Inkjet Printing Transparent and Conductive MXene (Ti3C2Tx) Films: A Strategy for Flexible Energy Storage Devices

Smart Energy Storage: W18O49 NW/Ti3C2Tx Composite-Enabled All Solid State Flexible Electrochromic Supercapacitors

Developing a highly efficient electrochromic energy storage device with sufficient color fluctuation and significant electrochemical performance is highly desirable for practical energy-saving applications. Here, to achieve a highly stable material with a large electrochemical storage capacity, a W18O49 NW/Ti3C2Tx composite has been fabricated and deposited on a pre-assembled Ag and W18O49 NW conductive network by Langmuir-Blodgett technique. The resulting hybrid electrode composed of 15 layers of W18O49 NW/Ti3C2Tx composite exhibits an areal capacitance of 125 mF cm-2, with a fast and reversible switching response. An optical modulation of 98.2% can be maintained at a current density of 5 mA cm-2. Using this electrode, a bifunctional symmetric electrochromic supercapacitor device having an energy density of 10.26 µWh cm-2 and a power density of 0.605 mW cm-2 is fabricated, with high capacity retention and full columbic efficiency over 4000 charge-discharge cycles. Meanwhile, the device displays remarkable electrochromic characteristics, including fast switching time (5 s for coloring and 7 s for bleaching), and a significant coloration efficiency of 116 cm2 C-1 with good optical modulation stability. In addition, the device exhibits significant mechanical flexibility and fast switching while being stable over 100 bending cycles, which is promising for real-world applications.

Exploring cyclic olefin copolymer (COC) for flexible silver nanowire electrode

The development of flexible electronic devices has been a primary focus in various fields, and silver nanowire (Ag NW) networks show significant promise due to their unique electrical and mechanical properties. However, achieving well-defined and stable nanowire coatings on polymer substrates remains challenging. This work presents a novel and simple approach for directly coating Ag NWs on cyclic olefin copolymer (COC) substrates utilizing ultraviolet/ozone (UVO) treatment, a method not previously demonstrated for this specific material system up to our knowledge. The compatibility of this approach with COC eliminates the need for complex pre- and post-treatment processes, making it a more straightforward and environmentally friendly way to improve adhesion between Ag NWs and COC. The Ag NWs/COC electrodes exhibited excellent optoelectrical performance, with a high optical transmittance of 84% and a low sheet resistance of 13 Ω/sq-metrics that compare favorably to industry standards for transparent conductive films. Additionally, the Ag NWs/COC electrodes displayed excellent mechanical stability, showing no changes in sheet resistance after both tape adhesion and film bending tests. The novelty of the presented Ag NW-COC system, combined with the simplicity and environmental benefits of the UVO coating approach, as well as the demonstrated performance and stability of the resulting electrodes, make this work a significant advancement towards realizing the commercial potential of flexible electronics for biocompatible and wearable device applications.

MnO2 Nanoparticles Decorated PEDOT:PSS for High Performance Stretchable and Transparent Supercapacitors

With the swift advancement of wearable electronics and artificial intelligence, the integration of electronic devices with the human body has advanced significantly, leading to enhanced real-time health monitoring and remote disease diagnosis. Despite progress in developing stretchable materials with skin-like mechanical properties, there remains a need for materials that also exhibit high optical transparency. Supercapacitors, as promising energy storage devices, offer advantages such as portability, long cycle life, and rapid charge/discharge rates, but achieving high capacity, stretchability, and transparency simultaneously remains challenging. This study combines the stretchable, transparent polymer PEDOT:PSS with MnO2 nanoparticles to develop high-performance, stretchable, and transparent supercapacitors. PEDOT:PSS films were deposited on a PDMS substrate using a spin-coating method, followed by electrochemical deposition of MnO2 nanoparticles. This method ensured that the nanosized MnO2 particles were uniformly distributed, maintaining the transparency and stretchability of PEDOT:PSS. The resulting PEDOT:PSS/MnO2 nanoparticle electrodes were gathered into a symmetric device using a LiCl/PVA gel electrolyte, achieving an areal capacitance of 1.14 mF cm-2 at 71.2% transparency and maintaining 89.92% capacitance after 5000 cycles of 20% strain. This work presents a scalable and economical technique to manufacturing supercapacitors that combine high capacity, transparency, and mechanical stretchability, suggesting potential applications in wearable electronics.

Simple and Efficient Synthesis of Ruthenium(III) PEDOT:PSS Complexes for High-Performance Stretchable and Transparent Supercapacitors

In the evolving landscape of portable electronics, there is a critical demand for components that meld stretchability with optical transparency, especially in supercapacitors. Traditional materials fall short in harmonizing conductivity, stretchability, transparency, and capacity. Although poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) stands out as an exemplary candidate, further performance enhancements are necessary to meet the demands of practical applications. This study presents an innovative and effective method for enhancing electrochemical properties by homogeneously incorporating Ru(III) into PEDOT:PSS. These Ru(III) PEDOT:PSS complexes are readily synthesized by dipping PEDOT:PSS films in RuCl3 solution for no longer than one minute, leveraging the high specific capacitance of Ru(III) while minimizing interference with transmittance. The supercapacitor made with this Ru(III) PEDOT:PSS complex demonstrated an areal capacitance of 1.62 mF cm-2 at a transmittance of 73.5%, which was 155% higher than that of the supercapacitor made with PEDOT:PSS under comparable transparency. Notably, the supercapacitor retained 87.8% of its initial capacitance even under 20% tensile strain across 20,000 cycles. This work presents a blueprint for developing stretchable and transparent supercapacitors, marking a significant stride toward next-generation wearable electronics.

Ionic Liquid-Enhanced Assembly of Nanomaterials for Highly Stable Flexible Transparent Electrodes

The controlled assembly of nanomaterials has demonstrated significant potential in advancing technological devices. However, achieving highly efficient and low-loss assembly technique for nanomaterials, enabling the creation of hierarchical structures with distinctive functionalities, remains a formidable challenge. Here, we present a method for nanomaterial assembly enhanced by ionic liquids, which enables the fabrication of highly stable, flexible, and transparent electrodes featuring an organized layered structure. The utilization of hydrophobic and nonvolatile ionic liquids facilitates the production of stable interfaces with water, effectively preventing the sedimentation of 1D/2D nanomaterials assembled at the interface. Furthermore, the interfacially assembled nanomaterial monolayer exhibits an alternate self-climbing behavior, enabling layer-by-layer transfer and the formation of a well-ordered MXene-wrapped silver nanowire network film. The resulting composite film not only demonstrates exceptional photoelectric performance with a sheet resistance of 9.4 Ω sq-1 and 93% transmittance, but also showcases remarkable environmental stability and mechanical flexibility. Particularly noteworthy is its application in transparent electromagnetic interference shielding materials and triboelectric nanogenerator devices. This research introduces an innovative approach to manufacture and tailor functional devices based on ordered nanomaterials.

3D Lightweight Interconnected Melamine Foam Modified with Hollow CoFe2O4/MXene toward Efficient Microwave Absorption

Considering the increasing severity of electromagnetic wave pollution, the development of high-performance low-filler-content microwave absorbers possessing wide frequency bands and strong absorption for practical applications is a demanding research hotspot. In this study, from the perspectives of the electromagnetic component coordination and structural design, a three-dimensional (3D) interconnected CoFe2O4/MXene-melamine foam (MF) was constructed via simple impregnation and a single freeze-drying step. By changing the absorber (CoFe2O4/MXene) concentration, the pore opening and electromagnetic properties of the 3D foams can be effectively adjusted. When the absorber concentration is sufficiently high to clog the internal pores, the microwave absorption is hindered. When the filler (CoFe2O4/MXene-MF) content is just ∼5.8 wt % (at a density of ∼33.3 mg cm-3), a minimum reflection loss (RLmin) of -72.1 dB is achieved at a matching thickness of 3.32 mm, and an effective absorption bandwidth (4.54 GHz) covering the whole X band is achieved at a thickness of 3 mm. CoFe2O4/MXene-MF, which possesses a 3D porous electromagnetic network structure, optimizes impedance matching and enhances multiple polarization relaxations and reflections/scattering, resulting in superior absorption capabilities. In particular, the continuous network structure ensures the uniform distribution of electromagnetic fields in the microstructure, achieving high absorption at low filler contents. This work provides a reference for subsequent 3D absorber concentration studies and a novel engineering strategy for preparing a low-filler-content, lightweight, and efficient electromagnetic wave absorber, which could be applied in the fields of radar security and information communications.

The Need for Smart Materials in an Expanding Smart World: MXene-Based Wearable Electronics and Their Advantageous Applications

As a result of the transformation of inflexible electronic structures into flexible and stretchy devices, wearable electronics now provide great advantages in a variety of fields, including mobile healthcare sensing and monitoring, human-machine interfaces, portable energy storage and harvesting, and more. Because of their enriched surface functionalities, large surface area, and high electrical conductivity, transition metal nitrides and carbides (also known as MXenes) have recently come to be extensively considered as a group of functioning two-dimensional nanomaterials as well as exceptional fundamental elements for forming flexible electronics devices. This Review discusses the most recent advancements that have been made in the field of MXene-enabled flexible electronics for wearable electronics. The emphasis is placed on extensively established nonstructural features in order to highlight some MXene-enabled electrical devices that were constructed on a nanometric scale. These attributes include devices configured in three dimensions: printed materials, bioinspired structures, and textile and planar substrates. In addition, sample applications in electromagnetic interference (EMI) shielding, energy, healthcare, and humanoid control of machinery illustrate the exceptional development of these nanodevices. The increasing potential of MXene nanoparticles as a new area in next-generation wearable electronic technologies is projected in this Review. The design challenges associated with these electronic devices are also discussed, and possible solutions are presented.

Unleashing the Potential of MXene-Based Flexible Materials for High-Performance Energy Storage Devices

Since the initial discovery of Ti3 C2 a decade ago, there has been a significant surge of interest in 2D MXenes and MXene-based composites. This can be attributed to the remarkable intrinsic properties exhibited by MXenes, including metallic conductivity, abundant functional groups, unique layered microstructure, and the ability to control interlayer spacing. These properties contribute to the exceptional electrical and mechanical performance of MXenes, rendering them highly suitable for implementation as candidate materials in flexible and wearable energy storage devices. Recently, a substantial number of novel research has been dedicated to exploring MXene-based flexible materials with diverse functionalities and specifically designed structures, aiming to enhance the efficiency of energy storage systems. In this review, a comprehensive overview of the synthesis and fabrication strategies employed in the development of these diverse MXene-based materials is provided. Furthermore, an in-depth analysis of the energy storage applications exhibited by these innovative flexible materials, encompassing supercapacitors, Li-ion batteries, Li-S batteries, and other potential avenues, is conducted. In addition to presenting the current state of the field, the challenges encountered in the implementation of MXene-based flexible materials are also highlighted and insights are provided into future research directions and prospects.

Printed High-Adhesion Flexible Electrodes Based on an Interlocking Structure for Self-Powered Intelligent Movement Monitoring

Two-dimensional transition metal carbide nitrides (MXenes) have been extensively explored in diverse areas, such as electrochemical energy storage and flexible electronics. Although the solution-processed MXene-based device has made significant achievements, it is still a challenge to develop large-scale and high-resolution printing methods for flexible printed electronics. In this work, we reported a novel strategy of a porous interlocking structure to obtain flexible MXene/laser-induced graphene (LMX) composite electrodes with enhanced adhesion and high printing resolution. In comparison to traditional printed MXene electrodes, the LMX electrode with an interlocking interface possesses enhanced mechanical properties (adhesive strength of 2.17 MPa) and comparable electrical properties (0.68 S/mm). Furthermore, owing to the outstanding stability and flexibility, the LMX-based triboelectric nanogenerator (TENG) can be used as a self-powered sensor to monitor finger-bending movement. A support vector machine (SVM)-assisted self-powered motion sensor can distinguish the bending angle with high recognition accuracy and can effectively identify different angles. The successful experience of directly printing flexible electrodes with excellent mechanical and electrical properties can be promoted to other solution-processed two-dimensional materials. Our strategy opens up a promising perspective to develop flexible and printed electronics.

Flexible Transparent Conductive Electrodes: Unveiling Growth Mechanisms, Material Dimensions, Fabrication Methods, and Design Strategies

Flexible transparent conductive electrodes (FTCEs) constitute an indispensable component in state-of-the-art electronic devices, such as wearable flexible sensors, flexible displays, artificial skin, and biomedical devices, etc. This review paper offers a comprehensive overview of the fabrication techniques, growth modes, material dimensions, design, and their impacts on FTCEs fabrication. The growth modes, such as the "Stranski-Krastanov growth," "Frank-van der Merwe growth," and "Volmer-Weber growth" modes provide flexibility in fabricating FTCEs. Application of different materials including 0D, 1D, 2D, polymer composites, conductive oxides, and hybrid materials in FTCE fabrication, emphasizing their suitability in flexible devices are discussed. This review also delves into the design strategies of FTCEs, including microgrids, nanotroughs, nanomesh, nanowires network, and "kirigami"-inspired patterns, etc. The pros and cons associated with these materials and designs are also addressed appropriately. Considerations such as trade-offs between electrical conductivity and optical transparency or "figure of merit (FoM)," "strain engineering," "work function," and "haze" are also discussed briefly. Finally, this review outlines the challenges and opportunities in the current and future development of FTCEs for flexible electronics, including the improved trade-offs between optoelectronic parameters, novel materials development, mechanical stability, reproducibility, scalability, and durability enhancement, safety, biocompatibility, etc.

Effective Surface Modification of 2D MXene toward Thermal Energy Conversion and Management

Thermal energy management is a crucial aspect of many research developments, such as hybrid and soft electronics, aerospace, and electric vehicles. The selection of materials is of critical importance in these applications to manage thermal energy effectively. From this perspective, MXene, a new type of 2D material, has attracted considerable attention in thermal energy management, including thermal conduction and conversion, owing to its unique electrical and thermal properties. However, tailored surface modification of 2D MXenes is required to meet the application requirements or overcome specific limitations. Herein, a comprehensive review of surface modification of 2D MXenes for thermal energy management is discussed. First, this work discusses the current progress in the surface modification of 2D MXenes, including termination with functional groups, small-molecule organic compound functionalization, and polymer modification and composites. Subsequently, an in situ analysis of surface-modified 2D MXenes is presented. This is followed by an overview of the recent progress in the thermal energy management of 2D MXenes and their composites, such as Joule heating, heat dissipation, thermoelectric energy conversion, and photothermal conversion. Finally, some challenges facing the application of 2D MXenes are discussed, and an outlook on surface-modified 2D MXenes is provided.

Material jetting of carbon nano onions for printed electronics

As an additive manufacturing process, material jetting techniques allow to selectively deposit droplets of materials in liquid or powder form through a small-diameter aperture, such as a nozzle of a print head. For the fabrication of printed electronics, a variety of inks and dispersions of functional materials can be deposited by drop-on-demand printing on rigid and flexible substrates. In this work, zero-dimensional multi-layer shell-structured fullerene material, also known as carbon nano-onion (CNO) or onion-like carbon, is printed on polyethylene terephthalate substrates using drop-on-demand inkjet printing. CNOs are produced using a low-cost flame synthesis technique and characterized by electron microscopy, Raman, x-ray photoelectron spectroscopy, and specific surface area and pore size measurements. The produced CNO material has an average diameter of ∼33 nm, pore diameter in the range ∼2-40 nm and a specific surface area of 160 m2.g-1. The CNO dispersions in ethanol have a reduced viscosity (∼1.2 mPa.s) and are compatible with commercial piezoelectric inkjet heads. The jetting parameters are optimized to avoid satellite drops and to obtain a reduced drop volume (52 pL), resulting in optimal resolution (220μm) and line continuity. A multi-step process is implemented without inter-layer curing and a fine control over the CNO layer thickness is achieved (∼180 nm thick layer after 10 printing passes). The printed CNO structures show an electrical resistivity of ∼600 Ω.m, a high negative temperature coefficient of resistance (-4.35 × 10-2°C-1) and a marked dependency on relative humidity (-1.29 × 10-2RH%-1). The high sensitivity to temperature and humidity, combined to the large specific area of the CNOs, make this material and the corresponding ink a viable prospect for inkjet-printed technologies, such as environmental and gas sensors.

MXene-Based Nanocomposites for Piezoelectric and Triboelectric Energy Harvesting Applications

Due to its superior advantages in terms of electronegativity, metallic conductivity, mechanical flexibility, customizable surface chemistry, etc., 2D MXenes for nanogenerators have demonstrated significant progress. In order to push scientific design strategies for the practical application of nanogenerators from the viewpoints of the basic aspect and recent advancements, this systematic review covers the most recent developments of MXenes for nanogenerators in its first section. In the second section, the importance of renewable energy and an introduction to nanogenerators, major classifications, and their working principles are discussed. At the end of this section, various materials used for energy harvesting and frequent combos of MXene with other active materials are described in detail together with the essential framework of nanogenerators. In the third, fourth, and fifth sections, the materials used for nanogenerators, MXene synthesis along with its properties, and MXene nanocomposites with polymeric materials are discussed in detail with the recent progress and challenges for their use in nanogenerator applications. In the sixth section, a thorough discussion of the design strategies and internal improvement mechanisms of MXenes and the composite materials for nanogenerators with 3D printing technologies are presented. Finally, we summarize the key points discussed throughout this review and discuss some thoughts on potential approaches for nanocomposite materials based on MXenes that could be used in nanogenerators for better performance.

Recent progress in energy, environment, and electronic applications of MXene nanomaterials

Since the discovery of graphene, two-dimensional (2D) materials have gained widespread attention, owing to their appealing properties for various technological applications. Etched from their parent MAX phases, MXene is a newly emerged 2D material that was first reported in 2011. Since then, a lot of theoretical and experimental work has been done on more than 30 MXene structures for various applications. Given this, in the present review, we have tried to cover the multidisciplinary aspects of MXene including its structures, synthesis methods, and electronic, mechanical, optoelectronic, and magnetic properties. From an application point of view, we explore MXene-based supercapacitors, gas sensors, strain sensors, biosensors, electromagnetic interference shielding, microwave absorption, memristors, and artificial synaptic devices. Also, the impact of MXene-based materials on the characteristics of respective applications is systematically explored. This review provides the current status of MXene nanomaterials for various applications and possible future developments in this field.

A review on accelerated development of skin-like MXene electrodes: from experimental to machine learning

Foreshadowing future needs has catapulted the progress of skin-like electronic devices for human-machine interactions. These devices possess human skin-like properties such as stretchability, self-healability, transparency, biocompatibility, and wearability. This review highlights the recent progress in a promising material, MXenes, to realize soft, deformable, skin-like electrodes. Various structural designs, fabrication strategies, and rational guidelines adopted to realize MXene-based skin-like electrodes are outlined. We explicitly discussed machine learning-based material informatics to understand and predict the properties of MXenes. Finally, an outlook on the existing challenges and the future roadmap to realize soft skin-like MXene electrodes to facilitate technological advances in the next-generation human-machine interactions has been described.

Free-standing reduced graphene oxide/carboxymethylcellulose-polyaniline (RGO/CMC-PANI) hybrid film electrode for high-performance asymmetric supercapacitor device

This work demonstrates a facile and effective strategy for the preparation of a reduced graphene oxide/carboxymethylcellulose-polyaniline (RGO/CMC-PANI) hybrid film electrode. Specifically, through the hydrogen bonding interaction between -OH of CMC molecules and -NH₂ of aniline monomer, PANI grows in an ordered manner on the surface of CMC, which effectively alleviates the structural collapse of PANI during the continuous charge/discharge process. After compounding with RGO, CMC-PANI bridges adjacent RGO sheets to form a complete conductive path, and opens the gap between RGO sheet layers to obtain fast ion channels. As a result, the RGO/CMC-PANI electrode exhibits excellent electrochemical performance. Moreover, an asymmetric supercapacitor was fabricated using RGO/CMC-PANI as the anode and Ti₃C₂T_x as the cathode. The results show that the device has a large specific capacitance of 450 mF cm^-2 (81.8 F g^-1) at 1 mA cm^-2 and a high energy density of 140.6 μWh cm^-2 at a power density of 749.9 μW cm^-2. Besides, 87.3 % initial capacitance and 100 % good coulombic efficiency can be maintained even after 20,000 GCD cycles. Therefore, the device has a broad application prospect in the field of new-generation microelectronic energy storage.

Bis(2-hydroxyethyl) Terephthalate-Modified Ti3C2Tx/Graphene Nanohybrids as Three-Dimensional Functional Chain Extenders for Polyurethane Composite Films with Strain-Sensing and Conductive Properties

Incorporation of functional nanofillers can unlock the potential of polymers as advanced materials. Herein, single-layered and three-dimensional reduced graphene oxide (rGO)/Ti₃C₂T_x (B-rGO@Ti₃C₂T_x) nanohybrids were constructed using bis(2-hydroxyethyl) terephthalate (BHET) as a coupling agent between rGO and Ti₃C₂T_x through covalent and hydrogen bonds. It is found that BHET can not only resist the weak oxidization of Ti₃C₂T_x to some degree but also prevent the self-stacking of Ti₃C₂T_x and rGO sheets. Then, B-rGO@Ti₃C₂T_x was used as a functional nanofiller and three-dimensional chain extender for preparing the waterborne polyurethane (WPU) nanocomposite through in situ polymerization. Compared with WPU nanocomposites with an equivalent amount of Ti₃C₂T_x/rGO@Ti₃C₂T_x, although containing an equivalent amount of BHET, WPU/B-rGO@Ti₃C₂T_x nanocomposites show significantly improved performance. For example, 5.66 wt % of B-rGO@Ti₃C₂T_x endows WPU with a high tensile strength of 36.0 MPa (improved by 380%), thermal conductivity of 0.697 W·m^-1·K^-1, electrical conductivity of 1.69 × 10^-2 S/m (enhanced by 39 times), good strain-sensing behavior, electromagnetic interference (EMI)-shielding performance of 49.5 dB in the X-band, and excellent thermal stability. Therefore, the construction of rGO@Ti₃C₂T_x nanohybrids with the aid of chain extenders may unlock new possibilities of polyurethane as smart materials.

Rational Design of Ti3C2Tx MXene Inks for Conductive, Transparent Films

Transparent conductive electrodes (TCEs) with a high figure of merit (FOM_e, defined as the ratio of transmittance to sheet resistance) are crucial for transparent electronic devices, such as touch screens, micro-supercapacitors, and transparent antennas. Two-dimensional (2D) titanium carbide (Ti₃C₂T_x), known as MXene, possesses metallic conductivity and a hydrophilic surface, suggesting dispersion stability of MXenes in aqueous media allowing the fabrication of MXene-based TCEs by solution processing. However, achieving high FOM_e MXene TCEs has been hindered mainly due to the low intrinsic conductivity caused by percolation problems. Here, we have managed to resolve these problems by (1) using large-sized Ti₃C₂T_x flakes (∼12.2 μm) to reduce interflake resistance and (2) constructing compact microstructures by blade coating. Consequently, excellent optoelectronic properties have been achieved in the blade-coated Ti₃C₂T_x films, i.e., a DC conductivity of 19 325 S cm^-1 at transmittances of 83.4% (≈6.7 nm) was obtained. We also demonstrate the applications of Ti₃C₂T_x TCEs in transparent Joule heaters and the field of supercapacitors, showing an outstanding Joule heating effect and high rate response, respectively, suggesting enormous potential applications in flexible, transparent electronic devices.

In Situ Growth of Ni-MOF Nanorods Array on Ti3C2Tx Nanosheets for Supercapacitive Electrodes

For the energy supply of smart and portable equipment, high performance supercapacitor electrode materials are drawing more and more concerns. Conductive Ni-MOF is a class of materials with higher conductivity compared with traditional MOFs, but it continues to lack stability. Specifically, MXene (Ti₃C₂T_x) has been employed as an electrochemical substrate for its high mechanical stability and abundant active sites, which can be combined with MOFs to improve its electrochemical performance. In this paper, a novel Ni-MOF nanorods array/Ti₃C₂T_x nanocomposite was prepared via a facile hydrothermal reaction, which makes good use of the advantages of conductive Ni-MOF and high strength Ti₃C₂T_x. The high density forest-like Ni-MOF array in situ grown on the surface of Ti₃C₂T_x can provide abundant active electrochemical sites and construct a pathway for effective ion transport. The formation of a "Ti-O···Ni" bond accomplished during an in situ growth reaction endows the strong interfacial interaction between Ni-MOF and Ti₃C₂T_x. As a result, the Ni-MOF/Ti₃C₂T_x nanocomposite can achieve a high specific capacitance of 497.6 F·g^-1 at 0.5 A·g^-1 and remain over 66% of the initial capacitance when the current density increases five times. In addition, the influence of the Ti₃C₂T_x concentration and reaction time on the morphology and performance of the resultant products were also investigated, leading to a good understanding of the formation process of the nanocomposite and the electrochemical mechanism for a supercapacitive reaction.

2D Titanium carbide printed flexible ultrawideband monopole antenna for wireless communications

Flexible titanium carbide (Ti₃C₂) antenna offers a breakthrough in the penetration of information communications for the spread of Internet of Things (IoT) applications. Current configurations are constrained to multi-layer complicated designs due to the limited conformal integration of the dielectric substrate and additive-free Ti₃C₂ inks. Here, we report the flexible ultrawideband Ti₃C₂ monopole antenna by combining strategies of interfacial modification and advanced extrusion printing technology. The polydopamine, as molecular glue nano-binder, contributes the tight adhesion interactions between Ti₃C₂ film and commercial circuit boards for high spatial uniformity and mechanical flexibility. The bandwidth and center frequency of Ti₃C₂ antenna can be well maintained and the gain differences fluctuate within ±0.2 dBi at the low frequency range after the bent antenna returns to the flat state, which conquers the traditional inelastic Cu antenna. It also achieves the demo instance for the fluent and stable real-time wireless transmission in bending states.

MXene-Based Ink Design for Printed Applications

MXenes are a class of two-dimensional nanomaterials with a rich chemistry, hydrophilic surface and mechano-ceramic nature, and have been employed in a wide variety of applications ranging from medical and sensing devises to electronics, supercapacitors, electromagnetic shielding, and environmental applications, to name a few. To date, the main focus has mostly been paid to studying the chemical and physical properties of MXenes and MXene-based hybrids, while relatively less attention has been paid to the optimal application forms of these materials. It has been frequently observed that MXenes show great potential as inks when dispersed in solution. The present paper aims to comprehensively review the recent knowledge about the properties, applications and future horizon of inks based on 2D MXene sheets. In terms of the layout of the current paper, 2D MXenes have briefly been presented and followed by introducing the formulation of MXene inks, the process of turning MAX to MXene, and ink compositions and preparations. The chemical, tribological and rheological properties have been deeply discussed with an eye to the recent developments of the MXene inks in energy, health and sensing applications. The review ends with a summary of research pitfalls, challenges, and future directions in this area.

Nature-Inspired 3D Spiral Grass Structured Graphene Quantum Dots/MXene Nanohybrids with Exceptional Photothermal-Driven Pseudo-Capacitance Improvement

Solar-thermal conversion is considered as a green and simple means to improve the performance of energy storage materials, but often limited by the intrinsic photothermal properties of materials and crude structure design. Herein, inspired by the unique light trapping effect of wide leaf spiral grass during photosynthesis, a biomimetic structural photothermal energy storage system is developed, to further promote the solar thermal-driven pseudo capacitance improvement. In this system, three-dimensional printed tortional Kelvin cell arrays structure with interesting light trapping property functions as "spiral leaf blades" to improve the efficiency of light absorption, while graphene quantum dots/MXene nanohybrids with wide photothermal response range and strong electrochemical activity serve as "chloroplast" for photothermal conversion and energy storage. As expected, the biomimetic structure-enhanced photothermal supercapacitor achieves an ideal solar thermal-driven pseudo capacitance enhancement (up to 304%), an ultrahigh areal capacitance of 10.47 F cm^-2 , remarkable photothermal response (surface temperature change of 50.1 °C), excellent energy density (1.18 mWh cm^-2 ) and cycling stability (10000 cycles). This work not only offers a novel enhancement strategy for photothermal applications, but also inspires new structure designs for multifunctional energy storage and conversion devices.

All-Printed Flexible Memristor with Metal–Non-Metal-Doped TiO2 Nanoparticle Thin Films

A memristor is a fundamental electronic device that operates like a biological synapse and is considered as the solution of classical von Neumann computers. Here, a fully printed and flexible memristor is fabricated by depositing a thin film of metal–non-metal (chromium-nitrogen)-doped titanium dioxide (TiO₂). The resulting device exhibited enhanced performance with self-rectifying and forming free bipolar switching behavior. Doping was performed to bring stability in the performance of the memristor by controlling the defects and impurity levels. The forming free memristor exhibited characteristic behavior of bipolar resistive switching with a high on/off ratio (2.5 × 10³), high endurance (500 cycles), long retention time (5 × 10³ s) and low operating voltage (±1 V). Doping the thin film of TiO₂ with metal–non-metal had a significant effect on the switching properties and conduction mechanism as it directly affected the energy bandgap by lowering it from 3.2 eV to 2.76 eV. Doping enhanced the mobility of charge carriers and eased the process of filament formation by suppressing its randomness between electrodes under the applied electric field. Furthermore, metal–non-metal-doped TiO₂ thin film exhibited less switching current and improved non-linearity by controlling the surface defects.

Flexible MXene-Based Composite Films: Synthesis, Modification, and Applications as Electrodes of Supercapacitors

MXenes, as a 2D planar structure nanomaterial, were first reported in 2011. Due to their large specific surface area, high ductility, high electrical conductivity, strong hydrophilic surface, and high mechanical flexibility, MXenes have been extensively explored in the development of various functional materials with desired performances. This review is aimed to summarize the current progress in synthesis, modification, and applications of MXene-based composite films as electrode materials of flexible energy storage devices. In the synthesis of MXenes, the evolution and exploration of etchants are emphasized. Furthermore, in order to develop MXene-based composite films, the components used to modify the MXene nanoflakes, including 0D, 1D, and 2D nanomaterials, are summarized, and the perspectives and research direction of such materials are also discussed.

Advances and emerging challenges in MXenes and their nanocomposites for biosensing applications

Two-dimensional materials have unique properties and their better functionality has created new paradigms in the field of sensing. Over the past decade, a new family of 2D materials known as MXenes has emerged as a promising material for numerous applications, including biosensing. Their metallic conductivity, rich surface chemistry, hydrophilicity, good biocompatibility, and high anchoring capacity for biomaterials make them an attractive candidate to detect a variety of analytes. Despite such notable properties, there are certain limitations associated with them. This review aims to present a detailed survey of MXene's synthesis; in particular, their superiority in the field of biosensing as compared to other 2D materials is addressed. Their low oxidative stability is still an open challenge, and recent investigations on MXene's oxidation are summarized. The hexagonal stacking network of MXenes acts as a distinctive matrix to load nanoparticles, and the embedded nanoparticles can bind an excess number of biomolecules (e.g., antibodies) thereby improving biosensor performance. We will also discuss the synthesis and corresponding performance of MXenes nanocomposites with noble metal nanoparticles and magnetic nanoparticles. Furthermore, Nb and Ti2C-based MXenes, and Ti3C2-MXene sandwich immunoassays are also reviewed in view of their importance. Different aspects and challenges associated with MXenes (from their synthesis to final applications) and the future perspectives described give new directions to fabricate novel biosensors.

Environmentally stable MXene ink for direct writing flexible electronics

MXene inks are promising candidates for fabricating conductive circuits and flexible devices. Here, MXene inks prepared from solvent mixtures demonstrate long-term stability and can be employed in commercial rollerball pens to write electronic circuits on flexible substrates. Such circuits exhibit a fast and accurate capacitive response for touch-boards and water level measurement, indicating the excellent potential of these MXene inks in electrical device fabrication.

Inkjet-Printed Silver Nanowire Ink for Flexible Transparent Conductive Film Applications

The development of flexible transparent conductive electrodes has been considered as a key issue in realizing flexible functional electronics. Inkjet printing provides a new opportunity for the manufacture of FFE due to simple process, cost-effective, environmental friendliness, and digital method to circuit pattern. However, obtaining high concentration of inkjet- printed silver nanowires (AgNWs) conductive ink is a great challenge because the high aspect ratio of AgNWs makes it easy to block the jetting nozzle. This study provides an inkjet printing AgNWs conductive ink with low viscosity and high concentration of AgNWs and good printing applicability, especially without nozzle blockage after printing for more than 4 h. We discussed the effects of the components of the ink on surface tension, viscosity, contact angle as well as droplet spreading behavior. Under the optimized process and formulation of ink, flexible transparent conductive electrode with a sheet resistance of 32 Ω·sq-1-291 nm·sq-1 and a transmittancy at 550 nm of 72.5-86.3% is achieved. We investigated the relationship between the printing layer and the sheet resistance and the stability of the sheet resistance under a bending test as well as the infrared thermal response of the AgNWs-based flexible transparent conductive electrode. We successfully printed the coupling electrodes and demonstrated the excellent potential of inkjet-printed AgNWs-based flexible transparent conductive electrode for developing flexible functional electronics.

A Simple Approach to MXene Micropatterning from Molecularly Driven Assembly

Here, a micropatterning strategy is demonstrated to achieve stable and selective MXene adsorption through the molecularly driven assembly. MXene flakes were assembled by strong interaction with a silicon substrate, which was functionalized by microcontact printing (μCP) to create an active surface. A clear micropattern was observed by scanning electron microscopy showing uniform coverage of MXene flakes. Atomic force microscopy revealed a pattern thickness of around 50 nm, much thinner than the patterns obtained by direct μCP. The obtained micropattern presents good stability against rinsing and sonication. X-ray photoelectron spectroscopy shows that this stability can be attributed to strong covalent bonding between MXene and active molecules on a silicon substrate. The sheet resistance of the as-formed MXene layer was measured at around 154.67 (Ω/□), which is lower than those of other published techniques with a similar thickness of around 50 nm. This method can achieve a well-defined MXene pattern around the sub-100 μm scale without requiring prior MXene surface modification. Therefore, MXene can retain its intrinsic surface property, allowing further molecule adsorption as a sensing platform. Moreover, this patterning technique does not require complicated control of ink preparation and offers possible application on a substrate of any geometry with few layers of thickness.

Ink Formulation and Printing Parameters for Inkjet Printing of Two Dimensional Materials: A Mini Review

Inkjet printing of two-dimensional (2D) material has been a center of interest for wearable electronics and has become a promising platform for next-generation technologies. Despite the enormous progress made in printed 2D materials, there are still challenges in finding the optimal printing conditions involving the ink formulation and printing parameters. Adequate ink formulation and printing parameters for target 2D materials rely on empirical studies and repeated trials. Therefore, it is essential to compile promising strategies for ink formulation and printing parameters. In this context, this review discusses the optimal ink formulations to prepare stable ink and steady ink jetting and then explores the critical printing parameters for fabricating printed 2D materials of a high quality. The summary and future prospects for inkjet-printed 2D materials are also addressed.