Proceedings of nanoGe Fall Meeting 2018 (NFM18)
Publication date: 6th July 2018
Perovskite-type oxynitrides AB(O,N)3 have been identified as promosing photoanode materials for a direct solar water splitting application, since they combine high chemical flexibility with high stability in comparsion to many other chemically modified binary materials.[1] The perovskite-type structure tolerates guided by the Goldschmidt tolerance factor t a vast range of elements on the cationic A- and B-site. Modifications on the anionic site by (partial) substitution of N3– or F– for O2– is in addition possible without sacrificing the inherent stability of the perovskite-type structure. A proper selection of the constituent elements allows the precise adjustment of the band positions (VBM and CBM) and hence the size of the band gap to the requirements given by the redox potential of the photocataclytic reaction and the solar spectrum.[2] Three different approaches for the synthesis of new oxynitride-based photoanode materials were evaluated: i) small cations with a low first quantum number on the A-site position together with abundant B-site cations such as Ti4+ and Zr4+,[3] ii) the application of Ta in unususal oxidations states such as Ta4+,[4,5] and iii) the double anionic substitution of O2– by N3– and F– for charge compensation.[6–8] In case i) the incorporation of Y3+ or Mg2+ in the perovskite-type structure was found to be difficult due to the large difference in the ionic radii (compared to classical A-site cations). For B = Ti4+ often the formation of TiN as impurity was observed negatively affecting the charge separation, while for B = Zr4+ due to the higher stability an insuffcient substitution of N occurred resulting in rather large band gaps of about 2.5–2.7 eV.[3] In case iii) the amount of substitution was too low to reduce the band gap efficiently. Additionally, a significant amount of optically active defects remained.[6] As most promising candidates the perovskite-type oxynitrides La1–xYxTaO2N were identified by optical band gap determination (DRS) and surface photovoltage spectroscopcy (SPS).[4,5]
[1] S. G. Ebbinghaus, H. P. Abicht, R. Dronskowski, T. Müller, A. Reller, A. Weidenkaff, Prog. Solid State Chem. 2009, 37, 173–205.
[2] I. E. Castelli, J. M. García-Lastra, F. Hüser, K. S. Thygesen, K. W. Jacobsen, New J. Phys. 2013, 15, DOI 10.1088/1367-2630/15/10/105026.
[3] M. Widenmeyer, C. Peng, A. Baki, W. Xie, R. Niewa, A. Weidenkaff, Solid Sate Sci. 2016, 54, 7–16.
[4] C. Bubeck, M. Widenmeyer, A. De Denko, G. Richter, M. Coduri, E. Salas Colera, A. Senyshyn, E. Goering, S. Yoon, F. Osterloh, et al., 2018, in preparation.
[5] C. Bubeck, M. Widenmeyer, G. Richter, M. Coduri, S. Yoon, A. Weidenkaff, 2018, in preparation.
[6] M. Widenmeyer, J. Häcker, S. Yoon, A. Weidenkaff, 2018, in preparation.
[7] S. Yoon, K. Son, S. G. Ebbinghaus, M. Widenmeyer, A. Weidenkaff, J. Alloys Compd. 2018, 749, 628–633.
[8] S. Yoon, A. E. Maegli, L. Karvonen, S. K. Matam, A. Shkabko, S. Riegg, T. Großmann, S. G. Ebbinghaus, S. Pokrant, A. Weidenkaff, J. Solid State Chem. 2013, 206, 226–232.
The authors are thankful for the financial support of the Deutsche Forschungsgemeinschaft within the SPP1613 (grant: WE2803/7-1).
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