Impact of thermal oxidation on the adsorptive properties and structure of porous silicon particles

Abstract

A combination of gas and probe molecule adsorption from aqueous solution have been applied to determine the adsorptive and structural properties of porous silicon (pSi) particles as a function of thermal oxidation in the range 473-1073 K. Gaseous nitrogen adsorption has shown a decrease in the BET specific surface area, mesopore volume, and diameter due to the increasing molecular volume of the oxidized silicon layer within the pores. Methylene blue was used as an adsorbate to establish the apparent surface area of pSi particles in aqueous solution and has shown adsorption capacity independent of oxidation in the range of 473-873 K. Comparable adsorption capacities at low oxidation temperatures are due to methylene-blue-induced oxidation which has been established by X-ray photoelectron spectroscopy, while increased adsorption at high temperatures is due to increased surface silanol concentrations. Comparisons between the nitrogen- and methylene-blue-determined surface areas demonstrate a difference in the adsorptive properties of gas and solution phase molecules onto pSi. These differences can be attributed to the different adsorption mechanisms, that is, physisorption for nitrogen and electrostatic attraction for methylene blue. Nitrogen and methylene blue adsorption probe different features of pSi, demonstrating modification of the structural and adsorptive properties with oxidation as well as highlighting the improved characterization that can be obtained from the combination of solution and gas phase adsorption. © 2008 American Chemical Society.