Molecular Dynamics,Phase Transition and Frequency‐Tuned Dielectric Switch of an Ionic Co‐Crystal

Dielectric switches that can be converted between high and low dielectric states by thermal stimuli have attracted much interest owing to their many potential applications. Currently one main drawback for practical application lies in the non‐tunability of their switch temperatures (T_S). We report here an ionic co‐crystal (Me₃NH)₄[Ni(NCS)₆] that contains a multiply rotatable Me₃NH⁺ ion and a solely rotatable one due to a more spacious supramolecular cage for the former one. This compound undergoes an isostructural order–disorder phase transition and it can function as a frequency‐tuned dielectric switch with highly adjustable T_S, which is further revealed by the variable‐temperature structure analyses and molecular dynamics simulations. In addition, the distinct arrangements and molecular dynamics of two coexisting Me₃NH⁺ ions confined in different lattice spaces as well as the notable offset effect on the promoting/hindering of dipolar reorientation after dielectric transition provide a rarely observed but fairly good model for understanding and modulating the dipole motion in crystalline environment.     

Self‐Optimization of the Active Site of Molybdenum Disulfide by an Irreversible Phase Transition during Photocatalytic Hydrogen Evolution

The metallic 1T‐MoS₂ has attracted considerable attention as an effective catalyst for hydrogen evolution reactions (HERs). However, the fundamental mechanism about the catalytic activity of 1T‐MoS₂ and the associated phase evolution remain elusive and controversial. Herein, we prepared the most stable 1T‐MoS₂ by hydrothermal exfoliation of MoS₂ nanosheets vertically rooted into rigid one‐dimensional TiO₂ nanofibers. The 1T‐MoS₂ can keep highly stable over one year, presenting an ideal model system for investigating the HER catalytic activities as a function of the phase evolution. Both experimental studies and theoretical calculations suggest that 1T phase can be irreversibly transformed into a more active 1T′ phase as true active sites in photocatalytic HERs, resulting in a “catalytic site self‐optimization”. Hydrogen atom adsorption is the major driving force for this phase transition.     

Quantifying Hydrogen‐Bond Populations in Dimethyl Sulfoxide/Water Mixtures

Dimethyl sulfoxide (DMSO) disrupts the hydrogen‐bond networks in water. The widespread use of DMSO as a cosolvent, along with its unusual attributes, have inspired numerous studies. Herein, infrared absorption spectroscopy of the S=O stretching mode combined with molecular dynamics and quantum chemistry models were used to directly quantify DMSO/water hydrogen‐bond populations in binary mixtures. Singly H‐bonded species are dominant at 10 mol %, due to strong DMSO–water interactions. We found an unexpected increase in non‐hydrogen‐bonded DMSO near the eutectic point (ca. 35 mol %) which also correlates with several abnormalities in the bulk solution properties. We find evidence for three distinct regimes: 1) strong DMSO–water interactions (<30 mol %), 2) ideal‐solution‐like (30–90 mol %), and 3) self‐interaction, or aggregation, regime (>90 mol %). We propose a “step in” mechanism, which involves hydrogen bonding between water and the DMSO aggregate species.     

Phosphorus‐Modulation‐Triggered Surface Disorder in Titanium Dioxide Nanocrystals Enables Exceptional Sodium‐Storage Performance

Structural modulation and surface engineering have remarkable advantages for fast and efficient charge storage. Herein, we present a phosphorus modulation strategy which simultaneously realizes surface structural disorder with interior atomic‐level P‐doping to boost the Na⁺ storage kinetics of TiO₂. It is found that the P‐modulated TiO₂ nanocrystals exhibit a favourable electronic structure, and enhanced structural stability, Na⁺ transfer kinetics, as well as surface electrochemical reactivity, resulting in a genuine zero‐strain characteristic with only approximately 0.1 % volume variation during Na⁺ insertion/extraction, and exceptional Na⁺ storage performance including an ultrahigh rate capability of 210 mAh g^?1 at 50 C and a strong long‐term cycling stability without significant capacity decay up to 5000 cycles at 30 C.