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Nanoparticle Synthesis Jimmy C. Yu Department of Chemistry Environmental Science Programme The Chinese University of Hong Kong. Nanotechnology:. • A technology. • Design, production, characterization, and applications of Nanostructured materials. (Size and Shape).

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Nanoparticle SynthesisJimmy C. YuDepartment of ChemistryEnvironmental Science ProgrammeThe Chinese University of Hong Kong



•A technology

•Design, production, characterization, and applications of Nanostructured materials

(Size and Shape)

•Also, fundamental understanding of physical properties and phenomena


•Study on fundamental relationships between physical properties and material dimensions on the nanometer scale

1. Nanotechnology and Nanoscience

(Royal Society of London, July 2004)


Things Natural

Things Manmade

1 cm

10 mm

10-2 m

MicroElectroMechanical devices

10 -100 mm wide

10-3 m

1 millimeter (mm)


~ 5 mm

0.1 mm

100 mm

10-4 m

Head of a pin

1-2 mm

Human hair

~ 10-50 mm wide

0.01 mm

10 mm


10-5 m

Red blood cells

with white cell

~ 2-5 mm

1 micrometer (mm)

10-6 m

0.1 mm

100 nm

10-7 m

Nanotube transistor

Nanotube electrode

0.01 mm

10 nm

10-8 m


1 nanometer (nm)

10-9 m

~10 nm diameter

Carbon nanotube

~2 nm diameter

ATP synthase

Quantum corral on copper surface

Corral diameter 14 nm

Atoms of silicon

spacing ~tenths of nm

10-10 m

0.1 nm


~2-1/2 nm diameter

The Scale of Things – Nanoworld and More

nanostructured materials
Nanostructured Materials


  • Nanoparticles (including quantum dots) exhibit quantum size
  • effects• Nanorods and nanowires• Thin films• Bulk materials made of nanoscale building blocks or consisting of nanoscale nanostructures
why nano
Why “nano”
  • Nanomaterials have superior properties than the bulk substances :
    • Mechanical strength
    • Thermal stability
    • Catalytic activity
    • Electrical conductivity
    • Magnetic properties
    • Optical properties
    • …….

A wide range of applications:

Quantum electronics, nonlinear optics, photonics, sensing, information storage and processing, adsorbents, catalysis, solar cells, superplastic ceramics…

New fields:

Nanofabrication, nanodevices, nanobiology, and nanocatalysis

two different approaches to nanofabrication

Bottom ⇨ Up:

•Building what you want by assembling it from building blocks ( such as atoms and molecules).

•Atom-by-atom, molecule-by-molecule, or cluster-by-cluster

Two Different Approaches to Nanofabrication
  • Top ⇨ Down:

•Start with the bulk material and “cut away material” to make the what you want


2. Nanoparticle Synthesis Strategies

2.1 Liquid-phase synthesis2.2 Gas-phase synthesis2.3 Vapor-phase synthesis

2 1 liquid phase synthesis
2.1 Liquid-Phase Synthesis

• Coprecipitation• Sol-gel Processing• Microemulsions• Hydrothermal/Solvothermal Synthesis• Microwave Synthesis• Sonochemical Synthesis • Template Synthesis• Biomimetic Synthesis


xAy+(aq) + yBx-(aq) AxBy(s)


Coprecipitation reactions involve the simultaneous

occurrence of nucleation, growth, coarsening, and/or agglomeration processes.

Coprecipitation reactions exhibit the following characteristics:

(i) The products are generally insoluble species formed under conditions of high supersaturation. (ii) Nucleation is a key step, and a large number of small particles will be formed. (iii) Secondary processes, such as Ostwald ripening and aggregation, dramatically affect the size, morphology, and properties of the products. (iv) The supersaturation conditions necessary to induce precipitation are usually the result of a chemical reaction.

Typical coprecipitation synthetic methods: (i) metals formed from aqueous solutions, by reduction from nonaqueous solutions, electrochemical reduction, and decomposition of metallorganic precursors; (ii) oxides formed from aqueous and nonaqueous solutions; (iii) metal chalconides formed by reactions of molecular precursors; (iV) microwave/sonication-assisted coprecipitation.

Example: Precipitation of ZnS nanoparticles from a solution containing thioacetamide and zinc acetate

Thioacetamide is used as a sulfide source.

Zn2+ + S2- ZnS

Murray C.B. et al., Annu. Rev. Mater. Sci. 2000, 30, 545.

sol gel processing
Sol-gel processing

The sol-gel process is a wet-chemical technique that uses either a chemical solution (sol short for solution) or colloidal particles (sol for nanoscale particle) to produce an integrated network (gel).

Metal alkoxides and metal chlorides are typical precursors. They undergo hydrolysis and polycondensation reactions to form a colloid, a system composed of nanoparticles dispersed in a solvent. The sol evolves then towards the formation of an inorganic continuous network containing a liquid phase (gel).

Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution.

After a drying process, the liquid phase is removed from the gel. Then, a thermal treatment (calcination) may be performed in order to favor further polycondensation and enhance mechanical properties.

example tio 2 nanoparticle mediated mesoporous film by sol gel processing


+H2O Stabilizer

Nanodisperse Oxide Sol

(Particulate or Polymeric)





T > 400 ºC



Porous TiO2

Example: TiO2 nanoparticle-mediated mesoporous film by sol-gel processing

100 nm

TiO2 nanoparticle-mediated mesoporous film (Yu, J. C. et al. Chem. Mater. 2004, 16, 1523.)


Microemulsions are clear, stable, isotropic liquid mixtures of oil, water and surfactant, frequently in combination with a cosurfactant.

The aqueous phase may contain salt(s) and/or other ingredients, and the "oil" may actually be a complex mixture of different hydrocarbons and olefins.

The two basic types of microemulsions are direct (oil dispersed in water, o/w) and reversed (water dispersed in oil, w/o).

Nanosized CdS-sensitized TiO2 crystalline photocatalyst prepared by microemulsion. (Yu, J. C. et al. Chem. Commun. 2003, 1552.)

hydrothermal solvothermal synthesis
Hydrothermal/Solvothermal Synthesis

In a sealed vessel (bomb, autoclave, etc.), solvents can be brought to temperatures well above their boiling points by the increase in autogenous pressures resulting from heating. Performing a chemical reaction under such conditions is referred to as solvothermal processing or, in the case of water as solvent, hydrothermal processing.



Yu, J. C. et al. J. Solid State Chem. 2005, 178, 321; Cryst. Growth Des.2007, 7, 1444

microwave assisted synthesis
Microwave-Assisted Synthesis

Microwaves are a form of electromagnetic energy with frequencies in the range of 300 MHz to 300 GHz. The commonly used frequency is 2.45G Hz.

Interactions between materials and microwaves are based on two specific mechanisms: dipole interactions and ionic conduction. Both mechanisms require effective coupling between components of the target material and the rapidly oscillating electrical field of the microwaves.

Dipole interactions occur with polar molecules. The polar ends of a molecule tend to re-orientate themselves and oscillate in step with the oscillating electrical field of the microwaves. Heat is generated by molecular collision and friction. Generally, the more polar a molecule, the more effectively it will couple with the microwave field.

conventional heating by conduction
Conventional Heating by Conduction
  • conductive heat
  • heating by
  • convection currents
  • slow and energy
  • inefficient process

The temperature on the outside surface is

in excess of the boiling point of liquid

heating by microwave irradiation
Heating by Microwave Irradiation
  • Solvent/reagent
  • absorbs MW energy
  • Vessel wall
  • transparent to MW
  • Direct in-core heating
  • Instant on-off

inverted temperature gradients !


Microwave (MW) rapid heating has received considerable attention as a new promising method for the one-pot synthesis of metallic nanostructures in solutions.

In this concept, advantageous application of this method has been demonstrated by using some typical examples for the preparation of Ag, Au, Pt, and AuPd nanostructures. Not only spherical nanoparticles, but also single crystalline polygonal plates, sheets, rods, wires, tubes, and dendrites were prepared within a few minutes under MW heating. Morphologies and sizes of nanostructures could be controlled by changing various experimental parameters, such as the concentration of metallic salt and surfactant polymer, the chain length of the surfactant polymer, the solvent, and the reaction temperature. In general, nanostructures with smaller sizes, narrower size distributions, and a higher degree of crystallization were obtained under MW heating than those in conventional oil-bath heating.

Tsuji M. et al.

example microwave assisted synthesis of zno nanoparticles
Example: Microwave-assisted synthesis of ZnO nanoparticles

1 mm

100 nm

Schematic representation and transmission electron microscope (TEM) images of ZnO-cluster nanoparticles prepared by microwave irradiation

Yu, J. C. et at., Adv. Mater. 2008, in press.

sonochemical synthesis
Sonochemical Synthesis

Ultrasound irradiation causes acoustic cavitation -- the formation, growth and implosive collapse of the bubbles in a liquid

The implosive collapse of the bubbles generates a localized hot spots of extremely high temperature (~5000K) and pressure (~20MPa).

The sonochemical method is advantageous as it is nonhazardous, rapid in reaction rate, and produces very small metal particles.

examples sonochemical synthesis of mesoporous tio 2 particles
Examples: sonochemical synthesis of mesoporous TiO2 particles

Mesoporous TiO2

20 kHz sonochemical processor

formation of mesoporous tio 2 by sonication
Formation of mesoporous TiO2 by sonication

TIP:Titanium isopropoxide

UIA: Ultrasound Induced Agglomeration

Titanium Oxide Sol Particle










Acetic acid modified TIP


Yu J. C. et al., Chem. Commun.2003, 2078.


Sono- and Photo-Chemical Deposition of Noble Metal Nanoparticles

40 kHz ultrasound

Cleaning Vessel

Yu J.C. et al., Adv. Funct. Mater. 2004, 14, 1178.

biomimetic synthesis
Biomimetic Synthesis

Nature is a school for material science and its associated discipline such as chemistry, biology, physics or engineering. Nature fascinates scientists and engineers with numerous examples of exceptionally building materials. These materials often show complex hierarchical organization from the nanometer to the macroscopic scale.

Learning from nature and imitating the growth and assembly processes found in nature enable new strategies for the design of nanoarchitectures. Biomimetic or bio-inspired processes generally occur under mild conditions such as room temperature, aqueous environment, and neutral pH, and thus are benign in comparison to traditional chemical reactions. Biologically inspired synthesis, hierarchical structuring, and stimuli-responsive materials chemistry may enable nanostructured materials systems with unprecedented functions.

Many exciting bioinspired materials concepts are currently under development, such as composite materials with nacre-like flaw tolerance, gecko-inspired reversible adhesives, and advanced photonic structures that mimic butterfly wings.

examples biomimetic synthesis
Examples: biomimetic synthesis

Model for silver crystal formation by silver-binding peptides.

biosynthetic silver nanoparticles.

(Stone M. O. et al. Nat. Mater.2002, 1, 169.)

Protein-encapsulated CoPt nanoparticles by bio-inspired synthesis

A protein of methanococcus jannaschii MjHsp

(Stone M. O. et al. Adv. Funct. Mater.2005, 15, 1489.)

2 2 gas phase synthesis
2.2 Gas-Phase Synthesis
  • Supersaturation achieved by vaporizing material into a background gas, then cooling the gas
    • Methods using solid precursors
      • Inert Gas Condensation
      • Pulsed Laser Ablation
      • Spark Discharge Generation
      • Ion Sputtering
    • Methods using liquid or vapor precursors
      • Chemical Vapor Synthesis
      • Spray Pyrolysis
      • Laser Pyrolysis/ Photochemical Synthesis
      • Thermal Plasma Synthesis
      • Flame Synthesis
      • Flame Spray Pyrolysis
      • Low-Temperature Reactive Synthesis
example gas phase chemical preparation of tio 2 ticl 4 g o 2 g tio 2 s 2cl 2 g tubular reactor
Example: Gas Phase Chemical Preparation of TiO2TiCl4 (g) + O2 (g) = TiO2 (s) + 2Cl2 (g)Tubular reactor
2 3 vapor phase synthesis
2.3 Vapor-Phase Synthesis
  • Same mechanism as liquid-phase reaction
  • Elevated temperatures + vacuum (low concentration of growth)
  • Vapor phase mixture rendered thermodynamically unstable relative to formation of desired solid material
    • “supersaturated vapor”
    • “chemical supersaturation”
    • particles nucleate homogeneously if
      • Degree of supersaturation is sufficient
      • Reaction/ condensation kinetics permit
  • Once nucleation occurs, remaining supersaturation is relieved by condensation, or reaction of vapor-phase molecules on resulting particles. This initiates particle growth phase.
  • Rapid quenching after nucleation prevents particle growth by removing source of supersaturation, or slowing the kinetics.
2 3 vapor phase synthesis continued
2.3 Vapor-Phase Synthesis (continued)
  • Coagulation rate proportional to square of number concentration (weak dependence on particle size)
  • Nanoparticles in gas phase always agglomerate. Loosely agglomerated particles may be re-dispersed. Hard (partially sintered) agglomerates cannot be fully redispersed.
  • size affections
    • reaction and nucleation

3. Challenges

  • Means to achieve monodispersity
  • Size and shape control
  • Reproducibility
  • Scale up
  • Building complex nanostructures