Photostimulated reactions at the surface of wide band-gap metal oxides (ZrO: And TiOa): Interdependence of rates of reactions on pressure-concentration and on light intensity

Article abstract of DOI:10.1021/jp9830373

In this study we examine photostimulated reactions taking place on the surface of wide band-gap metal oxides such as ZrO₂ and TiO₂. In particular, we explore the photostimulated adsorption (PhA) of oxygen on ZrO₂ upon illumination with UV light (?? < 260 nm) and its postphotostimulated photoadsorption (PPSPhA) on irradiation with visible light (?? > 400 nm). The emphasis is on an examination of the decay channels of PhA-active surface centers to rationalize the interdependencies of the rate r(p,p) on pressure and light intensity The distinguishing feature of O₂ PhA kinetics, dp/dt a?? p^mpⁿ, resides in the dependence of the order of the reaction on the order m with respect to photon flow (p^m) and on the order with respect to pressure (pⁿ). In other words, m a?? 1 if a?? 0 whereas n a??1 if m a??0. The reaction orders m and n are interdependent in the case of PhA of O₂ on ZrO₂. A general mechanism is described for the several stages implicated in both PhA and PPSPhA surface processes. A preliminary consideration is also given to expand the PhA pathway for O₂ on ZrO₂ to processes that take place in the photocatalyzed oxidation of phenol in irradiated TiO₂ aqueous dispersions and the interdependence of the rate of the photoreaction on the concentration of phenol and on the incident light intensity. ? 1998 American Chemical Society.

Full text of DOI:10.1021/jp9830373

10906
J. Phys. Chem. B 1998, 102, 10906-10916
Photostimulated Reactions at the Surface of Wide Band-Gap Metal Oxides (ZrO2 and
TiO2): Interdependence of Rates of Reactions on Pressure-Concentration and on Light
Intensity
A. V. Emeline,†,§ A. V. Rudakova,‡ V. K. Ryabchuk,*,‡ and N. Serpone*,†
Department of Chemistry and Biochemistry, Concordia UniVersity, 1455 deMaisonneuVe BlVd. West,
Montreal (Quebec), Canada H3G 1M8, and Department of Physics, UniVersity of St. Petersburg,
UlianoVskaya 1, St. Petersburg, 198904, Russia
ReceiVed: July 16, 1998
In this study we examine photostimulated reactions taking place on the surface of wide band-gap metal oxides
such as ZrO2 and TiO2. In particular, we explore the photostimulated adsorption (PhA) of oxygen on ZrO2
upon illumination with UV light (λ < 260 nm) and its postphotostimulated photoadsorption (PPSPhA) on
irradiation with visible light (λ > 400 nm). The emphasis is on an examination of the decay channels of
PhA-active surface centers to rationalize the interdependencies of the rate r(F,p) on pressure and light intensity
The distinguishing feature of O2 PhA kinetics, dp/dt Fmpn, resides in the dependence of the order of the
reaction on the order m with respect to photon flow (Fm) and on the order n with respect to pressure (pn). In
other words, m f 1 if n f 0 whereas n f 1 if m f 0. The reaction orders m and n are interdependent in
the case of PhA of O2 on ZrO2. A general mechanism is described for the several stages implicated in both
PhA and PPSPhA surface processes. A preliminary consideration is also given to expand the PhA pathway
for O2 on ZrO2 to processes that take place in the photocatalyzed oxidation of phenol in irradiated TiO2
aqueous dispersions and the interdependence of the rate of the photoreaction on the concentration of phenol
and on the incident light intensity.
Introduction
species via photocarrier localization by normally weakly bound
or poorly adsorbed molecules. In the second case, PhA is
considered as the interaction of free or adsorbed molecules with
surface-trapped charge carriers (i.e., with photoinduced surface-
active centers). In both cases, if the heterogeneous system
reaches adsorption equilibrium rapidly, the dependence of the
steady-state rate r of PhA either on gas pressure p or on the
concentration C of molecules and/or ions in a solvent medium
can then be expressed as:
Photostimulated Adsorption and Photocatalysis. Hetero-
geneous photocatalysis1-4 and photostimulated adsorption of
molecules (PhA)4,5 are particular cases of photostimulated
surface reactions in heterogeneous systems. There are several
processes that are common to both photostimulated adsorption
and photocatalysis: (i) light absorption, (ii) diffusion and drift
of charge carriers toward the surface, (iii) recombination and
trapping of charge carriers, (iv) formation of surface-active
centers, and (v) interaction of surface carriers with adsorbed
molecules. In particular, these common points between PhA and
photocatalysis manifest themselves in numerous linear correla-
tions between initial rates of PhA processes and rates of
photocatalytic reactions on metal oxides6,7 and on alkali halides.8
Thus, a study of photostimulated adsorption processes could
prove useful in understanding the mechanism of photoexcitation
of solids as a first step of surface photochemical processes and
photocatalysis. The main goal of the work presented here is a
kinetic analysis and experimental verification of the PhA
mechanism in which we examine different decay pathways of
active surface centers.
dp
dt
kKp
(1 + Kp)
r ) -
)
)
(1)
(2)
dC
dt
kKC
(1 + KC)
r ) -
where k is the apparent rate constant and K ) kads/kdes is the
adsorption equilibrium constant. Note that when C is the reactant
concentration, the dependence expressed by eq 2 resembles the
rate of a photocatalytic reaction of the Langmuir-Hinshelwood
type (LH).2,9
The experimental dependencies of the initial steady-state PhA
rate r(p) of simple molecules (O2, H2, CO, CH4) on gas pressure
have been approximated by eq 1 for a number of metal
oxides,5,10-12 alkali halides,13 and other solids.14,15 A rather
different treatment other than LH of r(p) dependencies for gas/
solid systems is used to interpret the PhA kinetics obtained at
relatively low pressure (p < 10 Pa) and at room temperature.
The concept of the photoinduced active center is applied in the
latter case. To our knowledge, the first and simplest example
of such a mechanism was given in the work of Rapoport and
co-workers10 for the PhA of H2 and CH4 on TiO2 particulates.
Langmuir-Hinshelwood and Eley-Rideal Kinetic Treat-
ments. To describe photostimulated adsorption of molecules
at a semiconductor (or insulator) surface, two relevant cases
are examined in the framework of the electronic theory of
catalysis.4 In the first case, PhA is treated as a disturbance of
the adsorption equilibrium due to formation of strongly bound
† Concordia University.
‡ University of St. Petersburg.
§ Permanent address: Department of Physics, University of St. Peters-
burg, Ulianovskaya 1, St. Petersburg, 198904, Russia.
10.1021/jp9830373 CCC: $15.00 © 1998 American Chemical Society
Published on Web 12/08/1998

Photosimulated Reactions at the Surface of ZrO2 and TiO2
J. Phys. Chem. B, Vol. 102, No. 52, 1998 10907
In this study10 (see also the study of Dolgikh et al.14), generation
of a photoinduced surface-active center S* was simplified to S
+ hν f S*. The rate of generation of S* is determined by the
absorption cross section kabs of some “potential” surface-active
center S. Decay of this active center, S* f S, can be described
as a spontaneous reaction with first-order kinetics, kd. Adsorption
of a gas molecule Mgas: (S* + Mgas f Mads) with the second-
order rate constant (ka), is represented as the collision of gaseous
molecules M (H2, CH4) with surface centers S*. Under steady-
state conditions for the exited state of the adsorption center (dS*/
dt ) 0), the initial PhA rate r(p) is given by
number of metal oxides.5 In the course of PhA kinetic studies,5
measurements of the r(F) dependence on photon flow were
carried out at high gas pressures (Kp . 1 in eq 1).
ZrO2 as a Photocatalyst and Photoadsorbent. Zirconia
(ZrO2; Ebg ≈ 5.0 eV) is an active and typical photoadsorbent
and photocatalyst among wide band gap metal oxides in
accordance with the empirical correlations between the metal
oxide’s PhA activity and its band-gap energy Ebg.5 The red limit
of the PhA for O2, H2, CH4, C2H6 on powdered ZrO26,26,27 is at
ca. 3.0 eV (λ ) 410 nm) in the extrinsic absorption band region.
PhA of methane and ethane is the first stage of their photooxi-
dation and conversion to other hydrocarbons.26,28 The quantum
yield Φ of propene photooxidation over powdered zirconia is
∼0.03;29 Φ of PhA of O2 reaches 0.02 on irradiation at the red
edge of the fundamental absorption band.30 ZrO2 powder is
photocolored under UV excitation due to formation of stable
kabsFSokap
(kd + kap)
dp
dttf0
r(p) ) -
)
(3)
31
defects (color centers) with trapped electrons (Zr3+, F+, F)
where F is the photon flow, So is the initial concentration of the
light absorbing centers, and p is the initial gas pressure. Equation
3 can be reduced to eq 1 by substituting kabsFSo for k and ka/kd
for K. This notwithstanding, the physical sense of the constant
K ) ka/kd differs from that in the LH kinetic treatment. The
parameter 1/kd ) τd in eq 3 refers to the lifetime of the surface-
and holes (O-•-like V centers).6,32 Color centers localized at
the surface serve as long-lived PhA centers (i.e. postirradiation
adsorption centers) for acceptor (O2) and donor (H2, CH4)
molecules, respectively. However, the activity of the PhA of
O2 under UV illumination of ZrO2 (∼90%) has been assigned
mostly to shallow trap centers (lifetime τd ≈ 10-2 to 10-3 s).7,32
Prior UV photocoloration of ZrO2 under vacuum shifts the
red limit of the PhA of O2 to ∼1.5 eV (λ ≈ 830 nm).6,7 PhA of
this kind may be described as a postphotostimulated photoad-
sorption (PPSPhA) effect; it needs two-step photoexcitation: (i)
prior UV coloration to produce surface and bulk color centers,
and (ii) subsequent photoionization of the bulk color centers
by visible radiation to induce further adsorption.6,33 Studies of
the PPSPhA of O2 undertaken herein provide additional support
for the proposed PhA mechanism. Comparative studies of light-
induced processes in micro- and nanoparticles of zirconia carried
out in vacuo and in a solvent medium show similarities between
the photophysical and photochemical behavior of zirconia
irrespective of the type of heterogeneous system examined.27
We also present a preliminary treatment of photocatalytic
processes in a solvent medium: the photooxidation of phenol
in aqueous TiO2 dispersions.
active center (S*), whereas the LH reverse constant 1/kdes
)
τdes refers to the lifetime of a molecule at the surface in an
adsorbed state. Taking ka as the rate constant for collision
between the surface center and a molecule, the lower limit of
τd can be estimated from approximations of the experimental
r(p) dependencies embodied in eq 3. The estimated τd values
for various systems scale in a wide time range from seconds to
microseconds (TiO2/H2/CH4,10 MgO/O2, CaO/O2,11 MgAl2O4/
H2,O2,12 KBr/O2,13 and AgBr/O214}. A similar mechanism has
been offered for the photooxidation of gaseous hydrogen by
oxygen prephotoadsorbed on KBr16 and on MF2, where M )
Mg, Ca, Sr, Ba.17 Lifetimes τd ranging from 10-2 to 10-4 s have
been determined for the spontaneous decay: O2*ads f O2 ads
1
of excited (presumably singlet ∆g O2-like) adsorbed species.
Note that the mechanism suggested by eq 3 is of the Eley-
Rideal type (ER).5,10,14,16,17 The resemblance of the experimental
kinetics predicted by LH and ER reaction mechanisms has been
discussed earlier.2,9,18,19 Whatever the type of mechanism, the
dependence of the photocatalytic reaction or the PhA rate on
photon flow r(F) (eqs 1 and 2) is referenced only by the k(F)
dependence on photon flow; k(F) ) kabsFSo in eq 3 is an
example. The orders of the rate of reaction or of the PhA process
(r pnFm) on photon flow m and on the reagent concentration
or gas pressure n are independent of each other in the LH and
ER mechanisms. To our knowledge, the only exception is the
work of Upadhya and Ollis20 which considered the degradation
of TCE (trichloroethylene) over TiO2 and proposed a mechanism
to explain the existence of the interdependence of the reaction-
rate dependencies on concentration and on light intensity.
Experimental Section
ZrO2 Sample. The powdered monoclinic form of ZrO2 used
(high-purity grade, IREA) in this study was prepared from
zirconium chloroxide. The major impurity in the sample is Fe
(100-120 mg L-1); total content of other metal impurities (Mn,
Cu, Co, Cr, Ti) was <100 ppm. The specific surface area of
the sample was determined by the BET method with nitrogen
gas: ∼7 m2 g-1. X-ray diffraction structure methods (X-ray
diffractometer DRON-2, Co KR line} confirmed that a high-
temperature pretreatment and preirradiation of the zirconia
samples caused no modifications to the crystalline form.
Sample Pretreatment. Ubiquitous organic impurities and
adsorbed molecules on the sample surface were removed by
thermal pretreatment (T ) 900 K) in an O2 atmosphere (P )
100 Pa) and in vacuo for a few days. Reproduction of the
original state of the samples between experiments was achieved
by heating the samples in oxygen for ca. 1 h.
The reaction order m has been determined for various
photoreactions primarily over titania.21 Typically, m varies
between 1 and 0.5. Similar results were obtained for photo-
catalysts other than TiO2.22,23 The half-order limit of m has been
ascribed to band-to-band recombination of free electrons and
holes or, otherwise, to the recombination of intermediate reaction
products (see, for example the studies21,24,25). Reaction orders
1
m < /2 have also been reported for photooxidative reactions
Experimental Setup. Powdered samples were contained in
a 5-mm path length quartz cell (illuminated cell area, 6 cm2)
connected to a high-vacuum setup equipped with an oil-free
pumping system; the final gas pressure in the reaction cell was
ca. 10-7 Pa. The volume of the reactor occupied by the gas
was 40 cm3. The cell was thermostated with air flow at T )
of organic substrates over titania/solution systems under ex-
tremely high photon flow. The diffusion limitation of the
reaction in a solvent medium was attributed to a sub-half-order
intensity dependence.21 A linear dependence of oxygen and
hydrogen PhA rates on photon flow has been established for a

10908 J. Phys. Chem. B, Vol. 102, No. 52, 1998
Emeline et al.
TABLE 1: Coefficients k and K and Lifetime of Active Centers τd at Different Relative Light Intensities
light intensity, relative units
0.1
coefficient k, Pa s-1 {mol s-1
}
coefficient K, Pa-1 {mol-1
}
lifetime τd, s
0.20 ( 0.03
0.00051 ( 0.00001
61.1 ( 8.3
{(83.8 ( 1.6) × 10-13
}
{(37.2 ( 5.0) × 108}
46.2 ( 5.0
0.15
0.00075 ( 0.00001
0.150 ( 0.015
0.075 ( 0.012
0.040 ( 0.006
0.029 ( 0.005
0.024 ( 0.004
0.047 ( 0.003
{(123.2 (1.6) × 10-13
}
{(28.1 ( 3.0) × 108}
23.3 ( 3.6
0.3
0.00149 ( 0.00005
{(245 ( 8) × 10-13
}
{(14.2 ( 2.2) × 108}
12.5 ( 1.7
0.5
0.00263 ( 0.00009
{(43.2 ( 1.5) × 10-12
}
{(7.6 ( 1.0) × 108}
9.1 ( 1.6
0.78
0.0040 ( 0.0002
{(65.7 ( 3.3) × 10-12
}
{(55.4 ( 9.7) × 107}
7.3 ( 1.5
1
0.0051 ( 0.0003
{(84 ( 5) × 10-12
}
{(44.4 ( 9.1) × 107}
14.7 ( 0.6
PPSPhA (λ > 400 nm)
0.00113 ( 0.00003
{(186 ( 5) × 10-13
}
{(89.5 ( 3.6) × 107}
293 K. A Pirani-type manometer (sensitivity, 18 mV Pa-1 for
O2) was used to measure gas pressures under photoadsorption
conditions. Analysis of gas composition was obtained using a
mass spectrometer MX-7301; it was also used to measure the
quantity of postirradiation adsorbed oxygen by means of
thermally programmed desorption techniques. Irradiation of
powdered ZrO2 samples was carried out with a 120-W high-
pressure mercury lamp (DRK-120; MELZ); the light irradiance
(6 mW cm-2) at wavelengths below 400 nm was measured
through a water filter using a thermoelement (IOFI; sensitivity,
1.5 V W-1). The maximum photon flow Fo at wavelengths below
260 nm was about 1 × 1015 photons cm-2 s-1. Attenuation of
Fo was achieved with nonselective “gray” metal-supported quartz
filters (Vavilov SOI). Cutoff filters from a standard set of
colored glass filters (Vavilov SOI) selected the excitation
spectral regions or the mercury spectral lines for the PPSPhA
studies (see below). For the latter studies, a 250-W lamp was
also used. Any experimental error in the PhA rate measurements
that may have been caused by the nonreproducibility of the
original state of the ZrO2 sample does not exceed 10% at,
respectively, high oxygen pressure (p > 1 Pa) and at the photon
flow used (F ) 1 × 1015 photons cm-2 s-1).
Figure 1. Dependencies of initial rates of oxygen photoadsorption rF(p)
on pressure p at different photon flows (Fo ) 1 × 1015 photons cm-2
s-1): (1) F ) Fo; (2) F ) 0.78Fo; (3) F ) 0.5Fo; (4) F ) 0.3Fo; (5) F )
0.1Fo.
PhA O2 rates rF versus initial oxygen pressure p at various
photon flows are plotted in Figure 1. Initial PhA rates r were
determined by differentiation of p(t) curves (dependence of
pressure on exposure time, p(t)). Note that initial PhA rates
determined in this manner were the highest for any p(t) curve
recorded. The highest photon flow (F ) 1 × 1015 photons cm-2
s-1 at λ < 260 nm) is taken as a unit of measure. The maximum
initial PhA rate at the highest oxygen pressure used (p ) 2.8
Pa) and maximum photon flow is 0.0047 Pa s-1, i.e., 7.7 ×
10-11 mol s-1 (or 4.6 × 1013 molecule s-1) from the volume of
the reactor and the temperature (see Experimental Section). All
curves of the set were approximated by eq 1, and values of the
constants k and K are presented in Table 1 and plotted in Figure
2.
TiO2 Sample. In preliminary experiments to study the
dependence of the photooxidation of phenol in aqueous solutions
on light intensity and concentration of phenol, we used
“Degussa” P25 TiO2 as the photocatalyst. Experiments were
carried out at pH ) 3 (HCl). Phenol concentration was measured
by liquid chromatography with an HPLC chromatograph
(Waters 501 pump and Waters µBondapak C18 column)
connected to a Shimadzu flow cell for absorption recording with
the Shimadzu UV-265 spectrophotometer interfaced to an IBM
PC computer. Recording parameter settings, data collection, and
data processing were carried out using the Spectroscopy
Interface Software, Version 3 (Shimadzu Scientific Instruments,
Inc.). Irradiation of a phenol solution with a TiO2 loading of
0.3 g L-1 was carried out with an Oriel 1000-W Hg/Xe lamp
in a sphere-like Pyrex reactor. An interference filter selected
the 365 nm actinic light. Nonselective gray metal-supported
Pyrex filters were used to attenuate the photon flow.
The results show that both constants in eq 1 depend on photon
flow: k depends linearly on photon flow, whereas the constant
K decreases with an increase in photon flow. In such case, K
cannot be considered a constant of the adsorption/desorption
equlibrium (K ) kads/kdes) in eq 1. It may be assumed that the
desorption constant kdes increases with temperature as a result
of the heating of the sample by the incident actinic light. To
verify this assumption, a few r(p) curves were obtained in the
temperature range T ) 300-450 °K at the highest photon flow
available under our conditions, i.e., for F/Fo ) 1. No significant
changes of K are evident. For example, K (450 K) is 6.7 ( 0.6
Pa-1, close to the value obtained at 300 K (see Table 1).
Results
Dependence of the Rate of PhA of O2 on Pressure and
UV Photon Flow. Extrinsic absorption of light by ZrO2 between
its red limit (λ ≈ 410 nm) and 260 nm causes a weak PhA of
O2, whereas band-to-band transitions lead to much stronger PhA
at wavelengths <260 nm. Intrinsic absorption dominates on
illumination of ZrO2 by the full emitted radiation of the mercury
lamp used in the set of experiments described below.
Comparison of eq 3 and eq 1 shows that K can be represented
as K ) ka/kd ) kaτd, where τd is the lifetime of the surface-

Photosimulated Reactions at the Surface of ZrO2 and TiO2
J. Phys. Chem. B, Vol. 102, No. 52, 1998 10909
Figure 4. Dependencies of initial rates of oxygen photoadsorption rp(F)
on photon flow F/Fo for Fo ) 1 × 1015 photons cm-2 s-1 at different
initial oxygen pressures: (1) p ) 2.8 Pa; (2) p ) 0.28 Pa; (3) p ) 0.11
Pa; (4) p ) 0.055 Pa; (5) p ) 0.028 Pa.
Figure 2. Dependencies of the LH (eq 1) parameters k (curve 1) and
K (curve 2) on photon flow F/Fo for Fo ) 1 × 1015 photons cm-2 s-1
.
Figure 5. Dependencies of the parameters a (curve 1) and b (curve 2)
on the initial oxygen pressure p (eq 4).
Figure 3. Dependence of the lifetime of surface-active centers τd on
photon flow F/Fo with Fo ) 1 × 1015 photons cm-2 s-1
.
approximated by the function
active center (see Introduction). Note that 1/K is also equal to
the characteristic pressure p′ at which the probability of
consumption of the active center in the reaction channel (ka p′)
is equal to the probability of its deactivation (kd) (eq 3). Lower
limits of τd estimated from K (Table 1) assumed that ka ) σV/
4κT, where σ ≈ 1 × 10-16 cm2 is the molecule/center collision
cross section, V ) 5 × 104 cm s-1 is the velocity of oxygen
molecules in the gas phase, and k is Boltzmann’s constant. The
dependence of the lifetime of surface-active centers plotted in
Figure 3 follows a hyperbolic decay such that both K and τd
scale with F-1. Thus, in contrast with the simplified PhA
mechanism,10,13,14 in the present instance kd ) 1/τd, which scales
with F and cannot be considered as the first-order rate constant
of spontaneous decay of the active center. The family of r(F)
curves at various oxygen pressures p was also obtained in the
subsequent set of experiments; they are portrayed in Figure 4.
It is clear that the PhA O2 rate r scales directly with photon
flow F only at high oxygen pressures (curve 1). Starting from
the high-intensity limit, the smaller the pressure, the greater is
the deviation of the r(F) curve from the straight line. At the
lowest initial oxygen pressure, p ) 0.028 Pa (curve 5), the r(F)
dependence becomes extremely sublinear. In fact, it is close to
saturation at F > 0.5Fo. In general, all the dependencies can be
abF
(1 + bF)
r(F) )
(4)
where a and b are constants at a given oxygen pressure p. The
dependencies of these constants on oxygen pressure are
presented in Figure 5, from which it is evident that the parameter
a depends linearly on oxygen pressure p, whereas b decreases
with increase in pressure via a hyberbolic decay function. Thus,
a scales with p and b scales with p-1. The corresponding values
of a and b are presented in Table 2.
Equation 4 is equivalent to eq 1 provided that a(p) and b(p)
in eq 4 satisfy the above-mentioned conditions, i.e., a p and
b
p-1; as well, eq 1 corresponds to eq 4 for k F and K
F-1 (see results above). In other words, the fucntions to describe
the experimental r(F,p) dependencies for PhA O2 rates (eqs 1
and 4) are equivalent. The r(F,p) dependence can thus be
generalized as
AFp
(BF + Cp)
r(F,p) )
(5)
where the coefficients A, B, and C do not depend on light

10910 J. Phys. Chem. B, Vol. 102, No. 52, 1998
Emeline et al.
n are interdependent in the case of the PhA of O2 on ZrO2.
Note that ZrO2 is a convenient adsorbent for PhA O2 kinetic
studies because the variation of m ranges from 0.16 to 1 under
moderate (10-fold) changes in photon flow at low pressures
(Figure 6, curve 4); m ) 1 is observed at high pressures (Figure
6, curve 1).
Spectral Dependence of the Lifetime of PhA Centers in
UV-Colored ZrO2. Pre-excitation of wide band-gap solids in
vacuo with UV light leads to their coloration as a result of
formation of color centers, i.e., carriers captured by deep traps.
The subsequent photoexcitation into the absorption bands of
color centers in a gas/solid system causes adsorption of the gas
molecules to the solid surface.33,34 As in the case of Solonitzin’s
effect, i.e., postirradiation adsorption in the dark,35 this kind of
photoadsorption appears after the preirradiation period but
subsequently also needs secondary irradiation. This is what is
referred to as the postphotostimulated photoadsorption effect
(hereafter referred to as PPSPhA).6,33,34 In PPSPhA processes,
the charge carriers appear at the surface because of photoion-
ization of the UV-generated color centers. The PPSPhA of O2
has been observed on 10 of 15 wide band-gap metal oxides
tested,34 including ZrO2.6 The red limit of PPSPhA of O2 on
ZrO2 is ca. 1.5 eV (λ ≈ 830 nm), in accordance with the position
of the red tail of the F center absorption band.31 The red limit
of the PPSPhA of H2 places around 2.14 eV (λ ≈ 580 nm),
where the hole V center band and electron F+ center band
overlap.
Figure 6. Dependencies of initial rates of oxygen photoadsorption {as
ln rp(F)} on photon flow F {as ln(F/Fo)} at different initial oxygen
pressures: (1) p ) 2.8 Pa; (2) p ) 0.28 Pa; (3) p ) 0.11 Pa; (4) p )
0.028 Pa. The straight line corresponds to the first-order process
dependence on photon flow.
TABLE 2: Coefficients a and b at Different Initial Oxygen
Pressures p
initial oxygen pressure,
coefficient a,
Pa {mol m-3
}
Pa s-1 {mol s-1
}
coefficient b
0.028
0.00075 ( 0.00002 7.9 ( 0.8
{1.15 × 10-5
0.055
}
}
}
}
}
}
}
{(123.2 ( 3.3) × 10-13
}
The dependencies of the initial PPSPhA O2 rates (rPPSPhA
)
0.00193 ( 0.00001 2.74 ( 0.02
on initial oxygen pressure p satisfy eq 1. The corresponding
coefficients k and K and τPPSPhA are presented in Table 1 (last
row). In this case, pre-UV-excitation of ZrO2 in vacuo was
carried out with the full emitted light of the DRK-120 mercury
lamp for 600 s. Illumination of the sample in oxygen was
conducted by the same lamp but with a cutoff filter having a
transmission threshold of ∼400 nm, i.e., in a wide range of
both F- and V-color-center absorption bands.
{2.26 × 10-5
0.11
{(317.1 ( 1.6) × 10-13
}
0.00321 ( 0.00015 1.62 ( 0.15
{4.52 × 10-5
0.28
{(52.7 ( 2.5) × 10-12
}
}
}
}
}
0.0158 ( 0.0004
0.45 ( 0.02
{1.15 × 10-4
1.1
{(259.6 ( 6.6) × 10-12
0.083 ( 0.006
0.058 ( 0.002
0.0404 ( 0.0005
0.0228 ( 0.0005
{4.52 × 10-4
1.6
{(136.3 ( 9.9) × 10-11
0.121 ( 0.002
{6.57 × 10-4
2.8
{(198.8 ( 3.3) × 10-11
0.214 ( 0.003
The lifetime τPPSPhA depends on the wavelength of the incident
actinic light within the range of the color centers absorption
(260-900 nm) in ZrO2; τPPSPhA ) 67 ( 5 ms from rPPSPhA versus
{1.15 × 10-3
{(351.6 ( 4.9) × 10-11
intensity and gas pressure. Note that eq 5 can be cast into the
Langmuir-Hinshelwood form (eq 1) when the photon flow F
is constant.
p curve under illumination by blue light (λ ) 436 nm, F436
)
7.2 ( 0.1 × 1014 photons cm-2 s-1). By contrast, rPPSPhA does
not depend on pressure in the pressure range p ) 0.02-5 Pa
for a UV-colored sample under illumination by red light (λ >
700 nm, F700 ≈ 5 × 1014 photons cm-2 s-1). In the latter case,
the characteristic pressure p′ (see above) is less than 0.02 Pa;
hence, τPPSPhA for red light is greater than 1 s. Note that τPPSPhA
for red light can, in principle, be greater than 104 s. The so-
called coefficient of postirradiation adsorption (Ω ) Na/Nph,
where Na is the number of post-irradiation adsorbed molecules
in vacuo for a given time period and Nph is the number of
photoadsorbed molecules during irradiation for the same
exposure time,5 was near unity for red light irradiation, in
contrast to that for blue light illumination (Ω436 ≈ 0.1). In other
words, Na ) Nph for oxygen adsorbed on UV-colored ZrO2
during a fixed exposure time by red light illumination. Also,
no change in Ω ) 1 was observed when the delay time between
the end of the red light exposure in vacuo and the introduction
of oxygen into the reaction cell was changed from 5 to 7 × 103
s. Hence, the delay time of ∼104 s can be taken as the lower
limit of τPPSPhA for red light illumination (λ > 700 nm). Results
of PPSPhA O2 studies on ZrO2 will be published in some detail
elsewhere.36
The PhA O2 rate can also be described by the function
r(F,p) ) (const)pnFm
(6)
where n and m are the orders of the reaction on pressure and
photon flow, respectively. The double logarithmic r(F) plot for
several fixed pressures p is shown in Figure 6. The high-pressure
(p ) 2.8 Pa) curve 1 has a slope m ) 0.98 (correlation
coefficient ) 0.9999). The remaining curves have variable
slopes, e.g., at the higher photon flow and at the smaller
pressures the smaller the slope for a given curve. For the lowest
pressure examined (p ) 0.028 Pa), the slope in the high photon
flow region (last four points of curve 4) is 0.16 (correlation
coefficient ) 0.9986). However, the slope of all curves is close
to 1 in the low photon flow region. Slopes estimated using the
first few data points (including those not shown for F < 0.1)
place m in the 0.75-1.25 range for the family of curves. The
reaction order m in eq 6 varies with pressure in accordance with
eq 5; m approaches unity as p f ∞ (i.e., for Cp . BF) and
conversely m f 0 if p f 0 (or if Cp , BI). Similarly, the
order n approaches unity if F f ∞ at BF . Cp and conversely
n f 0 if F f 0 provided that BF , Cp. In other words, m f
1 if n f 0, whereas n f 1 if m f 0. The reaction orders m and
Only a slight distinction in shape of the dioxygen (m/e )
32) thermally programmed desorption spectra was observed, and

Photosimulated Reactions at the Surface of ZrO2 and TiO2
J. Phys. Chem. B, Vol. 102, No. 52, 1998 10911
the ZrO2/O2 heterogeneous system (compare Figures 1 and 8.).
Thus, K cannot be considered a LH constant.
In the next set of experiments we assessed the dependencies
of the rate of phenol photooxidation on photon flow (Figure 9)
on irradiation at λ ) 365 nm for two different phenol
concentrations: C ) 128 (curve 1) and 4 µM (curve 2). Both
dependencies are approximated by eq 4 with a ) 0.28 ( 0.09
µM min-1 and b ) 0.24 ( 0.09 for C ) 128 µM (curve 1) and
a ) 1.76 ( 0.04 × 10-2 µM min-1 and b ) 7.2 ( 0.7 for C )
4 µM (curve 2). Note also that a increases with an increase in
the concentration of phenol, whereas b decreases with an
increase in the phenol concentration. At high phenol concentra-
tion, the dependence of the reaction rate on photon flow is nearly
linear, albeit the rate dependence approaches saturation; at small
phenol concentrations, the rate dependence on photon flow is
strongly sublinear (close to saturation) while the rate dependence
on concentration is almost linear, again similar to experimental
results obtained for the ZrO2/O2 heterogeneous system. The
existence of an interdependence of reaction orders on phenol
concentration and on light intensity can therefore be assumed
for the reaction of the phenol photooxidation in TiO2 aqueous
dispersions.
Figure 7. Thermoprogrammed desorption spectra of adsorbed oxygen
(m/e ) 32) after UV-induced photoadsorption, PhA (1), after postpho-
tostimulated photoadsorption, PPSPhA (λexc > 400 nm) (2), and after
post-irradiated adsorption of oxygen on UV preirradiated (in vacuo)
ZrO2 (3).
Discussion
General Description of PhA Mechanism for Wide Band-
Gap Solids. The mechanism of photostimulated adsorption has
been described earlier in connection with the effect of PhA on
photoinduced defect formation in metal oxides6,27,33,37 and in
alkali halides.6 Computer simulation of PhA kinetics (i.e.
temporal changes of pressure p(t)) at zero reaction order (n )
0) for the r(p) dependence has also been undertaken.36,37 The
emphasis in the present study is on an examination of the decay
channels of PhA active centers to rationalize the r(F,p) depend-
encies for the PhA of O2 on ZrO2. We consider the following
stages in the PhA mechanism
Λ + hν f e + h (kabs; photoexcitation of lattice Λ)
(stage 1)
R + e f R- (ket; trapping of electrons)
R- + h f R (kht; recombination of
(stage 2a)
Figure 8. Dependence of the rate of phenol photooxidation dC/dt over
TiO2 on phenol concentration C at different photon flows {as F/Fo}:
(1) F ) Fo ) 1.1 × 1017 photons cm-2 s-1; (2) F ) 0.12Fo.
the same desorption peaks were detected at 450, 580-590, and
700 K for PPSPhA under illumination by blue (λ ) 436 nm)
and red (λ > 700 nm) light (see Figure 7). Hence, the types of
PPSPhA centers and oxygen species formed on ZrO2 are
independent of the irradiation wavelength. A difference of a
few orders of magnitude in the lifetimes of surface-active
centers, i.e., between τPPSPhA for red and blue light illumination,
appears because the F centers that absorb the red light produce
only electrons whereas electron F+ centers and hole V centers
excited by blue light generate both free electrons and free holes,
respectively, in UV-colored ZrO2.
Photooxidation of Phenol in Aqueous Dispersions over a
Smaller Band-Gap Metal Oxide (TiO2). The dependencies of
the rate of phenol photooxidation on phenol concentration at
two different fixed light intensities F ) Fo ) 1.1 ( 0.3 × 1017
photon cm-2 s-1 (curve 1) and F ) 0.12 Fo (curve 2) at λ )
365 nm are presented in Figure 8. The experimental data can
be approximated by LH-like kinetics (eq 1) with k ) 6.0 ( 0.1
× 10-2 µM min-1 and K ) 7.5 ( 0.6 × 10-2 µM-1 at F ) Fo
and k ) 9.9 ( 0.2 × 10-3 µM min-1 and K ) 1.2 ( 0.3 µM-1
when F ) 0.12 Fo. Clearly, k increases with an increase in
photon flow, whereas K decreases with an increase in incident
photon flow. This is similar to the experimental results from
trapped e with free hole) (stage 2b)
Va + e f F (kF; formation of color centers)
Vc + h f V (kV; formation of color center)
S + e f S- (kc; formation of active surface center)
(stage 3a)
(stage 3b)
(stage 4)
S- + h f S (kR; recombination decay of active center)
(stage 5a)
S- + hν f S + e (kph; photoionization decay
of active center) (stage 5b)
S- f S + e (kth; thermal decay of active center) (stage 5c)
S- + M(gas) f M-(ads) (ka; adsorption of
gaseous molecule) (stage 6)
The absorption of photons (hν) by the regular crystal lattice
(Λ; note this symbol represents sites on the surface and in the
bulk) in the fundamental absorption band (absorption cross
section, kabs) creates free electrons (e) and free holes (h) in the
conduction and valence bands, respectively (stage 1). The main

10912 J. Phys. Chem. B, Vol. 102, No. 52, 1998
Emeline et al.
of adjacent trapped carriers by tunneling,38,39 is not treated in
this work.) To simplify the expression for r(F,p) (see below),
all three pathways (stages 5a-c) are considered for the same
PhA center S-, although the probability that all three processes
described by stages 5a-c occur is very small since these differ
strongly from each other for a given type of center in wide band-
gap solids. Which of these three stages predominates will depend
on whether the center S- is a shallow trap in which thermal
ionization dominates or a deep trap in which thermal ionization
is negligible. In turn, deep traps can be further subdivided into
recombination centers (rate of recombination is very high,
denoted above as R and R-) and color F and V centers (rate of
decay by recombination is moderate). Contrary to recombination
centers (R-), color centers F and V accumulate in the crystal
in sufficiently high concentrations and persist for long periods
after terminating irradiation. For simplicity, decay of color
centers through recombination and photoionization are also not
treated in the PhA mechanism described above. Photoionization
of color centers will be discussed subsequently in connection
with the PPSPhA of O2 on ZrO2. Suffice to mention that other
types of decay processes may be considered in relation to any
quasi-chemical particle or defect that has trapped a carrier; this
can include photoadsorbed molecular species. For example,
recombination of free holes with M- species (trapped electrons)
could break the surface/adsorbate bond. The latter can be
associated with indirect thermal or indirect photodesorption (via
excitation of the solid) depending on whether thermal or
photoliberation of holes takes place. Direct thermal and pho-
todesorption are associated with thermal and photoionization
of M- species, respectively. The simultaneous occurrence of
both photoadsorption and photodesorption under irradiation has
been demonstrated for metal oxides.40 However, the rate of
photodesorption was much smaller than the rate of photoad-
sorption (stage 6) at the beginning of the overall PhA process,
i.e., when initial PhA rates were measured.
Figure 9. Dependencies of the rate of phenol photooxidation over TiO2
on photon flow F/Fo {where Fo ) 1.1 × 1017 photons cm-2 s-1} at
different phenol concentrations: (a) C ) 128 µM; (b) C ) 4 µM.
channel of recombination of free carriers is their subsequent
trapping by the recombination centers R (stage 2). We presume
that electron/hole recombination occurs first by electron capture
(stage 2a, ket) and that recombination of holes (stage 2b, kht)
with trapped electrons (R-) occurs subsequently. Note that
trapping of the hole as a first step followed by recombination
with the electron or a consideration of both paths in the
mechanism has no influence on the final generalized results.
Note also that band-to-band recombination of free carriers (e
+ h f Λ) has not been included in the mechanism because
recombination of charge carriers through defects dominates in
very or moderately wide band-gap solids.
The photocoloration of solids is arbitrarily taken to originate
from trapping of free electrons and holes by bulk defects, e.g.,
by anion Va (stage 3a, kF) and cation Vc (stage 3b, kV) vacancies,
with formation of F and V color centers, respectively. In fact,
a variety of intrinsic and extrinsic lattice defects other than
vacancies can serve as deep carrier traps in the formation of
color centers. To simplify the quasi-chemical equations, herein
we shall adopt the notations valid for alkali halide M+X- ionic
crystals for which vacancies are singly charged in relation to
the lattice and for which F centers {Va+/e-}0 and V centers
{Vc-/h+}0 are neutral species. In the case of ZrO2 (see below),
the commonly accepted notations for F and F+ refer to the
neutral state {Va2+/2e-}0 and the positively charged {Va2+/e-}+
state, respectively. Similarly, the notation V refers to all kinds
of V-type color centers, V (Vc4-/jh+)j-4, where 1 e j e 4 (In
reality, j e 2 because of Coulombic repulsions between holes).
The V centers, especially those located at the metal oxide
surface, have also been described in photocatalysis as O-•
species.
In the quasi-stationary approximation (d[S-]/dt ) 0; d[e]/dt
) 0; d[h]/dt ) 0) the photoadsorption rate dp/dt (i.e. d[M]/dt)
can be expressed in terms of steady-state concentrations of
photoelectrons (e) and photoholes (h):
kcS[e]kap
dp
dt
-
)
(7)
(kR[h] + kphF + kth + kap)
Note that in the case of a dielectric, only the photocarriers
can be considered. This is also true for semiconductors in the
case of sufficiently high photon flow, i.e., when the concentra-
tion of photocarriers becomes much greater than the concentra-
tion of thermal majority and minority carriers.
The steady-state concentration of photoelectrons and photo-
holes is given by:
[e] ) kabsΛFτe ) RFτe ) gτe
[h] ) kabsΛFτh ) RFτh ) gτh
(8)
(9)
Formation of active PhA centers occurs via capture of free
electrons and holes by surface traps S. For completeness,
creation of electron PhA centers S- are indicated in stage 4
(kc). The centers interact with gas molecules M to yield adsorbed
under conditions that
kabsΛF . [kphS-F + kthS-]
(10)
-
-
-•
species M (stage 6, ka). For example, M can be the O2
radical anion in the case of the PhA of O2 on ZrO2.32
This condition (eq 10) is reasonable because the rate of
electron generation at band-to-band transitions (stage 1) is
usually much greater than the corresponding rates of their photo
(stage 5b) and thermal (stage 5c) detrapping.
Stages 5 describe three possible pathways for the decay of
PhA centers: (i) recombination with free carriers of opposite
sign (stage 5a, kR), (ii) photoionization of PhA centers by
incident photons (stage 5b, kph) and (iii) thermal ionization of
PhA centers (stage 5c, kth). (A fourth reaction, i.e., recombination
The absorption coefficient R (cm-1) is related to the
fundamental band such that g ) RF (cm-3 s-1) is the volume

Photosimulated Reactions at the Surface of ZrO2 and TiO2
J. Phys. Chem. B, Vol. 102, No. 52, 1998 10913
rate of generation of carriers
1
mechanism while others might also implicate the PhA O2 centers
on ZrO2.6,27 The sample is assumed to be illuminated uniformly,
whereas illumination of the active surface of the sample in the
cell used (see Experimental Section) is strongly irregular due
to light absorption and light scattering among the sample
particulates. Hence, kinetic constants evaluated from the
experimental data on the basis of the mechanism proposed
represent an average of the relevant constants over several
possible surface centers and over a distribution of photon flow.
The estimated lifetime of the PhA centers decreases from
0.2 to 0.02 s as the light intensity increases (Table 1). From eq
13 or 15 and Figure 3 we deduce that the term (RτhkRF + kphF)
τe )
(11)
ketR + kFVa + kcS
and
1
τh )
(12)
khtR- + kVVc + kRS-
are the lifetimes of free electrons (eq 11) and free holes (eq 12)
from the various channels of carrier trapping. In general, τe and
τh are interdependent, vary with time because of carrier trapping
by vacancies (see stages 3a and 3b) whose concentration is not
stationary, and depend on light intensity. Nevertheless, τe and
τh are taken as constants. This is a realistic assumption in the
case of wide band-gap solids irradiated by light of moderate
intensity within a narrow variation range. In such a case, when
the concentration of recombination centers (ca. 1017 to 1019
cm-3) is greater than the concentration of free carriers, the
lifetime of free carriers is determined by the magnitude of the
corresponding trapping cross-sections (also constants) of the
recombination centers.41 We must also point out that the present
studies deal with the initial stages of adsorption when the carrier
lifetime depends on the initial concentration of defects (see eqs
11, 12), which is the same for each experiment regardless of
the conditions used.
is greater than kth when F > 1.5 × 1014 photons cm-2 s-1
.
Moreover, according to the experimental results obtained at
different temperatures and at excitation in different spectral
regions, we can assume that kth ) 0 since kd ) 1/τd (see eqs 3
and 15) is independent of temperature and approaches zero
during red light irradiation. On the basis of kinetic studies alone,
it is difficult to separate the relative contributions of the
recombination term (RτhkRF) and photoionization term (kphF)
to the decay constant kd (eq 15). However, the photoionization
path is not likely to be responsible for the decay of PhA centers.
Using a large kph or otherwise an ionization cross section of
the surface-active centers of σph ) 1 × 10-16 cm2, we obtain a
lower limit of τd ) 10 s for F ) 1 × 1015 photons cm-2 s-1
.
Thus, the term kphF in eq 13 and in the subsequent expressions
can be neglected for the PhA of O2 on ZrO2.
On the basis of the above discussion and eqs 7-9, it follows
that the initial PhA rate can be described as
In contrast, the decay of PhA centers by a recombination path
seems reasonable. Let τd equal 1/σRV[hs] where σR is the
recombination cross section of PhA centers (in our notation S-),
V ≈ 107 cm s-1 is the velocity of carriers, and [hs] is the
concentration of photoholes at the surface. In deducing eq 13
we assumed that the concentration of both charge carriers in
the bulk of the solid (see the PhA mechanism, stages 1-3) and
at its surface (stages 4 and 5) was the same. In fact, the
concentration of photocarriers at the surface may be smaller
than in the bulk of the crystal lattice due to surface recombina-
tion of the carriers. Thus, although estimation of the hole
concentration [hs] by eq 9 may not reflect the actual concentra-
tion, the estimate of the uniform generation of carriers (g )
RF) does give reasonable values. Similarly, the term (RτhkRF)
in eq 15 qualitatively describes the rate of decay of PhA centers
through recombination and cannot be used for a numerical
estimation of τd times. The problem of determining the
concentration of photocarriers at the crystal surface was solved
in earlier studies devoted primarily to the photoconductivity of
solids41-43 as well as to photocatalysis.44-47 In the present
instance, the characteristic size of the crystalline particle (d ≈
10-4 cm) is nearly identical to the inverse of the absorption
coefficient R-1 (1 × 104 cm-1 at λ ) 254 nm for ZrO248). On
the basis of the results of earlier studies it can be shown49 that
in the absence of surface charge and at d ≈ R-1, the
concentration of holes at the surface may be determined from
eq 20
RFτekcSokap
RFτhkR + kphF + kth + kap
r ) -
(13)
where p is the initial concentration of the molecule M in the
gas phase and So is the initial concentration of surface traps
(stage 4). Equation 13 can be reduced to eq 3 by setting
Rτekc ) kabs
(14)
and
(RFτhkR + kphF + kth) ) kd
k ) RFτekcSo {i.e., k F}
(15)
and to eq 1 for
(16)
(17)
ka
K )
{i.e., K F-1}
RFτhkR + kphF + kth
or in the form of eq 4, for
RτekcSokap
a )
{i.e., a p}
(18)
(19)
τhkR + kph
RτhkR + k
kth + kap
b )
ph {i.e., b p-1}
RFL2
[hs] )
(20)
sL
D 1 +
as well as in the form of eq 5, where A ) RτekcSoka, B ) RτhkR
+ kph, and C ) kap with the additional constant kth in the sum
of the denominator in eq 5.
Mechanism of Oxygen Photoadsorption on ZrO2. The
experimental dependence r(F,p) for oxygen on ZrO2 (eqs 1, 2,
4, and 5) also satisfy eq 13 obtained from the PhA mechanism
described above. However, a few points are worth noting. For
simplicity, a single surface center (S, S-) is considered in the
(
)
D
where the diffusion length of the holes L is such that RL < 1,
D is the diffusion coefficient for holes, and s is the rate of
surface recombination. Let D equal µκT/e (10-2 cm2 s-1) at T
) 300 K, and the mobility of holes µ ≈ 1 cm s-1 V-1, typical
of carriers in wide band-gap ionic metal oxides.50 Estimated
values of s (ca. 105 cm s-1) and L (ca. 10-5 cm) are reasonable

10914 J. Phys. Chem. B, Vol. 102, No. 52, 1998
Emeline et al.
in the present case. Note that the diffusion length L given by
(Dτh)1/2 leads to realistic lifetimes for holes in the bulk of the
crystal lattice; namely, τh ≈ 10-8 s. For these s, D, and L
estimates and for a photon flow F ) 1 × 1015 photons cm-2
s-1, eq 20 yields [hs] ≈ 109 cm-3 and the lifetime of PhA centers
τd ) 1/σRV[hs] equals the experimentally found τd ) 2.3 × 10-2
s at F ) 1 × 1015 photon cm-2 s-1 (see Table 1) for σR ≈
10-14 cm2. Note that this recombination cross section σR is
typical for carrier trapping by Coulomb centers in solids. Hence,
the measured lifetimes (Table 1) coincide within an order of
magnitude with those expected for recombination decay of PhA
centers.
Whatever the magnitude of the estimated times τd presented
above are, results of PPSPhA studies show that decay of PhA-
O2 centers by recombination on ZrO2 does occur. The difference
in the lifetime τPPSPhA between a few tenths of milliseconds and
>104 s depends on whether blue light (λ ) 436 nm) or red
light (λ g 700 nm) induced oxygen photoadsorption on UV-
colored ZrO2 occurs. Absorption bands of electron F+ and hole
V-type centers overlap in the blue region of the UV-induced
color in ZrO2.6,27,31 In contrast, only F centers can be excited
by red light. To describe the photoexcitation of UV-colored
ZrO2, the set of quasi-chemical processes described by stages
1-6 should be supplemented with
deep PhA centers seems unlikely. As a result, τPPSPhA > 1 s
from the r(p) dependence on red light irradiation is not in
contradiction with reasonable τd values for photoionization of
PhA centers (see above). For the same reason, the coefficient
of postirradiation adsorption Ω is nearly unity for red light
illumination.
Our previous view of “short-lived” or “long-lived” PhA
6
centers in ZrO2 should be reexamined in connection with the
recombination decay of PhA centers observed in the present
study. Earlier it was thought that O2 interacts with short-lived
active centers primarily when UV light induces PhA of O2 on
ZrO2. Only 10% of molecules adsorb at long-lived PhA centers
in accordance with the coefficient of postadsorption Ω ≈ 0.1.6
The lifetime of these centers is longer than 104 s, as judged
from the delay time of postirradiation adsorption of oxygen.
From the effect of oxygen postadsorption on diffuse reflectance
spectra of UV preirradiated ZrO2, it was concluded that Zr3+
,
F+, and F centers localized at the surface were responsible for
postadsorption of oxygen.6 The lifetime of short-lived centers,
which scaled in the milliseconds range, was estimated from the
r(p) dependence under UV excitation of ZrO2 by light with a
photon flow F similar to that applied in this study. The short-
lived PhA centers in ZrO2 were attributed to some electron
shallow traps that thermally decay by first-order kinetics. The
distinction between short-lived and long-lived PhA centers
identical to that made for ZrO2 above was also made for the
systems KBr/O2 and KI/O2,13 MgAl2O4/O2/H2,12 and others.5
However, present results suggest that deep PhA centers can
display a bimodal behavior. Being accumulated at the surface
during UV preirradiation in vacuo, such centers persist in an
active state for extremely long times (hours to even days5)
whereas these same centers behave as short-lived adsorption
centers under permanent irradiation in a gas/solid system when
the decay of these PhA centers takes place by recombination
with photocarriers of the opposite sign. Interestingly, PhA
centers for H2 in Sc2O3 display a similar behavior.33
F+ + hν (436 nm) f
Va + e (photoionization of F+ center) (stage 7a)
V + hν (436 nm) f
Vc + h (photoionization of V center) (stage 7b)
F + hν (>700 nm) f
F+ + e (photoionization of F center) (stage 7c)
where the symbols for F and F+ centers and for anion vacancies
denote the point defects in the bulk of the crystal lattice because
introduction of oxygen into the reaction cell before the secondary
irradiation in the visible region leads to the annihilation of the
surface centers during the (dark) postirradiation adsorption
process. Both electrons and holes become free carriers (stages
7a and 7b) under irradiation with blue light. Carriers liberated
from F+ and V centers are involved in recombination (stages
2a and 2b), in trapping by bulk defects (stages 3a and 3b), and
in the formation of PhA O2 centers (stage 4). Note that in the
case of PPSPhA, the color centers of the same origin play
different roles, namely those localized in the bulk lattice absorb
light and emit photocarriers whereas those created at the sample
surface via capture of photocarriers by the corresponding empty
traps serve as PhA centers.6,27 Obviously, as occurs under UV
irradiation (stage 1), recombination decay of PhA centers occurs
(stage 5a) when PPSPhA is stimulated by blue light. As a result,
the lifetimes τPPSPhA measured under irradiation by monochro-
matic light at λ ) 436 nm (67 ms) or under irradiation with
light at λ > 400 nm (45 ms) are similar to those under UV
irradiation (Table 1). Similarly, the coefficients of postirradiation
adsorption in the case of UV preirradiation Ω ≈ 0.16 and Ω ≈
0.1 for irradiation at λ ) 436 nm are also nearly identical. In
contrast, only free electrons are generated under irradiation by
light at λ > 700 nm (stage 7c). Those electrons that avoided
trapping in the bulk lattice (stages 2a, 3a, and 7b) can be
captured by surface centers (stage 4). In the latter case,
recombination decay of PhA centers (stage 5a) does not take
place due to the absence of free photoholes. The only possible
pathway, namely photoionization, that might lead to decay of
Finally, oxygen adsorption primarily on stable PhA centers
(Zr3+, F+, and F6,31,34) determines the rate of the PhA of O2 on
ZrO2. The decay of PhA centers in an active state occurs via
recombination with photogenerated holes. Hence the dependence
of the initial rate of PhA of O2 on oxygen pressure and photon
flow can be described by eq 5.
Possible Appearance of Recombination Decay of Active
Centers and Intermediates in Photocatalytic Reactions.
Photooxidation of organic molecules over illuminated TiO2 in
aqueous media is representative of photocatalysis.1-3 Some
workers still maintain that photoholes are the oxidants in these
reactions. A few types of interactions between photoholes and
organic molecules, RH can be distinguished:
h + RHads f products
S+ + RHads f products
S+ + RHsol f products
(21)
(22)
(23)
Reaction 21 is of the LH-type where free photoholes (h) are
captured by adsorbed molecule (RHads). In the version of the
LH reaction 22, the mobility of adsorbed molecules is presumed
when RH interact with the trapped hole S+. Reaction 23 is of
the ER-type where a “free” molecule in the solvent medium
(RHsol) interacts with surface-trapped holes. Reaction 23 may
also implicate electron transfer through the solid/liquid interface
between the solid catalyst and the reagent molecules. In the

Photosimulated Reactions at the Surface of ZrO2 and TiO2
J. Phys. Chem. B, Vol. 102, No. 52, 1998 10915
Conclusions
latter case, S+ can be treated as any localized hole surface state
provided that it can recombine with free electrons (see below).
The distinguishing feature of oxygen photoadsorption kinetics,
dp/dt ≈ pnFm, on ZrO2 resides in the dependence of the order
of the reaction on photon flow Fm and in the dependence of the
order of reaction on oxygen pressure pn. Consideration of the
photon-flow-dependent pathways of the decay of photoinduced
surface centers, e.g., their recombination with photoholes,
affords a description of the kinetics of oxygen photoadsorption
on ZrO2. The photoadsorption mechanism proposed can also
be expanded to photocatalyzed oxidative reactions of phenol
in irradiated aqueous TiO2 dispersions.
•
Instead of trapped S+ or free holes, surface-adsorbed OH
radicals have also been taken as the oxidants in the version of
photocatalytic oxidation of organic substrates over irradiated
TiO2 particulates.18,24,47,51 These •OH radicals are generated by
hole capture by surface OH- species (or by adsorbed water):
s
OH-s + h f •OHs
(24)
The PhA mechanism treated earlier (see above) can easily
be adopted to these heterogeneous photocatalytic oxidations.
Indeed, stages 1 and 2 retain their validity. Stages 3a and 3b
are retained for common occurrence. (In addition, from a kinetic
point of view, stage 3a can represent the scavenging of electrons
by oxygen.) Stage 4 may be replaced by reaction 25:
Acknowledgment. Our research in Montreal was generously
supported by the Natural Sciences and Engineering Research
Council of Canada (NSERC,Ottawa). A.V.E. thanks NSERC
for a NATO Science Fellowship (1997, 1998). We also thank
Prof. D. Ollis of North Carolina State University for a preprint
of ref 20.
S + h f S+
(25)
References and Notes
where S is any surface hole trap including OH-s and S+ is either
a hole center (e.g., of the O-•s -type) or an •OHs radical. Stage
6 in the PhA mechanism is replaced by reactions 22, 23, or 24.
It is clear that expressions similar to eq 13 and others can be
obtained for the rate of oxidation of RH substrates over an
irradiated photocatalyst if the recombination decay of active
(1) Photocatalysis-Fundamentals and Applications; Serpone, N., Pel-
izzetti, E., Eds.; John Wiley & Sons: New York, 1989.
(2) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341.
(3) Photocatalytic Transformation of Solar Energy. Heterogeneous,
Homogeneous and Organized Molecular Structures; Zamaraev, K. I.,
Parmon, V. N., Eds.; Nauka; Novosibirsk, Soviet Union, 1991.
(4) Baru, V. G.; Volkenstein, Th. The effect of irradiation on surface
properties of semiconductors; Nauka: Moscow, Russia, 1978.
(5) Basov, L. L.; Kuzmin, G. N.; Prudnikov, I. M.; Solonitzyn, Yu. P.
Usp. Photoniki. 1977, 6, 82.
(6) Ryabchuk, V. K.; Burukina, G. V. SoV. J. Phys. Chem. 1991, 65,
1621.
(7) Burukina, G. V.; Basov, L. L.; Ryabchuk, V. K. Vestn. S.-Peterb.
UniV. 1993, 4, 8.
(8) Ryabchuk, V. K.; Basov, L. L.; Solonitsin, Yu. P. React. Kinet.
Catal. Lett. 1989, 39, 331.
•
centers (S+) or of OHs radicals is considered
S+ + e f S
(26)
(27)
•OHs + e f OH-
s
Note that the expressions for the photocatalytic reaction rate
r(F,C) will coincide with the corresponding r(F,p) if the
subscripts for electrons and holes in eq 13 are interchanged. In
reactions 26 and 27 e denotes free photoelectrons whose
concentration depends on the incident photon flow. The latter
point distinguishes this mechanism from those discussed earlier
where reaction 27 was treated47,51 or from those where the •OHs
radical decay was presented as a first-order reaction.18,23 It
should be emphasized also that regardless of the LH (in the
version presented by reaction 22) or ER (reaction 23) types of
reactions, the concept of recombination decay of active centers
(localized carriers) or of intermediates remains useful. Of course,
the definition of rate constants corresponding to reactions 22
and 23 will differ from each other and from the constant ka in
the PhA mechanism treated above. Nevertheless, the view of
photon-flow-dependent lifetimes of active centers or of inter-
mediates also remains valid in photocatalyzed reactions.
Results of preliminary studies on the rate of photooxidation
of phenol r(F,C) over TiO2 in aqueous solutions accord with
the reaction mechanism outlined above. It is clear that the r(F)
dependencies for phenol photooxidation over TiO2 are similar
to those for initial rates r(F) of the PhA of O2 on ZrO2 (compare,
e.g., curves 1 and 2 in Figure 9 and curves 1 and 5 in Figure
4). The mechanism proposed will be clarified during further
detailed studies of the photooxidation of phenol in solutions
over titania. In a preliminary and complementary study of
hydrogen photoadsorption on Degussa P-25 TiO2 particulates,
which were heat pretreated in a manner similar to ZrO2, show
that r(p) dependence on pressure is linear up to p ) 2 Pa at a
photon flow F ) Fo () 1 × 1015 photons cm-2 s-1) whereas at
F ) 0.1Fo the dependence is significantly sublinear (compare,
e.g., curves 1 and 2 for C < 20 µM, Figure 8). Full disclosure
of these studies shall be reported elsewhere.
(9) Pichat, P.; Herrmann, J.-M. In Photocatalysis-Fundamentals and
Applications; Serpone, N., Pelizzetti, E., Eds.; John Wiley & Sons: New
York, 1989; p 218-250
(10) Rapoport, V. L.; Antipenko, B. M.; Malkin, M. G. SoV. J. Kinet.
Catal. 1968, 9, 1306.
(11) Volodin, A. M.; Bolshov, V. A.; Konovalova, T. A. Mol. Eng. 1994,
4, 201.
(12) Emeline, A. V.; Ryabchuk, V. K. Russ. J. Phys. Chem. 1997, 71,
2085.
(13) Ryabchuk, V. K.; Basov, L. L.; Solonitzin, Yu. P. SoV. J. Chem.
Phys. 1989, 8, 1475.
(14) Dolgikh, Yu. K.; Basov, L. L.; Ryabchuk, V. K. React. Kinet. Catal.
Lett. 1986, 32, 119.
(15) Meshereryarov, E. P.; Ryabchuk, V. K.; Minakova, T. S. SoV. J.
Chem. Phys., 1991, 10, 1331.
(16) Solonitsin, Yu. P.; Basov, L. L.; Ryabchuk, V. K. SoV. J. Phys.
Chem. 1980, 54, 2619.
(17) Mesheryakov, E. P.; Emeline, A. V.; Ryabchuk, V. K.; Minakova,
T. S. Russ. J. Kinet. Catal. 1992, 33, 469.
(18) Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178.
(19) Terzian, R.; Serpone, N. J. Photochem. Photobiol.: A. Chem. 1995,
89, 163.
(20) Upadhya, S.; Ollis, D. F. J. AdV. Oxid. Technol. 1998, submitted
for publication.
(21) Ollis, D. F. In Photochemical ConVersion and Storage of Solar
Energy; Pelizzeti, E., Schiavello, M., Eds.; Kluwer: Dordrecht, 1991; pp
593-622.
(22) Tabata, S.; Ohnishi, H.; Yagasaki, E.; Ippommatsu, M.; Domen,
K. Catal. Lett. 1994, 28, 417.
(23) Al-Ekabi, H.; de Mayo, P. J. Phys. Chem. 1985, 89, 5815.
(24) Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. ReV. 1993, 22,
417.
(25) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. EnViron. Sci.
Technol. 1991, 25, 494.
(26) Burukina, G. V.; Basov, L. L.; Kuzmin, G. N.; Ryabchuk, V. K.
Vestn. Leningr. UniV. 1990, 4, 32.
(27) Emeline, A. V.; Kataeva, G. V.; Litke, A. S.; Rudakova, A. V.;
Ryabchuk, V. K.; Serpone N. Langmuir 1998, 14, 5011.
(28) Kotova, O. Ph.D. Thesis, Leningrad State University, Leningrad,
Soviet Union, 1986.

10916 J. Phys. Chem. B, Vol. 102, No. 52, 1998
Emeline et al.
(29) Pichat, P.; Herrman, J.-M.; Disdier, J.; Mozzanega, M.-N. J. Phys.
Chem. 1979, 83, 3122.
(30) Burukina, G. V.; Vitkovsky, G. E.; Ryabchuk V. K. Vestn.
S.-Peterb. UniV. 1990, 4, 92.
(31) Mikhailov, M. M.; Dvoretsky, M. I.; Kuznetsov, N. Y. Inorg. Mater.
1984, 20, 449.
(32) Burukina, G. V. Ph.D. Thesis, Leningrad State University, Lenin-
grad, Soviet Union, 1990.
(33) Emeline, A. V.; Petrova, S. V.; Ryabchuk, V. K.; Serpone, N. Chem.
Mater. 1998, 10, 3484.
(34) Burukina, G. V.; Emeline, A. V.; Ryabchuk, V. K. Vestn. S.-Petersb.
UniV. 1995, 4, 104.
(35) Solonitsin, Yu. P. SoV. J. Kinet. Catal. 1966, 7, 480.
(36) Emeline, A. V.; Ryabchuk, V. K.; Serpone, N. To be submitted
for publication.
(40) Kuznetsov, V. N.; Lisachenko, A. A. SoV. J. Kinet. Catal. 1986,
27, 557.
(41) Bonch-Bruevich, V. L.; Kalashnikov, S. G. Physics of Semiconduc-
tors; Nauka: Moscow, Russia, 1990.
(42) DeVore, H. B. Phys. ReV. 1956, 102, 86.
(43) Bube, R. H. PhotoconductiVity of Solids; John Wiley & Sons: New
York; 1960.
(44) Wolkenstein, Th. Th.; Karpenko, I. V. J. Appl. Phys. 1962, 33,
460.
(45) Albery, W. J.; Bartlett, P. N. J. Elecrochem. Soc. 1984, 31, 315.
(46) Curran, J. S.; Lamouche, D. J. Phys. Chem. 1983, 87, 5405.
(47) Gerischer, H. Electrochim. Acta 1993, 38, 3.
(48) Solomon, R. E.; Graven, W. M.; Adams, G. B. J. Chem. Phys.
1960, 32, 310.
(49) Emeline, A. V.; Ryabchuk, V. K. Vestn. S.-Petersb. UniV. 1998,
accepted for publication.
(50) Warman, J. M.; de Haas, M. P.; Pichat, P.; Koster, T. P. M.; van
der Zouwen-Assink, E. A.; Makor A.; Cooper, R. Radiat. Phys. Chem. 1991,
37, 433.
(37) Emeline, A. V. Ph.D. Thesis St.-Petersburg State University, St.-
Petersburg, Russia, 1995.
(38) Aristov, Yu. I.; Parmon, V. N.; Zamaraev, D. I. SoV. J. Phys. Chem.
1991, 65, 1575.
(39) Serpone, N.; Khairutdinov, R. F. Prog. React. Kinet. 1996, 21, 1.
(51) Riegel, G.; Bolton, J. R. J. Phys. Chem. 1995, 99, 4214.