Effects of calcination temperatures and additives on the photodegradation of methylene blue by tin dioxide nanocrystals

The tin dioxide (SnO 2 ) nanocrystals were synthesized by direct precipitation method and a series of SnO 2 samples were obtained via calcining SnO 2 at different temperatures. The characteristics of SnO 2 samples were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), Brunauer Emmett Teller (BET), photoluminescence spectroscopy (PL), and UV–vis diffuse reflectance spectroscopy (DRS). The effects of calcination temperatures and additives on the photodegradation of methylene blue (MB) on SnO 2 samples under UV-light irradiation have been researched. The results showed that the SnO 2 sample with low temperature-treated ( T ≤200   °C) exhibits the highest degradation efficiency, while with increasing temperature-treated (from 400   °C to 1000   °C) SnO 2 samples show decreasing degradation activities. The additives, such as methanol, sodium fluoride, Fe(III), Ag + , and terephthalic acid showed different influences on MB degradation in UV/SnO 2 systems. It has been demonstrated that in SnO 2 -MB systems both hydroxy radicals (OH ) generated on the surface of SnO 2 and photogenerated holes (h + vb ) in the valence band (VB) of SnO 2 jointly control the photo-oxidation process of MB. In SnO 2 system with increasing temperature-treated ( T ≥400   °C) the contribution of OH to MB degradation gradually decreased due to the diminution of surface-bonding hydroxyl groups (–OH) and chemisorbed water (H 2 O) molecules on the surface of SnO 2 , by comparison, the contribution of h + vb increased. The effects of Ag + and Fe(III) species on MB degradation and the formation of OH were investigated in detail and the mechanisms have been discussed. The presence of Ag + ion at low concentration promotes the degradation of MB by capturing the photogenerated electrons (e cb − ) on the surface of SnO 2 catalyst, suppressing the recombination of photogenerated electrons and holes. The presence of Ag + ion at high concentration observably reduces the degradation in catalyst system which the photodegradation process is controlled by OH , conversely, it intensively promotes the degradation in which the photodegradation process is controlled by h + vb . The effects of Fe(III) species on the degradation are similar to those of Ag + ions.