k \ (other terms) ] [exp([E /RT )/(exp([E /RT
o
o
] r exp([E /RT ))] (6)
d2
where E and E are the activation energies of steps 6 and 5
o
d2
in Scheme 1, respectively.
Approximation of the denominator of eqn. (6) by a function
exp[([r/(r ] 1))((E [ E )/RT)] leads to eqn. (7).
o
d2
ln k D (other terms) [ [r/(r ] 1)][(E [ E )/RT ]
(7)
o
d2
Then, the apparent activation energy E is expressed as a func-
a
tion of r.
E P [r/(r ] 1)][(E [ E )/R]
(8)
a
o
d2
Recombination of the photogenerated charge carriers is gener-
ally an ultra-fast process (\100 ns), which means that E is
d2
very small or E [ E [ 0. Taking r(Au/TiO ) \ r(Ag/TiO )
o
d2
2
2
into consideration, the decrease in E with Au loading in place
a
of Ag can be explained by eqn. (8).
Conclusions
Nanometre-sized Au (4.3È11 nm) and Ag particles (\5 nm)
were loaded onto the surface of TiO by the depositionÈ
2
precipitation method and the photodeposition method,
respectively. TiO photocatalytic reduction of bis(2-dipyridyl)
2
disulÐde to 2-mercaptopyridine by H O was enhanced signiÐ-
2
cantly with a small amount of Au (x D 0.3 wt.%). The kinetic
studies revealed that the rate constant of the Au/TiO system
2
is 2.6 times greater than of the Ag/TiO system and the activa-
2
tion energy of the former is smaller than that of the latter by a
Scheme 2 Energy diagram of the reaction system. In constructing
this, the following values were used: the work functions of Ag \ 4.0
eV and Au \ 5.1 eV;27 the electron energy for the normal hydrogen
factor of 1.6. This enhancing e†ect of Au could be attributed
to the increase in the charge separation efficiency, which is
achieved by restriction of back electron transfer due to the
high work function of Au.
electrode (NHE), [4.5 eV vs. NHE;28 the Ñat band potential of TiO
2
at pH 5.5, [0.45 V from the vacuum level;29 the band gap energy of
TiO (anatase), 3.2 eV;29 the oxidation potential of H O at pH 0, 1.23
2
2
V vs. NHE;29 the highest occupied molecular orbital (HOMO) of RS,
[6.4 eV.9
Acknowledgements
The authors express sincere gratitude to Dr. Masatake Haruta
and Dr. Susumu Tsubota (Osaka National Research Institute)
for helpful comments on the depositionÈprecipitation method,
Dr. Mitsunobu Iwasaki (Kinki University) for valuable dis-
be described as RS~ÈAu` (or RS~ÈAg`), which is conÐrmed
cussion and Ishihara Techno Co. for the gift of the TiO par-
by X-ray photoelectron spectroscopic measurements of
2
ticles (A-100). Finally, the authors thank the two reviewers for
SAMs.33 On the other hand, in the photoexcitation state, E
f
rises by several hundred meV (E@),34 which corresponds to the
f
constructive criticisms of an earlier version of this manuscript.
bonding energy of R@SSR@ on Au (D0.5 eV per R@S group).35 If
E@ exceeds the energy of the anti-bonding orbital (w ), it would
f
a
References
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In the Ag/TiO system, the E of Ag (4.0 ^ 0.15 eV) is close
1
2
3
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2
f
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2
Scheme 2, on illumination E increases evenly to a value of E@.
f
f
In the Au/TiO system, however, the E of Au (5.1 ^ 0.1 eV) is
4
5
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2
2
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6
7
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a
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2
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8
9
transfer from Au to the conduction band (CB) of TiO , while
2
it competes with electron transfer from the metal to w in the
a
Ag/TiO system. It follows that k (Au/TiO ) \ k (Ag/TiO ).
10 E. Cadot, M. Lacroix, M. Breysse and E. Arretz, J. Catal., 1996,
2
d2
2
d2
2
164, 490.
The larger k value for the Au/TiO system is thus ascribable
2
11 A. Ulman, Chem. Rev., 1996, 96, 1533 and references therein.
12 Y.-I. Inubushi, PhD Thesis, Kinki University, 1995.
13 S. Tsubota, M. Haruta, T. Kobayashi, A. Ueda and Y. Nakahara,
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to the increase in the efficiency of the charge separation, i.e.,
the increase in the term k [H O ]1@2/(k ] k [H O ]1@2) in
o
2
ad
d2
o
2 ad
eqn. (3). This equation can further be rewritten as eqn. (6) by
introducing a parameter r indicating the relative magnitude of
k
to k .
d2
o
Phys. Chem. Chem. Phys., 2001, 3, 1376È1382
1381