Energy-resolution Electron Energy-loss Spectroscopy Research Articles

Characterization of CeOx-decorated Pd/C Catalysts Synthesized By Controlled Surface Reactions for Hydrogen Oxidation in Anion Exchange Membrane Fuel Cells

The high cost and difficult task to eliminate platinum (Pt) catalyst in a Proton Exchange Membrane Fuel Cells (PEM-FCs) are one of the main obstacles preventing wide adoption of fuel cells. The anion exchange membrane fuel cell (AEM-FC) has been proposed as an alternative in which non-Pt metals may be employed. However, obtaining a high-power density Pt-free AEM-FC has been a challenging task because the Hydrogen Oxidation Reaction (HOR) in the electrocatalysis experiences a slow kinetics at the anode1–3. In the present study, with the aim of improving the efficiency of HOR catalysts, various ratios of CeOx/Pd catalysts were deposited onto a carbon support using Controlled Surface Reactions (CSR) process. A homogenous distribution of CeOx preferentially attached to Pd nanoparticles (NPs) was expected to achieve highly active CeOx-Pd/C catalysts for HOR. We present here a comprehensive characterization approach for the synthesized highly active catalyst, and correlate obtained structural/compositional parameters to the performance. The characterization of the catalysts was carried out via Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), X-ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HR-TEM), Scanning Transmission Electron Microscopy (STEM) - Energy Dispersive Spectroscopy (EDS), Electron Energy Loss Spectroscopy (EELS), and X-ray Photoelectron Spectroscopy (XPS) to confirm the bulk composition, phases present, morphology, elemental mapping, local oxidation state and surface chemical states, respectively. The HRTEM images indicated that Pd NPs were uniformly distributed on the carbon support with only some minor agglomeration. Additionally, the achieved high interfacial contact between CeOx and Pd acquired on single NPs was, for the first time, segmented and calculated using High-resolution STEM-EDS maps and Image J processing program by measuring the overlap intensities between Pd and Ce NPs; the results clearly showed that CeOx NPs were in intimate contact with Pd and their interfacial contact area increased with the addition of CeOx, reached a maximum at a ratio 0.38 CeOx-Pd/C, then decreased due to the formation of large CeOx islands upon the further addition of CeOx. The attained interfacial contact area also seems to be much higher than other previously reported Pd-CeO2 catalysts synthesized by other methods4–6, suggesting that the CSR method resulted in a higher selective deposition of CeOx on Pd.Moreover, the characterized two strong peaks shape resulting from electron transitions12 and the intensity ratio13 of the M4 to M5 edge bands of CeOx obtained from the highest energy resolution (1.1eV) EELS, suggested that most of the CeOx was in the form of CeO2 and Ce4+ was predominantly present in CeOx. However, the presence of Ce3+ cannot be ruled out, specially near the surface of the particles14. The study also found that the already mentioned interfacial contact area was directly correlated to the electrochemical performance reflected on the HOR activity of the CeOx-Pd/C catalysts. From this work, it can be said that when using the CSR method the highest recorded HOR exchange current density, compared to previously reported Pd-CeO2 catalysts, (51.5 mA mg–1 Pd)4–9 can be achieved. AEMFC performance obtained when using CeOx-Pd/C catalyst with Ce/Pd bulk atomic ratio of 0.38 was considerably higher than other cells with analogous HOR catalysts (peak power density of 1169 mW cm–2 mgPd –1 at 333 K)4,5,10,11. This results can be explained by the improved distribution of CeOx onto Pd enhancing OH– spillover from CeOx to Pd, the higher concentration of Pd (IV) sites, and the higher interfacial contact area between CeOx and Pd nanoparticles. REFERENCES: Davydova, E. S., Mukerjee, S., Jaouen, F. & Dekel, D. R. ACS Catal. 8, 6665–6690 (2018).Dekel, D. R. Curr. Opin. Electrochem. 12, 182–188 (2018).Dekel, D. R. J. Power Sources 375, 158–169 (2018).Miller, H. A. et al. Angew. Chemie - Int. Ed. 55, 6004–6007 (2016).Hamish Miller, Francesco Vizza, et al. Nano Energy, Elsevier 293–305 (2017) doi:10.1039/b000000x/1.Yu, H. et al. Nano Energy 57, 820–826 (2019).Yarmiayev, V., Alesker, M., Muzikansky, A., Zysler, M. & Zitoun, D. J. Electrochem. Soc. 166, F3234–F3239 (2019).Bellini, M. et al. ACS Appl. Energy Mater. 2, 4999–5008 (2019).Ralbag, N. et al. Electrochem. Soc. 167, 054514 (2020).Omasta, T. J. et al. J. Electrochem. Soc. 165, J3039–J3044 (2018).Alesker, M. et al. J. Power Sources 304, 332–339 (2016).Goris, B., Turner, S., Bals, S. & Van Tendeloo, G. ACS Nano 8, 10878–10884 (2014).Liu, J. Thesis, Univ. Birmingham (2017).Sims, C. M. et al. Nanotechnology 30, (2019).

Progress in ultrahigh energy resolution EELS

Electron energy loss spectroscopy (EELS) in the electron microscope has progressed remarkably in the last five years. Advances in monochromator and spectrometer design have improved the energy resolution attainable in a scanning transmission electron microscope (STEM) to 4.2 meV, and new applications of ultrahigh energy resolution EELS have not lagged behind. They include vibrational spectroscopy in the electron microscope, a field that did not exist 5 years ago but has now grown very substantially. Notable examples include vibrational mapping with about 1 nm spatial resolution, analyzing the momentum dependence of vibrational states in very small volumes, determining the local temperature of the sample from the ratio of energy gains to energy losses, detecting hydrogen and analyzing its bonding, probing radiation-sensitive materials with minimized damage by aloof spectroscopy and leap-frog scanning, and identifying biological molecules with different isotopic substitutions. We review the instrumentation advances, provide a summary of key applications, and chart likely future directions.

Surface plasmon resonances in a branched silver nanorod

The properties of surface plasmon-polariton (SPP) resonance modes were investigated in a branched silver nanorod consisting of two nanorods using high energy resolution electron energy-loss spectroscopy (EELS) combined with scanning transmission electron microscopy. The experimental EELS maps of the SPP excitation showed a specific distribution depending on the resonance mode. The analytical formulas of the EELS signal and the resonance conditions for SPP resonance in the branched nanorod were derived by expanding the method reported for a one-dimensional straight resonator, and these were applied to interpret the experimental results. The SPP resonance in the branched nanorod can be attributed to Fabry-Perot type resonance that propagates in any of the three different resonance paths. It was also demonstrated that the modes of SPP resonance are changed by the position of the crossing point in the branched structure, which indicates that the combination of the lengths of the respective branches can be used to control the propagation path and the resonance energy of SPP waves in branched structures.

The association of hydrogen with nanometre bubbles of helium implanted into zirconium

Electron energy-loss spectroscopy (EELS) is used to investigate the association of hydrogen with helium bubbles in zirconium. Conventional EELS data yield a signal at 13.5eV (similar to the hydrogen K-edge, 13eV), which is spatially distributed around the peripheries of bubbles and may correlate with the concentration of hydrogen/deuterium in the material. Ultra-high energy resolution EELS yields a signal at 148.6meV (comparable to a range of ZrH bonds, 130–156meV) from a region containing bubbles and no visible hydrides. These signals are interpreted in the context of either bubble surface chemisorption or bubble stress field trapping mechanisms.

Dielectric Properties of Photo-Luminescent CdSe/CdS Mono-Shell and CdSe/CdS/ZnS Multi-Shell Nanocrystals Studied by TEM-EELS

Semiconductor nanocrystals (NCs) of CdSe/CdS (core/shell) mono-shell and CdSe/CdS/ZnS (core/shell/shell) multi-shell having bright and stable luminescence were investigated using high energy-resolution electron energy-loss spectroscopy (EELS) based on transmission electron microscopy to evaluate their dielectric properties in the visible light energy region. EELS spectra of the mono-shell and multi-shell NCs showed two-step structures attributed to interband transitions near band gaps of individual CdSe core and CdS shell. EELS simulations based on the classical dielectric theory revealed that the two-step structures were results of dielectric response from CdSe core and CdS shell. The intensity profiles of the two-step structures depend on a thickness of CdS shells and mono/multi-shell structures of NCs. Besides, the quantum confinement effect was observed in the spectra of mono- and multi-shell NCs, where onset energy of the CdSe core was larger than that of bulk CdSe. The EELS spectra of multi-shell NCs, however, did not present an evident onset due to a bandgap of ZnS outer-shell suggesting defects or amorphous state in ZnS outer-shell. Such dielectric properties of NCs obtained from EELS analysis could provide valuable information to evaluate the crystallinity of individual core and shells in NCs.

Nanoscale Probing of Bandgap States on Oxide Particles Using Electron Energy-Loss Spectroscopy

Surface and near-surface electronic states were probed with nanometer spatial resolution in MgO and TiO2 anatase nanoparticles using ultra-high energy resolution electron energy-loss spectroscopy (EELS) coupled to a scanning transmission electron microscope (STEM). This combination allows the surface electronic structure determined with spectroscopy to be correlated with nanoparticle size, morphology, facet etc. By acquiring the spectra in aloof beam mode, radiation damage to the surface can be significantly reduced while maintaining the nanometer spatial resolution. MgO and TiO2 showed very different bandgap features associated with the surface/sub-surface layer of the nanoparticles. Spectral simulations based on dielectric theory and density of states models showed that a plateau feature found in the pre-bandgap region in the spectra from (100) surfaces of 60nm MgO nanocubes is consistent with a thin hydroxide surface layer. The spectroscopy shows that this hydroxide species gives rise to a broad filled surface state at 1.1eV above the MgO valence band. At the surfaces of TiO2 nanoparticles, pronounced peaks were observed in the bandgap region, which could not be well fitted to defect states. In this case, the high refractive index and large particle size may make Cherenkov or guided light modes the likely causes of the peaks.

Nanoscale probing of bandgap states on oxide particles using electron energy-loss spectroscopy

Surface and near-surface electronic states were probed with nanometer spatial resolution in MgO and TiO2 anatase nanoparticles using ultra-high energy resolution electron energy-loss spectroscopy (EELS) coupled to a scanning transmission electron microscope (STEM). This combination allows the surface electronic structure determined with spectroscopy to be correlated with nanoparticle size, morphology, facet etc. By acquiring the spectra in aloof beam mode, radiation damage to the surface can be significantly reduced while maintaining the nanometer spatial resolution. MgO and TiO2 showed very different bandgap features associated with the surface/sub-surface layer of the nanoparticles. Spectral simulations based on dielectric theory and density of states models showed that a plateau feature found in the pre-bandgap region in the spectra from (100) surfaces of 60nm MgO nanocubes is consistent with a thin hydroxide surface layer. The spectroscopy shows that this hydroxide species gives rise to a broad filled surface state at 1.1eV above the MgO valence band. At the surfaces of TiO2 nanoparticles, pronounced peaks were observed in the bandgap region, which could not be well fitted to defect states. In this case, the high refractive index and large particle size may make Cherenkov or guided light modes the likely causes of the peaks.

Vanadium Substitution in Li2MnSiO4/C as Positive Electrode for Li Ion Batteries

Vanadium substitution is an interesting approach to manipulate the properties of the poor electronic and ionic conducting lithium transition metal orthosilicates. Especially, if incorporated on the Si site, it could alter the highly insulating character of the SiO4 framework. This study addresses the feasibility and limitations of V substitution in Li2MnSiO4. Nominal compositions of Li2Mn1–xVxSiO4 (0 ≤ x ≤ 0.2) and Li2MnSi1–xVxO4 (0 ≤ x ≤ 0.3) were synthesized by a sol–gel method, and the structural evolution was analyzed by X-ray diffraction and transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). While the solid solubility of V on tetrahedral Mn sites was shown to be limited, substantial amounts of V entered the structure when intended to substitute Si. Elemental mapping by TEM showed that V was highly inhomogeneously distributed, and high energy resolution EELS demonstrated that the majority of V was present in a tetravalent state. The nominal compositions Li2Mn...

Measuring bandgap states in individual non-stoichiometric oxide nanoparticles using monochromated STEM EELS: The Praseodymium–ceria case

We describe a method to perform high spatial resolution measurement of the position and density of inter-band impurity states in non-stoichiometric oxides using ultra-high energy resolution electron energy-loss spectroscopy (EELS). This can be employed to study optical and electronic properties of atomic and nanoscale defects in electrically-conducting and optically-active oxides. We employ a monochromated scanning transmission electron microscope with subnanometer diameter electron probe, making this technique suitable for correlating spectroscopic information with high spatial resolution images from small objects such as nanoparticles, surfaces or interfaces. The specific experimental approach outlined here provides direct measurement of the Pr inter-band impurity states in Pr0.1Ce0.9O2−δ via valence-loss EELS, which is interpreted with valence-loss spectral simulation based on density of states data to determine the energy level and character of the inter-band state. Additionally, observation of optical color change upon chemically-induced oxygen non-stoichiometry indicates that the population of the inter-band state is accompanied by an energy level shift within the bandgap.

Extremely confined gap surface-plasmon modes excited by electrons.

High-spatial and energy resolution electron energy-loss spectroscopy (EELS) can be used for detailed characterization of localized and propagating surface-plasmon excitations in metal nanostructures, giving insight into fundamental physical phenomena and various plasmonic effects. Here, applying EELS to ultra-sharp convex grooves in gold, we directly probe extremely confined gap surface-plasmon (GSP) modes excited by swift electrons in nanometre-wide gaps. We reveal the resonance behaviour associated with the excitation of the antisymmetric GSP mode for extremely small gap widths, down to ~5 nm. We argue that excitation of this mode, featuring very strong absorption, has a crucial role in experimental realizations of non-resonant light absorption by ultra-sharp convex grooves with fabrication-induced asymmetry. The occurrence of the antisymmetric GSP mode along with the fundamental GSP mode exploited in plasmonic waveguides with extreme light confinement is a very important factor that should be taken into account in the design of nanoplasmonic circuits and devices.

Characteristic chemical shifts of quasicrystalline Zn–Mg–Zr alloys studied by EELS and SXES

Chemical shifts of the constituent atoms of primitive icosahedral quasicrystal (P-QC), face-centred icosahedral quasicrystal (F-QC) and 1/1-approximant (1/1-AP) of F-QC Zn–Mg–Zr alloys were investigated for the first time using high energy-resolution electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Among Zn M-shell and Mg L-shell excitation EELS spectra of P-QC, F-QC and 1/1-AP alloys, only the quasicrystalline alloys showed a chemical shift towards the larger binding energy side. In Zn-L and Zr-L emission SXES spectra, the P-QC and F-QC alloys showed a chemical shift towards larger binding energy side. The magnitudes of the shifts in the Zn-L emission spectra of the quasicrystalline alloys were almost the same as for ZnO. These results strongly suggest a decrease in valence charge in quasicrystalline states. Therefore, it should be concluded that bonding in quasicrystalline states involves a characteristic increase in covalency compared with bonding in corresponding approximant and standard metal crystals.

Characteristic chemical shift of quasicrystalline alloy Al53Si27Mn20 studied by EELS and SXES

Chemical shifts of all constituent atoms for amorphous (Am), quasicrystalline (QC) and crystalline (Cryst) alloys of Al53Si27Mn20 were investigated for the first time by high energy-resolution electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Among Al L-shell excitation EELS spectra of Am, QC and Cryst alloys, only QC alloy showed an apparent chemical shift to the larger binding energy side by 0.4 eV. In Al-Kα and Si-Kα emission SXES spectra of these alloys, only QC alloy showed a chemical shift to the larger binding energy side by 4 eV for Al-Kα and 6 eV for Si-Kα. These chemical shift values are comparable to those of corresponding metal oxides. This indicates a smaller amount of valence charge at Al and Si atomic sites in QC alloy. On the other hand, Mn-L SXES spectra did not show any chemical shift. Therefore, the decreased charge from Al and Si sites should be distributed between atomic sites, indicating the presence of covalent bonding nature for QC ordered alloy.

Electronic structure analyses of BN network materials using high energy‐resolution spectroscopy methods based on transmission electron microscopy

Electronic structures of boron-nitride (BN) nanotubes and a BN cone-structure material were studied by using a high energy-resolution electron energy-loss spectroscopy (EELS) microscope. A trial of the whole electronic structure study of hexagonal BN (h-BN), which consists of flat BN honeycomb layers, was conducted by a combination of EELS and X-ray emission spectroscopy (XES) based on transmission electron microscopy (TEM) (TEM-EELS/XES). The pi and pi+sigma plasmon energies of BN nanotubes (BNT) were smaller than those of h-BN. The pi+sigma energy was explained by the surface plasmon excitation. The spectrum of a two-wall BNT of 2.7 nm in diameter showed a new spectral onset at 4 eV. The valence electron excitation spectra obtained from the tip region of the BN cone with an apex angle of 20 degrees showed similar intensity distribution with those of BNTs. The B K-shell electron excitation spectra obtained from the bottom edge region of the BN cone showed additional peak intensity when compared with those of h-BN and BNT. The B K-shell electron excitation spectra and B K-emission spectra of h-BN were compared with a result of a LDA band calculation. It showed that high symmetry points in the band diagram appear as peak and/or shoulder structures in the EELS and XES spectra. Interband transitions appeared in the imaginary part of the dielectric function of h-BN experimentally obtained were assigned in the band diagram. The analysis also presented that the LDA calculation estimated the bandgap energy smaller than the real material by an amount of 2 eV. Those results of TEM-EELS/XES analysis presented that high energy-resolution spectroscopy methods combined with TEM is a promising method to analyze whole electronic structures of nanometer scale materials.

High Spatial and Energy Resolution EELS using a Monochromated STEM

The ability to directly correlate electron energy loss spectroscopy (EELS) with Z-contrast images in the scanning transmission electron microscope (STEM) provides a powerful tool to characterize nanoscale materials. The incorporation of a monochromator into the FEI Tecnai F20 UT at the National Center for Electron Microscopy (NCEM) extends the power of these techniques by increasing the sensitivity of core-loss spectra to small changes in fine-structure and by allowing the low-loss region of the spectrum to be analyzed in detail. Here, the configuration of the monochromated Tecnai will be discussed and examples of spectra obtained utilizing the highest spatial and energy resolution will be presented [1].

電子線結晶学と電子線分光学の研究 １．電子線結晶学を中心として

Our study of electron crystallography or convergent beam electron diffraction (CBED) and of electron spectroscopy or electron energy-loss spectroscopy (EELS), which were done from my student time until my retirement, is described together with other important results. All the crystal point groups can be determined unambiguously by CBED. Most crystal space groups are determined by using dynamical extinction effect and coherent CBED. The large angle technique enables us to determine the characteristic features of the lattice defects more reliably than the traditional electron microscope method. Using the energy filtering technique, the crystal structure refinement is successfully carried out by fitting the experimental CBED pattern with the theoretically calculated pattern. A case study of the determination of a charge density distribution is shown. A high energy resolution EELS instrument (< 0.1 eV) was developed for the study of electronic structures or the structure of the conduction band. A case study of a detailed band structure is shown with a theoretical calculation. A clean application to fullerenes is shown. A recent development of the wavelength dispersive spectroscopy attachment is mentioned.

A Sub-Ev Resolution Soft-X-Ray Spectrometer for a Transmision Electron Microscope to Obtain the Density of States of the Valence Band

Abstract We developed a high energy-resolution electron energy-loss spectroscopy (EELS) microscope to investigate electronic structures from specified small specimen areas [1]. EELS gives us information of the dielectric properties and the partial density of states (DOS) of the conduction band. Together with the DOS of the conduction band (unoccupied states), the DOS of the valence band (occupied states) is imperative to understand the whole electronic structure. Unfortunately, the DOS of the valence band cannot be reached directly by EELS. A partial DOS of the valence band can be obtained by fluorescent X-ray emission spectroscopy (XES). A conventional EELS instrument attached to a TEM usually takes spectra with an energy resolution of about 1 eV, by which the DOS of conduction bands has been studied using ELNES until now. Thus, an energy resolution of about 1 eV is at least necessary for the study of the DOS of the valence band.

Development of a high energy resolution electron energy-loss spectroscopy microscope.

We have developed a high energy resolution electron energy-loss spectroscopy (EELS) microscope, which can take spectra from specified small specimen areas and specified small reciprocal space areas to investigate detailed electronic structures. The EELS microscope is equipped with retarding Wien filters as the monochromator and the analyser. The filters are designed to achieve a stigmatic focus. The energy resolutions are 12 meV and 25 meV for cases without and with a specimen, respectively. Spatial and momentum resolutions are 30-110 nm in diameter and 1.1 nm-1 in angular diameter, respectively. EELS spectra are presented to show the performance of this instrument.