Competition between Organics and Bromide at the Aqueous Solution–Air Interface as Seen from Ozone Uptake Kinetics and X‑ray Photoelectron Spectroscopy

A more detailed understanding of the heterogeneous chemistry of halogenated species in the marine boundary layer is required. Here, we studied the reaction of ozone (O₃) with NaBr solutions in the presence and absence of citric acid (C₆H₈O₇) under ambient conditions. Citric acid is used as a proxy for oxidized organic material present at the ocean surface or in sea spray aerosol. On neat NaBr solutions, the observed kinetics is consistent with bulk reaction-limited uptake, and a second-order rate constant for the reaction of O₃ + Br^– is 57 ± 10 M^–1 s^–1. On mixed NaBr–citric acid aqueous solutions, the uptake kinetics was faster than that predicted by bulk reaction-limited uptake and also faster than expected based on an acid-catalyzed mechanism. X-ray photoelectron spectroscopy (XPS) on a liquid microjet of the same solutions at 1.0 × 10^–3–1.0 × 10^–4 mbar was used to obtain quantitative insight into the interfacial composition relative to that of the bulk solutions. It revealed that the bromide anion becomes depleted by 30 ± 10% while the sodium cation gets enhanced by 40 ± 20% at the aqueous solution–air interface of a 0.12 M NaBr solution mixed with 2.5 M citric acid in the bulk, attributed to the role of citric acid as a weak surfactant. Therefore, the enhanced reactivity of bromide solutions observed in the presence of citric acid is not necessarily attributable to a surface reaction but could also result from an increased solubility of ozone at higher citric acid concentrations. Whether the acid-catalyzed chemistry may have a larger effect on the surface than in the bulk to offset the effect of bromide depletion also remains open

Self-Assembly and Superexchange Coupling of Magnetic Molecules on Oxygen-Reconstructed Ferromagnetic Thin Film

We report on the distinctive molecular assembly and exchange coupling of paramagnetic manganese(III) tetraphenylporphyrin chloride (MnTPPCl) molecules on a metallic cobalt (Co(001)) and on an oxygen-reconstructed cobalt (O/Co(001)) substrate, the latter substrate being prepared by surfactant-mediated growth. For MnTPPCl, a ferromagnetic (FM) exchange coupling to Co(001) and an antiferromagnetic (AFM) exchange coupling to O/Co(001) were identified. The random adsorption of MnTPPCl on Co(001) is turned into a self-assembled and well-ordered 2D molecular domains on O/Co(001). Different oxidation states for Mn ions are found to exist on different substrates. The here presented spectromicroscopy-correlation approach, involving X-ray magnetic circular dichroism (XMCD) spectroscopy, X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM), demonstrates its strength to identify the mechanism involved in organic spintronic interfaces

Liquid–Vapor Interface of Formic Acid Solutions in Salt Water: A Comparison of Macroscopic Surface Tension and Microscopic in Situ X‑ray Photoelectron Spectroscopy Measurements

The liquid–vapor interface is difficult to access experimentally but is of interest from a theoretical and applied point of view and has particular importance in atmospheric aerosol chemistry. Here we examine the liquid–vapor interface for mixtures of water, sodium chloride, and formic acid, an abundant chemical in the atmosphere. We compare the results of surface tension and X-ray photoelectron spectroscopy (XPS) measurements over a wide range of formic acid concentrations. Surface tension measurements provide a macroscopic characterization of solutions ranging from 0 to 3 M sodium chloride and from 0 to over 0.5 mole fraction formic acid. Sodium chloride was found to be a weak salting out agent for formic acid with surface excess depending only slightly on salt concentration. In situ XPS provides a complementary molecular level description about the liquid–vapor interface. XPS measurements over an experimental probe depth of 51 Å gave the C 1s to O 1s ratio for both total oxygen and oxygen from water. XPS also provides detailed electronic structure information that is inaccessible by surface tension. Density functional theory calculations were performed to understand the observed shift in C 1s binding energies to lower values with increasing formic acid concentration. Part of the experimental −0.2 eV shift can be assigned to the solution composition changing from predominantly monomers of formic acid to a combination of monomers and dimers; however, the lack of an appropriate reference to calibrate the absolute BE scale at high formic acid mole fraction complicates the interpretation. Our data are consistent with surface tension measurements yielding a significantly more surface sensitive measurement than XPS due to the relatively weak propensity of formic acid for the interface. A simple model allowed us to replicate the XPS results under the assumption that the surface excess was contained in the top four angstroms of solution

The Extent of Platinum-Induced Hydrogen Spillover on Cerium Dioxide

Hydrogen spillover from metal nanoparticles to oxides is an essential process in hydrogenation catalysis and other applications such as hydrogen storage. It is important to understand how far this process is reaching over the surface of the oxide. Here, we present a combination of advanced sample fabrication of a model system and in situ X-ray photoelectron spectroscopy to disentangle local and far-reaching effects of hydrogen spillover in a platinum–ceria catalyst. At low temperatures (25–100 °C and 1 mbar H2) surface O–H formed by hydrogen spillover on the whole ceria surface extending microns away from the platinum, leading to a reduction of Ce4+ to Ce3+. This process and structures were strongly temperature dependent. At temperatures above 150 °C (at 1 mbar H2), O–H partially disappeared from the surface due to its decreasing thermodynamic stability. This resulted in a ceria reoxidation. Higher hydrogen pressures are likely to shift these transition temperatures upward due to the increasing chemical potential. The findings reveal that on a catalyst containing a structure capable to promote spillover, hydrogen can affect the whole catalyst surface and be involved in catalysis and restructuring

Centimeter-Sized Single-Orientation Monolayer Hexagonal Boron Nitride With or Without Nanovoids

Large-area hexagonal boron nitride (<i>h</i>-BN) promises many new applications of two-dimensional materials, such as the protective packing of reactive surfaces or as membranes in liquids. However, scalable production beyond exfoliation from bulk single crystals remained a major challenge. Single-orientation monolayer <i>h</i>-BN nanomesh is grown on 4 in. wafer single crystalline rhodium films and transferred on arbitrary substrates such as SiO₂, germanium, or transmission electron microscopy grids. The transfer process involves application of tetraoctylammonium bromide before electrochemical hydrogen delamination. The material performance is demonstrated with two applications. First, protective sealing of <i>h</i>-BN is shown by preserving germanium from oxidation in air at high temperatures. Second, the membrane functionality of the single <i>h</i>-BN layer is demonstrated in aqueous solutions. Here, we employ a growth substrate intrinsic preparation scheme to create regular 2 nm holes that serve as ion channels in liquids

Controlling the Dimensionality of On-Surface Coordination Polymers via Endo- or Exoligation

The formation of on-surface coordination polymers is controlled by the interplay of chemical reactivity and structure of the building blocks, as well as by the orientating role of the substrate registry. Beyond the predetermined patterns of structural assembly, the chemical reactivity of the reactants involved may provide alternative pathways in their aggregation. Organic molecules, which are transformed in a surface reaction, may be subsequently trapped via coordination of homo- or hetero­metal adatoms, which may also play a role in the molecular transformation. The amino-functionalized perylene derivative, 4,9-diamino­perylene quinone-3,10-diimine (DPDI), undergoes specific levels of dehydrogenation (−1 H₂ or −3 H₂) depending on the nature of the present adatoms (Fe, Co, Ni or Cu). In this way, the molecule is converted to an endo- or an exoligand, possessing a concave or convex arrangement of ligating atoms, which is decisive for the formation of either 1D or 2D coordination polymers

Optically Triggered Néel Vector Manipulation of a Metallic Antiferromagnet Mn₂Au under Strain

The absence of stray fields, their insensitivity to external magnetic fields, and ultrafast dynamics make antiferromagnets promising candidates for active elements in spintronic devices. Here, we demonstrate manipulation of the Néel vector in the metallic collinear antiferromagnet Mn2Au by combining strain and femtosecond laser excitation. Applying tensile strain along either of the two in-plane easy axes and locally exciting the sample by a train of femtosecond pulses, we align the Néel vector along the direction controlled by the applied strain. The dependence on the laser fluence and strain suggests the alignment is a result of optically triggered depinning of 90° domain walls and their motion in the direction of the free energy gradient, governed by the magneto-elastic coupling. The resulting, switchable state is stable at room temperature and insensitive to magnetic fields. Such an approach may provide ways to realize robust high-density memory device with switching time scales in the picosecond range