Photocatalytic reduction of CO2 on FeTiO3/TiO 2 photocatalyst

Article abstract of DOI:10.1016/j.catcom.2011.12.025

A uniform heterojunction photocatalyst of FeTiO₃/TiO₂ composite (FTC) was synthesized by a facile hydrothermal method. The structure, morphology, and spectral properties of FTC were characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and UV-Vis diffuse reflectance spectroscopy. The FeTiO₃ content can be easily tuned by tailoring the precursor amount. The FTC exhibits remarkable photocatalytic activity on CO₂ reduction to CH₃OH under both visible and UV-Vis light irradiation, presumably due to its unique band structures and the efficient charge transfer between two semiconductors as well as the low band gap of FeTiO₃.

Full text of DOI:10.1016/j.catcom.2011.12.025

Catalysis Communications 19 (2012) 85–89
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Photocatalytic reduction of CO2 on FeTiO3/TiO2 photocatalyst
a
a
a,
Quang Duc Truong a,b, , Jen-Yu Liu , Cheng-Chi Chung , Yong-Chien Ling
⁎
⁎
a
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 October 2011
Received in revised form 17 December 2011
Accepted 20 December 2011
Available online 29 December 2011
A uniform heterojunction photocatalyst of FeTiO3/TiO2 composite (FTC) was synthesized by a facile hydro-
thermal method. The structure, morphology, and spectral properties of FTC were characterized by X-ray
diffraction, Raman spectroscopy, transmission electron microscopy, and UV–Vis diffuse reflectance spectros-
copy. The FeTiO3 content can be easily tuned by tailoring the precursor amount. The FTC exhibits remarkable
photocatalytic activity on CO2 reduction to CH3OH under both visible and UV–Vis light irradiation, presum-
ably due to its unique band structures and the efficient charge transfer between two semiconductors as
well as the low band gap of FeTiO3.
Keywords:
FeTiO3
TiO2
© 2011 Elsevier B.V. All rights reserved.
CO2 reduction
CH3OH
Visible light response
1. Introduction
as electron trap providing active sites for CO2 reduction. Modifica-
tion of TiO2 with metals could inhibit charge recombination. How-
The rapid increase of atmospheric CO2 concentration has
drawn considerable attention at global level due to the inevitable
global warming and climate change. At the same time, human be-
ings are facing increased energy crisis due to the fossil fuel short-
age. The solar-driven photocatalytic reduction of CO2 to fuel offers
a novel opportunity to simultaneously manage global warming
and energy shortage problems. Many attempts have been devoted
to explore the photoreduction of CO2 into chemical fuels such as
CO, CH4, or CH3OH [1–11]. A variety of photocatalysts such as
metal oxides [1–4], InTaO4 [5], ZnGa2O4 [6], Zn2GeO4 [7,8], and
Ti-based materials [9–11] have been developed and studied for
CO2 conversion in liquid or gas phase. Among them, titanium
oxide (TiO2) semiconductor is one of the most promising candi-
dates owing to its availability, stability, efficiency, and long-term
durability in wide range of applications [12–18]. To expand TiO2
applications, current research is largely focusing on the enhance-
ment of photo-induced hole–electron pair separation and exten-
sion the photo-responsive region to the visible light region
[12–24].
ever, it does not help shifting the photo-responsive region. To
improve the photoactivity of TiO2 under visible light irradiation,
alternative methods have been proposed to shift its photo-
responsive region from UV to visible light region. Doping TiO2
with nonmetals such as nitrogen and iodine may lead band-gap
narrowing and altering the optical property of TiO2, resulting in
enhancement of CO2 reduction activity under visible light irradia-
tion [19,20]. Attaching photo-sensitizers such as metal complexes
also significantly improved TiO2 photocatalytic activity in visible
light region [21].
Aiming to simultaneous separate electron–hole pairs and im-
prove visible light response, narrow band-gap semiconductor com-
pounds such as AgI, CdS, Cu2O, and Bi2S3 was coupled with TiO2
[22,23]. Exploring the energy level difference (i.e. between valence
band (VB) and conduction band (CB)) between sensitizer and TiO2,
the photo-generated electrons and holes are driven to locate on
separated TiO2 host or loaded materials, respectively. For example,
FeTiO3 possessing lower VB than TiO2 has been loaded onto TiO2
[24]. As shown in Fig. 1A, electrons in TiO2 VB can readily transfer
to FeTiO3 VB; whereas the holes will subsequently be generated in
TiO2 CB. As a result, electrons on FeTiO3 are available for reductive
reactions; whereas holes on TiO2 have sufficient lifetime for photo-
oxidative reactions. Furthermore, electron–hole pairs can be gener-
ated upon visible light irradiation owing to the narrow band-gap
of the loading FeTiO3. To the best of our knowledge, such strategy of
coupling semi-conductor for CO2 reduction has not been investigated.
To prevent the recombination of photo-generated electrons
and holes, various noble and transition metals including Pt
[12,13], Pd [14], and Cu [15–18] were load onto TiO2 which serve
⁎
Corresponding author. Tel.: +886 3 5721484; fax: +886 3 5711082.
E-mail addresses: tqduc@mail.tagen.tohoku.ac.jp (Q.D. Truong),
ycling@mx.nthu.edu.tw (Y.-C. Ling).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.12.025

86
Q.D. Truong et al. / Catalysis Communications 19 (2012) 85–89
A
B
TiO2
-
e
FeTiO3
VB
-0.5
0.0
-
-
e
e
CO2 + 6e- + 6H+ CH3OH + H2O
E0red = 0.03 V vs. NHE
1.2
H2O
0.5 O2 + 2H+ + 2e-
E0ox = 1.23 V vs. NHE
2.6
2.7
CB
h+
h+
h+
UV light
Visible light
Fig. 1. Schematic illustrating the charge transfer between two semiconductors: (A) under visible light irradiation and (B) under UV–Vis light irradiation.
Herein, for the first time, we applied the FeTiO3/TiO2 for photo-induced
reduction of CO2 to CH3OH under UV-Vis and visible light irradiation.
Particularly, we propose a solution method to synthesize homoge-
neous nano-sized FTC. Chemical and physical properties as well as
photocatalytic activity by means of CO2 reduction was studied. The
effect of FeTiO3 amount in FTC on CO2 conversion efficiency was
also investigated in detail.
Jasco R-3000 Raman spectrometer in the backscattering geometry
using 732.2 nm wavelength and 90 W laser power for excitation. The in-
cident beam was focused on FTC using 100× microscope lenses to yield
2 μm spot. All samples were also subjected diffuse reflectance spectros-
copy (DRS) measurement. The experiment was carried by diffuse reflec-
tance scanning spectrophotometer (Shimadzu, UV 2450).
2.3. Photocatalytic reduction of CO2
2. Experimental section
The photoreduction of CO2 was carried out in an aqueous solution
under UV–Vis or visible light irradiation according to our previous report
[25]. Briefly, 0.05 g catalyst was dispersed in 30 ml distilled water con-
taining sodium bicarbonate (NaHCO3, 0.08 M). The solution was pre-
pared in a 30 cm×15 cm×5 cm reactor, N2 blow to remove oxygen,
stood in dark for 30 min until reaching adsorption–desorption equilibri-
um. The temperature of the solution was kept constant by a water bath.
UV–Vis and visible light irradiation was from a 500 W high-pressure Xe
lamp placed above the reactor. A Pyrex glass tube was placed on top of
the reactor to cut off light with wavelengths λb300 nm. A UV cut-off fil-
ter made of 2 M NaNO2 solution was applied to cut off entire UV region
(λb400 nm) [26]. The photocatalytic reaction lasted for 3 h and the re-
actor was allowed to cool down naturally for CH3OH desorption form
the FTC. The resultant sample was centrifuged and the thus-obtained so-
lution was analyzed by GC-FID with a 2-m Porapak Q column. The
photocatalytic activity of FTC was compared to that of commercial P25
(anatase/rutile=4/1) TiO2 photocatalyst.
2.1. Synthesis of FeTiO3/TiO2
FTC was prepared by a homogeneous solution method. Typically,
metallic titanium powder (5 mmol, 99.4%, Alfa Aesar, America) was
completely dissolved in a cold mixture of aqueous ammonia solution
(10 ml, 28%, J.T. Baker, Germany) and hydrogen peroxide solution
(40 ml, 30%, J.T. Baker, Germany). After aging at 353 K to remove ex-
cess reagent, a yellowish gel-like peroxo-titanic acid was obtained,
which was subsequently dissolved in 15 ml distilled water to produce
peroxo-titanic acid solution with pH 5. In a typical experiment, oxalic
acid (7.5 mmol, 99.5%, Merk, Germany) was added into the peroxo-
titanic acid solution. The solution color changed from yellow to red,
suggesting the formation of a titanium-oxalate complex. Ferric nitrate
(98%, Fluka, Japan) was added to the complex solution and its pH
changed to about 1.5. Different amounts of ferric nitrate were used
to prepare FTC with Fe/Ti mole ratio of 10%, 20%, 50%, and 70%, re-
spectively. The solution was diluted with distilled water to 20 ml,
placed into a Teflon-lined stainless steel autoclave, heated at 473 K
for 24 h for hydrothermal treatment. The autoclave was cooled
down to room temperature naturally. The white or brown solid pow-
der was separated after centrifugation and washed with distilled
water until a neutral pH was obtained. The obtained FTC was dried
at 353 K for 1 day.
The photoreduction of CO2 by various catalysts has been exten-
sively studied [1–21]. The following reaction has been proposed for
selective production of CH3OH:
2 CO2 þ 4 H2O → 2 CH3OH þ 3 O2:
ð1Þ
We carried out control experiment in dark first and no CH3OH was
detected for all tested catalysts. Similarly, no CH3OH was detected for
control experiment under light irradiation but without catalyst. Pre-
liminary results from these control experiments indicate that both
light irradiation and catalyst use are indispensible for CO2 reduction
to CH3OH.
2.2. Characterization of FeTiO3/TiO2
The crystalline phase was characterized using powder X-ray diffrac-
tion (XRD; Rigaku D/MAX-IIB, 40 kV and 30 mA) with Cu Kα radiation
(λ=1.5406 Å). The data were collected in 2θ–θ scanning mode at a
scan speed of 4°min−1 and a step-size of 0.02°. The morphology was ex-
amined using transmission electron microscopy (HR-TEM, Jeol-3010,
200 kV) which was conducted by dispersing FTC in ethanol and then
dropping onto Cu microgrid coated with a holey carbon film, followed
by evaporation of ethanol. N2 adsorption and desorption isotherms
were measured at 77 K (Micromeritics ASAP 2010) to yield Brunauer–
Emmett–Teller (BET) specific surface area. Phase composition was con-
firmed by Raman spectroscopy. The Raman spectra were acquired by a
3. Results and discussion
3.1. Structure, morphology, and spectral properties of FTC
Fig. 2A shows the XRD patterns of the synthesized TiO2 and FTC,
indicating that all synthesized samples composed of TiO2 or FTC.
The sample synthesized by hydrothermal treatment of the complex
solution is consisted of anatase as the main phase and trace amount

Q.D. Truong et al. / Catalysis Communications 19 (2012) 85–89
87
I
A
B
A
A
I
A,I
I
A
I
A I
A I
A
I
A(e)
(d)
(c)
100 nm 100 nm
C
(b)
(a)
TiO2
20 25 30 35 40 45 50 55 60 65 70 75 80
2θ/ degree, CuKα
[104]
B
FeTiO3
0.27nm
5 nm
Fig. 3. TEM (A, B) and HRTEM (C) images of the synthesized particles (A) TiO2 and
(B, C) 20% FeTiO3/TiO2.
(b)
(a)
FTC (Fig. 3B) contains both spherical NPs and rod NPs with an average
diameter of 20 nm and length of 100 nm. We also observed the deco-
ration of spherical NPs onto the surface of rod NPs, suggesting that
FeTiO3 tends to form rod NPs and from which TiO2 NPs grew on.
Fig. 3c shows the HR-TEM image of 20% FTC, presenting the lattice
fringe of TiO2 and FeTiO3 as well as the surface-phase junction in
FTC. Our observation suggests that FeTiO3/TiO2 heterojunction photo-
catalyst has been formed. Gao et al. was pioneering in synthesizing
FTC using hydrothermal method [24]. They used commercially avail-
able micrometer-sized FeTiO3 precursor which might limit the con-
tacting area between the two phases and degrade the homogeneity.
On contrast, the FTC synthesized by the hydrothermal route in this
study is more homogeneous and capable of providing more active
sites for catalytic reaction.
0
200
400
600
800 1000 1200
Raman shift / cm-1
Fig. 2. (A) XRD patterns of the synthesized NPs (a) TiO2, (b) 10% FeTiO3/TiO2, (c) 20%
FeTiO3/TiO2, (d) 50% FeTiO3/TiO2, and (e) 70% FeTiO3/TiO2. (A: anatase, I: ilmenite.)
(B) Raman spectra of the synthesized NPs (a) TiO2 and (b) 20% FeTiO3/TiO2.
of rutile. Hydrothermal treatment of the complex solution with
Fe(NO3)3 resulted in the formation of FTC with different FeTiO3 con-
tent. As shown in Fig. 2A, a set of reflections from ilmenite FeTiO3 at
2θ=24.14°, 33.16°, 35.66°, and 49.58°, corresponding to the (012),
(104), (110), and (024) diffraction peaks of the FeTiO3 rhombohedral
structure, [24,27] can be observed. We found that FeTiO3 content in-
creased with increasing Fe(NO3)3 amount, as evident from the rela-
tive height of the characteristics peaks of ilmenite and anatase
phases. However, when Fe(NO3)3 amount reaching 70% mole of Ti
(curve e in Fig. 2A), the FeTiO3 content does not increase compared
to 50% FeTiO3/TiO2, presumably due to the competitive growth of
FeTiO3 and TiO2 in hydrothermal reaction.
Diffuse reflectance spectra of the synthesized TiO2 and FTC in com-
parison with that of Dugussa P25 are shown in Fig. 4. The P25 (curve
The formation of FTC was further confirmed using Raman spec-
troscopy. Fig. 2B shows the Raman spectra of 20% FTC and TiO2.
Single-phase anatase has characteristics bands at 395 cm−1
,
520 cm−1, and 630 cm−1. The Raman spectrum of FTC (curve b in
Fig. 2B) shows additional characteristics bands at 300 cm−1 and
667 cm−1 which can be assigned to FeTiO3. Such characteristics
bands are not observed in the Raman spectrum of pure anatase TiO2
(curve a in Fig. 2B).
Fig. 3 shows the TEM images of TiO2 and 20% FTC. The synthesized
TiO2 nanoparticles (NPs) (Fig. 3A) uniformly disperse and display
spherical morphology with an average diameter of 15 nm. The 20%
Fig. 4. UV-Vis DRS spectra of the synthesized particles (a) TiO2, (b) 10% FeTiO3/TiO2,
(c) 20% FeTiO3/TiO2, (d) 50% FeTiO3/TiO2, and (e) P25.

88
Q.D. Truong et al. / Catalysis Communications 19 (2012) 85–89
a) shows the absorption on UV region only, whereas the synthesized
FTC show strong absorption in the entire UV and visible light region.
The visible light absorption intensity of FTC dominates that of TiO2
(i.e. pure anatase) NPs. We therefore conclude that FeTiO3 enhances
the visible light response in FTC. Furthermore, absorption of FTC in
visible light region increases with increasing FeTiO3 content and
50% FeTiO3/TiO2 (curve e) exhibits maximum absorption. The strong
visible light absorption behavior of FTC accompanied by its unique
charge transfer phenomena resulted in notable photocatalytic activity
as shown in Table 1.
speculation. Furthermore, it was found that TiO2 doped with nitrogen
or iodine shows an enhancement of photocatalytic activity for CO2 re-
duction [19,20]. Therefore, non-metal doping is an important factor
for the enhancements of catalytic efficiency.
All FTC exhibits significant improvement of photocatalytic activity
upon both UV–Vis and visible light irradiation, presumably benefiting
from the visible-light response and the junction effect between two
semiconductors. As mentioned above, FeTiO3 is a semiconductor
with band gap of about 2.6 eV and CB energy edge of 2.6 eV, approx-
imating to that of TiO2 (2.7 eV) (Fig. 1) [24]. Upon the irradiation of
visible light, the electrons in the VB of FeTiO3 is excited to CB. Thus,
the VB of FeTiO3 becomes partially vacant; consequently electrons
in VB of TiO2 can be transferred to that of FeTiO3 due to their similar-
ity in energy level, leaving behind the holes in VB of TiO2. On the
other hand, when the FTC is irradiated by UV–Vis light, electrons in
VB of TiO2 are excited to CB which can be transferred to CB of
FeTiO3 because the flat-band potential of TiO2 is 0.53 V higher than
that of FeTiO3. Consequently, electron–hole pairs are well-separated
and electrons in ilmenite surface has sufficient lifetime for CO2 reduc-
tion. Therefore, the photoactivity improvement along with increasing
content of FeTiO3 is reasonable owing to the increase of the active site
for the reduction of carbonate species. The lower activity of 50%
FeTiO3/TiO2 than that of 20% FeTiO3/TiO2 is possible that a high con-
centration of metal content in 50% FeTiO3/TiO2 may form recombina-
tion centers leading to reduced photocatalytic efficiency [12–14].
Furthermore, smaller BET specific surface area of 50% FeTiO3/TiO2 is
also responsible for reduced photocatalytic activity. Thus, the optimal
FeTiO3 content for the highest photocatalytic activity is 20 wt.%. Our
result is consistent with previous study that 28 wt.% FeTiO3/TiO2
showed the highest efficiency for the mineralization of 2-propanol
and 4-chlorophenol [24]. Furthermore, due to the low band gap of
FeTiO3, the FTC is highly responsive to visible light. This might explain
that CH3OH yield from the 50% FeTiO3/TiO2 under visible light irradi-
ation is even higher than that under UV–Vis irradiation. Similar find-
ing has been reported for iodine-doped TiO2 which has superior
photoreduction activity on CO2 under visible light irradiation than
that under UV–Vis light irradiation [20].
3.2. Photocatalytic reduction of CO2
The photocatalytic activity of our synthesized TiO2 and FTCs was
evaluated in terms of photoreduction of CO2 to CH3OH under visible
(λ>400 nm) and UV–Vis (λ>300 nm) light irradiation. Table 1 lists
the CH3OH yield and specific BET surface area of the corresponding
catalysts. Upon UV–Vis light irradiation, our synthesized TiO2 exhib-
ited comparable activity to that of P25. The CH3OH yield from
10% FeTiO3/TiO2 and 20% FeTiO3/TiO2 were 0.338 and
0.462 μmol g−1 h−1, respectively, which is twice or three times
higher than that from P25 or bare TiO2. The 50% FeTiO3/TiO2 unex-
pectedly showed lower photocatalytic activity with CH3OH yield of
0.298 μmol g−1 h−1, presumably due to smaller BET.
Under visible light irradiation, the photocatalytic activity of our
synthesized TiO2 was significantly increased compared to that of
P25. Surprisingly, FeTiO3/TiO2 remained its photocatalytic efficiency
comparing to that under UV–Vis light irradiation. The CH3OH yield
from 10% FeTiO3/TiO2 and 20% FeTiO3/TiO2 were 2.3 and 3.0 times
higher than that from bare TiO2. The CH3OH yield from 50% FeTiO3/
TiO2 becomes higher than that under UV–Vis irradiation. We noted
that under either UV–Vis or visible light irradiation the 20% FeTiO3/
TiO2 exhibited the highest photocatalytic activity. There are few re-
ports on photocatalytic production of CO2 under visible light irradia-
tion. For instance, Ozcan et al. have reported visible light-driven
photoproduction of CH4 using dye-sensitized Pt/TiO2 with a produc-
tion rate of 0.26 μmol g−1 h−1 [14]. Varghese et al. applied N-doped
TiO2 nanotube arrays for photoreduction of CO2 under the visible
light irradiation with a total evolution rate (CO, CH4 and other hydro-
carbons) of 0.33 μmol g−1 h−1 [19]. Zhang et al. recently reported
an impressive photoreduction rate under visible light irradiation
with a CO production rate of 2.4 μmol g−1 h−1 [20]. On comparison,
the photoproduction rate obtained in this work (CH3OH of
0.43 μmol g−1 h−1) is higher than that obtained using the dye-
sensitized or N-doped TiO2 [14,19].
The selective production of CH3OH in photoreduction of CO2 can
be understood from the electronic band structure of FTC. The photo-
generated holes in the VB can initiate the oxidation of water through
the half-reaction H2O→0.5 O2 +2H+ +2e−(E0ox =1.23 V vs. NHE)
(2). The edge of the VB of TiO2 is about 2.7 V (vs. NHE) which is
more positive than E0 (O2/H2O). Thus, the photooxidation reaction
is easily processed. On the other half-reaction, the photo-generated
electrons in CB can reduce CO2 through these reactions:
The remarkable activity of the bare TiO2 and FTC under visible
light irradiation may be partially attributed to accidently carbon dop-
ing. The DRS spectra shows that the absorption band of bare TiO2 NPs
(Fig. 4b) shifts toward longer wavelength compared to that of P25.
Previous study reported that 1–4 wt.% carbon was detected in the ti-
tania nano-sized powders synthesized from titanium complexes by
hydrothermal method [28]. Thus carbon doping is highly likely in
FTC. Further experimental works such as total carbon or x-ray photo-
electron spectroscopy measurements are warranted to confirm this
CO2 þ 8e− þ 8Hþ → CH4 þ 2H2O ðEr0ed ¼ −0:24Vvs:NHEÞ½6–8ꢀ
CO2 þ 6e− þ 6Hþ → CH3OH þ H2O ðEr0ed ¼ 0:03Vvs:NHEÞ:
ð3Þ
ð4Þ
As shown in Fig. 1, the edge of the CB of FeTiO3 is about 0.0 V (vs.
NHE) [24] that is higher than E0 (CO2/CH4) but lower than E0 (CO2/
CH3OH). Therefore, the photoreduction of CO2 may occur selectively
to produce CH3OH rather than CH4.
Photocatalytic reduction of CO2 may form a variety of products
such as CO, CH3OH, or CH4. Back to 1979, Inoue et al. have discovered
that reduction of CO2 with H2O on TiO2 under UV irradiation pro-
duced CH3OH, HCHO, and HCOOH [29]. Solymosi reported doping
TiO2 by Rhodium could improve photoreduction of CO2 to produce
CH3OH through electron transfer on Rh active site [30]. Rutile (110)
also exhibited photocatalytic activity on reduction of CO2 with H2O
producing CH3OH as main product [31]. The positive hole scavenger
such as 2-propanol could enhance reduction of CO2 to CH4 on TiO2
[32,33]. Photoreduction of carbonate in aqueous solution was signifi-
cantly improved in acidic solution with CH3OH as main product [34].
Table 1
BET specific surface area and CH3OH yield.
Photocatalyst
BET
CH3OH yield (μmol g−1 h−1
)
(m2 g−1
)
UV–Vis irradiation
λ>300 nm
Visible light irradiation
λ>400 nm
P25
TiO2
10% FeTiO3/TiO2
20% FeTiO3/TiO2
50% FeTiO3/TiO2
52.6
62.3
55.8
51.3
35.7
0.176
0.175
0.338
0.462
0.298
0.045
0.141
0.319
0.432
0.352

Q.D. Truong et al. / Catalysis Communications 19 (2012) 85–89
89
Recently, Wu et al. introduced steady-state optical-fiber reactor with
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ð5Þ
ð6Þ
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Financial support by the National Science Council, Taiwan (NSC95-
2113-M-007-044-MY3 and NSC98-2113-M-007-016-MY3) is grate-
fully acknowledged.