Polydopamine-Bi2WO6-Decorated Gauzes as Dual-Functional Membranes for Solar Steam Generation and Photocatalytic Degradation Applications

The dual-functional Bi2WO6/polydopamine (PDA)-modified gauze membrane has been developed for applications in photocatalytic degradation and solar steam generation. Two types of membrane were prepared by changing the growth sequence of Bi2WO6 nanomaterials and PDA on gauze substrates. The spatial distribution of Bi2WO6 and polydopamine has a great influence on light absorption, photocatalytic degradation, and solar steam generation performances. Bi2WO6 photocatalysts can absorb short-wavelength light for the photocatalytic decoloration of organic dyes. The photothermal polydopamine can convert light into heat for water evaporation. Besides, the gauze substrate provides water transport channels to facilitate water evaporation. The morphology, surface chemistry, and optical properties of Bi2WO6-PDA modified gauzes were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and diffuse reflectance spectra. The photothermal properties, wetting properties, and solar steam generation rates of the composite films were also studied. Degradation of 96% of indigo carmine was achieved after being irradiated for 120 min in the presence of G/PDA/BWNP. The water evaporation rates of the G/BWP/PDA sample under the irradiation of an Xe lamp (light intensity = 1000 W/m2) reached 1.94 kg·m−2·h−1.


Introduction
Developing practical approaches to obtain clean water and eliminate pollutants is an important issue for our society because of the increasing world population [1] and water contamination caused by various industrial production processes. Besides, since solar energy is a sustainable and renewable energy source, light energy has been widely utilized in photovoltaic [2,3], photothermal [4][5][6], and photocatalytic [7,8] applications. Solar energydriven water production is significant for people living in remote areas without electricity supply [9]. Solar steam generation is a promising process to produce clean water from seawater [10,11] or contaminated water sources [12,13]. Meanwhile, photocatalysts can be applied to the photocatalytic degradation of organic pollutants or the reduction of heavy metal ions [14,15]. Pollutants can also be degraded in the presence of heat (thermocatalysis) or light (photocatalysis, photoactivation and photothermocatalysis) [16]. Therefore, the development of dual-functional membranes for water treatment by the combined solar steam generation and photocatalytic degradation approaches is worth studying. Yang et al. [17] developed a novel dual-functional water-treatment system by integrating photothermal Ti 3 C 2 T x , La 0.5 Sr 0.5 CoO 3 photocatalysts, and polyvinyl alcohol/chitosan hydrogels. The system can achieve high photothermal conversion performance and photodegradation The effects of the spatial distribution of Bi2WO6 and polydopamine on the light absorpt properties, photocatalytic degradation, and solar steam generation performances of th films were studied.

Preparation of Dual-Functional Membrane
The non-sterile non-woven rayon/polyester gauze is provided by YASCO Enterpr Corp. Two types of dual-functional film were developed by the sequential growth Bi2WO6 nanomaterials and PDA on gauze substrates for applications in photocataly degradation and solar steam generation in this study.
For the first type of sample, polydopamine was grown on the gauze substrate by s polymerization. A dopamine solution was prepared by dissolving 0.25 g of dopam hydrochloride in 50 mL of deionized water. Then, a 2 M NaOH solution was used to tu the pH value of the solution to 8.5. The gauze substrate (2 cm × 2 cm) was immersed the dopamine solution at room temperature for 24 h to make the G/PDA sample. Then uniform Bi2WO6 nanoparticle layer was formed by the alternative immersion of G/PDA sample into separately placed Bi(NO3)3 (0.06 M, 50 mL) and Na2WO4 (0.06 M, mL) solution for 3 min. Before immersion in another solution, the gauze-based sam was immersed in D.I. water for 1 min. The alternative immersion was repeated 10 tim Then, the G/PDA/BWNp sample was obtained after washing by water and drying at °C for 24 h (Figure 1a).  For the second type of sample, G/BWP/PDA was prepared by the consecutive growth of Bi 2 WO 6 nanomaterials and PDA on gauze substrates (Figure 1b). The Bi 2 WO 6 nanomaterials were grown by a two-step process. At first, a uniform Bi 2 WO 6 nanoparticle layer was formed on the gauze by the alternate immersion of the gauze substrate (2 cm × 2 cm) Before the immersion in another solution, the gauze-based sample was rinsed with D.I. water for 1 min. The alternative immersion was repeated ten times. G/BWNp sample was obtained after washing by water and drying at 60 • C for 24 h. For the second step, Bi 2 WO 6 nanoplates (BWP) were grown on the previous G/BWNp substrate to make the G/BWP sample by a microwave-assisted hydrothermal method. 0.365 g of Bi(NO 3 ) 3 was dissolved in 20 mL of ethylene glycol to prepare solution A. 0.125 g of Na 2 WO 4 was dissolved in 20 mL of ethylene glycol to prepare solution B. The G/BWNp substrate (2 cm × 2 cm), solution A, and solution B were put in a EasyPrep Plus ® closed vessels (100 mL). The reaction mixture was sealed and heated by a Microwave Synthesis System (MARS 6, CEM, NC, USA) at 160 • C for 20 min to make the G/BWP sample. Then, polydopamine was grown on the G/BWP by self-polymerization to make the G/BWP/PDA sample. A dopamine solution was prepared by dissolving 0.25 g of dopamine hydrochloride in 50 mL of deionized water. Then, a 2 M NaOH solution was used to tune the pH value of the solution to 8.5. The G/BWP sample (2 cm × 2 cm) was immersed in the prepared dopamine solution for 24 h at room temperature. Then, the G/BWP/PDA sample can be obtained after being washed with deionized water three times and dried at 60 • C ( Figure 1b).

Solar Steam Generation
The solar steam generation test was performed to investigate the light-to-heat conversion performance of various samples (Figure 1c). A fluoro acrylate copolymer-based water repellent agent solution (JR7101, J-Young Technology Corp., Hsinchu city, Taiwan) was spray-coated on the top layer of the sample (2 cm × 2 cm) and dried at 60 • C for 10 min to make the sample floatable. In the solar steam generation experiment, 20 mL of water was added to a beaker with a diameter of 2.9 cm (height 4.1 cm), and the sample was floated on the water. The light intensity of the simulated solar light (Xenon lamp, PX350A, Prosper OptoElectronic Co., New Taipei City, Taiwan) was tuned at 1000 W/m 2 . The thermal images and surface temperature of the membranes floating on the solution were monitored by an infrared thermal imaging camera (FLIR-A320, FLIR SYSTEMS, Wilsonville, OR, USA). An analytical balance (GF2000, A&D, Tokyo, Japan) connected to a computer was used for real-time monitoring of the mass changes during the photothermal evaporation test. The whole evaporation process was carried out under the ambient temperature of 30 • C and relative humidity of 70%.

Photocatalytic Degradation
The photocatalytic activities were evaluated for the decoloration of the indigo carmine solution under light irradiation. The photocatalytic degradation of indigo carmine dye was performed in a reactor using 10 mL of a 60 ppm dye solution and the photocatalysts (2 cm × 2 cm). Before the irradiation, the solution was magnetically stirred for 60 min in the dark to reach an absorption-desorption equilibrium. These solutions were illuminated with a 350 W Xe lamp (Prosper HID). The temperature of the solution was maintained at 30 • C using a water bath. The dye degradation was monitored by a JASCO V-770 UV-vis spectrophotometer.

Characterization
The morphologies of samples were analyzed with a field scanning electron microscope (FESEM, HITACHI, S-4800) and a field-emission transmission electron microscope (TEM, JEOL, JEM-2100F). The surface chemical composition and elemental valence were identified from X-ray photoelectron spectra (XPS, ULVAC-PHI, PHI 5000 Versa Probe), using C 1s peak as a reference to calibrate binding energies. The surface wettability of the samples was evaluated with a contact-angle meter (CAM-100, Creating-Nanotech Co., Tainan city, Taiwan). The Raman spectrum was measured by a Raman microscope (MRI532S, Protrustech, Tainan city, Taiwan) with an emission wavelength of 532 nm (He-Ne laser). The detector integration time and laser irradiation power were 10 s and 2 mW. The diffuse reflectance

Morphology
The fiber surface morphology of the membrane changed after the growth of Bi 2 WO 6 nanoplates and polydopamine on the surface of gauze, as observed by the FESEM images. Figure 2 illustrates the FESEM images of gauze, G/PDA, G/PDA/BWNp, G/BWP, and G/BWP/PDA samples. As shown in Figure 2a,f, the fibers of pristine gauze show a smooth surface with a diameter ranging between 9 to 11 µm. The polydopamine was grown on the gauze substrate by self-polymerization to enable the G/PDA sample to act as a comparative sample. Parts of the surfaces of PDA-modified fibers for G/PDA are covered by some particles with the size ranging from 150 nm to 500 nm, due to the polymerization of dopamine (Figure 2b,g). Then, Bi 2 WO 6 nanoparticles (BWNp) were grown on the surface of G/PDA by a successive ionic layer adsorption and reaction process to prepare G/PDA/BWNp sample. As shown in Figure 2c,h, BWNp with the size of less than 100 nm was uniformly distributed on almost all the fiber surfaces of G/PDA/BWNp. The Bi 2 WO 6 nanomaterials were grown by a two-step process to achieve uniform modification of the fiber surface. The first step was a successive ionic layer adsorption and reaction route. The second step was a microwave-assisted hydrothermal process. The morphology of the G/BWP sample (Bi 2 WO 6 nanoplate modified gauze) is shown in Figure 2d,i. For the G/BWP sample, the surface of the fiber is fully covered with a lot of assembled twodimensional Bi 2 WO 6 nanoplates. The enlarged image of the G/BWP sample (Figure 2i) shows that Bi 2 WO 6 grows into two-dimension rectangular-plate-like morphologies with a side length of 5-10 µm and the width of 1.0-5.5 µm. The thickness of an individual Bi 2 WO 6 nanoplate is 0.22-1.1 µm. Bi 2 WO 6 nanoplates were randomly stacked together. Besides, the fiber diameter of G/BWP increases, ranging between 35 to 55 µm (Figure 2d). The increased fiber diameter results from the formation of large amounts of nanoplates on the fiber surface. Then, polydopamine was grown on the surface of G/BWP to make the G/BWP/PDA sample. Figure

Transmission Electron Microscopy Energy-Dispersive X-ray (TEM EDX)
The TEM energy-dispersive X-ray (EDX) spectrum of a line scan is presented in Figure 3a to investigate the elemental distribution for the powder scratched from the G/BWP/PDA composite sample. The Bi, W, O, and N elements are found in the G/BWP/ PDA sample. The elements of Bi, W, and O indicate the existence of Bi 2 WO 6 . The N element originates from dopamine due to the successful polymerization of dopamine on the fabrics. The signals of the Bi, W, O, and N elements are higher near the center of the line, indicating that the plate-like sample is Bi 2 WO 6 , and polydopamine is distributed well near Bi 2 WO 6 . Similar results were observed for the powder scratched from the G/PDA/BWNp composite sample (Figure 3b).

X-ray Photoelectron Spectra (XPS) Analysis
The XPS analysis of G/BWP/PDA sample was measured to analyze its oxidation state and chemical composition. Figure 4 shows the XPS (a) Bi 4f (b) W 4f (c) N 1s (d) O 1s spectra of G/BWP/PDA. The Bi 4f peaks of G/BWP/PDA at 158.9 and 164.2 eV can be assigned to Bi 4f 7/2 and Bi 4f 5/2 , indicating the Bi 3+ of Bi 2 WO 6 . The W 4f peaks of G/BWP/PDA at 37.8 and 34.7 eV are attributed to W 4f 5/2 and W 4f 7/2 , respectively, which are related to W 6+ of Bi 2 WO 6 [61,62]. The N 1s peak is deconvoluted into three peaks (Figure 4c). That peaks at 401.5, 399.8, and 398.7 eV can be attributed to primary amine (R-NH 2 ), secondary amine (R1-NH-R2), and tertiary amine (=N-R) groups, respectively [63,64]. The results are related to the chemical structures of polydopamine, possible intermediate species, and dopamine monomer [65]. The primary amine is related to dopamine. The secondary amine can be assigned to the intermediate species or polydopamine, while the tertiary amine can be attributed to tautomeric species of the intermediates. The O 1s spectrum of G/BWP/PDA was deconvoluted into two peaks. The peaks at 531.3 and 532.7 eV are assigned to Bi-O, and O-H, respectively. These results support the formation of Bi 2 WO 6 and polydopamine on the sample. Figure 4e-h present the XPS Bi 4f, W 4f, N 1s, and O 1s spectra of G/PDA/BWNp, respectively. The results of the G/PDA/BWNp sample were similar to those of G/BWP/PDA, except that the Bi-O peak was higher than the O-H peak in the O 1s spectrum. The TEM energy-dispersive X-ray (EDX) spectrum of a line scan is presented in Figure 3a to investigate the elemental distribution for the powder scratched from the G/BWP/PDA composite sample. The Bi, W, O, and N elements are found in the G/BWP/PDA sample. The elements of Bi, W, and O indicate the existence of Bi2WO6. The N element originates from dopamine due to the successful polymerization of dopamine on the fabrics. The signals of the Bi, W, O, and N elements are higher near the center of the line, indicating that the plate-like sample is Bi2WO6, and polydopamine is distributed well near Bi2WO6. Similar results were observed for the powder scratched from the G/PDA/BWNp composite sample (Figure 3b).

X-ray Photoelectron Spectra (XPS) Analysis
The XPS analysis of G/BWP/PDA sample was measured to analyze its oxidation state and chemical composition. Figure 4 shows the XPS (a) Bi 4f (b) W 4f (c) N 1s (d) O 1s spectra of G/BWP/PDA. The Bi 4f peaks of G/BWP/PDA at 158.9 and 164.2 eV can be assigned to Bi 4f7/2 and Bi 4f5/2, indicating the Bi 3+ of Bi2WO6. The W 4f peaks of G/BWP/PDA at 37.8 and 34.7 eV are attributed to W 4f5/2 and W 4f7/2, respectively, which are related to W 6+ of Bi2WO6 [61,62]. The N 1s peak is deconvoluted into three peaks (Figure 4c). That peaks at 401.5, 399.8, and 398.7 eV can be attributed to primary amine (R-NH2), secondary amine (R1-NH-R2), and tertiary amine (=N-R) groups, respectively [63,64]. The results are related to the chemical structures of polydopamine, possible intermediate species, and dopamine monomer [65]. The primary amine is related to dopamine. The secondary amine can be assigned to the intermediate species or polydopamine, while the tertiary amine can be attributed to tautomeric species of the intermediates. The O 1s spectrum of G/BWP/PDA was deconvoluted into two peaks. The peaks at 531.3 and 532.7 eV are assigned to Bi-O, and O-H, respectively. These results

Raman Spectra
The surface chemistry of G/BWP/PDA and G/PDA/BWN P was analyzed by the Raman spectra ( Figure 5). The peak at 709 cm −1 is associated with the asymmetric stretching mode of WO 6 octahedra for the vibrations of equatorial oxygen atoms within layers. The peak at 307 cm −1 was related to the simultaneous translational movement of Bi 3+ and the bending of WO 6 octahedra [66,67]. Besides, two broad peaks at 1343 and 1584 cm −1 were attributed to the catechol stretching vibration and deformation from the chains of polydopamine. Similar results were reported in the literature [68][69][70][71]. These peaks support the existence of polydopamine and Bi 2 WO 6 on G/BWP/PDA sample. Similar peaks were found for the G/PDA/BWN P sample. support the formation of Bi2WO6 and polydopamine on the sample. Figure 4e-h present the XPS Bi 4f, W 4f, N 1s, and O 1s spectra of G/PDA/BWNp, respectively. The results of the G/PDA/BWNp sample were similar to those of G/BWP/PDA, except that the Bi-O peak was higher than the O-H peak in the O 1s spectrum.

Raman Spectra
The surface chemistry of G/BWP/PDA and G/PDA/BWNP was analyzed by the Raman spectra ( Figure 5). The peak at 709 cm -1 is associated with the asymmetric stretching mode of WO6 octahedra for the vibrations of equatorial oxygen atoms within layers. The peak at 307 cm -1 was related to the simultaneous translational movement of Bi 3+ and the bending of WO6 octahedra [66,67]. Besides, two broad peaks at 1343 and 1584

Diffuse Reflection Spectra (DRS)
The light absorption properties of various samples were observed by a diffuse reflection spectrophotometer. G/BWP samples (Figure 6a) showed a strong light absorption in the UV region ranging from 200 to 380 nm. Compared with the G/BWP samples, G/BWP/PDA exhibits an increase in UV and visible light absorption ranging

Diffuse Reflection Spectra (DRS)
The light absorption properties of various samples were observed by a diffuse reflection spectrophotometer. G/BWP samples (Figure 6a) [72,73], indicating the formation of Bi 2 WO 6 in G/BWP/PDA.

Surface Hydrophilicity
An efficient water supply is essential to the preparation of an effective solar steam generation membrane. In this study, the SEM images of modified samples revealed that the porous structures of gauze substrates remained after the growth of polydopamine and Bi 2 WO 6 nanomaterials. These encapsulated materials should be hydrophilic to facilitate water supply and steam escape. The surface hydrophilicity also facilitates the contact between the photocatalyst and the organic pollutant. The surface hydrophilicity was evaluated by measuring the dynamic contact angles of water droplets on different samples (without coating the hydrophobic layer) to elucidate their potential applications in solar evaporation and photocatalytic degradation. As shown in Figure 8a, the complete wetting of a water droplet on pristine gauze was achieved within 6.83 s. However, G/PDA (Figure 8b), G/BWP (Figure 8c), G/BWP/PDA (Figure 8d), and G/PDA/BWN P (Figure 8e) samples exhibited rapid, complete wetting within 0.033 s. The surface hydrophilicity was further enhanced after the loading of polydopamine and Bi 2 WO 6 nanomaterials.

Surface Hydrophilicity
An efficient water supply is essential to the preparation of an effective solar steam generation membrane. In this study, the SEM images of modified samples revealed that the porous structures of gauze substrates remained after the growth of polydopamine and Bi2WO6 nanomaterials. These encapsulated materials should be hydrophilic to facilitate water supply and steam escape. The surface hydrophilicity also facilitates the contact

Photocatalytic Property
The activity of different immobilized photocatalysts (G/BWNP, G/BWP, G/BWP/PDA) under light illumination was investigated through photocatalytic decoloration experiments using indigo carmine (IC) as the pollutant model compound. The photocatalytic degradation performance of various samples is shown in Figure 8. The initial concentration of indigo carmine (60 ppm) and the size of the membrane (2 cm × 2 cm) were kept the same in all experiments for the comparison of different samples. For the Bi2WO6 nanoplates-based G/BWP sample, 39% of IC was adsorbed on the photocatalyst during the dark test (Figure 9a). The absorption peak of residual IC solution at 610 nm decreases rapidly under light irradiation, indicating the high photocatalytic

Photocatalytic Property
The activity of different immobilized photocatalysts (G/BWN P , G/BWP, G/BWP/PDA) under light illumination was investigated through photocatalytic decoloration experiments using indigo carmine (IC) as the pollutant model compound. The photocatalytic degra-dation performance of various samples is shown in Figure 8. The initial concentration of indigo carmine (60 ppm) and the size of the membrane (2 cm × 2 cm) were kept the same in all experiments for the comparison of different samples. For the Bi 2 WO 6 nanoplatesbased G/BWP sample, 39% of IC was adsorbed on the photocatalyst during the dark test (Figure 9a). The absorption peak of residual IC solution at 610 nm decreases rapidly under light irradiation, indicating the high photocatalytic degradation activity of G/BWP. 99% of IC can be degraded after the irradiation for 60 min in the presence of G/BWP. However, the IC adsorption property changed when PDA was grown on G/BWP. No noticeable decrease of IC was observed for the dark test of the G/BWP/PDA sample, indicating the adsorption of IC on the G/BWP/PDA was negligible (Figure 9b). Besides, the photocatalytic degradation activity of G/BWP/PDA was lower than that of G/BWP. The residual concentration of IC solution decreases gradually after light irradiation. The irradiation time required for degradation of 99% IC by G/BWP/PDA is 210 min. Such a decrease in the photocatalytic activity of the G/BWP/PDA sample may result from the coverage of B 2 WO 6 nanoplates by polydopamine, leading to a reduction in the exposed active sites of B 2 WO 6 nanoplates. Such a problem can be solved by changing the formation sequence of Bi 2 WO 6 and polydopamine on the gauze substrate. The G/PDA/BWN P photocatalyst was prepared by the deposition of polydopamine on the gauze substrate by self-polymerization, followed by the formation of a uniform Bi 2 WO 6 nanoparticle layer on top. Compared with G/BWP/PDA, the photocatalytic activity of the G/PDA/BWN P membrane was improved. Degradation of 96% of IC was achieved after being irradiated for 120 min in the presence of G/PDA/BWN P (Figure 8c). Photodegradation of indigo carmine (60 ppm) without photocatalyst was shown in Figure 9d as a blank test. Its decoloration rate was slower than other three curves with different photocatalysts. De Andrade et al. [74] studied the degradation of the indigo carmine dye in aqueous medium by the autoclaved cellular concrete/Fe 2 O 3 catalyst in Fenton-like and photocatalytic processes. The toxicity tests against Vero cells indicated that the toxicity of the degradation products, generated by both processes, is smaller or similar to the precursor dye. Vautier et al. [56] reported the photocatalytic degradation of indigo carmine using UV-irradiated titania-based catalysts. The oxidative agents are photo-produced holes h + and/or •OH radicals. A detailed degradation pathway, based on careful identification of intermediate products, is proposed. In addition to a prompt removal of the color, photocatalysis can mineralize carbon and of nitrogen and sulfur heteroatoms into innocuous compounds. In our previous study [53], the photocatalytic degradation by Ag/flower-like Bi 2 WO 6 photocatalysts with and without scavengers reveals that h + and •O 2 − are the major active species generated by the photocatalyst. The oxidation of water by holes can generate •OH. We believe the products of the photocatalytic degradation using Bi 2 WO 6 photocatalysts will not be more toxic than the parent compound, indigo carmine.

Solar-Steam Generation
The photothermal and solar steam generation performance of various samples (G/PDA, G/BWP/PDA, gauze) were evaluated. Typical infrared thermal images and the surface temperature changes of different membranes recorded by an infrared camera are shown in Figure 10a,b, respectively. Figure 10a reveals that the temperatures around the floated composite films are all higher than the bulk solution. Figure 10b shows the surface temperature profiles of G/PDA, G/BWP/PDA, G/PDA/BWN P , and gauze samples versus time under the simulated solar illumination. Before the light illumination, the initial surface temperature of all samples is about 30 • C. After the simulated solar illumination with a power density of 1000 W m −2 for 1 h, the surface temperature of the gauze is 36.1 • C. However, the surface temperatures of G/PDA and G/BWP/PDA samples reached 67.4 and 65.7 • C, respectively. These two samples with PDA on the top layer showed similar surface temperatures. The Bi 2 WO 6 nanoplates under the PDA polymer did not deteriorate the lightto-heat transform property of polydopamine. Han et al. [75] studied the dual functional polydopamine-modified CuS@HKUST for quick sterilization through enhanced photother-mal and photocatalytic ability. They found that the nanoparticles' ability to produce heat was improved after the coating of polydopamine. Compared with G/BWP/PDA, the G/PDA membrane exhibits higher light absorption ( Figure 6) and photothermal-induced temperature rise (Figure 10a). The surface temperature of G/PDA/BWN P (49.0 • C) is lower than that of G/PDA. The Bi 2 WO 6 nanomaterials exhibit lower light absorption than PDA (Figure 6). PDA also has excellent photothermal conversion properties. It may explain why the coverage of polydopamine by B 2 WO 6 nanoparticles leads to the decrease of surface temperature from 67.4 (G/PDA) to 49.0 • C (G/PDA/BWN P ). The distribution of PDA and Bi 2 WO 6 on the samples has some influences on the temperature increase. G/BWP/PDA with PDA-coated G/BWP structure exhibit a higher temperature increase than the G/PDA/BWN P sample with BWN P -coated G/PDA structure. The surface temperatures reached the maximum values for all samples after 10 min of irradiation (Figure 10b). The photothermal-induced temperature rise of gauze (as a comparative sample) was much lower than G/PDA and G/BWP/PDA. The results indicate the high light absorption and effective photothermal conversion properties of these PDA-based samples (G/PDA, G/BWP/PDA, G/PDA/BWN P ). Meanwhile, polydopamine is the critical component for the photothermal conversion properties of composite materials. Figure 10c presents the mass loss of water over time by G/PDA, G/BWP/PDA, gauze after simulated solar illumination at 1 kW m −2 . The water evaporation rates of the Bi 2 WO 6 -PDA based films did not follow the trend of the light-induced temperature rise. The water evaporation rates of the G/BWP/PDA sample under the irradiation of the Xe lamp (light intensity = 1000 W/m 2 ) reached 1.94 kg·m −2 ·h −1 . Although the G/PDA and G/BWP/PDA films exhibited similar light-induced temperature rise, G/BWP/PDA showed a higher water evaporation rate than the G/PDA film (1.68 kg·m −2 ·h −1 ). The water evaporation rates of the functional films depend on the photothermal effect of the material, film structure for water supply and steam escape, and thermal management [76]. As shown in the SEM images (Figure 2), there are interstices among the Bi 2 WO 6 nanoplates for G/BWP/PDA. Compared with the G/PDA film, the G/BWP/PDA sample with Bi 2 WO 6 nanoplate aggregates on the fiber surface may provide a better capillary structure for water supply and steam escape. Besides, both the G/PDA and G/BWP/PDA films showed higher water evaporation rates than the gauze (0.93 kg m −2 h −1 ). The G/PDA/BWN P membrane shows a higher water evaporation rate (1.83 kg m −2 h −1 ) than the G/PDA sample (Figure 10c). The Bi 2 WO 6 nanomaterials may contribute to water transport and steam escape. These photothermal membranes can convert light energy into localized heat and vaporize water across the microporous surface into steam.
Polymers 2021, 13, x FOR PEER REVIEW 12 of 17 than that of G/BWP. The residual concentration of IC solution decreases gradually after light irradiation. The irradiation time required for degradation of 99% IC by G/BWP/PDA is 210 min. Such a decrease in the photocatalytic activity of the G/BWP/PDA sample may result from the coverage of B2WO6 nanoplates by polydopamine, leading to a reduction in the exposed active sites of B2WO6 nanoplates. Such a problem can be solved by changing the formation sequence of Bi2WO6 and polydopamine on the gauze substrate. The G/PDA/BWNP photocatalyst was prepared by the deposition of polydopamine on the gauze substrate by self-polymerization, followed by the formation of a uniform Bi2WO6 nanoparticle layer on top. Compared with G/BWP/PDA, the photocatalytic activity of the G/PDA/BWNP membrane was improved. Degradation of 96% of IC was achieved after being irradiated for 120 min in the presence of G/PDA/BWNP (Figure 8c). Photodegradation of indigo carmine (60 ppm) without photocatalyst was shown in Figure  9d as a blank test. Its decoloration rate was slower than other three curves with different photocatalysts. De Andrade et al. [74] studied the degradation of the indigo carmine dye in aqueous medium by the autoclaved cellular concrete/Fe2O3 catalyst in Fenton-like and photocatalytic processes. The toxicity tests against Vero cells indicated that the toxicity of the degradation products, generated by both processes, is smaller or similar to the precursor dye. Vautier et al. [56] reported the photocatalytic degradation of indigo carmine using UV-irradiated titania-based catalysts. The oxidative agents are photoproduced holes h + and/or •OH radicals. A detailed degradation pathway, based on careful identification of intermediate products, is proposed. In addition to a prompt removal of the color, photocatalysis can mineralize carbon and of nitrogen and sulfur heteroatoms into innocuous compounds. In our previous study [53], the photocatalytic degradation by Ag/flower-like Bi2WO6 photocatalysts with and without scavengers reveals that h + and •O2 − are the major active species generated by the photocatalyst. The oxidation of water by holes can generate •OH. We believe the products of the photocatalytic degradation using Bi2WO6 photocatalysts will not be more toxic than the parent compound, indigo carmine.

Solar-Steam Generation
The photothermal and solar steam generation performance of various samples (G/PDA, G/BWP/PDA, gauze) were evaluated. Typical infrared thermal images and the

Conclusions
A Bi2WO6-polydopamine (PDA) modified gauze was developed as a dual-functional membrane for application in photocatalytic degradation and solar steam generation. The spatial distribution of Bi2WO6 and polydopamine has a great influence on the light absorption properties, photocatalytic degradation, and solar steam generation performances of G/BWP/PDA and G/PDA/BWNP. G/BWP/PDA with polydopamine on the top layer showing a higher photothermal temperature rise. G/PDA/BWNP with Bi2WO6 nanoparticles on top exhibit higher photocatalytic activity and similar solar steam generation performance when compared with G/BWP/PDA. Active sites of Bi2WO6 nanoparticles that were not covered by polydopamine may contribute to the improved photocatalytic performance of G/PDA/BWNP. For the G/BWP/PDA and G/PDA/BWNP with better solar steam generation performance than G/PDA, the Bi2WO6 nanomaterials may contribute to the water transport and steam escape. The membrane with an interconnected pore structure can facilitate the contact between Bi2WO6 and dye pollutants, and the capillary transport of water to the top surface. Degradation of 96% of IC was achieved after irradiation for 120 min in the presence of G/PDA/BWNP. The photothermal water evaporation rate and surface temperature of the G/BWP/PDA membrane under irradiation (1 sun) reached 1.94 kg m −2 h −1 and 65.7 °C, respectively. For the application of water production in remote areas without electricity supply, a dualfunctional membrane (G/BWP/PDA or G/PDA/BWPNp) using a ternary compound may be a good choice. People can collect clean water by condensing the solar-generated water vapor, and can degrade the pollutant before it flows into the water body.

Conclusions
A Bi 2 WO 6 -polydopamine (PDA) modified gauze was developed as a dual-functional membrane for application in photocatalytic degradation and solar steam generation. The spatial distribution of Bi 2 WO 6 and polydopamine has a great influence on the light absorption properties, photocatalytic degradation, and solar steam generation performances of G/BWP/PDA and G/PDA/BWN P . G/BWP/PDA with polydopamine on the top layer showing a higher photothermal temperature rise. G/PDA/BWN P with Bi 2 WO 6 nanoparticles on top exhibit higher photocatalytic activity and similar solar steam generation performance when compared with G/BWP/PDA. Active sites of Bi 2 WO 6 nanoparticles that were not covered by polydopamine may contribute to the improved photocatalytic performance of G/PDA/BWN P . For the G/BWP/PDA and G/PDA/BWN P with better solar steam generation performance than G/PDA, the Bi 2 WO 6 nanomaterials may contribute to the water transport and steam escape. The membrane with an interconnected pore structure can facilitate the contact between Bi 2 WO 6 and dye pollutants, and the capillary transport of water to the top surface. Degradation of 96% of IC was achieved after irradiation for 120 min in the presence of G/PDA/BWN P . The photothermal water evaporation rate and surface temperature of the G/BWP/PDA membrane under irradiation (1 sun) reached 1.94 kg m −2 h −1 and 65.7 • C, respectively. For the application of water production in remote areas without electricity supply, a dual-functional membrane (G/BWP/PDA or G/PDA/BWPNp) using a ternary compound may be a good choice. People can collect clean water by condensing the solar-generated water vapor, and can degrade the pollutant before it flows into the water body.