Journal of Alloys and Compounds
5
Facile synthesis of porous InNbO 4 nanofibers by electrospinning and their enhanced visible-light-driven photocatalytic properties 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 article info Article history: Received 27 August 2013 Received in revised form 30 December 2013 Accepted 31 December 2013 Available online 10 January 2014 Keywords: InNbO 4 Nanofibers Electrospinning Visible light Photocatalyst abstract Porous InNbO 4 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 InNbO 4 nanofibers by electrospinning is improved, in comparison to that of the InNbO 4 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. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Ever since Fujishima and Honda reported photoelectrochemical water splitting using a TiO 2 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 TiO 2 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 TiO 2 makes the utilization of visible light possi- ble, many researchers focus their efforts on the design and devel- opment of new non-TiO 2 and single-phase oxide photocatalysts with visible-light response [21–25]. InNbO 4 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 InXO 4 (X = Ta, Nb) loaded with NiO could split water directly into H 2 and O 2 under visible-light irradiation, and these photocatalysts were synthesized through calcining pre-dried In 2 O 3 and Nb 2 O 5 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 InNbO 4 photocatalyst at 200 °C for 24 h [30]. Photocatalytic performances of those InNbO 4 photocatalysts under visible light irradiation have been proven to be evidently improved. Moreover, InNbO 4 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 InNbO 4 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 InNbO 4 . Therefore, it is highly desirable to develop high-performance InNbO 4 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 TiO 2 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 TiO 2 nanofibers could be fabricated by coaxial electrospinning and di- rect annealing [42]. The short nanofiber membrane of InNbO 4 with good visible-light-driven photocatalytic function was synthesized 0925-8388/$ - see front matter Ó 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: [email protected] (X. Hu). Journal of Alloys and Compounds 592 (2014) 301–305 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
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: [email protected] (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).
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1 Introduction
2 Experimental
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: [email protected] (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).
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1 Introduction
2 Experimental
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