# 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  
Figure 1 shows the XRD patterns of BiOI and its family members as a result of the heating treatment. The different XRD patterns in the annealed BiOI films can be observed in Figure 1. The treatment at 100 \({}^{\circ}\)C displayed the existing BiOI crystal plane, which was matched to the Joint Committee on Powder Diffraction Standards (JCPDS) card of BiOI No. 00-73-2062. The crystal planes (001), (002), (102), (110), (013), (004), (200), (114), and (122) appeared at 20 around 9.6\({}^{\circ}\), 19.3\({}^{\circ}\), 29.8\({}^{\circ}\), 31.86\({}^{\circ}\), 36.8\({}^{\circ}\), 39.2\({}^{\circ}\), 45.8\({}^{\circ}\), 51.2\({}^{\circ}\), and 55\({}^{\circ}\), respectively. The similar pattern of annealed BiOI at 200 \({}^{\circ}\)C and 300 \({}^{\circ}\)C to the treated sample at 100 \({}^{\circ}\)C was also noticeable. An identical pattern was only observed in the heated BiOI at 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C with the different intensities of both materials. However, a slight decrease in the 20 shift was detected from the annealed BiOI at 300 \({}^{\circ}\)C in comparison to those treated samples at 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, whereas (102), (110), and (200) crystal planes were in 29.7\({}^{\circ}\), 31.8\({}^{\circ}\), and 45.7\({}^{\circ}\). In spite of its pattern being the same as the BiOI crystallinity at the lower temperature without any impurity peaks, we supposed that the distortion in BiOI crystal structure might happen during the heating treatment at 300 \({}^{\circ}\)C. Hence, it resulted in the smaller 20 compared with those of BiOI diffractograms. The expansion and distortion of BiOI structure can be attributed by the more bismuth and oxygen contained in the BiOI lattice crystal [29]. Regarding this phenomenon, we expected that the chemical structure of the annealed sample at 300 \({}^{\circ}\)C transformed to the different structure (Bi\({}_{7}\)O\({}_{9}\)I\({}_{3}\)), since a heating treatment allowed for the oxygen and iodine addition and subtraction. Some researchers claimed that it is difficult to distinguish BiOI and Bi\({}_{7}\)O\({}_{9}\)I\({}_{3}\) because both materials have similar XRD pattern [26]. We also believe that the annealed material at 300 \({}^{\circ}\)C was not Bi\({}_{4}\)O\({}_{5}\)I\({}_{2}\) since between Bi\({}_{7}\)O\({}_{9}\)I\({}_{3}\) and Bi\({}_{4}\)O\({}_{5}\)I\({}_{2}\) show the different pattern by XRD [28]. Additionally, the similar binding energy in the Bi and I region of BiOI and Bi\({}_{7}\)O\({}_{9}\)I\({}_{3}\)[32] made those materials difficult to differentiate by their X-ray photoelectron spectroscopy (XPS) spectra. To clarify, we found a different (110) d-spacing value in both materials, which indicated that the annealed material at 300 \({}^{\circ}\)C was dissimilar to the treated material at 100 \({}^{\circ}\)C. Both 0.285 and 0.280 nm d-spacing were obtained for the treated material at 300 \({}^{\circ}\)C and 100 \({}^{\circ}\)C, respectively. The d-spacing value of our BiOI was matched to the reported work by [PERSON] and co-workers [33], and the value for d-spacing of the treated sample at 300 \({}^{\circ}\)C was near from the Transmission Electron Microscopy (TEM) d-spacing material for Bi\({}_{7}\)O\({}_{9}\)I\({}_{3}\) (0.286 nm) [34]. Therefore, we propose that our annealed material at 300 \({}^{\circ}\)C was Bi\({}_{7}\)O\({}_{9}\)I\({}_{3}\), and it is important to note that the bigger d-spacing may be affected by the aforementioned distortion in BiOI standard structure.