Huolin L Xin

Publications (33) View all

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  Available from: Huolin L Xin

  Article: Channeling of a sub-angstrom electron beam in a crystal mapped to two-dimensional molecular orbitals

  Robert Hovden, Huolin L. Xin, David A. Muller
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  ABSTRACT: The propagation of high-energy electrons in crystals is in general a complicated multiple scattering problem. However, along high-symmetry zone axes the problem can be mapped to the time evolution of a two-dimensional (2D) molecular system. Each projected atomic column can be approximated by the potential of a 2D screened hydrogenic atom. When two columns are in close proximity, their bound states overlap and form analogs to molecular orbitals. For sub-angstrom electron beams, excitation of anti-symmetric orbitals can result in the failure of the simple incoherent imaging approximation. As a result, the standard resolution test and the one-to-one correspondence of atomic positions of a crystal imaged along a zone-axis with closely spaced projected columns ("dumbbells") can fail dramatically at finite and realistic sample thicknesses. This is demonstrated experimentally in high angle annular dark field scanning transmission electron microscope (HAADF STEM) images of [211]-oriented Si showing an apparent inter-column spacing of 1.28(+-.09) Angstroms, over 64% larger than the actual 0.78 Angstrom spacing. Furthermore, the apparent spacing can be tuned with sample thickness and probe size to produce a larger, smaller, or even the actual spacing under conditions when the peaks of two adjacent Si columns should not even have been resolved given the electron probe size.
  12/2012;
• Article: Selective Placement of Faceted Metal Tips on Semiconductor Nanorods.

  Hendrik Schlicke, Debraj Ghosh, Lam-Kiu Fong, Huolin L Xin, Haimei Zheng, A Paul Alivisatos
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  ABSTRACT: Squaring the circle: Carbon monoxide was used to grow faceted cube-like platinum tips on semiconductor nanorods. These novel hybrid structures reveal a new degree of synthetic control and might allow control over the catalytic activity of nanoscale photocatalysts by adding defined faceting.
  Angewandte Chemie International Edition 11/2012; · 13.45 Impact Factor
• Dataset: nmat3458-s1

  Deli Wang, Huolin L Xin, Robert Hovden, Hongsen Wang, Yingchao Yu, David A Muller, Francis J Disalvo, Héctor D Abruña
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  Article: Structurally ordered intermetallic platinum-cobalt core-shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts.

  Deli Wang, Huolin L Xin, Robert Hovden, Hongsen Wang, Yingchao Yu, David A Muller, Francis J Disalvo, Héctor D Abruña
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  ABSTRACT: To enhance and optimize nanocatalyst performance and durability for the oxygen reduction reaction in fuel-cell applications, we look beyond Pt-metal disordered alloys and describe a new class of Pt-Co nanocatalysts composed of ordered Pt(3)Co intermetallic cores with a 2-3 atomic-layer-thick platinum shell. These nanocatalysts exhibited over 200% increase in mass activity and over 300% increase in specific activity when compared with the disordered Pt(3)Co alloy nanoparticles as well as Pt/C. So far, this mass activity for the oxygen reduction reaction is the highest among the Pt-Co systems reported in the literature under similar testing conditions. Stability tests showed a minimal loss of activity after 5,000 potential cycles and the ordered core-shell structure was maintained virtually intact, as established by atomic-scale elemental mapping. The high activity and stability are attributed to the Pt-rich shell and the stable intermetallic Pt(3)Co core arrangement. These ordered nanoparticles provide a new direction for catalyst performance optimization for next-generation fuel cells.
  Nature Material 10/2012; · 32.84 Impact Factor
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  Article: Tuning Oxygen Reduction Reaction Activity via Controllable Dealloying: A Model Study of Ordered Cu(3)Pt/C Intermetallic Nanocatalysts.

  Deli Wang, Yingchao Yu, Huolin L Xin, Robert Hovden, Peter Ercius, Julia A Mundy, Hao Chen, Jonah H Richard, David A Muller, Francis J Disalvo, Héctor D Abruña
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  ABSTRACT: A promising electrocatalyst prototype of low Pt mole fraction, intermetallic nanoparticles of Cu(3)Pt, has been prepared using a simple impregnation-reduction method, followed by a post heat-treatment. Two dealloying methods (electrochemical and chemical) were implemented to control the atomic-level morphology and improve performance for the oxygen reduction reaction (ORR). The morphology and elemental composition of the dealloyed nanoparticles were characterized at angstrom resolution using an aberration-corrected scanning transmission electron microscope equipped with an electron energy loss spectrometer. We found that the electrochemical dealloying method led to the formation of a thin Pt skin of ca. 1 nm in thickness with an ordered Cu(3)Pt core structure, while chemical leaching gave rise to a "spongy" structure with no ordered structure being preserved. A three-dimensional tomographic reconstruction indicated that numerous voids were formed in the chemically dealloyed nanoparticles. Both dealloying methods yielded enhanced specific and mass activities toward the ORR and higher stability relative to Pt/C. The spongy nanoparticles exhibited better mass activity with a slightly lower specific activity than the electrochemically dealloyed nanoparticles after 50 potential cycles. In both cases, the mass activity was still enhanced after 5000 potential cycles.
  Nano Letters 09/2012; 12(10):5230-8. · 13.20 Impact Factor