TY - GEN A1 - Morales, Carlos A1 - Tschammer, Rudi A1 - Gouder, Thomas A1 - Choi, YongMan A1 - Anjum, Dalaver A1 - Baunthiyal, Aman A1 - Krisponeit, Jon-Olaf A1 - Falta, Jens A1 - Flege, Jan Ingo A1 - Idriss, Hicham T1 - Stabilization of Ce3+ cations via U-Ce charge transfer in mixed oxides: consequences on the thermochemical water splitting to hydrogen T2 - Journal of Physics: Energy N2 - The work's objective is to enhance the generation of H2 via the thermochemical water splitting (TCWS) reaction over nanocrystalline mixed oxide Ce1-xUxO2. While CeO2 is the most active and stable known reducible oxide for the TCWS reaction, it is below par to make it practical. This has motivated many works to enhance its reduction capacity and therefore increase its activity. In this work the presence of both metal cations (Ce4+ and U4+) has allowed for the charge transfer reaction to occur (Ce4+ + U4+ → Ce3+ + U5+) and therefore increased its capacity to generate oxygen vacancies, VO (2 Ce3+ + VO), needed for the TCWS reaction. Test reactions on the polycrystalline mixed oxides indicated that small atomic percentages of U (<10 %) were found to be optimal for H2 production due to a considerable increase of Ce3+ states. Further studies of the Ce-U interaction were performed on thin epitaxial Ce1-xUxO2 (111) films of about 6 nm deep. In situ X-ray photoelectron spectroscopy showed clear evidences of charge transfer at low U content. Moreover, it was found that while increasing the content of U decreased the charge transfer efficiency it protected reduced Ce3+ from being oxidized. Our computational results using the DFT + U method gave evidence of charge transfer at 3.5 and 6.2 at.% of U. In agreement with experiments, theoretical calculations also showed that the charge transfer is sensitive to the distribution of U4+ around the Ce4+ cations, which in turn affected the creation of VO needed for water splitting. Our results point out to the important yet often neglected effect of statistical entropy (cations distribution in the lattice), in addition to composition, in increasing the density of reduced states and consequently enhancing H2 production from water. KW - cerium uranium mixed oxide KW - charge transfer KW - TCWS KW - In situ x-ray photoelectron spectroscopy (in-situ XPS) KW - DFT KW - statistical entropy Y1 - 2025 U6 - https://doi.org/10.1088/2515-7655/adbad9 SN - 2515-7655 VL - 7 SP - 1 EP - 14 PB - IOP Publishing ER - TY - GEN A1 - Braud, Nicolas A1 - Buß, Lars A1 - Merte, Lindsay Richard A1 - Wallander, Harald A1 - Krisponeit, Jon-Olaf A1 - Schmidt, Thomas A1 - Lundgren, Edvin A1 - Flege, Jan Ingo A1 - Falta, Jens T1 - Growth and oxidation of ultra-thin Pt-Sn layers on Pt(111) by molecular and atomic oxygen T2 - Ultramicroscopy N2 - The preparation of ultra-thin PtSn-alloyed layers by molecular beam epitaxy was studied using low-energy electron microscopy (LEEM) and micro-diffraction (-LEED). Deposition at a sample temperature of 435 °C initially results in the formation of a PtSn/Pt(111) layer showing a (2 × 2) reconstruction. With continued Sn deposition, a PtSn/Pt(111) layer develops, showing a ()R30° reconstruction. An ultra-thin tin oxide was formed from the (2 × 2) surface by exposure to molecular oxygen at temperatures of 500 °C and 590 °C, respectively. LEED shows the evolution of a new surface structure, which could be identified as an incommensurate rectangular reconstruction with lattice parameters of a = (6.4 ± 0.1) Å and b = (8.6 ± 0.1) Å present in three domains rotated by 120° with respect to each other. This structure can be related to the zigzag reconstructions found for similar ultra-thin oxide systems. Contrarily, the ()R30° structure showed no oxide formation even after extensive exposure to molecular oxygen. The usage of atomic oxygen, however, allows for oxidation of this surface and the growth of thicker oxides on both types of overlayers. At 500 °C this process is accompanied by substantial roughening of the surface. KW - Tin KW - Platinum KW - Tin oxide KW - Platinum-tin KW - Oxidation KW - SnOx KW - Ultra-thin films KW - LEED KW - LEEM Y1 - 2025 U6 - https://doi.org/10.1016/j.ultramic.2025.114243 SN - 0304-3991 VL - 278 SP - 1 EP - 11 PB - Elsevier BV CY - Amsterdam ER -