Design of Superhydrophobic CoFe2O4 Solar Seawater Desalination Device and Its Application in Organic Solvent Removal

Environmental pollution and clean water production are challenges to the development of human society. In this paper, devices consisting of a superhydrophobic Ni-CoFe2O4 foam layer (floating layer), a hydrophilic channel and a superhydrophilic Ni-CoFe2O4 foam layer (photothermal conversion layer) were designed. The light energy was converted into heat on the photothermal layer, for which the hydrophilic channel provided a small amount of water. The superhydrophobic layer ensured the floating and selective adsorption of organic solvents on the water surface, whose contact angle reached 157°, and the steam production rate reached 1.68 kg·m−2·h−1. Finally, the LSV curve demonstrated that the Ni-CoFe2O4 foam prepared had a minimum starting potential, achieving the multifunctionality of the Ni foam.


Introduction
Environmental and drinking water safety is a global challenge that hinders social development [1,2]. Statistics show that water consumption will grow by >55% by 2050 with the increase in GDP [3]. Desalination is an important way to solve water shortage. At present, desalination methods mainly include thermal and membrane separation methods (RO), accounting for more than 90% of the market. Traditional RO methods have an energy consumption of 3.5 kWh elec /m 3 [4]. The enthalpy of evaporation (∆H) needs to be overcome for thermal methods, and the separation energy (∆G) needs to be overcome for filtration methods. ∆H is two orders of magnitude higher than ∆G [5]. Compared with seawater desalination methods that consume fossil fuels, only solar energy is used in solar desalination to produce clean water, which has attracted wide attention from researchers [6,7]. Through the modification of polydopamine and Ti 3 C 2 T x Mxenes, Chen et al. [8] used balsa wood as a raw material to obtain an evaporator, which had a high steam production speed of 2.08 kg·m −2 ·h −1 . Lin et al. [9] loaded poly(N-phenylglycine) and MoS 2 on a polyvinylidene fluoride (PVDF) film to form a photothermal layer. A steam production rate of 1.7 kg·m −2 ·h −1 was obtained for the photothermal device, assisted by hydrophilic channels and polystyrene foam. When photothermal technology is applied to sewage discharged from chemical plants, the problem of floating organic solvent pollutants needs to be solved. Superhydrophobic-superoleophilic materials have attracted research interests due to their ability to selectively adsorb organic solvents [10,11]. Liu et al. [12] prepared superhydrophobic silica floating on the oil-water interface, which could selectively adsorb the oil phase to achieve the purpose of oil-water separation. However, superhydrophobic silica is difficult to recover from the powder state.
In order to realize floating oil removal and clean water production, a multifunctional solar evaporator was designed, which is composed of a superhydrophobic floating layer, a filter paper layer and a CoFe 2 O 4 photothermal layer. The superhydrophobic layer has two functions: one is to selectively adsorb floating oil and the other is to support the floating of the device. Through the synergy of the three layers, a solar evaporation rate of 1.68 kg·m −2 ·h −1 was obtained by the device, providing a new solution for the application of solar evaporators in sewage treatment.  2 and Trichloro(1H, 1H, 2H, 2H-tridecafluoro-n-octyl)silane (AR, 97%) were provided by Aladdin Chemical Reagent Co., Ltd. Nickel foam was purchased from a local store.

Preparation of Solar Evaporator
The nickel foam was cut to the size of 3.5 cm × 3.5 cm and sonicated in 3 mol/L HCl, deionized water, acetone and ethanol for 20 min each. Amounts of 0.1592 g of FeCl 2 •4H 2 O, 0.4656 g of Co(NO 3 ) 2 •6H 2 O, 0.0592 g of NH 4 F and 0.2402g of urea were dissolved in 65 mL of deionized water and transferred to a 100 mL stainless-steel autoclave. Nickel foam was added, reacting at 140 • C for 8 h. After cooling, the samples were dried at 140 • C for 0.5 h and then sintered at 400 • C for 3 h (heating speed was 2 • C/min). The resulting superhydrophilic Ni-CoFe 2 O 4 foam was named the photothermal conversion layer. The floating layer was prepared as follows: the superhydrophilic Ni/CoFe 2 O 4 foam was added to 30 mL of silane ethanol solution (0.5 mL) and stirred at room temperature for 4 h. A superhydrophobic Ni-CoFe 2 O 4 foam floating layer was obtained after drying at 140 • C for 1 h. The structure of the solar evaporator was the superhydrophobic floating layer, the filter paper layer and the photothermal conversion layer from bottom to top.

Characterization
The structure of Ni, CoFe 2 O 4 and superhydrophobic CoFe 2 O 4 was obtained by TEM (FEI Tecnai F30) and XRD (Bruker D8w). The morphology of Ni, Ni-CoFe 2 O 4 and superhydrophobic Ni-CoFe 2 O 4 foams was obtained by FESEM (Zeiss Company, Ltd., Munich, Germany). The surface components of the superhydrophobic Ni-CoFe 2 O 4 foam were obtained by XPS (ESC PHI500). The surface wettability of the foam was measured by a contact angle gauge (JC2000C1). The LSV curves of the foams were obtained via an electrochemical workstation (Gamry Reference 3000). The surface temperature of the Ni-CoFe 2 O 4 foam was measured by an infrared camera (Fluke TiS20 + MAX).

Results and Discussion
The flexibility and three-dimensional structure of nickel foam make it an excellent carrier. The rough surface of Ni foam was observed from its morphology (Figure 1a). The surface of the Ni foam where CoFe 2 O 4 was grown became rougher. Importantly, on the basis of retaining the micron roughness, there was also nano roughness caused by the formation of nanosheets. Finally, micron-nano roughness was formed, which was beneficial to improving the hydrophobic performance ( Figure 1b). No obvious change could be seen in the morphology of Ni-CoFe 2 O 4 after the silane treatment ( Figure 1c). The element distribution was investigated based on EDS analysis. Elements including Fe, Co and F had the same shape as the Ni foam, proving that superhydrophobic Ni-CoFe 2 O 4 was successfully prepared (Figure 1d). and F had the same shape as the Ni foam, proving that superhydrophobic Ni-CoFe2O was successfully prepared (Figure 1d). To analyze the surface structure and composition of the composite, the results o XRD, TEM and XPS were analyzed. Notably, the CoFe2O4 powder used in the XRD and TEM analysis was obtained through ultrasonic dissection. As shown in Figure 2a  To analyze the surface structure and composition of the composite, the results of XRD, TEM and XPS were analyzed. Notably, the CoFe 2 O 4 powder used in the XRD and TEM analysis was obtained through ultrasonic dissection. As shown in Figure 2a Further fine spectral analyses of Co and Fe revealed that CoFe 2 O 4 had a mixed valent state consisting of Co 2+ , Co 3+ , Fe 2+ and Fe 3+ (Figure 3a,b). Relevant information of the deconvolution is presented in Table S1. The superhydrophobic Ni-CoFe 2 O 4 foam exhibited its superhydrophobicity under the synergy of the micron-nanoscale roughness and low surface energy. As shown in Figure 3c, the water droplets spread rapidly on the surface of the Ni-CoFe 2 O 4 foam, showing their superhydrophilicity. The inset shows that the surface exhibited a contact angle of 0 • on the water. After silane treatment, the surface of the superhydrophobic Ni-CoFe 2 O 4 foam showed a non-wetting state with a contact angle of 157 • . To verify the organic solvent removal property of the superhydrophobic Ni-CoFe 2 O 4 foam, cyclohexane was used as the target organic solvent. When the superhydrophobic Ni-CoFe 2 O 4 foam contacted the interface between cyclohexane and the water, cyclohexane was adsorbed, leaving a blank water surface. Further fine spectral analyses of Co and Fe revealed that CoFe2O4 had a mixed valent state consisting of Co 2+ , Co 3+ , Fe 2+ and Fe 3+ (Figure 3a,b). Relevant information of the deconvolution is presented in Table S1. The superhydrophobic Ni-CoFe2O4 foam exhibited its superhydrophobicity under the synergy of the micron-nanoscale roughness and low surface energy. As shown in Figure 3c, the water droplets spread rapidly on the surface of the Ni-CoFe2O4 foam, showing their superhydrophilicity. The inset shows that the surface exhibited a contact angle of 0° on the water. After silane treatment, the surface of the superhydrophobic Ni-CoFe2O4 foam showed a non-wetting state with a contact angle of 157°. To verify the organic solvent removal property of the superhydrophobic Ni-CoFe2O4 foam, cyclohexane was used as the target organic solvent. When the superhydrophobic Ni-CoFe2O4 foam contacted the interface between cyclohexane and the water, cyclohexane was adsorbed, leaving a blank water surface.   CoFe2O4 (a,b). Image of the Ni-CoFe2O4 foam fore and after silane modification (c). Selective organic solvent removal test for the Ni-Co foam (d).
Different from the traditional heating effect of solar energy on the whole amou water, interface heating only works on a small amount of water, improving the effici of steam production. A photo of the device is shown in Figure 4a. To verify the provement in the steam production efficiency, a comparative experiment was perform Different from the traditional heating effect of solar energy on the whole amount of water, interface heating only works on a small amount of water, improving the efficiency of steam production. A photo of the device is shown in Figure 4a. To verify the improvement in the steam production efficiency, a comparative experiment was performed. As shown in Figure 4b, the three-layer device had the highest steam production rate (1.68 kg·m −2 ·h −1 ) compared with pure water, the superhydrophobic Ni-CoFe 2 O 4 foam and the superhydrophobic Ni-CoFe 2 O 4 foam/filter paper composite. Experiments on water drops and friction stability were designed to verify the stability of the superhydrophobic Ni-CoFe 2 O 4 foam. The water drop was 3 cm above the surface of the superhydrophobic Ni-CoFe 2 O 4 foam. In the friction test, the sample size was 1.5 cm × 1.5 cm, and the load was 130 g. Sandpaper served as a friction surface. After the dripping of 1000 droplets and a friction test for 200 cycles, a contact angle of greater than 150 • remained on the surface of the superhydrophobic Ni-CoFe 2 O 4 foam, demonstrating an excellent superhydrophobic stability (Figure 4c). In addition, the CoFe 2 O 4 foam composite prepared could be used as a water oxidation electrocatalyst. A three-electrode system (the sample was taken as the working electrode, a saturated calomel electrode was used as the reference electrode and a graphite rod was used as the counter electrode) and an electrolyte of 1 M KOH were used in the LSV test. The conversion between the saturated calomel electrode (SCE) and the RHE was calculated through the following formula:  The inset of Figure 4b shows the surface temperature of the pure wate surface and the device given by the infrared camera. Inset of Figure 4d shows the phenomenon o oxygen production in the working electrode.

Conclusions
In this paper, CoFe2O4 was grown on Ni foam through hydrothermal synthesis, and a superhydrophobic Ni-CoFe2O4 foam was prepared through silane modification CoFe2O4 had a scaly morphology, and the crystal structure of CoFe2O4 was not changed after the hydrophobic modification. CoFe2O4 was proved to be the mixture valence state of Co 2+ , Co 3+ , Fe 2+ and Fe 3+ . The contact angle of the superhydrophobic Ni-CoFe2O4 foam reached 157°, which had an excellent selective organic solvent removal property. The device consisting of a superhydrophobic Ni-CoFe2O4 floating layer, a hydrophilic laye  Figure 4b shows the surface temperature of the pure water surface and the device given by the infrared camera. Inset of Figure 4d shows the phenomenon of oxygen production in the working electrode.
Due to the 1M KOH solution used in the testing process, the pH was 14. As shown in Figure 4d, the starting potential of the Ni-CoFe 2 O 4 foam was less than that of the Ni foam, and the phenomenon of oxygen production was observed in the working electrode.
Through device design, not only the removal of the floating organic solvent and clean water production, but also the multifunctionality of the device was realized.

Conclusions
In this paper, CoFe 2 O 4 was grown on Ni foam through hydrothermal synthesis, and a superhydrophobic Ni-CoFe 2 O 4 foam was prepared through silane modification. CoFe 2 O 4 had a scaly morphology, and the crystal structure of CoFe 2 O 4 was not changed after the hydrophobic modification. CoFe 2 O 4 was proved to be the mixture valence state of Co 2+ , Co 3+ , Fe 2+ and Fe 3+ . The contact angle of the superhydrophobic Ni-CoFe 2 O 4 foam reached 157 • , which had an excellent selective organic solvent removal property. The device consisting of a superhydrophobic Ni-CoFe 2 O 4 floating layer, a hydrophilic layer and a photothermal layer could reach a steam generation rate of 1.68 kg·m −2 ·h −1 . The hydrophobic stability of the superhydrophobic Ni-CoFe 2 O 4 foam was demonstrated through water dropping and friction experiments.

Data Availability Statement:
The data used to support the findings of this study are included within the article.