Initial Stages of Sintering of TiO 2 Nanoparticles: Variable-Charge Molecular Dynamics Simulations
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Initial Stages of Sintering of TiO2 Nanoparticles: Variable-Charge Molecular Dynamics Simulations Shuji Ogata,1 Hiroshi Iyetomi,2 Kenji Tsuruta,3 Fuyuki Shimojo,4 Aiichiro Nakano,5 Priya Vashishta,5 Rajiv K. Kalia,5 and Chun-K. Loong6 1 Department of Applied Sciences, Yamaguchi University, Ube 755-8611, Japan 2 Department of Physics, Niigata University, Niigata 950-2181, Japan 3 Department of Electrical and Electronic Engineering, Okayama University, Okayama 700-8530, Japan 4 Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan 5 Concurrent Computing Laboratory for Materials Simulations, Louisiana State University, Baton Rouge, LA 70803-4001, U.S.A 6 Argonne National Laboratory, Argonne, IL 60439, U.S.A.
ABSTRACT Variable-charge molecular dynamics simulation of 32 TiO2-nanoparticles with diameter 60Å is performed for 40 ps at 1 GPa and 1,400 K for both rutile and anatase phases, to investigate their phase-dependent sintering mechanisms. In the rutile case, the nanoparticles rotate around their centers during the first 20 ps. Varying degrees of neck formation between neighboring rutile-nanoparticles are found at ~ 40 ps. In the anatase case, the nanoparticles maintain their original orientations. Similar degrees of neck formation are observed at contacting regions of the anatase nanoparticles.
INTRODUCTION TiO2 is well known for its wide range of applications including photocatalysts [1-3], capacitors [3], and pigments [3]. Those useful applications stem from its high electric permittivity [3], ε ≈ 200 along 001 direction and ε ≈ 90 along 100 direction, indicating large degrees of dynamic charge-transfer between the atoms. The TiO2 assumes the rutile structure at the ground state; the anatase structure, at a metastable state [3]. TiO2 nanoparticles are synthesized by several methods. In the gas-condensation method [4], rutile nanoparticles with size ~ 14 nm are formed [5]. Anatase nanoparticles with 3 – 10 nm can be formed in the sol-gel method [6,7]. Those anatase-nanoparticles sinter at lower temperatures [8] (~ 900 K) than the sintering temperature (~ 1,300 K) of rutile nanoparticles. During the sintering of anatase nanoparticles, an anatase-to-rutile phase transformation has been found at grain boundaries [8]. Such low sintering temperatures are desirable for technical applications since grain growth is significantly suppressed during the sintering. Theoretical understanding of the different sintering mechanisms between the rutile and anatase nanoparticles will offer valuable information for improved production of nanophase materials. We have recently developed a variable-charge inter-atomic potential [9] for molecular dynamics (MD) simulations of TiO2 systems. In the potential, atomic charges vary dynamically depending on their environment to minimize the total potential energy [9,10]. The calculated B7.6.1
pressure- and orientation-dependent dielectric constants and surface energies of low-index planes for the rutile crystal as well as its cohesive energy, elastic
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