Final states after Ni2p photoemission in the nickel—oxygen system

The Ni2p, Ni3p, Li1s and O1s XPS spectra were measured for single crystal NiO and LixNi1−xO, cleaved and scraped, respectively, in a spectrometer. The cleaved surface had Ni2p32 peaks at 853.9 and 855.8 eV with an intensity ratio of 1:1, where the FWHM of the former was smaller than that of the latter. The intensity ratio decreased with doping of lithium. Although these peaks had previously been ascribed to di- and tri-valent nickel ions in their ground states, we interpret them here as follows. The peaks at 854 and 856 eV correspond to the final states of Ni2p3d9L and Ni2p3d10L2, where the underline denotes an electron hole. An O2p electron hole in the ground state is necessary to give the latter final state. The doping of lithium increases the density of electron holes and the intensity of the latter final state. The spectral profile of Ni3p did not change with the addition of lithium. The binding energies of Li1s from the particle surface and the bulk were 55.5 and 53.5 eV, respectively. The large shift is associated with surface contamination. The O1s spectrum of the cleaved NiO had two peaks at 529.5 and 531.5 eV with an intensity ratio of 1:0.1. The FWHM of the peaks at 529.5 eV for NiO and Li0.3Ni0.70 were 1.0 and 1.7 eV, respectively. The binding energy of the main peak for LiNiO2 was 528.7 eV.