Transcript of Journal of Alloys and Compounds
Facile synthesis of porous InNbO4 nanofibers by electrospinning and
their enhanced visible-light-driven photocatalytic
propertiesContents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier .com/locate / ja lcom
Facile synthesis of porous InNbO4 nanofibers by electrospinning and
their enhanced visible-light-driven photocatalytic properties
0925-8388/$ - see front matter 2014 Elsevier B.V. All rights
reserved. http://dx.doi.org/10.1016/j.jallcom.2013.12.261
⇑ Corresponding author. Tel.: +86 27 87558237; fax: +86 27
87558241. E-mail address: huxl@mail.hust.edu.cn (X. Hu).
Huiyu Feng, Dongfang Hou, Yunhui Huang, Xianluo Hu ⇑ State Key
Laboratory of Materials Processing and Die & Mould Technology,
School of Materials Science and Engineering, Huazhong University of
Science and Technology, Wuhan 430074, PR China
a r t i c l e i n f o a b s t r a c t
Article history: Received 27 August 2013 Received in revised form
30 December 2013 Accepted 31 December 2013 Available online 10
January 2014
Keywords: InNbO4
Porous InNbO4 nanofibers with diameters of 50–100 nm were prepared
by a facile electrospinning method combined with subsequent
annealing. The resulting photocatalyst was characterized by X-ray
diffraction (XRD), scanning electron microscopy (SEM), transmission
electron microscopy (TEM), UV– vis diffuse reflectance spectra
(UV–vis DRS), and nitrogen-sorption analysis. The photocatalytic
activity of the photocatalysts was evaluated by degradation of
Rhodamine B under visible-light irradiation. Results demonstrate
that the photocatalytic activity of the as-formed porous InNbO4
nanofibers by electrospinning is improved, in comparison to that of
the InNbO4 crystallites that were prepared by a high-temperature
solid-state reaction. The enhanced visible-light-driven
photocatalytic activity is attrib- uted to the porous nanofibrous
architecture, high surface area, and narrower band gap.
2014 Elsevier B.V. All rights reserved.
1. Introduction
Ever since Fujishima and Honda reported photoelectrochemical water
splitting using a TiO2 electrode in 1972 [1], semiconductor
photocatalysis has attracted much attention because of the great
potential in environmental remediation and hydrogen energy pro-
duction [2–8]. As the largest proportion of the solar spectrum or
artificial light sources is visible light, it is necessary to
develop highly active photocatalysts that work efficiently under a
wide range of visible-light irradiation conditions [9–15].
Considerable efforts have been contributed to the design and
development of hybrid materials based on TiO2 with visible light
response, such as doping with metal or nonmetal elements, coupling
with carbon materials or other narrow band-gap semiconductors
[16–20]. Although modified TiO2 makes the utilization of visible
light possi- ble, many researchers focus their efforts on the
design and devel- opment of new non-TiO2 and single-phase oxide
photocatalysts with visible-light response [21–25].
InNbO4 is considered to be a potential photocatalytic material for
water splitting and dye waste treatment owing to its layered
wolframite structure and photoinduced hydrophilicity [26–30]. Zou
et al. reported that InXO4 (X = Ta, Nb) loaded with NiO could split
water directly into H2 and O2 under visible-light irradiation, and
these photocatalysts were synthesized through calcining pre-dried
In2O3 and Nb2O5 at 1100 C for 2 d based on a solid-state
reaction [26–29]. Zhang and coworkers developed a nonaqueous
sol–gel route to prepare the nanocrystalline InNbO4 photocatalyst
at 200 C for 24 h [30]. Photocatalytic performances of those InNbO4
photocatalysts under visible light irradiation have been proven to
be evidently improved. Moreover, InNbO4 thin films could be
fabricated by a sol–gel method combined with subsequent annealing
at 950 C for 12 h [31]. Recently, a wet-chemical tech- nique has
been developed to synthesize InNbO4 photocatalysts for
decomposition of organic contaminants [32]. Despites these
advances, high reaction temperature or long reaction time is often
unavoidable during the synthesis of crystalline InNbO4. Therefore,
it is highly desirable to develop high-performance InNbO4 photo-
catalysts with well-defined nanostructures by a mild method.
One-dimensional (1D) nanostructured semiconductor photo- catalysts
have so far aroused much interest because of their novel
nanoarchitectures (controlled surface or porosity) and unique
physicochemical properties [33–38]. For instance, Zhang et al.
reported that hollow mesoporous 1D TiO2 nanofibers exhibited
enhanced photocatalytic activity towards photodegradation of
Rhodamine B (RhB) [33]. Among various methodologies, electros-
pinning is a most convenient and direct technique to fabricate con-
tinuous fibers with diameters down to the nanoscale. It has been
extensively investigated for preparing 1D nanostructured materi-
als due to the low cost, versatility, and ease of manufacturing
[38–41]. Tong and coworkers found that N, Fe and W doped TiO2
nanofibers could be fabricated by coaxial electrospinning and di-
rect annealing [42]. The short nanofiber membrane of InNbO4 with
good visible-light-driven photocatalytic function was
synthesized
through electrospinning followed by calcination [43]. Recently,
Bi4Ti3O12 nanofibers that exhibited both enhanced visible-
light-driven photocatalytic decomposition of RhB and favorable
recycling capability were fabricated through electrospinning
combined with subsequent calcination in our group [44]. Herein, we
report a facile electrospinning route to fabricate long porous
InNbO4 nanofibers with diameters of 50–100 nm on a large scale. The
resulting InNbO4 nanofibers exhibit enhanced visible-light-driven
photocatalytic activity for photodegradation of RhB that was as a
model organic compound.
2. Experimental
2.1. Material synthesis
Acetic acid, In2O3, Nb2O5 and N,N-dimethylformamide (DMF) were of
analytical grade, and were supplied by Shanghai Chemical Reagent
Co. Ltd. China. In(NO3)3, ethanol niobium and
poly(vinylpyrrolidone) (PVP, Mw 1,300,000) were obtained from
Sigma–Aldrich. All the chemicals were used as received without
further puri- fication. In a typical procedure, the precursor
solution for electrospinning was pre- pared by dissolving In(NO3)3
(0.2 g), ethanol niobium (0.17 mL), PVP (0.5 g) and acetic acid (1
g) in DMF (5 mL) at room temperature according to the
stoichiometric composition. After stirring for 3 h, a transparent
precursor of pH = 5.5 was obtained. The precursor solution was then
delivered into a plastic syringe equipped with a 20- gauge
stainless steel needle. The feeding rate was 1 mL h1 monitored by a
syringe pump. The metallic needle fixed with an electrode was
connected to a variable high-voltage power supply, and a collector
made of the aluminum foil was as a grounded counter electrode which
was 6 cm away from the tip of the needle. When a high voltage of 17
kV was applied, the nanofibers composed of In(NO3)3, ethanol
niobium and PVP were formed. Then the as-collected electrospun
fibers were cal- cined at 600 C for 5 h at a heating rate of 2 C
min1. The uncalcined and calcined samples of the nanofibers were
denoted as INO-PRE and INO-NF, respectively. For comparison, the
InNbO4 (INO) crystallites were also prepared by a solid-state reac-
tion method. Stoichiometric amounts of In2O3 (99.99%) and Nb2O5
(99.99%) were mixed and heated in a crucible in air at 1000 C for
12 h at a heating rate of 10 C min1. This product was denoted as
INO-SS.
2.2. Materials characterization
X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-RB
diffrac- tometer with Cu Ka radiation. Field-emission scanning
electron microscopy (FE- SEM, SIRION200, Holland; accelerating
voltage: 10 kV) was used to characterize the morphology of the
samples. Transmission electron microscopy (TEM) and high-resolution
transmission electron microscopy (HRTEM) images were obtained by a
JEOL JEM-2010F microscope. The thermogravimetric (TG) analysis
and
Fig. 1. SEM images of (a) the as-spun precursor nanofibers, (b) the
porous InNbO4 nan HRTEM image for the porous InNbO4 nanofibers
(INO-NF).
differential thermal analysis (DTA) were performed by a PerkinElmer
Diamond TG/DTA apparatus operated at a heating rate of 10 C min1 in
flowing air. The pho- toluminescence spectra of the samples were
recorded on a Hitachi F-4500 fluores- cence spectrophotometer at
room temperature. A SHIMADZU UV-2550 spectrophotometer with an
integrating sphere was used to record UV–vis diffuse reflectance
spectra. The Brunauer–Emmett–Teller (BET) surface area was detected
by nitrogen-sorption using a Micromeritics ASAP 2020
analyzer.
2.3. Activity evaluation
A photochemical reactor was used to study the photocatalytic
activity of the samples. The photochemical reactor was a self-made
cylindrical glass vessel coated with a water-cooling jacket. The
photocatalytic activities of the samples were eval- uated through
the degradation of RhB under visible-light irradiation of a 500-W
Xe lamp with a UV cut-off filter (k P 420 nm) at ambient
temperature. 60 mg of the photocatalyst was uniformly dispersed
into the glass vessel containing 60 mL of RhB solution (6 ppm). The
distance between the sample and the lamp was 12 cm, and the light
intensity is about 150 W cm2. Before irradiation, the suspension
(pH = 6.3) was stirred for 30 min in the darkness in order to
ensure the adsorp- tion–desorption equilibrium. At a 30-min
interval, 3 mL of the reaction solution was taken, centrifuged and
measured on a UV–vis spectrometer at a maximum absorption
wavelength of 554 nm.
3. Results and discussion
Fig. 1a displays the SEM images of the as-spun INO-PRE nanof-
ibers. Obviously, the surface of these nanofibers is very smooth,
and the diameters are in the range of 100–200 nm. Fig. 1b displays
the SEM images for the INO-NF nanofibers which were obtained by
annealing the INO-PRE nanofibers at 600 C. Clearly, the INO-NF
nanofibers exhibit shrinkage because of the decomposition of PVP
during the calcination process. Fig. 1c shows the XRD pattern of
the INO-NF nanofibers. All the diffraction peaks could be well in-
dexed to a monoclinic phase of InNbO4 (ICDD 33-619). No other
crystalline by-products were found in the pattern, indicating that
the INO-NF sample was pure crystalline InNbO4. Fig. 1d and e shows
the representative TEM images of the INO-NF nanofibers. Owing to
the shrinkage during calcination, the surface of the INO-NF
nanofibers becomes coarse. The nanoporous structural configuration
is clearly observed along the nanofibers. It is believed that the
nanoporous structure is caused by the decomposition of PVP during
calcination. PVP may act as both a binder and a pore- forming agent
during the formation of porous InNbO4 nanofibers:
ofibers (INO-NF) obtained at 600 C, (c) XRD pattern, (d and e) TEM
image and (f)
Fig. 2. (a) XRD pattern and (b and c) SEM images for InNbO4
crystallites (INO-SS) prepared by the solid-state reaction
method.
Fig. 3. Nitrogen adsorption–desorption isotherms for porous InNbO4
nanofibers (INO-NF).
H. Feng et al. / Journal of Alloys and Compounds 592 (2014) 301–305
303
(1) PVP was involved in the precursor solution for electrospinning,
and served as a binder when the fibrous In(NO3)3/Nb(C2H5O)/PVP
composite was electrospun; (2) during the subsequent annealing
process, PVP as a pore-forming agent was decomposed when the
as-spun In(NO3)3/Nb(C2H5O)/PVP fibers were heated at 600 C for 5 h.
The nanoporous architecture would be quite beneficial in the
photocatalyst design, since it not only increases the surface area
of the products but also offers a lot of channels for the reactants
and products in photocatalytic reactions to easily go through. Fig.
1f indicates the HRTEM image at the edge of an individual INO-NF
nanofiber. This HRTEM image supports the claim of the crystallinity
of the INO-NF nanofibers. The periodic fringe spacing of 3.7 Å
corresponds to interplanar spacing between the (0 1 1) planes of
InNbO4, which agrees well with the XRD result. For comparison, the
INO-SS crystallites were also prepared by a con- ventional
solid-state reaction process. The XRD pattern for the INO-SS
product is shown in Fig. 2a. All the diffraction peaks can be
indexed to the monoclinic phase of InNbO4 (ICDD 01-083- 1780). The
SEM images of Fig. 2b and c shows that the INO-SS product consists
of sub-micrometer-sized InNbO4 particles with diameters in the
range of 100–400 nm.
Fig. 3 shows nitrogen adsorption–desorption isotherms of the INO-NF
nanofibers. It can be seen that the isotherm of the INO- NF sample
is of type IV with a hysteresis loop in the range of (0.4–1.0)P/P0.
The specific Brunauer–Emmett–Teller (BET) surface area is 18.1 m2
g1. The pore size calculated from the desorption branch of the
nitrogen isotherm by the BJH (Barrett–Joyner–Halenda) method ranges
from 5 to 15 nm. In contrast, the InNbO4 crystal- lites obtained by
the traditional solid-state reaction method have a much smaller
specific surface area of 1.7 m2 g1. Fig. 4a and b shows the
UV–visible diffuse reflectance spectra in the wavelength range of
200–700 nm for the INO products. Based on the diffuse reflectance
spectrum, the bandgap energy of the InNbO4 nanofibers is estimated
to be 3.1 eV. The absorption edges of the two samples are nearly
the same. Compared with the InNbO4 parti- cles, however, the
UV–visible diffuse reflectance spectrum of the
InNbO4 nanofibers shows an absorption tail in the visible-light
range from 400 to 700 nm, indicating the visible-light response.
Fig. 4c shows the room-temperature photoluminescence spectra of the
porous InNbO4 nanofibers and the InNbO4 nanoparticles. The shape
and position of the two curves are similar. In contrast to the
InNbO4 crystallites, the emission intensity from the InNbO4
nanofibers is reduced, which suggests that the recombination of
photogenerated charge carriers can be inhibited.
The photocatalytic activity of the resulting photocatalysts was
evaluated under visible-light illumination (k > 420 nm) by using
RhB as the model pollutant. A typical temporal evolution of the
spectra during the RhB adsorption and photodecomposition over
INO-NF and INO-SS is shown in Fig. 5. Before irradiation, the
three
Fig. 4. (a and b) UV–visible diffuse reflectance spectra and (c)
photoluminescence spectra of INO-NF and INO-SS.
Fig. 5. Degradation profiles of RhB over different samples where C
is the concentration of the RhB. C0 is the initial concentration of
RhB after adsorption/ desorption equilibrium.
304 H. Feng et al. / Journal of Alloys and Compounds 592 (2014)
301–305
InNbO4 photocatalysts can adsorb RhB molecules in the darkness. The
highest adsorption ratio can reach 13%. After the adsorption–
desorption equilibrium, the INO-NF sample without using a UV filter
shows the highest photocatalytic activity. The photodegrada- tion
efficiency of this sample can reach 88.6% after 3.5 h. When there
is a UV filter, the photocatalytic efficiency of the INO-NF sam-
ple is also as high as 73.7% after 3.5 h. In addition, the
photocata- lytic activity of the INO-NF sample under visible-light
irradiation is higher than that of the INO-SS sample as well as the
commercial TiO2 (anatase) powder. Furthermore, we examined the XRD
pattern (data not shown) of the INO-NF nanofibers after
photocatalytic degradation of RhB. It is found that the diffraction
peaks agree well with those of the original InNbO4 nanofibers
before use. No other peaks for the impurities appeared, which
implies that the INO-NF sample has good stability.
It is known that InNbO4 possesses a layered wolframite-type
structure, and there are two kinds of octahedra (NbO6 and InO6) in
one cell. They form the layers by sharing the corner [26–32]. This
structure is assumed to motivate the separation of photogenerated
electro–hole pairs to improve the photocatalytic activity of the
photocatalyst [45]. Meanwhile, the band structure of InNbO4 is dif-
ferent from other oxides, due to its unusual crystal structure. The
band structure of oxides is generally defined by d-level and O
2p-level. For oxides that contain two kinds of octahedra like
InNbO4, however, the energy for the valence band should be
assumed from both the O 2p-leves of NiO6 and NbO6 octahedra, and is
negative than that of O 2p-levels [26–32]. This will lead to the
decrease in band gap and the visible-light response. In addition,
the porous fibrous nanoarchitecture with high surface area
contributes to both more active sites and more efficient transfer
of the photogenerated charges [9].
4. Conclusions
The porous InNbO4 nanofibers with diameters of 50–100 nm have been
successfully fabricated by electrospinning combined with subsequent
annealing. These nanofibers show much higher photocatalytic
activity for photodecomposition of RhB under visi- ble-light
irradiation than that of the InNbO4 crystallites synthe- sized by a
high-temperature solid-state reaction method. This may be assigned
to the porous nanofibrous architecture, high sur- face area and
relatively narrower band gap. This work provides a facile and
economical strategy to fabricate InNbO4 nanofibers on a large
scale. Also, we believe that this route can be extended to prepare
other porous nanostructured oxides for photocatalytic and
optoelectronic applications. Furthermore, the as-formed InNbO4
nanofibers are expected to be used as a monolith that can be
flexible and bendable for a variety of advanced optical and
photoelectric devices.
Acknowledgments
This work was supported by Natural Science Foundation of China
(Grant Nos. 21271078 and 51002057), PCSIRT (Program for Changjiang
Scholars and Innovative Research Team in University), and NCET
(Program for New Century Excellent Talents in University, No.
NECT-12-0223).
References
[1] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [2] L. Li,
P.A. Salvador, G.S. Rohrer, Nanoscale 6 (2014) 24–42. [3] C. Li, F.
Wang, J.C. Yu, Energy Environ. Sci. 4 (2011) 100–113. [4] S. Paul,
P. Chetri, A. Choudhury, J. Alloys Comp. 583 (2014) 578–586. [5]
J.C. Yu, L. Zhang, Z. Zheng, J. Zhao, Chem. Mater. 15 (2003)
2280–2286. [6] X. Liu, L. Pan, T. Lv, Z. Sun, J. Alloys Comp. 583
(2014) 390–395. [7] J.C. Yu, L. Yu, J. Zhao, Appl. Catal. B 36
(2002) 31–43. [8] X. Chen, S.S. Mao, Chem. Rev. 107 (2007)
2891–2959. [9] X. Chen, S. Shen, L. Guo, S.S. Mao, Chem. Rev. 110
(2010) 6503–6570.
[10] X.L. Hu, G.S. Li, J.C. Yu, Langmuir 26 (2010) 3031–3039. [11]
G. Liu, L. Wang, H.G. Yang, H.M. Cheng, G.Q. Lu, J. Mater. Chem. 20
(2010) 831–
843. [12] G. Liu, J.C. Yu, G.Q. Lu, H.M. Cheng, Chem. Commun. 47
(2011) 6763–6783. [13] Z. Liu, D.D. Sun, P. Guo, J.O. Leckie, Nano
Lett. 7 (2007) 1081–1085. [14] R. Asahi, T. Morikawa, T. Ohwaki, K.
Aoki, Y. Taga, Science 293 (2001) 269–271. [15] X. Liu, L. Pan, J.
Li, K. Yu, Z. Sun, C.Q. Sun, J. Colloid Interface Sci. 404
(2013)
150–154. [16] M.Q. Yang, N. Zhang, Y.J. Xu, ACS Appl. Mater.
Interface 5 (2013) 1156–1164.
H. Feng et al. / Journal of Alloys and Compounds 592 (2014) 301–305
305
[17] J. Yang, X. Zhang, B. Li, H. Liu, P. Sun, C. Wang, L. Wang, Y.
Liu, J. Alloys Comp. 584 (2014) 180–184.
[18] D.P. Subagio, M. Srinivasan, M. Lim, T.T. Lim, Appl. Catal. B:
Environ. 95 (2010) 414–422.
[19] X.J. Liu, L.K. Pan, T. Lv, Z. Sun, C.Q. Sun, RSC Adv. 2 (2012)
3823–3827. [20] O. Akhavan, R. Azimirad, S. Safa, M.M. Larijani, J.
Mater. Chem. 20 (2010) 7386–
7392. [21] K.B. Dermenci, B. Genc, B. Ebin, T. Olmez-Hanci, S.
Gurmen, J. Alloys Comp. 586
(2014) 267–273. [22] L. Zhang, W. Wang, L. Zhou, H. Xu, Small 3
(2007) 1618–1625. [23] J. Yu, A. Kudo, Adv. Funct. Mater. 16 (2006)
2163–2169. [24] A. Charanpahari, S.S. Umare, R. Sasikala, Catal.
Commun. 40 (2013) 9–12. [25] X. Bu, B. Wu, T. Long, M. Hu, J.
Alloys Comp. 586 (2014) 202–207. [26] Z. Zou, J. Ye, H. Arakawa,
Chem. Phys. Lett. 332 (2000) 271–277. [27] Z. Zou, J. Ye, K.
Sayama, H. Arakawa, Nature 414 (2001) 625–627. [28] Z. Zou, J. Ye,
K. Sayama, H. Arakawa, Mater. Res. Bull. 36 (2001) 1185–1193. [29]
J. Ye, Z. Zou, H. Arakawa, M. Oshikiri, M. Shimoda, A. Matsushita,
T. Shishido, J.
Photochem. Photobiol. A 148 (2002) 79–83. [30] T. Kako, J. Ye,
Langmuir 23 (2007) 1924–1927. [31] L.Z. Zhang, M. Niederberger, I.
Djerdj, M. Cao, M. Antonietti, M. Niederberger,
Adv. Mater. 19 (2007) 2083–2086.
[32] R. Ullah, H. Sun, S. Wang, H.M. Ang, M.O. Tade, Ind. Eng.
Chem. Res. 51 (2012) 1563–1569.
[33] X. Zhang, V. Thavasi, S.G. Mhaisalkarb, S. Ramakrishna,
Nanoscale 4 (2012) 1707–1716.
[34] M. Yu, Y. Long, B. Sun, Z. Fan, Nanoscale 4 (2012) 2783–2796.
[35] M.T. Buscaglia, M. Sennour, V. Buscaglia, C. Bottin, V.
Kalyani, P. Nani, Cryst.
Growth Des. 11 (2011) 1394–1401. [36] Y. Liu, L. Zhou, Y. Hu, C.F.
Guo, H.S. Qian, F.M. Zhang, X.W. Lou, J. Mater. Chem.
21 (2011) 18359–18364. [37] Z.G. Zhao, M. Miyauchi, Agnew. Chem.
Int. Ed. 47 (2008) 7051–7055. [38] N. Bhardwaj, S.C. Kundu,
Biotech. Adv. 28 (2010) 325–347. [39] Z. Dong, S.J. Kennedy, Y. Wu,
J. Power Sources 196 (2011) 4886–4904. [40] A. Greiner, J.H.
Wendorff, Angew. Chem. Int. Ed. 46 (2007) 5670–5703. [41] D. Li, Y.
Xia, Adv. Mater. 16 (2004) 1151–1170. [42] H. Tong, X. Tao, D. Wu,
X. Zhang, D. Li, L. Zhang, J. Alloys Comp. 586 (2014)
274–278. [43] L. Fu, Y. Wu, F. Li, B. Zhang, Mater. Lett. 109
(2013) 225–228. [44] D.F. Hou, W. Luo, Y.H. Huang, J.C. Yu, X.L.
Hu, Nanoscale 5 (2013) 2028–2035. [45] M. Kudo, S. Tauzuki, K.
Katsumata, A. Yasumori, Y. Sugahara, Chem. Phys. Lett.
393 (2004) 12–16.
1 Introduction
2 Experimental