Development of photocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) photocatalysts with a narrow bandgap is important for solar water splitting. Herein, narrow bandgap Sr2CoWO6 double perovskites with a light absorption edge of ≥700 nm are synthesized by a solid‐state reaction method varying the precursor ratios. The sample synthesized with a precursor Co/W ratio of 1:4 has a conduction band (CB) and valence band (VB) located at −0.82 and 0.95 V versus the normal hydrogen electrode (NHE) at pH = 7, respectively. As a result, both the photocatalytic OER and HER are observed even without loading any cocatalysts. After loading Pt and Rh cocatalysts, the average photocatalytic OER and HER rates are 188 μmol h−1 g−1 (apparent quantum efficiency of 3% at ≈420 nm) and 30 μmol h−1 g−1, respectively. Density functional theory calculations indicate that the OER active sites may shift from a high overpotential W‐site to a low overpotential Co‐site when the W content is increased, which renders high photocatalytic activity for W‐rich samples. Therefore, W‐rich Sr2CoWO6 double perovskite is identified as a novel narrow bandgap bifunctional semiconductor photocatalyst for photocatalytic OER and HER, which is rare for oxide semiconductor photocatalysts. This work opens up a new avenue for the development of oxide‐based double perovskite semiconductor photocatalysts for photocatalytic water splitting.

The paper reports the effects of lanthanum and aluminum ions, on the structural, electrical and magnetic properties of NiFe2O4 spinel ferrite nanoparticles.