Si-Based Metal–Insulator–Semiconductor Structures with RuO₂–(IrO₂) Films for Photoelectrochemical Water Oxidation

We report on the properties of metal–insulator–semiconductor (MIS) photoanodes for water oxidation employing a thin RuO2–(IrO2) film as a top catalytic layer. In this study, MIS photoanodes with the configurations RuO2/SiO2/n-Si and IrO2–RuO2/SiO2/n-Si were prepared and their photoelectrochemical (PEC) oxygen evolution under solar irradiation has been discussed. The thin SiO2 layers were prepared by the atomic layer deposition method and the RuO2–(IrO2) thin layers were deposited by the metal–organic chemical vapor deposition method. The photocurrent and photovoltage of these MIS photoanodes were studied in 1 M aq. H2SO4 (pH = 0), 0.5 M aq. Na2SO4 (pH = 6), and 1 M aq. KOH (pH = 14) electrolytes showing the trend acidic > alkaline > near-neutral pH conditions for both RuO2- and IrO2–RuO2-based structures. The RuO2/SiO2/n-Si photoanode exhibited a photovoltage of 0.49 V and was able to generate a photocurrent of ∼10 mA/cm2 at a thermodynamic water oxidation potential (1.23 V vs the normal hydrogen electrode, NHE) in 1 M aq. H2SO4 solution under 1 Sun intensity with AM 1.5 spectrum. A photovoltage of 0.42 V and a photocurrent of ∼4 mA/cm2 were achieved for the IrO2–RuO2/SiO2/n-Si photoanode under acidic conditions. The stability of the photoanodes was examined in 1 M aq. H2SO4 and 1 M aq. KOH solutions. Chronoamperometry measurements on the RuO2/SiO2/n-Si photoanode in acidic solution under an applied voltage of 1.23 V versus NHE showed the deterioration of the photoanode after 2 h of operation. Similarly, stability measurements were performed on IrO2–RuO2/SiO2/n-Si photoanodes in 1 M aq. H2SO4 solution. Under acidic conditions, at an applied bias of 1.23 V versus NHE, a photocurrent of ∼2 mA/cm2 was observed, which was stable for 24 h for the IrO2–RuO2-based photoanodes. The preparation, PEC activity, stability, and characterization of the RuO2/SiO2/n-Si and IrO2–RuO2/SiO2/n-Si have been discussed in our study.