A Generic Wet Impregnation Method for Preparing Substrate-Supported Platinum Group Metal and Alloy Nanoparticles with Controlled Particle Morphology

Quantification, Exchange, and Removal of Surface Ligands on Noble-Metal Nanocrystals

ConspectusSurface ligands are vital to the colloidal synthesis of noble-metal nanocrystals with well-controlled sizes and shapes for various applications. The surface ligands not only dictate the formation of nanocrystals with diverse shapes but also serve as a colloidal stabilizer to prevent the suspended nanocrystals from aggregation during their synthesis or storage. By leveraging the facet selectivity of some surface ligands, one can further control the sites for growth or galvanic replacement to transform presynthesized nanocrystals into complex structures that are otherwise difficult to fabricate using conventional methods. Furthermore, the presence of surface ligands on nanocrystals also facilitates their applications in areas such as sensing, imaging, nanomedicine, and self-assembly. Despite their popular use in enhancing the properties of nanocrystals and thus optimizing their performance in a wide variety of applications, it remains a major challenge to quantitatively determine the coverage density of ligand molecules, not to mention the difficulty of substituting or removing them without compromising the surface structure and aggregation state of the nanocrystals.In this Account, we recapitulate our efforts in developing methods capable of qualitatively or quantitatively measuring, exchanging, and removing the surface ligands adsorbed on noble-metal nanocrystals. We begin with an introduction to the typical interactions between ligand molecules and surface atoms, followed by a discussion of the Langmuir model that can be used to describe the adsorption of surface ligands. It is also emphasized that the adsorption process may become very complex in the case of a polymeric ligand due to the variations in binding configuration and chain conformation. We then highlight the capabilities of various spectroscopy methods to analyze the adsorbed ligands qualitatively or quantitatively. Specifically, surface-enhanced Raman scattering, Fourier transform infrared, and X-ray photoelectron spectroscopy are three examples of qualitative methods that can be used to confirm the absence or presence of a surface ligand. On the other hand, ultraviolet-visible spectroscopy and inductively coupled plasma mass spectrometry can be used for quantitative measurements. Additionally, the coverage density of a ligand can be derived by analyzing the morphological changes during nanocrystal growth. We then discuss how the ligands present on the surface of metal nanocrystals can be exchanged directly or indirectly to meet the requirements of different applications. The former can be done using a ligand with stronger binding, whereas the latter is achieved by introducing a sacrificial shell to the surface of the nanocrystals. Furthermore, we highlight three additional strategies besides simple washing to remove the surface ligands, including calcination, heating in a solution, and UV-ozone treatment. Finally, we showcase three applications of metal nanocrystals in nanomedicine, tumor targeting, and self-assembly by taking advantage of the diversity of surface ligands bearing different functional groups. We also offer perspectives on the challenges and opportunities in realizing the full potential of surface ligands.

Iridium and IrOx nanoparticles: an overview and review of syntheses and applications

Precious metals are key in various fields of research and precious metal nanomaterials are directly relevant for optics, catalysis, pollution management, sensing, medicine, and many other applications. Iridium based nanomaterials are less studied than metals like gold, silver or platinum. A specific feature of iridium nanomaterials is the relatively small size nanoparticles and clusters easily obtained, e.g. by colloidal syntheses. Progress over the years overcomes the related challenging characterization and it is expected that the knowledge on iridium chemistry and nanomaterials will be growing. Although Ir nanoparticles have been preferred systems for the development of kinetic-based models of nanomaterial formation, there is surprisingly little knowledge on the actual formation mechanism(s) of iridium nanoparticles. Following the impulse from the high expectations on Ir nanoparticles as catalysts for the oxygen evolution reaction in electrolyzers, new areas of applications of iridium materials have been reported while more established applications are being revisited. This review covers different synthetic strategies of iridium nanoparticles and provides an in breadth overview of applications reported. Comprehensive Tables and more detailed topic-oriented overviews are proposed in Supplementary Material, covering synthesis protocols, the historical role or iridium nanoparticles in the development of nanoscience and applications in catalysis.

In Situ Exsolution Catalyst: An Innovative Approach to Develop Highly Selective and Sensitive Gas Sensors

The gas sensing characteristics of oxide semiconductors can be enhanced by loading noble metal or metal oxide catalysts. The uniform distribution of nanoscale catalysts with high thermal stability over the sensing materials is essential for sensors operating at elevated temperatures. An in situ exsolution process, which can be applied to catalysts, batteries, and sensors, provides a facile synthetic route for developing second-phase nanoparticles with uniform distribution, excellent thermochemical stability, and strong adhesion to the mother phase. In this study, we investigated the effect of Co-exsolved nanoparticles on the gas sensing characteristics of La_0.43Ca_0.37Co_0.06Ti_0.94O_3-d (LCCoT). The amount and size of the Co-exsolved nanoparticles on the surface of the perovskite mother phase were adjusted depending on the reduction temperature of the exsolution process. The LCCoT with Co-exsolved nanoparticles prepared by reduction at 700 °C exhibited a response (resistance ratio) of 116.3 to 5 ppm ethanol at 350 °C, which was 10-fold higher than the response of a sensor without exsolution. The high gas response was attributed to the catalytic effect promoted by the uniformly distributed Co-exsolved nanoparticles and the formation of p-n junctions on the sensing surface during reduction. Additionally, we demonstrated the catalytic effect of Co-exsolved nanoparticles using a proton transfer reaction-quadrupole mass spectrometer. By controlling the amount and distribution of exsolved nanoparticles on semiconductor chemiresistors, a new pathway for designing high-performance gas sensors with enhanced thermal stability can be achieved.

CO and O2 Adsorption and CO Oxidation on Pt Nanoparticles by Indirect Nanoplasmonic Sensing

We used indirect nanoplasmonic sensing (INPS) coupled with mass spectrometry to study CO and oxygen adsorption as well as CO oxidation, on Pt nanoparticles, in the Torr pressure range. Due to an optimization of the physical parameters of our plasmonic sample, we obtain a highly sensitive probe that can detect gas adsorption of a few hundredths of a monolayer, even with a very low number density of Pt particles. Moreover and for the first time, a similarity is observed between the sign and the evolution of the localized surface plasmon resonance (LSPR) peak shift and the work function measurements for CO and oxygen chemisorption. Controlling the size, shape, and surface density of Pt particles, the turnover frequency (TOF) has also been accurately determined. For similar experimental conditions, the TOF is close to those measured on Pt/oxide powder catalysts and Pt(100) single crystals.

Surfactant-Assisted Synthesis of Palladium Nanosheets and Nanochains for the Electrooxidation of Ethanol

The synthesis of metal nanometer electrocatalysts with a two-dimensional (2D) structure or rich active sites has become a research hotspot in electrocatalysis. In this work, surfactant hexadecyltrimethylammonium bromide (CTAB) was used to assist the synthesis and assembly of Pd ultrathin nanosheet with the help of Mo(CO)₆ in the start system. Pd nanochain composed of nanoparticles is obtained under the same condition, replacing CTAB with carrageenan only. Electrochemical measurements showed that the catalytic peak current density for the electrooxidation of ethanol can reach 2145 mA mg^-1 for the Pd nanosheet assembly (NSA) and 1696 mA mg^-1 for Pd nanochains. Pd nanosheet assembly also has a lower electron-transfer barrier, better catalytic stability, and antipoisoning performance than that of Pd nanochains. The mechanism of Pd nanosheets and nanochains catalysts the enhanced electrocatalytic activity toward ethanol oxidation has been discussed based on the experimental data.

Selective Hydrogenation of Cinnamaldehyde over the Stepped and Plane Surface of Pd Nanoparticles with Controlled Morphologies by CO Chemisorption

Carbon monoxide (CO) molecules are attracting attention as capping agents that control the structure of metal nanoparticles. In this study, we aimed to control the shape and surface structure of Pd particles by reducing the supported Pd precursor with CO. The reduction of Pd nanoparticles with CO promoted the exposure of step sites and generated spherical and concave-tetrahedral Pd particles on carbon and SiO₂ supports. On the other hand, conventional H₂-reduced Pd particles show a flattened shape. The preferential exposure of the step sites by the adsorbed CO molecules was supported by the density functional theory-calculated surface energy and the Wulff construction. Morphology- and surface-controlled Pd nanoparticles were used to study the surface structure and morphology effects of Pd nanoparticles on cinnamaldehyde (CAL) hydrogenation. With an increase in the fraction of step sites on Pd nanoparticles, the hydrogenation activity and selectivity of hydrocinnamaldehyde (HCAL) increased. On step sites, the adsorption of the C═C bond of CAL proceeded preferentially, and HCAL was efficiently and selectively generated.

Stable Multimetallic Nanoparticles for Oxygen Electrocatalysis

Nanostructured catalysts often face an important challenge: poor stability. Many factors contribute to catalytic degradation, including parasitic chemical reactions, phase separation, agglomeration, and dissolution, leading to activity loss especially during long-term catalytic reactions. This challenge is shared by a new family of catalysts, multimetallic nanoparticles, which have emerged owing to their broad tunability and high activity. While significant synthesis-based advances have been made, the stability of these nanostructured catalysts, especially during catalytic reactions, has not been well addressed. In this study, we reveal the critical influence of a synthetic method on the stability of nanostructured catalysts through aprotic oxygen catalysis (Li-O₂ battery) demonstrations. In comparison to the conventional wet impregnation (WI) method, we show that the carbothermal shock (CTS) method dramatically improves the overall structural and chemical stability of the catalyst with the same elemental compositions. For multimetallic compositions (4- and 8-elements), the overall stability of the electrocatalysts as well as the battery lifetime can be further improved by incorporating additional noncatalytically active elements into the individual nanoparticles via CTS. The results offer a new synthetic path toward the stabilization of nanostructured catalysts, where additional reaction schemes beyond oxygen electrocatalysis are foreseeable.

Highly efficient hydrogen evolution of platinum via tuning the interfacial dissolved-gas concentration

We present a facile perfluorooctanesulfonate-modulation strategy with a precisely controlled dissolved-gas concentration at the electrode/gas/electrolyte interface for enhanced HER.

Gas-phase synthesis of morphology-controlled Pt nanoparticles and their impact on cinnamaldehyde hydrogenation

Pt nanoparticles of which morphology is controlled by gas-phase synthesis using carbon monoxide as a protective agent show high catalytic activity and selectivity for cinnamaldehyde hydrogenation.

Architectural Designs and Synthetic Strategies of Advanced Nanocatalysts

Advanced nanocatalysts with high compositional and structural tailorability have emerged as a new class of heterogeneous catalysts exhibiting many new technical merits over their conventional counterparts. Generally, preparation of such catalysts involves the integration of catalyst components with compositional, size, and shape controls into a larger material system in order to bring along collective and synergetic effects of individual components. Herein, a brief review of architectural designs and synthetic strategies for making these nanocatalysts is presented. Due to length constraints, only four major types of them are highlighted together with some general rules of design and synthesis. Finally, a critical outline of future perspective in this field is proposed.

Active Sites in Heterogeneous Catalytic Reaction on Metal and Metal Oxide: Theory and Practice

Active sites play an essential role in heterogeneous catalysis and largely determine the reaction properties. Yet identification and study of the active sites remain challenging owing to their dynamic behaviors during catalysis process and issues with current characterization techniques. This article provides a short review of research progresses in active sites of metal and metal oxide catalysts, which covers the past achievements, current research status, and perspectives in this research field. In particular, the concepts and theories of active sites are introduced. Major experimental and computational approaches that are used in active site study are summarized, with their applications and limitations being discussed. An outlook of future research direction in both experimental and computational catalysis research is provided.

Recent advances in the precise control of isolated single-site catalysts by chemical methods

The search for constructing high-performance catalysts is an unfailing topic in chemical fields. Recently, we have witnessed many breakthroughs in the synthesis of single-atom catalysts (SACs) and their applications in catalytic systems. They have shown excellent activity, selectivity, stability, efficient atom utilization and can serve as an efficient bridge between homogeneous and heterogenous catalysis. Currently, most SACs are synthesized via a bottom-up strategy; however, drawbacks such as the difficulty in accessing high mass activity and controlling homogeneous coordination environments are inevitably encountered, restricting their potential use in the industrial area. In this regard, a novel top-down strategy has been recently developed to fabricate SACs to address these practical issues. The metal loading can be increased to 5% and the coordination environments can also be precisely controlled. This review highlights approaches to the chemical synthesis of SACs towards diverse chemical reactions, especially the recent advances in improving the mass activity and well-defined local structures of SACs. Also, challenges and opportunities for the SACs will be discussed in the later part.

Low-cost palladium decorated on m-aminophenol-formaldehyde-derived porous carbon spheres for the enhanced catalytic reduction of organic dyes

A novel method for the synthesis of recyclable Pd@PCS catalyst was applied for the reduction of CV, EY, and SY.

Platinum-Based Nanowires as Active Catalysts toward Oxygen Reduction Reaction: In Situ Observation of Surface-Diffusion-Assisted, Solid-State Oriented Attachment

Facile fabrication of advanced catalysts toward oxygen reduction reaction with improving activity and stability is significant for proton-exchange membrane fuel cells. Based on a generic solid-state reaction, this study reports a modified hydrogen-assisted, gas-phase synthesis for facile, scalable production of surfactant-free, thin, platinum-based nanowire-network electrocatalysts. The free-standing platinum and platinum-nickel alloy nanowires show improvements of up to 5.1 times and 10.9 times for mass activity with a minimum 2.6% loss after an accelerated durability test for 10k cycles; 8.5 times and 13.8 times for specific activity, respectively, compared to commercial Pt/C catalyst. In addition, combined with a wet impregnation method, different substrate-materials-supported platinum-based nanowires are obtained, which paves the way to practical application as a next-generation supported catalyst to replace Pt/C. The growth stages and formation mechanism are investigated by an in situ transmission electron microscopy study. It reveals that the free-standing platinum nanowires form in the solid state via metal-surface-diffusion-assisted oriented attachment of individual nanoparticles, and the interaction with gas molecules plays a critical role, which may represent a gas-molecular-adsorbate-modified growth in catalyst preparation.