Bottom up strategy for nanomaterial fabrication involves with assembling of the nanoparticles from fundamental building blocks of matter: atoms and molecules. Unlike in top down approaches, where high power techniques are employed to physically decrease the size of the particles, top down methods use much more gentle approach. The top down approaches are typically driven by thermodynamic aspects and take use of much weaker forces like chemical bonding, intermolecular attraction, etc. Bottom up approaches are also classified in to two broad groups as, wet chemical methods and gas phase methods depending on the medium at which nanoparticles form.
Let’s first review some of the basic wet based bottom up methods as they are the most highly reported in the literature and much popular among the researchers.
The solvothermal process involves the use of molecular level precursors dissolved in a solvent to build nanoparticles from bottom up. The process is typically carried out in a closed reaction vessel that facilitate either decomposition or chemical reaction between precursors. To facilitate these interactions, high temperature and/or high pressure may be used. If water is used as the solvent, the process is reffered to as “hydrothermal synthesis”.
Solvothermal technique is very popular in the field of nanomaterial preparation and used in preparint variety of materials such as metals, metal oxides, semiconductors, ceramics and sometimes even polymers. This process is also used widely to make nanomaterials with different shapes, such as, spheres, rods, wires, etc. The shape, size and morphology of the nanomaterials produced in this method may depend on lot of parameters. Solvent type, temperature, pressure, surfactant used in the solvent are of great importance in determining nanomaterial properties among others. This process is adopted in making nanomaterials such as zeolites in the industrial scale already.
The process usually follows a liquid nucleation pathway. Which means that small nanocrystals are formed due to decomposition or as a byproduct of the reaction. Sometimes seed crystals are introduced to the solvent system. During the process, the nanomaterials will grow in the precursor solution producing the nanomaterials.
Chemical precursors that are dissolved or in a melt in a liquid phase are subjected to thermal motions. This sometimes can lead to a formation of an atomic assembly that would resemble a solid phase. Usually these assemblies can reversibly dissolve back to the liquid phase. However, if there’s a thermodynamic advantage of cluster formation, the assembly would not dissolve but become a distinct phase in medium that is stable. These assemblies are referred to as nuclei and can serve as the seeds for particle formation. However, at the moment when a solid phase is created, solid liquid interface is formed. Making a surface is energetically costly affair. Hence, there exists a minimum energetically favorable size for the nuclei. Thus it’s only favorable to form nuclei only above this limit. Below this optimum size clusters remain unstable and dissolves back to the solution. So a nuclei would persist to exist in the medium only if the local fluctuations can only make large enough clusters that are above this critical size.
A stable nuclei will continue to grow in size to minimize the energy loss related to the newly formed surface area. The growth occurs due to diffusion of monomers or precursor material towards the surface of the nanomaterial followed by reaction and stabilization. This type of growth, where the particle coarsening is controlled by mass transport or diffusion is the most predominant growth mechanism and referred to as Ostwald ripening process as a credit to the scientist who discovered it.
The sol gel process, as the name suggests, involves a formation of an inorganic network of colloidal suspension (sol) followed by gelation of the sol solution to form a continuous liquid phase (gel). Thus formed gel can be used to fabricate various nanomaterials and nanostructures such as powders, aerogels, xerogels, etc. There are number of precursors that can serve as sol forming constitutes such as, metal alkoxides, metal organic compounds, salts of inorganic acids, salts of organic acids, etc. However, the most commonly used precursors are metal alkoxides; compounds in which a metal is bonded to one or more alkyl groups through an intermediate oxygen atom.
In a typical process, precursor solution is dissolved in a solvent or a mixture of organic solvents. Addition of acid or base catalyst is also common in sol-gel processes to increase the rate of the reaction. Despite of the range of the precursors that can serve the function of precursor materials, they will undergo hydrolysis of and polycondensation processes to form M-O-M bonds. In the hydrolysis process, as the name suggests, precursor will hydrolyze and attains a hydroxyl group typically through the reaction with water. Polycondensation is the process in which hydrolyzed species combine to make a inorganic polymer like chains through elimination of a water molecule.
Depending on the process conditions and the chemical constitutes in the sol-gel bath, sol properties can be tuned. Typically the process conditions are designed in such a way to obtain short or long polymeric chains or colloidal particles. These differences in sol properties can be advantageous for specific applications such as thin film coating, powder preparation and spray pyrolysis.
The sol can be transformed in to a gel state through continuous polycondensation and solvent evaporation. The resulting gel is a system consisting a solid and three dimensional network of sol and the solvent. The gel can be further processed to remove the solvent residue and other chemical traces. Once the residue is removed, the sol system collapse in to an amorphous solid structure, which is known as xerogel. This material can be sintered in a furnace to transform the xerogel to a solid crystalline material.
Sol-gel process has number of unique advantages. Unlike other methods, sol-gel processes can be helpful to obtain materials with wide range of oxygen and other metallic composition. It also provides a wet based method to dope nanomaterials for improved performance. Sol-gel systems can be controlled to obtain desired particle size, shape and size distributions.