corresponds to the bonding energy of RASSRA on Au (ca. 0.5 eV
1
5
per RAS group). If E A exceeds the energy of the interfacial
f
antibonding orbital, it would be occupied by the two electrons
from Ag. The destabilizing energy will enable the desorption of
RS upon illumination, and further the photocatalytic cycle in
2
this reaction. We refer to this as photodesorption with an
upward shift of the Fermi energy. Essentially, the electro-
chemical reductive desorption of n-alkanethiol monolayers
from Ag and Au electrodes can be interpreted as desorption with
an upward shift of the Fermi energy induced by the application
of external electric voltage. Widrig et al. explained the fact that
the reductive desorption from Ag as compared to Au occurs at
0
.30 V more negative potential due to the differences in the
7
point of zero charge for the two metals. According to our
theory, this can alternatively be attributed to the differences in
the work function (Ag = 4.0 ± 0.15 eV, Au = 5.1 ± 0.1 eV),
8
assuming comparable interaction energy (DE in Fig. 2).
In conclusion, it has been demonstrated that a highly
Fig. 2 Energy diagram of the reaction system. The following values were
8
endothermic reduction of RSSR to RSH by H
2
O proceeds
used for its construction: work function of Ag = 4.0 eV; electron energy
9
selectively and efficiently using Ag/TiO as a photocatalyst.
for the standard hydrogen electrode (SHE) = 24.5 eV vs. SHE; flat band
2
1
0
potential of TiO
energy of TiO
2
at pH 5.5 = 20.45 V from the vacuum level; band gap
The essential reaction mechanism is presented on the basis of
the adsorption and kinetic data. Photodesorption with an
upward shift of the Fermi energy is proposed as a key process in
1
0
2
2
= 3.2 eV; oxidation potential of H O at pH 0 = 1.23 V
vs. SHE; HOMO of RS· radical = 26.4 eV (this value was obtained from
1
0
PM3 MO calculations).
2
the catalytic cycle. This action of Ag/TiO may open up a new
field of photocatalytic reactions of heterocompounds, whose
thermal catalytic reactions using transition metal catalysts are
usually not straightforward due to their poisoning effect.
Support of this work by the ESRI (Kinki University) under
the artificial photosynthesis program is gratefully acknow-
ledged.
the closest packing states were estimated to be ca. 0.16
2
21
2
21
nm group for the flat lying orientation and 0.1 nm group
for the vertical orientation using the PM3 optimized molecular
structure. Clearly, the RS groups adsorb on the surface of
nanometer-sized Ag particles in a close packed state analogous
2
to those in SAMs, while most of the TiO surface is directly in
contact with H O.
2
A plausible reaction mechanism is summarized as follows. In
the initial stage of the reaction, selective RSSR adsorption on
the surface of Ag accompanied by S–S bond cleavage takes
place. Electron–hole pairs are generated by the band gap
Notes and references
1
S. Oae, Organic Chemistry of Sulfur, Plenum, New York, 1977.
2 T. P. Johnston and R. D. Elliott, J. Org. Chem., 1967, 32, 2344.
3 TiO2 particles (anatase, BET surface area = 8.1 m2 21, Ishihara
Sangyo Co.) were used. Ag/TiO was prepared according to S.-i.
2
excitation of TiO . Most of the pairs are lost by recombination
g
but a portion of the electrons excited to the conduction band
2
Nishimoto, B. Ohtani, H. Kajiwara and T. Kagiya, J. Chem. Soc.,
Faraday Trans. 1, 1983, 79, 2685. As particles of < 5 nm were observed
(
(
CB) flow into Ag, while the holes are left in the valence band
VB) of TiO . The Schottky barrier at the Ag/TiO interface
2
2
to be dispersed on TiO
T. Ung, M. Giersig, D. Dunstan and P. Mulvaney, Langmuir, 1997, 13,
773.
2
by transmission electron microscopy.
would assist the charge separation. The hole has enough
4
+
+
2 2
potential to oxidize H O to yield H and O . The coupling of H
1
2
and RS , driven to desorb reductively by the excited electron,
5
6
A. Ulman, Chem. Rev., 1996, 96, 1533 and references therein.
J. Y. Gui, F. Lu, D. A. Stern and A. T. Hubbard, J. Electroanal. Chem.,
1990, 292, 245.
forms RSH. A similar reductive desorption of n-alkanethiols
7
from an Au electrode has been reported by Widrig et al. In the
TiO
2
system, the reduction of RSSR is thought to proceed in a
7 C. A. Widrig, C. Chung and M. D. Porter, J. Electroanal. Chem., 1991,
310, 335.
8 D. E. Eastman, Phys. Rev. B2, 1970, 1.
similar manner as in the Ag/TiO
2
system; however, the
alternative reduction site would be surface Ti3 ions. Three
factors for reaction enhancement effect of the Ag loading can be
proposed. The first is enhanced adsorption of RSSR, the second
+
9
F. Lohaman, Z. Naturforsch., Teil A, 1967, 22, 843.
1
1
0 M. Graetzel, in Energy Resources through Photochemistry and
Catalysis, Academic Press, New York, 1983.
1 For RASH, two types of dissociative adsorption on coinage metals (M)
are proposed [eqns. (2) and (3)] (P. E. Laibinis, G. Whitesides,
D. L. Allara, Y.-T. Tao, A. N. Parikh and R. G. Nuzzo, J. Am. Chem.
Soc., 1991, 113, 7152).
is the separation of reduction (Ag) and oxidation sites (TiO
and the third is that the oxidant (RSSR) and the reductant (H O)
selectively adsorb on the reduction and oxidation sites,
respectively, resulting in a highly selective reaction.
2
)
2
14
Since the turnover number is calculated to be ca. 6.7 3 10
2
2
0
2
I
(2)
(3)
molecules cm , this reaction is qualified to be catalytic. The
strangest and most intriguing question raised by this reaction is
why RS adsorbed on Ag desorbs from the surface upon
irradiation despite its strong adsorption strength as evidenced by
RASH + M ? RAS –M + 1/2 H
2
I
2
I
+
RASH + M ? RAS –M + H
6
3
21
In the case of RSSR, homolytic dissociated adosrption appears to be
plausible [eqn. (4)].
2
the large b value of 1.56 3 10 dm mol for Ag/TiO . Fig. 2
depicts the energy diagram of the reaction system. A couple of
bonding and antibonding orbitals are formed as the result of the
interaction between the highest occupied molecular orbital
0
2
I
RSSR + 2M ? 2RAS –M
(4)
(
HOMO) of RS·11 and an unoccupied molecular orbital (UMO)
1
2 R. Hoffmann, A Chemist’s View of Bonding in Extended Structures,
VCH, New York, 1993.
above the Fermi energy (E
f
) of Ag.12 In the ground state, the
bonding orbital is occupied by two electrons, each of which
originally belongs to RS· and Ag, respectively, leading to a
strong interfacial RS–Ag bond. The contribution of the HOMO
of RS· to the bonding orbital is much greater than that of the
UMO of Ag. Thus the interfacial bond can approximately be
13 G. K. Jennings and P. E. Labinis, J. Am. Chem. Soc., 1997, 119,
5208.
1
4 T. Sakata, T. Kawai and K. Hashimoto, Chem. Phys. Lett., 1982, 88,
0.
5 J. B. Schlenoff, M. Li and H. Ly, J. Am. Chem. Soc., 1995, 117,
2528.
5
1
1
2
+
described as RS –Ag , which is confirmed by XPS measure-
ments of SAMs.13 On the other hand, in the photoexcitation
1
4
state, E
346
f
is raised by several hundreds of meV (E
f
A), which
Communication 8/05820B
2
Chem Commun., 1998