## 3 Results and Discussion  
Bi\({}_{2}\)O\({}_{2}\)Se forms a tetragonal structure at room temperature. With a spatial group symmetry of 14/mmm (a = b = 3.88 A, c = 12.16 A), eight Bi protons are located at the top point of the cube, as illustrated in Figure 1a. The [Bi\({}_{2}\)O\({}_{2}\)]\({}_{n}\)\({}^{2n+}\) cation layer and [Se]\({}_{n}\)\({}^{2n-}\) anionic layer are alternately stacked along the C-axis by weak electrostatic interaction, with a layer thickness of 0.608 nm [29; 39]. Unlike the common van der Waals layered materials, Bi\({}_{2}\)O\({}_{2}\)Se is ion-layered, but it still exhibits the characteristics of typical 2D materials. The experiment utilized chemical vapor deposition (CVD) to synthesize two-dimensional Bi\({}_{2}\)O\({}_{2}\)Se, with Bi\({}_{2}\)O\({}_{3}\) and Bi\({}_{2}\)Se\({}_{3}\) powders as precursors. Since Bi\({}_{2}\)O\({}_{2}\)Se is the only stable phase on the eutectic line of Bi\({}_{2}\)Se\({}_{3}\) and Bi\({}_{2}\)O\({}_{3}\), the reaction will not result in the formation of other impurities or the occurrence of side reactions. Bi\({}_{2}\)O\({}_{3}\) powder is adsorbed and diffused onto the substrate to react with Bi\({}_{2}\)Se\({}_{3}\) to form Bi\({}_{2}\)O\({}_{2}\)Se; this mechanism can provide an improved local environment for high-quality growth. Typically, CVD preparation of 2D materials uses silicon dioxide as the substrate. The interlayer bonding force in Bi\({}_{2}\)O\({}_{2}\)Se is electrostatic. Using silicon dioxide as the substrate would promote the vertical growth of Bi\({}_{2}\)O\({}_{2}\)Se, preventing the formation of few-layer nanosheets. The choice of freshly cleaved fluorophlogopite mica [KMg\({}_{3}\)(AlSi\({}_{3}\)O\({}_{10}\))F\({}_{2}\)] as the substrate for preparing 2D Bi\({}_{2}\)O\({}_{2}\)Se is due to its flat surface, surface inertness, and high thermal stability. Additionally, there is a strong electrostatic interaction between the K\({}^{+}\) layers in fluorophlogopite mica and the Se\({}^{2-}\) layers in Bi\({}_{2}\)O\({}_{2}\)Se, which promotes the lateral growth of Bi\({}_{2}\)O\({}_{2}\)Se [40; 41]. A diagram of the CVD system processing the sample here with a double-temperature zone is shown in Figure 1b. Bi\({}_{2}\)Se\({}_{3}\) powder and Bi\({}_{2}\)O\({}_{3}\) powder are used as source materials and placed in a hot center for evaporation; the distance between the two quartz boats is 5 cm. Freshly cleaved fluorophlogopite mica [KMg\({}_{3}\)(AlSi\({}_{3}\)O\({}_{10}\))F\({}_{2}\)] is placed in the hot center of the second warm zone, 10-13 cm away from the center of the first warm zone. As illustrated in Figure S1, when the substrate temperature is 680 \({}^{\circ}\)C, the precursor absorption rate on the substrate is relatively fast, allowing for the formation of larger Bi\({}_{2}\)O\({}_{2}\)Se nanosheets [29]. As the temperature increases, the precursor absorption rate decreases, resulting in smaller Bi\({}_{2}\)O\({}_{2}\)Se nanosheets. Further increases in temperature can lead to nanosheet decomposition and poorer crystallinity. In the experiment, electrodes were prepared using electron beam evaporation. Given that the mask electrode spacing is 30 \(\upmu\)m, we chose to control the substrate temperature between 680 and 690 \({}^{\circ}\)C to obtain large-area Bi\({}_{2}\)O\({}_{2}\)Se nanosheets. The two-dimensional Bi\({}_{2}\)O\({}_{2}\)Se nanosheets selected for heterojunction fabrication are shown in Figure 1c. The morphology of individual Bi\({}_{2}\)O\({}_{2}\)Se nanosheets is characterized by regular square shapes, with a side length of approximately 35 \(\upmu\)m. The thickness of the nanosheets, as measured by atomic force microscopy (AFM), is approximately 6 nm, as shown in Figure 1d. The Raman spectra of two-dimensional Bi\({}_{2}\)O\({}_{2}\)Se is shown in Figure 1e. Due to the A\({}_{\text{1g}}\) vibration modes of bi, the spectra show a single characteristic peak with a center of \(\approx\)159 cm\({}^{-1}\), indicating good uniformity of the 2D Bi\({}_{2}\)O\({}_{2}\)Se [35; 36; 37].