Vapor Detection Via Tin Oxide Solid State Semiconductors

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Using solid state sensors with tin oxide semiconducting material is common method of vapor detection in process, environmental, and safety applications.  Solid state Tin Oxide (SnO2) vapor detectors are small, durable, cost effective and can be used in a number of environmental conditions.  Tin oxide semiconductor gas sensors have been around since they were patented in 1962.  These sensors have become ubiquitous in sensing carbon dioxide in residential, commercial and industrial applications.  Recent research and development is aimed at unlocking the full potential of tin oxide sensors for use in sensing and monitoring applications of gases and toxic substances other than carbon monoxide.

Fully utilizing the physical and chemical properties of tin oxide for the sensing of other gases and toxic substances requires improvements in the selectivity, stability, sensitivity and time response of these sensors.  One's first inclination is to reduce the size of the tin oxide particles to increase surface area, thus increasing sensitivity and and time response, but this can have a negative effect on stability, making the sensors impractical for many applications.  The challenge facing researchers is reducing the size of the tin oxide nanoparticles to 2-6 nm while maintaining the mechanical strength of the tin oxide crystallites.  Heat treatment methods, methods common in semiconductor processing, result in an increased tin oxide crystal size above the 10 nm range that is needed to make pure tin oxide sensors practical for sensing gases other than carbon monoxide.

Considerable research is being targeted at two different types of tin oxide sensors, those with a pure, treated tin oxide semiconductor layer and sensors with a tin oxide layer doped with noble metals like platinum or other metal oxides.  In both types of sensors, the tin oxide particle size is relatively similar, but how they sense gases is different.  In pure crystalline tin oxide sensors, the tin oxide is the n-type semiconducting layer.  The conductivity of this layer increases when it comes in contact with reducing gases such as carbon monoxide, while having the reverse effect in the presence of an oxidizing gas such as CO2.  Metal-doped tin oxide sensors recognize gases and toxins through one of two similar mechanisms.  One method of sensing is the Fermi energy control mechanism where a reducing gas reacts with the metal additive, releasing an electron to the tin oxide, thus changing the electron density near the surface of the tin oxide layer.  this increases the electrical resistance of the semiconducting layer.  The catalytic mechanism, the metal additive acts as a catalyst and transports the reducing gas to the surface of the tin oxide where it directly reacts with the oxygen in the tin oxide and directly releases an electron to the lower electron density region[1].

The tin oxide nanoparticles used in the sensors are manufactured by Sol-Gel processing, sputtering, combustion synthesis, pulsed laser ablation, emulsification or other methods depending on whether the tin oxide is doped with other metals and the desired physical attributes.  The fabrication methods used to manufacture the sensors depend on the type of sensor being created.  Simple porous plug and block sensors is prepared by sintering tin oxide particles onto a ceramic block with embedded electrode wires.  Tin oxide semiconducting layers are adhered to substrates based on the desired thickness of the semiconducting layer.  The tin oxide layers on thick film sensors, those with semiconducting layers on the order of 2-10 microns thick, are generally synthesized by screen printing methods.  Thin film sensors, those with semiconducting layers ranging in thickness from 50nm to 2 microns, are synthesized via Sol-Gel processing or sputtering methods[2].


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The metal oxide surface reacts with the contaminate sending a signal to the sensor that indicates both the presence or absence and the concentration of the contaminant. The large surface area to volume of the metal oxide prevents overloading or degrading the detection capacity of the sensor. [1]

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  1. Waitz T, Becker B, Wagner T, Sauerwald T, Kohl C-D, Tiemann M. Ordered nanoporous SnO2 gas sensors with high thermal stability. Sensors and Actuators B: Chemical. 2010 ;150(2):788 - 793.

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This technology has the potential to increase the reliability, functionality, and overall perform of environmental (air) monitoring systems.[1]

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  1. Waitz T, Becker B, Wagner T, Sauerwald T, Kohl C-D, Tiemann M. Ordered nanoporous SnO2 gas sensors with high thermal stability. Sensors and Actuators B: Chemical. 2010 ;150(2):788 - 793.

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There are very little risk to ecological or human health from the end-product, but there are simple risks associated with the manufacture and fabrication of the tin oxide nanoparticles and the sensors. The environmental and human health risks associated with the manufacture of the tin oxide nanoparticles are dependent on the type of metals used and the manufacturing process. Some of the metals used as catalysts, such as platinum group metals, have been shown toxic in birds, while small aspect ratio nanoparticles have been shown to be physiologically harmful to humans. Additionally, the chemical processes used to fabricate the sensors themselves are analogous to those used in the semiconductor industry and may pose similar risks.

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