## 3 Results and Discussion  
SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanode was fabricated through the combination of e-beam deposition and MOD method. First, SnO\({}_{2}\) was deposited on FTO with 50 and 100 nm thickness. The obtained samples were annealed at 450 \({}^{\circ}\)C for 1 h. WO\({}_{3}\) was deposited on SnO\({}_{2}\)/FTO using the same technique, followed by annealing at 500 \({}^{\circ}\)C for 1 h. BiVO\({}_{4}\) was coated on SnO\({}_{2}\)/WO\({}_{3}\) using the well-known MOD technique. X-ray diffraction patterns shown in Figure 1a confirm the formation of pure phases of SnO\({}_{2}\), WO\({}_{3}\), and BiVO\({}_{4}\). SEM images in Figure 1b-g reveal the morphology of triple planar heterojunction photoanode. As is clearly evident from the SEM images, the triple layers are intact with each other. There was no obvious change in the morphology of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) after inserting the WO\({}_{3}\) layer. Photoelectrochemical properties of triple planar heterojunction were evaluated in three electrode configurations, using aqueous Na\({}_{2}\)SO\({}_{3}\) with buffer solution as electrolyte (pH  
Figure 1: **(a)** XRD patterns of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\). Scanning electron microscopic (SEM) top images of **(b)** SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 100 nm/BiVO\({}_{4}\) **(c)** SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 50 nm/BiVO\({}_{4}\) **(d)** SnO\({}_{2}\) 100 nm/WO\({}_{3}\) 50 nm/BiVO\({}_{4}\). SEM cross-section images of **(e)** SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 100 nm/BiVO\({}_{4}\) on FTO substrate **(f)** SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 50 nm/BiVO\({}_{4}\) **(g)** SnO\({}_{2}\) 100 nm/WO\({}_{3}\) 50 nm/BiVO\({}_{4}\). LSV diagrams of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanode under one sun illumination. The photocurrent obtained from front and back illumination behavior was compared for different thicknesses of SnO\({}_{2}\). We have noticed that the photocurrent obtained from front and back illumination varies with the thickness of the SnO\({}_{2}\) and WO\({}_{3}\). Among various thicknesses of SnO\({}_{2}\) and WO\({}_{3}\) in SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\), SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 50 nm/BiVO\({}_{4}\) triple planar heterojunction exhibits superior performance compared to SnO\({}_{2}\) 100 nm/WO\({}_{3}\) 50 nm/BiVO\({}_{4}\), and SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 100 nm/BiVO\({}_{4}\). SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 50 nm/BiVO\({}_{4}\) shows photocurrent density of \(\sim\)2.01 mA/cm\({}^{2}\) at 1.23 V under front side illumination, while the photocurrent during back illumination is 1.80 mA/cm\({}^{2}\). As the thickness of WO\({}_{3}\) increases to 100 nm, photocurrent density decreases. However, SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 100 nm/BiVO\({}_{4}\) shows a high photocurrent under front illumination compared to its back illumination. It is interesting to note that the behavior of the front and back illumination is unaffected by increasing the thickness of WO\({}_{3}\). Figure 2b presents the LSV of SnO\({}_{2}\) 50 nm/BiVO\({}_{4}\) under directional illumination. SnO\({}_{2}\)/BiVO\({}_{4}\) photoanode shows \(\sim\)1.2 mA/cm\({}^{2}\) at 1.23 V during backside illumination, while it shows a photocurrent density of \(\sim\)0.5 mA/cm\({}^{2}\) under front side illumination. Photoelectrochemical properties of heterojunction SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 50 nm are shown in Figure 2c. LSV of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanode has been carried out in the absence of a hole scavenger, which is shown in Supplementary Material Figure S1a. SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) shows a reasonably good photocurrent of 1.5 mA/cm\({}^{2}\) under front illumination at 1.23 V vs. RHE. The typical transient photo response was shown in Supplementary Material Figure S1b under chopped illumination.