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1. pH, Nanosheet Concentration, and Antioxidant Affect the Oxidation of Ti ₃ C ₂ T _x and Ti ₂ CT _x MXene Dispersions

   https://doi.org/10.1002/admi.202000845  

   Zhao, Xiaofei; Vashisth, Aniruddh; Blivin, Jackson W.; Tan, Zeyi; Holta, Dustin E.; Kotasthane, Vrushali; Shah, Smit A.; Habib, Touseef; Liu, Shuhao; Lutkenhaus, Jodie L.; et al (August 2020, Advanced Materials Interfaces)

   Abstract The chemical stability of 2D MXene nanosheets in aqueous dispersions must be maintained to foster their widespread application. MXene nanosheets react with water, which results in the degradation of their 2D structure into oxides and carbon residues. The latter detrimentally restricts the shelf life of MXene dispersions and devices. However, the mechanism of MXene degradation in aqueous environment has yet to be fully understood. In this work, the oxidation kinetics is investigated of Ti₃C₂T_xand Ti₂CT_xin aqueous media as a function of initial pH values, ionic strengths, and nanosheet concentrations. The pH value of the dispersion is found to change with time as a result of MXene oxidation. Specifically, MXene oxidation is accelerated in basic media by their reaction with hydroxyl anions. It is also demonstrated that oxidation kinetics are strongly dependent on nanosheet dispersion concentration, in which oxidation is accelerated for lower MXene concentrations. Ionic strength does not strongly affect MXene oxidation. The authors also report that citric acid acts as an effective antioxidant and mitigates the oxidation of both Ti₃C₂T_xand Ti₂CT_xMXenes. Reactive molecular dynamic simulations suggest that citric acid associates with the nanosheet edge to hinder the initiation of oxidation. 

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2. Large Polarization and Susceptibilities in Artificial Morphotropic Phase Boundary PbZr _{1− x} Ti _x O ₃ Superlattices

   https://doi.org/10.1002/aelm.201901395  

   Lupi, Eduardo; Ghosh, Anirban; Saremi, Sahar; Hsu, Shang‐Lin; Pandya, Shishir; Velarde, Gabriel; Fernandez, Abel; Ramesh, Ramamoorthy; Martin, Lane W. (January 2020, Advanced Electronic Materials)

   Abstract The ability to produce atomically precise, artificial oxide heterostructures allows for the possibility of producing exotic phases and enhanced susceptibilities not found in parent materials. Typical ferroelectric materials either exhibit large saturation polarization away from a phase boundary or large dielectric susceptibility near a phase boundary. Both large ferroelectric polarization and dielectric permittivity are attained wherein fully epitaxial (PbZr_0.8Ti_0.2O₃)_n/(PbZr_0.4Ti_0.6O₃)_2n(n= 2, 4, 6, 8, 16 unit cells) superlattices are produced such that the overall film chemistry is at the morphotropic phase boundary, but constitutive layers are not. Long‐ (n≥ 6) and short‐period (n= 2) superlattices reveal large ferroelectric saturation polarization (P_s= 64 µC cm⁻²) and small dielectric permittivity (ε_r≈ 400 at 10 kHz). Intermediate‐period (n= 4) superlattices, however, exhibit both large ferroelectric saturation polarization (P_s= 64 µC cm⁻²) and dielectric permittivity (ε_r= 776 at 10 kHz). First‐order reversal curve analysis reveals the presence of switching distributions for each parent layer and a third, interfacial layer wherein superlattice periodicity modulates the volume fraction of each switching distribution and thus the overall material response. This reveals that deterministic creation of artificial superlattices is an effective pathway for designing materials with enhanced responses to applied bias. 

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3. Operando Tracking of Resistance, Thickness, and Mass of Ti ₃ C ₂ T _x MXene in Water‐in‐Salt Electrolyte

   https://doi.org/10.1002/aenm.202405028  

   Perju, Audrey; Zhang, Danzhen; Wang, Ruocun_John; Taberna, Pierre‐Louis; Gogotsi, Yury; Simon, Patrice (January 2025, Advanced Energy Materials)

   Abstract MXenes are among the fastest‐growing families of 2D materials, promising for high‐rate, high‐energy energy storage applications due to their high electronic and ionic conductivity, large surface area, and reversible surface redox ability. The Ti₃C₂T_xMXene shows a capacitive charge storage mechanism in diluted aqueous LiCl electrolyte while achieving abnormal redox‐like features in the water‐in‐salt LiCl electrolyte. Herein, variousoperandotechniques are used to investigate changes in resistance, mass, and electrode thickness of Ti₃C₂T_xduring cycling in salt‐in‐water and water‐in‐salt LiCl electrolytes. Significant resistance variations due to interlayer space changes are recorded in the water‐in‐salt LiCl electrolyte. In both electrolytes, conductivity variations attributed to charge carrier density changes or varied inter‐sheet electron hopping barriers are detected in the capacitive areas, where no thickness variations are observed. Overall, combining thoseoperandotechniques enhances the understanding of charge storage mechanisms and facilitates the development of MXene‐based energy storage devices. 

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4. Structural Stabilization and Piezoelectric Enhancement in Epitaxial (Ti _{1− x} Mg _x ) _0.25 Al _0.75 N(0001) Layers

   https://doi.org/10.1002/adfm.202001915  

   Wang, Baiwei; Aryana, Kiumars; Gaskins, John T.; Hopkins, Patrick E.; Khare, Sanjay V.; Gall, Daniel (June 2020, Advanced Functional Materials)

   Abstract Epitaxial (Ti_1−xMg_x)_0.25Al_0.75N(0001)/Al₂O₃(0001) layers are used as a model system to explore how Fermi‐level engineering facilitates structural stabilization of a host matrix despite the intentional introduction of local bonding instabilities that enhance the piezoelectric response. The destabilizing octahedral bonding preference of Ti dopants and the preferred 0.67 nitrogen‐to‐Mg ratio for Mg dopants deteriorate the wurtzite AlN matrix for both Ti‐rich (x< 0.2) and Mg‐rich (x≥ 0.9) alloys. Conversely,x= 0.5 leads to a stability peak with a minimum in the lattice constant ratioc/a, which is caused by a Fermi‐level shift into the bandgap and a trend toward nondirectional ionic bonding, leading to a maximum in the expected piezoelectric stress constante₃₃. The refractive index and the subgap absorption decrease withx, the optical bandgap increases, and the elastic constant along the hexagonal axisC₃₃= 270 ± 14 GPa remains composition independent, leading to an expected piezoelectric constantd₃₃= 6.4 pC N⁻¹atx= 0.5, which is 50% larger than for the pure AlN matrix. Thus, contrary to the typical anticorrelation between stability and electromechanical coupling, the (Ti_1−xMg_x)_0.25Al_0.75N system exhibits simultaneous maxima in the structural stability and the piezoelectric response atx= 0.5. 

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5. Ti ₂ CT _x MXene‐based all‐optical modulator

   https://doi.org/10.1002/inf2.12052  

   Yi, Jun; Li, Jie; Huang, Shuohan; Hu, Longyu; Miao, Lili; Zhao, Chujun; Wen, Shuangchun; Mochalin, Vadym N.; Rao, Apparao M. (December 2019, InfoMat)

   Abstract MXenes, a new class of 2D transition metal carbides, nitrides, and carbonitrides, have attracted much attention due to their outstanding properties. Here, we report the broadband spatial self‐phase modulation of Ti₂CT_xMXene nanosheets dispersed in deionized water in the visible to near‐infrared regime, highlighting the broadband nonlinear optical (NLO) response of Ti₂CT_xMXene. Using ultrafast pulsed laser excitation, the nonlinear refractive indexn₂and the third‐order nonlinear susceptibilityof Ti₂CT_xMXene were measured to be ∼10⁻¹³m²/W and ∼ 10⁻¹⁰esu, respectively. Leveraging the large optical nonlinearity of Ti₂CT_xMXene, an all‐optical modulator in the visible regime was fabricated based on the spatial cross‐phase modulation effect. This work suggests that 2D MXenes are ideal broadband NLO materials with excellent prospects in NLO applications. image 

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