Optically Active MXenes in Van der Waals Heterostructures

Modulating cell adhesion and infiltration in advanced scaffold designs based on PLLA fibers with rGO and MXene (Ti3C2T x )

The development of electrospun scaffolds that support cell adhesion and infiltration remains a critical challenge in tissue engineering. In this study, we investigate the influence of two-dimensional (2D) fillers-reduced graphene oxide (rGO) and MXene (Ti3C2T x )-incorporated into poly(L-lactic acid) (PLLA) electrospun fibers on their properties and osteoblast responses. The presence of fillers modified fiber arrangement and created varying inter-fiber spacing due to surface charge repulsion and agglomeration. Importantly, surface potential measurements via Kelvin probe force microscopy (KPFM) of PLLA fibers show a significant shift caused by the incorporation of Ti3C2T x to ∼400 mV compared to ∼50 mV for rGO. In vitro tests indicate that rGO-modified scaffolds support osteoblast infiltration up to ∼100 μm, unlike PLLA fibers, which limit cell infiltration to a maximum of ∼70 μm. However, Ti3C2T x promotes even deeper (∼120 μm) and more uniform cell's infiltration due to changes in scaffold architecture. High-resolution confocal imaging confirmed that PLLA-Ti3C2T x fosters larger, elongated adhesion site clusters of cells, whereas rGO increases cell's adhesion site density in relation to PLLA scaffolds without any filler. Our findings highlight the distinct roles of rGO and Ti3C2T x in modulating scaffold geometry, mechanical behavior, and cellular interactions. Tailoring the composition and distribution of conductive fillers in fibers offers a promising strategy for optimizing scaffold performance in tissue engineering applications.

MXene-Coated Liquid Metal Nanodroplet Aggregates

Combining droplets of liquid metal (LM) with nanomaterials often introduces synergistic thermal or electrical properties that are not found in the constituent materials alone. However, in these existing systems, LM droplets maintain a statistically uniform dispersion and are not capable of self-assembly or aggregation. These composites are limited by their need for high volume fractions of LM (>60 vol %) to achieve high thermal properties, introducing LM leaking as a drawback for thermal management and wearable electronic applications. In this work, we show that coating nanoscale droplets of eutectic gallium-indium (EGaIn) LM with small volume fractions of Ti3C2Tx MXenes (0.25 vol %) results in a unique LM morphology in which droplets self-assemble to form semisolid aggregates. This is accomplished by wrapping MXene sheets around individual LM droplets to create "sticky" particles that form self-assembled aggregates when mixed with a silicone oil. By introducing aggregation as a design parameter in soft LM composites, the thermal and electric resistance of the composite is shown to change dramatically. In contrast to silicone-based composites containing LM droplets or MXene nanosheets alone, these MXene-LM-silicone-based composites exhibit an exponential increase in thermal and electrical conductivity with decreasing interfacial thickness with significantly lower LM volume fractions (25 vol %) while avoiding LM rupture and bleed-out. This could enable more effective composites, reducing the amount of filler material required for thermal interface materials (TIM) and printed electronics.

Unraveling the Role of Interfacial Charge Transfer on Photoactivity and Anomalous Luminescence Quenching of V4C3Tx/Protonated g-C3N4 Heterostructures

Two-dimensional van der Waals heterostructures with exotic quantum phenomena have garnered a huge surge in the field of optoelectronic devices. Herein, we report spectroscopic evidence of efficient interfacial charge transfers at the interface of a novel 2D/2D V4C3Tx MXene/protonated g-C3N4 (PCN) heterostructured thin film, demonstrating robust photosensitivity and a large exciton activation energy of 139.5 meV. Through temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy, we unravel the photophysical mechanism driving efficient charge transfer and photosensitivity in V4C3Tx/PCN heterostructures. These heterostructures exhibit superior photosensitivity to white and UV light compared with either PCN or V4C3Tx pristine materials. Additionally, we observed significant PL quenching with unusual negative thermal quenching and extended charge carrier lifetime in the V4C3Tx/PCN heterostructures across a broad temperature range of 70-370 K. Notably, at the elevated temperature of 370 K, the carrier lifetime was enhanced by more than 2-fold, making the heterostructures promising for optoelectronic applications. This work provides critical insight into the charge transfer mechanism between V4C3Tx MXene and PCN, opening a new avenue for rationally designing g-C3N4-based heterostructures for highly photosensitive optoelectronic devices.

Ti3C2Tx MXene/MoS2 hybrid nanocomposites for synergistic smart corrosion protection of epoxy coatings

MXene nanosheets have recently become a focus of research for corrosion protection due to their two-dimensional, sheet-like structure and distinct physicochemical characteristics. Nevertheless, their susceptibility to restacking and oxidation restricts their practical applications. To address this, the study proposes a custom hybrid structure by growing molybdenum disulfide (MoS2) nanoparticles on the Ti3C2 MXene nanosheets (MX/MS) to prevent oxidation and restacking. This innovative structural design is essential for corrosion-protective coatings, as the sheet-like configuration enhances the barrier properties. The manufacturing of the MX/MS nanoparticles was verified by their characterization employing field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The barrier properties and self-healing functions of the nanoparticle-filled epoxy coatings were evaluated using electrochemical impedance spectroscopy (EIS) and salt spray tests. The epoxy resin including 0.5 wt% MX/MS nanoparticles exhibited outstanding corrosion resistance, with an impedance value (|Z|0.01Hz) of 23.77 GΩ.cm2 after 70 days of immersion. After 48 h of immersion, the coatings also showed a high impedance value (log|Z|0.01Hz = 4.24) and excellent self-healing capabilities in the scratched areas. Additionally, after 42 days, the filled nanohybrid coatings showed the least amount of rust and corrosion product according to salt spray analysis. The results of cathodic delamination and pull-off tests indicated that in comparison to the neat epoxy (11 mm and 70 %), the filled coatings containing the synthesized nanofiller had the lowest cathodic delamination radius (1.7 mm) and lowest adhesion loss (46 %). This study highlights the effectiveness of Ti3C2/MoS2 hybrid in enhancing the anticorrosive performance of organic coatings, offering a novel approach for designing high-performance additives with promising applications in various fields requiring corrosion protection.

Structurally-Modulated Substrate of MXene for Surface-Enhanced Raman Scattering Sensing

The distinctive structure of MXene offers exceptional electron transport properties, abundant surface chemistry, and robust mechanical attributes, thereby bestowing it with remarkable advantages and promising prospects in the realm of surface-enhanced Raman scattering (SERS). This review comprehensively outlines the evolution, synthesis methodologies, and characterization techniques employed for MXene-based SERS substrates. It delves into the intricacies of its SERS enhancement mechanism, substrate variants, and performance metrics, alongside showcasing its diverse applications spanning molecular detection, biosensing, and environmental monitoring. Furthermore, it endeavors to pinpoint the research bottlenecks and chart the future research trajectories for MXene-based SERS substrates.

Two-Dimensional Organic-Inorganic van der Waals Hybrids

Two-dimensional organic-inorganic (2DOI) van der Waals hybrids (vdWhs) have emerged as a groundbreaking subclass of layer-stacked (opto-)electronic materials. The development of 2DOI-vdWhs via systematically integrating inorganic 2D layers with organic 2D crystals at the molecular/atomic scale extends the capabilities of traditional 2D inorganic vdWhs, thanks to their high synthetic flexibility and structural tunability. Constructing an organic-inorganic hybrid interface with atomic precision will unlock new opportunities for generating unique interfacial (opto-)electronic transport properties by combining the strengths of organic and inorganic layers, thus allowing us to satisfy the growing demand for multifunctional applications. Here, this review provides a comprehensive overview of the latest advancements in the chemical synthesis, structural characterization, and numerous applications of 2DOI-vdWhs. Firstly, we introduce the chemistry and the physical properties of the recently rising organic 2D crystals (O2DCs), which feature crystalline 2D nanostructures comprising carbon-rich repeated units linked by covalent/noncovalent bonds and exhibit strong in-plane extended π-conjugation and weak interlayer vdWs interaction. Simultaneously, representative inorganic 2D crystals (I2DCs) are briefly summarized. After that, the synthetic strategies will be systematically summarized, including synthesizing single-component O2DCs with dimensional control and their vdWhs with I2DCs. With these synthetic approaches, the control in the dimension, the stacking modes, and the composition of the 2DOI-vdWhs will be highlighted. Subsequently, a special focus will be given on the discussion of the optical and electronic properties of the single-component 2D materials and their vdWhs, which will be closely relevant to their structures, so that we can establish a general structure-property relationship of 2DOI-vdWhs. In addition to these physical properties, the (opto-)electronic devices such as transistors, photodetectors, sensors, spintronics, and neuromorphic devices as well as energy devices will be discussed. Finally, we provide an outlook to discuss the key challenges for the 2DOI-vdWhs and their future development. This review aims to provide a foundational understanding and inspire further innovation in the development of next-generation 2DOI-vdWhs with transformative technological potential.

Wedge-Like Microstructure of Al2O3/i-Ti3C2Tx Electrode with "Nano-Pumping" Effect for Boosting Ion Diffusion and Electrochemical Defluoridation

Controlled synthesis and regulation of 2D nanomaterials with sufficient active sites are promising in electrochemical fluorine capture, but simultaneously achieving rapid rates and efficient activity of intercalation materials remains challengs. Herein, an integrated strategy of micro-regulation interlayer space and in situ modification of MXenes is proposed to enhance ion storage kinetics. The wedge-like microstructure of aluminum oxide/incomplete-Ti3C2Tx MXene (Al2O3/i-Ti3C2 Tx) is constructed by incomplete etching MAX and in situ derivation of A-layer element, in which the sub-nanoscale interlayer space is conducive to the small size ions intercalation, and the formation of "nanopump-like" effect boosted the ions diffusion. As evidenced by simulation calculations, Al2O3 nanoparticles not only shorten the migration distance of electrons/hydrated ions in interlayers but also contribute a lower adsorption energy barrier, bringing excellent capture kinetics and stability. Benefiting from the interfacial conversion-intercalation pseudocapacitance, such electrode is endowed with a high defluoridation capacity (69.9 mg g-1 at 1.6V) and an outstanding instantaneous adsorption rate (9.51 mg g-1 min-1), and shows satisfactory stability in more than 200 cycles. The physicochemical coupling strategy opens a novel approach to optimizing the interlayer structure and in situ modification interface of MXene, which also provids a universal idea for efficient capture of varisized ions of intercalation materials.

Prospects of Band Structure Engineering in MXenes for Active Switching MXetronics: Computational Insights and Experimental Approaches

MXenes, two-dimensional (2D) transition metal carbides and nitrides, have shown promise in a variety of applications. The use of MXenes in active electronic devices is restricted to electrode materials due to their metallic nature. However, MXenes can be modified to be semiconducting and can be used for next-generation channel materials. The inherent metallic characteristics of pristine Mn+1Xn-structured MXene can be tuned to semiconducting by (i) functionalizing MXenes with different moieties, (ii) applying external strain, and (iii) varying the composition. These strategies effectively modify the metallic electronic structure of MXene into a semiconducting one. This review focuses on the potential of tuning the electronic band structure of MXenes by surface functionalization, strain engineering, and compositional variation. The computational and experimental approaches to tuning the electronic band structure using these strategies are discussed in detail. In addition, the experimental methods which can be used to prepare semiconducting MXenes are described.

Nanoscale Surface and Bulk Electronic Properties of Ti3C2Tx MXene Unraveled by Multimodal X-Ray Spectromicroscopy

2D layered materials, such as transition metal carbides or nitrides, known as MXenes, offer an ideal platform to investigate charge transfer processes in confined environment, relevant for energy conversion and storage applications. Their rich surface chemistry plays an essential role in the pseudocapacitive behavior of MXenes. However, the local distribution of surface functional groups over single flakes and within few- or multilayered flakes remains unclear. In this work, scanning X-ray microscopy (SXM) is introduced with simultaneous transmission and electron yield detection, enabling multimodal nanoscale chemical imaging with bulk and surface sensitivity, respectively, of individual MXene flakes. The Ti chemical bonding environment is found to significantly vary between few-layered hydrofluoric acid-etched Ti3C2Tx MXenes and multilayered molten salt (MS)-etched Ti3C2Tx MXenes. Postmortem analysis of MS-etched Ti3C2Tx electrodes cycled in a Li-ion battery further illustrates that simultaneous bulk and surface chemical imaging using SXM offers a method well adapted to the characterization of the electrode-electrolyte interactions at the nanoscale.

Microwave-Assisted Hydrothermal Synthesis of Photocatalytic Truncated-Bipyramidal TiO2/Ti3CN Heterostructures Derived from Ti3CN MXene

TiO2/MXene heterostructure has garnered significant interest as a photocatalyst due to its large surface area and efficient charge carrier separation at the interface. However, current synthesis methods produce TiO2 without clear crystal faceting and often require complicated postprocessing step, limiting its practical applications. We demonstrate a facile and controlled microwave-assisted hydrothermal synthesis for transforming multilayered Ti3CN MXene to a truncated-bipyramidal TiO2/Ti3CN heterostructure. The resulting TiO2 nanocrystals at the Ti3CN surface exhibited crystalline anatase truncated bipyramids, exposing {001} and {101} facets. We further tailored an indirect optical band gap of the TiO2/Ti3CN heterostructure in the range of 3.17-3.23 eV by varying the hydrothermal synthesis time from 15 min to 5 h at a fixed temperature of 160 °C. Efficient charge separation allowed us to decompose 97% of methylene blue (MB) within 30 min of ultraviolet (UV) light irradiation, ∼3.9-fold faster than the benchmark P25, higher than any other TiO2/MXene heterostructures. With simulated white light, we achieved over 60% efficiency of the dye decomposition within 2 h of irradiation, which resulted in 1.5-fold faster kinetics than P25. We also observed a similar excellent performance of Ti3CN-derived TiO2 in decomposing various persistent synthetic dyes, including commercial textile dye, methyl orange, and rhodamine B. In conclusion, our study provides a strategy for utilizing MXene chemical reactivity to produce highly crystalline optically active TiO2/Ti3CN heterostructure. The developed heterostructure can serve as an efficient photocatalyst for the degradation of organic pollutants.

A Molecular Coordination Strategy for Regulating the Interface of MoS2 Field Effect Transistors

Chemically modifying monolayer two-dimensional transition metal dichalcogenides (TMDs) with organic molecules provides a wide range of possibilities to regulate the electronic and optoelectronic performance of both materials and devices. However, it remains challenging to chemically attach organic molecules to monolayer TMDs without damaging their crystal structures. Herein, we show that the Mo atoms of monolayer MoS2 (1L-MoS2) in defect states can coordinate with both catechol and 1,10-phenanthroline (Phen) groups, affording a facile route to chemically modifying 1L-MoS2. Through the design of two isomeric molecules (LA2 and LA5) comprising catechol and Phen groups, we show that attaching organic molecules to Mo atoms via coordinative bonds has no negative effect on the crystal structure of 1L-MoS2. Both theoretical calculation and experiment results indicate that the coordinative strategy is beneficial for (i) repairing sulfur vacancies and passivating defects; (ii) achieving a long-term and stable n-doping effect; and (iii) facilitating the electron transfer. Field effect transistors (FETs) based on the coordinatively modified 1L-MoS2 show high electron mobilities up to 120.3 cm2 V-1 s-1 with impressive current on/off ratios over 109. Our results indicate that coordinatively attaching catechol- or Phen-bearing molecules may be a general method for the nondestructive modification of TMDs.

Interfacial interactions of doped-Ti3C2 MXene/MAPbI3 heterostructures: surfaces and the theoretical approach

The work function (WF) of perovskite materials is essential for developing optoelectronic devices enabling efficient charge transfer at their interfaces. Perovskite's WF can be tuned by MXenes, a new class of two-dimensional (2D) early transition metal carbides, nitrides, and carbonitrides. Their variable surface terminations or the possibility of introducing elemental dopants could advance perovskites. However, the influence of doped-MXenes on perovskite materials is still not fully understood and elaborated. This study provides mechanistic insight into verifying the tunability of MAPbI3 WF by hybridizing with fluorine-terminated Ti3C2Tx (F-MXene) and nitrogen-doped Ti3C2Tx (N-MXene). We first reveal the interfacial interaction between MAPbI3 and MXenes via X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and photoluminescence spectroscopy (PL). UPS supported by density functional theory (DFT) calculations allowed the description of the influence of F and N on MXene's WF. Furthermore, we developed MAPbI3/MXene heterostructures using F- and N-MXenes. The F-MXenes extended the most WF of MAPbI3 from 4.50 eV up to 3.00 eV, compared to only a small shift for N-MXene. The underlying mechanism was charge transfer from low WF F-MXene to MAPbI3, as demonstrated by PL quenching in MAPbI3/F-MXene heterostructures. Altogether, this work showcases the potential of fluorine-doped MXenes over nitrogen-doped MXenes in advancing perovskite heterostructures, thus opening a door for efficient optoelectronic devices.

Ambient Processed rGO/Ti3CNTx MXene Thin Film with High Oxidation Stability, Photosensitivity, and Self-Cleaning Potential

Solution-based processing offers advantages for producing thin films due to scalability, low cost, simplicity, and benignity to the environment. Here, we develop conductive and photoactivated self-cleaning reduced graphene oxide (rGO)/Ti3CNTx MXene thin films via spin coating under ambient conditions. The addition of a thin rGO layer on top of Ti3CNTx resulted in up to 45-fold improvement in the environmental stability of the film compared to the bare Ti3CNTx film. The optimized rGO/Ti3CNTx thin film exhibits an optical transmittance of 74% in the visible region of the spectrum and a sheet resistance of 19 kΩ/sq. The rGO/Ti3CNTx films show high rhodamine B discoloration activity upon light irradiation. Under UV irradiation, the electrically conductive MXene in combination with in situ formed semiconducting titanium oxide induces photogenerated charge carriers, which could potentially be used in photocatalysis. On the other hand, due to film transparency, white light irradiation can bleach the adsorbed dye via photolysis. This study opens the door for using MXene thin films as multifunctional coatings with conductive and potentially self-cleaning properties.