Constructing novel WO 3 /Fe(III) nano ﬁ bers photocatalysts with enhanced visible-light-driven photocatalytic activity via interfacial charge transfer effect

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Constructing novel WO ₃ /Fe(III) nano ﬁ bers photocatalysts with enhanced visible-light-driven photocatalytic activity via interfacial charge transfer effect

Ge Ma

^a^,^c

, Zhen Chen

^a^,^d

, Zhihong Chen

^b^,^*

, Mingliang Jin

^a^,^c

, Qingguo Meng

^b

, Mingzhe Yuan

^b

, Xin Wang

^a^,^c^,^**

, Jun-Ming Liu

^d

, Guofu Zhou

^a

aInstitute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province, China

bShenyang Institute of Automation, Guangzhou, Chinese Academy of Sciences, Guangzhou 511458, China

cInternational Academy of Optoelectronics at Zhaoqing, South China Normal University, Zhaoqing, Guangdong Province, China

dInstitute of Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province, China

a r t i c l e i n f o

Article history:

Received 9 September 2016 Received in revised form 19 January 2017 Accepted 5 February 2017

Keywords:

Electrospinning Photocatalytic WO3/Fe(III) nanoﬁber

a b s t r a c t

In this work, we have prepared a series of efficient visible-light-driven WO3/Fe(III) nanofiber photocatalysts by electrospinning and impregnation method. The physical and optical properties of the prepared photocatalysts were characterized by field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and the nitrogen adsorption analysis. Under the irradiation of visible light, all the prepared WO3/ Fe(III) nanofiber photocatalysts exhibit much higher activity for degrading methyl orange (MO) solution than the pristine WO3nanofibers, and reaches 94.6% under visible light irradiation for 3 h when the relative grafting amount of Fe(III) ions was 0.003 by mass percentage. Grafting of Fe(III) ions on the surface of WO3nanofiber could enhance its visible-light absorption and reduce the photo-generated electron-hole pairs recombination due to the interfacial charge transfer (IFCT) effect, and thus improve its photocatalytic activity, which were further verified by the ultravioletevisible diffuse reflectance spectroscopy (UVeVis DRS), photoluminescence (PL) and the ns-level time-resolvedfluorescence decay spectrum measurements.

1. Introduction

Due to their stable physical properties, great photocatalytic degradation ability, economical and recyclable advantages, metal oxide semiconductors have been extensively studied in wastewater treatment[1e7]. Semiconductors photocatalysts can decompose the organic pollutants in water just by utilizing solar energy[8].

However, the most investigated semiconductors, such as titanium dioxide (TiO2) and zinc oxide (ZnO), can only be photo-excited by

the irradiation in the ultra-violet (UV) light-region, which accounts for only 4% of the total sunlight energy and thus greatly limits their practical applications [9e11]. Therefore, investigations on high- efﬁcient visible-light-driven photocatalysts have become hot is- sues in photocatalysis.

Tungsten trioxide (WO3) is one of the n-type metal oxide semiconductors, whose band gap energy is 2.2e2.8 eV, and thus can effectively absorb visible light. Recently, considerable re- searches on WO3have been carried out and it has been regarded as a promising substitute for the UV-light-driven semiconductors [12e16]. The major methods to prepare WO₃are thermal evaporation, solvothermal, chemical vapor deposition (CVD), electrospinning, etc.[17e21]. Among these methods, electrospinning is an effective technique used in the industrial production of extremely long nanoﬁbers with uniform diameters[22e24]. WO₃nanoﬁbers made by electrospinning and further calcination consist of many

*Corresponding author.

**Corresponding author. Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province, China.

E-mail addresses:chenzhihong1227@sina.com(Z. Chen),wangxin@scnu.edu.cn (X. Wang).

Contents lists available atScienceDirect

Materials Today Energy

j o u r n a l h o m e p a g e :w w w . j o u r n a l s . e l s e v i e r . c o m / m a t e r i a l s - t o d a y - e n e r g y /

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ordered monoclinic crystals, which result in a large speciﬁc surface area, and thus contribute to the improved photocatalytic activity [21,25]. However, electrospun WO3nanoﬁbers display low photocatalytic decomposition activity, which is due to the short diffusion length of the charge carriers, leading to the fast recombination of electron-hole pairs.

In recent years, the photoinduced interfacial charge transfer (IFCT) effect has become an active area in photocatalysis to enhance the photocatalytic efﬁciency of catalysts, and has gained increasing attentions[26e32]. Hiroshi reported that grafting Fe(III) ions onto the surface of WO₃via a facile impregnation method can improve the photocatalytic activity of WO3due to the IFCT effect between the Fe(III) ions and WO3[33]. The photo-generated electrons can be directly transferred from the valence band of WO₃to the Fe(III) species that serve as the active sites for multi-electron reaction, and thus enhance the photocatalytic activity of WO3. However, to the best of our knowledge, there has been no report examining the effect of photoinduced IFCT of Fe(III) ions into the electrospun WO₃ nanoﬁbers and their photocatalytic property.

Accordingly, we undertook such task and describe in this paper the successful preparation of uniform WO₃nanofibers by electrospinning, followed by grafting of Fe(III) ions to obtain an efficient photocatalyst, namely WO3/Fe(III) composite nanofibers. Scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and the nitrogen adsorption analyses were used to investigate the morphology, chemical composition and the textural properties of the WO3/Fe(III) composite nanofibers. The photocatalytic activity of the obtained samples was investigated by the photodegradation of methyl orange (MO) in aqueous solution at room temperature under visible light irradiation. Additionally, the role of the IFCT mechanism playing to improve the photocatalytic activities was confirmed by ultravioletevisible diffuse reflectance spectroscopy (UVeVis DRS), photoluminescence (PL) and the ns- level time-resolvedfluorescence decay spectrum measurements.

2. Experiment 2.1. Materials

Ammonium Metatungstate (AMT, Mw ¼ 2956.26), Poly- vinylpyrrolidone (PVP, Mw¼1300000) and methyl orange (MO, Mw¼327.33, 96% purity) were purchased from Aladdin (Ningbo, China). Iron chloride hexahydrate (FeCl₃$6H₂O) was purchased from the Tianjin City Branch Institute of Fine Chemicals (Tianjin, China).

2.2. Preparation of WO3nanoﬁbers by electrospinning

In a typical procedure, 2.5 g of AMT was dissolved in 5 ml of deionized water, which was followed by the addition of 1.0 g of PVP.

Then, the mixture was magnetically stirred for 12 h at room temperature to obtain a homogenous precursor solution for further electrospinning. In this experiment, the needle high-voltage was 20 KV, the distance between the needle tip and the rotating col- lector was 12 cm. Subsequently, the electrospun compositeﬁbers were calcined at 600C in air for 3 h. A slow heating rate of 2C/

min was needed to achieve the formation of the WO3nanoﬁbers.

2.3. Grafting Fe(III) ions onto the surface of WO₃nanoﬁbers

WO3/Fe(III) composite nanofibers were prepared using a simple impregnation technique. Briefly, 0.5 g of WO₃ nanofibers was dispersed in 5 ml deionized water, and followed by different amounts of FeCl3$6H2O (0.001 g, 0.002 g, 0.003 g, 0.004 g, and

0.005 g). The suspension was next heated at 90C for 1 h with stirring andﬁltered twice. The resulting residue is then dried at 110 C for 24 h to obtain the WO3/Fe(III) composite nanoﬁbers, which is denoted as WO3/Fe(III)-0.001 to WO3/Fe(III)-0.005, respectively.

2.4. Material characterization

The morphology of WO3 and WO3/Fe(III) nanofibers was examined byfield emission scanning electron microscopy (FESEM, S4800) and transmission electron microscopy (TEM, JEM-2100F, JEOL). Elemental analysis was carried out by X-ray photoelectron spectroscopy (XPS, VGESCA-LAB MKII) with a monochromatic Mg KaX-ray source. The crystalline structure of the samples was analyzed by X-ray diffraction (XRD, Bruker D8 advance with Cu Ka radiation). Additionally, the textural properties of the materials were analyzed by a nitrogen adsorption analyzer apparatus (Quantachrome Instruments Quadrasorb SI). UV-DRS measurement was performed using a Shimadzu spectrophotometer (UV-2450) to evaluate the band gap energy of WO3and WO3/Fe(III) nanofibers in the wavelength region of 360e800 nm.

2.5. Photodegradation experiments

The photodegradation properties of the obtained materials were studied by decomposing MO under visible-light at room temperature in aqueous solution. Before the photocatalytic degradation experiment, we conducted an adsorption/desorption experiment in the dark to achieve the adsorption/desorption equilibrium (Fig. S1).

Brieﬂy, 50 mg of the obtained materials were added into 150 ml of MO aqueous solution (10 mg/L) and the suspension was stirred in the darkness for 30 min to achieve the adsorption/desorption equilibrium. Subsequently, the suspension solution was irradiated for 3 h with a Xenon lamp (300 W) under continuous stirring with a ﬁlter with the cutoff wavelength before 420 nm. 5 ml of the solution were taken every half hour and the suspensions were removed by centrifugation. Then absorption spectrum of the obtained clear solution was measured and the absorption intensity of MO was determined at the wavelength of 464 nm.

3. Results and discussion 3.1. Morphological analysis

The morphological results of pure WO3and WO3/Fe(III) composite nanofibers by the FESEM are shown in Fig. 1. The FESEM image of pure electrospun WO₃ nanofibers (Fig. 1a) resembles a uniformly distributed web-like structure, and is more than 1 mm in length with an average diameter of 300 nm. The images with higher magnification shown inFig. 1b reveals that the WO₃nanofibers consist of nanocrystals with an average size of 20 nm. The FESEM images of the WO3/Fe(III)-0.003 nanofibers at different magnifica- tions (Fig. 1c and d) indicate that the grafted Fe(III) ions did not change the morphological structure and size of WO3 nanofiber.

Nevertheless, the surface of the WO3 nanoﬁbers became more smooth after Fe(III) ions grafted, which may be due to the fact that nanoparticles combine more closely when immersed in the FeCl₃ solution.

The nitrogen adsorption and desorption isotherms of WO3and WO₃/Fe(III)-0.003 composite nanoﬁbers are shown inFig. 2. The results indicate that the pore volume of WO3/Fe(III)-0.003 is smaller than that of pure WO3, and their speciﬁc surface areas calculated by the BET method were found decreased from 5.93 m²/ g to 3.31 m²/g, which are consistent with the FE-SEM results that the surface of WO3became smooth after Fe(III) grafting.

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TEM was used to characterize the detailed structural features of the samples and the TEM elemental mapping was performed to study the distribution of Fe(III) and other elements. Typical TEM mapping pictures, TEM images and the corresponding EDX pattern of WO3/Fe(III)-0.003 are given inFig. 3. The elements W (Fig. 3b), O (Fig. 3c), Fe (Fig. 3d) are all distributed uniformly in the WO₃/Fe(III) composite nanofiber. A HRTEM image (Fig. 3e) clearly reveals the lattice plane with spacing of 0.384 nm, corresponding to the (002) plane of WO₃. The corresponding EDX result (Fig. 3f) can confirm the presence of the Fe element. The successful grafting of Fe(III) ions were further confirmed by HRTEM. As shown in theFig. S2, the lattice spacing of the Fe species was measured to be about 0.188 nm, which corresponding to the (005) plane of FeOOH[33]. Besides, all the diameters of Fe(III) clusters for different content of iron were about 2 nm, which did not change with the different grafting contents. In addition, it was shown inFig. S2that Fe(III) ions are not inserted into WO3lattice, but grafting on the surface of WO3as the form of FeOOH clusters. Because IFCT is a surface phenomenon and the absorption electron number is not unlimited, the more grafting

Fe(III) ions would provide more“active-sites”to transfer electrons [28,34].

3.2. Phase analysis

The XRD patterns of pure WO3 and WO3/Fe(III) composite nanofibers with different Fe(III) ions grafting amounts are pre- sented inFig. 4. All the diffraction peaks of the prepared nanofibers at 23.1, 23.5, 24.3, 26.5, 28.9, 33.2, 34.1 and 41.9can be indexed to the (002), (020), (200), (120), (112), (022), (202) and (222) dif- fractions of WO₃(JCPDS card #43-1035), respectively. The diffraction peaks of the WO3 nanofibers did not change after grafting Fe(III) ions on its surface by impregnation, indicating that grafting does not alter the phase structure of the WO₃nanofibers. However, no any trance characteristic XRD peaks associated with the iron compounds were identified, due to the much lower amount of Fe(III).

XPS analysis was performed to examine the chemical states and composition of the surface elements. The XPS spectra of the WO3/ Fe(III)-0.003 nanoﬁbers are shown inFig. 5. The result inFig. 5b reveals that after grafting Fe(III) ions, the chemical-state of W atoms in the material remains as theþ6 oxidation state (35.6 and 37.4 eV for the W 4f7/2and W 4f5/2peaks), corresponding to WO3

[35]. While the chemical-state of O atoms remains as the2 state (530.5 eV and 533 eV for O1s) inFig. 5c. As shown inFig. 5d, the binding energy of Fe 2p3/2 (711.5 eV) and Fe 2p1/2 (724 eV) are found, which correspond to FeOOH and Fe₂O₃, respectively. In addition, the Fe ions exist in both ofþ3 (respond to the blue lines) andþ2 (responds to the pink lines) states, and the major oxidation state isþ3. Taking consideration of the relatively weak XPS peak of Fe₂O₃ and the HRTEM images, the grafting of Fe(III) ions are dominantly as the FeOOH clusters. Thus it can be considered that the grafting Fe (III) ions are like a FeOOH cluster on the surface of WO₃ nanofibers, which is well agree with the result of HRTEM (Fig. S1). The peak intensity is weak, due to the small grafting concentration of Fe(III). Besides, the specific contents of Fe (III) ions Fig. 1.FESEM images a) pure WO3nanofibers, b) high magnification image of a), c), WO3/Fe(III)-0.003 composite nanofibers and d) high magnification image of c).

Fig. 2.N2absorption-desorption isotherms of pure WO3nanoﬁbers and WO3/Fe(III)- 0.003 composite nanoﬁbers.

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on surface were investigated by XPS, which were 1.03%, 1.06%, 1.24%, 1.36%, 1.41%, respectively (Table 1).

3.3. Photocatalytic measurements

The photocatalytic decomposition results on degradation of MO under visible light (l>420 nm) at room temperature are displayed inFig. 6, and photocatalytic reaction results of different materials

are shown inFig. 7. Under visible-light irradiation, all the WO₃/ Fe(III) composite nanofibers exhibit higher photocatalytic activity on degradation of MO than pure WO3nanofibers, suggesting that grafting Fe(III) on the surface of WO₃nanofibers by impregnation method is a simple and feasible route to enhance the photocatalytic activity of WO3nanofibers. With the increase of Fe(III) ions grafting contents, the MO degradation performance of different WO₃/Fe(III) composites nanofibers increases first and then decreases with Fig. 3.a) TEM image of WO3/Fe(III)-0.003 composite nanofiber, b), c), d) the corresponding TEM mapping analysis of W, O, Fe, e) high-magnified TEM image of a), f) the corresponding EDX analysis of a).

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grafting concentration, while the maximum degradation percentage by WO3/Fe(III)-0.003 sample reaches 94.6% under visible-light irradiation for 3 h. Grafting Fe(III) ions on the surface of WO3

nanoﬁbers can not only decrease the recombination of the electron-hole pairs, but also increase the transport of electrons.

Therefore, the grafting of Fe(III) ions could improve the

photocatalytic activity. However, when the amount of Fe(III) ions added was higher than 0.003, the photocatalytic degradation of MO would gradually decrease, probably due to the formation of recombination sites by excess Fe(III) ions on the surface of WO3

nanofibers. In addition, the kinetics of the photocatalytic degradation was studied to compare the photocatalytic performances of the samples. The kinetic plots for MO degradation over WO3and WO₃/Fe(III) composite nanofibers are shown inFig. 8. The highest rate constant of 0.01524 min¹ was found on WO3/Fe(III)-0.003 nanofiber, whereas that of the photo-degradation rate of pure MO was nearly zero. The mechanism responsible for the improved photolytic property by IFCT effect will be discussed in the later section.

The stability of as-prepared WO₃/Fe(III)-0.003 was investigated by recycling the photocatalytic degradation of MO under visible light irradiation, which was shown inFig. 9. The degradation percentage by WO3/Fe(III)-0.003 sample was 94.6% under visible-light irradiation for 3 h. After experiencingﬁve cycling runs, the photocatalytic activity of WO3/Fe(III)-0.003 did not show distinct change. The good photocatalytic stability of as-prepared photocatalysts could be due to the stable host WO₃nanoﬁbers by electrospinning and strong attachment of Fe(III) ions with the WO3

nanoﬁbers through the impregnation method.

Fig. 4.XRD patterns of pure WO3 and WO3/Fe(III) with different Fe(III) grafting amount.

Fig. 5.a) Overview XPS spectrum of WO3/Fe(III)-0.003 nanoﬁbers. XPS spectra of the b) W 4f region, c) O 1s region and d) Fe 2p region of WO3/Fe(III)-0.003 nanoﬁbers.

Table 1

The specific iron ions contents, energy gaps, andfluorescence lifetimes of WO3nanofibers and WO3/Fe(III) composite nanofibers.

Samples WO3 WO3/Fe(III)

0.001

WO3/Fe(III) 0.002

WO3/Fe(III) 0.003

WO3/Fe(III) 0.004

WO3/Fe(III) 0.005

Iron ions content 0 1.03% 1.16% 1.24% 1.36% 1.41%

Energy gap (eV) 2.51 2.47 2.44 2.41 2.24 2.21

Fluorescence Lifetime (ns) 0.9437 1.9907 2.9125 3.2566 2.7334 2.2676

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3.4. UV-DRS

The UVeVis diffuse reflectance spectra of the pure WO3 and WO₃/Fe(III) nanofibers were measured from 300 nm to 800 nm to compare their optical properties, as depicted inFig. 10. Their band gaps were calculated using the KubelkaeMunk functions. The pure WO₃nanofiber exhibits an absorption edge at about 455 nm, corresponding to the band gap of 2.51 eV. After grafting Fe(III) ions, the composite nanofibers show a similar absorption edge but a broader absorption band in the visible region, and the energy gap decrease to 2.21 eV from 2.47 eV (Table 1). Thus, the improvement of the

visible-light absorption leads to the increase of the photocatalytic activity of the composite photocatalysts.

3.5. Photoluminescence study

The (PL) spectrum was used to investigate the charge recombination and transfer behavior. The PL spectra of pure WO3 and WO₃/Fe(III)-0.003 nanofibers with an excitation wavelength at 340 nm, are given in Fig. 11. All the WO3 nanofibers show a prominent emission band centered at about 480 nm, which cor- responds to the recombination of electron-hole pairs. In addition, we can clearly see that the pure WO₃ nanofibers show higher Fig. 7.Comparison of the degradation rate by photocatalysts with different Fe(III)

grafting amount.

Fig. 8.Kinetic studies of MO degradation with pure WO3and WO3/Fe(III) composite nanoﬁbers.

Fig. 9.Five cycle runs of WO3/Fe(III)-0.003 for the photodegradation of MO under the visible light irradiation.

Fig. 10.UVeVis diffuse reﬂection spectra of pure WO3and WO3/Fe(III) composite nanoﬁbers. Inset shows the band energy gaps of the corresponding materials.

Fig. 11.Photoluminescence spectra of pure WO3nanoﬁbers and WO3/Fe(III)-0.003 composite nanoﬁbers.

Fig. 6.Photocatalytic degradation of MO by pure WO3and WO3/Fe(III) nanoﬁbers.

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intensity than the WO3/Fe(III)-0.003 nanoﬁbers, which suggests that the pure WO3has higher recombination rate of electron-hole pair than that of the WO₃/Fe(III)-0.003 composite nanoﬁbers. The PL results illustrate that IFCT from the grafting of Fe ions could suppress the electron-hole recombination, thereby increasing the visible light photo-catalytic activity of WO₃.

3.6. Fluorescence decay and lifetime

To further understand the photo-physical characteristics of photo-excited charge carriers, we conducted the ns-level time- resolvedfluorescence decay spectra of WO₃nanofibers and WO₃/ Fe(III) composite nanofibers, as shown inFig. 12. Byfitting the decay spectra, the charge carriers radiative lifetimes of WO3and WO₃/Fe(III) composite nanofibers can be determined as 0.9437, 1.9907, 2.9125, 3.2566, 2.7334 and 2.2676 ns, respectively (Table 1), where the longest lifetime is observed onWO3/Fe(III)-0.003 sample.

The radiative lifetime of the charge carriers increases by grafting Fe(III) ions on the surface of WO₃, which will play an important role in improving the probability of their involvement in photocatalytic reaction before recombination. The excess grafting of Fe(III) ions would form the recombination sites on the surface of WO₃

nanoﬁbers, therefore the radiative lifetime decrease after 0.003.

Longer radiative lifetime leads to the stronger catalytic activity, and the ﬂuorescence decay spectra results are consistent with the photocatalytic results. This prolonged lifetime would be ascribed to the IFCT effect between surface-grafted Fe(III) and WO3, which may increase probability of holes that are captured by reactive sub- strates to initiate the photocatalytic reactions.

3.7. Mechanism of IFCT effect on the photocatalytic activity

The proposed photocatalytic mechanism for the WO3/Fe(III) nanofibers photocatalyst is shown inFig. 13. Under visible light irradiation, the pure WO3nanofibers were excited and generated electron-hole pairs are recombined much easier. For the WO3/ Fe(III) nanofibers photocatalyst, the photo-generated electrons can directly transferred from the valence band of WO3 to the Fe(III) species that served as the active sites for multi-electron reaction.

The electrons transferred to Fe(III) ions by IFCT would reduce Fe(III) to form Fe(II), and the formed Fe(II) easily converts back to Fe(III).

Therefore, the Fe(III) ions cluster provides an oxygen reduction site to inhibit the photo-recombination of the electron-hole pairs [36e38]. Moreover, the light-absorbance ability of WO₃nanoﬁbers photocatalyst will be enhanced after grafting Fe(III) ions on its surface because of the direct transfer of photo-generated electrons from valence band of WO₃to the Fe(III) species.

4. Conclusion

In this paper, we have prepared WO3 nanoﬁbers by electrospinning to serve as visible-light-driven catalyst, and grafting Fe(III) ions on the surface of WO₃to improve its photocatalytic property.

After irradiation under visible light for 3 h, the degradation rate reached 94.6% of the MO solution with WO3/Fe(III)-0.003. The key factor for improving the catalytic activity is that grafting Fe(III) ions could impress the photo-recombination of electron-hole pairs. The electrospun WO3nanofibers have a uniform morphology and the Fig. 12. ns-level time-resolvedfluorescence decay curves of WO3nanofibers and WO3/Fe(III) composite nanofibers.

Fig. 13.Schematic mechanism of charge transfer in the WO3/Fe(III) composite systems under visible-light irradiation.

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impregnation method to prepare WO3/Fe(III) is easy and feasible.

Thus, grafting Fe(III) ions on the surface of WO₃ nanoﬁbers is a feasible method which signiﬁcantly increases the photocatalytic activity by IFCT.

Acknowledgements

The authors acknowledge theﬁnancial support from the NSFC (Grant No. 51602111, 51561135014), the National Key Research and Development Program of China (NO.2016YFB0401502), Guangdong Province Grant NO. 2014A030308013, 2014B090915005, 2015A030310196, 2015B050501010, 14KJ13, the Pearl River S&T Nova Program of Guangzhou (201506040045), Guangdong Inno- vative Research Team Program (No. 2013C1022011D039), PCSIRT Project No. IRT13064, supported by Hundred Talent Program of Chinese Academy of Sciences (QG Meng), Guangzhou post-doctoral initial funding (ZH Chen) and the 111 Project.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttp://

dx.doi.org/10.1016/j.mtener.2017.02.003.

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