Surface Coordination Chemistry of Metal Nanomaterials

A stable rhodium single-site catalyst encapsulated within dendritic mesoporous nanochannels

Catalysis plays an essential role in the modern chemical industry. However, it still remains a great challenge to improve the efficiency of many heterogeneous catalysts based on a per metal atom basis. Single-site catalysts (SsCs) with isolated metal atoms/ions anchored to the supports are thus highly desirable, providing an innovative solution towards highly efficient usage of precious metal atoms in heterogeneous catalysts. Creating SsCs with high metal loading proves to be challenging because, without robust anchoring, atoms tend to diffuse to form large aggregates during catalytic reactions. We report a facile ligand exchange method to anchor a single-site Rh catalyst inside the individual channels of three-dimensional dendritic mesoporous silica nanospheres (MSNSs). The short porous channels inside MSNSs provide an easy access of reactants and the strong binding of the ligand prevents the aggregation of catalyst sites. The as-synthesized Rh₁@MSNS-NH₂ catalyst shows excellent activity, stability and reusability in the reduction of 4-nitrophenol. The same catalyst shows high regioselectivity in the hydrosilylation of terminal alkynes to yield α-vinylsilanes through the Markovnikov addition.

LSPR Tuning from 470 to 800 nm and Improved Stability of Au-Ag Nanoparticles Formed by Gold Deposition and Rebuilding in the Presence of Poly(styrenesulfonate)

Stability and precise control over functional properties of metal nanoparticles remain a challenge for the realization of prospective applications. Our described process of shell formation and rebuilding can address both these challenges. Template silver nanoparticles (AgNPs) stabilized by poly(styrenesulfonate) are first transformed with gold deposition, after which the resulting shell rebuilds with the replaced silver. The shell formation and rebuilding are accompanied by large shifts in localized surface plasmon resonance (LSPR) peak position, which enables LSPR tuning in a range from 470 to 800 nm. Furthermore, chemical stability of Au-AgNPs is significantly improved compared to AgNPs due to gold stability. Silver templates of different shapes and sizes were demonstrated to transform to AuAg composite NPs to further extend the accessible LSPR range tuning. Stabilization of template AgNPs with poly(styrenesulfonate), in contrast to commonly used poly(vinylpyrrolidone), was found to be a key factor for shell rebuilding. The developed Au-AgNPs were shown to be advantageous for surface plasmon resonance (SPR) detection and surface-enhanced Raman spectroscopy (SERS) owing to their tunable LSPR and enhanced stability.

Spectroscopic Evidence for Ligand Substitution Reactions at the Solid-Liquid Interface of a Sub-micrometer Gold(I) Carbene Complex

Colloidal coordination compounds in the sub-micrometer range were synthesized from a chloro gold(I) carbene complex via the anti-solvent procedure and stabilized by a surfactant shell of Tween 20. This compound was successfully applied as model system to monitor heterogeneous multiphase ligand substitution reactions at the solid-liquid interface.

Design of atomically precise Au2Pd6nanoclusters for boosting electrocatalytic hydrogen evolution on MoS2

A new Au–Pd alloy nanocluster (NC) – Au₂Pd₆S₄(PPh₃)₄(C₆H₄F₂S)₆is synthesized. The NC is applied to enhance the electrocatalytic HER activity of MoS₂compared with a single Pd or Au component.

Ligand-functionalized Pt nanoparticles as asymmetric heterogeneous catalysts: molecular reaction control by ligand–reactant interactions

Stereoselective control on amino acid functionalized supported Pt nanoparticles by means of dispersion interactions.

Porous organic cage stabilised palladium nanoparticles: efficient heterogeneous catalysts for carbonylation reaction of aryl halides

Porous organic cage stabilised palladium nanoparticles were prepared using methanol as a mild reductant and displayed high catalytic activity for the carbonylation reaction of aryl halides under mild conditions.

Concentration-dependent photophysical switching in mixed self-assembled monolayers of pentacene and perylenediimide on gold nanoclusters

The precise control and switching of photophysical processes such as singlet fission, electron transfer and excimer were performed using mixed SAMs of pentacene and perylenediimide units on Au nanoclusters.

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.

Simultaneous hetero-atom doping and foreign-thiolate exchange on the Au25(SR)18 nanocluster

Hetero-atom-Ag doping and foreign-ligand exchange have been synchronously achieved on the Au₂₅(SR)₁₈ template.

Degradation of Bisphenol A by Peroxymonosulfate Catalytically Activated with Mn1.8Fe1.2O4 Nanospheres: Synergism between Mn and Fe

A high-efficient, low-cost, and eco-friendly catalyst is highly desired to activate peroxides for environmental remediation. Due to the potential synergistic effect between bimetallic oxides' two different metal cations, these oxides exhibit superior performance in the catalytic activation of peroxymonosulfate (PMS). In this work, novel Mn_1.8Fe_1.2O₄ nanospheres were synthesized and used to activate PMS for the degradation of bisphenol A (BPA), a typical refractory pollutant. The catalytic performance of the Mn_1.8Fe_1.2O₄ nanospheres was substantially greater than that of the Mn/Fe monometallic oxides and remained efficient in a wide pH range from 4 to 10. More importantly, a synergistic effect between solid-state Mn and Fe was identified in control experiments with Mn₃O₄ and Fe₃O₄. Mn was inferred to be the primary active site in the surface of the Mn_1.8Fe_1.2O₄ nanospheres, while Fe(III) was found to play a key role in the synergism with Mn by acting as the main adsorption site for the reaction substrates. Both sulfate and hydroxyl radicals were generated in the PMS activation process. The intermediates of BPA degradation were identified and the degradation pathways were proposed. This work is expected to help to elucidate the rational design and efficient synthesis of bimetallic materials for PMS activation.

Surfactant dependent evolution of Au-Pd alloy nanocrystals from trisoctahedron to excavated rhombic dodecahedron and multipod: a matter of crystal growth kinetics

In wet chemical syntheses of noble metal nanocrystals, surfactants play crucial roles in regulating their morphology. To date, more attention has been paid to the effect of the surfactant on the surface energy of crystal facets, while less attention has been paid to its effect on the growth kinetics. In this paper, using the growth of Au-Pd alloy nanocrystals as an example, we demonstrate that different concentration of surfactant hexadecyltrimethyl ammonium chloride (CTAC) may cause the different packing density of CTA⁺ bilayers on different sites (face, edge or vertex) of crystallite surface, which would change the crystal growth kinetics and result in preferential crystal growth along the edge or vertex of crystallites. The unique shape evolution from trisoctahedron to excavated rhombic dodecahedron and multipod structure for Au-Pd alloy nanocrystals was successfully achieved by simply adjusting the concentration of CTAC. These results help to understand the effect of surfactants on the shape evolution of nanocrystals and open up avenues to the rational synthesis of nanocrystals with the thermodynamically unfavorable morphologies.

Hydrogen assisted synthesis of branched nickel nanostructures: a combined theoretical and experimental study

The selective adsorption of small molecules over specific facets plays an important role in morphology controlled synthesis of metal nanocrystals. In the present work, hydrogen is found to be a good capping agent for direct synthesis of branched nickel nanocrystals, i.e., secondary branching (Ni-SB) nanoparticles and multipods (Ni-MP). Using ab initio thermodynamics and the Wulff construction principle, it has been found that: (i) in the presence of hydrogen (P_H2 = 6 bar), the facet structure stability follows the order of Ni(100) > Ni(111) > Ni(110), resulting in competitive over-growth along the 〈111〉 and 〈110〉 directions; (ii) with increasing hydrogen pressure, the Ni deposition rate over the crystal surface increases as a result of more Ni²⁺ reduction; the competition between deposition and surface diffusion, therefore, becomes the vital factor for the nanocrystal growth process; (iii) the diffusion energy barrier of a surface Ni atom on Ni(111) is lower than that on Ni(110), especially on hydrogen covered surfaces, indicating that the kinetic over-growth only along the 〈111〉 direction producing Ni-MP will be dominant under P_H2 = 14 bar; (iv) the ab initio based Wulff construction principle predicts the shapes and morphologies at different hydrogen pressures which is further confirmed with HRTEM results. Finally, compared with nickel nanoparticles (Ni-NP) synthesized in the absence of hydrogen, the hydrogen assisted branched Ni nanomaterials, especially the Ni-MP, show higher catalytic activities for hydrogenation reactions of acetophenone and nitrobenzene.

Reduction rate as a quantitative knob for achieving deterministic synthesis of colloidal metal nanocrystals

Despite the incredible developments made to the synthesis of colloidal metal nanocrystals, it is still challenging to produce them in a reproducible and predictable manner. This drawback can be attributed to the fact that the protocols continue to be built upon qualitative observations and empirical laws. Because of the vast number of intricately entangled experimental parameters in a synthesis, it is almost impossible to predict and control the outcome by knowingly alternating these parameters. In this Perspective article, we discuss the recent efforts in pushing nanocrystal synthesis towards a deterministic process based upon quantitative measurements. In particular, we focus on how the reduction rate of a salt precursor can be used as a quantitative knob for predicting and controlling the outcomes of both nucleation and growth. We begin with a brief introduction to the techniques that have been used to extract the kinetic information of a synthesis and then discuss how the reduction rate is correlated with the defect structure, shape/morphology, and elemental distribution of the resultant nanocrystals. We conclude by highlighting some of the recent advances related to in situ probing of nanocrystal synthesis, with an emphasis on the real-time, quantitative aspects with regard to both nucleation and growth.

Ultrathin Vein-Like Iridium-Tin Nanowires with Abundant Oxidized Tin as High-Performance Ethanol Oxidation Electrocatalysts

Iridium (Ir) holds great promise for ethanol oxidation reaction (EOR), while its practical applications suffer from the limited shape-controlled synthesis due to its low-energy barrier for nucleation. To overcome this limitation, the preparation of a new class of ultrathin vein-like Ir-tin nanowires (IrSn NWs) with abundant oxidized Sn is reported. By tuning the ratio of Ir to Sn, the optimized Ir₆₇ Sn₃₃ /C exhibits the highest mass density of 95.6 mA mg^-1 Ir for EOR at low potential (0.04 V), which is 4.1-fold and 20-fold higher than that of Ir/C and the commercial Pt/C, respectively. It also exhibits the smallest Tafel slope of 153 mV dec^-1 and superior stability after 2 h chronoamperometric measurement. Electrochemical measurements and X-ray photoelectron spectra results confirm that the abundant oxidized Sn promotes a complete oxidization of ethanol into CO₂ at low potential. This work highlights the importance of non-noble metal on enhancing the EOR performance.

Ultrastable atomic copper nanosheets for selective electrochemical reduction of carbon dioxide

The electrochemical conversion of CO₂ and H₂O into syngas using renewably generated electricity is an attractive approach to simultaneously achieve chemical fixation of CO₂ and storage of renewable energy. Developing cost-effective catalysts for selective electroreduction of CO₂ into CO is essential to the practical applications of the approach. We report a simple synthetic strategy for the preparation of ultrathin Cu/Ni(OH)₂ nanosheets as an excellent cost-effective catalyst for the electrochemical conversion of CO₂ and H₂O into tunable syngas under low overpotentials. These hybrid nanosheets with Cu(0)-enriched surface behave like noble metal nanocatalysts in both air stability and catalysis. Uniquely, Cu(0) within the nanosheets is stable against air oxidation for months because of the presence of formate on their surface. With the presence of atomically thick ultrastable Cu nanosheets, the hybrid Cu/Ni(OH)₂ nanosheets display both excellent activity and selectivity in the electroreduction of CO₂ to CO. At a low overpotential of 0.39 V, the nanosheets provide a current density of 4.3 mA/cm² with a CO faradaic efficiency of 92%. No decay in the current is observed for more than 22 hours. The catalysts developed in this work are promising for building low-cost CO₂ electrolyzers to produce CO.

Engineering a red emission of copper nanocluster self-assembly architectures by employing aromatic thiols as capping ligands

Luminescent Cu nanoclusters (NCs) are potential phosphors for illumination and display, but the difficulty in achieving full-color emission greatly limits practical applications. On the basis of our previous success in preparing Cu NC self-assembly architectures with blue-green and yellow emission, in this work, Cu NC self-assembly architectures with strong red emission are prepared by replacing alkylthiol ligands with aromatic thiols. The introduction of aromatic ligands is able to influence the ligand-to-metal charge transfer and/or ligand-to-metal-metal charge transfer, thus permitting the tuning of the emission color and enhancing of the emission intensity. The emission color can be tuned from yellow to dark red by choosing the aromatic ligands with different conjugation capabilities, and the photoluminescence quantum yield is up to 15.6%. Achieving full-color emission Cu NC self-assembly architectures allows the fabrication of Cu NC-based white light-emitting diodes.

Interfacial charge distributions in carbon-supported palladium catalysts

Controlling the charge transfer between a semiconducting catalyst carrier and the supported transition metal active phase represents an elite strategy for fine turning the electronic structure of the catalytic centers, hence their activity and selectivity. These phenomena have been theoretically and experimentally elucidated for oxide supports but remain poorly understood for carbons due to their complex nanoscale structure. Here, we combine advanced spectroscopy and microscopy on model Pd/C samples to decouple the electronic and surface chemistry effects on catalytic performance. Our investigations reveal trends between the charge distribution at the palladium-carbon interface and the metal's selectivity for hydrogenation of multifunctional chemicals. These electronic effects are strong enough to affect the performance of large (~5 nm) Pd particles. Our results also demonstrate how simple thermal treatments can be used to tune the interfacial charge distribution, hereby providing a strategy to rationally design carbon-supported catalysts.Control over charge transfer in carbon-supported metal nanoparticles is essential for designing new catalysts. Here, the authors show that thermal treatments effectively tune the interfacial charge distribution in carbon-supported palladium catalysts with consequential changes in hydrogenation performance.

Analogous self-assembly and crystallization: a chloride-directed orientated self-assembly of Cu nanoclusters and subsequent growth of Cu2-xS nanocrystals

Self-assembly and crystallization are two common methods to control the morphologies of nanomaterials, which have many similarities. In this work, chloride is used to direct the self-assembly process of Cu nanoclusters and the subsequent growth of Cu_2-xS nanocrystals. Meaningfully, chloride both promotes the transformation of Cu nanocluster self-assembled architectures from one-dimensional (1D) to 2D, and facilitates the transformation of Cu_2-xS nanocrystals from nanorods to nanosheets. Such an influence is attributed to the selective adsorption of chloride ions on the specific facets of nanoclusters and nanocrystals, which alters the inter-nanocluster weak interactions during self-assembly and suppresses the activity of Cu_2-xS facets during nanocrystal growth. The current results indicate that the method used to direct the morphologies of nanocrystals is extendable to control the tendency of nanocluster self-assembly.

Thiols make for better catalysts: Au nanoparticles supported on functional SBA-15 for catalysis of Ullmann-type homocouplings

A strategy for arraying nanoparticles (NPs) on mesoporous supports modified with functional self-assembled monolayers is described. Au NPs supported on materials containing thiols exhibited thermal stability and catalytic competency well beyond that of unsupported, thiol-protected Au NPs of similar size.

Grand challenges for catalysis in the Science and Technology Roadmap on Catalysis for Europe: moving ahead for a sustainable future

This perspective discusses the general concepts that will guide future catalysis and related grand challenges based on the Science and Technology Roadmap on Catalysis for Europe prepared by the European Cluster on Catalysis.

Modulating photo-luminescence of Au2Cu6 nanoclusters via ligand-engineering

The luminescence of Au₂Cu₆ nanocluster is controlled by tailoring the ligand to metal charge transfer via engineering the phosphine ligands.

De-assembly of assembled Pt1Ag12 units: tailoring the photoluminescence of atomically precise nanoclusters

De-assembly of assembled Pt₁Ag₁₂-units renders a blue-shift of the photoluminescent emission as well as an enhancement of the quantum yield.