Pd-Functionalized ZnO Nanocube Loaded on WO3 Nanofibers for Exhaled Breath Analysis

Abstract

Semiconductor metal oxide (SMO)-based volatile organic compound sensors are getting much attention due to its future possibilities for portable applications with the low cost fabrication. Increase of surface area and catalytic sensitization should be accompanied to achieve highly sensitive and selective exhaled breath sensors using SMO-based sensing materials. Among the numerous nanomaterials, metal-organic frameworks (MOFs) are strong candidates for breath sensing materials because they have fascinating features such as ultrahigh porosity, and incredibly high surface area. In addition, noble metal nanoparticles (NPs) such as Pt and Pd which can enhance the sensitivity of the breath sensing materials can be encapsulated in the cavities of the MOFs. In this regard, MOFs are suitable materials for SMO-based breath analyzer. In this work, we propose catalyst-functionalized ZnO loaded on WO3 nanofibers (Catalyst-ZnO-WO3 NFs) synthesized by using ZIF-8 templates, which were prepared by solution process at room temperature. Subsequently, catalytic Pd NPs were encapsulated in the cavities of the ZIF-8 by reduction of catalytic metal ions. Electrospinning solution were prepared by dispersing Pd-functionalized ZIF-8 and polyvinylpirrolidone (PVP), and dissolving tungsten precursor [(NH4)6H2W12O40·xH2O] in DI-water solvent. Pd-ZnO-WO3 NFs were finally achieved by electrospinning and following calcination at 500 °C for 1 h. During the calcination, ZIF-8 were completely oxidized to ZnO, and porous WO3 NFs were synthesized by the decomposition of PVP and crystallization of the inorganic precursors. The sensing performances of porous WO3 NFs, ZnO-WO3 NFs, and Pd-ZnO-WO3 NFs were evaluated toward various biomarker gases (toluene, acetone, ethanol, hydrogen sulfide, nitric oxide, and ammonia) in high humid atmosphere. Pd-ZnO-WO3 NFs exhibited a twenty-four times higher toluene response, which is a biomarker for lung cancer, (Rair/Rgas=53.55 at 5 ppm) at 350 °C compared to that of porous WO­3 NFs. In addition, Pd-ZnO-WO3 NFs detected at 100 ppb of toluene with high response (Rair/Rgas=4.37 at 350 °C). Moreover, Pd-ZnO-WO3 NFs improved other sensing characteristics, in terms of response time and selectivity. These results demonstrates that the novel and facile synthesis of highly porous catalyst-loaded heterojunction SMO NFs by using MOF template can be applicable for exhaled breath analysis.