Polylactic acid Fiber

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 28955−28962

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Melt Electrospun Reduced Tungsten Oxide /Polylactic Acid Fiber Membranes as a Photothermal Material for Light-Driven Interfacial Water Evaporation Tolesa Fita Chala, Chang-Mou Wu,* Min-Hui Chou, and Zhen-Lin Guo Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607 Taiwan, R.O.C. ACS Appl. Mater. Interfaces 2018.10:28955-28962. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/18/18. For personal use only.

S Supporting Information *

ABSTRACT: The development of efficient photothermal materials is the most important issue in solar water evaporation. In this work, melt electrospun reduced tungsten oxide/polylactic acid (WO2.72/PLA) fiber membranes were successfully prepared with improved near-infrared (NIR) photothermal conversion properties owing to strong NIR photoabsorption by the metal oxide. WO2.72 powder nanoparticles were incorporated into PLA matrix by melt processing, following which the composites were extruded into wires using a single screw extruder. Subsequently, fiber membranes were prepared from the extruded wire of the WO2.72/PLA composite by melt electrospinning, which is a cost-effective technique that can produce fiber membranes without the addition of environmentally unfriendly chemicals. The melt electrospun WO2.72/PLA fiber membranes, floatable on water due to surface hydrophobicity, were systematically designed for, and applied to, vapor generation based on the interfacial concept of solar heating. With the photothermal WO2.72/PLA fiber membrane containing 7 wt % WO2.72 nanoparticles, the water evaporation efficiency was reached 81.39%, which is higher than that for the pure PLA fiber membrane and bulk water. Thus, this work contributes to the development of novel photothermal fiber membranes in order to enhance light-driven water evaporation performance for potential applications in the fields of water treatment and desalination. KEYWORDS: tungsten trioxide (WO3), polylactic acid (PLA), melt electrospinning, photothermal conversion, water evaporation energy economization.16 Hua et al. reported that NIR and visible (vis) light absorbing materials are desirable for solar water evaporation application because NIR and vis lights compose nearly 40% of total solar energy. NIR light responsive photothermal materials are attracting a great attention because the wavelength of NIR is larger than most water drops in cloud and thus avoid the blocking of cloud much easier than visible light. As a result, the energy harvesting from NIR light is promising for steam generation.2,17 In addition, Wang et al. employed Cu7S4 nanocrystals for water evaporation application under an infrared light irradiation because of excellent photothermal conversion properties of Cu7S4 nanocrystals in the NIR region.10 Hence, the development of appropriate NIRabsorbing solar radiation materials is considered to be a rational strategy to efficiently harvest solar energy for vapor generation via heat localization.18 Currently, a novel technique called “air−water interface solar heating” has attracted immense interest for water evaporation. In this process, lightabsorbing materials float on water and heat only the surface while avoiding homogeneously heating the bulk water. Target heating at the air−water interface can generate a sharper

1. INTRODUCTION Solar energy conversion is the most important aspect of solar energy harvesting technologies, enabling the effective utilization of renewable, clean, and sustainable energy for the development of human society.1−6 Water evaporation is an interfacial process, an essential part of the global water cycle, and can be accelerated using solar radiation as a source of heat, which is free and abundantly available on the earth. It is also an endothermic process, in which heat energy is absorbed.7,8 Recently, utilization of efficient solar energy for the production of other forms of energy is a critical factor for variety of applications, such as a solar photothermal autoclave for sterilization, desalination, and power generation.9,10 A material that absorbs light energy and converts it to thermal energy in a process also known as photothermal conversion can be employed for water evaporation.11,12 Photothermal materials can be harnessed for water evaporation to generate heat under light irradiation, further improve the efficiency of water evaporation without affecting water quality.2,13 Among several active photothermal nanomaterials, those tungsten oxide based are of particular interest owing to their strong local surface plasma resonances (LSPRs), which give rise to strong photoabsorption in a broad wavelength range of the nearinfrared (NIR) region.14,15 They are also suitable for solar energy collectors, heat-ray shielding, smart windows, and © 2018 American Chemical Society

Received: May 7, 2018 Accepted: July 27, 2018 Published: July 27, 2018 28955

DOI: 10.1021/acsami.8b07434 ACS Appl. Mater. Interfaces 2018, 10, 28955−28962

Research Article

ACS Applied Materials & Interfaces

ductivity, thermal resistivity, and thermal absorptivity of the fiber membranes, were discussed in this study.

localized temperature and realize a more efficient water evaporation efficiency, whereas a bulk water heating system delivers poor efficiencies.19−23 The physicochemical stability and increasing availability of transition-metal oxide nanostructures have also attracted great attention. In particular, there is interest in oxygen-deficient non-stoichiometric metal oxides such as WO3−x, MoO3−x, and TiOx whose strong photoabsorption properties in a broad wavelength region make them suitable for converting light energy to thermal energy to generate steam power.4,24,25 Thus, the WO2.72/polylactic acid (PLA) composite fiber membrane is a suitable candidate for the process of photothermal water evaporation because of its efficient light-to-heat conversion, strong NIR absorption ability, and floatability on water due to surface hydrophobicity.26 Previously, numerous photothermal materials with strong light absorbance in the solar spectrum range, including noble metal nanoparticles,13,27−29 conducting polymers of polypyrrole,30,31 carbon-based materials, graphene oxide, carbon fibers, carbon nanotubes,32−35 MXene,36 black metal oxides,37 and plasmonic absorbers,38,39 have been intensively studied and found to exhibit greater evaporation rate than that of bulk water under irradiation. However, to the best of our knowledge, melt electrospun WO2.72/PLA fiber membrane photothermal material has not yet been reported for light-driven water evaporation. Fibers are currently used in a variety of engineering applications in which a fibrous structure and a large surface area are required.40 A variety of materials, such as hybrid, inorganic−organic, or polymer composites with nanofibers, have been prepared using electrospinning techniques.41,42 Electrospinning techniques is an attractive method for producing ultrathin fibers with diameters ranging from the micro- to nanoscale.43,44 There are two types of electrospinning methods: Melt electrospinning and solution electrospinning, in which both microscale and nanoscale fibers are produced, respectively.45,46 Solution electrospinning requires a solvent to dissolve a given polymer type, whereas melt electrospinning involves the heating of the polymer to its melting point to produce a viscous fluid which can be drawn into fibers.47,48 Melt electrospinning holds some benefits over solution electrospinning in tissue engineering applications49 because this technique is free of toxic solvents, safer, more environmentally friendly, and economical and is a controllable method for producing higher efficiency nano-/ microfibers, compared to solution electrospinning.50−53 Therefore, melt electrospinning has recently been gaining more attention and a limited number of research studies have been conducted using this technique. In this work, reduced tungsten oxide (WO2.72) nanoparticles were incorporated into PLA by a melt compounding process and extruded into a wire using a single screw extruder. Subsequently, novel fiber membranes were fabricated from the extruded wire of WO2.72/PLA by melt electrospinning. Tungsten oxide nanoparticles were rationally selected as a photothermal material because of their strong absorption ability in the NIR region due to unusual oxygen defect structure and light to heat conversion properties, whereas PLA polymer is used as a matrix for fabrication of a fiber membrane because PLA is an eco-friendly polymer and can be easily electrospun into fiber membrane by melt possessing. The prepared photothermal melt electrospun WO2.72/PLA fiber membrane was employed for light-driven water evaporation via air−water interface solar heating. In addition, photothermal conversion and thermal properties, such as thermal con-

2. MATERIALS AND METHODS Tungsten trioxide (WO3) was obtained from Advanced Ceramics Nanotech Co. Ltd, Taiwan (molecular weight = 231.84 g/mol). PLA pellets (PLA 6210D, NatureWorks, USA) were used in this study. 2.1. Preparation of Melt Electrospun WO2.72/PLA Fiber Membrane. The preparation and characterization of WO2.72 nanoparticles were reported in our previous work.54 To prepare the melt electrospinning WO2.72/PLA fiber membrane first, PLA pellets were dried in a vacuum oven at 80 °C overnight to remove moisture prior to blending. Then, 4 and 7 wt % WO2.72 powders were meltblended with PLA using a twin-screw extruder (T-BEG-02042, Taiwan). The extruder temperature was set to 190 °C with a screw rotation speed of 17 rpm, and the melt-compounded sample was cooled in a water bath and pelletized. Subsequently, the pelletized composite samples were also dried in a vacuum oven at 80 °C for 12 h, and WO2.72/PLA wires were obtained using a single-screw extruder (FS-50J, Taiwan). Finally, using melt electrospinning, the WO2.72/ PLA wires were fabricated into fiber membranes under the following electrospinning conditions: nozzle temperature: 260 °C, electric field: 4 kV/cm, drum rotation speed: 100 rpm, and extrusion rate: 0.013 g/ min. The diagram of the melt electrospinning system developed in this work is shown in Figure 1.55 The as spun fiber membranes were

Figure 1. Image of the melt electrospun system (inset: processed fiber membrane). heat treated at 80 °C for 10 min to release the residual stress caused by stretching during fiber spinning and shows some shrinkage. Then, the thermal-treated fiber membranes become stable and never shrinkage under infrared light irradiation. 2.2. Water Evaporation Performance Measurement. The measurement of water evaporation performance was carried out under infrared light irradiation at a room temperature of 25 °C and a relative humidity of about 50%. First, the membrane sample with a diameter of 5 cm and an areal density of 57.1 g/m2 was placed on the surface of water in a 100 mL beaker and was irradiated by infrared light vertically from above with a light intensity of 0.294 W cm−2. In order to reduce the thermal loses, the beaker was wrapped by a polystyrene foam as a heat-insulating materials.56,57 The temperature distribution of the fiber membrane floating on water was recorded using an IR camera, and the weight loss of water due to evaporation was recorded using an electronic analytical balance (XS 105 Dual Range). Finally, the evaporation rate of water was determined by recording the mass change as a function of time. 2.3. Characterization. The morphology of the melt electrospun fiber membranes was analyzed by scanning electron microscopy (SEM; JSM-6390, Japan) and transmission electron microscopy (TEM, JEOL JEM-2010). To prepare the TEM samples, the fiber membrane was embedded in epoxy resin and after curing, the prepared samples were cut with an ultramicrotome for imaging (Leica EM UCT6 and EM FC6 and EM KMR2, Germany). The optical 28956


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ACS Applied Materials & Interfaces responses were measured in the wavelength range 300−2500 nm with a UV−vis−NIR-spectrophotometer (JASCO V-670, Keith Link Technology, Taiwan). The photothermal conversion properties of the fiber membrane were investigated under irradiation of 150 W infrared lamp, and temperature profile was monitored using an IR thermographic camera (FLIR P384A3-20, CTCT, Co. Ltd, Taiwan), which is shown in Figure S1. Contact angles of the fabricated melt electrospun fiber membrane were measured using a contact angle measuring device (contact angle goniometer, Sindatek model 100SB, Taiwan) at ambient temperature. Thermal properties such as thermal conductivity, thermal absorptivity, and thermal resistivity of the fiber membrane were also evaluated using an Alambeta device (Sensora Instruments, Czech Republic).

Figure 3. (a) TEM image and (b) HRTEM image of WO2.72/PLA nanofiber with 7 wt % WO2.72 nanoparticles.

3. RESULTS AND DISCUSSION 3.1. Morphology of the Melt Electrospun WO2.72/PLA Fiber Membranes. Representative images of the morphology of the melt electrospun WO2.72/PLA fiber membrane, containing 0, 4, and 7 wt % of WO2.72 nanoparticles, are shown in Figure 2a−c, respectively. Figure 2a is an image of

Figure 4. UV−vis-absorption spectra of WO2.72/PLA with (0−7 wt %) WO2.72 nanoparticles.

particularly in the NIR light region ranging from 780 to 2500 nm. The absorbance for pure PLA is limited in the NIR region; however, after the addition of WO2.72 nanoparticles, the absorbance in this region significantly increases. Furthermore, for water evaporation application, the most important requirements for photothermal materials are their light absorption ability in the entire region.58 As shown in Figure 4, WO2.72/PLA containing 7 wt % WO2.72 nanoparticles exhibits a short wavelength absorption band edge at ≈550 nm, with its band gap of Eg = 2.65 eV (Figure S3). Moreover, they also exhibit an increased photoabsorption in the NIR region. The reasons why tungsten oxides (WO3−x) give strong and wide absorption in the NIR region are originated from intervalence charge transfers, LSPR of free electrons, and small polaron absorption.15,59−61 In addition, the presences of narrow band gap with strong NIR light absorption of metal oxide may convert NIR light energy into heat. Light is absorbed by metal oxides such as MoO3−x, Fe3O4, WO3−x, Cu2−xS, and TiOx to induce electron−hole pairs. These electron−hole pairs would further recombine to generate heat.4,17,24,62,63 Therefore, the prepared material is suitable for light-driven water evaporation application. The photothermal conversion performance of the fabricated melt electrospinning fiber membranes was investigated under infrared light illumination. Figure 5 shows the temperature elevation of WO2.72/PLA containing 0, 4, and 7 wt % of WO2.72 nanoparticles. The results indicate that higher weight fractions of WO2.72 nanoparticles facilitate quicker and higher temperature increments. The temperature increment (ΔT) reached ΔT = 85.4 °C for 7 wt % and ΔT = 71.7 °C for 4 wt % after 3

Figure 2. FESEM images of (a) Neat PLA, (b) WO2.72/PLA with 4 wt % WO2.72, (c) WO2.72/PLA with 7 wt % WO2.72 nanoparticles, and (d) EDS map of WO2.72/PLA with 7 wt % WO2.72 nanoparticles.

the pure PLA fibers, which are cylindrical and uniform with a diameter of ∼8−13 μm. The diameter of the melt electrospun fibers containing WO2.72 was unaffected by the addition of WO2.72 nanoparticles. The energy-dispersive spectroscopy (EDS) scan shown in Figure 2d also indicates the presence of elements W, O, and C in the WO2.72/PLA composites, confirming the incorporation of tungsten oxide nanoparticles in the PLA matrix. Moreover, the incorporation of WO2.72 nanoparticles into the PLA matrix was also investigated using TEM. The TEM image shows that WO2.72 nanoparticles are uniformly dispersed and distributed in the PLA matrix (Figure 3a). The highresolution TEM image in Figure 3b shows a well-defined lattice fringe with a spacing of 0.37 nm, corresponding to the interplanar spacing of the (010) lattice plane of monoclinic WO2.72 nanoparticles. 3.2. Optical and NIR Photothermal Conversion Properties. Figure 4 shows the UV−visible−NIR absorbance spectra of WO2.72/PLA containing 0, 4, and 7 wt % of WO2.72 nanoparticles. The WO2.72/PLA material exhibits a strong broad absorption in the wavelength region of 300−2500 nm, 28957


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trapped inside the fiber membrane. The resistance to heat flow or thermal resistance of the WO2.72/PLA fiber membrane with different WO2.72 nanoparticle weight fractions was also examined under similar conditions. The results in Table 1 show that the thermal resistivity of the WO2.72/PLA melt electrospun fiber membrane decreases with increasing WO2.72 weight fraction. For water evaporation process, the photothermal material should be floatable on water to generate localized heat at the water−air interface to enhance evaporation rate. Hence, investigating the hydrophobicity of the surface of the photothermal WO2.72/PLA fiber membrane is important to confirm its floatability on water. The hydrophobicity of the WO2.72/PLA fiber membranes was determined by measuring contact angle. Figure 6 shows the water contact angle of the

Figure 5. Photothermal response of WO2.72/PLA electrospun fiber membrane with (0−7 wt %) WO2.72 in the presence and absence of infrared light.

min of irradiation. This shows that the prepared WO2.72/PLA melt electrospun fiber membrane can efficiently and rapidly convert absorbed light energy into local heat. For comparison, the temperature change using pure PLA melt electrospun fiber membrane was only ΔT = 15.1 °C after 3 min of irradiation. Furthermore, the temperature of the WO2.72/PLA fiber membrane containing 7 wt % WO2.72 nanoparticles increased dramatically, reaching ΔT = 56.7 °C after 10 s and ΔT = 67.5 °C after 30 s, then remained constant upon further prolonging the irradiation time to 180 s. This phenomenon can be attributed to faster heat loss at higher temperatures.59,64 In addition, when the source of illumination was turned off after 180 s, the temperature of the WO2.72/PLA melt electrospun fiber membrane quickly returned to ambient temperature. This explains why the higher temperature photothermal material undergoes rapid cooling. Such a quick response results from the strong light-to-heat converting ability of the WO2.72/PLA fiber membrane and enables a quick start of the water evaporation process.8,65 Thermal-related properties including thermal conductivity, absorptivity, and resistance of the WO 2.72 /PLA fiber membrane were measured by an Alambeta instrument. Table 1 presents the thermal characteristics of the WO2.72/PLA fiber

Figure 6. Water contact angle of WO2.72/PLA fiber membrane containing 7 wt % WO2.72 nanoparticles.

WO2.72/PLA fiber membrane containing 7 wt % WO2.72 nanoparticles. The membrane exhibits a strong hydrophobic character with a large contact angle of 136.7°. Therefore, because of its hydrophobicity and low density, the WO2.72/ PLA fiber membrane can easily float on water (as shown in Figure S1), enabling interfacial solar heating to increase the evaporation rate of water. 3.3. Water Evaporation Performance. Having confirmed the photothermal conversion properties, the WO2.72/ PLA fiber membrane was evaluated for water evaporation performance by floating it on water under infrared light irradiation. Figure 7a shows a schematic of the experimental setup used for this measurement. Temperature variation and thermal images were captured by an IR thermographic camera. Figure 7b,c shows surface temperatures and IR thermal images, respectively, of the WO2.72/PLA fiber membrane containing 0, 4, and 7 wt % WO2.72 nanoparticles floating on water and pure water without a floating fiber membrane. For the pure PLA fiber membrane, there was only a slow and small temperature rise of ΔT = 19.4 °C. In contrast, the WO2.72/PLA fiber membranes containing 4 and 7 wt % WO2.72 nanoparticles exhibited a rapid rise in temperature, which then remained constant over 5 min of irradiation. Temperature variation for the membrane containing 7 wt % WO2.72 reached ΔT = 75.3 °C and that for the 4 wt % sample reached ΔT = 44.7 °C. The rapid temperature rises for WO2.72/PLA fiber membranes indicate good photothermal conversion performance for the reduced tungsten oxide (WO2.72) nanoparticles, attributed to its strong optical absorption characteristics in the NIR wavelength region. The temperature rise for bulk water without a membrane was significantly smaller ΔT = 14.0 °C under similar condition.

Table 1. Thermal Properties of WO2.72/PLA Melt Electrospun Fiber Membrane Prepared with (0−7 wt %) WO2.72 Nanoparticles weight fraction of WO2.72 (wt %) thermal conductivity (mW m−1 K−1) thermal absorption (W s1/2 m−2 K−1) thermal resistance (m2 m kW−1)

0 29.00 57.13 37.10

4 29.50 59.80 26.40

7 33.00 63.13 24.00

membranes with 0, 4, and 7 wt % of WO2.72 nanoparticles. The thermal conductivity and absorption of the melt electrospun fiber membrane slightly increase as the weight fraction increases from 0 to 7 wt %. The incorporation of a lower weight fraction of 4 wt % WO2.72 yields no obvious changes in either thermal conductivity or absorption of the WO2.72/PLA fiber membrane compared with the pure PLA melt electrospun fiber. Thus, the WO2.72/PLA composite melt electrospun fiber membrane does not show significant improvement in thermal properties compared to the pure PLA melt electrospun fiber membrane. This is due either to the open pore structure and irregular pore network of the fiber membrane or to the air 28958


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Figure 7. (a) Schematic of water evaporation measurement with WO2.72/PLA fiber membrane floating on the air−water interface, (b) temperature of WO2.72/PLA fiber membrane containing 0, 4, and 7 wt % WO2.72 nanoparticles floating on water and that of pure water without the fiber membrane at ambient temperature, and (c) IR thermal images of WO2.72/PLA fiber membrane containing 0, 4, and 7 wt % WO2.72 nanoparticles and pure water without the fiber membrane, under infrared light.

fiber membrane, and the evaporation rate gradually increased with an increasing content of WO2.72 nanoparticles. This is attributable to an enhancement in photothermal performance of the WO2.72/PLA fiber membrane, contributed by the reduced tungsten oxide nanoparticles. Finally, the steam generation efficiency (η) of melt electrospun WO2.72/PLA fiber membranes was estimated by eq 28,66

To determine the potential applications of the prepared photothermal WO2.72/PLA melt electrospun fiber membrane, we investigated its water evaporation performance by floating it on water in a beaker under light irradiation. A 100 mL beaker containing sample membrane was kept on an electronic balance to measure the real-time mass loss of water. Infrared light with an intensity of 0.294 W cm−2 was shone directly on the WO2.72/PLA fiber membrane. For comparison, water evaporation performance without a membrane was measured under similar conditions. Figure 8a shows the weight loss of evaporated water as a function of the irradiation time. The results show that the evaporation of water increased linearly with irradiation time. With 7 wt % WO2.72 nanoparticles, water evaporation mass loss reached 1.340 kg m−2 after 35 min of irradiation. Without a fiber membrane, evaporation was 0.640 kg m−2 under similar conditions. Figure 8b shows the corresponding evaporation rate V, estimated by eq 1 V=

dm dt × S

η=

Qe Qs

(2) −2

where Qs is the incident light power of 0.294 W cm and Qe is the power of evaporation of water which can estimated by eq 3 Q e = V × He

(3)

where He is the heat of evaporation of water (≈2260 kJ kg−1) and V is the water evaporation rate estimated from eq 1. The corresponding water evaporation efficiency (η) is shown in Figure 8c. The calculation shows that the water evaporation efficiency (η) of the WO2.72/PLA fiber membrane containing 7 wt % WO2.72 nanoparticles reached 81.39%, which is significantly higher than that of pure water 39.09%. As a result, the water evaporation efficiency of the WO2.72/PLA fiber membrane is significantly improved in the presence of photothermal reduced tungsten oxide nanoparticles. Thus, this photothermal material could have large-scale practical applications in areas such as steam generation, sterilization of waste, and seawater desalination. Cycling test was conducted

(1)

where t is the time, m is the mass of evaporated water, and S is the surface area of the fiber membrane. The evaporation rates for 0, 4, and 7 wt % (WO2.72/PLA) fiber membranes after 35 min reached 2.45, 3.07, and 3.81 kg m−2 h−1, respectively, under infrared irradiation. By comparison, the evaporation rate for pure water reached only 1.830 kg m−2 h−1 under the same conditions. These results clearly indicate that water evaporation is strongly enhanced in the presence of a photothermal 28959


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to demonstrate the durability and stability of the WO2.72/PLA fiber membranes under infrared light irradiation. Figure 8d shows that the water weight loss was identical at each cycle tests, conforming that the WO2.72/PLA fiber membranes possesses the high photothermal stability and durability. Thus, the WO2.72/PLA fiber membranes can be an ideal option for practical vapor generation applications.

4. CONCLUSIONS In conclusion, melt electrospun WO2.72/PLA fiber membranes with strong NIR absorption capability were developed for the first time for highly efficient interfacial water evaporation. The photothermal conversion characteristics of the WO2.72/PLA fiber membranes were investigated under infrared irradiation. The composite WO2.72/PLA fiber membranes exhibit higher photothermal conversion than pure PLA fiber membranes. The as-prepared WO2.72/PLA fiber membrane also exhibits strong hydrophobic behavior with a large contact angle and easily floats on water. The evaporation rate for the WO2.72/PLA fiber membrane with 7 wt % WO2.72 nanoparticles calculated to be 3.81 kg m−2 h−1, which is significantly higher than that without a fiber membrane 1.83 kg m−2 h−1. The main reason for the enhancement of evaporation is a significant temperature increment of the WO2.72/PLA fiber membrane on irradiation because of strong broad band light absorption. Thus, it is expected that this fabricated melt electrospun fiber membrane has great potential for applications in photothermal energy conversion, such as water distillation, desalination, and evaporation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b07434.

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Photothermal experimental investigation of melt electrospun fiber and floatability of prepared melt electrospun fiber membrane on water (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +886-2-2737-6530. ORCID

Chang-Mou Wu: 0000-0001-8127-991X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology of Taiwan, ROC, financial support part of this work, under contract numbers: MOST 106-2221-E-011-138 and 106-2218-E-011-018.

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Figure 8. (a) Mass loss through water evaporation under infrared illumination as a function of the irradiation time for WO2.72/PLA fiber membranes containing 0, 4, and 7 wt % WO2.72 nanoparticles and pure water without the fiber membrane, (b) corresponding water evaporation rates, (c) evaporation efficiency, and (d) mass loss through water evaporation under infrared light irradiation for 35 min over six cycles showing the stability of melt electrospun WO2.72/PLA fiber membrane containing 7 wt % nanoparticles.

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