Fabrication of Zinc Oxide Film by Successive Ionic Layer Adhesion and Reaction for Carbon Monoxide Gas Sensing
Emmanuel A. Florido1*, Nicolas Aelric C. Dagaas1
1Institute of Mathematical Sciences and Physics, University of the Philippines Los Banos, Los Banos, Laguna, Philippines
* presenting author:Emmanuel Florido, email:eaflorido@up.edu.ph
This study was aimed to determine the carbon monoxide (CO) gas sensing capability of zinc oxide (ZnO) film fabricated by successive ionic layer adsorption and reaction (SILAR). The film was fabricated by alternate dipping of pre-cleaned glass substrates in a sodium zincate bath and in a 950C hot water bath. The sodium zincate solution was prepared by mixing zinc sulfate (ZnSO4 7H2O) in excess sodium hydroxide in aqueous solution. When the glass substrate was dipped in the zincate bath, Na2ZnO2 was deposited on the glass surface. When the glass substrate was next dipped in hot water bath, ZnO was subsequently produced. The process was then repeated several times with a dipping time duration of 2 seconds each in the solution and in the hot water bath. Two sets of film samples were fabricated. One set consisted of films deposited on only one side of the glass substrate. The second set consisted of films deposited on both sides of the glass substrates.

Films consisting of a mixture of flower-like clusters of ZnO nanorods and nanowires were observed using scanning electron microscopy (SEM) for the sample grown using 0.1M concentration of sodium zincate and 200 dippings. Current-voltage characterization of the samples showed an average resistivity of 13.0 -m. The sensing circuit for the zinc oxide film consisted of a Wheatstone bridge, microcontroller, and laptop computer. Carbon monoxide gas was synthesized by mixing formic acid and excess sulfuric acid in a beaker placed inside an airtight 3.4-Li gas chamber. CO gas concentrations of 100, 200, 300, 400, and 500 parts per million (ppm) v/v were obtained by using the corresponding amount of formic acid. Five trials were conducted for each concentration. Two sets of data were obtained. One set consisted of the voltage response of the one-sided film sensor while the other set were obtained from the double-sided film sensor.
Linear regression analysis of the voltage response for the single-sided film sensor showed an average sensitivity of 0.0038 volts per ppm. The maximum concentration the film can detect with a 5V supply is 1320ppm. For the double-sided film sensor the sensitivity of the film was 0.0024 volts per ppm. It is capable of detecting up to 1980 ppm with a 5V power supply. This shows that using the double film sensor is advantageous compared to single film sensor, because of its higher concentration range which is due to the larger surface area for the gas to interact. Moreover, the measured average resistance for the one-sided film was 10 MΩ while for the two-sided-film the average resistance was 5 MΩ.

Keywords: zinc oxide, carbon monoxide sensing, successive ionic layer adhesion and reaction, voltage response, sensor sensitivity