ion engineering techniques for the preparation of the highly effective tio2 photocatalysts operating...

Ion engineering techniques for the preparationof the highly effective TiO2 photocatalysts operatingunder visible light irradiation

Masato Takeuchi • Masaya Matsuoka •

Masakazu Anpo

Received: 2 October 2011 / Accepted: 28 November 2011 / Published online: 18 February 2012

� Springer Science+Business Media B.V. 2012

Abstract The successful application of ion engineering techniques for the

development of TiO2 photocatalysts operating under visible and/or solar light irradia-

tions has been summarized in this review article. First, we have physically doped various

transition metal ions within a TiO2 lattice on an atomic level by using an advanced metal

ion implantation method. The metal ion implanted TiO2 could efficiently work as a

photocatalyst under visible light irradiation. Some field tests under solar light irradiation

clearly revealed that the Cr or V ions implanted TiO2 samples showed 2–3 times higher

photocatalytic reactivity than the un-implanted TiO2. Second, we have developed

the visible light responsive TiO2 thin film photocatalyst by a single process using an

RF-magnetron sputtering (RF-MS) deposition method. The vis-type TiO2 thin films

showed high photocatalytic reactivity for various reactions such as reduction of NOx,

degradation of organic compounds, and splitting of H2O under visible and/or solar light

irradiations.

Keywords Photocatalyst � Visible light � Ion engineering techniques �Metal ion implantation method � RF-magnetron sputtering deposition method

Introduction

We are now facing various environmental issues on a large scale such as a greenhouse

effect caused by CO2 and pollution of air, water, and soil by toxic chemicals.

Additionally, prompt solutions for the energy issue caused by exhaustion of fossil fuels

have been required. From this viewpoint, photocatalysis has attracted much attention as

‘‘an environmentally benign catalyst’’ because photocatalysts possess a potential to

M. Takeuchi � M. Matsuoka � M. Anpo (&)

Department of Applied Chemistry, College of Engineering, Osaka Prefecture University,

1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan

e-mail: [email protected]

123

Res Chem Intermed (2012) 38:1261–1277

DOI 10.1007/s11164-011-0465-x

oxidize organic compounds into nontoxic CO2 and H2O, decompose NOx, and reduce

CO2 under UV light irradiation. Hence, the photocatalytic system is often represented as

‘‘an artificial photosynthesis’’ [1–5]. However, a TiO2 photocatalyst with a wide

bandgap larger than 3.2 eV necessitates UV light irradiation having a wavelength

shorter than 388 nm. In 1991, Gratzel et al. [6] reported a dye-sensitized solar cell using

a TiO2 electrode and photofunctional Ru-bipyridyl dye to absorb visible light. This

finding has opened a way to convert solar energy into electric energy using an

inexpensive solar cell. In 2001, nitrogen, carbon, and sulfur-doped TiO2 semiconductors

have been reported to work as a photocatalyst under visible light irradiation [7–9]. These

anion dopants are supposed to exist in the O2- site of TiO2 lattice and interact with O2p

orbital, with the result that the bandgap of the TiO2 semiconductor becomes smaller.

Recently, Domen et al. have reported that TaON or (Ga1-xZnx)(N1-xOx) work as a

visible light responsive photocatalyst for water splitting reaction [10–12]. The

development of such visible light responsive photocatalysts will give us a clear answer

to purify polluted environments and may give a critical breakthrough to convert

abundant solar energy into stockable chemical energy. However, research to develop

better photocatalysts operating under visible light irradiation is still in progress.

In this review, we deal with an innovative application of ion engineering

techniques for the preparation of well-defined TiO2 photocatalysts. The metal ion

implantation method is generally used to modify the electronic property of silicon

semiconductors. Highly accelerated ions having higher energies are implanted into

the deep bulk of solids without any significant damage, resulting in a successful

modification of the surface and electronic properties. So, we have applied this

advanced method in order to develop the visible light responsive TiO2 photocat-

alysts. Then, although TiO2 thin film photocatalysts have generally been prepared

by wet processes, we have applied an RF-magnetron sputtering deposition method

as a dry process to develop transparent UV-type and vis-type TiO2 thin film

photocatalysts. Such dry processes to prepare thin film materials under high vacuum

have some advantages, as follows: (1) contamination with impurities can be

avoided; (2) a dry preparation method does not require the use of any organic

solvents so that it can be considered as an ‘‘environmentally-benign process’’; (3)

thin films with high crystallinity and sufficient adhesion can be prepared onto

substrates without calcination at high temperatures; and (4) preparation conditions

are easily controlled.

Visible light responsive TiO2 photocatalysts

TiO2 semiconductor powder system

Since the 1970s, various approaches have been carried out to develop the visible

light responsive photocatalysts by adding second components such as metal oxides

or metal ions onto TiO2 surfaces. However, no remarkable results have been

obtained for several decades, because the added metal oxide or metal ions aggregate

on the surface and work as a recombination center for photo-formed electrons and

holes [13–16]. From this viewpoint, we have directed our attentions to an advanced

1262 M. Takeuchi et al.

123

metal ion implantation method in order to modify the electronic property of the

TiO2 semiconductor. In fact, many kinds of TiO2 photocatalysts implanted with

various transition metal ions, such as V, Cr, Mn, Fe, Co, Ni, Cu, and so on, have

been prepared [17–24]. Figure 1a shows UV–Vis absorption spectra of the Cr ion-

implanted TiO2 photocatalysts and a solar spectrum. Interestingly, the absorption

edges of TiO2 semiconductor itself smoothly shifted toward visible light regions

depending on the amounts of Cr ions implanted. This finding clearly means that the

Cr ion implanted TiO2 samples obviously absorb much more sunlight as compared

to the original TiO2 without Cr ion implantation. The same characteristic

phenomenon in the absorption spectra could also be observed by implanting other

transition metals such as V, Mn, Fe, Co, Ni, and Cu ions. In contrast, the

implantation of Mg, Ti, and Ar ions did not show any red-shift in the absorption

spectra of the TiO2 semiconductor. For comparison, Fig. 1b shows UV–Vis

absorption spectra of the TiO2 catalysts chemically doped with Cr ions by an

impregnation method. These catalysts showed a new absorption at 400–500 nm as a

Wavelength / nm

K. M

. fun

ctio

n / a

. u. Solar spectrum

200 400 600 800 1000

(a)

(b)

(c)(d)

Wavelength / nm

K. M

. fun

ctio

n / a

.u.

300 400 500 600

(a)(b’)

(c’)

(e’)

(d’)

(A)

(B)

Fig. 1 a Diffuse reflectanceUV–Vis absorption spectra ofthe TiO2 (a) and Cr ionimplanted TiO2 catalysts (b–d).Amount of Cr ions implanted(lmol/g-TiO2); (a) 0, (b) 0.22,(c) 0.66, (d) 1.3. b Diffusereflectance UV–Vis absorptionspectra of the TiO2 (a) and Crion impregnated TiO2 catalysts(b’–e’). Amount of Cr ionsimpregnated (wt%); (a) 0, (b’)0.01, (c’) 0.1, (d’) 0.5, (e’) 1.(0.1 wt% equals to 4.9 lmol/g-TiO2)

Ion engineering techniques for the preparation 1263

123

shoulder, but the absorption edge of TiO2 semiconductor at 388 nm did not change

at all. The intensity of the shoulder absorption increased depending on the amounts

of Cr ions doped, showing the formation of aggregated Cr-oxide species on the TiO2

surface. These results clearly indicate that the red-shift observed in the absorption

spectra of the metal ion implanted TiO2 is attributed to a chemical interaction

between TiO2 semiconductor and implanted metal ions but not to a change of

physical property arose from an introduction of lattice defects. Furthermore, such

red-shift in the absorption spectra of TiO2 photocatalysts could be achieved only by

applying the advanced metal ion implantation method.

We have confirmed that the metal ion implanted TiO2 samples work efficiently as

a visible light responsive photocatalyst for various reactions, such as decomposition

of NO, isomerization of cis-2-butene, degradation of organic compounds in water,

and the hydrogenation of methyl-acetylene with water. Figure 2 shows the reaction

time profiles for the decomposition of NO over the Cr ion implanted TiO2 and

original TiO2 under visible light irradiation. Although the un-implanted TiO2 did

not show any photocatalytic reactivity, the Cr ion implanted TiO2 efficiently

decomposed NO into N2 and N2O even under visible light irradiation. In addition,

these metal ion implanted TiO2 samples maintained a higher photocatalytic

performance than the un-implanted TiO2 under UV light irradiation. On the other

hand, the photocatalytic reactivity of the TiO2 chemically doped with metal ions

under visible and UV light irradiations was drastically depressed because of the

aggregated metal oxide species doped onto the TiO2 surface. In fact, we have

carried out fieldwork tests of NOx decomposition on the Cr or V ion implanted TiO2

photocatalysts under solar light irradiation. As shown in Fig. 3, the Cr or V ions

implanted TiO2 samples showed 2–3 times higher photocatalytic reactivity as

compared to the original TiO2 under solar light irradiation. SIMS (secondary ion

mass spectrometer) analyses have revealed that the implanted metal ions existed in

the deep bulk of TiO2, indicating that such metal ions in the TiO2 lattice efficiently

modify the electronic property of TiO2 semiconductor to realize visible light

Yie

lds

of N

2fo

rmat

ion

/ µm

ol•g

-TiO

2-1

Cr ion-implanted TiO2

Time / h

N2

N2O

0

0.5

1.0

1.5

-2 0 2 4 6 8 10

original TiO2

off offon onFig. 2 Reaction time profilesfor photocatalyticdecomposition of NO undervisible light (k[ 450 nm)irradiation over the Cr ionimplanted TiO2 and the originalTiO2


123

responsive photocatalysts. These findings have opened a new way to develop the

visible light responsive photocatalysts by applying ion engineering techniques.

Highly dispersed Ti-oxide species

We have already reported that the highly dispersed Ti-oxide species exhibit unique

photocatalytic property [25–31]. Figure 4a shows the diffuse reflectance UV

absorption spectra of Ti-containing mesoporous silica (Ti-MCM-41). As the Ti-

content decreased, the absorption shifted toward a shorter wavelength region [25].

Furthermore, various molecular spectroscopic investigations revealed that these

highly dispersed Ti-oxide species possess a characteristic local structure as

compared to powdered TiO2 semiconductors [26–31]. From the results of XAFS

(X-ray absorption fine structure) measurements, the TiO2 semiconductor was found

to consist of octahedral TiO6 units, on the other hand, the highly dispersed Ti-oxide

species were found to include tetrahedral TiO4 units within SiO2 or zeolite matrices.

The catalysts including these tetrahedral TiO4 species also exhibited characteristic

photoluminescence spectra at around 480 nm, when they were excited by UV light

irradiation at around 260 nm. From these results, the absorption and photolumi-

nescence spectra of the highly dispersed Ti-oxide species were attributed to the

ligand to metal charge transfer (LMCT) process and its reverse recombination

process of the correlated electron–hole pairs, respectively. The addition of O2 or NO

molecules led to efficient quenching of the photoluminescence spectra, suggesting

that the TiO4 units highly dispersed on the support surfaces as a photocatalytic

active site effectively interacted with those quencher molecules. UV light irradiation

of these highly dispersed Ti-oxide catalysts in the presence of NO could catalyze the

decomposition of NO as a photocatalytic reaction and mainly produce N2 instead of

N2O. Since the yield and the selectivity for N2 formation were in good agreement

PhotocatalystsY

ield

s of

NO

elim

inat

ion

/ mol

min

-1Cr/TiO2 V/TiO2TiO2

0

2

4

6

8x 10-9

Solar beam intensity : 38.5 mW/cm2

Amount of catalyst : 3.6 gFlow rate : 18 liter/min

Fig. 3 Photocatalytic reactivityfor decomposition of NO undersunlight irradiation over theTiO2, Cr- and V-ion implantedTiO2 catalysts


123

with the intensity of the photoluminescence spectra, the LMCT process observed for

the highly dispersed Ti-oxide catalysts is supposed to play an important role in the

photocatalytic decomposition of NO.

As mentioned above, the highly dispersed Ti-oxide species show a unique

photocatalytic performance, especially for the decomposition of NO into N2 and O2

[26–28] as well as the reduction of CO2 with H2O into CH3OH and CH4 [29–31].

However, the tetrahedral TiO4 species necessitate the UV light irradiation having a

wavelength shorter than 270 nm to utilize them as a photocatalyst. So, we have tried to

modify the electronic property of the highly dispersed Ti-oxide species by applying the

metal ion implantation method. Figure 4b shows the diffuse reflectance UV–Vis

absorption spectra of the V ion implanted Ti-MCM-41 samples (Ti content: 1 wt%). As

is the case with the powdered TiO2 semiconductor, the absorption spectra effectively

shifted toward visible light regions depending on the amount of V ions implanted. The V

or Cr ion implanted Ti-MCM-41 as well as Ti–Si binary oxide photocatalysts were

actually confirmed to decompose NO into N2 and O2 under visible light irradiation.

These results clearly suggest that the metal ion implantation method is very effective for

the modification of electronic properties not only in the TiO2 semiconductor but also in

200 250 300 350Wavelength / nm

K.M

. fun

ctio

n

200 300 400 500Wavelength / nm

K.M

. fun

ctio

n

(A)

(B)

(a)

(b)

(c)

(d)

(e)(f)

(g)

(h)

Fig. 4 a Diffuse reflectanceUV absorption spectra of theTi-MCM-41 with differentTi contents; (a) 0.15, (b) 0.60,(c) 0.85 and (d) 2.0 wt%.b Diffuse reflectance UV–Visabsorption spectra of the V-ionimplanted Ti-MCM-41 (Ti: 1.0wt%); amount of V ionsimplanted (lmol/g-cat): (e) 0,(f) 0.66, (g) 1.3, (h) 2.0


123

the highly dispersed Ti-oxide species, realizing the visible light responsive highly

dispersed Ti-oxide photocatalysts.

TiO2 thin film system

Transparent visible light responsive TiO2 thin film photocatalysts

Although ultra-fine TiO2 powders are one of the most useful candidates for

purification of polluted water and air, it is difficult to recycle the used catalysts from

the reaction systems. To address this problem, various methods to coat TiO2 onto

the substrates, such as a sol–gel method [32–35], a metal organic chemical vapor

deposition (MOCVD) method [36, 37], and a direct deposition technique from

aqueous solutions of (NH4)2TiF6 [38] or TiOSO4 [39], have been attempted. TiO2

powder is used as a very useful white color pigment because of its high refractive

index [40]. In contrast, TiO2 thin film shows a high transparency and does not lose

the design and texture of the substrates. Moreover, TiO2 thin films were reported to

exhibit a photo-induced high wettability in which the contact angle of water droplets

on the thin film surface can reach zero under UV light irradiation [41, 42]. This

characteristic feature of TiO2 thin films has been applied for anti-fogging mirrors.

Such TiO2 thin film photocatalysts, which can eliminate offensive odors and

decompose volatile organic compounds under UV light irradiation, have already

been utilized. This feature is also applied as a car body coating to prevent grease and

dirt accumulation and as a house wall coating with a self-cleaning effect [3, 5].

Presently, the coating of TiO2 on various substrates has mainly been carried out

by sol–gel methods such as a dip coating or a spin coating as a wet process [32–35,

38, 39]. Since metal alkoxides are used as precursor solutions, a calcination process

at high temperatures is necessary after coating the sol solution in order to obtain the

thin films with a sufficient adhesion on the substrates. Therefore, the materials

which do not have high heat resistance cannot be used as substrates for the wet

coating process. To overcome this problem, various coating solutions containing

TiO2 fine particles and inorganic binders which can solidify at relatively low

temperatures have been developed. However, these thin films do not always show

sufficient photocatalytic performances or strong adhesion onto the substrates.

In our research, the ion engineering techniques have been applied to prepare

transparent TiO2 thin films as a dry process due to their simplicity to control the

various preparation conditions [43, 44]. Several physical vapor deposition (PVD)

methods have been developed. In these methods, thin films are formed onto various

substrates by accumulating the atomic- or ionic-state vapors of starting materials

which are vaporized by resistance heating, electron beam irradiation, laser ablation,

or sputtering. Advantages of these dry processes are: (1) contamination with

impurities can be easily prevented due to a process using a high vacuum chamber;

(2) thin films with high crystallinity and strong adhesion onto substrates can be

easily prepared without calcination at high temperatures; and (3) the various

physical and chemical properties can be easily controlled.

In this section, we deal with the successful preparation of transparent TiO2 thin

films on quartz substrates by applying an ionized cluster beam (ICB) deposition


123

method [45, 46]. The schematic diagram of this method is shown in Fig. 5. Titanium

metal as a source material was heated to about 2,200 K in the crucible and titanium

vapor was introduced into the high vacuum chamber forming titanium metal

clusters. At this time, the titanium clusters reacted with O2 molecules in the vacuum

chamber and the stoichiometric TiO2 clusters were formed (oxygen pressure:

2 9 10-4 Torr). These TiO2 clusters ionized by electron beam irradiation were

accelerated by an electric field (acceleration voltage: 500 V) and bombarded onto

various substrates. Detailed characterizations of the prepared TiO2 thin films were

carried out by molecular spectroscopies such as UV–Vis absorption, XPS, XRD,

and XAFS measurements. The surface morphologies of the thin films were also

observed by AFM, SEM, TEM, and SIMS analyses. The photocatalytic reactivity

was evaluated by the decomposition of NO and the oxidation of acetaldehyde with

O2 under UV and/or visible light irradiations.

TiO2 thin films having the film thickness larger than 300 nm could be

characterized by XRD patterns. The TiO2 thin films prepared by an ICB deposition

method showed typical diffraction patterns due to the anatase and rutile phases. On

the other hand, the thin films with the thickness smaller than 200 nm did not show

any diffraction patterns due to the TiO2 structure. However, we have characterized

such very thin TiO2 films by using a XAFS measurement, which is one of the most

effective analysis methods to investigate catalysts with very low concentrations. As

shown in Fig. 6, even the thin films having a much smaller thickness could be

confirmed to consist of an anatase TiO2 structure from the XANES (extended X-ray

absorption fine structure) spectra [45]. Since the conventional TiO2 powder (Evonik,

P25) that is a mixture of anatase and rutile phases is well known to exhibits high

photocatalytic reactivity, it was expected that the TiO2 thin films prepared by

Substrate

+

(Quartz glass)

El i fi ld++

e

e

-e-

e- -Electronbeam

Electric field(500 V)O2 atmosphere

(2 x 10-4 Torr)

e

Ti metalTi metalin crucible

Fig. 5 Schematic diagram of the ionized cluster beam (ICB) deposition method


123

the ICB deposition method were also expected to show high photocatalytic

performances.

Figure 7 shows UV–Vis absorption spectra of the TiO2 thin films having

different film thicknesses. Clear interference fringes can be observed in the visible

light regions, indicating that the uniform and highly transparent thin films are

formed on the substrates. The absorption edges of these TiO2 thin films were found

to shift toward the shorter wavelength regions as the film thickness decreased. This

result can be attributed to the quantum size effect caused by the presence of nano-

sized TiO2 particles as a composition of the transparent thin films.

In the presence of NO, UV light (k[ 270 nm) irradiation of TiO2 thin films

smoothly catalyzed a photocatalytic decomposition of NO at 275 K. The formation

4940 4980 5020Energy / eV

Nor

mal

ized

Abs

orpt

ion

(a)

(b)

0 2 4 6

Distance / ÅF

T o

f k3 c

(k)

/ a.u

.

(A)

(B)

Fig. 6 Ti K-edge XANES (a, b) and Fourier transforms of EXAFS oscillation (A, B) of the TiO2 thinfilms prepared by an ICB deposition method. Film thickness: (a, A) 20, (b, B) 300 nm

200 400 600 800

Tra

nsm

ittan

ce /

a.u.

Wavelength / nm

30 %

(a)

(b)

(c)

(d)

Fig. 7 UV–Vis absorption(transmittance) spectra of theTiO2 thin films prepared by anICB method. Film thickness:(a) 20, (b) 100, (c) 300,(d) 1,000 nm


123

yield of N2 linearly increased with UV light irradiation time and no photocatalytic

reaction occurred under dark conditions. Thus, the TiO2 thin films prepared by the

ICB deposition method could be clearly confirmed to exhibit high photocatalytic

reactivity for the decomposition of NO. The photocatalytic reactivity of the TiO2

films was almost comparable or even higher as compared with TiO2 thin films

prepared by a typical sol–gel process. Moreover, it was found that the photocatalytic

reactivity strongly depended on the film thickness, BET surface area, and the

wavelength of absorption edge of these TiO2 thin films. As shown in Fig. 8, the

TiO2 thin films having a smaller thickness showed much higher photocatalytic

reactivity. As the film thickness increased, the photocatalytic reactivity became

lower. The BET surface area and the wavelength of the absorption edge showed

the same tendency toward the photocatalytic reactivity. These results indicate

that, when we want to get TiO2 thin films having high enough photocatalytic

performance, the film thickness and crystallinity might be one of the most important

factors in the preparation conditions.

It was clearly shown that the ion engineering technique is very useful for the

preparation of the transparent TiO2 thin film photocatalysts operating under UV

light irradiation. However, these pure TiO2 thin films cannot work as a visible light

responsive photocatalyst. So, we have applied the advanced ion implantation

method to modify the transparent TiO2 thin films as in the case with the TiO2

powder system [46]. UV–Vis transmittance spectra of the Cr ion implanted TiO2

thin films are shown in Fig. 9. The absorption edges of the thin films were found to

shift toward visible light regions as the amount of Cr ions implanted increased.

On the other hand, TiO2 thin films chemically-doped with Cr or V ions by an

impregnation method did not exhibit such a red-shift in the absorption spectra (not

shown here). These results indicate that the modification of TiO2 semiconductor to

absorb visible light can be achieved only by the ion implantation process but not by

any other chemical doping methods.

Yie

ld o

f N2

form

atio

n / µ

mol

BE

T s

urfa

ce a

rea

(TiO

2si

de o

nly)

/ m

2

Wav

elen

gth

of a

bsor

ptio

n ed

ge /

nm

Film thickness / nm

0.04

0.12

0 12000

0.08

400 800

30

25

10

20

15

280

300

360

320

340

Fig. 8 Effects of the TiO2 film thicknesses on the photocatalytic reactivity to decompose NO (circleplots) under UV light irradiation, the BET surface areas (diamond plots), and the wavelengths ofabsorption edge (square plots)


123

The Cr ion implanted TiO2 thin films were confirmed to catalyze NO decomposition

under visible light (k[ 450 nm) irradiation at 275 K. However, the un-implanted

TiO2 thin films and the TiO2 thin films chemically-doped with Cr ions did not show

any photocatalytic reactivity under the same irradiation conditions. The decompo-

sition reaction of NO also proceeded with a good linearity against the visible

light irradiation time. These results clearly suggest that the Cr ion implanted TiO2 thin

films do work as a highly reactive photocatalyst operating under visible light

irradiation [46].

In order to clarify the local structure of the Cr ions implanted into the TiO2

semiconductor, we have carried out XAFS measurements for the Cr ion implanted

TiO2 thin films [47, 48]. The sophisticated analytical method clearly revealed the

existence of highly dispersed octahedral Cr3? species within the TiO2 lattice, not

tetrahedral species. Furthermore, detailed analyses of the EXAFS oscillation could

make clear its local structure shown in Fig. 10, which has neighboring O atoms at

ca. 1.6–1.7 A in the first coordination sphere as well as Ti atoms at ca. 2.8–3.0 A in

the second coordination sphere. In addition, ESR measurements for the V ions

implanted TiO2 photocatalysts after calcination at 723–873 K revealed the existence

of typical vanadyl [V = O2?] species. Modification of the electronic property of

TiO2 semiconductor by the metal ion implantation is expected to be closely

associated with the formation of highly dispersed Cr3? or [V = O2?] species

replaced within the TiO6 octahedron. However, such highly dispersed transition

metal species might affect the formation of impurity energy levels within the band

gap of the TiO2 semiconductor. These findings have successfully opened one door

to design and develop TiO2 photocatalysts operating under visible light irradiation.

Visible light responsive TiO2 thin film photocatalysts prepared by an RF magnetronsputtering deposition method

In the previous section, development of the visible light responsive TiO2 thin films

by applying a metal ion implantation method was mentioned. However, this method

necessitates two different processes, the first is a preparation of transparent TiO2

200 300 400 500 600

(a)(b)

(c)Tra

nsm

ittan

ce /

a.u.

Wavelength / nm

Fig. 9 UV–Vis absorption(transmittance) spectra of theTiO2 thin film (a) and Cr ion-implanted TiO2 thin films (b, c).Amount of Cr ions implanted(ions/cm2); (a) 0, (b) 3 9 1016,(c) 6 9 1016


123

thin films by an ICB deposition method and the second is a modification of the

electronic property of the TiO2 semiconductor by a metal ion implantation method.

In this section, we will deal with an alternative and more practical method, an

RF-magnetron sputtering (RF-MS) deposition method, for the creation of highly

transparent TiO2 thin films operating under visible light irradiation [49–52]. The

schematic diagram of the RF-MS deposition method is shown in Fig. 11. The ring-

state Ar gas plasma induced by magnetic and electric fields sputters a target material

surface (in this study, a TiO2 plate) to produce the sputtered particles such as Ti4?

and O2- ions. These sputtered particles are accumulated onto the substrate surface

to form TiO2 thin films. In a general reactive sputtering method, oxide films are

prepared by sputtering a metallic target in the presence of O2 as a reactive gas.

However, since a TiO2 plate with rutile structure was applied as an ion source

O2-

Ti4+

Cr3+

1.6 – 1.7 Å

2.8 – 3.0 Å

Fig. 10 Proposed structure of the highly dispersed octahedral Cr ion implanted within the TiO2 lattice

HeaterSubstrate(Quartz glass)

Target (TiO2)

gas plasma(Ar+)

S NNMagnet

N SS

Fig. 11 Schematic diagram of a RF magnetron sputtering (RF-MS) deposition method


123

material (a sputtering target), only Ar was used for a sputtering gas without

coexisting O2 as a reactive gas. TiO2 thin films prepared by a RF-MS deposition

method were then characterized by XRD and UV–VIS absorption measurements.

Surface morphologies of the TiO2 thin films were observed by SEM analyses.

Furthermore, atomic compositions of the TiO2 thin films from surface to deep bulk

were estimated by using AES measurement.

Figure 12 shows UV–Vis transmittance spectra of the TiO2 thin films prepared

by a RF-MS deposition method. TiO2 thin films prepared at low temperatures

showed high transparency and specific interference fringes in visible light region,

indicating the formation of stoichiometric and transparent TiO2 thin films.

In contrast, the TiO2 thin films prepared at temperatures higher than 773 K show

an effective absorption in the visible light region. However, the TiO2 thin films

prepared by sputtering the TiO2 plate with O2 gas did not show any absorption in the

visible light region, only showing the absorption in UV light region at around

330 nm. These results indicate that the TiO2 thin films which effectively absorb

visible light can be obtained only when the TiO2 target is sputtered by Ar gas

plasma without O2 at relatively high temperatures.

The transparent TiO2 thin films prepared at temperatures lower than 473 K were

found to show an effective photocatalytic reactivity for decomposition of NO as well as

oxidation of acetaldehyde with O2 under UV light irradiation. Its photocatalytic

reactivity per surface area was almost comparable to a TiO2 powder (Evonik, P25)

which is known as one of the most reactive photocatalysts. On the other hand, the TiO2

thin films prepared at temperatures higher than 773 K were found to work as a

photocatalyst under visible light (k[ 450 nm) irradiation for both the above reactions.

These results clearly indicate that the TiO2 thin films prepared by the RF-MS deposition

method possess high photocatalytic performance not only to decompose NO but also to

oxidize organic compounds even under visible light irradiation.

The detailed mechanism for the efficient absorption of visible light has been

discussed by observations of the surface morphology of these thin films. As shown

in Fig. 13, obvious differences between the TiO2 thin films prepared at 473 K (UV-

type) and 873 K (Vis-type) could be observed in their cross-sectional SEM images.

The UV-TiO2 thin film had the structure in which nano-sized TiO2 particles

200 400 600 800T

rans

mitt

ance

Wavelength / nm

20 %

(a)

(b)

(c)

(d)

(e)

Fig. 12 UV–Vis absorption(transmittance) spectra of theTiO2 thin films prepared atdifferent preparationtemperatures; (a) 373, (b) 473,(c) 673, (d) 873, (e) 973 K


123

randomly sinter with each other. On the other hand, the Vis-TiO2 thin film was

found to possess the unique structure in which columnar TiO2 single crystals

(diameter: ca. 100 nm) are orderly aligned. This unique structure was observed only

when TiO2 thin films were prepared at temperatures higher than 773 K by using the

RF-MS deposition method. Moreover, AES measurements for the Vis-TiO2 thin

films revealed that the O/Ti atomic ratio gradually decreased from the surface of 2.0

for stoichiometric TiO2 to the deep bulk of 1.933. In contrast, the O/Ti atomic ratio

of the UV-TiO2 thin films was confirmed to be almost constant at 2.0 even in the

bulk. Such a decreased O/Ti atomic ratio observed for the Vis-TiO2 thin films to be

stable even after calcination at 773 K, because the stoichiometric TiO2 layer at the

near surface works as a passive layer to protect the bulk to effectively absorb visible

light. These results suggested that such a characteristic structure of the Vis-TiO2

thin films is supposed to play an important role in the modification of the electronic

property of TiO2 thin films enabling them to absorb visible light [49–53].

Photocatalytic separate evolution of H2 and O2 from H2O using Vis-TiO2 thin filmsprepared by the RF-MS deposition method [54–60]

Vis-TiO2 thin films were, thus, applied for photocatalytic decomposition of H2O

into H2 and O2 under sunlight irradiation. The photocatalytic device was prepared

by depositing the Vis-TiO2 film on one side of Ti foil with Pt particles on the

opposite side. The H-type reactor mounted with the photocatalytic device is

TiO2

(1.2 µm)

Quartz

TiO2

(1.2 µm)

Quartz

(A)

(B)

Fig. 13 Cross-sectional SEM images of the UV-TiO2 (A) and Vis-TiO2 thin films


123

illustrated in Fig. 14a. The Vis-TiO2 photocatalyst side was immersed in 1.0 M

NaOH solution and the Pt side was immersed in 0.5 M H2SO4 solution in order to

apply a chemical bias of ca. 0.8 V. As shown in Fig. 14b, sunlight irradiation on the

photocatalytic devise efficiently catalyzed an evolution of H2 on the TiO2 side and

O2 on the Pt side, while no reaction proceeded on the UV-TiO2 film under the same

reaction conditions. This experiment was carried out on a sunny day in March, but

the relative intensity of sunlight varied from hour to hour. The decreased evolution

rates of H2 and O2 after sunlight irradiation of 4–5 h was thus related to the decrease

in the sunlight intensity. The conversion efficiency of solar energy could be

estimated as ca. 0.1% from the initial evolution rate of H2. A novel photocatalytic

system for the separate evolution of H2 and O2 from H2O under sunlight irradiation

Photocatalyst

Pt Vis-TiO2 thin film

H+

H2

e-

OH-

O2

h+

Stainless steelStainless steel Nafion

Quartz

sunlight

Ti foil

(A)

H2

O2

Time / h

Am

ount

s of

evo

lved

H2

and

O2

/ µm

ol

0 2 4 60

5

10

15

20

25

8

(B)

Fig. 14 a Schematic diagram of the H-type reactor combined with the Vis-TiO2 photocatalytic devicefor water splitting reaction. b Reaction time profiles for separate evolution of H2 and O2 from H2O undersunlight irradiation. Sunlight was irradiated for 7 h from 0930 to 1630 hours


123

could be achieved by the visible light responsive TiO2 photocatalysts prepared by

the ion engineering techniques.

Conclusions

We have successfully applied various ion engineering techniques for the develop-

ments of highly functional TiO2 photocatalysts operating under visible and/or solar

light irradiations. Especially, the advanced metal ion implantation method to modify

the electronic property of semiconductors was of great importance to develop the

TiO2 photocatalysts operating under visible light irradiation. In addition, the RF-

magnetron sputtering (RF-MS) deposition method to prepare various oxide thin

films has created the possibility of developing such visible light responsive TiO2

thin film photocatalysts in a single process. From the viewpoint of green chemistry,

constructive application of such dry processes is necessary for widespread practical

uses and much safer manufacturing methods without using toxic chemicals. Ion

engineering techniques are, thus, expected to create a possible way to design further

new functional materials.

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