The Au NPs deposited BiVO4/WO3 heterostructure was fabricated using a modified electron-charging method for WO3, which is based on cathodic electrochromism. Scheme 1 illustrates the synthesis process for constructing the Au/BiVO4/WO3 ternary junction. The electron-charging process was performed in chronoamperometric mode using a three-electrode setup, with the BiVO₄/WO₃ acting as the working electrode, a platinum coil as the counter electrode, and an Ag/AgCl electrode (saturated with NaCl) serving as the reference. To determine the optimal charging voltage of the BiVO4/WO3, the cyclic voltammetry technique was used as seen in Figure , accordingly, the charging was carried out at -0.4 V vs. Ag/AgCl for 30 s, as shown in Figure . Following the charging process, the anode sample changed color from yellow to blue due to the intrinsic electrochromic property of WO3. The blue-colored BiVO4/WO3 was then immediately immersed into 50 μM of aqueous solution of gold chloride (HAuCl4.3H2O, Sigma Aldrich) for 4 h in the dark. The uniform brown coloration of Au/BiVO4/WO3 indicated that electrons stored in WO3 were successfully released to form Au NPs upon reduction of the Au(III) precursor in the solution (bleaching process). This electron-charging method essentially follows the literature protocols reported previously for WO3 . Scheme 1. Schematic representation for the fabrication of Au/BiVO4/WO3 ternary junction photoanode.