Introduction to Surface Chemistry and Catalysis

Renewable energy conversion and storage play an important role in our global efforts to limit the drastic effects of climate change. In particular, the electrocatalytic reduction of carbon dioxide to chemicals and fuels can bring us closer towards a closed-loop anthropogenic carbon cycle. Significant breakthroughs are often the result of deeper understandings of reaction mechanisms, material structures and surface sites. To this end, operando techniques have been invaluable in combining advanced characterization of a catalyst with simultaneous measurements of its activity and selectivity under real working conditions. This Review aims to highlight significant progress in the use of operando characterization techniques that enhance our understanding of heterogeneous electrocatalytic CO2 reduction. We provide a summary of the most recent mechanistic understanding using operando optical, X-ray and electron-based techniques, along with key questions that need to be addressed. We conclude by offering some insight on emerging directions and prospects in the field. The electrochemical reduction of carbon dioxide allows recycling of the greenhouse gas to produce chemicals and fuels. In this Review, Seh and co-workers discuss the progress in operando techniques applied to heterogeneous carbon dioxide electroreduction, highlighting the mechanistic insights that these techniques have provided.

Understanding heterogeneous electrocatalytic carbon dioxide reduction through operando techniques

Fundamentals Of Modern Vlsi Devices

Owing to their attractive application potentials in both non-volatile memory and unconventional computing, memristive devices have drawn substantial research attention in the last decade. However, major roadblocks still remain in device performance, especially concerning relatively large parameter variability and limited cycling endurance. The response of the active region in the device within and between switching cycles plays the dominating role, yet the microscopic details remain elusive. This Review summarizes recent progress in scientific understanding of the physical origins of the non-idealities and propose a synergistic approach based on in situ characterization and device modeling to investigate switching mechanism. At last, the Review offers an outlook for commercialization viability of memristive technology. Memristor as the fourth basic element of electric circuits has drawn substantial attention for developing future computing technologies. Sun et al. report the progress and the challenges facing researchers on understanding memristive switching, and advocate continuous studies using a synergistic approach.

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Understanding memristive switching via in situ characterization and device modeling.

Oxygen vacancies play crucial roles in determining the physical properties of metal oxides, representing important building blocks in many scientific and technological fields due to their unique chemical, physical, and electronic properties. However, oxygen vacancies are often invisible because of their dilute concentrations. Therefore, characterizing and quantifying their presence is of utmost importance for understanding and realizing functional metal oxide devices. This, however, is oftentimes a non-trivial task. In this Perspective paper, we discuss the relevant regimes of concentrations and associated phenomena arising from oxygen vacancies. We then focus on experimental techniques available for observing oxygen vacancies at widely different levels of concentrations. Finally, we discuss current challenges and opportunities for utilizing oxygen vacancies in metal oxides.

https://backend.orbit.dtu.dk/ws/files/209337425/Gunkel_APL_Perspective_DTU_2020_PREPRINT.pdf

Oxygen vacancies: The (in)visible friend of oxide electronics

Studying catalysts in situ is of high interest for understanding their surface structure and electronic states in operation. Herein, we present a study of epitaxial manganite perovskite thin films (Pr1−xCaxMnO3) active for the oxygen evolution reaction (OER) from electro-catalytic water splitting. X-ray absorption near-edge spectroscopy (XANES) at the Mn L- and O K-edges, as well as X-ray photoemission spectroscopy (XPS) of the O 1s and Ca 2p states have been performed in ultra-high vacuum and in water vapor under positive applied bias at room temperature. It is shown that under the oxidizing conditions of the OER a reduced Mn2+ species is generated at the catalyst surface. The Mn valence shift is accompanied by the formation of surface oxygen vacancies. Annealing of the catalysts in O2 atmosphere at 120 °C restores the virgin surfaces.

https://www.mdpi.com/2073-4344/4/2/129/pdf

In Situ XANES/XPS Investigation of Doped Manganese Perovskite Catalysts

Environmental transmission electron microscopy (ETEM) studies offer great potential for gathering atomic scale information on the electronic state of electrodes in contact with reactants. It also poses big challenges due to the impact of the high energy electron beam. In this article, we present an ETEM study of a Pr0.64Ca0.36MnO3 (PCMO) thin film electrocatalyst for water splitting and oxygen evolution in contact with water vapor. We show by means of off-axis electron holography and electrostatic modeling that the electron beam gives rise to a positive electric sample potential due to secondary electron emission. The value of the electric potential depends on the primary electron flux, the sample’s electric conductivity and grounding, and gas properties. We present evidence that two observed electrochemical reactions are driven by a beam induced electrostatic potential of the order of a volt. The first reaction is an anodic oxidation of oxygen depleted amorphous PCMO which results in recrystallization of...

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Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr0.64Ca0.36MnO3 Catalysts for Oxygen Evolution

Small polaron optical properties are studied comprehensively in thin film samples of the narrow bandwidth manganite $\mathrm{P}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ by optical absorption spectroscopy as a function of doping and temperature. A broad near infrared double-peak absorption band in the optical conductivity spectras is observed and interpreted in the framework of photon-assisted small polaron intersite hopping and on-site Jahn-Teller excitation. Application of quasiclassical small polaron theory to both transitions allows an approximate determination of polaron specific parameters like the polaron binding energy, the characteristic phonon energy, as well as the Jahn-Teller splitting energy as a function of temperature and doping. Based on electronic structure calculations, we consider the impact of the hybridization of $\mathrm{O}\phantom{\rule{0.28em}{0ex}}2p$ and Mn $3d$ electronic states on the Jahn-Teller splitting and the polaron properties. The interplay between hopping and Jahn-Teller excitations is discussed in the alternative pictures of mixed valence $\mathrm{M}{\mathrm{n}}^{3+}/\mathrm{M}{\mathrm{n}}^{4+}$ sites (Jahn-Teller polaron) and equivalent $\mathrm{M}{\mathrm{n}}^{(3+x)+}$ sites (Zener polaron). We give a careful evaluation of the estimated polaron parameters and discuss the limitations of small polaron quasiclassical theory for application to narrow bandwidth manganites.

Temperature- and doping-dependent optical absorption in the small-polaron system Pr1−xCaxMnO3.

The electronic structure of ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ has been investigated using a combination of first-principles calculations, x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), electron-energy loss spectroscopy (EELS), and optical absorption The full range of compositions, $x=0,1/2,1$, and a variety of magnetic orders have been covered Jahn-Teller as well as Zener polaron orders are considered The free parameters of the local hybrid density functionals used in this study have been determined by comparison with measured XPS spectra A model Hamiltonian, valid for the entire doping range, has been extracted A simple local-orbital picture of the electronic structure for the interpretation of experimental spectra is provided The comparison of theoretical calculations and different experimental spectra provide a detailed and consistent picture of the electronic structure The large variations of measured optical absorption spectra are traced back to the co-existence of magnetic orders (respectively, to the occupation of local orbitals) A consistent treatment of the Coulomb interaction indicates a partial cancellation of Coulomb parameters and supports the dominance of the electron-phonon coupling

Electronic structure of Pr 1 -x Ca x MnO 3

The mechanism of the electric-pulse induced resistance change effect in Au/Pr0.65Ca0.35MnO3/SrTi0.99Nb0.01O3 thin-film samples is studied by means of in situ electrical stimulation inside a transmission electron microscope. A detailed equivalent-circuit model analysis of the measured current–voltage characteristics provides crucial information for the proper interpretation of the microscopy results. The electrical transport data of the electron-transparent samples used for the in situ investigations is verified by comparison to measurements of unpatterned thin-film samples. We find comprehensive evidence for electrochemical oxygen vacancy migration affecting the potential barrier of the pn junction between Pr0.65Ca0.35MnO3 and SrTi0.99Nb0.01O3 as well as the resistance of the manganite bulk. The high-resistance state formation in the Pr0.65Ca0.35MnO3 bulk is frequently accompanied by structural transformations, namely detwinning and superstructure formation, most likely as the result of the joint impact of dynamic charge inhomogenities by oxygen vacancy migration and injection of high carrier densities at the electrodes.

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Stephanie Mildner

Papers

In Situ XANES/XPS Investigation of Doped Manganese Perovskite Catalysts

Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr0.64Ca0.36MnO3 Catalysts for Oxygen Evolution

Temperature- and doping-dependent optical absorption in the small-polaron system Pr1−xCaxMnO3.

Electronic structure of Pr 1 -x Ca x MnO 3

In situ TEM analysis of resistive switching in manganite based thin-film heterostructures