The doping of metal oxides is an interesting route to increase catalyst activity and lower activation temperatures in H2 dissociation to replace Pt in catalysts for electrochemical devices. In this process, the roles of both the matrix and dopant cations are fundamental to understanding and designing more efficient catalysts. In this work, we have investigated the reduction process in pure and doped CeO2 films. We followed the oxidation states of Ce and dopants (Cu and Fe) during H2 exposure at ambient pressure by combining X-ray absorption spectroscopy and gas chromatography on 5 nm films in the temperature range of 300–620 K. We have observed that Cu doping (at concentrations of 5 and 14 at. %) promotes the ceria reduction, while the addition of Fe seems to have a limited impact on the oxide chemical reactivity only at low temperatures. Moreover, thanks to the chemical sensitivity of operando X-ray absorption spectroscopy, we were able to follow simultaneously the evolution of Ce and Cu oxidation states during the reaction, which has permitted to identify two distinct reduction processes taking place above and below 500 K. These measurements show that at low temperatures, the H2 dissociation takes place at the Cu1+ sites, thus explaining the higher reactivity of the Cu-doped samples. The described mechanism can help in the design of Pt-free catalysts with enhanced performances.
From our users
Phys. Rev. Materials, 5, 104403, (2021)
Identification of hidden orbital contributions in the La0.65Sr0.35MnO3 valence band
F. Offi, K. Yamauchi, S. Picozzi, V. Lollobrigida, A. Verna, C. Schlueter, T.-L. Lee, A. Regoutz, D. J. Payne, A. Petrov, G. Vinai, G. M. Pierantozzi, T. Pincelli, G. Panaccione, and F. Borgatti
Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of La0.65Sr0.35MnO3 (LSMO), a landmark system for spintronics applications, a description based solely on Mn 3d and O 2p electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La 5p, Mn 4s, and O 2s orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general.
From our users
J. Phys. Chem. C, 124, 17783–17794, (2020)
Ga2Se3 Nanowires via Au-Assisted Heterovalent Exchange Reaction on GaAs
F. Berto, N. Haghighian, K. Ferfolja, S. Gardonio, M. Fanetti, F. Martelli, V. Mussi, V.G. Dubrovskii, I.V. Shtrom, A. Franciosi, and S. Rubini
Out-of-plane Ga2Se3 nanowires are grown by molecular beam epitaxy via Au-assisted heterovalent exchange reaction on GaAs substrates in the absence of Ga deposition. It is shown that at a suitable temperature around 560 degrees C the Audecorated GaAs substrate releases Ga atoms, which react with the incoming Se and feed the nanowire growth. The nanowire composition, crystal structure, and morphology are characterized by Raman spectroscopy and electron microscopy. The growth mechanism is investigated by X-ray photoelectron spectroscopy. We explore the growth parameter window and find an interesting effect of shortening of the nanowires after a certain maximum length. The nanowire growth is described within a diffusion transport model, which explains the nonmonotonic behavior of the nanowire length versus the growth parameters. Nanowire shortening is explained by the blocking of Ga supply from the GaAs substrate by thick, in-plane worm-like Ga2Se3 structures, which grow concomitantly with the nanowires, followed by backward diffusion of Ga atoms from the nanowires down to the substrate surface.
NFFA Thesis
University of Milan PhD Thesis, (2019)
Strain-mediated magneto electric coupling and beyond: case studies by in-operando spectroscopy
I explored the properties of systems that were fabricated aiming to exploit enhanced multiferroic behavior and potentially useful functionalities at room temperature. The systems of choice for this thesis were two prototypical multiferroic heterostructures composed by a ferromagnetic thin film deposited on a ferroelectric substrate: LSMO/BTO(001) and Fe,FeMn/PMN-PT(001). I focused on the magnetic response of the thin films to applied electric fields oriented perpendicular to the interface, and influencing the substrate. In both the chosen heterostructures the magnetic layers and ferroelectric substrates are all materials with high ordering temperature.
From our users
OSA Technical Digest, paper EW2B.5, (2018)
A Novel High Order Harmonic Source for Time- and Angle-Resolved Photoemission Experiments
P. Miotti, F. Cilento, R. Cucini, A. De Luisa, A. Fondacaro, F. Frassetto, D. Kopić, D. Payne, A. Sterzi, T. Pincelli, G. Panaccione, F. Parmigiani, G. Rossi, and L. Poletto
The design and characterization of a HHG source conceived for Time and Angle Resolved PhotoElectron Spectroscopy (TR-ARPES) experiments are presented. The harmonics are selected through a grating monochromator with an innovative design able to provide XUV radiation for two distinct TR-ARPES setups.
From our users
Chemical Physics Letters, 683. 135, (2017)
Time resolved resonant photoemission study of energy level alignment at donor/acceptor interfaces
R. Costantini, T. Pincelli, A. Cossaro, A. Verdini, A. Goldoni, S. Cichoň, M. Caputo, M.Pedio, G. Panaccione, M.G. Silly, F. Sirotti, A. Morgante, M. Dell'Angela
The knowledge of the picosecond dynamics of the energy level alignment between donor and acceptor materials in organic photovoltaic devices under working conditions is a challenge for fundamental material research. We measured by means of time-resolved Resonant X-ray Photoemission Spectroscopy (RPES) the energy level alignment in ZnPc/C60 films. We employed 800 nm femtosecond laser pulses to pump the system simulating sunlight excitation and X-rays from the synchrotron as a probe. We measured changes in the valence bands due to pump induced modifications of the interface dipole. Our measurements prove the feasibility of time-resolved RPES with high repetition rate sources.
NFFA Thesis
University of Milan PhD Thesis, (2017)
Probing electron correlation dynamics: a multi-technique study applied to the half-metallic oxide La3-xSrxMnO5
This thesis completes my work as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Università degli Studi di Milano that has been carried out, starting in November 4236, mostly at the Laboratorio TASC of IOM-CNR3 in the premises of the Elettra - Sincrotrone Trieste and FERMI@Elettra infrastructures4, in the framework of the NFFA and APE-beamline facilites5, as well as by accessing international large scale infrastructures and laboratories. The activity has addressed the development of experimental methodologies and novel instrumentation oriented to the study of the dynamical properties of highly correlated materials after high energy excitation. The science programme has been carried out by exploiting ultrafast femtosecond probes from the optical regime (Ti-Sa lasers, fibre laser oscillators) to the extreme UV-soft X rays at FERMI, to the picosecond hard X-rays from the SPring-: and Diamond synchrotron radiation source. The sample synthesis of correlated oxides and its characterization has been performed within the NFFA facility and APE-group collaboration in Trieste as well as the design and construction of the all new laser High Harmonic Generation beam line NFFA-SPRINT and its end station for time resolved vectorial electron spin polarimetry.
NFFA Thesis
University of Modena and Reggio Emilia Master's Thesis, (2014)
Towards Spin-resolved/Time-resolved Photoelectron Spectroscopy at the fs time scale: construction and commissioning of the ULTRASPIN apparatus
This thesis reports on the construction and commissioning tests of the novel experimental set-up needed for a long term research project, named ULTRASPIN, aiming at establishing time resolved spin-resolved photoemission measurements with ultra-short (10−14 s) photon pulses from Free Electron Laser beamlines or from table-top UV/Soft-X beamlines.
The ULTRASPIN project started in the summer 2013, building on competences and instrumentation in part available from the APE-beamline group of IOM-CNR at Elettra, and with the partial support of an European contract (EXSTASY-EXperimental STation for the Analysis of the Spin Dynamics, Grant agreement N.PIIF-GA-2012-326641) and related fellowship of a world-expert of Mott scattering.
I have been involved from the beginning in the final design, in the construction and commissioning of a novel stray-field free UHV apparatus for preparing and hosting atomically clean surfaces and for measuring the spin-polarization of the photo-emitted electrons with “single pulse” sensitivity down to the 10−14 s time scale, as well as in the standard high frequency spectroscopy mode. In the commissioning phase I have participated to test experiments on ULTRASPIN as well as to relevant experiments conducted in other apparatuses.
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