The device used for most measurements features comb-style interdigitated contact, which decreases the sample resistance compared to the four-contact geometry. A structure with the latter type of contacts and without a heat distribution plate was used for the stability tests.
The nanoparticle film is produced by spinning on a colloidal dispersion of 10 nm antimony-doped tin oxide (15 percent) particles in water obtained from Alfa Aesar[4,5]. Undoped tin oxide was also examined, but resulted in films with much higher resistance. The spin speed was 4000 rpm for 20 s. After the spin, the film was dry with some evidence of cracks, though none on the suspended hotplates. An estimate of the film thickness of about 100 nm was obtained by weighing similarly prepared glass slides before and after spin-coating. Films were annealed at 500°C using the device's microheater. Scanning electron microscopy images of the film show fine structure on the nanometer scale.
The high sensitivity of these devices is evident in the response from three devices operating at 350 °C in varying concentrations of methanol in dry air. It is interesting to note that there is an increase in the baseline conductance as the dilution flow rate changes. This is attributed to a cooling of the sensor surface by the dilution gas, which because of the small gas-test volume is a significant effect. That this occurs in spite of the constant heater temperature, assured by the heater control feedback, shows that significant temperature differences can occur between the sensing surface and the buried heater.
"Temperature programmed sensing" was used to produce a time-varying signal that enhances the capability of the device to recognize an analyte. For these experiments, the comb-contact device was used because of the lower sample resistance and thus smaller resistance-capacitance time-constant. A linear ramp of temperature pulses was applied to the microheater of a device, while the conductance is measured between the pulses. The temperature pulse ramp ranged from 30°C to 400 °C in steps of 10°C, with a pulse width of 300 ms, and a separation between pulses of 150 ms.
Shown is the transition between air and an exposure to methanol in air at 16 µmole/mole (16 ppm). The change in response patterns between air and methanol could be analyzed by various signal processing methods for chemical identification. Below is one cycle of conductance plotted against pulse temperature for each concentration, ranging from 64 µmole/mole (64 ppm) down to 10 nmole/mole (10 ppB) and also in dry air.
Catalysts are often used to enhance the sensitivity of tin oxide. A drawback, however, has been the potential for drift in sensitivity due to changes in the catalyst structure as a result of the heating during operation or contamination (coking or poisoning) from the detected gases during long exposures. A four-contact device with the nanoparticle film was tested for long exposures to ethanol and methanol. Each 5 minute gas exposure is followed by a 5-minute exposure to dry air. For each gas the concentration sequence was two exposures at 25 µmole/mole (25 ppm) and two exposures at 50 µmole/mole (50 ppm). Sensor operating temperature was 350 °C. Over the course of the 120-hour run, the sample exhibited no loss in sensitivity.
1. R.E. Cavicchi, R. M. Walton, M. Aquino-Class, J. D. Allen, B. Panchapakesan, "Spin-on nanoparticle tin oxide for microhotplate gas sensors", in 8th International Meeting on Chemical Sensors Basel, Switzerland, 2000, pp. 200.
2. G. Sakai, N. S. Baik, N. Miura and N. Yamazoe, "Gas sensing properties of tin oxide thin films fabricated from hydrothermally treated nanoparticles-Dependence of CO and H2 sensitivity on film thickness", in 8th International Meeting on Chemical Sensors Basel, Switzerland, 2000, pp. 93.
3. F. Tozzi, J. Xu, Q. Wu and C. C. Liu, "A nano-crystalline tin oxide thin film micro gas sensor", in 8th International Meeting on Chemical Sensors Basel, Switzerland, 2000, pp. 184.
4. Certain commercial instruments are identified to adequately specify the experimental procedure. In no case does such identification imply endorsement by the National Institute of Standards and Technology.
5. Although the film results may be different from those obtained from the suspension used in the experiments described here, the following paper describes a method of producing a nanoparticle Sb-doped colloidal suspension: T. Nutz, U. zum Felde and M. Haase, "Wet chemical synthesis of doped nanoparticles: Blue-colored colloids of n-doped SnO2:Sb", J. Chem. Phys. 110, 12142 (1999).