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X-ray absorption spectroscopy measured in transmission mode was used to study the effect of alkali and halide ions on the hydrogen-bonding (H-bonding) network of water. Cl- and Br- are shown to have insignificant effect on the structure of water while I- locally weakens the H-bonding, as indicated by a sharp increase of the main-edge feature in the x-ray absorption spectra. All alkali cations act as structure-breakers in water, weakening the H-bonding network. The spectral changes are similar to spectra of high density ices where the 2nd shell has collapsed due to a break-down of the tetrahedral structures, although here, around the ions, the breakdown of the local tetrahedrality is rather due to non-directional H-bonding to the larger anions. In addition, results from temperature-dependent x-ray Raman scattering measurements of NaCl solution confirm the H-bond breaking effect of Na+ and the effect on the liquid as similar to an increase in temperature. [ABSTRACT FROM AUTHOR]

Aqueous solutions of acetonitrile (MeCN) have been studied with oxygen K-edge x-ray Raman scattering (XRS) which is found to be sensitive to the interaction between water and MeCN. The changes in the XRS spectra can be attributed to water directly interacting with MeCN and are reproduced by density functional theory calculations on small clusters of water and MeCN. The dominant structural arrangement features dipole interaction instead of H-bonds between the two species as revealed by the XRS spectra combined with spectrum calculations. Small-angle x-ray scattering shows the largest heterogeneity for a MeCN to water ratio of 0.4 in agreement with earlier small-angle neutron scattering data. [ABSTRACT FROM AUTHOR]

Water in aerosol OT reverse micelles has been extensively studied as a model system for nanoconfined water. Results from previous vibrational studies suggest that water confined this way has slower dynamics compared to bulk water; however, the effects on the hydrogen bonding network are unclear. From our study of the structure of water hydrogen bonding using x-ray Raman spectroscopy, we found an increased fraction of weakened hydrogen bonds upon confinement, similar to the effect seen in temperature increase and NaCl solvation, as evidenced by the changes in specific spectral features. [ABSTRACT FROM AUTHOR]

The majority of research activities on LiCoPO 4 are focused on the phospho-olivine (space group Pnma ), which is a promising high-voltage cathode material for Li-ion batteries. In contrast, comparably little is known about its metastable Pna 2 1 modification. Herein, we present a comprehensive study on the structure–property relationships of 15–20 nm Pna 2 1 -LiCoPO 4 nanospheres prepared by a simple microwave-assisted solvothermal process. Unlike previous reports, the results indicate that the compound is non-stoichiometric and shows cation-mixing with Co ions on the Li sites, which provides an explanation for the poor electrochemical performance. Co L 2,3 -edge X-ray absorption spectroscopic data confirm the local tetrahedral symmetry of Co 2+ . Comprehensive studies on the thermal stability using thermogravimetric analysis, differential scanning calorimetry, and in situ powder X-ray diffraction show an exothermic phase transition to olivine Pnma -LiCoPO 4 at 527 °C. The influence of the atmosphere and the particle size on the thermal stability is also investigated. [ABSTRACT FROM AUTHOR]

The surface and near surface structure of copper-indium-gallium-selenide (CIGS) absorber layers is integral to the producing a high-quality photovoltaic junction. By using X-ray absorption spectroscopy (XAS) and monitoring multiple elemental absorption edges with both theory and experiment, we are able to identify several features of the surface of CIGS as a function of composition and surface treatments. The XAS data shows trends in the near surface region of oxygen, copper, indium and gallium species as the copper content is varied in the films. The oxygen surface species are also monitored through a series of experiments that systematically investigates the effects of water and various solutions of: ammonium hydroxide, cadmium sulfate, and thiourea. These being components of cadmium sulfide chemical bath deposition (CBD). Characteristics of the CBD are correlated with a restorative effect that produces as normalized, uniform surface chemistry as measured by XAS. This surface chemistry is found in CIGS solar cells with excellent power conversion efficiency (<19%). The results provide new insight for CIGS processing strategies that seek to replace CBD and/or cadmium sulfide. [ABSTRACT FROM AUTHOR]

A systematic investigation of the electronic structure of the CdS buffer layer of CIGS solar cells has been undertaken using S K- edge X-ray absorption spectroscopy (XAS), both experimentally and theoretically. We found from XAS that growing CdS films by chemical bath deposition (CBD) exhibits more long-range disorder when compared to single crystal CdS, CdS grown by atomic layer deposition (ALD) and theory. We investigated the significance of a variety of point defects and potential atomic substitutions in first-principles estimates of the differential S K- edge XAS sensitivity. We find that substituting some sulfur atoms with e.g. oxygen or selenium does not introduce significant changes to the partial S(3 p ) density of states of the conduction band in CdS and we infer that the electronic structure modifications of these defects are spatially localized and do not hybridize strongly with bands with significant sulfur p character. Moreover, by comparison with experimental XAS should be sensitive to CdSO 4 ; however this is not in the CdS layers of CIGS devices studied here. We also find that the CBD of CdS on FTO-glass and separately on CIGS does not produce any significant changes in the local atomic structure of CdS indicating no CIGS-influenced growth. [ABSTRACT FROM AUTHOR]

Understanding catalyst function to improve activity for the oxygen evolution reaction (OER) is key to increasing the overall efficiency of electrochemical water splitting, a promising method for sustainable and clean production of hydrogen. Using a straightforward and scalable sol–gel synthesis, we explore the effects of metal composition in Co x Ti 1−x O y on electrochemical activity, atomic structure, and electronic state. Physical and electronic characterization reveal that increased amounts of Ti stabilize the 2+ oxidation state of the Co precursor and lead to formation of less active CoO-like catalysts. Conversely, films with Co:Ti ratios of 1:1 or greater result in catalysts with high activity, correlating with greater Co 3+ character, as measured by ex situ XAS for samples as-prepared and after exposure to OER conditions. Additionally, decreasing the Ti content systematically shifts the Co redox potential from approximately 1.5 V vs. RHE with a 1:3 Co:Ti ratio to 1.0 V vs. RHE with no Ti, further evidence that Ti stabilizes Co in a lower oxidation state. Controlling the oxidation state of metals in metal-oxide OER catalysts can have a profound effect on catalytic activity. [ABSTRACT FROM AUTHOR]

For lithium intercalation battery electrodes, understanding of the electronic structure of bulk and surface is essential for their operation and functionality. Soft X-rays are excellent probes for such electronic structure information, but soft X-rays are predominantly surface sensitive and thus cannot probe the bulk. Moreover, soft X-rays hardly permit meaningful in situ and operando studies in battery assemblies. We show here how we penetrate with hard X-rays (>10 keV) in situ a lithium cell, containing a manganite-based cathode. Through X-ray Raman spectroscopy we extract the Mn 2p multiplet from the entire cathode material, thus obtaining bulk-sensitive electronic structure information during battery charging and discharging. [ABSTRACT FROM AUTHOR]

Robust methods to tune the unique electronic propertiesof grapheneby chemical modification are in great demand due to the potentialof the two dimensional material to impact a range of device applications.Here we show that carbon and nitrogen core-level resonant X-ray spectroscopyis a sensitive probe of chemical bonding and electronic structureof chemical dopants introduced in single-sheet graphene films. Inconjunction with density functional theory based calculations, weare able to obtain a detailed picture of bond types and electronicstructure in graphene doped with nitrogen at the sub-percent level.We show that different N-bond types, including graphitic, pyridinic,and nitrilic, can exist in a single, dilutely N-doped graphene sheet.We show that these various bond types have profoundly different effectson the carrier concentration, indicating that control over the dopantbond type is a crucial requirement in advancing graphene electronics. [ABSTRACT FROM AUTHOR]

Lithium‐ion batteries (LIBs) are increasingly encouraged to enhance their environmental friendliness and safety while maintaining optimal energy density and cost‐effectiveness. Although various electrolytes using greener and safer glyme solvents have been reported, the low charge voltage (usually lower than 4.0 V vs Li/Li+) restricts the energy density of LIBs. Herein, tetraglyme, a less‐toxic, non‐volatile, and non‐flammable ether solvent, is exploited to build safer and greener LIBs. It is demonstrated that ether electrolytes, at a standard salt concentration (1 m), can be reversibly cycled to 4.5 V vs Li/Li+. Anchored with Boron‐rich cathode‐electrolyte interphase (CEI) and mitigated current collector corrosion, the LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode delivers competitive cyclability versus commercial carbonate electrolytes when charged to 4.5 V. Synchrotron spectroscopic and imaging analyses show that the tetraglyme electrolyte can sufficiently suppress the overcharge behavior associated with the high‐voltage electrolyte decomposition, which is advantageous over previously reported glyme electrolytes. The new electrolyte also enables minimal transition metal dissolution and deposition. NMC811||hard carbon full cell delivers excellent cycling stability at C/3 with a high average Coulombic efficiency of 99.77%. This work reports an oxidation‐resilient tetraglyme electrolyte with record‐high 4.5 V stability and enlightens further applications of glyme solvents for sustainable LIBs by designing Boron‐rich interphases. [ABSTRACT FROM AUTHOR]

Printing has emerged as a technically feasible and economically viable approach to fabricate antennas on a wide range of rigid and flexible substrates. Printing offers several advantages such as rapid prototyping, minimal number of processing steps and minimal equipment needs relative to conventional microfabrication and other antenna manufacturing techniques, versatility in the choice of substrates and low cost. Here we use a plasma-based printing approach to print self-sintered copper and silver antennas. The morphology of the printed films is continuous with nanoparticles agglomerated and fused together under the influence of the plasma. The printed antennas exhibit a reflection coefficient of about −20 dB or lower and bandwidth over 10% at a resonant frequency around 2.4 GHz. The results show the feasibility of printing high performance antennas on flexible substrates such as polyimide using plasma printing technology. [ABSTRACT FROM AUTHOR]

Graphene quantum dots (GQDs) and Carbon dots (C-dots) have been widely studied in recent years due to their structural and optoelectrical properties. These properties have prompted the exploration of the role of these carbon-based materials in many potential applications. This includes solar cells, photodetectors, bioimaging, sensors, batteries and drug delivery. These properties and applications of GQDs and C-dots are highly dependent on their size, shape and surface functionality. In this work, GQDs and C-dots mixtures have been synthesized by an inexpensive wet chemical method by varying the synthesis temperature from 85°, 100° to 115 °C. The surface functionalities of the synthesized carbon-based materials were investigated by several analytical methods. We discovered a higher degree of oxidation at higher temperatures. The mechanism of formation of different sized GQDs and C-dots with different functionalities have been explained with the help of XPS and NEXAFS analysis. The influence of size and surface functionalities on the optical properties of these nanomaterials is analyzed by UV–Vis and PL spectroscopic techniques. This study demonstrates that physicochemical properties of GQDs and C-dots can be controlled by changing the synthesis temperature. Unlabelled Image • Synthesis temperatures affect the size and surface functionalities of GQDs. • NEXAFS study provides element specific characterization and electronic structure of GQDs. • The optical properties of GQDs highly depends on their surface functionalities. [ABSTRACT FROM AUTHOR]

Advances in mobile electronics and telecommunication systems along with 5G technologies have been escalating the electromagnetic interference (EMI) problem in recent years. Graphene-based material systems such as pristine graphene, graphene-polymer composites and other graphene-containing candidates have been shown to provide adequate EMI shielding performance. Besides achieving the needed shielding effectiveness (SE), the method of applying the candidate shielding material onto the object in need of protection is of enormous importance due to considerations of ease of application, reduced logistics and infrastructure, rapid prototyping and throughput, versatility to handle both rigid and flexible substrates and cost. Printing readily meets all these criteria and here we demonstrate plasma jet printing of thin films of graphene and its composite with copper to meet the EMI shielding needs. SE over 30 dB is achieved, which represents blocking over 99.9% of the incoming radiation. Graphene and its composite with copper yield higher green index compared to pure copper shields, implying reduced reflection of incoming electromagnetic waves to help reduce secondary pollution. [ABSTRACT FROM AUTHOR]

Li‐rich disordered rocksalt (DRS) oxyfluorides have emerged as promising high‐energy cathode materials for lithium‐ion batteries. While a high level of fluorination in DRS materials offers performance advantages, it can only be achieved via mechanochemical synthesis, which poses challenges of reproducibility and scalability. The definition of relationships between fluorination and structural stability is required to devise alternative methods that overcome these challenges. In this study, the thermal evolution of three highly fluorinated phases, Li2TMO2F (TM = Mn, Co, and Ni), is investigated in an inert atmosphere. Diffraction and spectroscopic techniques are utilized to examine their electronic and chemical states up until conditions of decomposition. The analysis reveals that the materials phase‐separate above 400 °C, at most. It is also observed that heat‐treated DRS materials exhibit intricate changes in the local coordination of the metals, including their spin, and ordering compared to the pristine states. The changes upon annealing are accompanied by a modulation of the voltage profile, including reduced hysteresis, when used as electrodes. These results provide an in‐depth understanding of the fundamental crystal chemistry of DRS oxyfluorides in view of their promising role as the next generation of Li‐ion cathodes. [ABSTRACT FROM AUTHOR]

The microstructure at the interface between the cocatalyst and semiconductor plays a vital role in concentrating photo‐induced carriers and reactants. However, observing the atomic arrangement of this interface directly using an electron microscope is challenging due to the coverings of the semiconductor and cocatalyst. To address this, multiple metal‐semiconductor interfaces on three TiO2 crystal facets (M/TiO2─N, where M represents Ag, Au, and Pt, and N represents the 001, 010, and 101 single crystal facets). The identical surface atomic configuration of the TiO2 facets allowed us to investigate the evolution of the microstructure within these constructs using spectroscopies and DFT calculations. For the first time, they observed the transformation of saturated Ti6c─O bonds into unsaturated Ti5c─O and Ti6c─O─Pt bonds on the TiO2─010 facet after loading Pt. This transformation have a direct impact on the selectivity of the resulting products, leading to the generation of CO and CH4 at the Ti6c─O─Pt and Pt sites, respectively. These findings pinpoint the pivotal roles played by the atomic arrangement at the M/TiO2─N interfaces and provide valuable insights for the development of new methodologies using conventional lab‐grade equipment. [ABSTRACT FROM AUTHOR]

Ether solvents are suitable for formulating solid-electrolyte interphase (SEI)-less ion-solvent cointercalation electrolytes in graphite for Na-ion and K-ion batteries. However, ether-based electrolytes have been historically perceived to cause exfoliation of graphite and cell failure in Li-ion batteries. In this study, we develop strategies to achieve reversible Li-solvent cointercalation in graphite through combining appropriate Li salts and ether solvents. Specifically, we design 1M LiBF4 1,2-dimethoxyethane (G1), which enables natural graphite to deliver ~91% initial Coulombic efficiency and >88% capacity retention after 400 cycles. We captured the spatial distribution of LiF at various length scales and quantified its heterogeneity. The electrolyte shows self-terminated reactivity on graphite edge planes and results in a grainy, fluorinated pseudo-SEI. The molecular origin of the pseudo-SEI is elucidated by ab initio molecular dynamics (AIMD) simulations. The operando synchrotron analyses further demonstrate the reversible and monotonous phase transformation of cointercalated graphite. Our findings demonstrate the feasibility of Li cointercalation chemistry in graphite for extreme-condition batteries. The work also paves the foundation for understanding and modulating the interphase generated by ether electrolytes in a broad range of electrodes and batteries. [ABSTRACT FROM AUTHOR]

"Giant" or core/thick‐shell quantum dots (gQDs) are an important class of solid‐state quantum emitter characterized by strongly suppressed blinking and photobleaching under ambient conditions, and reduced nonradiative Auger processes. Together, these qualities provide distinguishing and useful functionality as single‐ and ensemble‐photon sources. For many applications, operation at elevated temperatures and under intense photon flux is desired, but performance is strongly dependent on the synthetic method employed for thick‐shell growth. Here, a comprehensive analysis of gQD structural properties "from the inside out" as a function of shell‐growth method is reported: successive ionic layer adsorption and reaction (SILAR) and high‐temperature continuous injection (HT‐CI), or sequential combinations of the two. Key correlations across synthesis methods, structural features (interfacial alloying, stacking‐fault density and surface‐ligand identity), and performance metrics (quantum yield, single‐gQD photoluminescence under thermal/photo stress, charging behavior and quantum‐optical properties) are identified. Surprisingly, it is found that interfacial alloying is the strongest indicator of gQD stability under stress, but this parameter is not the determining factor for Auger suppression. Furthermore, quantum yield is strongly influenced by surface chemistry and can approach unity even in the case of high shell‐defect density, while introduction of zinc‐blende stacking faults increases the likelihood that a gQD exhibits charged‐state emission. [ABSTRACT FROM AUTHOR]

Nickel-rich layered oxide cathode materials are attractive near-term candidates for boosting the energy density of next generation lithium-ion batteries. The practical implementation of these materials is, however, hindered by unsatisfactory capacity retention, poor thermal stability, and oxygen release as a consequence of structural decomposition, which may have serious safety consequences. The undesired side reactions are often exothermic, causing complicated electro-chemo-mechanical interplay at elevated temperatures. In this work, we explore the effects of thermal exposure on chemically delithiated LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC-811) at a practical state-of-charge (50% Li content) and an over-charged state (25% Li content). A systematic study using a suite of advanced synchrotron radiation characterization tools reveals the dynamics of thermal behavior of the charged NMC-811, which involves sophisticated structural and chemical evolution; e.g. lattice phase transformation, transition metal (TM) cation migration and valence change, and lithium redistribution. These intertwined processes exhibit a complex 3D spatial heterogeneity and, collectively, form a valence state gradient throughout the particles. Our study sheds light on the response of NMC-811 to elevated temperature and highlights the importance of the cathode's thermal robustness for battery performance and safety. [ABSTRACT FROM AUTHOR]

We report small angle x-ray scattering data demonstrating the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in hydrophobic solvation. By increasing the side chain length of amphiphilic tetraalkyl-ammonium (CnH2n+1)4N+ (R4N+) cations in aqueous solution we observe diffraction peaks indicating association between cations at a solute size between 4.4 and 5 Å, which show temperature dependence dominated by hydrophobic attraction. Using O K-edge x-ray absorption we show that small solutes affect hydrogen bonding in water similar to a temperature decrease, while large solutes affect water similar to a temperature increase. Molecular dynamics simulations support, and provide further insight into, the origin of the experimental observations. [ABSTRACT FROM AUTHOR]

X-ray absorption spectroscopy (XAS) and small angle x-ray scattering (SAXS) were utilized to study the effect of fluoride (F-) anion in aqueous solutions. XAS spectra show that F- increases the number of strong H-bonds, likely between F- and water in the first hydration shell. SAXS data show a low-Q scattering intensity increase similar to the effect of a temperature decrease, suggesting an enhanced anomalous scattering behavior in F- solutions. Quantitative analysis revealed that fluoride solutions have larger correlation lengths than chloride solutions with the same cations but shorter compared to pure water. This is interpreted as an increased fraction of tetrahedral low-density structures in the solutions due to the presence of the F- ions, which act as nucleation centers replacing water in the H-bonding network and forming stronger H-bonds, but the presence of the cations restricts the extension of strong H-bonds. [ABSTRACT FROM AUTHOR]

X-ray Raman scattering (XRS) spectroscopy and small angle x-ray scattering (SAXS) are used to study water in aqueous solutions of NaCl, MgCl2, and AlCl3 with the particular aim to provide information about the structure of the hydration shells of the cations. The XRS spectra show that Na+ weakens the hydrogen bonds of water molecules in its vicinity, similar to the effect of increased temperature and pressure. Mg2+ and Al3+, on the other hand, cause the formation of short and strong hydrogen bonds between the surrounding water molecules. The SAXS data show that Mg2+ and Al3+ form tightly bound hydration shells that give a large density contrast in the scattering data. From the form factors extracted from the SAXS data, we found that Mg2+ and Al3+ have, respectively, an equivalent of one and one and a half stable hydration shells that appear as a density contrast. In addition, we estimated that the density of water in the hydration shells of Mg2+ and Al3+ is, respectively, ∼61% and ∼71% higher than in bulk water. [ABSTRACT FROM AUTHOR]

We report a new theoretical procedure for calculating Auger decay transition rates including effects of core-hole excited-state dynamics. Our procedure was applied to the normal and first resonant Auger processes of gas-phase water and compared to high-resolution experiments. In the normal Auger decay, calculated Auger spectra were found to be insensitive to the dynamics, while the repulsive character of the first resonant core-excited state makes the first resonantly excited Auger decay spectra depend strongly on the dynamics. The ultrafast dissociation of water upon O(1s)→4a1 excitation was analyzed and found to be very sensitive to initial vibrational distortions in the ground state which furthermore affect the excitation energy. Our calculated spectra reproduce the experimental Auger spectra except for the Franck-Condon vibrational structure which is not included in the procedure. We found that the Auger decay of OH and O fragments contributes to the total intensity, and that the contribution from these fragments increases with increasing excitation energy. [ABSTRACT FROM AUTHOR]

Abstract This study probes the electrochemical intercalation mechanism of Li into graphitic carbon using x-ray Raman spectroscopy (XRS), an inelastic x-ray scattering technique. Operando and high resolution spectra of electrochemically lithiated composite graphitic electrodes at discrete states of Li uptake (stages III, II and I) show gradually changing spectral features with Li intercalation, the end state agreeing well with chemically fully lithiated LiC 6 highly oriented pyrolytic graphite (HOPG). The two most dominant changes in the XRS spectrum of the C-K edge are the reduced intensity of the π* peak and shift the onset of the σ* states to lower energies upon full lithiation. The excellent instrumental energy resolution uncovered novel spectral features and, thus, electronic changes with varying lithium content. The general spectral changes with progressing Li intercalation agree well with our accompanying DFT calculations. [ABSTRACT FROM AUTHOR]

Surface properties of electrode materials play a critical role in the function of batteries. Therefore, surface modifications, such as coatings, have been widely used to improve battery performance. Understanding how these coatings function to improve battery performance is crucial for both scientific research and applications. In this study the electrochemical performance of coated and uncoated LiNi0.5Mn1.5O4 (LNMO) electrodes is correlated with ensemble-averaged soft X-ray absorption spectroscopy (XAS) and spatially resolved scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) to illustrate the mechanism of how ultrathin layer Al2O3 coatings improve the cycle life of LiNi0.5Mn1.5O4. Mn2+ evolution on the surface is clearly observed in the uncoated sample, which results from the reaction between the electrolytic solution and the surfaces of LiNi0.5Mn1.5O4 particles, and also possibly atomic structure reconstructions and oxygen loss from the surface region in LiNi0.5Mn1.5O4. The coating effectively suppresses Mn2+ evolution and improves the battery performance by decelerating the impedance buildup from the surface passivation. This study demonstrates the importance of combining ensemble-averaged techniques (e.g., XAS) with localized techniques (e.g., STEM-EELS), as the latter may yield unrepresentative information due to the limited number of studied particles, and sheds light on the design of future coating processes and materials. [ABSTRACT FROM AUTHOR]

We report on atom-specific activation of CO oxidation on Ru(0001) via resonant X-ray excitation. We show that resonant 1s core-level excitation of atomically adsorbed oxygen in the co-adsorbed phase of CO and oxygen directly drives CO oxidation. We separate this direct resonant channel from indirectly driven oxidation via X-ray induced substrate heating. Based on density functional theory calculations, we identify the valence-excited state created by the Auger decay as the driving electronic state for direct CO oxidation. We utilized the fresh-slice multi-pulse mode at the Linac Coherent Light Source that provided time-overlapped and 30 fs delayed pairs of soft X-ray pulses and discuss the prospects of femtosecond X-ray pump X-ray spectroscopy probe, as well as X-ray two-pulse correlation measurements for fundamental investigations of chemical reactions via selective X-ray excitation. [ABSTRACT FROM AUTHOR]

Carbon materials are used in a diverse set of applications ranging from pharmaceuticals to catalysis. Nitrogen modification of carbon powders has shown to be an effective method for enhancing both surface and bulk properties of as-received material for a number of applications. Unfortunately, control of the nitrogen modification process is challenging and can limit the effectiveness and reproducibility of N-doped materials. Additionally, the assignment of functional groups to specific moieties on the surface of nitrogen-modified carbon materials is not straightforward. Herein, we complete an in-depth analysis of functional groups present at the surface of ion-implanted Vulcan and Graphitic Vulcan through the use of X-ray photoelectron spectroscopy (XPS) and near edge X-ray adsorption fine structure spectroscopy (NEXAFS). Our results show that regardless of the initial starting materials used, nitrogen ion implantation conditions can be tuned to increase the amount of nitrogen incorporation and to obtain both similar and reproducible final distributions of nitrogen functional groups. The development of a well-controlled/reproducible nitrogen implantation pathway opens the door for carbon supported catalyst architectures to have improved numbers of nucleation sites, decreased particle size, and enhanced catalyst-support interactions. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Phase pure LiNiO2 was prepared using a solid-state method and the optimal synthesis conditions led to a remarkably high capacity of 200 mA h g−1 with excellent retention. The combination of bulk and surface characterization elucidated an essential role of the excess Li in phase formation during synthesis and the subsequent electrochemical performance. [ABSTRACT FROM AUTHOR]

In the spinel oxide cathode family, LiNi0.5Mn1.5O4 (LNMO) shows a high operating voltage (≈4.7 V vs Li/Li+) and excellent Li‐ion mobility with stable 3D conducting channels. Ni/Mn cation disordered and ordered phases usually coexist in LNMO materials, and they have distinct structural and electrochemical properties, resulting in different battery performances for LNMO materials with different phase compositions. Identifying the correlation between phase compositions and electrochemical properties is of significance to the improvement of battery performance and understanding of degradation mechanisms. Herein, the disordered/ordered phase compositions in LNMO materials are tailored by post‐annealing strategies and their impacts on electrochemical performance and degradation mechanisms from the surface to the bulk are systematically investigated. The ordered phase increases rapidly as Mn3+ is oxidized to Mn4+ through a post‐annealing process. LNMO with an intermediate fraction of disordered and ordered phases gives rise to improved cycling stability. This article further reports that a high ordered phase fraction can preferentially protect Ni from dissolution during cycling. However, these results suggest that the transition metal dissolution and surface structural change of LNMO do not exhibit a direct correlation with cycling stability. These results indicate the capacity fading mainly correlates with the bulk structural distortion, leading to decreased Li‐ion kinetics. [ABSTRACT FROM AUTHOR]

Aside from band gap reduction, little is understood about the effect of the tinfor-lead substitution on the fundamental optical and optoelectronic properties of metal halide perovskites (MHPs), especially when transitioning from 3D to lower dimensional structures. Herein, we take advantage of the spectroscopic isolation of excitons in 2D MHPs to study the intrinsic differences between lead and tin MHPs. The exciton's spectral fine structure indicates a larger polaron binding energy in tin MHPs. Additionally, the electroabsorption responses of the 2D MHPs demonstrates that tin MHPs have exciton binding energies 1.5-2× lower than that of their lead counterparts. Despite the lower binding energy, the excitons in tin MHPs are more Frenkel-like with small radii, small polarizabilities, and large dipole moments. These results are interpreted as consequences of small polaron formation and disorder-induced dipole moments. This work highlights the wide range of intrinsic differences between lead and tin MHPs as well as the complexity of excited states in these systems. [ABSTRACT FROM AUTHOR]

Chemical doping of graphene represents a powerful means of tailoring its electronic properties. Synchrotron-based X-ray spectroscopy offers an effective route to investigate the surface electronic and chemical states of functionalizing dopants. In this work, a suite of X-ray techniques is used, including near edge X-ray absorption fine structure spectroscopy, X-ray photoemission spectroscopy, and photoemission threshold measurements, to systematically study plasma-based chlorinated graphene on different substrates, with special focus on its dopant concentration, surface binding energy, bonding configuration, and work function shift. Detailed spectroscopic evidence of C-Cl bond formation at the surface of single layer graphene and correlation of the magnitude of p-type doping with the surface coverage of adsorbed chlorine is demonstrated for the first time. It is shown that the chlorination process is a highly nonintrusive doping technology, which can effectively produce strongly p-doped graphene with the 2D nature and long-range periodicity of the electronic structure of graphene intact. The measurements also reveal that the interaction between graphene and chlorine atoms shows strong substrate effects in terms of both surface coverage and work function shift. [ABSTRACT FROM AUTHOR]

The role of pre-electrode deposition annealing on the morphology and the fracture properties of polymer solar cells is discussed. We found an increase in adhesion at the weak P3HT:PCBM/PEDOT:PSS interface with annealing temperature, caused by increased interdiffusion between the organic layers. The formation of micrometer sized PCBM crystallites, which occurs with annealing above the crystallization temperature of PCBM, initially weakened the P3HT:PCBM layer itself. Further annealing improved the cohesion, due to a pull-out toughening mechanism of the growing PCBM clusters. Understanding how the morphology, tuned by annealing, affects the adhesive and cohesive properties in these organic films is essential for the mechanical integrity of OPV devices. [ABSTRACT FROM AUTHOR]

Understanding how various redox activities evolve and distribute in disordered rocksalt oxides (DRX) can advance insights into manipulating materials properties for achieving stable, high‐energy batteries. Herein, the authors present how the reaction kinetics and spatial distribution of redox activities are governed by the particle size of DRX materials. The size‐dependent electrochemical performance is attributed to the distinct cationic and anionic reaction kinetics at different sizes, which can be tailored to achieve optimal capacity and stability. Overall, the local charged domains in DRX particles display random heterogeneity caused by the isotropic delithiation pathways. Owing to the kinetic limitation, the micron‐sized particles exhibit a holistic "core‐shell" charge distribution, whereas sub‐micron particles show more uniform redox reactions throughout the particles and ensembles. Sub‐micron DRX particles exhibit increasing anionic redox activities yet inferior cycling stability. In summary, engineering particle size can effectively modulate how cationic and anionic redox activities evolve and distribute in DRX materials. [ABSTRACT FROM AUTHOR]

In order to assess the sensitivity and complementarity of X-ray absorption and emission spectroscopies for determining changes in the metal ligation sphere, a systematic experimental and theoretical study of iron model complexes has been carried out. A series of high-spin ferrous complexes, in which the ligation sphere has been varied from a three-coordinate complex, [LtBuFe(SPh)] (1) (where LtBu = bulky β-diketiminate ligand; SPh = phenyl thiolate) to four-coordinate complexes [LtBuFe(SPh)(X)] (where X = CNtBu (2); 1-methylimidazole (3); or N,N-dimethylformamide (DMF) (4)), has been investigated using a combination of Fe K-edge X-ray absorption (XAS) and Kβ X-ray emission (XES) spectroscopies. The Fe K XAS pre-edge and edge of all four complexes are consistent with a high-spin ferrous assignment, with the largest differences in the pre-edge intensities attributed to changes in covalency of the fourth coordination site. The X-ray emission spectra show pronounced changes in the valence to core region (V2C) as the identity of the coordinated ligand is varied. The experimental results have been correlated to density functional theory (DFT) calculations, to understand key molecular orbital contributions to the observed absorption and emission features. The calculations also have been extended to a series of hypothetical high-spin iron complexes to understand the sensitivity of XAS and XES techniques to different ligand protonation states ([LtBuFeII(SPh)(NHn)]3-n (n = 3, 2, 1, 0)), metal oxidation states [LtBuFe(SPh)(N)]n- (n = 3, 2, 1), and changes in the ligand identity [LtBuFeIV(SPh)(X)]n-(X = C4-, N3-, O2-; n = 2, 1, 0). This study demonstrates that XAS pre-edge data have greater sensitivity to changes in oxidation state, while valence to core (V2C) XES data provide a more sensitive probe of ligand identity and protonation state. The combination of multiple X-ray spectroscopic methods with DFT results thus has the potential to provide for detailed characterization of complex inorganic systems in both chemical and biological catalysis. [ABSTRACT FROM AUTHOR]

Semiconductor materials that can be doped both n-type and p-type are desirable for diode-based applications and transistor technology. Copper nitride (Cu3N) is a metastable semiconductor with a solar-relevant bandgap that has been reported to exhibit bipolar doping behavior. However, deeper understanding and better control of the mechanism behind this behavior in Cu3N is currently lacking in the literature. In this work, we use combinatorial growth with a temperature gradient to demonstrate both conduction types of phase-pure, sputter-deposited Cu3N thin films. Room temperature Hall effect and Seebeck effect measurements show n-type Cu3N with 1017 electrons/cm3 for low growth temperature (≈35 °C) and p-type with 1015 holes/cm3-1016 holes/cm3 for elevated growth temperatures (50 °C-120 °C). Mobility for both types of Cu3N was ≈0.1 cm2/Vs-1 cm2/Vs. Additionally, temperature-dependent Hall effect measurements indicate that ionized defects are an important scattering mechanism in p-type films. By combining X-ray absorption spectroscopy and first-principles defect theory, we determined that VCudefects form preferentially in p-type Cu3N, while Cuidefects form preferentially in n-type Cu3N, suggesting that Cu3N is a compensated semiconductor with conductivity type resulting from a balance between donor and acceptor defects. Based on these theoretical and experimental results, we propose a kinetic defectformation mechanism for bipolar doping in Cu3N that is also supported by positron annihilation experiments. Overall, the results of this work highlight the importance of kinetic processes in the defect physics of metastable materials and provide a framework that can be applied when considering the properties of such materials in general. [ABSTRACT FROM AUTHOR]

Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for these studies, since even simple binary non-ground state compounds of common elements may be awaiting discovery. However, there are significant research challenges related to non-equilibrium thin film synthesis and crystal structure predictions, such as small strained crystals in the experimental samples and energy minimization based theoretical algorithms. Here, we report on experimental synthesis and characterization, as well as theoretical first-principles calculations of a previously unreported mixed-valent binary tin nitride. Thin film experiments indicate that this novel material is N-deficient SnN with tin in the mixed ii/iv valence state and a small low-symmetry unit cell. Theoretical calculations suggest that the most likely crystal structure has the space group 2 (SG2) related to the distorted delafossite (SG166), which is nearly 0.1 eV/atom above the ground state SnN polymorph. This observation is rationalized by the structural similarity of the SnN distorted delafossite to the chemically related Sn3N4 spinel compound, which provides a fresh scientific insight into the reasons for growth of polymorphs of metastable materials. In addition to reporting on the discovery of the simple binary SnN compound, this paper illustrates a possible way of combining a wide range of advanced characterization techniques with the first-principle property calculation methods, to elucidate the most likely crystal structure of the previously unreported metastable materials. [ABSTRACT FROM AUTHOR]

Monolayer MoS2 is a promising semiconductor to overcome the physical dimension limits of microelectronic devices. Understanding the thermochemical stability of MoS2 is essential since these devices generate heat and are susceptible to oxidative environments. Herein, the promoting effect of molybdenum oxides (MoOx) particles on the thermal oxidation of MoS2 monolayers is shown by employing operando X‐ray absorption spectroscopy, ex situ scanning electron microscopy and X‐ray photoelectron spectroscopy. The study demonstrates that chemical vapor deposition‐grown MoS2 monolayers contain intrinsic MoOx and are quickly oxidized at 100 °C (3 vol% O2/He), in contrast to previously reported oxidation thresholds (e.g., 250 °C, t ≤ 1 h in the air). Otherwise, removing MoOx increases the thermal oxidation onset temperature of monolayer MoS2 to 300 °C. These results indicate that MoOx promote oxidation. An oxide‐free lattice is critical to the long‐term stability of monolayer MoS2 in state‐of‐the‐art 2D electronic, optical, and catalytic applications. [ABSTRACT FROM AUTHOR]

The structure of bulk liquid water was recently probed by x-ray scattering below the temperature limit of homogeneous nucleation (TH) of ~232 K [J. A. Sellberg et al., Nature 510, 381-384 (2014)]. Here, we utilize a similar approach to study the structure of bulk liquid water below TH using oxygen K-edge x-ray emission spectroscopy (XES). Based on previous XES experiments [T. Tokushima et al., Chem. Phys. Lett. 460, 381-400 (2008)] at higher temperatures, we expected the ratio of the 1b1' and 1b1" peaks associated with the lone-pair orbital in water to change strongly upon deep supercooling as the coordination of the hydrogen (H-) bonds becomes tetrahedral. In contrast, we observed only minor changes in the lone-pair spectral region, challenging an interpretation in terms of two interconverting species. A number of alternative hypotheses to explain the results are put forward and discussed. Although the spectra can be explained by various contributions from these hypotheses, we here emphasize the interpretation that the line shape of each component changes dramatically when approaching lower temperatures, where, in particular, the peak assigned to the proposed disordered component would become more symmetrical as vibrational interference becomes more important. [ABSTRACT FROM AUTHOR]

Achieving the targeted control of layered oxide properties calls for more fundamental studies to mechanistically probe their evolution during their synthesis. Herein, dopant distribution, phase propagation, and local chemical changes as well as their interplay in multielement‐doped LiNiO2 materials are investigated using spectroscopic, imaging, and scattering techniques. It is shown that dopants undergo dynamic redistribution in the Ni(OH)2 host lattice at the early stage of calcination (below 300 °C). Such redistribution behavior exhibits strong dopant‐dependent characteristics, allowing for targeted surface and bulk doping control. The Ni oxidation process exhibits depth‐dependent characteristics and the most rapid Ni oxidation takes place between 300 and 700 °C. Using Ni oxidation state as the proxy for the phase transformation, the buildup of heterogenous phase propagation in the early stage of calcination is shown, especially along the radial direction of secondary particles. The radial heterogenous phase distribution gradually decreases upon completing the calcination. However, a high degree of mosaic‐like heterogeneity may still be present in the final product, departing from the perfect layered oxide. The present study offers fundamental insights into manipulating multiscale materials properties during calcination for obtaining stable, high‐energy layered oxide cathodes. [ABSTRACT FROM AUTHOR]

Achieving the targeted control of layered oxide properties calls for more fundamental studies to mechanistically probe their evolution during their synthesis. Herein, dopant distribution, phase propagation, and local chemical changes as well as their interplay in multielement‐doped LiNiO2 materials are investigated using spectroscopic, imaging, and scattering techniques. It is shown that dopants undergo dynamic redistribution in the Ni(OH)2 host lattice at the early stage of calcination (below 300 °C). Such redistribution behavior exhibits strong dopant‐dependent characteristics, allowing for targeted surface and bulk doping control. The Ni oxidation process exhibits depth‐dependent characteristics and the most rapid Ni oxidation takes place between 300 and 700 °C. Using Ni oxidation state as the proxy for the phase transformation, the buildup of heterogenous phase propagation in the early stage of calcination is shown, especially along the radial direction of secondary particles. The radial heterogenous phase distribution gradually decreases upon completing the calcination. However, a high degree of mosaic‐like heterogeneity may still be present in the final product, departing from the perfect layered oxide. The present study offers fundamental insights into manipulating multiscale materials properties during calcination for obtaining stable, high‐energy layered oxide cathodes. [ABSTRACT FROM AUTHOR]

The effect of crystal growth conditions on the O K-edge x-ray absorption spectra of ice is investigated through detailed analysis of the spectral features. The amount of ice defects is found to be minimized on hydrophobic surfaces, such as BaF2(111), with low concentration of nucleation centers. This is manifested through a reduction of the absorption cross-section at 535 eV, which is associated with distorted hydrogen bonds. Furthermore, a connection is made between the observed increase in spectral intensity between 544 and 548 eV and high-symmetry points in the electronic band structure, suggesting a more extended hydrogen-bond network as compared to ices prepared differently. The spectral differences for various ice preparations are compared to the temperature dependence of spectra of liquid water upon supercooling. A double-peak feature in the absorption cross-section between 540 and 543 eV is identified as a characteristic of the crystalline phase. The connection to the interpretation of the liquid phase O K-edge x-ray absorption spectrum is extensively discussed. [ABSTRACT FROM AUTHOR]

We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li2SO4, Li2O, Li3N, and Li2CO3 using high resolution X-ray Raman spectroscopy (XRS) measurements and first-principles density functional theory calculations within the eXcited electron and Core Hole approach. Based on thermodynamic sampling via ab initio molecular dynamics simulations, we find calculated XAS in much better agreement with experiment than those computed using the rigid crystal structure alone. We show that local instantaneous distortion of the atomic lattice perturbs the symmetry of the Li 1s core-excitedstate electronic structure, broadening spectral line-shapes and, in some cases, producing additional spectral features. The excellent agreement with high-resolution XRS measurements validates the accuracy of our first-principles approach to simulating XAS, and provides both accurate benchmarks for model compounds and a predictive theoretical capability for identification and characterization of multi-component systems, such as lithium-ion batteries, under working conditions. [ABSTRACT FROM AUTHOR]

X-ray Raman spectroscopy (XRS) combined with small-angle x-ray scattering (SAXS) were used to study aqueous solutions of HCl and NaOH. Hydrated structures of H+ and OH- are not simple mirror images of each other. While both ions have been shown to strengthen local hydrogen bonds in the hydration shell as indicated by XRS, SAXS suggests that H+ and OH- have qualitatively different long-range effects. The SAXS structure factor of HCl (aq) closely resembles that of pure water, while NaOH (aq) behaves similar to NaF (aq). We propose that protons only locally enhance hydrogen bonds while hydroxide ions induce tetrahedrality in the overall hydrogen bond network of water. [ABSTRACT FROM AUTHOR]

A layered oxide cathode, LiNi0.6Mn0.2Co0.2O2, undergoes noticeable crystal expansion by losing significantly higher amounts of Li+ at the end of fast charging cycles. However, the bulk structure of the cycled NMC622 is restored back to its pristine discharged state when intercalated with enough lithium ions during an electrochemical process. [ABSTRACT FROM AUTHOR]

Poly(ethylene oxide) (PEO)‐based solid electrolytes are expected to be exploited in solid‐state batteries with high safety. Its narrow electrochemical window, however, limits the potential for high voltage and high energy density applications. Herein the electrochemical oxidation behavior of PEO and the failure mechanisms of LiCoO2‐PEO solid‐state batteries are studied. It is found that although for pure PEO it starts to oxidize at a voltage of above 3.9 V versus Li/Li+, the decomposition products have appropriate Li+ conductivity that unexpectedly form a relatively stable cathode electrolyte interphase (CEI) layer at the PEO and electrode interface. The performance degradation of the LiCoO2‐PEO battery originates from the strong oxidizing ability of LiCoO2 after delithiation at high voltages, which accelerates the decomposition of PEO and drives the self‐oxygen‐release of LiCoO2, leading to the unceasing growth of CEI and the destruction of the LiCoO2 surface. When LiCoO2 is well coated or a stable cathode LiMn0.7Fe0.3PO4 is used, a substantially improved electrochemical performance can be achieved, with 88.6% capacity retention after 50 cycles for Li1.4Al0.4Ti1.6(PO4)3 coated LiCoO2 and 90.3% capacity retention after 100 cycles for LiMn0.7Fe0.3PO4. The results suggest that, when paired with stable cathodes, the PEO‐based solid polymer electrolytes could be compatible with high voltage operation. [ABSTRACT FROM AUTHOR]

Ni,N‐doped carbon catalysts have shown promising catalytic performance for CO2 electroreduction (CO2R) to CO; this activity has often been attributed to the presence of nitrogen‐coordinated, single Ni atom active sites. However, experimentally confirming Ni−N bonding and correlating CO2 reduction (CO2R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile‐derived Ni,N‐doped carbon electrocatalysts (Ni‐PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO2R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X‐ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square‐planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts. [ABSTRACT FROM AUTHOR]

Ni,N‐doped carbon catalysts have shown promising catalytic performance for CO2 electroreduction (CO2R) to CO; this activity has often been attributed to the presence of nitrogen‐coordinated, single Ni atom active sites. However, experimentally confirming Ni−N bonding and correlating CO2 reduction (CO2R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile‐derived Ni,N‐doped carbon electrocatalysts (Ni‐PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO2R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X‐ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square‐planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts. [ABSTRACT FROM AUTHOR]

Arranging molecules into highly symmetric, topological crystal structures has been recognized as the best approach to functionalize electronic properties in molecular crystals, where the constituent molecules have been assumed to be rigid in shape. Here, in striking contrast, we demonstrate that the molecules in a monolayer organic crystal can undergo a significant deformation in proximity to the substrate, which is reflected by an asymmetry in the electron density profile. X-ray reflectivity and X-ray absorption spectroscopies in conjunction with density-functional theory calculations reveal that the highly planarized π-core are deformed into a bent shape, while the bulk lattice parameters are maintained. The molecular shape change is found to be perfectly suppressed in a bilayer single crystal, which leads to a 40% increase in mobility in the bilayer crystal. Our finding of a unique, sub-molecular scale shape change in monolayer single crystals can offer possibilities for functionalizing electrical properties via nano-scale physisorption. Organic molecules exhibit intrinsic semiconducting properties as well as increased flexibility in comparison to their silicon counterparts and consequently are playing an increasing role in semiconductor technologies. Here, the authors demonstrate that monolayers of organic crystals can undergo deformation at the molecular level when in proximity to a substrate and that this has an effect on their electronic structure. [ABSTRACT FROM AUTHOR]

Terminal copper-nitrenoid complexes have inspired interest in their fundamental bonding structures as well as their putative intermediacy in catalytic nitrene-transfer reactions. Here, we report that aryl azides react with a copper(I) dinitrogen complex bearing a sterically encumbered dipyrrin ligand to produce terminal copper nitrene complexes with near-linear, short copper–nitrenoid bonds [1.745(2) to 1.759(2) angstroms]. X-ray absorption spectroscopy and quantum chemistry calculations reveal a predominantly triplet nitrene adduct bound to copper(I), as opposed to copper(II) or copper(III) assignments, indicating the absence of a copper−nitrogen multiple-bond character. Employing electron-deficient aryl azides renders the copper nitrene species competent for alkane amination and alkene aziridination, lending further credence to the intermediacy of this species in proposed nitrene-transfer mechanisms. [ABSTRACT FROM AUTHOR]

Modification of the surface chemistry of carbon-based nanomaterials is often necessary in order to embrace their full potential. A wide variety of different post-fabrication treatments, such as acid, oxidizing plasma and heat treatments have been described in the literature. However, their specific effects on the materials surface chemistry is typically only vaguely disclosed. Here we report an in-situ method to functionalize tetrahedral amorphous carbon (ta-C) thin films by introducing high purity oxygen into the vacuum chamber during the film fabrication. Additionally, we analyze and compare the material properties of the resulting thin films to those of nitric acid and oxygen plasma treated as well as those with no treatment at all. Using x-ray absorption spectroscopy (XAS), we show that in-situ functionalizing decreases the sp2 content of the surface and increases the amount of carboxyl-like functionalities. Subsequent oxygen plasma treatment further decreases the sp2 fraction and ketone/aldehyde content as well as increases the amount of carboxyl groups. The same trends are observed with the reference ta-C exposed to oxygen plasma treatment. For both materials, a concentrated nitric acid treatment has only a subtle effect on the surface chemistry. Capitalizing on this knowledge, we can selectively produce materials with higher surface loading of specific functional groups, paving the way for application specific material fabrication. [ABSTRACT FROM AUTHOR]