MXenes represent a large family of functionalized two-dimensional (2D) transition-metal carbides and carbonitrides. However, most of the understanding on their unique structures and applications stops at the theoretical suggestion and lack of experimental support. Herein, the surface structure and intercalation chemistry of Ti3C2X are clarified at the atomic scale by aberration-corrected scanning transmission electron microscope (STEM) and density functional theory (DFT) calculations. The STEM studies show that the functional groups (e.g., OH(-), F(-), O(-)) and the intercalated sodium (Na) ions prefer to stay on the top sites of the centro-Ti atoms and the C atoms of the Ti3C2 monolayer, respectively. Double Na-atomic layers are found within the Ti3C2X interlayer upon extensive Na intercalation via two-phase transition and solid-solution reactions. In addition, aluminum (Al)-ion intercalation leads to horizontal sliding of the Ti3C2X monolayer. On the basis of these observations, the previous monolayer surface model of Ti3C2X is modified. DFT calculations using the new modeling help to understand more about their physical and chemical properties. These findings enrich the understanding of the MXenes and shed light on future material design and applications. Moreover, the Ti3C2X exhibits prominent rate performance and long-term cycling stability as an anode material for Na-ion batteries.

The brookite phase of TiO(2) is hardly prepared and rarely studied in comparison with the common anatase and rutile phases. In addition, there exist immense controversies over the cognition of the light-induced liveliness of this material. Here, a novel, low-basicity solution chemistry method was first used to prepare homogeneous high-quality brookite TiO(2) single-crystalline nanosheets surrounded with four {210}, two {101}, and two {201} facets. These nanosheets exhibited outstanding activity toward the catalytic degradation of organic contaminants superior even to that of Degussa P25, due to the exposure of high-energy facets and the effective suppression of recombination rates of photogenerated electrons and holes by these facets as the oxidative and reductive sites. In contrast, irregularly faceted phase-pure brookite nanoflowers and nanospindles were inactive in catalytic reactions. These results demonstrate that the photocatalytic activity of brookite TiO(2) is highly dependent upon its exposed facets, which offers a strategy for tuning the catalysts from inert to highly active through tailoring of the morphology and surface structure.

capacity of the modified oxide reaches 320 mAh g -1 in the initial cycle, 94.5% of which remains after 100 cycles. More importantly, the average discharge potential drops only by 136 mV in this process. Our findings illustrate the importance of inactivating the surface oxygen in suppressing the cation mixing in the bulk, providing an effective strategy for designing high-performance Li-rich cathode materials.