Co metal nanoparticles deposition inside or outside multi-walled carbon nanotubes via facile support pretreatment

•Co/MWCNT hybrids with controllable localization of Co nanoparticles are obtained.•Localization of Co nanoparticles depends on the nature and structure of MWCNTs.•Hydrophobic/hydrophilic properties of MWCNT surface strongly affect Co deposition.•Oxidized MWCNTs with thin walls stabilize Co nanoparticles mainly inside their channels.•Oxidation of MWCNTs leads to the decrease of Co nanoparticles average size.