Amphiphilic Janus patch-grafted hydrogels for salt-rejecting solar water desalination

Interfacial solar seawater desalination is considered as one of the most promising sustainable techniques for producing fresh water. Janus hydrogel evaporators with a hydrophobic top layer and a hydrophilic bottom layer have been demonstrated as an effective way to accelerate water evaporation and reject salt ions. However, the existing strategies make it difficult to achieve precise control over surface wettability and confine a thin hydrophobic layer to the hydrogel's surface, both of which were considered to have a significant impact on solar seawater desalination. Herein, we fabricated amphiphilic Janus patch-grafted hydrogel evaporators, in which ultrathin hydrophobic polystyrene patches were uniformly and discretely distributed on the top of the hydrogel and hydrophilic quaternized poly(4-vinyl pyridine)s (QP4VPs) were entangled within the hydrogel network. By the rational design of the Janus patch size and surface coverage, the wettability of Janus hydrogels could be precisely regulated. The Janus hydrogel resulted in optimized solar water evaporation performance with an evaporation rate of 3.2 kg m-2 h-1 and Janus patch surface coverage of similar to 60%. Moreover, the Janus hydrogel has a superior salt ion rejection ratio, which was attributed to the high ionic strength of the QP4VP-rich entangled layer. The amphiphilic Janus patch-grafted hydrogels outperformed all non-photothermal hydrogels and were comparable to photothermal hydrogels, in terms of water evaporation rate and salt ion rejection ratio, demonstrating their potential for solar seawater desalination. The wettability of Janus hydrogel could be precisely regulated via modulating the amphiphilic Janus patch size and surface coverage to develop an excellent evaporator with a superior solar evaporation rate and salt ion rejection ratio.