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Overview of Chemical Gas Sensors. Ashok K. Batra + Department of Physics, PO Box 1268, Alabama A&M University, Normal, AL 35762. NSF/RISE Workshop/Short Course on Development and Study of Advanced Sensors and Sensor Materials July 9 - July 13 2007. + [email protected]

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Overview of Chemical Gas Sensors

  • Ashok K. Batra+

  • Department of Physics, PO Box 1268,

  • Alabama A&M University, Normal, AL 35762

NSF/RISE Workshop/Short Course


Development and Study of Advanced Sensors and Sensor Materials

July 9 - July 13 2007

+ [email protected]

Overview of Chemical Gas Sensors


  • Chemical Sensor

  • Categories of Sensors

  • How does it work?

  • What we are doing…

Nature’s Creation

Five Senses: Eyes, Ears, Skin, Nose and Tongue


Microphone and Tape Recorder………mimics…………..Ears

Tactile Sensors……………………………………mimic………Skin

Chemical Sensors………………….mimic……….. Nose & Tongue

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.

  • Current


    Light Intensity


    Refractive Index



    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.

    Expected Qualities of an Excellent Chemical Sensor


    • Sensitivity

    • Stability

    • Selectivity

    • Minimum Hardware Requirements

    • Good Reversibility

    • Identification and Quantification of Multiple Species

    • Quick Response

    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.

    Four General Groups

    Chemical Sensors

    • Electrochemical Sensors

    • Mass Sensors

    • Optical Sensors

    • Chromatography and Spectrometry


    Sensing Material


    Electrochemical Sensor

    Based on Metal Oxides

    - Stoichiometry

    - Microstructure

    - Thickness

    - Phase

    - Temperature

    Sensitivity = Rgas / Rair

    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

    Chemical Sensors

    New Silicon-Based Metal-Oxide Chemical Sensor

    Microfabricated Metal-Oxide Chemical Sensor

    Chemical Sensors

    A Scheme of SnO2 acting as Semiconductor Sensing Material

    2e- + O2 2 O-

    O- + CO CO2

    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

    Additives Distribution Ways on Semiconductor Gas Sensors

    Electrochemical Sensors

    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

    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


    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.

    Mass Sensors

    Based on Cantilever: Nanotechnology

    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

    Optical Sensors


    Work by analyzing the color of the contaminated water that has been mixed with a particular reagent

    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

    Optical Setup for SPR


    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

    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


    Chromatography: Separation of Molecules

    Liquid Chromatography


    Gas Chromatography


    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-Flight is determined


    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

    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; SnO2:Pd; TiO2

    Sensors & Actuators B 122 (2007) 659-671

    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

    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 A&M University, Normal, AL 35762

    • *Instrumentation & Advanced Sensor Group,

    • NASA/Marshall Space Flight Center, AL 35812

    NSF/RISE Workshop/Short Course


    Development and Study of Advanced Sensors and Sensor Materials July 9-July 13, 2007

    + [email protected]

    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.

    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)

    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: SnO2:WO3; SnO2:In2O3; SnO2:ZnO

    Weighing Raw






    Pressed to











    SnO2 Sample Processing Steps

    Pellet / Sample Preparation

    Tablet Pressing Fixture

    2.5 cm

    2.5 cm

    Pressing Sleeve

    Pressing Sleeve

    1.3 cm

    1.3 cm



    5 mm

    5 mm

    5 mm

    5 mm



    4 cm



    2.9 cm

    2.9 cm

    Pressing Ram

    1.4 cm

    1.4 cm

    1.2 cm

    1.2 cm

    1.2 cm

    1.2 cm


    Sensor Configuration

    The SnO2 Sensor (pellet) shows shaded electroded regions on the top surface having a finite gap between these two physical regions.

    Test Sample

    GPIB-USB2 Interface

    Keithley 617


    Agilent 34401A

    6 ½ Digit Multimeter

    Gateway E-3301

    CPU running

    National Instruments

    LabVIEW Ver. 6.0

    Hot Plate w/

    Thermal Chamber

    QuadTech 7600

    LCR Meter

    Test Facility for VOC Chemical Sensors

    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.

    Decrease in Response Time

    Response Time is based on 1 (62.3% of saturation).

    IPA Response Time

    Decrease in Response Time Observed

    IPA Response Time*

    *Response Time is based on 1 or (62.3% of saturation).

    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.

    Thick Film














    Why Screen Printing?

    • Low Cost of Production.

    • Ease of Fabrication.

    • Compatible with semiconductor technology.

    • Films with large surface to volume ratio.

    • Porous films.



    Mixing (Paste)





    Functional & Porous Film

    Process of Thick-film Preparation by Screen-printing

    Organic Binder



    Nylon Screen



    Vacuum Chuck*


    Dowel Pins

    Base Plate




    *Fabricatedby Mr. Garland Sharp

    Fabrication of Thick-Film using Screen Printing Technology

    Screen Printing Set-up

    Design of a Typical Semiconductor Sensor

    “Heater-substrate-film-combination electrode” structure.

    S. Film



    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 Administrator’s 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.