Droplet-Pen Writing of Ultra-Uniform Graphene Pattern for Multi-Spectral Applications.

Artificial optical patterns bring wide benefits in applications like structural color display, photonic camouflage, and electromagnetic cloak. Their scalable coating on large-scale objects will greatly enrich the multimodal-interactive society. Here, a droplet-pen writing (DPW) method to directly write multi-spectral patterns of thin-film graphene is reported. By amphiphilicity regulations of 2D graphene nanosheets, ultra-uniform and ultrathin films can spontaneously form on droplet caps and pave to the substrate, thus inducing optical interference. This allows the on-surface patterning by pen writing of droplets. Specifically, drop-on-demand thin films are achieved with millimeter lateral size and uniformity up to 97% in subwavelength thickness (<100 nm), corresponding to an aspect ratio of over 30 000. The pixelated thin-film patterns of disks and lines in an 8-inch wafer scale are demonstrated, which enable low-emittance structural color paintings. Furthermore, the applications of these patterns for dual-band camouflage and infrared-to-visible encryption are investigated. This study highlights the potential of 2D material self-assembly in the large-scale preparation and multi-spectral application of thin film-based optical patterns.

Bio-Inspired Ultrathin Perfect Absorber for High-Performance Photothermal Conversion.

Ultrathin perfect absorber (UPA) enables efficient photothermal conversion (PC) in renewable chemical and energy systems. However, it is challenging so far to obtain efficient absorption with thickness significantly less than the wavelength, especially considering the common view that an ultrathin film can absorb at most 50% of incident light. Here, a highly light-absorbing and mechanically stable UPA is reported by learning from the honeycomb mirror design of the crab compound eyes. With the hollow apertures enclosed by the self-supporting ultrathin film of reduced graphene oxide and gold nanoparticles, the absorber achieves spoof-plasmon enhanced broadband absorption in solar spectrum and low radiative decay in infrared. Specifically, a strong absorption (87%) is realized by the apertures with cross-section thickness of 1/20 of the wavelength, which is 7.3 times stronger than a planar counterpart with the identical material. Its high PC efficiency up to 64%, with hot-electron temperature as high as 2344 K, is also experimentally demonstrated. Utilizing its low thermal mass nature, a high-speed visible-to-infrared converter is constructed. The absorber can enable high-performance PC processes for future interfacial catalysis and photon-detection.

Low-Grade Waste Heat Enables Over 80 L m- 2 h-1 Interfacial Steam Generation Based on 3D Superhydrophilic Foam.

Clean water scarcity and energy shortage have become urgent global problems due to population growth and human industrial development. Low-grade waste heat (LGWH) is a widely available and ubiquitous byproduct of human activities worldwide, which can provide effective power to address the fresh water crisis without additional energy consumption and carbon emissions. In this regard, 3D superhydrophilic polyurethane/sodium alginate (PU/SA) foam and LGWH-driven interfacial water evaporation systems are developed, which can precipitate over 80 L m-2  h-1 steam generation from seawater and has favorable durability for purification of high salinity wastewater. The excellent water absorption ability, unobstructed water transport, and uniform thin water layer formed on 3D skeletons of PU/SA foam ensure the strong heat exchange between LGWH and fluidic water. As a result, the heat-localized PU/SA foam enables the efficient energy utilization and ultrafast water evaporation once LGWH is introduced into PU/SA foam as heat flow. In addition, the precipitated salt on PU/SA foam can be easily removed by mechanical compression, and almost no decrease in water evaporation rate after salt precipitation and removal for many times. Meanwhile, the collected clean water has high rejection of ions of 99.6%, which meets the World Health Organization (WHO) standard of drinking water. Above all, this LGWH-driven interfacial water evaporation system presents a promising and easily accessible solution for clean water production and water-salt separation without additional energy burden for the society.

Sunlight-Coordinated High-Performance Moisture Power in Natural Conditions.

It is a challenge to spontaneously harvest multiple clean sources from the environment for upgraded energy-converting systems. The ubiquitous moisture and sunlight in nature are attractive for sustainable power generation especially. A high-performance light-coordinated "moist-electric generator" (LMEG) based on the rational combination of a polyelectrolyte and a phytochrome is herein developed. By spontaneous adsorption of gaseous water molecules and simultaneous exposure to sunlight, a piece of 1 cm2 composite film offers an open-circuit voltage of 0.92 V and a considerable short-circuit current density of up to 1.55 mA cm-2 . This record-high current density is about two orders of magnitude improvement over that of most conventional moisture-enabled systems, which is caused by moisture-induced charge separation accompanied with photoexcited carrier migration, as confirmed by a dynamic Monte Carlo device simulation. Flexible devices with customizable size are available for large-scale integration to effectively work under a wide range of relative humidity (about 20-100%), temperature (10-80 °C), and light intensity (30-200 mW cm-2 ). The wearable and portable LMEGs provide ample power supply in natural conditions for indoor and outdoor electricity-consuming systems. This work opens a novel avenue to develop sustainable power generation through collecting multiple types of natural energy by a single hybrid harvester.

Large-Scale Spinning Approach to Engineering Knittable Hydrogel Fiber for Soft Robots.

Efforts to impart responsiveness to environmental stimuli in artificial hydrogel fibers are crucial to intelligent, shape-memory electronics and weavable soft robots. However, owing to the vulnerable mechanical property, poor processability, and the dearth of scalable assembly protocols, such functional hydrogel fibers are still far from practical usage. Herein, we demonstrate an approach toward the continuous fabrication of an electro-responsive hydrogel fiber by using the self-lubricated spinning (SLS) strategy. The polyelectrolyte inside the hydrogel fiber endows it with a fast electro-response property. After solvent exchange with triethylene glycol (TEG), the maximum tensile strength of the hydrogel fiber increases from 114 kPa to 5.6 MPa, far superior to those hydrogel fiber-based actuators reported previously. Consequently, the flexible and mechanical stable hydrogel fiber is knitted into various complex geometries on demand such as a crochet flower, triple knot, thread tube, pentagram, and hollow cage. Additionally, the electrochemical-responsive ionic hydrogel fiber is capable of acting as soft robots underwater to mimic biological motions, such as Mobula-like flapping, jellyfish-mimicking grabbing, sea worm-mimicking multi-degree of freedom movements, and human finger-like smart gesturing. This work not only demonstrates an example for the large-scale production of previous infeasible hydrogel fibers, but also provides a solution for the rational design and fabrication of hydrogel woven intelligent devices.

Reduced Graphene Oxide-Based Spectrally Selective Absorber with an Extremely Low Thermal Emittance and High Solar Absorptance.

Carbon-based black materials exhibit strong solar absorptance (αsolar >0.90), which play key roles in transforming solar energy into available power for solar-thermal, thermophotovoltaic, thermoelectric, and many other systems. However, because of high thermal emittance (>95%), these carbon-based materials always cause huge energy loss that hinders the solar-thermal conversion efficiency tremendously. In this study, a reduced graphene oxide-based spectrally selective absorber (rGO-SSA) is demonstrated, which possesses a recorded low thermal emittance (≈4%) and high solar absorptance (αsolar ≈ 0.92) by easily regulating the reduction level of inner 2D graphene sheets. Compared to conventional carbon-based black materials, thermal emittance of this rGO-SSA is largely reduced by ≈95.8% and the cutoff wavelength of rGO-SSA is broadband-tunable that can range from 1.1 to 3.2 µm. More importantly, this simply sol-gel coated rGO-SSA has high temperature tolerance at 800 °C for 96 h that is hardly achieved by other cermet-based or photonic-based SSAs. Based on this rGO-SSA, ultrafast solar steam escape (0.94 mg cm-2 s-1) under concentrated solar irradiance is achieved directly. The insight from this study will provide a new strategy for constructing thermally stable carbon-based SSAs and greatly facilitate the solar-thermal practical significance.

Thermal Efficiency of Solar Steam Generation Approaching 100 % through Capillary Water Transport.

Solar-driven interfacial water evaporation yield is severely limited by the low efficiency of solar thermal energy. Herein, the injection control technique (ICT) achieves a capillary water state in rGO foam and effectively adjusts the water motion mode therein. Forming an appropriate amount of capillary water in the 3D graphene foam can greatly increase the vapor escape channel, by ensuring that the micrometer-sized pore channels do not become completely blocked by water and by exposing as much evaporation area as possible while preventing solar heat from being used to heat excess water. The rate of solar steam generation can reach up to 2.40 kg m-2 h-1 under solar illumination of 1 kW m-2 , among the best values reported. In addition, solar thermal efficiency approaching 100 % is achieved. This work enhances solar water-evaporation performance and promotes the application of solar-driven evaporation systems made of carbon-based materials.

A Microstructured Graphene/Poly(N-isopropylacrylamide) Membrane for Intelligent Solar Water Evaporation.

Intelligent solar water evaporation (iSWE) was achieved with a thermally responsive and microstructured graphene/poly(N-isopropylacrylamide) (mG/PNIPAm) membrane. As the solar intensity varies, the water evaporation is tuned through reversible transformations of microstructures reminiscent of the stomatal opening and closing of leaves. Consequently, this mG/PNIPAm membrane displays a high water evaporation rate change (ΔWER) of 1.66 kg m-2 h-1 under weak sunlight (intensity<1 sun) and a low ΔWER of 0.24 kg m-2 h-1 under intense sunlight (1 sun

Interface-mediated hygroelectric generator with an output voltage approaching 1.5 volts.

Hygroelectricity is proposed as a means to produce electric power from air by absorbing gaseous or vaporous water molecules, which are ubiquitous in the atmosphere. Here, using a synergy between a hygroscopic bulk graphene oxide with a heterogeneous structure and interface mediation between electrodes/materials with Schottky junctions, we develop a high-performance hygroelectric generator unit with an output voltage approaching 1.5 V. High voltage (e.g., 18 V with 15 units) can be easily reached by simply scaling up the number of hygroelectric generator units in series, enough to drive commercial electronic devices. This work provides insight for the design and development of hygroelectric generators that may promote the efficient conversion of potential energy in the environmental atmosphere to electricity for practical applications.