Effects of Size and Coalescence on the Interfacial Dynamics of Nanoparticles: A Molecular Dynamics Study

• Author / Creator

  Yang, Ying

• Normally chemically inert materials such as Au have been found to be catalytically active in the form of particles whose size is about 1 nm. Direct and indirect observations of various types of metal nanoparticles (NPs) in this size range, under catalytically relevant conditions for fuel-cell operation and catalysis, have indicated that such ‘small’ particles can exhibit large spontaneous shape fluctuations and significant changes in shape and chemical activity in response to alterations in environmental conditions. NPs also normally exhibit facile coalescence when in proximity, and these agglomations impact their stability and reactivity in applications. Our molecular dynamics simulations first focus on Ni nanoparticles, a commonly used NP in catalytic applications and carbon nanotube growth, in the ~ 1 nm size regime where large scale shape fluctuations have been observed experimentally. An analysis of the large scale shape fluctuations observed in our simulations of these ‘small’ NPs indicates that they are accompanied by collective migration motion of Ni atoms through the NP center and we quantify these dynamic structures and their impact on NP shape. In contrast, string-like collective atomic motion is confined to the NP interfacial region of NPs having a diameter greater than a few nm and, correspondingly, the overall NP shape remains roughly spherical, a case studied in our prior Ni NP simulations. Evidently, the large spontaneous NP shape fluctuations reflect a change in character of the collective atomic dynamics when the NPs become critically small in size. Meanwhile, we are aimed to investigate the NP sintering and coalescence phenomenon quantitatively to determine the string-like collective motions in the coalescence of ultra-small NPs and the crystallization process to form the crystalline nanoparticles. We identify the string-like collective motions accompany the evolvements of the local fcc structure, the local icosahedral structure and the liquid-like structure as well as the crystallization process, rather than the rotations of the NPs, in the coalescence of the multiple ultra-small NPs. The recognition of this phenomenon would help us understand the early stages of crystal growth and structural rearrangement of the relatively disordered ‘amorphous’ pre-nucleation clusters. Finally, we explore the NP behavior under the constraint of the graphene substrate as the supporting material. The results show that NP affected by the substrate has lower ratio of the local fcc structure than the single free NP, indicating that the substrate makes the NP more disordered and more active.

• Subjects / Keywords

  • Molecular-dynamics
  • Nanoparticle shape fluctuation
  • String collective atomic motion
  • Nanoparticles

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3RR1PS3P

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.