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

摘要

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 ℃) exhibits the highest degradation efficiency, while with increasing temperature-treated (from 400 ℃ to 1000 ℃) 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 ℃） the contribution of OH~? to MB degradation gradually decreased due to the diminution of surface-bonding hydroxyl groups (-OH) and chemisorbed water (H_20) 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.