Improvement of the crystallinity and photocatalytic property of zinc oxide as calcination product of Zn–Al layered double hydroxide

Zinc oxide with different degrees of crystallinity can be formed as Zn–Al-layered double hydroxide (Zn–Al–NO3–LDH) calcination products. ZnAl2O4 spinel is also formed in a range of calcination temperatures from 600 to 1000 °C from the LDH. X-ray diffraction patterns showed that the crystallinity of the ZnO phase increased as calcination temperatures increased. The LDH structure was fully collapsed at and above 400 °C. The photocatalytic activity was determined by UV–VIS–NIR diffuse reflectance spectroscopy. The band gap of the calcined samples increased as the calcination temperature increased. Electron spin resonance (ESR) spectra of the fresh and calcined LDH at room temperature demonstrated that oxygen vacancies in the ZnO and ZnAl2O4 were responsible for the generation of ESR signals. One BET specific surface area increased from 1 m2/g for the LDH to a maximum at 400 °C (43 m2/g) and decreased thereafter down to 6 m2/g at 1000 °C.

► ZnO phase and ZnAl2O4 spinel can be formed as Zn–Al–NO3–LDH calcination products. ► The crystallinity of ZnO phase increased with an increase of calcination temperature. ► The optical band gaps of ZnO were improved with an increase in temperature. ► The oxygen vacancies in ZnO and ZnAl2O4 generated the ESR signals.