Porous Janus materials with unique asymmetries and functionality

Abstract

Porous Janus materials have attracted widespread attention due to their asymmetry in wettability, charge, pore size or structure, thermal/electrical conductivity, and chemical activity. Multifunction integration and unique directional manipulation of liquid, ion, or gas within porous Janus materials enable rapid progress in diverse applications such as fog collection, personal moisture and healthcare management, energy conversion, water purification, sensor devices, and biomedical applications. Compared with conventional homogeneous materials, porous Janus materials not only showcase superior performance and energetic potential but also open up new applications by virtue of the synergistic or independent effects of asymmetry. This comprehensive review systematically analyzes the research progress of porous Janus materials, highlighting fabrication strategies, synergistic/independent work mechanisms, and emerging advanced applications. Finally, ongoing challenges and outlook for the future research of porous Janus materials are presented.

Introduction

Ever-increasing diversified and complex materials-based demands necessitate exploitation of novel materials with multifunction integration or advanced functionality [1], [2], [3], [4], [5]. Therein, artificial Janus materials (natural Janus materials are ubiquitous such as cell membranes for ionic transport [6]), named after the two-faced god Janus in ancient Roman mythology, have two faces with asymmetric properties and came to light after P. G. de Gennes’ Nobel Prize lecture in 1991 [7]. Originally, efforts were focused on fabricating materials on the micro-/nano-scale, such as Janus particles/nanosheets/cylinders with two different compositions, morphologies, or sizes, finding applications as solid emulsion stabilizers, optical probes, and catalysts [8], [9]. Afterward, this sector expanded Janus materials to the macroscopic scale, and Lin et al. [10] realized directional water-transfer by asymmetric wettability integration across material thickness, which cannot be accomplished by conventional homogeneous porous materials. To date, the exploited asymmetric properties of porous Janus materials at the macroscopic level include wettability, charge, pore size/structure, thermal/electrical conductivity, and chemical activity, but are not limited to these.

There are two working modes for asymmetric properties of porous Janus materials: synergistic and independent. In the synergistic mode, porous Janus materials will present unique diode performance. For asymmetric wettability such as hydrophobic/hydrophilic [11] and super-oleophobic/super-oleophilic [12], these porous Janus materials can realize directional liquid transport such as water/oil “diodes” [13], [14] (liquid penetrating from one side and being blocked in the reverse direction) when regulating appropriate wettability, pore size, and thickness of the functional layers, which promises applications in advanced separation [15], fog collection [16], healthcare [17], moisture absorption and perspiration [18]. Hydrophobic/hydrophilic Janus porous materials are also aerophilic/aerophobic in water, enabling realization of bubble unidirectional transport underwater (air ‘‘diode’’) [19], [20] and applications of water-splitting [21] or superior gas transport [22]. Distinguished from conventional homogeneous materials, asymmetric wettability provides an inner driving force for facilitating liquid or bubble transport, which improves operational energy efficiency [23], [24]. In addition, architectures with charge asymmetry enable porous Janus materials as ion diodes, which can capture osmotic energy from natural systems [25]. In the independent mode, porous Janus materials tend to be multifunctional and integrated, which showcase superior performance or open up new application fields. The asymmetric properties work separately without interfering with each other, which can realize the coexistence of various functionalities for a variety of applications in water purification, sensors, and in the biomedical field. For example, hydrophilic/hydrophobic Janus membranes can deliver functionality of both hydrophilic and hydrophobic membranes to separate or generate different emulsions [26], [27]. Integrating asymmetric properties can also decouple application processes to achieve superior performance. As an example, solar desalination can be broken down into water pumping by the hydrophilic layer and light-heat conversion and steam generation by the hydrophobic layer. The presence of the hydrophilic layer mitigates salt deposition, which has been an important obstacle to efficiency for conventional materials [28].

Other investigations have explored the combination of Janus character with other emerging functional materials such as graphene oxide (GO) [29], MXene [30], carbon nanotubes (CNTs) [31], or stimulation-responsive substances [13], [32], [33], which greatly enriches the function and application space of porous Janus materials. In addition, porous Janus materials can also possess two or more asymmetric properties in one unit at the same time, which results in tremendous variety and opportunities for multi-application [34], [35], [36]. These achievements demonstrate that porous Janus materials present enormous potential in constructing advanced and intelligent devices.

This review aims to provide a concise and comprehensive overview (Fig. 1), systematically summarizing recent advances in various asymmetry and fabrication strategies, highlighting the two working mechanisms of asymmetry of porous Janus materials, discussing their multi-functional application in water purification, fog collection, personal moisture and healthcare management, energy conversion, sensor devices, bubble manipulation, and biomedical applications in detail, and finally, offering the challenges and outlook for future development of porous macroscopic Janus materials.