Fabrication of a well-aligned TiO₂ nanofibrous membrane by modified parallel electrode configuration with enhanced photocatalytic performance

Abstract

Aligned nanofibers play a significant role in the organic or inorganic material relevant applications because of their remarkable anisotropy, high surface-to-volume ratio, enhanced mechanical properties and charge transfer efficiency. In this work, highly aligned TiO₂ nanofibers were continuously prepared over large areas via an updated electrospinning configuration consisting of two parallel electrodes and one additional assistant electrode. Because of the higher quantity and mechanical strength, the aligned TiO₂ nanofibers could form an independent membrane. The morphology, thermal stability, structure, optical properties and charge transfer efficiency of this membrane were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TG), X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), UV-visible (UV-vis) diffused reflectance spectroscopy (DRS) and electrochemical impedance spectroscopy (EIS). Compared with the traditional electrospun TiO₂, the well-aligned TiO₂ nanofibrous membrane exhibited excellent photoelectrochemical performance and enhanced photocatalytic degradation efficiency of rhodamine B, which means it is a promising material to replace traditional TiO₂ for solar light utilization and organic pollutants degradation.

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1. Introduction

Electrospinning, as a simple and low-cost method using an electric field to control the deposition of polymer fibers onto a collector, has generated considerable attention in recent years.^1–3 This electrostatic processing strategy can be employed to fabricate fibrous organic or inorganic mats composed of nano/microscale fibers. Due to their flexibility and excellent physicochemical properties, the one-dimensional (1-D) nanofibrous mats are widely applied to drug delivery,⁴ tissue engineering,^5,6 sensing,⁷ catalysis,⁸ energy storage,⁹ etc.

Recently, well-aligned electrospun nanofibers have attracted growing attention in the various application due to their improved properties such as anisotropy, high surface-to-volume ratio and enhanced mechanical stress when compared with the disordered counterparts.¹⁰ In the field of tissue engineering and regenerative medicine, aligned fibrous scaffolds were useful in replicating the ECM for a specific tissue type such as tendon, where collagen fibrils are aligned parallel to each other.¹¹ In the sensor application, the Ba_0.8Sr_0.2TiO₃ nanofiber arrays are reported to exhibit stronger ability of electron axial transmission than the disordered one, and therefore possess the improved humidity response property.¹² As an inorganic catalysis, well-aligned nanofibers not only exhibit the higher surface area and charge transfer efficiency,¹³ but also reduce the recombination sites of charge carriers at the fiber/fiber interface owing to their less fibrous overlap junctions.¹⁴

However, the controllability of the spatial orientation of electrospun nanofibers remains a significant challenge. On the one hand, the oriented structure is difficult to be fabricated by traditional electrospinning technique due to the inevitable bending instability of the highly charged jet.¹⁵ On the other hand, for the inorganic semiconductor materials, the aligned architectures can hardly maintain when the amount of aligned nanofibers is less, because of the fibrous shrink and embrittle during heat treatment^16,17 where the polymer needed to be removed.¹⁸ Therefore, how to prepare the oriented fibers with higher alignment degree and large-scale yield is the most urgent problem need to be settled.

To date, several feasible methods have been used to fabricate aligned fibers using electrospinning, which can be classified as follows: (1) using specially designed collectors, such as rotating mandrel,¹⁹ rotating wheel-like bobbin,²⁰ metal frame,²¹ parallel electrodes,^22,23 three-dimensional configuration,^24,25 etc.; (2) manipulating the electrical field;²⁶ (3) magnetic electrospinning²⁷. Although the orientation may be achieved to a certain extent by these methods, some drawbacks still exist and hinder the further practical application. For example, the degree of fibrous alignment from the rotating mandrel collector or adding the auxiliary electrodes is inferior and need to be greatly improved; the output of well-aligned electrospun fibers obtained from wheel-like bobbin, the parallel electrodes or metal frame collector configuration is low; when using magnetic electrospinning, the additional Fe₃O₄ nanoparticles could be introduced and therefore influence the purity of as-prepared fibers. In order to preferably solve those issues, in recent years, Beachley et al.²⁸ reported a novel parallel automated track collector basing on the principle of the parallel electrodes, in which the fibers with high alignment degree was collected continuously, uniformly, and indefinitely. Although some defects such as the complex operations and large power consumption still exist, inspired by this idea, the better aligned nanofibers over large areas would be gained by modifying and updating the original parallel electrodes configuration.

Here, based on the principle of parallel electrodes configuration, an updated device was built to fabricate the aligned PVP/TiO₂ electrospun fibers. The two Cu electrodes were horizontal placed in a gap, an additional assistant electrode was introduced to facilitate the continuous collection of fibers with improved alignment degree. The optimal parameters using this configuration were further determined via the overall consideration of the fibrous order degree, sizes and density.

Under the obtained optimal condition, highly aligned TiO₂ nanofibrous membranes were successfully prepared over large areas. Moreover, the aligned electrospun TiO₂ nanofibers displayed a better performance than the traditional disordered one in the photodegradation of organic contaminants.

2. Experimental section

Chemicals and materials

All chemicals were of analytical grade and used without further purification. Poly(vinylpyrrolidone) (PVP M_w ≈ 1.3 × 10⁶) was purchased from BASF chemical company in Germany. Tetrabutyl titanate (Ti(OC₄H₉)₄), acetic acid (CH₃COOH), methyl alcohol (CH₃OH) were purchased from Sinopharm Chemical Reagent Co. Ltd. Degussa P25 was purchased from Degussa AG Company in Germany. The fluorine-doped tin dioxide (FTO) glass (14 ohm square⁻¹, thickness of 2.2 mm) was obtained from Pilkington (Toledo, USA).

Preparation of aligned TiO₂ (A-TiO₂) nanofibrous membrane

Firstly, a mixture of 3.45 g Ti(OC₄H₉)₄, 18.5 g CH₃OH and 1.5 g CH₃COOH were magnetically stirred for 30 min to get homogeneity. Subsequently, 3 g PVP was added to this mixture and stirred for another 6 h at room temperature to form the electrospun precursor. 5 mL of the above precursor was further added in a 20 mL syringe with a 20 G stainless steel needle connecting to a high-voltage supply (BGG Bmei Co., Ltd.). In the process of electrospinning, the PVP/A-TiO₂ nanofibers were collected by a modified collector, as shown in Fig. 1a, made of two pieces of copper electrodes in a gap accompanying by an additional vertical copper plate as an assistant electrode. The foot point of spinneret was in the middle of the horizontal distance between the collector and assistant electrode. The detailed electrospinning parameters were performed as follows: a 14 kV applied voltage, spinneret-collector vertical distance (V) of 12 cm, collector-assistant electrode horizontal distance (H) of 12 cm, an electrode gap width (G) of 2 cm, and a 5 min collecting time. For achieving the automatic sequential operation, a home-made electric reciprocating machine with the digital pulse power was used to be the collector for the aligned fibers. The as-electrospun PVP/A-TiO₂ fibers were equably transferred to the surface of the glass substrate fixing on the collector by vertically moving this substrate through the gap. The second collection of fibers was similarly deposited on the first layer of fibers. After 5 times roboticized collection, a mass of PVP/A-TiO₂ nanofibers were gathered by layer-by-layer deposition, and then were calcined at 500 °C for 90 min (the heating rate of 10 °C min⁻¹) using a muffle furnace to form the a TiO₂ nanofibrous membrane.

Fig. 1 Modified electrospinning system for fabrication of PVP/A-TiO₂ fibers. (a) Schematic of the electrospinning system made of two copper electrodes in a gap and an individual assistant electrode. (b) The image recorded by a camera showing the oriented fibers between two electrodes after 5 min collection. (c) Optical micrograph of PVP/A-TiO₂ fibers, the image in the upper left corner showing the PVP/A-TiO₂ fibers on the glass substrate.

Preparation of non-aligned TiO₂ (N-TiO₂) nanofibrous membrane

The synthesis route of N-TiO₂ nanofibrous membrane was similar with that of A-TiO₂, except that the two copper electrodes were merged into one without a gap and the collecting time was changed into 25 min without repeated collection.

Characterization

Morphologies of the samples were obtained using a L2020 Optical microscope (OM) with a high-resolution optical CCD camera in transmitted illumination mode and a scanning electron microscopy (SEM) measurement performed on a JEOL JJSM-6390A instrument. More detailed information about the morphology and structure was obtained using transmission electron microscope (TEM) performed on a FEI-TECNAI-G20 instrument. In order to acquire the clearer image, the sample is uniformly dispersed in the ethanol solution by ultrasonic technique. Thermogravimetric analysis (TG) measurements were acquired on a Setaram Labsys Evo TG-DSCS-TYPE DSC sensor at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C under steady nitrogen flow. X-Ray Diffraction (XRD) patterns of the samples were collected on an X'pert MPDPro (PANalytical Co.) diffractometer using Cu Kα radiation (40 kV, 40 mA). Raman spectra at 514 nm excitation radiation were performed on a HORIBA JOBIN YVON HR 800 spectrometer, and the wavenumbers were calibrated to the highest peak of A-TiO₂ (144.8 eV). Electronic structures were recorded by X-ray photoelectron spectroscopy (XPS) on an Axis Ultra, Kratos (UK) at monochromatic Al Ka radiation (150 W, 15 kV and 1486.6 eV). The vacuum in the spectrometer was 10⁻⁹ Torr. Binding energies were calibrated relative to the C 1s peak (284.8 eV). UV-vis diffused spectroscopy (DRS) of the samples were obtained on a HITACHI UV4100 spectrometer operating between 800 nm and 245 nm wavelength, using BaSO₄ as a reference.

Photoelectrochemical measurements

The photoelectrochemical response and the impedance measurements were performed in the dark and under illumination by a CHI 660D electrochemical work station using a conventional three electrode system. This system was comprised of a working electrode, an Ag/Ag⁺ reference electrode and a Pt foil counter electrode in 1 M Na₂SO₄ aqueous electrolyte. The excitation light source was provided by a 375 nm LED at power intensity of 300.0 mW cm⁻². The photoanodes were prepared as follows: firstly, the FTO glass substrates were cleaned by successive sonicated in deionized water and acetone for 30 min each. Further, an organic interlayer made of 7.4 g CH₃OH, 0.6 g CH₃COOH and 0.6 g PVP was introduced by spin coating to increase adhesion between the sample and FTO. Finally, the PVP/A-TiO₂ and PVP/N-TiO₂ were collected onto the treated FTO under the same collecting time of 5 min. After interlayer was removed by calcining at 500 °C for 90 min, the photoanodes only containing the samples were successfully prepared to compare the photoelectrochemical performance.

Photocatalytic measurements

The photocatalytic activities of the as-prepared samples were evaluated by degradation of RhB model wastewater (10 mg L⁻¹) under UV irradiation using a 500 W Xe lamp with a cutoff filter (λ < 420 nm). The light source and the Pyrex glass containing 0.02 g catalysts and 50 mL RhB solution were placed at a distance of 15 cm. The mixture was firstly magnetic stirred in the dark for 30 min to reach adsorption/desorption equilibrium. In the process of experiment, 3 mL of the suspension was sampled at a 10 min interval and then centrifuged (4000 rpm, 10 min) to remove the catalysts. The supernatants were analyzed by an Agilent 8430 UV-visible spectrophotometer at the absorption band of 554 nm with deionized water as a reference sample.

3. Results and discussion

Design and optimization of electrospinning system

The modified electrospinning system designed for collecting the aligned fibers is illustrated in Fig. 1a. On the nonconductive stage, two copper electrodes were separated by a gap that served as the fiber collector. Notably, an individual assistant electrode connecting the electrospinning spinneret in parallel was vertically placed at an adjustable distance from the collector. The charged precursor jet was driven by the double electric-field force from the spinneret-collector and collector-assistant electrode, and then highly aligned across the gap. There are four distinct advantages to this modified system: (i) the introduction of the horizontal auxiliary electric field can improve the aligned degree of fibers and prolong the collecting time, which has been confirmed in the previous report;²⁹ (ii) the operation is simplified without reducing applied voltage as the assistant electrode is parallelly connected with spinneret; (iii) the location of the collector is far away in the side of the spinneret rather than just under it. This project is convenient for collecting the fibers automatically, continuously and indefinitely; (iv) the amount of the spinneret can be properly added to 2, 4 or more in the both sides of the collector. Moreover, aligned composite membranes may also be fabricated by loading different precursor solutions in the added syringes. The variability of this modified system means the tremendous application potential for the large-area and composite preparation of aligned fibers.

The image recorded by a camera in Fig. 1b demonstrates the PVP/A-TiO₂ fibers deposited across the gap after 5 min electrospinning collection. The combined action of the electrostatic repulsion among every positively charged fiber and the electrostatic tension existing in the two negatively charged electrodes achieved the independence and spatial orientation of fibers.²² After layer-by-layer deposition, a voidless aligned fibrous membrane was obtained on the glass substrate (the inset in Fig. 1c). The typical optical micrograph of PVP/A-TiO₂ fibers in Fig. 1c further verified the high degree of fiber alignment and uniformity.

In all the electrospinning systems using to produce the aligned fibers, electrospinning parameters played a significant role to dictate the formation, properties and alignment of the resultant fibers.^10,29 For the system setup in our study, an array of identified basic factors influencing the electrospinning process, such as applied voltage, spinneret-collector vertical distance, collector-assistant electrode horizontal distance, electrode gap width and collecting time, have been investigated with detail by various SEM images to evaluate the alignment, diameter and density of PVP/A-TiO₂ fiber (Fig. S1 and S2†). Ultimately, the optimal electrospinning parameters was determined and shown in the Experimental section. More interestingly, through adjusting the relevant electrospinning parameters (Table S1†) or calcination time (Fig. S3†), the diameter of the fibers or the size of the crystal particles that comprise the nanofibers could be controlled by simply varying the synthesis conditions, which is able to facilitate its further applications.

Morphology and characterization of A-TiO₂ membrane

To fabricate the A-TiO₂ nanofibrous membrane, the PVP/TiO₂ aligned fibers using the obtained optimal parameters were further calcined after 5 times layer-by-layer deposition. The image of obtained A-TiO₂ membrane had a flat surface morphology and regular shape compared with N-TiO₂ one (Fig. S4†). Notably, this aligned membrane had enough quantity and mechanical strength to be completely removed from the glass substrate and served as an independent material in the wider range of application such as photocatalytic degradation and sensor. Fig. 2a and b displays the SEM images of N-TiO₂ and A-TiO₂, respectively. In contrast to the disorganized one (Fig. 2c), A-TiO₂ maintained high alignment and continuity in spite of the previous calcination treatment. The magnified image in Fig. 2d implied that the average diameter of A-TiO₂ was ca. 385 nm. Furthermore, the TiO₂ arrayed nanofibers with other patterns, such as double layer (cross), three layers (snowflake) and sterepsinema (Fig. S5†), have be fabricated through adjusting the angle of the collection plate, which also expand the applicability of this novel method.

Fig. 2 The SEM images under different magnifications: (a and c) N-TiO₂, (b and d) A-TiO₂.

The more detailed information about the morphology and crystal structure of the A-TiO₂ nanofibrous membrane was acquired by TEM observations, as presented in Fig. 3. A low-magnification TEM image (Fig. 3a) showed the overall view of a single nanofiber detached from A-TiO₂ membrane, confirming the continuous 1-D morphology. Through the high-magnification TEM image (Fig. 3b), the particle arrangement in the fiber was disordered despite A-TiO₂ displayed the high degree of orientation on a macro scale. The average size of the particles embedded in the fibers was measured to be ca.11.0 nm, which is consistent with the previous outcomes of the XRD data illustrated in Fig. S3.† Furthermore, the HRTEM image of A-TiO₂ (Fig. 3c) uncovered the lattice fringes with d spacing of 0.36 nm, corresponding to the (101) lattice plane of anatase TiO₂. The selective area electron diffraction (SAED) pattern in the inset of Fig. 3c further certified that A-TiO₂ had a polycrystalline structure.

Fig. 3 (a and b) The TEM images of A-TiO₂ under different magnifications; (c) HRTEM image of A-TiO₂; the inset in the upper left is the selective area electron diffraction (SAED) pattern of A-TiO₂.

TG curves are performed to inspect the thermal stability of A-TiO₂ and N-TiO₂ nanofibers. As shown in Fig. 4a, both samples elucidated four obvious weight loss processes: (1) evaporation of moistures (before 100 °C); (2) evaporation of organic solvent in the metallic precursor, such as titanate, acetic acid and methyl alcohol (between 100 °C and 300 °C); (3) PVP decomposition (between 300 °C and 450 °C); (4) metal oxides residues (exceed 450 °C). A-TiO₂ exhibited an obvious less weight loss than N-TiO₂ before 300 °C, which was attributed to the less loss of both water and solvent. Compared to the traditional electrospun nanofibers, individual PVP/A-TiO₂ fibers between the parallel electrodes were allowed enough time to dry, because they hardly contact with each other on the collector.²⁸ Furthermore, the similar tendency of A-TiO₂ and N-TiO₂ between 300 °C and 800 °C illuminated that the thermal stability of TiO₂ have scarcely been influenced by improving the fibrous alignment.

Fig. 4 (a) Thermogravimetric analysis of the sample A-TiO₂ and N-TiO₂; (b) X-ray diffraction of the sample A-TiO₂ and N-TiO₂; (c) Raman spectra of the sample A-TiO₂ and N-TiO₂.

The crystal phase of the as-prepared samples was confirmed by XRD analyses. In Fig. 4b, all the diffraction peaks of A-TiO₂ and N-TiO₂ belonging to the anatase TiO₂ (JCPDS no. 21-1272). In addition, using {101} facet as a benchmark, relative intensity of {004}, {200}, {105} and {211} facet in the samples A-TiO₂ (I_A), N-TiO₂ (I_N) and the standard anatase TiO₂ (I) were calculated from XRD data (Table S2†). The order of them was summarized as follows: I_A < I < I_N, suggesting A-TiO₂ bears preferential oriented growth in the {101} direction. Interestingly, TiO₂ crystallites with exposed anatase {101} facets have been verified to possess relatively high photocatalytic activity for oxidative decomposition of organic compounds, because of the characteristics of the anatase {101} surface.³⁰

In Fig. 4c, Raman spectra once again proved that both samples possessed the similar TiO₂ characteristic waveform. Compared to N-TiO₂, the blue shift was observed in A-TiO₂ (the inset of Fig. 4c), because its excellent anisotropy changed the original compressive stress of TiO₂.³¹ This phenomenon led to the enhanced energy, and then the force constant would increase with the decrease in bond length of molecular, which would further increase the vibrational frequency. As a result, the bands of Raman spectrum were shifted to the higher frequency.

Electronic structures of A-TiO₂ and N-TiO₂ products were evaluated by XPS in Fig. 5. From the survey spectrum in Fig. 5a, it was observed that the peaks of Ti and O coexisted in both of the as-prepared samples. Fig. 5b showed the high-resolution Ti 2p spectrum of A-TiO₂ and N-TiO₂, and the binding energies of Ti 2p_3/2 and Ti 2p_1/2 peaks appeared at 458.8 eV and 464.7 eV, respectively. The exclusive presence of tetravalent titanium ion in the two samples was substantiated by the splitting energy of 5.9 eV between Ti 2p_3/2 and Ti 2p_1/2. In addition, the asymmetric peaks of A-TiO₂ appearing in O 1s spectrum in Fig. 5c were constituted by lattice oxygen at 530.1 eV and surface hydroxyl oxygen at 532.2 eV. According to both Ti 2p and O 1s spectrums, the peak position of A-TiO₂ did not shift compared with that of N-TiO₂, manifesting the highly aligned arrangement could barely affect the valence state and the electronic cloud distribution electronic structures of TiO₂.

Fig. 5 (a) XPS fully scanned spectra of sample A-TiO₂ and N-TiO₂; high resolution XPS spectrum for (b) Ti 2p, (c) O 1s for sample A-TiO₂ and N-TiO₂.

Fig. 6 compares the optical absorption properties of the synthesized samples in the UV-visible light region. It is evident that the absorption edge of both A-TiO₂ and N-TiO₂ occurred in the UV region. The data plots of absorption square versus energy in the absorption edge region are further estimated in the inset of Fig. 6. The square of absorption coefficient was linear with energy in the absorption edge region suggested that the absorption edges of A-TiO₂ and N-TiO₂ were caused by direct transitions.³² Band gap energies were deduced via extrapolating a straight line to the abscissa axis. The band gaps of A-TiO₂ and N-TiO₂ were estimated to be 3.09 eV and 3.34 eV, respectively. Compared with N-TiO₂, A-TiO₂ exhibited a narrower band gap and a higher absorption in the visible light absorption, due to its higher carbon content from XPS component analysis (Table S3†), which has been proved in the previous report.³³ The more compact structure of A-TiO₂ went against the heat transmission and the degradation of organics, and consequently improved the carbon content in A-TiO₂. But fortunately, the reduced band gap energy implied that A-TiO₂ was conducive to utilize the external photon in a wider range.³⁴ However, the absorption of A-TiO₂ was lower than that of N-TiO₂ in the UV light region. This phenomenon was attributed to the fact that the additional carbon covered the active sites in the surface of the original TiO₂ fibers, which would be unfavorable for the absorption of photon.

Fig. 6 UV-vis diffuse reflectance of the sample A-TiO₂ and N-TiO₂; the inset in the upper right is the square of absorption versus energy curve.

Photoelectrochemical and photocatalytic studies

In the past decades, 1-D TiO₂ have attracted intensive attentions in the fields of solar energy utilization due to its low cost, eco-friendliness, dimensional confinement and structurally well-defined physicochemical properties.^35–37 Therefore, the following experiments were carried out to verify the photoelectrochemical and photocatalytic performance of A-TiO₂. Fig. 7a shows the photocurrent of A-TiO₂ in 1 M Na₂SO₄ electrolyte and the comparison with N-TiO₂. Notably, A-TiO₂ possessed excellent photo-response and comparatively stable upon illumination. The average current density of A-TiO₂ (24.2 μA cm⁻²) was approximately twice higher than that of N-TiO₂ (13.2 μA cm⁻²). This result could be explained that the oriented structure of A-TiO₂ enhanced the charge transfer efficiency³⁸ and minimized photogenerated e⁻/h⁺ recombination by lessening the overlap area between fiber/fiber.¹⁴ Electrochemical impedance spectroscopy (EIS) was further measured in Fig. 7b. Under UV illumination, the diameter of the A-TiO₂ semicircle was smaller compared to that of N-TiO₂, due to the increased charge transfer and separation efficiency of e⁻/h⁺ pairs, suggesting A-TiO₂ would have a better photocatalytic performance than N-TiO₂.³⁹

Fig. 7 (a) The photocurrent vs. time curves of the sample A-TiO₂ and N-TiO₂ were measured at 0 V, in 1 M Na₂SO₄ electrolyte under illumination of 375 nm light at power density of 300.0 mW cm⁻²; (b) electrochemical impedance spectroscopy (EIS) were recorded in the range of 0.1 to 1000 Hz at a modulation frequency of 10 mV; (c) degradation profiles of RhB over the sample A-TiO₂, N-TiO₂ and P25; (d) kinetic linear simulation curves for the sample A-TiO₂, N-TiO₂ and P25.

The actual photocatalytic activities of A-TiO₂, N-TiO₂ and P25 (commercial TiO₂) are compared by the UV-light degradation of RhB aqueous solution (Fig. 7c). It was evident that the degradation rate of RhB by A-TiO₂ reached 95.4% after 180 min under UV light illumination, which was higher than both 84.4% by N-TiO₂ and 78.1% by P25 under the similar condition, demonstrating the betterment of these well-aligned TiO₂ nanofibers. The degradation kinetics by A-TiO₂, N-TiO₂ and P25 were further analyzed in Fig. 7d according to the k values, derived from ln(C₀/C) versus irradiation time (min) by the pseudo first-order equation. The photocatalytic rate of A-TiO₂ (k = 0.01629 min⁻¹) was 1.5 times faster than that of N-TiO₂ (k = 0.01012 min⁻¹), conforming to the previous result in Fig. 7a. All these studies suggest that the well-aligned TiO₂ nanofibers possess a better prospect of application in eliminating the organic contaminants than the traditional non-aligned TiO₂ nanofibers.

4. Conclusions

To conclude, highly aligned TiO₂ nanofibers has been successfully synthesized over large areas via an updated electrospinning configuration consisting of two parallel electrodes and one additional assistant electrode. In order to fabricate large-area and well-aligned nanofibers, the essential electrospinning parameters such as the applied voltages, spinneret-collector vertical distance, collector-assistant electrode horizontal distance, electrode gap width and electrospinning collection time were investigated, considering the fibrous order degree, sizes and density. The obtained aligned TiO₂ nanofibers, serving as an independent membrane, could be completely removed from the substrate due to its enough quantity and mechanical strength. Compared with the traditional N-TiO₂, A-TiO₂ exhibited the significant enhancement on both photoelectrochemical performance and photocatalytic degradation of RhB, which were attributed to the less fibrous overlap area, favorable crystal structure and increased charge transfer efficiency, profiting from its highly aligned structure. The aligned TiO₂ nanofibrous membranes fabricated with this updated electrospinning technique could be expected to be a promising material replacing the traditional TiO₂ for solar light utilization and organic pollutants degradation.

Acknowledgements

This work is supported by the China Postdoctoral Science Foundation (2015M582647) and the Fundamental Research Funds for the Central Universities of China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01606e

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