Xylem-Inspired Polyimide/MXene Aerogels with Radial Lamellar Architectures for Highly Sensitive Strain Detection and Efficient Solar Steam Generation

Efficient solar-driven crude oil cleanup via graphene/cellulose aerogel with radial and centrosymmetric design

Frequent oil spills pose significant threats to ecosystems; therefore, strict requirements are needed for prompt remediation and reclamation of spilled oil. Influenced by the structure of coniferous trees and their water transport, this experiment used cellulose nanofiber (CNF), polyvinyl alcohol (PVA), and methyltrimethoxysilane (MTMS) to prepare radially centrosymmetric aerogels. By utilizing the in-situ polycondensation reaction of MTMS, CNF, and PVA were connected, and the hydrophobicity and mechanical properties of the aerogel were greatly enhanced. Furthermore, the introduction of graphene oxide (GO), enshrouded within the cross-linked network, engenders heightened photo-thermal effects. The resultant composite aerogel exhibits expeditious oil absorption under solar irradiation and radial layered channel architecture, significantly curtailing the crude oil absorption timeframe (achieving a maximum absorption capacity of 51.7 g/g). Moreover, it demonstrates superior performance in rapidly and repeatedly adsorbing highly viscous crude oil, surpassing existing literature. Notably, continuous absorption of high-viscosity crude oil is achieved by integrating the composite aerogel with a peristaltic pump. This study offers a novel approach to the absorption and retrieval of high-viscosity crude oil, broadening the potential application horizons of CNF-based aerogels within environmental remediation.

Tree-Inspired Aerogel Comprising Nonoxidized Graphene Flakes and Cellulose as Solar Absorber for Efficient Water Generation

As global freshwater shortages worsen, solar steam generation (SSG) emerges as a promising, eco-friendly, and cost-effective solution for water purification. However, widespread SSG implementation requires efficient photothermal materials and solar evaporators that integrate enhanced light-to-heat conversion, rapid water transportation, and optimal thermal management. This study investigates using nonoxidized graphene flakes (NOGF) with negligible defects as photothermal materials capable of absorbing over 98% of sunlight. By combining NOGF with cellulose nanofibers (CNF) through bidirectional freeze casting, we created a vertically and radially aligned solar evaporator. The hybrid aerogel exhibited exceptional solar absorption, efficient solar-to-thermal conversion, and improved surface wettability. Inspired by tree structures, our design ensures rapid water supply while minimizing heat loss. With low NOGF content (∼10.0%), the NOGF/CNF aerogel achieves a solar steam generation rate of 2.39 kg m-2 h-1 with an energy conversion efficiency of 93.7% under 1-sun illumination, promising applications in seawater desalination and wastewater purification.

Tree-Inspired Structurally Graded Aerogel with Synergistic Water, Salt, and Thermal Transport for High-Salinity Solar-Powered Evaporation

Solar-powered interfacial evaporation is an energy-efficient solution for water scarcity. It requires solar absorbers to facilitate upward water transport and limit the heat to the surface for efficient evaporation. Furthermore, downward salt ion transport is also desired to prevent salt accumulation. However, achieving simultaneously fast water uptake, downward salt transport, and heat localization is challenging due to highly coupled water, mass, and thermal transport. Here, we develop a structurally graded aerogel inspired by tree transport systems to collectively optimize water, salt, and thermal transport. The arched aerogel features root-like, fan-shaped microchannels for rapid water uptake and downward salt diffusion, and horizontally aligned pores near the surface for heat localization through maximizing solar absorption and minimizing conductive heat loss. These structural characteristics gave rise to consistent evaporation rates of 2.09 kg m-2 h-1 under one-sun illumination in a 3.5 wt% NaCl solution for 7 days without degradation. Even in a high-salinity solution of 20 wt% NaCl, the evaporation rates maintained stable at 1.94 kg m-2 h-1 for 8 h without salt crystal formation. This work offers a novel microstructural design to address the complex interplay of water, salt, and thermal transport.

Biomimetic Design of 3D Fe3O4/V-EVOH Fiber-Based Self-Floating Composite Aerogel to Enhance Solar Steam Generation Performance

An interfacial solar steam generation evaporator for seawater desalination has attracted extensive interest in recent years. Nevertheless, challenges still remain in relatively low evaporation rate, unsatisfactory energy conversion efficiency, and salt accumulation. Herein, we have demonstrated a biomimetic bilayer composite aerogel consisting of bottom hydrophilic and vertically aligned EVOH channels and an upper hydrophobic conical Fe3O4 array. Thanks to the design merits, the 3D Fe3O4/V-EVOH evaporator exhibits a high evaporation rate of ∼2.446 kg m-2 h-1 and an impressive solar energy conversion efficiency of ∼165.5% under 1 sun illumination, which is superior to those of state-of-the-art evaporators reported so far. Moreover, the asymmetrical wettability not only allows the evaporator to self-float on the water but also facilitates the salt ion diffusion in the channels; thus, the evaporator shows no salt crystals on its surface and only a 6% decrease in evaporation performance even after the salt concentration increases from 0 to 10.0 wt %.

In Situ MXene Anchored Structure for Highly Durable Solar Steam Generation

As a promising fresh water harvesting technology, interfacial solar steam generation has attracted growing interest. Efficient solar absorption and long-term operational performance are critical requirements of this technology. However, developing robust evaporators to promote practical applications under extreme conditions is still a grand challenge. Herein, we propose a light-assisted strategy to in situ prepare a Ti3C2Tx MXene anchored structure (MXAS) for enhanced solar evaporation with superior mechanical properties (compressive strength of 78.47 MPa, which can withstand a pressure of 3.92 × 106 times its own weight). Light irradiation enlarges the interlayer spacing of MXene and improves the solar absorption capability. Under one sun, the three-dimensional MXAS evaporator exhibits a steam generation rate of 2.48 kg m-2 h-1and an evaporation efficiency of 89.3%, and it demonstrates long-term durability when testing in seawater. This strategy provides valuable insights into the potential application of a high-performance water evaporation system.

Anisotropic MXene/Poly(vinyl alcohol) Composite Hydrogels with Vertically Oriented Channels and Modulated Surface Topography for Efficient Solar-Driven Water Evaporation and Purification

Hierarchical structure and surface topography play pivotal roles in developing high-performance solar-driven evaporators for clean water production; however, there exists a notable gap in research addressing simultaneous modulation of internal microstructure and surface topography in hydrogels to enhance both solar steam generation performance and desalination efficiency. Herein, anisotropic poly(vinyl alcohol)/MXene composite hydrogels for efficient solar-driven water evaporation and wastewater purification are fabricated using a template-assisted directional freezing approach followed by precise surface wettability modulation. The resultant composite hydrogels exhibit vertically oriented channels that ensure fast water supply during evaporation, and their poly(vinyl alcohol) skeletons can reduce the vaporization enthalpy of the water in the hydrogels. The incorporation of MXene sheets enables efficient solar light absorption and solar-thermal conversion while providing structural reinforcement to the hydrogels. More importantly, the as-created undulating solar-thermal surface, featuring modulated hydrophilic troughs and hydrophobic crests, significantly enhances solar-thermal conversion efficiency, thereby boosting solar evaporation performances. As a result, the fabricated hydrogel-based evaporator exhibits an impressive evaporation rate of 2.55 kg m-2 h-1 under 1 sun irradiation, coupled with long-term durability and desalination stability. Notably, the outstanding mechanical robustness of the hydrogel further enables high portability through a readily achievable process of reversible dehydration/hydration.

Boosting the Viable Water Harvesting in Solar Vapor Generation: From Interfacial Engineering to Devices Design

Continuously increasing demand for the life-critical water resource induces severe global water shortages. It is imperative to advance effective, economic, and environmentally sustainable strategies to augment clean water supply. The present work reviews recent reports on the interfacial engineering to devices design of solar vapor generation (SVG) system for boosting the viability of drinkable water harvesting. Particular emphasis is placed on the basic principles associated with the interfacial engineering of solar evaporators capable of efficient solar-to-thermal conversion and resulting freshwater vapor via eliminating pollutants from quality-impaired water sources. The critical configurations manufacturing of the devices for fast condensation is then highlighted to harvest potable liquid water. Fundamental and practical challenges, along with prospects for the targeted materials architecture and devices modifications of SVG system are also outlined, aiming to provide future directions and inspiring critical research efforts in this emerging and exciting field.

Outstanding Synergy of Sensitivity and Linear Range Enabled by Multigradient Architectures

Flexible pressure sensors are devices that mimic the sensory capabilities of natural human skin and enable robots to perceive external stimuli. One of the main challenges is maintaining high sensitivity over a broad linear pressure range due to poor structural compressibility. Here, we report a flexible pressure sensor with an ultrahigh sensitivity of 153.3 kPa-1 and linear response over an unprecedentedly broad pressure range from 0.0005 to 1300 kPa based on interdigital-shaped, multigradient architectures, featuring modulus, conductivity, and microstructure gradients. Such multigradient architectures and interdigital-shaped configurations enable effective stress transfer and conductivity regulation, evading the pressure sensitivity-linear range trade-off dilemma. Together with high pressure resolution, high frequency response, and good reproducibility over the ultrabroad linear range, proof-of-concept applications such as acoustic wave detection, high-resolution pressure measurement, and healthcare monitoring in diverse scenarios are demonstrated.

Flexible, Thermally Stable, and Ultralightweight Polyimide-CNT Aerogel Composite Films for Energy Storage Applications

Polyimide (PI) aerogels are promising in various fields of application, ranging from thermal insulators to aerospace. However, they are typically in the form of a bulk monolith, which suffers from a lack of conformability and drapability. Moreover, their electrical conductivity is limited, and they mainly display an insulative behavior. These shortcomings can limit the applications of PI aerogels in energy storage systems, which require ultralightweight flexible conductive films, which at the same time offer high thermal stability, ultralow density, and high surface area. To overcome these obstacles, the present study reports the fabrication of PI-carbon nanotube (PI-CNT) aerogel composite films with varying CNT content prepared through a sol-gel preparation method, followed by a supercritical drying procedure. Compared to pristine PI aerogels, which displayed a large shrinkage and density of 18.3% and 0.12 g cm-3, respectively, the incorporation of only 5 wt % CNTs resulted in a significant reduction of both shrinkage and density to only 11.5% and 0.10 g cm-3, respectively. This suggests the importance of CNTs in improving the dimensional stability of aerogels and creating a robust network. Further characterizations showed that incorporation of 5 wt % CNTs also resulted in the highest pore volume (1.25 cm3 g-1), highest surface area (324 m2 g-1), highest real permittivity (80), highest electrical conductivity (3 × 10-1 S m-1), and ultrahigh service temperature (575 °C). It was also shown that the aerogel films can withstand a large degree of bending, can be twisted, and can be fully rolled with no obvious cracks propagated in the structure. The combined outstanding properties of the developed aerogel composite films make them promising potential candidates for supercapacitor electrodes. Therefore, the electrochemical performance of the devices based on aerogel electrodes was further studied. The device demonstrated a high energy density of 2.6 Wh kg-1 at a power density of 303.8 W kg-1. The total capacitance after 5000 cycles was 91.8% of the initial capacitance, which indicated excellent stability and durability of the device. Overall, this work provides a facile yet effective methodology for the development of high-performance aerogel materials for energy storage applications.

Advances in photothermal regulation strategies: from efficient solar heating to daytime passive cooling

Photothermal regulation concerning solar harvesting and repelling has recently attracted significant interest due to the fast-growing research focus in the areas of solar heating for evaporation, photocatalysis, motion, and electricity generation, as well as passive cooling for cooling textiles and smart buildings. The parallel development of photothermal regulation strategies through both material and system designs has further improved the overall solar utilization efficiency for heating/cooling. In this review, we will review the latest progress in photothermal regulation, including solar heating and passive cooling, and their manipulating strategies. The underlying mechanisms and criteria of highly efficient photothermal regulation in terms of optical absorption/reflection, thermal conversion, transfer, and emission properties corresponding to the extensive catalog of nanostructured materials are discussed. The rational material and structural designs with spectral selectivity for improving the photothermal regulation performance are then highlighted. We finally present the recent significant developments of applications of photothermal regulation in clean energy and environmental areas and give a brief perspective on the current challenges and future development of controlled solar energy utilization.

Directional-Freezing-Enabled MXene Orientation toward Anisotropic PVDF/MXene Aerogels: Orientation-Dependent Properties of Hybrid Aerogels

Polymer hybrid materials that contain reinforcements with a preferred orientation have received growing attention because of their unique properties and promising applications in multifunctional fields. Herein, anisotropic poly(vinylidene fluoride) (PVDF)/MXene hybrid aerogels with highly ordered delaminated MXene nanosheets and anisotropic porous structures were successfully fabricated by unidirectional freezing of thermoreversible gels followed by a freeze-drying process. The strong interfacial interactions between PVDF chains and abundant functional groups on the surface of MXene enabled the orientation of MXene nanosheets at the boundaries of ice crystals as the semicrystalline PVDF and delaminated MXene nanosheets are squeezed along the freezing direction. These aerogels display distinct properties along the freezing and perpendicular to the freezing (transverse) directions. These anisotropic aerogels are flexible and flame-retardant and possess an anisotropic compression performance, heat transfer, electrical conductivity, and electromagnetic interference (EMI) shielding. Further, by increasing the MXene loadings, the electrical conductivity and EMI shielding performances of hybrid aerogels were significantly improved. The PVDF aerogel showed sticky hydrophobicity with a contact angle of 139°, whereas the contact angle increased significantly in hybrid aerogels (153°) with low water adhesion, making them suitable as self-cleaning materials. The combination of the above characteristics makes these hybrid aerogels potential candidates for a wide range of electronic applications.

Cellulose-based evaporator with dual boost of water transportation and photothermal conversion for highly solar-driven evaporation

Two-dimensional (2D) evaporation systems could significantly reduce the heat conduction loss compared with the photothermal conversion materials particles during the evaporation process. But the normal layer-by-layer self-assembly method of 2D evaporator would reduce the water transportation performance due to the highly compact channel structures. Herein, in our work, the 2D evaporator with cellulose nanofiber (CNF), Ti₃C₂T_x (MXene) and polydopamine modified lignin (PL) by layer-by-layer self-assembly and freeze-drying methods. The addition of PL also enhanced the light absorption and photothermal conversion performance of the evaporator due to the strong conjugation and π-π molecular interactions. After the combination process of layer-by-layer self-assembly and freeze-drying process, the as-prepared freeze-dried CNF/MXene/PL (f-CMPL) aerogel film exhibited highly interconnected porous structure with promoted hydrophilicity (enhanced water transportation performance). Benefiting these favorable properties, the f-CMPL aerogel film showed enhanced light absorption performance (surface temperature could be reached to 39 °C under 1 sun irradiation) and higher evaporation rate (1.60 kg m^-2 h^-1). This work opens new way to fabricate cellulose-based evaporator with highly evaporation performance for the solar steam generation and provides a new idea for improving the evaporation performance of 2D cellulose-based evaporator.

Toward Smart Sensing by MXene

The Internet of Things era has promoted enormous research on sensors, communications, data fusion, and actuators. Among them, sensors are a prerequisite for acquiring the environmental information for delivering to an artificial data center to make decisions. The MXene-based sensors have aroused tremendous interest because of their extraordinary performances. In this review, the electrical, electronic, and optical properties of MXenes are first introduced. Next, the MXene-based sensors are discussed according to the sensing mechanisms such as electronic, electrochemical, and optical methods. Initially, biosensors are introduced based on chemiresistors and field-effect transistors. Besides, the wearable pressure sensor is demonstrated with piezoresistive devices. Third, the electrochemical methods include amperometry and electrochemiluminescence as examples. In addition, the optical approaches refer to surface plasmonic resonance and fluorescence resonance energy transfer. Moreover, the prospects are delivered of multimodal data fusion toward complicated human-like senses. Eventually, future opportunities for MXene research are conveyed in the new material discovery, structure design, and proof-of-concept devices.

Quasi-Homogeneous and Hierarchical Electronic Textiles with Porosity-Hydrophilicity Dual-Gradient for Unidirectional Sweat Transport, Electrophysiological Monitoring, and Body-Temperature Visualization

On-skin electronics based on impermeable elastomers and stacking structures often suffer from inferior sweat-repelling capabilities and severe mechanical mismatch between sub-layers employed, which significantly impedes their lengthy wearing comfort and functionality. Herein, inspired by the transpiration system of vascular plants and the water diode phenomenon, a hierarchical nonwoven electronic textile (E-textile) with multi-branching microfibers and robust interlayer adhesion is rationally developed. The layer-by-layer electro-airflow spinning method and selective oxygen plasma treatment are utilized to yield a porosity-hydrophilicity dual-gradient. The resulting E-textile shows unidirectional, nonreversible, and anti-gravity water transporting performance even upon large-scale stretching (250%), excellent mechanical matching between sub-layers, as well as a reversible color-switching ability to visualize body temperature. More importantly, the conducting and skin-conformal E-textile demonstrates accurate and stable detecting capability for biomechanical and bioelectrical signals when applied as an on-skin bioelectrode, including different human activities, electrocardiography, electromyogram, and electrodermal activity signals. Further, the E-textile can be efficiently implemented in human-machine interfaces to build a gesture-controlled dustbin and a smart acousto-optic alarm. Hence, this hierarchically-designed E-textile with integrated functionalities offers a practical and innovative method for designing comfortable and daily applicable on-skin electronics.

Cicada-Wing-Inspired Highly Sensitive Tactile Sensors Based on Elastic Carbon Foam with Nanotextured Surfaces

Electronic devices with tactile and pressure-sensing capabilities are becoming increasingly popular in the automatic industry, human motion/health monitoring, and artificial intelligence applications. Inspired by the natural nanotopography of the cicada wing, we propose here a straightforward strategy to fabricate a highly sensitive tactile sensor through nanotexturing of erected polyaniline (PANI) nanoneedles on a conductive and elastic three-dimensional (3D) carbon skeleton. The robust and compressible carbon networks offer a resilient and conducting matrix to catering complex scenarios; the biomimetic PANI nanoneedles firmly and densely anchored on a 3D carbon skeleton provide intimate electrical contact under subtle deformation. As a result, a piezoresistive tactile sensor with ultrahigh sensitivity (33.52 kPa^-1), fast response/recovery abilities (97/111 ms), and reproducible sensing performance (2500 cycles) is developed, which is capable of distinguishing motions in a wide pressure range from 4.66 Pa to 60 kPa, detecting spatial pressure distribution, and monitoring various gestures in a wireless manner. These excellent performances demonstrate the great potential of nature-inspired tactile sensors for practical human motion monitoring and artificial intelligence applications.

Coaxial Graphene/MXene Microfibers with Interfacial Buffer-Based Lightweight Distance Sensors Assisting Lossless Grasping of Fragile and Deformable Objects

Lossless and efficient robotic grasping is becoming increasingly important with the widespread application of intelligent robotics in warehouse transportation, human healthcare, and domestic services. However, current sensors for feedback of grasping behavior are greatly restricted by high manufacturing cost, large volume and mass, complex circuit, and signal crosstalk. To solve these problems, here, we prepare lightweight distance sensor-based reduced graphene oxide (rGO)/MXene-rGO coaxial microfibers with interface buffer to assist lossless grasping of a robotic manipulator. The as-fabricated distance microsensor exhibits a high sensitivity of 91.2 m^-1 in the distance range of 50-300 μm, a fast response time of 116 ms, a high resolution of 5 μm, and good stability in 500 cycles. Furthermore, the high-performance and lightweight microsensor is installed on the robotic manipulator to reflect the grasp state by the displacement imposed on the sensor. By establishing the correlation between the microsensing signal and the grasp state, the safe, non-destructive, and effective grasp and release of the target can be achieved. The lightweight and high-powered distance sensor displays great application prospects in intelligent fetching, medical surgery, multi-spindle automatic machines, and cultural relics excavation.

MXene-Based Porous Monoliths

In the past decade, a thriving family of 2D nanomaterials, transition-metal carbides/nitrides (MXenes), have garnered tremendous interest due to its intriguing physical/chemical properties, structural features, and versatile functionality. Integrating these 2D nanosheets into 3D monoliths offers an exciting and powerful platform for translating their fundamental advantages into practical applications. Introducing internal pores, such as isotropic pores and aligned channels, within the monoliths can not only address the restacking of MXenes, but also afford a series of novel and, in some cases, unique structural merits to advance the utility of the MXene-based materials. Here, a brief overview of the development of MXene-based porous monoliths, in terms of the types of microstructures, is provided, focusing on the pore design and how the porous microstructure affects the application performance.

Perspiration-Wicking and Luminescent On-Skin Electronics Based on Ultrastretchable Janus E-Textiles

Stretchable electronics have attracted surging attention for next-generation smart wearables, yet traditional flexible devices fabricated on hermetical elastic substrates cannot satisfy lengthy wearing comfort and signal stability due to their poor moisture and air permeability. Herein, perspiration-wicking and luminescent on-skin electrodes are fabricated on superelastic nonwoven textiles with a Janus configuration. Through the electrospin-assisted face-to-face assembly of all-SEBS microfibers with differentiated diameters and composition, porosity and wettability asymmetry are constructed across the textile, endowing it with antigravity water transport capability for continuous sweat release. Also, the phosphor particles evenly encapsulated in the elastic fibers empower the Janus textile with stable light-emitting capability under extreme stretching in a dark environment. Additionally, the precise printing of highly conductive liquid metal (LM) circuits onto the matrix not only equips the electronic textile with broad detectability for various biophysical and electrophysiological signals but also enables successful implementation of human-machine interface (HMIs) to control a mechanical claw.