Solar interfacial evaporation has been recognized as a versatile energy conversion protocol for cutting-edge applications such as water treatment and power generation (e.g., hydro voltaic effect). Recently, to enhance water evaporation rates, water temperature and evaporation area have been considered as essential ingredients, and thus photothermal materials and three-dimensional hierarchical structures have been developed to promote light-to-heat conversion efficiency and enhance interfacial evaporation. However, less attention has been paid to the airflow effect, because the interfacial floatability of photothermal membranes should be considered under air blast. Here, inspired from the stable interfacial floatability of lotus leaves, we report the airflow enhanced solar interfacial evaporation approach using a graphene-based Janus membrane. Laser-induced graphene (LIG) film was treated unilaterally by O2 plasma, forming a LIG/oxidized LIG (LIG-O) Janus membrane with distinct wettability on two sides. Higher water evaporation rate of 1.512 kg m–2 h–1 is achieved. The high solar interfacial evaporation performance can be attributed to the two advantages: (i) the combination of microscale capillary water transporting and nanoscale light trapping; (ii) hydrophobic/hydrophilic Janus membrane for stable interfacial floatability under airflow. Our approach is feasible for developing high-performance solar interfacial evaporation devices for practical clean energy utilization.

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ease of preparation. As a pioneer of this field, Gracias' group has proposed several impressive bilayer actuators. For instance, a thermoresponsive theragripper with photopatternable poly(propylene fumarate) (PPF)/poly(N-isopropylacrylamide-coacrylic acid) (pNIPAM-AAc) bilayer structure has been developed for drug delivery. [28] Besides, Yang and co-workers have successfully fabricated stimuliresponsive actuators including an intelligent spring, a smart gripper, and a new type of "microrobot" based on the singlewall carbon nanotube (SWCNT) and polyvinylidene fluoride (PVDF) bilayer structure. [29] Zhu and co-workers reported novel soft electrothermal bimorph actuators made of polyimide (PI) and silver nanowire (AgNW)/polydimethylsiloxane (PDMS), which could be used in selfwalkers and soft grippers. [30] Despite a rapid progress has been made in this field, continuous efforts have been devoted to exploring new functional materials for further promotion of their performance.Graphene oxide (GO) that features ultrafast water adsorption/desorption capability, tunable physical/chemical properties, and tractable solution processing property holds great promise for developing smart bimorph actuators. Significantly, there exist plenty of oxygen-containing groups (OCGs) on GO sheets, which make GO a very hydrophilic material. [31] Water molecules can be adsorbed by GO easily and transferred freely among the GO multilayers. In this case, GO is very promising for moisture-responsive actuators. Additionally, the OCGs on GO sheets can be selectively removed or modified through various chemical/physical strategies, which can endow the resultant materials with conductivity, light absorption property, and high electrothermal and photothermal conversion efficiency. [32] Therefore, bimorph actuators based on GO or its derivatives, for instance reduced GO (RGO), also enable light and electrical actuation. Recently, GO has emerged as a versatile material for actuator design. For example, Liu and coworkers prepared an electromechanical ring-shaped actuator by combining an RGO layer with a PDMS layer, in which the RGO layer serves as an electric-heated layer in the electrothermal actuator. [33] Tang et al. combined thermally expanding microspheres (TEMs) with RGO to fabricate an RGO-TEM-PDMS/ PDMS bilayer actuator. They realized remote construction of 3D structures upon light irradiation, and the photothermal effect Graphene oxide (GO) with tunable physical/chemical properties is a versatile material for smart bimorph actuators. Using GO or its derivatives (e.g., reduced GO, RGO) as an active material, actuators can be manipulated under various external stimuli including moisture, light, temperature, and electricity. However, most of these GO-based actuators respond to a solo stimulus, which limits its cutting-edge applications in soft robotics. Here, the programmable patterning of RGO/GO Janus paper using a threshold-controlled direct laser writing (DLW) technology is reported. By combining the RGO/GO Janus paper with a common thermal...

Graphene oxide (GO), which has many oxygen functional groups, is a promising candidate for use in moisture-responsive sensors and actuators due to the strong water-GO interaction and the ultrafast transport of water molecules within the stacked GO sheets. In the last 5 years, moisture-responsive actuators based on GO have shown distinct advantages over other stimuli-responsive materials and devices. Particularly, inspired by nature organisms, various moisture-enabled soft robots have been successfully developed via rational assembly of the GO-based actuators. Herein, the milestones in the development of moisture-responsive soft robots based on GO are summarized. In addition, the working mechanisms, design principles, current achievement, and prospects are also comprehensively reviewed. In particular, the GO-based soft robots are at the forefront of the advancement of automatable smart devices.

The strong interaction between water molecules and graphene oxide (GO) enables moisture‐responsive graphene actuators, revealing great potential for soft robots. However, current strategies for developing smart graphene actuators fail to tailor their material property gradient in a controlled manner, and the driving manner is usually limited to single stimulus actuation. Here, a facile preparation of humidity/thermal/light multiresponsive graphene actuators by sequential vacuum filtration of GO and reduced GO (RGO) aqueous solutions is reported. The photoreduction degree of RGO layer is tuned precisely beforehand by changing ultraviolet (UV) light irradiation time, and thus a pretailored reduction gradient along the normal direction of the GO/RGO bilayer paper would form in a highly controlled manner. Taking advantage of the competitive water adsorption between the GO and RGO layers, as well as the thermal‐, light‐promoted desorption, the GO/RGO bilayers deform in response to moisture, light, and temperature changes; and the deformation degree can be modulated by controlling the gradient of oxygen‐containing groups (OCGs). As a proof of principle, a humidity‐responsive graphene mimosa and a humidity/thermal/light multiresponsive graphene actuators are fabricated. The GO/RGO bilayer paper with pretailored reduction gradient holds great promise for easy fabrication of biomimetic actuators that enable performing predictable deformation.

Inspired by the robustness of nacre's structure, moisture-responsive actuators with high mechanical strength and self-healing properties were successfully developed based on graphene oxide and cellulose fiber hybrids.

Graphene-based actuators featuring fast and reversible deformation under various external stimuli are promising for soft robotics. However, these bimorph actuators are incapable of complex and programmable 3D deformation, which limits their practical application. Here, inspired from the collective coupling and coordination of living cells, we fabricated a moisture-responsive graphene actuator swarm that has programmable shape-changing capability by programming the SU-8 patterns underneath. To get better control over the deformation, we fabricated SU-8 micropattern arrays with specific geometries and orientations on a continuous graphene oxide film, forming a swarm of bimorph actuators. In this way, predictable and complex deformations, including bending, twisting, coiling, asymmetric bending, 3D folding, and combinations of these, have been achieved due to the collective coupling and coordination of the actuator swarm. This work proposes a new way to program the deformation of bilayer actuators, expanding the capabilities of existing bimorph actuators for applications in various smart devices.

a large surface to volume ratio [7] ) and have broad applications (e.g., energy conversion, [8] smart robots, [9] and biomedical devices [10] ). Recently, Ariga and co-workers achieved highly selective gas sensing using ionic liquid-intercalated graphene layers that were prepared by in situ reduction of graphene oxide layers in the presence of nonvolatile ionic liquids, and subsequent electrostatic layer-by-layer assembly. [11] Sadasivuni et al. presented the layer-bylayer spraying of modified graphene oxidefilled cellulose nanocrystals for proximity sensing. [12] Wang et al. reported ultrathin reduced graphene oxide (RGO) films with controllable thicknesses for noncontact relative humidity (RH) sensing. [13] In our previous works, an RGO with hierarchical micro-nanostructures prepared by two-beam-laser interference was employed to produce humidity-sensing devices. [14] However, in spite of these advancements, graphene-based sensing devices have not been well employed for smart device design. A possible reason for this gap would be the difficulty in tailoring highly permeable graphene nanostructures that permit spontaneous, timely, and reliable molecular discrimination. Moreover, complex experimental procedures or special instruments are generally necessary in device fabrication, which largely limits their integration with other devices. The current trend is to produce smart devices equipped with versatile sensors created in facile, reliable, cost-effective, and environmentally friendly manners. However, it is currently challenging to achieve this goal.Herein, we report a facile preparation of a graphene-based moisture detector for smart device design. A humidity-sensing device has been fabricated by a simple focused sunlight treatment of GO. The drastic removal of oxygen-containing groups (OCGs) not only mediates the controllable tuning of conductive properties but also leads to the formation of a highly porous RGO structure that is of benefit to the adsorption of molecules and humidity sensitivity. The RGO-based humidity-sensing device shows moisture recognition capability and excellent sensing performance, including high sensitivity, good repeatability, small humidity hysteresis, and fast response recovery at room temperature, enabling a series of humidity-sensing devices including a moisture controller, a humidity detector robot, and even a novel electronic harmonica. As a simple and chemical-free method, sunlight-mediated photoreduction of GO provides a very simple and cost-effective way to integrate humidity sensors in the development of versatile moistureresponsive smart devices.Here, a facile fabrication of graphene-based humidity sensors is reported for smart device design. Focused sunlight photoreduction of graphene oxide (GO) helps to remove most of the oxygen-containing groups on GO sheets, which not only recovers their conductivity but also leads to the formation of a highly porous nanostructure, enabling the manufacture of versatile humidity-sensing smart devices, such as moisture controllers, hu...