Real-Time Imaging of On-Surface Ullmann Polymerization Reveals an Inhibiting Effect of Adatoms

Ullmann coupling is a widely used reaction for the on-surface growth of low-dimensional carbon nanomaterials. The irreversible nature of this reaction prevents the "self-healing" of defects, and a detailed knowledge of its mechanism is therefore essential to enable the growth of extended ordered structures. However, the dynamics of the Ullmann polymerization remain largely unexplored, as coupling events occur on a timescale faster than conventional scanning probe microscopy imaging frequencies. Here, we reveal the dynamics of these surface events using high-speed variable-temperature scanning tunneling microscopy (STM) (10 frames per second). Performing the measurements at the onset reaction temperatures provides an unprecedented description of the evolution of organometallic (OM) and covalent surface species during the Ullmann polymerization of para-dibromobenzene on Cu(110). Our results demonstrate the existence of an intermediate OM phase with Cu adatoms that inhibits the polymerization. These observations now complete the picture of the pathways of on-surface Ullmann polymerization, which includes the complex interplay of the phenylene moieties and metal atoms. Our work demonstrates the unique capability of high-speed STM to capture the dynamics of molecular self-assembly and coupling.

Direct Atomic-Scale Insight into the Precipitation Formation at the Lanthanum Hydroxide Nanoparticle/Solution Interface

Understanding precipitation formation at lanthanum hydroxide (La(OH)₃) nanoparticle-solution interfaces plays a crucial role in catalysis, adsorption, and electrochemical energy storage applications. Liquid-phase transmission electron microscopy enables powerful visualization with high resolution. However, direct atomic-scale imaging of the interfacial metal (hydro)oxide nanostructure in solutions has been a major challenge due to their beam-driven dissolution. Combining focused ion beam and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we present an atomic-scale study of precipitation formation at La(OH)₃ nanoparticle interfaces after reaction with phosphate. The structure transformation is observed to occur at high- and low-crystalline La(OH)₃ nanoparticle surfaces. Low-crystalline La(OH)₃ mostly transformed and high-crystalline ones partly converted to LaPO₄ precipitations on the outer surface. The long-term structure evolution shows the low transformation of high-crystalline La(OH)₃ nanoparticles to LaPO₄ precipitation. Because precipitation at solid-solution interfaces is common in nature and industry, these results could provide valuable references for their atomic-scale observation.

Application of Scanning Tunneling Microscopy in Electrocatalysis and Electrochemistry

Abstract

Scanning tunneling microscopy (STM) has gained increasing attention in the field of electrocatalysis due to its ability to reveal electrocatalyst surface structures down to the atomic level in either ultra-high-vacuum (UHV) or harsh electrochemical conditions. The detailed knowledge of surface structures, surface electronic structures, surface active sites as well as the interaction between surface adsorbates and electrocatalysts is highly beneficial in the study of electrocatalytic mechanisms and for the rational design of electrocatalysts. Based on this, this review will discuss the application of STM in the characterization of electrocatalyst surfaces and the investigation of electrochemical interfaces between electrocatalyst surfaces and reactants. Based on different operating conditions, UHV-STM and STM in electrochemical environments (EC-STM) are discussed separately. This review will also present emerging techniques including high-speed EC-STM, scanning noise microscopy and tip-enhanced Raman spectroscopy.

Graphic Abstract

Potential-Dependent Morphology of Copper Catalysts During CO2 Electroreduction Revealed by In Situ Atomic Force Microscopy

Electrochemical AFM is a powerful tool for the real-space characterization of catalysts under realistic electrochemical CO2 reduction (CO2 RR) conditions. The evolution of structural features ranging from the micrometer to the atomic scale could be resolved during CO2 RR. Using Cu(100) as model surface, distinct nanoscale surface morphologies and their potential-dependent transformations from granular to smoothly curved mound-pit surfaces or structures with rectangular terraces are revealed during CO2 RR in 0.1 m KHCO3 . The density of undercoordinated copper sites during CO2 RR is shown to increase with decreasing potential. In situ atomic-scale imaging reveals specific adsorption occurring at distinct cathodic potentials impacting the observed catalyst structure. These results show the complex interrelation of the morphology, structure, defect density, applied potential, and electrolyte in copper CO2 RR catalysts.

Modeling of adsorption flux in nickel-contaminated synthetic simulated wastewater in the batch reactor

In the present investigation, physico-chemical characterization of composite material revealed the presence of fluffy surface structure with crystalline look and negatively charged surface functional groups. The study of adsorption flux by using dimensionless numbers φ (2.62), N_k (62.68) and λ (1.17 × 10^-5) proved that adsorption of nickel ions on the surface of composite material was mostly film diffusion-limited with maximum surface area coverage coupled with weakened surface tension. The results of intraparticle diffusivity and Boyd plot model showed that at the onset of process, film diffusion was the primary mechanism involved and at the later stage intraparticle diffusion played a critical role as rate governing step. The values of film (0.65 × 10^-8 cm² sec^-1) and pore diffusivity (1.8 × 10^-12 cm² sec^-1) coefficients showed that the adsorption process is dependent upon two different types of diffusion namely film and pore diffusion. Overall, transport and reshuffling mechanism had no substantial role in adsorption dynamics of nickel ions on the surface of composite material. Sorption isotherm and kinetics modeling showed higher values of regression coefficients for Langmuir isotherm (R² = 0.99) and pseudo-second-order kinetic model (R² = 0.99) compared to other models. This showed that sorption of nickel followed monolayer coverage with chemisorption at optimized process parameters like pH 6, biosorbent dose 0.1 g/L, temperature 50 °C, agitation rate180 rpm, adsorbate concentration100 mg/L and contact time 60 minutes. The positive value of enthalpy of adsorption (ΔH = + 10.41 kJ/mole) and entropy (ΔS = +58.19 J/mol K) showed that binding of nickel ions on the surface of the composite material was endothermic with improved randomness at solid-liquid interface. The negative value of (ΔG = -6.4 to -8.67 kJ/mol) showed spontaneous nature of nickel adsorption on composite material in the liquid phase.

Conjugated bilayer structure of the homogeneous solid–liquid interface of metals

The knowledge about the homogeneous solid–liquid interface is promoted in this work from traditional concept of “interface region” to a new “conjugated bilayer structure” that reveals the intrinsic structure of the interface.

Selective CO2 Electroreduction to Ethylene and Multicarbon Alcohols via Electrolyte-Driven Nanostructuring

Production of multicarbon products (C₂₊) from CO₂ electroreduction reaction (CO₂RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO₂RR catalysts that are highly selective for C₂₊ products via electrolyte‐driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO₂ into ethylene and multicarbon alcohols in aqueous 0.1 m KHCO₃ solution, with the iodine‐modified catalyst displaying the highest Faradaic efficiency of 80 % and a partial geometric current density of ca. 31.2 mA cm⁻² for C₂₊ products at −0.9 V vs. RHE. Operando X‐ray absorption spectroscopy and quasi in situ X‐ray photoelectron spectroscopy measurements revealed that the high C₂₊ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis, presence of subsurface oxygen and Cu⁺ species, and the adsorbed halides.

Density fluctuations as door-opener for diffusion on crowded surfaces

How particles can move on a catalyst surface that, under the conditions of an industrial process, is highly covered by adsorbates and where most adsorption sites are occupied has remained an open question. We have studied the diffusion of O atoms on a fully CO-covered Ru(0001) surface by means of high-speed/variable-temperature scanning tunneling microscopy combined with density functional theory calculations. Atomically resolved trajectories show a surprisingly fast diffusion of the O atoms, almost as fast as on the clean surface. This finding can be explained by a "door-opening" mechanism in which local density fluctuations in the CO layer intermittently create diffusion pathways on which the O atoms can move with low activation energy.

In Situ Analysis of Surface Catalytic Reactions Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

Electrochemistry and heterogeneous catalysis continue to attract enormous interest. In situ surface analysis is a dynamic research field capable of elucidating the catalytic mechanisms of reaction processes. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is a nondestructive technique that has been cumulatively used to probe and analyze catalytic-reaction processes, providing important spectral evidence about reaction intermediates produced on catalyst surfaces. In this perspective, we review recent electrochemical- and heterogeneous-catalysis studies using SHINERS, highlight its advantages, summarize the flaws and prospects for improving the SHINERS technique, and give insight into its future research directions.