Ultrafast Dynamics of Plasmon-Mediated Charge Transfer in Ag@CeO2 Studied by Free Electron Laser Time-Resolved X-ray Absorption Spectroscopy

Probing the Dynamic Landscape: From Static to Time-Resolved X-Ray Absorption Spectroscopy to Investigate Copper Redox Chemistry in Neurodegenerative Disorders

Copper (Cu), with its ability to exist in various oxidation states, notably Cu(I) and Cu(II), plays a crucial role in diverse biological redox reactions. This includes its involvement in pathways associated with oxidative stress in neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and Transmissible Spongiform Encephalopathies. This paper offers an overview of X-ray Absorption Spectroscopy (XAS) studies designed to elucidate the interactions between Cu ions and proteins or peptides associated with these neurodegenerative diseases. The emphasis lies on XAS specificity, revealing the local coordination environment, and on its sensitivity to Cu oxidation states. Furthermore, the paper focuses on XAS applications targeting the characterization of intermediate reaction states and explores the opportunities arising from recent advancements in time-resolved XAS at ultrabright synchrotron and Free Electron Laser radiation sources.

Plasmonic properties and stability of Au and Cu nanoparticles embedded in cerium oxide

With the aim of sensitizing cerium oxide-a very important catalytic material-to visible light, its coupling with Au and Cu nanoparticles is investigated. The samples are grown by physical synthesis by embedding a layer of nanoparticles between two cerium oxide films. The films are controlled in composition byin-situx-ray photoemission spectroscopy and in morphology byex-situscanning electron microscopy. The optical properties as a function of the oxide thickness, investigated by spectrophotometry in the UV-Vis range, are interpreted based on the results of the morphological characterization and of simulations based on the Maxwell Garnett model. The stability of chemical and optical properties after air exposure is also investigated. The results, indicating that stable materials with tuneable optical properties can be obtained, are important in view of the potential application of the investigated systems in photocatalysis.

Nanostructured CeO2 photocatalysts: optimizing surface chemistry, morphology, and visible-light absorption

Emerging photocatalytic applications of cerium dioxide (CeO2) include green hydrogen production, CO2 conversion to fuels, and environmental remediation of various toxic molecules. These applications leverage the oxygen storage capacity and tunable surface chemistry of CeO2 to photocatalyze the chosen reaction, but many open questions remain regarding the fundamental physics of photocatalysis over CeO2. The commonly ascribed 'bandgap' of CeO2 (∼3.1 eV) differs fundamentally from other photocatalytic oxides such as TiO2; UV light excites an electron from the CeO2 valence band into a 4f state, generating a polaron as the lattice distorts around the localized charge. Researchers often disregard the distinction between the 4f state and a traditional, delocalized conduction band, resulting in ambiguity regarding mechanisms of charge transfer and visible-light absorption. This review summarizes modern literature regarding CeO2 photocatalysis and discusses commonly reported photocatalytic reactions and visible light-sensitization strategies. We detail the often misunderstood fundamental physics of CeO2 photocatalysis and supplement previous work with original computational insights. The exceptional progress and remaining challenges of CeO2-based photocatalysts are highlighted, along with suggestions for further research directions based on the observed gaps in current understanding.

Effects of Oxygen Adsorption on the Optical Properties of Ag Nanoparticles

Plasmonic metal nanoparticles are efficient light harvesters with a myriad of sensing- and energy-related applications. For such applications, the optical properties of nanoparticles of metals such as Cu, Ag, and Au can be tuned by controlling the composition, particle size, and shape, but less is known about the effects of oxidation on the plasmon resonances. In this work, we elucidate the effects of O adsorption on the optical properties of Ag particles by evaluating the thermodynamic properties of O-decorated Ag particles with calculations based on the density functional theory and subsequently computing the photoabsorption spectra with a computationally efficient time-dependent density functional theory approach. We identify stable Ag nanoparticle structures with oxidized edges and a quenching of the plasmonic character of the metal particles upon oxidation and trace back this effect to the sp orbitals (or bands) of Ag particles being involved both in the plasmonic excitation and in the hybridization to form bonds with the adsorbed O atoms. Our work has important implications for the understanding and application of plasmonic metal nanoparticles and plasmon-mediated processes under oxidizing environments.

Injecting Electrons into CeO2 via Photoexcitation of Embedded Au Nanoparticles

The electron injection efficiency and the steady state absorptance at different photon energies for a composite system made of Au NPs embedded in a cerium oxide matrix are reported. Cerium oxide can be coupled with plasmonic nanoparticles (NPs) to improve its catalytic properties by visible-light absorption. The present work is a study of the ultrafast dynamics of excited states induced by ultraviolet and visible-light excitation in Au NPs combined with cerium oxide, aimed at understanding the excitation pathways. The data, obtained by femtosecond transient absorption spectroscopy, show that the excitation of localized surface plasmon resonances (LSPRs) in the Au NPs leads to an ultrafast injection of electrons into the empty 4f states of the surrounding cerium oxide. Within the first few picoseconds, the injected electrons couple with the lattice distortion forming a polaronic excited state, with similar properties to that formed after direct band gap excitation of the oxide. At sub-picosecond delay times, we observed relevant differences in the energetics and the time dynamics as compared to the case of band gap excitation of the oxide. Using different pump energies across the LSPR-related absorption band, the efficiency of the electron injection from the NPs into the oxide was found to be rather high, with a maximum above 30%. The injection efficiency has a different trend in energy as compared to the LSPR-related static optical absorptance, showing a significant decrease in low energies. This behavior is explained considering different deexcitation pathways with variable weight across the LSPR band. The results are important for the design of materials with high overall solar catalytic efficiency.

Epitaxial growth of highly symmetrical branched noble metal-semiconductor heterostructures with efficient plasmon-induced hot-electron transfer

Epitaxial growth is one of the most commonly used strategies to precisely tailor heterostructures with well-defined compositions, morphologies, crystal phases, and interfaces for various applications. However, as epitaxial growth requires a small interfacial lattice mismatch between the components, it remains a challenge for the epitaxial synthesis of heterostructures constructed by materials with large lattice mismatch and/or different chemical bonding, especially the noble metal-semiconductor heterostructures. Here, we develop a noble metal-seeded epitaxial growth strategy to prepare highly symmetrical noble metal-semiconductor branched heterostructures with desired spatial configurations, i.e., twenty CdS (or CdSe) nanorods epitaxially grown on twenty exposed (111) facets of Ag icosahedral nanocrystal, albeit a large lattice mismatch (more than 40%). Importantly, a high quantum yield (QY) of plasmon-induced hot-electron transferred from Ag to CdS was observed in epitaxial Ag-CdS icosapods (18.1%). This work demonstrates that epitaxial growth can be achieved in heterostructures composed of materials with large lattice mismatches. The constructed epitaxial noble metal-semiconductor interfaces could be an ideal platform for investigating the role of interfaces in various physicochemical processes.

Elettra-Sincrotrone Trieste: present and future

We present an overview of the Elettra-Sincrotrone Trieste research center, which hosts synchrotron and free-electron laser light sources. We review the current status, provide examples of recent achievements in basic and applied research and discuss the upgrade programs of the facility.

Fast synthesis of porous iron doped CeO2 with oxygen vacancy for effective CO2 photoreduction

The activity of photocatalytic CO₂ conversion to carbon-containing products is determined by the adsorption and activation of CO₂ molecules on the surface of catalyst. Here, iron doped porous CeO₂ with oxygen vacancy (PFeCe) was prepared by one-step combustion method. The amount of CO₂ adsorbed via using the porous structure has been significantly increased in the case of a relatively small specific surface area and CO₂ molecules are more easily captured and undergo a reduction reaction with photoinduced carriers. In addition, oxygen vacancies are formed in the iron doped CeO₂ lattice as the active sites for CO₂ reduction, which can form strong interactions with CO₂ molecules, thereby effectively activating CO₂ molecules. The reduction products of CO₂ over PFeCe composite are CO and CH₄, which is approximately 9.0 and 7.7 folds than that of CeO₂. This work offers insights for the construction of efficient ceria-based photocatalysts to further achieve robust solar CO₂ conversion.

Two-Dimensional Restructuring of Cu2O Can Improve the Performance of Nanosized n-TiO2/p-Cu2O Photoelectrodes under UV-Visible Light

p-Cu₂O/n-TiO₂ photoanodes were produced by electrodeposition of octahedral p-type Cu₂O nanoparticles over n-type TiO₂ nanotubes. The photoresponse of the composite p-n photoanodes was evaluated in photoelectrochemical cells operating at "zero-bias" conditions under either visible or UV-vis irradiation. In both operating conditions, the produced electrodes invariably followed the p-n-based photoanode operations but exhibited lower photoelectrochemical performance as compared to the bare n-TiO₂ photoanode under UV-vis light. The reported experimental analysis evidenced that such decreased photoactivity is mainly induced by the scarce efficiency of the nanosized p-n interfaces upon irradiation. To overcome such limitation, a restructuring of the originally electrodeposited p-Cu₂O was promoted, following a photoelectrochemical post-treatment strategy. p-Cu₂O, restructured in a 2D leaf-like morphology, allowed reaching an improved photoelectrochemical performance for the p-n-based photoanode under UV-vis light. As compared to the bare n-TiO₂ behavior, such improvement consisted of photoanodic currents up to three times larger. An analysis of the mechanisms driving the transition from compact (∼100 nm) octahedral p-Cu₂O to wider (∼1 μm) 2D leaf-like structures was performed, which highlighted the pivotal role played by the irradiated n-TiO₂ NTs.