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.