Characterization of Nanoparticles
1. Meaning of Characterization
Characterization means studying the
important properties/features of nanoparticles.
It helps us know:
- Composition –
nanoparticles mein kaunse elements/chemicals present hain.
- Structure –
atoms/molecules ka arrangement kaisa hai.
- Size
and shape – particle kitna small hai aur kis shape ka
hai.
- Physical
properties
- Electrical
properties
- Magnetic
properties
- Optical
properties
Why is characterization important?
Nanoparticles ki properties unke size aur shape ke
according significantly change ho sakti hain.
Therefore, accurate measurement of size, shape and
structure is very important for their application.
2. General Characterization Methods
A. Microscopy
1. Scanning
Electron Microscopy (SEM)
2. Transmission
Electron Microscopy (TEM)
3. Scanning
Tunneling Microscopy (STM)
B. Spectroscopy
1. X-ray
Diffraction (XRD)
2. Small
Angle X-ray Scattering (SAXS)
3. X-ray
Photoelectron Spectroscopy (XPS)
4. UV-Visible
Spectroscopy
5. FT-IR
Spectroscopy
A. MICROSCOPY
Microscopy techniques are used to visualize nanoparticles
and study their size, shape, morphology and structure.
1. Scanning Electron Microscopy — SEM
Principle
SEM uses a focused electron beam to scan the surface of
a sample.
When the electron beam strikes the sample, different signals
are produced, mainly:
- Secondary
electrons
- Back-scattered
electrons
- Characteristic
X-rays
These signals are detected and converted into an image.
Simple principle
Electron beam → strikes sample surface →
electrons/signals produced → detector → image
SEM is mainly useful for studying the surface of
nanoparticles.
What can we see with SEM?
1. Topography
It shows the texture/surface of the sample.
2. Morphology
It gives information about:
- Size
- Shape
- Arrangement/order
of particles
3. Composition
It can provide information about the elemental composition.
4. Crystalline structure
It can give information about the arrangement present within
the sample.
SEM Instrument
An electron gun produces the electron beam.
The beam is focused using electromagnetic lenses and
scanned across the sample surface.
The signals produced from the sample are detected and
converted into an image.
The electron gun can operate approximately at 1–30 kV for
SEM.
Magnification
SEM: approximately 10–500,000×
Advantages of SEM
1. Bulk
samples can be observed.
2. A
relatively large sample area can be viewed.
3. Produces
photo-like images.
4. Produces
high-resolution images.
5. Provides
useful information about sample purity and degree of aggregation.
Disadvantages of SEM
1. Sample
generally needs to have surface electrical conductivity.
2. Non-conductive
samples may need to be coated with a conductive layer.
3. It can be
time-consuming and expensive.
4. Sometimes
it is difficult to clearly differentiate nanoparticles from the substrate.
5. SEM
cannot resolve the internal structure of some domains.
2. Transmission Electron Microscopy — TEM
Principle
In TEM, a high-energy electron beam is transmitted through
a very thin sample.
The electrons interact with the sample. Differences in
electron transmission produce contrast and form an image.
Simple principle
Electron beam → thin sample →
transmitted/scattered electrons → detector → image
TEM is particularly useful when very high resolution is
required.
What can we see with TEM?
1. Morphology
It gives:
- Shape
- Size
- Order/arrangement
of nanoparticles
2. Crystalline structure
It can show:
- Arrangement
of atoms
- Defects
in crystalline structure
3. Composition
It can provide information about the elemental composition
of the sample.
TEM Instrument
An electron gun produces the electron beam.
The beam is focused using electromagnetic lenses and
transmitted through the specimen.
TEM generally operates at approximately 80–300 kV.
Magnification
TEM: approximately 2,000–1,000,000×
Advantages of TEM
1. Very high
resolution.
2. High-resolution
images are possible.
3. Additional
analysis such as X-ray spectrometry can be possible with TEM.
4. Image
construction can show internal/nanostructural information.
5. Useful
for studying interactions involving gases, liquids and biological structures
when appropriate sample preparation is used.
Disadvantages of TEM
1. Sample
must withstand the electron beam.
2. Sample
must be sufficiently thin.
3. Sample
preparation is necessary.
4. Mostly
used for 2-D images.
5. Preparation
and analysis can be time-consuming.
SEM vs TEM
|
Feature |
SEM |
TEM |
|
Main purpose |
Surface/morphology |
Internal structure + morphology |
|
Sample |
Bulk/surface sample |
Very thin sample |
|
Magnification |
~10–500,000× |
~2,000–1,000,000× |
|
Depth resolution |
10–1000 nm |
1–10 nm |
|
Lateral resolution |
1–10 nm |
0.1–1 nm |
|
Image |
Surface appearance |
Transmitted electron image |
|
Sample preparation |
Comparatively easier |
More demanding |
Easy way to remember:
SEM = Surface
TEM = Through the sample
3. Scanning Tunneling Microscopy — STM
Principle
STM is based on the concept of quantum tunneling.
When a conducting tip is brought extremely close to a
conducting or semi-conducting surface, a small voltage between the tip and
surface allows electrons to tunnel between them.
The tunneling current changes according to the distance
between the tip and surface.
These changes are converted into an image.
Simple principle
Sharp tip + very close surface + voltage →
tunneling current → image
The technique can provide extremely high-resolution surface
images.
Resolution
The PPT gives approximately:
- Lateral
resolution: ~0.1 nm
- Depth
resolution: ~0.01 nm
Advantages of STM
1. Very high
image resolution.
2. Capable
of observing/manipulating atoms.
3. Can be
used not only under ultra-high vacuum but also in air and various liquids.
4. Can
operate at ambient and elevated temperatures.
Disadvantages of STM
1. Tip
curvature can affect the result.
2. Tip
radius can introduce problems.
3. Very
sensitive to environmental vibrations.
4. Requires
extremely clean surfaces and sharp tips.
5. Mainly
suitable for conducting or semiconducting surfaces.
B. SPECTROSCOPY
Spectroscopy studies the interaction of radiation with
matter to obtain information about nanoparticles.
The PPT includes:
1. XRD
2. SAXS
3. XPS
4. UV-Visible
spectroscopy
5. FT-IR
spectroscopy
1. X-Ray Diffraction — XRD
Principle
XRD is based on the diffraction of X-rays by crystal planes.
When X-rays fall on a crystalline material, they are
diffracted by the regularly arranged atoms.
The diffraction condition is described by Bragg's law:
nλ = 2d sin θ
Where:
- n =
order of diffraction
- λ =
wavelength of X-ray
- d =
distance between crystal planes
- θ =
Bragg angle
What does XRD characterize?
XRD can provide information about:
1. Lattice
constant
2. d-spacing
3. Crystal
structure
4. Crystallite
size
5. Sample
orientation
6. Particle
size/grain size
Crystallite size
The PPT gives the Scherrer equation:
t = Kλ / B cosθ
Where:
- t =
thickness/crystallite size
- K =
shape constant
- λ =
wavelength
- B =
FWHM
- θ =
Bragg angle
Important point
Smaller crystals → broader XRD peaks
XRD can be time-consuming and may require a relatively large
amount of sample.
2. Small Angle X-Ray Scattering — SAXS
Principle
SAXS studies X-ray scattering produced by inhomogeneities/structures
at the nanometer scale.
It is useful for studying structures from several nanometers
to larger nanoscale dimensions.
What can SAXS characterize?
1. Particle
size
2. Specific
surface area
3. Morphology
4. Porosity
It can provide structural information over length scales
approximately from nanometers to larger dimensions, as described in the
PPT.
Important point
SAXS is useful for studying partially ordered systems
and nanoscale structures.
3. X-Ray Photoelectron Spectroscopy — XPS
XPS is a surface-sensitive characterization technique.
It provides information about the elemental composition and
chemical state of the surface.
Principle
X-rays strike the sample surface.
The X-rays cause emission of photoelectrons.
The energy of the emitted electrons is measured.
From this, information about the elements and their chemical
states can be obtained.
Simple flow
X-ray → sample → photoelectron emitted →
kinetic/binding energy measured → elemental & chemical information
What is XPS used for?
XPS can determine:
- What
elements are present on the surface
- Quantity
of elements
- Surface
contamination
- Empirical
formula
- Chemical
state of elements
- Binding
energy of electronic states
- Thickness
of different materials
- Density
of electronic states
XPS is generally sensitive to the near-surface region,
approximately 1–12 nm as stated in the PPT.
Materials analyzed by XPS
XPS can be used for:
- Inorganic
compounds
- Metals
- Semiconductors
- Polymers
- Ceramics
- Other
suitable materials
The PPT notes that organic chemicals are not routinely
analyzed because they may be degraded by X-ray energy or heat from
non-monochromatic X-ray sources.
Important limitation
XPS detects elements with atomic number > 2
according to the PPT.
It cannot normally detect hydrogen (Z = 1) and helium (Z =
2) because of their very small orbital diameter and extremely low detection
probability.
4. UV-Visible Spectroscopy
Principle
UV-Visible spectroscopy involves the absorption of near-UV
or visible light by a material.
It measures:
- Intensity
- Wavelength
It is commonly applied to molecules and inorganic ions in solution.
Beer-Lambert Law
The PPT gives:
A = −log(I/I₀) = a × b × c
Where:
- A =
absorbance
- I =
intensity of transmitted light
- I₀ =
incident light intensity
- a =
absorptivity
- b =
path length
- c =
concentration
Application
It can be used to determine the concentration of an analyte
from its absorbance.
For nanoparticles, UV-Vis spectroscopy can also help study
their optical properties.
5. FT-IR Spectroscopy
FT-IR = Fourier Transform Infrared Spectroscopy
Principle
Molecules can be considered as systems of atoms connected by
bonds.
These bonds can vibrate.
When infrared radiation interacts with the sample, it can be
absorbed when its frequency matches the vibrational frequency of a bond.
Therefore, FT-IR provides information about the chemical
bonds and functional groups present in the sample.
Simple flow
IR radiation → sample → molecular vibration → IR
absorption → spectrum → functional-group information
What can FT-IR characterize?
The PPT lists:
1. Composition
2. Concentration
3. Atomic
structure
4. Surrounding
environment/atomic arrangement
It is particularly useful for studying the chemical
environment and molecular/crystal vibrations.
Quick Revision Table
|
Technique |
Main information obtained |
|
SEM |
Surface morphology, size, shape, topography |
|
TEM |
Internal structure, morphology, crystal defects,
high-resolution images |
|
STM |
Atomic/surface structure, extremely high resolution |
|
XRD |
Crystal structure, d-spacing, crystallite size |
|
SAXS |
Nanoparticle size, surface area, morphology, porosity |
|
XPS |
Surface elements, chemical state, binding energy |
|
UV-Vis |
Absorption, optical properties, concentration |
|
FT-IR |
Functional groups, chemical bonds, molecular vibrations |
One-line memory trick
Microscopy → “How does the particle look?”
XRD → “How are atoms arranged in the crystal?”
SAXS → “What is the nanoscale structure/size?”
XPS → “Which elements and chemical states are on
the surface?”
UV-Vis → “How does it absorb UV/visible light?”
FT-IR → “Which bonds/functional groups are
present?”
This is the rewrite based on the uploaded PPT's
own classification and content—not a new classification.