Metal oxides experienced chemical transformation under operating conditions that limits their applications for photosynthesis. Liu et al. (doi.org/10.1038/s41560-021-00927-1) recently found that under illumination the Cu₂O concurrently undergoes reduction by photoelectrons and oxidation by holes in the material. A protection scheme was proposed where an Ag catalyst was used to accelerate electron transfer and a Z-scheme heterojunction to extract holes. The resulting photoelectrode demonstrated stable CO₂ reduction to ethylene with a 60% Faradaic efficiency, while bare Cu₂O failed within minutes.

a) Stabilizing Cu₂O with Ag catalyst and Z-scheme (ZS). b) Chronoamperometry of Cu₂O and Ag/Cu₂O photocathodes,  c) Chronoamperometry of Cu2O/ZS/FTO and Ag/Cu₂O/ZS/FTO photocathodes at −1.2 V versus Fc⁺/Fc.  under AM 1.5G simulated sunlight (100 mW cm⁻²) using 0.1 M Bu₄NPF₆ as electrolyte and 0.1 M TEOA as proton donor in AcCN solution (CO₂ saturated).

The experiments were conducted in aqueous (Na₂SO₄, KHCO₃, NaOH and deionized water), and non-aqueous (acetonitrile) electrolytes. The results showed that the dissolved Cu ions in KHCO₃ electrolyte are one order of magnitude higher than those in Na₂SO₄ in darkness. Under prolonged illumination, Cu⁰ and CuO/Cu(OH)₂ were formed on the surface Cu₂O in Na₂SO₄. In KHCO₃ solution, significantly enhanced Cu²⁺-OH species are formed under illumination within 10 min. Cu₂O showed rapid transformation into CuO/Cu(OH)₂ within 10 min in NaOH, relative to Na₂SO₄ solution. In contrast, Cu₂O barely changed without any solutions present or in acetonitrile non-aqueous solution under illumination for 4 h. This validate the hypothesis that the presence of OH− ions in aqueous solutions may be key in promoting the oxidative transformation of Cu₂O under illumination. The results show Cu2O degradation depends on the electrolyte and applied potentials.

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