As mentioned previously, the interface between Cu2O and overlayers often acts as a recombination center. In particular, it is well known that metal layers on semiconductors can cause severe charge recombination at the surface of semiconductors due to either the presence of surface states or a high density of states in the metal near the mid-gap energy of the semiconductor . To mitigate this, we introduced a 1 nm Al2O3 layer on the Cu2O surface using ALD to enhance charge separation efficiency through surface state passivation. The introduction of the 1 nm Al2O3 layer improved PEC performance of both Cu2O/Al2O3/NiFe and Cu2O/Al2O3/Au/NiFe electrodes, which indicates that the Al2O3 layer effectively functions as a passivation layer (Fig. ). To further explore the effect of the Al2O3 layer, we investigated its PEC performance by modulating the thickness of this layer. In LSV measurements, the photoanode with a 3 nm-thick Al2O3 layer showed a decreased PEC performance (Fig. ).