Janus smart materials with asymmetrical wettability for on-demand oil/water separation: a comprehensive review

Abstract

Increasing growth in complex industrial oil-containing wastewater has caused tremendous damage to the ecological environment and human health. Materials with special wettability have created a promising future for the purification of oily wastewater. Unfortunately, conventional oil–water separation materials with single wettability are hardly suitable for extremely complex oily wastewater systems. Therefore, innovative materials urgently need to be developed for on-demand and efficient clarification of oily wastewater. Recently, Janus materials have received considerable attention due to their switchable wetting characteristics for on-demand oil–water separation. So far, Janus materials have progressed the exploration from mechanical inversion to intelligent self-response. Specifically, the manual wettability switchable Janus materials artificially reverse the wetting surface depending on the specific type of oil–water mixture being separated, resulting in on-demand oil–water separation. Instead, smart self-response Janus materials switch surface wettability in response to external irritations, leading to on-demand oil–water separation. Both of them have demonstrated great potential for industrial applications in the treatment of oily wastewater. However, only a few systematic discussions and comprehensive summaries of such Janus materials are reported in the existing literature. In this review, innovative advances in oil–water separation made by Janus materials over the past five years are discussed. We highlight Janus materials with reversible wetting behaviors, including manually switchable Janus materials (asymmetric modification and asymmetric fabrication) and smart responsive Janus materials (pH-, photo-, thermal-, electro-, and gas-responsiveness). Correspondingly, the response mechanisms, construction principles, fabrication strategies, representative studies, application prospects and the respective advantages and disadvantages of Janus materials with different wettability transition modes are also evaluated. Finally, the challenges and future directions based on the Janus materials are presented.