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16.982 Bio-Inspired Structures
Spring 2009
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Chemical Sy
ynthetic Routes for
for
Metal and Semiconductor
Semiconductor
Nanoparticles and Structures
Structure
Structures
Additional Synthetic Approaches
Approache
• Sonochemical
• Electrochemical
• Photochemical
• Chemical Vapor Deposition
Aspects of Nanoparticle Growth in
S l ti
Solution
Solution
ti
Arrested precipitation
Precipitation under starving conditions: a large number
numbe
of nucleation centers are formed by vigorous mixing of
the reactant solutions.
If concentration growth is kept small, nuclei growth is
stopped
pp due to lack of material.
Particles had to be protected from Oswald Ripening by stabilizers
Oswald Ripening
Th growthh mechanism
The
h i where
h small
ll particles
i l dissolve,
di l
and are consumed by larger particles. As a result the
averagge nanopparticle size increases with time and the
particle concentration decreases. As particles increase
in size, solubility decreases.
Synthetic Approaches for Metal and
and
Semiconductor Nanoparticles
Nanoparticles
via Chemical Routes
Routes
1. Metal Compound
• Positively charge metal salt, or
• Metal centers of complexes
2. Solvents (depends on the nature of the salt)
• Water
• Polar organic solvents
• Non-polar organic solvents
3 Red
3.
R ducing
i agentt (determi
(d t
inedd by
b the
th nature
t
off the
th metal
t l compound)
d)
• Gaseous hydrogen
• Hydridic compounds
• Reducing organics, e.g. alcohols
Many others
Stabilization of Nanoclusters Against
Against
Aggregation
Aggregation
1. Electrostatic stabilization
Adsorption
Ad
i off ions
i
to the
h
+
+ +
+
+ 

surface. Creates an electrical
-- + + - - + + -double layyer which results in a
+ + + + + 
+
+ -- - --- - -Coulombic repulsion force
+
between individual particles
2. Steric Stabilization
S
Surrounding
di thhe metall center by layers of material that are stericallyy bulkyy, Examples: polymers, surfactants, etc
Synthetic Approaches for Metal and
and
Semiconductor Nanoparticles
N
Nanoparticles
anoparticle
via Chemical Routes
Routes
4. Stabilizers
Role of stabilizers:
Stabilizing agents/ligands/capping agents/passivating agents
• prevent uncontrollable growth of particles
• prevent particle aggregation
aggregatio
• control growth rate
• controls particle size
• Allows particle solubility in various solvents
Other Common Stabilizers
1. Organic ligands
• Thiols (thioethanol,
(thioethanol thioglycerol,
thioglycerol mercaptoethylamine
mercaptoethylamine, etc)
• Amines
• phosphates
2. Surfactants
3. Polymers
4 Solvents
4.
ether
thioether
5. Polyoxoanions
Schematic Procedure for Cluster Synthesis
Synthesis
+ ++
++ +
Metal cations
in solution
Ligand molecules (•)
Add
electron donor
•• ••••
• •••••• •• •
Metal atoms in
solution
••••• •••••••••••
•
••••••• ••••••••••
•••••••••••••••••• ••••••••••••
Isolation in solid form
•••••• •••••••
••••••••••••••••
Cluster
formation
•••••
•
•••••••
Schematic Procedure for Cluster
Cluster
Synthesis
Synthesis
++ + + +
+ ++ ++
Metal cations
in solution
Add
Ligand
molecules (•)
+ +
• +• • • +
•• •
• + +
•
Add
electron
donor
•••••• •••••• ••••••
•••••••••••••••••
••••• • •••••
• • •• •
•••••••••••••• •••••• ••••••••
••• •• ••• •• •••
••
••• •••
•••• •••••• ••••••
•• ••••
•••••••••••••••••• ••••
Stabilized
Metal particles
in solution
Synthesis of Metal Nanoparticles in
in
Organic Media
Media
Medi
Biphasic reduction procedure
Aqueous
solution of
metal salt
Add phase transfer reag
gent e.g. tetraoctyl
ammonium
bromide
(TOAB)
Extract
Add
R d i Reducing
agent
TEM Image of Au Nanoparticles Prepared
Prepared
in the Presence of a Surfactant (CTAB)
(CTAB)
Image removed due to copyright restrictions.
J. Phys. Chem. B. 2001, 105, 4065
CTAB = cetyltrimethylammonium bromide
Nucleation and Growth
Growth
Homogeneous nucleation occurs via a stepwise sequence of
bimolecular additions until a nucleus of critical size is obtained.
a. Nucleation from supersaturated solution
nS
Sn
b. Diffusion-Controlled Growth
Sn + S
Sn+1
LaMer et al. J. Am. Chem. Soc. 1950, 72, 4847
Highly monodisperse nanoparticles are formed if the processes
processes
of nucleation and growth can be successfully separated
• Nucleation
N l i process must be fast
f
• Growth process must be slow
Nucleation
Nucleation
Nucleus Radius is calculated as follows:
G = 4(r2 – [2r3 / 3r*])
Where r = nucleus radius
Where
r* = critical nucleus radius
 = surface tension
G(nucleus) = n(G formation, bulk – G formation, free atom) + A
Where A = particle surface area
Dendrimer-Templated Nanocluster
DendrimerNanocluster Synthesis
Synthesis
Structure of poly(amidoamine) dendrimer (PAMAM)
Generation 2 PAMAM Dendrimer
Dendrimer-Templated Nanocluster Synthesis
DendrimerPioneered in 1998, by Donald A. Tomalia (Michigan Molecular Institute
Richard M. Crooks (TAMU)
PAMAM +
1 x 10-66 mol
CuAc2
1 x 10-55 mol
Cu Nanoclsuters
Formation of Cu nanoclusters
can be monit
itoredd by
b UV-vis
UV i
spectrophotometry
0.6000
Abs
590 nm
0.4000
0.2000
a
b
c
d
0.0000
200.00
Hydrazine
1 x 10-4 mol
400.00
600.00
Wavelength, nm
800.00
Figure by MIT OpenCourseWare.
Reaction is pH dependent:
Presumably H+ ions compete
with Cu2+ ions for the tertiary
amine sites
J. Am. Chem. Soc. 1998, 120, 4877
J. Am. Chem. Soc 1998, 120, 7355
Reverse Micelles
Water-in-oil droplets
[H2O]
Water pool w =
[surfactant]
Particle size is controlled by the size of the water
wate
droplets in which synthesis takes place
Consider that:
Rw =
R = 3V/A
3Vaq[H2O]
[s]
[s]
V = volume , R = radius,
radius
A = surface area
 = head polar group area
Vaq = volume of water
Parameters Affecting Particle Growth/
Sh
Shape/
/ St
Structture
ure
•
•
•
•
Type of capping agent/stabilizers
Concentration of the reactant
reactants
pH value of the solution
D
Duration
i off hheat treatment
Sonochemical Approaches for
for
Nanoscale Particle Synthesis
Synthesis
Synthesi
Rectified Diffusion
‘area’ eff
ffect
Acoustic
preessure
+
-
‘shell’ effect
Bubble
Liquid shell
Bubble
Sonochemical Approaches for
for
Nanoscale Particle Synthesis
Synthesis
Synthesi
Step 1:
• Bubble expands when surrounding medium experiences –ve
pressure
• Bubble collapses when surrounding medium experiences +ve
pressure
• Bubble collapse leads to extreme temperatures (5,000 – 50, 000
K), and pressure (100 atm) within the bubble
Step 2:
Solvent or solute molecules present within the bubbles are
decomposed under these extreme conditions and generate highly
reactive radicals
Formation of Highly Reactive Radicals
Depending on the liquid medium, sonication leads to the
generation of oxidizing and reducing radicals
In aqueous solution
H2 O
H • + OH •
M+ + H •
M + H+
In solutes like alcohols, sonication leads to secondary radicals
RHOH + H• (OH •)
ROH •+ M+
ROH • + H2(H2O)
M + RO + H+
Examples of Metal Nanoparticles
Nanoparticles
Prepared by Sonication
Sonication
Sonicatio
Ag nanoparticles prepared in aqueous solution at 1 MHz
H2 O
H• + OH•
Ag+ + H •
Ag + H+
J. Phys. Chem. 1987, 91, 6687
Au nanoparticles prepared by sonication
AuCl4
ROH •
Au + Products
Langmuir 2002, 18, 7831-7836
Synthesis of CdS Nanoparticles
General:
Anionic or Cd(II) salt + Lewis basic + Sulfide source
polymers
Sodium
Cd(NO3)2.4H
4H2O + polyphosphate + Na2S
2 x 10-4 M
2 x 10-4 M
2 x 10-4 M
1-10 nm
CdS
CdS
Chem. Mater. 1999, 11, 3595
Synthesis of CdSe Nanoparticles
Nanoparticles
General:
High
Phosphine oxide
Cd(CH3)2 + Se reagent +
temperature
surfactant
Cd(CH3)2 + (C8H17)3PSe
HAD-TOPO-TOP
CdSe
CdSe
HAD-TOP-TOP
A
O O = hhexadecylamine-trioctylphosphine
d l i
i
l h hi oxide-trioctylphosphine
id i
l h hi
J. Am. Chem. Soc. 1993, 115, 8706
Synthesis of ZnSe Nanoparticles
Nanoparticles
Zn/Se
/ TOP
Solution
Zn(CH3CH2)2 + (C8H17)3PSe
o
Zn/Se
Z
/S TOP + Hexadecylamine 270 C
Solution
ZnSe
J. Phys. Chem. B. 1998, 102, 3655
TOPO binds too strongly to Z
Zn
TOP binds too weakly
Amines, however have intermediate strength
Synthesis of IIIIII-V Semiconductor
Semiconductor Nanoparti
Nanoparti
ticllles
tic
es
Synthesis of III-V
III-V semiconductor nanoparticles is quite complex
Requires high temperature
370 – 400 oC
GaP
GaCl3 + tris-(trimethylsilyl)phosphine + TOPO-TOP
GaP particles prepared in this manner lacked monodispersity
G Cl3 + As(SiMe
GaCl
A (SiM 3)3
~ 700 oC
GaAs
Chem. Mater. 1989, 1, 4
J. Am. Chem. Soc. 1990, 112, 9438
Synthesis of InP and InCl3
3
oC, TOP
260
InCl3 + [(
[(CH3)3Si]]3P
InP nanocrystals
y
Synthesis of InAs via Dehalosilylation
Me3SiCl
evolved
InCl3 + (Me3Si)3As
3 days, rt
Me3SiCl
evolved
oC
70-75
4 days
d
Me3SiCl
evolved
oC
150
4 days
d
InAs
Chem. Mater. 1989, 1, 4
Factors Affecting the Nature of the
the
Nanoparticle
Nanoparticle
•
•
•
•
Particle size and shape
S f
Surface
properti
ties
Particle-solvent interactions
Particle-particle interactions
Common Methods for Nanoparticle
Nanoparticle
Characterization
Characterization
Surface state
Particle
Size
Surface
Area
Electron Microscopy
X-ray diffraction
Magnetic Measurements
Surface
composition
Surface
Complexes
AES,
XPS,
SIMS,
EPMA,
A
EXAFS
Surface
structure;
Topography
LEED
SEM
TEM
EXAFS
IR, UV-Vis, ESR, NMR, Raman
UV-Visible Spectroscopy
UV• Particularly effective in characterizing semiconductor and
metal particles
• Useful for metal nanoparticle characterization whose surface
pllasmon ab
bsorb
bance lies
li in th
he viisibl
ible range, e.g. Cu, Au, Ag
• Can be used to determine particle size:
(For semiconductor nanoparticles: as the radius decreases, the
band gap increases and max shifts to lower energy.
• Particle aggregation
• Information
I f
i about
b
the
h surface,
f
e.g. presence off adsorbates
d b
Infrared Spectroscopy
Spectroscopy
IR has been used as a surface probe for nanostructures
Example illustrated by Bardley:
Adsorbing CO onto the metal nanoparticle surface resulted in IR
depending on particle size
siz
More edge:
M
d
linear CO is
stable
2.5 nm 4 nm
More face:
bridged CO
is stable
6 nm
As particle size increased, the ratio of terminal CO to bridging
CO decreased
Bradley,
y, et al. Chem. Mater. 1992,, 4,, 1234
Nuclear Magnetic Resonance (NMR)
Two uses:
1. Probing the ligands that surround metal core
2. Probing the intra-core metallic atoms (difficult)
Probingg the intra-core metallic atoms
Nuclear spin relaxation time, and nuclear resonance frequency, are sensitive to any metallic property the particle may exhibit
Change in frequency (known as ‘Knight shift’) is a consequence
of the interaction of the metal nucleus with the conduction band
electrons
If particles are very small, in favorable cases, the Knight shift allows resonances for surface and interior metal particles to be
b
identified
Microscopy
Microscopy
• TEM: High voltage beam passes through a very thin
sample The sample areas that do not allow passage of
sample.
electrons allow image to be presented
• STM: Involves dragging a sharp needlelike probe across a
sample very close to the surface. The tunneling current
between the sampple and probe tipp can be monitored . As
probe approaches an elevated portion, the probe moves up
and over, and produces a surface map.
• AFM: The probe tip is essentially touching the surface,
and the surface can be mapped by the weak interaction
between the tip and the sample.
sampl
Transmission Electron Microscopy
Microscopy
• Provides direct visual information of size, shape, dispersity,
structure and morphology
• Routine magnifications > 40,000 to 0.2 nm
Drawbacks
• Samples are dried and examined under high vacuum conditions
• Therefore, no direct information is gained on how particles exist in
solution
• Only a finite number of particles can be examined and counted,
which may not be a representative of the sample as a whole
• Requires electron beam in which case, some nanoparticles may
undergo structural rearrangement, aggregation or decomposition.
Scanning Tunneling Microscopy (STM)
(STM)
Makes possible the determination of the total diameter of the
nanoparticle including the stabilizing ligand shell
nanoparticle,
shel
Effective probe of the electronic properties of nanoparticles
Reetz et al. Science 1995, 267, 367
367-369
369
A combined STM/TEM study of Pd nanoparticles stabilized by
R4N+Br
B -.
Determined thickness of stabilizing ligand shell by subtracting
th STM determined
the
d t
i d diameter
di
t from
f
the
th TEM determined
d t
i d
diameter
Shortcomings to STM
STM
• Nanoparticles may not stick well to the substrate surface,
preventing good images from being obtained
• Geometry of the tip shape may lead to inaccurate measurements
t or artifacts
tif t in
i the
th image
i
• Tunneling mechanism is not well understood
• Samples have to be dried
•
drie
• Specific techniques applied to imaging are not mature,
mature i.e.
i e
standard literature protocols have not been established
Atomic Force Microscopy
• Technique is purely mechanical
– A cantilevered tip attached to a spring is dragged across
acros
a sample
– Increase or decrease in tip height is measured yielding a
surface height profile as a function of distance
– Can be carried out on non-conducting samples
Shortcomings
• Can reliably determine particle height but not diameter
• Cannot distinguish between subtle shape differences, or image particles that are not spatially close to each other