Ashok K. Batra

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Overview of Chemical Gas Sensors

• Ashok K. Batra
• Department of Physics, PO Box 1268,
• Alabama AM University, Normal, AL 35762

NSF/RISE Workshop/Short Course on Development and
Study of Advanced Sensors and Sensor
Materials July 9 - July 13 2007
ashok.batra_at_aamu.edu
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Overview of Chemical Gas Sensors
Outline

• Chemical Sensor
• Categories of Sensors
• How does it work?
• What we are doing

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Natures Creation
Five Senses Eyes, Ears, Skin, Nose and Tongue
CameramimicsEyes Microphone and
Tape Recordermimics..Ears Tactile
SensorsmimicSkin Chemical
Sensors.mimic.. Nose Tongue
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A Typical Sensor
An interactive material which interacts with
environment and generates a response.
Device which reads the response and converts it
into an interpretable and quantifiable term.

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Elements of chemical sensor
Current Voltage Light Intensity Mass Refractive
Index Capacitance Resistance

Transducer
Molecular recognition conversion into physical signal
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Some Definitions and Terms
Adsorption is a process that occurs when a gas
or liquid or solute (called adsorbate)
accumulates on the surface of a solid or more
rarely a liquid (adsorbent), forming a molecular
or atomic film (adsorbate). Chemisorption a
type of adsorption whereby a molecule adheres to
a surface through the formation of a chemical
bond.
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Expected Qualities of an Excellent Chemical
Sensor
3Ss

• Sensitivity
• Stability
• Selectivity
• Minimum Hardware Requirements
• Good Reversibility
• Identification and Quantification of Multiple
  Species
• Quick Response

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Application of Gas Sensors

• Safety
• Indoor Air Quality
• Environmental Control
• Food
• Industrial Production
• Medicine
• Automobiles

Chemical sensors play an increasingly important
role in our everyday life environmental
monitoring, industrial process control, quality
control of food and beverages, hazardous
chemicals, explosives detection and workplace
monitoring are just a few examples of their
widespread use. In all cases the driving force
behind the development of sensor technology is
the need for immediate and accurate analyses.
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Four General Groups
Chemical Sensors

• Electrochemical Sensors
• Mass Sensors
• Optical Sensors
• Chromatography and Spectrometry

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Electrochemical Sensor
Based on Metal Oxides
- Stoichiometry
- Microstructure - Thickness - Phase - Temperature
Sensitivity Rgas / Rair
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Design of a Typical Thick Film Semiconductor
Sensor
Heater-substrate-film-combination electrode
structure.
- Tin Oxide - Tungsten oxide - Zinc Oxide -
Indium oxide -------------
The combination of the sensor operating
temperature and composition of the metal oxide
yields different responses to various gases
Materials Science Engineering B 139 (2007) 1-23
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chemical Sensors
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Chemical Sensors
New
Silicon-Based Metal-Oxide Chemical Sensor
Microfabricated Metal-Oxide Chemical Sensor
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Chemical Sensors
A Scheme of SnO2 acting as Semiconductor Sensing
Material
2e- O2 2 O- O-
CO CO2
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Chemical Sensors
Role of Additives (Dopants)
Additives are used for sensitizing and increase
the response to particular gases i.e. enhance the
sensitivity, selectivity, decrease the response
time and operating temperature of sensitive
layer. Sensitizing SnO2 with Cu under oxidizing
condition Cu is present as CuO which is p-type
p-n junction is formed which results in electron
depletion at interface. Exposure to H2S converts
CuO to Cu2O that exhibits metallic character and
thus increases the conductivity of the system
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• Electrochemical Sensors

Additives Distribution Ways on Semiconductor Gas
Sensors
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Electrochemical Sensors
Polymer-absorption sensor
Metal Lead
Conductive Polymer
Chemical sensitive absorbent is deposited on a
solid phase that acts as an electrode When
chemical vapors come in contact with the
absorbent. The chemical absorbs into the polymer,
causing it to swell. The swelling changes the
resistance of electrode, which can be
measured The amount of swelling corresponds to
the concentration of the chemical vapors
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Catalytic Bead Sensor
It is comprised of a passive and active
element. The active element is coated with a
catalyst platinum and passive is coated with an
inert glass to act as a reference element Both
the elements are heated to a prescribed
temperature. When a combustible gas contacts the
elements, the vapor combusts on the active
element, and the active element increases in
temperature. As a result, the resistance of the
platinum coil changes. Two elements are connected
to a Wheatstone bridge circuit, so changes are
measured in voltage
Ceramic

Active Passive
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Mass Sensors
Surface Acoustic Wave Sensor
The velocity and attenuation of the signal are
sensitive to the viscoelasticity and mass of the
thin film which can allow for the identification
of the contaminant. Heating element under the
chemical film can also be used to desorb
chemicals from the device. A Signal pattern
recognition system is needed.
Piezoelectric Quartz Substrate
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Mass Sensors
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Mass Sensors
Based on Cantilever Nanotechnology


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• Optical Sensors

Infrared Sensors
Infrared sensors can be used to detect gases,
which have unique infrared absorption signatures
in the 2-14 µm range. The uniqueness of the gas
absorption spectra enables identification and
quantification
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• Optical Sensors

Colorimetry
Work by analyzing the color of the contaminated
water that has been mixed with a particular
reagent
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Surface Plasmon Resonance (SPR) Sensors SPR- A
Charge Density Oscillation that may exist at the
interface of two media.
The SPR technique is an optical method for
measuring the refractive index of very thin layer
of material adsorbed on a metal
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Optical Setup for SPR
P-reflectivity
Photons at certain angle are able to excite SP on
the adsorbate side of the metallic slab whenever
plasmon is excited, one photon disappears,
producing a dip in the reflected light angle is
dependent on refractive index of the adsorbate.
Angle of Incident
SPR Curves for Different Molecules
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Surface Enhances Raman Spectroscopy (SERS)

• Optical Sensors

SERS is based on finding the chemical composition
of a sample by irradiating it with laser and
measuring the light that scatters from it.
Surface Enhanced ( 1014) Raman Scattering is
observed for molecules found close to silver or
gold nanoparticles because of surface plasmon
resonance. Thus sensitivity increases many folds.
Plasmons are collective oscillations of the free
electron density, often at optical frequencies
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• Chromatography

Chromatography Separation of Molecules
Liquid Chromatography
Sorbent
Gas Chromatography
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• Spectrometry

Ion Mobility Spectrometry

• Time-of-Flight Measurement

When the gas has entered the spectrometer, it
will be ionized by a radioactive source The
resulting positive and negative charged species
are accelerated over a short distance Time-of-Fli
ght is determined
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• Spectrometry

Mass Spectrometry
The principle is similar to the ion mobility
spectrometer, except vacuum is required Gas
mixture is ionized, and charged fragments are
produced These fragments are sorted in a mass
filter according to their mass to charge
ratio. The ions are detected as electrical
signal with an electron multiplier
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Recent Advances
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Nano Structured Materials
Next Generation of Sensors ?
Because of the small size of nanotubes,
nanowires, or nanoparticles, a few gas molecules
are sufficient to change the electrical
properties of the sensing elements. This allows
the detection of a very low concentration of
chemical vapors.
Nanotechnology based chemical sensors provide
high sensitivity ( 3-4 orders), low power and low
cost portable tools for in-situ chemical
analysis. Operate at room temperature.
SnO2 ZnO In2O3 WO3 SnO2Pd TiO2
Sensors Actuators B 122 (2007) 659-671
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Optochemical Sensors
For H2, O2, O3, CO, CO2 and H2O detection in Air

• Absorbance and Reflectance
• Refractive Index
• Photoluminescence
• Photothermal
• Photoacoustic and related
• Surface Plasmon Resonance (SPR)
• Chemiluminescence

Trends in Analytical Chemistry 25(2006) 937-948
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Development of Nanoparticles-based Chemical Gas
Sensor

• A. K. Batra, J. R. Currie, Anup D. Sharma and
  R. B. Lal
• Department of Physics, PO Box 1268,
• Alabama AM University, Normal, AL 35762
• Instrumentation Advanced Sensor Group,
• NASA/Marshall Space Flight Center, AL 35812

NSF/RISE Workshop/Short
Course on Development and Study of Advanced
Sensors and Sensor Materials July 9-July
13, 2007
ashok.batra_at_aamu.edu
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A Typical Semiconductor Sensor
An interactive material which interacts with
environment and generates a response.
Device which reads the response and converts it
into an interpretable and quantifiable term.
Sensing material captures a molecule of vapor
with a certain selectivity that induces physical
change in the material because of captured
molecule's chemical interaction with the
material.
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Semiconductor Gas Sensors Mechanisms
Detect Gases Due to Change in Their Resistance
or Conductance
Changes in Conductance can result from
combination of several physical properties of
film

• Bulk defects ( interstitials and oxygen
  vacancies)
• Surface defects ( donor type oxygen
  vacancies)
• Catalytic elements ( breakdown of an incoming
  gases by catalyst on the surface of the sensing
  film)
• Microstructure and grain boundaries (
  smaller grains, large number of grain
  boundaries, high surface to volume ratio)
• Interface and three phase boundaries (changes
  in interface conductance due to incoming gases
  at triple point)

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Our Approach

• Microstructure and grain boundaries (
  smaller grains, large
• number of grain boundaries, high surface
  to volume ratio).
• 
  
• By use of Nanoparticles in fabrication of
  the thick-films.
• high surface reactivity larger density of
  molecules which can adsorb on the surface
  contribute to larger effect on electrical
  conductivity enhances sensor sensitivity.
• ? Binary composites SnO2WO3 SnO2In2O3
  SnO2ZnO

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SnO2 Sample Processing Steps
Pellet / Sample Preparation
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Tablet Pressing Fixture
2.5 cm
2.5 cm
Pressing Sleeve
Pressing Sleeve
1.3 cm
1.3 cm
Anvil
Anvil
5 mm
5 mm
5 mm
5 mm
Die
Die
4 cm
Piston
Piston
2.9 cm
2.9 cm
Pressing Ram
1.2 cm
1.4 cm
1.2 cm
1.4 cm
1.2 cm
1.2 cm
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Sensor Configuration
The SnO2 Sensor (pellet) shows shaded electroded
regions on the top surface having a finite gap
between these two physical regions.
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Test Facility for VOC Chemical Sensors
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Our Study

• Sensor elements were bombarded with helium
  particles to change the surface characteristics.
  In our study, a VOC sensor pellets of Tin
  Dioxide (SnO2) are fabricated then bombarded at
  various (2 MeV ) helium doses.
• A noteworthy result has been observed that the
  device has a decrease in response time when
  bombarded at about 1016 ion/cm2.
• The response time decreases with increase of
  fluence.
• A trend is seen whereby capacitance tends to
  
• decrease as fluence is increased.

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Decrease in Response Time
Response Time is based on 1? (62.3 of
saturation).
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IPA Response Time
Decrease in Response Time Observed
IPA Response Time
SnO2 Nanopowder Samples IPA Response Time (s)
No Bombardment 389
62088 Counts 241
621585 Counts 145
Response Time is based on 1? or (62.3 of
saturation).
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Why look at Thick Films?

• Bulk
• Not stable consume power not compatible with
  silicon technology and high operating cost.
• Thin-Film
• Compatible with micro-sensors rapid response
  compact low operating cost mechanically weak
  Dopants homogeneity???.
• Thick-Film

• More Robust than Thin Films.
• Lighter than Bulk Materials.
• Dopants Homogeneity.
• Consume less power.
• Compatible with silicon technology.

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Screen Printing
Liquid Phase Epitaxy
Melt Spinning
Dip Coating
Solution Casting
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• Why Screen Printing?

• Low Cost of Production.
• Ease of Fabrication.
• Compatible with semiconductor technology.
• Films with large surface to volume ratio.
• Porous films.

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Process of Thick-film Preparation by
Screen-printing
Organic Binder
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Fabrication of Thick-Film using Screen Printing
Technology
Screen Printing Set-up
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Design of a Typical Semiconductor Sensor
Heater-substrate-film-combination electrode
structure.
S. Film
Electrode
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Acknowledgements
The authors gratefully acknowledge the support of
the present work through SMDC grants
W9113M-05-1-0011, and NSF RISE grant
HRD-0531183. One of the authors (RBL) would
like to acknowledge NASA Administrators
Fellowship program. Further, technical
assistance of Mr. Garland Sharp and members of
the Center for Irradiation of Materials (CIM) at
Alabama A M University is greatly appreciated.
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Thank you .
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