Characterization of samples
Figures 1 and 2 show the X-ray diffraction patterns of the samples, containing initial titania, which were used for preparing photocatalysts by method 1, and samples 10–12, prepared by method 2, respectively.
Samples of TiO2, prepared by the sulfate method, contain only one phase, anatase (Fig. 1; Table 2). Degussa P25 contains nano-anatase and nano-rutile in the ~85: 15 ratio (Fig. 1; Table 2). Hombikat UV100 (Fig. 1) contains, along with nano-anatase, a phase with a characteristic diffraction reflection at 2θ ~ 12°. According to Dadachov (2006), this peak belongs to hydrous titania TiO2–x(OH)
yH2O, where y is about 1. Therefore, Hombikat UV100 contains ~9 % of hydrous titania (Table 2). According to technical information, Degussa P25 and Hombikat UV100 consist of nanoparticle ~20 and <10 nm in size, respectively. Our measurements with the use of SEM and comparative method based on nitrogen sorption capacity showed the sizes of the particles 28–30 nm for Degussa P25 and 12 nm for Hombikat UV100. Specific surface area measured by the comparative method was 53–54 m2/g for Degussa P25 and >123 m2/g for Hombikat UV100; volume of micro pores (<300 nm) was 0.163 and 0.344 cm3/g, respectively.
Impregnation with organic dyes and H2O2 by method 1 changes titania composition. Since modification of the titania samples was carried out in aqueous media, all modified samples contain hydrous titania (Table 2). Its quantity is small (1–2 %), when self-made anatase (A) and Degussa P25 are modified with MeB and large (up to 42 %), when the same initial titania samples are treated with MeR and H2O2. Content of hydrous titania in Hombikat UV100 decreases down to 1 % on treatment with MeR and increases up to 46 and 31 % on treatment with MeB and H2O2, respectively (Table 2). The anatase:rutile ratio in the samples based on Degussa P25 and sizes of coherent scattering regions in all samples prepared by method 1 do not vary significantly (Table 2).
Preparing sensitized titania by the new method for sensitizing tinania—method 2—resulted in formation of the samples containing η-TiO2 (83–85 %) and hydrous anatase (15–17 %) when using organic dyes, and containing anatase (81 %) and hydrous anatase (19 %) when using H2O2 (Fig. 2; Table 2).
Photocatalytic properties of sensitized titania under visible light
The MeB- and MeR-sensitized titania (A) samples, prepared by method 1 (samples 1 and 2, respectively), show no significant preference as compared to the commercial photocatalysts Degussa P25 (samples 4 and 5, respectively) and Hombikat UV100 (samples 7 and 8, respectively) (Table 1; Figs. 3, 4). The degree of MeO decolorization in the presence of P25-MeB sample is nonmonotonous; an increase of the dye concentration at 130–170 min can be caused by washing the pre-adsorbed dye off the surface of Degussa P25.
Sensitization of titania by method 2, developed in this work, with MeB (sample 10) and MeR (sample 11) results in higher photocatalytic activity (Table 1; Figs. 3, 4, curves 4); see “Discussion”. The best result for dye sensitizing was achieved with the use of MeB (sample 10).
After treatment with hydrogen peroxide (method 1), Degussa P25 turned pale yellow; the yellow color for A-H2O2 and UV100-H2O2 was more intensive. However, the samples on the basis of Hombikat UV100 decolorize on storage (even in darkness). The rate of the photoreaction was higher for A-H2O2 (sample 3) and UV100-H2O2 (sample 9) and lower for P25-H2O2 (sample 6) than for the corresponding dye-sensitized samples (Table 1; Fig. 5, curves 1–3). However, it was much lower than for samples 10 and 11, prepared by sensitizing titania with dyes by the new method, developed in this work (method 2).
The best visible light photocatalyst, H2O2–TiO2 (sample 12) was prepared by treatment of the reaction mixture with H2O2 while synthesizing titanium dioxide from titanyl sulfate (method 2). As shown in Fig. 5 (curve 4) complete decomposition of MeO in the presence of H2O2–TiO2 under visible light takes 60–80 min, which is better than for the samples sensitized with organic dyes (Figs. 3, 4). It is noteworthy that the sample contains no free or adsorbed H2O2 since it was completely removed during preparing sensitized TiO2. All peroxo groups are strongly coordinated by titanium atoms. Therefore, in contrast to Zou et al. (2009), the effect of destruction MeO with H2O2 is excluded.
The results of studying photocatalytic activity of sensitized titania under visible light and dependence of the rate constants on the lamp power are summarized in Figs. 6 and 7. For all the samples, increase of the lamp power first leads to increase of the reaction rate. However, for the samples prepared by method 2, the reaction rate is almost independent of the lamp power >60 W; this may be caused by stronger electron–hole pair recombination as compared to the samples prepared by method 1.
The H2O2–TiO2 (sample 12) photocatalytic activity exceeds the activity of the earlier reported visible light-responsible photocatalysts. The best previous results were the following: complete decomposition of MeO catalyzed by N/Ag- and S/Ag-doped titania took 1.5 and 4 h, respectively (Varma et al. 2011). Our photocatalyst (H2O2–TiO2) decreases the degradation time down to 1 h, although we used less powerful lamp along with lower catalyst content and MeO concentration.
Photocatalytic properties of sensitized titania under UV light
Sensitization should not affect photocatalytic properties of titania under UV radiation; however, they can be changed due to modification of the surface. That is why we compared photocatalytic activity of pure titania (self-made titania, Degussa P25, and Hombikat UV100) and the samples treated with MeB, MeR, and H2O2. In addition, photocatalytic activity of initial titania, pre-dispersed in water by stirring for 40 min, was also examined.
The results are summarized in Table 3 and Fig. 8.