Arylazopyrazole derivatives based on four core structures (4pzMe, 3pzH, 4pzH, and 4pzH-F2) and functionalized with a dodecanoate group were demonstrated to store thermal energy in their metastable Z isomer liquid phase and release the energy by optically triggered crystallization at −30 °C for the first time. Three heat storage−release schemes were discovered involving different activation methods (optical, thermal, or combined) for generating liquid-state Z isomers capable of storing thermal energy. Visible light irradiation induced the selective crystallization of the liquid phase via Z-to-E isomerization, and the latent heat stored in the liquid Z isomers was preserved for longer than 2 weeks unless optically triggered. Up to 92 kJ/mol of thermal energy was stored in the compounds, demonstrating remarkable thermal stability of Z isomers at high temperatures and liquid-phase stability at temperatures below 0 °C.

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We have studied atomic level interactions between single Pt atoms and the surface of monolayer MoS 2 using aberration-corrected annular dark field scanning transmission electron microscopy at an accelerating voltage of 60 kV. Strong contrast from single Pt atoms on the atomically resolved monolayer MoS 2 lattice enables their exact position to be determined with respect to the MoS 2 lattice, revealing stable binding sites. In regions of MoS 2 free from surface contamination, the Pt atoms are localized in S vacancy sites and exhibit dynamic hopping to nearby vacancy sites driven by the energy supplied by the electron beam. However, in areas of MoS 2 contaminated with carbon surface layers, the Pt atoms appear at various positions with respect to the underlying MoS 2 lattice, including on top of Mo and in off-axis positions. These variations are due to the Pt bonding with the surrounding amorphous carbon layer, which disrupts the intrinsic Pt−MoS 2 interactions, leading to more varied positions. Density functional theory (DFT) calculations reveal that Pt atoms on the surface of MoS 2 have a small barrier for migration and are stabilized when bound to either a single or double sulfur vacancies. DFT calculations have been used to understand how the catalytic activity of the MoS 2 basal plane for hydrogen evolution reaction is influenced by Pt dopants by variation of the hydrogen adsorption free energy. This strong dependence of catalytic effect on interfacial configurations is shown to be common for a series of dopants, which may provide a means to create and optimize reaction centers. KEYWORDS: Pt dopants, MoS 2 , ADF-STEM, 2D materials, catalysts, dopants T he presence of single isolated atom impurities and dopants either on the surface or directly bonded within the lattice structure of 2D materials influences the properties.1−10 Understanding the atomic structure that defines the bonding between single foreign atoms and the host 2D crystal is critical for developing accurate models that can predict the impacts of doping. Single metal atom doping of materials has also found significant recent interest in the development of catalysts by reducing the mass content of precious metals such as Pt, while maintaining high performance.11−13 A key aspect to single metal atom catalysts is preventing aggregation of the active species into clusters, reducing the loss of metal atoms by detachment, and providing the optimum bonding configuration for catalytic activity. Recent work has shown that doping 2D layered MoS 2 can lead to higher activities in the hydrogen evolution reaction, utilizing Co, Ni, and Pt single-metal dopants.14,15 MoS 2 is normally active only at edge sites, but the incorporation of isolated metal substitutional or surface dopants can activate the basal plane and lead to a large increase in the number of active catalytic sites. Therefore, resolving the atomic structure of single metal atoms and their structural interaction with MoS 2 helps reveal the stable configurations that dictate the catalytic...

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Thermal energy storage offers enormous potential for a wide range of energy technologies. Phase-change materials offer state-of-the-art thermal storage due to high latent heat. However, spontaneous heat loss from thermally charged phase-change materials to cooler surroundings occurs due to the absence of a significant energy barrier for the liquid–solid transition. This prevents control over the thermal storage, and developing effective methods to address this problem has remained an elusive goal. Herein, we report a combination of photo-switching dopants and organic phase-change materials as a way to introduce an activation energy barrier for phase-change materials solidification and to conserve thermal energy in the materials, allowing them to be triggered optically to release their stored latent heat. This approach enables the retention of thermal energy (about 200 J g−1) in the materials for at least 10 h at temperatures lower than the original crystallization point, unlocking opportunities for portable thermal energy storage systems.