# Study of Annealing Temperature Effect on the Photovoltaic Performance of BiOI-Based Materials
## 3 Results and Discussion
### Structure Analysis
#### 3.1.1 X-Ray Diffraction
To study more about the quantitative aspect of BiOI crystal, the average crystal size value of annealed BiOI at 100 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, and 300 \({}^{\circ}\)C was calculated using the Debye-Scherrer equation, \(L~{}=K\lambda/\beta Cos\theta\), in which \(L\) represents the crystallite size, \(K\) (0.9) is the constant, \(\lambda\) (0.154 nm) is the wavelength of Cu, \(\beta\) is the FWHM (full width at half maximum; in radian), and \(\theta\) is the Bragg angle. We obtained the average values of crystallite size around 29.97 nm, 33.06 nm, and 27.42 nm for annealed materials at 100 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, and 300 \({}^{\circ}\)C. Based on these results, it is noted that the heating process had an impact on crystal size distribution [35]. Normally, an increase in annealing temperature causes an increase in a grain size since a higher annealing temperature leads to crystal growth by producing the larger crystallites number. Hence, the crystal size increases as the consequence of the lesser crystal defect in the higher temperature condition. However, we observed that the smaller crystal size was acquired in the annealed material at 300 \({}^{\circ}\)C than in the annealed BiOI at 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C. These data are supported by the broadening FWHM peak of annealed BiOI at 300 \({}^{\circ}\)C, as is shown in Figure 1. We assume that the broader peak reflects the less number of BiOI crystallites owing to the BiOI transformation to bismuth and oxygen-rich material, like Bi\({}_{\gamma}\)O\({}_{9}\)I\({}_{3}\). Based on the Debye-Scherrer equation, we noted that the broader peak related to the declining in the crystal size value. The XRD patterns of the annealed BiOI at 450 \({}^{\circ}\)C and 550 \({}^{\circ}\)C reveal the crystal character of Bi\({}_{5}\)O\({}_{7}\)I and \(\beta\)-Bi\({}_{2}\)O\({}_{3}\) crystals. Referring to the JCPDS card No. 00-40-0548, it was found that Bi\({}_{5}\)O\({}_{7}\)I crystal planes were assigned to the (001), (312), (004), (204), (020), and (316) at 20 around 7.7\({}^{\circ}\), 28.2\({}^{\circ}\), 31.09\({}^{\circ}\), 33.02\({}^{\circ}\), 33.4\({}^{\circ}\), and 53.6\({}^{\circ}\). Moreover, Figure 1 shows the crystal plane (201) and (220) at 20 were around 27.9\({}^{\circ}\) and 32.6\({}^{\circ}\), which correspond to the tetragonal \(\beta\)-Bi\({}_{2}\)O\({}_{3}\) for the annealed sample at 550 \({}^{\circ}\)C. This is matched to the data in the JCPDS card No. 00-27-0050 and previous research [36]. Our Bi\({}_{5}\)O\({}_{7}\)I pattern is  
Figure 1: XRD pattern of annealed BiOI at varied temperatures: 100 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 450 \({}^{\circ}\)C, and 550 \({}^{\circ}\)C.  
in line with the reported research [37]. Overall, the heating treatment on BiOI changes its composition and structures, as is illustrated in the proposed reaction:  
\[7\text{BiOI}+\text{O}_{2}\rightarrow\text{Bi}_{7}\text{O}_{9}\text{I}_{3}+2 \text{I}_{2} \tag{1}\]  
\[5\text{Bi}_{7}\text{O}_{9}\text{I}_{3}+2\text{O}_{2}\rightarrow 7\text{Bi}_{5}\text{O}_{7}\text{I}+4\text{I}_{2} \tag{2}\]  
\[4\text{Bi}_{5}\text{O}_{7}\text{I}+\text{O}_{2}\rightarrow 10\text{Bi}_{2} \text{O}_{3}+2\text{I}_{2} \tag{3}\]