FIGURE 6 Schematic illustration of the fabrication of magnetic
photothermal evaporator (left) and the evaporation test system (right)
(a). The water evaporation rates of
Fe3O4,
Fe3O4@C and
Fe3O4@tC evaporators under dark and
light conditions respectively (b). Water evaporation rates of
photothermal evaporators at various distances between magnet to
nanoparticles (c). The evaporation rate and the durability of magnetic
evaporator (d). The images right showed the surface morphologies before
and after five cycling experiments.
The performance of magnetic evaporator on water evaporation was been
systematically evaluated based on a lab-made system that was composed of
a solar simulator with a fixed light intensity of 1000 W
m-2 and an electronic balance connected to PC (Figure
6a). Evaporation measurements were performed in a temperature and
humidity chambers to minimize the effect of conditions. The set
temperature and relative humidity were fixed at 25 °C and 50%,
respectively.
The evaporation performances with the assistance of magnetic
nanoparticles were shown in Figure 6b in comparison with the natural
evaporation. The evaporation rates of pure water were 0.49 kg
m-3 h-1and 0.12 kg
m-3 h-1 with and without light. As
the distance was fixed at 0.6 cm, evaporators assembled in magnetic
field accelerated the photothermal rates to 0.26, 0.24, 0.24 kg
m-3 h-1 respectively in dark and
1.28, 1.39, 1.31 kg m-3 h-1respectively under illumination. The higher evaporation rates of
magnetic evaporators in dark could be attributed to the enlargement of
evaporation interface area via spiny morphology. While, the good
photothermal ability of magnetic spines resulted in faster water
evaporation with light. Among the three kinds of nanoparticles,
Fe3O4@C showed the best performance on
photothermal evaporation. Such result was consistent with the solar
light absorptance data in Figure 5 and Figure S7.
The water evaporation performance could also be affected by the distance
between the spiny surface and magnet. As discussed above, the magnetic
field tended to be uniform at an appropriate distance. A relatively
integral spiny surface could be assembled under the induction of
magnetic field, even as the loading density of nanoparticles was as low
as 1 mg mm-2. For
Fe3O4@C, the water evaporation rates
could reach up to 1.36, 1.39, 1.29 and 1.26 kg m-3h-1, as the distance was changed from 0.1 to 0.6, 1.1
and 1.6 cm. At a narrow distance, the coverage region with nanoparticles
was incomplete under the non-uniform magnetic field. Exposed partial
sites reduced the light harvesting even in wet state, which caused the
lower evaporation rate. As the distance increased from 1.1 to 1.6 cm,
the evaporation rate declined continuously because of the decay of
magnetic field strength. A weak magnetic field could hardly remain the
spiny morphology against the strong surface tension of water in wet
state. Fe3O4 and
Fe3O4@tC showed a similar tendency for
water evaporation at different distances to magnet.
Furthermore, to evaluate the stability of magnetic evaporator, the
photothermal evaporation rates were measured under 12-hour illumination
alternating with 12-hour darkness. After 5 cycles, the evaporation rate
still kept up to about 1.33 kg m-3h-1, which was 96% of the initial rate (Figure 6d).
The stable performance on long-term photothermal evaporation could be
attributed to the good stability of the spiny morphology of
Fe3O4@C. Meanwhile, thanks to the
protection of carbon layer, Fe3O4@C
remained the crystalline structure of
Fe3O4 without obvious change which was
confirmed by XRD in Figure 7e. In this case, the
Fe3O4@C layer could maintain its
photothermal performance under long-term illumination for water
evaporation, thus enabling low-cost actual water desalination.
2.5. Solar-driven desalination of magnetic photothermal evaporator
To evaluate the performance of magnetic evaporator on seawater,
photothermal evaporation was measured with five concentrations of saline
solutions to simulate seawater from different seas, including the Baltic
Sea (0.8 wt%), the world ocean (3.5 wt%), the Red Sea (4.1 wt%) and
the Dead Sea (30 wt%). As shown in Figure 7a, the evaporation rate
under illumination decreased from 1.39 kg m-3h-1 for DI water to 1.37, 1.31, 1.26 and 0.98
m-3 h-1 for the simulated seawater
of the Baltic Sea, world ocean, the Red Sea and the Dead Sea,
respectively. Correspondingly, the evaporation rate of evaporator
decreased from 0.27 to 0.25, 0.19, 0.18 and 0.16 kg
m-3 h-1, respectively in dark. The
decrease in evaporation rate is due to the lower saturation vapor
pressure of seawater. Higher concentration of saline solution indicated
lower saturation vapor pressure, and further caused slower evaporation.
The real seawater from Bohai Bay (obtained at 38°53′N, 121°34′34′′E) was
also employed in measurement, and the evaporation rate with
Fe3O4@C evaporator was about 1.35 kg
m-3 h-1 (Figure 7a).