Understanding the Mechanism of Urea Oxidation from First-Principles Calculations.

Developing electrocatalysts for urea oxidation reaction (UOR) works toward sustainably treating urea-enriched water. Without a clear understanding of how UOR products form, advancing catalyst performance is currently hindered. This work examines the thermodynamics of UOR pathways to produce N2, NO2 -, and NO3 - on a (0001) ß-Ni(OH)2 surface using density functional theory with the computational hydrogen electrode model. Our calculations show support for two major experimental observations: (1) N2 favours an intramolecular mechanism, and (2) NO2 -/NO3 - are formed in a 1 : 1 ratio with OCN-. In addition, we found that selectivity between N2 and NO2 -/NO3 - on our model surface appears to be controlled by two key factors, the atom that binds the surface intermediates to the surface and how they are deprotonated. These UOR pathways were also examined with a Cu dopant, revealing that an experimentally observed increased N2 selectivity may originate from increasing the limiting potential required to form NO2 -. This work builds towards developing a more complete atomic understanding of UOR at the surface of NiOxHy electrocatalysts.

Low-Frequency Oscillations in Optical Measurements of Metal-Nanoparticle Vibrations.

Time-resolved optical measurements of vibrating metal nanoparticles have been used extensively to probe the ultrafast mechanical properties of the nanoparticles and of the surrounding liquid, but nearly all investigations so far have been limited to the linear regime. Here, we report the observation of a low-frequency oscillating signal in transient-absorption measurements of nanoparticles with octahedral gold cores and cubic silver shells; the signal appears at the difference of two mechanical vibrational frequencies in the particles, suggesting a nonlinear mixing process. We tentatively attribute this proposed mixing to a nonlinear coupling between a vibrational mode of the nanoparticle and its optical-frequency plasmon resonance. The optimization of this nonlinear transduction may enable high-efficiency opto-mechanical frequency mixing in the GHz-THz frequency regime.

Simulating electric field and current density in nanostructured electrocatalysts.

The electrocatalytic performance of nanostructured heterogeneous electrocatalysts can be tailored by adjusting their geometries due to the morphologically dependent physicochemical effects, such as field-induced reagent concentration near high-curvature nanoscale features and the confinement of reaction intermediates in a nanocavity. However, the theoretical studies on these physicochemical effects in various nanoscale structures are considerably less common in comparison to the density functional theory calculations on the atomic structure design due to the absence of consistent simulation protocols in this area. This tutorial review presents the theory, models, and protocols for the simulation of the electrochemical properties of nanoelectrocatalysts with complex morphologies using the finite element method (FEM), including the local electric field (E-field) and the current density in the electrolyte adjacent to the electrode (Jelectrolyte) and in the electrode (Jelectrode). In the E-field simulation, we demonstrate the significant screening effect of the EDL on the E-field distribution as well as the influence of the relative permittivity of the electrolyte on the screening strength. In the Jelectrolyte simulation, we illustrate the impact of the electrode kinetics on the electron transfer, which can significantly affect the Jelectrolyte profile. In the Jelectrode simulation, we reveal that the Jelectrode crowding can occur in constricted areas of nanostructures, which would cause the structural transformation via electromigration. Finally, we discuss the applicability and limitations of the theoretical models discussed in this tutorial, suggesting the focus of future work to develop advanced multiscale modelling approaches. We hope this tutorial will assist electrochemists in navigating how to conduct accurate electrochemical physics effect simulations for analyzing the catalytic performance of nanoelectrocatalysts and thereby contribute to a wider adoption of FEM simulations in the rational design of advanced electrocatalysts.

Reaction-intermediate-induced atomic mobility in heterogeneous metal catalysts for electrochemical reduction of CO2.

Improving the activity and selectivity of heterogeneous metal electrocatalysts has been the primary focus of CO2 electroreduction studies, however, the stability of these materials crucial for practical application remains less understood. In our work, the impact of the reaction intermediates (RIs) on the energetics and mechanism of metal-atom migration is studied with a combination of density functional theory (DFT) and ab initio molecular dynamics (AIMD) on pure transition metals Cu, Ag, Au, Pd, as well as three Cu4-xPdx (x = 1,2, and 3) alloys. Reaction intermediates (RIs) for the CO2 reduction reaction, H2 evolution, and O2 reduction were considered. The effect of adsorbed RIs was observed to facilitate metal atom migration generally by decreasing the kinetic barriers for migration. The atomic mobility trends in the commonly used CO2RR metal electrocatalysts in the course of electrolysis conditions were established. This study provides theoretical insight into understanding how the electrocatalyst may undergo promoted restructuring in the presence of RIs under realistic electrochemical conditions.

Surface patterning of nanoparticles with polymer patches.

Patterning of colloidal particles with chemically or topographically distinct surface domains (patches) has attracted intense research interest. Surface-patterned particles act as colloidal analogues of atoms and molecules, serve as model systems in studies of phase transitions in liquid systems, behave as 'colloidal surfactants' and function as templates for the synthesis of hybrid particles. The generation of micrometre- and submicrometre-sized patchy colloids is now efficient, but surface patterning of inorganic colloidal nanoparticles with dimensions of the order of tens of nanometres is uncommon. Such nanoparticles exhibit size- and shape-dependent optical, electronic and magnetic properties, and their assemblies show new collective properties. At present, nanoparticle patterning is limited to the generation of two-patch nanoparticles, and nanoparticles with surface ripples or a 'raspberry' surface morphology. Here we demonstrate nanoparticle surface patterning, which utilizes thermodynamically driven segregation of polymer ligands from a uniform polymer brush into surface-pinned micelles following a change in solvent quality. Patch formation is reversible but can be permanently preserved using a photocrosslinking step. The methodology offers the ability to control the dimensions of patches, their spatial distribution and the number of patches per nanoparticle, in agreement with a theoretical model. The versatility of the strategy is demonstrated by patterning nanoparticles with different dimensions, shapes and compositions, tethered with various types of polymers and subjected to different external stimuli. These patchy nanocolloids have potential applications in fundamental research, the self-assembly of nanomaterials, diagnostics, sensing and colloidal stabilization.

Enhanced electrocatalytic CO2 reduction via field-induced reagent concentration.

Electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO) is the first step in the synthesis of more complex carbon-based fuels and feedstocks using renewable electricity. Unfortunately, the reaction suffers from slow kinetics owing to the low local concentration of CO2 surrounding typical CO2 reduction reaction catalysts. Alkali metal cations are known to overcome this limitation through non-covalent interactions with adsorbed reagent species, but the effect is restricted by the solubility of relevant salts. Large applied electrode potentials can also enhance CO2 adsorption, but this comes at the cost of increased hydrogen (H2) evolution. Here we report that nanostructured electrodes produce, at low applied overpotentials, local high electric fields that concentrate electrolyte cations, which in turn leads to a high local concentration of CO2 close to the active CO2 reduction reaction surface. Simulations reveal tenfold higher electric fields associated with metallic nanometre-sized tips compared to quasi-planar electrode regions, and measurements using gold nanoneedles confirm a field-induced reagent concentration that enables the CO2 reduction reaction to proceed with a geometric current density for CO of 22 milliamperes per square centimetre at -0.35 volts (overpotential of 0.24 volts). This performance surpasses by an order of magnitude the performance of the best gold nanorods, nanoparticles and oxide-derived noble metal catalysts. Similarly designed palladium nanoneedle electrocatalysts produce formate with a Faradaic efficiency of more than 90 per cent and an unprecedented geometric current density for formate of 10 milliamperes per square centimetre at -0.2 volts, demonstrating the wider applicability of the field-induced reagent concentration concept.

Nickel-Catalyzed Urea Electrolysis: From Nitrite and Cyanate as Major Products to Nitrogen Evolution.

The electrochemical urea oxidation reaction (UOR) to N2 represents an efficient route to simultaneous nitrogen removal from N-enriched waste and production of renewable fuels at the cathode. However, the overoxidation of urea to NOx - usually dominates over its oxidation to N2 at Ni(OH)2 -based anodes. Furthermore, detailed reaction mechanisms of UOR remain unclear, hindering the rational catalyst design. We found that UOR to NOx - on Ni(OH)2 is accompanied by the formation of near stoichiometric amount of cyanate (NCO- ), which enabled the elucidation of UOR mechanisms. Based on our experimental and computational findings, we show that the formation of NOx - and N2 follows two distinct vacancy-dependent pathways. We also demonstrate that the reaction selectivity can be steered towards N2 formation by altering the composition of the catalyst, e.g., doping the catalyst with copper (Ni0.8 Cu0.2 (OH)2 ) increases the faradaic efficiency of N2 from 30 % to 55 %.

Interparticle gap geometry effects on chiroptical properties of plasmonic nanoparticle assemblies.

Chiral linear assemblies of plasmonic nanoparticles with chiral optical activity often show low asymmetry factors. Systematic understanding of the structure-property relationship in these systems must be improved to facilitate rational design of their chiroptical response. Here we study the effect of large area interparticle gaps in chiral linear nanoparticle assemblies on their chiroptical properties using a tetrahelix structure formed by a linear face-to-face assembly of nanoscale Au tetrahedra. Using finite-difference time-domain and finite element methods, we performed in-depth evaluation of the extinction spectra and electric field distribution in the tetrahelix structure and its dependence on various geometric parameters. The reported structure supports various plasmonic modes, one of which shows a strong incident light handedness selectivity that is associated with large face-to-face junctions. This works highlights the importance of gap engineering in chiral plasmonic assemblies to achieveg-factors greater than 1 and produce structures with a handedness-selective optical response.

Cu(ii)-nanoparticle-derived structures under CO2 reduction conditions: a matter of shape.

Uncovering the nature and formation mechanisms of active sites in electrocatalysts is crucial for advancing energy conversion technologies. Cu(ii)-derived electrodes show unique activity in CO2 electroreduction, but its origins are not fully understood. We investigate the structural evolution of Cu(OH)2 nanoparticle-derived electrodes and its effect on their performance in CO2 electroreduction.

Self-Assembly and Surface Patterning of Polyferrocenylsilane-Functionalized Gold Nanoparticles.

Chemical and topographic surface patterning of inorganic polymer-functionalized nanoparticles (NPs) and their self-assembly in nanostructures with controllable architectures enable the design of new NP-based materials. Capping of NPs with inorganic polymer ligands, such as metallopolymers, can lead to new synergetic properties of individual NPs or their assemblies, and enhance NP processing in functional materials. Here, for gold NPs functionalized with polyferrocenylsilane, two distinct triggers are used to induce attraction between the polymer ligands and achieve NP self-assembly or topographic surface patterning of individual polymer-capped NPs. Control of polymer-solvent interactions is achieved by either changing the solvent composition or by the electrooxidation of polyferrocenylsilane ligands. These results expand the range of polymer ligands used for NP assembly and patterning, and can be used to explore new self-assembly modalities. The utilization of electrochemical polymer oxidation stimuli at easily accessible potentials broadens the range of stimuli leading to NP self-assembly and patterning.

Ethanol-dependent expression of the NKG2D ligands MICA/B in human cell lines and leukocytes.

Alcohol consumption affects the human immune system, causing a variety of disorders. However, the mechanisms of development of these changes are not fully understood. We hypothesized that ethanol may influence the expression of MICA and MICB, stress-induced molecules capable of regulating the activity of cytotoxic lymphocytes through the interaction with receptor NKG2D, which substantially affects the functionality of cellular immunity. We analyzed the effects of ethanol on MICA/B expression in tumor cell lines and human leukocytes. In the cell line models, ethanol caused different changes in the surface expression of MICA/B; in particular, it induced the translocation of intracellular proteins MICA/B to the cell surface and shedding of MICA (in soluble and microparticle-associated forms) from the plasma membrane. The observed results are not linked with cell death in cultures, taking place only under higher doses of ethanol. Ethanol at physiologically relevant concentrations (and higher) stimulated expression of MICA/B genes in different cell types. The effect of ethanol was more pronounced in hepatocyte line HepG2 compared with hematopoietic cell lines K562, Jurkat, and THP-1. Among the tested leukocytes, the most sensitive to ethanol action were T cells activated ex vivo with IL-2, in which the increase of MICA/B mRNA expression was registered with the smallest dose of ethanol (0.125%). In human monocytes, ethanol may lead to elevations in surface MICA/B levels. Presumably, changes in MICA/B expression caused by ethanol can affect the functions of NKG2D-positive cytotoxic lymphocytes, modulating immune reactions at excessive alcohol consumption.

Shape-Dependent Interactions of Palladium Nanocrystals with Hydrogen.

Elucidation of the nature of hydrogen interactions with palladium nanoparticles is expected to play an important role in the development of new catalysts and hydrogen-storage nanomaterials. A facile scaled-up synthesis of uniformly sized single-crystalline palladium nanoparticles with various shapes, including regular nanocubes, nanocubes with protruded edges, rhombic dodecahedra, and branched nanoparticles, all stabilized with a mesoporous silica shell is developed. Interaction of hydrogen with these nanoparticles is studied by using temperature-programmed desorption technique and by performing density functional theory modeling. It is found that due to favorable arrangement of Pd atoms on their surface, rhombic dodecahedral palladium nanoparticles enclosed by {110} planes release a larger volume of hydrogen and have a lower desorption energy than palladium nanocubes and branched nanoparticles. These results underline the important role of {110} surfaces in palladium nanoparticles in their interaction with hydrogen. This work provides insight into the mechanism of catalysis of hydrogenation/dehydrogenation reactions by palladium nanoparticles with different shapes.

Linear assembly of patchy and non-patchy nanoparticles.

Linear assemblies of nanoparticles show promising applications due to their collective electronic, optical and magnetic properties. Rational design and controllable organization of nanoparticles in one-dimensional structures can strongly benefit from the marked similarity between conventional step-growth polymerization reactions and directional step-wise assembly of nanoparticles in linear chains. Here we show different aspects of the "polymerization" approach to the solution-based self-assembly of polymer-functionalized metal nanoparticles with different chemical compositions, shapes and dimensions. The self-assembly was triggered by inducing solvophobic attraction between polymer ligands, due to the change in solvent quality. We show that both anisotropic (patchy) nanoparticles and nanoparticles uniformly capped with polymer molecules can self-assemble in linear chains. We explore the control of chain length, morphology, and composition, discuss the ability to form isotropic and hierarchical structures and show the properties and potential applications of linear assemblies of plasmonic nanoparticles.

Colloidal analogs of molecular chain stoppers.

A similarity between chemical reactions and self-assembly of nanoparticles offers a strategy that can enrich both the synthetic chemistry and the nanoscience fields. Synthetic methods should enable quantitative control of the structural characteristics of nanoparticle ensembles such as their aggregation number or directionality, whereas the capability to visualize and analyze emerging nanostructures using characterization tools can provide insight into intelligent molecular design and mechanisms of chemical reactions. We explored this twofold concept for an exemplary system including the polymerization of bifunctional nanoparticles in the presence of monofunctional colloidal chain stoppers. Using reaction-specific design rules, we synthesized chain stoppers with controlled reactivity and achieved quantitative fine-tuning of the self-assembled structures. Analysis of the nanostructures provided information about polymerization kinetics, side reactions, and the distribution of all of the species in the reaction system. A quantitative model was developed to account for the reactivity, kinetics, and side reactions of nanoparticles, all governed by the design of colloidal chain stoppers. This work provided the ability to test theoretical models developed for molecular polymerization.

Self-assembled plasmonic nanostructures.

Self-assembly of plasmonic nanoparticles offers a labour- and cost-efficient strategy for the expansion of the library of plasmonic nanostructures with highly tunable, coupled optical properties. This review covers recent advances in solution-based self-assembly of plasmonic nanoparticles, modelling of the self-assembly process and of the optical properties of the resulting nanostructures, and potential applications of self-assembled plasmonic nanostructures.

Structural and optical properties of self-assembled chains of plasmonic nanocubes.

Solution-based linear self-assembly of metal nanoparticles offers a powerful strategy for creating plasmonic polymers, which, so far, have been formed from spherical nanoparticles and cylindrical nanorods. Here we report linear solution-based self-assembly of metal nanocubes (NCs), examine the structural characteristics of the NC chains, and demonstrate their advanced optical characteristics. In comparison with chains of nanospheres with similar dimensions, composition, and surface chemistry, predominant face-to-face assembly of large NCs coated with short polymer ligands led to a larger volume of hot spots in the chains, a nearly uniform E-field enhancement in the gaps between colinear NCs, and a new coupling mode for NC chains due to the formation of a Fabry-Perot resonator structure formed by face-to-face bonded NCs. The NC chains exhibited stronger surface-enhanced Raman scattering in comparison with linear assemblies of nanospheres. The experimental results were in agreement with finite difference time domain simulations.

Mechanistic Analysis of Urea Electrooxidation Pathways: Key to Rational Catalyst Design.

Urea electrolysis is an emerging approach to treating urea-enriched wastewater and an attractive alternative anodic process to the oxygen evolution reaction (OER) in electrochemical clean energy conversion and storage technologies (e. g., hydrogen production and CO2 electroreduction). While the thermodynamic potential for urea oxidation to dinitrogen is quite low compared to that of the OER, the catalysts reported to date require high overpotentials that far exceed those for the OER. Consequently, there is much room for improvement and rational catalyst design for the urea oxidation reaction (UOR). At the same time, due to the urea molecule having a more complex structure than water, UOR can lead to the formation of various products beyond the commonly assumed N2 and CO2. This concept article will critically assess recent efforts of the research community to decipher the formation mechanisms of UOR products focusing on the systematic analysis of the reaction selectivity. This work aims to analyze the current state of the art and identify existing gaps, providing an outlook for the future design of UOR catalysts with superior activity and selectivity by applying the knowledge of the molecular transformation mechanisms.

Structural transitions in nanoparticle assemblies governed by competing nanoscale forces.

Assembly of nanoscale materials from nanoparticle (NP) building blocks relies on our understanding of multiple nanoscale forces acting between NPs. These forces may compete with each other and yield distinct stimuli-responsive self-assembled nanostructures. Here, we report structural transitions between linear chains and globular assemblies of charged, polymer-stabilized gold NPs, which are governed by the competition of repulsive electrostatic forces and attractive poor solvency/hydrophobic forces. We propose a simple quantitative model and show that these transitions can be controlled by the quality of solvent, addition of a salt, and variation of the molecular weight of the polymer ligands.

The effect of tensile strain in Pd-Ni core-shell nanocubes with tuneable shell thickness on urea electrolysis selectivity.

Expanding our understanding of the structure-performance relationship in nanoscale electrocatalysts for urea electrolysis is crucial for efficient urea waste treatment and concomitant cathodic hydrogen production or CO2 reduction. Here, we elucidate the effect of the lattice strain in Pd-Ni core-shell nanocubes on the dominance of urea overoxidation pathway.

The role of hole localization in sacrificial hydrogen production by semiconductor-metal heterostructured nanocrystals.

The effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals is investigated. By tuning the energy balance at the semiconductor-ligand interface, we demonstrate that hydrogen production on Pt sites is efficient only when electron-donating molecules are used for stabilizing semiconductor surfaces. These surfactants play an important role in enabling an efficient and stable reduction of water by heterostructured nanocrystals as they fill vacancies in the valence band of the semiconductor domain, preventing its degradation. In particular, we show that the energy of oxidizing holes can be efficiently transferred to a ligand moiety, leaving the semiconductor domain intact. This allows reusing the inorganic portion of the "degraded" nanocrystal-ligand system simply by recharging these nanoparticles with fresh ligands.