Emission X-ray Photoelectron Spectroscopy Research Articles

Unveiling nano-scale chemical inhomogeneity in surface oxide films formed on V- and N-containing martensite stainless steel by synchrotron X-ray photoelectron emission spectroscopy/microscopy and microscopic X-ray absorption spectroscopy

Nano-scale chemical inhomogeneity in surface oxide films formed on a V- and N-containing martensite stainless steel and tempering heating induced changes are investigated by a combination of synchrotron- based hard X-ray Photoelectron emission spectroscopy (HAXPES) and microscopy (HAXPEEM) as well as microscopic X-ray absorption spectroscopy (μ-XAS) techniques. The results reveal the inhomogeneity in the oxide films on the micron-sized Cr2N- and VN-type particles, while the inhomogeneity on the martensite matrix phase exists due to localised formation of nano-sized tempering nitride particles at 600 °C. The oxide film formed on Cr2N-type particles is rich in Cr2O3 compared with that on the martensite matrix and VN-type particles. With the increase of tempering temperature, Cr2O3 formation is faster for the oxidation of Cr in the martensite matrix than the oxidation of Cr nitride-rich particles.

Characterization and electrochemical properties of TiO2-rNTs/SnO2–Sb/PbO2 electrodes for the mineralization of persistent organic pollutants using anodic oxidation coupled Electro-Fenton treatment: Effect of precursor selection

The present study compares the effect of using different solvents on the electrochemical properties of the reduced TiO2 nanotubes (TiO2-rNTs) layered Ti/TiO2-rNTs/SnO2–Sb/PbO2 anodes. The electrodes are prepared using three different solvent-based precursors: (i) isopropanol, (ii) ethylene glycol and citric acid (Pechini method), and (iii) 2-hydroxyethylammonium acetate (2HEAA) ionic liquid (IL) via the thermal decomposition route. The decomposition mechanism of precursor solutions was explored using the thermogravimetric (TGA) analysis. Further, the physicochemical properties of the electrodes are examined using Field emission Scanning Electron microscopy (FE-SEM), X-ray diffraction spectroscopy (XRD), and X-ray photoelectron emission spectroscopy (XPS). The results revealed that solvents with higher viscosity and slower decomposition rates support better film uniformity and higher stability of the electrode. The TiO2 -rNTs bottom layer and PbO2 top layer helped obtain higher film stability, increased working potential window (2.2 V vs. SHE) of the electrode, and the repeatability of the results. The performance of different electrodes based on the precursor solution is found as IL ≫ Pechini > Isopropanol. 4-chlorophenol (4-CP) is used as a model pollutant to test the performance of IL-Ti/TiO2-rNTs/SnO2–Sb/PbO2 anode in an anodic oxidation (AO) coupled electro-Fenton (EF) treatment. Further, the reliability of the electrode is evaluated by mineralizing other persistent organic pollutants (POPs) like tetracyclin, phenol, 2-chlorophenol (2-CP), and 2,4-dichlorophenol (2,4-DCP). Under the optimized conditions, the proposed system was able to mineralize the tetracyclin, phenol, 2-CP, 2,4-DCP, and 4-CP up to 78.91, 82.07, 74.96, 78.78, and 69.3 %, respectively. Moreover, the degradation mechanism of chlorophenols is proposed.

Thin film deposition of undoped and Ni-doped cobalt ferrite on MgO (110) substrates using pulsed laser deposition technique

The thin film samples of pristine and Ni-doped Cobalt ferrite(CFO) are deposited on MgO(110) substrates through pulsed laser deposition technique. The recorded X-ray diffraction scans of the samples confirms the thin film deposition of the said material on the given substrate. From the Atomic Force Microscopy studies, the surface roughness of CoFe1.7Ni0.3O4 thin film is found to be 0.62 nm, which is more than CoFe2O4 and CoFe1.5Ni0.5O4 (0.34 nm and 0.45 nm), attributed to the surface defects and recrystallization process. The x-ray photoelectron emission spectroscopy (XPS) survey spectra of the thin film samples confirm the presence of Co, Ni, Fe, O and C. In our case, the XPS-wide scans revealed the +2-oxidation state for Co and +2 as well as +3 oxidation states for Fe. The respective binding energy positions and the ratio of relative contribution to the overall intensity of Co and Fe ions at the octahedral and tetrahedral sites are also discussed in this paper. It has been shown that the Field Cooled-Zero Field Cooled magnetization curves do not merge until 400 K, which confirms that the Curie temperature Tc is higher than 400 K.

PICOSECOND PULSED LASER DEPOSITION OF MOS2 THIN FILMS

MoS2 thin films were grown by the pulsed laser deposition technique using a picosecond laser at a wavelength of 1030 nm. The plasma ion mean kinetic energy and density were estimated from the time-of-flight distributions measured using a Langmuir planar probe. It has been found that the mean kinetic energy decreases with increasing the laser pulse energy. This unusual effect is explained by the difference in the volatility of the vaporized species. Samples were structurally characterized by Raman spectroscopy and grazing angle X-ray diffraction. It was found that thin films of amorphous matrices containing MoS2 nanocrystallites were grown. Optical characterization carried out by UV-vis spectroscopy yielded transmittance values above 90% in the visible spectral range and an indirect electronic transition at 1.4 eV. Chemical oxidation states for molybdenum and sulfur were analyzed by means of X-ray photoelectron emission spectroscopy, which revealed Mo-S bonding states, confirming the growth of MoS2.

Effect of vacuum heat treatment on structural, optical, and magneto-electric properties in Bi-doped Y3Fe5O12 ceramics

Yttrium iron garnet (YIG; Y3Fe5O12) is an important material in the field of electronics because of its unique magnetic properties. This makes it ideal for use in devices such as microwave filters, amplifiers, sensors, and even magnetic storage devices. It is also used in spintronics, which is the study of the spin of electrons and how it can be manipulated for storage and computation. Additionally, it is highly stable and has low losses when exposed to electromagnetic fields, making it useful in applications such as controlling electric properties with magnetic fields or vice-versa, and magnetic resonance imaging (MRI). Herein, we have prepared YIG and Bi-doped YIG (Bi: YIG: Y2Bi1Fe5O12) nanoparticles (NPs) using the sol–gel auto-combustion method. The obtained garnet powders were sintered at 950 °C both in an air and vacuum environment. We have explored the structural, electrical, optical, magnetic, and magneto-electric (ME) properties of Bi-doped YIG sintered in a vacuum (YBIG-V) and compared it with YIG and Bi-doped YIG sintered in the air (YBIG-A). We found the enhancement in dielectric response, magnetic properties, and the reduction in leakage current for the YBIG-V ceramics than for YBIG-A, YIG ceramics. We have studied the magneto-electric (ME) coupling at room temperature and found that YBIG-V ceramics show the better coupling strength with a maximum coupling coefficient, αME of 354.3 mV/cm-Oe. The dielectric response of these samples significantly varies with the applied magnetic field, which will be positive in Bi-doped YIG ceramics and negative in pure YIG ceramics. Compared to YBIG-A, YBIG-V samples have better variation in dielectric response with the magnetic field, due to which they may be utilized for magnetic field sensing applications. We also observed that the resonance frequency varies with the applied magnetic field, which may be another parameter for field sensing applications. We attribute the enhancement of these properties in YBIG-V sample to the reduction in the average oxygen valency which is 1.63 for YBIG-A and 1.58 for YBIG-V. These values have been determined with the help of X-ray photoelectron emission spectroscopy data.

Structural and electrical investigations of novel CdFeO/(Bi,Pb)-2212 superconductor composite

ABSTRACT The current investigation reports the influence of the addition of CdFeO nanoparticles on the structural and superconducting properties of the (Bi,Pb)-2212 phase. The formation of an orthorhombic phase of Bi-2212 superconductor is confirmed by the X-ray powder diffraction measurements. The scanning electron microscope measurements show an improvement in the grain connectivity and a decline of voids due to the CdFeO nanoparticles addition. The Fourier transform infra-red spectroscopy is used to associate the absorption bands to equivalent functional groups. The DC-electrical resistivity and the I-V characteristic measurements show an enhancement of the superconducting transition temperature (Tc ) and the critical current density (Jc ) up to 0.02 wt% addition of CdFeO, respectively. The elemental composition and oxidation states are determined using X-ray photoelectron emission spectroscopy. The results suggest a correlation between the electric behavior and the cationic equilibrium reactions among Pb2+ ions in Bi3+ or Cu2+ sites.

Perpendicular Magnetic Anisotropic NiCo2O4 Epitaxial Films with Tunable Coercivity

Ferrimagnetic ${\mathrm{Ni}\mathrm{Co}}_{2}{\mathrm{O}}_{4}$ (NCO) films with perpendicular magnetic anisotropy, 12.5 nm in thickness, are epitaxially deposited on (001) ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ single-crystalline substrates by pulsed laser deposition. It is observed that the coercive field of the NCO films can be tuned systematically by about 20 times, while keeping the large perpendicular magnetic anisotropy, through simply increasing the deposition temperature from 250 to 350 \ifmmode^\circ\else\textdegree\fi{}C. The increase of resistivity and the increase of low-temperature metal-to-insulator transition temperature suggests increased concentration of defects in NCO films deposited at elevated temperatures. The magnetic and transport characteristics are discussed based on a systematic change in cation valence and coordination in NCO films, as revealed by x-ray photoelectron emission and x-ray absorption spectroscopy measurements. The perpendicular magnetic anisotropy with tunable coercivity makes NCO films promising for multifunctional applications.

Europium oxide: Growth guide for the first monolayers on oxidic substrates

Interfacial oxygen exchange at oxide interfaces bears huge potential in stabilizing metastable or novel phases of functional oxides down to the monolayer limit. By taking advantage of active oxygen supply of the substrate material, waiving any external oxygen dosage, high-quality, crystalline ultrathin films of the Heisenberg ferromagnet europium monoxide (EuO) were stabilized on YSZ (001). This so-called redox-assisted growth mode (or, vice versa, the extreme case of a distillation growth) was monitored end to end by in situ x-ray photoelectron emission spectroscopy and electron diffraction techniques. The evolution of Eu $3d$ core levels allows us to disentangle the processes of interfacial oxygen diffusion and vacancy formation in stabilizing the very first monolayers of EuO on YSZ (001). An expedient background correction analysis is presented, which allows us to quantify the critical ${\mathrm{Eu}}^{3+}/{\mathrm{Eu}}^{2+}$ ratio in the ultrathin film regime. We concluded on the key mechanisms of redox-assisted EuO/YSZ (001) thin film synthesis, which merge in a universal three-process growth model that may serve as guideline for redox-assisted synthesis of metastable low-dimensional oxides.

Chemical Structure of a Carbon-Rich Layer at the Wet-Chemical Processed Cu2ZnSn(S,Se)4/Mo Interface

The carbon-rich layer at the back-contact interface of a solution-processed Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ZnSn(S,Se) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (CZTSSe) absorber is investigated with a combination of surface-sensitive X-ray photoelectron and bulk-sensitive X-ray emission spectroscopy. For absorber deposition, an aqueous ammonium-thioglycolate (ATGL) solution was used, and the “buried” back-contact interface was accessed by cleaving in a liquid nitrogen environment. In the pertinent literature, it is reported that such a carbon layer at the absorber/back-contact interface could have beneficial effects, e.g., to reduce series resistance or increase the short circuit current. Here, a detailed picture of the chemical structure of this carbon-rich layer at the back contact is derived, which consists of carbon (74 ± 7%), selenium (19 ± 4%), and sulfur (7 ± 3%). The selenium in this layer is found as elemental inclusions, possibly from not fully reacted selenium during the absorber production. The sulfur content in this carbon-rich layer is twice that of sulfur in the absorber. A detailed analysis of the chemical environment suggests that residuals from the aqueous ATGL solution are the origin of sulfur in this carbon-rich layer. Furthermore, underneath the carbon-rich layer, S-Mo bonds are found at the Mo back contact.

Uranium-Incorporated Pyrochlore La2(UxMgxZr1-2x)2O7 Nuclear Waste Form: Structure and Phase Stability.

As efficient and stable nuclear waste forms, single-phase uranium (U6+)-incorporated La2Zr2O7 nanoparticles were designed and synthesized in an air atmosphere. To obtain a high U loading, divalent magnesium (Mg2+) was introduced to balance the extra charge from the substitution of tetravalent zirconium (Zr4+) by U6+ with a minimized impact to the lattice. There is a composition-driven phase transition from order pyrochlore to defect fluorite as the U concentration increases from 10 to 30 mol %, demonstrating both good solubility and stability of the La2Zr2O7 host for U and potentially for other actinides. La2(UxMgxZr1-2x)2O7 (x = 0-0.3) nanoparticles showed good dispersity and crystallinity with an average particle size of ∼48 nm. Furthermore, X-ray photoelectron spectroscopy, Raman spectroscopy, and emission spectroscopy revealed that U was stabilized in the hexavalent state in the form of a UO22+ ion. Spectroscopic methods also demonstrated that our samples caused a scintillating response with an orange emission (597 nm) by 230 nm excitation. In addition, density functional theory simulations were employed to investigate the atomic structures and electronic properties of the U-incorporated pyrochlores. The calculated bond lengths, atomic charges, and charge density confirm the existence of UO22+ ions. Supported by both experimental and computational results, a novel geometrical structure was proposed to explain the Mg2+-U6+ substitution. This work demonstrated the successful development of U-incorporated La2Zr2O7 nanoparticles and provided an efficient way to immobilize U in these ceramic waste matrixes.

(Invited) Nanoscale (XPEEM) Spectroscopy Unveils the Complex Local (Electro-) Chemical Surface Reactions of Cycled Electrodes in Li-Ion Batteries

Understanding the electrode-electrolyte interface evolution during cycling as well as the surface structural change of the active materials is crucial to improve the safety and the life-cycle of Li-ion batteries. Despite the large effort directed at elucidating the nature and origin of the surface reactivity of the electrodes in liquid organic electrolytes, there is still no complete knowledge of the various mechanisms that take place at such interfaces. The lack of agreement in the reported findings is caused mainly by the surface complexity of the commercial electrodes (multiple particles, high roughness and porosity) and by the intrinsic limitations of the commonly used surface characterization techniques, (mainly the poor lateral resolution). Thus, in this contribution the recent development on X-Ray Photoelectron Emission Spectroscopy (XPEEM) will be presented, as a surface analytical technique capable of providing local information on single particles of anode and cathode materials, while maintaining the complex formulation of composite electrodes. The unique combination of the nanoscale lateral resolution and the spectroscopic capability, confined to within the depth analysis range of 3-4 nm, enable us to provide the missing piece of the exact mechanism of the electrolyte-electrode interactions. Two commercially-relevant electrodes for Li-ion battery will be presented during the talk, namely Li4Ti5O12 (LTO) and the high-voltage cathode lithium rich Li1+x (NiaCobMn1-a-b)1-xO2 (Li-rich NCM) after cycling in the conventional liquid carbonate electrolytes. In the first system, the controversial surface reactivity of LTO when is cycled in carbonate-based electrolyte is resolved. Despite the theoretical prediction of a stable electrochemical interface, the electrolyte reduction is found to occurs solely on LTO particles during lithiation. With the support of density functional theory (DFT) calculations, we show that this behavior is caused by the solvents adsorbed on the LTO outer planes driven by the Li-ion insertion. The DFT calculation indicates that Li-ion insertion leads to a shift of the LUMO of the adsorbed solvents to energies below the Fermi level position of lithiated Li7Ti5O12 and thus to chemical instability.[1]In the second system, at the Li-rich NCM interface, two major parasitic reactions were identified at high potential. The first reaction is related to the electrolyte oxidation, whereas the second is associated with the Li-rich NCM surface degradation.[2] Those parasitic reactions are caused mainly by the oxidation of oxygen leading to the chemical oxidation of the electrolyte solvents and to the formation of a surface layer containing reduced transition metals (TMs).[3] The presence of electrolyte by-products and TMs "cross-talk" is also demonstrated between the cathode and the anode, creating on the latter a layer rich of micrometer-sized agglomerates of TMs. The impact of these parasitic reactions on the electrodes electrochemical cycling performance will be discussed. Finally, as a further development of the technique, operando XPEEM will be presented and discussed on all-solid-state batteries for the working electrode LiCoO2 mixed with (Li2S)3-P2S5 (LPS), used as a solid electrolyte. [1] D. Leanza, C. A. F. Vaz, I. Czekaj, P. Novák, M. El Kazzi, Journal of Material Chemistry A, 2018, 6, 3534–3542. [2] D. Leanza, C. A. F. Vaz, G. Melinte, X. Mu, P. Novák, M. El Kazzi, ACS Appl. Mater. Interfaces, 2019, 11, 6054−6065. [3] D. Leanza, M. Mirolo, C. A. F. Vaz, P. Novák, M. El Kazzi, Batteries & Supercaps, 2019, 2, 1-12.

Ni-Fe-Based Nanostructures for Oxygen Evolution Reaction in Alkaline Media

Controlling morphology, composition, and crystalline phase of Ni-Fe-base nanostructures is of great interests in the development of nanocatalysts for improving the sluggish kinetics of oxygen evolution reaction (OER) in alkaline media. This work develops a scalable, solution-based synthesis approach based on thermal decomposition of organometallic complexes that enables manipulation of both the morphology and crystalline phase of the Ni-Fe nanocatalysts. Different design of morphologies including core-shell and alloy nanostructures are synthesized via either sequential or simultaneous injection. Using fine control of reaction kinetics, the resulting Ni-Fe-based nanostructures exhibit high uniformity with ultrafine size. These uniform nanostructures allow us to use transmission electron microscopy (TEM) including high resolution TEM and electron energy loss spectroscopy (EELS) for in-depth structural and electronic characterization of single particles. The quantitative analysis of the elemental distribution on individual particles using EELS not only matches well with that of the bulk measurement using inductively coupled plasma mass spectrometry but also reveals additional information for the composition of nanoparticles with uneven elemental distribution. Together with the x-ray methods including x-ray power diffraction, x-ray photoelectron emission spectroscopy, x-ray absorption spectroscopy on the bulk sample, we elucidate the morphology, composition, and structure of individual particles for each of these nanostructures in detail. Semi-quantitative analysis of the electronic structures using EELS on single particles agrees with the results from the x-ray techniques. We then evaluate the OER performance for these well-defined, Ni-Fe-based nanostructures and compare with Ni and Fe alone nanoparticles. Through this study we establish a fundamental understanding of morphological, compositional, and structural influences on the electrocatalytic activity of Ni-Fe-based nanocatalysts for OER and identify the best performed nanocatalysts to guide the design of future OER nanocatalysts for anion exchange membrane based electrocatalysis.

Experimental and theoretical investigations of the influence of carbon on a Ho3+-TiO2 photocatalyst with Vis response

Due to their photon up-converting capability, lanthanide ions are ideal candidates dopants for semiconductors for developing visible light-driven photocatalytic activity. Of particular relevance, the low luminescence efficiency of Ln-based nanoparticles is one of the main factors that limits their further applications. Carbon, which is present on the surface of all TiO2 photocatalysts, can be responsible for luminescence quenching processes and, thus, decreasing the photocatalytic activity of TiO2. This article presents a systematic experimental and theoretical study of the effects of carbon on the photocatalytic performance of Ho3+-modified TiO2. Ho3+-TiO2 photocatalysts modified with various carbon contents (from 0.5 to 20 mol.%) were successfully prepared using a simple hydrothermal method. As-obtained samples were characterized by UV–Vis diffuse reflectance spectroscopy (DRS/UV–Vis), X-ray diffraction (XRD), X-ray photoelectron emission spectroscopy (XPS), N2 adsorption measurements, photoluminescent spectroscopy (PL), field-emission scanning electron microscopy (FE-SEM) and scanning transmission microscopy (STEM). The photodegradation efficiency of phenol was estimated for visible light (λ > 420 nm and λ > 455 nm). The XPS and XRD analyses and theoretical calculations revealed that the substitutional doping of holmium and carbon in the TiO2 anatase structure resulted in the appearance of a new sub-band-gap. Changes in the material texture, BET surface area and pore volume can be easily controlled via carbon content in samples. Doping of the Ho3+-TiO2 photocatalysts with carbon resulted in quenching of the emission of Ho3+ and, thus, the photodegradation of phenol, was observed in samples containing smaller amounts of carbon. Sixty minutes of irradiation resulted in 89% of phenol degradation under visible light (λ > 420 nm).

NiFe nanocatalysts for the hydrocracking heavy crude oil

In this work, NiFe nanocatalysts with different molar ratios 1:0.33, 1:1 and 1:3 were synthesized by a modified inverse microemulsion method at Soft Chemistry conditions in order to apply them to heavy crude oil (HCO) in situ hydroprocessing and to improve their physico-chemical properties. All samples were characterized by mean, atomic absorption (AA), small-angle X-ray scattering (SAXS), scanning transmission electron microscopy (STEM), X-ray energy dispersion spectroscopy (EDX), and X-ray photoelectron emission spectroscopy (XPS). Catalytic evaluation of nanocatalysts was performed in a batch reactor at 372 °C, 9.8 MPa of initial H2 pressure and 1 h of reaction time. The reaction products, were analyzed by means analysis of saturates, aromatics, resins, and asphaltenes (SARA) fractions, simulated distillation (SIMDIS), API gravity, rheology, and nitrogen and sulfur contents. Techniques applied reveal that nanoparticles of NiFe with spherical morphology with average sizes in the range (1–20) nm and agglomerates smaller than 50 nm were obtained. The nanocatalysts synthesized improved API gravity from 13.1 to 18.3, furthermore, the asphaltenes conversion was into 37.2–43.7%. Moderate sulfur and nitrogen removal was obtained. SARA analysis indicated that saturated and aromatics fractions increased while resins and asphaltenes were reduced.

Xpeem Reveals the Local (electro-) Chemical Surface Reactions of the Ncm Cathode and Their Impact on the Lto Counter Anode

Understanding the electrode-electrolyte interface evolution during cycling as well as the surface structural change of the active materials is crucial to improve the safety and the life-cycle of Li-ion batteries. Despite the large effort directed at elucidating the nature and origin of the surface reactivity of the electrodes in liquid organic electrolytes, there is still no complete knowledge of the various mechanisms that take place at such interfaces. The lack of agreement in the reported findings is caused mainly by the surface complexity of the commercial electrodes (multiple particles, high roughness, and porosity) and by the intrinsic limitations of the commonly used surface characterization techniques (poor lateral resolution). Thus, in this contribution we use X-Ray Photoelectron Emission Spectroscopy (XPEEM) as a surface analytical technique capable of providing localized information on single particles of composite anode and cathode materials, while preserving their working environment (as in commercial-like electrodes). The unique combination of the nanoscale lateral resolution and the spectroscopic capability, confined to within the depth analysis range of 3-4 nm, enable us to finally provide the missing piece of the exact mechanism of the electrolyte-electrode interactions. The surfaces of two types of electrodes are investigated and will be presented, Li(Mn,Ni,Co)O2-Li2MnO3 (NCM) and Li4Ti5O12 (LTO), as promising industrially relevant positive and negative electrodes, respectively, for Li-ion batteries. A systematic study of the potential dependency of the surface evolution was performed during the early stage and after long cycling. The contrast images in Figure 1a,d attest the good spatial resolution of XPEEM and its capability to localize the different particles present on the surface. The local XAS spectra (Figure 1b) at the transition metals (TMs)-L edges acquired on NCM particles allow us to monitor its oxidation states and the surface structure modification. At the same time, the C-K edge provides direct information on the chemical changes and the electrolyte byproducts decomposition.[1] We find that, despite cycling the NCM to a highly oxidative potential of 5.1 V vs. Li+/Li, there is no formation of an oxidized electrolyte byproduct layer on the surface of the cathode particles (C K-edges Figure 1.b, c); instead, we show that a homogeneous organic-inorganic surface layer builds-up across the particles of the LTO counter anode (C K-edges, Figure 1.e f). In addition, we observe that such surface layer incorporates, already from the first charge, micrometer-sized agglomerates of TMs explained as due to the instability of a reduced layer of Mn, Co and Ni at the NMC-electrolyte interface that is mainly formed during the cathode delithiation (Mn L-edge, Figure 1.b). These results demonstrate that two simultaneous reactions take place at the NCM-electrolyte interface subject to a high working potential, namely, surface degradation of the NCM active materials and decomposition of the electrolyte which affect directly the anode surface and the overall cell performance through a cross-talk process. [1] D. Leanza, C.A.F. Vaz, I. Czekaj, P. Novák, M. El Kazzi, Journal of Materials Chemistry A 6 (8), 3534-3542 (2018) Figure 1

The Orientation of Silver Surfaces Drives the Reactivity and the Selectivity in Homo-Coupling Reactions.

Original reaction pathways can be explored in the on-surface synthesis approach where small aromatic precursors are confined to the surface of single crystal metals. The bis-indanedione molecule reacted with itself on silver surfaces in different ways, through a Knoevenagel reaction or an oxidative coupling, leading to the formation of a variety of new molecular compounds and covalently-linked 1D or 2D networks. Noteworthy, original reaction products were obtained that cannot be synthesized in traditional solvent-based chemistry. The lowest activation temperature for the homo-coupling reactions was found on the Ag(111) surface. The Ag(110) was highly selective in terms of coupling reaction type, while on Ag(100) the temperature could finely control the selectivity. The on-surface synthesis approach is shown here to be particularly efficient to produce original compounds in mild conditions, using activation temperatures as low as 200 °C. The different structures were characterized by scanning tunnelling microscopy (STM) together with X-ray photoelectron emission spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS).

Transition metal nanocatalysts by modified inverse microemulsion for the heavy crude oil upgrading at reservoir

Abstract In this work, nanocatalysts as Ni, Mo, Fe nanoparticles (NPs) were synthesized by the modified inverse microemulsion method at room temperature conditions in order to apply them to heavy crude oil (HCO) in situ hydroprocessing and to improve their physicochemical properties. All samples of NPs were characterized by X-ray photoelectron emission spectroscopy (XPS), X-ray energy dispersion spectroscopy (EDX), small-angle X-ray scattering (SAXS), and scanning transmission electron microscopy (STEM). Catalytic evaluation of nanocatalysts was performed in a batch reactor at 372 °C, 9.8 MPa of initial H2 pressure and 1 h of reaction time. The reaction products were studied by analysis of saturates, aromatics, resins, and asphaltenes (SARA) fractions, simulated distillation (SIMDIS), API gravity, and both nitrogen and sulfur contents to determine their physicochemical properties. Applied techniques revealed that nanoparticles of Ni, Mo, and Fe with spherical morphology with average sizes in the range (2.5–20) nm and agglomerates smaller than 50 nm were obtained. The nanocatalysts synthesized improved from 13 to 18°API, also of an asphaltenes conversion in the range 20–43%, and moderate sulfur and nitrogen removal were obtained. SARA analysis indicated that saturated and aromatics fractions increased while resins and asphaltenes were reduced, increasing the possibility of a high mobility of crude oil into the reservoir.

Valence change detection in memristive oxide based heterostructure cells by hard X-ray photoelectron emission spectroscopy

The switching mechanism of valence change resistive memory devices is widely accepted to be an ionic movement of oxygen vacancies resulting in a valence change of the metal cations. However, direct experimental proofs of valence changes in memristive devices are scarce. In this work, we have employed hard X-ray photoelectron emission microscopy (PEEM) to probe local valence changes in Pt/ZrOx/Ta memristive devices. The use of hard X-ray radiation increases the information depth, thus providing chemical information from buried layers. By extracting X-ray photoelectron spectra from different locations in the PEEM images, we show that zirconia in the active device area is reduced compared to a neighbouring region, confirming the valence change in the ZrOx film during electroforming. Furthermore, we succeeded in measuring the Ta 4f spectrum for two different resistance states on the same device. In both states, as well as outside the device region, the Ta electrode is composed of different suboxides without any metallic contribution, hinting to the formation of TaOx during the deposition of the Ta thin film. We observed a reduction of the Ta oxidation state in the low resistance state with respect to the high resistive state. This observation is contradictory to the established model, as the internal redistribution of oxygen between ZrOx and the Ta electrode during switching would lead to an oxidation of the Ta layer in the low resistance state. Instead, we have to conclude that the Ta electrode takes an active part in the switching process in our devices and that oxygen is released and reincorporated in the ZrOx/TaOx bilayer during switching. This is confirmed by the degradation of the high resistance state during endurance measurements under vacuum.

Electrocatalytic performance evaluation of cobalt hydroxide and cobalt oxide thin films for oxygen evolution reaction

The development of an inexpensive, stable, and highly active electrocatalyst for oxygen evolution reaction (OER) is essential for the practical application of water splitting. Herein, we have synthesized an electrodeposited cobalt hydroxide on nickel foam and subsequently annealed in an air atmosphere at 400°C for 2h. In-depth characterization of all the films using X-ray diffraction (XRD), X-ray photoelectron emission spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) techniques, which reveals major changes for their structural, morphological, compositional and electrochemical properties, respectively. The cobalt hydroxide nanosheet film shows high catalytic activity with 290mV overpotential at 10mAcm−2 and 91mVdec−1 Tafel slope and robust stability (24h) for OER in 1M KOH electrolyte compared to cobalt oxide (340mV). The better OER activity of cobalt hydroxide in comparison to cobalt oxide originated from high active sites, enhanced surface, and charge transport capability.

Optical emission and surface characterization of stainless steel treated by pulsed microwave-atmospheric helium plasma jet

A pulsed microwave-excited atmospheric pressure plasma jet MWAPPJ in helium, has been investigated by optical emission spectroscopy OES. Beside helium spectral lines various excited species such as atomic oxygen OI, hydrogen H α line, N2 + ion and nitrogen molecule species N2 (C3Πu) which are responsible for metal surface activation. The rotational T rot and vibrational temperature T vib were evaluated using optical emission spectroscopy and their dependences on the pulsed microwave parameters such as duty factor (20–70%) and a repetition rate (4 to 90 kHz) are investigated. Spectroscopic measurements show that, T rot, T vib and relative emission intensity of excited species greatly increases with increasing pulse duty factor. Meanwhile they show weak dependences as repetition frequency increased. Surface characterization of the examined stainless steel samples were performed through contact angle, Atomic force microscope AFM and X-ray photoelectron emission spectroscopy XPS. The contact angle decreased from 71° to 20° after pulsed MWAPPJ helium plasma at duty factor of 70% treatment for 30 s, which it is mainly due to incorporation of oxygen-containing functional groups as inferred from XPS results.