In the last years tantalum oxide nitrides have attracted considerable attention with outstanding potential for visible light induced water splitting due to suitable band positions of the conduction and valence band [1]. The band positions are strongly influenced by the particular crystal structure type and the ratio between nitrogen and oxygen.

For pure TaON the so-called ß-modification, exhibiting the baddeleyite-type structure, is known for many years [2]. Recently, we successfully prepared powders of two new polymorphs: γ- and δ-TaON, crystallizing in the VO₂(B)- and anatase-type, respectively [3,4]. Expanding the field of investigation to quaternary phases with additional contents of magnesium or scandium, a variety of tantalum oxide nitride-based materials with different N/O ratios and crystal structure types (anatase, baddeleyite, bixbyite, rutile, anosovite) have been prepared in our group. Whereas the colors of pure TaON polymorphs are yellow (ß) or orange (γ, δ) – variation is an effect of crystal structure type – the complete range between yellow and red (optical band gaps between 2.6 and 2.0 eV) is covered by the abovementioned quaternary phases – variation is an effect of crystal structure type, N/O ratio, and the cationic dopants (Mg, Sc). Furthermore, ß-TaON was obtained in a yellow and green form, depending on the synthesis conditions.

The photocatalytic activity of the different tantalum oxide nitride materials was tested in the oxygen evolution reaction (OER) using Ag-nitrate as a sacrificial agent. Markedly different performance was observed depending on the presence of an additional dopant and the color of the pure TaON phases. In addition, the powders were deposited on FTO substrates by electrophoretic deposition in order to yield, after careful post-necking, TaON-based photoanodes. In line with the previous results the photoelectrocatalytic performance depends on color, doping and crystal structure of the TaON phase. The catalytic investigations were complemented by measurements of the charge carrier dynamics determined by optical-pump terahertz-probe and photoluminescence spectroscopy, giving differences among the tested materials. Preliminary results indicate a correlation of the initial OER rate and the charge carrier lifetime showing that an optimized solid state synthesis is a key to high-performance photocatalysts. In particular yellow/green ß- and orange γ-TaON showed substantial OER activity. 

This work is supported by the German Science Foundation (DFG) in the frame of the priority program 1613.

[1] W. Chun et al., J. Phys. Chem. B 2003, 5, 1798.

[2] M. Weishaupt, J. Strähle, Z. anorg. allg. Chem. 1977, 429, 261.

[3] H. Schilling et al., Angew. Chem. Int. Ed. Engl. 2007, 46, 2931.

[4] T. Lüdtke et al., Inorg. Chem. 2014, 53(21), 11691.