Effect of Carbon Dioxide to Oxygen Ratio on the Voltage Response of Zinc Oxide Film Grown by Successive Ionic layer Adhesion and Reaction
1Institute of Mathematical Sciences and Physics, University of the Philippines Los Banos, Los Banos, Laguna, Philippines
* presenting author:EMMANUEL FLORIDO, email:eaf_physics@yahoo.com
This study was aimed to determine the effect of carbon dioxide (CO2) to oxygen (O2) ratio on the voltage response 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 using an automated dipping machine to control the sequence and dipping time. 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.

Scanning Electron Microscopy (SEM) revealed a uniform film consisting of wurtzite ZnO nanorods 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 1.343 -m. The sensing circuit for the zinc oxide film consisted of a wheatstone bridge and instrumentation amplifier. The ZnO film was then exposed to a mixture of CO2 and O2 in varying proportions in a 450 ml air tight chamber equipped with a water trap at 1 atm pressure to determine the gas response. A mass flow controller was used to introduce synthetic air from a tank containing 21.9% O2 in nitrogen. Atmospheric air containing 0.033% carbon dioxide and 20.947% oxygen in nitrogen was introduced into the chamber using a gas syringe.

The ZnO film exhibited a sensitivity of -1.7 V/(1% CO2/O2) with a high linear correlation of 0.98. The negative sign means a decrease in the voltage response of the film with increasing CO2/O2 ratio due to a decrease in the zinc oxide film resistance. Oxygen gets adsorbed on the zinc oxide surface by binding to electrons from the zinc oxide resulting to band bending and increase in resistance. Molecules such as CO and CO2 have the ability to remove oxygen from the zinc oxide surface. When this happens, oxygen-bound electrons are returned to the zinc oxide film resulting to a decrease in the resistance of the film.

Keywords: zinc oxide, sensitivity, successive ionic layer adhesion and reaction