Bimetallic nanoparticle catalysts provide enhanced activity, as combining metals allows tuning of electronic and geometric structure, but the enhancement may vary during the reaction because the nanoparticles can undergo metal migration under catalytic reaction conditions. Using cyclic voltammetry to track the surface composition over time, we carried out a detailed study of metal migration in a well-defined model Au-Pd core-shell nanocatalyst. When subjected to electrochemical conditions, Au migration from the core to the shell was observed. The effect of Pd shell thickness and electrolyte identity on the extent of migration was studied. Migration of metals during catalytic ethanol oxidation was found to alter the particle's surface composition and electronic structure, enhancing the core-shell particles' activity. We show that metal migration in core-shell nanoparticles is a phenomenon common to numerous electrochemical systems and must be considered when studying electrochemical catalysis.

Aqueous solutions of FeCl3 have been widely studied to shed light on a number of processes from dissolution, mineralization, biology, electrocatalysis, corrosion, to microbial biomineralization. Yet there are little to no molecular level studies of the air–liquid FeCl3 interface. Here, both aqueous and glycerol FeCl3 solution surfaces are investigated with polarized vibrational sum frequency generation (SFG) spectroscopy. We also present the first ever extreme ultraviolet reflection–absorption (XUV-RA) spectroscopy measurements of solvated ions and complexes at a solution interface, and observe with both X-ray photoelectron spectroscopy (XPS) and XUV-RA the existence of Fe­(III) at the surface and in the near surface regions of glycerol FeCl3 solutions, where glycerol is used as a high vacuum compatible proxy for water. XPS showed Cl– and Fe­(III) species with significant Fe­(III) interfacial enrichment. In aqueous solutions, an electrical double layer (EDL) of Cl– and Fe­(III) species at 0.5 m FeCl3 concentration is observed as evidenced from an enhancement of molecular ordering of water dipoles, consistent with the observed behavior at the glycerol surface. At higher concentrations in water, the EDL appears to be substantially repressed, indicative of further Fe­(III) complex enrichment and dominance of a centrosymmetric Fe­(III) species that is surface active. In addition, a significant vibrational red-shift of the dangling OH from the water molecules that straddle the air–water interface reveals that the second solvation shell of the surface active Fe­(III) complex permeates the topmost layer of the aqueous interface.

The creation of hierarchical porosity in metal-organic frameworks (MOFs) could benefit various applications of MOFs such as gas storage and separation. Having single-crystalline microcrystals instead of poly-crystalline composites is critical for these potential applications of MOFs with hierarchical porosity. We developed a room temperature synthetic method to generate uniform hollow and mesoporous zeolitic imidazolate framework-8 (ZIF-8) microcrystals with a single-crystalline structure via overgrowing a ZIF-8 shell in methanol solution on a ZIF-8 core with water adsorbed in the pores. The cavities formed as a result of the different solvent micro-environment. This double-solvent mediated overgrowth method could be applied to prepare other MOFs with hierarchical porosity.

The selectivities and driving forces governing phosphate recognition by charged receptors at prevalent aqueous interfaces is unexplored relative to the many studies in homogeneous solutions. Here we report on electrostatic binding versus hydrogen-bond-assisted electrostatic binding of phosphate (H2PO4 –) for two important receptor classes in the unique microenvironment of the air–water interface. We find that the methylated ammonium receptor (U-Ammo + ) is dominated by electrostatic binding to phosphate anions and fails to be selective for phosphate binding over chloride, whereas the highly phosphate-selective guanidinium receptor (U-Guan + ) provides synergistic hydrogen-bonding and electrostatic interactions. Apparent binding constants were evaluated in situ for U-Ammo + and U-Guan + using temperature-controlled infrared reflection–absorption spectroscopy with Langmuir-type fitting. Thermodynamic quantities showed enthalpically driven binding affinities of U-Guan + and U-Ammo + receptors (ΔH°b = −71 (±9) kJ/mol and ΔH°b = −155 (±13) kJ/mol, respectively). U-Guan + revealed a nearly fourfold smaller entropic barrier to binding (ΔS°b = −132 (±34) J/mol K) than the U-Ammo + receptor (ΔS°b = −440 (±45) J/mol K), attributed to hydration differences. The larger entropic penalty for the U-Ammo + receptor is correlated with a molecular expansion shown in surface pressure–area isotherms, whereas the smaller entropic penalty of the U-Guan + receptor conversely correlated with no expansion. The U-Guan + receptor also revealed anti-Hofmeister selectivity for phosphate over chloride, while the non-hydrogen-bonding U-Ammo + receptor followed Hofmeister selectivity. Our results indicate that hydrogen bonding is an integral chemical design element for achieving anti-Hofmeister selectivity for phosphate.

Conventional gas sensors work upon changes in mechanical or conductive properties of sensing materials during a chemical process, which may limit availabilities of size miniaturization and design simplification. However, fabrication of miniaturized sensors with superior sensitivities in real-time and label-free probing of chemical reactions or catalytic processes remains highly challenging, in particular with regard to integration of materials into a desired smaller volume without losing the recyclability of sensing properties. Here, we demonstrate a unique bimetallic nanostructure, the Au−Pd− Au core−shell−frame nanobrick, as a promising archetype for fabrication of miniaturized sensors at nanoscale. Upon analysis of the aqueous synthesis, both ex situ and in situ, the formation of Au frames is consistent with selective deposition and aggregation of NaBH 4 -reduced Au nanoparticles at the corners and edges of cubic Pd shells, where the {100} surfaces, capped by iodide ions, are growth-limited. By virtue of the thin Pd shell (∼3.5 nm) sandwiched in-between the two Au layers of the core and the frame, the Au−Pd−Au nanobrick yields excellent optical sensitivity in hydrogen gas sensing, leading to a large 13 nm spectral shift of light scattering between Pd and PdH x . The composite nanostructure with a size of ∼60 nm offers an archetype for miniaturized sensors possessing label-free, real-time, and highresolution probing abilities and hence paves the way for fabrication of highly efficient nanosensors via sustainable methods.