On the relation between X-ray Photoelectron Spectroscopy and XAFS

Inversion model for extracting chemically resolved depth profiles across liquid interfaces of various configurations from XPS data: PROPHESY

PROPHESY, a technique for the reconstruction of surface-depth profiles from X-ray photoelectron spectroscopy data, is introduced. The inversion methodology is based on a Bayesian framework and primal-dual convex optimization. The acquisition model is developed for several geometries representing different sample types: plane (bulk sample), cylinder (liquid microjet) and sphere (droplet). The methodology is tested and characterized with respect to simulated data as a proof of concept. Possible limitations of the method due to uncertainty in the attenuation length of the photo-emitted electron are illustrated.

Theoretical Perspective on Operando Spectroscopy of Fluxional Nanocatalysts

Improvements in operando spectroscopy have enabled the catalysis community to investigate the dynamic nature of catalysts under operating conditions with increasing detail. Still, the highly dynamic nature of some catalysts, such as fluxional supported subnano clusters, presents a formidable challenge even for the most state-of-the-art techniques. The reason is that such fluxional catalytic interfaces contain a variety of thermally accessible states. Operando spectroscopies used in catalysis generally fall into two categories: ensemble-based techniques, which provide spectra containing the signals of the entire ensemble of states of the catalyst and are not necessarily dominated by the most active species, and localized techniques, which provide atomistic-level information about the dynamics of active sites in a very small area, which might not include the most active species. Combining many different kinds of techniques can provide detailed insight; however, we propose that effective utilization of specific computational techniques and approaches within the fluxionality paradigm can fill the gap and enable atomistic characterization of the most relevant catalytic sites.

Point-Defect Engineering: Leveraging Imperfections in Graphitic Carbon Nitride (g-C3 N4 ) Photocatalysts toward Artificial Photosynthesis

Graphitic carbon nitride (g-C₃ N₄ ) is a kind of ideal metal-free photocatalysts for artificial photosynthesis. At present, pristine g-C₃ N₄ suffers from small specific surface area, poor light absorption at longer wavelengths, low charge migration rate, and a high recombination rate of photogenerated electron-hole pairs, which significantly limit its performance. Among a myriad of modification strategies, point-defect engineering, namely tunable vacancies and dopant introduction, is capable of harnessing the superb structural, textural, optical, and electronic properties of g-C₃ N₄ to acquire an ameliorated photocatalytic activity. In view of the burgeoning development in this pacey field, a timely review on the state-of-the-art advancement of point-defect engineering of g-C₃ N₄ is of vital significance to advance the solar energy conversion. Particularly, insights into the intriguing roles of point defects, the synthesis, characterizations, and the systematic control of point defects, as well as the versatile application of defective g-C₃ N₄ -based nanomaterials toward photocatalytic water splitting, carbon dioxide reduction and nitrogen fixation will be presented in detail. Lastly, this review will conclude with a balanced perspective on the technical and scientific hindrances and future prospects. Overall, it is envisioned that this review will open a new frontier to uncover novel functionalities of defective g-C₃ N₄ -based nanostructures in energy catalysis.

Mechanochemistry for ammonia synthesis under mild conditions

Ammonia, one of the most important synthetic feedstocks, is mainly produced by the Haber-Bosch process at 400-500 °C and above 100 bar. The process cannot be performed under ambient conditions for kinetic reasons. Here, we demonstrate that ammonia can be synthesized at 45 °C and 1 bar via a mechanochemical method using an iron-based catalyst. With this process the ammonia final concentration reached 82.5 vol%, which is higher than state-of-the-art ammonia synthesis under high temperature and pressure (25 vol%, 450 °C, 200 bar). The mechanochemically induced high defect density and violent impact on the iron catalyst were responsible for the mild synthesis conditions.

A portable data-collection system for soft x-ray absorption spectroscopy in the Shanghai Synchrotron Radiation Facility

Based on the Experimental Physics and Industrial Control System, a portable data-collection system for soft x-ray absorption spectroscopy has been developed at the BL02B and BL08U beamlines of the Shanghai Synchrotron Radiation Facility. The data-collection system can be used to carry out total electron yield (TEY) and total fluorescence yield (TFY) experiments simultaneously. The hardware consists of current preamplifiers, voltage-to-frequency converters, and a multi-channel counter, which are aimed at improving the signal-to-noise ratio. The control logic is developed using Python and Java. The novelty of this control system is its designed portability while being extensible and readable and having low noise and high real-time capabilities. The oxygen K-edge absorption spectra of SrTiO₃ were obtained using the TEY and TFY technology at the BL02B beamline. Furthermore, the TEY and TFY spectra of the relaxor ferroelectric single-crystal of lead magnesium niobate-lead titanate measured by the present data-collection system have lower peak-to-peak noise amplitude than the ones measured by using a picoammeter. The experimental results show that the spectral signal-to-noise ratio recorded by the present system is 5.7-12.4 dB higher than that with the picoammeter detector.

Energy-Dependent Relative Cross Sections in Carbon 1s Photoionization: Separation of Direct Shake and Inelastic Scattering Effects in Single Molecules

We demonstrate that the possibility of monitoring relative photoionization cross sections over a large photon energy range allows us to study and disentangle shake processes and intramolecular inelastic scattering effects. In this gas-phase study, relative intensities of the carbon 1s photoelectron lines from chemically inequivalent carbon atoms in the same molecule have been measured as a function of the incident photon energy in the range of 300-6000 eV. We present relative cross sections for the chemically shifted carbon 1s lines in the photoelectron spectra of ethyl trifluoroacetate (the "ESCA" molecule). The results are compared with those of methyl trifluoroacetate and S-ethyl trifluorothioacetate as well as a series of chloro-substituted ethanes and 2-butyne. In the soft X-ray energy range, the cross sections show an extended X-ray absorption fine structure type of wiggles, as was previously observed for a series of chloroethanes. The oscillations are damped in the hard X-ray energy range, but deviations of cross-section ratios from stoichiometry persist, even at high energies. The current findings are supported by theoretical calculations based on a multiple scattering model. The use of soft and tender X-rays provides a more complete picture of the dominant processes accompanying photoionization. Such processes reduce the main photoelectron line intensities by 20-60%. Using both energy ranges enabled us to discern the process of intramolecular inelastic scattering of the outgoing electron, whose significance is otherwise difficult to assess for isolated molecules. This effect relates to the notion of the inelastic mean free path commonly used in photoemission studies of clusters and condensed matter.

Attenuation of slow (10-40 eV) electrons in soft nanoparticles: Size matters in argon clusters

Electron attenuation due to inelastic and elastic scattering in condensed media can often be described in terms of the effective attenuation length (EAL) of the electron. The EAL is thus an important parameter for describing electron transport processes as exemplified by dissipation of energy following radiolysis. Focusing on electrons at low electron kinetic energies (10-40 eV) in condensed argon, we determine EAL from x-ray photoelectron spectra of argon nanoparticles and compare to values obtained from valence ionization in thin argon films as well as from gas-phase electron-scattering data. EAL determined from argon clusters shows variation with cluster size. Moreover, the values are significantly lower than those obtained in valence-ionization studies and from scattering data. Our results corroborate recent x-ray photoelectron spectroscopy-based determination of EALs of water showing large differences to the EALs determined by other methods in amorphous ice at low kinetic energies of the photoelectron [Y.-I. Suzuki, K. Nishizawa, N. Kurahashi, and T. Suzuki, Phys. Rev. E 90, 010302 (2014)PLEEE81539-375510.1103/PhysRevE.90.010302], underlining that care must be taken when using EAL values from other sources for core-level electrons.

Interatomic scattering in energy dependent photoelectron spectra of Ar clusters

Soft X-ray photoelectron spectra of Ar 2p levels of atomic argon and argon clusters are recorded over an extended range of photon energies. The Ar 2p intensity ratios between atomic argon and clusters' surface and bulk components reveal oscillations similar to photoelectron extended X-ray absorption fine structure signal (PEXAFS). We demonstrate here that this technique allows us to analyze separately the PEXAFS signals from surface and bulk sites of free-standing, neutral clusters, revealing a bond contraction at the surface.