~
m
Solar Energy Materials
and Solar Cells
~
Solar Energy Materials and Solar Cells 38 (1995) 249-276
ELSEVIER
Photochemical production of hydrogen and oxygen from
water: A review and state of the art
Edmond Amouyal
Laboratoire de Physico-Chimie des Rayonnements (CNRS, URA 75), Bat 350, Universite Paris-Sud, 91405
Orsay, Cedex, France
Abstract
Photochemical hydrogen production is potentially one of the most fascinating ways for
solar energy conversion and storage. Since 1977, several homogeneous, quasi-homogeneous
or microheterogeneous model systems of hydrogen or oxygen generation from water, by
visible-light irradiation, have been proposed and are briefly reviewed. These half photosysterns are based on different approaches: (i) multimolecular systems, (ii) systems involving a
supramolecular structure of polyad type, and (iii) systems incorporated in organized and
constrained or confined media. A survey of the different attempts for complete water
splitting into hydrogen and oxygen is also made.
1. Introduction
The production of renewable and non-polluting fuels via the direct conversion
of solar energy into chemical energy remains a fascinating challenge for the end of
this century. Among various interesting reactions, the splitting of water into
molecular hydrogen and molecular oxygen by visible light (reaction 1) is potentially
one of the most promising ways for the photochemical conversion and storage of
solar energy [1-4].
visible light
H 20
~
H2 + 1/202'
(1)
Indeed hydrogen is a valuable fuel: the free enthalpy needed in reaction 1 to
produce one mole of H 2 , i.e., the energy stored per mole, is LlGg98 = 237.2
kJ . mol-I. Due to its small weight, the energy storage capacity of H2 per gram,
119000 J. g-I, is very high. It is, for example three times higher than the storage
capacity of oil (40 000 J. g-I). Moreover, the water-splitting process has two other
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250
E. Amouyal / Solar Energy Materials and Solar Cells 38 (J995) 249-276
advantages. First, the raw material, i.e., water, is abundant and cheap. Second, the
combustion of H2 in air (reverse of reaction 1) again gives water. In other words,
the overall process is cyclic and non-polluting. The main disadvantage is that H2
can react explosively with 02' However, the explosive limit of H2 in air is 4.00%,
less than that of butane: 1.86%. Hence, the utilization of hydrogen is no more
dangerous than that of natural gas, and like natural gas hydrogen can be quite
easily stored and transported.
Since water does not absorb visible light, intermediates are needed to achieve
the water photocleavage via a cyclic pathway (reaction 1). Numerous strategies
have been described and several model systems capable of producing separately
hydrogen and oxygen have been proposed. Attempts to generate simultaneously
H2 and O 2 have also been reported. In this paper, I present a general survey of
the principal homogeneous and microheterogeneous systems (multimolecular,
supramolecular, constrained or confined systems) in which the illumination of a
coloured compound acting as a photosensitizer gives rise to photoinduced redox
processes. Microheterogeneous systems involving the formation of electron-hole
pairs through direct excitation or photosensitization of semiconductor particles are
not considered. Several reviews on these semiconductor-based systems are available [1,5,6].
2. Multimolecular systems
2.1. Ideal functions
As emphasized in the introduction, hydrogen and/or oxygen production from
water by visible light requires one or several intermediates having ideally the
following functions:
(i)
visible light absorption,
(iO conversion of the excitation energy to redox energy (charges),
(iii) concerted transfer of several electrons to water leading to the formation of
H2 as energy-storage compound and/or to the formation of 02'
Indeed, one of the main difficulties in achieving the splitting of water by means
of light-induced redox processes is that hydrogen requires two electrons (reaction
2) while oxygen requires four electrons (reaction 3).
2H 20 + 2e--+ H2 + 20H-,
2H 20-+0 2 +4H++4e-,
EO (pH = 7) = -0.41 V versus NHE,
EO(pH=7) = +0.82VversusNHE.
(2)
(3)
This number of charges corresponds to the most favourable thermodynamic
conditions for reaction 1. In other words, this reaction is a multi-electron transfer
process which requires 1.23 eV per electron transferred. Hence, photons with
A < 1008 nm corresponding to a minimum energy of 1.23 eV can induce the
cleavage of water.
E. Amouyal / Solar Energy Materials and Solar Cells 38 (1995) 249-276
251
2.2. General schemes for H2 and O2 production
In a first approach, photochemical systems involving several compounds were
proposed. In these multimolecular systems, each function is fulfilled by one
molecule, namely, (i) a photosensitizer PS able to absorb visible light to generate
excited species PS • with useful redox properties (reaction 4):
hu·
PS --:spS' ,
(4)
(ii) a second compound R which can be reduced or oxidized by quenching of the
excited species PS' in electron transfer reactions leading to the formation of
charge pairs, such as PS +, R - in the case of the oxidative quenching of PS
(reaction 5):
(5)
(iii) and a third compound able to collect several electrons to facilitate the
exchange of two (reaction 6) or four electrons with water. This multi-electron
collection and transfer can be realized by a specific redox catalyst Cat.
Cat
2R-+ 2H+ ~2R + H 2 •
(6)
In such a system, the second compound R acts as an electron relay between the
photosensitizer PS and the catalyst Cat mediating the electron collection. The
redox potential of its reduced species R - must be less than - 0.41 V (versus NHE,
pH = 7) to take part in reaction 2.
In practice, difficulties arise from a fast recombination of charge pairs (reaction
7).
(7)
The main problem, for these multimolecular systems and more generally for
photochemical systems, is how to retard this back electron transfer reaction in
order to get a charge separation of long lifetime.
In the case of multimolecular systems, the back reaction should be prevented by
using a fourth compound, an electron-donor D, which scavenges the oxidized
photosensitizer PS + in a competitive electron transfer reaction to give the initial
PS and a donor oxidation product D + (reaction 8).
PS++ D
~
PS + D+,
D + ~ products.
(8)
(9)
The latter rapidly decomposes irreversibly (reaction 9), and such systems have
been qualified as "sacrificial". D is the only compound, apart obviously from water
(H+), which is consumed. The other compounds PS, R and Cat follow catalytic
cycles.
Two schemes for cyclic production of hydrogen from water can be envisaged [7].
The first is called the "oxidative quenching mechanism" because it involves
252
E. Amouyal/ Solar Energy Materials and Solar Cells 38 (1995) 249-276
/'.'
D~X~-X~~
(b)
0+
Fig. 1. Schematic representation of the redox catalytic cycles in the photoreduction of water to
hydrogen by visible-light irradiation of a four-component model system PS/R/D/Cat: (a) oxidative
quenching mechanism, (b) reductive quenching mechanism.
oxidation of the excited photosensitizer PS' to PS + by the electron relay R (Fig.
1a). It corresponds to reaction 4 to 9.
The second scheme involving reduction of the excited state photosensitizer PS'
by D is called the "reductive quenching mechanism" (Fig. 1b). This primary
reaction (reaction 10) yields the
(10)
reduced photosensitizer PS - and the oxidized donor D + which decomposes
irreversibly (reaction 9). In this way, PS - can accumulate and react with an
electron relay R to regenerate PS and to yield R - (reaction 11).
(11)
In the presence of a suitable catalyst, R - can lead to the formation of hydrogen
as in the first scheme (reaction 6).
It should be remarked that PS - is a more powerful reducing species than R -.
Hence, the reduction of water to H2 can be achieved directly by PS- itself in the
presence of a suitable catalyst. As a consequence, this scheme involves only three
components (PS, D, Cat) and the mechanism becomes simplified (Fig. 2).
E. Amouyal / Solar Energy Materials and Solar Cells 38 (1995) 249-276
253
0+
Fig. 2. Schematic representation of the redox catalytic cycles in the photoreduction of water to H 2, via
reductive quenching mechanism, for a three-component model system PS/D/Cat.
Similar three-component systems for O 2 production from water have been
proposed (Fig. 3). These systems require the formation, following visible-light
excitation of the photosensitizer PS, of a strong oxidizing species PS +, having a
redox potential EO(PS+ IPS) greater than 0.82 V (versus NHE, pH = 7). This can
be achieved by using an electron-acceptor A as quencher which, once reduced to
A - (reaction 12), decomposes irreversibly (reaction 13).
(12)
(13)
A - - decomposition products.
The oxidized PS + can thus accumulate and lead to oxygen evolution in the
presence of a suitable catalyst capable of facilitating the exchange of 4 electrons
with water (reaction 14).
(14)
/hV"'
~X02
A~PS"S '" 0
"2
A'
Fig. 3. Schematic representation of the redox catalytic cycles in the photooxidation of water to oxygen
by visible-light irradiation of a three-component model system PSI A/Cat.
254
E. Amouyal j Solar Energy Materials and Solar Cells 38 (1995) 249-276
o
Fig. 4. Schematic representation of the redox catalytic cycles in the photoreduction of water to H 2 , via
energy transfer (antenna effect), by visible-light irradiation of a five-component model system
PSjR.n /R/D/Cat.
Another approach (Fig. 4) consists in using the photosensitizer PS as an
antenna and transferring the excitation energy to a receptor molecule Ren (reaction 15).
(15)
+PS + R: n.
The receptor can subsequently react with the electron relay R via electron
transfer (reaction 16)
PS' + Ren
~
R:n+R~R:n+R-
(16)
to give a charge pair (R:n, R - for example). The reduction of water to H2 can be
achieved in the presence of a sacrificial electron-donor D and a suitable catalyst
(Fig. 4) as in the first scheme (Fig. la). In this five-component system
PS/Ren/R/D/Cat, the energy-transfer photosensitizer PS is not involved in any
redox processes as are the antenna molecules in natural photosynthesis, and the
receptor Ren acts as an energy-electron relay.
It should be noted that, although the multimolecular approach is the simplest
manner to achieve cyclic photochemical water cleavage, its accomplishment necessitates overcoming several difficulties. Indeed, the different components of such
systems must fulfil spectral, photophysical, thermodynamic and kinetic conditions.
Some of them have been mentioned here. The other requirements can be found in
earlier comprehensive reviews [3-6,8-10].
2.3. First model systems for H2 production
Several sacrificial model systems of H2 production from water have been
proposed since 1977. The first ones are listed in Table 1. They used acridine dyes
such as acridine yellow [11] as PS. But transition metal complexes, in particular
E. Amouyal / Solar Energy Materials and Solar Cells 38 (1995) 249-276
255
Table 1
First model systems for water photoreduction to hydrogen
System No. PS
1
2
3
4
Acridine Yellow
[Ru(bpY)3]2+
[Ru(bpY)3]2+
[Ru(bpY)3]2 +
Reference
R
D
Cat
Eu 3+ or y3+
[Rh(bpY)3P+
MV 2 +
My2+
Cysteine
TEOA
EDTA
TEOA
Pt0 2
Shilov group 1977 [11]
Lehn group 1977 [12]
K 2 PtCI 6
Colloidal Pt (PYA) Orsay groups 1978 [7]
Pt0 2
Gratzel group 1978 [13]
[Ru(bpY)3J2+ [7,12,13], then appeared to be remarkable photosensitizers with
respect to visible light absorption, excited state properties, redox potentials and
kinetic requirements [9]. The electron relay species first investigated include Eu3+
and y3+ salts [11], a transition metal complex [Rh(bpY)3P+ [12] which can transfer
two electrons, and methyl viologen MYz+ the most commonly used electron relay
[7,11,13]. Cysteine [11,13] and especially tertiary amines such as EDTA (ethylenediamine tetra-acetic acid) [7,11] and triethanolamine TEOA [11-13], which are
rapidly decomposed when oxidized, were used as sacrificial electron-donors. Platinum compounds [7,11-13] turned out to be suitable catalysts.
The first system (Table 1, system 1) has been described by Shilov et al. [11]. It
consists of acridine yellow A Y as PS, cysteine as sacrificial electron-donor, salicylate complexes of Eu3+ and V3+ as R and Adams' catalyst (PtO z) as Cat. They
also used EDTA, TEOA or HzS as D, MVz+ as Rand K zPtCl 6 as catalyst. The
mechanism assumed was of the "reductive" type (Fig. 1b), and the quantum yield
of Hz production in the case of AY /Eu 3+ /cysteine/PtO z model system was of
the order of 1%. This quantum efficiency was too low so that the validity of the
system was questioned at that time. Indeed, in the early 1970s, binuclear metal
complexes were considered as more promising candidates for PS than organic
compounds [14]. The two following systems, that of Lehn and Sauvage [12] (Table
1, system 2) and ours [7] (Table 1, system 3) used a metal complex, [Ru(bpY)3]2+, as
PS. The hydrogen quantum yields 0 (1/2 Hz) were much higher (> 10%) than
that of Shilov's system [11]. Consequently, it was easier to produce and characterize Hz, and even to detect the formation of Hz bubbles with the naked eye. In
addition, in the case of the Orsay system (Table 1, system 3), we have clearly
established the oxidative quenching mechanism for Hz production (Fig. 1a) by
laser flash spectroscopy [7,15]. So it is not surprising that these two systems [7,12]
which were presented at the International Conference on Photochemical Conversion and Storage of Solar Energy OPS-2) in 1978 at Cambridge [16], contributed
largely to convince the most sceptic "solar experts" of the interest of the multimolecular approach. These results were rapidly reproduced by many other laboratories.
It should be noted that in the Shilov system [11] and in the Lehn system [12], it
was assumed that Pt particles are formed in situ through the photosensitized
reduction of K zPtCI 6 , while in our system [7] it was demonstrated for the first time
that colloidal metals (Pt,Au), chemically prepared and stabilized by polymers
Bipyridinium ions:
Metal complexes of Ru, Cr, Os,
Ir, Pt ... :[Ru(bpY)3J2+
Metal porphyrins of Zn, Mg, Ru ... :
ZnTMPyp 4 +
Metal phtalocyanines of Zn, Co, Mg ...
MV 2 +
Cat
Enzymes: hydrogenase, nitrogenase
Supported metals: Pt-Ti0 2 , Rh-SrTi0 3 •
Ni-Ti0 2
Metal oxides: Ru0 2 , Pt0 2 , Ir0 2 ,
Pd0 2 , Ti0 2 , Fe 2 0 3
Supported metal oxides:
Ru0 2 + Ir0 2 /zeoJite
Colloidal metal systems: Ni-Pd
Metal powders: Pt, Ru, Ni
EDTA and glycine derivatives (NPG) Colloidal metals: group VIII metals
(Ir, Pt,NL),Au,Ag
Amines: TEOA, TEA.
Pt salts: K 2 PtCI 6 , K 2 PtCI 4
D
Metal ions: Eu 3 +, V 3 +, Cr 3 + Sulphur compounds: cysteine, thiols
(mercaptoethano\), H 2 S
Acridine dyes: acridine yellow, proflavine. Metal complexes of Rh, Co ... : Urea derivatives: allythiourea
[Rh(bpY)3]3+, [Co(Sep)]3+
Proteins: cytochrome c3
Xanthene dyes: fluorescein,
Amino acids
eosin Y
Cyanine dyes
Carbon compounds: ascorbate,
ethanol
Metal ions: Eu 2 +
Organic compounds:
poly(pyridine-2,5-diyJ)
Coenzymes: NADH, NADPH
Phenanthrolinium ions
R
PS
Table 2
Components of H 2-generating microheterogeneous systems
E. Amouyal / Solar Energy Materials and Solar Cells 38 (J995) 249-276
257
(polyvinyl alcohol PYA), can be used successfully as catalysts in a photochemical
model system.
Gratzel et al. [13] described at the same time a system (Table 1, system 4),
similar to the Orsay system (Table 1, system 3) [7,15], but using platinum oxide
Pt0 2 (Adam's catalyst) instead of colloidal platinum, and triethanolamine TEOA
or cysteine instead of EDTA. With these components (Pt0 2, TEOA) [13], the H2
yields are much lower than those obtained by visible-light irradiation of the Orsay
system [15,17].
2.4. Components of water photoreduction systems
It is hardly possible to give an exhaustive list of all the multimolecular systems
of H2 production from water described in the literature and based on the general
schemes (Figs. 1,2,4). However, I have categorized in Table 2 the different
constituents used in these systems as PS, R, D and Cat, and I shall describe only a
few systems, in particular the Orsay system.
2.4.1 Photosensitizers
Transition metal complexes of Ru, Cr, Os... [18,19], metalloporphyrins and
metallophthalocyanines [19,20] and acridine dyes [11,21-23] are the principal
classes of PS, [Ru(bpY)3]2+ being the most investigated. This complex is essentially
involved in the oxidative quenching mechanism. For the reductive mechanism, one
of the best candidates as PS is [Ru(bpz)3]2+ (bpz = 2,2'-bipyrazine) [24,25]. Hydrogen is produced with good yields when it is used in a photochemical system with
TEOA as D, My 2+ as R and a platinum compound as catalyst [26,27]. More
recently, we have shown that [Ru(bpY)2dppz]2+ (dppz = dipyrido [3,2-a:2',3'c]
phenazine) can be involved as PS either in an oxidative mechanism, with EDTA as
D, or in a reductive mechanism, with TEOA as quencher [28]. Interestingly, a rigid
copper (I) complex [Cu(dpp)2]+ (dpp = 2,9-diphenyl-1,1O-phenanthroline) has been
used as energy-transfer photosensitizer (Fig. 4) in a five-component system [29]. It
should be noted that high quantum yields for H2 formation with an optimum
cJ>(l/2 H 2) = 0.6 [30] have been found when aqueous solutions of a water-soluble
zinc porphyrin, Zn (II) tetrakis (N-methyl-4-pyridyI) porphyrin ZnTMPyp 4 +
[20,30-32], are irradiated with 550 nm light in the presence of My2+, EDTA and
colloidal Pt. However, these yields decrease dramatically within an irradiation time
of 4 hours [20,30]. More recently, cage complexes [33,34], cyanine dyes [35] and
organic compounds absorbing visible light such as poly{pyridine-2,5-diyI) [36] have
been tested as PS.
2.4.2. Electron relays
Bipyridinium ions, also called viologens, are the main compounds used as
electron relays R (Table 2), the most popular being methyl viologen My2+. They
also provide an extended range of redox potentials [37]. We have investigated
several homogeneous series of 2,2'-bipyridinium (or diquat), 4,4'-bipyridinium (or
paraquat) and 1,1O-phenanthrolinium ions as mediators for water photoreduction
258
E. Amouyal / Solar Energy Materials and Solar Cells 38 (1995) 249-276
[37-39], the most efficient electron relays being MYz+ and I,Y-dimethylene-4,4'dimethyl-2,2'-bipyridinium ion [37,38]. Transition metal complexes are also interesting mediators, in particular [Rh(bpY)3]3+ which can transfer two electrons
[12,40,41] and cage complexes such as [Co(sep)]3+ (sep = sepulchrate) [42,44].
[Co(sep)]3+ is, contrary to viologens, insensitive to hydrogenation, an undesired
side reaction which can occur at the catalyst surface. A natural electron mediator,
cytochrome c 3, which unlike My2+ is not at all toxic, has been tested in model
systems in association with hydrogenase as catalyst [23,45]. It is of interest to note
that the only compound used as energy-electron relay Ren in a five-component
system (Fig. 4) is 9-carboxylate anthracene anion [29,46].
2.4.3. Electron-donors
Krasna [21] thoroughly tested several classes of organic compounds as electrondonors D (Table 2), with proflavine as PS, MYz+ as R and the enzyme hydrogenase or Pt asbestos as Cat. With this model system, the most effective donors were
EDTA and 1,2-diaminocyclohexane tetra acetic acid.
Whitten et al. [47] found that triethylamine TEA which is not at all effective in
Krasna's systems, leads to high Hz yields (0.53) in a three-component system
[Ru(bpY)3]2+/TEA/PtO z, i.e., in the absence of MYz+, but in 25% wateracetonitrile mixtures. Coenzymes such as NADH and NADPH have been tested as
sacrificial electron-donors [45,48]. However, the photoinduced regeneration of
these natural reductants can also be achieved [48,49]. In photochemical systems
involving a sacrificial electron-donor, it is interesting, from a practical point of
view, to find and use electron-donors such as HzS [48] which are easily available as
waste industrial products.
2.4.4. Microheterogeneous catalysts
As regards the catalyst (Table 2), and since the first report on the catalytic
activity of colloidal Pt in a photochemical system of Hz production [7], we have
investigated systematically colloidal metals (of groups YIII and IB essentially),
metals deposited on solid supports (semiconductor, zeolite), metal and metal oxide
powders [50,51]. Iridium and platinum hydrosols are extremely efficient [50,51]. Pt
supported on TiO z [27,50-52] leads to similar high Hz yields. We have found that
ruthenium oxides, known to be good catalysts for 0z generation from water
(Section 2.6), efficiently mediate the photoreduction of water to Hz without
catalyzing the undesired hydrogenation of the electron relay [53,54]. Ru0 2 and
IrOz codeposited on zeolite give the highest H2 yields [50]. More recently, we have
observed an improvement of catalytic activity by an alloying effect in the case of
bimetallic sols of Ni-Pd [55]. It is of interest to note that the enzyme hydrogenase
which contains at least Fe 4 S4 -type clusters has been used as a natural catalyst
[21,23,45] but it is unstable and the H2 yields are lower than those obtained with
Pt compounds [21].
2.4.5. Homogeneous catalysts and homogeneous systems for H2 production
The great majority of catalysts described in the literature are heterogeneous.
Very few homogeneous catalysts and hence homogeneous systems for H 2 produc-
E. Amouyal / Solar Energy Materials and Solar Cells 38 (1995) 249-276
Table 3
Components and quantum yields for H 2-generating homogeneous systems
D
Homogeneous catalyst
pH
PS
Eu2+
[Ru(bpY)3]2+
[Ru(bpY)3]2 +
ascorbate
[Ru(bpY)3]2+
EDTA
[Ru(bpY)3]2+
TEOA
[Ru(bpY)3]2+
ascorbate
[Ru(bpY)3]2+
TEOA
[Ru(4,7-(CH 3)2phen)3]2+ TEOA
a
b
[Co(Me 6[14]diene N4 XH 2O)2]2+
[Co(Me6[14]diene N4 XH 2O)2]2+
[Rh(bpY)3P+
[Rh(bpY)3P+
[Co(bPY)nJ2 +
[Co(dimethylglyoxime)2]
[Co(bpY)3]2+
S
S
S.2
259
cfJ(H 2)
Reference
O.OS
0.0005
0.04 [41]
0.02
0.03
1979 [S6]
1979 [S6]
1979 [40]
1981 [41]
1981 [57]
1983 [S9]
1985 [58]
S
S
8.7 •
0.29
8
b
In DMF /H 20 or in neat organic media (DMF, acetone, acetonitrile ... ).
In SO % aqueous acetonitrile.
.
tion (Table 3) have been reported (see Section 2.6 for homogeneous catalysts for
O 2 production). In these systems, following visible-light excitation of the photosensitizer PS, one of the components is transformed into an unstable intermediate, for
instance a metal hydride, which in turn decomposes to yield Hz and the starting
component. Therefore, this component acts as homogeneous catalyst. Good candidates are inorganic compounds, such as homogeneous hydrogenation catalysts,
which have a metallic site able to present different oxidation states during the
catalytic cycle, and which can form an intermediate hydride, unstable in solution,
to provide a pathway for H2 release. Homogeneous catalysts (Table 3) which have
been first reported in 1979 are a macrocyclic cobalt (II) complex, [Co(Me 6[14]diene
N4 ) (H 20)2]2+ [56], and [Rh(bpY)3]3+ [40,41]. Other CoOI) complexes have been
proposed viz. [Co(bpY)3]2+ [57,58], cobaloxime [59] and other macrocyclic complexes [59]. These homogeneous systems (Table 3) consist of three components, the
photosensitizers PS being essentially [Ru(bpY)3]2+ [40,41,56,57,59] or [Ru(4,7(CH 3)2 phen)3]2+ (phen = 1,1O-phenanthroline) [58], and the electron donors
being EDTA [40,41], Eu(II) [56], ascorbate [56,57] or tertiary amines like TEOA
[41,58,59]. As in the case of microheterogeneous systems, the mechanism of H2
production involves either a reductive [56,57,59] or an oxidative [40,41,58] quenching of PS '. These systems are efficient, especially in organic media [58,59]. In the
case of the [Ru(4,7-(CH3)zphen)3]2+/TEOA/[Co(bpY)3]2+ system proposed by
Sutin et al. [58], the Hz yields increase from about 0.02 in HzO to 0.29 in 50%
CH 3CN-H 20 (Table 3).
2.5. The [Ru(bpY)3P + / MV 2 + / EDTA / colloidal Pt model system for
from water
H2
generation
The classical system proposed in 1978 for the first time by the Orsay groups
[7,15] (Table 1, system No.3) comprises [Ru(bpY)3]2+ as PS, My2+ as R, EDTA as
D, and colloidal Pt as catalyst [7,15] (Fig. 5). It has been known since 1934 [60] that
electron exchange between My 2+ and H2 is catalyzed by Pt. But for the first time,
it has been proved that catalyst hydrosols [61] used in a model photosystem [7]
mediate visible-light H2 generation from water. In previous studies [11,12] the
E. Amouya/ / Solar Energy Materials and Solar Cells 38 (J995) 249-276
260
products -
EOTA+
EOTA
Fig. 5. Schematic representation of the redox catalytic cycles in the photoreduction of water to H2 by
visible-light irradiation of the Orsay model system [Ru(bpY)3j2+ /MV2+ /EDTA/colloidal Pt proposed
by Moradpour et al. [7].
formation in situ of such colloids had been assumed through the reduction of Pt
salts. This system produces Hz very efficiently, leads to reproducible results, and is
well characterized as regards Hz formation quantum yields [50,62] and detailed
mechanism [15,37]. These are the reasons why this system has been thoroughly
studied by several groups [17,26,52,63-70] and is still considered as a reference for
testing new PS, R, D and catalysts (Table 2), and for evaluating solar photochemical reactors on a pilot level [71].
The system has been described in many reviews and books. I just wish to recall
some of its important features. When an aqueous solution containing [Ru{bpY)3F+,
My2+, EDTA, and colloidal Pt is irradiated with visible light (400 nm < A < 600
nm), an important Hz evolution is observed (Fig. 5) according to the following
mechanism established from laser flash spectroscopy experiments [7,15]. In deaerated solutions, the main reactions are:
[
12 +' ,
[Ru(bpYh 12+ hu
~ Ru(bpYh
[Ru(bpYh1
[Ru(bpYh1
z+' ko
~ [Ru(bpYh1
z+'
kq
2+
(17)
(18)
'
+ My 2 + ~ [Ru(bpYh1
3+
+ MY+-,
kb[ Ru(bpYh 12+ + MY 2+,
[Ru(bpYh 13+ + MY+- ~
[Ru(bpYh ] 3+ +
kox[
EDTA~
Ru(bpYh 12+ + EDTA +,
(19)
(20)
(21)
(22)
Net: EDTA + H+
hu Pt
~
EDTA ++ lj2H 2.
(23)
E. Amouyal / Solar Energy Materials and Solar Cells 38 (J995) 249-276
261
The proposed mechanism for EDTA degradation (reaction 24)
EDTA + ~ products (mainly glyoxylic acid) ,
(24)
and for the catalytic process on metallic particles (reaction 22) have been described
in detail in previous reports [15,37,51,72].
Besides the EDTA consumption (reaction 24), difficulties arise from undesirable reactions such as MY+ dimerization [72] (reaction 25) and the irreversible
2MY+'~
(MY+'h
(25)
hydrogenation of methyl viologen [15,37,72] (reaction 26)
(26)
The following rate constant values, as determined by flash photolysis [15] are in
good agreement with published ones [65]:
ko = 1.45 X 10 6
S-I
kq = 1.03 X 10 9 M- 1 S-I (for !-L
kb
= 0.018 M and dried MYz+)
= 2.8 X 10 9 M- 1 S-I
k ox = 1.1 X 10 8 M- 1 S-I
We have shown [37] that kq increases with the ionic strength p. of the solution
[73] with kg = 2 X 10 8 M -I S -1 extrapolated for zero p. [37].
In non-deaerated solutions, Hz formation rates and yields decrease due mainly
to reaction 27 with k = 8 X 10 8 M- 1 S-I [74].
(27)
This reaction leads to HzOz as a stable intermediate and to MYz+ degradation
products [75,76].
02" +H+~HOi,
(28)
2HOi ~ 0z + HzOz,
(29)
MY+'+ 02"(HOi)
~
MYO z ~ products.
(30)
The mechanism shows that Hz production depends on light intensity, pH and
concentration of the four constituents of the system. An optimum quantum yield
<PU/2 Hz) = 0.171 was found for the following optimized concentrations: pH = 5,
[Ru(bpY)3]2+ 5.65 X 10- 5 M, MYz+ 3 X 10- 3 M, EDTA 0.1 M and colloidal Pt
1.92 X 10- 5 M [50]. For the same optimized conditions but in the absence of Pt,
the MY+' quantum yield was <P(MY+') = 0.181 in good agreement with the value
calculated from the relationship (31)
(31)
provided that the cage-escape efficiency <Pee = 0.30 [50]. (<PT is the quantum yield
for intersystem crossing, <Pq is the efficiency of the quenching reaction 19, <Pee is
262
E. Amouya/ / Solar Energy Materials and Solar Cells 38 (1995) 249-276
the efficiency of net formation of [Ru(bpY)3]3+ and MY+' from the solvent cage
before the ions undergo back electron transfer [77]). We have also established that
the irreversible hydrogenation of MY+' (or My2+) catalyzed by colloidal Pt
(reaction 26) is a main limiting factor to the longevity of the system [15,78,79].
The catalytic turnover numbers TN for H2 production, which reflect the
stability of each constituent of the system, and particularly My2+, are for example
[15]:
TN([Ru(bpyh]2+) > 290
TN(My2+) = 115
TN( colloidal Pt) > 2900
Effectively, H 2 generation stops when the viologen is completely destroyed via
hydrogenation. Meanwhile [Ru(bpY)3]2+ is slightly decomposed and colloidal Pt
remains apparently intact. Thus these calculated TN values are lower limits of the
true TN for [Ru(bpY)3F+ and Pt. Indeed, an extrapolated TN [40] of 6000 has
been found [64a] for [Ru(bpY)3]2+. It should also be stressed that these TN were
not determined under optimal experimental conditions for H 2 production [78].
The inhibition of the catalytic hydrogenation is thus of major importance. Such an
undesired reaction may be prevented (i) by finding electron relays whose structure
would be less sensitive to hydrogenation such as HMy 2+ (l,1',2,2',6,6'-hexamethyl4,4'-bipyridinium ion) [80,81] and [Co(sep)]3+ [42-44], (ij) by using more specific
catalysts such as Ru0 2 [53], and/or (iii) by adding to the solution hydrogenation
poisons such as sulphur compounds, glutathione GSH in particular [50,51,64].
Indeed, in the presence of GSH, we have shown that colloidal Pt is operating with
100% efficiency. In particular, for [Pt] > 1.92 X 10- 5 M, we found that:
<I>(1/2)H2) = <I>(MY+') = 0.18.
Not only H2 formation rates but also H2 amounts are considerably improved in
the presence of a hydrogenation poison. With GSH, the My 2+ turnover reaches a
value TN(My 2+) = 750 [81].
In
a
five-com ponent
modified
version
(Fig. 4)
[Ru(bpY)3]2+ / AC-/My2+ /EDTA/colloidal Pt using 9-carboxylate anthracene
anion AC- as energy-electron relay Ren, Sasse et al. [46]b have achieved, to my
knowledge, the highest quantum yield for H 2 production [46]b,[72]:
<I>(1/2H2)
= 0.85.
This result reflects a cage escape efficiency epee (Eq. 31) of about 100% for the
charge pair (AC·, MY+')in reaction 16. Recently, a high turnover number TN
(MSPY) = 358 has been calculated for a zwitterionic viologen MSPY (3,3'-dimethyl-I,!'-bis(3 sulfonato-propyI)-4,4' -bipyridinium) in a modified system
[Ru(bpY)3]2+ /MSPY /TEOA/Ti0 2-Pt at pH = 7.6 [27]. This result reinforces the
interest of using microheterogeneous catalysts such as Ti0 2-Pt which was found to
be as efficient as colloidal Pt for H 2 generation from water (Table 2 of Ref. [50]).
E. Amouyal / Solar Energy Materials and Solar Cells 38 (1995) 249-276
263
2.6. Model systems for O 2 production
Contrary to half photosystems of Hz production, the number of model systems
of water photooxidation is much more limited. The first model systems for 0z
production from water by visible-light irradiation have been proposed in 1979 by
Lehn et al. [82] and a little later by Kalyanasundaram and Gratzel [83]. Also based
on the multimolecular approach, they consist of three components including
[Ru(bpY)3]2+ as PS, [Co(NH 3)sCl]z+ as sacrificial electron-acceptor, and RuO z as
catalyst. According to the general scheme (Fig. 3) the main reactions [32-35]
leading to 0z evolution are the following:
]z+· ,
[Ru(bpYh] Z+hU[
--+ Ru(bpyh
(32)
[Ru(bpyh]2+' + [Co(NH3)sCI]z+ --+ [Ru(bpYh]3+ + [Co(NH3)sCI] +,
(33)
(34)
(35)
The mechanism involves an oxidative quenching of the excited state of
[Ru(bpY)3]2+ by a Co(II1) complex, [Co(NH 3)sC!F+ (reaction 33). The rapid and
irreversible aquation of [Co(NH 3)sCI]+ (reaction 34) allows the accumulation of
[Ru(bpY)3P+ which is an oxidant strong enough to oxidize water to 0z in the
presence of a suitable redox catalyst like RuO z (reaction 35).
Modified versions of this system have been proposed with practically only
[Ru(bpY)3]2+ as PS because it is the most efficient, although [Ru(bpz)3]2+ [84] and
metalloporphyrins such as ZnTMPyp 4 + [85] also appear suitable for 0z generation. However, several sacrificial electron-acceptors have been examined including
[Co(NH 3)sCI]2+ [82-84,86-88], [Co(NH 3)sBr]2+ [83], [Co(C Z0 4 )3P- [83], T1 3+
[83,87], SzO~- [84,87-89], Ag+ [85,90], Fe 3+ [85,91] and a Mn (IV) pyrophosphate
complex [92]. Because redox catalysts represent an important key to the water
cleavage problem, they were investigated even more widely. Thus catalysts which
are active in photochemical systems include RuO z powder [82,83], colloidal RuO z
[83], RuO z supported on alumina [82] or TiO z [85], TiO z [85], CoS0 4 [88] and
Pruss ian Blue [93,94]. Quantum yields for 0z generation from the irradiation of
aqueous solutions of [Ru(bpY)3]2+ j[Co(NH 3)sCI]z+ jcatalyst systems at pH = 5
were found to be 0.03, 0.02 and 0.003 with colloidal RuO z, CoS0 4 and RuO z
powder respectively [88]. Lehn et al. [95] and Harriman et al. [96] tested several
metal oxide catalysts (RuO z, Ir0 z , PtO z MnO z, Mn z0 3 , Rh z0 3, C0 3 0 4 ,
NiCo z0 4 ). Iridium and ruthenium oxides supported on Y zeolites are much more
efficient than RuO z powder or RuO z supported on y-alumina and silica, the most
efficient being a mixture of IrOz and RuO z deposited on Y zeolite [95]. Pure
RuO z (stoichiometric) and PtO z are inactive for 0z generation [95]. It is of
interest to note, because of the involvement of Mn ions in the oxygen-evolving
264
E. Amouyal/ Solar Energy Materials and Solar Cells 38 (1995) 249-276
center in greenplant photosystem II, that MnO z [92,95], Mn Z0 3 [96] and a Mn(lV)
cluster bound to a lipid vesicle [97] are also efficient catalysts. Shafirovich et al.
[86] have claimed that visible-light irradiation of aqueous solutions of a two-component homogeneous system [Ru(bpy)~+ /[Co(NH 3)s0]2+ resulted in continuous
Oz generation at pH = 7. They assumed that [Co(H ZO)6]2+, the hydrolysis product
of [Co(NH 3)sO]+, acts as a redox catalyst. However, other laboratories [88,98] did
not observe any photochemical evolution of oxygen, although it has been reported
that hydroxo complexes of Co(ll), Fe(II), Fe(III), Ni(II) and CuOI) are active
catalysts for non-photochemical generation of Oz from water in alkaline solutions
using [M(bpY)3J3+ (M = Fe, Ru, Os) or (lr06)Z- as oxidants [86,99].
In a different approach, Jl.-oxo dinuclear complexes of transition metals have
been examined not only for their possible activity as homogeneous catalysts but
also because they may provide models for the oxygen-evolving site of photosystem
II in natural photosynthesis. For this purpose, the di-Jl.-oxotetrakis (2,2'-bipyridyl)
dim anganese (I1I,IV) perchlorate
[(bpyhMnIII(J.L - 0hMnIV(bpyh]3+ ,3004
has been proposed as model system by Calvin [100]. But, contrary to this first
report, Cooper and Calvin [101] did not succeed in producing Oz photochemically.
However, it was shown that dinuclear complexes of Mn as well as Jl.-oxo diruthenium complexes like
[(bpyh(HzO)RuIIIORuIII(HzO)(bpyh] 4+
first examined by Meyer et al. [102], are good catalysts for Oz generation from
water in a non-photochemical process [102-104]. It was postulated that Oz formation involves higher oxidation states of ruthenium, probably a Ru(V), Ru(V)
dinuclear complex [102,103]. More recently, similar water soluble Jl.-OXO dinuclear
complexes were proposed as homogeneous catalysts for thermal water oxidation
[105]. Moreover, photoinduced generation of oxygen from water was also observed,
according to the general scheme (Fig. 3), by using these complexes as catalysts,
Ru(II) tris(4,4'-dicarbethoxy-2,2'-bipyridine) as PS and SzO~- as a sacrificial
electron-acceptor [105].
Finally, I would like to emphasize another interesting approach which involves a
restricted geometry such as zeolite cages [106-109]. Calzaferri et al. [107-109] have
shown that irradiation of silver zeolites A dispersed in aqueous solution leads to
the reduction of silver cations Ag+ and to the production of oxygen with high
yields. The process may be schematically represented by reaction 36.
hu
(Ag+ ,HZO)zeolite -+ (AgO,H+)zeolite + Oz·
(36)
The system which is initially insensitive to visible light needs to be illuminated
with near UV-light (about 370 nm) before becoming active in the visible up to
about 600 nm. Clearly, Oz is produced by self-sensitization. This phenomenon is
interpreted as a quantum size effect. It can be explained by the photoinduced
formation of partially reduced silver clusters (probably Ag6'+' with m < 6, in
E. Amouyal/ Solar Energy Materials and Solar Cells 38 (1995) 249-276
265
zeolite A) which absorb at longer wavelength and act as new chromophores. More
recently, similar results have been obtained with mono grain layers of silver zeolite
A [109].
3. Supramolecular systems
In the multimolecular approach, the lifetime of the charge pair produced
initially (reaction 5), following light excitation, is very short because of the
occurrence of a back electron-transfer reaction (reaction 7). It results that the
charge recombination is too rapid to let the ions react with water at the catalyst
surface. In all the homogeneous and microheterogeneous systems with three, four
or five components described before, efficient charge separation is achieved by
means of an electron donor which is "sacrificed". To overcome this problem,
another approach has been explored which consists in the utilization of
supramolecular structures. Excellent reviews describing photoactive supramolecular systems for photoinduced charge separation appeared recently [110-114]. In
this section, I shall consider only a few supramolecular systems, in particular
among those involving several components covalently linked to one another. These
systems are called dyad, triad, tetrad, pentad ... according to whether the number of
components is two, three, four, five ... respectively. I shall use the generic term of
polyad for these muiticomponent molecular systems.
To my knowledge, the systems proposed by Gust, Moore et al. [115] and by
Nishitani, Mataga et al. [116] in 1983 are the first triads. In the triad of Gust and
Moore, of D-PS-A type, a donor D (carotenoid Car) and an electron acceptor A
(benzoquinone BQ) are covalently linked to a photosensitizer PS (porphyrin P),
whereas in the triad of Nishitani and Mataga, of PS-AI-A2 type, the photosensitizer PS (porphyrin P) is covalently linked to two successive acceptors At and A 2,
with A2 (trichlorobenzoquinone CIBQ) being a stronger electron acceptor than At
(benzoquinone BQ). Light excitation of PS leads to the formation of charge-separated states D+-PS-A - (reactions 37) and PS+-AcAi" (reactions 38), respectively, via two consecutive intramolecular electron transfer
hu
D-PS-A--+D-PS*-A--+D-
e
--+
PS+-A -
--+
e
-+
-AD+-PS
(37)
steps. In Table 4, different polyads are listed with the corresponding lifetimes 'Tcs
and quantum yields of formation <Pes of charge separated states, as well as the
energy stored Est. When the components are linked by flexible chains, as in the
case of the triad of Nishitani and Mataga [116], the lifetime 'Tes is very short (100
ps). With more rigid spacers, i.e., when there is a smaller chance for the oxidized
donor and the reduced acceptor to be in contact, lifetimes in the nanosecond and
microsecond time-scales have been determined (Table 4). The triad of Gust and
266
E. Amouyal / Solar Energy Materials and Solar Cells 38 (]995) 249-276
Table 4
Lifetimes Tes and quantum yields of formation of charge-separated states <Pes and energy stored Est for
several polyads
Polyad
Solvent
Tes
<Pes
Est (eY)
Reference
Car-P-BO
CH 2CI 2
butyronitrile
CH 2CI 2
CH 3CN
CH 2 CI 2
dioxane
300ns
2 Jl.s
460 ns
4 Jl.s
340 j.LS
300ps
400ps
100 ps
2.45 j.LS
150 ns
6.4 j.LS
160 ns
108 ns
174 ns
0.04
1.1
[115)
Car-P-NO-BO
Car-PJ-Pz-NO-BO
P- BO-CIBO
P-BO
DMA-P-NO
P -(My 2+)2
p-(My2+)4
(PTZ)2 - [Ru(bpY)3j2+ - D02+
lysine - ([Ru(bpY)31 2+, PTZ, My2+)
lysine - ([Ru(bpY)3j2+, PTZ, AO)
THF
butyronitrile
DMSO
DMSO
CH 2CI 2
CH 3CN
CH 3CN
0.23
[118)
0.15
[119)
[116)
0.71
1.39
0.33
0.26
0.34
0.26
1.29
1.17
1.54
[117)
[120)
[121)
[122)
[123)
[124)
[125)
Moore [115], which presents a long lifetime T es = 2 ~s in butyronitrile, played a
great part in the development of the supramolecular approach for solar energy
conversion and for the design of molecular systems that mimic the functions of
natural photosynthesis. Wasielewski et al. [120] obtained the best charge separation data with a rigid triad dimethylanilineporphyrin-naphthoquinone (DMA-PNQ) using triptycene as rigid spacers. In butyronitrile, the charge-separated state
is formed with a quantum efficiency of 71 % and it lasts 2.45 ~s, the energy stored
being estimated at 1.4 eV (Table 4). Table 4 also shows, in particular from the
works of Gust and Moore [111,115,118,119], that Tes can be increased by increasing
the number of components of the polyad. Thus a lifetime of 340 ~s has been
measured in CH 2Cl 2 in the case of a pentad carotenoid-porphyrin-porphyrinnaphthoquinone-benzoquinone [111,119]. Also recently, Meyer et al. [124,125]
proposed a new interesting type of polyad in which a photosensitizer {Ru{bPY)3f+,
an electron-donor (phenothiazine PTZ) and an electron-acceptor (My2+ [124] or
anthraquinone AQ [125]) are attached to an amino acid (L-Iysine). Although these
polyads are flexible, lifetimes of the order of a hundred nanoseconds (Table 4)
have been obtained for the charge-separated states.
Several supramolecular systems exhibiting efficient charge separation have been
proposed, but few of them have been tested for photochemical H2 or 02 production from water [35,114,126-128]. For instance, Okura et al. [126] synthesized a
series of dyads involving a porphyrin and My2+ covalently linked by flexible
chains of different length ({CH 2)n, n = 2-6). Visible-light irradiation of these
dyads in the presence of NADPH as electron donor and colloidal Pt or hydrogenase as catalyst gives rise to H2 evolution only in the case of the dyad with the
shortest chain (n = 2). It is of interest to note that, although the system only works
with UY light (A < 400 nm), a dyad proposed by Mau et al. [127,128] and
consisting of anthracene as PS and a Co (III) cage complex as electron relay, leads
E. Amouyal / Solar Energy Materials and Solar Cells 38 (]995) 249-276
267
to H 2 generation in the presence of EDTA as D and colloidal Pt as catalyst. High
H2 quantum yields CPU/2H 2) = 0.14 were determined at pH = 6.5 [128]. More
recently, Konigstein and Bauer [35] proposed a system involving a cyanine~MV2+
dyad which can form J-aggregates. When this dyad is illuminated by visible light
(A> 400 nm) in the presence of EDTA and a platinum catalyst, H2 is generated.
H2 yields are low ( < 1 %) and optimum for a J-aggregate containing two cyaninedye molecules per one cyanine-My 2+ dyad [129]. In this system, J-aggregates act
as an antenna allowing light harvesting and migration until the excitation energy
reaches the dyad. This induces a primary electron transfer reaction which leads to
an efficient intramolecular charge separation. According to Konigstein and Bauer
[35], charge migration and charge distribution in the J-aggregate are probably
responsible for the retardation of the back reaction.
4. Constrained / confined systems
The system of Konigstein and Bauer [35] uses the property of some cyanine dyes
to form spontaneously J~aggregates, in other words organized molecular systems.
Indeed the utilization of organized assemblies, constrained and confined media or
the combination of these media with, for instance, supramolecular systems, consti~
tutes other approaches which have attracted much interest with a view to retarding
the back electron~transfer reaction. I shall limit this overview to a few examples of
model systems in constrained media such as sol-gel silica glasses and zeolites.
Concerning photochemical conversion in organized media (micelles, vesicles,
monolayers, bilayers, Langmuir-Blodgett films, polymers, polyelectrolytes ... ), the
reader is referred to papers and review articles that appeared in the last few years
[5,112,130], Slama~Schwok et al. [131] described a two~component donor-acceptor
system consisting of an Ir (III) complex, [Ir(bpy>z(C 3 ,N')bpy]3+, and 1,4~di­
methoxybenzene (DMB). The excited state of Ir (III) trapped in a transparent,
inert porous Si0 2 glass by the "sol-gel" process, can be reductively quenched by
DMB dissolved in the water-filled pores of the glass. The back reaction of the
photoinduced charge-separated pair (Ir(II), DMB+) is retarded by four orders of
magnitude with respect to homogeneous solutions. This strong retardation is
attributed to the combined effects of trapping Ir(II) and adsorption of DMB +, i.e.,
to the restriction of the diffusional processes of the charge pair, leading to
long-lived charge separation at spatially separated sites. The back reaction is
sufficiently inhibited so that under acidic pH (1.3), Ir(II) is able to react with water
on the glass surface to produce H2 with a quantum yield of about 1%. The
turnover number for Ir(III), which acts both as a photosensitizer and a catalyst, is
estimated to be higher than 100 [131]. The same group proposed recently [132] a
three~component system of PSI Al/ A2 type involving pyrene (Py) as PS, N,N'~te­
tramethylene~2,2'-bipyridinium ion (DQ~+) as primary electron-acceptor A l , and
My2+ as secondary electron-acceptor A 2. Pyrene and My2+ are both immobilized in a porous sol~gel silica glass and the redox reaction is carried out by the
mediation of DQ~+ acting as a mobile charge carrier in the intrapore space.
268
E. Amouyal / Solar Energy Materials and Solar Cells 38 (J995) 249-276
solution
zeolite
Fig. 6. Schematic representation of unidirectional electron transfer cascade in a zeolite-based triad
proposed by Krueger et al. [[133D.
Under these conditions, the unidirectionality of the shuttling reaction is due to the
difference of redox potentials of DQ~+ and MV 2+, the latter being a better
electron-acceptor than the fonner. The spatial separation of Py+ and MY+'
inhibits strongly the back electron-transfer reaction between these two species.
Low yields of about 5% but very long lifetimes T cs of at least four hours were
found for the photoinduced charge-separated pair (Py+, MY+') [132].
In a similar way, but with zeolites as constraining media, Mallouk et al. [133]
proposed an interesting three-component system, of PS-A 1•••A 2 type, which
consists of a [Ru(R-bpY)3]2+-DQ~+ flexible dyad (R = H or CH 3 and DQ~+=
N,N'-dialkyl-2,2'-bipyridinium ion with n = 2 or 3) and benzylviologen By 2+ as a
secondary electron-acceptor (A 2), By 2+ being contained within the zeolite framework. The [Ru(R-bpY)3]2+ moiety of the dyad is too large to enter through the
7.1A window of zeolite L. Thus, only the DQ~+ moiety can enter the zeolite
anionic structure. This spatial arrangement (Fig. 6) restricts the motion of the
flexible ethylene chain and allows the contact of DQ~+ with By2+. When the
system is irradiated by visible light, an electron cascade occurs from the excited
state of [Ru(R-bpY)3]2+ to DQ~+ and then to the mobile By 2+ according to a
mechanism similar to reactions 38. A charge-separated state having a long lifetime
T cs = 37 ~s (see Table 4 for comparison with supramolecular systems) is fonned
with a quantum yield of 17% [133].
In the trimolecular system, of PSI AllA2 type, developed recently by Borja and
Dutta [134] and based on the same approach as that of Ottolenghi et al. [132], PS
and Al are encapsulated in zeolite Y whereas A2 is in the surrounding solution.
Here PS is [Ru(bpY)3]2+, Al is DQ~+ and A2 is the zwitterionic sulfonatopropyl
viologen SPY. Since SPY is neutral, it does not replace DQ~+ in the zeolite. A
unidirectional electron-transfer cascade takes place from the excited state of
[Ru(bpY)3]2+ to DQ~+ in zeolite, and then across the zeolite-solution interface
from the reduced DQrto SPY, according to a mechanism similar to reactions 38.
E. Amouyal / Solar Energy Materials and Solar Cells 38 (1995) 249-276
269
However, the long-lived SPV-· is formed in very low yields (5 X 10- 4 ) [134]. New
developments concerning this system are discussed elsewhere in this volume [135].
5. Attempts to generate simultaneously
irradiation
H2
and O 2 from water by visible-light
The incorporation of at least one component of a photochemical system in
organized assemblies and the utilization of constrained/confined media in order
to inhibit the back reaction is not really a new approach. Indeed, as· early as 1976,
Whitten et a1. [136] reported on the complete cleavage of water into H2 and O 2 by
visible light, using monolayer assemblies of surfactant derivatives of [Ru(bpY)3F+.
Although this observation could not be reproduced by the same group [137] and by
many other laboratories [138], it has drawn attention towards organized media
(monolayers, micelles, vesicles, microemulsions ... ) and towards the idea that microheterogeneous environments can strongly modify photochemical behaviour and
reactivity of compounds incorporated in these media.
Another system was developed by Van Damme, Fripiat et a1. [139] in 1983.
These authors used as support solid-state materials such as clays and related
minerals. Photoinduced water splitting was achieved by compartmentalizing the
system into two subsystems (Fig. 7). In the first of these, [Ru(bpY)3F+ as PS and
Ru0 2 as catalyst for O 2 evolution, were deposited on negatively charged colloidal
particles of a fibrous clay (sepiolite), whereas in the second colloid, which is
positively charged, Pt particles, as catalyst for H2 evolution, were deposited on
amorphous aluminium-europium mixed hydroxide Al x Eu 1 _ x (OH)3' Eu 3+ acting as
electron relay (Fig. 7). The two colloidal subsystems were prepared separately and
were then mixed. Their coupling occurs spontaneously through interparticular
association by electrostatic interactions. Visible-light irradiation of aqueous suspensions of the system leads to the oscillatory generation of H2 and O 2 via
oxidative quenching of [Ru(bpY)3F+· by Eu3+, but with a turnover number for the
Eu3+ relay of only 5. Although the catalytic nature of the process is not clear, this
confmed-system approach deserves more consideration ..
A system similar to that of Van Damme and Fripiat, but without any organized
solid support, was proposed before by Kalyanasundaram and Gratzel in 1979 [83].
This four-component system, based on the multimolecular approach (Section 2),
consists of [Ru(bpY)3F+ as PS, MV 2+ instead of Eu3+ as electron relay, colloidal
Pt and Ru0 2 as catalysts for H2 and O 2 generation, respectively. The idea was to
couple the two half photosystems (reactions 6 and 14) in order to avoid the use of
any sacrificial electron-donor or acceptor. It was claimed that visible-light irradiation of this system at pH = 4.7 leads to simultaneous generation of H2 and O 2. But
these observations were not reproduced by several laboratories
[15,71,91],[93]a,[14O]. Nevertheless, this system is described even in recent books
and review articles without specifying that it has given rise to much controversy. In
fact, this system does not work for at least one reason, viz. the obligation to use
very specific catalysts in order to avoid short circuit processes, principally the back
270
E. Amouyal / Solar Energy Materials and Solar Cells 38 (J995) 249-276
H2
Fig. 7. Schematic representation of the system [Ru(bpY)3J2+ /Eu 3+/Pt/RuO z proposed by Van
Damme and Fripiat [139] for the simultaneous photogeneration of Hz and 0z from water by
visible-light irradiation of this system incorporated in colloidal suspensions of a fibrous clay (sepiolite).
PS is [Ru(bpY)3]2+ and the electron relay is Eu3+ which is embedded into AI hydroxide colloids.
reaction (reaction 20). In the case of Ru0 2 which was considered at that time as a
highly specific catalyst for O 2 evolution, I have proved, for the first time that RU02
was also a good catalyst for H2 evolution in a four-component sacrificial system
[Ru(bpY)3F+ /MV2+/EDTA/Ru0 2 [53,54].
Several other questionable attempts to generate H2 and O 2 simultaneously
have been reported. For instance, in 1982, Kaneko et al. [93] described a microheterogeneous system involving [Ru(bpY)3F+ as PS, Prussian Blue colloids acting
both as electron mediator and catalyst, and K + or Rb + cations. Prussian blue
(PB), which is a mixed-valence iron complex {Fe~1l[Fell(CN)6h}, is known to form a
three-dimensional polymeric structure of zeoli tic nature. Visible-light irradiation of
aqueous solutions of this system results in the formation of H2 and O 2 with low
yields at the optimum pH = 2 and with a turnover number for [Ru(bpY)3]2+ of 11
after an irradiation time of 90 h. But under the same conditions, one can calculate
a turnover number for PB catalyst much lower than 1. In addition, Harriman et al.
[94] could not confirm the simultaneous production of H2 and 2 , but they did
show that PB can catalyze O 2 generation at pH = 7 with a quantum efficiency of
2% in a three-component system [Ru(bpY)3]2+ /Na 2 S 2 0S/PB using sodium persulfate as sacrificial electron-acceptor. More recently, in 1992, Katakis et al. [141]
described a two-component homogeneous system for water splitting based on a
dithiolene complex of tungsten, tris-[l-(4-methoxyphenyI)-2-phenyl-1,2-ethylenodithiolenic-S,s'] tungsten, acting as both photosensitizer and catalyst. In the
presence of MV 2 +, the authors claimed that the visible-light irradiation in the
°
E. Amouyal/ Solar Energy Materials and Solar Cells 38 (1995) 249-276
271
350-500 run range of acetone-water (70:30 vIv) solutions of the tungsten dithiolene results in H2 and O 2 evolution in stoichiometric proportions with an average
quantum yield for O 2 production (in equivalents) of 4%, and turnover numbers for
the tungsten dithiolene greater than 1000. We know from experience that the
greatest care must be taken with such water splitting results. Therefore, before
concluding as to the catalytic nature of this system, the observations of Katakis et
al. [141] should be repeated by other groups.
6. Conclusion and outlook
This review has illustrated different approaches for visible-light-induced water
splitting into H2 and/or O 2 , and their evolution from multimolecular to
supramolecular systems, and from homogeneous or quasi-homogeneous to organized, constrained or confined media. These approaches give rise to numerous
sacrificial model systems capable of producing H2 and O 2 separately from water.
It should be emphasized that, excepting the homogeneous system of Katakis et al.
[141] which await confirmation, no reproducible, complete system generating H2
and O 2 simultaneously is available up to now. Among the half photosystems, the
four-component [Ru(bpY)3]2+ /MV 2 +/EDTA/colloidal Pt classical model system
remains a reference for testing new photosensitizers, electron relays, electron
donors and catalysts. A large selection of such components is now available which
opens the possibility of designing "a la carte" photochemical systems, in particular
supramolecular systems. Thanks to these systems, remarkable progress has been
made in the effective retardation of the back electron-transfer reaction, and hence
in the achievement of charge separation with a long lifetime. This was also
accomplished by incorporating at least one component in organized molecular
assemblies or in constrained/confined media, and by a combination of such
multiphase media with supramolecular polyads. Moreover, such media introduce
organized microenvironments which it would be judicious to use in order to
compartmentalize the reducing and oxidizing processes. Indeed, it is not desirable,
on the microscopic level, to produce H2 and O 2 at the same catalytic site.
Although enormous progress, essentially in fundamental research, has been done
in less than twenty years, the development of practical devices for water splitting
still requires facing several problems. For example, (i) efficiency and selectivity of
catalyst for O 2 generation must be highly improved, (ii) separation of H2 and O 2
in the early stages of the reaction is required in order to avoid the catalytic
regeneration of water.
In conclusion, visible-light water splitting into H2 and O 2 remains one of the
most exciting challenges for scientists, especially photochemists. I am convinced
that there exist many, and perhaps all, elements for the design of a complete
water-splitting photosystem, and I am confident that one will succeed in designing
efficient photochemical devices for solar energy storage. Anyhow, such studies,
which are intrinsically multidisciplinary, have greatly contributed to novel and
fascinating aspects of modem chemistry.
272
E. Amouyal / Solar Energy Materials and Solar Cells 38 (J995) 249-276
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