Efficient Fabrication of Carbon Nanotube-Based Stretchable Electrodes for Flexible Electronic Devices.

Stretchable electrodes are highly demanded in various wearable and flexible electronic devices, whereas the efficient fabrication approach is still a challenge. In this work, an efficient shrinking method to fabricate carbon nanotube (CNT)-based stretchable electrodes is proposed. The electrode is a layer of anisotropic CNT wrinkling film coated on a latex balloon substrate (CNT@latex), whose resistivity remains stable after 25 000 stretching cycles of 0 to 50% tensile strain, and can survive up to 500% tensile train. The highly conductive electrode can be used as the current collector of a stretchable Zinc-ion battery, maintaining an output voltage of 1.3 V during the stretching process of 0 to 100%. The applications of the electrode in flexible triboelectric nanogenerators and Joule heating devices are also demonstrated, further indicating their good prospects in the field of stretchable electronic devices.

Hierarchical Wrinkles for Tunable Strain Sensing Based on Programmable, Anisotropic, and Patterned Graphene Hybrids.

Flexible, stretchable, wearable, and stable electronic materials are widely studied, owing to their applications in wearable devices and the Internet of Things. Because of the demands for both strain-insensitive resistors and high gauge factor (GF) strain-sensitive materials, anisotropic strain sensitivity has been an important aspect of electronic materials. In addition, the materials should have adjustable strain sensitivities. In this work, such properties are demonstrated in reduced graphene oxide (RGO) with hierarchical oriented wrinkle microstructures, generated using the two-step shrinkage of a rubber substrate. The GF values range from 0.15 to 28.32 at 100% strain. For device demonstrations, macrostructure patterns are designed to prepare patterned wrinkling graphene at rubber substrate (PWG@R). Serpentiform curves can be used for the constant-value resistor, combined with the first-grade wrinkles. Strip lines can increase the strain-sensing property, along with the second-grade wrinkles. The patterned sensor exhibits improved GF values range from 0.05 to 49.5. The assembled sensor shows an excellent stability (>99% retention after 600 cycles) with a high GF (49.5). It can monitor the vital signs of the throat and wrist and sense large motions of fingers. Thus, PWG@R-based strain sensors have great potential in various health or motion monitoring fields.

Recent Progress of Toxic Gas Sensors Based on 3D Graphene Frameworks.

Air pollution is becoming an increasingly important global issue. Toxic gases such as ammonia, nitrogen dioxide, and volatile organic compounds (VOCs) like phenol are very common air pollutants. To date, various sensing methods have been proposed to detect these toxic gases. Researchers are trying their best to build sensors with the lowest detection limit, the highest sensitivity, and the best selectivity. As a 2D material, graphene is very sensitive to many gases and so can be used for gas sensors. Recent studies have shown that graphene with a 3D structure can increase the gas sensitivity of the sensors. The limit of detection (LOD) of the sensors can be upgraded from ppm level to several ppb level. In this review, the recent progress of the gas sensors based on 3D graphene frameworks in the detection of harmful gases is summarized and discussed.

Bioinspired Multiscale Wrinkling Patterns on Curved Substrates: An Overview.

The surface wrinkling of biological tissues is ubiquitous in nature. Accumulating evidence suggests that the mechanical force plays a significant role in shaping the biological morphologies. Controlled wrinkling has been demonstrated to be able to spontaneously form rich multiscale patterns, on either planar or curved surfaces. The surface wrinkling on planar substrates has been investigated thoroughly during the past decades. However, most wrinkling morphologies in nature are based on the curved biological surfaces and the research of controllable patterning on curved substrates still remains weak. The study of wrinkling on curved substrates is critical for understanding the biological growth, developing three-dimensional (3D) or four-dimensional (4D) fabrication techniques, and creating novel topographic patterns. In this review, fundamental wrinkling mechanics and recent advances in both fabrications and applications of the wrinkling patterns on curved substrates are summarized. The mechanics behind the wrinkles is compared between the planar and the curved cases. Beyond the film thickness, modulus ratio, and mismatch strain, the substrate curvature is one more significant parameter controlling the surface wrinkling. Curved substrates can be both solid and hollow with various 3D geometries across multiple length scales. Up to date, the wrinkling morphologies on solid/hollow core-shell spheres and cylinders have been simulated and selectively produced. Emerging applications of the curved topographic patterns have been found in smart wetting surfaces, cell culture interfaces, healthcare materials, and actuators, which may accelerate the development of artificial organs, stimuli-responsive devices, and micro/nano fabrications with higher dimensions.

Facile synthesis of a BCN nanofiber and its ultrafast adsorption performance.

Boron carbonitride (BCN) nanofibers with rapid and efficient adsorption performance were prepared by electrospinning technology. TEM, XRD, XPS and N2 adsorption-desorption isotherms were performed to study the microstructure of the nanofibers. The results showed that the BCN fibers synthesized at 1000 °C (BCN-1000) have good crystallinity and high specific surface areas (403 m2 g-1). BCN-1000 nanofibers adsorb 70% of amino black 10B (AB-10B) within 10 minutes and reach adsorption equilibrium within 60 minutes. Compared with previous reports, it is found that the adsorption rate of BCN-1000 nanofibers to amino black (AB-10B) is much higher than that of other adsorbents. And BCN nanofibers exhibit a large adsorption capacity (625 mg g-1). In addition, the process of AB-10B adsorption on BCN nanofibers was systematically investigated, which was in accordance with the pseudo-second-order kinetics model and Langmuir isotherm model.

Magnetic MoS2: a promising microwave absorption material with both dielectric loss and magnetic loss properties.

In order to obtain magnetic MoS2 and investigate the influence of magnetic moment on the microwave absorption properties of MoS2, transition metal element Ni-doped MoS2 (0-30 at%) was obtained by a hydrothermal synthesis. The results revealed that the low doping concentration (<10 at%) did not significantly change the crystal structure of MoS2, and the Ni element formed a Ni x Mo1-x S2 compound within the MoS2 bulk phase. While the high doping concentration (10-30 at%) led to the formation of impurities. The hydrothermal products which were formed by the accumulation of pleated nanosheets looked like spherical flowers. As the doping concentration further increased, the spherical particles became more compact. The magnetization of MoS2 could be increased by proper amount of Ni doping. When doping with 3 at% Ni (Ni-3), the M s value increased from 0.53 emu g-1 for non-doped MoS2 to 0.93 emu g-1. When the doping ratio was further increased, the M s value of the material decreased. The zigzag edges and variations in the number of vacancies in the materials may be the root of changes in magnetic properties. The overall performance of Ni-3 was also the best in the examined doping range. Compared with non-doped MoS2, the matching thickness decreased from 3.50 to 2.05 mm, while the minimum reflection loss value decreased from -55.18 to -58.08 dB, and the effective absorption bandwidth (<-10 dB) increased from 3.05 to 5.19 GHz. The excellent absorption performance of the doped materials can be attributed to the change of complex dielectric constant and complex permeability of MoS2 and resulting in the improvement of loss capability. This study may introduce new opportunities for fully exploiting these nanocrystals for microwave absorption, even for diluted magnetic semiconductors.

Electrospun PAN/MAPbI3 Composite Fibers for Flexible and Broadband Photodetectors.

Methylammonium lead triiodide perovskite (CH3NH3PbI3, MAPbI3) has been emerging as an easy processing and benign defect material for optoelectronic devices. Fiber-like perovskite materials are especially in demand for flexible applications. Here we report on a kind of polyacrylonitrile (PAN)/MAPbI3 composite fiber, which was electrospun from the mixing solution of PAN and MAPbI3. The absorption edge and optical gap of the PAN/MAPbI3 composite fibers can be easily tuned as the ratio of the perovskite changes. Both the moisture stability and the thermal stability of the perovskite are improved with the protection of PAN polymers. Flexible photodetectors based on this perovskite fiber were fabricated and analyzed. The photoresponse of the detector was highly sensitive to broadband visible light, and reached 6.5 µA W-1 at 700 nm with a voltage bias of 10 V. Compared with pure MAPbI3 photodetectors, this composite fiber photodetector has much-improved stability and flexibility, which can even be used to detect motion-related angular changes.

Hierarchical Reduced Graphene Oxide Ridges for Stretchable, Wearable, and Washable Strain Sensors.

Recently, flexible and wearable devices are increasingly in demand and graphene has been widely used due to its exceptional chemical, mechanical and electrical properties. Building complex buckling patterns of graphene is an essential strategy to increase its flexible and stretchable properties. Herein, a facile dimensionally controlled four-dimensional (4D) shrinking method was proposed to generate hierarchical reduced graphene oxide (rGO) buckling patterns on curved substrates mimicking different parts of the uniforms. The reduced graphene oxide ridges (rGORs) generated on the spherical substrate seem isotropic, while those generated on the cylindrical substrate are obviously more hierarchical or oriented, especially when the cylindrical substrate are shrinking via two steps. The oriented rGORs are superhydrophobic and strain sensitive but obviously anisotropic along the axial and circumferential directions. The sensitivity of rGORs along the axial direction is much higher than those along the circumferential direction. In addition, the intrinsic solvent barrier property of graphene enables the crack-free rGORs an excellent chemical protective performance, withstanding DCM immersion for more than 2.5 h. The flexible rGORs-based strain sensors can be used to detect both large and subtle human motions and activities by achieving high sensitivity (maximum gauge factor up to 48), high unidirectional stretchability (300-530%), and ultrahigh areal stretchability (up to 2690%). Excellent durability was also demonstrated for human motion monitoring with resistance to hand rubbing, ultrasonic cleaning, machine washing, and chemical immersion.

Synthesis of hierarchical hollow sodium titanate microspheres and their application for selective removal of organic dyes.

Titanate-based materials are attractive inorganic adsorbents for wastewater treatment. In this study, hierarchical hollow sodium titanate microspheres (HHSTMs) were successfully synthesized via a template-assisted method. Silica microspheres were selected as hard templates, with a uniformly smooth TiO2 shell first grown onto the surface of the SiO2 cores. Then, through an alkaline hydrothermal process, the silica core was removed and the TiO2 shell gradually converted into a sodium titanate shell with a preserved morphology. The as-synthesized HHSTMs are constructed from twined nanobelts, with a high surface area of 308 m2 g-1. A typical organic dye, methylene blue, was employed to investigate the adsorption properties of the HHSTMs. The adsorption process matched well with the Langmuir isothermal model, with the maximum adsorption capacity of methylene blue reaching 443 mg g-1. Moreover, the resulting HHSTMs can be used to selectively capture of methylene blue from a cationic-anionic dye binary system due to their negatively charged surface. All adsorption processes were very fast and could complete in ten minutes.

Gyrification-Inspired Highly Convoluted Graphene Oxide Patterns for Ultralarge Deforming Actuators.

Gyrification in the human brain is driven by the compressive stress induced by the tangential expansion of the cortical layer, while similar topographies can also be induced by the tangential shrinkage of the spherical substrate. Herein we introduce a simple three-dimensional (3D) shrinking method to generate the cortex-like patterns using two-dimensional (2D) graphene oxide (GO) as the building blocks. By rotation-dip-coating a GO film on an air-charged latex balloon and then releasing the air slowly, a highly folded hydrophobic GO surface can be induced. Wrinkling-to-folding transition was observed and the folding state can be easily regulated by varying the prestrain of the substrate and the thickness of the GO film. Driven by the residue stresses stored in the system, sheet-to-tube actuating occurs rapidly once the bilayer system is cut into slices. In response to some organic solvents, however, the square bilayer actuator exhibits excellent reversible, bidirectional, large-deformational curling properties on wetting and drying. An ultralarge curvature of 2.75 mm-1 was observed within 18 s from the original negative bending to the final positive bending in response to tetrahydrofuran (THF). In addition to a mechanical hand, a swimming worm, a smart package, a bionic mimosa, and two bionic flowers, a crude oil collector has been designed and demonstrated, aided by the superhydrophobic and superoleophilic modified GO surface and the solvent-responsive bilayer system.

Hybrids of Reduced Graphene Oxide and Hexagonal Boron Nitride: Lightweight Absorbers with Tunable and Highly Efficient Microwave Attenuation Properties.

Sandwichlike hybrids of reduced graphene oxide (rGO) and hexagonal boron nitride (h-BN) were prepared via heat treatment of the self-assemblies of graphene oxide (GO) and ammonia borane (AB). TG-DSC-QMS analysis indicate a mutually promoted redox reaction between GO and AB; 900 °C is a proper temperature to transfer the hybrids into inorganic sandwiches. XRD, XPS, and Raman spectra reveal the existence of h-BN embedded into the rGO frameworks. High-resolution SEM and TEM indicate the layer-by-layer structure of the hybrids. The content of h-BN can be increased with increase of the mass ratio of AB and the highest heat treatment temperature. The complex permittivity and the microwave absorption are tunable with the variation of the content of h-BN. When the mass ratio of GO/AB is 1:1, the microwave absorption of the hybrid treated at 900 °C is preferable in the range of 6-18 GHz. A minimum reflection loss, -40.5 dB, was observed at 15.3 GHz for the wax composite filled with 25 wt % hybrids at the thickness of 1.6 mm. The qualified frequency bandwidth reaches 5 GHz at this thickness with a low surface density close to 1.68 kg/m2. The layer-by-layer structure of the hybrid makes great contributions to the increased approaches and possibilities of electron migrating and hopping, which has both highly efficient dielectric loss and excellent impedance matching for microwave consumption.

Room-temperature humidity-sensing performance of SiC nanopaper.

Centimeters-long SiC nanowire could be a strong "bridge" between microworld and macroworld due to its unique morphology, excellent chemical stability, and intriguing physical properties. Here we present a novel "paperlike" material-free-standing SiC nanopaper fabricated by acetone-assisted compression of centimeters-long SiC nanowires. The resistance of this new paperlike material linearly increases with increasing environmental relative humidity in a very short time. We suggest that adsorption of water molecules on SiC nanopaper surface led to fast electron transfer between SiC nanopaper and water layer, which indicates that SiC nanopaper could be applied to high-performance humidity sensor in harsh environment.