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Solar-powered potentially induced TiO2, ZnO and SnO2-catalyzed
iodine generation
C. Karunakaran n, P. Anilkumar, G. Manikandan, P. Gomathisankar
Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamilnadu, India
a r t i c l e i n f o
Article history:
Received 4 February 2009
Received in revised form26 November 2009
Accepted 19 January 2010Available online 16 February 2010
Keywords:
Photoelectrocatalysis
Semiconductor
Sunlight
Iodide ion
a b s t r a c t
Formation of iodine from air-saturated KI solution under natural sunlight (950725 W m 2) with
nanocrystals of TiO2 in different phase compositions, ZnO and SnO2, characterized by powder XRD,
UV-visible diffuse reflectance and impedance spectroscopies, increases by 39 fold on impressing acathodic bias of 0.2 V (vs NHE) to the semiconductors. The iodine generation essentially requires
light, the semiconductor and dissolved oxygen. An anodic bias of +0.2 V (vs NHE) to the illuminated
semiconductors fails to oxidize iodide ion from air-saturated KI solution. Under cathodic bias, mixtures
of ZnOTiO2 rutile, SnO2TiO2 rutile and ZnOSnO2 nanocrystals show higher photocatalytic activity,
pointing to an improved charge separation in the oxide mixtures. The mechanism and the energetics of
photocatalysis under cathodic bias are discussed. Also, the photocatalytic activities of the semiconductor
nanocrystals are analyzed in terms of their optical, electrical and crystal properties.
& 2010 Elsevier B.V. All rights reserved.
1. Introduction
Photoexcitation of semiconductor creates electronhole pairs,
electrons in the conduction band and holes in the valence band
[1,2]. While a fraction of these pairs diffuse out to the surface of
the crystal and react with the adsorbed electron donors and
acceptors leading to photocatalysis, the rest recombine resulting
in low photocatalytic efficiency [3]. Application of a potential bias
to semiconductor under band gap-illumination reduces the
recombination of the electronhole pairs and increases the
photocatalytic efficiency, and studies on TiO2-photoelectrocata-
lyzed mineralization of organics are many [48]. But here we
report semiconductor-photocatalysis with natural sunlight under
potential bias for the conversion of solar radiation into chemical
energy. Enhanced solar photoelectrocatalysis by particulate
semiconductor mixtures, due to improved charge separation, is
also observed; the enhancement of catalysis under UV light by
particulate semiconductor mixtures in absence of any potentialbias is known [913]. Semiconductor-photocatalysis evokes
interest due to its potential to utilize sunlight. However, such
studies with natural sunlight are rare, probably due to fluctuation
of solar irradiance during the experiment even on sunny days
under clear sky. We have overcome the same by carrying out set
of experiments under the required experimental conditions
simultaneously, side by side, thereby making the solar irradiance
in the set of experiments identical. Generation of energy bearing
chemicals by non-spontaneous reactions is the objective of solar
energy conversion and storage and iodide ion-oxidation, the title
study, is such a reaction. Also, the operational efficiency of iodide/
triiodide redox couple influences the light-to-electricity conver-sion efficiency of dye-sensitized solar cells [14]. Further, the
mechanism of TiO2-photocatalyzed iodide ion-oxidation is a well-
established one [1518]. TiO2, SnO2 and ZnO are the semiconduc-
tors largely employed in the fabrication of dye-sensitized solar
cells and hence are used in this study [19,20]. Photooxidation of
iodide ion on TiO2 [1518,2124], Pt-loaded TiO2 [25], phthalo-
cyanine- or flower pigment cyanidin-sensitized TiO2 [26,27],
MoO3, Fe2O3, ZnO and CeO2 [2123] have been reported but they
are in absence of any potential bias.
2. Experimental
2.1. Materials
TiO2 P25 (Degussa), TiO2 Hombikat (Fluka), TiO2 anatase
(Sigma-Aldrich), TiO2 rutile (Sigma-Aldrich), ZnO (Sigma-Aldrich)
and SnO2 (Sigma-Aldrich) nanoparticles were used as supplied.
2.2. Characterization
The powder X-ray diffraction patterns were obtained using a
Bruker D8 system employing Cu Ka radiation (l=1.5406 A) in a 2y
range of 5601 at a scan speed of 0.0501 s1 or a Rich. Siefert
model 3000 X-ray diffractrometer. A PerkinElmer Lambda 35
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journal homepage: www.elsevier.com/locate/solmat
Solar Energy Materials & Solar Cells
0927-0248/$- see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2010.01.015
n Corresponding author. Tel.: +91 9443481590; fax: +91 4144238145.
E-mail address: [email protected] (C. Karunakaran).
Solar Energy Materials & Solar Cells 94 (2010) 900906
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spectrophotometer was employed to record the UV-visible diffuse
reflectance spectra of the oxides. The impedance spectra of the
different forms of TiO2 nanocrystals were obtained using a HP
4284A Precision LCR meter at room temperature in air over the
frequency range of 1 MHz to 20 Hz; while the disk area was
0.5024 cm2 the thicknesses of anatase, rutile, P25, Hombikat TiO2pellets were 2.12, 1.53, 2.78, and 1.80 mm, respectively.
2.3. Method
Photooxidation of iodide ion with the semiconductor nano-
crystals was carried out under a potential bias with AM 1 sunlight
under clear sky in summer (MarchJuly) and the sunlight
intensity was measured using a Daystar Solar Meter, USA. For
each experiment, 50 mL of fresh air-saturated KI solution was
taken in wide cylindrical glass vessels of uniform diameter, fitted
with a stainless steel bottom which was completely covered with
the semiconductor [28,29]. Potential bias was applied to the
semiconductor bed using a DC power supply and was measured
against SCE with a potentiostat. Platinum electrode (Elico, India)
was used as the counter-electrode. The iodine formed was
determined from the absorbance of the 30-min illuminated KI
solution, measured at 350 nm using a Shimadzu 1650 PCspectrophotometer [2123]. The dissolved oxygen was deter-
mined with an Elico dissolved oxygen analyzer PE 135 and pre-
sonication was made with a Toshcon SW 2 ultrasonic bath
(3773 kHz, 100 W).
3. Results and discussion
3.1. Catalyst characteristics
The X-ray diffractogram of the anatase TiO2 used confirms its
structure. It matches with the standard JCPDS pattern of anatase
TiO2 (89-4921) and the rutile lines are absent. The XRD of the
used TiO2 rutile sample is in accordance with its crystal structure.
The observed XRD matches with the standard JCPDS pattern of
rutile TiO2 (894202) and the anatase peaks are not seen. The
X-ray diffraction pattern of the TiO2 P25 employed confirms the
presence of anatase and rutile phases in the sample; the standard
JCPDS patterns of anatase (00-021-1272) as well as rutile
(01-089-0553 (C)) are present. The phase percentages have been
obtained from the integrated intensity of the peaks at 2y value of
25.31 for anatase (1 0 1) and 27.41 for rutile (1 1 0). The
percentage of anatase is given by A(%)=100/{1+1.265(IR/IA)},
where IA is the intensity of the anatase peak at 2y=25.31 and IRis that of the rutile peak at 2y=27.41. The phase composition thus
obtained is 81% anatase and 19% rutile, which is in accordance
with that provided by the manufacturer. The diffraction pattern of
TiO2 Hombikat shows the presence of anatase as well as rutilephases; the peak fitting conforms to the JCPDS patterns of anatase
(89-4921) and rutile (1 1 0, 2 1 1, etc., peaks). The percentages of
anatase and rutile, determined using the intensities of 1 0 1 peak
of anatase and 1 1 0 peak of rutile, are 69 and 31, respectively. The
diffraction pattern of ZnO confirms its crystal structure. The peak
fitting is highly satisfactory with the JCPDS pattern (89-7102). The
XRD of SnO2 is in accordance with its structure; the observed
diffractogram totally matches with the standard JCPDS pattern
(88-0287).
The crystal size of the nanoparticles have been determined
from the half-width of the full-maximum (HWFM) of the most
intense peaks of the different nanocrystals using the Scherrer
equation, D=0.9l/b cos y, where D is the mean crystallite size, l is
the X-ray wavelength, y is the Bragg angle and b is the corrected
line broadening of the sample. The specific surface areas of the
nanocrystals have been obtained using the relationship, S=6/rD,where S is the specific surface area, D is the average particle size
and r is the material density. The results are presented in Table 2.The diffuse reflectance spectra of the semiconductors used are
shown in Fig. 1; the reflectance data are reported as F(R) value,
obtained by application of the KubelkaMunk algorithm. Fig. 2 is
the Tauc plots of [F(R)hn]1/2 vs hn, and the extrapolation of the
rising segment of [F(R)hn]1/2
to the abscissa at zero F(R) providesthe band gap energy, given in Table 2.
Defects in semiconductor nanocrystals strongly influence the
electrical properties of the materials and the photocatalytic
activity of the semiconductor depends on the extent of defects
in the crystals. Impedance spectroscopy (IS) is a relatively new
and powerful tool to probe the electrical properties of semi-
conductors [30]. It may be employed to investigate the dynamics
of the bound and mobile charges in the bulk or interfacial region
of the semiconductors. In polycrystalline materials, the overall
sample resistance may be a combination of the intragranular or
bulk crystal resistance and intergranular or grain boundary
resistance. The impedance data, in general, are analyzed in terms
of an equivalent circuit model; an electrode interface undergoing
an electron-transfer reaction is typically analogous to an electric
circuit consisting of a specific combination of capacitors and
resistors. By fitting the IS data to a model or an equivalent circuit
the electrical properties of the semiconductors may be inferred.
Fig. 3 presents the impedance spectra, the variation of impedance
with frequency, of anatase, rutile, P25 (81% anatase, 19% rutile),
and Hombikat (69% anatase, 31% rutile) TiO2. It shows decrease in
the impedance with the increase in frequency indicating the
capacitance of the semiconductors studied. The Nyquist plot, a
popular format of evaluating the impedance data, is presented in
Fig. 4. The semicircle in the Nyquist plot is the expected response
of the simple circuit shown in Fig. 5. The uncompensated or
ohmic resistance (RO) corresponds to the grain boundary or
intergranular resistance and the polarization or electron-transfer
(charge-transfer) resistance (RP or RCT) refers to the bulk crystal
or intragranular resistance; the latter is related to Warburg
resistance, the resistance to mass transfer, which is controlled
by the specific conductance, s. The constant phase element,C, may be associated with a non-uniform distribution of current
due to material heterogeneity and is equivalent to a double layer
capacitance. Although P25 and Hombikat are the blends of
250 300 350 400 450 500 550 600
0
5
10
15
20
25
30
35
F(R)
Wavelength, nm
TiO2 anatase
TiO2 hombikat
TiO2 P25TiO2 rutile
SnO2
ZnO
Fig. 1. DRS of the nanocrystals.
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anatase and rutile phases, they exhibit single semicircles in
the Nyquist plots pointing out homogeneity in the electrical
properties of the materials. The Nyquist plot of Hombikat displays
a large charge-transfer resistance with only a very limited
frequency region where mass transfer is a significant factor,
thus indicating the predominance of kinetic control; the oxide
may be photocatalytically sluggish. All other Nyquist plots showlarge regions of mass transfer and kinetic control at low
and high frequencies, respectively. Also, the Nyquist plots show
the existence of appreciable (but not large or insignificant)
series capacitance in the equivalent circuit and the Warburg
impedance is also important. Table 1 shows the determined
specific conductance, capacitance and the intergranular and
intragranular resistances of the different forms of TiO2employed; the specific conductance has been obtained from the
measured charge-transfer resistance and the capacitance has been
deduced employing the equations omax=1/CRCT and omax=2pf,where f is the frequency corresponding to the maximum of the
semicircle of the Nyquist plot. Among the TiO2 nanocrystals,
the specific conductance of anatase is larger than that of rutile.
But the benchmark photocatalyst P25 Degussa, which is a blend of
anatase (81%) and rutile (19%) phases, shows surprisingly higher
specific conductance than anatase and rutile. However, Hombikat,
which also contains anatase (69%) and rutile (31%) phases, fails to
exhibit similar characteristics; its specific conductance is even
less than that of rutile. Further, P25 Degussa exhibits the largestcapacitance among the semiconductors examined.
3.2. Obtaining solar results
The intensity of sunlight during the experiment, even under
clear sky on sunny days, fluctuates. Also, it differs from day to day.
Now, the solar irradiance for a set of experiments with sunlight, of
the required reaction conditions, has been kept identical by
carrying out the experiments simultaneously, side by side, thus
making possible to compare the solar results. The results of solar
oxidation of iodide ion with TiO2 anatase, TiO2 Hombikat, TiO2rutile, TiO2 P 25 Degussa, ZnO and SnO2 nanoparticles are
consistent. For each semiconductor-photocatalysis, a pair of solar
2 3 4 5 610
2
4
6
8
10
12
14
3.16 eV
3.03 eV
3.16 eV
3.14 eV
3.22 eV
3.58 eV
h, eV
TiO2 anatase
TiO2 hombikat
TiO2 P25
TiO2 rutile
SnO2ZnO
[F(R
)h
]
Fig. 2. Modified KM plots.
0 250 500 750 1000
0
100
200
300
Impedance,
k
Frequency, kHz
TiO2 P25
TiO2 anatase
TiO2 rutile
TiO2 hombikat
Fig. 3. IS of TiO2.
0 50 100 150 200 250
0
20
40
60
Z"k
Z' k
TiO2 P25
TiO2 anatase
TiO2 rutile
TiO2 hombikat
Fig. 4. The Nyquist plots.
Rs
Rct
Cdl
Fig. 5. The equivalent circuit.
Table 1Ohmic (RO) and charge-transfer (RCT) resistances, capacitance (C) and specific
conductance (s) of TiO2.
TiO2 RO (kO) RCT (kO) s (mS m1) C (pF)
Anatase 12 41 1.03 9.7
Rutile 20 70 0.38 7.1
P25 Degussa 4 37 1.50 43
Hombikat 9 203 0.18 8.2
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experiments of identical reaction conditions carried out simulta-
neously, side by side, yields results within 75% and this is so on
different days. This consistency is not surprising as the solar
irradiance is the same in the test and control experiments.
3.3. Solar photocatalysis under potential bias
Application of a cathodic bias to the semiconductor bed under
sunlight enhances the iodide ion-oxidation. Of the four experiments
conducted simultaneously in a set under identical solar irradiance for
each catalyst, cost effective stainless steel bottom was used for three,
which was completely covered with the catalyst [28,29]. A positive
potential bias was applied to one, a negative potential to another
and none to the third. While an anodic bias and no bias fail to
effect significant photocatalysis, a cathodic bias shows enhanced
photooxidation compared to the control experiment where the
catalyst was spread on a glass bottom without any bias (Table 2). The
photoelectrocatalysis was confirmed by carrying out the experiments
under identical conditions but in dark. Identical cathodic bias in dark
fails to effect comparable iodide ion-oxidation. Also, the solar-
electroxidation requires dissolved oxygen; the oxidation of iodide
ion in nitrogen-purged KI solution under cathodic bias is insignificant
compared to that in air-saturated KI solution. The anodic iodide ion-oxidation in the absence of semiconductor is also small under the
experimental conditions. These results unequivocally prove the
electrochemical-assistance of semiconductor-photocatalysis with
sunlight. All the oxide nanopowders show sustainable photo-
catalysis; recycling of the catalysts without any pre-treatment
provides identical results. Generally, the photocatalytic efficiency is
susceptible to the surface and size modification of the catalyst
particles. Sonication in aqueous solution causes rapid formation,
growth and collapse of cavities resulting in local high pressures and
temperatures that are responsible for surface and particle size
modification of the catalyst [31]. However, pre-sonication of the
solid catalysts or their suspension in KI solution does not influence
the photocatalytic efficiencies of all the nanosemiconductors studied;
the TiO2 anatase, TiO2 Hombikat, TiO2 rutile, TiO2 P 25, ZnO, andSnO2-catalyzed iodine-formations are not altered due to pre-
sonication for 10 min at 3773 kHz and 100 W (the experimental
conditions remain as in Table 2).
3.4. Enhanced photocatalysis by semiconductor mixtures under
potential bias
Mixed semiconductors enable a vectorial transfer of electrons
and holes from one semiconductor to another [913]. On illu-
mination, both the semiconductors are excited and the electrons
accumulate at the lower-lying conduction band of one semicon-
ductor while the holes move to the less anodic valence band.
These processes of charge separation are very fast and hence
enhance the photocatalytic efficiency. Although enhanced photo-
catalysis by mixed semiconductors is known [913], the improved
photoelectrocatalysis by semiconductor mixtures with natural
sunlight has not been reported so far. Under cathodic
bias, rutile TiO2ZnO, rutile TiO2SnO2 and ZnOSnO2 mixtures
(1:1 w/w) exhibit enhanced photocatalytic efficiency (Table 3);however, other combinations do not show improved photo-
catalysis. A possible reason for the observed enhancement by
rutile but not by anatase or anatase containing samples is adsorp-
tion of water. Anatase is more active than rutile in adsorbing
water molecules. The primary sheath of water molecules around
anatase particles could prevent physical contact with ZnO or SnO2crystals thus hindering interparticle charge transfer [32].
3.5. Mechanism
The diffuse reflectance spectra of all the semiconductor
nanocrystals used (Figs. 1 and 2) show that UV-A light is required
for band gap-excitation and hence the observed photocatalysis is
due to the UV light of the solar radiation which is about 5%. In theabsence of any bias, band gap-illumination of semiconductors on
glass bottom generates electronhole pairs, part of which reacts
with the adsorbed iodide ion and oxygen molecule, respectively,
forming iodine and superoxide radical ion, O2 . In aqueous
medium, O2 in turn generates highly reactive species such as
HO , HO2 and H2O2, which also oxidize the iodide ion [2123].
In absence of any potential bias, both oxidation and reduction
occur on the illuminated surface of the semiconductor. But, the
photocatalysis without bias becomes insignificant with semicon-
ductors on stainless steel bottom. This is likely due to short-
circuit of the photoformed charges by stainless steel. Anodic bias
on an illuminated semiconductor should lead to iodide ion-
oxidation at the semiconductor surface and naturally reduction of
O2
at the platinum electrode. The reverse may be the case under a
cathodic bias impressed upon an illuminated semiconductor.
Here, anodic bias fails to effect iodide ion-oxidation whereas a
cathodic bias does. That is, O2 adsorbed on the illuminated
semiconductor is reduced to superoxide radical ion and the iodide
ion is oxidized at the counter-electrode also (Fig. 6); the formation
of iodine at the Pt electrode is visible during illumination. The
possible reason is as follows. The reduction of O2 by the
illuminated semiconductor surface is slow and rate determining
and application of a cathodic bias eases the same; it has been
shown that in the photooxidation of methanol (1.6 M) electrons
on TiO2 particles persist for at least $1 min even in O2-saturated
Table 2
Iodide ion-oxidation with semiconductors under sunlight and potential biasa,b.
Nanocrystals D (nm) S (m2 g1) Eg (eV) I2 formed (mM)
Controlc No bias Anodic biasd Cathodic biase Cathodic bias in darke
TiO2 anatase 9 165 3.22 86 2 $0 248 (4) $0
TiO2 rutile 12 123 3.03 13 3 2 96 (1) $0
TiO2 P25 anatase:rutile::81:19 23 68.1 3.14 43 3 2 190 (3) $0
TiO2 Hombikat anatase:rutile::69:31 18 87.4 3.16 32 1 $0 128 (2) $0
ZnO 32 32.9 3.16 9 $0 $0 48 (1) $0
SnO2 27 31.1 3.58 11 $0 1 76 (1) $0
Nil $0 $0 $0 $0 $
a 50 mL 0.05 M KI, 31.2 mg L1 dissolved O2, 15.1 cm2 catalyst bed, 0.10 g catalyst covered on stainless steel bottom, 30 min sunshine at 950725 W m2.
b Values in a row correspond to identical solar irradiance.c Catalyst on glass bottom without bias.d +0.2 V vs NHEe0.2 V vs NHE; values in parenthesis correspond to N 2-purged solution (4.2 mg L
1
dissolved O2).
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solutions [33]. Also, the iodide ion-oxidation at the platinum
electrode with oxygen reduction on the illuminated semiconductor
is kinetically and thermodynamically favorable than the reduction
of oxygen at platinum electrode with iodide ion-oxidation on
the semiconductor; the reduction potentials of conduction band
electron (TiO2) and superoxide radical ion are 0.50 and 0.33 V,
respectively. Further, reduction of oxygen at the platinum
electrode requires an over potential whereas iodide ion-oxidation
at platinum surface or oxygen reduction at the oxide surface does
not require the same. The enhanced photoelectrocatalysis shownby mixed semiconductors is in line with the better charge
separation by mixed semiconductors. The larger photogeneration
of iodine by TiO2 rutileSnO2, TiO2 rutileZnO, and SnO2ZnO
mixtures under cathodic bias may be explained on the basis of
their band edge positions. The energy positions of the conduction
band edges and valence band edges of the semiconductors on
the absolute vacuum scale (AVS), presented in Fig. 7 [1], determine
the charge transfer between the particulate semiconductors; the
AVS is related to the normal hydrogen electrode (NHE) scale
by E(AVS) = E(NHE)4.5 [34]. The energy difference between the
conduction band electrons of the two semiconductors is the driving
force for the interparticle electron injection and the free energy
change is given by DG = e(ECB(SC1)ECB(SC2)) [35]. Regarding the
conduction band (CB) edges, the CB of SnO2 is least cathodic while
that of ZnO is the most cathodic [1]. Among the valence bands (VB),
the VB of SnO2 is the most anodic and that of TiO2 rutile is the least.
Hence on illumination of SnO2ZnO or SnO2TiO2 rutile mixture
the CB electrons will accumulate in SnO2 while the VB holes will
collect in ZnO or TiO2. In the case of ZnOTiO2 rutile mixture, the
VB holes will migrate to ZnO and the CB electrons to TiO2. It is
pertinent to state that both the conduction and valence bands do
not exist in reality and they refer only to the reduced and oxidized
states in the semiconductor. For example, in TiO2 or SnO2 the
conduction band electron corresponds to the reduced form of Ti4+
(i.e., Ti3+ ) or reduced form of Sn4+ (i.e., Sn3+ ) and the valence band
hole refers the oxidized form of O2 (i.e., O). The interparticle
charge transfer, the transfer of electron from the conduction band
of TiO2 to that of SnO2 refers to the electron jump from Ti3+ to
Sn4+ . Similarly, the hole-transfer from the valence band of SnO2 to
that of TiO2 corresponds to the hole-jump from O of SnO2 to O
2
of TiO2. That is, electron jumps from O2 of TiO2 to O
of SnO2. The
possibility of cross-electronhole combination, the transfer of
electron from the conduction band of one semiconductor (SC1) to
the valence band of the other (SC2), such as from Ti3+ of TiO2 to O
of SnO2 is very remote; the very low population of the excited
states make the electron transfer between two excited states
highly improbable. Under cathodic bias migration of holes becomes
less important as they are neutralized; the electron movement
between semiconductors enhance the activity.
3.6. Photoelectrocatalytic efficiencies
The quantities of iodine formed with TiO2 anatase, TiO2Hombikat, TiO2 rutile, TiO2 P 25, ZnO and SnO2, under identical
solar irradiance and experimental conditions, reveal TiO2 anatase
as the most efficient photocatalyst under potential bias; the order
of photoelectrocatalytic efficiency is: TiO2 anatase4TiO2P254TiO2 Hombikat4TiO2 rutile 4SnO24ZnO (conditions as
in Table 2). This order is not in accordance with the band gap
energies of the semiconductors. Comparing the photocatalysis by
SnO2 and ZnO, the band gap energy of SnO2 (3.58 eV) is larger
than that of ZnO (3.16 eV) and hence SnO2 requires shorter
wavelength of radiation for activation than ZnO. In the solar
spectrum, the intensity of sunlight of energy sufficient to activate
SnO2 (o346 nm) is less than that to excite ZnO (o392 nm).
Table 3
Enhanced photocatalysis by mixed semiconductors under cathodic bias a.
Catalyst I2 formed (mM)
ZnO 48
SnO2 76
ZnO/SnO2 118
TiO2 rutile 96
SnO2/TiO2 rutile 118
ZnO/TiO2 rutile 111
a 50 mL 0.05 M KI, 31.2 mg L1 dissolved O2, 15.1 cm2 catalyst bed, 0.10 g
catalyst loading (total), 0.2 V vs NHE, 30 min sunshine at 950725 W m 2.
KI Solution
O2O2
H+
I2
I
H2O
Pt
TiO2
HO2
H2O2
HO
Fig. 6. Photocatalytic mechanism under cathodic bias.
-3
-4
-5
-6
-7
-8
-9
SnO2
ZnO TiO2
E
(AVS),eV
Fig. 7. CB (less negative) and VB (more negative) energy levels of semiconductors.
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Hence some other factors predominate in determining the
photocatalytic activity. The specific surface areas of the two
oxides do not differ significantly and hence is not the reason for
the difference in the catalytic activity. Although the mean particle
size of SnO2 is less than that of ZnO, it is not reflected in the
photocatalysis in absence of potential bias. However, remarkable
difference in the photocatalytic activity is observed under
cathodic bias. Better adsorption of molecular oxygen on the
illuminated SnO2 surface under potential bias or effectiveinhibition of the photogenerated electronhole pair recombina-
tion under cathodic bias may be one of the possible reasons for
the observed higher photocatalytic activity of SnO2 compared to
ZnO. Comparing the photocatalytic activities of different forms of
TiO2 studied, the observed catalytic activities are in agreement
with neither the grain boundary resistances nor the charge-
transfer resistances. The determined specific conductances and
the capacitances are not in accordance with the measured iodine
generation. Further, the photocatalytic activities are not in
accordance with either the specific surface areas or with the
crystal sizes. The surface trapping of photogenerated electrons
and holes prior to recombination may be more efficient in small
crystals and hence smaller the crystal size, the larger should be
the photocatalytic activity. The photogenerated electronhole
pairs have a much shorter distance to travel to reach the surface
in a small particle. Once the electrons and holes have been
trapped at the interface, they may then participate in redox
reactions. However, the observed photocatalytic activities of TiO2are in agreement with the anatase content; larger the percentage
of anatase phase present, higher is the photocatalytic activity.
Anatase TiO2 is more photoactive than rutile TiO2 [2,32,36] and
this is because of larger photoadsorption and desorption of
oxygen on anatase and also to a lower electronhole recombina-
tion [36]. Further, anatase is more active than rutile in adsorbing
water and hydroxyl groups and the photocatalytic activity
depends on the surface-adsorbed water and hydroxyl groups [32].
3.7. Energetics
In the title reaction, for a cathodic bias of 0.2 V at the illumi-
nated semiconductor surface, one electron is transferred across the
electrodeelectrolyte interface. The reaction on the oxide surface is
O2 + e
-O2 and the counter reaction at the Pt anode is I -
I2 + e. But, two iodine molecules are produced for the one-
electron transfer. The possible reactions in solution that follow the
one-electron transfer across the electrodeelectrolyte interface to
satisfy the stoichiometric requirement are:O2 + H+-HO2
;HO2 +
H2O-H2O2 + HO ;HO + I -I2 + HO
; H2O2 + 2H++2I -
I2 + 2H2O. The net reaction is 4I + 3H+ + O2-2I2 + H2O + HO
.
The applied potential of0.2 V for a one-electron electrode reaction
results in an energy expenditure of 19.3 kJ which corresponds to
9.65 kJ mol1 of iodine produced (DG = 196487 C mol1
0.2 V = 19.3 kJ). Semiconductor-catalyzed iodine generation is
the reaction that converts light energy into chemical energy in
solar cells and the light energy stored as chemical energy per mole
of iodine generated is 103.2 kJ. The net energy storage in the
potentially induced iodine generation is 93.55 kJ mol1. Hence this
technique has the advantage of being faster towards solar energy
conversion and storage than that in absence of any potential bias.
4. Conclusions
Under natural sunlight at 950725 W m2, application of
a cathodic bias of 0.2 V (vs NHE) to anatase, rutile, P25,
and Hombikat TiO2, SnO2 and ZnO nanocrystals enhances iodide
ion-oxidation by 39 times. Under cathodic bias, mixtures of
ZnOTiO2 rutile, SnO2TiO2 rutile and ZnOSnO2 nanocrystals
show higher photocatalytic activity, pointing out the improved
charge separation in oxide mixtures. The observed photoelec-
trocatalytic activity is not in accordance with the specific
conductance or capacitance of the TiO2 nanocrystals. Also, it is
not in line with the crystal size or surface area of TiO2nanocrystals but is in agreement with the composition of anatase
phase in the crystals.
Acknowledgements
The authors thank the University Grants Commission (UGC),
New Delhi, for the financial support through major research Grant
no. F.12-64/2003 (SR) and Degussa for gifting TiO2 P25 sample.
P.A. is grateful to UGC for PF. The authors also thank Dr.
J. Jayabharathi, Annamalai University for the DRS facility.
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