Previous studies have established that photoanodes modified with a-TiO2 and metal co-catalysts exhibited enhanced PEC performance, attributed to the sufficient charge extraction from the TiO2 to the co-catalysts. However, even with various types of metal co-catalysts, we observed insufficient improvement and non-reproducibility in the PEC performance according to the work function of the used metal (Supplementary Fig. ). This result indicates that the photogenerated holes are unable to be transferred through the amorphous TiO2 layer when the thick TiO2 is directly deposited on BiVO4. On the contrary, BiVO4/PEI/TiO2 showed photoanodic current even without co-catalysts: photocurrent density of 1.08 mA cm -2 at 1.23 V vs. RHE (Fig. ). Since co-catalysts are generally required for water oxidation, 7 the photoanodic current may (partly) originate from the oxidation of the PEI layer at the interface between BiVO4 and TiO2 layer. Further discussion on the origin of the photocurrent in BiVO4/PEI/TiO2 will be discussed in a subsequent section.