Effect of calcination temperature on the photoactivities of ZnO/SnO₂ nanocomposites for the degradation of methyl orange

Calcination temperature plays a key role in the crystallinity and photocatalytic activities of semiconductor photocatalysts. In this work, ZnO/SnO₂ (ZS) nanocomposites were synthesized by co-precipitation method. The as prepared photocatalysts were divided in two parts; one part was subjected to thermal heating at 700, 800, 900 and 1000 °C for 1 h in the presence of nitrogen gas while the second part was first heated at 500 °C for 4 h and then followed by the same heat treatment as the first part. The calcined samples were characterized by different techniques such as XRD, SEM and EDX to determine the crystal phase, crystal size, morphology and elemental analysis. The presence of different functional groups was determined with the help of Fourier Transform Infra-Red (FTIR) spectroscopy. The charge separation and band gaps were determined with the help of photoluminescence (PL) and diffuse reflectance spectra (DRS) respectively. The photocatalytic activities were evaluated by degrading organic dye methyl orange (MO). Interestingly, as the calcination temperature was increased, the photodegradation activities were decreased. The samples calcined at 700 °C showed excellent photocatalytic activities due to enhanced charge separation via electron transformation from ZnO to SnO₂, small crystallite sizes and large surface areas while the efficiencies of the samples calcined at 1000 °C were poor due to the formation of new intermediate phases Zn₂SnO₄. Further, from trapping experiments, it was concluded that super oxide anions and photogenerated holes were involved in the photodegradation of MO.