Highly Correlated Size and Composition of Pt/Au Alloy Nanoparticles via Magnetron Sputtering onto Liquid

Fuel Cell Catalyst Layers with Platinum Nanoparticles Synthesized by Sputtering onto Liquid Substrates

Platinum (Pt) nanoparticles are widely used as catalysts in proton exchange membrane fuel cells. In recent decades, sputter deposition onto liquid substrates has emerged as a potential alternative for nanoparticle synthesis, offering a synthesis process free of contaminant oxygen, capping agents, and chemical precursors. Here, we present a method for the synthesis of supported nanoparticles based on magnetron sputtering onto liquid poly(ethylene glycol) (PEG) combined with a heat-treatment step for attachment of nanoparticles to a carbon support. Transmission electron microscopy imaging reveals Pt nanoparticle growth during the heat-treatment process, facilitated by the carbon support and the reducing properties of PEG. Following the heat treatment, a bimodal size distribution of Pt nanoparticles is observed, with sizes of 2.5 ± 0.8 and 6.7 ± 1.8 nm, compared to 1.8 ± 0.4 nm after sputtering. Synthesized Pt nanoparticles display excellent specific and mass activities for the oxygen reduction reaction, with 1.75 mA/cm2 Pt and 0.27 A/mgPt respectively, measured at 0.9 V vs the reversible hydrogen electrode. The specific activities reported herein outperform literature values of commercial Pt/C catalysts with similar loading and are on par with values of bulk Pt and mass-selected nanoparticles of comparable size. Also, the mass activities agree well with the literature values. The results provide new insights into the growth processes of SoL-synthesized carbon-supported Pt catalyst nanoparticles, and most crucially, the high performance of the synthesized catalyst layers, along with the possibility of nanoparticle growth through a straightforward heat-treatment step at relatively low temperatures, offer a scalable new approach for producing fuel cell catalysts with more efficient material utilization and new material combinations.

A review of physical, chemical and biological synthesis methods of bimetallic nanoparticles and applications in sensing, water treatment, biomedicine, catalysis and hydrogen storage

This article provides an in-depth analysis of various fabrication methods of bimetallic nanoparticles (BNP), including chemical, biological, and physical techniques. The review explores BNP's diverse uses, from well-known applications such as sensing water treatment and biomedical uses to less-studied areas like breath sensing for diabetes monitoring and hydrogen storage. It cites results from over 1000 researchers worldwide and >300 peer-reviewed articles. Additionally, the article discusses current trends, actionable recommendations, and the importance of synthetic analysis for industry players looking to optimize manufacturing techniques for specific applications. The article also evaluates the pros and cons of various fabrication methods, highlighting the potential of plant extract synthesis for mass production of capped BNPs. However, it warns that this method may not be suitable for certain applications requiring ligand-free surfaces. In contrast, physical methods like laser ablation offer better control and reactivity, especially for applications where ligand-free surfaces are critical. The report underscores the environmental benefits of plant extract synthesis compared to chemical methods that use hazardous chemicals and pose risks to extraction, production, and disposal. The article emphasizes the need for life cycle assessment (LCA) articles in the literature, given the growing volume of research on nanotechnology materials. This article caters to researchers at all stages and applies to various fields applying nanomaterials.

Magnetron sputtering onto nonionic surfactant for 1-step preparation of metal nanoparticles without additional chemical reagents

Plasma-based sputtering onto liquids (SoL) is a straightforward approach for synthesizing small metal nanoparticles (NPs) without additional stabilizing reagents. In this work, nonionic surfactant Triton X-100 was used for the first time as a host liquid for the SoL process and the production of colloidal solutions of gold, silver and copper NPs was demonstrated. The average diameter of spherical Au NPs lies in the range from 2.6 to 5.5 nm depending on the conditions. The approach presented here opens the pathway to the production of concentrated dispersions of metal NPs of high purity that can be dispersed in water for future usage, therefore extending further the reach of this synthesis pathway.

Controlled gold-palladium cores in ceria hollow spheres as nanoreactor for plasmon-enhanced catalysis under visible light irradiation

Visible-light-driven organic transformations boosting by localized surface plasmon resonance (LSPR) have been attracting considerable interests. Gold-palladium (Au-Pd) bimetallic nanoparticles (NPs) are considered as ideal plasmonic catalysts realizing efficient light-driven catalysis. Nevertheless, stability and adjustability of plasmonic Au-Pd NPs remain to be a challenging task. Herein, we designed the controlled Au-Pd cores in ceria (CeO₂) hollow spheres (Au-Pd@h-CeO₂) as nanoreactor for Suzuki cross-coupling reactions. Under visible light irradiation, the Au-Pd@h-CeO₂ exhibited remarkable photocatalytic performance with a turnover frequency (TOF) value as high as 797 h^-1. More impressively, the coupling reactions of aryl chlorides bearing electron-withdrawing groups proceeded better and afforded the corresponding desired products in good yields. Detailed structural, optical and photoelectrochemical characterizations unraveled that the enhanced photocatalytic efficiency of Au-Pd@h-CeO₂ was attributed to the LSPR effect of controllable Au-Pd cores and their synergetic effect of hollow CeO₂ shells. The merits of this hollow sphere architecture lied on as followed: (I) Incident light could be reflected and refracted between the inner cores and outer shells, which extended the trapping of incident light, and then enhanced the light harvesting efficiency; (II) the mesoporous architecture of CeO₂ hollow spheres provided a huge specific surface area and numerous mesoporous channels, which could enhance the absorption of reactants and provided more active sites; (III) LSPR excitation of Au-Pd NPs and band-gap excitation of CeO₂ simultaneously occurred under visible light illumination, inducing a more efficient separation and transfer of charge carriers. Furthermore, due to the confinment effect of CeO₂ shells, the Au-Pd@h-CeO₂ exhibited an excellent reusability after six cycles without significant deactivation of yield. Our findings provided a facile way to design highly efficient plasmonic-enhanced photocatalysts utilized for catalytic organic reactions.

Plasmonic Ag/Cu/PEG nanofluids prepared when solids meet liquids in the gas phase

Since the time of Faraday's experiments, the optical response of plasmonic nanofluids has been tailored by the shape, size, concentration, and material of nanoparticles (NPs), or by mixing different types of NPs. To date, water-based liquids have been the most extensively investigated host media, while polymers, such as poly(ethylene glycol) (PEG), have frequently been added to introduce repulsive steric interactions and protect NPs from agglomeration. Here, we introduce an inverse system of non-aqueous nanofluids, in which Ag and Cu NPs are dispersed in PEG (400 g mol^-1), with no solvents or chemicals involved. Our single-step approach comprises the synthesis of metal NPs in the gas phase using sputtering-based gas aggregation cluster sources, gas flow transport of NPs, and their deposition (optionally simultaneous) on the PEG surface. Using computational fluid dynamics simulations, we show that NPs diffuse into PEG at an average velocity of the diffusion front of the order of μm s^-1, which is sufficient for efficient loading of the entire polymer bulk. We synthesize yellow Ag/PEG, green Cu/PEG, and blue Ag/Cu/PEG nanofluids, in which the color is given by the position of the plasmon resonance. NPs are prone to partial agglomeration and sedimentation, with a slower kinetics for Cu. Density functional theory calculations combined with UV-vis data and zeta-potential measurements prove that the surface oxidation to Cu₂O and stronger electrostatic repulsion are responsible for the higher stability of Cu NPs. Adopting the De Gennes formalism, we estimate that PEG molecules adsorb on the NP surface in mushroom coordination, with the thickness of the adsorbed layer L < 1.4 nm, grafting density σ < 0.20, and the average distance between the grafted chains D > 0.8 nm. Such values provide sufficient steric barriers to retard, but not completely prevent, agglomeration. Overall, our approach offers an excellent platform for fundamental research on non-aqueous nanofluids, with metal-polymer and metal-metal interactions unperturbed by the presence of solvents or chemical residues.

Sputtering onto liquids: a critical review

Sputter deposition of atoms onto liquid substrates aims at producing colloidal dispersions of small monodisperse ultrapure nanoparticles (NPs). Since sputtering onto liquids combines the advantages of the physical vapor deposition technique and classical colloidal synthesis, the review contains chapters explaining the basics of (magnetron) sputter deposition and the formation of NPs in solution. This review article covers more than 132 papers published on this topic from 1996 to September 2021 and aims at providing a critical analysis of most of the reported data; we will address the influence of the sputtering parameters (sputter power, current, voltage, sputter time, working gas pressure, and the type of sputtering plasma) and host liquid properties (composition, temperature, viscosity, and surface tension) on the NP formation as well as a detailed overview of the properties and applications of the produced NPs.

Synthesis and potential applications of trimetallic nanostructures

The present review highlights the synthetic strategies and potential applications of TMNs for organic reactions, environmental remediation, and health-related activities.

Control of nanoparticles synthesized via vacuum sputter deposition onto liquids: a review

Sputter deposition onto a low volatile liquid matrix is a recently developed green synthesis method for metal/metal oxide nanoparticles (NPs). In this review, we introduce the synthesis method and highlight its unique features emerging from the combination of the sputter deposition and the ability of the liquid matrix to regulate particle growth. Then, manipulating the synthesis parameters to control the particle size, composition, morphology, and crystal structure of NPs is presented. Subsequently, we evaluate the key experimental factors governing the particle characteristics and the formation of monometallic and alloy NPs to provide overall directions and insights into the preparation of NPs with desired properties. Following that, the current understanding of the growth and formation mechanism of sputtered particles in liquid media, in particular, ionic liquids and liquid polymers, during and after sputtering is emphasized. Finally, we discuss the challenges that remain and share our perspectives on the future prospects of the synthesis method and the obtained NPs.

Gas Phase Synthesis of Multi-Element Nanoparticles

The advantages of gas-phase synthesis of nanoparticles in terms of size control and flexibility in choice of materials is well known. There is increasing interest in synthesizing multi-element nanoparticles in order to optimize their performance in specific applications, and here, the flexibility of material choice is a key advantage. Mixtures of almost any solid materials can be manufactured and in the case of core-shell particles, there is independent control over core size and shell thickness. This review presents different methods of producing multi-element nanoparticles, including the use of multiple targets, alloy targets and in-line deposition methods to coat pre-formed cores. It also discusses the factors that produce alloy, core-shell or Janus morphologies and what is possible or not to synthesize. Some applications of multi-element nanoparticles in medicine will be described.

Blurring the boundary between homogenous and heterogeneous catalysis using palladium nanoclusters with dynamic surfaces

Using a magnetron sputtering approach that allows size-controlled formation of nanoclusters, we have created palladium nanoclusters that combine the features of both heterogeneous and homogeneous catalysts. Here we report the atomic structures and electronic environments of a series of metal nanoclusters in ionic liquids at different stages of formation, leading to the discovery of Pd nanoclusters with a core of ca. 2 nm surrounded by a diffuse dynamic shell of atoms in [C₄C₁Im][NTf₂]. Comparison of the catalytic activity of Pd nanoclusters in alkene cyclopropanation reveals that the atomically dynamic surface is critically important, increasing the activity by a factor of ca. 2 when compared to compact nanoclusters of similar size. Catalyst poisoning tests using mercury and dibenzo[a,e]cyclooctene show that dynamic Pd nanoclusters maintain their catalytic activity, which demonstrate their combined features of homogeneous and heterogeneous catalysts within the same material. Additionally, kinetic studies of cyclopropanation of alkenes mediated by the dynamic Pd nanoclusters reveal an observed catalyst order of 1, underpinning the pseudo-homogeneous character of the dynamic Pd nanoclusters.

Establishing a structure-property relationship for nanoclusters and the link with their catalytic performance remain challenging. Here the authors show palladium nanocluster with a core of 2 nm surrounded by a diffuse dynamic shell of Pd atoms exhibit features of heterogeneous and homogenous catalyst at the same time.

Plasma-Induced Heating Effects on Platinum Nanoparticle Size During Sputter Deposition Synthesis in Polymer and Ionic Liquid Substrates

Nanoparticle catalyst materials are becoming ever more important in a sustainable future. Specifically, platinum (Pt) nanoparticles have relevance in catalysis, in particular, fuel cell technologies. Sputter deposition into liquid substrates has been shown to produce nanoparticles without the presence of air and other contaminants and the need for precursors. Here, we produce Pt nanoparticles in three imidazolium-based ionic liquids and PEG 600. All Pt nanoparticles are crystalline and around 2 nm in diameter. We show that while temperature has an effect on particle size for Pt, it is not as great as for other materials. Sputtering power, time, and postheat treatment all show slight influence on the particle size, indicating the importance of temperature during sputtering. The temperature of the liquid substrate is measured and reaches over 150 °C during deposition which is found to increase the particle size by less than 20%, which is small compared to the effect of temperature on Au nanoparticles presented in the literature. High temperatures during Pt sputtering are beneficial for increasing Pt nanoparticle size beyond 2 nm. Better temperature control would allow for more control over the particle size in the future.

Pt/Ag Solid Solution Alloy Nanoparticles in Miscibility Gaps Synthesized by Cosputtering onto Liquid Polymers

Pt/Ag solid solution alloy nanoparticles (NPs) with mean size below 3 nm were obtained with composition in miscibility gaps by cosputtering onto liquid polyethylene glycol (PEG, MW = 600). Adjusting the sputtering currents from 10 to 50 mA did not influence the particle sizes obviously but caused a substantial difference in the composition and distributions of Pt/Ag NPs. This is different from sputtered Pt/Au NPs where particle size is correlated with composition. For a pair of sputtering currents, the formed Pt/Ag alloy NPs have a range of compositions. The normal distribution with Pt of 60.2 ± 16.2 at % is observed for the Pt/Ag sample with a nominal Pt content of 55.9 at %, whereas Pt-rich (85.1 ± 14.0 at % Pt) and Ag-rich (19.8 ± 12.2 at % Pt) Pt/Ag samples with nominal Pt contents of 90.9 and 11.9 at % contain more pure Pt and pure Ag NPs, respectively. Different from NPs obtained in PEG, the sputtered NPs on TEM grids had more uniform composition for a longer sputtering time along with a significant increase of particle sizes. This reveals that PEG hindered the combination of NPs and clusters, resulting in small particle sizes even for long time sputtering and broader composition distributions. Thus, the samples obtained in PEG have the compositions mainly determined by the random atom combination in the vacuum chamber and possibly in initial landing of atom/clusters on the PEG surface.

Co-sputtering of gold and copper onto liquids: a route towards the production of porous gold nanoparticles

Effective methods for the synthesis of high-purity nanoparticles (NPs) have been extensively studied for a few decades. Among others, cold plasma-based sputtering metals onto a liquid substrate appears to be a very promising technique for the synthesis of high-purity NPs. The process enables the production of very small NPs without using any toxic reagents and complex chemical synthesis routes, and enables the synthesis of alloy NPs which can be the first step towards the formation of porous NPs. In this paper, the synthesis of gold-copper alloy NPs has been performed by co-sputtering gold and copper targets over pentaerythritol ethoxylate. The resulting solutions contain a mixture of gold, copper oxide, and alloy NPs having a radius of few angstroms. The annealing of these NPs, inside the solution, has been performed in order to increase their size and further induce the dealloying of the Au-Cu NPs. The resulting NPs exhibit either a nanoporous structure or are self-organized in an agglomerate of small NPs.