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.

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.

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.

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.

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.

Electrochemical Reduction of Carbon Dioxide at TiO2/Au Nanocomposites.

Herein, we report on the facile synthesis of nanocomposite consisting of TiO2 and Au nanoparticles (NPs) via a tailored galvanic replacement reaction (GRR). The electrocatalytic activity of the synthesized TiO2/Au nanocomposites for CO2 reduction was investigated in an aqueous solution using various electrochemical methods. Our results demonstrated that the TiO2/Au nanocomposites formed through the GRR process exhibited improved catalytic activities for CO2 reduction, while generating more hydrocarbon molecules than the typical formation of CO in contrast to polycrystalline Au. GC analysis and NMR spectroscopy revealed that CO and CH4 were the gas products, whereas HCOO-, CH3COO-, CH3OH, and CH3CH2OH were the liquid products from the CO2 reduction at different cathodic potentials. This remarkable change was further studied using the density functional theory (DFT) calculations, showing that the TiO2/Au nanocomposites may increase the binding energy of the formed ·CO intermediate and reduce the free energy compared to Au, thus favoring the downstream generation of multicarbon products. The TiO2/Au nanocomposites have high catalytic activity and excellent stability and are easy to fabricate, indicating that the developed catalyst has potential application in the electrochemical reduction of CO2 to value-added products.

Inductive effects in cobalt-doped nickel hydroxide electronic structure facilitating urea electrooxidation.

Electrochemical oxidation of urea provides an approach to prevent excess urea emissions into the environment while generating value by capturing chemical energy from waste. Unfortunately, the source of high catalytic activity in state-of-the-art doped nickel catalysts for urea oxidation reaction (UOR) activity remains poorly understood, hindering the rational design of new catalyst materials. In particular, the exact role of cobalt as a dopant in Ni(OH)2 to maximize the intrinsic activity towards UOR remains unclear. In this work, we demonstrate how tuning the Ni:Co ratio allows us to control the intrinsic activity and number of active surface sites, both of which contribute towards increasing UOR performance. We show how Ni90Co10(OH)2 achieves the largest geometric current density due to the increase of available surface sites and that intrinsic activity towards UOR is maximized with Ni20Co80(OH)2. Through density functional theory calculations, we show that the introduction of Co alters the Ni 3d electronic state density distribution to lower the minimum energy required to oxidize Ni and influence potential surface adsorbate interactions.

An aligned octahedral core in a nanocage: synthesis, plasmonic, and catalytic properties.

Plasmonic metal nanostructures with complex morphologies provide an important route to tunable optical responses and local electric field enhancement at the nanoscale for a variety of applications including sensing, imaging, and catalysis. Here we report a high-concentration synthesis of gold core-cage nanoparticles with a tethered and structurally aligned octahedral core and examine their plasmonic and catalytic properties. The obtained nanostructures exhibit a double band extinction in the visible-near infrared range and a large area electric field enhancement due to the unique structural features, as demonstrated using finite difference time domain (FDTD) simulations and confirmed experimentally using surface enhanced Raman scattering (SERS) tests. In addition, the obtained structures had a photoelectrochemical response useful for catalyzing the CO2 electroreduction reaction. Our work demonstrates the next generation of complex plasmonic nanostructures attainable via bottom-up synthesis and offers a variety of potential applications ranging from sensing to catalysis.

Hydrogen bonding vs. molecule-surface interactions in 2D self-assembly of [C60]fullerenecarboxylic acids.

The adsorption of C60-malonic derivatives C61(CO2H)2 and C66(CO2H)12 on Au(111) and a pentafluorobenzenethiol-modified Au substrate (PFBT@Au) has been investigated using scanning tunneling microscopy (STM) at a liquid-solid interface. Monofunctionalized C61(CO2H)2 forms a hexagonal close-packed overlayer on Au(111) and individual aligned dimers on PFBT@Au(111). The difference is attributed to the nature of the substrateC61(CO2H)2 interaction (isotropic π-Au bonding vs. anisotropic PFBTCOOH interactions). Surprisingly, in both cases, the directionality of the COOHCOOH motif is compromised in favor of synergistic van der Waals/H bonding interactions. Such van der Waals contacts are geometrically unfeasible in hexafunctionalized C66(CO2H)12 and its assembly on Au(111) leads to a 2D molecular network controlled exclusively by H bonding. For both molecules, the "free" CO2H groups on the monolayer surface can engage in out-of-plane H bonding interaction resulting in the epitaxial growth of subsequent molecular layers.