Investigating the effect of particle size on Volume-per-atom parameter of TiO2 nanoparticles

Nanoparticles are considered as one of the most important building blocks of nanotechnology. Due to the high surface to bulk ratio of nanoparticles, most of their properties and parameters differ significantly from their bulk counter parts. One of these parameters is volume of nanoparticle. In the present work, molecular dynamics (MD) simulation was used to explore the effect of amorphous surface layer on volume-per-atom (Ω) of TiO2 nanoparticles. Two most common phases of TiO2, rutile and anatase, were investigated. A novel method was proposed for studying the radial variation of Ω from the center to the surface of nanopaticles. It was found that for atoms located in the crystalline core of TiO2 nanoparticles, the value of Ω is radial independent and is the same as that of bulk TiO2. However, in the amorphous surface layer, Ω showed radial variation, with its average value larger than that of crystalline core. Moreover, it was observed that the average volume-per-atom (Ωave) of TiO2 nanoparticles is larger that that of bulk TiO2. For a more detailed examination, the radial variation of coordination number (CN) of titanium ion, from the center to the surface of nanoparticle, was calculated. It was concluded that the lower CN of titanium ions located in the amorphous surface layer, than that of conventional value, i.e. 6, is responsible for larger Ωave of TiO2 nanoparticles.