Lucia Amidani

Probing Long‐Lived Plasmonic‐Generated Charges in TiO2/Au by High‐Resolution X‐ray Absorption Spectroscopy

Exploiting plasmonic Au nanoparticles to sensitize TiO2 to visible light is a widely employed route to produce efficient photocatalysts. However, a description of the atomic and electronic structure of the semiconductor sites in which charges are injected is still not available. Such a description is of great importance in understanding the underlying physical mechanisms and to improve the design of catalysts with enhanced photoactivity. We investigated changes in the local electronic structure of Ti in pure and N-doped nanostructured TiO2 loaded with Au nanoparticles during continuous selective excitation of the Au localized surface plasmon resonance with X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). Spectral variations strongly support the presence of long-lived charges localized on Ti states at the semiconductor surface, giving rise to new laser-induced low-coordinated Ti sites.

A High-Valent Non-Heme μ-Oxo Manganese(IV) Dimer Generated from a Thiolate-Bound Manganese(II) Complex and Dioxygen

This study deals with the unprecedented reactivity of dinuclear non-heme MnII -thiolate complexes with O2 , which dependent on the protonation state of the initial MnII dimer selectively generates either a di-μ-oxo or μ-oxo-μ-hydroxo MnIV complex. Both dimers have been characterized by different techniques including single-crystal X-ray diffraction and mass spectrometry. Oxygenation reactions carried out with labeled 18 O2 unambiguously show that the oxygen atoms present in the MnIV dimers originate from O2 . Based on experimental observations and DFT calculations, evidence is provided that these MnIV species comproportionate with a MnII precursor to yield μ-oxo and/or μ-hydroxo MnIII dimers. Our work highlights the delicate balance of reaction conditions to control the synthesis of non-heme high-valent μ-oxo and μ-hydroxo Mn species from MnII precursors and O2 .

Contraction, cation oxidation state and size effects in cerium oxide nanoparticles

An accurate description of the structural and chemical modifications of cerium oxide nanoparticles (NPs) is mandatory for understanding their functionality in applications. In this work we investigate the relation between local atomic structure, oxidation state, defectivity and size in cerium oxide NPs with variable diameter below 10 nm, using x-ray absorption fine structure analysis in the near and extended energy range. The NPs are prepared by physical methods under controlled conditions and analyzed in morphology and crystalline quality by high resolution transmission electron microscopy. We resolve here an important question on the local structure of cerium oxide NPs: we demonstrate a progressive contraction in the Ce-O interatomic distance with decreasing NP diameter and we relate the observed effect to the reduced dimensionality. The contraction is not significantly modified by inducing a 4%-6% higher Ce3+ concentration through thermal annealing in high vacuum. The consequences of the observed average cation-anion distance contraction on the properties of the NPs are discussed.

Quantitative determination of In clustering in In-rich InxGa1−xN thin films

We investigated atomic ordering in In-rich InxGa1−xN epilayers in order to obtain an understanding of whether a deviation from a random distribution of In atoms in the group-III sublattice could be the origin of the strong carrier localization and defect-insensitive emission of these semiconductor alloys. This phenomenon can be exploited for application in optoelectronics. By coupling In K-edge x-ray absorption spectroscopy and high resolution x-ray diffraction, we were able to discard the hypothesis of significant phase separation into InN + GaN, in agreement with previous N K-edge absorption spectroscopy. However, we found an enrichment of In neighbours in the second atomic shell of In as compared to random statistics (clustering) for x = 0.82, while this is not the case for x = 0.46. This result, which is also supported by optical spectroscopy, is likely to stimulate new theoretical studies on InxGa1−xN alloys with a very high In concentration.

Appearance potential spectroscopy with a photon counting detector and multiple scattering spectral interpretation

We describe a soft x-ray appearance potential spectroscopy apparatus, which uses a windowless hyperpure Ge detector operated in the photon counting mode. Direct comparisons of recorded spectra with the self-convolution of x-ray absorption spectra and with ab initio simulations in the multiple scattering framework are reported and discussed.

Influence of the surface structure on the magnetic properties of Zn1−xCoxO

The surface of ferromagnetic Zn1−xCoxO wurtzite epilayers has been studied by coupling atomic force microscopy and advanced x-ray spectroscopy. We found that, even in high-quality epilayers, the formation of Co clusters and iso-space-group Co-rich regions can take place at the sample surface while the bulk maintains random Co distribution. Comparing structural characterization with magnetometry, we show that these surface modifications are not at the origin of the magnetic properties of the material. Quite the reverse, ferromagnetic behavior is enhanced in the sample characterized by the less defective surface.

Photo-electrochemical hydrogen production from neutral phosphate buffer and seawater using micro-structured p-Si photo-electrodes functionalized by solution-based methods

Solar fuels such as H2 generated from sunlight and seawater using earth-abundant materials are expected to be a crucial component of a next generation renewable energy mix. We herein report a syste ...

Element-specific channels in photo-excitation of V-doped TiO2 nanoparticles

Refined X-ray spectroscopies can be crucial in elucidating charge transfer phenomena which play a key-role in photo-catalysis and other processes relevant for clean energy production. A deep understanding of electron photo-dynamics is, in fact, essential to develop efficient knowledge-based devices. We developed a differential illumination RIXS and HERFD-XAS [1], method on ID26 @ ESRF to investigate charge transfer phenomena with chemical sensitivity; specifically, we studied V-doped TiO2 nanoparticles, a promising materials system for photo-catalysis, performing measurements around both the V Kα and Ti Kβ emissions. We found that visible light absorption induces the transfer of electrons from the V dopants to the host matrix cations in defective sites. With a steady state model, it was also possible to estimate the lifetime of the excited state. The value we obtained (around 1 ms) suggests that dopant-injected electrons can remain trapped near Ti atoms for a very long time. The procedure we used is completely general and can be successfully applied to detect any kind of long-living charge transfer phenomena in a wide range of possible devices [2].

XAS, XES and DFT simulations to bridge local and macroscopic properties in GaAsN

Although it is well known that dilute species often significantly modify the properties of solids in which they are hosted, a basic question remains open: how critical are their local properties in determining the macroscopic ones of the host material? Here, we address this issue by taking N in dilute III-V-N alloys as a paradigmatic case and propose an original approach based on synergic use of x-ray spectroscopies (XANES and valence – to – core XES at the N K-edge), density functional theory simulations and hydrogen exposure as a tool to modify N local bonding. Our results disclose the key role of the local N – Ga interactions in determining both Nrelated band states and the macroscopic effects of dilute N on the GaAs energy gap. The success of our approach indicates a new way to connect local and macroscopic properties.

Tuning of the optical properties of In-rich InxGa1-xN (x=0.82-0.49) alloys by light-ion irradiation at low energy

The effects of low-energy irradiation by light ions (H and He) on the properties of In-rich InxGa1−xN alloys are investigated by optical and structural techniques. H-irradiation gives rise to a remarkable blue-shift of light emission and absorption edge energies. X-ray absorption measurements and first-principle calculations address the microscopic origin of these effects.