Paola Luches - Profile on Academia.edu (original) (raw)

Papers by Paola Luches

Topics in Catalysis, Sep 16, 2016

Cerium oxide in combination with metals is a very important material for catalysis. We present an... more Cerium oxide in combination with metals is a very important material for catalysis. We present an indepth characterization of the growth and morphology of cerium oxide in the form of an inverse model catalyst on Pt(111). Using state of the art low energy electron microscopy (LEEM) and microprobe low energy electron diffraction we provide a detailed in-situ insight into the nucleation and growth dynamics of this model system in different experimental conditions. By probing the unoccupied band structure of the sample during the growth process using intensity-voltage LEEM we also obtain information on the growth dynamics of the deposited ceria overlayer. The results represent an important basis in view of reactivity studies on well controlled ceria epitaxial systems.

Journal of Physical Chemistry Letters, Jun 25, 2020

The ultrafast dynamics of excited states in cerium oxide are investigated to access the early mom... more The ultrafast dynamics of excited states in cerium oxide are investigated to access the early moments of polaron formation, which can influence the photocatalytic functionality of the material. UV transient absorbance spectra of photoexcited CeO2 exhibit a bleaching of the band edge absorbance induced by the pump and a photoinduced absorbance feature assigned to Ce 4f → Ce 5d transitions. A blue shift of the spectral response of the photoinduced absorbance signal in the first picosecond after the pump excitation is attributed to the dynamical formation of small polarons with a characteristic time of 330 fs. A further important result of our work is that the combined use of steady-state and ultrafast transient absorption allows us to propose a revised value for the optical gap for ceria (Eog = 4 eV), significantly larger than usually reported.

Injecting Electrons into CeO₂ via Photoexcitation of Embedded Au Nanoparticles

ACS Photonics, May 3, 2023

Journal of Chemical Physics, Mar 17, 2020

Wide band gap oxides can be sensitized to visible light by coupling them with plasmonic nanoparti... more Wide band gap oxides can be sensitized to visible light by coupling them with plasmonic nanoparticles. We investigate the optical and electronic properties of composite materials made of Ag nanoparticles (NP) embedded within cerium oxide layers of different thickness. The electronic properties of the materials are investigated by x-ray and ultraviolet photoemission spectroscopy, which demonstrate the occurrence of static charge transfers between the metal and the oxide and its dependence on the NP size. UV-Vis spectrophotometry measurements show that the materials have a strong absorption in the visible range induced by the excitation of localized surface plasmon resonances. The plasmonic absorption band can be modified in shape and intensity by properly tuning the NP size and density.

Journal of Physical Chemistry C, May 6, 2019

The functionality of cerium oxide, and in particular its reactivity, can be significantly altered... more The functionality of cerium oxide, and in particular its reactivity, can be significantly altered by the addition of diluted cationic species with different electronic properties as compared to cerium. We investigate the modifications induced by Ag and Cu as modifier cations in cerium oxide ultrathin epitaxial films. The reducibility is assessed by following the modifications of the oxidation state of surface Ce ions by x-ray photoemission spectroscopy, during thermal treatments in ultra-high vacuum and oxygen partial pressure. A significantly higher reducibility is observed in Ag-and Cu-modified films as compared to pure CeO 2 films of the same thickness. The thermal stability of the cation modifier concentration and the changes of the surface structure with the reducing treatments are also discussed. The modifications induced in the material are explained by comparison with density functional theory calculations, which indicate that the oxygen vacancy formation energy is significantly modified by the addition of Ag or Cu in the cerium oxide matrix. The obtained results are of help in view of a rational design of catalysts with optimized performance.

Nanotechnology, Sep 15, 2016

Ceria nanoparticles (NPs) are fundamental in heterogeneous catalysis because of their ability to ... more Ceria nanoparticles (NPs) are fundamental in heterogeneous catalysis because of their ability to store or release oxygen depending on the ambient conditions. Their oxygen storage capacity is strictly related to the exposed planes, crystallinity, density and distribution of defects. In this work a study of ceria NPs produced with a ligand-free, physical synthesis method is presented. The NP films were grown by a magnetron sputtering based gas aggregation source and studied by high resolution-and scanning-transmission electron microscopy and x-ray photoelectron spectroscopy. In particular, the influence of the oxidation procedure on the NP reducibility has been investigated. The different reducibility has been correlated to the exposed planes, crystallinity and density and distribution of structural defects. The results obtained in this work represent a basis to obtain cerium oxide NP with desired oxygen transport properties.

Initial stages of cobalt disilicide formation on silicon single crystals

The initial stages of cobalt disilicide formation on Si(111) and Si(100)surfaces are studied usin... more The initial stages of cobalt disilicide formation on Si(111) and Si(100)surfaces are studied using backscattered electron imaging of the near-surface atomicstructure. Both the reactive deposition and the solid phase epitaxy are investigatedin the coverage range of 1-10 ML of Co. The evidence for CoSi2 island formation atthe earliest stages of the process is found. The epitaxial orientations of disilicidelayers grown on Si(111) and Si(100) surfaces are determined

Journal of Physical Chemistry B, Jun 30, 2017

W e report an error in panel a of Figure of the original article, which reports ill-chosen Ce 3d ... more W e report an error in panel a of Figure of the original article, which reports ill-chosen Ce 3d XPS spectra after soaking in a 0.1 M solution of H 2 O 2 for 1 h and 4 days. We report below the correct version of Figure . The Ce 3+ /Ce 4+ ratio reported in panel c of the original figure was calculated using the correct spectra; therefore, the error made does not affect all of the considerations and conclusions of the article. Figure 7. Panel a: Ce 3d XPS spectra for BG_5.3Ce (red lines) and fitting curves (black lines) after different selected soaking times in 0.1 M H 2 O 2 solution. The spectra have been normalized in intensity to compare their shape.

Photoluminescence Spectroscopy of Cuprous Oxide: Bulk Crystal versus Crystalline Films

Physica Status Solidi A-applications and Materials Science, Mar 23, 2023

Advanced Materials Interfaces, 2020

To optimize the catalytic functionality of cerium oxide it is important to understand the structu... more To optimize the catalytic functionality of cerium oxide it is important to understand the structural modifications associated with reduction and the role of the proximity of metals, which are often coupled with the oxide in the applications. For this purpose, the evolution of the short-and long-range structure of cerium oxide ultrathin epitaxial films and nanostructures supported on Pt(111) is investigated using x-ray absorption spectroscopy at the Ce L3 edge and surface x-ray diffraction, during reduction by thermal treatments in vacuum. In epitaxial nanoislands reduction is associated with a contraction of the Ce-O distance and with the appearance of Ce-Pt bonds. The formation of a phase with a (2×2) periodicity after a thermal treatment at 1023 K is ascribed to the formation of a Pt5Ce alloy. Films of 3 nm thickness do not show, on average, significant structural modifications with the same thermal treatment, consistent with the hypothesis that the reduction involves only the topmost surface layers and it does not influence significantly the bulk structure of the material. This study demonstrates a strong interaction between cerium oxide and platinum, which has implications for the reactivity and stability of catalysts based on metals combined with reducible oxides.

Ceramics International, 2019

In this work, we investigate the ability of mesoporous bioactive glasses (MBGs) modified with cer... more In this work, we investigate the ability of mesoporous bioactive glasses (MBGs) modified with cerium ions (Ce 3+ /Ce 4+ ) to act as catalase and superoxide dismutase (SOD) mimetic materials. We have previously reported that the catalytic properties of bioactive Ce-containing glasses based on 45S5 Bioglass ® and synthesized via melting are influenced by: i) composition (presence/absence of P 2 O 5 ); ii) Ce 3+ /Ce 4+ molar ratio. The introduction of cerium species drastically decreased the bioactivity in terms of Hydroxyapatite formation during bioactivity tests in vitro. We thus decided to add cerium to MBGs, a class of glasses with improved bioactivity with respect to classical molten glasses. MBGs exhibit a high surface area and their reactivity is increased with respect to the molten glasses; they are able to induce the formation of Hydroxyapatite over the surface within shorter times with respect to the 45S5. The catalase and SOD mimetic activity tests revealed that the Ce-MGBs are able to act as mimetic materials for the two enzymes. Both Infrared Spectroscopy and X-ray diffraction analysis have confirmed the presence of Hydroxyapatite over both 80SiO 2 -15CaO-5P 2 O 5 , and 80SiO 2 -20CaO MBGs samples modified by 5.3% mol of CeO 2 ; simultaneously the glasses maintain a good catalase activity. Moreover, the 80SiO 2 -15CaO-5P 2 O 5 potential bioactive glasses showed SOD mimetic activity. These results highlight that it is possible to obtain a glass with both antioxidant and bioactivity properties.

Applied Surface Science, 2017

Films of Ni@CoO core-shell nanoparticles (NP Ni core size d ≈ 11 nm) have been grown on Si/SiO x ... more Films of Ni@CoO core-shell nanoparticles (NP Ni core size d ≈ 11 nm) have been grown on Si/SiO x and lacey carbon supports, by a sequential layer deposition method: a first layer of CoO was evaporated on the substrate, followed by the deposition of a layer of pre-formed, mass-selected Ni NPs, and finally an overlayer of CoO was added. The Ni NPs were formed by a magnetron gas aggregation source, and mass selected with a quadrupole mass filter. The morphology of the films was investigated with Scanning Electron Microscopy and Scanning Transmission Electron Microscopy. The Ni NP cores have a shape compatible with McKay icosahedron, caused by multitwinning occurring during their growth in the source, and the Ni NP layer shows the typical random paving growth mode. After the deposition of the CoO overlayer, CoO islands are observed, gradually extending and tending to merge with each other, with the formation of shells that enclose the Ni NP cores. In situ X-ray Photoelectron Spectroscopy showed that a few Ni atomic layers localized at the core-shell interface are oxidized, hinting at the possibility of creating an intermediate NiO shell between Ni and CoO, depending on the deposition conditions. Finally, Xray Magnetic Circular Dichroism at the Ni L 2,3 absorption edge showed the presence of magnetization at room temperature even at remanence , revealing the possibility of magnetic stabilization of the NP film.

Preface to the JPCM special issue on physical and chemical properties of reducible oxides

Journal of Physics: Condensed Matter

Microchimica Acta

Novel solid-phase microextraction coatings based on the use of multiwalled carbon nanotube–cyclod... more Novel solid-phase microextraction coatings based on the use of multiwalled carbon nanotube–cyclodextrin (MWCNT-CD) nanocomposites were developed for the determination of 16-priority polycyclic aromatic hydrocarbons at ultratrace levels in snow samples. The performance of both β- and γ-CD was tested to increase the detection capabilities towards the heaviest and most lipophilic compounds, i.e., five- and six-ring PAHs. To facilitate the interactions of MWCNTs with CDs, an oxidation procedure using both HNO3 and H2O2 was applied, obtaining superior results using MWCNTs-H2O2-γ-CD fiber. Detection and quantitation limits below 0.7 and 2.3 ng/L, RSD lower than 21%, and recoveries of 88(± 2)–119.8(± 0.4)% proved the reliability of the developed method for the determination of PAHs at ultratrace levels. The complexation capability of the γ-CD was also demonstrated in solution by NMR and fluorescence spectroscopy studies and at solid state by XRD analysis. Finally, snow samples collected in...

Journal of Applied Biomaterials & Functional Materials, 2016

Concept: Self-assembling of reduced graphene oxide (RGO) platelets ('segregated' arrangement), as... more Concept: Self-assembling of reduced graphene oxide (RGO) platelets ('segregated' arrangement), as a tailored interconnected network within natural rubber and butyl rubber matrices, is proposed as a mean for obtaining nanocomposites with significantly enhanced functional properties as compared to unloaded rubber, i.e. gas barrier properties and electric conductivity, even at very low filler contents. Interestingly, the prescribed spatial arrangement of the nanoparticles results to be much more effective in improving properties than homogeneous dispersion ('not segregated' arrangement) of platelets, even at low loadings. The 'segregated' structure originates from the confinement of platelets within the interstices of the coagulated latex particles, which act as a template for the network formation. The platelets are assembled on the latex particles giving rise to spheres with a core-shell structure, with a partial or complete covering depending on graphene amount. Conversely, the 'not-segregated' structure is obtained by destroying this interconnected network by further processing the RGO nanocomposite masterbatch via twin-roll mixing, thus determining a uniform orientation of exfoliated RGO platelets. The aim is exploiting self-assembling of RGO platelets to provide a cost-effective method to tailor the dispersion of nanoparticles inside the host matrix, allowing to significantly decrease the percolation threshold for electric conductivity and drastically change the dependence of gas barrier on nanofiller concentration. Results and discussion: The presence of RGO nanoplatelets radically affect the dependence of gas barrier properties on nanofiller content. An example is provided in Figure for the case of oxygen permeation in nanocomposites with natural rubber matrix where the 'segregated structure (filled symbols) is compared with 'not segregated' one (empty symbols). Taking the real part of complex conductivity at the lowest investigated frequency, σ'0, as representative of the DC conductivity one can estimate the percolation threshold for electric conductivity in a nanocomposite. The optimized spatial distribution of the nanoplatelets attained in the case of 'segregated' structure, for the case of butyl rubber matrix, promotes an enhancement of σ'0 of almost two orders of magnitude suggesting the building up of conductive paths of RGO throughout the rubber phase (see filled symbols in Figure ). Conversely, the system having a 'not-segregated' morphology displays no evident changes as compared to neat matrix (see empty symbols in Figure ). A value of critical RGO concentration as low as Φc = 0.4 vol.% has been estimated for the "segregated" system according to the percolation theory (inset of Figure ).

Cerium oxide is a very interesting material that finds applications in many different fields, suc... more Cerium oxide is a very interesting material that finds applications in many different fields, such as catalysis, energy conversion, and biomedicine. An interesting approach to unravel the complexity of real systems and obtain an improved understanding of cerium oxide-based materials is represented by the study of model systems in the form of epitaxial ultrathin films or nanostructures supported on single crystalline substrates. These materials often show interesting novel properties, induced by spatial confinement and by the interaction with the supporting substrate, and their understanding requires the use of advanced experimental techniques combined with computational modeling. Recent experimental and theoretical studies performed within this field are examined and discussed here, with emphasis on the new perspectives introduced in view of the optimization of cerium oxide-based materials for application in different fields.

On the ionisation mechanism of reflection (e,2e) events

AIP Conference Proceedings, 1996

The possibility of using the reflection grazing angle (e,2e) technique as a binding energy and/or... more The possibility of using the reflection grazing angle (e,2e) technique as a binding energy and/or momentum spectroscopy of surface states rests on the accurate knowledge of the ionisation process. Two possible mechanisms are envisaged that can generate pairs of correlated electrons in the reflection geometry: a single inelastic collision at large momentum transfer or a double collision (elastic plus inelastic). In this paper are presented the results of (e,2e) experiments that allow to elucidate the ionisation mechanism at intermediate energies (300 eV) and asymmetric kinematics. The measurements, performed on highly oriented pyrolitic graphite, also show that an overall energy resolution as good as 1.2 eV can be achieved.

Materials & Design, 2016

The effect of phosphate on the ability of Ce-containing bioactive glasses to inhibit oxidative st... more The effect of phosphate on the ability of Ce-containing bioactive glasses to inhibit oxidative stress was studied on compositions based on Hench (46.2%SiO 2 \ \24.3%Na 2 O\ \26.9%CaO\ \2.6P 2 O 5 , mol%) and Kokubo (50.0%SiO 2 \ \ 25.0%Na 2 O\ \25.0%CaO) glasses. In particular, the reduction of catalase mimetic activity of Ce-containing glasses due to the presence: i) of P 2 O 5 in the glass compositions, and ii) of phosphate groups in the solution employed for catalase mimetic activity tests was explained and rationalized by combining SEM, XPS, XRD, DTA, FT-IR and UVvis experiments with Molecular Dynamics simulations. The results suggest that the Ce ions play a different structural role in the two series of glasses. In particular, in phosphate free glasses Ce is coordinated by non-bridging oxygens (NBOs) originated from the disruption of the silicate network, whereas in phosphate containing glasses the NBOs around Ce ions belong to orthophosphate groups. The latter groups stabilize the Ce 3+ species subtracting them from the interconversion process between Ce 3+ and Ce 4+ , which is of fundamental importance for the exhibition of the catalase mimetic activity.

Materials Science and Technology, 2016

Ultrathin films of TiO 2 were grown on a W(100)-O(261) substrate and characterised with a combina... more Ultrathin films of TiO 2 were grown on a W(100)-O(261) substrate and characterised with a combination of scanning tunneling microscopy (STM) and low energy electron diffraction. In addition to islands of rutile TiO 2 (110) with (161) termination that were reported previously, we also observed rutile TiO 2 (110) islands with a (162) film termination. A lepidocrocite-like TiO 2 nanosheet was also observed on the W(100) surface. High resolution STM images show that the nanosheet grows in the principal orthogonal directions of the W(100) substrate and forms a commensurate (167) coincident cell.

Thickness-dependent strain in epitaxial MgO layers on Ag()

Surface Science, 2002

ABSTRACT Thin MgO films (1–20 ML) were synthesised by evaporating the metallic component in oxyge... more ABSTRACT Thin MgO films (1–20 ML) were synthesised by evaporating the metallic component in oxygen atmosphere on Ag(0 0 1). The reduced lattice misfit (3.1%) between the (0 0 1) surface unit meshes of the Ag fcc and MgO rock salt structure leads to the formation of epitaxial MgO layers with the (0 0 1)MgO∥(0 0 1)Ag and [1 0 0]MgO∥[1 0 0]Ag orientation relationships. The oxide layers were structurally characterized by LEED and modulated electron emission, while their surface morphology was studied by STM. In spite of the weak interaction between the oxide overlayer and the substrate, the occurrence of a significant tetragonal distortion of the MgO structure has been detected. We investigated in detail the strain in the oxide layer as a function of the thickness. Thin (&lt;6 ML) films exhibit an expansion along the growth axis with respect to the equilibrium phase. This vertical strain progressively reduces as the thickness increases and disappears at about 9 ML. A Poisson ratio larger than the bulk MgO value has been obtained for the 3 ML film, while the bulk value is appropriate films thicker than 6 ML. STM data indicate that an almost complete substrate coverage is obtained at 3 ML deposition.