Combination Of X-ray Photoelectron Spectroscopy Research Articles

Targeted Construction of Amorphous MoSx with an Inherent Chain Molecular Structure for Improved Pseudocapacitive Lithium-Ion Response.

Owing to low ion/electron conductivity and large volume change, transitional metal dichalcogenides (TMDs) suffer from inferior cycle stability and rate capability when used as the anode of lithium-ion batteries (LIBs). To overcome these disadvantages, amorphous molybdenum sulfide (MoSx ) nanospheres were prepared and coated with an ultrathin carbon layer through a simple one-pot reaction. Combining X-ray photoelectron spectroscopy (XPS) with theoretical calculations, MoSx was confirmed as having a special chain molecular structure with two forms of S bonding (S2- and S2 2- ), the optimal adsorption sites of Li+ were located at S2 2- . As a result, the MoSx electrode exhibits superior cycle and rate capacities compared with crystalline 2H-MoS2 (e.g., delivering a high capacity of 612.4 mAh g-1 after 500 cycles at 1 A g-1 ). This is mainly attributed to more exposed active S2 2- sites for Li storage, more Li+ transfer pathways for improved ion conductivity, and suppressed electrode structure pulverization of MoSx derived from the inherent chain-like molecular structure. Quantitative charge storage analysis further demonstrates the improved pseudocapacitive contribution of amorphous MoSx induced by fast reaction kinetics. Moreover, the morphology contrast after cycling demonstrates the dispersion of active materials is more uniform for MoSx than 2H-MoS2 , suggesting the MoSx can well accommodate the volume stress of the electrode during discharging. Through regulating the molecular structure, this work provides an effective targeted strategy to overcome the intrinsic issues of TMDs for high-performance LIBs.

All roads lead to Rome: Sodiation of different-stacked SnS2

Structural symmetry variations of two-dimensional materials may significantly affect their reaction process and kinetics with alkali ions. Here, SnS2 with different structural symmetry is used as the target to be comparatively investigated the phase transition pathway and ionic diffusion upon (de)sodiation in sodium-ion batteries using in situ transmission electron microscopy and ex situ x-ray photoelectron spectroscopy in combination with first-principles calculations. Different intermediate phases, AA1 NaSnS2 for trigonal SnS2 and AB1 NaSnS2 for hexagonal SnS2, are observed after sodium ions fully occupy all Oh sites. While the coming conversion/alloying reaction processes are relatively similar. The AA1/AB1 NaSnS2 sequentially transits to SnS, β-Sn and Na15Sn4 phases starting from Td sites of SnS2 being held. Reversible reactions occur among SnS, Na15Sn4 and Na2Sm (2<m<8) in following cycles. Our works provide a deep understanding of the ionic diffusion and electrochemical reaction mechanisms for two-dimensional materials with different symmetry, and offer some guidance for the design of the high energy two-dimensional electrode materials.

Effects of Annealing Temperature and Ambient on Metal/PtSe2 Contact Alloy Formation.

Forming gas annealing is a common process step used to improve the performance of devices based on transition-metal dichalcogenides (TMDs). Here, the impact of forming gas anneal is investigated for PtSe2-based devices. A range of annealing temperatures (150, 250, and 350 °C) were used both in inert (0/100% H2/N2) and forming gas (5/95% H2/N2) environments to separate the contribution of temperature and ambient. The samples are electrically characterized by circular transfer length method structures, from which contact resistance and sheet resistance are analyzed. Ti and Ni are used as metal contacts. Ti does not react with PtSe2 at any given annealing step. In contrast to this, Ni reacts with PtSe2, resulting in a contact alloy formation. The results are supported by a combination of X-ray photoelectron spectroscopy, Raman spectroscopy, energy-dispersive X-ray spectroscopy, and cross-sectional transmission electron microscopy. The work sheds light on the impact of forming gas annealing on TMD–metal interfaces, and on the TMD film itself, which could be of great interest to improve the contact resistance of TMD-based devices.

Resistance of nanoclay reinforced epoxy composites to hyperthermal atomic oxygen attack

Due to outstanding mechanical properties, heat resistance, and relatively facile production, nanoclay reinforced epoxy composites (NCRE composites) have been suggested as candidate materials for use on external surfaces of spacecraft residing in the low Earth orbit (LEO) environment. The resistance of the NCRE composites to bombardment by atomic oxygen (AO), a dominant component of the LEO environment, has been investigated. Four types of samples were used in this study. They were pure epoxy (0 wt% nanoclay content), and NCRE composites with different loadings of nanoclay—1 wt%, 2 wt%, and 4 wt%. Etch depths decreased with increasing nanoclay content, and for the 4 wt% samples it ranged from 28% to 37% compared to that of pure epoxy. X-ray photoelectron spectroscopy (XPS) indicates that after AO bombardment, relative area of C–C/C–H peak decreased, while the area of the C–O, ketones peaks increased, and the oxidation degree of surfaces increased. New carbon-related component carbonates were detected on nanoclay containing composite surfaces. Scanning electron microscopy indicates that aggregates formed on nanoclay-containing surfaces after AO bombardment. The sizes and densities of aggregates increased with nanoclay content. The combined erosion depths, XPS and SEM results indicate that although all the studied surfaces got eroded and oxidized after AO bombardment, the nanoclay containing composites showed better AO resistance compared to pure epoxy, because the produced aggregates on surface potentially act as a physical “shield”, effectively retarding parts of the surface from further AO etching.

Bulk In2O3 crystals grown by chemical vapour transport: a combination of XPS and DFT studies

High purity In2O3 crystals were synthesized by the chemical vapour transport (CVT) as confirmed by X-ray Photoelectron Spectroscopy (XPS) using survey, core-levels and valence band scans of the samples. Further, high energy resolution XPS spectra revealed perfectly bonded In 3d configuration and excluded the presence of metallic In0, concurrently, the dual band observed for O 1s configuration was interpreted in term of the additional defectiveness in the oxygen sublattice. The valence band as measured by XPS is in agreement with the calculated one using DFT approach, and an insignificant shift of valence band toward Fermi level (about 0.05 eV) caused by the presence of the defects was established. Moreover, the energetics of the primarily defects were calculated and the trends were set in the perspective of XPS measurements. Altogether we make CVT In2O3 crystals ready to be used for the onward studies and technical applications.

Neutrophils Activated by Nanoparticles and Formation of Neutrophil Extracellular Traps: Work Function Mapping and Element Specific Imaging.

Photoemission electron microscopy (PEEM) and imaging X-ray photoelectron spectroscopy (XPS) have over the years been powerful tools in classical surface physics and material sciences, and due to recent technological advances, their uses within other fields/disciplines are rapidly growing. Lately, the XPS/PEEM based elemental analysis and characterization in imaging mode, with exquisite spatial resolution and high sensitivity, has shown the potential to deliver new mechanistic insights in cell-biology/medicine. In this work, the aim was to visualize biological processes on the cellular level, with the additional dimension of topographical morphology and element specific information, mapping chemical composition and chemical states. This is hereby demonstrated by combined PEEM and imaging XPS investigation of neutrophils and their activation processes, where fluorescence microscopy commonly used in biology is used for benchmarking. Neutrophils are phagocytic cells and are vital components in the human immune system, with the fundamental role of fighting invading pathogens. They are capable of ingesting microorganisms or particles, and in order to capture and trap foreign objects, one of their strategies is to release nuclear DNA by the formation of extracellular web-like traps (NETs). Here, we report how neutrophils are triggered by controlled nanoparticle (NP) exposure. The neutrophils and NETs formation are imaged in the presence of NPs, and we report the elemental composition of single cells and the structure of NETs. Cellular uptake of nanoparticles is proven and the states just before and after NETs release are imaged, as well as visualization of the extraordinary capability for mass transport at distances 10 times or more than the size of the cell itself. This method paves the way for element specific imaging of biorelated cells on surfaces as well as nanoparticle tracking in the submicro- and nanoregions.

Nanoscale Surface Compositions and Structures Influence Boron Adsorption Properties of Anion Exchange Resins.

Boron adsorption properties of poly(styrene-co-divinylbenzene) (PSDVB)-based anion-exchange resins with surface-grafted N-methyl-d-glucamine (NMDG) depend strongly on their local surface compositions, structures, and interfacial interactions. Distinct boron adsorption sites have been identified and quantified, and interactions between borate anions and hydroxyl groups of NMDG surface moieties have been established. A combination of X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (NMR), and Fourier-transform infrared (FT-IR) spectroscopy were used to characterize the atomic-level compositions and structures that directly influence the adsorption of borate anions on the NMDG-functionalized resin surface. Surface-enhanced dynamic-nuclear-polarization (DNP)-NMR enabled dilute (3 atom % N) tertiary alkyl amines and quaternary ammonium ions of the NMDG groups to be detected and distinguished with unprecedented sensitivity and resolution at natural abundance 15N (0.4%). Two-dimensional (2D) solid-state 11B{1H}, 13C{1H}, and 11B{11B} NMR analyses provide direct atomic-scale evidence for interactions of borate anions with the NMDG moieties on the resin surfaces, which form stable mono- and bischelate complexes. FT-IR spectra reveal displacements in the stretching vibrational frequencies associated with the O-H and N-H bonds of NMDG groups that corroborate the formation of chelate complexes on the resin surfaces. The atomic-level compositions and structures are related to boron adsorption properties of resin materials synthesized under different conditions, which have important remediation applications.

Iron-Nitrogen-Carbon (Fe-N-C) Active Sites Imaging By Scanning Transmission Electron Microscopy (STEM)

Iron-nitrogen-carbon electrocatalysts (i.e. Fe-N-C) are of interest for the oxygen reduction reaction because they can solve the geological and economic constraints associated with the use of Pt-based electrocatalysts. However, the nature and the stability of their active sites is yet to be fully understood. The presence of nitrogen and atomically dispersed Fe is believed to result in the presence of various moieties, i.e. Fe-Nx, N-pyridinic, N-pyrrolic, etc. Such assumptions are based on the fitting of the N1s peak observed by X-ray photoelectron spectroscopy (XPS) 1,2 and, for the Fe-Nx moieties, extended X-ray absorption fine structure (EXAFS) 3. Imaging a non-model Fe-N-C electrocatalyst at an atomic scale could provide additional insights onto the exact structure of those moieties, and, in combination with electrochemical ageing, ease the assessment of their stability. Here, we discuss the multiplicity of actives sites in an Fe-N-C electrocatalyst synthesized by the sacrificial support method. We use a combination of XPS, EXAFS, scanning transmission electron micrographs (STEM) and electron energy loss elemental mapping (EELS – see Figure 1) to unveil the presence, additional to Fe-Nx, of other active sites, e.g. Fe-NxCy or nitrogen moieties. Then, we investigate the stability of said moieties (with a focus onto Fe-Nx) using a combination of step-cycles (10,000 cycles; 0.6 – 1.0 V vs. RHE, 3s – 3s, in Ar-saturated 0.1 M H2SO4, T = 80°C) and STEM (see Figure 1), showing that, in these experimental conditions and for this specific family of Fe-N-C electrocatalysts, the atomically dispersed Fe forms clusters – but not nanoparticles – during the ageing process. (1) Artyushkova, K.; Serov, A.; Rojas-Carbonell, S.; Atanassov, P. J. Phys. Chem. C 2015, 119, 25917–25928. (2) Matanovic, I.; Artyushkova, K.; Strand, M. B.; Dzara, M. J.; Pylypenko, S.; Atanassov, P. J. Phys. Chem. C 2016, 120 (29225–29232), 6–13. (3) Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F. Nat. Mater. 2015, 14 (September), 937–942. Figure 1

Efficient Electrochemical Reduction of CO2 by Ni–N Catalysts with Tunable Performance

Developing highly selective and durable catalysts for electrochemical CO2 reduction (ECR) to valuable chemicals is essential for curbing CO2 emissions into the atmosphere and to close the anthropogenic carbon cycle. Herein, we report facile and scalable synthesis of Ni/NC as an efficient low-cost catalyst for ECR to CO. The electrocatalytic properties of the catalysts can be readily tailored by tuning N configurations and the fraction of single Ni sites in Ni/NC via manipulation of the N precursor and annealing conditions. The Ni/NC catalyst designed by thermal treatment of a composite mixture of carbon black, Ni(II) acetate, and polyacrylonitrile achieved low overpotential CO2 to CO conversion with a Faradaic efficiency (FE) as high as 96.5% at −0.9 V (vs reversible hydrogen electrode, RHE), a current density reaching −12.6 mA cm–2 at −1.2 V (vs RHE) corresponding to a CO turnover frequency of up to 4760 h–1. In addition, high FEs for CO formation (≥84.7%) were obtained in a wide potential range from −0.8 to −1.2 V (vs RHE) on the Ni/NC electrode. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectra in combination with multiple electrochemical results suggested that the pyrrolic N and Ni–N structures concertedly play a determinant role in the catalytic performance of Ni/NC during the CO2 reduction reaction.

Preparation of Carbon Nanomembranes without Chemically Active Groups.

The electron-irradiation-induced synthesis of carbon nanomembranes (CNMs) from aromatic thiol-based self-assembled monolayers (SAMs) on gold substrate is a well-established method to form molecular thin nanosheets. These molecular two-dimensional materials can be prepared with tunable properties; therefore, they find a variety of applications in nanotechnology ranging from ultrafiltration to nanobiosensors. However, no chemically inert CNM was fabricated up to now, as the reactive thiol group is present on the membrane surface even after transferring it to other substrates. Here, we study the electron irradiation of carboxylic acid-based SAMs on a silver substrate as an alternative route for CNM formation. Our analysis, based on a combination of X-ray photoelectron spectroscopy and scanning electron microscopy demonstrates that for this type of SAMs, purely carbonaceous CNMs with tunable porosity can be obtained.

Ti-doped Sn2Se3 phase change material for improved thermal stability

We comprehensively study the influences of Ti doping on the phase change properties of Sn2Se3 film through experiments and ab initio molecular dynamics simulations. Both the crystallization temperature and the activation energy of Sn2Se3 increase significantly by introducing Ti atoms, indicating enhanced thermal stability. Based on the results of x-ray photoelectron spectroscopy in combination with theoretical calculations, we assume that Ti atoms tend to form covalent bonds with Se in amorphous Sn2Se3. Because of the stronger bond energy of Ti-Se and the change of the bonding environment around Ti atoms, it is more difficult to rearrange the atoms to form long-range order, thus giving rise to the thermal stability of the amorphous phase. Our work shows that Ti doped Sn2Se3 is a promising candidate for the application of next generation non-volatile memory.

Surface oxidation of amorphous Si and Ge slanted columnar and mesoporous thin films: Evidence, scrutiny and limitations for infrared optics

Issues on the superficial oxidation of mesoporous amorphous silicon and germanium photonic layers generated at oblique angles are reported. Such films were designed to improve the transmittance of optical elements over the mid-IR window (3.6–4.9 μm) by removing the light reflection. These nanostructures were deposited on silicon substrates by e-beam evaporation at room temperature. IR ellipsometry and spectrophotometry studies combined with models based on effective medium approximations predicted the presence of silicon or germanium oxides. Such oxidation was evidenced by combining X-ray photoelectron spectroscopy and advanced (scanning-)transmission electron microscopy studies based on energy-dispersive X-rays and electron energy-loss spectroscopies. Both techniques also allowed to prove the formation of core-shell-type architectures consisting of pure Si or Ge surrounded by oxidized species, even for a Ge layer subsequently capped with a dense MgF2 coating. The different approaches used for preparing electron transparent specimens (tripod polishing and focused ion-beams) confirmed a fast oxidation of the Ge nanocolumns even for short air exposure periods, and allowed comparing it with the level of oxidation promoted from other pollutant sources. This work sheds light on the spontaneous undesired oxidation in Si or Ge slanted nanorods which can diminish the performances and limit further development of optical devices.

MoS4]2--Intercalated NiCo-Layered Double Hydroxide Nanospikes: An Efficiently Synergized Material for Urine To Direct H2 Generation.

Substituting the energy-uphill water oxidation half-cell with readily oxidizable urea-rich urine, a ground-breaking bridge is constructed, combining the energy-efficient hydrogen generation and environmental protection. Hence, designing a robust multifunctional electrocatalyst is desirable for widespread implementation of this waste to fuel technology. In this context, here, we report a simple tuning of the electrocatalytically favorable characteristics of NiCo-layered double hydroxide by introducing [MoS4]2- in its interlayer space. The [MoS4]2- insertion as well as its effect on the electronic structure tuning is thoroughly studied via X-ray photoelectron spectroscopy in combination with electrochemical analysis. This insertion induces overall electronic structure tuning of the hydroxide layer in such a way that the designed catalyst exhibited favorable kinetics toward all the required reactions of hydrogen generation. This is why our homemade catalyst, when utilized both as a cathode and anode to fabricate a urea electrolyzer, required a mere ∼1.37 V cell potential to generate sufficient H2 by reaching the benchmark 10 mA cm-2 in 1 M KOH/0.33 M urea along with long-lasting catalytic efficiency. Other indispensable reason of selecting [MoS4]2- is its high-valent nature making the catalyst highly selective and insensitive to common catalyst-poisoning toxins of urine. This is experimentally supported by performing the real urine electrolysis, where the nanospike-covered Ni foam-based catalyst showed a performance similar to that of synthetic urea, offering its industrial value. Other intuition of selecting [MoS4]2- was to provide a ligand-based mechanism for hydrogen evolution half-cell [hydrogen evolution reaction (HER)] to preclude the HER-competing oxygen reduction. Another crucial point of our work is its potential to avoid the mixing of two explosive product gases, that is, H2 and O2.

Water Splitting on Ti-Oxide-Terminated SrTiO3(001)

Combining X-ray photoelectron spectroscopy with the standing wave technique, we investigated adsorption of a monolayer of water on Ti-oxide-terminated SrTiO3(001) in ultra-high vacuum (UHV). At room temperature, the surface is water-free but hydroxylated. A quarter monolayer of hydroxyl is tightly bound 1.85 ± 0.06 A above the TiO2 surface. Deposited at a low temperature, a monolayer of water adsorbs with the oxygen located 2.55 ± 0.2 A above the surface, apparently close to atop Ti, but H2O is unstable at 200 K. A fraction desorbs, in part under the X-ray beam, but a major fraction of H2O dissociates immediately, with the liberated hydrogen most likely attaching to a surface oxygen. The produced hydroxyls bind only loosely to the surface, are unstable at 200 K, and rapidly desorb once the surface is water-free. Although our study was conducted in UHV, the presented results suggest that Ti-oxide-terminated SrTiO3(001) may possess a high catalytic activity toward hydrolysis under realistic conditions.

Sponge Effect Boosting Oxygen Reduction Reaction at the Interfaces between Mullite SmMn2O5 and Nitrogen-Doped Reduced Graphene Oxide.

Exploring the effect of interfacial structural properties on catalytic performance of hybrid materials is essential in rationally designing novel electrocatalysts with high stability and activity. Here, in situ growth of mullite SmMn2O5 on nitrogen-doped reduced graphene oxide (SMO@NrGO) is achieved for highly efficient oxygen reduction reaction (ORR). Combining X-ray photoelectron spectroscopy and density functional theory calculations, interfacial chemical interactions between Mn and substrates are verified. Interestingly, as revealed by charge density difference, the interfacial Mn-N(C) bonds display a sponge effect to store and compensate electrons to boost the ORR process. In addition, bidentate adsorption of oxygen intermediates instead of monodentate ones is observed in hybrid materials, which facilitates the interactions between intermediates and active sites. Experimentally, the hybrid catalyst SMO@NrGO exhibits a half-wave potential as high as 0.84 V, being comparable to benchmark Pt/C and higher than that of the pure SMO (0.68 V). The Zn-air battery assembled with SMO@NrGO shows a high discharge peak power density of 244 mW cm-2 and superior cycling stability against noble metals.

Activation mechanism of peroxymonosulfate by biochar for catalytic degradation of 1,4-dioxane: Important role of biochar defect structures

Carbonaceous materials have been emerged as promising metal-free catalysts to activate peroxymonosulfate (PMS) for the degradation of organic contaminants. In the present study, a low-cost carbonaceous material, pine needle biochar was prepared under the oxygen limited pyrolyzation process and applied to activate PMS for the catalytic degradation of potentially carcinogenic 1,4-dioxane. Based on the batch experiments, the pyrolysis temperature has a significant influence on the potency of biochar to activate PMS. Within the biochar/PMS system, the degradation efficiencies of 1,4-dioxane were increased from 4.1 to 84.2% when the biochar pyrolysis temperatures were increased from 300 °C to 800 °C. Electron paramagnetic resonance (EPR) study and the quenching experiment verified that the dominant free radical within the system of biochar/PMS was OH. The EPR combined X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) and Raman analysis implied that the defect of biochar contributed to the generation of SO4− and OH and high pyrolysis temperatures would eliminate the excess of oxygen functional groups in biochar, which could modulate and transform sp3 carbons, generating more defect structures, therefore, enhancing the activation potency of biochar towards PMS. Simultaneously, five major intermediates were identified and three possible degradation pathways were proposed in this study. In addition, 71.4% and 57.5% of 1,4-dioxane removal rates were achieved in 1,4-dioxane contaminated tap water and groundwater, indicating that biochar activating PMS is a promising technique for the remediation of 1,4-dioxane contaminated water.

Yellowing of laser-cleaned artworks: Formation of residual hydrocarbon compounds after Nd:YAG laser cleaning of gypsum plates covered by lamp black

Abstract The removal of black crusts decaying the surface of artworks is an important concern for the conservation of cultural heritage. Nd:YAG laser cleaning of encrusted stones and plasters at 1064 nm is widely recognized as an effective restoration technique, but induces a noticeable yellowing of the treated surface. Several researches carried out on the effects of laser cleaning have been focused on the induced yellowing and how to visually mitigate this phenomenon. To this end, UV-B radiations were successfully used to lessen the laser-induced yellowing due to the removal of lamp black particles on gypsum. The mechanism at play for both the formation of the compounds yellowing the surface and their disappearance upon UV-B exposure remains, however, poorly understood. Within the frame of this research, we apply surface-sensitive characterization techniques to analyze the yellowed surface produced after Nd:YAG Q-Switched laser cleaning of lamp black deposit on a gypsum plate, and the same surface after UV-B exposure. A combination of X-ray photoelectron spectroscopy and Fourier-transformed infrared spectroscopy has been used to identify the residual carbon compounds responsible for the yellow coloration of the substrate. A nanoscale structural description of the ejected particles collected during the laser cleaning was finally performed with transmission electron microscopy. We found that the yellowing is due to partially oxidized hydrocarbons compounds deposited at the surface of the gypsum substrate. We propose that they form by reactions between carbon species emitted by the vaporization of the carbon particles, with hydrogen and oxygen produced by the dissociation of water molecules coming together from dehydration of the gypsum surface and from the water sprayed by the operator during cleaning.

Creation of a two-dimensional conducting path based on the engineering oxidation state of WO3 with Al2O3 at the interface

We report the dramatic manipulation of an oxide interface in Al2O3/WO3 from insulating to metallic state by creating a two-dimensional conducting path. The WO3 thin film is deposited on the SiO2 grown Si substrate using radio frequency magnetron sputtering and the amorphous Al2O3 layer is formed by atomic layer deposition on the WO3 layer. After rapid vacuum annealing at 400 °C, the interfacial conducting path of Al2O3/WO3 reveals an abnormally high sheet carrier density of 4.91 × 1016 cm−2 compared with <109 cm−2 for bare WO3. The X-ray photoelectron spectroscopy (XPS) shows that WO3 is partially reduced; and a W5+ oxidation state is observed at the interface. The schematic electronic band diagram of the interfacial region obtained by combined XPS and ultraviolet photoelectron spectroscopy analyses indicates that the downward band bending at the surface of WO3 induces a confined electron accumulation at the interface. Synergetic effects between the increased unpaired W 5d electrons in the W5+ oxidation state and the accumulative charge confinement at the Al2O3/WO3 interfaces lead to a two-dimensional conducting path. The metallic conduction is distinct from the insulating bulk properties. This study demonstrates an example of a short-range-ordered two-dimensional conducting path at two insulating oxide interfaces formed at low temperature, which is applicable to flexible oxide conductors with great conduction path stability.

On the presence of Ga2O sub-oxide in high-pressure water vapor annealed AlGaN surface by combined XPS and first-principles methods

We conducted X-ray photoelectron spectroscopy (XPS) and first-principles calculations based on density functional theory (DFT) to confirm the presence of Ga2O sub-oxide in high-pressure water vapor annealed AlGaN surface. We note that the Ga 3d XPS peak broadens and shifts towards higher binding energies, which suggests surface oxide formation. Deconvoluted Ga 3d XPS profiles between HPWVA-treated and reference samples reveal reasonable inclusion of Ga2O peak, suggesting formation of Ga2O sub-oxide. To theoretically confirm the presence of Ga2O, we calculated the Ga 3d core-level shift using initial state approximation. We obtained a 0.74 eV shift, in reasonable agreement with that of Ga2O. Moreover, based on the calculation of net charge on Ga using DFT, we also obtained a +1 oxidation state for Ga, indicating its existence in Ga2O form. By combining theory and experiment, therefore, we have explored the possibility of the formation of Ga2O sub-oxide, which may provide new avenues for obtaining highly stable operation in GaN-based devices.

Influence of Fe Substitution into LaCoO3 Electrocatalysts on Oxygen-Reduction Activity.

The development of commercially friendly and stable catalysts for oxygen reduction reaction (ORR) is critical for many energy conversion systems such as fuel cells and metal-air batteries. Many Co-based perovskite oxides such as LaCoO3 have been discovered as the stable and active ORR catalysts, which can be good candidates to replace platinum (Pt). Although researchers have tried substituting various transition metals into the Co-based perovskite catalysts to improve the ORR performance, the influence of substitution on the ORR mechanism is rarely studied. In this paper, we explore the evolution of ORR mechanism after substituting Fe into LaCoO3, using the combination of X-ray photoelectron spectroscopy, high-resolution X-ray microscopy, X-ray diffraction, surface-sensitive soft X-ray absorption spectroscopy characterization, and electrochemical tests. We observed enhanced catalytic activities and increased electron transfer numbers during the ORR in Co-rich perovskite, which are attributed to the optimized eg filling numbers and the stronger hybridization of transition metal 3d and oxygen 2p bands. The discoveries in this paper provide deep insights into the ORR catalysis mechanism on metal oxides and new guidelines for the design of Pt-free ORR catalysts.