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1. The Evolution of MXenes Conductivity and Optical Properties Upon Heating in Air

   https://doi.org/10.1002/smtd.202300568  

   Shamsabadi, Ahmad A. ; Fang, Hui ; Zhang, Danzhen ; Thakur, Anupma ; Chen, Cindy Y. ; Zhang, Aixi ; Wang, Haonan ; Anasori, Babak ; Soroush, Masoud ; Gogotsi, Yury ; et al ( July 2023 , Small Methods)

   MXenes, a family of 2D transition‐metal carbides and nitrides, have excellent electrical conductivity and unique optical properties. However, MXenes oxidize in ambient conditions, which is accelerated upon heating. Intercalation of water also causes hydrolysis accelerating oxidation. Developing new tools to readily characterize MXenes’ thermal stability can enable deeper insights into their structure–property relationships. Here, in situ spectroscopic ellipsometry (SE) is employed to characterize the optical properties of three types of MXenes (Ti₃C₂T_x, Mo₂TiC₂T_x, and Ti₂CT_x) with varied composition and atomistic structures to investigate their thermal degradation upon heating under ambient environment. It is demonstrated that changes in MXene extinction and optical conductivity in the visible and near‐IR regions correlate well with the amount of intercalated water and hydroxyl termination groups and the degree of oxidation, measured using thermogravimetric analysis. Among the three MXenes, Ti₃C₂T_xand Ti₂CT_x, respectively, have the highest and lowest thermal stability, indicating the role of transition‐metal type, synthesis route, and the number of atomic layers in MXene flakes. These findings demonstrate the utility of SE as a powerful in situ technique for rapid structure–property relationship studies paving the way for the further design, fabrication, and property optimization of novel MXene materials.
2. Scalable Synthesis of Ti ₃ C ₂ T _x MXene

   https://doi.org/10.1002/adem.201901241  

   Shuck, Christopher E. ; Sarycheva, Asia ; Anayee, Mark ; Levitt, Ariana ; Zhu, Yuanzhe ; Uzun, Simge ; Balitskiy, Vitaliy ; Zahorodna, Veronika ; Gogotsi, Oleksiy ; Gogotsi, Yury ( February 2020 , Advanced Engineering Materials)

   Scaling the production of synthetic 2D materials to industrial quantities has faced significant challenges due to synthesis bottlenecks whereby few have been produced in large volumes. These challenges typically stem from bottom‐up approaches limiting the production to the substrate size or precursor availability for chemical synthesis and/or exfoliation. In contrast, MXenes, a large class of 2D transition metal carbides and/or nitrides, are produced via a top‐down synthesis approach. The selective wet etching process does not have similar synthesis constraints as some other 2D materials. The reaction occurs in the whole volume; therefore, the process can be readily scaled with reactor volume. Herein, the synthesis of 2D titanium carbide MXene (Ti₃C₂T_x) is studied in two batch sizes, 1 and 50 g, to determine if large‐volume synthesis affects the resultant structure or composition of MXene flakes. Characterization of the morphology and properties of the produced MXene using scanning electron microscopy, X‐ray diffraction, dynamic light scattering, Raman spectroscopy, X‐ray photoelectron spectroscopy, UV–visible spectroscopy, and conductivity measurements show that the materials produced in both batch sizes are essentially identical. This illustrates that MXenes experience no change in structure or properties when scaling synthesis, making them viable for further scale‐up and commercialization.
3. High‐Speed Ionic Synaptic Memory Based on 2D Titanium Carbide MXene

   https://doi.org/10.1002/adfm.202109970  

   Melianas, Armantas ; Kang, Min‐A ; VahidMohammadi, Armin ; Quill, Tyler James ; Tian, Weiqian ; Gogotsi, Yury ; Salleo, Alberto ; Hamedi, Mahiar Max ( November 2021 , Advanced Functional Materials)

   Synaptic devices with linear high‐speed switching can accelerate learning in artificial neural networks (ANNs) embodied in hardware. Conventional resistive memories however suffer from high write noise and asymmetric conductance tuning, preventing parallel programming of ANN arrays. Electrochemical random‐access memories (ECRAMs), where resistive switching occurs by ion insertion into a redox‐active channel, aim to address these challenges due to their linear switching and low noise. ECRAMs using 2D materials and metal oxides however suffer from slow ion kinetics, whereas organic ECRAMs enable high‐speed operation but face challenges toward on‐chip integration due to poor temperature stability of polymers. Here, ECRAMs using 2D titanium carbide (Ti₃C₂T_x) MXene that combine the high speed of organics and the integration compatibility of inorganic materials in a single high‐performance device are demonstrated. These ECRAMs combine the speed, linearity, write noise, switching energy, and endurance metrics essential for parallel acceleration of ANNs, and importantly, they are stable after heat treatment needed for back‐end‐of‐line integration with Si electronics. The high speed and performance of these ECRAMs introduces MXenes, a large family of 2D carbides and nitrides with more than 30 stoichiometric compositions synthesized to date, as promising candidates for devices operating at the nexus of electrochemistry and electronics.
4. 2D MXene-containing polymer electrolytes for all-solid-state lithium metal batteries

   https://doi.org/10.1039/C8NA00206A  

   Pan, Qiwei ; Zheng, Yongwei ; Kota, Sankalp ; Huang, Weichun ; Wang, Shijun ; Qi, Hao ; Kim, Seyong ; Tu, Yingfeng ; Barsoum, Michel W. ; Li, Christopher Y. ( January 2019 , Nanoscale Advances)

   Nanocomposite polymer electrolytes (CPEs) are promising materials for all-solid-state lithium metal batteries (LMBs) due to their enhanced ionic conductivities and stability to the lithium anode. MXenes are a new two-dimensional, 2D, family of early transition metal carbides and nitrides, which have a high aspect ratio and a hydrophilic surface. Herein, using a green, facile aqueous solution blending method, we uniformly dispersed small amounts of Ti 3 C 2 T x into a poly(ethylene oxide)/LiTFSI complex (PEO 20 -LiTFSI) to fabricate MXene-based CPEs (MCPEs). The addition of the 2D flakes to PEO simultaneously retards PEO crystallization and enhances its segmental motion. Compared to the 0D and 1D nanofillers, MXenes show higher efficiency in ionic conductivity enhancement and improvement in the performance of LMBs. The CPE with 3.6 wt% MXene shows the highest ionic conductivity at room temperature (2.2 × 10 −5 S m −1 at 28 °C). An LMB using MCPE with only 1.5 wt% MXene shows rate capability and stability comparable with that of the state-of-the-art CPELMBs. We attribute the excellent performance to the 2D geometry of the filler, the good dispersion of the flakes in the polymer matrix, and the functional group-rich surface.
5. Assembling 2D MXenes into Highly Stable Pseudocapacitive Electrodes with High Power and Energy Densities

   https://doi.org/10.1002/adma.201806931  

   VahidMohammadi, Armin ; Mojtabavi, Mehrnaz ; Caffrey, Nuala Mai ; Wanunu, Meni ; Beidaghi, Majid ( December 2018 , Advanced Materials)

   Electrochemical capacitors (ECs) that store charge based on the pseudocapacitive mechanism combine high energy densities with high power densities and rate capabilities. 2D transition metal carbides (MXenes) have been recently introduced as high‐rate pseudocapacitive materials with ultrahigh areal and volumetric capacitances. So far, 20 different MXene compositions have been synthesized and many more are theoretically predicted. However, since most MXenes are chemically unstable in their 2D forms, to date only one MXene composition, Ti₃C₂T_x, has shown stable pseudocapacitive charge storage. Here, a cation‐driven assembly process is demonstrated to fabricate highly stable and flexible multilayered films of V₂CT_xand Ti₂CT_xMXenes from their chemically unstable delaminated single‐layer flakes. The electrochemical performance of electrodes fabricated using assembled V₂CT_xflakes surpasses Ti₃C₂T_xin various aqueous electrolytes. These electrodes show specific capacitances as high as 1315 F cm⁻³and retain ≈77% of their initial capacitance after one million charge/discharge cycles, an unprecedented performance for pseudocapacitive materials. This work opens a new venue for future development of high‐performance supercapacitor electrodes using a variety of 2D materials as building blocks.