Catalyst-decorated hollow WO3 nanotubes using layer-by-layer self-assembly on polymeric nanofiber templates and their application in exhaled breath sensor

In this work, highly porous WO3 nanotubes (NTs) were synthesized by facile layer-by-layer (LbL) self-assembly on polymeric nanofiber (NF) templates followed by calcination. Polymeric NFs using poly(methyl methacrylate) (PMMA) were prepared by electrospinning as sacrificial templates. Then, ionic polymers were coated on PMMA to modify the surface charge of PMMA NFs. Catalyst-loaded WO3 NTs were synthesized by self-assembly of tungsten precursor and catalytic precursor on the surface of the polymeric PMMA NFs followed by calcination at 500 °C for 1 h. Gas sensing performances were evaluated in highly humid atmosphere (90% RH) using pristine WO3 NTs, Pt-loaded WO3 NTs (Pt-WO3 NTs), and Pd-loaded WO3 NTs (Pd-WO3 NTs). Pristine WO3 NTs exhibited a high NO response (Rgas/Rair = 63.59 at 5 ppm) at 350 °C and cross-selectivity toward toluene (Rair/Rgas = 1.05 at 5 ppm). On the other hand, Pt-WO3 NTs and Pd-WO3 NTs exhibited a high toluene response (Rair/Rgas = 2.24 for the Pt-WO3 NTs and Rair/Rgas = 2.35 for the Pd-WO3 NTs at 5 ppm) at 400 °C and a negligible NO response (Rgas/Rair = 1.25 for the Pt-WO3 NTs and Rgas/Rair = 1.04 for the Pd-WO3 NTs at 5 ppm) at 400 °C. These results demonstrated that LbL synthesis is a highly promising method for producing hollow semiconductor metal oxide NTs functionalized with various catalysts, which leads to potential application in exhaled breath analysis for asthma and lung cancer diagnosis.