## 3 Result and Discussion  
### Material Characterization  
XRD was used to characterize the phase structure of the obtained samples. Figure 2 shows the XRD pattern of the BiVO\({}_{4}\) microspheres prepared for each calcination temperature. The XRD pattern at 400 \({}^{\circ}\)C indicates that the sample was mainly in the monoclinic phase, which exhibits high photocatalytic activity compared to other phases. In addition, a well-defined sharp peak can be seen after calcination at 300 \({}^{\circ}\)C. This indicates that the sample achieved a well-defined organizational structure, which leads to better photocatalytic activity.  
Figure 2: X-ray diffractometer (XRD) analysis for BiVO\({}_{4}\) microspheres under different calcination temperatures.  
As shown in the scanning electron microscopy (SEM) observation images in Figure 3a-c, after calcination at 400 \({}^{\circ}\)C, the products formed were uniform. The SEM images of commercial BiVO\({}_{4}\) can be seen as bulky, and not much surface area is available.  
At 500 \({}^{\circ}\)C (Figure 6), the mixture of flower-like structure and agglomerates started to form, while, at 600 \({}^{\circ}\)C, the particles started to form only agglomerates, and no flower-like structures were found.  
To further study the morphologic structure, we observed the samples under high magnification SEM (Figure 4a,b), and the flower-like structures of the sample were composed of numerous nanosheets with a thickness of about 100 nm. The nanosheets intercrossed to form a flower-like microsphere via self-assembly and the Ostwald ripening process. These flower-like structures resulted in an increased surface area, which leads to better photocatalytic performance compared to commercial BiVO\({}_{4}\), which has a bulky structure and low surface area. Besides that, the large surface area provided by the flower-like structure can allow more rapid diffusion of the reactants and products during the reaction process. If we increased the pH of the synthesis condition to more than 12, the flower-like structure could not form, and only microspheres/microcubes would be observed.  
We also proposed the formation mechanism of the flower-like BiVO\({}_{4}\) microspheres, as shown in Figure 5. A precipitation process occurred by the nucleation and growth of the second phase from a supersaturated solution. With a two-phase mixture composed of a dispersed second phase in a matrix, the mixture does not satisfy thermodynamic equilibrium. This is due to small particles with a higher surface ratio and a higher energy state leading to excess surface energy, which does not satisfy the requirement of minimum energy configuration. The total energy of this system can be decreased via an increase in the scale of the second phase, thus decreasing the total interfacial area. This will result in a continuous nucleation and growth process until a single precipitate particle forms, which introduces a new particle of a given size class, which, in this case, was the flower-like microsphere structure [8; 9].  
Figure 4: (**a,b**) High magnification of flower-like BiVO\({}_{4}\) and the results obtained under synthesis conditions of pH > 12.  
Figure 3: (**a–c**) Fourier-transform infrared spectroscopy images of flower-like BiVO\({}_{4}\); and (**c**) commercial BiVO\({}_{4}\).  
The mean particle size for the obtained samples was also studied using the zeta potential and particle analyzer. As shown in Figure 6, the smallest size was at 400 \({}^{\circ}\)C, which was 2566 nm with a slight error value. At 60 \({}^{\circ}\)C and 300 \({}^{\circ}\)C, the sample consisted of impurities, which led to a larger mean particle size. However, at 500 \({}^{\circ}\)C, the mean particle size was larger than at 400 \({}^{\circ}\)C, because a mixture of 3D microsphere structure and agglomerates started to form. The mean particle size became the largest, with a substantial error value at 60 \({}^{\circ}\)C due to most of the sample being made up of large agglomerates, contributing to poor photocatalytic performance.