Surface photovoltage and XPS studies of electronic structure in defective nickel oxide powders

Surface photovoltage measurements of polycrystalline powder samples of nickel oxide have revealed differences in band bending (V_s) related to the defect concentrations of the oxide. The powders prepared at 700°C and 1450°C in air consist of small single crystals with low concentrations ( <10⁹ cm^-2) of extended defects, but considerably different nickel vacancy concentrations. The defective NiO (700°C) gave V_s = 760 mV and the equilibrated NiO (1450°C) V_s = 230 mV under UV illumination, possibly due to Fermi level pinning of the Ni 3d⁸ band near V′_Ni and V″_Ni, respectively. These results are consistent with values of V_s (i.e. 630 and 240 mV, respectively) for these two samples obtained previously from XPS measurements of the dependence of surface charging on temperature. A visible transition at ≈ 2.5 eV, giving Δφ = 135 mV, is also found in the equilibrated NiO (1450°C) sample but not in the defective NiO (700°C) sample. The results are discussed in relation to other work on the electronic band structure and surface states of nickel oxide.