Investigations on the Origin of Topotactic Phase Transition of LaCoO Thin Films with In Situ XRD and Ambient Pressure Hard X-ray Photoelectron Spectroscopy.

With the applications of in situ X-ray diffraction (XRD), electrical - measurement, and ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES), the characteristics of the topotactic phase transition of LaCoO (LCO) thin films are examined. XRD measurements show clear evidence of structural phase transition (SPT) of the LCO thin films from the perovskite (PV) LaCoO to the brownmillerite (BM) LaCoO phases through the intermediate LaCoO phase at a temperature of 350 °C under high-vacuum conditions, ∼10 mbar. The reverse SPT from BM to PV phases is also found under ambient pressure (>100 mbar) of air near 100 °C.

Study of CO Adsorption Properties on the SrTiO(001) Surface with Ambient Pressure XPS.

The adsorption properties of CO on the SrTiO(001) surface were investigated using ambient pressure X-ray photoelectron spectroscopy under elevated pressure and temperature conditions. On the Nb-doped TiO-enriched (1 × 1) SrTiO surface, CO adsorption, i.e.

Effect of Bias Potential on the Interface of a Solid Electrolyte and Electrode during XPS Depth Profiling Analysis.

Depth profiling is an essential method to investigate the physical and chemical properties of a solid electrolyte and electrolyte/electrode interface. In conventional depth profiling, various spectroscopic tools such as X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) are utilized to monitor the chemical states along with ion bombardment to etch a sample. Nevertheless, the ion bombardment during depth profiling results in an inevitable systematic error, i.

Revealing CO dissociation pathways at vicinal copper (997) interfaces.

Size- and shape-tailored copper (Cu) nanocrystals can offer vicinal planes for facile carbon dioxide (CO) activation. Despite extensive reactivity benchmarks, a correlation between CO conversion and morphology structure has not yet been established at vicinal Cu interfaces. Herein, ambient pressure scanning tunneling microscopy reveals step-broken Cu nanocluster evolutions on the Cu(997) surface under 1 mbar CO(g).

Design of a Metal/Oxide/Carbon Interface for Highly Active and Selective Electrocatalysis.

Sustainable energy-conversion and chemical-production require catalysts with high activity, durability, and product-selectivity. Metal/oxide hybrid structure has been intensively investigated to achieve promising catalytic performance, especially in neutral or alkaline electrocatalysis where water dissociation is promoted near the oxide surface for (de)protonation of intermediates. Although catalytic promise of the hybrid structure is demonstrated, it is still challenging to precisely modulate metal/oxide interfacial interactions on the nanoscale.

How Rh surface breaks CO molecules under ambient pressure.

Utilization of carbon dioxide (CO) molecules leads to increased interest in the sustainable synthesis of methane (CH) or methanol (CHOH). The representative reaction intermediate consisting of a carbonyl or formate group determines yields of the fuel source during catalytic reactions. However, their selective initial surface reaction processes have been assumed without a fundamental understanding at the molecular level.

AP-XPS beamline, a platform for operando science at Pohang Accelerator Laboratory.

Beamline 8A (BL 8A) is an undulator-based soft X-ray beamline at Pohang Accelerator Laboratory. This beamline is aimed at high-resolution ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), soft X-ray absorption spectroscopy (soft-XAS) and scanning photoemission microscopy (SPEM) experiments. BL 8A has two branches, 8A1 SPEM and 8A2 AP-XPS, that share a plane undulator, the first mirror (M1) and the monochromator.

Correlation between structural phase transition and surface chemical properties of thin film SrRuO/SrTiO (001).

The correlation between the structural phase transition (SPT) and oxygen vacancy in SrRuO (SRO) thin films was investigated by in situ X-ray diffraction (XRD) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In situ XRD shows that the SPT occurs from a monoclinic SRO phase to a tetragonal SRO phase near ∼200 °C, regardless of the pressure environment. On the other hand, significant core level shifts in both the Ru and Sr photoemission spectra are found under ultrahigh vacuum, but not under the oxygen pressure environment.

Covalent-bonding-induced strong phonon scattering in the atomically thin WSe layer.

In nano-device applications using two-dimensional (2D) van der Waals materials, a heat dissipation through nano-scale interfaces can be a critical issue for optimizing device performances. By using a time-domain thermoreflectance measurement technique, we examine a cross-plane thermal transport through mono-layered (n = 1) and bi-layered (n = 2) WSe flakes which are sandwiched by top metal layers of Al, Au, and Ti and the bottom AlO substrate. In these nanoscale structures with hetero- and homo-junctions, we observe that the thermal boundary resistance (TBR) is significantly enhanced as the number of WSe layers increases.

Design Principle of Fe-N-C Electrocatalysts: How to Optimize Multimodal Porous Structures?

The effect of porous structures on the electrocatalytic activity of N-doped carbon is studied by using electrochemical analysis techniques and the result is applied to synthesize highly active and stable Fe-N-C catalyst for oxygen reduction reaction (ORR). We developed synthetic procedures to prepare three types of N-doped carbon model catalysts that are designed for systematic comparison of the porous structures. The difference in their catalytic activity is investigated in relation to the surface area and the electrochemical parameters.

Element selective oxidation on Rh-Pd bimetallic alloy surfaces.

The interactions between oxygen and Rh-Pd bimetallic alloy surfaces were investigated using surface sensitive experimental techniques and density functional theory calculations. The alloy surfaces were oxidized under 10-5 Torr and 100 mTorr oxygen upon heating above 250 °C. A thin Rh oxide layer was preferentially formed on a Rh1Pd9(100) surface, while a thin Pd oxide layer was formed on a Rh1Pd9(111) surface, though the Rh oxide is thermodynamically more stable irrespective of the surface orientation.

Adsorbate-driven reactive interfacial Pt-NiO nanostructure formation on the PtNi(111) alloy surface.

The origin of the synergistic catalytic effect between metal catalysts and reducible oxides has been debated for decades. Clarification of this effect, namely, the strong metal-support interaction (SMSI), requires an understanding of the geometric and electronic structures of metal-metal oxide interfaces under operando conditions. We show that the inherent lattice mismatch of bimetallic materials selectively creates surface segregation of subsurface metal atoms.

Catalytic CO oxidation over PdAu(111) alloy surfaces: spectroscopic evidence for Pd ensemble dependent activity.

Catalytic CO oxidation over Pd(111) and PdAu(111) surfaces was investigated by in situ spectroscopic observations to understand the alloying effect. The reaction behaviour on PdAu(111) is greatly different from that on Pd(111). Pd monomer and dimer ensembles can act as active centers, whereas triangular-shaped trimers and larger ensembles are inactive.

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 PtO 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.

Controlling the Temperature and Speed of the Phase Transition of VO2 Microcrystals.

We investigated the control of two important parameters of vanadium dioxide (VO2) microcrystals, the phase transition temperature and speed, by varying microcrystal width. By using the reflectivity change between insulating and metallic phases, phase transition temperature is measured by optical microscopy. As the width of square cylinder-shaped microcrystals decreases from ∼70 to ∼1 μm, the phase transition temperature (67 °C for bulk) varied as much as 26.

Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction.

Demand on the practical synthetic approach to the high performance electrocatalyst is rapidly increasing for fuel cell commercialization. Here we present a synthesis of highly durable and active intermetallic ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated with a "dual purpose" N-doped carbon shell. Ordered fct-PtFe NPs with the size of only a few nanometers are obtained by thermal annealing of polydopamine-coated PtFe NPs, and the N-doped carbon shell that is in situ formed from dopamine coating could effectively prevent the coalescence of NPs.

Graphene oxide-phosphor hybrid nanoscrolls with high luminescent quantum yield: synthesis, structural, and X-ray absorption studies.

Highly luminescent graphene oxide (GO)-phosphor hybrid thin films with a maximum quantum yield of 9.6% were synthesized via a simple chemical method. An intense luminescence emission peak at 537 nm and a broad emission peak at 400 nm were observed from the GO-phosphor hybrid films.

A high-pressure-induced dense CO overlayer on a Pt(111) surface: a chemical analysis using in situ near ambient pressure XPS.

We investigated the high-density CO adsorption phase formed on a Pt(111) surface when exposed to CO gas of pressure ranging from UHV to 100 mTorr using near-ambient-pressure (NAP)-XPS. Combined results from the NAP-XPS measurements and DFT calculations reveal the adsorption structure of CO molecules in the dense CO overlayer, which is stable under realistic conditions.

Excavated Fe-N-C sites for enhanced electrocatalytic activity in the oxygen reduction reaction.

Platinum (Pt) is the best electrocatalyst for the oxygen reduction reaction (ORR) in hydrogen fuel cells, but it is an extremely expensive resource. The successful development of a cost-effective non-Pt ORR electrocatalyst will be a breakthrough for the commercialization of hydrogen-air fuel cells. Ball milling has been used to incorporate metal and nitrogen precursors into micropores of carbon more effectively and in the direct nitrogen-doping of carbon under highly pressurized nitrogen gas in the process of the preparation of non-noble ORR catalysts.

Role of preferential weak hybridization between the surface-state of a metal and the oxygen atom in the chemical adsorption mechanism.

We report on the chemical adsorption mechanism of atomic oxygen on the Pt(111) surface using angle-resolved-photoemission spectroscopy (ARPES) and density functional calculations. The detailed band structure of Pt(111) from ARPES reveals that most of the bands near the Fermi level are surface-states. By comparing band maps of Pt and O/Pt, we identify that dxz (dyz) and dz(2) orbitals are strongly correlated in the surface-states around the symmetry point M and K, respectively.

An etched nanoporous Ge anode in a novel metal-air energy conversion cell.

We first report the successful synthesis of porous germanium with ordered hierarchical structures, via controlled etching, and show its performance as an anode in a new metal-air battery. Our experimental results demonstrate the potential use of porous germanium in a high power density Ge-air energy conversion cell, showing a stable long-term discharge profile at various current drains.

Active Surface Oxygen for Catalytic CO Oxidation on Pd(100) Proceeding under Near Ambient Pressure Conditions.

Catalytic CO oxidation reaction on a Pd(100) single-crystal surface under several hundred mTorr pressure conditions has been studied by ambient pressure X-ray photoelectron spectroscopy and mass spectroscopy. In-situ observation of the reaction reveals that two reaction pathways switch over alternatively depending on the surface temperature. At lower temperatures, the Pd(100) surface is covered by CO molecules and the CO2 formation rate is low, indicating CO poisoning.

Spectroscopic assessment of the role of hydrogen in surface defects, in the electronic structure and transport properties of TiO2, ZnO and SnO2 nanoparticles.

The interaction of metal oxides with gases is very important for the operation of energy devices such as fuel cells and gas sensors, and also relevant for materials synthesis and processing. The electronic transport properties of metal oxides for the aforementioned devices strongly depend on the chemistry of these gases and on the presence or absence of defects on the surface and in the bulk. The Debye screening length is in this respect a material specific property which becomes particularly significant when the material is comprised of nanoparticles.

The study of oxygen molecules on Pt (111) surface with high resolution x-ray photoemission spectroscopy.

By using high resolution x-ray photoelectron spectroscopy, we show that inelastic scattering of photoelectron at low temperature (30-50 K) generates two kinds of oxygen species on Pt (111) surface. Intense synchrotron radiation source dissociates oxygen molecules into chemisorbed atomic oxygen and induces the formation of PtO on the surface. Estimated coverage of dissociated atomic oxygen is 0.