Highly Sensitive Acetone Gas Sensors Based on in-Doped ZnO Quantum Dots Fabricated By Morphology Controlled Synthesis

Abstract

Introduction Detection of biomarkers for some diseases via exhaled breath is a promising method of a diagnosis without invasive blood sampling. Recently, over 1,000 gases are known to be included in human breath and some of endogenously produced VOCs provide valuable information about metabolism [1]. Since people with certain diseases show altered metabolic pathways, their composition of exhaled breath may exhibit some variation [2, 3]. Consequently, sensing technology for the detection of VOCs can be a new option of diagnosis.Various materials were investigated as candidates for acetone gas sensors. Among them, ZnO is considered as an appropriate sensing material due to its high band gap, various nanostructures and high sensitivity. Recently, sensors based on ZnO quantum dots (QDs) were reported to have a highly sensitive characteristic for the sensing of acetone by reducing size of particles and doping other elements[4-6]. In order to enhance their properties, we decided to introduce soft lithography technology. We report on the effect of morphologies on sensing performances by fabricating patterned IZO film for the detection of acetone Method A master mold was fabricated by photolithography (MJB6, SUSS MicroTec) using SU-8 as a negative photoresist. The pattern on photomask was a periodic line/space with pitch of 10 μm, that is the linewidth and space equals at 5 μm. The thickness of the photoresist was controlled to be similar size to linewidth/space, which makes the aspect ratio of the pattern to be close to 1, that is the thickness of the photoresist to be ~5um. Elastomeric polydimethylsiloxane (PDMS) stamp was replicated from the master mold prepared by photolithography. A 10:1 ratio by weight of base resin and curing agent (Sylgard 184, Dow) was manually mixed, and degassed for 1 hour in a desiccator to drive out air bubbles trapped in the mixture during the mixing. After pouring the mixture onto master mold and curing the sample 80 oC for 3 hrs, cured and patterned PDMS stamp was peeled off from the master mold. Finally, the PDMS stamp was cut into small (5×5 mm2) size for the following transfer molding step. Results and Conclusions We investigated on the effect of surface morphology of In-doped ZnO based sensors to their sensing performances. After synthesizing IZO QDs by a wet chemical method, we fabricated line patterned samples by stamping IZO solution mold, which were placed on top of PDMS stamps, on Pt electrodes. Further hydrothermal growth developed additional nanocolumns on the surface (Fig. 1). By testing performance of produced sensors under 10 ppm of acetone, we found that hydrothermally grown sensors for 0.5 hrs (G-0.5 hr) exhibited the best sensing response of 26,000 (Fig. 2). Similarly, the sensors also showed the shortest response time and the highest response at 0.1 ppm of acetone among all sensors (Fig. 3). The sensors achieved selectivity for acetone over other gases such as isoprene, NH3, CO, CH4 and H2 (Fig. 4). After close investigation by UV-Vis and XPS analyses, we could figure out that G-0.5hr sensors exhibited the highest the optical band gap energies (3.08 eV) and the highest ratio of O-deficient peaks (69 %). The general trend of sensing performances, optical band gap energies, and oxygen vacancies were all in similar shapes. Based on these results, our proposed mechanism for the detection of acetone was based on the change of electron depletion regions and number of active sites under air and acetone environments. This results demonstrated that fabricating sensing materials with complicated surface nanostructures can enhance performance of sensors dramatically and the potential of our gas sensors to be utilized for exhaled breath tests.