In-depth investigation of an In–Ni–Ta–O–N photocatalyst for overall water splitting under sunlight

• An auto-catalytic material In–Ni–Ta–O–N is synthesized via solid-state reaction.
• The In–Ni–Ta–O–N can produce NiOOH as an O2 evolution site on its surface.
• The In–Ni–Ta–O–N can split water into stoichiometric H2 and O2 under sunlight.
• The In–Ni–Ta–O–N is an n-type semiconductor with an Fermi level around 0.3 V.
• The In–Ni–Ta–O–N has perfect band edges for the overall water splitting.

A mixed oxynitride, with the formula In–Ni–Ta–O–N, has been examined and used in the preparation of photoelectrodes for water splitting. The material belongs to a monoclinic crystal system and has a layered structure. The nitrogen content is determined by a thermogravimetric method. The flat band of the material is determined with a Mott–Schottky plot and is located at +0.3 V vs. RHE. Cyclic voltammograms and XPS spectra prove that the material catalyzes a self-transformation, forming NiOOH at the surface, which acts as the active oxygen evolution site. Under simulated sunlight, the material exhibits stable hydrogen (62.4 μmol h−1 g−1) and oxygen (29.6 μmol h−1 g−1) evolution rates from water without any sacrificial reagent and without any additional co-catalyst. The open-circuit photovoltage measurement indicates that the low concentration of photogenerated charge carriers limits the performance of the material as a photoelectrode. Furthermore, two different sacrificial reagents are applied to show that the hydrogen evolution reaction is the rate-determining step in water splitting under visible light.

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