Study of Effect of Size on Iron Nanoparticle by Molecular Dynamics Simulation

We use the molecular dynamics simulation to study iron nanoparticles (NPs) consisting of 4000, 5000, 6000 atoms at temperatures of 300 and 900 K. The crystallization and microstructure were analyzed through the pair radial distribution function (PRDF), the potential energy per atom, the distribution of atom types and dynamical local structure parameters <fx>, where x is the bcc, ico or 14. The simulation indicated that amorphous NP contains a large number of ico-type atoms that play a role in preventing the crystallization. Amorphous NP is crystallized through transformations of f14 > 0 and fbcc = 0 type to bcc-type atoms when it is annealed at 900 K upon 40 ns. The growth of crystal clusters happens parallel with changing its microstructure. The behavior of the crystal cluster resembles the nucleation process described by classical nucleation theory. Furthermore, we found that the amorphous NP has two parts: the core has the structure similar to the one of amorphous bulk, in while the surface structure is more porous amorphous. Unlike amorphous NP, the crystalline NP also has three parts: the core is the bcc, the next part the distorted bcc and the surface is amorphous. Amorphous and crystalline NPs have part core which has the structure not depend on size. Doi: 10.28991/HIJ-2021-02-03-01 Full Text: PDF

The Structure and Crystallization Process of Amorphous Iron Nanoparticles

This paper studies the crystallization process and structure of amorphous iron nanoparticles by molecular dynamics method. The study shows that amorphous iron nanoparticles could not be crystallized at 300 K and 500 K. Iron nanoparticle, annealed at 900 K over a long time, was crystallized into a BCC crystal structure. The structure of crystallized iron nanoparticle at 900 K was analyzed through the pair radial distribution function and the number of crystal atoms upon various regions in nanoparticles. The simulation revealed that the first nuclei was formed most frequently in the area near the surface of the nanoparticle. Then the crystal cluster grew toward the centre of the nanoparticle. The completely crystallized nanoparticle had two components: the core with a BCC crystal structure and surface with an amorphous structure. As for the amorphous nanoparticle at 300 or 500 K, crystal-clusters were too small to grow large enough to crystallize the nanoparticle. Keywords Iron nanoparticle, crystallize, annealing, crystal atom, crystal cluster. References [1] J.D. Honeycutt, C.H. Andersen, Molecular dynamics study of melting and freezing of small Lennard-Jones clusters, Journal of Physical Chemistry 91 (1987) 4950-4963. https://doi.org/ 10.1021/j100303a014.[2] H. Shin, H.S. Jung, K.S. Hong and J.K. Lee, Crystallization process of TiO2 nanoparticles in an acidic solution, Chemistry letters 33 (2004) 1382-1383. https://doi.org/10.1246/cl.2004. 1382.[3] D. Shi, Z. Li, Y. Zhang, X. Kou, L. Wang, J. Wang, J. Li, Synthesis and characterizations of amorphous titania nanoparticles, Nanoscience and Nanotechnology Letters 1 (2009) 165-170. https://doi.org/10.1166/nnl.2009.1037.[4] D.N. Srivastava, N. Perkas, A. Gedanken, I. Felner, Sonochemical synthesis of mesoporous iron oxide and accounts of its magnetic and catalytic properties, The Journal of Physical Chemistry B 106 (2002) 1878-1883. https://doi. org/10.1021/jp015532w.[5] N. Zaim, A. Zaim and M. Kerouad, The hysteresis behavior of an amorphous core/shell magnetic nanoparticle, Physica B: Condensed Matter 549 (2018) 102-106. https://doi.org/ 10.1016/j.physb. 2017.10.071.[6] L. Gao and Q. Zhang, Effects of amorphous contents and particle size on the photocatalytic properties of TiO2 nanoparticles, Scripta materialia 44 (2001) 1195-1198. https://doi.org/ 10. 1016/S1359-6462(01)00681-9.[7] G. Madras, B.J. McCoy, Kinetic model for transformation from nanosized amorphous TiO2 to anatase, Crystal growth & design 7 (2007) 250-253. https://doi.org/10.1021/cg060272z.[8] C.I. Wu, J.W. Huang, Y.L. Wen, S.B. Wen, Y.H. Shen, M.Y. Yeh, Preparation of TiO2 nanoparticles by supercritical carbon dioxide, Materials Letters 62 (2008) 1923-1926. https://doi.org/10. 1016/j.matlet.2007.10.043.[9] C. Pan, P. Shen and S.Y. Chen, Condensation and crystallization and coalescence of amorphous Al2O3 nanoparticles, Journal of crystal growth 299 (2007) 393-398. https://doi.org/ 10. 1016/j.jcrysgro.2006.12.006.[10] M. Epifani, E. Pellicer, J. Arbiol, N. Sergent, T. Pagnier, J.R. Morante, Capping ligand effects on the amorphous-to-crystalline transition of CdSe nanoparticles, Langmuir 24 (2008) 11182-11188. https://doi.org/10.1021/la801859z.[11] P.H. Kien, M.T. Lan, N.T. Dung, P.K. Hung, Annealing study of amorphous bulk and nanoparticle iron using molecular dynamics simulation. International Journal of Modern Physics B 28 (2014) 1450155 (17 page). https:// doi.org/10.1142/S0217979214501550.[12] V.V. Hoang and N.H. Cuong, Local icosahedral order and thermodynamics of simulated amorphous Fe. Physica B: Condensed Matter 404 (2009) 340-346. https://doi.org/10.1016/ j.physb. 2008.10.057.

Molecular dynamic simulation of Fe nanoparticles

Fe nanoparticles have been investigated by means of molecular dynamics simulation. The nucleation and crystal growth is analyzed through the potential energy and number of different types of atoms. The simulation shows that when the amorphous sample is annealed at 900 K, it is crystallized into bcc phase. We found that as the crystal cluster has a size larger than some critical value, the mean potential energy of different types of atoms decreases in following orders: amorphous-atom → surface-crystal atom → crystal-atom. As a result, the crystal cluster is stable and tends to have a nearly spherical shape. Further, it was shown that small nuclei form frequently in the core and rarely in the surface area. After a long annealing time a cluster expands and reaches the critical radius. Then this cluster grows exponentially with times. The fully crystallized sample consists of the core with crystalline structure and surface shell with amorphous porous structure. The Fe nanoparticle has a number of polymorphs which are stable upon annealing at 300 K. We have analyzed the pair radial distribution function (PRDF) for obtained polymorphs. We found that as the fraction of crystal-atoms is less than 0.18, the PRDF is like those of amorphous metal. However, the left sub-peak is higher than right sub-peak when the fraction of crystal-atoms is less than 0.05.