Fig. 3. Photocatalytic degradation of RhB: a) the decolorization of RhB, b) the logarithmic curve of the decolorization, c) rate constant of first order reaction
Fig. 3a shows the photocatalytic activity of γ-Bi2O3/Fe2O3(x) composites for decolorization of RhB. The decomposition of RhB approximates to the first order kinetics concerning the concentration of RhB. That is, the photocatalytic reaction is simply described by:
ln(C0/C)=k1t.
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where C0, C, k1, and t represents initial concentration, final concentration, rate constant, and reaction time, respectively [14-15]. In order to characterize the photocatalytic activity more clearly, k1 is obtained from the slope of the logarithmic curve of the decolorization (Fig.3b) and presented in Fig.3c. The value of k1 for samples S1 to S9 is 0.0606 min-1, 0.2495 min-1, 0.0867 min-1, 0.0665 min-1, 0.0492 min-1, 0.0351 min-1, 0.0177 min-1, 0.0013 min-1, 0.0259 min-1, respectively. For pure γ-Bi2O3 photocatalyst, more than 90% of RhB was decolorized after 60 minutes under visible-light irradiation. However, the reaction time is only 20 minutes for γ-Bi2O3/Fe2O3(x) composite with 10 wt. % Fe2O3. Comparing with pure γ-Bi2O3, the photocatalytic activity is promoted with the x less than 30 %. If further increasing Fe2O3¬, the photocatalytic ability of γ-Bi2O3/Fe2O3(x) decreases gradually. Nevertheless, it remains better than TiO2-xNx until the proportion of Fe2O3¬ reaches 50 %.
Fig. 4. Energetic diagrams of Fe2O3 and Bi2O3
N. Serpone et al. [16-17] have reported the chemical evidence for electron and hole transfer between coupled semiconductors. Xintong Zhang et al. [18] have carried out research on the photo-induced interfacial charge transfer in TiO2/Fe2O3 heterostructured composite film and deduced a photoinduced inter-facial electron transfer mechanism. Considering the position of band edge, it is supposed that γ-Bi2O3/Fe2O3(x) shares similar mechanism with TiO2/Fe2O3 composites. As we known, γ-Bi2O3 demonstrates excellent photocatalytic ability to decompose RhB, but it can only absorb narrow visible light. Although, Fe2O3 shows its advantage in utilizing visible light, its photocatalytic activity is low due to the rapid recombination of its photoinduced electron-hole pair. Therefore, γ-Bi2O3 is regarded as a main photocatalyst, and the role of Fe2O3 is a sensitizer absorbing visible light. As indicated in Fig.4, the VB level of Fe2O3 is lower than that of γ-Bi2O3. With irradiation of visible-light, the electrons in the VB of Fe2O3 are excited to its CB. The photoinduced hole in VB of Fe2O3 can transfer to that of Bi2O3. As a result, electron-hole pair can separate successfully and the holes accumulate in VB of Bi2O3, which is advantageous for photocatalytic oxidation reaction. For holes generated in the VB of Fe2O3 at same time, the transportation to that of γ-Bi2O3 and recombination with electrons are two competitive process each other. With the increase of x, the latter process becomes dominant, so the photocatalytic activity deceases gradually.
4. Conclusions A series of γ-Bi2O3/Fe2O3(x) composites were prepared
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