More than just screening: combinatorial insights into titanium doping in hematite photoanodes Marco Salvi 1 , Alberto Piccioni 1,2 , Nicola Gilli 2 , Alessandro Puri 1,3 , Federico Boscherini 1,4 , Raffaello Mazzaro 1,2 , Luca Pasquini 1,2 1Department of Physics and Astronomy, Alma Mater Studiorum – Università di Bologna, Viale Berti Pichat 6/2, 40127, Bologna, Italy, 2 Istituto per lo Studio dei Materiali Nanostrutturati (ISMN)), Consiglio Nazionale delle Ricerche (CNR), Via P. Gobetti 101, Bologna 40129, Italy, 3 CNR- Istituto Officina dei Materiali (IOM), Grenoble c/o ESRF, The European Synchrotron, 71 Avenue des Martyrs, CS40220, 38043 GrenobleCedex9, France, 4 CNR - Istituto Officina dei Materiali (IOM), c/o AREA Science Park Basovizza, SS 14, Km 163.5 – 34149 Trieste – Italy High-throughput and combinatorial approaches are commonly regarded as powerful tools for discovering new material compositions or optimizing processing parameters. In this work, we demonstrate that these methods can also provide valuable insights into the fundamental mechanisms governing material behavior. Specifically, we investigated Ti-doped hematite (Ti:Fe 2 O 3 ) photoanodes by preparing two types of samples: one in which titanium was incorporated throughout the film via co-sputtering, and another in which titanium was deposited as a surface overlayer on hematite. Both samples were then subjected to thermal annealing under a lateral temperature gradient and characterized using a custom-built, low-cost scanning photoelectrochemical flow cell. This high-throughput strategy enabled us to rapidly identify critical temperatures at which photoelectrochemical performance changed significantly, guiding targeted structural characterization using XRD, TEM, and XAS. The resulting analysis revealed that titanium must be incorporated into the hematite film to enhance performance, reaching a common solubility limit at high temperatures on both samples. Beyond this limit, excess titanium segregates to the surface, forming Ti-rich phases that can ultimately hinder activity. Our results highlight how combinatorial synthesis and high-throughput screening can be extended beyond optimization tasks to uncover the mechanistic roles of dopants and other modifications in complex materials systems. References 1. S. K. Suram, L. Zhou, N. Becerra-Stasiewicz, K. Kan, R. J. R. Jones, B. M. Kendrick and J. M. Gregoire, Review of Scientific Instruments , DOI:10.1063/1.4914466 2. J. M. Gregoire, C. Xiang, X. Liu, M. Marcin, J. Jin, Review of Scientific Instruments , DOI:10.1063/1.4790419D. 3. Monllor-Satoca, M. Bärtsch, C. Fàbrega, A. Genç, S. Reinhard, T. Andreu, J. Arbiol, cd Markus Niederberger and J. Ramon, Energy Environ. Sci , 2015, 8 , 3242
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