Room temperature oxidation kinetics of Si nanoparticles in air, determined by x-ray photoelectron spectroscopy

Abstract

The air oxidation kinetics of low coverages of ϳ5 nm Si nanoparticles, deposited by pulsed excimer laser ablation (KrF, 248 nm) in He, have been characterized by x-ray photoelectron spectroscopy. A simple model, based on the evolution of the Si 2p spectral components during oxidation, has been developed to determine the nanoparticle oxide thickness. It is found that the short-term oxide thickness is greater, and the long-term room-temperature air oxidization rate of these nanoparticles is less, than those reported for bulk a-Si and c-Si. The results are also consistent with an earlier transmission electron microscope observation of the oxidation of larger Si particles at higher temperatures. The greater short-term oxide thickness may be attributed to surface defects on the prepared Si nanoparticles, and lower long-term oxidation rate is due to the nonlinear decrease of oxygen diffusion in spherical systems.

XPS Characterization of Au (Core)/SiO2 (Shell) Nanoparticles

“…One way is to use a simple attenuation factor through the shell where d is the thickness of the silica shell and λ S and λ C are the attenuation lengths of the core and the shell, respectively. The extended formula derived by Yang et al 33 is similar to eq 2 but also includes additional terms where κ(x) is another function containing several other terms. Two points are relevant to our discussion below: (i) the additional term in eq 3 is always smaller than 1, and (ii) eq 3 yields eq 1 when d ) 0.…”

“…31,32 In a very recent paper, the same group extended the formula, derived for a simple spherical particle by Wertheim and DiCenzo, for application to particles with a spherical core and a uniform shell and studied the oxidation kinetics of Si nanoparticles by XPS. 33 In most of the previous reports, only one element was probed by XPS to extract information about the structure of the core-shell nanoparticles. It is, however, desirable to probe by XPS different elements belonging to the core and the shell separately in order to extract more complete structural information, which eliminates many of the experimental sources of error.…”

“…21 Various reports have been published dealing with the use of XPS for characterization of various core-shell-type naostructures. [22][23][24][25][26][27][28][29][30][31][32][33] When these core-shell nanoparticles are deposited on smooth surfaces for XPS analysis, the photoelectrons created are severely attenuated as they pass through the core and the shell before they escape into the vacuum for their kinetic energy analysis. This attenuation is, in principle, well-known and can be modeled to extract structural and morphological information from the XPS data.…”

Freestanding silicon nanocrystals with extremely low defect content

“…This difference should not result from the differences in size of the NCs in the two samples. In previous studies, shorter induction periods have been observed for nanoparticles (curved Si surface) in comparison to flat surfaces of bulk silicon, 44 due to the increasingly higher number of oxidation sites present in increasingly curved surfaces. Thus, the somewhat smaller size of the NCs in the LIDD samples would result in a shortened induction period in comparison to MIDD Si-NCs, which is in clear contrast with our experimental results.…”

Characterization challenges for nanomaterials

“…Determining oxidation and corrosion rates for nanoparticles is somewhat more complex than determining corrosion rates for larger flat samples. [21] Here we compare the oxidation rates of two differently sized nanoparticles, prepared by two different processes, in DI water. Reactivity studies conducted on these particles show significantly different reaction rates.…”