The powder (NiOþMg) to ball ratio was fixed at
1:5 while 13.37 ml of ethanol (99.5%, Sigma Aldrich) was used as a
process control agent during ball milling. The optimal milling time
was 10 h which is the time that took for Mg turnings to flatten. The
ball-milled blend of NiO powder and Mg was combusted in the
presence of dry ice (solid CO2). The combustion product was washed with 1 M HCl (37%, Merck) to remove any impurities and
free-standing metal oxides' residue. The product was then filtered,
washed several times with deionized water and finally, dried under vacuum for 12 h at 100 °C. Please see Supplementary Data for
further details.