Colloidal Nanocrystals as Electrocatalysts with Tunable Activity and Selectivity

Photoactive p-Type Spinel CuGa2O4 Nanocrystals

Herein we report the synthesis and characterization of spinel copper gallate (CuGa₂O₄) nanocrystals (NCs) with an average size of 3.7 nm via a heat-up colloidal reaction. CuGa₂O₄ NCs have a band gap of ∼2.5 eV and marked p-type character, in agreement with ab initio simulations. These novel NCs are demonstrated to be photoactive, generating a clear and reproducible photocurrent under blue light irradiation when deposited as thin films. Crucially, the ability to adjust the Cu/Ga ratio within the NCs, and the effect of this on the optical and electronic properties of the NCs, was also demonstrated. These results position CuGa₂O₄ NCs as a novel material for optoelectronic applications, including hole transport and light harvesting.

Shape-Controlled Synthesis of Platinum-Based Nanocrystals and Their Electrocatalytic Applications in Fuel Cells

To achieve environmentally benign energy conversion with the carbon neutrality target via electrochemical reactions, the innovation of electrocatalysts plays a vital role in the enablement of renewable resources. Nowadays, Pt-based nanocrystals (NCs) have been identified as one class of the most promising candidates to efficiently catalyze both the half-reactions in hydrogen- and hydrocarbon-based fuel cells. Here, we thoroughly discuss the key achievement in developing shape-controlled Pt and Pt-based NCs, and their electrochemical applications in fuel cells. We begin with a mechanistic discussion on how the morphology can be precisely controlled in a colloidal system, followed by highlighting the advanced development of shape-controlled Pt, Pt-alloy, Pt-based core@shell NCs, Pt-based nanocages, and Pt-based intermetallic compounds. We then select some case studies on models of typical reactions (oxygen reduction reaction at the cathode and small molecular oxidation reaction at the anode) that are enhanced by the shape-controlled Pt-based nanocatalysts. Finally, we provide an outlook on the potential challenges of shape-controlled nanocatalysts and envision their perspective with suggestions.

Osmium and OsO x nanoparticles: an overview of syntheses and applications

Precious metal nanoparticles are key for a range of applications ranging from catalysis and sensing to medicine. While gold (Au), silver (Ag), platinum (Pt), palladium (Pd) or ruthenium (Ru) nanoparticles have been widely studied, other precious metals are less investigated. Osmium (Os) is one of the least studied of the precious metals. However, Os nanoparticles are interesting materials since they present unique features compared to other precious metals and Os nanomaterials have been reported to be useful for a range of applications, catalysis or sensing for instance. With the increasing availability of advanced characterization techniques, investigating the properties of relatively small Os nanoparticles and clusters has become easier and it can be expected that our knowledge on Os nanomaterials will increase in the coming years. This review aims to give an overview on Os and Os oxide materials syntheses and applications.

Copper Phosphonate Lamella Intermediates Control the Shape of Colloidal Copper Nanocrystals

Understanding the structure and behavior of intermediates in chemical reactions is the key to developing greater control over the reaction outcome. This principle is particularly important in the synthesis of metal nanocrystals (NCs), where the reduction, nucleation, and growth of the reaction intermediates will determine the final size and shape of the product. The shape of metal NCs plays a major role in determining their catalytic, photochemical, and electronic properties and, thus, the potential applications of the material. In this work, we demonstrate that layered coordination polymers, called lamellae, are reaction intermediates in Cu NC synthesis. Importantly, we discover that the lamella structure can be fine-tuned using organic ligands of different lengths and that these structural changes control the shape of the final NC. Specifically, we show that short-chain phosphonate ligands generate lamellae that are stable enough at the reaction temperature to facilitate the growth of Cu nuclei into anisotropic Cu NCs, being primarily triangular plates. In contrast, lamellae formed from long-chain ligands lose their structure and form spherical Cu NCs. The synthetic approach presented here provides a versatile tool for the future development of metal NCs, including other anisotropic structures.

The Native Oxide Skin of Liquid Metal Ga Nanoparticles Prevents Their Rapid Coalescence during Electrocatalysis

Liquid metals (LMs) have been used in electrochemistry since the 19th century, but it is only recently that they have emerged as electrocatalysts with unique properties, such as inherent resistance to coke poisoning, which derives from the dynamic nature of their surface. The use of LM nanoparticles (NPs) as electrocatalysts is highly desirable to enhance any surface-related phenomena. However, LM NPs are expected to rapidly coalesce, similarly to liquid drops, which makes their implementation in electrocatalysis hard to envision. Herein, we demonstrate that liquid Ga NPs (18 nm, 26 nm, 39 nm) drive the electrochemical CO₂ reduction reaction (CO₂RR) while remaining well-separated from each other. CO is generated with a maximum faradaic efficiency of around 30% at -0.7 V_RHE, which is similar to that of bulk Ga. The combination of electrochemical, microscopic, and spectroscopic techniques, including operando X-ray absorption, indicates that the native oxide skin of the Ga NPs is still present during CO₂RR and provides a barrier to coalescence during operation. This discovery provides an avenue for future development of Ga-based LM NPs as a new class of electrocatalysts.

Polymer Modification of Surface Electronic Properties of Electrocatalysts

Finding alternative ways to tailor the electronic properties of a catalyst to actively and selectively drive reactions of interest has been a growing research topic in the field of electrochemistry. In this Letter, we investigate the tuning of the surface electronic properties of electrocatalysts via polymer modification. We show that when a nickel oxide water oxidation catalyst is coated with polytetrafluoroethylene, stable Ni-CF _x bonds are introduced at the nickel oxide/polymer interface, resulting in shifting of the reaction selectivity away from the oxygen evolution reaction and toward hydrogen peroxide formation. It is shown that the electron-withdrawing character of the surface fluorocarbon molecule leaves a slight positive charge on the water oxidation intermediates at the adjacent active nickel sites, making their bonds weaker. The concept of modifying the surface electronic properties of an electrocatalyst via stable polymer modification offers an additional route to tune multipathway reactions in polymer/electrocatalyst environments, like with ionomer-modified catalysts or with membrane electrode assemblies.

The Interactive Dynamics of Nanocatalyst Structure and Microenvironment during Electrochemical CO2 Conversion

In the pursuit of a decarbonized society, electrocatalytic CO₂ conversion has drawn tremendous research interest in recent years as a promising route to recycling CO₂ into more valuable chemicals. To achieve high catalytic activity and selectivity, nanocatalysts of diverse structures and compositions have been designed. However, the dynamic structural transformation of the nanocatalysts taking place under operating conditions makes it difficult to study active site configurations present during the CO₂ reduction reaction (CO₂RR). In addition, although recognized as consequential to the catalytic performance, the reaction microenvironment generated near the nanocatalyst surface during CO₂RR and its impact are still an understudied research area. In this Perspective, we discuss current understandings and difficulties associated with investigating such dynamic aspects of both the surface reaction site and its surrounding reaction environment as a whole. We further highlight the interactive influence of the structural transformation and the microenvironment on the catalytic performance of nanocatalysts. We also present future research directions to control the structural evolution of nanocatalysts and tailor their reaction microenvironment to achieve an ideal catalyst for improved electrochemical CO₂RR.

Shaping Copper Nanocatalysts to Steer Selectivity in the Electrochemical CO2 Reduction Reaction

The carbon-neutral production of fuels and chemical feedstocks is one of the grand challenges for our society to solve. The electrochemical conversion of CO₂ is emerging as a promising technology contributing to this goal. Despite the huge amount of progress made over the past decade, selectivity still remains a challenge. This Account presents an overview of recent progress in the design of selective catalysts by exploiting the structural sensitivity of the electrochemical CO₂ reduction reaction (CO₂RR). In particular, it shows that the accurate and precise control of the shape and size of Cu nanocatalysts is instrumental in understanding and in discovering the structure-selectivity relationships governing the reduction of CO₂ to valuable hydrocarbons, such as methane and ethylene. It further illustrates the use of faceted Cu nanocatalysts to interrogate catalytic pathways and to increase selectivity toward oxygenates, such as ethanol, in the framework of tandem schemes. The last part of the Account highlights the role of well-defined nanocatalysts in identifying reconstruction mechanisms which might occur during operation. An outlook for the emerging paradigms which will empower the design of novel catalysts for CO₂RR concludes the Account.

Two-dimensional copper based colloidal nanocrystals: synthesis and applications

Two-dimensional (2D) semiconductor nanocrystals display unconventional physical and opto-electronic properties due to their ultrathin and unique electronic structures. Since the success of Cd-based photoemissive nanocrystals, the development of sustainable and low-cost nanocrystals with enhanced electronic and physical properties has become a central research theme. In this context, copper-based semiconductor 2D nanocrystals, the cost-effective and eco-friendly alternative, exhibit unique plasmonic resonance, transport properties, and high ionic conductivity beneficial for sensing, energy storage, conversion, and catalytic applications. This review summarizes recent progress in the colloidal synthesis, growth mechanisms, properties, and applications of 2D copper-based nanostructures with tunable compositions, dimensions, and crystal phases. We highlight the growth mechanisms concerning their shape evolution in two dimensions. We analyse the effectiveness of cation exchange as a tool to synthesize multinary nanocrystals. Based on the preparation of Cu-based chalcogenide and non-chalcogenide compositions, we discuss synthesis control achieved via colloidal approaches to allow dimension tunability, phase engineering, and plasmonic and thermoelectric property optimization. Furthermore, their potential in various applications of catalysis, energy storage, and sensing is reviewed. Finally, we address the current challenges associated with 2D Cu-based nanocrystal development and provide an outlook pertaining to unexplored research areas.

Noble Metal-Based Multimetallic Nanoparticles for Electrocatalytic Applications

Noble metal-based multimetallic nanoparticles (NMMNs) have attracted great attention for their multifunctional and synergistic effects, which offer numerous catalytic applications. Combined experimental and theoretical studies have enabled formulation of various design principles for tuning the electrocatalytic performance through controlling size, composition, morphology, and crystal structure of the nanoparticles. Despite significant advancements in the field, the chemical synthesis of NMMNs with ideal characteristics for catalysis, including high activity, stability, product-selectivity, and scalability is still challenging. This review provides an overview on structure-based classification and the general synthesis of NMMN electrocatalysts. Furthermore, postsynthetic treatments, such as the removal of surfactants to optimize the activity, and utilization of NMMNs onto suitable support for practical electrocatalytic applications are highlighted. In the end, future direction and challenges associated with the electrocatalysis of NMMNs are covered.

A polarizable coarse-grained model for metal, metal oxide and composite metal/metal oxide nanoparticles and its applications

We present a selected set of exemplifying applications of the novel polarizable coarse-grained model to various outstanding problems in the physics and chemistry of nanoparticles.

Ligand Dynamics in Nanocrystal Solids Studied with Quasi-Elastic Neutron Scattering

Nanocrystal surfaces are commonly populated by organic ligands, which play a determining role in the optical, electronic, thermal, and catalytic properties of the individual nanocrystals and their assemblies. Understanding the bonding of ligands to nanocrystal surfaces and their dynamics is therefore important for the optimization of nanocrystals for different applications. In this study, we use temperature-dependent, quasi-elastic neutron scattering (QENS) to investigate the dynamics of different surface bound alkanethiols in lead sulfide nanocrystal solids. We select alkanethiols with mono- and dithiol terminations, as well as different backbone types and lengths. QENS spectra are collected both on a time-of-flight spectrometer and on a backscattering spectrometer, allowing us to investigate ligand dynamics in a time range from a few picoseconds to nanoseconds. Through model-based analysis of the QENS data, we find that ligands can either (1) precess around a central axis, while simultaneously rotating around their own molecular axis, or (2) only undergo uniaxial rotation with no precession. We establish the percentage of ligands undergoing each type of motion, the average relaxation times, and activation energies for these motions. We determine, for example, that dithiols which link facets of neighboring nanocrystals only exhibit uniaxial rotation and that longer ligands have higher activation energies and show smaller opening angles of precession due to stronger ligand-ligand interactions. Generally, this work provides insight into the arrangement and dynamics of ligands in nanocrystal solids, which is key to understanding their mechanical and thermal properties, and, more generally, highlights the potential of QENS for studying ligand behavior.

Surfactant-Free Precious Metal Colloidal Nanoparticles for Catalysis

Colloidal syntheses of nanoparticles (NPs) are one of the preferred approaches to prepare precious metal catalysts. Unfortunately, most colloidal syntheses developed require stabilizing agents to avoid NP agglomeration and/or control NP size and morphology. While these surfactants can bring positive features, they typically block catalytically active sites on the NP surface. As a consequence, these additives often need to be removed by energy and/or time consuming steps, at the risk of complicating the synthesis, introducing irreproducibility and negatively altering the structure and properties of the prepared catalysts. Fortunately, several surfactant-free colloidal syntheses have been reported and are being developed. This Mini Review addresses the challenges in defining a surfactant-free colloidal synthesis of NPs and survey established and emerging strategies to obtain surfactant-free colloidal precious metal NPs. A focus is given to approaches that show promising features to bridge the gap between fundamental and applied research towards industrial applications.

Electrospray deposition for single nanoparticle studies

Single entity electrochemical (SEE) studies that can probe activities and heterogeneity in activities at nanoscale require samples that contain single and isolated particles. Single, isolated nanoparticles are achieved here with electrospray deposition of colloidal nanoparticle solutions, with simple instrumentation. Role of three electrospray (ES) parameters, viz. spray distance (emitter tip-to-substrate distance), ES current and emitter tip diameter, in the ES deposition of single Au nano-octahedra (Au ODs) is examined. The ES deposition of single, isolated Au ODs are analyzed in terms of percentage of single NPs and local surface density of deposition. The local surface density of ES deposition of single Au ODs was found to increase with decrease in spray distance and emitter tip diameter, and increase in ES current. While the percentage of single particle ES deposition increased with increase in spray distance and decrease in emitter tip size. No significant change in the single Au ODs ES deposition percentage was observed with change in ES current values included in this study. The most favourable conditions in the ES deposition of Au ODs in this study resulted in the local surface density of 0.26 ± 0.05 single particles per μm² and observation of 96.3% single Au OD deposition.

Pt-Co@Pt Octahedral Nanocrystals: Enhancing Their Activity and Durability toward Oxygen Reduction with an Intermetallic Core and an Ultrathin Shell

Despite extensive efforts devoted to the synthesis of Pt-Co bimetallic nanocrystals for fuel cell and related applications, it remains a challenge to simultaneously control atomic arrangements in the bulk and on the surface. Here we report a synthesis of Pt-Co@Pt octahedral nanocrystals that feature an intermetallic, face-centered tetragonal Pt-Co core and an ultrathin Pt shell, together with the dominance of {111} facets on the surface. When evaluated as a catalyst toward the oxygen reduction reaction (ORR), the nanocrystals delivered a mass activity of 2.82 A mg^-1 and a specific activity of 9.16 mA cm^-2, which were enhanced by 13.4 and 29.5 times, respectively, relative to the values of a commercial Pt/C catalyst. More significantly, the mass activity of the nanocrystals only dropped 21% after undergoing 30 000 cycles of accelerated durability test, promising an outstanding catalyst with optimal performance for ORR and related reactions.

Kinetically Controlled Synthesis of Rhodium Nanocrystals with Different Shapes and a Comparison Study of Their Thermal and Catalytic Properties

We report the synthesis of Rh nanocrystals with different shapes by controlling the kinetics involved in the growth of preformed Rh cubic seeds. Specifically, Rh nanocrystals with cubic, cuboctahedral, and octahedral shapes can all be obtained from the same cubic seeds under suitable reduction kinetics for the precursor. The success of such a synthesis also relies on the use of a halide-free precursor to avoid oxidative etching, as well as the involvement of a sufficiently high temperature to remove Br^- ions from the seeds while ensuring adequate surface diffusion. The availability of Rh nanocrystals with cubic and octahedral shapes allows for an evaluation of the facet dependences of their thermal and catalytic properties. The data from in situ electron microscopy studies indicate that the cubic and octahedral Rh nanocrystals can keep their original shapes up to 700 and 500 °C, respectively. When tested as catalysts for hydrazine decomposition, the octahedral nanocrystals exhibit almost 4-fold enhancement in terms of H₂ selectivity relative to the cubic counterpart. As for ethanol oxidation, the order is reversed, with the cubic nanocrystals being about three times more active than the octahedral sample.