The MXenes are a class of 2D materials composed of transition-metal sheets alternating with carbide/nitride sheets, stacked just a few atoms thick. MXenes discovered thus far also have a surface termination layer that is likely a mixture of hydroxides and fluorides. While reasonable structural models based on x-ray diffraction and transmission electron microscopy data exist, the exact nature and distribution of the surface termination species is not well understood. Here, 1 H, 19 F, and 13 C solid-state NMR spectroscopy is used to investigate the model MXene V 2 CT x , where T signifies the surface termination groups. 1 H NMR experiments provide direct proof of hydroxide moieties in the surface layer by measuring interactions with the MXene surface.Furthermore, 1 H NMR spectroscopy shows a significant amount of water hydrogen bonded to the surface hydroxide layer. 19 F NMR experiments show fluoride moieties bonded to the MXene surface, with extremely unusual 19 F spectra caused by strong interactions with the metallic/semi-conducting MXene. 13 C NMR observes the sample from the center of the MXene layer, and shows that the 13 C chemical shift is extremely sensitive to the MAX➞MXene transformation. Nuclear-spin magnetization transferred from 1 H nuclei in the hydroxide surface termination layer to 13 C nuclei in the center of the MXene sheet yields further evidence of this connectivity. The multinuclear NMR experiments provide direct experimental verification of the structural models, and depict the MXene V 2 CT x as infinite sheets of small-bandgap V 2 C sheets terminated by a mixed hydroxide/fluoride layer embedded in a matrix of strongly hydrogen-bonded water molecules.

The active phase responsible for low-temperature CO oxidation in nanoparticulate CuO/CeO2 catalysts was identified as surface-substituted CuyCe1-yO2-x. Contrary to previous studies, our measurements on a library of well-defined CuO/CeO2 catalysts have proven that the CuO phase is a spectator species while the surface-substituted CuyCe1-yO2-x phase is active for CO oxidation. Using in-situ X-ray absorption spectroscopy, we found that the copper ions in CuyCe1-yO2x remain at high oxidation states (Cu 3+ and Cu 2+ ) under oxygen-rich catalytic conditions without any evidence for Cu + . Artificial neural network potential Monte-Carlo simulations suggest that Cu 3+ and Cu 2+ preferentially segregate to the {100} surface of the CuyCe1-yO2-x nanoparticle, which is supported by aberration-corrected electron microscopy measurements. These results pave the way for understanding, at the atomic level, the mechanisms and descriptors pertinent for CO oxidation on these materials and hence the rational design of next generation catalysts.

The dielectric response of two-dimensional (2D) Ti 3 C 2 stacked sheets was investigated by high-resolution transmission electron energy-loss spectroscopy and ab initio calculations in the 0.2-30-eV energy range. Intense surface plasmons (SPs), evidenced at the nanometer scale at energies as low as 0.3 eV, are shown to be the dominant screening process up to at least 45-nm-thick stacks. This domination results from a combination of efficient free-electron dynamics, begrenzungs effect, and reduced interband damping. It is shown that, in principle, the SPs energies can be tuned in the mid-infrared, from 0.2 to 0.7 eV, by controlling the sheets' functionalization and/or thickness. This point evidences a new attribute of this new class of 2D materials.

A platform for producing stabilized Pt atoms and clusters through the combination of an N-doped graphene support and atomic layer deposition (ALD) for the Pt catalysts was investigated using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was determined, using imaging and spectroscopy techniques, that a wide range of N-dopant types entered the graphene lattice through covalent bonds without largely damaging its structure. Additionally and most notably, Pt atoms and atomic clusters formed in the absence of nanoparticles. This work provides a new strategy for experimentally producing stable atomic and subnanometer cluster catalysts, which can greatly assist the proton exchange membrane fuel cell (PEMFC) development by producing the ultimate surface area to volume ratio catalyst.

Fine-tuning nanocatalysts to enhance their catalytic activity and durability is crucial to commercialize proton exchange membrane fuel cells. The structural ordering and time evolution of ordered Pt3Fe2 intermetallic core-shell nanocatalysts for the oxygen reduction reaction that exhibit increased mass activity (228%) and an enhanced catalytic activity (155%) compared to Pt/C has been quantified using aberration-corrected scanning transmission electron microscopy. These catalysts were found to exhibit a static core-dynamic shell regime wherein, despite treating over 10,000 cycles, there is negligible decrease (9%) in catalytic activity and the ordered Pt3Fe2 core remained virtually intact while the Pt shell suffered a continuous enrichment. The existence of this regime was further confirmed by X-ray diffraction and the compositional analyses using energy-dispersive spectroscopy. With atomic-scale two-dimensional (2-D) surface relaxation mapping, we demonstrate that the Pt atoms on the surface are slightly relaxed with respect to bulk. The cycled nanocatalysts were found to exhibit a greater surface relaxation compared to noncycled catalysts. With 2-D lattice strain mapping, we show that the particle was about -3% strained with respect to pure Pt. While the observed enhancement in their activity is ascribed to such a strained lattice, our findings on the degradation kinetics establish that their extended catalytic durability is attributable to a sustained atomic order.

We report on the molecular beam epitaxial growth and structural characterization of self-organized AlGaN nanowire arrays on Si substrate with high luminescence efficiency emission in the deep ultraviolet (UV) wavelength range. It is found that, with increasing Al concentration, atomic-scale compositional modulations can be realized, leading to three-dimensional quantum confinement of charge carriers. By further exploiting the Anderson localization of light, we have demonstrated, for the first time, electrically injected AlGaN lasers in the deep UV band operating at room temperature. The laser operates at ∼289 nm and exhibits a threshold of 300 A/cm(2), which is significantly smaller compared to the previously reported electrically injected AlGaN multiple quantum well lasers.

Pt single atoms, stable on alumina under O2, form mobile clusters under H2, due to adsorbate- and nuclearity-dependent metal-support interaction.

Multifunctional hybrid-design nanomaterials appear to be a promising route to meet the current therapeutics needs required for efficient cancer treatment. Herein, two efficient heat nano-generators were combined into a multifunctional single nanohybrid (a multi-core iron oxide nanoparticle optimized for magnetic hyperthermia, and a gold branched shell with tunable plasmonic properties in the NIR region, for photothermal therapy) which impressively enhanced heat generation, in suspension or in vivo in tumours, opening up exciting new therapeutic perspectives.