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Origin and nature of spontaneous shape fluctuations in "small" nanoparticles.

Yang Y, Zhang H, Douglas JF - ACS Nano (2014)

Bottom Line: 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.NPs also normally exhibit facile coalescence when in proximity, impacting their stability and reactivity in applications.In contrast, stringlike collective atomic motion is confined to the NP interfacial region of NPs having a diameter greater than a few nanometers, and correspondingly, the overall NP shape remains roughly spherical, a case studied in our prior Ni NP simulations.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Materials Engineering, University of Alberta , Edmonton, Alberta T6G 2V4, Canada.

ABSTRACT
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, impacting their stability and reactivity in applications. We perform molecular dynamics simulations 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 motion of Ni atoms through the NP center, and we quantify these dynamic structures and their impact on NP shape. In contrast, stringlike collective atomic motion is confined to the NP interfacial region of NPs having a diameter greater than a few nanometers, 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.

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Numbers of mobile atoms, string atoms, and string length compared with <u2> as a function of t for N = 55 and T = 0.86Tm.
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fig8: Numbers of mobile atoms, string atoms, and string length compared with <u2> as a function of t for N = 55 and T = 0.86Tm.

Mentions: If mobile atoms in the form of stringlike cooperative atomic rearrangements are responsible for the peak in α2 and the hopping peak in Gs (r, t), as in previous studies of larger Ni NPs, then we should see particles moving, but staying within proximity of each other rather than wandering off on their own. Using criteria explained in Supporting Information and our previous papers,34,37,65 the extent of stringlike collective motion is then determined and the results of this analysis are shown in Figure 8. The majority of collective motion events observed in the current study is sequential, but we need to be careful in drawing general conclusions about the nature of the collective motion from these limited observations. As can be seen, the relative shape anisotropy, the number of mobile atoms, the number of atoms involved in the string, and the string length peak positions all match each other rather well. Stringlike collective motion clearly accompanies the NP shape changes.


Origin and nature of spontaneous shape fluctuations in "small" nanoparticles.

Yang Y, Zhang H, Douglas JF - ACS Nano (2014)

Numbers of mobile atoms, string atoms, and string length compared with <u2> as a function of t for N = 55 and T = 0.86Tm.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4334263&req=5

fig8: Numbers of mobile atoms, string atoms, and string length compared with <u2> as a function of t for N = 55 and T = 0.86Tm.
Mentions: If mobile atoms in the form of stringlike cooperative atomic rearrangements are responsible for the peak in α2 and the hopping peak in Gs (r, t), as in previous studies of larger Ni NPs, then we should see particles moving, but staying within proximity of each other rather than wandering off on their own. Using criteria explained in Supporting Information and our previous papers,34,37,65 the extent of stringlike collective motion is then determined and the results of this analysis are shown in Figure 8. The majority of collective motion events observed in the current study is sequential, but we need to be careful in drawing general conclusions about the nature of the collective motion from these limited observations. As can be seen, the relative shape anisotropy, the number of mobile atoms, the number of atoms involved in the string, and the string length peak positions all match each other rather well. Stringlike collective motion clearly accompanies the NP shape changes.

Bottom Line: 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.NPs also normally exhibit facile coalescence when in proximity, impacting their stability and reactivity in applications.In contrast, stringlike collective atomic motion is confined to the NP interfacial region of NPs having a diameter greater than a few nanometers, and correspondingly, the overall NP shape remains roughly spherical, a case studied in our prior Ni NP simulations.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Materials Engineering, University of Alberta , Edmonton, Alberta T6G 2V4, Canada.

ABSTRACT
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, impacting their stability and reactivity in applications. We perform molecular dynamics simulations 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 motion of Ni atoms through the NP center, and we quantify these dynamic structures and their impact on NP shape. In contrast, stringlike collective atomic motion is confined to the NP interfacial region of NPs having a diameter greater than a few nanometers, 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.

Show MeSH
Related in: MedlinePlus