FIGURE 3 The distribution of magnetic field strength at different distances from the center of the magnet in vertical direction (a). The red line and gray line represent the actual measured data and the simulated data respectively. The inserted image is schematic diagram of the magnet with a diameter of 30 mm. Optical photographs of surface morphology on magnetic evaporator before and after wetted by water with Fe3O4 (b), Fe3O4@C (c) and Fe3O4@tC (d) as photothermal materials, respectively. The scale bar was 5 mm. Simulated magnetic field distribution at various distances with the corresponding magnetic field strength (e, f). Spiny morphology of the photothermal layers formed by Fe3O4@C at various distances (g-j). The scale bar was 6 mm.
As shown in Figure 3a, the simulated magnetic strength of the magnet edge could reach over 150 mT. While the strength gradually decreased from edge to center down to just ~20 mT. Such result was generally consistent with the data from actual measurements (red line in Figure 3a and Figure S5). Because of the non-uniform magnetic field distribution, as the distance between evaporator surface and the magnet below was close (0.1 cm), Fe3O4nanoparticles were preferentially distributed on the edge with stronger magnetic fields. Comparatively, the central region with weaker magnetic field strength exposed the underlying air-laid paper due to fewer magnetic nanoparticles as the loading density of just 1 mg mm-2 (Figure 3b).
As Fe3O4 was replaced by Fe3O4@C, spines of the Fe3O4@C turned denser and smaller, and tended to form a more well-distributed spiny structure. The exposed region in the center of the magnet was relatively minimized (Figure 3c). It was speculated that the change in the distribution of carbon-coated nanoparticles was attributed to the decrease in magnetic intensity of Fe3O4@C, which resulted in deviation of the magnetic particle arrangement from the magnetic field (Figure S3). Owing to the weaker magnetic property, Fe3O4@tC assembled into fewer spines and covered the magnet surface completely.
During practical solar-driven evaporation, photothermal layer would be wetted by water, and it was necessary to investigate morphology in the wet state. After wetting, the exposed regions of the spiny surfaces composited by Fe3O4 and Fe3O4@C tended to be larger, and some small spines gathered into larger ones driven by the surface tension of water (Figure 3b-c). However, for Fe3O4@tC, there was no obvious change in its spiny morphology after wetting (Figure 3d).
As mentioned above, the spiny morphology of photothermal layer was adjustable via controlling the distance between magnetic nanoparticles and magnet. To investigate the effect of distance, the numerical simulation was used to determine the distribution of magnetic fields at four different distances of 0.1 cm, 0.6 cm, 1.1 cm, and 1.6 cm (Figure 3e). The corresponding magnetic field strengths at these distances were shown in Figure 3f. Enlarging the distance to magnet, the magnetic field strength gradually decreased especially at the edge. The magnetic induction intensity of the edge declined from 44 to 11, 6 and 3 mT with the distance increasing from 0.1 to 0.6, 1.1 and 1.6 cm. In comparison, there was a less decline in the center of magnet from 15 to 11, 7 and 4 mT. When the distance from the magnet was 0.6 cm, the magnetic field strength tended to be uniform.
For observing the spine structure conveniently, following the loading density used in this work, a quantity of Fe3O4@C nanoparticles was confined in a narrow gap between two glass sheets above the holder with magnet (Figure S6a). As shown in Figure 3g, non-uniform magnetic field distribution caused a denser distribution of Fe3O4@C spines at the edge and a relatively sparser spiny structure in the center in both dry and wet states. As the distance increased to 0.6 cm, following the uniform magnetic field, a well-distributed Fe3O4@C spines was formed (Figure 3h). However, as the distance got larger to 1.1 and 1.6 cm (Figure 3i-j), the corresponding magnetic field strength became weaker which failed to attract all the Fe3O4@C effectively, which caused the less obvious spiny morphology. Similarly, the same trend also existed in the structure composited by Fe3O4 (Figure S6b-c).