“…The high-resolution O1s spectrum can feed back the information including oxygen-metal bonding and oxygen vacancies. O1s spectra can be separated into three peaks three peaks at 529.3, 530.6, and 532.6 eV, which correspond to Fe-O, Bi-O bonds, and the surface-adsorbed water (Soltani and Lee, 2016;Gu et al, 2019). The bond energy was shifted from 530.6 to 530.2 eV, which indicates the presence of oxygen vacancies with different doping amounts of Ca 2+ ions (Ibrahim et al, 2007;Jaafar et al, 2019).…”