Abstract. Snowpack nitrate photolysis is a major source of nitrogen oxides and nitrous acid that control atmospheric oxidants in polar environments. It remains unclear at what temperature nitrate salts crystalize in snow and how this impacts the photolysis product yields. Here we show, using near‑edge X‑ray absorption fine structure (NEXAFS) spectroscopy, that sodium nitrate does not precipitate at temperatures down to 23 K below the eutectic temperature at ice surfaces, but does so in absence of ice. This indicates that strong interfacial supercooling or liquid‑like solvation persists. Complementary snow photolysis experiments demonstrate that such liquid-like nitrate shows a higher HONO to NO2 emission ratio than precipitated nitrate, which is consistent with the enhanced role of secondary chemistry in the aqueous phase. These findings show that supercooled nitrate might be present over wide ranges of Arctic, Antarctic, and upper tropospheric temperatures with a significant impact on the HONO and NO2 fluxes and thus the composition and chemistry in the air above snow or in contact with ice clouds.

Halide ions in oceans and sea-spray aerosol particles are an important source of reactive halogen species in the atmosphere that impact the ozone budget and radiative balance. The multiphase cycling of halogen species is linked to the abundance of halide ions at the aqueous solution-air interface. Ubiquitously present surface-active organic compounds may affect the interfacial abundance of halide ions. Here, we use liquid jet X-ray photoelectron spectroscopy and molecular dynamics (MD) simulations to assess the impact of surfactants with different headgroups on the abundance of bromide and sodium ions at the interface. Core level spectra of Br 3d, Na 2s, and O 1s are reported for solutions containing tetrabutylammonium, hexylamine (HA), and propyl sulfate. We used a photoelectron attenuation model to retrieve the interfacial concentration of bromide in the presence of these different surfactants. The experimental results confirm the previously reported strong enhancement of bromide in the presence of tetrabutylammonium at the interface. In turn, propyl sulfate had a minor impact on the abundance of bromide but led to a significantly enhanced concentration of sodium cations. The MD simulations performed for bromide solutions containing hexylammonium and propyl sulfate show an enhancement of the interfacial bromide and sodium concentrations, respectively, comparable to the experimental results. The difference between the measured enhancement of bromide for HA and the nearly nonexistent effect of HA on bromide in the MD simulations is ascribed to the small amounts of hexylammonium present in the experimental solution. The present work suggests an important role of electrostatic interactions at the interface, which may guide the assessment of anion and cation abundances in atmospheric particles more generally.

We present a fundamental study of the band alignment at the interface of HfZrO4 (HZO) with Ge-doped Ga2O3. Ge is an alternative n-type dopant for the wide band gap Ga2O3 due to its shallow donor level and favorable MBE growth conditions. In the perspective of using the ferroelectric polarization of hafnia based oxides, we have used a stack of HZO on highly Ge doped Ga2O3, the latter providing high carrier density. Electrical contacts were ensured by a TiN top electrode deposited on the HZO and an Au pad on the Ge:Ga2O3. The band alignment was measured by carrying out hard X-ray photoelectron spectroscopy (HAXPES) with in situ bias application across the HZO and following the evolution of both HZO and Ga2O3 energy level. Complementary high-resolution transmission electron microscopy (HRTEM) provided structural confirmation of the polar orthorhombic phase however electrical characterization showed that charge injection and trapping at the interface prevents stabilizing the ferroelectric polarization in HZO. The band alignment in the presence of a leaky HZO layer is therefore dominated by the bias induced band skewing.

In situ investigations of solid-liquid interfaces are crucial for gaining a fundamental understanding of electrocatalytic processes. Dip-and-pull ambient pressure X-ray photoelectron spectroscopy (APXPS) enables such investigations by analyzing an electrocatalyst surface (solid) through the covering electrolyte layer (liquid) with an applied electrochemical potential. This stable solid-liquid interface is created by vertically "dipping" and then "pulling" an electrode from a bulk electrolyte solution. The resulting electrolyte layer has a decreasing thickness toward the upper electrode, allowing in situ probing of the electrocatalyst surface in this upper region. However, detecting representative electrocatalyst surface changes remains challenging with dip-and-pull APXPS. To address the challenge, this study experimentally evaluates electrochemical and spectroscopic aspects of dip-and-pull APXPS by investigating thin films of Ni1-x Fe x O y oxygen evolution reaction (OER) electrocatalysts. Here, two technical limitations are revealed: (1) missing Faradaic reactions (i.e., redox and OER) in the electrocatalyst surface probing region and (2) low spectroscopic surface-sensitivity with the typically used tender X-rays. Limitation (1) is discovered with a modified electrode design that enables OER activity measurements, indicating limited ionic conductance along the vertically thinning electrolyte layer. This strongly suppresses the studied Faradaic reactions and hinders operando investigations of OER electrocatalyst thin films in the upper electrode region, where their surfaces are probed. To minimize limitations (1) and (2), the findings suggest changing electrochemical potential only when the electrode is completely dipped in the bulk electrolyte to enhance surface modifications and using lower energetic photons at higher flux to improve the surface-sensitivity. Moreover, cyclic voltammetry is presented as an electrochemical conditioning method to maximize the spectroscopic detectability of OER electrocatalyst surface changes. Overall, this dip-and-pull APXPS evaluation provides fundamental insights and suggestions that will further improve the technique for investigating OER electrocatalysts.

We present a study of Ge segregation at the surface of highly germanium-doped gallium oxide (2.5 x 10(20) cm(-3) nominal doping level) grown by molecular beam epitaxy. We probed the dopant concentration as a function of depth by hard x-ray photoelectron spectroscopy and standard laboratory photoemission spectroscopy. We notably found that there is germanium segregation within the top 2 nm where its concentration is 3 times the nominal doping level. This increased dopant concentration leads to a threefold enhancement of surface conductivity. The results suggest a reliable method for delta doping for power electronics applications.

Utilizing near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) under pressures up to 10 mbar, this study investigates the hydration behaviors of two swelling (negatively charged) smectite clays, specifically hydroxyhectorite and hydroxysaponite, under the influence of counterions Cs+, Na+, and Sr2+, which differ in size and affinity for water. NAP-XPS reveals differences in the electronic structures of hydroxyhectorite and hydroxysaponite that are related to the location of cation substitution within the negatively charged phyllosilicate layers. Importantly, this study provides atomic-scale insights into the interactions between water and these clays, focusing on changes in the counterion core-level binding energies due to hydration. We discuss the fact that the binding energy shift due to hydration results from the combination of two components: a physical component related to a change in the electrostatic potential (calculated after molecular dynamics simulation) and a chemical component related to a change in charge transfer between ligands and counterions due to the replacement of phyllosilicate oxygen atoms with water molecules (as suggested by quantum chemistry calculations). These insights-critical for understanding the electronic structure of swelling smectites-open promising avenues for future research into the hydration mechanisms of other nanostructured minerals using NAP-XPS.

The multi-phase oxidation of S(IV) plays a crucial role in the atmosphere, leading to the formation of haze and severe pollution episodes. We here contribute to its understanding on a molecular level by reporting experimentally determined pKa values of the various S(IV) tautomers and reaction barriers for SO2 formation pathways. Complementary state-of-the-art molecular-dynamics simulations reveal a depletion of bisulfite at low pH at the liquid-vapor interface, resulting in a different tautomer ratio at the interface compared to the bulk. On a molecular-scale level, we explain this with the formation of a stable contact ion pair between sulfonate and hydronium ions, and with the higher energetic barrier for the dehydration of sulfonic acid at the liquid-vapor interface. Our findings highlight the contrasting physicochemical behavior of interfacial versus bulk environments, where the pH dependence of the tautomer ratio reported here has a significant impact on both SO2 uptake kinetics and reactions involving NOx and H2O2 at aqueous aerosol interfaces. The complex equilibria of sulfur compounds at the liquid-vapor interface play key roles in atmospheric processes. Here, using X-ray photoelectron spectroscopy, Raman spectroscopy, and molecular dynamics simulations the authors determining pKa values and tautomer ratios at the air-vapor interface in a liquid microjet.

Iodine chemistry is implicated in atmospheric chemistry and can lead to the formation of several oxides such as HOI, I2, IO, OIO, and finally I2O5 or HIO3, which may nucleate as nanoparticles relevant for cloud formation in remote environments (Saiz-Lopez et al., 2012, Finkenzeller et al., 2022). These oxides can be formed through reaction with oxidants or other halogen compounds in the gas phase or the particle phase. Most of the iodide oxidation processes have been suggested to be enhanced at interfaces, similar to those involving other halogen species, either due to the surface propensity of intermediates (Artiglia et al., 2017) or the iodine species itself (Moreno and Beaza-Romero, 2019). However, no data are available about the surface concentration of iodine species other than iodide. After two decades of research into the surface propensity of iodide and bromide, the picture emerges that their surface propensity is not as extreme as initially thought (Jungwirth and Tobias, 2002; Ghosal et al., 2005; Olivieri et al., 2018).Liquid jet X-ray photoelectron spectroscopy (XPS) experiments have been carried out at the SIM beamline at the Swiss Light Source. Acquisition of kinetic energy dependent (thus at different probing depth) I3d, I4d core level and valence level spectra has been done for iodide, iodate and iodic acid. This allows to retrieve the surface propensity of these iodine species at the aqueous solution – air interface. HIO3, HOI and iodide surface propensity has also been investigated by Ab Initio Molecular Dynamics computation at the revPBE-D3/DZVP-SR level using CP2K software (Khüne et al., 2020).Artiglia et al., Nat. Commun., 8, 700 (2017). Finkenzeller et al., Nat. Chem. (2022). Ghosal et al., Science, 307, 563 (2005). Jungwirth and D. Tobias, J. Phys. Chem. B, 106, 25, 6361 (2002). D. Kühne et al., J. Chem. Phys., 152, 194103 (2020). Moreno and M. T. Baeza-Romero, 21, 19835 (2019). Olivieri et al., J. Phys, Chem. B, 122, 2, 910 (2018). Saiz-Lopez et al., Chem. Rev., 112, 1773 (2012).

Synchrotron radiation near ambient pressure X-ray photoemissionspectroscopy (SR NAP-XPS) has been an invaluable tool for examininggas/liquid and liquid/solid interfaces. Despite its benefits, concernshave emerged regarding beam damage in NAP-XPS experiments, particularlyinvolving condensed liquid water, because of the high dose rates,greater than 10(5) Gy & BULL;s(-1). This studyinvestigates the radiolytic effects on the chemistry of concentratedNaX sodium halide solutions (X = Cl, Br, I) and Mg-Cl solutionformed over the layered double hydroxide [Mg2Al(OH)(6)](+)[Cl-]. The formation of oxidizedspecies XO- as the radiolytic end product undersoft X-ray irradiation is discussed in detail. We examine the impactof known parameters (such as the dose rate) on the abundance of XO-. The observed scatter in the data likely arises fromstill unrecognized or insufficiently controlled parameters (such assolute concentration or solution hydrodynamics). Deciphering theseradiolytic effects in halide solutions allows us to propose guidelinesfor their better identification, understanding and control, ultimatelyimproving the reliability of synchrotron NAP-XPS analysis for interfacesrelevant to environmental chemistry and electrochemistry.

<p>Water is prevalent on all atmospheric particle surfaces, independent of material, phase state or water vapor saturation ratio. Under subsaturated relative humidity conditions, water occurs as reversibly adsorbed layers on solid surfaces or as concentrated aqueous solutions in aqueous particles, exhibiting different composition at the interface than in the bulk. The hydrogen bonding structure at these interfaces is affected by interactions with the substrate on solid surfaces and by ions and ionic or neutral surfactants on aqueous solutions. The water structure and its interplay with hydrated substrate features, hydrated free ions or neutral solutes or surfactant head groups are playing a key role in many chemical and physical processes at these interfaces. We have developed X-ray photoelectron spectroscopy (XPS) and electron yield near edge X-ray absorption fine structure (NEXAFS) spectroscopy to explore the interfacial water structure in situ. We will present examples of the structure of adsorbed water on different Feldspar surfaces, on silver iodide and titanium dioxide. We will also show cases related to the water structure on aqueous solutions containing salts or different surfactant species.</p>

Conspectus: Methane complete oxidation is an important reaction that is part of the general scheme used for removing pollutants contained in emissions from internal combustion engines and, more generally, combustion processes. It has also recently attracted interest as an option for the removal of atmospheric methane in the context of negative emission technologies. Methane, a powerful greenhouse gas, can be converted to carbon dioxide and water via its complete oxidation. Despite burning methane being facile because the combustion sustains its complete oxidation after ignition, methane strong C-H bonds require a catalyst to perform the oxidation at low temperatures and in the absence of a flame so as to avoid the formation of nitrogen oxides, such as those produced in flares. This process allows methane removal to be obtained under conditions that usually lead to higher emissions, such as under cold start conditions in the case of internal combustion engines. Among several options that include homo- and heterogeneous catalysts, supported palladium-based catalysts are the most active heterogeneous systems for this reaction. Finely divided palladium can activate C-H bonds at temperatures as low as 150 degrees C, although complete conversion is usually not reached until 400-500 degrees C in practical applications. Major goals are to achieve catalytic methane oxidation at as low as possible temperature and to utilize this expensive metal more efficiently.Compared to any other transition metal, palladium and its oxides are orders of magnitude more reactive for methane oxidation in the absence of water. During the last few decades, much research has been devoted to unveiling the origin of the high activity of supported palladium catalysts, their active phase, the effect of support, promoters, and defects, and the effect of reaction conditions with the goal of further improving their reactivity. There is an overall agreement in trends, yet there are noticeable differences in some details of the catalytic performance of palladium, including the active phase under reaction conditions and the reasons for catalyst deactivation and poisoning. In this Account we summarize our work in this space using well-defined catalysts, especially model palladium surfaces and those prepared using colloidal nanocrystals as precursors, and spectroscopic tools to unveil important details about the chemistry of supported palladium catalysts. We describe advanced techniques aimed at elucidating the role of several parameters in the performance of palladium catalysts for methane oxidation as well as in engineering catalysts through advancing fundamental understanding and synthesis methods. We report the state of research on active phases and sites, then move to the role of supports and promoters, and finally discuss stability in catalytic performance and the role of water in the palladium active phase. Overall, we want to emphasize the importance of a fundamental understanding in designing and realizing active and stable palladium-based catalysts for methane oxidation as an example for a variety of energy and environmental applications of nanomaterials in catalysis.

Noble metal-based catalysts are ubiquitous because of their high activity and stability. However, they irreversibly deteriorate over time especially in high-temperature applications. In these conditions, sintering is the main reason for deactivation, and understanding how sintering occurs gives the opportunity to mitigate these detrimental processes. Previous studies successfully distinguished between two fundamental sintering modes, namely, particle migration and coalescence (PMC) and Ostwald ripening (OR). However, differentiation between surface-and vapor mediated Ostwald ripening processes has not been demonstrated yet, even though it is crucial information to tune metal/support interactions and stabilize catalysts. Here, we demonstrate that surface-and vapor-mediated ripening occur in two distinct regimes of temperature with some overlap using Pt and Pd catalysts prepared from colloidal nanocrystals as precursors. By either co impregnating the two metal nanocrystals on the same grain of alumina support or by physically mixing powders of the two distinct metal catalysts, we tune the intermetal particle distance between nanometers and micrometers. We then use methane complete oxidation as a reporter reaction that occurs at higher rates on pure Pd and lower rates on alloyed Pd/Pt catalysts to trace the movement of Pt in the system. Aging the catalysts at different temperatures allows us to reveal that Pt initially sinters by surface mediated ripening until similar to 750 degrees C, but at temperatures above 800 degrees C, vapor-mediated ripening by PtO2 becomes the main sintering mechanism. This work demonstrates how colloidal catalysts allow unique insights into the working and deactivation mechanisms of supported systems.

The hydration of [Mg2Al(OH)6]+[Cl-, yH2O] LDH was investigated by volumetric water adsorption, X-ray diffraction, Fourier Transformed Infrared (FTIR) sepctroscopy, and Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). It was demonstrated that water uptake into interlayer region can be quantified by NAP-XPS. The LDH's electronic structure is assessed for increasing relative humidity (RH) up to 0.044. The Cl 2p level appears essentially unaffected by water uptake, but as supported by DFT calculation, H-bonds between chloride and water induce Cl 3p orbitals splitting. At RH = 0.7, LDH is covered by a water film that contains Mg and Cl species but no Al. Soft X-ray radiolysis of hydrated Cl species induces the formation of oxidized Cl species.

Supported palladium catalysts are the most active ones toward the complete oxidation of methane. However, the presence of relevant amounts of water hinders the catalytic activity and long-term stability, due to competition between methane and water for the active sites. Hence, understanding the inhibition effect of water on methane oxidation is mandatory to improve these catalysts. We present an in situ ambient pressure X-ray photoelectron spectroscopy study of methane oxidation on Pd/Al2O3 in presence and in absence of water. The inhibition effect of water is demonstrated by combining reactivity tests with electron microscopy and photoelectron spectroscopy measurements. In the presence of water, the redox activity of palladium decreases. Water competes with methane for the catalytically active sites, poisoning the surface with hydroxyl groups and hindering the generation of the coordinatively unsaturated palladium active sites. Multiple palladium oxide species displaying different reactivity are identified. A new cationic palladium species, assigned to adatoms, is detected, which shows higher reactivity with methane than particulate palladium oxide but also high inhibition by water.

Fenton chemistry, involving the reaction between Fe2+ and hydrogen peroxide, is well-known due to its applications in the mineralization of extremely stable molecules. Different mechanisms, influenced by the reaction conditions and the solvation sphere of iron ions, influence the fate of such reactions. Despite the huge amount of effort spent investigating such processes, a complete understanding is still lacking. This work combines photoelectron spectroscopy and theoretical calculations to investigate the solvation and reactivity of Fe2+ and Fe3+ ions in aqueous solutions. The reaction with hydrogen peroxide, both in homogeneous Fenton reagents and at the liquid-vapor interface, illustrates that both ions are homogeneously distributed in solutions and exhibit an asymmetric octahedral coordination to water in the case of Fe2+. No indications of differences in the reaction mechanism between the liquid-vapor interface and the bulk of the solutions have been found, suggesting that Fe3+ and hydroxyl radicals are the only intermediates.

Precipitation is mostly formed via the ice phase in mixed phase clouds, and ice clouds are very relevant for Earths’ climate. Freezing or prevention of freezing is common to everyday life, e.g. for food and drug storage, icing and de-icing, etc. However, the ice nucleation process is not well understood, since it occurs on the size scale of clusters of molecules and time scales of molecular fluctuations. In this study, we have taken a step toward nanoscale observation of particles that nucleate ice by developing a new ice nucleation instrument, referred as the INXcell, which couples an ice nucleation environmental cell to the scanning transmission X-ray microscope (STXM) at the Swiss Light Source. We employ near-edge X-ray absorption fine-structure spectroscopy (NEXAFS) to map in situ chemical composition of ice nucleating particles with 35 × 35 nm2 spatial resolution. The main technical challenge was control of temperature, T, and thus relative humidity, RH, while maintaining X-ray transparency. In the INXcell, X-rays are focused onto a sample through a temperature-controlled aperture, which was modified to host a jet of nitrogen cooled down to 170 K. The cold jet impinges on the back surface of a sample exposed to water vapor to control sample temperature and thus RH. We used our unique spectroscopic and ice nucleation capability and investigated the heterogeneous freezing ability of ferrihydrite particles with and without coatings of citric acid. Ferrihydrite is an amorphous or poorly crystalline iron oxyhydroxide abundant in mineral dust and is difficult to identify with conventional XRD analysis. We confirmed that ferrihydrite could nucleate ice via immersion freezing and deposition ice nucleation, depending on whether or not the particles first take up water, respectively. When coating ferrihydrite with citric acid, mimicking organic coatings that aerosol particles obtain throughout their atmospheric lifetime, we observed a reduction in the efficiency to nucleate ice following freezing point depression. Spectroscopic identification of the coated ferrihydrite structure emplyed the iron and carbon X-ray absorption L-edges and K-edge, respectively. We also investigated feldspar particles coated with xanthan gum, a surrogate for a highly ice active mineral with a highly viscous organic coating. We observed that deposition ice nucleation occurred only below the RH dependent glass transition of xanthan gum. Using a newly developed stochastic freezing model (SFM) based on solution water activity, we reproduced average conditions and data scatter of the RH and T at which ice formed. Additionally, we ran our model with atmospheric idealized air parcel trajectories and found overall that deposition ice nucleation was the dominant heterogeneous freezing mechanism. Homogeneous ice nucleation subsequent to water uptake out-performed immersion freezing.

Resorcinol and orcinol are simple members of the family of phenolic compounds present in particulate matter in the atmosphere; they are amphiphilic in nature and thus surface active in aqueous solution. Here, we used X-ray photoelectron spectroscopy to probe the concentration of resorcinol (benzene-1,3-diol) and orcinol (5-methylbenzene-1,3-diol) at the liquid-vapor interface of aqueous solutions. Qualitatively consistent surface propensity and preferential orientation was obtained by molecular dynamics simulations. Auger electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to probe the hydrogen bonding (HB) structure, indicating that the local structure of water molecules near the surface of the resorcinol and orcinol solutions tends towards a larger fraction of tetrahedrally coordinated molecules than observed at the liquid-vapor interface of pure water. The order parameter obtained from the molecular dynamics simulations confirm these observations. This effect is being discussed in terms of the formation of an ordered structure of these molecules at the surface leading to patterns of hydrated OH groups with distances among them that are relatively close to those in ice. These results suggest that the self-assembly of phenolic species at the aqueous solution-air interface could induce freezing similar to the case of fatty alcohol monolayers and, thus, be of relevance for ice nucleation in the atmosphere. We also attempted at looking at the changes of the O 1b1, 3a2 and 1b2 molecular orbitals of liquid water, which are known to be sensitive to the HB structure as well, in response to the presence of resorcinol and orcinol. However, these changes remained negligible within uncertainty for both experimentally obtained valence spectra and theoretically calculated density of states.

Ice nucleation is one of the most uncertain microphysical processes, as it occurs in various ways and on many types of particles. To overcome this challenge, we present a heterogeneous ice nucleation study on deposition ice nucleation and immersion freezing in a novel cryogenic X-ray experiment with the capability to spectroscopically probe individual ice nucleating and non-ice nucleating particles. Mineral dust type particles composed of either ferrihydrite or feldspar were used and mixed with organic matter of either citric acid or xanthan gum. We observed in situ ice nucleation using scanning transmission X-ray microscopy (STXM) and identified unique organic carbon functionalities and iron oxidation state using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the new in situ environmental ice cell, termed the ice nucleation X-ray cell (INXCell). Deposition ice nucleation of ferrihydrite occurred at a relative humidity with respect to ice, RHi, between ∼120-138% and temperatures, T ∼ 232 K. However, we also observed water uptake on ferrihydrite at the same T when deposition ice nucleation did not occur. Although, immersion freezing of ferrihydrite both in pure water droplets and in aqueous citric acid occurred at or slightly below conditions for homogeneous freezing, i.e. the effect of ferrihydrite particles acting as a heterogeneous ice nucleus for immersion freezing was small. Microcline K-rich feldspar mixed with xanthan gum was also used in INXCell experiments. Deposition ice nucleation occurred at conditions when xanthan gum was expected to be highly viscous (glassy). At less viscous conditions, immersion freezing was observed. We extended a model for heterogeneous and homogeneous ice nucleation, named the stochastic freezing model (SFM). It was used to quantify heterogeneous ice nucleation rate coefficients, mimic the competition between homogeneous ice nucleation; water uptake; deposition ice nucleation and immersion freezing, and predict the T and RHi at which ice was observed. The importance of ferrihydrite to act as a heterogeneous ice nucleating particle in the atmosphere using the SFM is discussed.

To advance the understanding of key electrochemical and photocatalytic processes that depend on the electronic structure of aqueous solutions, X-ray photoemission spectroscopy has become an invaluable tool, especially when practiced with liquid microjet setups. Determining vertical ionization energies referenced to the vacuum level, and binding energies referenced to the Fermi level, including the much-coveted reorganization energy of the oxidized species of a redox couple, requires that energy levels be properly defined. The present paper addresses specifically how the vacuum level "just outside the surface" can be known through the energy position of the rising edge of the secondary electrons, and how the Fermi level reference is uniquely determined via the introduction of a redox couple. Taking the case of the ferricyanide/ferrocyanide and ferric/ferrous couples, this study also tackles issues related to the electrokinetic effects inherent to the production of a liquid jet in a vacuum, which has become the standard water sample environment for photoemission experiments.