3. Conclusion
In summary, we fabricated a magnetic field induced photothermal evaporator assembled by carbon-coated Fe3O4(Fe3O4@C) nanoparticles, which revealed the adjustable spiny surface structure corresponding to the relative distance to magnet. At an appropriate relative distance, just 1 mg mm-2 of Fe3O4@C nanoparticles was necessary to form a well-distributed spiny surface. With the optimized surface structure, the solar absorbance of evaporator could reach up to 94.6% via multiple reflections. The Fe3O4@C assembled evaporator obviously improved the water evaporation as high as 1.39 kg m-2h-1 under the 1-sun illumination, about 2.8 times higher than natural evaporation. Meanwhile, the magnetic evaporator could also accelerate the seawater evaporation up to 1.31 kg m-2 h-1 with just a little performance degradation during long desalination owing to the good stability of Fe3O4@C. The work can be considered as a beneficial attempt to optimize the magnetic evaporator for application. Besides, the facile adjustment of the magnetic nanoparticles assembled surface via the position in magnetic field has high potential in not only sustainable solar-driven fresh water generation but more fields of water treatment.
Experimental Section
Preparation of the carbon-coated nanoparticles : The carbon-coated nanoparticles were synthesized by a typical hydrothermal method. Specifically, 0.3 g of Fe3O4nanoparticles, 1 mL of acrylic acid, and 40 mL of H2O were mixed with a certain amount of glucose in a 100 mL capped glass bottle. Different amounts of glucose led to different thicknesses of carbon layers around Fe3O4nanoparticles. The mixture was stirred vigorously at room temperature for 4 h and transferred to a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was heated at 200 ℃ for 12 h, then naturally cooled to room temperature at the end of the reaction. The black products were washed with DI water and anhydrous ethanol alternately three times. Subsequently, the products were dried at 60 °C for 6 h. The obtained samples were named Fe3O4@C, and Fe3O4@tC respectively according to 1.0 g/2.0 g of glucose.
Fabrication of magnetic photothermal evaporator : A 3D model of the evaporator was established by SOLIDWORKS software and was converted into object through 3D printer (raise3D Pro2 Plus) using PLA (polylactic acid) wire. This evaporator had two parts including an upper holder to support the magnetic nanoparticles and a lower screw plug equipped with a NdFeB magnet. The distance between magnet and magnetic nanoparticles could be adjusted by rotating the screw plug. The air-laid paper was cut into a specific shape serving as water transfer channels. Magnetic nanoparticles were deposited at the holder at the density of 1 mg mm-2, which could self-assemble into a spiny surface induced by magnetic field.