Unveiling the Interfacial and Structural Heterogeneity of Ti3C2Tx MXene Etched with CoF2/HCl by Integrated in Situ Thermal Analysis

Insights on updates in sodium alginate/MXenes composites as the designer matrix for various applications: A review

Advancements in two-dimensional materials, particularly MXenes, have spurred the development of innovative composites through their integration with natural polymers such as sodium alginate (SA). Mxenes exhibit a broad specific surface area, excellent electrical conductivity, and an abundance of surface terminations, which can be combined with SA to maximize the synergistic effect of the materials. This article provides a comprehensive review of state-of-the-art techniques in the fabrication of SA/MXene composites, analyzing the resulting structural and functional enhancements with a specific focus on advancing the design of these composites for practical applications. A detailed exploration of SA/MXene composites is provided, highlighting their utility in various sectors, such as wearable electronics, wastewater treatment, biomedical applications, and electromagnetic interference (EMI) shielding. The review identifies the unique advantages conferred by incorporating MXene in these composites, examines the current challenges, and proposes future research directions to understand and optimize these promising materials thoroughly. The remarkable properties of MXenes are emphasized as crucial for advancing the performance of SA-based composites, indicating significant potential for developing high-performance composite materials.

Tuning the Microenvironment of Water Confined in Ti3C2Tx MXene by Cation Intercalation

The local microenvironment has recently been found to play a major role in the electrocatalytic activity of nanomaterials. Modulating the microenvironment by adding alkali metal cations into the electrolyte can be used to either suppress hydrogen or oxygen evolution, thereby extending the electrochemical window of energy storage systems, or to tune the selectivity of electrocatalysts. MXenes are a large family of two-dimensional transition metal carbides, nitrides, and carbonitrides that have shown potential for use in electrochemical energy storage applications. Due to their negatively charged surfaces, MXenes can accommodate cations and water molecules between the layers. Nevertheless, the nature of the aqueous microenvironment in the MXene interlayer space is poorly understood. Here, we apply Fourier transform infrared spectroscopy (FTIR) to probe the hydrogen bonding of intercalated water in Ti3C2Tx as a function of intercalated cation and relative humidity. Substantial changes in the FTIR spectra after cation exchange demonstrate that the hydrogen bonding of water molecules confined between the MXene layers is strongly cation-dependent. Furthermore, the IR absorbance of the confined water correlates with resistivity estimated by 4-point probe measurements and interlayer distance calculated from XRD patterns. This work demonstrates that cation intercalation strongly modulates the confined microenvironment, which can be used to tune the activity or selectivity of electrochemical reactions in the interlayer space of MXenes in the future.

Energetic Stability and Interfacial Complexity of Ti3C2Tx MXenes Synthesized with HF/HCl and CoF2/HCl as Etching Agents

MXenes are ultra-thin two-dimensional layered early transition-metal carbides and nitrides with potential applications in various emerging technologies, such as energy storage, water purification, and catalysis. MXenes are synthesized from the parent MAX phases with different etching agents [hydrofluoric acid (HF) or fluoride salts with a strong acid] by selectively removing a more weakly bound crystalline layer of Al or Ga replaced by surface groups (-O, -F, -OH, etc.). Ti₃C₂T_x MXene synthesized by CoF₂/HCl etching has layered heterogeneity due to intercalated Al³⁺ and Co²⁺ that act as pillars for interlayer spacings. This study investigates the impacts of etching environments on the compositional, interfacial, structural, and thermodynamic properties of Ti₃C₂T_x MXenes. Specifically, compared with HF/HCl etching, CoF₂/HCl treatment leads to a Ti₃C₂T_x MXene with a broader distribution of interlayer distances, increased number of intercalated cations, and decreased degree of hydration. Moreover, we determine the enthalpies of formation at 25 °C (ΔH_f,25°C) of Ti₃C₂T_x MXenes etched with CoF₂/HCl, ΔH_f,25°C = -1891.7 ± 35.7 kJ/mol Ti₃C₂, and etched with HF/HCl, ΔH_f,25°C = -1978.2 ± 35.7 kJ/mol Ti₃C₂, using high-temperature oxidation drop calorimetry. These energetic data are discussed and compared with experimentally derived and computationally predicted values to elucidate the effects of intercalants and surface groups of MXenes. We find that MXenes with intercalated metal cations have a less exothermic ΔH_f,25°C from an increase in the interlayer space and dimension heterogeneity and a decrease in the degree of hydration leading to reduced layer-layer van der Waals interactions and weakened hydration effects applied on the MXene layers. The outcomes of this study further our understanding of MXene's energetic-structural-interfacial property relationships.

Thermal Properties of MXenes and Relevant Applications

The properties and applications of MXenes (a family of layered transition metal carbides, nitrides, and carbonitrides) have aroused enormous research interests for a decade since the successful synthesis of few-layer transition metal carbides in 2011. Though MXenes, as the building blocks, have already been applied in various fields (such as wearable electronics) owing to the distinctive optical, mechanical and electrical properties, their thermal stability and intrinsic thermal properties were less thoroughly investigated compared to other characteristics in early reports. The pioneering theoretical prediction of the thermoelectric nature of MXenes was performed in 2013 while the first experiment-based report concerning the degradation behavior of the 2D structure at elevated temperatures in a controlled atmosphere was published in 2015, followed by numerous discoveries regarding the thermal properties of MXenes. Herein, after a brief description of the synthesis, this Review summarized the latest insights into the thermal stability and thermophysical properties of MXenes, and further associated these unique properties with relevant applications by multiple examples. Finally, current hurdles and challenges in this field were provided along with some advices on potential research directions in the future.