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.

A Self-Assembly of Single Layer of Co Nanorods to Reveal the Magnetostatic Interaction Mechanism

In this work, we report a self-assembly method to fabricate a single layer of Co nanorods to study their magnetostatic interaction behavior. The Co nanorods with cambered and flat tips were synthesized by using a solvothermal route and an alcohol–thermal method, respectively. Both of them represent hard magnetic features. Co nanorods with cambered tips have an average diameter of 10 nm and length of 100 nm with coercivity of 6.4 kOe, and flat-tip nanorods with a 30 nm diameter and 100 nm length exhibit a coercivity of 4.9 kOe. They are further assembled on the surface of water in assistance of surfactants. The results demonstrate that the assembly type is dependent on the magnetic induction lines direction. For Co nanorods with flat tips, most of magnetic induction lines are parallel to the length direction, leading to an assembly that is tip to tip. For Co nanorods with cambered tips, they are prone to holding together side by side for their random magnetic induction lines. Under an applied field, the Co nanorods with flat tips can be further aligned into a single layer of Co nanorods. Our work gives a possible mechanism for the magnetic interaction of Co nanorods and provides a method to study their magnetic behavior.

Formation of Bi2Ir nanoparticles in a microwave-assisted polyol process revealing the suboxide Bi4Ir2O

Intermetallic phases are usually obtained by crystallization from the melt. However, phases containing elements with widely different melting and boiling points, as well as nanoparticles, which provide a high specific surface area, are hardly accessible via such a high-temperature process. The polyol process is one option to circumvent these obstacles by using a solution-based approach at moderate temperatures. In this study, the formation of Bi₂Ir nanoparticles in a microwave-assisted polyol process was investigated. Solutions were analyzed using UV-Vis spectroscopy and the reaction was tracked with synchrotron-based in situ powder X-ray diffraction (PXRD). The products were characterized by PXRD and high-resolution transmission electron microscopy. Starting from Bi(NO₃)₃ and Ir(OAc)₃, the new suboxide Bi₄Ir₂O forms as an intermediate phase at about 160 °C. Its structure was determined by a combination of PXRD and quantum-chemical calculations. Bi₄Ir₂O decomposes in vacuum at about 250 °C and is reduced to Bi₂Ir by hydrogen at 150 °C. At about 240 °C, the polyol process leads to the immediate reduction of the two metal-containing precursors and crystallization of Bi₂Ir nanoparticles.

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.

Advanced Thermoelectric Design: From Materials and Structures to Devices

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

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 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.

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.