Showing papers on "Chemical state published in 2020"

The role of in situ generated morphological motifs and Cu(I) species in C2+ product selectivity during CO2 pulsed electroreduction

Abstract: The efficient electrochemical conversion of CO2 provides a route to fuels and feedstocks. Copper catalysts are well-known to be selective to multicarbon products, although the role played by the surface architecture and the presence of oxides is not fully understood. Here we report improved efficiency towards ethanol by tuning the morphology and oxidation state of the copper catalysts through pulsed CO2 electrolysis. We establish a correlation between the enhanced production of C2+ products (76% ethylene, ethanol and n-propanol at −1.0 V versus the reversible hydrogen electrode) and the presence of (100) terraces, Cu2O and defects on Cu(100). We monitored the evolution of the catalyst morphology by analysis of cyclic voltammetry curves and ex situ atomic force microscopy data, whereas the chemical state of the surface was examined via quasi in situ X-ray photoelectron spectroscopy. We show that the continuous regeneration of defects and Cu(i) species synergistically favours C–C coupling pathways. Carbon dioxide can be reduced electrocatalytically to fuels using copper catalysts, but the key features that determine the selectivity of these materials to specific products remains uncertain. Here Aran–Ais et al. use in situ methods to explore the influence of morphology and oxidation state on the performance of copper catalysts.

Operando time-resolved X-ray absorption spectroscopy reveals the chemical nature enabling highly selective CO 2 reduction

Abstract: Copper electrocatalysts have been shown to selectively reduce carbon dioxide to hydrocarbons. Nevertheless, the absence of a systematic study based on time-resolved spectroscopy renders the functional agent-either metallic or oxidative Copper-for the selectivity still undecidable. Herein, we develop an operando seconds-resolved X-ray absorption spectroscopy to uncover the chemical state evolution of working catalysts. An oxide-derived Copper electrocatalyst is employed as a model catalyst to offer scientific insights into the roles metal states serve in carbon dioxide reduction reaction (CO2RR). Using a potential switching approach, the model catalyst can achieve a steady chemical state of half-Cu(0)-and-half-Cu(I) and selectively produce asymmetric C2 products - C2H5OH. Furthermore, a theoretical analysis reveals that a surface composed of Cu-Cu(I) ensembles can have dual carbon monoxide molecules coupled asymmetrically, which potentially enhances the catalyst's CO2RR product selectivity toward C2 products. Our results offer understandings of the fundamental chemical states and insights to the establishment of selective CO2RR.

The Role of in Situ Generated Morphological Motifs and Cu(i) Species in C2+ Product Selectivity During Co2 Pulsed Electroreduction

Abstract: The efficient electrochemical conversion of CO2 provides a route to fuels and feedstocks. Cu catalysts are well-known to be selective to multicarbon products although the role played by the surface architecture and the presence of oxides is not fully understood. Here, we report improved efficiency towards ethanol by tuning the morphology and oxidation state of the Cu catalysts via pulsed CO2 electrolysis. We establish a correlation between the enhanced production of C2+ products (76 % ethylene, ethanol and n-propanol at -1.0 V vs RHE) and the presence of (100) terraces, Cu2O, and defects on Cu(100). We monitored the evolution of the catalyst morphology by analysis of cyclic voltammetry curves and ex situ atomic force microscopy data, while the chemical state of the surface was examined via quasi in situ X-ray photoelectron spectroscopy. We show that the continuous (re-)generation of defects and Cu(I) species synergistically favors the C-C coupling pathways.

Effect of the Presence of Carbon in Ti4O7 Electrodes on Anodic Oxidation of Contaminants.

Abstract: Magneli phase titanium suboxide, Ti4O7, has attracted increasing attention as a potential electrode material in anodic oxidation as a result of its high efficiency and (electro)chemical stability. Although carbon materials have been amended to Ti4O7 electrodes to enhance the electrochemical performance or are present as an unwanted residual during the electrode fabrication, there has been no comprehensive investigation of how these carbon materials affect the electrochemical performance of the resultant Ti4O7 electrodes. As such, we investigated the electrochemical properties of Ti4O7 electrodes impregnated with carbon materials at different contents (and chemical states). Results of this study showed that while pure Ti4O7 electrodes exhibited an extremely low rate of interfacial electron transfer, the introduction of minor amounts of carbon materials (at values as low as 0.1 wt %) significantly facilitated the electron transfer process and decreased the oxygen evolution reaction potential. The oxygen-containing functional groups have been shown to play an important role in interfacial electron transfer with moderate oxidation of the carbon groups aiding electron uptake at the electrode surface (and consequently organic oxidation) while the generation of carboxyl groups-a process that is likely to occur in long-term operation-increased the interfacial resistance and thus retarded the oxidation process. Results of this study provide a better understanding of the relationship between the nature of the electrode surface and anodic oxidation performance with these insights likely to facilitate improved electrode design and optimization of operation of anodic oxidation reactors.

High-performance formaldehyde sensing realized by alkaline-earth metals doped In2O3 nanotubes with optimized surface properties

Abstract: Although many formaldehyde sensing materials have been developed, it is still a great challenge to get sensing materials with good selectivity. To achieve better sensing selectivity, tuning the chemical state and the amount of oxygen absorbed on the surface of semiconductor metal oxides are effective strategies. In this work, alkaline-earth elements doped In2O3 nanotubes are prepared by an electrospinning method. According to the XPS results, work function of the material and the chemical adsorption analysis, it can be concluded that the Fermi level, the basicity of the material, the activity and the amount of oxygen chemisorbed on In2O3 can be tuned by the additional alkaline-earth elements and enhance the sensing selectivity. The sensor based on 5% Ca-In2O3 shows the highest response of 116 to 100 ppm formaldehyde which is about 4.5 and 10 times higher than those of ethanol and acetone at 100 ppm and becomes the most selective among those products.

Unravelling the Role of Structural Geometry and Chemical State of Well-Defined Oxygen Vacancies on Pristine CeO2 for H2O2 Activation.

Abstract: Although H2O2 has been often employed as a green oxidant for many CeO2-catalyzed reactions, the underlying principle of its activation by surface oxygen vacancy (Vo) is still elusive due to the irreversible removal of postgenerated Vo by water (or H2O2). The metastable Vo (ms-Vo) naturally preserved on pristine CeO2 surfaces was adopted herein for an in-depth study of their interplay with H2O2. Their well-defined local structures and chemical states were found facet-dependent affecting both the adsorption and subsequent activation of H2O2. It is concluded that a strong adsorption of H2O2 on ms-Vo may not guarantee its subsequent activation. The ms-Vo can be only free for the next catalytic cycle when the electron density of surface Ce is high enough to reduce/break the O-O bond of adsorbed H2O2. This explains the 211.8 and 35.8 times enhancement in H2O2 reactivity when the CeO2 surface is changed from (111) and (110) to (100).

Concepts for chemical state analysis at constant probing depth by lab‐based XPS/HAXPES combining soft and hard X‐ray sources

Abstract: Funding information Swiss National Science Foundation, Grant/ Award Number: 206021_182987 The greater information depth provided in hard X-ray photoelectron spectroscopy (HAXPES) enables nondestructive analyses of the chemistry and electronic structure of buried interfaces. Moreover, for industrially relevant elements like Al, Si, and Ti, the combined access to the Al 1s, Si 1s, or Ti 1s photoelectron line and its associated Al KLL, Si KLL, or Ti KLL Auger transition, as required for local chemical state analysis on the basis of the Auger parameter, is only possible with hard X-rays. Until now, such photoemission studies were only possible at synchrotron facilities. Recently, however, the first commercial XPS/HAXPES systems, equipped with both soft and hard X-ray sources, have entered the market, providing unique opportunities for monitoring the local chemical state of all constituent ions in functional oxides at different probing depths, in a routine laboratory environment. Bulk-sensitive shallow core levels can be excited using either the hard or soft X-ray source, whereas more surface-sensitive deep core-level photoelectron lines and associated Auger transitions can be measured using the hard X-ray source. As demonstrated for thin Al2O3, SiO2, and TiO2 films, the local chemical state of the constituting ions in the oxide may even be probed at near-constant probing depth by careful selection of sets of photoelectron and Auger lines, as excited with the combined soft and hard X-ray sources. We highlight the potential of lab-based HAXPES for the research on functional oxides and also discuss relevant technical details regarding the calibration of the kinetic binding energy scale.

Tuning the Chemical State of Silver on Ag─Mn Catalysts to Enhance the Ozone Decomposition Performance

Abstract: Ag–Mn catalysts with excellent water resistance and ozone decomposition activity were successfully synthesized by simple precipitation and impregnation methods. Under a relative humidity of 65% and...

The matrix effect in TOF-SIMS analysis of two-element inorganic thin films

Abstract: The matrix effect, i.e. the dependence of element ion yield on the surrounding chemical state, is very often considered as a negative and limiting factor in elemental characterization. In fact, it is the main reason making Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) a non-quantitative technique as element ionization efficiency can span over several orders of magnitude depending on the matrix. Despite that, even small chemical variations of an experimental setup can cause interpretation of TOF-SIMS depth profiles a challenging task. However, the sensitivity of element ionization to the neighboring atoms can also be very beneficial as ion yields can be enhanced in the presence of particular species such as oxygen, cesium, water and fluorine. In this work, we make an attempt to estimate the matrix effect in two-element Zr-containing alloys using TOF-SIMS. The Zr ionization efficiency as well as its response to the surface and interface contaminants was investigated depending on Al, Si and Cu matrices. It was observed that Zr ionization efficiency is over four times higher in the Si matrix than in the Cu matrix and over two times higher when compared to the results obtained in the Al matrix.

Electronic Structure of Nitrogen- and Phosphorus-Doped Graphenes Grown by Chemical Vapor Deposition Method

Abstract: Heteroatom doping is a widely used method for the modification of the electronic and chemical properties of graphene. A low-pressure chemical vapor deposition technique (CVD) is used here to grow pure, nitrogen-doped and phosphorous-doped few-layer graphene films from methane, acetonitrile and methane-phosphine mixture, respectively. The electronic structure of the films transferred onto SiO2/Si wafers by wet etching of copper substrates is studied by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy using a synchrotron radiation source. Annealing in an ultra-high vacuum at ca. 773 K allows for the removal of impurities formed on the surface of films during the synthesis and transfer procedure and changes the chemical state of nitrogen in nitrogen-doped graphene. Core level XPS spectra detect a low n-type doping of graphene film when nitrogen or phosphorous atoms are incorporated in the lattice. The electrical sheet resistance increases in the order: graphene < P-graphene < N-graphene. This tendency is related to the density of defects evaluated from the ratio of intensities of Raman peaks, valence band XPS and NEXAFS spectroscopy data.

Development of a reactor for the in situ monitoring of 2D materials growth on liquid metal catalysts, using synchrotron x-ray scattering, Raman spectroscopy, and optical microscopy.

Abstract: Liquid metal catalysts (LMCats) (e.g., molten copper) can provide a new mass-production method for two-dimensional materials (2DMs) (e.g., graphene) with significantly higher quality and speed and lower energy and material consumption. To reach such technological excellence, the physicochemical properties of LMCats and the growth mechanisms of 2DMs on LMCats should be investigated. Here, we report the development of a chemical vapor deposition (CVD) reactor which allows the investigation of ongoing chemical reactions on the surface of a molten metal at elevated temperatures and under reactive conditions. The surface of the molten metal is monitored simultaneously using synchrotron x-ray scattering, Raman spectroscopy, and optical microscopy, thereby providing complementary information about the atomic structure and chemical state of the surface. To enable in situ characterization on a molten substrate at high temperatures (e.g., ∼1370 K for copper), the optical and x-ray windows need to be protected from the evaporating LMCat, reaction products, and intense heat. This has been achieved by creating specific gas-flow patterns inside the reactor. The optimized design of the reactor has been achieved using multiphysics COMSOL simulations, which take into account the heat transfer, fluid dynamics, and transport of LMCat vapor inside the reactor. The setup has been successfully tested and is currently used to investigate the CVD growth of graphene on the surface of molten copper under pressures ranging from medium vacuum up to atmospheric pressure.

Investigations on microstructure, optical, magnetic, photocatalytic, and dielectric behaviours of pure and Co-doped ZnO NPs

Abstract: In this research, an effective usage of quantum size effect is a foregrounded strategy to produce and employ excellent visible light-driven photocatalytic materials. Zn100−xCoxO (x = 0, 2.5, 7.5) particles at nanoscale were synthesized successfully by chemical co-precipitation method and then investigated by some complimentary analytical techniques. Physical structure and chemical states were analysed by X-ray diffraction and X-ray photoelectron spectroscopy. XRD and HRTEM analysis confirmed the formation of wurtzite-type structure without any traces of secondary phase. XPS spectra exhibited the incorporation of Co2+ ions into ZnO lattice. Optical spectra of Zn100−xCoxO nanoparticles (NPs) indicated that bandgap is significantly narrowed with increasing Co concentration. Ferromagnetic behaviour was observed at room temperature in pure and Co-doped ZnO, due to the effect of quantum confinement and of Co2+ ions incorporation. The frequency and composition dependence of dielectric constant and dielectric loss have been analysed in detail using Maxwell–Wagner model and Cole–Cole plots. The activity of synthesized photocatalysts under simulated sunlight irradiation was characterized through decomposition of methylene blue (MB) dye in aqueous suspension, which revealed that Co2+ ions incorporation in ZnO greatly enhanced the photocatalytic degradation efficiency and hence employable in environmental cleaning.

Molecular Beam Epitaxy of Two-Dimensional Vanadium-Molybdenum Diselenide Alloys

Abstract: Two-dimensional (2D) alloys represent a versatile platform that extends the properties of atomically thin transition-metal dichalcogenides. Here, using molecular beam epitaxy, we investigate the growth of 2D vanadium-molybdenum diselenide alloys, VxMo1-xSe2, on highly oriented pyrolytic graphite and unveil their structural, chemical, and electronic integrities via measurements by scanning tunneling microscopy/spectroscopy, synchrotron X-ray photoemission, and X-ray absorption spectroscopy (XAS). Essentially, we found a critical value of x = ∼0.44, below which phase separation occurs and above which a homogeneous metallic phase is favored. Another observation is an effective increase in the density of mirror twin boundaries of constituting MoSe2 in the low V concentration regime (x ≤ 0.05). Density functional theory calculations support our experimental results on the thermal stability of 2D VxMo1-xSe2 alloys and suggest an H phase of the homogeneous alloys with alternating parallel V and Mo strips randomly in-plane stacked. Element-specific XAS of the 2D alloys, which clearly indicates quenched atomic multiplets similar to the case of 2H-VSe2, provides strong evidence for the H phase of the 2D alloys. This work provides a comprehensive understanding of the thermal stability, chemical state, and electronic structure of 2D VxMo1-xSe2 alloys, useful for the future design of 2D electronic devices.

X-ray photoelectron spectroscopy study of chromium and magnesium doped copper ferrite thin film.

Abstract: X-ray photoelectron spectroscopy (XPS) is a surface-sensitive quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements. Moreover, XPS is also widely used in semiconductor applications as the main characterization technique. In this work, chromium and magnesium doped copper ferrite thin films were prepared on the glass substrates by spin-coating technique at room temperature. The stoichiometry 1:2 ratio was used for copper (II) acetate monohydrate and iron (III) nitrate nonahydrate with different doping percentages from 5%, 10%, 15% to 20% of chromium and magnesium. After spin-coated, the films were annealed at 120°C, 200°C, 300°C and 400°C in air. The surface properties of the post-annealed thin films can be characterized by XPS technique due to their surface sensitivities. Therefore, the surface properties of different doped amounts of chromium and magnesium thin films were systematically investigated for different annealing temperatures. For varying anneal temperatures, the oxidation numbers of copper and iron ions were different. At low temperature, the oxidation number of copper was Cu+ and iron was a mixture of Fe2+ and Fe3+ while at high-temperature, the copper and iron ions showed Cu2+ and iron Fe3+ characteristics, respectively. After etching, the oxidation number of copper and iron ions at the surface shifted to lower oxidation state.

In situ unraveling of the effect of the dynamic chemical state on selective CO2 reduction upon zinc electrocatalysts

Abstract: Unraveling the reaction mechanism behind the CO2 reduction reaction (CO2RR) is a crucial step for advancing the development of efficient and selective electrocatalysts to yield valuable chemicals. To understand the mechanism of zinc electrocatalysts toward the CO2RR, a series of thermally oxidized zinc foils is prepared to achieve a direct correlation between the chemical state of the electrocatalyst and product selectivity. The evidence provided by in situ Raman spectroscopy, X-ray absorption spectroscopy (XAS) and X-ray diffraction significantly demonstrates that the Zn(II) and Zn(0) species on the surface are responsible for the production of carbon monoxide (CO) and formate, respectively. Specifically, the destruction of a dense oxide layer on the surface of zinc foil through a thermal oxidation process results in a 4-fold improvement of faradaic efficiency (FE) of formate toward the CO2RR. The results from in situ measurements reveal that the chemical state of zinc electrocatalysts could dominate the product profile for the CO2RR, which provides a promising approach for tuning the product selectivity of zinc electrocatalysts.

Physical and Chemical Synthesis of Au/CeO2 Nanoparticle Catalysts for Room Temperature CO Oxidation: A Comparative Study

Abstract: In many heterogeneous catalytic reactions, such as low-temperature CO oxidation, the preparation conditions, and the role of the CeO2 support (oxygen vacancies and redox properties) in the dispersion and the chemical state of Au, are considered critical factors for obtaining gold nanoparticle catalysts with high catalytic performance. In this work, the physical and chemical preparation methods were compared, aiming at understanding how the preparation method influences the catalytic activity. The Au/CeO2 nanoparticle catalysts with 5% Au loading were prepared via the Physical Laser Vaporization Controlled Condensation method (LVCC), and the chemical Deposition-Precipitation method (DP) was used to investigate the effect of synthesis methods on the structure and the catalytic activity toward the CO oxidation. In this manuscript, we compare the activity of nanostructured Au/CeO2 catalysts. The structure and the redox properties of the catalysts were investigated by the XRD, SEM, TEM, TPR, and XPS. The catalytic activity for low-temperature CO oxidation was studied using a custom-built quartz tube flow reactor coupled with an infrared detector system at atmospheric pressure. The study reveals that the prepared CeO2-supported Au nanoparticles’ catalytic activity was highly dependent on the preparation methods. It showed that the sample prepared by the DP method exhibits higher catalytic efficiency toward CO oxidation when compared with the sample prepared by the LVCC method. The high catalytic activity could be attributed to the small particle size and shape, slightly higher Au concentration at the surface, surface-active Au species such as Au1+, along with the large interface between Au and CeO2. This study suggests that the stability, dispersion of Au nanoparticles on CeO2, and strong interaction between Au and CeO2 lead to strong oxidation ability even below room temperature. Considering the universal character of different physical and chemical methods for Au/CeO2 preparation, this study may also provide a base for supported Au-based catalysts for many oxidation reactions in energy and environmental applications.

Glass surface evolution following gas adsorption and particle deposition from indoor cooking events as probed by microspectroscopic analysis

Abstract: Indoor surfaces are extremely diverse and their interactions with airborne compounds and aerosols influence the lifetime and reactivity of indoor emissions Direct measurements of the physical and chemical state of these surfaces provide insights into the underlying physical and chemical processes involving surface adsorption, surface partitioning and particle deposition Window glass, a ubiquitous indoor surface, was placed vertically during indoor activities throughout the House Observations of Microbial and Environmental Chemistry (HOMEChem) campaign and then analyzed to measure changes in surface morphology and surface composition Atomic force microscopy-infrared (AFM-IR) spectroscopic analyses reveal that deposition of submicron particles from cooking events is a contributor to modifying the chemical and physical state of glass surfaces These results demonstrate that the deposition of glass surfaces can be an important sink for organic rich particles material indoors These findings also show that particle deposition contributes enough organic matter from a single day of exposure equivalent to a uniform film up to two nanometers in thickness, and that the chemical distinctness of different indoor activities is reflective of the chemical and morphological changes seen in these indoor surfaces Comparison of the experimental results to physical deposition models shows variable agreement, suggesting that processes not captured in physical deposition models may play a role in the sticking of particles on indoor surfaces

Nitrogen-Terminated Polycrystalline Diamond Surfaces by Microwave Chemical Vapor Deposition: Thermal Stability, Chemical States, and Electronic Structure

Abstract: In this work, we investigate the interaction of microwave (MW) N2 plasma with polycrystalline diamond surfaces. Optical emission spectroscopy (OES) was used to identify different species present in...

Structure and Chemical State of Cesium on Well-Defined Cu(111) and Cu2O/Cu(111) Surfaces

Abstract: The deposition of cesium (Cs) onto the metallic and oxidized surfaces of Cu(111) was investigated using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density func...

XPS of the surface chemical environment of CsMAFAPbBrI trication-mixed halide perovskite film

Abstract: Perovskite-based materials have been considered as the most promising materials in several solar-driven processes, especially photovoltaics. Some features such as high sunlight harvesting and improved carrier transport have been highlighted to have an impact on the efficiency of the above topics, but limited studies have pointed out their surface composition, which mainly influence the above abilities. As a starting point to recognize the surface environment and chemical states of these materials, x-ray photoelectron spectroscopy measurements were performed. Trication-mixed halide perovskite films based on Cs0.08MA0.18FA0.78PbBr0.42I2.58 were prepared by a one step process and deposited on an indium tin oxide substrate by spin-coating. Survey spectra, Pb 4f, I 3d, Br 3d, Cs 3d, C 1s, N 1s, Pb 4d, and Pb 5d core levels, and valence band spectra were measured for the semiconductor sample. Results exhibit symmetrical peaks, indicating that a homogenous solid solution was achieved. Main chemical states of this kind of perovskites such as Cs-Br and Pb-I species are also shown. We deduced that the later signals are associated with the [PbI6]4− octahedra existing in the perovskite lattice, while the former one is related to Cs+ bounded to [PbBr6]4− units on the perovskite surface. Interestingly, the NR4+ signal was also identified, associated with the presence of methylammonium and formamidinium cations.