Control of the anisotropic shape of cobalt nanorods in the liquid phase: from experiment to theory… and back

Anisotropy in magnetic materials for sensors and actuators in soft robotic systems

The field of soft intelligent robots has rapidly developed, revealing extensive potential of these robots for real-world applications. By mimicking the dexterities of organisms, robots can handle delicate objects, access remote areas, and provide valuable feedback on their interactions with different environments. For autonomous manipulation of soft robots, which exhibit nonlinear behaviors and infinite degrees of freedom in transformation, innovative control systems integrating flexible and highly compliant sensors should be developed. Accordingly, sensor-actuator feedback systems are a key strategy for precisely controlling robotic motions. The introduction of material magnetism into soft robotics offers significant advantages in the remote manipulation of robotic operations, including touch or touchless detection of dynamically changing shapes and positions resulting from the actuations of robots. Notably, the anisotropies in the magnetic nanomaterials facilitate the perception and response with highly selective, directional, and efficient ways used for both sensors and actuators. Accordingly, this review provides a comprehensive understanding of the origins of magnetic anisotropy from both intrinsic and extrinsic factors and summarizes diverse magnetic materials with enhanced anisotropy. Recent developments in the design of flexible sensors and soft actuators based on the principle of magnetic anisotropy are outlined, specifically focusing on their applicabilities in soft robotic systems. Finally, this review addresses current challenges in the integration of sensors and actuators into soft robots and offers promising solutions that will enable the advancement of intelligent soft robots capable of efficiently executing complex tasks relevant to our daily lives.

Gaussian attractive potential for carboxylate/cobalt surface interactions

Ligand-decorated metal surfaces play a pivotal role in various areas of chemistry, particularly in selective catalysis. Molecular dynamics simulations at the molecular mechanics level of theory are best adapted to gain complementary insights to experiments regarding the structure and dynamics of such organic films. However, standard force fields tend to capture only weak physisorption interactions. This is inadequate for ligands that are strongly adsorbed such as carboxylates on metal surfaces. To address this limitation, we employ the Gaussian Lennard-Jones (GLJ) potential, which incorporates an attractive Gaussian potential between the surface and ligand atoms. Here, we develop this approach for the interaction between cobalt surfaces and carboxylate ligands. The accuracy of the GLJ approach is validated through the analysis of the interaction of oxygen with two distinct cobalt surfaces. The accuracy of this method reaches a root mean square deviation (RMSD) of about 3 kcal/mol across all probed configurations, which corresponds to a percentage error of roughly 4%. Application of the GLJ force field to the dynamics of the organic layer on these surfaces reveals how the ligand concentration influences the film order, and highlights differing mobility in the x and y directions, attributable to surface corrugation on Co(112̄0). GLJ is versatile, suitable for a broad range of metal/ligand systems, and can, subsequently, be utilized to study the organic film on the adsorption/desorption of reactants and products during a catalytic process.

Magnetic Nanoparticles: Synthesis, Anisotropy, and Applications

Anisotropy is an important and widely present characteristic of materials that provides desired direction-dependent properties. In particular, the introduction of anisotropy into magnetic nanoparticles (MNPs) has become an effective method to obtain new characteristics and functions that are critical for many applications. In this review, we first discuss anisotropy-dependent ferromagnetic properties, ranging from intrinsic magnetocrystalline anisotropy to extrinsic shape and surface anisotropy, and their effects on the magnetic properties. We further summarize the syntheses of monodisperse MNPs with the desired control over the NP dimensions, shapes, compositions, and structures. These controlled syntheses of MNPs allow their magnetism to be finely tuned for many applications. We discuss the potential applications of these MNPs in biomedicine, magnetic recording, magnetotransport, permanent magnets, and catalysis.

DockOnSurf: A Python Code for the High-Throughput Screening of Flexible Molecules Adsorbed on Surfaces

We present the open-source python package DockOnSurf which automates the generation and optimization of low-energy adsorption configurations of molecules on extended surfaces and nanoparticles. DockOnSurf is especially geared toward handling polyfunctional flexible adsorbates. The use of this high-throughput workflow allows us to carry out the screening of adsorbate-surface configurations in a systematic, customizable, and traceable way, while keeping the focus on the chemically relevant structures. The screening strategy consists in splitting the exploration of the adsorbate-surface configurational space into chemically meaningful domains, that is, by choosing among different conformers to adsorb, surface adsorption sites, adsorbate anchoring points, and orientations and allowing dissociation of (acidic) protons. We demonstrate the performance of the main features based on varying examples, ranging from CO adsorption on a gold nanoparticle to sorbitol adsorption on hematite. Through the use of the presented program, we aim to foster efficiency, traceability, and ease of use in research within tribology, catalysis, nanoscience, and surface science in general.

Selection Criteria for Metal Precursors and Solvents for Targeted Synthesis of Metallic Nanostructures Via Kinetic Control in the Polyol Process

Development of a technology for the synthesis of monometallic or multimetallic nanoparticles is exceptionally vital for the preparation of novel magnetic, optical. and catalytic functional materials. In this context, the polyol process is a safe and scalable method for preparation of metal nanoparticles with controlled sizes and shapes in large scales. However, there is no systematic investigation that discusses the criteria for the selection of metal salt and solvent type that determine the kinetics of reduction reaction that influences the morphology of the particles. Consequently, the design of metallic nanoparticles, which is controlled by the kinetics and thermodynamics of the reduction reaction, has become difficult. In this paper, the selection criterion for metal salt precursor is established based on the presumption that the ligand of the metal precursor promotes the formation of active species of the solvent, and the criterion for the selection of the solvent type is based on the highest occupied molecular orbital (HOMO) energy value estimated using molecular orbital theory. The results suggested that the dissociation constants of metal salt precursors and HOMO energy of the polyol solvent can be tuned to control the kinetics of the reduction reaction. The reduction potential of polyol depends on the number of carbon atoms and the location of hydroxyl ligands within the molecule. Among the polyols considered in this study, 1,4-butanediol had the highest reduction potential. The predictions have been experimentally verified by synthesizing metallic Co and Fe nanoparticles. The findings could be extended to other techniques such as thermal decomposition and alcohol reduction for the synthesis of noble metal-transition metal magnetic and catalytic nanoparticles with novel properties.

On the importance of the crystalline surface structure on the catalytic activity and stability of tailored unsupported cobalt nanoparticles for the solvent-free acceptor-less alcohol dehydrogenation

Unsupported nanoparticles are now recognized as model catalysts to evaluate the intrinsic activity of metal particles, irrespectively of that of the support. Co nanoparticles with different morphologies, rods, diabolos and cubes have been prepared by the polyol process and tested for the acceptorless catalytic dehydrogenation of alcohols under solvent-free conditions. Rods crystallize with the pure hcp structure, diabolos with a mixture of hcp and fcc phases, while the cubes crystallize in a complex mixture of hcp, fcc and ε-Co phases. All the cobalt particles are found to be highly selective towards the oxidation of a model secondary alcohol, octan-2-ol, into the corresponding ketone while no significant activity is found with octan-1-ol. Our results show the strong influence of particle shape on the activity and catalytic stability of the catalysts: Co nanorods display the highest conversion (85%), selectivity (95%) and recyclability compared to Co diabolos and Co cubes. We correlate the nanorods excellent stability with a strong binding of carboxylate ligands on their {1 1 2¯ 0} facets, preserving their crystalline superficial structure, as evidenced by phase modulation infrared reflection absorption spectroscopy.

Magnetic quantification of single-crystalline Fe and Co nanowires via off-axis electron holography

Investigating the local micromagnetic structure of ferromagnetic nanowires (NWs) at the nanoscale is essential to study the structure-property relationships and can facilitate the design of nanostructures for technology applications. Herein, we synthesized high-quality iron and cobalt NWs and investigated the magnetic properties of these NWs using off-axis electron holography. The Fe NWs are about 100 nm in width and a few micrometers in length with a preferential growth direction of [100], while the Co NWs have a higher aspect-ratio with preferential crystal growth along the [110] direction. It is noted that compact passivation surface layers of oxides protect these NWs from further oxidation, even after nearly two years of exposure to ambient conditions; furthermore, these NWs display homogeneous ferromagnetism along their axial direction revealing the domination of shape anisotropy on magnetic behavior. Importantly, the average value of magnetic induction strengths of Fe NWs (2.07 {±} 0.10 T) and Co NWs (1.83 {±} 0.15 T) is measured to be very close to the respective theoretical value, and it shows that the surface oxide layers do not affect the magnetic moments in NWs. Our results provide a useful synthesis approach for the fabrication of single-crystalline, defect-free metal NWs and give insight into the micromagnetic properties in ferromagnetic NWs based on the transmission electron microscopy measurements.

Enhanced Magnetic Behavior of Cobalt Nano-Rods Elaborated by the Polyol Process Assisted with an External Magnetic Field

Cobalt nano-rods with the hexagonal close-packed (hcp) structure were prepared by reduction of the long-chain carboxylate Co (II) precursor in polyol. The application of an external magnetic field (µ₀H = 1.25 T) during the nucleation and growth steps resulted in a noticeable modification of the mean aspect ratio (length/diameter) of the particles. The particle morphology was also modified as the nano-rods did not exhibit conical heads at their extremities anymore, which are observed for particles prepared without application of an external magnetic field. Besides, the stacking faults density along the c axis of the hcp structure in the cobalt nano-rods has been found to decrease with the increase in the applied magnetic field. The coercive field of randomly oriented nano-rods increased with the aspect ratio, showing the highest value (i.e., 5.8 kOe at 300 K) for the cobalt nano-rods obtained under the highest applied magnetic field. For partially oriented Co nano-rods in toluene solution, the magnetic properties were significantly enhanced with a coercive field of 7.2 kOe at 140 K, while the magnetization saturation reached 92% of the bulk. The M_R/M_S value was about 0.8, indicating a good orientation of the anisotropic particles relative to each other, making them suitable for the preparation of permanent magnets via a bottom-up approach.

Importance of the decoration in shaped cobalt nanoparticles in the acceptor-less secondary alcohol dehydrogenation

Ligands matter for shaped decorated Co nanoparticles, at the frontier between homogeneous and heterogeneous catalysis.

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation

The one-step seed-mediated synthesis is widely used for the preparation of ferromagnetic metal nanoparticles (NPs) since it offers a good control of particle morphology. Nevertheless, this approach suffers from a lack of mechanistic studies because of the difficulties of following in real time the heterogeneous nucleation and predicting structure effects with seeds that are generated in situ. Here, we propose a complete scheme of the heteronucleation process involved in one-pot seed-mediated syntheses of cobalt nanoparticles in liquid polyols, relying on geometrical phase analysis (GPA) of high-resolution high-angle annular dark field (HAADF)-STEM images and in situ measurements of the molecular hydrogen evolution. Cobalt particles of different shapes (rods, platelets, or hourglass-like particles) were grown by reducing cobalt carboxylate in liquid polyols in the presence of iridium or ruthenium chloride as the nucleating agent. A reaction scheme was established by monitoring the H₂ evolution resulting from the decomposition of metal hydrides, formed in situ by β-elimination of metal alkoxides, and from the polyol dehydrogenation, catalytically activated by the metal particles. This is a very good probe for both the noble metal nucleation and the heterogeneous nucleation of cobalt, showing a good separation of these two steps. Ir and Ru seeds with a size in the range 1-2 nm were found exactly in the center of the cobalt particles, whatever the cobalt particle shape, and high-resolution images revealed an epitaxial growth of the hcp Co on fcc Ir or hcp Ru seeds. The microstructure analysis around the seeds made evident two different ways of relaxing the lattice mismatch between the seeds and the cobalt, with the presence of dislocations around the Ir seeds and compression zones of the cobalt lattice near the Ru seeds. The relationship between the nature of the nucleating agent, the reaction steps, and the microstructure is discussed.

The first one-pot synthesis of undoped and Eu doped β-NaYF4 nanocrystals and their evaluation as efficient dyes for nanomedicine

Polygonal-shaped about 75 nm sized and highly crystallized Eu³⁺-doped β-NaYF₄ particles were directly prepared under mild conditions using the polyol process. A set of operating parameters were optimized for such a purpose. A conventional heating under reflux for 30 min of a mixture of Y(III) and Eu(III) acetate, ammonium fluoride, sodium hydroxide and oleic acid (OA) dissolved in ethyleneglycol offered a pertinent material processing route if a large excess of NH₄F and an enough amount of OA were used. Typically, the former parameter provides an exclusive stabilization of the desired β allotropic form, while the latter allows a significant size decrease of the particles. Thanks to their coating by a double OA layer, the produced particles exhibited a hydrophilic surface feature when dispersed in water and when excited under UV light they emitted a very intense red photoluminescence. Additionally, they did not evidence any accurate cytotoxicity when incubated with healthy human foreskin fibroblast (BJH) cells for doses as high as 50 μg·mL^-1 and contact time as long as 48 h, highlighting the ability of the prepared particles to be used as efficient down-converter light sources for cell labelling.

The polyol process: a unique method for easy access to metal nanoparticles with tailored sizes, shapes and compositions

After about three decades of development, the polyol process is now widely recognized and practised as a unique soft chemical method for the preparation of a large variety of nanoparticles which can be used in important technological fields. It offers many advantages: low cost, ease of use and, very importantly, already proven scalability for industrial applications. Among the different classes of inorganic nanoparticles which can be prepared in liquid polyols, metals were the first reported. This review aims to give a comprehensive account of the strategies used to prepare monometallic nanoparticles and multimetallic materials with tailored size and shape. As regards monometallic materials, while the preparation of noble as well as ferromagnetic metals is now clearly established, the scope of the polyol process has been extended to the preparation of more electropositive metals, such as post-transition metals and semi-metals. The potential of this method is also clearly displayed for the preparation of alloys, intermetallics and core-shell nanostructures with a very large diversity of compositions and architectures.

Air-Stable Anisotropic Monocrystalline Nickel Nanowires Characterized Using Electron Holography

Nickel is capable of discharging electric and magnetic shocks in aerospace materials thanks to its conductivity and magnetism. Nickel nanowires are especially desirable for such an application as electronic percolation can be achieved without significantly increasing the weight of the composite material. In this work, single-crystal nickel nanowires possessing a homogeneous magnetic field are produced via a metal-organic precursor decomposition synthesis in solution. The nickel wires are 20 nm in width and 1-2 μm in length. The high anisotropy is attained through a combination of preferential crystal growth in the ⟨100⟩ direction and surfactant templating using hexadecylamine and stearic acid. The organic template ligands protect the nickel from oxidation, even after months of exposure to ambient conditions. These materials were studied using electron holography to characterize their magnetic properties. These thin nanowires display homogeneous ferromagnetism with a magnetic saturation (517 ± 80 emu cm^-3), which is nearly equivalent to that of bulk nickel (557 emu cm^-3). Nickel nanowires were incorporated into carbon composite test pieces and were shown to dramatically improve the electric discharge properties of the composite material.

Unsupported shaped cobalt nanoparticles as efficient and recyclable catalysts for the solvent-free acceptorless dehydrogenation of alcohols

Unsupported cobalt nanoparticles are active and chemoselective catalysts for the solvent-free acceptorless dehydrogenation of alcohols.

Applications, Surface Modification and Functionalization of Nickel Nanorods

The growing number of nanoparticle applications in science and industry is leading to increasingly complex nanostructures that fulfill certain tasks in a specific environment. Nickel nanorods already possess promising properties due to their magnetic behavior and their elongated shape. The relevance of this kind of nanorod in a complex measurement setting can be further improved by suitable surface modification and functionalization procedures, so that customized nanostructures for a specific application become available. In this review, we focus on nickel nanorods that are synthesized by electrodeposition into porous templates, as this is the most common type of nickel nanorod fabrication method. Moreover, it is a facile synthesis approach that can be easily established in a laboratory environment. Firstly, we will discuss possible applications of nickel nanorods ranging from data storage to catalysis, biosensing and cancer treatment. Secondly, we will focus on nickel nanorod surface modification strategies, which represent a crucial step for the successful application of nanorods in all medical and biological settings. Here, the immobilization of antibodies or peptides onto the nanorod surface adds another functionality in order to yield highly promising nanostructures.

Prediction of Co nanoparticle morphologies stabilized by ligands: towards a kinetic model

The morphology variation of hcp cobalt nanoparticles as a function of ligand concentration is predicted using a novel kinetic method.

Dense arrays of cobalt nanorods as rare-earth free permanent magnets

We demonstrate in this paper the feasibility to elaborate rare-earth free permanent magnets based on cobalt nanorods assemblies with energy product (BH)max exceeding 150 kJ m(-3). The cobalt rods were prepared by the polyol process and assembled from wet suspensions under a magnetic field. Magnetization loops of dense assemblies with remanence to a saturation of 0.99 and squareness of 0.96 were measured. The almost perfect M(H) loop squareness together with electron microscopy and small angle neutron scattering demonstrate the excellent alignment of the rods within the assemblies. The magnetic volume fraction was carefully measured by coupling magnetic and thermogravimetric analysis and found in the range from 45 to 55%, depending on the rod diameter and the alignment procedure. This allowed a quantitative assessment of the (BH)max values. The highest (BH)max of 165 kJ m(-3) was obtained for a sample combining a high magnetic volume fraction and a very large M(H) loop squareness. This study shows that this bottom-up approach is very promising to get new hard magnetic materials that can compete in the permanent magnet panorama and fill the gap between the ferrites and the NdFeB magnets.

Synthesis of shape-controllable cobalt nanoparticles and their shape-dependent performance in glycerol hydrogenolysis

Metallic Co nanorods were fabricated using Ir as a heterogeneous agent. The nanorods with mainly exposed {10−10} facets showed much higher activity and 1,3-propanediol selectivity in glycerol hydrogenolysis.