Rollable and Ventilated Net‐Based Solar Thermal Water Evaporator for Casting on Water Surface

The solar‐thermal evaporator provides a sustainable freshwater production strategy, of which the large‐scale floating applications on river or sea still confront the challenge of the highly efficient flexible evaporator. However, existing flexible evaporators often use porous gel to load solar‐thermal materials, which only allow water to evaporate out from outermost surface but waste heat by dissipating into bulky water. Herein, a rollable and ventilated flexible solar‐thermal evaporator based on dendritic net substrate is presented, which can be cast in a scalable way on waving water surface. Compared to that of the porous gel, the dendritic structure can not only capture photons efficiently, but also enlarge the ratio of water evaporation area to heat dissipation area by over 109 times, with efficient water transfer path of merely 0.3 cm length in average for even 1 m2 water evaporation area. The flexible evaporator can achieve a photothermal evaporation temperature of 108 °C and an evaporation rate of >1.46 kg m−2 h−1@1 sun illumination. A sustainable ecological circulation evaporation system is demonstrated to show its potential for scalable floating‐on‐sea applications.

DOI: 10.1002/sstr.202300008 The solar-thermal evaporator provides a sustainable freshwater production strategy, of which the large-scale floating applications on river or sea still confront the challenge of the highly efficient flexible evaporator. However, existing flexible evaporators often use porous gel to load solar-thermal materials, which only allow water to evaporate out from outermost surface but waste heat by dissipating into bulky water. Herein, a rollable and ventilated flexible solar-thermal evaporator based on dendritic net substrate is presented, which can be cast in a scalable way on waving water surface. Compared to that of the porous gel, the dendritic structure can not only capture photons efficiently, but also enlarge the ratio of water evaporation area to heat dissipation area by over 109 times, with efficient water transfer path of merely 0.3 cm length in average for even 1 m 2 water evaporation area. The flexible evaporator can achieve a photothermal evaporation temperature of 108°C and an evaporation rate of >1.46 kg m À2 h À1 @1 sun illumination. A sustainable ecological circulation evaporation system is demonstrated to show its potential for scalable floating-on-sea applications.
photosynthetic unit of the large-area algae leaf system for nutrients and products transporting ( Figure 1a). It is noteworthy that the net-based solar-thermal evaporators can be further connected together to form flexible floating networks. Inside the solar-thermal evaporator, the large-area dendritic structure can serve for coating the photothermal layer, while being directly in contact with the atmosphere. Moreover, water molecules only need to flow through an efficient dendritic pathway along the hydrophilic polymer layer coating on the surface of dendrites. The water can be rapidly transferred all over the large solarthermal evaporation area and quickly evaporate under sunlight when flexible networks float on the undulating water surface.
The working principle of the rollable and ventilated net-based solar-thermal evaporator for water evaporation is shown in Figure 1b, S1, and Section S1, Supporting Information.
First, sunlight harvesting is a key point for efficient solarthermal water evaporation. As shown in Figure 1c,d, micrometal dendrites were grown on a flexible conductive net with a diameter of around 100 μm. The micrometal dendrites with a height below 450 μm are composed of small branches with a diameter of tens of micrometers. The inset of the Figure 1d shows the photo of the micrometal dendrites deposited on the flexible net with an average height of 400 μm, and the dendrites array can be properly arranged by finely controlling the synthesis conditions. As shown, light reflection could be reduced by the diffuse reflection of micro dendrites to form a photocage, which means that it would help to enhance photons harvesting and avoid photons escaping out of the dendritic photothermal layer. [8] Second, for efficient photothermal conversion, WO 2.72 particles with an average size of 105 nm in diameter ( Figure 1e) were chosen as the photothermal oxide due to its strong localized surface plasmon resonance (LSPR) effect, which was coated on the large surface area provided by micrometal dendrites. The crystalline structure of WO 2.72 was confirmed by X-ray diffraction (XRD) (Figure 1f ). The observed characteristic peaks at 23.3°, 26°, and 48.2°corresponded to the (010), (210), and (020) plane diffractions, respectively. [9] The interplane spacing of WO 2.72 (W 18 O 49 ) crystal as also indicated in the inset high resolution transmission electron microscope (HRTEM) image is 0.37 nm. It corresponds to the (010) plane of the monoclinic crystal structure. In addition, the X-ray photoelectron spectroscopy (XPS) spectrum was applied to explore the chemical composition and valence states of WO 2.72 . Figure 1g shows the complex energy distribution of W4f photoelectrons. W represents tungsten atoms, 4 represents  the principal quantum number, and f represents the core or inner atomic orbital. As is indicated, the W4f core-level spectrum could be fit to three spin-orbit doublets. They can be attributed to three different oxidation states of W atoms and are typically found in the WO 2.72 catalyst. The result of the selected-area electron diffraction pattern is also shown in Figure 1h, confirming that W and O elements distribute evenly throughout the WO 2.72 layer. [10] After the solar irradiation was efficiently absorbed by the photon cage, the solar energy was converted into heat by the WO 2.72 photothermal oxide as far as possible, leading to a rapid temperature increase in the dendritic photothermal layer. Furthermore, the water continuously flows from the bottom of the net to a thin layer of the solar evaporator floating on the water surface. Instead of heating the entire water body, the water is heated only when it reaches a small region in the thin layer of the solar evaporator.
Third the water and heat transfer in the net-based solar-thermal evaporator is very efficient. Taking a 1 cm 2 net-based solarthermal system as an example, the specific surface area of the dendrites coated with photothermal oxide and hydrophilic polymer composite can reach up to 46.8 cm 2 , which serves as a large water evaporation area. Meanwhile, the bottom grid part of the net-based solar-thermal system with an area of only 0.43 cm 2 directly contacts with bulky water, which serves as a small heat dissipation area. There before, the ratio of water evaporation area to heat dissipation area reaches 109 times. As is known, the energy loss through heat conduction is proportional to the heat dissipation area. The flat-type solar evaporator depends on a layer of thermal insulation material to avoid unnecessary energy loss. For our dendritic evaporator, the energy loss through heat conduction into the water only accounts for a small proportion, compared with the energy for water evaporation. In another word, the heat is concentrated in the thin dendritic layer, which is favorable for effective water evaporation.
The details for the calculation are summarized in Section S2, Supporting Information. What's more, the net-based solarthermal evaporator provides a super-short water transfer path of merely %0.3 cm for even 1 m 2 water evaporation area.
As a result, with the help of the algae-like structure, the netbased solar-thermal evaporator possesses a large photothermal evaporation area, a small heat dissipation area, and efficient water transfer path, leading to water evaporation efficiently under solar irradiation even in large-scale applications.

Structure Optimization of the Dendritic Net Substrate
Serving as the framework to support photothermal oxide/hydrophilic polymer composite, a flexible conductive net grown with metal dendrites was prepared through a template-free electrodeposition process using a homemade all-around reactor, [11] while the conductive net can be woven from metal, carbon, or metalcoated wires.
The mechanism of the dendrite's growth can be described as fractal growth. [12] At the onset of electrolysis, small branches will first emerge on the flexible net. Then, new small branches would grow from elder branches. As small branches grow bigger, new but smaller branches will then grow out as the next generation. This procedure can be repeated generation by generation, to form micrometal dendrites with a fractal structure similar to the branch or vein of plants. It is found that only a layer of metal grains piled on the flexible net at a low voltage, while a porous structure formed through the crosslink of dendrites with each other at a high voltage. As is discussed with more details in Section S3, Supporting Information, the formation of metal dendrites is caused by a selforganization mechanism during electrodriven ion migration.
To achieve more efficient water evaporation, structure optimization of micrometal dendrites has been carried out on factors, including growth time, growth voltage, current distribution on electrode, flow field, et al.
First, as indicated in Figure 2a,b, both the height and fractal dimension of micrometal dendrites elevated with growth time, given the same growth voltage. The average height of the micrometal dendrites increased from 0.15 mm at 30 s to 4.90 mm at 360 s. During this, the fractal dimension increased from 1.17 to 1.36 (Figure 2c). At a dendrite growth time of 240 s, a flexible net-based solar-thermal evaporator floating on water reaches the highest evaporation temperature ( Figure S2, Supporting Information) Notwithstanding, it is difficult for the photons to reach the bottom of the dendrites, if the crown of dendrites gets too crowded to reduce the sunlight utilization. A similar process can be observed for a tree crown in nature as the shadow effect. [13] Second, as indicated in Figure 2d,e, both the height and fractal dimension of micrometal dendrites increased with the growth voltage, given the same growth time. The height of the micrometal dendrites would increase from 0.42 mm at 24 V to 3.46 mm at 32 V, while the fractal dimension increased from 1.12 to 1.38. Accordingly, as shown in Figure S3, Supporting Information, for the dendrite growth voltage of 32 V, a flexible net-based solar-thermal evaporator floating on water reaches the highest evaporation temperature at 1 sun illumination.
Third, other factors including current distribution on electrode and flow-field direction were also investigated. On the one hand, metal dendrite growth is more favored for the intersection sites of the net ( Figure S4, Supporting Information), of which the current density was seven times higher than that at other locations according to our ANSYS simulation ( Figure S5, Supporting Information). On the other hand, different types of dendrites, including single-arrayed dendrites, axial-arrayed dendrites, and 2D arrayed dendrites ( Figure S6, Supporting Information), were prepared by controlling the flow-field direction in the reactors ( Figure S7, Supporting Information).

Heat and Mass Transfer Behavior Inside Solar-Thermal Evaporator
The high evaporation efficiency of the net-based solar-thermal evaporator can be also attributed to efficient photon capture capability, the large ratio of water evaporation area to heat dissipation area, and efficient water transfer path.
As indicated in Figure 3a, a flexible net-based solar-thermal evaporator with an optimized dendritic framework structure can decrease the reflectivity markedly from 85% to 20%, compared with that of a flexible metal net substrate without dendritic structure, which means a > 5 times increase of photon capture. Furthermore, after optimization works on loading amount of WO 2.72 -type photothermal materials, the highest evaporation temperature of 108°C at one sun illumination can be achieved   in an evaporation test floating on water, as shown in Figure 3b. For comparison, the highest evaporation temperature for metal net substrate and paper substrate loaded with the same amount and state of WO 2.72 only reach up to 50 and 73°C, respectively ( Figure S8, Supporting Information). As shown in Figure 3c and also in Section S4, Supporting Information, the heat has been concentrated in a thin photothermal layer of 400 μm, by taking advantage of efficient photons capture capability. Furthermore, the heat is evenly distributed along the branches of the micrometal dendrites, indicating that the forest of micrometal dendrites functions as a photon cage due to the interior diffuse reflection, which then enables a very efficient photon-to-heat conversion.
In addition, thermal management is very important for the design of high-efficiency evaporators. [14] For the flat-type sandwichlike solar evaporator, the water evaporation area into atmosphere and heat dissipation area into water is more or less the same, which usually depends on a layer of thermal insulation material to passively avoid unnecessary heat loss. For our dendritic evaporator, the unique dendritic structure is demonstrated to possess a large ratio of up to 109 times for water evaporation area to heat dissipation area. It provides another way to avoid excessive heat dissipation into the water, which is favorable for effective water evaporation.
Moreover, the photon capture capability can also be affected by the aggregation state of coated WO 2.72 catalyst. However, for the tradition dip-coating method, it is difficult to achieve a uniform layer of WO 2.72 catalyst on the curved surface of micrometal dendrites, as indicated in Figure S11, Supporting Information. Thus, the electrophoresis method was employed to improve the coating uniformity of WO 2.72 catalyst. As summarized in Figure S12, Supporting Information, after systematic optimization on the solvent, voltage, time, and concentration of the electrophoresis process, a %1 μm layer of WO 2.72 catalyst nanoparticles (105 nm in diameter) was evenly coated without blocking the dendritic structure, which further confirmed the photon cage effect of the dendrite and its excellent photothermal performance.
As shown in Figure 3d,e, the water transfer is essential for large-scale applications of the flexible solar-thermal evaporator. For the flat-type flexible solar-thermal evaporators, hydrophilic materials such as filter paper for water transfer over the photothermal layer (Figure 3d) are used. In large-scale applications, the water transfer path includes two sections, one is vertical section L1 and the other is horizontal section L2. The water transfer through horizontal section L2 is usually time-consuming. For 1 m 2 flat-type solar-thermal evaporators, the length of L 2 reaches 0.5 m, and it will cost more than 200 min for water transfer over the photothermal layer, giving a water transfer rate of 0.25 cm min À1 . During the water transfer process, water loss and heat waste are inevitable due to the long path of water transmission, leading to the low-efficiency utilization of the photothermal area.
For the dendritic-type flexible solar-thermal evaporators, the hydrophilic polymer coated on microdendrites surface is used, instead of the filter paper, for water transfer over the photothermal layer (Figure 3e). In large-area applications, the water transfer path only includes vertical section L1, from the metal net substrate to the top of dendrites. For an example of a 1 m 2 dendritic-type flexible solar-thermal evaporators, the average length of L 1 still remains below 0.3 cm, which is theoretically same as that of small solar-thermal evaporators of even 1 cm 2 . It will cost only %1 min for water transfer over the photothermal layer, giving a same water transfer rate of 0.3 cm min À1 . Therefore, the time for the water transfer over the photothermal layer of the 1 m 2 dendritic-type flexible solar-thermal evaporators is only 1/200 of that of 1 m 2 flat-type flexible solar-thermal evaporators.
The fast water transfer ability of the dendritic-type flexible solar-thermal evaporators is confirmed in Movie S1, Supporting Information, and the results at different stages are indicated in Figure 3f and S13, Supporting Information, which are conducive to the evaporation efficiency in large-area applications. In a word, for the dendritic-type flexible solar-thermal evaporators, no matter how large the evaporators are, the length of the water transfer still remains the same.
Except for the high evaporation efficiency, the dendritictype solar-thermal evaporator showed excellent flexibility and mechanical stability. As shown in Figure 3g,h, the dendritic-type solar-thermal evaporator can be stable at a bending angle of 150°, and no obvious mass loss of WO 2.72 was observed after over 100 bending cycles. It is interesting to find that gaps in between the dendrites substrate of the dendritic-type solar-thermal evaporator could allow a large relative shift. Furthermore, the dendritic-type solar-thermal can endure pressure of over 200 kPa (Figure 3i and S14, Supporting Information). As indicated in Figure 3j, the external mechanic pressure could be well-shared by branches of the micrometal dendrites, when these micrometal dendrites with a diameter of tens of micrometers are aligned in an array.

Water Evaporation Performance of the Solar-Thermal Evaporator
The photothermal water evaporation capability of the net-based solar-thermal evaporator was quantified by a floating experiment on the water surface under infrared light irradiation. Figure 4a shows the sectional-view thermal images of a net-based solarthermal evaporator under light irradiation captured by an IR thermographic camera. It shows that the interior temperatures quickly rose from 25 to 108°C in 5 min. As shown in Figure 4b, the evaporator with dendritic structure reached up to a water evaporation rate of >1.46 kg m À2 h À1 , while the water evaporation mass loss of the evaporator on the net substrate without dendritic structure can only achieve 0.35 kg m À2 h À1 under the same condition. Meanwhile, the evaporation rate of the dendritic-type solar-thermal evaporator can reach 1.46 and 1.32 kg m À2 h À1 in distilled water and salty water (3.5 wt% same as the seawater), respectively ( Figure S15, Supporting Information), which corresponds to an evidently promoted high water evaporation efficiency for the net-based solar-thermal evaporator (Figure 4c, S16 and Section S5, Supporting Information).
The flexible net-based solar-thermal evaporator shows its potential for applications in large-scale scenes, including seawater desalination, steam generation, sterilization, et al. A sustainable ecological circulation evaporation system has been constructed to demonstrate its potential for scalable floating-on-sea applications ( Figure 4d). As shown in Figure 4d, it contains three parts, a flexible net-based solar-thermal evaporator as the bottom floating on water, a transparent condensation chamber as the roof, and a tank for collecting evaporated water. www.advancedsciencenews.com www.small-structures.com Under solar illumination, the steady steam generated from the net-based solar-thermal evaporator will be condensed at the roof and then be collected by the tank. Besides water evaporation, the net-based solar-thermal evaporator not only allows oxygen passing through for underwater lives, as indicated in Figure 4e, but also has little effect on the temperature of the water under solar illumination, as shown in Figure 4f.
As shown by the long-term evaporation testing result of sustainable ecological circulation evaporation system, the dendritic structure of the net-based solar-thermal evaporator can lead to a > 400 times increase on the water evaporation under natural solar illumination (Figure 4g). By connecting together to form a floating network on water, the net-based solar-thermal evaporator can work steadily even under the disturbing and impact of wind and waves, as shown in Figure 1h, Movie S2, and S3, Supporting Information. Furthermore, the water evaporation rate after 30 days of test showed no efficiency loss, as indicated in Figure 4i, showing excellent stability for on-water applications. Figure S17, Supporting Information, shows the evaporation rate of 1.24 kg m À2 h À1 after 15 days' test, indicating the same excellent stability in salty water (3.5 wt% same as seawater). In addition, the metal leaching of our evaporator was very slow according to our metal leaching experiments (Table S1, Supporting Information).

Conclusion
In summary, a type of rollable and ventilated net-based solar thermal evaporator was developed which possesses a large photothermal evaporation area, small heat dissipation area, and efficient water transfer. With a merely 0.3 cm length of average water transfer path for even 1 m 2 water evaporation area, the flexible net-based solar thermal evaporator can achieve a photothermal evaporation temperature of 108°C and an evaporation rate of >1.46 kg m À2 h À1 under sunlight illumination. The dendritic structure of the net-based solar-thermal evaporator is stable, rollable, and ventilated, which can be cast on a waving water surface in a scalable way. Thus, it is expected that this fabricated netbased solar thermal evaporator has great potential for large-scale applications such as on-sea ecologically sustainable self-rescue systems, which show great potentials for solving the fresh water crisis, especially in uncultivated or isolated areas. www.advancedsciencenews.com www.small-structures.com

Experimental Section
Fabrication of the Net-Based Solar Thermal Evaporator: All reagents were of analytical grade, and a water purification system (HMC-WS10, Korea) was employed to prepare deionized water. The fabrication process of the net-based solar thermal evaporator consisted of two steps: 1) the electrogrowth of the micrometal dendrites and 2) the electrophoretic coating of nanoreduced tungsten oxide (WO 2.72 ) catalysts. For the first step, micrometal dendrites were directly deposited at a suitable range of current density using self-made all-around reactors on metal or metal-coated flexible net, which could be denoted as a template-free electrodeposition process ( Figure S18, Supporting Information). [15] For the seconds step, WO 2.72 was coated on the surface of micrometal dendrites by electrophoresis method. However, WO 2.72 was previously prepared from pure tungsten trioxide via reduction in a tube furnace under an atmosphere of carbon monoxide. [16] Then, the net-based solar thermal evaporator was dried for 10 h at 60°C. More details of the fabrication process are presented in Section S6, Supporting Information.
Characterization Techniques: XPS was conducted on an X-ray photoelectron spectroscope (Kratos Axis Ultra DLD, Japan).XRD was measured using a diffractometer (D/MAX-IIIIIIC, Rigaku, Japan). Meanwhile, SEM (S4800, Hitachi, Japan) and TEM (JEM-10°CX, JEOL, Japan) were used to assess the structure and morphology of each sample. UV-vis spectra of the samples were obtained on an UV-vis spectrophotometer (UV-3600, SHIMADZU, Japan). The calculation of fractal dimensions of the micrometal dendrites was based on the image analysis software of Image Pro Plus 6.0, using the classic box-counting algorithm. The water covered on the surface of net-based solar thermal evaporator was observed by a digital microscope (JT14008). The leached metal ions concentration was tested by inductively coupled plasma-mass spectrometry (NexION 5000, Perkinelmer, American).
Solar-Driven Interfacial Evaporation Experiment: A homemade optical container was designed to perform the interfacial evaporation experiment under ambient conditions. The temperature and relative humidity during the evaporation experiment process were maintained at 25 AE 1°C and 50 AE 5%, respectively. Unless specified, solar evaporation experiments were performed under an intensity of 1000 W m À2 , as 1 sun illumination. The temperature of the net-based solar-thermal evaporator was measured by a thermographic IR camera (Ti9, Fluke). The mass change of water was monitored by an electric balance (ME204). However, the water evaporation rate V (kg m À2 h À1 ) and solar thermal conversion efficiency (η) were calculated, as shown in Section S5, Supporting Information. The water evaporation rate was defined by Equation (1) as follows.
V ¼ Àdm=ðS Â dtÞ (1) dm is the mass change related with water evaporation (kg), S is the area of solar irradiation (m 2 ), and dt is the solar irradiation time (h). The solar thermal conversion efficiency (η) is expressed by Equation (2) as follows.
Q i represents the incident light power of 1000 W m À2 , and Q e represents the water evaporation power, which is estimated by Equation (3) as follows.
H e is the water evaporation heat, and V is estimated from Equation (1) as the water evaporation rate.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.