Electrospinning preparation of a H₄SiW₁₂O₄₀/polycaprolactam composite nanofibrous membrane and its greatly enhanced photocatalytic activity and mechanism

Abstract

H₄SiW₁₂O₄₀/polycaprolactam (PA6) composite nanofibrous membranes with a minimum average diameter of ∼70 nm were prepared by an electrospinning technique. Characterization with Fourier transform infrared spectroscopy (FT-IR), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) indicated that H₄SiW₁₂O₄₀ has been successfully loaded into the PA6 membrane and its Keggin structure was intact. The as-prepared H₄SiW₁₂O₄₀/PA6 membranes exhibited greatly enhanced photocatalytic efficiency (≥96.0%) and excellent reusability in the degradation of methyl orange (MO) under ultraviolet irradiation, which may be attributed to the synergistic effect of PA6 and H₄SiW₁₂O₄₀. The photocatalytic process was likely to be driven by the reductive pathway with a much faster degradation rate due to the electron donation from PA6 to H₄SiW₁₂O₄₀. The hydrogen bonds between H₄SiW₁₂O₄₀ and PA6 could enhance the stability of H₄SiW₁₂O₄₀ in the membrane, so that there was nearly no loss of photocatalytic activity of the catalyst after three cycles of reactions. In view of this, H₄SiW₁₂O₄₀/PA6 composite nanofibrous membranes exhibit potential for practical applications to eliminate organic pollutants from wastewater.

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1 Introduction

Organic dyes are one of the most important pollutants in wastewater released from the textile and other chemical industries.^1,2 Within the category of dyestuffs, azo dyes constitute a significant portion and cannot be readily degraded by conventional chemical and biochemical treatment processes.^3,4 Due to their harm to the ecosystem and human health,^5,6 the removal of azo dyes from wastewater has attracted extensive attention during the last few years. Polyoxometalates (POMs) are well-defined transition metal–oxygen clusters with unique structural characteristics and can function as photocatalysts through the photoexcitation of the oxygen-to-metal charge transfer bands to separate the electron–hole pair used for reductive and oxidative reactions with the surrounding molecules.^7,8 Earlier investigation has illustrated that there were many similarities between POMs and semiconductors.^9–11 Besides, POMs featured some advantages such as optical stability, adjustable oxidizability, more stable chemical structure, non-toxicity and inexpensiveness,¹² which aroused considerable interest in studying POMs as photocatalysts and applying them for degrading organic dyes in recent years.¹³

Unfortunately, POMs are soluble in aqueous solution and thus hard to be separated for recovery.^14–16 For the purpose of practical applications, it is desirable to develop a heterogeneous photocatalytic system by combining POMs with supporting materials to make them more recoverable. Driven by such desire, considerable interests have been dedicated to the coupling of POMs with many supports such as silica, activated carbons, TiO₂, mesoporous molecular sieve and polymeric membranes.^1,17–20 From the standpoint of separation and recovery of catalysts in practical wastewater treatment, preparation of POM-containing films is an effective way because no separation process is needed for the films. While there have been some studies on the photocatalytic properties of POM-containing composite films prepared by layer-by-layer self-assembly method²¹ and sol–gel technique,²² few reports have been made on photocatalytic application of POM-containing composite films prepared by electrospinning.

Electrospun nanofibrous membrane has many remarkable characteristics, such as large specific surface area, high porosity and high permeability.^23,24 Moreover, electrospun nanofibrous membrane could present much improved performance by blending two or more materials in which advantage of each component were synergized. Zhou²⁵ prepared Ag/PVA/SiW₁₂ tri-component nanohybrids by using electrospinning and photoreduction methods. The photocatalytic activity of the nanohybrids was signally improved due to the synergistic effect of three components. However, as a water-soluble polymer, PVA nanofibers should be treated to avoid dissolution before its utilization in water. Due to the high level exposure of the photocatalysts and easy separation,²⁶ electrospun nanofibrous membranes would be promising supports for the immobilization of photocatalysts.

In this work, H₄SiW₁₂O₄₀/polycaprolactam (PA6) composite nanofibrous membrane was prepared by electrospinning in which PA6 acted as the support of H₄SiW₁₂O₄₀. PA6 with high mechanical strength and good spinnability can interact with H₄SiW₁₂O₄₀ through hydrogen bonds, which would inhibit the leakage of H₄SiW₁₂O₄₀ from the support during the photocatalytic process. In addition, as a water-tolerant polymer, PA6 could make the composite membrane stable in water and thus reusable. More importantly, the synergistic effect of PA6 and H₄SiW₁₂O₄₀ would enhance the photocatalytic degradation rate significantly. To the best of our knowledge, this is the first report on the preparation of H₄SiW₁₂O₄₀/PA6 composite nanofibrous membrane and its photocatalytic properties and mechanism. Results presented in this report shall be useful for further study on the design of nanofibrous membrane for photocatalytic treatment of practical waste effluents.

2 Experimental

2.1 Materials

H₄SiW₁₂O₄₀, formic acid (HCOOH) and perchloric acid (HClO₄) which were of analytical reagent were supplied by Sinopharm Chemical Reagent Co., Ltd. Polycaprolactam (PA6) (M_w = 30 000) was obtained from Hunan Yueyang Baling Petrochemical Co., Ltd. (Hunan, China). Methyl orange (MO) were purchased from Aladdin Industrial Corporation. All the aqueous solutions were prepared by deionized water.

2.2 Preparation of H₄SiW₁₂O₄₀/PA6 composite nanofibrous membrane

Electrospinning solution was prepared by dissolving 2.0 g PA6 in 7 mL HCOOH and stirring until PA6 completely dissolved at ambient temperature. Then H₄SiW₁₂O₄₀ powder (0.25 g, 0.5 g, 0.75 g and 1.0 g, respectively) was dissolved in 3 mL HCOOH and dropped into the above solution slowly. The mixed solution was stirred for additional 4 h. The as-prepared electrospinning solution was added to a 5 mL glass syringe with a needle tip (0.5 mm diameter). The flow rate of the solution was 0.5 mL h⁻¹, which was controlled by micro syringe pump. A high voltage power supply was used to apply an electrical potential of 20 kV. The collection distance between the needle tip to aluminum foil was 13 cm. Since PA6 is hygroscopic, all the samples fabricated for testing in this work were stored in a sealed dry box to avoid any atmospheric moisture.

2.3 Photocatalytic activity test

The photocatalytic activity of the composite nanofibrous membranes were evaluated for decomposition of harmful methyl orange (MO) aqueous. A 300 W high pressure mercury lamp with a double walled quartz glass tube (for water cooling) was suspended vertically (the distance between quartz glass reactor and lamp was 10 cm). Photodegradation of MO was carried out at atmospheric pressure and room temperature. In a typical experiment, 50 mL MO aqueous solution (10 mg L⁻¹, pH was adjusted to 1.0 using HClO₄) was placed in the reactor, and the composite nanofibrous membrane (0.25 g) was immersed in the solution. Prior to the irradiation, the reactor was shaken in dark for 30 min to establish adsorption–desorption equilibrium between the organic molecules and the catalyst surface. Decreases in the concentrations of MO were analyzed by UV-vis spectrophotometer at λ = 510 nm. At given intervals of illumination, the samples (5.0 mL) of the reaction solution were taken out and analyzed.

2.4 Instruments and characterization

The injection rate of electrospinning solution was controlled by a 78-9100C syringe pump (Cole Palmer Instrument Company). The nozzle was connected to a high-voltage regulated DC power supply (DW-P503-4ACCD, Tianjin Dongwen High Voltage Power Supply Plant). FT-IR spectra was recorded using a VERTEX 70 spectrometer, which was obtained at a resolution of 4 cm⁻¹. Microstructure of the composite nanofibrous membranes were analyzed using a Hitachi S-4800 field emission-scanning electron microscope (FE-SEM) and with an energy dispersive X-ray spectrometry (EDX, Noran7 EDX spectrometer). XPS measurement was carried out with a ESCALAB 250Xi spectrometer, with the non-monochromatised Al Kα X-radiation (hν = 1486.6 eV) and a power of 150 W (10 mA × 15 kV). Concentrations of the MO solutions were measured by a UV-8000S UV-vis spectrophotometer (Shanghai Metash Instruments Co., Ltd) over the wavelength range of 200–800 nm.

3 Results and discussion

3.1 Characterization of the composite nanofibrous membrane

Fig. 1a–d shows the morphology of H₄SiW₁₂O₄₀/PA6 nanofibers with different mass ratio of H₄SiW₁₂O₄₀ to PA6. It can be observed that these randomly oriented fibers have smooth and uniform surface. The average diameter of the composite nanofibers increased slightly with the increase of H₄SiW₁₂O₄₀ content. 70, 96, 122 and 181 nm (average diameter) were obtained for the composite nanofibers in which H₄SiW₁₂O₄₀ : PA6 were 0.25 : 2.0, 0.5 : 2.0, 0.75 : 2.0 and 1.0 : 2.0, respectively. Composition of the composite nanofibrous membrane was confirmed by EDS spectrum analysis, as shown in Fig. 1e. The result shows that C, N, O, Si and W elements exist in the composite nanofibrous membrane, supporting the presence of H₄SiW₁₂O₄₀ in the membrane.

Fig. 1 SEM images of H₄SiW₁₂O₄₀/PA6 composite nanofibers with different mass ratio of H₄SiW₁₂O₄₀ to PA6 (a) 0.25 : 2.0, (b) 0.5 : 2.0, (c) 0.75 : 2.0, (d) 1.0 : 2.0, insets are the high magnification. (e) EDS spectrum of the composite nanofibrous membrane (1.0 : 2.0).

Fig. 2 shows the FT-IR spectra of PA6, H₄SiW₁₂O₄₀ and H₄SiW₁₂O₄₀/PA6 composite nanofibrous membrane. For PA6, the peak at 3302 cm⁻¹ corresponds to the N–H stretching of amide group. Peaks at 1642 cm⁻¹ and 1542 cm⁻¹ are attributed to amide I (C=O stretch) and amide II (C–N stretch and CO–N–H bend).²⁷ For H₄SiW₁₂O₄₀, the characteristic absorption peaks of Keggin unit at 1017 cm⁻¹, 980 cm⁻¹, 922 cm⁻¹ and 792 cm⁻¹ are attributed to the ν_as(Si–O_a), ν_as(W=O_d), ν_as(W–O_b–W) and ν_as(W–O_c–W), respectively.²⁵ The FT-IR spectrum of H₄SiW₁₂O₄₀/PA6 composite nanofibrous membrane has nearly all the key features of PA6 and H₄SiW₁₂O₄₀ with minor shifts of some peaks. It is obvious that the composite sample displays four discernible peaks between 790 cm⁻¹ and 1100 cm⁻¹, agreeing with Keggin unit well, which indicates that the Keggin structure of H₄SiW₁₂O₄₀ remains intact in the composite membrane.²⁸ There are shifts observed in peaks corresponding to ν_as(W=O_d) from 980 cm⁻¹ to 970 cm⁻¹, and ν_as(W–O_c–W) from 792 cm⁻¹ to 802 cm⁻¹, respectively. The intensity of the peak related to N–H has weakened significantly. All the changes indicate that there may be weak interaction such as hydrogen bonds between PA6 and H₄SiW₁₂O₄₀, which would inhibit the leakage of H₄SiW₁₂O₄₀ from the support during the photocatalytic process.

Fig. 2 FT-IR spectra of H₄SiW₁₂O₄₀, PA6 and H₄SiW₁₂O₄₀/PA6 composite nanofibrous membrane.

Surface chemical composition and the valence state of W in the composite nanofibrous membrane were obtained by XPS analysis. Fig. 3a shows the survey spectrum of H₄SiW₁₂O₄₀/PA6 composite nanofibrous membrane, indicating the presence of W, C, N and O as expected. From Fig. 3b, two different chemical states of W are observed. The spin–orbit doublet with binding energies for the W 4f_7/2 and W 4f_5/2 core levels of 35.7 and 37.8 eV account for approximately 83.3% of the total spectral area. These values are typical of the presence of W(VI), which is ascribed to H₄SiW₁₂O₄₀ in the composite membrane. A second doublet at 34.2 and 36.4 eV accounts for the remaining area. It was reported that these values indicated the existence of “perturbed tungstate environments” corresponding to tungsten atoms in terminal W=O bonds that directly coordinate to the support.²⁹ For O 1s spectrum (Fig. 3c), the peak at 531.1 eV is attributed to the lattice oxygen in the Keggin structure (W–O–W), in good agreement with the results reported elsewhere.³⁰ The component with higher binding energy at 532.7 eV can be assigned to C=O arising from PA6. The spectrum of C 1s sample is deconvoluted into three components (Fig. 3d), corresponding to C–C (284.8 eV), C–N (285.8 eV) and C=O (287.8 eV) of PA6, respectively.

Fig. 3 (a) XPS spectra of H₄SiW₁₂O₄₀/PA6 composite nanofibrous membrane. (b) W 4f scan. (c) O 1s scan. (d) C 1s scan.

3.2 Photocatalytic property

3.2.1 Photocatalytic activity. The photocatalytic activities of H₄SiW₁₂O₄₀/PA6 composite nanofibrous membranes were tested by degrading aqueous methyl orange (MO) under UV irradiation. As shown in Fig. 4a, in the presence of the composite nanofibrous membrane (H₄SiW₁₂O₄₀ : PA6 was 0.5 : 2.0), the major absorption peaks of MO at around 510 nm declined rapidly as the irradiation time was increased. At the same time, the color of the aqueous MO solution changed from red to nearly colorless, indicating a nearly complete degradation of MO.

Fig. 4 (a) UV-vis spectra of MO vs. photoreaction time catalyzed by the composite nanofibrous membrane (0.25 g); (b) photodegradation of MO over different catalysts against irradiation time; photodegradation of MO against irradiation time (c) and kinetic linear fitting curves (d) of the composite nanofibrous membranes with different mass ratio of H₄SiW₁₂O₄₀ to PA6.

As shown in Fig. 4b, no obvious degradation of MO was observed in the absence of photocatalyst under UV irradiation. Only 4.1% of MO was degraded in the presence of pure PA6 nanofibrous membrane, indicating that the removal process was absolute physical adsorption. The degradation efficiency of MO was closed to 100% for the composite nanofibrous membrane after 30 min UV light irradiation, while it was only about 18.8% for H₄SiW₁₂O₄₀ (SiW₁₂). The possible reason was that a completely different photocatalytic mechanism took place owing to the synergistic effect of PA6 and H₄SiW₁₂O₄₀, resulting in the enhancement of the degradation rate (see below).

Fig. 4c shows the photocatalytic activities of H₄SiW₁₂O₄₀/PA6 composite nanofibrous membranes with different mass ratio of H₄SiW₁₂O₄₀ to PA6. 95.6%, 99.2%, 99.3% and 96.8% degradation of MO was observed after 30 min irradiation for the composite membranes in which H₄SiW₁₂O₄₀ : PA6 were 0.25 : 2.0, 0.5 : 2.0, 0.75 : 2.0 and 1.0 : 2.0, respectively. All the samples exhibited excellent photocatalytic activities. For further investigation, the kinetics linear simulation curves of MO photocatalytic degradation over different composite membranes are displayed in Fig. 4d. The results show that the above degradation reactions follow a Langmuir–Hinshelwood apparent first-order kinetics model due to the low initial concentration (10 mg L⁻¹) of the reactant.³¹ The model can be expressed by the following equations:

where C₀ is the initial concentration of the reactant (mg L⁻¹), C is the concentration of the reactant at time t (mg L⁻¹), t is the UV-light illumination time, and k_app is the apparent first-order rate constant (min⁻¹).³² The determined k_app are 0.109, 0.170, 0.177 and 0.119 for the composite membranes in which H₄SiW₁₂O₄₀ : PA6 were 0.25 : 2.0, 0.5 : 2.0, 0.75 : 2.0 and 1.0 : 2.0, respectively. The photocatalytic activity of the membranes increased with the increase of H₄SiW₁₂O₄₀ content initially and then reached a maximum value as the mass ratio of H₄SiW₁₂O₄₀ to PA6 was 0.75 : 2.0. Further increase of H₄SiW₁₂O₄₀ content had an inverse effect on the photocatalytic activity. This may be attributed to the agglomeration of excess H₄SiW₁₂O₄₀ which led to a decrease of the specific surface area of the catalyst.

3.2.2 Reusability and stability. Cycling uses as well as maintaining high photocatalytic activity was a critical issue for long-term use in practical applications of the catalyst. In view of this, two factors were needed to be considered: one was how easy the catalyst could be separated from the reaction system; the other was the stability of the catalyst to maintain its high activity over time.³³ Due to their non-woven mesh forms, the as-prepared H₄SiW₁₂O₄₀/PA6 membranes could be directly separated from the aqueous MO solution without any sophisticated separation technique. From Fig. 5a, there was nearly no loss of photocatalytic activity of the catalyst after three cycles of reactions. The great performance of the as-prepared membrane may be attributed to the stability of H₄SiW₁₂O₄₀ in the membrane. As shown in Fig. 5b, there is a characteristic absorption peak of H₄SiW₁₂O₄₀ at a wavelength of 260 nm. However, during the three photocatalytic cycles, no peak was observed at 260 nm (Fig. 5c), suggesting the high stability of H₄SiW₁₂O₄₀ in the membrane. To further confirm this conclusion, the composite nanofibrous membrane was removed from the reaction system after 10 min irradiation. As seen in Fig. 5d, no obvious degradation of MO was observed in the absence of the membrane, demonstrating that there was no catalytically active species leaching into the system.

Fig. 5 (a) Photocatalytic activity of H₄SiW₁₂O₄₀/PA6 nanofibrous membrane (H₄SiW₁₂O₄₀ : PA6 was 0.75 : 2.0) for MO degradation with three times of cycling uses. (b) UV-vis spectra of H₄SiW₁₂O₄₀. (c) UV-vis spectra of MO vs. photoreaction time catalyzed by the composite membrane (H₄SiW₁₂O₄₀ : PA6 was 0.75 : 2.0) after a three-cycle experiment. (d) Membrane catalyst removal test.

3.2.3 Possible photocatalytic mechanism. Photocatalytic degradation of azo dyes with POM can be driven by either oxidative or reductive pathway. The photooxidative processes is in analogy to TiO₂,³⁴ which took place via the reaction through OH radicals or direct reaction of the excited POM with the substrate.^10,35 It was reported that in the presence of alcohol (methanol, ethanol and isopropanol) as sacrificial electron donor, the whole process was driven by the reductive pathway with much faster degradation rate.^1,14,36,37 Our previous work has proved that polyvinyl alcohol (PVA) could also enhance the photocatalytic activity of POM as sacrificial electron donor and its performance was more outstanding than isopropanol. Along this direction, the effect of PA6 and other related substances on the photocatalytic activity of H₄SiW₁₂O₄₀ was carried out. As shown in Fig. 6, 18.8%, 98.6%, 36.6%, 6.3%, 2.6% and 36.9% degradation of MO was observed after 30 min irradiation for H₄SiW₁₂O₄₀ (SiW₁₂), SiW₁₂ + PA6, SiW₁₂ + N-methylacetamide (NMA), SiW₁₂ + ethylenediamine (en), SiW₁₂ + acetone (Ace) and SiW₁₂ + isopropanol (i-PrOH) redox systems, respectively. Ace and en had an inverse effect on the photocatalytic activity of H₄SiW₁₂O₄₀. On the contrary, NMA, i-PrOH and PA6 enhanced the degradation rate and PA6 exhibited more efficiently. It was reported that the α-C atoms of PA6 can be oxidized,^38,39 suggesting that PA6, like alcohols, could donate electron to POM and make MO undergo photoreductive decomposition.

Fig. 6 Photodegradation of MO by different H₄SiW₁₂O₄₀ redox systems (H₄SiW₁₂O₄₀ powder, 0.05 g).

According to the literature¹³ and the experimental results, the possible mechanistic scheme for the degradation of MO on H₄SiW₁₂O₄₀/PA6 nanofibrous membrane under UV irradiation is proposed, as shown in Fig. 7. H₄SiW₁₂O₄₀ absorbs light and mediates the electron transfer from PA6, the sacrificial donor, to MO. Reactions (1) and (2) describe the reduction of H₄SiW₁₂O₄₀ to H₅SiW₁₂O₄₀ via accepting electron donated by PA6, while reaction (3) describes a fast reoxidation of H₅SiW₁₂O₄₀ in the presence of MO molecules. The synergistic effect of PA6 and H₄SiW₁₂O₄₀ may play an important role in the rate of H₅SiW₁₂O₄₀ production and hence, in the degradation rate of azo dyes.

Fig. 7 Photocatalytic mechanism for the degradation of MO on H₄SiW₁₂O₄₀/PA6 composite nanofibrous membrane. Color code: O, red; W, green; Si, yellow.

4 Conclusions

H₄SiW₁₂O₄₀/PA6 composite nanofibrous membranes were prepared by electrospinning technique. IR, EDX and XPS characterization confirmed that H₄SiW₁₂O₄₀ existed in the membrane and its Keggin structure was not destroyed in the process. The composite nanofibrous membrane exhibited greatly enhanced photocatalytic activity in the decomposition of MO, which may be attributed to the synergistic effect of PA6 and H₄SiW₁₂O₄₀. More importantly, the nanofibrous membranes containing H₄SiW₁₂O₄₀ were stable in water, so that they could be easily separated and reused without losses in photocatalytic activity. The photocatalytic process was driven by the reductive pathway with much faster degradation rate. In view of this, H₄SiW₁₂O₄₀/PA6 composite nanofibrous membrane is promising as a photocatalyst to remove organic pollutants from wastewater.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51172062 and 51472074), the Science and Technology Project of Hebei Province (15273627), the Scientific Research Foundation for Colleges and Universities of Hebei Province under grant for Project (QN2014051), the Hundred Talents Program of Hebei Province of China (E2012100005) and the Engagement fund of North China University of Science and Technology (GP201517), to which the authors would like to express grateful appreciation.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25550c

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