Janus graphene aerogels with asymmetric wettability and integrated structure for efficient solar steam generation

Janus evaporators with asymmetric wettability, strong mechanical structure, and simple preparation are essential for efficient solar steam generation and seawater desalination in large-scale. In this work, a Janus graphene aerogel (JGA) with asymmetric wettability is prepared through a facile ice-templating method. The lower vertical assembled layer (VAL) of the JGA features a vertically aligned microstructure with superhydrophilicity (a droplet can quickly impregnate into the surface in just 172.4 ms), which continuously supplies water for evaporation and provides good thermal insulation. The upper horizontal assembled layer (HAL) with a horizontally stacked microstructure shows hydrophobicity (the receding water contact angle≈ 109°), enhancing solar absorption and providing horizontal heat spreading at the interface. Moreover, due to the C-N covalent bonding, the integrated structure shows good interfacial bonding strength between the two layers. This innovative design effectively enhances the heat localization and the steam generation efficiency. The evaporation rate remains stable at 1.53 kg m⁻² h⁻¹ after ten cycles of testing under one sun illumination (1 kW m⁻²), with an energy conversion efficiency of 91.9 %. The simplicity, structure robustness, and continuous operational capability makes JGA a promising candidate as solar evaporators to tackle the growing issue of water scarcity.