After confirming the uniform deposition of the TiO2 protection layer, we evaluated the PEC performance and the stability of the photoanodes. Linear sweep voltammetry (LSV) was carried out in 0.5 M potassium phosphate (KPi) buffer solution (pH 8). First, the BiVO4/TiO2 photoanode exhibited an extremely low photocurrent density of 0.0008 mA cm -2 at 1.23 V vs. RHE under 1 sun illumination (Fig. ). Even after the modification with CoOOH co-catalyst (BiVO4/TiO2/CoOOH), the photocurrent only remained in the microampere (µA) range at 1.23 V vs. RHE. The ALD TiO2 used here has been previously shown to transmit holes on silicon photoanodes, and thus the typical "leaky" TiO2 is not suitable for BiVO4.