## 3 Results and Discussion
Therefore, it is quite obvious  
Figure 3: (**a**) Electron impedance spectra (EIS) of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\); (**b**) Zoomed region of EIS spectra of (**a**) for SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanode, measured using a three-electrode configuration set up in an aqueous phosphate buffer (pH 7.0) with 0.5 M Na\({}_{2}\)SO\({}_{3}\) at 1.23 V vs. Reversible hydrogen electrode (RHE). that there is no significant change in the morphology of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoelectrode after 5 h of photostability measurement. IPCE of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanode is shown in Figure 4b. Remarkably high external quantum efficiency of \(\sim\)80% during back side and \(\sim\)90% upon front side illumination has been witnessed for SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) at 1.23 V vs. RHE. The photocurrent obtained by integrating the IPCE spectra (1.5 mA/cm\({}^{2}\)) is found to be in consistent with that of LSV photocurrent density. This is the highest IPCE value obtained for SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanode to date. Previous report demonstrated that the photoelectrochemical property of WO\({}_{3}\)/SnO\({}_{2}\)/BiVO\({}_{4}\) is inferior to that of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\)[30]. Therefore, we have restricted our study only to the SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) system. It is known from the literature that introducing SnO\({}_{2}\) under the BiVO\({}_{4}\) layer enhances pec performance [26; 27; 28]. The SnO\({}_{2}\) layer enhances the charge transfer properties of BiVO\({}_{4}\) by suppressing the possible recombination that can occur at the interface of SnO\({}_{2}\)/BiVO\({}_{4}\). When the WO\({}_{3}\) layer is introduced between SnO\({}_{2}\)/BiVO\({}_{4}\), the charge separation can be further enhanced, as seen from Figure 2a, where the photocurrent increased after the insertion of WO\({}_{3}\) layer. It is evident from the published results that the heterojunction of nanostructured WO\({}_{3}\)/BiVO\({}_{4}\) performs better than either of the individual materials due to the formation of type II heterojunction [35; 36]. Since the band positions of both materials are suitable to construct type II heterojunction, significant separation of the photogenerated electron hole pairs can be achieved to enhance pec performance. The photogenerated electrons are transferred from BiVO\({}_{4}\) to WO\({}_{3}\), which has excellent electron transport property compared to BiVO\({}_{4}\). While WO\({}_{3}\) improves the electron transport properties, BiVO\({}_{4}\) enhances the overall optical property of WO\({}_{3}\) in heterojunction of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\). It is also proved in the literature that the lifetime of the photogenerated holes increases in the heterojunction compared to the individual material [37]. However, the photoelectrochemical enhancement in WO\({}_{3}\)/BiVO\({}_{4}\) is limited by the efficient transfer of the electrons from WO\({}_{3}\) to BiVO\({}_{4}\). From this point of view, the double heterojunctions of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) would certainly enhance pec performance, as can be seen from the LSV curves of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\). The schematic diagram depicting the mechanism of water splitting is demonstrated in Figure 5.  
Figure 4: (**a**) Stability measurement for SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\); (**b**) Incident photon to current conversion efficiency (IPCE) of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanode measured using a three-electrode configuration set up in an aqueous phosphate buffer (pH 7.0) with 0.5 M Na\({}_{2}\)SO\({}_{3}\) at 1.23 V vs. RHE. Band alignments of double heterojunction SnO\({}_{2}\)/BiVO\({}_{4}\) also improves separation efficiency due to the lower laying valence band position [27; 38; 39]. Charge carrier transport has been depicted in Figure 5 for SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\), which has been constructed according to the previous report [30].