Total Reflection Fluorescence X-ray Absorption Research Articles

Can amine ligands atomically disperse Cu atoms on TiO2(110)? Cu deposition on TiO2(110) premodified with o-anthranilic acid

Abstract The three-dimensional (3D) structure of evaporated Cu atoms on a TiO2(110) surface premodified with o-anthranilic acid (o-AA) was examined using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) to elucidate the effect of the premodification on the Cu dispersion over the TiO2(110) surface. Cu was found to be atomically dispersed with the formation of N–Cu–O bonds (where the N is provided by the o-AA and the O is present in the TiO2 lattice) on the surface, and the factors for controlling the atomic dispersion are discussed.

Bent-crystal Laue analyser combined with polarization-dependent total reflection fluorescence X-ray absorption fine structure (BCLA-TRFXAFS) and its application to electrode surface

Bent-crystal Laue analyser combined with polarization-dependent total reflection fluorescence X-ray absorption fine structure (BCLA-TRFXAFS) and its application to electrode surface

State of the Active Site in La1–xSrxCoO3−δ Under Oxygen Evolution Reaction Investigated by Total-Reflection Fluorescence X-Ray Absorption Spectroscopy

Recent developments in hydrogen energy devices have furthered the research on sustainable hydrogen production methods. Among these, the water splitting process has been considered a promising hydrogen production method, particularly, in alkaline media. The lack of information on the reaction active sites under the conditions of the oxygen evolution reaction (OER) hinders establishing guidelines for catalyst development. In the case of powder catalysts, many operando techniques also measure bulk information, and therefore, extracting information on the reaction active sites is challenging. Accordingly, film electrodes were used in this study, and the electrochemical performance and reaction kinetics of perovskite-type La1–xSrxCoO3−δ films as OER catalysts synthesized by pulsed laser deposition were investigated. By combining ex situ X-ray absorption spectroscopy (XAS) and operando total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS), we succeeded in observing a significant oxidation state change on the surface of La0.6Sr0.4CoO3−δ, which indicated that the active surface sites were formed upon applying the OER potential. This surface reconstruction resulted in numerous active sites at the reaction interface, thereby enhancing the OER activity. This study provides definitive evidence for the surface reconstruction of OER catalysts, which enhances the fundamental understanding of OER catalyst behaviors, and can inspire the design of active OER catalysts by suitable surface modulation.

Development of Operando Polarization-Dependent Total Reflection Fluorescence X-ray Absorption Fine Structure Technique for Three-Dimensional Structure Determination of Active Metal Species on a Model Catalyst Surface under Working Conditions

Development of <i>Operando</i> Polarization-Dependent Total Reflection Fluorescence X-ray Absorption Fine Structure Technique for Three-Dimensional Structure Determination of Active Metal Species on a Model Catalyst Surface under Working Conditions

Extracting the local electronic states of Pt polycrystalline films surface under electrochemical conditions using polarization-dependent total reflection fluorescence x-ray absorption near edge structure spectroscopy

The local atomic information about the interface between the 30 nm-thick Pt polycrystalline films and the solution with and without perfluorosulfonic acid polymers (Nafion®) for the model cathode catalyst of fuel cell has been captured under electrochemical conditions using polarization-dependent total reflection fluorescence x-ray absorption near edge structure spectroscopy (PTRF–XANES). The results show that the formation of sub-monolayer-equivalent PtO or adsorbed hydrogen/oxygen species in the surface region can be successfully observed in the PTRF–XANES spectra when the thickness of the solution layer and the incidence angle are properly controlled. This capability enables us to examine the metal /(Nafion®/) solution interface structure through XANES together with other surface analysis methods, which will enhance comprehensive understanding of the nature of the interface of the fuel cell system.

Bent crystal Laue analyser combined with total reflection fluorescence X-ray absorption fine structure (BCLA + TRF-XAFS) and its application to surface studies.

A bent crystal Laue analyser (BCLA) is an X-ray energy analyser used for fluorescence X-ray absorption fine-structure (XAFS) spectroscopy to separate the fluorescence X-ray emission line of a target atom from the elastic scattering X-rays and other fluorescence emission lines. Here, the feasibility of the BCLA for total reflection fluorescence XAFS (TRF-XAFS), which has a long X-ray footprint on the substrate surface owing to grazing incidence, was tested. The focal line of the BCLA was adjusted on the X-ray footprint and the XAFS signal for one monolayer of Pt deposited on a 60 nm Au film with high sensitivity was obtained. Although range-extended XAFS was expected by the rejection of Au fluorescence arising from the Au substrate, a small glitch was found in the Au L3 edge because of the sudden change of the complex refraction index of the Au substrate at the Au edge. This abnormal spectrum feature can be removed by reflectivity correction using Au foil absorption data. BCLA combined with TRF-XAFS spectroscopy (BCLA + TRF-XAFS) is a new technique for the in situ surface analysis of highly dispersed systems even in the presence of a liquid overlayer.

Premodified Surface Method to Obtain Ultra-Highly Dispersed Metals and their 3D Structure Control on an Oxide Single-Crystal Surface.

Precise control of the three-dimensional (3D) structure of highly dispersed metal species such as metal complexes and clusters attached to an oxide surface has been important for the development of next-generation high-performance heterogeneous catalysts. However, this is not easily achieved for the following reasons. (1) Metal species are easily aggregated on an oxide surface, which makes it difficult to control their size and orientation definitely. (2) Determination of the 3D structure of the metal species on an oxide powder surface is hardly possible. To overcome these difficulties, we have developed the premodified surface method, where prior to metal deposition, the oxide surface is premodified with a functional organic molecule that can strongly coordinate to a metal atom. This method has successfully provided a single metal dispersion on an oxide single-crystal surface with the 3D structure precisely determined by polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). Here we describe our recent results on ultra-high dispersions of various metal atoms on TiO2 (110) surfaces premodified with mercapto compounds, and show the possibility of fine tuning and orientation control of the surface metal 3D structures.

Reaction Stoichiometry and Mechanism of Pt Deposition via Surface Limited Redox Replacement of Copper UPD Layer on Au(111)

The stoichiometry and reaction mechanism of Pt deposition via surface limited redox replacement (SLRR) of Cu underpotential-deposited (UPD) monolayer on Au(111) was studied using in situ polarization dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). We proposed that Pt deposition via SLRR of Cu UPD monolayer leads to formation/deposition of Pt-surface species mainly consisting of Pt(II) chloride with a square planar local structure [PtCl4] parallel to the Au surface (60%) which has a strong interaction of the Pt complex with the Au substrate. The rest (40%) was one-monolayer Pt metal cluster. This result provides a new understanding into the mechanism and stoichiometry of the SLRR reaction, which has a wide application for synthesis of monolayer catalysts.

Polarization-dependent Total Reflection Fluorescence X-ray Absorption Fine Structure (PTRF-XAFS) Studies on the Structure of a Pt Monolayer on Au(111) Prepared by the Surface-limited Redox Replacement Reaction

We studied the initial stage of a Pt monolayer produced by surface-limited redox replacement (SLRR) using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). Different from the widely accepted understanding that a metallic monolayer island is formed, our XAFS showed that the Pt monolayer, initially present on the Au(111) substrate, was mainly in the form of a planar [PtCl4]2- complex with its molecular plane parallel to the Au(111). This result provides a new insight into the mechanism of SLRR.

The Enhancement of the Interface Reaction in LiFePO4 Two-Phase System

LiFePO4 is considered as a high rate capability cathode material for lithium-ion battery. During charging and discharging, the material undergoes a two-phase reaction between Li-rich and Li-poor phases1. The rate-determining step in the phase transition of LixFePO4 is expected to be the nucleation reaction2. Recently, it is reported that surface modification of LiFePO4 with nitrogen improves the rate capability3. However, the origin of the improvement in rate performance still has not been understood well. This is because it is difficult to analyze the interfacial reaction using composite electrodes, which have complex structures of mixture with active material, carbon, and binder. To discuss correlation between the surface modification and reaction kinetics of LiFePO4, we made surface modified LiFePO4 thin-film electrode with nitrogen, and investigated the surface electronic structure of the electrode by total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS).LiFePO4 thin-film (Bare-LFP) was prepared on a polycrystalline Au substrate by pulsed laser deposition method. The deposition was conducted under Ar atmosphere (0.0001 Pa) at 923K for 30 min. Surface modified LiFePO4 thin-film with nitrogen (N-LFP) was prepared by heating the Bare-LFP under NH3 atmosphere at 923K for 10 min. For the electrochemical analysis, three-electrode cells employing Li metal as a counter and a reference electrode, and 1 M LiClO4 in propylene carbonate as a electrolyte were prepared. Bare-LFP was characterized by cyclic voltammetry and transmission electron microscope (TEM). N-LFP was characterized by N-K edge XAS. The rate capability of Bare-LFP and N-LFP estimated charge/discharge measurements from 1C to 100C.We also discuss the reaction kinetics of LiFePO4 with anion surface modification to enhance rate performance. The reaction kinetics of total process included charge transfer was calculated by cyclic voltammetry (CV) and of phase transition by potentiostatic intermittent titration technique (PITT). The result of CV indicated better kinetics by nitrogen surface modification than by undoped and of PITT corresponded with them.The results support improvement of nucleation kinetics.Here we report a characterization of surfaces of LiFePO4 thin-film model electrodes by TRF-XAS that shows the slightly higher oxidation state of iron by nitrogen surface modification than by undoped. This result indicates that the existence of Fe(2+δ)+at the surface decreases the nucleation energy as well as moderates the lattice strain, leading to better battery performance. The facilitation of phase transition, rather than particle size minimization or intimate carbon contact reforming bulk structures, is a novel approach to enhance reaction kinetics.1. Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc., 144, 1188,(1997). 2. Delmas, C.; Maccario, M.; Croguennec, L.; Le Cras, F.; Weill, F. Nat Mater, 7, 665,(2008). 3. Park, K. S.; Xiao, P. H.; Kim, S. Y.; Dylla, A.; Choi, Y. M.; Henkelman, G.; Stevenson, K. J.; Goodenough, J. B. Chem Mater, 24, 3212,(2012). This work was partially supported by KAKENHI Grant Number 25249141.

A New Indicator for Single Metal Dispersion on a TiO2(110) Surface Premodified with a Mercapto Compound

Ni and Pt structures evaporated onto a TiO2(110) surface premodified with ortho-mercaptobenzoic acid (o-MBA) were studied using polarization-dependent total reflection fluorescence X-ray absorption fine structure analysis to determine the effects of the premodification on the dispersion of the metal atoms over the TiO2(110) surface. Ni was found to be atomically dispersed with the formation of S–Ni–O bonds (where the S is provided by the o-MBA and the O is present in the TiO2 lattice) on the surface. In contrast, Pt underwent aggregation to form small clusters. The varying behavior of these metals on the o-MBA-modified TiO2(110) surface is discussed based on the energy difference between sulfur–metal–oxygen and metal–metal bond formations, and we propose a new indicator for single metal dispersion on the TiO2(110) surface.

Local Structural Analysis of Amorphous In-Ga-Zn-O Thin Films by Total Reflection Fluorescence X-ray Absorption Fine Structure Technique

Local Structural Analysis of Amorphous In-Ga-Zn-O Thin Films by Total Reflection Fluorescence X-ray Absorption Fine Structure Technique

Portable ultrahigh-vacuum sample storage system for polarization-dependent total-reflection fluorescence x-ray absorption fine structure spectroscopy

A portable ultrahigh-vacuum sample storage system was designed and built to investigate the detailed geometric structures of mass-selected metal clusters on oxide substrates by polarization-dependent total-reflection fluorescence x-ray absorption fine structure spectroscopy (PTRF-XAFS). This ultrahigh-vacuum (UHV) sample storage system provides the handover of samples between two different sample manipulating systems. The sample storage system is adaptable for public transportation, facilitating experiments using air-sensitive samples in synchrotron radiation or other quantum beam facilities. The samples were transferred by the developed portable UHV transfer system via a public transportation at a distance over 400 km. The performance of the transfer system was demonstrated by a successful PTRF-XAFS study of Pt4 clusters deposited on a TiO2(110) surface.

Effect of an Electrolyte Additive of Vinylene Carbonate on the Electronic Structure at the Surface of a Lithium Cobalt Oxide Electrode under Battery Operating Conditions

Lifetimes of lithium-ion batteries are often affected by deterioration of positive electrodes. It is well-known that the deterioration of the positive electrodes can be reduced by using electrolyte additives; however, the mechanism underlying this cyclability improvement needs to be clarified. In this study, we investigate electronic structure at the electrode/electrolyte interface using in situ total-reflection fluorescence X-ray absorption spectroscopy to elucidate the mechanism underlying the cyclability improvement of a LiCoO2 electrode upon addition of vinylene carbonate (VC) to the electrolyte. The results indicate that the reduction of cobalt ions at the surface of the LiCoO2 electrode, which occurs upon soaking in the electrolyte in the absence of VC, is suppressed by the presence of the VC additive. The VC additive also suppresses irreversible change in the electronic structure of the cobalt ions at the LiCoO2 surface during successive charge/discharge processes. The effects of the VC additive can be attributed to the formation of a layer of decomposed VC molecules at the LiCoO2/electrolyte interface, which plays an important role in the suppression of the irreversibility at the LiCoO2 surface during the charge/discharge processes.

In Operando Analysis of Cathode Reaction By Synchrotron X-Ray Absorption and Diffraction

1. Introduction in operando analysis enables us to observe non-equilibrium behavior occurring in an operating battery condition without touching its components and so actual processes associated with charging/discharging can be elucidated. In this study, in operando techniques are applied to investigate two phenomena; (1) non-equilibrium phase transition behavior (2) stability at electrode / electrolyte interface. 2. Snapshot Observation of Phase Transition Dynamics in LiFePO4 and LiNi0.5Mn1.5O4 [1-4] Phase transitions of solid state materials are widely utilized in lithium-ion batteries (LIBs). Phase transitions are explained using a phase diagram with physicochemical parameters, e.g. concentration and temperature. Phase diagrams, including the stability of phase transitions, are explained using the concept of Gibbs-energy, based on the equilibrium between stable phases on either side of a phase transition. For phase transitions in practical devices such as LIBs, the actual phase transition takes place under non-equilibrium conditions, which can result in phenomena different to those expected from thermodynamics. The difficulty in in situ analysis has been low time resolutions. Recently the measuring probes have been much improved and observation at practical charging/discharging rate to 50 C is now available. In this study, in situ techniques by using synchrotron X-ray method and its application to lithium battery analysis using their measurement characteristics are demonstrated.The phase transition between LiFePO4 and FePO4 under non-equilibrium battery operation was tracked in real-time using time-resolved X-ray diffraction. A metastable crystal phase appears in conjunction with increasing current density, in addition to the thermodynamically stable LiFePO4 and FePO4 phases. The metastable phase gradually diminishes under open circuit conditions following electrochemical cycling. We propose a phase transition path that passes through the metastable phase, and posit the new phase’s role in decreasing nucleation energy, accounting for the excellent rate capability of LiFePO4. We examined LiNi0.5Mn1.5O4 as another example. It has been known that there are two phase transitions in its charging processes, namely LiNi0.5Mn1.5O4 (Li1) to Li0.5Ni0.5Mn1.5O4 (Li0.5) and Li0.5 to Ni0.5Mn1.5O4 (Li0). In the in situ measurement, it turned out that the Li0.5 phase is clearly observed during charging and is obscure during discharging, showing the thermodynamically reversible but kinetically asymmetric behavior. Using potential step experiments and kinetic analysis, this phenomenon is ascribed to the slow phase transition between Li0.5 and Li0, compared to that between Li1 and Li0.5. The high time resolution was effective in data acquisition during the potential step experiments. 3. in operando X-ray Absorption Spectroscopic Study on Stability at Electrode / Electrolyte Interface [5-7] In the previously proposed deterioration mechanisms of LIBs, the electrode/electrolyte interface is thought to play a significant role. For example, the formation of a solid electrolyte interface, changes in the crystal structure on the surface of active materials during Li ion extraction/insertion, and improvement in the cyclic performance through surface coating of the active materials have all been reported previously. However, determining the specific cause of the interface stability is quite difficult because the geometric and electronic structures must be tracked with a resolution of less than a few nanometers to reveal the phenomena at the electrode/electrolyte interface. To achieve further elucidation of the deterioration mechanism of LIBs, clarification of the electronic structure at the electrode/electrolyte interface under operation conditions is required.We investigated the effects of the electronic structure at the electrode/electrolyte interface on the cyclic performance of cathode materials via in-situ total-reflection fluorescence X-ray absorption spectroscopy. In a LiCoO2 thin-film electrode that exhibits gradual deterioration upon subsequent Li ion extractions and insertions (cycling), the reduction of Co ions at the electrode/electrolyte interface was observed upon immersion in an organic electrolyte followed by irreversible behavior after cycling. In contrast, in a LiFePO4 thin-film electrode, the electronic structure at the electrode/electrolyte interface was stable and reversible upon electrolyte immersion with subsequent cycling. The increased stability of the electronic structure at the LiFePO4/electrolyte interface impacts its cycling performance. Acknowledgement This work was supported by RISING battery project from NEDO, Japan.

Analysis for the Electronic Structure at Cathode/Electrolyte Interface in Lithium-Ion Battery Via In Situ Total Reflection Fluorescence X-ray Absorption Spectroscopy

Several research groups have reported that the cathode/electrolyte interface plays an essential role for the deterioration of lithium ion batteries [1, 2]. To further enhance the cycle time of lithium-ion batteries, it is important to understand the reactions at the cathode/electrolyte interface. However, the mechanisms of such interface reactions have not been fully understood due to the difficulty of their direct observation at the nanometer scale. Recently, we have developed a new technique named in-situ total-reflection fluorescence X-ray absorption spectroscopy (in-situ TRF-XAS) to analyze directly the electronic structure at the cathode/electrolyte interface under operating condition [3, 4]. By using in-situ TRF-XAS, we investigate the electronic structure of LiCoO2 and/or LiFePO4/electrolyte interface under battery operation condition and discuss the correlation between the electronic structure at the interface and durability of the active materials. LiCoO2 is one of the most popular active materials whose capacity easily fades with the long charge/discharge cycling [5]. On the other hand, LiFePO4 exhibits a more stable capacity with cycling compared to LiCoO2 [6].LiCoO2 and LiFePO4 thin-films were prepared on Pt and Au polycrystalline substrate by pulsed layer deposition (PLD), respectively. In-situ TRF-XAS measurements were performed at the BL01B1, BL28XU and BL37XU at SPring-8 (Japan) using a home-made cell with a counter electrode of Li foil (Fig. 1). The separator (CELGARD®3501) was immersed with 1 mol dm-3 LiClO4 in a 1:1 volumetric mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). The TRF-XAS measurements were conducted under setting the X-ray incident angle at 0.17-0.20° and the generated fluorescence X-ray was detected with a solid state detector. In-situ TRF XAS spectra corresponding to the information at cathode/electrolyte interface of the LiCoO2 and LiFePO4 thin films were analyzed before and after electrolyte immersion. The XANES spectrum of the LiCoO2 surface was shifted to lower energy upon electrolyte immersion, meaning the reduction of the Co ions at the surface. In contrast to LiCoO2, the XANES spectrum of the LiFePO4 surface did not show any shift upon electrolyte immersion, meaning that no reduction of the Fe ions at the surface occurred. At the electrode/electrolyte interface, the electrochemical potential gap between the two phases should be compensated. The potential change at the electrode side and the electrolyte side forms space charge layer and electrical double layer, respectively. For LiCoO2, the reduction of the surface means the formation of the space charge layer via electron transfer form the electrolyte and therefore the potential gap is compensated by both the space charge layer and the electrical double layer (Fig. 2 (a)). As for LiFePO4, the space charge layer hardly forms at the surface because the surface was not reduced via electron transfer from the electrolyte. Therefore, the electrical double layer contributes locally to the potential compensation for LiFePO4 (Fig. 2(b)). The durability of the electrode materials depends on the formation of the space charge layer. Acknowledgments This work was partially supported by New Energy and Industrial Technology Department Organization (NEDO), Japan, for Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING) project.

Improved Cyclic Performance of Lithium-Ion Batteries: An Investigation of Cathode/Electrolyte Interface via In Situ Total-Reflection Fluorescence X-ray Absorption Spectroscopy

For the further development of lithium-ion batteries, improvement of their cyclic performance is crucial. However, the mechanism underlying the deterioration of the battery cyclic performance is not fully understood. We investigated the effects of the electronic structure at the electrode/electrolyte interface on the cyclic performance of the cathode materials via in situ total-reflection fluorescence X-ray absorption spectroscopy. In a LiCoO2 thin-film electrode that exhibits gradual deterioration upon subsequent Li ion extractions and insertions (cycling), the reduction of Co ions at the electrode/electrolyte interface was observed upon immersion in an organic electrolyte, with subsequent irreversible changes after cycling. In contrast, in a LiFePO4 thin-film electrode, the electronic structure at the electrode/electrolyte interface was stable and reversible upon electrolyte immersion with subsequent cycling. The increased stability of the electronic structure at the LiFePO4/electrolyte interface affect...

Thin Film Structures of Metal-Organic Framework [Cu3(BTC)2(H2O)3]n on TiO2(110)

Thin film structures of [Cu3(BTC)2(H2O)3]n (HKUST-1), which is one of Metal-organic frameworks (MOFs), prepared via step-by-step liquid phase epitaxy (LPE) method, on a TiO2(110) surface were investigated by polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). For the ultra thin film, PTRF-XAFS enlightened that HKUST-1 eventually formed even at 1 cycle LPE. For further growth, XRD and SEM verified well-oriented < 111 > HKUST-1 thin films on the surface. These results proposed that the adsorption structure of linkers on the surface plays an important role to control the orientation of MOF films.

High potential durability of LiNi0.5Mn1.5O4 electrodes studied by surface sensitive X-ray absorption spectroscopy

Phenomena at electrode/electrolyte interface of LiNi0.5Mn1.5O4 are studied by in situ total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS), ex situ X-ray photoelectron spectroscopy (XPS), and electrochemical tests. Flat and well-defined thin films of LiNi0.5Mn1.5O4 prepared by pulsed laser deposition (PLD) are used as model electrodes to facilitate the observation of the interface. The thin-film LiNi0.5Mn1.5O4 electrode showed good cycling characteristics at around 4.7 V vs. Li/Li+. The TRF-XAS measurements reveal that nickel and manganese species at the surface have almost the same chemical states and local environments as those in the bulk when in contact with organic electrolyte solutions (1 mol dm−3 LiClO4 in a 1:1 volumetric mixture of ethylene carbonate and diethyl carbonate). This is in sharp contrast to the behavior of a LiCoO2 electrode, in which the surface cobalt species is irreversibly reduced by soaking to the organic electrolyte solutions, leading to gradual material deterioration during the delithiation/lithiation cycling (D. Takamatsu et al., Angew. Chem. Int. Edit., 51 (2012) 11597). It is suggested that the electrolyte decomposition products detected by XPS form a protective layer to restrict the reduction of the surface species of LiNi0.5Mn1.5O4, leading to good cycling characteristics of LiNi0.5Mn1.5O4 in spite of its high operating potential.

Anisotropic growth of a nickel trimer formed on a highly-stepped TiO2(1 1 0) surface

The structures of Ni clusters formed on a highly-stepped TiO2(110) surface were studied by polarization-dependent total reflection fluorescence X-ray absorption fine structure analysis. When 0.8 monolayers of Ni were deposited, three-dimensional Ni clusters with 1–2nm diameters and heights less than 1nm were formed. Conversely, when 0.07 monolayers of Ni were deposited, an anisotropic Ni trimer with a Ni–Ni distance of 0.260nm was created at the [001] step. We revealed that the surface modification to enhance the metal–anion interaction can control the deposited metal structure.