@misc{MoralesKostoTschammeretal., author = {Morales, Carlos and Kosto, Yuliia and Tschammer, Rudi and Henkel, Karsten and Flege, Jan Ingo}, title = {Reduction by H2 exposure at room temperature of ceria ultrathin films grown by atomic layer deposition}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Atomic layer deposition (ALD) exhibits a high potential for integration as a scalable process in microelectronics, allowing well-controlled layer-by-layer deposition and conformal growth on 3D structures. Yet, the ALD technique is also well known to lead to amorphous and defective, non-stoichiometric films, potentially resulting in modified materials properties that, in the case of ultra-thin deposits, can also be affected by film/substrate interaction. Interestingly, initial in situ X-ray photoemission spectroscopy (XPS) measurements of ceria ALD-deposits on Al2O3/Si, sapphire, and SiO2 substrates confirm a Ce3+/Ce4+ mixture dependent on the substrate interaction, deposit thickness, and morphology. Using near-ambient pressure XPS, we have significantly reduced ultrathin (< 10 nm) ceria films grown by ALD by exposing them to different O2/H2 partial pressures at moderate temperatures (< 525K). Notably, the total amount of reduction to Ce3+ is found to depend on the deposit thickness and initial ceria/substrate interaction. Furthermore, the intrinsic defects related to the ALD method seem to play a critical role in the reversible reduction at room temperature.}, language = {en} } @misc{TschammerBussMoralesetal., author = {Tschammer, Rudi and Buß, Lars and Morales, Carlos and Senanayake, Sanjaya D. and Falta, Jens and Flege, Jan Ingo}, title = {In situ characterization of cerium oxide on Au(111) under reducing and oxidizing conditions by low-energy electron microscopy}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {The development of novel catalysts for a variety of applications is a key challenge for modern catalysis. Inverse metal oxide catalysts consisting of oxide nanoparticles dispersed on a metal support have recently attracted much attention, showing higher activity and selectivity compared to traditional catalytic systems, harnessing synergistic effects attributed to the so-called metal-support interaction. To gain further insights, we deposited cerium oxide nanoparticles on Au(111) and studied this system by low-energy electron microscopy (LEEM) and low-energy electron diffraction (LEED). The prepared samples demonstrate a distinct correlation between the deposition temperature and the structural order of the nanoparticles. This has been expanded upon by exploring the changes induced by reduction with H2 and reoxidation with O2 or CO2, again exhibiting a connection between structural order and activity, while also showing the influence of the oxide-metal interaction on the stability of cerium oxide under reducing conditions.}, language = {en} } @misc{AbdeldayemBakiMoralesetal., author = {Abdeldayem, Mohamed and Baki, Aykut and Morales, Carlos and Flege, Jan Ingo and Klimm, Detlef and Fiedler, Andreas and Bierwagen, Oliver and Schwarzkopf, Jutta}, title = {Potential of La-doped SrTiO3 thin films grown by metal-organic vapor phase epitaxy for thermoelectric applications}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Conversion of waste heat energy into electrical energy by exploiting the thermoelectric effect in solids promises a great contribution to energy harvesting concepts. However, most thermoelectric materials use toxic Pb or Te. Recently, La-doped SrTiO3 has gained a lot of interest as a potential candidate for thermoelectric devices for its good thermoelectric properties, chemical and thermal stability. In this paper, we report the homoepitaxial growth of La-doped SrTiO3 thin films by metalorganic vapor phase epitaxy (MOVPE) technique, which works at high oxygen partial pressures and offers upscaling potential for industry. The adjustment of charge carrier concentration, necessary for thermoelectric power factor optimization, was performed by introducing a defined amount of the metal-organic precursor La(tmhd)3 and tetraglyme to the liquid precursor solution. X-ray diffraction and atomic force microscopy verified a pure perovskite phase with high structural quality. The electrical conductivity increases linearly with the La concentration in the gas phase, which is attributed to the substitution of La+3 ions on the Sr+2 sites inferred from photoemission spectroscopy.}, language = {en} } @misc{PożarowskaPleinesEwertetal., author = {Pożarowska, Emilia and Pleines, Linus and Ewert, Moritz and Prieto, Mauricio J. and Tănase, Liviu Christian and Souza Caldas, Lucas de and Tiwari, Aarti and Schmidt, Thomas and Falta, Jens and Krasovskii, Eugene and Morales, Carlos and Flege, Jan Ingo}, title = {Preparation and stability of the hexagonal phase of samarium oxide on Ru(0001)}, series = {Ultramicroscopy}, volume = {250}, journal = {Ultramicroscopy}, issn = {0304-3991}, doi = {10.1016/j.ultramic.2023.113755}, abstract = {We have used low-energy electron microscopy (LEEM), micro-illumination low-energy electron diffraction (µLEED) supported by ab initio calculations, and X-ray absorption spectroscopy (XAS) to investigate in-situ and in real-time the structural properties of Sm2O3 deposits grown on Ru(0001), a rare-earth metal oxide model catalyst. Our results show that samarium oxide grows in a hexagonal A-Sm2O3 phase on Ru(0001), exhibiting a (0001) oriented-top facet and (113) side facets. Upon annealing, a structural transition from the hexagonal to cubic phase occurs, in which the Sm cations exhibit the +3 oxidation state. The unexpected initial growth in the A-Sm2O3 hexagonal phase and its gradual transition to a mixture with cubic C-Sm2O3 showcases the complexity of the system and the critical role of the substrate in the stabilization of the hexagonal phase, which was previously reported only at high pressures and temperatures for bulk samaria. Besides, these results highlight the potential interactions that Sm could have with other catalytic compounds with respect to the here gathered insights on the preparation conditions and the specific compounds with which it interacts.}, language = {en} } @misc{MoralesTschammerGouderetal., author = {Morales, Carlos and Tschammer, Rudi and Gouder, Thomas and Choi, YongMan and Anjum, Dalaver and Baunthiyal, Aman and Krisponeit, Jon-Olaf and Falta, Jens and Flege, Jan Ingo and Idriss, Hicham}, title = {Stabilization of Ce3+ cations via U-Ce charge transfer in mixed oxides: consequences on the thermochemical water splitting to hydrogen}, series = {Journal of Physics: Energy}, volume = {7}, journal = {Journal of Physics: Energy}, publisher = {IOP Publishing}, issn = {2515-7655}, doi = {10.1088/2515-7655/adbad9}, pages = {1 -- 14}, abstract = {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.}, language = {en} } @misc{TschammerBussPożarowskaetal., author = {Tschammer, Rudi and Buß, Lars and Pożarowska, Emilia and Morales, Carlos and Senanayake, Sanjaya D. and Prieto, Mauricio J. and Tănase, Liviu C. and de Souza Caldas, Lucas and Tiwari, Aarti and Schmidt, Thomas and Ni{\~n}o, Miguel A. and Foerster, Michael and Falta, Jens and Flege, Jan Ingo}, title = {High-temperature growth of CeOx on Au(111) and behavior under reducing and oxidizing conditions}, series = {The journal of physical chemistry C}, volume = {129}, journal = {The journal of physical chemistry C}, number = {7}, publisher = {American Chemical Society (ACS)}, address = {Washington, DC}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.4c08072}, pages = {3583 -- 3594}, abstract = {Inverse oxide-metal model catalysts can show superior activity and selectivity compared with the traditional supported metal-oxide architecture, commonly attributed to the synergistic overlayer-support interaction. We have investigated the growth and redox properties of ceria nanoislands grown on Au(111) between 700 and 890 °C, which yields the CeO2-Au(111) model catalyst system. We have observed a distinct correlation between deposition temperature, structural order, and oxide composition through low-energy electron microscopy, low-energy electron diffraction, intensity-voltage curves, and X-ray absorption spectroscopy. Improved structural order and thermal stability of the oxide have been achieved by increasing the oxygen chemical potential at the substrate surface using reactive oxygen (O/O2) instead of molecular O2 during growth. In situ characterization under reducing (H2) and oxidizing atmospheres (O2, CO2) indicates an irreversible loss of structural order and redox activity at high reduction temperatures, while moderate temperatures result in partial decomposition of the ceria nanoislands (Ce3+/Ce4+) to metallic cerium (Ce0). The weak interaction between Au(111) and CeOx would facilitate its reduction to the Ce0 metallic state, especially considering the comparatively strong interaction between Ce0 and Au0. Besides, the higher reactivity of atomic oxygen promotes a stronger interaction between the gold and oxide islands during the nucleation process, explaining the improved stability. Thus, we propose that by driving the nucleation and growth of the ceria/Au system in a highly oxidizing regime, novel chemical properties can be obtained.}, language = {en} } @misc{PożarowskaPleinesPrietoetal., author = {Pożarowska, Emilia and Pleines, Linus and Prieto, Mauricio J. and Tănase, Liviu C. and de Souza Caldas, Lucas and Tiwari, Aarti and Schmidt, Thomas and Falta, Jens and Morales, Carlos and Flege, Jan Ingo}, title = {The relationship between Sm alloying and structure sensitivity of ceria(111)- and (100)-oriented nanoislands on Cu(111)}, series = {Physical chemistry, chemical physics}, volume = {27}, journal = {Physical chemistry, chemical physics}, number = {29}, publisher = {Royal Society of Chemistry (RSC)}, address = {Cambridge}, issn = {1463-9076}, doi = {10.1039/D5CP01171J}, pages = {15691 -- 15703}, abstract = {We have investigated the complex dynamics of samarium deposition on ceria islands of different orientations, namely (111) and (100), grown side by side on a Cu(111) single-crystal substrate, followed by post-oxidation and annealing under ultra-high vacuum conditions. Only the (100)-oriented ceria islands undergo substantial initial reduction upon samarium deposition at 740 K via a pathway similar to the strong Ce-ceria interfacial interaction, while the (111)-oriented islands remain in the Ce4+ oxidation state. This remarkable structure sensitivity is explained by the different energies required for oxygen vacancy formation for both oxide orientations. Subsequent mild re-oxidation with O2 results in the complete recovery of the Ce4+ oxidation state in the (100)-oriented islands, indicating the complete healing of oxygen vacancies. In contrast, extended annealing at moderate temperatures likely induces persistent samarium incorporation into the cerium oxide matrix. Our results provide new insights into the complex structure-activity relationships in mixed rare-earth metal oxide systems and have promising implications for optimizing catalytic reactions over such compounds in reducing environments.}, language = {en} }