CO₂ Sensors Based on Nanocrystalline SnO₂ Doped With CuO

John H. Glenn Research Center

Nanocrystalline tin oxide (SnO₂) doped with copper oxide (CuO) has been found to be useful as an electrical-resistance sensory material for measuring the concentration of carbon dioxide in air. SnO₂ is an n-type semiconductor that has been widely used as a sensing material for detecting such reducing gases as carbon monoxide, some of the nitrogen oxides, and hydrocarbons. Without doping, SnO₂ usually does not respond to carbon dioxide and other stable gases. The discovery that the electrical resistance of CuO-doped SnO₂ varies significantly with the concentration of CO₂ creates opportunities for the development of relatively inexpensive CO₂ sensors for detecting fires and monitoring atmospheric conditions. This discovery could also lead to research that could alter fundamental knowledge of SnO₂ as a sensing material, perhaps leading to the development of SnO₂-based sensing materials for measuring concentrations of oxidizing gases.

Prototype CO₂ sensors based on CuO-doped SnO₂ have been fabricated by means of semiconductor-microfabrication and sol-gel nanomaterial-synthesis batch processes that are amendable to inexpensive implementation in mass production. A fabrication process like that of the prototypes includes the following major steps:

Platinum interdigitated electrodes are microfabricated on a quartz substrate.
Nanocrystalline SnO₂ is synthesized in a partial sol-gel process. CuO dopant is synthesized through a precipitation process. The dopant and the sol-gel are mixed in proportions chosen to obtain the desired composition of the final product. One composition found to be suitable is a molar ratio of 1:8 CuO:SnO₂.
The dopant and sol-gel mixture is deposited in drops on (and across the gaps between) the electrodes.
The workpiece is heated at a temperature of 700°C, converting the dopant and sol-gel mixture to a film of nanocrystalline CuO doped SnO₂.

The Electrical Resistance of a 1:8 CuO:SnO₂ film fabricated as described in the text was found to decrease as the concentration of CO₂ in air increased. R_air signifies the resistance of the film in pure air; R_CO2 signifies the resistance of the film at the indicated concentration of CO₂.

In operation, a sensor of this type is heated to a temperature of 450°C while it is exposed to the CO to be detected and the electrical resistance of the film between the electrodes is measured. Preliminary results of tests on a sensor containing a film of 1:8 CuO:SnO₂ showed an approximately linear response at CO₂ concentrations from 1 to 4 percent (see figure). In subsequent research and development efforts, it may be possible to increase sensitivities and/or reduce operating temperatures by combining CuO-doped SnO₂ with solid-electrolyte materials.

This work was done by Jennifer C. Xu and Gary W. Hunter of Glenn Research Center and Chung Chiun Liu and Benjamin J. Ward of Case Western Reserve University.

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