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Cleavage of Water by Visible-Light Irradiation of Colloidal CdS Solutions; Inhibition of Photocorrosion by RuO2.

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Cleavage of Water by Visible-Light
Irradiation of Colloidal CdS Solutions;
Inhibition of Photocorrosion by RuOz[**l
By Kuppuswarny Kalyanasundaram, Enrico Borgarello,
Dung Duonghong, and Michael Gratzel["
The conversion of light into electricity or chemical fuels
in photoelectrochemical devices equipped with semiconductor electrodes is of great current interest'']. A serious
problem associated with the use of materials with valenceband gaps appropriate for solar energy exploitation, is
their inherent instability. For example, n-type CdS or GaP
are subject to undesirable decomposition upon irradiation[". Holes (h +) produced in the valence band upon irradiation migrate to the surface where photocorrosion occurs :
CdS + 2 h ' -t Cd"
GaP + 3 H 2 0 + 8 h ' - G a l +
+S
+ PO; + 6 H '
(1)
(2)
This undesired process may be prevented in the presence
of suitable reducing agents such as, Fe(CN):-,
S2-,
SO:-, or hydr~quinone[~]
which scavenge the hole at the
semiconductor/electrolyte interface before lattice dissolution can occur. However, in order to be effective, the holetransfer across the interface requires a large driving force
which consumes a significant fraction of the absorbed light
energy.
In investigations of microheterogeneous systems capable
of decomposing water into Hz and 0, by visible light[4a1,
we discovered that ultrathin layers of R u 0 2 on CdS inhibit
photocorrosionr4h1:reaction ( I ) is replaced by oxidation of
water.
+ 2HzO
4h'(CdS)
RuOz
___*
0 2
+ 4H'
Precipitation of CdS from aqueous solutions of Cd2+
and (NH4),S in the presence of maleic anhydridelstyrene
copolymer produces a perfectly transparent, intensely yellow colored sol, whose absorption spectrum is shown in
Figure 1 . Noteworthy is the upper band limit at 520 nm,
whose position coincides with the 2.4eV band gap of CdS.
The band rises towards the UV in a very steep fashion, typical of electronic transitions in semiconductor^^^^. From
these characteristics it is concluded that microcrystalline
CdS particles of colloidal dimensions are present in solution.
The CdS particles themselves do not afford photodecomposition of water. Active catalysts are obtained only
after they have been loaded with Pt and
Sustained
water cleavage by visible light (/z>400 nm) was observed
with such colloidal solutions, e.g. from a 25 mL solution
(2.75 mg of CdS, 1 or 0.5 mg of Pt and 0.2 rng of RuO,) a
total of 2.8 mL of H, and 1.4 ml of 0, was produced after
44 h of irradiation (cf. Fig. 2); this corresponds to turnover
numbers of 6, 25, and 85 for CdS, Pt, and Ru02, respectively. Clearly the reaction is catalytic with respect to all
three constituents. In particular, no appreciable photodegradation of CdS occurs. To further confirm this surprising
and important effect we examined carefully the Cd" concentration before and after 72 h of photolysis. Polarographic analysis showed no detectable increase in ion concentration during this period, which precludes any significant
contribution of the anodic dissolution reaction (1) to the
overall photoprocess. Since, on the other hand, the generation of oxygen by photolysis can be demonstrated readily
and unambiguously it can be inferred that oxidation of water occurs exclusively, and at the expense of photocorrosion.
(3)
We now describes the design and performance of colloidal
CdS microelectrodes which exhibit surprisingly high activity as water-cleaving catalysts.
\
\
\
\
\
\
\
0
-
600
500
400
hInml
Fig. I . Absorption spectrum of the colloidal CdS solution (330 mg/L), optical path length 0.2 cm.
I*] K. Kalyanasundararn, E. Borgarello, D. Duonghong, M. Gratzel
Institut de Chimie Physique, Ecole Polytechnique Federale Ecublens
CH-I015 Lausanne (Switzerland)
[**I This work was supported by the Schweizerischer Nationalfonds and by
Ciba-Geigy, Basel.
Angew Chem Int Ed Engl 20 (1981)
No. I 1
Fig. 2. Production of H2and O2 by irradiation of a 8 . 4 ~1 0 - a M CdS sol at
room temperature. Particles loaded with Pt and R u 0 2 . O 2 determined by
flow procedures ( 1 11.
Further experiments established that the stabilization of
CdS is brought about by the RuOz catalyst. Particles only
loaded with Pt decompose rapidly under illumination with
formation of sulfur. A similar observation was made with
dispersions of commercial CdS powdersr4b1.
0 Verlag Chemie GmbH. 6940 Weinhelm, 1981
0570-0833/81/1111-0987 $ 02.50/0
987
The reaction temperature has a decisive influence on the
efficiency of the light induced water cleavage. Thus, at
75 C the rate of H2 formation (0.1 mL/h) is approximately
twice as fast than at room temperature.
Compared to a macrodispersion, decreasing the particle
size to colloidal dimensions increases the yield of Hz per
unit mass of catalyst by a factor of 50. These observations
may be rationalized in terms of electron/hole pair-formation in the colloidal CdS particles under band gap excitation. The electrons migrate to Pt sites where H, production
from water occurs. The flat band potential of CdS is located at ca. -900 mV (us. standard H, electrode at
pH = 7), which is sufficiently cathodic for water reduction.
On the other hand the valence band edge of n-type CdS is
ca. 1.5 V and holes trapped by R u 0 2 sites on the surface
are subsequently used to produce oxygen from water:
since the driving force for water oxidation is therefore
more than 600 mV this process is sufficiently fastr7]to compete with surface corrosion. Thus, the role of the RuO, is
to promote the transfer of holes from the valence band to
the aqueous solution where O2 evolution occurs.
Moreover, we have found that the catalytic properties of
the CdS sols (also withour Pt!) described here are also particularly suitable for the photolytic decomposition of H2S
into H2 and sulfur[9b1.
Our results clearly identify the decisive influence of
RuO, on valence band processes in irradiated semiconductor particles. In the case of CdS a striking augmentation of
the photoactivity is observed when the particle size is reduced to colloidal dimensions'']. Work is now in progress
to substitute these microsystems by polycrystalline semiconductor electrodes by covering the electrode with an ultrathin layer of RuO, and by coupling it to a Pt counterelectrode, such a device should afford local separation of
the H2- and 0,-generating
+
Experimental
100 mg of maleic anhydride/styrene copolymer (50/50,
TNO, Utrecht) is dissolved in 100 mL of alkaline water
(pH = 10, adjusted with 1 N NaOH) under constant stirring
at 50°C for several hours. When dissolution is complete, 0.15 mL of a 10%(W/v) solution of (NH,),S is added. A stock solution of CdS0, (2 g/L) is prepared, and 10
mL is slowly (microsyringe!) injected. After being stirred
for ca. 1 h the appearance of an intense yellow color indicates formation of the CdS sol. The pH of the solution is
subsequently adjusted to 3 (HCI), and excess sulfide removed as H2S by slushing the solution with Nz overnight.
The final concentration is 110 mg CdS/L ( 8 . 4 ~1 0 - 4 ~ ) .
CdS is loaded with RuOz via decomposition of RuO,
(RuO4-RuO2+O2) by injecting 0.5 mL of a stock solution
of R u 0 2 in water (0.5 g/L) into 20 mL of the CdS solution.
988
0 Verlag Chemie GmbH. 6940 Weinheim, 1981
An ultrathin deposit of RuOz is formed when the mixture
is stirred for 30 min. H2PtCI, (3 mg) and 1 mL of a 40%
aqueous solution of formaldehyde are subsequently directly added to the CdS/Ru02 solution. After deoxygenation, Pt is deposited onto the CdS particles via photoplat i n i ~ a t i o n [lo]
~ .and excess formaldehyde is subsequently removed under in uacuo. The final concentrations are 40 mg/
L Pt, 8 mg/L RuOz, and 110 mg CdS/L.
Photolysis experiments were carried out with a xenon
lamp (Osram XBO-450W; filter for I R region and A<400
nm) in a pyrex flask (25 mL of solution) equipped with optically flat entry- and exit-windows. Hz production was
monitored by gas chromatography (GOW MAC, carbosieve column, 35 "C, N2 carrier gas). 0, was analyzed using
a Teledyne B1 oxygen specific microfuel
Received: January 28, 1981,
publication delayed at the authors' request (2 820 IE]
German version: Angew. Chem. 93, 1012 (1981)
[ I ] A . J . Nozik. Ann. Rev. Phys. Chem. 21. 189 (1978); R . Memming, Philips
Techn. Rev. 38. 160 (1978179); A. Heller. B. Miller. Electrochim. Acta
25. 24 (1980); Faraday Discuss. Photoelectrochemistry, Oxford, September 1980.
121 R . Williams. J. Chem. Phys. 32, 1505 (1960); H. Gerischer, J. Electroanal. Chem. Interfacial Electrochem. 58, 263 (1975); M . Matsumura. K .
Yamamoto, H . Tsubomura. J. Chem. SOC.Japan 1976. 399.
[3j H. Gerischer. E. Meyer, Z. Phys. Chem. (Frankfurt am Main) 74. 302
(1971); A. Fujishrma. E . Sugiyama. K . Honda. Bull. Chem. SOC.Jpn. 44,
304 (1971); F. Sitabkhan. Eer. Bunsenges. Phys. Chem. 76. 389 (1972);
A. B. Ellis. S. W. Kaiser. M . S. Wrighton, J. Am. Chem. SOC.98. 1635
(1976); G. Hodes, J. Manassen, D . Cohen. Nature 261, 403 (1976); 8.
Miller. A. Heller. Nature 262. 680 (1976): T. Inone. T. Watanabe. A. Fumishima, K . Honda. K . Kohayakawa, J. Electrochem. SOC. 12. 719
(1977).
I41 a) K . Kalyanasundaram. M. Griitzel. Angew. Chem. 91. 759 (1979); Angew. Chem. Int. Ed. Engl. 18. 701 (1979): J. Kiwi, E. Borgarello. E. Pelizzetti. M. Visca. M. Grutzel, ibid. 92. 663 (1980): 19, 646 (1980); E. Borgarello. J . Kiwi. E. Pekzetti. M . Visca, M . Gratzel. Nature 289. 158
(1981); J. Am. Chem. SOC.,in press; b) K . Kalyunasundurum. E. Borgurello. M. Grutrel. Helv. Chim. 64. 362 (1981).
151 P. Saluador. Solar Energy Mat. 2. 413 (1980).
161 The loading does not significantly change the absorption behavior of the
CdS sol.
171 RuO, is known to be an excellent electrocatalyst for water oxidation: S.
Trasatfr. G . Burzanca. Electroanal. Chem. 29. App. 1 (1971); D . Calm
rioli. F. Tuntardine. S. Trasatti, J. Appl. Electrochem. 4. 57 (1974); D.
Galizzioli. F. Tantardini. S . Trasatti. J. Appl. Electrochem. 5.203 (1975);
G . Lodi. G . Zucchini. A . de Buttisti. E. Siuieri. S. Trasaffi. Mater. Chem.
3. 179 (1978).
[S] Darwent and Porter have observed light-induced H I evolution on Ptloaded CdS particles. In this case ethylenediaminetetracetic acid
(EDTA) instead of water is oxidized. We thank these authors for communicating their results prior to publication. Cf. F. D. Saeua. G . R . Olin.
J. R. Harbour, J. Chem. SOC.Chem. Commun. 1980. 401, and references
cited therein.
[9] a) M. Neumann-Spullart. K . Kulyunasundaram, M . Griifzel.unpublished
results. b) M . Grafzel, E. Borgarello, K . Kalyanasundaram. E. Pelizzefti.
unpublished results.
[lo] B. Krueutler. A. J . Bard. J. Am. Chem. SOC.100. 4318 (1978).
[I I ] D. Duonghong, E. Borgarello, M. Gratzel. J. Am. Chem. SOC. 103, 4685
( I98 1).
0570-0833/81/111 I-0988 $02.50/0
Angew Chem. Inr Ed. Engl. 20 ( I Y H I j No. I 1
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water, solutions, colloidal, cleavage, inhibition, ruo2, light, photocorrosion, irradiation, cds, visible
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