The influences of dealloying temperature and time on the morphology, structure, and magnetic properties of porous Co nanoparticles

The Co-61.8 wt% Al nanoparticles (C-ANs) were prepared by hydrogen plasma-metal reaction method. These spherical nanoparticles possess core/shell structure with an average particle size of 45 nm. The particle core belongs to single crystalline B2-CoAl compound of about 37 nm, and the particle shell is made of Al with a thickness of 4-6 nm. Three kinds of porous Co nanoparticles (PCNs) were produced via dealloying these C-ANs with sodium hydroxide solution at 323 K for 10 min, 323 K for 60 min, and 353 K for 10 min, respectively. The nanoparticles dealloyed at 323 K are composed of both fcc-Co and hcp-Co phases, and the high dealloying temperature of 353 K promotes the formation of single fcc-Co phase. These dealloyed nanoparticles are porous and change into polycrystalline structure with grain size ranging from 2 to 5 nm. The residual Al content in PCNs decreases with increasing dealloying time or temperature, and it is crucial to prevent the porous structure from collapsing. The BET surface area of PCNs dealloyed at 323 K for 10 min is 44.1 m²/g, and it decreases with increasing time and temperature. The elevated dealloying temperature improves the saturation magnetization of the dealloyed nanoparticles, and it reaches 109.9 emu/g for the sample dealloyed at 353 K. The coercive force of PCNs decreases with the increase of dealloying time and temperature due to the decrease of micro defects.