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  1. Free, publicly-accessible full text available November 10, 2023
  2. Although scanning transmission electron microscopy (STEM) images of individual heavy atoms were reported 50 years ago, the applications of atomic-resolution STEM imaging became wide spread only after the practical realization of aberration correctors on field-emission STEM/TEM instruments to form sub-Ångstrom electron probes. The innovative designs and advances of electron optical systems, the fundamental understanding of electron–specimen interaction processes, and the advances in detector technology all played a major role in achieving the goal of atomic-resolution STEM imaging of practical materials. It is clear that tremendous advances in computer technology and electronics, image acquisition and processing algorithms, image simulations, and precision machining synergistically made atomic-resolution STEM imaging routinely accessible. It is anticipated that further hardware/software development is needed to achieve three-dimensional atomic-resolution STEM imaging with single-atom chemical sensitivity, even for electron-beam-sensitive materials. Artificial intelligence, machine learning, and big-data science are expected to significantly enhance the impact of STEM and associated techniques on many research fields such as materials science and engineering, quantum and nanoscale science, physics and chemistry, and biology and medicine. This review focuses on advances of STEM imaging from the invention of the field-emission electron gun to the realization of aberration-corrected and monochromated atomic-resolution STEM and its broad applications.
  3. Single-atom catalysts (SACs) exhibit excellent performance for various catalytic reactions but it is still challenging to have adequate total activity for practical applications. Here we report the high-valence, square planar Pt 1 –O 4 as an active site that enables significantly to increase the total activity of the Pt 1 /Fe 2 O 3 SAC with a Pt loading of only ∼30 ppm, which is similar to that of a 1.0 wt% nano-Pt/Fe 2 O 3 , for CO oxidation at 350 °C. Density functional theory calculations reveal that Pt 1 –O 4 catalyzes CO oxidation through a non-classical Mars–van Krevelen mechanism. The adsorbed O 2 on Pt 1 atoms activates the coordination oxygen in the Pt 1 –O 4 configuration, and then a barrierless O 2 dissociation occurs on the Pt 1 –Fe 2 triangle to replenish the consumed coordination oxygen by the cooperative action of Pt 5d and Fe 3d electrons. This work provides a new fundamental understanding of oxidation catalysis on stable and active SACs, providing guidance for rationally designing future heterogeneous catalysts.