p - t y p e semiconductor electrodes was a n a logous to t h a t found with n - t y p e electrodes. Typical
cyclic v o l t a m m o g r a m s a r e shown in Fig. 7 and 8. Unlike oxidations at the n - t y p e material, some c u r r e n t
a t t r i b u t a b l e to d a r k r e d u c t i o n of the s u b s t r a t e is o b served at the p - t y P e material, b u t only at potentials
negative of w h e r e the r e d u c t i o n w a v e is found at Pt.
Oxidations at potentials positive of V~b occur r e a d i l y
and the electrode shows essentially m e t a l l i c behavior.
Table Ill. Summary of ECL experiments on the photoinducedup-conversionat p- and n-type InP and GaAs
Electrode
n.GaAs
System
TMPD/DPA
1O-MP/DPA
DMAA
Oxidation" *
Reduction* *
Epa
Epc
Ep c
--0.38
-- 0.17
0.25
0.16
--0.34
+ 0.09
0.23
0.29
0.52
--0.22
0.29
0.29
0.47
--0.17
0.30
Epa
AEEOL*"*
Emission
1.~1.7
Blue
Stable, b r i g h t , ~EEeL < ET
2.1-2.2
2.1-2.3
Blue
Yellow
2.0-2.1
1.9-2.1
1.9-2.1
Blue
Yellow
Blue
M o d e r a t e l y stable, b r i g h t emission
B r i g h t i n t r a m o l e c u l a r exeiplex
emission, cation unstable
Mad. stable, b r i g h t , AEEen < ET
Bright, unstable
Stable, b r i g h t
2.2-2.4
2.2-2.5
Blue
Blue
n-InP
10-MP/FLU
RUB
TMPD/DPA
p-GaAs
10-MP/FLU
PER
-- 1.45
--1.18
DPA
-- 1.46
- 1.52
2.7-2.8
Blue
PER
- 1.30
-- 1.06
2.3-2.5
Blue
DPA
-1.37
--1.14
2.6-2.8
Blue
p-InP
- 1.48
-1.31
Commentst
Stable, b r i g h t , EECL -~ ET
B r i g h t , m a d . stable, cooxidation of
electrode
B r i g h t , u n s t a b l e , cooxidation of
electrode
B r i g h t , mad. stable, s o m e oxidation
of I n P
B r i g h t , unstable, cooxidation of I n P
* 0.1M T B A P / a c e t o n i t r i l e solution.
** C u r r e n t p e a k p o t e n t i a l s , volts vs. SCE. E l e c t r o d e i l l u m i n a t e d w i t h h~ ~ 1.7 eV.
*** P o t e n t i a l excursiozzs n e e d e d to o b t a i n ECL on i l l u m i n a t e d s e m i c o n d u c t o r electrode.
t StabiUty r e f e r s to s e n n c o n d u e t o r e l e c t r o d e w i t h r e s p e c t t o d e g r a d a t i o n .
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J. Electrochem. Sac.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y
418
I
/jCtooA
?
i
I
I
1
I
O
Volts vs SCE
I
I
-1
I
-2
Fig. 7. Cyclic voltammetry of 1 mM RUB in 0.1M TBAP/ACNBZ(I:I); scan rate, 200 mV/sec. Ca) On platinum, (b) on p-lnP in
the dark; (c) on p-lnP illuminated with h~ ~ 1.7 eV.
I
100 pA
/
|
I
1
~
I
Volts vs SCE
|
-1
I
|
-2
Fig. 9. ECL-potential curve obtained by anodic linear potential
sweep voltammetry at 2 V/sec with 2.5 mM FLU and 1.5 mM 10MP in 0.1M TBAP/ACN. (a) On platinum from --1.60V vs. SCE
(Gain, • 100); (b) on platinum from --2.1V vs. SCE; (c) on p-GaAs
illuminated with 3 mW He-Ne laser from --1.60V vs. SCE.
positive Ez/~(RIR +") values w e r e oxidized concurr e n t l y with the semiconductor e l e c t r o d e lattice and in
these systems the ECL i n t e n s i t y t e n d e d to d e g r a d e
w i t h continuous pulsing. F o r example, the o x i d a t i o n
of D P A occurs at potentials w h e r e semiconductor l a t tice o x i d a t i o n is o b s e r v e d and t h e ECL emission w i t h
this system was r a t h e r short-lived. A s y s t e m e m p l o y ing 10-MP which has a s o m e w h a t less positive
4"4--1
March 1979
jb
+
E 1 / ~ ( R / R ' ) v a l u e was quite stable w i t h continuous
potential pulsing and i l l u m i n a t i o n of the p - t y p e s e m i conductor electrodes e x a m i n e d however. As o b s e r v e d
on the n - t y p e semiconductor electrodes, traces of H20
h a d a deleterious effect u p o n the semiconductor elect r o d e stability, especially w h e n r a t h e r positive p o t e n tials w e r e a p p l i e d to the electrode.
m~.
'
I
I
-2
Volts vs SCE
Fig. 8. Cyclic voltammetry of ,~2.5 ~mM FLU and 1.5 mM 10-MP
in 0.1M TBAP/ACN; scan rate, 200 mV/sec. (a) On platinum; (b)
on p-GaAs in the dark; (c) on p-GaAs illuminated with h~ < 1.7
eV.
W h e n the electrode at potentials n e g a t i v e of Vfb is i l l u m i n a t e d w i t h b a n d g a p e n e r g y light, electrons e x cited into the conduction b a n d can reduce the solution
acceptor species. Again, some surface p h e n o m e n o n
which allows the p h o t o i n d u c e d reduction of suitable
solution species w i t h solution e n e r g y levels above the
conduction band edge w i t h a considerable "negative
o v e r p o t e n t i a l " must be involved (14).
The ECL results w i t h p - t y p e semiconductor electrodes are s u m m a r i z e d in Table III; a t y p i c a l e x p e r i m e n t is shown in Fig. 9. A l t h o u g h the reduced species
( F L U h e r e ) could be g e n e r a t e d in the d a r k at v e r y
negative potentials (i.e., b e y o n d ca. --2.5V), the A was t y p i c a l l y g e n e r a t e d at potentials positive of w h e r e
it is t h e r m o d y n a m i c a l l y accessible on p l a t i n u m b y ill u m i n a t i n g the semiconductor electrode. As illustrated,
no ECL could be observed on p l a t i n u m u n d e r these
conditions.
T h e ECL from the systems e x a m i n e d was quite
intense and visible to the eye. Species w i t h r a t h e r
Discussion
The results p r e s e n t e d h e r e d e m o n s t r a t e t h a t u p conversion b y the E C L - s e m i c o n d u c t o r electrode
scheme, in a process w h e r e no m a t e r i a l is consumed
(except p e r h a p s b y parasitic side reactions), is possible. Moreover, photoassistance of the electrode p r o c ess allows a coupling of the light e n e r g y and electrical
e n e r g y i n p u t to p r o d u c e emission at an e n e r g y w h e r e
n e i t h e r the light nor electrical e n e r g y alone w o u l d be
sufficient. I n t h e systems e x a m i n e d n e g a t i v e o v e r p o tentials of 400-600 mV could be attained. W h i l e i t is
t r u e t h a t the t r i p l e t - t r i p l e t annihilation step, Eq. [4],
was responsible for the e n e r g y gain, in some of the
cases e x a m i n e d n e i t h e r the p o t e n t i a l stimulus nor the
e n e r g y of the i l l u m i n a t i o n light alone was sufficient to
p o p u l a t e the lowest t r i p l e t level. F o r example, for the
10 MP F L U system at n - I n P , ET for F L U is 2.30 eV,
w h i l e the a p p l i e d potential to the s y s t e m was only 2.02.1V.
The results r e p o r t e d h e r e suggest t h a t the f a b r i c a tion of devices e m p l o y i n g small b a n d g a p (<1.5 eV)
semiconductor electrodes to u p - c o n v e r t n e a r i n f r a r e d
light to h i g h e r e n e r g y light in the visible region m a y
be possible. The e m i t t e d light was quite intense and
the semiconductor electrodes were quite stable u n d e r
strictly a n h y d r o u s conditions upon r e p e a t e d cycling,
although most ECL systems are k n o w n to d e g r a d e over
a v e r y long time period. A device built on these p r i n ciples would r e q u i r e r a t h e r low voltages (,~3V) and
would have a spacial resolution d e t e r m i n e d b y the
n a r r o w w i d t h of the r a d i c a l ion a n n i h i l a t i o n zone.
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ELECTROGENERATED CHEMILUMINESCENCE
Vol. 126, No. 3
Acknowledgment
We gratefully acknowledge the A r m y Research Ofrice and the National Science Foundation for the support of this research. We are indebted to Paul Kohl
for the flatband potential measurements and many
helpful discussions concerning this work.
Manuscript submitted May 17, 1978; revised manuscript received J u l y 19, 1978.
Any discussion of this paper will appear in a Discussion Section to be published in the December 1979
JOURNAL. All discussions for the December 1979 Discussion Section should be submitted by Aug. 1, 1979.
Publication costs o~ this article were assisted by the
University of Texas at Austin.
REFERENCES
1. S. N. F r a n k and A. J. Bard, J. Am. Chem. Soc., 97,
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7531 (1977).
4. R. Landsberg, P. Janietz, and R. Dehmlow, Z. Phys.
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Jpn., 50, 783 (1977).
6. M. M. Nicholson, This Journal, 119, 461 (1972).
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(1975).
8. K. G. Boto and A. J. Bard, J. Electroanal. Chem.
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419
9. L. R. Faulkner, in "MTP Int. Rev. Sci. Phys. Chem.
Ser. Two," Vol. 9, D. R. Herschbach, Editor,
Chap. 6, and ref. therein, Medical and Technical
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10. L. R. Faulkner and A. J. Bard, in "Etectroanalytical
Chemistry," Vol. 10, A. J. Bard, Editor, Chap. 1
and ref. therein, Marcel Dekker, Inc., New York
(1977).
11. H. Gerischer, in "Advances in Electrochemistry
and Electrochemical Engineering," Vol. 1, P.
Delahay and C. Tobias, Editors, Chap. 4, Interscience, New York (1961); H. Gerischer, in
Physical Chemistry," Vol. 9A, H. Eyring, D.
Henderson, and W. Jost, Editors, Chap. 5, Academic Press, New York (1970); H. Gerischer,
Photochem. Photobiol., 16, 243 (1972); H. Gerischer, Ber. Bunsenges. Phys. Chem., ~'~, 771
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12. L. R. Faulkner, H. Tachikawa, and A. J. Bard, J.
Am. Chem. Soc., 94, 691 (1972).
13. J. D. Luttmer and A. J. Bard, This Journal, 125,
1423 (1978).
14. P. A. Kohl and A. J. Bard, Paper submitted to ibid.
15. S. M. Park, Ph.D. Dissertation, University of Texas
at Austin (1975).
16. C. P. Keszthelyi, A. T. Kessel, W. B. Edwards, and
G. D. Clark, Proc. Louis. Acad. Sci., 39, 65 (1976).
17. S. L. Murov, "Handbook of Photochemistry," M a r cel Dekker, New York (1973).
18. D. K. K. Liu and L. R. Faulkner, J. Am. Chem. Soc.,
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19. K. Itaya and S. Toshima, Chem. Phys. Lett., 51,
447 (1977).
Semiconductor Properties of Iron Oxide Electrodes
S. M. Wilhelm,* K. S. Yun,** L. W. Ballenger,* and N. Hackerman***
Department of Chemistry, Rice University, Houston, Texas f7001
ABSTRACT
The semiconductor electrode properties of thermally grown iron oxide
and anodic oxide films on iron are compared. The bandgaps are 2.1 and 1.9
eV, respectively. Donor densities and flatband potentials are reported. Measurement of the currents produced by chopped illumination (f ~ 100 Hz) has
allowed evaluation of electrode parameters and determination of reaction
pathways.
The recent interest in iron oxide electrodes centers
around their potential application to photoassisted electrolysis of water. These materials are promising because of their relatively narrow bandgap and of possible use because of their stability. The n - t y p e polycrystalline ~Fe208 (1-5) behaves much as does single
crystal aFe203 (6) (see Table I, P a r t A) and has the
advantage of being easy to prepare.
The anodic oxide or passive film on iron is also reported to be a semiconductor (7-9) (see Table I, Part
B). Photocurrent data, however, has not been reported.
In the present investigation, the semiconducting character of thermally grown polycrystalline iron oxide
and anodically produced oxide films on iron are compared directly with regard to photocurrent, differential
capacitance, and polarization measurements.
Experimental
All measurements were made in a solution 0.075N in
H3BO3, 0.075N in Na2B4OT, and 1N in NaNO3 (pH 8.4).
The K3Fe (CN) 6/K4Fe (CN) 6 couple was added in certain experiments. All solutions were prepared from
* Electrochemical
** Electrochemical
* * * Electrochemical
Society
Society
Society
Student Member.
Active Member.
Honorary
Member.
Key words: passivity, photocurrent, electrolysis, ion oxides.
triply distilled water and were saturated with purified N2. All chemicals were reagent grade or better.
A two compartment Pyrex cell containing a platinum
gauze counterelectrode and a saturated calomel reference electrode (SCE) was used. All potentials are reported vs. SCE. The working electrode was approximately 0.5 cm from a Pyrex window through which
the light was focused on the electrode surface in
photocurrent experiments.
Electrodes were constructed from zone-refined iron
(99.99% Fe). The thermally grown iron oxide electrodes were prepared as described previously (5). The
bare metal electrodes were polished to a mirror finish
and cleaned with methanol prior to use. For each
working electrode a 0.5-1.0 cm 2 planar surface was exposed to solution by insulating the rest of the electrode
with silicone adhesive (Dow Corning). Anodic oxide
films were formed by first cathodically reducing any
air formed oxide at --0.80V in pH 8.4 buffer solution
for 1 hr, replacing with fresh solution under nitrogen
and then polarizing at -F0.80V for 12 hr. The film
thickness was estimated by galvanostatic reduction
(~0).
The photocurrent measurements (Fig. 1) employed
a 200W tungsten lamp for polyehromatie experiments.
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