Binding Energy Shifts Research Articles

Adsorption Behavior of PGM-Free Catalysts By Near-Ambient Pressure X-Ray Photoelectron Spectroscopy

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are an important component in the portfolio of renewable energy technologies that are important for our society. While being green technology, PEMFCs rely on precious metals to catalyze the oxygen reduction reaction (ORR) on the cathode side. Several approaches have been explored in order to reduce the amount of precious metals, including core-shell structures, extended thin films, and various alloys with non-precious metals. Potential solutions also include PGM-free catalysts made with earth abundant, inexpensive elements. One promising area of PGM-free catalyst development is the integration of nitrogen and iron into high surface area carbon nanostructures.1,2 These materials have significantly improved in activities over the past decade, yet their heterogeneous nature motivates further study into the nature of the active sites, with the goal of improving activity, active site density, and stability. Multi-technique, multi-scale characterization of these materials, under ex situ and in situ conditions, provides opportunity to better understand surfaces of the materials under relevant conditions and to optimize various interfaces in these systems for optimum performance. In this work near-ambient pressure x-ray photoelectron spectroscopy (nAP-XPS) is used to investigate adsorption and desorption behavior of a diverse set of catalysts and supports, which contain nitrogen and iron sites of interest. Challenges associated with analysis of nAP-XPS data from adsorption and desorption studies include: i) presence of different types of potential sites and the multitude of binding energy shifts that could be happening upon adsorption on these various sites, ii) subtle changes in the spectra associated with adsorption/desorption due to low number of potential sites for adsorption, and iii) sampling depth limitations of the technique.3 To overcome these challenges, we explore less conventional approaches to analysis of the XPS N 1s and O 1s spectra, and complemented XPS with other in situ techniques such as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS). Figure 1. XPS N1s difference spectra comparing adsorption and desorption processes on a PGM-free FeNC material. Jaouen, F., Proietti, E., Lefèvre, M., Chenitz, R., Dodelet, J.P., Wu, G., Chung, H.T., Johnston, C.M., and Zelenay, P., Energy Environ. Sci. 4, 114 (2011).Wood, K. N., O’Hayre, R., and Pylypenko, S. Energy Environ. Sci. 7, 1212 (2014).Matanovic, I., Artyushkova, K.; Strand, M. B., Dzara, M. J., Pylypenko, S., Atanassov, P., Core Level Shifts of Hydrogenated Pyridinic and Pyrrolic Nitrogen in the Nitrogen-Containing Graphene-Based Electrocatalysts: In-Plane vs Edge Defects. J. Phys. Chem. C 120, 51 (2016) Figure 1

Polaron effect influenced by thicknesses of GaAs film and AlxGa1−xAs substrate

The binding energy and effective mass of a polaron confined in a GaAs film deposited on AlxGa1-xAs substrate are studied theoretically within the framework of the fractional-dimensional approach and by using a correctional length of confinement. The numerical results for the polaron binding energy and effective mass in the GaAs film deposited on AlxGa1-xAs substrate are obtained as functions of the film thickness and substrate thickness. Through the correctional length of confinement, the problem of the original fractional-dimensional approach for the jumps of the polaron binding energy and mass shift with increasing the film thickness is solved. Our calculations show that the polaron binding energy and mass shift decreases monotonously as the film thickness increases. It is also shown that the polaron binding energy and mass shift both have their maxima as the substrate thickness increases.

On the Surface Nature of Bimetallic PdZn Particles Supported on a ZnO–CeO2 Nanocomposite for the Methanol Steam Reforming Reaction

CO adsorption—as a molecular probe—was studied by transmission IR spectroscopy on pre-reduced Pd and bimetallic PdZn nanoparticles. Palladium was supported (2 wt% Pd) on pure CeO2, ZnO and a ZnO–CeO2 composite (atomic ratio Zn:Ce = 1:2). The Pd 3d5/2 binding energy shift, together with the formation of metallic zinc were consistent with the development of a PdZn alloy over the zinc-containing supports at increasing reduction temperature, as revealed by XPS. Following H2 reduction at 623 K the bimetallic particles showed only linear CO adsorption (COL) at initial contact time (10 Torr CO, 298 K), giving rise to a convoluted IR band ascribed to different Pd sites, where it was assumed that the Pd–Pd distances were larger than for pure Pd crystallites, indicating the presence of a PdZn alloyed surface. However, for longer exposure time to CO and/or higher superimposed pressure, the appearance of bridge and hollow coordinated CO (COB and COH, respectively) on the Pd sites suggested the degradation of the PdZn surface alloy, most likely due to the segregation of Pd surface patches. The temperature-programmed, dynamic isobaric adsorption of CO (TPA-CO), under flowing CO(1%)/He on the catalysts pre-reduced at 623 K (that is, for similar conditions to those found in the methanol steam reforming—MSR-process) showed faster desorption of COL as compared to COB + COH species for supported Pd/CeO2, as expected. However, the TPA-CO results on Pd/ZnO–CeO2 were atypical: even under the superimposed, low CO partial pressure, and for a temperature range similar to those found at high methanol conversion in the MSR reaction, the PdZn bimetallic surface nature was recovered, which could be an explanation of the good selectivity to CO2 of Pd/ZnO-based catalysts and—in particular—of the catalytically stable Pd/ZnO–CeO2 materials.

Bond−Energy−Electron Relaxation of BeN Nanoclusters and BeX Alloys

AbstractBy combining bond order‐length‐strength (BOLS) notion and density functional theory (DFT) calculations, the bond−energy−electron relaxtion of BeN nanoclusters and BeX (C, S, B, Cl, Cu, Co, Te) alloys are studied. The results show that the size and shape of the Be nanoclusters will cause the binding energy (BE) shift, the local densification of electrons and quantum entrapment. Actions exemplified the atomic coordination ratio and the BE of BeX alloys can predict the quantitative information regarding the change of the bond length, local bond strain, bond energy density and atomic cohesive energy.

Electronic behaviour of Au-Pt alloys and the 4f binding energy shift anomaly in Au bimetallics- X-ray spectroscopy studies

The electronic structure and charge redistribution of 6s conduction charge and 5d charge in Au and Pt alloys, Au9Pt and AuPt9 have been investigated using a charge compensation model. It is found that, both the Au and Pt 4f binding energy (BE) exhibits a negative shift in the alloys relatively to the pure metal in apparent disagreement with electroneutrality considerations (Au is the most electronegative metallic element); more interestingly, the negative Au 4f BE shift in Au-Pt alloy is in contrast to previous observations for a large number of Au bimetallic systems with more electropositive hosts in which the more electropositive the host„ the more positive the Au 4f BE shift. This anomaly is counter intuitive to electronegativity considerations. This dilemma was resolved by the charge compensation model in which both electronegativity and charge neutrality can be satisfied and the overall charge flow δ, onto Au is small and positive and δ arises from charge flow of 6s conduction charge, Δnc onto Au site, which is partially compensated by the depletion of 6d charge Δnd at the Au site (δ = Δnc+ Δnd ∼0.1 >0). The much larger Coulomb interaction between 4f and 5d than that between 4f and 6s results in positive 4f BE shifts. The Au 4f BE shift in Au-Pt alloys together with 193Au Mössbauer data were used in the charge compensation model analysis which shows that the model is still valid in that the Au 4f shift in Au-Pt alloy arises from mainly conduction charge gain with little depletion of d charge at the Au site. The model also works for Pt. The Au and Pt 5d character in the alloys have been examined with valence band spectra which show both maintain their d characteristic in dilute alloys with Pt d piling up at the Fermi level, and the top of the Au valence band being pushed toward the Fermi level; this is confirmed with DFT densities of state calculations. When Pt is diluted in Au, it gains d charge as evident from the reduction in whiteline intensity at the Pt L3-edge XANES. What emerges from this work is a picture in which the s-d charge compensation in Au bimetallic alloys is triggered by electronegativity difference between Au and the host. For Au-Pt and Au-Pd systems, the difference in electronegativity is very small, conduction charge transfer dominates, and the Au 4f shift is negative whereas in most Au bimetallics, the larger the electronegativity difference, the larger the compensation and the larger the Au 4f shifts.

Extracting Chemical Information from XPS Spectra: A Perspective

Important mechanisms that lead to features, often complex, in X-ray photoelectron spectroscopy (XPS) spectra are defined and described. It is shown that there is much information in an XPS spectrum that can be obtained by examining these features rather than examining only the shifts of main peaks between different materials. These mechanisms are presented with a focus on describing the underlying chemical and physical phenomena responsible for features of the XPS and on showing how these XPS features can be related to the properties and electronic structure of the material studied. While it is necessary to consider certain quantum mechanical rules, the mathematical formalism is not discussed. However, a general awareness of multiplet splittings, which are a result of angular momentum coupling combined with ligand field and spin–orbit splittings, and of covalent mixings in the metal–ligand bond of oxides is essential to properly interpret the significance of XPS features. A conceptual framework of shake excitation from bonding to anti-bonding orbitals is introduced to provide an understanding of the significance of XPS satellites. While the coupling of theory and measurement is required to extract quantitative information from XPS, it may be possible to obtain useful qualitative information directly from features of the XPS spectra provided that one takes into account more than only shifts of the XPS binding energies. A correct analysis of XPS features may require a careful treatment of many-body effects that distribute intensity over many individual, unresolved final states.

Growth and coverage dependent electronic structure of MgO on Ag(001)

Growth and coverage dependent electronic structure of MgO films on Ag(001) have been studied using low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopic (ARPES) techniques. The appearance of mosaic structure in the LEED pattern has been observed around 10 ML coverage due to misfit dislocations in the metal-oxide interface. The XPS core level spectra show a substantial amount of binding energy shift relative to their bulk values, indicating the reduction of the on-site Coulomb (U) and change transfer (Δ) energies in oxide layers. This binding energy shift is mainly associated with the image charge screening effect of the highly polarizable metallic substrate. The effect of image charge screening is also reflected in the valence band electronic structure as the bands are shifted to lower binding energies with decreasing film coverage maintaining the bulk MgO band gap till 5 ML. However, for the film coverages below 5 ML, the band gap is reduced substantially in presence of O 2p-Ag 5sp hybridized bands with the finite size effects in proximity to the Ag(001) substrate. Furthermore, our measured valence band electronic structure has been compared with the theoretical calculations and a good agreement has been observed between them for the bulk MgO, however, a more detailed calculation is necessary to explain the monolayer MgO band structure properly.

In Situ XPS Reveals Voltage Driven Asymmetric Ion Movement of an Ionic Liquid through the Pores of a Multilayer Graphene Electrode

Under application of a voltage bias, asymmetric ion movement of an ionic liquid (IL) through a multilayered graphene (MLG) electrode has been detected by X-ray photoelectron spectroscopy, via recording the intensity of the two nitrogen peaks. Accordingly, we observe that upon increasing the bias, the two peaks representing the cationic and anionic fragments of the IL start appearing with increasing intensity, together with an asymmetry in their ratio, differing from unity by about 10%. Bias-dependent binding energy shifts followed through atomic features of the IL (F 1s, N 1s, and C 1s) and the graphene electrode (C 1s) indicate that a distinct solid–liquid interface develops throughout the entire intercalation process with an additional and pertinent evidence for finite potential drops across the two electrical double layers. This evidence is bolstered by the fact that the measured binding energy difference between the F 1s of the liquid and C 1s peak of the semisolid MLG electrode is only about half of ...

Surface plasmon resonance induced enhancement of photoluminescence and Raman line intensity in SnS quantum dot-Sn nanoparticle hybrid structure

In this article, we report on enhancement in photoluminescence and Raman line intensity of SnS quantum dots embedded in a mesh of Sn nanostructures. SnS nanoparticles synthesized by homogenous precipitation method show strong quantum confinement with a band gap of ∼2.7 eV (blue shift of ∼1 eV compared to bulk SnS particles). The optical band gap of SnS quantum dots is controlled by varying the pH (∼0 to 2.25), ageing time (24 to 144 h) and molarity (0 to 2 M) of the precursors. These SnS nanoparticles are embedded in a mesh of Sn nanostructures which are synthesized from tin chloride by using sodium borohydride as reducing agent. The Sn nanostructures have a morphology dependent, tunable surface plasmon resonance (SPR), ranging from UV (∼295 nm) to visible region (∼400 nm) of the electromagnetic spectrum. In the SnS-Sn nanohybrids, the excitons are strongly coupled with plasmons leading to a shift in the excitonic binding energy (∼400 meV). The pure SnS quantum dots have a very weak photoluminescence peak at ∼560 nm and Raman shift of low intensity at 853.08 cm−1, 1078.17 cm−1, 1255.60 cm−1, 1466.91 cm−1. The coupling of SnS nanoparticles with Sn nanoparticles results in strong exciton-plasmon interactions leading to enhanced photoluminescence and Raman line intensity. The nanohybrids formed using Sn nanosheets whose SPR matches with absorption onset of the SnS nanoparticles shows an enhancement of ∼104 times higher than pure SnS nanoparticles. Thus, Sn nanosheet with surface plasmon resonance in visible region (400 nm) like Au and Ag is a promising material for surface enhanced Raman spectroscopy, plasmon assisted fluorescence imaging and for enhancing the emission intensity of semiconductors with weak emission intensity.

Probing the correlation between Pt-support interaction and oxygen reduction reaction activity in mesoporous carbon materials modified with Pt-N active sites

Pt nanoparticles (NPs) on nitrogen-functionalized mesoporous carbon have proved to be interesting materials for oxygen reduction reaction. In this paper, we employed a new synthetic route to simultaneously deposit Pt NPs and introduce N-doping in the carbon support, by using dichloro(1,10-phenanthroline)platinum(II) or dichloro(2,2′-bipyridyl)platinum(II) as a precursor of both platinum and nitrogen. Pt NPs (average size 2.5 nm) were successfully synthesized by solid state reduction of Pt(II) under N2/H2 flow at 650 °C on a commercial mesoporous carbon. The synthesis was also carried out by employing PtCl2 or Pt(acac)2 as platinum precursors and 1,10-phenanthroline or 2,2′-bipyridyl as nitrogen dopant agents and the resulting materials were taken for comparison. XPS analysis confirmed that during the thermal treatment the ligand degrades and the nitrogen is embedded in the mesoporous carbon structure, resulting in a surface modification of the carbon support with preservation of bulk conductivity and thermal stability. Electrochemical characterization at a rotating disk electrode in O2-saturated 0.1 M HClO4 revealed a superior catalytic activity for ORR, in terms of E1/2 and mass activity, in those catalysts showing higher PtN interaction, expressed as binding energy shift of nitrogen components. Furthermore, cyclic voltammetry analysis in Ar purged 0.1 M HClO4 electrolyte showed that nitrogen doping appears to inhibit PtO formation. The influence of nitrogen incorporation in the carbon matrix on Pt NPs/support interaction was also rationalized on the basis of density functional theory simulations.

Investigation of gaseous wet methyl iodide adsorption on Ag nanoparticles embedded in organic–inorganic hybrid silica gels

X-ray photoelectron spectroscopy (XPS), single-particle photoluminescence, and lifetimes were adopted to investigate the interaction properties between gaseous wet methyl iodide (CH3I) and Ag nanoparticles (Ag NPs), which are embedded into a propylethylenediamine (NH2CH2CH2NHCH2CH2CH2-, PEDA) anchored silica gel. Based on the obtained XPS resolutions and shifts of binding energies, a stronger interaction of CH3I than Ag NPs toward SiO2, methylation of amines and silanols by CH3I, the partial interaction in the Ag NPs and CH3I interface, condensation of SiO2 and PEDA, and stabilization of Ag NPs by PEDA were explained. In addition, an enhanced luminescence of Ag NP-embedded PEDA-anchored SiO2 gel (Ag NP-PEDA gel) by surrounding CH3I, and Ag NP stabilization by PEDA and CH3I were also observed from the single-particle photoluminescence and lifetimes.

Enhanced sunlight-driven photocatalytic performance of Bi-doped CdMoO4 benefited from efficient separation of photogenerated charge pairs

In this paper, to further boost the photocatalytic performance of CdMoO4, Bi3+ was successfully doped into CdMoO4 by a facile microwave hydrothermal method. The Bi-doped CdMoO4 photocatalysts prepared were characterized by Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), UV–Vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), electron spin-resonance (ESR) and surface photovoltage spectroscopy (SPS). The results exhibit that doping Bi3+ into CdMoO4 remarkably boosts the separation rate of photoinduced charge pairs and the specific surface area, decrease the crystal size, narrows the band gap of the CdMoO4 and induces the binding energy shift of Cd, all these advantageous factors result in the promoted photocatalytic performance of CdMoO4. Using rhodamine B (RhB) as model toxic pollutant, the photocatalytic activities of the photocatalysts were evaluated under a 500 W Xe lamp irradiation. When the molar ratio of Bi/Cd is 0.2%, Bi-CdMoO4 prepared displays the best photocatalytic performance, the photocatalytic performance of the 0.2% sample is more than twice of that of the reference CdMoO4.

Light clusters in warm stellar matter: Explicit mass shifts and universal cluster-meson couplings

In-medium modifications of light cluster properties in warm stellar matter are studied within the relativistic mean-field approximation. In-medium effects are included by introducing an explicit binding energy shift analytically calculated in the Thomas-Fermi approximation, supplemented with a phenomenological modification of the cluster couplings to the $\sigma$ meson. A linear dependence on the $\sigma$ meson is assumed for the cluster mass, and the associated coupling constant is fixed imposing that the virial limit at low density is recovered. The resulting cluster abundances come out to be in reasonable agreement with constraints at higher density coming from heavy ion collision data. Some comparisons with microscopic calculations are also shown.

(Invited) Design of Electrocatalysts with Ad-Atoms, Single Crystals and Supported Nano-Particles for the Applications to Fuel Cells

For the wide applications of any types of fuel cells, the performance, durability and cost-reduction are essential. We have worked on developments of the materials, structures and the relating mechanisms for DMFC, PAFC, SOFC and PEFC. In this presentation, the author will briefly review first some major discoveries noted topics 1 and 2 below, but mainly discuss on the recent results relating to topic 3, obtained by e. g. “HiPer-FC Project”, targeted the development of high performance & durable electrocatalysts and hydrocarbon type PEMs based on mechanistic studies of the enhancements or degradations. Development of catalysts for direct methanol fuel cells (DMFCs) DMFCs are attractive by the high volumetric power-density, but they were accompanied with a large potential-loss at the anode even at Pt with the highest activity among pure catalysts. We discovered that in the classification of alloying components into relatively active A-metals (Pt, Ir, Pd, Rh etc.) and inert B-metals (Ru, Os, Re, etc.), AB alloys exhibit a noticeable enhancement and its optimum composition of about 1/1 for the methanol oxidation and AA alloys showed just a dilution effect. We proposed the “Bi-Functional Mechanism” on the catalysis, where each A and B atom contributes to the enhancement by playing a different role in the reaction scheme and generalized the theory by extending to several types “Electrocatalysis by Ad-Atoms” as convenient tools for new catalyst developments. The preparation of nano-sized Pt-Ru supported on carbon black (CB), invented by us, and the performance have widely been referred and the same catalyst has been uniquely applied to both of commercialized DMFCs and polymer electrolyte fuel cells (PEFCs) in residential co-generation systems. Development of less-expensive Pt-alloy catalysts for anode/cathode Much tolerance to the poisoning by CO remained in reformed H2 fuel is needed to the anode Pt catalyst for commercialized PEFCs. On the other hand, a management of the improvement of performance and the reduction of Pt loading at the cathode are essential because O2 reduction reaction (ORR) is much slower than that of H2 oxidation reaction (HOR). We discovered that a superior tolerance even to 100ppm CO for HOR can be achieved by alloys between non-precious transient-metal groups such as Fe, Co, Ni etc. with the other precious metal groups such as Pt, Pd, Rh etc. by the lowered CO adsorbing strength and coverage on their surfaces. The alloys exhibit a positive core-level shift in Pt4f binding energy, which is inversely proportional to Pt-CO bonding energy, vice vasa at non-CO tolerant alloys. Thus, a guiding principle for the development of CO tolerant anode catalysts is established. On the other hand, we also discovered an enhanced ORR at Pt alloyed with Co etc, which is more than one order of magnitude at their optimum compositions of 30-50%, compared with pure Pt. By efficient and unique applications of the electrochemical method (EC) together with modern experimental systems, we have clarified that the alloys are covered with a “Pt skin” consist of 1-2 atomic-layer modified in the electronic structure by that of underlying alloy, resulting in the noticeable enhancement of ORR as mentioned above. Development of highly dispersed catalysts and the performances and durability Reduction of Pt loading to less than 1/10 from the presently used level, corresponding to the amounts for the treatment of exhausting-gas from ICE cars, i.e., 10gPt/100kW, is essential for the wide penetration to fuel cell vehicles (FCVs) market. We aimed to overcome the difficult hurdle, i.e., the increase of the mass activity MA which is proportional to the specific surface area S and the specific area activity J, by nano-sizing (to ca. d=2nm in diameter) and alloying (as mentioned above) of the advanced catalysts, respectively. We proposed an original “Territory Theory” and demonstrated experimentally there is no reduction in the MA caused by the so-called “Particle Size Effect” even at d=2nm as long as the inter-crystallite distance is kept larger than ca. 15nm on the supporting substrates, resulting in the MA increased proportionally to increased S-value. Based on the above results, we invented a “Nano-Capsule Method” for the preparation of nano-sized pure and alloy catalysts, which are controlled strictly in the size, composition and dispersion state on the supporting materials and has been demonstrating their extremely high performance and durability in comparison with widely commercial catalysts and the same level durability as that of larger-size ones, independent of the particle sizes. We have developed HC-type PEMs and already demonstrated single PEFC operations for 5,000 hrs with these advanced PEMs.

Amine Capped Gold Colloids at Oxidic Supports: Their Electronic Interactions

Abstract Colloidal deposition of noble metal nanoparticles on oxidic supports is a recent approach for the fabrication of heterogeneous catalyst materials. We present studies on the interaction of different amine ligands with gold nanoparticles before and after deposition on several oxidic supports (titania, silica, alumina, magnesia or zinc oxide), using X-ray photoelectron and Auger spectroscopy, and high-resolution transmission electron microscopy. The adsorption of amines on thin gold films as well as on nanoparticles leads to a decrease in metal photoelectron binding energies. Usually, this is explained by donor-acceptor interactions via the amine group. By additional analysis of Auger signals, which are more sensitive to changes in the oxidation state than photoelectron spectra, we demonstrate that these shifts are due to a final state effect, namely, the increased photoelectron hole screening in presence of amine adsorbates. It will be shown, that this effect is not sensitive neither to the nanoparticle size nor the sterical properties of the capping amine. After deposition on oxide supports, the photoelectron binding energies are even further decreased. The presented findings exhibit that care has to be taken to interpret binding energy shifts simply with charging, which has impact on understanding the local electronic situation on the surface of metal-loaded oxides, crucial for heterogeneous catalysis.

(Invited) Platinum Group Metal-Free Oxygen Reduction Electrocatalysts: Structure-to-Property Relationships and Design Directions

Platinum group metal-free (PGM-free) electrocatalysts draw much attention these days as low-temperature (polymer electrolyte) fuel cell technology is approaching its broad market introduction. Among several competing chemistries, catalysts based on metal-nitrogen-carbon (M-N-C) materials system are by far the most popular choice. UNM have developed and commercialized catalysts from this class obtained by pyrolysis of N-containing organic precursor (N and C source) and transition metal salt (Fe, Co, Mn or combination of more than one metal). The pyrolysis is carried after integration of the precursors with a sacrificial support (usually mono-dispersed silica powder) through ball-milling. After the pyrolysis, the silica support is removed by leaching with HF. The catalysts are usually re-pyrolyzed and a second acid wash is carried out to remove remaining of metallic, oxide or carbide particles from the nano-composite. The resulting catalysts show high graphitic content (exceeding 80%) and atomically dispersed transition metal (usually less than 1%). Both nitrogen and transition metal moieties are integrated into the carbonaceous structure as defects in graphene. Design for performance of this type of catalysts depends on an understanding the nature and of these transition metal and nitrogen sites and the ways they participate in the oxygen reduction reaction (ORR). The structure-to-property relationships in these materials will be discussed here through correlating of theirs observed electrocatalytic activity in Rotating Ring-Disk Electrode (RRDE) and performance in single Membrane Electrode Assembly (MEA) polymer electrolyte fuel cells with results of X-ray photoelectron (XPS), X-ray absorption (XAS) and Mössbauer spectroscopy charaterization. We have introduced Density Functional Theory (DFT) calculations of core level binding energy shifts as a powerful tool for interpretation of XPS spectra of such catalysts.1-3 A combination of XPS and first principles calculations of nitrogen-containing model electrocatalysts4-5 was further used to elucidate the chemical nature of the nitrogen defects. We will discuss here how hydrogenation and protonation of different nitrogen-containing defects play a major role in ORR activity of the PGM-free catalysts across different pH ranges. This paper will place a particular emphasis on elucidating the role of nitrogen- and Fe-N-containing in-plane and edge defects on the acid-base steps in ORR catalysis and thus regulation PGM-free catalysts reactivity and selectivity. The nature of Fe-N-containing in-plane and edge defects pre-determines their association with the micro-pores (edge defects) or with the extended graphene surface (in-plane defects). This sets the stage for a “first order” dependence of chemistry of the active site and the morphology of the carbonaceous material, which is the catalysts’ backbone (see the Figure for conceptual illustration). We have shown that these correlations are more pronounced when MEA performance data are being used. In this case, we observe a strong anti-correlation of the overall performance with the “stacking number” of the graphitic domains in these catalysts, which lead us to conclude that in-plane defects are the predominant contributors to catalytic activity in fuel cells.6 Further discussion will bring the importance of these two classes of active sites in different modes of MEA performance loss and will serve as a guide for both active and durable PGM-free Polymer Electrolyte Fuel Cell (PEMFC) cathode catalysts and catalyst layer design. K. Artyushkova, et al., Chem. Commun., 49 (2013) 2539-2541S. Kabir, et al., Phys. Chem. Chem. Phys., 17 (2015) 17785-17789S. Kabir, et al., Surf. Interface Anal., 48 (2016) 293-300I. Matanovic, et al., J. Phys. Chem. C, 120 (2016) 29225-29232K. Artyushkova, et al., J. Phys. Chem. C, 121 (2017) 2836–2843M. Workman, et al., ACS Energy Letters, 2 (2017) 1489-1493 Figure 1

Argon atoms insertion in diamond: New insights in the identification of carbon C 1s peak in X-ray photoelectron spectroscopy analysis

The C 1s peak's electron binding energy position in X-ray photoelectron spectroscopy (XPS) analysis of single crystal diamond (SCD) and crystalline and polycrystalline diamond and graphite films has been extensively investigated. A key issue is the experimental identification of C 1s peaks position from graphite sp2 and diamond sp3 C-atoms bonding in the diamond lattice, based on expected energy shifts of C 1s peaks from theory. Because of material charging upon electron photoemission, the absolute binding energy of the C 1s/sp3 related peak cannot be determined. The systematic study of C 1s peaks from XPS analysis of crystalline diamond, polycrystalline diamond films and graphite show key findings: 1) Ar+ ion bombardment is a reliable technique to characterize Diamond vs graphite materials during XPS analysis; 2) A low energy peak reported as C 1s/sp2 bonding, is attributed to the C 1s/sp3 in the presence of Ar atoms inserted in the lattice. 3) The data show direct correlation between the energy of the Ar+ ion beam used for surface sputter-cleaning and binding energy shifts of XPS C 1s peaks.

Fractal dimension study of polaron effects in cylindrical GaAs/Al x Ga1-xAs core–shell nanowires

Polaron effects in cylindrical GaAs/Al x Ga1-xAs core–shell nanowires are studied by applying the fractal dimension method. In this paper, the polaron properties of GaAs/Al x Ga1-xAs core–shell nanowires with different core radii and aluminum concentrations are discussed. The polaron binding energy, polaron mass shift, and fractal dimension parameter are numerically determined as functions of shell width. The calculation results reveal that the binding energy and mass shift of the polaron first increase and then decrease as the shell width increases. A maximum value appears at a certain shell width for different aluminum concentrations and a given core radius. By using the fractal dimension method, polaron problems in cylindrical GaAs/Al x Ga1-xAs core–shell nanowires are solved in a simple manner that avoids complex and lengthy calculations.

Aromatic Ring Size Effect of a Surface Modification Agent on Platinum Nanoparticle Electrocatalysts for Oxygen Reduction Reaction

Platinum nanoparticles (NPs) modified with alkylamines containing various aromatic rings (benzyl, naphtyl, anthryl, and pyrenyl) were synthesized as model cathode catalysts for fuel cells to investigate the role of aromatic surface modification agents on catalytic performance. Surface modification of Pt was confirmed by nuclear magnetic resonance spectra for NPs and thermogravimetric analysis for carbon-supported catalysts. Surface modification of Pt NPs with aromatic alkylamines improved the electrocatalytic activities for the oxygen reduction reaction, whose activity values increased by increasing the number of aromatic rings. The effect of surface modification on the electronic properties of the catalyst was evaluated by X-ray photoelectron spectroscopy. A large positive shift in the Pt 4f binding energy of surface-modified catalyst, especially for anthryl and pyrenyl modifications, was observed, indicating that one of the important factors is metal–support interaction facilitated by the aromatic groups through the π–π interactions with carbon support.

A Study on Organic Thin-Film Transistors Using Hf–La Oxides With Different La Contents as Gate Dielectrics

The effects of La content in HfLaO gate dielectric on the performance of pentacene organic thin-film transistor (OTFT) fabricated on Si have been studied. The OTFT with Hf0.103La0.897O y gate dielectric shows high performance such as high carrier mobility of 3.45 cm $^{\textsf {2}}\cdot \textsf {V}^{-\textsf {1}}\cdot \textsf {s}^{-\textsf {1}}$ (132 times and 40 times higher than those of devices using Hf oxide and La oxide, respectively), small threshold voltage of −2.09 V, and negligible hysteresis of −0.029 V. Binding-energy shift of Hf 4f peak in the X-ray photoelectron spectroscopy spectrum indicates that La incorporation can passivate the oxygen vacancies in HfO2. Atomic force microscope reveals that the La incorporation can reduce the surface roughness of the gate dielectric by suppressing the crystallization of HfO2. Therefore, by using Hf0.103La0.897O y as gate dielectric, OTFT with high carrier mobility and small threshold voltage can be obtained.