Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy

We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals (NCs) in toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L3-edge and the Cs L2-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes are observed at the Cs L2-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs+ cation plays no role in the charge transport, at least beyond 80 ps. These results can explain the rather modest charge carrier mobilities in these materials.Comment: 19 pages, 3 figure

Catálisis multifuncional (cuatro pasos) promovida por efectos de disolvente

Trabajo presentado a la Reunión de la Sociedad Española de Catálisis: "Catálisis, confluencia interdisciplinar: modelos, catalizadores y reactores", celebrada en Barcelona (España) del 13 al 15 de julio de 2015.Epoxidación y rotura de alquenos: Se había descrito en algunos casos la rotura del doble enlace C=C de alquenos, con dudas sobre el origen de la rotura (¿epóxido → diol → rotura?).Agradecimientos: Ministerio de Economía y Competitividad (España); Gobierno de Aragón; European Research Institute of CatalysisPeer Reviewe

Multifunctional catalysis promoted by solvent effects: Ti-MCM41 for a one-pot, four-step, epoxidation-rearrangement-oxidation-decarboxylation reaction sequence on stilbenes and styrenes

Titanium sites grafted on several siliceous supports are able to act as multifunctional catalytic centers, activating tert-butyl hydroperoxide for oxidation reactions, as redox centers, and promoting rearrangements, as Lewis acids. In the one-pot, four-step conversion of stilbene into benzophenone, the best results were obtained over Ti-MCM41. Under suitable conditions, the catalyst promotes a tandem sequence: alkene epoxidation, epoxide rearrangement, aldehyde oxidation, and oxidative decarboxylation. α,α,α-Trifluorotoluene and a fluorinated glycerol-derived solvent were the optimal solvents for this tandem process, due to their polar aprotic character that allows the efficient oxidation reactions and a poor coordinating ability to prevent any deactivation of the Lewis acid character of the sites. The result of the tandem sequence of reactions is a ketone with loss of a carbon atom that, depending on the starting alkene, is the same result as that of an ozonolysis but under safer and milder conditions. Interesting and new insights on the mechanism of the different reactions involved are also described.Financial support from the Spanish Ministerio de Economia y Competitividad (Project CTQ2011-28124) and the Diputacion General de Aragon (E11 Group cofinanced by the European Regional Development Funds) is gratefully acknowledged. M.G. thanks the European Research Institute of Catalysis (ERIC; http://www.eric-aisbl.eu/) for a short-mobility grant.Peer Reviewe

Hunting for the elusive shallow traps in TiO2 anatase

Understanding electron mobility on TiO2 is crucial because of its applications in photocatalysis and solar cells. This work shows that shallow traps believed to be involved in electron migration in TiO2 conduction band are formed upon band gap excitation, i.e., are not pre-existing states. The shallow traps in TiO2 results from large polarons and are not restricted to surface

Multifunctional Catalysis Promoted by Solvent Effects: Ti-MCM41 for a One-Pot, Four-Step, Epoxidation–Rearrangement–Oxidation–Decarboxylation Reaction Sequence on Stilbenes and Styrenes

Titanium sites grafted on several siliceous supports are able to act as multifunctional catalytic centers, activating <i>tert</i>-butyl hydroperoxide for oxidation reactions, as redox centers, and promoting rearrangements, as Lewis acids. In the one-pot, four-step conversion of stilbene into benzophenone, the best results were obtained over Ti-MCM41. Under suitable conditions, the catalyst promotes a tandem sequence: alkene epoxidation, epoxide rearrangement, aldehyde oxidation, and oxidative decarboxylation. α,α,α-Trifluorotoluene and a fluorinated glycerol-derived solvent were the optimal solvents for this tandem process, due to their polar aprotic character that allows the efficient oxidation reactions and a poor coordinating ability to prevent any deactivation of the Lewis acid character of the sites. The result of the tandem sequence of reactions is a ketone with loss of a carbon atom that, depending on the starting alkene, is the same result as that of an ozonolysis but under safer and milder conditions. Interesting and new insights on the mechanism of the different reactions involved are also described

Direct observation of charge separation on Au localized surface plasmons

Plasmonic nano-structures of d(10) metals are suggested to be the future of photo-voltaics and photo-catalysis under solar irradiation, thanks to their large light absorption cross-section, versatility, and stability. We investigated the impact of continuous plasmon excitation at 532 nm on the density of states of gold nanoparticles, and found an increase of the unoccupied density of d states of gold nanoparticles at the Fermi level, consistent with the formation of electron-hole pairs. Some of those electrons have sufficient energy to overcome the Schottky barrier, and be injected into the TiO2 conduction band. The results confirm that d(10) metal plasmonic structures can act as light sensitizers and photo-catalysts

Time-resolved Element-selective Probing of Charge Carriers in Solar Materials

We review our recent results on the implementation of picosecond (ps) X-ray absorption spectroscopy to probe the electronic and geometric structure of centres formed by photoexcitation of solar materials such as TiO2 polymorphs and inorganic Cs-based perovskites. The results show electron localization at Ti defects in TiO2 anatase and rutile and small hole polaron formation in the valence band of CsPbBr3, all within 80 ps. This method is promising for the study of the ultrafast time scales of such processes, especially with the advent of the Swiss X-ray Free Electron Laser (SwissFEL)

Time-resolved Element-selective Probing of Charge Carriers in Solar Materials

We review our recent results on the implementation of picosecond (ps) X-ray absorption spectroscopy to probe the electronic and geometric structure of centres formed by photoexcitation of solar materials such as TiO2 polymorphs and inorganic Cs-based perovskites. The results show electron localization at Ti defects in TiO2 anatase and rutile and small hole polaron formation in the valence band of CsPbBr3, all within 80 ps. This method is promising for the study of the ultrafast time scales of such processes, especially with the advent of the Swiss X-ray Free Electron Laser (SwissFEL)

Transient mid-IR study of electron dynamics in TiO2 conduction band

The dynamics of TiO2 conduction band electrons were followed with a novel broadband synchrotron-based transient mid-IR spectroscopy setup. The lifetime of conduction band electrons was found to be dependent on the injection method used. Direct band gap excitation results in a lifetime of 2.5 ns, whereas indirect excitation at 532 nm via Ru-N719 dye followed by injection from the dye into TiO2 results in a lifetime of 5.9 ns