Lab-based XPS measurements were carried out to investigate the chemical composition and electronic structure of LNO with different amounts of oxygen vacancies. The XPS core-level spectra were used to investigate the chemical state and surface composition of the prepared thin film samples. Since the Ni 2p3/2 is overlapping with La 3d3/2, we used the Ni 3p region to calculate the ratio of Ni 2+ to Ni 3+ . Figure shows the highresolution Ni 3p and La 3d5/2 spectra of LNO, LNO-Vo and LNO-Vo'. The peak components in Figure at 66.4 eV (A) and 68.3 eV (B) correspond to Ni 3+ 3p3/2 and Ni 3+ 3p1/2, while the peaks at 70.0 eV (C) and 71.9 eV (D) can be assigned to Ni 2+ 3p3/2 and Ni 2+ 3p1/2. The spin-orbit splitting distance between Ni 3p3/2 and Ni 3p1/2 for Ni 2+ and Ni 3+ is 1.9 eV, which is close to the theoretical value of 2 eV. The area of the fitted components of the Ni 3p region was employed to determine the ratio of Ni 2+ and Ni 3+ , reported in Table . The results show that introducing oxygen defects in LNO in different ways leads to a decrease in Ni 3+ concentration. It is well known that oxygen vacancies induce n-type doping, resulting in extra electrons in the system, which further reduces the Ni 3+ to Ni 2+ . Therefore, the concentration of Ni 3+ in LNO decreases when oxygen vacancies exist in the LNO. In addition, the concentration of Ni 2+ is higher in LNO-Vo' than LNO-Vo, which indicates a higher concentration of surface oxygen vacancies in LNO-Vo'. The fitting peaks in La 3d5/2 show that there are two species of La2O3 and La(OH)3 in all LNO samples, where I corresponds to La2O3, II corresponds to La(OH)3, and III and IV correspond to the satellite peaks of La2O3 and La(OH)3. The surface composition ratios of Ni 2+ /Ni 3+ and Ni/La are shown in Table . In addition, the O 1s spectra of LNO, LNO-Vo and LNO-Vo' are shown in Figure . There are three species in the O 1s spectra, namely lattice oxygen (A), adsorbed hydroxyl (B) and adsorbed water (C). The results show that the intensity lattice oxygen peak (A) decreased in the LNO-Vo and LNO-Vo' compared to the LNO, which is attributed to the oxygen vacancy. More importantly, the intensity of lattice oxygen in the LNO-Vo' is much lower than in LNO-Vo, which indicates that the H2 annealing process produces more oxygen vacancies on the surface of the thin film samples. The more oxygen vacancies in LNO-Vo' is consistent with the higher Ni 2+ concentration shown in Table1.