Design of Visible-Light Photocatalysts by Coupling of Narrow

Registered Charity Number 207890

Accepted Manuscript

This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication.

Accepted Manuscripts are published online shortly after acceptance, which is prior to technical editing, formatting and proof reading. This free service from RSC Publishing allows authors to make their results available to the community, in citable form, before publication of the edited article. This Accepted Manuscript will be replaced by the edited and formatted Advance Article as soon as this is available.

To cite this manuscript please use its permanent Digital Object Identifier (DOI), which is identical for all formats of publication.

More information about Accepted Manuscripts can be found in the Information for Authors.

Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.

www.rsc.org/catalysis

Catalysis Science & Technology

ISSN 2044-4753

Volume 1 | N

umber x | 2010

MedC

hemCom

m

Pages 00000000

www.rsc.org/catalysis Volume 1 | Number 1 | 2011 | Pages 00010100

CatalysisScience&Technology

View Article OnlineView Journal

1

Design of Visible-Light Photocatalysts by Coupling of Narrow

Bandgap Semiconductors and TiO2: Effect of Their Relative Energy

Band Positions in Photocatalytic Efficiency

Sher Bahadur Rawal,a Sandipan Bera,

a Daeki Lee,

b Du-Jeon Jang

b and Wan In Lee*

a

Abstract

According to relative energy band positions between TiO2 and visible-light-absorbing

semiconductors, three different types of heterojunction were designed, and their visible-light

photocatalytic efficiencies were analyzed. In Type-A heterojunction, the conduction band (CB)

level of sensitizer is positioned more negative side than that of TiO2, whereas in Type-B system

its valence band (VB) level is more positive than that of TiO2 and in Type-C system the

sensitizer energy level is located between the CB and VB of TiO2. In evolving CO2 from the

gaseous 2-propanol (IP) under visible-light irradiation, the Type-B systems such as FeTiO3/TiO2,

Ag3PO4/TiO2, W18O49/TiO2, and Sb-doped SnO2 (ATO)/TiO2 demonstrated noticeably higher

photocatalytic efficiency than the Type-A such as CdS/TiO2 and CdSe/TiO2, while the Type-C

such as NiTiO3/TiO2, CoTiO3/TiO2, and Fe2O3/TiO2 did not show any appreciable improvement.

Remarkably high visible-light photocatalytic activity of Type-B heterojunction structures could

be explained by inter-semiconductor hole-transfer mechanism between the VB of sensitizer and

that of TiO2. The evidence for the hole-transport between sensitizer and TiO2 was also obtained

by monitoring the hole-scavenging reactions with iodide (I-) and 1,4-terephthalic acid (TA).

Keywords: Photocatalyst, Visible light, Heterojunction, Sensitizers, TiO2, 2-propanol, CO2

evolution.

Page 1 of 32 Catalysis Science & Technology

Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

2

1. Introduction

Remediation of environmental pollutants by photocatalytic reaction has attracted extensive

attention for the last a few decades.1,2

TiO2 has been known as the most efficient photocatalyst

among various semiconductors,3-6

but it cannot utilize the photons in the visible-light range,

occupying major portion of solar spectrum due to its wide bandgap (Eg = 3.2 eV).7-11

Thus,

development of photocatalysts functional under visible-light will be a crucial issue.

Thus far, several strategies, including the substitution of various transition elements to the Ti

site,12-14

several anions such as N, C, B, and S to the oxygen site,15-18

and incorporation of

carbon nanomaterials such as carbon nanotubes and graphene,19,20

have been attempted to

extend the band edge of TiO2 up to the visible-light range. Another promising strategy will be

coupling of TiO2 with other narrow bandgap semiconductors capable of harvesting the photons

in the visible range.21-24

Conceptually, according to the relative energy band location between the

sensitizer and TiO2, the heterojunction structures can be classified by the following three

different types.

First, as described in Scheme 1a, the conduction band (CB) of sensitizer is positioned more

negative side than that of TiO2 (denoted as Type-A heterojunction). For example, several metal

chalcogenide quantum dots or molecular dyes are loaded on the TiO2 surface to form Type-A

heterojunction.25-27

With visible-light irradiation to this system, the sensitizer is excited, and the

electrons are then transported to the CB of TiO2, since the CB level of TiO2 is lower than that of

the sensitizer. These electrons can induce various reduction reactions or participate in

decoloration reactions of organic dyes.28-30

Complete oxidation of organic pollutants is also

possible by forming the O2- and HO2, as shown in equation 1-3.30-32

O2 + e- O2

-, E0 = -0.284 V (vs. NHE) (1)

O2- + H

+ HO2, E0 = -0.046 V (vs. NHE) (2)

HO2 + Organic compounds CO2 + H2O (3)

Page 2 of 32Catalysis Science & Technology

Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

3

Second, as described in Scheme 1b, valence band (VB) of the sensitizer is located more

positive side than that of the TiO2 (Type-B heterojunction). With irradiation of visible-light to

this coupled system, the electrons in the sensitizer VB are excited to its CB. Thereby the holes in

the sensitizer VB can be transferred to that of TiO2. As a result, holes are generated in the VB of

TiO2 by inter-semiconductor hole-transfer mechanism,33-35

and in turn they initiate various

oxidation reactions by generating the OH radical on the TiO2 surface, as described in equation 4-5.36

(H2O)ads + h+ H

+ + OH (4)

(OH-)ads + h

+ OH (5)

Considering the powerful oxidative ability of the holes generated in the VB of TiO2, efficient and

complete decomposition of organic compounds could be achieved.

Third, as described in Scheme 1c, the CB and VB of sensitizer are located between those of the

TiO2 (Type-C heterojunction). Under visible light irradiation the electrons in the VB of sensitizer

are excited to its CB, but neither electrons in CB nor holes in VB of sensitizer can be transferred

to the CB or VB of TiO2, due to unfavorable energy band matching. Hence, no synergetic effect

enhancing visible-light photocatalytic activity is expected to this system.37

Thus far various coupled systems were investigated to design efficient visible-light

photocatalysts, but most of them were limited to the Type-A systems, and the other types were

scarcely studied. In the present study, various narrow bandgap semiconductors such as CdS,

CdSe, Sb-doped SnO2 (ATO), Ag3PO4, W18O49, FeTiO3, NiTiO3, CoTiO3 and Fe2O3 were

prepared, and they were coupled with TiO2 to form the Type-A, Type-B and Type-C

heterojunction structures, respectively. Correlation of the relative energy band positions between

sensitizer and TiO2 and the resultant visible-light photocatalytic activities have been

systematically investigated. We found that relative energy band positions are highly important in


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

4

determining the visible-light photocatalytic activity. The obtained results will provide the

insights in designing highly efficient visible-light photocatalysts based on heterojunction

structures as well as in understanding the photocatalytic reaction mechanism.

2. Experimental section

2.1. Preparation of Type-A Heterojunction Structures (CdS/TiO2 or CdSe/TiO2)

CdS and CdSe quantum dots (QDs) were synthesized by the procedures reported in literature.38,39

In a typical syntheses, 0.1 mmol CdO (Aldrich), 1.2 mmol oleic acid (OA, Aldrich) (or 2.4 mmol

trioctylphosphine oxide (TOPO, Aldrich) for CdSe synthesis), and 3.0 ml 1-octadecene (ODE,

Aldrich) were mixed in a three-neck flask, and heated to 300oC under Ar flow. In a separate flask,

a solution of sulfur (0.05 mmol, Daejung Chem. Co.) in 1.91 ml ODE or 0.24 mmol selenium

(Aldrich) with 0.96 mmol trioctylphosphine (TOP, Aldrich) in ODE was prepared, and injected

swiftly to the hot solution. The temperature of the mixture was then adjusted to ~260C for the

growth of CdS or CdSe QDs. After 5 min, the solution was immediately cooled down by adding

the 30 ml cold toluene to obtain OA-capped CdS or TOPO-capped CdSe QDs.

The OA and TOPO groups on the CdS and CdSe surfaces, respectively, were exchanged to

mercapto propionate (MPA) group,33,40

in order to anchor the CdS and CdSe QDs onto the TiO2

surface. Typically, 0.2 mmol MPA (Aldrich) was dissolved in 10 mL anhydrous methanol, and

the pH was adjusted to 11.4 by adding tetramethylammonium hydroxide (TMAH, Aldrich). 40

mg OA-capped CdS or TOPO-capped CdSe was then suspended in this solution, followed by

heating at 63C under dry Ar atmosphere for ~24 h. The formed MPA-capped CdS or CdSe QD

was precipitated by adding the mixture of ethyl acetate and diethyl ether (1:1 in volume). The

collected precipitate was washed several times with ethyl acetate to remove residual MPA, OA

or TOPO.

To anchor the CdS or CdSe QDs onto TiO2 surface, 50 ml ethanol solution containing 0.5 g

TiO2 and the stoichiometric amount of the MPA-capped CdS (or CdSe) was magnetically stirred

at 60oC for 6 h. The prepared CdS/TiO2, or CdSe/TiO2 (Type-A heterojunctions) suspended in


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

5

the solution were then precipitated by centrifugation, and the collected powders were

subsequently annealed at 250oC for 3 h.

2.2. Preparation of Type-B and Type-C Heterojunction Structures

Antimony doped tin oxide (ATO), Ag3PO4, W18O49, FeTiO3, NiTiO3, CoTiO3, and Fe2O3

particles were prepared by the procedures reported in the literatures.33-44

10 mol% Sb-doped

SnO2 was prepared by co-precipitation of SnCl45H2O and SbCl3, followed by post heat

treatment method.33

FeTiO3 nanodisc was synthesized by the hydrothermal reaction of FeSO47H2O, KOH, titanium(IV) isopropoxide (TTIP, Aldrich) stabilized in aqueous solution of

tetrabutylammonium hydroxide (TBAH, Aldrich) at 220oC.

34 Ag3PO4 nanoparticle (NP) was

synthesized by ion-exchange reaction between AgNO3 and Na3PO4 in solid phase.41

W18O49

nanorods were synthesized by heating the reaction mixture of WCl4, oleic acid and oleylamine at

350oC under argon environment.

42 NiTiO3 (or CoTiO3) particle was prepared by co-precipitation

of nickel acetate (or cobalt acetate), titanium(IV) butoxide (Aldrich) and citric acid, followed by

subsequent heat treatment method.43

Fe2O3 NP was synthesized by a hydrothermal reaction of

FeCl36H2O stabilized in 25/75 ammonia/water solution at 180oC.

44

For the formation of Type-B or Type-C heterojunction, 3.67 g titanium isopropoxide (97%,

Aldrich) was stabilized in the mixed solution of 40 ml ethanol, 1 ml concentrated nitric acid, and

1 ml water, and the mixture was then gently stirred for 1 h. Stoichiometric amount of each

sensitizer particle was added to this solution. For example, to obtain 5/95 ATO/TiO2 (in wt%

ratio), 53 mg ATO particle was added and gently stirred overnight. The amorphous titania-coated

samples were then dried at 80oC for 24 h, and subsequently heat treated at 300

oC for 3 h to

crystallize the TiO2. As a blank sample, bare TiO2 was prepared by the same procedure without

adding the sensitizer particle.

2.3. Characterizations


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

6

X-ray diffraction (XRD) patterns were obtained for the heterojunction composite powder

samples by using a Rigaku Multiflex diffractometer with monochromatic light-intensity Cu K

radiation. XRD scanning was performed under ambient conditions over 2 region of 2070o

at a

rate of 2o/min (40 kV, 20mA). UV-visible diffuse reflectance spectra were acquired by a Perkin-

Elmer Lambda 40 spectrophotometer. BaSO4 was used as the reflectance standard. Field

emission transmission electron microscope (FE-TEM) images were obtained by a JEOL

JEM2100F operated at 200 kV. One milligram of the synthesized particles was dispersed in 50

mL of ethanol, and a drop of the suspension was then spread on a holey amorphous carbon film

deposited on the copper grid.45

2.4. Evaluation of photocatalytic activity

The visible-light photocatalytic efficiencies of the photocatalytic samples were estimated by

monitoring the evolved amount of CO2 by decomposing 2-propanol (IP) in gas phase. An

aqueous suspension containing 8.0 mg of photocatalytic sample was spread on a 2.52.5 cm2

Pyrex glass in a film form and subsequently dried at room temperature. The gas reactor system

used for this photocatalytic activity has been described elsewhere.45

The net volume of the gas-

tight reactor was 200 mL, and the photocatalytic film was located at the center of the reactor. The

entire area of the photocatalytic film (2.5 cm2.5 cm) was irradiated by a 300 W Xe lamp

through a UV cut-off filter (

7

condition, the IP and H2O remained in the vapor phase. After a certain irradiation interval, 0.5

mL of the gas in the reactor was automatically picked up and sent to a gas chromatograph

(Agilent Technologies, Model 6890N) using an auto sampling valve system. For CO2 detection, a

methanizer was installed between the GC column outlet and the FID detector.

3. Results and discussion

The X-ray diffraction patterns in Fig. 1a indicate that the prepared CdS and CdSe QDs are in

the cubic (JCPDS, No. 75-0581) and hexagonal (JCPDS, No. 88-2346) phases, respectively, with

no impurity peaks. The average crystallite sizes of CdS and CdSe QDs, as determined from the

corresponding (111) peaks by applying Scherrer equation, were 3.8 and 5.5 nm, respectively. Fig.

2a shows the UV-vis diffuse absorbance spectra of CdS and CdSe QDs loaded on the TiO2

particles. The absorption edge of CdS and CdSe QDs appeared at ~500 and ~660 nm,

respectively, exhibiting the capability of sensitizing the visible-light.

Antimony doped tin oxide (Sn0.9Sb0.1O2, ATO), Ag3PO4, W18O49, and FeTiO3 particles were

prepared by the procedures reported in the literature.33,34,41,42

X-ray diffraction patterns in Fig. 1b

confirm that the prepared sensitizers are in the pure phase. The average crystallite size of ATO,

Ag3PO4, W18O49, and FeTiO3 particles, as determined from the (110) peak of ATO, (210) peak

of Ag3PO4, (010) peak of W18O49 and (104) peak of FeTiO3 by applying Scherrer equation, were

49 nm, 62 nm, 43 nm and 46 nm, respectively, clearly indicating their high crystallinity. UV-vis

diffuse absorbance spectra of the prepared sensitizers are shown in Fig. 2b. Ag3PO4 reveals an

absorption band edge at ~560 nm, whereas ATO, W18O49, and FeTiO3 exhibited profound

absorbance in the entire visible region. The NiTiO3, CoTiO3, and Fe2O3 particles were also

prepared by the reported methods in the literature.43,44

The prepared samples were in the pure

phase with high crystallinity, as shown in the XRD patterns in Fig. 1c. UV-vis diffuse

absorbance spectra in Fig. 2c indicate that the prepared NiTiO3, CoTiO3, and Fe2O3 particles also

exhibit profound absorption in the visible region. NiTiO3 showed the absorption edges at ~550

nm and ~410 nm, caused by the transition of Ni2+

Ti4+

and O2-

Ti4+

,43

respectively. CoTiO3


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

8

revealed a broad absorption peak at ~610 nm with the absorption band edges at ~670 nm and

~490 nm, respectively, inherent from the transition of Co2+

Ti4+

and O2-

Ti4+

.43

Fe2O3

powder with a bandgap of 2.20 eV displays an absorption band edge at ~610 nm.

Table 1 illustrates the CB and VB energy levels and the bandgaps of the synthesized

sensitizers reported in the literatures.46-50

Herein we also calculated the bandgaps of these

sensitizers from the Kubelka Munk (KM) or Tauc plots versus wavelength. The determined

values, shown in Fig. S1 and S2, were quite close to the reported ones. The sensitizers were

categorized as Sensitizer-A (Sen-A), Sensitizer-B (Sen-B), and Sensitizer-C (Sen-C), according

to their energy band positions relative to those of TiO2. Considering that the CB and VB levels of

TiO2 at pH 7 are -0.5 and +2.7 V (vs. NHE), respectively, CdS and CdSe are classified to Sen-A,

whereas FeTiO3, Ag3PO4, W18O49, and ATO belong to Sen-B, and Fe2O3, CoTiO3 and NiTiO3

belong to Sen-C. Each sensitizer was then coupled with TiO2 to form the three different types of

heterojunctions, categorized as Type-A, Type-B, and Type-C. As shown in Scheme 2a, a few

nanometer-sized Sen-A QDs were loaded on the surface TiO2 (Degussa P25) to form Type-A

heterojunction. In order to form Type-B or Type-C heterojunction, relatively large Sen-B or Sen-

C particles were fully covered with TiO2, as described in Scheme 2b.

Fig. 3 shows the TEM images of the Type-A heterojunction structures prepared by anchoring

the MPA capped CdS or CdSe QDs onto the surface of Degussa P25. The images in Fig. 3a and

3b shows the 2/98 CdS/TiO2 and 1/99 CdSe/TiO2 (both are in molar ratio), respectively, clearly

indicating that the individual CdS and CdSe QDs with a size of ~4 and ~5 nm, respectively, are

attached on the TiO2 surface. The dotted rectangular part in Fig. 3a was further magnified, as

shown in the inset. The d-spacing of 0.335 nm was identified to be the (111) plane of CdS. Fig. 4

shows the TEM images of the Type-B heterojunction structures prepared by sol-gel method in

depositing the TiO2 on the surface of large sensitizer particles. TEM images in Fig. 4a, 4c, 4e

and 4f show the 5/95 ATO/TiO2, 7/93 W18O49/TiO2, 3/97 Ag3PO4/TiO2, and 5/95 FeTiO3/TiO2

(all are in wt% ratio), respectively. It is clear that large sensitizer particles are located in the core

and their surfaces are fully covered by small TiO2 grains. The high resolution TEM images were


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

9

also obtained for the dotted rectangular parts of Fig. 4a and 4c. Their characteristic fringe

patterns, as shown in Fig. 4b and 4d, certify the presence of crystallized ATO and W18O49,

respectively, in the 5/95 ATO/TiO2 and 7/93 W18O49/TiO2 composites. Type-C heterojunction

structures were also prepared by the same sol-gel method. TEM images of the 3/97 NiTiO3/TiO2,

3/97 CoTiO3/TiO2, and 3/97 Fe2O3/TiO2 (all are in wt% ratio) are shown in Fig. 5a-c, revealing

that the large Sen-C particles are fully covered with TiO2. High resolution TEM image of 3/97

Fe2O3/TiO2 in Fig. 5d showed the fringe patterns, identified to be the (110) plane of Fe2O3 and

(101) of TiO2.

Photocatalytic efficiencies of three different heterojunction structures, that is, Type-A

(CdS/TiO2, and CdSe/TiO2), Type-B (ATO/TiO2, Ag3PO4/TiO2, W18O49/TiO2, and FeTiO3/TiO2),

and Type-C (NiTiO3/TiO2, CoTiO3/TiO2, and Fe2O3/TiO2) were evaluated by monitoring the

decomposition of 2-propanol (IP) in gas phase under visible-light irradiation (422nm). The

amount of CO2 evolved in 2 h was monitored to evaluate the photocatalytic activity of each

system. More detailed photocatalytic data for the individual heterojunction structures are shown

in Fig. S3. First, as shown in Fig. 6a, the Type-A systems exhibited considerably higher catalytic

activity than the bare TiO2. The amounts of evolved CO2 with 2/98 CdS/TiO2 and 1/99

CdSe/TiO2 (both are in molar ratio) were 4.3 and 3.1 ppm, respectively, whereas that with the

bare TiO2 was only 0.95 ppm and those with the pure CdS and CdSe were 0.65 and 0.80 ppm,

respectively.

Second, visible-light photocatalytic activities of the Type-B systems were illustrated in Fig. 6b.

The amount of CO2 evolved in 2 h was remarkably enhanced for all Type-B systems. The

optimum ratios between the sensitizer and TiO2, with respect to photocatalytic efficiency, were

determined to be 5/95 for ATO/TiO2, 3/97 for Ag3PO4/TiO2, 7/93 for W18O49/TiO2, and 5/95 for

FeTiO3/TiO2 (all are in wt% ratio). As shown in Fig. 6b, the amounts of CO2 evolved in 2 h with

ATO/TiO2, Ag3PO4/TiO2, W18O49/TiO2, and FeTiO3/TiO2 were 8.3, 11.8, 6.8, and 8.1 ppm,

respectively.

Third, visible-light photocatalytic activities of the 3/97 NiTiO3/TiO2, 3/97 CoTiO3/TiO2, and


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

10

3/97 Fe2O3/TiO2 (all are in wt% ratio), classified as Type-C heterojunction, were also examined.

As shown in Fig. 6c, the amounts of CO2 evolved in 2 h for the NiTiO3/TiO2, CoTiO3/TiO2, and

Fe2O3/TiO2 were 0.85, 0.88, 0.96 ppm, respectively, indicating that the coupling of Type-C

sensitizer with TiO2 did not induce appreciable enhancement in photocatalytic activity.

Recently, Kubacka et al. reported that the photocatalytic activities of the coupled systems are

influenced by the size of sensitizer, due to the energy band modification by the quantum size

effect.51

However, the particle sizes of the metal oxides prepared in this work were in the range

of 4070 nm, which does not offer any quantum size effect. Thus the sizes of sensitizers in these

ranges would not be critical in determining the catalytic activity. From the observed trends in the

visible-light photocatalytic activities of the Type-B and Type-C heterojunctions, it is obvious that

relative energy band position between the sensitizer and TiO2 is a crucial factor. Hence, in

designing efficient catalytic system based on heterojunction structure, the transport of electrons

or holes from the sensitizer to TiO2 is regarded to be the most important issue, since the

electron/hole pairs are generated in the sensitizer and the catalytic sites are formed on the TiO2

surface.

In the Type-B heterojunction, the electrons in the sensitizer VB are excited to its CB under

visible-light irradiation. The generated holes in the sensitizer VB will be transported voluntarily

to that of TiO2. By this inter-semiconductor hole-transport mechanism, holes are generated on

the TiO2 VB, followed by formation of OH radicals through the equation 4-5. Hence, the

generated OH radicals can completely decompose the organic compounds to CO2 and H2O.

For the case of Type-C heterojunction, the electrons in the sensitizer VB are excited to its CB

under visible-light irradiation, but the photogenerated electrons and holes cannot be transported

to TiO2 CB and VB, respectively, since the sensitizers energy levels lies in between the CB and

VB of TiO2. Thus the coupling of these two semiconductors will not bring any synergetic effect

in charge separations. As a result, NiTiO3/TiO2, CoTiO3/TiO2, and Fe2O3/TiO2, belonging to

Type-C heterojunction, did not show appreciable visible-light photocatalytic activity.

In the Type-A heterojunction system, sensitizers are excited under visible-light irradiation,


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

11

followed by the transportation of the photogenerated electrons to TiO2 CB. As described in

equations 1-3, the electrons in the TiO2 CB are transferred to O2 to form O2- radicals, and further

converted to HO2 by reacting with proton. Even though HO2 is not as strong as OH in

oxidation ability, it is able to induce the complete mineralization of organic compounds.30,46

This

explains why CdS/TiO2 and CdSe/TiO2 systems showed considerably enhanced visible-light

photocatalytic activities, compared with the bare TiO2.

Herein it was found that Type-B heterojunction systems exhibited relatively higher catalytic

activity than the Type-A systems. Moreover, some of the Type-B systems showed comparable or

even higher activity than the typical N-doped TiO2. Basically, direct comparison of activity

among the different photocatalytic systems is not simple, since the several factors such as

particle size of sensitizer, contact and charge transport between sensitizer and TiO2, and others,

are involved in determining the photocatyalytic activity. Nonetheless, Type-B systems seem to

have a significant advantage in photocatalytic oxidation reactions due to the availability of OH

radicals in the TiO2 VB. The produced OH radicals, known as the most powerful oxidant, can

induce fast and complete decomposition of organic pollutants, rationalizing the enhanced

photocatalytic efficiency of Type-B systems.

In order to confirm the hole-transfer mechanism between the VB of sensitizer and TiO2 in the

Type-B heterojunction, the evidence for the generation of holes in TiO2 VB was investigated by

monitoring the chemical reaction of the iodide ion (I-), known as a hole scavenger. As a Type-B

heterojunction system, ATO/TiO2 was used in this experiment. Generally, I-/I3- redox couple has

been used as electrolyte mediating the charges in the dye-sensitized solar cells, and the role of I-

ions is accepting the holes from the HOMO of dye.52,53

Therefore, it is deduced that the I- ions

can be oxidized to triiodide (I3-) by reacting with the generated holes in the ATO or TiO2, as

shown in equation 7, since the redox potential of I-/I3- is +0.536 V,

54 which lies much higher

than the VB position of ATO (+3.6 V)33

or TiO2 (+2.7 V).46

2h+(VB) + 3I

-(aq) I3

-(aq) (7)


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

12

40 mL KI (0.01 M) solution containing ATO, ATO/TiO2 or TiO2 (20 mg each) was irradiated

for 2 h under visible-light (422nm). Then the I3-

formed in the solution could be identified

from its characteristic absorption peak at 286 nm and 345 nm. Fig. 7 showed the UVvis

absorption spectra of KI solution, after the photocatalytic reaction with ATO, ATO/TiO2 and TiO2.

First, pure 0.01 M KI solution, after visible-light irradiation for 2h, did not show any

characteristic absorption peak in 250-500 nm range. Second, when KI solution was irradiated in

presence of the bare ATO, interestingly, there was no characteristic absorption peak of I3-,

indicating that the bare ATO cannot oxidize I- under visible-light irradiation. In case of pure ATO,

electron and hole pairs will be generated in its CB and VB, respectively, but the holes generated

in ATO did not seem to be consumed for the formation of I3-, presumably due to the faster

electron-hole recombination than the reaction between the hole and I-. Third, when KI solution

was irradiated in presence of the bare TiO2 under visible-light, there was no characteristic

absorption peak of I3-, but under UV light strong absorption peaks appeared at 286 nm and 345

nm, indicating the presence of I3-. It is deduced that the TiO2 itself cannot be excited by visible-

light due to its wide band gap but that the generated holes in the VB by UV-light can induce the

formation of I3-, as described in equation 7. Fourth, when the KI solution was irradiated in

presence of the ATO/TiO2, noticeably, the characteristic absorption peak of I3- was observed (Fig.

7), indicating that the holes are generated in the TiO2 VB. Therefore, the obtained result strongly

supports that the visible-light photocatalytic activity of Type-B heterojunction systems originates

from the inter-semiconductor hole-transport.

The presence of OH radicals on the ATO/TiO2 surface during the visible-light irradiation

was also monitored in order to support the hole transfer mechanism.55,56

That is, 20 mg of ATO,

TiO2 or ATO/TiO2 was suspended in 60 mL aqueous solution containing 0.01 M NaOH and 3

mM 1,4-terephthalic acid (TA). Before exposure to visible-light, the suspension was stirred in

dark for 30 min. Then, 5 mL of the solution was taken after every 1 h for the fluorescence

measurements. It is known that OH radical reacts with TA in basic solution and generates 2-


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

13

hydroxy terephthalic acid (TAOH), which emits the unique fluorescence peak at 426 nm.56

Bare

ATO as well as TiO2, suspended in TA solution, did not show any appreciable fluorescence peaks

upon visible-light (422 nm) irradiation, as shown in Fig. 8a and 8b, suggesting that bare TiO2

or ATO cannot produce holes nor utilize them in producing OH radicals. Contrarily, the

ATO/TiO2 shows the characteristic fluorescence peak, and its intensity was increased with elapse

of irradiation time, as shown in Fig. 8c. This clearly indicates that the holes are formed at the

TiO2 side and they were transported from the ATO by the inter-semiconductor hole-transport

mechanism.

For the Fe2O3/TiO2 system, we also performed the OH radical test using TA in order to

check the possibility of hole-transfer between Fe2O3 and TiO2. Fig. 9 shows the fluorescence

spectra of TAOH for the suspensions with TiO2, Fe2O3/TiO2, and Fe2O3, after 2 h irradiation of

visible-light. It was found that the fluorescence peaks of the TAOH with the bare Fe2O3 and

Fe2O3/TiO2 were not appreciably different, suggesting that the hole transport from Fe2O3 to TiO2

was blocked in the Fe2O3/TiO2 system. The obtained result rationalize the reason for the low

catalytic efficiency of Type-C systems including Fe2O3/TiO2.

In order for the Type-B system to be efficient catalyst, the excited electrons in the CB of

sensitizer have to be scavenged. Considering the CB level of the sensitizer, direct electron

transfer to the oxygen molecules, requiring -0.284 V (vs. NHE), will be difficult in general

(equation 1).34

Thus it is considered that the electrons in the sensitizer CB are transported to the

oxygen species through the processes described in equation 8 and 9.54,57,58

O2 + H+ + e- HO2, E0 = -0.046 V (8)

O2 + 2H+ + 2e- H2O2, E0 = +0.682 V (9)

The visible-light catalytic activity of Type-B systems, achieved in the present work, is

comparable or even higher than that of N-doped TiO2, which are well-known visible-light

photocatalyst, but we believe that the catalytic activity can be enhanced much more, if the


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

14

appropriate sensitizers are developed and the interface control between the sensitizer and TiO2

are optimized for the efficient hole-transport. In this regards, several properties will be required

to the sensitizers for the design of efficient Type-B systems. The VB level of sensitizer has to be

sufficiently lower than that of TiO2, and the inherent hole-transport from sensitizer to TiO2 have

to be efficient and fast with less charge recombination. Small band gap will be favorable for

utilization of visible-light, but its CB level needs to be high enough for effective scavenging of

the electrons in its CB.

4. Conclusions

Nine heterojunction systems (CdS/TiO2, CdSe/TiO2, FeTiO3/TiO2, Ag3PO4/TiO2, W18O49/TiO2,

ATO/TiO2, NiTiO3/TiO2, CoTiO3/TiO2, and Fe2O3/TiO2) were fabricated, and they were

classified to three types, according to the relative energy band positions between sensitizer and

TiO2. Among them Type-B systems such as FeTiO3/TiO2, Ag3PO4/TiO2, W18O49/TiO2, and

ATO/TiO2 exhibited relatively higher visible-light photocatalytic activity in evolving CO2 from

the gaseous IP under visible-light irradiation. Especially, Ag3PO4/TiO2 showed significantly

higher catalytic efficiency than the typical N-doped TiO2. It is deduced that higher photocatalytic

activities of Type-B systems originates from the inter-semiconductor hole-transport. That is, the

generated holes in the sensitizer VB is transported to that of TiO2, resultantly inducing the

formation of OH radicals. The evidence for the hole-transport between sensitizer and TiO2 was

also investigated by monitoring the reaction with iodide (I-). By irradiating visible-light in

presence of ATO/TiO2, it was found that I- was converted to I3

-, clearly indicating that the holes

are generated in the TiO2 VB. It was also found that ATO/TiO2 system can convert the TA to

TAOH, which is evident for the formation of OH radicals in TiO2 side under visible-light

irradiation.

Acknowledgments


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

15

This work has been supported by the National Research Foundation of Korea (Project No. 2011-

0002995), and Korean Center for Artificial Photosynthesis (KCAP) funded by the Ministry of

Education, Science, and Technology (NRF-2011-C1AAA001-2011-0030278).

Notes and References

aDepartment of Chemistry, Inha University, Incheon 402-751, Republic of Korea

Fax: +82-32-867-5604; Tel: +82-32-863-1026; E-mail: [email protected]

bSchool of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea

Electronic Supplementary Information (ESI) available: [Detailed photocatalytic activities,

bandgaps, and PL spectra]. See DOI: 10.1039/b000000x/

1. T. Inoue, A. Fujishima, S. Konishi, K. Honda, Nature, 1979, 277, 637.

2. M. R. Hoffmann, Chem. Rev., 1995, 95, 69.

3. E. S. Jang, J. H. Won, S. J. Hwang, J. H. Choy, Adv. Mater., 2006, 18, 3309.

4. Z. Ding, G. Q. Lu, P. F. Greenfield, J. Phys. Chem. B, 2000, 104, 4815.

5. Y. Ou, J. Lin, S. Fang, D. Liao, Catal. Comm. 2007, 8, 936.

6. Y. Huang, Z. Zheng, Z. Ai, L. Zhang, X. Fan, Z. Zou, J. Phys. Chem. B, 2006, 110, 19323.

7. C. Burda, Y. Lou, X. Chen, A. C. S. Samia, J. Stout, J. L. Gole, Nano Lett. 2003, 3, 1049.

8. V. Stengl, V. Houskova, S. Bakardjieva, N. Murafa, ACS Appl. Mater. Interfaces., 2010, 2,

575.

9. S. Sakthivel, H. Kisch, Chem. Phys. Chem., 2003, 4, 487.

10. K. Y. Song, M. K. Park, Y. T. Kwon, H. W. Lee, W. J. Chung, W. I. Lee, Chem. Mater.

2001, 13, 2349.

11. L.-L. Tan, S.-P. Chai, A. R. Mohamed, ChemSusChem., 2012, 5, 1868.

12. M. Anpo, M. Takeuchi, J. Catal. 2003, 216, 505.

13. B. K. Vijayan, N. M. Dimitrijevic, J. Wu, K. A. Gray, J. Phys. Chem. C, 2010, 114, 21262.


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

16

14. P. Bouras, E. Stathatos, P. Lianos, Appl. Catal. B: Environ. 2007, 73, 51.

15. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science, 2001, 293, 269.

16. S. U. M. Khan, M. Al-Shahry, W. B. Ingler, Science, 2002, 297, 2243.

17. W. Zhao, W. Ma, C. Chen, J. Zhao, Z. Shuai, J. Am. Chem. Soc., 2004, 126, 4782.

18. J. C. Yu, W. Ho, J. Yu, H. Yip, P. K. Wong, J. Zhao, Environ. Sci. Technol., 2005, 39,

1175.

19. W. J. Ong, M. M. Gui, S. P. Chai, A. R. Mohamed, RSC Advances, 2013, 3, 4505.

20. Y. Zhang, N. Zhang, Z. R. Tang, Y. J. Xu, Phys. Chem. Chem. Phys., 2012, 14, 9167.

21. D. Liu, P. V. Kamat, J. Electroanal. Chem., 1993, 347, 451.

22. L. Spanhel, H. Weller, A. Henglein, J. Am. Chem. Soc., 1987, 109, 6632.

23. S. Y. Chai, Y. J. Kim, M. H. Jung, A. K. Chakraborty, D. Jung, W. I. Lee, J. Catal., 2009,

262, 144.

24. S. B. Rawal, S. D. Sung, W. I. Lee, Catal. Commun., 2012, 17, 131.

25. S. N. Frank, A. J. Bard, J. Phys. Chem., 1977, 81, 1484.

26. W. Ho, J. C. Yu, J. Mol. Catal. A: Chem., 2006, 247, 268.

27. E. Bae, W. Choi, J. Park, H. S. Shin, S. B. Kim, J. S. Lee, J. Phys. Chem. B, 2004, 108,

14093.

28. X. Yu, Q. Wu, S. Jiang, Y. Guo, Mater. Charact., 2006, 57, 333.

29. J. C. Kim, J. K. Choi, Y. B. Lee, J. H. Hong, J. I. Lee, J. W. Yang, W. I. Lee, N. H. Hur,

Chem. Commun., 2006, 5024.

30. Y. Bessekhouad, N. Chaoui, M. Trzpit, N. Ghazzal, D. Robert, J. V. Weber, J. Photochem.

Photobiol. A: Chem., 2006, 183, 218.

31. A. G. Agrios, P. Pichat, J. Appl. Electrochem., 2005, 35, 655.

32. U. I. Gaya, A. H. Abdullah, J. Photochem. Photobiol. C, 2008, 9, 1.

33. S. B. Rawal, A. K. Chakraborty, Y. J. Kim, H. J. Kim, W. I. Lee, RSC Advances, 2012, 2,

622.


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

17

34. Y. J. Kim, B. Gao, S. Y. Han, M. H. Jung, A. K. Chakraborty, T. Ko, C. Lee, W. I. Lee, J.

Phys. Chem. C, 2009, 113, 19179.

35. S. Shamaila, A. K. L. Sajjad, F. Chen, J. Zhang, J. Colloid Interface Sci., 2011, 356, 465.

36. H. Gnaser, M. R. Savina, W. F. Chalaway, C. E. Tripa, I. V. Veryovkin, M. J. Pellin, Int. J.

Mass Spectrom., 2005, 245, 61.

37. B. Gao, Y. J. Kim, A. K. Chakraborty, W. I. Lee, Appl. Catal. B: Environ., 2008, 83, 202.

38. W. W. Yu, X. Peng, Angew. Chem. Int. Ed., 2002, 41, 2368.

39. Q. Dai, D. Li, H. Chen, S. Kan, H. Li, S. Gao, Y. Hou, B. Liu, G. Zou, J. Phys. Chem. B,

2006, 110, 16509.

40. K. S. Leschkies, R. Divakar, J. Basu, E. E. Pommer, J. E. Boercker, C. B. Carter, U. R.

Kortshagen, D. J. Norris, E. S. Aydil, Nano Lett., 2007, 7, 1793.

41. Z. Yi, J. Ye, N. Kikugawa, T. Kako, S. Ouyang, H. S. Williams, H. Yang, J. Cao, W. Luo, Z.

Li, Y. Liu, R. L. Withers, Nat. Mater., 2010, 9, 559.

42. J. W. Seo, Y. W. Jun, S. J. Ko, J. W. Cheon, J. Phys. Chem. B, 2005, 109, 5389.

43. Y. J. Lin, Y. H. Chang, W. D. Yang, B. S. Tsai, J. Non-Cryst. Solids, 2006, 352, 789.

44. J. Ma, J. Lian, X. Duan, X. Liu, W. Zheng, J. Phys. Chem. C, 2010, 114, 10671.

45. Y. T. Kwon, K. Y. Song, W. I. Lee, G. J. Choi, Y. R. Do, J. Catal., 2000, 191, 192.

46. N. Serpone, P. Maruthamuthu, P. Pichat, E. Pelizzetti, H. Hidaka, J. Photochem. Photobiol.

A: Chem., 1995, 85, 247.

47. P. V. Kamat, Chem. Rev., 1993, 93, 207.

48. Y. Xu, A. A. Schoonen, Am. Mineral., 2000, 85, 543.

49. S. B. Rawal, S. Bera, W. I. Lee, Catal. Lett., 2012, 142, 1482.

50. Z. Q. Li, Y. L. Yin, X. D. Liu, L. Y. Li, H. Liu, Q. G. Song, J. Appl. Phys., 2009, 106,

083701.

51. A. Kubacka, M. Fernndez-Garca, G. Coln, Chem. Rev., 2012, 112, 1555.

52. M. Gratzel, Nature, 2001, 414, 338.


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

18

53. M. Wang, N. Chamberland, L. Breau, J.E. Moser, R. H. Baker, B. Marsan, S.M.

Zakeeruddin, M. Gratzel, Nat. Chem., 2010, 2, 385.

54. Handbook of Chemistry and Physics, 77th ed.; Weast, C. R., Ed.; CRC Press: Boca Raton,

FL, 1996.

55. T. Hirakawa, Y. Nosaka, Langmuir, 2002, 18, 3247.

56. G. Liu, C. Sun, L. Cheng, Y. Jin, H. Lu, L. Wang, S. C. Smith, G. Q. Lu, H. M. Cheng, J.

Phys. Chem. C, 2009, 113, 12317.

57. A. B. Anderson, T. V. Albu, J. Am. Chem. Soc., 1999, 121, 11855.

58. H. Irie, S. Miura, K. Kamiya, K. Hashimoto, Chem. Phys. Lett., 2008, 457, 202.

Table 1. CB and VB potential levels (vs. NHE) and bandgaps of various visible-light-absorbing

semiconductors measured at pH 7.44-48

Sensitizers Type CB (V) VB (V) Eg (eV)

CdS A -0.85 +1.55 2.40

CdSe A -0.60 +1.10 1.70

Ag3PO4 B +0.45 +2.90 2.45

ATO B +0.95 +3.60 2.55

W18O49 B +0.61 +3.21 2.60

FeTiO3 B +0.20 +3.00 2.80

CoTiO3 C +0.14 +2.39 2.25

NiTiO3 C +0.20 +2.38 2.18

Fe2O3 C +0.28 +2.48 2.20

TiO2 -- -0.50 +2.70 3.20


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

19

FIGURE CAPTIONS

Scheme 1. Schematic diagrams of the photo-induced charge flow under visible-light irradiation

for Type-A (a), Type-B (b), and Type-C (c) heterojunction structures.

Scheme 2. Schematic diagrams describing the preparation method for Type-A (a) and Type-B or

Type-C (b) heterojunction structures.

Fig. 1. X-ray diffraction patterns for the sensitizers belonging to Sen-A (a), Sen-B (b), and Sen-C

(c).

Fig. 2. UV-vis diffuse absorbance spectra for the sensitizers belonging to Sen-A (a), Sen-B (b),

and Sen-C (c).

Fig. 3. TEM images of CdS/TiO2 (a) and CdSe/TiO2 (b). High resolution (HR) image is shown in

the inset of (a), obtained from the dotted rectangular parts in (a).

Fig. 4. TEM images of ATO/TiO2 (a), W18O49/TiO2 (c), Ag3PO4/TiO2 (e), and FeTiO3/TiO2 (f).

HR-TEM images of ATO/TiO2 (b) and W18O49/TiO2 (d) obtained from the dotted rectangular

parts in (a) and (c), respectively.

Fig. 5. TEM images of NiTiO3/TiO2 (a), CoTiO3/TiO2 (b), and Fe2O3/TiO2 (c). HR-TEM image

of Fe2O3/TiO2 (d) obtained from the dotted rectangular parts in (a).

Fig. 6. Visible-light photocatalytic activities of several Type-A (a), Type-B (b), and Type-C (c)

heterojunction structures. The amounts of CO2 evolved under visible-light irradiation in 2 h were


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

20

monitored.

Fig. 7. UV-vis spectra of 0.01 M KI aqueous solution in presence of several catalytic systems

after visible-light (422nm) or UV light irradiation for 2 h.

Fig. 8. Fluorescence spectra measured for visible-light (310540 nm) irradiated bare TiO2 (a),

ATO/TiO2 (b), and ATO (c) suspensions in 3 mM TA. Wavelength of the excitation light for

obtaining the fluorescence spectra was 320 nm.

Fig. 9. Fluorescence spectra of TAOH for the 3 mM TA suspensions with TiO2, Fe2O3/TiO2, and

Fe2O3 after 2 h irradiation of visible-light. Wavelength of the excitation light for obtaining the

fluorescence spectra was 320 nm.


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

+2.7 V

TiO2

(a)

Sen-A

-0.5 V

e

Visible

light

e

h+

CB

VB

(b)

Visible

light

Sen-B TiO2

h+

h+

CB

VB

e

-0.5 V

+2.7 V

Visible

light

Sen-C TiO2

e

h+

CB

VB x

(c)

+2.7 V

-0.5 V x

Scheme 1. Schematic diagrams of the photo-induced charge flow under

visible-light irradiation for Type-A (a), Type-B (b), and Type-C (c)

heterojunction structures.


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

Dry, 60oC

MPA-capped Sen-A Type-A TiO2 NP

(a)

+ Heat, 250oC

(b)

Sen-B or Sen-C

Ti-sol + Dry, 80oC Heat, 300oC

Type-B or Type-C

Scheme 2. Schematic diagrams describing the preparation method for Type-A

(a) and Type-B or Type-C (b) heterojunction structures.


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

ATO

FeTiO3

Ag3PO

4

W18

O49

40

0

32

1

32

0

22

2

31

0

22

0

21

1

21

0

20

0

11

0

30

01

24

01

8

11

6

02

4

11

3

11

010

4

01

2

02

0

01

0

31

0

00

2

22

0

21

1

21

0

11

120

0

10

1

(b)

Inte

ns

ity

(A

.U.)

11

0

20 30 40 50 60

02

4

00

6

30

02

14

01

811

6

02

4

11

311

0

10

4

01

2

Fe2O

3

CoTiO3

NiTiO3

(c)

Two Theta (Degree)

01

2

10

4

11

0

11

3 11

6

01

8 12

43

00

20

2

11

0

12

1

11

0

12

0

22

0

23

1

23

3

13

02

11

CdSe

CdS

31

122

0

(a)

11

11

11

22

0

31

1

Fig. 1. X-ray diffraction patterns for the sensitizers belonging to Sen-A (a), Sen-

B (b), and Sen-C (c).


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

CdS

CdSe

(a)

Ag3PO

4

ATO

W18

O49

FeTiO3

(b)

Ab

so

rba

nc

e (

A.U

.)

400 500 600 700 800 900

NiTiO3

CoTiO3

Fe2O

3

Wavelength (nm)

(c)

Fig. 2. UV-vis diffuse absorbance spectra for the sensitizers belonging to Sen-A (a),

Sen-B (b), and Sen-C (c).


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

1 0 0 n m1 0 0 n m 30 nm

(a)

TiO2

20 nm CdSe

(b)

Fig. 3. TEM images of CdS/TiO2 (a) and CdSe/TiO2 (b). High resolution (HR) image

is shown in the inset of (a), obtained from the dotted rectangular parts in (a).


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

Fig. 4. TEM images of ATO/TiO2 (a), W18O49/TiO2 (c), Ag3PO4/TiO2 (e), and

FeTiO3/TiO2 (f). HR-TEM images of ATO/TiO2 (b) and W18O49/TiO2 (d) obtained

from the dotted rectangular parts in (a) and (c), respectively.

(b)

50 nm50 nm50 nm

(a)

(d)

TiO2

Ag3PO4

TiO2

(e)

TiO2

FeTiO3

(f)

20 nm

TiO2

TiO2

(c)


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

Fig. 5. TEM images of NiTiO3/TiO2 (a), CoTiO3/TiO2 (b), and Fe2O3/TiO2 (c). HR-

TEM image of Fe2O3/TiO2 (d) obtained from the dotted rectangular parts in (a).

100 nm100 nm50 nm

Fe2O3

TiO2

(c)

2 nm2 nm

(d)

10 nm10 nm

TiO2

CoTiO3

20 nm

(b)

TiO2 NiTiO3

50 nm

(a)


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

03

6

9

12

N-d

op

ed

TiO

2

Fe

TiO

3/T

iO2

W1

8O

49/T

iO2

Ag

3P

O4/T

iO2

AT

O/T

iO2

TiO

2

(b)

Ev

olv

ed

CO

2 (

pp

m)

0

3

6

9

12

TiO

2

Fe

2O

3/T

iO2

Co

TiO

3/T

iO2

NiT

iO3/T

iO2

(c)

Ev

olv

ed

CO

2 (

pp

m)

0

3

6

9

12

Cd

Se

Cd

S

Cd

Se/T

iO2

Cd

S/T

iO2

TiO

2

(a)

Evo

lved

CO

2 (

pp

m)

Fig. 6. Visible-light photocatalytic activities of several Type-A (a), Type-B (b), and

Type-C (c) heterojunction structures. The amounts of CO2 evolved under visible-

light irradiation in 2 h were monitored.


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

300 400 500

Ab

so

rba

nc

e (

A.

U.)

KI, Visible

KI + TiO2, Visible

KI + TiO2, UV

KI + ATO, Visible

KI + ATO/TiO2, Visible

Wavelength (nm)

Fig. 7. UV-vis spectra of 0.01 M KI aqueous solution in presence of several

catalytic systems after visible-light (422nm) or UV light irradiation for 2 h.


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

Fig. 8. Fluorescence spectra measured for visible-light (310540 nm) irradiated

bare TiO2 (a), ATO/TiO2 (b), and ATO (c) suspensions in 3 mM TA. Wavelength

of the excitation light for obtaining the fluorescence spectra was 320 nm.

350 400 450 500

(b)

ATO/TiO2 in 0 h

ATO/TiO2 in 1 h

ATO/TiO2 in 2 h

Flu

ore

scen

ce In

ten

sity

(A

. U.)

350 400 450 500

TiO2 in 0 h

TiO2 in 1 h

TiO2 in 2 h

(a)

350 400 450 500

Wavelength (nm)

ATO in 0 h

ATO in 1 h

ATO in 2 h

(c)


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

350 400 450 500

Flu

ore

scen

ce In

ten

sity

(A

. U.)

Wavelength (nm)

TiO2

Fe2O

3/TiO

2

Fe2O

3

Fig. 9. Fluorescence spectra of TAOH for the 3 mM TA suspensions with TiO2,

Fe2O3/TiO2, and Fe2O3 after 2 h irradiation of visible-light. Wavelength of the

excitation light for obtaining the fluorescence spectra was 320 nm.


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

Design of Visible-Light Photocatalysts by Coupling of Narrow Bandgap Semiconductors

and TiO2: Effect of Their Relative Energy Band Positions in Photocatalytic Efficiency Sher Bahadur Rawal, Sandipan Bera, Daeki Lee, Du-Jeon Jang and Wan In Lee

Graphic Abstract

According to relative energy band positions between TiO2 and visible-light-absorbing semiconductors,

three different types of heterojunctions were designed, and their visible-light photocatalytic efficiencies

were analyzed.

Ev

olv

ed

CO

2 (

pp

m)

0

2

4

6

8

10

12

14

TiO2

Type-A

Type-B

Type-C

N-doped TiO2

Photocatalytic

activity in

visible-light

2-propanol

CO2

e-

Type-A TiO2

Sen-A

TiO2

h+

Sen-B

Type-B

TiO2 Sen-C

Type-C


Cata

lysi

s Sc

ien

ce &

Tech

no

logy

A

ccep

ted

Man

usc

ript

Dow

nloa

ded

by U

nive

rsity

of Y

ork

on 1

7/05

/201

3 11

:04:

49.

Publ

ished

on

02 M

ay 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/C3CY

00004D

View Article Online

Design of Visible-Light Photocatalysts by Coupling of Narrow

Documents

Transcript of Design of Visible-Light Photocatalysts by Coupling of Narrow