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1. Defect engineering over anisotropic brookite towards substrate-specific photo-oxidation of alcohols

Author

Hejazi, S. M. Hossein, Shahrezaei, Mahdi, Błoński, Piotr, Allieta, Mattia, Sheverdyaeva, Polina M., Moras, Paolo, Baďura, Zdeněk, Kalytchuk, Sergii, Mohammadi, Elmira, Zbořil, Radek, Kment, Štěpán, Otyepka, Michal, Naldoni, Alberto, and Fornasiero, Paolo

Subjects
Condensed Matter - Materials Science, Physics - Chemical Physics
Abstract

Generally adopted design strategies for enhancing the photocatalytic activity are aimed at tuning properties such as the visible light response, the exposed crystal facets, and the nanocrystal shape. Here, we present a different approach for designing efficient photocatalysts displaying a substrate-specific reactivity upon defect engineering. The defective anisotropic brookite TiO2 photocatalyst functionalized with Pt nanocrystals are tested for alcohol photoreforming showing up to an 11-fold increase in methanol oxidation rate, compared to the unreduced one, whilst presenting much lower ethanol or isopropanol specific oxidation rates. We demonstrate that the alcohol oxidation and hydrogen evolution reactions are tightly related, and when the substrate-specific alcohol oxidation ability is increased, the hydrogen evolution is significantly boosted. The reduced anisotropic brookite shows up to twenty-six-fold higher specific photoactivity with respect to anatase and brookite with isotropic nanocrystals, reflecting the different type of defective catalytic sites formed depending on the TiO2 polymorph and its crystal shape. Advanced in-situ characterizations and theoretical investigations reveal that controlled engineering over oxygen vacancies and lattice strain produces large electron polarons hosting the substrate-specific active sites for alcohol photo-oxidation.

Published
2023
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2. Lattice small polarons and magnetic interactions drive preferential nanocrystal growth in silicon doped hematite

Author

Allieta, Mattia, Marelli, Marcello, Coduri, Mauro, Stefan, Mariana, Ghica, Daniela, Morello, Giorgio, Malara, Francesco, and Naldoni, Alberto

Subjects
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Materials Science
Abstract

Understanding the interplay between the structural, chemical and physical properties of nanomaterials is crucial for designing new devices with enhanced performance. In this regards, doping of metal oxides is a general strategy to tune size, morphology, charge, lattice, orbital and spin degrees of freedoms and has been shown to affect nanomaterials properties for photoelectrochemical water splitting, batteries, catalysis, magnetic applications and optics. Here we report the role of lattice small polaron in driving the morphological transition from nearly isotropic to nanowire crystals in Si doped hematite ($\alpha-Fe_2O_3$). Lattice small polaron formation is well evidenced by the increase of hexagonal strain and degree of distortion of $FeO_6$ showing a hyperbolic trend with increasing Si content. Local analysis via pair distribution function highlights an unreported crossover from small to large polarons, which affects the correlation length of the polaronic distortion from short to average scales. Ferromagnetic double exchange interactions between $Fe^{2+}/Fe^{3+}$ species is found to be the driving force of the crossover, constraining the chaining of chemical bonds along the [110] crystallographic direction. This promotes the increase in the reticular density of Fe atoms along the hematite basal plane only, which boosts the anisotropic growth of nanocrystals with more extended [110] facets. Our results show that magnetic and electronic interactions drive preferential crystallographic growth in doped metal oxides, thus providing a new route to design their functional properties.

Published
2019

6. On the role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO3

Author

Allieta, Mattia, Scavini, Marco, Spalek, Leszek, Scagnoli, Valerio, Walker, Helen C., Panagopoulos, Christos, Saxena, Siddharth, Katsufuji, Takuro, and Mazzoli, Claudio

Subjects
Condensed Matter - Strongly Correlated Electrons
Abstract

Up to now the crystallographic structure of the magnetoelectric perovskite EuTiO3 was considered to remain cubic down to low temperature. Here we present high resolution synchrotron X-ray powder diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal I4/mcm, involving TiO6 octahedra tilting, in analogy to the case of SrTiO3. The temperature evolution of the tilting angle indicates a second-order phase transition with an estimated Tc=235K. This critical temperature is well below the recent anomaly reported by specific heat measurement at TA\sim282K. By performing atomic pair distribution function analysis on diffraction data we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated Tc, the average model symmetry is fully compatible with the local environment distortion but the former is characterized by a reduced value of the tilting angle compared to the latter. At T=240K data show the presence of local octahedra tilting identical to the low temperature one, while the average crystallographic structure remains cubic. On this basis, we propose intrinsic lattice disorder to be of fundamental importance in the understanding of EuTiO3 properties., Comment: 13 pages, 8 figures, 2 tables

Published
2011
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7. Black TiO 2 and Oxygen Vacancies: Unraveling the Role in the Thermal Anatase-to-Rutile Transformation.

Author

Allieta, Mattia, Coduri, Mauro, and Naldoni, Alberto

Subjects
TITANIUM dioxide, PHASE transitions, METALLIC oxides, UNIT cell, VALENCE bonds, OXYGEN
Abstract

Understanding the role of oxygen vacancies in the phase transformation of metal oxide nanomaterials is fundamental to design more efficient opto-electronic devices for a variety of applications, including sensing, spintronics, photocatalysis, and photo-electrochemistry. However, the structural mechanisms behind the phase transformation in reducible oxides remain poorly described. Here, we compare P25 and black TiO2 during the thermal anatase-to-rutile transformation using in situ synchrotron powder diffraction. The precise measurement of the phase fractions, unit cell parameters, and Ti-O bond sheds light on the phase transformation dynamics. Notably, we observe distinct temperature-dependent shifts in the relative phase fractions of anatase and rutile in both materials highlighting the role of the oxygen vacancy in promoting the phase transformation. We employ bond valence concepts for structural modeling, revealing unique trends in temperature evolution of Ti-O distances of black rutile, confirming that this TiO2 phase is preferentially reduced over anatase. These findings not only enhance our understanding of phase transitions in TiO2 but also open new ways for the design of advanced photocatalytic materials through targeted phase control. [ABSTRACT FROM AUTHOR]

Published
2024
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16. Local Structure and Magnetism of Fe2O3 Maghemite Nanocrystals: The Role of Crystal Dimension

Author

Coduri, Mauro, primary, Masala, Paolo, additional, Del Bianco, Lucia, additional, Spizzo, Federico, additional, Ceresoli, Davide, additional, Castellano, Carlo, additional, Cappelli, Serena, additional, Oliva, Cesare, additional, Checchia, Stefano, additional, Allieta, Mattia, additional, Szabo, Dorothee-Vinga, additional, Schlabach, Sabine, additional, Hagelstein, Michael, additional, Ferrero, Claudio, additional, and Scavini, Marco, additional

Published
2020
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17. ms16-03 Impact of intense electric fields on the structure of centrosymmetric relaxor ferroelectric SR0.85PR0.15TIO3

Author

Checchia, Stefano, Cabassi, Riccardo, Allieta, Mattia, Coduri, Mauro, Giacobbe, Carlotta, and Scavini, Marco

Subjects
antiferrodistorivity, Condensed Matter::Materials Science, Ferroelectricity, perovskites, synchrotron high-resolution diffraction, Stontium Titanate, cristallography under electric field, relaxor ferroelectrics
Abstract

Crystallography under electric field (EF) has been recently applied to reveal macroscopic and microscopic structural response of ferroelectric functional materials to intense EF. Here we focus on the effect of applied EF on the structure of a relaxor ferroelectric oxide. While in classic ferroelectrics clear signatures of polar distortion instabilities allow straightforward interpretation of diffraction data in terms of increased electric dipole moment, the centrosymmetric structure of relaxor ferroelectrics inhibits long-range polarization However, their susceptibility to distortion under EF points to local polar fluctuations and structural changes. We present the case of Pr-doped strontium titanate (Sr0.85Pr0.15TiO3.). Its structure shows high-temperature stabilization of the centrosymmetric antiferrodistortive (AFD) tetragonal phase observed in SrTiO3 below 105 K. The structural modifications induced by applied EF were studied by synchrotron high-resolution diffraction combined with Pair Distribution Function analysis. EF induces inhomogeneous strain and preferential orientation. Concomitant increases of tetragonal strain and octahedral TiO6 tilting angle do not break the initial long-range centrosymmetry. Conversely, PDF reveals that Ti is off-centered within its octahedral cage already at zero field, forming polar regions spanning up to 2 nm. The electric field enhances polarization within the polar nanoregions with no effect on their size, suggesting that they are pinned to charged defects introduced by Pr-doping. The large tetragonal strain found at the local scale may contribute to the ordering of the tilting angle at larger scales. This may imply a coupling between the local erroelectric and the long-range AFD instabilities.

Published
2019

25. DIFFRACTION STUDIES ON STRONGLY CORRELATED PEROVSKITE OXIDES

Author

ALLIETA, MATTIA

Abstract

In recent years, a great interest has been devoted to the so called strongly correlated systems containing perovskite building blocks. These systems exhibit a complex interplay between charge, spin, orbital and lattice degrees of freedom paving the way for very attractive applications. In this work, entitled ?Diffraction Studies on Strongly Correlated Perovskite Oxides? the use of x-ray diffraction techniques to investigate the coupling between the structure and the physical properties of several bulk material based on perovskite structure is presented. The thesis is organized in five chapters. Introduction presents a very general overview on strongly correlated perovskite oxides and the scope of the thesis. The first chapter reports technical details of the diffraction techniques involved in all the structural studies performed during the PhD. Chapter 2 reports an accurate investigation performed on the magnetoresistive cobaltite GdBaCo2O5+d (d=0) using single-crystal and synchrotron powder X-ray diffraction. In this work, we assign the correct space group and we demonstrate that a very small tetragonal-to-orthorhombic lattice distortion is coupled to magnetic phase transition. In Chapter 3, we show the study of the temperature induced insulator-to-metal transition for GdBaCo2O5+d (d>0.5). By using a combined approach between electron paramagnetic resonance and powder diffraction techniques we provide new interesting features about the spin ? lattice interaction occurring in these systems. Chapter 4 presents synchrotron X-ray powder diffraction study on EuTiO3 system. We show for the first time the existence of a new structural phase transition occurring in EuTiO3 below room temperature. In addition, by performing the atomic pair distribution function analysis of the powder diffraction data, we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material and we propose that the lattice disorder is of fundamental importance to understand the EuTiO3 properties. Finally, beyond the scope of the thesis, in Chapter 5, we review the basic procedure to get the differential pair distribution function obtained by applying the anomalous X-ray diffraction technique to total X- ray scattering method. We show an example of the application of this procedure by presenting the case of gadolinium doped ceria electrolytes. This work will show that use of the powder diffraction techniques provides a powerful tool to unveil the coupling between the structure and the physical properties in strongly correlated perovskite oxides.

Published
2012
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42. Phase Transformations in the CeO2–Sm2O3System: A Multiscale Powder Diffraction Investigation

Author

Coduri, Mauro, Masala, Paolo, Allieta, Mattia, Peral, Inma, Brunelli, Michela, Biffi, Carlo Alberto, and Scavini, Marco

Abstract

The structure evolution in the CeO2–Sm2O3system is revisited by combining high resolution synchrotron powder diffraction with pair distribution function (PDF) to inquire about local, mesoscopic, and average structure. The CeO2fluorite structure undergoes two phase transformations by Sm doping, first to a cubic (C-type) and then to a monoclinic (B-type) phase. Whereas the C to B-phase separation occurs completely and on a long-range scale, no miscibility gap is detected between fluorite and C-type phases. The transformation rather occurs by growth of C-type nanodomains embedded in the fluorite matrix, without any long-range phase separation. A side effect of this mechanism is the ordering of the oxygen vacancies, which is detrimental for the application of doped ceria as an electrolyte in fuel cells. The results are discussed in the framework of other Y and Gd dopants, and the relationship between nanostructuring and the above equilibria is also investigated.

Published
2018
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47. EuTi03 magnetic structure studied by neutron powder diffraction and resonant x-ray scattering.

Author

Scagnoli, Valerio, Allieta, Mattia, Walker, Helen, Scavini, Marco, Katsufuji, Takuro, Sagarna, Leyre, Zaharko, Oksana, and Mazzoli, Claudio

Subjects
*MAGNETIC structure, *NEUTRON diffraction, *X-ray scattering, *MAGNETOELECTRIC effect, *EUROPIUM compounds, *ANTIFERROMAGNETISM, *SINGLE crystals, *TEMPERATURE effect
Abstract

We combine neutron powder diffraction and x-ray single-crystal magnetic diffraction at the Eu L2 edge to scrutinize the magnetic motif of the Eu ions in magnetoelectric EuTi03. Our measurements are consistent with an antiferromagnetic G-type pattern with the Eu magnetic moments ordering along the a,b -plane diagonal. Recent reports of a novel transition at 2.75 Κ with a flop of magnetic moments upon poling the sample in an electric field cannot be confirmed for a nonpoled sample. Our neutron diffraction data do not show any significant change of the structure below the Néel temperature. Magnetoelastic coupling, if present, is therefore expected to be negligible. [ABSTRACT FROM AUTHOR]

Published
2012
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48. Electron Paramagnetic Resonance Analysis of La1-xMxMnO3+δ (M = Ce, Sr) Perovskite-Like Nanostructured Catalysts.

Author

Oliva, Cesare, Allieta, Mattia, Scavini, Marco, Biffi, Cesare, Rossetti, Ilenia, and Forni, Lucio

Subjects
*PEROVSKITE, *ELECTRON paramagnetic resonance, *MAGNETIC resonance, *ELECTRON mobility, *INORGANIC chemistry
Abstract

The physical-chemical properties of some nanostructured perovskite-like catalysts of general formula La1-xMxMnO3+δ (M = Ce, Sr) have been investigated, in particular by using the electron paramagnetic resonance (EPR) technique. We show that the interplay between the -O-Mn3+-O-Mn4+-O- electron double-exchange and the electron mobility is strictly dependent on the dopant nature and the annealing conditions in air. A relationship between the observed properties of these samples and their activity in the methane flameless catalytic combustion is proposed. [ABSTRACT FROM AUTHOR]

Published
2012
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49. Role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO3.

Author

Allieta, Mattia, Scavini, Marco, Spalek, Leszek J., Scagnoli, Valerio, Walker, Helen C., Panagopoulos, Christos, Saxena, Siddharth S., Katsufuji, Takuro, and Mazzoli, Claudio

Subjects
*PHASE transitions, *CRYSTAL structure, *MAGNETOELECTRIC effect, *PEROVSKITE, *TITANATES, *EUROPIUM compounds, *CRYSTALLOGRAPHY, *LOW temperatures
Abstract

Up to now, the crystallographic structure of the magnetoelectric perovskite EuTiO3 has been considered to remain cubic down to low temperature. Here we present high-resolution synchrotron x-ray powder-diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal lAlmcm, involving TiO6 octahedra tilting, in analogy to the case of SrTiOs. The temperature evolution of the tilting angle and of the full width at half maximum of the (200) cubic reflection family indicate a critical temperature Tc = 235 K. This critical temperature is well below the recent anomaly reported by specific-heat measurement at TA ∼ 282 K. By performing atomic pair distribution function analysis on diffraction data, we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated Tc, the average model symmetry is fully compatible with the local environment distortion, but the former is characterized by a reduced value of the tilting angle compared to the latter At T = 240 K, data show the presence of local octahedra tilting identical to the low-temperature one, while the average crystallographic structure remains cubic. On this basis, we propose that intrinsic lattice disorder is of fundamental importance in the understanding of EuTiOa properties. [ABSTRACT FROM AUTHOR]

Published
2012
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50. Effect of Nature and Location of Defects on Bandgap Narrowing in Black TiO2 Nanoparticles.

Author

Naldoni, Alberto, Allieta, Mattia, Santangelo, Saveria, Marelli, Marcello, Fabbri, Filippo, Cappelli, Serena, Bianchi, Claudia L., Psaro, Rinaldo, and Dal Santo, Vladimiro

Subjects
*TITANIUM dioxide nanoparticles, *BAND gaps, *ELECTRONIC structure, *CHEMICAL reduction, *CRYSTALLIZATION, *NANOPARTICLES analysis, *CRYSTAL defects
Abstract

The increasing need for new materials capable of solar fuel generation is central in the development of a green energy economy. In this contribution, we demonstrate that black TiO2 nanoparticles obtained through a one-step reduction/crystallization process exhibit a bandgap of only 1.85 eV, which matches well with visible light absorption. The electronic structure of black TiO2 nanoparticles is determined by the unique crystalline and defective core/disordered shell morphology. We introduce new insights that will be useful for the design of nanostructured photocatalysts for energy applications. [ABSTRACT FROM AUTHOR]

Published
2012
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