High visible-light active Ir-doped-TiO2 brookite photocatalyst synthesized by hydrothermal microwave-assisted process

Article abstract of DOI:10.1016/j.cattod.2014.01.032

Iridium-doped TiO₂ having a brookite phase was synthesized as a visible light-active photocatalyst. It was prepared by using a hydrothermal assisted-microwave process as an improved competitive production method over the traditional hydrothermal process. The prepared materials were tested for decomposing acetaldehyde or toluene in gas phase under visible light irradiation (λ 455 nm), and they showed a strong photocatalytic activity response. The activity of the prepared brookite TiO₂ material was optimized by adjusting the concentration of iridium ions as a dopant. The photocatalytic efficiencies of all of the prepared Ir-doped-TiO₂ photocatalysts were higher than the values obtained from commercially available visible-light responsive photocatalysts under the same experimental conditions.

Full text of DOI:10.1016/j.cattod.2014.01.032

Catalysis Today 230 (2014) 214–220
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
High visible-light active Ir-doped-TiO2 brookite photocatalyst
synthesized by hydrothermal microwave-assisted process
Víctor M. Menéndez-Floresa,b,∗, Teruhisa Ohnob,c,d
a Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
b Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensuicho, Tobata, Kitakyushu 804-8550, Japan
c JST, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
d JST, ACT-C, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2013
Received in revised form 22 January 2014
Accepted 26 January 2014
Available online 20 February 2014
Iridium-doped TiO2 having a brookite phase was synthesized as a visible light-active photocatalyst. It was
prepared by using a hydrothermal assisted-microwave process as an improved competitive production
method over the traditional hydrothermal process. The prepared materials were tested for decomposing
acetaldehyde or toluene in gas phase under visible light irradiation (ꢀ 455 nm), and they showed a strong
photocatalytic activity response. The activity of the prepared brookite TiO2 material was optimized by
adjusting the concentration of iridium ions as a dopant. The photocatalytic efficiencies of all of the pre-
pared Ir-doped-TiO2 photocatalysts were higher than the values obtained from commercially available
visible-light responsive photocatalysts under the same experimental conditions.
Keywords:
Doping
Nanoparticles
Photocatalysts
Titanium dioxide
Visible light activity
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
[10]. Photocatalysis and nanotechnology have been closely related
to develop different doping semiconductor strategies for achieving
Many attempts have been made to obtain safe and efficient
semiconductor materials with high conducting properties. An
appropriate material is the polymorph titanium dioxide due to its
reduction–oxidation qualities, safety and stability [1]. The three
TiO2 crystal phases, anatase, rutile and brookite, have been rig-
orously studied. Rutile TiO2 and anatase TiO2 have been mainly
studied. Although the structure of brookite, TiO2 was determined
by Pauling and Sturdivant in 1928, preparation of pure brookite
TiO2 powder required longer investigations [2–9]. The difficulty
in preparing brookite TiO2 having both high purity and a large
surface area is probably one of the reasons for the limited appli-
cation of brookite TiO2 as a photocatalyst. Nevertheless, the TiO2
brookite phase has recently received attention as a photocatalyst
material because of its band gap, morphology and optical char-
acteristics. It has to be particularly pointed out the importance
of brookite TiO2 phase, since it contains the highest oxidation
potential in comparison with the other two different TiO2 crystal
phases brookite (−0.46 V), anatase (−0.45 V) and rutile (−0.37 V)
solar range applications in order to use visible light irradiation. In
this present study, we prepared visible light-responsive brookite
TiO2 by iridium doping. Iridium oxides have various applications,
including applications in cardiovascular medicine and neural stim-
ulation, because of their charge injection properties and non-toxic
effects [11]. The photocatalytic efficiency of the newly prepared
photocatalyst was evaluated under visible light irradiation > (ꢀ
455 0.1 nm). Iridium doped TiO2 having a brookite phase was
prepared by a microwave-assisted hydrothermal process [12–15].
2. Experimental
2.1. Chemicals
All chemical reagents used in the present study were commer-
cial products without further treatments. A titanium precursor
(titanium-ethoxide) was purchased from Sigma-Aldrich. Other
chemicals were purchased from Wako Co. Ltd. (all of reagent
grade). Glycolic acid was employed to control the crystallinity and
morphology of brookite TiO2 particles. IrCl3 was used as dopant
precursor. Commercial photocatalysts N- or S-doped TiO2 anatase
phase materials were used as reference for the comparison of
acetaldehyde decomposition. The surface areas of N-doped TiO2
∗
Corresponding author at: Graduate School of Engineering, Nagoya University,
Chikusa-ku, Nagoya 464-8603, Japan. Tel.: +81 52 789 2587; fax: +81 52 789 5299.
E-mail addresses: Victor.Menendez@apchem.nagoya-u.ac.jp,
menendez 99@yahoo.com (V.M. Menéndez-Flores),
tohno@che.kyutech.ac.jp (T. Ohno).
0920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2014.01.032

V.M. Menéndez-Flores, T. Ohno / Catalysis Today 230 (2014) 214–220
215
1.0
0.8
0.6
0.4
0.2
0.0
(Sumitomo Chemical Co. Ltd.) and S-doped TiO2 (TOHO Titanium
Co. Ltd.) were 97.4 m2 g−1 and 82.5 m2 g−1, respectively.
2.2. Preparation of Ir-doped TiO2 having a brookite phase
Brookite TiO2 was prepared in a microwave apparatus (Wave
Magic Eyela MWO-1000S/500W). One cm3 of titanium (IV) ethox-
ide and ethanol was added to Milli-Q water with vigorous stirring,
and the mixture was stirred for 30 min at room temperature. The
resulting precipitate was centrifugally separated from the solution
and dried under reduced pressure. The obtained titanium hydrox-
ide particles were dispersed in Milli-Q water and then irradiated
by ultrasonication. Then 30% hydrogen peroxide was added, and
yellow peroxo titanic acid (PTA) solution was obtained. Ammo-
nium (NH3) solution as a pH-adjusting agent and glycolic acid as a
shape-control reagent were then added to the solution. After stir-
ring the solution at ca. 60 ◦C for 24 h, the solution in a Teflon bottle
sealed with a stainless jacket was introduced into the microwave
apparatus with diverse IrCl3 concentrations.
300
400
500
600
700
800
λ (nm)
Fig. 1. Diffuse reflectance spectra of bare brookite TiO2 (—), N-doped TiO2 (· – ·),
S-doped TiO2 (· · ·), and 0.5% (w/w) Ir-doped brookite TiO2 (- - -).
Diverse conditions like reaction time, stirring speed, pH, power
and glycolic acid as well as content of iridium were tested to opti-
mize the photonic efficiency of Ir-TiO2 brookite.
The reaction in the microwave apparatus was carried out at
200 ◦C without stirring, adjusted to pH10 using a power energy of
500 watts for 7 min.
1.5
3.14 eV
2
1.0
2.3. Characterization
1.88 eV
0.5
The specific surface area was determined with a surface area
analyzer (Quantachrome, Autosorb-1) by using the Brunauer-
Emmett-Teller (BET) method. The crystal structures of the samples
were confirmed by using an X-ray diffractometer (Rigaku, Mini-
Flex II) with Cu K ␣ radiation (ꢀ 1.5405 A). The morphology of the
samples was observed by transmission electron microscopy (TEM;
Hitachi, H-9000NAR).
0.0
2
3
1
0
˚
2.4. Photocatalytic decomposition of acetaldehyde or toluene in
gas phase
1
2
3
4
Energy Photon (eV)
Photocatalytic activities of the photocatalysts were evaluated
by the decomposition of acetaldehyde or toluene in gas phase. Ir-
doped TiO2 particles (100 mg) were spread on a glass dish, and the
glass dish was placed into a Tedlar bag (AS ONE Co. Ltd.) with a vol-
ume of 125 cm3. Then 500 ppm of gaseous acetaldehyde or 100 ppm
toluene was injected into the Tedlar bag in a mixture of synthetic
air (20% O2 and 80% N2). Photo-irradiation was carried out at room
temperature after the acetaldehyde or toluene adsorption equilib-
rium had been reached. An LED lamp (ꢀ 455 0.1 nm) was used as
a light source. The light intensity was adjusted to 1 mW cm−2. Con-
centrations of acetaldehyde, toluene and carbon dioxide (CO2) were
estimated by gas chromatography auto-sampler Agilent Technolo-
gies 3000A Micro GC (Thermal conductivity detector). Before the
evaluation, all of the TiO2 samples were pretreated by black light
irradiation in order to remove contaminants on the TiO2 surface.
Fig. 2. Relationship between photo energy and [F(R)(hꢁ)]1/2 of bare TiO2 brookite.
Inseted graph shows impurity energy level in the lattice of Ir-doped TiO2 brookite.
higher than those of reference photocatalysts (S-doped TiO2 and
N-doped TiO2). Among the photocatalysts, the Ir-doped brookite
TiO2 showed the highest specific surface. The surface area was
increased by doping TiO2 with Ir due to the interstitial spaces
formed in the TiO2 lattice by insertion of Ir atoms.
Fig. 1 shows the normalized diffuse reflectance spectra of bare
brookite TiO2, N-doped TiO2, S-doped TiO2 and Ir-doped TiO2
brookite synthesized photocatalysts (Ir-TiO2). A clear red shift in
the absorption edge of the Ir-doped brookite TiO2 was observed
compared to the bare brookite TiO2.
The band gap energy of the prepared bare brookite TiO2 material
(3.14 eV) (Fig. 2) was determined by the Kubelka-Munk function
[F(R)hꢁ]1/2. Moreover, the impurity energy level of the Ir-doped
brookite TiO2 was found to be in the visible region (1.88 eV) (Fig. 2
inserted graph). High absorption of visible light by the Ir-doped
brookite TiO2 photocatalyst can be predicted.
The flatband potentials of semiconductor electrodes at the
semiconductor/electrolyte junction can be obtained from Mott-
Schottky plots (measured in the dark) [Eq. (1)] [16,17].
3. Results
3.1. Photocatalyst characterization
The specific surface areas of Ir-doped TiO2 brookite mate-
rials 0.25; 0.5; 0.75 and 1.5 wt% corresponding, respectively
to (172.13 m2 g−1); (208.41 m2 g−1
)
(219.67 m2 g−1
)
and
(231.15 m2 g−1) or the undoped brookite (128.18 m2 g−1) were
measured by the BET method and compared to the commercially
available visible light-responsive TiO2 photocatalysts (S-doped
TiO2 and N-doped TiO2). The surface area of bare brookite TiO2 was
ꢀ
ꢁ
1
C2
2
kT
e0
=
·
E − EFB
−
,
(1)
εTiO · ε0 · e0 · ND
2

216
V.M. Menéndez-Flores, T. Ohno / Catalysis Today 230 (2014) 214–220
(d)
(c)
(b)
(a)
ICSD# 036408
60 70
-0.80
-0.75
-0.70
-0.65
-0.60
20
30
40
50
E (V) (vs. Ag/AgCl)
2θ (deg)
Fig. 3. Mott-Schottky plot at 100 Hz for the TiO2 film electrode prepared with
brookite nanoparticles presents a flatband potential EFB of −0.766 V vs Ag/AgCl.
TiO2 film/Ag/AgCl/Pt in 0.1 KCl, pH 7.0.
Fig. 4. XRD patterns of Ir-doped TiO2 brookite particles, with the different iridium
dopant ratios; undoped (a); 0.25% (b), 0.5% (c) and 0.75% (d).
experiments were performed in aqueous 0.1 M KCl solutions at pH
7.
where ε0 is the permittivity of free space, εTiO is the permittivity
The Mott-Schottky plot for TiO2 brookite nanoparticles is shown
in Fig. 3. The results indicate that the EFB is −0.766 V (vs Ag/AgCl).
Kandiel et al. [9] obtained similar results of quasi-Fermi brookite
levels of −0.77 V (vs NHE). In this study after doping brookite with
iridium no significant EFB change was found presumably due to the
low iridium doping weight percent.
The crystal structure of Ir-doped TiO2 was assigned to the
brookite phase by XRD analyses (Fig. 4). The XRD pattern of
Ir-doped-TiO2 having a brookite phase shows a high level of crys-
tallinity after the microwave-assisted hydrothermal process at
200 ◦C for 7 min. With higher concentrations than 0.5% of irid-
ium as dopant, the intensity of the brookite peaks can be modified
(Fig. 4(d)).
2
of the semiconductor electrode, e0 is the elementary charge, ND is
donor density, E is applied potential, EFB is flatband potential, k is
the Boltzmann constant, T is temperature of the operation, and C is
space charge capacitance.
The flatband potentials have been measured by impedance spec-
troscopy using Mott-Schottky plots. TiO2 nanomaterial paste for
the fabrication of electrodes for impedance measurements was
obtained by mixing 1 mL of water and 100 mg of TiO2 nanomaterial
powder homogeneously by sonication. An appropriate amount of
the TiO2 suspension was spread on a conductive fluorine-tin oxide
glass with a glass rod, followed by drying in air at 100 ◦C for 2 h. The
electrical contact was formed on the uncoated area of the substrate
using silver paint and copper wire. The electrochemical setup for
impedance measurements consisted of three electrodes: the work-
ing electrode (TiO2 film), a platinum wire used as counter electrode,
and Ag/AgCl, NaCl (3.0 M) electrode as reference (+0.21 vs NHE). The
The morphology of synthesized TiO2 nanoparticles was studied
by TEM analyses. There is no clear difference between the pre-
pared doped or undoped brookite TiO2 materials. The Ir-doped
brookite TiO2 showed a rod-like shape with an average size of
Fig. 5. TEM images of (a) unmodified bare TiO2 brookite and (b) 0.5% Ir-doped-TiO2 brookite particle.

V.M. Menéndez-Flores, T. Ohno / Catalysis Today 230 (2014) 214–220
217
500
400
300
200
100
0
0
2
4
6
8
10
12
14
16
18
20
70
68
66
64
62
60
58
56
time (h)
Binding Energy (eV)
Fig. 7. Photocatalytic activities of Ir-doped brookite TiO2 with different Ir concen-
trations, 0.25% (- ꢀ -), 0.5% (- ꢁ -), 0.75 (- ꢂ -) and 1.5 (- ᭹ -), compared to those of
commercial N-doped TiO2 (- ꢂ -) and S-doped TiO2 (- ꢃ-) photocatalysts as assessed
by degradation of acetaldehyde (500 ppm) for 21 h under 1 mW cm−2 visible-light
LED lamp (ꢀ 455 nm) irradiation.
Fig. 6. XPS peak of Ir-doped brookite TiO2 assigned to Ir 4f7/2 and Ir 4f5/2 doublets
relative to the different Ir spectral electrons of IrIII (- - -) and IrIV (· · ·) after Ar etching
treatment for 60 s.
10 nm (Fig. 5). The size of Ir-doped brookite TiO2 nanoparticles
is due to the microwave process with a short reaction time. An
advantage of this particles size is their large surface area.
500
400
300
200
100
0
The XPS peaks of the prepared Ir-doped brookite TiO2 were
performed in order to elucidate the Ir cation doping condition in
the TiO2 lattice. XPS peaks assigned to Ir cations were observed.
After Ar etching, the peaks of Ir cations remained (Fig. 6). These
results strongly indicate that Ir cations should be located in the lat-
tice of TiO2. The best fit of the peaks yielded two oxidation states,
which were attributed, respectively to IrIII at 59.6 and to IrIV at 61.5.
These results are shifted 1.1 eV from the corresponding 61.7 eV and
62.6 eV due to the doping procedure.
3.2. Photocatalytic decomposition of acetaldehyde under visible
light using Ir-doped brookite TiO2
The contents of Ir cations in TiO2 were optimized by using dif-
ferent Ir concentrations, from 0.25% up to 1.5% in relation to TiO2
weight. When 0.5 wt% of Ir cations was doped into the bookite TiO2,
the highest photocatalytic activity of Ir-doped brookite TiO2 was
observed for decomposition of acetaldehyde in gas phase under
visible light (ꢀ 455 nm) (Fig. 7) [Eq. (2)]. In all cases the activity of
the self-prepared Ir-doped brookite TiO2 was superior to that of
the commercial S- or N-doped TiO2 photocatalysts under the same
reaction conditions. A possible reason for the fast initial decomposi-
tion reaction by the application of brookite TiO2 phase compared to
anatase or ruti•le−might be due to its higher oxidation potential able
to produce O2 radicals and formally trapped Ti-•OH (Fig. 8). On
the other hand decomposition processes with commercial photo-
catalysts takes place mainly only on the surface of the materials.
hꢁ,Ir-TiO
0
2
4
6
8
10
12
14
16
18
20
time (h)
Fig. 8. Photocatalytic activities of 0.5% Ir-doped TiO2 with different TiO2 phases,
brookite (- ꢁ -), anatase (- ᭹ -) and rutile (- ꢀ -) by the degradation of acetaldehyde
(500 ppm) for 21 h under 1 mW cm−2 visible-light LED lamp (ꢀ 455 nm) irradiation.
(3)]. Photocatalysts with higher iridium concentrations showed
lower photocatalytic decomposition rates.
hꢁ,Ir-TiO
C6H5CH3 + 9O2 −→ 27CO2 + 4H2O
(3)
3.4. Photonic efficiencies by decomposition of acetaldehyde or
toluene
CH3CHO + 5 O2 −→ 22CO2 + 2H2O
(2)
2
3.3. Photocatalytic decomposition of toluene under visible light
using iridium-doped brookite TiO2
Photonic efficiency is defined as the ratio of the initial degrada-
tion rate (k) divided by the incident photon flux [Eqs. (4) and (5)]
[18,19]. The incident photon flow was determined from UV-A light
meter measurements (1 mW cm−2) by using an LED lamp with ꢀ
455 nm. Photonic efficiencies were calculated for the decomposi-
tion of acetaldehyde and toluene for comparison of the different
photocatalysts.
Photodegradation of toluene in gas phase over TiO2 was also
evaluated under visible light irradiation. Toluene was selected as
a toxic and recalcitrant compound to show the important poten-
tial application of the developed photocatalytic material. Brookite
TiO2 doped with 0.5 wt% Ir cations was found to be the most
active photocatalyst among the photocatalysts with different irid-
ium quantities (0.25 wt%, 0.5 wt%, 0.75 wt% and 1.5 wt%), (Fig. 9) [Eq.
I · ꢀ
J0
=
,
(4)
NA · h · c

218
V.M. Menéndez-Flores, T. Ohno / Catalysis Today 230 (2014) 214–220
300
200
100
and, therefore, the surface states of photocatalysts. Manipula-
tion of surface states depends on control of the surface atomic
structure [20]. Although precise control of the surface atomic struc-
ture is very difficult, it is possible to develop some strategies to
increase/decrease the number of surface states. Many studies have
been devoted to improvement of photocatalytic efficiency of TiO2
by using various methods such as deposition of noble metals [19].
The deposition of a noble metal on semiconductor nanoparticles
is an essential factor for maximizing the efficiency of photocat-
alytic reactions. A noble metal (e.g., Pd), which acts as a sink for
photoinduced charge carriers, promotes interfacial charge-transfer
processes.
The TiO2 lattice with metal ions such as iridium cations intro-
duces new energy levels in the band gap which can be tailored
to extend photosensitivity in the visible light region. By doping of
metal or nonmetal ions, the properties of the functional materials
are modified. For example, S- or N-doped TiO2 has shown a rel-
atively high photoactivity under visible light irradiation [21–23].
However, Ir-doped brookite TiO2 showed the highest photonic effi-
ciency among the materials tested under visible light irradiation.
The importance of modifying the shape of a TiO2 structure by
controlling the morphology of TiO2 particles was confirmed in pre-
vious works [24], which promotes an effective electron–hole pair
charge separation. In this work, we synthesized Ir-doped brookite
TiO2 nanoparticles with high crystallinity by using a microwave
synthesis process. The surface area of the prepared TiO2 was
increased by increasing the amount of Ir cations as a dopant. A
higher photocatalytic activity than that of the tested commercial
doped photocatalysts was exhibited presumably due to the high
crystallinity and high surface area. With a high sp•e−cific surface
0
0
2
4
6
8
10
12
14
16
18
20
time (h)
Fig. 9. Photocatalytic decomposition of toluene (100 ppm) for 21 h under
1 mW cm−2 visible light (LED ꢀ 455 nm) using TiO2 brookite doped with different
iridium concentrations: 0.25 wt% (- ꢀ -), 0.5 wt% (- ꢁ -), 0.75 wt% (- ꢂ -) and 1.5 wt%
(- ᭹ -).
k · c0 · V
PE =
· 100.
(5)
J0 · A
J0 = photon flux intensity [mol s−1 cm−2]; I = incident light inten-
sity [J s−1 cm−2]; ꢀ = wavelength [nm]; NA = Avogadro’s number
[6.022 × 1023 mol−1]; h = Planck constant [6.63 × 10−34 J s]; c = light
velocity [m s−1]; k = rate constant [s−1]; A = area [cm2]; and
c0 = initialAcetaldehyde/Toluene concentration [mol L−1]; V = volume [L]
Table 1 shows the photonic efficiencies of the photocatalysts
under visible light LED lamp (ꢀ 455 nm) irradiation used for degra-
dation of acetaldehyde and toluene, respectively. The pollutant
concentration in the case of acetaldehyde was 500 ppm and that
in the case of toluene was 100 ppm by using 0.1 g of the photo-
catalyst in each case. As can be seen in Table 1, a comparison of
commercial N-doped TiO2 and S-doped TiO2 photocatalysts with
the self-prepared photocatalysts showed that a considerably higher
photonic efficiency (0.69%) could be obtained by using the 0.5%
Ir-doped TiO2 photocatalyst.
area, the possibility of O2 reduction producing O2
radicals is
higher [25]. In addition to this, brookite TiO2 has been reported
to show good performance under UV light for decomposition of
acetaldehyde [26]. In the present study, Ir-doped brookite TiO2 as
a photocatalyst could decompose acetaldehyde or toluene under
pure visible light irradiation (ꢀ 455 nm).
4.2. Dopant and light absorption band
UV–vis spectra were used to clarify the light absorption edge of
the TiO2 brookite material as well as band gap calculation through
the reflectance function for an indirect semiconductor. The band
gaps of the prepared samples were 3.14 eV and 1.88 eV before and
after 0.5 wt% Ir-doping, respectively. Experimental band gap ener-
gies of brookite TiO2 have been reported to range from 3.14 eV
[6,27,28] to 3.4 eV [8]. The obtained band gap of the pure brookite
TiO2 in this study is consistent with results of previous studies
[6,27,28]. Ir cations were shown to be incorporated in the lattice
of brookite TiO2 by XPS analyses after Ar etching treatment. Sev-
eral runs were conducted with the same Ir-doped TiO2 brookite
material without any detriment in its photocatalytic performance.
4. Discussion
4.1. Origin of visible light activity
The surface states of semiconductor materials can play an
important role in the photocatalytic activity. Surface states are
caused by incomplete covalent bonds at the surface of the semicon-
ductor and result in electron energy levels within the energy band
gap. When electrons and holes are trapped in the surface states,
the spatial overlap of charge carriers is reduced, retarding their
recombination due to the localized nature of local states. There-
fore, abundant surface states can greatly promote the separation
of photoexcited electron–hole pairs. Morphology, size, defects and
dopants are important factors affecting the surface atomic structure
4.3. Proposed mechanism
In relation to our characterized self-synthesized Ir-doped TiO2
photocatalyst and according to oxidation potential values of
Table 1
Photonic efficiencies under 1 mW cm−2 visible light (455 nm) irradiation for decomposition of acetaldehyde (500 ppm) or toluene (100 ppm) with 0.1 g of photocatalyst.
a
a
Material
0.25% Ir-TiO2
0.5% Ir-TiO2
0.75% Ir-TiO2
1.5% Ir-TiO2
N-TiO2
S-TiO2
Photonic efficiencyb (%)
Photonic efficiencyc (%)
0.31
0.1
0.69
0.18
0.09
0.04
0.05
0.02
0.04
–
0.01
–
a
Commercial visible light active photocatalyst.
Acetaldehyde decomposition.
Toluene decomposition.
b
c

V.M. Menéndez-Flores, T. Ohno / Catalysis Today 230 (2014) 214–220
219
Fig. 10. Scheme of proposed mechanism of Ir-doped TiO2 brookite for decomposition of pollutants in gas phase under visible light irradiation (ꢀ 455 nm) (at pH 7).
(Ti-OH)(s) + Ir-(Ti3+) → (Ti-OH−)(s) + Ir-(Ti4+).
(8)
toluene (1.276 V vs NHE) [29] and acetaldehyde (0.6 V vs NHE) [30],
a photocatalytic reaction mechanism under visible light irradia-
tion was proposed. The linked pathways are confirmed from the
Electron transfer reactions:
− + O2 → O−,
(9)
obtained CO2 evolution. When Ir-doped TiO2 was irradiated under
e
2
visible light, h+ from the photogenerated pairs e−/h+ in the doped
particles is quickly trapped in the deep trapping sites, that is, the
doping sites [23]. Meanwhile, the photogenerated conduction band
electrons will migrate to the surface to the reduction site of the par-
ticle, where the main process of photo-reduction of O2 takes place
O•2− + H+ → HO•2
(10)
The superoxide radicals and Ti-•OH should be important reac-
tants that interact and promote decomposition of pollutants on
the surface. It has been reported that TiO2 particles after H2O2
treatment increase the absorption range up to visible light due
to generation of surface peroxide species [38]. For this reason,
Equation (7) represents particular importance to the proposed
mechanism. From the oxidation potential and reaction potential, Ti-
•OH is thought to be predominant active species for degradation of
organic compounds. In a previous work was shown the importance
of oxidation sites on the surface [39].
•−
to produce superoxide O2 radicals (Fig. 10).
The flat band potential of the brookite TiO2 was estimated by
a Mott-Schottky plot to be −0.766 V (vs Ag/AgCl). Therefore, the
calculated conduction band (CB) should be −0.51 V (vs NHS) at pH
7, the potential of which is sufficient to reduce O2 adsorbed on
the surface of Ir-doped brookite TiO2, resulting in production of
superoxide radicals under visible light irradiation. The produced
superoxide radicals are thought to be responsible for the pro-
duction of surface-adsorbed H2O2 molecules [31], which might
produce the formally trapped •OH radical, Ti-•OH. Therefore, super-
oxide radicals play an important role in decomposition of organic
compounds. It has been reported that N-doped TiO2 [32,33] and
S-doped TiO2 [34] cannot generate free •OH radicals under visible
light irradiation and therefore have limited oxidative power.
It is plausible that free •OH radicals diffuse through an oxida-
tive path to the gas phase from the photocatalytic TiO2 surface
under UV light irradiation [31]. In our case, it might not be possible
to produce free •OH radicals by the midgap state due to the lack
of oxidation potential under visible light. Highly reactive free •OH
radicals require strong redox potential conditions to be produced
and released. A compound suitable for producing free •OH radicals
through the oxidative valence band holes is, for example, modified
WO3 with TiO2 or Pt as co-catalysts under visible light, because its
band gap lies on an appropriate redox potential position [35,36].
A certain amount of IrIII cations in the lattice of TiO2 was reduced
to generate IrII cations by photogenerated electrons. After irradia-
tion, the color of the Ir-doped TiO2 photocatalyst changed from
yellow to an almost brown color. The difference in the color is
probably due to the change in oxidation level of iridium [37]. This
phenomenon was confirmed by XPS and UV spectra before and after
irradiation.
5. Conclusions
Visible light-active Ir-doped brookite TiO2 photocatalysts were
successfully synthesized by using different concentrations of irid-
ium metal in a hydrothermal assisted-microwave system. The
synthesis procedure by application of a microwave can be consid-
ered a green process with low energy and fast synthesis reactions
within 7 min to obtain brookite TiO2 instead of the 48 h required
by the traditional hydrothermal method. The reaction period in
the microwave is relevant since the brookite structure will be well
formed by 7 min. A shorter time of reaction does not form a crys-
talline brookite. The reaction time can be increased over 7 min and
the brookite sharpness increases but no significant change was
observed.
A wide range of visible light absorption was confirmed by
reflectance spectra analyses and by band-gap calculations. TEM
images as well as XRD patterns showed a high level of crys-
tallinity of the doped photocatalysts. These results indicate that
crystallinity of the prepared TiO2 is one of the important factors
for exhibiting high photocatalytic activity.According to the BET
study, the doped photocatalysts showed a much larger surface area
than that of the undoped brookite TiO2. For photocatalytic decom-
position of acetaldehyde or toluene in gas phase, Ir-doped TiO2
brookite showed remarkable photocatalytic activities under visible
light irradiation (ꢀ 455 nm). The highest photonic efficiency of Ir-
doped brookite TiO2 over commercial available photocatalysts was
observed under visible light when 0.5 wt% Ir cations were doped
into the lattice of brookite TiO2.
The initial steps in the photocatalytic process prior the initiation
of acetaldehyde or toluene degradation are as follows [Equations
(6)–(10)].
Charge separation:
Ir-(TiO2)v−is→-hꢁe− + h+.
(6)
(7)
Acknowledgements
Charge transfer of positive charge:
This work was supported by the JST PRESTO program and the
JST ACT-C program.
h+ + OH(−s) → (Ti − OH)(s)
,

220
V.M. Menéndez-Flores, T. Ohno / Catalysis Today 230 (2014) 214–220
[21] T. Ohno, Z. Miyamoto, K. Nishijima, H. Kanemitsu, F. Xueyuan, Appl. Catal. A
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