Formation processes of nickel oxide nanoparticles in SiO2 by metal-ion implantation combined with thermal oxidation

Formation processes of nickel oxide (NiO) nanoparticles (NPs) in silica glass (SiO2) by implantation of 60 keV Ni− ions combined with thermal oxidation are studied using cross-sectional transmission electron microscopy (XTEM) and Rutherford backscattering spectrometry. In as-implanted state, Ni metallic NPs form within the surface layer of ∼80 nm thick. The mean depth 〈z〉 and the standard deviation 〈(Δz)2〉1/2 of Ni atom distribution determined by XTEM were 43 and 15 nm, respectively. After the oxidation at 800 °C for 1 h, 〈z〉 and 〈(Δz)2〉1/2 became 47 nm and 20 nm, respectively, i.e. the distribution was almost the same except a small diffusional shift to the deeper region. Mean volume of Ni- and NiO-NPs after heat treatments at 800 °C were 27 and 43 nm3, respectively. The larger mean volume of NiO-NPs is explained from the fact that NiO-NPs include both Ni and O atoms, i.e. approximately 2N atoms, while Ni-NPs include Ni atoms only, i.e. N atoms. Both the Ni- and NiO-NPs include 2.4 × 103 Ni atoms per NP in average. These results indicate that NiO-NPs are formed by oxidation of Ni-NPs without pronounced redistribution of Ni atoms.