Photoactivity of hierarchically nanostructured ZnO–PES fibre mats for water treatments

Abstract

A brush-like ZnO nanorods shell is grown by Chemical Bath Deposition on electrospun Zn(Ac)₂ doped polyethersulfone fibres. The obtained mats are water resistant and photocatalytically active, thus resulting suitable for applications in water purification. The used procedure is simple, cost-effective and scalable to large volumes. The use of supported ZnO nanostructures onto fibrous mats paves the way to easily reusable photocatalyst that benefits the high surface area of nanostructures and limits the drawbacks associated to the use of nanoparticles and nanopowders (i.e. water turbidity, irradiation efficiency and photocatalyst recover), thus resulting suitable for use in membrane technology.

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Introduction

Membrane water treatment plays an important role in areas such as drinking water purification, brackish and seawater desalination, wastewater treatment and reuse, as it offers high stability and efficiency, low energy requirement, and easy operation.^1,2

Nowadays, filtration technology is a widely used approach for making a healthier and cleaner environment, especially if “smart” materials with multiple functions are used. Currently, polymeric membranes are mostly used for filtering applications due to their straightforward pore forming mechanism, higher flexibility, smaller footprints required for installation and relatively low costs if compared to inorganic membranes.^3,4 Accordingly, filtration mechanisms of these membranes are mostly based on mechanical driven sieving effects, thus no chemical removal of toxic substances can be achieved resulting also in fouling effects limiting membrane performances.

Recently, the emerging interest towards water management and control is demanding for development of advanced membranes for active water filtering, characterised by combined sieving and photocatalytic properties.^5,6 Advanced Oxidation Processing (AOP) based on heterogeneous photocatalytic oxidation represents an intriguing approach to purify water by converting a large number of organic pollutant molecules into less harmful compounds.⁷ This photoactivated oxidative process has several advantages such as eco-compatibility, low temperature and mild oxidation conditions and applicability to a wide selection of contaminants. However, the implementation of AOP at an industrial level requires to address several drawbacks related to (i) process scale up, (ii) compatibility with on-site facilities and materials, (iii) costs for chemicals, (iv) energy consumption and recovery of photocatalyst (if it is dispersed in water as nanoparticles or nanopowders). In this regard, it has to be remarked that the need of photocatalyst removal from water is a serious issue when the primary goal for water treatment is to obtain safe drinking water. Accordingly, a good photocatalyst must be photoactive to visible and/or near UV light, biologically and chemically inert, inexpensive and non-toxic.^8,9

Wide band gap semiconducting oxides, like ZnO and TiO₂ nanoparticles, possess photodegradation abilities (under ultra-violet irradiation) towards many organic pollutants present in water, due to formation of hydroxyl radicals by photoinduced electron pair generation inside the metal oxide.^10–12 They act as sensitizers for light-induced redox processes due to the electronic structure of the metal atoms, characterised by a filled valence band and an empty conduction band.¹³ Upon irradiation, electrons in the valence band are promoted to the conduction band leaving a hole behind. These electron–hole pairs can either recombine or interact separately with other molecules. The holes may react either with electron donors in the solution or with hydroxide ions to produce powerful oxidising species like hydroxyl (oxidation potential 2.8 V) or superoxide radicals.¹⁴ Although TiO₂ in the anatase form has been used for many environmental applications, ZnO is a suitable alternative because it absorbs a larger fraction of solar spectrum than TiO₂. The quantum efficiency of ZnO powder is also significantly larger than that of TiO₂ powder, with higher catalytic efficiencies reported in the literature for ZnO.¹⁵ The direct dispersion of ZnO or TiO₂ nanoparticles into water poses the problem of their recover. To overcome this limit these nanoparticles can be grafted onto standard ultrafiltration polymeric membranes to obtain a nanoparticles coating film over the polymeric surface. These hybrid polymeric–inorganic membranes are able to act as mechanical and chemical filters.^16–20

To enhance the efficiency of these hybrid polymeric–inorganic membranes improvements of surface area over supported nanoparticulate films are sought by the development of novel nanoarchitectures. Recently, the development of polymeric nanofibers covered by nanograins of TiO₂ was achieved by combining electrospinning with subsequent low temperature sputtering deposition.²¹ However, TiO₂ can mainly be obtained in form of nanoparticles or nanocrystalline layers unless suitable templates are used. This limits the range of nanostructures achievable. On the contrary, ZnO grows in a wide variety of nanostructures²² (nanorods, nanoneedles, nanosprings, nanoflowers etc.) using different approaches among them vapour phase deposition²³ or solution based growth,²⁴ which open a full range of options for improving the active surface area.

This work confirmed the potential of polymeric electrospun membranes as support for the growing of ZnO nanostructures.

In particular, Chemical Bath Deposition is an advantageous low-cost, simple, easily scalable and “green” strategy to grow ZnO nanostructures onto a wide variety of substrates, including electrospun polymeric fibres.^25–27

Nanocomposite electrospun nanofiber membranes are interesting for environmental remediation because of their high porosity and surface area presenting an extraordinary permeability (thereby an energy efficiency) and selectivity.²⁸ Electrospinning is advantageous over other methods for nanonofiber production for its easiness and industrial scalability.^29,30 Production yields up to 6.5 kg h⁻¹ are reported for multi-nozzle electrospinning from the company Finetex.³¹ These developments resulted in the commercial exploitation of filter systems based on electrospun nanofibers ranging from air to liquid filtration such as the products: FILTRIQ™, NF-MBRANE®, NanoTrap filter, SETA™, Fineweb™ etc.

Electrospun fiber diameters are several orders of magnitude smaller (e.g. submicron or nanometers) than those of fibres obtained by conventionally extrusion processes for commercial membranes (e.g. micrometers): accordingly, combination of electrospun polymeric nanofibres with ZnO nanostructures leads to fabrication of hierarchical nanostructured hybrid materials that benefit of flexibility and high surface area of the polymeric nanofibres as well as of additional functionalities of ZnO nanostructures.

ZnO nanostructures decorate a wide selection of electrospun polymeric fibres, such PVA,³² polyamide,³³ polyimide,³⁴ nylon,³⁵ and many other polymeric composites usually employed as electrospinning precursor solutions.

In this study, ZnO nanorods have been grown by a Chemical Bath Deposition process, on the surface of undoped and Zn(Ac)₂ doped polyethersulfone (PES) electrospun fibres. In fact, most of the commercial ultrafiltration membranes are made using polyethersulfone (PES) because of the excellent chemical and physical stabilities of these polymers.^36,37

We proposed a cost-effective, simple and easily scalable process both in term of materials choice, process time and cost, system versatility and, even more relevant, pollutants photodegradation (herein methylene blue (MB) dye). The obtained ZnO–PES hybrid mats were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectrometry (EDS), X-Rays Diffraction, Thermal analysis and their photocatalytic performances were evaluated by following methylene blue degradation under UV irradiation.

Results

Zn(Ac)₂ doped PES fibres have been electrospun on glass fibre support starting from a Zn(Ac)₂-PES solution (using DMF : toluene (1 : 1) as solvent).

SEM images in Fig. 1 show morphology of the obtained undoped and Zn(Ac)₂ doped PES electrospun mats and their related diameter distribution histograms.

Fig. 1 SEM images and related diameter distributions of electrospun (a, b) undoped and (c, d) Zn(Ac)₂ doped PES fibres (scale bar 10 μm).

Fibre morphology is not significantly affected by the presence of the zinc dopant. The average fibre diameter is 539 nm ± 223 nm for undoped PES fibres and 481 nm ± 263 nm for Zn(Ac)₂ doped fibres.

The successful inclusion of Zn(Ac)₂ into the polymeric fibres is validated by the presence of Zn signal in EDX analysis of as spun fibres (not shown) and confirmed by the possibility to obtain pure ZnO nanofibres after annealing at T > 400 °C (Fig. 2).^38–40

Fig. 2 ZnO fibres formed after annealing at 400 °C of Zn(Ac)₂ doped PES fibres.

TGA analysis shows an increased thermal stability of these mats due to the presence of Zn(Ac)₂ which causes a considerable rise in the initial decomposition temperature (T_i) (Fig. 3).

Fig. 3 TG degradation curves, at 10 °C min⁻¹, in air of undoped (black line) and Zn(Ac)₂ doped (red line) PES fibres.

In particular, for undoped fibres degradation starts at ∼390 °C and finishes at ∼590 °C, following a two steps pathway. On the contrary, the maximum weight loss of Zn(Ac)₂ doped PES fibres occurs in one step between 540 °C and 590 °C, thus pointing at a delayed degradation of the polymeric component. We can rationalize this effect by considering the high thermal conductivity and heat capacity of ZnO nanofibres formed during annealing (see Fig. 2),⁴¹ thus responsible for the delay of PES degradation.⁴² The residue weight percentage calculated for Zn(Ac)₂ doped fibres degraded in air is higher than that obtained for undoped PES fibres, due to formation of ZnO fibres concomitant to PES degradation (see Fig. 2).

Zn(Ac)₂ doped PES fibres have been dipped in Zn(Ac)₂ aqueous solution (0.15 M) for a time ranging from 30 minutes to 1 hour and then dried in oven overnight at 110 °C. Zinc acetate hydrolysis is responsible for the formation onto the fibres surface of an insoluble zinc hydroxide precipitate, partially converted into ZnO by the following thermal treatment (Fig. 4).⁴³

Fig. 4 XRD of Zn(Ac)₂ doped PES fibres before and after seeding and annealing at 110 °C overnight.

In the XRD patterns reported in Fig. 4, the peaks at 31.73°, 34.45° and 36.24° are associated to (100), (002) and (101) ZnO reflections, while the one at 33.05° is related to the presence of trace of ε-Zn(OH)₂.⁴⁴

Many authors used preformed ZnO nanoparticles, prepared by hydrothermal approaches, as crystal seeds to grow ZnO nanorods by CBD.^45,46 Herein, we can easily demonstrate that Zn(Ac)₂ aqueous solutions are suitable to form an effective seed layer to promote ZnO nanorods (ZnO NRs) growth on PES fibres.²⁵ This approach benefits the absence of other contaminants (i.e. organic ligands) in the seeding solution.

After seeding, CBD growth onto seeded fibres is performed in a nutrient bath containing Zn(Ac)₂ and EDA aqueous solutions (0.025 M) at 80 °C for 3 hours.

The special support, designed to dip at constant height of PES mats into Zn(Ac)₂ seed solution as well as CBD nutrient bath, is shown in Fig. 5. The position inside the nutrient solution is kept controlled by placing the mats (Fig. 5a) in a holder (Fig. 5b) kept at a fixed position inside the bath (Fig. 5c). This approach allows for obtaining reproducible conditions for the preparation of the functionalized mats.

Fig. 5 Experimental set-up to treat Zn(Ac)₂ doped PES fibres.

Noteworthy, this procedure can be easily scaled up to large fibre mats dimension, compatible with the used roll-to-roll electrospinning procedure (Fig. 5a).

SEM images of fibrous mats after CBD growth, upon varying the seeding time (Fig. 6), clearly indicate that short seeding times do not lead to an effective CBD growth.

Fig. 6 CBD growth of ZnO NRs onto Zn(Ac)₂ doped PES fibres pre-treated using different seeding time: (a) 30 minutes and (b) 60 minutes. Scale bar is 10 μm.

In particular, short seeding time (30 minutes) is not enough to promote a massive growth of ZnO NRs onto the surface of polymeric fibres, as clearly visible in Fig. 6a. ZnO NRs do not uniformly cover fibres surface: in fact, the initial fibres morphology is still visible and the overall fibres dimension and mat porosity are not significantly varied. On the contrary, by prolonging the seeding time (60 minutes) a homogeneous coverage of ZnO NRs is achieved, as clearly shown in Fig. 6b. The mat morphology is now significantly changed, large and hierarchically complex fibres are visible and, due to the presence of ZnO NRs external shell, the voids within fibres entanglement are reduced.†

Fig. 7 shows high and low magnification SEM images of brush-like ZnO NRs grown on Zn(Ac)₂ doped and undoped PES fibres: ZnO nanostructures grown on Zn(Ac)₂ doped PES (Fig. 7a and a′) are larger than those grown on undoped PES fibres (Fig. 7b and b′).

Fig. 7 High magnification (scale bar 200 nm) and low magnification (scale bar 1 μm) SEM images of ZnO NRs on (a, a′) Zn(Ac)₂ doped PES and (b, b′) undoped PES (after 1 hour seeding).

Noteworthy, dimensions of ZnO NRs modify the overall diameter of Zn(Ac)₂ doped PES fibres after CBD process, as clearly evident by comparison of Fig. 7a′ and 7b′.

EDS analysis further confirms the growth of smaller ZnO nanostructures onto undoped PES fibre mats, thus resulting in an external shell thinner than the one grown on the Zn(Ac)₂ doped PES fibrous mats. Fig. 8 shows the EDS spectrum of the fibrous mats after CBD growth: the C and S signals related to PES core are still visible for undoped samples (black line), while for Zn(Ac)₂ doped PES fibre mat the thick ZnO shell hinders the contribution of the polymeric internal core and only ZnO shell related signals are visible in the recorded spectrum (red line).

Fig. 8 EDS spectra of undoped (black line) and Zn(Ac)₂ doped PES fibre (red line) mats after CBD growth.

However, Zn(Ac)₂ presence inside PES fibres does not affect the crystallinity of ZnO nanorods in the external shell: in fact XRD patterns of polymeric fibres after CBD growth, shown in Fig. 9, possess all reflection peaks indexed to hexagonal wurtzite ZnO (JCPDS card no. 36-1451).

Fig. 9 XRD patterns of undoped (black line) and Zn(Ac)₂ doped (red line) electrospun fibre mats after CBD process.

These brush-like ZnO nanorods show XRD patterns displaying a more intense (101) diffraction peak than the (002) one due either to the growth of nanostructures with {10−10} planes as basal facets or related to signal arising from tilted oriented nanorods with respect to supporting surface orientation.²³ In fact, we have already observed that CDB growth (in similar experimental conditions) on nanostructured ZnO seed layers obtained by MOCVD results in c-axis preferentially oriented nanostructures.²⁴

Thermogravimetric analysis (TGA) of fibre mats after CBD growth is reported in Fig. 10.

Fig. 10 TG degradation curves, at 10 °C min⁻¹, in air of undoped (black line) and Zn(Ac)₂ doped (red line) PES fibres after CBD growth of ZnO NRs brush-like shell.

ZnO CBD growth further improves thermal stability of polymer: moreover, the massive growth of ZnO nanostructures onto polymeric scaffold is confirmed by the significant decrease of polymeric fraction with respect to ceramic one as indicated by the faint degradation yield, now reduced at less than 10% (see, for comparison, TG data reported in Fig. 3). From these data we can estimate the weight percentage of the external ZnO shell with respect to the polymeric contribution, important for evaluation of photocatalytic properties of this ceramic–polymeric material.

Photocatalytic tests

The use of these hybrid oxide–polymer mats as active materials for water purification has been tested by evaluating their capability to degrade MB aqueous solutions.^47–51 Photodegradation of MB is evaluated by monitoring its absorption spectra as a function of irradiation time in the presence of bare and ZnO NRs decorated fibres: in particular, normalized change in its concentration (C₀/C) upon varying irradiation time is reported to determine the degradation efficiency. The standard procedure used requires to test the MB solution degradation, in presence of photocatalytic material, under dark to evaluate the effect of dye physisorption. Our experiments have been started after running overnight a control test in dark. The MB concentration remains almost unchanged (less than 1% variation of the initial absorbance, using 1 × 10⁻⁵ M MB solution). It is important to note that the hydrophilicity of Zn(Ac)₂ doped PES mats changes significantly in presence or in absence of ZnO nanorods external shell, as shown in Fig. 11.

Fig. 11 Quartz cuvettes used to irradiate MB aqueous solutions containing (a) bare and (b) ZnO NRs decorated Zn(Ac)₂ doped PES nanofibres.

In fact, as clearly visible in Fig. 11a, in the absence of ZnO shell the hydrophobic mat (measured water contact angle = 128°) remains at solution surface totally crumpled, to minimize the contact with aqueous solution: consequently, exposed surface area and light absorption are significantly limited. Accordingly, we can expect a reduction of catalytic performances, strongly dependant on exposed surface area. On the contrary, in presence of ZnO nanorods external shell (Fig. 11b) the mat remains well stretched in solution and can be easily dipped in dye aqueous solution. This behaviour can be related to the increased mat density due to the presence of ZnO NR external shell associated to the improved wettability (measured WCA = 106°).

Fig. 12 reports the normalized (C/C₀) methylene blue (MB) degradation after exposure to UV light.

Fig. 12 Photocatalytic MB degradation under UV light irradiation.

The measurements were performed at neutral pH:³⁸ at pH close to 7, ZnO surface is positive (since the pH of the zero point of charge of ZnO is 9.0) and accordingly only a faint electrostatic attraction between the photocatalyst and the cationic dye is expected.

Zn(Ac)₂ doped PES mats (red dots) show an improved photocatalytic activity with respect to undoped PES mats (blue triangles), comparable with that observed on TiO₂ covered PES fibres (thus providing another important insight to develop photocatalytic materials by controlling costs and process complexity).²¹

After ZnO NRs CBD growth, photocatalytic activity increased and a MB degradation of 78% is observed. The obtained results are well reproducible, thus validating the possibility to scale up the solution growth at larger volumes. Noteworthy, MB solution remains clear during the overall degradation experiment and no filtration is required to monitor the photocatalytic degradation of MB. This aspect is important, as highlighted by a recent review focused on zinc oxide based photocatalyst for water treatments, that discusses about drawbacks associated to solution turbidity and agglomeration phenomena caused by high ZnO nanoparticles loading.⁴⁷ The present approach, on the contrary, allows for triggering the dimensionality of external nanorod to increase the photocatalyst loading without affecting solution clearness (work is on-going to optimize CBD growth to guarantee the highest photocatalytic activity).

Another important result attains the possibility to reuse, after washing in water and drying overnight at 100 °C, these photocatalytic mats without a significant reduction (about 10%) of their efficiency (Fig. 12, purple triangles).

Experimental

Materials

Commercial polyethersulphone (PES) with a molecular mass of about 20 000 Da has been selected as material for membrane production because of its widely accepted use in the membrane field. Zinc acetate dehydrate (Zn(Ac)₂·2H₂O), dimethylformamide (DMF), toluene and ethylenediamine (EDA) were purchased from Sigma-Aldrich.

PES fibres production

PES fibres were produced by Electrospinning (NANON-01A MECC CO., LTD) starting from 0.25 g ml⁻¹ toluene : DMF (1 : 1) solution: the solution has been magnetically stirred until the polymer was completely solubilized by heating. Zn(Ac)₂ doped PES solution was prepared by firstly dissolving 1 g of Zn(Ac)₂·2H₂O in 10 ml of toluene : DMF (1 : 1) mixture. 2.5 g of PES were added only after the complete dissolution of the salt was achieved. The Zn(Ac)₂/PES solution was kept under stirring for 12 hours. 5 ml of this solution were loaded into a syringe and an electrical field of several kV cm⁻¹ applied to the drop ejected from the needle. The fibres produced were collected on a glass fibres support.

Thermogravimetric analysis

Thermal degradations were performed in a Mettler Thermogravimetric Analyzer TGA 1 Star System. The temperature calibration of equipment was made according to the method suggested by Mettler and reported elsewhere.⁵² Samples of about 7 mg were put into open alumina crucibles and heated in the temperature range 25–700 °C, at the heating rate of 10 °C min⁻¹, in static air atmosphere. In order to correct the error in the mass determination due to the reduction of the buoyancy force on increasing temperature, we used the blank method, recommended by the ICTAC Kinetics Committee and extensively reported by Vyazovkin et al.⁵³ A thermogravimetric (TG) run with an empty pan (blank) was preliminarily performed in the same experimental conditions used for samples. The so obtained blank curve was subtracted from those of samples, so obtaining corrected degradation TG curves. At the end of each experiment these data were used to plot the percentage of undegraded sample, (1 − D)%, as a function of temperature, where D = (W_o − W)/W_o, and W_o and W were the masses at the starting point and during scanning.

Chemical bath deposition process

ZnO nanorods were grown onto the surface of electrospun PES fibres by a Chemical Bath Deposition (CBD) process.²⁴ Seeding procedure consists of pre-treatment of PES based mats in Zn(Ac)₂/H₂O solution (0.5 M). A proper support has been used to allow for a repeatable sample positioning inside the seeding solution. Dipping time ranges from 30 minutes to 1 hour. After dipping, fibres were dried in an oven at 110 °C overnight.

Seeded fibres were immersed, using the same support employed for seeding procedure, in a 1 : 1 Zn(Ac)/H₂O and EDA solution (0.025 M), and kept magnetically stirred at 80 °C for 3 h. Finally, core shell ZnO NR/PES fibres were washed with distilled water and dried at 110 °C for 1 h.

Sample characterization

Size and morphology of the aligned ZnO nanorods were observed by ZEISS SUPRA-55 VP Field Emission Scanning Electron Microscopy equipped with an Oxford EDS solid state detector for Energy Dispersed X-rays Spectroscopy (EDS). The crystallographic structure of ZnO nanorods was analyzed by a X-ray Diffractometer (XRD, Brucker theta/2 theta geometry).

MB photocatalytic degradation

Doped and Zn(Ac)₂ doped PES mats (12 mg) were dipped in a quartz cuvette containing methylene blue (MB) aqueous solution (3 ml, 1 × 10⁻⁵ M, pH = 7) and irradiated by UV lamp (Black-RayB-100 A, 365 nm).⁵⁴ The cuvette was covered by a box wrapped around by an aluminium foil for reflection of UV light back into cuvette. The irradiated solution was measured at regular time intervals with an UV-VIS spectrophotometer (JASCO V-630) in a wavelength range of 200–800 nm.

The degradation of MB was evaluated by the absorbance peak at 664 nm in the Lambert–Beer regime. The photodegradation rate was calculated from the linear plot of ln(C₀/C) versus the irradiation time. The decomposition of the MB dye in the absence of any photocatalyst materials was checked as a reference. Control experiments in the dark overnight were conducted to clarify the contribution of the adsorption of the MB at the sample surface.

Conclusions

Zn(Ac)₂ doped PES fibrous mats are obtained by roll-to-roll electrospinning of polymer solutions doped with zinc acetate. The presence of Zn acetate contributes to thermally stabilize the polymeric composite, due to formation of ZnO during thermal treatments. An easy and cost effective seeding strategy allows for a high yield growth of ZnO nanostructures onto the obtained polymeric mats. In particular, brush-like ZnO nanorods are grown on the obtained sub-micrometer PES fibres by Chemical Bath Deposition to obtain highly nanostructured surfaces with a high level of crystallinity. These hierarchical mats are water resistant and photocatalytic actives. All these aspects, combined with the process simplicity and scalability to larger volumes are extremely important in the perspective of fabrication of active filters for water purification.

Acknowledgements

Authors would like to acknowledge Bio-nanotech Research and Innovation Tower (BRIT) project that supports this research activity.

Notes and references

1. Y. Cao, J. Chen, H. Zhou, L. Zhu, X. Li and Z. Cao, Nanotechnology, 2015, 26, 024002.
2. C. Zhao, J. Xue, F. Ran and S. Sun, Prog. Mater. Sci., 2013, 58, 76–150.
3. L. Y. Ng, A. W. Mohammad, C. P. Leo and N. Hilal, Desalination, 2013, 308, 15–33.
4. G. M. Geise, H. Lee, D. J. Miller, B. D. Freeman, J. E. Mcgrath and D. R. Paul, J. Polym. Sci., Part B: Polym. Phys., 2010, 48, 1685–1718.
5. S. Leong, A. Razmjou, K. Wang, K. Hapgood, X. Zhang and H. Wang, J. Membr. Sci., 2014, 472, 167–184.
6. M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Marinãs and A. M. Mayes, Nature, 2008, 452, 310.
7. R. Andreozzi, V. Caprio, A. Insola and R. Marotta, Catal. Today, 1999, 53, 51–59.
8. X. Chang, Y. Zhang, M. Tang and B. Wang, Nanoscale Res. Lett., 2013, 8, 51.
9. L. K. Adams, D. Y. Lyon and P. J. J. Alvarez, Water Res., 2006, 40, 3527–3532.
10. J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo and D. W. Bahnemann, Chem. Rev., 2014, 114, 9919–9986.
11. D. Beydoun, R. Amal, G. Low and S. McEvoy, J. Nanopart. Res., 1999, 1, 439–458.
12. M. A. Lazar, S. Varghese and S. S. Nair, Catalysts, 2012, 2, 572–601.
13. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahenemann, Chem. Rev., 1995, 95, 69–96.
14. W. Z. Tang and A. Huren, Chemosphere, 1995, 31, 4157–4170.
15. S. Vhakrabarti and B. K. Dutta, J. Hard Mater., 2004, 112, 269–278.
16. Y. Zhang, M. Lee, S. An, S. Sinha-Ray, S. Khansari, B. Joshi, S. Hong, J. Hong, J. Kim, B. Pourdeyhimi, S. S. Yoon and A. L. Yarin, Catal. Commun., 2013, 34, 35–40.
17. S. Homaeigohar and M. Elbahri, Materials, 2014, 7, 1017–1045.
18. E. Cossich, R. Bergamasco, M. T. Pessoa de Amorim, P. M. Martins, J. Marques, C. J. Tavares, S. Lanceros-Méndez and V. Sencadas, Mater. Chem. Phys., 2015, 164, 91–97.
19. I. H. Chowdhury, S. Ghosh and M. K. Naskar, Ceram. Int., 2016, 42, 2488–2496.
20. Y. Chen, C. Kuo, B. Chen, P. Chiu and P. Tsai, J. Polym. Sci., Part B: Polym. Phys., 2015, 53, 262–269.
21. A. Alberti, C. Bongiorno, G. Pellegrino, S. Sanzaro, E. Smecca, G. G. Condorelli, A. E. Giuffrida, G. Cicala, A. Latteri, G. Ognibene, A. Cassano, A. Figoli, C. Spinella and A. La Magna, RSC Adv., 2015, 5, 73444–73450.
22. Z. L. Wang, Mater. Today, 2004, 7, 26–33.
23. M. E. Fragalà, A. Di Mauro, G. Litrico, F. Grassia, G. Malandrino and G. Foti, CrystEngComm, 2009, 11, 2770–2775.
24. M. E. Fragalà, Y. Aleeva and G. Malandrino, Thin Solid Films, 2011, 519, 7694–7701.
25. T. J. Athauda, U. Butt and R. R. Ozer, RSC Adv., 2013, 3, 21431.
26. H. J. Kim, H. Raj Pant, C. Hee Park, L. D. Tijing, N. Jung Choi and C. Sang Kim, Ceram. Int., 2013, 39, 3095–3102.
27. W. Teo and S. Ramakrishna, Compos. Sci. Technol., 2009, 69, 1804–1817.
28. S. Homaeigohar and M. Elbahri, Materials, 2014, 7, 1017–1045.
29. S. Ramakrishna, K. Fujihara, W.-E. Teo, T. Yong, Z. Ma and R. Ramaseshan, Mater. Today, 2009, 9, 40–50.
30. J. Sundaramurthy, N. Li, P. S. Kumar and S. Ramakrishna, Biofuel Res. J., 2014, 2, 44–54.
31. C. J. Luo, S. D. Stoyanov, E. Stride, E. Pelan and M. Edirisinghe, Chem. Soc. Rev., 2012, 41, 4708–4735.
32. A. Gupta, D. V. Nandanwar and S. R. Dhakate, Adv. Mater. Lett., 2015, 6, 706–710.
33. T. J. Athauda, U. Butt and R. R. Ozer, MRS Commun., 2013, 3, 51–55.
34. Z. Chang, Chem. Commun., 2011, 47, 4427–4429.
35. H. J. Kim, H. R. Pant, C. Park, L. Tijing and N. Choi, Ceram. Int., 2013, 39, 3095–3102.
36. L. Abate, I. Blanco, G. Cicala, R. La Spina and C. L. Restuccia, Polym. Degrad. Stab., 2006, 91, 924–930.
37. L. Abate, I. Blanco, G. Cicala, G. Recca and A. Scamporrino, Polym. Eng. Sci., 2010, 49, 1477–1483.
38. A. Di Mauro, M. Zimbone, M. Scuderi, G. Nicotra, M. E. Fragalà and G. Impellizzeri, Nanoscale Res. Lett., 2015, 10, 484–490.
39. A. Di Mauro, M. Zimbone, M. E. Fragalà and G. Impellizzeri, Mater. Sci. Semicond. Process., 2016, 42, 98–101.
40. X. Yang, C. Shao, H. Guan, X. Li and J. Gong, Inorg. Chem. Commun., 2004, 7, 176–178.
41. X. W. Sun, Z. J. Liu, Q. F. Chen, H. W. Lu, T. Song and C. W. Wang, Solid State Commun., 2006, 140, 219–224.
42. S. Masia, P. D. Calvert, W. E. Rhine and H. K. Bowen, J. Mater. Sci., 1989, 24, 1907–1912.
43. E. A. Meulenkamp, J. Phys. Chem. B, 1998, 102, 5566.
44. J. Wang and L. Xiang, J. Cryst. Growth, 2014, 401, 279–284.
45. J. Zhao, Z. GuoJin, T. Li and X. Liu, J. Eur. Ceram. Soc., 2006, 26, 2769–2775.
46. G. Kenanakis, D. Vernardou, E. Koudoumas and N. Katsarakis, J. Cryst. Growth, 2009, 311, 4799–4804.
47. K. M. Lee, C. W. Lai, K. S. Ngai and J. C. Juan, Water Res., 2016, 88, 428–448.
48. F. Xu, Y. Shen, L. Sun, H. Zeng and Y. Lu, Nanoscale, 2011, 3, 5020–5025.
49. T. Reimer, I. Paulowicz, R. Roder, S. Kaps, O. Lupan, S. Chemnitz, W. Benecke, C. Ronning, R. Adelung and Y. K. Mishra, ACS Appl. Mater. Interfaces, 2014, 6, 7806–7815.
50. S. Kuriakose, B. Satpati and S. Mohapatra, Phys. Chem. Chem. Phys., 2015, 17, 25172–25181.
51. Y. K. Mishra, G. Modi, V. Cretu, V. Postica, O. Lupan, T. Reiner, I. Paulowicz, V. Hrkac, W. Benecke, L. Kienle and R. Adelung, ACS Appl. Mater. Interfaces, 2015, 7, 14303–14316.
52. I. Blanco, F. A. Bottino and L. Abate, Thermochim. Acta, 2016, 623, 50–57.
53. S. Vyazovkin, A. K. Burnham, J. M. Criado, L. A. Pérez-Maqueda, C. Popescu and N. Sbirrazzuoli, Thermochim. Acta, 2011, 520, 1–19.
54. A. Mills, Appl. Catal., B, 2012, 128, 144–149.

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Footnote

† Noteworthy, SEM analysis has been performed without any sample metallization: accordingly, if ZnO nanorods do not efficiently cover PES fibres, charging effect arises (Fig. 6a). On the contrary, a massive growth of ZnO nanorods totally avoids accumulation of static electric charges on the PES fibre mats surface (Fig. 6b).

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