Non-innocent role of fluorine as an electron donor in oxides

Engineering of reducible oxides is generally focused on the cation sites. As such, anion doping remains an underutilized tool despite its unique potential in altering the defect chemistry and steering redox processes. In this contribution, we explore the possibilities offered by substitution on the anion site on the case of a prototypical reducible oxide, namely cerium oxide, doped with fluorine. The choice of fluorine is motivated by the general stability of fluorine in oxide lattices and the fact that it can be readily incorporated in these up to very high concentration with minimal structural distortion [1]. Utilizing photoemission spectroscopy in combination with density functional theory [2], we show that the general notion of fluorine acting as a straightforward ionic donor fails to capture the intricacies of electronic interactions at play. Specifically, we provide evidence for covalent hybridization in the nominally ionic fluorine-cerium interaction that allows for altering the anion derived electron density in cerium oxide beyond the oxygen 2p band (see Figure 1), contrary to the simplified picture of solely introducing a deeper-laying fluorine 2p band [3]. The emergent electronic configuration can be further coupled to standard valence band engineering methods, such as strain manipulation, to provide an unprecedented playground for designing the oxide properties. Our results also demonstrate the practicality of interatomic resonant photoemission spectroscopy as a gauge of non-trivial electronic effects of ligand origin, allowing to efficiently probe the above-mentioned effects. We note that fluorine doping represents a complement to oxygen vacancy engineering and highlight the fact that, unlike oxygen vacancies, the electronic effects generated by fluorine can persist in an oxidizing environment. The latter represents an important contribution the electronic modification of mixed-anion oxides can provide to a breadth of fields, ranging from superoxide stabilization to resistive switching. Please click Additional Files below to see the full abstract

Origin of the surface-orientation dependence of the reduction kinetics of ultrathin ceria

Performance of catalytic redox reactions depends crucially on the oxygen storage and release capability of the catalyst and with that the catalyst’s defect chemistry. Here, we show that the surface defect chemistry of cerium oxide, a prototypical reducible oxide, differs markedly between two surface terminations. The results are in good agreement with density functional theory calculations and provide important guiding factors for rational design of industrially relevant catalysts. The study is conducted by preparing (100) and (111) terminated nanoislands of cerium oxide next to each other on Cu(111). Leveraging the benefits of full-field imaging capability of photoemission electron microscopy (PEEM), we follow the structural and chemical properties of the nanoislands under reducing hydrogen atmosphere simultaneously and in situ. The results, summarized in Figure 1, directly reveal different overall reducibility that can be traced to equilibrium oxygen vacancy concentrations via a kinetic model. The density functional theory calculations provide further details regarding the equilibrium co-ordination of oxygen vacancies for both surface planes. Conjoining the two, the unique simultaneous nature of the PEEM-facilitated structure–activity relationship study allows us to separate the thermodynamics of reduction from the kinetics of oxygen exchange, revealing the fact that the difference in reducibility of the two surfaces of ceria is not determined by the kinetic rate constants of the reduction reaction, but rather by the equilibrium concentration of oxygen vacancies, an information that has not been provided by the isolated model system approach to date. Surprisingly, the reason for the different reducibilities is a purely geometric one: the creation of nearest neighbor oxygen vacancies. Please click Additional Files below to see the full abstract

Selective versus routine lymphadenectomy in the treatment of liver metastasis from colorectal cancer: a retrospective cohort study

Abstract Background Limited data are available on the importance of routine lymphadenectomy of the hepatoduodenal ligament in the treatment of liver metastasis from colorectal cancer in the literature. Methods A single center retrospective cohort study was conducted to evaluate morbidity and long-term survival in patients who had undergone selective versus routine lymphadenectomy during surgery for colorectal liver metastasis. From January 2006 to December 2009, eighty-one patients undergoing radical resection due to liver metastasis from colorectal cancer were included. The combination of two surgical teams with different approaches to hepatoduodenal ligament lymphadenectomy at our institution allowed us to select two cohorts of patients undergoing selective or routine lymphadenectomy. Results No significant differences between the cohorts were found in age, American Society of Anesthesiology score or Fong’s prognostic criteria. Patients with pN+ disease had significantly inferior survival compared to patients with pN0 disease (hazard ratio [HR] = 6.33, 95% CI 2.16–18.57, p = 0.0001). No significant difference in postoperative morbidity was observed in the group undergoing routine opposed to selective lymphadenectomy (13.63% vs. 8.69%, p = 0.36). There was no difference in long-term survival between the groups (HR = 0.90, 95% CI 0.52–1.58, p = 0.70). There were also no significant differences in the subgroup of patients with pN0 stage (HR = 1.17, 95% CI 0.6–2.11, p = 0.60). Conclusions These data suggest that there is no survival benefit from the use of routine lymphadenectomy during surgery for colorectal liver metastasis, but these data should be confirmed in a prospective randomized study

Tuning the physical properties of La0.7Sr0.3MnO3-δ via oxygen off-stoichiometry using thermal annealing

The oxygen off-stoichiometry in La0.7Sr0.3MnO3-δ (LSMO) thin films on SrTiO3 (STO) substrates has been investigated employing Al-assisted vacuum annealing. The gradual deoxygenation during annealing induces a topotactic phase transition from the as-prepared Perovskite (PV, ABO3) phase to a layered oxygen-vacancy-ordered Brownmillerite (BM, ABO2.5) phase. The structural change is monitored by XRD. A metal-to-insulator and simultaneously a ferromagnetic (FM)-to-antiferromagnetic (AF) transition is found. The variation of the manganese oxidation state is characterized using XAS. The BM phase shows in magnetization vs. temperature curves a peculiar peak above room temperature which cannot be explained within the usual AF ordering at low temperatures. Moreover, to elucidate the role of the strain to the substrate, bulk-like LSMO powder samples were prepared and annealed at similar conditions as the film samples. Also here the PV-BM phase transition is achieved

Reconfigurable lateral anionic heterostructures in oxide thin films via lithographically defined topochemistry

Laterally structured materials can exhibit properties uniquely suited for applications in electronics, magnetoelectric memory, photonics, and nanoionics. Here, a patterning approach is presented that combines the precise geometric control enabled by lithography with topochemical anionic manipulation of complex oxide films. Utilizing oxidation and fluorination reactions, striped patterns of SrFeO2.5/SrFeO3,SrFeO2.5/SrFeO2F, and SrFeO3/SrFeO2F have been prepared with lateral periodicities of 200, 20, and 4 μm. Coexistence of the distinct chemical phases is confirmed through x-ray diffraction, optical and photoemission microscopies, and optical spectroscopy. The lateral heterostructures exhibit highly anisotropic electronic transport and also enable transience and regeneration of patterns through reversible redox reactions. This approach can be broadly applied to a variety of metal-oxide systems, enabling chemically reconfigurable lateral heterostructures tailored for specific electronic, optical, ionic, thermal, or magnetic functionalities

On the growth mechanisms of polar (100) surfaces of ceria on copper (100)

We present a study of temperature dependent growth of nano-sized ceria islands on a Cu (100) substrate. Low-energy electron microscopy, micro-electron diffraction, X-ray absorption spectroscopy, and photoemission electron microscopy are used to determine the morphology, shape, chemical state, and crystal structure of the grown islands. Utilizing real-time observation capabilities, we reveal a three-way interaction between the ceria, substrate, and local oxygen chemical potential. The interaction manifests in the reorientation of terrace boundaries on the Cu (100) substrate, characteristic of the transition between oxidized and metallic surface. The reorientation is initiated at nucleation sites of ceria islands, whose growth direction is influenced by the proximity of the terrace boundaries. The grown ceria islands were identified as fully stoichiometric CeO2 (100) surfaces with a (2 × 2) reconstruction

Interfacial Electrochemistry in Liquids Probed with Photoemission Electron Microscopy

Studies of the electrified solid–liquid interfaces are crucial for understanding biological and electrochemical systems. Until recently, use of photoemission electron microscopy (PEEM) for such purposes has been hampered by incompatibility of the liquid samples with ultrahigh vacuum environment of the electron optics and detector. Here we demonstrate that the use of ultrathin electron transparent graphene membranes, which can sustain large pressure differentials and act as a working electrode, makes it possible to probe electrochemical reactions in operando in liquid environments with PEEM

In Aqua Electrochemistry Probed by XPEEM: Experimental Setup, Examples, and Challenges

Recent developments in environmental and liquid cells equipped with electron transparent graphene windows have enabled traditional surface science spectromicroscopy tools, such as scanning X-ray photoelectron microscopy, X-ray photoemission electron microscopy (XPEEM), and scanning electron microscopy to be applied for studying solid–liquid and liquid–gas interfaces. Here, we focus on the experimental implementation of XPEEM to probe electrified graphene–liquid interfaces using electrolyte-filled microchannel arrays as a new sample platform. We demonstrate the important methodological advantage of these multi-sample arrays: they combine the wide field of view hyperspectral imaging capabilities from XPEEM with the use of powerful data mining algorithms to reveal spectroscopic and temporal behaviors at the level of the individual microsample or the entire array ensemble