## 3 Result and Discussion
### Optical Properties  
The UV-Visible optical absorption spectrum of BiVO\({}_{4}\) microspheres was measured and compared with commercial BiVO\({}_{4}\) and synthesized BiVO\({}_{4}\) microspheres, as shown in Figure 7. A broad absorption spectrum could be seen from 300 to 400 nm. At around 420 nm, an abrupt cutoff absorption edge was observed. The absorption band contains a tail extending rightward until about 800 nm. This may be a result of crystal defects formation during the growth of the BiVO\({}_{4}\) microspheres. The band gap energy was determined by Kubelka-Munk derivation (Figure 7). The band gap of this photocatalyst is 2.60 eV, and it has yellowish colored powder. This shows that this photocatalyst can work efficiently under visible light irradiation. In addition, the presence of the monoclinic phase which has superior photocatalytic activity contributed to the enhancement of the photocatalytic character of this photocatalyst. Figure 5: Schematic illustration of the proposed formation of flower-like BiVO\({}_{4}\) microsphere structure. Figure 6: Particle size analysis of BiVO\({}_{4}\) microspheres under calcination temperatures of 60 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C and 600 \({}^{\circ}\)C.  
Photoluminescence spectroscopy was studied at the excitation wavelength \(\lambda_{\text{ex}}\) = 325 nm. As shown in Figure 8, a strong peak can be seen at around 560 nm for both synthesized microspheres and commercial BiVO\({}_{4}\). The strong emission at 560 nm corresponds to recombination of the hole formed from the hybrid orbitals of Bi 6s and O 2p and the electron generated from the V 3d orbitals. In addition, the emission bands of BiVO\({}_{4}\)-m have a blue shift compared to commercial BiVO\({}_{4}\), which shows that it has a lower recombination rate. Raman spectroscopy was also investigated, as shown in Figure 9. The Raman bands at 313 cm\({}^{-1}\) are the typical vibrations of BiVO\({}_{4}\) and can be assigned to the asymmetric and symmetric deformation modes of the VO\({}_{4}\)\({}^{3-}\) tetrahedron. It has been reported that there were two Raman bands, at 710 cm\({}^{-1}\) and 805 cm\({}^{-1}\), which were attributed to the stretching modes of two different types of Vanadium-Oxygen bands in the Raman spectrum of BiVO\({}_{4}\). However, only one band around 805 cm\({}^{-1}\) was found in our case. The absence of a 710 cm\({}^{-1}\) band might be attributed to the local structure of porous microspheres, which were composed of nanosheets. Additionally, the Raman band at 517 cm\({}^{-1}\) indicates the characteristics of monoclinic scheelite BiVO\({}_{4}\) that made up these microsphere structures. Figure 8: Photoluminescence spectra at the excitation wavelength \(\lambda_{\text{ex}}\) = 325 nm for BiVO\({}_{4}\) microspheres at 400 \({}^{\circ}\)C.  
Figure 7: UV-Vis diffuse reflectance spectra of BiVO\({}_{4}\) microspheres. From the Fourier-transform infrared spectroscopy analysis we conducted (Figure 10), we can understand the bonding group presence in these BiVO\({}_{4}\) microspheres. At 1021 cm\({}^{-1}\), there is an unshared V = O stretching vibration, followed by 833 cm\({}^{-1}\), which shows the presence of an asymmetric stretching vibration of the bound oxygen that is shared by two vanadium atoms (V-O-V). Furthermore, at 523 cm\({}^{-1}\), a symmetric stretching mode of the V-O-V unit is present, and finally, at 3402 and 1627 cm\({}^{-1}\), it shows an O-H stretching and bending vibration of the lattice water molecules [10]. We also conducted a zeta potential analysis to study the stability of this flower-like BiVO\({}_{4}\) in terms of the colloidal dispersion, as shown in Figure 11. This shows that it has high stability compared to commercial BiVO\({}_{4}\), since it possesses a higher zeta potential value. When the potential is small, attractive forces can exceed this repulsion and the dispersion can break and flocculate.