Protection Materials on III–V Semiconductors for Photoelectrochemical CO Reduction

Tantalum oxide (TaO _<i>x</i> ) and amorphous titanium dioxide (TiO₂) are employed as protection materials for commercial GaInP/GaAs/Ge triple-junction solar cells to facilitate the unbiased photoelectrochemical reduction of carbon monoxide on nanostructured copper particles. It has been found that a photoelectrode protected by a 150 nm-thick layer of TiO₂, capped with 8 nm TaO _<i>x</i> , and decorated with copper nanocubes in the size of 150 nm can successfully drive the photoelectrochemical conversion of carbon monoxide to ethylene. Implemented into a photoelectrochemical flow reactor, which continuously supplies the active interface with CO-saturated electrolyte, the device achieves a faradaic efficiency of 24% under AM1.5G conditions. Direct attachment of the copper nanocubes to a protection layer of TiO₂ in the absence of TaO _<i>x</i> results in a strong hydrogen evolution reaction (HER) and no CO reduction products are found. This unexpected loss in selectivity is studied via post-operando X-ray photoemission spectroscopy and ion-scattering spectroscopy. No modifications in the redox state of TiO₂ or signs of H intercalation are found, while the preferential redeposition of small Cu nanoparticles is considered possible. This increase in HER appears specific for TiO₂, as additional, purely electroanalytical experiments using tantalum oxide or carbon as a support layer for Cu nanocubes can produce ethylene effectively.