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

Nemšák, Slavomír; Shavorskiy, Andrey; Karslıoğlu, Osman; Zegkinoglou, Ioannis; Rattanachata, Arunothai; Conlon, C.; Keqi, A.; Greene, Peter K.; Burks, Edward C.; Salmassi, Farhad; Gullikson, Eric M.; Yang, See‐Hun; Liu, Kai; Bluhm, Hendrik; Fadley, C. S.

doi:10.1038/ncomms6441

Cited by 102 publications

(73 citation statements)

References 36 publications

(37 reference statements)

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“…[62][63] An additional approach includes the use of X-ray lenses to produce standing waves, increasing the depth resolution of sample/interface analysis. 61,64 Finally, novel pump-probe techniques using AP-XPS coupled to laser systems (and the free-electron laser) can also provide unique time-resolved information. 65 Such studies can help assess the presence and energetics of reaction intermediates and further build further mechanistic insight in electrocatalysis.…”

Section: Discussionmentioning

confidence: 99%

Insights into Electrochemical Reactions from Ambient Pressure Photoelectron Spectroscopy

et al. 2015

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ConspectusThe understanding of fundamental processes in the bulk and at the interfaces of electrochemical devices is a prerequisite for the development of new technologies with higher efficiency and improved performance. One energy storage scheme of great interest is splitting water to form hydrogen and oxygen gas, and converting back to electrical energy by their subsequent recombination with only water as a by-product. However, kinetic limitations to the rate of oxygen-based electrochemical reactions hamper the efficiency in technologies such as solar fuels, fuel cells, and electrolyzers. For these reactions, the use of metal oxides as electrocatalysts is prevalent due to their stability, low cost, and ability to store oxygen within the lattice. However, due to the inherently convoluted nature of electrochemical and chemical processes in electrochemical systems, it is difficult to isolate and study individual electrochemical processes in a complex system. Therefore in situ characterization tools are required for observing related physical and chemical processes directly at the places where and while they occur, and can help elucidate the mechanisms of charge separation and charge transfer at electrochemical interfaces.X-ray photoelectron spectroscopy (XPS), also known as ESCA (electron spectroscopy for chemical analysis) has been used as a quantitative spectroscopic technique that measures the elemental composition, as well as chemical and electronic state of a material. Building from extensive ex situ characterization of electrochemical systems, initial in situ studies were conducted at (near) UHV conditions (≤10 −6 Torr) to probe solid-state electrochemical systems. However through the integration of differential-pumping stages, XPS can now operate at pressures in the Torr range, comprising a technique called ambient pressure XPS (AP-XPS).In this Account, we briefly review the working principles and current status of AP-XPS. We use several recent in situ studies on model electrochemical components as well as operando studies performed by our groups at the Advanced Light Source (ALS) at Lawrence Berkeley National Lab to illustrate that AP-XPS is both a chemically and electrically specific tool since photoelectrons carry information on both the local chemistry and electrical potentials. The applications of AP-XPS to oxygen electrocatalysis shown in this Account span well defined studies of (1) the oxide/oxygen gas interface, (2) the oxide/water vapor interface, and (3) operando measurements of half and full electrochemical cells. Using 2 specially designed model devices, we can expose and isolate the electrode or interface of interest to the incident X-ray beam and AP-XPS analyzer to relate the electrical potentials to the composition/chemical state of the key components and interfaces. We conclude with an outlook on new developments of AP-XPS endstations, which may provide significant improvement in the observation of dynamics over a wide range of time scale, higher spatial resolution, and improved characteri...

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Section: Discussionmentioning

confidence: 99%

Insights into Electrochemical Reactions from Ambient Pressure Photoelectron Spectroscopy

et al. 2015

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“…In fact, our above-and below-resonance SW technique has in more recent work been successfully applied to the system of LaCoO 3 and SrTiO 3 [61], two nonferroelectric materials which have been found to exhibit ferroelectric behavior at their interface [62]. As other examples looking ahead, we note the recent application of soft x-ray SW photoemission to the liquid-solid interface [13,63], with the above-and below-resonance methods described here promising to yield much more precise characterization of such interfaces in electrochemical cells [63] or gated devices involving a solid oxide and a liquid electrolyte [10]. …”

Section: Conclusion and Future Outlookmentioning

confidence: 99%

“…It is thus not an overstatement to repeat that "the interface is the device" [8,9], in many important applications areas. These include, for example, interfaces involving a solid oxide and a liquid electrolyte that have been studied in gated devices [10], but which are also crucial in energy production, electrochemistry, corrosion, and environmental science [11][12][13]; such solid/liquid interfaces have been studied for many years but are still not fully understood. The quantitative characterization of solid/solid or solid/liquid interfaces is thus a challenge in many areas of science and technology.…”

Section: Fig 1 (A)mentioning

confidence: 99%

Energetic, spatial, and momentum character of the electronic structure at a buried interface: The two-dimensional electron gas between two metal oxides

Nemšák¹,

Conti²,

Gray³

et al. 2016

Phys. Rev. B

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The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications and are the subject of intense study in many disciplines. We here apply experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of SrTiO 3 (STO) and GdTiO 3 (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form [6 unit cells (u.c.) STO/3 u.c. of GTO] 20 using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been exploited as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG in detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG and suggesting that oxygen vacancies are not the origin of it. Similar multitechnique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry.

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“…Another problem resulting from the liquid film thickness variations is a complicated control of the concentration and pH of the re-hydrated solution. While alkali halide solution films have successfully been formed by re-hydrating either dropcast or evaporated alkali halide films inside a humiditycontrolled vacuum chamber [45,46], the exact control of the concentration and pH of the film remains challenging.…”

Section: Probing Liquid Interphasesmentioning

confidence: 99%

Functional materials for information and energy technology: Insights by photoelectron spectroscopy

Müller

Nemšák

Pluciński

et al. 2016

Journal of Electron Spectroscopy and Related Phenomena

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The evolution of both information and energy technology is intimately connected to complex condensed matter systems, the properties of which are determined by electronic and chemical interactions and processes on a broad range of length and time scales. Dedicated photoelectron spectroscopy and spectromicroscopy experiments can provide important insights. We discuss some recent methodological developments with application to relevant questions in spintronics, and towards in-operando studies of resistive switching and electrochemical processes.

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

Cited by 102 publications

References 36 publications

Insights into Electrochemical Reactions from Ambient Pressure Photoelectron Spectroscopy

Insights into Electrochemical Reactions from Ambient Pressure Photoelectron Spectroscopy

Energetic, spatial, and momentum character of the electronic structure at a buried interface: The two-dimensional electron gas between two metal oxides

Functional materials for information and energy technology: Insights by photoelectron spectroscopy

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