Photocatalytic decolourization of methylene blue using [Zn-Al] layered double hydroxides synthesized at different molar cationic ratios

[Zn-Al] layered double hydroxides (LDH) with cationic molar ratios of R = Zn/Al 1-5 were synthesized by the coprecipitation method at constant pH = 10. The samples synthesized and their derived forms obtained after calcination at 500 degrees C and at 900 degrees C (denoted Zn-Al-R, Zn-Al-R-500 and Zn-Al-R-900, respectively), were characterized by X-ray diffraction (XRD), inductively coupled plasma-mass spectrometry, scanning electron microscopy, energy-dispersive X-ray spectroscopy, diffuse reflectance spectroscopy and nitrogen physisorption at -196 degrees C. The XRD study showed: (1) the presence of accessory ZnO with the LDH in samples synthesized with R > or = 3; and (2) the lamellar structure was destroyed at 500 degrees C which made room for a poorly ordered ZnO phase, while calcination at 900 degrees C yielded well crystallized ZnO and ZnAl (sub 2) O (sub 4) The photocatalytic activity of the calcined and the unheated samples was evaluated for the decolourization of methylene blue. The photocatalytic activity was dependent on the cationic ratio R and on the calcination temperature. The sample Zn-Al-3 displayed maximum photocatalytic activity. Calcination at 500 and 900 degrees C improved the photocatalytic activity of LDH synthesized at R = 1 and 2.