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
J. Phys.: Condens. Matter, 35, 405601, (2023)
Anisotropic hybridization probed by polarization dependent x-ray absorption spectroscopy in VI3 van der Waals Mott ferromagnet
R. Sant, A. De Vita, V. Polewczyk, G.M. Pierantozzi, F. Mazzola, G. Vinai, G. van der Laan, G. Panaccione and N.B. Brookes
Polarization dependent x-ray absorption spectroscopy was used to study the magnetic ground state and the orbital occupation in bulk-phase VI3 van der Waals crystals below and above the ferromagnetic and structural transitions. X-ray natural linear dichroism and x-ray magnetic circular dichroism spectra acquired at the V $L_{2,3}$ edges are compared against multiplet cluster calculations within the frame of the ligand field theory to quantify the intra-atomic electronic interactions at play and evaluate the effects of symmetry reduction occurring in a trigonally distorted VI6 unit. We observed a non zero linear dichroism proving the presence of an anisotropic charge density distribution around the V3+ ion due to the unbalanced hybridization between the vanadium and the ligand states. Such hybridization acts as an effective trigonal crystal field, slightly lifting the degeneracy of the $t_{2g}^2$ ground state. However, the energy splitting associated to the distortion underestimates the experimental band gap, suggesting that the insulating ground state is stabilized by Mott correlation effects rather than via a Jahn–Teller mechanism. Our results clarify the role of the distortion in VI3 and establish a benchmark for the study of the spectroscopic properties of other van der Waals halides, including emerging 2D materials with mono and few-layers thickness, whose fundamental properties might be altered by reduced dimensions and interface proximity.
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J. Phys. Chem. C, 126, 27, 11174–11181, (2022)
Lifetime of Photogenerated Positive Charges in Hybrid Cerium Oxide-Based Materials from Space and Mirror Charge Effects in Time-Resolved Photoemission Spectroscopy
J.S. Pelli Cresi, E. Spurio, L. Di Mario, P. O’Keeffe, S. Turchini, S. Benedetti, G.M. Pierantozzi, A. De Vita, R. Cucini, D. Catone, and P. Luches
Space and mirror charge effects in time-resolved photoemission spectroscopy can be modeled to obtain relevant information on the recombination dynamics of charge carriers. We successfully extracted from these phenomena the reneutralization characteristic time of positive charges generated by photoexcitation in CeO2-based films. For the above-band-gap excitation, a large fraction of positive carriers with a lifetime that exceeds 100 ps are generated. Otherwise, the sub-band-gap excitation induces the formation of a significantly smaller fraction of charges with lifetimes of tens of picoseconds, ascribed to the excitation of defect sites or to multiphoton absorption. When the oxide is combined with Ag nanoparticles, the sub-band-gap excitation of localized surface plasmon resonances leads to reneutralization times longer than 300 ps. This was interpreted by considering the electronic unbalance at the surface of the nanoparticles generated by the injection of electrons, via localized surface plasmon resonance (LSPR) decay, into CeO2. This study represents an example of how to exploit the space charge effect in gaining access to the surface carrier dynamics in CeO2 within the picosecond range of time, which is fundamental to describe the photocatalytic processes.
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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.
NFFA Thesis
University of Milan Master's Degree Thesis, (2021)
Transient grating spectroscopy with a magnetic probe: a study on phonon-driven magnons in a ferromagnetic thin film
M. Brioschi (Supervisors: G. Rossi, G. Panaccione, R. Cucini)
Materials and heterostructures that exhibit coupling between elastic and magnetic degrees of freedom are of both fundamental and technological interest. In particular, they have great potential for novel energy-efficient spintronic devices because acoustic waves can generate coherent and long-living spin waves through inverse magnetostriction, which consists in variations in the magnetization due to lattice deformations. As optical methods are versatile, non-invasive and contactless, an all-optical approach has been implemented and applied to study magnetoelastic coupling in a ferromagnetic film on a glass substrate. The present thesis work was performed at the NFFA-SPRINT facility of IOM-CNR in the Fermi@Elettra hall at Trieste, where I actively contributed to the realization and characterization of an all new experimental setup which is able to combine transient grating spectroscopy with a time-resolved Faraday polarimetry.
From our users
Small, 17, 2100050, (2021)
Quantitative Ultrafast Electron-Temperature Dynamics in Photo-Excited Au Nanoparticles
M. Sygletou, S. Benedetti, M. Ferrera, G.M. Pierantozzi, R. Cucini, G. Della Valle, P. Carrara, A. De Vita, A. di Bona, P. Torelli, D. Catone, Gi. Panaccione, M. Canepa, F. Bisio
The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications.
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.
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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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