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9 pags., 5 figs., 3 tabs. ; The temperature-induced structural changes of Fe−, Co−, and Ni−Au core−shell nanoparticles with diameters around 5 nm are studied via atomically resolved transmission electron microscopy. We observe structural transitions from local toward global energy minima induced by elevated temperatures. The experimental observations are accompanied by a computational modeling of all core−shell particles with either centralized or decentralized core positions. The embedded atom model is employed and further supported by density functional theory calculations. We provide a detailed comparison of vacancy formation energies obtained for all materials involved in order to explain the variations in the restructuring processes which we observe in temperature-programmed TEM studies of the particles. ; This research has been supported by the Austrian Science Fund (FWF) under Grant No. P 29893-N36, the FWF and the Christian Doppler Research Association (CDG) under Grant No. PIR8-N34, the Horizon 2020 research program of the European Union under Grant No. 823717-ESTEEM3, and the Spanish Agencia Estatal de Investigacion (AEI) and the Fondo ́ Europeo de Desarrollo Regional (FEDER, UE) under Grant No. MAT2016-75354-P. The authors acknowledge the use of HPC resources provided by the ZID of Graz University of Technology and by the Vienna Scientific Cluster (VSC). Further support by NAWI Graz is gratefully acknowledged. The CESGA supercomputer center (Spain) is also acknowledged for having provided computational resources. ; Peer reviewed

12 pags., 6 figs. -- Open Access funded by Creative Commons Atribution Licence 3.0 ; The decoration of semiconductors with subnanometer-sized clusters of metal atoms can have a strong impact on the optical properties of the support. The changes induced differ greatly from effects known for their well-studied, metallic counterparts in the nanometer range. In this work, we study the deposition of Cu5 clusters on a TiO2 surface and investigate their influence on the photon-absorption properties of TiO2 nanoparticles via the computational modeling of a decorated rutile TiO2 (110) surface. Our findings are further supported by selected experiments using diffuse reflectance and X-ray absorption spectroscopy. The Cu5 cluster donates an electron to TiO2, leading to the formation of a small polaron Ti3+ 3d1 state and depopulation of Cu(3d) orbitals, successfully explaining the absorption spectroscopy measurements at the K-edge of copper. A monolayer of highly stable and well fixated Cu5 clusters is formed, which not only enhances the overall absorption, but also extends the absorption profile into the visible region of the solar spectrum via direct photo-induced electron transfer and formation of a charge-separated state. ; This work has been partly supported by the Spanish Agencia Estatal de Investigacion (AEI) and the Fondo Europeo de Desarrollo Regional (FEDER, UE) under Grant No. MAT2016- 75354-P, the Austrian Science Fund (FWF) under Grant P29893-N36, the COST Action CM1405 "Molecules in Motion" (MOLIM), La Caixa Foundation (LCF/PR/PR12/11070003), the Ramon Areces Foundation (Project CIVP18A3940), European Union's Horizon 2020 Research and Innovation Programme (Grant Agreement No. Bac-To-Fuel 825999), the MINECO, Spain (MAT2015-67458-P – conanced with FEDER Funds – and CTQ2013-44762-R), the Xunta de Galicia, Spain (GRC ED431C2017/22), and the ANPCyT (PICT 2015-2285) and UNLP (Project 11/X790), Argentina. The CESGA Supercomputing Center (Spain) is acknowledged for having provided the computational resources used in this work. The partial support by the Laboratorio Nacional de Luz Sıncrotron (LNLS) under proposals 20170352 and 20180123 is also acknowledged. D. B. expresses gratitude for a postdoctoral grant from the Xunta de Galicia, Spain (ED481D 2017/021). ; Peer reviewed

The decoration of semiconductors with subnanometer-sized clusters of metal atoms can have a strong impact on the optical properties of the support. The changes induced differ greatly from effects known for their well-studied, metallic counterparts in the nanometer range. In this work, we study the deposition of Cu5 clusters on a TiO2 surface and investigate their influence on the photon-absorption properties of TiO2 nanoparticles via the computational modeling of a decorated rutile TiO2 (110) surface. Our findings are further supported by selected experiments using diffuse reflectance and X-ray absorption spectroscopy. The Cu5 cluster donates an electron to TiO2, leading to the formation of a small polaron Ti3+ 3d1 state and depopulation of Cu(3d) orbitals, successfully explaining the absorption spectroscopy measurements at the K-edge of copper. A monolayer of highly stable and well fixated Cu5 clusters is formed, which not only enhances the overall absorption, but also extends the absorption profile into the visible region of the solar spectrum via direct photo-induced electron transfer and formation of a charge-separated state ; This work has been partly supported by the Spanish Agencia Estatal de Investigación (AEI) and the Fondo Europeo de Desarrollo Regional (FEDER, UE) under Grant No. MAT2016-75354-P, the Austrian Science Fund (FWF) under Grant P29893-N36, the COST Action CM1405 "Molecules in Motion" (MOLIM), La Caixa Foundation (LCF/PR/PR12/11070003), the Ramon Areces Foundation (Project CIVP18A3940), European Union's Horizon 2020 Research and Innovation Programme (Grant Agreement No. Bac-To-Fuel 825999), the MINECO, Spain (MAT2015-67458-P – cofinanced with FEDER Funds – and CTQ2013-44762-R), the Xunta de Galicia, Spain (GRC ED431C2017/22), and the ANPCyT (PICT 2015-2285) and UNLP (Project 11/X790), Argentina. The CESGA Supercomputing Center (Spain) is acknowledged for having provided the computational resources used in this work. The partial support by the Laboratório Nacional de Luz Síncrotron (LNLS) ...

21 pags., 5 figs. ; Subnanometer-sized metal clusters often feature a molecule-like electronic structure, which makes their physical and chemical properties significantly different from those of nanoparticles and bulk material. Considering potential applications, there is a major concern about their thermal stability and susceptibility towards oxidation. Cu clusters of only 5 atoms (Cu5 clusters) are first synthesized in high concentration using a new-generation wet chemical method. Next, it is shown that, contrary to what is currently assumed, Cu5 clusters display nobility, beyond resistance to irreversible oxidation, at a broad range of temperatures and oxygen pressures. The outstanding nobility arises from an unusual reversible oxidation which is observed by in situ X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy on Cu5 clusters deposited onto highly oriented pyrolitic graphite at different oxygen pressures and up to 773 K. This atypical property is explained by a theoretical approach combining different state-of-the-art first principles theories. It reveals the essential role of collective quantum effects in the physical mechanism responsible for the nobility of Cu5 clusters, encompassing a structural 'breathing' through concerted Cu–Cu elongations/contractions upon O2 uptake/release, and collective charge transfer as well. A predictive phase diagram of their reversible oxidation states is also delivered, agreeing with the experimental observations. The collective quantum effects responsible of the observed nobility are expected to be general in subanometer-sized metal clusters, pushing this new generation of materials to an upper level. ; This work has been partly supported by the European Union's Horizon 2020 Research and Innovation Programme un-der Grant Agreement No. 825999; the Spanish Agencia Estatal de Investigación (AEI) and the Fondo Europeo de Desarrollo Regional (FEDER, UE) under Grant No. MAT2016-75354-P; the Austrian Science Fund (FWF) under Grant P29893-N36; the CMST COST Action CM1405 "Molecules in Motion" (MOLIM); the Xunta de Galicia, Spain (Grupos Ref. Comp.ED431C 2017/22 and AEMAT ED431E 2018/08); Obra Social Fundación La Caixa: Ref.LCF/PR/PR12/11070003; ANPCyT PICT (2017-1220 and 2017-3944) and UNLP (Project11/X790), Argentina. ; No