Impact of Catalysis-Relevant Oxidation and Annealing Treatments on Nanostructured GaRh Alloys.

In this study, we examine the surface-derived electronic and chemical structures of nanostructured GaRh alloys as a model system for supported catalytically active liquid metal solutions (SCALMS), a novel catalyst candidate for dehydrogenation reactions that are important for the petrochemical and hydrogen energy industry. It is reported that under ambient conditions, SCALMS tends to form a gallium oxide shell, which can be removed by an activation treatment at elevated temperatures and hydrogen flow to enhance the catalytic reactivity. We prepared a 7 at. % Rh containing the GaRh sample and interrogated the evolution of the surface chemical and electronic structure by photoelectron spectroscopy (complemented by scanning electron microscopy) upon performing surface oxidation and (activation treatment mimicking) annealing treatments in ultrahigh vacuum conditions. The initially pronounced Rh 4d and Fermi level-derived states in the valence band spectra disappear upon oxidation (due to formation of a GaOx shell) but reemerge upon annealing, especially for temperatures of 600 °C and above, i.e., when the GaOx shell is efficiently being removed and the Ga matrix is expected to be liquid. At the same temperature, new spectroscopic features at both the high and low binding energy sides of the Rh 3d5/2 spectra are observed, which we attribute to new GaRh species with depleted and enriched Rh contents, respectively. A liquefied and GaOx-free surface is also expected for GaRh SCALMS at reaction conditions, and thus the revealed high-temperature properties of the GaRh alloy provide insights about respective catalysts at work.

Elucidating the Complex Oxidation Behavior of Aqueous H3PO3 on Pt Electrodes via In Situ Tender X-ray Absorption Near-Edge Structure Spectroscopy at the P K-Edge.

In situ tender X-ray absorption near-edge structure (XANES) spectroscopy at the P K-edge was utilized to investigate the oxidation mechanism of aqueous H3PO3 on Pt electrodes under various conditions relevant to high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) applications. XANES and electrochemical analysis were conducted under different tender X-ray irradiation doses, revealing that intense radiation induces the oxidation of aqueous H3PO3 via H2O yielding H3PO4 and H2. A broadly applicable experimental procedure was successfully developed to suppress these undesirable radiation-induced effects, enabling a more accurate determination of the aqueous H3PO3 oxidation mechanism. In situ XANES studies of aqueous 5 mol dm-3 H3PO3 on electrodes with varying Pt availability and surface roughness reveal that Pt catalyzes the oxidation of aqueous H3PO3 to H3PO4. This oxidation is enhanced upon applying a positive potential to the Pt electrode or raising the electrolyte temperature, the latter being corroborated by complementary ion-exchange chromatography measurements. Notably, all of these oxidation processes involve reactions with H2O, as further supported by XANES measurements of aqueous H3PO3 of different concentrations, showing a more pronounced oxidation in electrolytes with a higher H2O content. The significant role of water in the oxidation of H3PO3 to H3PO4 supports the reaction mechanisms proposed for various chemical processes observed in this work and provides valuable insights into potential strategies to mitigate Pt catalyst poisoning by H3PO3 during HT-PEMFC operation.

Unveiling the Potential of Ambient Air Annealing for Highly Efficient Inorganic CsPbI3 Perovskite Solar Cells.

Here, we report a detailed surface analysis of dry- and ambient air-annealed CsPbI3 films and their subsequent modified interfaces in perovskite solar cells. We revealed that annealing in ambient air does not adversely affect the optoelectronic properties of the semiconducting film; instead, ambient air-annealed samples undergo a surface modification, causing an enhancement of band bending, as determined by hard X-ray photoelectron spectroscopy measurements. We observe interface charge carrier dynamics changes, improving the charge carrier extraction in CsPbI3 perovskite solar cells. Optical spectroscopic measurements show that trap state density is decreased due to ambient air annealing. As a result, air-annealed CsPbI3-based n-i-p structure devices achieved a 19.8% power conversion efficiency with a 1.23 V open circuit voltage.

Oxidation of Aqueous Phosphorous Acid Electrolyte in Contact with Pt Studied by X-ray Photoemission Spectroscopy.

The oxidation of the aqueous H3PO3 in contact with Pt was investigated for a fundamental understanding of the Pt/aqueous H3PO3 interaction with the goal of providing a comprehensive basis for the further optimization of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Ion-exchange chromatography (IEC) experiments suggested that in ambient conditions, Pt catalyzes H3PO3 oxidation to H3PO4 with H2O. X-ray photoelectron spectroscopy (XPS) on different substrates, including Au and Pt, previously treated in H3PO3 solutions was conducted to determine the catalytic abilities of selected metals toward H3PO3 oxidation. In situ ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) combined with the "dip-and-pull" method was performed to investigate the state of H3PO3 at the Pt|H3PO3 interface and in the bulk solution. It was shown that whereas H3PO3 remains stable in the bulk solution, the catalyzed oxidation of H3PO3 by H2O to H3PO4 accompanied by H2 generation occurs in contact with the Pt surface. This catalytic process likely involves H3PO3 adsorption at the Pt surface in a highly reactive pyramidal tautomeric configuration.

Unravelling the Surface Oxidation-Induced Evolution of the Electronic Structure of Gallium.

Gallium is widely used in liquid metal catalyst fabrication, and its oxidized species is a well-known dielectric material. In the past decades, these two species have been well studied separately. However, the surface oxide layer-induced impact on the chemical and electronic structure of (liquid) gallium is still mostly unclear because of the extreme fast formation of thermodynamically stable surface Ga2O3. In this study, we used a combination of direct and inverse photoemission complemented by scanning electron microscopy to examine the surface properties of Ga and Ga oxide (on a SiOx/Si support) and the evolution of the surface structure upon stepwise oxidation and subsequent reduction at an elevated temperature. We find oxidation time-dependent self-limited formation of a substoichiometric Ga2O3-δ surface layer on the Ga nanoparticles. The valence band maximum (conduction band minimum) for this Ga2O3-δ is located at -3.8 (±0.1) eV [1.4 (±0.2) eV] with respect to the Fermi level, resulting in an electronic surface band gap of 5.2 (±0.2) eV. Upon annealing in ultrahigh vacuum conditions, the Ga2O3-δ surface layer can efficiently be removed when using temperatures of 600 °C and higher. This study reveals how the surface properties of Ga nanoparticles are influenced by stepwise oxidation-reduction, providing detailed insights that will benefit the optimization of this material class for different applications.

Influence of X-Ray Irradiation During Photoemission Studies on Halide Perovskite-Based Devices.

Metal halide perovskites (MHPs) are semiconductors with promising application in optoelectronic devices, particularly, in solar cell technologies. The chemical and electronic properties of MHPs at the surface and interfaces with adjacent layers dictate charge transfer within stacked devices and ultimately the efficiency of the latter. X-ray photoelectron spectroscopy is a powerful tool to characterize these material properties. However, the X-ray radiation itself can potentially affect the MHP and therefore jeopardize the reliability of the obtained information. In this work, the effect of X-ray irradiation is assessed on Cs0.05 MA0.15 FA0.8 Pb(I0.85 Br0.15 )3  (MA for CH3 NH3 , and FA for CH2 (NH2 )2 ) MHP thin-film samples in a half-cell device. There is a comparison of measurements acquired with synchrotron radiation and a conventional laboratory source for different times. Changes in composition and core levels binding energies are observed in both cases, indicating a modification of the chemical and electronic properties. The results suggest that changes observed over minutes with highly brilliant synchrotron radiation are likely occurring over hours when working with a lab-based source providing a lower photon flux. The possible degradation pathways are discussed, supported by steady-state photoluminescence analysis. The work stresses the importance of beam effect assessment at the beginning of XPS experiments of MHP samples.

Isolated Rh atoms in dehydrogenation catalysis.

Isolated active sites have great potential to be highly efficient and stable in heterogeneous catalysis, while enabling low costs due to the low transition metal content. Herein, we present results on the synthesis, first catalytic trials, and characterization of the Ga9Rh2 phase and the hitherto not-studied Ga3Rh phase. We used XRD and TEM for structural characterization, and with XPS, EDX we accessed the chemical composition and electronic structure of the intermetallic compounds. In combination with catalytic tests of these phases in the challenging propane dehydrogenation and by DFT calculations, we obtain a comprehensive picture of these novel catalyst materials. Their specific crystallographic structure leads to isolated Rhodium sites, which is proposed to be the decisive factor for the catalytic properties of the systems.

Polycapillary-boosted instrument performance in the extreme ultraviolet regime for inverse photoemission spectroscopy: erratum.

We correct values and figures for the resolution of the spectrometer, as proposed in [Opt. Express25, 31840 (2017)10.1364/OE.25.031840OPEXFF1094-4087]. The new results take into account previously unknown, incoherent phase fluctuations, caused by the polycapillary lens (PCL), and estimate the realistic performance of the instrument.

Prospect of making XPS a high-throughput analytical method illustrated for a Cu x Ni1-x O y combinatorial material library.

Combinatorial material science crucially depends on robust, high-throughput characterization methods. While X-ray photoelectron spectroscopy (XPS) may provide detailed information about chemical and electronic properties, it is a time-consuming technique and, therefore, is not viewed as a high-throughput method. Here we present preliminary XPS data of 169 measurement spots on a combinatorial 72 × 72 cm2 Cu x Ni1-x O y compositional library to explore how characterization and evaluation routines can be optimized to improve throughput in XPS for combinatorial studies. In particular, two quantification approaches are compared. We find that a simple integration (of XPS peak regions) approach is suited for fast evaluation of, in the example system, the [Cu]/([Cu] + [Ni]) ratio. Complementary to that, the time-consuming (XPS peak-) fit approach provides additional insights into chemical speciation and oxidation state changes, without a large deviation of the [Cu]/([Cu] + [Ni]) ratio. This insight suggests exploiting the fast integration approach for 'real time' analysis during XPS data collection, paving the way for an 'on-the-fly' selection of points of interest (i.e., areas on the sample where sudden composition changes have been identified) for detailed XPS characterization. Together with the envisioned improvements when going from laboratory to synchrotron-based excitation sources, this will shorten the analysis time sufficiently for XPS to become a realistic characterization option for combinatorial material science.

Origin of Interface Limitation in Zn(O,S)/CuInS2-Based Solar Cells.

Copper indium disulfide (CuInS2) grown under Cu-rich conditions exhibits high optical quality but suffers predominantly from charge carrier interface recombination, resulting in poor solar cell performance. An unfavorable "cliff"-like conduction band alignment at the buffer/CuInS2 interface could be a possible cause of enhanced interface recombination in the device. In this work, we exploit direct and inverse photoelectron spectroscopy together with electrical characterization to investigate the cause of interface recombination in chemical bath-deposited Zn(O,S)/co-evaporated CuInS2-based devices. Temperature-dependent current-voltage analyses indeed reveal an activation energy of the dominant charge carrier recombination path, considerably smaller than the absorber bulk band gap, confirming the dominant recombination channel to be present at the Zn(O,S)/CuInS2 interface. However, photoelectron spectroscopy measurements indicate a small (0.1 eV) "spike"-like conduction band offset at the Zn(O,S)/CuInS2 interface, excluding an unfavorable energy-level alignment to be the prominent cause for strong interface recombination. The observed band bending upon interface formation also suggests Fermi-level pinning not to be the main reason, leaving near-interface defects (as recently observed in Cu-rich CuInSe2) as the likely reason for the performance-limiting interface recombination.

Fluoride Chemistry in Tin Halide Perovskites.

Tin is the frontrunner for substituting toxic lead in perovskite solar cells. However, tin suffers the detrimental oxidation of SnII to SnIV . Most of reported strategies employ SnF2 in the perovskite precursor solution to prevent SnIV formation. Nevertheless, the working mechanism of this additive remains debated. To further elucidate it, we investigate the fluoride chemistry in tin halide perovskites by complementary analytical tools. NMR analysis of the precursor solution discloses a strong preferential affinity of fluoride anions for SnIV over SnII , selectively complexing it as SnF4 . Hard X-ray photoelectron spectroscopy on films shows the lower tendency of SnF4 than SnI4 to get included in the perovskite structure, hence preventing the inclusion of SnIV in the film. Finally, small-angle X-ray scattering reveals the strong influence of fluoride on the colloidal chemistry of precursor dispersions, directly affecting perovskite crystallization.

Dynamic Effects and Hydrogen Bonding in Mixed-Halide Perovskite Solar Cell Absorbers.

The organic component (methylammonium) of CH3NH3PbI3-xClx-based perovskites shows electronic hybridization with the inorganic framework via H-bonding between N and I sites. Femtosecond dynamics induced by core excitation are shown to strongly influence the measured X-ray emission spectra and the resonant inelastic soft X-ray scattering of the organic components. The N K core excitation leads to a greatly increased N-H bond length that modifies and strengthens the interaction with the inorganic framework compared to that in the ground state. The study indicates that excited-state dynamics must be accounted for in spectroscopic studies of this perovskite solar cell material, and the organic-inorganic hybridization interaction suggests new avenues for probing the electronic structure of this class of materials. It is incidentally shown that beam damage to the methylamine component can be avoided by moving the sample under the soft X-ray beam to minimize exposure and that this procedure is necessary to prevent the creation of experimental artifacts.

Chemical Interaction at the MoO3/CH3NH3PbI3-xClx Interface.

The limited long-term stability of metal halide perovskite-based solar cells is a bottleneck in their drive toward widespread commercial adaptation. The organic hole-transport materials (HTMs) have been implicated in the degradation, and metal oxide layers are proposed as alternatives. One of the most prominent metal oxide HTM in organic photovoltaics is MoO3. However, the use of MoO3 as HTM in metal halide perovskite-based devices causes a severe solar cell deterioration. Thus, the formation of the MoO3/CH3NH3PbI3-xClx (MAPbI3-xClx) heterojunction is systematically studied by synchrotron-based hard X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. Upon MoO3 deposition, significant chemical interaction is induced at the MoO3/MAPbI3-xClx interface: substoichiometric molybdenum oxide is present, and the perovskite decomposes in the proximity of the interface, leading to accumulation of PbI2 on the MoO3 cover layer. Furthermore, we find evidence for the formation of new compounds such as PbMoO4, PbN2O2, and PbO as a result of the MAPbI3-xClx decomposition and suggest chemical reaction pathways to describe the underlying mechanism. These findings suggest that the (direct) MoO3/MAPbI3-xClx interface may be inherently unstable. It provides an explanation for the low power conversion efficiencies of metal halide perovskite solar cells that use MoO3 as a hole-transport material and in which there is a direct contact between MoO3 and perovskite.

Oxidation induced restructuring of Rh-Ga SCALMS model catalyst systems.

Supported catalytically active liquid metal solutions have been receiving increasing attention recently. We investigated the oxidation behavior of macroscopic Rh-Ga alloy droplets and Rh-Ga model catalyst nanoparticles supported on SiO2/Si(100) with low Rh content (<2.5 at. %) by x-ray photoelectron spectroscopy in ultra-high vacuum and under near-ambient pressure conditions using different photon energies and also using transmission electron microscopy. The experiments are accompanied by computational studies on the Ga oxide/Rh-Ga interface and Rh-Ga intermetallic compounds. For both Rh-Ga alloy droplets and Rh-Ga model catalyst nanoparticles, exposure to molecular oxygen leads to the formation of an oxide shell in which Rh is enriched. Transmission electron microscopy on the Rh-Ga nanoparticles confirms the formation of an ∼4 nm thick gallium oxide film containing Rh. Based on ab initio molecular dynamics and computational studies on the Ga2O3/Ga interface, it is concluded that Rh incorporation into the Ga2O3 film occurs by substituting octahedrally coordinated Ga.

NaF/RbF-Treated Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers: Distinct Surface Modifications Caused by Two Different Types of Rubidium Chemistry.

The underlying beneficial mechanism of heavy alkali postdeposition treatment (PDT) of Cu(In,Ga)Se2 thin-film solar cell absorbers that led to new record efficiencies in recent years is studied using photoelectron spectroscopy. Excitation energies between 40.8 eV and 6 keV were used to examine the near-surface region of Cu(In,Ga)Se2 thin-film solar cell absorbers that underwent NaF and combined NaF/RbF PDT. The already Cu-deficient surface region after NaF PDT, which is modeled as a Cu:(In + Ga):Se = 1:5:8 phase, shows further depletion after NaF/RbF PDT and seems to incorporate some Rb. Additionally, we have found strong indications for the NaF/RbF PDT-induced formation of a Rb-In-Se-type compound with a 1:1:2 stoichiometry partially covering the absorber surface. The electronic Cu(In,Ga)Se2 structure is modified due to the RbF treatment, with a pronounced shift in the valence band maximum away from the Fermi level in the immediate vicinity of the surface.

Impact of SnF2 Addition on the Chemical and Electronic Surface Structure of CsSnBr3.

We report on the chemical and electronic structure of cesium tin bromide (CsSnBr3) and how it is impacted by the addition of 20 mol % tin fluoride (SnF2) to the precursor solution, using both surface-sensitive lab-based soft X-ray photoelectron spectroscopy (XPS) and near-surface bulk-sensitive synchrotron-based hard XPS (HAXPES). To determine the reproducibility and reliability of conclusions, several (nominally identically prepared) sample sets were investigated. The effects of deposition reproducibility, handling, and transport are found to cause significant changes in the measured properties of the films. Variations in the HAXPES-derived compositions between individual sample sets were observed, but in general, they confirm that the addition of 20 mol % SnF2 improves coverage of the titanium dioxide substrate by CsSnBr3 and decreases the oxidation of SnII to SnIV while also suppressing formation of secondary Br and Cs species. Furthermore, the (surface) composition is found to be Cs-deficient and Sn-rich compared to the nominal stoichiometry. The valence band (VB) shows a SnF2-induced redistribution of Sn 5s-derived density of states, reflecting the changing SnII/SnIV ratio. Notwithstanding some variability in the data, we conclude that SnF2 addition decreases the energy difference between the VB maximum of CsSnBr3 and the Fermi level, which we explain by defect chemistry considerations.

The Doping Mechanism of Halide Perovskite Unveiled by Alkaline Earth Metals.

Halide perovskites are a strong candidate for the next generation of photovoltaics. Chemical doping of halide perovskites is an established strategy to prepare the highest efficiency and most stable perovskite-based solar cells. In this study, we unveil the doping mechanism of halide perovskites using a series of alkaline earth metals. We find that low doping levels enable the incorporation of the dopant within the perovskite lattice, whereas high doping concentrations induce surface segregation. The threshold from low to high doping regime correlates to the size of the doping element. We show that the low doping regime results in a more n-type material, while the high doping regime induces a less n-type doping character. Our work provides a comprehensive picture of the unique doping mechanism of halide perovskites, which differs from classical semiconductors. We proved the effectiveness of the low doping regime for the first time, demonstrating highly efficient methylammonium lead iodide based solar cells in both n-i-p and p-i-n architectures.

Interface Formation between CdS and Alkali Postdeposition-Treated Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers-Key To Understanding the Efficiency Gain.

A combination of X-ray photoelectron/Auger electron spectroscopy and soft X-ray emission spectroscopy has been employed to investigate the impact of different alkali postdeposition treatments (PDTs) on the chemical structure of the (buried) CdS/Cu(In,Ga)Se2 heterojunction: the key interface in chalcopyrite-based thin-film solar cells. Chemical bath deposited (CBD) CdS layers of different thicknesses on NaF PDT (CIGSeNaF) and NaF + KF PDT (CIGSeNaF+KF) Cu(In,Ga)Se2 absorbers prepared at low temperature (to facilitate the use of flexible, e.g., polyimide, substrates) were studied. While we find the CdS/CIGSeNaF interface to be mainly free of significant chemical interaction, in the proximity of the CdS/CIGSeNaF+KF interface, an elemental redistribution involving Cd, In, K, S, and Se is revealed. For the early stages of the CBD-CdS process, our findings are in agreement with the conversion of the K-In-Se-type layer present on the CIGSeNaF+KF surface into a mixed Cd-In-(O,OH,S,Se)-type layer, probably having some Cd-In and (S,O)-Se composition gradients. For long CBD times-independent of employed PDT-we find the buffer material to be best described by a Cd(O,OH,S)-like species rather than by a pure CdS buffer. These findings shed light on the observed performance leap of corresponding CdS/CIGSeNaF+KF-based solar cells.

Tunability of MoO3 Thin-Film Properties Due to Annealing in Situ Monitored by Hard X-ray Photoemission.

The chemical and electronic structure of MoO3 thin films is monitored by synchrotron-based hard X-ray photoelectron spectroscopy while annealing from room temperature to 310 °C. Color-coded 2D intensity maps of the Mo 3d and O 1s and valence band maximum (VBM) spectra show the evolution of the annealing-induced changes. Broadening of the Mo 3d and O 1s spectra indicate the reduction of MoO3. At moderate temperatures (120-200 °C), we find spectral evidence for the formation of Mo5+ and at higher temperatures (>165 °C) also of Mo4+ states. These states can be related to the spectral intensity above the VBM attributed to O vacancy induced gap states caused by partial filling of initially unoccupied Mo 4d-derived states. A clear relation between annealing temperature and the induced changes in the chemical and electronic structure suggests this approach as a route for deliberate tuning of MoO3 thin-film properties.