Osman Karslıoğlu

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.

Synchrotron-based Ambient Pressure X-ray Photoelectron Spectroscopy

Solid/vapor and liquid/vapor interfaces govern many processes in the environment and atmosphere, energy generation, and electrochemical devices as well as heterogeneous catalysis. Examples include catalytic converters in automobiles [1], solid oxide fuel cells [2], cloud droplet nucleation on atmospheric aerosol particles [3], as well as the interaction of trace gases with polar snow packs [4]. A fundamental understanding of the molecular processes at these interfaces requires experimental methods that allow the investigation of samples under as close to operating conditions as possible. This kind of investigation has become increasingly important over the last few decades, leading to the development of a number of surface-sensitive, in-situ spectroscopies and microscopies, including infrared spectroscopy (IR) [5, 6]; vibrational sum-frequency generation (VSFG) [7, 8]; X-ray emission spectroscopy (XES) [9]; surface X-ray diffraction (SXRD) [10]; scanning force microscopy (SFM) in both contact [11] and non-contact [12] modes; scanning tunneling microscopy (STM) [13]; as well as transmission electron microscopy [14] and scanning electron microscopy [15].

Ambient pressure photoelectron spectroscopy: Practical considerations and experimental frontiers

Over the past several decades, ambient pressure x-ray photoelectron spectroscopy (APXPS) has emerged as a powerful technique for in situ and operando investigations of chemical reactions under relevant ambient atmospheres far from ultra-high vacuum conditions. This review focuses on exemplary cases of APXPS experiments, giving special consideration to experimental techniques, challenges, and limitations specific to distinct condensed matter interfaces. We discuss APXPS experiments on solid/vapor interfaces, including the special case of 2D films of graphene and hexagonal boron nitride on metal substrates with intercalated gas molecules, liquid/vapor interfaces, and liquid/solid interfaces, which are a relatively new class of interfaces being probed by APXPS. We also provide a critical evaluation of the persistent limitations and challenges of APXPS, as well as the current experimental frontiers.

In Situ Characterization of the Initial Effect of Water on Molecular Interactions at the Interface of Organic/Inorganic Hybrid Systems

Probing initial interactions at the interface of hybrid systems under humid conditions has the potential to reveal the local chemical environment at solid/solid interfaces under real-world, technologically relevant conditions. Here, we show that ambient pressure X-ray photoelectron spectroscopy (APXPS) with a conventional X-ray source can be used to study the effects of water exposure on the interaction of a nanometer-thin polyacrylic acid (PAA) layer with a native aluminum oxide surface. The formation of a carboxylate ionic bond at the interface is characterized both with APXPS and in situ attenuated total reflectance Fourier transform infrared spectroscopy in the Kretschmann geometry (ATR-FTIR Kretschmann). When water is dosed in the APXPS chamber up to 5 Torr (~28% relative humidity), an increase in the amount of ionic bonds at the interface is observed. To confirm our APXPS interpretation, complementary ATR-FTIR Kretschmann experiments on a similar model system, which is exposed to an aqueous electrolyte, are conducted. These spectra demonstrate that water leads to an increased wet adhesion through increased ionic bond formation.

Unravelling the Chemical Influence of Water on the PMMA/Aluminum Oxide Hybrid Interface In Situ

Understanding the stability of chemical interactions at the polymer/metal oxide interface under humid conditions is vital to understand the long-term durability of hybrid systems. Therefore, the interface of ultrathin PMMA films on native aluminum oxide, deposited by reactive adsorption, was studied. The characterization of the interface of the coated substrates was performed using ambient pressure X-ray photoelectron spectroscopy (APXPS), Fourier transform infrared spectroscopy in the Kretschmann geometry (ATR-FTIR Kretschmann) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The formation of hydrogen bonds and carboxylate ionic bonds at the interface are observed. The formed ionic bond is stable up to 5 Torr water vapour pressure as shown by APXPS. However, when the coated samples are exposed to an excess of aqueous electrolyte, an increase in the amount of carboxylate bonds at the interface, as a result of hydrolysis of the methoxy group, is observed by ATR-FTIR Kretschmann. These observations, supported by ToF-SIMS spectra, lead to the proposal of an adsorption mechanism of PMMA on aluminum oxide, which shows the formation of methanol at the interface and the effect of water molecules on the different interfacial interactions.

Ambient-Pressure X-ray Photoelectron Spectroscopy (APXPS)

X-ray photoelectron spectroscopy (XPS) is a powerful technique for studying surfaces, including those of heterogeneous catalysts, through its ability to quantitatively analyze the elemental and chemical composition with high surface sensitivity. The understanding of heterogeneous catalytic processes under realistic conditions requires measurements at elevated pressures, far from the high-vacuum conditions under which the majority of XPS measurements are conducted. The investigation of surfaces using XPS at or near relevant pressures poses challenges due to scattering of electrons by gas molecules, which have been overcome through the development of ambient-pressure XPS (APXPS). In this chapter, we will review technical approaches for conducting XPS at higher pressures and discuss other experimental challenges that need to be addressed in APXPS investigations. At the end of the chapter, examples of APXPS experiments of CO oxidation over Ru and Pd, as well as the oxidation of other small hydrocarbons are shown.

Characterization of free-standing InAs quantum membranes by standing wave hard x-ray photoemission spectroscopy

Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a (Si/Mo) multilayer mirror substrate are characterized by hard x-ray photoemission spectroscopy (HXPS), and by standing-wave HXPS (SW-HXPS). Information on the chemical composition and on the chemical states of the elements within the nanoribbons was obtained by HXPS and on the quantitative depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves to x-ray optical calculations, the chemical depth profile of the InAs(QM) and its interfaces were quantitatively derived with angstrom precision. We determined that: i) the exposure to air induced the formation of an InAsO

Direct Mapping of Band Positions in Doped and Undoped Hematite during Photoelectrochemical Water Splitting

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Structure of Copper–Cobalt Surface Alloys in Equilibrium with Carbon Monoxide Gas

layer on top of the stoichiometric InAs(QM); ii) the top interface between the air-side InAsO

Reversed interfacial fractionation of carbonate and bicarbonate evidenced by X-ray photoemission spectroscopy

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