Gas phase synthesis

Introduction

Gas phase nanoparticle preparation methods have attracted huge interest over the years due to number of benefits that they can deliver over other methods. These techniques are typically characterized by the ability to accurately control the process parameters to be able to tune shape, size and chemical composition of the nanostructures.

Although, means and methods can differ, almost all gas phase nanomaterial production methods follows following sequence

• Suspending the precursor materials in a gas phase

• Transforming the precursor material to small clusters

• Enforcing the growth of these clusters to a nanoparticles

• Method to collect prepared nanoparticles.

The growth of small nuclei clusters to nanoparticles is referred to as condensation and it occurs only when the precursor vapor is supersaturated. Condensation process can be driven by both physical and chemical methods and will be discussed in the following.

Inert gas condensation

This method is the most rudimentary of all the gas phase fabrication techniques. The method is simple as heating a material inside a furnace usually under an inert gas such as Nitrogen or Helium. This method however, is only appropriate for the materials that have low vapor pressure. Or in other words material is only vaporized at elevated temperatures sometimes even 2000 degrees. This method is quite useful in preparation of metallic nanoparticles, as these materials show reasonable rates of evaporation at practical temperatures. Although the process is typically carried out at inert gas, reactive gases can also introduced to the heated chamber to encourage reactions. This is particularly useful in making metal oxide and metal halide nanoparticles. Nanoparticle attributes such as shape, size and the distribution is mainly controlled by the rate of evaporation (heating rate/temperature), rate of condensation (cooling rate) and the gas flow.

Pulsed laser ablation

In pulse laser ablation technique, more confined plume of the material is vaporized instead of the entire sample to produce vapor. To achieve this, high energy laser is focused on to a much localized position. The laser exposure is made in pulses thus the name pulsed laser. This rapidly heats up a small spot of the material to very high temperature at which the material is vaporized. Due to the small weight size of the sample being vaporized, this generally can be used to make small amount of nanomaterials. However, the technique is quite useful in synthesizing nanomaterials of the materials that cannot be evaporated otherwise. The method is excessively used to make metal oxide nanoparticles than other types.

Chemical vapor synthesis

In Chemical Vapor Synthesis (CVS) chemical vapors of precursor materials are brought to reaction in a reaction chamber. The reaction chamber is typically heated using joule heating. The method is similar to chemical vapor deposition, however instead of deposition of nanomaterial as a thin film, CVS process encourage formation of nanoparticles. Hence, the process parameters are adjusted appropriately during the synthesis in order to suppress film formation and to encourage nucleation of particles in the gas phase. Typically, the resident time of the precursor in the reaction chamber is the most critical parameter to determine whether the film or parameter will be formed.

This method is well recognized for the flexibility it provides as a nanomaterial synthesis process. The precursors can be in solid, liquid or gas phase at the ambient temperature but are delivered to the hot wall reactor at the vapor phase. The method has been adopted to fabricate wide range of nanomaterials from variety of precursor materials. CVS process has embraced much of the precursor chemistries developed from CVD processes and has contributed significantly for CVS process to stand as its own.

The main process parameters of the CVS process are the residence time of precursors, gas flow rate, pressure different between inlet and the main chamber and temperature of the hot wall. In the simplest form of CVS, metal organic precursor is introduced to the hot wall reactor at a controlled rate.

The reactor chamber can be filled with reactive gases to produce the respective metal oxide, halide, nitride or carbide during the process. In other technique, mixtures of nanoparticles of two phases can be fabricated by supplying two or more precursors simultaneously. The same strategy was used to make doped nanomaterials as well. Some researchers have used CVS process to make coated or core shell nanoparticles. This is typically achieved by supplying the second precursor at a later stage of the reactor. CVS is also regarded as a high throughput process as the production is continuous. Even a small scale reactor can produce considerably high amount of nanomaterials compared to other manufacturing techniques.

Flame assisted synthesis

In flame assisted synthesis process, the energy required to particle nucleation is given by a flame instead of a supplying the energy externally from a secondary heat source. In the flame assisted synthesis method, particle nucleation and the growth will occur inside the flame.

This method has the most widespread acceptance among all the synthesis procedures and amounts to millions of tons per year production in carbon black, fullerenes and metal oxides. The most frequently produced metal oxide from this method is silicon dioxide or silica. This method is particularly useful in making metal oxides as the conditions inside the flame and the environment surrounds it is quite oxidizing.

However, the greatest challenge with the flame synthesis is still the precise control of the complex processes that involve in this process. Large body of research is dedicated to widen the scope of materials that can be produced by this method and to obtain greater control over the particle morphology.

Sputtering

Sputtering is a gas phase method of fabrication of nanomaterials that involves vaporizing of a solid precursor material by bombarding with a high velocity ions of an inert gas. This produce a cloud of atoms and atom clusters that get deposited in to a substrate subsequently. Sputtering is typically carried out under vacuum environment as at higher pressures mobility of sputtered materials are hindered. The most commonly used sputter sources are ion guns or hollow cathode plasma generators. The method is advantages as the composition of the sputtered material is same as the target material.