Role of Electronic Structure on Nitrate Reduction to Ammonium: A Periodic Journey.

Electrocatalysis is a promising approach to convert waste nitrate to ammonia and help close the nitrogen cycle. This renewably powered ammonia production process sources hydrogen from water (as opposed to methane in the thermal Haber-Bosch process) but requires a delicate balance between a catalyst's activity for the hydrogen evolution reaction (HER) and the nitrate reduction reaction (NO3RR), influencing the Faradaic efficiency (FE) and selectivity to ammonia/ammonium over other nitrogen-containing products. We measure ammonium FEs ranging from 3.6 ± 6.6% (on Ag) to 93.7 ± 0.9% (on Co) across a range of transition metals (TMs; Ti, Fe, Co, Ni, Ni0.68Cu0.32, Cu, and Ag) in buffered neutral media. To better understand these competing reaction kinetics, we develop a microkinetic model that captures the voltage-dependent nitrate rate order and illustrates its origin as competitive adsorption between nitrate and hydrogen adatoms (H*). NO3RR FE can be described via competition for electrons with the HER, decreasing sharply for TMs with a high work function and a correspondingly high HER activity (e.g., Ni). Ammonium selectivity nominally increases as the TM d-band center energy (Ed) approaches and overcomes the Fermi level (EF), but is exceptionally high for Co compared to materials with similar Ed. Density functional theory (DFT) calculations indicate Co maximizes ammonium selectivity via (1) strong nitrite binding enabling subsequent reduction and (2) promotion of nitric oxide dissociation, leading to selective reduction of the nitrogen adatom (N*) to ammonium.

Magnetic Field Landscapes Guiding the Chemisorption of Diamagnetic Molecules.

It is shown that the self-assembly of diamagnetic molecule submonolayers on a surface can be influenced by magnetic stray field landscapes emerging from artificially fabricated magnetic domains and domain walls. The directed local chemisorption of diamagnetic subphthalocyaninatoboron molecules in relation to the artificially created domain pattern is proved by a combination of surface analytical methods: ToF-SIMS, X-PEEM, and NEXAFS imaging. Thereby, a new method to influence self-assembly processes and to produce patterned submonolayers is presented.

Metal Doping Regulates Electrocatalysts Restructuring During Oxygen Evolution Reaction.

High-efficiency and low-cost catalysts for oxygen evolution reaction (OER) are critical for electrochemical water splitting to generate hydrogen, which is a clean fuel for sustainable energy conversion and storage. Among the emerging OER catalysts, transition metal dichalcogenides have exhibited superior activity compared to commercial standards such as RuO2, but inferior stability due to uncontrolled restructuring with OER. In this study, we create bimetallic sulfide catalysts by adapting the atomic ratio of Ni and Co in CoxNi1-xSy electrocatalysts to investigate the intricate restructuring processes. Surface-sensitive X-ray photoelectron spectroscopy and bulk-sensitive X-ray absorption spectroscopy confirmed the favorable restructuring of transition metal sulfide material following OER processes. Our results indicate that a small amount of Ni substitution can reshape the Co local electronic structure, which regulates the restructuring process to optimize the balance between OER activity and stability. This work represents a significant advancement in the development of efficient and noble metal-free OER electrocatalysts through a doping-regulated restructuring approach.

Molecular suction pads: self-assembled monolayers of subphthalocyaninatoboron complexes on gold.

Subphthalocyaninatoboron complexes with six long-chain alkylthio substituents in their periphery are applicable for the formation of self-assembled monolayers (SAMs) on gold. Such films are prepared from solution with the axially chlorido-substituted derivatives and characterised by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results are in accord with the formation of SAMs assembled by the chemisorption of both covalently bound thiolate-type as well as coordinatively bound thioether units. The adsorbate molecules adopt an essentially flat adsorption geometry on the substrate, resembling a suction pad on a surface.

Phthalocyaninato complexes with peripheral alkylthio chains: disk-like adsorbate species for the vertical anchoring of ligands on gold surfaces.

Thin metalorganic films were prepared on gold by self-assembly of thioether-functionalised phthalocyaninato complexes from solution. The phthalocyaninato ligands used contain eight peripheral, ß-positioned, alkylthio substituents SR (1a: R = n-C(8)H(17), 1b: R = n-C(12)H(25)), which serve as headgroups for surface binding and promote lateral assembly, while the disk-like phthalocyaninato core offers the scope for the attachment of axial ligands to the adsorbed molecules. This process was mimicked by coordination of pyridine (Py) to [Zn(1a)] and [Zn(1b)], respectively. The crystal structures of the products [Zn(1a)(Py)] and [Zn(1b)(Py)] were determined. The crystal structures of 4,5-bis(octylthio)phthalodinitrile and 4,5-bis(dodecylthio)phthalodinitrile were also determined. The films fabricated from [Mn(1a)Cl] and [Mn(1b)Cl] on gold were characterised by XPS, ToF-SIMS and NEXAFS spectroscopy, which revealed the presence of well-defined and homogeneous self-assembled monolayers (SAMs), whose constituents are bound to the substrate by thioether-gold linkages. The orientation of the macrocycles is predominantly parallel to the surface. Strong electronic interaction of the manganese(III) centre with the substrate leads to Cl loss upon adsorption and its reduction to Mn(II).

In Situ X-ray Absorption Spectroscopy Studies of Nanoscale Electrocatalysts.

Nanoscale electrocatalysts have exhibited promising activity and stability, improving the kinetics of numerous electrochemical reactions in renewable energy systems such as electrolyzers, fuel cells, and metal-air batteries. Due to the size effect, nano particles with extreme small size have high surface areas, complicated morphology, and various surface terminations, which make them different from their bulk phases and often undergo restructuring during the reactions. These restructured materials are hard to probe by conventional ex-situ characterizations, thus leaving the true reaction centers and/or active sites difficult to determine. Nowadays, in situ techniques, particularly X-ray absorption spectroscopy (XAS), have become an important tool to obtain oxidation states, electronic structure, and local bonding environments, which are critical to investigate the electrocatalysts under real reaction conditions. In this review, we go over the basic principles of XAS and highlight recent applications of in situ XAS in studies of nanoscale electrocatalysts.

Ambient-Pressure X-ray Photoelectron Spectroscopy Characterization of Radiation-Induced Chemistries of Organotin Clusters.

Advances in extreme ultraviolet (EUV) photolithography require the development of next-generation resists that allow high-volume nanomanufacturing with a single nanometer patterning resolution. Organotin-based photoresists have demonstrated nanopatterning with high resolution, high sensitivity, and low-line edge roughness. However, very little is known regarding the detailed reaction mechanisms that lead to radiation-induced solubility transitions. In this study, we investigate the interaction of soft X-ray radiation with organotin clusters to better understand radiation-induced chemistries associated with EUV lithography. Butyltin Keggin clusters (ß-NaSn13) were used as a model organotin photoresist, and characterization was performed using ambient-pressure X-ray photoelectron spectroscopy. The changes in relative atomic concentrations and associated chemical states in ß-NaSn13 resists were evaluated after exposure to radiation for a range of ambient conditions and photon energies. A significant reduction in the C 1s signal versus exposure time was observed, which corresponds to the radiation-induced homolytic cleavage of the butyltin bond in the ß-NaSn13 clusters. To improve the resist sensitivity, we evaluated the effect of oxygen partial pressure during radiation exposures. We found that both photon energy and oxygen partial pressure had a strong influence on the butyl group desorption rate. These studies advance the understanding of radiation-induced processes in ß-NaSn13 photoresists and provide mechanistic insights for EUV photolithography.

Fabrication of a Flexible Amperometric Glucose Sensor Using Additive Processes.

This study details the use of printing and other additive processes to fabricate a novel amperometric glucose sensor. The sensor was fabricated using a Au coated 12.7 µm thick polyimide substrate as a starting material, where micro-contact printing, electrochemical plating, chloridization, electrohydrodynamic jet (e-jet) printing, and spin coating were used to pattern, deposit, chloridize, print, and coat functional materials, respectively. We have found that e-jet printing was effective for the deposition and patterning of glucose oxidase inks with lateral feature sizes between ~5 to 1000 µm in width, and that the glucose oxidase was still active after printing. The thickness of the permselective layer was optimized to obtain a linear response for glucose concentrations up to 32 mM and no response to acetaminophen, a common interfering compound, was observed. The use of such thin polyimide substrates allow wrapping of the sensors around catheters with high radius of curvature ~250 µm, where additive and microfabrication methods may allow significant cost reductions.

Adsorption of water dimer on platinum(111): identification of the -OH···Pt hydrogen bond.

The fundamental structure of an isolated water dimer on Pt(111) was determined by means of a spectroscopic method using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Two water molecules on adjacent atop sites form a dimer through a hydrogen bond, and they rotate even at a substrate temperature of 5 K. Action spectroscopy using STM (STM-AS) for water dimer hopping allows us to obtain the vibrational spectrum of a single water dimer on Pt(111). Comparisons between the experiments and theory show that one of the OH groups of the acceptor water molecule points toward the surface to form an -OH···Pt hydrogen bond.

X-Ray Photoelectron Spectroscopy Investigation of the Nitrogen Species in Photoactive Perfluorophenylazide-Modified Surfaces.

X-ray Photoelectron Spectroscopy (XPS) was used to characterize the nitrogen species in perfluorophenylazide (PFPA) self-assembled monolayers. PFPA chemistry is a novel immobilization method for tailoring the surface properties of materials. It is a simple route for the efficient immobilization of graphene, proteins, carbohydrates and synthetic polymers onto a variety of surfaces. Upon light irradiation, the azido group in PFPA is converted to a highly reactive singlet nitrene species that readily undergoes CH insertion and C=C addition reactions. Here, the challenge of characterizing the PFPA modified surfaces was addressed by detailed XPS experimental analyses. The three nitrogen peaks detected in the XPS N1s spectra were assigned to amine/amide (400.5 eV) and azide (402.1 and 405.6 eV) species. The observed 2:1 ratio of the areas from the 402.1 eV to 405.6 eV peaks suggests the assignment of the peak at 402.1 eV to the two outer nitrogen atoms in the azido group and assignment of the peak at 405.6 eV to the central nitrogen atom in the azido group. The azide decomposition as the function of x-ray exposure was also determined. Finally, XPS analyses were conducted on patterned graphene to investigate the covalent bond formation between the PFPA and graphene. This study provides strong evidence for the formation of covalent bonds during the PFPA photocoupling process.