High-Performance Humidity Sensor Based on Urchin-Like Composite of Ti3C2 MXene-Derived TiO2 Nanowires

Ti3C2Tx MXene Functionalized via Boron Doped MoS2 Quantum Dots: A Synergy of Heterojunctions and Doping Effect Enabling Ultrasensitive SO2 Detection at Room Temperature

The design of mixed-dimensional heterostructures has emerged to be a new frontier of research as it induces exciting physical/chemical properties that extend beyond the fundamental properties of single dimensional systems. Therefore, rational design of heterostructured materials with novel surface chemistry and tailored interfacial properties appears to be very promising for the devices such as the gas sensors. Here, a highly sensitive gas sensor device is constructed by employing heterostructures of boron doped molybdenum disulfide quantum dots (B-MoS2 Qdots) assembled into the matrix of Ti3C2Tx MXene. Functionalization of MXene surface with B-MoS2 Qdots as a result of strong electrostatic attraction leads to improved charge migration behavior, active site exposure and abundant specific surface area. As a result, the Ti3C2Tx/B-MoS2 sensor device shows ultra-high response (28,998.3% @ 3 ppm), ultra-fast response rate (23.1% s-1), sub-ppm (10 ppb lowest) detection of sulfur dioxide (SO2) gas and excellent reversibility at room temperature. Density functional theory-based calculations indicate that enhanced SO2 sensing performance results from synergy of the 2D-0D heterostructure formation and preferential adsorption of SO2, induced by doped boron (B) heteroatoms in Qdots. Finally, a portable and wireless SO2 monitoring system is demonstrated for real-time detection of SO2 leakage and quantification under certain circumstances.

Fully Flexible Humidity Sensor with Fast Response and High Responsivity Based on rGO/MoS2 for Human Respiration Monitoring and Nontouch Switches

Humidity sensors have been widely used to monitor humidity in daily life, agriculture fields, and so on. However, conventional sensors are not suitable for wearable devices because of their large dimensions and rigid substrates. Hence, we report a fast response, highly sensitive, and fully flexible humidity sensor on a PI substrate based on the composite material of reduced graphene oxide (rGO)/MoS2, with a response time of 0.65 s and a sensitivity of 96.7% in the relative humidity range of 11% RH-95% RH. A fully flexible wearable device was realized by integrating the prepared flexible humidity sensor with a flexible printed circuit, which was successfully applied for human breathing monitoring, motion monitoring, and nontouch switches. These results show that the rGO/MoS2 composite is a good candidate for humidity sensing and has the potential to be used in the fields of wearable devices and nontouch switching.

Toward Integrated Multifunctional Laser-Induced Graphene-Based Skin-Like Flexible Sensor Systems

The burgeoning demands for health care and human-machine interfaces call for the next generation of multifunctional integrated sensor systems with facile fabrication processes and reliable performances. Laser-induced graphene (LIG) with highly tunable physical and chemical characteristics plays vital roles in developing versatile skin-like flexible or stretchable sensor systems. This Progress Report presents an in-depth overview of the latest advances in LIG-based techniques in the applications of flexible sensors. First, the merits of the LIG technique are highlighted especially as the building blocks for flexible sensors, followed by the description of various fabrication methods of LIG and its variants. Then, the focus is moved to diverse LIG-based flexible sensors, including physical sensors, chemical sensors, and electrophysiological sensors. Mechanisms and advantages of LIG in these scenarios are described in detail. Furthermore, various representative paradigms of integrated LIG-based sensor systems are presented to show the capabilities of LIG technique for multipurpose applications. The signal cross-talk issues are discussed with possible strategies. The LIG technology with versatile functionalities coupled with other fabrication strategies will enable high-performance integrated sensor systems for next-generation skin electronics.

Sodium alginate-based coaxial fibers synergistically integrate moisture actuation, length tracing, humidity sensing, and electric heating

The development of wearable electronics has driven the need for smart fibers with advanced multifunctional synergy. In this paper, we present a design of a multifunctional coaxial fiber that is composed of a biopolymer-derived core and an MXene/silver nanowire (AgNW) sheath by wet spinning. The fiber synergistically integrates moisture actuation, length tracing, humidity sensing, and electric heating, making it highly promising for portable devices and protective systems. The biopolymer-derived core provides deformation for moisture-sensitive actuation, while the MXene/AgNW sheath with good conductivity enables the fiber to perform electric heating, humidity sensing, and self-sensing actuation. The coaxial fiber can be programmed to rapidly desorb water molecules to shrink to its original length by using the MXene/AgNW sheath as an electrical heater. We demonstrate proof-of-concept applications based on the multifunctional fibers for thermal physiotherapy and wound healing/monitoring. The sodium alginate@MXene-based coaxial fiber presents a promising solution for the next-generation of smart wearable electronics.

Meshed, Flexible, and Self-Supported Humidity Sensors by Direct-Writing with Multifunctional Applications

Flexibility endows humidity sensors with new applications in human health monitoring except for traditionally known environmental humidity detection in recent years. In this study, a flexible, mesh-structured, and self-supported humidity sensor was designed and manufactured by direct writing in a homemade two-dimensional stepping numerical control workstation. Bacterial cellulose with humidity sensitivity and good film-forming properties was applied as the self-supporting substrate, in which conductive activated carbon and water-absorptive magnesium chloride (MgCl2) were incorporated. The humidity sensing performance of the printed sensor was measured and optimized. Besides, the fundamental insight into the sensing mechanism of the printed humidity sensor was analyzed by a complex impedance spectrum. The multifunctional applications of the self-supported humidity sensor were demonstrated by human breathing detection, noncontact distance sensing, and speaking recognition. The simple self-supported structure combined with the meshed attribute of the flexible sensor showed large use potential in real-time monitoring of human respiration, voice detection, environmental humidity monitoring, and noncontact switches.

3D hierarchical Ti3C2/TiO2 composite via in situ oxidation for improved lithium-ion storage

To address the intrinsic limitations of both TiO2 and MXenes, we propose an effective strategy for the engineering of a 3D Ti3C2/TiO2 nanorod hybrid, where the in situ synthesized TiO2 nanorods are homogeneously decorated onto the surface of 3D Ti3C2 MXene via simple oxidation. As the LIB anode, it demonstrates exceptional long-term cycling stability with a specific capacity of 384.1 mA h g-1 after 600 cycles at 1.0 A g-1.

Graphene oxide intercalated Alk-MXene adsorbents for efficient removal of Malachite green and Congo red from aqueous solutions

Currently, adsorbents with high adsorption performance for eliminating pollutants from discharged wastewater have received many researchers' attention. To this aim, a novel AMXGO absorbent was fabricated by intercalating graphene oxide (GO) into alkalized MXene (Alk-MXene) layer which exhibited high efficacy for the removal of cationic Malachite Green (MG) and anionic Congo Red (CR). Analysis of FTIR, XRD, SEM and TG presented that AMXGO absorbent have a typical three-dimensional layer by layer structure and abundant oxygen-containing groups and its thermal stability was remarkably improved. BET results elucidated that AMXGO1 adsorbent has larger specific surface area and pore volume (16.686 m2 g-1, 0.04733 cm3 g-1) as compared to Alk-MXene (4.729 m2 g-1, 0.02522 cm3 g-1). A dependence of adsorption performance on mass ratio between Alk-MXene and GO, initial dye concentration, contact time, temperature and pH was revealed. Maximum adsorption capacity of MG (1111.6 mg/g) and CR (1133.7 mg/g) were particularly found for AMXGO1 absorbent with a mass ratio of 3:1 and its removal for both dyes were higher than 92%. The adsorption process of AMXGO1 adsorbent for both MG and CR complies with pseudo-second-order kinetic model and Freundlich isotherm model. In addition, adsorption mechanism was explored that synergism effects as electrostatic attraction, π-π conjugates, intercalation adsorption and pore filling were the main driving force for the high adsorption performance of dye. Therefore, AMXGO adsorbent has a potential application prospect in the purification of dye wastewater.

Preparation and Mechanism Analysis of High-Performance Humidity Sensor Based on Eu-Doped TiO2

TiO2 is a typical semiconductor material, and it has attracted much attention in the field of humidity sensors. Doping is an efficient way to enhance the humidity response of TiO2. Eu-doped TiO2 material was investigated in both theoretical simulations and experiments. In a simulation based on density functional theory, a doped Eu atom can increase the performance of humidity sensors by producing more oxygen vacancies than undoped TiO2. In these experiments, Eu-doped TiO2 nanorods were prepared by hydrothermal synthesis, and the results also confirm the theoretical prediction. When the doping mole ratio is 5 mol%, the response of the humidity sensor reaches 23,997.0, the wet hysteresis is 2.3% and the response/recovery time is 3/13.1 s. This study not only improves the basis for preparation of high-performance TiO2 humidity sensors, but also fills the research gap on rare earth Eu-doped TiO2 as a humidity-sensitive material.

MXene Key Composites: A New Arena for Gas Sensors

With the development of science and technology, the scale of industrial production continues to grow, and the types and quantities of gas raw materials used in industrial production and produced during the production process are also constantly increasing. These gases include flammable and explosive gases, and even contain toxic gases. Therefore, it is very important and necessary for gas sensors to detect and monitor these gases quickly and accurately. In recent years, a new two-dimensional material called MXene has attracted widespread attention in various applications. Their abundant surface functional groups and sites, excellent current conductivity, tunable surface chemistry, and outstanding stability make them promising for gas sensor applications. Since the birth of MXene materials, researchers have utilized the efficient and convenient solution etching preparation, high flexibility, and easily functionalize MXene with other materials to prepare composites for gas sensing. This has opened a new chapter in high-performance gas sensing materials and provided a new approach for advanced sensor research. However, previous reviews on MXene-based composite materials in gas sensing only focused on the performance of gas sensing, without systematically explaining the gas sensing mechanisms generated by different gases, as well as summarizing and predicting the advantages and disadvantages of MXene-based composite materials. This article reviews the latest progress in the application of MXene-based composite materials in gas sensing. Firstly, a brief summary was given of the commonly used methods for preparing gas sensing device structures, followed by an introduction to the key attributes of MXene related to gas sensing performance. This article focuses on the performance of MXene-based composite materials used for gas sensing, such as MXene/graphene, MXene/Metal oxide, MXene/Transition metal sulfides (TMDs), MXene/Metal-organic framework (MOF), MXene/Polymer. It summarizes the advantages and disadvantages of MXene composite materials with different composites and discusses the possible gas sensing mechanisms of MXene-based composite materials for different gases. Finally, future directions and inroads of MXenes-based composites in gas sensing are presented and discussed.

Recent Advances in Nanowire-Based Wearable Physical Sensors

Wearable electronics is a technology that closely integrates electronic devices with the human body or clothing, which can realize human-computer interaction, health monitoring, smart medical, and other functions. Wearable physical sensors are an important part of wearable electronics. They can sense various physical signals from the human body or the surrounding environment and convert them into electrical signals for processing and analysis. Nanowires (NW) have unique properties such as a high surface-to-volume ratio, high flexibility, high carrier mobility, a tunable bandgap, a large piezoresistive coefficient, and a strong light-matter interaction. They are one of the ideal candidates for the fabrication of wearable physical sensors with high sensitivity, fast response, and low power consumption. In this review, we summarize recent advances in various types of NW-based wearable physical sensors, specifically including mechanical, photoelectric, temperature, and multifunctional sensors. The discussion revolves around the structural design, sensing mechanisms, manufacture, and practical applications of these sensors, highlighting the positive role that NWs play in the sensing process. Finally, we present the conclusions with perspectives on current challenges and future opportunities in this field.

Plasmon assisted Ti3C2Tx grafting and surface termination tuning for enhancement of flake stability and humidity sensing performance

Humidity sensors play a critical role in monitoring human activities, environmental health, food processing and storage, and many other fields. Recently, some 2D materials, particularly MXenes, have been considered as promising candidates for creating humidity sensors because of their high surface area, surface-to-bulk ratio, and excellent conductivity, arising from the high concentration and mobility of free electrons. In this work, we propose the plasmon-assisted surface modification and termination tuning of common MXene (Ti3C2Tx) to enhance their response to humidity and increase their stability against oxidation. Hydrophobic (-C6H4-CF3) and hydrophilic (-C6H4-COOH) chemical moieties were covalently grafted to the Ti3C2Tx surface using plasmon-mediated diazonium chemistry. In situ Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) measurements, performed at different humidity levels indicate that surface modification significantly affects penetration of water molecules in Ti3C2Tx films. As a result, the sensitivity of the flakes to the presence of water molecules was significantly altered. Additionally, proposed surface grafting commonly proceeds on the less stable MXene surface sites, where flake oxidation commonly initiates. As a result of the modification, such "weak" and more chemically active sites were blocked and Ti3C2Tx stability was significantly enhanced.

Recent progress and applications of cellulose and its derivatives-based humidity sensors: A review

Cellulose and its derivatives, which are low-cost, degradable, reproducible and highly hydrophilic, can serve as both substrate and humidity sensitive materials, making them more and more popular as ideal biomimetic materials for humidity sensors. Benefiting from these characteristics, cellulose-based humidity sensors cannot only exhibit high sensitivity, excellent mechanical performance, wide humidity response range, etc., but also can be applied to fields such as human health, medical care and agricultural product safety monitoring. Herein, cellulose-based humidity sensors are first classified according to the different conductive active materials, such as carbon nanotubes, graphene, electrolytes, metal compounds, and polymer materials, based on which the latest research progress is introduced, and the roles of different types of conductive materials in cellulose-based humidity sensors are analyzed and summarized. Besides, the similarities and differences in their working mechanisms are expounded. Finally, the application scenarios of cellulose-based humidity sensors in human movement respiration and skin surface humidity monitoring are discussed, which can make readers quickly familiarize the current preparation method, working mechanism and subsequent development trend of cellulose-based humidity sensors more effectively.

High-performance paper-based humidity sensors with Nafion/AgNWs hybrid electrodes

In the past decade, the development of medical health and human-computer interfaces has put forward requirements for the non-contact application of flexible electronics. Among them, flexible humidity sensors play an important role in the field of non-contact sensing by virtue of their rapid response to humidity changes. In this paper, a flexible paper-based humidity sensor with high performance was fabricated by embedded Au@AgNWs electrodes on filter paper through spraying and electroplating (EP) methods. Benefitting from the moisture-sensitive properties of the paper and the tight integration of the electrodes into the filter paper, the sensor shows the humidity monitoring range of 33-100% RH, large response value of I/I0 = 1958, excellent linearity of R2 = 0.99662 and hysteresis performance under the low excitation voltage of only DC 1 V. In addition, the good biocompatibility of the paper-based humidity sensor endows it with multifunctional applications for breath detection, non-contact applications and food security monitoring. Easy access to raw materials and convenient preparation methods of this work provide new ideas for the development and commercialization of flexible humidity sensors.

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Heterostructures of MXenes and transition metal oxides for supercapacitors: an overview

MXenes are a large family of two dimensional (2D) materials with high conductivity, redox activity and compositional diversity that have become front-runners in the materials world for a diverse range of energy storage applications. High-performing supercapacitors require electrode materials with high charge storage capabilities, excellent electrical conductivity for fast electron transfer, and the ability of fast charging/discharging with good cyclability. While MXenes show many of these properties, their energy storage capability is limited by a narrow electrochemically stable potential window due to irreversible oxidation under anodic potentials. Although transition metal oxides (TMOs) are often high-capacity materials with high redox activity, their cyclability and poor rate performance are persistent challenges because of their dissolution in aqueous electrolytes and mediocre conductivity. Forming heterostructures of MXenes with TMOs and using hybrid electrodes is a feasible approach to simultaneously increase the charge storage capacity of MXenes and improve the cyclability and rate performance of oxides. MXenes could also act as conductive substrates for the growth of oxides, which could perform as spacers to stop the aggregation of MXene sheets during charging/discharging and help in improving the supercapacitor performance. Moreover, TMOs could increase the interfacial contact between MXene sheets and help in providing short-diffusion ion channels. Hence, MXene/TMO heterostructures are promising for energy storage. This review summarizes the most recent developments in MXene/oxide heterostructures for supercapacitors and highlights the roles of individual components.

Highly Sensitive Self-Powered Humidity Sensor Based on a TaS2/Cu2S Heterostructure Driven by a Triboelectric Nanogenerator

Self-powered humidity sensors with high response and good stability have attracted extensive interest in environmental monitoring, medical and health care, and sentiment detection. Because of its high specific surface area and good conductivity, two-dimensional material has wide application in the field of humidity sensing. In this work, we proposed a novel self-powered high-performance TaS₂/Cu₂S heterostructure-based humidity sensor driven by a triboelectric nanogenerator (TENG) made with the same structure. The TaS₂/Cu₂S heterostructure was prepared via the chemical vapor deposition method, and then, electrolytic and ultrasound treatments were introduced to further increase the surface area. The fabricated humidity sensor showed ultrahigh sensitivity (S = 3.08 × 10⁴), fast response (2 s), low hysteresis (3.5%), and great stability. First-principles calculation results demonstrated the existence of an electron transport channel with a low energy barrier (-0.156 eV) from the Cu₂S to TaS₂ layer in the heterostructure, which improves the surface charge transfer of the material. The TaS₂/Cu₂S heterojunction-based TENG can generate an output voltage of 30 V and an output current of 2.9 μA. Furthermore, the proposed self-powered humidity sensor verified the potential ability of detecting human respiratory frequency, skin humidity, and environmental humidity. This work provides a new and feasible path for research in the field of humidity sensors and promotes the application development of self-powered electronic devices.

Flexible Humidity Sensor with High Sensitivity and Durability for Respiratory Monitoring Using Near-Field Electrohydrodynamic Direct-Writing Method

The humidity of breath can serve as an important health indicator, providing crucial clinical information about human physiology. Significant progress had been made in the development of flexible humidity sensors. However, improving its humidity sensing performance (sensitivity and durability) is still facing many challenges. In this work, near-field electrohydrodynamic direct writing (NFEDW) was proposed to fabricate humidity sensors with high sensitivity and durability for respiration monitoring. Due to the applied electric field, dense carbon nanotube/cellulose nanofiber (CNT/CNF) networks formed during the printing process that enhance the sensitivity of the sensor. The prepared sensor showed excellent humidity responses, with a maximum response value of 61.5% (ΔR/R₀) at 95% relative humidity (RH). Additionally, the sensitivity film prepared by the NFEDW method closely fits the poly(ethylene terephthalate) (PET) substrate, endowing the sensor with outstanding bending (with a maximum curvature of 4.7 cm^-1) and folding durability (up to 50 times). The sensitivity of the prepared sensor under different simulated conditions, namely, nose breathing, mouth breathing, coughing, yawning, breath holding, and speaking, was excellent, demonstrating the potential of the sensor for the real-time monitoring of human breath humidity. Thus, the high-performance flexible humidity sensor is suitable for human respiration and health monitoring.

Recent Progress in Emerging Novel MXenes Based Materials and their Fascinating Sensing Applications

Early transition metals based 2D carbides, nitrides and carbonitrides nanomaterials are known as MXenes, a novel and extensive new class of 2D materials family. Since the first accidently synthesis based discovery of Ti₃ C₂ in 2011, more than 50 additional compositions have been experimentally reported, including at least eight distinct synthesis methods and also more than 100 stoichiometries are theoretically studied. Due to its distinctive surface chemistry, graphene like shape, metallic conductivity, high hydrophilicity, outstanding mechanical and thermal properties, redox capacity and affordable with mass-produced nature, this diverse MXenes are of tremendous scientific and technological significance. In this review, first we'll come across the MXene based nanomaterials possible synthesis methods, their advantages, limitations and future suggestions, new chemistry related to their selected properties and potential sensing applications, which will help us to explain why this family is growing very fast as compared to other 2D families. Secondly, problems that help to further improve commercialization of the MXene nanomaterials based sensors are examined, and many advances in the commercializing of the MXene nanomaterials based sensors are proposed. At the end, we'll go through the current challenges, limitations and future suggestions.

Application of Titanium Carbide MXenes in Chemiresistive Gas Sensors

The titanium carbide MXenes currently attract an extreme amount of interest from the material science community due to their promising functional properties arising from the two-dimensionality of these layered structures. In particular, the interaction between MXene and gaseous molecules, even at the physisorption level, yields a substantial shift in electrical parameters, which makes it possible to design gas sensors working at RT as a prerequisite to low-powered detection units. Herein, we consider to review such sensors, primarily based on Ti3C2Tx and Ti2CTx crystals as the most studied ones to date, delivering a chemiresistive type of signal. We analyze the ways reported in the literature to modify these 2D nanomaterials for (i) detecting various analyte gases, (ii) improving stability and sensitivity, (iii) reducing response/recovery times, and (iv) advancing a sensitivity to atmospheric humidity. The most powerful approach based on designing hetero-layers of MXenes with other crystals is discussed with regard to employing semiconductor metal oxides and chalcogenides, noble metal nanoparticles, carbon materials (graphene and nanotubes), and polymeric components. The current concepts on the detection mechanisms of MXenes and their hetero-composites are considered, and the background reasons for improving gas-sensing functionality in the hetero-composite when compared with pristine MXenes are classified. We formulate state-of-the-art advances and challenges in the field while proposing some possible solutions, in particular via employing a multisensor array paradigm.

Advances in Humidity Nanosensors and Their Application: Review

As the technology revolution and industrialization have flourished in the last few decades, the development of humidity nanosensors has become more important for the detection and control of humidity in the industry production line, food preservation, chemistry, agriculture and environmental monitoring. The new nanostructured materials and fabrication in nanosensors are linked to better sensor performance, especially for superior humidity sensing, following the intensive research into the design and synthesis of nanomaterials in the last few years. Various nanomaterials, such as ceramics, polymers, semiconductor and sulfide, carbon-based, triboelectrical nanogenerator (TENG), and MXene, have been studied for their potential ability to sense humidity with structures of nanowires, nanotubes, nanopores, and monolayers. These nanosensors have been synthesized via a wide range of processes, including solution synthesis, anodization, physical vapor deposition (PVD), or chemical vapor deposition (CVD). The sensing mechanism, process improvement and nanostructure modulation of different types of materials are mostly inexhaustible, but they are all inseparable from the goals of the effective response, high sensitivity and low response-recovery time of humidity sensors. In this review, we focus on the sensing mechanism of direct and indirect sensing, various fabrication methods, nanomaterial geometry and recent advances in humidity nanosensors. Various types of capacitive, resistive and optical humidity nanosensors are introduced, alongside illustration of the properties and nanostructures of various materials. The similarities and differences of the humidity-sensitive mechanisms of different types of materials are summarized. Applications such as IoT, and the environmental and human-body monitoring of nanosensors are the development trends for futures advancements.

Ultra-high performance humidity sensor enabled by a self-assembled CuO/Ti3C2T X MXene

An ultra-high performance humidity sensor based on a CuO/Ti3C2T X MXene has been investigated in this work. The moisture-sensitive material was fabricated by a self-assembly method. The morphology and nanostructure of the fabricated CuO/Ti3C2T X composites were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectra. The humidity sensing abilities of the CuO/Ti3C2T X sensor in the relative humidity (RH) range from 0% to 97% were studied. The results showed that the humidity sensor had a high sensitivity of 451 kΩ/% RH, short response time (0.5 s) and recovery time (1 s), a low hysteresis value, and good repeatability. The CuO/Ti3C2T X sensor exhibited remarkable properties in human respiration rate monitoring, finger non-contact sensing, and environmental detection. The moisture-sensitive mechanism of CuO/Ti3C2T X was discussed. The fabricated CuO/Ti3C2T X showed great potential in the application of moisture-sensitive materials for ultra-high-performance humidity sensors.

Collaborative Enhancement of Humidity Sensing Performance by KCl-Doped CuO/SnO2 p-n Heterostructures for Monitoring Human Activities

In this study, a high-performance humidity sensor based on KCl-doped CuO/SnO2 p-n heterostructures was fabricated by a ball milling-roasting method. The morphology and nanostructure of the fabricated KCl-CuO/SnO2 composite were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and nitrogen sorption analysis. The results showed that the humidity sensor had a high sensitivity of 194 kΩ/%RH, short response and recovery times of 1.0 and 1.5 s, a low hysteresis value, and good repeatability. The energy band structure and complex impedance spectrum of the KCl-CuO/SnO2 composite indicated that the excellent humidity sensing performance originated from the ionic conductivity of KCl, the formation of heterojunctions, the change in the Schottky barrier height, and the depletion of electronic depletion layers. The KCl-CuO/SnO2 sensor has great potential in respiratory monitoring, noncontact sensing of finger moisture, and environmental monitoring.

A review related to MXene preparation and its sensor arrays of electronic skins

MXenes have been flourishing over the last decade as a high-performance 2D material, which combines the advantages of high electrical conductivity, photothermal conversion, and easy dispersion. They have been used to create soft, highly conductive, self-healing, and tactile-simulating electronic skins (E-skins). However, these E-skins remain generally limited to one or two functions with a complex preparation process. Next-generation E-skins necessitate not only large-scale fabrication using simple and fast methods but also the integration of multiple sensing functions and signal analysis components in order to provide functionality that was not unattainable in the past. Starting with the synthesis of pure MXenes, we walk through the steps of designing MXene sensors, integrating electronic skin arrays, and determining the function of MXene-based electronic skins. We also summarise the problems with existing MXene-based E-skins and possible futuristic directions.

Formation of a Nanorod-Assembled TiO2 Actinomorphic-Flower-like Microsphere Film via Ta Doping Using a Facile Solution Immersion Method for Humidity Sensing

This study fabricated tantalum (Ta)-doped titanium dioxide with a unique nanorod-assembled actinomorphic-flower-like microsphere structured film. The Ta-doped TiO2 actinomorphic-flower-like microsphere (TAFM) was fabricated via the solution immersion method in a Schott bottle with a home-made improvised clamp. The samples were characterised using FESEM, HRTEM, XRD, Raman, XPS, and Hall effect measurements for their structural and electrical properties. Compared to the undoped sample, the rutile-phased TAFM sample had finer nanorods with an average 42 nm diameter assembled to form microsphere-like structures. It also had higher oxygen vacancy sites, electron concentration, and mobility. In addition, a reversed double-beam photoacoustic spectroscopy measurement was performed for TAFM, revealing that the sample had a high electron trap density of up to 2.5 μmolg-1. The TAFM showed promising results when employed as the resistive-type sensing film for a humidity sensor, with the highest sensor response of 53,909% obtained at 3 at.% Ta doping. Adding rGO to 3 at.% TAFM further improved the sensor response to 232,152%.

Towards Greener and More Sustainable Synthesis of MXenes: A Review

The unique properties of MXenes have been deemed to be of significant interest in various emerging applications. However, MXenes provide a major drawback involving environmentally harmful and toxic substances for its general fabrication in large-scale production and employing a high-temperature solid-state reaction followed by selective etching. Meanwhile, how MXenes are synthesized is essential in directing their end uses. Therefore, making strategic approaches to synthesize greener, safer, more sustainable, and more environmentally friendly MXenes is imperative to commercialize at a competitive price. With increasing reports of green synthesis that promote advanced technologies and non-toxic agents, it is critical to compile, summarize, and synthesize the latest development of the green-related technology of MXenes. We review the recent progress of greener, safer, and more sustainable MXene synthesis with a focus on the fundamental synthetic process, the mechanism, and the general advantages, and the emphasis on the MXene properties inherited from such green synthesis techniques. The emerging use of the so-called green MXenes in energy conversion and storage, environmental remediation, and biomedical applications is presented. Finally, the remaining challenges and prospects of greener MXene synthesis are discussed.

Tuning thermal and electrical properties of MXenes via dehydration

Recently, MXenes (a class of two-dimensional transition metal carbides) have attracted great attention in various applications such as humidity sensors, owing to their unique electrical and thermal properties. However, previous studies of MXenes mostly focus on their humidity-sensing characteristics such as the mechanical response, and only few reports on their electrical and thermal response are available. Herein, we present novel transient electrothermal experiments to demonstrate that a transition from a negative to a positive resistance-temperature relationship can take place when the MXene sample becomes fully dehydrated. This surprising and unusual phenomenon was elucidated through non-equilibrium molecular dynamics simulations and attributed to water absorption/desorption onto the chemically active MXene surface. A linear relationship was also found between electrical/thermal properties and environmental humidity, which could be related to water adsorption on the surface of the MXene sensor. We further decomposed the total measured thermal conductivity and found that phonons were the dominant thermal carriers in the MXene sample. The main breakthrough of this work is the discovery of the unusual resistance-temperature relationship, which should be applicable to the design of MXene-based sensors for various applications.

Hierarchical MXene/transition metal oxide heterostructures for rechargeable batteries, capacitors, and capacitive deionization

2D MXenes have attracted considerable attention due to their high electronic conductivity, tunable metal compositions, functional termination groups, low ion diffusion barriers, and abundant active sites. However, MXenes suffer from sheet stacking and partial surface oxidation, limiting their energy storage and water treatment development. To solve these problems and enhance the performance of MXenes in practical applications, various hierarchical MXene/transition metal oxide (MXene/TMO) heterostructures are rationally designed and constructed. The hierarchical MXene/TMO heterostructures can not only prevent the stacking of MXene sheets and improve the electronic conductivity and buffer the volume change of TMOs during the electrochemical reaction process. The synergistic effect of conductive MXenes and active TMOs also makes MXene/TMO heterostructures promising electrode materials for energy storage and seawater desalination. This review mainly introduces and discusses the recent research progress in MXene/TMO heterostructures, focusing on their synthetic strategies, heterointerface engineering, and applications in rechargeable batteries, capacitors, and capacitive deionization (CDI). Finally, the key challenges and prospects for the future development of the MXene/TMO heterostructures in rechargeable batteries, capacitors, and CDI are proposed.

Evaluating Different TiO2 Nanoflower-Based Composites for Humidity Detection

Unique three-dimensional (3D) titanium dioxide (TiO2) nanoflowers (TFNA) have shown great potential for humidity sensing applications, due to their large surface area-to-volume ratio and high hydrophilicity. The formation of a composite with other materials could further enhance the performance of this material. In this work, the effect of different types of composites on the performance of a TNFA-based humidity sensor was examined. NiO, ZnO, rGO, and PVDF have been explored as possible composite pairing candidates with TiO2 nanoflowers, which were prepared via a modified solution immersion method. The properties of the composites were examined using field emission electron spectroscopy (FESEM), X-ray diffractometry (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), current-voltage (I-V) analysis, Hall effect measurement, and contact angle measurement. The performance of the humidity sensor was assessed using a humidity sensor measurement system inside a humidity-controlled chamber. Based on the result, the combination of TiO2 with rGO produced the highest sensor response at 39,590%. The achievement is attributed to the increase in the electrical conductivity, hydrophilicity, and specific surface area of the composite.

Multifunctional Flexible Humidity Sensor Systems Towards Noncontact Wearable Electronics

In the past decade, the global industry and research attentions on intelligent skin-like electronics have boosted their applications in diverse fields including human healthcare, Internet of Things, human-machine interfaces, artificial intelligence and soft robotics. Among them, flexible humidity sensors play a vital role in noncontact measurements relying on the unique property of rapid response to humidity change. This work presents an overview of recent advances in flexible humidity sensors using various active functional materials for contactless monitoring. Four categories of humidity sensors are highlighted based on resistive, capacitive, impedance-type and voltage-type working mechanisms. Furthermore, typical strategies including chemical doping, structural design and Joule heating are introduced to enhance the performance of humidity sensors. Drawing on the noncontact perception capability, human/plant healthcare management, human-machine interactions as well as integrated humidity sensor-based feedback systems are presented. The burgeoning innovations in this research field will benefit human society, especially during the COVID-19 epidemic, where cross-infection should be averted and contactless sensation is highly desired.

Graphene oxide quantum dots attached on wood-derived nanocellulose to fabricate a highly sensitive humidity sensor

Herein, cellulose nanofibril (CNF) with various carboxyl amounts were prepared via regulating its oxidation degree using TEMPO oxidation. The CNF dispersion was dropped onto the interdigital electrode to be capacitive humidity sensor by the subsequent vacuum freeze-drying. Pure CNF-7 (NaClO content of 7 mmol/g) humidity sensor involves in orderly porous structure, which displays better performance than other CNFs for its moderate carboxyl content and dimension. As uniformly adding appropriate content of graphene oxide quantum dots (GOQD) with larger surface area and active sites, it can be attached on the CNF to construct a three-dimensional interconnected porous structure for their excellent aqueous dispersity as well as differences in morphology and size. Consequently, the CNF/GOQD sensor exhibits the sensitivity as high as 51,840.91 pF/% RH, short response time (30 s)/recovery time (11 s) and excellent reproducibility. The proposed method can provide effective guidance for the design of humidity sensors based on nanomaterials.

Flexible Humidity Sensors Based on Multidimensional Titanium Dioxide/Cellulose Nanocrystals Composite Film

A humidity sensor is a crucial device in daily life; therefore, in the present study, a novel humidity sensor was designed to increase its specific surface area to improve its humid sensing capacity and conductivity. Titanium dioxide nanoparticles (TiNP) consisting of zero-dimensional nanospheres and one-dimensional nanotubes were prepared by anodic oxidation. Rod-shaped cellulose nanocrystals (CNCs) with average length and diameter of 60 nm and 800 nm, respectively, were obtained by enzymatic hydrolysis and high pressure homogenization. TiNP/CNC composite films exhibited superior hydrophilicity and large specific surface areas based on Fourier transform infrared spectroscopy and nitrogen adsorption-desorption results. The humidity sensing characteristics of sensors based on TiNP/CNC flexible composite films with varying contents of TiNP were investigated under a relative humidity range of 11-97%. The 6% TiNP/CNC-based humidity sensor exhibited high humidity response, rapid response/recovery speed, and high stability. Furthermore, the humidity sensing mechanism of TiNP/CNC composite films was analyzed based on the density functional theory. TiNP/CNC-based humidity sensors could be applied in flexible and wearable electronics.

High-Sensitivity and Environmentally Friendly Humidity Sensors Deposited with Recyclable Green Microspheres for Wireless Monitoring

The reliable, high-sensitive, wireless, and affordable requirements for humidity sensors are needed in high-precision measurement fields. Quartz crystal microbalance (QCM) based on the piezoelectric effect can accurately detect the mass changes at the nanogram level. However, water-capture materials deposited on the surface of QCM generally show disadvantages in either cost, sensitivity, or recyclability. Herein, novel QCM-based humidity sensors (NQHSs) are developed by uniformly depositing green microspheres (GMs) of natural polymers prepared by the chemical synthesis of the emulsification/inner gel method on QCM as humidity-sensitive materials. The NQHSs demonstrate high accuracy and sensitivity (27.1 Hz/% RH) owing to the various hydrophilic groups and porous nano-3D deposition structure. Compared with the devices deposited with a smooth film, the frequency of the NQHSs shows almost no changes during the cyclic test and exhibits long-term stability. The NQHSs have been successfully applied to non-contact sensing human activities and remote real-time humidity monitoring via Bluetooth transmission. In addition, the deposited humidity-sensitive GMs and QCM substrate are fully recycled and reused (72% of the original value). This work has provided an innovative idea to construct environmental-friendly, high-sensitivity, and wireless humidity sensors.

Optical Microfibers for Sensing Proximity and Contact in Human-Machine Interfaces

The monitoring of proximity-contact events is essential for human-machine interactions, intelligent robots, and healthcare monitoring. We report a dual-modal sensor made with two functionalized optical microfibers (MFs), which is inspired by the somatosensory system of human skin. The integrated sensor with a hierarchical structure gradationally detects finger approaching and touching by measuring the relative humidity (RH) and force-triggered light intensity variations. Specifically, the RH sensory part shows enhanced evanescent absorption, achieving a sensitive RH measurement with a fast response (110 ms), a high resolution (0.11%RH), and a wide working range (10-100%RH). Enabled by the transition from guided modes into radiation modes of the waveguiding MF, the force sensory part exhibits a high sensitivity (6.2%/kPa) and a fast response (up to 1.5 kHz). By using a real-time data processing unit, the proximity-contact sensor (PCS) achieves continuous detection of the full-contact events, including finger approaching, contacting, pressing, releasing, and leaving. As a proof of concept, the electromagnetic-interference-free PCS enables a smart switch system to recognize the proximity and contact of bare/gloved fingers. Moreover, skin humidity detection and respiration monitoring are realized. These initial results pave the way toward a category of optical collaborative devices ranging from human-machine interfaces to multifunctional on-skin healthcare sensors.

UV illumination-enhanced ultrasensitive ammonia gas sensor based on (001)TiO2/MXene heterostructure for food spoilage detection

Ammonia has been used as an important marker to indicate the extent of food spoilage. However, current gas sensors for ammonia suffer from either insufficient sensitivity and selectivity or unsatisfactory levels of automation, impeding their practical application for on-site and real-time monitoring of food quality. To overcome these limitations, we propose here the design of a sensing material by in-situ growing (001)TiO₂ onto a two-dimensional transition-metal carbide (Ti₃C₂T_x, MXene). In this design, TiO₂ with a highly active (001) crystal plane provides efficient photogeneration under UV irradiation, while Ti₃C₂T_x can store holes through Schottky junction formed at the interface with TiO₂, which greatly promotes the separation of electron-hole pairs, thereby enhancing ammonia sensing performance. By further introducing UV light for electron excitation, the (001)TiO₂/Ti₃C₂T_x based sensor shows 34 times higher sensitivity for ammonia (30 ppm) than that of Ti₃C₂T_x. The density functional theory further revealed that the (001) plane of TiO₂ and Ti₃C₂T_x composite configuration exhibited the highest adsorption affinity towards ammonia. Finally, an integrated circuit alarm system including near-field communication and a micro-controller system was designed to detect the decay process of fresh pork, fish, and shrimp. We believe such a sensing technology holds great promise in food quality monitoring.

The Effect of Surface Hydroxyls on the Humidity-Sensitive Properties of LiCl-Doped ZnSn(OH)6 Sphere-Based Sensors

Pure zinc hydroxystannate (ZnSn(OH)₆) and LiCl-doped ZnSn(OH)₆ have been synthesized through a facile wet chemical method. The LiCl-doped samples keep their original spherical morphology as pure ZnSn(OH)_6, with some LiCl particles stuck to its surface, providing more active sites for the adsorption and desorption of water molecules. The influence of LiCl doping on the humidity-sensing properties was explored by varying the dopant concentration. The 16 wt% LiCl/ZnSn(OH)₆ showed a better humidity-sensing performance than that of the pure ZnSn(OH)₆ and other doped samples, including a high resistive sensitivity, a relatively small hysteresis, and a fast response speed. Through the FTIR analysis, the number of hydroxyl groups on the surface structure after aging has been found to decline markedly. These hydroxyl groups provide a platform for the adsorption of water molecules on the surface and promote the dissociation of water molecules. The detriment of aging to sensor performance should not be underrated. The complex impedance spectrum explains the mechanism of the sensor. These results demonstrate that ZnSn(OH)₆ has potential application in fabricating humidity sensors, and the sensing performance of the sensor is enhanced by the dopant LiCl.

Self-Powered Monitoring of Ammonia Using an MXene/TiO2/Cellulose Nanofiber Heterojunction-Based Sensor Driven by an Electrospun Triboelectric Nanogenerator

Real-time monitoring of harmful gases is of great significance to identify the environmental hazards to people's lives. However, this application scenario requiring low-power consumption, superior sensitivity, portability, and self-driven operation of gas sensors remains a challenge. Herein, an electrospun triboelectric nanogenerator (TENG) is synthesized using highly electronegative and conducting MXene nanofibers (NFs) paired with biodegradable cellulose acetate NFs (CA-NFs) as triboelectric layers, which supports a sufficient power density (∼1361 mW/m²@2 MΩ) and shows a self-powered ability to operate the chemiresistive gas sensor fabricated in this work. Further, by using cellulose nanofibers (C-NFs) as a substrate, a new kind of MXene/TiO₂/C-NFs heterojunction-based sensory component is developed for detection of NH₃. This sensor exhibits excellent reproducibility, high selectivity, and sensitivity toward NH₃ (1-100 ppm) along with a fast response/recovery time (76 s/62 s) at room temperature. Finally, a monitoring system comprising a TENG-powered sensor, an equivalent circuit, and an LED visualizer has been assembled and successfully demonstrated as a fully self-powered device for NH₃ leakage detection. Thus, this work pushes forward the intelligent gas sensing network self-driven by human motion energy, dispensing the external battery dependence for environment monitoring to reduce the possible health effects.

MXene Heterostructures as Perspective Materials for Gas Sensing Applications

This paper provides a summary of the recent developments with promising 2D MXene-related materials and gives an outlook for further research on gas sensor applications. The current synthesis routes that are provided in the literature are summarized, and the main properties of MXene compounds have been highlighted. Particular attention has been paid to safe and non-hazardous synthesis approaches for MXene production as 2D materials. The work so far on sensing properties of pure MXenes and MXene-based heterostructures has been considered. Significant improvement of the MXenes sensing performances not only relies on 2D production but also on the formation of MXene heterostructures with other 2D materials, such as graphene, and with metal oxides layers. Despite the limited number of research papers published in this area, recommendations on new strategies to advance MXene heterostructures and composites for gas sensing applications can be driven.

Touchless Sensing Interface Based on the Magneto-Piezoresistive Effect of Magnetic Microstructures with Stacked Conductive Coating

Robotics capable of human-like operations need to have electronic skin (e-skin) with not only tactile sensing functions but also proximity perception abilities. Especially, under the current widespread of COVID-19 pandemic, touchless interfaces are highly desirable. Magnetoreception, with inherent specificity for magnetic objects, is an effective approach to construct a non-contact sensing e-skin. In this work, we propose a new touchless sensing mechanism based on the magneto-piezoresistive effect. The substrate of the sensor is made of hierarchically microstructured ferromagnetic polydimethylsiloxane, coated with a three-dimensional (3D) piezoresistive network. The 3D network is constructed by stacked layers of reduced graphene oxide and carbon nanotubes through layer-by-layer deposition. With this integrated design, a magnetic force induced on the ferromagnetic substrate can seamlessly be applied to the piezoresistive layer of the sensor. Because the magnetic force relates strongly to the approaching distance, the position information can be transduced into the resistance change of the piezoresistive network. The flexible proximity sensor exhibits an ultrahigh spatial resolution of 60 μm, a sensitivity of 50.47 cm^-1, a wide working range of 6 cm, and a fast response of 10 ms. The repeatable performance of the sensor is shown by over 5000 cycles of approaching-separation test. We also demonstrate successful application of the sensor in 3D positioning and motion tracking settings, which is critical for touchless tactile perception-based human-machine interactions.

Efficient Anchoring of Polysulfides Based on Self-Assembled Ti3C2Tx Nanosheet-Connected Hollow Co(OH)2 Nanotubes for Lithium-Sulfur Batteries

Designing sulfur host materials with unique functions such as physical constraint or chemical catalysis to suppress the shuttle effect and promote the fast conversion of polysulfides is a prerequisite for lithium-sulfur batteries (LSBs). Herein, we construct hollow Co(OH)₂ nanotubes connected by Ti₃C₂T_x nanosheets (denoted as Co(OH)₂@Ti₃C₂T_x) as host materials for sulfur through a simple self-assembly method at room temperature. The large void spaces of Co(OH)₂ nanotubes not only confine higher sulfur loading but also mitigate the volumetric expansion in the process of lithiation. Moreover, the conductive Ti₃C₂T_x layers facilitate fast electron transfer and catalyze the transition of sulfur based on the terminations on the surface. Combining those two materials can also act as an efficient polysulfide anchor to enable outstanding electrochemical performance. The Co(OH)₂@Ti₃C₂T_x@S cathode presents a high discharge capacity of 1400 mAh g^-1 at 0.1C and long-cycling stability at 1C for 500 cycles. Moreover, the obtained capacity of Li₂S precipitation and the dissolution capacity reach 193.3 and 291.1 mAh g^-1, respectively. Consequently, this work demonstrates a facile strategy to design multifunctional materials that effectively confine the polysulfides and enhance the performance of LSBs.

2D Molybdenum Carbide MXenes for Enhanced Selective Detection of Humidity in Air

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo2 CTx flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-µm MXene multilayer for different organic vapors and humidity at 101 -104  ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H2 O down to 10 ppm. Moreover, humidity suppresses the response of Mo2 CTx to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10-2 -106  Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo2 CTx chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo2 C surface interaction with organic analytes and H2 O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.

A High-Wet-Strength Biofilm for Readable and Highly Sensitive Humidity Sensors

Low-cost and flexible biofilm humidity sensors with good wet strength are crucial for humidity detection. However, it remains a great challenge to integrate good reversibility, rapid humidity response, and robust humid mechanical strength in one sensor. In this respect, we report a facile method to prepare a sustainable biofilm (named MC film) from sisal cellulose microcrystals (MSF-g-COOH) and citric acid (CA). After cross-linking with CA, the MC film exhibits excellent wet strength and rapid humidity response. More importantly, MC film can be used over a wide temperature range with excellent durability and reversibility for humidity detection. A highly sensitive humidity sensor fabricated from the MC film exhibits high reversibility and excellent water resistance and can be applied in humidity and personalized breath health monitoring. Our work fills the gap between biomaterial design and high-performance sensing devices.

Sensing with MXenes: Progress and Prospects

Various fields of study consider MXene a revolutionary 2D material. Particularly in the field of sensors, the metal-like high electrical conductivity and large surface area of MXenes are desirable characteristics as an alternative sensor material that can transcend the boundaries of existing sensor technology. This critical review provides a comprehensive overview of recent advances in MXene-based sensor technology and a roadmap for commercializing MXene-based sensors. The existing sensors are systematically categorized as chemical, biological, and physical sensors. Each category is then classified into various subcategories depending on the electrical, electrochemical, structural, or optical sensing mechanism, which are the four fundamental working mechanisms of sensors. Representative structural and electrical approaches for boosting the performance of each category are presented. Finally, factors that hinder commercializing MXene-based sensors are discussed, and several breakthroughs in realizing commercially available MXene-based sensors are suggested. This review provides broad insights pertaining to previous and existing MXene-based sensor technology and perspectives on the future generation of low-cost, high-performance, and multimodal sensors for soft-electronics applications.

Electronic and Optical Property Control of Polycation/MXene Layer-by-Layer Assemblies with Chemically Diverse MXenes

MXenes, 2D nanomaterials derived from ceramic MAX phases, have drawn considerable interest in a wide variety of fields including energy storage, catalysis, and sensing. There are many possible MXene compositions due to the chemical and structural diversity of parent MAX phases, which can bear different possible metal atoms "M", number of layers, and carbon or nitrogen "X" constituents. Despite the potential variety in MXene types, the bulk of MXene research focuses upon the first MXene discovered, Ti₃C₂T. With the recent discovery of polymer/MXene multilayer assemblies as thin films and coatings, there is a need to broaden the accessible types of multilayers by including MXenes other than Ti₃C₂T_z; however, it is not clear how altering the MXene type influences the resulting multilayer growth and properties. Here, we report on the first use of MXenes other than Ti₃C₂T_z, specifically Ti₂CT_z and Nb₂CT_z, for the layer-by-layer (LbL) assembly of polycation/MXene multilayers. By comparing these MXenes, we evaluate both how changing M (Ti vs Nb) and "n" (Ti₃C₂T_zvs Ti₂CT_z) affect the growth and properties of the resulting multilayer. Specifically, the aqueous LbL assembly of each MXene with poly(diallyldimethylammonium) into films and coatings is examined. Further, we compare the oxidative stability, optoelectronic properties (refractive index, absorption coefficient, optical conductivity, and direct and indirect optical band gaps), and the radio frequency heating response of each multilayer. We observe that MXene multilayers with higher "n" are more electrically conductive and oxidatively stable. We also demonstrate that Nb₂CT_z containing films have lower optical band gaps and refractive indices at the cost of lower electrical conductivities as compared to their Ti₂CT_z counterparts. Our work demonstrates that the properties of MXene/polycation multilayers are highly dependent on the choice of constituent MXene and that the MXene type can be altered to suit specific applications.

Self-sensing coaxial muscle fibers with bi-lengthwise actuation

Artificial muscle fibers as a promising biomimetic actuator are needed for such applications as smart soft robots, muscle function restoration, and physical augmentation. Currently developed artificial muscle fibers have shown attractive performance in contractile and torsional actuations. However, the contractile muscle fibers do not have the capability of stimulus-responsive elongation, and real-time identifying their contractile position by themselves is still challenging. We report herein the preparation of a Ti₃C₂T_x MXene/single walled carbon-nanotubes (SWCNTs)-coated carbon nanotube (CNT)@polydimethylsiloxane (PDMS) coaxial muscle fiber that integrates the important features of self-position sensing and bi-lengthwise actuation. The bi-lengthwise actuation is realized by utilizing the large expansion coefficient difference of PDMS in response to solvent and heat, which results in ∼5% maximum elongation by n-heptane adsorption and ∼19% maximum contraction by electric heating under the optimal conditions. Meanwhile, due to the piezoresistive effect of the MXene/SWCNTs layer, the resistance change of this coating layer is almost linearly dependent on the contraction of the coaxial muscle fiber, providing a function of real-time self-position sensing. Furthermore, an application of using a bundle of these multifunctional coaxial muscle fibers for a bionic arm has been demonstrated, which provides new insights into the design of integrated intelligent artificial muscles with synergistic multiple functions.

MXene derivatives: synthesis and applications in energy convention and storage

Transition metal carbides or nitrides (MXene) have shown promising applications in energy convention and storage (ECS), owing to their high conductivity and adjustable surface functional groups. In the past several years, many MXene derivatives with different structures have been successfully prepared and their impressive performance demonstrated in ECS. This review summarizes the progress in the synthesis of MXene and typical Ti3C2T x MXene derivatives with different morphologies, including 0D quantum dots, 1D nanoribbons, 2D nanosheets and 3D nanoflowers. The mechanisms involved and their performance in photocatalysis, electrocatalysis and rechargeable batteries are also discussed. Furthermore, the challenges of MXene derivatives in ECS are also proposed.