Characteristics Of Chemoresistive Sensors
Characteristics Of Chemoresistive Sensors
Chemoresistive sensors (or chemoresistors) can be high-temperature chemoresistors (200~600°C) with semiconductor metal oxide coatings or low-temperature chemoresistors (room temperature) with polymeric and organic coatings.
Characteristics of Mixed Metal Oxide Semiconductor Sensors
In mixed metal oxide semiconductor (MMOS) sensors, the large resistance change is caused by a loss or a gain of surface electrons as a result of adsorbed oxygen reacting with the target gas. If the oxide is an n-type, there is either a donation (reducing gas) or subtraction (oxidizing gas) of electrons from the conduction band. When oxidizing gases (e.g., NO2 and O3) are present, the n-type oxides increase their resistance; if reducing gases (e.g., CO, CH4, and C2H5OH) are present, the n-type oxides reduce their resistance. Commercially, only a few oxides are available due to the requirement for a unique combination of resistivity, sensitivity (magnitude of resistance change in target concentration), thermal and humidity effects, and wide bandgap semiconductors. The commonly used materials for MMOS sensors include ZnO, TiO2,Cr2TiO3,WO3, SnO2, and In2O3. These materials have higher sensitivity, quicker response, and enhanced capability to detect gases at low concentrations compared to thin-film materials. Furthermore, doping with noble metals (Au, Pd, Pt) can effectively improve the sensitivity, selectivity, and response time of the nanomaterial gas sensors at low operating temperatures. These metal oxides can be deposited onto thin or thick films of semiconductor substrates. In recent years, novel nanostructures such as nanowires, nanotubes, nanorods, nanonails, nanocages, nonosheets, nonocables, and nanobelts, are increasingly used in chemoresistive sensor designs due to their large surface-to-volume ratios, single crystalline structures, and great surface activities. Figure 1 shows the change in conductance (in terms of electron density) of an individual SnO2 nanowires as a function of CO concentration at three different temperatures, where the electron density (in arbitrary units—a.u.) is proportional to CO partial pressure.
Semiconductor metal oxide sensors usually respond to almost any analyte (carbon monoxide, nitrogen oxide, hydrogen, or hydrocarbon), but they are not very selective. One way to modify the selectivity pattern is surface doping the metal oxide with catalytic metals such as platinum, palladium, gold, and iridium. Metal oxide sensors are more stable at higher operating temperatures. Under lower temperatures (e.g., 200°C or below), polymer sensors work better.
Characteristics of Polymer or Organic Material Sensors
Conducting polymers, such as polypyrroles, polyaniline, and polythiophene, change their electrical resistance when exposed to certain targets. They are broadly used in making chemoresistive sensors to monitor a variety of polar organic volatiles such as ethanol and methanol. Other conducting polymers, such as carbon black, can be dispersed in nonconducting polymers. When the polymer absorbs vapor molecules, it swells and the particles are pushed further apart, causing a decrease in conductivity. Figure 2 shows how the conductance of an H2O sensor changes as ethanol concentration changes at constant humidity and constant temperature. For instance, if the measured conductance change is 8%, the ethanol concentration is 24 ppm.
Conducting polymer-based chemoresistors are most commonly used in vapor or odor sensing due to the wide range of available polymer combinations and their ease of deposition, ability to operate at room temperature, low power consumption, and sensitivity to a broad range of volatile organic compounds and organic solvents (e.g., hydrocarbons, chlorinated compounds, and alcohols). The characteristics of polymer chemoresistive sensors depend upon the polymer material, electrode geometry, temperature, and ambient humidity. Some disadvantages of polymeric chemoresistors include their batch-to-batch variation in baseline resistance, large temperature and humidity coefficients, long-term drift, and small signal-to-noise ratio (common for most types of gas sensors).