(< 100 nm) particles. Slurry milling in Vertrel XF
(Fig. 1(d)) produces a more homogeneous particle size
distribution with few particles less than 100 nm.
Similarly, slurry milling in methanol produces a
particle size distribution with few particles below
100 nm. These qualitative results support the BET
surface area measurements which indicate only modest
surface area increases with slurry milling.
Upon dry milling, the color of the NiO samples
shifts from green to dark brown. Since stoichiometric
NiO is bright green and non-stoichiometric NixOy is
dark brown [34], it is suspected that dry milling either
changes the bulk crystal structure of the NiO or
changes the surface chemistry of the NiO. The former
possibility can be excluded because X-ray diffraction
does not provide any evidence for a change in bulk
crystal structure upon milling for 480 min with or
without additives