Fabrication of TiO2 nanofiber assembly from nanosheets (TiO2-NFs-NSs) by electrospinning-hydrothermal method for improved photoreactivity

Article abstract of DOI:10.1016/S1872-2067(19)63470-4

Hierarchically structured nanomaterials have attracted much attention owing to their unique properties. In this study, TiO₂ nanofibers assembled from nanosheets (TiO₂-NFs-NSs) were fabricated through electrospinning technique, which was followed by hydrothermal treatment in NaOH solution. The effect of hydrothermal reaction time (0–3 h) on the structure and properties of TiO₂ nanofibers (TiO₂-NFs) was systematically studied, and TiO₂-NFs was evaluated in terms of the photocatalytic activity toward photocatalytic oxidation of acetone and the photoelectric conversion efficiency of dye-sensitized solar cells. It was found that (1) hydrothermal treatment of TiO₂-NFs in NaOH solution followed by acid washing and calcination results in the formation of TiO₂-NFs-NSs; (2) upon extending the hydrothermal reaction time from 0 h to 3 h, the BET surface area of TiO₂-NFs-NSs (T3.0 sample) increases 3.8 times (from 28 to 106 m² g^?1), while the pore volume increases 6.0 times (from 0.09 to 0.54 cm³ g^?1); (3) when compared with those of pristine TiO₂-NFs (T0 sample), the photoreactivity of the optimized TiO₂-NFs-NSs toward acetone oxidation increases 3.1 times and the photoelectric conversion efficiency increases 2.3 times. The enhanced photoreactivity of TiO₂-NFs-NSs is attributed to the enlarged BET surface area and increased pore volume, which facilitate the adsorption of substrate and penetration of gas, and the unique hollow structure of TiO₂-NFs-NSs, which facilitates light harvesting through multiple optical reflections between the TiO₂ nanosheets.

Full text of DOI:10.1016/S1872-2067(19)63470-4

Chinese Journal of Catalysis 41 (2020) 209–218
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article (Special Issue on Photocatalytic H
2
Production and CO Reduction)
2
Fabrication of TiO
TiO -NFs-NSs) by electrospinning-hydrothermal method for
improved photoreactivity
2
nanofiber assembly from nanosheets
(
2
a
a
b,#
c
a,
Yachao Lu , Xiaoyu Ou , Wenguang Wang , Jiajie Fan , Kangle Lv *
a
Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education, College of Resources and Environmental Science, South-Central
University for Nationalities, Wuhan 430074, Hubei, China
School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, Guangdong, China
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, Henan, China
b
c
A R T I C L E I N F O
A B S T R A C T
Article history:
Hierarchically structured nanomaterials have attracted much attention owing to their unique prop-
Received 23 June 2019
Accepted 9 August 2019
Published 5 January 2020
erties. In this study, TiO
through electrospinning technique, which was followed by hydrothermal treatment in NaOH solu-
tion. The effect of hydrothermal reaction time (0–3 h) on the structure and properties of TiO nano-
fibers (TiO -NFs) was systematically studied, and TiO -NFs was evaluated in terms of the photo-
catalytic activity toward photocatalytic oxidation of acetone and the photoelectric conversion effi-
ciency of dye-sensitized solar cells. It was found that (1) hydrothermal treatment of TiO -NFs in
NaOH solution followed by acid washing and calcination results in the formation of TiO -NFs-NSs;
2) upon extending the hydrothermal reaction time from 0 h to 3 h, the BET surface area of
2
nanofibers assembled from nanosheets (TiO
2
-NFs-NSs) were fabricated
2
2
2
Keywords:
2
TiO nanofiber
2
2
Electrospinning
Photocatalytic oxidation
Acetone
(
2
–1
TiO -NFs-NSs (T3.0 sample) increases 3.8 times (from 28 to 106 m g ), while the pore volume
2
3
–1
increases 6.0 times (from 0.09 to 0.54 cm g ); (3) when compared with those of pristine TiO
2
-NFs
-NFs-NSs toward acetone oxidation increases
.1 times and the photoelectric conversion efficiency increases 2.3 times. The enhanced photoreac-
-NFs-NSs is attributed to the enlarged BET surface area and increased pore volume,
which facilitate the adsorption of substrate and penetration of gas, and the unique hollow structure
of TiO -NFs-NSs, which facilitates light harvesting through multiple optical reflections between the
TiO nanosheets.
Dye-sensitized solar cell
(
T0 sample), the photoreactivity of the optimized TiO
2
3
tivity of TiO
2
2
2
©
2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1.
Introduction
Semiconductor photocatalysis is a sustainable technology
[1–6], reduce CO
splitting water [11–15], and convert solar energy to electricity
[16–19]. Among all the semiconductor photocatalysts, TiO is
2
into hydrocarbon fuels [7–10], produce H
2
by
2
for solving the problems related to environmental pollution
and energy crisis, as it can be used to oxidize organic pollutants
the most widely studied owing to its strong oxidation power,
excellent photoelectrochemical stability, and good biocompati-
*
#
Corresponding author. Tel: +86-27-67841369; Fax: +86-27-67843918; E-mail: lvkangle@mail.scuec.edu.cn
Corresponding author. Tel/Fax: +86-20-39322570; E-mail: wenguangwang2005@163.com
This work was supported by the National Natural Science Foundation of China (51672312, 21373275) and the Fundamental Research Funds for the
Central Universities, South-Central University for Nationalities (CZT19006).
DOI: S1872-2067(19)63470-4 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 1, January 2020

210
Yachao Lu et al. / Chinese Journal of Catalysis 41 (2020) 209–218
bility [20–22]. However, the photoreactivity of TiO
enough to satisfy the requirements for practical applications,
which is attributed to its low quantum efficiency [23–25]. It is
2
is not high
catalytic oxidation, reactive oxygen species (ROS) such as hy-
droxyl radicals (•OH) and super oxygen radicals (•O
–) form on
the surface of UV-illuminated TiO photocatalyst, and these ROS
2
2
generally accepted that the photoreactivity of TiO
2
is a function
can be used to oxidize organic pollutants. Therefore, in this
research, we systematically studied the effect of hydrothermal
reaction time on the photocatalytic performance and photoe-
of many parameters such as Brunauer-Emmett-Teller (BET)
surface area, crystallinity, crystalline phases, morphology, and
pore structure [26]. As these parameters are usually in conflict
lectric conversion efficiency of TiO -NFs-NSs.
2
of each other, optimizing the photoreactivity of TiO
complex. For example, reducing the particle size of TiO
2
is very
can
2
2. Experimental
facilitate the adsorption of photosensitizers, which improves
the photoelectric conversion efficiency of a dye-sensitized solar
2
2.1. Synthesis of TiO -NFs
cell (DSSC). However, if the TiO particle size is too small, the
2
photoanode film will become transparent with little light scat-
tering, which is not good for the light absorption properties of
TiO -NFs was fabricated by electrospinning technique [29].
2
Briefly, 3.4 g of tetrabutyl titanate was added dropwise into a
beaker containing 15.0 g of absolute ethanol and 4.0 g of glacial
acetic acid under vigorous magnetic stirring. Then, 1.0 g of
polyvinylpyrrolidone (Mw = 1,300,000) was added into the
mixed solution. After further stirring for 6 h, the produced vis-
cous solution was spun using an electrospinning device
(QZNT-E01, Foshan Lepton, China) with a flow rate of 0.1 ml
TiO
2
photoanode film [27,28].
Although zero-dimensional (0D) TiO
2
nanoparticles usually
exhibit a high photoreactivity owing to the large BET surface
area, they are very hard to collect and reuse [29]. Therefore,
much attention has been paid to the synthesis of
one-dimensional (1D) TiO
2
nanowires [17] or nanotubes
nanosheets [32,33] or ar-
−
1
[30,31], two-dimensional (2D) TiO
2
min and an applied voltage of 22 kV. The distance between
the needle tip and the Al foil board used for collecting fibers
was kept at about 4.0 cm. The collected nanofibrous membrane
rays [18], and even three-dimensional (3D) TiO
2
hollow mi-
crospheres [28,34] and hollow nanoboxes [3,35].
Electrospinning provides a technique to fabricate 1D struc-
tured semiconductors such as nanofibers from discrete 0D
nanoparticles, which can improve the photoreactivity of the
photocatalyst by retarding the recombination of carriers as the
photogenerated electrons and holes are supposed to migrate
along different directions [29,36–38]. In addition, the gas per-
meability of the nonwoven materials obtained from electro-
spinning facilitates photocatalytic purification of air owing to
the high porosity.
was further calcined at 500 °C for 2 h to obtain TiO
sample)
2
-NFs (T0
2.2. Fabrication of TiO -NFs-NSs
2
2
Roller brush-like TiO -NF assembly was synthesized from
nanosheets by hydrothermal treatment of pristine TiO
2
-NF (T0
sample) nanofibrous membranes in an alkaline solution, which
was followed by acid washing and calcination. Typically, 2.5 g
of the as-spun nanofibrous membrane was immersed in 160 ml
of NaOH solution (10 M), which was then transferred into a 200
ml Teflon-lined autoclave and kept in an oven at 120 °C for 3 h.
After cooling to room temperature, the supernate was re-
moved, while the collected white powders were re-dispersed in
900 ml of diluted HCl solution (0.1 M). After magnetic stirring
for 12 h, the suspensions were filtrated, and the resulted cake
was washed with distilled water until the pH became neutral.
Finally, the cake was calcined at 400 °C for 1 h to obtain
To further increase the BET surface area and porosity of the
TiO
they were treated in NaOH solution. In the form of anatase, TiO
nanoparticles (TiO -NPs) can react with NaOH to produce so-
dium titanate nanosheets (Reaction (1)), which can then trans-
form into anatase nanosheets (TiO -NSs) after acid washing
and calcination (Reactions (2) and (3)) [28,39]. Using this
2
2
nanofibers (TiO -NFs) synthesized by electrospinning,
2
2
2
strategy, TiO
TiO -NFs-NSs).
Anatase (TiO
Na TiO
Na TiO
TiO
TiO
Anatase (TiO
2
-NF assembly can be obtained from TiO
2
-NSs
(
2
2
nanoparticles) + 2NaOH →
(nanosheets) + H
(nanosheets) + H SO
(nanosheets) + Na SO
(nanosheets) + heat →
nanosheets) + H
TiO
For comparison, a series of TiO
pared under identical conditions, except for the change in hy-
drothermal reaction time. The prepared TiO -NFs-NS samples
2
-NFs-NSs (T3.0 sample).
2
3
2
O
(1)
(2)
(3)
2
-NFs-NS samples were pre-
2
3
2
4
→
H
H
2
3
2
4
2
2
3
were denoted as “Tx”, where “x” represents the hydrothermal
reaction time (h) in NaOH solution (Table 1).
2
2
O
The study by Lin et al. [40] showed that, when applied in a
DSSC, TiO -NFs-NSs exhibited a higher photoelectric conver-
sion efficiency than pristine TiO -NFs. However, the depend-
ence of the photoelectric efficiency of TiO -NFs-NSs on hydro-
2
2.3. Characterization
2
2
The morphology of TiO nanofibers was observed by field
2
thermal reaction time was not systematically studied. In addi-
tion, the performance of a DSSC is based on a photosensitiza-
tion mechanism, whereby only the surface adsorbed photosen-
emission scanning electron microscopy (SEM, Hitach, Japan) at
10 kV acceleration voltage and by transmission electron mi-
croscopy (TEM; Tecnai G20, USA) at 200 kV acceleration volt-
age. The phase structure of the photocatalyst was identified by
X-ray powder diffraction (XRD; Bruker) at 15 kV acceleration
voltage and 20 mA applied current. The light-harvesting ability
sitizer (dye) in TiO
light irradiation to produce high-energy electrons, which then
transfer to the conduction band of TiO . However, in photo-
2
photoanode film is excited under visible
2

Yachao Lu et al. / Chinese Journal of Catalysis 41 (2020) 209–218
211
Table 1
Physical properties of the TiO nanofibers.
2
b
Photovoltaic parameters c
Crystalline size
Nitrogen sorption results
Sample
Reaction time (h)
a
2
–1
3
–1
sc (mA cm–2)
2.25
(
nm)
17.1
17.1
16.1
14.4
14.7
14.4
13.4
S
BET (m g ) PV (cm g ) APS (nm)
I
V
oc (V)
FF
Η (%)
T0
0
28
34
0.09
0.13
0.31
0.40
0.41
0.53
0.54
13.6
15.4
19.8
18.8
17.7
20.1
20.0
0.780
0.778
0.764
0.760
0.765
0.746
0.747
0.649
0.678
0.630
0.717
0.724
0.729
0.735
1.14
1.33
1.45
2.30
2.56
2.65
1.51
T0.5
T1.0
T1.5
T2.0
T2.5
0.5
1.0
1.5
2.0
2.5
3.0
2.52
3.01
4.22
4.62
4.87
2.75
62
84
91
104
106
T3.0
a
Crystalline size of TiO
S
V
2
sample was calculated based on Scherrer equation.
BET, PV and APS mean BET surface area, pore volume and average pore size of the photocatalyst, respectively.
OC, ISC, FF are open circuit voltage, short circuit current density and fill factor, respectivel. η represents photoelectric conversion efficiency of the
b
c
photoanode film.
of the photocatalyst was measured by UV-visible diffuse reflec-
as the electrolyte. Before measurement of the photocurrent, the
working electrode was prepared by coating TiO slurry onto
the surface of indium tin oxide conductive glasses. First, 30 mg
of TiO nanofibers was dispersed in 0.75 mL of ethanol solution
(50 v/v%), which was followed by sonication for 30 min. Sec-
ondly, 20 µL of Nafion, which was used as the agglomerant, was
added into the above dispersion, which was followed by ultra-
sonication for 30 min to obtain a homogeneous slurry.
tance spectroscopy (UV-2550, Shimadzu) by using BaSO
4
as the
2
reference. The BET specific surface area and pore structure of
the photocatalyst were analyzed with a nitrogen adsorption
apparatus (ASAP2020, Micromeritics, USA). The calculation of
the BET surface area was based on the adsorption data ob-
tained by multipoint BET method in the relative pressure range
2
0.05–0.3, whereas Barret-Joyner-Halender method was used to
determine the pore size distribution of the photocatalyst from
the data of the adsorption isotherm. To obtain information on
the separation and migration of photogenerated carriers in the
photocatalyst, the photoluminescence (PL) spectrum was rec-
orded by a fluorescence spectrophotometer (F-7000, Hitachi,
Japan) at the excitation wavelength of 315 nm.
2.5. Photocatalytic oxidation of acetone
The photocatalytic activity of TiO nanofibers was evaluated
2
for photocatalytic oxidation of acetone [33]. The volume of the
reactor was 15 L and an UV lamp that mainly emitted 365 nm
radiation was used as the light source [41]. 0.3 g of the photo-
catalyst well-dispersed in 30 mL distilled water was distributed
evenly in three watch glasses. Those watch glasses coated with
2.4. Photocurrent measurement
The photocurrent response of the photocatalyst was meas-
burr-like TiO nanofibers were dried overnight at 60 °C and
2
ured by an electrochemical workstation (CHI760e, Shanghai,
China) with the help of the standard three-electrode photoe-
lectrochemical cell, where a Pt-wire electrode, Ag/AgCl elec-
trode, and the prepared samples were used as the counter
electrode, reference electrode, and working electrodes, respec-
tively. A 365 nm LED (3 W; Shenzhen Lamplic, China) was used
then placed under the UV lamp. After that, 10 μL of acetone was
injected into the reactor, in which the system was sealed. After
about 30 min, the adsorption of acetone reached equilibrium
and its concentration reached about 300 ppm. The concentra-
tions of acetone and carbon dioxide in the reactor could be
detected on-line through a photoacoustic IR multigas monitor
(INNOVA Air Tech Instruments, Model 1412).
as the light source, and Na
2
SO solution (0.4 M) was employed
4
Scheme 1. Schematic diagram illustrating the fabrication of TiO
2
nanofibers assembly from nanosheets (TiO -NFs-NSs).
2

212
Yachao Lu et al. / Chinese Journal of Catalysis 41 (2020) 209–218
2.6. Measurement of the photoelectric conversion efficiency
The photoelectric conversion efficiency of TiO
measured based on the photovoltaic current density-voltage
I-V) curve. First, we prepared a TiO -NF film-based pho-
toanode by pasting TiO -NF slurry onto the surface of FTO glass
by screen printing [28,42]. In order to evaporate the organic
solvent, the air-dried FTO glass coated with a layer of TiO -NF
2
-NFs was
(
2
2
2
film was placed inside an oven and heated at 80 °C for 10 min.
After cooling to room temperature, the film was placed in a
tubular muffle furnace to completely remove the organics from
the substrate by calcination at 450 °C for 30 min. Secondly, the
calcined FTO glass coated with 10-layered TiO -NF film was
2
immersed in N719 dye anhydrous ethanol solution (0.5 mM)
for 24 h to adsorb the sensitizer. Finally, the photoanode film
was rinsed with ethanol to remove the physically adsorbed
sensitizer, which was then heated in an oven at 80 °C for 2 h to
evaporate the solvent.
To assemble DSSCs, the sensitized TiO -NF film-based pho-
2
toanode was clamped together with the Pt counter electrode.
After injection of the electrolyte, the photoelectric conversion
efficiency was immediately determined.
3
.
Results and discussion
.1. Morphology and phase structure
Fig. 1 shows the SEM images of T0 sample, which was not
3
treated with NaOH solution. It can be seen that T0 sample con-
tains aggregates of nanofibers with diameters of about 80–100
nm (Fig. 1(a)). From the enlarged SEM image (Fig. 1(b)), we can
Fig. 2. SEM images TiO
T2.0 (f), T2.5 (g) and T3.0 (h) samples. Arrows indicating the presence
of TiO nanoparticles.
2
nanofibers for T0.5 (a, b), T1.0 (c, d), T1.5 (e),
see that these TiO
ter treatment in NaOH solution for 0.5 h, it appears that the
morphology of the TiO nanofibers (T0.5 sample) almost re-
2
-NFs are assembled from nanoparticles. Af-
2
2
mains unchanged (Fig. 2(a)). However, careful inspection re-
veals that the surface of T0.5 sample is coarse. Some
The transformation of TiO
2
nanoparticles to TiO
2
nanosheets after the treatment in NaOH solution can also be
more clearly observed in the corresponding TEM images of T0
sample (Fig. 3(a) and Fig. 3(b)) and T3.0 sample (Fig. 3(c) and
Fig. 3(d)). Elemental mapping was also used to record the
nanosheets are attached to the surface of TiO -NFs (Fig. 2(b));
2
they should have originated from the transformation of TiO
2
nanoparticles (Eq. (1)–(3)) [28].
The surface of the obtained TiO
when the reaction time is extended to 1.0 h (Fig. 2(c)), which is
ascribed to the growth of TiO -NSs (Fig. 2(d)). These TiO -NFs
become curly brush-like upon further increasing the reaction
time to 1.5 h (Fig. 2(e)), 2.0 h (Fig. 2(f)), and 2.5 h (Fig. 2(g)).
2
-NFs becomes much coarser
transformation processes from anatase (Fig. 4(a)) to Na
(Fig. 4(b)), H TiO (Fig. 4(c)), and anatase TiO (Fig. 4(d)). From
Fig. 4, we can see that the NaOH-treated TiO sample contains a
high concentration of Na element, which is due to the formation
of Na TiO (Eq. (1)). After acid washing, the concentration of Na
sharply decreases because of the production of H TiO (Eq.
(2)). Therefore, by using this strategy, we successfully trans-
formed primary TiO -NFs into TiO -NFs-NSs. This transfor-
mation can not only sharply increase the BET surface area of
TiO -NFs to provide more active sites for adsorption and pho-
2
TiO
3
2
3
2
2
2
2
2
3
2
The fibrous structure of TiO -NFs can hardly be observed for
2
3
2
T3.0 sample owing to the overgrowth of TiO -NSs (Fig. 2(h)).
2
2
2
toreaction, but also facilitate the light absorption owing to the
multi-reflection of light between nanosheets [43], which will be
shown below.
The XRD characterization results show that all the nano-
fibers are mainly anatase, with a small amount of rutile phase
(Fig. 5). Generally speaking, anatase exhibits a higher photore-
Fig. 1. SEM images of TiO
solution.
2
-NFs (T0 sample) before treated in NaOH
activity than rutile phase, and the presence of a small amount of
rutile is believed to be beneficial to the photocatalytic activity

Yachao Lu et al. / Chinese Journal of Catalysis 41 (2020) 209–218
213
Fig. 5. XRD patterns of the TiO
2
nanofibers photocatalysts, together
with the expected peaks for anatase TiO .
2
time from 0 to 3.0 h. Poor crystallization means more defects,
which is detrimental to the photoreactivity of photocatalytic
TiO materials.
2
Fig. 3. TEM (a, c) and high resolution TEM images (b, d) of TiO -NFs
2
before (a, b) and after treated in NaOH solution (c, d).
3.2. Nitrogen sorption and UV-visible absorption spectrum
Specific surface area and pore structure are important fac-
tors affecting the photoreactivity of a photocatalyst. Therefore,
nitrogen sorption-desorption isotherms are obtained and
compared. In this research, from Fig. 6, we can see that the ad-
sorption isotherm steadily shifts upward with an increase in
the hydrothermal reaction time, which indicates an increase in
the BET surface area of TiO
area of TiO -NFs (T0 sample) is only 28 m g , which increases
to 62, 91, and 106 m g after treatment in NaOH solution for
.0, 2.0, and 3.0 h, respectively (Table 1). Therefore, the in-
crease in the BET surface area of TiO -NFs was achieved by
treating TiO -NFs in NaOH solution, which was followed by acid
2
-NFs-NSs [28]. The BET surface
2
–1
2
2
–1
1
2
2
washing and calcination.
Careful inspection suggests that the hysteresis loop of the
adsorption-desorption isotherm in the relative pressure range
Fig. 4. Elemental maps of the samples for TiO -NFs before (a) and after
2
treated in NaOH solution (b), washed in HCl solution (c), and followed
by calcination (d).
of anatase owing to the formation of a TiO homojunction [44].
2
From Fig. 5, we can also see that the XRD diffraction peaks,
particularly the (101) facet of anatase, become wider and wider
with the increase in the hydrothermal reaction time, which
indicate reduced crystallization. By using Scherrer equation, we
calculated the average crystalline sizes of the TiO
the results are shown in Table 1. It can be seen that the crystal-
lite size of TiO steady decreases from 17.1 nm (T0 sample) to
3.4 nm (T3.0 sample) with the increase in the hydrothermal
2
samples, and
Fig. 6. Comparison of the nitrogen sorption-desorption isotherms and
corresponding pore size distribution curves (inset) of the photocata-
lysts.
2
1

214
Yachao Lu et al. / Chinese Journal of Catalysis 41 (2020) 209–218
0
.6–0.9 evolves from type H2, which originates from bottle-like
tion time from 0 to 2.5 h. The photoreactivity of T2.5 sample
–1
pores, to type H3, which is ascribed to the formation of narrow
slit-shaped pores. This is consistent with the shape evolution of
the building blocks of TiO
nanosheets (T3.0) [45,46]. The inset of Fig. 6 compares the
pore size distribution curves of the photocatalyst. It can be seen
that the pore volume of the sample steady increases with the
increase in the hydrothermal reaction time (Table 1), which is
reaches 3.41 ppm min , which is 3.1 times that of T0 sample
–1
(1.11 ppm min ). However, upon further increasing the hy-
drothermal reaction time to 3.0 h, the reactivity of TiO -NFs
reduces to 3.32 ppm min . The drop in the photoreactivity for
2
-NFs from nanoparticles (T0) to
2
–1
T3.0 sample is possibly a result of its reduced TiO
2
crystallite
size (Table 1).
ascribed to the growth of TiO
2
-NSs. The pore volume of T3.0
3.4. UV-visible absorption spectrum, PL, and photocurrent
3
–1
sample is as high as 0.54 cm g , which is 6.0 times that of T0
3
-1
sample (0.09 m g ). The increased pore volume can facilitate
the penetration of gas during the photocatalytic reaction, which
is beneficial to the photoreactivity [47].
To account for the improved photoreactivity of
TiO
photocatalyst. From the UV-visible absorption spectra shown in
Fig. 8(A), we can see that the absorption of TiO -NFs in the UV
2
-NFs-NSs, we compared the light-harvesting abilities of the
2
3
.3. Photocatalytic oxidation of acetone
region steadily increases with increasing hydrothermal reac-
tion time, which is possibly due to the multi-reflection of light
between the nanosheets. Further observation shows that all the
samples exhibit weak absorptions at wavelengths above 410
Acetone is a widely used organic solvent in industry that is
also a typical hazardous volatile organic compound (VOC) that
can cause nervous disorders [48,49]. Here, photocatalytic oxi-
dation of acetone under UV irradiation was carried out to eval-
uate the photocatalytic activity of TiO
acetone is based on the following equation (Eq. (4)).
O + O = 3CO + 3H
Fig. 7(A) displays the photocatalytic oxidation curves of ac-
etone over different TiO -NFs. It can be seen, by using T0 sam-
ple as an example, that 32 ppm of acetone is decomposed
within 30 min, and simultaneously, 77 ppm of CO is produced,
which indicate that the reduced acetone is mineralized instead
of being adsorbed on the surface of the photocatalyst upon
irradiation.
The oxidation curve of acetone can be fitted with the pseu-
do-zero-order kinetic equation (Eq. (5)).
nm. This is possibly caused by the impurities present in TiO
the precursor of TiO -NFs contains some organics such as poly-
vinylpyrrolidone that may not have been completely removed
during calcination. The absorption of TiO -NFs in the visible
2
, as
2
2
-NFs. The oxidation of
2
C H
3 6
2
2
2
O
(4)
region is found to steadily decrease as the hydrothermal reac-
tion time increases, which indicates that the hydrothermal re-
2
action between TiO
impurities from the TiO
From the corresponding PL spectra shown in Fig. 8(B), we
can see that the PL intensity of TiO -NFs also steadily increases
2
-NFs and NaOH facilitates the removal of
2
-NFs.
2
2
with the increase in the hydrothermal reaction time. These PL
peaks in the visible region, ranging from 400 to 500 nm, origi-
nate from surface O vacancies and defects [50]. The increased
PL intensity of TiO -NFs-NSs can also be attributed to the im-
2
dC/dt = K
(5)
proved light-harvesting ability in the UV region.
where C (ppm) is the concentration of acetone at irradiation
Fig. 9 compares the transient PL spectra of pristine TiO -NFs
2
–1
time t (min), and K (ppm min ) is the rate constant.
Then, the relative photocatalytic activity of TiO -NFs can be
compared with the corresponding rate constant. From Fig.
(B), we can clearly see that the photoreactivity of TiO -NFs
steady increases with the increase in the hydrothermal reac-
(T0 sample) and TiO
time of the carriers for TiO
which is slightly shorter than that of pristine TiO
Because only those pairs that are charged very fast can recom-
bine, the shorter average carrier lifetime of TiO -NFs-NSs im-
2
-NFs-NSs (T3.0 sample). The average life-
-NFs-NSs is calculated to be 4.3 ns,
-NFs (6.7 ns).
2
2
2
7
2
2
Fig. 7. Photocatalytic oxidation curves of acetone (A) and comparison of the photoreactivity between different photocatalysts (B).

Yachao Lu et al. / Chinese Journal of Catalysis 41 (2020) 209–218
215
Fig. 8. UV-visible diffuse reflectance spectra (A) and PL spectra (B) of the photocatalysts.
plies enhanced exciton dissociation [11]. Therefore, the trans-
formation of TiO nanoparticles to TiO nanosheets is beneficial
to the charge dissociation and migration.
ꢄ ꢇꢈꢆꢉꢉ
ꢅꢆ
ꢀ
ꢁ%ꢂ ꢃ
ꢍ100
ꢊꢋꢌ
2
2
(6)
where VOC, ISC, FF, and Pin are the open circuit voltage, short
circuit current density, fill factor of the film, and energy of inci-
dent light, respectively.
Considering that the separation/migration rate of photo-
generated carriers is of great importance to the photoreactivity
of semiconductor photocatalysts, we further compared the
photocurrents of the photocatalyst. It is well accepted that, the
larger the photocurrent, the more efficient is the separation
and migration of photogenerated carriers [51]. The photocur-
Fig. 11 compares the photovoltaic current density-voltage
–2
rent of T2.0 sample reaches 0.156 µA cm , which is almost 2
–2
times that of T0 sample (0.085 µA cm ). Therefore, it is not
strange to observe a higher photoreactivity for TiO -NFs-NSs,
compared with that of pristine TiO -NFs (Fig. 10).
2
2
3.5. Photoelectric conversion
The photoreactivity of TiO -NFs was also evaluated in terms
2
of the photoelectric conversion efficiency of DSSCs.
The photoelectric conversion efficiency (η) was calculated
according to Eq. (6) [22,27]:
Fig. 10. Photocurrents of the photocatalysts.
Fig. 9. Comparison of the transient photoluminescence spectra be-
tween pristine TiO
2
-NFs (T0 sample) and hierarchical TiO
2
-NFs-NSs
Fig. 11. Current-voltage curves of TiO nanofibers-based photoanode
2
(T3.0 sample). Conditions: λex = 310 nm and λem = 410 nm.
DSSCs.

216
Yachao Lu et al. / Chinese Journal of Catalysis 41 (2020) 209–218
effects of an enlarged BET surface that provides more active
sites for substrate adsorption and photoreaction, increased
pore volume that is beneficial to the diffusion/penetration of
gas, improved light-harvesting ability owing to multi-reflection
of light between the nanosheets, and a more efficient separa-
tion of the photogenerated carriers of TiO nanosheets, com-
2
pared to the case of TiO nanoparticles.
2
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Graphical Abstract
Chin. J. Catal., 2020, 41: 209–218 doi: S1872-2067(19)63470-4
Fabrication of TiO nanofiber assembly from nanosheets (TiO
2
2
-NFs-NSs) by electrospinning-hydrothermal method for
improved photoreactivity
Yachao Lu, Xiaoyu Ou, Wenguang Wang *, Jiajie Fan, Kangle Lv *
South-Central University for Nationalities; Guangdong University of Technology; Zhengzhou University
TiO2
nanofibers assembly from nanosheets:
(
(
1) improved photoelectric conversion efficiency
2) enhanced photoreactivity towards acetone oxidation
Ti(OC
4H9)4
PVP
15 mL of
H O (30 wt.%)
2
2
V
(1) alkali heating
(
(
2) acid washing
3) calcination
calcination
Step 2
Step 3
Amorphous TiO
2-NFs
Anatase TiO
2-NFs
Anatase TiO -NFs-NSs
2
A TiO
2
nanofiber assembly was prepared from nanosheets (TiO
2
-NFs-NSs) by hydrothermal reaction of TiO
2
nanofibers in NaOH solution
2
followed by acid washing and calcination. The unique structures enable TiO -NFs-NSs to exhibit improved photoelectric conversion effi-
ciency and enhanced photoreactivity toward acetone oxidation.

218
Yachao Lu et al. / Chinese Journal of Catalysis 41 (2020) 209–218
[51] L. Zhang, C. Yang, K. L. Lv, Y. C. Lu, Q. Li, X. F. Wu, Y. H. Li, X. F. Li, J. J.
[52] H. X. Li, Z. F. Bian, J. Zhu, D. Q. Zhang, G. S. Li, Y. N. Huo, H. Li, Y. F.
Fan, M. Li, Chin. J. Catal., 2019, 40, 755–764.
Lu, J. Am. Chem. Soc., 2007, 129, 8406–8407.
静电纺丝-碱热法制备高活性由纳米片组装的TiO 纳米纤维
2
a
a
b,#
c
a,*
陆亚超 , 欧小雨 , 王文广 , 范佳杰 , 吕康乐
a
中南民族大学资源与环境学院, 催化转化与能源材料化学教育部重点实验室, 湖北武汉 430074
b
广东工业大学材料与能源学院, 广东广州 510006
c
郑州大学材料科学与工程学院, 河南郑州 450001
2
摘要: 半导体光催化有望解决日益严峻的环境污染与能源危机, 因而得到广泛重视. 纳米TiO 因为其强的氧化能力和良好
的(光)化学稳定性与生物相容性, 成为了最受欢迎的半导体光催化材料. 到目前为止, 材料科学家们制备了多种形貌的
TiO
2
光催化材料, 如纳米棒(线)、纳米片和空心微球等.
2
作为染料太阳能电池的光阳极材料, 小颗粒尺寸的纳米TiO 具有大的比表面积, 有利于敏化剂的吸附, 从而增强太阳
能电池的光电转换性能. 但是尺寸太小的TiO 颗粒不利于光散射, 导致入射的太阳光直接穿透光阳极薄膜而不利于吸收和
2
2
利用太阳光. 为了解决敏化剂吸附和增强光散射这对矛盾, 本文设计制备了由纳米片组装的TiO 纳米纤维: (1)首先通过静
电纺丝法制备TiO 纳米纤维前躯体; (2)将TiO 纳米纤维前驱体在500 °C焙烧, 去除有机物, 得到晶化度良好的由纳米颗粒
2
2
组装的TiO 纳米纤维; (3)将TiO 纳米纤维进行NaOH碱热处理, 使TiO 纳米颗粒转化成钛酸盐纳米片, 然后经历酸洗和焙
2
2
2
2 2
烧, 得到由纳米片组装的TiO 纳米纤维. 染料敏化太阳能电池的性能测试结果显示, 碱热2.5 h所得TiO 样品的光阳极薄膜
的光电转化效率提升了2.3倍; 同时, 利用丙酮光催化分解的活性来评价纳米纤维的光催化活性, 发现碱热2.5 h所得纳米纤
维上光催化降解丙酮的活性提升了3.1倍.
结构表征结果显示, 随着碱热时间的延长, 从纤维表面生长出来的纳米片逐渐变长, 催化剂的比表面积和孔容不断增
加. 大的比表面积有利于底物的吸附, 纳米片结构有利于增强光散射, 通过延长光程增强对光的利用效率, 从而提升纳米
纤维的光活性. 光电流测试的结果显示, 与碱热前的TiO
纳米片结构减小了扩散距离, 有利于光生载流子快速转移到催化剂表面, 引发丙酮的光催化氧化.
关键词: TiO 纳米纤维; 静电纺丝; 光催化氧化; 丙酮; 染料敏化太阳能电池
2 2
纳米纤维相比, 碱热后的TiO 纳米纤维光电流显著增强, 这是由于
2
收稿日期: 2019-06-23. 接受日期: 2019-08-09. 出版日期: 2020-01-05.
*
通讯联系人. 电话: (027)67841369; 传真: (027)67843918; 电子信箱: lvkangle@mail.scuec.edu.cn
#
通讯联系人. 电话/传真: (020)39322570; 电子信箱: wenguangwang2005@163.com
基金来源: 国家自然科学基金(51672312和21373275); 中南民族大学中央高校基本科研业务费专项资金(CZT19006).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).