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

3D-Printing of Freestanding Pure MXene Microarchitectures

Since their discovery, titanium-based MXenes (Ti3C2Tx) have attracted significant attention. Several studies have presented versatile, cost-effective, and scalable approaches for fabricating Ti3C2Tx-based functional components. However, most previous studies only allowed the realization of 2D patterns or required diverse additives to produce 3D architectures. Herein, a 3D-Printing approach for producing 3D microarchitectures composed entirely of Ti3C2Tx. Ti3C2Tx additive-free aqueous ink consists of 0.1 wt.% Ti3C2Tx nanosheets is proposed. The diameter (ds) of the printed Ti3C2Tx 3D microarchitectures can be determined by controlling the meniscus channel size, which is influenced by the diameter (dp) of the micropipette opening and pipette-pulling rate (v). Through optimized control of the pipette, a minimum ds of 1.3 µm is obtained, and complex shapes such as zigzag, helix, bridge, and pyramid shapes can be implemented. To demonstrate the feasibility of realizing functional Ti3C2Tx 3D components, three electrical components are demonstrated: 3D micro-interconnects and 3D transducers for photodetectors and humidity sensors. It is believed that this facile approach can be used for nano 3D-Printing as well as micro printing of Ti3C2Tx architectures.

External Electric Field Enhanced Ti3C2 MXene Surface Passivation for Realizing Ultra-Long Cycling Stability

External electric field (EEF), as a stimulating factor, is an effective method for optimizing the surface composition and structure of materials. Ti3C2 MXene surface enriched with negatively charged functional groups (─OH, ─O, etc.) will exhibit high sensitivity to EEF. However, the impact of EEF on the interaction mechanisms between the guest ions and MXene surface remains unclear and requires further investigation. Herein, the density functional theory (DFT) is employed to simulate the adsorption energies between butyl trimethylammonium ion (BTA+) and MXene surfaces under different intensities of EEFs (±0.9, ±0.7, ±0.5, ±0.3, ±0.1, and 0 V Å-1), indicating EEF can effectively regulate adsorption. It will increase the encapsulation degree of BTA+ on the MXene surface, thereby enhancing surface passivation. Based on theoretical predictions, quaternary-ammonium ions with different chain-lengths (BTA+, DTA+, STA+) are selected as guest ions to unveil the mechanism of EEF on MXene surface passivation. The applied-EEF promotes the formation of Ti─O─N bonds between ─OH and ammonium groups to construct more-denser protective layer, leading to enhancement of surface passivation and obviously increasing the capacitance retention after 100,000 cycles (50.8% to 97.5%). This work provides a new pathway and theoretical support for the surface passivation of MXene.

Highly Electrically Conductive PEDOT:PSS Films via Layer-By-Layer Electrostatic Self-Assembly

Electrically conductive films of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) are usually formed by spin coating of aqueous dispersions with PEDOT:PSS nanoparticles. To better understand the film formation, the adsorption conditions are investigated using dip coating and a flow cell with different flow rates. Multilayer films are formed by sequential adsorption of oppositely charged macromolecules or nanoparticles. PEDOT:PSS serves as polyanion, and PDADMA is the polycation. In the dip coating process, the first layer consists of a ≈70 nm thick PEDOT:PSS nanoparticle monolayer. Subsequent PDADMA/PEDOT:PSS bilayers have a constant thickness (9.5 nm). Using the flow cell (0.2 mL/min) for film preparation led to constant PDADMA/PEDOT:PSS bilayer thickness (7.5 nm). PEDOT:PSS nanoparticle monolayers were only observed after PEDOT:PSS adsorption when the washing step was omitted. The electrical conductivity is independent of the number of deposition cycles for both preparation methods. Films prepared by dip coating show low conductivity (26 kS/m) and high surface roughness, whereas films prepared by flow cell show high conductivity (230 kS/m) and low roughness (2-4 nm). We propose that the adsorption in a flow cell leads to a flat orientation of the PEDOT molecules, which increases charge carrier mobility. It is hoped that a better understanding of the relationship between adsorption conditions and carrier mobility will further improve electrical conductivity.

Highly Active MXene Quantum Dots/CuSe n-p Plasmonic Heterostructures for Ultrafast Photocatalytic Removal of Cr(VI) under Full Solar Spectrum

Identifying effective plasmonic photocatalysts exhibiting robust activities across the entire solar spectrum poses a significant challenge. CuSe, with its local surface plasmon resonance (LSPR) effect, has garnered attention as a prospective plasmonic photocatalyst. However, severe charge recombination and insufficient light absorption limit its photocatalytic performance. To enhance the performance, constructing CuSe-based n-p plasmonic semiconductor heterostructures is a potential strategy. MXene quantum dots (MQDs), a kind of n-type plasmonic semiconductor with metallic conductivity and a high LSPR effect, are a promising candidate to couple with p-type CuSe. According to the complementary principle, we designed a 0D/2D MQDs/CuSe n-p plasmonic semiconductor, achieved by wrapping CuSe nanosheets with MQDs. This n-p plasmonic heterostructure exhibits a synergistic effect on an enhanced electronic field, facilitating charge transfer and separation, thereby enhancing charge excitation, carrier migration, and photothermal effect. Furthermore, optimizing the MQD loading content leads to an ultrafast photocatalytic reaction rate, achieving 100% Cr(VI) reduction efficiency within just 60 min with a reaction kinetics of 0.069 min-1, surpassing the performance of bare CuSe. Our work presents a promising approach for developing advanced n-p plasmonic heterostructures based on MQDs for wastewater treatment and other photocatalytic applications.

On-Demand MXene-Coupled Pyroelectricity for Advanced Breathing Sensors and IR Data Receivers

MXene-inspired two-dimensional (2D) materials like Ti3C2Tx are widely known for their versatile properties, including surface plasmon, higher electrical conductivity, exceptional in-plane tensile strength, EMI shielding, and IR thermal properties. The MXene nanosheets coupled poly(vinylidene fluoride) (PVDF) nanofibers with d33 ∼-26 pm V-1 are able to capture the smaller thermal fluctuation due to a superior pyroelectric coefficient of ∼130 nC m-2 K-1 with an improved (∼7 times with respect to neat PVDF nanofibers) pyroelectric current figure of merit (FOMi). The significant enhancement of the pyroelectric response is attributed to the confinement effect of 2D MXene (Ti3C2Tx) nanosheets within PVDF nanofibers, as evidenced from polarized Fourier transform infrared (FTIR) spectroscopy and scanning probe microscopy (SPM). In subsequent studies, the practical applications of self-powered pyroelectric sensors of MXene-PVDF have been demonstrated. The fabricated flexible, hydrophobic pyroelectric sensor could be utilized as an excellent pyroelectric breathing sensor, a proximity sensor, and an IR data transmission receiver. Further, supervised machine learning algorithms are proposed to distinguish different types of breathing signals with ∼98% accuracy for healthcare monitoring purposes.

Pillaring Behavior of Organic Molecules on MXene: Insights from Molecular Dynamics Simulations

Pillaring MXene with organic molecules is an effective approach to expand the interlayer spacing and increase the accessible surface area for enhanced performance in energy storage applications. Herein, molecular dynamics simulations are employed to explore the pillaring effect of six organic molecules on Ti3C2O2. The interlayer spacing and structural characteristics of MXene after the insertion of different organic molecules are examined, and the influence of the type and quantity of organic molecules on the pillared MXene structure is systematically investigated. The results demonstrate that the inserted molecules are influenced by interactions between MXene layers, resulting in a thinner morphology. Effective pillar support on MXene is achieved only when a specific quantity of organic molecules is inserted between the layers. Furthermore, different organic molecules occupy distinct surface areas on MXene when acting as pillars. Pillaring molecules with a Pi-conjugated ring structure require a larger surface area on MXene, whereas those with a branched structure occupy a smaller surface area. Additionally, organic molecules containing oxygen functional groups tend to aggregate due to hydrogen bonding, impeding their diffusion within MXene sheets. Considering the interlayer expansion of MXene, surface area occupation, and diffusion characteristics, the isopropylamine demonstrates the most favorable pillaring effect on MXene. These findings provide valuable insights into the design and application of pillared MXenes in energy storage and other applications. Further studies on the properties and applications of the optimized pillared MXene structures will be conducted in the future.

Toward Smart Sensing by MXene

The Internet of Things era has promoted enormous research on sensors, communications, data fusion, and actuators. Among them, sensors are a prerequisite for acquiring the environmental information for delivering to an artificial data center to make decisions. The MXene-based sensors have aroused tremendous interest because of their extraordinary performances. In this review, the electrical, electronic, and optical properties of MXenes are first introduced. Next, the MXene-based sensors are discussed according to the sensing mechanisms such as electronic, electrochemical, and optical methods. Initially, biosensors are introduced based on chemiresistors and field-effect transistors. Besides, the wearable pressure sensor is demonstrated with piezoresistive devices. Third, the electrochemical methods include amperometry and electrochemiluminescence as examples. In addition, the optical approaches refer to surface plasmonic resonance and fluorescence resonance energy transfer. Moreover, the prospects are delivered of multimodal data fusion toward complicated human-like senses. Eventually, future opportunities for MXene research are conveyed in the new material discovery, structure design, and proof-of-concept devices.

Poly(ionic liquid)-Armored MXene Membrane: Interlayer Engineering for Facilitated Water Transport

Two-dimensional (2D) MXene-based lamellar membranes bearing interlayers of tunable hydrophilicity are promising for high-performance water purification. The current challenge lies in how to engineer the pore wall's surface properties in the subnano-confinement environment while ensuring its high selectivity. Herein, poly(ionic liquid)s, equipped with readily exchangeable counter anions, succeeded as a hydrophilicity modifier in addressing this issue. Lamellar membranes bearing nanochannels of tailorable hydrophilicity are constructed via assembly of poly(ionic liquid)-armored MXene nanosheets. By shifting the interlayer galleries from being hydrophilic to more hydrophobic via simple anion exchange, the MXene membrane performs drastically better for both the permeance (by two-fold improvement) and rejection (≈99 %). This facile method opens up a new avenue for building 2D material-based membranes of enhancing molecular transport and sieving effect.

Layer-by-Layer Materials for the Fabrication of Devices with Electrochemical Applications

The construction of nanostructured materials for their application in electrochemical processes, e.g., energy storage and conversion, or sensing, has undergone a spectacular development over the last decades as a consequence of their unique properties in comparison to those of their bulk counterparts, e.g., large surface area and facilitated charge/mass transport pathways. This has driven strong research on the optimization of nanostructured materials for the fabrication of electrochemical devices, which demands techniques allowing the assembly of hybrid materials with well-controlled structures and properties. The Layer-by-Layer (LbL) method is well suited for fulfilling the requirements associated with the fabrication of devices for electrochemical applications, enabling the fabrication of nanomaterials with tunable properties that can be exploited as candidates for their application in fuel cells, batteries, electrochromic devices, solar cells, and sensors. This review provides an updated discussion of some of the most recent advances on the application of the LbL method for the fabrication of nanomaterials that can be exploited in the design of novel electrochemical devices.

Recent Trends in Synthesis and Applications of porous MXene Assemblies: A Topical Review

MXene possesses high conductivity, excellent hydrophilicity, rich surface chemistry, hence holds great potential in various applications. However, MXene materials have low surface area utilization due to the agglomeration of ultrathin nanosheets. Assembling 2D MXene nanosheets into 3D multi-level architectures is an effective way to circumvent this issue. Incorporation of MXene with other nanomaterials during the assembly process could rationally tune and tailor the specific surface area, porosity and surface chemistry of the MXene assemblies. The complementary and synergistic effect between MXene and nanomaterials could expand their advantages and make up for their disadvantages, thus boost the performance of 3D porous MXene composites. Herein, we summarize the recent progress in fabrication of porous MXene architectures from 2D to 3D, and also discuss the potential applications of MXene nanostructures in energy harvesting systems, sensing, electromagnetic interference shielding, water purification and photocatalysis.