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Schottky defects arise when some lattice points in a crystal, such as those in NaCl, remain unoccupied, creating lattice vacancies without disturbing the overall electrical neutrality of the crystal. This defect is common in ionic crystals where the positive and negative ions are similar in size, as seen in sodium chloride and cesium chloride. The presence of Schottky defects enables the crystal to conduct electricity to a small extent through an ionic mechanism. Electric fields cause nearby...
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Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses
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Self-diffusion in nanocrystalline alloys.

Zbigniew Kaszkur1, Wojciech Juszczyk, Dariusz Łomot

  • 1Institute of Physical Chemistry PAS, Kasprzaka 44/52, 01-224, Warszawa, Poland. zbig@ichf.edu.pl.

Physical Chemistry Chemical Physics : PCCP
|April 28, 2015
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Summary

This study monitored surface segregation in palladium-silver (Pd-Ag) nanocrystals using operando X-ray diffraction/mass spectrometry (XRD/MS). Researchers observed reversible segregation of Pd and Ag, revealing insights into diffusion mechanisms in nanomaterials.

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Area of Science:

  • Materials Science
  • Nanotechnology
  • Surface Science

Background:

  • Nanocrystalline alloys are crucial for catalysis, but their surface composition dynamics are not fully understood.
  • Understanding surface segregation in bimetallic nanoparticles is key to controlling catalytic activity and selectivity.
  • Previous studies often lack in-situ methods to observe dynamic surface changes at the nanoscale.

Purpose of the Study:

  • To investigate the dynamic surface segregation of palladium (Pd) and silver (Ag) in a nanocrystalline Pd-Ag alloy under operando conditions.
  • To elucidate the diffusion mechanisms governing surface segregation in metal nanoclusters.
  • To demonstrate the potential for engineering nanoparticle surfaces for specific catalytic applications.

Main Methods:

  • Operando X-ray diffraction (XRD) and mass spectrometry (MS) experiments were performed on a 10 wt% Pd(70%)Ag(30%) alloy supported on silica.
  • Experiments were conducted at 673 K under controlled atmospheres (CO and He) to monitor surface segregation.
  • Structural changes were analyzed using XRD data, and diffusion phenomena were modeled.

Main Results:

  • Reversible surface segregation of Pd (in CO) and Ag (in He) was observed at 673 K.
  • XRD data revealed structural changes consistent with diffusion within nanoclusters.
  • Qualitative differences in segregation rates suggest distinct diffusion mechanisms, with Pd segregation involving vacancy depletion.

Conclusions:

  • Operando XRD/MS is a sensitive technique for detecting subtle surface changes in nanoclusters.
  • Surface segregation processes can be controlled by temperature, allowing for surface engineering.
  • This approach enables the preparation of well-defined nanoparticle surfaces for tailored catalytic reactions.