MXene Materials for Designing Advanced Separation Membranes

MXenes are emerging rapidly as a new family of multifunctional nanomaterials with prospective applications rivaling that of graphenes. Herein, a timely account of the design and performance evaluation of MXene-based membranes is provided. First, the preparation and physicochemical characteristics of MXenes are outlined, with a focus on exfoliation, dispersion stability, and processability, which are crucial factors for membrane fabrication. Then, different formats of MXene-based membranes in the literature are introduced, comprising pristine or intercalated nanolaminates and polymer-based nanocomposites. Next, the major membrane processes so far pursued by MXenes are evaluated, covering gas separation, wastewater treatment, desalination, and organic solvent purification. The potential utility of MXenes in phase inversion and interfacial polymerization, as well as layer-by-layer assembly for the preparation of nanocomposite membranes, is also critically discussed. Looking forward, exploiting the high electrical conductivity and catalytic activity of certain MXenes is put into perspective for niche applications that are not easily achievable by other nanomaterials. Furthermore, the benefits of simulation/modeling approaches for designing MXene-based membranes are exemplified. Overall, critical insights are provided for materials science and membrane communities to navigate better while exploring the potential of MXenes for developing advanced separation membranes.     

Functional Materials in Food Nanotechnology

The Institute of Food Technologists has issued this Scientific Status Summary to update readers on the applications of nanotechnology in the food industry.     

Nanolaminates Utilizing Size‐Dependent Homogeneous Plasticity of Metallic Glasses

Homogeneous plasticity in metallic glasses is generally only observed at high temperatures or in very small structures (less than ≈100 nm), so their applications for structural performance have been very limited. Here, nanolaminates with alternating layers of Cu₅₀Zr₅₀ metallic glass and nanocrystalline Cu are synthesized and it is found that samples with an optimal composition of 112‐nm‐thick metallic‐glass layers and 16‐nm‐thick Cu layers demonstrate a maximum strength of 2.513 GPa, a value 33% greater than that predicted by the rule‐of‐mixtures and 25% better than that of pure Cu₅₀Zr₅₀ metallic glass. Furthermore, ≈4% strain at fracture is achieved, suppressing the instantaneous catastrophic failure often associated with metallic glasses. It is postulated that this favorable combination of high strength and deformability is caused by the size‐dependent deformation‐mode transition in metallic glasses, from highly localized plasticity, leading to immediate failure in larger samples to homogeneous extension in the smaller ones.     

Optical and microstructural properties of ZnO/TiO2 nanolaminates prepared by atomic layer deposition

ZnO/TiO₂ nanolaminates were grown on Si (100) and quartz substrates by atomic layer deposition at 200°C using diethylzinc, titanium isopropoxide, and deionized water as precursors. All prepared multilayers are nominally 50 nm thick with a varying number of alternating TiO₂ and ZnO layers. Sample thickness and ellipsometric spectra were measured using a spectroscopic ellipsometer, and the parameters determined by computer simulation matched with the experimental results well. The effect of nanolaminate structure on the optical transmittance is investigated using an ultraviolet–visible-near-infrared spectrometer. The data from X-ray diffraction spectra suggest that layer growth appears to be substrate sensitive and film thickness also has an influence on the crystallization of films. High-resolution transmission electron microscopy images show clear lattice spacing of ZnO in nanolaminates, indicating that ZnO layers are polycrystalline with preferred (002) orientation while TiO₂ layers are amorphous.     

Oriented Growth of Al2O3:ZnO Nanolaminates for Use as Electron‐Selective Electrodes in Inverted Polymer Solar Cells

Atomic layer deposition is used to synthesize Al₂O₃:ZnO(1:x) nanolaminates with the number of deposition cycles, x, ranging from 5 to 30 for evaluation as optically transparent, electron‐selective electrodes in polymer‐based inverted solar cells. Al₂O₃:ZnO(1:20) nanolaminates are found to exhibit the highest values of electrical conductivity (1.2 × 10³ S cm^?1; more than six times higher than for neat ZnO films), while retaining a high optical transmittance (≥80% in the visible region) and a low work function (4.0 eV). Such attractive performance is attributed to the structure (ZnO crystal size and crystal alignment) and doping level of this intermediate Al₂O₃:ZnO film composition. Polymer‐based inverted solar cells using poly(3‐hexylthiophene) (P3HT):phenyl‐C₆₁‐butyric acid methyl ester (PCBM) mixtures in the active layer and Al₂O₃:ZnO(1:20) nanolaminates as transparent electron‐selective electrodes exhibit a power conversion efficiency of 3% under simulated AM 1.5 G, 100 mW cm^?2 illumination.     

Effect of the composition and porosity of sintered titanium nanolaminates on their mechanical properties at high temperatures

By the methods of micro-and macroindentation, uniaxial compression, and scanning electron microscopy, we study the behavior of the structure and mechanical properties of mono-and biphase Ti₃SiC₂, Ti₃AlC₂, and Ti₄AlN₃ titanium nanolaminates (in-situ composites) prepared by the method of reaction sintering in compact and porous states. The regularities, specific features, and mechanisms of deformation and fracture processes are established for each material within the temperature range 20–1300°C. The temperature-strain and force boundaries of their existence in the plastic state are determined. The comparative analysis of the mechanical properties of nanolaminates is performed. It is shown that the strength characteristics of nanolaminates and their strain and creep resistance at medium and high temperatures increase in the following sequence: Ti₃AlC₂-Ti₄AlN₃-Ti₃SiC₂. The obtained picture of high-temperature properties of these materials is explained. It is shown that the procedure of prestraining of a porous material by ε = 4–8% may result in a significant increase in its specific high-temperature strength up to values higher than those typical of the compact material. __________ Translated from Problemy Prochnosti, No. 6, pp. 79–94, November–December, 2006.     

Switchable Polar Nanotexture in Nanolaminates HfO2-ZrO2 for Ultrafast Logic-in-Memory Operations

Nontrivial topological polar textures in ferroelectric materials, including vortices, skyrmions, and others, have the potential to develop ultrafast, high-density, reliable multilevel memory storage and conceptually innovative processing units, even beyond the limit of binary storage of 180° aligned polar materials. However, the realization of switchable polar textures at room temperature in ferroelectric materials integrated directly into silicon using a straightforward large area fabrication technique and effectively utilizing it to design multilevel programable memory and processing units has not yet been demonstrated. Here, utilizing vector piezoresponse force and conductive atomic force microscopy, microscopic evidence of the electric field switchable polar nanotexture is provided at room temperature in HfO₂-ZrO₂ nanolaminates grown directly onto silicon using an atomic layer deposition technique. Additionally, a two-terminal Au/nanolaminates/Si ferroelectric tunnel junction is designed, which shows ultrafast (≈83 ns) nonvolatile multilevel current switching with high on/off ratio (>10⁶), long-term durability (>4000 s), and giant tunnel electroresistance (10⁸%). Furthermore, 14 Boolean logic operations are tested utilizing a single device as a proof-of-concept for reconfigurable logic-in-memory processing. The results offer a potential approach to “processing with polar textures” and addressing the challenges of developing high-performance multilevel in-memory processing technology by virtue of its fundamentally distinct mechanism of operation.