Chemoresistive properties of photo-activated thin and thick ZnO films

Room temperature operation is an extremely valuable goal in gas sensing research. Among the metal-oxides that have shown various chemoresistive properties, zinc oxide plays an important role as a wide gap semiconductor (Eg = 3.37 eV) whose chemical activity at surface can be enhanced by proper electromagnetic radiation. This property is fundamental to design gas sensing devices and to operate at room temperature. In this work, nanosized zinc oxide powder was synthesized by sol–gel technique through two different routes. After structural and morphological characterization, the materials were deposited onto miniaturized alumina substrates; one as a thick film by screen printing technique, the other one as a thin film by spin coating. The response to nitrogen dioxide was deeply investigated as an important gas target in air monitoring for human safety. Once identified the optimal activation energy for the as-prepared devices, the two types of gas sensors were tested in photo-activated mode with UV-LED of 385 nm peak wavelength. In order to collect information on the role of oxygen coverage in NO2 sensing mechanism with ZnO, thin and thick films were tested in air and in nitrogen conditions. Finally, measurements with several gases were performed in air conditions to compare the performances of thin and thick films. The manufactured sensors resulted suitable for alcohols detection, and thick films yielded negligible responses in comparison with their thin films counterpart. We interpreted this evidence in terms of extinction length of light in ZnO