Room temperature synthesized BaTiO3 for photocatalytic hydrogen evolution

Article abstract of DOI:10.1016/j.jallcom.2018.04.300

As a photocatalyst, barium titanate (BaTiO₃) has shown a great potential in photocatalytic water splitting for hydrogen evolution. In this work, BaTiO₃ nanoparticles were synthesized at room temperature conditions under ambient pressure. The small particle size below 10 nm plays a key role in suppressing the recombination of photo-induced carriers, and thus promoting the photocatalytic activity. The photocatalytic hydrogen evolution rates of BaTiO₃ (BTO-R) synthesized at room temperature are 8 and 2.9 times that of commercial BaTiO₃ (BTO-C) in the presence and absence of triethanolamine (TEOA) as a sacrificial agent, respectively. This work provides a good example on size control, low-cost synthesis and photocatalysts' design.

Full text of DOI:10.1016/j.jallcom.2018.04.300

Accepted Manuscript
Room temperature synthesized BaTiO for photocatalytic hydrogen evolution
3
Tao Chen, Jie Meng, Shiyan Wu, Jingyun Pei, Qingyun Lin, Xiao Wei, Jixue Li, Ze
Zhang
PII:
S0925-8388(18)31634-7
10.1016/j.jallcom.2018.04.300
JALCOM 45938
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 13 January 2018
Revised Date: 24 April 2018
Accepted Date: 26 April 2018
Please cite this article as: T. Chen, J. Meng, S. Wu, J. Pei, Q. Lin, X. Wei, J. Li, Z. Zhang, Room
temperature synthesized BaTiO for photocatalytic hydrogen evolution, Journal of Alloys and
3
Compounds (2018), doi: 10.1016/j.jallcom.2018.04.300.
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Room temperature synthesized BaTiO3 for Photocatalytic
Hydrogen Evolution
Tao Chen,1 Jie Meng,1 Shiyan Wu,1 Jingyun Pei, Qingyun Lin, Xiao Wei*,
Jixue Li, Ze Zhang*
Center of Electron Microscopy, State Key Laboratory of Silicon Materials, and School
of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR
China
*
Corresponding author. Tel. +86 571 87952797; fax: +86 571 87952797;
E-mail address: mseweixiao@zju.edu.cn (X. Wei); zezhang@zju.edu.cn (Z. Zhang).
1 These authors contributed equally to this work
ABSTRACT
As a photocatalyst, barium titanate (BaTiO3) has shown a great potential in
photocatalytic water splitting for hydrogen evolution. In this work, BaTiO3
nanoparticles were synthesized at room temperature conditions under ambient
pressure. The small particle size below 10 nm plays a key role in suppressing the
recombination of photo-induced carriers, and thus promoting the photocatalytic
activity. The photocatalytic hydrogen evolution rates of BaTiO3 (BTO-R) synthesized
at room temperature are 8 and 2.9 times that of commercial BaTiO3 (BTO-C) in the
presence and absence of triethanolamine (TEOA) as a sacrificial agent, respectively.
This work provides a good example on size control, low-cost synthesis and
photocatalysts’ design.
Keywords
BaTiO3; Room-temperature synthesis; Photocatalysis; Hydrogen evolution;
Nanoparticle.

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1 Introduction
Photocatalytic hydrogen evolution has been drawing enormous attention in recent
years for its low cost and environmental friendship in solving the energy shortage
through facile solar to chemical conversion [1-4]. An ideal photocatalyst must meet
the requirements that include wide range optical utilization, effective separation of
photo-generated carriers and high-efficient surface redox reaction [5-8]. The
photocatalytic efficiency of semiconductors is mainly determined by the coordination
and balance of thermodynamics and kinetics [9-11]. Titanium dioxide (TiO2) as a
popular and first reported photocatalyst used in hydrogen evolution, CO2 reduction
and environmental applications has been extensively investigated [12-15]. TiO2-based
photocatalysts supply the merits of low cost, low toxicity, and high physicochemical
stability. Various works of advanced TiO2-based semiconductors have been conducted
in relation to increasing photo-conversion efficiency [16]. However, by contrast with
perovskite-type oxides (ABO3), the limited diversities in chemical compositions and
internal structures hinder the development and application of TiO2 in environment and
energy fields [17]. Therefore, the perovskite oxides are regarded as one of promising
materials on photocatalysis and have gained extensive attention in recent years [18].
For example, novel perovskite SrTiO3-Ba2FeNbO6 Solid Solution [19], La/Cr
co-doped CaTiO3 [20], LaCoO3 co-catalyst for CO2 reduction [21], PbTiO3/TiO2 as
ferroelectric oxide heterostructures [22] and so on.
Barium titanate (BaTiO3), a common perovskite oxides with a band gap energy of
about 3.2 eV, has the ability to split water into hydrogen and oxygen for its
appropriate conduction- and valence- band position compared with the redox
potentials of H+/H2 and O2/H2O [23, 24]. Electrons with high reduction power in the
conduction band (CB) and holes with sufficient oxidation ability in the valence band
(VB) are generally required for efficient photocatalytic reactions [25]. Due to its
unique physical and chemical properties, a mass of researches have been carried out
to overcome the limitations that hamper the enhancement of photocatalytic efficiency
[24, 26, 27]. Among the factors that affect the photocatalytic activities of BaTiO3, a

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relatively high recombination rate of photo-excited holes and electrons is the main
reason that leads to a poor efficiency of photocatalytic reaction [28]. Thus, some
effective approaches have been explored to promote the separation and migration of
photo-generated carriers, such as noble metal loading, ion doping, heterogeneous
structure and size/morphology control [23, 25, 29-31].
High specific surface area through controlling the morphology or grain size could
increase the surface accessibility of photocatalysts, and thus improve the utilization of
photo-induced electrons and holes [32-34]. Small size particles or low-dimension
materials enable shorter diffusion distance of electrons and holes which could
facilitate the surface redox reaction as compared to their bulk counterparts [35, 36].
Various synthetic techniques have been applied in controlling particle size, including
solvothermal, sonochemical and wave-assisted hydrothermal methods in recent years
[37-39]. It is well-known that most of these methods demand harsh reaction
conditions such as good quality equipment, high temperature or high pressure [40, 41].
Therefore, for the purpose of meeting the requirements of industrial application, it is
highly competitive to make use of a facile and simple synthetic technology at room
temperature under ambient pressure conditions. Our group has demonstrated that
BaTiO3 can be prepared at room temperature [42]. However, the photocatalytic
activity of this room temperature synthesized BaTiO3 nanoparticle has not been
discussed.
Herein, in this work, BaTiO3 nanoparticles have been facilely synthesized at room
temperature conditions under ambient pressure using ethanediamine (EDA) as solvent.
The small particle size below 10 nm plays a key role in suppressing the recombination
of photo-induced carriers, and thus promoting the photocatalytic activity compared
with commercial BaTiO3. The photocatalytic hydrogen evolution rates of
room-temperature synthesized BaTiO3 (BTO-R) are 8 and 2.9 times that of
commercial BaTiO3 (BTO-C) in the presence and absence of triethanolamine (TEOA)
as a sacrificial agent, respectively. The specific characterizations and photocatalytic
mechanisms have been discussed. This work provides a better reference for other
works on the size control, crystal growth and design of photocatalysts.

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2 Experimental section
2.1 Preparation
BaTiO3 nanoparticles were synthesized at room temperature using
enthylenediamine (EDA) as solvent. All reagents were analytical stage without further
purification. Typically, 50 mmol of tetrabutyl titanate (C16H36O4Ti) was firstly
dissolved in 100 mL ethylene glycol monomethyl ether (HOCH2CH2OCH3), followed
by the addition of ammonium hydroxide (NH3·H2O) to obtain titanium hydroxide.
The titanium hydroxide precipitate was filtered and washed with distilled water
several times for subsequent use. At the same time, Ba(CH3COO)2 aqueous solution
(20 mL 2.5 M) was dispersed in a 300 mL of EDA stirring for 4 h. Then
Ba(CH3COO)2/EDA solution and KOH (20 mL 2.5 M) were put into the titanium
hydroxide precipitate in sequence under stirring. After stirring for 48 h at room
temperature under ambient pressure, the resultant product was washed with distilled
water several times, and then dried at 80°C for 12 h, namely BTO-R. The Commercial
BaTiO3 (BTO-C) bought from SIGMA-ALDRICH Corporation was used as a
comparison sample.
2.2 Characterization
The crystalline phase of as-obtained BaTiO3 nanoparticles was examined by X-Ray
powder diffraction (XRD) on a PANalytical X’Pert PRO X-ray powder diffractometer
with monochromatic Cu Kα radiation in the range of 10-90°. The field emission
scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
with an energy dispersive X-ray spectrometer (EDX) for elemental analysis were
performed on Hitachi SU-70 and Tecnai G2 F20, respectively. UV-vis diffuse
reflectance spectra were collected with a spectrophotometer (Persee TU-1900) using
BaSO4 as the reflectance standard. Photoluminescence (PL) spectra were recorded by
fluorescence spectrometer (Edinburgh instruments FLS920) with an excitation
wavelength of 325 nm. The specific surface area was determined by the the Brunauer-
Emmett-Teller (BET) method from N2 adsorption isotherms (Quantachrome

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AUTOSORB-IQ2-MP). X-Ray photoelectron spectra (XPS) were recorded on an
Escalab 250Xi for the surface electronic states.
2.3 Photocatalytic reaction
Photocatalytic reaction was carried out in a closed circulation system mainly
consisting of vacuum system, cooling water, and gas analysis device (Labsolar-IIIAG
photocatalytic system, Beijing Perfectlight Technology Co. Ltd). The samples (0.05 g)
were dispersed in a quartz reaction cell filled with 100 mL of aqueous solution. Prior
to irradiation, the reaction system was evacuated and then flowed with Ar gas to
remove the dissolved oxygen. The reaction was proceeded under the illumination of a
UV Xe lamp (200-400 nm). The produced hydrogen was analyzed through an on-line
gas chromatograph with a TCD detector (GC9790, FULI). The Pt loading on the
photocatalysts was prepared by chemical reduction method using H2PtCI6·6H2O and
NaBH4 as precursor and reducing agent, respectively. After continuous ultrasonic
oscillation for 30 min, the samples were washed by water and ethanol several times
and dried at 80 °C in the oven for overnight.
3 Results and discussion
Fig. 1 shows the XRD patterns of BaTiO3 samples. The diffraction peaks of BTO-C
and BTO-R can be well indexed to the perovskite phase (JCPSD No. 74-1967) with
the space group of Pm-3m. The BTO-R exhibits broad diffraction peaks compared
with BTO-C, indicating the formation of small size nanoparticles. For BTO-R
prepared at room temperature, the lower diffraction peak intensity can be ascribed to
low crystalline degree compared with BTO-C, which is common in materials with
small crystal sizes. The average crystallite size was about 7 nm calculated by the
Debye-Scherrer formula. Furthermore, it could be observed that the diffraction peaks
of BTO-R shift toward lower angles, which is related to their synthesis process and
the corresponding different crystal parameters.

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Fig.1 XRD patterns of (a) all samples, (b) room temperature synthesized BaTiO3 (BTO-R) (the
blue lines represent the positions of standard diffraction patterns of the BaTiO3 (JCPSD NO.
74-1967)).
The difference in crystalline size for the sample BTO-C and BTO-R could also be
seen in SEM images obviously (Fig. 2a and Fig. S1). The BTO-C sample consists of
large-size crystalline particles (around 500 nm) with irregular morphology (Fig. S1).
It is obvious that the crystal size of BTO-R was smaller than that of BTO-C as shown
in Fig. 2a. TEM was employed to further study the microstructure of BTO-R. A
low-magnification TEM image was shown in Fig. 2b, from which we can see that
room temperature synthesized BaTiO3 (BTO-R) was composed of mutually
aggregated nanoparticles with the size of 5-10 nm. This result is consistent with XRD
analysis. According to our precious study, the high activity of the titanium hydrous gel
and the high basicity of the solvent play a significantly important role in room
temperature synthesis. The organic solvent of EDA adsorbed on the surface of
particles was able to confine the crystal growth. Meanwhile, the ambient temperature
results in a low diffuse coefficient compared with high temperature reaction, thus
leading to the BaTiO3 particle with small size.[42] The particle size plays a key role in
promoting photocatalytic activities which can be seen in the following section. The
high-resolution TEM (HRTEM) reveals the lattice fringes of about 0.297 nm

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corresponding to the (110) plane of cubic BaTiO3 (Fig. 2c). The interplanar crystal
spacing of BTO-R is larger than that of PDF card (0.287 nm), which is in consistent
with XRD results according to Bragg equation. The indexed selected area electron
diffraction pattern (SAED) reveals that the room temperature synthesized
nanoparticles are well crystallized with the cubic perovskite phase, which is aslo
confirmed by HRTEM result. The contrast on crystal size are also reflected in the
surface area (Fig. S2). The specific surface area of BTO-R sample is 120.98 cm2/g,
which
is
larger
than
the
BTO-C
sample
(2.11
cm2/g).
Fig. 2 the electron microscopy images of room temperature synthesized BaTiO3 (BTO-R): (a)
SEM, (b) low-magnification TEM, (c) high-resolution TEM and (d) selected area electron
diffraction (SAED).
In order to evaluate the surface state of BaTiO3 samples, X-ray photoelectron
spectroscopy (XPS) measurements were carried out and the corresponding results are
displayed in Fig. 3. The peaks of Ba, Ti and O are presented in the survey spectrum
which can be seen in Fig. 3a. Two band peaks located at 793.9 eV and 779.6 eV can

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be assigned to Ba 3d3/2 and Ba 3d5/2 signals, respectively (Fig. 3b) [43]. In addition,
the spectrum of Ti 2p is mainly made up of two characteristic peaks, Ti 2p1/2 at 463.8
eV and Ti 2p3/2 at 458.0 eV, indicating the chemical valence of Ti4+ in the as-obtained
room temperature synthesized BaTiO3 samples (Fig. 3c) [44]. The O 1s signal
2-
displays three components corresponding to oxygen in BaTiO3, CO3 and hydroxyls
(Fig. 3d) [45]. Based on the above analysis, the results of the XPS spectra
demonstrate the formation of well-crystalline BaTiO3 in the room temperature
condition.
Fig. 3 XPS spectra of samples: a) survey, (b) Ba 3d, (c) Ti 2p and (d) O 1s.
The optical properties of the obtained BaTiO3 samples were characterized by
UV-vis diffuse reflectance spectra at room temperature. As shown in Fig. 4a, the
absorption edges of BTO-C and BTO-R locate at about 394 nm and 384 nm,
respectively. It is interesting that BTO-R shows a lower optical absorption range
compared with BTO-C because of the size effect of nanomaterials. Meanwhile, the
band gaps of samples are estimated from the Tauc plot of (αhν) ꢀ/ꢁ versus hν

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according to
(αhν) ꢂ = A(hν − Eꢃ)
where α, h, ν, A, and Eꢃ represent the absorption coefficient, Plank constant, light
frequency, proportionality and band gap, respectively [46]. The factor ꢄ is related to
the optical transition of a semiconductor where ꢄ=1/2 for direct transition and ꢄ=2
for indirect transition. The value of ꢄ=2 is corresponding to our samples because
cubic BaTiO3 is an indirect semiconductor confirmed by previous report [47]. Herein,
the band gaps of BTO-C and BTO-R are estimated to be 3.27 eV and 3.38 eV,
respectively (Fig. 4b). PL spectra were provided to investigate the migration and
separation efficiency of photo-induced holes and electrons. In Fig 4c, the broad
emission band centeres at around 438 nm for both BaTiO3 samples with an excitation
wavelength of 325 nm. The as-prepared BTO-R sample exhibits a lower PL intensity
compared with BTO-C sample, implying that the recombination rate of photo-excited
carriers is efficiently restrained. For these nanoparticles with smaller size, more
photo-generated carriers are easy to migrate from inner part of particles and take part
in surface redox reaction, further promoting the photocatalytic performance.
Fig. 4 optical properties of samples: (a) diffuse reflectance spectra, (b) the optical
transition type determination and (c) photoluminescence spectra (Pt loaded samples).
The photocatalytic hydrogen evolution of BaTiO3 samples were studied under UV
light (200-400 nm) illumination (Fig. 5a). With the assistance of TEOA as a
sacrificial reagent, the H2 production rate of BTO-R sample is 789.7 umol/g/h, which
is about 8 times that of commercial BaTiO3 (BTO-C, 98.5 umol/g/h). Meanwhile,
BTO-R sample exhibits a better photocatalytic hydrogen production from pure water

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without any sacrificial agent as shown in Fig. 5b. Furthermore, BTO-R sample
performs a better photocatalytic H2 evolution after 4 cycles under UV light irradiation
compared with BTO-C sample (Fig. 5c). By comparing the XRD patterns and TEM
images before and after UV irradiation (Fig. S3), it can be found that the BTO-R
sample keeps a good crystal structural integrity after long time illumination. The peak
of BaCO3 may result from the reaction of Ba2+ and CO2. It is well known that the
optical response range and the recombination rate have a significant impact on the
final photocatalytic efficiency. Because of the size effect, in comparison with BTO-C
sample, the BTO-R shows a lower optical absorption ability which is negative to the
photocatalytic performance. Usually, materials with low crystallinity always have a lot
of defects that influence the final photocatalytic activity [48]. If we normalize the
specific surface area, it can be found that the photocatalytic hydrogen generation rate
per SBET of BTO-R (6.5 umol/g/h/SBET) is far lower than BTO-C (46.7 umol/g/h/SBET).
So the defects in the BTO-R act as charge carrier recombination center and suppress
the photocatalytic performance to some extent [49]. Based on the above analysis, for
the BTO-R sample, its light absorption range and structure defects have adverse effect
on photocatalytic performance. Therefore, the small particle size and large specific
surface area play a key role in enhancing its hydrogen evolution rate by promoting the
separation of photo-induced carriers and providing more reactive sites. As a matter of
fact, in the previous reports the BaTiO3 photocatalyst was mainly studied for the uses
in dye degradation [25, 28, 29, 50]. So this work provides an important example for
BaTiO3 as a photocatalyst for splitting water into H2 with low-cost synthesis.
Fig. 5 photocatalytic hydrogen evolution of samples (catalyst: 50 mg, 0.38 wt.% Pt as
co-catalyst): (a) in the presence of TEOA (10 vol.%), (b) without TEOA, and (c) cycle

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experiments
4 Conclusions
In summary, BaTiO3 nanoparticles (~7 nm) have been synthesized at room
temperature conditions under ambient pressure. Because the particle size is lower than
that of commercial BaTiO3, there are more photo-induced carriers migrating from
inner part of particles to participate in the surface photocatalytic reduction reaction.
Under UV light irradiation, the room temperature synthesized BaTiO3 (BTO-R) shows
a better photocatalytic hydrogen evolution ability, which is 8 and 2.9 times that of
commercial BaTiO3 in the presence and absence of TEOA as a sacrificial agent,
respectively. This work provides a good example on size control, low-cost synthesis
and photocatalysts’ design.
Acknowledgments
This work was supported by National Natural Science Foundation of China (grant
number: 11234011, 11327901, 51102208), and the Fundamental Research Funds for
the Central Universities (2014QNA4008，2017QNA4011).
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ACCEPTED MANUSCRIPT
ꢀ BaTiO3 nanoparticles have been facilely synthesized at room temperature under
ambient pressure.
ꢀ The particle size below 10 nm plays a key role in promoting the photocatalytic
performance.
ꢀ This work provides a better reference for other works on the size control, crystal
growth of photocatalysts.