*2.4. Construction of Bi2S3/BiVO4/FTO Photoelectrochemical Sensor*

Prior to modification, the FTO electrode was washed with water and ethanol to remove any adsorbed species on the surface. The FTO plates were then sustained until the temperature equilibrated at 100 ◦C to the deposition of the BiVO4 material. A volume of 35 <sup>μ</sup>L of BiVO4 suspension was dropped into 0.7 × 1.0 cm2 of the FTO heated plate and a BiVO4 film was formed after few minutes. This procedure was repeated three times to completely cover the FTO electrode surface. The BiVO4/FTO platform was annealed at 500 ◦C for 1 h using an oven with a heating rate of 10 ◦C min−1. After annealing, the BiVO4/FTO electrode was allowed to cool to room temperature. Next, a Bi2S3 film was electrodeposited onto the BiVO4/FTO platform, according to the method proposed by [23] with few adaptations. An amount of 0.04 mol L−<sup>1</sup> Na2S2O3 was diluted in a solution containing 4.5 mL purified water and 0.5 mL HCl. After the formation of colloidal sulfur, the solution was kept still at room temperature for 24 h to allow the sulfur particles to precipitate at the bottom of the solution. The supernatant solution was then used for the preparation of another solution containing 0.006 mol L−<sup>1</sup> Bi(NO3)3 and 0.006 mol L−<sup>1</sup> EDTA. The Bi2S3 electrodeposition onto the BiVO4/FTO electrode surface was then performed by amperometry, applying a potential of −0.9 V vs. Ag/AgCl/KClsat for 300 s at room temperature. The Bi2S3/BiVO4/FTO platform was annealed using a hot plate at 200 ◦C for 30 min.
