Application of chemiluminescent probe to monitoring superoxide radicals and hydrogen peroxide in TiO2 photocatalysis

Article abstract of DOI:10.1021/jp970400h

A chemiluminescent probe, luminol, was successfully applied to monitoring Superoxide ions (O₂^?-) and hydrogen peroxide (H₂O₂) produced on photocatalytic reaction in aqueous TiO₂ suspension. Two chemiluminescent reactions were distinguished from the decay profile after the end of the irradiation, and the reaction mechanism was analyzed. The fast decay component gives information about O₂^?- and the slow one provides the amount of H₂O₂. The rate constant for the reaction of O₂^?- with luminol was found to be 1 x 10⁴ M^-1 s^-1. The amount of O₂^?- during the irradiation on TiO₂ in alkaline solution could be estimated to be on the order of 10^-13 M. Detection of H₂O₂ in concentrations as small as 10^-9 M was demonstrated in the photocatalytic water oxidation.

Full text of DOI:10.1021/jp970400h

5822
J. Phys. Chem. B 1997, 101, 5822-5827
Application of Chemiluminescent Probe to Monitoring Superoxide Radicals and Hydrogen
Peroxide in TiO2 Photocatalysis
Yoshio Nosaka,* Yoshifumi Yamashita, and Hiroshi Fukuyama
Department of Chemistry, Nagaoka UniVersity of Technology, Kamitomioka, Nagaoka 940-21, Japan
ReceiVed: January 31, 1997; In Final Form: April 23, 1997X
A chemiluminescent probe, luminol, was successfully applied to monitoring superoxide ions (O2•-) and
hydrogen peroxide (H2O2) produced on photocatalytic reaction in aqueous TiO2 suspension. Two chemilu-
minescent reactions were distinguished from the decay profile after the end of the irradiation, and the reaction
mechanism was analyzed. The fast decay component gives information about O2•- and the slow one provides
the amount of H2O2. The rate constant for the reaction of O2 with luminol was found to be 1 × 104 M-1
•-
s-1. The amount of O2 during the irradiation on TiO2 in alkaline solution could be estimated to be on the
•-
order of 10-13 M. Detection of H2O2 in concentrations as small as 10-9 M was demonstrated in the
photocatalytic water oxidation.
Photocatalytic reaction by using semiconductor powders has
been paid much interest because of its possible applications to
solar energy storage and mineralization of waste water. On
band-gap irradiation of semiconductor powder in photocatalysis,
electrons and holes are produced in the conduction band and
valence band, which reduce and oxidize the molecules at the
surface, respectively. In photocatalysis in air, active oxygen
species such as hydroxyl radical (OH.), superoxide ions (O2•-),
and hydrogen peroxide (H2O2) have been noticed as key species
to initiate the reaction.1,2 The efficiency of the photocatalytic
reaction is limited by the reduction kinetics of dissolved O2.3
The formation of O2•- in the photocatalytic system was certified
by means of spin-trapping ESR spectroscopy.4 Although, the
preaddition or postaddition. In some postaddition experiments,
MCLA (methoxy cypridina luciferin analogue, 2-methyl-6-(p-
methoxyphenyl)-3,7-dihydroimidazo[1,2-a]pyrazin-3-one, To-
kyo Kasei, Ltd.)10,11 was employed in place of luminol to detect
superoxide ions.
Irradiation at the wavelength of 387 ( 11 nm (fwhm) was
performed with a 150-W xenon lamp through two glass filters
(Hoya, U-330 and L39). The intensity of light incident on the
cell was measured with a power meter to be 13 mW. After
switching off the light irradiation, CL was measured at a side
of the cell through a light guide, which was connected to a
photon-counting photomultiplier tube (Hamamatsu Photonics
Ltd., R2693P) mounted in a cooled housing (C659-S). Between
the sample cell and the light guide, an interference filter and
another shutter were set. The shutter placed in front of the light
guide is to reduce the remaining signal of the photomultiplier
tube associated with the activation by scattered light during the
irradiation. In place of the interference filter, a sharp-cut filter
of 420 nm (Hoya, L42) was used in some experiments. The
photon-counting apparatus used was assembled with a discrimi-
nator (Hamamatsu, C3866) and a counting board (Hamamatsu,
M3949) controlled by a personal computer. Photons were
counted every 20 ms until 20 s after the end of the irradiation.
The two shutters were driven with the personal computer
through a digital pulse programmer (Stanford Research System,
DG535).
•-
lifetime of O2 is rather long in alkaline solution,5 Gerischer
assumed the steady concentration of O2•- to be negligible.6 The
spin-trapping method gave no information on the lifetime of
•-
O2
.
As an alternative method of detecting the active oxygen, it
is interesting to use chemiluminescence (CL) in semiconductor
photocatalytic reactions. Luminol is a representative CL probe,
which is known to be sensitive to any oxidation reaction and
the presence of hydrogen peroxide. Merenyi and co-workers
•-
investigated the mechanism of CL of luminol,7-9 where O2
takes an important role of producing the electronically excited
state. Since the luminescence detection is more sensitive than
the ESR method, new information is obtained on the behavior
of superoxide ions in photocatalysis. As far as we know, the
present article is the first report of utilizing CL probes in the
investigation of semiconductor photocatalytic reactions.
Results and Discussion
Postaddition of Probe Reagents. Figure 1 is a superposition
of the time profiles of the CL intensity on three batches of
experiments where the addition of luminol was performed at 1,
21, and 41 s after the end of each 60-s irradiation. The weak
signal observed in the short time region near 0 s is attributable
to the luminescence of TiO2 powder. Although the irradiation
duration was 60 s for Figure 1, a similar intensity of CL was
observed for the duration of more than 10 s. In order to confirm
that the observed light originates from the CL of luminol, the
luminescence at 1 s after the irradiation was measured at various
wavelengths. Figure 2 is the plot of the summation of the
counted photons at each wavelength. The curve in Figure 2 is
the photoluminescence spectrum of 3-aminophthalate (AP),
which has been identified as the light-emitting species in luminol
CL.
Materials and Method
The sample was an aqueous solution of 3.5 mL (L ) dm3)
containing typically 0.01M (M ) mol dm-3) NaOH and 15 mg
of TiO2 powder (Ishihara Sangyo, Ltd., ST-01). The pH was
usually 11.0. The solution was stirred with a magnetic bar in
a Pyrex cell 1 cm × 1 cm in size, and the cell was placed in a
dark box. An aqueous solution (50 µL) of 7 mM luminol (5-
amino-2,3-dihydro-1,4-phthalazinedione, Nacalai Tesque, Ltd.)
was added with a syringe into the solution before or after the
irradiation. We distinguish this experimental procedure as
X Abstract published in AdVance ACS Abstracts, July 1, 1997.
S1089-5647(97)00400-8 CCC: $14.00 © 1997 American Chemical Society

Application of Chemiluminescent Probe
J. Phys. Chem. B, Vol. 101, No. 30, 1997 5823
Figure 1. Superposition of three traces of CL intensity, where the
times of adding 0.1 mM luminol are 1, 21, and 41 s after the end of
the 60-s irradiation on 3.5 mL of an aqueous solution containing 15
mg of TiO2 and 0.01 M NaOH.
Figure 3. Integrated intensity of CL of luminol (0) and MCLA (O)
as a function of the delay time of probe mixing in postaddition
experiments at pH 11, as shown in Figure 1. The curve is the
exponential function fitted to the data of luminol.
order process in pure aqueous solution with the rate constant
of 6 × 1012[H+] M-1 s-1 at pH > 6.5 The concentration of
•-
O2 is less than the amount of incident photons, which is
estimated to be 10-4 M. The lifetime of O2•- is then expected
to be larger than 102 s. The observed decay time may originate
from the reaction with TiO2 particles.
The life time of the active species was measured at different
pH. At pH 10 and 12, the lifetimes were 13 and 18 s,
respectively. Increase in the luminescence intensity with pH
was observed as well. This observation is consistent with the
•-
fact that the active species is O2
.
In order to confirm the existence of O2•-, MCLA was used
in place of luminol, because its CL occurs in the reaction with
•-
1
O2 and singlet molecular oxygen, O2.10,11 In Figure 3 is
plotted the integrated number of photons observed on the
addition of MCLA at different delay times after the end of the
each irradiation. Since MCLA suffers autoxidation with air in
alkaline solution, a constant CL was observed. Then the sharp
peak of the CL observed immediately after the mixing was
counted. The decay of the reactant shows a rate similar to that
Figure 2. Spectrum of CL measured by changing interference filters
(O) in postaddition experiments as shown in Figure 1 with the mark
×. The curve is the fluorescence spectrum with excitation wavelength
at 350 nm for aqueous solution of 3-aminophthalic acid containing 0.01
M NaOH.
The excited state of 3-aminophthalate, AP*, can be induced
by the two reactions (eq 1 and 2).7
1
observed with luminol. Since luminol CL with O2 was not
reported, the present observation shows that the CL in postad-
k ) 2.3 × 108
M-1 s
L
-18 f AP*
(1)
(2)
•-
•- + O2
•-
dition experiments is caused by O2
.
The two-electron-oxidized form of luminol, L, rapidly
k ) 5 × 107
M-1 s
decomposes on reaction with OH-, as eq 4.13
•-
L + HO2
-18 f AP*
k ) 4 × 106
M-1 s
L + OH-
-18 decomposition
(4)
where L•- is luminol radicals and L is two-electron-oxidized
luminol or 5-aminophthalazine-1,4-dione. Reaction 2 is equiva-
lent stoichiometrically to the reaction 1, although the yield of
Reaction 2 then appears to be less plausible as the CL
mechanism in the postaddition experiment where the amounts
of L and HO2 are small.
-
AP* is rather low (Φ ) 0.012).7 Hydroperoxide ion HO2 is
-
formed by the acid-base equilibrium of H2O2 in alkaline
solution, where H2O2 is formed by oxidation of H2O or reduction
of O2 in photocatalytic reaction.12
There are several possible routes for the formation of L•-
radicals in eq 1. Although OH• radicals or surface holes are
candidates of L•- formation, the following experiment denied
the possibility.14 Figure 4 shows the integrated CL intensity
as a function of I- concentration. By adding I- the CL intensity
increased at first and reached a steady value, indicating that
the steady-state concentration of the oxidizing species increased
with I- ions. Since I- is oxidized by OH• radicals or surface
holes, then if these species oxidize luminol, the CL intensity
should be decreased. The life time of these oxidants is too short
to react with the luminol added after the end of the irradiation.
The increase in the CL with I- is explained by the increase of
H2O2 7pK ) 11.78 HO2- + H+
(3)
a
Figure 1 suggests that the amount of the chemical species
inducing the CL decreased in the time range of 10 s without
luminol. In order to know the lifetime of this species, the
luminescence intensity was measured for other delay times of
luminol mixing, and all obtained data were plotted in Figure 3.
Each datum is the average of more than three experiments. The
decay could be fit with an exponential function by a usual least-
squares method. The lifetime of the chemical species was 17
s at pH 11. Since the lifetime was rather long, it is plausible
that the active species is O2•-, which disappears by a second-
•-
O2 concentration, which originates from the suppression in
•-
the oxidation of O2
.
Other possible processes for the formation of L•- radicals in
the postaddition experiments are the autoxidation of luminol in

5824 J. Phys. Chem. B, Vol. 101, No. 30, 1997
Nosaka et al.
Figure 4. Effect of the presence of NaI in an aqueous TiO2 suspension
on the integrated CL intensities observed in the postaddition experi-
ments.
Figure 6. Integrated CL intensity observed after the end of the
irradiation of 1 s (×), 5 s ([), 10 s (O), 20 s (2), 40 s (9), and 80 s
(b), as a function of the number of repeated irradiation on the sample
under aerobic conditions (preaddition experiment). The irradiated
sample was 3.5 mL of an aqueous suspension of 15 mg of TiO2
containing 0.1 mM luminol and 0.01 M NaOH.
Figure 5. Decay rate of CL as a function of the concentration of
luminol added after the irradiation on an aqueous TiO2 suspension at
pH 11.
•-
alkaline solution (eq 5),9 the oxidation with O2 (eq 6),8 and
Figure 7. Typical time profile of the CL intensity in the preaddition
experiment. The two curves are the components of the decay obtained
from the curve fitting. See text for the details of the fitting procedure.
the oxidation with H2O2 (eq 7).15
k ) 1.1 × 10-8 M-1 s
LH- + O2 + OH-
LH- + O2•- 7
-18 L•- + O2•- + H2O
to the rate of AP* formation, the intensity is proportional to
the product of concentrations [O2•-][L•-] based on eq 1 for fast-
decay CL. This means that when [O2•-] , [L•-], the decay of
CL is controlled by the decay of [O2•-], and the decay rate is
proportional to [L•-]. In another case, when [L•-] , [O2•-],
the decay rate is proportional to [O2•-] or the concentration of
other reactants for L•-. The decay rate of CL is unchanged
with the amount of O2•-, as shown in Figure 1. Figure 5
indicates a linear relationship between the decay rate and [LH-].
Then the forward reaction of eq 6 is most plausible as the
reaction controlling the decay of CL intensity. Since the decay
rate is about 1 s-1 with 0.1 mM LH-, the rate constant of
reaction 6, k6, is estimated to be about 104 M-1 s-1. Further
discussion including the estimation of radical concentration will
be described in the last section.
Preaddition of Luminol. Figure 6 shows the total number
of photons observed 20 s after the end of the irradiation for
various durations ranging from 1 to 80 s. The abscissa of the
figure is the number of repeated irradiations. With the repeat
of irradiation, the CL intensity increased, indicating that some
species contributing to the CL is accumulated during the
irradiation of TiO2.
(5)
K ) 10-4 M
-18 L•- + H2O2 + OH-
(6)
(7)
LH- + H2O2 f L•- + OH• + H2O
where LH- is an ionized form of luminol in alkaline solution
under the equilibrium of eq 8.9,16
LH2 7pK ) 6.78 LH- + H+
(8)
a
In the presence of 0.1 M H2O2, luminol solution shows
continuous CL. This observation seems to justify the contribu-
•-
tion of eq 7. However, since HO2- is in equilibrium with O2
at pH 11 (eq 9),17
HO2- + O2 + OH- 7K ) 4 × 10-98 2O2•- + H2O
(9)
•-
a small amount of O2 may be produced with a large amount
of H2O2. Then oxidation can be attributable to the reaction with
•-
O2 (eq 6). Furthermore, reaction 7 has not been referred to
by Merenyi et al.,9 because of the energetical point of view.
Thus the reaction 7 is ignored in the further discussion.
Figure 5 shows the dependence of decay rate on the
concentration of luminol. Since the CL intensity is proportional
Figure 7 shows a time profile of luminescence in the
preaddition experiments. In the analysis of the data, the region
later than 7 s was fitted first. This part of the data lies on the
second-order decay curve. The calculated value of the fitted

Application of Chemiluminescent Probe
J. Phys. Chem. B, Vol. 101, No. 30, 1997 5825
Figure 9. Effect of the presence of NaI in an aqueous TiO2 suspension
on the integrated CL intensity of the fast (0) and slow (O) decay
components in the preaddition experiment.
was measured in the presence of the oxidizable agent I-. Figure
9 shows each CL intensity as a function of the concentration of
NaI. The CL intensity of both components increased with the
addition of I- similarly to that observed in the postaddition
experiment. This observation indicates that L in the present
observation was produced by another route than that with OH•
radicals or surface holes.
Since luminol is present during the irradiation in the pread-
dition experiments, L•- is formed by photocatalytic reaction and
the amount of L•- may be enough for producing L by
disproportionation (eq 10).7
Figure 8. Integrated CL intensity (a) and rate constants (b) of the fast
(0) and slow (O) decay components in the preaddition experiment as
a function of the number of repeated 60-s irradiations, where 0.2 mM
H2O2 was added at the end of 5 irradiations.
k ) 5 × 108
M-1 s
L
•- + L•- + H2O
-18 L + LH- + OH- (10)
Since the amount of H2O2 is on the order of 10-4 M and then
[HO2-] = 10-5 M at pH 11, the time scale of reaction 2 is on
the order of 1 ms. Then, the rate-determining step in CL of
the slow-decay component is reaction 10. Since the CL intensity
is proportional to [L], the second-order decay profile of the CL
can be explained by reaction 10. From the observed rate
constant of C0k′ in Figure 8b, the initial concentration of L•- is
estimated by equating the reported rate constant of eq 10 to k′.
They are 6 × 10-10 and 2 × 10-10 M before and after the
addition of 0.2 mM H2O2, respectively. This decrease in the
amount of L•- is caused by the competitive photocatalytic
oxidation of H2O2 during the irradiation.
curve was subtracted from the observed data. Then, the
remaining fast part of CL decay can be fit by an exponential
decay curve. Both components originate from luminol CL,
because the spectra of each component fit to that of the
photoluminescence of AP. The integrated number of photons
was calculated from the amplitude and decay rate for each
component.
Figure 8a shows the integrated CL intensity of each compo-
nent as a function of the number of repeated irradiations. The
component of slower decay increased with the number of
repeated irradiations, while the increment of the fast component
is small. When 0.2 mM H2O2 was added after the five 10-s
irradiations, only the intensity of the slow component increased.
The decay rate constant of each CL is shown in Figure 8b, where
the left-side scale is the pseudo-first-order rate constant of the
fast decay. In the right-side scale, C0 is the initial concentration
of the reactant, and k′ is the second-order rate constant. The
reactant will be identified in the later discussion. The rate for
the slow component C0k′ decreased with the addition of H2O2,
while that for the fast component remains almost constant. The
relatively large scattering of rate constants in the preaddition
experiment may be caused by possible adsorption of reaction
products on the TiO2 surface.
In aqueous solution at pH 11, the rate constants for the
reaction of OH• radicals with LH- (eq 11)18 and HO2- (eq 12)19
are the same order of magnitude.
k ) 9 × 109
M-1 s
OH• + LH-
-18 L•- + H2O
(11)
(12)
k ) 5.6 × 109
M-1 s
-18 O2•- + H2O
-
OH• + HO2
-
For HO2 of reaction 12, the rate constant reported is larger
than that for H2O2 by 2 orders of magnitude.19 Then, the amount
of H2O2 added (0.2 mM) is effective as HO2-, of which amount
is calculated to be 0.03 mM at pH 11 from eq 3. Then the rate
of reaction 12 is calculated to be 2 × 105 s-1. Provided that
[LH-] ) 0.1 mM, the rate of reaction 11 is 9 × 105 s-1. In the
experimental observation in Figure 8, the amount of L-
decreased by a factor of 3 in the presence of H2O2. Then, the
rate of reaction (12) is expected to be twice that of reaction 11.
The difference in the ratio of the competitive reactions by the
factor 9 shows that the environment of OH• radicals at the
catalyst surface is different from that in the bulk solution or
the species were oxidized by surface holes.
Since the decay rate of the fast component is similar to that
observed in the postaddition experiment, CL of this component
is probably induced by L•- with O2•- (eq 1). On the other hand,
Figure 8a shows that the chemiluminescent reaction of slow
decay may involve the accumulated species, and it may be H2O2
as indicated by eq 2. Thus, CL observed as the slow component
-
is attributable to the reaction of L with HO2 (eq 2).
When luminol is present during the irradiation of TiO2, it
may suffer oxidation to L by the reaction with surface holes or
OH• radicals. The luminescence intensity of two components

5826 J. Phys. Chem. B, Vol. 101, No. 30, 1997
Nosaka et al.
Figure 11. Simulated time profile of the amount of unstable species
in a 0.1 mM luminol solution of pH ) 11.0 after mixing with 5 ×
10-13 M O2•-: (a) d[AP*]/dt in units of 10-16 M s-1, (b) [O2•-] in
units of 5 × 10-13 M, (c) [L•-] in units of 5 × 10-12 M, and (d) [L] in
units of 10-19 M.
Figure 10. Total CL intensity in the preaddition experiment for an
aqueous TiO2 suspension containing 0.1 mM luminol and 0.01 M NaOH
in the absence of oxygen plotted as a function of the number of repeated
60-s irradiations. An interference filter of 430 nm was used for
observation.
approximation to eqs 5 and 10 for [L•-] and then eqs 2, 4, and
10 for [L], the following relations were obtained.
Although the slow-decay component of CL is attributable to
the reaction of L with HO2- (eq 2), this reaction competes with
the decomposition of L with OH- in alkaline solution (eq 4).
Then the CL intensity of the slow component is related to the
solution pH and concentrations of H2O2 and L•-, in a complex
manner.
As mentioned above, in the postaddition experiment where
luminol was mixed after the photoirradiation, CL of the slow-
decay component was not observed. This was attributed to the
small amount of L and H2O2. Then, in the presence of H2O2,
the slow-decay component of CL should be observed even in
the postaddition experiment. When 0.2 mM H2O2 was added
before irradiation in the postaddition experiment, a part of the
fast-decay changed to the slow-decay component.
[L•-] ) {(k5[O2] + k6[O2•-])[LH-]/k10}1/2
(14)
(15)
-
[L] ) k10[L•-]2/(k2[HO2 ] + k4[OH-])
By adopting the above mentioned parameters to eqs 14 and 15,
the steady-state concentrations for the unstable intermediates
were obtained to be [L•-] ) 1 × 10-12 M and [L] ) 9 × 10-20
M.
The formation rate of AP*, which is proportional to the
observed CL intensity, is obtained from eqs 1 and 2.
-
d[AP*]/dt ) k1[L•-][O2•-] + Φk2[L][HO2 ] (16)
Although most of the present experiments were performed
in aerated condition, the reaction may be sensitive to the
existence of oxygen. Then, we tried a preaddition experiment
under vacuum to detect the oxygen formation from water by
photocatalytic reaction. The sample cell was evacuated by
freeze-and-thaw cycles. Although the intensity was very low,
CL of the slower decay was observed. Figure 10 shows that
the total CL increases with the repeat of the 10-s irradiation.
The increment of the number of photons is smaller by a factor
of 104 than that observed in the addition of 0.2 mM H2O2 of
Figure 8a. Then, the amount of produced H2O2 is calculated
to be on the order of 10-9 M or 10-11 mol. This means that
the chemiluminescent probe is useful to detect the oxidation of
water in photocatalytic reaction.
By using the radical concentrations obtained for 0.1 mM luminol
with 0.2 mM H2O2 at pH 11.0, the steady-state formation rate
of AP* is calculated to be 1.6 × 10-18 M s-1. In the dark, the
number of photons observed for this solution was 72 count/(20
ms) in the present experimental setup. Since it was 0.3 count/
(20 ms) in the absence of oxygen and H2O2, the 72 count/(20
ms) observed corresponds to the formation rate of 1.6 × 10-18
M s-1. Thus the formation rate of AP* in the postaddition
experiment in Figure 1 is calculated to be 5.6 × 10-17 M s-1
,
based on the CL intensity of 2500 count/(20 ms) at 1 s after
the end of the irradiation. Although the second term in eq 16
is dominant in the CL for the solution containing H2O2 in the
dark, the first term is dominant in the postaddition experiment
because of low H2O2 concentration. Then, from eq 16 the
product of the concentration [L•-][O2•-] is estimated to be about
2.4 × 10-25 M2. Based on these considerations, the time profile
of CL in the postaddition experiment was simulated for the AP*
formation rate as shown in Figure 11. The parameters newly
introduced in the simulation are only k6 and the initial [O2•-];
the others are reported in the literature. Thus, the concentration
of O2•- during the TiO2 irradiation in water at pH 11 is estimated
to be about 5 × 10-13 M.
Estimation of Radical Concentration. Since the rate
constants of related reactions have been reported, we can
estimate steady-state concentration in the dark for the chemical
species involved in the CL. When the steady-state approxima-
•-
tion was applied to eqs 1, 5, 6, and 10 for O2 radicals, we
obtain
-
[O2•-] ) (k5[LH-][O2] + 2k9[HO2 ][O2])/(k1[L•-] +
k6[LH-] + kS) (13)
Acknowledgment. The authors are grateful to Prof. N.
Suzuki for his stimulating discussion. The present work is partly
defrayed by the Grant-in-Aid for Scientific Research on Priority-
Area-Research (Nos. 07228223 and 08218223) from the Japa-
nese Ministry of Science, Education and Culture.
where kS is decay rate constant of O2•- in the absence of luminol.
The forward rate constant for reaction 9, k9, is calculated to be
7 × 10-11 M-1 s-1, from the equilibrium constant reported17
and the backward rate constant of 60 M-1 s-1 at pH 11.0.5 Since
[L•-] < 10-9 M and k6 = 104 M-1 s-1, by adopting [O2] ) 3
References and Notes
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•-
13, the concentration of O2 is calculated to be 3 × 10-16
M
before the irradiation. When we applied the steady-state

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J. Phys. Chem. B, Vol. 101, No. 30, 1997 5827
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