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