Nothing Special   »   [go: up one dir, main page]
Scribd Logo
Upload

Welcome to Scribd!

  • 55 views

    Lecture 13q 2

    Uploaded by

    Aahil Aleem
    The document discusses various methods for synthesizing nanoparticles, including top-down and bottom-up approaches. It describes growth mechanisms for nanoparticles, including nucleation and growth processes. Different fabrication methods are covered, such as sol-gel processing and hydrothermal synthesis. Sol-gel involves forming a colloidal solution and gel from molecular precursors, while hydrothermal synthesis uses high temperatures and pressures to crystallize insoluble compounds.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd

    Lecture 13q 2

    Uploaded by

    Aahil Aleem
    55 views46 pages
    The document discusses various methods for synthesizing nanoparticles, including top-down and bottom-up approaches. It describes growth mechanisms for nanoparticles, including nucleation and growth processes. Different fabrication methods are covered, such as sol-gel processing and hydrothermal synthesis. Sol-gel involves forming a colloidal solution and gel from molecular precursors, while hydrothermal synthesis uses high temperatures and pressures to crystallize insoluble compounds.

    Original Description:

    Experimental

    Copyright

    © © All Rights Reserved

    Available Formats

    PDF, TXT or read online from Scribd

    Did you find this document useful?

    Is this content inappropriate?

    The document discusses various methods for synthesizing nanoparticles, including top-down and bottom-up approaches. It describes growth mechanisms for nanoparticles, including nucleation and growth processes. Different fabrication methods are covered, such as sol-gel processing and hydrothermal synthesis. Sol-gel involves forming a colloidal solution and gel from molecular precursors, while hydrothermal synthesis uses high temperatures and pressures to crystallize insoluble compounds.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    55 views46 pages

    Lecture 13q 2

    Uploaded by

    Aahil Aleem
    The document discusses various methods for synthesizing nanoparticles, including top-down and bottom-up approaches. It describes growth mechanisms for nanoparticles, including nucleation and growth processes. Different fabrication methods are covered, such as sol-gel processing and hydrothermal synthesis. Sol-gel involves forming a colloidal solution and gel from molecular precursors, while hydrothermal synthesis uses high temperatures and pressures to crystallize insoluble compounds.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    You are on page 1of 46

    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

    Fabrication Approaches
    Nanomaterials
    Section No.2

    Experimental Methods

    1- Top-Down Approach
    Creates nanostructures out of
    macrostructures
    by
    breaking
    down matter into more basic
    building blocks
    2- Bottom- Up Approach
    Building complex systems by
    combining simple atomic level
    components
    through
    self
    assembly
    of
    atoms
    or
    molecules into nanostructures

    Two strategies: 1: Top-Down

    Two strategies: 2: Bottom Up

    Kinetics of Growth mechanism

    Growth mechnism of nanoparticles


    The processes of nucleation
    and subsequent growth.

    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.

    Growth mechnism of nanoparticles


    When 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 nuclei
    The 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, r0, the radius
    difference, r, decreases as time increases or particles grow
    bigger, according to:
    r = r0 r0/r ....... (2)
    Combining with equation (1) we have:
    r = r0 r0 /(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 coprecipitation, 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 charged
    For 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 () potential
    .
    The pH for which =0 is called the
    isoelectrical point (IEP)
    The stability of a colloid depends on
    ; the larger the 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


    condensation
    The size of the surface potential 0 depends on the difference between pH
    and PZC.

    Colloidal gold nanoparticles

    Ostwalds ripening
    molecules 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 (< 100C, < 1 atm)
    Usually carried out below 300C (ionic product kw has value
    around 275-300C)
    Critical values for water are 374C 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 supersaturation
    based 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; 5chromelalumel 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 medium


    High-temperature thermal response characteristics of pressurized water are

    1) An important reaction between the ion


    acceleration
    2) 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 properties
    Vapor 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 highpressure 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

    Morphological Characterizations & Growth


    Mechanism of ZnMn2O4 Nanostructures
    Section No.3

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

    Results and Discussion

    Growth Mechanism

    Multifunctional probes

    You might also like