FIGURE 4 Water contact angle images of Fe3O4 (a), Fe3O4@C (b) and Fe3O4@tC (c) after touching 4 μl of water droplet. (d) Microstructure of the photothermal layer formed by Fe3O4@C. Schematic images of the spiny structures of Fe3O4 (e), Fe3O4@C (f) and Fe3O4@tC (g) contacted with 10 μl of water, the spiny structures could capture water quickly by capillary action. An air-laid paper was laid on the surface of photothermal layer formed by Fe3O4@C, and tested the water transmission capacity along the photothermal layer (h).
The commercial Fe3O4 nanoparticles were hydrophilic, and the hydrophilicity gradually decreased as the carbon layer outside the Fe3O4 nanoparticles became thickened (Figure 4a-c). The instantaneous water contact angles of flat surfaces of Fe3O4@C and Fe3O4@tC were 107.1 and 122.2°, respectively. However, with the spiny structure, all three photothermal surfaces composited by Fe3O4 and carbon-coated nanoparticles enabled to capture water in a short time (Figure 4e-g). Figure 4d shows morphology of the photothermal layer formed by Fe3O4@C that featured a multi-gap structure with spins. Such structure had capillary channels all over the interior, which could provide excellent paths for efficient water transport,23 and water could efficiently transport from the bottom to the top in a short period (about 140 s) by capillary force alone (Figure 4h).
2.3. Light absorptance of magnetic photothermal evaporator
As a fine semiconductor, the band gap of black Fe3O4 was just 0.1 eV, and such a quite narrow band gap ensured the nearly full spectrum of solar absorption.37 Under the evaporation environment with high temperature and humidity, Fe3O4 is gradually oxidized into Fe2O3 which has lower solar-light absorption due to the increase in bandgap.35 In response to this issue, a carbon layer as good light absorber was coated around the magnetic particles to maintain the performance on light harvesting for stable photothermal evaporation. The light harvesting in solar spectrum was carefully investigated based on the UV–vis–NIR spectra in the range of 280–2500 nm.
Without magnetic field, magnetic particles with and without coating were respectively fixed on glass slices to form flat surfaces. As shown in Figure 5a, the diffuse reflectance of Fe3O4@C and Fe3O4@tC were almost near the result of pristine Fe3O4 but with a slight decrease at visible and infrared regions. It indicated that the carbon layer displayed a comparable ability in light absorption to Fe3O4. The diffuse reflectance of Fe3O4@C was ~4.4% in UV region (280–400 nm), ~10.1% in visible region (400–780 nm), and ~11.6% in near-infrared region (780–2500 nm). As the nanoparticles assembled into a spiny morphology in magnetic field, there was a significant reduction in diffuse reflection. Fixing the distance to magnet at 0.6 cm, the diffuse reflectance of spiny surface composited by Fe3O4@C was down to ~1.6% in UV region, ~5.6% in visible region, and ~6.6% in near-infrared region (Figure 5c). The decrease of diffuse reflectance was attributed to the specific surface microstructure caused by magnetic field. In the gaps of spiny surface, part of the reflected light was re-captured by the Fe3O4@C spines during multiple reflections.34