## 3 Results and Discussion
The band positions have been adopted from the recent works on SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\), which were determined by ultraviolet photoelectron spectroscopy and optical band gap [30; 40; 41]. The described band position and electron transfer in type II heterojunction, as shown in Figure 5, match well with the reported literature [36; 42]. All the three layers form staggered band alignment and thereby achieve type II heterojunction, facilitating the efficient charge transport across the interface. The improved pec performance of this double heterojunction proves that there is an intimate contact between the two layers. Unlike the previous report on SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) heterojunction, this work sheds light on photocurrent behavior under directional illumination [30]. It can clearly be seen that the performance under front and back illumination has improved after the fabrication of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) double heterojunction. It is known that the directional dependent photoelectrochemical properties are governed by the electron transport efficiency of the materials, along with the morphology of the nanostructure [22]. In the present study, all three layers are planar and hence the directional photoelectrochemical property could be decided by both the electron transport property and the thickness of the heterojunction. The photoelectrochemical performance under front side illumination can easily be realized if the thickness is less than 100 nm, which is very close to the hole diffusion length. In the case of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\), introducing SnO\({}_{2}\) is advantageous, as it forms cascade alignment of the band positions. The electron-hole transport also improves as the SnO\({}_{2}\) layer acts as a hole mirror. The middle layer WO\({}_{3}\) in SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) helps effectively in charge transport, which results in improved photoactivity of BiVO\({}_{4}\). It is observed from SnO\({}_{2}\)/BiVO\({}_{4}\) heterojunction that the performance under back illumination enhances, while in SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) front illumination the photocurrent is increased compared to that of back illumination. This change in the performance under directional illumination could be due to the insertion of the WO\({}_{3}\) layer. In the case of SnO\({}_{2}\)/BiVO\({}_{4}\) heterojunction, majority carrier transportation was limited and hence the photogenerated electron at the interface can easily reach the contact without undergoing recombination under back illumination. However, when the WO\({}_{3}\) layer is introduced between SnO\({}_{2}\)/BiVO\({}_{4}\) heterojunction, photocurrent density behavior  
Figure 5: Schematic diagram for SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\), depicting the mechanism of water splitting. changes. WO\({}_{3}\) improves charge separation efficiency, as demonstrated by previous reports [43]. Adding SnO\({}_{2}\)/WO\({}_{3}\) beneath the BiVO\({}_{4}\) effectively reduces carrier recombination. Performance under front illumination is enhanced in SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\), as the hole blocking layer prevents recombination. Therefore, photogenerated holes at the surface are effectively extracted to electrolyte for water oxidation while it facilitates electron diffusion to the back contact without being vulnerable to recombination. Therefore, the front illumination photocurrent has been enhanced in SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\).