FIGURE 5 Diffuse reflection spectra of magnetic samples in the
wavelength range of 280-2500 nm (a). AM 1.5 G solar spectrum and
UV-Vis-NIR absorption spectra (b). Diffuse reflection spectra of the
Fe3O4@C flat and photothermal layer
formed by Fe3O4@C at 0.6 cm in the
wavelength range of 280-2500 nm (c). AM 1.5 G solar spectrum and
UV-Vis-NIR absorption spectra (d). The IR thermal images of magnetic
evaporator under 1-sun illumination (e).
The low diffuse reflectance of photothermal layer corresponded to the
higher light absorption with negligible transmittance. Sunlight
absorptance in the range of 280–2500 nm was used to evaluate the
photothermal evaporator, which was calculated as below:
\(\text{Sunlig}ht\ \text{absorptance}=\frac{\int_{280}^{2500}{I(\lambda)\times A(\lambda)\text{dλ}}}{\int_{280}^{2500}{I(\lambda)\text{dλ}}}\)(1)
where I(λ) is the sunlight intensity function at the wavelength (λ) from
280 to 2500 nm, A(λ) is the absorptance of samples at the corresponding
wavelength (Figure 5b-d). The sunlight absorptance of flat
Fe3O4@C surface was around 89.6%. In
comparison, the absorptance of spiny surface could be enhanced over
94.6% as the distance was 0.6 cm. Thanks to the high absorptance of
sunlight, the photothermal layer formed by
Fe3O4@C at a distance of 0.6 cm from
magnet revealed high performance on the light-to-thermal conversion.
After being exposed to the simulated solar light (~1000
W m-2) for 30 mins, spiny surface exhibited a higher
surface temperature of surface up to 80.1 °C on average than the flat
surface of just 73.7 °C (Figure 5e).
For testing convenience, the data was mainly collected from the center
of the spiny surface. Adjusting the distance to the magnet, spiny
surface showed changes in light harvesting. As the distances were 0.1 cm
and 1.1 cm, the diffuse reflectances were similar to the result at 0.6
cm. While, as the distance enlarged to 1.6 cm, there was an obvious
increase in diffuse reflectance, which was as high as
~12 % in visible region (Figure S7a-b).
During photothermal evaporation, the evaporator was wetted by water, and
it was necessary to investigate the light absorption of photothermal
layer in the wet state. After being wetted by water, the diffuse
reflectances of all photothermal layers were further decreased (Figure
S7c-d). This could be attributed to the presence of a thin water layer
around the wettable Fe3O4@C that
optimized the optical path following the Fresnel
equation.34,38
2.4. Photothermal evaporation performance of magnetic photothermal
evaporator
The photothermal evaporator for measurement was assembled as the
inserted image in Figure 6a shown. Magnetic nanoparticles were located
on a 3D-printed holder to form the photothermal layer. Between them,
there was a piece of air-laid paper for water supply. To adjust the
location of photothermal layer in magnetic field, a disc magnet was
placed on the top of screw plug, which could be rotated to change the
distance between the nanoparticles and magnet.