The ternary junction of Au/BiVO4/WO3 was integrated by a set of syntheses procedures where BiVO4 NPs were introduced on hydrothermally grown WO3 nanobricks, followed by the deposition of Au NPs through electrochromic activation of WO3. Depending on the amount of BiVO4 deposition, the number of BiVO4 drop-casting cycles was systematically varied from 1 to 3 on FTO and WO3 to achieve the best PEC water splitting performance. As described in the experimental section, the BiVO4 and BiVO4/WO3 thin films with 1, 2, and 3 layers of BiVO4 were denoted as BiVO4-1c/WO3, BiVO4-2c/WO3, and BiVO4-3c/WO3, respectively. The PEC performance of Au/BiVO4/WO3 was compared with the best-performing photoanodes of BiVO4-2c and BiVO4-2c/WO3 in their irrespective series as provided in the supplementary information (Figure and). Therefore, BiVO4-2c and BiVO4-2c/WO3 were used for the main characterization and referred to as BiVO4 and BiVO4/WO3 in the following sections, respectively. Figure displays the SEM images of the as-grown WO3 on FTO substrate. The asgrown WO3 is primarily composed of rectangular prism-shaped nanobricks, ranging from 1-2 μm in length (measured from the top side). These nanobricks are perpendicularly grown to the FTO substrate. After heat treatment, as can be seen in Figure , the length of WO3 nanobricks was significantly reduced to 500-1000 nm, while the large voids between them preserved. nm of FTO conductive layer. Figure shows the SEM image of Au/BiVO4/WO3 in backscattered electron mode. The mass contrast sensitivity in the backscatter mode is higher than in the secondary emission so that Au NPs appear shinier than surrounding BiVO4 and WO3 textures. In addition, the EDX analysis of Au/BiVO4/WO3 photoanode is presented in Figure Using this linear relationship, the absorption coefficient of BiVO4 at 420 nm (𝛼 420 = 5.25 × 10 3 cm -1 ) was calculated from the slope of the absorbance-thickness plot in Figure . The 𝛼 420 value is in a good agreement with the previously reported results . Besides, the absorbance of BiVO4/WO3 at 420 nm is slightly increasing with the gradually increasing BiVO4 layers, as can , proving that the loading amount of BiVO4 NPs onto WO3 nanobricks progressively increases with respect to increase in the number of BiVO4 drop-casting cycles. In fact, the calculated thickness corresponds to the rough thickness of the heterostructures.