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 (Fig. 2). All of the peaks present in the X-ray diffraction spectra of NiO milled for 480 min are in the same location as the peaks for unmilled NiO. These peaks are consistent with reported X-ray
diffraction patterns for NiO, indicating that milling does not induce changes to the crystal structure of the bulk materials [35]. Peak broadening is observed for dry milled NiO, but not for the slurry milled NiO (Fig. 2). This broadening is attributed primarily to crystallite size reduction with possible contributions from milling induced strain [35]. This interpretation is consistent with the fact that significant particle size reduction is observed in BET for dry milled NiO, but not for slurry milled NiO.