Figure 3. (a) Schematic
diagram of the photothermal antibacterial mechanism of AM/Ch gel. Image
of bacterial colonies formed by E. coli (b) and S. aureus(c) treated with PBS buffer solution, Ch gel and AM/Ch gel under
irradiation (0.2 W cm-2, 30 min). Curve of
antibacterial activity of E. coli (d) and S. aureus (e)
after treatment with PBS buffer solution, chitosan hydrogel and AM/Ch
gel with time increasing. (f) Comparison of the concentration of
Ag+ ions released
with time under irradiation (0.2 W cm-2, 30 min). (g)
Schematic diagram for self-cleaning process of AM/Ch gel. (h) Digital
photo of AM/Ch gel soaked in a bacterial solution. (i) Comparison of the
concentration of Ag+ ions released in dark.
Antibacterial tests: (j) spread plate results of E. coli from the
original bacterial solution, (k) the untreated bacterial solution for 24
hour, and (l) the bacterial solution after treatment with AM/Ch gel for
24 hour; spread plate results of (m) S. aureus from the original
bacterial solution, (n) the untreated bacterial solution for 24 hour,
and (o) the bacterial solution after treatment with AM/Ch gel for 24
hour.
2.4. The water evaporation property
of AM/Ch gel solar
evaporator
Considering
the superior photothermal performance of the Ag/MXene composites,
the solar evaporation performance of
the AM/Ch gel evaporator was investigated.
As illustrated in Figure
4 a, the water evaporation system was fabricated by the growth of the
hydrogel on the top of expandable polyethylene (EPE) foam as a thermal
insulator. An electronic analytical balance was utilized to quantify the
change in water mass caused by the vapor escape. The filled AM/Ch
gel in the two holes of EPE foam
could act as sustained water transportation channels, mimicking the
roots of trees in nature. During
the water evaporation test, an IR camera was used to record the surface
temperature of solar steam generator under light irradiation (incident
angle: 0°). As shown in Figure 4b,
the weight of water almost kept constant in dark, indicating that the
natural evaporation could be negligible. And under light irradiation,
the water evaporation was enhanced and the presence of chitosan hydrogel
brought no further enhancement of steam generation. In comparison, the
water weight in the presence of AM/Ch gel exhibited an almost
leaner decrease as the irradiation
time prolonged, an indicator of constant and stable steam generation.
Figure 4c showed that the evaporation rate of
the AM/Ch gel evaporator reached
3.22 kg m−2h−1, which was
approximately 3.7 times for the Ch gel evaporator (0.86 kg
m−2 h−1) and 3.88 times compared to
pure water control sample (0.83 kg m−2h−1).
Similarly,
the evaporation rate of the AM/Ch
gel evaporator in seawater obtained
from the Yellow Sea, China, was 3.10 kg m−2h−1, which was also much higher than the rate of
natural evaporation (Figure S7a-b, Supporting Information).
As shown in Figure 4d-e, the
surface temperature of the device increased quickly and then stabilized
for the irradiation period from 0 to 60 min. In particular, the surface
temperature increased to 52.3 °C (T = 30.0 °C) over AM/Ch gel, which was
considerably higher than Ch gel and pure water due to the strong
photothermal effect of Ag/MXene composites.
To assess the energy utilization
potential of AM/Ch gel evaporator, the solar evaporation efficiency (η)
was also calculated (Supporting
Information). According to Figure
S8a-b (Supporting Information), the
evaporation efficiency of the AM/Ch gel evaporator could reach 94.9%,
which was much higher than that of the Ch gel evaporator (22.8%) and
even most photothermal evaporators previously reported in the past five
years (Figure 4f and Table S1, Supporting Information).
Such an excellent evaporation rate
and energy conversion efficiency demonstrated the potential of AM/Ch gel
for durable and sustainable water distillation,
which could be mostly attributed to
its outstanding photothermal conversion capability, lowered evaporation
enthalpy, and sustained water molecules transportation in the networks.
The
vapor generated by light irradiation was condensed, and the liquid water
was collected to assess the purification effectiveness of AM/Ch gel
evaporator.
After
being measured by inductively coupled plasma-optical emission
spectrometry (ICP-OES),
the concentrations of four
primary ions (Na+,
Mg2+, K+ and Ca2+)
were greatly reduced by three to four orders of magnitude after
desalination when we used the seawater samples from the Yellow Sea,
China, (Figure 4g), which meet the requirement for drinking water
quality on World Health
Organization,[36] revealing the excellent
performance of AM/Ch gel evaporator
for seawater desalination.
Furthermore, water evaporation
experiments were also conducted using wastewater containing dyes as
simulated contaminants, including methyl orange and methylene blue
dyes.
Figure 4h showed the color of the sewage turned transparent, and the
strong absorption peaks of methyl orange (~ 465 nm) and
methylene blue (~ 665 nm) were almost eradicated after
purification with AM/Ch gel evaporator, showing its potential for clean
water production from sewage.