Ambient Pressure Photoemission Spectroscopy Research Articles

Solution Combustion Synthesis and Characterization of Magnesium Copper Vanadates.

Magnesium vanadate (MgV2O6) and its alloys with copper vanadate were synthesized via the solution combustion technique. Phase purity and solid solution formation were confirmed by a variety of experimental techniques, supported by electronic structure simulations based on density functional theory (DFT). Powder X-ray diffraction combined with Rietveld refinement, laser Raman spectroscopy, diffuse reflectance spectroscopy, and high-resolution transmission electron microscopy showed single-phase alloy formation despite the MgV2O6 and CuV2O6 end members exhibiting monoclinic and triclinic crystal systems, respectively. DFT-calculated optical band gaps showed close agreement in the computed optical bandgaps with experimentally derived values. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements confirmed a systematic variation in the optical bandgap modification and band alignment as a function of stoichiometry in the alloy composition. These data indicated n-type semiconductor behavior for all the samples which was confirmed by photoelectrochemical measurements.

Hybrid Cathodic/Anodic Electrosynthesis of Phase Pure Ag4V2O7 Thin Films

Here, we demonstrate a two-step electrosynthesis approach for the preparation of silver pyrovanadate, Ag4V2O7 in thin-film form. In the first, cathodic step, polycrystalline Ag was deposited on fluorine doped tin oxide (FTO) substrate from a non-aqueous bath. Aqueous pyrovanadate species were then generated by aging of a CO2-infused sodium orthovanadate (Na3VO4) solution for three weeks. Silver ions were subsequently generated in situ in this medium using anodic stripping of the Ag/ITO films from the first step. Interfacial precipitation of the Ag+ ions with the pyrovanadate species afforded the targeted product in phase pure form. The various stages of the electrosynthesis were monitored in situ via the combined use of voltammetry, electrochemical quartz crystal nanogravimetry (EQCN), and coulometry. The Ag4V2O7 thin films were characterized by a variety of experimental techniques, including X-ray diffraction, laser Raman spectroscopy, diffuse reflectance spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements afforded information on the energy band structure of the p-type Ag4V2O7 semiconductor. Finally, the electrochemical and photoelectrochemical properties of the electrosynthesized Ag4V2O7 thin films were studied in both aqueous and non-aqueous electrolytes.

Depth distribution of alkali metal ions on supported graphene in the presence of water

The interaction of alkali ions with multilayer graphene is critical in many applications, for example in energy storage devices. This requires a detailed understanding of ion interactions with carbonaceous layers. The mechanism of ion intercalation into graphene can be different from that observed for hard graphite. In this paper, we present an investigation of the vertical alkali ion (Na, K, Cs) distribution on multilayer graphene deposited onto SiO2 in vacuum and in the presence of water vapor using Standing Wave Ambient Pressure Photoemission Spectroscopy. It was found that Cs, K, and Na ions do not intercalate into multilayer graphene under vacuum conditions. The most likely reasons for this behavior are the reversibility of the process due to large inter-sheet spacing or lack of time for intercalation. When exposed to water vapor, Na ions intercalate soft carbon whereas Cs ions do not. This is a clear indication for the difference in the intercalation mechanisms on hard graphite and soft graphene.

Prospects for the expansion of standing wave ambient pressure photoemission spectroscopy to reactions at elevated temperatures

Standing wave ambient pressure photoemission spectroscopy (SWAPPS) is a promising method to investigate chemical and potential gradients across solid-vapor and solid-liquid interfaces under close-to-realistic environmental conditions, far away from high vacuum. Until now, these investigations have been performed only near room temperature, but for a wide range of interfacial processes, chief among them being heterogeneous catalysis, measurements at elevated temperatures are required. One concern in these investigations is the temperature stability of the multilayer mirrors, which generate the standing wave field. At elevated temperatures, degradation of the multilayer mirror due to, for example, interdiffusion between the adjacent layers, decreases the modulation of the standing wave field, thus rendering SWAPPS experiments much harder to perform. Here, we show that multilayer mirrors consisting of alternate B4C and W layers are stable at temperatures exceeding 600 °C and are, thus, promising candidates for future studies of surface and subsurface species in heterogeneous catalytic reactions using SWAPPS.

Electronic Work Function of Carbon Nanocomposite Films According to Vacuum and Atmospheric Photoemission

Using the methods of optically stimulated electron emission (OSEE) and ambient pressure photoemission spectroscopy (APS), the energy characteristics of carbyne-containing films on copper and silicon substrates have been studied. The average contact potential difference and the work function were determined, and the positions of the Fermi level for carbyne-containing films of various thicknesses were calculated. It was found that the electrons work function and the position of the Fermi level of the studied film samples do not depend on the film thickness and the type of substrate. The information content of the OSEE and APS methods for evaluating the quality of carbon coatings on substrates is shown.

Работа выхода электрона углеродных нанокомпозитных пленок по данным вакуумной и атмосферной фотоэмиссии

The energy characteristics of carbyne-containing films on copper and silicon substrates have been studied using the methods of optically stimulated electron emission (OSEE) and ambient pressure photoemission spectroscopy (APS). The average contact potential difference and the work function were determined, and the positions of the Fermi level for carbonaceous films of various thicknesses were calculated. It was found that the electron work function and the position of the Fermi level of the studied film samples are independent of the film thickness and the type of substrate. The information capabilities of the OSEE and APS methods for assessing the quality of carbon coatings on various substrates are shown.

Effect of Structural Properties on the Photoelectrochemical Performance of Transition Metal Dichalcogenides

Layered transition metal dichalcogenides (TMDCs) have become promising candidates for photoelectrochemical (PEC) studies from the 80’s. Both n- and p-type MoSe2, and WSe2 materials were studied in photoelectrocatalytic reactions using model redox couples and hydrogen evolution reaction. (1-2) Exfoliation of the bulk, layered crystals to single- and few-layered two-dimensional (2D) nanosheets leads to new physical phenomena, different from their bulk counterparts. (3) The electrochemical properties of 2D nanoflakes −which define the performance of these materials in energy-related applications− depends on their structural properties. These structural features include the number of layers, the basal/edge planes, and the defect density. (4-5) Therefore, to employ 2D nanosheets for energy conversion and storage we need to understand their fundamental PEC properties, using an approach with spatial resolution. In our study, TMDCs samples, MoSe2, WSe2, and MoWSe2 nanosheets were mechanically exfoliated to get bulk, few-layered, and monolayer specimens. The number of layers, and defect density in the separated nanosheets were characterised by Raman spectroscopy and atomic force microscopy. Our recently developed custom-designed microdroplet cell-based photoelectrochemical approach was applied to identify the structural parts and to measure the PEC activity of the flakes (10-50 μm droplets in diameter). PEC measurements, including photovoltammetry, photocurrent transient analysis, and quantum efficiency were carried out to reveal the role of structural properties on the light harvesting, charge transport, and recombination properties of TMDCs. We have determined the band diagrams of these materials using bandgap values from PEC studies and work functions achieved from ambient pressure photoemission spectroscopy (APS) and surface photovoltage spectroscopy (SPS) measurements. Our photoelectrochemical microscopy technique also allowed to probe the PEC reactivity of novel member of 2D family, such as MoWSe2 mixed transition metal dichalcogenide. Finally, I demonstrate the use of model reversible redox species (K4[Fe(CN)6], (NH4)2[IrCl6]) to mimic photoelectrocatalytic processes. (1) F. R. F. Fan, H. S. White, B. L. Wheeler, A. J. Bard; J. Am. Chem. Soc. 1980, 102, 5142-5148 (2) W. Kautek, H. Gerischer; Ber. Bunsenges. Phys. Chem. 1980, 84, 645-653 (3) K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, A. K. Geim; PNAS, 2005, 102, 10451-10453 (4) P. S. Toth, A. T. Valota, M. Velický, I. A. Kinloch, K. S. Novoselov, E. W. Hill, R. A. W. Dryfe; Chem. Sci., 2014, 5, 582-589 (5) M. Velický, M. A. Bissett, C. R. Woods, P. S. Toth, T. Georgiou, I. A. Kinloch, K. S. Novoselov, R. A. W. Dryfe; Nano Lett., 2016, 16, 2023-2032

The Electronic Properties of Silicon Nanowires during Their Dissolution under Simulated Physiological Conditions

Silicon nanowires are considered promising future biomedical sensors. However, their limited stability under physiological conditions poses a challenge in sensor development and necessitates a significantly improved knowledge of underlying effects as well as new solutions to enhance silicon nanowire durability. In the present study, we deduced the dissolution rates of silicon nanowires under simulated physiological conditions from atomic force microscopy measurements. We correlated the relevant change in nanowire diameter to changes in the electronic properties by examining the I-V characteristics of kinked silicon nanowire p–n junctions. Contact potential difference measurements and ambient pressure photoemission spectroscopy additionally gave insights into the electronic surface band structure. During the first week of immersion, the Fermi level of n-type silicon nanowires shifted considerably to higher energies, partly even above the conduction band edge, which manifested in an increased conductivity. After about a week, the Fermi level stabilized and the conductivity decreased consistently with the decreasing diameter caused by continuous nanowire dissolution. Our results show that a physiological environment can substantially affect the surface band structure of silicon nanowire devices, and with it, their electronic properties. Therefore, it is necessary to study these effects and find strategies to gain reliable biomedical sensors.

Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy.

Water adsorption and reaction on pure and Ni-modified CoOOH nanowires were investigated using ambient pressure photoemission spectroscopy (APPES). The unique capabilities of APPES enable us to observe water dissociation and monitor formation of surface species on pure and Ni-modified CoOOH under elevated pressures and temperatures for the first time. Over a large range of pressures (UHV to 1 Torr), water dissociates readily on the pure and Ni-modified CoOOH surfaces at 27 °C. With an increase in H2O pressure, a greater degree of surface hydroxylation was observed for all samples. At 1 Torr H2O, ratios of different oxygen species indicate a transformation of CoOOH to CoOxHy in pure and Ni-modified CoOOH. In temperature dependent studies, desorption of weakly bound water and surface dehydroxylation were observed with increasing temperature. Larger percentages of surface hydroxyl groups at higher temperatures were observed on Ni-modified CoOOH compared to pure CoOOH, which indicates an increased stability of surface hydroxyl groups on these Ni-modified surfaces.

(Invited) Surface Chemistry of Gap in Solar-Driven Pyridine-Catalyzed CO2 Reduction

Atypical high selectivity for the conversion of CO2 to methanol has been reported when pyridine is dissolved in the electrolyte of a photoelectrochemical cell employing a p-type gallium phosphide (GaP) photocathode.[1] Despite considerable electrochemical characterization and theoretical consideration, there is no consensus on the mechanism by which pyridine catalyzes CO2-to-methanol conversion. A fundamental understanding of the origin of high selectivity in this system is important for continued optimization of solar-driven CO2 reduction. Our research combines a surface science approach with in situ and operando spectroscopy to investigate GaP surface chemistry and determine the role of heterogeneous processes in this catalysis. Ambient pressure photoemission spectroscopy (APPES) was used to study the adsorption of H2O [2] on GaP(110) and pyridine (Py) on clean and hydrided GaP(110). The stable adsorption of both H and pyridine on GaP(110) characterizes the precursor state for the formation of adsorbed dihydropyridine (DHP), which could be a key hydride-shuttling catalyst for heterogeneous CO2 reduction. Using APPES we also observed dissociative adsorption of methanol on GaP(111), which leads to the formation of surface-bound methoxy (CH3O) species, while simultaneous formation of surface-bound formate (HCOO) and methoxy occurs on the more reactive GaP(110) surface. Low temperature scanning tunneling microscopy (LT-STM) was used to observe orbital-resolved bonding of adsorbed pyridine on the GaP(110) surface.[3] By examining the distribution of unoccupied molecular orbitals with high spatial and energy resolution, we have identified the sites on pyridine susceptible to nucleophilic attack that lead to the formation of hydrogenated species such as dihydropyridine. O perando FTIR studies of pyridine hydrogenation at a Pt electrode under relevant electrochemical conditions was used to observe the formation of a hydrogenated piperidinium (PyH7 +) near-surface species, which implies that dihydropyridinium (PyH3 +), the protonated form of dihydropyridine, must exist transiently during CO2 reduction.[4] 1. Barton, E. E.; Rampulla, D. M.; Bocarsly, A. B., J. Am. Chem. Soc. 130, 6342 (2008) 2. Kronawitter, C. X.; Lessio, M.; Zhao, P.; Riplinger, C.; Boscoboinik, A.; Starr, D. E.; Sutter, P.; Carter, E. A.; Koel, B. E., J. Phys. Chem. C 119, 17762 (2015) 3. Kronawitter, C. X.; Lessio, M.; Zahl, P.; Muñoz-García, A. B.; Sutter, P.; Carter, E. A.; Koel, B. E., J. Phys. Chem. C 119, 28917 (2015) 4. Kronawitter, C.; Chen, Z.; Zhao, P.; Yang, X.; Koel, B. E., Catal. Sci. Tech., 7, 831 (2017)

Measurements of Natural and Synthetic Diamond Samples Using Kelvin Probe, Surface Photovoltage and Ambient Pressure Photoemission Techniques

ABSTRACTDiamond is a promising wide band-gap semiconductor material for use in devices; therefore a thorough understanding of the surface electronic structure is important. The Kelvin Probe (KP), Surface Photovoltage / Surface Photovoltage Spectroscopy (SPV/SPS) and Ambient Pressure Photoemission Spectroscopy (APS) techniques are commonly applied to traditional and organic semiconductor materials. The application of these techniques to synthetic and natural diamond samples provides some challenges: surface charge on the samples and atypical capacitive interaction with the KP tip. In this study, measurements using a combination of KP, SPV/SPS and APS techniques are taken of samples of natural and synthetic diamond samples to investigate their surface electronic structure and compare their different properties. These techniques are all non-contact and non-destructive. The Fermi Level position of the diamond samples was found to vary, typically between 4.3 – 4.9 eV, depending on the light illumination. For example, when a natural diamond sample was illuminated with 400 nm light from a 150W Quartz Tungsten Halogen light source, there was a surface photovoltage response of ∼250 mV. The oxygen terminated synthetic diamond sample required near continuous illumination at low visible wavelengths in order to retain sufficient conductivity to allow measurement with the Kelvin Probe. By contrast, the natural diamond samples measured showed good conductivity in the layers underneath the top surface. In summary, the KP, SPV/SPS and APS measurement techniques provided some interesting information on the diamond samples and an initial investigation of their surface electronic states is performed.

Oxidation of Small Supported Platinum-based Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen

The investigation of nanocatalysts under their working conditions of pressures and temperatures represents a real strategy toward a realistic understanding of their chemical reactivity and related issues. Additionally, the reduction of Pt load in the catalysts while maintaining their optimum performances is essential to large scale practical applications. Here, we show that small PtZn bimetallic nanoparticles (NPs) supported on the rutile and reduced TiO2(110)-(1 × 1) surface can be prepared by a two step consecutive deposition process where Pt was deposited first and followed by Zn. In situ synchrotron-based near ambient pressure photoemission spectroscopy experiments are used to monitor the evolution of the oxidation states and surface elemental composition of pure Pt and PtZn NPs under high exposure to O2 pressure. The formation of stable Pt surface oxide was evidenced for both pure and PtZn NPs. While a sizeable encapsulation of pure Pt NPs by TiOx was seen after annealing at 440 K under 1 mbar of O2, no such effect was noticed for PtZn NPs. The formation of a zinc oxide layer on PtZn NPs enhances the stability of the NPs and induces a partial reduction of the TiO2(110) surface. Spontaneous formation of a Pt–Zn alloy phase at room temperature was seen in PtZn NPs.

Surface Chemistry of Electrocatalysts for CO2 Reduction to Fuels

The surface chemistry of electrocatalysts for the production of fuels by CO2 reduction was explored by surface science model studies using high-resolution spectroscopy and atomic scale imaging, as well as by electrochemical analysis of active electrodes. Using an experimental surface science approach, ultrahigh vacuum conditions were used to facilitate the fabrication of highly characterizable electrode-adsorbate systems. We have utilized a range of techniques for surface analysis to determine the structure and properties of single-crystal surfaces and unique alloys formed by vapor deposition under controlled conditions. Specifically, XPS, LEIS, LEED and STM were used to characterize the composition and structure of III-V semiconductors and bimetallic model catalysts. TPD, UPS, and vibrational spectrocopy using HREELS and IRAS were used to study the binding and electronic structure of adsorbates. Many of our experiments are designed to explain the role of the electrode surface in the high selectivity observed in pyridine and other N-heterocycle-catalyzed CO2 reduction systems. We examined the preferential adsorption sites, bonding interactions, and reactivities of these molecular catalysts and other relevant adsorbates on the surfaces of photoactive electrode materials. Ambient pressure photoemission spectroscopy (APPES), STM, and vibrational spectroscopies were used to study the interaction of water and CO2 reduction catalysts with the surfaces of III-V semiconductors. We have applied these techniques to also study bimetallic alloy electrocatalysts. Bimetallic alloys have been shown to exhibit lower overpotentials for CO2 reduction to fuels when compared to their metal constituents. By tuning the composition of the alloys, one can influence the binding of adsorbates and therefore reaction selectivity. We synthesized bimetallic surface alloys in controlled UHV environments to investigate the relationship between surface composition and electronic structure. We studied the interaction of relevant adsorbed species with the bimetallic alloy surfaces using TPD, IRAS, HREELS, and UPS to begin to understand the surface chemistry of these electrodes. In parallel to these surface science investigations, we studied the electrochemical behavior of single crystals and bimetallic alloys. The purpose of these studies is to correlate CO2 reduction product selectivity to the quantitative measurements of surface chemistry made in UHV discussed above. Specifically we examined the relationship between electrochemical activity, surface composition, and the binding strengths of intermediates along CO2 reduction pathways. This work aids development of a molecular-level understanding of the heterogeneous processes involved the reaction mechanisms of efficient electrocatalytic fuel generation. This material is based upon work supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012455.

Concentration and chemical-state profiles at heterogeneous interfaces with sub-nm accuracy from standing-wave ambient-pressure photoemission.

Heterogeneous processes at solid/gas, liquid/gas and solid/liquid interfaces are ubiquitous in modern devices and technologies but often difficult to study quantitatively. Full characterization requires measuring the depth profiles of chemical composition and state with enhanced sensitivity to narrow interfacial regions of a few to several nm in extent over those originating from the bulk phases on either side of the interface. We show for a model system of NaOH and CsOH in an ~1-nm thick hydrated layer on α-Fe2O3 (haematite) that combining ambient-pressure X-ray photoelectron spectroscopy and standing-wave photoemission spectroscopy provides the spatial arrangement of the bulk and interface chemical species, as well as local potential energy variations, along the direction perpendicular to the interface with sub-nm accuracy. Standing-wave ambient-pressure photoemission spectroscopy is thus a very promising technique for measuring such important interfaces, with relevance to energy research, heterogeneous catalysis, electrochemistry, and atmospheric and environmental science.

Dual Mode Kelvin Probe: Featuring Ambient Pressure Photoemission Spectroscopy and Contact Potential Difference

We describe a novel dual-mode Kelvin probe featuring ambient pressure Photoemission Spectroscopy (PES), which yields information on the absolute work function (Φ) of a metal and the Ionisation Potential (IP) of a semiconductor, coupled with a high resolution Contact Potential Difference capability which can be extended to Surface Photovoltage measurements. The relative energy resolution are 50 meV for PES and 1-3 meV for CPD. To surmount the limitation of electron scattering in air the incident photon energy is rastered rather than applying a variable retarding electric field as is used UPS. We propose a mechanism of atmospheric ion generation and show that for the metal photoresponse obeys Fowler Theory. The relationship between CPD and photoelectric threshold is a useful tool in characterizing the electrical behavior of materials. We illustrate this with native oxide covered Cu and n-type Si. Further we show that the photoresponse can be used to generate the near Fermi-level Density of States (DOS) in Iron and Nickel-Phthalocyanine.

Dynamic High Pressure: a novel approach toward near ambient pressure photoemission spectroscopy and spectromicroscopy

A Dynamic High Pressure (DHP) system has been developed, tested and implemented in the scanning photoelectron microscope (SPEM) operated at ESCAmicroscopy beamline at Elettra synchrotron. The system consists of a compact gas injection set up that allows experiments with local pressure near the sample several orders of magnitude higher that the allowable pressure for X-ray photoelectron spectroscopy setups. The DHP setup controls the amount of gas injected toward the sample by fine tuning the time and spatial profiles using a pulsed valve and a nozzle, respectively. The DHP functionality and effectiveness has been demonstrated by in operando oxidation experiments of Ru and Si. The obtained results confirmed that using the DHP the gas exposure onto the sample is equivalent to a static pressure between 10−3 and 10−2 mbar, about 3 orders of magnitude higher than the maximum gas pressure for the XPS machines under operation.

New ambient pressure photoemission endstation at Advanced Light Source beamline 9.3.2

During the past decade, the application of ambient pressure photoemission spectroscopy (APPES) has been recognized as an important in situ tool to study environmental and materials science, energy related science, and many other fields. Several APPES endstations are currently under planning or development at the USA and international light sources, which will lead to a rapid expansion of this technique. The present work describes the design and performance of a new APPES instrument at the Advanced Light Source beamline 9.3.2 at Lawrence Berkeley National Laboratory. This new instrument, Scienta R4000 HiPP, is a result of collaboration between Advanced Light Source and its industrial partner VG-Scienta. The R4000 HiPP provides superior electron transmission as well as spectromicroscopy modes with 16 microm spatial resolution in one dimension and angle-resolved modes with simulated 0.5 degrees angular resolution at 24 degrees acceptance. Under maximum transmission mode, the electron detection efficiency is more than an order of magnitude better than the previous endstation at beamline 9.3.2. Herein we describe the design and performance of the system, which has been utilized to record spectra above 2 mbar.

Photoelectron spectroscopy under ambient pressure and temperature conditions

We describe the development and applications of novel instrumentation for photoemission spectroscopy of solid or liquid surfaces in the presence of gases under ambient conditions of pressure and temperature. The new instrument overcomes the strong scattering of electrons in gases by the use of an aperture close to the surface followed by a differentially-pumped electrostatic lens system. In addition to the scattering problem, experiments in the presence of condensed water or other liquids require the development of special sample holders to provide localized cooling. We discuss the first two generations of Ambient Pressure PhotoEmission Spectroscopy (APPES) instruments developed at synchrotron light sources (ALS in Berkeley and BESSY in Berlin), with special focus on the Berkeley instruments. Applications to environmental science and catalytic chemical research are illustrated in two examples.

Soft X-ray microscopy and spectroscopy at the molecular environmental science beamline at the Advanced Light Source

We present examples of the application of synchrotron-based spectroscopies and microscopies to environmentally relevant samples. The experiments were performed at the molecular environmental science beamline (11.0.2) at the Advanced Light Source, Lawrence Berkeley National Laboratory. Examples range from the study of water monolayers on Pt(1 1 1) single crystal surfaces using X-ray emission spectroscopy and the examination of alkali halide solution/water vapor interfaces using ambient pressure photoemission spectroscopy, to the investigation of actinides, river water biofilms, Al-containing colloids and mineral–bacteria suspensions using scanning transmission X-ray spectromicroscopy. The results of our experiments show that spectroscopy and microscopy in the soft X-ray energy range are excellent tools for the investigation of environmentally relevant samples under realistic conditions, i.e., with water or water vapor present at ambient temperature.