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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Five Senses: Eyes, Ears, Skin, Nose and Tongue
Microphone and Tape Recorder………mimics…………..Ears
Chemical Sensors………………….mimic……….. Nose & Tongue
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.
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.
Based on Metal Oxides
Sensitivity = Rgas / Rair
“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
New Silicon-Based Metal-Oxide Chemical Sensor
Microfabricated Metal-Oxide Chemical Sensor
A Scheme of SnO2 acting as Semiconductor Sensing Material
2e- + O2 2 O-
O- + CO CO2
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
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
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
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.
Based on Cantilever: Nanotechnology
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
Work by analyzing the color of the contaminated water that has been mixed with a particular reagent
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
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
Ion Mobility Spectrometry
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
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
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
For H2, O2, O3, CO, CO2 and H2O detection in Air
Trends in Analytical Chemistry 25(2006) 937-948
NSF/RISE Workshop/Short Course
Development and Study of Advanced Sensors and Sensor Materials July 9-July 13, 2007
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.
Detect Gases Due to Change in Their Resistance or Conductance
Changes in Conductance can result from combination of several physical properties of film:
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
SnO2 Sample Processing Steps
Pellet / Sample Preparation
The SnO2 Sensor (pellet) shows shaded electroded regions on the top surface having a finite gap between these two physical regions.
6 ½ Digit Multimeter
LabVIEW Ver. 6.0
Hot Plate w/
Test Facility for VOC Chemical Sensors
decrease as fluence is increased.
Response Time is based on 1 (62.3% of saturation).
Decrease in Response Time Observed
IPA Response Time*
*Response Time is based on 1 or (62.3% of saturation).
Functional & Porous Film
Process of Thick-film Preparation by Screen-printing
*Fabricatedby Mr. Garland Sharp
Fabrication of Thick-Film using Screen Printing Technology
Screen Printing Set-up
“Heater-substrate-film-combination electrode” structure.
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.