The Zn 2p3/2 spectra for both the pure and the Al-doped ZnO ceramics include two components corresponding to metallic Zn (Zn–Zn) and Zn in the oxidized state (Zn–O) [21]. Moreover, the peaks of Zn 2p3/2 for Al-doped sample were shifted to a lower binding energy, revealing that more oxygen vacancies are produced by the introduction of Al.