Cobalt oxide (CoOx), an earth-abundant and low-cost oxygen evolving catalyst (OEC), has notable advantages as a top protection layer of photoanodes for solar-driven water oxidation because of its desirable durability. However, cobalt oxides exist as various phases, such as Co(II)O, Co2(III)O3, Co3(II,III)O4, and the (photo)electrochemical properties of CoOx are significantly governed by its phase. Atomic layer deposition (ALD) is a suitable method to form a multifunctional layer for photoelectrochemical (PEC) water splitting because it allows direct growth of a conformal high-quality film on various substrates as well as facile control over its chemical phases by adjusting the deposition conditions. Here, a well-controlled CoOx/SiOx/n-Si heterojunction prepared by ALD is demonstrated for solar-driven water splitting. The phase of the ALD CoOx films can be easily controlled from CoO to Co3O4 by varying the deposition temperature. In addition, this systematic study reveals that its energetic as well as electrochemical properties are changed significantly with the phase. Whereas CoO grown at 150 °C produces high photovoltage by building desirable hole-selective heterojunctions with n-Si, Co3O4 formed at 300 °C has a better catalytic property for water oxidation. To address this competitive correlation, we developed a double-layered (DL) ALD CoOx film that has advantages of both CoO and Co3O4. The DL ALD CoOx/SiOx/Si heterojunction photoanode produces a photocurrent density of 3.5 mA/cm2 without a buried junction and maintains a saturating current density of 32.5 mA/cm2 without noticeable degradation during 12 h in 1 M KOH under a simulated 1 sun illumination.
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