Sr2CoTaO6 Double Perovskite Oxide as a Novel Visible-Light-Absorbing Bifunctional Photocatalyst for Photocatalytic Oxygen and Hydrogen Evolution Reactions

The development of efficient and stable semiconductor photocatalysts for water splitting is regarded as a viable means to produce renewable hydrogen using abundant solar energy. Although some oxide semiconductors have already been demonstrated as efficient and stable photocatalysts for overall water splitting under UV light irradiation, no oxide semiconductor photocatalysts have been reported so far to split water efficiently under visible light irradiation. Hence, screening of visible-light-absorbing oxide semiconductors is highly demanded for photocatalytic water splitting. Herein, Sr2CoTaO6 double perovskite oxide has been identified as a visible-light-absorbing semiconductor photocatalyst with an electronic band structure suitable for overall water splitting by density of states (DOS) analysis. To demonstrate the capability of photocatalytic water splitting properties of Sr2CoTaO6, cubic-shaped Sr2CoTaO6-F and irregular-shaped Sr2CoTaO6-S were synthesized by the facile flux method (F) and the conventional high-temperature solid-state reaction method (S). The experimental analysis of the band structure indicates that the synthesized Sr2CoTaO6 is an n-type semiconductor with a bandgap of 2.7 eV, and the minimum of conduction band (CB) positions and the maximum of valence band (VB) positions are located at -0.87 and +1.83 V vs normal hydrogen electrode (NHE, pH = 7), respectively. Sr2CoTaO6-F showed much higher photocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activities than Sr2CoTaO6-S, indicating the advantage of the flux method in synthesizing double perovskite oxides in control of crystallinity and morphology. Without loading any cocatalysts, Sr2CoTaO6-F showed bifunctional photocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activities in the presence of sacrificial reagents under visible light irradiation, which is rarely reported for metal-oxide-based photocatalysts. The photocatalytic OER and HER activities could be further enhanced by loading RuO2 and Rh cocatalysts, respectively. This work further supports that visible-light-absorbing double perovskite oxide semiconductors are promising candidates worth exploring for photocatalytic water splitting.