Nylon Fabric/GO Based Self-Powered Humidity Sensor Based on the Galvanic Cell Principle with High Air Permeability and Rapid-Response.

Flexible humidity sensors have received more and more attention in people's lives, and the problems of gas permeability and power supply issues of the device have long been areas in need of improvement. In this work, inspired by the high air permeability of daily wear clothing and galvanic batteries, a self-powered humidity sensor with high air permeability and fast response is designed. A nylon fabric/GO net (as a humidity sensitive layer and solid electrolyte) is obtained by spraying technique. This structure enables the sensor to have fast response/recovery (0.78 s/0.93 s, calculated at 90% of the final value), ultra-high response (0.83 V) and excellent stability (over 150 cycles) at 35 °C. Such sensors are useful for health monitoring, such as non-contact monitoring of human respiratory rate before and after exercise, and monitoring a level of humidity in the palms, arms, and fingers. This research provides an idea for developing a flexible wearable humidity sensor that is both breathable and self-powered and can also be mass-produced similar to wearable clothing.

Pressure-Regulated Nanoconfined Channels for Highly Effective Mechanical-Electrical Conversion in Proton Battery-Type Self-Powered Pressure Sensor.

Battery-sensing-based all-in-one pressure sensors are generally successfully constructed by mimicking the information transfer of living organisms and the sensing behavior of human skin, possessing features such as low energy consumption and detection of low/high-frequency mechanical signals. To design high-performance all-in-one pressure sensors, a deeper understanding of the intrinsic mechanisms of such sensors is required. Here, a mechanical-electrical conversion mechanism based on pressure-modulated nanoconfined channels is proposed. Then, the mechanism of ion accelerated transport in graphene oxide (GO) nanoconfined channels under pressure is revealed by density functional theory (DFT) calculation. Based on this mechanism, a proton battery-type self-powered pressure sensor MoO3 /GO[CNF/Ca] /activated carbon (AC) is designed with an open-circuit voltage stabilization of 0.648 V, an ultrafast response/recovery time of 86.0 ms/93.0 ms, pressure detection ranges of up to 60.0 kPa, and excellent static/dynamic pressure response. In addition, the one-piece device design enables self-supply, miniaturization, and charge/discharge reuse, showing application potential in wearable electronics, health monitoring, and other fields.

3D Porous Structure in MXene/PANI Foam for a High-Performance Flexible Pressure Sensor.

The fields of electronic skin, man-machine interaction, and health monitoring require flexible pressure sensors with great sensitivity. However, most microstructure designs utilized to fabricate high-performance pressure sensors require complex preparation processes. Here, MXene/polyaniline (PANI) foam with 3D porous structure is achieved by using a steam-induced foaming method. Based on the structure, a flexible piezoresistive sensor is fabricated. It exhibits high sensitivity (690.91 kPa-1 ), rapid response, and recovery times (106/95 ms) and outstanding fatigue resistance properties (10 000 cycles). The MXene/PANI foam-based pressure sensor can swiftly detect minor pressure and be further used for human activity and health monitoring.

A Zinc-Ion Battery-Type Self-Powered Pressure Sensor with Long Service Life.

Accurate and continuous pressure signal detection without external power supply is a key technology to realize the miniaturization of wearable electronic equipment, the internet of things, and artificial intelligence. However, it is difficult to be achieved by using current sensor technologies. Here, a new one-body strategy, i.e., zinc-ion battery pressure (ZIB-P) sensor technology, which designs the rechargeable solid-state ZIB itself as a flexible pressure sensor is reported. In the device, an isolation layer is introduced into the sandwich configuration solid-state battery to realize the change of device internal resistance by pressure during the transformation of the mechanical signal to the electrical signal. This battery pressure sensor possesses good flexibility, fast response/recovery time (76.0/88.0 ms), stable long-term response, excellent cycle stability (100 000 times), and wide pressure detection range (2.0 to 3.68 × 105  Pa). Especially, the excellent charge-discharge performance in the ZIB-P sensor endows it with the real-time detection ability of human vital signs (pulse, limb movement, etc.) and ultrahigh stability without degradation even under 100 000 times pressure stimulation. The ZIB-P sensor strategy provides a new solution for the future development of miniaturized wearable electronic devices.

Flexible MXene/Bacterial Cellulose Film Sound Detector Based on Piezoresistive Sensing Mechanism.

Flexible pressure sensors have aroused extensive attention in health monitoring, human-computer interaction, soft robotics, and more, as a staple member of wearable electronics. However, a majority of traditional research focuses solely on foundational mechanical sensing tests and ordinary human-motion monitoring, ignoring its other applications in daily life. In this work, a paper-based pressure sensor is prepared by using MXene/bacterial cellulose film with three-dimensional isolation layer structure, and its sensing capability as a wearable sound detector has also been studied. The as-prepared device exhibits great comprehensive mechanical sensing performance as well as accurate detection of human physiological signals. As a sound detector, not only can it recognize different voice signals and sound attributes by monitoring movement of throat muscles, but also it will distinguish a variety of natural sounds through air pressure waves caused by sound transmission (also called sound waves), like the eardrum. Besides, it plays an important role in sound visualization technology because of the ability for capturing and presenting music signals. Moreover, millimeter-scale thickness, lightweight, and degradable raw materials make the sensor convenient and easy to carry, meeting requirements of environmental protection as well.

Studying Plasmon Dispersion of MXene for Enhanced Electromagnetic Absorption.

2D metal carbides and nitrides (MXene) are promising candidates for electromagnetic (EM) shielding, saturable absorption, thermal therapy, and photocatalysis owing to their excellent EM absorption. The plasmon resonances in metallic MXene micro/nanostructures may play an important role in enhancing the EM absorption; however, their contribution has not been determined due to the lack of a precise understanding of its plasmon behavior. Here, the use of high-spatial-resolution electron energy-loss spectroscopy to measure the plasmon dispersion of MXene films with different thicknesses is reported, enabling accurate analysis of the EM absorption of complex MXene structures in a wide frequency range via a theoretical model. The EM absorption of MXene can be excited at the desired frequency by controlling the momentum (e.g., the sizes of the nanoflakes for EM excitation) as the strength can be enhanced by increasing the layer number and the interlayer distance in MXene. For example, a 3 nm interlayer distance can nearly double the plasmon-enhanced EM absorption in MXene nanostructures. These findings can guide the design of advanced ultrathin EM absorption materials for a broad range of applications.

High-Performance Flexible Pressure Sensor with a Self-Healing Function for Tactile Feedback.

High-performance flexible pressure sensors have attracted a great deal of attention, owing to its potential applications such as human activity monitoring, man-machine interaction, and robotics. However, most high-performance flexible pressure sensors are complex and costly to manufacture. These sensors cannot be repaired after external mechanical damage and lack of tactile feedback applications. Herein, a high-performance flexible pressure sensor based on MXene/polyurethane (PU)/interdigital electrodes is fabricated by using a low-cost and universal spray method. The sprayed MXene on the spinosum structure PU and other arbitrary flexible substrates (represented by polyimide and membrane filter) act as the sensitive layer and the interdigital electrodes, respectively. The sensor shows an ultrahigh sensitivity (up to 509.8 kPa-1 ), extremely fast response speed (67.3 ms), recovery speed (44.8 ms), and good stability (10 000 cycles) due to the interaction between the sensitive layer and the interdigital electrodes. In addition, the hydrogen bond of PU endows the device with the self-healing function. The sensor can also be integrated with a circuit, which can realize tactile feedback function. This MXene-based high-performance pressure sensor, along with its designing/fabrication, is expected to be widely used in human activity detection, electronic skin, intelligent robots, and many other aspects.

Roles of MXene in Pressure Sensing: Preparation, Composite Structure Design, and Mechanism.

Flexible pressure sensors are one of the most important components in the fields of electronic skin (e-skin), robotics, and health monitoring. However, the application of pressure sensors in practice is still difficult and expensive due to the limited sensing properties and complex manufacturing process. The emergence of MXene, a red-hot member of the 2D nanomaterials, has brought a brand-new breakthrough for pressure sensing. Ti3 C2 Tx is the most popular studied MXene in the field of pressure sensing and shows good mechanical, electrical properties, excellent hydrophilicity, and extensive modifiability. It will ameliorate the properties of the sensitive layer and electrode layer of the pressure sensor, and further apply pressure sensing to many fields, such as e-skin flexibility. Herein, the preparation technologies, antioxidant methods, and properties of MXene are summarized. The design of MXene-based microstructures is introduced, including hydrogels, aerogels, foam, fabrics, and composite nanofibers. The mechanisms of MXene pressure sensors are further broached, including piezoresistive, capacitive, piezoelectric, triboelectric, and potentiometric transduction mechanism. Moreover, the integration of multiple devices is reviewed. Finally, the chance and challenge of pressure sensors improved by MXene smart materials in future e-skin and the Internet of Things are prospected.

Ti3C2Tx MXene-Based Flexible Piezoresistive Physical Sensors.

MXenes have received increasing attention due to their two-dimensional layered structure, high conductivity, hydrophilicity, and large specific surface area. Because of these distinctive advantages, MXenes are considered as very competitive pressure-sensitive materials in applications of flexible piezoresistive sensors. This work reviews the preparation methods, basic properties, and assembly methods of MXenes and their recent developments in piezoresistive sensor applications. The recent developments of MXene-based flexible piezoresistive sensors can be categorized into one-dimensional fibrous, two-dimensional planar, and three-dimensional sensors according to their various structures. The trends of multifunctional integration of MXene-based pressure sensors are also summarized. Finally, we end this review by describing the opportunities and challenges for MXene-based pressure sensors and the great prospects of MXenes in the field of pressure sensor applications.

Tensible and flexible high-sensitive spandex fiber strain sensor enhanced by carbon nanotubes/Ag nanoparticles.

Although the wearable strain sensors have received extensive research interest in recent years, it remains a huge challenge conforming the requirements in both of ultrahigh stretchability and high strain coefficient (gauge factor). Herein, a stretchable and flexible spandex fiber strain sensor coupled with carbon nanotubes (CNTs)/Ag nanoparticles (Ag NPs) that assembled through an efficient and large-scale layer-by layer self-assembly is presented. To ensure CNTs and Ag NPs can attach well to the spandex fiber without falling off, achieving high sensitivity under large tensile, sodium dodecyl benzene sulfonate, polyvinyl alcohol, and polystyrene sulfonic acid are introduced to improve the adhesion via the molecular entanglement and other interactions between them. Consequently, the strain sensor exhibits remarkable performance, such as an ultrahigh gauge factor of 58.5 in the low-strain range from 0% to 20%, a wide strain range (0%-200%), a fast response time of 42 ms and good working stability (>5000 stretching-releasing cycles). Subsequently, detailed mechanism of the sensor and its use in full range of human motion monitoring are further studied. It is worth noting that with the distinctive mechanism and structure, the special spandex fiber sensor is able to monitor minimum strain as low as 0.053%, showing tremendous prospect for the field of smart fabrics and wearable health care devices.

Interior and Exterior Decoration of Transition Metal Oxide Through Cu0/Cu+ Co-Doping Strategy for High-Performance Supercapacitor.

Although CoO is a promising electrode material for supercapacitors due to its high theoretical capacitance, the practical applications still suffering from inferior electrochemical activity owing to its low electrical conductivity, poor structural stability and inefficient nanostructure. Herein, we report a novel Cu0/Cu+ co-doped CoO composite with adjustable metallic Cu0 and ion Cu+ via a facile strategy. Through interior (Cu+) and exterior (Cu0) decoration of CoO, the electrochemical performance of CoO electrode has been significantly improved due to both the beneficial flower-like nanostructure and the synergetic effect of Cu0/Cu+ co-doping, which results in a significantly enhanced specific capacitance (695 F g-1 at 1 A g-1) and high cyclic stability (93.4% retention over 10,000 cycles) than pristine CoO. Furthermore, this co-doping strategy is also applicable to other transition metal oxide (NiO) with enhanced electrochemical performance. In addition, an asymmetric hybrid supercapacitor was assembled using the Cu0/Cu+ co-doped CoO electrode and active carbon, which delivers a remarkable maximal energy density (35 Wh kg-1), exceptional power density (16 kW kg-1) and ultralong cycle life (91.5% retention over 10,000 cycles). Theoretical calculations further verify that the co-doping of Cu0/Cu+ can tune the electronic structure of CoO and improve the conductivity and electron transport. This study demonstrates a facile and favorable strategy to enhance the electrochemical performance of transition metal oxide electrode materials.

Revealing the Phase-Transition Dynamics and Mechanism in a Spinel Li4Ti5O12 Anode Material through in Situ Electron Microscopy.

Spinel Li4Ti5O12 is considered as a promising anode material for long-life lithium-ion batteries because of the negligible volumetric variation during the insertion and extraction of Li ions. Phase transition is an inevitable process during the migration of Li ions, and the transition process and mechanism need detailed investigation down to the atomic scale. In this study, we investigated the behavior and mechanism on the phase transition of Li4Ti5O12 through in situ transmission electron microscopy (TEM). It has been found that the spinel-structured Li4Ti5O12 was gradually transformed to a rock salt structure under electron beam irradiation. A sharp interface with an epitaxial relationship was observed between the transformed rock salt phase and the parent spinel phase. Furthermore, the heterostructure with different crystal structures of Li4Ti5O12 has been precisely tailored with electron beam irradiation. Our detailed in situ TEM results and theoretical calculations led to unprecedented level on the understanding of phase-transition mechanism in Li4Ti5O12. This study demonstrates a possible approach to precisely engineer the crystal structure of materials and to realize a well-designed heterostructure in electrode materials.

Graphene Aerogel Broken to Fragments for a Piezoresistive Pressure Sensor with a Higher Sensitivity.

The porous and elastic reduced graphene aerogel (rGA) is a promising active material for piezoresistive pressure sensors (PRSs) to realize an electronic skin. Due to the specific working mechanism and the limitation of the rGA's monolithic conductive network, the PRSs based on rGA suffer from a limited change of resistance with mechanical deformation, so they show poor sensitivity and cannot detect low pressures. Here we aim to improve the sensitivity of the PRS and make it suitable for a low-pressure system (0.5-8 kPa) through an effective method. The monolithic rGA is broken into small pieces by cutting (named as CGA). The sensitivity of the PRS based on CGA can be improved by 10 times that of the uncut rGA (named as UCGA). The resistance variation ratio of CGA (0.96) is 1.45 times of the resistance variation ratio of the UCGA (0.66). By using a package of elastic polypropylene thin films (PP), the cycle stability performance of CGA remains stable after 4200 cycles. The CGA can detect the pulse of a human being with sensitivity higher than the UCGA and the ordinary sensors. This method is simple, effective, and universal to improve the sensitivity of PRS based on porous and elastic materials.

In Situ TEM Observation of Crystal Structure Transformation in InAs Nanowires on Atomic Scale.

In situ transmission electron microscopy investigation of structural transformation in III-V nanowires is essential for providing direct insight into the structural stability of III-V nanowires under elevated temperature. In this study, through in situ heating investigation in a transmission electron microscope, the detailed structural transformation of InAs nanowires from wurtzite structure to zinc-blende structure at the catalyst/nanowire interface is witnessed on the atomic level. Through detailed structural and dynamic analysis, it was found that the nucleation site of each new layer of InAs and catalyst surface energy play a decisive role in the growth of the zinc-blende structure. This study provides new insights into the growth mechanism of zinc-blende-structured III-V nanowires.

3D Synergistical MXene/Reduced Graphene Oxide Aerogel for a Piezoresistive Sensor.

A piezoresistive sensor based on ultralight and superelastic aerogel is reported to fabricate MXene/reduced graphene oxide (MX/rGO) hybrid 3D structures and utilize their pressure-sensitive characteristics. The MX/rGO aerogel not only combines the rGO's large specific surface area and the MXene's (Ti3C2 T x) high conductivity but also exhibits rich porous structure, which leads to performance better than that of single-component rGO or MXene in terms of the pressure sensor. The large nanosheets of rGO can prevent the poor oxidization of MXene by wrapping MXene inside the aerogel. More importantly, the piezoresistive sensor based on the MX/rGO aerogel shows extremely high sensitivity (22.56 kPa-1), fast response time (<200 ms), and good stability over 10 000 cycles. The piezoresistive sensor based on the MX/rGO hybrid 3D aerogel can easily capture the signal below 10 Pa, thus clearly testing the pulse of an adult at random. Based on its superior performance, it also demonstrates potential applications in measuring pressure distribution, distinguishing subtle strain, and monitoring healthy activity.

Highly Self-Healable 3D Microsupercapacitor with MXene-Graphene Composite Aerogel.

High-performance microsupercapacitors (MSCs) with three-dimensional (3D) structure provide an effective approach to improve the ability of energy storage. Because the electrodes with 3D structure are generally easily destroyed under mechanical deformation in practical applications, we fabricated a self-healable 3D MSC consisting of MXene (Ti3C2T x)-graphene (reduced graphene oxide, rGO) composite aerogel electrode by wrapping it with a self-healing polyurethane as an outer shell. The MXene-rGO composite aerogel combining large specific surface area of rGO and high conductivity of the MXene can not only prevent the self-restacking of the lamella structure but also resist the poor oxidization of MXene to a degree. The MSC based on a 3D MXene-rGO aerogel delivers a large area specific capacitance of 34.6 mF cm-2 at a scan rate of 1 mV s-1 and an outstanding cycling performance with a capacitance retention up to 91% over 15 000 cycles. The 3D MSC presents an excellent self-healing ability with specific capacitance retention of 81.7% after the fifth healing. The preparation of this self-healable 3D MSC can provide a method for designing and manufacturing next-generation long-life multifunctional electronic devices further to meet the requirements of sustainable development.

Piezoresistive Pressure Sensor Based on Synergistical Innerconnect Polyvinyl Alcohol Nanowires/Wrinkled Graphene Film.

Piezoresistive sensor is a promising pressure sensor due to its attractive advantages including uncomplicated signal collection, simple manufacture, economical and practical characteristics. Here, a flexible and highly sensitive pressure sensor based on wrinkled graphene film (WGF)/innerconnected polyvinyl alcohol (PVA) nanowires/interdigital electrodes is fabricated. Due to the synergistic effect between WGF and innerconnected PVA nanowires, the as-prepared pressure sensor realizes a high sensitivity of 28.34 kPa-1 . In addition, the device is able to discern lightweight rice about 22.4 mg (≈2.24 Pa) and shows excellent durability and reliability after 6000 repeated loading and unloading cycles. What is more, the device can detect subtle pulse beat and monitor various human movement behaviors in real-time.

A highly flexible and sensitive piezoresistive sensor based on MXene with greatly changed interlayer distances.

Since the successful synthesis of the first MXenes, application developments of this new family of two-dimensional materials on energy storage, electromagnetic interference shielding, transparent conductive electrodes and field-effect transistors, and other applications have been widely reported. However, no one has found or used the basic characteristics of greatly changed interlayer distances of MXene under an external pressure for a real application. Here we report a highly flexible and sensitive piezoresistive sensor based on this essential characteristics. An in situ transmission electron microscopy study directly illustrates the characteristics of greatly changed interlayer distances under an external pressure, supplying the basic working mechanism for the piezoresistive sensor. The resultant device also shows high sensitivity (Gauge Factor ~ 180.1), fast response (<30 ms) and extraordinarily reversible compressibility. The MXene-based piezoresistive sensor can detect human being's subtle bending-release activities and other weak pressure.

Recent Progress in Micro-Supercapacitors with In-Plane Interdigital Electrode Architecture.

Due to the boom of miniaturized electronic devices in the last decade, there are great demands for ultrathin and flexible on-chip rechargeable energy storage microdevices. Supercapacitor, as one of the most hopeful appearing energy storage devices, can provide a wonderful alternative to batteries or electrolytic capacitors, owing to its fast charge and discharge rates, high power density, and long cycling stability. Especially for the recently developed micro-supercapacitors, the unique in-plane interdigital electrode architecture can fully meet the integration requirements of rapidly developed miniaturized electronic devices, and improve the power density of the unit via shortening the ionic diffusion distance between the interdigital electrodes. This concept introduces the recent advances on the design, fabrication, and application of planar micro-supercapacitors for on-chip energy storage from an overall perspective. Moreover, challenges and future development trends are discussed.

Ag nanoparticles modified large area monolayer MoS2 phototransistors with high responsivity.

Monolayer MoS2 is considered to be one of the best candidates for next generation electronics because of its ultra-thin body and direct band gap. However, MoS2 based transistors have relatively low photoresponsivity, field effect mobility and narrow response spectrum range, which hinder the application of MoS2 in optoelectronic devices. Here, based on the enhancement of localized surface plasmon resonance (LSPR), a simple method of depositing Ag nanoparticles on the MoS2 surface is used. By adjusting the size of Ag nanoparticles, the response spectral range of phototransistor is broadened from red to near ultra-violet. The photoresponsivity gains an increase of 470% up to 2.97 × 104 A W-1 at 610 nm, and the response time also shows a decrease to some extent. The enhanced responsivity is comparable to those of devices encapsulated with high-quality dielectrics, and superior over other reported monolayer MoS2 in ambient conditions. The high responsivity and working current enables a wide range of device applications. This work provides a viable route towards performance enhancement of two-dimensional phototransistors.