Karunakaran Poetencial Solar

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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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Contents lists available at ScienceDirect

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.

C. Karunakaran et al. / Solar Energy Materials & Solar Cells 94 (2010) 900906 901


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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

C. Karunakaran et al. / Solar Energy Materials & Solar Cells 94 (2010) 900906902


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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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