Tuning surface wettability of MoS₂ to enhance solar-driven evaporation rates

Molybdenum disulfide (MoS₂), a promising two-dimensional photothermal material, possesses the capability to convert solar irradiation into thermal energy. This conversion is beneficial for processes like wastewater treatment and seawater desalination. Nevertheless, the influence of the surface chemical properties of MoS₂, particularly its wettability, on the evaporation rate of water confined to the surface remains unexplored. In our study, we demonstrate that the wettability degree of MoS₂ significantly influences its water evaporation rate performance. Interestingly, this parameter can be simply and accurately modulated by altering the synthesis time of MoS₂. We synthesized MoS₂ via a simple hydrothermal method at three different durations: 16, 20, and 24 h, at 200°C. Subsequently, we impregnated the resultant MoS₂ onto air-laid paper (ALP), forming a solar-driven evaporator system. We found that the MoS₂ nanosheets synthesized within the shortest duration (MoS₂-16 h) exhibited the highest evaporation rate of 1.77 kg m⁻² h⁻¹, along with 92% energy efficiency. MoS2-16 h resulted in MoS₂ rich in defects, featuring the largest surface area and the smallest contact angle. The hydrophilic areas of MoS₂-16 h facilitated the continuous diffusion of water molecules through defect sites, treating them as contact lines. Additionally, the expansive surface area introduced a larger region for light absorption, enhancing water-solid interactions. The presence of molybdenum oxide on the surface of the nanosheet system also contributed to superior wettability behavior. Notably, all the tested MoS₂/ALP systems in this study displayed excellent performance in salt rejection and heavy metal ion concentration reduction.