Synthesis Techniques for Fabrication of Nanofibers

Chapter # 02

Synthesis Techniques

Introduction

The different synthesis techniques for fabrication of nanofibers and nanoparticles will be discussed briefly in this chapter. The selected techniques and the experimental procedure for the synthesis of the nanoparticles and composite nanofibers will be discussed in detail.

Synthesis Techniques for Nanoparticles

Synthesis method being used for the preparation of nanomaterials is one of the most important factors that affect the different properties of the nanomaterials.

There are two major classes of synthesis techniques used for preparation of nanoparticles

• Bottom-up approach
• Top-down approach

Bottom-up Approach

Bottom-up approach refers to the synthesis techniques in which nanoparticles are created from atomic scale to nanoscale. There are different techniques which refer to bottom-up approach

• Co-precipitation technique
• Sol-gel technique

Wet-Chemical Methods

In these methods the precursor solutions with appropriate stoichiometric ratios are mixed together and processed through heating resulting in the desired product. Wet chemical methods are advantageous because the particle size and shape is controllable and also the homogeneity in particle size distribution can be achieved by these methods [ref]. these factors play a vital role in enhancing the different properties of the material such as structural properties, thermal properties, electrical properties etc.

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The given examples of the bottom-up method are categorized as wet-chemical techniques. In co-precipitation method, the precursor solutions are mixed together and are heated at an appropriately selected temperature. The nucleation occurs precipitates are formed. The by by-products are washed away and the resulted powder is further processed for the different desired characterizations.

Both the methods are advantageous in their own ways but sol-gel method is more convenient and efficient method for the preparation of nanomaterials at appropriately selected conditions. Also the major drawback of co-precipitation method is the non-uniformity of particles due to the intense agglomeration during the synthesis [ref]

Top-down Approach

In top-down approach, the material is processed from bulk state to nano regime by a number of methods. Solid state reaction method is one of top-down methods.

Solid-state Reaction Method

In solid state reaction method the precursors are directly mixed and ground into fine powder. The powder is further processed through hydraulic press and is pelletized for high heat treatment generally said to be sintering. The sintered pellet is characterized to check the desired properties of the sample.

The process occurs at the interface of the solids (precursors) at higher temperature. The appropriate processing temperature and reaction time is selected particularly for the precursors used. At higher temperature, the diffusion of reactants starts causing the reaction to begin. For faster reaction, high surface contact area of the solids and small diffusion distance for the reactant is required which is attained by the well-mixing of the precursors [ref].

Although this is a low-cost method but the major drawback of this method is the non-homogeneity in particle distribution and size. Impurity in desired phase is also introduced during the grinding procedure of the precursors. The desired nanostructure is also not easy to achieve by this process [ref 54].

Sol-gel Synthesis Technique for Na_xLi_1-xCoO₂ (x=0.0,0.5,1.0)

Sol-gel method is one of the wet-chemical methods for the preparation of the various nanostructures. The variety of nanostructures can be produced by controlling the various parameters of sol-gel process such as the type of precursors, solvent type, processing temperature etc. the major advantage of the sol-gel process over other methods is the phase purity of the product as well as the homogeneity in the particle size and particle size distribution.

This process involves two major steps

• Hydrolysis
• Condensation

Hydrolysis of the selected metal oxide precursor occurs forming the particular hydroxides. The condensation occurs to form a network of linked hydroxides in the form of dense porous gel.

In sol-gel process the precursor solutions are mixed together with a suitable gelling agent in appropriately selected ratio. The solution is then mixed homogeneously and heated until the gel is formed. The temperature is further raise to burn the gel which results in powder formation. The powder is heat treated and characterized to study the desired properties.

Experimental Procedure

Sol-gel method is used for the preparation of composition Na_xLi_1-xCoO₂ (x=0.0,0.5,1.0). the precursors used are: Li₂SO₄.H₂O (99.99% purity), Cu(NO₃)₂.6H₂O (99.99% purity) and NaNO₃ (99.99% purity). The selected precursors were added in appropriate stoichiometric amounts into ethylene glycol. Ethylene glycol was used as gelling agent. The molar ratio between the total mass of precursors to the volume of the gelling agent was kept 1:14 to achieve the homogeneity. The solution was initially magnetically stirred at room temperature to get the homogeneous solution. The solution was then heated at 100±2ºC until the formation of the gel. The temperature of the gel was further increased upto 150±2ºC which caused the gel to burn.

The resulted powder was hen processed through hydraulic press to prepare pellets of dimensions 13mm x 3mm. the sintering of the pellets was done at 550 ºC for two hours to achieve high phase purity.

The flow diagram of the experimental procedure is given in figure 2.1

Synthesis Techniques for Nanofibers

There are a number of techniques used to fabricate nanofibers, some of them are mentioned

1. Drawing
2. Template Synthesis
3. Phase Separation
4. Self-Assembly
5. Electrospinning

Brief detail of all these techniques is given below

Drawing

Drawing is the techniqus to fabricate long single nanofiber one-by-one from the droplet of polymer. Following steps are involved in this technique

1. a substrate material is applied a millimeter drop of polymer solution
2. a micropipette is moved towards the drop.
3. When micropipette comes in contact with the drop, it is pulled back with a certain rate. which depends upon the nature of the polymer solution.
4. A long nanofiber is drawn from the liquid.

The diameter of the resultant nanofiber depends on the type of the polymer, its composition, drawing velocity and speed of evaporation of the solvent.

The major drawback of this technique is that only a strong viscoelastic material that can undergo strong deformation during this process when stress is applied while pulling the nanofiber, can be used in this process. So, choice of material is limited in this process.

Template Synthesis

In template synthesis, a metal oxide membrane having pores of nanoscale diameter is used. The metal oxide membrane is placed over a solidifying solution. Polymer solution is extruded by the membrane by applying high water pressure over it. Polymer solution after passing through the membrane comes in contact with solidifying solution which converts the polymer solution into nanofibers. The diameter of the nanofibers depends on the diameter of the pores of membrane. Fig. 2.2 show the different steps involved in this process.

Phase separation

In this mechanism, separation of phases is involved due to the physical incompatibility. Following are the steps involved

1. A polymer is mixed with a suitable solvent
2. Gelation occurs in this mixture
3. The final step involves the separation of phases. One of the phases-which is that of the solvent- is extracted leaving behind the other remaining phase. The remaining phase is the nanofibrous structure.

Self-Assembly

Self-assembly processing involves the smaller molecules as basic building blocks to build-up the nanofibers. Molecules are spontaneously organized into an individual and stable structure with preprogrammed non covalent bonds.

Nanofibers of very thin diameter can be fabricated using this process but it requires very complicated procedures. The low productivity is another limitation of this method.

Electrospinning

This is the selected synthesis technique for the current research work. Electrospinning is the most efficient and simple technique to produce ultra-thin nanofibers.

There are different components of the electrospinning setup which include

1. High voltage source
2. Syringe
3. Needle
4. Metal Collector

Schematic of electrospinning process

High voltage source is used in this procedure. The positive end of the source is connected with the needle while the metal collector is grounded by connecting it with the negative end of the voltage source. This creates the potential difference between the two ends which accelerates the polymer solution from the needle towards the collector in the form of solution jet. Before the solution jet reaches the collector surface, the solvent of the solution is evaporated and is collected as an interconnected web of the fibers. The polymer solution in the needle is held due to its surface tension, which induces a charge on the solution surface. The charge repulsion and contraction among the surface charge and its counter electrode causes a force that is directly opposite to the surface tension.

As the intensity of the electric field increases, the hemispherical surface of the solution at the tip of the needle elongates to form a conical shape called Taylor cone. As the electric field is further increased, the repulsive electrostatic force overcomes the surface tension of the solution and jet of the solution is ejected from the Taylor cone. The ejected polymer solution undergoes instability and elongation process which allows the jet to become very long and thin. Meanwhile the solvent is evaporated from the polymer solution, leaving behind a charged polymer fiber.

In electrospinning technique there are different processing parameters which affect the diameter of the nanofibers. These parameters include voltage, needle tip to collector distance, feed rate and the concentration.

This method is advantageous over other methods due to its versatility, cost effectiveness and also by controlling the different processing parameters; the dimensions of the nanofibers can be controlled.

Sol-gel Combined Electrospinning Technique

Combining sol-gel method with electrospinning is the most convenient technique to prepare composite nanofibers.the different precursors involved in the sol-gel technique have different hydrolysis rate which leads to the in homogeneity of sol. To avoid such probles, electrospinning is combined with the sol-gel process. The problem is solved by electrospinning procedure due to the confinement of the different sol-gel reaction stages (hydrolysis, condensation and gelation) withing extremetly small space i-e the spinning jet and the final nanofiber [ref].

Experimental Procedure:

Sol-gel combined electrospinning technique is used for the preparation of composite nanofibers with Na_xLi_1-xCoO₂ (x=0.0,0.5,1.0). The precursors used are: Li₂SO₄.H₂O (99.99% purity), Cu(NO₃)₂.6H₂O (99.99% purity) and NaNO₃ (99.99% purity). The polymers selected for this purpose were Poly vinyl Pyrrolidone (PVP) with a molecular weight of 40,000 g/mol and Poly vinyl alcohol (PVA) having a molecular weight of 89,000 g/mol.

Polymer Solution Preparation

Polymer solution was prepared by firstly selecting the appropriate solvents for the selected polymers. The solvent used for the PVP polymer was Ethanol and that for PVA polymwer was DI water. The molarity of PVP and PVA polymer solution were calibrated as 0.008M and 0.0006M respectively. Both polymers were separately added to their respective solvents and the solutions were magnetically stirred for 1 hour at room temperature. After acquiring the homogeneity, both solutions were mixed together and stirred at room temperature for 1 hour.

Precursors Solution Preparation

The precursors solution was separately prepared. The precursors were added to their respective solvents separately and magnetically stirred at room temperature. DI water was used as a solvent for Li₂SO₄.H₂O while ethanol was used for Cu(NO₃)₂.6H₂O and NaNO₃. 0.2M solutions were prepared for each precursor separately. After acquiring the homogeneous solutions, all three solutions were mixed together and magnetically stirred for 45 min.

After achieving a homogeneous precursors solution, the polymer solution was added to it. the final solution was heated and stirred at 100±5ºC for 1 hour. The resultant solution was then loaded to the syringe and was further processed through electrospinning.

The figure shows the flow diagram of the discussed method