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.