Accurate prediction of the optical properties of nanoalloys with both plasmonic and magnetic elements.

The alloying process plays a pivotal role in the development of advanced multifunctional plasmonic materials within the realm of modern nanotechnology. However, accurate in silico predictions are only available for metal clusters of just a few nanometers, while the support of modelling is required to navigate the broad landscape of components, structures and stoichiometry of plasmonic nanoalloys regardless of their size. Here we report on the accurate calculation and conceptual understanding of the optical properties of metastable alloys of both plasmonic (Au) and magnetic (Co) elements obtained through a tailored laser synthesis procedure. The model is based on the density functional theory calculation of the dielectric function with the Hubbard-corrected local density approximation, the correction for intrinsic size effects and use of classical electrodynamics. This approach is built to manage critical aspects in modelling of real samples, as spin polarization effects due to magnetic elements, short-range order variability, and size heterogeneity. The method provides accurate results also for other magnetic-plasmonic (Au-Fe) and typical plasmonic (Au-Ag) nanoalloys, thus being available for the investigation of several other nanomaterials waiting for assessment and exploitation in fundamental sectors such as quantum optics, magneto-optics, magneto-plasmonics, metamaterials, chiral catalysis and plasmon-enhanced catalysis.

Single-Atoms on Crystalline Carbon Nitrides for Selective C─H Photooxidation: A Bridge to Achieve Homogeneous Pathways in Heterogeneous Materials.

Single-atom catalysis is a field of paramount importance in contemporary science due to its exceptional ability to combine the domains of homogeneous and heterogeneous catalysis. Iron and manganese metalloenzymes are known to be effective in C─H oxidation reactions in nature, inspiring scientists to mimic their active sites in artificial catalytic systems. Herein, a simple and versatile cation exchange method is successfully employed to stabilize low-cost iron and manganese single-atoms in poly(heptazine imides) (PHI). The resulting materials are employed as photocatalysts for toluene oxidation, demonstrating remarkable selectivity toward benzaldehyde. The protocol is then extended to the selective oxidation of different substrates, including (substituted) alkylaromatics, benzyl alcohols, and sulfides. Detailed mechanistic investigations revealed that iron- and manganese-containing photocatalysts work through a similar mechanism via the formation of high-valent M═O species. Operando X-ray absorption spectroscopy (XAS) is employed to confirm the formation of high-valent iron- and manganese-oxo species, typically found in metalloenzymes involved in highly selective C─H oxidations.

Metastable Ni(I)-TiO2-x Photocatalysts: Self-Amplifying H2 Evolution from Plain Water without Noble Metal Co-Catalyst and Sacrificial Agent.

Decoration of semiconductor photocatalysts with cocatalysts is generally done by a step-by-step assembly process. Here, we describe the self-assembling and self-activating nature of a photocatalytic system that forms under illumination of reduced anatase TiO2 nanoparticles in an aqueous Ni2+ solution. UV illumination creates in situ a Ni+/TiO2/Ti3+ photocatalyst that self-activates and, over time, produces H2 at a higher rate. In situ X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy show that key to self-assembly and self-activation is the light-induced formation of defects in the semiconductor, which enables the formation of monovalent nickel (Ni+) surface states. Metallic nickel states, i.e., Ni0, do not form under the dark (resting state) or under illumination (active state). Once the catalyst is assembled, the Ni+ surface states act as electron relay for electron transfer to form H2 from water, in the absence of sacrificial species or noble metal cocatalysts.

Full Spectroscopic Characterization of the Molecular Oxygen-Based Methane to Methanol Conversion over Open Fe(II) Sites in a Metal-Organic Framework.

Iron-based enzymes efficiently activate molecular oxygen to perform the oxidation of methane to methanol (MTM), a reaction central to the contemporary chemical industry. Conversely, a very limited number of artificial catalysts have been devised to mimic this process. Herein, we employ the MIL-100(Fe) metal-organic framework (MOF), a material that exhibits isolated Fe sites, to accomplish the MTM conversion using O2 as the oxidant under mild conditions. We apply a diverse set of advanced operando X-ray techniques to unveil how MIL-100(Fe) can act as a catalyst for direct MTM conversion. Single-phase crystallinity and stability of the MOF under reaction conditions (200 or 100 °C, CH4 + O2) are confirmed by X-ray diffraction measurements. X-ray absorption, emission, and resonant inelastic scattering measurements show that thermal treatment above 200 °C generates Fe(II) sites that interact with O2 and CH4 to produce methanol. Experimental evidence-driven density functional theory (DFT) calculations illustrate that the MTM reaction involves the oxidation of the Fe(II) sites to Fe(III) via a high-spin Fe(IV)═O intermediate. Catalyst deactivation is proposed to be caused by the escape of CH3• radicals from the relatively large MOF pore cages, ultimately resulting in the formation of hydroxylated triiron units, as proven by valence-to-core X-ray emission spectroscopy. The O2-based MTM catalytic activity of MIL-100(Fe) in the investigated conditions is demonstrated for two consecutive reaction cycles, proving the MOF potential toward active site regeneration. These findings will desirably lay the groundwork for the design of improved MOF catalysts for the MTM conversion.

Capabilities of Grazing Incidence X-ray Diffraction in the Investigation of Amorphous Mixed Oxides with Variable Composition.

X-ray Diffraction has been fully exploited as a probe to investigate crystalline materials. However, very little research has been carried out to unveil its potentialities towards amorphous materials. In this work, we demonstrated the capabilities of Grazing Incidence X-ray Diffraction (GIXRD) as a simple and fast tool to obtain quantitative information about the composition of amorphous mixed oxides. In particular, we evidenced that low angle scattering features, associated with local structure parameters, show a significant trend as a function of the oxide composition. This evolution can be quantified by interpolating GIXRD data with a linear combination of basic analytical functions, making it possible to build up GIXRD peak-sample composition calibration curves. As a case study, the present method was demonstrated on Ta2O5-SiO2 amorphous films deposited by RF-magnetron sputtering. GIXRD results were validated by independent measurement of the oxide composition using Rutherford Backscattering Spectrometry (RBS). These materials are attracting interest in different industrial sectors and, in particular, in photovoltaics as anti-reflection coatings. Eventually, the optical properties measured by spectroscopic ellipsometry were correlated to the chemical composition of the film. The obtained results highlighted not only a correlation between diffraction features and the composition of amorphous films but also revealed a simple and fast strategy to characterize amorphous thin oxides of industrial interest.

Structural and mechanistic insights into low-temperature CO oxidation over a prototypical high entropy oxide by Cu L-edge operando soft X-ray absorption spectroscopy.

High entropy oxides (HEOs) are an emerging class of materials constituted by multicomponent systems that are receiving special interest as candidates for obtaining novel and desirable properties. In this study we present a detailed investigation of the relevant intermediates arising at the surface of the prototypical HEO Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O during low-temperature CO oxidation. By combining Cu L2,3-edge operando soft X-ray absorption spectroscopy (soft-XAS) with density functional theory simulations and in situ FT-IR spectroscopy, we propose that upon HEO exposure to CO at 235 °C reduced Cu(I) sites arise mostly coordinated to activated CO molecules and partly to bidentate carbonate species. When the HEO surface is then exposed to a stoichiometric mixture of CO + 1/2O2 at 250 °C, CO2 is produced while bidentate carbonate moieties remain interacting with the Cu(I) sites. We structurally characterize the carbonate and CO preferential adsorption geometries on the Cu(I) surface metal centers, and find that CO adopts a bent conformation that may energetically favor its subsequent oxidation. The unique surface, structural and electronic sensitivity of soft-XAS coupled with the developed data analysis work-flow and supported by FT-IR spectroscopy may be beneficial to characterize often elusive surface properties of systems of catalytic interest.

The Missing Piece: The Structure of the Ti3C2Tx MXene and Its Behavior as Negative Electrode in Sodium Ion Batteries.

The most common MXene composition Ti3C2Tx (T = F, O) shows outstanding stability as anode for sodium ion batteries (100% of capacity retention after 530 cycles with charge efficiency >99.7%). However, the reversibility of the intercalation/deintercalation process is strongly affected by the synthesis parameters determining, in turn, significant differences in the material structure. This study proposes a new approach to identify the crystal features influencing the performances, using a structural model built with a multitechnique approach that allows exploring the short-range order of the lamella. The model is then used to determine the long-range order by inserting defective elements into the structure. With this strategy it is possible to fit the MXene diffraction patterns, obtain the structural parameters including the stoichiometric composition of the terminations (neutron data), and quantify the structural disorder which can be used to discriminate the phases with the best electrochemical properties.

Dielectric Effects in FeO x -Coated Au Nanoparticles Boost the Magnetoplasmonic Response: Implications for Active Plasmonic Devices.

Plasmon resonance modulation with an external magnetic field (magnetoplasmonics) represents a promising route for the improvement of the sensitivity of plasmon-based refractometric sensing. To this purpose, an accurate material choice is needed to realize hybrid nanostructures with an improved magnetoplasmonic response. In this work, we prepared core@shell nanostructures made of an 8 nm Au core surrounded by an ultrathin iron oxide shell (≤1 nm). The presence of the iron oxide shell was found to significantly enhance the magneto-optical response of the noble metal in the localized surface plasmon region, compared with uncoated Au nanoparticles. With the support of an analytical model, we ascribed the origin of the enhancement to the shell-induced increase in the dielectric permittivity around the Au core. The experiment points out the importance of the spectral position of the plasmonic resonance in determining the magnitude of the magnetoplasmonic response. Moreover, the analytical model proposed here represents a powerful predictive tool for the quantification of the magnetoplasmonic effect based on resonance position engineering, which has significant implications for the design of active magnetoplasmonic devices.

Operando X-ray Absorption Spectroscopy (XAS) Observation of Photoinduced Oxidation in FeNi (Oxy)hydroxide Overlayers on Hematite (α-Fe2O3) Photoanodes for Solar Water Splitting.

An FeNi (oxy)hydroxide cocatalyst overlayer was photoelectrochemically deposited on a thin-film hematite (α-Fe2O3) photoanode, leading to a cathodic shift of ∼100 mV in the photocurrent onset potential. Operando X-ray absorption spectroscopy (XAS) at the Fe and Ni K-edges was used to study the changes in the overlayer with potential in the dark and under illumination conditions. Potential or illumination only had a minor effect on the Fe oxidation state, suggesting that Fe atoms do not accumulate significant amount of charge over the whole potential range. In contrast, the Ni K-edge spectra showed pronounced dependence on potential in the dark and under illumination. The effect of illumination is to shift the onset for the Ni oxidation because of the generated photovoltage and suggests that holes that are photogenerated in hematite are transferred mainly to the Ni atoms in the overlayer. The increase in the oxidation state of Ni proceeds at potentials corresponding to the redox wave of Ni, which occurs immediately prior to the onset of the oxygen evolution reaction (OER). Linear combination fitting analysis of the obtained spectra suggests that the overlayer does not have to be fully oxidized to promote oxygen evolution. Cathodic discharge measurements show that the photogenerated charge is stored almost exclusively in the Ni atoms within the volume of the overlayer.

Dewetting of PtCu Nanoalloys on TiO2 Nanocavities Provides a Synergistic Photocatalytic Enhancement for Efficient H2 Evolution.

We investigate the co-catalytic activity of PtCu alloy nanoparticles for photocatalytic H2 evolution from methanol-water solutions. To produce the photocatalysts, a few-nanometer-thick Pt-Cu bilayers are deposited on anodic TiO2 nanocavity arrays and converted by solid-state dewetting via a suitable thermal treatment into bimetallic PtCu nanoparticles. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results prove the formation of PtCu nanoalloys that carry a shell of surface oxides. X-ray absorption near-edge structure (XANES) data support Pt and Cu alloying and indicate the presence of lattice disorder in the PtCu nanoparticles. The PtCu co-catalyst on TiO2 shows a synergistic activity enhancement and a significantly higher activity toward photocatalytic H2 evolution than Pt- or Cu-TiO2. We propose the enhanced activity to be due to Pt-Cu electronic interactions, where Cu increases the electron density on Pt, favoring a more efficient electron transfer for H2 evolution. In addition, Cu can further promote the photoactivity by providing additional surface catalytic sites for hydrogen recombination. Remarkably, when increasing the methanol concentration up to 50 vol % in the reaction phase, we observe for PtCu-TiO2 a steeper activity increase compared to Pt-TiO2. A further increase in methanol concentration (up to 80 vol %) causes for Pt-TiO2 a clear activity decay, while PtCu-TiO2 still maintains a high level of activity. This suggests improved robustness of PtCu nanoalloys against poisoning from methanol oxidation products such as CO.

Role of Synthetic Parameters on the Structural and Optical Properties of N,Sn-Copromoted Nanostructured TiO2: A Combined Ti K-Edge and Sn L2,3-Edges X-ray Absorption Investigation.

Sn-modification of TiO2 photocatalysts has been recently proposed as a suitable strategy to improve pollutant degradation as well as hydrogen production. In particular, visible light activity could be promoted by doping with Sn2+ species, which are, however, thermally unstable. Co-promotion with N and Sn has been shown to lead to synergistic effects in terms of visible light activity, but the underlying mechanism has, so far, been poorly understood due to the system complexity. Here, the structural, optical, and electronic properties of N,Sn-copromoted, nanostructured TiO2 from sol-gel synthesis were investigated: the Sn/Ti molar content was varied in the 0-20% range and different post-treatments (calcination and low temperature hydrothermal treatment) were adopted in order to promote the sample crystallinity. Depending on the adopted post-treatment, the optical properties present notable differences, which supports a combined role of Sn dopants and N-induced defects in visible light absorption. X-ray absorption spectroscopy at the Ti K-edge and Sn L2,3-edges shed light onto the electronic properties and structure of both Ti and Sn species, evidencing a marked difference at the Sn L2,3-edges between the samples with 20% and 5% Sn/Ti ratio, showing, in the latter case, the presence of tin in a partially reduced state.

Stabilization by Configurational Entropy of the Cu(II) Active Site during CO Oxidation on Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O.

The mechanisms of CO oxidation on the Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O high-entropy oxide were studied by means of operando soft X-ray absorption spectroscopy. We found that Cu is the active metal and that Cu(II) can be rapidly reduced to Cu(I) by CO when the temperature is higher than 130 °C. Co and Ni do not have any role in this respect. The Cu(II) oxidation state can be easily but slowly recovered by treatment of the sample with O2 at ca. 250 °C. However, it should be noted that CuO is readily and irreversibly reduced to Cu(I) when it is treated with CO at T > 100 °C. Thus, the main conclusion of this work is that the high configurational entropy of Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O stabilizes the rock-salt structure and permits the oxidation/reduction of Cu to be reversible, thus permitting the catalytic cycle to take place.

Understanding Solid-Gas Reaction Mechanisms by Operando Soft X-Ray Absorption Spectroscopy at Ambient Pressure.

Ambient-pressure operando soft X-ray absorption spectroscopy (soft-XAS) was applied to study the reactivity of hydroxylated SnO2 nanoparticles toward reducing gases. H2 was first used as a test case, showing that the gas phase and surface states can be simultaneously probed: Soft-XAS at the O K-edge gains sensitivity toward the gas phase, while at the Sn M4,5-edges, tin surface states are explicitly probed. Results obtained by flowing hydrocarbons (CH4 and CH3CHCH2) unequivocally show that these gases react with surface hydroxyl groups to produce water without producing carbon oxides and release electrons that localize on Sn to eventually form SnO. The partially reduced SnO2 - x layer at the surface of SnO2 is readily reoxidized to SnO2 by treating the sample with O2 at mild temperatures (>200 °C), revealing the nature of "electron sponge" of tin oxide. The experiments, combined with DFT calculations, allowed devising of a mechanism for dissociative hydrocarbon adsorption on SnO2, involving direct reduction of Sn sites at the surface via cleavage of C-H bonds and the formation of methoxy- and/or methyl-tin species at the surface.

Nanoaggregates of iron poly-oxo-clusters obtained by laser ablation in aqueous solution of phosphonates.

Laser ablation in liquid (LAL) emerged as a versatile technique for the synthesis of nanoparticles with various structures and compositions, although the control over products remains challenging in most cases. For instance, it is still difficult to drive the size of metal oxide crystalline domains down to the level of few atom clusters with LAL. Here we demonstrate that laser ablation of a bulk iron target in aqueous solution of phosphonates gives phosphonate-grafted iron oxo-clusters polymerized into nanoaggregates with Fe:ligand ratio of 2:1, instead of the usual nanocrystalline iron oxides. We attribute this result to the strong ability of phosphonate groups to bind iron oxide clusters and prevent their further growth into crystalline iron oxide. These laser generated poly-oxo-clusters are biocompatible and trackable by magnetic resonance imaging, providing interesting features for use in biological environments, such as nano-vehicles for iron administration. Besides, this method is promising for the generation of atom-scale metal-oxide clusters, which are ubiquitary in chemistry and of interest in biochemistry, catalysis, molecular magnetism and materials science.

Structure and Stability of a Copper(II) Lactate Complex in Alkaline Solution: a Case Study by Energy-Dispersive X-ray Absorption Spectroscopy.

Energy-dispersive X-ray absorption spectroscopy was applied, aimed at solving the problem of the structure and stability of a copper(II) lactate complex in alkaline solution, used as a precursor for the electrodeposition of Cu2O. The application of multiple scattering calculations to the simulation of the X-ray absorption near-edge structure part of the spectra allowed an accurate resolution of the structure: the copper(II) cation is surrounded by four lactate ions in a distorted tetrahedral environment, with the lactate anions acting as monodentate ligands. This results in an atomic arrangement where copper is surrounded by four oxygen atoms located at quite a short distance (ca. 1.87 Å) and four oxygen atoms located quite far apart (ca. 3.1-3.2 Å). The complex was finally found to be stable in a wide range of applied potentials.

Enhanced Electrocatalytic Oxygen Evolution in Au-Fe Nanoalloys.

Oxygen evolution reaction (OER) is the most critical step in water splitting, still limiting the development of efficient alkaline water electrolyzers. Here we investigate the OER activity of Au-Fe nanoalloys obtained by laser-ablation synthesis in solution. This method allows a high amount of iron (up to 11 at %) to be incorporated into the gold lattice, which is not possible in Au-Fe alloys synthesized by other routes, due to thermodynamic constraints. The Au0.89 Fe0.11 nanoalloys exhibit strongly enhanced OER in comparison to the individual pure metal nanoparticles, lowering the onset of OER and increasing up to 20 times the current density in alkaline aqueous solutions. Such a remarkable electrocatalytic activity is associated to nanoalloying, as demonstrated by comparative examples with physical mixtures of gold and iron nanoparticles. These results open attractive scenarios to the use of kinetically stable nanoalloys for catalysis and energy conversion.

Correction: Dependence of the Ce(iii)/Ce(iv) ratio on intracellular localization in ceria nanoparticles internalized by human cells.

Correction for 'Dependence of the Ce(iii)/Ce(iv) ratio on intracellular localization in ceria nanoparticles internalized by human cells' by Daniela Ferraro, et al., Nanoscale, 2017, 9, 1527-1538.

Observation of charge transfer cascades in α-Fe2O3/IrOx photoanodes by operando X-ray absorption spectroscopy.

Electrochemical devices for energy conversion and storage are central for a sustainable economy. The performance of electrodes is driven by charge transfer across different layer materials and an understanding of the mechanistics is pivotal to gain improved efficiency. Here, we directly observe the transfer of photogenerated charge carriers in a photoanode made of hematite (α-Fe2O3) and a hydrous iridium oxide (IrOx) overlayer, which plays a key role in photoelectrochemical water oxidation. Through the use of operando X-ray absorption spectroscopy (XAS), we probe the change in occupancy of the Ir 5d levels during optical band gap excitation of α-Fe2O3. At potentials where no photocurrent is observed, electrons flow from the α-Fe2O3 photoanode to the IrOx overlayer. In contrast, when the composite electrode produces a sustained photocurrent (i.e., 1.4 V vs. RHE), a significant transfer of holes from the illuminated α-Fe2O3 to the IrOx layer is clearly demonstrated. The analysis of the operando XAS spectra further suggests that oxygen evolution actually occurs both at the α-Fe2O3/electrolyte and α-Fe2O3/IrOx interfaces. These findings represent an important outcome for a better understanding of composite photoelectrodes and their use in photoelectrochemical systems, such as hydrogen generation or CO2 reduction from sunlight.