Lecture 12-13

Synthesis of nanoparticles

Synthesis techniques: Top down versus bottom-up

top down ⇒ lithography

bottom up ⇒ self assembly, molecular recognition

concepts / principles + examples

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Fabrication ApproachesNanomaterials

Section No.2 Experimental Methods

1- Top-Down Approach

Creates nanostructures out ofmacrostructures by breakingdown matter into more basicbuilding blocks

2- Bottom- Up Approach

Building complex systems bycombining simple atomic levelcomponents through selfassembly of atoms ormolecules into nanostructures

Two strategies: 1: Top-Down

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Two strategies: 2: Bottom Up

Kinetics of Growth mechanism

Growth mechnism of nanoparticles

• The nucleation occurs only when the supersaturation reaches a certain value above the solubility, which corresponds to the energy barrier for the formation of nuclei. • After the initial nucleation, the concentration or supersaturation of the growth species decreases and the change of volume Gibbs free energy reduces.

The processes of nucleation and subsequent growth.

Growth mechnism of nanoparticlesWhen the concentration decreases below this certain concentration, which corresponds to the critical energy, no more nuclei would form, whereas the growth will proceed until the concentration of growth species reached the equilibrium concentration or solubility.

Growth of nucleiThe size distribution of nanoparticles is dependent on the subsequent growth process of the nuclei. The growth process of the nuclei involves multi-steps and the major steps are (1) Generation of growth species, (2) Diffusion of the growth species from bulk to the growth surface, (3) Adsorption of the growth species onto the growth surface, and (4) Surface growth through irreversible incorporation of growth species onto the solid surface.

Growth of nuclei

The above steps can be further grouped into two processes.Supplying the growth species to the growth surface is termed as diffusion.Diffusion includes the generation, diffusion, and adsorption of growth species onto the growth surface. Incorporation of growth species adsorbed on the growth surface into solid structure is denoted as growth. A diffusion-limited growth would result a different size distribution of nanoparticles as compared with that by growth limited process.

Growth controlled by diffusion When the concentration of growth species reduces below the minimum concentration for nucleation, nucleation stops, whereas the growth continues. If the growth process is controlled by the diffusion of growth species from the bulk solution to the particle surface, the growth rate is given by:

dr/dt = D(C-Cs)Vm/r where r is the radius of spherical nucleus, D is the diffusion coefficient of the growth species, C is the bulk concentration, Cs is the concentration on the surface of solid particles, and Vm is the molar volume of the nuclei.

AE Nielsen, Kinetic of Precipitation, MacMillan, New York, 1964.

Growth controlled by diffusion By solving this differential equation and assuming the initial size of nucleus, r0, and the change of bulk concentration negligible, we have:

r2 = D(C-Cs)Vmt + r02 or

r2 = kDt + r02 ....... (1)

where kD = D(C-Cs)Vm. For two particles with initial radius difference, δr0, the radius difference, δr, decreases as time increases or particles grow bigger, according to:

δr = r0 δr0/r ....... (2)Combining with equation (1) we have:

δr = r0 δr0 /(kDt + r02)1/2 .......(3)

Growth controlled by surface process When the diffusion of growth species from the bulk to the growth surface is sufficiently rapid, the growth rate is controlled by the surface process. Two mechanisms for the surface processes:mononuclear growth and poly-nuclear growth. For the mononuclear growth, the growth proceeds layer by layer; the growth rate is thus proportional to the surface area:

dr/dt = km(C) r2 ........(4)where km(C) is a proportionality constant, dependent on the concentration of growth species.

Growth mechanism

Sol-gel method• The sol-gel process may be described as:”Formation of an oxide network through polycondensation reactions

of a molecular precursor in a liquid.”

A sol is a stable dispersion of colloidal particles or polymers in a solvent.

The particles may be amorphous or crystalline. An aerosol is particles in a gas phase, while a sol is particles in a liquid.

A gel consists of a three dimensional continuous network, which encloses a liquid phase.

In a colloidal gel, the network is built from agglomeration of colloidal particles.In a polymer gel the particles have a polymeric sub-structure made by aggregates of sub-colloidal particles. Generally, the sol particles may interact by van der Waals forces or hydrogen bonds.

• The idea behind sol-gel synthesis is to “dissolve” the compound in a liquid in order to bring it back as a solid in a controlled manner.

• Multi component compounds may be prepared with a controlled stoichiometry by mixing sols of different compounds.

• The sol-gel method prevents the problems with co-precipitation, which may be inhomogeneous, be a gelation reaction.

• Enables mixing at an atomic level.• Results in small particles, which are easily sinterable.

Sol• A sol consists of a liquid with colloidal particles

which are not dissolved, but do not agglomerate or sediment.

• Agglomeration of small particles are due to van der Waals forces and a tendency to decrease the total surface energy. Van der Waals forces are weak, and extend only for a few nanometers.

• In order to counter the van der Waals interactions, repulsive forces must be established.

Point of Zero charge (PZC)• Stabilization due to electrostatic repulsion are due to formation of a

double layer at the particle.• The surface of a particle is covered by ionic groups, which determines

the surface potential. Counter ions in the solution will cover this layer, shielding the rest of the solution from the surface charges.

• Fro hydroxides the surface potential will be determined by reactions with the ions H+ and OH-. Thus, the surface potential is pH dependent.

• The pH where the particle is neutral is called PZC, point of zero charge.

For pH > PZC the surface is negatively chargedFor pH < PZC the surface is positively charged.Typical values: MgO 12, Al2O3 9.0, TiO2 6.0, SnO2 4.5, SiO2 2.5

Double layer: for a positively charged surface

• In an electric field the particle will move toward the electrode with the opposite charge. It will carry the adsorbed layer and part of the counter ions.

• The “slip plane” divides the part that moves with the particle and the solution. The potential at the slip plane is called the zeta (ζ) potential φζ.

• The pH for which φζ=0 is called the isoelectrical point (IEP)

• The stability of a colloid depends on φζ; the larger the φζ the more stable the colloid. Should be > 30-50 mV.

• Given the same surface potential, the repulsive forces will increase with the particle size.

May be accomplished by:

Electrostatic repulsion. By adsorption of charged species onto the surface of the particles, repulsion between the particles will increase and agglomeration will be prevented. Most important for colloidal systems.

Steric hindrance. By adsorbing a thick layer of organic molecules, the particles are prevented from approaching each other reducing the role of the van der Waals forces. Works best in concentrated dispersions. Branched adsorbates works best. Usual for nanomaterials.

Depends somewhat on the size of the particle and the degree of condensationThe size of the surface potential φ0 depends on the difference between pH and PZC.

Colloidal gold nanoparticles

Ostwald‘s ripeningmolecules on the surface of particles are more energetically unstable than those within the particle.The unstable surface molecules often go into solution shrinking the particle over time and increasing the number of free molecules in solution.In case of supersaturated solution those free molecules will redeposit on the larger particles.Small particles decrease in size until they disappear and large particles grow even larger. The shrinking and growing of particles will result in a larger mean diameter of a particle size distribution

Hydrothermal Synthesis:

Hydrothermal simulation of the 19th century geologists began to study the role of natural mineralization of 1900 years later,

scientists have established the theory of hydrothermal synthesis, functional materials and later turned to the study now has been prepared by hydrothermal method over a hundred kinds of crystals.

Hydrothermal method, also known as hydrothermal method, is a liquid chemical areas is in a sealed pressure vessel, with water as solvent, under conditions of high temperature and pressure of the chemical reaction based on the hydrothermal reaction water can be divided into different types of thermal oxidation, hydrothermal reduction, hydrothermal deposits, hydrothermal synthesis, hot water solution and hydrothermal crystallization of which hydrothermal crystallization is the one mostly used.

Hydrothermal synthesis• Crystal growth under high temperature and high pressure

conditions of water using substance that are insoluble at normal conditions (< 100°C, < 1 atm)

• Usually carried out below 300°C (ionic product kw has value around 275-300°C)

• Critical values for water are 374°C and 22.1 MPa respectively• The solvent properties for many compounds, such as

dielectric constant and solubility, change dramatically under supercritical conditions

• The dielectric constant of water is 78 at room temperature, where polar inorganic salts can be soluble in water.

• The dielectric constant of water decreases with increasing temperature and decreasing pressure. The dielectric constant is below 10 under supercritical conditions; the contribution of the dielectric constant to the reaction rates becomes remarkable based on the electrostatic theory.

• Supercritical water gives a favorable reaction field for particle formation, owing to the enhancement of the reaction rate and large supersaturationbased on the nucleation theory, due to lowering the solubility.

A scheme of setup for conducting of ultrasonically assisted hydrothermal synthesis: 1-titanium high pressure reactor

autoclave; 2-ultrasonic generator UZG 3-4; 3-magnetostricton transducer PMS-1-1;4-reaction vessel; resistance furnace; 5-chromelalumel thermocouple; 6-autoclave lid; 7-Titanium plug

Fundamentals:

Hydrothermal solution is the use of high temperature and pressure conditions in the atmosphere for those insoluble or insoluble substances dissolved or the dissolution of the substance reaction product solution by controlling the temperature within the

autoclave so that convection in the formation of super saturation and precipitation method of crystal growth natural hydrothermal mineralization is at a certain temperature and pressure, hydrothermal ore-forming materials in the process of precipitation from the solution. Hydrothermal synthetic gemstones that simulate the natural process of crystal-forming hydrothermal of growth.

Principle and mechanism of hydrothermal synthesis device:

Hydrothermal methods are extensively used for the synthesis of nanostructures. These are the kind of approaches towards crystal growth, in which the crystal is grown by chemical transport reactions (reaction between the ion acceleration) directly from the high pressure or high temperature. Both elemental and compounds nanostructures can be obtained via this method by choosing appropriate starting materials and suitable reaction conditions. Main devices used in the hydrothermal Synthesis are , the autoclave hanging seed, and filled mineralizers as shown in Fig below.

Main device used in the hydrothermal Synthesis.

Schematic diagram of principle and mechanism of hydrothermal synthesis.

Characteristics of hydrothermal method:

The synthesis of crystals with crystal faces, less thermal stress, fewer defects within its inclusions is very similar to natural gemstones. 1-Airtight container 2- Cannot observe the growth process, not intuitive. 3- Equipment requirements (high temperature high pressure steel and corrosion-resistant lining)4- Technical difficulties (strict control of temperature and pressure, high cost and poor safety performance for others).Hydrothermal method is characterized by the production of particles of high purity, good dispersion, good crystal and can be controlled, low production cost. Hydrothermal powders prepared by the general without sintering, which can be avoided during sintering and grain grow impurities easily mixed with other disadvantages. Hydrothermal synthesis of the impact of factors: high and low temperature, heating rate, stirring speed and reaction time.

The nature of the reaction mediumHigh-temperature thermal response characteristics of pressurized water are

1) An important reaction between the ion acceleration2) Increased hydrolysis 3) Significant changes in redox potential.

High-temperature high-pressure hot water system, water properties

High-temperature high-pressure hot water system, water propertiesVapor pressure becomes high, low density, low surface tension, low precision; ion product to high pressure at high temperature under hydrothermal conditions, room temperature, the reaction of substances soluble in water, ion-induced reactions can also be alive to promote the reaction, the reaction increased due to increased ionization constant of water.

The role of high-temperature high-pressure water

1- As chemical composition, and promote chemical reactions and rearrangement reaction accelerator. 2- Pass from the pressure of the role of media. 3- Improve the solubility of the material from the low melting point .4- Non-toxic.5-Sometimes react with the container

See-urchin Type Cluster self assembled with Pen-type nanoneedles Growth Mechanism

Morphological Characterizations & Growth Mechanism of ZnMn2O4 Nanostructures

Section No.3 Results and Discussion

Multifunctional probes