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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Production of Nanoparticles
by Electrosprays
Apple group
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Introduction to Electrosprays of Nanoparticles
• Nanoparticles are of interests because their chemical and
physical behavior is unprecedented, making way for applications
in electronics, chemical and mechanical industries, drug
delivery, magnetic materials, as well as a variety of others
• Monodispersed Particles preferred
• Goal: Want a process in which particles having controlled
characteristics such as size, morphology and composition can
be produced, at the lowest cost and highest yield.
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Use of the Aerosol Process and Methods of Preparation
• There are at least tow routes for the preparation of ultrafine
particles by aerosol process.
• Gas to Particle Conversion (Build up Method)
– Advantages: Small particle size, narrow distribution, high
purity of particles produced
– Disadvantages: Formation of hard agglomerates in the gas
phase, difficulty in separating multi-component materials,
non-uniform composition
• Liquid to Solid Conversion (Break Down Method)
– Advantages: Multicomponent materials prepared well,
comparatively low cost
– Disadvantages: Not very well understood
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
CVD vs. PVD
• For Gas to Particle Conversion
• CVD: Chemical Vapor Deposition
– The vapor evaporated from the solution precursors is
thermally decomposed or reacted w/ another precursor
vapor or surrounding gas. Solids are produced during
nucleation, condensation and coagulation
• PVD: Physical Vapor Deposition
– Evaporation of a solid or liquid is the source of vapor
– In the cooling stage nucleation and condensation of the
saturated vapor take place and solid particles are formed
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Electrospray Physics
Taylor cone
Electrospray jet
Capillary
To colloid source
E-field
Voltage Src.
Colloid solution (electrolyte)
Under normal laminar flow
Collector/ voltage
sink
Solvent
Particle
From
colloid
Solution, no particle
yet formed
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
The Taylor Cone-Jet
Accumulation of excess
surface charges
Formation
of charged droplets
Electric field required for onset of electrospray:
.5
 2 cos(49 ) 
E0  

 0 rc


Surface tension
Vacuum
permittivity
Taylor cone forms along direction of electric field under
the competing influences of charge accumulation and surface
tension
•Liquids with high surface tension (e.g. water) require an enormous electric field
•Leads to problems with discharging
•Cone-jet mode for low flow rates and small electrolyte concentrations
•Charged droplet formation is statistical (no electrochemistry needed)
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Capillary
radius
Smith, 1989
Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Droplet Evolution
 Qf  


K


1/ 3
Radius of an evolved droplet:
Charge of an evolved droplet:
Ri
qi
Qf  vol. flow rate
K  solution conductivity

3 1/ 2
0.7 8  0 R 

Colloid
pathway
To reaction
Chamber / drier
Newly formed droplet
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Solvent Evaporation
~ 100 ms to Raleigh limit
Coulombic explosion
(droplet fission)
Accelerated
to counter
electrode
Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
\
Rayleigh Limit
Rayleigh limit: the equilibrium state
at which further addition of charge will cause the
drop to become unstable and break
qlim  8  0 R

3 1/ 2
nm range
Excess charge
Coulomb fission
Daughter droplets not equal!
•Offspring usually carry 2% of mass and 15%
of charge
ERy   4 /  0 R 
1/ 2
As large drops evaporate new Rayleigh limits are
reached until drops of the nanosize are predominant
When all solvent evaporates, we are left with our
particles (charged solid residues)
For very small drops, ion evaporation occurs.
This is more favorable than successive fissions
past some critical drop size.
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E-field required for fission (as opposed to
ion evaporation)
Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Limits of Electrospray
Particles produced by complete evaporation of droplets
(solid residue formation) require VERY LOW concentrations for
small sized particles:
10 mm
100 nm
Example: Produce a 100 nm particle of density r=2000 g/cm3 by
evaporating drops of 10 mm. What is the necessary solution
concentration (in g/mL) of solute? Assume the solute is nonvolatile.
3
d

  particle   1e  6  0.0001%
 d

Vdrop
 drop 
V
g
r particle  0.002 3
Vdrop
cm
V particle
This is a fairly low concentration. Electrosprays can produce droplets
from about 200 nm to 10 mm. Concentrations can become prohibitively
low for smaller particles (10 nm2e-6 g/cm3).
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\
Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Colloid Sprays
Electrosprays can atomize colloid solutions down to 1 particle/drop resolution!
Dispersed particles
(colloid)
Evaporation
Maintains integrity of colloidal particle
Coulombic
explosion
Drop size dependent on pre-reaction,
not size of droplet (below resolution)
Sample Dry Particle Generator
Reactants
Heat
Collector
Particle size
depends on residence
time in reactor and reaction
inside drop
Colloidal
mixture
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Voltage
Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Reactive Sprays
Sometimes we wish to produce small droplets of reactants,
then ‘turn on’ the reaction when the droplets are small enough.
Example: Production of ZnS particles via electrospray (de la Mora)
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Reactive Sprays (continued)
Particle size depends on reactant concentration (rate), temperature, pressure, gas phase
composition, drop size, residence time in reactor… and many more variables.
In this example:
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HEAT
Zn  NO3 2 +  NH2 2 SC 
 ZnS
Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Applications of Electrosprays in Nanoparticle
Production
• Ionization for mass spectrometry of large
biomolecules
• Fine metal powder production
• Deposition of ceramics
• Electrospray pyrolysis for metal salts production
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Ionization for mass spectrometry of large biomolecules
•
Mass spectrometry
ES ionization for MS
Transform individual molecules into
ions in vacuo and measure the
trajectories in electric or magnetic
fields.
•
Classical methods of
ionization
Based on gas-phase encounters of
molecule to be ionized with
electrons, photons or other ions.
Electrostatic lenses
Quadrupole mass
spectrometer
Cylindrical
electrode
Capillary
Needle
Liquid
sample
Skimmer
•
Difficulties in large
biomolecules
Vaporization together with
extensive, catastrophic
decomposition
Drying gas
John B. Fenn et al, Science 1989
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Ionization for mass spectrometry of large biomolecules
(continued)
Evaporation
1
nL
Initial droplet
Desolvation in the
self-generated
electric field
Quasi-molecular ion
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2   dp
KE  e
2
3 2
Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Fine metal powder production
• Classical method
Impacting a liquid metal stream
by either gas or liquid jets
consumable
electrode
+H.V
Molten tip
E-field
• Other methods
Rotating electrode, centrifugal
atomization, gas evaporation
• Advantages of ES method
Extractor
electrode
Droplet beam
Annular
electron
emitter
Sufficient yields and efficiency
John F. Mahoney et al, IEEE
transactions on industry applications.
1987
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Liquid from
pump
Deposition of Ceramics
•
Aim of ceramic manufacturing
process:
Metallic capillary nozzle
E.H.T
Fine particles will be deposited one at
a time at high speed
•
Comparison of ES method
with jet printers w.r.t
volumetric resolution:
Infinite plate
Resistor
W.D. Teng et al, J. Amer. Ceram. Soc. 1997
Jet printer
Conditions
Droplet size
# particles per droplet
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Ring
electrode
ES atomization
0.2 μm diameter
particle, 5 vol%
0.2 μm diameter
particle, 5 vol%
0.74 μm diameter
particle, 5 vol%
50 μm
<=2 μm
<=2 μm
0.8 million
50
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Electrospray pyrolysis for metal salt nanoparticle production
General spray pyrolysis process in
metal salt nanoparticle production:
Conventional spray
pyrolysis:
Solvent containing metal salt
Size
determination
Spray methods
Droplets
Introduced into furnace
Final
particles
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Very hard to get particles with
diameter less than 100 nm.
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Electrospray pyrolysis:
A method to generate
ultrafine droplets.
Evaporation,
diffusion, drying,
precipitaion,
reaction or sintering
Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Analyzing sizes of Colloidal Nanoparticles
• Transmission/Scanning electron microscopy
(TEM/SEM)
• Scanning near-field optical microscopy
• Scanning probe microscopy
• Atomic force microscopy
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
What is a DMA?
• Two charges concentric cylinders with an inlet slit &
sampling slit
• Separates particles based on their electrical mobility
• Aerosol particles are inserted and carried by clean air
through the annular region
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Concerns with Analysis techniques
• Sampling quality
• Operators ability
• Time spent on procedures
• Characterizing the size of isolated nanoparticles imbedded
within an oxide layer or substrates
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
New techniques for Sizing
• Transferring the colloidal particles from the liquid into
gas phase using electrosprays
• Particle sizing in the gas phase using an inertial
impactor or a differential mobility analyzer (DMA)
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Adjustment proposal
• Need to use the new
techniques in series
• Need for a high charge on
the particles
• Need to account for the large
particle losses in the flow
lines between the detector &
electrospray
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• Need less deformation of the
size distribution
• Need a method for
“unknown” sizes as well as
smaller ones
• Assessed by analyzing
colloids’ spheres with
diameters from 21 to 74
nanometers
Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Determination of the Stability Domain
• Measure the typical curves of the electric current vs.
the applied voltage of the spraying solutions
• Increase the voltage gradually
• Reduce the liquid meniscus at the capillary tip to a
dripping mode, then a pulsating mode
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Size distribution of Naturally Dried Colloids
• Raw material dries naturally prior to TEM/SEM analysis
• An average is taken with the collected nanoparticles
• Some particles are encapsulated by a large amount of
surfactants
• Surfactants generate a residue around the particles during the
solvent evaporation
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Particle Size Distribution of Electrosprayed Colloids
•
•
•
•
Measured using the ES-DMA system
Ambient temperature of 25oC & liquid flow rate at its minimum
Initial concentration influences final particle size distribution
At high concentrations, produces more than one particle per
droplet
• At low concentrations, changes electrical conductivity but size of
particles barely change
• At medium concentrations, provides proper concentration per
droplet ( defines conditions of 1 particle/droplet)
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Production of Nanoparticles by Electrosprays
Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman
Homework Problem
Why is the particle distribution the same over all tested concentration ranges??
Assume that the only variable changed is the reactant concentration.
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