Visible Light-Activated Photocatalytic Effect of Iron-Containing Silicate Glass - A Review

In this review, we introduce a relationship between the local structure and visible light activated photocatalytic effect of two types of iron containing silicate glasses, i.e., 15Na(2)O center dot 15CaO center dot xFe(2)O(3)center dot(70-x)SiO2 glass (x = 10-50 in mass %, abbreviated as xNCFS) prepared by melt quenching method and xFe(2)O(3)center dot(100-x)SiO2 glass (x = 10-50 in mass %, abbreviated as xFS) prepared by sol-gel method. They were investigated by applying Fe-57-Mossbauer spectroscopy (FeMS), X-ray diffractometry (XRD) and ultraviolet-visible light absorption spectroscopy (UV-VIS). In the case of decomposition test using methyleneblue aqueous solution (MBaq) and 5ONCFS annealed at 1000 degrees C for 100 min (50NCFSa) without visible light irradiation, peak top of the UV-VIS spectra originally observed at the wavelength of 664 rim was shifted to 615 nm with the decrease in the absorbance (Abs.) from 0.866 to 0.495 after 10 days. In contrast, no peaks in the Abs. could be observed for the UV-VIS spectra of MBaq decomposed with 5ONCFSa for one-day under visible light irradiation. First order rate constant (k) of MBaq decomposition using 5ONCFSa under the visible light was calculated to be 2.87x 10(-2) h(-1). 57Fe-Mossbauer spectrum of 5ONCFSa consisted of two magnetic sextets due to alpha-Fe2O3 (isomer shift (delta) : 0.36(+/- 0.01) mm s(-1), internal magnetic field (H-int) : 51.8(+/- 0.1) T) and Fe-1.833(OH)(0.5)O-2.5 (delta : 0.34(+/- 0.01) mm s(-1), H-int : 37.9(+/- 0.6) T). These results indicate that alpha-Fe2O3 precipitated in 5ONCFSa causes the visible light activated photocatalytic effect. On the other hand, MBaq decomposition test using 50FS annealed at 1000 degrees C for 3 hour (50FSa) resulted in a remarkable decrease in Abs. of MB from 1.54 to 0.45 only after 2 h under the visible light, while no significant change in Abs. was observed using 10FSa and 30FSa, respectively of which showed precipitation of fayalite (Fe2SiO4) and magnetite (Fe3O4). The k value of MBaq decomposition using 50FSa under the visible light was estimated to be 6.48x10(-1) h(-1), which is much larger than that of 5ONCFSa. From Fe-57-Mossbauer spectrum of 50FSa, only a magnetic sextet due to alpha-Fe2O3 was detected. Tauc's plot obtained from UV-VIS-NIR spectra of 5ONCFSa and 50FSa yielded the band gap energy (E-g) of 2.2 and 2.1 eV, respectively. Because E-g value of alpha-Fe2O3 was estimated to be 2.1 eV, precipitation of alpha-Fe2O3 is essential for the visible light activated photocatalytic effect of 5ONCFSa and 50FSa. It is concluded that iron containing silicate glasses after annealing such as 5ONCFSa and 50FSa exhibit visible light activated photocatalytic effect due to precipitation of alpha-Fe2O3. They can be applicable for environmental purification.