High-entropy-alloy nanoparticles (HEA-NPs) are nanoparticles having five or more elements alloyed in a single-phase solid solution structure. [1] HEA-NPs possess a wide range of compositional library, distinct alloy mixing structure, and nanoscale size effect, giving them huge potential in catalysis, energy, environmental, and biomedical applications.
HEA-NPs are a structural analog to bulk high-entropy alloys (HEAs), [2] [3] but synthesized at the nanoscale. The formation of HEAs typically requires high temperature for multi-element mixing; however, high temperature acts against nano-material synthesis due to high-temperature-induced structure aggregation and surface reconstruction.
In 2018, HEA-NPs were firstly synthesized by a carbothermal shock synthesis. [1] (The material and technology are patented. [4] [5] ) The carbothermal shock employs a rapid high-temperature heating (e.g. 2000 K, in 55 ms) to enable the non-equilibrium synthesis of HEA-NPs with uniform size and homogeneous mixing despite containing immiscible combinations. Although rapid quenching is desired to maintain the solid-solution state, too fast cooling rate can hinder structural ordering. Therefore, the cooling rate should be chosen carefully based on the temperature-time-transformation diagram. [6]
Another guide that can be used for the synthesis is the Ellingham diagram. Elements at the top of the diagram are easily reduced and tend to form HEA-NPs, while elements at the bottom of the diagram tend to form high-entropy oxide NPs. [6]
Later, other similar non-equilibrium "shock" methods were also introduced to synthesize HEA-NPs and other types of high entropy nanostructures. [7] [8] [9] Recently, a low temperature synthesis through simultaneous multi-cation exchange (below 900 K) has been demonstrated for high-entropy metal sulfide NPs, which may be applied to metal selenides, tellurides, phosphides, and halides as well. [10]
In 2024 a study showed that induction plasma can be used as a one-step method that enables the continuous synthesis of HEA-NPs directly from elemental metal powders via in-flight alloying [11] .
Due to the random distribution of elements in HEA-NPs, in addition to conventional characterization methods, other methods with higher resolution are needed for their structural analysis. To analyze the random mixing of multiple elements, atomic electron tomography can be used, which provides positional precision of 21 pm and identification of atoms by periods. [12] Furthermore, X-ray absorption spectroscopy can give information on local coordination environments, while extended X-ray absorption fine structure can be used to get coordination numbers and bond distances. [13] Combined with hard X-ray photoelectron spectroscopy or X-ray absorption near-edge structure, these analyses can be used to explore structure–property relationships in HEA-NPs. [14] In addition, due to the immense number of possibilities of compositions and surfaces (i.e., terrace, edge, and corner) available for HEA-NPs, simulations such as density functional theory calculations are also popularly used for their analysis. [15]
HEA-NPs have a large compositional library, which enables tunability in chemical composition, structure, and associated properties. In HEA-NPs, the same type of atoms can have different local density of states because their neighboring atom compositions can be different. [14] Such variations in local environment lead to diverse and tunable adsorption energy levels, which can be beneficial to satisfy the Sabatier principle especially for complex reactions. [6]
In addition, owing to the high entropy structure, HEA-NPs typically show improved structural stability. One suggested mechanism for the enhanced structural stability is through prevention of phase separation due to lattice distortions from different sized elements acting as diffusion barriers. [16] With the above merits, HEA-NPs have been used as high-performance catalysts for both thermochemical and electrochemical reactions, such as ammonia oxidation, decomposition, and water splitting. [1] [17] [18] [19] High throughput and data mining approaches are being implemented toward accelerated materials discovery in the multi-dimensional space of HEA-NPs. [20] [21]
Colloidal gold is a sol or colloidal suspension of nanoparticles of gold in a fluid, usually water. The colloid is coloured usually either wine red or blue-purple . Due to their optical, electronic, and molecular-recognition properties, gold nanoparticles are the subject of substantial research, with many potential or promised applications in a wide variety of areas, including electron microscopy, electronics, nanotechnology, materials science, and biomedicine.
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Laser ablation synthesis in solution (LASiS) is a commonly used method for obtaining colloidal solution of nanoparticles in a variety of solvents. Nanoparticles (NPs,), are useful in chemistry, engineering and biochemistry due to their large surface-to-volume ratio that causes them to have unique physical properties. LASiS is considered a "green" method due to its lack of use for toxic chemical precursors to synthesize nanoparticles.
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Gold nanoparticles in chemotherapy and radiotherapy is the use of colloidal gold in therapeutic treatments, often for cancer or arthritis. Gold nanoparticle technology shows promise in the advancement of cancer treatments. Some of the properties that gold nanoparticles possess, such as small size, non-toxicity and non-immunogenicity make these molecules useful candidates for targeted drug delivery systems. With tumor-targeting delivery vectors becoming smaller, the ability to by-pass the natural barriers and obstacles of the body becomes more probable. To increase specificity and likelihood of drug delivery, tumor specific ligands may be grafted onto the particles along with the chemotherapeutic drug molecules, to allow these molecules to circulate throughout the tumor without being redistributed into the body.
Transparent wood composites are novel wood materials which have up to 90% transparency. Some have better mechanical properties than wood itself. They were made for the first time in 1992. These materials are significantly more biodegradable than glass and plastics. Transparent wood is also shatterproof, making it suitable for applications like cell phone screens.
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