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).