Watching Ternary Oxides with Dual Eyes: two-colour xes studies of chemical transformations & electronic structure in ferric pseudobrookite (Fe 2 TiO 5 ) photoanodes Devi Prasad Adiyeri Saseendran 1 , Sergey Peredkov 2 , Carlos A. Triana 1 , Daniel Abbott 3 , Victor Mougel 3 , Serena DeBeer 2 , Greta R. Patzke 1 1 Department of Chemistry, University of Zurich, Switzerland, 2 Department of Inorganic Spectroscopy, Max Planck Institute for Chemical Energy Conversion Germany, 3 Department of Chemistry and Applied Biosciences, Switzerland The design of highly efficient, robust, and green water oxidizing catalysts is one of the most critical challenges in sustainable energy research. [1] Solar-assisted photo-electrocatalytic (PEC) water splitting has emerged as a promising approach to producing clean chemical fuels and energy resources. An ideal photo-electrode material encompasses a suitable energy bandgap for efficient sunlight absorption, electrical conductivity, chemical stability, and non-toxicity. [2] α-Fe 2 O 3 had gained considerable interest in the field due to its suitable bandgap (1.9-2.2 eV) and earth abundance. Doping α-Fe 2 O 3 with Ti 4+ , and Zr 4+ had been found to increase its carrier conductivity, thereby improving the PEC performance. [3] In this regard, ternary oxide materials have emerged as potential candidates for photoanode materials, as they provide diverse strategies for tuning the composition and electronic structure of photoanode materials compared to their binary counterparts. [4] Among these, the ferric pseudobrookite: Fe 2 TiO 5 has received significant attention owing to its high thermodynamic phase stability, phase stability in a wide pH range, and suitable bandgap (1.9- 2.1 eV) for efficient solar light absorption. [5–7] We report the fabrication of Fe 2 TiO 5 inverse opals photoanodes which have shown improved photo-current density under solar light irradiation. Understanding multiple metal active sites concomitantly, is of utmost importance to unravel the synergistic role of metal centers in driving the activation process. In this regard, herein we demonstrate the use of two-colour X-ray Emission Spectroscopy (XES) in identifying the intermediates and electronic structure changes involved in solar water oxidation catalyzed by Fe 2 TiO 5 photoanodes, by simultaneously tracking Fe and Ti sites under operational PEC conditions. In situ XES studies indicated that the Ti sites act as oxo-coordination sites whereas the Fe site behaves as a redox regulator. Post-catalytic XAS and XPS investigations also confirms change in local coordination at the Ti center, suggesting its active role in driving the O-O bond formation. References 1. B. You, Y. Sun, Acc Chem Res 2018 , 51 , 1571–1580. 2. Gurudayal, P. S. Bassi, T. Sritharan, L. H. Wong, J Phys D Appl Phys 2018 , 51 , 473002. 3. D. K. Lee, D. Lee, M. A. Lumley, K. S. Choi, Chem Soc Rev 2019 , 48 , 2126–2157. 4. Q. Liu, J. He, T. Yao, Z. Sun, W. Cheng, S. He, Y. Xie, Y. Peng, H. Cheng, Y. Sun, Y. Jiang, F. Hu, Z. Xie, W. Yan, Z. Pan, Z. Wu, S. Wei, Nat Commun 2014 , 5 , 1–7. 5. E. Courtin, G. Baldinozzi, M. T. Sougrati, L. Stievano, C. Sanchez, C. Laberty-Robert, J Mater Chem A Mater 2014 , 2 , 6567–6577. 6. M. Osada, K. Nishio, K. Lee, M. Colletta, B. H. Goodge, W. J. Kim, L. F. Kourkoutis, H. Y. Hwang, Y. Hikita, ACS Appl Energy Mater 2021 , 4 , 2098–2106.
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