Efficient Water Transport and Solar Steam Generation via Radially

Jun 24, 2019 - When the sunlight irradiation is applied to the PAAm-radial aerogel during the ... and exhibits a similar capacity for energy absorptio...
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Efficient Water Transport and Solar Steam Generation via Radially, Hierarchically Structured Aerogels Weizhong Xu,†,∥ Yun Xing,†,∥ Jian Liu,† Huaping Wu,‡ Ying Cui,§ Dewen Li,§ Daoyou Guo,† Chaorong Li,† Aiping Liu,*,† and Hao Bai*,§ Downloaded via UNIV OF SOUTHERN INDIANA on July 18, 2019 at 08:30:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Center for Optoelectronics Materials and Devices, Key Laboratory of Optical Field Manipulation of Zhejiang Province, Faculty of Mechanical Engineering & Automation, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China ‡ Key Laboratory of E&M (Ministry of Education & Zhejiang Province), Zhejiang University of Technology, Hangzhou 310014, P. R. China § State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China S Supporting Information *

ABSTRACT: A nature-inspired water-cycling system, akin to trees, to perform effective water and solar energy management for photosynthesis and transpiration is considered to be a promising strategy to solve water scarcity issues globally. However, challenges remain in terms of the relatively low transport rate, short transport distance, and unsatisfactory extraction efficiency. Herein, enlightened by conifer tracheid construction, an efficient water transport and evaporation system composed of a hierarchical structured aerogel is reported. This architecture with radially aligned channels, micron pores, and molecular meshes is realized by applying a radial ice-template method and in situ cryopolymerization technique. This nature-inspired design benefits the aerogel excellent capillary rise performance, realizing a long-distance (>28 cm at 190 min) and quick (>1 cm at 1 s, >9 cm at 300 s) antigravity water transport on a macroscopic scale, regardless of clean water, seawater, sandy groundwater, or dye-including effluent. Furthermore, an efficient water transpiration and collection is performed by the bilayer-structured aerogel with a carbon heat collector on an aerogel top, demonstrating a solar steam generation rate of 2.0 kg m−2 h−1 with the energy conversion efficiency up to 85.7% under one solar illumination. This biomimetic design with the advantage of water transport and evaporation provides a potential approach to realize water purification, regeneration, and desalination. KEYWORDS: hierarchically structured aerogel, radial ice-template method, water transport, solar steam generation, nature-inspired design ticles,13−15 one-dimensional nanowires,16,17 two-dimensional nanosheets,18−21 three-dimensional gels, and their composites,22−25 have been used to enhance the conversion efficiency. However, challenges remain in terms of the relatively low transport rate and unsatisfactory extraction efficiency for sewage, seawater, and groundwater. Waterharvesting and transport capabilities, especially long-distance liquid spreading, and material stability in a water-containing environment are still highly challenging but also urgently needed. Trees play a critical role in the global water cycle by relying on water transport via xylem vessels and channels and water

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ncreasing water scarcity, especially freshwater shortage, is a global issue that needs to be considered seriously. Solar steam generation is one of the most promising techniques to realize water purification, regeneration and desalination.1−3 Among the factors that affect the efficiency of solar steam generation, water transportation and evaporation is of very high importance and recently has attracted tremendous attention. The biomimetic strategy based on natural systems such as shorebird,4 spider silk,5 cactus spine,6 nepenthes alata,7 and sarracenia trichome8 provides the possibility of achieving small amounts of pure water through directional short-distance transportation. More recently, solar-irradiation driven water evaporation and extraction have been of great interest, and tremendous efforts have been devoted to enhancing sunlight absorption and thermal management, localizing heat generation, and improving water evaporation efficiency.9−12 A variety of materials, including zero-dimensional nanopar© XXXX American Chemical Society

Received: March 25, 2019 Accepted: June 24, 2019 Published: June 24, 2019 A

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structure by using a radial freezing and in situ cryopolymerization technique (low-temperature cross-linking).29−31 In addition, the multiwalled carbon nanotube (MWCNT) layer was used on the top part of aerogel as the solar absorber to driven water evaporation under solar irradiation (Figure 1d). Such a bilayer PAAm-radial aerogel with architectural features similar to those of T. distichum can realize powerful antigravity water transportation through the interconnected radially aligned channels, micron pores, wrinkled internal surface, and molecular meshes (Figure 1e−g) and highly efficient water evaporation under solar irradiation, whether with fresh water, seawater, sandy groundwater, or dye-including effluent. The transport distance of >28 cm at 190 min, 1.08 cm transport distance in the first second, and a high water evaporation rate of 2.0 kg m−2 h−1 under one sun irradiation verify the advantage of our designed materials in water transport and evaporation for water shortage issue solution.

transpiration through leaves under solar irradiation. Inspired by this, many biomimetic designs are carried out to obtain purified water via renewable solar energy collection and a thermal management system.9 As one type of conifer plants,26,27 the towering Taxodium distichum (or redwood) has radially aligned microchannels and wavy porous networks (Figure 1a−c). Furthermore, its torus margo with pits presents

RESULTS AND DISCUSSION The ice-templated method,32 also known as freeze-casting, has been considered as a powerful technique to make ordered porous structures, including honeycomb,33−35 lamellar,36−40 and radial (with graded channels),29,30,41,42 by controlling the nucleation and growth of ice crystals. The typical radial freezing and in situ radical cryopolymerization process to prepare the PAAm-radial aerogel is illustrated in Figure 2a. First, a given concentration of monomer (acrylamide, AAm), cross-linker (MBAA), and initiator (V-50) was adequately mixed and poured into a precooled copper tube (1 cm in diameter and 15−40 cm in length) vertically immersed in cryogenic ethyl alcohol (−90 °C). Hence, the copper tube was subjected to a temperature gradient along the radial directions,29 and anisotropic ice crystals grew preferentially along the temperature gradient by expelling the solute from the solidifying water (Figure S1a−d). After that, the sample was in situ polymerized under ultraviolet (UV) light (365 nm) and freeze-dried at −80 °C, resulting in a hierarchical structure

Figure 1. Images and schematic of taxodium distichum and the bilayer PAAm-radial aerogel. (a) Optical image and SEM images of (b) cross-section and (c) longitudinal section of taxodium distichum that show the radially and vertically aligned channels and porous structures. (d) Schematic of solar vapor generation based on the bilayer PAAm-radial aerogel. The CNTs coating layer acts as an efficient light absorber layer, while the bottom aerogel layer is hydrophilic, which promotes rapid water transport for continuous solar vapor generation. The antigravity water transportation is realized through (e) interconnected microchannels, (f) multilevel apertures, and (g) molecular meshes of PAAm-radial aerogel.

superior hydraulics for minimizing the flow resistance, which would favor faster and longer water transportation.28 Herein, we artificially designed a three-dimensional polyacrylamide (PAAm) based aerogel with a radial and centrosymmetric

Figure 2. Preparation, structure characterization, and morphology of the bilayer PAAm-radial aerogel. (a) Illustration of a PAAm-radial aerogel fabricated by radial freezing and in situ cryopolymerization process. (b−d) SEM images of PAAm-radial aerogel at different magnifications showing hierarchical structures including (b, c) radially aligned channels and micron pores and (d) wrinkled internal surface. (e) SEM image of MWCNTs layer coated on the surface of PAAm-radial aerogel. Photos showing (f) the top view of PAAm-radial aerogel, (g) the side view of PAAm-radial aerogel and bilayer aerogel, and various lengths and shapes of PAAm-radial aerogels: (g) and (i) single aerogels; (h) and (k) seamed grafted aerogels; (j) seamless grafted aerogel. B

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Figure 3. Water transport in the PAAm-based aerogels. The schematics and water transport status at different time point for (a) PAAm-radial aerogel, (b) PAAm-random aerogel, and (c) PAAm-hydrogel aerogel. (d) Long-distance water transport at different status for single PAAmradial aerogel. (e) Transport distance of water over time for PAAm-based aerogels. (f) Transport speed as a function of time. Inset: initial transport speed of water during the first second. (g) Transport distance and speed of water over time for long PAAm-radial aerogel without and with sunlight irradiation. The errors in (e)−(g) were calculated through six repeated experiments.

with radially aligned channels, micron pores, wrinkled internal surface, and molecular meshes (polymer network) (Figure 2b− d, Figure S2). When the freezing temperature was changed from −30 to −196 °C, the PAAm-radial aerogels showed a more hierarchical porous structure (Figure S3). This architecture would be helpful for the improvement of water transportation capability through capillarity and branched diffusion (Figure 1d−g).6,7 Combining seamless or seamed grafting technology (see details in the Methods), different lengths and shapes of aerogels can be easy obtained (Figure 2f−k). In order to increase the light absorption and improve the solar steam generation, the MWCNTs layer was compactly adsorbed on the top of the aerogel with a low mass percentage of 0.3 wt % (Figure 2g). The randomly oriented MWCNTs on the bilayer aerogel were verified by scanning electron microscopy (SEM, Figure 2e), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and Raman spectroscopy measurements (Figure S4). In order to verify the characteristics of radially aligned channels, two reference aerogel materials were designed. The PAAm-random aerogel was prepared by placing the solution in the same sized transparent plastic mold for random freezing at −90 °C and then polymerizing it in situ (Figure S5a). The PAAm-hydrogel aerogel was synthesized by in situ polymerizing the solution (UV light irradiation for 5 min) in same sized transparent plastic mold and then was radially frozen in the freeze-drying device (Figure S5b). It was found that different preparation methods led to different microstructures. Because the plastic

mold was surrounded by the cold source and ice crystals grew from the outside to the inside of the solution in various directions, the PAAm-random aerogel shows a random architecture with a disordered porous structure (the porosity of 79.17 ± 2.72%) and wrinkled internal surface (Figure S6a,b). The PAAm-hydrogel aerogel shows an ordered and gradient porous structure quite different from the PAAm-radial one with the pore diameter increasing from the center to the margin (Figure S6c, the porosity of 62.34 ± 5.21%) and the pore wall thicker and nonporous (Figure S6d). The results of Brunauer−Emmett−Teller (BET) surface area and Barrett− Joyner−Halenda (BJH) pore-size distribution indicate that the PAAm-radial aerogel possesses a surface area of 4.910 m2 g−1, and most of mesoporosity is distributed in the 2−10 nm range (Figure S7) with relatively large porosity of 83.32 ± 2.09%. This means that the PAAm-radial and PAAm-random aerogels have thinner (about 2 μm, Figure 2d and Figure S6b) and compacted polymer networks when compared to the PAAmhydrogel aerogel with thicker (about 5−10 μm, Figure S6d) and looser polymer networks. This could be attributed to the restricted growth of ice crystals by the formed polymer network with high viscosity for the PAAm-hydrogel aerogel,43,44 which makes the polymer networks hard to squeeze. Directional water transport is critical in the water collection and extraction.4−8 Many previous reports involve shortdistance vertical transport or long-distance horizontal transport of water by controlling the micro-/nanostructure, wettability, C

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Figure 4. Water transport behavior of different types of water in the PAAm-radial aerogel. The water transport status at different time point from (a) seawater (from the East China Sea), (b) groundwater, and (c) branch water through biomimetic forked branches. Transport distance and speed of water over time from (d) seawater, (e) groundwater, and (f) branch water through the PAAm-radial aerogels, respectively. The errors in (d)−(f) were calculated through six repeated experiments.

and response to external stimuli of materials.45−48 Additionally, the ability to resist deformation under water should be considered to provide necessary mechanical support. In order to reduce the hygroscopic strain of the aerogels, the aerogels were preswelled and then dried by lyophilization (Figure S8). The pretreated PAAm-radial aerogel is self-standing under dry and wet environments (Figure S9) with excellent deformation resistance in water by the compressive tests (10 cycles at 60% strain) when compared to the other two aerogels (Figure S10). The results of mechanical simulation also fit well with our experimental ones (Figure S11, movie S1), demonstrating the importance of pretreatment in keeping the volume and performance stability for subsequent long-term use. Furthermore, the photos and CLSM (confocal laser scanning microscope) images of the PAAm-radial aerogel under the wet environment confirm its structure stability even upon its tenth cyclical use (Figure S12). Parts a−c of Figure 3 present the transport status of fresh water (rhodamine B dye is used for visual contrast) in three aerogel materials during different transport times. The water can be effectively transported to the top of PAAm-radial aerogel (about 10 cm in length) in a relatively short period of time (about 300 s) under capillary

force (Figure 3a, movie S2). The longest transport distance of 9.90 cm at 300 s, 1.08 cm in the first second and a relatively high transport speed even in the late stage (∼0.01 cm/s) (Figure 3e,f) are achieved for the PAAm-radial aerogel. The power law of D ∼ t0.406 is further obtained by plotting the cures of capillary rise distance (D) versus transport time (t) (Figure 3e; Figure S13), which is completely different from the capillary rise into a deformable porous material like cellulose sponges (D ∼ t0.5 for early stages and D ∼ t0.2 for late stages).49 The increase in power exponent is attributed to the abnormally abundant and well-designed capillaries, which is more beneficial for wicking dynamics throughout the phase. It is clear that faster and longer water transport is achieved in a PAAm-radial aerogel. That is, the hierarchical porous structures, including radially aligned channels, micron pores, wrinkled internal surface, and molecular meshes, contribute to powerful capillary rise of water in the PAAm-radial aerogel. On the contrary, the water transport in the PAAm-random aerogel with random channels is very slow, showing only 6.53 and 0.55 cm transport distance within 300 s and in the first second, respectively (Figure 3b,f, movie S3). The transport behavior can be attributed to the increased transport distance of water in D

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Figure 5. Solar vapor generation performances. (a) Schematic of solar vapor generation by using the solar vapor generation device based on the bilayer PAAm-radial aerogel arrays under one sun irradiation. (b) Surface temperature change of aerogel samples and water relative to heating time under one sun irradiation. Insets: infrared images showing the temperature distribution after irradiation for 0, 300, 1800, and 3600 s. (c) Cumulative mass change due to water evaporation over time for water and aerogels under one sun irradiation. (d) Comparison of the water evaporation rate and energy efficiency for water alone and with PAAm-based aerogels. (e) Comparison of vapor generation performances obtained from different materials under one sun irradiation. The errors in (d) were calculated through six repeated experiments.

Sea with blue dye for visual contrast) in PAAm-radial aerogel decrease slightly due to the probable interference by the salt crystal movement and blockage (Figure 4a, movie S6), giving the transport distance of 10.10 cm at 420 s, 1.05 cm in the first second, and 0.01 cm/s final transport speed (Figure 4d). To mimic the sandy groundwater, an appropriate amount of water was added in bilayer sand (upper coarse sand and lower fine sand). The successful extraction of sandy groundwater is demonstrated (Figure 4b, movie S7) with the transport distance of 11.45 cm within 420 s, 1.09 cm in the initial second, and 0.01 cm in the last second, respectively (Figure 4e). Additionally, the dye-including effluent successes transport from the bottom (root) to the trunks and branches of a biomimetic forked tree (made with hot melt adhesive and fishing line, Figure 4c, movie S8) at different angles and transport velocities since the angle of inclination of branch will affect the transport rate of water (see more details in Figure S20). Water transport through branches needs more time due to the gap existence between grafted branches, giving a transport distance of 11.51 cm at 420 s and 1.03 cm during the first second, respectively (Figure 4f). The repeatability test of the PAAm-radial aerogel also exhibits imperceptible change in transport distance at 300 s and 10% loss in the first second at the tenth recycling process (Figure S21), indicating its excellent capability of reduplicative water transport. In order to collect the transmissive water to the branch top, he natural evaporation process of water was designed via the renewable solar energy (Figure 1d and Figure 5a). A CNTs coating layer which can efficiently absorb the solar illumination was deposited on the top of PAAm-radial aerogel (Figure 2e,g) to increase the evaporation action and continuous water transport. The optical properties of the bilayer aerogel were measured by the UV−vis spectrometer in the range of 200− 2500 nm. Compared to the single PAAm-radial aerogel, the bilayer aerogel could minimize the reflection and transmission of incident light and maximize light absorption (over 94%)

the random channels with micron pores (Figure S6a,b). The PAAm-hydrogel aerogel shows the worst water transport with only 0.59 and 0.43 cm transport distance within 300 s and in the first second, respectively (Figure 3c, movie S4) due to the lack of adequate capillaries for the smooth, thick pore walls (Figure S6c,d). The PAAm-radial aerogel also shows excellent water absorption ability with a unit mass water absorption of 15.66 ± 1.13 gg−1 (Figure S14). Changing the freezing temperatures from −30 to −196 °C or fabricating large-scale aerogel (5 cm in diameter and 14 cm in length) can further improve water-transport performance of PAAm-radial aerogels (Figure S15−S17). Therefore, we can conclude that the structure of the PAAm-radial aerogel would better benefit the continued rise of water over a long distance. In order to verify our speculation, we prepared long PAAm-radial aerogels or single (Figure 2i, Figure 3d, Figure S18a), seamless grafted (Figure S18b), or seamed grafted aerogels (Figure S18c). A >28 cm transport distance, a 0.01 cm/min final transport speed, and a longest transport distance of 32.86 cm within 770 min (Figure 3d,g, Figure S18, movie S5) suggest that the synergistic effect of micron- and nanosized capillary channels and pores favors the PAAm-radial aerogel an outstanding ability of long-distance water transport. When the sunlight irradiation is applied to the PAAm-radial aerogel during the water-transport process, the water-transport distance and transport speed do not change significantly (Figure 3d,g, Figure S18a,d). This indicates that the sunlight irradiation does not significantly affect the water transport performance of PAAm-radial aerogel due to its good thermal insulation ability. Therefore, the capillary force of PAAm-radial aerogel with excellent wettability (Figure S19) could be the prime driving force for the effective antigravity transport. Furthermore, we researched the transport behavior of different types of water in the PAAm-radial aerogel, such as seawater, sandy groundwater, and dye-including effluent. The transport rate and distance of seawater (from the East China E

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flows along the glass surfaces into the bottom of condensing receptacle after about 10 h (Figure S27d). Furthermore, a real seawater sample (from the East China Sea) was used for desalination. All of the ions (Na+, Mg2+, K+, and Ca2+) show four orders of ionic concentration decrement after desalination (Figure S28a). Though it exists the salt crystals on the surface of channels after seawater transport and evaporation, the transport distance, transport velocity and evaporation rate are almost unaffected in a 7 day service process due to the relative small size of the salt crystals compared to one of the channels (Figure S29). Meanwhile, the effect of dyeincluding effluent treatment (methyl orange, rhodamine B, and methylene blue) was examined in the same way, which shows negligible contaminant in the collected water as evidenced by the nearzero optical absorbance of the condensated water (Figure S28b−d). Therefore, the bilayer PAAm-radial aerogel is potentially reliable for solar vapor generation to realize water purification, regeneration, and desalination and to solve water scarcity issues globally.

throughout the solar spectra to convert to heat effectively (Figure S22). The photothermal behavior of the bilayer aerogel under one solar irradiation (1 kWm−2) was evaluated, and the temperature variation with illumination time for the bilayer aerogel in a glass bottle (40 mL) with 30 mL water was carefully traced (Figure 5b). The surface temperature of bilayer aerogel increases to about 38 °C within a short time (about 600 s) and 42 °C within 3600 s under one solar irradiation (see infrared photographs after irradiation, Figure 5b). The corresponding xerogels (without water) could be identically heated to ∼60 °C in a short period of time (Figure S23c,d). In contrast, the temperature variation of single PAAm-radial aerogel is gentler and exhibits a similar capacity for energy absorption just like that of pure water. This means that the absorbed energy of bilayer aerogel is mainly utilized to heat water (radiation heating) until the energy balance between water heating and water evaporation is reached. The overall mass change due to water evaporation under one solar irradiation was recorded within 3600 s (Figure 5c). Water evaporation in a dark environment is very slow with an evaporation rate about 0.08 kg m−2 h−1, which is therefore subtracted as baseline for evaporation rate calculation. The mass change of bilayer aerogel is more efficient with the evaporation rate of 2.0 kg m−2 h−1 when compared to pure water (0.36 kg m−2 h−1) and single PAAm-radial aerogel (0.88 kg m−2 h−1) at one sun irradiation. The corresponding energy efficiency (η) of the bilayer aerogel can be calculated using the following formula20,21 η = mhLV /CoptP0

CONCLUSIONS In summary, by mimicking the taxodium distichum and conifer tracheid, we successfully designed and fabricated a catching PAAM aerogel with hierarchical and radial architectures via the radial ice-templated assembly and UV-initiated crypolymerization. The as-prepared aerogel had a hierarchical structure with radially aligned channels, micron pores, wrinkled internal surface, and molecular meshes, which not only provided sufficient mechanical support performance but also exhibited excellent capillary rise function, realizing long-distance (>28. cm) and fast (1.08 cm at 1s, 9.90 cm at 300 s) antigravity water transport in macroscopic scale. Meanwhile, we also implemented various forms of water transport, including pure water, contaminative water, seawater, and sandy groundwater even through the biomimetic forked tree. Furthermore, we constructed an efficient solar vapor generation device based on the bilayer PAAm-radial aerogel with compact CNTs absorption, giving a solar vapor generation rate of 2.0 kg m−2 h−1 with energy efficiency up to 85.7%. This significant water transporting and evaporating system could serve as a source of inspiration in the field of microfludics, water transportation and collection, energy management, and pollution abatement.

(1)

where m is the evaporation rate, hLV is the total enthalpy of the liquid−vapor phase transition including the sensible heat, and Copt is the optical concentration on the absorber surface. Note that P0 refers to the nominal solar irradiation power of 1 kWm−2. The energy efficiency under one sun is calculated to be 22.5%, 47.3%, and 85.7% for pure water, single PAAm-radial aerogels, and bilayer aerogels, respectively (Figure 5d). Evaporation measurements demonstrate a reduced vaporization enthalpy compared with that of pure bulk water (Figure S24).24,25,50 The bilayer PAAm-radial aerogels prepared at different freezing temperatures and the large-scale bilayer PAAm-radial aerogel also show impressive water evaporation ability (Figure S25). This evaporation performance (water evaporation rate and energy efficiency) of bilayer aerogel is comparable and even superior when compared with many recently reported materials (Figure 5e). The evaporation rate almost keeps in the range from 1.8 to 2.0 kg m−2 h−1, and even the bilayer aerogel continually works at least 50 cycles under one sun irradiation (Figure S26), demonstrating its excellent stability. The real performance of this bilayer aerogel under natural sunlight outdoors (various solar flux, air temperature, internal humidity, and incident angles, Figure S27a,b) was also evaluated from 08:00 am to 20:00 pm (25 Sep 2018). The water evaporation rate exceeded 1.5 kg m−2 h−1 when the solar irradiation was strongest (∼700 W m2−) from 10:00 am to 15:00 pm (Figure S27b). Finally, a device for water transport and collection was developed by using a large-scale bilayer aerogel array (3 × 3, the total area of ∼4.5 cm2) placed at the center of the chamber (a custom glass container) (Figure S27c). The water steam was generated and then condensated when it arrived at the cold wall of the container after 30 min (movie S9). Under the action of gravity, the condensed water

METHODS Preparation of the Radial Freezing Device. A copper tube (mold) with a diameter of 1 or 5 cm (0.1−0.5 cm in thickness) and a length of 15 or 40 cm was chosen. The bottom of this cylindrical was filled with plastic foam (∼1 cm thick). When the copper tube was immersed vertically in cryogenic ethyl alcohol (−30, −60, and −90 °C) or liquid nitrogen (−196 °C), a radial temperature gradient from outside to inside was generated. Fabrication of PAAm-Radial Aerogel. The radially aligned PAAm aerogel (PAAm-radial aerogel) was fabricated by a two-step method. In brief, a precursor solution containing deionized water (8.9 mL), AAm (acrylamide, 0.9 g), chemical cross-linker N,Nmethylenebis (acrylamide) (MBAA, 0.0875g), and photoinitiator 2,2-azo-bis(2-methylpropionamidine) (V-50, 0.1125g) was poured into the radial freezing device (precooling for 30 min before use). After the dispersion was completely frozen in the freeze-casting step, the frozen samples were carefully demolded and then placed under UV light (λ = 365 nm) for 6 h to complete the in situ radical cryopolymerization in a cold trap at −80 °C. The samples were freeze-dried for more than 48 h at −80 °C with a freeze-dryer (Shanghai Leewen Scientific Instrument Co., Ltd.) under a pressure F

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SS150). The solar flux was measured using an optical power meter (BroL ght). The absorption, reflectance, and transparency were measured by a UV−vis−NIR spectrometer (UH4150, HITACHI) with an automated reflectance measurement unit and integrating sphere unit. The temperature and infrared images were measured using an IR camera (FLIR E4), and the mass change of water loss was recorded with an electronic mass balance with 0.1 mg accuracy. The evaporation rate in dark conditions was obtained after stabilization for 1 h. All evaporation rates were measured after stabilization under one sun irradiation for 30 min.

of 1 Pa. All of the samples needed preswelling and freeze-dried again to avoid large hygroscopic strain before use. Fabrication of PAAm-Random and PAAm-Hydrogel Aerogels. The PAAM-random aerogel was prepared by placing the solution in same sized transparent plastic mold in a freezer at −90 °C to obtain a random porous structure and then polymerizing it at the same condition. The PAAm-hydrogel aerogel was prepared by an opposite two-step method. In brief, a precursor solution was poured into the same sized transparent plastic mold, which was subjected to UV light irradiation for 5 min to form a PAAm hydrogel. Then, the hydrogel sample was transferred into a radial freezing device. After the hydrogel was frozen entirely, the sample was demolded and freezedried. Fabrication of Bilayer PAAm-Radial Aerogel. The PAAmradial aerogel was cut to 5 cm in length. The MWCNT suspension (5 mg/mL) was prepared by dispersing MWCNTs and surfactant sodium dodecyl sulfate (SDS, 1:2 wt %) in deionized water and sonicating for 30 min. Then the suspension was deposited on the top of PAAm-radial aerogel (the height of CNTs layer was about 0.5 cm) repeatedly to ensure complete coverage and then freeze-dried for more than 24 h. Characterization. The morphology of the aerogel was observed by scanning electron microscopy (SEM, S-4800, Hitachi, Tokyo, Japan) at an acceleration voltage of 5 kV. X-ray diffraction (XRD, Bruker D8) was recorded over the range of 10−90°. Fourier transform infrared (FTIR) (Thermo Scientific, Nicolet iS50 Series) data were recorded over the range of 500−4000 cm−1. Raman spectroscopy (Horiba Jobin Yvon LabRAM HR-Evolution Raman) was done with a 532 nm laser at ambient conditions. Brunauer− Emmett−Teller (BET) surface area and Barrett−Joyner−Halenda (BJH) pore-size distribution analysis were carried out by a surface area analyzer (Autosorb-iQ). The porosity of aerogels was measured by the ethanol replacement method. For a compressive test, the samples were cut to test cylinder size (8 mm in height, 8 mm in diameter) and soaked with water overnight. Then the samples were compressed in the direction perpendicular to the radial temperature gradient direction in an Instron 5943 at a displacement rate of 1 mm/ min. The microstructures of the PAAm-based aerogels under a wet environment were observed using a confocal laser scanning microscope (CLSM, IX83, OLYMPUS, Japan). Dynamic water contact angles were measured by a contact angle geometer (OCA 50 AF, Dataphysices, Germany) with a high speed camera (PCO. dimax HD, Germany). The concentration of ions in seawater was tracked by inductively coupled plasma mass spectrometry (ICP-MS, Elan DRC-e, PerkinElmer SCIEX, USA). The optical absorption of the dye-including effluent after treatment was measured by a UV−vis spectrometer (U-3900, HITACHI). Water-Transport Experiments. All samples (except for the PAAm-hydrogel aerogel) with preswelled and freeze-dried were fixed by the nipper and iron support, then water was pour into a glass dish until water touched the bottom of material. A small quantity of red dye (Rhodamine B) or blue dye (Methylene blue) was added in D.I. water to increase the visual contrast. To mimic long-distance water transport, several same materials were spliced seamless or with hot melt adhesive (seamed). To mimic a hostile environment, seawater (from the East China Sea), sandy groundwater (little water in the sand), and sewage (with methyl orange, rhodamine B, or methylene blue) were adopted in water transport. Continuous videos were recorded to determine the location of the water and then screenshot at different time points. The distance−time relations were acquired by analyzing printscreens with Image j software. The speed of water transport was calculated according to the distance in the corresponding time period. Each determination was repeated three times to evaluate the variability of the results. Solar Vapor Generation Experiments. The bilayer PAAmradial aerogel 8 mm in diameter was located on a glass bottle (40 mL) with 30 mL of water (about 1 cm distance from the interface of water/air) and immobilized through the preservative film, hot melt adhesive, and rubber band. The water evaporation experiment was executed using one solar flux (1 kWm−2) from a solar simulator (Zolix

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b02331. Experimental details and additional characterization data (XRD, FTIR, and Raman data, SEM images, compressive stress−strain curves, water transport photos, reflectance, transmittance, and absorption spectra, water evaporation rate) (PDF) Mechanical simulation of the structure evolution of the PAAm-radial aerogel under large geometry deformation (by ABAQUS 6.13) (MP4) Water transport behavior in the PAAm-radial aerogel (AVI) Water transport behavior in the PAAm-random aerogel (AVI) Water transport behavior in the PAAm-hydrogel aerogel (AVI) Water transport behavior in the long PAAm-radial aerogel (AVI) Transport behavior of seawater in the PAAm-radial aerogel (AVI) Transport behavior of groundwater in the PAAm-radial aerogel (AVI) Transport behavior of branch water in the PAAm-radial aerogel (AVI) Water steam generation and condensation by using the solar vapor generation device with bilayer PAAm-radial aerogel arrays under one sun irradiation (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Huaping Wu: 0000-0003-4505-7062 Daoyou Guo: 0000-0002-6191-1655 Aiping Liu: 0000-0002-2338-062X Hao Bai: 0000-0002-3348-6129 Author Contributions ∥

W.X. and Y.X. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51572242 and 21674098) and the Zhejiang Outstanding Youth Fund (No. LR19E020004). G

DOI: 10.1021/acsnano.9b02331 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano

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DOI: 10.1021/acsnano.9b02331 ACS Nano XXXX, XXX, XXX−XXX