*3.2.2 Photocatalytic and optical properties of Ba*2*Tb(Bi,Sb)O*<sup>6</sup> *samples*

Next, the visible-light induced decomposition experiment using IPA gas was carried out, to examine photocatalytic activities of the powder samples.

In **Figure 4a**, the temporal variation of evolved CO2 concentration after visible light irradiation are shown for the Ba2TbBiO6 citrate and solid state samples. For comparison, the data for the Sb50% substituted Ba2Tb(Bi0*:*5,Sb0*:*5)O6 sample are also given. The CO2 concentrations for both the citrate and solid state x = 0 samples rapidly rise at the initial 20 min and then show a gradual increase at further illumination time. For the Sb50% substituted sample, no clear evolution of CO2 was detected. It is expected that the heavy substitution of Sb ion at the B-site causes the band gap opening and considerably reduces the formation of electron–hole pairs, resulting in a strong suppression of the photocatalytic reaction processes. In our previous study [38], we investigated the influence of the band gap opening due to the Sb substitution on the basis of first-principles electric structure calculation. The Sb substitution at Bi site removes the Bi-orbitals and makes the corresponding band gap enlarged. The photocatalytic activity exhibits strong dependence of the Sb substitution, which is associated with the enhancement of the band gap energy. It is true that the photocatalytic behavior for the solid state sample is similar to that of the citrate sample. However, the evolved CO2 quantities of the *x*=0 citrate sample are about twice as large as the data of the corresponding solid state sample, as shown in **Figure 4a**. The improved photocatalytic activity of the Ba2TbBiO6 citrate sample is attributed to its morphology, where fine polycrystalline grains with a sub micron order are homogeneously dispersed.

Furthermore, we demonstrated photocatalytic degradation of methylene blue (MB) vs. visible light irradiation time for the end-member samples with Ba2(Pr,Tb) BiO6 and Ba2PrSbO6 compositions. For comparison, the MB data of the Ba2PrSbO6 solid-state sample are given. Here, the MB degradation rate (%) is given by ð Þ *C*ð Þ� 0 *C t*ð Þ *=C*ð Þ� 0 100, where the peak intensities located around *λ* = 665 nm at the initial and final concentrations at different time intervals are defined as *C*(0) and *C*(t), respectively. **Figure 9a** shows the MB degradation rate after the visible light irradiation for Ba2PrBiO6 and Ba2PrSbO6 samples. In **Figure 9b** and **c**, the typical absorbance spectra and corresponding MB solutions for Ba2PrSbO6 at different irradiation time intervals are also displayed. The MB degradation rates under visible light irradiation show rapid increases due to Sb-substitution. For the Sb50% substituted Ba2Tb(Bi0*:*5,Sb0*:*5)O6 sample, the highest performance of MB degradation was observed. The citrate parent sample of Ba2PrSbO6 exhibits higher degradation in comparison with the data of the solid-state sample with the identical composition. For Ba2PrBiO6, the effect of Sb-substitution on the photocatalytic degradation of MB is in direct contrast to that on the IPA decomposition under visible light irradiation.

Finally, we carried out the optical measurements on the Ba2Tb(Bi1�*<sup>x</sup>*,Sb*x*)O6 (*x*=0 and 0.5) powder samples by the diffuse reflectance method. In the first step, we transformed the observed reflectance data to the absorption coefficient *α*KM by applying the conventional Kubelka-Munk function. In the next step, extrapolating the tangent line to the *ε<sup>p</sup>* axis near the band edge, we evaluate the energy band gap from the intersection following the equation

### **Figure 9.**

*(a) Photocatalytic degradation of methylene blue (MB) vs visible light irradiation time for Ba*2*(Pr,Tb)(Bi,Sb) O*<sup>6</sup> *citrate pyrolysis samples. For comparison, the MB data of the Ba*2*PrSbO*<sup>6</sup> *solid-state sample are given. (b) Absorbance spectra as a function of visible light irradiation time for the MB degradation in the case of Ba*2*PrSbO*<sup>6</sup> *citrate pyrolysis sample. (c) Photocatalytic variations of the MB solutions at different irradiation time intervals.*

### **Figure 10.**

*(a) Optical properties of Ba*2*Tb(Bi*<sup>1</sup>�*x,Sbx) O*6*(x=0 and 0.5). α*KM*ε<sup>p</sup>* <sup>2</sup> *vs ε<sup>p</sup> for the x* ¼ 0*:*5 *sample are plotted as a function of εp. The inset shows plots of α*KM*ε<sup>p</sup>* <sup>1</sup>*=*<sup>2</sup> *vs εp. For the x = 0 sample, Eg is estimated from an intersection point of base line and straight line by extrapolation. (b) Band gap energies vs Sb content (x) for Ba*2*Tb(Bi*<sup>1</sup>�*x,Sbx) O*<sup>6</sup> *(x=0 and 0.5). The data of Ba*2*Pr(Bi,Sb)O*<sup>6</sup> *are cited from our previous work [24]. (c) Schematic illustration between the semiconductor band positions and the the surface redox reactions in photocatalysis. The valence band (VB) is located to facilitate oxidation, but the lower conduction band (CB1) is not sufficiently positioned to facilitate reduction. The higher conduction band (CB2) is suitable for promoting effective reduction process.*

*Functional Materials Synthesis and Physical Properties DOI: http://dx.doi.org/10.5772/intechopen.100241*

$$(a\varepsilon\_p)^n \propto \left(\varepsilon\_p - E\_\mathfrak{g}\right),\tag{3}$$

where *α*, *εp*, and *Eg* are the absorption coefficient, the photon energy, and the band gap energy. The exponents, *n* ¼ 2 and *n* ¼ 1*=*2, are responsible for direct and indirect optical transitions, respectively [18, 19]. In **Figure 10a**, the absorption coefficient, *α*KM*ε<sup>p</sup>* <sup>2</sup> is plotted as a function of *ε<sup>p</sup>* for the *x*=0.5 sample. In the inset of **Figure 10a**, *α*KM*ε<sup>p</sup>* <sup>1</sup>*=*<sup>2</sup> vs. *ε<sup>p</sup>* are shown for the *x*=0 sample. We obtain that *Eg* = 0.92 eV at *x*=0 and 2.45 eV at *x*=0.5, assuming indirect and direct photon transitions, respectively. The magnitude of the energy band gap is enhanced due to the Sb substitution.

In **Figure 10c**, schematic view between the semiconductor band positions and the the surface redox reactions in photocatalysis is presented. The valence band (VB) is located to facilitate oxidation, but the lower conduction band (CB1) is not sufficiently positioned to facilitate reduction. The higher conduction band (CB2) is needed to promote reduction process. Applying the present band model to the photocatalytic activities observed for Ba2Tb(Bi,Sb)O6, the parent compounds with smaller energy band gap of nearly 1 eV are responsible for the lower energy level of conduction band (CB1) edge. On the other hand, from the effect of band gap opening due to Sb substitution, we believe that the CB2 band model is valid in the heavily Sb substituted samples. The former is closely related to facilitate oxidation reactions at the surface for the IPA decomposition process. In the latter compounds with the larger energy band gaps of �2.5 eV, the MB degradation is strongly promoted in contrast to that of the former compounds. These findings indicate that the conduction band edge of the heavily Sb substituted samples is optimized.
