Physical Properties of Metal Nanoparticles
MNPs have unique physical properties that differ from their bulk metal counterparts since their size and shape can be precisely controlled during synthesis. The physical properties of metal nanoparticles are highly tunable. The size and shape of the nanoparticles affect their electronic, optical, and magnetic properties. Because of their higher surface area-to-volume ratio they become more reactive and enhance their properties in specific applications.
MNPs can exhibit unique colors and fluorescence properties. for example, Au NPs exhibit a characteristic red color due to their plasmonic properties. Some MNPs, such as iron, cobalt, and nickel, exhibit magnetic properties that can be exploited in applications such as magnetic data storage and biomedical imaging. The melting point of metal nanoparticles can be lower than that of the bulk metal due to the presence of surface defects and lattice distortions. The surface chemistry of MNPs can be tuned by modifying their surface with ligands or coatings, which can affect their properties in applications such as catalysis and drug delivery.
Synthesis/Preparation of Metal Nanoparticles
Chemical Reduction Method: In this method, a reducing agent is used to reduce metal ions into nanoparticles. For example, sodium borohydride is commonly used to reduce metal ions such as gold, silver, and platinum to form nanoparticles.
In Electrochemical Method, metal ions are reduced electrochemically to form nanoparticles. The size and shape of the nanoparticles can be controlled by adjusting the voltage and current during the electrochemical process.
In Microwave-Assisted Method: In this method, metal ions are mixed with a reducing agent and then exposed to microwave radiation. The heat generated by the microwave radiation promotes the reduction of the metal ions to form nanoparticles.
Green Synthesis Method, here the plant extracts or other natural sources are used as reducing agents to prepare metal nanoparticles. This method is considered environmentally friendly as it avoids the use of harsh chemicals and solvents.
Electro Chemical Method
Electrochemical Preparation Of Metal Nanoparticles
Electrochemical methods are widely used for the preparation of metal nanoparticles due to their simplicity, scalability, and ability to control the size and shape of the nanoparticles. The electrochemical methods for the preparation of metal nanoparticles can be classified into two categories: anodic and cathodic methods.
Anodic Methods: In anodic methods, the metal ions are reduced at the electrode surface to form metal nanoparticles. Anodic stripping voltammetry (ASV) is an example of an anodic method used to prepare metal nanoparticles. In ASV, the metal ions are first adsorbed onto the electrode surface and then reduced to form metal nanoparticles.
Cathodic Methods: In cathodic methods, the metal ions are reduced at the cathode surface to form metal nanoparticles. Cathodic reduction can be carried out in the presence of a stabilizing agent, such as a surfactant or polymer, to prevent the aggregation of nanoparticles. Electrochemical deposition (ECD) is an example of a cathodic method used to prepare metal nanoparticles. In ECD, a metal salt solution is electrolyzed using an appropriate cathode material to form metal nanoparticles.
Both anodic and cathodic methods can be used to prepare metal nanoparticles with a high degree of control over their size and shape. The electrochemical methods also offer the advantage of being scalable and easy to perform. The choice of method depends on the specific metal, the desired size and shape of the nanoparticles, and the intended application.
Photochemical methods are another popular approach for the preparation of metal nanoparticles. These methods involve the use of light to drive the reduction of metal ions to form nanoparticles. The photochemical methods can be divided into two categories: direct and indirect photochemical methods.
In direct photochemical methods, the metal ions are reduced (in the presence of a reducing agent) directly by a photochemical reaction induced by UV light Which lead to the formation of metal nanoparticles.
In indirect photochemical methods, a photosensitive metal precursor is first synthesized, which upon exposure to light, undergoes a chemical transformation to form the metal nanoparticles. Photochemical methods offer several advantages over other synthesis methods, including the ability to prepare metal nanoparticles at low temperatures, high yields, and good control over the size and shape of the nanoparticles. Moreover, photochemical methods are eco-friendly as they do not require any harsh reducing agents or high temperatures. However, these methods require a high-intensity light source, and the synthesis process can be time-consuming.
Synthesis of Metal Oxide Nanoparticles
Metal oxide (MO) nanoparticles are nanoscale particles made up of metal and oxygen atoms (metal cations and O anions) that exhibit unique physical and chemical properties due to their small size and high surface-to-volume ratio. In MO nanostructures, closely packed Oxygen anions form interstitial sites which are occupied by metal cations. The partially d-shells of transition metal ions facilitate enhanced electronic transitions, wide band gaps, superior electrical characteristics, and high dielectric constants. They have been studied and developed for a range of applications in fields such as energy, catalysis, sensing, and electronics. The size and shape of metal oxide nanoparticles can be precisely controlled, which allows their properties to be tailored for specific applications such as solar cells. Some examples of metal oxide nanoparticles include titanium dioxide (TiO2), zinc oxide (ZnO), and iron oxide (Fe2O3). These materials have been studied for a range of applications, such as photocatalysis, gas sensing, environmental remediation, and bioimaging, among others.
Synthesis of nanomaterials with tailored properties involves the control of size, shape, structure, composition, and purity of their constituents. Hence the nanomaterial properties can be tuned as desired via precisely controlling the size, shape, synthesis conditions, and appropriate functionalization.
The synthesis of oxide nanoparticles typically involves a chemical or physical process that produces nanoparticles of a desired oxide material. The common methods used for synthesizing oxide nanoparticles are:
Laser ablation method
This method involves the hydrolysis of metal alkoxides in a solution to form a gel, which is then dried and calcined to form oxide nanoparticles. This method is commonly used to synthesize metal oxide nanoparticles, such as titanium dioxide and zinc oxide.
It is a chemical process for producing solid materials from small molecules. It involves the transformation of a sol (a colloidal suspension of nanoparticles) into a gel-like network of interconnected particles that eventually forms a solid material.
In the sol-gel process, a precursor (usually a metal alkoxide) is hydrolyzed in the presence of a solvent and a catalyst. This hydrolysis reaction forms an intermediate product called a sol, which is a suspension of nanoparticles in the liquid phase. The sol then undergoes further reactions, such as condensation, that cause the nanoparticles to bond together and form a solid network.
The sol-gel method offers several advantages over other traditional methods of producing solid materials, such as ceramics and glasses. E.g., it allows for the formation of complex shapes and structures, and the resulting materials have high purity, homogeneity, and porosity. The process can also be performed at relatively low temperatures, which reduces energy consumption and enables the production of materials that are difficult to make using other methods. This method is used in a variety of applications, such as the synthesis of catalysts, optical materials, and biomaterials. It is also used in the production of coatings, such as anti-reflective coatings for glass, and in the manufacture of sensors and electronic devices.