Perovskite-Type Oxynitride Nanofibers Performing Photocatalytic Oxygen and Hydrogen Generation
Authors: Hofmann Anja , Weiß Morten , Timm Jana , Marschall Roland 
Published: February 2024

(111)-layered perovskites such as Ba5Ta4O15 or Ba5Nb4O15 are known for their promising activity in hydrogen evolution and overall water splitting.[1] Their activity can be further enhanced by the preparation of Ba5Ta4O15-Ba3Ta5O15-BaTa2O6 composites.[2,3] However these materials can only absorb UV-light, which prevents them to be used in solar-light induced photocatalysis. Therefore, the preparation of visible-light absorbing photocatalysts is very important for future application in solar-light induced hydrogen evolution and overall water splitting.
Perovskite-type oxynitrides AB(O,N)3 are one group of materials which can absorb visible light, i.e. BaNbO2N with an absorption edge up to 740 nm.[4-7] These perovskite-type oxynitrides can be quite easily prepared by ammonolysis of (111)-layered perovskites, due to their structural similarity of 6-fold coordinated B cations, corner-shared BO6 octahedra, and the 12-fold oxygen coordinated A cations in both crystal structures.[8] However, this conversion often results in micrometer-sized particles due to the high temperatures used in the ammonolysis process, thus resulting in small surface areas.[6,7] Therefore, the nanostructuring of such perovskite-type oxynitrides can be of interest, due to the higher surface area and shorter diffusion pathways of the minority charge carriers. One possibility to nanostructure photocatalysts and enhance the photocatalytic activity is the electrospinning of nanofibers. Hildebrandt et al. and Bloesser et al. reported the preparation of highly crystalline Ba5Nb4O15, Ba5Ta4O15, and Ba5Nb2Ta2O15 (111)-layered perovskite nanofibers and the tailoring of the nanofiber diameter of Ba5Ta4O15 and Ba5Ta4O15-Ba3Ta5O15 nanofibers by electrospinning.[9-11]
Herein, we present a novel approach for the preparation of nanostructured complex perovskite oxynitrides.[12] Ba5Nb4O15 (111)-layered perovskite nanofibers with tailored nanofiber diameters were electrospun and converted via ammonolysis. Most importantly, the nanofiber morphology was maintained during ammonolysis, and the nanofiber diameter could be adjusted as well. A thorough characterization, including Rietveld refinement, revealed the formation of a novel BaNbO2N-Ba2NbO3N perovskite oxynitride composite. UV-vis measurements further supported the results of a BaNbO2N-Ba2NbO3N composite formation and revealed a reduction of the band edge by more than 2 eV from 3.9 eV for Ba5Nb4O15 to 1.7 eV for the converted nanofiber composites. A diameter-dependent hydrogen and oxygen evolution activity of the BaNbO2N-Ba2NbO3N composite nanofibers after deposition of Pt or CoNbO4, respectively, with an optimum nanofiber diameter will be presented.
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