186,485 research outputs found
Melting of cooperative Jahn-Teller distortion in KMg0.2Cu0.8F3
No full textKCuF3 is a Mott-Hubbard insulator with a distorted perovskite structure (space group I4/mcm). The structural distortion is due to orbital ordering (OO) associated with cooperative Jahn-Teller effect (JT) [1]. and corresponds to an alternate occupation of Cu-3dy2-z2 and Cu-3dx2-z2 hole states on Cu(3d9) ion [2]. The orbital configuration results in quasi one-dimensional magnetic properties. Nearest-neighbour superexchange (NN-SE) interactions are strong and antiferromagnetic (AF) along the c axis and, for T>TN=38 K, weak and ferromagnetic in the ab plane. Debate is open in the literature on the actual driving force (i.e. either OO or JT) of the structural distortion and of the related electronic and magnetic properties. We have recently found experimental evidence of the ideal situation in which OO is melted while the JT distortion is still present:, in fact OO is expected to be very sensitive to slight changes in the electronic structure. Electron paramagnetic resonance investigations revealed melting of OO at room temperature in the KCu1-xMgxF3 system for x=0.1 [3]. We presents here a Synchrotron Radiation X-ray powder diffraction (XRPD) study in a sample with composition KCu0.8Mg0.2F3, which at room temperature is isostructural with KCuF3, a prototypical system for studying Orbital Order (OO). This sample can be considered a realisation of the ideal situation in which OO is melted while the cooperative JT distortion is still present. The melting of the cooperative JT distortion is observed in this system for T~600 K. This result is discussed in the framework of the different energy scales for OO and cooperative JT distortion.
[1] L.F. Feiner, A.M. Oleś, J. Zaanen, Phys. Rev. Lett., 1997, 78, 2799. [2] R. Caciuffo, L. Paolasini, A. Sollier, P. Ghigna, E. Pavarini, J. van den Brink, M. Altarelli Phys.Rev., 2002, B65, 174425
[3] C. Oliva, M. Scavini, S. Cappelli, C. Bottalo, C. Mazzoli, P. Ghigna, J.Phys.Chem., 2007, B111, 597
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
Study of photoelectrochemical behavior of copper oxides based materials using X-ray absorption spectroscopy
No full textThe use of sunlight to convert water into fuel is very attractive and ambitious since H2 is considered to be the energy carrier of the future thanks to its high mass energy density and its environmental friendliness [1,2]. Copper oxides-based photocathodes are attractive for their absorption in the visible range, low cost, high abundance and easy synthetic protocols as well as high photoactivity [3,4].
Two p-type semiconducting copper based materials has been prepared, characterized and tested as a photocathode for H2 production: CuO and Cu2O. The first one is prepared by thermal treatment of nanocrystalline CuI, which shows high efficiency in light conversion and interesting self-protection properties [5].
Cu2O instead was prepared by electrochemical deposition from a lactate-stabilized Cu++ bath [3]. Viceversa the main drawback of Cu(I) oxide is its lack of stability during photoelectrochemical conditions. For this material the influence of a metallic underlayer (Au, Cu) between the semiconductor itself and the FTO support was studied, together with the presence of a small load of Pt catalyst.
In-situ and in-operando techniques like X-ray absorption near edge structure (XANES), Extended X-Ray Absorption Fine Structure (EXAFS) and Fixed Energy X-ray Absorption Voltammetry (FEXRAV) [6] allow us to better understanding materials behavior. We observe changes in copper oxidation states upon light and/ or applied potential. Moreover, the role of methanol as hole-scavenger during photoelectrochemical experiment has been studied. FEXRAV measurements allow following the material degradation processes and defining the stability windows. With differential light and dark XANES spectra, we investigated the local changes in electronic structure upon spectroelectrochemical conditions. These results will allow us obtaining more stable system for photoelectrochemical hydrogen production.
References
[1] G. Centi, S. Perathoner, ChemSusChem. 3 (2010) 195–208.
[2] F. Malara, A. Minguzzi, M. Marelli, S. Morandi, R. Psaro, V. Dal Santo, A. Naldoni, ACS Catal. 5 (2015) 5292–5300.
[3] A. Paracchino, V. Laporte, K. Sivula, M. Grätzel, E. Thimsen, Nat. Mater. 10 (2011) 456–461.
[4] C. Li, T. Hisatomi, O. Watanabe, M. Nakabayashi, N. Shibata, K. Domen, J.-J. Delaunay, Energy Environ. Sci. 8 (2015) 1493–1500.
[5] T. Baran, S. Wojtyła, C. Lenardi, P. Ghigna, E. Achilli, S. Rondinini, A. Minguzzi, ACS Appl. Mater. Interfaces. (submitted).
[6] A. Minguzzi, O. Lugaresi, C. Locatelli, S. Rondinini, F. D’Acapito, E. Achilli. P. Ghigna. 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
Germanium K edge in GeO2 polymorphs. Correlation between local coordination and electronic structure of germanium
No full textThe electronic properties of two room Temperature persistent phases of germanium dioxide have been studied by means of
experimental and theoretical techniques. We collected the Ge-K edge XANES spectra of these materials at the GILDA beamline of
ESRF. The Density Of States of the two crystal phases, obtained from fully periodic Hartree-Fock and Density Functional calculations,
is taken as the reference term to rationalise and assign the manifolds of the XANES spectra. Although this scheme requires a number of
severe approximations, we obtained a good overall agreement between experiment and theory. The topological analysis of the
theoretical electron density distribution in the crystals gave further information regarding the electronic properties of germanium
dioxide
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
Magnetic phase diagram of Nd1.85Ce0.15CuO4+ delta from magnetization and muon spin relaxation measurements
No full text"Unusual Ln3+ substitutional defects": The local chemical environment of Pr3+ and Nd3+ in nanocrystalline TiO2 by Ln-K edge EXAFS
No full textThe local chemical environment of the trivalent lanthanide cations in anatase TiO(2) nanopowders doped with 1mol% of Pr or Nd, prepared via a sol-gel technique, has been studied by means of EXAFS at the Pr and Nd-K edge. Titanium dioxide can be considered an "unusual'' host for doping with Ln(3+) ions due to the large mismatch of both charge and ionic radii between the dopant and the host constituent cations. However, it can be demonstrated that the lanthanide ions enter the anatase structure as substitutional defects with respect to Ti, but that the amount of disorder around the substitutional defects is very large. For both Pr(3+) and Nd(3+) ions, the Ln-O and Ln-Ti distances have been found to increase by about 0.45 angstrom, with respect to what is found for the Ti-O and Ti-Ti distances in pure anatase. Valence-bond calculations have been used to validate the Ln-O distances obtained by the EXAFS fitting. Finally, no evidences for oxygen vacancies clustering around the substitutional defects have been found. Luminescence spectroscopy has shown that the lanthanide ions do not segregate in oxide or pyrochlore impurities phases
Local Chemical Environment of Pr3+ Substitutional Defects in Bulk and Nanocrystalline Gd3Ga5O12: A Joint EXAFS and Luminescence Study
No full textA joint approach using EXAFS (extended X-ray absorption fine structure) and luminescence results has been used to obtain information on the local structure and the electron-phonon coupling of the Pr3+ ion in bulk and nanocrystalline gadolinium gallium garnet samples. The nanocrystalline sample is prepared with a combustion synthesis, whereas the bulk sample is prepared by a sintering procedure using the nanocrystalline powders. EXAFS measurements show that the Pr-O distances are similar for the bulk and the nanocrystalline samples but a notable amount of static disorder in the first coordination sphere of the dopant is found to be present in the nanocrystalline sample. From the behavior of the emission line width and shift of a Pr3+ emission band as a function of the temperature, the electron-phonon coupling strength of the Pr3+ ion has been evaluated. The electron-phonon coupling strength results to be notably stronger for the nanocrystalline sample than for the bulk one. This behavior is attributed to the fact that the local surrounding of the Pr3+ ion is softer for the nanocrystalline sample than for the bulk one
Susceptibility and evidences of charge carriers interaction above T-c in Nd1.85Ce0.15CuO4+delta
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