Synthesis of Bi-doped TiO2 nanotubes and enhanced photocatalytic activity for hydrogen evolution from glycerol solution

Article abstract of DOI:10.1002/cjoc.201201151

Bi-doped TiO₂ nanotubes with variable Bi/Ti ratios were synthesized by hydrothermal treatment in 10 mol·L^-1 NaOH (aq.) through using Bi-doped TiO₂ particles derived from conventional sol-gel method as starting materials. The effects of Bi content on the morphology, textural properties, photo absorption and photocatalytic activity of TiO₂ nanotubes were investigated. The scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) observations of the obtained samples revealed the formation of titanate nanotube structure doped with Bi, which exists as a higher oxidation state than Bi³⁺. Bi-doping TiO₂ nanotubes exhibited an extension of light absorption into the visible region and improved photocatalytic activities for hydrogen production from a glycerol/water mixed solution as compared with pure TiO₂ nanotubes. There was an optimal Bi-doped content for the photocatalytic hydrogen production, and high content of Bi would retard the phase transition of titanate to anatase and result in morphology change from nanotube to nanobelt, which in turn decreases the photocatlytic activity for hydrogen evolution. Copyright

Full text of DOI:10.1002/cjoc.201201151

FULL PAPER
DOI: 10.1002/cjoc.201201151
Synthesis of Bi-doped TiO2 Nanotubes and Enhanced Photo-
catalytic Activity for Hydrogen Evolution
from Glycerol Solution
Wenxi Zhao,b Xitao Wang,*,a Huanxin Sang,b and Kang Wanga
a Tianjin Key Laboratory of Applied Catalysis Science and Technology, College of Chemical Engineering and
Technology, Tianjin University, Tianjin 300072, China
b Tianjin Academy of Environmental Sciences, Tianjin 300191, China
Bi-doped TiO2 nanotubes with variable Bi/Ti ratios were synthesized by hydrothermal treatment in 10 mol•L－
1
NaOH (aq.) through using Bi-doped TiO2 particles derived from conventional sol-gel method as starting materials.
The effects of Bi content on the morphology, textural properties, photo absorption and photocatalytic activity of
TiO2 nanotubes were investigated. The scanning electron microscopy (SEM), transmission electron microscopy
(TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) observations of the obtained samples
revealed the formation of titanate nanotube structure doped with Bi, which exists as a higher oxidation state than
Bi3＋. Bi-doping TiO2 nanotubes exhibited an extension of light absorption into the visible region and improved
photocatalytic activities for hydrogen production from a glycerol/water mixed solution as compared with pure TiO2
nanotubes. There was an optimal Bi-doped content for the photocatalytic hydrogen production, and high content of
Bi would retard the phase transition of titanate to anatase and result in morphology change from nanotube to nano-
belt, which in turn decreases the photocatlytic activity for hydrogen evolution.
Keywords Bi-doped, TiO2 nanotubes, photocatalytic hydrogen evolution, glycerol solution
Introduction
photocatalytic research.[11-16] Zhang et al. report the
controllable and reproducible synthesis of a Bi-doped
ordered mesoporous TiO2.[15] They found that Bi species
exist in various forms in the doped samples, and that the
suitable doping of Bi significantly enhanced photocata-
lytic oxidation of phenol and reduction of chromium in
aqueous suspension. Ji et al. prepared the Bi-doped an-
atase TiO2 nanobelts from layer-structural titanate
nanobelts using two-step hydrothermal treatment ap-
proach.[16] The ultraviolet-visible (UV-vis) absorption
In the last two decades, a number of efforts have
been made to explore the photocatalysts used for water
splitting to generate hydrogen by absorbing solar en-
ergy.[1-3] TiO2 is the most investigated photocatalytic
material due to its particular properties.[4-6] Nevertheless,
the ability of water splitting of TiO2 is limited due to its
wide band gap and narrow spectra range for light re-
sponse. In order to make better use of solar energy,
many attempts have been proposed to sensitize TiO2 in
the visible-light region.[7-9] Recently, the fabrication of
TiO2 nanotubes has attracted enormous attention owing
to its highly ordered tube structure, high surface-to-
volume ratio and large specific surface area. Besides
that, they would enhance the light absorption, and bene-
fit the efficiency for electron transportation.
Bi2O3 is a significant metal-oxide semiconductor
with a direct band gap of 2.8 eV, which can be excited
by visible light, but the photocatalytic activity of Bi2O3
is low due to the photocorrosion and fast recombination
of photogenerated electron-hole pairs.[10] In recent years,
the development of Bi2O3-TiO2 composite photocatalyst
that can work effectively under visible light irradiation
with photochemical stability is a very hot topic in
spectra show that the absorption edge for the samples
＋
with Bi3 has red shift as compared with that of un-
doped TiO2 nanobelts, and correspondingly, the photo-
catalytic degradation of methylene blue under visible-
light illumination is enhanced with the increase of
Bi-doping content.
Although a lot of researches have reported the Bi-
doped TiO2 with highly photocatalytic activities under
the visible-light irradiation, there is no work focusing on
the preparation and photocatalytic activity of Bi-doped
TiO2 nanotubes for hydrogen production. Therefore, in
the present work, we systematically studied the effects
of Bi-doping amount on the morphology, photo absorp-
tion and photocatalytic hydrogen production from a
glycerol/water mixed solution under UV and solar-sim-
*
E-mail: wangxt@tju.edu.cn
Received November 20, 2012; accepted January 13, 2013; published online March 1, 2013.
Chin. J. Chem. 2013, 31, 415—420
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Zhao et al.
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ulated light irradiation over nanotube in a fixed bed
flow-type reactor.
mg catalyst mixed with 5 mL quartz sand was loaded
into the reactor and afterwards 10 mL 5% glycerol solu-
tion was added to the reactor dropwise. Argon gas flow
of 20 mL/min was made to pass through the reactor to
collect and transfer gaseous products to the online gas
chromatography equipped with a thermal conductivity
detector.
Experimental
Preparation of photocatalysts
Bi-doped TiO2 nanoparticles with different Bi/Ti
molar ratios were prepared by sol-gel method as de-
scribed in our previous work.[17] Bi-doped TiO2 nano-
tubes were prepared by hydrothermal method. In a
typical synthesis, 0.5 g of the as-prepared Bi-TiO2
Results and Discussion
Characterization of photocatalysts
nanoparticle was dispersed in 10 mol•L－ NaOH (aq.)
The Bi/Ti molar ratios determined by ICP-AES and
XPS and the BET surface area of Bi-TiO2 nanotubes are
summarized in Table 1. It can be seen that the element
of Bi is detected in Bi-doped samples, which shows that
Bi element has been successfully doped into the TiO2
structure. For three Bi-doped samples, the Bi/Ti molar
ratios obtained by ICP-AES and XPS are very close,
showing that the Bi element is uniformly distributed in
TiO2 structure. However, the content of Bi determined
by ICP and XPS is obviously lower than the calculated
data, indicating the loss of Bi element. This can result
from the dissolving in the hydrothermal alkali treatment
or acid-washing during the preparation of nanotubes.
The specific surface areas of TiO2 and Bi-TiO2 nano-
tubes are in the range of 270－290 m2/g, which is much
higher than both Degussa P25 and Bi-doped TiO2 nano-
paticles prepared by the conventional sol-gel method.[19]
The specific surface areas of samples are almost not
changed when Bi doping amount is lower than 1.0% (A
slight drop of surface areas over 0.4-BT NTs and
0.7-BT NTs sample may be due to the experiment er-
rors), and then increase obviously with the further in-
crease in Bi content. The increase of surface areas for
2.0-BT NTs and 5.0-BT NTs samples can be due to the
formation of nanobelt structure.
1
and then was transferred into a 100 mL Teflon-lined
autoclave. The autoclave was held at 403 K for 48 h.
After cooling down to the room temperature, the sample
was washed by deionized water, and then treated by 0.5
mol•L－1 HCl (aq.) for 12 h. The suspension was filtered
and washed with deionized water again and dried at 393
K. According to the Bi content of starting materials, the
Bi-doped TiO2 nanotubes were named x-BT NTs (x%＝
Bi/Ti molar ratio).
Characterization of photocatalysts
The concentration of Bi was measured by ICP-AES
(Varian, VISTA-MPX). The measurement of the spe-
cific surface area (SBET) was carried out at 77 K on
Quantachrome SI instrument by using the nitrogen ad-
sorption/desorption method. Morphologies of the pre-
pared samples were observed with a Nova Nano430
scanning electron microscopy (SEM) and JEOL 2010F
transmission electron microscopy (TEM). X-ray diffrac-
tion (XRD) data of the samples were recorded on a
D/MAX-2500 automatic powder diffractometer equip-
ped with the Cu Kα radiation (λ＝0.15418 nm). Raman
spectra were obtained using an FT-Raman spectrometer
(Bruke, RFS 100/S). X-ray photoelectron spectroscopy
(XPS) measurements were measured on a Thermo
SCIENTIFIC ESCALAB 250 with a resolution of 0.3－
1.0 eV, using nonmonochromatized Al Kα X-ray as the
excitation source. Binding energy was calibrated with
respected to the signal for C 1s of 284.8 eV. UV-visible
diffusive reflectance spectra (UV-Vis DRS) were re-
corded on a Perkin Elmer-Lambda 35 UV-Vis spec-
trometer.
Table 1 Bi/Ti molar ratios, BET surface areas of Bi-TiO2
nanotubes
Bi/Ti molar ratio x/%
SBET
/
Sample
Obtained
XPS
0
(m2•g－
)
1
Caculated
ICP
0
TiO2 NTs
0
275.7
271.1
273.3
277.6
286.4
288.2
0.4-BT NTs 0.4
0.7-BT NTs 0.7
1.0-BT NTs 1.0
2.0-BT NTs 2.0
5.0-BT NTs 5.0
0.28
0.45
0.76
1.18
2.16
—
Photoactivity measurement
—
Photocatalysis tests were carried out under UV and
solar-simulated light irradiation. The reaction apparatus
were the same with the reference.[18] Two similar appa-
ratuses were used for the photocalytic activity test, each
consisting of light source, cooling system, inner irradia-
tion-type quartz photoreactor and online analysis system.
Two different light sources were used in the present
study: (a) a UV light source (high-pressure Hg lamp,
125 W), (b) a solar simulated light source (Xe lamp, 500
W). The temperature of the irradiated surface of the
catalyst was monitored by a thermocouple directly in-
serted into the catalyst bed. In a typical reaction, an 80
0.80
1.30
2.42
The SEM and TEM images of Bi-doped TiO2 nano-
tubes are shown in Figure 1. The tubular structure is
multi-wall nanotube with an inter-shell spacing of
around 1－1.5 nm. It can be seen from TEM and
HRTEM images that no particle was observed on the
wall of nanotubes, indicating the Bi cations may have
been doped into the crystal structure of TiO2. Figure 1
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Synthesis of Bi-doped TiO2 Nanotubes and Enhanced Photocatalytic Activity
(A)－(E) also showed that the nanobelt structure began
to appear and have gradually increased with the increase
of Bi doping amount. The 5-BT NTs sample exists al-
most in the form of complete nanobelt structure. These
results show that the excessive amount of Bi will re-
strain a sheet folding or wrapping to a nanotube structure.
sample are consistent with pure anatase phase (JCPDS:
21-1272). For TiO2 NTs with lower Bi content (Figure
2A b－e), the crystalline phase is mainly composed of
anatase TiO2. However, the magnified patterns (Figure
2B) show that the (101) peak at 25.0° shifts to a lower
degree with the increase of Bi content. This is because
＋
the radius of Bi3 ions (0.103 nm) is larger than that of
＋
Ti4 ions (0.061 nm). As a result, the distance of near-
＋
est-neighbor crystalline plane becomes wider when Bi3
＋
ions replacing Ti4 in TiO2.[16] Such results demon-
strate that Bi cations have been doped into the crystal
lattice of TiO2. In addition, the weak diffraction peaks at
23.9º, 27.8º, 31.7º and 47.7º corresponding to
H2Ti2O5•H2O phase (JCPDS: 47-0124) can be observed
with the increase of Bi content, and only H2Ti2O5•H2O
phase can be found in 5-BT NTs sample, suggesting that
the addition of Bi cation hindered the transformation of
H2Ti2O5•H2O phase to TiO2 anatase phase during the
acid-washing treatment.
Figure 2 XRD patterns (A), magnified patterns (B) of Bi-TiO2
nanotubes. (a) TiO2 NTs; (b) 0.4-BT NTs, (c) 0.7-BT NTs, (d)
1.0-BT NTs; (e) 2.0-BT NTs; (f) 5.0-BT NTs.
Figure 3 shows the Raman spectra of Bi-doped TiO2
NTs. The Raman peaks at 143.3, 395.3, 513.2 and 639.0
cm−1 are attributed to the typical anatase active modes,
and the peaks at 272.5, 452.6, 666.4 cm−1 are assigned
to the titanate nanotubes.[15] This illustrates that the
as-prepared TiO2 NTs are pure anatase phase, and the
5-BT NTs are pure titanate phase. However, other
Bi-doped TiO2 NTs samples contain a mixed phase of
titanate and anatase. Besides that, the inset pattern in
Figure 3 shows that the Raman peak at 143 cm－1 corre-
sponding to Ti-O-Ti frame becomes weak gradually and
shifts to a higher position with the increase of Bi content.
The shift and weakening of the Raman peaks of Bi-
doped titania should be interpreted as the effects of Bi
doping, which destroys the symmetry of the Ti-O-Ti
network partly by the formation of Ti-O-Bi bond.
Figure 1 SEM, TEM and HRTEM images of Bi-doped TiO2
nanaotubes. (A/A') 0.4-BT NTs, (B/B') 0.7-BT NTs, (C/C')
1.0-BT NTs, (D/D') 2.0-BT NTs, (E/E') 5.0-BT NTs.
To obtain further insight into the surface constitution
and the oxidation state of the bismuth dopant in the
samples, the Ti 2p and Bi 4f binding energy was investi-
gated. The corresponding spectra are plotted in Figure 4.
The XRD patterns of the Bi-doped TiO2 NTs are
shown in Figure 2A. The diffraction peaks of TiO2 NTs
Chin. J. Chem. 2013, 31, 415—420
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Zhao et al.
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than the values of pure Bi2O3. The positive shift is at-
tributed to a higher oxidation state of the bismuth
＋
＋
)
x
dopant (Bi(3
state).[15] It indicates a strong interac-
＋
tion between Bi and TiO2, and that the Bi3 centers are
＋
＋
oxidized to Bi(3 ). In addition, Figure 4B shows that
x
the binding energy of Ti 2p has a slight decrease with
the increase of Bi content. The slight decrease in bind-
＋
ing energy of Ti 2p suggests that some Ti4 ions are
converted to a lower oxidation state due to the Bi sub-
stitutions.[15] The change of oxidation state of Ti and Bi
energy may be attributed to the formation of Bi-O-Ti
bonds in the framework of titania. These results are
consistent with the Raman results.
Figure 5 shows the UV-vis diffuse reflectance ab-
sorption spectra of Bi-doped and pure TiO2 nanotubes.
In comparison with that of the undoped TiO2 NTs, Bi-
doped TiO2 samples have a tail or shoulder absorbance
in the visible light region and the tail is more obvious
with increasing Bi content, which is consistent with the
yellow color of the sample. This result indicates that Bi
doping can enlarge the wavelength response range and
the effect of Bi doping on visible-light response should
be related to the form of Ti-O-Bi in the network of the
titania nanotube.
Figure 3 FT-Raman of TO2 and Bi-TiO2 nanotubes. (a) TiO2
NTs; (b) 0.4-BT NTs, (c) 0.7-BT NTs, (d) 1.0-BT NTs; (e)
2.0-BT NTs; (f) 5.0-BT NTs.
Figure 5 UV-Vis DRS spectra of Bi-TiO2 nanotubes.
Photocatalytic activities
The photocatalytic performances of the as-prepared
samples were evaluated by photocatalytic H2 evolution
rates from glycerol aqueous solution (volume percent-
age＝5%) under the UV light (Hg-Lamp) and solar-
simulated light (Xe-Lamp) irradiation. The product
analysis showed that only H2, CO2, and glycerol were
detected, and no any other organic compounds could be
found in both gas and liquid products. It is surprising
that no oxygen was detected during the splitting of wa-
ter. However, such phenomena are reasonable in the
presence of sacrificial or reducing agents such as glyc-
erol or methanol.[21,22] It is observed that the rates of
hydrogen production over all the samples reach a max-
imum at ca. 30 min and then show a steady state even
Figure 4 XPS spectra of (A) Bi 4f, (B) Ti 2p. (a) 1.0-BT NTs,
(b) 2.0-BT NTs, (c) 5.0-BT NTs.
According to the previous reports, the binding energies
of Bi 4f7/2 and Bi 4f5/2 for pure Bi2O3 at 158.6 and 163.9
eV, respectively, were accepted as standard for the XPS
measurement.[15,20] Figure 4A shows that the intensity of
the Bi 4f peaks increases and the binding energies of the
Bi 4f peaks shift obviously toward higher values with
the increase of Bi content. The binding energies of Bi
4f7/2 and Bi 4f5/2 for three Bi doped samples are higher
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Chin. J. Chem. 2013, 31, 415—420

Synthesis of Bi-doped TiO2 Nanotubes and Enhanced Photocatalytic Activity
after 4 h prolonged irradiation. Figure 6 illustrates the
steady rates of the photocatalytic H2 production from
glycerol solution in the presence of various samples. It
can be seen clearly that the Bi-doped TiO2 nanotubes
exhibit a higher photocatalytic activity than pure TiO2
nanotubes. The photocatalytic hydrogen evolution rate
over the Bi-TiO2 nanotubes first increases, and then de-
creases with the increase of Bi content. The 2.0-BT NTs
sample exhibits a maximum H2 production of 4708
μmol•h−1•gcat−1 under UV light irradiation. Under so-
lar-simulated light irradiation, the 2.0-BT NTs sample is
still the most effective catalyst as it results in a maxi-
mum of 514 μmol•h−1•gcat−1. Although the 5.0-BT NTs
sample has the highest surface area, the hydrogen evo-
lution rate is lower than that of other Bi-doped TiO2
nanotubes, revealing that the surface area is not a key
factor for photocatalytic hydrogen product. According
to the discussion on characterizations mentioned above,
the decrease in photocatalytic performance of the 5-BT
NTs sample can be attributed to the inferior photocata-
lytic activity of titanate phase than that of anatase phase
and the morphology and structure of the nanobelt.
tron-hole pairs. After electron-hole separation, the elec-
trons can be scavenged by H2O chemisorbed on the
surface of catalyst, which in turn is reduced to H2. The
holes can be capture by surface hydroxyl to produce OH
radicals, which can further oxidize the glycerol to CO2
and H2O. Of course, the holes can also directly oxidize
the organic molecules.
Energy
CB
H2O
Photo-reductive
pathway
e- e- e- e- e- e-
Bi3+
hv
H2
Bi(3+x+)
h+ h+ h+ h+ h+
H2O
OH
Photo-oxidation
+ Glycor
TiO2
CO2 + H2O
OH
VB
Scheme1 Energy band structure and mechanism of pho-
to-generated carriers transfer of Bi-TiO2 nanotubes.
Conclusions
The Bi-doped-TiO2 nanotubes with variable Bi ratios
have been successfully synthesized by using hydro-
thermal treatment in 10 mol•L−1 NaOH (aq.) through
using Bi-doped TiO2 particles derived from conven-
tional sol-gel method as starting materials. Bi-doped-
TiO2 samples showed an extension of light absorption
into the visible region, which mainly originates from the
＋
＋
)
x
doping process with the formation of new Bi(3
spe-
cies and energy level of Bi ion, and reduction of the
electron-hole recombination rate. The appropriate Bi-
doped content has positive effects on the photocatalytic
activity of the TiO2 nanotubes. Among the as-prepared
samples, the 2-BT NTs display the maximum H2 pro-
duction under UV and solar-simulated light irradiation.
Excessive doping of Bi ions in the crystal structure of
TiO2 results in the transformation of nanotubes with
anatase phase to nanobelt composed entirely of titanate
phase, which would decrease the catalytic activity for
hydrogen production.
Figure 6 rates of the photocatalytic H2 evolution from glycerol
solution (5%) under UV light (A) at different temperatures and
solar-simulated light (B) irradiation at 333 K over Bi-TiO2 nano-
tubes.
Acknowledgement
We acknowledge the financial supports from the Na-
tional Natural Science Foundation of China (Nos.
21276190 and 20806059).
Compared with pure TiO2 nanotubes, the superior
photocatalytic performance of Bi-doped TiO2 nanotubes
may be ascribed to the Bi-doped ions into TiO2 nano-
tubes, which favors the photo absorption and the separa-
tion of photo-induced electron-hole pairs. In Bi-doped
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