Perovskite-type Nb-based oxynitrides (ANbO 2 N, A = Ba, Sr) exhibiting absorption of long light wavelengths up to (A = Sr) or even above 700 nm (A = Ba) were synthesized from stoichiometric perovskite-type oxides (ANbO 3 ) by a typical thermal ammonolysis method. The preparation of BaNbO 2 N using BaNbO 3 having an identical cubic-perovskite-type structure was demonstrated to effectively improve the surface and bulk crystallinity of the oxynitride because of the absence of a structural transition during nitridation and the stoichiometric 1:1 Ba/Nb ratio between the oxides and oxynitrides. The resulting BaNbO 2 N photoanode exhibited photoelectrochemical water oxidation activity with a current density of 0.85 mA cm −2 at 1.23 V RHE and a faradaic efficiency close to unity under simulated solar illumination (AM 1.5G). The associated incident photon-to-current efficiency in the range of 740−480 nm showed that BaNbO 2 N produces a photoanodic current at 740 nm, the longest response wavelength ever reported for a photoanode based on an (oxy)nitride.

Flux synthesized SrNbO2N photoanode exhibited a photocurrent density of 1.5 mA cm−2 at 1.23 VRHE under AM 1.5G irradiation.

Single crystalline BaNbO2N particles with crystal habits were obtained by appropriate flux treatments.

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