Composition and electronic structure of porous silicon nanoparticles after oxidation under air- or freeze-drying conditions

•Electronic structure and composition of differently dried porous Si nanoparticles.•Synchrotron investigation on porous Si nanoparticles surface composition.•A significant effect of drying conditions on the porous Si nanoparticles.•Freeze-drying process prevents deep oxidation of the porous silicon surface.•Porous silicon nanoparticles drying allow to fine control their surface composition. A controlled change in the composition and electronic structure of porous silicon nanoparticles by variation the drying conditions is proposed. Two types of nanopowders were obtained from mechanically milled porous silicon films with subsequent air- or freeze-drying processing. Obtained nanoparticles surface morphology and structure were investigated. Silicon atoms local surrounding specificity and electronic structure probing of studied powders were performed with the use of synchrotron ultrasoft X-ray absorption near edge structure spectroscopy technique. Noticeable changes in surface composition and structure are observed. The air-dried porous silicon nanoparticles were characterized by the presence of a thick surface oxide layer, while the presence of unoxidized silicon atoms at a depth of less than 3 nm was demonstrated for the lyophilized ones.