FIGURE 6 Schematic illustration of the fabrication of magnetic photothermal evaporator (left) and the evaporation test system (right) (a). The water evaporation rates of Fe3O4, Fe3O4@C and Fe3O4@tC evaporators under dark and light conditions respectively (b). Water evaporation rates of photothermal evaporators at various distances between magnet to nanoparticles (c). The evaporation rate and the durability of magnetic evaporator (d). The images right showed the surface morphologies before and after five cycling experiments.
The performance of magnetic evaporator on water evaporation was been systematically evaluated based on a lab-made system that was composed of a solar simulator with a fixed light intensity of 1000 W m-2 and an electronic balance connected to PC (Figure 6a). Evaporation measurements were performed in a temperature and humidity chambers to minimize the effect of conditions. The set temperature and relative humidity were fixed at 25 °C and 50%, respectively.
The evaporation performances with the assistance of magnetic nanoparticles were shown in Figure 6b in comparison with the natural evaporation. The evaporation rates of pure water were 0.49 kg m-3 h-1and 0.12 kg m-3 h-1 with and without light. As the distance was fixed at 0.6 cm, evaporators assembled in magnetic field accelerated the photothermal rates to 0.26, 0.24, 0.24 kg m-3 h-1 respectively in dark and 1.28, 1.39, 1.31 kg m-3 h-1respectively under illumination. The higher evaporation rates of magnetic evaporators in dark could be attributed to the enlargement of evaporation interface area via spiny morphology. While, the good photothermal ability of magnetic spines resulted in faster water evaporation with light. Among the three kinds of nanoparticles, Fe3O4@C showed the best performance on photothermal evaporation. Such result was consistent with the solar light absorptance data in Figure 5 and Figure S7.
The water evaporation performance could also be affected by the distance between the spiny surface and magnet. As discussed above, the magnetic field tended to be uniform at an appropriate distance. A relatively integral spiny surface could be assembled under the induction of magnetic field, even as the loading density of nanoparticles was as low as 1 mg mm-2. For Fe3O4@C, the water evaporation rates could reach up to 1.36, 1.39, 1.29 and 1.26 kg m-3h-1, as the distance was changed from 0.1 to 0.6, 1.1 and 1.6 cm. At a narrow distance, the coverage region with nanoparticles was incomplete under the non-uniform magnetic field. Exposed partial sites reduced the light harvesting even in wet state, which caused the lower evaporation rate. As the distance increased from 1.1 to 1.6 cm, the evaporation rate declined continuously because of the decay of magnetic field strength. A weak magnetic field could hardly remain the spiny morphology against the strong surface tension of water in wet state. Fe3O4 and Fe3O4@tC showed a similar tendency for water evaporation at different distances to magnet.
Furthermore, to evaluate the stability of magnetic evaporator, the photothermal evaporation rates were measured under 12-hour illumination alternating with 12-hour darkness. After 5 cycles, the evaporation rate still kept up to about 1.33 kg m-3h-1, which was 96% of the initial rate (Figure 6d). The stable performance on long-term photothermal evaporation could be attributed to the good stability of the spiny morphology of Fe3O4@C. Meanwhile, thanks to the protection of carbon layer, Fe3O4@C remained the crystalline structure of Fe3O4 without obvious change which was confirmed by XRD in Figure 7e. In this case, the Fe3O4@C layer could maintain its photothermal performance under long-term illumination for water evaporation, thus enabling low-cost actual water desalination.
2.5. Solar-driven desalination of magnetic photothermal evaporator
To evaluate the performance of magnetic evaporator on seawater, photothermal evaporation was measured with five concentrations of saline solutions to simulate seawater from different seas, including the Baltic Sea (0.8 wt%), the world ocean (3.5 wt%), the Red Sea (4.1 wt%) and the Dead Sea (30 wt%). As shown in Figure 7a, the evaporation rate under illumination decreased from 1.39 kg m-3h-1 for DI water to 1.37, 1.31, 1.26 and 0.98 m-3 h-1 for the simulated seawater of the Baltic Sea, world ocean, the Red Sea and the Dead Sea, respectively. Correspondingly, the evaporation rate of evaporator decreased from 0.27 to 0.25, 0.19, 0.18 and 0.16 kg m-3 h-1, respectively in dark. The decrease in evaporation rate is due to the lower saturation vapor pressure of seawater. Higher concentration of saline solution indicated lower saturation vapor pressure, and further caused slower evaporation. The real seawater from Bohai Bay (obtained at 38°53′N, 121°34′34′′E) was also employed in measurement, and the evaporation rate with Fe3O4@C evaporator was about 1.35 kg m-3 h-1 (Figure 7a).