We show that the HF acid etch commonly used to prepare SrTiO 3 (001) for heteroepitaxial growth of complex oxides results in a non-negligible level of F doping within the terminal surface layer of TiO 2 . Using a combination of x-ray photoelectron spectroscopy and scanned angle x-ray photoelectron diffraction, we determine that on average ~13 % of the O anions in the surface layer are replaced by F, but that F does not occupy O sites in deeper layers. Despite this perturbation to the surface, the Fermi level remains unpinned, and the surface-state density, which determines the amount of band bending, is driven by factors other than F doping. The presence of F at the STO surface is expected to result in lower electron mobilities at complex oxide heterojunctions involving STO substrates because of impurity scattering. Unintentional F doping can be substantially reduced by replacing the HF-etch step with a boil in deionized water, which in conjunction with an oxygen tube furnace anneal, leaves the surface flat and TiO 2 terminated.
Lithium-sulfur batteries (LSBs) are still limited by the shuttle of lithium polysulfides (LiPS) and the slow LiÀ S reaction. Herein, we demonstrate that when using cobalt sulfide as a catalytic additive, an external magnetic field generated by a permanent magnet can significantly improve the LiPS adsorption ability and the LiÀ S reaction kinetics. More specifically, the results show both experimentally and theoretically how an electron spin polarization of Co ions reduces electron repulsion and enhances the degree of orbital hybridization, thus resulting in LSBs with unprecedented performance and stability. Under an external magnetic field, LSBs with 0.0084 % per cycle decay rate at 2 C during 8150 cycles are produced. Overall, this work not only demonstrates an effective strategy to promote LiPS adsorption and electrochemical conversion in LSBs at no additional energy cost but also enriches the application of the spin effect in the electrocatalysis fields.
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