Electrospun Mesoporous InVO4/TiO2 Nanobelts with Enhanced Photocatalytic Properties

Article abstract of DOI:10.1111/php.13115

In the present paper, mesoporous InVO₄/TiO₂ nanobelts with diameter about 400?nm have been synthesized by elaborately designed electrospinning process. The microstructures of InVO₄/TiO₂ nanobelts are characteriz

Full text of DOI:10.1111/php.13115

Article type
: Research Article
Electrospun Mesoporous InVO /TiO Nanobelts with Enhanced
4
2
Photocatalytic Properties
Linbing Yao, Enyan Guo, Mingzhi Wei, Qinyu Wang, Qifang Lu*
Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass &
Functional Ceramics, School of Material Science and Engineering, Qilu University of
Technology (Shandong Academy of Sciences), Jinan, 250353, P. R. China
Corresponding author’s e-mail: luqf0110@126.com (Q. F. Lu)
Abstract
In the present paper, mesoporous InVO /TiO nanobelts with diameter about 400 nm have
4
2
been synthesized by elaborately designed electrospinning process. The microstructures of
InVO /TiO nanobelts are characterized in detail and their photoelectrocatalytic properties are
4
2
comprehensively investigated by the photocatalytic degradation tests with tetracycline (TTC)
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which may
lead to differences between this version and the Version of Record. Please cite this article as
doi: 10.1111/php.13115
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and rhodamine B (RhB) waste water. Furthermore, the energy band gap and density of states
of orthorhombic InVO and anatase TiO are modeled and analyzed by density functional
4
2
theory. Compared with single InVO nanobelts and TiO nanofibers, mesoporous InVO /TiO
2
4
2
4
nanobelts possess the extraordinary photocatalytic efficiencies and exceptional cycle
performances, which may be ascribed to the successful construction of hererostructures
between InVO and TiO and unique one-dimensional belt structures.
4
2
Keywords：Electrospinning, InVO /TiO , Nanobelts, Photocatalysis, Tetracycline
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INTRODUCTION
As a potentially facile and efficient way, solar light photocatalytic degradation has been
adopted for the treatment of water pollution in order to further implement the strategy of
sustainable development all over the world (1). TiO has been considered to be a potential
2
photocatalyst, since Honda and Fujishima discovered the photocatalytic splitting of water on
TiO electrode in 1972 (2, 3). However, the limitations due to the wide bandgap (about 3.2
2
eV) make TiO ineffective for many practical applications (4), so the focuses of researchers
2
have turned to alter the photocatalysts with visible light response, such as nitrides (5, 6),
phosphates (7), vanadates (8-10) and tungstate (11, 12). Among them, vanadates have been
extensively studied and shown the excellent photocatalytic properties in recent years (13).
Orthorhombic InVO nanomaterials have been known as a kind of ideal photocatalyst
4
for the degradation of organic pollutants under visible light irradiation, due to the wide light
absorption, narrow band gap (2.0 eV), and no secondary pollutants (13, 14). Unfortunately,
–
+
the high recombination rate of photoelectron/hole (e /h ) pairs of pure InVO nanomaterials
4
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is detrimental to the photocatalytic efficiency, therefore, it has been an urgent issue that has
to be solved for the low photocatalytic efficiency caused by photon-induced carrier
recombination.
Based on the semiconductor band engineering, the intergration of two semiconductors
with different band gaps could be an efficient way for the separation of photoinduced
carriers, delaying the recombination of electron-hole pairs and maximizing the photocatalytic
efficiencies (14-16). Although the researches on InVO /TiO heterostructures with different
4
2
morphologies, such as films (17, 18), nanofibers (19) and nanocomposites (20) have been
explored, only a limited number of literatures are covered about one-dimensional (1D)
InVO -based nanobelts, which could inhibit the carrier recombination in photocatalysis thus
4
improving the photocatalytic activity (21, 22).
In the present work, InVO /TiO heterostructured nanobelts with remarkable
4
2
photocatalytic performance have been successfully prepared, and the energy band structure
and DOS of InVO and TiO have been modeled and analyzed by density functional theory
4
2
(
DFT), respectively. The heterostructures constructed between InVO and TiO could restrain
4 2
the recombination of photoelectron-hole pairs, which could enhance the oxidation-reduction
ability and photocatalytic performance of InVO /TiO nanobelts.
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MATERIALS AND METHODS
Preparation of precursor sols. All of the chemical reagents were used without further
purification.
In a typical experimental procedure, In(NO ) ·4.5H O, Ti(SO ) ·9H O and NH VO with the
3
3
2
4 2
2
4
3
same stoichiometric ratio (0.8 mmol) were dissolved into 10 mL deionized water, and then
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9
.5 mmol (2.000 g) citric acid and 1 mL hydrochloric acid (12 M) were added during the
magnetic stirring process, which was labeled as solution A. 1.600 g PVP (K-90) as the
structure-directing reagent was dissolved in 20 mL absolute ethanol to obtain the solution B,
after that 6 mL solution A was removed and added to solution B with magnetic agitation for 2
h to obtain the precursor sols.
Synthesis of InVO /TiO nanobelts. The obtained precursor sols were transferred into the
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2
plastic injector (20 mL) attached to a stainless steel needle with inner diameter of 0.6 mm.
The applied voltage, deposition distance, and extrusion rate of the propeller was set to 20 kV,
−
1
2
5 cm and 3.6 mL h , respectively. The final InVO /TiO nanobelts were acquired via
4 2
−1
calcination at 500 °C for 2 h with a heating rate of 1 °C min in the air atmosphere. As
comparison, InVO nanobelts and TiO nanofibers were also prepared by similar procedures,
4
2
respectively. The typical experimental flowchart of InVO /TiO nanobelts was shown in Fig.
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2
S1.
Characterization. A Rigaku D/max-2000 X-ray diffractometer (Cu-Kα, λ=0.15418 nm, 40
kV, 30 mA) was used to record the phase compositions of products. The microstructure and
morphology of the samples were observed by scanning electron microscopy (SEM) (Hitachi
S-520, JXA-840) and transmission electron microscope (TEM, JEOL JEM-2100). The
surface area and porosity of the samples were analyzed by N adsorption-desorption
2
isothermal curve with a micromeritics ASAP 2020M. The UV-vis diffuse reflectance spectra
(DRS) were analyzed by spectrophotometer (Shimadzu, UV-2550) in the wavelength range
of 200-800 nm. Electron paramagnetic resonance (EPR) spectra were collected on an
ELEXSYS E500 spectrometer (Bruker, Germany). Photoluminescence (PL) spectra were
measured using F-4500 spectrophotometer (Hitachi, Japan) at an excitation at 360 nm. The
total organic carbon (TOC) contents of tetracycline (TTC) and rhodamine B (RhB) solution
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after degradation process were determined with a multi TOC/TN Analyzer NC 2100 s
Analytik Jena AG).
(
Photocatalytic test. The photocatalytic experiments were performed using 800 W xenon lamp
with wavelength range of 200-1200 nm (Fig. S2) as the light source, and TTC or RhB
solution was used to detect the photocatalytic activity of the samples, respectively. In a
−
1
−1
typical experiment, 40 mL TTC (conc. 50 mg L ) or RhB solution (conc. 20 mg L ) and 60
mg InVO /TiO photocatalysts were mixed in the test tube. Before starting the
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2
photodegradation experiments, the tubes were placed in the dark by magnetic stirring for 30
min in order to achieve the adsorption-desorption equilibrium between TTC (or RhB) and
photocatalysts. At fixed irradiation intervals, each 4 mL of mixed solution was taken out and
centrifuged to remove the sample powders. The photodegradation rate was calculated by the
maximum absorption intensity at the characteristic peak. After the reaction, the used
photocatalysts were collected for cycling experiments.
Photoelectrochemical test. Mott-Schottky measurement, photocurrent test and impedance
spectroscopy were performed using an electrochemical workstation (CHI 760E, Chenhua
Instru. Co. Ltd., Shanghai) and a standard three-electrode. Pt foil and saturated Ag/AgCl was
used as counter and reference electrode in 0.2 M Na SO electrolyte, respectively, and 500 W
2
4
xenon lamp was used as the light source. 10 mg photocatalysts were added to 0.5 mL D521
Nafion dispersion and the resulted slurry products were coated on 2.5 cm×1.0 cm conductive
glass sheets and dried at 60 °C for 2 h, which was used as a working electrode.
Computational details. The density functional theory (DFT) calculations were carried out by
using the Vienna Ab initio Simulation Package (VASP) code. The projector augmented wave
(PAW) of the generalized gradient approximation (GGA) was used as the
exchange-correlation function. For pure InVO and TiO , the Monkhorst-Pack k-point mesh
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2
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of 3×3×3 was both used to perform geometry optimization. The energy cutoff was 400 eV.
After the geometry optimization, the band structures and the density of states (DOS) of
InVO and TiO were calculated, respectively.
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RESULTS AND DISCUSSION
XRD was used to analyze the phase composition and crystal structure of the samples calcined
at 500 °C for 2 h. All of the diffraction peaks in Fig. 1a correspond to the orthorhombic
InVO (JCPDS No. 48-0898) (Fig. 1b) and anatase TiO (JCPDS No. 21-1272) (Fig. 1c),
4
2
respectively, which proves that the prepared samples have the pure two phases: anatase TiO2
and orthogonal InVO after calcined at 500 °C for 2 h.
4
As shown in Fig. 2a and b, InVO /TiO precursor nanobelts with diameter of about 1 μm
4
2
possess the smooth surface due to the presence of structure-directing reagent (21, 22). After
calcined at 500 °C for 2 h, all the samples have the rough surface consisting of the
nanoparticles with irregular morphology (Fig. 2c-d and Fig. S3). Interestingly, the width of
InVO /TiO nanobelts has been significantly reduced to about 350±50 nm and the thickness
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2
of InVO /TiO nanobelts is about 80 nm due to the decomposition of organics and
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2
crystallization of inorganics (22, 23), but 1D belt structure could still be well preserved. In
addition, it is worth mentioning that the irregular mesopores around 10 nm are discovered on
the nanobelt surface resulting from the aggregation of nanoparticles (24). Fig. 2e illustrates
that InVO /TiO nanobelts have the uniform elemental distributions along the single
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2
nanobelt, which can be easily distinguished from the different assigned colors of Ti, V and In
elements.
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To further confirm the deep insights, the heterostructured InVO /TiO nanobelts were further
4
2
analyzed using TEM and HRTEM. The samples with the diameter of about 350±50 nm
maintain the fine belt structure, which is in line with the SEM results (Fig. 3a). The
magnified view of single nanobelt indicates that there exist some irregular mesopores (Fig.
3
b). N adsorption-desorption isotherm and aperture distribution curve of InVO /TiO
2 4 2
2
nanobelts shown in Fig. 3c indicate that the specific surface area of the sample is about 15 m
−
1
g , and distribution of aperture is mainly around 5-15 nm. The presence of mesopores may
serve as the transport paths for organic pollutant molecules. The HRTEM image (Fig. 3d)
taken from the nanobelt demonstrates that the lattice fringes with d-spacings of 0.270 nm and
.287 nm correspond to (112) and (200) planes of InVO , respectively, and 0.243 nm is
0
4
ascribed to (103) planes of anatase TiO . Especially, the grain boundary between InVO and
2
4
TiO could be clearly described, and similarly, the different lattice planes of InVO and TiO
2
4
2
can be identified by the diffraction rings, proving the successful construction of
heterojunction, as well as the polycrystalline structure of the nanobelts. The quantum size
effect resulting from the existence of mesoporous and nanoparticles with the size less than the
De Broglie wavelength often leads to the excellent redox ability of the material due to the
physical confinements of electrons and holes in potential wells defined with the crystallite
boundaries (25, 26).
As we all know, UV-vis diffuse reflection spectra are used to analyze the optical absorption
property of photocatalysts (Fig. 4a). InVO /TiO nanobelts have a wide absorption range and
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2
strong absorption strength compared with TiO nanofibers and InVO nanobelts, illustrating
2
4
the higher optical absorption efficiency. Furthermore, the calculated band gaps of TiO2
nanofibers, InVO nanobelts and InVO /TiO nanobelts are approximately 3.16, 2.32 and
4
4
2
2
.45 eV, respectively (see the inset of Fig. 4a). Additionally, the oxygen vacancy (Ov) and
defects state of InVO /TiO nanobelts were provided by the solid state EPR spectra. As
4
2
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shown in Fig. 4b, the signals (g=1.98) correspond to the oxygen vacancy (27), and the
amounts of oxygen vacancy increases with the construction of heterostructures, implying the
enhancement of photoinduced electron generation and separation through InVO /TiO
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2
interface (27).
The energy band structures, DOS and optimized geometry structures of InVO and TiO were
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2
investigated based on VASP (Fig. 5). The analog values of band gap for InVO and TiO are
4
2
discrepancy from the experimental value. The exchange-correlation potential effect is treated
by the Perdew-Burke-Ernzerhof (PBE) at a level of the Generalized Gradient Approximation
(GGA), which maybe lead to some errors (28). However, this difference hardly interferes the
calculations of other important parameters (29), just as DOS of InVO and TiO can be
4
2
accurately described by VASP. It is obvious that the conduction band (CB) of TiO is mainly
2
composed of Ti 3d and O 2p states and valence band (VB) is composed of Ti 3d state. The
CB of InVO is mainly derived from V 3d, O 2p and a small amount of In 5p electron orbits,
4
and the composition of VB is mainly provided by O 2p and V 3d electron orbits.
The photodegradation efficiencies and ultraviolet-visible absorption spectra of TTC solution
using InVO /TiO nanobelts, TiO nanofibers, InVO nanobelts as photocatalysts under
4
2
2
4
simulated sunlight illumination were shown in Fig. 6 and Fig. S4. As illustrated in Fig. 6a,
after irradiation for 90 min, the degradation efficiency of TTC is 89.4%, 61.8% and 37.4% in
the presence of InVO /TiO heterostructured nanobelts, InVO nanobelts and TiO
4
2
4
2
2
nanofibers, respectively, illustrating that the photocatalytic activity of InVO /TiO
4
heterostructured nanobelts has been greatly enhanced. The same results are also shown in the
kinetic fit diagrams for photocatalytic reactions (Fig. 6b). The photocatalytic experiments for
degradation of RhB solution were also carried out simultaneously and shown in Fig. S5 and
S6. These observations offer the powerful evidences for the photocatalytic activity of
InVO /TiO heterostructured nanobelts, which may be attributed to the fact that the
4
2
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successful construction of hererostructures between InVO and TiO could effectively inhibit
4
2
the combination of photogenerated electrons and holes, and more importantly, the unique 1D
belt structure and existence of the mesopores may also provide more active sites and transfer
the photoelectron-holes to the photocatalyst surface to participate in the photocatalytic
reaction (30).
<figure 6>
In order to analyze the organic mineralization during the photocatalytic process, TOC and
HPLC tests were carried out and shown in Fig. 7. Similar to the result of photodegradation,
the removal rate of organic carbon gradually increases with the prolongation of degradation,
and after simulated sunlight irradiation for 90 min, the removal efficiency is 73.5% (Fig. 7a),
lower than the photodegradation rate (89.4%), indicating that TTC may be degraded into
other organic compounds and the longer time is required to achieve a complete photocatalytic
mineralization (31). It is clearly observed from Fig. 7b that the peak at 6.10 min gradually
decreases with the increase of degradation time, showing that TTC solution is gradually
degraded with the degradation time prolonging (32). TOC removal and HPLC
chromatograms of RhB solution were shown in Fig. S7.
<figure 7>
Transient photocurrent measurements and electrochemical impedance spectra (EIS) were
performed to elucidate the photocatalytic activities and electrical conductivity of the different
samples. Fig. 8a displays that InVO /TiO nanobelts possess the highest photoresponsivity
4
2
than TiO nanofibers and InVO nanobelts, suggesting the more photoinduced electrons and
2
4
holes are produced due to the synergistic effect between InVO and TiO . Fig. 8b illustrates
4
2
the decreased arc radius of InVO /TiO nanobelts, meaning the reduced electronic impedance
4
2
and improved charge mobility. The CV curve of InVO /TiO nanobelts displays the largest
4
2
current and oxidative and reduction peaks at different potentials compared with those of TiO2
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nanofibers and InVO nanobelts (Fig. 8c), demonstrating the enhancement of
4
electron-transfer rate (33). Variations of photocurrent density of different samples with the
applied potential in the range of 0.7-1.0 V are exposed in Fig. 8d. The photocurrent density of
InVO /TiO nanobelts is apparently highest than those of TiO nanofibers and InVO
4
2
2
4
nanoflowers, proving that the photogenerated electron-hole pairs are efficiently separated in
space, so that InVO /TiO heterostructures have the strong ability of oxidation and reduction
4
2
to degrade the organic pollutants (34).
In general, PL test was used to understand the photocatalytic mechanism. As is known to all,
the peak intensity corresponds to the high or low electron-hole coincidence rate. Fig. 9a
clearly demonstrates that the heterostructured InVO /TiO nanobelts exhibit the lowest
4
2
photoluminescence intensity compared with TiO nanofibers and InVO nanobelts, which
2
4
indicates that InVO /TiO nanobelts could effectively suppress the recombination of the
4
2
electron-hole pairs, and then more electrons and holes participate in the photocatalysis
process. The contribution of each active radical species (including O ·– radicals, ·OH
2
+
–
radicals, h and e ) in the photodegradation is validated by the reactive species trapping
experiments. As shown in Fig. 9b, the photocatalytic degradation of TTC decreases
significantly after adding isopropyl alcohol (IPA, ·OH scavenger) and ammonium oxalate
+
(
AO, h scavenger), and is not affected by the addition of p-benzoquinone (BQ, O ·–
2
scavenger). It is concluded that ·OH and holes play a more important role in photocatalysis,
and electron comes second. The ·OH and O ·– can be in situ measured by DMPO spin
2
trapping experiments by EPR spectroscopy (35). It can be clearly seen that all photocatalysts
possess a unique 1:2:2:1 signal attributable to DMPO-·OH adducts, which also proves the
producing of ·OH radicals (Fig. 9c). Even so, the EPR signal intensities of InVO /TiO
4
2
heterostructures are stronger than those of InVO nanobelts and TiO nanofibers, making it
4
2
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clear that construction of heterostructures is beneficial to the formation of ·OH radicals. As
illustrated in Fig. 9d, six characteristic peaks of DMPO-O ·– adducts obviously appear for the
2
suspension of TiO nanofibers, InVO nanobelts and InVO /TiO nanobelts. However, the
2
4
4
2
signal intensity of InVO /TiO nanobelts is more higher than those for TiO nanofibers and
4
2
2
InVO nanobelts, implying that the construction of heterostructures could promote more O ·–
4
2
radicals (27).
<figure 9>
In order to detect the recycling performance of InVO /TiO nanobelts, the cyclic degradation
4
2
experiments were carried out. The InVO /TiO nanobelts still remain the superior
4
2
photodegradation efficiency (87.4%) after four consecutive photodegradation cycles (Fig.
0a), and the slight decrease may be ascribed to the loss of photocatalysts in the processes of
1
sampling and filtration. XRD pattern and SEM image of the sample after the fourth cycle
illustrate that the crystal phase and belt structure of the sample do not change (Fig. 10b).
These results indicate that InVO /TiO nanobelts possess the excellent photocatalytic activity
4
2
and better cycle stability in the photocatalytic process.
<figure 10>
Based on the Mott-Schottky plots, the flat band potential (E ) of TiO nanofibers and InVO
4
fb
2
nanobelts are –0.69 and –0.77 V vs. SCE (pH=7), respectively. The CB minimum position
ECBM) for TiO nanofibers and InVO nanobelts can be estimated by the following equation:
(
2
4
ECBM (NHE, pH=0)=E (SCE, pH=7)+0.24+0.0591×pH–0.2
(1)
fb
The results show that the ECBM of TiO nanofibers and InVO nanobelts are about –0.24 and
2
4
–
0.32 V vs. NHE, respectively. According to the band gaps (3.16 and 2.32 eV for TiO2
nanofibers and InVO nanobelts, respectively), the VB maximum position (EVBM) of TiO2
4
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and InVO are estimated to 2.92 and 2.00 V vs. NHE, respectively. A schematic illustration
4
of photo-induced electrons and holes transferring and separation processes between InVO4
and TiO is shown in Fig. 11b. Under the simulated sunlight irradiation, InVO and TiO are
2
4
2
excited respectively, and the electrons tend to transfer from InVO phase to TiO phase, and
4
2
holes move toward InVO , driven by the potential difference caused by the differing band
4
levels of InVO and TiO , which leads to the great suppression of photoelectron-hole
4
2
recombination and the separation of photogenerated charges at the interface between TiO2
and InVO is more effectively realized.
4
CONCLUSIONS
In summary, 1D InVO /TiO nanobelts were prepared by electrospinning, which possessed
4
2
the excellent photocatalytic properties compared with InVO nanobelts and TiO nanofibers
4
2
under the simulated sunlight irradiation. InVO /TiO photocatalysts had the superior
4
2
photocatalytic activity and better cycle stability in the photocatalytic process. The greatly
enhanced performance was mainly attributed to the unique morphologies and
heterostructured system, leading to the effective charge separation.
ACKNOWLEDGEMENTS
This study was funded by the Shandong Provincial Natural Science Foundation (Grant No.
ZR2016BM22, ZR2018LF012 and ZR2016EMB23).
Compliance with ethical standards
Conflict of interest: The authors declare that they have no conflict of interest.
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Figure Caption
Figure 1. XRD patterns of InVO /TiO nanobelts (a), InVO nanobelts (b) and TiO
2
4
2
4
nanofibers (c) calcined at 500 °C for 2 h, respectively.
Figure 2. FESEM images of InVO /TiO precursor nanobelts (a, b), InVO /TiO nanobelts
4
2
4
2
calcined at 500 °C for 2 h (c, d) and EDS elemental mappings of Ti, V and In, respectively
e).
(
Figure 3. TEM images (a, b), N adsorption-desorption isotherm curve (c) and HRTEM
2
image (d) of InVO /TiO nanobelts calcined at 500 °C for 2 h. The inset in (c) and (d) is BJH
4
2
pore size distribution curve and selected-area electron diffraction (SAED), respectively.
Figure 4. UV-vis DRS (a), corresponding Kubelka-Munk function plots (inset) and solid
state EPR spectra of TiO nanofibers, InVO nanobelts and InVO /TiO nanobelts (b),
2
4
4
2
respectively.
Figure 5. Energy band structures (a), DOS (b) and optimized geometry structures of InVO4
c) and TiO (d), respectively.
(
2
Figure 6. Photocatalytic degradation efficiency (a) and corresponding kinetic linear
simulation curves (b) of TTC solution under simulated sunlight illumination using TiO2
nanofibers, InVO nanobelts and InVO /TiO nanobelts as photocatalysts.
4
4
2
Figure 7. TOC removals (a) and HPLC chromatograms (b) of TTC solution
photocatalytically degraded by InVO /TiO nanobelts at different time intervals under
4
2
simulated sunlight irradiation.
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Figure 8. Transient photocurrent responses (a), EIS Nyquist curves (b), CV curves (c) and
variation of photocurrent density versus applied potential (d) of the different samples.
Figure 9. PL spectra of the different samples (excitation wavelength: 360 nm) (a) and
trapping experiments of active species during photocatalytic degradation of TTC over
InVO /TiO nanobelts (b). DMPO spin-trapping EPR spectra for InVO /TiO nanobelts under
4
2
4
2
simulated sunlight irradiation (c, d).
Figure 10. Recycling TTC photodegradation results over InVO /TiO nanobelts (a), XRD
4
2
pattern (b) and SEM image (inset) of InVO /TiO nanobelts after the four times
4
2
photocatalytic reaction.
Figure 11. Mott-Schottky plots for TiO and InVO electrodes in Na SO solution (a) and
2
4
2
4
schematic illustration of photoelectron-hole pair separation mechanism for InVO /TiO2
4
nanobelts under simulated sunlight illumination (b).
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This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.