Bimetallic Nanoalloy Catalysts for Green Energy Production: Advances in Synthesis Routes and Characterization Techniques

Bimetallic Nanoalloy catalysts have diverse uses in clean energy, sensing, catalysis, biomedicine, and energy storage, with some supported and unsupported catalysts. Conventional synthetic methods for producing bimetallic alloy nanoparticles often produce unalloyed and bulky particles that do not exhibit desired characteristics. Alloys, when prepared with advanced nanoscale methods, give higher surface area, activity, and selectivity than individual metals due to changes in their electronic properties and reduced size. This review demonstrates the synthesis methods and principles to produce and characterize highly dispersed, well-alloyed bimetallic nanoalloy particles in relatively simple, effective, and generalized approaches and the overall existence of conventional synthetic methods with modifications to prepare bimetallic alloy catalysts. The basic concepts and mechanistic understanding are represented with purposely selected examples. Herein, the enthralling properties with widespread applications of nanoalloy catalysts in heterogeneous catalysis are also presented, especially for Hydrogen Evolution Reaction (HER), Oxidation Reduction Reaction (ORR), Oxygen Evolution Reaction (OER), and alcohol oxidation with a particular focus on Pt and Pd-based bimetallic nanoalloys and their numerous fields of applications. The high entropy alloy is described as a complicated subject with an emphasis on laser-based green synthesis of nanoparticles and, in conclusion, the forecasts and contemporary challenges for the controlled synthesis of nanoalloys are addressed.

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.

Microplasma Synthesis of Antibacterial Active Silver Nanoparticles in Sodium Polyacrylate Solutions

The great demand for functional, particularly biologically active, metal nanoparticles has led to the search for technologically effective, green, and controlled methods of synthesizing these metal nanoparticles. Plasma glow discharge is one of the most promising techniques in this direction. The results of studies based on the synthesis of colloidal solutions of stabilized silver nanoparticles (AgNPs) by the microplasma method in solutions of a nontoxic surfactant sodium polyacrylate (NaPA) are presented. It is shown that AgNPs with a size of 2-20 nm are formed in solutions of 0.05-0.2 mmol·L-1 AgNO3 + 5 g·L-1 NaPA at U = 250 V by tungsten cathode plasma glow discharge. At 20°C, the yellow solutions are formed with λ max ≈ 410 nm, which are stable during long-term storage. It was found that the process of AgNPs formation corresponds to a first-order reaction on the AgNO3 concentration. Its value has little effect on the geometry of nanoparticles, so the Ag(I) concentration in solution is one of the main factors influencing the rate of microplasma synthesis of AgNPs. The antimicrobial activity of synthesized AgNPs solutions against strains of Escherichia coli, Staphylococcus aureus, and Candida albicans was established.

Insights into Characterization Methods and Biomedical Applications of Nanoparticle-Protein Corona

Nanoparticles (NPs) exposed to a biological milieu will strongly interact with proteins, forming "coronas" on the surfaces of the NPs. The protein coronas (PCs) affect the properties of the NPs and provide a new biological identity to the particles in the biological environment. The characterization of NP-PC complexes has attracted enormous research attention, owing to the crucial effects of the properties of an NP-PC on its interactions with living systems, as well as the diverse applications of NP-PC complexes. The analysis of NP-PC complexes without a well-considered approach will inevitably lead to misunderstandings and inappropriate applications of NPs. This review introduces methods for the characterization of NP-PC complexes and investigates their recent applications in biomedicine. Furthermore, the review evaluates these characterization methods based on comprehensive critical views and provides future perspectives regarding the applications of NP-PC complexes.

Preparation and Growth Mechanism of Pt/Cu Alloy Nanoparticles by Sputter Deposition onto a Liquid Polymer

We use a green sputtering technique to deposit a Pt/Cu alloy target on liquid polyethylene glycol (PEG) to obtain well-dispersed and stable Pt₂₉Cu₇₁ alloy nanoparticles (NPs). The effects of sputtering current, rotation speed of the stirrer, sputtering time, sputtering period, and temperature of PEG on the particle size are studied systematically. Our key results demonstrate that the aggregation and growth of Pt/Cu alloy NPs occurred at the surface as well as inside the liquid polymer after the particles landed on the liquid surface. According to particle size analysis, a low sputtering current, high rotation speed for the stirrer, short sputtering period, and short sputtering time are found to be favorable for producing small-sized single crystalline alloy NPs. On the other hand, varying the temperature of the liquid PEG does not have any significant impact on the particle size. Thus, our findings shed light on controlling NP growth using the newly developed green sputtering deposition technique.

Langmuir Nanoarchitectonics from Basic to Frontier

Methodology to combine nanotechnology and these organization processes has been proposed as a novel concept of nanoarchitectonics, which can fabricate functional materials with nanolevel units. As an instant nanoarchitectonics approach, confining systems within a two-dimensional plane to drastically reduce translational motion freedom can be regarded as one of the rational approaches. Supramolecular chemistry and nanofabrication and their related functions at the air-water interface with the concept of nanoarchitectonics would lead to the creation of a novel methodology of Langmuir nanoarchitectonics. In this feature article, we briefly summarize research efforts related to Langmuir nanoarchitectonics including the basics for anomalies in molecular interactions such as highly enhanced molecular recognition capabilities. It is also extended to frontiers including the fabrication of supramolecular receptors and two-dimensional patterns with subnanometer-scale structural regulation, manual control of molecular machines and receptors by hand-motion-like macroscopic actions, and the regulation of cell fates at nanoarchitected arrays of nanocarbon assemblies and at direct liquid interfaces.

Materials nanoarchitectonics at two-dimensional liquid interfaces

Much attention has been paid to the synthesis of low-dimensional materials from small units such as functional molecules. Bottom-up approaches to create new low-dimensional materials with various functional units can be realized with the emerging concept of nanoarchitectonics. In this review article, we overview recent research progresses on materials nanoarchitectonics at two-dimensional liquid interfaces, which are dimensionally restricted media with some freedoms of molecular motion. Specific characteristics of molecular interactions and functions at liquid interfaces are briefly explained in the first parts. The following sections overview several topics on materials nanoarchitectonics at liquid interfaces, such as the preparation of two-dimensional metal-organic frameworks and covalent organic frameworks, and the fabrication of low-dimensional and specifically structured nanocarbons and their assemblies at liquid-liquid interfaces. Finally, interfacial nanoarchitectonics of biomaterials including the regulation of orientation and differentiation of living cells are explained. In the recent examples described in this review, various materials such as molecular machines, molecular receptors, block-copolymer, DNA origami, nanocarbon, phages, and stem cells were assembled at liquid interfaces by using various useful techniques. This review overviews techniques such as conventional Langmuir-Blodgett method, vortex Langmuir-Blodgett method, liquid-liquid interfacial precipitation, instructed assembly, and layer-by-layer assembly to give low-dimensional materials including nanowires, nanowhiskers, nanosheets, cubic objects, molecular patterns, supramolecular polymers, metal-organic frameworks and covalent organic frameworks. The nanoarchitecture materials can be used for various applications such as molecular recognition, sensors, photodetectors, supercapacitors, supramolecular differentiation, enzyme reactors, cell differentiation control, and hemodialysis.

Catalytic activity of nickel nanoparticles stabilized by adsorbing polymers for enhanced carbon sequestration

This work shows the potential of nickel (Ni) nanoparticles (NPs) stabilized by polymers for accelerating carbon dioxide (CO₂) dissolution into saline aquifers. The catalytic characteristics of Ni NPs were investigated by monitoring changes in diameter of CO₂ microbubbles. An increase in ionic strength considerably reduces an electrostatic repulsive force in pristine Ni NPs, thereby decreasing their catalytic potential. This study shows how cationic dextran (DEX), nonionic poly(vinyl pyrrolidone) (PVP), and anionic carboxy methylcellulose (CMC) polymers, the dispersive behaviors of Ni NPs can be used to overcome the negative impact of salinity on CO₂ dissolution. The cationic polymer, DEX was less adsorbed onto NPs surfaces, thereby limiting the Ni NPs’ catalytic activity. This behavior is due to a competition for Ni NPs’ surface sites between the cation and DEX under high salinity. On the other hand, the non/anionic polymers, PVP and CMC could be relatively easily adsorbed onto anchoring sites of Ni NPs by the monovalent cation, Na⁺. Considerable dispersion of Ni NPs by an optimal concentration of the anionic polymers improved their catalytic capabilities even under unfavorable conditions for CO₂ dissolution. This study has implications for enhancing geologic sequestration into deep saline aquifers for the purposes of mitigating atmospheric CO₂ levels.

Surface Modifications of Nanoparticles for Stability in Biological Fluids

Due to the high surface: volume ratio and the extraordinary properties arising from the nanoscale (optical, electric, magnetic, etc.), nanoparticles (NPs) are excellent candidates for multiple applications. In this context, nanoscience is opening a wide range of modern technologies in biological and biomedical fields, among others. However, one of the main drawbacks that still delays its fast evolution and effectiveness is related to the behavior of nanomaterials in the presence of biological fluids. Unfortunately, biological fluids are characterized by high ionic strengths which usually induce NP aggregation. Besides this problem, the high content in biomacromolecules—such as lipids, sugars, nucleic acids and, especially, proteins—also affects NP stability and its viability for some applications due to, for example, the formation of the protein corona around the NPs. Here, we will review the most common strategies to achieve stable NPs dispersions in high ionic strength fluids and, also, antifouling strategies to avoid the protein adsorption.

Effect of the Sodium Polyacrylate on the Magnetite Nanoparticles Produced by Green Chemistry Routes: Applicability in Forward Osmosis

Aqueous dispersions of magnetic nanocomposites have been proposed as draw electrolytes in forward osmosis. One possible approach for the production of nanocomposites based on magnetite nanoparticles and sodium polyacrylate is the synthesis of the magnetic iron oxide by coprecipitation or oxidative precipitation in the presence of an excess of the polymer. In this work, we explored the effect of the polymer proportion on the nanomaterials produced by these procedures. The materials obtained were compared with those obtained by the coating of magnetite nanocrystals produced beforehand with the same polymer. The samples were characterized by chemical analysis, photon correlation spectroscopy, thermogravimetry, X-ray diffraction, infrared spectroscopy, transmission electron microscopy, and magnetometry. The general trend observed is that the polymers heavily modify the texture of the magnetic material during the synthesis, with a drastic reduction of the particle size and magnetic response. The polycrystalline texture that is generated permits the incorporation of the polymer both on the external surface and in the intergranular space. The aqueous dispersions of the nanocomposites were highly stable, with a hydrodynamic size that was roughly independent of the polymer/magnetite ratio. Such dispersions show an osmotic pressure that is proportional to the concentration of the polymer. Interestingly, the proportionality constant was similar to that of the free polymer only in the case of the samples prepared by oxidative precipitation, being lower in the case of the samples prepared by coprecipitation. Finally, the possibilities of using these materials as draw electrolytes in forward osmosis will be briefly discussed.

The W alloying effect on thermal stability and hardening of nanostructured Cu-W alloyed thin films

In order to achieve desired mechanical properties of alloys by manipulating grain boundaries (GBs) via solute decoration, it is of great significance to understand the underlying mechanisms of microstructural evolution and plastic deformation. In this work, nanocrystalline (NC) Cu-W alloyed films with W concentrations spanning from 0 to 40 at% were prepared by using magnetron sputtering. Thermal stability (within the temperature range of 200 °C-600 °C) and hardness of the films were investigated by using the x-ray diffraction, transmission electron microscope (TEM) and nanoindentation, respectively. The NC pure Cu film exhibited substantial grain growth upon all annealing temperatures. The Cu-W alloyed films, however, displayed distinct microstructural evolution that depended not only on the W concentration but also on the annealing temperature. At a low temperature of 200 °C, all the Cu-W alloyed films were highly stable, with unconspicuous change in grain sizes. At high temperatures of 400 °C and 600 °C, the microstructural evolution was greatly controlled by the W concentrations. The Cu-W films with low W concentration manifested abnormal grain growth (AGG), while the ones with high W concentrations showed phase separation. TEM observations unveiled that the AGG in the Cu-W alloyed thin films was rationalized by GB migration. Nanoindentation results showed that, although the hardness of both the as-deposited and annealed Cu-W alloyed thin films monotonically increased with W concentrations, a transition from annealing hardening to annealing softening was interestingly observed at the critical W addition of ∼25 at%. It was further revealed that an enhanced GB segregation associated with detwinning was responsible for the annealing hardening, while a reduced solid solution hardening for the annealing softening.

Ligand free green plasma-in-liquid synthesis of Au/Ag alloy nanoparticles

Au/Ag alloy nanoparticles were successfully prepared by a microwave-induced plasma in liquid process without any organic protecting or reducing agents.

Matrix Sputtering Method: A Novel Physical Approach for Photoluminescent Noble Metal Nanoclusters

Noble metal nanoclusters are believed to be the transition between single metal atoms, which show distinct optical properties, and metal nanoparticles, which show characteristic plasmon absorbance. The interesting properties of these materials emerge when the particle size is well below 2 nm, such as photoluminescence, which has potential application particularly in biomedical fields. These photoluminescent ultrasmall nanoclusters are typically produced by chemical reduction, which limits their practical application because of the inherent toxicity of the reagents used in this method. Thus, alternative strategies are sought, particularly in terms of physical approaches, which are known as "greener alternatives," to produce high-purity materials at high yields. Thus, a new approach using the sputtering technique was developed. This method was initially used to produce thin films using solid substrates; now it can be applied even with liquid substrates such as ionic liquids or polyethylene glycol as long as these liquids have a low vapor pressure. This revolutionary development has opened up new areas of research, particularly for the synthesis of colloidal nanoparticles with dimensions below 10 nm. We are among the first to apply the sputtering technique to the physical synthesis of photoluminescent noble metal nanoclusters. Although typical sputtering systems have relied on the effect of surface composition and viscosity of the liquid matrix on controlling particle diameters, which only resulted in diameters ca. 3-10 nm, that were all plasmonic, our new approach introduced thiol molecules as stabilizers inspired from chemical methods. In the chemical syntheses of metal nanoparticles, controlling the concentration ratio between metal ions and stabilizing reagents is a possible means of systematic size control. However, it was not clear whether this would be applicable in a sputtering system. Our latest results showed that we were able to generically produce a variety of photoluminescent monometallic nanoclusters of Au, Ag, and Cu, all of which showed stable emission in both solution and solid form via our matrix sputtering method with the induction of cationic-, neutral-, and anionic-charged thiol ligands. We also succeeded in synthesizing photoluminescent bimetallic Au-Ag nanoclusters that showed tunable emission within the UV-NIR region by controlling the composition of the atomic ratio by a double-target sputtering technique. Most importantly, we have revealed the formation mechanism of these unique photoluminescent nanoclusters by sputtering, which had relatively larger diameters (ca. 1-3 nm) as determined using TEM and stronger emission quantum yield (max. 16.1%) as compared to typical photoluminescent nanoclusters prepared by chemical means. We believe the high tunability of sputtering systems presented here has significant advantages for creating novel photoluminescent nanoclusters as a complementary strategy to common chemical methods. This Account highlights our journey toward understanding the photophysical properties and formation mechanism of photoluminescent noble metal nanoclusters via the sputtering method, a novel strategy that will contribute widely to the body of scientific knowledge of metal nanoparticles and nanoclusters.

Small Nanosized Oxygen-Deficient Tungsten Oxide Particles: Mechanistic Investigation with Controlled Plasma Generation in Water for Their Preparation

Production of oxygen-deficient tungsten oxide nanoparticles with a diameter of around 10 nm have been successfully developed using a microwave-induced plasma in liquid technique. The prepared blue-green nanoparticles exhibit strong absorption in the visible region; thus, these could be efficient visible-light photocatalysts. The high-angle annular dark-field images revealed the dislocation of tungsten, which causes oxygen deficiencies.