The selective adsorption of tellurium in the aluminosilicate regions of AFI- and MOR-type microporous crystals

Optical study of Te8 ring clusters: comparison with density functional theory and a step towards materials design using nanoporous zeolite space

The Te8 ring molecule (cluster) is poorly investigated due to the lack of experimental data. Here, we report an experimental and theoretical study of a regular array of oriented Te8 rings formed in the ∼1.14 nm diameter cavities of zeolite LTA, which are arranged in a cubic lattice with a spacing of ∼1.2 nm. Single crystals of LTA with encapsulated tellurium (LTA-Te) were studied using Raman spectroscopy (RS) and optical absorption spectroscopy (OAS). The experimental LTA-Te spectra were found to be in agreement with those calculated using density functional theory (PBE0 hybrid functional and def2-TZVP basis sets) for the crown-shaped Te8 ring molecule with D4d symmetry. Using polarization-orientation RS, we show that the Te8 rings are oriented by their major axes along the 4-fold axes of cubic LTA. We also show that the site symmetry of Te8 in LTA-Te is lower than D4d. Te8 bond-bending modes are well described in the harmonic approximation, while bond-stretching modes are mixed due to the reduced ring symmetry and, probably, anharmonicity. Importantly, OAS data of LTA-Te display dependence on the Te8 concentration, implying the interaction of the rings from neighbouring LTA cavities with the generation of the valence and conduction electron bands of such a cluster crystal.

Optical properties of extreme tellurium nanowires formed in subnanometer-diameter channels

Single tellurium (Te) chains attract much attention as extreme nanowires with unique electronic and spintronic properties. Here, we encapsulate Te from a melt into channels of zeolites AFI (∼0.73 nm-channel diameter) and mordenite (MOR, ∼0.67 × 0.7 nm² channel cross-section) via high-pressure injection. Using polarized Raman and optical absorption spectra (RS and OAS) of zeolite single crystals with Te (AFI-Te and MOR-Te), we discriminate between features of Te chains and rings formed in the zeolites. We demonstrate good agreement of AFI-Te-chain RS and OAS with the calculated single Te-helix phonon and electron spectra. This suggests a very weak interaction of the AFI-Te-chain with the zeolite and its nearly perfect helix structure lacking inversion/mirror symmetry. An AFI-Te OAS feature, attributed to the electron transitions between Te-helix-Rashba-split valence and conduction bands confirms its 1D-electron-band origin with predicted possibilities of identifying Majorana fermions, manipulating spin transport and realizing topological superconductivity.

Changing water affinity from hydrophobic to hydrophilic in hydrophobic channels

The behavior of water at hydrophobic interfaces can play a significant role in determining chemical reaction outcomes and physical properties. Carbon nanotubes and aluminophosphate materials have one-dimensional hydrophobic channels, which are entirely surrounded by hydrophobic interfaces. Unique water behavior was observed in such hydrophobic channels. In this article, changes in the water affinity in one-dimensional hydrophobic channels were assessed using water vapor adsorption isotherms at 303 K and grand canonical Monte Carlo simulations. Hydrophobic behavior of water adsorbed in channels wider than 3 nm was observed for both adsorption and desorption processes, owing to the hydrophobic environment. However, water showed hydrophilic properties in both adsorption and desorption processes in channels narrower than 1 nm. In intermediate-sized channels, the hydrophobic properties of water during the adsorption process were seen to transition to hydrophilic behavior during the desorption process. Hydrophilic properties in the narrow channels for both adsorption and desorption processes are a result of the relatively strong water-channel interactions (10-15 kJ mol(-1)). In the 2-3 nm channels, the water-channel interaction energy of 4-5 kJ mol(-1) was comparable to the thermal translational energy. The cohesive water interaction was approximately 35 kJ mol(-1), which was larger than the others. Thus, the water affinity change in the 2-3 nm channels for the adsorption and desorption processes was attributed to weak water-channel interactions and strong cohesive interactions. These results are inherently important to control the properties of water in hydrophobic environments.