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Preparation method of precious metal nanomaterials

Edit: Ccdanni 2020-01-30 Mobile

  Precious metals are a particularly special category of non-ferrous metals. The key is that the word "precious" is full of money. Precious metal nanomaterials include gold, silver, platinum, palladium, ruthenium, rhodium, osmium, and iridium. Due to their outstanding catalytic, electrical, magnetic, and optical properties, their applications have almost spread across electronics, chemicals, medicine, and energy. , Metallurgy, ceramics and other industries, is one of the most dynamic branch disciplines in the field of nanotechnology.

  

  Noble metal nanoparticles can catalyze the cleavage of H—H, C—H, C—C and C—O bonds under appropriate conditions, and are widely used as catalysts

  However, as everyone knows, no matter how good the ingredients are, they also need excellent chefs to cook exquisite dishes. The physical and chemical properties, morphology, size, and cost of precious metal nanomaterials are also inseparable from their preparation methods. If you want to learn more about this "Gao Fushuai" nanomaterial, you must first start with the preparation method.

  The preparation of precious metal nanoparticles can be divided into two categories in general: "top-to-bottom" methods (mainly physical methods, such as mechanical pulverization, ultrasonic pulverization, etc.) and "bottom-to-top" (mainly chemical methods, precursor reactions The metal is generated by chemical reduction, photolysis, pyrolysis and other methods to aggregate metal atoms into nano metal particles), which will be summarized by the classification of the reaction medium below.

  I. Gas phase inert gas evaporation condensation method Inert gas condensation method (IGC method for short) is a method of heating metals in low-pressure Ar, He and other inert gases to cause rapid condensation after evaporation to form nano-powder. It is the most direct and effective method for preparing metal nanoparticles. Methods. At present, a few industrial developed countries such as Japan, the United States, France, and Russia have adopted this law to partially realize the industrial production of certain precious metal nanomaterials.

  The advantages of this method are: controllable particle size, high product purity, can produce metal nanoparticles with a particle size of 5-10nm and have a clean surface, few particles agglomerate, high block purity, and high relative density .

  Gas-phase chemical reaction

  Gas-phase chemical reaction method for preparing metal nanoparticles is to use the vapour of volatile metal compounds to generate the required compounds through chemical reactions, and to quickly condense in a protective gas environment to prepare nanoparticles of various substances. For example, metal nanoparticles such as Fe, Co, Ni, etc. can be prepared by reacting with CO to form volatile carbonyl compounds and decomposing into metal and CO when the temperature rises.

  The advantages of this method are high particle purity, small and uniform particle size, good dispersibility, and high chemical reactivity and activity. However, this method has a narrow application range due to the limitations of precursors.

  Second, the liquid phase method

  Liquid chemical reduction

  The liquid phase chemical reduction method refers to a method in which a metal salt solution is directly reduced by a reducing agent under the protection of a medium under normal pressure, normal temperature conditions or hydrothermal conditions. Metal salts are usually soluble salts such as chlorides, sulfates, or nitrates. Precious metal nanomaterials that have been successfully prepared using liquid-phase chemical reduction methods include nanometal clusters such as Pd, Pt, Ru, Au, Ag, and Co.

  The advantages of this method are: low preparation cost and simple equipment requirements; easy reaction control; crystal shape and particle size control can be achieved by optimizing process parameters such as temperature, time, and reducing agent; the disadvantage is that high purity reagents, The introduction of impurities is not allowed.

  Inverse microemulsion method When the concentration of the surfactant dissolved in the organic solution exceeds the critical micelle concentration, spherical microemulsion particles with hydrophilic groups facing inward and hydrophobic groups facing outward between several to several tens of nm will be formed. It can maintain a certain stable small size under certain conditions, and is an ideal microenvironment for preparing uniform small size particles. Since Boutonnet et al. First produced monodisperse metal nanoparticles from microemulsions, people have used this method to produce metal nanoparticles such as Fe, Co, Au, and Ag.

  The most outstanding advantage of this method is that, because the reaction is performed in a nanoreactor, the reactants are in a highly dispersed state, which prevents local supersaturation. Therefore, the resulting particles are usually very small and monodispersed; and because of the surface coating of the product With a layer of surfactant, it is not easy to agglomerate. However, the agglomeration of the sol-to-gel and powder drying processes must be strictly controlled when using this method.

  Electrochemical method

  Electrochemical method means that under the condition of using quaternary ammonium salt as electrolyte and stabilizer, the metal is oxidized at the anode and the ions are reduced at the cathode to produce metal nanoparticles. This method can produce a lot of high-purity metal nanoparticles that cannot be prepared or difficult to prepare by ordinary methods, especially metal nanoparticles with large electronegativity.

  The advantages of this method are: ① the particle size can be controlled by changing the current density (the higher the current density, the smaller the particle); ② the nanoparticles are easily separated after precipitation from the solvent; ③ the yield is high, exceeding 95%. Because the preparation of the particles and the surface coating are completed simultaneously, the resulting particles are highly dispersive and resistant to oxidation.

  Radiation synthesis

  The principle of radiation synthesis is that ionizing radiation ionizes and excites water to generate reduced particle H radicals, eaq- with strong reducing ability, and oxidized particle OH radicals. When free radical scavengers such as methanol and isopropanol are added, H-removal reactions occur to remove oxidative OH radicals, and the organic radicals generated are also reducing. These reducing particles gradually reduce metal ions to metal atoms or low Valence metal ions, the generated metal atoms aggregate into a nucleus, and eventually grow into nanoparticles.

  Advantages: simple preparation process, short preparation cycle, high yield, controllable product size; particle generation and coating are performed simultaneously to prevent particle agglomeration. However, the product obtained in this method is in the state of discrete colloids, and collection is difficult, and it is often used in combination with hydrothermal crystallization method and inverse microemulsion method.

  Ultrasonic method

  The effect of ultrasound comes from acoustic cavitation, which refers to the formation, oscillation, growth, contraction, and collapse of tiny bubble nuclei in a liquid, causing physical and chemical changes. When the cavitation bubble collapses, an instantaneous high temperature (~ 5000K) and high pressure (~ 1.8 × 108Pa) and a cooling rate exceeding 1010K · s-1 will be generated in a very small space around the cavitation bubble in a very short time. Strong shock waves and / or jets with speeds up to 400 Km and discharge luminescence.

  The extremely high energy in the ultrasonic process can promote the formation of new phases and has a unique role in the preparation of nano-sized ultrafine particles. In recent years, it has developed into a new technology for preparing nano-materials. Many nanometer-sized precious metal particles have been prepared by the ultrasonic method, such as: Au, Pd, Ag, and Pt.

  Microwave method

  Microwave method has the advantages of fast, energy saving, uniform heating, convenient control, small metal cluster particles, and narrow distribution. This is the first continuous synthesis method in the synthesis of nano metal particles. example. This method has stable operation and good repeatability. The metal colloid (cluster) microwave synthesis method has become a new methodological achievement, which is widely introduced and cited alongside the classical chemical reduction method, electrochemical reduction method, γ-radiolysis method, and sonochemical method.

  Photon quantum reduction

  Photon quantum reduction is a very important method for the preparation of precious metal colloids. The basic principle is to make the solution generate hydrating electrons eaq- and reducing free radicals by light, and eaq- or free radicals can reduce metal ions in the solution and make them show an unusual valence state. Such as: Ag ++ eaq- = Ago, continuous accumulation of Ago can form larger particles. By using a polymer or other medium to stabilize the formed particles, nanomaterials of different sizes can be prepared.

  Three, solid phase method

  Many organometallic compounds can be thermally decomposed to form corresponding zero-valent metals, thus opening up a new way to prepare metal nanoparticles by solid-phase method. Pd and Pt organosols can be obtained by pyrolysis of precursors such as palladium acetate, palladium acetylacetonate, and platinum chloride. However, the disadvantage of this method is that the solvent used has a high boiling point, and there is no stabilizer during synthesis, and the particle size distribution is wide. Large particles can often be observed, so it has not been promoted so far.

  Summary

  There are a variety of methods for preparing precious metal nanoparticles, and the process is relatively simple, which is one of the important reasons for its wide application. The unique properties brought by nanometer size make precious metal nanomaterials like water in many fields. Among them, 1 ~ 10nm precious metal nanoparticles with new electronic and catalytic properties have attracted much attention. More on the application of precious metal nanomaterials, I will continue to organize in the next article.

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