ABSTRACT
TiO2 nanohelices (NHs) have attracted extensive attention owing to their high aspect ratio, excellent flexibility, elasticity, and optical properties, which endow promising performances in a vast range of vital fields, such as optics, electronics, and micro/nanodevices. However, preparing rigid TiO2 nanowires (TiO2 NWs) into spatially anisotropic helical structures remains a challenge. Here, a pressure-induced hydrothermal strategy was designed to assemble individual TiO2 NWs into a DNA-like helical structure, in which a Teflon block was placed in an autoclave liner to regulate system pressure and simulate a cell-rich environment. The synthesized TiO2 NHs of 50 nm in diameter and 5-7 mm in length approximately were intertwined into nanohelix bundles (TiO2 NHBs) with a diameter of 20 µm and then assembled into vertical TiO2 nanohelix arrays (NHAs). Theoretical calculations further confirmed that straight TiO2 NWs prefer to convert into helical conformations with minimal entropy (S) and free energy (F) for continuous growth in a confined space. The excellent elastic properties exhibit great potential for applications in flexible devices or buffer materials.
ABSTRACT
Efficient thermal energy harvesting using phase change materials (PCMs) has great potential for thermal energy storage and thermal management applications. Benefiting from these merits of pore structure diversity, convenient controllability, and excellent thermophysical stability, SiO2-based composite PCMs have comparatively shown more promising prospect. In this regard, the microstructure-thermal property correlation of SiO2-based composite PCMs is still unclear despite the significant achievements in structural design. To enrich the fundamental understanding on the correlations between the microstructure and the thermal properties, we systematically summarize the state-of-the-art advances in SiO2-based composite PCMs for tuning thermal energy storage from the perspective of tailoring chemistry strategies. In this review, the tailoring chemistry influences of surface functional groups, pore sizes, dopants, single shell, and hybrid shells on the thermal properties of SiO2-based composite PCMs are systematically summarized and discussed. This review aims to provide in-depth insights into the correlation between structural designs and thermal properties, thus showing better guides on the tailor-made construction of high-performance SiO2-based composite PCMs. Finally, the current challenges and future recommendations for the tailoring chemistry are also highlighted.
ABSTRACT
Operation of the nonaqueous Li-O2 battery critically relies on the reversible oxygen reduction/evolution reactions in the porous cathode. Carbon and polymeric binder, widely used for the construction of Li-O2 cathode, have recently been shown to decompose in the O2 environment and thus cannot sustain the desired battery reactions. Identifying stable cathode materials is thus a major current challenge that has motivated extensive search for noncarbonaceous alternatives. Here, RuO x /titanium nitride nanotube arrays (RuO x /TiN NTA) containing neither carbon nor binder are used as the cathode for nonaqueous Li-O2 batteries. The free standing TiN NTA electrode is more stable than carbon electrode, and possesses enhanced electronic conductivity compared to TiN nanoparticle bound with polytetrafluoroethylene due to a direct contact between TiN and Ti mesh substrate. RuO x is electrodeposited into TiN NTA to form a coaxial nanostructure, which can further promote the oxygen evolution reaction. This optimized monolithic electrode can avoid the side reaction arising from carbon material, which exhibits low overpotential and excellent cycle stability over 300 cycles. These results presented here demonstrate a highly effective carbon-free cathode and further imply that the structure designing of cathode plays a critical role for improving the electrochemical performance of nonaqueous Li-O2 batteries.
