Plasmonic silver nanoparticle-decorated electrospun nanofiber membrane for interfacial solar vapor generation

Abstract

Interfacial solar vapor generation as an emerging technique has great potential in solving water shortage and pollution problems. Electrospun nanofiber membrane with high porosity, mechanical flexibility, numerous microsized channels for fast water transport, and low thermal conductivity offers an ideal platform for solar vapor generation. In this research work, plasmonic silver nanoparticles (Ag NPs) were utilized as photothermal materials and electrospun polyacrylonitrile (PAN) nanofiber membranes as substrates to fabricate Ag nanoparticles-uniformly decorated PAN (Ag@PAN) nanofiber membranes by electroless plating method. The morphology and chemical composition of the membranes were characterized by field emission scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffractometry. By varying the volume ratios of glucose and silver ammonia solution, the sizes of Ag NPs as well as the light-absorption ability of corresponding nanofiber membrane were regulated. As a result, the optimal Ag@PAN nanofiber membrane demonstrated a high light-absorption efficiency of 92.8% in the range of 280–2500 nm wavelength. The evaporation rate reached 1.34 kg m⁻² h⁻¹ and 5.83 kg m⁻² h⁻¹ under 1 sun and 5 sun irradiations, respectively. The plasmonic nanofiber membrane also exhibited long-term use stability, without any degradation in solar vapor generation performance even after 10 cycle tests. This work paves the way for the design and development of plasmonic nanofiber membranes as high-performance interfacial solar vapor generators.

Keywords

Electrospun nanofiber solar vapor generation plasmonic structure Ag nanoparticles photothermal conversion

An essential resource for human life and production, fresh water has been unable to meet the needs of human beings as a consequence of rapid population growth and serious water pollution.¹ Seawater desalination and sewage treatment, including membrane distillation, reverse osmosis, and electrodialysis, have become effective ways to solve the water shortage problem.^2–5 However, these water treatment methods consume energy and need expensive infrastructure.

In recent years, the emerging interfacial solar vapor generation technology, which can realize the production of clean water just by utilizing solar energy, has attracted more attention.^6–9 In the process of interfacial solar vapor generation, photothermal materials absorb the sunlight and convert it into heat, thereby accelerating water evaporation. Efficient light absorption and heat localization have critical influence on the performance of solar vapor generation technology. Based on this research, carbon-based materials,^10–12 semiconductors,^13–15 conductive polymers,^16–19 and plasmonic metal nanoparticles^20–25 are used to improve the photothermal conversion properties. Nanostructured carbon-based materials and conductive polymers can absorb light over the visible and near-infrared wave bands and convert the light into heat by relaxation of photo-excited electrons. Semiconductors make use of bandgap energy to achieve the light–heat conversion, although the complex preparation process and cost have restricted its practical application.

Compared with the above photothermal materials, plasmonic metal nanoparticles with excellent photothermal effect ascribing to the excitation of surface localized plasmons have attracted tremendous attention in the field of solar vapor generation. Various metal nanoparticles, including gold, silver, palladium, and aluminum, are deposited on different substrates for interfacial solar vapor generation.^26–28 For instance, Zhu et al.²² decorated Pd, Au, and Ag on a natural wood matrix to achieve a comparable high solar vapor conversion efficiency and the evaporation rate of ∼1 kg m⁻² h⁻¹ under 1 sun irradiation. Zhou et al.²⁹ fabricated self-assembly plasmonic absorbers by combining metallic nanoparticles and nanoporous anodic aluminum oxide templates, and nearly 99% absorption among full solar spectrum was attained. However, the lack of mechanical flexibility and structure regulation ability of those substrates has restricted their widespread utilization. Instead, electrospun nanofiber membrane, with high porosity, mechanical flexibility, numerous microsized channels for fast water transport, and low thermal conductivity, offers an ideal platform for solar vapor generation.^30–33 Nonetheless, plasmonic nanofiber membrane has seldom been reported, as the commonly used blending electrospinning method encountered a dispersing problem with metal nanoparticles, thereby leading to a low solar vapor generation property.³⁴ To improve solar vapor generation performance, it is necessary to prepare new membrane-based materials taking advantage of plasmonic metal nanoparticles and electrospun nanofiber.

In this paper, we present the assembly of plasmonic polyacrylonitrile (PAN) nanofiber membranes with uniformly distributed plasmonic silver nanoparticles (Ag NPs) via an electroless plating method and study the interfacial solar vapor generation performance.^35–37 It was found that the size of Ag NPs played a vital role in solar light absorption. Through manipulating the size of Ag NPs, a maximum light-absorption efficiency of 92.8% across the wavelength from 280 nm to 2500 nm and evaporation rate of 1.34 kg m⁻² h⁻¹ under 1 sun irradiation were obtained. Furthermore, the as-produced membranes showed long-term use stability due to the strong combination between PAN nanofibers and Ag NPs. Integration of flexibility, high performance, and durability makes the plasmonic nanofiber membrane a promising candidate for practical use in water treatment.

Experiments

Materials

Polyacrylonitrile powder (PAN, M_w = 86,000) was purchased from the Shanghai Chemical Fibers Institute. N,N-Dimethylformamide (DMF, AR) and D-(+)-Glucose were supplied by Sinopharm Chemical Reagent Co., Ltd., China. Silver nitrate (AgNO₃) was purchased from Shanghai Chemical Reagent Co., Ltd., China. Hydroxylamine (NH₂OH), ammonia water (NH₃·H₂O), tin (II) chloride dihydrate (SnCl₂·2H₂O, ≥98.0%), palladium chloride (PdCl₂), and potassium hydroxide (KOH) were provided by Aladdin Chemical Reagent Co., Ltd., China. Polystyrene (PS) foam was purchased from Shanghai Wang bang Packaging Materials Co., Ltd. All reagents were used as received without further purification. Deionized water was used for all the experiments.

Electrospinning PAN nanofiber membranes

A home-made electrospinning platform, including the parts of high-voltage power supply, liquid supply pump, syringe with 18G needle, and flat receiving plate, was used to prepare nanofiber membranes. PAN powder was dissolved in DMF solvent under magnetic stirring for 10 h to prepare 12 wt.% PAN spinning solution. The spinning solution was then loaded into the syringe and electrospun for 10 h. The feeding speed was set as 0.6 mL/h, the spinning voltage was 10 kV, and the receiving distance was 15 cm. Temperature and humidity during electrospinning were controlled at 24 ± 1°C and 50 ± 5%, respectively.

Fabrication of Ag-decorated PAN nanofiber membranes

Ag NPs were deposited on the surface of PAN nanofibers using a modified electroless plating method.^36,38 This included pretreatment, sensitization, and activation of the PAN nanofiber membranes, followed by deposition in an electroless plating solution. Figure 1 illustrates the fabrication processes of Ag NPs-decorated PAN nanofiber membrane (Ag@PAN).

Figure 1.

Schematic diagram of fabricating Ag@PAN nanofiber membrane and assembling into solar vapor generator with PS foam.

Specifically, the PAN nanofiber membranes were first treated with 1 M aqueous hydroxylamine solution at 70°C for 5 min to obtain the amidoxime surface-functionalized PAN (AM-PAN) nanofiber membranes, followed by sensitization and activation using 3 mM SnCl₂ and PdCl₂ solution, respectively, to get membranes with Pd seeds deposition (Pd-PAN).

An aqueous solution of 0.1 M silver nitrate was prepared, followed by adding an appropriate amount of ammonia until the precipitate disappeared. Then 0.8 M KOH aqueous solution and appropriate ammonia were mixed in until the solution became transparent again to obtain the silver ammonia solution. Finally, 0.25 M glucose as reduced agent and as-prepared silver ammonia solution were mixed for 20 s to obtain silver plating solutions. The volume ratios of glucose and silver ammonia solution were controlled as 1:500, 1:100, and 1:10.

Pd-PAN membranes were immersed in the above three silver plating solutions for 1 min to deposit Ag NPs, and the corresponding samples were denoted as Ag@PAN-1, Ag@PAN-2, and Ag@PAN-3 nanofiber membranes, respectively. All samples were rinsed three times by deionized water and dried for 12 h after each treatment step.

Characterization

Field emission scanning electron microscopy (FE-SEM, Hitachi SU-8010) was employed to characterize the morphology of the nanofiber membranes, and element composition of the membranes with Ag NPs deposition was evaluated using energy dispersive spectrometry (EDS, Quanta250). The Fourier transform infrared (FTIR, Nicolet 6700) spectra of the membranes were measured from 500 cm⁻¹ to 4000 cm⁻¹. X-ray diffractometry (XRD, D2 phaser, Germany) was utilized to obtain the diffraction patterns of the membranes at the scanning rate of 5° per minute using Cu-Kα radiation. Ultraviolet-Visible-Near-Infrared spectrometry (UV-Vis-NIR, Shimadzu UV-3600) with an integrating sphere was used to measure the absorption efficiency of the Ag@PAN nanofiber membranes in wet states from 280 nm to 2500 nm. The wettability of the membrane was measured with an optical contact angle device (OCA15EC, Dataphysics, Germany). A modern digital multimeter (UT39C, UNI-T) was used to measure the electric resistance of the membrane at the same position before and after sonication.

Solar vapor generation

The interfacial solar vapor generation experiments were conducted in a room with the temperature at 25 ± 1°C and the relative humidity at 50 ± 5%. As-prepared Ag@PAN samples were assembled into solar vapor generators for the whole test as illustrated in Figure 1. The membrane was first cut into a cross-sample with a specific size. The center of the sample wrapped the square PS foam with a width of 2 cm and the four corners touching the sides were used to suck water. The structure was then placed into a round foam (0.8 cm in thickness and 4.8 cm in diameter) with a square hole of 2 cm in width in the middle to get the interfacial solar vapor generator. The generator was floated at the surface of water loaded in a 100 mL beaker to measure the evaporation rate. A balance (FA 2004, LICHEN Technology Co., Ltd., China) with an accuracy of 0.0001 g was connected to a computer to record the mass change in real time. A solar simulator (Solar-500, Beijing CEAulight Technology Co., Ltd., China) with an air mass 1.5 global (AM 1.5G) filter was used to irradiate the solar vapor generator directly, and the different solar intensity was calibrated using a solar power meter (CEL-FZ-A, Beijing CEAulight Technology Co., Ltd., China). For comparison, water evaporation without a membrane was also measured under similar condition. An IR camera (Fortric 226) was applied to monitor temperature change of the surface of the membranes.

Results and discussion

Morphology and composition of the membrane

Ag NPs-decorated electrospun PAN nanofiber membrane was successfully prepared by the electroless plating method. The thickness of the membrane was controlled at ca. 70 µm to ensure no transmitting of light. The deposition of Ag NPs on the surface of nanofibers can improve light absorption and photothermal conversion properties compared with blending Ag NPs inside the nanofibers, thus contributing to the improvement of solar vapor generation performance.

The morphologies of the functionalized PAN-based nanofiber membranes are shown in Figure 2. At first, the as-spun flexible and lightweight PAN nanofiber membrane was composed of smooth and continuous nanofibers with the average diameter of ca. 340 nm (Figure 2(a)). Such a porous structure resulted from the nanofiber interlaced stacking that contributed to water transportation and vapor escape.³⁹ To enhance the combination of nanofibers and Ag NPs, the PAN nanofiber membrane was first treated with NH₂OH solution to obtain the AM-PAN nanofiber membrane. After treatment for up to 5 min, the resulting AM-PAN membrane still remained flexible with a close-packed morphology, and the nanofibers curled slightly and partially adhered to the adjacent ones (Figure 2b). The average diameter of the nanofibers increased to 358 nm. This may be ascribed to the situation in which -OH existing in the amidoxime group caused swelling of nanofibers by absorbing water.⁴⁰

Figure 2.

Representative FE-SEM images of (a) PAN, (b) AM-PAN: PAN nanofiber membrane treatment in 1 M NH₂OH for 5 min, (c) SnCl₂-treated AM-PAN, (d) Pd-PAN: PAN nanofiber membrane with Pd seeds deposition and (e) Ag@PAN-2 nanofiber membranes, (f) EDS mapping of the Ag@PAN nanofiber membranes.

The following treatment with SnCl₂ solutions had almost no obvious effect on the morphology of the nanofibers. In this process, the amidoxime group (-C(NH2) =N-OH) promoted the affinity of nanofibers with tin (II) ions by coordinating reaction. As shown in Figure 2(c), the average diameter of the nanofibers was maintained at ca. 365 nm. Similarly, the morphology of nanofibers with Pd seeds deposition, represented in Figure 2(d), was almost unchanged. The small amount of Pd seeds with catalytic function was beneficial for the following Ag deposition. The process of electroless plating caused the deposition of Ag NPs onto the surface of the nanofibers, as shown in Figure 2(e). More specific and enlarged images will be shown in next section. Moreover, as shown in Figure 2(f), the EDS mapping verified that numerous Ag NPs had deposited onto the nanofibers.

To further confirm the constituent change after each process, FTIR test was conducted to analyze the nanofiber membranes. The results of FTIR are shown in Figure 3(a) and the lines from A to D represent PAN, AM-PAN, SnCl₂-treated PAN, and Pd-PAN nanofiber membranes, respectively. The peaks appearing at 1656 cm⁻¹ and 2243 cm⁻¹ represent the cyano group (C≡N) existing in PAN. Compared with the as-spun PAN nanofiber membrane, the characteristic peak at 920 cm⁻¹ as shown in line B–D could be assigned to N-O of amidoxime group. The new peaks located at 3362 cm⁻¹ and 3474 cm⁻¹ can be attributed to both N-H and O-H of amidoxime group. These groups are hydrophilic and prone to water transportation.⁴⁰

Figure 3.

(a) FTIR spectra of (A) PAN, (B) AM-PAN, (C) SnCl₂-treated PAN, and (D) Pd-PAN nanofiber membranes; (b) XRD patterns of the PAN and Ag@PAN-2 nanofiber membranes.

Besides, XRD was used to characterize the crystal structures of the PAN and Ag@PAN nanofiber membranes, and the results are shown in Figure 3(b). There was a characteristic peak at 17° attributed to the crystallized plane of PAN. The XRD pattern of the Ag@PAN-2 nanofiber can be well indexed by JCPDS 00-004-0783 to confirm its structure. A series of new characteristic diffraction peaks appearing at 38.1°, 44.3°, 64.5°, and 77.4° were attributed to the (111), (200), (220), and (311) planes of face-centered cubic structure of Ag NPs, respectively, indicating Ag NPs were deposited on the PAN nanofibers.⁴¹

Effect of Ag NPs’ size on the properties

To investigate the effect of the size of Ag NPs on solar vapor generation performance, different volume ratios of silver ammonia and glucose solution were used to control the deposition of Ag NPs. In the process of electroless plating, the Pd seeds on PAN nanofibers acted as active sites for the uniform nucleation and growth of Ag NPs. The silver ions accepted electron from the glucose solution and were reduced to Ag atoms depositing on the nanofibers. Under the same reaction time of 1 min, the velocity of the reaction was controlled by changing the volume ratios of silver ammonia and glucose solution, which promoted the deposition of Ag NPs with different size distributions.

Figure 4 shows a series of FE-SEM images of Ag NPs-decorated PAN nanofiber membranes with different particle size distributions. The representative low-magnification FE-SEM images of Ag@PAN-1, Ag@PAN-2, and Ag@PAN-3 nanofiber membranes (Figure 4(a–c)) show that all of the samples preserved the porous morphology of the electrospun PAN nanofiber membranes. These microsized channels are beneficial for light absorption and vapor escape. The minor brighter parts that appear in the images come from the deposition of self-reaction Ag nanoparticle clusters in the solution, and had little influence on the photothermal properties. The insets clearly show the color change of the different nanofiber membranes. High-magnification FE-SEM micrographs, as shown in Figure 4(d–f), demonstrate the detailed surface morphology of the Ag@PAN-1, Ag@PAN-2, and Ag@PAN-3 nanofibers. The corresponding statistic size distributions of Ag NPs are shown in the insets.

Figure 4.

Morphologies of Ag@PAN nanofiber membranes prepared with different volume ratios of silver ammonia and glucose solution. (a) Low-magnification FE-SEM images of the Ag@PAN-1, (b) Ag@PAN-2, and (c) Ag@PAN-3 nanofiber membranes, and the insets show the optical photographs of the corresponding membranes; (d) high-magnification FE-SEM images of the Ag@PAN-1, (e) Ag@PAN-2, and (f) Ag@PAN-3 nanofiber membranes, and the insets represent the statistic distributions of the size of Ag NPs.

As shown in Figure 4(a) and Figure 4(d), for the Ag@PAN-1 nanofiber membrane, with the volume ratio of glucose and silver ammonia solution of 1:500, the membrane was dark yellow in color and the average diameter of Ag NPs was ca. 20 nm. Increasing the amount of reducing agent speeded up the reaction rate and the deposited Ag NPs size was ca. 32 nm, which caused the membrane to appear a gray–black color, as shown in Figure 4(b) and 4(e). When increasing the amount of reducing agent further to one-tenth of silver ammonia solution, the reaction process was the fastest. As shown in Figure 4(c), the Ag@PAN-3 nanofiber membranes showed a metallic silver gloss. This resulted from the phenomenon in which the Ag NPs with an average diameter up to ca. 67 nm connected together to form a coarse silver layer on the nanofibers Figure 4(f).

The apparent color may reflect the absorption of the nanofiber membranes on the visible light. However, in the process of interfacial solar vapor generation, a more important pursuit for photothermal material is broadband and efficient absorption across the entire solar spectrum. The broadband light-absorption capabilities of the Ag@PAN nanofiber membranes in a wet state were measured by a UV-Vis-NIR spectrometer. The absorption rate results obtained by subtracting the reflectivity and transmissivity are shown in Figure 5(a). The Ag@PAN-2 nanofiber membrane had the highest absorptance up to 92.8% across the wavelength from 280 nm to 2500 nm, weighted by the AM 1.5G solar irradiation spectrum. The Ag@PAN-1 and Ag@PAN-3 nanofiber membranes showed inferior absorption of 75.4% and 80.5%. The difference came from the different sizes of Ag NPs on nanofibers. Just comparing the Ag@PAN-1 and Ag@PAN-2 nanofiber membranes, the increase of the Ag NPs size was beneficial for light absorption. For the Ag@PAN-3 nanofiber membrane, the Ag NPs had attached together to form a coarse silver layer, which caused more reflection and reduced light absorption. Moreover, an adequate water supply to the heated region plays an important role in solar vapor generation. As shown in Figure 5(b), static contact angle confirmed that the Ag@PAN-2 nanofiber membrane was a good hydrophilic material where the droplets spread out as they touched the membrane and infiltrated completely within 5 s.

Figure 5.

(a) Absorption spectrum of the different Ag@PAN nanofiber membranes in the wavelength range of 280–2500 nm, (b) static contact angle of the Ag@PAN-2 nanofiber membrane at different shooting time.

Performance of solar vapor generation

The solar vapor generation performance of the sample was evaluated under simulated solar illumination using the measurement system as illustrated in Figure 6(a). The assembled solar vapor generator floating on a beaker filled with water was placed on an electronic balance connected to a computer to record the mass changes in real time. Figure 6(b) shows the mechanism of the interfacial solar vapor generation process utilizing Ag@PAN nanofiber membrane. Incident light is mainly absorbed by the nanofiber membrane, except the parts of reflection and transmission. Ag NPs on the surface of nanofibers play an important role in converting solar energy to thermal energy, taking advantage of the localized surface plasmonic resonance effect.^42,43 The generated heat increases the temperature of the water at the interface between the air and the membrane. Driven by the high energy state, water molecules in the liquid water side are transformed into the vapor phase.¹⁶ In addition to evaporation, the heat is lost through convection and radiation to the environment and conduction to the underlying water.

Figure 6.

(a) The schematic diagram of the system for solar vapor generation performance measurement. (b) Diagram of mechanism of interfacial solar vapor generation and energy conversion process. (c) The surface temperature changes of Ag@PAN nanofiber membranes and water under 1 sun irradiation. (d) The infrared pictures of Ag@PAN-2 nanofiber membrane before and after irradiation for 300 s.

The temperature variation of the sample surface and infrared pictures under 1sun irradiation were captured by an IR camera. Figure 6(c) displays the surface temperature changes of water and the three Ag@PAN nanofiber membranes. Besides, the representative infrared pictures of Ag@PAN-2 nanofiber membrane before and after irradiation for 300 s are shown in Figure 6(d). Obviously, once the light was on, the surface temperature of the all samples went up from the original temperature of water immediately. The temperature rise of water without a membrane was slow and inapparent. The equilibrium temperatures of water, Ag@PAN-1, Ag@PAN-2, and Ag@PAN-3 nanofiber membranes were around 28.4°C, 39.7°C, 41.3°C, and 40.4°C, respectively. The temperature change was positively correlated with light absorption of the corresponding nanofiber membranes. Rapid temperature rise was attributed to the excellent photothermal conversion properties of Ag NPs and the low thermal conductivity of the nanofiber membrane. In the process of solar vapor generation, heat is crucial in promoting vapor generation.⁴⁴

Figure 7(a) shows the dependence of the water mass change on irradiation time for different samples under 1 sun. The evaporation rate of water without membrane was 0.4 kg m⁻² h⁻¹, showing that the pure water evaporated at a quite low rate under normal solar irradiation. When the photothermal membranes were placed on the surface of water, the evaporation rates enhanced significantly. Specifically, Ag@PAN-2 nanofiber membrane had the highest evaporation rate up to 1.34 kg m⁻² h⁻¹, while the evaporation rates were 1.01 kg m⁻² h⁻¹ and 1.09 kg m⁻² h⁻¹ for Ag@PAN-1 and Ag@PAN-3 nanofiber membranes. The solar vapor conversion efficiency was then calculated according to the following equation:^22,45

η = \frac{ṁ H_{L V}}{Q_{i n} C_{opt}}

where η represents the solar vapor conversion efficiency, ṁ denotes the evaporation rate, which subtracts the evaporation rate at dark field (∼0.12 kg m⁻² h⁻¹) to eliminate the influence of intrinsic water evaporation. H_LV is the total enthalpy, including phase change enthalpy called latent heat and sensible heat (∼2260 kJ kg⁻¹), Q_in is the normal incident solar irradiation intensity (1 kW m⁻²), and C_opt denotes the optical concentration. The corresponding calculated results of solar vapor conversion efficiency based on the evaporation rates are shown in Figure 7(b). The solar vapor conversion efficiency of pure water was 25.5% under 1 sun, whereas in the presence of photothermal membranes, the solar vapor conversion efficiencies rose to 55.4%, 76.0%, and 60.4% for Ag@PAN-1, Ag@PAN-2, and Ag@PAN-3 nanofiber membranes, respectively. Figure 7(c) shows the comparison of evaporation rate of this work with other recently reported membrane-based solar vapor generators. The solar vapor generation performance of Ag@PAN nanofiber membrane with flexibility and structure regulation ability stands among the highest level and is more suitable for practical applications.

Figure 7.

Solar vapor generation performance of the Ag@PAN-1, Ag@PAN-2, Ag@PAN-3, and pure water under 1 sun irradiation. (a) The mass change of the solar vapor generator within 60 min. (b) The corresponding solar vapor conversion efficiency. (c) Comparison of evaporation rate of this work with other recently reported membrane-based solar vapor generators.

Furthermore, the evaporation of Ag@PAN-2 nanofiber membrane under different values of solar intensity irradiation was investigated. Figure 8(a) represents the optical photographs of the solar vapor generator under 1 sun, 3 sun, and 5 sun irradiations. A thin wisp of vapor can be seen on the surface of water obviously when the solar intensity increases from 1 kW m⁻² to 3 kW m⁻². When the light intensity reached up to 5 kW m⁻², a continuous thick vapor column was generated, implying that evaporation became faster. As shown in Figure 8(c), the evaporation rates were 3.65 kg m⁻² h⁻¹ and 5.83 kg m⁻² h⁻¹ under 3 sun and 5 sun irradiations, respectively. The results originate from higher temperatures on the membrane surface. As shown in Figure 8(b), the temperatures can reach up to 70.6°C and 84.3°C after irradiation for 300 s.

Figure 8.

Solar vapor generation performance of Ag@PAN-2 nanofiber membrane under 1 sun, 3 sun, and 5 sun irradiations. (a) Optical photographs showing the vapor generation. (b) Infrared images after 3 sun and 5 sun irradiations for 300 s. (c) The mass change of the solar vapor generator within 60 min under 1 sun, 3 sun, and 5 sun irradiations.

The long-term use stability and durability of the membrane is vital for its practical application. Compared with the membrane with photothermal material coating on surface, better contact and adhesion between the particles and nanofibers is achieved in the present membranes. As shown in Figure 9(a), after the Ag@PAN-2 nanofiber membrane was immersed in water and sonicated for 1 h, the membrane was still black and the color of water had no change. The FE-SEM image (Figure 9(b)) of the membrane after sonication shows that the nanofiber morphology remained unchanged and the Ag NPs were still adhered on the surface of nanofibers. As shown in Figure 9(c), electric resistance change rate (R/R₀) of the Ag@PAN-2 nanofiber membrane before and after sonication always kept at about 1 in multiple tests, further proving the stability of the membrane. A solar vapor generation cycle test was conducted to evaluate the stability of Ag@PAN-2 nanofiber membrane. As shown in Figure 9(d), the evaporation rate remained stable after 10 cycle tests under 1 sun, 3 sun, and 5 sun irradiations, respectively. This implies the Ag@PAN-2 nanofiber membrane has good durability.

Figure 9.

(a) Optical photograph of the Ag@PAN-2 nanofiber membrane before and after sonication for 1 h in water. (b) FE-SEM image of the Ag@PAN-2 nanofiber membrane after sonication for 1 h in water. (c) Electric resistance change rate (R/R₀) of the Ag@PAN-2 nanofiber membrane before and after sonication. (d) Cycling stability of the Ag@PAN-2 membrane through 10 cycles under 1 sun, 3 sun, and 5 sun irradiations.

Conclusions

In summary, Ag nanoparticles-decorated PAN nanofiber membranes were prepared using a modified electroless plating method and used for interfacial solar vapor generation. The amidoxime surface-functionalized electrospun PAN nanofiber membranes with following deposition of plasmonic Ag NPs retained flexibility, microsized channels, and hydrophilicity of nanofiber membrane. By varying the volume ratios of glucose and silver ammonia solution, the sizes of Ag nanoparticles as well as the light-absorption ability of the corresponding nanofiber membrane were regulated. When the volume ratios increased from 1:500 to 1:100, the average diameter of Ag NPs increased from 20 nm to 32 nm. The corresponding Ag@PAN-2 nanofiber membrane could obtain 92.8% light absorption across the entire solar spectrum. Under 1 sun irradiation, the evaporation rate was 1.34 kg m⁻² h⁻¹ and the solar vapor conversion efficiency was 76.0%. Together with excellent stability, the plasmonic nanofiber membrane is a promising candidate for practice application for seawater desalination and wastewater treatment.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interests with respect to the research, authorship, and/or publications of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partly supported by the Fundamental Research Funds for the Central Universities (2232020D-15, 2232020A-08, 2232020G-01, 2232020D-14 and 2232019D3-11) and grants (51773037, 51973027, 51803023, 52003044 and 61771123) from the National Natural Science Foundation of China. This work has also been supported by the Chang Jiang Scholars Program and the Innovation Program of Shanghai Municipal Education Commission (2019-01-07-00-03-E00023) to Prof. Xiaohong Qin, the Shanghai Sailing Program (19YF1400700), the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201906SIC), Young Elite Scientists Sponsorship Program by CAST and DHU Distinguished Young Professor Program to Prof. Liming Wang.

ORCID iDs

Liming Wang

Xiaohong Qin

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