Weakly bound capping agents on gold nanoparticles in catalysis: Surface poison?

Controlled synthesis of silver/silicon hybrid nanostructures enables enhanced photocatalytic CO2 reduction

Localized surface plasmon resonances in plasmonic nanoparticles enable effective photon harvesting and generate energetic electrons and holes. However, fast charge recombination and surface contamination due to reactant-involved synthesis can significantly hinder the activity. Herein, we report a facile, solution-processed synthesis of hybrid uncoated silver nanoparticles combined with silicon nanocrystals to address these issues. The resulting hybrid nanocolloid exhibits enhanced photocatalytic CO2 reduction, achieving 1552 μmol g-1 and 100% selectivity for CO production.

The Effect of Capping Agents on Gold Nanostar Stability, Functionalization, and Colorimetric Biosensing Capability

Capping agents (organic ligands, polymers, and surfactants) are pivotal for stabilizing nanoparticles; however, they may influence the surface chemistry, as well as the physico-chemical and biological characteristics, of gold nanostar (AuNS)-based biosensors. In this study, we proved that various capping agents affected capped and bioconjugated AuNS stability, functionality, biocatalysis, and colorimetric readouts. Capped and bioconjugated AuNSs were applied as localized surface plasmon resonance (LSPR)-based H2O2 sensors using glucose oxidase (GOx) as a model enzyme. Furthermore, our analyses revealed that the choice of capping agent influenced the properties of the AuNSs, their stability, and their downstream applications. Our analyses provide new insights into factors governing the choice of capping agents for gold nanostars and their influences on downstream applications with conjugated enzymes in confined environments.

Ionic Liquid Aggregation Mechanism for Nanoparticle Synthesis

Nanoparticle synthesis with silylamine reversible ionic liquids (RevILs) has been previously demonstrated to offer unique alternatives to traditional nanoparticle syntheses, allowing for size control and facile deposition onto support surfaces via the switchable nature of the IL. However, the mechanism of nanoparticle synthesis remains uncharacterized. The use of RevILs facilitates the synthesis of size-controlled nanoparticles without the use of additional stabilizing agents (i.e., surfactants, ligands, and polymers) that passivate the nanoparticle surface, which are traditionally required to control the nanoparticle size. Traditional techniques often require harsh activation steps that ultimately impact nanoparticle size and morphology. While RevIL syntheses offer an excellent alternative, as they do not require additional activation steps, the mechanism through which nanoparticles are synthesized in these systems has not been studied previously. Preceding work hypothesized nanoparticles prepared with RevILs are formed via a reverse micelle mechanism, in which nanoparticles are stabilized and templated within the aqueous core of the organized micelle structures. In this work, DOSY-NMR is used to demonstrate that nanoparticles synthesized with 3-aminopropyltriethylsilane RevIL are not formed through a reverse micelle mechanism but rather a switchable aggregation mechanism that affords control over the nanoparticle size via manipulation of the RevIL structure and concentration. Furthermore, it is shown that the addition of water to RevIL systems has detrimental effects on the aggregation behavior of the ionic liquid molecules in solution, causing disassembly of the ion pairs. However, because nanoparticle reduction likely occurs faster than the disassembly of the ion pairs, nanoparticle size is unaffected by the addition of water during nanoparticle reduction.

Mechanism of visible light enhanced catalysis over curcumin functionalized Ag nanocatalysts

There is little research on the visible light photocatalytic properties of the hybrids of plasmonic metals and organic molecules (OM) with the HOMO-LUMO gap in the visible range. Here, we investigate the mechanism of the visible light enhanced reduction of p-nitrophenol (PNP) by glycerol (a green reductant) at ambient temperature over curcumin functionalized Ag nanoparticles (c-AgNPs). The catalytic activity got significantly boosted under visible light irradiation. Reaction kinetics indicated that the catalytic mechanism followed under visible light and in the dark were different. DFT calculations showed that in the ground state, the HOMO resides on Ag while the LUMO is on the curcumin part of the composite. TD-DFT calculations demonstrated the transfer of charge from Ag to curcumin on photo-excitation. Based on this information, we propose a mechanism for understanding the role of curcumin in this photocatalytic phenomenon.

Template-Free and Spontaneous Formation of Vertically Aligned Pd Nanofiber Arrays at the Liquid-Liquid Interface between Redox-Active Ionic Liquid and Water

Vertically aligned Pd nanofiber arrays (NFAs) have been prepared at the liquid-liquid interface between redox-active ionic liquid (RAIL) and water (W) via a template-free manner. The RAIL with high hydrophobicity, (ferrocenylmethyl)dodecyldimethylammonium bis(nonafluorobutanesulfonyl)amide, plays dual roles of reducing agent for Pd precursor ions and the hydrophobic liquid phase simultaneously, and the RAIL|W interface has been utilized as the formation site for the spontaneous growth of Pd NFAs. The Pd NFAs consist of three parts: layers formed by partly connected particles on the top, NFAs in the middle, and firm sheetlike layers on the bottom. Because of the top and bottom supporting layers, the antideformation ability and durability of the Pd NFAs with a length reaching several micrometers are enhanced. A possible mechanism for the formation of the Pd NFAs has been discussed. The Pd NFAs show a good stability and a higher electrocatalytic activity toward the ethanol oxidation reaction than a commercial Pd/C catalyst. The present study provides a new strategy for the template-free and spontaneous formation of Pd NFAs.

Colloidal Au Catalyst Preparation: Selective Removal of Polyvinylpyrrolidone from Active Au Sites

Colloids with controlled dimensions, morphology and composition can be used to obtain supported metal catalysts with desired characteristics. Yet, removal of capping agents which block active metal sites in such catalysts can be challenging: mild methods often result in incomplete removal of capping agents, whereas harsher methods can cause change in particle size or morphology or cause metal segregation for bimetallic particles. Here we provide evidence that polyvinylpyrrolidone used as a stabilizing agent for gold colloids is present both on the metal and the support surface after colloid deposition on the TiO₂ support. The polymer adsorbed on Au sites blocks their catalytic activity if it cannot be desorbed/decomposed under reaction conditions. Polyvinylpyrrolidone can be removed completely from the active gold surface of Au/TiO₂ without particle growth using a number of mild treatment methods described in this work, despite only partial removal (≈45 %) of the stabilizer from the bulk of Au/TiO₂ can be achieved. The remaining >50 % of polyvinylpyrrolidone resides exclusively on the TiO₂ support and has no effect on the optical properties and catalytic activity of gold nanoparticles. The treated catalysts demonstrate catalytic activity and selectivity similar to those of a catalyst prepared by impregnation. These findings are important for further advancing the preparation of well‐defined supported catalysts using metal colloids.

The role and fate of capping ligands in colloidally prepared metal nanoparticle catalysts

In this Perspective article, we highlight emerging opportunities for the rational design of catalysts upon the choice, exchange, partial removal or pyrolysis of ligands.

Shape-controlled Pt nanocubes directly grown on carbon supports and their electrocatalytic activity toward methanol oxidation

Synthesis of shape-controlled Pt nanocrystals is substantial and important for enhancing chemical and electrochemical reactions. However, the removal of capping agents, shape-controlling chemicals, on Pt surfaces is essential prior to conducting the catalytic reactions. Here we report a facile one-pot synthesis of Pt nanocubes directly grown on carbon supports (Pt nanocubes/C) with modulating the kinetic reaction factors for shaping the nanocrystals, but without adding any capping agents for preserving the clean Pt surfaces. Well-dispersed Pt nanocubes/C shows enhanced activity and long-term stability toward methanol oxidation reaction compared to the commercial Pt/C catalyst.

A Generalized Method for the Synthesis of Ligand-Free M@SiO2 (M = Ag, Au, Pd, Pt) Yolk-Shell Nanoparticles

A universal method is reported for the synthesis of ligand free noble metal M@SiO₂ (M = Ag, Au, Pd, Pt) yolk-shell nanoparticles (YSNs). Mesoporous hollow silica shells (mHSS) are used as smart nanoreactors for the synthesis of noble metal yolk-shell nanoparticles. The nanocavity of a mHSS and anionic metal ions play a critical role in the formation of yolk-shell nanoparticles. The synthesis mechanism can be tuned by simply varying the pH of the noble metal precursor aqueous solution. A critical pH ≥ 4 is required for the formation of YSNs. The anionic metal ions can pass freely through the mesopores of mHSS and eventually lead to the formation of YSNs, whereas the cationic metal ions can show strong interaction with the surface of mHSS which hinders the formation of YSNs. The syntheses of YSNs are achieved without using any external capping ligands and reducing agents in the reaction.

Evolution of the PVP-Pd Surface Interaction in Nanoparticles through the Case Study of Formic Acid Decomposition

Palladium nanoparticles (Pd NPs) were synthesized by the reduction-by-solvent method using polyvinylpirrolidone (PVP) as capping agent. The nonstatic interaction between PVP and the metallic surface may change the properties of the NPs due to the different possible interactions, through either the O or N atoms of the PVP. In order to analyze these effects and their repercussions in their catalytic performance, Pd NPs with various PVP/Pd molar ratios (1, 10, and 20) were prepared, deposited on silica, and tested in the formic acid decomposition reaction. The catalytic tests were conducted using catalysts prepared by loading NPs with three different time lapses between their purification and their deposition on the silica support (1 day, 1 month, and 6 months). CO adsorption, FTIR spectroscopy, XPS, and TEM characterization were used to determine the accessibility of the Pd NPs surface sites, the electronic state of Pd, and the average NPs size, respectively. The H₂ production from the formic acid decomposition reaction has a strong dependence on the Pd surface features, which in turn are related to the NPs aging time due to the progressive removal of the PVP.

Peptide Binding for Bio-Based Nanomaterials

Peptide-based strategies represent transformative approaches to fabricate functional inorganic materials under sustainable conditions by modeling the methods exploited in biology. In general, peptides with inorganic affinity and specificity have been isolated from organisms and through biocombinatorial selection techniques (ie, phage and cell surface display). These peptides recognize and bind the inorganic surface through a series of noncovalent interactions, driven by both enthalpic and entropic contributions, wherein the biomolecules wrap the metallic nanoparticle structure. Through these interactions, modification of the inorganic surface can be accessed to drive the incorporation of significantly disordered surface metal atoms, which have been found to be highly catalytically active for a variety of chemical transformations. We have employed synthetic, site-directed mutagenesis studies to reveal localized binding effects of the peptide at the metallic nanoparticle structure to begin to identify the biological basis of control over biomimetic nanoparticle catalytic activity. The protocols described herein were used to fabricate and characterize peptide-capped nanoparticles in atomic resolution to identify peptide sequence effects on the surface structure of the materials, which can then be directly correlated to the catalytic activity to identify structure/function relationships.

Gold nanoparticles supported on mesoporous silica: origin of high activity and role of Au NPs in selective oxidation of cyclohexane

Homogeneous immobilization of gold nanoparticles (Au NPs) on mesoporous silica has been achieved by using a one-pot synthesis method in the presence of organosilane mercapto-propyl-trimethoxysilane (MPTMS). The resultant Au NPs exhibited an excellent catalytic activity in the solvent-free selective oxidation of cyclohexane using molecular oxygen. By establishing the structure-performance relationship, the origin of the high activity of mesoporous supported Au catalyst was identified to be due to the presence of low-coordinated Au (0) sites with high dispersion. Au NPs were confirmed to play a critical role in the catalytic oxidation of cyclohexane by promoting the activation of O₂ molecules and accelerating the formation of surface-active oxygen species.

Controlled synthesis of PVP-based carbon-supported Ru nanoparticles: synthesis approaches, characterization, capping agent removal and catalytic behavior

PVP-capped Ru nanoparticles were synthesized, immobilized on several carbon supports and tested in galactose hydrogenation.

Improved synthesis and hydrothermal stability of Pt/C catalysts based on size-controlled nanoparticles

A novel method for the preparation of stable Pt/C catalysts with size-controlled nanoparticles has been developed.

Electrostatic assembly of gold nanorods on a glass substrate for sustainable photocatalytic reduction via sodium borohydride

Gold nanorods were adhered onto a glass substrate for use as a sustainable, reusable photocatalyst to reduce 4-nitroaniline with sodium borohydride.

Electrosynthesis of diphenyl carbonate by homogeneous Pd electrocatalysts using Au nanoparticles on graphene as efficient anodes

Electrochemical carbonylation of phenol (PhOH) with CO (1 atm) to form diphenyl carbonate (DPC) was studied by using a Pd-(in situ NHC) electrocatalyst and graphene-supported Au nanoparticle (Au NPs/GR) anodes.

Recyclable glucose-derived palladium(0) nanoparticles as in situ-formed catalysts for cross-coupling reactions in aqueous media

Renewable sugar-derived palladium(0) nanoparticles (PdNPs) are effective as in situ formed catalysts for cross-coupling reactions in aqueous solutions.

Differential role of PVP on the synthesis of plasmonic gold nanostructures and their catalytic and SERS properties

Differential role of PVP modified with halide ions has been meticulously studied for in situ tuning of Au nanoparticle growth utilizing XRD measurements together with FTIR data, thus quantifying their catalysis and SERS applications.

Two-dimensional gold nanostructures with high activity for selective oxidation of carbon-hydrogen bonds

Efficient synthesis of stable two-dimensional (2D) noble metal catalysts is a challenging topic. Here we report the facile synthesis of 2D gold nanosheets via a wet chemistry method, by using layered double hydroxide as the template. Detailed characterization with electron microscopy and X-ray photoelectron spectroscopy demonstrates that the nanosheets are negatively charged and [001] oriented with thicknesses varying from single to a few atomic layers. X-ray absorption spectroscopy reveals unusually low gold-gold coordination numbers. These gold nanosheets exhibit high catalytic activity and stability in the solvent-free selective oxidation of carbon-hydrogen bonds with molecular oxygen.

A simple method for the preparation of ultra-small palladium nanoparticles and their utilization for the hydrogenation of terminal alkyne groups to alkanes

A simple and convenient method for the preparation of ultra-small palladium nanoparticles (Pd-NPs) by a modified digestive ripening method is described. These nanoparticles catalyse the hydrogenation of the terminal alkyne groups to alkanes selectively, and show no effect on other labile protecting and internal alkyne or internal/external alkene groups present in the molecule.

Preparation of TiO2-supported twinned gold nanoparticles by CO treatment and their CO oxidation activity

Au/TiO₂ prepared by CO treatment showed high catalytic activity for CO oxidation due to twinned structure of Au nanoparticles.

Factors influencing the catalytic oxidation of benzyl alcohol using supported phosphine-capped gold nanoparticles

The nature of Au cluster precursor and activation treatments affect catalyst activity in aerobic benzyl alcohol oxidation.

Synthesis of highly dispersed Pd nanoparticles supported on multi-walled carbon nanotubes and their excellent catalytic performance for oxidation of benzyl alcohol

A simple approach to synthesize highly dispersed Pd nanoparticles on CNTs without a capping agent is presented which exhibits high activity and selectivity for selective oxidation of benzyl alcohol.

Adsorption of methylamine on mackinawite (FES) surfaces: a density functional theory study

We have used density functional theory calculations to investigate the interaction between methylamine (CH3NH2) and the dominant surfaces of mackinawite (FeS), where the surface and adsorption properties of mackinawite have been characterized using the DFT-D2 method of Grimme. Our calculations show that while the CH3NH2 molecule only interacts weakly with the most stable FeS(001), it adsorbs relatively strongly on the FeS(011) and FeS(100) surfaces releasing energies of 1.26 eV and 1.51 eV, respectively. Analysis of the nature of the bonding reveals that the CH3NH2 molecule interacts with the mackinawite surfaces through the lone-pair of electrons located on the N atom. The electron density built up in the bonding region between N and Fe is very much what one would expect of covalent type of bonding. We observe no significant adsorption-induced changes of the FeS surface structures, suggesting that amine capping agents would not distort the FeS nanoparticle surfaces required for active heterogeneous catalytic reactions. The vibrational frequencies and the infrared spectra of adsorbed methylamine have been calculated and assignments for vibrational modes are used to propose a kinetic model for the desorption process, yielding a simulated temperature programmed desorption with a relative desorption temperature of <140 K at the FeS(011) surface and <170 K at FeS(100) surface.