87,124 research outputs found
Electronic and spectral properties of clean and C60-covered atom-thick Chromium oxide at the Fe(001) surface
No full textChemisorption of a single atomic layer of oxygen on the Fe(001) surface yields a highly ordered and reproducible benchmark substrate [1] for theoretical and experimental studies, and for the epitaxial growth of metal oxides, including atom-thick CrxOy layers, and hybrid interfaces with foreseen applications e.g. in organic spintronics.
This talk initially presents ab initio investigations that have supplemented microscopy and spectroscopy experiments of the electronic and magnetic properties of two-dimensional Chromium oxides of Cr3O4 and Cr4O5 stoichiometry grown on Fe(001), featuring antiferromagnetic magnetic configurations with underlying Fe(001) [2,3]. Despite Cr / CrO systems are notoriously difficult for mean field approaches, generalized-gradient results are found to explain most experimental findings, with a rigid shift of oxygen bands accounting for electronic correlation effects.
We eventually consider the effect of inserted Cr4O5 layers at the interface between the prototypical C60 organic semiconductor and Fe(001), which is shown to enhance the magnetic hybridization between the molecule and the surface through x-ray magnetic circular dichroism (XMCD) [4,5]. By means of ab initio calculation we characterize the local interface morphology, the magnetic configuration of the surface and the induced spin dependent electronic properties of the molecule, the latter reflecting the magnetic electronic properties of the surface at the relevant energy range. As seen from the substrate, adsorbates can influence the magnitude and even orientation of surface Cr magnetic moments. The interest in this interface is then twofold: on one side the thin magnetic oxide allows tailoring the magnetic properties of the organic layer, on the other side the adsorption of C60 can be envisioned as a tool to control the magnetic ordering of Cr atoms at the interface.
[1] A. Picone, M. Riva, A. Brambilla, A. Calloni, G. Bussetti, M. Finazzi, F. Ciccacci, L. Duò, Surface Science Reports 71, 32 (2016).
[2] A. Picone, G. Fratesi, M. Riva, G. Bussetti, A. Calloni, A. Brambilla, M. I. Trioni, L. Duò, F. Ciccacci, and M. Finazzi, Phys. Rev. B 87, 085403 (2013).
[3] A. Calloni, G. Fratesi, S. Achilli, G. Berti, G. Bussetti, A. Picone, A. Brambilla, P. Folegati, F. Ciccacci, and L. Duò, Phys. Rev. B 96, 085427 (2017).
[4] A. Brambilla, A. Picone, D. Giannotti, A. Calloni, G. Berti, G. Bussetti, S. Achilli, G. Fratesi, M. I. Trioni, G. Vinai, P. Torelli, G. Panaccione, L. Duò, M. Finazzi, and F. Ciccacci, Nano Lett. 17, 7440 (2017).
[5] A. Brambilla, A. Picone, S. Achilli, G. Fratesi, A. Lodesani, A. Calloni, G. Bussetti, M. Zani, M. Finazzi, L. Duò, and F. Ciccacci, Journal of Applied Physics 125, 142907 (2019)
Observation of charge transfer cascade in α-Fe2O3/IrO2 photoanodes by in-operando X-rays absorption spectroscopy
No full textIn this work we show the direct observation, by means of spectro-photoelectrochemical experiments, of charge transfer between a semiconductor (-Fe2O3) and a metal oxide overlayer (hydrous IrOx) as a photoanode architecture in photoelectrochemical water splitting.1 The aim is to clarify the ambiguous role of oxygen evolving catalysts used as overlayers on top of photoanodes in photoelectrochemical water splitting cells. Previous literature suggested that the real benefit of covering hematite with overlayers like iridium or cobalt oxides is not due to an increase of reaction kinetics but the decrease of the electron density in the hematite2 or the storage of photogenerates holes.3 These effects are likely more important when hydrous overlayer, that can act as adapting catalysts,4 are considered. All these hypothesis can explain the observed improved hole lifetime and reduce recombination with electrons.
The present experimental approach is similar to the one that allowed our recent disclosure of the oxidation states assumed by hydrous IrOx as catalyst for water oxidation.5 In the present case, FEXRAV6 and XANES have been used to probe changes in the charge state of Ir while the hematite was illuminated with 410nm radiation. Thanks to this in-operando setup, we were able to observe an increase of the density of empty Ir 5d states during hematite illumination and in correspondence of water spitting in the photoelectrochemical cell. The main conclusion is that a charge (hole) transfer between hematite and iridium occurs only when the hematite is illuminated. Hydrous iridium oxide is therefore capable of withdrawing holes from the semiconductor thus increasing the probability of interface reaction rather than charge recombination.
1 Minguzzi A., Lugaresi O., Achilli E., D'Acapito F., Naldoni A., Malara F., Locatelli C., Vertova A., Rondinini S., Ghigna P., In preparation
2 Badia-Bou L., Mas-Marza E., Rodenas P., M. Barea E., Fabregat-Santiago F., Gimenez S., Peris E., Bisquert J., J. Phys. Chem. C, 2013, 117, 3826−3833
3 Lin F., Boettcher S.W. Nature Materials, 2014, 13, 81-86
4 Barroso M., Mesa C.A., Pendlebury S.R. , Cowana A.J., Hisatomi T., Sivula K., Grätzel M., Klug D.R., Durrant J.R. PNAS, 2012, 109, 15640–15645
5 Minguzzi A., Lugaresi O., Achilli E., Locatelli C., Vertova A., Ghigna P., Rondinini S., Chem. Sci., 2014, 5, 3591-3597
6 Minguzzi, A.; Lugaresi, O.; Locatelli, C.; Rondinini S.; d'Acapito, F.; Achilli, E.; Ghigna, P. Anal. Chem. 2013, 85, 7009-7013
Electro- and photo-electrochemical water splitting as studied by In-Operando X-Rays Absorption Spectroscopy
No full textIn this work we show our most recent results obtained by in-operando X-Ray absorption spectroscopy on hydrous/amorphous [1] and on crystalline/dry [2] iridium oxide electrodes as electrocatalysts for the oxygen evolution reaction (OER). In all cases, XAS evidenced the role of Ir active sites, and the relevant oxidation states assumed during the catalytic cycle. Moreover, the local structure is not significantly influenced by the applied potential, thus suggesting a negligible reorganization energy of the catalyst.On the bases of these results, we were able to directly observe, by means of spectro-photoelectrochemical experiments, the charge transfer between a semiconductor (α-Fe2O3) and hydrous IrOx, the latter used as overlayer for generating a high performance photoanode architecture in photoelectrochemical water splitting[3]. The aim is to clarify the ambiguous role of oxygen evolving catalysts used as overlayers on top of photoanodes in photoelectrochemical water splitting cells. Previous literature suggested that the real benefit of covering hematite with overlayers like iridium or cobalt oxides is not due to an increase of the reaction rate but to a decrease of the electron density in the hematite[4] or to the storage of photogenerates holes[5]. These effects are likely more important when hydrous overlayer, that can act as adapting catalysts[6], are considered. All these hypotheses can explain the observed improved hole lifetime and reduce recombination with electrons. The experimental approach is similar to the one adopted to study Ir oxide particles electrocatalysts[1,2]. In the present case, FEXRAV [7] and XANES have been used to probe changes in the charge state of Ir while the hematite was illuminated with a 410nm diode. Thanks to this setup, we were able to observe an increase of the density of empty Ir 5d states during hematite illumination and in correspondence of water spitting in the photoelectrochemical cell. The main conclusion is that a charge (hole) transfer between hematite and iridium occurs only when the hematite is illuminated. Hydrous iridium oxide is therefore capable of withdrawing holes from the semiconductor thus increasing the probability of interface reaction rather than charge recombination.
References
[1] A. Minguzzi, O. Lugaresi, E. Achilli, C. Locatelli, A. Vertova, P. Ghigna, Rondinini S., Chem. Sci., 2014, 5, 3591-3597
[2] A. Minguzzi, C. Locatelli, O. Lugaresi, E. Achilli, G. Cappelletti, M. Scavini, M. Coduri, P. Masala, B. Sacchi, A. Vertova, P. Ghigna, S. Rondinini, submitted
[3] A. Minguzzi, O. Lugaresi, E. Achilli, F. D'Acapito, A. Naldoni, F. Malara, C. Locatelli, A. Vertova, S. Rondinini, P. Ghigna, In preparation
[4] M. Barroso, C.A. Mesa, S.R. Pendlebury, A.J. Cowana, T. Hisatomi, K. Sivula, M. Grätzel, D.R. Klug, J.R. Durrant PNAS, 2012, 109, 15640–15645
[5] L. Badia-Bou, E. Mas-Marza, P. Rodenas, E M. Barea., F. Fabregat-Santiago, S. Gimenez, E. Peris, J. Bisquert, J. Phys. Chem. C, 2013, 117, 3826−3833
[6] F. Lin, S.W. Boettcher Nature Materials, 2014, 13, 81-86
[7] A. Minguzzi, O. Lugaresi, C. Locatelli, S. Rondinini, F. d'Acapito, E. Achilli, P. Ghigna, Anal. Chem. 2013, 85, 7009-7013
Vertical movements along the northwestern side of Messina straits, in the 1967-1970 period
No full textThe results of a comparison of the spirit levelling data made by I.G.M. in 1967<br />and in 1970, are presented. An analysis of these data shows that the altitude<br />variations of bench marks in the considered triennial period seem to originate from<br />the collective rotation of an inclined plane, which intersects the geoid surface with<br />an azimuth of 43° to the North.<br />This is in accordance with the orientation of the fault pattern given by local<br />geology.<br />The fact that the existing bench marks were sited along the coastline, and the<br />lack of a comparison with data relative to motion on more compact geological<br />structures, lead necessarily to provisional conclusions
In-situ X-ray absorption spectroscopy on (photo-)electrocatalysts for water oxidation: towards new insights on the reaction mechanism
No full textHere we introduce the Fixed Energy X-Ray Absorption Voltammetry (1), a novel in-situ/in-operando X-Ray Absorption Spectroscopy (XAS) technique for fast and easy preliminary characterization of electrodes and photoelectrodes which consists in recording the absorption coefficient at a fixed energy while varying at will the electrode potential. The energy is chosen close to a core level absorption edge, in order to give the maximum contrast between different oxidation states of an element. It follows that any shift from the initial oxidation state determines a variation of the X-ray absorption coefficient. In this work we demonstrate that FEXRAV allows to quickly map the variation of the oxidation states of the element under consideration in a desired potential window. At this purpose, we use high surface area electrodes to attain a high surface/volume ratio (nanoparticles, nanostructures, highly hydrated films) and be more sensible to any chemical phenomena occurring at the surface.
We show that FEXRAV gives important information by itself but can also serve as a preliminary screening of the potential window or, more generally, for choosing the best experimental conditions for a better targeted XAS analysis. In fact, this work includes a detailed XAS study aimed to clarify the mechanism of iridium oxide as catalyst for water oxidation: for the first time we directly observed the co-existence of more than one Ir oxidation state at E >1.23V (RHE), that is consistent with the role of Ir as center of a catalytic cycle. This represents a crucial point for a better understanding of water electrolysis and photoelectrochemical (PEC) water splitting (2).
We completed this study by time-resolved energy dispersive XAS for better understanding the time-dependence of the interfacial phenomena occurring during pseudocapacitance charge/discharge and during the water oxidation catalysis.
(1) Minguzzi, A.; Lugaresi, O.; Locatelli, C.; Rondinini S.; d'Acapito, F.; Achilli, E.; Ghigna, P. Anal. Chem. 2013, 85, 7009-7013.
(2) Minguzzi A., Lugaresi O., Achilli E., Locatelli C., Vertova A., Ghigna P., Rondinini S., Chem. Sci. 2014, 5, 3591-3597
Mitogenomes from Two Uncommon Haplogroups Mark Late Glacial/Postglacial Expansions from the Near East and Neolithic Dispersals within Europe
No full textThe current human mitochondrial (mtDNA) phylogeny does not equally represent all human populations but is biased in favour of representatives originally from north and central Europe. This especially affects the phylogeny of some uncommon West Eurasian haplogroups, including I and W, whose southern European and Near Eastern components are very poorly represented, suggesting that extensive hidden phylogenetic substructure remains to be uncovered. This study expanded and re-analysed the available datasets of I and W complete mtDNA genomes, reaching a comprehensive 419 mitogenomes, and searched for precise correlations between the ages and geographical distributions of their numerous newly identified subclades with events of human dispersal which contributed to the genetic formation of modern Europeans. Our results showed that haplogroups I (within N1a1b) and W originated in the Near East during the Last Glacial Maximum or pre-warming period (the period of gradual warming between the end of the LGM, ~19 ky ago, and the beginning of the first main warming phase, ~15 ky ago) and, like the much more common haplogroups J and T, may have been involved in Late Glacial expansions starting from the Near East. Thus our data contribute to a better definition of the Late and postglacial re-peopling of Europe, providing further evidence for the scenario that major population expansions started after the Last Glacial Maximum but before Neolithic times, but also evidencing traces of diffusion events in several I and W subclades dating to the European Neolithic and restricted to Europe
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