Ambient Pressure X-ray Photoemission Spectroscopy Research Articles

Hydrogenation of graphene on Ni(111) by H2 under near ambient pressure conditions

Graphene hydrogenation on Ni(111) in the mbar region has been investigated by combining Near Ambient Pressure X-ray Photoemission Spectroscopy and Density Functional Theory calculations. A complete and nearly perfect graphene single layer and an incomplete defective monolayer were exposed to a methanation mixture (H2 + CO and H2 + CO2, respectively) with a large excess of H2. The observed behavior is quite different for the two systems. In the former case, the C 1s spectrum of the initially strongly interacting graphene shows the appearance of new components corresponding to hydrogenated C atoms and their first nearest neighbors. The defective carbon layer, grown in the presence of sulfur contamination and consisting of a mixture of strongly and weakly interacting graphene with a significant amount of dissolved C, initially shows the same C species and then evolves with the formation of sp3 carbon and patches of Ni oxide/hydroxide. NiO forms by CO2 dissociation, while sp3 carbon is indicative of a rumpling of the graphene adlayer induced by hydrogenation. Both species disappear when annealing in H2. Dissociation of H2 and graphene hydrogenation is possible thanks to the presence of the Ni substrate which makes the reaction weakly exothermic and significantly reduces the barrier for H2 dissociation.

Direct Work Function Measurement Using in-situ Ambient Pressure X-ray Photoemission Spectroscopy and Its Application on Copper Oxidation Process.

The work function (WF) measurement plays a critical role in engineering energy materials and energy devices. However, the ultra-high vacuum (UHV) environments of photoemission method limit the practical application for absolute work function measurements of materials, especially under complex working conditions. To understand the energy level of materials under complex chemical environments, the in-situ measurements of work function is necessary in complex metal/semiconductor system for various application. In this paper, we describe the utilization of ambient pressure X-ray photoemission spectroscopy (APXPS) with utilization of low photon energy X-ray for absolute WF measurements at BL02B of the Shanghai Synchrotron Radiation Facility. We herein present the WF measurement during oxygen adsorption on Pt(111) and oxidation of Cu(111) in ambient oxygen environment as demonstration of the APXPS capability for WF measurement. After oxygen chemisorption on Pt and formation of Cu2O, the WF will increase. This is due to charge transfer from metal to chemisorbed oxygen atoms. After the formation of bulk Cu2O and CuO, the WF value almost remain at ~5.5 eV. We believe the direct measurement of absolute work function via APXPS could help bridge the gap between the physical properties and the surface chemical species for metal/semiconductor materials.

The dehydration mechanism of Na and K birnessites: a comprehensive multitechnique study.

The structural, spectroscopic and electronic properties of Na and K birnessites were investigated from ambient conditions (birA) to complete dehydration, and the involved mechanisms were scrutinized. Density Functional Theory (DFT) simulations were employed to derive structural models for lamellar A0.33MnO2·xH2O (A = Na+ or K+, x = 0 or 0.66), subsequently compared with the experimental results obtained for Na0.30MnO2·0.75H2O and K0.22MnO2·0.77H2O materials. Thermal analysis (TGA-DSC), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, and Near Ambient Pressure X-ray Photoemission Spectroscopy (NAP-XPS) measurements were conducted for both birnessites. Dehydration under vacuum, annealing, or controlled relative humidity were considered. Results indicated that complete birnessite dehydration was a two-stage process. In the first stage, water removal from the interlayer of fully hydrated birnessite (birA) down to a molar H2O/A ratio of ∼2 (birB) led to the progressive shrinkage of the interlayer distance (3% for Na birnessite, 1% for K birnessite). In the second stage, water-free (birC) domains with a shorter interlayer distance (20% for Na birnessite, 10% for K birnessite) appeared and coexisted with birB domains. Then, birB was essentially transformed into birC when complete dehydration was achieved. The vibrational properties of birA were consistent with strong intermolecular interactions among water molecules, whereas partially dehydrated birnessite (birB) showed a distinct feature, with 3 (for Na-bir) and 2 (for K-bir) vibrations that were reproduced by DFT calculations for organized water into the interlayer (x = 0.66). The study also demonstrated that the electronic structure of Na birnessite depends on the interlayer water content. The external Na+ electronic level (Na 2p) was slightly destabilized (+0.3 eV binding energy) under near ambient conditions (birA) compared to drier conditions (birB and birC).

Engineering lanthanum into Pt doped CeO2 for Intermediate temperature solid oxide fuel cells

Platinum doped oxides with extremely low utilization of noble metal and excellent catalytic activity have gained attention in high-temperature electrochemical cells. However, platinum ions tend to be reduced in reducing atmospheres, resulting in the catalyst deactivation. In this work, lanthanum is introduced into Pt-CeO2, which aims to deal with the above problem and provides an approach to promote Pt-CeO2 based catalysts for intermediate temperature solid oxide fuel cells. In specific, lanthanum doped Pt-CeO2 catalysts are prepared and infiltrated into the anode. The results show the highest peak power density with 10 mol% lanthanum doped Pt-CeO2 among all samples. In-situ ambient pressure X-ray photoemission spectroscopy investigation reveals that the reducibility of CeO2 can be tailored by lanthanum dopants, which leads to exceptional stability of active Pt2+ in reducing atmospheres. Our results foster the development of more stable and inexpensive Pt-CeO2 for intermediate temperature solid oxide fuel cells.

In situ observation of an atomically modified glassy carbon surface: From synthesis of an Fe-N-C catalyst to intrinsic combined reactions of hydrogen dissociation and the oxygen reduction reaction

We carried out an in situ ambient-pressure X-ray photoemission spectroscopy (AP-XPS) study on the catalytically active surface of glassy carbon subjected to nitrogen-ion implantation and subsequent iron deposition. We also used argon ions to intentionally induce carbon defects on the glassy carbon to compare the effects of substrate defects. Notably, water was produced spontaneously on the modified glassy carbon surface. We compared and clarified the surface structures of the samples in situ by AP-XPS and observed some differences among the pristine glassy carbon (PGC), Fe and N co-doped PGC, Ar-sputtered glassy carbon (AGC), and Fe and N co-doped AGC samples. These differences reveal the potential active site of oxygen reduction reaction and the inherent source for spontaneous dissociation of hydrogen molecules on the atomically modified surface of glassy carbon. The analysis illuminates the new property of glassy carbon itself and offer new direction of Fe-N-C catalyst preparation. • Spontaneous dissociative behavior of hydrogen molecules is possible on Ar sputtered glassy carbon. • Glassy carbon with N 2 sputtering and Fe thermal evaporation have catalytic ability for oxygen reduction. • Using AP-XPS, glassy carbon with treatments is observed in situ.

Boudouard reaction under graphene cover on Ni(1 1 1)

We investigated the interaction of CO with graphene/Ni(111) and the Boudouard reaction at 3.7 mbar by Near Ambient Pressure X-Ray Photoemission Spectroscopy (NAPXPS), i.e. at one order of magnitude higher pressure than previously explored in-operando conditions. In this regime, CO intercalates under the graphene layer causing its partial detachment from the Ni substrate. The so-obtained high local CO coverage opens the way to CO2 formation via the Boudouard reaction. Its onset is witnessed by observing physisorbed CO2 accumulating below the graphene cover. The so-generated additional carbon atoms transform carbide into graphene, causing the expansion of the graphene islands. In addition, CO adsorption occurs on the strongly interacting areas of the graphene layer, confirming previous results obtained by some of us at low temperatures and in ultra-high vacuum conditions.

Oxidation of a Platinum–Tin Alloy Surface during Catalytic CO Oxidation

We have investigated the surface composition of a well-ordered Pt3Sn(111) surface during CO oxidation with ambient pressure X-ray photoemission spectroscopy. Oxidation of tin in the surface coincides with the onset of catalytic conversion, and we observe significant differences in the oxidation state and morphology of the oxide formed depending on the gas composition, with an oxygen-rich mixture leading to formation of 2D wetting layers and a CO-rich mixture leading to formation of 3D oxide islands. Spontaneous oscillations in conversion are observed at 300 °C in the oxygen-rich gas mixture and attributed to the combined effects of site blocking by tin oxides and by CO. The results highlight the importance of gas–surface interactions in determining the nature of oxides formed and thus the type and number of interfacial sites under reaction conditions.

Understanding the Surface Structure and Catalytic Activity of SnOx/Au(111) Inverse Catalysts for CO2 and H2 Activation

Carbon dioxide hydrogenation is a promising approach for the reduction of greenhouse gas pollution via the production of fuels and high-value chemicals utilizing C1 chemistry. In this process, the activation of nonpolar molecules, CO2 and H2, at mild conditions is challenging. Herein, we report a well-defined inverse SnOx/Au(111) catalyst that shows the ability to activate both CO2 and H2 at room temperature. Scanning tunneling microscopy (STM) and ambient pressure X-ray photoemission spectroscopy (AP-XPS) are combined to understand the surface structure, growth mode, chemical state, and activity of SnOx/Au(111) surfaces. Nanostructures of SnOx at the sub-monolayer level were prepared by depositing Sn on Au(111) followed by O2 oxidation. For the as-prepared SnOx/Au(111), two-dimensionally formed SnOx thin films on a Au(111) substrate were observed with STM of two different moieties, discernible based on their height: clusters (∼0.4 Å) and nanoparticles (NPs, 1–2.5 Å), which are assigned to Sn–Au alloys and SnOx, respectively, in corroboration with XPS analysis. Furthermore, SnOx/Au(111) was annealed under UHV to test its thermal stability. Upon annealing at 400–600 K, a disappearance of SnOx NPs and reappearance of highly dispersed Sn clusters were clearly noticeable from the STM and XPS results, identifying the thermal decomposition of SnOx and subsequent formation of Sn–Au alloys on the surface due to the recombination of Sn clusters with Au. We investigated the reactivity of the SnOx/Au(111) surfaces toward CH4, CO2, and H2. The SnOx/Au(111) surfaces have excellent CO2 and H2 activation abilities even at room temperature with negligible reactivity for methane activation. Our AP-XPS results show that H2 can be activated on the SnOx NPs by the reduction to Sn. For CO2, the activation and further dissociation are identified by a reoxidation of Sn with newly formed Sn–O bonds and the formation of surface carbon. Therefore, we propose that SnOx is a potential catalyst or additive to achieve CO2 hydrogenation under mild conditions.

Selective Methane Oxidation to Methanol on ZnO/Cu2O/Cu(111) Catalysts: Multiple Site-Dependent Behaviors.

Because of the abundance of natural gas in our planet, a major goal is to achieve a direct methane-to-methanol conversion at medium to low temperatures using mixtures of methane and oxygen. Here, we report an efficient catalyst, ZnO/Cu2O/Cu(111), for this process investigated using a combination of reactor testing, scanning tunneling microscopy, ambient-pressure X-ray photoemission spectroscopy, density functional calculations, and kinetic Monte Carlo simulations. The catalyst is capable of methane activation at room temperature and transforms mixtures of methane and oxygen to methanol at 450 K with a selectivity of ∼30%. This performance is not seen for other heterogeneous catalysts which usually require the addition of water to enable a significant conversion of methane to methanol. The unique coarse structure of the ZnO islands supported on a Cu2O/Cu(111) substrate provides a collection of multiple centers that display different catalytic activity during the reaction. ZnO-Cu2O step sites are active centers for methanol synthesis when exposed to CH4 and O2 due to an effective O-O bond dissociation, which enables a methane-to-methanol conversion with a reasonable selectivity. Upon addition of water, the defected O-rich ZnO sites, introduced by Zn vacancies, show superior behavior toward methane conversion and enhance the overall methanol selectivity to over 80%. Thus, in this case, the surface sites involved in a direct CH4 → CH3OH conversion are different from those engaged in methanol formation without water. The identification of the site-dependent behavior of ZnO/Cu2O/Cu(111) opens a design strategy for guiding efficient methane reformation with high methanol selectivity.

Inelastic electron scattering by the gas phase in near ambient pressure XPS measurements

X‐ray photoemission spectroscopy (XPS) measurements in near‐ambient pressure (NAP) conditions result in a signal loss of the primary spectrum as a result of inelastic scattering of photoelectrons in the gas phase. The inelastic scattering of the primary electrons gives rise to a secondary signal that can result in additional and often unwanted features in the measured spectrum. In the present work, we derive equations that can be used to model the resulting signal and provide equations that can be used to simulate or remove the inelastic scattering signal from measured spectra. We demonstrate this process for photoemission spectra of a wide range of kinetic energies, measured from Au, Ag, and Cu, in a variety of gases (N2, He, H2, and O2). The work is supplemented with an open‐source software in which the algorithms described here have been implemented and can be used to remove the gas phase inelastic scattering signal.

Atomic Layer Deposition of Hafnium Oxide on InAs: Insight from Time-Resolved in Situ Studies

III-V semiconductors, such as InAs, with an ultrathin high-κ oxide layer have attracted a lot of interests in recent years as potential next-generation metal-oxide-semiconductor field-effect transistors, with increased speed and reduced power consumption. The deposition of the high-κ oxides is nowadays based on atomic layer deposition (ALD), which guarantees atomic precision and control over the dimensions. However, the chemistry and the reaction mechanism involved are still partially unknown. This study reports a detailed time-resolved analysis of the ALD of high-κ hafnium oxide (HfOx) on InAs(100). We use ambient pressure X-ray photoemission spectroscopy and monitor the surface chemistry during the first ALD half-cycle, i.e., during the deposition of the metalorganic precursor. The removal of In and As native oxides, the adsorption of the Hf-containing precursor molecule, and the formation of HfOx are investigated simultaneously and quantitatively. In particular, we find that the generally used ligand exchange model has to be extended to a two-step model to properly describe the first half-cycle in ALD, which is crucial for the whole process. The observed reactions lead to a complete removal of the native oxide and the formation of a full monolayer of HfOx already during the first ALD half-cycle, with an interface consisting of In-O bonds. We demonstrate that a sufficiently long duration of the first half-cycle is essential for obtaining a high-quality InAs/HfO2 interface. (Less)

Ammonia Oxidation over a Pt25Rh75(001) Model Catalyst Surface: An Operando Study

The reaction of ammonia oxidation over PtRh binary alloy has been studied with a surface science approach by operando techniques such as Near Ambient Pressure X-Ray Photoemission Spectroscopy (NAP-...

Influence of the ZrO2 Crystalline Phases on the Nature of Active Sites in PdCu/ZrO2 Catalysts for the Methanol Steam Reforming Reaction—An In Situ Spectroscopic Study

In this work, the electronic properties of the metal sites in cubic and monoclinic ZrO2 supported Pd and PdCu catalysts have been investigated using CO as probe molecule in in-situ IR studies, and the surface composition of the outermost layers has been studied by APXPS (Ambient Pressure X-ray Photoemission Spectroscopy). The reaction products were followed by mass spectrometry, making it possible to relate the chemical properties of the catalysts under reaction conditions with their selectivity. Combining these techniques, it has been shown that the structure of the support (monoclinic or cubic ZrO2) affects the metal dispersion, mobility, and reorganization of metal sites under methanol steam reforming (MSR) conditions, influencing the oxidation state of surface metal species, with important consequences in the catalytic activity. Correlating the mass spectra of the reaction products with these spectroscopic studies, it was possible to conclude that electropositive metal species play an imperative role for high CO2 and H2 selectivity in the MSR reaction (less CO formation).

Operando study of Pd(100) surface during CO oxidation using ambient pressure x-ray photoemission spectroscopy

The surface chemical states of Pd(100) during CO oxidation were investigated using ambient pressure x-ray photoelectron spectroscopy and mass spectroscopy. Under the reactant ratio of CO/O2 = 0.1, i.e. an oxygen-rich reaction condition, the formation of surface oxides was observed with the onset of CO oxidation reaction at T = 525 K. As the reactant ratio (CO/O2) increased from 0.1 to 1.0, ∼ 90 % surface oxides remains on surface during the reaction. Upon the formation of surface oxides, the core level shift of oxygen gas phase peak was observed, indicating that change of surface work function. As CO oxidation takes places, i.e. making a transition from CO covered surface to the oxidic surface, the work functions of surface oxide on Pd(100) and Pt(110) display opposite behavior.

Notable Reactivity of Acetonitrile Towards Li2O2/LiO2 Probed by NAP XPS During Li–O2 Battery Discharge

One of the key factors responsible for the poor cycleability of Li–O2 batteries is a formation of byproducts from irreversible reactions between electrolyte and discharge product Li2O2 and/or intermediate LiO2. Among many solvents that are used as electrolyte component for Li–O2 batteries, acetonitrile (MeCN) is believed to be relatively stable towards oxidation. Using near ambient pressure X-ray photoemission spectroscopy (NAP XPS), we characterized the reactivity of MeCN in the Li–O2 battery. For this purpose, we designed the model electrochemical cell assembled with solid electrolyte. We discharged it first in O2 and then exposed to MeCN vapor. Further, the discharge was carried out in O2 + MeCN mixture. We have demonstrated that being in contact with Li–O2 discharge products, MeCN oxidizes. This yields species that are weakly bonded to the surface and can be easily desorbed. That’s why they cannot be detected by the conventional XPS. Our results suggest that the respective chemical process most probably does not give rise to electrode passivation but can decrease considerably the Coulombic efficiency of the battery.

Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces: Metal-Support Interactions and Water-Enabled Catalytic Conversion by Site Blocking.

The transformation of methane into methanol or higher alcohols at moderate temperature and pressure conditions is of great environmental interest and remains a challenge despite many efforts. Extended surfaces of metallic nickel are inactive for a direct CH4 → CH3OH conversion. This experimental and computational study provides clear evidence that low Ni loadings on a CeO2(111) support can perform a direct catalytic cycle for the generation of methanol at low temperature using oxygen and water as reactants, with a higher selectivity than ever reported for ceria-based catalysts. On the basis of ambient pressure X-ray photoemission spectroscopy and density functional theory calculations, we demonstrate that water plays a crucial role in blocking catalyst sites where methyl species could fully decompose, an essential factor for diminishing the production of CO and CO2, and in generating sites on which methoxy species and ultimately methanol can form. In addition to water-site blocking, one needs the effects of metal-support interactions to bind and activate methane and water. These findings should be considered when designing metal/oxide catalysts for converting methane to value-added chemicals and fuels.

Chemical states of surface oxygen during CO oxidation on Pt(1 1 0) surface revealed by ambient pressure XPS

The study of CO oxidation on Pt(1 1 0) surface is revisited using ambient pressure x-ray photoemission spectroscopy. When the surface temperature reaches the activation temperature for CO oxidation under elevated pressure conditions, both the α-phase of PtO2 oxide and chemisorbed oxygen are formed simultaneously on the surface. Due to the exothermic nature of CO oxidation, the temperature of the Pt surface increases as CO oxidation takes place. As the CO/O2 ratio increases, the production of CO2 increases continuously and the surface temperature also increases. Interestingly, within the diffusion limited regions, the amount of surface oxide changes little while the chemisorbed oxygen is reduced.

Correlation between active layer thickness and ambient gas stability in IGZO thin-film transistors

Decreasing the active layer thickness has been recently reported as an alternative way to achieve fully depleted oxide thin-film transistors for the realization of low-voltage operations. However, the correlation between the active layer thickness and device resistivity to environmental changes is still unclear, which is important for the optimized design of oxide thin-film transistors. In this work, the ambient gas stability of IGZO thin-film transistors is found to be strongly correlated to the IGZO thickness. The TFT with the thinnest IGZO layer shows the highest intrinsic electron mobility in a vacuum, which is greatly reduced after exposure to O2/air. The device with a thick IGZO layer shows similar electron mobility in O2/air, whereas the mobility variation measured in the vacuum is absent. The thickness dependent ambient gas stability is attributed to a high-mobility region in the IGZO surface vicinity with less sputtering-induced damage, which will become electron depleted in O2/air due to the electron transfer to adsorbed gas molecules. The O2 adsorption and deduced IGZO surface band bending is demonstrated by the ambient-pressure x-ray photoemission spectroscopy results.

Observation of in situ oxidation dynamics of vanadium thin film with ambient pressure X-ray photoemission spectroscopy

The evolution of oxidation/reduction states of vanadium oxide thin film was monitored in situ as a function of oxygen pressure and temperature via ambient pressure X-ray photoemission spectroscopy. Spectra analysis showed that VO2 can be grown at a relatively low temperature, T ∼ 523 K, and that V2O5 oxide develops rapidly at elevated oxygen pressure. Raman spectroscopy was applied to confirm the formation of VO2 oxide inside of the film. In addition, the temperature-dependent resistivity measurement on the grown thin film, e.g., 20 nm exhibited a desirable metal-insulator transition of VO2 with a resistivity change of ∼1.5 × 103 times at 349.3 K, displaying typical characteristics of thick VO2 film, e.g., 100 nm thick. Our results not only provide important spectroscopic information for the fabrication of vanadium oxides, but also show that high quality VO2 films can be formed at relatively low temperature, which is highly critical for engineering oxide film for heat-sensitive electronic devices.

Dry Reforming of Ethane and Butane with CO2 over PtNi/CeO2 Bimetallic Catalysts

Dry reforming is a potential process to convert CO2 and light alkanes into syngas (H2 and CO), which can be subsequently transformed to chemicals and fuels. In this work, PtNi bimetallic catalysts have been investigated for dry reforming of ethane and butane using both model surfaces and supported powder catalysts. The PtNi bimetallic catalyst shows an improvement in both activity and stability in comparison to the corresponding monometallic catalysts. The formation of PtNi alloy and the partial reduction of Ce4+ to Ce3+ under reaction conditions are demonstrated by in situ ambient-pressure X-ray photoemission spectroscopy (AP-XPS), X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) measurements. A Pt-rich bimetallic surface is revealed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) following CO adsorption. Combined in situ experimental results and density functional theory (DFT) calculations suggest that the Pt-rich PtNi bimetallic surface structure would weaken ...